An electrically heated wide tuning range narrow linewidth DFB laser and a manufacturing method thereof

Abstract

The application discloses a kind of electric heating wide tuning range narrow line width DFB laser and its manufacturing method, its structure includes DFB epitaxial structure, negative electrode metal, positive electrode metal and heating electrode metal, the positive side of epitaxial structure is equipped with ridge waveguide;Epitaxial structure includes phase shift region and gain region along the cavity length direction of DFB laser, one end of cavity length direction is reflection end face, wherein phase shift region is close to reflection end face;The epitaxial structure of gain region includes grating, the epitaxial structure of phase shift region does not include grating, positive electrode metal covers more than 70% of gain region and phase shift region ridge waveguide, heating electrode metal is located in the side of phase shift region ridge waveguide.The application introduces heating electrode metal in chip reflection end, and realizes current control's fast and wide range laser tuning in combination with part of grating epitaxial design, realizes the function of fine mode-hop-free frequency tuning.

Description

Technical Field

[0001] This invention belongs to the technical field of lasers, specifically relating to an electrically heated wide-tuning-range narrow-linewidth DFB laser and its manufacturing method. Background Technology

[0002] LiDAR employing Frequency Modulated Continuous Wave (FMCW) technology offers advantages such as simultaneous distance and velocity measurement and strong resistance to ambient light interference, making it an important development direction for lidar. FMCW radar requires lasers with narrow linewidths (approximately 100kHz), fast mode-hopping-free operation, and wide tuning ranges (approximately 20GHz). Narrow linewidth semiconductor lasers are small in size and highly efficient, making them ideal for application in the field of FMCW radar light sources.

[0003] Conventional narrow-linewidth semiconductor lasers mainly include: distributed feedback lasers (DFB), distributed Bragg reflector lasers (DBR), and external cavity semiconductor lasers (ECL). Narrow-linewidth DFBs have advantages in large-scale mass production and low cost due to their relatively simple and mature epitaxial and manufacturing processes. However, their mode-hopping-free tuning range is generally small, making it difficult to meet the requirements of a wide tuning range.

[0004] Existing heating techniques heat the entire cavity of a DFB laser, altering the period and equivalent refractive index of the DFB grating and providing a wide tuning range. However, these techniques are prone to mode hopping and tuning instability, and also introduce negative effects such as reduced internal quantum efficiency, decreased output power, and increased threshold voltage. These issues limit their application in FMCW laser radar. Summary of the Invention

[0005] This invention addresses the shortcomings of existing technologies by providing an electrically heated wide-tuning-range, narrow-linewidth DFB laser and its fabrication method.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A DFB laser with wide tuning range and narrow linewidth, characterized by: comprising a DFB epitaxial structure, a negative electrode metal, a positive electrode metal, and a heating electrode metal; the epitaxial structure has a ridge waveguide on its front side; the epitaxial structure includes a phase shift region and a gain region along the cavity length direction of the DFB laser, with one end of the cavity length direction being a reflecting end face, wherein the phase shift region is close to the reflecting end face; the epitaxial structure of the gain region includes a grating, while the epitaxial structure of the phase shift region does not include a grating; the positive electrode metal covers more than 70% of the ridge waveguides of the gain region and the phase shift region; and the heating electrode metal is located on one side of the ridge waveguide of the phase shift region.

[0008] Optionally, the epitaxial structure includes, from bottom to top, a substrate, a lower cladding layer, an active layer, an upper cladding layer, and a contact layer, wherein the active layer penetrates the phase shift region and the gain region, and the grating is disposed above or below the active layer in the gain region.

[0009] Optionally, the upper or lower cladding layer includes a grating forming layer, the grating forming layer in the gain region is etched to form a grating pattern, and the phase shift region does not contain a grating forming layer or contains a grating forming layer but is not etched.

[0010] Optionally, the negative electrode metal is an N-type metal; the negative electrode metal is disposed on the back side of the substrate, or the epitaxial structure exposes the n-type doped region of the epitaxial structure by etching the front side, and the negative electrode metal is disposed on the n-type doped region.

[0011] Optionally, the phase-shifting region occupies 5% to 20% of the cavity length of the laser.

[0012] Optionally, the front side of the epitaxial structure is further covered with a passivation layer, the passivation layer having an opening on the ridge waveguide, the positive electrode metal and the heating electrode metal being disposed on the passivation layer, and the positive electrode metal contacting the ridge waveguide through the opening.

[0013] Optionally, the passivation layer is a silicon dioxide or silicon nitride thin film with a thickness of 100nm-1000nm; the heating electrode metal is a platinum or titanium metal thin film with a thickness of 100-1000nm, a width of 5-50um, a length accounting for more than 70% of the length of the phase shift region, a heating electrode resistance greater than 100Ω, and a distance of less than 50um between the heating electrode and the ridge waveguide.

[0014] Optionally, the type of grating includes a uniform grating, a quarter-phase shift grating, a multiphase shift grating, and a periodic modulation grating, wherein the period of the grating is determined according to the emission wavelength of the laser.

[0015] A method for fabricating the above-mentioned electrically heated DFB laser with a wide tuning range and narrow linewidth includes the following steps:

[0016] 1) A primary epitaxial layer for a DFB laser is grown on a substrate, defining a phase-shifting region and a gain region. A partial grating pattern is fabricated by electron beam exposure, and a partial grating is formed by etching. The partial grating is located in the gain region.

[0017] 2) Perform secondary extension;

[0018] 3) Fabricate the ridge waveguide;

[0019] 4) Deposit metal onto the ridge waveguides in the phase-shift and gain regions and the corresponding metal wire bonding regions as positive electrode metal;

[0020] 5) Deposit metal on one side of the ridge waveguide in the phase-shift region to create the heating electrode metal;

[0021] 6) Deposit negative electrode metal.

[0022] Steps 4) through 6) are not in any particular order.

[0023] Optionally, between steps 3) and 4), a step is also included to deposit a passivation layer on the surface of the structure formed in step 3) and to open the passivation layer on the ridge waveguide.

[0024] The beneficial effects of this invention are as follows:

[0025] 1) The epitaxial structure forms a gain region and a phase shift region near the reflection end through the design of a partial grating. A heating electrode metal is introduced in the phase shift region, and the waveguide without grating is heated by electrothermal power to form a change in refractive index, which in turn causes a change in phase and wavelength. This enables fast and wide-range laser tuning with current control, and realizes the function of fine mode-hopping frequency tuning, thereby improving the tuning range and tuning rate of the DFB laser to meet the needs of FMCW radar applications.

[0026] 2) Introducing heating electrode metal only at the reflecting end has a small impact on the junction temperature of the entire active layer, and a small impact on the threshold and slope efficiency; the partial grating structure design preserves the integrity of the active region in the cavity and maintains the function of the phase shift region to generate gain, which is beneficial to reducing the threshold of the laser and increasing the power.

[0027] 3) The manufacturing process is simple and the cost is low. Attached Figure Description

[0028] Figure 1 This is a top view of the electrically heated DFB laser with a wide tuning range and narrow linewidth, as described in Example 1.

[0029] Figure 2 for Figure 1 Cross-sectional view along the a-a' direction;

[0030] Figure 3 for Figure 1 Cross-sectional view along the b-b' direction;

[0031] Figure 4 This is a cross-sectional schematic diagram of the electrically heated DFB laser with a wide tuning range and narrow linewidth in Example 2 (corresponding to...). Figure 2 (cross section). Detailed Implementation

[0032] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments. The accompanying drawings are merely illustrative to facilitate understanding of the invention, and their specific proportions can be adjusted according to design requirements. The vertical relationships of relative elements and the definitions of front / back in the graphics described herein should be understood by those skilled in the art to refer to the relative positions of the components; therefore, they can all be flipped to present the same component, and all of this should fall within the scope disclosed in this specification.

[0033] Example 1

[0034] refer to Figures 1 to 3 The electrically heated DFB laser with a wide tuning range and narrow linewidth in Embodiment 1 has an epitaxial structure 1 comprising, from bottom to top, a substrate 11, a lower cladding layer 12, an active layer 13, an upper cladding layer 14, and a contact layer 15. A ridge waveguide 16 is formed on the front side of the epitaxial structure 1 along the cavity length direction (shown as the x-direction in the figures). The epitaxial structure 1 includes a phase shift region A and a gain region B along the cavity length direction. One end of the DFB laser cavity length direction is a reflecting end face HR, and the phase shift region A is close to the reflecting end face HR. A passivation layer 2 is covered on the contact layer 15, and the passivation layer 2 has an opening on the ridge waveguide 16. The opening length can be, for example, the entire cavity length. A positive electrode metal 3 covers the entire cavity length direction of the ridge waveguide 16 and contacts the ridge waveguide 16 through the opening. In this embodiment, the positive electrode metal 3 extends from the ridge waveguide 16 to the surfaces of the passivation layers 2 on both sides in the gain region B, and extends from the ridge waveguide 16 to the surface of the passivation layer 2 on one side in the phase shift region A. The heating electrode metal 4 is disposed on the passivation layer 2 on the other side of the ridge waveguide in phase-shift region A. The epitaxial structure of gain region B includes a grating 17, while the epitaxial structure of phase-shift region A does not include a grating. The negative electrode metal 5 is located on the back side of substrate 11. Gain region B provides optical gain and basic mode selection functions via the grating, while phase-shift region A provides fine, mode-hopping-free frequency tuning. The length of phase-shift region A is 1 / 10 of the cavity length.

[0035] A conventional DFB laser epitaxial structure can be applied to this embodiment. The active layer 13 extends through the phase-shift region A and the gain region B, covering the entire length of the laser cavity. The grating 17 can be located above or below the active layer 13 in the gain region B. For example, the upper cladding layer 14 includes a grating forming layer, a grating cladding, and an upper waveguide layer. When fabricating the grating using etching processes such as etching the grating forming layer, a partial etching process is used, etching only the gain region B and not the phase-shift region A, thus forming a partial grating structure. Grating types include uniform gratings, quarter-phase-shift gratings, multi-phase-shift gratings, and periodic modulation gratings, etc., with the grating period determined according to the emission wavelength of the laser. Furthermore, in other structures, the phase-shift region A may not contain a grating forming layer.

[0036] A conventional DFB laser ridge waveguide structure can be applied to this embodiment. For example, two trenches 18 are etched on the front side of the epitaxial structure 1, with the bottoms of the two trenches 18 formed at any position between the contact layer 15 and the active layer 13, and a ridge waveguide 16 is formed between the two trenches 18. The width of the ridge waveguide 16 is, for example, about 2-3 μm, and the width of the trenches 18 is about 15 μm. In this embodiment, in the phase shift region A, the positive electrode metal 3 is located on one side of the ridge waveguide 16, covering the surface of one side of the trench 18, the surface of the ridge waveguide 16, and extending to cover at least a portion of the bottom of the other side of the trench 18. The positive electrode metal 3 covers the reflective end face edge of the surface of the phase shift region A to avoid light absorption. The heating electrode metal 4 is located on the surface outside the trench 18 on the other side of the phase shift region A, extending along the cavity length direction and having positive and negative access regions at both ends. The heating electrode metal 4 is a strip-shaped platinum metal thin film with a thickness of approximately 500 nm, a width of approximately 10 μm, and a length accounting for more than 70% of the phase-shift region length. The heating electrode resistance is greater than 100 Ω, and the distance between it and the ridge waveguide 16 is 20 μm. The passivation layer 2 is a silicon dioxide or silicon nitride thin film with a thickness of approximately 300 nm. The passivation layer is an insulator and has the function of isolating current.

[0037] When the active layer of the DFB is not modulated, the period and equivalent refractive index of the DFB grating remain stable, and the resonant frequency is mainly affected by the cavity length, specifically expressed as: υ*( <n a >*La+ <n p >*Lp)=m*c / 2. Where υ is the resonant frequency, <na> 、 <np>Let be the equivalent refractive indices of the DFB gain region and phase-shift region, respectively; La and Lp be the lengths of the DFB gain region and phase-shift region, respectively; c be the speed of light in vacuum; and m be the longitudinal mode number. Keeping the temperature and current of the DFB gain region constant, then... <n a *La remains stable. When the refractive index of the material in the phase-shift region is slightly changed, mode m remains unchanged, and the resonant frequency is tuned without mode skipping.

[0038] The refractive index of the phase-shift region is affected by temperature, and its refractive index can be controlled by heating, thereby achieving laser tuning. In this embodiment, a thin metal heating element is introduced at the high-reflectivity (HR) end of the chip, combined with a partial grating epitaxial design. Electrothermal power is used to heat the gratingless portion of the waveguide, causing a change in refractive index, which in turn induces changes in phase and wavelength, achieving fast and wide-range laser tuning controlled by current. Because the heating element is only introduced at the HR end, the impact on the junction temperature of the entire active layer is relatively small, as is the impact on the threshold and slope efficiency. This improves the tuning range and tuning rate of the DFB laser to meet the requirements of FMCW radar applications.

[0039] In this embodiment, the phase-shift region is a gratingless active waveguide. The active region structure of the phase-shift region is the same as that of the gain region, maintaining the integrity of the intracavity active region. It can be formed with only one epitaxial growth. On the one hand, it is compatible with conventional DFB processes without the need for passive waveguide docking growth. On the other hand, it also maintains the gain generation function of the phase-shift region, which is beneficial for reducing the laser threshold and increasing power.

[0040] The fabrication method of the above-mentioned electrically heated DFB laser with a wide tuning range and narrow linewidth includes the following steps:

[0041] 1) Grow a primary epitaxial layer of a DFB laser on a substrate, define the phase shift region and the gain region, fabricate a partial grating pattern on the primary epitaxial layer by electron beam exposure, form a partial grating by etching, the partial grating is located in the gain region, and the phase shift region is not etched.

[0042] 2) Perform secondary epitaxy, including ridge waveguide structural materials and contact layers, etc.

[0043] 3) The ridge waveguide is fabricated using conventional chip manufacturing processes;

[0044] 4) Deposit a passivation layer, opening the passivation layer on the ridge waveguide;

[0045] 5) Deposit metal onto the ridge waveguide in the phase shift region and gain region and the corresponding metal wire bonding region to form the positive electrode metal, which is in contact with the contact layer of the ridge waveguide;

[0046] 6) Deposit metal on one side of the ridge waveguide in the phase-shift region to create the heating electrode metal;

[0047] 7) Deposit negative electrode metal on the back side of the substrate.

[0048] Based on the conventional DFB structure, this embodiment adopts a partial grating design and utilizes the electric heating effect at the HR end to achieve fast and wide-range current tuning. It does not require connection to a passive waveguide, and the manufacturing method is similar to that of existing DFBs, resulting in low cost.

[0049] Conventionally, the positive electrode metal is a p-type metal and the negative electrode metal is an n-type metal. In other embodiments, the n-type doped region of the epitaxial layer (such as substrate 11 or lower cladding 12) can be exposed by etching the epitaxial layer on the front side, and the negative electrode metal can be disposed on the n-type doped region to provide another circuit connection method.

[0050] Example 2

[0051] refer to Figure 4 The difference between Example 2 and Example 1 is that the opening of the passivation layer 2 on the ridge waveguide does not extend to the edge of the light-emitting end (the end with the antireflection coating AR, which is opposite to the reflecting end face HR in the cavity length direction), but to 10 μm from the edge. That is, the passivation layer 2 retains a portion without opening. This structure can prevent electrical injection and improve catastrophic cavity surface damage (COD) at the laser end face, which is beneficial to improving the reliability of the laser.

[0052] The above embodiments are only used to further illustrate the electric heating wide tuning range narrow linewidth DFB laser and its manufacturing method according to the present invention. However, the present invention is not limited to the embodiments. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the technical solution of the present invention.< / np> < / na>

Claims

1. A DFB laser with wide tuning range and narrow linewidth, characterized in that: The laser comprises a DFB epitaxial structure, a negative electrode metal, a positive electrode metal, and a heating electrode metal. A ridge waveguide is provided on the front side of the epitaxial structure. Along the cavity length of the DFB laser, the epitaxial structure includes a phase-shifting region and a gain region. One end of the cavity length is a reflecting end face, and the other end is a emitting end face. The phase-shifting region is located close to the reflecting end face. The length of the phase-shifting region accounts for 5% to 20% of the cavity length of the laser. The epitaxial structure of the gain region includes a grating and an active layer. The active layer extends through the phase shift region and the gain region. The grating is only located above or below the active layer in the gain region. The phase shift region does not include the grating. The front side of the epitaxial structure is covered with a passivation layer, and the passivation layer has an opening on the ridge waveguide, with the opening of the passivation layer on the ridge waveguide extending 10 μm from the edge of the light-emitting end. The positive electrode metal and the heating electrode metal are disposed on the passivation layer. The positive electrode metal covers more than 70% of the ridge waveguide in the gain region and the phase shift region and contacts the ridge waveguide through the opening. The heating electrode metal is located on the passivation layer on one side of the ridge waveguide in the phase shift region, with a length of more than 70% of the length of the phase shift region, a distance of less than 50 μm from the ridge waveguide, and a resistance of more than 100 Ω. The negative electrode metal is disposed on the back side of the substrate, or exposed on the n-type doped region of the epitaxial structure by etching on the front side.

2. The electrically heated DFB laser with a wide tuning range and narrow linewidth according to claim 1, characterized in that: The epitaxial structure, from bottom to top, includes a substrate, a lower cladding layer, an active layer, an upper cladding layer, and a contact layer.

3. The electrically heated DFB laser with a wide tuning range and narrow linewidth according to claim 2, characterized in that: The upper or lower cladding layer includes a grating forming layer. The grating forming layer in the gain region is etched to form a grating pattern. The phase shift region does not contain a grating forming layer or contains a grating forming layer but is not etched.

4. The electrically heated DFB laser with a wide tuning range and narrow linewidth according to claim 1, characterized in that: The ridge waveguide is formed along the cavity length direction. Two trenches are etched on the front side of the epitaxial structure, and the ridge waveguide is formed between the two trenches. The width of the ridge waveguide is 2-3 μm.

5. The electrically heated DFB laser with a wide tuning range and narrow linewidth according to claim 1, characterized in that: The passivation layer is a silicon dioxide or silicon nitride thin film with a thickness of 100nm-1000nm; the heating electrode metal is a platinum or titanium metal thin film with a thickness of 100-1000nm and a width of 5-50um.

6. The electrically heated DFB laser with a wide tuning range and narrow linewidth according to claim 1, characterized in that: The length of the phase shift region is 1 / 10 of the cavity length.

7. The electrically heated DFB laser with a wide tuning range and narrow linewidth according to claim 1, characterized in that: The positive electrode metal extends from the ridge waveguide to the surface of the passivation layers on both sides in the gain region, and extends from the ridge waveguide to the surface of the passivation layer on one side in the phase shift region; the heating electrode metal is disposed on the passivation layer on the other side of the ridge waveguide in the phase shift region.

8. The electrically heated DFB laser with a wide tuning range and narrow linewidth according to claim 1, characterized in that: The types of gratings include uniform gratings, quarter-phase shift gratings, multiphase shift gratings, and periodic modulation gratings, and the period of the grating is determined according to the emission wavelength of the laser.

9. A method for manufacturing a DFB laser with a wide tuning range and narrow linewidth via electric heating as described in any one of claims 1 to 8, characterized in that, Including the following steps: 1) A primary epitaxial layer of a DFB laser is grown on a substrate, defining a phase-shifting region and a gain region. A partial grating pattern is fabricated by electron beam exposure, and a partial grating is formed by etching. The partial grating is located only in the gain region. 2) Perform secondary extension; 3) Fabricate the ridge waveguide; 4) Deposit a passivation layer on the surface of the structure formed in step 3) and open the passivation layer on the ridge waveguide. The opening of the passivation layer on the ridge waveguide extends to a distance of 10 μm from the edge of the light-emitting end. 5) Deposit metal onto the ridge waveguides in the phase-shift and gain regions and the corresponding metal wire bonding regions as positive electrode metal; 6) Deposit metal onto the passivation layer on one side of the ridge waveguide in the phase-shift region to fabricate the heating electrode metal; 7) Deposit negative electrode metal.

10. The method for fabricating an electrically heated DFB laser with a wide tuning range and narrow linewidth according to claim 9, characterized in that: The heating electrode metal is selected from platinum or titanium, with a thickness of 100-1000 nm and a width of 5-50 μm. Its lateral distance from the ridge waveguide is less than 50 μm, its length accounts for more than 70% of the length of the phase shift region, and its resistance is greater than 100 Ω.

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