We present a compact high-power continuous-wave tunable neodymium-doped double cladding fiber laser operating on three-level 4F3/2 - 4I9/2 transition with a maximum output power up to 810 mW. At 926.7 nm, it has a maximum slope efficiency of 49.3% against absorbed 808-nm pump. By compressing the fiber Bragg grating, 15-nm tuning range is achieved

Fu, L.B.

Grudinin, A.B.

Ibsen, M.

Nilsson, J.

Payne, D.N.

Richardson, D.J.

English

Southampton (e-Prints Soton)

306 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 2, FEBRUARY 2005Compact High-Power Tunable Three-Level Operationof Double Cladding Nd-Doped Fiber LaserL. B. Fu, M. Ibsen, D. J. Richardson, J. Nilsson, D. N. Payne, and A. B. GrudininAbstract—We present a compact high-power continuous-wavetunable neodymium-doped double cladding ﬁber laser operatingon three-level 4F3 2–4I9 2 transition with a maximum outputpower up to 810 mW. At 926.7 nm, it has a maximum slope efﬁ-ciency of 49.3% against absorbed 808-nm pump. By compressingthe ﬁber Bragg grating, 15-nm tuning range is achieved.Index Terms—Double-cladding ﬁber laser, Nd ﬁber laser, three-level transition, tunable laser.I. INTRODUCTIONTHE OPERATION of three-level transition F – I(890–950 nm) of Nd-doped ﬁber laser is of great in-terest as it has many applications in water-vapor absorptioncharacterization, frequency doubling, lidar, spectroscopy, etc.In these applications, narrow spectral width, high power,and tunable output is highly desirable. However, achievinghigh-power tunable laser output has long been a great challengein three-level transition in Nd-doped ﬁber lasers. Similar to thewell-known challenges in three-level transition operation ofYb-doped double cladding ﬁber lasers [1], operating three-leveltransition in Nd-doped double cladding ﬁber lasers also hasgreat difﬁculty not only due to the existence of undesiredextremely serious competition with four-level F – Itransition when pumped at 808 nm, but also due to the verypoor overlap between pump and signal. Hence, laser operationsof the F – I transition in Nd-doped optical ﬁbers havebeen limited to low power.Previously,laseroperationpowerofNd-dopedﬁberlaserwasreported to be 40 mW using a core pumping scheme [2]. Al-though under very low temperature (Nd-doped double claddingﬁbers cooled by liquid nitrogen), 2.1 W has been demonstratedusing a master oscillation optical ampliﬁer conﬁguration [3].Further, recently 11 W at 938 nm has been demonstrated in afollow-upexperimentusingalownumericalaperture(NA)ﬁberand higher pump power [4]; this has changed the 938-nm oper-ation to a four-level transition under very low temperature, butsuch operations at low temperature are not desirable in manyapplications. Using a cladding-pumped ampliﬁer which con-sists of Nd-doped ﬁbers with a core structure of two concen-tric zones, Kane recently demonstrated a 3-W blue source fromManuscript received August 9, 2004; revised October 2, 2004.L.B.FuwaswithORC-UniversityofSouthampton,SouthamptonSO171BJ,U.K. He is now with CUDOS, School of Physics, University of Sydney, SydneyNSW 2006, Australia (e-mail: libin@physics.usyd.edu.au).M. Ibsen, D. J. Richardson, J. Nilsson, and D. N. Payne are with ORC-Uni-versity of Southampton, Southampton SO17 1BJ, U.K.A. B. Grudinin is with the Fianium-NewOptics Ltd., Southampton, U.K.(e-mail: abg@ﬁanium.com).Digital Object Identiﬁer 10.1109/LPT.2004.840034Fig. 1. Experimental setup (L1, L2: lens; M1: mirror).a 914-nm Nd YVO passively -switched laser [5]. Never-theless, the highest output power of tunable laser ever reportedis only 32 mW (from 896 to 939.5 nm) using core pumping[6]. Most recently, although by combining two 808-nm pumpdiodesandbulkdiffractivegratings,2.4-Wpeakpowerhasbeenachieved,thelaserisbulky,expensive,andfarfromcompact[7].Inthisletter,wereportaverycompact,highlystable,robust,andas high as 810-mW output from a double cladding Nd-dopedﬁber laser with a tuning range of 15 nm, which we conﬁdentlyexpect to be further extended. When operating at 926.7 nm, thelaser has a threshold of 1.3 W (launched pump power) and aslope efﬁciency of 17% against launched high-brightness pumpat 808 nm (49.3% against absorbed pump). In order to suppressthe 1060-nm emission, we have designed the Nd-doped ﬁberwith a W-type refractive index and doping proﬁle [8]. In addi-tion, the laser spectral width is very narrow (full-width at half-maximum is about 0.11 nm), a result that we attribute mainly tothecompact,narrow-bandwidth,andtunableﬁberBragggratingas the high reﬂective mirror, which is based on the compressiontunable ﬁber grating technique [9], [10].II. EXPERIMENTAL SETUPThe experimental setup is shown in Fig. 1. Pump laser beamfromahighbrightnessbroad-stripe808-nmpumpdiodeiscolli-mated using a lens L1 and then launched into Nd-doped doublecladdingﬁber.Themaximumpumppowerfromthebroad-stripe808-nm diode is about 7.5 W. Mirror M1 has 98% transmissionat pump wavelength and 99% reﬂectivity at 900–1100 nm. Thetotal launching efﬁciency is about 84% for the pump.The ﬁbers employed in this experiment are high concentra-tion Nd-doped alumino–silicate ﬁbers. To enhance the pumpabsorption efﬁciency, the inner cladding is designed to bea square shape with 100 m side coated with low-indexpolymer, giving a nominal NA to be 0.4. The core radius is5.5 m with an NA of 0.1. More fabrication details aboutthis Nd-doped ﬁber can be found in [11]. High Nd-dopingenables high pump absorption (about 0.3 dB/m) and, thereby,short length can be used. Besides, 1060-nm band is greatly sup-pressedbyW-typerefractiveindexanddopingproﬁle.Sincethesignal at 1060 nm is more loosely conﬁned than that at 930 nm,1041-1135/$20.00 © 2005 IEEEAuthorized licensed use limited to: UNIVERSITY OF SOUTHAMPTON. Downloaded on July 1, 2009 at 10:20 from IEEE Xplore.  Restrictions apply.FU et al.: COMPACT HIGH-POWER TUNABLE THREE-LEVEL OPERATION OF DOUBLE CLADDING Nd-DOPED FIBER LASER 307Fig. 2. Spontaneous emission from 6.1-m Nd-doped double cladding ﬁber.the Nd-doped ﬁbers are coiled with a bending radius around2.5 cm to further suppress the four-level F – I transitionat 1.06 m when pumped at 808 nm. When the Nd-doped ﬁberis pumped by 808-nm high brightness broad-stripe diode, thespontaneous emission at 1060 nm is greatly suppressed, as isshown in Fig. 2. Note that due to the codoped aluminum, thepeak of the ﬂuorescence of this Nd-doped ﬁber is blue-shiftedto be around 932 nm [12]. In three-level transition F – I ,there is a balance between signal reabsorption and pump uti-lization. Initially, we chose a Nd-doped ﬁber of 8 m. Then, bycutting back some of the Nd-doped ﬁbers and characterizingthe maximal slope efﬁciency for each step, the optimal lengthis given to be around 6.1 m without utilizing pump feedback.Thereafter, the ﬁber is spliced to a tunable ﬁber grating withan initial wavelength at 926.7 nm and the splicing loss is esti-mated to be around 1.4 dB. The splice loss is high because theNA of this Nd-doped ﬁber is only 0.10, while the photosensitiveﬁber used to write a ﬁber Bragg grating is 0.12, leading to modemismatch between the two ﬁbers. However, if we design a pho-tosensitive ﬁber with an NA of 0.10, the splice loss 0.1 dB canbe easily achieved. Hence, much higher slope efﬁciency can beachieved.The ﬁber Bragg grating is 1 cm long with a reﬂectivity of98.3% at 926.7 nm (3-dB full bandwidth 0.37 nm). To form acavity,theﬁberatthepumpendisﬂatlycleavedtomakeamirrorof 4% reﬂectivity. The compression tuning technique used inthis experiment is the same as that used in [9] and [10]. Thebasic idea is that when a ﬁber Bragg grating is mounted on abeam, it will experience different strain relative to the neutralaxis by varying bending radius, enabling compression tuning ofthe Bragg wavelength through elastic–optic effect.III. EXPERIMENTAL RESULTS AND DISCUSSIONWhen the ﬁber Bragg grating is set at its original Braggwavelength at 926.7 nm, the laser has got maximal outputwith launched pump power of 6.3 W. Fig. 3 demonstratesthe optical spectrum when the laser is operating at this wave-length. The spectrum shows that the main desired signal at926.7 nm is 17 dB higher than that at 1061 nm. We estimate thebending loss to be around 0.5 dB for 1060 nm and 0.05 dB for926 nm. Because the high splicing loss point has reﬂection forFig. 3. Optical spectrum of the laser (resolution: 0.2 nm), the inset is thespectrum around 926.7 nm, (resolution: 0.05 nm).Fig. 4. Slope efﬁciency of the double cladding Nd-doped ﬁber laser at926.7 nm.1060 nm and it forms a cavity together with the other cleavedend, 1060-nm lasing still occurs at high pump power. Whenthe pump power is lower, the 1061-nm signal disappears. Ifsplicing is optimized, lasing at 1060 nm can be completelyeliminated. Alternatively, we could bend the ﬁbers with smallerradius, which however, would result in more pump loss. Hence,we keep a bending radius at 2.5 cm. Meanwhile, the slopeefﬁciency is characterized as is shown in Fig. 4. The lasingthreshold is around 1.3 W (launched pump power). We attributethe high threshold to two factors. First, signiﬁcant ground stateabsorption exists in this transition. Second, the mode mismatchbetween the Nd-doped ﬁber and the single-mode ﬁber forgrating writing leads to high splicing loss. The maximal slopeefﬁciency is 17% against launched pump power while it is49.3% against absorbed pump power. The power variations arebelow 1% of the average output power in 10 min when the laseris operating at the highest pump power of 6.3 W. To investigatethe output beam qualities, we have checked the of theAuthorized licensed use limited to: UNIVERSITY OF SOUTHAMPTON. Downloaded on July 1, 2009 at 10:20 from IEEE Xplore.  Restrictions apply.308 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 2, FEBRUARY 2005Fig. 5. Output spectra during tuning (RB: 0.5 nm).output beam to be 1.1 when the launched pump is around 6 W.The laser output beam is indeed single spatial transverse mode.Then we tuned the Bragg wavelength of the ﬁber grating toinvestigate the lasing properties of this laser at different wave-lengths. Under the same pump power (launched power of 6 W),the output is characterized as is depicted in Fig. 5. When thelaser is tuned to 911.8 nm, the ampliﬁed spontaneous emis-sion begins to increase. If we optimize the splice between theNd-doped ﬁber and the ﬁber Bragg grating, the cavity loss willbe even lower and the laser can continue to work below 910 nmwhen the grating is further compressed. When the laser is tunedto 911.8 nm, its threshold is around 4.6 W and its slope efﬁ-ciency against launched power is 2.4%.The current tuning range has been limited mainly by theBragg wavelength of the phase masks available to our labora-tory. If theBraggwavelength determinedbythe phasemask canbe 950 nm, we shall be able to easily tune the laser from 950 to910 nm by using the afore-mentioned bending technique as theBragg wavelength of a ﬁber Bragg grating can be tuned over100-nm range using a simple compression tuning technique[13].We have also brieﬂy investigated the power output of thedouble-clad ﬁber laser employing a bulk dichroic mirror as acomplete reﬂective mirror both at pump and signal wavelength.In this case, there is no splice loss in the cavity. The Nd-dopeddouble-clad ﬁber length is extended to 9.2 m to enable enoughpump absorption to avoid any possible damage to the broad-stripepump diodes causedbyreﬂected pumpfrom thereﬂectivemirror. The lasing threshold is around 1.42 W and the slope ef-ﬁciency is 33.8% against launched pump power. The maximumpoweris around1.55 W ata launched pumppowerof6.3 W andthe lasing longitudinal modes are between 920–932 nm.If a better dichroic mirror is available, which allows hightransmission at 800 nm but at the same time high reﬂectionat 900–980 nm and low reﬂection at 1000–1100 nm, we caneasily change the output to a low reﬂection tunable ﬁber Bragggrating side. This conﬁguration facilitates easy connection tohigh-power Nd-doped optical ﬁber ampliﬁers.IV. CONCLUSIONWe have demonstrated a very compact, robust, ultrastable,continuous-wave tunable Nd-doped double cladding ﬁberlaser based on the transition F – I with output as highas 810 mW. To overcome the very strong competition fromfour-level F – I transition, we employed a high bright-ness broad-stripe pump diode and designed the ﬁber withW-type refractive index and doping proﬁle plus bending loss tosuppress 1060-nm band emission. The laser shows a maximumslope efﬁciency of 49.3% at 926.7 nm against absorbed pumppower. In addition, it is operating in a single spatial mode.By compression-tuning the ﬁber Bragg grating, 15-nm tuningrange is achieved. With such a simple, low-cost, compact, andhighly reliable compression tuning technique, we conﬁdentlyexpect that a further extension of the tuning range to be as highas 40 nm can be implemented.REFERENCES[1] R. Selvas, J. K. Sahu, L. B. Fu, J. N. Jang, J. Nilsson, A. B. Grudinin, K.H. Ylä-Jarkko, S. A. Alam, P. W. Turner, and J. Moore, “High-power,low-noise, Yb-doped, cladding-pumped, three-level ﬁber sources at 980nm,” Opt. Lett., vol. 28, no. 13, pp. 1093–1095, Jul. 2003.[2] P. Dragic and G. Papen, “Fiber ampliﬁcation in the 940-nm water vaporabsorption band using the F – I transition in Nd,” IEEE Photon.Technol. Lett., vol. 9, no. 11, pp. 1478–1480, Nov. 1997.[3] J. Dawson, R. Beach, A. Drobshoff, Z. Liao, D. Pennington, and S.Payne,“938nmNd-dopedhighpowercladdingpumpedﬁberampliﬁer,”in 2003 Advanced Solid-State Photonics Tech. 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Aluminum or phosphorous co-doping effects on the ﬂuorescence and structural properties of neodymium-doped silica glass,”

blue source based on 914-nm Nd:YVO4 passively-Q-switched laser ampliﬁed in claddingpumped Nd ﬁber,” presented at the

Cladding pumped Nd-doped ﬁber laser tunable from 908 nm to 938 nm,”in Conf.LasersElectro-Optics/Int.Quantum ElectronicsConf. (CLEO/IQEC)

Diode-laser-pumped tunable 896–939.5-nm neodymium-doped ﬁber laser with 43-mW output power,”

Fiber ampliﬁcation in the 940-nm water vapor absorption band using the F – I transition in Nd,”

Fiber Bragg gratings compression-tuned over 110 nm,”

High-power, low-noise, Yb-doped, cladding-pumped, three-level ﬁber sources at 980 nm,”

Modal characteristics for W-Type and M-Type dielectric proﬁle ﬁbers,”

Wideband compression-tuned all-ﬁber DFB laser: analysis and characterization,”inOFC2003,vol.1,Atlanta,GA,PaperTUL3,pp.235–236.

Widely tunable optical ﬁlters based on ﬁber Bragg gratings,”

Compact high-power tunable three-level operation of double cladding Nd-doped fiber laser

Compact high-power tunable three-level operation of double cladding Nd-doped fiber laser

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