DFB semiconductor laser

Abstract

The utility model discloses a DFB semiconductor laser, which comprises a laser resonant cavity, a grating is arranged in the laser resonant cavity, the grating comprises a plurality of standard periods along the cavity length direction, and each standard period comprises a first refractive index area and a second refractive index area; k empty cycles are inserted every n standard cycles, the length of the empty cycles is the same as that of the standard cycles, and the empty cycles are first refractive index regions or second refractive index regions; wherein the direction from the center of the resonant cavity to the end face is changed for multiple times in n or k so as to form the grating with the non-uniform coupling coefficient. By changing the pattern of the grating, the light intensity and carrier distribution in the whole cavity are more uniform, so that the space hole burning effect is slowed down, and the problems of Kink, mode hopping, multimode and the like caused by the space hole burning effect are avoided.

Description

Technical Field

[0001] This utility model belongs to the technical field of lasers, specifically relating to a DFB semiconductor laser. Background Technology

[0002] During the testing of long-cavity DFB (Distributed Feedback Laser) semiconductor lasers, "kinks" are often observed in the output power-current (LI) curve, accompanied by abnormal phenomena such as spectral mode hopping or multimode oscillations. These phenomena affect the stability and performance of the laser, limiting its use in high-power and long-distance applications. The main reason for this phenomenon is the non-uniformity of gain distribution caused by the spatial distribution of light intensity and charge carriers in the long-cavity laser, leading to spatial hole burning and the generation of kinks, mode hopping, and multimode oscillations. Existing technologies mainly address this by adjusting the epitaxial structure and strengthening testing and screening. Optimizing the epitaxial structure to mitigate the spatial hole burning effect usually has a ripple effect, leading to a deterioration in the performance of other parameters. Furthermore, each epitaxial wafer can only undergo one DOE (Design of Experiments), resulting in long iteration cycles and high costs. Adjusting the duty cycle requires adjusting the exposure dose, which is complex in the fabrication process and has poor precision controllability. Strengthening testing and screening can also cause significant yield losses, thereby increasing chip costs. Utility Model Content

[0003] The purpose of this invention is to provide a DFB semiconductor laser to mitigate the space hole burning effect.

[0004] To achieve the above objectives, the technical solution of this utility model is as follows:

[0005] A DFB semiconductor laser includes a laser resonant cavity with a grating inside. The grating includes multiple standard periods along the cavity length, each standard period comprising a first refractive index region and a second refractive index region. K empty periods are inserted every n standard periods, the length of each empty period being the same as the length of a standard period. Each empty period is either the first or second refractive index region, where n and k are natural numbers, n ≥ 3 and k ≥ 1. The direction from the center of the resonant cavity to the end face varies with n or k multiple times to form a grating with a non-uniform coupling coefficient. The refractive indices of the first and second refractive index regions are different.

[0006] When n < 3, the grating period will change, which will also affect the lasing wavelength and thus the laser's emission parameters, and the adjustability will be poor. Therefore, the number of standard period intervals n ≥ 3.

[0007] Optionally, the trend of change is the same for n or k changes in the direction from the center of the resonant cavity to the end face; the trend of change is either increasing or decreasing.

[0008] Optionally, k is constant in the direction from the center of the resonant cavity to the end face, and the number of intervals n of the standard period varies in an arithmetic or non-arithmic manner; wherein the value of n is always greater than 2k.

[0009] Optionally, n is constant in the direction from the center of the resonant cavity to the end face, and the number of insertions k of the empty period varies in an arithmetic or non-arithmic manner; wherein the value of k is always less than n / 2.

[0010] Among the values ​​mentioned above, when k > n / 2, the resonance of the grating may be insufficient, preventing the formation of a mode selection effect. Furthermore, the adjustment range is limited, making it impossible to gradually adjust according to changes in the light field intensity. Therefore, k should be within the range of 2 / n.

[0011] Optionally, in the standard period, the proportion of the first refractive index region is 40% to 60%.

[0012] Optionally, one end face of the DFB semiconductor laser is provided with an anti-reflection film, and the other end face is provided with a high-reflection film.

[0013] Optionally, the change of n or k from the center of the resonant cavity to one end of the antireflective film may be the same as or different from the change of n or k to one end of the high-reflective film.

[0014] Optionally, the epitaxial structure of the laser includes an active region, a grating layer, and a grating cap layer arranged in sequence. The grating layer has a plurality of grating strips arranged at intervals. The grating cap layer is disposed on the grating layer and fills the intervals between the grating strips. The grating strips form the first refractive index region, and the grating cap layer forms the second refractive index region.

[0015] Optionally, the empty period is formed by continuous grating strips or by intervals between continuous grating strips.

[0016] Optionally, the cavity length of the laser ranges from 150 to 3000 μm, and the length of the standard period ranges from 170 to 270 nm.

[0017] The beneficial effects of this utility model are as follows:

[0018] In the resonant cavity of a laser, by changing the grating pattern, a grating design with a weak coupling coefficient is used in areas with high light intensity distribution, while a grating design with a strong coupling coefficient is used in areas with low light intensity distribution. This makes the light intensity and carrier distribution more uniform throughout the cavity, thereby mitigating the spatial hole burning effect and avoiding problems such as kinking, mode hopping, and multimode. Grating design allows for multiple designs to be implemented on a single wafer without changing the specific epitaxial structure, greatly accelerating the R&D cycle and reducing R&D costs. The design can be achieved by changing the grating pattern without adjusting the exposure dose, resulting in high controllability and precision. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the initial light intensity distribution within the resonant cavity of the DFB semiconductor laser in the first to third embodiments;

[0020] Figure 2 This is a schematic diagram of the grating distribution of the DFB semiconductor laser in the first embodiment;

[0021] Figure 3 This is a schematic diagram of a portion of the epitaxial structure of the DFB semiconductor laser in the first embodiment;

[0022] Figure 4 This is a schematic diagram of a portion of the epitaxial structure of the DFB semiconductor laser in the second embodiment;

[0023] Figure 5 This is a schematic diagram of the grating distribution of the DFB semiconductor laser in the third embodiment. Detailed Implementation

[0024] 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 present 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.

[0025] refer to Figure 1 The DFB semiconductor laser in this embodiment includes a laser resonant cavity. One end face of the laser is coated with an anti-reflection film (AR) along the cavity length, and the other end face has a high-reflection film (HR). In a DFB laser, the light field distribution typically exhibits a weak distribution in the middle and a strong distribution at the HR and AR ends. This distribution trend is more pronounced in long-cavity DFB lasers (cavity length greater than 500 μm). The main reasons for this distribution include:

[0026] Gain Distribution: In DFB lasers, the distribution of the gain medium is usually uniform. However, due to the propagation and reflection of light within the cavity, the intensity of the light field in the center of the cavity is suppressed. This is because when light propagates in the cavity, it undergoes multiple reflections and diffractions, and the light intensity in the center of the cavity cancels out due to phase differences, resulting in a weaker light intensity in the central region.

[0027] Boundary conditions: The HR endface and AR endface have high reflectivity and anti-reflectivity characteristics, respectively. The HR endface can effectively reflect light back into the cavity, enhancing the light intensity inside the cavity, while the AR endface allows some light output, while also creating a certain light intensity inside the cavity. This boundary condition setting makes the light field inside the cavity stronger at the HR endface.

[0028] Mode competition: In DFB lasers, multiple modes may compete for light. Certain modes may have weaker intensity in the cavity center but stronger intensity at the HR and AR ends. This mode competition leads to the enhancement of specific modes, thus affecting the optical field distribution.

[0029] for Figure 1 The light intensity distribution within the resonant cavity is shown in the first embodiment of the DFB semiconductor laser, referenced. Figure 2 The laser resonator cavity contains a grating, which includes multiple standard periods A and multiple empty periods B arranged along the cavity length. Each standard period A includes a first refractive index region 11 and a second refractive index region 21, and each empty period B consists entirely of the second refractive index region 22. The first refractive index of the first refractive index region 11 is greater than the second refractive index of the second refractive index region 21. The period lengths of the standard periods A and empty periods B are the same, both denoted by d. The arrangement is as follows: along the cavity length from the center to the end face, k empty periods B are inserted every n standard periods A, where n and k are natural numbers, with k being constant and n gradually decreasing (in the example in the diagram, n3 < n2 < n1). That is, towards the end face, the light intensity distribution increases, the number of empty periods B also increases, the modulation capability of the light decreases, and the corresponding coupling coefficient decreases; while at the center, where the light intensity distribution is smaller, the standard periods dominate, the modulation capability of the light is stronger, and the corresponding coupling coefficient is larger. By gradually changing the arrangement of empty period B, a uniform carrier and light intensity distribution is obtained in the cavity, thereby reducing the spatial hole burning effect and mitigating Kink, multimode, and mode hopping.

[0030] The standard period mentioned here refers to the period of a uniform grating set to correspond to the target wavelength of the laser. In the standard period, the proportion of the first refractive index region is 40%–60%, preferably 50%. An empty period refers to a region with the same length as the standard period, but entirely composed of the first or second refractive index, thus lacking the periodic resonance modulation effect of the grating. In this embodiment, with constant k and decreasing n, k ≥ 1, and n1, n2, n3… are distributed in an arithmetic sequence. The values ​​of k, n, and the difference between n are specifically designed based on the intensity difference and variation law between the cavity length center and the end face, but the value of n is always greater than 2k. For example, from the cavity length center to the end face, the variation law is: 2 empty periods every 30 standard periods, 2 empty periods every 25 standard periods, 2 empty periods every 20 standard periods, 2 empty periods every 15 standard periods… and so on. The cavity length of the laser ranges from 150 to 3000 μm, and the length of the standard period ranges from 170 to 270 nm. Preferably, n < 100; if n is too large, the effect will be insignificant.

[0031] In other embodiments where k is constant and n decreases, k ≥ 1, and n1, n2, n3... are distributed in an arithmetic progression. The values ​​of k, n, and the difference between n are specifically designed based on the light intensity difference and variation pattern between the cavity length center and the end face. In this case, the range of the difference in n can be 0. Preferably, n decreases in a stepwise manner. For example, from the cavity length center to the end face, the variation pattern is as follows: 2 empty cycles every 25 standard cycles, 2 empty cycles every 25 standard cycles, 2 empty cycles every 20 standard cycles, 2 empty cycles every 20 standard cycles, 2 empty cycles every 15 standard cycles, 2 empty cycles every 15 standard cycles, and so on. In other embodiments, n decreases in a non-arithmetic progression, and the difference gradually decreases. For example, when there is a long light intensity trough at the center, the variation pattern from the cavity center to the end face is as follows: there are 2 empty cycles every 200 standard cycles, 2 empty cycles every 80 standard cycles, 2 empty cycles every 70 standard cycles, 2 empty cycles every 60 standard cycles, and so on.

[0032] The structure of the grating described above is achieved by spacing the grating strips. For example... Figure 3As shown, the epitaxial structure of the DFB semiconductor laser in this embodiment includes an active region 3, a grating layer 1, and a grating capping layer 2 in sequence. The grating layer 1 has a plurality of grating strips spaced apart, which constitute the first refractive index region 11. The grating capping layer 2 is disposed on the grating layer 1 and fills the gaps between the grating strips, forming the second refractive index regions 21 and 22. The material of the grating layer 1 is InGaAsP, and the material of the grating capping layer 2 is InP. During fabrication, the grating layer 1 is first formed by epitaxial deposition and other processes. The surface of the grating layer 1 is then etched to form grating strips arranged at intervals. The grating strips and the gaps form a standard period. Some periods of the grating layer 1 are completely etched away, creating continuous gaps that form empty periods. That is, empty periods can be considered as removing a portion of the grating strips from the uniformly arranged grating strips of the standard period. Thus, only the pattern of the photomask needs to be changed, without adjusting the exposure energy, etc.

[0033] In a second embodiment, as Figure 4 As shown, the empty period B can also be entirely the first refractive index region 12, which forms a continuous grating strip through a partial period of the grating layer 1 without etching, thus achieving a similar technical effect.

[0034] refer to Figure 3 In a third embodiment of the grating pattern, k empty periods B are inserted every n standard periods A along the cavity length from the center of the resonant cavity to the end face, where n and k are natural numbers, with n being constant and k gradually increasing (e.g., k1 < k2 < k3 in the figure). Similarly, towards the end face, the light intensity distribution increases, the number of empty periods B also increases, the modulation capability of light decreases, and the corresponding coupling coefficient decreases; while at the center where the light intensity distribution is smaller, standard periods dominate, the modulation capability of light is stronger, and the corresponding coupling coefficient is larger, achieving a similar technical effect. In this embodiment, where n is constant and k increases, 3 ≤ n ≤ 100, and k1, k2, k3... are distributed in an arithmetic or unequal sequence. The value of k can start from 0 and not exceed n / 2. The values ​​of k, n, and the difference between k are specifically designed based on the light intensity difference and variation law between the cavity length center and the end face. For example, from the center of the cavity length to the end face, the variation pattern is as follows: there is 1 empty cycle every 20 standard cycles, 2 empty cycles every 20 standard cycles, 3 empty cycles every 20 standard cycles, 4 empty cycles every 20 standard cycles, and so on.

[0035] In addition, in other embodiments, the light intensity variation from the center of the laser resonator to the AR end face may be the same as or different from the light intensity variation from the center to the HR end face. When they are different, different variation settings can be adopted to ultimately achieve overall light intensity uniformity.

[0036] Furthermore, in other embodiments, if the distribution of the gain medium is uneven, it may lead to increased light intensity in the center of the cavity. For example, if the concentration of the gain medium is higher in the center of the cavity, the light gain in that region will be stronger, resulting in increased light intensity in the central region. Interference from an external light source within the cavity can also cause changes in the light field distribution. For example, if the wavelength of the external light source is similar to the resonant wavelength within the cavity, it may lead to increased light intensity in the center of the cavity. In these cases, the light intensity distribution along the cavity length may exhibit a pattern of higher intensity in the middle and lower intensity at both ends, gradually decreasing from the middle to both ends. In such cases, a grating structure design opposite to the above embodiments can achieve a similar technical effect.

[0037] The above embodiments are only used to further illustrate a DFB semiconductor laser of the present invention, but 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.

Claims

1. A DFB semiconductor laser, characterized in that: The system includes a laser resonant cavity, within which a grating is provided. The grating comprises multiple standard periods along the cavity length, each standard period including a first refractive index region and a second refractive index region. K empty periods are inserted every n standard periods, the length of each empty period being the same as the length of the standard period. Each empty period is either the first refractive index region or the second refractive index region, where n and k are natural numbers, n≥3 and k≥1. Furthermore, n or k varies multiple times in the direction from the center of the resonant cavity to the end face to form a grating with a non-uniform coupling coefficient.

2. The DFB semiconductor laser according to claim 1, characterized in that: The trend of change is the same for n changes from the center of the resonant cavity to the end face, or the trend of change is the same for k changes; the trend of change is either increasing or decreasing.

3. The DFB semiconductor laser according to claim 2, characterized in that: The direction from the center of the resonant cavity to the end face is constant, and the number of intervals n of the standard period varies in an arithmetic or non-arithmic manner; where the value of n is always greater than 2k.

4. The DFB semiconductor laser according to claim 1, characterized in that: The direction from the center of the resonant cavity to the end face is constant, and the number of insertions k of the empty period varies in an arithmetic or non-arithmic manner; where the value of k is always less than n / 2.

5. The DFB semiconductor laser according to claim 1, characterized in that: In the standard period, the proportion of the first refractive index region is 40% to 60%.

6. The DFB semiconductor laser according to claim 1, characterized in that: The DFB semiconductor laser has an anti-reflection film on one end face and a high-reflection film on the other end face.

7. The DFB semiconductor laser according to claim 6, characterized in that: The change of n or k from the center of the resonant cavity to one end of the antireflective film may be the same as or different from the change of n or k to one end of the high reflective film.

8. The DFB semiconductor laser according to claim 1, characterized in that: The epitaxial structure of the laser includes an active region, a grating layer, and a grating cap layer arranged in sequence. The grating layer has a plurality of grating strips arranged at intervals. The grating cap layer is disposed on the grating layer and fills the intervals between the grating strips. The grating strips form the first refractive index region, and the grating cap layer forms the second refractive index region.

9. The DFB semiconductor laser according to claim 8, characterized in that: The empty period is formed by the formation of continuous grating strips or by the intervals between continuous grating strips.

10. The DFB semiconductor laser according to claim 1, characterized in that: The cavity length of the laser ranges from 150 to 3000 μm, and the length of the standard period ranges from 170 to 270 nm.

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