T h e in f r a r e d d e te c to r s u s e d in th e p r e s e n t in v e s tig a tio n w ere p h o to co n d u c tiv e devices. T h ese d ev ices a re se m ic o n d u c to rs w hose r e s i s t a n c e d e c r e a s e s w h e n e x p o s e d to i n f r a r e d r a d i a t i o n . Photoconductive devices h av e a m ode of o p eratio n so m ew h at s im ila r to t h a t of o th e r p h o to n d e te cto rs, su c h as p h o to m u ltip liers. T he in fra re d p h o to n s h o w ev er do n o t h av e su fficie n t e n erg y to co m p letely rem ove electro n s from a su rface, as w ith th e p h o to m u ltip lier. T h ey do how ever h av e enough en erg y to free a n electro n from th e c ry s ta l la ttic e a n d so in c re a se th e n u m b e r of free electro n s a n d /o r holes t h a t a ct as c u rre n t c a rrie rs. T he process h a s b een d escrib ed a s a n in te r n a l p h o to electric effect. T his effect is am plified by cooling th e sem iconductor a n d th ere b y red u cin g th e n u m b e r of th e rm a lly excited electrons. H gC dTe d etecto rs w ere chosen for th e ir h ig h specific d e te ctiv ity a n d fa s t resp o n se tim e a t 10.6 |im .
I n i ti a l e x p e r im e n ts w e re c o n d u c te d u s in g a B o sto n e le c tro n ic s co rporation h ig h speed uncooled in fra re d la s e r d e tecto r (m odel R005-2). T h is is a room te m p e r a tu r e h ig h sp e ed H gC dT e p h o to co n d u c tiv e device, w ith a n active a re a of 1 m m 2, a rise tim e of 1 n s a n d optim ized for th e 10.6 qm C 0 2 la s e r r a d ia tio n . L o sse s in th e M ich elso n in te rfe ro m e te r (reflectio n s off th e w indow s, c o m p e n sa to rs a n d b e am s p litte r etc), re s u lte d in ap p ro x im ately 250 mW of C 0 2 la s e r ra d ia tio n fa llin g on th e d e te c to r. B e ca u se o f th e s e low p o w e rs, a n d th e re s p o n s iv ity of th e device b e in g o n ly 20 m V/W a t 10.6 |im , a
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p e a k —to—p e ak o u tp u t of only 1-4 mV w as o b ta in e d (th is p ro b lem w as am plified as th e se n sitiv ity d ecreased as th e te m p e ra tu re of th e d etecto r su rfa c e in c re a s e d due to a b so rp tio n of th e in f r a r e d ra d ia tio n ). T h is o u tp u t w as u n a c c e p ta b ly low in th e e le ctrica lly n oisy e n v iro n m e n t in w h ich th e e x p e rim e n t w as ta k in g place. In a n a tte m p t to rec tify th is pro b lem , a h ig h sp eed a m p lifie r w as b u ilt u sin g th e in te g ra te d c irc u it S E /N E 5 9 2 . T h is is a w ide b a n d video a m p lifie r w ith a 120 M H z b a n d w id th w ith a d ju s ta b le g a in from 0 to 400. T he c h a ra c te ris tic s of th is a m p lifier w ere stu d ie d , a n d it w as found n o t to be lin e a r over th e e n tire freq u en cy ra n g e of in te r e s t. F u rth e rm o re , i t w as fo u n d t h a t e le c tr ic a l n o ise w a s r e a d ily a m p lifie d a n d c a u s e d c o n s id e ra b le p ro b lem s a t th e trig g e rin g stag e, p re -trig g e rin g of th e d ig ita l reco rd in g device b e in g a com m on p ro b lem . M uch w ork w as u n d e r ta k e n w ith F a r a d a y sh ie ld in g in a n a tte m p t to re d u c e n o ise a s se e n b y th e a m p lifie r, b u t i t w a s n o t p o ssib le to re d u c e th e n o ise below th e e x p e rim e n ta l sig n al w hich w as of o rd er 1 mV.
To overcom e th e low s ig n a l a n d a m p lific a tio n p ro b le m s, a second d e te c to r w a s o b ta in e d . T h is w as a n E G & G J u d s o n in f r a r e d in c o rp o ra te d , h ig h sp e e d th e rm o e le c tric a lly cooled in f r a r e d la s e r d e te c to r (m odel J15T E 4/10-M C 31G -SO 1M ). A g ain th is is a H gC dT e photoconductive device w ith a n active a re a of 1 m m 2, a rise tim e of 1 n s a n d optim ized for th e 10.6 Jim C 0 2 la s e r ra d ia tio n . T he resp o n siv ity of th is device how ever w as so m ew h at h ig h e r a t 2 V/W. F u rth e rm o re , due to th e th e rm o e le c tric cooling, th e in h e r e n t n o ise (d u e to th e r m a l e le ctro n s) w as g re a tly red u ced . T he J u d s o n in fra re d d e te cto r, b e in g th e rm o e le c tric a lly cooled, re q u ire d a h e a t sin k a t th e b ack su rfa ce of th e cooler capable of d issip a tin g a t le a s t 10 W a tts w ith less th a n 2 K
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te m p e r a tu re ris e (th e m a x im u m h e a t sin k te m p e r a tu r e w as n o t to exceed 323 K). D ue to th e h ig h cost of th e device, coupled w ith th e fact t h a t o v e rh e a tin g w ould cau se p e rm a n e n t d a m a g e in a s h o rt tim e , a w a te r cooled copper h e a t sin k w as c o n stru c te d a ro u n d th e d e te cto r. T he d esig n of th is h e a t sin k is p re s e n te d in fig u re 4.6. T he in fra re d d e te c to r w as “g lu e d ” to th e h e a t sin k w ith a th e rm a lly c o n d u ctin g p a ste .
T he co o la n t w a s a m ix tu re of d is tille d w a te r (w ith a n a n ti- a lg a e a d d itiv e ) a n d e th e ly e n e glycol, k e p t b y tw o r e frig e ra tio n u n its a t a te m p e ra tu re of 278 K. T his w as p u m p ed th ro u g h th e h e a t sin k a t a r a te of 5 lite rs /m in a n d m a in ta in e d th e surface te m p e ra tu re of th e h e a t sin k a t 278 K. To elim in a te an y co n d en satio n t h a t m ay hav e occurred on th e d e te c to rs o p tic al su rfa c e , a n a ir - tig h t cover w a s b u ilt a ro u n d th e d e te c to r so t h a t n itro g e n could be p u rg e d th ro u g h , a n d su b s e q u e n tly sealed. A ru b b e r “b la d d e r” w as in c o rp o ra te d in to th e n itro g e n su p p ly lin e to co m p e n sa te for volum e c h an g es t h a t occur as a r e s u lt of th e n itro g e n cooling a n d th e re b y rem oved an y u n d u e m ech an ical s tre s s on th e d etector. All w irin g from th e d e te cto r p a sse d th ro u g h a hole in th e en clo su re, w hich w as se a le d a fte r fittin g . A sim p le b ia s c irc u it w as a tta c h e d d ire c tly to th e b a c k of th e h e a t s in k in o rd e r to h a v e th e d etecto r as close as possible to th e b ias electronics. T h is w as done in a n a tt e m p t to m in im iz e e le c tr o m a g n e tic p ic k u p a n d e a r t h lo o p s. E le c tr o m a g n e tic n o ise p ro v e d to be a s e rio u s p ro b le m in th e e x p e r im e n t. O p e r a tin g w ith v e ry low s ig n a l le v e ls , s e v e r a l e le c tro m a g n e tic so u rces, (ele ctric a l noise from th e C 0 2 la s e r p o w er supply, th e b ias electro n ics a n d tw o local FM rad io s ta tio n s o p e ra tin g on n e a rb y b a n d s), w ere p ick ed u p a n d co nvoluted w ith th e in f r a r e d
DC Power Bias Voltage Signal Input Signal Output DMM Thermistor N2 Purge Input/Output ZnSe Window Coolant Input/Output Internal Schematic of Copper Heat Sink
F igure 4.6
The design of the infrared detector housing showing the water cooled, copper heat sink, bias electronics and N2 purge system.
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s ig n a l. F a r a d a y s h ie ld in g a n d “t r u e e a r t h s ” w e re e m p lo y e d to e lim in a te th e se problem s. T he pow er to th e th e rm o e le c tric cooler w as p ro v id ed by a c o n s ta n t c u rr e n t su p p ly w ith le s s t h a n 2% rip p le . A d ig ita l m u ltim e te r w as u se d to m o n ito r th e th e r m is to r re s is ta n c e , w hich w as in d ic ativ e of th e d e te cto r te m p e ra tu re . T he th erm o ele ctric cooler w as o p e ra ted typically a t a te m p e ra tu re of a p p ro x im ate ly 190 K. W hen th e CW C 0 2 la s e r w as im ag ed onto th e d etector, th e te m p e ra tu re of th e d e te cto r w ould a g a in rise, c au sin g th e se n sitiv ity to d ecrease and in d ee d ap p ro ach ed th e 1 mV o u tp u t of th e B oston electronics uncooled in fra re d device. T h is h e a tin g process h a d a tim e c o n s ta n t of th e ord er of 5 seconds, a n d hence a p p ro p ria te m illiseco n d s h u tte r in g o f th e C 0 2 la s e r, e lim in a te d th is problem . W ith th e h e a t sin k o p e ra tio n a l, th e th erm o electric cooler w as able to cool to te m p e ra tu re s some 50 K low er th a n w h en th e d etecto r w as a ir cooled, re s u ltin g in a sig n al in c re a se of some 30%.