Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1996-05-28
1998-04-21
Negash, Kinfe-Michael
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359158, 359139, H04J 1402
Patent
active
057424152
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND TO THE INVENTION
The present invention relates to a non-linear optical switch, and in particular to a switch incorporating a semiconductor optical gain medium which imposes a switchable phase change on an optical signal, and to optical circuits incorporating such a switch.
Non-linear optical switches of the type the present invention is concerned with, are potentially of use in a wide variety of fields. For example, in optical telecommunications systems, such switches may form the core of a demultiplexer for use with an OTDM (optical time division multiplexing) pulse stream. A demultiplexer using a semiconductor non-linear element in a non-linear loop mirror configuration can operate at bit rates as high as 10 GHz. Also in the field of optical telecommunications systems, the use of semiconductor non-linear elements has been proposed for clock-recovery. Our co-pending International application PCT/GB 93/00863 discloses and claims such clock-recovery circuits incorporating non-linear elements in the cavity of a mode-locked laser system.
Hitherto, using conventional semiconductor non-linear elements, the performance of such optical switches has been limited significantly by the recovery time of the semiconductor optical gain medium. High electrical bias fields have been used with the semiconductor laser medium to maximise the recovery speed, but even so for some applications, such as clock recovery, it has still not proved possible to obtain the recovery times necessary for operation at the highest data rates.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a non-linear optical phase switch including a semiconductor optical gain medium arranged to switch a signal at a first wavelength (.lambda..sub.3) in response to a control signal at a second wavelength (.lambda..sub.2) applied to the optical gain medium, the refractive index of the gain medium being modified in response to the control signal, characterised by means for applying an optical holding signal at a third wavelength (.lambda..sub.1) to the gain medium, the optical holding signal pumping the semiconductor optical gain medium thereby fixing the Fermi level of the gain medium.
The present invention uses a third light beam applied to the semiconductor gain medium to provide a non-linear switching element with a reduced recovery time, and a recovery time which moreover can be controlled directly simply by varying the intensity of the holding beam. The holding beam, by clamping the Fermi level of the conduction band in the gain medium, ensures that the response of the non-linear switching element to the control signal is uniform, even where the control signal itself is irregular. This is of particular significance in clock-recovery applications, where the control signal is a data stream which may, for example, include a long sequence of zeros.
The system may be operated with wavelength-degeneracy, that is to say the wavelength of the holding beam may be equal to one or both of the other wavelengths. However it is much preferred that the three wavelengths should all be different. This makes possible optimisation of the performance of the device by selecting suitable wavelengths in accordance with the wavelength-gain characteristic of the amplifier. It also makes possible separation of the different beams using, for example, WDM couplers.
There are few restrictions on the wavelengths which may be selected for the signal .lambda..sub.3. The closer it is to the band-gap equivalent wavelength, the better the phase modulation, the nearer it is to the holding beam the better the amplitude modulation. It may also be placed above the holding beam energy (i.e. in loss)--this was the case in recent successful clock recovery experiments at 10 and 20 GHz. A further alternative possible position for the signal wavelength is below the band-gap energy of the semiconductor material. The signal is then in transparency.
The optical holding signal may be a continuous wave (cw) signal, or alternatively may be a
REFERENCES:
patent: 5357359 (1994-10-01), Uchiyama et al.
patent: 5369520 (1994-11-01), Avramopoulos
patent: 5444560 (1995-08-01), Bernsley
patent: 5548433 (1996-08-01), Smith
patent: 5592319 (1997-01-01), Lee et al.
Ellis Andrew D.
Manning Robert J.
Patrick David M.
Spirit David M.
British Telecommunications public limited company
Negash Kinfe-Michael
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