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Experimental investigation of an unstable ringresonator with 90-deg beam rotation for achemical oxygen iodine laser

Yuqi Jin, Bailing Yang, Fengting Sang, Dazheng Zhou, Liping Duo, and Qi Zhuang

We report the experimental results of an unstable ring resonator with 90-deg beam rotation for a kilowattclass chemical oxygen iodine laser ~COIL!. The distributions of near-field phase and far-field intensitywere measured. A beam quality of 1.6 was achieved when the COIL average output power was approx-imately 5 kW. © 1999 Optical Society of America

OCIS codes: 140.1550, 140.3300, 140.4780.

1. Introduction

The chemical oxygen iodine laser ~COIL! is an elec-ronic transfer laser first demonstrated by McDer-ott at the Air Force Weapons Laboratory in 1978.1

It is the shortest wavelength high-power chemicallaser to date. Recently, development of the COIL atTRW has been remarkable with an average outputpower of greater than several hundred kilowatts al-ready produced by use of a supersonic flow.2 Beamquality is also an important characteristic for a high-power laser. However, experimental investigationfor improvement of the COIL beam quality hasscarcely been reported.

For good beam quality of a high-power COIL sys-tem, it is important to have an optical resonator thatis suitable for a large mode volume and a trans-versely flowing gain medium. It is well known thata stable resonator can be used for any type of gainmedium and oscillates easily. However, the diame-ter of the fundamental mode is small, it oscillates inmultimode in a large bore laser cavity, and the beamquality is poor, making the stable resonator unsuit-able for a large mode volume laser. On the otherhand, an unstable resonator does not only achieve alarge controllable mode volume but it also has goodtransverse mode discrimination. Hence it is best

The authors are with the Dalian Institute of Chemical Physics,Post code 116023, 457 Zhongshan Road, Dalian, China. Thee-mail address for Y. Jin is yqjin@rose.dicp.ac.cn.

Received 7 May 1998; revised manuscript received 25 August1998.

0003-6935y99y153249-04$15.00y0© 1999 Optical Society of America

suited for high round-trip gain. For low round-tripgain, the unstable resonator requires use of a lowmagnification resonator to extract the availablepower efficiently, but a low magnification conven-tional unstable resonator produces output with asmall clear aperture-to-obscuration ratio. This typeof far-field energy distribution results in a much re-duced rather than a filled in beam with the samedimensions. So a novel unstable ring resonator with90-deg beam rotation3–6 has been proposed to avoidobscuration in the output beam. The first descrip-tion of the properties of unstable resonators with fieldrotation was reported in 1979.7 This type of resona-tor has been shown to have good beam quality andalignment insensitivity, and is particularly suitablefor a transversely flowing gas and chemical laser withsmall signal gain.

To our knowledge, we report the first experimentalresults of an unstable ring resonator with 90-degbeam rotation for the COIL including forward modeoutput power and beam quality. Reverse mode sup-pression and the actual polarization will be reportedin a future paper.

2. Experimental Apparatus

A. Chemical Oxygen Iodine Laser Experimental Setup

The COIL experimental setup is shown in Fig. 1.The basic composition of the supersonic COIL systemconsists mainly of a gas supply system, a basic hy-drogen peroxide preparation system, a rotating disksinglet oxygen generator, a gas–liquid separator, awater vapor cold trap, an iodine vapor generator, asupersonic oxygen iodine mixing nozzle array, a gain

20 May 1999 y Vol. 38, No. 15 y APPLIED OPTICS 3249

IT

1

Con

90 P

3

cell with an optical resonator, and a vacuum systemwith roots and a mechanical pump.

The principle of COIL operation is based on energytransfer from singlet oxygen O2~1D! to ground-stateatomic iodine and lasing as a result of the I~2P1y2!3~2P3y2! transition at a laser wavelength of 1.315 mm.he singlet oxygen O2~1D! is chemically generated:

2KOH 1 H2 O2 1 Cl23 2KCl 1 2H2 O 1 O2~1D!. (1)

Fig. 1. Experimental setup of the COIL with the UR90.

Fig. 2. Schematic of the configuration of the UR90.

Table 1. Operating

Flow Rate of Cl2~mmolys!

Flow Rate of I2

~mmolys!

Flow Rate ofPrimary Helium

~mmolys!

1000 ;10 4000

Table 2. UR

MagnificationM

EquivalentFresnel Number

Neq

ConcaveRadius~mm!

1.28 2.375 11343

aThe length is in millimeters.bParameters defined in text.

250 APPLIED OPTICS y Vol. 38, No. 15 y 20 May 1999

The singlet oxygen O2~ D! causes the iodine mole-cules to dissociate into iodine atoms:

nO2~1D! 1 I23O2~

3S! 1 2I ~2 # n # 5!. (2)

This is followed by rapid energy transfer and theestablishment of a population inversion in atomiciodine and then lasing at a wavelength of 1.315 mm:

O2~1D! 1 I~2P3y2!3O2~

3S! 1 I~2P1y2!, (3)

I~2P1y2! 1 hn3 I~2P3y2! 1 2hn ~l 5 1.315 mm!.

(4)

The operating conditions of the COIL are listed inTable 1.

B. Description of the Unstable Ring Resonator with90-deg Beam Rotation

A schematic of the unstable ring resonator with 90-deg beam rotation ~UR90! is illustrated in Fig. 2.The UR90 is composed of rooftop mirrors, an off-axisexpanding telescope, and a scraper. The rooftopmirror that consists of two orthogonal reflectingplates lightly glued together is a key optical assemblyproviding beam rotation. The rooftop mirror rotatesthe beam 90 deg and translates it both horizontallyand vertically. All the mirrors are coated for highreflectivity at 1315- and 633-nm wavelength, the lat-ter of which is needed for alignment. We used amagnification of 1.28. The equivalent length isgiven by3

Leq 5 S1 11

M2DSLa 1Lb

M1

Lc

M2D , (5)

where M is the magnification, La is the distance inthe forward direction from the scraper to the convexmirror, Lb is the distance from the convex mirror tothe concave mirror, and Lc is the distance from the

ditions of the COIL

Flow Rate ofSecond Helium

~mmolys!Cavity Size

~mm!Gain Medium Length

~mm!

550 40 3 100 500

arametersa

ConvexRadius~mm! La

b Lbb Lb

b ab

8835 2848 1254 4074 10.41

concave mirror to the scraper. The equivalentFresnel number Neq 5 2.375 is calculated from

Neq 5a2~M4 2 1!

2lM4Leq5

a2~M2 2 1!

2l~M2La 1 MLb 1 Lc!, (6)

Fig. 3. Schematic of the left-hand side view of the UR90.

Fig. 4. Optical layout of the BQ measurement.

Fig. 5. Typical waveform of the COIL output power.

Fig. 6. Plot of the far-field intensity profile pattern of the C

where a is the distance from the optical axis to theedge of the scraper mirror. The actual parameter ofthe UR90 can be seen in Table 2. Alignment of theresonator is important. To ensure that the geomet-ric beam size matches the flux aperture of the gaincell, we must create O1O3 ' O2O3 and O1O3 5 O2O3as shown in Fig. 3.

3. Results and Discussions

The laser beam quality characterized by beam focus-ability in the far field and the far-field intensity pro-file pattern depends on the near-field beam intensityand phase distribution. We concentrated on mea-surements of the near-field phase distribution withthe Shack–Hartmann wave-front sensor, and the far-field beam intensity profile pattern with the ModelLBA-100A laser beam analyzer made by Spiricon,Inc. The optical beam train for the diagnostics isshown in Fig. 4.

The two beam splitters were uncoated and wedgedto provide beam attenuation. The first surface re-flection of the first wedge mirror was directed to thecalorimeters, and the second surface reflection wasdirected down the beam train to the second wedgemirror. The first surface reflection of the secondwedge mirror was sent to the Shack–Hartmannwave-front sensor, and the second surface reflectionwas sent to the laser beam analyzer. The experi-mental results of the average output power wave-form, the far-field beam intensity profile pattern, andthe near-field beam phase distribution are illustratedin Figs. 5, 6, and 7, respectively. The average outputpower of 5 kW with a beam quality of b 5 1.6 6 0.2has been achieved, where b is defined as

b 5uexperiment

utheory, (7)

where u is the beam divergence and utheory is thedivergence of a beam with uniform amplitude andphase at the output scraper.

Figure 7 shows the near-field beam phase distribu-tion measured by the Shack–Hartmann wave-frontsensor; the cross-sectional units are in wavelengths.The reason the plot is cut off at the edge is that theactual beam size measured is smaller than the avail-

with the UR90. The focusing lens has a 5-m focal length.

OIL

20 May 1999 y Vol. 38, No. 15 y APPLIED OPTICS 3251

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able aperture of the equipment. Also, as is obviousfrom Fig. 7, the wave error of the rms is 0.173 wave-length and the wave error of the peak to valley is 1.01wavelength. On the basis of the rms value, we canobtain the laser beam quality ~BQ! defined by the near-field wave-front error:

BQ 5 Î 1Sr

5 1.8, (8)

Sr 5 expF2S2p

l D2

DF2G , (9)

where Sr is the Srehl ratio and DF is the wave errorf the RMS. This BQ value as defined by the near-eld wave-front error satisfies the value of b definedy the far-field beam divergence.Although the UR90 has many advantages com-

ared with a conventional unstable resonator, it doesot completely average the optical aberrations, ther-al distortion, and heterogeneity of a supersonic flow

ain medium by beam rotation. The main sources ofave error of 1.01 wavelength from peak to valley are

hermal distortion and heterogeneity of a supersonicow. A beam quality of 1.06 as noted in Ref. 4 wasbserved for the CO2 laser. Inasmuch as the wave-

length of the CO2 laser is much longer than that forthe COIL, based on a comparison of a long and a short

Fig. 7. Plot of the near-field beam phase distribution of the COILwith the UR90. The wave error unit is in wavelengths and thebeam size is 35.16 3 17.53 mm2.

252 APPLIED OPTICS y Vol. 38, No. 15 y 20 May 1999

wavelength, we discovered that the long wavelengthis insensitive to the above factors that influence BQ.

4. Conclusions

Compared with conventional unstable resonators,8the aberrations or imperfect optical elements, ther-mal distortion, and heterogeneity of the gain mediumin the UR90 can be mitigated by beam rotation. Ourexperimental results prove that the UR90 produces afilled-in output beam with good beam quality. Anaverage output power of 5 kW with a BQ of b 5 1.6

as been achieved with the COIL system.

The authors thank Cui Tieji, Zhao Tong, Xu Wen-an, Ma Yueren, Wang Ke, Guo Jingwei, and Conghiqiang for their invaluable help with the opticallignment, the optical diagnostic, and operation ofhe COIL system.

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3. A. H. Paxton and W. P. Latham, Jr., “Unstable resonators with90° beam rotation,” Appl. Opt. 25, 2939–2946 ~1986!.

4. S. Holswade, R. Riviere, C. A. Huguley, C. M. Clayton, and G. C.Dente, “Experimental evaluation of an unstable ring resonatorwith 90° beam rotation: HiQ experimental results,” Appl. Opt.27, 4396–4405 ~1988!.

5. C. M. Clayton, “Diagnostics for a developmental iodine laser,” inLaser Beam Radiometry, A. A. Sanders, ed., Proc. SPIE 888,28–33 ~1988!.

6. W. P. Latham, A. H. Paxton, and G. C. Dente, “Laser with90-degree rotation,” in Optical Resonators, D. A. Holmes, ed.,Proc. SPIE 1224, 265–282 ~1990!.

7. Yu A. Anan’ev, V. L. Kuprenyuk, and V. E. Sherstobitov, “Prop-erties of unstable resonators with field rotation,” Sov. J. Quan-tum Electron. 9, 1105–1114 ~1979!.

8. Jin Yuqi, Sun Yizhu, Sang Fengting, Yang Bailing, and ZhuangQi, “Experimental investigation of unstable resonator for a cwsupersonic COIL,” High Power Laser Particle Beams 9 ~2!, 227–232 ~1997!.