TRANSIENT SUPPRESSION IN OPTICAL WAVELENGTH DIVISION MULTIPLE NETWORK

Abstract: A wavelength division multiplexed optical communications network has one or more semiconductor laser amplifiers (1) which are adapted to suppress transient amplitude fluctuations in traffic signals. Such fluctuations are caused by adding or dropping of traffic channels at one or more points in the optical communications network.

Description: TRANSIENT SUPPRESSION IN OPTICAL WAVELENGTH DIVISION MULTIPLEX NETWORK Technical Field This invention relates to the suppression of transients which can occur in conventional optical wavelength division multiplex (WDM) communications systems. Background Art It has been shown in J.L. Zyskind et al.. "Fast power transients in optically amplified multi- wavelength optical networks". Optical Fibre Communication Conference. Vol. 2, OSA Technical Digest Series, PD31-1. San Jose. 1996. that power transients in chains of highly pumped, deeply saturated Erbium-doped fibre amplifiers (EDFA's) were demonstrated to occur on time scales much faster than those for an individual amplifier. Such fast power transients constitute a major issue for optical networks in which channels are added and dropped either due to network reconfiguration or failures. The surviving channels will suffer error bursts if their powers exceed the dynamic range of the terminal receiver. Protection against such error bursts must be fast enough to limit the surviving channel power excursions. Previously reported work has demonstrated dynamic gain control at time scales of microseconds. The present invention seeks to provide an improved protection for such a system. Disclosure Of Invention According to the present invention there is provided a wavelength division multiplexed optical communications network comprising one or more semiconductor laser amplifiers adapted to suppress transient amplitude fluctuations in traffic signals caused by the adding or the dropping of traffic channels at one or more points in the optical communications network. Preferably, one or more semiconductor laser amplifiers are adapted for operation in a saturated state, wherein the transient amplitude fluctuations are suppressed through the semiconductor laser amplifier frequency response. In the present invention, semiconductor laser amplifiers are used to suppress large power excursions generated by adding or removing channels from long optically amplified links. The time response of this technique exceeds all those reported earlier by almost an order of magnitude. The fast time response of these devices makes them suitable to protect transoceanic size optical links with more than 100 optical amplifiers. Brief Description of Drawings Examples of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a schematic block diagram of part of an optically amplified multi- wavelength network: Figure 2 shows a schematic block diagram of part of an optically amplified multi- wavelength network which includes an optical cross-connect; Figure 3 shows a schematic block diagram of part of a submerged branching unit: Figure 4 shows an experimental set-up for testing the effectiveness of a semiconductor laser amplifier in suppressing fast transients; Figure 5 illustrates the response of the experimental set-up both with and without transient suppression control; and, Figure 6 illustrates the effectiveness of a semiconductor laser amplifier in the suppression of fast transients. Detailed Description In Figure 1. a semiconductor laser amplifier (SLA) 1 in saturation acts as a limiting amplifier to control the light intensity into a terminal 2 of a network. Due to the fast dynamic response of the SLA 1 and strong high pass filtering characteristic in saturation, optical transients, up to a few nano-seconds. are going to be effectively suppressed into the nominal channel power. Any faster transients can be corrected using forward error correction (FEC) in the terminal 2. The SLA 1 is used in the place of a pre-amplifier and it can clamp the power of the channel before detection when fast optical transients occur. If the SLA 1 can work in deep saturation for a sufficiently large range of input powers, no extra control circuitry will be necessary. As shown in Figure 2. in the case of more complicated optical networks, an alternative arrangement may employ a similar configuration with an SLA 5 implemented immediately after an optical cross-connect 3 to protect long optically amplified links 4. A suitable SLA is the hermetically sealed semiconductor optical amplifier manufactured bv E-TEK DYNAMICS. INC. In Figure 3, SLAs 6 and 7 are placed in the drop and trunk output branches 8 and 9. respectively, of a submerged branching unit. At this location, without some form of protection any optical transients generated in different parts of the network can be routed into long optically amplified links and so degrade the traffic quality. The SLA 6 in the drop path 8 can replace the usual drop amplifier in the standard branching unit configuration and so protect the traffic on the link. The trunk and drop amplifiers 10 and 1 1 secure the saturation of the SLAs 6 and 7, respectively. Figure 4 shows the set-up for a transmission experiment conducted to demonstrate the potential use of an SLA to suppress power transients. Four out of eight channels used in the experiment were removed and added periodically using an acousto-optic modulator 12 operating at a few KHz. Channel 1 (Txl) was optically modulated using a Mach-Zehnder modulator 13 at a line rate of 2.5 Gbs " 1 with pseudo random bit sequence data from a bit error rate test transmitter 14. This provided a channel for monitoring the effect on bit error rates (see below). After passing through a series of 18 erbium doped fibre amplifiers 15 j to 15 1 the signal was detected using a fixed wavelength optical filter 16, and a variable wavelength optical filter 17 which tracks the wavelength of the incoming signal and adjusts accordingly. After passing through these filters the signal is detected by a 10 GHz PIN detector 22 and continues to a bit error rate test receiver 23. An arm 18 from an optical splitter 19 having a 100 MHz PIN detector 24 provides a trigger for a sampling oscilloscope 20 where the optical transient was monitored. Figure 5 records the optical transient response after passing through the 18 optical amplifier 15, to 15 18 . The y axis represents the relative intensity in one of the surviving channels. In the absence of transient control the change in surviving signal exceeds 9 dB (4.5 dB in terms of optical power). As shown, the presence of a saturated SLA 21 placed immediately before the optical splitter 19 can successfully limit the intensity excursion of the surviving channel to less than 0.5 dB. Note that the rise time is in the range of 200 μs after the 18 optical amplifiers compared with a few milliseconds after one optical amplifier. Zyskind repoπs that for 100 plus optical amplifiers the transient speed could be less than 100 ns. To test the effectiveness of an SLA to suppress such fast transients an optical transient with a rise time of 100 ns was created using a second Mach Zehnder modulator (not shown) which superimposed a single pulse on the PRBS data on Channel one. The intensity variation represents removal or addition of half of the link traffic without considering the effects of channel pre-emphasis. The rise time of the pulse was defined by suitable low-pass filtering. The oscilloscope trace shown in Figure 6 demonstrates full restoration of the channel power is achieved with the SLA 21 when channels are dropped or added in the optically amplified link. To test the effect of the transient on the channel performance the system was loaded with noise to achieve a BER of approximately 10 "10 with and without the SLA 20 in the set-up. The fast power excursion resulted in a BER degradation from 8.4x 10 " ' ' to 4. x10 ' with no transient control. On the other hand when the SLA 20 was present the measured change in the BER was from 1.9xl0 "10 to 3.9xl0 "10 . suggesting no penalty due to the optical transient. The SLAs can affect the system performance due to their polarisation sensitivity and pulse response when they operate in deep saturation. The extent of the penalty will depend on the SLA type and the number of devices in the network. In the present invention, SLAs are used to suppress large power excursions generated by adding or removing channels from long optically amplified links. The time response of this technique exceeds all those reported earlier by almost an order of magnitude. The fast time response of these devices makes them suitable to protect transoceanic size optical links with more than 100 optical amplifiers.

Claims: CLAIMS: 1. A wavelength division multiplexed optical communications network comprising one or more semiconductor laser amplifiers adapted to suppress transient amplitude fluctuations in traffic signals caused by the adding or the dropping of traffic channels at one or more points in the optical communications network. 2. A wavelength division multiplexed optical communications network as claimed in claim 1. wherein said one or more semiconductor laser amplifiers are adapted for operation in a saturated state, wherein the transient amplitude fluctuations are suppressed through the semiconductor laser amplier frequency response. 3. A wavelength division multiplexed optical communications network as claimed in claim 2. wherein at least one of said one or more semiconductor laser amplifiers is a preamplifier for traffic signals entering a receiving station of the network. 4. A wavelength division multiplexed optical communications network as claimed in claim 2 or claim 3. wherein at least one of said one or more semiconductor laser amplifiers is located at an optical cross connect of the network. 5. A wavelength division multiplexed optical communications network as claimed in claim 2 or claim 3. wherein at least one of said one or more semiconductor laser amplifiers is located within a submerged branching unit. 6. A wavelength division multiplexed optical communications network as claimed in claim 5. wherein the branching unit comprises a semiconductor laser amplifier in each of a drop path and a main trunk output path. 7. A wavelength division multiplexed optical communications network as claimed in any one of claims 2 to 6, wherein said one or more semiconductor laser amplifiers are adapted for operation as limiting amplifiers.

Inventors: HAXELL IAN, SIAN SURINDER SINGH, SIMEONIDOU DIMITRA, HAXELL, IAN, SIAN, SURINDER, SINGH, SIMEONIDOU, DIMITRA
Applicants: HAXELL IAN, SIAN SURINDER SINGH, SIMEONIDOU DIMITRA, STC SUBMARINE SYSTEMS LTD, HAXELL, IAN, SIAN, SURINDER, SINGH, SIMEONIDOU, DIMITRA, STC SUBMARINE SYSTEMS LIMITED
Application year: 1997
Main classification: H04J14
IPC codes: H01S5, H04B10, H04J14