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100-megawatt Q-switched Er-glass laser

John Taboada*, John M. Taboada*, David J. Stolarski+, Justin J. Zohner+, Lucas J. Chavey+, Harvey M. Hodnett+, Gary D. Noojin+, Robert J. Thomas#,

Clarence P. Cain+, and Semih S. Kumru#

* Taboada Research Instruments, Inc., 1923 N. New Braunfels, San Antonio, TX 78208

+ Northrop Grumman, 4241 Woodcock Dr., Suite B-100, San Antonio, TX 78228-1330

# U. S. Air Force AFRL/HEDO, Brooks City-Base, TX 78235-5278

ABSTRACT

A very high energy Q-switched Er-glass laser is reported. We incorporated a rotating resonant mirror-Porro cavity reflector optical arrangement to achieve very high shutter speeds on the cavity Q of a laser designed for energetic, flashlamp-pumped 600-µs 1540-nm pulses. Reproducible 3.75-J, 35-ns, 1533-nm laser pulses were obtained at a repetition rate less than 1 minute. Our work shows that reliable, very high energy, Q-switched, Er-glass laser pulses at 1533 nm can be generated mechanically with no apparent damage to laser cavity components. We demonstrate the applications of this “eye safe” wavelength to energetic processes such as LIBS and materials processing. The la ser could also serve as a new tool for bioeffects studies and targeting applications.

1. INTRODUCTION

Substantial progress has been made in the development of so called “eye safe” Er-glass lasers operating at a wavelength of 1540 nm for application in laser range finding [1-5]. Short range-finding applications usually require laser pulses of a few mJ and the term “eye safe” is appropriate as this radiation is absorbed by the hydrated tissue of the anterior eye segment and does not penetrate to the more sensitive retina. The extinction coefficient at 1540 nm in water is 11.83 cm-1 , implying that a beam would experience an attenuation of about 5x10-11 through the ocular media which is primarily water [6]. Higher laser fluences, which could expand the range of applications of this Er-glass laser, have been made possible through the development of improved laser gain media such as the co-doping of the glass with Erbium and Ytterbium and increased glass resiliency against fracture [7]. Also, several Q-switching methods [2,3,8] have been developed to perfect the operation of the mJ-energy, “eye safe” lasers, but these have not been applied to a possible joule-energy-level device. Our work has extended for the first time, Q-switching methods into the multi-joule level resulting in 100-megawatt pulses at 1533 nm. The laser has been used to explore a new regime for laser-biological material interaction [9]. A well known approach for Q-switching solid state lasers is the use of rapidly rotating Porro prisms as a cavity reflector [10]. This requires rotational rates of approximately 24,000 rpm to operate in the fast Q-switch regime where the occurrence of secondary pulses is reduced or eliminated. Optimum Q-switch design also points to maximizing the logarithmic signal gain-to-round-trip-loss ratio to achieve the highest energy output. To accomplish this requires maintaining low losses and large apertures in the cavity as well as short cavity lengths.

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2. EXPERIMENTAL

The present approach to develop a high-power, Q-switched, Er-glass laser was to modify an existing laser already optimized for producing very energetic flashlamp pumped, free running pulses. A convenient system was the 77-J, Er-glass laser manufactured by Megawatt Laser, Inc. This system was comprised of a 10-mm diameter by 200 mm cylindrical Er-Yb-doped and chemically-strengthened laser rod produced by Kigre and pumped by two linear flashlamps (model QS/ER). The laser emitted free-running pulses with energy as high as 77 J in 1200 µs from a basic plano-concave resonator. We combined this laser system with a cavity modification to include a novel rotating mirror-Porro prism rear reflector in the resonator as shown in Figure 1.

Figure. 1. Schematic of 100-megawatt, Q-switched, Er-glass laser

With this resonator design, we obtained mechanical spindle wobble compensation and at the same time derived a factor of 2 increase in the effective cavity Q-switch angular speed. That is, spindle speed need only be one-half the typical speed of a simple Porro rotating rear reflector. Good Q-switching results were obtained with a 200-Hz rotation rate which represented a simple Porro reflector rotation rate of 24,000 rpm. This also allowed a reduction in the rotor mass and an increase in the cavity limiting apertures associated with rotating a relatively larger 19-mm diameter mirror instead of a Porro prism of comparable aperture. In Figure 1, a signal from the cavity reflector servo controller triggered by the servo motor Hall sensor synchronized the flashlamp driver through a time-delay generator to excite two flashlamps with approximately 400 J each for a fixed 1.5-ms flash duration. The cavity length was approximately 70 cm. The laser output energy per pulse and duration were observed with variations in lamp energy, flashlamp timing and rotating reflector angular rate.

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3. RESULTS AND DISCUSSION

Initially, the laser flashlamp pump energy was set to 600 J per lamp, a value that would normally yield approximately 20 J in a free-running pulse. The cavity reflector rotation rate was set at 200 Hz. With these initial conditions, a number of pulse characteristics were observed as the flashlamp timing was varied over 5 ms, the rotating reflector rotational time period. For example, at 6.5-ms time delay from the Hall sensor reference pulse, a double pulse was observed with a relaxation period of approximately 200 ns as shown in Figure 2.

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Figure. 2. Laser pulse oscilloscope trace for delay at 6.5 ms. Angular rate: 200 Hz , Trace speed: 100ns/division.

The observed pulse duration for the first pulse was approximately 36 ns. Setting the flashlamp time delay to 2.1 ms and placing the cavity alignment event close to the front end of the flashlamp pulse interval, yielded reproducible single pulses as shown in Figure 3. The pulse duration was approximately 37 ns. From the oscilloscope trace, a certain asymmetry is noted indicating that the effective Q-switch rotation speed may still be lower than optimum. At a time delay of 2.1 ms and rotation rate of the cavity reflector of 200 Hz, a study of the pulse energy versus pumping energy is shown in Figure 4.

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Figure. 3. Laser pulse oscilloscope trace for delay at 2.1 ms. Angular rate: 200 Hz, Trace speed: 50ns/division.

Q-Switch Output vs. Total Energy Input

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Figure. 4. Output Q-switch pulse energy versus total flashlamp pump energy. Rotating reflector angular rate

200 Hz, flashlamp delay 2.1 ms.

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A saturation of the output is observed at approximately 800-J energy input. The pulse energy for fixed settings had a reproducibility of σ = ± 1% at pump energies above 800 J. At an increased rotational rate of the cavity reflector of 300 Hz, which corresponds to a simple shutter rotational speed of 36,000 rpm, a shorter pulse duration was observed as shown in Figure 5. The observed pulse duration was approximately 35.2 ns. This setting produced the highest output of about 3.65 J with 800 J total pump energy.

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Figure. 5. Pulse temporal profile for cavity reflector rotational rate of 300 Hz. Total lamp pump energy: 800 J. Delay: 1.96 ms.

The Q-switched laser pulse had a reasonably Gaussian spatial distribution as shown in Figures 6 and 7 with an approximate 5.74 mm diameter in the near field. The (full-angle) beam divergence was 1.2 mrad in the horizontal direction and 1.10 mrad in the vertical direction. The M-squared parameter was calculated based on spot size measurements at various points on the beam path after a 100-cm focal length lens. This yielded a value of 4.2 which is short of an ideal diffraction limit but within the range of wavefront correction techniques. The spectral distribution of the high-power, Q-switch output increased with lamp pumping as shown in Figure 8. Spectrally, the laser pulse was centered at 1533 nm with a spectral bandwidth of 14 nm at 350 J/lamp pumping, increasing to 62 nm at 450 J/lamp pumping.

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Figure. 6. Near-field spatial distribution of Q-switched laser beam. Width at half-height is approximately 5.74 mm.

Figure. 7. Cross sectional energy distribution in the near field.

Spectral Curves with 2.1ms Delay, 200 Hz Spindle Speed

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Figure. 8. Spectral distribution of the high-power, Q-switched, Er-glass laser.

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To summarize, our experimental high-power, Q-switched, Er-glass laser has the parameters listed in Table 1.

Nominal pulse duration 35 ns Nominal energy per pulse 3.5 J Central wavelength 1533 nm Mean spectral bandwidth 38 nm Divergence 1.2 mrad M2 4.2 Beam diameter 5.74 mm Pulse-to-pulse reproducibility ± 1%

Table. 1. High-power Q-switched Er-glass laser parameters.

4. POTENTIAL APPLICATIONS

The new high output power capability of the Er-glass laser reported here opens the applications of “eye-safe” wavelengths to material working and laser induced breakdown spectroscopy. For example, focusing the 3.5-J, Q-switched pulse on cadmium-plated steel yielded a significant pit, approximately 0.5 mm deep as shown in Figure 9 with just two pulses.

Figure. 9. Photograph of laser pulse imprint on cadmium-plated steel. Scale

marks: 1 mm.

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If the pulse is focused in air, an intense plasma is obtained, which is useful for laser-induced breakdown spectroscopy. Focusing lenses of varying focal lengths were used to vary the peak irradiance at the focus of the beam. This allowed for an attempt to determine the threshold for producing an LIB event in air. Using the focal length of the lens and the measured beam divergence to calculate the beam diameter at the focus of the lens and the nominal pulse energy for the parameters used, an approximate threshold for LIB in air was obtained. Based on a limited number of measurements, we find the threshold to be about 100 GW/cm2. Figure 10 shows a typical air breakdown spectrum.

Spectrum of Air

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Argon 462.8 nm

Oxygen 777.4 nm

Nitrogen 567.4 nm

Hydrogen 656.3 nm

Figure. 10. Laser-induced breakdown spectrum of air. Some of the air composition spectra are identified.

Spectral lines from some of the air components are noted. Focusing on solid surfaces yielded similar highly structured spectra. Optimization would require time-dependent sampling of the plasma.

5. CONCLUSIONS

A high-power, Er-glass laser in the 100-megawatt regime has been demonstrated for the first time. Reproducible and long-surviving operation of the laser has been noted. In its present flashlamp-pumped design, useful fluences are made available for new biophotonic studies, material working, and laser induced breakdown spectroscopy. A more efficient diode-pumped design is currently under development for long distance 2,000-km ranging.

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REFERENCES

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2. E. Tanguy, et al., “Mechanically q-switched codoped Er-Yb glass laser under Ti-sapphire and laser diode pumping,” Electr. Lett., vol. 31, no. 6, pp. 458-460, 1995.

3. E. Georgiou, et al., “High-efficiency and high-output pulse energy performance of a laser diode-pumped Er:Yb:glass 1.54 µm laser,” Appl. Phys. B 70, pp. 755-762, 2000.

4. U. Nilsson, et al., “High-power and tunable operation of Erbium-Ytterbium co-doped cladding-pumped fiber lasers,” IEEE J. Quant. Elect., vol. 39, pp. 987-994, 2003.

5. D. Y. Shen, et al., “Highly efficient Er, Yb-doped fiber with 188 W free-running and > 100 W tunable output power,” Opt. Express, vol. 13, pp. 4916-4921, 2005.

6. G. M. Hale and M. R. Querry, “Optical constants of water in the 200 nm to 200 µm wavelength region,” Appl. Opt., vol. 12, pp. 553-563, 1973.

7. S. Jiang, et al., “Laser and thermal performances of a new erbium doped phosphate laser glass,” SPIE, vol. 2138, pp. 166-174, 1994.

8. R. Wu, et al., “Eye-safe erbium laser transmitter study q-switched with cobalt spinel,” SPIE, vol. 5412, pp. 117-125, 2004.

9. C. P. Cain, et al. "Porcine skin visible lesion thresholds for near-IR lasers, including modeling, at two pulse durations and wavelengths," Journal of Biomedical Optics, 2006 (accepted for publication).

10. W. Koechner, Solid State Laser Engineering, Ch.8, Springer-Verlag, Berlin, 1996.