THE FREQUENCY MEASUREMENT OF VISIBLE LIGHT

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THE FREQUENCY MEASUREMENT OF VISIBLE LIGHT

K. Evenson, D. Jennings, F. Petersen

To cite this version:

K. Evenson, D. Jennings, F. Petersen. THE FREQUENCY MEASUREMENT OF VISIBLE LIGHT.

Journal de Physique Colloques, 1981, 42 (C8), pp.C8-473-C8-483. �10.1051/jphyscol:1981853�. �jpa- 00221749�

JOURNAL DE PHYSIQUE

CoZZoque C8, suppZdment au nO1 2, Tome 42, de'cembre 1981 page (28-473

THE FREQUENCY MEASUREMENT OF V I S I B L E L I G H T *

K.M. Evenson, D.A. Jennings and F.R. Petersen

Laser Frequency MetroZogy Group, Time and Frequency Division, National Bureau of Standards, Boulder, Colorado 80303, U. S.A.

Abstract. - ^A discussion o f the extension o f absolute frequency measure- ments t o the v i s i b l e w i l l be given along w i t h some new measurements o f v i s i b l e frequency d i fferences using the MIM diode. Future frequency measurements and the r e d e f i n i t i o n o f the meter w i l l also be discussed.

Introduction. - The f i r s t absolute frequency measurement o f v i s i b l e r a d i a t i o n was made about two and one-half years ago [I]. A1 though i t was n o t performed w i t h high accuracy, i t demonstrated the f e a s i b i 1 i t y o f d i r e c t frequency measurements i n t h e v i s i b l e . Many standards l a b o r a t o r i e s i n the world are now i n the process o f making h i g h l y accurate absolute v i s i b l e frequency measurements t o e s t a b l i s h accurate v i s i b l e frequency references, which might a l s o serve as length standards i n a possible r e d e f i n i t i o n o f the meter. The r e d e f i n i t i o n o f the meter may be considered a t the next General Assembly o f the I n t e r n a t i o n a l Committee on Weights and Measures. The r e d e f i n i t i o n proposed by the Consultative Committee f o r the D e f i n i t i o n o f the Meter [2] states: "The meter i s the l e n g t h equal t o the d i s - tance t r a v e l e d $ n a time i n t e r v a l o f 1/299 792 458 o f a second by plane e l e c t r o - magnetic waves i n vacuum." With t h i s r e d e f i n i t i o n , the speed o f l i g h t , c, w i l l be fixed, and t h e meter w i l l be r e a l i z e d v i a a frequency measurement o f a s t a b i l - i z e d laser. I t i s the advent o f the absolute frequency measurement o f v i s i b l e l i g h t w i t h the i n h e r e n t l y high accuracy o f frequency measurement which has made t h i s r e d e f i n i t i o n possible. V i s i b l e r a d i a t i o n i s most important f o r r e a l i z i n g t h e meter because o f alignment ease and higher accuracy l e n g t h measurement a t these shorter wavelengths.

The extension o f absolute frequency measurements t o the v i s i b l e requires f i r s t , stab1 e osci 11 ators and, secondly, harmoni c generating techniques f o r frequency synthesis purposes (eventually, the Cs frequency standard must be m u l t i p l i e d some 60 000 times). An a l t e r n a t i v e t o harmonic generation would be a d i v i d i n g technique; however, the technique proposed by Wineland [3] has n o t y e t been proven operational.

"Contribution o f the U. S. Government, not subject t o copyright.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981853

C8-474 JOURNAL DE PHYSIQUE

S t a b i l i z e d o s c i l l a t o r s are provided by lasers locked t o atomic and molecular resonances. This f i e l d has progressed r a p i d l y i n the l a s t decade, and some o f t h e s t a b i l i z e d o s c i l l a t o r s are challenging the fundamental cesium standard. The various techniques and the progess made are n i c e l y summarized by H a l l [4].

Techniques f o r harmonic generation have not been as e f f i c i e n t above 1 THz as a t microwave frequencies where up t o 1000 harmonics have been generated. A maximum o f 12 harmonics has been achieved w i t h lasers. Consequently, "chains1' o f o s c i l l a t o r s are used t o reach the v i s i b l e . The most useful harmonic generator- mixer operating between 1 and 200 THz discovered so f a r i s the tungsten-nickel point-contact diode. This same diode has been used as a t h i r d - o r d e r mixer i n t h e v i s i b l e a t frequency differences up t o 2.5 THz ( t o be described l a t e r ) .

I n t h i s paper, we w i l l attempt t o review progress i n the l a s e r frequency measurement f i e l d , present some recent r e s u l t s , and make a few comments about f u t u r e experiments.

The Measurement o f the Frequencies o f Various S t a b i l i z e d Lasers. - Various "well- behaved1' 1 asers have been used t o probe narrow atomic and mol ecul a r resonances, and the narrawest o f these have been used f o r frequency s t a b i l i z a t i o n . Hence, it i s the narrow atomic o r molecular t r a n s i t i o n frequency which i s being measured.

Three sets o f these s t a b i 1 iz e d lasers have emerged and w i l l be described:

1, The CO, l a s e r s t a b i l i z e d t o a number o f molecular absorptions.

2. The 3.39 pm helium-neon l a s e r s t a b i l i z e d t o methane.

3. The various sources o f v i s i b l e coherent l i g h t s t a b i l i z e d t o iodine.

Carbon Dioxide Frequencies. - The carbon dioxide l a s e r i s unique since a l l o f the l i n e s ( w i t h the exception o f the sequence bands) can be s t a b i l i z e d t o Doppler- f r e e absorption features i n CO, i t s e l f by the saturated f 1 uorescence technique [5l. The frequencies o f l a s i n g t r a n s i t i o n s i n seven CO, isotopes, 12C160 2 9

13C1602, 12C180 ^{2 9} 13C180 ^2, 12C160180, 14C1602, and 14C180,, have now been measured t o w i t h i n an uncertainty o f a few p a r t s i n lo9 [6]. The measurements are r e l a t e d t o the Cs standard v i a chains which measured the absolute frequencies o f RII(lO) and RI(30) i n 12C1602 [7]. Figure 1 shows good agreement among measurements o f RI(30) made by various standards l a b o r a t o r i e s throughout the world [7-121. With the improvement i n l a s e r s t a b i l i z a t i o n and i n the frequency measurement chains, u n c e r t a i n t i e s have been reduced t o l e s s than two p a r t s i n 1010 [123. The NRC measurement i s d i f f e r e n t i n t h a t i t was made by m u l t i p l i c a t i o n o f CO, l a s e r d i f f e r e n c e frequencies w i t h only CO, lasers i n the chain, and the p r e l i m i n a r y r e s u l t here shows the f e a s i b i l i t y o f t h i s technique [Ill.

Future improvements i n t h e CO, l a s e r reference frequencies w i l l r e q u i r e improvement i n the l a s e r s t a b i l i z a t i o n i n a d d i t i o n t o the frequency measurement.

F i ure 1. - Frequency o f the saturated-f 1 uorescence stabi 1 iz e d R (30), 12~160, 1 a:er 1 i ne. A1 1 measurements are uncorrected f o r base1 i ne s l dpe , pressure, power, o r other frequency s h i f t s . The GS + LPTF measurement combines the GS measurement o f the OsO, s t a b i l i z e d P (14) l i n e and the 050, - ^P (14) and PI(14) -

R (30) frequency d i f f e r e n c e measurdments by LPTF. The NRC deasurement demon- s t r a t e s use o f an a1 1 CO, l a s e r chain and i s a p r e l i m i n a r y r e s u l t .

1442(13m

ZBUZ(U380 (kWl

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Figure 2. - Frequency measurements o f methane-stabilized He-Ne lasers a t 3.39 pm.

References are: NBS [7], NPL [a], GS 191, LPTF [121, NRC L131, NPL D41, and GS [15], i n order.

FREQUENCY OF THE SATURATED-FLUORESCENCE-STABILIZED RI (30), I2c LASER LINE

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FREQUENCY OF METHANE-STABILIZED He-Ne LASER at 3.39 urn

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C8-476 JOURNAL DE PHYSIQUE

Selection of heavy molecules such as Os04 which have frequency overlaps w i t h CO, have provided a thousand-fold reducution i n l i n e w i d t h (1 kHz w i t h a corresponding improvemel;t i n the frequency r e p r o d u c i b i l i t y ) . A judicious s e l e c t i o n o f absorp- t i o n 1 ines i n these molecules could provide an improved frequency reference g r i d over the intermediate i n f r a r e d band [lo]. U n t i l t h a t time, the approximately 600 CO, l a s e r frequencies known t o a few p a r t s i n l o 9 are serving we1 1 as secondary frequency standards i n the i n f r a r e d .

Methane Frequencies. - The measurements of methane a t 88 THz [7-9,12-151 and the u n c e r t a i n t i e s (1-0) are shown i n f i g u r e 2 versus the year each measurement was made. Good agreement between the values i s seen, w i t h the discrepancies being due t o possible power s h i f t s from the hyperfine s t r u c t u r e o f methane. The next measurements o f t h i s molecule probably w i l l be made on lasers which can resolve t h e hyperf in e s t r u c t u r e t o eliminate t h i s problem. The smallest reported uncer- t a i n t y i s & 2 x 10-ll. This frequency measurement i s b e t t e r than best wavelength comparison by about a f a c t o r o f ten, and hence, t h i s l a s e r could be used as a length standard as i s . As was mentioned e a r l i e r , however, i t i s simpler and more accurate t o use v i s i b l e radiation.

V i s i b l e Frequencies. - The absolute measurement o f the iodine absorption frequency (520 THz) a t twice the strong 1.15 pm l a s e r frequency i n ,ONe was accomplished w i t h the chain 1161 o f frequency measurements shown i n f i g u r e 3. The W-Ni diode was used as the nonlinear element up t o 197 THz, b u t harmonic generation was

1 im i t e d t o harmonics of the C02 l a s e r i n the 88 THz measurement. Subsequently, 148 THz r a d i a t i o n was synthesized i n our laboratory i n a point-contact diode w i t h f i v e harmonics o f a CO, laser.

The 260 THz frequency was synthesized by f i r s t summing two s t a b i l i z e d CO, l a s e r frequencies i n the nonlinear c r y s t a l , CdGeAs,, and subsequently summing the r e s u l t a n t 5 pm r a d i a t i o n and the 1.5 ym r a d i a t i o n from a ,ONe l a s e n i n a AgAsS, c r y s t a l (proustite). The synthesized r a d i a t i o n and the r a d i a t i o n from a Lamb-dip s t a b i l i z e d 1.15 pm 20Ne l a s e r were then mixed on a photodiode and the beat ob- served on a spectrum analyzer.

The f i n a l step [I] was performed i n a j o i n t experiment w i t h the National Research Council i n Ottawa, Canada i n which t e n hyperfine t r a n s i t i o n s i n 12712 near 520 THz were measured by comparing the 1.15 pm r a d i a t i o n a t one-half the frequency o f the I,l i n e s w i t h the frequency o f the Lamb-dip s t a b i l i z e d pure ,ONe l a s e r a t 260 THz. The yellow-green l i g h t a t 520 THz, generated ( i n the Hanes NRC laser) by i n t r a c a v i t y doubling i n LiNbO, o f the 260 THz r a d i a t i o n from a He-Ne 1 aser, was servo-1 ocked t o i n d i v i d u a l h y p e r f i ne components o f 12712 observed i n saturated absorption, and frequencies were determined simply by measurement o f the beat frequencies o f the two r a d i a t i o n s a t 260 THz.

LASER

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Figure 3. - Old "cesium1' t o "iodine" frequency chain

JOURNAL DE PHYSIQUE

The frequency measurements were done simply by combining the 260 THz beams from the two lasers on a high-speed photodiode and measuring the rf beat between them. The frequency, fa, of the a-component of the hyperfine structure is given by

The mean value of the o-component beat frequency was fbeat = fo/2 - ^{fNe =} 154.3 MHz,

and the estimated 1-0 error was 0.5 MHz. The frequency of the o-component of the P(62), 17-1 band of 1271, is thus

fo = 2[fNe + (fo/2 - ^{fNe)l = 520 206 837} + ^{60 MHz.}

A preliminary measurement of the wavelength of this component gave A = 576 294 758

* 6 fm [I], from which we calculate fo = c/h = 520 206 811 * ⁶ MHz. The agree- ment between the above values for the frequency is satisfactorily within the error limits.

This extension of absolute frequency measurements to the visible paves the way for highly accurate measurements for this portion of the electromagnetic spectrum. The rather large error limit on fa is due to the free-running 197 THz He-Ne laser used in the measurement of the Lamb-dip stabilized 20Ne, 1.15 ym laser. In this chain, fourteen lasers and six klystrons were used in seven steps, each terminated by a laser actively stabilized to a Doppler-free absorp- tion line when possible.

A simpler chain, shown in figure 4, is now being constructed at NBS, Boulder as an alternative technique for connecting the 12712 stabilized 520 THz laser to the Cs frequency standard. The multiplication factor of 48020 is accomplished with six lasers and four klystrons, a significant reduction from the number in the previous chain. A major change in this chain is the substitution of the color-center laser tuned to half the frequency of the 1.15 pm 20Ne line. This color-center laser, being developed by C. R. Pollock, will use (F, + )A centers in lithium-doped KC2 pumped with 1.3 pm cw YAG radiation [171. A ring configuration (shown in figure 5) has been developed, and output powers of 100 mW have been obtained. This chain should have a measurement capability with a fractional frequency uncertainty between 10-lo and 10-l1 for the I, transition at 520 THz.

A preliminary measurement will be made from the stabilized CO, laser to the visible before the final chain is completed. This measurement will be limited by the fractional uncertainty in the CO, reference frequency (- a 1 x lo-$).

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FREQUENCY SYNTHESIS

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C8-480 JOURNAL DE PHYSIQUE

Figure 5 - Color center ring laser.

COLOR CENTER RING LASER

EXPERIMENTAL SET - ^UP

Dye-Lwr Dye-Laser Figure 6 . - Experimental ar-

rangement t o t e s t the MIM diode in the v i s i b l e .

co, hnrp FIR

wowguida ^v3 Point Contact Diode

Loser Laser (MIMI

Figure 7. - Visible beat signal-to-noise ratio versus frequency differ- ence from the MIM diode.

This s t a b i l i z e d frequency a t 520 THz appears t o be an i d e a l reference from which t o synthesize other standard frequencies i n the v i s i b l e spectrum. These frequencies may be synthesized by mixing the 520 THz r a d i a t i o n w i t h an appropri- a t e i n f r a r e d l a s e r i n a nonlinear c r y s t a l such as AgGaS?. This c r y s t a l has been produced w i t h a wide, low-absorption transmission band (a < 0.5 cm-I f o r 0.5 < A < 10 pm) and has been used t o up-convert 10.6 pm r a d i a t i o n i n t o v i s i b l e sidebands i n the green spectral range. The c r y s t a l , however, does n o t phase match i n t h e e n t i r e v i s i b l e region, and the MIM diode might be used there, as w i l l be described i n the next section.

V i s i b l e Difference Frequencies Measured a t 2.5 THz. - A major breakthrough i n the measurement of v i s i b l e frequency differences occured t h i s year. I n a j o i n t experiment between the Max-Planck I n s t i t u t e f o r Quantum Optics, Garching, W.

Germany and the Time and Frequency D i v i s i o n o f NBS, Boulder, Colorado, R. E.

D r u l l i n g e r , J. C. Bergquist, D. A. Jennings, F. R. Petersen, Lee Burkins, U l r i c h Daniel, and K, M. Evenson demonstrated t h a t the MIM diode (W-Ni o r W-Co) could be used t o measure the d i f f e r e n c e frequency between two v i s i b l e dye lasers separated by as much as 2.5 THz. We a l s o p r e d i c t the p o s s i b i l i t y o f extending t h a t d i f f e r - ence up t o 30 THz, i.e., up t o the frequency o f t h e CO, laser. Before these measurements were made, frequency d i f f e r e n c e measurements i n t h e v i s i b l e had been made i n three d i f f e r e n t ways: 1) w i t h photodiodes a t difference frequencies up t o about 50 GHz, 2 ) w i t h Schottky b a r r i e r diodes a t d i f f e r e n c e frequencies up t o 200 GHz, and 3) w i t h nonlinear bul k mixing.

The experiment was i n s p i r e d by the success o f Daniel g G . [ I 8 1 who used t h e point-contact diode t o measure frequency differences up t o 122 GHz. The experimental setup i s shown i n f i g u r e 6. From 0.5 t o 2 mW o f cw r a d i a t i o n from a f a r i n f r a r e d l a s e r was mixed w i t h 15 mW o f cw r a d i a t i o n from two d i f f e r e n t dye lasers which were tuned t o a frequency difference approximately equal t o the f a r i n f r a r e d frequency. Three d i f f e r e n t f a r i n f r a r e d frequencies were used: 0.425, 1.8, and 2.5 THz. Two tunable, s t a b i l i z e d , r i n g dye lasers were used t o generate t h e two v i s i b l e frequencies near 575 nm. The spectral l i n e w i d t h o f each was about 200 kHz, Beat signals o f a few hundred MHz were observed w i t h a conven- t i o n a l spectrum analyzer. The observed signal-to-noise r a t i o s are p l o t t e d i n f i g u r e 7. Four data p o i n t s a t microwave frequencies a r e a l s o shown, one a t 10 and one a t 74 GHz, and two obtained by the Max-Planck group, one a t 88 GHz and another a t 170 GHz. The signal-to-noise i s observed t o f a l l o f f a t 2.3 dB per octave. The reason f o r t h i s f a l l - o f f i s not known. To compensate f o r a possible decrease i n coupling e f f i c i e n c y a t high frequencies, the power was adjusted t o g i v e a constant r e c t i f i e d voltage. Also, t h e c a p a c i t i v e shunting o f t h e diode i s expected t o be a t l e a s t -3 dB per octave, b u t i s not expected t o begin f a l l i n g

0 0

o f f u n t i l about 7 THz f o r a 400 A t i p and 30 THz f o r a 100 A t i p . The r a d i a t i o n s from both dye lasers were combined on a beam s p l i t t e r and focused w i t h a s i n g l e

JOURNAL DE PHYSIQUE

microscope objective. The wavelength o f each dye l a s e r was measured w i t h a A-meter t o about f 50 MHz. I n a f i n a l experiment, the dye lasers were tuned 25 THz apart (a CO, l a s e r frequency), b u t a beat note was not observed.

Only the best signal-to-noise r a t i o s were p l o t t e d ; they v a r i e d from 12 t o 22 dB a t 10 GHz, possibly depending on the sharpness o f the diode. Electron micrographs were n o t taken o f t h e diodes used i n the experiments, b u t previous

0 0

micrographs have revealed r a d i i varying from less than 100 A t o 600 A. This v a r i a t i o n i n t i p radius might cause the observed v a r i a t i o n i n s e n s i t i v i t y .

The experiment was repeated again w i t h much f a s t e r s t a b i l i z a t i o n o f the dye lasers, which produced a l i n e w i d t h o f less than 10 kHz. However, again, a signal was not observed. I n t h i s case, the signal-to-noise obtainable a t X-band increased by about 13 dB (a maximum o f 35 dB was observed). It was n o t possible t o use one o f these "super" diodes tested a t 10 GHz f o r the 25 THz measurement, since during the time required t o retune the dye lasers, the contact resistance i n v a r i a b l y changed. When the diode was readjusted, i t o f t e n produced very prom- i s i n g r e c t i f i e d voltages from each r a d i a t i o n , b u t no beat was observed. With the W-Ni diode, the r e c t i f i e d voltage was generally negative a t frequencies below 200 THz, and p o s i t i v e i n the v i s i b l e . The p o l a r i t y o f the r e c t i f i e d voltage from the diode w i t h the cobalt base was less predictable, b u t t h i s diode performed equally w e l l i n the successful d i f f e r e n c e frequency measurements.

We believe t h a t these experiments were n o t conclusive and should be repeated under more c o n t r o l l e d conditions, i.e., w i t h a measurement o f the whisker t i p radius and w i t h a check on the absolute accuracy o f the A-meter. We t h i n k t h a t the point-contact diode can probably be used i n the v i s i b l e a t frequency d i f f e r - ences up t o 30 THz. This w i l l f a c i l i t a t e the measurement o f v i s i b l e frequency differences i n steps o f about 30 THz and allow one t o obtain a comb o f 7 o r 8 known frequencies covering the e n t i r e v i s i b l e spectrum.

Conclusion. - Secondary frequency standards extending from the microwave t o the v i s i b l e are becoming available; hence w i t h improvements i n the frequency measure- ment chains, the accuracies o f spectral measurements o f electromagnetic r a d i a t i o n w i l l soon be l i m i t e d by t h e s t a b i l i t i e s o f the sources themselves. It i s also f a i r l y c e r t a i n t h a t the meter w i l l be defined i n terms o f the second w i t h the value o f c fixed. Thus, the wavelength standard w i l l not be the l i m i t a t i o n i n the measurement o f length as i s now the case w i t h the krypton primary standard.

I n conclusion, we see t h a t i n 20 years, t h e l a s e r has caused a r e a l revolu- t i o n i n frequency and wavelength metrology as w e l l as i n the f i e l d o f fundamental constants; i.e., one o f the fundamental contants (the meter) may soon be defined i n terms o f another (the second).

References.

1. Baird, K. M., Evenson, K. M., Hanes, G. R., Jennings, D. A., and Petersen, F. R., Opt. Lett. 4 (1979) 263.

2. Comite Consultatif Pour La D e f i n i t i o n du Metre, 6 t h Session, Sevres, France (1979).

3. Wineland, D. J., J. Appl.'Phys. 2 (1979) 2528.

4. Hal 1, J. L. , Proc. 2nd I n t e r n a t i o n a l Conference on Precision Measurement and Fundamental Constants (1981) (Abstract).

5. Freed, C. and Javan, A., Appl. Phys. L e t t . 11 (1970) 53.

6. Freed, C., Bradley, L. C., and OIDonnell, R. G., IEEE J. Quantum Electron.

9E-16 (1980) 1195.

7. Evenson, K. M., Wells, J. S., Petersen, F. R., Danielson, B. L., and Day, G.

W., Appl. Phys. L e t t . 22 (1973) 192.

Blaney, T. G., Edwards, G. J., J o l l i f f e , B. W., Knight, D. J. E., and Wood, P. T., J. Phys. D 2 (1976) 1323.

Domnin, Yu. S., Koshelyaevskii, N. B., Tatarenkov, V. M., and Shumyatskii, P.

S., JETP L e t t . 30 (1980) 253.

Clairon, A, VanLerberghe, A., Salornon, C., Ouhayoun, M. and Bord6, C. J, Opt. Commun. 35 (1980) 368

Whitford, 6 . G., IEEE Trans. Instrum. Meas. IM-29 (1980) 168.

Clairon, I$., Dahmani, B., and Rutman, J., IEEE Trans. Instrum. Meas. IM-29

(1980) 268.

Whitford, B. G. and Smith, D. S. , Opt. Commun. 2 (1977) 280.

Knight, D. J. E., Edwards, G. J., Pearce, P. R., and Cross, N. R., IEEE Trans. Instrum. Meas. IM-29 (1980) 257.

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