Chirp parameter testing method for high-speed semiconductor laser

Abstract

The application discloses a method for testing chirp parameters of a high-speed semiconductor laser, and belongs to the field of optoelectronic technology. The method is aimed at overcoming the problems of additional auxiliary parameter testing and non-unique solution of multi-parameter fitting in the current sweep frequency testing method. The method comprises the following steps: obtaining a fiber transfer function curve through sweep frequency testing; adjusting a bias current of the high-speed semiconductor laser to be tested to change the output optical power of the high-speed semiconductor laser to be tested, so that the frequency of the power minimum point of the fiber transfer function curve at a high frequency no longer changes with the output optical power of the high-speed semiconductor laser to be tested; calculating the fiber dispersion parameter and the line width enhancement factor of the high-speed semiconductor laser through the frequency of the power minimum point of the fiber transfer function curve at the high frequency; using a least square fitting method to obtain the adiabatic chirp coefficient of the high-speed semiconductor laser; and finally calculating the chirp parameters of the high-speed semiconductor laser.

Description

Technical Field

[0001] This invention belongs to the field of optoelectronic technology, specifically relating to a method for testing the chirp parameters of a high-speed semiconductor laser. Background Technology

[0002] Intensity modulation / direct detection systems based on high-speed semiconductor lasers offer a highly cost-effective optical transmission solution. Compared to external modulation systems using electroabsorption modulators or Mach-Zehnder modulators, direct modulation systems are characterized by lower cost, lower power consumption, higher output power, and smaller footprint. However, high-speed semiconductor lasers exhibit significant chirp effects, including instantaneous chirp related to instantaneous power changes and adiabatic chirp related to power values. Signal distortion caused by the interaction between the frequency chirp of the high-speed semiconductor laser and fiber dispersion greatly limits transmission distance. Therefore, accurately measuring the chirp parameters of high-speed semiconductor lasers is crucial for device manufacturing and applications.

[0003] Currently, the methods for testing the chirp parameter of high-speed semiconductor lasers are mainly divided into two categories: spectroscopic methods and electrical spectroscopic methods. The spectroscopic method is the simplest method (T. Zhang, NH Zhu, BH Zhang, et al. Measurement of chirp parameter and modulation index of a semiconductor laser based on optical spectrum analysis[J]. IEEE Photonics Technology Letters, 2007, 19(4): 227-229.). It uses the power ratio between the optical carrier sideband, the first-order sideband, and the second-order sideband, combined with a graphical method, to obtain the analytical solution of the chirp parameter. However, this method ignores the influence of adiabatic chirp and only measures the linewidth enhancement factor of the high-speed semiconductor laser. Therefore, testing the adiabatic chirp coefficient, which has a greater impact on low frequencies, requires an extremely high-resolution spectrometer to obtain the modulation spectrum information at low frequencies. Electrical spectroscopic methods mainly include optical frequency discrimination and transfer function methods combined with fiber dispersion. The optical frequency discrimination method (J. Provost and F. Grillot. Measuring the chirp and the linewidth enhancement factor of optoelectronic devices with a Mach–Zehnder interferometer[J], IEEE Photonics Journal, 2011, 3(3): 476-488.) uses a Mach-Zehnder interferometer to extract the amplitude and phase response of the high-speed semiconductor laser under test for the calculation of the linewidth enhancement factor and the adiabatic chirp coefficient. However, the interference structure is easily affected by the environment, and the experimental results are not accurate enough. The transfer function method (RCSrinivasan and JCCartledge. On using fiber transfer functions to characterize laser chirp and fiber dispersion[J], IEEE Photonics Technology Letters, 1995, 7(11): 1327-1329.) is a more commonly used method. Fiber dispersion introduces asymmetric phase into the double-sideband optical signal, causing the power detected by the photodetector to exhibit periodic power fading. Due to the influence of adiabatic chirp in high-speed semiconductor lasers, the frequency of the power minimum point in the fiber transfer function curve cannot be directly analyzed to obtain the chirp parameter. Instead, the fiber dispersion parameter, the linewidth enhancement factor, and the adiabatic chirp coefficient of the high-speed semiconductor laser under test can only be extracted by three-parameter fitting of the fiber transfer function curve.The method of obtaining fiber dispersion parameters by building an additional test system based on an external modulator, and then fitting the linewidth enhancement factor and adiabatic chirp coefficient of the high-speed semiconductor laser under test, can reduce the number of fitting parameters, but the test system and steps are more complex. In addition, fitting the three parameters by the descending simplex method (LANeto, D. Erasme, N. Genay, et al. Simple estimation of fiber dispersion and laser chirp parameters using the downhill simplex fitting algorithm[J], Journal of Lightwave Technology, 2013, 31(2):334-342.) can improve the accuracy of fitting, but this type of three-parameter fitting often faces problems such as the data processing results changing with the frequency range and the lack of a unique solution. At present, the test method for the chirp parameters of high-speed semiconductor lasers still needs further simplification and research. Summary of the Invention

[0004] The purpose of this invention is to provide a method for testing the chirp parameters of high-speed semiconductor lasers, which overcomes the problems of existing frequency sweep testing methods that require additional fiber dispersion testing and lack a unique solution for three-parameter fitting. This method has the advantages of simple structure and convenient testing.

[0005] A method for testing the chirp parameters of a high-speed semiconductor laser includes the following steps: Step 1: Construct a chirp parameter testing device for a high-speed semiconductor laser, including a DC signal source, an RF signal source, a high-speed semiconductor laser under test, an optical isolator, a first fiber optic connector, a single-mode fiber, a second fiber optic connector, an optical power meter, a photodetector, a data acquisition module, and a control and data processing module. The DC signal source and the RF signal source are electrically connected to the high-speed semiconductor laser under test. The high-speed semiconductor laser under test, the optical isolator, the first fiber optic connector, the single-mode fiber, the second fiber optic connector, and the photodetector are sequentially optically connected. The second fiber optic connector is optically connected to the optical power meter. The photodetector is electrically connected to the data acquisition module. The control and data processing module is data-connected to the RF signal source and the data acquisition module. Step 2: Set the DC signal source output current to be greater than the threshold current of the high-speed semiconductor laser under test, and set the RF signal source output frequency to f. mA sinusoidal signal is applied to the RF port of the high-speed semiconductor laser under test. The modulated optical signal from the high-speed semiconductor laser under test passes sequentially through an optical isolator, a first fiber optic connector, a single-mode fiber, and a second fiber optic connector, finally entering a photodetector to generate a photocurrent signal by beating the frequency. The data acquisition module measures the frequency f in the photocurrent signal output by the photodetector. m The power; Step 3: Perform frequency sweep control on the RF signal source through the control and data processing module, and obtain the frequency f from the data acquisition module. m The power is transmitted to the control and data processing module for data processing and analysis, and the frequency f is recorded. m The power curve A1 shows the change in power of the optical power meter as a function of the frequency of the radio frequency signal source; after turning off the output of the radio frequency signal source, record the reading P1 of the optical power meter at this time. Step 4: Remove the single-mode fiber, connect the first and second fiber connectors directly, repeat steps 2 and 3, and obtain the power curve A2; turn off the RF signal source output and record the optical power meter reading P2 at this time; Step 5: Normalize curve A1 using the optical power values ​​P1 and P2 obtained in steps 3 and 4 and curve A2 to obtain the fiber transfer function curve A, i.e., A = P2·A1 / (P1·A2). Step 6: Adjust the output current of the DC signal source to change the output optical power of the high-speed semiconductor laser under test, so that the frequency of the power minimum point of the fiber transfer function curve at high frequency no longer changes with the output optical power of the high-speed semiconductor laser under test, and record the data of the fiber transfer function curve A and the output optical power P0 of the high-speed semiconductor laser under test at this time. Step 7: Find the frequency f corresponding to the power minimum point on curve A. n (n = 1, 2, 3, ..., f) n (≠0), remove the smallest value f1, according to f n By relating the square of the sum of the squares and the minimum point index n, the dispersion parameter DL of the single-mode fiber and the linewidth enhancement factor α of the high-speed semiconductor laser under test can be calculated. H ; Step 8: Perform least squares fitting on curve A to obtain the adiabatic chirp coefficient κ of the high-speed semiconductor laser under test.

[0006] Compared with the prior art, the present invention has the following beneficial effects: I. This invention eliminates the need for additional fiber dispersion parameter testing. By adjusting the bias current, the dispersion parameters of a single-mode fiber and the linewidth enhancement factor of the high-speed semiconductor laser under test can be directly calculated using the frequency of the power minimum point on the fiber transfer function curve at high frequencies. Second, this invention only requires fitting the adiabatic chirp coefficient of the high-speed semiconductor laser under test, reducing two fitting parameters, simplifying the data processing process, and improving the accuracy of the test results. Attached Figure Description

[0007] Figure 1 This is a schematic diagram of the testing system of the present invention.

[0008] Figure 2 This is a comparison chart of the experimental data in the examples and the fiber transfer function curve fitted based on the test results. Detailed Implementation

[0009] The following is a further description of the present invention in conjunction with the accompanying drawings and embodiments. It should be noted that the scope of protection claimed by the present invention is not limited to the scope described in the embodiments, and should not be construed as limiting the scope of protection of the present invention.

[0010] like Figure 1 As shown, the output optical signal of the high-speed semiconductor laser under test passes sequentially through an optical isolator, a first fiber optic connector, a single-mode fiber, and a second fiber optic connector before entering a photodetector for photoelectric conversion to generate an electrical signal. The data acquisition module detects the frequency f. m The power of the signal source was measured by controlling the frequency sweep of the RF signal source through the control and data processing module, and the frequency f was obtained by testing. m The power curve A1 shows the power variation of the RF signal source frequency. The single-mode fiber is removed, and the first and second fiber connectors are directly connected to obtain the power curve A2. The electrical connection between the RF signal source and the high-speed semiconductor laser under test is disconnected, and the power values ​​of the optical power meter with and without the single-mode fiber connection are measured as P1 and P2. Based on the measured optical power values ​​P1, P2, and curve A2, curve A1 is normalized to obtain the fiber transfer function curve A. The output current of the DC signal source is changed to make the frequency f of the power minimum point of the fiber transfer function curve A at a high frequency... n (n = 2, 3, ...) precisely no longer changes with the output optical power P0 of the high-speed semiconductor laser under test. At this point, the dispersion parameter DL of the single-mode fiber and the linewidth enhancement factor α of the high-speed semiconductor laser under test can be directly calculated by analyzing the fiber transfer function curve A. H Finally, the adiabatic chirp coefficient κ of the high-speed semiconductor laser under test is obtained by least squares fitting of the fiber transfer function curve A.

[0011] To better understand the invention, the measurement principle is briefly described below:

[0012] The high-speed semiconductor laser under test passes through a frequency of f m The modulated output light field is represented as Where ω0 represents the center angular frequency of the output optical carrier, and m represents the intensity modulation coefficient.

[0013] After obtaining the power curve A1 through frequency sweep testing and performing normalization, the fiber transfer function can be expressed as: Where λ represents the center wavelength of the output light of the high-speed semiconductor laser under test, and c represents the speed of light.

[0014] As shown in equation (2), the fiber transfer function curve A is affected by both instantaneous chirp and adiabatic chirp. Adiabatic chirp has two effects on the fiber transfer function curve A: firstly, it compensates for the power at the first power minimum, preventing the power from approaching zero; secondly, it causes the power minimum at high frequencies to occur at frequency f. n The frequency f of the power minimum point cannot be directly determined by the change in (n=2,3,…). n The fiber dispersion parameter DL and the linewidth enhancement factor α of the high-speed semiconductor laser under test were calculated. H However, with the modulation frequency f m The increase of P0 or the decrease of the output optical power P0 of the high-speed semiconductor laser under test will cause the frequency f of the power minimum point at high frequencies to be induced by the adiabatic chirp. n The variation of (n=2,3,…) becomes smaller and smaller. Therefore, when the power minimum point of the fiber transfer function curve A at a high frequency is reached, the frequency f… n When (n=2,3,…) no longer varies with the output optical power P0 of the high-speed semiconductor laser under test, the effect of adiabatic chirp can be ignored, and equation (2) can be simplified to

[0015] At this point, in the fiber transfer function curve A, after removing the minimum value f1, f n The linear relationship between the sum of squares and the index n of the minimum point can be expressed as:

[0016] Equation (4) can be used to express the fiber dispersion parameter DL and the linewidth enhancement factor α of the high-speed semiconductor laser under test. H for Where a and b represent the slope and zero point value of formula (4), respectively.

[0017] Next, the adiabatic chirp coefficient κ of the high-speed semiconductor laser under test is obtained by least squares fitting of the fiber transfer function curve A. Example

[0018] The output center wavelength of the high-speed semiconductor laser under test is 1545.62nm (corresponding to a frequency of 193.96THz). First, the output current of the DC signal source is set to 32mA, greater than the threshold current of 7.50mA. Then, the frequency sweep range of the sinusoidal modulation signal from the RF signal source is set to 10MHz~25GHz. The modulated optical signal passes through an optical isolator and a 40km single-mode fiber. Finally, the control and data processing module processes the data and records the power curve A1, showing the change in fundamental frequency power detected by the photodetector as a function of the RF signal source frequency. The single-mode fiber is then removed, and the first and second fiber connectors are directly connected to obtain the power curve A2. The electrical connection between the RF signal source and the high-speed semiconductor laser under test is then disconnected. The power values ​​of the optical power meter with and without the single-mode fiber connection are P1 = 1.38mW and P2 = 4.84mW, respectively. Then, the measured optical power values ​​P1, P2, and curve A2 are used to normalize curve A1 to obtain the fiber transfer function curve A.

[0019] When the output current of the DC signal source is set to 20mA, the frequency f of the power minimum point on the fiber transfer function curve A at a high frequency is... n (n = 2, 3, ...) exactly no longer changes with the output optical power P0 of the high-speed semiconductor laser under test. At this time, the optical power meter measures the output optical power P0 of the high-speed semiconductor laser under test to be 2.28mW.

[0020] In the fiber transfer function curve A, the frequencies of the 2nd, 3rd, and 4th power minima are f2 = 14.57 GHz, f3 = 20.09 GHz, and f4 = 24.29 GHz, respectively. Substituting these into formula (4), we obtain the slope a = 188.86 and the zero-point value b = 24.25. From formulas (5) and (6), we can calculate the fiber dispersion parameter DL = 665.46 ps / nm and the linewidth enhancement factor α of the high-speed semiconductor laser under test. H =2.34. Next, the least squares method was used to fit the fiber transfer function curve A to obtain the adiabatic chirp coefficient κ = 5.10 GHz / mW for the high-speed semiconductor laser under test. Finally, the test results of the fiber dispersion parameter DL and the chirp parameter of the high-speed semiconductor laser under test were substituted into formula (2) for fitting, and verified to be consistent with the experimental data of the fiber transfer function curve A, such as... Figure 2 As shown.

Claims

1. A method for testing the chirp parameters of a high-speed semiconductor laser, characterized in that, Includes the following steps: Step 1: Construct a chirp parameter testing device for a high-speed semiconductor laser, including a DC signal source, an RF signal source, a high-speed semiconductor laser under test, an optical isolator, a first fiber optic connector, a single-mode fiber, a second fiber optic connector, an optical power meter, a photodetector, a data acquisition module, and a control and data processing module. The DC signal source and the RF signal source are electrically connected to the high-speed semiconductor laser under test. The high-speed semiconductor laser under test, the optical isolator, the first fiber optic connector, the single-mode fiber, the second fiber optic connector, and the photodetector are sequentially optically connected. The second fiber optic connector is optically connected to the optical power meter. The photodetector is electrically connected to the data acquisition module. The control and data processing module is data-connected to the RF signal source and the data acquisition module. Step 2: Set the DC signal source output current to be greater than the threshold current of the high-speed semiconductor laser under test, and set the RF signal source output frequency to f. m A sinusoidal signal is applied to the RF port of the high-speed semiconductor laser under test. The modulated optical signal from the high-speed semiconductor laser under test passes sequentially through an optical isolator, a first fiber optic connector, a single-mode fiber, and a second fiber optic connector, finally entering a photodetector to generate a photocurrent signal by beating the frequency. The data acquisition module measures the frequency f in the photocurrent signal output by the photodetector. m The power; Step 3: Perform frequency sweep control on the RF signal source through the control and data processing module, and obtain the frequency f from the data acquisition module. m The power is transmitted to the control and data processing module for data processing and analysis, and the frequency f is recorded. m The power curve A1 shows the change in power of the optical power meter as a function of the frequency of the radio frequency signal source; after turning off the output of the radio frequency signal source, record the reading P1 of the optical power meter at this time. Step 4: Remove the single-mode fiber, connect the first and second fiber connectors directly, repeat steps 2 and 3, and obtain the power curve A2; turn off the RF signal source output and record the optical power meter reading P2 at this time; Step 5: Normalize curve A1 using the optical power values ​​P1 and P2 obtained in steps 3 and 4 and curve A2 to obtain the fiber transfer function curve A, i.e., A = P2·A1 / (P1·A2). Step 6: Adjust the output current of the DC signal source to change the output optical power of the high-speed semiconductor laser under test, so that the frequency of the power minimum point of the fiber transfer function curve at high frequency no longer changes with the output optical power of the high-speed semiconductor laser under test, and record the data of the fiber transfer function curve A and the output optical power P0 of the high-speed semiconductor laser under test at this time. Step 7: Find the frequency f corresponding to the power minimum point on curve A. n (n = 1, 2, 3, ..., f) n (≠0), excluding the smallest value f1, according to f n By relating the square of the sum of the squares and the minimum point index n, the dispersion parameter DL of the single-mode fiber and the linewidth enhancement factor α of the high-speed semiconductor laser under test can be calculated. H ; Step 8: Perform least squares fitting on curve A to obtain the adiabatic chirp coefficient κ of the high-speed semiconductor laser under test.

2. The method for testing the chirp parameters of a high-speed semiconductor laser according to claim 1, characterized in that, In step 8, the single-mode fiber dispersion parameter DL obtained from the test and the linewidth enhancement factor α of the high-speed semiconductor laser under test are used. H Substituting the values ​​into the fiber transfer function expression, the adiabatic chirp coefficient κ is then fitted to the fiber transfer function curve.

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