Takashi Nakashima

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.

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.

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.

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.

Temporal coherence properties of a dispersively propagating beam in a fiber-optic interferometer

The temporal coherence properties of a dispersively propagating optical beam are clarified experimentally by means of a two-beam interferometer composed of optical fibers with 1.3- and 1.5-μm zero-chromatic-dispersion wavelengths. Experimental results fully explain and reflect theoretically predicted ones concerning both the magnitude of the degree of coherence and the shape of the coherence curve with respect to the optical path difference.

Efficient multiple visible light generation in a polarization‐preserving optical fiber pumped by a 1.064‐μm yttrium aluminum garnet laser

Visible light generation by sum‐frequency mixings in a polarization‐preserving fiber has been observed with pumping by a 1.064‐μm Q‐switched, and mode‐locked neodymium:yttrium aluminum garnet laser. The visible light has been efficiently generated when the fast axis of the test fiber was excited. The total conversion efficiency from the pump to all visible wavelengths amounts to 1% with a mode‐locked pump peak power of 18 kW. By fusion splicing two fibers with different visible emission lines, we have succeeded in combining those two visible spectra into one spectrum.

An optoelectronic self-oscillatory circuit with an optical fiber delayed feedback and its injection locking technique

An optoelectronic self-oscillatory circuit with a constant time delay and its injection locking have been theoretically and experimentally described. The oscillation circuit incorporates a delayed feedback path by utilizing an optical fiber and is characterized by a differential-difference equation. The oscillation waveform and amplitude have been investigated by computing the equation. It is also shown that a stable frequency locking region exists and expands with an increase in the injection amplitude. By employing a carrier signal, two methods for optoelectronic oscillation, an AM and an FM type, are proposed and have been demonstrated to obtain more stable oscillation.

Effect of stimulated Raman scattering on pulse-compression characteristics.

The effect of stimulated Raman scattering on pulse-compression characteristics has been studied experimentally and theoretically. When an optical pulse is compressed by an optical fiber and a grating pair, the generation of stimulated Raman scattering becomes important. When stimulated Raman scattering occurs, the compressed pulse width changes asymmetrically with respect to the magnitude of the anomalous dispersion created by the grating pair. The compressed pulse has much residual energy in its wings, and it does not have a smooth envelope for large anomalous dispersion.

Configuration of the optical transmission line using stimulated Raman scattering for signal light amplification

Expansion of a repeater spacing of optical transmission line is investigated using forward stimulated Raman scattering generated by a 1.34-μm Nd:YAG laser to amplify the signal light. Achievable repeater spacing at the 1.42-μm wavelength transmission is numerically estimated on the basis of coupled power equations and experimentally obtained Raman gain for various core-cladding index difference. A transmission distance of more than 200 km is predicted for an input signal light power of 100 μw and a pump power of 200 mW using a fiber with a relative index difference of 1 percent.

Efficient optical pulse compression using a pair of Brewster-angled TeO 2 crystal prisms

We describe an optical pulse-compression technique that uses a pair of Brewster-angled TeO2 crystal prisms. The negative group-velocity dispersion of the TeO2 prisms is much larger than that of fused quartz or other dense flint glasses. This permits the distance between the prisms to be shortened considerably, approaching that of a grating (1200 lines/mm) pair. The transmission for 633-nm monochromatic light through the prism is 99.8%, and the total transmission through the prism pair is higher than 90%. Optical pulses of 800-fsec duration from a synchronously pumped cw dye laser have been successfully compressed to 120 fsec.