Optical waveguides – Directional optical modulation within an optical waveguide – Electro-optic
Patent
1994-10-07
1996-08-27
Bovernick, Rodney B.
Optical waveguides
Directional optical modulation within an optical waveguide
Electro-optic
385 2, 385 14, G02B 610
Patent
active
055509398
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical switching devices and finds particular application in high bit rate communications links.
2. Related Art
Communications links which can carry high data rates are advantageous because they can transmit increased levels of information and/or can provide links via a single physical connection which service higher numbers of customers. For instance, time division multiplexed signals on a communications link can provide a higher number of time slots, and potentially therefore service a higher number of customers, where the link itself carries a higher data rate.
In order to access the data on a communications link, it is necessary to down load the information on the link to a receiver. In time division multiplexing, one particular customer will require information from only one or more selected time slots to be down loaded. To do this, switching devices may be used, the speed of switching of the device being commensurate with the capability of the link for carrying high speed traffic. In optical communications, it is envisaged that data rates may be achieved as high as 100G bits/s in the foreseeable future. This might be carried in ten time slots, providing ten channels at 10G bits/s.
As well as potentially increasing the number of customers using a communications link, ultra high bit rate links in future networks may enable customers to be offered new large bandwidth services as well as giving telecommunications companies greater flexibility in managing their networks. The present invention is concerned with an important part of achieving ultra high bit rate links, providing a type of switch capable of demultiplexing a 10G bit/s bit stream from a 100G bit/s optical time division multiplexed (OTDM) signal. An optical clock signal might be used to switch out every tenth bit. Hence, to recover all the data on the link, ten switching devices might be used in parallel.
Switching devices are known, including switching devices which can be controlled by an optical input. For instance, in "130 ps" Recovery of All-optical Switching in a GaAs Multi Quantum Well Directional Coupler" published in Applied Physics Letters volume 58 number 19 on 13th May 1991 by Li Kam Wa et al (Ref. (i)), a recovery time of 130 ps is reported in a zero gap directional coupler using multiple quantum wells. However, it has now been discovered in making the present invention that much faster recovery times can be achieved. Reduced recovery times provide potentially faster switches, recovery time being a limiting factor.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a semiconductor optical switching device, for use in optical communications systems operating at high data rates, the switching device comprising an optical waveguide, including a p-n junction, provided on a substrate, the waveguide having input and output ports for receiving a data signal at a first wavelength from a communications link and a control signal at a second wavelength, the control signal operating to modify the refractive index of at least part of the waveguide at the first wavelength, by creating electrical carriers in the region of the p-n junction, so as to change the optical path length of the data signal in the device there being significant capacitance across the device in use.
The purpose of the significant capacitance is to sweep carriers out of the region of the junction as quickly as possible after their creation by input of the control signal to the device. This is achieved because the significant capacitance provides a low impedance recombination path for photogenerated carriers, thus speeding up the recovery of the device, and therefore its potential switching speed.
Although the data and control signal need not both be guided through the device, so that for example the data signal could be guided by the waveguide while the control signal is input at right angles to the waveguide and is not guided by it, preferably both the dat
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Mak et al, "High-Speed Bulk InGaAsP-InP Electroabsorption Modulators with Bandwidth in Excess of 20 GHz", IEEE Photonics Technology Letters, vol. 2, No. 10, pp. 730-733.
LinkamWa et al, "130 ps recovery of all-optical switching in a GaAs multipuantum well directional coupler", Appl. Phys. Lett. 58(19), pp. 2055-2057.
Moodie David G.
Rodgers Paul M.
Wake David
Bovernick Rodney B.
British Telecommunications public limited company
Sanghavi Hemang
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