High-power tunable pulsed and CW mid-infrared fiber gas laser sources in acetylene-filled hollow-core fibers, to the best of our knowledge, are demonstrated for the first time. By precisely tuning the wavelength of the pump source, an amplified tunable 1.5 μm diode laser, to match different absorption lines of acetylene, the laser output is step-tunable in the range of 3.09~3.21 μm with a maximum pulse average power of ~0.3 W (~0.6 μJ pulse energy) and a maximum CW power of ~0.77 W, making this system the first watt-level tunable fiber gas laser operating at mid-infrared range. The output spectral and power characteristics are systemically studied, and the explanations about the change of the ratio of the P over R branch emission lines with the pump power and the gas pressure are given, which is useful for the investigations of mid-infrared fiber gas lasers.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
There have been strong research interests in mid-infrared emission from fiber lasers which have the advantages of excellent beam quality, high conversion efficiency, long interaction length and compact system configuration [1,2]. Extending the operation wavelengths of fiber lasers from near-infrared to mid-infrared region has been developing rapidly, due to potential applications in many aspects. For example, because of the high absorption of the O-H bond in water in the spectral region at 1.94 μm and 2.7 μm, laser ablation could be employed in soft-tissue medicine with minimal collateral damage . Besides, the mid-infrared wavelengths near atmospheric windows could be used for remote monitoring and free space communication . Solid-core fiber lasers based on silicate glasses doped with rare-earth ions, such as erbium and thulium, are usually constructed due to low loss, high tenability and strong strength [1, 2]. However, phonon energy of silica can be up to 1100 cm−1, which limits the emission wavelength to be below ~2.2 μm [2, 4], only covering the near-infrared laser radiation (1 μm, 1.5 μm and 2 μm). For longer wavelengths, alternative systems based on fluoride fibers and chalcogenide fibers have drawn significant attention due to their lower phonon energy and broader transmission window [1,2]. Nevertheless, due to some drawbacks such as nonlinearity, low damage threshold, and weak heat handling capacity, power scaling of such systems would be limited, which is far below the power level at shorter wavelengths based on silicate glasses . In addition, the promising chalcogenide glasses are too fragile to purify and fabricate into fibers easily. The advent of silica-based hollow-core fibers (HCFs) with low loss at mid-infrared wavelengths [5, 6] provided an alternative novel method—gas-filled HCF lasers [1, 7]. The majority of light in the HCF is guided through small hollow core areas usually filled with gas molecules as gain medium, which has tight confinement for the light. Advantages of using HCFs rather than traditional gas cells in developing gas lasers include long interaction length, low threshold, and high efficiency . Because of the ultra-low optical overlap with the thin silica cladding tube walls of HCFs, gas-filled HCF lasers have higher damage thresholds and higher thresholds for detrimental nonlinear optical effects compared with solid core fiber lasers .
Since the mid-IR fiber gas laser was firstly demonstrated in 2011 , a variety of gas-filled HCF lasers including C2H2, CO, CO2, HCN and I2 have been intensively investigated [8–15], widely extending the wavelength range. Among them, an optically pumped acetylene-filled HCF laser based on population inversion was firstly reported to generate pulsed 3.1 μm mid-infrared emission, but the pulse energy is only several tens of nJ with a very low slope efficiency (~1%), due to a very high fiber loss (20 dB/m) . Later, similar gas fiber lasers with improved performance were demonstrated , which reached about 20% slope efficiency using pulsed optical parametric amplification (OPA) as a pump source, but the average laser power is at microwatt-level. In 2014, by using diode laser as pump source firstly, up to 30% conversion efficiency with respect to absorbed pump power was recorded , which provides an effective route to obtain compact mid-infrared fiber lasers, while the average power is <10 mW. In 2016, the first ring-cavity based mid-infrared laser oscillation operating at both pulsed and continuous-wave (CW) regime was demonstrated in acetylene-filled HCF, while the output power was only several mW, only focusing on the strongest pump line of P(9) . In 2017, the first watt-level CW mid-infrared laser output at 3 μm in a single-pass configuration was reported, while also only the strongest absorption line of P(9) was studied, and the change of the ratio of P(9) over R(7) with the pump power and the gas pressure was not researched . The tunable mid-infrared lasers are required in many applications such as environmental monitoring, infrared interference and confrontation, remote sensing and so on. We previously reported preliminary results of a tunable pulsed mid-infrared fiber gas laser with average power of only several mW in a conference paper .
In this paper, to the best of our knowledge, we firstly report watt-level tunable diode-pumped mid-infrared fiber gas laser sources. The characteristics of pulsed and CW mid-infrared emission from acetylene-filled HCFs pumped by a modulated and amplified diode laser are systematically investigated. By precisely tuning the diode laser to different P branch absorption lines of acetylene, the laser wavelength is step-tunable over the range of 3.09~3.21 μm. The output spectral and power characteristics are systemically studied. We get a maximum laser pulse average power of ~0.3 W (~0.6 μJ pulse energy) by P(9) pump transition at 0.9 mbar gas pressure with a power conversion efficiency of ~16%, and a maximum CW laser power of ~0.77 W by P(15) pump transition at 1.5 mbar gas pressure with a power conversion efficiency of ~13%.
2. Fundamental principles
An acetylene molecule is comprised of two carbon atoms attached to two hydrogen atoms through electrostatic forces between the positively charged atomic nuclei and the electrons of the combined atomic system . In general, the electronic system and the translational, rotational, and vibrational degrees of freedom of the molecule determine four sets of discrete energy levels. But for mid-infrared emission, where typically optical transitions correspond to energies less than 1 eV, we usually neglect the electronic energy and assume that the electrons stay in the electronic ground state .
Acetylene molecules vibrate and rotate simultaneously leading to discrete and quantized energy levels. Therefore, a change of the vibrational state can be accompanied by a change in the rotational state leading to vibration-rotation spectra. The five kinds of vibrational normal modes include three stretching modes and two bending modes of acetylene , as sketched in Fig. 1(a). The v1 and v3 modes correspond to the symmetric and anti-symmetric C-H stretch; the v2 mode corresponds to the symmetric C-C stretch; and the v4 and v5 modes correspond to the symmetric and anti-symmetric C-H bends . For certain vibrational state, there is a rotational ladder, the spacing of which to first order is independent of the vibrational state and is given by 2B(j + 1), where B is the rotational constant of the particular vibrational state and j is the rotational quantum number . From HITRAN database , the absorption spectrum of acetylene molecules around 1.5 μm is displayed in Fig. 1(b). Due to the symmetric requirements of the total multi-particle wave function under exchange of identical nuclei , intensities of the odd absorption lines are much stronger. Each absorption line has a finite linewidth and characteristic determined by three line broadening process—natural broadening, collisional broadening and Doppler broadening. And the first two mechanisms give rise to homogeneous broadening while the Doppler effect gives rise to inhomogeneous broadening . In most practical situations the broadening of gas molecules is dominated by Doppler broadening and pressure (collisional) broadening [8,17]. For gas pressure of a few mbar and below, the linewidth ∆v is about several hundreds of MHz, which is much narrower than typical transitions of dopants in solid core fibers .
In principle, all the absorption lines can be selected as the pump wavelengths on any of the rotation-vibration transitions from the ground state (v0 vibrational state) to the upper level (v1 + v3 vibrational state). This creates an immediate population inversion between the upper level and the v1 vibrational state (the lower laser level), since the thermal population of v1 vibrational state at room temperature is negligible . Then the acetylene molecules can leave the v1 + v3 vibrational state through radiation transition to the essentially empty v1 vibrational state , from where lasing occurs according to selection rules ∆j = + 1 (or ∆j = −1) referred to as R branch (P branch) transitions. The R and P branch transitions are usually labeled as R(j) and P(j) where j is the rotational quantum number of the lower state, as illustrated in Fig. 2. But the relaxation transition from the lower level to the ground level is dipole forbidden  and relies on non-radiative transitions mechanisms to depopulate the lower laser level . Alternatively, the intermolecular collisions or collisions with the fiber core wall not only could depopulate both upper and lower laser levels, decreasing the laser efficiency , but also assist repopulation of ground state, playing a valuable role in maintaining CW laser operation . Usually, the intermolecular collision is dominant at pressures above 2.5 mbar while the contribution of wall collisions could not be neglected at pressure below 0.4 mbar [11,14]. It is worthwhile pointing out that Fig. 2 only shows lasing transitions pumped with odd P branch absorption lines for the sake of brevity. And for even P branch absorption lines and R branch absorption lines, acetylene molecules have similar lasing transitions within vibrational and rotational state.
3. Experimental setup
To demonstrate laser emission in acetylene-filled HCFs both in pulsed and CW regime, the experimental platform was set up in a single-pass configuration due to the high gain resulting from the tight confinement of pump and signal light together with the active gas, as shown in Fig. 3(a). The key component of the setup is the low loss anti-resonant HCF which traps light in the air core by the microstructure. Design parameters of the fiber are chosen, such that the fiber can guide both the pump and the lasing wavelengths, as illustrated in Fig. 3(b). It can be seen that the fiber has an outer diameter of 255 μm and a core diameter of 70 μm surrounded by six untouched cladding silica tubes with wall thickness of ~1.8 μm. The thin cladding tube walls have higher refractive index, acting as Fabry-Perot resonators, which can enhance (decrease) the confinement of light in the core under different anti-resonant (resonant) conditions . A standard cut-back method from 30 m to 3.8 m gives about 0.08 dB/m and 0.13 dB/m attenuation at 1.5 μm and 3 μm spectral regions respectively (displayed in Fig. 3(c)). Both ends of the HCF are enclosed inside gas cells sealed within 5 mm thick windows to guarantee fine air-tightness due to the experimental requirement of nearly vacuum conditions. Experiments were performed with a piece of 10 m long HCF that can be filled with acetylene to required pressure which need a few hours to equilibrate.
By optimizing the coupling system, the pump beam is coupled into the HCF with >90% maximum coupling efficiency, which is higher than our previous experiments [11,13]. The incident pump power could be controlled by a continuously tunable attenuator and monitored after being reflected to a power meter by M3 on a flipper mount. Consequently, after interaction between the pump light and acetylene in the HCF, both the laser output and residual pump light would pass through W2 and be collimated and focused. Through M4 on a flipper mount, the mixed light could not only be sent to power meter after separating laser output from pump through infrared bandpass filter (IBF) (~90% transmittance only at laser wavelengths), but also delivered to a grating monochromator for detecting output laser spectra.
As illustrated in the left dashed frame in Fig. 3(a), we used pulsed or CW tunable pump laser which is tunable from 1525 nm to 1565 nm, covering the P branch absorption lines of acetylene. The pump system has weaker ASE background when tuned to longer wavelength absorption lines due to the emission characteristics of customized EDFA, as shown in Fig. 4. Adjusting AOM to different pulse duration and repetition rates, we found that 50 ns and 500 kHz give the optimum pump laser performance in terms of power and ASE properties. Measured by a Fabry-Perot interferometer, the linewidth of both pulsed pump laser and CW pump laser is less than 300 MHz, which is much narrower than the Doppler broadening of absorption lines (~480 MHz for 12C2H2, room temperature, central wavelength 1.53 μm, ignoring pressure broadening) . The narrow linewidth of pump laser is useful for the generation of step-tunable mid-infrared laser emission when the wavelengths of pump are precisely tuned to the absorption lines of acetylene.
4. Experimental results and discussion
Output spectra pumped at different P branch absorption lines, shown in Fig. 5(a), contain two laser peaks of different intensity at each absorption line due to the transition selection rules. And the inset shows a good beam quality of the laser output. Explained by Fig. 2, the separate lasing wavelengths change gradually from 3.1 μm to 3.2 μm with the changing odd absorption lines from P(1) to P(25), also shown in the left table of Fig. 2. Usually, for each pump absorption line, the two laser emission peaks of acetylene consist of P branch transition with longer wavelength and R branch transition with shorter wavelength. The relative strengths of P branch and R branch indicate competitions between the two transitions sharing a common upper level  and insufficient time for molecules in the excited state to mix through intermolecular collisions before the onset of lasing and vibrational relaxation . When pumped by P(9) absorption line, for instance, two spectral components, P(9) and R(7), will be generated from j = 8 rational state (v1 + v3 vibrational state) to j = 9 and j = 7 rational state (v1 vibrational state). And the ratio of output P(9) to R(7) as a function of incident pump power (average power, using pulsed pump laser) taken at various acetylene pressure is illustrated in Figs. 5(b)-5(d). We can see that the intensity of P(9) is always stronger than that of R(7). That is because P(9) has higher gain than R(7) due to the larger emission cross section of P(9) than R(7) [21,22]. At low incident pump power, P(9) dominates the mid-infrared laser output. While with the increasing of the pump power, the ratio of P(9) over R(7) decreases, which is caused by the gain reducing of transition P(9) with the population building up in its lower lasing level, until it is exceeded by the gain of the competing line (with a lower lasing level that is still empty), allowing this second line to lase as well. From Figs. 5(b)-5(d), it can also be seen that there is an optimal pressure for a given length of HCF, which is 0.9 mbar for P(9) pump line in our experiments. At the optimal pressure, both the emission transitions of P(9) and R(7) are relatively strong (as shown in Fig. 5(c)), resulting in a maximum laser efficiency, agreeing with output laser power properties, as illustrated in Figs. 6 and 7. As we all know, the transition gain can be given by g = Δn⸱σ21 , where Δn is the inversion population between the transition levels and σ21 is the emission cross section corresponding to the transition. The inversion population Δn is nearly linearly proportional to the gas pressure and the pump power before saturation, while σ21 is inversely proportional to the pressure due to the gas collisional broadening , resulting in a optimal gas pressure.
Figure 6(a) indicates the measured laser emission energy for different gas pressure pumped with P(9) absorption line using pulsed pump laser. It can be seen that at low pressure levels (below 0.5 mbar), the output shows onset of saturation as the pump energy is increased, indicating that the pump is only partially absorbed and the gain is low. At higher pressure levels (0.7 mbar-1.5 mbar), the laser pulse energy increases with the rising pump pulse energy beyond the threshold, reaching a maximum value of 0.6 μJ (0.3 W average power) due to a higher gain. As the gas pressure is further increased (above 2.2 mbar), intermolecular collisions become more frequent, and such rapid relaxation process will significantly reduced the lifetime of laser upper level, decreasing the gain, leading to reduced output laser energy. With given coupled average pump power at different absorption lines, Fig. 6(b) plots the laser energy and residual pump energy at different acetylene pressure. It can be seen that different absorption lines have similar optimum pressure of about 0.9-1.5 mbar. When the pump source is tuned to different absorption lines, the laser energy and the energy conversion efficiency (Given by the ratio of the output laser pulse energy to the coupled pump pulse energy) with respect to coupled pump energy at the suitable pressure of 0.9 mbar is shown in Figs. 6(c) and 6(d). We can see that P(9) pump line gives the highest laser slope efficiency due to the strongest absorption. And the conversion efficiency increases sharply at low pump energy, approaching saturation at high pump energy with the decreasing growth rate. The maximum conversion efficiency of ~16% is observed with P(9) absorption line.
Figure 7 shows similar relation like Fig. 6 when using CW pump laser. Displayed in Figs. 7(a) and 7(d), about 0.77 W laser output power and 13% power conversion efficiency are measured when tuned to P(15) at 1.5 mbar. The lasing threshold, defined as the minimum coupled pump power necessary to observe mid-infrared laser output, increases quickly with the increasing of the acetylene pressure that lead to the enhanced collision between acetylene molecules. As sketched in Fig. 7(c), due to the trade-off between the absorption intensity and the ASE background of absorption lines, P(15) has the higher laser slope efficiency rather than stronger absorption line P(13). From Fig. 7(b), we can see that there is an optimum pressure for the given fiber length and pump power. As the pressure increases from vacuum, the increasing molecular density in the HCF results in more pump absorption and gain giving higher lasing output. Beyond a certain pressure, the increased intermolecular collision rate increases the internal losses and decreased the gain, leading to reduced output power and an increased laser threshold.
We have demonstrated here watt-level tunable mid-infrared fiber gas laser sources at both pulsed and CW regimes for the first time. Pumped with a precisely tunable amplified diode laser, step-tunable mid-infrared emission of 3.09~3.21 μm is generated in acetylene-filled HCFs. The output power of the mid-infrared emission has been characterized with various absorption lines and gas pressure, and we get a maximum laser pulse average power of ~0.3 W (~0.6 μJ pulse energy) with a power conversion efficiency of ~16%, and a maximum CW laser power of ~0.77 W with a power conversion efficiency of ~13%. Tunable mid-infrared fiber sources have many applications in environmental monitoring, infrared interference and confrontation, remote sensing and so on. One useful future work is to make all-fiber gas laser systems, which are compact and robust. By properly designing the HCF and selecting other gases as the active medium, such system can provide a potential way to extend wavelengths of fiber lasers to broader mid-infrared regions.
National Natural Science Foundation of China (NSFC) (Grant No. 11504424).
We thank Dr. Yingying Wang from Beijing University of Technology for providing the HCF for our experiments. We are also grateful to Dr. Zhihong Li, Dr. Xiaoming Xi and Dr. Chengmin Lei for useful discussions.
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