DIODE-PUMPED solid-state lasers are attractive light

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1 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 3, NO. 1, FEBRUARY Analytical and Experimental Studies on the Characteristics of Composite Solid-State Laser Rods in Diode-End-Pumped Geometry Masaki Tsunekane, Noboru Taguchi, Tadashi Kasamatsu, and Humio Inaba, Fellow, IEEE Abstract In this paper, we report analytical and experimental studies on the characteristics of end-pumped composite laser rods with undoped end, using mainly Nd:YAG rods as an example. It is found that the peak temperature rise in a composite rod decreases to <70% of that in a noncomposite crystal. Thermal stress is dramatically reduced to <60% by employing the composite rod structure. We also demonstrate high-power operation of the diode-end-pumped composite Nd:YVO 4 rod and a maximum CW output power of 9.3 W was achieved, which is about 1.5 times higher than that in the noncomposite rod. This high-power performance of the composite rod is primarily attributed to the reduction of thermal stress inside the rod. Index Terms Finite-element methods, integrated optics, neodymium:yag lasers, stress, thermal factor. I. INTRODUCTION DIODE-PUMPED solid-state lasers are attractive light sources for many applications because of the high brightness, high efficiency, high reliability, and compact size. These sources have not only simply replaced arc-lamp pumped systems, but they have often provided new capabilities such as much more stable single-frequency emission and much shorter -switched and mode-locked pulses. In recent years, many novel techniques have been developed to improve the output power and conversion efficiency of diode-pumped lasers. The most familiar pumping techniques are classified into endand side-pumping schemes. Both of these techniques have some advantages and disadvantages each other. The endpumping scheme offers the advantage over side-pumping of high efficiency and good beam quality due to the superior spectral and spatial overlap of pump beam and the resonator mode. Efficient, TEM mode, CW diode-end-pumped lasers at power levels up to 60 W have been developed. The most deleterious problem obstructing efficient highpower TEM mode operation is the thermal effect which results from quantum inefficiencies of deposited pump energy in an active medium. One important effect, referred to as thermal lensing, results from temperature-induced changes in the refractive index of a gain medium. Heating of a rod also leads to beam depolarization effects as a consequence of the Manuscript received November 11, 1996; revised December 19, M. Tsunekane and N. Taguchi are with Biophotonics Information Laboratories, Yamagata 990, Japan. T. Kasamatsu is with NEC Opto-electronics Research Laboratories, Kawasaki 216, Japan. H. Inaba is with Tohoku Institute of Technology, Sendai 982, Japan. Publisher Item Identifier S X(97)03760-X. thermal stress-induced birefringence. The thermal lensing and thermal stress-induced birefringence make the laser cavity unstable and cause significant optical losses in the system which requires linear polarization state because of the use of the brewster plate, electrooptical -switching device, nonlinear harmonic generation crystal, and others. The final and certainly the most fatal limitation on power scaling is thermal fracture. For a given rod and pump geometry, as the pumping power increases the thermal stresses increase as well which ultimately cause fracture of a rod at the maximum stress point. It is more difficult to reduce these stresses in an end-pumped geometry than in a side-pumped geometry where the axial dimension of the rod may be scaled linearly to achieve linear reductions in thermal stress. Recently, the concept of combining doped and undoped components for improving thermal uniformity and controlling the extent or location of laser active media is applied to solid-state laser rods or slabs. Bowman et al. [1] presented the first evaluation of composite Tm,Ho:YAG laser rods with undoped ends for pump input with high peak and average power operation at 2 m in a diode-side-pumped geometry. Their comparative experiments demonstrated that the output power was doubled and the amount of the thermal lensing effect reduced by 30% by the use of composite rods. Jani et al. [2] also demonstrated the operation of diode-side-pumped Ho,Tm:YLF lasers at pulse repetition rate up to 10 Hz using a composite laser rod with undoped ends. Such a composite laser rod of a three-level laser material in a side-pumped geometry allows pumping the full length of the doped section while avoiding ground state absorption losses in the unpumped ends of a laser rod. A composite laser rod in a diode-end-pumped geometry was first demonstrated by Hanson [3]. Efficient pulsed roomtemperature laser operation at 946 nm was achieved using a composite Nd:YAG rod end-pumped through the undoped end with multiple diode array bars. Significant improvements in power scaling were realized by the high heat extraction of the composite rod, which suppresses ground state absorption loss. At almost the same time, authors [4] also examined the new feature of composite laser rods and estimated the temperature rise in the composite laser rod in an end-pumped geometry. Recently, high-power operations using diode-end-pumped composite laser rods have been performed at Lawrence Livermore National Laboratory (LLNL). An Nd:YAG laser with a CW output power of 155 W using the composite rod was X/97$ IEEE

2 10 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 3, NO. 1, FEBRUARY 1997 Fig. 1. Concept of the composite laser rod structure in end-pumped geometry. The arrows show thermal flows. reported [5]. Beach et al. [6] of LLNL demonstrated a 2- m Tm :YAG laser system capable of generating 26 W of CW output power using the composite rod, and Bibeau et al. [7] of LLNL have applied the composite approach and coaxed 131 W of CW power from a diode-end-pumped Yb:YAG laser. Though these excellent experimental results were reported, few theoretical or analytical discussions of the characteristics of composite laser rods have been presented. In this paper, we first discuss analytical and experimental studies of the characteristics of composite laser rods, diffusion-bonded with an undoped end in an end-pumped geometry. The finite-element method was used to calculate the temperature distribution and stress in end-pumped laser rods. Composite laser rod samples were fabricated and tested to compare with the theoretical estimation. heat flow or the temperature distribution which constructs the thermal lens or thermally induced stress/birefringence in composite rods is quite different from that in face-cooled rods from the theoretical and experimental studies discussed below, and some characteristics of composite rods are rather similar to conventionally cooled rods. This is interesting and seems to be strange, but the following explanations will help understanding. The composite laser rods are cooled only at the edge of the rods. The undoped face (pumped face) is not cooled. Then all of the heat generated in the Nd-doped section is finally removed, only from the edge of the rod even if the heat flows through the undoped section. In these situations, the temperature distribution in the Nd-doped section is similar to that in face-cooled rods because the undoped section acts as an effective heatsink, but the temperature distribution in the undoped section is quite different from face-cooled rods because the heat, taken from the face of the Nd-doped section, is not removed from the undoped face and still remains in the undoped section until it diffuses to the edge of the rod. We emphasize that the temperature distribution in the undoped section strongly affects the thermal and optical characteristics of composite rod. In other words, the undoped section is inactive as a laser material but active thermally and optically. If the different host material, which is transparent at pumping and lasing wavelength and has much higher thermal conductivity and much lower than the laser material, could be used for the same undoped host material, the characteristics of the composite rod would approach to that of face cooled rods. II. CONCEPT OF END-PUMPED COMPOSITE LASER ROD Fig. 1 shows the concept of the composite laser rod in an end-pumped geometry. In this figure, a composite Nd:YAG rod is shown as an example, but other laser host materials and other doping species are also applicable. The thermal energy deposited in the pumped region of Nd:YAG diffuses not only to the radial direction of the rod, but to the axial direction, i.e., into an undoped region where there is no absorption of pump energy, in particular, the undoped section faces the hottest doped region near the end face which is exposed pump light with maximum intensity, then deposited thermal energy is diffused effectively to the cool undoped region. The decrease of temperature rise in a laser rod will lead to reduction of thermal effects which degrade the laser performance in highpower operation. In addition to this, as the optical coating on the pumped rod surface of the undoped section is unaffected by the thermal effects directly, it will be not suffered from damage due to the thermal expansion or deformation of the surface. Here we assume that the lasing medium and undoped cap are the same host crystal. Because the difference of refractivity can be ignored and the lattice constant and thermal expansion coefficient are almost the same, they will have little problem during the diffusion bonding process and reliability after bonding. The composite rods are expected to have the same characteristics and advantage of face-cooled ( active mirror ) rods exhaustively analyzed by Cousins [8], but we show that the III. SAMPLE PREPARATION The composite Nd:YAG laser rod in our experiments was fabricated by diffusion-bonding an cut 1.1 at.% neodymium doped YAG (5-mm-diameter, 1-mm-long) crystal to a cut undoped piece (5-mm-diameter, 3-mmlong), with precise alignment of the direction of both components. The segment endfaces were precisely polished to a surface flatness of, and after both the segments were optically contacted, it was heat-treated at a temperature a little below the crystal melting point to enhance bond strength. No inorganic or organic bonding aid or adhesive is used. Before the heat treatment, light scattering at the bonding interface could be observed using a He Ne laser beam as a probe, but after the heat treatment, it could not be observed by the naked eye. It seems that dangling bonds of both the YAG crystal surfaces at the interface are thermally activated and rearranged or recombined through the high temperature heating. The integral structure will result in efficient thermal diffusion through the interface comparing to simple contact or optical contact structure. As a control, nonbonded Nd:YAG rod with cut 1.1 at.% neodymium doping (5-mmdiameter, 1-mm-long) is used. Endfaces of both the control and composite Nd:YAG rods are antireflection-coated to have 0.1% reflectivity at 1064 nm and 0.5% at 810 nm. We performed a comparative analysis and experiment by using these two rods in this paper. In the composite rod, the 3-mm-long undoped YAG will be enough to remove the heat generated in the thin active segment

3 TSUNEKANE et al.: STUDIES ON THE CHARACTERISTICS OF COMPOSITE SOLID-STATE LASER RODS 11 (a) (b) Fig. 2. (c) (d) Calculated temperature rise in half an axial cross section of diode-end-pumped (a) composite Nd:YAG and (b) noncomposite Nd:YAG rods. of the 1-mm-long Nd-doped one and reduce the temperature rise effectively. In addition to this, under the condition of the pump beam diameter being almost constant in the thin active segment, experimental values will agree with the theoretical results. So we consider this to be a suitable sample to characterize the thermal properties of a composite structure. The basic characteristics of the composite rod discussed in this paper will be almost common to composite rods with another dimensions of the active medium. IV. CHARACTERISTICS OF COMPOSITE STRUCTURED Nd:YAG LASER RODS A. Temperature Rise In Fig. 2, we show the calculated temperature rise in half an axial cross section of the cylindrical laser rod of the composite and control Nd:YAG rods using the finite-element method [4]. In this calculation, 10 W of optical pumping light is assumed to illuminate uniformly on a 1-mm-diameter area of the center of composite and control rods with an absorption coefficient of 5 cm. With the use of fiber-coupled laser diodes, as described in the next section, the pumping intensity distribution can be approximately constant over the central portion of the rod. Thermal conduction coefficient is assumed to be 13 W/m/K. More over, we made an assumption that 30% of absorbed pump power is converted to the heat in the laser media and temperature of the rod edge is clamped to be 300 K. It is seen in Fig. 2(a) that the effective heat diffusion is achieved in the composite rod pumped through an undoped end. While the peak temperature rise in the control rod is 32 C as shown in Fig. 2(b), the composite rod exhibits the temperature rise of 22 C. Significant heat flow through the end face of the doped section to the undoped section decreases temperature rise to less than 70% when compared to that of a nonbonded Nd:YAG rod. From these results, it is confirmed that an undoped end section acts a very effective heat diffuser and can dramatically reduce temperature rise in a active medium. We have confirmed the validity of our finite-element calculations by comparing with the calculated temperature distributions using other methods. A good agreement is achieved between the calculated results by the finite-element method and by analytic method or finite difference method when we calculated the same samples. Fig. 3 shows calculated peak temperature rise in the composite rods as a function of undoped YAG crystal length. The rod diameter, 3 and 5 mm, and Nd:YAG crystal length, 1 and 5 mm, which are commonly used dimensions, are selected as parameters. The pumping condition is the same as in Fig. 2. As shown in this figure, at any dimensions, undoped YAG length of 1 mm is sufficient to decrease peak temperature rise to less than 70%, which is almost constant at its minimum, compared to the temperature rise of control rods at the undoped YAG length of zero. As the composite rod fabricated for the experiments has 3-mm length of the undoped YAG, it is sufficient to suppress the temperature rise inside the rod. We compared experimentally the temperature rise in both the composite and control rods. The result is shown in Fig. 4 where the rod circumference (edge) temperature dependence

4 12 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 3, NO. 1, FEBRUARY 1997 Fig. 3. Calculated peak temperature rise in composite Nd:YAG rods as a function of undoped crystal length. The parameters are the rod diameter and length of Nd:YAG crystal. (a) Fig. 4. Comparison of rod circumference temperature dependence of emission peak ( nm) intensity between composite and noncomposite Nd:YAG laser rods. of emission (fluorescence) peak intensity at nm is indicated under the same pumping configuration. The temperature was controlled and changed by a thermo-electric cooler. Laser rods were singly end-pumped by CW fiber-coupled laser diodes (OPC-030-mmm-FC, Opto Power Corporation). The fibers were drawn into round bundles of 1.55-mm diameter and a numerical aperture of The focused pump-spot radius was 320 m using a lens of 60-mm focal length. In this measurement, the composite rod was pumped from the Nd:YAG side and the emitted light was monitored from the same side using an optical spectrum analyzer, because accurate comparison of temperature rise in both the rods requires the coincidence of pump volume, intensity, and an optical path from the activated region to the detector. The input pump power is increased to 22 W to obtain the appreciable difference of the emitted light intensity between the rods. In this figure, the emission intensity of the control rod at circumference temperature of 20 C is almost equal to that of the composite rod at 60 C. This means that the temperature rise in the composite rod is lower by approximately 40 C than that in the control rod. This value is twice higher than the calculated one using the finite-element method with the pump power of 22 W. We consider that this difference is attributed to the spot size of the pump beam. The tightly focused pump beam used in the experiments will cause a higher temperature rise in laser rods than in the calculations in which we assumed 500- m spot size. (b) Fig. 5. Input output characteristics of (a) the composite Nd:YAG and (b) the noncomposite Nd:YAG rods. From these results, we confirmed that the composite structure can reduce the peak temperature rise in an active media to 70% or less of that of a noncomposite rod. This rod structure is also expected to improve the laser performance of the three-level laser materials such as Ho,Tm:YAG, Yb:YAG, and Nd:YAG lasing at 946 nm, or chromium-doped materials such as Cr:LiSAF and Cr:Forsterite. These materials have the lasing characteristics which are strongly dependent on the crystal temperature. B. Thermal Lensing From the calculated temperature profiles in Fig. 2, we can estimate an optical pass difference in these rods which indicates the change of integrated optical pass length of the rod to the axial direction due to temperature rise. The calculated optical path difference of the composite rod is shorter than that of the control rod, but the difference is only 1% or less even at the center of the rod [9]. The distribution of the optical pass difference offers the magnitude of the thermal lens of the rod. Then the difference of 1% or less of the optical pass difference between the composite and control rods indicates that the magnitude (focal length) of the thermal lens is almost identical between the rods. The reason, we think, is that the undoped section of the composite rod diffuses the heat generated in a doped section mainly to the axial direction. In Fig. 5, the input output characteristics of the composite and control rods are shown. In these figures, an absorbed pump power in a rod is indicated in abscissa. The absorbed pump power is determined as the difference between the input pump power and the transmitted pump power which is not

5 TSUNEKANE et al.: STUDIES ON THE CHARACTERISTICS OF COMPOSITE SOLID-STATE LASER RODS 13 absorbed. The thickness of the active Nd-doped section in these Nd:YAG rods we used is only 1 mm, then a large amount of pump power (about 60% or higher of the input power) transmits through the crystal and does not contribute to the performance at all. Addition to this, the absorption ratio significantly changes with the pumping wavelength, i.e., diode laser drive current. It causes large aberration in the estimation of the laser performance. Then we measured the input and transmitted pump power and calculated the absorbed pump power at each input pump power point for proper estimation of the input output characteristics. The absorbed pump power of 8 W corresponds to the input pump power of 22 W from the fiber out. These lasing experiments were performed with a linear standing-wave cavity configuration. Laser rods were singly end-pumped by CW fiber-coupled laser diodes mentioned previously, through a concave highreflection mirror at 1064 nm (1-m radius-of-curvature). The output coupler (95% reflectivity at 1064 nm) is a flat mirror. Three cases of cavity length of 100, 150, and 200 mm are illustrated in both the figures. Threshold pump power and slope efficiency are almost identical between the composite and control rods. This fact appears to demonstrate that the optical loss at the interface of the composite rod is considerably low. Output power over 5 W is obtained at an absorbed pump power of 8 W in the cases of 100- and 150-mm cavity length. But in the case of 200-mm cavity length, saturation of output power is observed in both the rods because a focal length of thermal lens becomes comparable to the cavity length. The absorbed pump power, at which output power begins to saturate, is a little higher in the composite rod than that for the control one. At pump power of 8 W, the output power of the control rod nearly stops oscillating, whereas the composite rod still exhibits oscillating behavior. This result shows that the thermal lensing effect is a little improved by employing a composite structure. Fig. 6 shows the measured focal length of both the rods as a function of absorbed pump power. The focal length is estimated experimentally from the cavity length at which the output power begins to saturate. The focal length of the composite rod is approximately 10% longer than that of the control rod in this pump power range. We have confirmed the estimated focal length of the thermal lens by stability of the cavity is sufficiently accurate to within 10% from the reproducibility of our experimental results. We think that the formed thermal lens could be thought to be an almost ideal thin lens because the rods are thin itself and are pumped from the end face and the cavity length is much longer than the length of the rods. The rods are situated very close to one of the cavity mirrors to reduce the aberration of the evaluated focal length in the stability experiments. The amount of the measured improvement in thermal lensing is much larger than that estimated theoretically. It is considered that this is due to the thermal distortion near the end of the rod which we did not take into account in the calculation. The end effect will be discussed in the later section. We also measured values of the laser output. As the pump power increases, the value increases proportionally. The value of the control rod at the absorbed pump power Fig. 6. Measured focal length of the thermal lens of the composite and noncomposite Nd:YAG laser rods as a function of absorbed pump power. of 8 W was 3.5, while that of the composite rod was 20% lower. This advantage of the composite rod was unchanged even at the lower pump power. The improvement of the transverse mode quality might be due to the reduction of thermal lensing effect or also be due to the change in mode overlap between the pump beam and the laser mode. It is important to keep an of in experiments for their future practical applications, but we emphasize that the purpose of this paper is to characterize the composite laser rods by comparison with noncomposite conventional rods and to estimate the advantage in the laser performance as a first report. We think that the basic characteristics and advantage of composite rods estimated from the theoretical and experimental results presented in this paper will be unchanged qualitatively and we believe that the quantitative results will be also important to suggest the possibility of composite rods as a promising structure for high-power solid-state lasers, even if an of is not accomplished. Here we can conclude that the thermal lensing effect is reduced and the output performance is improved by the composite rod with undoped end. However, the amount of the improvement is 10% or less, which seems not so significant, as estimated from the calculations of optical path difference. C. Thermally Induced Birefringence Thermally induced birefringence is known to be caused by the photoelastic effect of thermal stress. The refractive index change is associated with the temperature gradient inside the material. Theoretically, both the thermal lensing and thermally induced birefringence are a function of heat generation ratio per unit volume [9]. The former is inversely proportional to the heat generation ratio, and the latter is proportional to that ratio. As discussed before, the composite rod is almost equivalent to the control rod in the amounts of the thermal lensing effect. Accordingly, the amounts of thermally induced birefringence are also inferred to be almost the same in both the rods. Fig. 7 shows the experimental test of thermally induced birefringence in the composite and control rods. In these measurements, the end-pumped rod is placed between two polarizers arranged perpendicular to the crystal axis (crossed polarizers) and a linearly polarized He Ne laser beam is used as a probe for the detection of the amount of birefringence. As seen in these figures, the contrast and pattern of transmitted

6 14 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 3, NO. 1, FEBRUARY 1997 (a) (b) Fig. 7. Comparison of thermally induced birefringence in (a) the composite Nd:YAG and (b) the noncomposite Nd:YAG rods. light monitored by a CCD camera are almost same for both the rods, so that the amounts of thermally induced birefringence is considered to be equal. Future possibility of combination between different host materials, such as a laser medium and a transparent substrate which has larger thermal conductivity or smaller, will provide far more significant improvement in thermal lensing and thermally induced birefringence compared to a noncomposite laser rod. D. Stress Fracture The stress fracture of a crystal occurs at the point where thermal stress exceeds its own fracture limit. Thermal stress is a function of the temperature gradient and is almost proportional to the pump power. As discussed previously, the composite structure can reduce the temperature rise effectively, then significant improvement of the limitation of thermal stress and pump input power by using a composite rod will be achieved. Fig. 8 indicates the calculated thermal stress in end-pumped composite and noncomposite Nd:YAG rods by the finiteelement method. The numbers in the figure show the sum of the calculated stress vectors in three axial directions. Pumping condition of 10-W input power is assumed to be the same as that in Fig. 2. As shown in this figure, the maximum stress in the composite rod is reduced to 60% of that in the control rod, due to the decrease of temperature gradient. The maximum stress occurs at the center of the rod in the control rod, whereas it occurs in a ring shape in the composite one. Fracture stress of Nd:YAG is known to be ˆ9 dyne/cm [10]. It is about five times larger than the calculated stress maximum in the control rod and nine times larger than that in the composite rod at the same input pump power. Then the composite rod is estimated to work up to 90 W. More improvement of the maximum pump power can be achieved by optimizing the pumping design, especially by broadening the spot size. We also calculated the distortion of the flatness at the rod end due to the thermal stress. The results show that the displacement difference between the rod center and the edge on the pumped end surface in the composite rod is reduced to 30% of that in the control rod. We think that this dramatic improvement of the end effects contributes mainly to the improvement of thermal lensing in the composite rod. The effect of thermal lensing consists of bulk effects due to temperature/stress-dependent variation of the refractive index and end effects due to the physical distortion of the flatness of the rod ends. The latter end effects constitute the minor contribution of the thermal lensing. Then the 70% dramatic improvement of the end distortion contributes only a 10% improvement of the thermal lensing as a whole. The reduction of the distortion will also prevent the coating on the rod surface from failing. The reduction of thermal stress in a composite rod using a Nd:YVO, as a sample which has lower capability of the pump power, will also be described in the next section. V. CHARACTERISTICS OF COMPOSITE Nd:YVO LASER ROD Nd:YVO is one of the efficient laser host crystals currently used for diode-pumped solid-state laser [11]. It has a large emission cross section, approximately four times that of Nd:YAG, and a high absorption coefficient over a broad pumping wavelength bandwidth. Owing to these properties, highly efficient single-frequency operation of diode-pumped miniature Nd:YVO lasers has been successfully demonstrated [12], [13]. Recently, multiwatt diode-pumped intracavitydoubled green lasers have been developed using Nd:YVO as a laser crystal [14], [15]. However, the thermal conductivity of Nd:YVO is approximately half that of Nd:YAG, and a large amount of thermal energy converted from absorbed pump power is accumulated near the pumped crystal surface. Then large temperature gradient and, consequently, a large amount of stress are produced at the pump end of the crystal. Highpower operation is, therefore, not feasible owing to the crystal fracture by thermal stress W of TEM output is to date the highest power achieved using Nd:YVO with a doubly

7 TSUNEKANE et al.: STUDIES ON THE CHARACTERISTICS OF COMPOSITE SOLID-STATE LASER RODS 15 (a) Fig. 8. (b) Calculated thermal stress in (a) the composite Nd:YAG and (b) the noncomposite Nd:YAG rods. end-pumped design [16]. If the thermal stress in the crystal could be reduced, Nd:YVO would become a more promising laser crystal at higher power range. We have first tried to improve the thermal stress by employing the composite rod structure of Nd:YVO [17]. The composite laser rod was fabricated by diffusion-bonding axis cut 1.1 at.% neodymium-doped YVO (3-mm diameter, 3-mm length) crystal to the same size -axis cut undoped piece, with precise alignment of the axis of both the components. The noncomposite (control) Nd:YVO rod was axis cut 1.1 at.% neodymium-doped (3-mm diameter, 3-mm length). It is much harder to fabricate Nd:YVO rods than blocks but a block of Nd:YVO would have a problem at the fabrication process of composite rods. The degradation of the surface flatness near the corner is unavoidable at polishing if a block is used. We are afraid that incomplete face-toface bonding at the corner of the composite blocks degrades the performance. But we think if the composite blocks can fabricated well, it is interesting because they must have a different stress distribution from the composite cylindrical rods. We estimated the temperature rise in the same pumping condition in Fig. 2 using the finite-element method. Peak temperature rise in the control Nd:YVO rod is calculated to be 170 C, which is five times higher than that of Nd:YAG control rod with the same doping density and size. Such a large temperature gradient causes strong stress in the laser rod and thereby large stress-induced optical distortion. In the contrast, the peak temperature rise in the composite Nd:YVO rod is

8 16 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 3, NO. 1, FEBRUARY 1997 Fig. 9. Input output characteristics of the composite and noncomposite Nd:YVO 4 laser rods. estimated as low as 110 C. Significant heat flow through the end face of the doped section to the undoped section decreases the temperature rise to 70% when compared to that of a nonbonded commercially available Nd:YVO rod [18]. The anisotropic thermal conductivity and of YVO suggests that it should not be cooled isotropically. We think that the evaluation of the effect of the anisotropic cooling for the performance must be next subject. The lasing experiments were performed with the endpumping configuration of linear standing-wave cavity. Fig. 9 shows the input output characteristics of diode-pumped composite and control Nd:YVO rods. Laser rods were singly end-pumped by CW fiber-coupled laser diodes through a concave high-reflection mirror (1000 mm radius-of-curvature, 99.7% at 1064 nm). The output coupler ( 5% at 1064 nm) is a flat mirror. Cavity length was fixed to as short as 50 mm to prevent thermal focusing from affecting the laser performance in high pumping power region. Laser rods were antireflection-coated for 1064 and 810 nm on both end surfaces. As seen in this figure, the threshold pump power and slope efficiency of both the rods are almost identical. This fact means that the interface between the doped and undoped rod sections do not appreciably contribute to loss in the laser performance due to its scattering or reflection. As an aperture for transverse mode control was not inserted in the cavity, the laser was operated in multi-transverse mode ( 4). Maximum CW output power of 9.3 W was obtained in the composite Nd:YVO rod. It is about 1.5 times higher than that in the control rod which emitted the maximum power of 6 W. To our knowledge, this is the highest CW output power achieved to date with a singly end-pumped Nd:YVO. The optical conversion efficiency at the 9.3 W is 46%. In both the rods, however, once the pump power reached the maximum, the output power rapidly dropped and output characteristics were not reproducible on decreasing pump power. Then after the measurements, we inspected the crystals carefully. Fig. 10 shows the photographs of Nd:YVO rods with and without composite structure. In the composite Nd:YVO rod, several small cracks on the diffusion-bonded interface roundly positioned near the circumference were observed. On the other hand, a large dislocation which runs from rod center to the circumference and perpendicular to the axis of the crystal was observed in the noncomposite rod. (a) (b) Fig. 10. Photographs of (a) the composite Nd:YVO 4 and (b) the noncomposite Nd:YVO 4 rods after fracture. Our results suggests that the maximum laser output power is limited by stress fracture of laser rods and improvement of laser performance of the composite rod is mainly due to the decrease of thermal stress. Further improvement will be achieved by optimization of pumping design including pumping spot size and cavity configuration or by doubly end-pumping, such as performed in [16].

9 TSUNEKANE et al.: STUDIES ON THE CHARACTERISTICS OF COMPOSITE SOLID-STATE LASER RODS 17 VI. CONCLUSION Analytical and experimental studies on the characteristics of composite laser rods in diode-end-pumped geometry were reported in detail using mainly Nd:YAG rods as an example. The undoped end section is found to act as an effective heat diffuser and then peak temperature rise decreases to 70% compared with that of the noncomposite crystals. Thermal lensing is improved due to reduction of distortion of the flatness on the rod surface. Thermal stress is demonstrated to be dramatically reduced to 60%. We have also confirmed the high-power operation of a diode-end-pumped composite Nd:YVO rod and its maximum CW output power of 9.3 W, which is about 1.5 times higher than that in the noncomposite rod. This high-power performance of the composite rod is attributed to reduction of the thermal stress. The composite structure can improve the fracture power of the whole solid-state laser rods and spread the application area to higher power range with easy modification (diffusion bonding). Especially the improvement of laser performance of recently developed laser materials, for example, Yb:YAG, Tm,Ho:YAG, etc., which have high sensitivity for operating temperature is thought to be significant because composite structure can also reduce the temperature rise in the rods effectively. Then we believe that it is a hopeful technique for high-power solid-state lasers. in Extended Abstracts, 43rd Spring Meet. Jpn. Soc. Appli. Phys., 1996, paper 27p-ZH-2 (in Japanese). [10] W. Koechner, Solid-State Laser Engineering, 3rd ed. New York: Springer-Verlag, [11] R. A. Fields, M. Birnbaum, and C. L. Fincher, Highly efficient Nd:YVO 4 diode-laser end-pumped laser, Appl. Phys. Lett., pp , [12] T. Sasaki, T. Kojima, A. Yokotani, O. Oguri, and S. Nakai, Singlelongitudinal operation and second-harmonics generation of Nd:YVO 4 microchip lasers, Opt. Lett., vol. 16, pp , [13] T. Taira, A. Mukai, Y. Nozawa, and T. Kobayashi, Single-mode oscillation of laser-diode-pumped Nd:YVO 4 microchip lasers, Opt. Lett., vol. 16, pp , [14] W. L. Nighan and J. Cole, Jr., >6 W of stable, 532 nm, TEM 00 output at 30% efficiency from an intracavity-doubled, diode-pumped multiaxial mode Nd:YVO 4 laser, in Advanced Solid-State Lasers, OSA Tech. Dig.. Washington, DC: Opt. Soc. Amer., 1996, paper PD4. [15] M. D. Selker, T. J. Johnston, G. Frangineas, J. L. Nightingale, and D. K. Negus, >8.5 Watts of single frequency 532 nm light from a diode pumped intra-cavity doubled ring laser, in Conf. Lasers and Electro-Optics, OSA Tech. Dig., Washington, DC, 1996, paper CPD21. [16] W. L. Nighan, Jr., D. Dudley, and M. S. Keirstead, Diode-bar-pumped Nd:YVO 4 lasers with >13 W TEM 00 output at >50% efficiency, in Conf. Lasers and Electro-Optics, OSA Tech. Dig., Washington, DC, 1995, paper CMD5. [17] M. Tsunekane, N. Taguchi, and H. Inaba, High power operation of diode end-pumped Nd:YVO 4 laser using the composite rod with undoped end, in 16th Annu. Meet. Laser Soc. Jpn., Tech. Dig., 1996, paper 25a VII 3 (in Japanese). [18], High power operation of diode-end pumped Nd:YVO 4 laser using composite rod with undoped end, Electron. Lett., vol. 32, pp , ACKNOWLEDGMENT The authors gratefully acknowledge the expert technical assistance provided by the Aoshima brothers at NAMU TECH Corporation in Nakano-ku, Tokyo, Japan, in fabrication of the laser rods with the undoped diffusion bonded end, and they also thank Y. Kuwano of NEC Corporation for helpful discussions about the optimum diffusion-bonding condition of Nd:YVO. REFERENCES [1] S. R. Bowman, J. G. Lynn, S. K. Searles, B. J. Feldman, J. M. McMahon, W. Whitney, C. Marquart, D. Epp, G. J. Quarles, and K. J. Riley, Power scaling of diode pumped 2 micron lasers, in IEEE Lasers and Electro- Optics Society 1993 Annu. Meet., San Jose, CA, Nov , 1993, paper SSL2.2. [2] M. G. Jani, N. P. Barnes, K. E. Murray, and G. E. Lockard, Diodepumped, long pulse length Ho:Tm:YLF 4 laser at 10 Hz, in Advanced Solid State Lasers, OSA Tech. Dig.. Washington, DC: Opt. Soc. Amer., 1995, pp [3] F. Hanson, Improved laser performance at 946 and 473 nm from the composite Nd:Y 3 Al 5 O 12 rod, Appl. Phys. Lett., vol. 66, pp , [4] M. Tsunekane, M. Ihara, N. Taguchi, and H. Inaba, Improvement method of the thermal conduction by composite structure of Nd:YAG crystals, Extended Abstracts, 56th Autumn Meet. Jpn. Soc. Appl. Phys., 1995, paper 27a-D-3 (in Japanese). [5] S. G. Anderson, Diode-pumped Nd:YAG laser delivers 155 W (column of diode pumping), Laser Focus World, p. 18, Nov [6] R. J. Beach, S. B. Sutton, J. A. Skidmore, and M. A. Emanuel, High power 2 m wing-pumped Tm:YAG laser, in Advanced Solid State Lasers, Washington, DC, 1996, paper ThE2. [7] C. Bibeau and R. Beach, CW and Q-switched performance of a diode end-pumped Yb:YAG laser, presented at the IEEE Conf. Lasers and Electro-Optics Soc., Washington, DC, 1996, paper CDP-23. [8] A. K. Cousins, Temperature and thermal stress scaling in finite-length end-pumped laser rods, IEEE J. Quantum Electron., vol. 28, pp , [9] M. Tsunekane, N. Taguchi, and H. Inaba, Improvement method of the thermal conduction by composite structure of solid-state laser crystals, Masaki Tsunekane was born in Takamatsu, Japan, in He received the B.E. and M.E. degrees in electrical engineering from Nagoya Institute of Technology, Nagoya, Japan, in 1984 and 1986, respectively. In 1986, he joined NEC Corporation, Kawasaki, Japan, and had engaged in the development of highpower individually addressable AlGaAs laser diode arrays as optical disk light sources for parallel, high data transfer. In 1992, he had engaged in the development of diode-pumped intracavity-doubled Nd:YVO 4 compact green lasers for optical disk light sources. In 1994, he joined Biophotonics Information Laboratories, Yamagata, Japan, and has been working on the research and development of CW single-frequency, all-solidstate tunable lasers for biomedical applications. Mr. Tsunekane is a member of the Japan Society of Applied Physics and the Laser Society of Japan. Noboru Taguchi was born in Tokyo, Japan, in He received the B.E. and M.E. degrees in electrical engineering from Tohoku University, Sendai, Japan, in 1963 and 1965, respectively. In 1965, he joined NEC Corporation, Kawasaki, Japan, where he engaged in the development of electron tubes, gas lasers, and solid-state lasers. In 1993, he joined Biophotonics Information Laboratories, Yamagata, Japan, and has been working on the development of all-solid-state tunable lasers. Mr. Taguchi is a member of the Japan Society of Applied Physics and the Laser Society of Japan.

10 18 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 3, NO. 1, FEBRUARY 1997 Tadashi Kasamatsu was born in Osaka, Japan, in He received the B.E. and M.E. degrees in electrical engineering from Kyoto University, Kyoto, Japan, in 1990 and 1992, respectively. In 1992, he joined NEC Corporation, Kawasaki, Japan, and had been working on the research and development of a narrow bandwidth ArF excimer laser injection-locked by a fourth harmonic seed source of Ti:sapphire laser for lithography light sources for next generation. He is now working on the research and development of high power, diodepumped solid-state lasers. Mr. Kasamatsu is a member of the Japan Society of Applied Physics and the Laser Society of Japan. Humio Inaba (SM 65 F 85) was born in Tokyo, Japan, in He received the B.S. degree in geophysics and the Ph.D. degree in applied physics from Tohoku University, Sendai, Japan, in 1951 and 1962, respectively. He joined the Research Institute of Electrical Communication, Tohoku University, in 1957, was appointed a Professor of Quantum Electronics in 1965, and the Director of the Institute in In 1992, he became a Professor Emeritus of Tohoku University and moved to the Tohoku Institute of Technology, where he is a Professor of Electronics. He is also a Member of the Board of Directors at Biophotonics Information Labs., Inc., Yamagata, Japan. From 1961 to 1962, he was invited by Stanford Electronics Laboratories, Stanford University, Stanford, CA, working on lasers and optical heterodyning. His current research interests include laser science and technology, photonics, laser radar and optical remote sensing, laser imaging and processing, ultraweak biophoton phenomena, and biophotonics with novel applications of lasers. From 1986 to 1992, he served concurrently as the Director of INABA Biophoton Project administered by the Research Development Corporation of Japan (JRDC) in the Exploratory Research for Advanced Technology (ERATO) Program. He has served as chairman on numerous organizing and program committees for major international conferences, including the 6th and 17th International Laser Radar Conferences (1974 and 1994), the 4th IOOC (1983), and the 16th IQEC (1988). Dr. Inaba has received over twenty prizes and awards for his original contributions and achievements in the fields of quantum and optical electronics and biophotonics, which include the Matsunaga Prize (1972), the Shimazu Prize (1985), the Excellent Paper Awards from IEICE of Japan (1989 and 1992), the Toray Science and Technology Prize (1991), the Award from the Minister of Science and Technology Agency of Japan (1991), and the Medal with Purple Ribbon from the Prime Minister of Japan (1994) for outstanding contributions to science and the culture of Japan. He is a fellow of the Optical Society of America and a member of the Engineering Academy of Japan.