OPTICS IN 1989 of about 20. The accompanying figure shows the differ- ence between the ATI electron energy distribution when linearly (solid curve) and circularly (dashed curve) polar- ized 10 m light is used to ionize xenon. This additional factor multiplies the 2000 already mentioned. (Further- more, if the ion species is not of interest, an additional variable is introduced that can add even greater flexibili- ty!) Although we have emphasized the implications of MPI for electrons, it is clear that ion motion is frozen on the time scale of an ultrashort laser pulse. The plasma density profile will be determined by the initial gas density profile. Thus, like the electron temperature, the ion density is a parameter under experimental control. One important implication of these results is the pro- duction of plasmas for recombination x-ray lasers. If the plasma electron temperature is less than about 10% of the ionization potential, transient inversions are predicted for hydrogen-like or lithium-like ions. Furthermore, the small scale of plasmas that can be produced by MPI is very fa- vorable for all cooling mechanisms and will facilitate qua- si-steady state gain on transitions above the resonance lev- el. Real-time enhancement of submicron defects using photorefractives L. Hesselink, Stanford University W e have developed a new approach using photore- fractives for real-time inspection of periodic masks or cracks and defects in non-periodic objects. 1-2 The ap- proach is based on Fourier transform, holographic record- ing of the object, filtering, and phase-conjugate readout using a photorefractive crystal. These processes are per- formed simultaneously to allow real-time operation. The object to be inspected (top part of the figure) is placed in the input plane, and the defect-enhanced image in the bot- tom part appears at the output plane, in a time limited only by the time constant of the photorefractive material. This time constant is material and light intensity depen- dent and ranges in our experiments from 50 to 250 msecs. This method differs from previous approaches in that all operations are carried out in the Fourier domain, no ob- ject dependent mask is needed, and real-time operation is achieved without the need for careful alignment of filters and masks. The technique for performing real-time defect enhance- ment is based on two observations. First, the Fourier Defect size Coordinates ( m 2 ) (hor., vert.) 100 X 100 (22, 7.5) 50 X 100 (10,13.5) 100 X 50 (12.5,25) 10 X 100 (15, 20.5) 25 X 100 (24, 27.5) 100 X 25 (25.5, 22) 100 X 10 (25.5,15) Optical surface inspection using Fourier transform ho- lography in photorefractives. transform of a periodic object is an array of spikes, where- as the Fourier transform of a small defect is a low ampli- tude, broad signal. The second observation is that the dif- fraction efficiency of volume phase holograms formed in a photorefractive medium is maximized when the amplitude of the interfering beams is approximately equal, and de- creases as the difference in intensity increases. These obser- vations are used in our apparatus, and defects are en- hanced by tuning the amplitude of the reference wave to the amplitude of the weak defect signal. This increases the signal strength of the phase conjugated defect signal rela- tive to the periodic background. As a result, a small spot is visible in the output plane at the defect location, and the periodic pattern has been erased from the image, as shown in the lower part of the figure. Defects ranging from 10 to 100 m 2 are thus easily located and may be inspected in more detail by subsequent digital or optical processing. More recently we have extended this approach to in- clude detection of submicron features. 2 As an example, we have detected cracks as small as 0.14 m in diskheads of magnetic recording devices. The optical signal may be fur- ther enhanced by simple digital processing. REFERENCES 1. E. Ochoa, J.W. Goodman, and L. Hesselink, Real-time enhancement 36OPTICS NEWS • DECEMBER 1989