A grating surface emitting semiconductor laser

Abstract

The present invention discloses a grating surface-emitting semiconductor laser, belonging to the field of laser technology. The laser comprises a surface metal electrode layer, a ridge waveguide layer, a p-type doped cladding layer, a p-type doped light confinement layer, an active region, a first-order grating layer, an n-type doped light confinement layer, an isolation layer, and a substrate layer, arranged in sequence. A high-order surface grating is provided above the active region. The ridge waveguide has a gradually varying width in the overlapping region with the high-order surface grating, forming a gradual change in refractive index, so that the laser emission angle forms an angle with the normal direction of the laser surface. A relatively small change in the ridge waveguide width can achieve a relatively large equivalent refractive index change, resulting in a good equivalent "chirp" effect, which allows the laser emission direction to deviate from the normal direction of the laser surface.

Description

Technical Field

[0001] The present invention belongs to the technical field of lasers, and more particularly relates to a horizontal cavity surface emitting semiconductor laser with tunable light emission angle. Background Art

[0002] With the advancement of science and technology, laser technology, as a representative of high-tech, has gained increasingly widespread application, including military applications such as laser guidance, lidar, and laser weapons, as well as civilian applications such as laser communications, laser cosmetology, and laser processing. Laser technology has brought tremendous economic benefits to human society and has great significance for the development of laser light sources. Semiconductor lasers, due to their high energy conversion efficiency and excellent beam performance, have gradually replaced traditional gas and solid-state laser sources and become the mainstream laser light source in the market.

[0003] Semiconductor lasers can be divided into two types according to their light emission direction: edge-emitting lasers and surface-emitting lasers. An edge-emitting laser refers to a laser whose light emission direction is parallel to the active layer (a characteristic layer of a semiconductor laser), that is, in the horizontal direction, and emits light from the end face. The edge-emitting laser has a large beam divergence angle, and the light spot is usually elliptical. The coupling efficiency with the optical fiber is low, and lenses or other discrete optical components are usually required to couple to the optical fiber. In addition, it needs to be cleaved into bars during testing, which increases the testing cost. Another type of laser that is different from the edge-emitting laser is the surface-emitting laser. The light emission direction of the surface-emitting laser is perpendicular to the active layer of the laser, and the light is emitted in the vertical direction, that is, from the surface. The surface-emitting laser has the advantages of a small beam divergence angle, a circular light spot, and high coupling efficiency with the optical fiber. It can also be tested directly on the wafer, reducing manufacturing costs and improving laser performance. In recent years, high-order surface grating-coupled surface emitting semiconductor lasers (SELs) that utilize low-cost, highly damage-resistant deep-etched surface SELs to achieve surface emission have attracted attention from academia and industry, and numerous research results have been reported. SELs have broad market prospects in the future.

[0004] In practical applications, semiconductor laser light sources need to form a functional system with other optical devices for use. Therefore, they need to meet the requirements of high-efficiency optical coupling with other devices. For example, when used in optical communication systems, semiconductor lasers need to be coupled with optical fibers; in integrated photonic chip applications, lasers need to be coupled with waveguide gratings, etc. When coupled with these devices, semiconductor lasers will have a large back reflection of light, resulting in a series of problems such as reduced optical coupling power, and echo light noise affecting the stability of laser operation. In order to solve this problem, some additional processes need to be added in the actual application of lasers. For example, when semiconductor lasers are coupled with optical fibers, an additional 8° inclined surface is usually processed on the end face of the optical fiber to achieve efficient coupling and suppress back reflection. This measure undoubtedly increases costs. However, in some application areas, such as integrated photonic chip applications, it is not possible to make an 8° inclined surface on the chip surface for coupling, resulting in strong light reflection from the coupling surface and low optical coupling efficiency.

[0005] If the light emission angle of the laser can be designed and adjusted to deviate from the normal direction of its light-emitting surface, it will be beneficial to the coupling of the laser with other devices, increase the coupling efficiency, and greatly expand the application scenarios of the laser. Due to the limitations of its working principle and spatial structure, it is difficult for edge-emitting lasers to achieve a light beam emission angle that deviates from the normal direction of its end face. In surface-emitting semiconductor lasers, researchers have greatly improved the light emission performance of the laser by introducing chirp into the grating (grating chirp refers to the change in the grating period along the spatial position). On the one hand, it has achieved a small angle between the laser and the normal direction of the laser surface, thereby improving the light coupling efficiency. However, the chirped grating structure is difficult to manufacture and is expensive. This makes the chirped grating surface-emitting semiconductor laser expensive and difficult to manufacture. Summary of the Invention

[0006] To address one or more of the aforementioned issues, the present application aims to provide a low-cost, easily fabricated grating surface-emitting semiconductor laser (GSSEL) with an output beam angled relative to the laser surface normal. By utilizing a graded-ridge waveguide structure and a high-order surface grating structure, this laser achieves an equivalent "chirped" GSSEL. This laser requires only a single epitaxial growth and conventional photolithography techniques to achieve an angle between the laser output angle and the laser surface normal, and is simple to fabricate at a low cost.

[0007] In order to achieve the above objectives, the technical solutions of this application are as follows:

[0008] A grating surface emitting semiconductor laser comprises a surface metal electrode layer, a ridge waveguide layer, a p-type doped cladding layer, a p-type doped light confinement layer, an active region, an n-type doped light confinement layer, an isolation layer, and a substrate layer, which are sequentially arranged. A first-order grating layer is arranged between the active region and the n-type doped light confinement layer. A high-order surface grating layer is arranged above the active region, extending from the p-type doped light confinement layer to the ridge waveguide layer. The ridge waveguide in the overlapping region between the grating strips of the high-order surface grating and the ridge waveguide layer has a gradually varying width structure, wherein the width of the gradually varying width structure gradually increases along the optical resonance direction of the laser.

[0009] Furthermore, the length of the high-order surface grating is 9 μm to 100 μm.

[0010] Furthermore, the period of the first-order grating layer is 200nm to 280nm, the duty cycle is 50%, and the depth is 20nm to 80nm.

[0011] Furthermore, the period of the high-order surface grating is 1 μm to 5 μm, the duty cycle is 20% to 90%, and the depth is 0.1 μm to 3 μm.

[0012] Furthermore, the high-order surface grating has a period of 1440 nm, a duty cycle of 0.5, and a depth of 1.9 μm.

[0013] Furthermore, the grating order of the high-order surface grating is 2 to 14.

[0014] Furthermore, the grating order of the high-order surface grating is 6, 8 or 10.

[0015] Furthermore, the resonant cavity length of the grating surface emitting semiconductor laser is 250 μm to 500 μm, and the end face power reflectivity is 3% to 5%.

[0016] Furthermore, the width of the short side of the width gradient structure is 1 to 1.5 μm, and the width of the long side is 2 to 3.5 μm.

[0017] Furthermore, the width of the short side of the width gradient structure is 1 μm, and the width of the long side is 3 μm.

[0018] The grating surface emitting semiconductor laser proposed in this application has the following advantages compared with existing designs:

[0019] (1) Because the ridge waveguide in the high-order surface grating region adopts a width gradient structure, the high-order surface grating with a fixed grating period forms an equivalent "chirped" grating effect (the light field when transmitting along the high-order surface grating has different equivalent refractive indices), so that the direction of laser emission deviates from the normal direction of the laser surface, which can reduce backlight reflection when coupling with other optical devices. Moreover, this method is low-cost and simpler to process than actually changing the grating period of the high-order surface grating.

[0020] (2) High-order surface gratings have a large refractive index difference and a long grating period. A small change in the ridge waveguide width can achieve a large equivalent refractive index change, resulting in a good equivalent "chirp" effect. In addition, the production of high-order surface gratings does not require EBL technology. Ordinary optical lithography and nanoimprinting technologies can be used for large-scale production, greatly reducing the difficulty and requirements of production and lowering the cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the epitaxial structure of the grating surface emitting semiconductor laser of this application;

[0022] Figure 2 A longitudinal cross-sectional schematic diagram of the grating surface emitting semiconductor laser of the present application;

[0023] Figure 3 Schematic diagram of the graded ridge waveguide structure of the grating surface emitting semiconductor laser of this application;

[0024] Figure 4 Schematic diagram of the change of the equivalent refractive index of the laser in this application with the width of the ridge waveguide. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0026] like Figure 1 、 Figure 2 As shown, the present application provides a grating surface emitting semiconductor laser, comprising a surface metal electrode layer, a ridge waveguide layer, a p-type doped cladding layer, a p-type doped light confinement layer, an active region, an n-type doped light confinement layer, an isolation layer and a substrate layer arranged in sequence, wherein a first-order grating layer is provided between the active region and the n-type doped light confinement layer; a high-order surface grating layer is provided above the active region, wherein the high-order surface grating layer extends from the p-type doped light confinement layer to the ridge waveguide layer, and the ridge waveguide in the overlapping region of the grating strips of the high-order surface grating and the ridge waveguide layer has a width gradient structure, wherein the width of the width gradient structure gradually increases along the optical resonance direction of the laser.

[0027] Figure 2 The figure shows a schematic diagram of the longitudinal cross-section of the laser along the resonant cavity. The z direction is the horizontal direction of the laser (the direction of the resonant cavity). The laser resonant cavity is composed of two end faces in the longitudinal direction. The first-order grating layer is distributed in the direction of the entire laser resonant cavity, while the high-order surface grating is located in a window in the middle of the resonant cavity and does not cover the entire resonant cavity. The x direction is the vertical direction of the laser (the epitaxial direction). From top to bottom, the laser epitaxial layers are the surface metal electrode layer, ridge waveguide layer, p-type doped cladding layer, p-type doped light confinement layer, active region, first-order grating layer, n-type doped light confinement layer, isolation layer and substrate layer.

[0028] The surface metal electrode is used to inject external current (lasers require current to operate), and the substrate is the base of the epitaxially grown laser. The active region is a quantum well or quantum dot active region, where carrier recombination generates laser light. The optical confinement layer located above and below it is used to vertically confine the laser light, preventing the laser light generated by the active region from leaking out of the laser. The optical confinement layer greatly improves the laser's light extraction efficiency. The ridge waveguide layer is used to confine the laser light in the cross section of the laser perpendicular to the laser resonator.

[0029] Unlike traditional lasers, the laser of the present application is designed with two sections of gratings. In order to ensure the best optical coupling effect, the two sections of gratings are placed on both sides of the active area, namely the first-order grating layer below the active area and the high-order surface grating extending from the ridge waveguide layer to the p-type light confinement layer above the active area. The first-order grating layer provides optical feedback and frequency selection to achieve single-mode output of the laser. Above the active area, at the window position in the middle of the laser resonant cavity, there is a section of high-order surface grating with a length of (9-100) microns. The grating order is 2 to 14 orders, preferably sixth-order, eighth-order and tenth-order surface gratings. This section of high-order surface grating is used to couple a portion of the light energy that resonates along the direction of the laser resonant cavity (horizontal direction) to the output in the vertical direction, that is, to achieve surface laser emission.

[0030] like Figure 2 、 Figure 3 As shown in the figure, the grating strips of the high-order surface grating and the ridge waveguide layer have an overlapping area. The ridge waveguide in the overlapping area has a gradient width structure, forming an equivalent "chirped" grating, which makes the laser originally emitted along the normal direction of the laser surface emit at a certain angle to the normal direction. The width of the gradient structure gradually increases along the optical resonance direction of the laser, as shown in the figure. Figure 3 As shown, the width of the short side is 1-1.5 μm, and the width of the long side is 2-3.5 μm. The short side length is preferably 1 μm, and the long side length is preferably 3 μm. The ridge waveguide on each grid strip has a trapezoidal structure, and its width gradually changes along the optical resonance direction of the laser.

[0031] The working principle of the grating surface emitting laser in this application is as follows:

[0032] Carriers are injected into the laser through metal electrodes and gathered in the laser active layer, where they recombine with holes to generate laser light. Confined by the confinement layer, the laser light propagates back and forth within the laser resonator. Through optical feedback and mode selection from the first-order gratings distributed throughout the resonator, located in the grating layer, a stable traveling wave mode is formed within the laser resonator, propagating in both the forward and reverse directions. This resonant light has a fixed wavelength (dependent on the Bragg wavelength and laser operating conditions) and is typically detuned from the Bragg wavelength (dependent on current injection conditions). When the traveling wave mode propagating within the laser resonator passes through a high-order surface grating located in the center of the laser resonator above the active region, due to its radiative properties, it couples a portion of the traveling wave mode into the radiative mode, forming a surface output. The output intensity depends on the laser structural parameters and the parameters of the high-order surface grating. Through optimized design, the radiative mode can be made to output a single-peak circular spot with high coupling efficiency to standard single-mode fiber. Since the high-order surface grating overlaps with the ridge waveguide, and the ridge waveguide in the overlapping area has a width gradient structure, the equivalent refractive index of the grating is changed, resulting in the high-order surface grating with uniform period having the effect of an equivalent "chirped" grating structure.

[0033] In the embodiment of the present application, the material selected for the laser is a III-V group material, the central wavelength of the laser emission is selected to be 1560 nm, and the thickness of each layer of the laser is a typical value.

[0034] In a specific embodiment, the cavity length of the laser resonant cavity is 250 μm to 500 μm, preferably 300 μm, and the end face power reflectivity is 3% to 5%, preferably 3%.

[0035] In another specific embodiment, the period of the first-order grating layer is 200nm-280nm, the duty cycle is 50%, and the depth is 20nm-80nm. The period of the first-order grating layer is preferably 244nm, the duty cycle is preferably 0.5, and the grating etching depth is preferably 40nm.

[0036] In another specific embodiment, the period of the high-order surface grating is 1 μm to 5 μm, the duty cycle is 20% to 90%, and the depth is 0.1 μm to 3 μm. The period of the high-order surface grating is preferably 1440 nm, the duty cycle is preferably 0.5, and the grating etching depth is preferably 1.9 μm.

[0037] Figure 4 The equivalent refractive index of the laser changes with the width of the ridge waveguide. The width gradient structure of the ridge waveguide in this application changes the equivalent refractive index of the grating, resulting in a high-order surface grating with a uniform period having an equivalent "chirped" grating structure effect.

[0038] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A grating surface emitting semiconductor laser comprising a surface metal electrode layer, a ridge waveguide layer, a p-type doped cladding layer, a p-type doped light confinement layer, an active region, an n-type doped light confinement layer, an isolation layer, and a substrate layer, wherein: A first-order grating layer is provided between the active region and the n-type doped light confinement layer; a high-order surface grating layer is provided above the active region, the high-order surface grating layer extending from the p-type doped light confinement layer to the ridge waveguide layer, the ridge waveguide in the overlapping region between the grating strips of the high-order surface grating and the ridge waveguide layer is a width gradient structure, and the width of the width gradient structure gradually increases along the optical resonance direction of the laser; The first-order grating layer has a period of 200 nm to 280 nm, a duty cycle of 50%, and a depth of 20 nm to 80 nm. The period of the high-order surface grating is 1 μm to 5 μm, the duty cycle is 20% to 90%, and the depth is 0.1 μm to 3 μm.

2. The grating surface emitting semiconductor laser according to claim 1, wherein The length of the high-order surface grating is 9 μm to 100 μm.

3. The grating surface emitting semiconductor laser according to claim 1, wherein The high-order surface grating has a period of 1440 nm, a duty cycle of 0.5, and a depth of 1.9 μm.

4. The grating surface emitting semiconductor laser according to claim 1, wherein The grating order of the high-order surface grating is 2 to 14.

5. The grating surface emitting semiconductor laser according to claim 4, wherein The grating order of the high-order surface grating is 6 orders, 8 orders or 10 orders.

6. The grating surface emitting semiconductor laser according to claim 1, wherein The resonant cavity length of the grating surface emitting semiconductor laser is 250 μm to 500 μm, and the end face power reflectivity is 3% to 5%.

7. The grating surface emitting semiconductor laser according to claim 1, wherein The width of the short side of the width gradient structure is 1-1.5 μm, and the width of the long side is 2-3.5 μm.

8. The grating surface emitting semiconductor laser according to claim 7, wherein: The width of the short side of the width gradient structure is 1 μm, and the width of the long side is 3 μm.

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