This paper describes the all-fiber dual-wavelength thulium ytterbium co-doped fiber laser by using a newly developed octagonal shaped double-clad thulium-ytterbium co-doped fiber as gain medium in conjunction with spatial filtering effect. The thulium ytterbium co-doped fiber laser operates at wavelengths of 1990.64 nm and 1998.92 nm with a signal to noise ratio of more than 34 dB as the 980 nm multimode pump power is increased above a threshold value of 1.5 W. The slope efficiency of the dual-wavelength laser is measured to be around 26.16%, which is comparable with the conventional thulium laser pumped by 800 nm single mode pump

Ahmad, H

Babar, I M

Bhadra, Shyamal Kumar

Das, S

Halder, A

Harun, S W

Latiff, A A

Paul, Mukul Chandra

Shamsudin, H

IR@CGCRI - Central Glass and Ceramic Research Institute (CSIR)

Vol. 130 (2016) ACTA PHYSICA POLONICA A No. 6Highly Efficient Cladding Pumped Dual-WavelengthThulium Ytterbium Co-Doped Fiber LaserA.A. Latiffa, I.M. Babarb, H. Shamsudina, M.C. Paulc, A. Halderc, S. Dasc,S.K. Bhadrac, H. Ahmada and S.W. Haruna,b,∗aPhotonics Research Centre, University of Malaya, 50603 Kuala Lumpur, MalaysiabDepartment of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, MalaysiacFiber Optics and Photonics Division, Central Glass and Ceramic Research Institute, CSIR, Kolkata, India(Received June 21, 2016; in final form December 2, 2016)This paper describes the all-fiber dual-wavelength thulium ytterbium co-doped fiber laser by using a newlydeveloped octagonal shaped double-clad thulium-ytterbium co-doped fiber as gain medium in conjunction withspatial filtering effect. The thulium ytterbium co-doped fiber laser operates at wavelengths of 1990.64 nm and1998.92 nm with a signal to noise ratio of more than 34 dB as the 980 nm multimode pump power is increasedabove a threshold value of 1.5 W. The slope efficiency of the dual-wavelength laser is measured to be around26.16%, which is comparable with the conventional thulium laser pumped by 800 nm single mode pump.DOI: 10.12693/APhysPolA.130.1332PACS/topics: 42.55.Wd, 42.60.Fc, 42.70.–a1. IntroductionFiber lasers operating in 2 µm region have attractedgreat research interest in recent years because of theirpotential scientific applications in various fields such asin remote sensing, spectroscopy and biomedical applica-tion [1, 2]. The wavelength also overlaps with many ab-sorption lines of several gas molecules such as hydrogenbromide (HBr) and carbon dioxide (CO2) [3], which cre-ates the possibility of constructing cost-effective gas sen-sors. Strong water absorption in this wavelength rangemakes the light source extremely desirable in biomed-ical applications. It has been shown that 2 µm laseroutperform 1.0 µm and 1.5 µm sources in dermatol-ogy and surgery, serving as precise and efficient opticalscalpels [4, 5]. The 2 µm fiber laser can be realized us-ing thulium-doped fiber (TDF) as the gain medium inconjunction with 794 nm pumping. However, the lasingthreshold for achieving an efficient 3F4 and 3H6 transi-tion is relatively high and a power of the available com-mercial single mode 794 nm laser diode is limited toaround 180 mW.Of late, a thulium ytterbium co-doped fiber laser(TYDFL) was also demonstrated for oscillation around2 µm based on ytterbium (Yb) to thulium (Tm) energytransfer [6]. It uses a commercially available and cheaperlaser diode operating within 905 to 980 nm as a pumpsource. This is due to the 3H5 level of Tm3+, which is(quasi-) resonant with the excited Yb3+ level (2F5/2) andthus allows the energy transfer to create a population in-version in Tm3+ [7, 8]. In the previous work, Jeong et al.demonstrated a TYDF operating in D-shaped geometrymultimode with efficiency of around 32% [9]. To find the∗corresponding author; e-mail: swharun@um.edu.mybest laser performance which is appropriate to specificapplications, various 2 µm fiber laser configurations havedeveloped. Dual-wavelength running fiber lasers is one ofthe laser that has been under intensive research recentlydue to its wide application potential in ranging, commu-nication, microwave photonics, terahertz generation, andoptical instrument testing [10, 11]. Up to date, there weremany techniques have been proposed and demonstratedto achieve a dual-wavelength lasing such as by employ-ing a dual-cavity configuration and twin-peak reflectiongrating. For instance, Zhou et al. [12] demonstrated aroom temperature all-fiber dual-wavelength TDFL basedon cascaded fiber Bragg grating (FBG) array. However,the FBG structure requires an expensive UV laser to befabricated and the conventional FBG cannot operate attemperature higher than 300 ◦C.In this paper, a room temperature all-fiber dual-wavelength TYDFL operating in 2 µm region is demon-strated for the first time using a newly developed anorthogonal shaped double-clad thulium-ytterbium co-doped fiber (TYDF) as a gain medium in a half-openedlinear laser cavity. In this cavity, one fiber end is con-nected to a reflector whereas the other end is open. Inthis work, a high-reflectivity optical mirror is used asa reflector to reduce the threshold pump power while a980 nm multimode laser diode is used to pump the TYDFvia a multimode combiner. The dual-wavelength opera-tion is realized in the proposed laser based on spatialfilter generated due to mismatch at the splicing points ofTYDF.2. TYDF fabrication and their working principleIn this work, a proposed double-clad octagonal shapedTYDF, which was coated by a low refractive index poly-mer, was used as the gain medium. It was drawn by apreform, which was fabricated by the modified chemicalvapor deposition (MCVD) process in conjunction with(1332)Highly Efficient Cladding Pumped. . . 1333Fig. 1. The microscopic cross-sectional view of an oc-tagonal shaped double-clad fiber used as the gainmedium.the solution doping technique. Figure 1 shows the mi-croscopic view of fiber cross-section, which indicates ournewly developed double-clad structure with a core andinner-cladding diameters of 5.96 µm and 123.86 µm, re-spectively. The numerical aperture (NA) of the fabri-cated TYDF core is measured to be 0.23.The preform was made by deposition of two porousunsintered SiO2 soot layers inside a pure silica glass tubeat temperature around 1550±10 ◦C. The porous layerswere soaked by an alcoholic solution containing dopingelements i.e. Tm, Yb, Y, and Al for about 30 min toachieve efficient doping. Then, dehydration and oxida-tion were performed at the temperature around 900 to1000 ◦C. Sintering of the un-sintered layers was also doneby slow increase of the temperature from 1500 to 2000 ◦Cusing the conventional MCVD technique. Upon comple-tion of sintering as well as oxidation, the tube was slowlycollapsed to convert it into optical preform. Based onelectron probe micro analyser (EPMA) result, we foundout that the fabricated optical preform consists of Al2O3,Y2O3, Tm2O3 and Yb2O3 dopants with average weightpercentage of 5.5, 3.3, 0.7, and 4.0 wt%, respectively.The composition of dopants in this fiber was optimized toallow the formation of smaller phase-separated nanopar-ticles of 3–5 nm, which functions to reduce the back-ground loss of the fiber as well as to increase the energytransfer efficiency from Yb to Tm ions. The presence ofAl2O3 and Y2O3 helps to decrease the phonon energy ofalumino-silica glass, which assists in preventing the clus-tering of Yb and Tm ions into the core glass matrix andthus increases the probability of radiative emission.The fabricated circular preform is converted to octago-nal shape through grinding followed by polishing method.The geometrically modified preform is then drawn attemperature of 2050 ◦C to obtain an octagonally shapedfiber with outer cladding diameter of 125 µm. The fiberis then coated with a low refractive index polymer toensure the robustness of the fiber. The doping levelsof Tm3+ and Yb3+ ions in the fabricated TYDF are4.85×1019 ions/cc and 27.3×1019 ions/cc, respectively.The Tm3+ and Yb3+ cladding absorptions of the fiberare measured to be around 0.325 and 3.3 dB/m at 790 nmand 976 nm, respectively.As opposed to the conventional single-clad fiber, wherethe pump light is coupled directly into the core, the pumplight travels down the fiber in the inner cladding for thedouble-clad fiber. When the pump light overlaps withthe core, it gets absorbed by the dopants. Such octagonalgeometry of the cladding improves the pump absorptionefficiency. Octagonal geometrical shape has more brokencircular geometry (eight sides) and symmetrical cross-section compared to the conventional hexagonal shape.The higher number of broken circular geometry couldprovide more pump absorption, which in turn increasesthe laser output power [13]. In addition, more symmetri-cal cross-section in the octagonal geometry could providea lower splicing loss.Fig. 2. Energy diagram levels for both ytterbium andthulium ions in TYDF showing an energy transfer.The TYDFL operates based on the energy transitionas shown in Fig. 2. Since, the active fiber has ytter-bium ions as sensitizer, it employs up-conversion processto generate laser at 2 µm region. The pumping was car-ried out using a 980 nm multimode laser diode, whichis available in high power, easily obtained and relativelycheaper compared to other wavelength range. The pumppropagates inside inner cladding of the fiber and the oc-tagonally shaped fiber is expected to increase the overlap-ping factor of the pump and ions. As the pump photonsare absorbed by the ytterbium ions, it excites the ions1334 A.A. Latiff et al.from ground state 2F7/2 to 2F5/2. As the ion relaxes toground state, energy transfer process happens to neigh-boring thulium ion. When the thulium ions in groundstate (3H6) absorb the donated photons it got elevated to3H5 level before it irradiatively relaxes to 3F4. Thuliumions that populate this state level drops to ground stateagain generating the 2 µm laser.3. TYDFL experimental configurationFigure 3 shows the experimental setup for dual-wavelength TYDFL with a half-opened linear cavity con-figuration, which is formed by the broadband mirror andan output coupler reflector. A 15 m long homemadeTYDF, which is pumped by a 980 nm multimode laserdiode (LD) via a multimode combiner (MMC), is used asthe gain medium. The 15 m is the optimized length forthe available pump power. The output coupler reflectoris formed by the 4% Fresnel reflection from the perpen-dicular cleaved fiber end. The laser output is measuredby using an optical spectrum analyzer (OSA) with a res-olution of 0.02 nm and an optical power meter (OPM).Fig. 3. Experimental setup of the proposed dual wave-length TYDFL.4. Results and discussionsThe laser operation is supposed to be grounded on theTYDF gain and the resonance between broadband mir-ror and a 4% Fresnel reflection from the perpendicularcleaved fiber end. When the TYDF is pumped by 980 nmlaser to generate population inversion of ytterbium ions,the emitted energy is then transferred to thulium ionsto generate an amplified spontaneous emission (ASE) in2 µm region via spontaneous and stimulated emissionprocess. The ASE oscillates in the linear cavity to gen-erate laser when the gain overcomes the total cavity loss.With increase of power of the pump, dual-wavelengthlaser is generated due to the effect of the phase mismatchat the splicing points of a TYDF birefringence.As the geometry of TYDF is significantly differentwith both fibers spliced to it, the power oscillations oc-cur within the TYDF, thus producing some wavelength-dependent losses. As a results of specific transmissionfunction of the spatial mode-beating filter, the signalsat two different wavelengths can oscillate in the cavitywhenever the difference of the cavity losses for them aresomewhat reduced. At sufficient pump power, the cavitylosses for at least two wavelength reduced to seek dual-wavelength oscillations with the aid of the TYDF, whosegain broadening behaves inhomogeneously.Fig. 4. Output spectrum of the proposed dual-wavelength TYDFL at multimode pump power of1.5 W.Fig. 5. Output power characteristic against the pumppower.The output spectrum of the proposed laser as recordedby an OSA is shown in Fig. 4 when the multimode 980 nmpump is fixed at 1.5 W, respectively. As seen in thefigure, a dual-wavelength output lines are obtained at1990.64 and 1998.92 nm with a signal to noise ratio ofmore than 34 dB. The 3 dB bandwidth of both lasers ismeasured to be less than 0.2 nm. The power differencebetween the two peaks is less than 1 dB. The relationbetween the output powers of the dual-wavelength laseragainst the multimode pump power is shown in Fig. 5.The TYDFL starts to lase at pump power of 400 mWto generate a random laser operating at around 1995 nmregion. The output power of the laser is observed to in-crease linearly with the increment of multimode pumppower up to pump power of 1200 mW with a slope effi-ciency of 12.62%. The output power suddenly drops atpump power above 1200 mW to enable dual-wavelengthlaser generation.The dual-wavelength laser starts to lase at thresholdpower of 1550 mW before both output peaks power in-creases linearly with the increment of multimode pumppower up to pump power of 2750 mW. The slope effi-ciency of the dual-wavelength laser is measured to bearound 26.16%, which is comparable with the conven-tional TDFL pumped by 800 nm single mode pump.Highly Efficient Cladding Pumped. . . 1335For TYDFL system, this is the highest slope efficiency,which has been reported so far. At the multimode pumppower of 2750 mW, the dual-wavelength laser producesthe maximum total output power of 366 mW. As thepump power is increased above 2750 mW, the total out-put power drops due to more number of lines are gener-ated in the laser.5. ConclusionLet us summarize the main results of this report. Anall-fiber dual-wavelength TYDFL operating at 2 µm re-gion is demonstrated based on spatial filtering effect.It uses a newly developed octagonal shaped double-cladTYDF as gain medium. The laser developed by us gener-ates the dual-wavelength output lines at wavelengths of1990.64 and 1998.92 nm as the 980 nm multimode pumppower is increased above a threshold value of 1.5 W. Thesignal to noise ratios of the both laser lines are measuredto be more than 34 dB and the slope efficiency of thelaser is 26.16%. The use of chirped FBG as a reflectormay enhance the laser performance.References[1] Y. Zhang, Y. Tian, W. Wang, B. Yao, LaserPhys. Lett. 7, 225 (2010).[2] P. Zhang, W. Ma, T. Wang, Q. Jia, C. Wan,Chin. Opt. Lett. 12, 111403 (2014).[3] W. Zeller, L. Naehle, P. Fuchs, F. Gerschuetz,L. Hildebrandt, J. Koeth, Sensors (Switzerland) 10,2492 (2010).[4] N.M. Fried, K.E. Murray, J. Endourol. 19, 25 (2005).[5] R. Szlauer, R. Götschl, A. Razmaria, L. Paras,N.T. Schmeller, Eur. Urol. 55, 368 (2009).[6] A. Halder, M.C. Paul, S.S.A. Damanhuri,N.A.D. Huri, A. Hamzah, S.W. Harun, H. Ah-mad, S. Das, M. Pal, S.K. Bhadra, J. Mod. Opt. 59,527 (2012).[7] S.W. Harun, N. Saidin, S.S.A. Damanhuri, H. Ah-mad, A. Halder, M.C. Paul, S. Das, M. Pal,S.K. Bhadra, Laser Phys. Lett. 9, 50 (2012).[8] M.Y. Jeon, N. Kim, J. Shin, J.S. Jeong, S. Han,C.W. Lee, Y.A. Leem, D. Yee, H.S. Chun, K.H. Park,Opt. Expr. 18, 12291 (2010).[9] Y. Jeong, P. Dupriez, J.K. Sahu, J. Nilsson, D. Shen,W.A. Clarkson, in: Proc. Europ. Symp. on Opticsand Photonics for Defence and Security, 2004, p. 28.[10] H. Ahmad, M.Z. Zulkifli, A.A. Latiff, S.W. Harun,Opt. Commun. 282, 4771 (2009).[11] H. Li, H. Ding, K.T. Chan, Electron. Lett. 33, 52(1997).[12] P. Zhou, X.L. Wang, Y.X. Ma, K. Han, Z.J. Liu,Laser Phys. 21, 184 (2011).[13] N. Saidin, M.S.Z. Abidin, S.S.A. Damanhuri,A. Haider, M.C. Paul, S.W. Harun, S. Das, M. Pal,S.K. Bhadra, in: Proc. IEEE 6th Int. Conf. on Pho-tonics, 2016, p. 1.

Highly Efficient Cladding Pumped Dual-Wavelength Thulium Ytterbium Co-Doped Fiber Laser

http://cgcri.csircentral.net/4145/1/mukul.pdf

Highly Efficient Cladding Pumped Dual-Wavelength Thulium Ytterbium Co-Doped Fiber Laser

Abstract

Similar works

Full text

Available Versions

IR@CGCRI - Central Glass and Ceramic Research Institute (CSIR)