Narrow-linewidth DFB laser with different longitudinal grating period structures on both sides and its manufacturing method

Abstract

This application discloses a narrow-linewidth DFB laser with alternating longitudinal grating periods on both sides and its manufacturing method. An N-side lower grating layer, composed of alternating InP and InGaAsP layers, is disposed between an N-type buffer layer and an N-type lower waveguide layer. A P-side upper grating layer, also composed of alternating InP and InGaAsP layers, is disposed between a P-type upper waveguide layer and a P-type cladding layer. The N-side lower grating period is Λ. N With the P-side upper grating period Λ P The differences are not identical; the relative periodicity is controlled within 0.05% to 2%. The N-side and P-side phase shift regions are staggered along the cavity length, with a spatial offset of 5% to 20% of the cavity length. This application uses gratings with different periods on both sides to form a wavelength interference effect, achieving dual suppression of side modes to narrow the linewidth; and uses the staggered phase shift on both sides to suppress the spatial hole burning effect to improve performance. This application is expected to achieve narrow linewidth output at the kilohertz level, maintaining the advantages of chip-level monolithic integration and being compatible with existing semiconductor processes.

Description

Technical Field

[0001] This application relates to the field of semiconductor optoelectronic technology, specifically to a distributed feedback (DFB) semiconductor laser, and more particularly to a narrow linewidth DFB laser with different longitudinal grating period structures on both sides and its manufacturing method. Background Technology

[0002] Narrow-linewidth semiconductor lasers hold an irreplaceable core position in cutting-edge fields such as coherent optical communication, high-precision fiber optic sensing, microwave photonics, and quantum information processing. Linewidth is a key indicator for measuring the spectral purity and phase noise of a laser—the narrower the linewidth, the better the monochromaticity of the laser, the lower the phase noise, the more complex the high-order modulation formats it can support, and the larger the communication bandwidth.

[0003] Current mainstream narrow-linewidth DFB lasers mainly rely on the following technical approaches: (1) Long-cavity DFB lasers: The Scholomance-Towns linewidth limit is reduced by increasing the cavity length. However, a longer cavity results in a greater number of longitudinal modes, leading to difficulties in mode selection and decreased single-mode stability. In addition, long-cavity lasers require a larger chip area, which is not conducive to high-density integration and cost reduction.

[0004] (2) Apodized coupled grating DFB laser: By reducing the coupling coefficient near the phase shift region and gradually increasing it towards both ends, the uniformity of photon density distribution is improved, thereby suppressing the spatial hole burning effect and reducing linewidth broadening. However, this technology is essentially still modulating the coupling strength of a single grating, and the degree of freedom in controlling the intracavity optical mode is limited.

[0005] (3) External cavity semiconductor laser: An external cavity is formed by introducing external optical feedback elements (such as fiber gratings, volume gratings, FP etalons, etc.), and the frequency selection characteristics of the external cavity are used to achieve narrow linewidth output. However, this solution has disadvantages such as large size, poor vibration resistance, and high cost, and it is difficult to achieve full monolithic integration.

[0006] (4) Multi-period modulation structure DFB laser: By inserting multiple periodic modulation gratings at different positions of a uniform grating, the carrier density and field strength distribution along the cavity length are made more uniform. However, the periodic difference of this scheme exists at different longitudinal positions of the same grating layer (segmented), and its control is still limited to the inside of a single-sided grating.

[0007] A search revealed that the existing technical literature "Distributed feedback lasers with two-layer phase-shifted moiré gratings" (IEEE Journal of Quantum Electronics, 2019) discloses a DFB laser with a double-layer grating, the technical solution of which is as follows: Epitaxial structures include standard DFB epitaxial structures such as InP substrate, N-InP buffer layer, active region (multiple quantum well), and P-InP waveguide layer; A first grating layer (N-side grating) is disposed between the N-side waveguide layer below the active region and the substrate. A second grating layer (P-side grating) is set between the P-side waveguide layer and the cladding above the active region. Both sides employ a phase-shifting grating structure (λ / 4 phase shift); The two-layer grating is formed by double-exposure laser interference lithography using moiré grating technology.

[0008] The shortcomings of this existing technology are: (1) Although double-sided gratings are set, their period design is based on the double exposure formation principle of moiré gratings. The periods of the double-sided gratings are similar or have a moiré fringe relationship. The independent design dimension of "different periods on both sides" is not revealed or utilized. (2) Its technical effect focuses on improving single-mode stability and process compatibility (avoiding electron beam lithography), and does not take the significant narrowing of laser linewidth as its core purpose; (3) Its phase shift structure is a double-sided aligned design (both located at the center of the cavity), and no technical solution for double-sided phase shift misalignment is proposed; (4) Its working principle is moiré fringe phase modulation, but it does not reveal the physical mechanism of "wavelength interference effect" (constructive / destructive interference).

[0009] Currently, the linewidths of commercially available DFB lasers are generally in the MHz range, while the demand for narrow linewidths in sub-megahertz and even kilohertz ranges is becoming increasingly urgent for next-generation coherent optical communication and quantum technologies. How to further narrow the linewidth of DFB lasers while maintaining the advantages of short-cavity chips is a core technological challenge currently facing this field. Summary of the Invention

[0010] The main objective of this application is to provide a DFB laser capable of achieving narrow linewidth output at sub-megahertz or even kilohertz levels on a monolithic integrated platform, and a method for manufacturing the same.

[0011] This application provides a narrow-linewidth DFB laser with different longitudinal grating period structures on both sides, including: The epitaxial structure, from bottom to top, comprises: a substrate, an N-type buffer layer, an N-side grating layer, an N-type lower waveguide layer, an active region, a P-type upper waveguide layer, a P-side grating layer, a P-type cladding layer, and an ohmic contact layer. The N-side grating layer comprises a first material layer and a second material layer arranged periodically alternately along the cavity length direction. The first material layer is an InP layer, and the second material layer is an InGaAsP layer. Together, they form a first Bragg grating with alternating high and low refractive indices and a first grating period Λ. N ; The P-side grating layer includes a third material layer and a fourth material layer arranged periodically and alternately along the cavity length direction. The third material layer is an InP layer, and the fourth material layer is an InGaAsP layer. The two layers form a second Bragg grating with alternating high and low refractive indices and have a second grating period Λ. P ; The first grating period Λ N With the second grating period Λ P Satisfy: Λ N ≠Λ P .

[0012] Furthermore, the relative difference in grating periods between the N-side grating layer and the P-side grating layer is... Controlled within the range of 0.05% to 2%, of which .

[0013] Furthermore, the relative difference in the grating period is controlled within the range of 0.1% to 1%.

[0014] Furthermore, the N-side grating layer includes an N-side phase-shifting region, and the P-side grating layer includes a P-side phase-shifting region; the N-side phase-shifting region and the P-side phase-shifting region are spatially misaligned along the cavity length direction, forming a predetermined spatial offset.

[0015] Furthermore, the spatial offset between the N-side phase shift region and the P-side phase shift region is 5% to 20% of the cavity length of the DFB laser.

[0016] Furthermore, the N-side grating layer and / or the P-side grating layer are λ / 4 phase-shifting gratings.

[0017] Furthermore, the N-side grating layer and / or the P-side grating layer are CPM gratings with a periodically modulated gradient along the cavity length direction.

[0018] Furthermore, the N-side grating layer and / or the P-side grating layer are formed using a reconstruction-equivalent chirp technique, and the first grating period Λ is defined on the uniform seed grating by sampling gratings with different sampling periods. N and the second grating period Λ P .

[0019] Another object of this application is to provide a method for manufacturing a narrow-linewidth DFB laser as described above, comprising the following steps: (a) An N-type buffer layer is grown on a substrate, and then an N-side grating layer is formed on the N-type buffer layer, which is composed of InP layers and InGaAsP layers arranged periodically alternately along the cavity length direction. The N-side grating layer has a first grating period Λ. N ; (b) An N-type lower waveguide layer, an active region, and a P-type upper waveguide layer are sequentially grown on the N-side lower grating layer; (c) A P-side grating layer is formed on the P-type upper waveguide layer, consisting of InP layers and InGaAsP layers arranged periodically alternately along the cavity length direction, wherein the P-side grating layer has a second grating period Λ. P , of which Λ P ≠Λ N ; (d) Based on the fabrication of the P-side grating layer, a P-type cladding layer and an ohmic contact layer are grown sequentially to form an epitaxial wafer; (e) Perform ridge waveguide process, ridge windowing process, metal process, N-plane process and chip process on the epitaxial wafer to obtain a single DFB laser chip.

[0020] Furthermore, in steps (a) and / or (b), the grating pattern is defined using holographic exposure or nanoimprint lithography. The chip process includes: cleaving a wafer into strips, depositing a high-reflectivity film on the backlight end face, depositing an anti-reflection film on the light-emitting end face, and then cleaving it into a single chip.

[0021] The advantages of this application regarding a narrow-linewidth DFB laser with different longitudinal grating period structures on both sides and its fabrication method are as follows: (1) Significantly narrow laser linewidth: The wavelength constructive / destructive interference effect formed by the P / N double-sided gratings with different periods achieves dual suppression of side modes, greatly improving the equivalent quality factor of the cavity. With optimized design, it is expected to achieve narrow linewidth output at the level of hundreds of kilohertz or even tens of kilohertz, which is 1 to 2 orders of magnitude higher than the existing MHz-level DFB laser.

[0022] (2) Enhanced single-mode stability: The constructive / destructive mechanism in the wavelength dimension effectively increases the threshold gain difference between the main mode and the side mode, and the side mode suppression ratio can be significantly improved, ensuring that the laser maintains stable single-longitudinal mode operation over a wide temperature and current range.

[0023] (3) Maintain the advantages of chip-level monolithic integration: All structures in this application are monolithically integrated on the chip, without relying on external optical feedback components. They have the advantages of small size, vibration resistance, and high reliability, and are suitable for mass production and optoelectronic integration.

[0024] (4) High degree of design freedom: By flexibly selecting the period values ​​and period difference of the N-side and P-side gratings, the linewidth and spectral characteristics of the laser can be customized according to different application requirements.

[0025] (5) Strong process feasibility: The gratings with different periods on both sides can be defined by two photolithography processes to define the grating periods on the N side and P side respectively, which is compatible with existing mature semiconductor laser manufacturing processes.

[0026] (6) Suppressing spatial hole burning effect: By using the double-sided phase shift misalignment design, the optical field distribution in the cavity is more uniform, further suppressing the spatial hole burning phenomenon and improving the performance stability of the laser under high power injection. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the chip cross-sectional structure in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the structure of the double-sided CPM grating in Embodiment 3 of the present invention; The meanings of the labels in the attached diagram are as follows: 1-Ohmic contact layer; 2-P-type cladding layer; 3-P-side grating layer; 4-P-type upper waveguide layer; 5-Active region; 6-N-type lower waveguide layer; 7-N-side grating layer; 8-N-type buffer layer; 9-Substrate. Detailed Implementation

[0028] This application aims to solve the following four key technical problems: (1) Existing DFB lasers have limited means of narrowing linewidth, making it difficult to achieve sub-megahertz level narrow linewidth output while maintaining the advantages of short cavity chips; (2) Existing single-sided grating DFB lasers have limited ability to suppress side modes, resulting in insufficient side mode suppression ratio and linewidth broadening; (3) Existing double-sided grating schemes (such as moiré grating DFB) do not fully utilize the control potential of double-sided gratings in the wavelength dimension, and lack the technical approach of using different periods on both sides to generate wavelength interference effects; (4) Existing DFB lasers have obvious spatial hole burning effect under high power injection, causing uneven refractive index distribution and linewidth broadening, which limits the high power performance and single-mode stability of the laser.

[0029] To address the aforementioned technical problems, this application employs the following technical means: (1) Double-sided longitudinal grating structure.

[0030] Grating structure layers are respectively set between the N-type buffer layer and the N-type lower waveguide layer (referred to as the N-side) and between the P-type upper waveguide layer and the P-type cladding layer (referred to as the P-side). The N-side grating layer includes a first material layer and a second material layer arranged periodically and alternately along the cavity length direction. The first material layer is an InP layer and the second material layer is an InGaAsP layer. The two form a first Bragg grating with alternating high and low refractive indices. The P-side grating layer includes a third material layer and a fourth material layer arranged periodically and alternately along the cavity length direction. The third material layer is an InP layer and the fourth material layer is an InGaAsP layer. The two form a second Bragg grating with alternating high and low refractive indices.

[0031] (2) The gratings on both sides have different periods.

[0032] The first grating period Λ of the N-side grating layer N The second grating period Λ of the P-side grating layer P Different values, i.e., Λ N ≠Λ P Preferably, the relative difference in their periods is... It is controlled within the range of 0.05% to 2%, more preferably 0.1% to 1%.

[0033] (3) Double-sided phase shift misalignment design.

[0034] The N-side grating layer contains an N-side phase shift region, and the P-side grating layer contains a P-side phase shift region. The N-side phase shift region and the P-side phase shift region are spatially misaligned along the cavity length, and the spatial offset is preferably 5% to 20% of the cavity length.

[0035] First, in this application, the N-side grating layer and the P-side grating layer each have different grating periods Λ. N and Λ P According to the Bragg condition λ=2n_effΛ, the Bragg wavelengths corresponding to gratings with different periods are slightly different. Therefore, the N-side grating layer and the P-side grating layer provide the strongest distributed feedback for light waves with slightly different center wavelengths.

[0036] When the light wave generated in the active region propagates in the resonant cavity, it will be simultaneously subjected to feedback from the N-side grating layer and the P-side grating layer: Wavelength constructive interference effect: For a specific wavelength (dominant mode wavelength), the feedback phase provided by the N-side grating layer and the P-side grating layer just meets the constructive interference condition. The feedback light waves on both sides enhance each other, so that the wavelength obtains the strongest comprehensive feedback and the lowest threshold, thereby stabilizing lasing.

[0037] Wavelength cancellation effect: For other wavelengths (side modes) that deviate from the main mode wavelength, there is a phase mismatch in the feedback provided by the grating structures on both sides. The feedback light waves undergo destructive interference, the overall feedback intensity is significantly weakened, the threshold is increased, and thus it is effectively suppressed.

[0038] This "wavelength-dimensional constructive / destructive interference" mechanism is equivalent to establishing a high-Q composite spectral filter within the cavity. Through the wavelength interference effect formed by gratings with different periods on both sides, dual suppression of the side modes is achieved (simultaneously bearing the detuning loss of one grating and the destructive interference loss of the other), significantly improving the cavity's equivalent quality factor and thus significantly narrowing the laser linewidth (expected to achieve output at the hundreds of kilohertz level). Simultaneously, it increases the threshold gain difference between the main mode and the side modes, enhancing single-mode stability.

[0039] Secondly, this application designs the relative difference in grating periods between the N-side and P-side gratings to be within the range of 0.05% to 2%, forming a critical window for wavelength interference effects. The direct consequence of this difference in grating periods is a slight separation in the Bragg wavelengths of the two gratings. Let the Bragg wavelength on the N-side be λ. N = 2n_eff·Λ N The P-side Bragg wavelength is λ P = 2n_eff·Λ P .

[0040] When the difference is too small (<0.05%): λ N With λ P The two sides almost overlap, with the feedback spectra highly overlapping. Although there is still feedback from both sides, it is essentially equivalent to a "widened single-peak filter," which cannot form a clear constructive / destructive interference beat, resulting in limited improvement in wavelength selectivity and insignificant linewidth narrowing effect.

[0041] When the difference is in the range of 0.05% to 2%: λ N With λ P There is a moderate separation between the two sides, and the feedback spectra on both sides partially overlap. In the overlapping region, the feedback on both sides interferes—constructive interference at the center wavelength (enhancing the overall feedback) and destructive interference off-center (weakening the overall feedback). This composite filter spectrum with a "convex center and concave sides" is equivalent to a high-Q bandpass filter, which can significantly suppress side modes and narrow the linewidth.

[0042] When the difference is too large (>2%): λ N With λ P Complete separation, with no overlap in the feedback spectra on both sides. At this point, the laser tends to be at λ. N or λ P When lasing occurs separately at different locations, or when dual-mode lasing occurs, the feedback on both sides no longer coherently superimposes, the wavelength interference effect fails, and the linewidth may actually broaden.

[0043] Based on the design of different periods of the double-sided gratings and a relative period difference of 0.05% to 2%, this application achieves the following technical effects: (1) Improved side mode suppression ratio: The side mode simultaneously bears the detuning loss of one side grating and the destructive interference loss of the other side grating, thus achieving dual suppression; (2) Linewidth narrowing is achieved: the equivalent cavity Q value is increased and the Sholo-Towns linewidth limit is reduced, enabling narrowing from MHz to hundreds of kHz. (3) Enhanced single-mode stability: The threshold gain difference between the main mode and the side mode increases, maintaining a single longitudinal mode over a wide temperature / current range; (4) Improved design flexibility: By adjusting the period difference value, the filter line width can be "customized" to match different application requirements; (5) Avoid dual-mode lasing: A window of 0.05% to 2% ensures that the feedback spectra on both sides partially overlap rather than completely separate, thus ensuring single-mode operation.

[0044] Finally, this application incorporates a phase-shift misalignment design for the dual-sided grating structure, the core function of which is to suppress the spatial hole-burning effect. In conventional phase-shifted DFB lasers, the λ / 4 phase shift results in a centrosymmetric distribution of the optical field within the cavity, with the maximum light intensity at the center. During high-power injection, a large number of carriers are consumed in the central region, while the carrier concentration at both ends is relatively high, resulting in a spatially non-uniform distribution—the spatial hole-burning effect.

[0045] Consequences of the spatial hole burning effect: It causes non-uniform distribution of refractive index along the cavity length → equivalent chirp → spectral broadening → linewidth broadening; it may induce side-mode lasing, reduce single-mode stability; and limit the high-power output capability of the laser.

[0046] When the N-side phase shift region and the P-side phase shift region are spatially misaligned: (1) Peak splitting of optical field: The N-side grating and the P-side grating define their respective standing wave distributions. Due to the different phase shift positions, the positions of their respective standing wave nodes / antinodes are shifted. The final synthesized optical field in the cavity exhibits two (or more) peaks, rather than a single peak concentrated at the center.

[0047] (2) Homogenization of carrier consumption distribution: Multiple light field peaks mean enhanced stimulated emission in multiple regions, avoiding excessive carrier consumption in a single central region. The carrier consumption at different locations is more balanced, and the spatial hole burning effect is suppressed.

[0048] (3) More uniform refractive index distribution: homogenization of carrier distribution → more uniform refractive index change caused by plasma effect → reduced equivalent chirp → suppression of linewidth broadening.

[0049] When the spatial offset is less than 5%, the two phase shift regions are too close, and the optical field peak is still concentrated near the center, resulting in limited SHB suppression. When the spatial offset is greater than 20%, they may be too close to the end face, introducing end face loss, increasing the threshold, and potentially disrupting single-mode stability. Only when the spatial offset is controlled within the range of 5% to 20% can the two peaks be moderately separated, covering the main area of ​​the cavity length, resulting in the best SHB suppression effect.

[0050] Based on the spatial misalignment of the N-side and P-side phase shift regions and the control of the spatial offset within 5% to 20% of the cavity length, this application achieves the following technical effects: (1) Suppress spatial hole burning: the peak of the optical field is split and the carrier consumption is homogenized; (2) Reduced linewidth broadening: The refractive index distribution is more uniform, and the equivalent chirp is reduced; (3) Improved high-power performance: The uniform carrier distribution is maintained even under high injection, and the output characteristics are stable; (4) Lowering the threshold (indirectly): The light field distribution and gain distribution are more matched, and the threshold gain is reduced.

[0051] The aforementioned two techniques—the double-sided grating structure with different periods and the double-sided phase shift misalignment—act on different linewidth broadening mechanisms, forming complementary enhancements: the wavelength interference effect achieved by the double-sided grating with different periods reduces the linewidth limit related to the equivalent cavity Q-value by mitigating quantum noise; the double-sided phase shift misalignment design mitigates the spatial hole-burning effect, suppressing linewidth broadening caused by nonlinearity; the combined effect of these two techniques significantly narrows the linewidth of the laser beam. Specifically, (1) Complementary light field modulation.

[0052] The "wavelength interference" effect generated by different periods on both sides compresses the spectral width in the frequency domain; the phase shift misalignment on both sides optimizes the optical field distribution in the cavity and homogenizes carrier consumption in the spatial domain; the simultaneous optimization in the frequency domain and spatial domain achieves a better linewidth narrowing effect.

[0053] (2) Stability under high power injection.

[0054] When using different periods on both sides alone, if the spatial via burning is severe, linewidth broadening may still occur under high injection levels. However, by introducing phase shift dislocation, spatial via burning is suppressed, and the wavelength interference effect can still be effectively maintained even at higher injection levels. That is, phase shift dislocation creates a stable gain / carrier environment for the wavelength interference effect.

[0055] (3) Synergistic enhancement of single-mode stability.

[0056] The wavelength interference mechanism increases the gain difference between the main and side modes from the perspective of "spectral mode selection"; the phase shift misalignment avoids the oscillation of higher-order modes due to spatial effects from the perspective of "suppressing spatial hole burning"; the superposition of the two makes single-mode operation more stable, especially in high power and wide temperature range.

[0057] Example 1 This embodiment provides a narrow-linewidth DFB laser with different longitudinal grating period structures on both sides.

[0058] I. Extensional Structure like Figure 1 As shown, the DFB laser of this embodiment includes several structural layers stacked sequentially from bottom to top, as shown in Table 1.

[0059] Table 1

[0060] II. Grating Structure Parameters N-side grating layer: Grating period: Λ N =200~280nm (the specific value is determined according to the target lasing wavelength); Duty cycle: 0.3-0.7; Phase shift: A λ / 4 phase shift is set at the center of the resonant cavity.

[0061] P-side grating layer: Grating period: Λ P =200~280nm (the specific value is determined according to the target lasing wavelength); Duty cycle: 0.3-0.7; Phase shift: A λ / 4 phase shift is set at the center of the resonant cavity.

[0062] Bilateral periodic differences: Λ N With Λ P Satisfy Λ N ≠Λ P And the relative differences in their cycles The variation should be controlled within the range of 0.05% to 2%. In this embodiment, the preferred relative variation in the cycle is 0.1% to 1%.

[0063] For example, if the target lasing wavelength λ≈1550nm and the effective refractive index n_eff≈3.2, then Λ N ≈242nm, Λ P It can be set to approximately 243nm (with a difference of approximately 0.4%), or other difference values ​​can be selected according to specific design requirements.

[0064] III. Manufacturing Method The manufacturing method of the DFB laser in this embodiment includes the following steps: Step 1: Fabrication of the N-area grating.

[0065] An N-type buffer layer 8 and an N-side grating layer 7 are sequentially grown on a substrate 9. Then, an N-side grating structure, consisting of alternating InP and InGaAsP layers arranged periodically along the cavity length, is fabricated on the N-side grating layer 7 using photolithography and etching processes. Specifically, the grating pattern is defined using holographic exposure or nanoimprint lithography, and the periodic grating structure is formed by dry or wet etching, followed by removal of the photoresist. This N-side grating layer 7 has a first grating period Λ. N A λ / 4 phase shift is set at the center of the resonant cavity.

[0066] Step 2: Growth of the active region and subsequent layers.

[0067] On a substrate with N-sided grating layers 7, an N-type lower waveguide layer 6, an active region 5, and a P-type upper waveguide layer 4 are sequentially grown by MOCVD. The active region 5 adopts a strain-compensated InGaAsP or InGaAsAl multi-quantum-well structure.

[0068] Step 3: P-area grating fabrication.

[0069] A P-side grating layer is grown on the P-type upper waveguide layer 4. Then, a P-side grating structure is fabricated on the P-side grating layer using photolithography and etching processes, giving it a second grating period Λ. P (Λ) P ≠Λ N ), and a λ / 4 phase shift is set at the center of the resonant cavity. In this step, the grating period Λ P It can be defined through independent photomask design.

[0070] Step 4: Epitaxial wafer completed.

[0071] On a substrate with a P-side grating layer 3, the P-side grating layer 3, the P-type cladding layer 2, and the ohmic contact layer 1 are grown sequentially by MOCVD to obtain a complete epitaxial wafer.

[0072] Step 5: Ridge waveguide process.

[0073] The ridge waveguide structure is fabricated through steps such as mask deposition, photolithography, etching, resist removal, dry / wet etching, and mask removal.

[0074] Step 6: Ridge window opening process.

[0075] The passivation layer at the top of the waveguide is removed by passivation layer deposition, photolithography, dry / wet etching, and resist removal to facilitate the subsequent fabrication of metal electrodes.

[0076] Step 7: Metalworking.

[0077] Metal electrodes and pads that form ohmic contacts are fabricated through steps such as photolithography, vapor deposition, stripping, and alloying.

[0078] Step 8: N-face process.

[0079] The wafer is thinned by grinding, N-side metal is deposited by vapor deposition, and alloying is performed to complete the N-side process.

[0080] Step 9: Chip manufacturing process.

[0081] The wafer is cleaved into bars, a high reflectivity (HR) film is deposited on the backlight end face, an antireflection (AR) film is deposited on the light-emitting end face, and then cleaved into individual chips.

[0082] By following the steps above, a narrow-linewidth DFB laser chip with different longitudinal grating periodic structures on both sides can be obtained.

[0083] IV. Working Principle and Effects In this embodiment, due to the grating period Λ of the N-side grating layer 7 N The grating period Λ of the P-side grating layer 3 P The two are different, with slight differences in their corresponding Bragg wavelengths. When the light wave generated in active region 5 propagates in the resonant cavity, it is simultaneously subjected to distributed feedback from the grating structures on both sides.

[0084] For the dominant mode wavelength, the feedback light waves provided by the grating structures on both sides satisfy the constructive interference condition, resulting in the strongest overall feedback and the lowest lasing threshold. For the side mode wavelength, there is a phase mismatch in the feedback from both sides, leading to destructive interference, which weakens the overall feedback and increases the threshold. This wavelength interference effect is equivalent to forming a high-Q filter within the cavity, providing dual suppression of the side modes and thus significantly narrowing the linewidth.

[0085] Experiments show that the DFB laser in this embodiment can achieve narrow linewidth output at the level of hundreds of kilohertz under optimized design, which is 1 to 2 orders of magnitude higher than that of traditional single-sided grating DFB lasers (the linewidth is usually at the MHz level), while maintaining the advantages of monolithic integration and miniaturization.

[0086] Example 2: Two-sided phase shift misalignment scheme Based on Embodiment 1, this embodiment designs a staggered spatial position of the phase shift region between the N-side grating layer 7 and the P-side grating layer 3.

[0087] Specifically: The center of the λ / 4 phase shift region of the N-side grating layer 7 is located at the center of the resonant cavity (or at a certain set position off the center). The center of the λ / 4 phase shift region of the P-side grating layer 3 and the center of the N-side phase shift region are offset by a certain spatial offset along the cavity length direction; The spatial offset is preferably 5% to 20% of the cavity length.

[0088] For example, if the laser resonant cavity length is L=300μm, the center of the N-side phase shift region can be set at the center of the cavity (150μm), and the center of the P-side phase shift region can be set at a distance of 20μm from the center (i.e., 170μm or 130μm), with an offset of approximately 6.7% of the cavity length.

[0089] Working principle and effects: The double-sided phase-shift misalignment design makes the optical field distribution within the cavity more uniform, avoiding excessive concentration of optical energy at the cavity center caused by double-sided aligned phase shifts. This design effectively suppresses the spatial hole-burning effect, resulting in a more uniform carrier distribution and further reducing linewidth broadening caused by spatial hole-burning. Simultaneously, the uniform optical field distribution also helps improve the laser's output power and high-temperature stability.

[0090] Example 3: Dual-sided CPM grating In an alternative embodiment, the N-side grating layer 7 and / or the P-side grating layer 3 can employ a CPM grating (Corrugation-Pitch-Modulated Grating) with a periodically modulated shape along the cavity length. This means that by changing the period or duty cycle of the grating region, an equivalent λ / 4 phase shift or other phase shift distribution is formed, thereby replacing the traditional λ / 4 phase shift structure. Figure 2 As shown.

[0091] The advantages of using a dual-sided CPM composite grating are: the period gradient design can further optimize the intracavity light field distribution and reduce the non-uniformity of intracavity photon density, thereby further narrowing the linewidth and suppressing spatial hole burning based on the interference effect of different periods on both sides.

[0092] Example 4: Using REC technology In another alternative implementation, a reconstruction-equivalent-chirp (REC) technique can be used to fabricate the N-side grating layer 7 and the P-side grating layer 3.

[0093] The specific method is as follows: First, a uniform seed grating is fabricated on the wafer using holographic exposure or nanoimprint lithography; Then, sampling grating masks with different sampling periods are designed; By using a double exposure (exposure on the sampling grating mask), the required equivalent grating periods Λ for the N-side and P-side are defined on the seed grating respectively. N and Λ P .

[0094] The advantage of this alternative method is that it can use a uniform seed grating with different sampling periods to achieve a grating structure with different periods on both sides, avoiding the high cost and complexity of electron beam exposure for two different fine period gratings, significantly reducing the difficulty of the process and the manufacturing cost, and improving production efficiency.

[0095] Explanation of the relationship between Example 2 and Examples 3 and 4 The above embodiments two, three, and four can be implemented in combination. For example, based on the double-sided gratings with different periods fabricated using REC technology, a double-sided phase shift misalignment design and / or a CPM period gradient design can be further introduced to achieve better linewidth narrowing and optical field modulation performance. Those skilled in the art can flexibly select and combine the above technical features according to specific application requirements.

[0096] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Those skilled in the art can make various improvements and modifications without departing from the spirit and principles of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A narrow-linewidth DFB laser with a double-sided longitudinal grating period structure, characterized in that, It includes an epitaxial structure, which, from bottom to top, comprises a substrate, an N-type buffer layer, an N-side grating layer, an N-type lower waveguide layer, an active region, a P-type upper waveguide layer, a P-side grating layer, a P-type cladding layer, and an ohmic contact layer. The N-side grating layer comprises a first material layer and a second material layer arranged periodically alternately along the cavity length direction. The first material layer is an InP layer, and the second material layer is an InGaAsP layer. Together, they form a first Bragg grating with alternating high and low refractive indices and a first grating period Λ. N ; The P-side grating layer includes a third material layer and a fourth material layer arranged periodically and alternately along the cavity length direction. The third material layer is an InP layer, and the fourth material layer is an InGaAsP layer. The two layers form a second Bragg grating with alternating high and low refractive indices and have a second grating period Λ. P ; The first grating period Λ N With the second grating period Λ P Satisfy: Λ N ≠Λ P The relative difference in grating periods between the N-side grating layer and the P-side grating layer Controlled within the range of 0.05% to 2%, of which .

2. The narrow linewidth DFB laser according to claim 1, characterized in that, The relative difference in the grating period is controlled within the range of 0.1% to 1%.

3. The narrow linewidth DFB laser according to claim 1, characterized in that, The N-side grating layer includes an N-side phase shift region, and the P-side grating layer includes a P-side phase shift region; the N-side phase shift region and the P-side phase shift region are spatially misaligned along the cavity length direction, forming a set spatial offset.

4. The narrow linewidth DFB laser according to claim 3, characterized in that, The spatial offset between the N-side phase shift region and the P-side phase shift region is 5% to 20% of the cavity length of the DFB laser.

5. The narrow linewidth DFB laser according to claim 1, characterized in that, The N-side grating layer and / or the P-side grating layer are λ / 4 phase-shift gratings.

6. The narrow linewidth DFB laser according to claim 1, characterized in that, The N-side grating layer and / or the P-side grating layer are CPM gratings with a periodically modulated gradient along the cavity length direction.

7. The narrow linewidth DFB laser according to claim 1, characterized in that, The N-side grating layer and / or the P-side grating layer are formed using a reconstruction-equivalent chirp technique, and the first grating period Λ is defined on the uniform seed grating by sampling gratings with different sampling periods. N and the second grating period Λ P .

8. A method for manufacturing a narrow-linewidth DFB laser as described in any one of claims 1 to 7, characterized in that, Includes the following steps: (a) An N-type buffer layer is grown on a substrate, and then an N-side grating layer is formed on the N-type buffer layer, which is composed of InP layers and InGaAsP layers arranged periodically alternately along the cavity length direction. The N-side grating layer has a first grating period Λ. N ; (b) An N-type lower waveguide layer, an active region, and a P-type upper waveguide layer are sequentially grown on the N-side lower grating layer; (c) A P-side grating layer is formed on the P-type upper waveguide layer, consisting of InP layers and InGaAsP layers arranged periodically alternately along the cavity length direction, wherein the P-side grating layer has a second grating period Λ. P , of which Λ P ≠Λ N ; (d) Based on the fabrication of the P-side grating layer, a P-type cladding layer and an ohmic contact layer are grown sequentially to form an epitaxial wafer; (e) Perform ridge waveguide process, ridge windowing process, metal process, N-plane process and chip process on the epitaxial wafer to obtain a single DFB laser chip.

9. The manufacturing method according to claim 8, characterized in that, In step (a) and / or step (b), the grating pattern is defined by holographic exposure or nanoimprint lithography. The chip process includes: cleaving a wafer into strips, depositing a high-reflectivity film on the backlight end face, depositing an anti-reflection film on the light-emitting end face, and then cleaving it into a single chip.

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