3412 OPTICS LETTERS / Vol. 34, No. 21 / November 1, 2009

In-fiber resonantly pumped Q-switchedholmium fiber laser

David G. Lancaster1,* and Stuart D. Jackson2

1Electro Optical Technology Group, Defence Science and Technology Organisation, Edinburgh,South Australia, Australia

2Institute of Photonics and Optical Science, School of Physics, University of Sydney, Camperdown,New South Wales 2006, Australia

*Corresponding author: david.lancaster@dsto.defence.gov.au

Received July 31, 2009; revised September 29, 2009; accepted September 29, 2009;posted October 5, 2009 (Doc. ID 115111); published October 29, 2009

We demonstrate a double-clad fiber laser that incorporates a pump-light absorbing coaxially locatedTm3+-doped outer core that reaches threshold and resonantly pumps an Ho3+-doped inner core. Under cwdiode pumping, the output from the laser displays a pulse modulated behavior at wavelengths correspondingto the Tm3+ transition and short gain switched pulses from the Ho3+ core that have mode-locked character-istics. The Ho3+ and Tm3+ fiber lasers simultaneously produced 1.5 W across 2040–2140 nm and operate ata pulse repetition frequency of �80 kHz and at pulse widths as short as 330 ns. © 2009 Optical Society ofAmerica

OCIS codes: 140.3510, 060.3510.

Fiber lasers offer a wide variety of emission wave-lengths and operation modes while providing highbeam quality in a near-diffraction-limited beam. Thesuccess of these devices largely relates to the ex-tended longitudinal dimension of the gain mediumthat provides efficient cooling of the fiber to high-power levels. A large number of innovations, how-ever, relate to the transverse dimension of the fiberwhereby a large mode area [1], ring doping [2], or po-larization control can be produced with specific re-fractive index and/or doping profiles across the fiber.One innovation, proposed some time ago [3] involvesthe incorporation of a rare earth metal doped clad-ding surrounding a rare earth metal doped activecore that absorbs the pump light and emits at awavelength that is absorbed by the active core. Withthis design the active core is pumped at awavelength—from a commercial diode laser, forexample—that is outside the absorption bands of therare-earth-doped active core. To the best of ourknowledge, this proposed device or a variation of ithas not been demonstrated, possibly owing to the factthat the cladding is required to be entirely doped,which is a complex and expensive process using tra-ditional fabrication techniques such as modifiedchemical vapor deposition (MCVD).

In this Letter we report a fiber configurationwhereby a Tm3+-doped annulus or outer core sur-rounds an Ho3+-doped inner core in an otherwisestandard double-clad fiber laser (DCFL) arrange-ment. Unlike the original proposal [3], the entirecladding is not doped and the fiber was fabricated us-ing standard fabrication techniques. Our modifiedDCFL architecture involves a cavity designed to fullyresonate the Tm3+ transition to radiatively pump theHo3+ inner core at moderate power levels derivedfrom a diode pump source. The advantage of this de-sign is the brightness increase as a result of the two-

step laser process, allowing a much larger cladding to

0146-9592/09/213412-3/$15.00 ©

be employed to allow pumping from low brightnessand hence cheaper fiber-coupled diodes. This ap-proach has a particular benefit to three-level lasertransitions because the core-to-cladding area ratiomust be optimized for efficient operation, whichplaces a limit on the size of the cladding to efficientlypump a small core that will support a single trans-verse mode.

Our pumping method contrasts nonradiativepumping of Ho3+ in Tm3+,Ho3+-codoped fiber laser ar-rangements, which are limited by the energy transferupconversion that impedes the performance of the la-ser [4]. With radiative pumping, the efficiency of eachtransition can be separately addressed by optimizingthe concentration levels and diameters of each core.While the slope efficiency of our demonstration was amoderate 10%, we nevertheless show that the Tm3+

outer-core laser resonates across the 1980–2070 nmspectral region and in turn pumps the Ho3+-doped in-ner core that emits a comparatively brighter mode at�2130 nm, consistent with the refractive index pro-file of the waveguide. We demonstrate that thecoupled coaxial cores lead to pulsed Tm3+ emission,which is modulated by the Ho3+ ions that behave assaturable absorbers; the Ho3+ emission is in turngain switched leading to pulses as short as �330 ns.

The double-clad optical fiber was fabricated at theformer Optical Fibre Technology Centre using MCVDand solution doping. A photograph of the fiber isshown in the inset to Fig. 1. The noncircular core ge-ometry relates to the difference in the softening tem-peratures of the glasses used to compose the preform.The core noncircularity may have had a detrimentalimpact on the efficiency of the laser, but the beamquality of the Ho3+ emission was not seriously af-fected. Figure 1 shows the measured refractive indexprofile of the preform. The substrate tube was placedin a glass working lathe, and 12 layers of glass, index

matched to silica, were deposited on the inside sur-

2009 Optical Society of America

November 1, 2009 / Vol. 34, No. 21 / OPTICS LETTERS 3413

face of the tube. A silica flocculent layer was then de-posited, and solution doped with Tm3+ ��2%� andAl3+ (16%) salts was dissolved in ethanol. The lowTm3+ concentration was necessary to prevent a largerefractive index step; however, this compromised theefficacy of cross relaxation. The doped flocculent layerwas sintered forming a doped glass layer on the inte-rior surface of the tube. The deposition–doping–sintering process was repeated twice, forming a thickTm3+-doped aluminosilicate layer. Eight germanosili-cate layers were then deposited to separate theTm3+-doped outer core from the Ho3+-doped innercore, and, to prevent Ge from diffusing into the innercore, two pure silica layers were deposited. A finalflocculent layer was deposited and solution dopedwith a Ho3+ (0.4%), Al3+, and La3+ (4%) salt solutionbefore sintering and the tube collapsed to form thepreform. The preform was milled into a hexagon anddrawn with a flat-to-flat separation of 274 �m. Theaverage radii and V numbers of the Ho3+-doped innercore and Tm3+-doped outer core were estimated fromthe refractive index profiles giving �12 and 23 �m,and �8 and 13, respectively.

A fiber-coupled (400 �m, NA=0.22) 790 nm diodelaser (LIMO, �350 W) was used to pump the laser.The delivery fiber was tapered down to 200 �m andthe output imaged onto the 274 �m diameter dual-core fiber with �90% launch efficiency. Standard cut-back measurements gave the absorption coefficient at790 nm to be 9.1 dB/m (or �=2.1 m−1). The fiber endswere cleaved perpendicularly to the fiber axis andeach end was held in a conduction-cooled copperchuck. The fiber was coiled to a diameter of 10 cmand forced-air cooled. A broadly reflecting mirror(highly reflective at 1.7–2.2 �m) with a high trans-mission at 790 nm was butt coupled to the fiber laserinput; and, for outcoupling the light, three optionswere available: (a) the 4% Fresnel reflection from thefiber end face, (b) a �55% broadband reflector outputcoupler (OC) mirror �1.9–2.2 �m�, and (c) a long-pass

Fig. 1. Measured refractive index profile of the fiber pre-form. The inset shows an optical microscope image of theholmium �Ho3+� inner-core/germanosilicate doped barrierregion and thulium �Tm3+� outer-core double-clad opticalfiber.

OC mirror that was HR at ��2050 nm, designed to

fully resonate the Tm3+ transition and outcouple theHo3+ transition using a reflectivity of �73% at 2125nm; see Fig. 2(b) for the transmission characteristicsof this mirror.

Using a fiber length of L=100 cm, the highest slopeefficiency was achieved when operated in the highlyresonant arrangement, i.e., using the long-pass OCmirror. The measured slope efficiency was �10%,with up to 1.5 W of the output power measured,which included light from both the Tm3+- andHo3+-doped cores. The slope efficiencies for 135 and153 cm fiber lengths were �6% and �3%, respec-tively. Figure 2(b) displays the measured laser spec-trum showing emission across the broad Tm3+ tran-sition and emission from the Ho3+ transition. Withhighly resonant pumping, the Tm3+ transition oper-ates on random wavelengths between 1980 and 2060nm (observed because of some leakage through thelong-pass OC mirror) and the Ho3+ transition oper-ated near 2130 nm. The inset to Fig. 2(b) shows a Py-rocam (Spiricon) image of the fiber end face (with thehexagonal fiber end face highlighted) and confirmsthat the emission is predominantly confined to thecenter of the fiber, with M2=2.8 measured for thisprimarily Ho3+ output.

When L=100 cm and the �45% reflecting OC mir-ror is used, the slope efficiency reached 27% and upto 2 W of the output was produced. With this ar-rangement, the Tm3+ transition dominated the out-put; see the spectrum shown in Fig. 2(a). As expected,the beam quality of the emission was lower �M2

Fig. 2. (Color online) (a) Measured laser spectrum usingthe broadband (BB) OC for L=100 cm, showing the emis-sion from the Tm3+ and Ho3+ transitions. The transmissioncharacteristics of the BB OC mirror are shown. The insetshows an image of the fiber end face, showing laser emis-sion from both cores, with the hexagonal cladding high-lighted. (b) Measured laser spectrum using the long-passOC mirror and L=100 cm fiber, showing dominant emis-sion from the Ho3+ transition. The transmission character-istics of the long-pass OC are shown. The inset shows animage of the fiber end face, when the laser emission was

predominantly from the inner core.

3414 OPTICS LETTERS / Vol. 34, No. 21 / November 1, 2009

=7.3�; an image of the laser end face is shown in theinset to Fig. 2(a). The more moded output indicatesthat laser emission originates mainly from the Tm3+

outer core in modes that avoid interaction with the“lossy” Ho3+ inner core. When 4% Fresnel reflectionwas used with L=100 cm, the laser emitted at 1975nm, with no Ho3+ transition output, and produced anannular output beam originating from theTm3+-doped region of the fiber.

The temporal characteristics of the output dis-played modulated behavior. Figure 3 shows the tem-poral behavior when the output from the fiber laser(using the long-pass OC) was spectrally separated,using spatial and spectral filtering, into the Tm3+ andHo3+ transitions and simultaneously recorded usingfast detectors (Vigo Pem-10.6, �1 ns response time)and a 1 GHz bandwidth oscilloscope (Tektronix5104). As can be seen from Fig. 3(b), the highly reso-nant Tm3+ transition was unstably modulated at apulse repetition frequency (PRF) of �96 kHz havingpulse widths between �1 and 3 �s. Figure 3(a) showsthe temporal characteristics of the Ho3+ transition,which has a lower PRF of �81 kHz and shorter gainswitched pulses with widths between 0.4 and 1 �s.The inset to Fig. 3(a) shows that the Ho3+ pulse com-

Fig. 3. (a) Measured temporal output of the Ho3+ transi-tion using the long-pass OC; the inset shows the mode-locked characteristics of the pulse. (b) Measured temporalcharacteristics of the output of the Tm3+ transition re-corded simultaneously with the Ho3+ output.

prises of mode-locked subpulses with a period of�9.6 ns corresponding to the round trip time of thecavity.

As shown in Fig. 3, there was no apparent correla-tion measured between the gain switched Ho3+ andTm3+ transition pulses that typically occur with gainswitched lasers. The modal and spectral instabilitiesassociated with the pulses from the highly multimodeTm3+ outer core do not provide a consistent excitationof the Ho3+ ions because of the arbitrary spectral andspatial overlaps. Thus the Ho3+ transition reachesthe threshold after some number of Tm3+ pumppulses. The comparatively shorter (by a factor of �3)pulses derived from the Ho3+ transition relates to thefewer round trips required to deplete the populationinversion because of the comparatively smaller gainvolume. In addition, in power-scaled versions of thelaser thermal management of the fiber laser may berequired because of the extended core size and shortfiber length.

In conclusion, we have demonstrated in-fiber ra-diative pumping of Ho3+ using a double-clad fiber ge-ometry that has the Tm3+- and Ho3+-doped regionsoriented so that an outer core that was Tm3+ dopedpumped an inner core that was Ho3+ doped. The bestslope efficiency was 10% with a measured beam qual-ity of M2=2.8. The laser produced short Q-switchedand mode-locked pulses. The use of nonoverlappingcoaxially doped regions in double-clad fibers allows avariety of operating regimes to exist, depending onthe length and reflection characteristics of the reso-nator.

The authors acknowledge the Australian ResearchCouncil and Defence Science and Technology Organi-sation for financial support. The authors gratefullyacknowledge the fiber fabrication skills of PeterHenry.

References

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2. J. Nilsson, R. Paschotta, J. E. Caplen, and D. C.Hanna, Opt. Lett. 22, 1092 (1997).

3. D. Hanna, “Optical fibre with doped core and dopedinner cladding, for use in an optical fibre laser,” U.S.patent 5,291,501 (March 1, 1994).

4. S. D. Jackson, IEEE Photon. Technol. Lett. 18, 1885

(2006).