Donald R. Herriott

Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses

A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.

Off-Axis Paths in Spherical Mirror Interferometers

When a spherical mirror interferometer is illuminated by an off-axis ray of light, the repeated reflections cause the ray to trace a path which lies on the surface of a hyperboloid, with the points of reflection on the mirrors on ellipses. Under special conditions, these ellipses may become circles, with the points of reflection displaced by an angle 2θ after every round trip. When 2νθ = 2μπ, ν and μ being integers, the rays retrace their paths. These ray paths give rise to additional resonances which were observed. Pictures of the points of reflection are reproduced. The theory is in good agreement with the experimental observations. In laser amplifiers these ray paths enable one to obtain long effective path lengths in the active medium which may be contained in a thin annular cylindrical or hyperboloidal shell.

Folded Optical Delay Lines

A long optical path has been folded between two 7.5-cm diam spherical or aspherical mirrors to provide an output beam which can be well separated from previous reflections with 1000 or more passes between the mirrors. The 3000-m path provides 10 μsec of delay. This system can be used as a dispersionless optical delay line for use in filtering or storage of information modulated onto the light beam. The pattern of beams between the two mirrors is obtained in one of two ways. A small perturbing mirror may be inserted to give a series of offset ellipses, or one or both of the mirrors can be made astigmatic to give a Lissajous pattern of spots on each mirror. The output beam can be separated from others by discriminating in both angle and position. The diffraction losses of the system are much lower than those for an open beam because of the periodic focusing of the spherical mirrors. The extreme dependence of the loss of the delay line upon the absorption and scattering loss of the mirrors makes the system dependent upon very low loss mirrors and also makes the system a suitable method for measuring mirror loss. Block diagrams are shown for some possible filtering and storage applications.

EBES: A practical electron lithographic system

An electron beam exposure system (EBES) has proven to be practical and economic for generating high-quality fine-featured integrated circuit masks. It is also capable of exposing patterns directly on resist-coated silicon wafers and, when so used, is an effective tool with which to develop new semiconductor devices. EBES combines continuous translation of the mask or wafer substrate with periodic deflection of the electron beam in a raster-scan mode of exposure. Substrate position is monitored by means of laser interferometers. The strategy permits both the electronic and mechanical subsystems to work well within their limits of capability and contributes to system reliability. It also permits the system to be stepped up to higher resolution and faster exposure as brighter electron sources, more sensitive resist, and faster data processing techniques are developed.

A Scanning Spherical Mirror Interferometer for Spectral Analysis of Laser Radiation

The results of a theoretical and experimental investigation of a scanning spherical mirror interferometer designed specifically for analysis of laser radiation are given. It is shown that the high degree of spatial coherence and monochromaticity of laser radiation makes it possible to excite individual interferometer modes selectively. A theory of single-mode excitation is presented, and a specific example treated. The mechanical construction of the instrument and the dynamics of the scanning system are described. The capability of the instrument for observation of laser mode separation, amplitudes, and frequency shifts is illustrated by specific experiments. It is shown that the instrument can easily be used in both the visible and infrared. Results of simultaneous observation of beats between laser modes with an rf spectrum analyzer and of the laser optical field with the scanning interferometer are given, and it is shown that the combined system, in certain cases, will permit determination of the relative phases of the laser modes. Experiments illustrating the use of the scanning interferometer for observation of mode competition and the effects of magnetic fields on laser output are recounted, as is also the operation of an active scanning interferometer with which resolving powers of 2.5 × 109 and finesses of 1040 were obtained. Effects of coupling between laser and interferometer are shown, and a polarization isolator for decoupling is described.

Optical Properties of a Continuous Helium-Neon Optical Maser*

A continuous optical maser has been operated at five wavelengths in the near infrared. Such a maser oscillator consists of a medium having optical amplification at the wavelength of interest and a Fabry-Perot interferometer as a resonant cavity.The optical amplification is provided by maser action in a discharge through a mixture of helium and neon gas. The Fabry-Perot interferometer is constructed within the gas volume using two very flat, and highly reflecting, parallel, silica plates.The transmission of the Fabry-Perot plates, although small, allows a beam to pass through each end window, with four milliwatts of continuous output power in the strongest transition at 1.153-μ wavelength.Examination of the beam shows that it is almost diffraction limited for its one-centimeter diameter. The spectral line shape at each transition is made up of three or more components each less than a few hundred cycles in width separated by the spacing of orders in the interferometer.

Resonant Optical Faraday Rotator

Faraday rotation of the plane of polarization of light can be greatly enhanced by placing the Faraday rotating material in a resonant cavity. The transmission characteristics of a resonant rotator are analyzed, and observation of resonant rotation is reported. Under suitable conditions, the resonant rotation can be two orders of magnitude larger than the single-pass Faraday rotation. Bandwidth limitations are discussed, and a resonant rotation-bandwidth product is determined.

Multiple-Wavelength Multiple-Beam Interferometric Observation of Flat Surfaces

The use of multiple-beam interferometry makes it possible to observe very small deviations in the flatness of optical surfaces. A method is described which reduces the contour interval between multiple beam fringes to between 1/20 and 1/100 wavelength. This method has been applied to the critical examination of a quantity of Fabry-Perot plates for use in continuous helium-neon optical masers.

Long-Path Multiple-Wavelength Multiple-Beam Interference Fringes*

Multiple-beam interference fringes between two closely spaced highly reflecting surfaces have long been used for sensitive examination of surface contours. The spacing between the two surfaces has been limited to a few millimeters at most to minimize the walk-off effect of the beam between the two slightly angled surfaces and to obtain fringes having a narrow width compared to their spacing, usually referred to as a high finesse.A system consisting of two spherical surfaces with a lens between them gives sharp multiple-beam fringes at a spacing of 50 cm. Fringes with a contour interval of λ/6 result from the use of the separate wavelengths of a helium–neon laser as a source.This long-path system can be used to examine the index homogeneity in a sizeable thickness of transparent solids, plasmas, or gases. The system should be useful for examining thermal, pressure, or composition gradients in wind tunnels, shock tubes, or plasma studies with about ten times normal sensitivity.

Recording Electronic Lens Bench

The measurement of lens characteristics by the method of contrast rendition of a sine wave target has been studied extensively in recent years. This method has not, however, been widely used because of the time required to measure a lens at many field angles and focal positions as well as the time required to reduce the data to suitable form. These limitations are caused by poor signal-to-noise ratio from the photoreceptor when operating at reduced lens aperture or with wavelength limiting filters. A system is described which uses sweep frequency sine wave targets with rectification and filtering of the electrical signal to reduce the required band width thereby overcoming these difficulties. Rapid scanning with oscilloscope presentation is used for preliminary adjustment of lens parameters. An X-Y recorder combines curves for a number of lens adjustments on preprinted paper for permanent records at a rate of ten seconds per curve.