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 DESIGN IMPROVEMENTS IN THE 
 COLFTAR COOLED CRYSTAL LIGHT-VALUE 
 
 by 
 
 Stanley John Kopec, Jr. 
 
 July, 19lh 
 
 LIBRARY OF THE 
 
 SEP 2 3 1974 
 
 UNIVERSITY OF ILLINOIS 
 
 DEPARTMENT OF COMPUTER SCIENCE 
 UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN 
 
 URBANA, ILLINOIS 
 
UIUCDCS-R-T 1 +-669 
 
 DESIGN IMPROVEMENTS IN THE 
 COLFTAR COOLED CRYSTAL LIGHT -VALUE 
 
 by 
 
 STANLEY JOHN KOPEC, JR. 
 
 July, 19lh 
 
 Department of Computer Science 
 University of Illinois 
 Urbana, Illinois 6l801 
 
 This work was supported in part by the Department of Computer Science 
 and submitted in partial fulfillment for the degree of Master of Science in 
 Electrical Engineering, 197^ • 
 
Digitized by the Internet Archive 
 in 2013 
 
 http://archive.org/details/designimprovemen669kope 
 
Ill 
 
 ACKNOWLEDGEMENT 
 
 The author wishes to thank all the members of the Information 
 Engineering Lab for continued support in both technical and non-tech- 
 nical areas. In particular, a special note of thanks is due Professor 
 W. J. Poppelbaum, the guiding force behind the Lab, and Professor 
 W. J. Kubitz, advisor on the COLFTAR project. The Electro-Optic 
 Section of the Lab, and most of all Doug Sand, deserve much credit 
 for the work upon which this thesis is based. 
 
 Finally, Evelyn Huxhold is to be thanked for typing the manu- 
 script, and Greta for typing the draft. 
 
IV 
 
 TABLE OF CONTENTS 
 
 Page 
 
 1. INTRODUCTION 1 
 
 2. ELECTRO-OPTIC EFFECT 3 
 
 3. SYSTEM BASICS ..... 6 
 
 3.1 Ardenne Tube 6 
 
 3.2 Cooling Apparatus 8 
 
 3.3 Electron Guns 11 
 
 3.4 System Electronics 11 
 
 4. SYSTEM IMPROVEMENTS 16 
 
 4.1 Temperature Sensors 16 
 
 4.2 Electron Guns 26 
 
 4.3 Synchronization 26 
 
 4.4 Dynamic Focus 29 
 
 5. SUGGESTED CHANGES 34 
 
 5.1 Substrate Mounting 34 
 
 5.2 System Optics 05 
 
 6. PRESENT SYSTEM CAPABILITY AND FUTURE APPLICATIONS 37 
 
V 
 
 LIST OF FIGURES 
 
 Figure Page 
 
 1. Pockels Effect Light Modulator .... 4 
 
 2. Transmission-Mode Ardenne Tube 6 
 
 3. KD*P Dielectric Constant vs. Temperature 7 
 
 4. COLFTAR Ardenne Tube 9 
 
 5. COLFTAR System Blocks 12 
 
 6. Thermoelectric Module 18 
 
 7. Thermilinear Component Array 22 
 
 8. Temperature Amplifier 23 
 
 9. Temperature Multiplexor 24 
 
 10. Flying-Spot Scanner Synchronization Circuit 28 
 
 11. Photon-Coupler Preamp-Driver 31 
 
 12. Focus Modulator 33 
 
 13. COLFTAR Live Television Image 38 
 
VI 
 
 LIST OF TABLES 
 
 Table Page 
 
 1. Typical Thermistor Resistance vs. Temperature 19 
 
 2. Iron-Constantan Thermocouple Output Voltage 
 
 vs. Temperature 20 
 
1. INTRODUCTION 
 
 The efficient and widespread usage of optical data processing, 
 with its high degree of parallelism, has been limited by, among other 
 things, the inability to produce a high-resolution image at any rea- 
 sonable data input rate . . . say video rates of 30 frames per second. 
 The utilization of the electro-optic properties of certain materials 
 to transform an electronic input into an optical one would appear to 
 be one possible answer. At the Information Engineering Laboratory of 
 the University of Illinois under the direction of Professor W. J. Pop- 
 pelbaum, such an electro-optic data processing system has been in a 
 constant state of development for several years. Originally designated 
 OLFT (On-Line Fourier Transform) and later COLFTAR (Cooled On-Line 
 Fourier Transform and Reconstruction), this system employs at present 
 an Ardenne tube, or Pockels cell, consisting of a l" x l" x .005" KD*P 
 crystal cooled to near its Curie temperature of -52° C, with an off -axis 
 arrangement of write and erase electron guns which allow the formation 
 and erasure of any arbitrary video signal frame on the crystal surface. 
 Once such a "charge raster" is deposited on the crystal surface, the 
 electric field developed between the charges and a transparent electrode 
 on the opposite face of the crystal (held at ground) modulates a colli- 
 mated laser beam by the longitudinal linear electro-optic, or Pockels 
 effect. An "instant transparency" is the result. By use of a coherent 
 light source such as a laser beam, phase and amplitude modulation of the 
 beam may be obtained, and operations such as the Fourier transform may 
 take place with the use of only a single additional lens. The possibili- 
 ties such an apparatus opens up are obviously enormous* 
 
2 
 The view of the system just given is overly simple. Each of the 
 components of the system is really a complex subsystem, and the molding 
 of these subsystems into a unified, operational data processing system 
 
 has gone through a long process of evolution. Originally, the OLFT sys- 
 
 (M 
 tern utilized a KD*P crystal operated at room temperature. But such 
 
 operation leads to early crystal failure and poor resolution. To avoid 
 
 these problems, it was decided to cool the crystal to near its Curie 
 
 temperature in order to achieve improved resolution and crystal lifetime. 
 
 The thermal problems (to be discussed in more detail later) that result 
 
 from cooling a 5 mil crystal and supporting substrate to approximately 
 
 70° C below ambient are severe. 
 
 The work currently being completed to upgrade the COLFTAR system is 
 
 described in the following text, and may be basically divided into these 
 
 sections : 
 
 (1) Improved crystal-temperature sensors 
 
 (2) A new, higher capacity write gun 
 
 (3) Synchronization of all system electronic signals 
 
 (U) Addition of dynamic focusing to the write gun circuitry 
 Also, several other questions will be addressed, such as that of 
 
 the quality of system optical elements, and crystal-substrate mounting 
 
 problems. 
 
2. ELECTRO-OPTIC EFFECT 
 
 The Ardenne Tube used in the COLFTAR system employs the linear 
 electro-optic, or Pockels effect. This phenomenon has been treated 
 extensively in the literature. The crystal utilized is KD*P, 
 
 a deuterated isomorph of KDP ( KH 9 PO), ) • This is a uniaxial material, 
 and for the linear electro-optic effect, the light propogates along the 
 crystallographic c-axis (optic axis). The two allowed modes of propaga- 
 tion are linearly polarized with respect to the (x' ,y' ) axis, where 
 (x' ,y' ) are rotated U5 about the c-axis from the crystallographic (a,b) 
 axes. For each mode the index of refraction is given by 
 
 3 
 
 n ' 
 
 X 
 
 n 
 
 - n ~ r c? E 
 a 2 63 
 
 n i 
 
 y 
 
 3 
 n 
 
 = n + -f- r._ E 
 a 2 63 
 
 where n is ordinary index of refraction, r^„ is the electro-optic co- 
 efficient, and E the local electro-static field strength. After propa- 
 
 Zi 
 
 gating through the crystal a distance L, the electric field vectors E ' 
 and E ' of the two optical modes are of the form 
 
 y 
 
 E ' = A ' exp j(k n 'L) 
 X X ox 
 
 E ' = A ' exp j(k n »L) 
 y y o y 
 
 where A ' and A ' are the initial complex amplitudes of the two modes and 
 x y 
 
 k = 2tt/A . 
 o o 
 
 A simple amplitude modulation is shown in Figure 1. The input 
 
 polarizer is oriented so that the initial amplitudes are equal: 
 
 A ' = A ' = A/1/2 
 x y 
 
When the light emerges from the crystal it is circularly polarized 
 with components 
 
 E = (E ' + E ' )//2 
 x x y 
 
 E = (E ' - E ')//2 
 
 y y x 
 
 The output polarizer selects E , which is 
 
 y 
 
 E = jAe j(k o n a L ) sin (V/2) 
 
 where V = k n r^„ E L 
 o a 63 z 
 
 Incident 
 
 S 
 
 light beam 
 
 Input 
 polarizer 
 (lltox) 
 
 Transmitted 
 light beam 
 
 Output 
 polarizer 
 
 (II toy) 
 
 Figure 1. Pockels Effect Light Modulator 
 
The amplitude has then been modulated by the factor 
 j sin (V/2) = J sin u(V/V l/2 ) 
 
 where V = E L and V _. the "half-wave" voltage is 
 
 z y2' 
 
 3 3 
 
 V n , = Tr/k n r^ = X /2 n r^ 
 1/2 o a 63 o a 53 
 
 At room temperature the half-wave voltage of KD*P is UkV for 
 X = 633 nm. If we initially have our polarizers crossed as in the sim- 
 ple modulation shown we would get little light through the system with 
 V = 0. However, if V = V is applied across the crystal, the electric 
 field vector is rotated 90°, and a maximum amount of light is transmitted. 
 Any modulating system must therefore be able to develop the half-wave 
 voltage across the crystal if the entire range of the crystal's modulat- 
 ing ability is to be employed. 
 
3. SYSTEM BASICS 
 
 3.1 Ardenne Tube 
 
 The electro-optic crystal in the Ardenne tube is a l" x l" x .005" 
 KD*P crystal mounted on an approximately 1/2" thick x 1 1/2" diameter 
 CaF substrate for mechanical stability. The face of the crystal which 
 abuts the substrate is coated with a thin, transparent conducting layer 
 which acts as a grounded electrode in the crystal system. An arbitrary 
 pattern of electric charges may be deposited on the other face of the 
 crystal by an electron gun and the field developed between these charges 
 and the transparent electrode acts as the modulating field. The entire 
 assembly is enclosed in a vacuum chamber with optical windows for light 
 beam transmission. 
 
 The components of the Ardenne tube used in the COLFTAR system are 
 arranged in a transmission, (as shown in Figure 2) as opposed to reflex, 
 
 POl_AR.lZ.ER P. 
 
 iMCiOEMT 
 
 
 LIGHT 
 
 
 
 
 l_l_>HT 
 
 
 lOUROt 
 
 
 ElECTROI. 
 BEAM 
 
 t: transparent 
 conducting, 
 electrode 
 
 ANAL.YZ.ER P z 
 
 TRANSMITTED 
 L.KS.HT 
 
 Substrate 
 
 'CRYSTAL. 
 
 k: cathode 
 
 Figure 2. Transmission-Mode Ardenne Tube 
 
mode. That is, the light beam travels through the crystal only once in 
 the transmission mode, whereas in the reflex mode, it is reflected back 
 through the crystal by a reflective coating. A discussion of why the 
 transmission mode has been selected for COLFTAR is given in Sand. 
 
 The electro-optic properties of KD*P are greatly enhanced if the 
 crystal can be operated near its transition temperature of about -52° C. 
 As shown in Figure 3, the dielectric constant of KD*P shows an exponen- 
 tial increase immediately above the transition temperature, which re- 
 sults in an increased crystal time constant. The crystal time constant 
 is a measure of how fast a charge pattern deposited on the crystal sur- 
 face will disperse. By operating near the Curie temperature, a charge 
 time constant of extremely long duration may be obtained, and a charge 
 pattern may be maintained for essentially any arbitrary length of time. 
 
 10 s - 
 
 10' 
 
 w- 
 
 10 2 - 
 
 10- 
 
 /l 1 — I — I I I — I — I — I — I — I — 1 I I I I I ■ ■ I 
 
 -50 50 y 100 °C 
 
 Figure 3. KD*P Dielectric Constant vs. Temperature 
 
8 
 The long time constant of the crystal, when operated near the tran- 
 sition temperature, serves to reinforce the need for an erase electron 
 gun apparatus. This gun need not be focused or even precisely aligned. 
 All that is required is that the entire surface of the crystal be ex- 
 posed, or flooded, with a stream of electrons. Each electron is accele- 
 rated to a sufficient level to dislodge several other electrons by 
 secondary emission. These dislodged electrons are collected by a col- 
 lector ring placed around the edge of the crystal which is held at a 
 small positive potential with respect to the grounded crystal face. A 
 net discharge of the crystal surface can thus take place. Without this 
 provision, not only would successive video frames tend to blur the image 
 transmitted through the crystal, but additionally, the accumulation of 
 charge would soon exert such a repulsive force as to preclude further 
 
 m 
 
 charge deposition and image formation. 
 
 3 .2 Cooling Apparatus 
 
 In order to exploit fully the electro-optic properties of KD*P, it 
 is necessary to cool the crystal to approximately -52° C. The problems 
 inherent in such an arrangement are many. Any cooling scheme must pre- 
 vent the development of large temperature gradients across the crystal. 
 The resultant temperature differences could very easily shatter a 5 mil 
 crystal. The cooling apparatus must be relatively small to allow easy 
 incorporation into the Ardenne tube apparatus. The present crystal 
 chamber arrangement is shown in Figure h. Ideally, mounting and de- 
 mounting of the crystal assemblies should be relatively simple. Some 
 of these problems have been addressed by Lin and Sand. The fol- 
 lowing points show how they have been dealt with to some extent. 
 
COPPER BLOCK 
 (HEAT SINK) 
 
 ERASE 
 GUN 
 
 KD*P 
 CRYSTAL 
 
 TEMPERATURE 
 CONTROL 
 
 OPTIC 
 AXIS 
 
 Figure k. COLFTAR Ardenne Tube 
 
10 
 The CaF substrate has been chosen specifically because of its high 
 rrnal conductivity and good match to the thermal characteristics of 
 KD*P. The substrate thus acts as a thermal damper, evening out tempera- 
 ture variations before they reach the crystal. Since the KD*P is bonded 
 permanently to the substrate, severe stress that might damage the CaF 
 must be avoided. However, this is less of a problem than in the case of 
 the KD*P crystal, if only due to the much greater bulk and dimensions of 
 the substrate. 
 
 Many types of cooling apparatus were considered, but eventually two- 
 stage thermoelectric modules (TEMs), or Peltier cells, were decided upon. 
 
 (6) 
 Lin has analyzed rather extensively the thermal problems associated 
 
 with cooling the crystal. Basically, the cooling assembly consists of 
 four two-stage TEMS bonded to the back side of the substrate near its 
 periphery. The bonding is accomplished presently with silver-loaded 
 epoxy, and the problems inherent in this arrangement will be discussed 
 later. The modules directly bonded to the substrate, or secondary modu- 
 les, are each controlled independently by means of a temperature sensor, 
 amplifier and current driver. The temperature sensor is mounted on the 
 TEM surface near the TEM-substrate interface. In this manner, the tem- 
 peratures in the four quadrants of the substrate, and therefore the 
 crystal, may be matched quite closely. The primary modules, whose "cold" 
 sides are bonded to the hot sides of the secondary modules, in turn have 
 their hot sides bonded to a large finned-copper heat sink which is cooled 
 by forced air. The primary modules are operated in series electrically, 
 as independent control is not necessary. As long as the temperature dif- 
 ferences across the secondary modules are kept within the limit of the 
 
11 
 
 ability of the secondary modules to pump heat from the crystal-substrate 
 assembly, precise control is not necessary. 
 
 3-3 Electron Guns 
 
 The electron guns in the system have been one of the troublesome 
 points in the system; the write gun much more so than the erase gun since 
 the erase gun requires no focusing or great beam current density. 
 
 The write gun of the system has been a Westinghouse model with a 
 coated-powder-cathode which has a good resistance to up-to-air cycling 
 and a longer lifetime at higher operating temperatures. This gun is 
 rated at about 1 uA beam current in a . 5 - • 6 mil beam spot size. This 
 gun is operated with its cathode depressed approximately 15kV below 
 ground, as the target crystal is near ground. Deflection is magnetic 
 and focus electrostatic. Focus potential is about UkV with respect to 
 cathode potential. 
 
 The erase gun is also a coated-powder-cathode device, with no 
 focusing or deflection apparatus. Good secondary emission is obtained 
 with the cathode of this device operated at about 1 to 2kV below ground. 
 The collector ring, as related earlier, consists of a metal ring placed 
 around the crystal and held at a small positive potential, say +25 Volts 
 with respect to ground. 
 
 3.U System Electronics 
 
 There are many electronic subsystems within the system, and I will 
 attempt only to briefly outline the method of operation and not give de- 
 tailed circuit analysis. The present system configuration is shown in 
 Figure 5 • 
 
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13 
 
 Video information is coupled into the write gun through the cathode 
 of the device. A video amplifier operated at cathode potential ampli- 
 fies video signals which are passed to it by a video isolation amplifier. 
 The video isolation amplifier is rather unique in that the coupling from 
 ground to cathode potential is performed by an optical coupler, or photon 
 coupler. These devices consist of a transmitting infrared light-emitting 
 diode and receiving photodiode, mounted in a hollow insulating tube about 
 5 inches long. A transparent light-pipe, with shaped convex ends for 
 energy collection and dispersion, is mounted within the hollow area of 
 the pipe between the transmitter and receiver. A very linear relation 
 between LED current and photodiode current allows the use of these photon 
 couplers to transmit video information. A voltage-to-current amplifier 
 couples the signal into the LED and a current-to-voltage amplifier takes 
 the signal from the photodiode and supplies it to the video amplifier at 
 cathode potential. A rise time of less than 100 ns for the system allows 
 a bandwidth of greater than 10 MHz, or standard video bandwidth. 
 
 As mentioned, it is necessary to erase a given video charge pattern 
 in order to prevent blurring and build up of a repelling field. However, 
 it is not necessary to write only a single video frame on the crystal 
 and erase it before another may be deposited. In fact, a continuous 
 series of video fields may be written on the crystal before erasure, as 
 long as they are not too numerous and the video signal does not change 
 appreciably and cause blurring. The COLFTAR system incorporates a video 
 field counter which allows up to 15 video fields to be coupled to the 
 write gun before lockout occurs (as described below). This has the added 
 advantage that, should the image prove too dim on an initial write cycle, 
 a double frame may be written the next time and the half-wave voltage, 
 with the accompanying better transmission, approximated closer. 
 
14 
 
 As Sand mentions, transient protection of the crystal is essential. 
 Voltage transients which cause severe over-writing on the crystal can 
 cause piezoelectric strains which can shatter the crystal. As a result, 
 gating and lockout circuitry have been incorporated into the system. 
 Among these is a circuit which normally clamps the grid of the write gun 
 at about -200V with respect to cathode, thus effectively cutting off the 
 write gun. The grid of the gun is brought into its operating range by a 
 circuit triggered by an "unblank" pulse, which is coupled into the video 
 isolation amplifier by another photon coupler. After a single continuous 
 series of video fields has been coupled into the write gun, the grid of 
 the gun is clamped in the cut off region for at least 5 ms. That is, a 
 video sequence may not be immediately followed by another. The unblank 
 pulse is generated by the video field counter. 
 
 The deflection system for the write gun consists of a pair of de- 
 flection coils, driven by voltage-to-current amplifiers which in turn 
 are driven by twin sawtooth voltage generators, one for the vertical 
 sweep and one for the horizontal sweep. These generators are triggered 
 from an external signal source, such as a video camera, video monitor, or 
 flying spot scanner. The vertical sync and horizontal sync of the in- 
 coming video signal is separated from the composite sync signal by a 
 circuit and fed to the sweep generators. 
 
 The erase gun is operated with its cathode about 1.5 kV below 
 ground. The erase gun is triggered by a TTL compatible input signal. 
 The erase pulse duration is adjustable from 0.6 to 2 msec, and is con- 
 trolled by a one-shot. 
 
 This then is an overview of the cooled-crystal light valve as it 
 stood before the current modifications. In the following section, some 
 
15 
 
 of the components of the various subsystems will be looked at in more 
 detail and the rationale behind the improvements will be given. 
 
16 
 
 k. SYSTEM IMPROVEMENTS 
 
 k . 1 Temperature Sensors 
 
 It is clear that a very accurately calibrated set of temperature 
 sensors is an integral part of the cooled-crystal light valve. These 
 sensors must be calibrated in both a relative and absolute sense, that 
 is, there must be a close match in indicated temperature at the same 
 temperature for all units and the calibration of a given unit relative 
 to a standard themometric scale must be known accurately. These re- 
 quirements, along with that of small size and low power proved diffi- 
 cult to satisfy. 
 
 The temperature sensors sought would ideally have an absolute 
 tolerance of +_ 0.1° C. This tolerance range would minimize potentially 
 destructive temperature differences across the substrate crystal assem- 
 bly adequately. Size would be limited to approximately l/V on a side, 
 in order to fit well into the light valve chamber. Economy would of 
 course be a consideration. A linear relation between temperature and 
 output parameter would be highly desirable to facilitate read-out and 
 a stable feed-back-amplifier system. 
 
 Initially, Sand utilized transistor base-emitter junctions cali- 
 brated against a reference thermocouple and thermistor as temperature 
 sensors. The outputs of these pn junctions were fed to four temperature 
 sensor amplifiers, four temperature difference amplifiers, and four 
 voltage-controlled current drivers. The temperature sensed by each pn 
 junction was compared to a reference temperature (set manually) and if 
 a difference resulted, current was supplied to the TEMs to cool, or 
 stopped to warm the substrate-crystal quadrant not at the correct tem- 
 perature . 
 
17 
 
 The transistor temperature sensors did fulfill the requirement of 
 
 being small and producing a very linear output -volt age input temperature 
 relationship. From the Ehers-Moll equations, the base-to-emitter voltage 
 V is a linear function of temperature at constant I and V = 0. 
 
 However, the temperature sensors did suffer from several drawbacks. 
 The junction were originally calibrated against a thermocouple and two 
 precision termistors over the temperature range of -60° C to -20° C, 
 with increments of 1° C, and over the range - 20° C to +25° C with 2° 
 increments. The cooling apparatus for the calibration procedure con- 
 sisted of a single two-stage TEM (similar to that mounted in the light- 
 valve chamber) mounted on a large copper block with a bell jar over the 
 apparatus to allow evacuation of the assembly. Tolerance in the cali- 
 bration of the thermistor is +0.1° C which introduced some error. More 
 importantly, local variation of temperature across the surface of the 
 TEM may have amounted to this much error if not more. Due to the manner 
 in which the TEMs are constructed, as shown in Figure 6, a periodic tem- 
 perature distribution across the "cold" side may be expected. This was 
 not taken into account during calibration. 
 
 All calibration had to take place after all lead soldering, etc., had 
 been performed to avoid device characteristic drift. This was time con- 
 suming and difficult. The individual characteristics of the transistors 
 also meant tailoring circuit parameters to the particular device in 
 question. This is obviously somewhat annoying, and leads to lengthy 
 circuit calibration procedures. A standard matched set of devices would 
 be preferable. 
 
 Finally, the junction can detect stray rf radiation, which can raise 
 the junction temperature as much as 1° C above ambient as the detected 
 power is dissipated. 
 
BISMUTH TELLURIDE 
 ELEMENTS WITH 'N' 
 AND "P" TYPE PROPERTIES 
 
 ELECTRICAL CONDUCTOR 
 ELECTRICAL INSULATOR 
 
 HEAT ABSORBED 
 
 (COLD JUNCTION) 
 
 rzz 
 
 £Z 
 
 ZZ2T 
 
 777777 
 
 ZZZ 
 
 zzzzza Mini/ut nzznm vinrun Xnmr 
 
 TIL 
 
 T7TTT7 
 
 VUUljllP F* 
 
 F 
 
 HEAT REJECTED 
 
 (HOT JUNCTION) 
 
 *#!■ 
 
 DC SOURCE 
 
 Figure 6. Thermoelectric Module (side view) 
 
 18 
 
 During initial test operation of a new write gun, arcing occurred 
 within the write gun, which was later attributed to a dirty inner sur- 
 face of the glass tube which housed the gun. The arcing produced shorts 
 in k pn junctions disabling them. Had the arcing occurred with the 
 crystal cooled to operating temperature of about -50° C, instead of at a 
 higher temperature, crystal fracture could easily have resulted. As it 
 was, damage to the TEM and substrate assemblies, which proved a chronic 
 problem in later system operation, resulted from the brief period of un- 
 controlled and nonuniform cooling. This failure will be discussed in 
 more detail later. 
 
19 
 
 The shorted pn sensors initiated a renewed search for small pre- 
 cision, reliable temperature sensors. Platinum resistance sensors could 
 not he located quickly at a cost and size which would make them useful. 
 Standard thermistors in general were not calibrated well enough, or as 
 is true of all thermistors, had a highly non-linear temperature resis- 
 tance relationship. In the temperature range of interest (+25 C to 
 
 -55 C) a typical thermistor's resistance spans the range 10K to 600K 
 
 (7) 
 as shown the Table 1. This is undesirable, as already noted. 
 
 Resistance Temperature 
 (°C) 
 
 201K -50 
 
 101K -40 
 
 53K -30 
 
 29K -20 
 
 16K -10 
 
 9796 
 
 5971 +10 
 
 3748 +20 
 
 3000 +25 
 
 Table 1. Typical Thermistor Resistance vs. Temperature 
 
 (YSI - 44030 precision thermistor) 
 
 Choice of temperature sensors was finally narrowed down to thermo- 
 couples or a component set manufactured by the Yellow Springs Instrument 
 
 (7) • 
 
 Company known as Thermilinear components. Thermocouples offer the 
 
20 
 
 optimum choice as far as space and calibration are concerned, their 
 
 characteristics being tabulated by the National Bureau of Standards. 
 
 Table 2 shows some values for the output of an Iron-Const antan (Copper- 
 
 ( R) 
 Nickel) thermocouple over the range of interest. Variation is only 
 
 3 mV or so over the entire range. This opens up a myriad of problems 
 as far as sensitivity of amplifiers, drift of circuits, stray radiation, 
 etc., are concerned. A chopper-stabilized amplifier scheme could help 
 some of the drift problems, but additionally, the implementation of a 
 reference junction or equivalent correction circuit would compound prob- 
 lems. The choice was made clear by these problems. The Thermilinear 
 components were chosen. 
 
 Output Voltage 
 (millivolts) 
 
 Temperature 
 (°C) 
 
 -2.431 
 
 -50 
 
 -1.960 
 
 -40 
 
 -1.481 
 
 -30 
 
 -0.995 
 
 -20 
 
 -0.501 
 
 -10 
 
 0.000 
 
 
 
 +0.507 
 
 +10 
 
 +1.019 
 
 +20 
 
 +1.277 
 
 +25 
 
 Reference junction at C 
 
 Table 2. Iron-Constantan Thermocouple 
 Output Voltage vs. Temperature 
 
21 
 
 The YSI Thermilinear components consist of three precision thermis- 
 tors mounted in one package approximately .280" long "by .125" in diameter. 
 While slightly long by the criteria previously given, the important dimen- 
 sion is the diameter, and the package mounts well on the TEM surface near 
 the substrate-TEM interface. When used in conjunction with an array of 
 0.1% precision resistors as shown in Figure 7» the array shows a linearity 
 deviation from the equation 
 
 E QUT = (-0.005591^9 E IN ) T + 0.59300 E JN 
 
 of at most +_ 0.15° C. This is, however, a worst case approximation and 
 since units are matched quite closely, relative error may be expected to 
 be quite small. 
 
 In order to avoid development of new circuitry which would be time 
 consuming and unnecessary, it was decided to utilize as much of the 
 existing temperature controlling circuitry as possible. The four tem- 
 perature sensor amplifier circuits for the pn sensor arrangement had 
 supplied to the temperature difference amplifiers a voltage equal to the 
 temperature of the sensor in degrees Centigrade divided hy 10, e.g., a 
 sensor temperature of -30° C would cause an output of -3.00 V. It was 
 decided to develop a circuit which would take the outputs of the Thermi- 
 linear component arrays and transform them to such levels. This output 
 could then be fed to the temperature difference amplifier as before. 
 
 The circuit shown in Figure* 8 transforms the Thermilinear array 
 output into such a voltage. A precision voltage source consisting of a 
 yA723 voltage regulator and associated components provides a constant 
 E j = 2.500 Volts to the Thermilinear arrays. With this input to the 
 array, a look at the equation for E above shows that it is necessary 
 to subtract a bias term of 0.59300 x 2.500, or 1.U83V., and multiply the 
 
22 
 
 IN 
 
 RESISTORS 0.1% METAL FILM 
 
 r 
 
 L 
 
 THERMISTOR 
 COMPOSITE 
 
 OUT 
 
 Eout = (-0.00559149 E, N )T + 0.59300E 
 
 IN 
 
 IN 
 
 Figure J. Thermilinear Component Array 
 
23 
 
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24 
 
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 resultant term by T.15U to get E = T/10. The LM30U negative voltage 
 regulator provides a -1.U83 Volts to the summing amplifier circuit con- 
 sisting of a yATTT precision operational amplifier and associated com- 
 ponents. A second uA777 amplifier provides isolation "between the Thermi- 
 linear array and summing amplifier. 
 
 This circuit has proven extremely stable and no problems have been 
 encountered with this portion of the temperature control circuitry. The 
 Thermilinear components have also been extremely reliable, and no signi- 
 ficant variation between devices have been noted. 
 
 For ease of operation, the circuit in Figure 9 has been added to the 
 system. During a typical cooling cycle of the crystal- substrate, the 
 four temperatures of the TEMs are monitored closely to make sure no great 
 temperature gradients occur. Formerly it was necessary to manually con- 
 nect a digital voltmeter to test points at the outputs of the tempera- 
 ture amplifiers to get a readout. The circuit shown acts as an analog 
 signal multiplexer, cycling through the four temperature amplifiers 
 outputs continuously, displaying each for a period variable from one to 
 8 seconds. The multiplexing can be stopped at any TEM desired if 
 observation for a longer period is desired. This circuit has helped 
 immeasurably in making system operation easier. 
 
 As one final note, the thermistor composites of the Thermilinear 
 arrays have been mounted on the TEMs near the junction with the sub- 
 strate rather than on the substrate proper to facilitate the changing of 
 crystal assemblies. Differences in temperature across the interface at 
 steady-state are considered insignificant and operation has been satis- 
 factory in this manner. 
 
26 
 
 U.2 Electron Guns 
 
 The write gun used in the system previously was a gun rated at 1 pA 
 beam current in a .5 - -6 mil spot size. This emission was found to be 
 too low and in order to get sufficient beam current for adequate trans- 
 mission, it was necessary to operate the gun at times with the grid 
 positive with respect to the cathode. The gun then operates outside its 
 design range with poorer spot size. In tests of the system, insufficient 
 transmission led to the conclusion that the half-wave voltage was not 
 being obtained. A new gun, with higher emission, was sought. 
 
 Attempts were initially made to obtain a gun with a Phillips, or 
 barium dispenser cathode material. Reports of these guns obtaining very 
 high emission were very encouraging. However, upon checking into avail- 
 able sources of demountable electron guns (principally, Westinghouse, 
 the original supplier), it was learned that dispenser cathode guns were 
 not obtained without actually funding a subcontract for development. A 
 less-than-ideal higher-current gun was then considered. 
 
 A representative of Westinghouse was able to suggest a gun with a 
 high (1+0 uA) beam current in a l.k mil spot size. While this spot size 
 is in the marginal area of acceptable performance for a l" square crys- 
 tal, it was decided to use the gun and see what level of performance 
 could be obtained. Even with the increased beam size, beam current den- 
 sity is about k to 5 times greater than in the former gun. This should 
 allow the half-wave voltage to be developed with a minor loss in spot 
 size. The overall effect should be a net gain in contrast. 
 
 k. 3 Synchronization 
 
 While improving the systems capabilities , it was decided to use a 
 constant video source, as opposed to a varying one such as a live 
 
 
27 
 
 television broadcast, to help determine when resolution quality changed. 
 A flying spot scanner would seem to be ideal for this application. How- 
 ever, the flying spot scanner on hand had no provision for synchroniza- 
 tion to an external signal source. This caused a serious problem, as 
 all other signals in the system could be synchronized to signals from 
 the vidicon used to pick up the light-valve output. It was decided to 
 modify the flying spot scanner so it could be synchronized to an extern- 
 nal sync source, or run off its own internal sync sources. 
 
 To prevent damage to the image tube should the external sync source 
 fail, a system had to be devised which would automatically utilize the 
 internal sync should failure occur. This was not easy in the case of 
 the vertical sweep circuit, as the vertical sweep in the flying spot 
 scanner is triggered by the 60 Hz line voltage of about 300 Volts peak- 
 to-peak. Such a signal is difficult to override or simulate with solid- 
 state circuitry. 
 
 Eventually, the circuit shown in Figure 10 was decided upon to con- 
 trol the vertical sweep. If no external sweep is present, the circuit 
 operates as it would if unmodified. Should the external sweep signal 
 appear, the U0327 transistor effectively grounds the grid of the ver- 
 tical discharge tube, and any external sync signal with it. The other 
 transistor pair in the cathode of the tube, which normally ground it, 
 are triggered on and off by the three one-shots, which are in turn trig- 
 gered by the external sync signal. In either case, the vertical sweep 
 is generated in the same manner: when the tube is "off," a capacitor 
 is charged by the constant current source producing a very linear ramp 
 voltage; when the tube turns "on," the capacitor is discharged. The 
 arrangement for the horizontal sweep, since the trigger voltage is 
 
28 
 
29 
 
 generated internally and is only a few volts in magnitude, is simply 
 an emitter follower which produces, upon application of an external hori- 
 zontal sync, a signal which overrides the internal signal. Via these 
 two circuit systems, the flying spot scanner can be triggered by any 
 external TTL compatible sync signals of the approximate frequencies re- 
 quired. 
 
 k.k Dynamic Focus 
 
 One of the serious handicaps to better resolution in the light- 
 valve system has been the lack of a dynamic focus capability. As shown 
 in Figure U, the write gun is inclined at an angle of 30° to the optic 
 axis of the light valve. As a result, the throw of the write gun should 
 be changed as a scan of the raster is made from top to bottom of the 
 crystal. That is, the focus voltage of the write gun (normally a d.c. 
 voltage) must be modulated by a linear ramp signal at the vertical 
 sweep frequency. Without this provision, it is impossible to get the 
 entire raster in focus at a single time, i.e., if the top of the raster 
 is in focus, the bottom will be out of focus, and vice versa. 
 
 The obvious way to construct a dynamic focus system would be to use 
 a magnetic focus write gun. In this way, since focusing is provided by 
 a coil placed around the write gun barrel external to the gun, no high- 
 voltage isolation is necessary. A relatively low-level d.c. current pro- 
 vides the basic focus field, and a small current ramp impressed on this 
 d.c. term provides the dynamic focus. This was initially the method 
 pursued. However, as this approach was examined, several difficulties 
 presented themselves. To design a circuit which would produce a good 
 linear current ramp into an inductive load such as the focus coil would 
 be difficult. The electron gun size, coupled with the space required 
 
30 
 
 for the magnetic deflection coil caused severe space problems with re- 
 spect to positioning the focus soil correctly. Finally, and most im- 
 portantly, a source of demountable write guns with the right charac- 
 teristics and magnetic focus could not be found. These limitations 
 ultimately led to the decision to implement the dynamic focus with an 
 electrostatic focus gun as before and to provide high-voltage isolation 
 via the photon coupler assembly to be described. 
 
 Through experimentation, it was found that approximately a 150 Volt 
 modulating ramp was necessary on top of the normal HkV focus level to 
 get good dynamic focus. A subsystem was designed to impress this ramp 
 on the focus voltage. 
 
 The dynamic focus subsystem actually consists of two parts, the 
 
 photon-coupler pre amp-driver , and the actual focus modulator, which are 
 
 (9) 
 connected by one of the photon couplers previously described. The 
 
 problem of modulating the focus voltage is not an easy one, as the 
 ground potential of the focus supply is the write gun cathode potential, 
 or approximately 15kV below ground. To get the isolation necessary, the 
 photon couplers, tested for at least 30kV of isolation, worked very 
 well. The highly linear input-output relation of the couplers also 
 made their use possible. 
 
 The photon-coupler preamp-driver consists of three uA7^1 opera- 
 tional amplifiers, as shown in Figure 11. The first accepts the vertical 
 ramp signal from the vertical sweep generator, zeroes out any bias term, 
 and then amplifies and inverts the signal. The second stage reinverts 
 the signal and provides isolation. Finally, the last stage converts the 
 voltage to a current output which drives the LED input of the photon 
 coupler. The transfer characteristic of the photon-coupler gives 
 
31 
 
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32 
 
 approximately .6 uA of output current for each milliamp of input cur- 
 rent. Peak drive to the LED input is uaually adjusted for about 10mA 
 of input current. 
 
 The focus modulator, being mounted within the focus supply, has to 
 supply its own power, os a +lk Volt power supply is part of the design. 
 Figure 12 shows the focus modulator circuit. A uAT23 positive voltage 
 regulator regulates the positive supply, and an LM30U negative voltage 
 regulator regulates the negative supply. Both of these monolithic 
 regulators supply extremely well regulated power to the focus modulator. 
 
 The output current of the photodiode is transformed to a voltage 
 by the resistance network, to give a nominal 300mV output. The first 
 uAT^l is used for isolation. Its high input resistance (nominally 
 > 2UQ ) prevents the shunting of the input current around the resistance 
 network. The voltage follower drives another yA7^1 which amplifies the 
 signal to approximately a 3 Volt peak level. The ramp is a.c. coupled 
 to a grid of the regulator tube by a capacitor. 
 
 The regulator tube forms part of a differential amplifier in the 
 regulator section of the focus power supply. At the grid of one section 
 of this dual triode tube, the output vernier of the focus supply is con- 
 nected. It is at this point the dynamic focus is capacitively coupled 
 in also. The output vernier control provides an output adjustment of 
 up to lkV. The application of the low-level signal from the focus modu- 
 lator at the grid of this tube varies the output of the supply over the 
 required range. 
 
 This arrangement then allows the entire raster pattern to be brought 
 into focus at one time, giving a much more uniform image. 
 
33 
 
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34 
 
 5. SUGGESTED CHANGES 
 
 5.1 Substrate Mounting 
 
 The mounting of the crystal-substrate assembly to the TEMs has been 
 done with silver-loaded epoxy. This has generally proven satisfactory, 
 except on one occasion. During the shorting-out of the original pn tem- 
 perature sensors, half of the substrate assembly was cooled about 5° C 
 cooler than the other half due to the uncontrolled cooling of two of the 
 TEMs. At the time, no damage was apparent to the light valve assembly. 
 However, as the system was operated a number of times after the new tem- 
 perature sensors were installed, a seeming inefficiency in one of the 
 TEMs became more and more apparent. This was manifest as a much slower 
 cooling rate in one quadrant of the substrate, which eventually reached 
 the point that the crystal assembly could not be cooled to less than 
 -30° C (in order to prevent dangerous temperature differences across 
 the assembly which might cause the substrate and crystal to crack). 
 
 Upon opening the chamber for inspection, it was discovered that a 
 hairline crack of the substrate had occurred near the interface with 
 the TEM which had been thought to be malfunctioning. In fact, the frac- 
 ture had acted as a high impedance, limiting the cooling of that par- 
 ticular quadrant of the sbustrate. It is surmised the fracture was 
 initially caused by the pn junction short out and the accompanying high 
 temperature gradient. After that, repeated temperature cyclings with 
 the accompanying expansion and contraction of the elements aggravated 
 the problem until cooling was no longer even marginal. 
 
 A new crystal is installed in the chamber now. Barring any similar 
 catastrophic failure of the cooling apparatus, the silver-loaded epoxy 
 bonds should be adequate for system operation. To minimize strain on 
 
35 
 
 the crystal and substrate, a less rigid arrangement has "been investigated, 
 
 which would also allow easy replacement of crystals. (At present, it is 
 necessary to pry the crystal assembly loose, which normally causes some 
 damage to the substrate edges). Perhaps a silver-loaded silicon grease 
 along the TEM-substrate interfaces would allow sufficient heat conduction 
 and yet allow expansion and contraction of the components freely. To 
 utilize such a system would require a non-heat conducting retaining 
 ring or clips to be used to hold the crystal assembly in place. Space 
 within the existing chamber is quite limited, and no suitable material 
 for crystal retention is available presently. Should such materials be 
 found, a "floating" crystal assembly would be preferable. 
 
 5 .2 System Optics 
 
 The primary difficulty with the system optics at present consists 
 of a nonuniform antireflection coating on the optical windows leading 
 into and out of the chamber. Due to cleaning of the windows over a 
 period of time, part of the coating has been worn away in the central 
 area of the windows. This irregularity causes a variation in background 
 light across the image displayed on the system monitor. To get more 
 uniform images, the old coatings should be removed entirely and new coat- 
 ings installed. The alternative is the installation of entirely new 
 coated windows. 
 
 Another minor difficulty has been the mount for the mirror which re- 
 directs the laser beam down the optic axis of the chamber. At present, 
 the laser axis and optic axis of the chamber are at an angle of about 
 l60° . The mount for the mirror is at present a very simple three-point 
 mounting system with two thumb screws for adjustment. This is a very 
 
36 
 
 difficult system to align. It is necessary to get very accurate align- 
 ment of the incoming laser beam with the face of the KD*P crystal in 
 order to get a full extinction ratio, i.e., the laser beam must be as 
 perpendicular to the face of the crystal as possible. This angle works 
 out to 90° + .05° for good extinction (see Sand ) . This degree of 
 alignment is difficult at best. A more accurate system allowing trans- 
 lation and angular adjustment in all dimensions would make alignment 
 much easier. If the optical processing ability of the system is to be 
 exploited fully, a superior mirror mount should be high on the list of 
 required equipment. 
 
37 
 
 6. PRESENT SYSTEM CAPABILITY AND FUTURE APPLICATIONS 
 
 At present, resolution in the COLFTAR light valve system is in the 
 area of standard video: that is, approximately 500 lines per raster scan. 
 Figure 13 is a picture of a typical live television broadcast as dis- 
 played on the crystal. This is limited by the electronics of the video 
 amplification system, deflection system and photon coupling system. 
 Quality of video sources available (standard video or flying spot scanner) 
 also make a determination of system resolution difficult. Theoretically, 
 with improved electronics resolution of up to 1000 lines is possible. To 
 get above the 10 MHz/500 line barrier might take considerable effort, and 
 it might be better to explore some of the actual data processing capa- 
 bility of the system as it stands first. 
 
 The system should have as its priority improvements the following 
 items: 
 
 First, as mentioned earlier, the chamber windows should be recoated 
 and/or replaced. This will help reduce scattered light which degrades 
 image quality and diffraction effects of the laser beam which highlight 
 any irregularity. 
 
 Second, improved optical alignment equipment should be installed to 
 improve beam alignment. 
 
 Third, a more effective cooling mechanism for the copper block 
 heat sink of the chamber should be found. A heat-pipe device might be 
 the answer which would allow the piping of heat pumped from the cooling 
 assembly to a location where it could be more easily dissipated. At 
 present, space limitations on the optic bench and the necessity to avoid 
 any mechanical vibration permit better local heat dissipation. 
 
38 
 
 Should the above improvements be implemented, the ease of system 
 operation and effectiveness will increase immensely. The availability 
 of an on-line transparency will be realized and the use of optical data 
 processing at reasonable data rates a reality. 
 
 Figure 13. COLFTAK Live Television Image 
 
LIST OF REFERENCES 
 
 1. Born, M. , and Wolf, E. , "Principles of Optics," Pergamon Press, 
 
 Oxford, 1964. 
 
 2. Yariv, A., "Quantum Electronics," John Wiley & Sons, New York, 
 
 1967. 
 
 3. Sand, D. S., "A Theoretical Analysis of the Modulation Charac- 
 
 teristics of an Electro-Optic Light Valve" (M. S. Thesis), 
 Report No. 303, Department of Computer Science, University 
 of Illinois, Urbana, Illinois, January, 1969. 
 
 4. Casasent, D. P., "An On-Line Electro-Optic Video Processing 
 
 System" (Ph.D. Thesis), Report No. 331, Department of Computer 
 Science, University of Illinois, Urbana, Illinois, May, 1969. 
 
 5. Sand, D. S.,"C0LFTAR: A Real-Time Electro-Optical System for 
 
 Two-Dimensional Fourier Transforms" (Ph. D. Thesis) , Depart- 
 ment of Computer Science, University of Illinois, Urbana, 
 Illinois, December, 1972. 
 
 6. Lin, C, "Design Factors for a Transition Temperature Pockels 
 
 Tube" (M. S. Thesis), Report No. 413, Department of Computer 
 Science, University of Illinois, Urbana, Illinois, August, 
 1970. 
 
 7. "YSI Precision Thermistors," Yellow Springs Instrument Co. , Yellow 
 
 Springs, Ohio, 1972. 
 
 8. "Omega Temperature Measurement Handbook," Omega Engineering, Inc., 
 
 Stanford, Connecticut, 1972. 
 
 9. Sand, D., Quarterly Technical Progress Report, Section 2.2, Depart- 
 
 ment of Computer Science, University of Illinois, Urbana, 
 Illinois, First Quarter, 1972. 
 
BIBLIOGRAPHIC DATA 
 SHEET 
 
 1. Report No. 
 
 UIUCDCS-R-7 1 +-669 
 
 4. Title and Subtitle 
 
 DESIGN IMPROVEMENTS IN THE COLFTAR 
 COOLED CRYSTAL LIGHT-VALUE 
 
 7. Author(s) 
 
 Stanley John Kopec, Jr. 
 
 9. Performing Organization Name and Address 
 
 Department of Computer Science 
 
 University of Illinois at Urbana-Champaign 
 
 Urbana, Illinois 6l801 
 
 12. Sponsoring Organization Name and Address 
 
 Department of Computer Science 
 
 University of Illinois at Urbana-Champaign 
 
 Urbana, Illinois 618OI 
 
 'Pplcmentary Notes 
 
 3. Recipient's Accession No. 
 
 5. Report Date 
 
 July, 197^ 
 
 8. Performing Organization Rept. 
 No. 
 
 10. Project/Task/Work Unit No. 
 
 11. Contract/Grant No. 
 
 13. Type of Report & Period 
 Covered 
 
 technical 
 
 14. 
 
 16. Abstracts 
 
 This report describes four improvements to the electronics of the COLFTAR 
 (Cooled On-Line Fourier Transform and Reconstruction) System. These improve- 
 ments are: the addition of more reliable crystal -temperature sensors, a new 
 higher capacity write gun, synchronization of the entire system, and the 
 addition of dynamic focusing to the write gun circuitry. These changes improve 
 the reliability of the system and provide for improved resolution and contrast. 
 
 17. Key Words and Document Analysis. 17a. Descriptor 
 
 ptors 
 
 Pockels Cell 
 
 Light Value 
 
 Video Fourier Transform 
 
 Electronic Spatial Filter 
 
 Electro -Optical Processor 
 
 Coherent Optical Processor 
 
 7b. Identifiers 'Open-Ended Terms 
 
 7c. COSATI Field/Group 
 
 8. Availability Statement 
 
 Unlimited distribution 
 
 ORM NTIS-35 ( 10-70) 
 
 19. Security Class (This 
 Report) 
 
 UNCLASSIFIED 
 
 20. Security Class (This 
 
 Page 
 UNCLASSIFIED 
 
 21. No. of Pages 
 
 k6 
 
 22. Price 
 
 USCOMM-DC 40329-P71 
 

 
m * *,i>v 
 
 UNIVERSITY OF ILLINOIS-URBANA 
 110 14 ILtR no COO? no 667 tl?(Wi 
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