Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1997-07-23
1999-12-07
Chan, Jason
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359139, 359138, 359128, 359158, H04J 1408, H04J 1402, H04B 1000
Patent
active
05999293&
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND TO THE INVENTION
The present invention relates to an optical switch which might be used, for example, as a demultiplexer for adding or dropping OTDM (optical time-division multiplexing)channels in an optical telecommunications system.
It has been recognised that in order to achieve the highest possible bit rates (e.g. of 100 Cbit/s or more) in optical transmission systems it is desirable that functions such as adding or dropping a channel should be carried out entirely in the optical domain. It has previously been proposed to carry out such functions using a non-linear loop mirror (NOLM) in which the non-linearity is provided by a semiconductor gain medium which is positioned asymmetrically in the loop. Incoming optical data is split into two counter-propagating components. Because the semiconductor gain medium is positioned asymmetrically, one of these components passes through it before the other. Accordingly, if an optical control signal is applied to the gain medium to switch it after the first component has passed through but before the second counter-propagating component has arrived, then the two components experience different phase shifts through the semiconductor gain medium, producing a differential phase shift which may used to switch the signal to determine whether it is transmitted or reflected by the NOLM.
Although, as described in the present applicant's co-pending International application PCT/GB94/02240, it is found to be possible to operate such as NOLM at switching rates higher than the saturation recovery time of the semiconductor optical gain medium, nonetheless there remains a fundamental upper limit to the possible switching speeds determined largely by the transit time associated with the semiconductor gain medium. In use, there has to be time for the first pulse to pass through the gain medium, then for the gating pulse to pass through, and subsequently for the second pulse to pass through the medium. This limits the possible bit-rate which can be handled by the switch.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, an optical switch comprising a semiconductor gain medium and means for applying an optical control signal to the semiconductor gain medium is characterised by a pair of arms arranged in a Mach-Zehnder configuration with a respective semiconductor gain medium connected in each arm, in use respective control signals being applied to each semiconductor gain medium with a delay between the control signals selected so that a differential phase shift is applied to an input optical signal propagating through the arms of the switch, and by a phase control element connected in one of the arms of the switch and arranged to cancel a phase offset between pulses propagating in the two arms.
The present invention provides an optical switch using a semiconductor optical gain medium arranged in such a way that the switching rate is not limited by the transit time of the gain medium. To this end, a pair of semiconductor gain elements are used in the two arms of a Mach-Zehnder interferometer. By appropriate control of the period and relative delay of a sequence of control pulses applied to the respective gain media, a differential phase shift can be applied to a selected one or more of the pulses in an optically encoded data stream so that selected pulses are output from a different port of the switch. The switching rate is not limited by the transit time of the semiconductor gain media and so higher rates can be achieved than hitherto has been possible. Furthermore, the switch achieves contrast/extinction ratios superior to those of conventional NOLM switches when used at high bit-rates.
As will be further described below, while the selected one or more pulses experience a differential phase shift, e.g. of .pi., the other pulses experience a small differential phase shift from the two semiconductor gain media correlated to the delay between the two control sequences. However, the present inventors have found that this residual differential
REFERENCES:
patent: 4912706 (1990-03-01), Eisenberg et al.
patent: 5309267 (1994-05-01), Huang
patent: 5548433 (1996-08-01), Smith
British Telecommunications public limited company
Chan Jason
Sedighian Mohammad
LandOfFree
Optical switch does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical switch, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical switch will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-830547