A current-tuned semiconductor laser frequency correction system and method

Abstract

The application discloses a current tuning semiconductor laser frequency modulation correction system and a correction method thereof, and belongs to the technical field of laser frequency modulation. The device is composed of a DFB laser driving module, a self-heterodyne Mach-Zehnder interferometer module, a frequency discriminator and phase detector based on FPGA and an adaptive loop filter module. The application designs an optoelectronic digital mixed phase-locked loop closed-loop correction system based on FPGA control, uses the self-heterodyne Mach-Zehnder interferometer and the DFB laser to build an equivalent voltage-controlled oscillator, uses the frequency discriminator and phase detector to obtain the phase error signal of the beat frequency signal, and then uses the adaptive loop filter based on PI control to process the error signal and load the feedback quantity to the driving signal to realize the real-time correction of the DFB laser. The system replaces the traditional charge pump phase-locked loop, has wider applicability, adjustable parameters, good control effect, and reduces the nonlinearity degree in the wide-range and fast frequency modulation process of the current tuning DFB laser.

Description

Technical Field

[0001] This invention belongs to the field of laser frequency modulation technology, specifically a frequency modulation correction system and correction method for a current-tuned semiconductor laser. Background Technology

[0002] Linear tunable laser technology works by emitting a linear sawtooth, triangular, or sine wave frequency-modulated light signal and interfering with the frequency-modulated light signal reflected from the target. By measuring the frequency difference caused by the time delay, the position and even velocity information of the target can be obtained. In practice, there is an important application need for obtaining an ideal linear frequency-modulated laser.

[0003] As a high-quality light source for tunable laser technology, the output wavelength of a DFB laser is mainly affected by temperature and driving current. Current tuning can achieve a higher frequency modulation rate. Theoretical analysis shows a linear relationship between the injected current and the output wavelength, while laser wavelength is inversely proportional to frequency. Therefore, even with a linear current, linearly frequency-modulated lasers cannot be obtained. Furthermore, the temperature changes during current tuning also affect the output laser frequency.

[0004] Tunable laser technology, as a key technology in fields such as FMCW lidar, short-range OFDR, and synthetic aperture radar, directly affects the demodulation, accuracy, and resolution performance of the system. Currently, for frequency modulation nonlinearity correction of tunable lasers, one widely used method is to use an optical comb as a precise frequency modulation network, but this has drawbacks such as easy phase confusion and high cost. Another method is to achieve linear frequency modulation and coherent enhancement of tunable lasers through servo control or optical phase-locked loops. Summary of the Invention

[0005] The purpose of this invention is to provide a system and correction method that can reduce the nonlinearity problem in the triangular wave frequency modulation process of a current-tuned semiconductor laser, so as to solve the problems mentioned in the background art and realize wide-range, fast, and linear frequency modulation of the current-tuned DFB laser, thereby achieving high-precision and high-resolution optical FMCW spatial positioning.

[0006] The present invention employs a current-tuned semiconductor laser frequency modulation correction system, the core of which is an FPGA-based optoelectronic digital-analog hybrid closed-loop correction technology. The system includes: a DFB laser (1), an optical isolator (2), a first coupler (3), a second coupler (4), a third coupler (5), an optical balance detector (6), a high-speed comparator (7), a frequency and phase detector (8), an adaptive loop filter (9), a digital-to-analog converter (10), and a voltage-to-current converter (13).

[0007] The technical solution adopted in this invention is as follows: The modulated voltage signal output by the digital-to-analog converter is used as the driving signal of the DFB laser by the voltage-to-current converter. After the DFB laser outputs frequency-modulated laser light, it is split into two paths after passing through the first coupler. One path is used as a detection signal to realize the sensing of the optical FMCW. The other path enters the second coupler and is split into two paths. One path goes directly into the third coupler, and the other path goes into the third coupler after passing through the delay fiber. The second coupler and the third coupler together constitute a self-heterodyne Mach-Zehnder interferometer. After the two signals interfere at the third coupler, they enter the photoelectric balance detector to realize the conversion of optical signals into electrical signals and photoelectric balance. The electrical signal detected by the detector is the beat frequency signal, the frequency of which is related to the delay arm length of the self-heterodyne Mach-Zehnder interferometer. The electrical beat frequency signal is converted into a square wave signal with frequency and phase information after passing through a high-speed comparator. After passing through a frequency and phase detector, the phase error sequence is output. The phase error sequence is quantized by a time-to-digital converter circuit. The quantized phase error is output as a correction signal after passing through an adaptive loop filter. The correction signal output by an arbitrary signal generator is output by an adder. The modulation signal and the bias voltage signal output by the DC voltage source are combined to form the total output signal, which serves as the driving voltage signal for the DFB laser and drives the DFB laser through a voltage-to-current converter.

[0008] Furthermore, the second and third couplers together constitute a self-heterodyne Mach-Zehnder interferometer, which plays a differentiating role in the optical FMCW frequency modulation process, converting the laser frequency, which cannot be detected by electrical devices, into a beat frequency with time delay information.

[0009] Furthermore, an equivalent evaluation index for the frequency modulation nonlinearity of a DFB laser was constructed using a Mach-Zehnder interferometer and a photoelectric balanced detector. The frequency modulation nonlinearity of the DFB laser can be evaluated by the magnitude of the fluctuation in the time-frequency curve of the beat frequency signal and the bandwidth of the center frequency of the power spectrum curve of the beat frequency signal.

[0010] Furthermore, during the frequency modulation process of a DFB laser, amplitude modulation is inevitably introduced. In order to avoid the influence of beat frequency signal amplitude modulation on subsequent processing, a high-speed comparator is used to connect the analog circuit section and the digital circuit section to convert the analog beat frequency signal into a TTL signal carrying its frequency and phase information.

[0011] Furthermore, a frequency and phase detector and an adaptive loop filter were designed in the FPGA, and a time-to-digital conversion method was used to quantize the phase error sequence output by the frequency and phase detector for easier further processing.

[0012] Furthermore, in order to balance the bandwidth and locking rate of the optoelectronic hybrid digital-analog loop, the adaptive loop filter is designed with different proportional-integral control parameters, dividing the working state of the phase-locked loop into over-capture region, fast capture region, transition region, slow capture region, and lock-in region.

[0013] Compared with the prior art, the present invention has the following technical effects:

[0014] 1. The system structure of this invention is complete and the design is reasonable. Unlike the charge pump phase-locked loop used in traditional optoelectronic hybrid digital-analog lasers, this invention designs an optoelectronic hybrid digital-analog phase-locked loop based on FPGA, which has higher stability, easier parameter adjustment, and is easy to port to various types of current-tuned DFB lasers.

[0015] 2. This invention achieves dynamic, real-time, and high-performance correction of DFB laser frequency modulation, reducing the nonlinearity in the DFB laser frequency modulation process. Attached Figure Description

[0016] Figure 1 This is a structural block diagram of the present invention;

[0017] Figure 2 This is a schematic diagram of the structure of the present invention;

[0018] Figure 3 This is a schematic diagram of the adaptive loop control principle;

[0019] Figure 4 It is the power spectrum curve of the beat frequency signal before correction;

[0020] Figure 5 It is the power spectrum curve of the corrected beat frequency signal.

[0021] The components include: 1. DFB laser; 2. Optical isolator; 3. First coupler; 4. Second coupler; 5. Third coupler; 6. Photoelectric balance detector; 7. High-speed comparator; 8. Frequency and phase detector; 9. Adaptive loop filter; 10. Digital-to-analog converter; 11. DC voltage source; 12. Adder; 13. Voltage-to-current converter. Detailed Implementation

[0022] To address the shortcomings of existing technologies, the present invention aims to provide a more widely applicable frequency modulation nonlinear correction system for current-tuned DFB lasers, enabling wide-range, rapid, and linear frequency modulation of current-tuned DFB lasers, thereby achieving high-precision and high-resolution optical FMCW spatial positioning.

[0023] The present invention will be further described below with reference to the accompanying drawings;

[0024] Please see the appendix Figure 1 With appendix Figure 2 A frequency modulation and correction device for a current-tuned semiconductor laser, the device mainly consists of three parts:

[0025] The first part is the DFB laser module and driving part, including: DFB laser 1, voltage-to-current converter 13, digital-to-analog converter (10) and DC voltage source (11). The digital-to-analog converter (10) and DC voltage source (11) together constitute the voltage signal source for modulating, correcting and driving the DFB laser (1). The voltage signal is then converted into a current signal by the voltage-to-current converter 13 to directly drive the DFB laser (1). The center wavelength of the laser is λ = 1550nm.

[0026] The second part is the self-heterodyne Mach-Zehnder interferometer, including: a second coupler (4) and a third coupler (5). Between the couplers are two optical paths: one is a reference arm, and the other is a delay arm of an extended optical fiber with a fixed time delay compared to the reference arm. The frequency-modulated laser interferes after passing through the self-heterodyne Mach-Zehnder interferometer, and the expression for its interference signal is:

[0027]

[0028] The interference signal in the formula includes a DC component, a harmonic component, a difference frequency component, a sum frequency component, and a difference frequency component. The signal that the photoelectric balance detector (6) can detect is the difference frequency component in the interference signal, which is the beat frequency signal.

[0029] The third part is the correction section, which includes: a high-speed comparator (7), a frequency and phase detector (8), and an adaptive loop filter (9). The high-speed comparator (7) converts the analog beat frequency signal into a TTL square wave signal with its frequency and phase information, and avoids the amplitude modulation effect brought about by the laser frequency modulation process. The frequency and phase detector (8) and the adaptive loop filter (9) are designed in the FPGA. The frequency and phase detector (8) compares the phase of the output signal of the high-speed comparator (7) with the ideal beat frequency signal. The output phase error sequence is bidirectionally quantized by the time-to-digital converter (TDC) in the FPGA. Then, the adaptive loop filter (9) controls the loop to output the correction signal, which is output through the digital-to-analog converter (10). The control function of the adaptive loop filter (9) is much higher than the filtering function, so it is also the adaptive loop control part. Its schematic diagram is shown below. Figure 3 As shown, unlike the traditional charge pump phase-locked loop, the adaptive loop control section, while ensuring the stability of the phase-locked loop, adaptively changes its proportional-integral control parameter PI according to the magnitude of the phase error between the reference signal generated inside the FPGA and the beat frequency signal generated by the frequency modulation of the DFB laser (1). When the phase error is in the locked region and the over-capture region, the control voltage is cleared to zero respectively. In the locked region, it is ensured that the phase-locked loop will not lose lock due to further correction. In the over-capture region, it is avoided that the phase detector (8) continuously outputs disordered phase, causing the accumulation of erroneous feedback, which further improves the stability of the system.

[0030] Figure 2 A schematic diagram of a hybrid photoelectric-analog phase-locked loop system for achieving wide-range, linear frequency scanning is shown. This system also includes an open-loop soft correction section based on an iterative algorithm. In the open-loop correction section, a standard triangular wave current signal is first used to drive the DFB laser (1). The beat frequency signal output by the photoelectric balance detector (6) is acquired using an oscilloscope (OS). The beat frequency signal is then subjected to Hilbert transform in the computer, and the time-frequency curve of the beat frequency signal is analyzed. Based on the relationship between the laser current and the output laser frequency, the pre-correction drive current is obtained. The pre-correction drive current signal is pre-input into the FPGA. The DDS signal output by the FPGA and the digital-to-analog converter drives the DFB laser. This process is repeated until the fluctuation and bandwidth of the beat frequency signal are minimized.

[0031] Assume the instantaneous frequency emitted by the laser can be expressed as:

[0032] f out =f o +i(t)K[i(t)] (1)

[0033] Among them, f o Let be the initial frequency of the laser emission, i(t) be the driving current of the laser, and K[i(t)] be the sweep rate of the laser emission frequency as a function of the current. Based on the sweep characteristics of the DFB laser and the characteristics of the self-heterodyne Mach-Zehnder interferometer, the condition for the DFB laser to output a linearly swept laser can be expressed as follows:

[0034]

[0035] In the soft calibration process of DFB laser, after driving the DFB laser with a triangular wave drive signal of fixed slope, the laser emitted from the laser generates an interference signal after passing through the self-heterodyne MZI. By testing the frequency modulation characteristics of the laser, the ideal beat frequency of the DFB laser can be predicted after fixing the length of the reference arm fiber of the self-heterodyne MZI. After the photoelectric balance detector collects the beat frequency signal, the computer analyzes the beat frequency signal to obtain time and frequency information. According to equation (2), the nonlinear tuning amount of the laser can be obtained by using the time-frequency information of the beat frequency signal and the ideal beat frequency. After determining the nonlinear tuning amount of the laser, the triangular wave drive signal is reshaped according to equation (2) and the laser is driven for frequency modulation again.

[0036] Specific implementation method

[0037] In specific experiments, this invention first uses a 100Hz triangular wave signal (modulation period T = 10ms) as the modulation signal for DFB laser 1. The voltage drive signal uses an external power supply of 3.5 to 6.9V, of which the bias voltage provided by the DC voltage source is 5.2V. The digital-to-analog converter uses a dual-channel high-speed AD9764 module, which can output a 14-bit parallel DA signal. The output signal is a pre-formulated triangular wave signal with open-loop soft correction output by the DDS arbitrary signal generator in the FPGA, with a voltage range of -1.7 to 1.7V. By measuring the dynamic output spectrum of DFB laser 1, its modulation bandwidth B can be obtained.

[0038] In a self-heterodyne Mach-Zehnder interferometer, the delay arm is 5m longer than the reference arm, i.e., the delay τ = 40ns. According to the formula...

[0039]

[0040] The ideal beat frequency, i.e. the reference signal frequency of the frequency and phase detector, can be calculated.

[0041] The frequency and phase detector adopts a frequency and phase detector structure based on timing logic and state transition, which greatly reduces the glitches of the input signal, i.e. the output signal of the high-speed comparator. Then, a time-to-digital converter circuit based on high-frequency clock signal counting is used to quantize the phase error sequence output by the frequency and phase detector.

[0042] The photoelectric balance detector acquires the beat frequency signal and performs photoelectric conversion before it enters the high-speed comparator. The high-speed comparator uses a TLV3501 module with a rise time of 4.5ns and outputs a TTL logic signal for FPGA recognition.

[0043] The adaptive loop filter section adopts an adaptive controller based on PI control. By establishing and analyzing the stability of the optoelectronic hybrid digital-analog model, and setting different proportional-integral control parameters, a balance between the system's response rate and stability can be achieved, resulting in a more efficient phase-locked loop dynamic response.

[0044] Finally, the nonlinearity of the beat frequency signal output by the photoelectric balanced detector is analyzed. The power spectra of the beat frequency signal before and after correction are as follows: Figure 4 , Figure 5 Analysis of the center frequency and 3dB bandwidth of the beat frequency signal reveals that the frequency modulation nonlinearity of the DFB laser before correction was 16.55%, while that after correction was 0.078%, indicating that the system achieved a good correction effect.

[0045] The specific embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the concept and scope of the invention. Those skilled in the art will understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the invention. Therefore, this means that all changes and modifications falling within the scope of the invention are included in the appended claims.

Claims

1. A frequency modulation correction system for a current-tuned semiconductor laser, characterized in that... include: The DFB laser module and driving section, the self-heterodyne Mach-Zehnder interferometer section and the correction section; the DFB laser module and driving section includes a DFB laser (1), a voltage-to-current converter (13), a digital-to-analog converter (10) and a DC voltage source (11); the self-heterodyne Mach-Zehnder interferometer section includes a second coupler (4) and a third coupler (5). Among them, the output end of the DFB laser (1) is connected to the input end of the optical isolator (2), the output end of the optical isolator (2) is connected to the input end of the first coupler (3), the first output end of the first coupler (3) is output to the detection system of each optical FMCW, the second output end of the first coupler (3) is connected to the input end of the second coupler (4), the first output end of the second coupler (4) is connected to the first input end of the third coupler (5) through a single-mode optical fiber, the second output end of the second coupler (4) is directly connected to the third input end of the third coupler (5), and the output end of the third coupler (5) is connected to the input end of the photoelectric balance detector (6); the output end of the photoelectric balance detector (6) is connected to the input end of the photoelectric balance detector (6). The output terminal is connected to the high-speed comparator (7). The output signal of the high-speed comparator (7) enters the FPGA and is first processed in the frequency and phase detector (8). The output signal of the frequency and phase detector enters the adaptive loop filter (9). The output signal of the adaptive loop filter (9) is output through the digital-to-analog converter (10). The output signal of the digital-to-analog converter (10) and the output signal of the DC voltage source (11) enter the input terminal of the adder (12). The output terminal of the adder (12) is connected to the input terminal of the voltage-current converter (13). The output terminal of the voltage-current converter (13) is connected to the input terminal of the DFB laser (1), thus forming a current-tuned semiconductor laser tuning closed-loop correction system.

2. The frequency modulation and correction system for a current-tuned semiconductor laser according to claim 1, characterized in that: The digital-to-analog converter (10) and the DC voltage source (11) together constitute the voltage signal source for modulating, correcting and driving the DFB laser (1). The driving voltage signal is then converted into a current signal by the voltage-to-current converter (13) to directly drive the DFB laser (1). The DFB laser (1) is a current-tuned 1550nm butterfly DFB laser.

3. The frequency modulation and correction system for a current-tuned semiconductor laser according to claim 1, characterized in that: A self-heterodyne Mach-Zehnder interferometer with unequal arm lengths is constructed using a second coupler (4) and a third coupler (5). One arm is a reference arm, and the other is a delay arm of an extended optical fiber with a fixed time delay compared to the reference arm. The frequency-modulated laser interferes after passing through the self-heterodyne Mach-Zehnder interferometer, and the expression for its interference signal is: The interference signal in the formula includes a DC component, a harmonic component, a sum frequency component and a difference frequency component. The frequency signal that the photoelectric balance detector (6) can detect is the difference frequency component in the interference signal, which is the beat frequency signal.

4. The frequency modulation and correction system for a current-tuned semiconductor laser according to claim 1, characterized in that: The high-speed comparator (7) converts the analog beat frequency signal into a TTL square wave signal with its frequency and phase information. A frequency and phase discriminator (8) and an adaptive loop filter (9) are designed in the FPGA. The frequency and phase discriminator (8) compares the output signal of the high-speed comparator with the ideal beat frequency signal in phase. The output phase error sequence is bidirectionally quantized by the time-to-digital converter (TDC) in the FPGA. Then, the loop is controlled by the adaptive loop filter (9) to output a correction signal, which is then output through the digital-to-analog converter.

5. A method for frequency modulation correction of a current-tuned semiconductor laser, characterized in that: Using the current-tuned semiconductor laser frequency modulation correction system according to any one of claims 1-4, the modulation voltage signal output by the digital-to-analog converter is used as the driving signal of the DFB laser (1) through the voltage-to-current converter. After the DFB laser (1) outputs the frequency-modulated laser, it is split into two paths after passing through the first coupler (3). One path is used as the detection path optical signal to realize the optical FMCW sensing, and the other path is used as the correction path optical signal to enter the second coupler (4) and the third coupler (5) to form a self-heterodyne Mach-Zehnder interferometer with an arm length difference of 5m, outputting the optical FMCW beat frequency optical signal. The photoelectric balance detector (6) obtains the optical... The beat frequency signal is converted into an electrical beat frequency signal. The high-speed comparator (7) identifies the electrical beat frequency signal as a TTL signal with beat frequency and phase information. The obtained TTL signal enters the frequency and phase detector (8) in the FPGA and is compared with the pre-made reference beat frequency signal. The output phase difference sequence enters the adaptive loop filter (9) in the FPGA to generate a correction sequence. Finally, the adaptive loop filter outputs a correction signal to the digital-to-analog converter to drive the DFB laser (1) and complete the modulation correction. The system as a whole constitutes a photoelectric digital-to-analog hybrid phase-locked loop structure, realizing the closed-loop frequency modulation nonlinear correction of the DFB laser.

6. The frequency modulation correction method for a current-tuned semiconductor laser according to claim 5, characterized in that: The self-heterodyne Mach-Zehnder interferometer with an arm length difference of 5m converts the high-frequency laser frequency into a beat frequency signal that can be detected by the photoelectric balance detector (6). At the same time, the time-frequency curve of the beat frequency signal can be obtained through Hilbert transformation, realizing the equivalent evaluation index of frequency modulation linearity.

7. The frequency modulation correction method for a current-tuned semiconductor laser according to claim 5, characterized in that: The driving signal of the DFB laser (1) consists of three parts: bias voltage, triangular wave driving signal generated by DDS arbitrary signal generator, and correction signal output by adaptive loop filter (9).

8. The frequency modulation correction method for a current-tuned semiconductor laser according to claim 5, characterized in that: The phase error sequence output by the frequency and phase detector (8) is processed by a PI controller to achieve frequency modulation nonlinear correction of the DFB laser (1). The PI parameters in the adaptive loop filter (9) are divided into over-capture region, fast capture region, transition region, slow capture region and lock region according to the magnitude of the phase error sequence, so as to achieve a balance between the loop locking speed and the loop bandwidth.

9. The frequency modulation correction method for a current-tuned semiconductor laser according to claim 5, characterized in that: The phase error sequence output by the frequency and phase detector (8) is quantized by the TDC, and bidirectional quantization of the phase error sequence is achieved according to the phase lead and lag.

10. The frequency modulation correction method for a current-tuned semiconductor laser according to claim 5, characterized in that: The conversion method between electrical beat frequency signal and TTL signal is to use a high-speed comparator (7) to extract the frequency and phase of the electrical beat frequency signal and output the TTL signal.

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