We report on an ultra-long range optical frequency domain reflectometry (OFDR) using a coherence-enhanced highly linear frequency-swept fiber laser source based on an optoelectronic phase-locked loop (OPLL). The frequency-swept fiber laser is locked to an all-fiber-based Mach-Zehnder interferometer (MZI) to suppress sweep nonlinearity and enhance the laser coherence, leading to a high coherence linear frequency sweep of 1 GHz in the duration time of 25 ms. This enables the OFDR to realize an ultra-long range measurement with a high spatial resolution. As a result, we obtain a 10 cm transform-limited spatial resolution at a 20 km fiber within 25 ms measurement time, and a 72 cm spatial resolution over an entire 200 km fiber link within 5 ms measurement time. The proposed reflectometry provides a high-performance solution with both high spatial resolution and ultra-long measurement range for field real-time fiber network monitoring and sensing applications.

We present optical frequency-domain reflectometry with high performance in terms of an ultralong range window, high spatial resolution, and precision enabled by a high-order optical phase-locked loop (OPLL)-assisted commercial fiber laser. Due to elaborate loop designs that allow matching with the nonlinear frequency modulation response of the laser, the potential tuning capability can be fully exploited, leading to efficient optimization of the frequency chirp range and rate in addition to chirp linearization and an enhancement of the dynamic coherence. A nearly Fourier-transform-limited spatial resolution is realized over multiples of the intrinsic coherence length. Practically, a cm-level spatial resolution is demonstrated along the entire fiber link. We achieved a spatial resolution of ~3.2 cm and ~4.3 cm for backscatter no more than 130 km and reflections at 242 km, respectively, corresponding to a high range resolution factor of up to 1.78 × 10 ^-7 in the latter case. The precision of the backscatter measurement is verified to be at least 0.5 dB throughout the entire measurement range. Moreover, the limiting factors resulting in a degeneration of the spatial resolution are carefully investigated. The proposed approach can be readily applied to other lasers, offering a powerful tool for a variety of applications.

We investigate the performance of coherent free-space optical (FSO) system in terms of bit error rate (BER) evaluation by adopting the modified Rician distribution based coherent channel model which allows taking into consideration the composite effects of both Rician turbulence, including amplitude fluctuation and optical phase distortion, and pointing errors (PEs). By expanding the Rician distribution, a mathematically traceable expression of the probability density function (PDF) for the composite channel is derived in the form of the Meijer-G function. Based on the composite channel PDF, the exact BER expression is obtained, allowing the analysis of BER performance for single-input single-output (SISO) links. This analysis is extended to single-input multi-output (SIMO) links with maximal ratio combining (MRC). With the help of the moment generating function (MGF), the exact BER expression can be simplified into a single integral, facilitating the analysis with high accuracy and reducing calculation complexity. Engineering insights including high-SNR approximated channel PDF, asymptotic BER expression, coding and diversity gains, are investigated and cross-validated for both SISO and SIMO links. Through both analytical and numerical verifications, the impairment due to PEs as well as the effect of modal compensation on the BER performance are discussed in detail, unveiling the fact that their inner relations should be taken into account for optimization. These verify the effectiveness of our models for both SISO and SIMO links with a wide range of different conditions and can be feasibly applied for different types of coherent FSO links.

We report on the generation of a highly coherent broadband optical linear frequency sweep (LFS) using mode-spacing swept comb and multi-loop composite optical phase-locked loop (OPLL). We exploit a specially designed agile opto-electronic frequency comb as a sweeping reference, whose mode-spacing is capable of arbitrary frequency sweep while preserving a stable phase and power distribution per mode. By locking a continuous-wave (CW) laser to any of its modes using composite OPLL with a large loop bandwidth, it allows the extraction of the optical LFS at high-order modes in a coherent manner with a multiplied sweep range and rate. With such capability, only intermediate frequency LFS with smaller bandwidth is required to yield a broadband LFS while inheriting the coherence and precision from the comb. We achieve optical LFS of 60 GHz at 6 THz/s sweep rate with a nine-folded sweep bandwidth of the driving signal. Fourier transform-limited spatial resolution at more than 80 times of the intrinsic coherence length of the CW laser is demonstrated in an OFMCW interferometry, verifying the high coherence with more than 4 orders of magnitude improvement in spatial resolution. The characteristics in terms of agility, coherence, and precision are discussed together with the potential limitations. The proposed method is capable of generating arbitrary frequency-modulated optical waveforms with a multiplied bandwidth, showing attractive potential in future metrology applications.

A photonic generation scheme of a programmable coherent linear frequency modulated (LFM) signal is proposed and experimentally demonstrated. By heterodyne beating an agile optical frequency comb (OFC) with a fixed OFC, LFM signals in different bands can be obtained. The two OFCs are generated independently from two arms of a single dual-drive Mach-Zehnder modulator (DDMZM) and are then propagated through the same optical path, thus naturally ensuring the coherence of the LFM pulses at the minimum cost. The generation of the agile OFC only requires a sweeping intermediate frequency (IF) signal with the far lower center frequency and smaller bandwidth than the generated LFM signal. And by precisely controlling this IF signal, parameters of the LFM signal can be flexibly adjusted such as the center frequency, bandwidth, time duty, and sweeping pattern. In addition, envelope-tailorable LFM pulsed signals are further realized by pre-distorting the IF signal, supporting future multifunctional radar applications. We then establish a complete X-band radar system. The pulse-to-pulse phase coherent integration of the generated LFM signal enhances the signal-to-noise ratio of echo signals by ∼24 dB. A 5.8 cm range resolution is finally achieved, which is comparable to more complex photonic radar systems. This compact DDMZM based scheme not only improves the radar performance by realizing coherence pulses but also provides full control of these pulses, evidently benefiting the system integration and multifunctionality in future advanced radar applications.

Coherent fading has long been regarded as a critical issue in phase-sensitive optical frequency domain reflectometry ( $\varphi $ -OFDR) based distributed fiber-optic sensing. Here, we report on an approach for fading noise suppression in $\varphi $ -OFDR using multi-core fiber (MCF). By exploiting the independent nature of the randomness in the distribution of the refractive index in each of the cores, the phase fluctuations due to the fading phenomena can be effectively alleviated by applying a weighted vectorial averaging for the Rayleigh backscattering (RBS) traces obtained from each of the cores with their distinct fading distribution. The proposed $\varphi $ -OFDR with a commercial seven-core fiber has achieved quantitative distributed vibration sensing with ~2.2 nm length precision and 2 cm sensing resolution along a 500 m MCF, corresponding to a resolution-to-range factor as high as ${\sim }4\times 10^{-5}$ . Featuring long distance, high sensitivity, high spatial resolution, and fading robustness, it shows promising potential in various sensing techniques for a wide range of practical scenarios.