Single mode laser

Details Single mode laser Single mode laser

1993-04-06

1995-01-10

Epps, Georgia Y.

Coherent light generators

Particular beam control device

Mode discrimination

372 20, 372 21, 372 69, 372 34, 372 71, 372 27, 372105, H01S 3098

Patent

active

053814274

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to a single-mode laser, in particular in the form of a diode-pumped solid state laser, whereby this laser can also be used as a frequency-doubled laser, optionally for single-mode operation or multimode operation.
For a laser to have single-mode operation, i.e. operation involving only the basic transversal mode and a longitudinal mode in accordance with the wavelength of the active laser medium, the resonator must meet certain criteria. To limit the transversal modes to the basic mode one can use mode screens or adapt the volume of the pumped light to the volume of the basic mode of the resonator.
The invention is based on the problem of providing a laser characterized by particularly stable single-mode operation and designed as a frequency-doubled laser in a special embodiment, also in multimode operation.


SUMMARY OF THE INVENTION

This problem is solved by a single-mode laser, in particular in the form of a diode-pumped solid state laser, which has the following features:
a pumped light source,
a resonator having a first mirror,
a laser medium,
a polarizer,
a crystal, and
a second mirror serving as an output mirror.
Single-mode operation is possible if the crystal is disposed as a birefringent crystal with its optical axes at an angle of 45.degree. to the polarizing direction produced by the polarizer, in such a way that the losses are substantially equal for the ordinary and extraordinary rays.
The birefringent crystal polarizes the light elliptically, and the polarizer permits the loss-free passage of only the light with the "right" polarizing direction.
This design of the resonator of the laser results in a Lyot filter. The laser light passes through the resonator repeatedly before leaving the resonator via the output mirror. Since the light thus passes repeatedly through the Lyot filter one obtains a very narrow-band filter so that single-mode operation of the laser is possible if the free spectral range of the resonator is large enough.
The Lyot filter formed by the inventive measures has transmission peaks that depend on the difference in refractive index of the crystal, the wavelength and the length of the crystal. The product of the difference in refractive index and crystal length must be adapted to the amplification curve of the laser wavelength. A 5-mm KTP crystal with an intersection angle of .THETA..about.26.degree. typically provides a free spectral range of .delta..nu.=3.2.times.10.sub.11 Hz, which is clearly above the half-width value of the amplification curve of Nd-YAG of .DELTA..nu.=1.6.times.10.sup.11 Hz.
The free spectral range of the Lyot filter must be larger than the half-width value of the amplification curve since two longitudinal modes can otherwise occur.
The invention ensures that no other losses occur for the extraordinary ray than for the ordinary ray. This is obtained e.g. by designing the second mirror as a plane mirror or as a mirror with a very large radius of curvature, the birefringent crystal being located between the polarizer formed as a Brewster window and the output mirror.
The pumped light is preferably adapted to the basic mode TEM.sub.oo with the aid of a lens.
As mentioned above, the refractive indices as well as the length of the crystal play a special part in single-mode operation in fixing the wavelength of the laser or the laser frequency. Since both the refractive indices and the length of the crystal are temperature-dependent, the exact laser frequency is dependent on the temperature of the crystal.
The invention accordingly provides for a temperature control means to keep the temperature of the resonator, in particular the crystal, at a constant value. A thermoelectric cooling means is preferably provided for keeping the temperature constant. If the temperature is kept constant within narrow limits both the refractive indices and the length of the crystal remain constant and stable single-mode operation is obtained. For single-mode operation of the laser one specially selects an output mirro

REFERENCES:
patent: 4617666 (1986-10-01), Liu
patent: 4618957 (1986-10-01), Liu
patent: 4809291 (1989-02-01), Byer et al.
patent: 5249190 (1993-09-01), Kortz et al.
Wolfgang Demtroder Laser Spectroscopy Basic Concepts and Instrumentation, Springer-Verlag Berlin Heidelberg New York 1982 pp. 178-365.

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