Spectral-linewidth broadening in synchronous Raman fiber amplification caused by cross-phase modulation effects and its suppression.

When coherent light is amplified by a synchronous Raman fiber amplifier pumped by output pulses from a high-power erbium-doped fiber amplifier operating in an unsaturated gain regime, the detection efficiency with the heterodyne method is degraded. This degradation is caused mainly by cross-phase modulation effects. It is confirmed that using power-flattened pump pulses can suppress these effects.

Absolute frequency synthesis of pulsed coherent light waves through phase-modulation active optical feedback.

A novel method for the broadband absolute frequency synthesis of pulsed coherent lightwaves is demonstrated. It is based on pulse recirculation around an active optical feedback ring containing a delay-line fiber, an external phase modulator, an acousto-optic frequency shifter (AOFS), and a high-finesse Fabry-Perot étalon. The modulation frequency F(M) and the frequency shift F(AO) that are due to AOFS are designed so that their sum or difference equals the free-spectral range of the étalon and F(AO) is set at larger than the half-width at full maximum of its resonant peaks. If one of the peak frequencies is tuned to the frequency of the initial pulse, the frequency of the recirculating pulse jumps to the next peak for each round trip. In the experiment the absolute frequency is synthesized over a frequency span of 700 GHz around the initial stabilized frequency of the master laser.

Measurement of distributed strain and temperature in a branched optical fiber network by use of Brillouin optical time-domain reflectometry.

A new scheme is proposed for sensing distributed strain and temperature in optical fibers. This scheme uses Brillouin scattering as the sensing mechanism and a branched optical fiber network as the sensing fibers. Brillouin optical time-domain reflectometry makes it possible to distinguish Brillouin-scattered light waves from different optical fiber branches in the network when the different Brillouin frequency shifts are assigned to each branch. The technical feasibility of this scheme is confirmed experimentally for a branched optical network composed of two branches connected to a trunk optical fiber.

Frequency translation of light waves by propagation around an optical ring circuit containing a frequency shifter: II. Theoretical analysis.

Theoretical aspects of the frequency-translation ring circuit are considered through numerical simulations. We analyze the signal and noise propagation around an optical ring circuit that contains a frequency shifter, an erbium-doped fiber amplifier and a bandpass filter (BPF). The relations between the frequency-translation limit and some important parameters such as the BPF bandwidth and the polarization state are clarified. Numerical results for the frequency-translation limit are compared with reported experiments and a frequency translation of more than 100 GHz is predicted.

Frequency-difference stabilization of distributed-feedback laser diodes by use of an optical frequency translation circuit.

Frequency-difference stabilization over a 100-GHz range is demonstrated for two distributed-feedback laser diodes (LD's) by utilizing a loss-compensated optical loop with a frequency shifter. A light wave emitted from a master distributed-feedback LD is modulated into a pulse. The pulse is then circulated around the optical loop to shift its frequency to the difference required between the master and slave distributed-feedback LD's. A continuous-wave pulse from the slave LD is mixed with the frequency-shifted pulse, and their beat frequency is measured with a conventional low-speed heterodyne receiver. The frequency of the slave laser is adjusted by controlling the injection current so that the heterodyne beat frequency is kept constant. The error in the center frequency difference was estimated to be within the submegahertz range for a short period of 10 min.

Frequency translation of light waves by propagation around an optical ring circuit containing a frequency shifter: I. Experiment.

A technique for the external frequency translation of light waves is reported. The technique permits the stepwise sweeping of an optical frequency over a wide range with high linearity with respect to time. The frequency translator is composed of an optical pulse modulator and an optical ring circuit containing an acousto-optic frequency shifter and an optical amplifier. The pulse launched into the ring circuit undergoes a constant frequency shift for each circulation around the circuit and the frequency can be translated to a considerable degree from that of the original input pulse. We report a stepwise frequency translation over approximately 68 GHz for a 1.5-µm light wave with a strictly constant frequency-sweep rate and an approximately constant intensity.

Technique for translating light-wave frequency by using an optical ring circuit containing a frequency shifter.

A new technique for the external frequency translation of light waves is proposed. The technique enables the stepwise sweeping of an optical frequency in time over a wide range. The frequency translator is composed of an optical pulse modulator and an optical ring circuit that contains an acousto-optic frequency shifter and an optical amplifier. The pulse launched into the ring circuit undergoes a frequency shift for each complete trip around the ring circuit, and the frequency is translated considerably from the original input pulse. We confirm a frequency translation of as much as 8 GHz for a 1.5-microm light wave, in which the sweeping is strictly linear with respect to time.