Masamitsu Tokuda

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.

Photon probe fault locator for single-mode optical fiber using an acoustooptical light deflector

A new backscattering technique for diagnosing the attenuation characteristics, spatial imperfections with length (fault location), and splice loss in a single-mode optical fiber has been developed by using a TeO 2 acoustooptical light deflector operating at 120 MHz. Due to the small insertion loss and high extinction ratio of the deflector, the dynamic range of the backscattered signal has been increased by at least 10 dB, which corresponds to the extension of 5 km in measurable length for fiber loss of 1 dB/km, compared with the conventional back-scattering technique in which the beam splitter and polarizer-analyzer combination are utilized. Another advantage of this technique is in that the saturation of the amplifier is avoided by arbitrarily cutting off a large power in the early stage of the Rayleigh scattering signal. A single-mode fiber of 19.2 km in length has been examined, and the distance for fault location up to 18.4 km was obtained.

Measurement and analysis on polarization properties of backward Rayleigh scattering for single-mode optical fibers

Fluctuation components on a backward Rayleigh scattered signal measured by the polarization optical time domain reflectometer (POTDR) have been investigated in detail by means of the least squares method and the power spectra. As a result, it is revealed that the fluctuation component for the single-mode optical fibers is attributable to the inherent polarization beat length determined by the difference of the phase velocities between the orthogonal e HE 11 and o HE 11 modes. The power spectrum of the fluctuation is sharply peaked at the Fourier frequency determined by the polarization beat length of approximately 15 m, which is in good agreement with the analysis. The utilization of the power spectrum presents a new method to diagnose the polarization properties along the optical fiber.

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.

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).

Optical loss measurement in graded-index fiber using a dummy fiber

The utility of an auxiliary fiber, called a dummy fiber, is investigated for optical fiber loss measurements. The dummy fiber is spliced and used to excite the test fiber. Excess loss caused by undesirable modes is found to be reduced to less than 0.05 dB by using a 500-m dummy fiber and choosing the test fiber cut length to be 2 m for reference. Loss linearity to the fiber length is examined on 6-km spliced fibers, and satisfactory agreement is obtained between the total loss and the sum of the individual fiber and splice losses.

130-km-long fault location for single-mode optical fiber using 1.55-μm Q-switched Er 3+ :glass laser

An optical-time-domain reflectometer has been constructed with a 1.55-microm Er(3+):glass laser, a TeO(2) acousto-optical directional coupler, and a cooled Ge P-I-N photodiode. With it, a maximum detectable fault-location length of 130 km for single-mode optical fiber has been successfully achieved at a 1.55microm wavelength.

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.

Analyses of optical time-domain reflectometry for single-mode fibers and of polarization optical time-domain reflectometry for polarization-maintaining fibers

Optical time-domain reflectometry for single-mode optical fibers has been theoretically discussed in terms of the Lorentz reciprocity principle and of a correlation function of the refractive-index fluctuation. Through an application of analysis to polarization-maintaining fibers, the properties of the polarization optical time-domain reflectometry have been made clear.

Self‐sustained intensity oscillation of a laser diode introduced by a delayed electrical feedback using an optical fiber and an electrical amplifier

A self‐sustained oscillation of laser light intensity emitted by a laser diode was newly observed by a delayed network using an optical fiber and an electrical feedback system. The time delay (transit time) of the light beam through the optical fiber plus the time delay of the electrical signal in the electrical feedback system determines the oscillation frequency of the light intensity. The oscillation for the fiber length of 7 km was obtained by using an electrical amplifier with large gain. With a wide‐band amplifier, harmonic (multimode) oscillations took place where the waveform was almost square.