Ring laser

Details Ring laser Ring laser

1991-11-13

1993-06-22

Scott, Jr., Leon

Coherent light generators

Particular resonant cavity

Folded cavity

372 92, 372 66, 372 71, 372108, 372100, 359669, H01S 3083

Patent

active

052220940

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to a ring laser.
2. Discussion of Prior Art
Ring lasers are described in for example British Patent No. 1,237,255, British Patent Appln. No. 2,030,760A and European Patent Appln. No. 0,071,007 (equivalent to U.S.A. Pat. No. 4,556,320). These documents generally speaking disclose lasers having triangular optical paths defined by a respective reflector (mirror or prism) at each triangle apex. In each case, the laser medium is located between two reflectors in a position corresponding to one side of the relevant triangle. The lasers are employed as rotation sensors. Two counter-rotating beams pass round the triangle in each case. Rotation of a ring laser about its central axis perpendicular to its plane sets up a beat frequency in the laser output proportional to the rotation rate. This arises because one counter-rotating beam is affected differently to the other.
Ring lasers with triangular paths suffer from the disadvantage of difficulty of alignment. It is necessary to position three mirrors (or other reflecting devices) so that reflection is from a first mirror to a second, from the second to a third, from the third back to the first and so on along the same path round the ring once more. If one mirror is misaligned, there is no laser beam to detect and no means of identifying which mirror requires adjustment. The arrangement does not lend itself to a straightforward alignment procedure. Similar remarks apply to ring lasers employing four or more mirrors to define square or figure-of-eight optical geometry.
Another form of ring laser, the unidirectional ring laser, is disclosed in U.S. Pat. No. 4,578,793 to Kane et al and also by Trutna et al in Optics Letters, Vol 12, page 248, April 1987. This form of laser operates in an entirely different way. It is arranged to generate only one beam, not two. Its purpose is to avoid the formation of standing waves. The reason for this is as follows. A conventional laser produces a standing wave pattern within its cavity, and within its laser medium in particular. At the antinodes of the standing wave pattern, where light intensity is high, the laser medium gain is depleted. However, the gain is depleted very little in the regions of standing wave pattern nodes. In consequence, gain depletion is inhomogeneous in the direction of the laser light path, i.e. longitudinally of the laser medium. This is commonly referred to as "spatial hole burning". The undepleted gain regions remain capable of supporting laser action. Consequently, the laser outputs more than one longitudinal mode; i.e. it has multiple output wavelengths. To overcome this problem, unidirectional ring lasers were developed. In such a laser, the optical losses are arranged to be higher for one beam rotation or propagation direction than for the other. The laser then operates in the lowest loss or highest gain mode only. Moreover, there is no standing wave pattern in the laser. Instead, a travelling wave propagates in one direction round the ring. Both Kane et al and Trutna et al (referred to above) disclose a unidirectional ring laser consisting of a monolithic Nd:YAG crystal block. Reflections occur at four surfaces of the block, one surface being curved and the others planar. The curved surface acts as a partially transmitting laser cavity end mirror through which a pump beam enters and an output beam exits. The laser light path within the block is from the curved surface to the first, second and third planar surfaces in succession and then back to the mirror. Total internal reflection occurs at the planar surfaces, each of which acts as a non-transmitting laser cavity mirror. The light path sections between the curved surface and the first and third planar surfaces lie in a plane. However, the light path section between the first and second planar surfaces is out of that plane, as is that between the second and third planar surfaces. The result of this is that light leaving the curved surface experiences a net rotation of pola

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patent: 4764933 (1988-08-01), Kozlovsky et al.
patent: 4797896 (1989-01-01), Kane
patent: 4829532 (1989-05-01), Kane
patent: 5007065 (1991-04-01), Trutna, Jr.
patent: 5052815 (1991-10-01), Nightingale

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