Shinichi Furukawa

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>>

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.

Enhanced coherent OTDR for long span optical transmission lines containing optical fiber amplifiers

We have newly constructed an enhanced coherent optical time domain reflectometer (C-OTDR) for use in testing optical cable spans in transmission lines containing erbium-doped fiber amplifiers (EDFAs), which is based on heterodyne detection using acousto-optic (AO) switches. In order to avoid any optical surges in the EDFAs in the transmission lines, optical dummy pulses were added between the signal pulses by an AO switch to keep the probe power from the C-OTDR as uniform as possible. We achieved a large single-way dynamic range of 42 dB with 5 dBm less probe power. The measurable portion of the fiber spans was more than 80 km in optical transmission lines containing EDFAs. This is twice the previously reported value.<<ETX>>

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. >

Optical fiber cables for residential and business premises

This paper describes a new drop cable, a new cluster-type drop cable for residential premises, a new indoor cable, a new termination cable and a new riser cable for fiber distribution in residential and business premises. The structural design of these cables is investigated and the optimum structures for cost reduction of cable and installation are obtained. Based on the results, these cables are manufactured and their transmission and mechanical characteristics are confirmed to be stable.

High-accurate fault location technology using FSK-ASK probe backscattering reflectometry in optical amplifier submarine transmission systems

This paper proposes a new modulation format for optical time domain reflectometry (OTDR) to eliminate optical surge and improve OTDR performance in optical amplifier submarine transmission systems. The modulation format, FSK-ASK, uses a short high-power probe pulse and a long dummy pulse. Thanks to the slow gain dynamics of erbium-doped fiber amplifiers, both pulses experience an identical gain, equal to the loss of a span, so that the probe pulse maintains its high power and does not develop into an optical surge. Fault location experiments verify a theoretical prediction that FSK-ASK improves the signal-to-noise ratio (SNR) of OTDR by an amount as large as the power ratio of the probe to dummy signal. They also confirm the elimination of the optical surge caused by conventional OTDR using a solitary probe pulse. An FSK-ASK OTDR is applied in a commercial submarine amplifier transmission system which has a total transmission length of 890 km and a repeater spacing of 90 km. These field trial results show that subtle fiber anomalies can be located, with a spatial resolution of 1 km, along the entire length of the amplifier transmission system from a terminal end.

Propagation characteristics of a single-polarization optical fiber with an elliptic core and triple-clad

This paper gives the propagation characteristics of a single-polarization optical fiber with an elliptic core and triple-clad. This fiber is designed so as to satisfy the zero total dispersion at the operating wavelength of 1.55 /spl mu/m in the single-polarization region. Thus far, we had studied the n propagation characteristics of single-polarization optical fibers with pits between an elliptic core and the outer cladding (Type-1) and with pits alongside the major axis of an elliptic core (Type-2). In this paper, it is found that a single-polarization optical fiber with an elliptic core and triple-clad has better propagation characteristics compared with Type-1 and Type-2. When the relative index difference between the core and the outer cladding is 0.5%, the main comparison results are as follows: 1) The minimum wavelength sensitivity of the total dispersion can be reduced by approximately 1/3 of those for Type-1 and Type-2, and 2) the single-polarization bandwidth is 2.2 times larger than those of Type-1 and Type-2.

An optical fiber amplifier for wide-band wavelength range around 1.65 mu m

The optical amplification characteristics of a 0.781- mu m pumped thulium-doped fiber in the wavelength range of 1.6-1.7 mu m are discussed. A maximum net gain of 2.0 dB was obtained for 1.69- mu m operation. This optical fiber amplifier is suitable for in-service monitoring and identifying fibers operating at 1.2-1.6 mu m.<<ETX>>

A single-polarization optical fiber of hollow pit type with zero total dispersion at wavelength of 1.55 /spl mu/m

This paper gives a design method for the single-polarization optical fiber which satisfies simultaneously the wide single-polarization bandwidth, the large modal birefringence, and the zero total dispersion at the wavelength of 1.55 /spl mu/m. As a type of single-polarization fiber, the optical fiber with two hollow circular pits across the core-clad interface is proposed and designed. The normalized power (=/spl Delta/ the power in the core/the total power) in the core is also evaluated for the designed fibers. It was found from the numerical analysis that when the zero total dispersion is satisfied at the wavelength of 1.55 /spl mu/m, the maximum modal birefringence of 1.133/spl times/10/sup -3/ and the maximum single-polarization bandwidth of 100.6 nm are attained for the relative index difference of 1.6%. Then the normalized power in the core is 0.859. >