Coherent light generators – Particular beam control device – Modulation
Patent
1992-05-06
1993-05-11
Lee, John D.
Coherent light generators
Particular beam control device
Modulation
331135, H01S 310
Patent
active
052107639
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a single frequency oscillator, and more particularly, although not exclusively, to one that may operate at high frequencies.
An oscillator is an electric circuit arranged such that electrical oscillations occur freely, and either at one frequency, or at a frequency which is tunable over a band of frequencies. Oscillators of various kinds are well known. They include, for example, oscillators incorporating tuned electrical circuits, quartz crystals and surface acoustic wave (SAW) devices operative at modest frequencies. Gunn diodes, klystrons and magnetrons are known for use at microwave frequencies and lasers for use at optical frequencies.
Of those oscillators that operate at microwave frequencies, klystrons and magnetrons are physically large and operate at high voltages and/or powers; Gunn diodes are small, operate at low power but are relatively unstable. In addition, it is difficult to produce feedback elements for use in these devices with Q values greater than 10.sup.4. A further problem is that, for an electrical oscillator to operate at radio frequency (RF), each electrical component is required to operate at RF, which at best proves expensive. There is therefore a need for physically small, electrically stable, high Q, preferably low cost oscillators to operate at high frequencies.
Oscillators producing multiple frequencies are also known in the prior art. One such is disclosed in U.S. Pat. No. 4,297,656. It employs a circuit which is part electrical and part optical, the latter incorporating a fibre optic delay line filter. It is unsuitable for single frequency operation.
The present invention provides an oscillator for generating an operating frequency, and comprising a light source, means for modulating the light source output at the operating frequency, and a feedback loop responsive to the light modulation and arranged to provide positive feedback to the modulating means, characterised in that the feedback loop includes:
a) optical delay line filtering means having a plurality of optical fibr with differing lengths arranged to define a fundamental frequency and associated harmonic frequencies,
b) secondary filtering means arranged to select one of the fundamental and harmonic frequencies to be the operating frequency of the oscillator.
The invention has a relatively small number of electrical components that are required to operate at high frequency, since it at least partially consists of optical components. In addition it is potentially capable of high stability and operation over a wide range of frequencies with appropriate components. The frequency range may extend to the microwave region. The delay line filtering means may be arranged to provide a Q factor in the region of 10.sup.6.
The delay line filtering means may be a single fibre optic filter, or alternatively it may consist of first and second fibre optic filters in series. In the latter case, light output from the first filter is coupled to the second filter such that light from each fibre of the first filter illuminates each fibre of the second filter. The effect of the two filters is therefore multiplicative.
The light source output may be modulated by means of positive feedback to an electrical power supply to the light source. Alternatively, the light source may be modulated by an electro-optic modulator located between the light source and the delay line filtering means. In this case, the modulator receives positive feedback from the feedback loop.
The light source may be a laser diode.
The feedback loop may include a photodiode which is responsive to the light modulation.
The oscillator may include optical means for directing an output light beam modulated at the operating frequency to remote receiving means.
In order that the invention might be more fully understood, embodiments thereof will now be described, with reference to the accompanying drawings, in which:
FIG. 1 is a schematic drawing of an oscillator of the invention;
FIG. 2 is a schematic view of optical coupling between a laser diode and
REFERENCES:
patent: 3333212 (1967-07-01), Vasseur et al.
patent: 3401357 (1968-09-01), D'Asaro
patent: 4297656 (1981-10-01), Pan
patent: 4345482 (1982-08-01), Adolfsson et al.
Electronics Letters, vol. 22, No. 19, Sep. 19, 1986 (Stevenage, GB) M. F. Lewis et al "Novel Narrowband Fibre-Optice Microwave Filter" pp. 1016-1017.
Patent Abstract of Japan, vol. 9, No. 240 (E-345) (1963), Sep. 26, 1985 & JP, A, 6090443 (Fujikura Densen K.K.), May 21, 1985.
Patent Abstract of Japan, vol. 12, No. 135 (E-604), Apr. 23, 1988 & JP, A, 62258528 (NEC Corp.), Nov. 11, 1987.
International Search Report.
Lewis Meirion F.
Wight David R.
Lee John D.
The Secretary of State for Defence in her Britannic Majecty's Go
Wise Robert E.
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