We demonstrate a 2.8 μm gas Raman laser in a methane-filled hollow-core negative-curvature fiber with average power of 113 mW, pulse energy of 113 μJ and estimated peak power of 9.5 MW. Raman quantum efficiency of 40% has been reached from the pump source at 1.064 μm to the 2nd order vibrational Stokes at 2.812 μm using 1.8 MPa methane gas. To our knowledge, this is the first high peak power fiber-based gas Raman laser in mid-infrared region and a range of applications in supercontinuum generation, laser surgery, molecular tracing and gas detection are in prospect.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
2-5 μm mid-infrared (MIR) lasers are currently at the center of intense study due to a plethora of applications in infrared countermeasures, free-space communications, laser surgery and gas detection [1–3]. Interest has been particularly focused in 2.7-3 μm spectral range firstly because of the overlap of the water absorption peak which could facilitate bio-medical applications [4,5] and secondly as an appropriate pump source for MIR wavelength extension . Specifically, a 2.7-3 μm picosecond laser with high peak power is highly sought-after for precision surgery that requires minimal heat-affected zone to reduce collateral damage [7,8], for destroying enemy detectors in countermeasures  and for broadband MIR supercontinuum generation via nonlinear conversion . Quantum cascade laser is commercially available but could only provide CW output . Optical parametric oscillator and amplifier systems are more versatile but bulky and inefficient. Recently, ultrafast fiber lasers based on holmium or erbium-doped ZBLAN fibers have been developed showing high superiority but the peak power is limited to kW level [11–13] (e.g. nJ pulse energy for fs pulse duration). The issue of optical damage, fiber strength and nonlinearity poses restrictions on fiber lasers based on soft glass material .
Gases are important alternatives to solid state media with indispensable attributes of high damage threshold and wide transparency window for achieving high output power at unusual wavelength. Recent progress on hollow core fibers (HCF) regenerate people’s interest in gas lasers, since previous concerns on low efficiency and bulky setup has been conquered by the HCF platform. Indeed, a high performance HCF, for example a hollow-core negative curvature fiber (HC-NCF) [15,16] with low transmission loss, broad bandwidth and quasi-single modeness could provide a versatile and compact platform for gas-laser interaction. Gas lasers based on HCF could be competitive to and sometimes better than solid state lasers. In recent years, optically pumped gas lasers at 3 μm has been developed using acetylene gas enabling 1W level CW output  and 1.4 μJ pulsed emission  respectively. Stimulated Raman scattering of gases is another promising approach for long wavelength conversion with flexible wavelength tunability and high efficiency. Previous work has been concentrated in visible and near IR region [19,20] where high peak power of 400 kW and 150 kW has been achieved at 1.5 μm  and 1.9 μm [22,23] respectively using ethane and hydrogen gases. At MIR spectral range, the only demonstration is a 4.4 µm gas Raman laser [24,25] with quantum efficiency of 36%, average power of 250 mW and peak power of only 2 kW, too low for many applications.
In this paper, we demonstrate a 2.8 μm gas Raman laser in a methane-filled MIR-guiding HC-NCF with a Raman quantum efficiency from 1.064 μm to 2.8 μm reaching 40%, an average power of 113 mW, a pulse energy of 113 μJ and an estimated peak power of 9.5 MW (in assumption of similar pulse duration of 12 ps with the pump laser). To our knowledge, this is the first high peak power fiber-based gas Raman laser in MIR region.
2. Octave-spanning HC-NCF for MIR spectral range
Figure 1(a) shows the scanning electron microscope (SEM) image of the in-house fabricated HC-NCF consisting of one ring of 6 untouched thin tubes forming a negative-curvature hollow-core. The fiber is fabricated via modified stack-and-draw technique with an inscribed core diameter of 68 μm, a tube diameter of 34 μm, a membrane thickness of 585 nm and an outer diameter of 200 μm. Our recently developed multi-layered model  elaborately explains the guidance mechanism in this type of HCF.
The fiber’s transmission and attenuation performance is characterized by a cut-back measurement and numerical simulation is carried out coordinately for assistance. For the near-IR region from 800 to 1700 nm, a supercontinuum source (NKT Photonics, SuperK Compact) is butt-coupled to a 5 m long fiber and the output spectrum is recorded by an optical spectrum analyzer (Yokogawa, AQ6370C) as shown in the green curve in Fig. 1(b). The low transmission band from 1100 to 1243 nm is a consequence of the first order resonance in the glass membrane . Due to the large core size and high bending sensitivity in this spectral range, we couldn’t determine the fiber’s attenuation via the cut-back measurement. Numerical simulation based on a finite-element mode solver (Comsol Multiphysics) together with an optimized mesh size and a perfectly matched layer indicates a confinement loss of <0.03 dB/m at 1064 nm, neglectable for our experiment. To measure the attenuation in the MIR region, an optical parametric oscillator system (OPO, EKSPLA PL2210) tunable from 1.55 μm to 3.35 μm is coupled into a 36 m piece of fiber with a bend radius of 16 cm via a CaF2 lens. An energy meter records the input and output energy before and after the fiber respectively at each wavelength. Here, the wavelength of the OPO is manually tuned with an interval between 5 nm to 20 nm depending on the studied wavelength. At absorption peaks of OH- or HCl, a higher resolution of 5 nm is adopted. Totally 150 points are recorded from 1.55 μm to 3.35 μm. The fiber is then cutback to 6 m and the readings on the energy meter are recorded again before and after the fiber. Here the two sets of input energy is used to deduce the stability of the OPO system which shows less than 1% fluctuation and the data after the 36 m and 6 m fiber are used for calculating the optical attenuation. Figure 1(b) shows the attenuation spectrum with a minimum value of 50 dB/km ± 3 dB/km at 2.45 μm. A high loss region appears from 2.55 μm to 2.85 μm corresponding to absorption peaks of CO2 and OH-  in the air and the measured loss at our Raman emission wavelength of 2.812 μm is 0.41 dB/m. Note that this loss figure is greatly affected by the water absorption peak while in our follow-up Raman laser experiment, the high pressure of methane could expel most of the water in the air and hence substantially reduce the transmission loss at this wavelength. The simulation results indicates a confinement loss level of 0.015 dB/m at 1.544 μm and 0.045 dB/m at 2.812 μm. In the simulation, the effect of the silica material loss [insert of Fig. 1(b)] has been taken into account by multiplying a coefficient that corresponds to the spatial overlap of the mode in the hollow core with the silica (2 × 10−5). In this spectral range, the bulk material’s absorption has neglectable influence to the confinement loss thanks to the low spatial overlap. Previously, MIR guiding ice-cream shaped NCF has been reported showing a similar level of loss but in a much narrower bandwidth of ~500 nm due to the thicker glass membrane [28,29]. Our broadband guiding NCF for the MIR spectral range could find a range of applications in molecular tracing, gas detection, pulse compression and gas lasers.
3. 2.8 μm Raman laser emission
Figure 2 shows the experimental setup for the fiber based gas Raman laser. The pump source is a home-made diode-pumped Nd:YAG laser emitting pulses at 1064 nm with duration of 12 ps, spectral linewidth of 0.24 nm, repetition rate of 1 kHz, peak power of 91 MW and average power of 1.1 W. The pump laser passes through a set of half wave plate (λ/2), polarization beam splitter (PBS) and quarter-wave plate for adjusting the power and the polarization and is coupled into a 3 m long fiber via a 50 mm coated plano convex lens with a coupling efficiency of 76%. The fiber is kept straight to avoid bending loss and is sealed at both ends by two gas chambers for methane gas filling. CaF2 optical windows with 90% transmissivity from 0.5 μm to 3 μm are assembled in the gas chambers for optical alignment. In this experiment, we use CH4 gas because of its suitable Raman frequency shift of 2917 cm−1 via ν1 vibration mode and its high steady state gain coefficient of approximately 0.3 cm/GW [30,31]. This could enable the first order Stokes Raman line at 1.544 μm and the second order Stokes Raman line at 2.812 μm. At the output, the spectrum is recorded by a monochromator (Horiba, iHR320) and the mode profile is recorded by a camera (Ophir-Spiricon Pyrocam III Beam Profiler, Model PY-III-C-A). To measure the output power, the output beam is filtered by two bandpass filters centered at 1550 nm and 2750 nm with bandwidth of 12 nm and 500 nm, transmissivity of 40% (Thorlabs, FB1550-12) and 70% (Thorlabs, FB2750-500) respectively before entering the thermal power meter.
In a hollow-core fiber based gas Raman laser, a comprehensive understanding of the stimulated Raman scattering (SRS) process has been provided by . Here, we only provide a simple estimation on the Raman energy threshold based on [31–33]. The dephasing time T2 of CH4 for a pressure of 18MPa is roughly 26 ps  and here with pump pulse duration τ = 12 ps, we are clearly operating in the transient regime with high degree of molecular coherence and an energy threshold :31]. This yields a Raman energy threshold of roughly 1.9 μJ, much lower than the threshold using a capillary waveguide . In our measurement, due to the sensitivity of the monochromator and a number of indispensable loss, we observed the Raman signal at 1.544 μm at an input coupled energy of 7 μJ (average power 7 mW) and the 2nd order Stokes at 2.812 μm appears at input coupled energy of 26 μJ (average power 26 mW). Figure 3 depicts the measured spectrum at input coupled power of 381 mW and gas pressure of 1.5 MPa showing the pump, the 1st and 2nd order Stokes lines. Weak anti-Stokes emission at visible wavelength (1st, 2nd, 3rd, and 4th order anti-Stokes lines at 812 nm, 656 nm, 552 nm, 475 nm) are also observed in our experiment but not recorded here. Due to the different sensitivities of the monochromator at the three wavelength, the intensity depicted in Fig. 3 are not scalable. Though the fiber has a large core size of 70 μm, we observed quasi-single mode output at both the pump and the Stokes wavelengths thanks to the proper coupling of the pump laser.
In Fig. 4(a), the evolution of the output power (normalized to the total transmitted power) of the residual pump, the 1st Stokes and the 2nd Stokes are plotted as a function of the coupled input pump power at an optimized pressure of 1.8 MPa. This optimized pressure is obtained by altering the pressure in the gas chamber at several different pump powers for maximum conversion efficiency at 2.812 μm. We observed slightly decreased conversion efficiency at higher pressures probably because of the saturation of Raman gain. For the recorded output power, the optical loss of the CaF2 window and bandpass filters have both been taken into account. At low input coupled powers below 220 mW, the 1st order Stokes at 1.544 μm is stronger than the 2nd order one at 2.812 μm. With the increase of the pump power, the 2.812 μm Raman emission surpasses that of the 1.544 μm and increases linearly with the pump power. In this cascaded Raman process, the 1st order Stokes acts as a pump for the 2nd order one and saturates in a low quantum efficiency [Fig. 4(b)] while the 2nd order Stokes experience a notable growth. Similar phenomenon has been observed in [24,34]. At a coupled input power of 766 mW, the output power at 1.544 μm and 2.812 μm are 71 mW and 113 mW and the quantum efficiency reach 14% and 40% respectively. No saturation on the output power or quantum efficiency is observed, indicating further power scaling may be possible by using a higher power pump source. The 113 mW Raman emission at 2.812 μm corresponds to the maximum pulse energy of 113 μJ. Due to the lack of proper equipment, we couldn’t determine the pulse duration but from the literature [21,23], the Stokes lines usually have a similar and sometimes even shorter pulse duration than the pump. If we estimate the pulse duration for the 4.4 μm laser line is 12 ps (similar to the pump), this corresponds to a peak power of 9.5 MW.
In conclusion, a 2.8 μm fiber based gas Raman laser is demonstrated with MW level high peak power. We consider it as a promising alternative to solid state lasers with the possibility of enabling high peak power MIR supercontinuum generation if a proper nonlinear media is selected and many military and bio-medical applications are in prospect.
National Natural Science Foundation of China (NSFC) (No. 61377098, 61675011, 61527822, 61535009).
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