Mode field separation high-polarization semiconductor laser and application thereof

Abstract

The invention relates to a mode field separation high-polarization semiconductor laser and application thereof, and aims to solve the problem of low polarization degree caused by stress of an existing laser. The laser comprises a main mode field region and a coupling region which are stacked along an epitaxial growth direction. The main mode field region is used for limiting and lasing a first polarization optical mode; the coupling region is made of a high-loss material and is used for absorbing and attenuating the second polarization optical mode. The two meet the following matching conditions: the effective refractive index of the second polarization optical mode of the coupling region is matched with the effective refractive index of the specific m-order second polarization optical mode of the main mode field region, so that the second polarization optical mode is optically coupled and expanded to the coupling region to be dissipated; the effective refractive index of the first polarization optical mode of the coupling region is mismatched with the effective refractive index of the specific m-order first polarization optical mode of the main mode field region, so that the coupling region is limited in the main mode field region for stable lasing. According to the invention, a polarization-related mode filtering mechanism is utilized, so that the polarization degree and the process robustness of the laser are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser technology, and more specifically to a mode-field separation high-polarization semiconductor laser that utilizes a polarization-dependent coupling mechanism to achieve high polarization output and its applications. Background Technology

[0002] Semiconductor lasers, due to their small size and high efficiency, are core light sources in fields such as fiber laser pumping, solid-state laser pumping, and industrial materials processing. In these high-power applications, to overcome the power limitations of a single laser, polarization combining (PBC) of multiple laser beams is typically required. The efficiency of beam combining and the stability of the system directly depend on the degree of polarization (DOP) of each individual laser chip. However, achieving high polarization degrees in current technologies, especially for semiconductor lasers in the 780nm band, faces significant challenges: (1) Constraints of intrinsic material properties: AlGaAs / GaAsP quantum well structures commonly used in the 780nm band usually exhibit tensile strain. Under tensile strain, the transition from electronic states to light holes in the quantum well is dominant. This transition mechanism can simultaneously excite TE (transverse electric) mode and TM (transverse magnetic) mode, which is very easy to cause dual-mode coexistence, resulting in low polarization degree.

[0003] (2) Interference from external process stress: Parasitic stress is inevitably introduced during chip fabrication, coating, and packaging (such as solder solidification). These stresses will randomly change the refractive index ellipsoid of the material through photoelasticity, breaking the originally fragile gain balance. As a result, the polarization state of the laser fluctuates drastically with changes in temperature, current, or stress, which seriously affects the stability of the downstream beam combining system.

[0004] Traditional solutions, such as adding external polarizers, increase size and losses, while etching metal gratings on the chip surface is extremely complex and yields low results. Therefore, how to achieve high-purity and high-stability polarization control through internal chip structure design without changing the standard epitaxial process flow is a pressing technical problem to be solved in this field. Summary of the Invention

[0005] The technical problem to be solved by this invention is to address the low polarization degree and poor stability of semiconductor lasers in the prior art due to gain competition of TE / TM modes and interference from packaging stress, and to provide a mode-field separated high polarization semiconductor laser and its application.

[0006] To address the aforementioned technical problems, this mode-field separated highly polarized semiconductor laser includes a main mode field region and a coupling region stacked along the epitaxial growth direction. The main mode field region is used to confine and lase a first polarization optical mode. The coupling region is used to absorb and attenuate a second polarization optical mode through optical coupling. The main mode field region and the coupling region are configured to satisfy the following optical matching conditions: the effective refractive index of the second polarization optical mode in the coupling region matches the effective refractive index of a specific m-order second polarization optical mode in the main mode field region, causing the second polarization optical mode within the main mode field region to undergo optical coupling and extend into the coupling region where it is absorbed and lost; the effective refractive index of the first polarization optical mode in the coupling region is mismatched with the effective refractive index of a specific m-order first polarization optical mode in the main mode field region, causing the first polarization optical mode within the main mode field region to be confined within the main mode field region and preferentially lased.

[0007] Furthermore, the first polarization optical mode is a TM polarization mode, and the second polarization optical mode is a TE polarization mode; the coupling region is composed of a semiconductor material layer with high optical absorption loss characteristics to suppress the TE polarization mode entering the region.

[0008] Furthermore, the epitaxial structure of the laser, from bottom to top, includes an N-type side electrode, an N-type substrate region, an N-type confinement region, an N-type photonic crystal region, an N-type waveguide region, an active region, a P-type waveguide region, a P-type confinement region, a P-type contact region, an insulating region, and a P-type side electrode.

[0009] Specifically, the main mode field region is composed of the N-type confinement region, the N-type photonic crystal region, the N-type waveguide region, the active region, and the P-type waveguide region; the coupling region is composed of the P-type confinement region and the P-type contact region.

[0010] Furthermore, the N-type photonic crystal region has a structure with periodically changing refractive index, which is used for lateral confinement of the light field, filtering of higher-order modes, adjustment of the effective refractive index of the main mode field region, and pinning of crystal defects.

[0011] This invention discloses a mode-field separated high-polarization semiconductor laser and its application, which solves the technical problems of low polarization degree and poor stability caused by TE / TM mode gain competition and packaging stress interference in existing semiconductor lasers. By introducing a polarization-related selective coupling mechanism, it achieves active screening and suppression of specific polarization modes. Attached Figure Description

[0012] The following description, in conjunction with the accompanying drawings, further illustrates a mode-field separated highly polarized semiconductor laser of the present invention and its applications: Figure 1 This is a schematic diagram of the cross-sectional structure of the mode-field separated highly polarized semiconductor laser in Example 1; Figure 2 This is a schematic diagram showing the relationship between the mode field distribution and the refractive index distribution in the laser in Example 2; Figure 3 This is a graph showing the output power versus polarization (PI) degree test results in Example 2.

[0013] In the picture: 10 - Main mode field region, 20 - Coupling region; 101 - N-type side electrode, 102 - N-type substrate region, 103 - N-type confinement region, 104 - N-type photonic crystal region, 105 - N-type waveguide region, 106 - active region, 107 - P-type waveguide region, 108 - P-type confinement region, 109 - P-type contact region, 110 - insulating region, 111 - P-type side electrode. Detailed Implementation

[0014] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0015] Example 1: As Figure 1 As shown, this mode-field separated high-polarization semiconductor laser differs from traditional lasers that rely solely on the gain difference in the active region to maintain polarization differences. This laser employs a waveguide structure design, constructing two stacked functional regions: a lower main mode field region 10 and an upper coupling region 20. The main mode field region 10 is used to confine and lase the first polarization optical mode (TM mode in this embodiment); the coupling region 20 is used to absorb and attenuate the second polarization optical mode (TE mode in this embodiment) through optical coupling.

[0016] The principle lies in utilizing a directional coupling filtering mechanism in the vertical direction (x-direction): by precisely controlling the refractive index and thickness of each epitaxial layer, the effective refractive index (corresponding to the optical propagation constant) of the TE mode in the coupling region 20 is matched with the effective refractive index of a specific m-order TE mode in the main mode field region 10. In this embodiment, the specific m-order is the 0th order, i.e., the fundamental mode. According to coupled-mode theory, when the propagation constants of two waveguides are similar, energy will be strongly exchanged between them. Therefore, the TE mode optical field originally generated in the main mode field region 10 will extend to the upper coupling region 20. The coupling region 20 contains a heavily doped P-type contact region 109, which has a large free carrier absorption loss for the optical field and cannot form lasing oscillations. Conversely, for the TM mode, because its electromagnetic field continuity condition at the boundary is different from that of the TE mode, the effective refractive index of the TM mode in the coupling region 20 is mismatched with the effective refractive index of the specific m-order TM mode in the main mode field region 10. The TM mode cannot meet the coupling conditions, so it is minimally affected by the coupling region 20 and is firmly confined within the low-loss main mode field region 10, thereby achieving high gain and stable single-mode lasing.

[0017] The epitaxial structure of the laser, from bottom to top, includes: an N-type side electrode 101 for providing N-side electrical injection; an N-type substrate region 102 for supporting the chip; an N-type confinement region 103 for limiting the leakage of the optical field to the substrate; an N-type photonic crystal region 104 as an optional functional layer (for providing additional lateral confinement of the optical field and high-order mode filtering functions, and by changing its duty cycle and other parameters, the effective refractive index of the main mode field region 10 can be finely adjusted, thereby assisting in achieving the above-mentioned refractive index matching conditions; it can also pin growth defects from the substrate and improve material quality); an N-type waveguide region 105 for extending the optical field; an active region 106 (containing a multi-quantum well structure) as the core for generating photons; a P-type waveguide region 107 for extending the optical field; a P-type confinement region 108 for confining the optical field; and a P-type contact region 109 for forming an ohmic contact with the P-type side electrode 111. In this embodiment, the physical structure of the main mode field region 10 is mainly composed of an N-type confinement region 103, an N-type photonic crystal region 104, an N-type waveguide region 105, an active region 106, and a P-type waveguide region 107; while the physical structure of the coupling region 20 is mainly composed of a P-type confinement region 108 and a P-type contact region 109.

[0018] Example 2: This example performs simulation verification for the case where m=0 (i.e., the fundamental mode) described in Example 1, as follows. Figure 2 As shown in the figure (the black solid line represents the refractive index distribution along the epitaxial growth direction, i.e., the x-direction, and the gray curve represents the light field distribution), the fundamental TM polarization mode (gray dashed line) has its main peak perfectly localized in the main mode field region 10 (i.e., the middle high refractive index region), with very little distribution in the coupling region 20 on the right (corresponding to the upper part). This indicates that the TM mode experiences extremely low absorption loss, ensuring low threshold lasing. The fundamental TE polarization mode (gray dashed line) has its peak significantly shifted to the right, with a large amount of energy leaking and extending into the coupling region 20 (P-type confinement region and contact region). Due to the high loss characteristics of the P-type contact region, the TE mode will suffer huge modal losses, thus being effectively suppressed.

[0019] like Figure 3 As shown in the figure (which displays the measured power-current curve, i.e., the PI curve), under a large current injection of 10A, the TM mode output power (dominantly represented by the gray dashed line) reaches 10.28W, while the TE mode (gray dotted line) is only 0.40W. Calculations show that the device's degree of polarization (DOP) is as high as 96.3%, and remains stable with changes in current, demonstrating the superiority of this structural design.

[0020] Example 3: This mode-field separated highly polarized semiconductor laser, due to its excellent polarization purity and stress stability, is particularly suitable for high-power laser pump sources. When used as a pump source for fiber lasers or solid-state lasers, the high polarization degree can significantly improve the efficiency of polarization beam combining (PBC) and reduce heat generation in the combining module. Simultaneously, in the field of industrial materials processing, this laser can provide more consistent processing results and improve the smoothness of the cut surface. Finally, a highly polarized laser seed source can improve the mode purity of external cavity narrow-linewidth lasers in the quantum information field, thereby increasing the energy utilization efficiency of atomic level transitions.

[0021] This mode-field separated highly polarized semiconductor laser achieves active screening and suppression of specific polarization modes by introducing a polarization-dependent selective coupling mechanism. Specifically, (1) A polarization-dependent optical coupling mechanism was constructed, overcoming the limitations of material gain: instead of relying solely on the gain difference of the quantum well itself, the refractive index matching difference between the coupling region and the main mode field region under different polarization states was utilized. By ensuring that the TE mode satisfies the phase matching condition, it is filtered out by entering the high-loss coupling region through the optical coupling effect; while the TM mode is stably confined to the low-loss main mode field region for lasing due to mismatch. This mechanism breaks the degeneracy competition between the TE / TM modes, achieving high-purity output of a single polarization state, with a measured polarization degree of over 96%.

[0022] (2) It achieves the reuse of structural functions and has great advantages in mass production: It utilizes the P-type confinement layer and P-type contact layer (heavily doped layer) that are originally necessary in the epitaxial structure of lasers as coupling regions. It uses its natural high free carrier absorption characteristics to suppress TE mode without the need to introduce additional light-absorbing materials or complex secondary epitaxial processes, and is compatible with existing standard semiconductor laser production lines.

[0023] (3) The stress-induced polarization state lock is removed, improving the robustness of the device: By physically separating the TE and TM modes in spatial distribution (the TE mode is pulled into the coupling region and loses power, while the TM mode is localized in the main mode field region and lases), the polarization characteristics of the laser are mainly determined by the waveguide geometry, rather than solely by the material gain that is susceptible to stress. This greatly reduces the interference of packaging stress on the polarization degree and improves the device's operational stability under complex conditions.

[0024] The foregoing description illustrates the main features, basic principles, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments or examples described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the above embodiments or examples should be considered exemplary and not restrictive. The scope of the present invention is defined by the appended claims rather than the foregoing description, and therefore all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0025] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A mode-field separated highly polarized semiconductor laser, characterized in that: It includes a main mode field region (10) and a coupling region (20) stacked along the epitaxial growth direction; wherein, the main mode field region (10) is used to confine and lasing a first polarization optical mode, and the coupling region (20) is used to absorb and attenuate a second polarization optical mode through optical coupling, and the main mode field region (10) and the coupling region (20) are configured to satisfy the following optical matching conditions: The effective refractive index of the second polarization optical mode in the coupling region (20) matches the effective refractive index of the specific m-order second polarization optical mode in the main mode field region (10), so that the second polarization optical mode in the main mode field region (10) undergoes optical coupling and extends into the coupling region (20) where it is absorbed and lost. The effective refractive index of the first polarization optical mode in the coupling region (20) is mismatched with the effective refractive index of the specific m-order first polarization optical mode in the main mode field region (10), so that the first polarization optical mode in the main mode field region (10) is confined within the main mode field region (10) and preferentially lased.

2. The mode-field separated highly polarized semiconductor laser according to claim 1, characterized in that: The first polarization optical mode is the TM polarization mode, and the second polarization optical mode is the TE polarization mode; the coupling region (20) is composed of a semiconductor material layer with high optical absorption loss characteristics, so as to couple and suppress the TE polarization mode of the main mode field region (10).

3. The mode-field separated highly polarized semiconductor laser according to claim 2, characterized in that: The epitaxial structure of the laser, from bottom to top, includes an N-type side electrode (101), an N-type substrate region (102), an N-type confinement region (103), an N-type photonic crystal region (104), an N-type waveguide region (105), an active region (106), a P-type waveguide region (107), a P-type confinement region (108), a P-type contact region (109), an insulating region (110), and a P-type side electrode (111).

4. The mode-field separated highly polarized semiconductor laser according to claim 3, characterized in that: The main mode field region (10) is composed of the N-type confinement region (103), the N-type photonic crystal region (104), the N-type waveguide region (105), the active region (106), and the P-type waveguide region (107); the coupling region (20) is composed of the P-type confinement region (108) and the P-type contact region (109).

5. The mode-field separated highly polarized semiconductor laser according to claim 3, characterized in that: The N-type photonic crystal region (104) has a structure with a periodically changing refractive index, which is used to laterally confine the light field, filter higher-order modes, adjust the effective refractive index of the main mode field region (10), and pin crystal defects; the N-type photonic crystal region (104) is an optional structure.

6. The mode-field separated highly polarized semiconductor laser according to claim 3, characterized in that: The coupling region (20) may also be located in the N-type confinement region (103) or the P-type confinement region (108) and is composed of a multilayer waveguide structure.

7. The mode-field separated highly polarized semiconductor laser according to claim 3, characterized in that: The N-type substrate region (102), N-type confinement region (103), N-type photonic crystal region (104), and N-type waveguide region (105) are N-type doped GaAs, AlGaAs, or AlGaInP materials; the active region (106) contains a quantum well structure with tensile strain characteristics; the P-type waveguide region (107), P-type confinement region (108), and P-type contact region (109) are P-type doped GaAs, AlGaAs, or AlGaInP materials.

8. The mode-field separated highly polarized semiconductor laser according to claim 3, characterized in that: The laser has a ridge waveguide structure, which is formed by dry etching or wet etching to the P-type confinement region (108) or the P-type waveguide region (107).

9. The mode-field separated highly polarized semiconductor laser according to claim 1, characterized in that: The laser is pumped either electrically or optically.

10. The mode-field separated highly polarized semiconductor laser according to claim 1, characterized in that: The mode-field separated highly polarized semiconductor laser is the mode-field separated highly polarized semiconductor laser according to any one of claims 1 to 9, and it is applied to high-power laser pump sources, industrial material processing, or quantum information fields.

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