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Lasing of HCN and C2H2 in the 3-micron region was demonstrated with a ns pump emitting in the telecommunication C band (1.5 micron region). The observed laser lines correspond to transitions from the terminal pump vibration-rotation state to a combination vibrational state.
©2010 Optical Society of America
1. Introduction
Optically pumped gas lasers have gained interest as wavelength converters for generating coherent radiation in the mid-infrared, a spectral region of great interest in applications such as remote sensing and imaging through the atmosphere. These lasers can potentially also be used as a means to combine the output of several incoherent laser sources, such as fiber or diode lasers, into one coherent output beam. In principle, with molecular lasers the emission can be shifted further into the infrared region with larger Stokes shifts compared to atomic vapor lasers [1–3]. Several optically pumped molecular lasers (OPML) have been demonstrated in the past. Among them are an HBr laser [4] pumped by a 1.339-μm Nd:YAG laser, an NH3 laser [5] pumped by a 9.29-μm CO2 laser, a CF4 laser [6] and OCS laser [7] pumped by a 9.6-μm CO2 laser to name a few examples. Spectrally narrow pump sources are necessary to match the absorption line widths of the molecule, which is typically smaller than a few GHz. In recent years, considerable progress has been made to spectrally narrow high-power diode lasers [8,9] and fiber lasers [10] and thus these lasers are now well suited to optically pump gases [11]. For example, diode laser pumped atomic vapor lasers [12] have reached output powers exceeding 40 W [13] in the near infrared spectral region.
For beam combining it is most desirable to consider molecules whose absorption spectra match available narrow band fiber and/or diode lasers. In this respect, C2H2 and HCN are attractive active gas candidates. Both have absorption bands near 1.5 μm that overlap with the telecommunication C band where diode and fiber optics technology is well advanced.
Few attempts have been made to explore lasing in C2H2 either using discharge pumping [14] or optical pumping [15,16]. Lasing in HCN using discharge pumping has also been reported [17]. CW laser emission from C2H2 has been observed at 8 μm in an electric discharge CO-C2H2 laser [14]. Here energy transfer occurred from vibrationally excited CO to C2H2. Pulsed operation at ~8 μm has been achieved in a CO-C2H2 laser by resonant vibrational-to-vibrational energy transfer from CO gas optically pumped by a frequency-doubled CO2 TEA laser at 4.8 μm [15]. Tapalian et al. [16] demonstrated super radiant emission in C2H2 at 13.6 μm and 15.6 μm when optically pumped by a narrow band optical parametric oscillator at 3 μm. Wang et al. observed 3.85-μm lasing in an HF-HCN laser [17]. The excitation occurred through vibrational energy transfer from optically (2.5 μm) excited HF molecules to HCN molecules.
The aim of this paper is to demonstrate 12C2H2 and H13CN lasers emitting in the 3-micron spectral region that are optically pumped by a ns OPO. A pump wavelength of about 1.5 μm was chosen to mimic the emission of readily available (CW) fiber based laser systems operating in the telecommunication C-band. In order to attain the gain required for 3 micron lasing, a nanosecond parametric oscillator (OPO) was used as the pump source. An assessment of the cw lasing potentials of these molecules can be found in reference [18].
The vibrational normal modes of C2H2 and HCN are sketched in Figs. 1 and 2 together with simplified energy level diagrams. Associated with each vibrational level labeled (v1,v2,v3,v4,v5) for C2H2 and (v1,v2,v3) for HCN is a rotational ladder. The spacing of rotational energy levels to first order is independent of the vibrational state and is given by 2B(j + 1), where B is the rotational constant of the vibrational state and j is the rotational quantum number. For example, the rotational constant for C2H2 for the (10100) state is ~35.0 GHz [19], and B ~42.6 GHz for the (002) state of HCN [20]. At room temperature, according to the Boltzmann distribution, maximum population occurs in the rotational state with j = 9 for C2H2 and j = 8 for HCN.
Fig. 1 Normal vibrational modes of C2H2 and simplified energy level diagram. An R branch pump transition from j = 7 to j = 8 and observed laser transitions at about 3120 nm and 3160 nm are shown.
Fig. 2 Normal vibrational modes of HCN and simplified energy level diagram. An R branch pump transition from j = 9 to j = 10 and observed laser transitions at about 3100 nm and 3170 nm are shown (solid arrows). Dashed vertical arrows indicate some other possible laser transitions that were not observed in our experiments. The dotted vertical arrow corresponds to the observed cw fluorescence transition (see text).
2. Experiment
The layout of the OPML is shown in Fig. 3 . For the C2H2 laser, an 80-cm gas cell filled with 2.2 torr of C2H2 was optically pumped by a nanosecond OPO (~5 ns, bandwidth ~3.5 GHz) tuned to the R(7) absorption line at ~1.521 μm. The incident energy, after accounting for windows and coupling losses was ~6.8 mJ. Using the known absorption coefficient [23], the small signal absorption at line center was expected to be 98%. Due to the bandwidth of the OPO (3.5 GHz versus absorption line width of about 500 MHz), the absorbed pump energy was about 2.3 mJ. The OPO pump was focused to the center of the cell by a 2-m focal length lens producing a spot of diameter about 4 mm. The 1.4-m long cavity consisted of two 5-m concave mirrors transparent to the pump wavelength and having a reflectivity of 0.9 in the 3.15 μm region. The laser threshold occurred at about 100 μJ of absorbed pump pulse energy. The laser output was detected from the reflection off an uncoated CaF2 window inside the laser cavity (the mirror substrate was BK7 and therefore had a low transmission at the laser wavelength).
Fig. 3 Schematic diagram of the C2H2 and HCN laser cavity. M1, M2: cavity mirrors, L- lens, D- fast detector. F is a filter to block the pump beam. W is a CaF2 window to pick the laser output. For spectral measurements, the detector D was been replaced by an infrared scanning spectrometer.
The HCN laser consisted of a 4-cm gas cell filled with 5 torr of HCN. The gas was optically pumped by the nanosecond OPO at ~1.536 μm to excite the R(9) absorption line. The cavity length was 73 cm. The pump pulse was focused to the center of the cell using a 1-m focal length lens resulting in a spot diameter of about 2 mm. The 0.5-m concave front cavity and the rear end mirrors were transparent to the pump wavelength and had a reflectivity of 0.9 in the 3 micron region. The incident energy was 8 mJ. The absorbed pump energy for the 4-cm cell was about 140 μJ. The laser threshold occurred at 40 μJ of absorbed pump pulse energy. A fast detector was employed to record the temporal profile of the laser output. Spectra were recorded using a NIR scanning spectrometer.
In addition, the optical emission of the gases without cavity (mirrors) was also investigated. In these experiments, the OPO pump was focused to the center of the cells by a 1-m focal length lens producing spot of diameters of about 2 mm. The lengths of the cells were 80 cm and 1 m for C2H2 at 2 torr and HCN at 5 torr, respectively. Amplified spontaneous emission (ASE) was observed for absorbed pump energies exceeding 200 μJ (C2H2) and 415 μJ (HCN), respectively.
3. Results, Interpretation and discussion
The observed spectrum of the C2H2 laser is shown in Fig. 4(a) , an example temporal laser profile is depicted in Fig. 4(b). The corresponding ASE traces are shown as insets. The laser emission shows two peaks separated by about 40 nm in the 3140 nm region. The emission spectrum remained same when the pump energy was reduced by a factor of 2. The two peaks correspond to the R(7) and P(9) transition. Both originate at the upper pump level with j = 8 of the (10100) vibrational state and terminate at the (10000) vibrational state.
Fig. 4 a) Spectrum and, b) temporal profile of the C2H2 laser pumped with 5-ns pulses at 1.521 μm. The insets show the ASE spectrum and temporal profile. The origin of time axis corresponds to an arbitrary trigger.
The two spectral laser components do not represent different transitions from the allowed rotational states adjacent to the pumped level to the (10000) vibrational level. Their wavelength separation would be about 9 nm, which is estimated from the known value of the rotational constant for the (10100) state [19]. The rotational relaxation time of the pumped level is about 18 ns for a pressure of 2.2 torr [21], hence one would not expect any laser transitions to occur from the neighboring states during the pump pulse. It is instructive to note the absence of any emission signature from the lower laser state (10000) to the ground state (00000), which would be at about 3 μm. This transition is not dipole allowed [22].
From HITRAN [23] data the absorption cross section for the pump transition is ~8 × 10−18 cm2. The emission cross section for the (10100) to (10000) transition is not known. One can expect that this cross section is of the same order of magnitude as the cross section for a similar transition (00100) to (00000), which is estimated to be about 2.2 × 10−16 cm2 using the known Einstein A coefficient [23].
The exact temporal profile of the laser output fluctuated somewhat from shot to shot and was slightly dependent on the cavity alignment. It reflects a superposition of longitudinal and transverse modes and showed the typical mode beating frequencies. The fast modulation can be attributed to longitudinal mode beating. The measured period of 9.3 ns agrees well with the beat frequency of 109 MHz obtained from our cavity geometry. The slower modulation with a period of 36.6 ns is due to transverse mode beating. For a cavity with mirrors of radius of curvature R1 and R2 separated by a distance L, the transverse mode beating frequency is given by (c/2Lπ) × cos−1([g1g2]0.5) [24] where the parameter g is defined as g1,2 = (1-L/R1,2). Using our cavity parameters the expected transverse mode beat frequency is 26 MHz, which explains the observed period of 36.6 ns. At 2.2 torr, the gain profile has a pressure broadened width of about 260 MHz [25,26], while the longitudinal mode spacing is about 109 MHz.
Figure 5 shows the spectral and temporal profile of the HCN laser and ASE. The spectrum in Fig. 5(a) shows a main peak at ~3165 nm. The peak corresponds to the P(11) transition originating from the initially populated level j = 10 of the (002) vibrational state. The observed structure in the short-wavelength tail of the peak is likely from P line emission originating at adjacent rotational states (j = 8 and j = 9) because of the short (~5ns) rotational relaxation time [27].
Fig. 5 (a) Spectrum and (b) temporal profile of an HCN laser pumped with 5-ns pulses at 1.536 μm. The inset shows the corresponding ASE data.
The exact relaxation pathway from the lower laser level to the ground state and relevant time constants are not completely known. There is a possibility for another laser transition between vibrational levels (001) and (010) producing radiation at about 4 μm [28]. There is also another potential laser transition from level (001) to the ground state (000), which would emit at about 3 microns [28]. None of these lasing lines were observed. Likely reasons are insufficient pump energy and inadequate bandwidth of the cavity mirrors.
The ASE spectrum shows an additional (weaker) spectral component at about 3.1 μm, which most likely represents the R(9) transition. In the laser, due to gain competition, the weaker line is suppressed. The spectral components in the short-wavelength tail of the laser line can originate from P(9) and P(8) transitions. Similar emission was not observed for C2H2. The reason is that the rotational relaxation constant in HCN is about 10 times larger [27]. It should also be mentioned that the HCN fluorescence excited by a cw diode laser tuned to the P(11) absorption transition showed a weak signal at about 4 microns. This signal could be from the (002) to (011) transition, see Fig. 2.
The absorption cross-section for the pump transition is ~2 × 10−18 cm2 [20]. Emission cross-section for (002) to (001) transition is not known. However, one can expect this cross section is of the same order of magnitude as for a similar transition (001) to (000), which is estimated to be ~1.8 × 10−16 cm2 from the known Einstein coefficient [23] for the transition.
The temporal profile of the HCN laser output is distinctly different from the C2H2 laser output. Owing to the shorter gas cell of the former, more cavity roundtrips are needed before the pulse peak. Thus mode competition for the available gain is more pronounced and almost entirely suppresses higher-order modes. Thus only small modulation (mode beating) in the HCN laser output is visible. The beat frequencies of about 240 MHz and 71 MHz, respectively are what is expected for the longitudinal and transverse mode spacing from the cavity parameters.
Since the purpose of the experiment was to demonstrate the first molecular gas laser(s) which can be pumped by a laser source emitting in the telecommunication C – band, no attempt was made to optimize the laser output by finding optimum gas pressures and cavity configurations. The observed laser efficiencies in terms of absorbed pump energies were of the order of 10% for the C2H2 and HCN laser.
4. Summary
We have demonstrated lasing of C2H2 and HCN near 3 μm when pumped with a nanosecond OPO in the telecommunication C-band (1.5-micron band). The laser lines likely originate from transitions from the terminal pump states to combination states of C2H2 and HCN. The absorption band of both gases in the telecommunication C band region makes these gases potentially attractive for combining coherently the output of mutually incoherent diode and or fiber lasers. Simulations suggest that CW laser operation is possible with laser output that is spectrally close to the pump radiation [18].
Acknowledgement
The authors acknowledge funding from the Army Research Office (W911NF-08-1-0332), the National Science Foundation (PHY-0722622) and the Joint Technology Office (W911NF-05-1-0507). We are grateful to Kristan Corwin and Brian Washburn (Kansas State University) and to Michael Heaven (Emory University) for helpful discussions. We are grateful to Kurt Vogel (Precision Photonics) for providing the dichroic mirrors.
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