Optical Scattering using DUV

Photon Systems has developed a unique instrument to measure the deep UV non-specular scatter properties of optical elements, including gratings, mirrors, filters, etc. Extraneous scattering may adversely affect the performance of spectrometers and other deep UV instruments, even with a grating separator on the laser beam to clean-up  laser lines and various plasma lines from the laser.

Until now it has been expensive or difficult to measure optical scattering in the deep UV, as commercial instruments do not exist in the deep UV, and other deep UV lasers are expensive.  To better understand the scatter performance of the optical elements used in our spectrometers, we have created a deep UV scatterometer that is based on our compact and cost effective 248.6nm hollow cathode laser and our single channel boxcar integrator and averager with combined PMT.

The graph to the right shows the photon count average of 20 laser pulses using a Photon Systems single channel boxcar integrator and averager. This instrument enables photon counting down to about 2 to 3 photons during a single laser pulse and has a dynamic range from about 50 million, or over 7 orders or magnitude. This is accomplished in the combined PMT with PSI’s gated boxcar integrator, synchronized with a Photon Systems’ 248.6 nm laser.

To enable detection over this wide a dynamic range we employed a series of filters over the gated PMT detector including a Semrock 248.6 nm laser line filter, an Edmund Optics bandpass filter 248/10, and three UV neutral density filter in series, including an OD7, OD1, and OD1.9. This enabled the gated PMT detector to directly observe the laser output during a pulse, and to automatically adjust the sensitivity when the scatter angle was off-axis. They dynamic range is provided by the Photon Systems single channel gated boxcar integrator, which has 3 orders of magnitude in integration capacitor range, and over 6 orders of magnitude range in gain of the PMT.

This data in the graph to the right shows that mirrors are typically 100X lower in scatter than gratings with the Newport mirror having nearly 5x less scattering than the Lambda XHR. To be fair, the Newport mirror is a simple UV enhanced aluminum mirror and the Lambda is a much higher efficiency narrow band mirror centered around 248 nm. Among the gratings, the Newport/Richardson 3600 g/mm SSI grating has highest scatter, followed by the Edmund 3600 g/mm grating, followed by the SSI 1200 g/mm and SSI 3600 g/mm grating.

You will note that at high scattering angles, the mirrors approach the dark signal of the detector, which is near 5 photons. This simple Photon Systems deep UV scatterometer capability is only available in one other organization worldwide, to the best of our knowledge. This is at the Fraunhofer Institute for Applied Optics and Precision Engineering in Jena, Germany.

Optical schematic and photo of deep UV reflectometer and scatterometer setup operating at 248.6 nm for use with gratings, dichroic filters, mirrors, and other optical surfaces. Theata1 is fixed at 10 deg. The only moving element is the optic, which is rotated around the vertical axis only. For gratings, the grating is mounted in a vertical Littrow so that the rotation of the grating is in a “conical” mount configuration.

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