Keiji Takagi

Gain saturation characteristics of Raman amplification in silica and fluoride glass optical fibers

Abstract Raman amplification in silica and fluoride glass fibers is studied experimentally and theoretically. Two tunable dye lasers are employed to provide pump and signal light. In an 80 m long silica glass fiber, a small-signal gain of 39 dB has been achieved at a pump wavelength of 580 nm. The gain bandwidth is 127 cm-1 in small-signal amplification and 250 cm-1 in saturated amplification. In an 18 m long fluoride glass fiber, a small-signal gain of 9.3 dB has been obtained. The measured Raman-gain coefficient at 580 nm is (2±0.5) × 10-11 cm/W in the fluoride glass fiber.

1/f Noise in Metal Contacts and Granular Resistors

To measure the current noise in metal contacts and granular resistors, we arranged a noise measuring system with a switched-capacitor filter. The samples are crossbar contacts and grphite thick-film resistors. All noise spectra are the 1/f type. The noise of crossbar contacts and thick-film resistors increases in proportion to the fifth and third power of the resistance, respectively. A model for the noise of metal contacts and granular resistors is proposed. It is based on the spontaneous fluctuation of temperature of the small Conducting spots between metal surfaces and granular boundaries, respectively. The results of the analysis are qualitatively in agreement with those of measurement.

Short-pulse ultraviolet fiber Raman laser pumped by a XeCl excimer laser

Short-pulse generation by an ultraviolet fiber Raman laser has been studied both experimentally and by computer analysis. A XeCl excimer laser generating 3-nsec pulses pumped a multimode silica fiber. The fiber produced five orders of Stokes pulses in the range 312–331 nm at 2-GW/cm2 input. The duration of these Stokes pulses was controlled by input intensity. The shortest pulse durations were between 1.4 and 1.7 nsec. The pulse waveforms were analyzed by solving coupled differential equations. The calculated results are in agreement with the experimental ones. The effects of modal and wavelength dispersion, four-photon mixing, and two-photon absorption are discussed.

Wavelength dependence of two-photon absorption properties of silica optical fibers.

Two-photon absorption coefficients in ultraviolet-grade optical fibers are measured in the range of 219-260 nm. A frequency-doubled tunable dye laser is used as the light source. The two-photon absorption coefficient increases exponentially with an increase in the photon energy. The exciton peak does not appear in the spectrum. Theoretical values are calculated by the Keldysh formula and compared with the experimental results.

Noise in silicon and FET's at high electric fields

Abstract The noise temperature of n -type silicon was measured on an epitaxial layer up to a field strength of 10 KV/cm from 1 to 20 MHz at 77°K. The result was applied to obtain an approximate form for the drain noise of an FET in the high electric field. We found that the noise increases in proportion to the product of the drain current and voltage in high electric fields. The noise of the FET was also measured and the experimental data show reasonable agreement with the theoretical expressions.

Phase noise and a low-frequency noise reduction method in bipolar transistors

A low-frequency amplitude and phase-noise measuring system was set up and both noises were measured. It is noted they were correlated in a bipolar transistor. Both noises were from the same origin: diffusion coefficient fluctuation in the base. A demonstration was carried out on the amplitude noise reduction by using the correlation between both noises.

Current Dependence of 1/f Noise in Junction Diodes*

Experimental evidence shows that the noise current is proportional to the flowing current. The samples are the P-N junction, and light-emitting and laser diodes. Noise analysis is carried out using Hooges equation. The low-frequency noise agrees qualitatively with the analytical result.

Two-photon absorption in silica optical fibers measured with A XeBr excimer laser

Abstract Two-photon absorption in ultraviolet-grade silica optical fibers is measured with a 282-nm XeBr excimer laser. The two-photon absorption coefficient is 5x10 -6 ±20% cm/MW. The two-photon absorption is not resonance-enhanced but intrinsic at this wavelength. The calculated value based on Keldysh formula is also presented and compared with the measured one.

A study on phase-noise reduction method in phase-locked loop systems

Experimental studies are carried out on phase noise and the correlation coefficient between the phase and average current noises of the voltage-controlled oscillator in phased-locked loop (PLL) systems. The precise phase stabilization technique is discussed, and new methods to reduce the phase noise are described in PLL systems, using the correlation.

A method to reduce the phase noise in bipolar transistor circuits

We measured the phase and amplitude noises and their correlation coefficient in the collector current. The noise-generating mechanism due to the diffusion coefficient fluctuation in the base is discussed, using the distributed constant circuit model. A possible noise reduction method is proposed, using the correlation between the phase and amplitude noises.