Passively Q-switched Tm3�-doped silica fiber lasers

Stuart D. Jackson*Optical Fiber Technology Centre, The University of Sydney, 206 National Innovation Centre, Australian Technology Park,

1430 Eveleigh, Sydney, Australia

*Corresponding author: s.jackson@oftc.usyd.edu.au

Received 19 January 2007; revised 28 February 2007; accepted 1 March 2007;posted 1 March 2007 (Doc. ID 79202); published 15 May 2007

By splicing on a length of Ho3�-doped silica fiber onto a diode-pumped double-clad Tm3�-doped silica fiber,stable passive Q switching of the Tm3�-doped silica fiber laser is demonstrated. The formation ofQ-switched pulses was found to depend on both the length and the position of the Ho3�-doped silica fiberthat was inserted into the fiber laser cavity. For stable Q-switched pulse generation, Ho3�-doped silicafiber lengths shorter than twice the absorption depth must be used. For long Ho3�-doped silica fiberlengths, randomly generated pulses are observed at operating wavelengths longer than 2090 nm, whichare attributed to intracavity pumping of the Ho3�-doped silica fiber. © 2007 Optical Society of America

OCIS codes: 060.2320, 140.3510, 140.3540.

1. Introduction

High power Tm3�-doped silica fiber lasers which offeroutput in the 1.8 �m–2.05 �m range could be consid-ered one of the most important sources of mid-infrared laser radiation that has been developed. Theoutput power from the directly diode pumped ver-sions of these sources has steadily increased to wellover the 100 W level [1] at optical-to-optical efficien-cies of �39%. A wide range of applications includingoptical pumping, infrared countermeasures and ma-terials processing will benefit using such a lightsource.

Despite the augmentation in both the output powerand efficiency there have been only a small number ofstudies reporting pulsed output from these sources.These efforts have related to active Q-switching[2–4], passive Q-switching (using external crystals)[5], mode locking [6,7] and gain switching [8]. Anopportunity exists for the development of passivelyQ-switched 2 �m fiber lasers using an all fiber ar-rangement.

Passive Q-switching offers many advantages e.g.,simplicity and robustness compared to active Q-switching. For passively Q-switched fiber lasers it isalso desirable to have the saturable absorber in

fiber form [9–11] so that the aforementioned at-tributes are further enhanced. The use of the Ho3�

ion as the saturable absorber material for the Tm3�

ion was demonstrated some time ago for bulk crystal-line solid state lasers [12] and subsequently a range ofHo3�-based crystalline saturable absorbers have beentested [13,14]. The use of crystalline Ho3�-based satu-rable absorbers for passively Q-switched Tm3�-dopedsilica fiber lasers is achievable, however, the additionallosses and complexity incurred by extending the fiberlaser cavity will reduce the overall efficiency and ro-bustness.

In this investigation, we continue the above inves-tigations by demonstrating the passive Q-switchingof a Tm3�-doped silica fiber laser using a section ofHo3�-doped silica fiber as the saturable absorber. Weshow that the position and length of the Ho3�-dopedsilica fiber that was spliced onto the double-cladTm3�-doped silica fiber were critical to the passiveswitching of the laser.

2. Experiment

The experimental arrangements tested in this workare shown in Fig. 1. The double-clad Tm3�-doped sil-ica fiber was manufactured using the standard mod-ified chemical vapour deposition and solution dopingtechniques. The double-clad Tm3�-doped silica fiberwas set at a length �Lg� that was equivalent to2��eff �� 6.7 m� where �eff is the effective absorption

0003-6935/07/163311-07$15.00/0© 2007 Optical Society of America

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coefficient, determined by cutbacks, of the entiredouble-clad fiber. The core diameter, Tm3� concentra-tion and numerical aperture (NA) of the fiber were20 �m, 3.2 wt.% and 0.11, respectively. The fiber washexagonally-shaped, had a flat-to-flat separation of300 �m and had was coated with a low refractiveindex polymer to provide an NA for the pump core of�0.45. The fiber laser was excited with a high power

(42 W) diode laser system (Fisba Optik, Switzerland).The diode laser operated at a wavelength of 805 nmand was focussed into the fiber with an efficiency of�75% with a NA � 0.35 lens system.

Figure 1(a) shows a schematic diagram that hasthe diode laser pumping the Tm3�-doped silica fiberin the co-propagating or forward direction. The out-put characteristics of the Tm3�-doped silica fiber la-ser in this arrangement and without the Ho3�-dopedsilica fiber in place are shown in Fig. 2. The slopeefficiency was measured to be 43% with respect to thelaunched pump power, PL. To cool the fiber, the fiberwas immersed in water. The inset to Fig. 2 shows themeasured spectrum of the output. The groupings dis-played in the spectrum relate to the fringes arisingfrom modal interference between the lowest ordersymmetric modes of the fiber [15].

The Ho3�-doped silica fiber was also fabricated inhouse. The fiber was single clad with a core diameterand NA of �9 �m and 0.17, respectively. The Ho3�

concentration was �0.5 wt%. The absorption coeffi-cient ��a� of the Ho3�-doped silica fiber using thepump �Tm3�� fiber laser discussed above was 4.1 m�1.The absorption spectrum of a preform sample ofHo3�-doped aluminosilicate glass is shown in Fig. 3.The preform tested was not identical to the preformused to make the Ho3�-doped silica fiber and thereappears some evidence in the spectrum of OH con-tamination as a result of the additional peaks appear-ing at �1400 nm and �1850 nm. We see that the 5I7absorption spectrum is at least 300 nm wide and hasa peak absorption coefficient that is �2–3 timesstronger than the corresponding 5I6 absorption peak.The position of the 2051 nm emission from the Tm3�-doped silica fiber laser is on the trailing edge of the 5I7absorption feature and has a coefficient �0.5 timesthe peak 5I7 absorption coefficient.

Fig. 1. Schematic diagram of the experimental setup for (a) thepassively Q-switched Tm3�-doped silica fiber laser which hasthe pump light propagating in the forward direction (setup A), (b)the extended cavity that has the pump light propagating in thebackward direction (setup B) and (c) the cavity used to extra-cavitypump the Ho3�-doped silica fiber laser.

Fig. 2. Measured output power, Pout as a function of the launchedpump power, PL, from the Tm3�-doped silica fiber laser pumped inthe forward direction. The inset displays the spectrum of the out-put measured at full pump power. Spectrometer resolution was0.3 nm.

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3. Results

Initially, the light from the Tm3�-doped silica fiberlaser was used to pump a Ho3�-doped silica fiberlaser, see schematic diagram in Fig. 1(c). Two objec-tive lenses were used to image the pump light ontothe 0.5-m long Ho3�-doped silica fiber and an Au-coated mirror was used for feedback. A dielectricbeam splitter mirror, which transmitted �90% of thepump light and which reflected 10% of the 2.1 �mlight, was used to feed out the emission from theHo3�-doped silica fiber laser. The output characteris-tics of the fiber laser are shown in Fig. 4, which showsthe measured output power as a function of PL. TheHo3�-doped silica fiber was not cooled and the aver-age slope efficiency of �66% is lower than previousdemonstrations [16] for reasons that may relate toadditional losses in the Ho3�-doped silica fiber. Theinset to Fig. 4 shows that the output from the

Ho3�-doped silica fiber laser is spectrally broad andfor this fiber length covers a range from 2090 nm to2115 nm.

A section of Ho3�-doped silica fiber, of length La,was spliced onto the Tm�-doped double-clad silicafiber using a standard fusion splicer (Fujikura FSM-40S). The splice was produced after a 0.1 m end sec-tion of the Tm3�-doped silica double clad fiber wasetched down to 130 �m. The splice loss ranged be-tween 0.12–0.34 dB. Index-matching gel was used tofeed out the unabsorbed diode laser light before thesplice. The characteristics of the output from theexperimental arrangement shown in Fig. 1(a), withLa � 0.88 m is shown in Fig. 5(a). The maximumoutput power and slope efficiency from the fiber laserincorporating the dielectric mirror were approxi-mately 7.3 W and 30%, respectively. The maximumoutput power is �60% of the maximum Tm3�-dopedsilica fiber laser output (see Fig. 2) which can beexpected given the high Stokes efficiency limit of�98% and the fact that the Ho3�-doped silica fiberwas �3.6 times longer than the passive absorptiondepth ��1��a�.

The temporal characteristics of the output for La

� 0.88 m is shown in Fig. 5(b). The output was char-acterised by randomly pulsed behaviour that is char-acteristic of unstable self-pulsing. The three-levelnature of the laser transition, the long fiber lengthand resonator arrangement allows saturable absorp-tion to occur at the under-pumped region of theHo3�-doped silica fiber. The inset displays a singlepulse that has a pulsewidth of 0.5 �s.

The spectrum of the output from the fiber laser forLa � 0.88 m and for other La values that were testedis shown in Fig. 6. These measurements were takenfor a constant splice loss of 0.34 dB. It can be ob-served that the fiber laser operated on the 5I7 → 5I8

transition of Ho3� for La � 0.88 m �� 3.5��a� becausethe emission wavelengths were longer than 2100 nm.For La � 2.5��a, the emission from the fiber laserrelated to the 3F4 → 3H6 transition of Tm3�. TheTm3�-doped silica fiber laser has been shown [17] tooperate at 2090 nm, albeit at very low efficiency.From an observation of Fig. 6 we can see that thespectrum, for La � 0.88 m, may comprise of bothTm3� emission and Ho3� emission because the spec-trum is composed of two peaks. It is difficult, how-ever, to remark precisely on the origins of the spectrafor these values of La from an observation of thespectrum alone. With this arrangement, however, thevalues for La allowing pumping of the Ho3�-dopedsilica fiber laser and not passive Q-switching of theTm3�-doped silica fiber laser are longer as comparedto extra-cavity pumping.

Figure 7 displays the output measured without themirror in place, i.e., feedback was obtained fromFresnel reflection only. Optical damage to the dielec-tric mirror occurred for low PL with this arrangement.It can be observed from Fig. 7(a) that the outputpower increased by �20% when La � 0.19 m com-pared to La � 0.88 m case and that the operating

Fig. 3. Measured absorption spectrum in the 1000 nm to 2200 nmrange of Ho3�-doped aluminosilicate glass. A Cary 5 UV-Vis Spec-trophotometer with a resolution of 1 nm was used.

Fig. 4. Measured output from the Ho3�-doped silica fiber laserpumped in the backward direction with the 2.05 �m output fromthe Tm3�-doped silica fiber laser. The length of the Ho3�-dopedsilica fiber was 0.5 m. The inset displays the spectrum of the out-put measured at full pump power.

1 June 2007 � Vol. 46, No. 16 � APPLIED OPTICS 3313

wavelength, which is now closer to 2020 nm, indi-cates that the 3F4 → 3H6 transition of Tm3� is lasing,see Fig. 6. The temporal characteristics of the outputare shown in Fig. 7(b). We observe stable pulse for-mation that relates to passive Q-switching of theTm3�-doped silica fiber laser. The pulsewidth, tp andpulse repetition frequency, R, measured as a functionof power are shown in Fig. 8. The observed charac-teristics are typical of Q-switched pulses because tp

decreases and R increases with the increase in PL. Aswe increase La to 0.305 m, we note that the tP valuesare shorter and the R-values are lower as comparedto the La � 0.19 m case.

To reduce the intensity impinging on the dielec-tric mirror used above and maintain unidirectionaloutput, an experimental arrangement that had thecavity extended using a microscope objective (trans-mission of �85% at 2 �m) was constructed; seeFig. 1(b). The pump light propagated in the back-

ward direction and a cleave angle of �10° wasapplied to the Ho3�-doped silica fiber. The mea-surements were taken for a constant splice loss of0.12 dB.

Figure 9 displays the output power measured fromthe fiber laser as a function of the launched diodelaser light. The slope efficiency is observed to be ap-proximately constant ranging from 25% to 28% withno apparent correlation with La. The launched pumppower at threshold, on the other hand, showed astrong correlation with La as can be observed in theinset to Fig. 9. With the increase in La we observe thatPth increases as expected given that an increasingloss was introduced into the cavity of the Tm3�-dopedsilica fiber laser. There is a smooth transition in thevalue of Pth as we move from the Tm3�-doped silicafiber laser to the intracavity-pumped Ho3�-doped sil-ica fiber laser.

Figure 10 displays the temporal characteristics ofthe output plotted for four values of La. For all valuesof La, the fiber laser in setup B arrangement did notproduce a stable Q-switched pulse train. For longvalues of La, the output is characterised by randompulses similar to the case for the intracavity-pumpedHo3�-doped silica fiber laser discussed above. As La isshortened and we move to the Tm3�-doped silica fiberlaser system, the output is oscillatory showing nosigns of Q-switching. The cavity arrangement andposition of the Ho3�-doped silica fiber within theTm3�-doped silica fiber laser cavity are important tothe passive Q-switching process. Figure 11 displaysthe spectrum of the output from this fiber laser ar-rangement for various values of La.

4. Discussion

For all of the fiber laser arrangements tested, whichinvolved splicing the Ho3�-doped silica fiber onto theTm3�-doped silica fiber, we were unable to achieve cwoperation of the laser. In a recent investigation [18]stable cw (to within 2%) operation of a diode-pumped

Fig. 5. Measured output from the intracavity-pumped Ho3�-dopedsilica fiber laser in setup A arrangement when La � 0.88 m dis-playing (a) the output power, Pout, measured as a function of thelaunched diode laser power, PL, and (b) the temporal characteris-tics with the inset displaying the characteristics for a single pulse.

Fig. 6. Measured spectrum of the output from the Tm3�-dopedsilica fiber laser in setup A arrangement which has a section ofHo3�-doped silica fiber spliced into the cavity displayed as a func-tion of La. Spectrometer resolution was 0.3 nm.

3314 APPLIED OPTICS � Vol. 46, No. 16 � 1 June 2007

Tm3�, Ho3�-doped silica fiber laser was achieved afterthe fiber was pumped from both ends. In the presentstudy the fiber could only be pumped from one end,which is an arrangement that leads to a proportion ofthe Tm3�- and Ho3�-doped silica fibers being under-pumped. Replacing the single-clad Ho3�-doped silicafiber with double-clad Ho3�-doped silica fiber wouldallow dual-end pumping. Since the Ho3�-doped silicafiber is transparent to the pump wavelength forTm3�, the Ho3� ions in this proposed arrangementwould act as saturable absorbers much in the sameway as they do in the present study. Thus powerscaling and more stable operation of the intracavity-pumped and passively Q-switched fiber lasers is pos-sible.

The emission from the fiber laser output did notdivide into clear separate emissions relating to the5I7 → 5I8 transition of Ho3� at �2100 nm and the3F4 → 3H6 transition of Tm3� at 2050 nm. For La �0.88 m in setup A arrangement, it is apparent that

both transitions maybe be lasing, see Fig. 6 because asmall peak appears at 2090 nm along with the largepeak at 2102 nm. The very broad 2082 to 2100nm emission for La � 0.86 m in setup B arrange-ment may also indicate that both transitions arelasing simultaneously. This result would alignwith intracavity-pumped Ho:YAG lasers [19–21]whereby some residual transmitted emission fromthe Tm3�:YAG(or YLF) pump laser is observed. In thepresent case and for the longest values of La, theemission wavelength of the Tm3�-doped silica fiberlaser would shift to long wavelengths, in a similar wayto the crystalline lasers [21], so that the absorption ofthe pump light from the Tm3�-doped silica fiber laseris lowered and some pump transmission results. Un-

Fig. 7. Measured (a) output power, Pout as a function of thelaunched diode power, PL for the passively Q-switched Tm3�-dopedsilica fiber laser in setup A arrangement that employed Fresnelreflection only, La � 0.19 m and (b) the temporal characteristics forthe same fiber laser arrangement when PL � 28.6 W.

Fig. 8. Measured pulse width, tp and repetition rate R of the pulsesgenerated from the Q-switched Tm3�-doped silica fiber laser in setupA arrangement plotted as a function of the launched diode laserpower, PL. Two Ho3�-doped silica fiber lengths, La are represented.

Fig. 9. Measured output power, Pout from the fiber laser in setupB arrangement plotted as a function of the launched diode laserpower PL and the Ho3�-doped silica fiber length, La, �s � 0.12 dB.The inset shows the launched diode laser power at threshold as afunction of the splice loss, �s and the length of Ho3�-doped silicafiber, La.

1 June 2007 � Vol. 46, No. 16 � APPLIED OPTICS 3315

der these conditions the long pump wavelength stillexcites the Ho3� ion and causes it to lase.

For intermediate values of La, the Tm3�-doped sil-ica and Ho3�-doped silica fiber laser emissions maycoincide. This situation is possible because the lowenergy side of the 3F4 → 3H6 transition of Tm3�-dopedsilica is degenerate with the high-energy side of the5I7 → 5I8 transition of Ho3�-doped silica. If both tran-sitions were oscillating simultaneously, then cou-pling between the population inversions of Tm3� andHo3� would occur with one broadband intracavityfield.

We did not observe laser oscillation of the Ho3�-doped silica fiber laser with the passively Q-switchedfiber laser despite the fact that large pulses at 2020 nmto 2030 nm were emitted by the Tm3�-doped silicafiber laser when 0.19 m � La � 0.305 m. Output withthis wavelength range is strongly absorbed by theHo3� ions, see the absorption spectrum in Fig. 3, andone could expect gain-switched operation of theHo3�-doped silica fiber laser. We are currently carry-

ing out spectrally resolved temporal measurementsof the fiber laser output in order to fully elucidate theprocesses behind the operation of the laser at all val-ues of La.

5. Summary

In this study, we have demonstrated a simple andefficient method of passively Q-switching a moderatepower Tm3�-doped silica fiber laser. By splicing asection of singly Ho3�-doped silica fiber onto outputside of the Tm3�-doped silica double-clad fiber, wehave produced �1 �s Q-switched pulses at pump-power-dependent repetition rates ranging from 10kHz to �110 kHz. �10 W of average power wasproduced from the passively Q-switched fiber laser,which represented more than 80% of the free-runningoutput. It was established that the position of theHo3� saturable absorber was critical to the formationof the passively Q-switched pulses.

The author thanks Andy Clarkson of the Optoelec-tronics Research Centre for the useful discussionsthat initiated this work. The Australian ResearchCouncil is thanked for providing financial support.

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3316 APPLIED OPTICS � Vol. 46, No. 16 � 1 June 2007

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