Precise Frequency Comparison System Using Bidirectional Optical Amplifiers

Precise frequency comparisons are becoming more urgent given the recent rapid progress in next-generation frequency standards. This paper describes a new type of bidirectional optical amplifier that overcomes the fiber loss limits that have prevented accurate frequency comparisons between widely separated places; such comparison is realized by bidirectionally transmitting wavelength-division-multiplexed (WDM) signals along a single fiber. The proposed 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 good optical isolation of 65 dB. A radio-frequency reference signal can directly be sent by simple intensity modulation and direct detection (IM-DD) devices in the 1550-nm region widely used in telecommunication networks. Phase comparisons of the received signals and the frequency standards at each terminal are used for frequency comparisons. The amplifier is tested in the field using two hydrogen masers. A 120-km fiber with loss of 52.5 dB is used to connect the National Metrology Institute of Japan (NMIJ) to the University of Tokyo. Because of this loss, an amplifier is needed to realize sufficient receiving power. The frequency stability of the system with a 10-MHz direct transmission is evaluated by returning the optical signal from a halfway point [55 km from the NMIJ] where the amplifier is installed. The result is 2.6 x 10(-16) (Allan deviation) with the averaging time of 7 x 10(4) s. The laboratory result is 8.7 x 10(-17) (tau = 4 x 10(4) s). The amplifier's long-term stability is promising for stable frequency dissemination in addition to precise frequency comparisons.