OPTICAL FREQUENCY SYNTHESIS AT NPL

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Colloque C8, supple'ment au n012, Tome 42, de'cembre 1981 page C8-495

OPTICAL FREQUENCY SYNTHESIS A T NPL

D . J . E . Knight

Division o f &uantwri Metrology, National Physical Laboratory, Teddington, Middlesex, TW11 OLW, U.K.

A b s t r a c t

C u r r e n t a c t i v i t i e s r e l e v a n t t o l a s e r f r e q u e n c y measurement a r e d i s c u s s e d . The main emphasis i s a n e x t e n d i n g f r e q u e n c y measurement from t h e 3 . 3 9 p m r e g i o n t o t h e v i s i b l e , f o r which a p a r t i c u l a r f r e q u e n c y s y n t h e s i s scheme i s p r e s e n t e d . Experimental r e s u l t s a r e g i v e n on phase l o c k i n g g a s l a s e r s t o g e t h e r and on c h e c k i n g t h e e q u i v a l e n c e o f f r e q u e n c y d o u b l i n g i n a

phase-matched c r y s t a l and i n a p o i n t c o n t a c t d e v i c e . T e c h n i c a l improvements o f Josephson j u n c t i o n d e v i c e s a r e mentioned.

About two y e a r s ago we o b t a i n e d a n a c c u r a t e r e s u l t f o r t h e f r e q u e n c y o f a m e t h a n e - s t a b i l i z e d l a s e r a t 3.39pm. and were a b l e t o r e f e r t h e r e s u l t s t o t h e mean frequency of s e v e r a l such l a s e r s ' . The t e c h n i q u e i n v o l v e d making c o u n t s o f f o u r b e a t f r e q u e n c i e s s i m u l t a n e o u s l y under c a l c u l a t o r c o n t r o l t o a v o i d e r r o r s from f r e q u e n c y i n s t a b i l i t i e s o f t h e t r a n s f e r - o c i l l a t o r l a s e r s , end t h e f i n a l u n c e r t a i n t y was e s t i m a t e d a t 5 3 p a r t s i n 10". Ve hoped t h e r e s u l t would p r o v i d e a b a s e f o r e x t e n d i n g frequency measurements towards t h e v i s i b l e and a c t a s a n i n t e r i m f r e q u e n c y s t a n d a r d f o r l e n g t h measurements.

T h i s hope r e m a i n s , a l t h o u g h c o n t i n u i t y o f o p e r a t i o n o f m e t h a n e - s t a b i l i z e d l a s e r s was l o s t a t NPL s h o r t l y a f t e r t h a t experiment was f i n i s h e d . Our p r e s e n t main o b j e c t i v e is t o measure t h e f r e q u e n c y o f a v i s i b l e s t a b i l i z e d l a s e r by r e f e r e n c e t o t h e methane s t a b i l i z e d l a s e r o r t o t h e caesium s t a n d a r d . D r G J Edwards h a s proposed a scheme t o r e a c h t h e 633 nm I2 s t a b i l i z e d He-Ne l a s e r by u s i n g two s u c c e s s i v e n o n - l i n e a r mixing s t a g e s from an F - c e n t r e l a s e r a t 2 . 5 3 ~ ~ 1 . However, o u r e x p e r i m e n t a l r e s u l t s h e r e d e s c r i b e d come from two s u b s i d i a r y experiments: (i) a f e a s i b i l i t y s t u d y on phase-locking two l a s e r s t o g e t h e r ( a t a n o f f s e t f r e q u e n c y ) , and ( i i ) a n i n v e s t i g a t i o n o f t h e e x a c t n e s s o f f r e q u e n c y d o u b l i n g i n a phase-matched c r y s t a l , a s compared w i t h a p o i n t - c o n t a c t d e v i c e . The former e x p e r i m e n t i n v o l v e d phase-locking a s m a l l 3 . 3 9 p m He-Ne l a s e r and a 6.2 m-long one t o g e t h e r a t a 5 MHz o f f s e t f r e q u e n c y , and t h e l a t t e r was performed w i t h CO

l a s e r r a d i a t i o n doubled t o t h e 5 m r e g i o n . 2

?

The main p r a c t i c a l i n t e r e s t , of c o u r s e , i n measuring t h e f r e q u e n c i e s o f v i s i b l e s t a b i l i z e d l a s e r s i s t o p r o v i d e working l e n g t h s t a n d a r d s under t h e proposed new d e f i n i t i o n o f t h e m e t r e , i n which t h e speed o f l i g h t w i l l become f i x e d . There a r e a number o f v i s i b l e s t a b i l i z e d l a s e r s used a s l a b o r a t o r y l e n g t h s t a n d a r d s i n d i f f e r e n t p a r t s o f t h e s p e c t r u m , and it i s a l s o i n t e r e s t i n g t o d e v i s e means o f measuring t h e f r e q u e n c y d i f f e r e n c e s between them, f o r t h i s i s i n t r i n s i c a l l y more powerful t h a n u s i n g p r e c i s e

i n t e r f e r o m e t r y t o c a l i b r a t e t h e s t a n d a r d s f o r p r e c i s e i n t e r f e r o m e t r y i t s e l f .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981855

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Table I shows some frequency intervals between visible stabilized lasers, from which it appears that C02, frequency-doubled C02, CO or far-infrared optically-pumped lasers can provide suitable radiations if non-linear elements such as phase-matchable crystals can be found for the addition or subtraction of the frequencies.

Table I

.-

Visible s t a b i l i z e d l a s e r s

Visible i n t e r v a l Infrared l a s e r residual

Ref. beat

No. Laser Molecule h/nm. colour (THz gas l i n e f (THZ) (GHZ)

1 ~ r * I2 515 green C02 9.62 p ~ ( 2 8 ) 31.1595 x 2 - 35.3

CO 1 - 0 P(22) 62.2768 + 6.9

"0"

2 He-Ne 576 yellow CO 2 - ¹ ~ ( 1 0 ) 62.2711 + 12.6

r2 - *3 "CO, 9.88 pm P(8) 30.3150 + 11.5

3 He-Ne I2 612 orange

(30.3265) 13c1802 9.88 pm P(20) 30.3398 - 13.3 4 He-Ne

'2 - '6 C02 9.35 pm R ( 6 ) 32.0482 x 2 + 14

5 He-Ne 4 640

}

6 dye ~ a ( b e a m ) 657 1 ^{f 4} - f5 CD30H 55.56 pm 5.396 (26) - ( 2 9 6 )

(5.394) CH30H 55.37 pm 5.4143 - 20

One of my own activities, and an increasingly arduous one, is to maintain a list of far-infrared CW laser lines as sources of coherent radiation in that part of the spectrum2. Histograms of the distribution with frequency are shown in figure 1. The number of optically-pumped lines reported is still increasing, for another 450 entries now await adding to the list. However, there is little sign of a change in the distribution to fill the "gapn between about 4 THz and 24 THz in which only 8% of the lines fall. The extensive coverage by CO radiation starts just above 24 THZ~. It must be admitted therefore that $he list may not be of great use in finding sources at a particular frequency near, say, 17 THz, although it will list what there is. (The main demand for the list is from molecular spectroscopists using far infrared lasers).

Fig.1 Spectral distribution of far-infrared cw laser lines.

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Laser phase-locking at 3.39pm

A student, J C Coveney from Cambridge University, has helped carry out some experiments on phase-locking two lasers t0gether.A 6.2-171 long He-Ne laser used as a power transfer oscillator, and a short (0.3-m) laser were phase locked together via a quartz-crystal-oscillator reference at 5 MHz.

Fig.2 Scheme for phase-locking two 3 . 3 9 p m He-Ne lasers together.

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A t f i r s t a f r e q u e n c y o f f s e t l o c k b a s e d o n a f r e q u e n c y t o v o l t a g e c o n v e r t e r was u s e d t o I 1 a s s i s t t 1 a weak b u t f a s t phase-lock l o o p (PLL) b a s e d on d r i v i n g a LiNbO c r y s t a l i n t h e l o n g l a s e r c a v i t y ( s u g g e s t e d by G J Edwards). However a f r e q u ~ n c y - l o c k s y s t e m h a s z e r o e r r o r s i g n a l u n d e r p h a s e - l o c k e d c o n d i t i o n s a n d s o c a n n o t i n c r e a s e t h e l o c k h o l d r a n g e a g a i n s t d r i f t , f o r example. Hence t h e f r e q u e n c y l o c k was p u t a s i d e a n d a s e c o n d , s l o w , p h a s e - e r r o r f e e d b a c k s i g n a l was a p p l i e d t o a p i e z o e l e c t r i c t r a n s d u c e r (PZT) d r i v i n g a n end m i r r o r o f t h e small laser, see F i g . 2 . The PZT h a d r e s o n a n c e s i n t h e 2 kHz r e g i o n w h i c h h a d n o t t o b e e x c i t e d . The PLL now h a d two d r i v e s i n p a r a l l e l , which w e r e c o u p l e d v i a t h e laser b e a t f r e q u e n c y , s o i t was n e c e s s a r y t o u s e t i m e c o n s t a n t s s l o w e n o u g h t o a v o i d r e l a t i v e p h a s e s h i f t s between them i n t h e r e g i o n where t h e r e i s n e t g a i n . With a fffast" s e c o n d o r d e r l o o p , h a v i n g )I,, - 34 x l o 3 r a d / s , a n d a s l o w l o o p w i t h i n t e g r a t i o n t i m e c o n s t a n t ' 0 . 8 s . i t Gas p o s s i b l e t o a c h i e v e a p h a s e - l o c k jump-in r a n g e o f - 140 kHz ( f r o m t h e f a s t l o o p ) , a n d a h o l d - i n r a n g e o f - 10 t o 17 MHz ( f r o m t h e s l o w l o o p ) , t h u s o f f e r i n g h o l d t i m e s o f a b o u t 1 0 0 s w i t h a r e l a t i v e f r a c t i o n a l d r i f t of 10-'/s b e t w e e n t h e lasers. A s f i n a l l y s e t u p , t h e f a s t PLL a l o n e c o u l d o n l y m o m e n t a r i l y c a t c h p h a s e l o c k on i t s own, b u t w i t h b o t h l o o p s c o n n e c t e d t h e s y s t e m was s t a b l e enough t o f l i p f r o m t h e + 5 MHz t o t h e - 5 MHz l o c k s i d e b a n d s when t h e end p l a t e o f a l a s e r was pushed. The n s l o w l l l o o p was found t o b e c a r r y i n g s i g n i f i c a n t f e e d b a c k s i g n a l s o u t t o a b o u t 1 kHz.

\

\

- \ ^\ between two 3.39 p m

\ He-Ne l a s e r s . Data

f o r F o u r i e r f r e q u e n c i e s below a b o u t 100 Hz i s i n t e r p o l a t e d from s q u a r e - r o o t A l l a n v a r i a n c e p l o t s . The W-locus i n t e r c e p t g i v e s a measure o f t h e FWHM o f t h e s p e c t r u m 7 .

\

The performance of the double PLL was constrained on the one hand by all the perturbations affecting the lase s and on the other hand by the speed of the Ithigh voltage* driver amplifiers', each having about 10 kHz bandwidth.

Once the loop was locked it was possible to investigate a number of sources of perturbations of the lasers. The laboratory was the same as that used for the methane-stabilized laser frequency measurement1 and is on the second floor above ground of an "office-block" style building, with fan and vacuum-pump noise, but most of the frequency deviations from these causes fell below about 400 Hz, in the region where the loops have substantial gain.

The perturbations affecting phase-locking were mainly associated with the dc-plasma excitation of the lasers

-

intervals) were also capable of upsetting the quality of phase-lock, if not of unlocking the system.

The spectral quality of the phase lock is shown in the phase spectral density plot Fig.3, where it is compared with the free-running and frequency-locked conditions. The data is derived from both square-root Allan variance measurements and from the spectrum of the lock d beat. The FWHM from phase-noise, W , ( - Halford's "fast line width" 'i ) is Z 1 Hz phase locked and

- 20 kHz frequency-locked or free-running. The free-running linewidth FWHM on a spectrum analyser was about 100 kHz when taking about 0.1 s to cross the signal.

It seems likely, but has yet to be proved, that ordinary microwave

phase-locked boxes could usefully phase lock lasers in this way, provided a dual feedback system is attached, and offset-locking beyond the usual range of frequency to voltage converters is clearly possible. In our trials the beat detector limited the useful beat frequency, but by correct biassing this can-be extended to hundreds of MHz with the same type of diode.

Test of "exactness" of frequency doubling in a crystal

This experiment involves the frequency doubling of the radiation of a CO laser in a crystal and subsequent 2nd-harmonic mixing with a second lase?, in an MIM diode. The scheme is shown in Fig.4 and the work has mainly been done by G J Edwards and P R.Pearce assisted by a student, R J H Jennings,.from Bristol University. The doubling crystal was of CdGeAs , and we are grateful to N ~ e n ~ u k s o f MIT Lincoln Laboratory for supplying itZ.

The experiment was much more difficult to set up and operate than had been anticipated. For example it was necessary to spatially filter the ( - 8 W) output of the laser incident on the doubling crystal, to convert its

"doughnutv output mode to a near-Gausian one, before adequate doubling efficiency could be realized. An iris also had to be fitted in this laser's cavity to prevent multimoding when the PZT tuning was adjusted.

With about 1 mW of 5 p m radiation from the doubling crystal, a beat with

- 30 dB signal/noise in 100 kHz bandwidth was obtained by 2nd harmonic mixing with the second C02 laser in an MIM diode.

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SPATIAL snrm!

FILTER I

Fig.4 Scheme for comparison of frequency doubling in a crystal and

-

r

Simultaneous frequency counting by two high-precision counters controlled by a calculator was used to test for differences between twice the fundamental frequency beat, 2 A f , and the second harmonic beat, A(2f). A f was set at 5 MHz by a frequency-lock loop and counting was done via filters at 5 and 10 MHz having bandwidths of about 2 MHz. The program took 50 pairs of 1-second counts, alternately reversing connections to the counters.

Fourteen such sets have been obtained in experiments on three different days, within a week. Counts were also made for 10 s averaging times, and test

counts were taken in each run by putting the same, weaker, 10 MHz beat ~ ( 2 f ) into both count channels. Earlier checks were made using a synthesiser

modulated with "white-frequencyn f.m.

The test counts showed a system resolution of 0.02+0.04 Hz. The overall result for [&(2f) - 2 A f ] gave closely the same result, showing that any net difference arising in the two freggency-doubling processes has a fractional magnitude less than 7 parts in 10

.

"calibrationn needs to be introduced, perhaps by introducing a time-dependent phase shift through a rise or decrease in temperature of a transmission element, for example.

F-centre l a s e r a t 2.5 p m and scheme t o r e a c h 633 nm

The scheme o f t h e experiment i s shown i n F i g u r e 5 , and was d e v i s e d by D r G J Edwards, who i s mainly r e s p o n s i b l e f o r t h e F-centre l a s e r and t h e two n o n - l i n e a r s t a g e s t o 633 nm. A f t e r frequency d o u b l i n g t h e 2 . 5 3 ~ ~ 1 r a d i a t i o n i n LiNbO , p o s s i b l y i n s i d e t h e F - c e n t r e l a s e r c a v i t y , t h e 1 . 2 7 p m r a d i a t i o n e n t e r s a a o t h e r LiNbO c r y s t a l i n s i d e t h e c a v i t y o f a l a r g e He-Ne l a s e r o p e r a t i n g a t 633 nm, which acts a s a near-degenerate p a r a m e t r i c a m p l i f i e r . 3 The b e a t between s i g n a l and paramp i d l e r a t 1 . 2 7 p m t h e n measures t h e 633 nm pump frequency.

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Fig.5 Scheme t o r e a c h 633 nm from a c o l o u r - c e n t r e t u n a b l e l a s e r a t 2 . 5 ~ ~ .

On p r o g r e s s , t h e F - c e n t r e l a s e r is working and t h e b a s i c equipment is b u i l t . However n e i t h e r frequency measurement o f i t s r a d i a t i o n a g a i n s t a CO

harmonic, n o r a d e q u a t e frequency d o u b l i n g h a s y e t been demonstrated: n o r h a s t h e 633 nm paramp-pump l a s e r (6-m l o n g ) y e t been r u n s i n g l e mode w i t h t h e c r y s t a l i n t h e c a v i t y .

A l l t h e s e s t a g e s must be demonstrated b e f o r e t h e f r e q u e n c y s y n t h e s i s

experiment c a n be p u t t o g e t h e r , i n t u r n b e f o r e any f r e q u e n c y measurements c a n b e made. It seems u n l i k e l y t h a t w i t h o u r p r e s e n t s t a f f o f r a t h e r l e s s t h a n t h r e e p e o p l e we can make a v i s i b l e f r e q u e n c y measurement by t h i s r o u t e b e f o r e t h e recommendations on t h e new d e f i n i t i o n o f t h e metre a r e formulated n e x t y e a r , b u t we s h a l l t r y .

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Development o f p o i n t - c o n t a c t J o s e p h s o n j u n c t i o n d e v i c e s

Work h a s been c a r r i e d o u t i n t h e g r o u p , u n d e r T G B l a n e y , p r i m a r i l y by N R Cnoss and Wu P e i Heng, a g u e s t w o r k e r from Nanking U n i v e r s i t y , o n ( i ) c o n s t r u c t i o n o f e f f i c i e n t l y - c o u p l e d d e v i c e s as m i x e r s f o r s u b m i l l i m e t r e a s t r o n o m y , and ( i i ) o n making up g l a s s - e n c a p s u l a t e d p o i n t c o n t a c t s w h i c h c a n b e t e m p e r a t u r e c y c l e d w i t h o u t s e r i o u s d e g r a d a t i o n .

The e f f i c i e n t m i x e r j u n c t i o n s make u s e o f c o n d u c t i o n - c o o l e d h e l i u m c r y o s t a t s w h i c h ( i ) remove t h e j u n c t i o n f r o m t h e l i q u i d h e l i u m and s o a v o i d s

p e r t u r b a t i o n o f t h e i n p u t c o u p l i n g by b u b b l i n g , and ( i i ) o f f e r good a c c e s s t o , a n d a g r e a t e r volume f o r , t h e mounting o f t h e j u n c t i o n . The c r y o s t a t s u s e d are a l s o much less b u l k y t h a n t h o s e u s e d b e f o r e , and c a n e a s i l y b e f i t t e d o n a bench. They a r e a b o u t 500 mm h i g h by a b o u t 300 mm i n d i a m e t e r , a n d t h e 1.5 l i t r e s o f h e l i u m l a s t s f o r 15 h o u r s .

The e f f i c i e n c y o f c o u p l i n g r a d i a t i o n t o t h e d e v i c e , and t h e d e g r e e o f a b s e n c e o f n o i s e i n t h e o p e r a t i o n i s d e m o n s t r a t e d by C r o s s t r e c e n t l y o b t a i n i n g a n o i s e t e m p e r a t u r e n e a r 400 K , d o u b l e s i d e b a n d , a t 230 GHz, and n e a r 2200 K a t 460 GHz, f o r f u n d a m e n t a l - f r e q u e n c y h e t e r o d y n e m i x i n g . T h i s was done w i t h a room-temperature I F a m p l i f i e r and a r a t h e r p o o r I F l i n e .

We p l a n e x p e r i m e n t s t o t r y t o e x t e n d f r e q u e n c y measurements beyond o u r p r e v i o u s l i m i t o f 4.25 THz u s i n g t h i s new a p p a r a t u s , and p o s s i b l y h i g h e r 6 energy-gap materials.

The program t o t r y t o make t e m p e r a t u r e - c y c l a b l e i . e . "permanent" Nb-Nb p o i n t c o n t a c t s h a s met w i t h a c e r t a i n amount of s u c c e s s . P r e s e t c o n t a c t s a r e s e a l e d i n t o g l a s s t u b e s i n a n i n e r t a t m o s p h e r e . Some 4 o r 5 s u c h j u n c t i o n s now e x i s t t h a t e x h i b i t J o s e p h s o n b e h a v i o u r when i r r a d i a t e d w i t h HCN l a s e r r a d i a t i o n a t 8 9 0 GHz and which h a v e been immersed i n l i q u i d h e l i u m o n s e v e r a l o c c a s i o n s . They a r e t o be t e s t e d f o r m e a s u r i n g t h e laser's f r e q u e n c y by harmonic m i x i n g w i t h microwaves.

The g r o u p a l s o l o o k s a h e a d t o t h e p o s s i b i l i t y o f f a b r i c a t i n g e v a p o r a t e d f i l m d e v i c e s s u i t a b l e f o r f r e q u e n c y measurements a t THz f r e q u e n c i e s .

C o n c l u s i o n s and Comments

The r e c e n t e x p e r i m e n t s h a v e shown up i n a d e q u a c i e s i n o u r e q u i p m e n t , p a r t l y b e c a u s e i t i s g e t t i n g o l d a n d t h e r e a r e f e w e r p e o p l e now t o k e e p it g o i n g , a n d p a r t l y b e c a u s e t h e u s e o f n o n - l i n e a r c r y s t a l s i n t h e CO

f r e q u e n c y - d o u b l i n g e x p e r i m e n t , f o r e x a m p l e , makes g r e a t e r demands on t h e 2 q u a l i t y o f t h e e q u i p m e n t t h a n m i x i n g w i t h p o i n t - c o n t a c t d e v i c e s . The s t i f f n e s s o f t h e m o u n t i n g t a b l e ( n o t o u r b e s t ) and t h e d e g r e e o f i s o l a t i o n a c h i e v e d between c r y s t a l and l a s e r were s u c h t h a t f l u c t u a t i o n s o f 2nd h a r m o n i c a m p l i t u d e c o u l d be q u i t e l a r g e .

However, we h a v e e s t a b l i s h e d t h a t p h a s e - l o c k i n g o u r l a s e r s t o g e t h e r i s n o t t o o d i f f i c u l t , and p r o d u c e s a d r a m a t i c r e d u c t i o n i n t h e be!k s p e c t r a l w i d t h , and we h a v e o b t a i n e d a u s e f u l u p p e r l i m i t o f 7 p a r t s i n 10 f o r a n y

d i f f e r e n c e s i n f r e q u e n c y d o u b l i n g between a p o i n t - c o n t a c t and a

phase-matched, b u l k , d e v i c e . T h e r e i s room f o r hope now t h a t we c a n make faster p r o g r e s s w i t h t h e s y n t h e s i s scheme t o t h e v i s i b l e u s i n g t h e c o l o u r - c e n t r e laser.

We welcome e f f o r t s i n o t h e r l a b o r a t o r i e s t o e x t e n d t h e f r e q u e n c y r a n g e o f p o i n t - c o n t a c t d e v i c e s as m i x e r s i n t h e v i s i b l e , and we a l s o hope t h a t a n e w l y - a r r i v e d g u e s t w o r k e r w i l l h e l p u s e x t e n d t h e r a n g e o f m i x i n g i n J o s e p h s o n j u n c t i o n s beyond 4 THz, t o w a r d s C02 f r e q u e n c i e s .

References

1. KNIGHT D. J. E., EDWARDS G. J., PEARCE P. R. and CROSS N. R., IEEE Trans. Instr. Meas. IM-29 (1980) 257-264.

2. KNIGHT D. J. E., NPL Report no. QU 45 (1st revision) Feb. 1981, (National Physical Laboratory, Teddington, Middx. TWll OLW, U.K.)

3. FREED C., BRADLEY L. C. and O'DONNELL R.G., IEEE J. Quant. Electron.

aE-16 (1980) 1195-1206.

4. SHOTTON K.C. and ROWLEY W. R. C., NPL Report QU 28 Jan. 1975, (National Physical Laboratory, Teddington, Middx. TWll OLW, U.K.)

5. MENYUK N., ISELER G. W. and MOORADIAN A., Appl. Phys. Lett. a (1976) 422-424.

6. BLANEY T. G., CROSS N. R., KNIGHT D. J. E., EDWARDS G.J. and PEARCE P.

R.. J. Phys. D: Appl. Phys. 2 (1980) 1365-1370.

7. HALFORD D., Proc. Frequency Standards and Metrology Seminar, Lava1 University, Quebec, Canada, Aug-Sept. 1971, (Produced at Nat. Bur. Stand.

Boulder CO 80302, USA) pp 431-466.

See also: HOME D. A . , NBS Tech. Note 679 Mar 1976 (Nat. Bur. Stand.

Boulder CO 80302, USA).