Shigeyuki Seikai

Fiber‐optic logic gate

Operation of a fiber‐optic logic gate is demonstrated for the first time. The principle of operation is based upon the intensity‐dependent polarization rotation in birefringent fibers. Using a birefringent fiber combined with polarizers in the crossed state, the ON/OFF state of an AND operation is clearly observed with an extinction ratio of approximately 10 dB. The fiber‐optic logic gate would be a promising device for optical information processing in the future because of the feasibility of compact configuration and very fast‐speed operation.

Optical sampling using an all‐fiber optical Kerr shutter

An all‐fiber optical Kerr shutter consisting of a short single‐mode fiber and a highly birefringent fiber which serves not only as a polarizer but also as a wavelength filter to reject pump light is successfully realized. By utilizing the Kerr shutter, optical sampling is demonstrated. Time‐varying signal light from a laser diode at λ=0.84 μm is sampled into a train of pulses through the Kerr shutter which is driven by an intense light pulse at λ=1.064 μm. The applications to high‐speed signal processing and the generation of picosecond optical pulse are promising because of the fiber’s fast response of optical Kerr effect.

Arc-fusion splicing of single-mode fibers. 1: Optimum splice conditions.

The optimum conditions for arc-fusion splicing of single-mode fibers with core eccentricity of a few microns are investigated in detail. A narrow, quick fusion method, where the fiber-fused region is restricted by the narrowing of the electrode gap and the arc-discharge time is short compared with the conventional multi-mode fiber splice, is presented that will reduce the surface tension effect. The optimum values of electrode gap, prefusion time, and discharge duration are found to be 0.7 mm, 0.2 sec, and 1 sec, respectively, when the discharge current is 18 mA. The optimum pressing stroke of a fiber after the fiber end faces contact each other is determined to be 20 microm. Splice loss <0.1 dB is achieved in the present conditions for fibers with 2-microm core eccentricity and +/-3-microm o.d. discrepancy.

Dependence of Raman gain on relative index difference for GeO(2)-doped single-mode fibers.

Raman gain has been measured for five single-mode fibers with different relative index differences up to 1.0%. Using an experimental equation relating the mode-field radius to the relative index difference, a simple and practical relation between the Raman gain and the relative index difference has been established. The Raman-gain constant for pure SiO(2) has been evaluated to be 7.4 x 10(-14) m/W.

Arc-fusion splicing of single-mode fibers. 2: A practical splice machine.

A new splice machine for single-mode fiber has been developed. Alignment mechanisms are constructed for high-precision alignment of the core axis. Alignment precision of +/-0.1 microm is possible with a movable stroke of +/-10 microm, which enables low-loss splicing even for fibers with relatively large core eccentricity. An average 0.08-dB splice loss and 0.04-dB standard deviation were obtained in a laboratory experiment. A 76% splice loss probability of Delta0.1 dB was achieved. Effect of fiber end face inclination on splice loss was investigated experimentally and theoretically, and it was found that the experimental splice loss is explained by the loss due to end face inclination, core metamorphosis, and core axis misalignment.

Theoretical limit of repeater spacing in an optical transmission line utilizing Raman amplification

The limit of repeater spacing is investigated theoretically for optical transmission lines utilizing stimulated Raman scattering to amplify the signal light. Achievable repeater spacing is numerically estimated on the basis of coupled power equations and measured fiber characteristics for various signal wavelengths and relative index differences. Three types of transmission line configurations are considered, i.e., utilization of forward only, backward only, and bidirectional amplification. In the third case, a transmission distance of more than 400 km is predicted for an input signal light power of 100 μW, a signal wavelength of 1.57 μm, and a pump power of 0.5 W using a fiber with a relative index difference of 1 percent for signal light amplification.

Thermal characteristics of optical pulse transit time delay and fiber strain in a single-mode optical fiber cable

Thermal characteristics of optical pulse transit time delay and fiber strain in a single-mode optical fiber cable are investigated theoretically and experimentally. Measurements of the transit time delay shift are made by a spatial interference technique using a 1.5-in long fiber, six-fiber unit, and cable. Experimental results for a jacketed fiber whose fiber axis is well centered in nylon coating are in good agreement with those predicted from the theory. A jacketed fiber whose fiber axis is positioned eccentrically from the jacket center exhibits a small change in fiber strain at low temperature due to fiber buckling compared with that for the well-centered jacketed fiber. The loss increase for the off-centered jacketed fiber is explained by the buckling model. Furthermore, thermal characteristics of the unit-type cable examined here are found to coincide with those for the constituent six-fiber unit.

Raman amplification in 1.4–1.5-μm spectral region in polarization-preserving optical fibers

Saturation characteristics, polarization, and wavelength dependences of the stimulated Raman gain in polarization-preserving fibers have been studied to construct an active transmission line. In a backward-pump configuration, a Raman gain coefficient of 1.1 × 10−11 cm/W and a gain as high as 25 dB have been attained with a combination of a 1.34-μm pump and a 1.42-μm signal pulse, where the pump power is 35 W and the interaction length is 100 m. A 3-dB Raman gain at 1.51 μm has also been obtained under the same conditions. It was also found that there is a nonlinear polarization mode coupling that couples the vertical axis to the pump axis. This type of nonlinear coupling seems to be large when the linear mode-coupling coefficient of the fiber is large.

Stress‐induced frequency tuning for stimulated four‐photon mixing in a birefringent single‐mode fiber

In stimulated four‐photon mixing using a stress‐induced birefringent fiber, the frequency shift of the stimulated emission has been tuned by changing the modal birefringence with an external force applied to the fiber. The frequency shift tuning with the range up to 140 cm−1 has been observed for an external stress of 0.3 kg/cm in the test stress‐induced birefringent fiber.

65-femtosecond pulse generation from a synchronously pumped dye laser without a colliding-pulse mode-locking technique

Optical pulses as short as 65 fsec have been successfully generated from a synchronously pumped dye laser that does not employ a colliding pulse mode-locking technique. The laser consists of a linear cavity with two dye jets (a gain jet with R6G and a saturable absorber jet with DODCI and DQOCI) and a four-Brewster-angled-prism sequence. A spatial filter between the pairs of prisms plays an important role in achieving saturable-absorber mode locking around 615 nm. The pulse has a spectral width of 8.0 nm, and the average power is 100 mW at a repetition rate of 76 MHz for a pump power of 1.2 W at 532 nm.