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Te~hnolo~~~, Vol. 29. No. 4. pp. 21 (’ 1997 Elsevier

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003&3992:97 PII:

I 215.1997

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ELSEVIER

High power CW solid-state multilaser system I. PESHKO, G. GALICH,

M. LOPIYCHUK,

A. KHIZHNYAK,

V. NAKVASUK

A laser system containing four CW Nd : YAG double-rod lasers, transforming optics and a fibre with input-output optics has been designed and investigated. It is shown that reduction of the beam parameter product of the non-Gaussian beams in the image space of the lens has made it possible to collect the emission of four lasers to the same fibre. Independent laser operation has provided the output power control in the range of IO-900 W with a stable light spot size on the input end of the fibre. @ 1997 Elsevier Science Ltd. KEYWORDS: lasers (Nd : YAG), beam parameter product, fibre

Technological achievements

solid-state lasers: and problems

difference between the axis and the surface areas, stresses appear in the laser rod and, as a result, thermal lensing takes place. In addition, the stresses induce birefringence and mechanical deformation of the rod. Increasing a number of the rods in the cavity solves the high-power problem’, but decreases the total efficiency of a laser system. Moreover, the power growth leads to the thermal effects in all optical elements of the cavity. Usually, a level of power inside the cavity is 5-7 times higher as compared with output power. So, for a lens or a mirror substrate, material absorption has to be less than 0.01% to avoid lensing or other deformations of these elements.

Today, high power CW solid-state lasers achieve some kW of output power. Such lasers are used for technological purposes such as instruments for welding, cutting, and drilling of metal sheets some centimetres thick. In spite of the great success of the diode pumped Nd : YAG lasers in the l-5 kW power range, the lamppumped systems remain more practically used’. Because of their relatively low efficiency (about 3-5%) these lasers need significant levels of input power. As a result, the cooling system has to be very efficient. Usually, a two-circuit water-cooling system is used, which needs the additional expenditure of electric power and distilled water. However, such lasers are quite attractive from many points of view. Their relatively compact size and solid-state design make them easy to include into most technological systems. The volume size of a modern 1 kW-class solid-state laser is only about 0.5 m3. The wavelength of a YAG laser is near 1 urn, which potentially allows one to realize fine focusing of emission and to achieve a CW power density of lo7 W cmm2. At this wavelength a modern fibre has losses of some dB per km. The output optics design is light enough to be driven by a three-dimensional moving system with fine accuracy. The apparatus for welding in difficult and inaccessible places, for example inside the boiler of an atomic power station, may be constructed and manufactured.

For neutralization of thermal lens action, special adaptive mirrors were used which had curvature identical to the phase front of the waves4. Another variant of the lensing compensation was manufacturing of negative lenses on the ends of the rod3. These methods reduced the divergence of emission but unfortunately decreased the output power. Moreover, the compensation was possible at certain input powers because the lens refractive power depends on the pump level. Amplitude modulation of laser flux at low frequencies takes place because of mechanical vibrations of the laser cavity elements and refractive index modulation of air or the gain crystal’. The changes of the dissipated power result in changes of thermal lens refractive power. The instability of the discharge in the plasma modulates the pump intensity, which leads to thermal lens parameter instability. As a result, the optical axis position and refractive power of the thermal lens [email protected] with frequencies of about 1- 10 Hz. ABCD-matrix formalism was used to search the cavity parameter combinations that provided the minimum influence of thermal lens fluctuations on the output parameters’ stability6. It was shown that simultaneous stabilization of mode size on the thermal lens and curvature of the wave phase front near the active rod is impossible.

For a high power crystal laser, the main problem is heat processes in the active rod2,3. Because of the temperature IP, GG and ML are in the Institute of Physics National Academy of Sciences of Ukraine 46, Science Ave., 252650, Kyiv, Ukraine. AK is in the International Centre ‘institute of Applied Optics’, National Academy of Sciences of Ukraine, 10-G, Kudryavskaya Str. 254053, Kyiv, Ukraine. VN is in the Institute of Welding, National Academy of Sciences of Ukraine, 11, Bozhenko Str., 252650, Kyiv-5, Ukraine. Received 5 June 1996. Accepted 10 December 1996.

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Naturally, the solutions of the thermal problem with individual boundary conditions and parameters of heat generation in different experiments are different. However, the general character of the thermal lens behaviour can be known. A non-linear (steady-state) model of thermal effects in Nd : YAG has been suggested7, which took into account the real temperature variations of the thermal conductivity of the active medium. It has been shown that the formulas contained new relationships and corresponded to the experiment very much closer than did those of the linear model. Some important conclusions can be made from the nonlinear solution. First, the temperature distribution in the central area of the rod is not parabolic and the lens refractive power is higher in the axial region. Second, the temperature gradient between the centre and the surface is proportional to the surface temperature and depends on cooling intensity. Calculated lens power is higher for a non-linear model, even when the thermal resistance of heat irradiation is equal to zero. Finally, in contrast to the linear model, which gives a linear dependence of the back focal length on heat generation, the non-linear approximation explains experimental results2,3 with a higher power of this dependence. Moreover, the temperature distribution in Nd : YAG laser rods deviates from a parabolic function form not only due to the temperature dependence of thermal conductivity but to the inhomogeneity of pumping light distribution8. An analytical expression was derived which relates the resulting radial dependent refractive power to the shape of the pumping light distribution. It has been shown that the refractive power has approximately a parabolic radial dependence. As far as a technological laser is concerned, it should be remembered that during the welding or cutting process the beam is reflected from the detail or from the plasma plume and a significant portion of it may be returned to the cavity. This means that strong modulation of laser quality and changes of the intensity and divergence of the beam take place. A simultaneous fast increase of the intensity and divergence above the average values causes the destruction of the input and output ends of the fibre. General composition multilaser system

of the

Both the amplitudes of the birefringence and thermal lens aberration depend on heat generation intensity inside the rod. These effects do not have additive properties. The laser with two simple-lamp pumping cavities can provide the same amplification as one rod with two lamps, but the total thermal lens effect is reduced for two low-pumped rods compared to a single high-pumped rod. In our 1 kW-class system, used as a basic laser, we used a double-rod Nd : YAG CW laser. It is well known that l-2 kW YAG lasers’ have 4-6 pump cavities in the same laser optical cavity. Such a configuration is quite simple but a very high intracavity power requires excellent optical quality of the laser elements. We propose using the parallel scheme where the emission of four lasers was collected to the same fibre. The intracavity power was four to five times lower in this case. Moreover, the special time-intensity distribution may be realized for the parallel scheme. For example, the

multilaser

system: I. Peshko et al.

Fig. 1 Scheme of the technological laser equipment: (1) doublerod lasers; (2) lenses; (3) prisms; (4) input lens; (5) fibre holder; (6) HeNe pilot laser; (7) beam handling robot; (8) beam focusing head; (9) silicon-silicon fibre. Beam intensity distributions at the points a-d are shown in Fig. 5

emission of four independent lasers which work in CW, quasi-CW, free-running pulsed and a Q-modulated manner are mixed in the same fibre. Such a complex time-intensity distribution is impossible to get in a single laser but may be required in some technological processes. Finally, one low-pumped laser provides tolerant operation with some watts of output, and the sequential switching of the lasers and increasing the pump changes the output power from some watts up to kilowatts. A general view of the laser system is demonstrated in Fig. 1. The two lasers of the first row had the optical axis 5 mm higher than the second row of the lasers. The beams were focused by the lenses (2) on the plane in front of the input lens (4). The prisms (3) turned the beams to the fibre input lens. The beam of the HeNe pilot-laser (6) was directed between the power beams along the axis of symmetry. Fine angular and linear alignment of the beam position were realized by threedimensional (3-D) displacement of the lenses (2). The fibre holder (5) additionally had angular rotations, and the fibre (9) had a length of about 6 m. The output objective (8) was driven by a 6-D robot (7). The fibre output objective had four lenses and protecting glass, devices for air cooling the lenses and protected gas connection. The total energy loss along the whole optical path was nearly 16%. The light spot size was changeable in the range 0.6-1.1 mm. A maximum output power of 900 W was achieved. Figure 2 shows the scheme of beam collection to the fibre. For the single beam the relation &F 6 rf has to be satisfied, where & = half divergence angle, rr = fibre core radius, F = focal length of the objective. To enter into the fibre the total work aperture of the objective, rt transmitting four beams has to be rt x F tan 8r x F x NA, where 8f is the half angle of entrance into the fibre and NA = sin & is the numerical aperture. Consequently, the possible value of the input lens radius is rt Z rfNA/&. On the other hand, on the plane in front of the objective (Fig. 2) the relation

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are the Rayleigh ranges of both the beams. It has been shown’ that a certain lens position ,Si can be found, which provides equality of the divergences of both the beams in the image space of the lens &I = 0;2. The same transformation of the waist radii takes place and the measured values dimaxand fi’irnaxsimultaneously reduce. Consequently, the parameter product may be decreased.

Fig. 2 Scheme of the laser beams’ fibre coupling: (l)-(4) cross-sections of four power beams on the plane of the lens; (5). (6) beam of HeNe pilot-laser; (7) fibre; F focal length of the lens; d gap between the beams. To simplify the picture some of the rays after the lens are not shown

+ 2~ takes place, where d is the distance between the edges of the neighbour beams in the horizontal or vertical directions. From this relation l^h= (v!%, - n) x (2 + fi)“2. Substitution of the numerical values of experimental parameters rf = 0.35 mm, & = 0.014 rad, NA = 0.21, d = 1 mm gives rb z 1.9 mm. Fibre input parameter product p = Ofrf is 210 mrad x 0.35 mm x 70 mrad mm. So the critical beam divergence angle is 01, = p/rt % 13 mrad. For Gaussian beams the angle of full power propagation relates to the angle containing 0.86 of the total power, as eo.99/eo.86= 1.33. This means that the fibre core really has to be 1.33 times wider than the calculated value according to the metrology standard. In this case the full divergence angle has to be 2 x 0.75 x 13 mrad z 20 mrad. However, the beam full divergence available was nearly 2X-30 mrad. The beam parameter product needed to be compressed approximately 1.5 times to enter the fibre successfully. 2v, = (2(2rb + d)2)“’

Divergence

transformation

Two main phenomena in the high pumped active rod cause the deformation of the Gaussian beam intensity distribution. These are thermally induced birefringence and radial inhomogeneity of the thermal lens refractive power. First, from the aforementioned processes, the appearance of different thermal lens refractive powers for radial and tangential light polarization components occurs. The cavity with such a ‘double lens’ can be imagined as two coinciding cavities with different lenses, which cause the simultaneous appearance of the coaxial beams with different divergences. As mentioned above, a thermal lens in the active rod has refractive power with a parabolic radial dependence. In this case, the laser cavity configuration may also be simulated as a superposition of some cavities with thermal lenses of different refractive power and different effective lens diameters. Consequently. the resulting beam would be presented as a superposition of some Gaussian beams but with different divergences and waist diameters. For the simplest case the laser emission can be imagined as a sum of two components with two different divergence values Bol and H02.The lens with focal length .f’ for which the conditions ZOI

The higher is the pump power, the stronger parabolic dependence of the refractive power takes place and the larger is the difference between 001 and Ho2.For such cases, the maximum of the relation OO~~O/Qimax~~~imax is higher. For low pump levels, this difference disappears and the above mentioned relation becomes equal to one everywhere. As mentioned above, the space intensity distribution of the beams has an elliptical form with a complex internal structure. For the lasers investigated, the best reduction of p (2-2.2 times in the horizontal direction) was observed for the laser providing 330 W of output power. The typical value for other lasers with output powers 220-260 W was about 1.4- 1.5 times. The measurements of the beam parameters by the power-meter placed behind the circle diaphragm gave the average value of power emitted in all directions. In this case the maximum product reduction for the power measurements was about 1.4- 1.8 times for different laser specimens. In Table 1, experimentally measured parameter products are demonstrated for some pump powers and focal lengths of the lens. The minimum of p was observed for a lens with focal length 24 cm. For a lens with 30 mm focal length the aberrations play a significant role. In the case of a four-beam structure, the aberrations spread the focal spot mainly in the radial direction. However, the divergence acts in all directions. Figure 3 depicts the calculated distribution of the rays near the position of maximum ray concentration after the input lens (Fig. 1, (4)). The initial picture had four symmetrically deposited beams. For the beams with low divergence (1 mrad) the concentration power is high, but the distribution is obviously inhomogeneous-which may be the source of stresses on the surface of the fibre end. For the 15 mrad divergence angle, much of the energy falls out of the fibre core, but the 10 mrad case seems near optimum. The beams with such divergence were used after compression of the output beams to enter, with minimum losses, into the fibre. Figure 4 illustrates the experimentally measured relative power (normalized to the total power PJ emitted in space angle A0 versus the propagation angle 8: (a) before the external lens; and (b) after the lens. Comparison of Table pump

1. Beam parameter product power and lens focal length

Parameter product

versus

(cm)

(mm x mrad) at pump power 7 kW

Parameter product (mm x mrad) at pump power 10 kW

16 24 30

14 11 14

22 16 19

Lens focal length

214

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Distance Tom focal plane

0.6 - I.lmm

Fig. 3 Caiculated intensity distribution on the focal plane of the input lens (central column), before (left column) and after (right column) the focal plane for half divergence angle 15 mrad (upper row), 10 mrad (middle row) and 1 mrad (lower row). Box size is 1 mm x 1 mm, circle diameter corresponds to the fibre core 0.7 mm

(4 8 0.96P I

0

12

@I

00.96P

22mm

Fig. 5 Intensity distribution in the points marked in Fig. 1: (a, b) far field; (c) near field; (d) distribution at 5.5 cm from the focal point of the output objective. Pump power was about 8 kW for all figures

distribution is not ideally symmetric because the left and right peaks of Fig. 5(d) represent the emission from different lasers. At low pump levels the own-beam divergence is relatively low and the Rayleigh range is larger than the focal length of the external lens (Fig. 1, (2)). This means that the lens increases the beam divergence. At high pump levels the beam divergence becomes high and the Rayleigh range becomes shorter than the lens focal length. Therefore, the lens decreases the beam divergence. It was measured that the own-beam divergence changes approximately ten times against the pump power, but after the lens this value changes by no more than + 12%. This effect gives the possibility of stabilizing the light spot size on the input fibre end and successfully coupling the output beams and fibre in a wide pump range.

24

Conclusions Propagation angle (mmd)

Propagation angle (mrad)

Fig. 4 Output power AP, normalized to total power P,, and emitted in the space conic angle [email protected] = 3 mrad versus average propagation angle 8. &.ssp indicates that the propagation angle accumulated 96% of the total output power

both diagrams shows the redistribution of the beam’s structure. After the lens, the direction angle of the maximum power propagation becomes two times less and the angle containing 96% of output power becomes 1.5 times less. This means that the mean part of emission is collected by the objective closer to the centre of the fibre end. Such preliminary compression of the beam divergence gave the possibility of using it successfully as an input objective single lens. Figure 5 demonstrates the intensity distributions in the near and far fields along the optical path of the system in the places marked by the arrows in Fig. 1. The length of the fibre was not sufficient to mix well the four-beam emission in the radial direction. At distances of some centimetres after the focal plane of the fibre output optics, the intensity distribution assumed a ring-shaped form (Fig. 5(d)). The cross-section of the intensity

A laser system consisting of four CW Nd : YAG lasers, beam transforming and collecting optics, a fibre and input-output optics has been designed and investigated. It has been shown that reduction of the beam parameter product took place for non-Gaussian beams, which made it possible to collect the emission of four lasers into the same fibre. The lens optical system stabilized the half divergence angle of the beams at a level of 6.5 + 0.8 mrad in the entire pump range available. The sequential switching of the lasers and the independent operation provided an output power control in the range of lo-900 W. After the output objective, the intensity distribution assumed a ring-shaped form. Acknowledgements The authors wish to thank Dr J. Jabchinski from the Military University of Technology, Institute of Optoelectronics, Warsaw, Poland for calculation of the four beams’ propagation through the fibre input lens. The authors would also like to thank Professor 0. Nazarenko from the Institute of Welding, National Academy of Sciences of Ukraine, Kyiv, Ukraine for financial support and fruitful discussions.

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References 1 Ishide, T., Matsumoto, O., Nagura, Ya., Nagashima, T. Optical

2 3 4

fiber transmission of 2 kW cw YAG laser and its practical application to welding, High-Power Solid State Laser and Applications, SPIE Proc, 1277 (1990) 188- 198 Koechner, W. Thermal lensing in an Nd : YAG laser rod, Appl Opf, 9 (1970) 2548-2553 Forster, J.D., Osterink, L.M. Thermal effects in an Nd : YAG laser, J Appl Phys, 41 (1970) 3656-3663 Apollonov, V.V., Vdovin, G.V., Ostrovskaya, L.N., Roden, V.N., Chetkin, S.A. Active correction of a thermal lens of a solid-state laser. I. A metal mirror with controlled curvature of central region of the reflecting surface (in Russian), SOP &an/ Electron, 18 (1991) 128-130

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Koechner, W. Output fluctuation of CW-pumped Nd : YAG lasers, IEEE J Quant Electron, 8 (1972)656-66 1 Siliehev, 0.0. Problem of stabilization of laser radiation parameters, Sov J Quantum Electron (USA), 13 (1983) 172-177 Rozanov, A.G. Nonlinear model of thermal effects in YAG : Nd laser crystals, Sov J Quantum Electron (USA). 21 (199 I) 1074- 1076 Hodgson, N., Weher, H. Influence of spherical aberration of the active medium on the performance of Nd : YAG laser, IEEE J Quant Elecrron, 29 (1993) 2497-2507 Khizhnyak, A.I., Lopiychuk, M.M., Peshko, 1. K. Lens transformations of high power solid-state laser beams (in press)

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