Optical communications – Transmitter and receiver system – Including synchronization
Reexamination Certificate
2002-05-23
2004-11-02
Wong, Don (Department: 2828)
Optical communications
Transmitter and receiver system
Including synchronization
C372S006000, C372S094000
Reexamination Certificate
active
06813447
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed to recovering clock pulses of wavelength division multiplexed optical signals and to regeneration of wavelength division multiplexed optical signals. In particular, it relates to simultaneous clock recovery and regeneration of many wavelength division multiplexed optical signals.
2. Technical Background
As the capacity of wavelength division multiplexed (WDM) transmission systems increases in response to the increasing demand for communication, the maximum reach of each transmission system is diminished. Regenerators are therefore required at regular intervals along a transmission link in addition to any regenerators associated with network nodes where traffic routing takes place. It may be argued that regenerators are necessary within switching nodes to provide traffic routing and grooming functions, though this is not always the case when traffic on a given wavelength is routed straight through the node. However, the use of regenerators between nodes increases the network cost without contributing additional functionality. A cost-effective means of regenerating WDM signals is therefore required as an alternative to full WDM demultiplexing and opto-electronic regeneration. System manufacturers indicate that this is particularly necessary for 40 Gbit/s data rate systems with a target reach of 3000 km but a practical transmission limit around 1500 km.
All-optical regenerators which provide for the individual regeneration of each wavelength in a WDM system have been proposed (see, for example, Electronics Letters vol 32 no. 6, pp567, 1996 “Error free operation of a 40 Gbit/s all-optical regenerator” by Pender, Widdowson, Ellis; Electronics Letters vol 24 no. 14, pp848, 1998 “All-optical regenerator” by Giles, Li, Wood, Burrus, Miller). However, such systems require the WDM signals to be demultiplexed, after which each channel is processed by a respective optical regenerator. Such single channel fiber-based regenerators may perform clock recovery using a fiber ring laser mode-locked through cross-phase modulation or non-linear polarization rotation and a decision gate based on similar non-linear properties. The output wavelength is determined by the local pulse source in the ring laser section and is in general substantially different to the incoming wavelength. It is well known that the non-linearity of optical fibers is broadband, and so the device is tunable over a large wavelength range. However, since the device is based on cross-phase modulation or its derivative effect, non-linear polarization rotation, attempts to operate with several wavelengths simultaneously inevitably result in unwanted crosstalk between the channels through the same cross-phase modulation effects.
It has also been suggested that RZ (return to zero) formatted WDM signals may be simultaneously regenerated using soliton transmission and synchronous modulation. However, in this scheme it is necessary to ensure that all WDM signals arrive at the synchronous modulator with the same phase. This offers many practical difficulties arising from different clock sources, different propagation paths and small-scale drift of laser wavelengths coupled with residual dispersion. These difficulties are exaggerated when a wavelength-routed network is contemplated, and the requirements of soliton transmission are taken into account.
Therefore there is a need for an improved method and apparatus for use in all-optical clock recovery and signal regeneration, which can simultaneously process a plurality of WDM signals.
BRIEF SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided an apparatus for recovering clock pulses of wavelength division multiplexed optical signals passed therethrough. The apparatus comprises an optically-pumped laser cavity defining a cavity length and comprising a nonlinear medium pumped at a wavelength selected to give efficient parametric amplification within the medium. The cavity length corresponds to an integer multiple of bit periods of at least one of the multiplexed optical signals. The optical signals co propagate through the medium with the pump radiation. The apparatus further comprises an optical path for recirculating a proportion of the output from the laser cavity back through the laser cavity. In this way, idler waves are generated symmetrically about the pump wavelength by four wave mixing with the at least one of the multiplexed optical signals and recirculated through the laser cavity to be amplified by parametric amplification in order to recover wavelength division multiplexed clock pulses.
By pumping the dispersion-shifted nonlinear medium to give efficient parametric amplification, the input data signals interact efficiently with the pump to generate corresponding wavelength converted signals symmetrically spaced either side of the pump. At other non-symmetric wavelengths, group velocity dispersion destroys the phase matching, and results in a weak four wave mixing interaction. Depending on the relative phase of the pump and the symmetrically placed signals, the latter may extract energy from the pump by parametric amplification. Recirculation of the idler waves through the laser cavity enables generation of further idler waves where data pulses may have been absent during previous recirculations, so recovering wavelength-converted clock pulses.
The cavity may be formed between a plurality of reflectors, at least one of which reflectors is a wavelength-selective reflector which is partially reflective at the wavelength of at least one of the idler waves for partially reflecting such idler waves back through the cavity after passage through the cavity. In this way, a proportion of the wavelength converted clock pulse radiation may be recirculated through the cavity with the correct phase, to ensure strong parametric amplification of the idler waves (as described above) and recovery of clock pulses.
Alternatively, the cavity may be formed in a ring laser or a sigma laser configuration. Use of a ring laser configuration overcomes the disadvantages arising from non-linear effects and formation of sub-cavities by reflections inherent in linear cavity configurations.
Preferably, the apparatus further comprises a filter to prevent further transmission of radiation at the pump and signal wavelengths after passage through the nonlinear medium. The filter may comprise a band pass filter within the optical cavity. By blocking the pump and signal wavelengths after passage through the dispersion-shifted medium, one is left with idler waves corresponding to data pulses of the optical signal. Without a means to prevent further transmission of radiation at the pump and signal wavelengths after passage through the nonlinear medium, the recirculated pump and signals would need to be in phase.
Preferably, the apparatus further comprises an adjuster for adjusting the cavity length to correspond to an integer multiple of bit periods of at least one of the multiplexed signals. An adjuster would be required for apparatus using longer cavity lengths where environmental fluctuations are sufficient to induce a measurable change in cavity length. The adjuster may comprise an adjustable fiber delay line. This would provide sufficient phase matching accuracy for a short cavity.
Preferably, the adjustable fiber delay line is actively stabilised to compensate for environmental fluctuations in the cavity.
Preferably, the cavity further comprises dispersion slope compensation. This enables correct cavity length adjustment for a greater range of simultaneous signal wavelengths. Preferably, the dispersion slope compensation has mirror image dispersion characteristics to those of the nonlinear medium, Suitably, the dispersion slope compensation comprises dispersion-compensating fiber. Alternatively, the dispersion slope compensation and band-pass filter comprise at least one fiber grating, and may include a number of gratings per wavelength.
Preferably, the residual dispersion after dispersion compe
Ellis Andrew D.
Kuksenkov Dmitri V.
Li Shenping
Corning Incorporated
Menefee James
Paglierani Ronald J.
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