Hisashi Izumita

Fading noise reduction in coherent OTDR

The authors describe a reduction in fading noise which causes amplitude fluctuation on a backscattered trace in coherent OTDR (optical time-domain reflectometry). In order to reduce the fading noise, the optical frequency of the DFB-LD (distributed feedback laser diode) is changed by changing the LD temperature during the integrations of the backscattered signals. At the same time, the state of polarization of the launched signal pulses is changed. The amplitude fluctuation has been reduced to as low as 0.06 dB. Measurements were performed on a 10 km length of conventional single-mode fiber.<<ETX>>

Fault location technique for in-service branched optical fiber networks

A fault location technique using an optical time-domain reflectometer (OTDR) for branched optical fiber networks is described. The required dynamic range of the OTDR for locating a fiber failure occurring after N-way optical power splitting has been studied. In addition, it has been demonstrated that by using an OTDR operating at 1.65 mu m, it is possible to locate one fiber failure among 16-branched fibers without disrupting the 1.31- mu m and 1.55- mu m services.<<ETX>>

In-Service Line Monitoring System in PONs Using 1650-nm Brillouin OTDR and Fibers With Individually Assigned BFSs

This paper introduces an in-service line monitoring technique that can locate a fault in branched optical fibers in passive optical networks (PONs) from a central office. This technique can upgrade the conventional optical fiber line testing and monitoring system. To distinguish a backscattered signal from each branched PON fiber, we employ the fibers with individually assigned Brillouin frequency shifts (BFSs) fibers as drop cables and a 1650-nm Brillouin optical time-domain reflectometer (B-OTDR) instead of the OTDR used in the conventional testing system. This paper describes the design of the BFS taking the outside plant environment into consideration, and discusses the performance of the proposed testing system. We also demonstrate an experimental measurement that locates a fault in a branching fiber after a 1 × 8 optical splitter, and in-service line monitoring for gigabit Ethernet PON (GE-PON) transmission systems.

Stochastic amplitude fluctuation in coherent OTDR and a new technique for its reduction by stimulating synchronous optical frequency hopping

The dynamic range of optical time-domain reflectometry (OTDR) can be extended by employing self-heterodyne coherent detection. However, with coherent detection OTDR (C-OTDR) there is a problem of amplitude fluctuation in the C-OTDR trace caused by (1) the fading noise resulting from the interference between the Rayleigh backscattered lights, (2) the polarization dependent fluctuation of the optical detection efficiency, and (3) the heterodyne detection efficiency fluctuation due to the relative phase change between the Rayleigh backscattered signals and the local oscillator (LO). This paper provides a stochastic description of the amplitude fluctuation using probability density functions and the calculated amplitude fluctuation with M integrations when reduction techniques are applied. We have found theoretically that it is difficult to reduce the amplitude fluctuation effectively by the optical frequency domain integration technique using the asynchronous optical frequency hopping of the source. This is because of an inclination increase in the C-OTDR trace which reduces the measurement accuracy. We propose a synchronous optical frequency hopping technique in which an RF current pulse is induced in the drive current of the laser diode (LD) during the LD temperature change. This effectively reduces the amplitude fluctuation without any increase in the inclination. The amplitude fluctuation for a 1 /spl mu/s pulse width is reduced experimentally to 1/7 that with the LD temperature stabilized. For 100 and 30 ns pulse widths, it is reduced to 1/11 of that with the LD temperature stabilized. These experimental results are in good agreement with the calculated ones.

The performance limit of coherent OTDR enhanced with optical fiber amplifiers due to optical nonlinear phenomena

This paper theoretically and experimentally clarifies the limit of incident optical pulse power in coherent optical time-domain reflectometry (C-OTDR) enhanced with optical fiber amplifiers. The critical pulse power, at which the performance of C-OTDR is degraded by the effect of optical nonlinear phenomena in a single-mode optical fiber, depends on the amplified optical pulse waveform and the pulse width. For a pulse width of 1 /spl mu/s or longer, the incident pulse power is limited by the effect of self-phase modulation (SPM). When an optical pulse having a power gradient within the pulse width is incident to a single-mode optical fiber, the optical frequency of the backscattered signal is shifted by SPM, and the center frequency of the signal moves outside the receiver band, so the sensitivity of C-OTDR is degraded. For a pulse width of 100 ns, the incident optical pulse power is limited by four-wave mixing (FWM) which transfers the energy from the incident optical pulse to Stokes and anti-Stokes light as a result of the interaction between the incident optical pulse and amplified spontaneous emission. This paper also demonstrates the high performance of C-OTDR enhanced with EDF As with 48, 44, 39, and 29 dB single-way dynamic ranges for pulse widths of 10 /spl mu/s, 4 /spl mu/s, 1 /spl mu/s, and 100 ns, respectively, limited by the effect of SPM or FWM. These results are believed to be the best performance of C-OTDR with EDFAs. >

Design of Identification Fibers With Individually Assigned Brillouin Frequency Shifts for Monitoring Passive Optical Networks

We propose a method for monitoring branched sections in passive optical networks (PONs), where identification fibers are used to distinguish each branched section. We clarify the required characteristics for the separation of the Brillouin frequency shifts of each identification fiber. We design and fabricate identification fibers with well-controlled nuB values for PON monitoring based on the required characteristics. The realized Brillouin frequency shift difference is less than 40 MHz from the target, and we achieve a 1-GHz Brillouin frequency shift separation and a 0.06-dB splice loss for standard single-mode fiber. We also demonstrate fault location in a PON using identification fibers connected to a 4-branched splitter. The bending loss in the branched section is successfully located by using the designed identification fibers and a Brillouin optical time domain reflectometer

Maintenance method using 1650-nm wavelength band for optical fiber cable networks

This paper proposes a maintenance method for optical fiber cable networks that uses 1650-nm wavelength band light. We determined the maintenance wavelength of 1650-nm after considering the maintenance system requirements and the future trends in optical transmission systems. We improved the conventional maintenance system by adding appropriate system components including a narrow wavelength light source and a fiber-grating filter for the 1650-nm wavelength band. We designed the system and undertook manufacturing trials. The characteristics of our proposed maintenance method indicate good practical performance levels.

Brillouin OTDR employing optical frequency shifter using side-band generation technique with high-speed LN phase-modulator

In order to measure spontaneous Brillouin backscattered lights, which are related to fiber-distributed tensile strain and temperature, by using a highly sensitive self-heterodyne coherent detection technique, a wideband optical frequency shifter is required to reduce the beat signal frequency between the Brillouin backscattered signal and the LO light to the IF bandwidth of the conventional coherent receiver. We constructed a highly stable optical frequency shifter with a 14 GHz bandwidth and 20 kHz frequency resolution which employs side-band light generated by a high-speed LN phase-modulator. The structure is simple and the insertion loss is low. We applied this shifter to B-OTDR and measured the distributed tensile strain in a 30-km dispersion-shifted optical fiber with a 100-m spatial resolution.

Over 31.5 dB dynamic range optical fiber line testing system with optical fiber fault isolation function for 32-branched PON

We describe a prototype optical fiber line testing system based an H-OTDR and optical filters using fiber Bragg grating technologies for a PON with a dynamic range of over 31.5 dB that can isolate fiber faults in a PON with a 32-branch splitter.

Extended optical fiber line testing system using new eight-channel L/U-band crossed optical waveguide coupler for L-band WDM transmission

This paper describes the system design for an extended optical fiber line testing system that uses a new L/U-band crossed optical waveguide coupler and a fiber Bragg grating filter for L-band wavelength-division multiplexing transmission. We describe the reflection characteristic required for optical filters located in central offices in order to suppress the ghost signal caused by multireflection in the optical time-domain reflectometry (OTDR) trace. We design and evaluate an eight-channel crossed optical waveguide coupler with a new thin dielectric film filter that separates a 1650-nm test light from the L-band communication light, and confirm that there was no degradation caused by multireflections in the OTDR trace. We also demonstrate the in-service line monitoring of a 10-Gb/s L-band transmission with no degradation in the transmission quality.