Weilin Xie

Phase drift cancellation of remote radio frequency transfer using an optoelectronic delay-locked loop.

In this Letter, we propose a phase drift cancellation method for remote radio frequency transfer. Phase fluctuation along the transmission fiber, which is induced by temperature and pressure changes, is measured and compensated by a heterodyne optoelectronic delay-locked loop. The control loop consists of a heterodyne optoelectronic phase detector, a microwave delay module, and the loop filter. We demonstrate the concept by transmitting a 10 GHz microwave frequency over 50 km single-mode fiber, with subpicosecond jitters measured at the remote end.

Distribution of high-stability 10 GHz local oscillator over 100 km optical fiber with accurate phase-correction system

We have developed a radio-frequency local oscillator remote distribution system, which transfers a phase-stabilized 10.03 GHz signal over 100 km optical fiber. The phase noise of the remote signal caused by temperature and mechanical stress variations on the fiber is compensated by a high-precision phase-correction system, which is achieved using a single sideband modulator to transfer the phase correction from intermediate frequency to radio frequency, thus enabling accurate phase control of the 10 GHz signal. The residual phase noise of the remote 10.03 GHz signal is measured to be -70  dBc/Hz at 1 Hz offset, and long-term stability of less than 1×10⁻¹⁶ at 10,000 s averaging time is achieved. Phase error is less than ±0.03π.

Distribution of high-stability 100.04 GHz millimeter wave signal over 60 km optical fiber with fast phase-error-correcting capability

We demonstrate a phase-stabilized remote distribution of 100.04 GHz millimeter wave signal over 60 km optical fiber. The phase error of the remote millimeter wave signal induced by fiber transmission delay variations is detected by dual-heterodyne phase error transfer and corrected with a feedback system based on a fast response acousto-optic frequency shifter. The phase noise within the bandwidth of 300 Hz is effectively suppressed; thus, the fast transmission delay variations can be compensated. The residual phase noise of the remote 100.04 GHz signal reaches -56  dBc/Hz at 1 Hz frequency offset from the carrier, and long-term stability of 1.6×10(-16) at 1000 s averaging time is achieved. The fast phase-noise-correcting capability is evaluated by vibrating part of the transmission fiber link.

Coherence enhancement of a chirped DFB laser for frequency-modulated continuous-wave reflectometry using a composite feedback loop.

We demonstrate efficient coherence enhancement of a chirped distributed feedback (DFB) laser for frequency-modulated continuous-wave (FMCW) reflectometry. Both sweep nonlinearity and broadband stochastic frequency noises during the laser chirp are efficiently suppressed by a composite feedback loop. The residual frequency error relative to a perfect linear chirp is shown to be about 89 kHz for a laser chirp of 50 GHz in 100 ms, compared with 44 MHz with the loop open. The broadband frequency noise suppression of the frequency-swept laser greatly improves its coherence, leading to a higher signal-to-noise ratio and a significantly extended measurement range in FMCW reflectometry ranging. We demonstrate a 2 mm transform-limited spatial resolution at a range window of 50 m and a 17.5 cm spatial resolution at an extended measurement range of 750 m, which is about 15 times the intrinsic laser round-trip coherence length.

Compensation of phase error in optical frequency-domain reflectometry using delay-matched sampling

Abstract. We proposed and experimentally demonstrated a delay-match sampling method to measure and compensate the laser phase error in optical frequency-domain reflectometry system. By using the error signal extracted from a simple auxiliary Mach-Zehnder interferometer with only a 10-ns delay, the laser phase error is effectively compensated. Considerable improvement is achieved in spatial resolution from 200 m to 7 cm at a measurement distance over 10 times the round-trip laser coherence length.

Fast Spectrum Analysis for an OFDR Using the FFT and SCZT Combination Approach

Spectrum analysis is a significant process for many measurement applications which is usually implemented by fast Fourier transform (FFT). Nevertheless, FFT is not suitable to deal with big data because of extra burden of computation. Moreover, FFT fails to provide enough accuracy for signals with a very sparse and broadband spectral distribution. In this letter, we propose a combination approach called FFT-segmented chirp-Z transform that allows to analyze a long-time signal, while the data are received, achieving faster speed, better resolution with only small memory size which shows great potential in real-time performance. With the help of this approach, zoom bands are detected, and optimal parameters are established to guarantee peaks in a broadband spectrum can be found in short time with high precision. We implement this approach in a high spatial resolution optical frequency-domain reflectometry to realize high speed and high precision of components localization in optical fiber. The experimental result shows that 2-mm spatial resolution is achieved at a distance of 54 m and the processing time was less than 2 s for 107 data points.

Dynamic frequency-noise spectrum measurement for a frequency-swept DFB laser with short-delayed self-heterodyne method

We proposed and experimentally demonstrated a short-delayed self-heterodyne method with 15.5m delay to get a large-frequency-range laser frequency-noise spectrum over 10Hz to 50 MHz, and an averaging approach to extract the intrinsic frequency noise of a frequency-swept laser. With these two techniques, dynamic frequency-noise spectrum of a frequency-swept DFB laser when free running and servo-controlled are both measured. This measurement method permits accurate and insightful investigation of laser stability.

Photonic generation of low phase noise arbitrary chirped microwave waveforms with large time-bandwidth product.

We present a photonic approach for generating low phase noise, arbitrary chirped microwave waveforms based on heterodyne beating between high order correlated comb lines extracted from frequency-agile optical frequency comb. Using the dual heterodyne phase transfer scheme, extrinsic phase noises induced by the separate optical paths are efficiently suppressed by 42-dB at 1-Hz offset frequency. Linearly chirped microwave waveforms are achieved within 30-ms temporal duration, contributing to a large time-bandwidth product. The linearity measurement leads to less than 90 kHz RMS frequency error during the entire chirp duration, exhibiting excellent linearity for the microwave and sub-THz waveforms. The capability of generating arbitrary waveforms up to sub-THz band with flexible temporal duration, long repetition period, broad bandwidth, and large time-bandwidth product is investigated and discussed.

Coherent Comb Generation With Continuous Sweep of Repetition Rate Over One-Octave

We proposed and demonstrated a coherent optical frequency comb generation with broadband continuous sweep of repetition rate based on cascaded phase and intensity modulators. Broadband phase matching between electrical drive signals applied on the cascaded phase and intensity modulators is achieved by compensating propagation delay skew between optical signal path and electrical signal path. Therefore, phase mismatch induced flatness deterioration is effectively suppressed. A flat-top 19-line optical frequency comb with repetition rate continuously sweeping over one-octave from 8.5 to 19 GHz is obtained, where the overall power deviation is .

Fourier transform-limited optical frequency-modulated continuous-wave interferometry over several tens of laser coherence lengths.

We report on a versatile optical frequency-modulated continuous-wave interferometry technique that exploits wideband phase locking for generating highly coherent linear laser frequency chirps. This technique is based on an ultra-short delay-unbalanced interferometer, which leads to a large bandwidth, short lock time, and robust operation even in the absence of any isolation from environmental perturbations. In combination with a digital delay-matched phase error compensation, this permits the achievement of a range window about 60 times larger than the intrinsic laser coherence length with a 1.25 mm Fourier transform-limited spatial resolution. The demonstrated configuration can be easily applied to virtually any semiconductor laser.