Grating element and external resonator type light emitting device

Abstract

A grating element includes: a support substrate; an optical material layer; a ridge optical waveguide having an incidence surface on which a laser light is incident and an emission end from which an emission light with a desired wavelength is emitted; and a Bragg grating including concave and convex portions formed within the optical waveguide. The optical waveguide includes an incident portion between the incidence surface and the Bragg grating, and a tapered portion between the incident portion and the Bragg grating. In the Bragg grating, a propagation light propagates in single mode. The width W.sub.in of the optical waveguide in the incident portion is larger than the width W.sub.gr of the optical waveguide in the Bragg grating. The width W.sub.t of the optical waveguide in the tapered portion is decreased from the incident portion toward the Bragg grating. The relationships represented by formulas (1) to (3) are satisfied.

Claims

1. An external resonator type light emitting device, comprising: a light source for oscillating a laser light; and a grating element forming an external resonator together with said light source, wherein said light source includes an active layer oscillating said laser light whose transverse mode is of single mode; said grating element comprising: a support substrate; an optical material layer disposed over said support substrate; a ridge optical waveguide disposed in said optical material layer, said ridge optical waveguide having an incidence surface to which a laser light is incident and an emission end from which an emission light with a desired wavelength is emitted; and a Bragg grating comprising concave and convex portions formed within said ridge optical waveguide, wherein said ridge optical waveguide comprises an incident portion disposed between said incidence surface and said Bragg grating, and a tapered portion disposed between said incident portion and said Bragg grating, wherein a propagating light propagates in said ridge optical waveguide in a single mode, wherein said tapered portion has a first end having a first width and a second end having a second width less than the first width, the first end being in contact with said incident portion, wherein a width of said ridge optical waveguide in said incident portion is larger than a width of said ridge optical waveguide in said Bragg grating, wherein a width of said ridge optical waveguide in said tapered portion is decreased from said incident portion toward said Bragg grating, wherein said width of said ridge optical waveguide in said incident portion is 1.5 times or more of a mode-field diameter in a horizontal direction of said laser light, and wherein relationships represented by formulas (1) to (5) below are satisfied:
0.8 nm.sub.G6.0 nm(1)
10 mL.sub.b300 m(2)
20 nmtd250 nm(3)
n.sub.b1.8(4)
L.sub.WG500 m(5) .sub.G in said formula (1) is a full width at half maximum of a peak of a Bragg reflectance in said Bragg grating; L.sub.b in said formula (2) is a length of said Bragg grating; t.sub.d in said formula (3) is a depth of each of said concave and convex portions forming said Bragg grating; n.sub.b in said formula (4) is a refractive index of a material forming said optical material layer; and L.sub.WG in said formula (5) is a length of said grating element.

2. The external resonator type light emitting device of claim 1, wherein a thickness of said optical material layer in said incident portion is larger than a thickness of said optical material layer in said Bragg grating, and wherein a thickness of said optical material layer in said tapered portion is decreased from said incident portion toward said Bragg grating.

3. The external resonator type light emitting device of claim 1, wherein W.sub.inv/ is not less than 2 nor more than 3 where is a wavelength of said laser light, and W.sub.inv is a thickness of said optical material layer at least in said incident portion, and wherein said width of said ridge optical waveguide in said Bragg grating is not less than 3 nor more than 5.

4. The external resonator type light emitting device of claim 3, wherein W.sub.grv/ is not less than 1 nor more than 2 where W.sub.grv is a thickness of said optical material layer in said Bragg grating, and is said wavelength of said laser light.

5. The external resonator type light emitting device of claim 1, wherein a single-layer film is formed on at least one of said incidence surface and said emission end, said single-layer film being made of a material having a lower refractive index than a refractive index of said material forming said optical material layer.

6. The external resonator type light emitting device of claim 1, wherein a thickness of said optical material layer in said incident portion is 1.5 times or more of a mode-field diameter in a vertical direction of said laser light.

7. The external resonator type light emitting device of claim 1, wherein a number of wavelengths capable of satisfying a phase condition of lasing within a full width at half maximum .sub.G is not less than 2 nor more than 5.

8. The external resonator type light emitting device of claim 1, wherein a relationship represented by formula (6) below is satisfied: $\begin{matrix} .Math. \frac{d_{G}}{d T} - \frac{d_{TM}}{d T} .Math. 0.03 nm / C . & (6) \end{matrix}$ wherein said formula (6), d.sub.G/dT is a temperature coefficient of a Bragg wavelength, and d.sub.TM/dT is a temperature coefficient of a wavelength satisfying a phase condition of an external resonator laser.

9. A An external resonator type light emitting device, comprising: a light source for oscillating a laser light; and a grating element forming an external resonator together with said light source, wherein said light source includes an active layer oscillating said laser light whose transverse mode is of single mode; said grating element comprising: a support substrate; an optical material layer disposed over said support substrate; a ridge optical waveguide disposed in said optical material layer, said ridge optical waveguide having an incidence surface on which a laser light is incident and an emission end from which an emission light with a desired wavelength is emitted; and a Bragg grating comprising concave and convex portions formed within said ridge optical waveguide, wherein said ridge optical waveguide comprises an incident portion disposed between said incidence surface and said Bragg grating, and a tapered portion disposed between said incident portion and said Bragg grating, wherein a propagation light propagates in said ridge optical waveguide in a single mode, wherein said tapered portion has a first end having a first width and a second end having a second width less than the first width, the first end being in contact with said incident portion, wherein a width of said ridge optical waveguide in said incident portion is larger than a width of said ridge optical waveguide in said Bragg grating, wherein a width of said ridge optical waveguide in said tapered portion is decreased from said incident portion toward said Bragg grating, wherein said width of said ridge optical waveguide in said incident portion is 1.5 times or more of a mode-field diameter in a horizontal direction of said laser light, wherein relationships represented by formulas (1), (2), (3) and (5) below are satisfied, and wherein a material forming said optical material layer is selected from said group consisting of gallium arsenide, lithium niobate single crystal, tantalum oxide, zinc oxide, aluminum oxide, lithium tantalate, magnesium oxide, niobium oxide, and titanium oxide,
0.8 nm.sub.G6.0 nm(1)
10 mL.sub.b300 m(2)
20 nmtd250 nm(3)
L.sub.WG500 m(5) where .sub.G in said formula (1) is a full width at half maximum of a peak of a Bragg reflectance in said Bragg grating; L.sub.b in said formula (2) is a length of said Bragg grating; t.sub.d in said formula (3) is a depth of each of said concave and convex portions forming said Bragg grating; and L.sub.WG in said formula (5) is a length of said grating element.

10. The external resonator type light emitting device of claim 9, wherein a thickness of said optical material layer in said incident portion is larger than a thickness of said optical material layer in said Bragg grating, and wherein a thickness of said optical material layer in said tapered portion is decreased from said incident portion toward said Bragg grating.

11. The external resonator type light emitting device of claim 9, wherein W.sub.inv/ is not less than 2 nor more than 3 where is a wavelength of said laser light, and W.sub.inv is a thickness of said optical material layer at least in said incident portion, and wherein said width of said ridge optical waveguide in said Bragg grating is not less than 3 nor more than 5.

12. The external resonator type light emitting device of claim 11, wherein W.sub.grv/ is not less than 1 nor more than 2 where W.sub.grv is a thickness of said optical material layer in said Bragg grating, and is said wavelength of said laser light.

13. The external resonator type light emitting device of claim 9, wherein a single-layer film is formed on at least one of said incidence surface and said emission end, said single-layer film being made of a material having a lower refractive index than a refractive index of said material forming said optical material layer.

14. The external resonator type light emitting device of claim 9, wherein a thickness of said optical material layer in said incident portion is 1.5 times or more of a mode-field diameter in a vertical direction of said laser light.

15. The external resonator type light emitting device of claim 9, wherein a number of wavelengths capable of satisfying a phase condition of lasing within a full width at half maximum .sub.G is not less than 2 nor more than 5.

16. The external resonator type light emitting device of claim 9, wherein a relationship represented by formula (6) below is satisfied: $\begin{matrix} .Math. \frac{d_{G}}{d T} - \frac{d_{TM}}{d T} .Math. 0.03 nm / C . & (6) \end{matrix}$ wherein said formula (6), d.sub.G/dT is a temperature coefficient of a Bragg wavelength, and d.sub.TM/dT is a temperature coefficient of a wavelength satisfying a phase condition of an external resonator laser.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1(a) and 1(b) are schematic plan views showing grating elements 2A and 2B, respectively.

(2) FIG. 2 (a) is a schematic plan view showing the grating element 2A, and FIG. 2(b) is a schematic side view of the grating element 2A.

(3) FIG. 3 (a) is a schematic plan view showing the grating element 2B, and FIG. 3(b) is a schematic side view of the grating element 2B.

(4) FIG. 4 is a perspective view of the grating element.

(5) FIG. 5 is a cross-sectional view of the grating element.

(6) FIG. 6 is a cross-sectional view of another grating element.

(7) FIG. 7 is a cross-sectional view of a further grating element.

(8) FIG. 8 is a schematic plan view showing an external resonator type light emitting device.

(9) FIG. 9 is a schematic side view showing the external resonator type light emitting device.

(10) FIG. 10 is a schematic plan view showing another external resonator type light emitting device.

(11) FIG. 11 is a schematic plan view showing a further external resonator type light emitting device.

(12) FIG. 12 is a schematic plan view showing an external resonator type light emitting device according to a further embodiment.

(13) FIG. 13 is a schematic longitudinal sectional view of the external resonator type light emitting device shown in FIG. 12.

(14) FIG. 14 is a diagram for explaining lasing conditions.

(15) FIG. 15 is a diagram for explaining the form of mode hopping in a conventional example.

(16) FIG. 16 is another diagram for explaining the form of the mode hopping in the conventional example.

(17) FIG. 17 is an example of a mode field pattern.

(18) FIG. 18 is another example of a mode field pattern.

(19) FIG. 19 is a graph showing the relationship between the width and effective index of a ridge optical waveguide.

(20) FIG. 20 is a graph showing the relationship between the width of the ridge optical waveguide and the propagation efficiency in the waveguide.

(21) FIG. 21 shows an example of discrete phase conditions in the present invention.

DESCRIPTION OF EMBODIMENTS

(22) As shown in FIGS. 1(a) and 1(b), each of grating elements 2A and 2B includes an optical material layer 1 that has an incidence surface 1a on which a laser light is incident and an emission surface 1b from which a light with a desired wavelength emits. A ridge optical waveguide 3A is provided within the optical material layer 1, and a ridge optical waveguide 3B is provided within the optical material layer 1.

(23) The ridge optical waveguide 3A includes an incident portion 3a, a tapered portion 3b, a coupling portion 3c, a grating portion 3d, and an emission portion 3e. The grating portion 3d has a Bragg grating 20 formed therein. The width of the emission portion 3e is set constant. The ridge optical waveguide 3B includes the incident portion 3a, the tapered portion 3b, the coupling portion 3c, the grating portion 3d, the emission portion 3e, and a coupling portion 3f located between the grating portion and the emission portion 3e. The grating portion 3d has the Bragg grating 20 formed therein. While the width of the coupling portion 3f is set constant, the width of the emission portion 3e is gradually decreased as it approaches the emission surface.

(24) As illustrated in FIGS. 2(a) and 2(b), for the grating element 2A, the optical material layer 1 is formed over a support substrate 5 with a lower cladding layer 4 interposed between them. The optical material layer 1 has an upper surface 1c. The cladding layer may be made of any material that has a smaller refractive index than the optical material layer and thus may be an adhesive layer. When the cladding layer is made of the adhesive layer, a bottom surface 1d of the optical material layer is bonded onto the support substrate 5.

(25) As illustrated in FIGS. 3(a) and 3(b), for the grating element 2B, the optical material layer 1 is formed over the support substrate 5 with the lower cladding layer 4 interposed between them. The optical material layer 1 has the upper surface 1c. The lower cladding layer may be made of any material that has a smaller refractive index than the optical material layer and thus may be an adhesive layer. When the cladding layer is made of the adhesive layer, the bottom surface 1d of the optical material layer is bonded to the support substrate 5.

(26) FIG. 4 schematically illustrates a perspective view of the grating element 2A. The same goes for the grating element 4B.

(27) FIGS. 5, 6, and 7 respectively illustrate cross-sectional views of the grating elements taken along the Bragg gratings.

(28) In an example illustrated in FIG. 5, the optical material layer 1 is formed over the support substrate 5 with an adhesive layer 7 and the lower buffer layer 4 interposed between them, and an upper buffer layer 8 is formed over the optical material layer 1. For example, a pair of ridge grooves 9 is formed in the optical material layer 1, and the grating portion 3d of the ridge optical waveguide is formed between the adjacent ridge grooves 9.

(29) In this case, the Bragg grating may be formed on the upper surface 1c side or on the bottom surface 1d side. To reduce variations in the shapes of the Bragg grating and the ridge groove, the Bragg grating is preferably formed on the flat bottom surface 1d side, thereby positioning the ridge grooves 9 on the opposite side of the substrate from the Bragg grating.

(30) The example shown in FIG. 6 is substantially the same as the example shown in FIG. 5. Note that the optical material layer 1 is formed over the support substrate 5 with the lower buffer layer 4 interposed between them. The upper buffer layer 8 is formed on the optical material layer 1.

(31) In an example shown in FIG. 7, the optical material layer 1 is formed over the support substrate 5 via the adhesive layer 7 and the lower buffer layer 4, and the upper buffer layer 8 is formed on the optical material layer 1. For example, a pair of ridge grooves 9 is formed on the support substrate 5 side of the optical material layer 1, and the ridge optical waveguide 3d is formed between the adjacent ridge grooves 9. In this case, the Bragg grating may be formed on the flat upper surface 1c side or at the bottom surface 1d side with the ridge grooves formed thereat. To reduce variations in the shapes of the Bragg grating and the ridge groove, the Bragg grating is preferably formed on the flat upper surface 1c side, thereby positioning the ridge grooves on the opposite side of the substrate from the Bragg grating. Further, the upper buffer layer 8 may not be formed. In this case, an air layer can be in direct contact with the grating. With this arrangement, the presence and absence of the grating grooves can increase a difference in the refractive index, so that the reflectance can be increased in a short grating length.

(32) However, when the refractive index of the support substrate 5 is higher than the refractive index of the optical material layer 1, the upper buffer layer is preferably formed in terms of reducing the propagation loss of the light in the waveguide.

(33) A light emitting device 10 as schematically shown in FIGS. 8 and 9 includes a light source 11 that oscillates the laser light, and the grating element 2A. The light source 11 and the grating element 2A are mounted on a common substrate 17.

(34) The light source 11 includes an active layer 12 that oscillates the semiconductor laser light. In this embodiment, the active layer 12 is provided on a base substrate 15. A reflective film 16 is provided at the outer end face of the light source, while an anti-reflective layer 13A is formed at the end face of the active layer 12 on the grating element side.

(35) The incident portion of the grating element faces the active layer 12 with a gap 14 interposed between them. An anti-reflective film 13B is provided on the incidence surface side of the optical material layer 1, while an anti-reflective film 13C is provided on the emission surface side of the optical material layer 1.

(36) The anti-reflective layers 13A, 13B, and 13C need only to have a reflectance lower than the reflectance of the grating, and more preferably have a reflectance of 0.1% or less. However, if the reflectance of the end face of the optical material layer is lower than the reflectance of the grating, the anti-reflective layer may not be required, and a reflective film can also be provided in place of the anti-reflective layer.

(37) In this case, the lasing wavelength of the laser light is determined by the wavelength of light reflected by the Bragg grating. When the light reflected by the Bragg grating and the light reflected from the end face of the active layer on the grating element side exceed a gain threshold of the laser, the oscillation conditions are satisfied. In this way, the laser light with higher wavelength stability can be obtained.

(38) To enhance the wavelength stability, the amount of feedback from the grating needs only to be increased. From this point of view, the reflectance of the grating is preferably set higher than the reflectance of the end face of the active layer.

(39) As a light source, a GaAs-based or InP-based laser with high reliability is suitable for use. In an application of the structure in the present application, for example, when intended to oscillate green laser light as a second harmonic wave using a non-liner optical element, the GaAs-based laser is used that oscillates at a wavelength of around 1064 nm. The GaAs-based or InP-based laser has high reliability, and thus light sources, including a laser array formed by arranging such lasers in one dimension, can be implemented. The laser may be a superluminescent diode or a semiconductor optical amplifier (SOA).

(40) As the wavelength of the laser light becomes longer, changes in the Bragg wavelength depending on the temperature become significant. To enhance the wavelength stability, particularly, the lasing wavelength of the laser is preferably 990 nm or less. On the other hand, as the wavelength of the laser light becomes short, changes in the refractive index Ana of the semiconductor become too significant. To enhance the wavelength stability, particularly, the lasing wavelength of the laser is preferably 780 nm or more. Material of the active layer and the wavelength of light for the active layer can be selected as appropriate.

(41) Note that a method for stabilizing power by a combination of a semiconductor laser and a grating element is disclosed in the following document. (Non-Patent Document 3: Furukawa Review, No. 105, January, 2000, p 24-29)

(42) The ridge optical waveguide can be obtained by physically processing and forming, for example, by a cutting process with a peripheral cutting edge, or a laser ablation process.

(43) The Bragg grating can be formed by physical or chemical etching in the following way.

(44) Specifically, a metal film made of Ni, Ti, Cr, etc. is deposited on a substrate with a high refractive index, and windows are formed periodically by photolithography, thereby forming an etching mask. Then, periodic grating grooves are formed by a dry etching device for reactive ion etching and the like. Finally, the metal mask is removed, whereby the Bragg grating can be formed.

(45) The ridge optical waveguide can also be formed in the same way as the grating grooves.

(46) To further improve the optical damage resistance of the optical wave guide, the optical material layer may contain one or more kinds of metal elements selected from the group consisting of magnesium (Mg), zinc (Zn), scandium (Sc), and indium (In). In this case, magnesium is particularly preferable. Crystals of the optical material layer can contain a rare-earth element as a doping component. Suitable rare-earth elements particularly include Nd, Er, Tm, Ho, Dy, and Pr.

(47) Material for the adhesive layer may be an inorganic adhesive, an organic adhesive, or a combination of the inorganic adhesive and the organic adhesive.

(48) The optical material layer may be deposited and formed over the support base by a thin-film formation method. Suitable thin-film formation methods can include sputtering, vapor deposition, and CVD. In this case, the optical material layer is formed directly on the support base. Alternatively, after forming a lower buffer layer on the support base, the optical material layer can be formed thereon.

(49) Examples of material for such a support base are not particularly limited to, but including lithium niobate, lithium tantalate, glass, such as fused quartz, crystal, Si, etc.

(50) When the refractive index of the support base is higher than the refractive index of the optical material layer, a lower cladding layer is essential, but additionally an upper cladding layer is preferably provided in terms of reducing the light loss in the waveguide.

(51) The lower and upper cladding layers may be lower and upper buffer layers, respectively, and they have lower refractive index than the optical material layer.

(52) The reflectance of the anti-reflective layer needs to be lower than the grating reflectance. Materials suitable for use in deposition of the anti-reflective layer can include a laminated film made of oxides, such as silicon dioxide, tantalum pentoxide, or oxide of magnesium fluoride, and metals.

(53) The respective end faces of the light-source element and the grating element may be obliquely cut to suppress the end face reflection. Bonding between the grating element and the support substrate is fixed with the adhesive in the example shown in FIG. 2, but may be direct bonding.

(54) In a preferred embodiment, a single-layer film is formed on at least one of the incidence surface and emission surface, the single-layer film having a lower refractive index than the refractive index of material forming the optical material layer. The thickness of such a single-layer film is not necessarily determined strictly, unlike an AR coat. The mere formation of the single-layer film can reduce the end face reflection. Here, if the film is made of a multi-layer, the reduction of the reflection is degraded or eliminated depending on the relationship of the refractive index and thickness between layers of the multi-layer film, which requires the control of the thickness of each layer in the multi-layer film. Thus, the single-layer film is superior. Because of this, the reflectance of the end face of the grating element can be surely reduced, compared to the reflectance of a grating element without the single-layer film. The thickness of the single-layer film is preferably 1 m or less.

(55) In an example shown in FIG. 10, the grating element 2A is optically coupled to the light source 11, while an optical waveguide 22 of another optical waveguide substrate 21 is optically coupled to the emission surface of the emission portion 3e of the grating element 2A.

(56) In an example shown in FIG. 11, the grating element 2B is optically coupled to the light source 11, while the optical waveguide 22 of another optical waveguide substrate 21 is optically coupled to the emission surface of the emission portion 3e of the grating element 2B.

(57) In examples shown in FIGS. 12 and 13, the light source 11 and the grating element 22 are mounted on a common substrate 24. Adhesive layers 7 and 25 can also be provided.

(58) The ridge optical waveguide 23A of the grating element 22 includes an incident portion 23a, a tapered portion 23b, a grating portion 23d, and an emission portion 23e. The grating portion 23d has the Bragg grating 20 formed therein. As viewed from the top side of the element (see FIG. 12), an optical waveguide width W.sub.in in the incident portion 23a is larger than an optical waveguide width W.sub.gr in the Bragg grating. An optical waveguide width W.sub.t in the tapered portion 23b is monotonically decreased from W.sub.in toward W.sub.gr. In this example, an optical waveguide width W.sub.out in the emission portion 23e is set constant.

(59) As viewed from the lateral side of the element (see FIG. 13), an optical material layer thickness W.sub.inv in the incident portion 23a is set larger than an optical material layer thickness W.sub.grv in the Bragg grating. An optical material layer thickness W.sub.tv in the tapered portion 23b is monotonically decreased from W.sub.inv toward W.sub.grv. In this example, an optical material layer thickness W.sub.outv in the emission portion 23e is set constant.

(60) Now, the conditions of the present invention will be further described with reference to the configuration illustrated in FIG. 14.

(61) It is noted that since mathematical formulas are hard to understand because of the abstractness, first, the direct comparison between a typical form in the related art and one embodiment of the present invention will be made and thereby the features of the present invention will be described. Then, various conditions of the present invention will be described.

(62) First, the lasing condition of the semiconductor laser is determined by the product of the gain condition and the phase condition, given by the following formula.
[Formula 6]
(C.sub.out.sup.2).sup.4|r.sub.1r.sub.2|exp{(.sub.tg.sub.th.sub.a)L.sub.a.sub.bL.sub.b}exp{j(.sub.1.sub.22L.sub.a)}=1 Formula (2-1)

(63) The gain condition is represented by the following formula derived from Formula (2-1).

(64) $\begin{matrix} _{t} g_{th} =_{a} L_{a} +_{b} L_{b} + \frac{1}{L_{a}} \ln (\frac{1}{.Math. r_{1} .Math. .Math. r_{2} .Math. C_{out}^{2}}) & Formula (2 - 2) \end{matrix}$

(65) Note that in the formulas, .sub.a, .sub.g, .sub.wg, and .sub.gr are loss coefficients in the active layer, the gap between the semiconductor laser and the waveguide, the waveguide portion on the input side without any grating, and the grating portion, respectively; L.sub.a, L.sub.g, L.sub.wg, and L.sub.gr are the lengths of the active layer, the gap between the semiconductor laser and the waveguide, the waveguide portion on the input side without any grating, and the grating portion, respectively; r.sub.1 and r.sub.2 are mirror reflectances (where r.sub.2 is the reflectance of the grating); C.sub.out is a coupling loss between the grating element and the light source; .sub.tg.sub.th is a gain threshold of the laser medium; .sub.1 is a phase change of a reflective mirror on the laser side; and .sub.2 is a phase change on the grating portion.

(66) Formula (2-2) shows that when the gain .sub.tg.sub.th of the laser medium (gain threshold) exceeds the loss therein, the lasing is performed. A gain curve (wavelength dependency) of the laser medium has a full width at half maximum of 50 nm or more and exhibits the broad characteristics. The loss terms (on the right side), except for the reflectance of the grating, hardly have the wavelength dependency, so that the gain condition is determined dominantly by the grating. Because of this, in a comparison table, the gain condition can be considered only by the grating.

(67) On the other hand, the phase condition is defined by the following formula derived from Formula (2-1). Note that .sub.1 is zero.
.sub.2+2.sub.aL.sub.a+2.sub.gL.sub.g+2.sub.wgL.sub.wg=2p(p is an integer number.) Formula (2-3)

(68) The external-cavity laser, which includes an external resonator that utilizes a quartz-based glass wave guide and an FBG, has been manufactured. As shown in FIGS. 15 and 16, the conventional design concept is that the reflection characteristics of the grating are set to satisfy .sub.G=approximately 0.2 nm and a reflectance of 10%. This leads to the fact that the length of the grating portion is 1 mm. On the other hand, the phase condition is designed such that the satisfactory wavelengths become discrete, and that the number of wavelengths satisfying Formula (2-3) is set at two or three within .sub.G. For this reason, the active layer in the laser medium is required to have a greater length. The active layer with a length of 1 mm or more is used.

(69) For the glass waveguide or FBG, the temperature dependency of the wavelength .sub.G is very small, and d.sub.G/dT is approximately 0.01 nm/ C. (d.sub.G/dT=approximately 0.01 nm/ C.). This shows that the external resonator type laser has the feature of having high wavelength stability.

(70) However, compared to this case, the temperature dependency of the wavelength satisfying the phase condition is large, and d.sub.S/dT is approximately 0.05 nm/ C. (d.sub.S/dT=approximately 0.05 nm/ C.), whereby a difference between them is 0.04 nm/ C.

(71) In general, the temperature T.sub.mh at which mode hopping occurs can be considered to be represented by the following formula based on Non-Patent Document 1 (where T.sub.a is supposed to be T.sub.f (i.e. T.sub.a=T.sub.f)).

(72) G.sub.TM is a spacing between the wavelengths (longitudinal mode spacing) that satisfies the phase condition of the external resonator type laser.

(73) $\begin{matrix} T_{mh} = \frac{G_{TM}}{.Math. \frac{d_{G}}{d T} - \frac{d_{TM}}{d T} .Math.} & Formula (2 - 4) \end{matrix}$

(74) Thus, in the related art, the temperature T.sub.mh at which mode hopping occurs is approximately 5 C. This is why mode hopping is more likely to occur. Therefore, once mode hopping occurs, the power fluctuates based on reflection characteristics of the grating, and specifically it fluctuates by 5% or more.

(75) As can be seen from the above, in an actual operation, the external resonator type laser employing the conventional glass waveguide or FBG controls its temperature by using a Peltier element.

(76) In contrast, the present invention uses the grating element that makes a denominator of Formula (2-4) small as a precondition. The denominator in Formula (2-4) is preferably set at 0.03 nm/ C. or lower. Specifically, examples of material for the optical material layer preferably include gallium arsenide (GaAs), lithium niobate (LN), lithium tantalate (LT), tantalum oxide (Ta.sub.2O.sub.5), zinc oxide (ZnO), aluminum oxide (Al.sub.2O.sub.3), magnesium oxide (MgO), niobium oxide (Nb.sub.2O.sub.5), and titanium oxide (TiO.sub.2).

(77) When five or less wavelengths satisfying the phase condition exist within the .sub.G, the laser can operate under the stable lasing conditions even if mode hopping occurs.

(78) That is, in the configuration of the present invention, for example, when using polarized light in the z axis of LN, the lazing wavelength changes at a rate of 0.1 nm/ C. relative to the change in temperature, based on the temperature characteristic of the grating, which makes it less likely to cause power variation even if mode hopping occurs. In the configuration of the present application, to increase .sub.G, the grating length L.sub.b is set, for example, at 100 m, whereas to increase G.sub.TM, L.sub.a is set, for example, at 250 m.

(79) Note that the description of differences from Patent Document 6 will also be supplemented.

(80) The present application is based on the premise that the temperature coefficient of the grating wavelength is made closer to the temperature coefficient of the gain curve of the semiconductor. From this point of view, the material having a refractive index of 1.8 or more is used. Furthermore, the groove depth t.sub.d of the grating is set at not less than 20 nm and not more than 250 nm, the reflectance thereof is set at not less than 3% nor more than 60%, and the full width at half maximum .sub.G thereof is set not less than 0.8 nm nor more than 250 nm. This arrangement can make the resonator structure compact and can achieve the temperature independency without adding any element. Patent Document 6 describes respective parameters as follows, all of which fall within the related art.

(81) .sub.G=0.4 nm

(82) Longitudinal Mode Spacing G.sub.TM=0.2 nm

(83) Grating Length L.sub.b=3 mm

(84) LD Active Layer Length L.sub.a=600 m

(85) Propagation Portion Length=1.5 mm

(86) The following respective conditions in the present invention will be described more specifically below.
0.8 nm.sub.G6.0 nm(1)
10 mL.sub.b300 m(2)
20 nmt.sub.d250 nm(3)

(87) The refractive index n.sub.b of the material forming the Bragg grating is preferably 1.7 or more, and more preferably 1.8 or more.

(88) In the related art, material having a lower refractive index, such as quartz, is commonly used. However, in the idea of the present invention, the refractive index of the material forming the Bragg grating is enhanced. The reason for this is that the material having a high refractive index tends to significantly change its refractive index depending on the temperature, whereby T.sub.mh of Formula (2-4) can be increased, and as mentioned above, the temperature coefficient d.sub.G/dT of the grating can be increased. From this point of view, n.sub.b is further preferably 1.9 or more. The upper limit of n.sub.b is not particularly limited. However, any excessive refractive index n.sub.b leads to an excessively small grating pitch in design, making it difficult to form the grating. Thus, the refractive index n.sub.b is preferably 4 or less.

(89) The full width at half maximum .sub.G of the peak of the Bragg reflectance is set at 0.8 nm or more (Formula 1), where .sub.G is the Bragg wavelength. That is, as shown in FIGS. 15 and 16, when the longitudinal axis represents the reflectance, and the lateral axis represents the reflection wavelength due to the Bragg grating, the wavelength at which the reflectance is maximized is referred to as the Bragg wavelength. The full width at half maximum .sub.G is a difference between two wavelengths at which its reflectance is equal to half the maximum reflectance at the peak with the Bragg wavelength positioned at the center.

(90) The full width at half maximum .sub.G of the peak of the Bragg reflectance is set at 0.8 nm or more (Formula 1). This is to make the peak of the reflectance broad. From this point of view, the full width at half maximum .sub.G is preferably set to 1.2 nm or more, and more preferably 1.5 nm or more. The full width at half maximum .sub.G is set to 5 nm or less, preferably 3 nm or less, and more preferably 2 nm or less.

(91) The length L.sub.b of the Bragg grating is set at 300 m or less (see Formula 2 and FIG. 9). The length L.sub.b of the Bragg grating is a grating length in the optical axis direction of light propagating through the optical waveguide. The length L.sub.b of the Bragg grating is set to 300 m or less, which is shorter than in the related art. This is the premise on which the design idea of the present invention is based. That is, to suppress the occurrence of mode hopping, a spacing between wavelengths satisfying the phase condition (longitudinal mode spacing) needs to be larger. For this reason, it is necessary to shorten the length of the resonator, so that the length of the grating element is decreased. From this point of view, the length L.sub.b of the Bragg grating is more preferably 200 m or less.

(92) Shortening the length of the grating element leads to the reduced loss thereof, which can decrease the lazing threshold. Consequently, the laser can be driven at low current and with low heat generation and low energy.

(93) The length L.sub.b of the grating is preferably 5 m or more to attain the reflectance of 3% or higher, and more preferably 10 m or more to attain the reflectance of 5% or higher.

(94) In Formula (3), t.sub.d is a depth of each of the concave and convex portion forming the Bragg grating (see FIG. 4). By setting t.sub.d in a range of 20 nm to 250 nm (20 nmt.sub.d250 nm), .sub.G can be set at not less than 0.8 nm nor more than 250 nm. The number of the longitudinal modes within the range .sub.G can be adjusted to be not less than 2 nor more than 5. From this point of view, t.sub.d is more preferably set at 30 nm or more, and further preferably 200 nm or less. To set the full width at half maximum at 3 nm or less, t.sub.d is preferably 150 nm or less.

(95) In the preferred embodiments, in terms of promoting the lasing, the reflectance of the grating element is preferably set at not less than 3% nor more than 40%. The reflectance is more preferably 5% or more to further stabilize the output power, and more preferably 25% or less to increase the output power.

(96) As shown in FIG. 14, the lasing conditions are configured by the gain condition and the phase condition. The wavelengths satisfying the phase condition are discrete, and shown in, for example, FIG. 21. That is, in the configuration of the present application, the temperature coefficient of the gain curve (0.3 nm/ C. for GaAs) is made closer to the temperature coefficient d.sub.G/dT of the grating, so that the lasing wavelength can be fixed within the range .sub.G. Further, when the number of longitudinal modes within the range .sub.G is not less than 2 nor more than 5, the lasing wavelength repeatedly shows mode hopping within the range .sub.G, whereby the possibility of lasing outside the range .sub.G can be reduced. This can prevent the occurrence of significant mode hopping, and can operate the laser at a stable wavelength and with stable output power.

(97) In the preferred embodiments, the length L.sub.a of the active layer is set at 500 m or less (see FIG. 9). From this point of view, the length L.sub.a of the active layer is more preferably 300 m or less. The length L.sub.a of the active layer is preferably set at 150 m or more in terms of increasing the output from the laser.

(98) In Formula (6), d.sub.G/dT is a temperature coefficient of the Bragg wavelength.

(99) Furthermore, d.sub.TM/dT is the temperature coefficient of the wavelength satisfying the phase condition of the external resonator type laser.

(100) Here, .sub.TM is the wavelength satisfying the phase condition of the external resonator type laser, that is, the wavelength satisfying the phase condition of Formula (2-3) described above. This is called the longitudinal mode in the present specification.

(101) Now, the description of the longitudinal mode will be supplemented.

(102) In Formula (2-3), is 2n.sub.eff/ (i.e. =2n.sub.eff/)), n.sub.eff is an effective index therein, and satisfying this condition is .sub.TM. .sub.2 is a change in phase of the Bragg grating, and .sub.TM is shown in FIG. 15.

(103) G.sub.TM is a spacing between the wavelengths (longitudinal mode spacing) satisfying the phase condition of the external resonator type laser. There is a plurality of the wavelengths .sub.TM, which means the presence of a plurality of differences between the wavelengths .sub.TMs.

(104) Thus, by satisfying Formula (6), the temperature at which mode hopping occurs can be increased, thereby suppressing mode hopping in reality. Thus, the value of Formula (6) is more preferably 0.025 nm/ C.

(105) In the preferred embodiments, the length L.sub.WG of the grating element is set at 600 m or less (see FIG. 9). L.sub.WG is preferably 400 m or less, and more preferably 300 m or less. Furthermore, L.sub.WG is preferably 50 m or more.

(106) In the preferred embodiments, a length L.sub.g between the emission surface of the light source and the incidence surface of the optical waveguide is set at not less than 1 m nor more than 10 m (see FIG. 9). Thus, the stable oscillation is possible.

(107) In the preferred embodiments, an entire length L.sub.m of the incident portion and tapered portion is set at 100 m or less. Thus, the stable oscillation is promoted. The lower limit of the length L.sub.m of the propagation portion is preferably 10 m or more, and more preferably 20 m or more.

(108) In terms of increasing the tolerance of axial misalignment relative to the light source, the width W.sub.in of the ridge optical waveguide in the incident portion (see FIGS. 10, 11, and 13) is preferably 1.5 times or more as large as the mode-field diameter W.sub.h in the horizontal direction of the laser light. The width W.sub.in of the ridge optical waveguide in the incident portion is more preferably 2.5 times or less as large as the mode-field diameter W.sub.h in the horizontal direction of the laser light.

(109) In terms of increasing the tolerance of axial misalignment relative to the light source, the thickness W.sub.inv of the optical material layer in the incident portion (see FIG. 13) is preferably 1.5 times or more as large as the mode-field diameter W.sub.v in the vertical direction of the laser light. The thickness W.sub.inv of the optical material layer in the incident portion is preferably 2.5 times or less as large as the mode-field diameter W.sub.v in the vertical direction of the laser light.

(110) The mode-field diameters in the horizontal direction and the vertical direction of the laser light are measured in the following way.

(111) The term mode-field diameter as used herein is generally defined as a width of 1/e.sup.2 (where e is the base of natural logarithm: 2.71828) of the maximum (normally corresponding to the center of a core) in the light intensity distribution of the laser light, which is obtained by measurement. With regard to the laser light, the mode field has different sizes in the respective horizontal and vertical directions of the laser element, and thus the mode field is defined for each of the horizontal and vertical directions. For a concentric circular structure, like an optical fiber, the mode-field diameter is defined as the diameter of the structure.

(112) With regard to the measurement of the optical intensity distribution, generally, the measurement of a beam profile using a near-infrared camera or the measurement of optical intensities by a knife edge can produce the light intensity distribution of spots of the laser light.

(113) In the preferred embodiments, when W.sub.inv is the thickness of the optical material layer located in the incident portion, W.sub.inv/ is set at 2 or more. A too thick optical material layer leads to a significant coupling loss. Thus, W.sub.inv/ is preferably 3 or less.

(114) When the wavelength of the laser light is 0.85 m, W.sub.inv is not less than 1.7 m nor more than 2.55 m.

(115) In the present invention, the width W.sub.in of the ridge optical waveguide located in the incident portion is larger than the width W.sub.gr of the ridge optical waveguide in the grating portion. Note that the term width of the ridge optical waveguide as used herein means an interval between two corners on the cross section of a ridge part at its upper surface, the ridge part forming the optical waveguide (see FIG. 5).

(116) From the viewpoint of the present invention, W.sub.in/W.sub.gr is preferably 1.5 or more, and more preferably 2 or more. Any excessive W.sub.in/W.sub.gr tends to increase the substrate radiation in the Bragg grating. Thus, W.sub.in/W.sub.gr is preferably set at 3.5 or less.

(117) For example, when the wavelength of the laser light is 0.85 m, the width of the ridge optical waveguide in the incident portion is preferably 5 m or more, and also preferably 10 m or less. The ridge width of the grating portion is preferably not less than 3 nor more than 5. When the wavelength of the laser light is 0.85 m, the ridge width is preferably 2.55 m or more, and also 4.25 m or less.

(118) The incident portion and the grating portion are coupled together by the tapered portion. In the tapered portion, the ridge width W.sub.t is preferably decreased gradually from the incident portion toward the grating portion, and more preferably decreased in the form of linear function in the element longitudinal direction.

(119) It is noted that the width W.sub.out of each of the emission portions 3e and 23e may be the same as the width W.sub.gr of the ridge optical waveguide in the grating element, but may be smaller than W.sub.gr. W.sub.out/W.sub.gr is preferably 1.0 or less, and may be 0.5 or less. W.sub.out/W.sub.gr is preferably 0.7 or more in terms of the propagation efficiency.

(120) If the width of the ridge optical waveguide in the grating portion is too decreased, it is found that the mode shape will be distorted, causing the substrate radiation and reducing the reflectance of the grating.

(121) That is, when decreasing the ridge width to produce a single mode in the grating portion, the propagation light forms the spot shape such as that illustrated in FIG. 17. FIG. 17 shows the spot shape formed when the wavelength is 0.85 m, and the ridge width is 2 m. At this time, optical electric field leaks into the substrate, resulting in reduced optical electric field at the upper surface of the ridge. Thus, when forming the grating in the upper surface of the ridge waveguide, the stepped portion formed by the groove is less susceptible to the optical electric field, whereby the reflectance cannot be high.

(122) In contrast, FIG. 18 shows the spot shape formed when the width of the ridge optical waveguide is 3 m. In this case, it shows that the spot shape is ellipsoidal and no leakage occurs into the substrate in the substrate mode.

(123) Meanwhile, if the ridge width of the grating portion is too large, the laser is brought into the multi-mode, which means that there exists the optimal ridge width.

(124) FIG. 19 shows calculated values of the effective indexes of the waveguide in the fundamental mode that are obtained by changing the ridge width from 1 to 10 m with the thickness W.sub.inv of the optical material layer set at 2 m and T.sub.r set at 1.2 m. Based on this result, a region with the ridge width of 1 m or 2 m is an initial rise region of the effective index, which is close to a cutoff region.

(125) FIG. 20 is a calculated value of the propagation efficiency with the same ridge widths. As can be seen from the result, for the ridge widths 1 m and 2 m, the propagation efficiency is reduced due to radiation into the substrate in the substrate mode. For the ridge width of 5 m or more, the propagation efficiency tends to decrease, which is due to the multi-mode.

(126) To further improve the single mode characteristics of the grating portion, the thickness of the optical material layer in the grating portions 3d or 23d is set smaller than that of the optical material layer in the incident portions 3a or 23a (see FIG. 3(b) and FIG. 13).

(127) From this viewpoint, the ratio of the thickness of the optical material layer in the grating portions 3d or 23d to the thickness of the optical material layer in the incident portions 3a and 23a (W.sub.grv/W.sub.inv) is preferably 1.0 or less, and may be 0.7 or less. In terms of the propagation efficiency, the ratio is preferably 0.3 or more.

EXAMPLES

Example 1

(128) Grating elements shown in FIGS. 1(a), 2 and 4 was fabricated in the following way.

(129) Specifically, SiO.sub.2 was deposited in a thickness of 1 m by a sputtering device on a support substrate 5 made of quartz, and Ta.sub.2O.sub.5 was deposited thereon in a thickness of 2 m to form an optical material layer 1. Then, Ti was deposited on the optical material layer 1, and a grating pattern was fabricated by the photolithography technique.

(130) Thereafter, grating grooves were formed at a pitch interval of 205.4 nm with a length L.sub.b of 25 m by fluorine reactive ion etching using the Ti pattern as a mask. The groove depth td of the grating was set to 100 nm. Further, a waveguide was patterned by the photolithography technique to form a pattern therein, and a ridge groove process was performed by reactive ion etching to form the incident portion with the width W.sub.in of 8 m and the thickness T.sub.r of 1 m as well as the grating portion with the width W.sub.gr of 3 m and the thickness T.sub.r of 1 m. The length from the incidence surface to the starting point of the grating portion was set at 25 m.

(131) Thereafter, the input side and output side of the element were etched down to the quartz substrate by dry etching to thereby make mirror surfaces at end faces thereof. Finally, SiO.sub.2 was formed by sputtering to form a single-layer film of 90 nm in thickness at both end faces. At this time, the reflectance of the end face was 3%. The element size was set to have 1 mm width and 100 m length L.sub.wg.

(132) Regarding the optical characteristics of the grating element, the reflection characteristics of the grating element were evaluated from the transmission characteristics by inputting the light in the TE mode into the grating element using the superluminescent diode (SLD), which was a broadband wavelength light source, followed by analyzing the outgoing light therefrom with an optical spectrum analyzer.

(133) The measured reflection center wavelength of the grating element was 850 nm.

(134) Next, as illustrated in FIGS. 8 and 9, a laser module was mounted. As a light source element, a conventional GaAs-based laser was used.

(135) Specifications of the Light Source Element:

(136) Center Wavelength: 847 nm

(137) Output: 50 mW

(138) Full width at half maximum: 0.1 nm

(139) Length of laser element: 250 m

(140) Specifications of Mounting:

(141) L.sub.g: 1 m

(142) W.sub.in: 8 m

(143) W.sub.h: 3 m

(144) W.sub.in/W.sub.h: 2.7

(145) W.sub.inv: 2 m

(146) W.sub.v: 1 m

(147) W.sub.inv/W.sub.v: 2

(148) After being mounted, the laser module was driven under current control (Automatic Current Control: ACC) without using a Peltier element. The laser module had the laser characteristics that it oscillated at a center wavelength of 850 nm, which corresponded to the reflection wavelength of the grating, and its output was 30 mW, which was smaller than without such a grating element. Variations in output were within 1%, so that the stable output characteristics were obtained. Subsequently, the temperature dependency of the laser module at the lasing wavelength was measured at operating temperatures in a temperature range from 20 C. to 40 C. As a result, the temperature coefficient of the laser module at the lasing wavelength was 0.05 nm/ C.

Comparative Example 1

(149) A grating element having the same structure was formed in the same way as in Example 1. However, a ridge groove process was performed to form a light-source input portion with the width W.sub.in of 8 m and the thickness T.sub.r of 1 m as well as the grating portion with the width W.sub.gr of 8 m and the thickness T.sub.r of 1 m. The length from the input end face to the starting point of the grating portion was set at 25 m.

(150) Thereafter, the input side and output side of the element were etched down to the quartz substrate by dry etching to thereby make mirror surfaces at end faces thereof. Finally, SiO.sub.2 was formed by sputtering to form a single-layer film of 90 nm in thickness at both end faces. At this time, the reflectance of the end face was 3%. The element size was set to have 1 mm width and 100 m length L.sub.wg.

(151) Regarding the optical characteristics of the grating element, the reflection characteristics of the grating element were evaluated from the transmission characteristics by inputting the light in the TE mode into the grating element using the superluminescent diode (SLD), which was a broadband wavelength light source, followed by analyzing the outgoing light therefrom with the optical spectrum analyzer.

(152) The measured reflection center wavelength of the grating element was 850 nm. However, reflection was also measured on the shorter wavelength side than this, i.e. at a plurality of wavelengths 845 nm, 840 nm, and 836 nm.

(153) Next, as illustrated in FIG. 9, a laser module was mounted. As a light source element, a normal GaAs-based laser was used.

(154) Specifications of the Light Source Element:

(155) Center Wavelength: 847 nm

(156) Output: 50 mW

(157) Full width at half maximum: 0.1 nm

(158) Length of laser element: 250 m

(159) Specifications of Mounting:

(160) L.sub.g: 1 m

(161) W.sub.in: 8 m

(162) W.sub.h: 3 m

(163) W.sub.in/W.sub.h: 2.7

(164) W.sub.inv: 2 m

(165) W.sub.v: 1 m

(166) W.sub.inv/W.sub.v: 2

(167) After being mounted, the laser module was driven under current control (ACC) without using a Peltier element. The laser module had the laser characteristics that it oscillated at a center wavelength of 845 nm, which corresponded to the reflection wavelength of the grating, and its output was 30 mW, which was smaller than without such a grating element, but the substantially the same as in Example 1. Subsequently, the temperature dependency of the laser module at the lasing wavelength was measured at operating temperatures in a temperature range from 20 C. to 40 C. As a result, the temperature coefficient of the laser module at the lasing wavelength was 0.05 nm/ C. However, at around 30 C., the lasing wavelength significantly changed to 850.4 nm, causing a large change in output.

Example 2

(168) A grating element having the same structure was formed in the same way as in Example 1. However, a ridge groove process was performed to form the light-source input portion with the width W.sub.in of 8 m and the thickness T.sub.r of 1 m as well as the grating portion with the width W.sub.gr of 2 m and the thickness T.sub.r of 1 m. The length from the input end face to the starting point of the grating portion was set at 25 m.

(169) Thereafter, the input side and output side of the element were etched down to the quartz substrate by dry etching to thereby make mirror surfaces at end faces thereof. Finally, SiO.sub.2 was formed by sputtering to form a single-layer film of 90 nm in thickness at both end faces. At this time, the reflectance of the end face was 3%. The element size was set to have 1 mm width and 100 m length L.sub.wg.

(170) Regarding the optical characteristics of the grating element, the reflection characteristics of the grating element were evaluated from the transmission characteristics by inputting the light in the TE mode into the grating element using the superluminescent diode (SLD), which was a broadband wavelength light source, followed by analyzing the outgoing light therefrom with the optical spectrum analyzer. The measured reflection center wavelength of the grating element was 849.5 nm.

(171) Next, as illustrated in FIGS. 8 and 9, a laser module was mounted. As a light source element, a normal GaAs-based laser was used.

(172) Specifications of the Light Source Element:

(173) Center Wavelength: 847 nm

(174) Output: 50 mW

(175) Full width at half maximum: 0.1 nm

(176) Length of laser element: 250 m

(177) Specifications of Mounting:

(178) L.sub.g: 1 m

(179) W.sub.in: 8 m

(180) W.sub.h: 3 m

(181) W.sub.in/W.sub.h: 2.7

(182) W.sub.inv: 2 m

(183) W.sub.v: 1 m

(184) W.sub.inv/W.sub.v: 2

(185) After being mounted, the laser module was driven under current control (ACC) without using a Peltier element. The laser module had the laser characteristics that it oscillated at a center wavelength of 849.5 nm, which corresponded to the reflection wavelength of the grating, and its output was 10 mW, which was smaller than in Example 1. This is because the propagation loss became significant. However, variations in output were within 1%, so that the stable output characteristics were obtained. Subsequently, the temperature dependency of the laser module at the lasing wavelength was measured at operating temperatures in a temperature range from 20 C. to 40 C. As a result, the temperature coefficient of the laser module at the lasing wavelength was not varied, like Example 1, and was 0.05 nm/ C.

Example 3

(186) Elements shown in FIGS. 1 (a) and 3 were fabricated in the following way.

(187) Specifically, SiO.sub.2 was deposited in a thickness of 2 m by a sputtering device on a support substrate 5 made of quartz, and Ta.sub.2O.sub.5 was deposited thereon in a thickness of 2 m to thereby form an optical material layer 1. Then, Ti was deposited on the optical material layer 1, and a grating pattern was fabricated by the photolithography technique. Thereafter, grating grooves were formed at a pitch interval of 205.4 nm with a length L.sub.b of 25 m by the fluorine reactive ion etching using the Ti pattern as a mask. The groove depth t.sub.d of the grating was set to 100 nm. Further, a waveguide was patterned by the photolithography technique to form a pattern therein, and a ridge groove process was performed by the reactive ion etching to form the incident portion with the width W.sub.in of 8 m and the thickness T.sub.r of 1.6 m as well as the grating portion with the width W.sub.gr of 3 m and the thickness T.sub.r of 1.6 m. The length from the input end face to the starting point of the grating portion was set at 25 m.

(188) Thereafter, the grating portion was subjected to the reactive ion etching while masking the input portion and the tapered portion, thereby reducing the thickness of the Ta.sub.2O.sub.5 film to 1 m as a whole through the etching. In this way, as shown in FIG. 3(b), the thickness of the optical material layer in the grating element was smaller than that of the optical material layer in the incident portion and tapered portion. At this time, the groove depth td of the grating was set at 40 nm, while the groove depth of the ridge waveguide was set at 0.6 m.

(189) Subsequently, the input side and output side of the element were etched down to the quartz substrate by dry etching to thereby make mirror surfaces at end faces of the element. Finally, SiO.sub.2 was formed by sputtering to form a single-layer film of 90 nm in thickness at both end faces. At this time, the reflectance of the end face was 3%. The element size was set to have 1 mm width and 100 m length L.sub.wg.

(190) Regarding the optical characteristics of the grating element, the reflection characteristics of the grating element were evaluated from the transmission characteristics by inputting the light in the TE mode into the grating element using the superluminescent diode (SLD), which was a broadband wavelength light source, followed by analyzing the outgoing light therefrom with the optical spectrum analyzer.

(191) The measured reflection center wavelength of the grating element was 848 nm.

(192) Next, as illustrated in FIG. 9, a laser module was mounted. As a light source element, a normal GaAs-based laser was used.

(193) Specifications of the Light Source Element:

(194) Center Wavelength: 847 nm

(195) Output: 50 mW

(196) Full width at half maximum: 0.1 nm

(197) Length of laser element: 250 m

(198) Specifications of Mounting:

(199) L.sub.g: 1 m

(200) W.sub.in: 8 m

(201) W.sub.h: 3 m

(202) W.sub.in/W.sub.h: 2.7

(203) W.sub.inv: 2 m

(204) W.sub.v: 1 m

(205) W.sub.inv/W.sub.v: 2

(206) W.sub.grv: 1 m

(207) W.sub.inv/Wgrv: 2

(208) After being mounted, the laser module was driven under current control (ACC) without using a Peltier element. The laser module had the laser characteristics that it oscillated at a center wavelength of 848 nm, which corresponded to the reflection wavelength of the grating, and its output was 30 mW, which was smaller than without such a grating element. Variations in output were within 1%, so that the stable output characteristics were obtained. Subsequently, the temperature dependency of the laser module at the lasing wavelength was measured at operating temperatures in a temperature range from 20 C. to 40 C. As a result, the temperature coefficient of the laser module at the lasing wavelength was 0.05 nm/ C.

(209) As an additional experiment, the tolerance of axial misalignment between the semiconductor laser and the grating element was measured. First, the semiconductor laser as a light source element and the grating element was installed on an optical alignment device, and the optical axis were adjusted to maximize the amount of light output from the grating element. From this state, the axis was shifted every 0.1 m only in the horizontal direction, followed by measuring changes in the amount of light. The tolerance of axial misalignment in the horizontal direction was defined as an amount of axial misalignment produced when the amount of light was 25 mW. The tolerance of axial misalignment in the vertical direction was also measured in the same way, thus measuring the amount of axial misalignment.

(210) The measurement results in Examples 1, 3, and Comparative Example 1 were as follows.

(211) TABLE-US-00001 TABLE 1 Tolerance of axial misalignment Horizontal Vertical direction direction Example 1 2 0.8 Example 3 2 0.8 Comparative 2 0.8 Example 1

(212) Units (m)

Comparative Example 2

(213) A grating element was formed in the same way as in Example 1, and then the laser module illustrated in FIG. 9 was mounted.

(214) Specifications of the Light Source Element:

(215) Center Wavelength: 847 nm

(216) Output: 50 mW

(217) Full width at half maximum: 0.1 nm

(218) Length of laser element: 250 m

(219) Specifications of Mounting:

(220) L.sub.g: 1 m

(221) W.sub.in: 3 m

(222) W.sub.h: 3 m

(223) W.sub.in/W.sub.h: 1

(224) W.sub.inv: 2 m

(225) W.sub.v: 1 m

(226) W.sub.inv/W.sub.v: 2

(227) As a result, the measurements of the tolerance of axial misalignment for this element were 1.3 m in the horizontal direction and 0.8 m in the vertical direction.

(228) After being mounted, the laser module was driven under current control (ACC) without using a Peltier element. The laser module had the laser characteristics that it oscillated at a center wavelength of 850 nm, which corresponded to the reflection wavelength of the grating, and its output was 30 mW. Variations in output were within 1%, so that the stable output characteristics were obtained. Subsequently, the temperature dependency of the laser module at the lasing wavelength was measured at operating temperatures in a temperature range from 20 C. to 40 C. As a result, the temperature coefficient of the laser module at the lasing wavelength was not varied, like Example 1, and was 0.05 nm/ C.

Comparative Example 3

(229) Next, an element illustrated in FIGS. 1(a) and 3 was fabricated, and then a laser module was mounted as illustrated in FIG. 9.

(230) Specifications of the Light Source Element:

(231) Center Wavelength: 847 nm

(232) Output: 50 mW

(233) Full width at half maximum: 0.1 nm

(234) Length of laser element: 250 m

(235) Specifications of Mounting:

(236) L.sub.g: 1 m

(237) W.sub.in: 3 m

(238) W.sub.h: 3 m

(239) W.sub.in/W.sub.h: 1

(240) W.sub.inv: 2 m

(241) W.sub.v: 1 m

(242) W.sub.inv/W.sub.v: 2

(243) W.sub.grv: 1 m

(244) W.sub.inv/W.sub.grv: 2

(245) As a result, the measurements of the tolerance of axial misalignment for this element were 1.3 m in the horizontal direction and 0.8 m in the vertical direction.

(246) After being mounted, the laser module was driven under current control (ACC) without using a Peltier element. The laser module had the laser characteristics that it oscillated at a center wavelength of 848 nm, which corresponded to the reflection wavelength of the grating, and its output was 30 mW. Variations in output were within 1%, so that the stable output characteristics were obtained. Subsequently, the temperature dependency of the laser module at the lasing wavelength was measured at operating temperatures in a temperature range from 20 C. to 40 C. As a result, the temperature coefficient of the laser module at the lasing wavelength was 0.05 nm/ C.

Comparative Example 4

(247) A grating element was formed in the same way as in Example 1, and then the laser module illustrated in FIG. 9 was mounted.

(248) Specifications of the Light Source Element:

(249) Center Wavelength: 847 nm

(250) Output: 50 mW

(251) Full width at half maximum: 0.1 nm

(252) Length of laser element: 250 m

(253) Specifications of Mounting:

(254) L.sub.g1 m

(255) W.sub.in: 3 m

(256) W.sub.h: 3 m

(257) W.sub.in/W.sub.h: 1

(258) W.sub.inv: 1 m

(259) W.sub.v: 1 m

(260) W.sub.inv/W.sub.v: 1

(261) As a result, the measurements of the tolerance of axial misalignment for this element were 1.3 m in the horizontal direction and 0.4 m in the vertical direction.

(262) After being mounted, the laser module was driven under current control (ACC) without using a Peltier element. The laser module had the laser characteristics that it oscillated at a center wavelength of 848 nm, which corresponded to the reflection wavelength of the grating, and its output was 35 mW. Variations in output were within 1%, so that the stable output characteristics were obtained. Subsequently, the temperature dependency of the laser module at the lasing wavelength was measured at operating temperatures in a temperature range from 20 C. to 40 C. As a result, the temperature coefficient of the laser module at the lasing wavelength was not varied, like Example 1, and was 0.05 nm/ C.

Grating element and external resonator type light emitting device

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Cpc classification

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G02B6/122

PHYSICS

Classification Explorer

G02B6/124

PHYSICS

Classification Explorer

H01S5/141

ELECTRICITY

Classification Explorer

G02B6/42

PHYSICS

Classification Explorer

G02B6/305

PHYSICS

Classification Explorer

G02B6/1228

PHYSICS

Classification Explorer

G02B6/12016

PHYSICS

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G02B6/12019

PHYSICS

International classification

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H01S5/14

ELECTRICITY

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G02B6/122

PHYSICS

Classification Explorer

G02B6/124

PHYSICS

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G02B6/30

PHYSICS

Classification Explorer

G02B6/42

PHYSICS

Classification Explorer

G02B6/12

PHYSICS

Abstract

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Description