Abu Hassan, Muhammad Rosdi

Bateman, Samuel

Knight, Jonathan

Love, Adrian

Wadsworth, William

Yu, Fei

A new generation of very low loss hollow fibers (~30 dB/km at 3 µm) in the mid-IR has opened up the possibilities for fiber gas lasers and laser beam delivery in this challenging spectral region

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        Citation for published version:Wadsworth, W, Love, A, Bateman, S, Yu, F, Abu Hassan, MR & Knight, J 2015, 'Lasers Using Low Loss HollowOptical Fibres in the Mid-Infrared' Paper presented at OECC, Shanghai, China, 28/06/15 - 2/07/15, .Publication date:2015Document VersionPeer reviewed versionLink to publicationPublisher RightsUnspecifiedUniversity of BathGeneral rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Download date: 07. Dec. 2019Lasers Using Low Loss Hollow Optical Fibres  in the Mid-Infrared  W.J. Wadsworth, A.L. Love, S.A. Bateman, F. Yu, M.R. Abu-Hassan and J.C. Knight Centre for Photonics and Photonic Materials, Department of Physics University of Bath Bath, UK W.J.Wadsworth@bath.ac.uk   Abstract—A new generation of very low loss hollow fibers (~30 dB/km at 3 µm) in the mid-IR has opened up the possibilities for fiber gas lasers and laser beam delivery in this challenging spectral region. Keywords—optical fiber; mid-infrared; gas laser; fiber laser I.  INTRODUCTION  Hollow optical fibers confining light in a hollow core by a photonic bandgap formed in an intricate periodic cladding were first developed 15 years ago [1]. Such fibers promised a revolution in the way one can think about optical fiber materials. No longer was one limited to solid materials forming a fiber core, but gases became possible, and one could imagine that one also might not be limited in the wavelength range of operation by the stringent material transparency requirements of solid optical fibers, or limited in power level by damage or nonlinearity in the fiber. Since then much of this promise has been realized in high energy pulse transmission [2,3] and nonlinear interactions in gases [4]. Unfortunately, whilst excellent performance has been demonstrated and great physical insights have been gained, such fibers remain difficult to fabricate and so are uncommon and costly. Many of the advantages of hollow-core photonic bandgap fibers can however be achieved in simpler structures using anti-resonant features to confine light in a hollow core. This process of simplification has culminated in the development over the last couple of years of hollow fibers with the functional cladding structure reduced to the boundary of the hollow core, formed by a thin glass wall whose thickness and shape defines the fiber properties [5,6,7]. In a relatively short space of time these new fiber designs have revolutionized the way in which one can think of using hollow optical fibers. The simplicity of the structure (Fig. 1) and broad transmission bands makes fabrication much less challenging for any particular application. Furthermore these hollow core silica fibers have been shown to have extremely low attenuation, particularly in the mid-infrared (of the order of 30 dB/km at 3 µm [6,8]) where one would expect a silica fiber to be highly attenuating (bulk attenuation 50 dB/m). Here we consider two applications taking advantage of this new generation of long, low-loss, large core, mid-infrared fibers – fiber gas lasers with electrical and optical pumping. These applications illustrate the properties of the fiber which are also useful for high power laser beam delivery, gas sensing, nonlinear Raman wavelength conversion and other uses in the mid-IR.    (a) (b) Fig. 1. Hollow-core fibres designed for mid-IR guidance. Fibre (b) has less suceptability to bend loss. The fabrication and guidance of these fibres are considered in [6] and [7]. II. GAS DISCHARGE LASERS Electrically-pumped lasers using DC glow discharges were amongst the first lasers to be demonstrated and were among the early commercial laser successes. For the neutral noble gas lasers (notably He:Ne, but also neutral Xe, Ar and Kr) it was shown that the inversion and gain is enhanced at higher pressure in smaller diameter discharge tubes [9]. Unfortunately there has always been a limit to exploring this effect as diffraction of a free-space laser beam becomes significant for diameters of the order of 1 mm. Grazing incidence reflection in a thick walled capillary can provide some measure of guidance, but with high loss and extreme sensitivity to bending. Nevertheless lasers were demonstrated using gas discharges in specially straightened short capillaries [10,11]. A hollow core optical fiber immediately overcomes the guidance limitation, but can present difficulties with achieving the electrical discharge – breakdown voltages rise for small tube diameters, which is compounded by the fact that the optimum gas pressure required for lasers increases for small tubes and the breakdown voltage rises further for higher pressures. Many of the first hollow-core optical fibers, particularly those with photonic bandgaps, have very small cores, a few times the wavelength. The new generation of fibers using anti-resonant guidance (of the types shown in Fig. 1) have much larger cores, typically of This work was supported in part by EPSRC grant No EP/I011315/1the order of 30 . Furthermore there are particularly strong laser transitions in the mid-infrared (He:Ne at 3.39 µm, Xe at 3.51 µm), where the relatively long wavelength also means the fibers are physically larger. Overall these two effects mean that the fiber core diameter is large enough for the glow discharge to be initiated and maintained relatively easily, whilst the structure is still far smaller than previous attempts and can also be long and flexible. Using a fiber with a structure like Fig. 1(b) we have initiated gas discharges up to 1 m in length with discharge voltages of the order of 15 kV in He:Xe mixtures at 12 mbar [12]. The fiber core diameter was ~120 μm with an attenuation <0.2 dB/m at 3.5 µm. Fig. 2 shows the super-linear dependence of single-pass output with fiber length, demonstrating gain. 024681012140 10 20 30 40 50 60 70 80 90 100 110Signal at 3.5μm (arb. units)Length of Glow Discharge (cm) Fig. 2. Single pass output signalat 3.5 µm for a 5:1 mixture of He:Xe at 12 mbar pressure for 0.25 mA discharge current. III. OPTICALLY-PUMPED GAS LASERS Optically pumped gas lasers have gained interest recently, partly because the precisely controlled pump lasers required to address the narrow pump transitions of gas lasers can now be remarkably compact. By placing the gas in an optical fiber one may increase the length and hence gain and potentially output power of the laser. Many of the useful transitions are in the mid-IR [13,14], where the new fibers excel. Also this application benefits from the large core diameter as this reduces collisional broadening of the atomic transitions at low pressure, and the low attenuation allows for a long length to achieve pump absorption in the low pressure gas. We have demonstrated high single-pass gain and high conversion efficiency at 3.16 µm in a hollow core fiber filled with acetylene [15] pumped with a fiber amplified telecommunications tunable DFB laser at 1530.37 nm. The stability achieved by standard current and temperature control in the DFB laser is sufficient to remain on resonance with the Doppler-broadened acetylene transitions without need for any active locking. Indeed the absolute accuracy of the diodes, even several years after factory calibration, is within a few hundred megahertz of the molecular transitions before fine tuning. The pump laser was modulated into nanosecond pulses before amplification, as the acetylene transitions are self-terminating and the lower laser level must be depopulated by molecular collisions or wall collisions. Fig. 3 shows the output spectrum. The single pass conversion efficiency is high; for a 10.5 m fiber length at 0.7 mbar the threshold pump energy was 50 nJ, and the conversion efficiency >20% at 2 µJ pump energy. IV. CONCLUSION Hollow core silica fibers using anti-resonant structures are a key new technology enabling exciting new openings in fiber lasers. We have demonstrated strong ASE and gain in electrically- and optically-pumped gases in the mid-IR based on these fibers and many more opportunities arise including nonlinear conversion in gases and high power beam delivery.  3.05 3.10 3.15 3.20 3.25020406080100120Wavelength (m)Energy (a.u.)  R(7): 3.12 mP(9): 3.16 m(a) Fig. 3. Measured output optical spectrum of 12C2H2 at 0.7 mbar pressure, 10.5 m length, and 4.2 μJ incident pump energy. REFERENCES [1] R.F. Cregan, B.J. Mangan, J.C. Knight, T.A. Birks, P.StJ. Russell, P.J. Roberts and D.C. Allan, “Single-mode photonic band gap guidance of light in air,” Science, vol. 285, p. 1537, 1999. [2] F. Luan et al., “Femtosecond soliton pulse delivery at 800nm wavelength in hollow-core photonic bandgap fibers,” Opt. Express, vol. 12, pp. 835-840, 2004. [3] F. Gerome, P. Dupriez, J.C. Clowes, J.C. Knight, and W.J. Wadsworth, “High power tunable femtosecond soliton source using hollow-core photonic bandgap fiber, and its use for frequency doubling,” Opt. Express,vol. 16, pp. 2381-2386, 2008. [4] F. Benabid, J.C. Knight, G. Antonopoulos and P.St.J. Russell, “Stimulated Raman scattering in hydrogen-filled hollow-core photonic crystal fiber,” Science, vol. 298, pp. 399-402, 2002. [5]  A.N. Kolyadin, A.F. Kosolapov, A.D. Pryamikov, A.S. Biriukov, V.G. Plotnichenko and E.M. Dianov, “Light transmission in negative curvature hollow core fiber in extremely high material loss region,” Opt. Express, vol. 21, pp. 9514-9519, 2013. [6] F. Yu, W.J. Wadsworth and J.C. Knight, “Low loss silica hollow core fibres for 3–4 μm spectral region,” Opt. Express, vol. 20, pp 11153-11158, 2012. [7] W. Belardi and J.C. Knight, “Hollow antiresonant fibers with low bending loss,” Opt. Express, vol.  22, pp. 10091-10096, 2014. [8] F. Yu and J.C. Knight, “Spectral attenuation limits of silica hollow core negative curvature fiber,” Opt. Express, vol. 21, pp 21466-21471, 2013. [9] C.S. Willett, “Introduction to Gas Lasers: Population Inversion Mechanisms,” Oxford, Clarendon Press, 1974. [10] P.W. Smith and P.J. Maloney, “A self-stabilized 3.5-µm waveguide He-Xe laser,” Appl. Phys. Lett. vol. 22, pp. 667-669, 1973. [11] P.W. Smith, “A waveguide gas laser,” App. Phys. Lett., vol. 19, pp. 132-134, 1971. [12] S.A. Bateman, W. Belardi, F. Yu, C.E. Webb and W.J. Wadsworth, “Gain from helium-xenon discharge in hollow optical fibres at 3 to 3.5 μm,” in CLEO:Science and Innovations, (Optical Society of America, San Jose, USA, 2014), paper STh5C.10, 2014. [13] A. M. Jones, C. Fourcade-Dutin, C. Maoc, B. Baumgart, A. V. V. Nampoothiric, N. Campbell, Y. Wang, F. Benabid, W. Rudolph, B. R. Washburn, and K. L. Corwin, "Characterization of mid-infrared emissions from C2H2, CO, CO2, and HCN-filled hollow fiber lasers," SPIE, 8237: 82373Y, 2012. [14] Andrew M. Jones, A. V. Vasudevan Nampoothiri, Amarin Ratanavis, Tobias Fiedler, Natalie V. Wheeler, François Couny, Rajesh Kadel, Fetah Benabid, Brian R. Washburn, Kristan L. Corwin and Wolfgang Rudolph, "Mid-infrared gas filled photonic crystal fiber laser based on population inversion," Opt. Express, 19(3): 2309–2316 (2011). [15] Z.F. Wang, W. Belardi, F. Yu, W.J. Wadsworth, and J.C. Knight, "Efficient diode-pumped mid-infrared emission from acetylene-filled hollow-core fiber," Opt. 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Lasers Using Low Loss Hollow Optical Fibres in the Mid-Infrared

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Lasers Using Low Loss Hollow Optical Fibres in the Mid-Infrared

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