High Speed Inexpensive Photodiode AssemblyGene H. McCall Citation: Review of Scientific Instruments 43, 865 (1972); doi: 10.1063/1.1685791 View online: http://dx.doi.org/10.1063/1.1685791 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/43/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Inexpensive highspeed intensified CID camera controller Rev. Sci. Instrum. 64, 946 (1993); 10.1063/1.1144147 The Si photodiode: An inexpensive though highperforming α detector Am. J. Phys. 51, 452 (1983); 10.1119/1.13232 A High-Precision Inexpensive Photodiode Gate for Electronic Timers Am. J. Phys. 40, 1168 (1972); 10.1119/1.1986779 High Speed Semiconductor Photodiodes Rev. Sci. Instrum. 33, 994 (1962); 10.1063/1.1718049 An Inexpensive Stroboscope for High Speed Photography Rev. Sci. Instrum. 14, 273 (1943); 10.1063/1.1770186

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THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 43. NUMBER 6 JUNE 1972

High Speed Inexpensive Photo diode Assembly*

GENE H. MCCALL

Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico 87544 (Received 24 January 1972; and in final form, 28 February 1972)

A photodiode assembly is described which incorporates a Hewlett-Packard 5082-4220 diode in an impedance matched holder to produce a unit with a rise time of approximately 100 psec and a fall time of approximately 300 psec. The assembly was designed for detecting fast laser pulses, but it can be used for monitoring any high speed optical signal.

INTRODUCTION

Several high speed photodiode assemblies which have subnanosecond response times are commercially available,! but in general the cost of distributing these units through­out a laser system would be quite large. The assembly de­scribed in this paper incorporates a Hewlett-Packard 5082-4220 photodiode in an impedance matched holder to produce a unit which is competitive in speed with the commercial units at a considerable saving in cost. The output current is lower than that attainable from a vacuum photodiode, but it is sufficient to drive a Tektronix 519 oscilloscope.

DESCRIPTION

A diagram of the assembly is shown in Fig. 1. The con­nector is a Tektronix 017-0033-00 if a 125 Q load is used, or a General Radio 874-PB for a 50 Q load. The two circular plates separated by a sheet of 0.089 mm thick Mylar or polyethylene form a low inductance parallel plate capacitor of 1500 pF when mated with nylon screws. A modification of the assembly uses hard anodizing on one electrode and soft anodizing on the other to form the insulator.2 To detect pulses longer than 100 nsec, silver­mica capacitors may be connected between the plates. The rear plate which contains the connector is at ground potential, and the front plate is maintained at + 150 V through a 1 MQ resistor. Although 150 V is above the rated voltage for the 4220, a number of these units have been operated daily for more than a year with no diode failures, and the high supply voltage improves the linearity when driving low impedance loads.

The diode case is common with the cathode in the 4220, and the mounting hole in the front plate is made slightly undersize and reamed during assembly to make a pressed fit for good electrical contact. The anode lead is soldered to the connector center conductor before the plates are mated.

The barrel on the front plate is threaded and retaining rings are designed to hold a ground glass diffuser and an interference filter. Because of the small active area of the diode (0.2 mm2) , a diffuser reduces the alignment difficulty considerably for high intensity sources such as mode­locked and Q-switched lasers. The length of the barrel

865

is such that the entire area of a diffuser mounted at the front is within the acceptance angle of the diode.

PERFORMANCE

Figure 2 shows a pulse from a mode-locked Nd-YAG laser displayed on a Tektronix type 556 oscilloscope with a type lS2 sampling plug-in. The laser was mode-locked using a flowing dye cell and Kodak 9740 dye. Two-photon fluorescence measurements showed a full width at half­maximum of approximately 25 psec. The laser was run at approximately 10 pulses per second and the trace shown was produced by averaging points taken from a manual time scan of the sampling unit. When the lS2 internal pulse generator (50 psec risetime) was displayed, a trace risetime of 130 psec was observed. This indicates that approximately 80 psec should be subtracted from the times determined from Fig. 2. The rise and fall times (10%-90%) determined in this way are 120 and 320 psec, respectively. The connector on the diode assembly was plugged directly into the oscilloscope connector, and it was observed that as little as 1.8 m of RG-9 or RG-63 cable degraded the response time noticeably.

The long fall time is a characteristic of the diode. The minimum response time (tr+I,) can be reduced to approxi­mately 200 psec with a loss in measurement accuracy of approximately 5% using shunt peaking techniques.3 One

NO. 3-48. 1/4 SCREWS

N

'" ..:

TEKTRONIX. NO. 017-0033-00

0.6351f 3.81 1 0.8-32.1/2

II 27 NYLON SCREWS . /0.0051 1 0.0076

MVLAR 7.620.0. .0.476 1.0.

:=+-OIFFUSER

'" 'I' ~

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POS.

FIG. 1. Diagram of photodiode assembly. Connector shown is for 125!J cable. HP-4220 diode is shown in place. Dimensions are in centimeters.

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866 GENE H. McCALL

I-

V ~ r----

FIG. 2. Pulse from a mode-locked Nd­YAG laser displayed on a lS2 sampling os­cilloscope. Horizon­tal scale-200 psec/ div j vertical scale -O.S V/div.

method is to connect a 2oo 12, 1 W carbon resistor from the center conductor to ground inside the connnector with the lead length adjusted to give the desired in­ductance. For many purposes, however, the effort required to properly adjust the peaking circuit is not justifiable.

The high supply voltage increases the fall time by about 20%, but the linearity is better than 10% for peak currents between 20 and loo rnA.

The diode is typically used to observe pulse trains from a mode-locked Nd-YAG or ND-glass laser. For an energy of approximately 0.02 m] in 10-30 psec incident on the diffuser, an output signal of approximately 10 V peak is produced in 125 12.

THE REVIEW OF SCIENTIFIC INSTRUMENTS

This assembly has demonstrated its reliability and versatility in this and other laboratories over the past two years. Because of the low diode leakage current (2 nA) it can be operated for long periods from a 22.5 V transistor radio battery mounted on the ground plane with only a slight degradation in perfonnance. Because of its high sensitivity (1 JLA/mW /cm2) it can be used without the diffuser for applications such as detection of light from fast sparks.

ACKNOWLEDGMENTS

The author would like to thank D. H. Gill and P. N. Mace for helpful comments and suggestions, and D. L. Barker and D. P. Campbell for technical assistance.

* Work supported by the U. S. Atomic Energy Commission. 1 A. J. DeMaria, W. H. Glenn, M. J. Brienza, and M. E. Mack,

Proc. IEEE 57, 2 (1969). 2 F. Franklin, Sandia Laboratories (private communication). 3 J. Millman and H. Taub, Pulse and Digital Circuits (McGraw­

Hill, New York, 1956).

VOLUME 43. NUMBER 6 JUNE 1972

A Motorized Drive for the Double-Tilting Specimen Holder of an Electron Microscope

S. KRITZINGER, D. J. MARAIS, AND T. MONACI

Merensky Institute for Physics, University of Stellenbosch, Stellenbosch, Republic of South Africa

(Received 31 January 1972)

A detailed description is given of a very compact motorized drive for the double-tilting specimen stage of an electron microscope. Tilting is effected by foot controls at a speed which is continuously variable between wide limits. A warning system indicates when it is safe to lift the specimen cartridge from the stage table and a friction clutch protects the delicate tilting stage against possible hazards. Manual tilting is possible by simply turning a lever to disconnect the motorized drive. No alterations to the microscope column are necessary to receive the motorized unit.

INTRODUCTION

For the electron microscopic investigation of all kinds of crystalline materials it is of the utmost importance that the orientation of the specimen should be accurately variable with respect to the incident electron beam to comply with the required Bragg diffracting conditions. In most com­mercially available electron microscopes this can be achieved by the rotation of one or two drives which emerge from the specimen chamber through the column. If these drives have to be rotated manually, the seated operator has to keep one or both of his hands above shoulder height, which is extremely tiresome. Tilting of the specimen, especially at high magnification, almost invariably results in some degree of movement of the image on the fluorescent screen, so that compensation has to be made with the aid of the specimen traverse controls when a certain object is viewed. Furthermore, tilting of the specimen through large

angles mostly gives rise to defocusing of the image on the screen, which has to be corrected by adjusting the ob­jective lens current. When doing precision tilting, the above-mentioned changing conditions have to be corrected in an almost continuous cycle, which means that the operator's two hands have to keep moving in turn between the positions of the two tilt drives (above shoulder height), the two specimen traverse controls (above the microscope desk), and the objective control knob (on the microscope desk).

By using foot-operated motorized drives for the tilting stage the operator is able to keep both his hands almost constantly on the specimen traverse controls, not only to keep a particular area of a specimen under observation, but also to scan much larger areas, for example a whole grain, while tilting continuously. Occasionally only his one hand has to adjust the objective control for refocusing. Since

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