Novel mode-locked carbon monoxide laser system achieves high accuracy
Extending the emission range of gas lasers is an attractive proposition, for example, in laser spectroscopy, atmosphere sensing, laser media diagnostics, initiating chemical reactions, and separating elemental isotopes. In this context, carbon monoxide (CO) lasers offer several advantages over other sources of IR radiation, including control of output energy and pulse duration over a wide dynamic range. Moreover, they ensure high average power in repetitively pulsed modes. An electric discharge CO laser can operate on every spectral line from among a few hundred in both fundamental (∼ 4.7–8.2μm)1 and first-overtone (∼ 2.5–4.2μm) spectral bands.2–4 In addition, parametric frequency conversion (i.e., frequency doubling, or sum or difference frequency generation) of CO laser radiation using a single nonlinear crystal can cover both mid- and far-IR spectral ranges. However, to achieve high-frequency conversion efficiency, CO laser radiation must have high peak power. The first mode-locked (i.e., pulsing) laser was reported decades ago. But it had a peak power of only a few kilowatts because it operated at room temperature. No efforts have been made since then to obtain substantially higher power.
We have developed a cryogenically cooled, electron-beam-sustained-discharge (EBSD) mode-locked CO laser that produces a train of ∼5–15ns full width at half-maximum spikes with a pulse repetition rate of 10MHz in the mid-IR range of ∼5μm.5 We obtained maximum output power of up to 120kW for a multiline mode and 70kW for a single-line mode of operation. As suggested elsewhere,6 such radiation could be used to pump an optical parametric amplifier for stochastic cooling in a relativistic heavy-ion collider (see Figure 1).
However, the increased output power of this CO laser destabilized the mode locking. To increase the peak power of the spikes and, ultimately, the efficiency of frequency conversion, we decided to amplify the CO laser pulses. Accordingly, we developed a master oscillator-power amplifier (MOPA) system using a cryogenically cooled wide-aperture EBSD CO laser installation with a 1.2m active medium.7, 8 The master oscillator cavity had an optical length of 15m and was composed of a diffraction grating (240 grooves/mm) installed in a so-called Littrow configuration and a plane output mirror with reflectivity 30% in the wavelength range of 5.0–7.0μm. To increase the master oscillator active volume and spectral selectivity, we used a combination of a concave spherical mirror with a curvature radius of 1m and a convex spherical mirror with a curvature radius of 0.3m as an intracavity telescope. We positioned a germanium acousto-optical modulator with aperture of 8.0mm and antireflection coating in the spectral range of 4.5–5.5μm near the output mirror. The mode-locked multi- or single-line master oscillator emitted a train of nanosecond spikes (TNS) with a pulse repetition rate of 10MHz. Figure 2 shows the temporal behavior of the CO laser power at the entrance and exit of the amplifier and dependence of gain (α) versus time.
The low-intensity output of the master oscillator stabilized its mode locking. Comparison of the left and right diagrams in Figure 2 reveals gain saturation for the signal at the entrance of the power amplifier without attenuation (left) compared with the much weaker input signal (right). Following amplification of the TNS, its maximal peak power rose to 380kW for multiline (see Figure 3) and 130kW for single-line modes of operation. The minimum spike duration was 5ns. The efficiency of the CO laser MOPA system was two and 1.5 times higher than that of the master oscillator for single- and multiline mode, respectively, and reached 1.6 and 5.3%.
For experimental purposes, we estimated the time of rotational relaxation of a CO molecule to be 1.7ns, which was less than the duration of any single spike. By comparing the experimental and calculated temporal behavior of nanosecond spikes at the exit of the power amplifier, we estimated the saturation intensity to be IS=14±4kW/cm2 for vibrational-rotational transition 9–8 P(11) of the CO molecule. Application of the MOPA system for frequency conversion of CO laser radiation in nonlinear crystal zinc germanium diphosphide (ZnGeP2) into the second harmonic (frequency) resulted in enhancement of the internal efficiency of the conversion by 3.5 times, from 7% 7 to 25%. 8
Taking into account the high efficiency of our CO laser in the fundamental and first overtone bands, the frequency conversion efficiency of its radiation in ZnGeP2 and, perhaps, in gallium selenide crystals into the mid-IR, and their physical properties, we suggest that frequency-converted emission of CO lasers should be able to cover an extremely wide spectral range from 1.25 to ∼3000μm. In particular, we can obtain coherent emission in the 1.25–2.1μm range by second harmonic generation of overtone CO laser lines. Generating difference frequency when mixing fundamental and overtone (or second harmonic of the fundamental) band radiation enables coverage of the interband spectral range between 4.0 and 5.0μm, which agrees well with one of the main atmospheric ‘transparency windows.’ As of yet, no high-power laser sources operate within this spectral range, though they could successfully be used to efficiently transport laser energy through the atmosphere. High-power laser sources of between ∼5:3 and ∼16μm radiation used to separate uranium isotopes 9 can also be developed directly or based on frequency conversion of CO laser radiation. Our next steps will be aimed at enhancing CO laser peak power, developing a single-pulse MOPA system, and obtaining lasing on sum and difference frequencies of the MOPA system.
Andrey Ionin is professor of laser physics and chief scientist and head of the Gas Lasers Laboratory of the Lebedev Institute. His field of scientific interest is high-power lasers and their applications, low-temperature plasma, nonlinear optics, and laser-matter interactions. He has published more than 300 papers, nine patents, and two monographs.
Igor Kinyaevskiy graduated from the National Research Nuclear University, Russia (2010). He is a PhD candidate. His present research interests are high-power gas lasers including pulsed, repetitively pulsed, and continuous-wave lasers, their applications, and nonlinear optics. He has published more than 20 papers.
Yuriy Klimachev is a senior scientist. His current research focuses on high-power gas lasers including pulsed, repetitively pulsed, and continuous-wave lasers, their applications, and low-temperature plasma. He has more than 120 publications, including four patents.
Andrey Kotkov is a senior scientist. His research interests focus on electrical-discharge molecular-gas lasers including pulsed, repetitively pulsed, and continuous-wave lasers, their applications, and low-temperature plasma. He has more than 180 publications, including four patents.
Andrey Kozlov graduated from the Moscow Engineering and Physics Institute, Russia (2002). He is a junior scientist. His present research interests are high-power gas lasers including pulsed, repetitively pulsed, and continuous-wave lasers, their applications, and low-temperature plasma. He has published more than 40 papers.
