Optically Controlled Selector

A selector in accordance with this invention and an experiment to demonstrate its operation will now be described with reference to the accompanying drawings in which:

FIG. 1 is a diagram of the selector and the experimental arrangement;

FIG. 2 is a graph of the gain against wavelength characteristic of the selector with light of two different powers; and,

FIG. 3 is a graph showing the change of amplifier resonance wavelength against control signal power for input signals in three different wavelength bands.

A selector in accordance with this invention comprises an optical fibre coupler 1 having a first input 2 for an input optical signal, a second input 3 for a control signal and an output for a combined optical signal, and a laser amplifier 5. The laser amplifier is of the Fabry-Perot type and configured as a double channel planer buried hetrostructure laser such as that described in an article in Electronics Letters May 23, 1985, Vol. 21, No. 11, pages 493-494, entitled "High Performance DC-PBH Lasers at 1.52 micrometres by a Hybrid MOVPE/LPE Process", by Nelson A W, Wong S, Regnault J C, Hobbs R E, Murrel D L, and Walling R H. The facet reflectivities of the laser are reduced by the application of anti-reflection coating until they have a reflectivity of between 3 and 4%. The laser is biased at 93% of its threshold current. The fibre coupler 1 is a directional coupler and the output fibre 4 terminates in a taper and is coupled via lens 51 to the laser amplifier 5.

For the purpose of an experiment to demonstrate the operation of the selector a second output 6 is taken out of the coupler 1 to enable an optical input signal applied to the second input 2 to be monitored. Light emitted from the second output 6 is received on a photodiode 7. An optical signal input is generated by a tunable source 8 comprising an external-cavity laser such as that described in Electronics Letters Feb. 3, 1983, Vol. 19, No. 3, pages 110-112, in an article entitled "10 kHz Linewidth 1.5 micrometre InGaAsP external Cavity Laser with 55 nm Tuning Range", by Wyatt R, and Devlin W J. The external cavity of this laser is defined by a rotatable diffraction grating and the laser is tuned by rotating its grating with a stepping motor. The tunable source 8 also includes an optical feedback loop which controls the bias current to keep the output of the laser constant at 3 microwatts. The output is fed via a chopper wheel 9 through the first input 2 of the coupler 1. An output 10 from the amplifier 5 via lens 52 is fed to a monochromator 11 which is coupled to the tunable source 8 so that it always transmits light of the same wavelength as that emitted by the tunable source 8. A second photodiode 12 is located downstream from the monochromator 11 to monitor light that passes through the monochromator 11.

A control signal is provided at the second input 3 of the coupler 1 by a distributed feedback laser 13 operating at 1526.5 nm. The light output from this laser is coupled via attenuator 14 to the second input 3 of the coupler 1.

To demonstrate the effectiveness of the selector its gain was measured against wavelength by tuning the tunable source 8 and the monochromator 11 together and recording the chopped signals monitored by the photodiodes 7 and 12. Such a scan was repeated with different levels of control signal and about different center wavelengths. Two of the scans are shown in FIG. 2. From the series of such scans the change of resonant wavelength against control signal power was plotted as shown in FIG. 3.

The amplifier acted as an active filter with a half-power bandwidth of 0.11 nm (14 GHz) and peak fibre-to-fibre gain of 10 dB. The bandwidth depends upon the bias current and the facet reflectivities and may be increased or reduced as required. Increasing the control signal power increases the rate of stimulated emission and thus reduces the carrier density and raises the refractive index in the amplifier. An increase from 1.4 to 25 microwatts tuned the amplifier resonance by 0.11 nm.

FIG. 3 illustrates a decreasing tuning effect as the power of the control signal is increased. This is because an amplifier resonance is moving away from a fixed control signal wavelength, thus reducing the amplification and effect of the control signal. A larger tuning range is obtainable with a more powerful control signal at a slightly longer wavelength.