MICROLASERS - Q-switched microchip lasers find real-world application

The short cavity lengths of Q-switched microchip lasers allow them to produce pulses with a duration comparable to that obtained with modelocked systems. At the same time, they take full advantage of the gain medium's ability to store energy, resulting in energetic pulses with high peak powers.

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John Zayhowski

The short cavity lengths of Q-switched microchip lasers allow them to produce pulses with a duration comparable to that obtained with modelocked systems. At the same time, they take full advantage of the gain medium's ability to store energy, resulting in energetic pulses with high peak powers.

The high output intensities of these lasers make possible the construction of extremely compact nonlinear optical systems capable of operating at any wavelength from 5 µm to 190 nm. Systems based on passively Q-switched microchip lasers, like the lasers themselves, are small, efficient, robust, and potentially low cost, making them ideally suited for applications ranging from time-of-flight three-dimensional (3-D) imaging to environmental monitoring.

Saturable absorber enables Q-switching

The principle behind the operation of a passively Q-switched laser is that an intracavity saturable absorber prevents the onset of lasing until the average inversion density within the cavity reaches a critical threshold value. At that point, the onset of lasing produces a high intracavity optical field that saturates the saturable component of the optical loss, increasing the cavity Q and resulting in a Q-switched output pulse.

In their simplest embodiment, the passively Q-switched microchip lasers developed at MIT Lincoln Laboratory (Lexington, MA) are constructed by diffusion-bonding a thin, flat wafer of Nd:YAG gain medium to a similar wafer of Cr:YAG saturable absorber.1 The composite structure is polished flat and parallel on the two faces normal to the optic axis. Cavity mirrors are deposited directly onto the polished faces. The YAG cavity is completed by dicing the wafer into small squares, typically 1-2 mm on a side. The cavity is mounted on a heat sink and longitudinally pumped with a diode laser. The simplicity of the passively Q-switched microchip laser and its small amount of material give it the potential for inexpensive mass production; the nearly monolithic construction results in robust devices.

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FIGURE 1. Complete passively Q-switched microchip laser system includes a black box containing the pump diode and all control electronics and an optical head coupled with a multimode fiber. The turnkey system consumes 8 W of electrical power at room temperature.
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Several variations of the 1-W-pumped (low-power) passively Q-switched microchip laser have been built for a variety of applications. These devices typically have a cavity length between 0.75 and 1.5 mm. They produce output pulses as short as 218 ps, pulse energies as high as 14 µJ, and peak powers up to 30 kW, with time-averaged powers up to 120 mW. Although some of these lasers operate at pulse-repetition rates in excess of 70 kHz, the high-performance lasers typically operate at repetition rates between 8 and 15 kHz.

All of these lasers oscillate in a single longitudinal mode with transform-limited spectral performance, in the fundamental transverse mode with diffraction-limited divergence, and in a linear polarization. The pulse-to-pulse amplitude fluctuations have been measured to be <0.05%. Pulse-to-pulse timing jitter tracks fluctuations in the output of the pump diode.

High-power passively Q-switched microchip lasers are pumped with the output of high-power diode-laser arrays and typically have a cavity length between 6 and 12 mm. When pumped with high-brightness, 10-W fiber-coupled diode-laser arrays, they produce pulsewidths as short as 310 ps, pulse energies as high as 250 µJ, and peak powers up to 565 kW, at pulse-repetition rates of several kilohertz. Like the low-power devices, the high-power passively Q-switched microchip lasers have nearly ideal mode properties and excellent pulse-to-pulse stability.

Nonlinear frequency conversion

The high peak intensities of the passively Q-switched microchip lasers allow for efficient nonlinear frequency generation. By placing appropriate nonlinear optical crystals near the output facet of the 1-W-pumped lasers, Lincoln Laboratory has generated 7 µJ of 532-nm (second harmonic), 1.5 µJ of 355-nm (third-harmonic), 1.5 µJ of 266-nm (fourth-harmonic), and 50 nJ of 213-nm (fifth-harmonic) light, at a typical pulse-repetition rate of 10 kHz. In each case, the crystals are polished flat on the faces normal to the optic axis and butt-coupled to each other without any intervening optics, allowing for simple, inexpensive fabrication.

The optical head, including the infrared (IR) microchip laser and the nonlinear crystals, is packaged in a 1-cm-diameter x 2.5-cm-long stainless-steel can. The only input to the optical head is a multimode optical fiber carrying the continuous-wave diode pump light (see Fig. 1).

When the output of the microchip laser is coupled into a single-mode optical fiber, the intensities in the fiber core exceed 10 GW/cm2. This leads to efficient cascaded stimulated Raman scattering and other nonlinear effects, resulting in the generation of an extremely broadband continuum. A 100-m length of 10-µm-core fiber pumped with 1.064-µm light produces a relatively featureless output spectrum extending from 850 nm to 2.25 µm. The spectrum obtained from a 100-m length of 4.6-µm-core fiber pumped with 532-nm light extends from the green to 950 nm.

The high-power microchip lasers can be harmonically converted to produce high-power UV output. Devices producing more than 19 µJ of 355-nm output, or 12 µJ of 266-nm output, at 5 kHz, have been packaged in robust, 8-cm-long cans. The high-power lasers can also pump optical parametric amplifiers and oscillators. Optical parametric devices and their harmonics extend the spectral coverage of microchip-laser systems continuously from approximately 5 µm to 250 nm. By frequency-summing the output of optical parametric devices with the harmonics of the microchip laser, the wavelength coverage is extended to about 190 nm.

Applications

Time-of-flight optical ranging is one application for the microchip laser. A 200-ps optical pulse can provide a range resolution (minimum separation between two resolvable objects) of 3 cm. When the shape of the optical pulse is repeatable, as it is for microchip lasers, the accuracy of the system can be much better. Together with Cyra Technologies Inc. (Oakland, CA), Lincoln Laboratory developed a compact time-of-flight optical transceiver using a low-power frequency-doubled microchip laser attenuated to the Class-II eye-safe level.

The system is able to range to objects, including black felt, with a single-pulse range accuracy of 1 mm at distances up to 50 m. Coupled to a two-dimensional scanning system, the high repetition rate of the laser makes it possible to obtain a high-resolution 3-D image in minutes (see Fig. 2). Applications include automated production, civil engineering, construction, architecture, and virtual reality.

Passively Q-switched microchip lasers can be used to perform laser-induced breakdown spectroscopy (LIBS). The diffraction-limited output of even the low-power microchip lasers can be focused to intensities sufficient to break down metals and other absorptive solids. The resulting plasma contains highly excited ions, atoms, and molecules-each emitting a unique spectrum as it recombines. The small plasma generated by the microchip laser emits very little continuum radiation, allowing measurements of metallic impurities down to 100 parts per million without any of the standard temporal or spatial gating that conventional LIBS systems use. Applications include the identification of heavy-metal contaminants such as lead, mercury, cadmium, chromium, and zinc.

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FIGURE 2. Built around a frequency-doubled passively Q-switched microchip laser, Cyra Technologies' tripod-mounted, battery-operated optical transceiver enables high-precision 3-D imaging (left). For example, from the ship's deck (center) the transceiver is used to image the mast of an aircraft carrier (right) at a 60-m distance.
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As the power of the microchip laser increases, so too do the sensitivity of the resulting LIBS system and the variety of materials that can be examined. Devices pumped with 3-W diode-laser arrays easily break down transparent media, including glasses and water; the focused output of the highest-power devices is sufficient to break down clean air, with potential applications in the monitoring of effluents and closed-loop process control.

The high peak powers of microchip lasers also can be used for micromachining. The output of a low-power microchip laser has cut clean 5-µm-wide lines in the metallization on semiconductor wafers and drilled holes through the substrate. Higher-power devices have been used to bore holes in glass and to scribe alumina, for example. Applications in microsurgery also are being investigated.

Remote detection of chemicals

Ultraviolet laser-induced fluorescence spectroscopy is capable of measuring small concentrations of chemical species, including pollutants. Remote detection can be performed with UV light delivered to the remote area with an optical fiber. However, because optical fibers transmit UV light poorly, powerful lasers are required and sensitivity is dependent on the fiber length. These limitations are overcome by using a multimode fiber to deliver easily transmitted near-IR diode-laser pump radiation to a remote head containing a UV frequency-converted passively Q-switched microchip laser.

Lincoln Laboratory constructed a 2.5-cm-diameter x 7-cm-long sensor head for use in a cone penetrometer to characterize subsurface contamination at depths up to 50 m. The head contains a frequency-quadrupled low-power passively Q-switched microchip laser and collection optics. The laser light is filtered to remove the IR and visible light before being focused outside through a sapphire window. Fluorescence from material contacting the window is collected in a 500-µm-core return fiber for spectral and temporal analysis. The short duration of the excitation pulse allows measurement of the decay times of even short-lived compounds. This laser-probe technology has been field-tested, demonstrating in situ, real-time characterization of soils and groundwater in a robust, compact, inexpensive package (see Fig. 3). 2

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FIGURE 3. Cone penetrometer contains a fluorescence sensor head built around a UV frequency-quadrupled passively Q-switched microchip laser. The penetrometer is pushed into the ground to obtain fluorescence spectra of chemicals, including environmental pollutants (left). Time-wavelength spectra obtained from the sensor head at depths of 10.5-12.5 ft show a concentration of jet fuel centered at 11.5 ft (right). Intensity information is represented in false color.
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Ultraviolet laser-induced fluorescence spectroscopy has also been used for the detection of airborne biological particles. The spectral characteristics of the fluorescence of tryptophan, NADH, and flavins distinguish biological particles from nonbiological particles and can be used to perform some classification of biological particles. In recent field tests, Lincoln Laboratory's battery-powered, port able bioaerosol fluorescence sensor, built around a low-power frequency-quadrupled micro chip laser, proved to be effective in the detection of biological-warfare simulants. Other potential applications of this technology include monitoring the air in hospitals and public buildings to help control the spread of airborne communicable diseases.

Active Impulse Systems (Natick, MA; recently acquired by Philips Analytical, Eindhoven, The Netherlands) has developed a thin-film measurement system around a low-power passively Q-switched microchip laser. Using impulse-stimulated thermal scattering, the system-intended primarily to characterize semiconductor process wafers-can make nondestructive measurements of thin-film thickness to an accuracy of 5 nm, with a transverse resolution of 10 µm. It can also measure anisotropic elastic moduli and thermal diffusivity, as well as determine whether or not there is delamination of the film, all at about 1000 measurements per second. In addition to semiconductor manufacturing, applications of this technology include checking for defects in painted or laminated surfaces, and monitoring the curing of epoxies and resins.

The broad spectrum generated by microchip-laser-pumped cascaded stimulated Raman scattering in fibers has applications in absorption, reflection, and excitation spectroscopy; active 3-D hyperspectral imaging; and white-light interferometry. The high-power UV devices can be used for photoionization spectroscopy, matrix-assisted laser desorption and ionization, and stereolithography. Potential applications for the deep-UV systems include the alignment and calibration of optics for excimer lasers and excimer-laser systems. New applications continue to emerge as this technology becomes more readily available. (Note: Low-power passively

Q-switched microchip lasers are currently available from Uniphase Lasers & Fiberoptics, a division of Uniphase Corp., San Jose, CA; Synoptics, a division of Litton Airtron, Charlotte, NC; and Nanolase, Meylan, France.)

ACKNOWLEDGEMENT

Work described here was sponsored by the US Department of the Air Force under Air Force Contract #F19628-95-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Air Force.

REFERENCES

  1. J. J. Zayhowski, Rev. Laser Eng. 26, 841 (1998); J. J. Zayhowski and C. Dill III, Opt. Lett. 19, 1427 (1994).
  2. J. Bloch et al., Appl. Spectrosc. 52, 1299 (1998).

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