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A technique for the alignment of a ring laser cavity
Technical
note
A technique for the alignment of a ring laser cavity ZUN-JIANG
ZHANG
An alignment technique for a ring laser cavity is presented which is suitable for a resonator into which auxiliary laser beams cannot pass. KEYWORDS: lasers,alignment, resonators,ring lasercavities
Introduction In order to make an active ring cavity laser, the resonator has to be perfectly aligned so that the beam passing through the axis of the discharge tube, after a ‘round-trip’, retraces exactly the same optical contour. This is often difficult due to the critical alignment of optical components. Fig. 1 shows a ring laser cavity with two HeNe discharge tubes, DT,, DT,, two plane mirrors, M,, M,, two spherical mirrors, M,, M, and a high loss optical element, F. The four mirrors are all completely reflecting to optical beams having a wavelength of 632.8 nm. Thus, the techniques reported earlier1,2 are unsuitable. In this paper an alignment technique for this kind of ring laser cavity is presented.
discharge tubes are collinear in the sense that the beam coincides with both of their axes through M, and M,. If they are also collinear through M, and M,, the ring laser cavity is in perfect alignment. This is the principle of this alignment technique. After the above condition is fulfilled, mirror M is set between mirrors M, and M,, as shown in Fig. lb, and is adjusted so that the beam can travel along the axis of discharge tube DT,. Auxiliary mirror M is a coated completely reflecting mirror. Since the cavity is divided into two portions to be aligned, it is unnecessary for the reference beam to pass through M to close the Aperture,
Optical ,
element,
F
Alignment procedure On an optical table, an auxiliary HeNe laser is used to produce a reference beam parallel to the surface of the table. As shown in Fig. la mirrors M, to M, and the plane mirror M are carefully positioned: mirror M is for alignment only and is not part of the ring. A small aperture, A, is placed close to mirror M,. The beam from the auxiliary laser hits mirror M and travels clockwise via mirror M, and aperture A, then via mirrors M, to M,, and mirror M, forming a ring optical contour. The ring can be made in a plane parallel to the surface of the table by adjusting every mirror. Discharge tube DT, and the optical element F are then placed between mirrors M, and M 1. By tuning the tube the beam can be made to travel along its axis. A family of concentric diffraction circles can be observed on a screen behind mirror M. In the same way, discharge tube DT, is inserted and adjusted between mirrors M, and M, and a similar diffraction pattern occurs on the screen. The two The author is in the Department of Precision Instruments, Tsinghua University, Beijing, PRC. Received 22 January 1988.
0030-3992/88/060321-02/$03.00 VOL 20 NO 6 DECEMBER 1988
Mpirror 3
Discharge
a
, M
tube,
2
DT2
Aperture,
,
Optical
element,
F
Screen Mirror
’
-Mirror, 3
Discharge
tube,
M
2
DT2
b L
Fig. I Schematic mirrors. M,, M, - plane mirrors. alignments of the
of a ring laser cavity with four completely reflecting - spherical mirrors with radius of 2 m; M,, M, a - adjustments of discharge tubes; b ring cavity
@ 1988 Butterworth & Co (Publishers) Ltd 321
optical contour. The wedge and thickness of M are therefore of no importance. Reflected by mirror M,, the beam does not overlap with aperture A in general, nevertheless one can make it do so by tuning mirror M,. Since the centre of aperture A is a point on the axis of discharge tube DT,, the beam reflected by M, can be made to coincide completely with this axis by adjusting M, without affecting the positional overlap of the beam and aperture A. In theory, this procedure ensures that the ring laser is in perfect alignment. Aperture A and mirror M can be removed and the power supply of the discharge tubes can be turned on. At this point laser action will be obtained. However, due to adjustment inaccuracy or observational errors in judging whether the beam and the axes of the tubes are collinear or not, in practice the alignment of the ring resonator is not
322
perfect. Sometimes very careful trimming is necessary for laser action. Conclusion A technique for the alignment of a ring laser cavity has been presented. With this technique, laser action was easily obtained in the ring laser cavity. Experiments show that it is very effective, especially when auxiliary laser beams cannot pass into the ring through mirrors. In the technique the adjustments of mirrors M, and M, are quite independent. References 1 Fang, Z-H., Thienpoot, H. ‘An alignment 2
technique for a ring laser cavity’Opt Laser Technoll6 (1984) 269-270 Uppal, J.S., Monga, J.C., Bhawakar, D.D. ‘A simple technique for the alignment of a ring resonator’ Opr Laser Technoll7 (1985) 215-216
OPTICS AND LASER TECHNOLOGY
note
A technique for the alignment of a ring laser cavity ZUN-JIANG
ZHANG
An alignment technique for a ring laser cavity is presented which is suitable for a resonator into which auxiliary laser beams cannot pass. KEYWORDS: lasers,alignment, resonators,ring lasercavities
Introduction In order to make an active ring cavity laser, the resonator has to be perfectly aligned so that the beam passing through the axis of the discharge tube, after a ‘round-trip’, retraces exactly the same optical contour. This is often difficult due to the critical alignment of optical components. Fig. 1 shows a ring laser cavity with two HeNe discharge tubes, DT,, DT,, two plane mirrors, M,, M,, two spherical mirrors, M,, M, and a high loss optical element, F. The four mirrors are all completely reflecting to optical beams having a wavelength of 632.8 nm. Thus, the techniques reported earlier1,2 are unsuitable. In this paper an alignment technique for this kind of ring laser cavity is presented.
discharge tubes are collinear in the sense that the beam coincides with both of their axes through M, and M,. If they are also collinear through M, and M,, the ring laser cavity is in perfect alignment. This is the principle of this alignment technique. After the above condition is fulfilled, mirror M is set between mirrors M, and M,, as shown in Fig. lb, and is adjusted so that the beam can travel along the axis of discharge tube DT,. Auxiliary mirror M is a coated completely reflecting mirror. Since the cavity is divided into two portions to be aligned, it is unnecessary for the reference beam to pass through M to close the Aperture,
Optical ,
element,
F
Alignment procedure On an optical table, an auxiliary HeNe laser is used to produce a reference beam parallel to the surface of the table. As shown in Fig. la mirrors M, to M, and the plane mirror M are carefully positioned: mirror M is for alignment only and is not part of the ring. A small aperture, A, is placed close to mirror M,. The beam from the auxiliary laser hits mirror M and travels clockwise via mirror M, and aperture A, then via mirrors M, to M,, and mirror M, forming a ring optical contour. The ring can be made in a plane parallel to the surface of the table by adjusting every mirror. Discharge tube DT, and the optical element F are then placed between mirrors M, and M 1. By tuning the tube the beam can be made to travel along its axis. A family of concentric diffraction circles can be observed on a screen behind mirror M. In the same way, discharge tube DT, is inserted and adjusted between mirrors M, and M, and a similar diffraction pattern occurs on the screen. The two The author is in the Department of Precision Instruments, Tsinghua University, Beijing, PRC. Received 22 January 1988.
0030-3992/88/060321-02/$03.00 VOL 20 NO 6 DECEMBER 1988
Mpirror 3
Discharge
a
, M
tube,
2
DT2
Aperture,
,
Optical
element,
F
Screen Mirror
’
-Mirror, 3
Discharge
tube,
M
2
DT2
b L
Fig. I Schematic mirrors. M,, M, - plane mirrors. alignments of the
of a ring laser cavity with four completely reflecting - spherical mirrors with radius of 2 m; M,, M, a - adjustments of discharge tubes; b ring cavity
@ 1988 Butterworth & Co (Publishers) Ltd 321
optical contour. The wedge and thickness of M are therefore of no importance. Reflected by mirror M,, the beam does not overlap with aperture A in general, nevertheless one can make it do so by tuning mirror M,. Since the centre of aperture A is a point on the axis of discharge tube DT,, the beam reflected by M, can be made to coincide completely with this axis by adjusting M, without affecting the positional overlap of the beam and aperture A. In theory, this procedure ensures that the ring laser is in perfect alignment. Aperture A and mirror M can be removed and the power supply of the discharge tubes can be turned on. At this point laser action will be obtained. However, due to adjustment inaccuracy or observational errors in judging whether the beam and the axes of the tubes are collinear or not, in practice the alignment of the ring resonator is not
322
perfect. Sometimes very careful trimming is necessary for laser action. Conclusion A technique for the alignment of a ring laser cavity has been presented. With this technique, laser action was easily obtained in the ring laser cavity. Experiments show that it is very effective, especially when auxiliary laser beams cannot pass into the ring through mirrors. In the technique the adjustments of mirrors M, and M, are quite independent. References 1 Fang, Z-H., Thienpoot, H. ‘An alignment 2
technique for a ring laser cavity’Opt Laser Technoll6 (1984) 269-270 Uppal, J.S., Monga, J.C., Bhawakar, D.D. ‘A simple technique for the alignment of a ring resonator’ Opr Laser Technoll7 (1985) 215-216
OPTICS AND LASER TECHNOLOGY