fibers Article Development of a Thulium Fiber Laser for an Atomic Spectroscopy Experiment Ronnie Currey *, Ali Khademian and David Shiner Department of Physics, University of North Texas, Denton, TX 76203, USA; [email protected] (A.K.); [email protected] (D.S.) * Correspondence: [email protected] Received: 20 December 2019; Accepted: 9 February 2020; Published: 15 February 2020 Abstract: A convenient thulium fiber laser source is described with 3 W of output power operating at a wavelength of 2059 nm with a slope efficiency of 49% with respect to input pump power and 60% with respect to absorbed pump power. The laser was applied in an atomic helium spectroscopy 3 4 1 experiment to quench He (2058.63 nm) and He (2058.69 nm) meta-stable singlets (2 S0), allowing for further investigation of the helium fine structure. The customized laser effectively eliminates the singlet counts to well below a background level (1%). A simplified analysis describes the basic laser performance with fitted constants in reasonable agreement with previous work. Keywords: fiber; laser; fiber laser; thulium; thulium fiber laser; optics; rare-earth elements; spectroscopy 1. Introduction The key benefits of double clad single transverse mode fiber lasers have been well established, such as efficient and powerful pumping, alignment ease and stability, good heat dissipation due to large surface area to volume ratio, and excellent beam quality [1,2]. For spectroscopic applications, small diameters (typically 250 µm) and long fiber lengths allow for both compact designs and dense mode spacing to conveniently excite very narrow spectral lines. Fiber Bragg gratings are frequently employed to create the laser cavity inside the fiber and select the necessary wavelength and bandwidth. Grating centers can be tuned with temperature for fine control of output wavelength ( 0.02 nm/C for ≈ 2 µm). ≈ Silica (Si) with rare-earth (RE) do-pants provide very broad homogeneous fluorescent emission. When placed in a silica glass host, thulium (Tm) has several different absorption bands, located at 3 3 790 nm, 1210 nm, and 1670 nm [3,4]. The emission spectrum for the F4- H6 transition spans about 400 nm, ranging between 1700 nm to 2100 nm [5]. This emission range extends into the eye-safe regime and has many applications such as laser detection and ranging (LIDAR) for wind shear detection radar, remote sensing, atmospheric pollution monitoring, surgery, range finders, as well as for inducing transitions in atomic spectroscopy experiments [6]. Fiber coupled laser diodes emitting around 790 nm are capable of generating high signal powers especially when employing cladding pumped schemes [7–9].Thulium can be pumped at any of the wavelength bands shown in Figure1, but the 790 nm band is of particular interest because at high doping ( 4%–6%) it yields a near 2 for 1 cross relaxation process populating the upper lasing level, ≈ where the cross relaxation efficiency can be defined as the average number of lasing levels created per pump photon absorbed and increases the overall efficiency that would otherwise be limited to the usual quantum defect between lasing and pump frequencies [10]. Efficiencies have been reported to exceed 60% [3,11]. Fibers 2020, 8, 12; doi:10.3390/fib8020012 www.mdpi.com/journal/fibers Fibers 2020, 8, 12 2 of 7 Fibers 2020, 8, x 2 of 7 FigureFigure 1.1. Energy Levels of Thulium.Thulium. 2. Materials and Methods 2. Materials and Methods 2.1. Laser Design 2.1. Laser Design The laser described in this report is pumped with a diode module centered at 793 nm with a maximumThe laser output described power in of this 8 W report (BWT Beijing),is pumped see wit Figureh a diode2. This module pump lasercentered is fiber at 793 coupled nm with with a 105maximumµm/125 outputµm NA power= 0.15 of fiber 8 W and (BWT has aBeijing), bandwidth see Figure of 3 nm. 2. ItThis also pump is built laser with is feedback fiber coupled protection with between105 µm/125 1900–2000 µm NAnm. = 0.15 The fiber diode and module has a bandwidth is fused to of a 3 (2 nm.+ 1) It also1 multi-mode is built with pump feedback combiner protection (ITF × Technologies)between 1900 to–2000 facilitate nm. The launching diode themodule pump is powerfused intoto a the(2 + active 1) × 1 fiber. multi For-mode this pump experiment, combiner only (ITF one ofTechnologies) the combiner’s to facilitate input ports launching was used. the pump The input power was into designed the active to match fiber. For the pumpthis experiment, laser fiber only and soone had of the 105 combiner’sµm/125 µm input NA = ports0.15 fiberwas used. on the The input input pump was side.designed The signalto match side the had pump 10 µ mlaser/125 fiberµm lowand indexso had double 105 µm/125 clad NAµm =NA0.46 = 0.15 fiber fiber to matchon the theinput active pump fiber. side. We The measured signal side 95% had pump 10 µm/125 beam µm low index double clad NA = 0.46 fiber to match the active fiber. We measured 95% pump beam combiner efficiency and 99% transmission efficiency between the combiner signal ports (called Tc below).combiner Launching efficiency the and pump 99% powertransmission from the effic combineriency between into the the 10 µcombinerm/130 µm signal low indexports Tm(called active Tc fiberbelow). was Launching more diffi thecult pump than we power had expected.from the combiner The 3 cm into of the low 10 index µm/130 recoat µm was low determined index Tm active to be ≈ responsiblefiber was more for mostdiffic ofult the than coupling we had loss.expected. With improvementsThe ≈3 cm of low in cleanlinessindex recoat and was technique, determined the to loss be wasresponsible reduced for to most 5%, as ofverified the coupling by cutback loss. With experiments. improvements Nevertheless, in cleanliness simple and observation technique, withthe loss an opticalwas reduced microscope to 5%, indicates as verified the by index cutback uniformity experiments. of the recoated Nevertheless, fiber was simple not nearlyobservation as uniform with an as theoptical commercial microscope fiber. indicates The active the index fiber consists uniformity of 2.15 of the m ofrecoated thulium fiber doped was 10 notµ mnearly/130 µ asm uniform low index as the commercial fiber. The active fiber consists of 2.15 m of thulium doped 10 µm/130 µm low index NA = 0.46 fiber (Nufern, East Granby, CT, USA) designed to give a truly single-mode profile (TEM00). TheNA thulium= 0.46 fiber fiber (Nufern has a, claddingEast Granby, absorption CT, USA of) 4.6designed dB/m atto 793give nm. a truly This single fiber-mode is expected profile to (TEM absorb00). nominallyThe thulium 90% fiber of the has pump a clad lightding launched absorption into of the 4.6 gain dB/m fiber at (7%793 wasnm. measuredThis fiber exitingis expected the back to absorb which isnominally consistent 90% with of athe 90% pump absorption light launched and the subsequentinto the gain losses fiber through (7% was grating measured and exiting recoat sections).the back Thewhich gain is lengthconsistent was with chosen a 90% basically absorption by balancing and the the subsequent cost of lost losses pump through photons grating versus and the costrecoat of additionalsections). The Tm gain fiber. length The frequency was chosen mode basically spacing by ofbalancing the overall the cavitycost of length lost pump (c/2nL photons= 150 MHz)versus was the alreadycost of additional dense enough Tm tofiber. overlap Thethe frequency broad spectral mode featurespacing (300 of the MHz overall FWHM), cavity so nolength additional (c/2nL length = 150 MHz) was already dense enough to overlap the broad spectral feature (300 MHz FWHM), so no was needed. The absorbed pump power Pabs in terms of the diode power Pdiode is given by: additional length was needed. The absorbed pump power Pabs in terms of the diode power Pdiode is given by: P = ηp P (1) abs ∗ diode 푃abs = 휂p ∗ 푃diode (1) Fibers 2020, 8, x 3 of 7 Fiberswith 2020an ,absorption8, 12 efficiency ηp of 81%, given by the product of the beam combiner (0.95), launch3 of 7 (0.95),Fibers 20 and20, 8 , absorptionx (0.90) efficiencies. 3 of 7 withwith anan absorptionabsorption eefficiencyfficiency ηηpp of 81%,81%, givengiven byby thethe productproduct ofof thethe beambeam combinercombiner (0.95),(0.95), launchlaunch (0.95),(0.95), andand absorptionabsorption (0.90)(0.90) eefficiencies.fficiencies. Figure 2. Thulium Fiber Laser Schematic. The laser cavity is created by employing a highly reflective fiber Bragg gating (R1 ≈ 99.7% FigureFigure 2.2. Thulium Fiber Laser Schematic.Schematic. inferred from an optical spectrum analyzer measurement of T1 = 0.3% as supplied by the vendor, O/E Land).The It laseris centered cavity isat created2058.45 by nm employing with a bandwidth a highly reflective of 1.2 nm. fiber The Bragg grating gating is fused (R1 to99.7% the back inferred end The laser cavity is created by employing a highly reflective fiber Bragg gating≈ (R1 ≈ 99.7% fromof the an active optical fiber spectrum and the analyzer Fresnel measurement reflection of ofR2T =1 3.3%= 0.3% forms as supplied the output by the coupler vendor, at O the/E Land).front end. It is inferred from an optical spectrum analyzer measurement of T1 = 0.3% as supplied by the vendor, O/E centeredThe fiber at Bragg 2058.45 grating nm with (FBG) a bandwidth was designed of 1.2 to nm. insure The gratingthe laser is bandwidth fused to the easily back endoverlapped of the active the Land). It is centered at 2058.45 nm with a bandwidth of 1.2 nm.