Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Patent
1988-01-19
1989-11-07
Griffin, Robert L.
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
330308, 372 70, 455608, 455618, H04B 900
Patent
active
048797618
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Communications and data transmission systems which transmit information signals in the form of optical pulses over a dielectric waveguide such as an optical fibre are now commonplace. Whilst improvements in the sources of the optical pulses and in the optical fibre waveguides have increased the range over which such signals can be transmitted to between 100 and 200 kilometers it is still necessary to regenerate the signals when they are transmitted over greater distances and when their power is reduced by beam splitting or being switched or otherwise handled. In a conventional regenerator, the optical signal is received by a photodiode and converted to an electrical signal. This electrical signal is then amplified and reshaped in an electronic regenerator circuit before being converted by an optical source into an optical pulse once again for onward transmission along the next optical fibre transmission line.
SUMMARY OF THE INVENTION
In this specification, the term optical is intended to refer to the visible region of the electromagnetic spectrum together with those parts of the infrared and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
According to this invention an optical signal regenerator comprises an optical coupler having a first input for an optical information signal, a second input for optical clock pulses and an output for a combined signal, and a resonant laser amplifier downstream of the output of the coupler and arranged to receive the combined signal and, in use, output a regenerated optical information signal. The bias current applied to the laser amplifier, in use, biases the amplifier to a level below its lasing threshold, and the power of the optical clock pulses is just below the bistable threshold of the amplifier, so that, when an optical information signal is fed into the first input during the application of an optical clock pulse to the second input, the bistable threshold of the amplifier is exceeded causing a sudden jump in the power of the optical output of the amplifier to provide a regenerated optical information signal.
It should be noted that the lasing threshold and the bistable threshold apply to distinct phenomena. In the optical regenerator, the electrical bias applied to the laser amplifier is below the lasing threshold bias current and consequently the amplifier does not lase. The sudden amplification occurs because the optical power of an input optical signal is made to exceed an optical power bistable threshold for the amplifier.
When the optical power input to a semiconductor laser amplifier is increased, the extra stimulated emission raises the recombination rate. The carrier density is correspondingly reduced. As a result, the effective refractive index of the active region of a resonant laser amplified increases with the optical power passing through it. The amplifier resonances are thereby tuned to longer wavelengths, and the gain at a given wavelength therefore varies. The power transfer characteristics of such an amplifier are consequently non-linear and, at appropriate input wavelengths, bistable operation is made possible.
Regeneration is achieved by combining an input signal with an optical clock signal and coupling them both into the resonant optical amplifier.
The clock signal consists of a train of optical pulses at the desired regeneration rate and with a wavelength at which the amplifier is bistable. The peak power of the clock signal is held marginally below the bistable threshold optical power level at which the amplifier will jump into a higher gain state.
With a low level input signal the output of the amplifier is in a lower gain state and its output comprises the slightly amplified clock signal. When the input signal increases to a high enough level such that the extra power in the combined input and clock signals is sufficient to exceed the bistable threshold then suddenly the resonant wavelength of the laser am
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Beek L. Van
British Telecommunications plc
Griffin Robert L.
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