An experimental infrared video transmitter, capable of sending 20 simultaneous televi­sion ciiannels or 20,000 voice channels over line-of-sight distances on a single beam of intense infrared light. Heart of device is a tiny crystal of gallium arsenide, size of a pin-head mounted near bottom end of black rod seen through circular window, that gen­erates a modulated infrared light beam with an intensity up to 2,500 watts per square

centimeter

this ga l l i um a r sen ide d iode shou ld be ab le t o p e n e t r a t e the ionized p l a s m a shea th t h a t bui lds u p a r o u n d a space capsu le d u r i n g re -en t ry in the a t m o s ­p h e r e . T h i s n e w opt ical c o m m u n i c a t i o n t e chn ique , if it can be su i t ab ly e m ­p loyed , migh t m a k e it possible to avoid t he b l a c k o u t t h a t occur s d u r i n g re-fentry

w h e n r a d i o f requenc ies a r e used for c o m m u n i c a t i o n .

G a l l i u m a r sen ide d iodes w h i c h h a v e t hus far been o p e r a t e d h a v e an ac t ive a r ea on ly as big as t he h e a d of a c o m ­m o n pin a n d p r o d u c e a p e a k light p o w e r o u t p u t of 3 wa t t s . S ince these d iodes r a d i a t e u p to 2 ,500 wa t t s p e r s q u a r e cen­

t ime te r , it is qu i t e r e a s o n a b l e t o expec t th^ t l a r g e d iodes can be m a d e wi th p e a k o u t p u t s of 15,000 w a t t s o r m o r e .

T h e h igh -qua l i t y v ideo t r ansmi s s ions in M a y w e r e m a d e o n the roof of L in ­co ln L a b o r a t o r y ( a d i s t ance of a b o u t 275 feet). W i t h a m o r e refined bu t still re la t ive ly s imple sys tem for focuss ing a n d p o i n t i n g t h e l ight b e a m , the p r e sen t g e n e r a t o r shou ld p r o d u c e a b e a m tha t c a n be rece ived c lea r ly a n d dis t inct ly at a d i s t ance of 30 mi les o r m o r e . Opt ica l filters will p e r m i t o p e r a t i o n in day l igh t .

T h e s e ga l l i um a r sen ide d iodes a re smal l a n d s imple in a p p e a r a n c e . T h e y cons is t of a th in wafe r of s ingle-crys ta l ga l l i um a r sen ide wi th a t iny spot of z inc difi'used in to o n e su r face , and t w o wires to wh ich the e lec t r ic inpu t is c o n n e c t e d . T h e input c u r r e n t suppl ies the o p e r a t i n g p o w e r a n d a l so ca r r i e s t he signal t o be t r a n s m i t t e d . T h e m o d u l a t e d inpu t cur ­r en t is c o n v e r t e d d i rec t ly in to m o d u l a t e d inf ra red l ight o u t p u t , by a p rocess cal led " b a n d - t o - b a n d r e c o m b i n a t i o n r a d i a t i o n . "

T h e d iodes g e n e r a t e l ight at w a v e ­l eng ths in the n e a r in f ra red reg ion of t he op t ica l s p e c t r u m , just b e y o n d the red r eg ion of t he vis ible l ight s p e c t r u m . T h e l ight is conf ined to a n a r r o w b a n d of w a v e l e n g t h s , on ly a few h u n d r e d ang­s t r o m s wide , d e p e n d i n g o n the t e m p e r a ­t u r e at wh ich the d i o d e is o p e r a t e d . ( T h e lower the t e m p e r a t u r e , t he n a r r o w e r the s p e c t r u m , a n d the be t t e r the efficiency a n d p o w e r ou tpu t . )

T o o b t a i n o p t i m u m efficiency the p r e sen t d iodes a r e o p e r a t e d at a t e m ­p e r a t u r e of 77 Κ ( a b o u t 370 degrees be­low z e r o F a h r e n h e i t ) , w h i c h is t he t em­p e r a t u r e of l iquid n i t r ogen t h a t is used t o cool the c o n t a i n e r in w h i c h the d iode o p e r a t e s .

Milky W a y G a l a x y mysteries probed by U.C. Laboratory at Hat Creek

T h e quie tes t ( in t e r m s of r a d i o in ter fer ­e n c e ) access ib le val ley in N o r t h e r n C a l i ­forn ia , was chosen in 1959 for the U n i ­versi ty of Ca l i fo rn i a ' s new R a d i o A s t r o n o m y L a b o r a t o r y , wh ich f o r m a l l y wen t in to service in H a t C r e e k Va l l ey , 20 mi les n o r t h of Lassen V o l c a n i c N a ­t iona l P a r k in J u n e of this year , a n d is n o w in full o p e r a t i o n .

Wi th a pa i r of " e a r s " a i m e d at o u t e r space , the l a b o r a t o r y is p e r f o r m i n g signi-ficiently in a b r o a d p r o g r a m s u p p o r t e d in la rge p a r t by the Office of N a v a l R e ­search . T h e p r o g r a m will seek answer s to ques t ions tha t h a v e puzz led m a n for cen tu r i e s : H o w a re the s tars f o r m e d ? W h a t " s h a p e " is the un ive r se? Is t h e r e life on o t h e r p l ane t s? In keep ing wi th the Univers i ty ' s educa t i ona l ro le , the new facility will serve b o t h ins t ruc t ion and re ­s e a r c h : g r a d u a t e s tuden t s in a s t r o n o m y a re at w o r k on a r e sea rch projec t a long wi th U . C . facul ty m e m b e r s a n d visi t ing scient is ts .

T h e $1.2 mi l l ion facili ty inc ludes 3 3 -foot a n d 85-foot t e lescope " d i s h e s " c a p ­ab le of de tec t ing r a d i o waves f r o m far b e y o n d the l imits of visual a s t r o n o m y . T h e y w e r e des igned , deve loped and bui l t in P a l o A l t o by the P h i l c o C o r p . , a sub ­

s id iary of t h e F o r d M o t o r C o . T h e t w o sky - sea rch ing d i shes—fu tu r i s t i c l and ­m a r k s in the w i l d e r n e s s — a r e loca ted 400 feet a p a r t on an eas t -wes t base l ine ; t he t w o mass ive i n s t r u m e n t s a r e m a d e u p of th in a l u m i n u m reflectors m o u n t e d in a l u m i n u m a n d steel f r ames a n d sup­p o r t e d o n steel t ower s .

T h e la rge te lescope we ighs a b o u t 2 0 0 tons , s t a n d s 110 feet a b o v e the g r o u n d , and is bui l t to w i t h s t a n d w i n d s of u p to 100 mi les p e r h o u r . By m e a n s of r e ­m o t e l y con t ro l l ed e lec t r ic m o t o r a r ives and la rge gea r s , the te lescopes c a n be po in t ed at any spot in the sky a n d c a n t r ack a r a d i o sou rce as the e a r t h ro t a t e s .

Ad jacen t to each te lescope is a l a b o r a ­to ry bu i ld ing c o n t a i n i n g con t ro l ins t ru­m e n t s a n d sensi t ive , specia l ly des igned r a d i o rece ivers . T h e rece ivers a re con ­nec ted to r e c o r d i n g devices w h e r e the in­c o m i n g r a d i o s ignals a r e t r a c e d as w a v y l ines on a m o v i n g p a p e r s t r ip . D u r i n g the L a b o r a t o r y ' s t u n i n g - u p pe r iod , t h e sma l l e r t e lescope soon af ter it was e rec ted m a p p e d a p o r t i o n of the n o r t h e r n Mi lky W a y . T h i s o b s e r v a t i o n r evea l ed n e w de ­tai ls a b o u t t he s t a r - s tudded spira l a r m s of t he wh i r l i ng g a l a x y in w h i c h t h e sun is a m i n o r s tar .

In its init ial tes ts , the 85-foot t e l escope r e c o r d e d r a d i o s ignals f r o m d e e p wi th in t he ga lac t ic c e n t e r — t h e h u b of o u r i s land un ive r se t ha t is c o m p l e t e l y h i d d e n f rom visua l o b s e r v a t i o n . T h e d i s t r i bu t ion o t

Atop a portable elevator, technician makes adjustments on antenna horn at focus of U.C.'s new 85-foot steerahle radio telescope. Radio waves from space, reflected into the coaxial horn, travel to receivers in a nearby laboratory where

they are recorc

756 E L E C T R I C A L E N G I N E E R I N G S E P T E M B E R 1962

Construction scene: workmen place the 4-legged ''feed" into the 6,000-square-foot re­flector dish for radio telescope, now in operation at Hat Creek, Calif. Earlier, the re­

flector itself was hoisted onto its tower the same way

gaseous m a t e r i a l in t h e g a l a x y a n d t h e m o t i o n s of th is m a t e r i a l will be of g rea t in teres t to scient is ts e n g a g e d in t he s t udy . " F r o m the s tudy of these aspec t s of t h e gas we h o p e t o l e a r n m o r e a b o u t t h e s t n i c t o e a n d d y n a m i c s of the ga l axy , a n d a b o u t t he w a y it c h a n g e s wi th t i m e , " D r . H a r o l d W e a v e r , a Berke ley a s t r o n o ­m e r , says .

T h e first o b s e r v i n g p r o g r a m wi th t he 33 - foo l t e l e scope h a s a l r e a d y p r o d u c e d a sky m a p of t he n o r t h e r n M i l k y W a y s b o w i ^ . the p a t t e r n of t h e r m a l r a d i a t i o n arisfrig f r o m h y d r o g e n gas . F r o m th is m a p t h e a s t r o n o m e r s will d e d u c e t h e d i s ­t r i bu t i on of ion ized h y d r o g e n — k n o w n to

be a s soc ia t ed wi th ho t , b r i g h t s t a r s — a n d will t r a c e t h e p a t t e r n of t hese s t a r s in t h e spi ra l a r m s of the ga l axy .

S tud ies of the ga l ac t i c c e n t e r s a r e be ­ing m a d e a t t w o r a d i o f r e q u e n c i e s : a t 8 ,000 m c the t e l e scope p r o v i d e s a c l ea r ly defined p i c tu r e of t he gas in the n u c l e u s as seen p r o j e c t e d aga ins t t he sky ; a n d at 1,420 m c it r evea l s i n f o r m a t i o n o n t h e m o t i o n s of t he gas .

R a d i o a s t r o n o m y , a y o u n g a n d p r e ­coc ious sc ience , h a s b e g u n t o ske t ch t h e first a c c u r a t e p i c t u r e of t h e M i l k y W a y : it h a s focused o n severa l s u p e r n o v a e , t h e g i an t r e m n a n t s of e x p l o d i n g s t a r s , a n d e x p l a i n e d the i r i n t ense r a d i a t i o n in t e r m s

of a n e l e c t r o n - a c c e l e r a t i n g " c y c l o t r o n " effect; it h a s p e e r e d in to t h e cen t e r of t he ga l axy , t he r e g i o n h i d d e n t o op t i ca l t e l escopes , a n d has b e g u n t o d e v e l o p in­f o r m a t i o n o n the ga lac t i c c i r c u l a t i o n of m a t t e r ; it ha s d e t e c t e d a pa i r of ga laxies in co l l i s ion ; a n d has l i s t e n e d — s o far w i t h o u t s u c c e s s — f o r messages f r o m u n ­k n o w n c iv i l i za t ion o n s o m e d i s t an t p l ane t .

C l o s e r to h o m e , it has inc reased m a n ' s k n o w l e d g e of t h e sun , t he m o o n , a n d n e i g h b o r i n g p l a n e t s in the so la r sys tem, i n c l u d i n g a n e w e s t i m a t e of t h e sur face t e m p e r a t u r e of V e n u s w h i c h is expec t ed t o be of g r ea t v a l u e in fu tu re space ex­p l o r a t i o n . It ha s a l so p i o n e e r e d a n e w s y s t e m of ce les t ia l n a v i g a t i o n ( t h e r a d i o s e x t a n t ) t h a t c a n be used t o guide m a r i n e r s a n d i n t e r c o n t i n e n t a l rocke t s .

E l e c t r o m a g n e t i c w a v e s of every l eng th a r e p r o b a b l y t r a v e l i n g t h r o u g h o u t e r s p a c e ; t h o s e t h a t r e a c h the e a r t h en te r t h r o u g h t w o " w i n d o w s " ; t he n a r r o w o p ­t ical w i n d o w t h a t a l lows us t o see t h e l ight of ce les t ia l ob jec ts , a n d the m u c h b r o a d e r r a d i o w i n d o w o p e n to waves in t h e t e lev i s ion a n d p a r t s of t he m i c r o w a v e a n d s h o r t - w a v e r a d i o f r equenc ies ( f r o m a b o u t Vi i nch t o 100 feet l o n g ) . In s im­plified o u t l i n e , r a d i o a s t r o n o m y can be c o m p a r e d t o a c o m b i n a t i o n of opt ica l a s t r o n o m y a n d t h e n o w fami l i a r h o m e r e c e p t i o n of r a d i o o r te lev is ion s ignals . T h e p a r a b o l i c d i sh , o r o t h e r type of re ­flector, se rves t o focus t he waves f rom a p rec i se spo t in the sky o n t o the a n t e n n a , in t h e s a m e w a y t h a t op t i ca l t e l e scope re ­flectors focus r a y s of l ight .

B o r n of e l ec t ron i c s a n d b r ing ing a f resh a p p r o a c h t o a n o ld sc ience , r a d i o a s t r o n o m y h a s o n l y b e g u n to s h o w its full a p p r o a c h t o capab i l i t i e s in e x p a n d ­ing m a n ' s k n o w l e d g e of the un ive r se .

O t h e r O N R - s p o n s o r e d pro jec t s inc lude a n 85- foot r a d i o t e l e scope o p e r a t e d by t h e U n i v e r s i t y of M i c h i g a n ; o n e at O w e n s Va l l ey , Calif . , o p e r a t e d by t h e C a l i f o r n i a In s t i t u t e of T e c h n o l o g y ; a n d o n e n e a r D a n v i l l e , 111. T h e H a t C r e e k p ro j ec t is o n e of o u r l a rges t .

Lasers advance in accuracy of detecting motion: improve in power A grea t a d v a n c e in t h e a c c u r a c y of d e ­tec t ing a n d m e a s u r i n g m o t i o n h a s b e e n m a d e by a laser d o p p l e r r a d a r m o d e l , a n d a t e c h n i q u e is n o w used for mu l t i ­p ly ing the p o w e r of a laser , a c c o r d i n g t o r ecen t a n n o u n c e m e n t s .

LASER D O P P L E R R A D A R

Feas ib i l i ty of a laser d o p p l e r r a d a r t h a t can de tec t a n d m e a s u r e m o t i o n 10 ,000 t imes m o r e a c c u r a t e l y t h a n the bes t m i ­c r o w a v e sys tems k n o w n has been d e m o n ­s t ra ted by the S p e r r y G y r o s c o p e C o m ­p a n y .

A w o r k i n g l a b o r a t o r y m o d e l e m p l o y ­ing a m o v a b l e m i r r o r as a t a r g e t h a s s h o w n t h a t a r a d a r c a n m e a s u r e ve loc i ty wi th equa l a n d a b s o l u t e p rec i s ion f r o m orb i t a l inject ion speeds of 5 mi l e s p e r second all t h e w a y d o w n to a v i r t ua l s t o p — l e s s t h a n o n e t e n - t h o u s a n d t h of an inch p e r second . Such a r a d a r s h o w s p r o m i s e for b r o a d use as a space veh ic l e

r e n d e z v o u s a id a n d for t r a c k i n g miss i les a n d sa te l l i tes .

M i c r o w a v e d o p p l e r e c h o e s a r e a c c u ­r a t e o n l y at h igh r a d a r - t o - t a r g e t ve loc i ­t ies . B u t a l a se r d o p p l e r r a d a r o p e r a t i n g at a f r e q u e n c y of h u n d r e d s of t r i l l ions of cycles p e r s e c o n d c a n eas i ly d e t e c t a n d m e a s u r e t he d o p p l e r shift p r o d u c e d by a veh ic le edg ing u p to a s p a c e s t a t ion even at a smal l f r ac t ion of a n inch p e r s econd . A c o n t r o l s y s t e m e m p l o y i n g a s ignal o u t p u t of th is p r ec i s ion w o u l d a l l ow a sh ip to d o c k as l ight ly as a f ea the r . It is sa id t o b e t he first dev ice of its k ind .

A l s o , t h e phys ica l d i v e r g e n c e of a l aser b e a m , m u c h n a r r o w e r t h a n t h e s p r e a d of m i c r o w a v e e n e r g y , w o u l d p u t a g r e a t e r p r o p o r t i o n of t h e t r a n s m i t t e d e n e r g y d i rec t ly o n the t a r g e t s t a t i on as the space sh ip e n t e r e d its l a n d i n g p a t t e r n t h o u s a n d s of mi les f r o m t o u c h d o w n .

S p e r r y ' s m o d e l uses a n e w h e l i u m -

n e o n c o n t i n u o u s w a v e gas laser . A m o d i ­fied M i c h e l s o n i n t e r f e r o m e t e r spli ts t he l a s e r o u t p u t in t w o . ha l f def lect ing 9 0 d e g r e e s t o b o u n c e off a s t a t i o n a r y m i r r o r a n d the o t h e r ha l f pa s s ing s t r a igh t t h r o u g h t o reflect off a m o v a b l e t a r g e t m i r r o r . T h e l ight f r o m the s t a t i o n a r y m i r r o r c o m e s b a c k t h r o u g h the b e a m sp l i t t e r a n d is de t ec t ed by a p h o t o m u l t i -p l ie r t u b e . T h e b e a m f rom t h e t a r g e t m i r r o r is deflected b a c k off the b e a m sp l i t t e r a t 9 0 d e g r e e s a n d a l so h i t s t h e p h o t o t u b e .

W i t h t h e t a r g e t m o t i o n l e s s , t h e t w o b e a m s s t r ike t he p h o t o m u l t i p l i e r at t he s a m e f r e q u e n c y . But m o v e m e n t of t h e t a r g e t p r o d u c e s a d o p p l e r shift b e t w e e n t h e d e t e c t e d b e a m s — a f r e q u e n c y shift s o m e 10 ,000 t imes s h a r p e r in r e s o l u t i o n t h a n a m i c r o w a v e shift .

E N E R G Y C O N C E N T R A T I O N

A n e w t e c h n i q u e for c o n c e n t r a t i n g t he e n e r g y of a l a se r t o m u l t i p l y its ins tan­t a n e o u s p o w e r as m u c h as 1,000 t imes h a s been d e v e l o p e d by G e n e r a l D y n a m i c s C o r p . By the s a m e m o d u l a t i o n m e t h o d , a l a se r b e a m h a s a l so b e e n swi tched

S E P T E M B E R 1962 · E L E C T R I C A L E N G I N E E R I N G 757