基于高阶边带注入法布里-珀罗激光器的多路线性扫频光源

Optical Source with Multichannel Linearly Swept Frequency Based on High⁃Order Sideband Injected Fabry⁃Perot Laser

基于高阶边带调制和光注入法布里-珀罗(F-P)激光器的波长选择性放大理论,设计并实现了一种具有较宽扫频范围和较窄线宽特性的多路线性扫频光源(SOS)。由窄线宽激光器阵列产生的多个光波经过线性扫频电信号调制后,产生多路高阶边带扫频光信号。利用同步电流补偿,将光信号注入并锁定F-P激光器的多个纵模,经放大边带信号后能获得多路线性扫频光源。实验使用了4个窄线宽激光器,将光信号注入F-P激光器锁定三阶边带,可以同时产生4路线性扫频光源,其扫频范围和扫频速率比一阶边带数值提高了3倍,分别为39.8~44.2 GHz和9.28 THz/s,实现线性度超过99.9992%和线宽低于1 kHz。所提结构可以应用于调频连续波(FMCW)激光雷达和光频域反射(OFDR)分布式光纤传感。

Objective Broadband, narrow-linewidth, and ultralinearly swept optical sources are critical in several applications, including fiber network diagnostics, environmental monitoring, laser radar imaging, and phase-coherent optical communication. Current methods for optical frequency sweeping include the inner-cavity tuning of a tunable laser and the external modulation of a continuous-wave(CW)light wave. One of the most recent solutions involves employing a wavelength-tunable laser array to achieve a wide sweep range.However, the complex internal-cavity tuning control results in sweeping linearly differences and broad linewidths. By contrast,external high-order modulation uses a swept radio frequency(RF) signal combined with a four-wave mixing(FWM) process to achieve ultralinearly frequency sweeps and millimeter resolutions. Nevertheless, phase noise increases in FWM, thus complicating the structure for implementing a multichannel swept source. Herein, we report the development of a multichannel ultralinearly swept optical source(SOS) with broadbands and narrow linewidths based on high-order sideband modulation and an optical injection-locked Fabry-Perot(F-P) laser. This approach enables wide sweep ranges, high resolutions, and multiple channels to be obtained. We hypothesize that the injection-locked F-P laser can provide multichannel frequency sweeps as well as serve as an optical filter and amplifier. The SOS is particularly suitable for precision LiDAR and remote, optical frequency-domain reflectometry(OFDR) systems.Methods A tunable laser array comprising multiple narrow-linewidth tunable lasers converted a frequency-swept RF signal into the optical domain. To expand the optical sweeping range, high-order sidebands were generated by increasing the injected RF power and using low-Vπ intensity modulators(IM). According to high-order modulation theory, the frequency shift Δω of the RF-driven signal corresponded to the frequency shift nΔω of the nth sideband. The injection-locking setup was utilized as an optical filter to prevent two neighboring sidebands from overlapping and to isolate the desired sideband. Furthermore, to generate multiple frequency-swept signals simultaneously, an F-P laser with different modes was used to replace the distributed feedback laser(DFB) laser as a slave laser(SL). Additionally, by controlling the F-P laser drive current with synchronized current compensation, the injection-locking range was increased, thus resulting in a wider frequency sweep. In other words, multiple optical waves from a narrow-linewidth laser array were processed with a chirped RF signal to generate multichannel high-order sideband signals, which can subsequently be enlarged using multiple injection-locking F-P modes. We experimentally generated a four-channel linearly sweeping laser source using a four-channel narrow-linewidth laser source with injection-locking third-order modulation sidebands. The frequency-sweeping range of this scheme can be derived from a fiber asymmetric Mach-Zehnder interferometer.Results and Discussions The triangular sawtooth wave of the pre-compensated signal has the same shape as the arbitrary waveform generator(AWG) output signal, with a period of 5.5 ms and a peak-to-peak value of 120 mV [Fig. 4(a)]. Based on measurements, the current applied to the SL can achieve continuous injection locking with a circuit tuning range of 50-70 mA, a frequency increment of approximately 1 GHz requiring a 1 mA current increment, and a tuning linearity of 0.9998 [Fig. 4(b)]. An example of the four third-order sideband signal injection-locking four longitudinal modes of the custom-developed F-P laser is presented in Fig. 5(d). Considering a center wavelength of 1550 nm as an example, the output power is 6 dBm, and the power ratio between the injection-locked sideband and the neighboring sideband is 30.8 dB, which is more than 18 dB higher than that without injection locking. Within the current adjustment range of 50 mA to 75 mA, a locking range from 0.34 to 0.39 nm for four injectionlocked sidebands is observed(Fig. 6). Based on calculations, the frequency sweep range is 39.8-44.2 GHz, which is approximately thrice that of the RF variation. After filtering, we extract each individually swept signal with a side-mode suppression ratio(SMSR)and a swept-frequency linearity exceeding 32 dB(Fig. 7) and 99.9992%(Fig. 8), respectively.Conclusions Using an optical injection-locked F-Perot laser and external high-order modulation, we designed a novel frequencyswept optical source to be used in multichannel, long-range, and high-resolution measurement systems. The source offers several advantages: 1) the F-P laser features multiple longitudinal modes and uses a synchronous compensation current to maintain stable optical injection locking, which extends the sweep range;2) the nth sideband-modulation technique preserves the linearity of the swept frequency and the stability of the RF signal while increasing the frequency sweep range and speed by a factor of n;3) this scheme provides a simple and effective method without requiring phase-noise compensation or optimization technology;4) the injection-locked SL provides a narrow linewidth that is approximately equal to that of the master laser, which implies a large measurement range. In the future, we plan to develop a modulator with a lower Vπ to minimize the comparatively high injected power of the carrier and firstorder sideband to obtain sidebands exceeding the fifth order. The proposed structure is suitable for frequency-modulated continuouswave(FMCW) laser radars and OFDR-distributed optical fiber sensing.