Elliptical ridge waveguide laser and preparation method thereof

Abstract

The invention belongs to the technical field of semiconductor lasers, and discloses an elliptical ridge waveguide laser and a preparation method thereof. The elliptical ridge-shaped waveguide laser comprises a laser body, the ridge-shaped structure of the laser body is an elliptical ridge-shaped waveguide structure, and the top surface of the elliptical ridge-shaped waveguide structure is elliptical; the ellipse shape of the top surface of the elliptical ridge-shaped waveguide structure meets an elliptic equation, the distance between two focuses of the ellipse of the top surface of the elliptical ridge-shaped waveguide structure is the same as the cavity length of the laser body, and the length of the short axis of the ellipse of the top surface of the elliptical ridge-shaped waveguide structure is smaller than the width of the top surface of the epitaxial substrate of the laser body. The elliptical ridge-shaped waveguide structure is formed through photoetching and etching. The problem that a laser with high optical cavity gain, low threshold current, single mode, high power and low polarization ratio is difficult to manufacture is solved, and reliable technical guarantee is provided for commercial and industrial application of the high-power laser.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductor lasers and relates to an elliptical ridge waveguide laser and a preparation method thereof. Background Art

[0002] Semiconductor lasers have the advantages of high power and conversion efficiency, low threshold, small size, light weight, and easy integration, and have become a research hotspot in recent decades. Among them, gallium nitride (GaN)-based ridge waveguide lasers are the best choice for blue and green light sources in the three primary colors of high-power laser display. They have important application value and broad market prospects in the fields of laser lighting and display, optical information storage, quantum technology, underwater communication, metal welding, and laser additive manufacturing. In semiconductor lasers, the ridge waveguide plays a role in limiting the light field and current injection. The geometric dimensions of the ridge waveguide directly affect the optoelectronic properties of the laser and are the key research structures of semiconductor lasers. The ridge shape of traditional ridge lasers is mostly rectangular, where the length of the ridge is defined as the laser cavity length. The two parallel planes formed after cleavage along the direction perpendicular to the cavity length are a pair of reflection surfaces of the laser, namely the laser cavity facets. The cavity length and the parallel cavity facets form the laser's resonant cavity, which is called the Fabry-Perot resonant cavity (FP cavity). The current is vertically injected into the quantum well through the rectangular top contact surface to generate spontaneous radiation light, which forms self-excited oscillation after multiple reflections on the cavity surface, and finally forms laser emission in the longitudinal direction perpendicular to the cavity surface to obtain a ridge waveguide semiconductor laser.

[0003] The ridge structure of the ridge waveguide laser determines the optical performance of the laser. High optical output power requires a wide ridge, but a wide ridge will lead to problems such as an increase in the laser threshold current and an increase in the number of lasing modes, reducing the optoelectronic performance of the laser. The threshold current characterizes the optical cavity loss (including internal loss and cavity surface loss) of the laser. A small threshold current indicates low optical cavity loss. An optical cavity with low optical cavity loss and high quality factor is an inevitable requirement for high-power lasers to pursue higher output power and stability. Single-mode high-power semiconductor lasers have attracted attention due to their advantages such as high beam quality, energy concentration, high conversion efficiency, and easy coupling. According to the physical properties of the semiconductor laser itself, its refractive index decreases at high temperatures, resulting in a decrease in the light field confinement factor. It is necessary to increase the injection current to increase the output power. The increase in laser gain leads to the simultaneous oscillation of the fundamental mode and the higher-order mode, resulting in a deterioration in the beam quality. At present, the problem of balancing the output power and beam quality of high-power edge-emitting lasers is mainly solved by the growth of device epitaxial materials and the design of ridge structures. Traditional III-V single-transverse mode semiconductor lasers use narrow ridge waveguides (ridge width of about 1.5um-3um), which is conducive to the output of single-transverse mode light spots and improves the efficiency of beam coupling. However, due to the large injection current density, poor heat dissipation and low cavity surface optical catastrophe (COD) threshold, narrow ridge waveguides cannot achieve high-power laser output. Multi-mode wide waveguide lasers significantly increase the heat dissipation area and COD threshold of the waveguide, greatly improving the output power, but due to the existence of multiple transverse modes, the application scope of the laser is limited. In addition, in the field of laser display, the high coherence of the laser will lead to the formation of stray light with uneven brightness next to the laser output spot, which is called laser speckle. The laser speckle phenomenon will degrade the imaging effect and hinder the development of laser display. Summary of the invention

[0004] 1. Technical problems solved:

[0005] The purpose of the present invention is to innovatively invent an elliptical ridge waveguide laser in response to the above technical difficulties and in combination with the needs of industrial production. The present invention creatively designs and prepares a GaN-based elliptical ridge waveguide edge-emitting laser, and uses an elliptical ridge waveguide structure to increase the optical gain of the resonant cavity and reduce the threshold current of the laser, which is the basis for realizing a wide ridge high-power laser. In view of the multi-mode oscillation existing in the wide waveguide ridge laser, the elliptical ridge waveguide structure is used to increase the mode selection ability of the optical cavity to suppress the high-order mode oscillation. The elliptical ridge waveguide structure not only increases the area of ​​the injection electrode to achieve high current operation, but also improves the heat dissipation capacity of the chip, solves the problem of reduced waveguide limiting capacity caused by temperature rise due to the refractive index, and is the technical guarantee for realizing single-mode high-power and long-life lasers. Compared with the high polarization ratio of traditional ridge waveguide lasers, elliptical ridge waveguide lasers have two oscillation modes, namely FP oscillation and WGM oscillation mode, which can effectively weaken the polarization characteristics of the laser, that is, weaken the coherence of the laser, thereby suppressing the generation of laser speckle and improving the display effect. In addition, the elliptical ridge waveguide structure increases the oscillation optical path of the light beam in the laser FP cavity, effectively reducing the length of the traditional resonant cavity, thereby increasing the number of chips and reducing costs. In summary, the present invention provides a reliable technical guarantee for the commercial and industrial applications of high-power lasers.

[0006] (II) Technical solution:

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] An elliptical ridge waveguide laser comprises a laser body, wherein the ridge structure of the laser body is an elliptical ridge waveguide structure, and the top surface of the elliptical ridge waveguide structure is elliptical.

[0009] Furthermore, the top surface of the elliptical ridge waveguide structure satisfies the elliptical equation The distance between the two foci F1 and F2 of the top surface ellipse of the elliptical ridge waveguide structure is the same as the cavity length L of the laser body, and the length of the short axis B1B2 of the top surface ellipse of the elliptical ridge waveguide structure is smaller than the width of the top surface of the epitaxial substrate of the laser body.

[0010] Furthermore, the elliptical ridge waveguide structure is formed by photolithography and etching.

[0011] Another technical problem to be solved by the present invention is to provide a method for preparing an elliptical ridge waveguide laser, comprising the following steps:

[0012] Step 1: deposit a contact layer on the laser epitaxial wafer, prepare an elliptical contact layer through photolithography and etching processes, and anneal. The function of the contact layer is to form an ohmic contact between the injection electrode and the semiconductor to reduce the contact resistance.

[0013] Step 2: Deposit a hard mask on the wafer surface of the laser epitaxial wafer and complete the hard mask pattern of the elliptical structure through photolithography and etching methods, which is used for the etching and sidewall modification process of the laser ridge waveguide structure. The contact layer of step 1 is located between the wafer and the hard mask.

[0014] Step 3: Implement an elliptical ridge waveguide structure by etching, and use an alkaline solution to repair the surface damage of the elliptical ridge etched sidewall (etched sidewall of the elliptical ridge waveguide structure).

[0015] Step 4: Deposit an insulating layer on the surface of the wafer, and use photolithography and etching methods to precisely "open windows" in the insulating layer above the elliptical ridges. The formed windows serve as current injection channels.

[0016] Step 5: Prepare P-side electrode, thin the wafer and prepare N-side electrode.

[0017] Step 6: The wafer is subjected to cavity surface cleavage, cavity surface coating and single tube cleavage to complete the preparation process of the elliptical ridge waveguide laser chip.

[0018] Furthermore, the top surface of the elliptical ridge waveguide structure described in step 3 satisfies the elliptical equation, the distance between the two foci F1 and F2 of the top surface ellipse of the elliptical ridge waveguide structure is the same as the cavity length L of the laser body, and the short axis B1B2 of the top surface ellipse of the elliptical ridge waveguide structure is smaller than the width of the laser device.

[0019] Furthermore, the surface damage repair in step 3 is performed by immersion etching in an alkaline solution, the alkaline solution is a developer (THAM) or the like, the immersion temperature is 60-90° C., and the immersion time is 30-120 min.

[0020] Furthermore, the elliptical shape of the contact layer in step 1 satisfies the elliptical equation, and the top surface of the contact layer of the elliptical ridge waveguide structure in step 1 is compatible with the top surface of the elliptical ridge waveguide structure in step 3 and the "window" structure of the insulating layer in step 4.

[0021] (III) Beneficial effects:

[0022] Compared with the prior art, the present invention provides an elliptical ridge waveguide laser and a method for preparing the same, which have the following beneficial effects:

[0023] This method creatively designs and prepares GaN-based elliptical ridge waveguide edge-emitting lasers, which solves the difficulties in manufacturing high optical cavity gain, low threshold current, single-mode high power, and low polarization ratio lasers, and provides reliable technical support for its civil and industrial fields such as laser display lighting, metal welding, and additive manufacturing.

[0024] 1. The elliptical ridge waveguide laser and its preparation method increase the optical gain of the resonant cavity by utilizing the elliptical ridge waveguide structure. It not only has the optical gain of the traditional FP cavity, but also the deformed part provides a new optical oscillation gain path, thereby improving the quality factor of the optical resonant cavity and reducing the threshold of the laser, which is beneficial to improving the optoelectronic performance of high-power semiconductor lasers.

[0025] 2. The elliptical ridge waveguide laser and its preparation method, in view of the multi-mode oscillation existing in the traditional wide waveguide ridge laser, forms a specific optical oscillation mode in the optical cavity through the unique focal properties of the elliptical ridge waveguide structure, that is, the mode selection of the laser, which is beneficial to the mode suppression of high-power lasers, obtains high beam quality and more concentrated energy distribution of the output laser, and improves the efficiency of beam coupling.

[0026] 3. The elliptical ridge waveguide laser and its preparation method, through the elliptical ridge structure, not only increases the area of ​​the injection electrode and improves the level of the maximum injectable current, but also significantly improves the heat dissipation capacity of the laser, solves the problem of decreased waveguide limiting capacity caused by increased temperature, is conducive to achieving mode suppression of high-power devices and improving the stability and life of the devices.

[0027] 4. Compared with the high polarization ratio of traditional ridge lasers, the elliptical ridge waveguide laser and its preparation method obtain a laser with adjustable TE or TM ratio through the design of the elliptical ridge structure, realize the control of the laser polarization ratio, and are beneficial to the application of high-power devices in the display field.

[0028] 5. The elliptical ridge waveguide laser and its preparation method, the elliptical ridge waveguide structure increases the oscillation optical path of the light beam in the laser FP cavity, which can effectively reduce the length of the traditional resonant cavity, increase the chip output of a single wafer, and achieve the cost reduction goal. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A schematic diagram of an elliptical ridge waveguide laser according to the present invention;

[0030] Figure 2 A schematic diagram of a ridge structure of an elliptical ridge waveguide laser according to the present invention;

[0031] Figure 3 A structural side view of an elliptical ridge waveguide laser according to the present invention;

[0032] Figure 4 It is a three-dimensional schematic diagram of an elliptical ridge waveguide laser of the present invention;

[0033] Figure 5Schematic diagram of a laser having the same cavity length and cavity face ridge width as the elliptical ridge waveguide of the present invention and the traditional rectangular ridge waveguide structure, wherein the solid line is the elliptical ridge waveguide structure and the dotted line is the traditional rectangular ridge waveguide structure;

[0034] Figure 6 Schematic diagram of a laser with the same cavity length and different cavity ridge widths of the elliptical ridge waveguide and the traditional rectangular ridge waveguide structure of the present invention, wherein the solid line is the elliptical ridge waveguide structure and the dotted line is the traditional rectangular ridge waveguide structure;

[0035] Figure 7 Schematic diagram of the transverse wave vibration mode of the traditional ridge laser, where a is the schematic diagram of the transverse wave vibration mode of the chip cavity surface, and b is the schematic diagram of the transverse wave vibration mode of the chip top surface;

[0036] Figure 8 It is a schematic diagram of the transverse wave vibration mode of the elliptical ridge waveguide laser of the present invention; DETAILED DESCRIPTION

[0037] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] The manufacturing method of the elliptical ridge waveguide edge emitting semiconductor laser of this embodiment is specifically divided into the design and preparation of the elliptical ridge waveguide structure. First, the design of the elliptical ridge waveguide structure uses the properties of the elliptical focus. The light passing through the focus will realize stimulated oscillation in the elliptical cavity and be emitted from the light-emitting surface. The design requires that the elliptical ridge waveguide structure must satisfy the elliptical equation Where a is the semi-major axis length, b is the semi-minor axis length, the distance between the two foci F1 and F2 of the ellipse is equal to the cavity length L of the laser, that is, L = 2c, c represents the focal length, the minor axis B1B2 of the ellipse is the maximum width of the ellipse ridge, that is, B1B2 = 2b, the major axis of the ellipse is A1A2, that is, A1A2 = 2a, and the ridge width W of the laser light-emitting cavity surface is calculated by the ellipse equation, that is,

[0039] Example 1

[0040] See also Figure 1 , Figure 2 , Figure 3 , Figure 4 An elliptical ridge waveguide laser comprises a laser body, the ridge structure of the laser body is an elliptical ridge waveguide structure, and the top surface of the elliptical ridge waveguide structure is elliptical.

[0041] In this embodiment, the top surface of the elliptical ridge waveguide structure satisfies the elliptical equation a=600.5μm, b=25μm, c=600μm, the ridge width W of the laser light-emitting cavity end is 2.1μm, the distance between the two foci F1 and F2 of the top surface ellipse of the elliptical ridge waveguide structure is the same as the cavity length L of the laser body, that is, F1F2=L=2c=1200μm, the short axis B1B2=2b=50μm of the top surface ellipse of the elliptical ridge waveguide structure, and the width of the single laser body is 250μm.

[0042] Another technical problem to be solved by the present invention is to provide a method for manufacturing an elliptical ridge waveguide laser, comprising the following steps:

[0043] Step 1: depositing a contact layer on the laser epitaxial wafer, preparing an elliptical contact layer by photolithography and etching processes, and annealing;

[0044] Step 2: Deposit a hard mask on the wafer surface of the laser epitaxial wafer and complete the hard mask pattern of the elliptical structure through photolithography and etching methods, which is used for the etching and sidewall modification process of the laser ridge waveguide structure;

[0045] Step 3: Implement an elliptical ridge waveguide structure by etching, and use an alkaline solution to repair surface damage on the elliptical ridge etched sidewall;

[0046] Step 4: Deposit an insulating layer on the surface of the wafer, and precisely “open windows” the insulating layer above the elliptical ridges through photolithography and etching methods to form a current injection channel;

[0047] Step 5: Prepare P-side electrode, thin the wafer and prepare N-side electrode;

[0048] Step 6: The wafer is subjected to cavity surface cleavage, cavity surface coating and single tube cleavage to complete the preparation process of the elliptical ridge waveguide laser chip.

[0049] When this embodiment is used, step 2 uses an alkaline solution to repair the surface damage of the ridge-type etched sidewall by immersion etching in an alkaline solution, the alkaline solution is a developer (THAM), etc., the immersion temperature is 60-90°C, and the immersion time is 30-120min.

[0050] When this embodiment is in use, the elliptical shape of the contact layer in step 1 satisfies the elliptical equation, and the top surface of the contact layer of the elliptical ridge waveguide structure in step 1 is compatible with the top surface of the elliptical ridge waveguide structure in step 3 and the insulating layer "window" structure in step 4.

[0051] Comparative Example 1: Same cavity length and light output ridge width

[0052] A conventional rectangular ridge waveguide laser with a cavity length of 1200 μm and a ridge width of 2.1 μm was manufactured, and the other aspects were the same as those in Example 1.

[0053] Comparative Example 1 and Comparative Example 1, as Figure 5 As shown, the solid line is an elliptical ridge waveguide, and the dotted line is a traditional rectangular ridge waveguide. Under the condition of the same cavity length and the same light-emitting surface ridge width, the area of ​​the elliptical ridge waveguide is larger than that of the traditional rectangular ridge waveguide. The area of ​​the ridge waveguide corresponding to the area of ​​the contact layer is the same. By adopting the elliptical ridge structure, not only the area of ​​the injection electrode is increased, the level of the maximum injectable current is improved, but also the heat dissipation capacity of the laser is significantly improved, which solves the problem of decreased waveguide limitation caused by increased temperature, which is conducive to the mode suppression of high-power devices and improves the stability and life of the devices.

[0054] Comparative Example 2: Same cavity length and short axis width

[0055] A conventional rectangular ridge waveguide laser with a cavity length of 1200 μm and a ridge width of 50 μm was manufactured, and the other aspects were the same as those in Example 1.

[0056] By comparing Example 1 with Comparative Example 2, Figure 6 As shown, the solid line is the traditional rectangular ridge waveguide, and the dotted line is the elliptical ridge waveguide. Under the same cavity length and maximum ridge width, the ridge width of the laser light output cavity of the elliptical ridge waveguide is W = 2.1μm<<50μm (the ridge width of the traditional rectangular ridge waveguide), which is conducive to single-mode lasing and threshold current reduction, and can effectively improve the quality factor of the laser.

[0057] Embodiment 2: The top surface of the elliptical ridge waveguide structure is elliptical and satisfies the elliptical equation a=152.4μm, b=25μm, c=150μm, the ridge width W of the laser light-emitting cavity end is 8.2μm, the distance between the two foci F1 and F2 of the top surface ellipse of the elliptical ridge waveguide structure is the same as the cavity length L of the laser body, that is, F1F2=L=2c=300μm, and the short axis B1B2=2b=50μm of the top surface ellipse of the elliptical ridge waveguide structure.

[0058] Comparative Example 3: Same output ridge width and current injection area

[0059] A conventional rectangular ridge waveguide laser with a cavity length of 1456 μm and a ridge width of 8.2 μm at the laser output end was manufactured, and the rest was the same as in Example 1.

[0060] By comparing Example 1 with Comparative Example 3, under the conditions of the same output end ridge width and current injection area, the laser cavity length L=300 μm of the elliptical ridge waveguide is smaller than the traditional rectangular ridge waveguide laser cavity length of 1456 μm. On the basis of meeting the requirements of cleavage into a single device, the area of ​​a single device is greatly reduced, which is beneficial to improving chip production and is an excellent solution for reducing costs.

[0061] Laser polarization control: There are two types of transverse wave vibrations in the emission direction of the traditional ridge laser cavity surface, transverse electric wave (TE) and transverse magnetic wave (TM), such as Figure 7 As shown. Due to the ridge-type mode selection, the TM mode is structurally suppressed, so the TE mode intensity of the laser oscillation is greater than the TM intensity, and the laser has a high polarization ratio, where the polarization ratio I TE and I TM are the light intensities of TE and TM, respectively. Compared with the traditional ridge waveguide laser structure, the light passing through the focus of the elliptical ridge waveguide laser forms oscillations in the elliptical cavity, in which the TE mode produces loss under the condition of changing the propagation direction of the light and is suppressed by the structure, while the light in the TM vibration direction is not affected, such as Figure 8 As shown, this increases the TM oscillation intensity and reduces the polarization ratio of the laser, which is beneficial to the application of the laser in display.

[0062] As for other processes after the epitaxial wafer growth is completed and annealed, such as: electrode formation process, exposure and etching process, protective film formation process and cleavage process, etc., they belong to the recognized field scope for ordinary technicians in this field and will not be elaborated in this article.

[0063] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An elliptical ridge waveguide laser, comprising a laser body, characterized in that: The ridge structure of the laser body is an elliptical ridge waveguide structure, and the top surface of the elliptical ridge waveguide structure is elliptical.

2. The elliptical ridge waveguide laser according to claim 1, characterized in that: The elliptical shape of the top surface of the elliptical ridge waveguide structure satisfies the elliptical equation, the distance between the two foci of the top surface ellipse of the elliptical ridge waveguide structure is the same as the cavity length of the laser body, and the short axis length of the top surface ellipse of the elliptical ridge waveguide structure is less than the width of the top surface of the epitaxial substrate of the laser body.

3. The elliptical ridge waveguide laser according to claim 1, characterized in that: The elliptical ridge waveguide structure is formed by photolithography and etching.

4. A method for preparing an elliptical ridge waveguide laser, characterized by comprising the following steps: Step 1: depositing a contact layer on the laser epitaxial wafer, preparing an elliptical contact layer by photolithography and etching processes, and annealing; Step 2: Deposit a hard mask on the wafer surface of the laser epitaxial wafer and complete the hard mask pattern of the elliptical structure through photolithography and etching methods, which is used for the etching and sidewall modification process of the laser ridge waveguide structure. The contact layer of step 1 is located between the wafer and the hard mask; Step 3: Obtain an elliptical ridge waveguide structure by etching, and use an alkaline solution to repair the surface damage of the elliptical ridge etched sidewall; Step 4: Deposit an insulating layer on the surface of the wafer, and precisely open a window on the insulating layer above the elliptical ridge by photolithography and etching methods. The formed window serves as a current injection channel. Step 5: Prepare P-side electrode, thin the wafer and prepare N-side electrode; Step 6: Perform cavity surface cleavage, cavity surface coating and single tube cleavage on the wafer to complete the preparation process of the elliptical ridge waveguide laser chip.

5. The method for preparing an elliptical ridge waveguide laser according to claim 4, characterized in that: The elliptical shape of the top surface of the elliptical ridge waveguide structure described in step 3 satisfies the elliptical equation, the distance between the two foci of the top surface ellipse of the elliptical ridge waveguide structure is the same as the cavity length of the laser body, and the short axis length of the top surface ellipse of the elliptical ridge waveguide structure is less than the width of the top surface of the epitaxial substrate of the laser body.

6. The method for preparing an elliptical ridge waveguide laser according to claim 4, characterized in that: The surface damage repair in step 3 is performed by immersion etching in an alkaline solution.

7. The method for preparing an elliptical ridge waveguide laser according to claim 6, characterized in that: The immersion temperature of the alkaline solution in the alkaline solution immersion etching is 60-90° C., and the immersion time is 30-120 min.

8. The method for preparing an elliptical ridge waveguide laser according to claim 4, characterized in that: The elliptical shape of the contact layer in step 1 satisfies the elliptical equation, and the top surface of the contact layer in step 1 is compatible with the top surface of the elliptical ridge waveguide structure in step 3 and the window of the insulating layer in step 4.

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