We report the first distributed acoustic sensing (DAS) results over >1,000 km on a field-lab hybrid link using chirped-pulses with correlation detection and 20× frequency-diversity, achieving a sensitivity of 100 p ∈ / Hz at 20-meters spatial resolution.
Waveleiigth-division multiplexing (WDM) networks using optical adddrop multiplexers and optical switches are effective for increasing not only capabilities but also transparency, survivability against fiber cuts, etc."* In systems employing erbium-doped fiber amplifiers (EDFAs), the per-channel output power control by a wavelength monitor is an important function, because a change in the number of channels caused by network reconfigurations or faults may bring a power transition in EDFA output.A wavelength monitor3 for an optical amplifier with cascaded fiber gratings and delay lines of optical fibers has been reported, but the number of channels that can be detected is limited because of the fu;ed wavelength of the fiber gratings. We have recently developed an EDFA repeater for WDM networks that has a wavelength monitor for arbitrary wavelength and uses an acousto-optic tunable filter (AOTF).4 This repeater keeps the per-wavelength power constant when the number of channels changes. Figure 1 shows the wavelength monitor used for EDFA gain control. The monitor uses a polarization-independent AOTF that operates over a wide tunable wavelength range (> 110 nm), with a high extinction ratio -Tuning acoustic frequency (MHz) J85.0 1788 173.1 -Time Wavelength,(&) 5 -30 /I450 1500 1550 WDM signal input TuEl characteristics. AOTF: acousto-optic tunable filter; PD: photo diode.Fig. 1. Block diagram for wavelength monitor and measured AOTF TuElBlock diagram for a dynamic gain-controlled EDFA repeater. AOTF: acousto-optic tunable filter; PD: photo diode; CPL optical coupler; ALC automatic level controller. Fig. 2. detected WDM pulses 1 detected WDM pulses -.-E c a time (10msIdiv) TuEl EDFA.Fig. 3. Detected WDM pulses from AOTF and output waveforms from(>15 dB). The AOTF is driven by the pulses sweeping frequencies from 172-178 MHz (from 1530-1570 nm). A WDM signal is converted to separate pulses (in time) corresponding to the wavelength. The channel number, wavelength band, and power for each wavelength are obtained from the number, position, and level of the detected pulses, respectively. The AOTF selects wavelength continuously, so that the wavelength monitor can detect any wavelength. Figure 2 shows the block diagram of the dynamic gain-controlled EDFA repeater. The repeater consists of a wavelength monitor circuit, a gain control circuit, and a wideband EDFA with a high aluminum codoped EDF pumped by a 0.98 p m laser diode (LD) (forward) and a 1.48 p m LD (backward). To keep per-channel output power constant, the gain of the EDFA repeater was controlled by tuning the pump LD current according to the number of channels detected. To maintain the quality of service on surviving channels, it is also important to suppress the power transient overshoot5 that occurs in the surviving channels' waveform after a change in number of channels. A frequency equalizer is connected to the LD drive circuit to increase response speed for EDFA gain control and limit the overshoot.
Wavelength-division multiplexing (WDM) technology is considered to have great potential for use in such optical networks as optical crossconnect systems and optical add-drop multiplexing (ADM) systems."' But the issue of monitoring (to ensure effective system maintenance in the practical operation of such networks) has yet to be fully resolved. Specifically, while effective monitoring is available for the regenerators SDH/SONET Regenerator A 1 EDFA Repeater Tho2 Fig. 1. Supervision and control for SDH/SONET regenerator and EDFA repeater. SDH synchronous digital hierarchy; SONET: synchronous optical network; O/E optical-electrical converter; RST regenerator section termination; E/O: electrical-optical converter; OA&M: operation, administration and maintenance; EDFA erbium-doped fiber amplifier; SNR signal-to-noise ratio.used in SDH/SONET system^,^ the only available performance monitors for Er-doped fiber amplifiers (EDFAs) in WDM systems are spectrum analyzers, which are neither fast nor reliable enough. And just as the monitoring of bit errors is important for guaranteeing quality in SDH/ SONET systems, so signal-to-noise ratio (SNR) is important to EDFA monitoring in WDM systems.Tn this paper, we present an optical performance monitor that employs an acousto-optic tunable filter (AOTF), which has previously been utilized as a wavelength channel counter for per-channel output power control in an EDFA repeater? This monitor, which features low cost, high reliability, and high-speed operations, can detect SNRs, output levels per channel, and the number of channels, information which can be used in the supervision and control of EDFA repeaters. Figure 2 shows the optical performance monitor, which employs a polarization-independent AOTF that operates over a wide tunable wavelength range (>110 nm), with a high extinction ratio (>15 dB). The AOTF is driven by pulses ranging in frequencyfrom 172-178 MHz (from Tho2 Fig. 2. Optical performance monitor built-in EDFA repeater. EDFA erbium-doped fiber amplifier; AOTF: acousto-optic tunable filter; PD: photo diode; S(xi): signal power of channel A, ; total power; PXi-bar: channel Xi removed power; N: noise power. 2 -40 -30 -20 -1 0 EDFA Input Power per Channel :Pin [dBm] Tho2 Fig. 3. Experimental setup and results. AWG: arrayed wave guide; LN Mod: LiNb03 external modulator; ATT: optical attenuator; EDFA erbiumdoped fiber amplifier; Pin: EDFA input power per channel; Rx: optical receiver.
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