Kazumasa Takada

New measurement system for fault location in optical waveguide devices based on an interferometric technique

A new measurement system for fault location in optical waveguide devices is presented. The system consists of a fiber-optic Mach-Zehnder and a bulk-type Michelson interferometers. The spatial resolution of the scatter distribution is <380 microm, which is limited by the averaging time. The minimum detectable backscattered power is -116 dB relative to the light power propagating in the waveguides. Preliminary experimental results using single-mode fibers <10 cm long are demonstrated.

Polarization-independent interferometric optical-time-domain reflectometer

A polarization-independent interferometric optical-time-domain reflectometer is proposed and demonstrated. The experimental setup is composed of a superluminescent diode and a fiber coupler which combines a polarization-maintaining fiber and a conventional single-mode fiber to achieve a polarization-independent spatial profile of the reflectance in a single-mode fiber or waveguide sample. The polarization independence and the reliability of the measured reflectivity, were confirmed, and the measurement of loss and birefringence in an optical waveguide from its reflectance profile is demonstrated. The influence of dispersion in the fibers on the spatial resolution of the reflectometer is also discussed. >

Phase‐noise and shot‐noise limited operations of low coherence optical time domain reflectometry

This letter shows theoretically and experimentally that Fresnel end reflection of the waveguide under test degrades the sensitivity of low coherence optical time domain reflectometry (OTDR). Optical mixing of end reflection and reference light in the OTDR produces the phase noise in proportion to end reflectivity. With the balanced detection technique, the excess photon noise is subdued and the phase noise becomes the dominant source of sensitivity degradation. At 3.2% end reflection and 300 μA mean photocurrent, the noise floor due to the phase noise is −138 dB/Hz. By reducing the fiber end reflection with matching oil, a shot‐noise limited sensitivity of −140 dB at a 3 Hz bandwidth has been demonstrated at submillimeter resolution.

Optical low coherence method for characterizing silica-based arrayed-waveguide grating multiplexers

We describe a powerful method for the precise measurement of optical path phase errors and power-distribution coefficients in silica-based arrayed-waveguide grating multiplexers through the use of Fourier transform spectroscopy. The theoretical accuracies for measuring these parameters are /spl plusmn/1/spl deg/ and /spl plusmn/5%, respectively, when the geometrical step increment of the arrayed waveguides is greater than 48 /spl mu/m. The method reveals the random distributions of optical path phase errors in two multiplexers with channel spacings of 100 and 10 GHz, and proves that these distributions are the main origins of the channel crosstalk. The method predicts that when optical path phase errors are sufficiently reduced, the channel crosstalk values will decrease to -39 and -29 dB, respectively, and these limits are due to slightly deformed power distributions.

1-GHz-spaced 16-channel arrayed-waveguide grating for a wavelength reference standard in DWDM network systems

We fabricated a 1-GHz-spaced 16-channel arrayed-waveguide grating (AWG) by using a new AWG configuration where the path of each arrayed waveguide winds backward and forward across a 4-in diameter wafer without crossing any other waveguides. The ultra-narrow (< 1 GHz) and stable transmission bands of this AWG can be used to construct a wavelength reference standard covering the S, C, and L bands in the dense wavelength-division-multiplexing network systems whose frequency deviation is /spl plusmn/160 MHz.

Noise in optical low-coherence reflectometry

It is theoretically and experimentally shown that Fresnel end reflection at a waveguide under test can degrade the sensitivity of an optical low-coherence reflectometer (OLCR) with a balanced detection scheme. Optical mixing of the local oscillator (LO) light and the end reflection produces beat noise whose current noise spectral density is represented by 2(1+P/sup 2/) //spl delta//spl nu/, where P and /spl delta//spl nu/ are the degree of polarization and the effective linewidth of the light, respectively, and and are the total mean photocurrents of the LO light and the end reflection at the balanced mixer, respectively. The balanced detection technique suppresses the intensity noise of the light and the beat noise becomes the dominant source of sensitivity degradation. The minimum detectable reflectivity is derived which includes the effect of sensitivity degradation caused by beat noise.

Measurement of spatial distribution of mode coupling in birefringent polarization-maintaining fiber with new detection scheme

We present a new detection scheme for measuring the spatial distribution of mode coupling in birefringent, polarization-maintaining fibers. The new method canl improve the signal-to-noise ratio by a factor of 100 compared with the previously reported heterodyne-detection scheme. As a result, origin of the mode coupling in a low-cross-talk, birefringent fiber is clarified.

Precision measurement of modal birefringence of highly birefringent fibers by periodic lateral force

Modal birefringence of highly birefringent fibers can be measured nondestructively by the elastooptic modulation method. Based on this modulation method, a new method for precisely measuring the wavelength dependence of modal birefringence in highly birefringent fibers is presented using an incoherent light source such as a fiber Raman laser.

Rayleigh backscattering measurement of single‐mode fibers by low coherence optical time‐domain reflectometer with 14 μm spatial resolution

An optical time‐domain reflectometer (OTDR) with −136 dB minimum detectable reflectivity and 14 μm spatial resolution is developed based on low coherence interference. The high sensitivity was accomplished by using a high‐power superluminescent diode module with a 1.5 mW fiber output and a new system configuration, both for effectively operating the balanced heterodyne detection. The reflectivity of −136 dB is only 6 dB above the shot noise limit. The first observation of 1.3 μm wavelength Rayleigh backscattering in single‐mode fibers was successfully made by the OTDR with a 17 dB dynamic range and 14 μm spatial resolution.

New fiber-optic depolarizer

A new type of single mode fiber-optic depolarizer is proposed and demonstrated. The new depolarizer consists of a Mach-Zehnder interferometer with one arm used as a delay line with respect to the other. Light having a 100-MHz spectral width, for example, can be depolarized with high coupling efficiency to single mode fibers by using a polarization-maintaining fiber-delay line of a few meters in length.