FIGURE 6.123 Ratio of difference voltage to sum voltage vs. displacement for Sitek 1L10 continuous PSD.

The disadvantages due to small laser beam sizes and small displacements are overcome by the continuous PSDs. Since these devices are typically much larger (available in several-inch diameters), they typically have longer risetimes than the other PSDs. However, the Sitek 1L10, with a 10 mm linear active range has a measured upper half-power frequency of 3 MHz.

In applications where the output signal from a PSD must be linearly proportional to the displacement of the beam, analog-divider operational amplifiers to obtain Vd/Vs in real time are used to extend the range of linearity of the device. Unfortunately, the frequency response of these amplifiers are often the frequency-response-limiting factors of the PSD system. In cases where high-frequency response is important, Vd alone can often be used if care is taken to operate in the linear range of the device. For large static displacements that are of the order of the size of the detector, Vd and Vs can be recorded with computer-controlled data acquisition, the calibration characteristic numerically fitted to a polynomial, and then any voltage from the detector can be related to beam position.

The noise limitations in OBD sensing are due to the laser, the nature of the reflecting surface, and the PSD. Lasers with good amplitude stability are to be preferred, but this is not an important contribution to noise when Vd/Vs is used to infer displacement. Laser beam-pointing stability, on the other hand, is important. If the reflecting surface is that of a typical solid, then negligible noise is introduced on reflection; this may not be true for a reflector such as a pellicle, where Brownian motion of the surface may be significant. The noise limitations of the PSD are the usual ones associated with the photodetector and the amplifiers.

References

1.A. C. Boccara, D. Fournier, and J. Badoz, Appl. Phys. Lett., 30, 933, 1983.

2.G. C. Wetsel, Jr. and S. A. Stotts, Appl. Phys. Lett., 42, 931, 1983.

3.e.g.: D. Fournier and A. C. Boccara, Scanned Image Microscopy, E. A. Ash, Ed., London: Academic Press, 1980, 347-351; J. C. Murphy and L. C. Aaamodt, Appl. Phys. Lett., 39, 519, 1981; G. C. Wetsel, Jr. and F. A. McDonald, Appl. Phys. Lett., 41, 926, 1982.

4.M. A. Olmstead, S. Kohn, N. M. Amer, D. Fournier, and A. C. Boccara, Appl. Phys. A, 132, 68, 1983.

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