We investigate the mid-IR laser beam characteristics from an acetylene-filled hollow-core optical fiber gas laser (HOFGLAS) system. The laser exhibits near-diffraction limited beam quality in the 3 μm region with M2 = 1.15 ± 0.02 measured at high pulse energy, and the highest mid-IR pulse energy from a HOFGLAS system of 1.4 μJ is reported. Furthermore, the effects of output saturation with pump pulse energy are reduced through the use of longer fibers with low loss. Finally, the slope efficiency is shown to be nearly independent of gas pressure over a wide range, which is encouraging for further output power increase.
© 2017 Optical Society of America
Hollow-core Optical Fiber Gas Lasers (HOFGLAS) offer the high average power, high damage threshold and very good slope efficiency of gas lasers [1–5] along with the merits of a fiber-based laser . The first successful demonstration of HOFGLAS by Jones et al. [7,8] heralded a new era in gas and fiber laser research. Since then, scientists have demonstrated HOFGLAS in a variety of gasses including HCN, CO2 and I2 [8–12], spanning a wide wavelength range, and even demonstrating CW operation [12,13]. Much of this success is owed to a new branch of hollow-core photonic crystal fibers (HC-PCF) based on an inhibited coupling (IC) mechanism [14,15]; with the introduction of a hypocycloidal (i.e. negative curvature) core contour , record-setting propagation losses of 17 dB/km around 1 µm  and 7.7 dB/km at 750 nm  have been demonstrated. Such fibers have formed the basis of efficient HOFGLAS systems.
Acetylene-filled HOFGLAS is of specific interest because it can operate in the eye-safe mid-IR spectral region at 3 μm. It has a large scope for applications in remote sensing, communications, trace gas sensing and molecular fingerprinting. The lasing mechanism in acetylene that brings about stimulated emission at 3 μm has already been well discussed [5,9]. An improved realization of acetylene HOFGLAS by Jones et al. was reported [9,11] in which an optical parametric amplifier (OPA) pumped an acetylene-filled HOFGLAS that achieved laser operation at 3 μm with more than 20% slope efficiency. Recently, improved performance at 3 μm has also been demonstrated using a modulated, fiber-amplified diode laser as the pump laser . Hassan et al.  added a feedback fiber to the traditional HOFGLAS system, thereby reducing the pump power required to produce 3 μm output.
Our work here highlights the power scalability and beam quality that can be achieved from a stable OPA-pumped acetylene-filled HOFGLAS configuration. We report the highest 3 μm pulse energy output achieved from an acetylene-filled pulsed HOFGLAS . With a HOFGLAS system comprised of a 10.9 m length of IC kagome fiber filled with acetylene at 9.8 torr, we were able to achieve the maximum 3 μm pulse energy of 1.4 μJ when acetylene absorbed 8.2 μJ of OPA pump pulse energy along the P(13) absorption line at 1.53 μm. To the best of our knowledge, this output laser pulse energy is nearly two times higher than the pulse energy reported in the most recent work . The performance of the laser in terms of the highest 3 μm pulse energy that it can deliver is expected to increase further once higher pump pulse energies are employed. Also, the laser operated at a nearly constant slope efficiency of ~20% with respect to the absorbed pump pulse energy, nearly independent of the acetylene pressure inside the fiber.
The beam quality of the laser output was characterized using the finite slit method described in [21,22] to measure the beam waist along the focus and thereby obtain the M squared (M2) of the 3-μm laser output. This first-time investigation of the beam quality of the output from acetylene-filled HOFGLAS yields M2 of 1.15 ± 0.02 for the 3-μm output beam reflecting the near-diffraction limited performance of the laser. Given that beam combining results with M2 of ~1.3 have been considered nearly ideal [23,24], the measured M2 of our HOFGLAS system makes it an excellent candidate for coherent beam combining. The good beam quality also makes it a promising laser source when it comes to other applications in machining, medicine and telecommunications [23–25].
2. Experimental setup
The OPA-pumped acetylene-filled HOFGLAS setup is shown in Fig. 1(a). The key component of the setup is the 10.9 m long IC kagome-structured fiber. The low loss of the kagome-structured fibers over broad bandwidths makes them a suitable choice for fiber gas lasers that require a long interaction length. The core of these IC kagome fibers is hypocycloidal, which enhances the coupling inhibition between the core and cladding modes [16,17]. The fiber has an outer diameter of 360 μm and the hypocycloid core diameter varies between 60 µm and 72 µm, as shown in Fig. 1(c).
The design parameters of the fiber are chosen such that the fiber provides spatial guidance and bandwidth for the pump and lasing wavelengths, shown in Fig. 2(a), thereby enabling coherent laser emission. The fiber loss was measured to be 0.08 dB/m at the pump wavelength of 1.53 μm and 1.13 ± 0.05 dB/m at the average lasing wavelength of 3.1 μm. The measured loss is consistent with the empirical scaling laws reported in  for inhibited coupling HC-PCF.
The fiber is supported with its ends enclosed in vacuum chambers that can be filled with acetylene to required pressures (1.2 to 14 torr). The gas is allowed to reach equilibrium through the length of the fiber over a span of a few hours. The acetylene in the fiber is then pumped using an OPA that can produce a maximum pulse energy of 21 μJ with ~1 ns pulse duration, spectral width of 440 MHz based on the time-bandwidth product, and a repetition rate of 30 Hz at 1.53 μm. A combination of half-wave plate and polarizing beam splitter acts as a variable attenuator to regulate the pump pulse energy for the HOFGLAS setup. An anti-reflection (AR) coated BK7 lens is used to couple the pump pulse into the core of the fiber. An AR coated BK7 window on one of the vacuum chambers allows the pump to be coupled into the fiber and a CaF2 window on the other chamber enables both the pump and laser wavelengths to be coupled out of the chamber efficiently.
The focal length of the lens that couples the OPA pump beam into the fiber is chosen such that the mode of the OPA pump beam has maximum overlap with the fundamental mode of the fiber. Improved mode quality of the homebuilt OPA output played a crucial role in obtaining a good energy coupling into the fiber. An average pump coupling efficiency of 52% was obtained from this configuration. The output beam is collimated using a CaF2 lens of focal length 150 mm. A germanium filter at the output separates the laser output from the residual pump for laser diagnostics.
3. OPA pumped acetylene-filled HOFGLAS characteristics
The performance of the laser is characterized for various acetylene pressures in the fiber in terms of the spectral content of the laser output, the output pulse energy and laser beam quality, the results of which are discussed in the subsections that follow.
The seed diode laser for the OPA allows tunability of the pump wavelength on and off resonance with the P(13) transition of acetylene at 1.53 μm. The seed laser for the OPA is tuned on resonance with the ν1 + ν3 P(13) transition line of acetylene. This results in laser emission along the P(13) and R(11) transitions.
3.1 Spectral content
The simple molecular structure of acetylene and the lasing transitions that lead to emission at 3 μm are shown in the inset of Fig. 2(b). To initiate lasing, the pump wavelength is tuned to the P(13) absorption line in acetylene, corresponding to the transition between the vibrational ground state and the ν1 + ν3 vibrationally excited state at 1.53 μm, which is Doppler-broadened to ~475 MHz and further pressure-broadened by ~11 MHz/torr . Then population inversion between the rotational-vibrational states results in lasing at 3.11 μm and 3.17 μm which are the R(11) and P(13) lines in acetylene corresponding to the transitions between the excited state and the ν1 vibrational state, as shown in Fig. 2(b).
3.2 Output pulse energy and laser efficiency
The use of sensitive pyroelectric energy meters allowed the characterization of the laser efficiency over a broad range of operating parameters. The 1.53 μm and 3 μm pulse energies were measured at different positions in the laser configuration. To measure pulse energy at 3 μm, a Ge filter placed just before the detector effectively blocked the residual 1.53 μm pump. Figure 3(a) shows the 3 μm output pulse energy as a function of the pump pulse energy coupled into the fiber when a coupling lens of focal length f = 75 mm was used. By measuring the input OPA pump pulse energy and the residual pump pulse energy after the vacuum chambers, it is possible to estimate the total pump pulse energy absorbed by the gas when the pulse is on resonance with the P(13) transition, distinct from of the pulse energy lost to imperfect fiber guidance or to imperfect coupling optics. The 3 μm output pulse energy from the laser versus the total pump pulse energy absorbed by acetylene in the fiber can hence be estimated for various acetylene pressures, Fig. 3(b).
A similar curve appearing in  shows slightly smaller quantities on the x-axis for estimated absorbed pump pulse energy. The discrepancy is due to a difference in the way the coupling efficiency was estimated when accounting for fiber loss. Figure 3 reflects our best understanding of those losses at the present time. It can be observed from Fig. 3 that for pressures below 9 torr, the 3 μm output energy increases but eventually saturates as the pump pulse energy absorbed by the acetylene gas increases. The saturated output pulse energies are plotted against the pressure of acetylene in Fig. 4(a). Also the slope efficiency of the laser is obtained by fitting the linear region of each curve in Fig. 3(b), and is plotted against acetylene pressure in Fig. 4(b) (purple squares).
For the range of pump pulse energy achievable from the current OPA, when acetylene pressure is above 9 torr the laser operates without seeing any saturation behavior, and produces a maximum 3 μm pulse energy of 1.41 μJ at 9.8 torr of acetylene pressure. At this highest output pulse energy the slope efficiency of the laser is ~17% and the overall efficiency with respect to the incident pump pulse energy is ~14%. Figure 4(a) clearly shows that as the acetylene pressure increases, the pump energy at which saturation occurs increases. This may be due to linear dependence on pressure of the number of molecules participating in stimulated emission. The plot also shows the extrapolation of a linear fit to the measured values of saturated output pulse energies, suggesting that the output of the laser may reach 2 μJ without saturation when sufficient pump power can be realized. We expect this predicted power scaling at higher acetylene pressures provided that factors like pressure broadening, fiber loss, and other effects do not limit the laser performance at the higher pump powers.
We have also compared our laser efficiency with that of a diode pumped HOFGLAS configuration reported in , as shown in Fig. 4(b). It can be seen that while the laser efficiency of the diode-pumped configuration varies over a wide range from ~8% to ~30%, the slope efficiency of our OPA pumped HOFGLAS system stays fairly consistent and stable at ~20% over the same range of acetylene pressures for which the laser performance was investigated. The slope efficiency of the laser is nearly independent of the pressure of acetylene in the hollow core fiber, indicating that collisional relaxation is not a limitation on laser performance in this regime. This behavior is encouraging for scaling to higher pressures and pump powers. Further investigations are underway to understand the dependence of slope efficiency on the fiber loss at 3 μm and the length of the fiber.
3.3 M2 of 3 μm output beam
The laser mode quality is characterized in terms of M2, which provides information about the beam’s propagation properties and achievable brightness. The M2 is determined by measuring the beam profiles of the 3 μm laser output through the focus of a fixed position lens in free space when the laser operated at an acetylene pressure of 10 torr and coupling between the fiber and the 1.53 μm pump beam was optimized for single mode operation. In this case, 1.15 μJ was the maximum observed 3 μm pulse energy.
The beam widths reported here were measured using a CaF2 plano-convex lens of 150 mm focal length and a 20-μm wide slit. The slit was scanned along the transverse axis (x) of the output beam to obtain beam profiles at different positions along the propagation axis (z). An example of this data set is shown in Fig. 5, for which a series of beam profiles was measured at the highest 3-µm pulse energy produced by the laser. This method of measuring the M2 of the beam by scanning the beam using a slit at various axial positions typically requires a few hours owing to the 30 Hz repetition rate of our OPA pump in the current laser configuration. The laser operated stably through the measurement, which can be seen in the smoothness of the curves comprising the cascade plot of Fig. 5 and the small error bars in Fig. 3.
To extract M2, the beam widths for each profile in Fig. 5 were obtained using the ISO standard D4σ method and were fitted to the definition of M2 [21,22] with an averaged wavelength of 3.143 μm. Figure 6(a) shows beam widths calculated as the second moment of each beam profile curve in Fig. 5, and the fit that yields an M2 value of 1.15 ± 0.02. The error in the M2 value comes from fitting the measured beam widths to the definition of M2 in , and represents the uncertainty on an average of 50 shots. The uncertainty on a measurement of individual pulses, assuming random noise, is expected to be ~0.14.
The M2 of the laser output was also measured for other pump pulse energies when the HOFGLAS operated with acetylene pressure at 10 torr. The M2 measurements at other pulse energies, shown in Fig. 6(b), indicate that the beam quality of the laser output stayed relatively uncompromised.
M2 measurement reproducibility was checked at two powers using a 50 μm wide slit. In both cases, M2 values were reproduced as shown in Fig. 6(b). This shows the near-Gaussian beam profile of the produced 3μm output and hence the near-diffraction limited performance of the acetylene-filled HOFGLAS system. This clearly indicates that the laser operates stably with a good diffraction-limited beam quality.
We have demonstrated near-diffraction limited performance of an OPA-pumped acetylene-filled HOFGLAS system at high pulse energy near 3.1 μm. The HOFGLAS with a kagome-structured fiber and a hypocycloidal core of 10.9 m operates without any saturation at a slope efficiency of ~20% up to pulse energy of 1.4 μJ. The stable 3 μm output comes with a good mode quality (M2 = 1.15) and improved performance over the previous realizations of acetylene-filled HOFGLAS systems with higher pulse energies and reduced dependence of slope efficiency on pressure.
Air Force Office of Scientific Research (AFOSR) (FA9550-14-1-0024). Air Force Research Laboratory (AFRL) (FA9451-17-2-0011). Agence Nationale de la Recherche (ANR) (PHOTOSYNTH) and (Σ_LIM Labex Chaire). La région Limousin.
We thank Wolfgang Rudolph and Vasudevan Nampoothiri for useful discussions, and Sajed Hosseini-Zavareh and the staff of the James R. Macdonald laboratory for helpful technical contributions. We also thank the PLATINOM platform for the help in the fiber fabrication.
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