Study of Frequency Transfer via Optical Fiber in the Microwave Domain

Technical issues are investigated for a precise frequency transfer system using two-way signals by wavelength division multiplexing (WDM) in a single fiber. Bi-directional optical amplifiers are necessary to make the distance longer. Frequency stability is shown in the tandem optical amplifier link where the amplified spontaneous emission (ASE) noises are accumulated. Increasing transmission speed is effective for improving the system performance; however, chromatic dispersion of the fiber degrades the frequency stability significantly in an experiment with 10 GHz signal and 50-km fiber. The degradation can be improved by using 1550 nm zero-dispersion-shifted fiber (DSF) instead of SMF. Effects of polarization mode dispersion (PMD) and polarization scrambling are experimentally shown with a differential group delay (DGD) generator. It is also important to use stable oscillators for stability evaluation, since the time difference between the original and the received signal at the far end degrades the performance if the phase noise of the OSC source is not small enough. INTRODUCTION Time and frequency standards are widely used in a broad area of applications in industry, science, navigation, and telecommunications. In accordance with the progress of next generation frequency standards, excellent performances of frequency transfer systems using optical fiber have been demonstrated by many groups (for example, [1-8]). For instance, it is reported that the frequency stability of 10 -15 at 1 s and 10 -18 at a 1-day averaging time for optical link in the microwave domain [6]. This paper describes our transfer system configurations with a newly developed bi-directional amplifier and technical items to be solved for the systems. The experimental results are shown in terms of several factors, such as input optical power to photo-detectors, transmission frequencies, fiber types, stability of the source oscillator (OSC) for improving frequency stability, and precise evaluation. 41 st Annual Precise Time and Time Interval (PTTI) Meeting 46 SYSTEM CONFIGURATIONS The systems for both frequency comparison and distribution use bi-directionally transmitting densewavelength-division-multiplexed (D-WDM) signals along a single fiber. A continuous wave (CW) light source in 1550 nm region is directly or externally modulated with microwave reference signals. The wavelength separation of the two signals is 0.8 nm (i.e. 100 GHz), which is compatible with values in the recommendation by the International Telecommunication Union and is widely used in recent telecommunication networks. The light signal is directly detected by a photodiode. The comparison system should be capable of long-distance transfer between two frequency standards. The distance will be several hundred kilometers for regional applications and 10,000 km in the ultimate sense for international applications. Round trip time becomes 0.1 s in that case and this makes it difficult to realize a precise phase compensation system. In our system, phase comparisons of the received signals and the frequency standards at each terminal are used for frequency transfer without the phase compensation (offline data processing). As for frequency distribution system, realtime use is a must, since the signal is used as a timing signal for particle accelerators or radio astronomy applications. The distance is 50 km at most in those applications. TECHNICAL ISSUES FOR THE SYSTEM There are technical items to be solved for the precise transfer systems. We show investigated results about bi-directional amplifiers to compensate the fiber loss for long-distance transfer, accumulated noise effects from the tandem optical amplifiers, system performance improvement by high-speed signal, degradation caused by fiber chromatic dispersion, and polarization mode dispersion. BI-DIRECTIONAL OPTICAL AMPLIFIER AND THE NOISE EFFECT The developed optical amplifier has an optical isolator in each two-way channel divided by wavelength filters to suppress the optical reflection that causes amplification instability. The additional insertion optical loss due to this method is about 1.5 dB. The optical gain greater than 30 dB is obtained for both signals, with a good optical isolation of 65 dB. The loss of the long-distance fiber can be compensated by the tandem amplifiers; however, amplified spontaneous emission (ASE) noise, as shown in Fig. 1, from each erbium-doped fiber amplifier (EDFA) degrades the received signal-to-noise ratio. The Allan deviation at averaging time (τ) of 1 s is shown in Fig. 2 (dashed lines) for K = 1, 10, and 100 at 10 MHz and 10 GHz, respectively (K is the total number of amplifiers). The optical amplifier gain is 20 dB, with a noise figure of 6 dB in this calculation. The optical filter bandwidth, Δf, is set to 100 GHz so as to make 10-GHz signal transmission possible. The Allan deviation improves as the input power is increased, as shown in Fig. 2; however, the slope approaches -1/2 in the log-log plot in the high input power region (e.g., over about 0 dBm for K = 1), because the Signal-ASE beat noise becomes dominant. It is the interference (beat) noise between the signal light component and the ASE noise. As K is increased, the Allan deviation degrades, as shown in Fig. 2. The figure shows the Allan deviation on the order of 10 -15 at τ = 1 s is possible for 10 GHz and K = 100 when the optical averaged input power is over -20 dBm or so. A hundred optical amplifiers (K = 100) can make a 10,000 km transfer with an amplifier spacing of 100 km. 41 st Annual Precise Time and Time Interval (PTTI) Meeting 47 SHORT-TERM STABILITY IMPROVEMENT BY HIGH -SPEED SIGNAL As already pointed in [1], increasing the modulation signal frequency f0 as well as increasing SNR is effective to improve the short-term stability. Figure 3 shows the calculated stability taking account of the signal shot noise and the thermal noise of the receiver. The ASE noise is not considered here. The experimental results were obtained in a setup by an optical transmitter to a detector with a variable attenuator instead of a long fiber (the fiber effect is not considered). If f0 is 100 MHz, the Allan deviation on the order of 10 -14 at τ = 1 s is possible if the input power is around -10 dBm, as shown in Fig. 3. It becomes 2×10 -15 (+5 dBm input power) for a 1-GHz signal. The value was improved to 8×10 -16 (+3 dBm) by using a 10-GHz signal. The measured values were worse than the calculated values in the high-frequency region. The possible reasons for this are measurement system noise limit (4×10 -16 for 1 GHz, 1×10 -16 for 10 GHz at τ = 1 s), not enough modulation depth, Fig. 2. Frequency stability at an averaging time of 1 s in the tandem optical amplifier system. Optical Averaged Input Power (dBm)‏ A ll a n D ev ia ti o n , σ y ( τ ), τ = 1 s -30 -20 -10 0 +10 +20 Calculated f0=10 MHz With optical amplifier 1 10 =100 K=0 K=0 =0 1 10 =100 10