基于频移反馈激光器的高精度射频调制激光雷达 - CNRS - Centre national de la recherche scientifique Accéder directement au contenu
Thèse Année : 2019

Research on a high-precision RF-modulated lidar using a frequency-shifted feedback laser

Recherches sur un lidar à modulation RF de haute précision utilisant un laser à contre-réaction décalée en fréquence

基于频移反馈激光器的高精度射频调制激光雷达

Résumé

Conventional lasers rely on multiple passes through a gain medium to reinforce a preselected frequency, thereby obtaining near-monochromatic output. For many purposes, such as optical arbitray waveform generation, it is desirable to have light spread over a broader range of frequencies. One means of fulfilling that objective is to introduce a frequency shifting element into the laser feedback loop such that successive passes of a wavepacket take place with different carrier frequencies. The operation of such a frequency shifting feedback (FSF) laser has been investigated by several research groups [1–46] and it has been used for a number of practical applications, including the ultrahigh repetition rates pulse, optical real-time Fourier transformation, RF-chirped waveform generation. In the early 1970’s, the concept of frequency shifted feedback loop incorporating an active medium was introduced for pulsed lasers as a possibility for electronic frequency control. The prediction was that cavity modes were chirped because of the frequency shift introduced during each transit through the cavity. By satisfying a resonant condition between the applied frequency shift and the loop fundamental frequency, it was possible to generate a pulsed output with a repetition rate equal to the modulator frequency. And a theoretical work has shown the possibility of further increasing the repetition rate by adjusting the frequency shift and the loop fundamental frequency as the ratio of two coprimes. Then some scholars provided an experimental demonstration of the idea by injecting a dye FSF cavity with a dye single-mode seed laser: the generation of 6-ps Fourier-transform-limited pulses was demonstrated with repetition rates tunable by steps of 80 MHz between 0.24 GHz and 36.6 GHz, this is, over two orders of magnitude. Finally, franch scholar H.Guillet de Chatellus explains this result in depth by providing an extensive description of the fractional Talbot effect in cw-seeded FSF lasers. Then we called the resonant condition as the integer Talbot condition and the fractional resonant condition as the fractional Talbot condition. When the modulating frequency is slightly detuned away from the integer Talbot case, the FSF loop seeded by a single CW laser is demonstrated to generate the RF chirp wavform. Based on these unique properties of the FSF laser, I propose some new methods to enhance the accuracy of the velocity and distance measurement by using the frequency shifting loop. Besides, I also do some researches on the dual-sidebands frequency-shifting loop (FSL) and find some originally results, such as pulse doublets, optical real-time Fourier transformation and temporal fractional Talbot effect. In the following, I will present these academic researches systematically. The investigation of the coherent dual-frequency lidar-radar system. We construct a frequency-shift Mach-Zehnder Interferometer (MZI) to generate a fibered dual-frequency laser source and apply it to range measurement. Different from the traditional Mach-Zehnder interferometer in the case of long delay time, we theoretically investigates the beat phase noise under the case that delay length is far shorter than the laser coherent length. The effect of laser phase noise and acousto-optic frequency shifter (AOFS) noise in the beat note phase noise of the dual-frequency laser is theoretically investigated. General expressions are presented for the power spectral density and phase spectral density of the beat note. From the simulations, we find that high AOFS noise introduces a much greater phase jitter into the beat note noise conversion. Based on the theoretical results, an outdoor experiment is conducted to demonstrate the ability of the phase-shift method in the absolute distance measurement and beat note Doppler frequency-shift method in the velocity measurement. By conducting the experiment in the outdoor environment, we finally obtain 8 mm range resolution at a distance of 1.5 km and the velocity resolution is better than 0.5 m/s. The investigation on the RF up-conversion and waveform generation using a frequency-shifting amplifying fiber loop, application to Doppler velocimetry. RF-modulated laser based on the frequency-shifting amplifying loop has the ability of generating high-order harmonics. Here we focus on a direct application of this source: a velocity measurement with different order harmonics. The emitted wave from the frequency-shifting amplifying loop is directed through a fiber coupler with a splitting ratio of 1:99. The 99% output is connected to a fiber circulator, and the other one (1%) is directly detected by photodetector PD2 (3.5 GHz bandwidth) and used as a monitor signal. The light beam emitted by the circulator is collimated by a 10-cm-diameter lens and transmitted to the moving target. The target moves toward and away from the laser in one cycle at a speed ν adjustable from 0.1 m/s to 0.8 m/s. We find (i) a good linear dependency of the frequency shift versus motor velocity, and (ii) the expected n-fold enhancement of the slopes. This measurement confirms that RF up-conversion permits to significantly improve the experimental accuracy at low velocities. The investigation on a novel hybrid TOF/phase-shift method for absolute distance measurement using a falling-edge RF-modulated pulsed laser. Traditional range-finding techniques often take advantage of two methods: pulsed TOF laser ranging and phase-shift laser ranging. The pulsed TOF laser ranging technique is appropriate for long-distance measurement because of the long unambiguous distance and high peak power. The range resolution, which is determined by the pulse width, is typically on the order of meters. In comparison, phase-shift laser ranging offers good precision but exhibits "2π" phase ambiguity, making it suitable for measurements of incremental displacement. To address the limited ambiguous distance, two or more measurement rulers are usually used in the phase-shift rangefinder. Therefore, the TOF method has the characteristic of a long unambiguous distance, and the phase-shift method has the characteristic of a high precision. They are thus complementary in terms of the unambiguous distance and accuracy. Combining the characteristics of the long, unambiguous distance of the pulsed TOF method and the accuracy of the phase-shift measurement, a hybrid TOF/phase-shift method may achieve long-distance and high-accuracy absolute distance measurement. Then we investigate a falling-edge RF-modulated pulsed laser by using a frequency-shifted amplifying loop. A single-frequency fiber laser (seed laser) is coupled into a fiber link through coupler 1. The output port of coupler 1 is sent through an amplifier, followed by an acousto-optic (AO) switch back to the input of coupler 2. The second coupler allows us to extract a fraction of the optical field in the pulsed frequency-shifted amplifying loop. The AO switch is used as a frequency shifter and beam chopper. A Yb3+-doped fiber amplifier is inserted to compensate for the loss in the loop. The RF modulation within the pulse results from the interference of the frequency-shifted pulse with the seed laser. Then we directly use the falling-edge RF-modulated pulsed laser to measure the object distance. To verify the capability for long-range absolute distance measurement, we make a distance measurement inside the fiber by inserting a 2-km fiber in the receiving port. The experimental results show that the TOF/phase-shift method can compensate for the timing error caused by the TOF method and significantly improve the distance precision. Finally, we show experimentally that the hybrid TOF/phase-shift method is a promising technique that achieves 3 mm precision in the 1.5 km measurement range (in fiber), corresponding to a relative precision of 2×10-6. The investigation on the temporal and spectral properties of frequency-shifting loop. Frequency-shifting loops usually rely on the use of an acousto-optic frequency-shifter. While it features high frequency conversion efficiency in the sub-100 MHz range, AOFS have limited efficiency in the GHz range, and offer limited tunability. In this respect, EOM offer much higher modulation frequency and bandwidth. Besides, EOM are compact and easy to integrate with other fibered devices. Thus we propose to investigate an all-fibered frequency-shifted feedback loop when a widely tunable common electro-optic amplitude or phase modulator (EOAM or EOPM) is employed. Instead of the single side-band AO frequency-shift, the EO loop will produce at each round-trip two side-bands with opposite frequency-shifts. The carrier will also circulate together with the multiple frequency-shifted sidebands. In order to predict the time response of the FSL, a general time-delayed interference model is deduced under the cases of the integer Talbot condition and fractional Talbot condition. When the integer Talbot condition is satisfied (f_m⁄f_c =p), the output traces show a periodic pulse. However, different modulators also show some differences. The phase-modulating FSL is demonstrated to achieve the optical real-time Fourier transformation. And the amplitude-modulating FSF loop features double-pulse regime which the interval between the two pulses can be continuously adjusted by changing the static phase retardance and the modulation depth. Besides, when the fractional Talbot condition is satisfied (f_m⁄(f_c=p⁄q)), the time response of the amplitude-modulating FSF loop could observe the temporal fractional Talbot effect, that is a periodic pulse with the repetition rate qf_s=pf_c. Phase-modulating FSF loop features the arbitrary waveform generation due to the destructive interference of higher-order harmonics. Experiments involving the temporal and spectral properties of the amplitude-modulating FSL and the phase-modulating FSL are conducted to demonstrate the above results.
射频调制激光雷达是以射频调制激光作为探测载波,对目标进行测距、测速和其他特征探测的一种激光雷达,其以激光作为探测载波,具有激光雷达空间分辨率高的特点,同时又利用拍频产生的微波信号进行探测,具有微波雷达较强的抗大气干扰能力,是一种结合了激光雷达和微波雷达各自优点,又在一定程度上克服二者不足的一种新型激光雷达。本论文通过分析相干双频激光(一种特殊类型的射频调制激光)的拍频特性,提出了基于多测尺相位测距法和拍频多普勒测速法的相干双频激光探测体制,完成了相干双频激光雷达原理样机的搭建,并进行了外场实验验证。通过详细分析相干双频激光雷达的测距、测速原理,我们发现:1)当目标距离接近或者等于整数倍测尺时,容易发生相位跳变而引起距离测量误差;2)低速目标的超低多普勒频移问题(mHz - Hz)。为了改善上述问题,本文通过分析频移反馈环路的时频特性,创新性地提出了基于频移反馈激光的高阶谐波多普勒测速法和复合式脉冲/相位测距法,探索了射频调制激光雷达在目标速度、距离测量等领域的一些新方法,为这种新体制激光雷达的发展应用奠定了一定的技术基础。除此之外,本论文在详细分析频移反馈环路的基础上,进一步研究了强度调制、相位调制频移反馈激光的时频响应,首次发现了基于电光强度调制频移反馈环路的双脉冲激光、时域分数Talbot效应以及基于电光相位调制频移反馈环路的光域实时傅里叶变换等。本论文的主要研究内容包括以下几方面: (1)开展了相干双频激光雷达系统研究。相干双频激光是利用声光移频器构建移频马赫曾德尔干涉仪(MZI),将单频种子激光调制成相干双频激光源。根据非平衡MZI相位噪声模型、随机过程理论以及高斯白噪声模型,论文详细分析了激光相位噪声、声光射频噪声对相干双频激光拍频的影响,推导了拍频信号的功率谱密度和相位谱密度。理论和实验结果同时表明,在短光纤延时的MZI系统中,射频相位噪声可以直接转换为拍频的相位噪声,是影响拍频噪声的最主要因素。本文通过对相干双频激光雷达系统的工作原理和各组成部分进行深入的分析与研究,搭建起了一套相干双频激光雷达原理样机,并在外场实现了1.5 km目标的8 mm的测量精度,显著提高了外场测量距离。同时,利用相干双频激光外差干涉产生的射频信号实现了移动目标的速度测量,测量精度优于0.5 m/s。 (2)开展了基于频移反馈环路的射频调制激光及其高阶谐波多普勒测速研究。首先,本论文根据声光频移反馈环路的延时、频移特性建立了延迟自外差干涉理论模型,数值仿真了激光环路长度、调制频率对于射频波形的影响,以及放大器增益系数对高阶谐波阶次的影响。其次在实验上,通过精确控制声光调制频率与频移反馈环路长度的关系,时域上产生了脉冲、三角波、方波等波形,频域上观察到了宽谱频率梳。最后根据拍频Doppler测速原理,分别利用15th谐波和基带信号对移动目标进行了多普勒频移测量。实验结果表明高阶谐波能有效改善低速目标的极低多普勒频移问题,显著提高最小可探测速度和速度分辨率。 (3)开展了基于频移反馈环路的射频调制脉冲激光及其高精度距离测量研究。本人提出了基于脉冲下降沿射频调制的复合式脉冲/相位激光测距技术,该方法结合了经典的脉冲测距法(利用脉冲上升沿粗略测量)和相位测距法(利用脉冲下降沿的射频信号精确测量)进行距离测量,有效地提高了脉冲测距的距离分辨率。为了有效地结合时间飞行法和相位测距法,这要求激光源需同时具有脉冲和射频调制特性。在此背景下,本人利用声光斩波器构建频移反馈环路,实现了稳定的脉冲内射频调制。实验上,通过对比分析复合式脉冲/相位激光测距技术和脉冲测距技术在不同回波功率下的测距分辨率,证明了复合式脉冲/相位激光测距技术能够有效补偿不同回波功率带来的测量误差,最终实验上实现了1.5 km(光纤)距离的3 mm分辨率的高精度激光测距。该复合式脉冲/相位测距方法是对相干双频激光测距技术的有效补充,同时对于高精度激光测距的发展也提供了一些理论和实验基础。 (4)开展了频移反馈激光的时频特性研究。本论文在广泛调研常规频移反馈环路的时频特性基础上,根据不同调制类型的频移反馈环路的延时、移频特性,创新性地提出了基于频移反馈环路的通用延时自外差干涉理论模型。在此理论模型基础上,分别从理论和实验上研究了整数Talbot条件和分数Talbot条件下的强度调制和相位调制频移反馈环路的时频特性,提出了基于电光强度调制频移反馈环路的双脉冲激光、时域分数Talbot效应和基于电光相位调制频移反馈环路的光域实时傅里叶变换特性。
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tel-02892043 , version 1 (07-07-2020)

Identifiants

  • HAL Id : tel-02892043 , version 1

Citer

Hongzhi Yang. 基于频移反馈激光器的高精度射频调制激光雷达. Optics / Photonic. Beijing Institute of Technology, 2019. Chinese. ⟨NNT : ⟩. ⟨tel-02892043⟩
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