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Absolute frequency measurement of a CF2CH2 absorption line near 890 GHz using an HCN laser
Volume 32A, number 2
A
CF2CH
PHYSICS
ABSOLUTE 2 ABSORPTION
LETTERS
FREQUENCY LINE NEAR
MEASUREMENT 890 GHz USING
15 June 1970
OF AN HCN
LASER
C. C. B R A D L E Y and D. J. E. KNIGHT National Physical Laboratory, Teddington, Middlesex, UK
Received 4 May 1970
The absolute frequency of a molecular absorption line in difluoroethylene has been determined directly as 890 759.6 + 0.2 MHz by a harmonic mixing technique. In addition the centre emission f r e quencies of two HCN lasers have been measured and found to be 890 760.2 :e 0.2 MHz.
T h e 890 GHz (337 g m ) HCN l a s e r l i n e is i m portant as a transfer oscillator for extending frequency measurements from microwave standa r d s to the 10 g m r e g i o n and b e y o n d [1, 2], p a r t i c u l a r l y in c o n n e c t i o n with a d e f i n i t i v e m e a s u r e m e n t of the s p e e d of light. T h e l a s e r a l s o might be used for Josephson voltage standards. S i n c e t h e r e p r o d u c i b i l i t y of the l a s e r c e n t r e frequency is not better than 1 in 106 it would be valuable to be able to lock it to a more stable reference. A transition in 1,1 difluoroethylene (CF2 =CH2) has been found to have a usable absorption coefficient and to be offset by about -0.4 MHz from the laser centre frequency [3,4]. This letter reports an absolute measurement of the centre frequency of the difluoroethyleneabsorption. Measurements were also made on the centre emission frequencies of two HCN lasers used. The apparatus for observing the absorption was a modificationof that used by Duxbury and Jones [4]. The laser output was split into three beams. The main beam was focussed into a frequency-mixingdiode using a Cassegrainian reflector, and two weaker beams were used for differential detection $ of the absorption using Golay cells [5]. The laser frequency was observed as a 30 MHz beat signal against the 12th harmonic of a 74.3 GHz klystron using a crossed-waveguide silicon point-contact mixer [6]. Power levels of a few mW from the laser (about I0 mW produced a few mV of rectified dc) and about 50 mW from The signal r e c e i v e d via the absorbing gas was subtracted from that r e c e i v e d on a reference beam. This was arranged to give a zero net signal over the l a s e r tuning range with the absorption cell evacuated.
the k l y s t r o n w e r e r e q u i r e d . T h e k l y s t r o n w a s p h a s e - l o c k e d d i r e c t l y to the 152nd h a r m o n i c of 488 MHz ( c h e c k e d to 1 in 108) f r o m a H P 5 1 0 5 A s y n t h e s i s e r . T h i s n o v e l l o c k i n g of a 4 m m k l y s t r o n , by m e a n s of a c o m b g e n e r a t o r f o l l o w e d by a 4 t h - h a r m o n i c 1N53 m i x e r in the r e f erence chain, simplified the frequency measu r e m e n t . P h a s e n o i s e f r o m t h e s y n t h e s i s e r and m u l t i p l i e r s c o n t r i b u t e d to the l a s e r b e a t s i g n a l a width of 300 kHz at h a l f - v o l t a g e h e i g h t but l e s s than 50 kHz at t h e top. T h e b e a t s i g n a l w a s d i s p l a y e d on a c a l i b r a t e d s p e c t r u m - a n a l y s e r ( d i s p e r s i o n 500 kHz p e r cm) v i a a 30 MHz l o w - n o i s e preamplifier. T h e f i r s t H C N l a s e r u s e d w a s 10 cm d i a m e t e r and 6 m l o n g with a n e a r - c o n f o c a l c a v i t y . T h e r a d i a t i o n w a s c o u p l e d out t h r o u g h a 2 cm d i a m e t e r hole. A 1A d i s c h a r g e f r o m a w a t e r - c o o l e d h o l l o w c a t h o d e in a 50 : 50 m i x t u r e of CH 4 and N 2 w a s c a r e f u l l y a d j u s t e d to c o n s i s t of s t a b l e s t r i a t i o n s o v e r the w h o l e of its length. T h i s c o n d i t i o n is e s s e n t i a l f o r m i n i m i s i n g the f r e q u e n c y width of the output [7, 8]. T h e l a s e r e m i s s i o n w a s l e s s than about 50 kHz w i d e (judged f r o m the b e a t s p e c t r u m ) and c o u l d be tuned o v e r 2.4 MHz to half p o w e r . A s e c o n d l a s e r w a s u s e d w h i c h d i f f e r e d in h a v i n g a h e m i s p h e r i c a l c a v i t y 4 m long, m a x i m u m d i a m e t e r 15 c m and b e a m - s p l i t t e r output coupling. T h i s had a m u c h g r e a t e r tuning width than the f i r s t l a s e r but the d i s c h a r g e w a s not a s s t a b l e and s o m e a c c u r a c y w a s l o s t b e c a u s e the b e a t s i g n a l w a s b r o a d e n e d to 200 kHz at the top. T h e f r e q u e n c y of the d i f l u o r o e t h y l e n e a b s o r p tion w a s o b t a i n e d by r e a d i n g t h e f r e q u e n c y of t h e l a s e r b e a t f r o m the s p e c t r u m a n a l y s e r when t h e l a s e r w a s tuned with a m i c r o m e t e r to g i v e a p e a k in the d i f f e r e n t i a l a b s o r p t i o n s i g n a l . T h e 59
Volume 32A, n u m b e r 2
PHYSICS
LETTERS
15 June 1970
Table 1 Absorption frequency m e a s u r e m e n t s and c o r r e c t i o n s . difluoroethylene absorption Laser No.
L a s e r tuning width * (MHz)
Absorption width * (MHz)
Number of frequency readings
Apparent peak (MHz)
Calculated correction
1
2.4
1.5 - 1.6
9
-0.41 i 0.05
-0.2
2
6
1.5 - 2
22
-0.50 ± 0.09
-0.05
Centre frequency (MHz)
-0.6 ± 0.2
height. ~ AtR e fhalf e r r e d to 890 760.20 MHz w i d t h of t h i s a t h a l f h e i g h t v a r i e d b e t w e e n 1.5 a n d 2.4 MHz o v e r a p r e s s u r e r a n g e of 0.05 to 0.25 t o r r in t h e a b s o r p t i o n c e l l . A s i g n a l / n o i s e r a t i o of a b o u t 50 a l l o w e d s e t t i n g on t h e m a x i m u m to a b o u t 0.1 MHz. S i n c e t h e a b s o r p t i o n i s c o m p a r a b l e in w i d t h w i t h t h e l a s e r t u n i n g p r o f i l e and is offset from the laser centre, the absorption observed using the differential technique has its peak shifted towards the centre. A correction for this was calculated. The absorptionfrequency measurements and corrections app l i e d a r e s u m m a r i s e d in t a b l e 1. T h e c o r r e c t i o n for the second laser was much smaller than for t h e f i r s t b e c a u s e of i t s g r e a t e r t u n i n g w i d t h . T h e f i n a l a s s i g n e d e r r o r i n c l u d e s a n u n c e r t a i n t y of a b o u t 50% in t h e c o r r e c t i o n b e c a u s e of u n r e l i a b l e c u r v e f i t s to t h e l a s e r a n d a b s o r p t i o n l i n e p r o files. The combined measurement for the 1,1 d i f l u o r o e t h y l e n e a b s o r p t i o n i s 890 759.6 + 2 MHz. No p r e s s u r e s h i f t s w e r e r e s o l v e d o v e r t h e r a n g e of p r e s s u r e s t a t e d a b o v e . T h e c e n t r e f r e q u e n c i e s of b o t h H C N l a s e r s w e r e w i t h i n 0.2 M H z of 8 9 0 7 6 0 . 2 M H z , w h i c h l i e s b e t w e e n two v a l u e s (1.2 MHz a p a r t ) o b t a i n e d b y E v e n s o n et al. [ 1 , 7 ] . T h e v a r i a t i o n c o u l d r e s u l t f r o m t h e u s e of d i f f e r e n t c a v i t y a n d d i s c h a r g e c o n d i t i o n s a n d i n d i c a t e s t h e v a l u e of the difluoroethylene absorption as a reference. T h i s i s r e l e v a n t to t h e w o r k of F i n n e g a n e t al. who compared Josephson-junction voltage steps
60
generated by an HCN laser and by a microwave s o u r c e to a n a c c u r a c y of 1 in 106 [9]. T h e a u t h o r s a r e g r a t e f u l to D r . K. D. F r o o m e , a n d D r . N. W. B. S t o n e f o r a d v i c e a n d e n c o u r a g e m e n t , a n d to M r . J . D r o m e y f o r t e c h n i c a l a s s i s t ance. T h i s w o r k i s on t h e r e s e a r c h p r o g r a m m e of the National Physical Laboratory.
References [1] K. M. Evenson, J.S. Wells, L.M. M a t a r r e s e and L. B. Elwell, Appl. Phys. L e t t e r s 16 (1970) 159; K. M. Evenson, J.S. Wells and L. M. M a t a r r e s e , Appl. Phys. L e t t e r s 16 (1970) 251. [21 V. Daneu, D. Sokoloff, A, Sanchez and A. Javan, Appl. Phys. L e t t e r s 15 (1969) 398. [3] G. Duxbury and W. J. Burroughs, J. Phys. Soc. B 3 (1970) 98. [4] G. Duxbury and R. G. Jones, Phys. L e t t e r s 30A (1969) 498. [5] C. C. Bradley, G. Duxbury, R. G. Jones and J. Birch, Nature, to be published. [6] L.O. Hocker, A. Javan, D. R a m a c h a n d r a - R a o , L. Frenkel and T. Sullivan, Appl. Phys. L e t t e r s 10 (1967) 147. [7] K.M. Evenson, H . P . B r o i d a , J.S. Wells, R . J . Mahler and M. Mizushima, Phys. Rev. L e t t e r s 21 (1968) 1038. [8] D. W. E. Fuller, J. Hines and B. Compton, Electr. L e t t e r s 5 (1969) 448. [9] T. F. Finnegan, A. Denenstein, D. N: Langenberg, J. C. McMenamin, D. E. Novoseller and L. Cheng, Phys. Rev. L e t t e r s 23 (1969) 229.
A
CF2CH
PHYSICS
ABSOLUTE 2 ABSORPTION
LETTERS
FREQUENCY LINE NEAR
MEASUREMENT 890 GHz USING
15 June 1970
OF AN HCN
LASER
C. C. B R A D L E Y and D. J. E. KNIGHT National Physical Laboratory, Teddington, Middlesex, UK
Received 4 May 1970
The absolute frequency of a molecular absorption line in difluoroethylene has been determined directly as 890 759.6 + 0.2 MHz by a harmonic mixing technique. In addition the centre emission f r e quencies of two HCN lasers have been measured and found to be 890 760.2 :e 0.2 MHz.
T h e 890 GHz (337 g m ) HCN l a s e r l i n e is i m portant as a transfer oscillator for extending frequency measurements from microwave standa r d s to the 10 g m r e g i o n and b e y o n d [1, 2], p a r t i c u l a r l y in c o n n e c t i o n with a d e f i n i t i v e m e a s u r e m e n t of the s p e e d of light. T h e l a s e r a l s o might be used for Josephson voltage standards. S i n c e t h e r e p r o d u c i b i l i t y of the l a s e r c e n t r e frequency is not better than 1 in 106 it would be valuable to be able to lock it to a more stable reference. A transition in 1,1 difluoroethylene (CF2 =CH2) has been found to have a usable absorption coefficient and to be offset by about -0.4 MHz from the laser centre frequency [3,4]. This letter reports an absolute measurement of the centre frequency of the difluoroethyleneabsorption. Measurements were also made on the centre emission frequencies of two HCN lasers used. The apparatus for observing the absorption was a modificationof that used by Duxbury and Jones [4]. The laser output was split into three beams. The main beam was focussed into a frequency-mixingdiode using a Cassegrainian reflector, and two weaker beams were used for differential detection $ of the absorption using Golay cells [5]. The laser frequency was observed as a 30 MHz beat signal against the 12th harmonic of a 74.3 GHz klystron using a crossed-waveguide silicon point-contact mixer [6]. Power levels of a few mW from the laser (about I0 mW produced a few mV of rectified dc) and about 50 mW from The signal r e c e i v e d via the absorbing gas was subtracted from that r e c e i v e d on a reference beam. This was arranged to give a zero net signal over the l a s e r tuning range with the absorption cell evacuated.
the k l y s t r o n w e r e r e q u i r e d . T h e k l y s t r o n w a s p h a s e - l o c k e d d i r e c t l y to the 152nd h a r m o n i c of 488 MHz ( c h e c k e d to 1 in 108) f r o m a H P 5 1 0 5 A s y n t h e s i s e r . T h i s n o v e l l o c k i n g of a 4 m m k l y s t r o n , by m e a n s of a c o m b g e n e r a t o r f o l l o w e d by a 4 t h - h a r m o n i c 1N53 m i x e r in the r e f erence chain, simplified the frequency measu r e m e n t . P h a s e n o i s e f r o m t h e s y n t h e s i s e r and m u l t i p l i e r s c o n t r i b u t e d to the l a s e r b e a t s i g n a l a width of 300 kHz at h a l f - v o l t a g e h e i g h t but l e s s than 50 kHz at t h e top. T h e b e a t s i g n a l w a s d i s p l a y e d on a c a l i b r a t e d s p e c t r u m - a n a l y s e r ( d i s p e r s i o n 500 kHz p e r cm) v i a a 30 MHz l o w - n o i s e preamplifier. T h e f i r s t H C N l a s e r u s e d w a s 10 cm d i a m e t e r and 6 m l o n g with a n e a r - c o n f o c a l c a v i t y . T h e r a d i a t i o n w a s c o u p l e d out t h r o u g h a 2 cm d i a m e t e r hole. A 1A d i s c h a r g e f r o m a w a t e r - c o o l e d h o l l o w c a t h o d e in a 50 : 50 m i x t u r e of CH 4 and N 2 w a s c a r e f u l l y a d j u s t e d to c o n s i s t of s t a b l e s t r i a t i o n s o v e r the w h o l e of its length. T h i s c o n d i t i o n is e s s e n t i a l f o r m i n i m i s i n g the f r e q u e n c y width of the output [7, 8]. T h e l a s e r e m i s s i o n w a s l e s s than about 50 kHz w i d e (judged f r o m the b e a t s p e c t r u m ) and c o u l d be tuned o v e r 2.4 MHz to half p o w e r . A s e c o n d l a s e r w a s u s e d w h i c h d i f f e r e d in h a v i n g a h e m i s p h e r i c a l c a v i t y 4 m long, m a x i m u m d i a m e t e r 15 c m and b e a m - s p l i t t e r output coupling. T h i s had a m u c h g r e a t e r tuning width than the f i r s t l a s e r but the d i s c h a r g e w a s not a s s t a b l e and s o m e a c c u r a c y w a s l o s t b e c a u s e the b e a t s i g n a l w a s b r o a d e n e d to 200 kHz at the top. T h e f r e q u e n c y of the d i f l u o r o e t h y l e n e a b s o r p tion w a s o b t a i n e d by r e a d i n g t h e f r e q u e n c y of t h e l a s e r b e a t f r o m the s p e c t r u m a n a l y s e r when t h e l a s e r w a s tuned with a m i c r o m e t e r to g i v e a p e a k in the d i f f e r e n t i a l a b s o r p t i o n s i g n a l . T h e 59
Volume 32A, n u m b e r 2
PHYSICS
LETTERS
15 June 1970
Table 1 Absorption frequency m e a s u r e m e n t s and c o r r e c t i o n s . difluoroethylene absorption Laser No.
L a s e r tuning width * (MHz)
Absorption width * (MHz)
Number of frequency readings
Apparent peak (MHz)
Calculated correction
1
2.4
1.5 - 1.6
9
-0.41 i 0.05
-0.2
2
6
1.5 - 2
22
-0.50 ± 0.09
-0.05
Centre frequency (MHz)
-0.6 ± 0.2
height. ~ AtR e fhalf e r r e d to 890 760.20 MHz w i d t h of t h i s a t h a l f h e i g h t v a r i e d b e t w e e n 1.5 a n d 2.4 MHz o v e r a p r e s s u r e r a n g e of 0.05 to 0.25 t o r r in t h e a b s o r p t i o n c e l l . A s i g n a l / n o i s e r a t i o of a b o u t 50 a l l o w e d s e t t i n g on t h e m a x i m u m to a b o u t 0.1 MHz. S i n c e t h e a b s o r p t i o n i s c o m p a r a b l e in w i d t h w i t h t h e l a s e r t u n i n g p r o f i l e and is offset from the laser centre, the absorption observed using the differential technique has its peak shifted towards the centre. A correction for this was calculated. The absorptionfrequency measurements and corrections app l i e d a r e s u m m a r i s e d in t a b l e 1. T h e c o r r e c t i o n for the second laser was much smaller than for t h e f i r s t b e c a u s e of i t s g r e a t e r t u n i n g w i d t h . T h e f i n a l a s s i g n e d e r r o r i n c l u d e s a n u n c e r t a i n t y of a b o u t 50% in t h e c o r r e c t i o n b e c a u s e of u n r e l i a b l e c u r v e f i t s to t h e l a s e r a n d a b s o r p t i o n l i n e p r o files. The combined measurement for the 1,1 d i f l u o r o e t h y l e n e a b s o r p t i o n i s 890 759.6 + 2 MHz. No p r e s s u r e s h i f t s w e r e r e s o l v e d o v e r t h e r a n g e of p r e s s u r e s t a t e d a b o v e . T h e c e n t r e f r e q u e n c i e s of b o t h H C N l a s e r s w e r e w i t h i n 0.2 M H z of 8 9 0 7 6 0 . 2 M H z , w h i c h l i e s b e t w e e n two v a l u e s (1.2 MHz a p a r t ) o b t a i n e d b y E v e n s o n et al. [ 1 , 7 ] . T h e v a r i a t i o n c o u l d r e s u l t f r o m t h e u s e of d i f f e r e n t c a v i t y a n d d i s c h a r g e c o n d i t i o n s a n d i n d i c a t e s t h e v a l u e of the difluoroethylene absorption as a reference. T h i s i s r e l e v a n t to t h e w o r k of F i n n e g a n e t al. who compared Josephson-junction voltage steps
60
generated by an HCN laser and by a microwave s o u r c e to a n a c c u r a c y of 1 in 106 [9]. T h e a u t h o r s a r e g r a t e f u l to D r . K. D. F r o o m e , a n d D r . N. W. B. S t o n e f o r a d v i c e a n d e n c o u r a g e m e n t , a n d to M r . J . D r o m e y f o r t e c h n i c a l a s s i s t ance. T h i s w o r k i s on t h e r e s e a r c h p r o g r a m m e of the National Physical Laboratory.
References [1] K. M. Evenson, J.S. Wells, L.M. M a t a r r e s e and L. B. Elwell, Appl. Phys. L e t t e r s 16 (1970) 159; K. M. Evenson, J.S. Wells and L. M. M a t a r r e s e , Appl. Phys. L e t t e r s 16 (1970) 251. [21 V. Daneu, D. Sokoloff, A, Sanchez and A. Javan, Appl. Phys. L e t t e r s 15 (1969) 398. [3] G. Duxbury and W. J. Burroughs, J. Phys. Soc. B 3 (1970) 98. [4] G. Duxbury and R. G. Jones, Phys. L e t t e r s 30A (1969) 498. [5] C. C. Bradley, G. Duxbury, R. G. Jones and J. Birch, Nature, to be published. [6] L.O. Hocker, A. Javan, D. R a m a c h a n d r a - R a o , L. Frenkel and T. Sullivan, Appl. Phys. L e t t e r s 10 (1967) 147. [7] K.M. Evenson, H . P . B r o i d a , J.S. Wells, R . J . Mahler and M. Mizushima, Phys. Rev. L e t t e r s 21 (1968) 1038. [8] D. W. E. Fuller, J. Hines and B. Compton, Electr. L e t t e r s 5 (1969) 448. [9] T. F. Finnegan, A. Denenstein, D. N: Langenberg, J. C. McMenamin, D. E. Novoseller and L. Cheng, Phys. Rev. L e t t e r s 23 (1969) 229.