Yi Dong

Frequency noise of frequency-swept laser fundamentally governs the performance of optical frequency-modulated continuous-wave (OFMCW) interferometry based technologies. Here, we proposed general and versatile analytic model capable of throughout analyzing and accurately predicting the performance of OFMCW system for lasers without assuming specific analytical forms of frequency noise spectrum (FNS). Compared to conventional noise models that rely on specific analytical expression or empirical parameter fitting, this approach establishes a direct link between the laser FNS and the performance of OFMCW system. It allows for highly accurate prediction of the beat note spectrum regardless of laser FNS characteristics with a computational complexity of O(NlogN), and thus enables assessing the impact of laser FNS on key performance metrics such as carrier-to-noise ratio, effective distance, and spatial resolution. Both theoretical and experimental demonstrations for laser with irregular FNS by incorporating optical phase-locked loop (OPLL) have validated an excellent agreement between the predictions and actual measurements with an average R-square of 0.9910. For a practical usage, the analysis permitted by this model reveals the dominance of low-frequency noise, confirming the important role of OPLL in improving the performance. With the verified accuracy, versatility, efficiency, and reliability, the proposed model provides a comprehensive and practical tool for design, characterization, and optimization of OFMCW systems.

In distributed interferometric antenna arrays, accurate measurement and long-term stabilization of transmission delay are essential for coherent combining and accurate target localization. This paper presents a fiber-based delay measurement and stabilization system. The employed dual-tone probe enables accurate measurement of delay variations and the absolute delay simultaneously, while a phase-locked loop provides long-term delay stabilization. Experimental results demonstrate a delay measurement accuracy better than 20 fs and a stabilized delay fluctuation of 15.7 fs (standard deviation) over a 45 km fiber link for 3600 s. The proposed system realizes highly stable microwave transmission with accurately measured absolute delay, making it well-suited for coherent combining and localization in distributed antenna arrays.

The modified Gerchberg-Saxton (GS) algorithm provides an effective solution for electrical dispersion pre-compensation (pre-EDC) in short reach intensity modulation and direct detection (IM/DD) optical fiber transmission systems. To further enhance its performance under bandwidth-limited IM/DD systems, a joint bandwidth limitation and electrical dispersion pre-compensation (pre-BL-EDC) scheme is proposed. In this scheme, an equalization filter for bandwidth limitation (BL) is integrated into the modified GS pre-EDC, and a 2-tap finite impulse response (FIR) filter is introduced to improve the pre-compensation performance. In this way, simultaneous pre-compensation of BL and chromatic dispersion (CD) can be achieved at the transmitter, eliminating the need for any post-equalization at the receiver. Simulation results in a C-band 56 Gbit/s on-off keying (OOK) system demonstrate that the pre-BL-EDC can improve the bit error ratio (BER) performance and reduce device bandwidth requirements. Under back-to-back (BTB) transmission, bandwidth limitation pre-compensation (pre-BLC) improves receiver sensitivity by 2.2 dB, 1.8 dB, and 1.4 dB at the hard-decision forward error correction (HD-FEC) threshold for 24 GHz, 32 GHz, and 40 GHz bandwidths, respectively, compared to without pre-compensation. Under 80 km transmission, the receiver sensitivity of pre-BL-EDC is 3.2 dB, 2.8 dB and 1.0 dB higher than that of modified GS pre-EDC for the above corresponding bandwidths. Moreover, pre-BL-EDC allows for longer transmission distances in bandwidth-limited systems. Taking 32 GHz bandwidth as an example, for transmission distances ranging from 10 km to 100 km, the BER can be below the HD-FEC threshold using pre-BL-EDC, whereas it cannot be achieved using modified GS pre-EDC. The proposed scheme is further validated by an experimental system with 3-dB bandwidth of 9 GHz at different baud rates over a 50 km transmission. For OOK signals at 20 Gbit/s, 28 Gbit/s and 32 Gbit/s, the receiver sensitivity gains of pre-BL-EDC compared to modified GS pre-EDC are 1.0 dB, 1.0 dB and 2.0 dB at the HD-FEC threshold, respectively.

Conventional cross-correlation is commonly used in the estimation of the spectral shift for optical frequency-domain reflectometry (OFDR) based distributed sensing. However, the potential errors in cases of relatively large shifts have severely limited the sensing dynamic range. We report a modified longest common substring algorithm that exploits two-dimensional image processing for more accurate shift estimation. Theoretical study for the conventional cross-correlation is conducted, which reveals quantitatively the underlying mechanism for the emergence of the correlation errors. By adopting the relative error function as the similarity function in connection with the binarization and normalized projection, it allows for a reduction for the possibility of large estimation error in a two-dimensional image processing manner, leading to a substantial improvement in the sensing range. Experimental demonstrations have verified a remarkable enhancement of about 18.7 times in strain sensing range, which accounts for ∼56.2% of the effective sweep range. The proposed algorithm not only permits opportunities in OFDR based distributed sensing, but also promises enhanced fidelity for applications where cross-correlation is concerned.

As a fundamental limiting factor in the space division multiplexing (SDM) system based on weakly coupled multi-core fiber (MCF), inter-core crosstalk (IC-XT) requires measuring conveniently and efficiently during MCF deployment. In this paper, we propose an effective IC-XT measurement method based on optical time domain reflectometer (OTDR). By connecting the designed bridge single-mode fibers (SMFs) structure at the beginning end of the MCF and launching optical pulse signal into one core under test at the far end, multiple loopback IC-XT peaks can be read on the OTDR, from which the IC-XT levels from the core under test to all other cores can be simultaneously calculated. Two bridge schemes for IC-XT measurement are proposed and experimentally verified on a 41 km 7-core fiber. The first scheme supports bidirectional IC-XT measurement, while the second performs better in the OTDR dynamic range and computational complexity. Both schemes demonstrate good measurement accuracy, with < 1 dB of errors. We believe that the proposed method will certainly provide convenient and efficient IC-XT evaluation during future MCF deployment.

Measurement range, as an important performance metric in distributed fiber-optic sensing, is inherently limited due to the ±π range in most of the phase demodulation based sensing techniques. In this work, we report a dual-wavelength phase-sensitive optical frequency-domain reflectometry (ϕ-OFDR) that permits an enhanced measurement range based on direct laser sweep synchronization. By exploiting the broadband frequency sensitivity, a common unbalanced Mach-Zehnder interferometer is shared by two individual lasers to extract the frequency errors during their respective frequency sweeps regardless of their frequency interval. Based on this, common-referenced optical phase-locked loops for each of the lasers are established, allowing synchronizing the two sweeps at arbitrary frequency interval. By constructing virtually a synthetic probe of a longer wavelength, equivalently an enhanced phase demodulation range is achieved for the phase difference of the two lasers. This thus enables an enhancement in the measurement range using direct frequency-swept lasers with a significant reduction of system complexity. Demonstrations based on direct modulation fiber lasers testified the permitted enhancement in strain measurement range as large as ∼190 times. The performance such as the precision has been assessed at different intervals along the distance. This method provides an efficient, cost-effective, and compact implementation of ϕ-OFDR based distributed sensing with an enhanced measurement range from a practical point of view.

Fourier transform-limited spatial resolution (TLSR) is regarded as a fundamental limit in optical frequency-modulated continuous-wave (OFMCW) systems. Here, we report on a spatial resolution enhanced ranging method by using a pulse-tailored OFMCW generated by cutting the traditional OFMCW signal with optical pulse trains. Taking advantage of the narrow full-width-at-half-maximum for the auto-correlation of the pulses, a finer spatial resolution can be achieved by scanning the delay of the reference arm while taking the spectral power at the original beat frequencies. The featuring low detection bandwidth inherited from the OFMCW system is preserved. Theories and numerical modelling are firstly provided, and followed by experiments that have efficiently demonstrated a significant improvement of more than 10 times in the achieved spatial resolution compared to the theoretical TLSR in traditional OFMCW systems, even at long distance. Finally, some factors that may affect the measurements have been discussed. The testified spatial resolution enhancement is found quite appealing in various ranging scenarios.

We report on an interval-locked dual-frequency phase-sensitive optical frequency-domain reflectometry relying on a common-reference optical phase-locked loop. With a shared unbalanced interferometry, this design allows for synchronizing the frequency drift of two lasers, leading to a steadily stabilized dual frequency with an arbitrary interval. Equivalently to a longer synthetic wavelength, their phase difference is utilized to demodulate the ambient changes of interest with an enhanced dynamic range and long-term stability. With a stabilized interval of 1 THz, it allows for an enhancement in a strain measurement range of up to 193-fold in theory. Demonstration in terms of distributed strain sensing covering a distance of 500 m with a 10 cm spatial resolution has been verified, showing a significant extension in the achievable strain dynamic range with a preserved sensitivity over 1 h.

We demonstrate an ultra-stable wideband signal dissemination scheme suitable for distributed systems. By inserting a 26 GHz probe signal whose round-trip delay variation is precisely measured with double optical mixing, the link length variation has been accurately detected. Assisted with a well-designed homodyne phase locked loop, the probe has time stability on the order of femtosecond for hours and fractional frequency stability of 8.3×10 ^-18 at 1000 s averaging time. Experimentally, a tunable single-frequency signal ranging from 8 GHz to 12 GHz is disseminated from a local end to 2 remote ends via 20 km fiber links. The root-mean-square delay jitters are between 10 to 20 femtoseconds within an hour, indicating exceptional stability. Additionally, an X-band 500 Mbps quadrature phase shift keying signal is also tested as a true wideband signal. The relative delay jitter is 1.88 picoseconds, which is close to the measurement noise floor. The proposed wideband signal dissemination scheme is highly desirable for distributed systems that require high stability and strict coherence.