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Continuous wave dual rod Nd:YLF laser with dynamic tensing compensation Herman Vanherzeele

E. I. Du Pont de Nemours & Company, Ltd., Central Research & Development Department, Experimental Sta­tion, P.O. Box 80356, Wilmington, Delaware 19880-0356. Received 12 July 1989. Sponsored by George I. Stegeman, University of Arizona. 0003-6935/89/194042-03$02.00/0. © 1989 Optical Society of America. A cw pumped dual rod Nd:YLF laser with a dynamic

compensation of the thermally induced tensing effects is described. Compared with a conventional Nd:YLF laser, this system generates twice as much power with an in­creased stability and superior beam quality.

Recently, we have shown in a series of papers1-3 that Nd:YLF outperforms Nd:YAG in well designed oscillator and amplifier systems for generating short pulses with high peak power. The main advantages of Nd:YLF over Nd:YAG include (1) a larger bandwidth, which allows generation of pulses shorter by a factor of 2-3 and (2) a strong natural birefringence, which overwhelms the thermally induced bi­refringence and thus eliminates thermal depolarization problems. Radiation from Nd:YLF is naturally polarized: either σ(E ⊥ c) at 1053 and 1313 nm or π(E || c) at 1047 and 1320 nm. Henceforth we restrict the discussion to the 1053-nm transition. Although Nd:YLF is at least an order of magnitude more athermal than Nd:YAG, a recent study1 of the thermal lensing properties of a Nd:YLF rod revealed the presence of a strong astigmatism. This effect gives rise to elliptically distorted beam profiles both inside and outside the resonator, thereby reducing the maximum extractable TEM00 power from the rod. Earlier, we proposed a static compensation of the thermal lensing astigmatism by a cylin­drical intracavity lens, and we demonstrated that in this way one obtains about the same average mode-locked power from a Nd:YLF rod as from a Nd:YAG rod with the same dimen­sions: typically 10 W for a 4- × 79-mm rod cw pumped by a single krypton lamp (Quantronix 116 laser head).2 It is the purpose of this Letter to demonstrate a significantly im­proved Nd:YLF laser, for which a dynamic lensing compen­sation by two rods in tandem ensures twice as much output power with increased beam quality and better stability. Uses of this new high power source are also discussed.

In principle, one can follow different routes to increase the output power of a cw pumped system. One obvious way is to increase the pump power, e.g., by adding a second lamp to the laser head. Another method consists in using a longer rod. Both methods or their combination pose serious problems. Not only a costly redesign of power supply and/or laser head

MENT FROM THE OSA FELLOW (who in effect has served as the referee and whose sponsorship will be indicated in the published Letter), A COMMITMENT FROM THE AUTHOR'S INSTITUTION TO PAY THE PUBLICATIONS CHARGES, and the signed COPYRIGHT TRANS­FER AGREEMENT. The Letter will be published without further refereeing. The latest Directory of OSA Members, including Fellows, is published in the July 1988 issue of Optics News.

where P (kW) is the input power per rod and D (m) is the (effective) distance between the image principal plane of the first rod and the object principal plane of the second rod. In this manner one thus achieves a dual goal. The system is free from astigmatism for any input power, and in addition it has virtually zero dioptric power. The latter property is illustrated in Fig. 1, where the dioptric power of a single rod system is compared with that of the dual rod one (D = 20 cm) for a wide range of input power. In practice, one may find that each rod has a somewhat different thermal lensing. In that case, it is still possible to realize a fully compensating structure by balancing the respective pump power for each

4042 APPLIED OPTICS / Vol. 28, No. 19 / 1 October 1989

is involved, but in addition one expects a deterioration of the beam quality (the product of beam divergence and the spot size w) as well as a decrease in resonator stability. This is caused by the increase in thermal dioptric power (S = 1/ƒ) resulting from an increase in either pump power and/or length of the rod. Moreover, the poorer optical quality of Nd:YLF (compared to Nd:YAG) restricts the use of long rods. To avoid problems typical of increased thermal lens­ing power, Eggleston proposed the use of periodic resona­tors.4 For these resonators, the output power scales linearly with the number of periods. However, while the transverse eigenmodes are preserved in such a structure, the longitudi­nal modes are not, and this is a problem for mode locking. We built a dual rod system (two Quantronix 116 laser heads), although not a periodic one, with a cavity length matching our 100-MHz mode locker.

As shown below, the use of two Nd:YLF rods (at 1053 nm) allows one to design a system that is virtually free from thermal lensing. For a σ-polarized beam propagating along the a-axis of the rod, the thermal dioptric power S in the a-b plane has nearly the same magnitude as the one in the a-c plane but both differ in sign. For a 4- × 79-mm rod (σ-polarization)1

where P is the lamp input power in kW and S is in m -1. (To convert to lamp current for a Quantronix 116 system, one has to divide Pby 130 V.) The validity of Eq. (1) was checked for several rods and found to be good within ±25%. Equation (1) clearly reveals the origin of the thermal lensing astigma­tism in Nd:YLF. These equations also predict that in a dual rod system the thermal lensing of one rod can be (dynamical­ly) compensated by that of the other rod provided the c axes of both rods are orthogonal. A halfwave plate, sandwiched between the rods, is then required to maintain the proper polarization (E ⊥ c) in each rod. Assuming that two identi­cal rods are oriented in this way, the resulting dioptric power of this system (in both the a-b and a-c planes) is given by

Fig. 1. Dioptric power (m-1) for 4- × 79-mm Nd:YLF rods as a function of electrical input power (kilowatts) to the lamp; (a) σ-polarized light in the a-b plane (single rod); (b) σ-polarized light in the a-c plane (single rod); and (c) σ-polarized light for a dynamically

compensated dual rod system.

Fig. 2. Schematic representation of the Nd:YLF resonator dis­cussed in the text: HR, flat high reflector; OC, 12% output coupler (R = -120 cm); L, spherical lens (ƒ = 38 cm); ML, 100-MHz mode-locker; PH, pinhole; WP, halfwave plate; D1 = 47 cm, D2 = 21 cm.

rod. We examined both theoretically and experimentally sev­

eral dual rod resonator candidates to ensure (1) optimum rod filling in TEM 0 0 operation and (2) small beam divergence. While several configurations performed well, we picked the one shown in Fig. 2 for a number of practical reasons, includ­ing ease of maintenance (i.e., the possibility of replacing the lamp and rod in each Quantronix 116 head without the need to remove intracavity components) and good performance in the mode-locking regime. The 1.5-m long resonator consists of a flat high reflector and a — 1.2-m output coupler. The transmission of the output coupler is 12% (a standard value for Nd:YAG lasers). No attempt was made to optimize this transmission. A spherical lens (ƒ = 38 cm) located at 47 cm from the high reflector ensures a good filling of both 4- × 79-mm rods in TEM 0 0 operation: the average spot size (HW1/ e2M) in the rods is ≃1 mm. A pinhole is required to obtain the lowest order transverse mode. Both heads are mounted so that a symmetric pumping scheme is achieved (one lamp on either side of the cavity axis). No intracavity polarizer is needed to restrict lasing to 1053 nm since the cavity is unsta­ble for operation at 1.047 μm (because of the different ther­mal lensing for π-polarized radiation1). The output beam has an excellent Gaussian profile without ellipticity and is linearly polarized. The polarization direction is determined by the c-axis of the rod nearest the output coupler. The beam divergence is <1.5 mrad (FWl/e2M). The beam char­acteristics are independent of pump power as a result of the dynamic compensation of the thermal lensing effects. The output power as a function of electrical input power is shown in Fig. 3 and is typically twice that of our single rod cavity.2

Both the temporal and pointing stability of the system are superior compared to our single rod design. When harmoni­cally modulated, the mode-locking characteristics of the sys­tem (as a function of cavity length detuning) are similar to those of the single rod laser described in Refs. 2 and 3. In particular, the active stabilization schemes described in Ref. 3 work well in the dual rod laser without modification.

The high average power (typically >18 W) combined with the relatively short pulse duration (typically <45 ps FWHM) suggests many new applications for this Nd:YLF system. One example is the generation (in cascade) of the third harmonic at 351 nm with sufficient average power to syn­chronously pump a UV-VIS dye laser. Another example, for which we actually developed this new Nd:YLF laser, is the

Fig. 3. Average output power (watts) as a function of electrical input power (kilowatts) to each lamp. The dots represent the

experimental data.

generation of 100-MHz trains of continuously tunable pico­second pulses in the IR. For this application, ~50% of the laser output is frequency doubled in KTiOPO4 (KTP)5 to produce 2.5 W of green for pumping a synchronously mode-locked dye laser. Our dye laser is tunable with a birefringent filter from 560 to 1030 nm (using three sets of mirrors and five different dyes) and typically generates 2-4-ps (FWHM) pulses with 150-250-mW average output power over this wavelength range. The output of the dye laser (from 560 to 850 nm) is then collinearly mixed with the remaining 50% of the 1053-nm beam of the Nd:YAG laser in a 3-mm thick KTP crystal to produce the difference frequency. In this way, one obtains 2-4-ps (FWHM) IR pulses tunable from 1.2 to 4.5 μm (onset of an absorption band in KTP) with average power in the milliwatt range.6 This is more than 3 orders of magni­tude larger than previous results obtained with a similar setup7 and makes this scheme attractive as an alternative for color center lasers. Further amplification (at lower repeti­tion rates) of these picosecond pulses over the entire wave­length range of 560 nm-4.5 μm is possible in a KTP paramet­ric amplifier.8 '9 The latter is pumped by a Nd:YLF regenerative amplifier, seeded by a small fraction of the output of the cw mode-locked Nd:YLF laser. A more de­tailed description of this part of our laser system will be the subject of a separate paper.

In conclusion, a high power cw mode-locked Nd:YLF laser has been described. The key feature of this system is a dynamic thermal lensing compensation provided by the use

1 October 1989 / Vol. 28, No. 19 / APPLIED OPTICS 4043

of two rods with orthogonal c-axes and a halfwave plate sandwiched between them. The laser, which is virtually free from thermal lensing, typically generates twice as much pow­er as an optimized single rod laser. The high output power combined with the short pulse durations and the improved stability make this system suitable for many applications including nonlinear frequency mixing. Finally, it should be noted that our dynamic lensing compensation scheme also is applicable at 1313 nm. For the latter transition the gain is much lower than at 1053 nm, and, therefore, a boost in power resulting from a dual rod configuration will be beneficiary for many applications at this technologically interesting wave­length.

We would like to acknowledge the valuable technical assis­tance of J. Kelly. We also thank G. Meredith for supporting this work.

References 1. H. Vanherzeele, "Thermal Lensing Measurement and Compen­

sation in a Continuous-Wave Mode-Locked Nd:YLF Laser," Opt. Lett. 13, 369-371 (1988).

2. H. Vanherzeele, "Optimization of a cw Mode-Locked Frequency-Doubled Nd:LiYF4 Laser," Appl. Opt. 27, 3608-3615 (1988).

3. H. Vanherzeele, "Characterization and Active Stabilization of a Harmonically Modulated Continuous Wave Nd:LiYF4 Laser," Rev. Sci. Instrum. 60, 592-597 (1989).

4. J. M. Eggleston, "Periodic Resonators for Average-Power Scaling of Stable-Resonator Solid-State Lasers," IEEE J. Quantum Elec­tron. QE-24, 1821-1824 (1988).

5. F. C. Zumsteg, J. D. Bierlein, and T. E. Gier, "KxRb1-xTiOPO4: A New Nonlinear Optical Material," J. Appl. Phys. 47, 4980-4985 (1976).

6. H. Vanherzeele, "Generation of Tunable Infrared Picosecond Pulses at 100 MHz by Difference-Frequency Mixing in KTiO-PO4," Opt. Lett. 14, 728-730 (1989).

7. D. Cotter and K. I. White, "Picosecond Pulse Generation and Detection in the Wavelength Range 1200-1600 nm," Opt. Com-mun. 49, 205-209 (1984).

8. J. D. Bierlein and H. Vanherzeele, "Potassium Titanyl Phos­phate: Properties and New Applications," J. Opt. Soc. Am. B 6, 622-633 (1989).

9. H. Vanherzeele, "Recent Advances in the Generation of Picosec­ond Tunable Infrared Radiation," Proc. Soc. Photo-Opt. In­strum. Eng. 1104 (to be published), 000-000 (1989).

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