Precision Alignment of Laser Mirrors

J. N. Bradford and R. C. Eckardt Naval Research Laboratory, Washington, D.C. 20390. Received 22 August 1968.

A practical problem in laser research is the precise alignment of laser cavity reflectors. Of the few successful solutions to the problem, the most accurate is the precise autocollimator. If

480 APPLIED OPTICS / Vol. 8, No. 2 / February 1969

Fig. 1. Diagram of apparatus for interferometric alignment of laser reflectors. M1 and M2 are the laser cavity reflectors, and M3 is a mirror to deflect the coincident reflected beams to an observing telescope. The hatched area surrounding the laser

rod represents the flashlamp housing.

Fig. 2. Ray paths at reflector M1 in the interference alignment technique: (a) reflected incident beam; (b) reflected return beam.

Fig. 3. Photograph of the interference fringes with noncritical alignment of cavity reflectors.

conditions are favorable, the autocollimator may have an error limit of as little as 0.01 sec of arc. Unfortunately, the large aperture and broadband light sources of autocollimators cannot readily cope with common laser apertures or the light absorption that is characteristic of laser materials like ruby and neodymium glass. As a consequence, the autocollimator may be less accurate than is required or possibly even useless for a given laser mirror alignment.

In order to meet these limitations, we have devised an inter­ference technique for aligning plane mirrors in a laser cavity. I t

is simple, it offers precision compatible with the figures of most laser reflectors, and it has wide applicability. The apparatus arrangement (Fig. 1), applied to a neodymium-glass laser, has three components in addition to the laser cavity: a He-Ne laser light source with a beam-expanding collimator, a mirror (M3), and a telescope or ground glass screen. The laser cavity serves as a two-beam interferometer in which at least one of the reflectors (M1) must be wedged in the manner of an interferom­eter flat. The primary reflecting surface of M1 is initially ori­ented so that it is nearly perpendicular to the He-Ne laser beam. The laser light is largely transmitted by M1, but a small amount is reflected by the front and back surfaces in succession [Fig. 2(a)]. Since the back surface is not parallel to the front, the reflected beam emerges in a direction somewhat different from that of the transmitted beam. The transmitted beam traverses the laser rod and is returned by the mirror M2. If M2 is approximately parallel to M1, the return beam reaches M1 along a direction al­most parallel to that of the transmitted beam. I t is reflected by the back surface of M1 along a path almost parallel to that of the original reflected beam [Fig. 2(b)]. The two reflected beams may be deflected by a mirror (M3) at a convenient point and viewed with the aid of a telescope or screen. By focusing on the end of the laser rod, one sees a diffraction-free circular or elliptical field of fringes on a larger background (Fig. 3), the large field representing the initially reflected beam and the circular or elliptical part, the return.

When true parallelism of the mirrors is achieved, the two re­flected beams are coincident and the fringes vanish. We have estimated that we can adjust to within a tenth of a fringe, the tolerance limit for most laser reflectors. This represents an angular uncertainty of

where λ is the wavelength of the test light (6.33 × 10 - 5 cm) and D is the aperture diameter (1.0 cm). This is nearly two orders of magnitude smaller than the published value of about 5 × 1 0 - 4

rad obtained by superimposing reflected gas laser spots on a hole in a screen.1

In the critical adjustment region near parallelism, where the fringes broaden to fill the field, some means of scanning the order of interference is needed. I t is difficult to determine otherwise whether the mirrors are truly parallel. The condition for perfect alignment is that the field darken or brighten uniformly and not from edge to edge as the order is scanned. One way to accomplish the scanning is deliberately to change the laser cavity length and consequently the light frequency. This can be done readily, as in our laser, with a piezoelectrically or magnetostrictively driven laser mirror. Lacking such control, one may induce a tempera­ture drift in the laser that will cause the resonant cavity to expand or contract and effect the same sort of frequency tuning. Either method will bring about a fringe scan.

Since there is a large difference in the path lengths of the two beams, some care may be necessary. Our alignment is done with a single frequency helium-neon laser, Spectra-Physics model 119, and we find no restrictions on the cavity length. However, if one uses a source laser that supports two or more oscillating modes, conditions are imposed on the optical length of the cavity being aligned. The condition for interference in the cavity interfer­ometer in that situation is related to the condition defining the laser modes. In a laser with cavity length l, the modes are gen­erated according to the relation mλ = 2l, where m is the integral mode order and λ is the mode wavelength. Of course, the wave­length is constrained to lie within the spectral emission line of the laser medium. If the laser generates only one mode the test cavity may be any length, but if two or more modes are emitted, the optical length of the cavity L must approximate an integral

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number of laser lengths.2 - 4 As the number of modes increases, the approximation must become better.4 Using a He-Ne laser of 30-cm length that supports two or three modes, we found fringe contrast was adequate when L = n 30 cm ± 3 cm, where n = 1, 2, 3, . . . .

This alignment technique may readily be applied to any laser cavity with flat mirrors, provided the mirrors are transparent and have low reflectance at the test wavelength. Some complica­tion arises if one of the reflectors is a flat with parallel faces or if one or both end surfaces of the laser medium are perpendicular to the cavity axis. With care, this complication can be at least partially overcome.

With birefringent materials such as ruby care must be taken so that there is no rotation of the plane of polarization of the re­turn beam after two passes through the material. With homo­geneous material this can be accomplished by properly adjusting the initial plane of polarization of the He-Ne laser beam. If the plane of polarization of the return beam is rotated fringe visibility will be reduced.

References 1. P . N . Everett , Rev. Sci. Instrum. 37, 375 (1966). 2. D. R. Herriott, J. Opt. Soc. Amer. 52, 31 (1962). 3. K. A. Stetson and R. L. Powell, J. Opt. Soc. Amer. 56, 1161

(1966). 4. E. F . Erickson and R. M. Brown, J. Opt. Soc. Amer. 57, 367

(1967).

482 APPLIED OPTICS / Vol. 8, No. 2 / February 1969