Method for measuring coupling factor of DFB laser chip

Abstract

The invention provides a method for measuring a coupling factor of a DFB (Distributed Feedback) laser chip, belongs to the technical field of semiconductor laser chips, and is used for solving the technical problem that a coupling factor obtained by modeling / simulation is greatly different from a real coupling factor. The method comprises the following steps: testing spectrums of a plurality of DFB laser chips in the same batch of chips, and counting a wavelength median; calculating the effective refractive index of the DFB laser chip according to the wavelength median; a coupling factor is calculated according to the effective refractive index, and the corresponding relation between the coupling factor and the wavelength difference of the dual-mode chip under the corresponding grating condition is obtained through a transmission matrix method; finding out the dual-mode chip, and testing the wavelength difference of the dual-mode chip; and substituting the wavelength difference into the corresponding relation to obtain a coupling factor. According to the method, the coupling factor of the DFB laser chip can be accurately obtained by combining actual test data with a transmission matrix method; moreover, test data is not influenced by current and temperature, and the method has very good universality and stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor laser chips, and particularly relates to the measurement of a coupling factor. Background Art

[0002] In 1962, laser was successfully lasered with semiconductor materials as the working substance. After multiple process and design upgrades, semiconductor lasers have gradually developed into laser light sources with small volume, light weight, high efficiency, and low price. The DFB laser chip introduces periodic refractive index changes in the semiconductor material to form a grating structure, thereby realizing the selective amplification of a specific wavelength. The DFB laser chip has good monochromaticity, narrow linewidth, high side mode suppression ratio, and high stability, and is widely used in fields such as optical fiber communication, data centers, lidar, and laser ranging.

[0003] The coupling factor of the DFB laser chip refers to the coupling strength between the grating and the waveguide mode. The magnitude of the coupling factor directly affects performance parameters such as the threshold current, power, front and rear output ratio, and single-mode yield of the DFB laser chip, and is crucial for the design / fabrication of the DFB laser chip.

[0004] However, the coupling factor of the DFB laser chip cannot be directly obtained through performance testing methods. However, to obtain the coupling factor by establishing a laser model, it is necessary to first obtain the waveguide structure of the laser, the distance from the grating to the active region, the grating layer thickness, the grating duty cycle, the grating composition, the grating type, the grating parameters, etc.; then, based on the above parameters, modeling / simulation is carried out; finally, the coupling factor is obtained. However, there are certain differences between the actual data of the above parameters and the ideal values in the model, resulting in a large difference between the coupling factor obtained by modeling / simulation and the actual coupling factor. Summary of the Invention

[0005] Aiming at the technical problem that there is a large difference between the coupling factor obtained by modeling / simulation and the actual coupling factor, the present invention proposes a method for measuring the coupling factor of a DFB laser chip. This method can accurately obtain the coupling factor of the DFB laser chip by combining actual test data with the transfer matrix method; moreover, the test data is not affected by current and temperature, and this method has very good universality and stability.

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

[0007] A method for measuring the coupling factor of a DFB laser chip includes the following steps:

[0008] (1) Confirm the grating type and parameters of the DFB laser chip, test the spectra of several DFB laser chips in the same batch, and count the median wavelength of the DFB laser chips;

[0009] (2) Calculate the effective refractive index of the DFB laser chip according to the median wavelength.

[0010] (3) Calculate the coupling factor according to the effective refractive index, and use the transfer matrix method to obtain the corresponding relationship between the coupling factor and the wavelength difference of the dual-mode chip under the corresponding grating conditions.

[0011] (4) Test this batch of DFB laser chips to find the dual-mode chips, and test the wavelength difference of the dual-mode chips.

[0012] (5) Substitute the wavelength difference of the dual-mode chip obtained in step (4) into the corresponding relationship obtained in step (3) to obtain the corresponding coupling factor, and this coupling factor is the true coupling factor of this batch of chips.

[0013] The grating type of the DFB laser chip is uniform grating, partial grating, phase-shifted grating or periodic modulation grating.

[0014] The grating parameters of the DFB laser chip are the cavity length of the DFB laser chip, the grating period or the duty cycle of the grating.

[0015] The waveguide of the DFB laser chip is ridge waveguide or masked heterojunction.

[0016] In step (1), the number of tested chips in the same batch is not less than 100.

[0017] In step (2), the effective refractive index n eff = λ / (2*W); where λ is the median wavelength and W is the grating period.

[0018] The coupling factor where L is the cavity length of the DFB laser chip, n eff is the effective refractive index, △n is the change in the effective refractive index caused by the grating, and λ is the median wavelength.

[0019] The transfer matrix method is completed through an operation software to establish a model, an optical software or a laser software.

[0020] In step (4), the method for finding the dual-mode chips is: test the spectrum of the DFB laser and judge according to the side mode suppression ratio.

[0021] In step (4), the wavelength difference of the dual-mode chips is obtained by testing with a spectrometer or a wavelength meter.

[0022] Advantages of the present invention: Through the technical solution combining actual test data with the transfer matrix method, the present invention can accurately obtain the coupling factor of the DFB laser chip. The accurate coupling factor provides a core quantitative basis for the grating structure design of the DFB laser chip. By analyzing its relationship with parameters such as grating period and refractive index modulation depth, the design scheme can be optimized to ensure that the laser achieves key performance indicators such as stable single-mode output and high wavelength stability, and improve the design accuracy. As a quantitative scale for process accuracy, the coupling factor can monitor the preparation process links such as lithography and etching in real time. When the measured value deviates from the theoretical value, the process deviation can be quickly located, guiding the adjustment of process parameters, effectively ensuring the consistency of performance during the chip preparation process, and reducing process errors; by comparing the measured results and theoretical design values of the coupling factor of different versions of chips, the performance optimization direction can be accurately locked. Whether it is the improvement of material characteristics or the reconstruction of structural parameters, the coupling factor provides clear quantitative guidance for the optimization strategy, promoting the continuous advancement of the performance of DFB laser chips in the fields of optical communication, optical sensing, etc., and helping product iteration and upgrading. Description of the Drawings

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0024] Figure 1 It is the appearance of the DFB laser chip.

[0025] Figure 2 It is the change of the lasing wavelength of the DFB laser chip under different random phases.

[0026] Figure 3 It is the corresponding relationship between the coupling factor and the wavelength difference in the double-mode of the laser.

[0027] Figure 4 It is the spectrum of the DFB laser chip at different temperatures and different currents, (a) 25°C, (b) 45°C, (c) 65°C, (d) 85°C. Detailed Embodiments

[0028] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0029] A method for measuring the coupling factor of a DFB laser chip, comprising the following steps:

[0030] (1) Confirm the grating type and parameters of the DFB laser chip, test the spectra of several DFB laser chips in the same batch, and count the median wavelength of the DFB laser chips;

[0031] (2) Calculate the effective refractive index of the DFB laser chip according to the median wavelength;

[0032] (3) Calculate the coupling factor according to the effective refractive index, and use the transfer matrix method to obtain the corresponding relationship between the coupling factor and the wavelength difference of the dual-mode chip under the corresponding grating condition;

[0033] (4) Test this batch of DFB laser chips, find the dual-mode chip, and test the wavelength difference of this dual-mode chip;

[0034] (5) Substitute the wavelength difference of the dual-mode chip obtained in step (4) into the corresponding relationship obtained in step (3) to obtain the corresponding coupling factor, which is the true coupling factor of this batch of chips.

[0035] Specifically, in step (1), the grating type is a uniform grating, a partial grating, a phase-shifted grating or a periodic modulation grating.

[0036] The grating parameters of the DFB laser chip are the cavity length, grating period or grating duty cycle of the DFB laser chip.

[0037] The waveguide of the DFB laser chip is a ridge waveguide or a masked heterojunction.

[0038] The grating period and the effective refractive index determine the lasing wavelength through the Bragg condition; grating duty cycle: generally, the grating duty cycle is 1:1, but due to process or design reasons, the grating duty cycle will change, thus affecting the coupling factor. In actual tests, it is necessary to combine process parameters and spectral data to optimize the extraction method of the effective refractive index.

[0039] Test the spectra of DFB laser chips in the same batch.

[0040] Test conditions: Ensure that all chips are tested at the same temperature (such as 25 °C) and drive current (above the threshold current). Use a spectral analyzer (such as Agilent 86142B), and the resolution should be less than 0.01 nm.

[0041] Test steps: Fix the chip on the test fixture, connect the drive circuit and the temperature control module. Drive the chip above the threshold current and record the spectral data. Repeat the test for multiple chips in the same batch (not less than 100 pieces) to ensure data representativeness.

[0042] Data acquisition: Extract the main peak wavelength from the spectra of each chip, sort the wavelength data, and take the median value.

[0043] The effective refractive index n in step (2) eff = λ / (2*W); where λ is the median wavelength and W is the grating period.

[0044] The coupling factor where L is the cavity length of the DFB laser chip, n eff is the effective refractive index, △n is the change in the effective refractive index caused by the grating, and λ is the median wavelength.

[0045] The transfer matrix method mentioned above is implemented through operation software to establish models, optical software, or laser software. Using operation software to establish models: Operation software such as MATLAB and Python can be programmed and modeled according to the principle of the transfer matrix method. Taking MATLAB as an example, the characteristic matrices of each layer of the medium in the optical system can be defined by writing functions, and then these matrices are multiplied in sequence according to the order of light propagation to obtain the transfer matrix of the entire system. In this way, various complex optical systems can be flexibly simulated, and the propagation characteristics of light in them can be deeply studied.

[0046] With the help of optical software or laser software: For example, professional optical design software such as Zemax and Code V, as well as some specialized laser design software, which integrate relevant algorithms and functional modules of the transfer matrix method internally. Users only need to input the parameters of the optical system, such as the refractive index and thickness of the medium, according to the interface prompts of the software, and the software can automatically calculate using the transfer matrix method and give the results of light propagation, such as light intensity distribution and phase change.

[0047] The method for finding the dual-mode chip in step (4) is: Test the spectrum of the DFB laser and determine according to the side mode suppression ratio. The dual-mode of the distributed feedback (DFB) laser chip means that the chip can output two different modes of laser simultaneously. By testing the spectrum of the laser and determining whether the DFB laser chip is in the dual-mode working state according to the side mode suppression ratio (SMSR), the specific principle and method are as follows:

[0048] Spectrum test principle: Spectrum test is an important means to analyze the wavelength and intensity distribution of the light output by the laser. For a DFB laser, when it operates in single mode, the spectrum should show a main peak, corresponding to the main oscillation mode of the laser, and the intensity is much higher than other possible weak modes. When the laser is in the dual-mode working state, in addition to the original main peak in the spectrum, a relatively strong secondary peak will appear, and this secondary peak represents another oscillation mode. Through a high-resolution spectrum analyzer, the wavelength position and intensity information of this peak can be accurately measured.

[0049] Side Mode Suppression Ratio (SMSR) determination: The side mode suppression ratio refers to the ratio of the main mode intensity to the strongest side mode intensity of a laser, usually expressed in decibels (dB). In the ideal case of single-mode operation, the SMSR is very high, meaning that the main mode intensity is much greater than the side mode intensity, and the side modes can be almost ignored. Generally speaking, when a DFB laser chip is operating normally in single mode, its SMSR can reach above 30 dB. When dual-mode operation occurs, the intensities of the two modes are relatively close, resulting in a decrease in SMSR. If two peaks with relatively close intensities are observed in the spectrum and the calculated SMSR is significantly lower than the typical value in single-mode operation, for example, lower than 20 dB, then it can be preliminarily determined that the laser is in dual-mode operation.

[0050] However, it should be noted that relying solely on SMSR to determine dual-mode is not completely accurate, and it is necessary to comprehensively judge by combining factors such as the shape and width of the peaks in the spectrum and the operating conditions of the laser. For example, in some cases, due to external environmental interference or some abnormal factors inside the laser, some weak side modes may appear in the spectrum, reducing the SMSR, but it does not necessarily mean that the laser is in a stable dual-mode operation state.

[0051] The wavelength difference of the dual-mode chip in step (4) is obtained by testing with a spectrometer and a wavelength meter.

[0052] Embodiment

[0053] A method for measuring the coupling factor of a DFB laser chip includes the following steps:

[0054] Step 1: Confirm the grating type and parameters of the DFB laser chip;

[0055] As Figure 1 shown, select a 10G 1370nm DFB laser chip with a laser chip cavity length of 200 μm; the grating is a uniform grating, the front cavity surface is coated with an antireflection film, and the rear cavity surface is coated with a high-reflection film.

[0056] Step 2: Test the spectra of a batch of DFB laser chips to obtain the wavelength median of the batch of DFB laser chips;

[0057] Use a laser chip test device to test 150 chips; the wavelength median of the batch of DFB laser chips at room temperature is obtained as 1372 nm.

[0058] Step 3: Obtain the effective refractive index of the laser chip waveguide according to the grating period and the wavelength median of the batch of DFB laser chips;

[0059] For a DFB laser chip with a uniform grating, the effective refractive index n of the waveguide eff= λ / (2*W); where λ is the wavelength of 1372 nm and W is the grating period of 212.47 nm, and the effective refractive index of the waveguide is finally obtained as 3.229.

[0060] Step Four: Using the transfer matrix method, obtain the "coupling factor" and the "wavelength when the laser is in dual mode" for the corresponding grating case

[0061] In the DFB laser chip, the effective refractive index at the position with the grating is n eff + Δn, and the effective refractive index at the position without the grating is n eff - Δn. The coupling factor of the laser chip is where L is the cavity length of the laser chip, 200 μm, dn = (n eff + Δn) - (n eff - Δn) = 2*Δn, Finally, it is obtained that

[0062] Using the transfer matrix method, obtain the variation of the lasing wavelength of the DFB laser chip under different random phases, as Figure 2 shown. Further, obtain the corresponding relationship between the "coupling factor" and the "wavelength difference when the laser is in dual mode", as Figure 3 shown.

[0063] Step Five: Test different DFB laser chips to find the chips in dual mode;

[0064] Test different DFB laser chips to find the chips with obvious side modes. The chip ID is S05-L41.

[0065] Step Six: Test the wavelength difference of this chip;

[0066] As Figure 4 shown, test the spectra of the DFB laser chip at different temperatures and different currents; different current values above the threshold current can be obtained at 25 / 45 / 65 / 85 °C, and the wavelength difference is stable at around 2.22 nm. According to the test data, this method has very good universality and stability.

[0067] Step Seven: According to the relationship confirmed in Step Four, obtain the coupling factor of the DFB laser chip.

[0068] According to Figure 3 , through the corresponding relationship between the "coupling factor" and the "wavelength difference when the laser is in dual mode", the coupling factor of the DFB laser chip is obtained to be approximately 2.9.

[0069] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for measuring the coupling factor of a DFB laser chip, characterized in that, It includes the following steps: (1) Confirm the grating type and parameters of the DFB laser chip, test the spectra of several DFB laser chips in the same batch of chips, and count the median wavelength of the DFB laser chips; (2) Calculate the effective refractive index of the DFB laser chip according to the median wavelength; (3) Calculate the coupling factor according to the effective refractive index, and use the transfer matrix method to obtain the corresponding relationship between the coupling factor and the wavelength difference of the dual-mode chip for the corresponding grating case; (4) Test the DFB laser chips in this batch to find the dual-mode chips and test the wavelength difference of the dual-mode chips; (5) Substitute the wavelength difference of the dual-mode chip obtained in step (4) into the corresponding relationship obtained in step (3) to obtain the corresponding coupling factor, and this coupling factor is the true coupling factor of the chips in this batch.

2. The measurement method of the coupling factor of the DFB laser chip according to claim 1, wherein The grating type of the DFB laser chip is a uniform grating, a partial grating, a phase-shifted grating or a periodically modulated grating.

3. The measurement method of the coupling factor of the DFB laser chip according to claim 2, characterized in that, The grating parameters of the DFB laser chip are the cavity length of the DFB laser chip, the grating period or the duty cycle of the grating.

4. The measurement method of the coupling factor of the DFB laser chip according to claim 3, wherein The waveguide of the DFB laser chip is a ridge waveguide or a masked heterojunction.

5. The measurement method of the coupling factor of the DFB laser chip according to any one of claims 1-4, characterized in that, In step (1), the number of chips tested in the same batch is not less than 100.

6. The measurement method of the coupling factor of the DFB laser chip according to claim 5, wherein The effective refractive index n in the step (2) eff = λ / (2*W); where λ is the median wavelength and W is the grating period.

7. The measurement method of the coupling factor of the DFB laser chip according to claim 6, characterized in that, The coupling factor where L is the cavity length of the DFB laser chip, n eff is the effective refractive index, Δn is the change in the effective refractive index caused by the grating, and λ is the median wavelength.

8. The measurement method of the coupling factor of the DFB laser chip according to claim 7, characterized in that, The transfer matrix method is completed by establishing a model through operation software, optical software or laser software.

9. The measurement method of the coupling factor of the DFB laser chip according to claim 8, characterized in that The method for finding the dual-mode chip in step (4) is: test the spectrum of the DFB laser and judge according to the side mode suppression ratio.

10. The measurement method of the coupling factor of the DFB laser chip according to claim 9, characterized in that, The wavelength difference of the dual-mode chip in step (4) is obtained by testing with a spectrometer or a wavelength meter.

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