Bateman, Samuel

Belardi, Walter

Love, Adrian

Wadsworth, William

Webb, Colin E.

Gain is observed in a double pass of a Helium-Xenon gas DC discharge in a 90cm long flexible hollow core fibre. Output at 3.5μm increases with discharge current up to the maximum of 0:55m

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        Citation for published version:Love, A, Bateman, S, Belardi, W, Webb, CE & Wadsworth, W 2015, 'Double Pass Gain in Helium-XenonDischarges in Hollow Optical Fibres at 3.5 m' Paper presented at CLEO:2015, San Jose, USA United States,10/05/15 - 15/05/15, .Publication date:2015Document VersionPeer reviewed versionLink to publicationUniversity 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. 2019Double Pass Gain in Helium-Xenon Discharges inHollow Optical Fibres at 3.5 µmA.L. Love1∗, S.A. Bateman1, W. Belardi1†, C.E. Webb2, W.J. Wadsworth11Centre for Photonics and Photonic Materials, Department of Physics, University of Bath, Bath, BA2 7AY, UK2Department of Physics, University of Oxford, Clarendon Laboratory, Parks Rd., Oxford, OX1 3PU, UK†Now at the Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, UK∗A.Love@Bath.ac.ukAbstract: Gain is observed in a double pass of a Helium-Xenon gas DC discharge in a90cm long flexible hollow core fibre. Output at 3.5µm increases with discharge current up tothe maximum of 0.55mA.© 2014 Optical Society of AmericaOCIS codes: 060.2390 Fiber optics, Infrared; 060.3510 Lasers, fiber; 140.1340 Atomic gas lasers.Electrically pumped gas discharge lasers have exploited the abundance of laser lines in atomic and molecular gasesto produce sources at a wide range of wavelengths from the UV to the infrared. It has been shown that when usinga neutral noble gas as a medium, narrower bore tubes can produce more gain and output power [1, 2]. However, asconventional gas discharge tubes require to light to propagate as a free space beam, the dimensions of the tube havebeen limited by Gaussian beam optics to around 1 mm in diameter. One attempt to overcome these limits was touse specially straightened sub-mm capillary waveguides [1]. While these were effective, they are still dimensionallylimited as the waveguide loss scales with r−3 and they are extremely bend sensitive, restricting the tubes to diametersgreater than 250 µm and lengths of 5–30 cm. For this reason gas lasers still had the image of requiring long, rigorouslystraight glass tubes, that is until recent development of low-loss hollow core optical fibre technology [3] opened upthe possibility of having a flexible gas discharge laser, capable of having a very narrow discharge tube without anyfundamental limits on length besides the voltages required to sustain the discharge. The narrower tube diametershave however made the discharge parametres much harder to achieve. While several groups have attempted to createdischarges in very narrow bore capillaries and fibres with a variety of methods including DC, RF and microwavecoupled discharges [4, 5], recent breakthroughs were made in creating DC-excited glow discharges in fibres over1 m in length [6]. Using a mixture of HeXe, strong indications of gain on the three xenon transitions of 3.11 µm,3.37 µm and 3.51 µm were observed over a variety of fibre lengths. Here we continue with that work with a doublepass experiment with these discharges, as a step towards a full laser cavity.3-4 µmSpectrometerH. V.SourceBallastResistorAnodeCathodeHollow CoreFibreTo VacuumPump andGas InputSphericalMirrorSignal100mm(a) (b) 50µm×300Fig. 1. (a) Experimental setup. (b) SEM image of the fibre.The system shown in Fig. 1 (a) was used to produce and sustain DC glow discharges within a 90 cm long hollowcore optical fibre. The ends of the fibre were held inside a pair of gas cells for the evacuation and delivery of gas tothe system, inside which a pair of electrodes were positioned near the fibre ends to deliver a DC high voltage of up to40 kV with a variable current. The anode gas cell featured a large calcium fluoride window perpendicular to the fibre.The fibre used (Fig. 1 (b)) had a hollow core inner diameter of approximately 120 µm, and a low loss guidance bandfrom 3–4 µm (< 0.18 dB/m at 3.5 µm). This type of fibre has a low bend loss of ∼0.3 dB per turn of radius 8 cm [7].The spectrometer (Bentham DTMc300) had a grating with 300 grooves/mm and a cryogenically cooled InSb detectorfor the range at 3–4 µm.Discharges were performed with a 5 : 1 mixture of HeXe at a total pressure of 12 mbar which produced a strongemission at the 3.51 µm transition. By positioning a 100 mm radius of curvature circular mirror at the anode end ofthe fibre, light was reflected back into the fibre to perform a double pass. Assuming that the amount of light comingout of each end of the fibre is equal and that the coupling efficiency between the mirror and the fibre is 100%, then inthe absence of gain the best possible increase of signal in the double pass is a factor of two. Fig. 2 (a) shows the signalfor a single and double pass against the discharge current normalised to the signal from a single pass with a current of0.2 mA. These were recorded by covering and uncovering the mirror while stepping the current up then down for thesame discharge. From this is it clear that the factor difference between the single and double passes is greater than two,at its highest reaching a factor of 2.6, which indicates gain as the actual coupling efficiency will be considerably lessthan 100%. The small increase is also an indication that the system may be close to saturation by amplified stimulatedemission even from a single pass. Fig. 2 (b) shows the emission spectrum of a single and double pass for a singledischarge at a current of 0.25 mA, which shows no noticeable increase in signal on the 3.11 µm and 3.37 µm emissionlines, showing a gain on the 3.51 µm line and not on any of the other emission lines.0.2 0.3 0.4 0.5 0.6123456789Current / mASignal / arbitrary units  Single PassDouble Pass(a)3 3.1 3.2 3.3 3.4 3.5 3.600.511.522.5Wavelength / µmSignal / arbitrary units  Single Pass 1Double Pass 1Single Pass 2Double Pass 2(b)Fig. 2. (a) Signal at various currents for single (dashed) and double (solid) pass discharges in 90 cmof fibre. (b) Spectra for single (dashed) and double (solid) pass discharges in 90 cm of fibre with adischarge current of 0.25 mA and 20 nm resolution.We have shown gain in the 3.51 µm Xe transition in HC-PCF gas discharges by performing a double passexperiment, and in doing so we have demonstrated that it is possible to couple light from these discharges backinto the fibre for an electrically excited fibre-based gas laser.References[1] P.W. Smith, “A waveguide gas laser,” Applied Physics Letters, 19, 132 (1971)[2] D. Schuo¨cker, W. Reif and H. Lagger, “Theoretical Description of Discharge Plasma and Calculation ofMaximum Output Intensity of He-Ne Waveguide Lasers as a Function of Discharge Tube Radius,” IEEE J.Quantum Electronics, QE-15, 232 (1979)[3] F. Yu, W. Wadsworth and J. C. Knight, “Low loss silica hollow core fibres for 3–4 µm spectral region,” OpticsExpress, 20, 11153, (2012)[4] X. Shi et al, “Investigation of glow discharge of gas in hollow-core fibres,” Appl. Phys. B, 91, 377 (2008)[5] B. Debord et al., “Generation and confinement of microwave gas-plasma in photonic dielectric microstructure,”Opt. Express, 21, 25509 (2013)[6] S. A. Bateman et al., “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[7] W. Belardi and J.C. Knight, “Hollow antiresonant fibres with low bending loss,” Opt. Express, 22, 10091 (2014)

Double Pass Gain in Helium-Xenon Discharges in Hollow Optical Fibres at 3.5 μm

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Double Pass Gain in Helium-Xenon Discharges in Hollow Optical Fibres at 3.5 μm

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