Premature gain switch initiation by amplifier interaction in a TEA CO2 oscillator-amplifier laser system

Volume

12, number

3

OPTICS COMMUNICATIONS

PREMATURE

GAIN SWITCH INITIATION

November

BY AMPLIFIER

IN A TEA CO, OSCILLATOR-AMPLIFIER

1974

INTERACTION

LASER

SYSTEM

A.D. CRAIG and R.M. PERKIN* Euratom-UKAEA

Association for Fusion Research, Culham Laboratory,

Received

9 September

Abingdon,

Oxon, OX14 3DB, UK

1974

It is found that a premature initiation of the gain switch in a TEA CO, laser oscillator teraction from an adjacent amplifier with the consequence that the peak oscillator power three.

We describe results which demonstrate a premature initiation of the gain switch in a TEA CO, laser oscillator by optical interaction from an adjacent TEA CO, amplifier with the consequence that the peak oscillator power is reduced by a factor of three. In several previous reports the possibility of such interaction has not been considered, e.g. [ 11. A simple four-level model of the dynamics of the oscillator and amplifier predicts a much smaller degree of interaction. The experimental arrangement is shown schematically in fig. 1. Oscillator and amplifier discharges were identical and involved Rogowski profiled electrodes (70 cm active length, 2.5 cm spacing) of the type described by Lamberton and Pearson [2]. Two tungsten trigger wires running parallel and adjacent to the anode and connected to earth by 250 pF trigger capacitors provided preionization for the main discharge which was excited by a 0.1 I.IF capacitor charged to typically 45 kV and switched by a low inductance spark gap. The current rise time was 3 0 nsec with a pulse width, Y fwhm, of 400 nsec. A continuously flowing gas mixture, CO, 2.5%, N2 20%, He 55%, at one atmosphere pressure was passed through the discharge regions. The 1 m optical resonator was formed by a gold coated beryllium copper mirror (reflectivity 99%, radius of curvature 10 m) and a flat germanium output mirror (reflectivity 36%). A 12 mm diameter aperture

* Royal

256

Holloway

College,

Egham,

Surrey,

U.K.

is produced by optical inis reduced by a factor of

Gc photon drag detector PDz

-1

TEA CO2 Oscillator

(h Gold mirror (R = 99 ‘,.)

,’

I

-I :: Beam splwzr

spllttor (5%)

(5%)

Gk’mirror (R = 36%)

Fig. 1. Schematic

-1

0

of apparatus.

1 Oscillator -amplifier

tining

2 1, (ps)

Fig. 2. Amplified (PD? X 20) and unamplified (PD, X 20) power levels and oscillator pulse delay time plotted as function of oscillator amplifier timing t,.

Volume

12, number

3

OPTICSCOMMUNICATIONS

adjacent to the output mirror limited the transverse mode structure of the oscillator output. AR coated germanium windows were used in the amplifier. The power levels following the oscillator and amplifier were measured by sampling 5% of the beam and focusing on to photon drag detectors. In fig. 2 the peak amplified (PD, X 20) and unamplified (PD, X 20) power levels are plotted as a function of the relative timing, f,, between the start of the oscillator and amplifier discharge currents (t, is taken to be positive when the amplifier discharge starts earlier). Also plotted is the delay time of the peak of the laser pulse after the start of the oscillator current. The rise time of the laser pulse (- 30 nsec) is independent of the pulse delay time. For t, less than - 100 nsec the operation of the oscillator is unaffected by the presence of the amplifier. However, for rr > 100 nsec the amplifier interacts back on the oscillator such that the peak oscillator power drops by a factor of up to three and the pulse delay is reduced by up to 50 nsec. We note therefore that the maximum useful gain of the amplifier, i.e. the maximum amplified output divided by the oscillator output (in the absence of any amplifier interaction) (useful G,, = 2 at t, = 100 nsec) is lower than the real maximum gain x 6 at fr = 1 psec). It does not follow, (real G,, however, that if the interaction were not present, a gain of 6 would still be achieved at t, = 1 psec because of the possibility of saturation effects. We have examined the effect of varying the amount of optical interaction between amplifier and oscillator by inserting apertures of different diameters at the beam centre (t, futed at 500 nsec). Only aperture diameters of less than I mm measureably reduced the interaction. The degree of interaction is not dependent on the position of the aperture within the beam. Spontaneous emission in the amplifier will be amplified in a single pass along the amplifier discharge and will enter the oscillator cavity. We consider two ways in which this amplifier radiation might effect the oscillator dynamics. (i) The radiation will to some extent deplete the inversion in the oscillator. The effect of such a depletion would be to reduce the power of the laser pulse but also to increase the delay of the pulse relative to the start of the oscillator current. (ii) A small proportion of the amplifier radiation, directed close to the optical axis of the oscillator, will combine with spontaneously emitted radiation from

November

1974

the oscillator to initiate the gain switch. The effect of such amplifier radiation would be to reduce :he delay of the gain switch and hence decrease the puise power because of the lower inversion present at the earlier gain switch. We believe this to be the explanation of the observed result and now discuss it quantitatively using a simple four level theoretical model described by Gilbert et al. [3] which has successfully predicted the essential features of the operation of a TEA CO, oscillator. Well established transfer rate coefficients are used in simplified equations describing the oscillator population densities in the upper laser level (n,u), the lower laser level (nbu) and the v = 1 vibrational level of N, (neu). These are solved numerically together with the equation describing the photon density in the cavity, i.e.

where Z is the radiation density, u, the effective radiation cross-section, To the cavity decay time. The last term is the spontaneous emission term with ys the inverse radiative lifetime of the laser transition and F (- h$& (2~Irr~T~Av,)-~, wheref, is the Boltzmanr factor of the emission line, A the beam area, Au, the collisional half width) the fraction of photons being emitted into the small angular aperture of the mode within the spectral width of a single axial mode. We have extended this model to include the population levels in an amplifier discharge in order to include, in the above oscillator photon density equation, an additional term representing the amplifier spontaneous emission entering the oscillator mode within the appropriate angular and spectral aperture. This term is given by

(1 - Rk,y,F &LO

LexpcaLAl - ’ 1,

where R is the reflectivity of the oscillator output mirror, LA is the amplifier discharge length, Lo is the oscillator cavity length, 01(= oe(fl,A - nbA), where n,A and ?$,A are amplifier population densities) is the small signal gain coefficient of the amplifier. This can greatly exceed the oscillator spontaneous emission term in the early stages of the oscillator discharge for positive values oft, due to higher n, and a significant gain during the single pass along the amplifier discharge 257

Volume

12. number

(a)

(b)

3

OPTICS COMMUNICATIONS

*I $E

6-

% x c

4-

Pulsedelay Peak power (ns) 640tlO

(MW

600

-0.8

-04

I 0

04

0.8

1.2 1, (lid

14

1.6

Fig. 3. (a) Typical theoretical dependence of number densities in upper and lower level (pumping time = 0.5 ps, t, = 0.4 ps). (b) Theoretical variation of peak pulse power (dashed curves) and pulse delay (solid curves) versus t, for several values of amplifier spontaneous emission.

entering the oscillator. Fig. 3a shows typical theoretical time dependence of population densities in the oscillator and amplifier with t, = 400 nsec. The gain switch occurring at t = 1.01 ~.ts,i.e. 0.61 ps after the start of the oscillator current, is seen as the sharp decrease in inversion. A pumping period of 0.5 MS,i.e. f the current pulse, is approximately the duration assumed during which the p I!pulation densities rise exponentially, (n 0~[exp (fit) - l] ) to values based on the figures of Gilbert et al. 133 and the relative gas ratios of the two laser systems. We have examined the effect of varying t, on the oscillator performance. Fig. 3b shows theoretical predictions of the peak power of the pulse (= 0.5 chvZ( 1 -R)) and pulse delay time plotted as a function of I,. The pumping levels and /3 in the pumping function are suitably chosen such that in the absence of any amplifier interaction, i.e.

before

258

November

1974

t, < - 0.63 ps, theory and experiment are in good agreement with regard to pulse amplitude and pulse delay. Curve (i), in which the amplifier spontaneous emission is as given by the above equation, shows an interaction that is considerably smaller than the experimental result. We also show curves, (ii), (iii) and (iv), where this spontaneous emission term is increased by factors 10, lo2 and lo3 respectively. The theoretically predicted interaction for much larger amplifier systems is therefore also relatively small. To obtain the experimentally observed reduction in pulse delay time of 50 nsec requires an increase of - lo3 [curve (iv)] . The reduction in pulse amplitude by a factor of 3 is, however, not obtained. Theory predicts an amplitude proportional to the inversion (naO - ?$,O) at the time of the onset of the pulse and thus the reduction in amplitude is proportional to the time differential of (fTaO- nbO) at the time of the pulse. To increase this time differential to obtain a result consistent with the experiment, i.e. (naO - nbO) changing by a factor of 3 in SO nsec, would require an unrealistically high value of the excited nitrogen density. We should also recall the result that, experimentally, when the amount of amplifier spontaneous emission reaching the oscillator was reduced by a factor of 100 by aperturing down, the effect was negligible. We must conclude that some physical process not included in our simplified four-level theoretical model is, in practice, of vital importance. We consider two possible approaches to eliminate this interaction, and hence realise the full potential gain available from the amplifier. (i) A suitable saturable absorber placed between the oscillator and amplifier will in principle absorb the lower level spontaneous emission from the amplifier, but will saturate and hence not significantly absorb the laser pulse from the oscillator. However, in some preliminary tests with a series of absorbing gases, SF,, CC12F2, C,H,, when the gas pressure was sufficiently high to absorb the amplifier spontaneous emission the absorbtion of the main pulse was found to be unacceptably high. (ii) An electro-optic shutter placed between the oscillator and amplifier may be opened at the moment of the onset of the laser pulse [4]. We wish to thank Dr. A.C. Seldon, Dr. J.W.M. Paul and Dr. R.J. Bickerton for some valuable discussions and Mr. G.A. Baxter for his technical assistance.

Volume

12, number

3

OPTICS COMMUNICATIONS

References [l] A. Girard, H. Pepin, Opt. Commun. 8 (1973) 68. [2] H.M. Lamberton, P.R. Pearson, Electronics Letters 141.

7 (1971)

November

1974

[3] J. Gilbert, J.L. Lachambre, F. Rheault, R. Fortin, Can. J. Phys. 50 (1972) 2523. [4] L.6. Champagne, F. O’Neill, W.T. Whitney, Opt. Commun. 11 (1974) 11.

259