1. Field of the Invention
The present invention relates to methods and apparatus for lasers and laser resonating cavities, and in particular to methods and apparatus involving the use of a diffraction grating to tune the output of a laser.
2. Description of the Technology
Any periodic arrangement of diffracting bodies can be referred to as a diffraction grating, but in practice gratings usually consist of equidistant rulings made by diamond on a plate or mirror, or a replica of such a ruled grating. Many of the important properties of diffraction gratings are associated with the interference effects between disturbances from corresponding parts of the separate elements. That is to say, they depend more on the periodicity of the diffracting elements than on the shape of the individual elements.
If the elements of a grating are narrow slits, they act as sources of disturbances which, since they are narrow, radiate uniformly. When the elements of the grating are not narrow slits they can still be regarded as sources but they do not radiate uniformly, and the result is that diffracted light intensity peaks of almost exactly the same shape are formed in the same parts of the pattern as before, but with different relative intensities.
A typical diffraction grating, then, is an optical element consisting of a very large number of closely and uniformly spaced grooves ruled on some sort of substrate. In the case of a reflection grating, the grating surface is coated with a reflecting metal so that parallel monochromatic light incident on the grating is diffracted from the grating surface to produce an intensity pattern which depends on the angle at which the grating is viewed. Roughly speaking there are intensity peaks at given angular intervals which arise from the constructive interference of light wavelets emanating from the regularly spaced reflecting elements. The "order" of the diffracting peak depends on the phase difference between the successively scattered wavelets that combine to produce the total light intensity at a given angle. In general the larger the phase difference (in multiples of 2.tau./.lambda. where .lambda. is the wavelength), the weaker the diffracted peak. A so-called "blazed grating" utilizes reflections from a regular array of angled surfaces, and has the advantage that the predetermined groove form concentrates most of the diffracted light within a narrow range of directions.
Diffraction gratings when used with monochromatic light are useful for changing the direction of a beam. In particular, if the light is diffracted back on itself, a diffraction grating can serve as a retroreflective element. However, as in the case of a retroreflecting mirror or prism, a retroreflecting diffraction grating must be aligned; the parallel grating lines have to be perpendicular to the optical axis of the apparatus of which the grating is a part. If a blazed grating is used in tuning to spectral lines having different wavelengths, the grating iines must in addition be parallel to the axis of rotation that determines which spectral line is selected.
FIGS. 1 and 2 show a diffraction grating 15 mounted on an axis of rotation 24 for purposes of tuning the wavelength of the light diffracted in a particular direction (as in a spectral line selector in a laser resonator). There are three degrees of freedom of alignment:
(1) the tuning angle of rotation, .theta. 16,
(2) the angle .alpha. in the plane of the grating 15 between the grating lines and the projection of tuning axis of rotation 24, as shown in FIG. 2a, and
(3) the tilt angle .beta. between the tuning axis of rotation 24 and the plane of the grating 15, as shown in FIG. 2b. (If the grating is a concave grating the angles .theta., .alpha., and .beta. can be suitably redefined.)
An important case is the one in which the diffracted light travels back along the path of the incident beam (the so-called Littrow condition). For this case an analysis of the problem yields (to first order in .alpha. and .beta.) the following equation for the alignment error of the diffracted light when the tuning angle is changed to select a different wavelength: EQU .epsilon.=2.alpha.(sin .theta.-sin .theta..sub.0)+2.beta.(cos .theta.-cos .theta..sub.0), (1)
where .epsilon. is the total alignment error of the diffracted light (as shown in FIG. 3), .theta..sub.0 is the tuning angle at a reference wavelength where the system is correctly aligned, .theta. 16 is the tuning angle for a desired wavelength, .alpha. is the alignment angle in the plane of the grating 15 between the grating lines and the projection of the tuning axis of rotation on the plane of the grating 15 (see FIG. 2a), and .beta. is the tilt angle between the plane of the grating 15 and the tuning axis of rotation 24 (see FIG. 2b).
Previous apparatus and methods of grating alignment require stringent machining and assembly tolerances with respect to adjustments for the two angles of alignment of the grating 15 to the axis of tuning rotation 24. The usual method of grating alignment consists of aligning the system as a whole at one reference wavelength and then trusting that the angles .alpha. and .beta. are sufficiently small that the diffracted light error angle .epsilon. will not become unacceptably large as the grating 15 is tuned to other wavelengths.
The approach of providing adjustments for both .alpha. and .beta. and developing an alignment procedure to reduce them to sufficiently small values has serious problems associated with it. There is no easy way of measuring either .alpha. or .beta.. Even if there were convenient ways of measuring .alpha. and .beta., such an approach would require adjustments for both angles.
There has been a long felt but unsatisfied need for the design and construction of lasers whose output wavelength can be tuned rapidly and accurately to particular spectral lines. Tunable CO.sub.2 lasers, for example, will become more important in the future, especially as sources for remote sensing spectrometers for chemical detection. Remote detection of specific types of chemical contamination will certainly find many uses.