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Photodiode arrays: A convenient tool for laser diagnosticsW. Seka and J. Zimmermann Citation: Review of Scientific Instruments 45, 1175 (1974); doi: 10.1063/1.1686841 View online: http://dx.doi.org/10.1063/1.1686841 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/45/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A compact and high repetitive photodiode array detector for the JT-60U Thomson scattering diagnostic Rev. Sci. Instrum. 70, 747 (1999); 10.1063/1.1149435 Onedimensional parallel Josephsonjunction arrays as a tool for diagnostics Appl. Phys. Lett. 65, 2102 (1994); 10.1063/1.113041 A combined Thomson–Rayleigh scattering diagnostic using an intensified photodiode array Rev. Sci. Instrum. 63, 3369 (1992); 10.1063/1.1142554 Multilayer Targets as a Diagnostic Tool for LaserProduced Plasmas J. Appl. Phys. 42, 315 (1971); 10.1063/1.1659592 THE LASER INTERFEROMETER AS A DIAGNOSTIC TOOL IN SHOCKTUBE EXPERIMENTS Appl. Phys. Lett. 8, 59 (1966); 10.1063/1.1754482
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1175 Notes
(a)
(b)
FIG. 2. The free induction decay of 8lBr (off resonance) in crystalline NH.Br. The 90° pulse width is 16 ).tsec. Oscilloscope sweep is 10 ).tsec/ div. (a) saturation of the rf amplifier system causes a long recovery time for the de baseline anrI (b) using the amplifier described the de baseline is restored immediately after the pulse.
This post-amplifier has a gain of ten and a bandwidth from 1 kHz to 15 MHz. The low frequency cutoff is used to filter out any 60 Hz pickup in the pre-amplifiers. Figure 2 (a) and (b) show the improvement in dead time when the postamplifier is used as a part of the rf amplillcation system.
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FIG. 3. The free induction decay of 81Br after detection with a phase coherent detector. (a) without and (b) with use of amplifier described. :\'ote: baseline recovery and increased dead time in (a). Dead time is 6 ).tsec. In (b) dead time is 2 ).tsec.
Figure 3(a) and (b) show the improvement in baseline shift owing to ac coupling when another post-amplifier is used after the phase coherent detection system.
*We wish to acknowlege support for this project by the National Research Council of Canada.
Photodiode arrays: A convenient tool for laser diagnostics* W. Seka and J. Zimmermann
Institute of Applied Physics. University of Berne. Switzerland
(Received 21 May 1974)
We report on a variety of useful applications of photodiode arrays in on-line diagnostics of pulsed lasers. These diagnostics include beam cross section monitoring and psec-pulse duration monitoring.
In laser research it is often desirable if not necessary to check a number of laser parameters in an accurate and possibly on-line fashion. Such parameters may be the beam cross section, beam divergence, pulse duration of ultrashort pulses, etc. So far, these characteristics could be measured only through laborious and time consuming photography or by means of expensive TV cameras. We would like to present here an inexpensive alternative employing commercially available photodiode arrays. These simply replace the film and yield instant 1-dimensional time-integrated images on a scope.
FIG. 1. Laser beam cross section of a single mode-locked Nd-YAG pulse taken with a photodiode array at 1 m from the exi t mirror.
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In Fig. 1 a laser beam cross section is reproduced. We used a single pulse out of a mode-locked r\"d-YAG laser but the technique works for any pulse duration. It is thus easy to measure the divergence as well as the exact beam cross section. The spikes visible in the wings of Fig. 1 are due to electronic clock noise but diode signal measurements may be made well within this region. The intensity distribution near the focal spot of a lens may also be easily investigated in this way although the spatial resolution of the diode arrays appears to be limited to about 25 jJ. at present. However, the dynamic range of the tested array (Reticon RL256E/17) was about 500 as may be seen from Fig. 2 where we have reproduced the diffraction pattern of a slit. Note that in the upper trace of Fig. 2 the 'scope sensitivity has been increased by a factor of 25.
Another important application of this technique is an online check of the duration of picosecond pulses. Although
Rev. Sci. lnstrum., Vol. 45, No.9, September 1974 Copyright © 1974 by the American Institute of Physics
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1176
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FlG. 2. Diffraction pattern of a slit taken with a He-Ne laser. N.B.: The electron clock noise in the lower trace (sharp needles between diode signals covers the whole of the photograph in the upper trace but measurements may be made well within this region).
TPF and fast Kerr cell shutter determinations of the pulse duration are afflicted by a number of short comings, they nevertheless furnish very useful information. Typical TPF and Kerr cell photographsl of single psec pulses using a photodiode array are shown in Fig. 3(a) and (b). Their respective calibration curves are drawn on the right. These curves depend on the wavelength and the pulse duration but they may be taken independent of the experimental setup in which they are used ultimately. As a check we have included in Fig. 3(c) a fast streak camera (Imacon 600) photograph and densitometer curve of the same pulse. The advantage of this method for the ultimate detail of the pulse shape is clear but for routine checks its correlation with Fig. 3 (a) or (b) is satisfactory. [The fine structure observed in Fig. 3(c) could be related to the dye cell thickness and corresponding structure was also found in the spectrogram of the pulse.]
The electronics required for the photodiode array is in part also commercially available. The rest has to be adapted to the particular laser system and was made in-house. A
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FlG. 3. Picosecond pulse duration· measurements. (a) TPF pattern taken with a photodiode array; (b) fast Kerr cell photograph of a psec pulse taken at A=0.53 ~; (c) corresponding fast streak camera photograph and densitometer curve. The experimental calibration curves are shown on the right-hand side.
convenient scanning rate for this array was 500 kHz resulting in a 500 J.!sec total scal1ning period. The sensitivity of the array in our TPF experiments was equivalent to ASA 200 film. Since the spatial resolution of 25 or 50 J.! is much higher than necessary for this application, a lateral increase in diode surface and thus of its sensitivity by an order of magnitude appears possible without noticeably deteriorating the TPF temporal resolution.
*This work was supported in part by the Swiss National Science Foundation and the Swiss Armament Group.
1M. A. Duguay and J. H. Hansen, "Measurements of the nonlinear index n2 of glass using psec pulses," in Damage in Laser Materials (NBS, Washington, 1970), Special Publication 341, pp. 45-50.
Pulse control unit for repetitive, slow-scan far infrared interferometry
U. Strom, P. C. Taylor, and B. D. McCombe
Naval Research Laboratory, Washington, D.C. 20375
(Received 15 February 1974; and in fmal form, 19 April 1974)
An inexpensive electronic interface which controls the flow of a phase-detected signal from a slow-scan Michelson interferometer to a signal averager is described. This interface is designed to add the advantages of signal averaging techniques to an existing slow scan interferometer system which employs phase sensitive detection and time constant averaging.
Fourier transform (FT) spectroscopy has become a useful tool for the investigation of the properties of materials in the far infrared. The primary disadvantage of FT spectroscopy is that the set of data obtained from the interferometer (interferogram) must be processed by computer. However, this disadvantage is more than offset by two major advantages.1 The first of these is increased throughput compared
to grating spectrometers because of the absence of slits and less restrictive filtering requirements. The second advantage is the so-called multiplex or Fellgett advantage.2 This advantage arises because in the grating spectrometer only one spectral element at a time is incident on the detector, while in the FT spectrometer all spectral elements are incident on the detector at all times throughout the measurement. In
Rev, Sci. Instrum" Vol. 45, No.9, September 1974 Copyright © 1974 by the American Institute of Physics
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