
- Chinese Optics Letters
- Vol. 23, Issue 6, 060605 (2025)
Abstract
Keywords
1. Introduction
Coherent Doppler wind lidars (CDWLs) have been widely applied in aviation security[1], climate modeling[2], wind-farm project optimization, and other fields. 1.55 µm single-frequency fiber lasers with low noise, narrow linewidth, high beam quality, and high integration have become primary light sources for CDWLs. For a high single-pulse energy output of single-frequency pulsed fiber lasers, the main oscillating power amplifier (MOPA) structure using erbium–ytterbium-doped fibers is well adapted[3]. However, fiber amplifiers easily excite nonlinear effects owing to their small mode field area and high power density, and stimulated Brillouin scattering (SBS) is a nonlinear process with the lowest threshold because of approximately kilohertz linewidth of the light source, limiting the peak power of the output lasers[4–6]. Coherent beam combining (CBC) allows the combination of the energy of serial fiber amplifiers and output light sources while simultaneously maintaining the linewidth, beam quality, and polarization degree of the subbeam. This is an important method to overcome the bottleneck of single-frequency pulsed fiber laser energy[7,8].
In a continuous CBC system, the phase compensation direction is determined by the sampling intensity value. However, the intensity modulation of the pulsed laser affects the judgment of the phase difference[9–11]. Therefore, extracting the phase difference between each subbeam is a research focus for pulsed CBC systems. Currently, for low-repetition (
This study presents the active CBC of a pulsed laser based on sampling the intrapulse evaluation function for CBC with low repetition, avoiding a CW light leakage. Meanwhile, the pulsed CBC system based on intrapulse sampling is simpler than using a low-power CW light leak. Phase locking is accomplished by directly sampling a fixed time point of the pulsed light as the feedback signal of the active phase control evaluation function. The experimental results demonstrated that the pulsed CBC technology is applicable for improving the pulse energy of a CDWL without degrading performance.
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2. Principle and Method
The experimental setup of the active pulsed CBC is shown in Fig. 1, which includes part of the CBC optical path and pulsed active phase control. For the CBC optical path, a 1550 nm distributed Bragg reflector (DBR) seed laser with a power of 10 mW is modulated using an acoustic-optic modulator (AOM) to produce 500 ns pulses at a repetition frequency of 10 kHz. The output of the pulsed laser is then split into two arms using a 50/50 coupler. One arm is coupled to a phase modulator (PM) with a half-wave voltage of 4 V, which is controlled to achieve phase synchronization. The two arms are coupled to a two-stage fiber amplifier (AMP). The first stage is an Er-doped fiber amplifier, and the second stage is an ErYb-co-doped fiber amplifier. The average laser power in the two channels is 70 mW and the single-pulse energy is 7 µJ. The amplified outputs of the two laser channels are combined using a
Figure 1.Experimental setup for active pulsed CBC.
In pulsed active phase control, to generate the phase-control signal in the pulsed active phase controller, the feedback arm is attenuated to an appropriate range using an attenuator (VOA) and connected to a photodetector (PD). The intensity detected by the PD is defined as evaluation function
- 1)Set phase control voltage
, then obtain the corresponding evaluation function ; generate disturbance voltage ; - 2)Compare
with . If , keep phase control voltage unchanged and acquire real-time evaluation function to compare with ; If , carry out step 3) of the procedure; - 3)Set phase control voltage
; then obtain the corresponding evaluation function ; - 4)Compare
with . If , apply the control voltages with to the PM in the next disturbances, return to step 2) of the procedure; if , apply the control voltages with to the PM in the next disturbances, then obtain the corresponding evaluation function ; - 5)Compare
with . If , apply the control voltages with to the PM in the next disturbances; repeat procedures 2) to 5).
The above phase control processes require real-time acquisition of evaluation functions
Figure 2.Control timing diagram of pulsed CBC.
3. Results of Pulsed CBC
In the pulsed CBC system based on intrapulse sampling, two factors can introduce additional perturbations in the acquisition of the evaluation function
Figure 3.Sampled intensity of the two pulsed subbeams at the fixed time point.
The temporal profiles of the amplified and combined beams are shown in Fig. 4. The consistency of the two fiber amplifiers is ensured to the maximum possible extent via the fabrication process, including the gain and fiber length (difference of less than 0.2 m). Therefore, the two pulsed lasers overlap in the time domain, and the combined pulse waveforms are consistent with those of the subbeam. Figure 5 shows the changes in the power and normalized cost function
Figure 4.Results. (a) Pulse shapes of two channels and combined beam in one cycle; (b) pulse shape of combined beam in multicycle.
Figure 5.Results. (a) Output powers in open loop and closed loop; (b) normalized cost functions in open loop and closed loop.
4. Application of Pulsed CBC in CDWL
Pulsed CBC can be applied to improve the emission energy. According to the lidar equation as shown in Eq. (2)[14],
The setup of the CDWL based on the pulsed CBC is shown in Fig. 6. The circulator (CIR) separates the transmitted pulsed laser and the continuous optical atmospheric backscatter signal in opposite directions. The output light source is sent to the atmosphere via port 2 of the CIR. The transceiver system adopts an integrated configuration and uses a 100 mm aperture telescope. The optical atmospheric backscatter signal will be received by the telescope. In the receiving system, the backscattered signal is mixed with a local oscillator light using a
Figure 6.Setup for coherent Doppler wind lidar based on pulsed CBC.
Triple light sources are compared, including single-beam with 5.4 µJ (source A, listed in Table 1), single-beam with 10.8 µJ (source B, listed in Table 1), and two-beam CBC light source with 10.8 µJ (source C, listed in Table 1). Figure 7 shows the narrowband SNR of CDWL for three different light sources. Given the higher power, source C exhibits a 4 dB higher SNR than source A. Meanwhile, the SNR of source C is comparable to that of source B.
Figure 7.Results of SNR. (a) Average SNR; (b) SNR difference.
Source | Value (µJ) | Illustrate |
---|---|---|
A | 5.4 | single-beam |
B | 10.8 | single-beam |
C | 10.8 | two-beam CBC |
Table 1. Parameters of Three Light Sources
Figure 8 shows the wind velocity profiles of the lidar for the three different light sources. Figure 8(a) denotes source A, Fig. 8(b) denotes source B, and Fig. 8(c) denotes source C. According to Eq. (2), the energy of a single pulse increases by a factor of 2, and the detection range can be increased by
Figure 8.Wind velocity profiles. (a) Source A; (b) source B; (c) source C.
5. Discussion
The pulsed CBC based on intrapulse sampling requires accurate acquisition timing control, and a narrower pulse width requires higher acquisition timing precision. Currently, electronic components, such as A/D chips, are limited to the gigahertz range, limiting the accuracy of acquisition timing to the nanosecond level. Therefore, this scheme is more suitable for pulsed CBC systems with pulse widths exceeding 100 ns. Currently, CDWLs with all-fiber structured light sources and detection ability exceeding 10 km, the pulse widths are designed to be 200 ns–1 µs to achieve the requirement of submillijoule-level emission energy[15]. Therefore, the above emission source can serve as a subbeam in an amplification array, compatible with the pulsed CBC technique based on intrapulse sampling. The fiber coupler with limited power handling is used as the beam combiner in Fig. 1 and can be replaced by a polarization coherent beam combiner with higher power handling. Then, on the current output level of the 1.5 µm single-frequency pulsed fiber laser, the multibeam CBC can be achieved to break through output single pulse energy up to the millijoule level and coherent detection at a longer distance.
6. Conclusion
In conclusion, an active CBC of pulsed laser based on sampling the intrapulse evaluation function is demonstrated. By precisely controlling the trigger sequence of the ADC, the fixed time point of the pulse light is sampled as the evaluation function. Then the hill-climbing algorithm is applied to correct the phase errors. In the experiment, the active CBC of two fiber amplifiers with a pulse width of 500 ns, a repetition rate of 10 kHz, and a phase error of
References
[14] F. Takashi, F. Tetsuofukuch. Laser Remote Sensing(2005).
[15] R. William, G. Didier, V. Matthieu et al. Beyond 10 km range wind-speed measurement with a 1.5 µm all-fiber laser source. Proceedings of the 2014 Conference on Lasers and Electro-Optics (CLEO) -Laser Science to Photonic Applications(2014).

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