A fitting method for ultra-low reflectivity of optical coatings

Abstract

The present invention proposes a fitting method for the ultra-low reflectivity of optical coatings to solve the technical problem of large measurement errors in the ultra-low reflectivity of optical coatings. The steps of the present invention are as follows: simulating and calculating the effective refractive index of the waveguide of an uncoated FP laser chip at both ends to obtain the reflectivity of the uncoated FP laser chip and air; performing a P-I test on the uncoated FP laser chip at both ends to obtain a threshold current; performing an ultra-low optical coating on one end of the uncoated FP laser chip to obtain a laser chip with one end coated; applying different current values ​​to the laser chip with one end coated according to the threshold current and testing the corresponding spectral curves; using the spectral curves under different current values, the reflectivity of the ultra-low optical coating is obtained according to the reflectivity fitting. The present invention can accurately estimate the ultra-low reflectivity of the optical coating, playing a decisive role in the optimization of SLD and SOA products.

Description

Technical Field

[0001] The present invention relates to the technical field of optical coating reflectivity fitting, and in particular to a method for fitting ultra-low reflectivity of optical coatings. Background Art

[0002] Superluminescent diodes (SLDs) are widely used in fiber optic gyroscopes, optical coherence tomography, atomic force microscopes, fiber optic sensors, optical time-domain reflectometry, laser displacement meters, laser scales, and other fields. Furthermore, demand for SLDs in medical imaging is rapidly increasing, and the industry holds promising prospects for future development.

[0003] Semiconductor Optical Amplifier (SOA) has the advantages of high bandwidth, low power consumption, high gain, miniaturization and easy integration, and plays an increasingly important role in all-optical network communications and sensor networks.

[0004] Ultra-low reflectivity optical coatings are a key process for the performance of SLDs and SOAs. Therefore, accurate estimation of the reflectivity of optical films is crucial. Conventional reflectivity measurement methods (such as spectrophotometers) are highly inaccurate for ultra-low reflectivity optical coatings.

[0005] Patent application number 201010191207.3 discloses a surface coating method for improving the temperature stability of semiconductor lasers. This method adjusts the laser wavelength to within the rising bandwidth of the reflectivity-wavelength curve. Because the laser heats up during operation, the lasing wavelength of the semiconductor laser undergoes a red shift as the temperature rises. Consequently, the reflectivity of the optical film is higher during operation, improving the high-temperature characteristics of the laser. However, this patent does not allow for the measurement of reflectivity. Summary of the Invention

[0006] In order to solve the technical problem of large measurement error of ultra-low reflectivity of existing optical coatings, the present invention proposes an ultra-low reflectivity fitting method to achieve the purpose of accurately estimating the ultra-low reflectivity of optical coatings.

[0007] In order to achieve the above object, the technical solution of the present invention is implemented as follows: a fitting method for ultra-low reflectivity of optical coating, the steps of which are as follows:

[0008] Step 1: Simulate and calculate the effective refractive index of the waveguide of the uncoated FP laser chip at both ends, and then obtain the reflectivity of the uncoated FP laser chip and air;

[0009] Step 2: Perform PI test on the FP laser chip with no coating at both ends to obtain the threshold current;

[0010] Step 3: Perform ultra-low optical coating on one end of the FP laser chip that is not coated at both ends to obtain a laser chip with one end coated;

[0011] Step 4: Apply different current values ​​to the laser chip coated at one end according to the threshold current and test the corresponding spectral curve;

[0012] Step 5: Using the spectral curves at different current values, the reflectivity of the ultra-low optical coating is obtained by fitting the reflectivity in step 1.

[0013] Preferably, the FP laser chip is a ridge waveguide structure or a buried heterojunction structure; the FP laser chip is InP-based or GaAs-based.

[0014] Preferably, the effective refractive index in step 1 is obtained by simulation using the FDTD or BPM method.

[0015] Preferably, the method for obtaining the reflectivity of the uncoated FP laser chip and air is: using the FDTD method in combination with the thickness / refractive index of the epitaxial layer and the depth / width of the ridge waveguide to obtain the mode field distribution in the ridge waveguide, and fitting to obtain the effective refractive index n of the ridge waveguide. eff ;Reflectivity of uncoated FP laser chip and air Here, n0 is the refractive index of air.

[0016] Preferably, the threshold current is obtained according to a test curve of a PI test by a 2-point method, a 4-point method, a second differential method or a 1 / N method.

[0017] Preferably, the ultra-low optical density coating method is electron beam evaporation, ion beam sputtering or electron cyclotron resonance.

[0018] Preferably, the spectrum curves of the laser chip coated at one end with different current values ​​are tested by a spectrometer; the current values ​​are near the threshold currents of the FP lasers without coating at both ends.

[0019] Preferably, the method for fitting the reflectivity of the ultra-low optical coating in step 5 is:

[0020] The amplification factor of light traveling one cycle within the laser Among them, R1 is the reflectivity of the uncoated end of the FP laser chip, R2 is the reflectivity of the ultra-low reflectivity optical coating of the FP laser chip, g + 、g - are the gains of light transmitted to the left and right, respectively; 2βL is the phase change of light during one cycle of transmission in the laser chip; when 2βL is an odd or even multiple of π, the amplification factor a corresponds to the minimum and maximum values ​​in the spectrum curve of the FP laser chip, respectively;

[0021] The transfer matrix method is used to calculate the FP laser chip and we can get Among them, P min 、P max are the minimum and maximum values ​​in the spectrum curve of the FP laser chip, respectively;

[0022] According to the spectral curves at at least three current values, the value of the amplification factor |a| at the corresponding current is obtained;

[0023] Gain g + 、g - It is approximately a linear function of the current; combined with R2<<1, we get Where γ is the threshold current I th The gain coefficient of the laser is at, L is the cavity length of the laser, I is the test current, I th is the threshold current of the uncoated FP laser;

[0024] A line is obtained by fitting at least three different current values and ln|a|; when I=I th When , we get the value of ln|a| of the fitting line, which is The reflectivity R2 of the ultra-low reflectivity optical coating is obtained based on the reflectivity R1 of the uncoated end of the FP laser chip.

[0025] Compared with existing technologies, the present invention offers the following advantages: Optical film reflectivity is a key factor in the performance of SLDs and SOAs, making accurate estimation of optical coating reflectivity crucial. This invention calculates the reflectivity of an uncoated FP laser chip and air by fitting the effective refractive index of the waveguide. By applying a current near the threshold current to a laser chip coated at one end and measuring the spectral curve, the reflectivity of the ultra-low reflectivity optical coating is calculated. This allows for accurate estimation of ultra-low reflectivity, playing a crucial role in optimizing SLD and SOA products. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 Schematic diagram of the light field distribution and effective refractive index of the ridge waveguide of the present invention.

[0028] Figure 2 This is the PI test result of the uncoated FP laser chip of the present invention.

[0029] Figure 3 The present invention is fitted and the straight line of ln|a|. DETAILED DESCRIPTION

[0030] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without creative work are within the scope of protection of the present invention.

[0031] Example 1

[0032] like Figure 1 As shown in FIG, a fitting method for ultra-low reflectivity, the steps are as follows:

[0033] Step 1: Simulate and calculate the effective refractive index of the waveguide of the uncoated FP laser chip at both ends, and then obtain the reflectivity of the uncoated FP laser chip and air.

[0034] FP laser chips are ridge waveguide structures or buried heterojunction structures, realizing two technical routes for lasers. FP laser chips are based on InP or GaAs.

[0035] The effective refractive index can be calculated by FDTD or BPM simulation.

[0036] Step 2: Perform PI test on the FP laser chip with uncoated ends to obtain the threshold current.

[0037] The threshold current can be obtained by the 2-point method, the 4-point method, the second differential method or the 1 / N method.

[0038] Step 3: Perform ultra-low optical coating on one end of the FP laser chip that is not coated at both ends to obtain a laser chip with one end coated.

[0039] For the purpose of testing the spectrum below, ultra-low optical coating is applied to one end of the spectrum. The ultra-low optical coating method is electron beam evaporation, ion beam sputtering or electron cyclotron resonance.

[0040] Step 4: Apply different current values ​​to the laser chip coated at one end according to the threshold current and test the corresponding spectral curve.

[0041] The spectrometer measures the spectral curves of different current values ​​applied to a laser chip with one end coated. To ensure the accuracy of the ultra-low reflectivity fitting, the current value needs to be close to the threshold current of the FP laser with no coating at both ends.

[0042] Step 5: Using the spectral curves at different current values, the specific value of the reflectivity of the ultra-low optical film is obtained according to the reflectivity fitting in step 1.

[0043] Example 2

[0044] A fitting method for achieving ultra-low reflectivity involves epitaxially growing a lower confinement layer, an AlGaInAs MQW (Multiple Quantum Well), an upper confinement layer, an InP (Indium Phosphide) layer, an InGaAsP etch-stop layer, a P-InP layer, and finally a P-InGaAs layer on an InP substrate. During fabrication, a ridge waveguide is fabricated using processes such as photolithography and etching. This ridge waveguide structure can achieve lateral confinement of the light field and serve as a channel for current injection. A SiO2 dielectric layer is formed using SiO2 deposition, photolithography, and SiO2 etching, and a SiO2 window is opened in the ridge waveguide. The SiO2 dielectric layer provides electrical isolation, while the SiO2 window serves as a channel for current injection into the ridge waveguide. P-side metal is deposited using processes such as sputtering, photolithography, and lift-off. This P-side metal serves as the positive electrode connection during device packaging. N-side metal is deposited using thinning / polishing and sputtering. This N-side metal serves as the negative electrode connection during device packaging. Finally, a scribing process is performed to obtain the uncoated FP laser chip. The specific steps of the present invention are:

[0045] (1) Using the FDTD method, combined with the thickness / refractive index of the epitaxial layer and the depth / width of the ridge waveguide, the effective refractive index of the ridge waveguide is obtained by fitting. Figure 1 The figure shows the mode field distribution in the ridge waveguide, and the effective refractive index n of the ridge waveguide is obtained. eff ; Then the reflectivity of the uncoated FP laser chip and air is obtained Where n0 is the refractive index of air.

[0046] (2) Perform PI test on the uncoated FP laser chip. The test curve is as follows: Figure 2 As shown. According to the test curve, the threshold current I is obtained using the 4-point method. th It is 11.9mA.

[0047] (3) Electron beam evaporation is used to coat one end of the FP laser chip with an ultra-low reflectivity optical film.

[0048] (4) Apply current values ​​of 13.9 mA, 16.9 mA, and 19.9 mA to the laser chip with one end coated, and measure the corresponding spectral curves.

[0049] (5) The amplification factor of light traveling one cycle within the laser Among them, R1 is the reflectivity of the uncoated end of the FP laser chip, R2 is the reflectivity of the ultra-low reflectivity optical coating of the FP laser chip, 2βL is the phase change of light transmitted in the laser chip, g + 、g - are the gains of light transmitted to the left and right, respectively. When 2BL is an odd or even multiple of π, the amplification factor a corresponds to the minimum and maximum values ​​in the spectrum curve of the FP laser chip, respectively.

[0050] Using the transfer matrix method to calculate the FP laser chip, we can get Among them, P min 、P max These are the minimum and maximum values ​​in the spectral curve of the FP laser chip, respectively. Based on the spectral curves at currents of 13.9mA, 16.9mA, and 19.9mA, the amplification factor |a| at the corresponding current can be obtained.

[0051] Since the test currents are all near the threshold current of the uncoated FP laser, the gain g + 、g - It can be approximated as a linear function of current; combined with R2<<1, we can get Where γ is the threshold current I th The gain coefficient of the laser is at, L is the cavity length of the laser, I is the test current, I th is the threshold current of the uncoated FP laser. According to different current values, a line can be fitted to obtain and ln|a|, such as Figure 3 shown; when I=I th When , the value of ln|a| corresponding to the fitting line is obtained; at this time, ln|a| is The reflectivity R2 of the ultra-low reflectivity optical coating is obtained to be 0.008.

[0052] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for fitting ultra-low reflectivity of optical coatings, characterized in that: The steps are as follows: Step 1: Simulate and calculate the effective refractive index of the waveguide of the uncoated FP laser chip at both ends, and then obtain the reflectivity of the uncoated FP laser chip and air; Step 2: Perform PI test on the FP laser chip with no coating at both ends to obtain the threshold current; Step 3: Perform ultra-low optical coating on one end of the FP laser chip that is not coated at both ends to obtain a laser chip with one end coated; Step 4: Apply different current values ​​to the laser chip coated at one end according to the threshold current and test the corresponding spectral curve; Step 5: Using the spectral curves at different current values, the reflectivity of the ultra-low optical coating is obtained by fitting the reflectivity in step 1.

2. The method for fitting ultra-low reflectivity of optical coating according to claim 1, characterized in that: The FP laser chip is a ridge waveguide structure or a buried heterojunction structure; the FP laser chip is an InP-based or GaAs-based one.

3. The method for fitting ultra-low reflectivity of an optical coating according to claim 1 or 2, wherein: The effective refractive index in the step 1 is obtained by simulation using the FDTD or BPM method.

4. The method for fitting ultra-low reflectivity of optical coating according to claim 3, characterized in that: The method for obtaining the reflectivity of the uncoated FP laser chip and air is as follows: using the FDTD method to combine the thickness / refractive index of the epitaxial layer and the depth / width of the ridge waveguide to obtain the mode field distribution in the ridge waveguide, and fitting to obtain the effective refractive index n of the ridge waveguide. eff ;Reflectivity of uncoated FP laser chip and air Here, n0 is the refractive index of air.

5. The method for fitting ultra-low reflectivity of optical coating according to claim 4, characterized in that: The threshold current is obtained according to a test curve of a PI test by a 2-point method, a 4-point method, a second differential method or a 1 / N method.

6. The method for fitting ultra-low reflectivity of optical coating according to claim 5, characterized in that: The ultra-low optical coating method is an electron beam evaporation method, an ion beam sputtering method or an electron cyclotron resonance method.

7. The method for fitting ultra-low reflectivity of an optical coating according to claim 5 or 6, wherein: The spectrum curves of the laser chip coated at one end were tested by a spectrometer with different current values ​​applied; the current values ​​were near the threshold currents of the FP laser without coating at both ends.

8. The method for fitting ultra-low reflectivity of optical coating according to claim 7, characterized in that: The method for fitting the reflectivity of the ultra-low optical coating in step 5 is: The amplification factor of light traveling one cycle within the laser Among them, R1 is the reflectivity of the uncoated end of the FP laser chip, R2 is the reflectivity of the ultra-low reflectivity optical coating of the FP laser chip, g + 、g - are the gains of light transmitted to the left and right, respectively; 2βL is the phase change of light during one cycle of transmission in the laser chip; when 2βL is an odd or even multiple of π, the amplification factor a corresponds to the minimum and maximum values ​​in the spectrum curve of the FP laser chip, respectively; The transfer matrix method is used to calculate the FP laser chip and we can get Among them, P min 、P max are the minimum and maximum values ​​in the spectrum curve of the FP laser chip, respectively; According to the spectral curves at at least three current values, the value of the amplification factor |a| at the corresponding current is obtained; Gain g + 、g - It is approximately a linear function of the current; combined with R2<<1, we get Where γ is the threshold current I th The gain coefficient of the laser is at, L is the cavity length of the laser, I is the test current, I th is the threshold current of the uncoated FP laser; A line is obtained by fitting at least three different current values and ln|a|; when I=I th When , we get the value of ln|a| of the fitting line, which is The reflectivity R2 of the ultra-low reflectivity optical coating is obtained based on the reflectivity R1 of the uncoated end of the FP laser chip.

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