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Lasers & Sources

Compact-slab radio-frequency discharge CO laser

A novel repetitively pulsed first-overtone laser produces an average output power of 0.3W and a laser efficiency of 0.5% at operating wavelengths of 2.5–4.0μm.
26 May 2009, SPIE Newsroom. DOI: 10.1117/2.1200905.1626

Carbon monoxide (CO) lasers1 are very attractive sources of mid-IR radiation for laser spectroscopy and multicomponent-gas analysis. The spectral range (2.5–4.2μm) of first-overtone (FO) CO lasers (characterized by vibrational transitions from V+2→V) covers those of the well-established hydrogen fluoride and deuterium fluoride lasers, but with significantly smaller vibrational-rotational line spacings. Their full output spectra may contain more than 400 emission lines,2 many of which coincide with absorption lines exhibited by a wide range of organic and inorganic materials such as, H2O, CO2, CH4, NO2, NO, SO2, acetone, benzene, and methanol. FO CO lasers operating at these wavelengths can thus be used to assess the impact of resonances on various media in fields as diverse as, nonlinear spectroscopy, atmospheric remote sensing, and laser chemistry. FO CO lasers are among the best devices for laser-based spectroscopic analysis of multicomponent-gas mixtures.3 In addition, the longer-wavelength section of their output spectrum coincides with the atmospheric ‘transparency window.’

Successful applications require the development of compact, sealed-off FO CO lasers. Present-generation FO CO lasers use either fragile glass components, e.g., for continuous-wave (cw) gas-flow high-voltage DC discharge, or very complicated, large-scale designs (e.g., for high-voltage electron-beam-sustained discharge or supersonic cooling), and are not suitable for integration in compact multiwavelength-laser gas analyzers.3 One of the most promising approaches to producing compact gas lasers uses radio-frequency (RF) discharge in a slab geometry to pump an active medium. We developed a cryogenically cooled slab RF-discharge CO laser4,5 (see Figure 1). Significant operational advantages of our FO CO laser include its slab RF-discharge pumping operation, robust compact stainless-steel design, sealed-off performance, cryogenic cooling of the electrode system combined with discharge-chamber walls at room temperature, and a relatively low operational voltage.

Figure 1. Slab radio-frequency discharge first-overtone carbon monoxide (CO)-laser setup. (a) General view. (b) Open view.

The laser's active volume is 3×30×250mm3 (the discharge chamber's total internal volume is ~8l). The electrode system is cooled by liquid nitrogen to ~120K (using ~5L/hr). An RF power supply with a carrier frequency of 81 or 60MHz, operating in a repetitively pulsed mode, is used for discharge ignition. The power supply produces up to 1000W of peak-power rectangular pulses of variable duration, τ= (0.01–0.99)×F−1, where F= (100–500)Hz is the pulse-repetition rate. The discharge chamber is filled with gas mixtures CO:O2:N2:He of different component concentrations at a total gas pressure of 15–22Torr. Note that gas mixtures with high oxygen content are needed for stable long-term operation.4,5 We use a stable laser resonator with very-high-reflectivity mirrors (>99% in the spectral range 2.5–4.2μm). Two sets of mirrors optimized for either 2.5–4.2 or 2.5–3.2μm operation can be used, depending on the requisite output spectrum, which strongly depends on the laser's output-mirror characteristics (see Figure 2).

Figure 2. Vertical bars: Typical first-overtone CO-laser spectra obtained with two laser-resonator mirror sets (laser power, PLAS, in mW). Curves: Characteristic reflectivity of the rear mirror (black) and the output mirrors for the first and second sets (blue and red, respectively).

The maximum FO CO-laser average output power obtained with the first laser-resonator mirror set was ~0.32W in the ~2.55–3.15μm spectral range. The output spectrum included more than 40 vibrational-rotational lines, corresponding to overtone vibrational bands from 8→6 up to 23→21 (see the blue bars in Figure 2). The laser efficiency under these conditions was ~0.5%. Using the second set of mirrors, FO CO lasing was observed at ~3.05–3.95μm. The output spectrum consisted of more than 60 vibrational-rotational lines, corresponding to overtone vibrational bands from 22→20 up to 36→34 (see the red bars in Figure 2). The full spectrum obtained using both sets of resonator mirrors included more than 100 vibrational-rotational lines in the 2.5–4.0μm wavelength range, with a maximum single-line average output power of ~12mW. Our next objective is to increase the laser efficiency to ~5–10%, which has been achieved in facilities using other types of electric discharges.2

Andrey Ionin, Andrey Kozlov, Leonid Seleznev, Dmitry Sinitsyn
Gas Lasers Laboratory
P. N. Lebedev Physical Institute
Russian Academy of Sciences
Moscow, Russia

Andrey A. Ionin is chief scientist and head of the Gas Lasers Laboratory. His scientific interests focus on high-power gas lasers, including pulsed, repetitively pulsed, and cw CO2, CO, and N2O lasers, their applications, low-temperature plasma, and nonlinear optics. He has published more than 300 papers, eight patents, and two monographs, and worked in German and US laser centers.

Andrey Kozlov graduated from the Moscow Engineering and Physics Institute (Russia) in 2002. He is a junior scientist. His present research interests are in high-power gas lasers including pulsed, repetitively pulsed, and cw lasers, their applications, and low-temperature plasma. He has published more than 40 articles.

Leonid Seleznev is a senior scientist. His current research interests are in high-power gas lasers including pulsed, repetitively pulsed, and cw lasers, their applications, and low-temperature plasma. He has more than 100 publications including two patents.

Dmitry Sinitsyn is a senior scientist. His research interests focus on electrical-discharge molecular-gas lasers including pulsed, repetitively pulsed, and cw lasers, their applications, and low-temperature plasma. He has more than 160 publications including three patents.