Coherent Doppler wind lidar (CDWL) has become an essential tool for wind velocity measurement in various fields, including wind resource assessment, aviation safety, and meteorological research. In applications like turbulence monitoring and aircraft wake vortex detection, where fine-scale wind field analysis is crucial, enhanced range resolution and improved velocity measurement precision are required. Traditional pulsed CDWL systems employ short pulses to achieve high-range resolution. However, shorter pulses compromise frequency resolution, leading to a decline in wind velocity measurement precision. Phase-coded modulation schemes offer a potential solution by decoupling range and frequency resolutions. However, in these schemes, the pulse width is typically constrained by spread spectrum crosstalk if the modulation format is not appropriately selected. To overcome these limitations, we propose a novel long-pulsed CDWL system based on minimum shift keying (MSK) modulation. Due to the effective crosstalk suppression of MSK signals, the advantages of a longer coding sequence are fully utilized. Consequently, the range resolution is determined by the chip duration, while the extended pulse duration ensures high-frequency resolution and signal-to-noise ratio, contributing to precise wind velocity measurement.

We employ an all-fiber coherent receiving architecture. The signal beam is frequency-shifted and gated by an acousto-optic modulator (AOM) and subsequently encoded by an I/Q electro-optic modulator for MSK modulation. The amplified probe pulse is transmitted into the atmosphere via an optical antenna. The Mie backscattering from aerosols is received by the same antenna and then coherently detected. Through digital signal processing, the radial wind velocity at various ranges is finally retrieved from the Doppler frequency shift. Phase or frequency coding modulation leads to spectral spreading. Therefore, in the decoding process, the scattered signal is despread when multiplied by a decoding sequence with different time delays. Based on this architecture, theoretical analysis, simulations, and experiments are conducted. The crosstalk suppression performance of MSK modulation is first explained through theoretical evaluation. Subsequent simulations are conducted based on available experimental conditions, comparing the MSK scheme with non-coded and classical binary phase shift keying (BPSK) schemes. The clearer spectral peaks and higher precision in wind velocity estimation further demonstrate the low crosstalk characteristic of MSK-modulated signals. In the experiments, a comparative measurement is conducted between a 63-bit MSK-coded pulse and a non-coded pulse with a duration of 300 ns to validate the effectiveness of the MSK coding scheme. Additionally, the MSK scheme is compared with the BPSK scheme under the same conditions to prove the superior performance of the MSK scheme.

The simulation results demonstrate that the MSK-modulated pulse offers better frequency estimation performance than the other two pulses due to its effective crosstalk suppression (Fig. 4). To evaluate the precision and accuracy of wind velocity estimation, the standard deviation (SD) and root mean square error (RMSE) are calculated for different modulation schemes. As a result, the MSK-modulated scheme not only has an advantage in range resolution over the 300 ns non-coded pulse, but also achieves higher wind velocity estimation precision, accuracy, and a longer reliable detection range compared with both the non-coded pulse and the BPSK modulation (Fig. 5). In the experimental measurements, the superiority of MSK modulation is further demonstrated. Compared to the BPSK-modulated pulse, the MSK-modulated pulse provides more stable wind velocity estimates in regions with significant velocity variation, which results in smaller velocity SD across multiple measurements (Fig. 8). Especially, 3 m range resolution and 0.20 m/s wind velocity precision within a 450 m detection range are achieved in the MSK modulation scheme, using a pulse peak power of only 20 W. Despite the promising results, there is still room for improvement in the current system. The reflection of the optical antenna directly causes a detection blind zone because of the deployment of a monostatic transceiver configuration. Therefore, in applications where the blind zone needs to be minimized, a bistatic system with separate antennas for transmission and reception should be considered. Furthermore, future work will aim to optimize the telescope diameter and receiver efficiency to extend the detection range.

We introduce a novel MSK-modulated CDWL system that effectively resolves the trade-off between range and frequency resolutions of pulsed CDWL. Due to its crosstalk suppression performance, a longer pulse duration can be applied to wind velocity measurements. Therefore, the signal-to-noise ratio gain provided by the long-coded pulse reduces the reliance on high peak power in pulsed CDWL systems. Both simulation and experimental results consistently show that the MSK scheme, with its superior crosstalk suppression, outperforms BPSK in terms of wind velocity measurement precision and detection range under the same conditions. Moreover, thanks to its phase continuity, the proposed scheme requires a lower bandwidth, which allows simplification of the CDWL system architecture. Given an optimized peak power and optical antenna telescope size, MSK modulation can fully exploit its potential at extended detection ranges, offering a promising approach to enhancing range resolution and velocity measurement precision in pulsed CDWL systems.