Iodine stabilised He-Ne Laser

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Pramgna, Vol. 12, No. 5, May 1979, pp. 465--474, cg) printed in India

Iodine stabilised H e - N e Laser

V D D A N D W A T E , P N P U N T A M B E K A R a n d D S E N National Physical Laboratory, New Delhi 110 012

MS received 13 November 1978; revised 30 January 1979

Abstract. A single mode He-Ne laser operating at 6328 A is used with an iodine cell in the cavity to detect the absorption components of iodine falling within the gain curve of the laser line. Experimental details are given for locking the frequency of the laser line with one of the hyperfine components of the iodine absorption line, using a servo-control system. The system uses the technique of detecting the first and third harmonics of the modulation frequency.

Keywords. Frequency stabilisation; wavelength standard; He-Ne laser.

1. Introduction

A m a j o r i m p r o v e m e n t in the technique o f stabilisation o f the f r e q u e n c y o f H e - N e lasers o p e r a t i n g at 633 nm, t o o k place after the e x p e r i m e n t o f I-Ianes a n d D a h l s t r o m (1969). T h e y r e p o r t e d the coincidence o f hyperfine c o m p o n e n t s o f R(127) line o f 11-5

+ X I Z + electronic transition o f iodine w i t h that o f 633 n m n e o n b a n d o f the B3II0u <-- 0g

line o f the H e - N e laser. U n d e r the condition o f s a t u r a t e d a b s o r p t i o n , they o b s e r v e d 14 c o m p o n e n t s o f the iodine line, within the tuning r a n g e o f the single f r e q u e n c y H e - N e laser. D u e to v e r y narrow width o f a b o u t 4.5 M H z , they suggested using these c o m p o n e n t s as fixed references on which the laser f r e q u e n c y m i g h t be stabilised.

Later, a n u m b e r o f laboratories (Chartier e t a l 1976) a d o p t e d this scheme a n d developed iodine stabilised H e - N e lasers as future international length or f r e q u e n c y standard. Recently, s o m e o f these laboratories h a v e i n t e r c o m p a r e d the frequencies o f these lasers to e s t i m a t e their reproducibility a n d stability. It is found t h a t the reproducibility between t w o similar lasers operating u n d e r similar conditions o f iodine t e m p e r a t u r e a n d m o d u l a t i o n amplitude, is o f the o r d e r o f 2 × 10 -11 a n d stability o f the o r d e r o f 5 × 10 -12 for 10 see integration time.

We r e p o r t in this p a p e r the w o r k on development o f the iodine stabilised H e - N e laser carried out in o u r l a b o r a t o r y . T h e m a i n e m p h a s i s is o n the e x p e r i m e n t a l aspect o f locking the laser frequency to one o f the a b s o r p t i o n features o f iodine with the technique o f d e t e c t i o n o f the first and third h a r m o n i c s o f the m o d u l a t i o n frequency.

2. Principle

T o build a f r e q u e n c y stabilised laser, one has to o p e r a t e the laser in a single f r e q u e n c y m o d e , w i t h a n iodine a b s o r p t i o n cell in the laser cavity. As the frequency o f the laser 465

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466 V D Dandwate, P N Puntambekar and D Se:t

is varied over the Doppler broadened gain curve o f the laser line, saturated absorption peaks are observed in the power output o f the laser. These peaks stand about 0.1 ~ higher than the background and can conveniently be detected by phase-sensitive detection technique. The laser frequency can be servo-locked to any one o f these absorption components taking it as a reference. T o do this, the deviations o f the laser frequency from the reference frequency caused by any disturbance, are detected as an error signal and applied back through the servo-control system to retune the laser frequency to the reference.

The phase-sensitive detector output voltage as a function o f laser frequency scanning, is the first derivative o f the power output curve and shows the broad feature o f power output with a superimposed structure due to iodine absorption component.

However, it is difficult to identify the centre o f each component and with conventional zero-seeking servo-control system, arbitrary off-sets o f zero are necessary for stabilisation to a particular component. O f course, this zero off-set becomes redundant when an absorption component fails conveniently at the peak o f the power output curve and provides the absorption feature at the zero crossing o f the main curve.

However, the problem of identifying the centres o f other absorption components may be overcome, using a technique which basically differentiates the power output curve successively thrice (Wallard 1972). This ensures an asymmetric discriminant shape and also eliminates the large background slope. The differentiation may be performed by detecting the third harmonic content o f the fundamental modulation frequency in the power output o f the laser. The third harmonic component represents the rate o f change curvature o f the iodine feature. Using one o f the components as a reference and the deviation of the laser frequency from it as the error signal, allows the servo-control to lock the laser frequency at a point corresponding to the maximum rate o f change o f iodine absorption curve.

This enables location o f the centre o f the absorption curve for locking the laser frequency to it.

3. Theory

According to theory, the minimum fractional spectral width o f the output of a single mode laser is extremely small. For a low power gas laser in the visible region o f the spectrum a typical value is __. 10 -15.

This extremely sharp frequency can be varied over the entire band-width o f the Doppler broadened gain curve o f the laser transition. This gain curve is denoted by ACv, Vo) and is well approximated by the Gaussian profile, given by,

A (v, %) ---= A 0 exp - - [(V/Vo)/AVD]2

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where v 0 is the laser emission frequency at the centre o f the line profile and /kv D is the full width at height Ao/e.

In the presence o f an absorber, say iodine vapour in the cavity, this gain curve profile is slightly modified due to the appearance o f saturated absorption peak at a frequency up. The shape of this may be adequately represented by a Lorentzian function Bp (v, up) such as,

B,, 0', ,',,) = ,~ [ ~ + (,,-,',,)~]-~ (2)

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Iodine stabilised He-Ne laser

467 where ~ is the half-width at half-maximum of the peak. Thus, when the absorber saturates only weakly, the output of the laser power curve is quite well represented by the function I (v, re, up) defined as (Cerez and Brillet 1977),

i (v. v0. v,) = A

(~, v.)

[1 + ~ B,

(~, %)]

(3) where k is the relative size of the absorption peak.

As the power curve represented by (3) is formed by a slowly varying background curve and the sharp absorption feature, the resulting intensity changes, when the laser frequency is modulated by a low frequency sinusoidal signal (o~), are to be analysed as two different cases (Hanes

et al

1973).

Case

1 : Let the slowly varying background power curve be denoted by F (v). Then, with the modulation signal applied, the laser intensity is given by (Wallard 1972)

I = F (v -}- a sin oJt) (4)

where a is the amplitude of modulation.

Expanding the expression as Taylor's series, and considering only the important term from the sum of the coefficients of sin

mot

we get the coefficient of sin

mot

as,

a" F" (v) 2 "-x n !

where

F"(v)

is the nth derivative of

F(v).

This term approximately gives the amplitude of the phase-sensitive detector signal when the detection is canied out at nth harmonic of the modulation signal. As

F(v)

is given by

A o { e x p - - ( v - - v ° l 2,--~vnl ~"

we have the coefficient of sin

noJt

proportional to

(a! AvD)", a

being much smaller than

Av n

the higher order terms fall off very rapidly.

Case

2: In the neighbourhood of iodine absorption feature, the power curve is modified by the presence of inverted Lamb dip, which for not too large saturation may be represented by a Lorentzian profile given by (2). The harmonic content in this case has been analysed by Arndt (1965), and is applied to the case o f iodine absorption component by Hanes

et al

(1973). They have shown that if the modulation amplitude bandwidth is of the order of Lorentzian semi-half-width w, the signal due to background falls off relatively to that from the narrow iodine component as

(w/A~v)".

As this ratio is of the order of 100 for most of the iodine features falling within the gain curve of the 6328 .K He-Ne laser, detection at third harmonic, instead of first, gives a large reduction in the background signal.

4. Experimental details

The resonator structure is made from invar bars of length 450 mm and diameter 25 m m with mirror mounts on either side. The mirror mounts are provided with

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468 V D Dandwate, P N Puntambekar and D Sen

very fine adjustment screws to align the mirrors. The laser tube and the iodine cell are m o u n t e d on two independent adjustable holders which in turn are saddled on the two invar bars.

T h e concave mirrors used have radii o f curvature o f 1000 m m and transmission o f 99.5 ~o a n d 99.8 ~ . One mirror is m o u n t e d on a pair o f P Z T tubes: one o f which is used f o r m o d u l a t i o n and the other one to scan and servo-control the laser cavity length.

The Brewster angle laser tube has an active length o f 170 m m and the iodine cell length is 100 m m . The tube is filled with H e - N e mixture o f 9:1 ratio at a pressure o f 3 m m o f Hg. A high voltage stabilised power supply which is modified to work as a constant current source (Herngvist 1969; G o l d s b a r o u g h 1972) is used to run the laser. T h e ripple content o f the power supply is less than 30 mV.

T h e laser tube shows the presence o f regular and irregular oscillations in the power o u t p u t at certain values o f current (Wallard and W o o d s 1974). Hence, suitable current ranges are chosen such that the laser noise is minimum. A F a b r y - P e r o t inter- f e r o m e t e r with 80 m m mirror spacing is used to check the single m o d e operation o f the laser.

T h e phase lock amplifier for the first and third h a r m o n i c detection o f the modulation frequency a n d other electronic units for servo-control system are made in the l a b o r a t o r y based on the circuits given by S h o t t o n and Rowley (1975) and Wallard and Wilson (1974).

Figure 1 shows the block diagram o f the experimental set-up. A m o d u l a t i o n signal o f 775 H z ( f ) f r o m an oscillator is applied to P Z T No. 1. A saw-tooth scanning voltage f r o m a cathode ray oscilloscope (CRO) or a function generator is also applied to P Z T N o . 2 t h r o u g h the high tension ( H T ) amplifier.

T h e changes in intensity due to the scanning are detected by the photo-multiplier tube ( P M T ) and are displayed on the CRO beam No. 2 or on a chart recorder channel

PhOto m~tlpll|

tube

[

Invar rod spacer .~ _ P I E Z O

~odin

N~_]He- Ne

gain tube /

: Ref.

3 f

3f

t Channel ~

Chaon , No.Z f Figure 1. Block diagram of the experimental set-up.

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lodine stabilised He-Ne laser 469 N o . 2. T h e same P M T o u t p u t is fed to the phase-sensitive d e t e c t o r (psd) t h r o u g h n a r r o w b a n d filters f o r frequencies f o r 3 f . A reference signal o f frequency f o r 3 f f r o m the oscillator is also p r o v i d e d to the psd. The resulting psd o u t p u t a f t e r passing t h r o u g h integrator is displayed on the b e a m N o . 1 o f the C R O or on the channel N o . 1 o f the chart recorder. Thus, we get the p o w e r o u t p u t curve a n d also its first derivative or third derivative display.

As explained earlier, the laser frequency can be servo-locked to those frequencies where the first or third derivatives cross the zero. Usually, during the scan, the first derivative crosses the zero only once, while the third deriw.tive crosses it at each p e a k o f the a b s o r p t i o n c o m p o n e n t s . T h e servo-control is switched o n b y the switch S which changes the H T amplifier input f r o m function generator to the i n t e g r a t o r o u t p u t at t h a t m o m e n t during the scanning when the derivative o f a p a r t i c u l a r c o m p o n e n t

crosses zero.

5. Results and discussion

T h e u p p e r trace o f figure 2 shows the power output o f the laser as a function o f cavity tuning (intensity decreasing downward). The lower trace is the p s d - i n t e g r a t o r o u t p u t a n d is the first derivative o f the u p p e r curve. It can be seen t h a t the seven s a t u r a t e d a b s o r p t i o n p e a k s o f iodine j, i, h and g, f, e, d which are s u b m e r g e d in noise in the p o w e r o u t p u t curve are revealed in this curve. T h e s a m e seven a b s o r p t i o n c o m p o n e n t s recorded on a chart recorder are shown in figure 3.

By reducing the scanning amplitude only a small p a r t o f the gain curve can be scanned to get better details. Figure 4 shows the group o f f o u r a b s o r p t i o n c o m p o - nents g, f, e, d which fall conveniently near the centre o f the single frequency tuning

Figure 2. (Upper trace) Power output of the laser as a function of cavity tuning.

(intensity increasing downward) (Lower trace) psd integrator output as a function of cavity tuning. (first derivative of the upper curve) Frequency increases from left to right. The three features at the top of the curve are the j, i, h, components and the lower four are g, f, e, d. The zero crossing is in the middle of the trace.

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470 V D Dandwate, P N Puntambekar and D Sen 1 0 - -

8 ^~

i-

5 ~

F i g u r e 3.

1 I

I0 8 O _

S a m e as figure 2 b u t o n a c h a r t - r e c o r d e r s h o w i n g a series o f these curves.

range o f the laser. The laser frequency can be stabilised with reference to one of these components. The same figure also gives the corresponding power o u t p u t curve, with power increasing downward. (It m a y be noted that there is some lateral shift in the two traces for a simultaneous event.)

Figure 5 shows the power output and its first derivative f o r two scans o f the laser cavity and also when the servo-lock is closed at the f c o m p o n e n t during the t h i r d scan. The d u r a t i o n o f one scan is about 20 sec, and the duration o f the power o u t p u t record in locked condition is for I0 min.

N o special precautions could be taken for the t e m p e r a t u r e control o f the laser cavity and the I z cell, and its mechanical and acoustical isolation. The laser was m o u n t e d on a f o a m r u b b e r cushioned table-top and the experiment was carried out in an iso- lated r o o m . T h e slow decrease in power o u t p u t o f the laser as seen in the power output trace is due to the slow increase in iodine a b s o r p t i o n with the rise in temperature.

T h e third h a r m o n i c detection technique was also used considering its advantages and figure 6 shows the result. The upper curve in figure 6 is the p o w e r o u t p u t curve

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4 7 1

_

7 ~

6 -

5 - Iodine stabilised He-Ne laser

1 - -

I I I

8 O - - 6 4

Figure 4. Curves showing the power output (intensity increasing downward) vs cavity tuning and its corresponding first derivative, showing only the four absorption features of iodine. The zero line for the power output curve is at the top of the figure and that for first derivative is at the centre.

(increasing d o w n w a r d ) and the lower one is its third derivative. T h e same traces recorded on a chart recorder are depicted in figure 7. T h e zero-crossing f o r all the seven c o m p o n e n t s can be clearly seen.

A s the cavity length in this case is a b o u t 450 m m , the adjacent laser m o d e s are spaced b y a b o u t 300 M H z , while the frequency separation between a a n d j c o m p o - nents is m o r e t h a n 300 M H z . Scanning o f a wider range to get the o t h e r a b s o r p t i o n c o m p o n e n t s (a, b a n d c) is possible only in our next set-up where the cavity length is reduced to 350 m m .

T h e noise in the third derivative curve is partly due to a small 3rd h a r m o n i c content in the m o d u l a t i o n signal ( f ) and also partly due to uncontrolled e n v i r o n m e n t a l conditions.

W o r k on the setting u p o f the two identical iodine stabilised lasers w i t h a cavity length o f 350 m m is n o w in progress. These lasers will be r u n u n d e r controlled e n v i r o n m e n t a l conditions and their stability a n d reproducibility will be d e t e r m i n e d

b y the usual beat frequency method.

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t I I I. I 1 10 8 O__ 6 4 2 0 Figure 5. A series of curves, showing the power output and its first derivative for a few scans of the cavity and when the laser frequency is locked, to one of the absorption feature.

:5 % 0 ¸

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Iodine stabilised He-Ne laser 4 7 3

Figure 6. The upper trace shows the laser power output as a function of cavity tuning (intensity increasing downward) and the corresponding third derivative showing the seven absorption components of iodine line.

1

9 - -

8 - -

7 - -

4 - -

Figure 7. Same as figure 6 but recorded on a chart recorder.

absorption features is reverse to that in figure 6. The order of the

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474 V D Dandwate, P N P u n t a m b e k a r and D Sen Acknowledgements

T h e a u t h o r s w i s h t o t h a n k t h e i r c o l l e a g u e s , M e s s r s V T C h i t n i s , V G K u l k a r n i , B K R o y , R a m N a r a i n a n d A K K a n j i l a l f o r h e l p d u r i n g t h i s w o r k .

References

Arndt R 1965 J. Appl. Phys. 36 2522 Cerez P and Brillet A 1977 Metrologia 13 29

Chartier J M, Helmake J and Wallard A J 1976 1EEE Trans. lnstrum. Meas. 25 450

Goldsbarough J P 1972 Laser Handbook eds F T Arecchi and E O Schulz-Doubois (Amsterdam:

North Holland Pub. Co.) 1 612

Hanes G R, Baird K M and De Remigis 1973 AppL Opt. 12 1600 Hanes G R and Dahlstrom C E 1969 Appl. Phys. Lett. 14 362 Herngvist K G 1969 RCA Rev. 30 429

Shotton K C and Rowley W R C 1975 NPL Teddington Report QU 28 Wallard A J 1972 J. Phys. E5 926

Wallard A J and Wilson D C 1974 J. Phys. E7 161 Wallard A J and Woods P T 1974 J. Phys. E7 209