CN109768468B - Semiconductor laser - Google Patents
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Abstract
The invention discloses a semiconductor laser, comprising: along the growth direction of the epitaxial layer, the epitaxial layer consists of a lower flat waveguide and an upper ridge waveguide; the optical fiber is composed of a back side ridge width invariable area and a front side ridge width narrowing area along the optical transmission direction, wherein the ridge width of a ridge waveguide on the upper part of the ridge width invariable area is kept invariable, the ridge width of the ridge waveguide on the upper part of the ridge width narrowing area is gradually narrowed, the ridge width of the wide ridge end of the ridge width narrowing area is equal to the ridge width of the ridge width invariable area, the ridge width of the narrow ridge end of the ridge width narrowing area is smaller than the ridge width of the wide ridge end, and the narrow ridge end is an optical output end; in the ridge width narrowing region, the equivalent refractive index of the upper ridge waveguide gradually decreases as the ridge width gradually narrows until the equivalent refractive index of the lower slab waveguide is approached at the narrow ridge end, and the longitudinal and transverse dimensions of the fundamental mode at the narrow ridge end are increased, so that the longitudinal and transverse divergence angles of the output light are reduced. The divergence angle of the output light of the semiconductor laser provided by the invention is smaller than that of the semiconductor laser with the common ridge waveguide structure.
Description
Technical Field
The invention relates to the technical field of semiconductor lasers, in particular to a semiconductor laser.
Background
In the coming era of interconnection of everything, the demand for people-to-people, people-to-things, and things-to-things communication has increased dramatically, so that optical communication systems play an increasingly important role in modern information society. Various living, service and industrial applications derived based on developed communication infrastructures are related to national technological advancement, national strength improvement and more convenient life of people. The reliable and high-quality performance of the key basic components in the optical communication system, such as the transmitter and the receiver, determines whether the optical communication system can operate stably, orderly and efficiently for a long time with low energy consumption.
In an optical communication system, after signal light is generated, the signal light needs to be coupled with an optical fiber with high efficiency and low loss so as to transmit information to a receiving subsystem, and the semiconductor laser has the advantages of small volume, easiness in integration, low power consumption and the like, so that the semiconductor laser is determined to be a very suitable light source. However, the common ridge waveguide structure of the quantum well semiconductor laser determines that the light spot output by the laser is a small-sized elliptical light spot, i.e. the laser with the structure has larger transverse and longitudinal far-field divergence angles, namely, the divergence angle is about 20 degrees multiplied by 40 degrees. In order to improve the coupling efficiency with the single-mode fiber, generally, a semiconductor laser adopts a corresponding packaging strategy according to the quality of a divergence angle, for example, a semiconductor laser with a larger far-field divergence angle is packaged by a large ball lens; the end face of the single mode fiber coupled with the optical fiber is processed to a certain degree, such as a lens fiber, but the coupling efficiency is still not high, and great energy waste occurs. In view of the above, researchers at home and abroad have proposed a series of solutions to reduce the far field divergence angle of the semiconductor laser, thereby reducing the packaging cost and reducing unnecessary energy waste.
For example, V.Vusirika et al proposed a Low Loss, high Alignment tolerance dilute ridge Waveguide semiconductor Laser (GaAs-AlGaAs QW partitioned Waveguide Laser with Low Loss, Alignment-Laser Coupling to a Single-Mode Fiber, IEEE Photonics Technology Letters, Vol.8, No.9, September 1996), by reducing the refractive index difference between the core layer and the cladding layer, the optical field limiting factor of the fundamental Mode is reduced, the spot size of the fundamental Mode is increased, and the purpose of reducing the far field divergence angle is achieved.
For example, a slab-coupled waveguide structure (US 6928223B2) proposed by Joseph p. donnelly et al, based on the slab-coupled waveguide theory, reduces the optical field limiting factor of the fundamental mode while ensuring that only the fundamental mode is supported, obtains a large-size fundamental mode light spot, and finally obtains a smaller far-field divergence angle. The common point of the two methods is that the purpose of reducing the far field divergence angle is achieved by reducing the light field limiting factor of the fundamental mode. However, a smaller optical field limiting factor means a smaller mode gain, and therefore, the two types of semiconductor lasers have larger threshold currents, and require longer device lengths and higher injection currents for proper operation.
The laser with beam shape modification (CN 104380545B, US 9401582B2) proposed by cristia stagarrescu et al does not need to reduce the optical field limiting factor of the fundamental mode nor regrow a core layer with a lower refractive index at the light exit end, but achieves the purpose of improving the far-field divergence angle by etching or integrating the inclined raise, step, reflective ceiling and reflective sidewall at the light exit end, but this method easily causes a sub-peak with low intensity in the far-field spot, and reduces the spot quality to some extent.
The narrow divergence angle ridge waveguide semiconductor laser (CN 104466675B) proposed by Chinese researchers, such as car-shines, introduces the expanding waveguide layer as described in the patent, and can reduce the transverse and longitudinal divergence angles of the device by utilizing the stretching effect of the expanding waveguide layer below the ridge waveguide on the fundamental mode. However, there is still much room for improvement in the smaller divergence angle obtained by this method.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to solve the problems of larger threshold current, longer device length, secondary epitaxial growth and butt joint, poorer far-field quality, less ideal small divergence angle and the like caused by improving the divergence angle of a semiconductor laser in the prior art.
To achieve the above object, the present invention provides a semiconductor laser including: the transverse structure of the semiconductor laser is formed by a lower flat waveguide and an upper ridge waveguide along the growth direction of an epitaxial layer;
dividing the semiconductor laser into a back-side ridge width invariant region and a front-side ridge width narrow-narrowing region along a light transmission direction, wherein the light transmission direction is vertical to the growth direction of the epitaxial layer, the ridge width invariant region consists of a part of slab waveguide and a ridge waveguide on the upper part of the slab waveguide, and the ridge width narrow-narrowing region consists of another part of slab waveguide and a ridge waveguide on the upper part of the slab waveguide; in the ridge width invariable area, the ridge width of the ridge waveguide on the upper part of the ridge width invariable area is kept invariable, in the ridge width narrowing area, the ridge width of the ridge waveguide on the upper part of the ridge width invariable area is gradually narrowed along the light output direction, the ridge width of the wide ridge end of the ridge width narrowing area is equal to the ridge width of the ridge width invariable area, and the ridge width of the narrow ridge end of the ridge width narrowing area is smaller than the ridge width of the wide ridge end of the ridge width narrowing area, wherein the narrow ridge end is the light output end of the semiconductor laser;
in the ridge width narrowing region, along the light output direction, along with the gradual narrowing of the ridge width, the equivalent refractive index of the upper ridge waveguide gradually decreases until the equivalent refractive index of the upper ridge waveguide approaches the equivalent refractive index of the lower slab waveguide at the narrow ridge end, and the longitudinal dimension and the transverse dimension of the fundamental mode perpendicular to the light transmission direction at the narrow ridge end are both increased relative to the ridge width invariant region, so that the longitudinal divergence angle and the transverse divergence angle of the output light of the semiconductor laser are both decreased relative to the ridge width invariant region.
Specifically, the fundamental mode is an optical mode in which the single transverse mode optical waveguide can maintain the best beam quality. The equivalent refractive index of the upper ridge waveguide gradually decreases until it is close to that of the lower slab waveguide, which means that the equivalent refractive index of the upper ridge waveguide decreases to approximately equal the equivalent refractive index of the lower slab waveguide. It will be understood by those skilled in the art that the equivalent refractive index of the upper ridge waveguide is theoretically reduced to be equal to that of the lower slab waveguide, but it is difficult to achieve complete equivalence through practical experiments and simulations, and therefore, the approach expression is used herein.
In one possible example, the end plane of one side of the ridge width unchanged area is plated with a high reflection film, the other side of the ridge width unchanged area is connected with the wide ridge end of the ridge width narrowed area, and the end plane of the narrow ridge end of the ridge width narrowed area is plated with an anti-reflection film.
In one possible example, the lower slab waveguide comprises, in order along the epitaxial layer growth direction: substrate layer, buffer layer, lower waveguide sandwich layer, lower spacer layer.
In one possible example, the upper ridge waveguide comprises, in order along the epitaxial layer growth direction: a lower spacer layer, a lower barrier layer, a lower confinement layer, an active layer, an upper confinement layer, an upper barrier layer, an upper spacer layer, an etch stop layer, an upper cladding layer, a sub-cladding layer, and a top cladding layer.
In one possible example, the length of the ridge width invariant region is 150um, the ridge width is 2um, the length of the ridge width narrowing region is 150um, the ridge width of the wide ridge end is 2um, and the ridge width of the narrow ridge end is 0.5 um.
In one possible example, the narrowing of the ridge width narrowing region is linear, polygonal, parabolic or exponential.
In one possible example, the semiconductor laser outputs light having a longitudinal divergence angle and a lateral divergence angle that are both less than 15 °.
In one possible example, the lower waveguide core layer may be a single layer medium or a multilayer index-graded medium.
Generally, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects:
1. easy to manufacture and compatible with the existing process
Compared with a semiconductor laser with a common ridge waveguide structure, the growth of the lower waveguide core layer and the growth of the lower spacing layer are only needed to be increased in the growth of the epitaxial layer; during post-processing, only the pre-designed mask plate with the ridge width narrowing pattern needs to be replaced, other process flows are completely the same as those of a semiconductor laser with a common ridge waveguide structure, and mode converters with different core layers do not need to be grown in an additional mode converter in a butt joint mode at the light emitting end.
2. Lower threshold, small size
Different from a semiconductor laser adopting a dilute ridge waveguide structure and a slab coupling waveguide structure, the semiconductor laser provided by the invention does not need to reduce the optical field limiting factor of a fundamental mode deliberately so as to increase the size of a fundamental mode light spot and further obtain a smaller far-field divergence angle. By adjusting the structural parameters of the lower waveguide, the lower waveguide basically has no influence on the fundamental mode in the ridge width unchanged area, most of the fundamental mode is still limited in the ridge waveguide on the upper part of the device, and the laser part in the area has an optical field limiting factor, namely a mode gain, which is equivalent to that of a semiconductor laser with a common ridge waveguide structure; in the ridge width narrowing region, the optical field limiting factor of the fundamental mode is reduced due to the gradual increase of the size of the fundamental mode, but the threshold current of the device cannot reach 100mA or even more due to the smaller increase of the material loss and the ridge width unchanged region which can be properly lengthened. Meanwhile, the larger spot size at the light exit end of the device also reduces the risk of catastrophic optical damage to the end face of the device.
3. Smaller far field divergence angle and low packaging cost
In the ridge width narrowing region, the equivalent refractive index of the upper waveguide, namely the upper ridge waveguide, is gradually reduced along with the gradual narrowing of the ridge width until the equivalent refractive index of the upper waveguide is close to that of the lower waveguide. In the process, as can be known from the coupling waveguide principle, the fundamental mode gradually transits from a mode mainly concentrated in the upper ridge waveguide to a super mode (even mode) distributed in both the upper ridge waveguide and the lower slab waveguide, that is, the longitudinal size of the narrow ridge end fundamental mode is increased; meanwhile, due to the narrowing of the ridge width, the transverse limitation of the ridge to light is gradually weakened, so that the transverse size of the base mode at the end of the narrow ridge is also increased. If the longitudinal and transverse dimensions of the fundamental mode of the narrow ridge end, which is the light output end, are simultaneously increased, the longitudinal and transverse divergence angles of the output light will be reduced.
After a series of optimization design is carried out on the structure provided by the invention, for example, on the premise of ensuring the two advantages, the lower waveguide core layer structure, the thickness and the refractive index of the lower spacing layer are properly adjusted, and the simulation verification of optical commercial software is adopted, so that the effect of improving the far-field divergence angle is better compared with a narrow divergence angle ridge waveguide semiconductor laser (CN 104466675B) provided by Chinese researchers, such as a car glow and the like, and the transverse and longitudinal far-field divergence angles can be below 15 degrees and even reach about 10 degrees. Such a small divergence angle allows a small ball lens to be used in packaging, thereby reducing packaging costs.
Drawings
Fig. 1 is a three-dimensional structural view of a semiconductor laser having a small divergence angle provided by the present invention;
fig. 2 is a top view of a semiconductor laser provided by the present invention;
fig. 3(a) is a lateral divergence angle curve of a semiconductor laser corresponding to embodiment example 1 of the present invention;
fig. 3(b) is a longitudinal divergence angle curve of a semiconductor laser corresponding to embodiment 1 of the present invention;
fig. 4(a) is a lateral divergence angle curve of a semiconductor laser corresponding to embodiment 2 of the present invention;
fig. 4(b) is a longitudinal divergence angle curve of a semiconductor laser corresponding to embodiment 2 of the present invention;
the same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein: i is a ridge width narrowing region, II is a ridge width invariant region, 1 is a substrate layer, 2 is a buffer layer, 3 is a lower waveguide core layer, 4 is a lower spacer layer, 5 is a lower barrier layer, 6 is a lower separate confinement layer, 7 is an active layer, 8 is an upper separate confinement layer, 9 is an upper barrier layer, 10 is an upper spacer layer, 11 is an etch stop layer, 12 is an upper cladding layer, 13 is a sub-cladding layer, and 14 is a top cladding layer.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
The invention provides a semiconductor laser, which provides an optional excellent scheme with easy manufacture, low cost and better performance for key components of an optical communication system. In order to obtain a semiconductor laser with a small far-field divergence angle without introducing complicated processes and without causing possible adverse effects, a semiconductor laser having a dual waveguide structure consisting of a slab waveguide in a lower part of the device and a ridge waveguide in an upper part of the device has been proposed.
The invention adopts the following technical scheme:
the invention provides a semiconductor laser with small divergence angle, based on the coupling waveguide principle, the structure is as follows: along the growth direction of the epitaxial layer, the device consists of a slab waveguide at the lower part of the device and a ridge waveguide at the upper part of the slab waveguide; the light-transmitting direction includes a ridge width-constant region on the back side and a ridge width-narrowing region on the front side.
The lower slab waveguide sequentially comprises the following components in the growth direction of the epitaxial layer: the waveguide structure comprises a substrate layer, a lower buffer layer, a lower waveguide core layer and a lower spacing layer.
The upper ridge waveguide sequentially comprises along the growth direction of the epitaxial layer: a lower spacer layer, a lower barrier layer, a lower confinement layer, an active layer, an upper confinement layer, an upper barrier layer, an upper spacer layer, an etch stop layer, an upper cladding layer, a sub-cladding layer, and a top cladding layer.
The core layer of the lower slab waveguide can be a single-layer medium or a multi-layer medium with gradually changed refractive index.
The lower slab waveguide is separated from the upper ridge waveguide by a single or multiple layers of dielectric material of suitable thickness and suitable refractive index.
The single-layer or multi-layer dielectric material is generally referred to as a lower spacing layer or an N-type spacing layer in the invention, and the lower spacing layer is an upper cladding layer of a lower flat waveguide of the device and a lower cladding layer of an upper ridge waveguide of the device.
The ridge waveguide on the upper part of the device is similar to the transverse structure of a semiconductor laser with a common ridge waveguide structure.
The ridge width of the ridge width invariant region on the back side of the device is of the double-waveguide structure, and the ridge width of the ridge waveguide on the upper portion of the ridge width invariant region is invariant along the light transmission direction of the laser.
The ridge width narrowing region on the front side of the device has a transverse structure of the double-waveguide structure, the ridge width of the ridge waveguide on the upper portion of the device is gradually narrowed along the light transmission direction of the laser, and the ridge width of the ridge width unchangeable region is narrowed to another narrower ridge width. The narrowing of the ridge width can be linear, broken, parabolic or exponential.
The semiconductor laser with small divergence angle provided by the invention has the ridge width invariant region and the ridge width narrowing region, the same post-process treatment is simultaneously carried out on the same substrate, and the epitaxial growth sequence of the substrate is as follows in sequence: the waveguide structure comprises a substrate layer, a buffer layer, a lower waveguide core layer, a lower spacing layer, a lower barrier layer, a lower respectively limiting layer, an active layer, an upper respectively limiting layer, an upper barrier layer, an upper spacing layer, an etching stop layer, an upper cladding layer, a secondary cladding layer and a top cladding layer. In subsequent processing, the upper cladding layer, the sub-cladding layer and the top cladding layer form the ridge of the device after appropriate etching.
The invention provides a three-dimensional structure diagram of a semiconductor laser with a small divergence angle, which is shown in figure 1. It can be seen that the substrate used by the device is of a dual-waveguide structure, and is composed of a slab waveguide at the lower part of the device and a ridge waveguide at the upper part of the device, and the two waveguides are separated by a dielectric material with proper thickness and proper refractive index; the ridge width invariant regions and the ridge width narrowed regions have the same lateral structure, except that the ridge width of the ridge width narrowed regions is gradually reduced. The substrate comprises the following components in sequence from the substrate layer to the top covering layer: the waveguide structure comprises a substrate layer 1, a buffer layer 2, a lower waveguide core layer 3, a lower spacing layer 4, a lower barrier layer 5, a lower separate limiting layer 6, an active layer 7, an upper separate limiting layer 8, an upper barrier layer 9, an upper spacing layer 10, an etch stop layer 11, an upper cladding layer 12, a secondary cladding layer 13 and a top cladding layer 14.
Fig. 2 is a top view of the device, and it can be seen that the device is divided into a ridge width narrowing region I and a ridge width invariant region II, where the initial ridge width of the ridge width narrowing region I is the same as the ridge width of the ridge width invariant region II, and the ridge width at the narrow end is an analog optimized value, such as 0.5 um. In order to reduce the loss of the device, the ridge width narrowing region also needs to be covered by an electrode. And, the left end of the device is plated with a high anti-reflection film HR, and the right end of the device, namely the narrow end, is plated with an anti-reflection film AR.
The structure, technical solutions and performance advantages of the present invention will be explained below with reference to the accompanying drawings and examples. It should be noted that the following examples are for the purpose of explaining the present invention, are only two specific examples of the present invention, and are not intended to limit the present invention.
The semiconductor laser in this example has an operating wavelength of 1310nm, and its three-dimensional structure diagram is shown in fig. 1, and the three-dimensional structure diagram sequentially includes: the waveguide grating comprises a substrate layer 1, a buffer layer 2, a lower waveguide core layer 3, a lower spacing layer 4, a lower barrier layer 5, a lower separate limiting layer 6, an active layer 7, an upper separate limiting layer 8, an upper barrier layer 9, an upper spacing layer 10, an etch stop layer 11, an upper cladding layer 12, a secondary cladding layer 13 and a top cladding layer 14, wherein the materials, doping types and thicknesses of the layers are as shown in Table 1:
table 1 table of materials, doping types and thicknesses used for the layers of example 1
Wherein, the upper cladding 12, the secondary cladding 13 and the top cladding 14 form the ridge of the device through etching, and the bottom of the substrate layer 1 and the top of the top cladding 14 are respectively plated with an electrode layer.
FIG. 2 is a top view of the example, where region II is a ridge width invariant region, providing most of the gain of the device, with a length of about 150um, a ridge width of 2um, and a left wide end coated with a high reflective film; district I is the ridge width and receives the narrow district, provides the great exit light spot of size, and length is about 150um, and wide end ridge width is 2um, and narrow end ridge width is about 0.5um, and the right side narrow end plates anti-reflection coating. The two areas can be formed by adopting a pre-designed mask plate to carry out etching treatment without a butt joint growth process with a complex process.
The far-field divergence angle curves of the emergent light spots of the light-emitting end of the structure are obtained by simulation of optical commercial software, and are shown in fig. 3(a) and 3(b), wherein fig. 3(a) is a transverse divergence angle curve, fig. 3(b) is a longitudinal divergence angle curve, and the transverse far-field divergence angle and the longitudinal far-field divergence angle are respectively 8 degrees and 12 degrees.
Example 2, a second semiconductor laser with a small divergence angle according to the present invention.
The operating wavelength of the semiconductor laser in this example is also 1310nm, and its three-dimensional structure diagram is shown in fig. 1, and the following are sequentially from the substrate layer to the top cladding layer: substrate layer 1, buffer layer 2, lower waveguide core layer 3, lower spacer layer 4, lower barrier layer 5, lower confinement layer 6, active layer 7, upper confinement layer 8, upper barrier layer 9, upper spacer layer 10, etch stop layer 11, upper cladding layer 12, sub-cladding layer 13, top cladding layer 14, the materials, doping types and thicknesses used for each layer are as in table 2:
table 2 table of materials, doping types and thicknesses used for the layers of example 2
| Dielectric layer | Material | Doping type | Thickness (nm) | Remarks for | |
| Substrate layer | |||||
| 1 | InP | N type | --- | ||
| Buffer layer 2 | InP | N type | 300 | ||
| Lower waveguide core layer 3 | InGaAsP | N type | 1300 | Multi-layer graded-index medium | |
| Lower spacer layer 4 | InGaAsP | N type | 3000 | ||
| Lower barrier layer 5 | | N type | 40 | ||
| Lower limiting layer 6 | InAlGaAs | --- | 60 | ||
| Active layer 7 | InAlGaAs | --- | --- | 10 barrier 9 well overlap growth | |
| Upper respective limiting layer 8 | InAlGaAs | --- | 60 | ||
| Upper barrier layer 9 | | P type | 40 | ||
| |
| P type | 80 | ||
| Etch stop layer 11 | InGaAsP | P type | 16 | ||
| Upper cladding layer 12 | InP | P type | 1600 | ||
| Secondary coating 13 | InGaAsP | P type | 0.1 | ||
| Top cover layer 14 | InGaAs | P type | 0.15 |
Wherein, the upper cladding 12, the secondary cladding 13 and the top cladding 14 form the ridge of the device through etching, and the bottom of the substrate layer 1 and the top of the top cladding 14 are respectively plated with an electrode layer.
FIG. 2 is a top view of the example, where region II is a ridge width invariant region, providing most of the gain of the device, with a length of about 150um, a ridge width of 2um, and a left wide end coated with a high reflective film; district I is the ridge width and receives the narrow district, provides the great facula of emergence of size, and length is about 150um, and wide end ridge width is 2um equally, and narrow end ridge width is about 0.5um, and the right side narrow end plates anti-reflection coating. The two areas can be formed by adopting a pre-designed mask plate to carry out etching treatment without a butt joint growth process with a complex process.
The far-field divergence angle curves of the emergent light spots of the light-emitting end of the structure are obtained by simulation of optical commercial software, and are shown in fig. 4(a) and 4(b), wherein fig. 4(a) is a transverse divergence angle curve, fig. 4(b) is a longitudinal divergence angle curve, and the transverse far-field divergence angle and the longitudinal far-field divergence angle are respectively 12 degrees and 14 degrees.
The semiconductor laser with small divergence angle has double-waveguide structure and one section of gradually changed ridge waveguide area. The structure can reduce the transverse divergence angle and the longitudinal divergence angle of the semiconductor photoelectric device with the common ridge waveguide structure to a greater extent, and can effectively solve the problems of large far-field divergence angle, low coupling efficiency and the like of the semiconductor photoelectric device with the common ridge waveguide structure. Moreover, the invention has the characteristics of small volume, compatibility with the existing manufacturing process, low packaging cost and the like.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (4)
1. A semiconductor laser is characterized in that the lateral structure of the semiconductor laser is formed by a lower flat waveguide and an upper ridge waveguide along the growth direction of an epitaxial layer; the slab waveguide of lower part includes in proper order along epitaxial layer growth direction: the waveguide structure comprises a substrate layer, a buffer layer, a lower waveguide core layer and a lower spacing layer; the ridge waveguide on the upper part sequentially comprises the following components in the growth direction of the epitaxial layer: a lower spacer layer, a lower barrier layer, a lower confinement layer, an active layer, an upper confinement layer, an upper barrier layer, an upper spacer layer, an etch stop layer, an upper cladding layer, a sub-cladding layer, and a top cladding layer; the lower spacing layer is a component of the slab waveguide at the lower part and a component of the ridge waveguide at the upper part;
dividing the semiconductor laser into a back-side ridge width invariant region and a front-side ridge width narrow-narrowing region along a light transmission direction, wherein the light transmission direction is vertical to the growth direction of the epitaxial layer, the ridge width invariant region consists of a part of slab waveguide and a ridge waveguide on the upper part of the slab waveguide, and the ridge width narrow-narrowing region consists of another part of slab waveguide and a ridge waveguide on the upper part of the slab waveguide; in the ridge width invariable area, the ridge width of the ridge waveguide at the upper part is kept invariable, in the ridge width narrowing area, the ridge width of the ridge waveguide at the upper part is gradually narrowed along the light output direction, the ridge width of the wide ridge end of the ridge width narrowing area is equal to the ridge width of the ridge width invariable area, the wide ridge end is connected with the ridge width invariable area, and the ridge width of the narrow ridge end of the ridge width narrowing area is smaller than the ridge width of the wide ridge end, wherein the narrow ridge end is the light output end of the semiconductor laser;
in the ridge width narrowing region, along the light output direction, as the ridge width is gradually narrowed, the equivalent refractive index of the upper ridge waveguide is gradually reduced until the equivalent refractive index of the upper ridge waveguide is equal to the equivalent refractive index of the lower slab waveguide, so that at the narrow ridge end, the fundamental mode is uniformly distributed in the upper ridge waveguide and the lower slab waveguide, the longitudinal size of the fundamental mode is increased, the transverse limitation of the ridge to the light is weakened due to the narrowed ridge width, the transverse size of the fundamental mode at the narrow ridge end is also increased, the longitudinal size and the transverse size of the fundamental mode perpendicular to the light transmission direction at the narrow ridge end are both increased relative to the ridge width unchanged region, and the longitudinal divergence angle and the transverse divergence angle of the output light of the semiconductor laser are both smaller than 15 degrees.
2. The semiconductor laser as claimed in claim 1, wherein the end plane of the side of the ridge width invariant region not connected with the wide ridge end is plated with a high reflection film, and the end plane of the narrow ridge end of the ridge width narrowing region is plated with an anti-reflection film.
3. The semiconductor laser of claim 1 or 2, wherein the narrowing of the ridge width narrowing region is linear, polygonal, parabolic, or exponential.
4. A semiconductor laser as claimed in claim 1 wherein the core layer of the lower slab waveguide is a single layer medium or a multilayer graded index medium.
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