Optical communications – Receiver – Heterodyne
Reexamination Certificate
2000-11-06
2004-05-04
Chan, Jason (Department: 2633)
Optical communications
Receiver
Heterodyne
Reexamination Certificate
active
06731883
ABSTRACT:
FIELD OF INVENTION
The invention comprises a system and method for demultiplexing a multiplexed signal in a high speed optical communication system such as a fibre optic based communication system.
BACKGROUND TO INVENTION
The availability of a stable, compact and robust ultra-fast switch is essential for the future development of high speed optical communication systems based on optical time division multiplexing (OTDM). To achieve demultiplexing in very high capacity OTDM systems it is necessary to use all-optical switching devices based on instantaneous optical non-linearities. Two high speed all-optical demultiplexers already demonstrated are the non-linear optical loop mirror (NOLM) based on the Kerr effect in optical fibres, and the terahertz optical asymmetric demultiplexer (TOAD) based on non-linearities associated with carrier depletion in semiconductor optical amplifiers (SOAs). NOLM and TOAD-based demultiplexers have demonstrated all-optical demultiplexing in systems operating at up to 640 Gbit/s. However high speed switching in the NOLM requires speciality fibre and precise wavelength control of signal and control pulses about the fibre zero dispersion wavelength, while with TOAD gain depletion in the SOA has been shown to limit the minimum control pulsewidth and thus the maximum switching speed.
SUMMARY OF INVENTION
In broad terms in one aspect the invention comprises a demultiplexer in an optical communication system, comprising an opto-electronic device in which two photon absorption can occur configured in circuit, and means to apply the received optical multichannel signal to the device, and means to apply a control optical signal to the device at the repetition rate of and synchronised to the selected channel to be detected, to generate by non-linear two photon absorption effect in the device a current similar to the original signal.
In broad terms in another aspect the invention comprises a method of demultiplexing a selected channel in an optical communication system, comprising applying the received optical multichannel signal to an opto-electronic device in which two photon absorption can occur configured in circuit and applying a control optical signal to the device which is at the repetition rate of and is synchronised to the selected channel to be detected, to generate by non-linear two photon absorption effect in the device a current similar to the original signal.
The invention utilises two photon absorption effect or TPA in an OTDM demultiplexing system and method. TPA is a non-linear phenomenon in opto-electronic devices in which two photons are absorbed simultaneously to generate a single electron hole pair. The opto-electronic switching device may be any form of optical detector or opto-electronic device such as laser diode for example, or any other suitable semiconductor wave guide structure that generates electrical carriers across a junction when exposed to light. The wavelength of the non-linear device should be less than the wavelength of the received light so that the energy required to overcome the band gap in the junction of device is greater than the energy in the light that will be received in use—so that significant current is only generated in the device by two photon absorption in accordance with the invention rather than linearly.
In the invention the high speed OTDM signal and the control optical pulses at the repetition rate of the individual channel in the OTDM signal are simultaneously injected into the opto-electronic switching device. The wavelength of the OTDM signal and control pulses is such that the injected photons do not contain enough energy to generate electron-hole pairs via linear absorption. However, electron-hole pairs will be generated by two photon absorption effect in the device, and due to the non-linear nature of TPA the number of earners generated in the external circuit is proportional to the square of the incident power. The peak power of the signal pulses is such that the electrical signal generated when they are injected into the diode on their own is negligible. With the higher optical power control pulse train synchronised with one of the data signal pulse trains in the OTDM signal, the signal pulses will generate a significant electrical signal in the external circuit due to the non-linear TPA effect.
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Asobe, M., et al; “Applications of Highly Nonlinear Chalcogenide Glass Fibers in Ultrafast All-Optical Switches”; IEEE Journal of Quantum Electronics vol. 29, No. 8, (1993).
Aitchison et al; “The Effect of Two-Photon Absorption on the Operation of an Ultrafast Semiconductor Switching Device”; LEOS 89 17-20 (1989); pp. 51, 52, 15.
Barry Liam
Dudley John
Harvey John
Auckland Uniservices Limited
Chan Jason
Li Shi K.
Nixon & Vanderhye
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