Coherent light generators – Particular beam control device – Mode locking
Patent
1992-09-08
1994-07-19
Scott, Jr., Leon
Coherent light generators
Particular beam control device
Mode locking
372 28, 372 32, 372 38, H01S 3098
Patent
active
053316487
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to laser systems incorporating microchip lasers.
BACKGROUND OF THE INVENTION
Single-frequency microchip lasers are already known and have a cavity length which is extremely short (usually measured in microns) such that only a single longitudinal mode falls under the gain curve of the gain medium when pumped by a laser diode. These microchip lasers have good inherent frequency stability, but where very high frequency stability is required (such as optical communications) they may require frequency locking to a stable reference interferometer in order to overcome frequency fluctuations arising from intensity variations in the pump diode output, acoustic noise, or mechanical vibration of the microchip structure.
Locking of laser frequencies is itself known by diverting at least some of the laser output through an electro-optic phase modulator driven at a modulation frequency and at a very low modulation index (of the order 0.1) to generate sidebands which are applied to the reference interferometer in order to provide an error signal when the sideband signal is reflected from the reference interferometer. The error signal is then fed back to the laser to strain a piezo-electric mirror mount and thereby correctively change the cavity length and hence the frequency output of the microchip laser.
Electro optic phase modulators utilize birefringent crystals such as ADP, KDP, LiNbO.sub.3 each of which is expensive to manufacture, requires very accurate angular orientation of its birefringent axes with respect to the optical axis of the laser system, in effect attenuates the available laser system output power, requires a polarized output from the laser, and a large drive power (typically of the order of 1 W), even for the low modulation index required because at the modulation frequencies concerned (10-100 MHz) such, crystals have significant capacitive impedance.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a new and improved laser system incorporating a microchip laser.
According to one aspect of the present invention there is provided a laser system incorporating a microchip laser having a piezo-electric mounted mirror and pumped by a laser diode, the system including an oscillator operating substantially at one of the mechanical resonance frequencies of the piezo-electric mounted mirror and connected via coupling means to drive the mount, means for diverting at least some of the laser output power to a reference interferometer having a resolution less than the resonance frequency, a photo-electric detector for converting the output of the interferometer, a phase-sensitive detector coupled to the output of the photo-electric detector and referenced to the output of the oscillator for providing a frequency locking signal, and means for applying the frequency locking signal to the coupling means whereby the mirror mount is controlled to function as both a frequency modulator and a frequency correction element.
Preferably the coupling means comprises a capacitive branch connected to the output of the oscillator and an inductive branch connected to the output of the applying means to provide mutual isolation between the oscillator and the applying means.
Preferably the interferometer comprises a Fabry Perot optical cavity aligned with the optical axis of the laser system, the output from the cavity being separated from the incoming beam by a quarter wave plate and a polarizing beam splitter. Alternatively, a Faraday isolator may be used. As a further alternative, the interferometer cavity may be off-axis whereby the cavity output is separated from the incoming beam by also being off-axis. Various other forms of interferometers may be utilised such as a confocal interferometer or a ring interferometer. The interferometer may utilise optical fibres.
Preferably the diverting means comprises a lens to match the diverted laser output to a mode of the optical cavity.
Another aspect the present invention provides a laser system incorporat
REFERENCES:
patent: 3711786 (1973-01-01), Vautier
patent: 4905243 (1990-02-01), Lokai et al.
"The Fast Frequency Stabilisation of an Argon Laser to an Optical Resonator Using an Extra-Cavity Electro-Optic Modulator", by Kerr et al., pp. 11-16.
"Laser Phase and Frequency Stabilization Using an Optical Resonator"; by R. W. P. Drever, pp. 97-105.
"Frequency-Stabilized Laser-Diode-Pumped Nd:YAG Laser", by Shoemaker et al., pp. 609-611.
Jr. Leon Scott
University of Strathclyde
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