Laser module

Abstract

A laser module, includes: an optical fiber; n laser diodes LD.sub.i (i=1, 2, . . . , n) arranged in an order corresponding to a descending order of optical path lengths LO.sub.i of optical paths respectively extending from the laser diodes LD.sub.i to the optical fiber; and n collimating lenses SL.sub.i respectively disposed in the optical paths to be distant from the respective laser diodes LD.sub.i and from the optical fiber. When a distance from each of the laser diodes LD.sub.i to a corresponding one of the collimating lenses SL.sub.i is defined as a collimation length LC.sub.i, at least one of a collimation length LC.sub.1 and a collimation length LC.sub.n differs from a certain distance SL that is set with respect to each of the collimating lenses SL.sub.i.

Claims

1. A laser module, comprising: an optical fiber; n laser diodes LD.sub.i (i=1, 2, . . . , n) arranged in an order corresponding to a descending order of optical path lengths LO.sub.i of optical paths respectively extending from the laser diodes LD.sub.i to the optical fiber, wherein the laser diodes LD.sub.i, respectively emit laser beams LB.sub.i; and n S-axis collimating lenses SL.sub.i respectively disposed in the optical paths to be distant from the respective laser diodes LD.sub.i and from the optical fiber, wherein each of the laser diodes LD.sub.i and each of the S-axis collimating lenses SL.sub.i constitute a part of n unit optical systems which respectively couple the laser beams LB.sub.i and a laser beam propagated through the optical fiber, a collimation length LC.sub.i is defined as a distance from each of the laser diodes LD.sub.i to a corresponding one of the S-axis collimating lenses SL.sub.i, the laser module is configured to have either an odd number or an even number of the unit optical systems, such that: in a case where a total number of the unit optical systems is the odd number, a certain distance SL is defined as a collimation length LC.sub.i of a predetermined unit optical system whose optical path length LO.sub.i is an intermediate value of optical path lengths among the unit optical systems, and in a case where the total number is the even number, a certain distance SL is defined as a collimation length LCi of a predetermined unit optical system whose optical path length LOi is closest to an intermediate value of optical path lengths among the unit optical systems, at least one of a collimation length LC.sub.1 and a collimation length LC.sub.n differs from the certain distance SL, such that the largest one of incident angles β.sub.i, at which rays of a laser beam LB.sub.1 or laser beam LB.sub.n enter the optical fiber, is reduced, the collimation length LC.sub.1 corresponds to a laser diode LD.sub.1 and the collimation length LC.sub.n corresponds to a laser diode LD.sub.n, the certain distance SL is set such that optical paths of rays of a laser beam emitted from each of the laser diodes LD.sub.i included in the predetermined unit optical system are parallel to one another or diverged after the rays have passed through the corresponding one of the S-axis collimating lenses SL.sub.i included in the predetermined unit optical system, an S-axis collimating lens SL.sub.1 is disposed to satisfy SL<LC.sub.1, a laser diode LD.sub.m is defined as a laser diode that is included in a unit optical system whose optical path length LO.sub.i is longer than an optical path length of the predetermined unit optical system and is shorter than the optical path length LO.sub.1, and an S-axis collimating lens SL.sub.j corresponding to a laser diode LD.sub.j (2≤j≤m, 2≤m≤n−1) is disposed to satisfy SL<LC.sub.m<LC.sub.j<LC.sub.1.

2. The laser module as set forth in claim 1, wherein an S-axis collimating lens SL.sub.n is disposed to satisfy LC.sub.n<SL.

3. The laser module as set forth in claim 2, wherein a laser diode LD.sub.1 is defined as a laser diode that is included in a unit optical system whose optical path length LO.sub.i is shorter than an optical path length of the predetermined unit optical system and is longer than the optical path length LO.sub.n, and an S-axis collimating lens SL.sub.k corresponding to a laser diode LD.sub.k (1≤k≤n−1) is disposed to satisfy LCn<LCk≤LC1<SL.

4. The laser module as set forth in claim 1, further comprising: n mirrors M.sub.i that are respectively disposed in the optical paths extending from the laser diodes LD.sub.i to the optical fiber, wherein each of the mirrors M.sub.i is (i) distant from a corresponding one of the laser diodes LD.sub.i and from the optical fiber and (ii) disposed between a corresponding one of the S-axis collimating lenses SL.sub.i and the optical fiber, and each of the mirrors M.sub.i is disposed to bend, at a predetermined angle, a corresponding one of the optical paths extending from the laser diodes LD.sub.i to the optical fiber.

5. The laser module as set forth in claim 4, further comprising: a substrate that has a mounting surface S on which the laser diodes LD.sub.i the S-axis collimating lenses SL.sub.i and the mirrors M.sub.i are mounted, wherein the mounting surface S includes n sub-mounting surfaces SS.sub.i having a step-like configuration in which heights of the sub-mounting surfaces SS.sub.i descend toward the optical fiber, on each of the sub-mounting surfaces SS.sub.i a corresponding one of the laser diodes LD.sub.i a corresponding one of the S-axis collimating lenses SL.sub.i and a corresponding one of the mirrors M.sub.i are mounted.

6. A laser module, comprising: an optical fiber; n laser diodes LD.sub.i (i=1, 2, . . . , n) arranged in an order corresponding to a descending order of optical path lengths LO.sub.i of optical paths respectively extending from the laser diodes LD.sub.i to the optical fiber, wherein the laser diodes LD.sub.i respectively emit laser beams LB.sub.i; and n S-axis collimating lenses SL.sub.i respectively disposed in the optical paths to be distant from the respective laser diodes LD.sub.i and from the optical fiber, wherein each of the laser diodes LD.sub.i and each of the S-axis collimating lenses SL.sub.i constitute a part of n unit optical systems which respectively couple the laser beams LB.sub.i and a laser beam propagated through the optical fiber, a collimation length LC.sub.i is defined as a distance from each of the laser diodes LD.sub.i to a corresponding one of the S-axis collimating lenses SL.sub.i, the laser module is configured to have either an odd number or an even number of the unit optical systems, such that: in a case where a total number of the unit optical systems is the odd number, a certain distance SL is defined as a collimation length LC.sub.i of a predetermined unit optical system whose optical path length LO.sub.i is an intermediate value of optical path lengths among the unit optical systems, and in a case where the total number is the even number, a certain distance SL is defined as a collimation length LCi of a predetermined unit optical system whose optical path length LOi is closest to an intermediate value of optical path lengths among the unit optical systems, at least one of a collimation length LC.sub.1 and a collimation length LC.sub.n differs from the certain distance SL, such that the largest one of incident angles β.sub.i, at which rays of a laser beam LB.sub.1 or laser beam LB.sub.n enter the optical fiber, is reduced, the collimation length LC.sub.1 corresponds to a laser diode LD.sub.1 and the collimation length LC.sub.n corresponds to a laser diode LD.sub.n, the certain distance SL is set such that optical paths of rays of a laser beam emitted from each of the laser diodes LD.sub.i included in the predetermined unit optical system are converged after the rays have passed through the corresponding one of the S-axis collimating lenses SL.sub.i included in the predetermined unit optical system, an S-axis collimating lens SL.sub.n is disposed to satisfy SL<LC.sub.n, a laser diode LD.sub.m is defined as a laser diode that is included in a unit optical system whose optical path length LO.sub.i is longer than an optical path length of the predetermined unit optical system and is shorter than the optical path length LO.sub.n, and an S-axis collimating lens SL.sub.j corresponding to a laser diode LD.sub.j (m≤j≤n−1, 2≤m≤n−1) is disposed to satisfy SL<LC.sub.m≤LC.sub.j<LC.sub.n.

7. The laser module as set forth in claim 6, wherein an S-axis collimating lens SL.sub.1 is disposed to satisfy LC.sub.1<SL.

8. The laser module as set forth in claim 7, wherein a laser diode LD.sub.1 is defined as a laser diode that is included in a unit optical system whose optical path length LO.sub.i is longer than an optical path length of the predetermined unit optical system and is shorter than the optical path length LO.sub.1, and an S-axis collimating lens SL.sub.k corresponding to a laser diode LD.sub.k (2≤k≤1) is disposed to satisfy LC1<LCk≤LC1<SL.

9. The laser module as set forth in claim 6, further comprising: n mirrors M.sub.i that are respectively disposed in the optical paths extending from the laser diodes LD.sub.i to the optical fiber, wherein each of the mirrors M.sub.i is (i) distant from a corresponding one of the laser diodes LD.sub.i and from the optical fiber and (ii) disposed between a corresponding one of the S-axis collimating lenses SL.sub.i and the optical fiber, and each of the mirrors M.sub.i is disposed to bend, at a predetermined angle, a corresponding one of the optical paths extending from the laser diodes LD.sub.i to the optical fiber.

10. The laser module as set forth in claim 9, further comprising: a substrate that has a mounting surface S on which the laser diodes LD.sub.i, the S-axis collimating lenses SL.sub.i, and the mirrors M.sub.i are mounted, wherein the mounting surface S includes n sub-mounting surfaces SS.sub.i having a step-like configuration in which heights of the sub-mounting surfaces SS.sub.i descend toward the optical fiber, on each of the sub-mounting surfaces SS.sub.i a corresponding one of the laser diodes LD.sub.i, a corresponding one of the S-axis collimating lenses SL.sub.i and a corresponding one of the mirrors M.sub.i are mounted.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a perspective view illustrating a laser module of Embodiment 1 of the present invention.

(2) FIG. 2 is a three-view drawing of the laser module illustrated in FIG. 1.

(3) FIG. 3A is a plan view schematically illustrating a unit optical system in a case of n=4 among unit optical systems included in the laser module illustrated in FIG. 1. FIG. 3B is a plan view schematically illustrating a unit optical system in a case of n=1 among unit optical systems included in a laser module of Comparative Example illustrated in FIG. 7. FIG. 3C is a plan view schematically illustrating a unit optical system in a case of n=1 among the unit optical systems included in the laser module illustrated in FIG. 1.

(4) FIG. 4A is a view illustrating an illuminance distribution of each laser beam at an entrance face of a condenser lens which is included in both the laser module illustrated in FIG. 1 and the laser module of Comparative Example illustrated in FIG. 7. FIG. 4B is a view illustrating an angle distribution of each laser beam at an entrance face of an optical fiber included in the laser module of Comparative Example illustrated in FIG. 7. FIG. 4C is a view illustrating an angle distribution of each laser beam at an entrance face of an optical fiber included in the laser module illustrated in FIG. 1.

(5) FIG. 5A is a graph illustrating a correlation between a coupling efficiency and a collimation length of a unit optical system included in a laser module of Example 1 of the present invention. FIG. 5B is a graph illustrating a collimation length for each unit optical system included in the laser module of Example 1 of the present invention.

(6) FIG. 6A is a graph illustrating a correlation between a coupling efficiency and a curvature of an S-axis collimating lens of a unit optical system included in a laser module of Example 2 of the present invention. FIG. 6B is a graph illustrating a curvature for each unit optical system included in the laser module of Example 2 of the present invention.

(7) FIG. 7 is a perspective view of a conventional laser module.

DETAILED DESCRIPTION

Embodiment 1

(8) (Configuration of Laser Module)

(9) The following will discuss a configuration of a laser module 1 in accordance with Embodiment 1 of the present invention, with reference to FIG. 1. FIG. 1 is a perspective view illustrating a laser module 1.

(10) The laser module 1 includes seven laser diodes LD.sub.1 to LD.sub.7, seven F-axis collimating lenses FL.sub.1 to FL.sub.7, seven S-axis collimating lenses SL.sub.i to SL.sub.7, seven mirrors M.sub.1 to M.sub.7, a condenser lens FL, and an optical fiber OF, as illustrated in FIG. 1. The laser diodes LD.sub.1 to LD.sub.7, the F-axis collimating lenses FL.sub.1 to FL.sub.7, the S-axis collimating lenses SL.sub.i to SL.sub.7, the mirrors M.sub.1 to M.sub.7, and the F-axis condenser lens FL are mounted on a bottom plate B of a housing of the laser module 1. The optical fiber OF passes through a side wall of the housing of the laser module 1 such that an end portion including an entrance end surface OF.sub.I of the optical fiber OF extends into the housing of the laser module 1. Note that the side wall of the housing of the laser module 1 is not illustrated in FIG. 1.

(11) The bottom plate B, which corresponds to “substrate” recited in the Claims, is constituted by a pair of main surfaces facing each other and four side surfaces. One of the pair of main surfaces of the bottom plate B facing each other which one is located on a positive side along a z axis in a coordinate system shown in FIG. 1 will be hereinafter referred to as a mounting surface S. The mounting surface S includes at least seven sub-mounting surfaces SS.sub.i. The sub-mounting surfaces SS.sub.i are each a flat surface that extends along (in Embodiment 1, parallel to) an xy plane in the coordinate system shown in FIG. 1. The sub-mounting surfaces SS.sub.i are provided in a step-like configuration such that respective heights of the sub-mounting surfaces SS.sub.i descend toward the entrance end surface OF.sub.I of the optical fiber OF. In other words, the bottom plate B is configured such that (i) the height of a sub-mounting surface SS.sub.i located furthest from the entrance end surface OF.sub.I is the greatest, (ii) the heights of the sub-mounting surfaces SS.sub.i gradually descend toward the entrance end surface OF.sub.I, and (iii) the height of a sub-mounting surface SS.sub.7 located closest to the entrance end surface OF.sub.I is the smallest.

(12) On the sub-mounting surfaces SS.sub.i, respective laser diodes LD.sub.i are mounted. The laser diode LD.sub.i (i=1, 2, . . . , 6, 7) are arranged in this order in accordance with a descending order of an optical path length LO.sub.i, which is a length of an optical path extending from an exit end surface of the laser diode LD.sub.i to the entrance end surface OF.sub.I of the optical fiber OF. That is, a laser diode LD.sub.1 is located furthest from the entrance end surface OF.sub.I and has the longest optical path length LO.sub.1, and a laser diode LD.sub.7 is located closest to the entrance end surface OF.sub.I and has the shortest optical path length LO.sub.1.

(13) On the sub-mounting surface SS.sub.i, not only the laser diode LD.sub.i but also an S-axis collimating lens SL.sub.i, an F-axis collimating lens FL.sub.i, and a mirror M.sub.i, each of which corresponds to the laser diode LD.sub.i, are mounted. That is, the laser diode LD.sub.i, the S-axis collimating lens SL.sub.i, the F-axis collimating lens FL.sub.i, and the mirror M.sub.i are mounted on the sub-mounting surface SS.sub.i. A unit optical system which couples a laser beam LB.sub.i to a laser beam which propagates through the optical fiber OF is constituted by (i) the laser diode LD.sub.i, the S-axis collimating lens SL.sub.i, the F-axis collimating lens FL.sub.i, and the mirror M.sub.i mounted on each single sub-mounting surface SS.sub.i, (ii) the condenser lens FL, and (iii) the optical fiber OF.

(14) The laser diodes LD.sub.i (where i is a natural number; 1≤i≤7) are light sources which respectively emit laser beams LB.sub.1 to LB.sub.7. In Embodiment 1, each laser diode LD.sub.i is a laser diode which is arranged such that in the coordinate system illustrated in FIG. 1, an active layer is parallel to the xy plane and an exit end surface is parallel to a zx plane. A laser beam LB.sub.i emitted from each laser diode LD.sub.i travels in a direction (traveling direction) corresponding to a positive direction of a y axis. The laser beam has a Fast axis (F axis) parallel to the z axis and a Slow axis (S axis) parallel to an x axis. These laser diodes LD.sub.i are mounted on respective sub-mounting surfaces SS.sub.i which differ in height as described above. The laser diodes LD.sub.1 to LD.sub.7 are arranged such that an exit end surface of each laser diode LD.sub.i is aligned on a certain plane parallel to the zx plane. Accordingly, optical axes of the laser beams LB.sub.i extend along (in Embodiment 1, parallel to) the respective sub-mounting surfaces SS.sub.i.

(15) The F-axis collimating lens FL.sub.i is provided in an optical path of each laser beam LB.sub.i. In Embodiment 1, F-axis collimating lenses FL.sub.1 to FL.sub.7 are identical in configuration. In Embodiment 1, the F-axis collimating lens FL.sub.i is a plano-convex cylindrical lens which is arranged such that in the coordinate system shown in FIG. 1, a flat surface (entrance face) faces in a negative direction of the y axis and a curved surface (exit face) faces in the positive direction of the y axis. The F-axis collimating lens FL.sub.i is arranged so as to have an arc-like outer edge of a cross section parallel to a yz plane on a positive side along the y axis. Then, the F-axis collimating lens FL.sub.i collimates the laser beam LB.sub.i diverging in an F-axis direction.

(16) In an optical path of the laser beam LB.sub.i having passed through the F-axis collimating lens FL.sub.i, an S-axis collimating lens SL.sub.i is provided. In Embodiment 1, S-axis collimating lenses SL.sub.1 to SL.sub.7 are identical in configuration. In Embodiment 1, the S-axis collimating lens SL.sub.i is a plano-convex cylindrical lens which is arranged such that in the coordinate system shown in FIG. 1, a flat surface (entrance face) faces in the negative direction of the y axis and a curved surface (exit face) faces in the positive direction of the y axis. The S-axis collimating lens SL.sub.i is provided so as to have an arc-like outer edge of a cross section parallel to the xy plane on a positive side along the y axis. Then, the S-axis collimating lens SL.sub.i collimates the laser beam LB.sub.i diverging in an S-axis direction, which laser beam LB.sub.i has been emitted from the laser diode LD.sub.i.

(17) In an optical path of the laser beam LB.sub.i having passed through the S-axis collimating lens SL.sub.i, the mirror M.sub.i is provided. The mirror M.sub.i has a reflective surface whose normal vector is perpendicular to the z axis and makes an angle of 45° with respect to a positive direction of the x axis and with respect to the negative direction of the y axis. The reflective surface of the mirror M.sub.i reflects the laser beam LB.sub.i so as to convert (bend) the traveling direction of the laser beam LB.sub.i from the positive direction of the y axis to the positive direction of the x axis and also to convert the laser beam LB.sub.i from a state in which the S axis is parallel to the x axis to a state in which the S axis is parallel to the y axis.

(18) These mirrors M.sub.i are arranged such that optical path lengths LM.sub.i from the laser diodes LD.sub.i to respectively corresponding mirrors M.sub.i are identical. Then, respective optical axes of the laser beams LB.sub.i having been reflected by the mirrors M.sub.i are parallel to one another in a plane parallel to the zx plane.

(19) In an optical path of the laser beam LB.sub.i having been reflected by the mirror M.sub.i, the condenser lens FL is provided. In Embodiment 1, the condenser lens FL is a plano-convex lens which is arranged such that in the coordinate system shown in FIG. 1, a curved surface (exit face) faces in a negative direction of the x axis and a flat surface (entrance face) faces in the positive direction of the x axis.

(20) The condenser lens FL is arranged so as to have an arc-like outer edge of a cross section parallel to the xy plane on a negative side along the x axis and also to have an arc-like outer edge of a cross section parallel to the zx plane on a negative side along the x axis. Then, the condenser lens FL (i) condenses the laser beams LB.sub.i, which have been reflected by the mirrors M.sub.i, so that the optical axes of these laser beams LB.sub.i intersect with one another at one point and at the same time, (ii) condenses each of the laser beams LB.sub.i so that a diameter of each of the laser beams LB.sub.i reduces.

(21) At an intersection of the optical axes of the laser beams LB.sub.i having passed through the condenser lens FL, the entrance end surface OF.sub.I of the optical fiber OF is provided. The optical fiber OF is provided such that the entrance end surface OF.sub.I faces in the negative direction of the x axis. The laser beams LB.sub.i having been condensed by the condenser lens FL enter the optical fiber OF via this entrance end surface OF.sub.I. That is, the laser beams LB.sub.i emitted from the laser diodes LD.sub.i are optically coupled to a laser beam which propagates through the optical fiber.

(22) (Arrangement of F-Axis Collimating Lens FL.sub.i and Coupling Efficiency)

(23) A characteristic feature of a laser module in accordance with Embodiment 1 is arrangement of the F-axis collimating lens FL.sub.i. More specifically, a characteristic feature of the laser module 1 in accordance with Embodiment 1 is arrangement of the F-axis collimating lens FL.sub.1 corresponding to the laser diode LD.sub.1. This characteristic feature of the laser module 1 will be described below with reference to FIGS. 2 to 4C.

(24) FIG. 2 is a three-view drawing (a plane view, a front view, and a left side view) of the laser module 1. Among the side surfaces of the laser module 1 illustrated in FIG. 2, (1) a side surface located on a negative side along the y axis and extending along the zx plane is referred to as a front surface of the laser module 1 and (2) a side surface located on a positive side along the x axis and extending along the yz plane is referred to as a left side surface of the laser module 1.

(25) FIG. 3A is a view schematically illustrating a unit optical system in a case of i=4 among unit optical systems included in the laser module 1. FIG. 3B is a view schematically illustrating a unit optical system in a case of i=1 among unit optical systems included in the laser module 101 of Comparative Example illustrated in FIG. 7. FIG. 3C is a plan view schematically illustrating a unit optical system in a case of i=1 among the unit optical systems included in the laser module 1. The unit optical system in the case of i=4 is a unit optical system whose optical path length LO.sub.i has an intermediate value among the unit optical systems included in the laser module 1 and which serves as a reference in designing of the laser module 1. The unit optical system in the case of i=1 is a unit optical system whose optical path length LO.sub.i is the longest among the unit optical systems included in the laser module 1 and which includes features of the laser module 1. As such, in Embodiment 1, arrangement of the F-axis collimating lens FL.sub.i will be described with reference to the unit optical system in the case of i=1 and the unit optical system in the case of i=4.

(26) Note that FIGS. 3A-3C do not contain any F-axis-related information of optical paths of the unit optical systems. Accordingly, FIGS. 3A-3C do not illustrate the F-axis collimating lenses FL.sub.i included in the respective unit optical systems.

(27) A line RR′ shown in each of FIGS. 3A-3C indicates a position of a reflective surface of the mirror M.sub.i of each unit optical system, to simplify the plane view and thus help understand the optical path of the laser beam LB.sub.i. Accordingly, in FIGS. 3A-3C, optical paths for the unit optical systems do not appear bent but unfolded straight.

(28) A line EE′ shown in each of FIGS. 3A-3C indicates a position of the exit end surface of the laser diode LD.sub.i included in each unit optical system. A line II′ shown in each of FIGS. 3A-3C indicates a position of the entrance end surface OF.sub.I of the optical fiber OF. In each of FIGS. 3A-3C, an arrow shown on the line EE′ indicates a spot size SP.sub.E of the laser beam LB.sub.i at the exit end surface of the laser diode LD.sub.i, an arrow shown inside the condenser lens FL indicates a spot size SP.sub.F of the laser beam LB.sub.i at the entrance face of the condenser lens FL, and an arrow shown on the line II′ indicates a spot size SP.sub.I of the laser beam LB.sub.i at the entrance end surface OF.sub.I. The spot size SP.sub.E is in other words a width of a near-field pattern of the laser beam LB.sub.i along an S-axis (the x axis in the coordinate system shown in FIG. 1) and corresponds to “emitter size ES.sub.1” recited in the Claims. In Embodiment 1, the spot size SP.sub.F is a spot size of the laser beam LB.sub.i at the entrance face of the condenser lens FL. Alternatively, the spot size SP.sub.F may be a spot size of the laser beam LB.sub.i at the exit face of the condenser lens FL. Note that the spot size SP.sub.F is also schematically shown in FIG. 2.

(29) FIG. 4A is a view illustrating an illuminance distribution of the laser beam LB.sub.i at the entrance face of the condenser lens FL which is included in both the laser module 1 and the laser module 101 illustrated in FIG. 7. FIG. 4B is a view illustrating an angle distribution of the laser beam LB.sub.i at the entrance face OF.sub.I of the optical fiber OF included in the laser module 101. FIG. 4C is a view illustrating an angle distribution of each laser beam at the entrance face OF.sub.I of the optical fiber OF included in the laser module 1. The laser module 101 is a Comparative Example of the laser module 1. The following description will be given partly with reference to the laser module 101. FIG. 7 is a perspective view of the laser module 101.

(30) The following description will first discuss an issue of the laser module 101 and then explain how the laser module 1 deals with the issue.

(31) As illustrated in FIG. 2, a distance from the laser diode LD.sub.i to the S-axis collimating lens SL.sub.i is defined to be a collimation length LC.sub.i.

(32) In the laser module 101, the S-axis collimating lenses SL.sub.1 to SL.sub.7 are arranged such that each of the collimation lengths LC.sub.1 to LC.sub.7 coincides with a focal length (corresponding to “certain distance SL” recited in the Claims) of the S-axis collimating lens SL.sub.i. This allows rays of each of the laser beams LB.sub.1 to LB.sub.7 emitted from the laser diodes LD.sub.1 to LD.sub.7 to be collimated by the S-axis collimating lenses SL.sub.1 to SL.sub.7 such that respective optical paths of the rays of each of the laser beams LB.sub.1 to LB.sub.7 are parallel to one another.

(33) This will be discussed in more detail with reference to, for example, the unit optical system of i=4 illustrated in FIG. 3A. Rays of a laser beam LB.sub.4, which rays have been emitted from each point of an emitter of a laser diode LD.sub.4 and have a predetermined divergence angle, enter an entrance face of an S-axis collimating lens SL.sub.4. The rays are refracted at the entrance face and a reflective face of the S-axis collimating lens SL.sub.4 at predetermined angles so as to be emitted from the S-axis collimating lens SL.sub.4 via the exit face of the S-axis collimating lens SL.sub.4 in a state where respective optical paths of the rays are parallel to one another. Here, an angle between (i) a propagation direction of each of the rays of the laser beam LB.sub.4 which rays have been emitted from an end portion of the emitter and converted by the S-axis collimating lens SL.sub.4 into parallel rays and (ii) a propagation direction of a chief ray of the laser beam LB.sub.4 is defined to be an angle α. Note that rays whose optical paths spread outward (gradually become distant from one another) as in the case of the rays of the laser beam LB.sub.4 which rays have been emitted from a certain point of the emitter will be hereinafter referred to as divergent rays, whereas rays whose optical paths gather inward (gradually approach one another) will be hereinafter referred to as convergent rays.

(34) The rays of the laser beam LB.sub.4 which rays have been converted by the S-axis collimating lens SL.sub.4 into the parallel rays enter the entrance face of the condenser lens FL, which is distant from the S-axis collimating lens SL.sub.4 by a lens-to-lens distance LL.sub.4. The rays are refracted at the entrance face and a reflective face of the condenser lens FL at predetermined angles and emitted, as convergent rays whose optical paths gather inward, from the condenser lens FL via an exit end surface of the condenser lens FL.

(35) The spot size SP.sub.F at the condenser lens FL increases due to the above-described angle α and in accordance with the lens-to-lens distance LL.sub.i. This is readily understood from the fact that LL.sub.i.Math.tan α increases in accordance with the lens-to-lens distance LL.sub.i. Therefore, the spot size SP.sub.F is the smallest in a case where the lens-to-lens distance LL.sub.i is the smallest (i=7), and the spot size SP.sub.F is the largest in a case where the lens-to-lens distance LL.sub.i is the largest (i=1) (see FIG. 4A).

(36) Note that although the above description refers to the case of i=4, the same description applies also to each case of i=1, 2, 3, 5, 6, 7, except that the spot size SP.sub.F is different between each of those cases and the case of i=4 due to a difference in the lens-to-lens distance LL.sub.i. As described above, the laser beam LB.sub.i emitted from the laser diode LD.sub.i and a laser beam which propagates through the optical fiber OF are optically coupled to each other. As used herein, a “coupling efficiency” refers to a coupling efficiency between the laser beam LB.sub.i emitted from the laser diode LD.sub.i and a laser beam which propagates through the optical fiber OF.

(37) In the laser module 101 having the configuration described above, the spot size SP.sub.F is the smallest in a case where the lens-to-lens distance LL.sub.i is the smallest (i=7), and is the greatest in a case where the lens-to-lens distance LL.sub.i is the greatest (i=1). Accordingly, when the largest one of incident angles at which the rays of the laser beam LB.sub.i respectively enter the entrance face OF.sub.I is defined to be an angle β.sub.i, an angle β.sub.1 is the largest among angles β.sub.1 to β.sub.7.

(38) A coupling efficiency between the laser beam LB.sub.i and a laser beam which propagates through the optical fiber OF tends to decrease as an incident angle at which the laser beam LB.sub.i enters the entrance face OF.sub.I increases. Accordingly, when respective coupling efficiencies of the laser beams LB.sub.1 to LB.sub.7 are compared, the coupling efficiency of each of the laser beams LB.sub.1 to LB.sub.3 tends to be lower than that of the laser beam LB.sub.4 (the coupling efficiency which serves as a reference in designing of the laser module 1). Further, when the coupling efficiencies of the laser beams LB.sub.1 to LB.sub.3 are compared, the laser beam LB.sub.1 has the lowest coupling efficiency, followed by the laser beam LB.sub.2 and then by the laser beam LB.sub.3.

(39) The laser module 1 is therefore configured, with particular attention to the laser beam LB.sub.1 having the lowest coupling efficiency in the laser module 101, such that a position of the S-axis collimating lens SL.sub.1 is shifted toward a positive side along the y axis so that the collimation length LC.sub.1 of the S-axis collimating lens SL.sub.1 is longer than the focal length of the S-axis collimating lens SL.sub.i (see FIG. 2).

(40) That is, the unit optical system in the case of i=1 in the laser module 1 is configured such that, as illustrated in FIG. 3C, a distance between the position of the S-axis collimating lens SL.sub.1 and the exit end surface of the laser diode LD.sub.1 is greater than that of the unit optical system in the case of i=1 in the laser module 101 illustrated in FIG. 3B.

(41) As such, after rays of the laser beam LB.sub.1 emitted from the laser diode LD.sub.1 are refracted by the S-axis collimating lens SL.sub.1, the rays are not parallel rays but convergent rays that make an angle α with a propagation direction of a chief ray of the laser beam LB.sub.1. Accordingly, the unit optical system in the case of i=1 in the laser module 1 has a spot size SP.sub.F smaller than that of the unit optical system included in the case of i=1 in the laser module 101. That is, the angle β.sub.i of the unit optical system in the case of i=1 in the laser module 1 is smaller than that of the unit optical system in the case of i=1 in the laser module 101. This is readily understood from a comparison between FIGS. 4B and 4C.

(42) Therefore, the laser module 1 having the above-described configuration achieves an improved coupling efficiency as compared with the laser module 101 in which the collimation lengths LC.sub.1 to LC.sub.7 uniformly coincide with the focal length of the S-axis collimating lens SL.sub.i.

(43) (Other Configurations)

(44) The above description of Embodiment 1 has explained that arranging the S-axis collimating lens SL.sub.1 so as to satisfy SL<LC.sub.1 enables improving the coupling efficiency, in a configuration in which the certain distance SL coincides with the focal length of the S-axis collimating lens SL.sub.i (i.e., in a configuration in which rays of a laser beam become parallel rays after passing through the S-axis collimating lens).

(45) Note that the laser module 1 may be configured such that the certain distance SL is set to be shorter than the focal length of the S-axis collimating lens SL.sub.i (i.e., such that rays of a laser beam become divergent rays after passing through the S-axis collimating lens). Also in this case, arranging the S-axis collimating lens SL.sub.1 so as to satisfy SL<LC.sub.1 enables improving the coupling efficiency.

(46) Further, the above description of Embodiment 1 has discussed a case in which only the position of the S-axis collimating lens SL.sub.1 is set to satisfy SL<LC.sub.1. However, a laser module may be configured such that, when a laser diode that is neither the laser diode LD.sub.1 nor the laser diode LD.sub.n (e.g., the laser diode LD.sub.7) is defined to be a laser diode LD.sub.m, an S-axis collimating lens SL.sub.j corresponding to each laser diode LD.sub.i (2≤j≤m, 2≤m≤n−1) is arranged so as to satisfy SL<LC.sub.m≤LC.sub.j<LC.sub.1. For example, in a case of m=3, the S-axis collimating lenses SL.sub.1 to SL.sub.3 may be arranged so as to satisfy SL<LC.sub.3<LC.sub.2<LC.sub.1.

(47) With this configuration, not only the laser beam LB.sub.1 but also the laser beams LB.sub.2 to LB.sub.m can achieve a reduction in the angle β.sub.i, which is a maximum value of an incident angle of a laser beam. This enables improving the coupling efficiency not only of the laser beam LB.sub.1 but also of the laser beams LB.sub.2 to LB.sub.m.

(48) Further, a laser module may be configured such that the certain distance SL is set such that (i) respective optical paths of rays of a laser beam emitted from each of the laser diodes LD.sub.i are parallel to one another or diverged after the rays have passed through a corresponding one of the collimating lenses SL.sub.i and (ii) at least an S-axis collimating lens SL.sub.n (e.g., the S-axis collimating lens SL.sub.7) is arranged so as to satisfy LC.sub.n<SL (e.g., LC.sub.7<SL).

(49) The angle β.sub.i of the laser beam LB.sub.i and the spot size of the laser beam LB.sub.i are in a trade-off relationship. Thus, the angle β.sub.i and the spot size of the laser beam LB.sub.i cannot be reduced to below their limits at the same time. As such, an excessively small angle β.sub.i results in an excessively large spot size of the laser beam, which may cause a decrease in the coupling efficiency of the laser beam LB.sub.i.

(50) The above-described configuration enables preventing the angle β.sub.i of the laser beam LB.sub.i from becoming too small and thus preventing a decrease in the coupling efficiency of the laser beam LB.sub.i. Note that the angle β.sub.i of the laser beam LB.sub.i can be adjusted by adjusting a curvature of the condenser lens FL. This adjusting technique, however, may excessively reduce an angle β.sub.i of one of the laser beams LB.sub.i (e.g., an angle β.sub.n of a laser beam LB.sub.n) since the laser module 1 includes a single condenser lens FL which collectively condenses the n laser beams LB.sub.i. Such an issue is prevented by employing the above-described configuration which reduces a difference between incident angles of the laser beams LB.sub.i (approximating the incident angles to one another) to thereby achieve the angle β.sub.i and the spot size SP.sub.I that are both suitable for preventing a decrease in the coupling efficiency.

(51) Further, a laser module may be configured such that (i) the certain distance SL is set such that rays of the laser beam LB.sub.i emitted from the laser diode LD.sub.i become convergent rays after passing through the S-axis collimating lens SL.sub.i and (ii) at least the S-axis collimating lens SL.sub.n is arranged so as to satisfy SL<LC.sub.n.

(52) There can be a case in which the certain distance SL is set such that rays of the laser beam LB.sub.i becomes convergent rays after passing through the S-axis collimating lens SL.sub.i. In such a case where the certain distance SL is set so as to cause rays of the laser beam LB.sub.i to become convergent rays, the spot size SP.sub.F of the laser beam LB.sub.i at the condenser lens FL after the laser beam LB.sub.i has passed through the S-axis collimating lens SL.sub.i may decrease as the lens-to-lens distance LL.sub.i increases.

(53) The laser diode LD.sub.n is arranged so as to have a lens-to-lens distance LL.sub.n (an optical path length LOU) shorter than a lens-to-lens distance of each of the other laser diodes LD.sub.1 to LD.sub.n-1. As such, in a case where the certain distance SL is set so as to cause rays of the laser beam LB.sub.i to become convergent rays, a laser beam emitted from the laser diode LD.sub.n tends to have a spot size (i.e., an incident angle) larger than that of a laser beam emitted from each of the laser diodes other than the laser diode LD.sub.n.

(54) The above configuration enables reducing an incident angle (i.e., the angle β.sub.i) at which the laser beam LB.sub.n enters the entrance face OF.sub.I, in a case where the certain distance SL is set so as to cause rays of the laser beam LB.sub.i to become convergent rays. This enables reducing a difference between respective incident angles at which the laser beams LB.sub.i, which differ in the optical path length LO.sub.i, enter the entrance face OF.sub.I.

(55) Further, the laser module in which the certain distance SL is set so as to cause rays of the laser beam LB.sub.i to become convergent rays may be configured such that when a laser diode that is neither the laser diode LD.sub.1 nor the laser diode LD.sub.n is defined to be a laser diode LD.sub.m, an S-axis collimating lens SL.sub.j corresponding to each laser diode LD.sub.j (m≤j≤n−1, 2≤m≤n−1) is arranged so as to satisfy SL<LC.sub.m≤LC.sub.j<LC.sub.n. For example, in a case of m=5, the S-axis collimating lenses SL.sub.5 to SL.sub.7 may be arranged so as to satisfy SL<LC.sub.5<LC.sub.6<LC.sub.7.

(56) With this configuration, not only the laser beam LB.sub.n but also the laser beams LB.sub.m to LB.sub.n-1 can achieve a reduction in the incident angle (i.e., the angle β.sub.i) at which a laser beam enters the entrance face OF.sub.I. This enables improving the coupling efficiency not only of the laser beam LB.sub.n but also of the laser beams LB.sub.m to LB.sub.n-1.

(57) Further, the laser module in which the certain distance SL is set so as to cause rays of the laser beam LB.sub.i to become convergent rays may be configured such that at least the S-axis collimating lens SL.sub.1 is arranged so as to satisfy LC.sub.1<SL.

(58) The laser diode LD.sub.1 is arranged such that the optical path length LO.sub.1 is longer than the optical path length of each of the other laser diodes LD.sub.2 to LD.sub.n. Accordingly, with a configuration in which rays of the laser beam LB.sub.i become convergent rays after passing through the S-axis collimating lens SL.sub.i, a laser beam emitted from the laser diode LD.sub.1 tends to have an angle β.sub.i smaller than that of a laser beam emitted from each of the laser diodes other than the laser diode LD.sub.1.

(59) The above-described configuration prevents the angle β.sub.1 of the laser beam LB.sub.1 from becoming too small and thus prevents a decrease in the coupling efficiency of the laser beam LB.sub.1. Note that the angle β.sub.i of the laser beam LB.sub.i can be adjusted by adjusting a curvature of the condenser lens FL. This adjusting technique, however, may excessively reduce an angle β.sub.i of one of the laser beams LB.sub.i (e.g., the angle β.sub.i of the laser beam LB.sub.1) since the laser module 1 includes a single condenser lens FL which collectively condenses the n laser beams LB.sub.i. Such an issue is prevented by employing the above-described configuration which reduces a difference between incident angles of the laser beams LB.sub.i (approximating the incident angles to one another) to thereby achieve the angle β.sub.i and the spot size SP.sub.I that are both suitable for preventing a decrease in the coupling efficiency.

(60) Further, as described above, the laser module 1 may include the n mirrors M.sub.i each arranged such that a corresponding one of the optical paths of the laser beams LB.sub.i is bent at a predetermined angle (in Embodiment 1, 90°).

(61) With this configuration, an optical path from each laser diode LD.sub.i to the optical fiber OF is bent by the mirror M.sub.i at a predetermined angle. This enables reducing a length of the laser module 1. Note that the “length” of the laser module 1 refers to one of dimensions of the laser module which one is measured along a direction (the x axis direction in the coordinate system shown in FIG. 1) parallel to a central axis of the optical fiber OF.

(62) Further, as described above, the laser module 1 may be configured such that the laser module 1 further includes a substrate having a mounting surface S on which the laser diodes LD.sub.i, the S-axis collimating lenses SL.sub.i, and the mirrors M.sub.i are mounted, the mounting surface S including at least n sub-mounting surfaces SS.sub.i provided in a step-like configuration such that respective heights of the sub-mounting surfaces SS.sub.i descend toward the optical fiber, on each of the sub-mounting surfaces SS.sub.i, a corresponding one of the laser diodes LD.sub.i, a corresponding one of the S-axis collimating lenses SL.sub.i, and a corresponding one of the mirrors M.sub.i being mounted.

(63) This configuration, in which each laser diode LD.sub.i, each S-axis collimating lens SL.sub.i, and each mirror M.sub.i are mounted on a corresponding one of the sub-mounting surfaces SS.sub.i which differ in height, allows laser beams LB.sub.i, which have been emitted from the respective laser diodes LD.sub.i and passed through the respective S-axis collimating lenses SL.sub.i and whose optical paths have been bent by the respective mirrors M.sub.i, to be at respective different heights.

(64) Further, a laser module may be configured such that when a curvature of each of the S-axis collimating lenses SL.sub.i is defined to be a curvature r.sub.i, at least one of a curvature r.sub.1 of an S-axis collimating lens SL.sub.1 corresponding to a laser diode LD.sub.1 and a curvature r.sub.n of an S-axis collimating lens SL.sub.n corresponding to a laser diode LD.sub.n differs from a curvature of each of the other ones of the collimating lenses SL.sub.i.

(65) A laser module may have a configuration modified from the configuration of the laser module 1 such that one of the curvature r.sub.1 and the curvature r.sub.n differs from a curvature of each of the other ones of the collimating lenses SL.sub.i, unlike in the laser module 1 in which one of the collimation length LC.sub.1 and the collimation length LC.sub.n differs from the certain distance SL. This configuration allows the laser module to provide the same effect as the laser module 1 in which one of the collimation length LC.sub.1 and the collimation length LC.sub.n differs from the certain distance SL.

(66) Further, the above laser module may be configured such that at least the curvature r.sub.1 is smaller than the curvature of each of the other ones of the collimating lenses SL.sub.i.

(67) With this configuration, in a case where rays of each of the laser beams LB.sub.i become parallel rays or divergent rays after passing through a corresponding one of the S-axis collimating lenses SL.sub.i, it is possible to provide the same effect as a configuration in which at least the S-axis collimating lens SL.sub.1 are arranged so as to satisfy SL<LC.sub.1.

(68) Further, a laser module may be configured such that when a size of an emitter included in each of the laser diodes LD.sub.i is defined to be an emitter size ES.sub.i, at least one of an emitter size ES.sub.1 and an emitter size ES.sub.n differs from an emitter size of each of the other ones of the laser diodes LD.sub.i.

(69) A laser module may have a configuration modified from the configuration of the laser module 1 such that one of the emitter size ES.sub.1 and the emitter size ES.sub.n differs from a emitter size of each of the other ones of the laser diodes LD.sub.i, unlike in the laser module 1 in which one of the collimation length LC.sub.1 and the collimation length LC.sub.n differs from the certain distance SL. This configuration allows the laser module to provide the same effect as the laser module 1 in which one of the collimation length LC.sub.1 and the collimation length LC.sub.n differs from the certain distance SL.

(70) Further, a laser module may be arranged such that at least the emitter size ES.sub.1 is smaller than the emitter size of each of the other ones of the laser diodes LD.sub.i.

(71) This configuration allows the laser module to provide the same effect as the laser module 1 in which at least the S-axis collimating lens SL.sub.1 is arranged so as to satisfy SL<LC.sub.1, in a case where rays of each of the laser beams LB.sub.i become parallel rays or divergent rays after passing through a corresponding one of the S-axis collimating lenses SL.sub.i.

Example 1

(72) The following description will discuss a laser module 1 in accordance with Example 1 of the present invention with reference to FIGS. 5A-5B. The laser module 1 of Example 1 was obtained by modifying the configuration of the laser module 1 illustrated in FIG. 1 such that the number of the unit optical systems is increased from n=7 to n=13.

(73) FIG. 5A is a graph illustrating a correlation between a coupling efficiency and a collimation length LC.sub.i of a unit optical system included in the laser module 1 of Example 1. FIG. 5B is a graph illustrating a collimation length LC.sub.i for each unit optical system included in the laser module 1 of Example 1.

(74) As FIG. 5A indicates, causing each collimation length LC.sub.i (i=1, 3, 7, 10, 13) to change within a range of 9 mm to 10.5 mm causes a change in the coupling efficiency of a laser beam LB.sub.i. From results illustrated in FIG. 5A, a collimation length LC.sub.i for each case of i (i=1, 3, 7, 10, 13) was taken and plotted with respect to i as illustrated in FIG. 5B. In FIG. 5B, a line obtained by fitting those collimation lengths LC.sub.i by the least-square method is indicated with a dotted line.

(75) As FIG. 5B indicates, an increase in coupling efficiency is achieved by setting the collimation length LC.sub.i to be shorter as the value of i increases and to be longer as the value of i decreases, with respect to a collimation length LC.sub.7 (=9.9 mm) for i=7 as a reference value, which collimation length LC.sub.7 served as a reference in designing of the laser module 1.

(76) Note that the above description of Embodiment 1 discussed a case in which a position of a part of S-axis collimating lenses SL.sub.i differs from a certain distance SL. Alternatively, a laser module 1 may employ a configuration in which positions of all of the S-axis collimating lenses SL.sub.i differ from the certain distance SL. In such a case, the respective collimation lengths LC.sub.i may satisfy LC.sub.1>LC.sub.2> . . . >LC.sub.i> . . . >LC.sub.12>LC.sub.13.

Example 2

(77) The following description will discuss a laser module in accordance with Example 2 of the present invention with reference to FIGS. 6A-6B. The laser module 1 of Example 2 was obtained by modifying the configuration of the laser module 101 illustrated in FIG. 7 such that the number of the unit optical systems is increased from n=7 to n=13 and the curvature r.sub.i of the S-axis collimating lens SL.sub.i is changed.

(78) FIG. 6A is a graph illustrating a correlation between a coupling efficiency and a curvature r.sub.i of a unit optical system included in the laser module of Example 2. FIG. 6B is a graph illustrating a curvature r.sub.i for each unit optical system included in the laser module 1 of Example 2.

(79) As FIG. 6A indicates, causing each curvature r.sub.i (i=1, 3, 7, 10, 13) to change within a range of 6.2 mm to 6.9 mm causes a change in the coupling efficiency of a laser beam LB.sub.i. From results illustrated in FIG. 6A, a curvature r.sub.i for each case of i (i=1, 3, 7, 10, 13) was taken and plotted with respect to i as illustrated in FIG. 6B. In FIG. 6B, a line obtained by fitting those curvatures r.sub.i by the least-square method is indicated with a dotted line.

(80) As FIG. 6B indicates, an increase in coupling efficiency is achieved by setting the curvature r.sub.i to be longer as the value of i increases and to be shorter as the value of i decreases, with respect to a curvature r.sub.i (=6.7 mm) for i=7 as a reference value, which curvature r.sub.i served as a reference in designing of the laser module 1.

(81) Note that the above description of Embodiment 2 discussed a case in which the curvature r.sub.i of a part of the S-axis collimating lenses SL.sub.i is different from a certain distance SL. Alternatively, a laser module 1 may employ a configuration in which the curvatures r.sub.i of all of the S-axis collimating lenses SL.sub.i are different from one another. In such a case, the respective curvatures r.sub.i may satisfy r.sub.1<r.sub.2< . . . <r.sub.i< . . . <r.sub.12<r.sub.13.

(82) One or more embodiments of the present invention can also be expressed as follows:

(83) A laser module 1 is a laser module, including: an optical fiber OF; n laser diodes LD.sub.i (i=1, 2, . . . , n) arranged in this order in accordance with a descending order of an optical path length LO.sub.i, the optical path length LO.sub.i being a length of an optical path extending from each of the laser diodes LD.sub.i to the optical fiber OF; and n collimating lenses SL.sub.i provided in the respective optical paths so as to be distant from the respective laser diodes LD.sub.i and from the optical fiber OF. In the laser module 1, when a distance from each of the laser diodes LD.sub.i to a corresponding one of the collimating lenses SL.sub.i is defined to be a collimation length LC.sub.i, at least one of a collimation length LC.sub.1 corresponding to a laser diode LD.sub.1 and a collimation length LC.sub.n corresponding to a laser diode LD.sub.n differs from a certain distance SL which is set with respect to each of the collimating lenses SL.sub.i.

(84) The laser module is configured such that laser beams which have passed through the respective collimating lenses SL.sub.i after being emitted from the respective laser diodes LD.sub.i are collectively caused to enter the optical fiber. Each of the laser diodes LD.sub.i includes an emitter which has a certain, though small, size. Due to this size, rays of each of the laser beams enter the optical fiber at respective incident angles in a certain wide range within a predetermined angular range including a right angle. This certain width of the incident angle can decrease a coupling efficiency between the laser beam emitted from each of the laser diodes LD.sub.i and a laser beam which propagates through the optical fiber.

(85) In the laser module, at least one of the collimation length LC.sub.1 corresponding to the laser diode LD.sub.1 and the collimation length LC.sub.n corresponding to the laser diode LD.sub.n differs from the certain distance SL. This configuration enables reducing at least one of an incident angle at which a laser beam emitted from the laser diode LD.sub.1 enters the optical fiber and an incident angle at which a laser beam emitted from the laser diode LD.sub.n enters the optical fiber. In other words, the above configuration enables narrowing the predetermined angular range including a right angle. Therefore, the laser module achieves an improved coupling efficiency between laser beams emitted from the laser diodes and a laser beam which propagates through the optical fiber as compared with conventional laser modules.

(86) Further, a laser module 1 may be configured such that: the certain distance SL is set such that respective optical paths of rays of a laser beam emitted from each of the laser diodes LD.sub.i are parallel to one another or diverged after the rays have passed through a corresponding one of the collimating lenses SL.sub.i; and at least a collimating lens SL.sub.1 is arranged so as to satisfy SL<LC.sub.1.

(87) In many cases, the certain distance SL is set such that respective optical paths of rays of a laser beam emitted from each of the laser diodes LD.sub.i are parallel to one another after the rays have passed through a corresponding one of the collimating lenses SL.sub.i. There can be also a case in which the certain distance SL is such that the optical paths are diverged.

(88) The laser diode LD.sub.1 is arranged such that the optical path length LO.sub.1 is longer than the optical path length of each of the other laser diodes LD.sub.2 to LD.sub.n. Accordingly, in a case where the certain distance SL is set so as to cause the optical paths to be parallel to one another or diverged, a laser beam emitted from the laser diode LD.sub.1 has an incident angle greater than that of a laser beam emitted from each of the laser diodes other than the laser diode LD.sub.1.

(89) The above configuration enables reducing an incident angle of a laser beam emitted from the laser diode LD.sub.1, in a case where the certain distance SL is set so as to cause the optical paths to be parallel or diverged.

(90) Further, a laser module 1 may be configured such that when a laser diode that is neither the laser diode LD.sub.1 nor the laser diode LD.sub.n is defined to be a laser diode LD.sub.m, a collimating lens SL.sub.j (2≤j≤m) corresponding to each laser diode LD.sub.j is arranged so as to satisfy SL<LC.sub.m≤LC.sub.j<LC.sub.1.

(91) In the above configuration, not only the collimating lens SL.sub.1 but also the collimating lenses SL.sub.2 to SL.sub.m corresponding to the respective laser diodes LD.sub.2 to LD.sub.m are arranged such that the collimation lengths LC.sub.2 to LC.sub.m are each longer than the certain distance SL. Also, the collimation lengths LC.sub.2 to LC.sub.m are set to decrease in this order. This configuration enables reducing an incident angle of not only a laser beam emitted from the laser diode LD.sub.1 but also an incident angle of each of laser beams emitted from the respective laser diodes LD.sub.2 to LD.sub.m.

(92) Further, a laser module 1 may be configured such that: the certain distance SL is set such that respective optical paths of rays of a laser beam emitted from each of the laser diodes LD.sub.i are parallel to one another or diverged after the rays have passed through a corresponding one of the collimating lenses SL.sub.i; and at least a collimating lens SL.sub.n is arranged so as to satisfy LC.sub.n<SL.

(93) The laser diode LD.sub.n is arranged so as to have an optical path length LO.sub.n shorter than an optical path length of each of the other laser diodes LD.sub.1 to LD.sub.n-1. As such, in a configuration in which respective optical paths of rays of each laser beam are parallel to one another after the rays have passed through a corresponding one of the collimating lenses SL.sub.i, a laser beam emitted from the laser diode LD.sub.n tends to have an incident angle smaller than that of a laser beam emitted from each of the laser diodes other than the laser diode LD.sub.n.

(94) Note here that an incident angle of a laser beam (i.e., a numerical aperture of the laser beam) and a spot size of the laser beam are in such a relationship that reducing one of the incident angle and the spot size inevitably results in increasing the other. As such, an excessively small incident angle of a laser beam emitted from the laser diode LD.sub.n results in an excessively large spot size of the laser beam, which may cause a decrease in a coupling efficiency between the laser beam emitted from the laser diode LD.sub.n and a laser beam which propagates through the optical fiber.

(95) The above-described configuration enables preventing the incident angle of the laser beam emitted from the laser diode LD.sub.n from becoming too small.

(96) Further, a laser module 1 may be configured such that: the certain distance SL is set such that respective optical paths of rays of a laser beam emitted from each of the laser diodes LD.sub.i are converged after the rays have passed through a corresponding one of the collimating lenses SL.sub.i; and at least a collimating lens SL.sub.n is arranged so as to satisfy SL<LC.sub.n.

(97) There can be a case in which the certain distance SL is set such that respective optical paths of rays of a laser beam emitted from each of the laser diodes LD.sub.i are converged after the rays have passed through a corresponding one of the collimating lenses SL.sub.i. In such a case where the certain distance SL is set so as to cause the optical paths to be converged, the spot size of the laser beam having passed through the S-axis collimating lens SL.sub.i may decrease as a distance from the collimating lens SL.sub.i increases.

(98) The laser diode LD.sub.n is arranged so as to have an optical path length LO.sub.n shorter than an optical path length of each of the other laser diodes LD.sub.1 to LD.sub.n-1. As such, in a case where the certain distance SL is set so as to cause the optical paths to be converged, a laser beam emitted from the laser diode LD.sub.n tends to have a spot size larger than that of a laser beam emitted from each of the laser diodes other than the laser diode LD.sub.n.

(99) The above configuration enables reducing an incident angle of a laser beam emitted from the laser diode LD.sub.n, in a case where the certain distance SL is set so as to cause the optical paths to be converged.

(100) Further, a laser module 1 may be configured such that: when a laser diode that is neither the laser diode LD.sub.1 nor the laser diode LD.sub.n is defined to be a laser diode LD.sub.m, a collimating lens SL.sub.j (m≤j≤n−1) corresponding to each laser diode LD.sub.j is arranged so as to satisfy SL<LC.sub.m≤LC.sub.j<LC.sub.n.

(101) In the above configuration, not only the collimating lens SL.sub.n but also the collimating lenses SL.sub.m to SL.sub.n-1 corresponding to the respective laser diodes LD.sub.m to LD.sub.n-1 are arranged such that the collimation lengths LC.sub.m to LC.sub.n-1 are each longer than the certain distance SL. Also, the collimation lengths LC.sub.m to LC.sub.n-1 are set to decrease in this order. This configuration enables reducing an incident angle not only of a laser beam emitted from the laser diode LD.sub.n but also of each of laser beams emitted from the respective laser diodes LD.sub.m to LD.sub.n-1.

(102) Further, a laser module 1 may be configured such that: the certain distance SL is set such that respective optical paths of rays of a laser beam emitted from each of the laser diodes LD.sub.i are converged after the rays have passed through a corresponding one of the collimating lenses SL.sub.i; and

(103) at least a collimating lens SL.sub.i is arranged so as to satisfy LC.sub.1<SL.

(104) The laser diode LD.sub.1 is arranged so as to have an optical path length LO.sub.1 longer than an optical path length of each of the other laser diodes LD.sub.2 to LD.sub.n. As such, in a configuration in which respective optical paths of rays of each laser beam are converged after the rays have passed through a corresponding one of the collimating lenses SL.sub.i, a laser beam emitted from the laser diode LD.sub.1 tends to have an incident angle smaller than that of a laser beam emitted from each of the laser diodes other than the laser diode LD.sub.1.

(105) As such, an excessively small incident angle as described above may cause a decrease in a coupling efficiency between the laser beam emitted from the laser diode LD.sub.1 and a laser beam which propagates through the optical fiber. The above-described configuration enables preventing an excessively small incident angle of a laser beam emitted from the laser diode LD.sub.1.

(106) Further, a laser module 1 may be configured such that the laser module 1 further includes n mirrors M.sub.i each provided in a corresponding one of the optical paths extending from the respective laser diodes LD.sub.i to the optical fiber, so as to be (i) distant from a corresponding one of the laser diodes LD.sub.i and from the optical fiber and (ii) between a corresponding one of the collimating lenses SL.sub.i and the optical fiber, each of the mirrors M.sub.i being arranged such that a corresponding one of the optical paths extending from the laser diodes LD.sub.i to the optical fiber is bent at a predetermined angle.

(107) With the above configuration, an optical path from each laser diode LD.sub.i to the optical fiber is bent by the mirror M.sub.i at a predetermined angle. This enables reducing a length of the laser module. Note that the “length” of the laser module refers to one of dimensions of the laser module which one is measured along a direction parallel to a central axis of the optical fiber.

(108) Further, a laser module 1 may be configured such that the laser module 1 further includes a substrate B having a mounting surface S on which the laser diodes LD.sub.i, the collimating lenses SL.sub.i, and the mirrors M.sub.i are mounted, the mounting surface S including at least n sub-mounting surfaces SS.sub.i provided in a step-like configuration such that respective heights of the sub-mounting surfaces SS.sub.i descend toward the optical fiber OF, on each of the sub-mounting surfaces SS.sub.i, a corresponding one of the laser diodes LD.sub.i, a corresponding one of the collimating lenses SL.sub.i, and a corresponding one of the mirrors M.sub.i being mounted.

(109) This configuration, in which each laser diode LD.sub.i, each collimating lens SL.sub.i, and each mirror M.sub.i are mounted on a corresponding one of the sub-mounting surfaces SS.sub.i which differ in height, allows laser beams, which have been emitted from the respective laser diodes LD.sub.i and passed through the respective collimating lenses SL.sub.i and whose optical paths have been bent by the respective mirrors M.sub.i, to be at respective different heights.

(110) A laser module 1 is a laser module, including: an optical fiber OF; n laser diodes LD.sub.i (i=1, 2, . . . , n) arranged in this order in accordance with a descending order of an optical path length LO.sub.i, the optical path length LO.sub.i being a length of an optical path extending from each of the laser diodes LD.sub.i to the optical fiber OF; and n collimating lenses SL.sub.i provided in the respective optical paths so as to be distant from the respective laser diodes LD.sub.i and from the optical fiber. In the laser module 1, when a curvature of each of the collimating lenses SL.sub.i is defined to be a curvature r.sub.i, at least one of a curvature r.sub.i of a collimating lens SL.sub.i corresponding to a laser diode LD.sub.1 and a curvature r.sub.n of a collimating lens SL.sub.n corresponding to a laser diode LD.sub.n differs from a curvature of each of the other ones of the collimating lenses SL.sub.i.

(111) A laser module 1 may have a configuration in which one of the curvature r.sub.1 and the curvature r.sub.n differs from a curvature of each of the other ones of the collimating lenses SL.sub.i, instead of a configuration in which one of the collimation length LC.sub.1 and the collimation length LC.sub.n differs from the certain distance SL. This configuration provides the same effect as a configuration in which one of the collimation length LC.sub.1 and the collimation length LC.sub.n differs from the certain distance SL.

(112) Further, a laser module 1 may be configured such that at least the curvature r.sub.1 is smaller than the curvature of each of the other ones of the collimating lenses SL.sub.i.

(113) With this configuration, in a case where respective optical paths of rays of each laser beam become parallel to one another or diverged after the rays have passed through a corresponding one of the S-axis collimating lenses SL.sub.i, it is possible to provide the same effect as a configuration in which at least the S-axis collimating lens SL.sub.1 is arranged so as to satisfy SL<LC.sub.1.

(114) A laser module 1 is a laser module, including: an optical fiber OF; n laser diodes LD.sub.i (i=1, 2, . . . , n) arranged in this order in accordance with a descending order of an optical path length LO.sub.i, the optical path length LO.sub.i being a length of an optical path extending from each of the laser diodes LD.sub.i to the optical fiber OF; and n collimating lenses SL.sub.i provided in the respective optical paths so as to be distant from the respective laser diodes LD.sub.i and from the optical fiber. In the laser module 1, when a size of an emitter included in each of the laser diodes LD.sub.i is defined to be an emitter size ES.sub.i, at least one of an emitter size ES.sub.1 and an emitter size ES.sub.n differs from an emitter size of each of the other ones of the laser diodes LD.sub.i.

(115) A laser module 1 may have a configuration in which one of the emitter size ES.sub.1 and the emitter size ES.sub.n differs from a emitter size of each of the other ones of the laser diodes LD.sub.i, instead of a configuration in which one of the collimation length LC.sub.1 and the collimation length LC.sub.n differs from the certain distance SL. This configuration provides the laser module to provide the same effect as the laser module in which one of the collimation length LC.sub.1 and the collimation length LC.sub.n differs from the certain distance SL.

(116) Further, a laser module 1 may be configured such that at least the emitter size ES.sub.1 is smaller than the emitter size of each of the other ones of the laser diodes LD.sub.i.

(117) With this configuration, in a case where respective optical paths of rays of each laser beam become parallel to one another or diverged after the rays have passed through a corresponding one of the S-axis collimating lenses SL.sub.i, it is possible to provide the same effect as a configuration in which at least the S-axis collimating lens SL.sub.1 is arranged so as to satisfy SL<LC.sub.1.

(118) Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims.

REFERENCE SIGNS LIST

(119) 1 laser module LD.sub.i laser diode FL.sub.i F-axis collimating lens SL.sub.i S-axis collimating lens M.sub.i mirror FL condenser lens OF optical fiber B bottom plate (substrate) S mounting surface SS.sub.i sub-mounting surface