Multichannel grating spectrometer for millimeter wavesJ. A. Pasour and S. P. Schlesinger Citation: Review of Scientific Instruments 48, 1355 (1977); doi: 10.1063/1.1134890 View online: http://dx.doi.org/10.1063/1.1134890 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/48/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Multichannel millimeter wave interferometer for W7-AS Rev. Sci. Instrum. 68, 1162 (1997); 10.1063/1.1147878 Application of blazed gratings to millimetersubmillimeter wave gyrotrons J. Appl. Phys. 74, 2197 (1993); 10.1063/1.354726 A new Fourier transform millimeter wave spectrometer Rev. Sci. Instrum. 63, 4108 (1992); 10.1063/1.1143220 Broadband Millimeter Wave Paramagnetic Resonance Spectrometer Rev. Sci. Instrum. 31, 551 (1960); 10.1063/1.1931248 A Grating Spectrometer for Millimeter Waves Rev. Sci. Instrum. 19, 586 (1948); 10.1063/1.1741332

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I A (without noise gate) = 100.4/s ± 1.6/s A (with noise gate) = 99.8/s ± 3.0/s The fraction of counts lost with the gate in is 0.006.

II A (without noise gate) = 107.4/s ± 2.2/s A (with noise gate) = 106.6/s ± 2.2/s The fraction of counts lost with the gate in is 0.007. The agreement with the theory is quite acceptable.

Finally, the above circuit is by no means unique for this task; it has been designed to take care of some sys­temic peculiarities of an electron scattering experiment. Ideally, MV 1 and MV2 should have output pulses as narrow as possible. Also, it has been found that by moni­toring the reject count rate, noise conditions can be recognized and corrections made.

Multichannel grating spectrometer for millimeter waves J. A. Pasour")

Department of Physics. North Carolina State University. Raleigh. North Carolina 27607

S. P. Schlesingerb)

U. S. Naval Research Laboratory. Washington. DC 20375

(Received 25 March 1977; in final form. 2 June 1977)

A multichannel grating spectrometer for use with high-power (~1 MW), short-duration (S 5 ns) pulses of millimeter-wavelength radiation is described.

Recently, millimeter-wavelength pulses of about 2-3 ns duration at power levels of about 1 MW have been produced by reflecting a 500-kW, 9.3-GHz microwave pulse from the front of a relativistic electron beam at the Naval Research Laboratory.! To analyze the spectral content of these bursts of radiation, a multichannel grating spectrometer, employing both optical and micro­wave techniques, was designed and fabricated. Although grating spectrometers have been used previously in the far infrared and millimeter-wave regions, they have typically been single-channel devices for use with low­power, long-pulse (or cw) radiation. 2

The spectrometer, as shown schematically in Fig. 1, consists of an entrance horn and collimating lens, an aluminum echelette grating, a spherical, copper-clad mirror, and an array of detectors. Although not required for the experiment mentioned above, filtering of higher orders may be accomplished by inserting mesh filters into the input line to the spectrometer. The entire as­sembly is enclosed in an absorber-lined, aluminum box of dimensions 1.2 m x 1.5 m x 25 cm.

The conical aluminum horn opens from a diameter of 7 mm (corresponding to a cutoff wavelength of 12 mm) to a diameter of 12.7 cm. A hyperbolic Lucite lens placed over the horn collimates the radiation onto a grating. Three 18 x 30-cm gratings have been fabricated to cover wavelengths from 1-8 mm, the only difference between them being the groove spacing (d! = 5.1 mm, d2 = 2.8 mm, d3 = 1.4 mm). The gratings are blazed at an angle of 30° and are typically operated at an inci­dence angle of 60°. Diffraction angles ranging from - 5° to +45° are used to span the desired wavelength range in first order. These values of the various grating parame­ters were chosen to provide a relatively large angular

1355 Rev. Sci. Instrum., Vol. 48, No. 10, October 1977

dispersion. A grating with blaze angle 10°, identical to that described in Ref. 2, and groove spacing 8 mm was also tested, but it proved to be inferior both in first­order intensity and in resolution when operated at a constant angle of incidence over the one-octave range from 5-10 mm.

The spherical mirror has a 1.5-m radius of curvature and is 80 cm long by 20 cm high. It was molded from plaster and then covered with a sheet of copper, thus providing an inexpensive mirror sufficiently smooth for use at millimeter wavelengths. At the focal points of the mirror, various types of horns or light pipes can be positioned to carry the dispersed radiation to an array of detectors. Typically, standard microwave horns with a width of about 1 cm have been used. A calibrated, variable attenuator is connected to each horn to lower the incoming pulse to a level suitable for detection by

LENS GRATlN~ SPHERICAL MIRROR

FIG. I. Schematic diagram of spectrometer.

Notes

::::cJ :::Q

DETECTORS

:::C]

::::J

1355

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the broadband crystal detector. As many as eight channels can be monitored simultaneously.

The spectrometer was calibrated with swept sources from 2.5 to 8 mm, using the two coarsest gratings. A linearly polarized input with the E-vector parallel to the grating grooves was used in all the measurements. The insertion loss (signal detected through a I-cm-wide horn placed at the optimum position versus signal incident on the spectrometer) and the resolving power AI ~A are plotted versus wavelength in Fig. 2. Here, ~A is the difference in wavelength between two monochromatic sources whose detected signals are down by 3 dB where they overlap. The drop in resolution at the longer wavelengths is probably due to diffraction effects as well as to spherical aberration or imperfections in the corresponding por­tion of the mirror. The increased insertion loss at these wavelengths is due partially to the lower resolution and partially to the intensity profiles of the gratings. At the shorter wavelengths the resolving power is limited slightly by the size of the detector horn.

Acknowledgments. The authors greatly appreciate the help of Dr. D. Zuckerman of Bell Laboratories, Holmdel, NJ, where the spectrometer was calibrated, and of Dr. V. L. Granatstein and Dr. R. K. Parker and the Naval Research Laboratory, where the experiments were per­formed. This work partially supported by AFOSR under Contract F49620-76-C-0007.

PUBLICATIONS

CD 30 u

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GRATING 1 (d'5.1 mm)

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4 567 WAVELENGTH (mm)

GRATING 2

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3.0 3.5 4.0 WAVELENGTH (mm)

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FIG. 2. Insertion loss (solid line) and resolving power (dashed line) as functions of wavelength for two different gratings.

al On temporary assignment at NRL. hi Permanent address: Department of Electrical Engineering and

Computer Science, Columbia University, New York, NY 10027. IV. L. Granatstein, P. Sprangle, R. K. Parker, J. Pasour, M.

Herndon, S. P. Schlesinger, and J. L. Seftor, Phys. Rev. A 14, 1194 (1976).

2 S. S. Sesnic, IEEE Trans. Nuc!. Sci. NS-18 (4),365 (197\).

All books received in the office of the editor will be acknowledged by listing in the Books Received column of this section. Books pertaining directly to instrumentation or closely allied subjects will be selected for review as space permits. Reviews of books on physics or engineering, other than instru­mentation, will be carried in Physics Today or other journals published by the American Institute of Physics.

Books Received

Photon Correlation Spectroscopy and Velocimetry. Edited by H. Z. Cummins and E. R. Pike. Pp. 589 + ix. lIIus. Plenum Press, 227 West 17th St., New York 100\ I, 1977. Price $59.50.

Structure Determination by X-ray Crystallography. M. F. C. Ladd and R. A. Palmer. Pp. 393 + xvi. lIIus. Plenum Press, 227 West 17th St., New York 10011, 1977. Price: $29.50.

Interpretive Techniques for Microstructural Analysis. Edited by

1356 Rev. Sci. [nstrum., Vol. 48, No. 10, October 1977

James L. McCall and P. M. French. Pp. 201 + vii. lIIus. Plenum Press, 227 West 17th St., New York 10011, 1977. Price: $25.00.

Analytical Calorimetry. Vol. 4. Edited by Roger S. Porter and Julian F. Johnson. Pp. 251 + ix. lIIus. Plenum Press, 227 West 17th St., New York 10011, 1977. Price: $27.50.

Proton Synchrotron Accelerator Theory. E. J. N. Wilson. CERN77-07. Pp. 49 + vi. IlIus. CERN, Service d'information scientifique­RD/227, Geneva, Switzerland. No price listed (softcover).

Copyright © 1977 American Institute of Physics 1356

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