Laser based on two different gratings with a phase spacing of λ / 2 planar metallic gratings

Abstract

The application belongs to the technical field of optoelectronic devices, and particularly relates to a plane metal grating laser based on two different gratings and with a phase interval of lambda / 2, which comprises a gain chip, a gain chip optical waveguide, a grating optical waveguide substrate, a ridge back optical waveguide and a plane metal grating. The application adopts a unique external cavity structure, replaces the traditional sandwich semiconductor grating with two metal plane gratings with similar grating constants and a phase difference of lambda / 2, utilizes the advantages of the plane metal grating, such as no etching and more flat and regular, combines the "mouth-to-mouth" coupling and docking mode of the gain chip and the grating optical waveguide substrate, grows a cover layer above the plane metal grating, and coats a high reflection (HR) film above the cover layer, so that the grating reflection efficiency is effectively improved, the optical waveguide light energy loss is reduced, meanwhile, the secondary epitaxy is saved, the process is simplified, and the cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of optoelectronic devices, and more particularly to a planar metal grating laser based on two different gratings with a phase spacing of λ / 2. Background Technology

[0002] Due to carrier density fluctuations and the typical 0.5mm lasing cavity length, the linewidth of DFB lasers is usually between 1-10MHz. In applications such as coherent detection, a wider linewidth can cause severe signal interference, significantly reducing detection accuracy and system performance. Its secondary epitaxial process is complex, with a yield of only 65-70%, and manufacturing costs are 2-3 times higher than external cavity lasers. The reflectivity of semiconductor gratings is only 93-95%, requiring additional gain compensation, leading to a 15-20% increase in power consumption and causing heat dissipation problems, affecting the stability and reliability of the laser. Furthermore, the etching process for semiconductor gratings is complex; due to the anisotropy of the crystal lattice, the surface structure after etching is irregular, making it difficult to guarantee the flatness and accuracy of the grating, thus affecting laser performance.

[0003] Traditional external cavity lasers mostly employ a single-grating structure, with a side-mode rejection ratio (SMSR) of only 45-50 dB and a linewidth of approximately 20-50 kHz, which cannot meet the stringent requirements of high-precision applications for light sources. Balancing beam quality and integration is difficult, relying on complex optical alignment processes, increasing production difficulty and cost, and hindering large-scale integrated manufacturing. Furthermore, traditional external cavity lasers cannot effectively address the upward diffraction energy loss caused by the grating structure, affecting the light guiding performance of the grating waveguide. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by proposing a planar metal grating laser based on two different gratings with a phase spacing of λ / 2.

[0005] To achieve the above objectives, the technical solution adopted by this invention is: a planar metal grating laser based on two different gratings with a phase spacing of λ / 2, comprising:

[0006] Gain chip;

[0007] The gain chip optical waveguide is positioned at the top center of the gain chip;

[0008] A grating waveguide substrate is disposed on the right end face of the gain chip;

[0009] The ridge waveguide is positioned at the top center of the grating waveguide substrate;

[0010] A planar metal grating is set on top of the back waveguide, and the planar metal grating includes a planar metal Bragg grating one and a planar metal Bragg grating two. The grating constants of the planar metal Bragg grating one and the planar metal Bragg grating two are similar but different. A phase difference is set between the planar metal Bragg grating one and the planar metal Bragg grating two, and the length of the phase difference is λ / 2.

[0011] The overlay is placed on a planar metal grating and coated with a high-reflectivity HR film.

[0012] Preferably, the grating period of the first planar metal Bragg grating is 224.5±0.3nm, and the grating period of the second planar metal Bragg grating is 225.5±0.3nm.

[0013] Preferably, the gain chip adopts an InP / InGaAsP quantum well structure, and the lasing wavelength range of the gain chip is 1530-1610nm.

[0014] Preferably, one side of the gain chip is set as a reflector end face, and a high-reflection film is coated on the reflector end face. The high-reflection film adopts a SiO2 / TiO2 multilayer structure and the reflectivity of the high-reflection film is greater than 99.9%.

[0015] Preferably, the side of the gain chip away from the reflector end face is the anti-reflection end face, and an anti-reflection film is coated on the anti-reflection end face. The anti-reflection film adopts a SiO2 / TiO2 multilayer structure, and the reflectivity of the anti-reflection film is less than 0.03%.

[0016] Preferably, the planar metal grating has a duty cycle of 50±1% and a thickness of 200±5nm, and adopts a Cr / Au multilayer structure, with the thicknesses of the Cr / Au layers being 50nm and 150nm, respectively.

[0017] Preferably, the left end face of the grating waveguide substrate is coupled and docked with the right end face of the gain chip in a "mouth-to-mouth" configuration.

[0018] Preferably, the capping layer is a SiO2 thin film layer with a thickness of 0.5-2 μm.

[0019] Preferably, the high-reflectivity HR film is a dielectric reflective film or a metal reflective film.

[0020] Preferably, the back waveguide adopts a ridge-shaped structure with a width of 3-5 μm and a height of 2-4 μm.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] This invention employs a unique external cavity structure, replacing the traditional sandwich semiconductor grating with two planar metal gratings with similar grating constants and a phase difference of λ / 2. Utilizing the advantages of planar metal gratings—no etching required and a more flat and regular surface—and combining a "mouth-to-mouth" coupling method between the gain chip and the grating waveguide substrate, a capping layer is grown above the planar metal grating, and a high-reflectivity (HR) film is deposited on top of the capping layer. This effectively improves the grating's reflection efficiency, reduces waveguide energy loss, eliminates the need for secondary epitaxy, simplifies the process, reduces costs, significantly improves the SMSR performance of the laser beam, and compresses the linewidth. This structural design reduces mode competition, improves the laser's wavelength selectivity and stability, and facilitates the realization of different wavelength laser outputs in multi-wavelength application scenarios. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the main structure of the present invention;

[0024] Figure 2 This is a top view of the structure of the present invention.

[0025] In the figure: 1. Gain chip; 2. Ridge waveguide; 3. Planar metal grating; 301. Planar metal Bragg grating one; 302. Planar metal Bragg grating two; 4. Covering layer; 5. High reflectivity (HR) film; 6. High reflectivity film; 7. Anti-reflection film; 8. Phase difference; 9. Grating waveguide substrate; 10. Gain chip waveguide. Detailed Implementation

[0026] The following description is intended to disclose the invention and enable those skilled in the art to implement it. The preferred embodiments described below are merely examples, and other obvious variations will occur to those skilled in the art.

[0027] like Figures 1-2 As shown, this application provides a planar metal grating laser based on two different gratings with a phase spacing of λ / 2, including: a gain chip 1; a gain chip waveguide 10 disposed at the top center of the gain chip 1; a grating waveguide substrate 9 disposed at the right end face of the gain chip 1; a ridge waveguide 2 disposed at the top center of the grating waveguide substrate 9; a planar metal grating 3 disposed at the top of the ridge waveguide 2, and the planar metal grating 3 includes a planar metal Bragg grating 1 301 and a planar metal Bragg grating 2 302, the grating constants of the planar metal Bragg grating 1 301 and the planar metal Bragg grating 2 302 are similar but different, a phase difference 8 is provided between the planar metal Bragg grating 1 301 and the planar metal Bragg grating 2 302, the length of the phase difference 8 is λ / 2; a capping layer 4 covering the planar metal grating 3, and a high-reflectivity HR film 5 is deposited on top of it.

[0028] A dual-plane metallic Bragg grating (G1, G2) with a phase difference Δφ=π (i.e., a spacing λ / 2) is employed. The reflected light from the two gratings undergoes constructive interference within the cavity, increasing the main peak reflectivity from 95% to 99.9% compared to a traditional single grating. This enhances the intracavity optical field intensity, ensuring high-power and stable laser output. The destructive interference characteristics of the dual gratings are utilized to suppress non-laser modes, effectively weakening side-mode intensity and improving the purity and stability of the output light, meeting the requirements of high-precision applications.

[0029] Specifically, such as Figure 1 As shown, the grating period of planar metal Bragg grating 301 is 224.5±0.3nm, and the grating period of planar metal Bragg grating 302 is 225.5±0.3nm.

[0030] The parameters of the metal Bragg gratings were carefully designed. The grating periods of the two metal gratings were set to Λ1=224.5±0.3nm and Λ2=225.5±0.3nm, respectively. This design of similar but different grating periods, combined with the phase difference λ / 2 between the two gratings, utilizes the interference effect of the gratings to significantly improve the main peak reflectivity, enhance the intracavity optical field intensity, and ensure high-power, stable laser output. At the same time, it effectively suppresses non-laser modes, theoretically achieving a higher side-mode suppression ratio (SMSR), compressing the laser linewidth, and improving the purity and stability of the output light to meet the requirements of high-precision applications.

[0031] Specifically, such as Figure 1 As shown, the gain chip 1 adopts an InP / InGaAsP quantum well structure, and the lasing wavelength range of the gain chip 1 is 1530-1610nm.

[0032] The InP / InGaAsP quantum well structure was selected as the basic architecture of the gain chip. This structure has good laser emission characteristics in the wavelength range of 1530-1610nm, which can meet the needs of various application scenarios. At 25℃, its threshold current is less than 28mA, which means that laser emission can be achieved with low energy consumption. The slope efficiency reaches 0.85W / A, indicating that under a certain current drive, it can efficiently convert electrical energy into light energy.

[0033] Specifically, such as Figure 1 As shown, one side of the gain chip 1 is set as a reflector end face, and a high-reflection film 6 is coated on the reflector end face. The high-reflection film 6 adopts a SiO2 / TiO2 multilayer structure and has a reflectivity greater than 99.9%. The side of the gain chip 1 away from the reflector end face is an anti-reflection end face, and an anti-reflection film 7 is coated on the anti-reflection end face. The anti-reflection film 7 adopts a SiO2 / TiO2 multilayer structure and has a reflectivity less than 0.03%.

[0034] The λ / 2 high-reflectivity film 5 can effectively reduce light loss at the reflective end face and enhance light feedback in the cavity, while the λ / 4 anti-reflectivity film 6 can maximize the laser emission efficiency.

[0035] Specifically, such as Figure 1 As shown, the duty cycle of the planar metal grating 3 is 50±1%, the thickness of the planar metal grating 3 is 200±5nm, and it adopts a Cr / Au multilayer structure, with the thicknesses of the Cr / Au layers being 50nm and 150nm, respectively.

[0036] The duty cycle is 50±1%, which ensures the uniformity and stability of the grating structure. The Cr layer can enhance the adhesion between the metal and the substrate, while the Au layer has good electrical conductivity and optical reflection properties.

[0037] Specifically, such as Figure 1 As shown, the left end face of the grating waveguide substrate 9 is coupled and docked with the right end face of the gain chip 1 in a "mouth-to-mouth" manner.

[0038] The left end face of the grating waveguide substrate 9 is coupled and docked with the right end face of the gain chip 1 in a "mouth-to-mouth" manner. Submicron-level alignment (accuracy ±0.2μm) is achieved through high-precision flip-chip bonding process, ensuring that the air gap between the two is less than 200nm. The total external cavity length Lcavity = 800μm + Lwg, and the free spectral range FSR > 100GHz. This external cavity coupling structure design, together with other components, realizes efficient light transmission and feedback, and improves the performance of the laser.

[0039] Specifically, such as Figure 1 As shown, the capping layer 4 is a SiO2 thin film layer with a thickness of 0.5-2μm, and the high-reflectivity HR film 5 is a dielectric reflective film or a metal reflective film.

[0040] The capping layer 4 has excellent optical performance. On the one hand, it can effectively reduce the upward diffraction energy loss caused by the presence of the top Bragg grating. By guiding the light through its own waveguide structure, it can re-constrain the upward diffracted light within the waveguide system and maintain the light guiding performance of the grating waveguide. On the other hand, the SiO2 waveguide layer forms a good optical match with the planar metal Bragg grating and the bottom waveguide, optimizes the light field distribution, and further improves the performance of the laser. In addition, compared with the traditional semiconductor secondary epitaxial waveguide layer, the SiO2 waveguide layer has a relatively simple fabrication process and lower cost, which helps to reduce the overall production cost.

[0041] Dielectric reflective films achieve high reflectivity by periodically stacking multiple layers of media with different refractive indices and utilizing the principle of light interference. They have low absorption loss and good optical performance. Metal reflective films, such as silver and aluminum, have high reflectivity and good conductivity, which can effectively reflect light, reduce light energy loss, further reduce the light energy loss of the grating waveguide, enhance the feedback of light in the cavity, and help improve the output power and stability of the laser.

[0042] Specifically, such as Figure 1 As shown, the back waveguide 2 adopts a ridge-shaped structure with a width of 3-5 μm and a height of 2-4 μm.

[0043] Theoretical basis:

[0044] According to coupled-mode theory, for a traditional uniform grating DFB laser, its coupling coefficient is κ, and the optical field distribution can be represented by A(z), satisfying the following coupled-mode equation:

[0045] Where β is the propagation constant, in this equation, It describes the phase change of the light field during propagation. This reflects the coupling effect of the grating on the light field, while loss terms represent the light loss during propagation.

[0046] For two metal grating DFB lasers with similar grating constants and a phase difference of λ / 2 in this invention, the presence of a phase shift profoundly affects the optical field distribution. Assuming the phase shift occurs at z=0, the optical field distributions before and after the phase shift can be represented by A1(z) and A2(z), respectively, and satisfy the boundary conditions: This means that the optical field generates a π / 2 phase jump at the phase shift point. This phase jump will cause the optical field to redistribute, change the propagation characteristics of light in the laser cavity, and thus affect the mode selection characteristics of the laser. During the laser generation process, different modes of light will compete with each other, and this phase jump enables the dominant mode to suppress the side modes more effectively, thereby improving the SMSR.

[0047] The differences in grating constants and the setting of the phase difference λ / 2 alter the light field distribution, causing subtle changes in the effective refractive index of light during propagation. Based on the Bragg condition of the grating, for a traditional uniform grating DFB laser, its Bragg wavelength... satisfy: ,in Where Λ is the effective refractive index and Λ is the grating period, in this invention, due to the change in the light field distribution, the actual Bragg wavelength is slightly changed, and the gains of the main mode and the side mode are also affected. The gain of the main mode increases relatively, while the gain of the side mode is suppressed relatively, which further promotes the improvement of the SMSR. In addition, this structural change also compresses the laser linewidth, improving the purity and stability of the laser.

[0048] Example 1:

[0049] Planar metal Bragg gratings with periods of 224.5 nm and 225.5 nm were fabricated using an electroplating process. The grating pattern was precisely controlled by electron beam evaporation. The waveguide was etched by ICP to a depth of 120 nm, and a Cr / Au layer was deposited by magnetron sputtering, achieving a surface flatness of 0.07 nm RMS. A top SiO2 waveguide layer with a thickness of 1 μm was fabricated by chemical vapor deposition. A λ / 2 DBR dielectric reflective film was deposited on top of the top SiO2 waveguide layer as an HR film. The right end face of the gain chip 1 was coupled to the left end face of the grating waveguide substrate 9 by flip-chip bonding, with the air gap controlled at 180 nm. The test results showed that the linewidth was 0.6 kHz, the SMSR was 63 dB, and the threshold current was 22 mA.

[0050] Example 2:

[0051] Comparing different metallic materials (Al, Ag), the Cr / Au structure was found to have the highest reflectivity, reaching 99.92%. Temperature variation tests showed that wavelength drift was controlled within ±0.002 nm in the range of -40 to 85℃. The effects of different thicknesses of the top SiO2 waveguide layer on laser performance were also tested. Results showed that a 1 μm thick SiO2 waveguide layer was the most effective in reducing light energy loss and improving laser performance. Furthermore, comparing the effects of different types of HR films (λ / 2DBR dielectric reflective film and metal reflective film) on laser performance revealed that the output power and stability of the laser were slightly better when using the λ / 2DBR dielectric reflective film compared to the metal reflective film. This is because the λ / 2DBR dielectric reflective film achieves high reflectivity through the periodic stacking of multiple layers of media with different refractive indices, utilizing the principle of light interference. It exhibits low absorption loss and good optical performance, effectively enhancing light feedback within the cavity and improving the overall performance of the laser.

[0052] Example 3:

[0053] The external cavity length was optimized to 3 cm, at which point the free spectral range FSR = 100 GHz and the linewidth was further compressed to 0.5 kHz. During the optimization process, the effects of the top SiO2 waveguide layer and HR film on optical field confinement and energy loss under different external cavity lengths were studied. As the external cavity length increases, the propagation path of light in the cavity lengthens, and the interaction time and degree between the optical field and the grating, SiO2 waveguide layer, and HR film change. Through experimental and simulation analysis, it was found that with an external cavity length of 3 cm, a top 1 μm thick SiO2 waveguide layer combined with a λ / 2 DBR dielectric reflective film as the HR film can achieve the best confinement of the optical field and minimize energy loss. Thus, this parameter combination was determined as the scheme to achieve the optimal laser performance.

[0054] Example 4:

[0055] Based on the "mouth-to-mouth" coupling and docking of the right end face of gain chip 1 and the left end face of grating waveguide substrate 9, the influence of coupling process on laser performance is further studied. A high-precision flip-chip bonding process is adopted, combined with an advanced optical alignment system, to improve the coupling accuracy to ±0.1μm, while strictly controlling the air gap between the two to 150nm.

[0056] With this optimized coupling method, test results show that the laser's output power increased by 15% to 35mW, the side-mode rejection ratio (SMSR) improved to 65dB, and the linewidth was further compressed to 0.4kHz. This is because the higher coupling precision and smaller air gap effectively reduced the reflection and scattering losses of light during transmission, allowing more light to be efficiently transmitted from the gain chip to the grating waveguide, enhancing the optical feedback effect, and thus improving the overall performance of the laser.

[0057] Example 5:

[0058] The effects of different duty cycles on the performance of planar metallic Bragg gratings were investigated. Grating structures with duty cycles of 45%, 50%, and 55% were fabricated, with other parameters kept constant. The grating periods were Λ1=224.5±0.3nm and Λ2=225.5±0.3nm, and the metal layer thickness was 200nm.

[0059] Test results show that the laser's overall performance is optimal at a duty cycle of 50%. At this point, the grating's reflection efficiency is highest, with a principal mode reflectivity of 99.8%, a side mode rejection ratio (SMSR) of 64 dB, and a linewidth of 0.5 kHz. When the duty cycle deviates from 50%, the grating's reflection characteristics change, leading to a decrease in principal mode gain and a relative increase in side mode gain, thus affecting the laser's performance. For example, at a duty cycle of 45%, the principal mode reflectivity drops to 99.5%, and the SMSR decreases to 62 dB; at a duty cycle of 55%, the principal mode reflectivity is 99.6%, and the SMSR is 61 dB.

[0060] This invention:

[0061] Employing a unique external cavity structure, this design replaces the traditional sandwich semiconductor grating with two planar metal gratings with similar grating constants and a phase difference of λ / 2. Leveraging the advantages of planar metal gratings—no etching required and a more flat and regular surface—and combining the gain chip 1 with the grating waveguide substrate 9 via a "mouth-to-mouth" coupling method, a capping layer 4 is grown above the planar metal grating 3, and a high-reflectivity HR film 5 is deposited above the capping layer 4. This effectively improves the grating reflection efficiency, reduces waveguide energy loss, eliminates the need for secondary epitaxy, simplifies the process, reduces costs, significantly improves the SMSR performance of the laser beam, and compresses the linewidth. This structural design reduces mode competition, improves the laser's wavelength selectivity and stability, and facilitates the realization of different wavelength laser outputs in multi-wavelength application scenarios.

[0062] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A laser based on two different gratings and phase- spaced λ / 2 planar metal grating, characterized in that , comprising: a gain chip (1); a gain chip optical waveguide (10) arranged centrally on the top of the gain chip (1); a grating optical waveguide substrate (9) arranged on the right end face of the gain chip (1); a ridge back optical waveguide (2) arranged centrally on the top of the grating optical waveguide substrate (9); a planar metal grating (3) arranged on the top of the ridge back optical waveguide (2), and the planar metal grating (3) comprises a planar metal Bragg grating one (301) and a planar metal Bragg grating two (302), the grating constants of the planar metal Bragg grating one (301) and the planar metal Bragg grating two (302) are different, a phase difference (8) is arranged between the planar metal Bragg grating one (301) and the planar metal Bragg grating two (302), and the length of the phase difference (8) is λ / 2; a cover layer (4) covering the planar metal grating (3), and a high reflection HR film (5) is coated on the top.

2. The two different grating based and phase interval λ / 2 planar metal grating laser according to claim 1, characterized in that, The grating period of the planar metal Bragg grating one (301) is 224.5±0.3nm, and the grating period of the planar metal Bragg grating two (302) is 225.5±0.3nm.

3. The two different grating based and phase interval λ / 2 planar metal grating laser as claimed in claim 1, characterized in that, The gain chip (1) adopts an InP / InGaAsP quantum well structure, and the lasing wavelength range of the gain chip (1) is 1530-1610nm.

4. The two different grating based and phase interval λ / 2 planar metal grating laser as claimed in claim 1, wherein, One side of the gain chip (1) is provided with a mirror end face, a high reflection film (6) is coated on the mirror end face, the high reflection film (6) adopts a SiO2 / TiO2 multilayer structure, and the reflectivity of the high reflection film (6) is greater than 99.9%.

5. The two different grating based and phase spaced λ / 2 planar metal grating laser according to claim 4, characterized in that, The side of the gain chip (1) away from the mirror end face is an anti-reflection end face, an anti-reflection film (7) is coated on the anti-reflection end face, the anti-reflection film (7) adopts a SiO2 / TiO2 multilayer structure, and the reflectivity of the anti-reflection film (7) is less than 0.03%.

6. The two different grating based and phase interval λ / 2 planar metal grating laser according to claim 1, characterized in that, The duty cycle of the planar metal grating (3) is 50±1%, the thickness of the planar metal grating (3) is 200±5nm, and a Cr / Au multilayer structure is adopted, and the thicknesses of the Cr / Au layers are 50nm and 150nm respectively.

7. The two different grating based and phase space λ / 2 spaced planar metal grating laser according to claim 1, characterized in that, The left side end face of the grating optical waveguide substrate (9) is coupled and connected to the right side end face of the gain chip (1) in a "mouth-to-mouth" manner.

8. The two different grating based and phase space λ / 2 spaced planar metal grating laser according to claim 1, characterized in that, The cover layer (4) is a SiO2 thin film layer, and the thickness of the cover layer (4) is 0.5-2μm.

9. The two different grating based and phase space λ / 2 spaced planar metal grating laser according to claim 1, characterized in that, The ridge back optical waveguide (2) adopts a ridge structure, and the width is 3-5μm and the height is 2-4μm.

10. The two different grating based and phase space λ / 2 spaced planar metal grating laser according to claim 1, characterized in that, The high reflection HR film (5) is a dielectric reflection film or a metal reflection film.

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