A gradually changing frequency-stabilized mode-locked, ultra-narrow linewidth, high-power fundamental mode external cavity semiconductor laser

Abstract

The application relates to a gradually changing frequency-stabilized mode-locked, super-narrow-line-width, high-power base-mode external-cavity semiconductor laser, and belongs to the technical field of semiconductor lasers. The semiconductor laser comprises a laser chip and a mode-locked element, a waveguide is arranged on the laser chip, the mode-locked element is arranged outside the light-emitting cavity surface of the laser chip, and the light refracted by the light-emitting cavity surface enters the mode-locked element vertically to the end surface of the mode-locked element. The application is characterized in that the light reflected by the light-emitting cavity surface cannot be transmitted back to the ridge waveguide, and only the light reflected by the external mode-locked element can be transmitted back to the waveguide to form F-P oscillation, so that the overall side mode suppression ratio of the device is effectively improved, and the mode jumping phenomenon of the external-cavity laser is greatly reduced.

Description

Technical Field

[0001] This invention relates to a gradient-mode-locked, ultra-narrow linewidth, high-power fundamental mode external cavity semiconductor laser, belonging to the field of semiconductor laser technology. Background Technology

[0002] High-power semiconductor lasers are increasingly widely used in pumping, optical communication, medical and other fields. With the expansion and segmentation of application fields, the performance requirements of semiconductor lasers are also getting higher and higher. As a semiconductor laser used for pumping and communication, it has very high requirements for the spectral linewidth of the wavelength. It needs to have a very narrow linewidth without the wavelength changing, i.e. mode hopping. The spectrum of a typical semiconductor laser is Gaussian, with a wide spectral width, and its center wavelength changes with temperature. Therefore, it is necessary to mode lock the light emitted by the semiconductor laser. The so-called mode locking usually refers to wavelength locking and linewidth compression. Currently, there are two main types of commonly used mode locking mechanisms: internal cavity structure and external cavity structure.

[0003] The internal cavity structure mode-locking approach refers to achieving linewidth compression and wavelength locking by etching microstructures such as Bragg gratings on the chip itself. It can be mainly divided into DFB semiconductor lasers and DBR semiconductor lasers. However, due to the small period of the Bragg grating, the design and manufacturing of its grating structure have very high precision requirements, resulting in high manufacturing costs and low yield.

[0004] External cavity structures typically utilize externally connected fiber optic gratings (FBGs) or other mode-locking elements with linewidth compression and wavelength locking as external cavity feedback. This allows light of a specific wavelength to be fed back into the FP cavity of the semiconductor laser chip, further amplifying that wavelength while suppressing other wavelengths. This achieves the goal of compressing the linewidth of the oscillating light within the chip, thus realizing the purpose of linewidth compression and frequency stabilization mode-locking in a uniform external cavity laser. FBGs, as optical feedback components in external cavity structures, offer advantages such as simple fabrication, narrow reflection spectrum, and stable center wavelength. However, since the reflectivity of a semiconductor laser cavity surface cannot be zero (i.e., there is some residual reflectivity), some light is reflected back from the cavity surface and enters the FP cavity within the chip for amplification. Especially in high-power operating modes, the feedback light caused by the residual reflectivity of the cavity surface can easily trigger mode competition within the semiconductor laser chip, ultimately leading to mode switching and device failure.

[0005] Chinese patent document CN105406356B discloses a folded external cavity ultra-narrow linewidth semiconductor laser, which effectively reduces the package size by replacing the straight waveguide with a folded external cavity and effectively filters out higher-order modes through the bent waveguide to achieve the transmission of the fundamental mode. However, it does not describe the longitudinal mode. At the same time, this method requires the design and fabrication of the folded external cavity, which is relatively complex. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a gradient-mode-locked, ultra-narrow linewidth, high-power external cavity semiconductor laser, which eliminates longitudinal mode competition caused by residual reflectivity of the cavity surface, effectively improves the side-mode suppression ratio of the external cavity laser, and solves the mode-hopping problem that occurs in ultra-narrow linewidth external cavity semiconductor lasers.

[0007] The technical solution of the present invention is as follows:

[0008] A gradient-mode-locked, ultra-narrow linewidth, high-power fundamental mode external cavity semiconductor laser includes a laser chip and a mode-locking element. A waveguide is disposed on the laser chip, and a mode-locking element is disposed outside the output cavity surface of the laser chip. The light refracted from the output cavity surface enters the mode-locking element perpendicularly to the end face of the mode-locking element.

[0009] Preferably, the laser chip includes, from bottom to top, an N-face electrode, a substrate, an N-type confinement layer, an N-type waveguide layer, an active region, a P-type waveguide layer, and a P-type confinement layer, with a waveguide disposed on the P-type confinement layer.

[0010] More preferably, the waveguide is a three-segment waveguide, which includes a vertical waveguide, an arc waveguide, and a tilted waveguide. The vertical waveguide extends to the non-emitting cavity surface of the laser chip, and the vertical waveguide is connected to the tilted waveguide via the arc waveguide. The tilted waveguide extends to the emitting cavity surface of the laser chip. The three-segment waveguide is an integral structure without any partitions in the middle.

[0011] Preferably, the three-segment waveguide is a ridge waveguide, and the junction between the tilted waveguide and the output cavity surface is located in the middle of one side of the laser chip, which facilitates subsequent coupling.

[0012] Preferably, the horizontal extensions of the vertical waveguide and the tilted waveguide do not overlap.

[0013] In a further preferred embodiment, an insulating layer is provided on the P-type confinement layer except for the ridge region, and P-surface electrodes are provided on the ridge region and the insulating layer.

[0014] Preferably, the ridge waveguide is formed by etching away the P-type confinement layer except for the ridge region using a wet etching process or a dry etching process. The effective refractive index of the ridge region is higher than that of the two sides, and the magnitude of the effective refractive index difference is mainly related to the etching depth on both sides of the ridge. Under the premise of not completely etching away the P-type confinement layer, the greater the etching depth, the greater the effective refractive index difference.

[0015] Preferably, the width of the vertical waveguide conforms to the following relationship:

[0016]

[0017] Where n eff1It is the effective refractive index of the waveguide below the ridge, n eff2 λ is the effective refractive index of the waveguide on both sides of the ridge, W is the width, and λ is the center wavelength of the laser.

[0018] More preferably, the width of the tilted waveguide is 1 to 2 times the width of the vertical waveguide.

[0019] Preferably, the arc waveguide is a width-gradient waveguide, with one end having the same width as the vertical waveguide and the other end having the same width as the tilted waveguide, thus achieving a seamless connection between the vertical waveguide and the tilted waveguide.

[0020] Further preferred, the maximum curvature of the arc-shaped waveguide causes total internal reflection of light propagating parallel to the direction perpendicular to the waveguide.

[0021] Preferably, the tilt angle of the mode-locking element is the same as the tilt angle of the tilted waveguide, ensuring that the light emitted from the output cavity surface enters the mode-locking element in a direction perpendicular to the end face of the mode-locking element, thereby achieving docking and coupling between the mode-locking element and the tilted waveguide.

[0022] Preferably, both the end face of the mode-locking element and the light-emitting cavity surface of the laser chip are coated with a high-transmittance film. The mode-locking element is a grating fiber or a photonic crystal device. The mode-locking element is a wavelength-locking element selected according to the actual situation.

[0023] The beneficial effects of this invention are as follows:

[0024] 1. This invention sets an angle between the ridge waveguide and the laser chip's output cavity surface, so that the light reflected back from the output cavity surface cannot be transmitted back to the ridge waveguide. Only the light reflected back through the external mode-locking element can be transmitted back into the waveguide to form FP oscillation, which effectively improves the overall side-mode suppression ratio of the device and greatly reduces the mode hopping phenomenon of the external cavity laser.

[0025] 2. The present invention adopts a ridge waveguide structure with a gradually varying width. The ridge waveguide at the light-emitting cavity surface has a larger width, while the ridge waveguide at the non-light-emitting cavity surface has a smaller width. This can effectively improve the COD threshold at the light-emitting cavity surface. Furthermore, the narrower ridge waveguide at the non-light-emitting cavity surface is used for mode control, ensuring that the output light is in single-mode mode.

[0026] 3. This invention connects the tilted waveguide and the vertical waveguide through an arc waveguide. The curvature of the arc waveguide is designed based on the principle of total internal reflection to ensure that the fundamental mode light can achieve total internal reflection at the interface of the arc waveguide, while the light of higher-order modes cannot achieve total internal reflection and is refracted out of the ridge waveguide and lost, further suppressing higher-order mode lasing and improving the maximum power threshold of single-mode output. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the structure of the present invention;

[0028] Figure 2 This is a schematic diagram of the waveguide plane of the present invention;

[0029] Figure 3 This is a schematic diagram of the structure of the present invention;

[0030] Figure 4 This is a diagram showing the effective refractive index distribution of the waveguide in this invention.

[0031] Figure 5 This is the optical path diagram of the present invention;

[0032] Figure 6 This is a partial optical path diagram of the present invention;

[0033] Wherein: 1. Vertical waveguide; 2. Curved waveguide; 3. Tilted waveguide; 4. Mode-locking element; 5. P-face electrode; 6. Insulating layer; 7. P-type confinement layer; 8. P-type waveguide layer; 9. Active region; 10. N-type confinement layer; 11. N-type waveguide layer; 12. N-face electrode; 13. Fundamental mode light; 14. Total internal reflection light; 15. Refracted outgoing light; 16. Reflected loss light; 17. First normal; 18. Second normal; 19. Third normal. 20. Normal; 21. Effective refractive index of the ridge region; 22. Effective refractive index on both sides of the ridge region; 23. Outgoing cavity surface; 24. Non-outgoing cavity surface; 25. Laser chip; 26. Mode-locking element end face; 27. Angle of incidence of the fundamental mode light at the tilted waveguide cavity surface; 28. Angle of refraction of the fundamental mode light at the tilted waveguide cavity surface; 29. ​​Angle of incidence of the refracted outgoing light at the end face of the mode-locking element; 20. Reflection angle of the fundamental mode light at the tilted waveguide cavity surface. Detailed Implementation

[0034] The present invention will be further described below with reference to the embodiments and accompanying drawings, but is not limited thereto.

[0035] Example 1:

[0036] like Figure 1-6 As shown, this embodiment provides a gradient-mode-locked, ultra-narrow linewidth, high-power fundamental mode external cavity semiconductor laser, including a laser chip 24 and a mode-locking element 4. A waveguide is disposed on the laser chip 24, and a mode-locking element is disposed on the outer side of the laser chip's output cavity surface. The light refracted from the output cavity surface enters the mode-locking element perpendicularly to the end face of the mode-locking element.

[0037] The laser chip comprises, from bottom to top, an N-face electrode 12, a substrate, an N-type confinement layer 10, an N-type waveguide layer 11, an active region 9, a P-type waveguide layer 8, and a P-type confinement layer 7, with a waveguide disposed on the P-type confinement layer 7.

[0038] The waveguide is a three-segment waveguide, which includes a vertical waveguide 1, an arc waveguide 2, and a tilted waveguide 3. The vertical waveguide 1 extends to the non-emitting cavity surface 23 of the laser chip. The vertical waveguide is connected to the tilted waveguide via the arc waveguide. The tilted waveguide extends to the emitting cavity surface of the laser chip. The three-segment waveguide is an integral structure without any partitions in the middle. The three waveguides together constitute the FP cavity that generates laser oscillation.

[0039] The three-segment waveguide is a ridge waveguide. The junction between the tilted waveguide 3 and the output cavity surface 22 is located in the middle of one side of the laser chip, facilitating subsequent coupling. The ridge waveguide forms the optical waveguide for laser propagation inside the chip through the waveguide layer below it. The ridge waveguide is a self-built refractive index waveguide, and the optical waveguide is formed by the lateral effective refractive index spatial distribution, which is a weak optical waveguide confinement.

[0040] The horizontal extensions of vertical waveguide 1 and inclined waveguide 3 do not overlap. This prevents the laser from bypassing the curved waveguide and directly passing through the overlapping position to perform FP oscillation, thus negating the purpose of eliminating residual reflectivity.

[0041] An insulating layer 6 is disposed on the P-type confinement layer except for the ridge region for electrical isolation. P-surface electrodes 5 are disposed on the ridge region and the insulating layer, such as... Figure 3 As shown.

[0042] The ridge waveguide is formed by etching away the P-type confinement layer 7, excluding the ridge region, using either wet or dry etching processes. The effective refractive index spatial distribution in the lateral direction of the ridge waveguide is as follows: Figure 4 As shown, the dark area represents the ridge region. The effective refractive index 20 of the ridge region is higher than that of the effective refractive index 21 on both sides of the ridge region. The magnitude of the effective refractive index difference is mainly related to the etching depth on both sides of the ridge (refer to Zhang Qi, Zhao Yihao, Dong Zhen. 980nm high-power fundamental transverse mode distributed feedback laser [J]. Chinese Journal of Lasers. 2016.02). Under the premise of not completely etching away the P-type confinement layer, the greater the etching depth, the greater the effective refractive index difference. Therefore, in the lateral direction, it can be simplified as a three-layer planar waveguide along the horizontal direction.

[0043] The width of the vertical waveguide conforms to the following relationship, which is based on the fundamental mode transmission condition of a three-layer planar waveguide. (Refer to Zhao Jianyi, Chen Xin, Zhou Ning, et al. Reliability study of DFB laser fabricated by nanoimprint technology[J]. Acta Optica Sinica, 2014, 34(2): 0206003.)

[0044]

[0045] Where n eff1 It is the effective refractive index of the waveguide below the ridge, n eff2 λ is the effective refractive index of the waveguide on both sides of the ridge, W is the width, and λ is the center wavelength of the laser.

[0046] The three-segment waveguide has a vertical waveguide 1 that serves as the waveguide at the non-emitting cavity surface 23. Most of the light generated by the FP oscillation is reflected back into the FP cavity at the non-emitting cavity surface. Therefore, this vertical cavity surface segment is used as the waveguide segment that limits the output of the fundamental mode, and its width conforms to the fundamental mode transmission condition in the above relationship.

[0047] The width of the tilted waveguide 3 is 1 to 2 times the width of the vertical waveguide 1. According to the theory of conical lasers (refer to Pu Taofei, Zhang Jing. High-power, high-beam-quality conical semiconductor lasers [J]. Journal of Changchun University of Technology (Natural Science Edition). 2015.04), the waveguide at the non-output cavity surface is used as the waveguide segment that restricts the output of the fundamental mode. The light of higher-order modes is lost here after being reflected back. At the same time, the waveguide width at the output cavity surface is larger, which can effectively reduce the optical power density at the output cavity surface, thereby reducing the COD threshold of the laser chip and ultimately effectively improving the maximum output power of the laser chip.

[0048] Arc waveguide 2 is a width-gradient waveguide. One end of the arc waveguide has the same width as the vertical waveguide, and the other end has the same width as the tilted waveguide, achieving a seamless connection between the vertical waveguide and the tilted waveguide.

[0049] The maximum curvature of the arc waveguide 2 ensures total internal reflection of light propagating parallel to or perpendicular to the waveguide direction. The maximum curvature of the arc waveguide 2 guarantees total internal reflection of the fundamental mode light 13 propagating within the arc waveguide at the first normal 17, reducing the loss of the fundamental mode light due to refraction within the arc waveguide. Figure 5 As shown, the two conditions for total internal reflection are: 1. Light must travel from an optically denser medium to an optically less dense medium; 2. The angle of incidence must be greater than the angle of total internal reflection. This embodiment uses a ridge structure, which satisfies condition 1, as shown... Figure 4 As shown, according to the formula for the critical angle of total internal reflection:

[0050] φ = arcsin(n1 / n2)

[0051] φ is the critical angle for total internal reflection, n1 is the effective refractive index of the waveguide on both sides of the ridge, n2 is the effective refractive index of the waveguide below the ridge, and neff2 is the effective refractive index of the waveguide below the ridge. The angle of total internal reflection is the minimum angle of incidence at which total internal reflection can occur. When light parallel to the vertical waveguide is incident into the curved waveguide, the angle of incidence is as follows: Figure 5As shown by the first normal, the angle of incidence is to the left of the normal. Total internal reflection can only occur at this point when the angle of incidence is greater than the critical angle of total internal reflection. The critical angle of total internal reflection is calculated according to the above formula. According to the relationship between curvature, normal and angle of incidence, the curvature must be small enough to ensure total internal reflection of the incident light. The curvature of the arc waveguide 2 is small enough to ensure that the two vertical waveguides 1 and the tilted waveguide 3, whose extensions do not coincide, can be connected.

[0052] The tilt angle of mode-locking element 4 is the same as the tilt angle of the tilted waveguide, ensuring that the light emitted from the output cavity surface enters the mode-locking element in a direction perpendicular to the end face 25 of the mode-locking element, such as... Figure 5 and Figure 6 As shown, the mode-locking element is coupled to the tilted waveguide. The fundamental mode light is reflected and refracted at the output cavity surface. The reflected light is lost because it cannot undergo total internal reflection at the third normal 19 at the tilted waveguide. The fundamental mode light is refracted at the output cavity surface 22 and enters the external mode-locking element in a direction perpendicular to the end face of the external mode-locking element 4 that receives light.

[0053] Due to limitations in laser materials, the fundamental mode light emitted by a laser has a Gaussian spectrum containing multiple longitudinal modes. In a typical ridge-type external cavity semiconductor laser, the ridge is a vertical waveguide. Since the reflectivity of the laser cavity surface cannot be reduced to zero over a wide wavelength range, a portion of the Gaussian mode light, including multiple longitudinal modes, is reflected back from the cavity surface. Especially under high power conditions, the light reflected back by the residual reflectivity of the cavity surface will cause mode competition among the longitudinal modes, triggering lasing of other longitudinal modes, ultimately resulting in mode hopping and failure of the external cavity laser.

[0054] Based on the principles of reflection and optical path reversibility, this embodiment ensures that the light reflected from the cavity surface is lost through refraction and scattering. Only the light reflected after being mode-locked by the external mode-locking element 4 can enter the laser chip waveguide for FP oscillation. This eliminates the longitudinal mode competition caused by the light reflected back from the cavity surface due to the residual reflectivity. Consequently, the mode-locked light is continuously amplified, while other wavelengths are compressed and suppressed. This eliminates the longitudinal mode competition caused by the residual reflectivity of the cavity surface, effectively improves the side-mode suppression ratio of the external cavity laser, and greatly reduces the problem of mode hopping in the external cavity laser under high-power operating conditions.

[0055] The three-segment waveguide ensures that the waveguide at the light-emitting end is tilted, while the waveguide at the non-light-emitting end is perpendicular to the cavity surface. The principle behind this is as follows:

[0056] 1. The light emitted by the laser chip is confined to the fundamental mode by the vertical waveguide 1;

[0057] 2. The waveguide at the light output point is a wide waveguide, which greatly reduces the optical power density of the light output cavity surface and effectively improves the COD threshold of the semiconductor laser chip;

[0058] 3. By using a waveguide with a gradually varying arc width, the fundamental mode light is ensured to undergo total internal reflection within the arc waveguide, thereby reducing the reduction in external quantum efficiency caused by refraction and other factors, or even preventing the formation of a laser oscillation FP cavity;

[0059] 4. By using an arc-shaped waveguide, the light of higher-order modes can be prevented from undergoing total internal reflection, effectively filtering out the light of higher-order modes, thereby further suppressing higher-order mode lasing and increasing the maximum power threshold of single-mode output.

[0060] Example 2:

[0061] A gradient-mode-locked, ultra-narrow linewidth, high-power fundamental mode external cavity semiconductor laser is described in Example 1. The difference is that a high-transmittance film is deposited on both the end face of the mode-locking element and the output cavity surface of the laser chip, so that the transmittance of the two end faces to the center wavelength of the mode-locking element reaches more than 99.9%. The mode-locking element is a grating fiber or a photonic crystal device, and the mode-locking element is a wavelength-locking element selected according to the actual situation.

Claims

1. A graded-frequency mode-locked, ultra-narrow linewidth, high-power fundamental mode external cavity semiconductor laser, characterized in that, It includes a laser chip and a mode-locking element. A waveguide is set on the laser chip, and a mode-locking element is set on the outside of the laser chip's output cavity surface. The light refracted from the output cavity surface enters the mode-locking element perpendicularly to the end face of the mode-locking element. The laser chip comprises, from bottom to top, an N-face electrode, a substrate, an N-type confinement layer, an N-type waveguide layer, an active region, a P-type waveguide layer, and a P-type confinement layer, with a waveguide disposed on the P-type confinement layer; The waveguide is a three-segment waveguide, which includes a vertical waveguide, an arc waveguide, and a tilted waveguide. The vertical waveguide extends to the non-output cavity surface of the laser chip. The vertical waveguide is connected to the tilted waveguide via the arc waveguide. The tilted waveguide extends to the output cavity surface of the laser chip. The three-segment waveguide is an integral structure without any partitions in the middle. The three-segment waveguide is a ridge waveguide, and the junction between the tilted waveguide and the output cavity surface is located in the middle of one side of the laser chip. The horizontal extensions of the vertical waveguide and the tilted waveguide do not coincide; An insulating layer is provided on the P-type confinement layer except for the spine region, and P-surface electrodes are provided on the spine region and the insulating layer; The width of a vertical waveguide conforms to the following relationship: in, n eff1 It is the effective refractive index of the waveguide below the ridge. n eff2 λ is the effective refractive index of the waveguide on both sides of the ridge, W is the width, and λ is the center wavelength of the laser. The width of the tilted waveguide is 1 to 2 times the width of the vertical waveguide.

2. The gradient-mode-locked, ultra-narrow linewidth, high-power fundamental mode external cavity semiconductor laser as described in claim 1, characterized in that, Ridge waveguides are formed by etching away the P-type confinement layer, excluding the ridge region, using either wet etching or dry etching processes.

3. The gradient-mode-locked, ultra-narrow linewidth, high-power fundamental mode external cavity semiconductor laser as described in claim 2, characterized in that, The arc waveguide is a width-gradient waveguide. One end of the arc waveguide has the same width as the vertical waveguide, and the other end has the same width as the tilted waveguide.

4. The gradient-mode-locked, ultra-narrow linewidth, high-power fundamental mode external cavity semiconductor laser as described in claim 3, characterized in that, The maximum curvature of an arc waveguide causes total internal reflection of light propagating parallel to the direction perpendicular to the waveguide.

5. The gradient-mode-locked, ultra-narrow linewidth, high-power fundamental mode external cavity semiconductor laser as described in claim 4, characterized in that, The tilt angle of the mode-locking element is the same as the tilt angle of the tilted waveguide.

6. The gradient-mode-locked, ultra-narrow linewidth, high-power fundamental mode external cavity semiconductor laser as described in claim 1, characterized in that, Both the end face of the mode-locking element and the light-emitting cavity surface of the laser chip are coated with high-transmittance films. The mode-locking element is a grating fiber or a photonic crystal device.

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