We demonstrate the first Tm-doped yttria planar waveguide laser to our knowledge, grown by pulsed laser deposition. A maximum output power of 35 mW at 1.95 µm with 9% slope efficiency was achieved from a 12 µm-thick film grown on a Y3Al5O12 substrate

Eason, Robert W.

Mackenzie, Jacob I.

Parsonage, Tina L.

Sloyan, Katherine

Szela, Jakub

Optics Express

English

Jakub W. Szela

Katherine A. Sloyan

Tina L. Parsonage

Jacob I. Mackenzie

Robert W. Eason

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Laser operation of a Tm:Y_2O_3 planar waveguide

Southampton (e-Prints Soton)

20 30 40 50 60 70 8010100100010000100000CB(563)(451)DA(741)/(811)/(554)(652)(800) substrate(444)(433)(431)(332)(400) substrate(222)Counts per second2 theta (degrees)(a)20 30 40 50 60 70 8010100100010000100000DGFAECounts per second2 theta (degrees)(400) substrate(800) substrate (222)(444)(b)Fig. 2. XRD spectra of (a) the 12 µm-thick Tm3+:Y2O3 waveguide and (b) a 50 nm-thickTm3+:Y2O3 ﬁlm. Very small peaks corresponding to growth in orientations other than theprimary (222) and (444) orientation can be observed, particularly in case (a). Unassignedﬁlm peaks are observed around 26.4◦ (A) and 17.6◦ (E). Peaks arising from substrate con-tributions may also be observed: peaks labelled B and C may correspond to orientations ofcubic yttria, while peaks D, F and G likely correspond to orientations of YAlO3.deposition, both where the ﬁlm and substrate were lattice matched (for example in the caseof growth of yttria on sapphire, where the lattice mismatch was ∼5% [8]), and in the case ofgrowth on amorphous substrates (where there is no lattice match, growth in the cubic phaseand {111} orientation minimises the surface free energy [15]). A number of peaks at least twoorders of magnitude smaller than the primary (222) orientation peak can also be observed. Thisindicates a small amount of growth in other orientations and possibly in a non-cubic phase(corresponding to the peak labelled A). This latter peak could not be assigned directly fromdatabasevalues, althougha numberofnon-cubicorientationsofY2O3 exhibitpeaksat a similarvalue of 2θ [16].For comparison,a 50 nm Tm3+:Y2O3 ﬁlm was grown under the same deposition conditions,the XRD spectrum of which is shown in Fig. 2(b). In this case, only peaks correspondingto the(222) and (444)planes of cubic yttria can be observed,as well as additional small peaks around17.6◦ (E) and 26.4◦ (A). This suggests that the ﬁlm grows primarily in the (222) orientationeven during the ﬁrst tens of nanometres of growth, while growth in other orientations becomesmore prevalent as the ﬁlm thickness increases. A very small peak likely corresponding to thenon-cubicyttria phase is also observedaround26.4◦, indicatingsome growth in this phase evenat these small thickness values.The growthmechanisms that lead to the formationof cubic and non-cubicphases in differentorientations, as well as their changing signiﬁcance as ﬁlm thickness increases, are currentlyunder investigation. It should be noted that due to the small thickness of the ﬁlm, the XRDspectrum is dominated by the substrate contribution, and peaks resulting from the ﬁlm itself(i.e. small amounts of growth in other phases/orientations) may be lost in the noise. It shouldalso be noted that the presence of peaks other than (400) and (800) orientations of YAG in thesubstrate spectra indicates that the substrates themselves are not perfect crystal, which may beinﬂuencing subsequent ﬁlm growth. The peaks labelled B and C in Fig. 2(a) may correspondtothe(211)and(440)orientationsofcubicyttria,whilepeakslabelledD,F andGinFigs. 2(a)and2(b) likely correspond to the (222), (111) and (221) orientations of YAlO3 (YAP) respectively.#186939 - $15.00 USD Received 13 Mar 2013; revised 2 May 2013; accepted 3 May 2013; published 14 May 2013(C) 2013 OSA 20 May 2013 | Vol. 21,  No. 10 | DOI:10.1364/OE.21.012460 | OPTICS EXPRESS  12463Ti:sapphireChopperMMMMNDﬁlterThermaldetectorRG1000ﬁlter L1L2 L3 IC OC Tm :Y O3+2 3waveguideFig. 5. Setup of the Tm3+:Y2O3 waveguide lasing experiments. Abbreviations are as fol-lows: MM – metal mirror; L1 – in-plane collimating cylindrical lens; L2 – aspheric lens;IC – input coupling mirror; OC – interchangeable output coupling mirror; L3 – collectionlens.the proximity of the Tm3+ to neighbouringdopant ions, hence the higher RE ion concentrationfor yttria per atomic substitution (compared to YAG) should enable sufﬁcient cross-relaxationrates at lower doping levels as reportedly used in early bulk crystal experiments [22].The experimental setup of the end-pumpedlaser is depicted in Fig. 5. It consists of a tunableTi:sapphire pump laser, a 100 mm focal length cylindrical lens, an 11 mm focal length asphericlens, a thin pump input coupling mirror (IC), the waveguide, and various interchangeable bulkoutput couplingmirrors (OC). A collection lens (NA = 0.16)was used after the output couplingmirror to transfer the 2 µm light to a thermal detector covered by a 3 mm-thick longpass ﬁlterto reject the transmitted pump light.The near-diffraction-limitedpump source was tuned to match the maximum absorption peakaround 797 nm and its optical power set by a variable metallic neutral density ﬁlter, providinga maximum incident power of 600 mW after the variable ﬁlter. In the waveguide’s fast-axis(guiding direction), the incident pump light was focused down to a waist of 3.6 µm at the inputfacet, signiﬁcantly smaller than half the thickness of the active layer. The NA of the input pumplens was 0.25, much less than the acceptance NA of the waveguide, hence our assumption fora 100% launch efﬁciency (neglecting interface reﬂections). In the other (unguided) directionthe beam was either collimated, with a beam waist of 90 µm positioned approximatelymidwayalong the waveguide, or diverging from the pump focus at the input facet i.e. a beam waist of3.6 µm. The former conﬁguration, achieved by placing the cylindrical lens before the inputcoupling aspheric, provided a good overlap between the pump beam width and cavity mode inthe unguidedaxis throughoutthe whole waveguide.Thelatter conﬁguration,however,providedhigher inversion density at the input facet, at the expense of the overlap efﬁciency: the pumpbeam radius spread to ∼350 µm at the exit facet of the waveguide (calculated assuming nolensing effects in the plane), and was hence signiﬁcantly larger than the ∼75 µm-radius cavitymode.A quasi-monolithicoptical resonator was made by butt-couplingplane parallel mirrors to theuncoated waveguide facets with a Fluorinert FC-70 ﬂuid interface for the thin mirrors, holdingthem in place and acting as an index matching ﬂuid. Four different OCs were investigated intotal,with reﬂectancevaluesof∼99.5%,94%,90%and85%at thelaserwavelength.Thepumptransmittance of the HR IC was 92% and each of the other mirrors also had high transmissionat this wavelength, leading to a single pass of the pump light. The waveguide was ﬁxed to analuminium pedestal on a 3-axis translation stage and was not actively cooled.#186939 - $15.00 USD Received 13 Mar 2013; revised 2 May 2013; accepted 3 May 2013; published 14 May 2013(C) 2013 OSA 20 May 2013 | Vol. 21,  No. 10 | DOI:10.1364/OE.21.012460 | OPTICS EXPRESS  12466

Laser operation of a Tm:Y<sub>2</sub>O<sub>3</sub> planar waveguide

Laser operation of a Tm:Y<sub>2</sub>O<sub>3</sub> planar waveguide

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Southampton (e-Prints Soton)