Yukiyasu Negishi

Active transmission line: light amplification by backward-stimulated Raman scattering in polarization-maintaining optical fiber

The possibility of an active transmission line with a polarization-maintaining optical fiber has been investigated by means of backward-stimulated Raman gain. The fourth Stokes line of the stimulated Raman scattering is placed near 1.3 μm with pumping light of 1.06-μm wavelength, in which Raman gain at 1.30 μm is produced in terms of a strong pump that is due to the third Stokes line at 1.24 μm. A laser diode (InGaAsP/InP) operating at 1.30 μm is used as signal light to meet the Raman gain. As a result, a Raman gain as high as 20 dB and a gain coefficient of 2.0 × 10−12 cm/W have been obtained. It is shown experimentally that it is important to meet exactly the polarization directions between the pump and the signal pulses to obtain a large Raman gain.

Loss increase for optical fibers exposed to hydrogen atmosphere

Loss spectrum changes for optical fibers exposed to a hydrogen atmosphere in the 15-200\deg C temperature range are measured. Loss increase due to molecular hydrogen dissolved into fibers is investigated from the loss peak at 1.24 μm, and that due to hydroxyl group formation from the loss peak at 1.41 μm. The loss increase due to molecular hydrogen is fully explained by physical solubility theory and diffusion equation. The empirical formula for time, temperature, and hydrogen-pressure dependences of the loss increase due to hydroxyl group formation is evaluated from the experimental results. The loss increase at 1.3- and 1.5-μm wavelength band at room temperature are estimated.

Measurements of polarization mode couplings along polarization-maintaining single-mode optical fibers

A new technique for measuring the polarization mode coupling along a polarization-maintaining optical fiber is demonstrated. Additional analyses of optical time-domain reflectometry signals are used. Using a 1.34-μm Nd3+:YAG laser as a light source and an acousto-optical light switch to reduce the Fresnel reflection at the input end of the fiber, we have examined characteristics of the polarization mode couplings for four fibers with different extinction ratios. The extinction ratios evaluated by the present method are in good agreement with those obtained by a conventional technique within ±0.5 dB.

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.

Measurement of polarization mode coupling along a polarization-maintaining optical fiber using a backscattering technique.

A new technique for measuring the polarization mode coupling of a polarization-maintaining optical fiber has been proposed that uses a modified optical time-domain reflectometry in which two kinds of backscattered signals coming through each principal axis of the fiber are utilized. This technique shows how the mode coupling occurs along the fiber. The extinction ratio evaluated by the present technique is in good agreement with that obtained by a conventional technique (within +/-0.5 dB).

Characteristics of dispersion-shifted dual shape core single-mode fibers

The structural design, bending loss, and dispersion characteristics of the dual shape core (DSC) single-mode fiber with zero dispersion at 1.55 μm are described. It is clarified that the bending loss of the DSC fiber is less than one tenth of that of a step-index fiber, and that the dispersion sensitivity due to a small change of the core radius is less than half of that of a step-index fiber. The explanation for the small bending loss is presented. A greater reduction of bending loss is expected from the replacement of the center core profile with a Gaussian or triangular profile.

Performance of optical cable composed of dispersion-shifted single-mode fibers

A low-loss dispersion-shifted single-mode fiber cable has been fabricated. The index-profile of the fiber is nearly parabolic and the relative index difference is 0.8 percent. An average cable loss is 0.229 dB/km at 1.55 μm, and excellent loss stability has been achieved in the cabling process and in the temperature and mechanical test.

Residual elongations of submarine optical-fiber cable laid on the sea bottom

Fiber elongations during laying and residual fiber-elongation strain after laying of submarine optical-fiber cables have been reported. The fiber elongations have been measured by the optical-pulse-delay method. It has been found that the residual fiber-elongation strains are closely related to cable slack and the cable tension at the sea bottom. Therefore, the residual strains can be minimized by controlling the slack and the tension. The relation between necessary proof-test strain and allowed residual strain has also been mentioned. If the proof-test strain is 1 percent, the allowed residual strain due to laying becomes 0.26 percent.

Low temperature characteristics of UV-curable resin coated optical fiber

Transmission characteristics of coated optical fiber depend on coating materials at low temperature. Loss increase phenomena are compared for UV-curable resin coated and silicone-nylon coated fibers. Different mechanisms induce loss increase depending on coating materials. Lateral forces are caused by shrinkage and Youngs modulus increase at low temperature for UV-curable resin coated fibers. These forces cause fiber microbending for correlation lengths of not more than 1 mm. However, for silicone-nylon coated fibers, axial forces cause fiber buckling for correlation lengths longer than 5 mm. The relationship between the UV-coating structure and loss increase at low temperature is also described.

Infrared loss increase phenomenon of coated optical fibers at high temperatures

The loss increase phenomenon of coated optical fibers at high temperature has been studied. The wavelength dependent loss increase, observed for plastic-coated fibers at 200°C, is found to be irreversible. During heating, the absorption peak of second overtone of Ge-OH preferentially appeared. The dependence of the loss increase on temperature, heating time, and dopant is also examined. The loss increase level is strongly dependent on phosphorous concentration. The experimental results indicate that the loss increase is caused by chemical reactions between fiber constituent materials and hydrogen generated from coating materials. It is also confirmed that the heating test of secondary coated fiber is a practical, useful method to evaluate the hydroxyl loss increase of optical fibers.