PHASE-COHERENT FREQUENCY SYNTHESIS SUITABLE FOR DIRECT FREQUENCY MEASUREMENTS UP TO 30 THz

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PHASE-COHERENT FREQUENCY SYNTHESIS SUITABLE FOR DIRECT FREQUENCY

MEASUREMENTS UP TO 30 THz

E. Bava, A. Godone, A. de Marchi

To cite this version:

E. Bava, A. Godone, A. de Marchi. PHASE-COHERENT FREQUENCY SYNTHESIS SUITABLE FOR DIRECT FREQUENCY MEASUREMENTS UP TO 30 THz. Journal de Physique Colloques, 1981, 42 (C8), pp.C8-463-C8-472. �10.1051/jphyscol:1981852�. �jpa-00221748�

JOURNAL DE PHYSIQUE

Colloque C8, suppZ6ment au n012, Tome 42, ddcembre 1981 page C8-463

PHASE-COHERENT FREQUENCY SYNTHESIS S U I T A B L E FOR DIRECT FREQUENCY MEASUREMENTS UP TO 3 0 THz

E. Bava, A. Godone and A. De Marchi

I s t i t u t o Elettrotecnieo NazionaZe "GaliZeo Ferrarisf', ToTorino, I t a l y

Abstract.- Great advances have been made in the recent years in the field of phase-coherent frequency synthesis into the infrared region of the electromagnetic spectrum.

Most important are the improvements achieved in the phase noise of the reference oscillators, in the modulation and control capa- bilities of important intermediate oscillators such as optically pumped FIR lasers, and in general in the understanding of the problems to be dealt with, both of noise level and of multiplica- tion efficiency.

In this paper a short review is given of the progress occurred in the field in the last years, and a projection is attempted for the ultimate possibilities of the techniques presented. The im- portance of reaching the 30 THz region with a coherent signal is discussed in view of a coherent frequency scale from VHF to the infrared.

Introduction.- Research in the field of infrared and optical frequency

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synthesis has shown great progress in the last five years. Both from the point of view of the understanding of the various theoretical and technical problems involved, and from the point of view of the realizg tions reported by different laboratories, the advances have been im- pressive in all the relevant aspects of the field. In fact theoretical studies and experimental results have been tightly interconnected.

Better insight has been obtained into the basic problem of spectral purity of the synthesized signal, for which the role of phase noise in the multipliers and in the seference oscillator [1,2,3] and the role of distortion efficiency of non-linear devices has been analyzed 141.

At the same time oscillators of unprecedented spectral purity have been studied, realized and made available commercially; moreover an extremely attractive solution has been proposed for a high spectral purity quartz oscillator making use of electrodeless crystal which can stand very high rf power [5]. Multipliers have also been studied and improved from the noise point of view, and synthesis chains capable of maintaining the phase coherence of the signal up to 30 THz are now available [3 ,6 3.

Stable intermediate oscillators, suitable for amplification purposes where needed along the chain,have also been developed and of particular importance are in this sense the studies, realizations, stability mea- surements [ 7 , 8 ] and frequency modulation [9,10,11] achievements in the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981852

C8-464 JOURNAL DE PHYSIQUE

f i e l d of o p t i c a l l y pumped FIR l a s e r s . I n a d d i t i o n t o t h e s e , e l e c t r o n i c t u b e s have been manufactured which o s c i l l a t e a t f r e q u e n c i e s of s e v e r a l hundred GHz (BWO), and can be frequency swept and s t a b i l i z e d .

With t h e s e r e a l i z a t i o n s , and with t h e a v a i l a b i l i t y of h i g h l y s t a b i l i z e d l a s e r s conveniently spaced i n frequency, i t appears t h a t frequency

s y n t h e s i s and phase coherent connection up t o t h e i n f r a r e d should now have passed t h e o b s t a c l e of b a s i c d i f f i c u l t i e s .

1. Phase noise t h e o r y

The v a r i o u s problems connected with t h e i n c r e a s e of phase n o i s e a l o n g a m u l t i p l i e r c h a i n have been r e c e n t l y analysed t h e o r e t i c a l l y and experimentally t o a g r e a t e x t e n t . P a r t i c u l a r l y important f o r t h e s p e c t r a l p u r i t y of t h e s y n t h e s i z e d s i g n a l a t f r e q u e n c i e s i n t h e THz r e g i o n i s t h e phase n o i s e modulating t h e d r i v i n g o s c i l l a t o r and/or i n t r o d u c e d by t h e f i r s t s t a g e s of t h e m u l t i p l i e r . The shape of t h e s y n t h e s i z e d s i g n a l spectrum i s g e n e r a l l y a c a r r i e r emerging over a modulating phase n o i s e p e d e s t a l , and, i f S w , e f f ( y o ) i s t h e e f f e c t i v e phase n o i s e power spectrum i n the white s e c t i o n of t h e p e d e s t a l , r e - f e r r e d t o t h e i n p u t frequency yo of t h e m u l t i p l i e r , t h e signal-to-phase- n o i s e r a t i o S / N ~ i n a bandwidth Bw a t t h e o u t p u t of t h e c h a i n i s given by C31

s

-

[

- -

4

Bw~g7

%)

where n i s t h e m u l t i p l i c a t i o n f a c t o r ,

Q!

0

power i n t h e p e d e s t a l a t t h e i n p u t frequency and ^0(. i s t h e r o l l - o f f

s l o p e of t h e phase n o i s e modulating process a t h i g h F o u r i e r f r e q u e n c i e s . It can be s e e n from ( 1 ) t h a t S/N+ d e c r e a s e s i n i t i a l l y w i t h n-2 t o c o l l a p s e t h e n r a p i d l y a t t h e c a r r i e r c o l l a p s e frequency ycOll when nS@o-'/2. Also e s s e n t i a l i s t h a t a t all f r e q u e n c i e s a l o n g t h e c h a i n t h e s i g n a l power P be high enough f o r t h e thermal n o i s e t o be much lower t h a n t h e phase n o i s e so t h a t t h e s p e c t r a l p u r i t y be preserved. This c o n d i t i o n i s s a t i s f i e d i f :

where kTe i s t h e n o i s e power v a i l a b l e from t h e c i r c u i t s i n a bandwidth of 1 Hz.

2. S p e c t r a l p u r i t y r e s u l t s

Phase-noise power s p e c t r a l d e n s i t y curves e x p e r i m e n t a l l y obtained f o r two t y p i c a l h i g h s p e c t r a l p u r i t y designs of q u a r t z c r y s t a l con- t r o l l e d o s c i l l a t o r s a r e shown i n f i g . 1. The two a r e i n a s e n s e a t t h e two ends of t h e spectrum of p o s s i b l e r e s o n a t o r s o l u t i o n s . I n f a c t curve 1 corresponds t o a 5 MHz AT c u t plan0 -convex c r y s t a l s e l e c t e d f o r h i g h

Fig. 1 : Phase n o i s e power s p e c t r a l d e n s i t y curves f o r :

1 ) High s p e c t r a l p u r i t y 5 MHz q u a r t z c r y s t a l o s c i l l a t o r 1 ' ⁾ The same m u l t i p l i e d t o 120 MHz ( t h e e f f e c t of two-pole

q u a r t z f i l t e r i n g a t 120 MHz i s shown) 2) SAW o s c i l l a t o r a t 120 MHz

Q and e x c i t e d with h i g h r f power a t t h e expense of i t s a g i n g r a t e , while curve 2 corresponds t o a 120 MHz s u r f a c e a c u s t i c wave device [12].

Curve 1 ' i s r e l a t i v e t o a s i g n a l a t 120 MHz obtained from t h e same o s c i l l a t o r of curve 1 w i t h a n o i s e l e s s m u l t i p l i e r . The e f f e c t of q u a r t z f i l t e r i n g a t 120 MHz i s a l s o shown. R e l a t i v e m e r i t s of t h e two p r o f i l e s appear e v i d e n t . I n f a c t c a r r i e r l i n e w i d t h i n t h e THz r e g i o n can be ex- pected t o be much g r e a t e r f o r t h e SAW o s c i l l a t o r , while a lower c a r r i e r c o l l a p s e frequency can be obtained from t h e 5 MHz o s c i l l a t o r . Levels of phase n o i s e introduced by t h e m u l t i p l i e r t h a t a r e s u i t a b l e f o r h a n d l i n g t h e l a t t e r without s p e c t r a l p u r i t y degradation have been achieved [3]

and a measurement of S+ of t h e c h a i n a t 9.5 GHz i s g i v e n i n f i g . 2.

Extremely narrow, though h a r d l y measurable, c a r r i e r l i n e w i d t h s a r e obtained with t h i s c o n f i g u r a t i o n i n t h e THz r e g i o n and a c a r r i e r c o l l a p s e frequency of t h e o r d e r of 30 THz i s c a l c u l a t e d . This i s

evidenced i n f i g . 3 where t h e good agreement between t h e o r e t i c a l calcu- l a t i o n s and measurements of signal-to-phase-noise r a t i o ( i n 1 Hz band- width) i s a l s o shown.

JOURNAL DE PHYSIQUE

16' lo0 ^10' ld 10' to4 1 (Hz) I@

Fig. 2 : Power s p e c t r a l d e n s i t y of t h e phase n o i s e i n t r o d u c e d by t h e

--

Fig. 3 : S i g n a l t o white phase n o i s e r a t i o i n 1 H z bandwidth f o r s i g - n a l s a t d i f f e r e n t f r e q u e n c i e s s y n t h e s i z e d w i t h t h e m u l t i p l i e r of f i g . 2.

-

o Experimental p o i n t s : p o i n t s up t o 10 GHz have been ob- t a i n e d by comparison of two e q u a l s y n t h e s i z e d s i g n a l s ; i n t h e f a r i n f r a r e d o p t i c a l l y pumped l a s e r s have been used.

3. Non-linear devices

Recent technological refinement and theoretical understanding of non-linear devices usable in the far infra red range of frequencies, from tens of GHz to tens of THz, have made the state-of-the-art capable of handling in a satisfactory way also in that range all functions that are essential to signal processing in general and to frequency synthesis in particular: detection, mixing, harmonic generation.

Micron size GaAs-Pt Schottky barrier diodes are now well understood and available for use up to around 1 THz; where their intrinsic high frequency detection roll-off of fe4 [4] ^and the low level of signals obtainable by harmonic generation or mixing sharply limit their useful- ness. Fig. 4, a and b, and Fig. 5 illustrate this situation: it appears that mixing conversion loss and multiplication efficiency even for small multiplication numbers make signals hardly usable when 1 THz is ap- proached, although arrays of diodes should improve the situation, and better efficiency is expected from varactor type multipliers [14].

Metal-oxide-metal junctions can be used at higher frequencies up to 200 THz and provide low order harmonic mixing signals that are suitable for obtaining lock in control loops and for frequency measure- ment s.

Harmonic mixers that make use of Josephson junctions can also be used to obtain beat notes directly between low level signals in the tens of GHz region and intermediate oscillators in the THz region [15].

a >

Fig. 4 Conversion loss of a Schottky barrier mixer a) as a function of the harmonic number n

(PLO 0.4-W) and b) as a function of the local oscillator power

(loss relative to a)).

JQURNAL DE PHYSIQUE

Fig. 5 : Power radiated at different frequencies by a Schottky barrier multiplier driven by various oscillators:

100 G H z klystron o 145 G H z carcinotron

A 300 G H z carcinotron

In this way phase lock of these latter oscillators is possible directly from a microwave output multiplier chain, without the use of any micro- wave intermediate source.

4. Intermediate oscillators

Oscillators to be used in a phase-lock scheme for amplification purposes along the synthesis chain were lacking a decade ago in the region between 100 GHz and a few THz, with few exceptions. Thousands of laser emission frequencies have been demonstrated since, by the use of the optically pumped FIR laser scheme.

The f r e q u e n c y s t a b i l i t y of t h e s e d e v i c e s h a s been measured b o t h by comparing two of them and by b e a t i n g one of them w i t h s y n t h e s i z e d r e f e r e n c e s i g n a l s , and h a s been found t o be l a r g e l y independent of t h e s t r u c t u r e and l a s i n g l i n e used. T y p i c a l s t a b i l i t y c u r v e s a r e shown i n f i g . 6 C7,81

F i g . 6 : A l l a n v a r i a n c e of r e l a t i v e f r e q u e n c y f l u c t u a t i o n s of two CH30H

--

o = 428 GHz; t h e r e f e r e n c e i s a s y n t h e s i z e d s i g n a l

A = 4.25 T H z ; b e a t n o t e between two l a s e r s [ 7 ]

The e x p e r i m e n t a l r e s u l t s s u g g e s t t h e i n d i c a t e d i n t e r p o l a t i o n . F a s t f r e q u e n c y modulation of t h e s e s o u r c e s s u i t a b l e f o r phase- l o c k i n g p u r p o s e s h a s been o b t a i n e d i n d i f f e r e n t ways and a summary of

e x p e r i m e n t a l r e s u l t s i s g i v e n i n Table 1. A l l t h e methods proposed a p p e a r t o be c a p a b l e of m a i n t a i n i n g phase-lock of FIR l a s e r s w i t h r e s p e c t t o t h e s y n t h e s i z e d s i g n a l t o which we r e f e r i n f i g . 3 . I n f a c t t h e f r e q u e n c y d e v i a t i o n AY n e c e s s a r y f o r a p r o b a b i l i t y p of u n l o c k i n g d u r i n g t h e t i m e T i s [8]

where sW i s t h e w h i t e f r e q u e n c y n o i s e power ( p e r Hz) of t h e l a s e r and B i s t h 8 d e s i r e d phase-lock bandwidth.

P r o b l e m a t i c a p p e a r s i n s t e a d t o be t h e wide t u n i n g of? t h e s e s o u r c e s , which h a s been demonstrated o n l y f o r v e r y few l i n e s by t h e u s e of t h e S t a r k e f f e c t . Other s o u r c e s i n t h e hundreds of G E z r e g i o n a r e more e a s i l y tuned. They a r e backward wave t y p e e l e c t r o n i c t u b e s , making u s e of p r e c i s e l y machined d e l a y l i n e s , known by t h e name " C a r c i n o t r o n " .

C8-470 JOURNAL DE PHYSIQUE

Tab. 1 : C h a r a c t e r i s t i c s and measured p a r a m e t e r s f o r d i f f e r e n t f a s t

--

modulation t e c h n i q u e s of FIR l a s e r s .

-

Modulation t e c h n i q u e

S t a r k e f f e c t

Locking by i n j e c t i o n w i t h a s y n t h e s i z e d s l g n a l

S c h o t t k y d i o d e v a r a c t o r

Cold plasma

X

These v a l u e s of f r e q u e n c y d e v i a t i o n a r e p e c u l i a r of t h e equipment u s e d i n t h e e x p e r i m e n t s [ I l l . They can be i n c r e a s e d by t h e u s e of improved d e v i c e s .

T h e i r r e a l i z a t i o n however p o s e s some problems t o t h e r e s e a r c h l a b o r a t o - r y and t h e i r a v a i l a b i l i t y i s s c a r c e because t h e l i m i t e d ma.rket h a s n o t j u s t i f i e d t o t h i s p o i n t l a r g e s c a l e p r o d u c t i o n .

Frequency d e v i a t i o n 2 MHz

Q l i m i t e d 100 kHz

Power and Q l i m i t e d 15 kHzX Capacitance sviing &

c o u p l i n g l i m i t e d 6 k ~ z * E l e c t r o n d e n s i t y

l i m i t e d

5. Coherent s y n t h e s i s toward t h e v i s i b l e

E x t e n s i o n t o t h e i n f r a r e d of f r e q u e n c y s y n t h e s i s w i t h i n p u t - o u t p u t phase coherence s t a r t i n g from h i g h l y pure VHF s i g n a l s a p p e a r s now pos- s i b l e w i t h t h e use of a c l e a n and s t a b l e m u l t i p l i e r , such as t h e one r e f e r r e d t o i n t h e t e x t , and a c o n v e n i e n t l y s t a b i l i z e d F I R o s c i l l a t o r t o compensate f o r t h e h i g h m u l t i p l i c a t i o n l o s s of n o n - l i n e a r d e v i c e s i n t h a t range. The a v a i l a b i l i t y of a h i g h l y s t a b i l i z e d l a s e r a t a f r e - quency as low as 30 THz ( a C02 l a s e r l o c k e d t o the s a t u r a t e d a b s o r p t i o n p a t t e r n produced i n a C02 c e l l ) may a l l o w t h e e x t e n s i o n of c o h e r e n t s y n t h e s i s t o a t l e a s t 88 THz by t h e u s e of t h e scheme i n d i c a t e d i n f i g . 7. I n f a c t a non-col.lapsed s y n t h e s i z e d s i g n e l a t 30 THz can be u s e d a s a r e f e r e n c e f o r a slow s e r v o of t h e o f f s e t s t a b i l i z e d C 0 2 l a s e r , s o t h a t t h e l o n g t e r m s t a b i l i t y of t h e C s s t a n d a r d i s t r a n s f e r r e d t o a s i g n a l a t 30 THz which h a s t h e spectrum of t h e s t a b i l i z e d l a s e r . T h i s s i g n a l can t h e n be more e a s i l y u s e d t o s l o w l y s e r v o a HeNe l a s e r sta- b i l i z e d i n o f f s e t w i t h r e s p e c t t o t h e s a t u r a t e d a b s o r p t i o n p a t t e r n of CH4, which would r e a l i z e c o h e r e n t s y n t h e s i s t o 88 THz.

A q u e s t i o n which s t i l l n e e d s t o be answered i n t h i s p e r s p e c t i v e c o n c e r n s t h e medium and l o n g t e r m and t h e environmental s t a b i l i t y of t h e m u l t i p l i e r s u s e d i n t h i s s y n t h e s i s . These need most l i k e l y t o be

Bandwidth 2 XHz Q l i m i t e d

100 kHz

Power and Q l i m i t e d

2 MHz

€2 l i m i t e d

20 kHz

Plasma recombination time l i m i t e d

improved i f t h e l o n g term s t a b i l i t y of C s i s t o be t r a n s f e r r e d t o t h e i n f r a r e d r e g i o n . A d i f f e r e n t scheme, which i s commonly u s e d f o r f r e - quency measurements w i t h t h e t r a n s f e r - o s c i l l a t o r concept, can a l s o be a d a p t e d t o c o h e r e n t s y n t h e s i s by t h e u s e of cascaded slow phase-lock l o o p s as i l l u s t r a t e d above. The most s t a b l e o s c i l l a t o r i n t h e c h a i n

c a n t h e n be adopted a s t h e r e f e r e n c e t o which a l l t h e o t h e r s a r e s l o w l y l o c k e d .

S T A B I L I Z E D L A S E R S

C H A I N PLATE

FAST PHASE LOCK SLOW PHASE LOCK SLOW PHASE LOCK

Fig. 7 : Suggested scheme f o r e x t e n s i o n of c o h e r e n t s y n t h e s i s t o 88 THz.

The slow phase-lock c o n t r o l l i n g o f f - s e t s t a b i l i z e d l a s e r s a c t s as a f i l t e r improving t h e s p e c t r a l p u r i t y of s y n t h e s i z e d s i g - n a l s .

6. Conclusion

With t h e r e c e n t development i n t h e f i e l d s of s p e c t r a l l y pure o s c i l l a t o r s and f r e q u e n c y m u l t i p l i e r s , of F I R s o u r c e s and of non l i n e a r d e v i c e s

-

-

S y n t h e s i z e d s i g n a l s have been o b t a i n e d i n t h e THz r e g i o n which have t h e same s p e c t r a l p u r i t y of t h e X band s i g n a l s u s e d i n primary C s s t a n d a r d s , and t h e c a p a b i l i t y of s y n t h e s i z i n g non-collapsed s i g n a l s a t 30 THz h a s been demonstrated.

Questions remain t o be answered about t h e phase s t a b i l i t y of t h e m u l t i p l i e r s f o r t i m e s l o n g e r t h a n 1 s.

Very u s e f u l c a n a l s o be t h e t r a n s f e r o s c i l l a t o r t e c h n i q u e which i s now made usab1.e i n any f r e q u e n c y range by t h e a v a i l a b i l i t y of v e r y s t a b l e o s c i l l a t o r s , p a r t i c u l a r l y f o r t h e e x t e n s i o n of t h e s y n t h e s i s t o h i g h e r f r e q u e n c i e s and e s p e c i a l l y where o n l y f r e q u e n c y measurement i s

sought.

-

C8-472 JOURNAL DE PHYSIQUE

7. R e f e r e n c e s

[ I ] WALLS F.L., DE MARCH1 A., IEEE Trans. Instrum. Meas. IM-24 (1975) 210.

[2] BAVA E., BAVA G.P., DE MARCH1 A., GODONE A., IEEE Trans. Instrum.

Meas. IM-26 (1977) 33.

131 DE MARCH1 A , , GODONE A., BAVA E.

,

[4] GODONE A., BAVA E., The p-size S c h o t t k y d i o d e s a s d e t e c t o r s and harmonic g e n e r a t o r s i n t h e 100 GHz

-

.

[5] BESSON R . J . , SBminaire s u r l e s B t a l o n s de f r g q u e n c e , l e u r c a r a c t g - r i s a t i o n e t l e u r u t i l i s a t i o n , Besanqon 1981, M-1.

[6] GODONE A., DE MARCH1 A., BAVA E., Proc. 33rd Ann. Symp. on Frequen- cy C o n t r o l , A t l a n t i c C i t y (1979) 498.

[7] JIMENEZ J.J., PETERSEN F.R., I n f r a r e d P h y s i c s

11

[8] BAVA E., GODONE A., DE MARCH1 A., LESCHIUTTA S., Comparison of d i f - f e r e n t f a s t modulation t e c h n i q u e s f o r FIR l a s e r s t a b i l i z a t i o n . I n t . J. of I n f r a r e d and mm waves (1981) (To be p u b l i s h e d ) .

[g] STEIN S.R., RISLEY A.S., Van de STADT H., STRUPIIA F., Appl. O p t i c s 16 (1977) 1893.

-

[ l o ] BAVA E., GODONE A., DE MARCH1 A., IEEE Trans. Instrum. leas.IM-29 (1980) 273.

[l I ] GODONE A., BAVA E., DE MARCHI A., IEEE Trans. I n s t n u n . Meas.IM-29 (1980) 277.

[ I 21 PENAVAIRE L., SEGUIGNES D., LARDAT C., BONMIER J. J . , CHEVALIER J . Y., BESSON Y., IEEE U l t r a s o n i c s Symp. Boston (1980).

[13] PYEE M., P r i v a t e communication.

[14] BAVA E., BAVA G.P., GODONE A., RIETTO G., IEEE Trans. Microwave Theory Techn. MTT-29 (Nov. 1981 ) (To be p u b l i s h e d )

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