Improving the Beam Quality of Solid-State Systems Using Both Outside and Inside Cavity Devices with Variable Optical Characteristics Along the Cross Section

IMPROVING THE BEAM QUALITY OF SOLID-STATE SYSTEMS USING BOTH OUTSIDE AND INSIDE CAVITY DEVICES WITH VARIABLE OPTICAL CHARACTERISTICS ALONG THE CROSS SECTION

S. G. Lukishova, N. R. Minhuey Mendez, V. V. Ter-Mikirtychev, and T. V. Tulajkova

This paper presents the results of application to different laser installations of soft or apodized apertures (AA) [1-3]* with smooth transmission profiles decreasing from center to edges. Two types of AA, which were made of CaF~:Pr crystals, have been used: induced absorption (IA) AM and photooxidation (PhO) AA. The --3-45-mm-diameter IA and PhO AA with smooth monotonic fiat-top profiles have been used in 1.06-#m laser amplifier systems to suppress hard-edge Fresnel diffraction rings in beam cross section and to increase the second harmonic conversion efficiency. The ~3-4-mm-diameter PhO AA with bell-like transmission profiles were placed inside the 2.94-pm and 1.06-lzm resonators of master oscillators. The tendency of the output energy to increase by 1.3-1.8 times and the decrease in beam divergence in single-mode lasing as compared with a hard-edge aperture have been observed in the experiments described below.

1. INTRODUCTION

There are two applications of apertures with variable optical characteristics along the cross section in laser physics: 1) as outside-cavity elements to suppress hard-edge Fresnel diffraction ripples; 2) as inside-cavity ones to improve the mode composition, suppress the side lobes, and diminish beam divergence. In the last 15 years the term "apodization" [1] has become popular in the physics of high-power neodymium glass laser installations used in plasma heating experiments. Application in laser amplifier physics is largely connected with Fresnel diffraction [4-6]. Analysis reveals that the spatial scale of nonuniformities attributable to hard-edge Fresnel diffraction in standard laser installations is within an order of magnitude of the spatial scale most easily focused due to small-scale self-focusing in glass. An isolated diffraction ring may in this case be unstable with respect to decomposition at the azimuthal angle and will decay into spatial cells [7, 8], so that the light spot appears as the central part with "hot points" and a circular aureole of radiation scattered at large angles (up to 80% of the energy) at the output of the amplifier system [7, 8]. The glass damage as rod axial damage and concentric Fresnel diffraction rings are representative of the types of damage in laser rods observed at mean intensities substantially below the glass threshold intensities.. One of the simplest methods of eliminating hard-edge Fresnel diffraction effects on light beam propagation in a laser amplifier system is to employ apodization. An AA is placed in front of the output hard-edge aperture and makes the beam profile rounded at the edges, so that as the beam travels within the working zone of the installation no diffraction intensity flashes or ripples occur.

*See also A. A. Mak, L. N. Soms, B. A. Fromzel, and V. E. Yashin, Neodymium Glass Lasers [in Russian], Nauka, Moscow (1990), pp. 137--157.

Institute of Radioengineering and Electronics, Academy of Sciences of the USSR. Institute of General Physics, Academy of Sciences of the USSR. Translated from Preprint No. 17 of the Institute of General Physics, Moscow, 1991.

0270-2010/91/1203-0295S12.50 9 Plenum Publishing Corporation 295

296 [43, 441. These devices have been used in different types of lasers, e.g., Nd:YAG [15, 22-24, 26-32, 36-39, 41, 44], CO 2 [18, 20, 21, 25, 33, 35, 36], ruby and alexandrite [17]; excimer [401, and dye-lasers [44]. Frosted-edge quartz apertures (QuanteI International), liquid crystals AA [45] (Opton), and Pockels cells with nonuniform electric field (State Optical Institute, Leningrad) have been proposed for use inside the cavity. Published works show that the main results of using elements with variable optical characteristics inside the resonators (in most cases unstable) rather than hard-edge ones are: 1) generation of diffraction-limited beams without side lobes; 2) decrease in beam divergence and even sometimes increase in output energy in single-mode lasing; 3) increase in second harmonic conversion efficiency [15, 24]. tt should also be mentioned that another modification of the cavity aperture edge such as serrating has been proposed theoretically for a cavity in [46] and has been tested experimentally in [47, 48]. Tile taper zone required is a/(2Feq), there a is the aperture radius and Feq is the Fresnel number. It was also proposed to use resonators with step reflectivity or phase tapering of mirrors to reduce diffraction effects and improve the output characteristics of the oscillator, e.g., [46, 49], or with aperture shaping by choosing the appropriate mirror boundary shape [471. Another approach to reducing the effect of edge diffraction is the use of zero equivalent length owing to the self-imaging aperture [50]. In this paper we report on the investigations of self-made AA on the basis of induced absorption (IA) [3, 51-58] and photooxidation (PhO) [3, 56-60] in both cases: in 1.06-r amplifier systems to suppress diffraction ripples; inside the 2.94- and 1.06-pro resonators to increase brightness of laser radiation in single mode lasing.

2. BRIEF DESCRIPTION OF AA DESIGN

The IA and PhO AA design have been considered in detail in [3, 51-59]. These AA were made of CaF2:Pr crystals. The technology of producing IA AA is based on increasing the absorption capacity of the edges of transparent crystals when they are subjected to ionizing radiation which penetrates at a definite depth inside the crystal, In contrast to the IA AA, the PhO (or photodestruction) AA represent whole-volume ~,-colored crystal samples, the transparent central part of which is created by the tong-time exposition of short-wavelength cw laser radiation or the light of a powerful mercury tamp with stable bleaching of the crystal. The peculiarity of CaFz:Pr crystals is the sufficiently high value of the induced absorption coefficient K > 2 era-t in the near-infrared region (only a few materials have a high value of K in this spectral region). Figure 1 shows the absorption spectra of a CaF2:Pr sample of 7 mm thickness before (curve 1) and after exposure to ~,- radiation with a dose of ~108 R (curve 2), The Pr concentration was 0.2 tool,%. The absorption coefficient at wavelength ), = 1.06 t*m is K ~ 1.8 cm-1 (some samples of CaF x with unknown impurities had K ~ 6 cm-I at k = 1.06 ~tm); at A = 1.3 etm, K ~ 1.3 cm-t; at ,t = 2.7-2.94 pro, K ~ 2.7 cm-1. In the region ~1,4-1.6 pm, we cannot use CaF2:Pr c~stals, but in this region, e.g., at ,~ = 1.54 pro, CaFz:Nd crystals are suitable, Figure 2 shows photographs of the IA AA (at left) and PhO AA (at right) for the amplifier systems. A photograph of the PhO AA lbr inside-cavity application is shown in Fig, 3.

3. INVESTIGATIONS OF IA AND PhO AA IN VARIOUS LASER AMPLIFIER SYSTEMS

3.1. Effect of Smoothing the AA Edges on Cross-Section Homogeneities of the Output Beam

Tile IA and PhO AA were used in various single-pulse lasers for high-temperature heating of plasma at the Institute of General Physics and at the I. V. Kurchatov Institute of Atomic Energy (Troitsk, Moscow Region). Figure 4 shows the burn pattern on photosensitive paper of the beam cross section at the output from an aperture --2.5 cm in diameler and 3 cm in length at a distance immediately behind the aperture (a) as well as at a distance of 3.5 m (b) at a radiation energy density of 3 J/cm 2 (pulse duration 3 nsec). The aperture is mounted in a Teflon mount to eliminate diffraction rings owing to the upper crystal surface. A portion of the Teflon mount was eliminated from the upper right sector of the outer edge of the AA for visual demonstration in Fig. 4b. Figure 4b clearty reveals the lack of diffraction rings from the soft aperture even at a distance of 3.5 m, whereas diffraction rings are already clearly evident from the sharp edge (the upper right sector). Figure 5 shows the transmission profiles at ,l = 1.06 ~m of two CaFa:Pr IA AA (diameter of crystals 9 mm, length 30 ram). The cross~sectional intensity distribution of the beam was measured by a photoelectronic method, where the light beam is scanned through a small-diameter (0.2 ram) diaphragm placed in front of an FEU-62 photomultiplier sensitive to near-infrared radiation. As indicated by the measurements, curve 2 is approximated by the relation exp[--(r/r0)4.d], where r 0 = 3 mm, and curve

297

a b

b a c .,'//.)I/~

ZZ.Z

! t I l ! I I 1 r 1 r 7 Z J z/ 5 ~,/;7~7z Fig. 12. Transmission profiles of PhO AA for intracavity application at 2 = 2.94-/~m.

.,,.j~ s Y/TC I ,r _ ~(/7c/z, _!

Fig. 13. Setup of Er:YAG oscillator.

2JJ

ZNJ

~:Jg D ,'/,7 \",,\ ',

J 7 g J 4.- 5 5 7 8 G n7,'~7 Fig. 14. Intensity profiles of laser beam at the output of Er:YAG oscillator with AA.

4. INVESTIGATION OF PhO AA INSIDE THE LASER RESONATOR TO IMPROVE TttE OUTPUT CHARACTERISTICS OF THE OSCILLATOR

Figure 3 shows a number of PhO AA about 05-4 mm in diameter and 5-7 mm in thickness.

4.l, Experimental Results for a 2.94./~m Er:YAG Laser Resonator in Free Running Oscillation

The PhO AA, whose transmission profile at 2 = 2.94/zm is shown in Fig. 12, has been used inside a semiconfocal resonator of a flash-pumped Er:YAG laser (2 = 2.94 btm) in free running oscillation (Fig. 13). The AA transmission profile was measured by scanning 0.1-0.2 mm 2.94-pm laser beam relative to the pyrosensor. The pump cavity housing a 12 cm • 6.3 mm rod

301 80

! \ : / \ 1 \ 1 \ 1 \ ,/ \ / \ \

7 2 : ~:::::: Fig, 15, Intensity profiles of laser beam at the output of Er:YAG oscillator with hard-edge aperture.

2'oJ

::700 :ZOO" ::/:r ,:i:'i:'J

Fig. 16. OutPu~ oscillator energy versus pumping voltage,

was of a reflective Quantron type. T/he resonator is made of a concave corper hard mirror with a curvature radius of +3 m and a plane Si plate of high optical quality with thickness ~300/~m and reflectance from one face --30%, In multimode lasing the beam reflected from another race of the Si plate may appear, and we may see another light spot near the main one. In single-mode lasing the second spot disappears. The resonator length is 150 cm. Pulse duration was 150 izsec, and repetition rate 0~4 Hz. The PhO AA was placed at a distance of 47 cm from the Si plate, and the Er:YAG rod was placed 65 cm from the corper mirror. To record the intensity distribution of the laser output beam, a pyroelectric sensor and a 200-#m slit were used. Figure 14 shows the intensity profile at 10 cm (curve 1) and t05 cm (curve 2) from the output of this oscillator with the PhO AA of Fig. 12 inside the cavity. In this case we observe single-mode lasing. As we put a hard-edge aperture inside this resonator in place of AA, single-mode lasing was observed only with a hard- edge aperture of diameter less than 3 mm~ Figure 15 shows the output intensity profiles at 10 cm (curve 1) and !30 cm (curve 2) from the oscillator with hard-edge aperture and diameter 3 mm. In both cases the pumping electrical energy was about 2(30 L The output energy was measured by a pyroelectrie energy meter. Figure 16 shows the results of the dependence of output oscillator energy versus the pumping voltage. Curve t is for the PhO AA and curve 2 is for the hard-edge aperture 3 mm in diameter, tt should be mentioned that PhO AA has Freshet reflecti0n losses about 6% at its faces and a small value of absorption in the center (the transmittance at the center of the AA was -87%), but in spite of these losses the output energy in single-mode fusing with AA was greater than in the case of hard-edge aperture. To compensate for the Fresnel reflection losses we placed the CaF2 sample inside the resonator ~th hard-edge aperture. Figure i7 shows the output cavity energy in single-rhode lasing versus the pumping voltage. Curve 1 is for PhO AA and curve 2 is for a hard-edge aperture 3 mm in diameter with the CaF2 sample inside the cavity. We can eliminate the Fresne! losses by the use of an antireflection coating and also some of absorption at the center of the PhO AA by a longer time interval of irradiation with the UV cw-taser beam being manufactured [3, 57].

302 25Of f 7"f~ ~ e::7,:0 t Z

"kl

,:dTaF~9' ' Z2ffz7 74~-Jz7 75gz7

Fig, 17, Output oscillator energy versus pumping voltage with compensation of difference in Fresnel reflection losses of AA and hard-edge aperture.

JQO

I...... I

Fig. 18. Far-field beam inlensity distribution for Er:YAG oscillator with AA and hard-edge aperture.

Fig. 19, Setup of the Nd:YAG oscillator for energy measurements.

The far-field was taken at the plane of minimum spot size of converging lenses (with focuses 20 or 30 cm) rather than at its focal plane. Figure 18 shows the radial scan measurements of the far-fieId intensity" profiles of the output beam from the oscillator with the AA (solid line) and the hard-edge aperture (dotted line). The measurements were made with a 100-/zm slit. The divergence angte at level 1/2 in the case of AA is about 2.10 mrad; in the case of a hard-edge 3-ram aperture, about 3.2 mrad from these measurements. The results of Fig. 18 and Figs. 14 and 15 were obtained with different output energies but the same pumping voltage,

303 zl#)~

o. 5 l/6"a~ :f.,.)

j i I .1 ...... I, , i ..... I 7 Z J ~,rmrl Fig. 20. Transmission profiles of PhO AA for the Nd:YAG resonator.

~s ~7~ Poo~'eZs ~

")e~','wl- ZOc,~ __~

Fig. 21. Setup for the qualitative comparative analysis of Nd:YAG oscillator divergence with the use of AA and hard-edge aperture.

4.2. Preliminary Experimental Results for the Q-Switched 1.06.pro Nd:YAG Laser Resonator

A diagram of the resonator is shown in Fig. 19. The PhO AA was placed inside the Nd:YAG oscillator cavity with plane mirrors. The flash-pumped cavity housing the 6 x 60 mm rod was of a reflective Quantron type. One face of the Nd:YAG rod was an 8% reflectance mirror. Another mirror had nearly 100% reflectivity. The Pockels ceil provided 20 ns pulse duration. Repetition rate was 12.5 Hz. A single-mode lasing of such an oscillator took place if the hard-edge aperture of 2.5 mm diameter was inserted inside the cavity, but in this case we obtain only about 30% of the output energy without aperture insertion, When we placed theAA with transmission profile at 2 = 1.06/zm shown in Fig. 20 (curve 1), the output energy in single-mode lasing was already about 55% of the multimode case. (The output energy with the AA was 0.1 J with 40 J electrical energy of pumping.) The transmittance at the center of the AA was 92% at 2 = 1.06 ~m. Concerning divergence measurements, we can only report on it quantitatively. The experimental setup is shown in Fig. 21. We also used plane-mirror cavities with 30 and nearly 100% refiectivity. We used similar Quantron-type housing, but the 6 x 60 mm Nd:YAG rod had faces with antireflection coating. A Pockels cell was also used in this case. We observed single-mode lasing with a 3-ram hard-edge aperture. Using the number of filters and burning spots on the photosensitive paper, we see that in this case the beam close to the Gaussian was at the output of the oscillator. In the case of the AA of curve 1 of Fig. 20, we obtained a beam profile closer to uniform than in the case of a hard aperture. We also observed a 1.06-pro light spot on the surface of the tuminescing substrate at the output of the oscillator and at a distance of about 6 m from it. In the case of the hard-edge aperture, the --t-mm spot became .-10 mm at such a distance, tn the case of the AA, we observed an -2-ram bright spot and a very weak aureole of -5 mm in diameter at the output of the oscillator. The aureole was removed when we placed inside this resonator another AA with the transmission profile shown in Fig. 20 (curve 2) together with the first AA. In the case of the AA inside the cavity we observed the bright spot of --5 mm diameter at a distance of 6 m from the oscillator output. It should be mentioned that we used equal pumping in both cases: with the hard- edge aperture and with the AA.

304 5. CONCLUSIONS

According to the results of our experiments we can say that the AA presented here shows great potential: for both outside- cavity elements in laser amplifiers (we used CaF2:Pr samples up to 200 mm in diameter), where the AA suppressed the appearance of hard-edge Fresnel diffraction ripples. The high damage thresholds of such an AA makes them potentially useful in single-pulse high-power laser systems. As to inside-cavity applications of these AA, the first results show that they can improve the output characteristics of the oscillator in single-mode lasing, but it is necessary to examine them in another regime of oscillation and inside the different ~pes of resonators, e.g., unstable. The PhO AA for an unstable resonator has the shape of an absorbing ring on the transparent substrate. The main advantages of the IA and PhO AA are their simplicity of manufacture and use, and also the wide spectral band in operation.

6. ACKNOWLEDGMENTS

The authors express their gratitude to V, A. Sokotov, E. A. Simun, and V. K. Karpovich of the State Optical Institute (Leningrad) for fabricating the CaF2:Pr samples; V. D. Terekhov and V. K. Ivanchenko of the Yd. M. Karpov Physical and Chemical Institute (Moscow); B. M. Terentiev from the All-Union Research Institute of Radiation Technique (Moscow); M. A. Plemev of the V. t. Lenin All.Union Electrotechnical (Moscow) for technical assistance in producing the AA for the ionizing radiation sources; V. A. Konjushkin of the General Physics Institute for producing the second inside-cavity AA for the 1.06-/~m oscillator; L. V. Chernysheva, Yu. K. Nizienko, M. V. Brenner, and V. V. Aleksandrov of the I. V. Kurchatov Institute of Atomic Energy (Troitsk); B, V. Gorshkov, Yu. V. Korobkin, A. V. Kil'pio, K. I. Vodopianov, V. A. Chikov, and S. B. Mirov of the General Physics Institute; O, Yu. Nosach and L. D. Mikheev of the P. N, Lebedev Physical Institute (Moscow) for the chance to study the AA in laser systems; Yu, IC Danileiko of the General Physics Institute for a discussion of the divergence measurements. The photographs of the AA units were taken by S. I. Gorchev of Sovetsky Khudozhnik Publisher (Moscow). We also wish to thank Dr. P. D. Beresin of the P. N, Lebedev Physical Institute for help in publishing the manuscript. The PMT preparation of half-tone photographs was made in the Editorial and Publication Department of the P. N. Lebedev Physical Institute by A. F. Kolesnichenko. We also used the translation of some sentences from Russian into English by K. S. Hendzet of Nova Science Publishers and some references from the paper [36].

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