Modulators – Amplitude modulator – Nonlinear device controlled by modulating signal
Patent
1987-05-14
1989-06-13
Moskowitz, Nelson
Modulators
Amplitude modulator
Nonlinear device controlled by modulating signal
370 3, 372 32, H01S 313, H04J 100
Patent
active
048396141
DESCRIPTION:
BRIEF SUMMARY
There is an increasing requirement particularly in optical wideband networks closely to control the wavelength or frequency of the transmitted signals. Indeed, the more channels which are transmitted around the network the more important it is to control the frequency of the signals.
In accordance with one aspect of the present invention, a method of referencing the frequencies of radiation from a plurality of sources comprises guiding the radiation to a filter element having a replicated set of passbands spaced apart in frequency; monitoring radiation transmitted through the filter element; and controlling the sources so that the radiation transmitted through the filter element is maintained substantially constant, the bandwidths of the radiation being smaller than the corresponding passbands of the filter element.
In accordance with a second aspect of the present invention, a frequency referencing system comprises a plurality of sources of radiation of different frequencies; a filter element on to which the radiation is incident, the element having a replicated set of passbands spaced apart in frequency; detection means for detecting radiation passing through the filter element and for providing corresponding output signals; and control means responsive to the output signals from the detection means to control the sources so that the radiation transmitted through the filter element is maintained substantially constant, the bandwidths of the radiation being smaller than the corresponding passbands of the filter element.
The invention provides a particularly simple way to lock the frequencies of the radiation from the sources relatively to one another. It is thus particularly suitable, when the radiation is optical radiation, in optical wideband networks as described above.
The invention is also applicable to radiation at non-optical frequencies such as radio and microwaves.
The monitoring step may comprise monitoring the intensity of the transmitted radiation. In this case the bandwidths of the radiation are preferably offset from the central frequency of the corresponding passbands of the filter element and particularly conveniently they are positioned close to the edge of the corresponding passbands. The advantage of this is that any change in relative position between the radiation frequency and the filter element passband will correspond with an increase or decrease in the intensity of the radiation transmitted through the filter element depending on the direction of the change.
Alternatively, the phase of the transmitted radiation may be monitored. In this case the radiation bandwidth may be substantially centered on the corresponding passband and any change in the relative positions of the radiation frequency and the passband will result in a phase change in the transmitted beam the type of phase change differing with the direction of movement between the radiation frequency and passband. The advantages of this are that there is no attenuation since the beam is centered on a passband, and any dc drifts will not affect performance since phase and not intensity is monitored.
In some cases, the passbands of the filter element will shift together in response for example to temperature changes. Preferably, therefore one source comprises a reference source for generating a reference beam, the method further comprising monitoring transmitted radiation corresponding to the reference beam, and adjusting the filter element to maintain the transmission characteristic of the reference beam substantially constant. For example, the system may include means for adjusting the filter element in response to output signals from the detection means corresponding to radiation from the reference source.
The adjusting means may comprise means for physically moving or stressing the filter element such as a piezo transducer or a stepper motor or means for adjusting the temperature of the filter element or its support such as a Peltier cooler.
Preferably, the passbands of the filter element are substantially equally spaced a
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Hill Godfrey R.
Smith David W.
British Telecommunications public limited company
Moskowitz Nelson
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