Sub-terahertz photonic frequency divider with a large division ratio based on phase locking.

We present a photonic frequency divider with a large division ratio for microwave signals up to sub-terahertz. A high-operating frequency and a large frequency division ratio have both been achieved by phase-locking a Fabry-Perot frequency comb to the input signal that is to be divided. The input signals ranging from 50.10 GHz to 200.10 GHz are all divided to 2.5 GHz signals, which can be further divided into lower- frequency signals easily. The proposed divider is free of high-speed electrical devices, thanks to the intermediate-frequency detection and feedback control in the phase locking process. Moreover, the phase noise caused by the photonic frequency division is negligible at low offset frequencies, proving that the divider has superior long-term stability. This flexible, cost-efficient, and stable photonic frequency divider is an ideal candidate for frequency division at the remote end of a high-precision frequency transfer system.

Theoretical analysis for fiber-optic distribution of RF signals based on phase-locked loop.

We establish an analytical model for the stable dissemination of radio-frequency (RF) signals via fiber-optic links. Based on the phase-locked loop theory, the contributions from the photonic RF source, transmission-path, and additional system noise have been taken into account, leading to the quantitative analysis of the phase noise evolution in the transmission link. Furthermore, the theoretical analysis reveals the relation between the system instability and the frequency of the transmitted signal, which is further verified. Assisted with the proposed model, the optimization for stabilized dissemination of RF signals with a certain length of transmission link or any specified noise floors can be achieved with minimized timing jitter performance, testifying the potential high stability obtained thanks to the higher transmitted signal frequencies. This quantitative model, enabling precise prediction of the frequency instability and timing jitter from the residual phase noise, can be a useful guide in designing a fiber-optic distribution system and evaluating its fundamental limits.

Distribution of optical-comb-based multi-frequency microwave signals over 100†km optical fiber with high phase stability.

We demonstrate a long-distance multi-frequency microwave distribution system over an optical fiber link with high phase stability based on transferring an optical frequency comb (OFC). The phase fluctuation induced by the transmission link variations is detected by applying a reference OFC and is then compensated with the proposed optical voltage-controlled oscillator (OVCO) by adjusting the phase of the repetition rate of the transmitted OFC. By applying the OVCO, we perform the OFC-based multi-frequency microwave distribution over a 100 km standard single-mode fiber. The performance of the transmission system can be exhibited by evaluating the repetition rate (10.015 GHz) and second harmonic frequency (20.03 GHz) signals achieved at the remote end. The residual phase noise of the 10.015 GHz and 20.03 GHz signal is -64 dBc/Hz and -58 dBc/Hz at 1 Hz frequency offset from the carrier, respectively. The fractional frequency instability is 1.4×10-16 and 2.4×10-16 at 10000 s averaging time, respectively. And the timing jitter in the frequency range from 0.01 Hz to 1 MHz reaches 88 fs and 87 fs, respectively. Based on the phase-locked loop theory, we conduct a simulation model of the transmission system and the simulated results match well with experiments. It shows that by detecting the phase fluctuation with higher harmonic frequency signals in the simulation system, the performance of the transmission system can be further improved.

Ultra-long range optical frequency domain reflectometry using a coherence-enhanced highly linear frequency-swept fiber laser source.

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.

Photonics-based coherent wideband linear frequency modulation pulsed signal generation.

A photonics-based scheme is proposed to generate wideband linear frequency modulation pulses with broadly tunable carrier frequencies for coherent radars. The approach integrates the concept of the microwave-photonic multiplication and coherent beating to enable reconfiguration of the bandwidth and carrier frequency of the generated pulses. The phase fluctuation between two beating arms is suppressed by a stabilization technique based on an optical phase-locked loop to maintain the pulse-to-pulse phase coherence. Further, we also demonstrate a coherent radar system, including the generation, wireless transmission and detection of the radar echo signal in the Ka-band. The coherent integration of several echo signals is achieved. About 8 dB of signal-to-noise ratio improvement is obtained with every ten-fold of integration times. The range resolution of the radar system is ∼3.75 cm, which is close to the theoretical prediction.

Distribution of a phase-stabilized 100.02 GHz millimeter-wave signal over a 160 km optical fiber with 4.1 × 10-17 instability.

We report a long-distance phase-stabilized millimeter-wave distribution over optical fibers, where the optical-link-induced phase noise is compensated with a high-precision photonic-generated millimeter-wave (mm-wave) voltage-controlled oscillator (VCO). The mm-wave VCO is realized based on pre-filtering and re-modulating optical spectral lines of an optical frequency comb (OFC). By adjusting the frequency spacing of the optical spectral lines extracted from the OFC, the phase error of the transmitted optical mm-wave signal can be compensated precisely. Using the mm-wave VCO, we demonstrate a distribution of a 100.02 GHz signal over spooled optical fibers and the fractional frequency instability of the system at different transmission distances is exhibited. The residual phase noise of the remote mm-wave signal after being transferred through a 160-km fiber link is measured to be -59 dBc/Hz at 1 Hz frequency offset from the carrier, and the RMS timing jitter in the frequency range from 0.01 Hz to 1 MHz reaches 62 fs. The long-term fractional frequency instability of 4.1 × 10-17 at 10000 s averaging time is achieved, and the maximum timing drift is within 0.93 ps (peak to peak) during 4 hours.

Photonic generation of phase-stable and wideband chirped microwave signals based on phase-locked dual optical frequency combs.

A photonics-based scheme is presented for generating wideband and phase-stable chirped microwave signals based on two phase-locked combs with fixed and agile repetition rates. By tuning the difference of the two combs' repetition rates and extracting different order comb tones, a wideband linearly frequency-chirped microwave signal with flexible carrier frequency and chirped range is obtained. Owing to the scheme of dual-heterodyne phase transfer and phase-locked loop, extrinsic phase drift and noise induced by the separated optical paths is detected and suppressed efficiently. Linearly frequency-chirped microwave signals from 5 to 15 GHz and 237 to 247 GHz with 30 ms duration are achieved, respectively, contributing to the time-bandwidth product of 3×108. And less than 1.3×10-5 linearity errors (RMS) are also obtained.

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.

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.

Power-area method to precisely estimate laser linewidth from its frequency-noise spectrum.

We proposed a precise and simple method to estimate the laser linewidth from its frequency power spectral density, which is termed as power-area method (PAM). We applied this method to determine the full width at half-maximum (FWHM) of white-frequency noise and flicker-frequency noise, and the error was less than 7%. Then we successfully estimated the FWHM of the beat note of delayed self-homodyne/heterodyne interferometry with this method. Lastly we investigated the selection of loop gain and loop bandwidth using PAM to achieve a better result in linewidth compression with servo-loop control.

Photonic radio-frequency dissemination via optical fiber with high-phase stability.

We demonstrate a photonic radio-frequency transmission system via optical fiber. Optical radio-frequency signal is generated utilizing a Mach-Zehnder modulator based on double-side-band with carrier suppression modulation scheme. The phase error induced by optical fiber transmission is transferred to an intermediate frequency signal by the dual-heterodyne phase error transfer scheme, and then canceled by a phase locked loop. With precise phase compensation, a radio frequency with high-phase stability can be obtained at the remote end. We performed 20.07-GHz radio-frequency transfer over 100-km optical fiber, and achieved residual phase noise of -65 dBc/Hz at 1-Hz offset frequency, and the RMS timing jitter in the frequency range from 0.01 Hz to 1 MHz reaches 110 fs. The long-term frequency stability also achieves 8×10(-17) at 10,000 s averaging time.

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.