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.
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.
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.
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.
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.
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