interferometric laser beam scanner

1
Interferometric Laser Beam Scanner D. E. Brown Institute of Science and Technology, The University of Michigan, Ann Arbor, Michigan 48107. Received 15 July 1968. Myers and Pole have caused the output beam of a hollow cathode mercury ion laser to scan by Q-control at a focal plane (the Scanlaser). 1 A similar result has been demonstrated here using a variable spacing, in-cavity, Fabry-Perot interferometer. The optical arrangement is shown in Fig. 1. Fig. 1. Interferometric laser beam scanner. Maximum transmission through the interferometer, and hence lasing, is permitted for those incidence angles (θ) which satisfy: nλ = 2d cosθ. Here, n is the order of interference, λ is the lasing wavelength, and d is the plate separation. By varying d, the allowed lasing directions may be correspondingly varied. The device is not a beam deflector, rather it controls the direction in which the laser is allowed to oscillate. The tantalum hollow cathode of our mercury discharge tube 2 is 20 cm long by 3.2 cm outside diameter. Mercury enriched to 90% of the 202 isotope is used. Although maximum trans- mission through the interferometer is only 75%, no particular difficulty is experienced in attaining lasing with it in the cavity. The interferometer must be tilted so that lasing cannot occur from it acting as a flat mirror. A piezoelectric cylinder is used to move one plate with respect to the other. With proper initial plate spacing, a change of approximately λ/2 serves to scan the output by 40 mrad. Using a tube with greater length to width ratio, a smaller scan angle is attainable. The output spot spacing to spot width ratio (i.e., the number of unambiguous single order resolution elements) corresponds closely to the finesse of the interferometer ( 12). This is a severe limitation, since a finesse of 100 is about all that can be attained. One way to obtain a greater number of resolution elements would be to gang two interferometers, in which case, the number could closely correspond to the product of the finesse of each. The scan rate of this device is limited by the rate at which the plate separation can be changed. It is tempting to consider use of an electrooptic spacer for the variable Fabry-Perot. How- ever, for the configuration shown, the electric field required exceeds the dielectric strength of presently available materials. The extended variable interferometer of Buck and Holland 3 may provide the answer, although the tilt required will be a problem. This work was sponsored by the U.S. Air Force, Wright- Patterson Air Force Base, Air Force Avionics Laboratory, under a contract. References 1. R. A. Myers and R. V. Pole, IBM J. 11, 502 (1967). 2. H. Wieder, R. A. Myers, C. L. Fisher, C. G. Powell, and J. Colombo, Rev. Sci. Instrum. 38, 1538 (1967). 3. W. E. Buck and T. E. Holland, Appl. Phys. Lett. 8, 198 (1966). 2422 APPLIED OPTICS / Vol. 7, No. 12 / December 1968

Upload: d-e

Post on 03-Oct-2016

212 views

Category:

Documents


0 download

TRANSCRIPT

Interferometric Laser Beam Scanner D. E. Brown

Institute of Science and Technology, The University of Michigan, Ann Arbor, Michigan 48107. Received 15 July 1968.

Myers and Pole have caused the output beam of a hollow cathode mercury ion laser to scan by Q-control at a focal plane (the Scanlaser).1 A similar result has been demonstrated here using a variable spacing, in-cavity, Fabry-Perot interferometer. The optical arrangement is shown in Fig. 1.

Fig. 1. Interferometric laser beam scanner.

Maximum transmission through the interferometer, and hence lasing, is permitted for those incidence angles (θ) which satisfy: nλ = 2d cosθ. Here, n is the order of interference, λ is the lasing wavelength, and d is the plate separation. By varying d, the allowed lasing directions may be correspondingly varied. The device is not a beam deflector, rather it controls the direction in which the laser is allowed to oscillate.

The tantalum hollow cathode of our mercury discharge tube2

is 20 cm long by 3.2 cm outside diameter. Mercury enriched to 90% of the 202 isotope is used. Although maximum trans­mission through the interferometer is only 75%, no particular difficulty is experienced in attaining lasing with it in the cavity. The interferometer must be tilted so that lasing cannot occur from it acting as a flat mirror. A piezoelectric cylinder is used to move one plate with respect to the other.

With proper initial plate spacing, a change of approximately λ/2 serves to scan the output by 40 mrad. Using a tube with greater length to width ratio, a smaller scan angle is attainable.

The output spot spacing to spot width ratio (i.e., the number of unambiguous single order resolution elements) corresponds closely to the finesse of the interferometer (≃12). This is a severe limitation, since a finesse of 100 is about all that can be attained. One way to obtain a greater number of resolution elements would be to gang two interferometers, in which case, the number could closely correspond to the product of the finesse of each.

The scan rate of this device is limited by the rate at which the plate separation can be changed. It is tempting to consider use of an electrooptic spacer for the variable Fabry-Perot. How­ever, for the configuration shown, the electric field required exceeds the dielectric strength of presently available materials. The extended variable interferometer of Buck and Holland3

may provide the answer, although the tilt required will be a problem.

This work was sponsored by the U.S. Air Force, Wright-Patterson Air Force Base, Air Force Avionics Laboratory, under a contract.

References 1. R. A. Myers and R. V. Pole, IBM J. 11, 502 (1967). 2. H. Wieder, R. A. Myers, C. L. Fisher, C. G. Powell, and J.

Colombo, Rev. Sci. Instrum. 38, 1538 (1967). 3. W. E. Buck and T. E. Holland, Appl. Phys. Lett. 8, 198

(1966).

2422 APPLIED OPTICS / Vol. 7, No. 12 / December 1968