FIBER-COUPLED DIODE LASER MODULE AND METHOD OF ITS ASSEMBLING

Abstract

A pigtailed diode laser module is configured with a case housing a plurality of multimode chips which are arranged in at least one row and output respective beams in one direction. Each output beam is collimated in upstream fast and downstream slow axes collimators which are spaced from one another in the one direction. The collimated output beams are incident on respective mirrors redirecting the incident output beams in another direction which is transverse to the one direction. Propagating further one above another, the output beams constitute a combined beam which diverges in the slow axis while propagating towards at least one lens which focuses the combined beam in the slow axis in the focal plane thereof. The output fiber is mounted to the case such that its core end is located coplanar with the smallest cross-section of the focused combined beam spaced downstream from the focal plane at a predetermined distance.

Claims

1. A pigtailed diode laser module, comprising: a case housing: spaced multimode (MM) chips outputting respective parallel beams along a path; an optical system configured to collimate parallel output beams in respective slows axes, wherein the collimated beams define a combined beam which diverges along the path; at least one focusing lens focusing the combined beam in a focal plane thereof; and an output fiber coupled to the case and having a core end downstream from the focal plane, wherein the combined beam, coupled into the core end, has a cross-section smaller than that of the combined beam in the focal plane.

2. The pigtailed diode laser module of claim 1, wherein the optical system includes a plurality of slow-axis collimators (SAC) each located between and optically coupled to the MM chip and one focusing lens and configured to collimate the output beam in the slow axis.

3. The pigtailed diode laser module of claim 2, further comprising a plurality of fast-axis collimators coupled between respective chips and SACs, the MM chips being arranged in at least one row and emitting respective output beams in a first direction.

4. The pigtailed diode laser module of claim 3, wherein the optical system further includes a plurality of angularly adjustable mirrors each located between the SAC and one focusing lens and deflecting the collimated output beam in a second direction transverse to the first direction, the focusing lens being configured to focus the combined beam in both fast and slow axes.

5. The pigtailed diode laser module of claim 3 further comprising at least one second focusing lens spaced upstream from the one focusing lens and configured to focus the combined beam in the fast axis.

6. The pigtailed diode laser module of one of claim 1, wherein the core end is spaced downstream from the focal plane of the one lens at a distance corresponding to a difference between distances of respective smallest and largest cross-sections of output beams, which are emitted by respective first and last MM chips, from the one focusing lens, with the first MM chip being closest to the lens, and the last MM chip being farthest from the lens.

7. The pigtailed diode laser module of one of claim 1, wherein the core end is spaced downstream from the focal plane of the one lens at a distance corresponding to a mean value of distances between the one focusing lens and respective smallest cross-sections of output beams which are located downstream from the one focusing lens, wherein the MM chips are spaced from the one focusing lens at respective distances which are different from one another.

8. A method of manufacturing the pigtailed diode laser module, comprising: energizing a plurality of MM chips, thereby outputting respective parallel beams; collimating the parallel beams each in a slow axis in a SACs optically coupled to the MM chip and located downstream therefrom, wherein the collimated beams propagate along a path and define a combined beam diverging along the path; focusing the diverging combined beam in a focal plane of a one focusing lens; and displacing the one focusing lens and a beam receiving core end of an output fiber away from one another at a predetermined distance such the combined beam is coupled into the receiving core end, wherein the focused combined beam has a cross-section at an entrance of the receiving core end smaller than the cross section of the beam in the focal plane.

9. The method of claim 8, wherein the one focusing lens focuses the diverging combined beam in a slow-axis.

10. The method of claim 9 further comprising collimating output beams each in a fast axis by a fast-axis collimator (FAC) located upstream from the SAC, and focusing the diverging combined beam in the fast axis by the one focusing lens.

11. The method of claim 10 further comprising selectively adjusting an angular position of selective mirrors located between the second focusing lens and respective SACs to adjust a focal plane of the one focusing lens, located in the optimal position, in a fast axis of the combined beam to be coplanar with the upstream core end of the output fiber.

12. The method of claim 8 further comprising collimating the output beams in respective fast axes by a plurality of FACs each located upstream from the SAC, and focusing the diverging combined beam in the fast axis by a second focusing lens located between the MM chips and one focusing lens.

13. The method of claim 8 further comprising: locating smallest spaced cross-sections of respective two output beams downstream from the one focusing lens, the two output beams being emitted by respective MM chips with one of the MM chips being closest to and the other MM chip being farthest from the one focusing lens, determining a distance between the located smallest cross-sections; and displacing the one focusing end upstream at the determined distance.

14. The method of claim 8 further comprising: locating smallest cross-sections of respective output beams downstream from the one lens, determining a distance as a mean value of distances between the one focusing lens and respective located cross-sections; and displacing the one focusing lens upstream at the determined distance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The above and other aspects and features will become more readily apparent from the following drawings, in which:

[0017] FIGS. 1 and 2 illustrate a pigtailed MEMM diode laser module of the known prior art.

[0018] FIG. 3 is a ray diagram associated with an individual extended light source;

[0019] FIGS. 4 and 5 are respective ray diagrams illustrating the operation of the individual extended diode laser at first and second distances between a SAC and SAOL, wherein the second distance is greater than the first one;

[0020] FIGS. 6 and 7 illustrate respective optical schematics of the disclosed pigtailed MEMM diode laser module; and

[0021] FIG. 8 illustrates the desired locations of minimal cross-sections of respective beams depending from distances at which the chips are spaced from the SAOL in FIGS. 6 and 7.

[0022] FIGS. 9 and 10 illustrate respective perspective views of optical schematics of respective FIGS. 6 and 7.

SPECIFIC DESCRIPTION

[0023] In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure.

[0024] Referring to FIGS. 6 and 7, a laser module 50 includes a plurality of spaced diode lasers or chips 12 (12.sub.1 . . . 12n) outputting respective parallel beams 14 in the first direction. The chips 12 are each associated with a beam-shaping optic including FAC 16, SAK 18 and mirror 20. Each chip 12 is aligned with designated FAC 16, SAC 18 and mirror 20 in the first direction and together these components constitute a group 32 (FIG. 7). The beams 14 each are collimated first in the fast axis by FAC 16 and then in the slow axis by SAC 18. In the fast axis, beam 14 is SM while in the slow axis this beam includes multiple spatial modes (MM).

[0025] The beams 14 are further redirected by respective mirrors 20 in a second direction, which is transverse to the first direction, and form a combined beam 24. The groups 32 are enclosed in case 34 having a bottom 15 which is made of heat-dissipating material and have respective chips 12 each coupled to mount 33 also made from heat dissipating material. The groups 32 (FIG. 7) may be mounted on a common mount 33 or on respective individual mounts 33 (FIG. 2) which are in contact with bottom 15. The architectures of module 50 illustrated in respective FIGS. 6 and 7 are well known from U.S. Pat. Nos. 7,764,723 and 8,711,894 which are incorporated herein in their entirety by reference.

[0026] Specifically, FIG. 6 illustrates chips 12.sub.1-12n mounted in a row. Typically, module 50 is configured with two rows of chips 12 of FIG. 7 which are mounted on respective opposite sides of combined beam 24 such that groups 32 of one row are not aligned with respective groups 32 of the other row in the first direction. The output fiber 30 is mounted in a ferrule (not shown) and aligned with fast axis objective lens 26 (FAOL) and SAOL 22 in the second direction.

[0027] It should be noted that combined beam 24 is astigmatic in which smallest cross-sections or waists in respective slow and fast axes are spaced from one another. Astigmatism may be corrected by installing FAOL 26 upstream from SAOL 22, as shown in FIG. 6 such that respective focal planes of these lenses located in the same plane. Alternatively, it is possible to use, among others, a single spherical, aspherical, cylindrical lens 36, as shown in FIG. 7. Each of the schematics of FIGS. 6 and 7 may be configured with multiple objective lenses or a single one as explained in somewhat greater detail below. However, beam 24 may remain astigmatic since the waist along the fast axis is very deep (Raleigh parameter is ˜1 mm.) Thus as long as mirrors 20 focus combined beam 24 on the fiber's core, the astigmatism may not be critical.

[0028] The distance between any of SACs 18 and SAOL 22 in both FIGS. 6 and 7 increases with the increased number of chips addressing the demands for higher output powers. The experiments show that generally when SAC 18 is configured with a focal length exceeding, for example, about 6 mm, the beam may significantly diverge in the slow axis.

[0029] In accordance with one of the aspects of the disclosure, SAOL. 22 is displaced upstream from its original position, in which the SAOL, focal length f2 and original focal plane Fo-Fo all each are shown in dash lines, to its new optimal position, in which SAOL 22 along with focal length f2 and new focal plane Fn-Fn are shown in solid lines. A distance 1 between the original and optimal positions ranges between about 50 and 500 μm and may be determined in accordance with the disclosed method discussed below in reference to FIG. 8. The output fiber 30 remains intact with the receiving end thereof lying in the original focal plane Fo-Fo. The focal plane of FAOL 26 coincides with the original focal plane Fo-Fo of SAOL 22 before the latter is shifted upstream. The desired distance at which SAOL 22 is displaced upstream from its original position is determined so that the smallest cross-section of the combined beam in the slow axis also lies in the original focal plane Fo-Fo. In other words, SAOL 22 and the receiving core end are spaced at a distance equal to the focal length of the SAOL and a newly determined distance D, as explained hereinbelow. The schematic of FIG. 6 can also be seen in FIG. 9.

[0030] Referring specifically to FIG. 7, diode module 50 has an additional row of chips 12. As mentioned above, only one lens 36 functioning simultaneously as FAOL and SAOL is utilized in the shown configuration. According to the above-discussed salient feature of the disclosure, lens 36 is shifted upstream from its original position shown in dash lines and including the receiving end of fiber 30, to an optimal position at the determined distance D) for the reasons explained above. The optical schematic of FIG. 7 is also shown in FIG. 10.

[0031] Referring to FIGS. 5-7, beams 14.sub.1 . . . 14.sub.n are output by respective chips 12.sub.1 . . . 12n and propagate over different optical paths before impinging upon SAOL 22. Due to different optical paths, the region of SAOL 22 impinged by multiple beams 14 varies. The region with the smallest area is impinged by beam 14.sub.1, which propagates over the shortest optical path since chip 12.sub.1 is the closest to SAOL 22 or 36, whereas the largest area is covered by beam 14n which is emitted from chip 12n most distant from the SAOL 22. As a consequence, beams 14.sub.1-14n are “focused” in slow axis at respective different distances downstream from focal plane F-F corresponding to the original position of SAOL 22 and including the receiving end of fiber 30. The distance between the small beam cross-sections of respective beams 14.sub.1 and 14n emitted by first and last chips of module 10 determines the distance D at which SAOL 22 is shifted upstream from its original position. Alternatively, distance D may be determined as the mean of all distances of respective smallest cross-sections of beams 14.sub.1 . . . 14.sub.n.

[0032] FIG. 8 considered in light of FIGS. 5-7 helps explain the location adjustment of the SAOL in the context of the present disclosure. As one of ordinary skilled in the semiconductor arts readily understands, in mass production once a sample, such as a MEMM diode laser module, is tuned up, subsequent modules are each easily adjusted in accordance with data obtained during the tuning of the sample. Thus, the determined distance D, at which the SAOL is shifted upstream from its original position is once determined, is subsequently used in all other modules.

[0033] Accordingly, selectively turning either each of chips 12 in the tested module or just two chips—the closest to and most distant from the SAOL—it is possible to determine minimal cross-sections of respective beams incident on fiber 30. As can be seen in FIG. 8, curves 1 through 6 correspond to respective beams 14.sub.1 . . . 14n of FIGS. 5-7. The smallest cross-section of each beam corresponds to a bottom region of the associated curve. Thus, curve 1 corresponding to beam 14.sub.1 from chip 12.sub.1, which is located at the shortest distance upstream from the SAOL, has its smallest cross-section downstream from focal plane F-F at the shortest a distance. The beam 14n emitted from distant chip 12n corresponds to curve 6 and has its smallest cross-section at a second distance greater than that one of beam 14.sub.1. The distance D between the smallest cross-sections of respective beams 14.sub.1 and 14.sub.n relative to the SAOL is the desired uniform distance for all subsequently tunable modules at which the SAOL is shifted upstream from its original position. The curve 7 illustrates the behavior of all beams after combined beam is focused in FAOL 36. As can be seen, SM beams 14.sub.1 . . . 14.sub.n have respective beam spots in fast axis lying in the same plane as the receiving core end of output fiber 30. In other words, in the fast axis beams 14 each are focused in focal plane F-F of SAOL 22 before the latter is shifted at distance D to its optimal position.

[0034] Referring to the configuration with single lens 36 of FIG. 7, care has to be taken not only of the lens adjustment in the slow axis, but also in the fast axis. The displacement of lens 36 from its original position to the optimal position in the slow axis at distance D detrimentally affects the beam spot of the combined beam in the fast axis because when lens 36 is in its original position, the smallest beam spot in the fast axis is located in original focal plane F-F. However, the angular adjustment of mirror or mirrors 20 can effectively compensate for the shift of lens 36. The mirrors 20 can be angularly adjusted such that beams 14.sub.1 . . . 14.sub.n, incident on the lens 36, open up at a greater angle and thus could be focused in focal plane F-F of lens 36 when it is located in its original position. The angular position of the mirrors, like distance ID, can be used for adjusting subsequent diode laser modules in mass production.

[0035] As one of ordinary skill readily realizes the above and further disclosed features of the inventive module and method can be used in any possible situation and all together. Certain obvious modifications of the disclosed module can be easily surmised by one of ordinary skill in the laser arts without compromising the scope of the invention. For example, the disclosed chips may be mounted so that respective output beams propagate in the same direction along the entire path until the combined beam is collimated in a slow axis and coupled into the fiber. This can be realized by arranging collimating and beam guiding optics in a configuration apparent to one of ordinary skill. The inventive module may function without FACS. Thus although shown and disclosed is what is believed to be the most practical and preferred embodiments, it is apparent that departures from the disclosed configurations and methods will suggest themselves to those skilled in the art and may be used without departing from the spirit and scope of the invention.