1486 OPTICS LETTERS / Vol. 32, No. 11 / June 1, 2007

Stable high-power continuous-wave Yb3+-dopedsilica fiber laser utilizing a point-by-point

inscribed fiber Bragg grating

Nemanja Jovanovic,1,* Alexander Fuerbach,1 Graham D. Marshall,1 Michael J. Withford,1 andStuart D. Jackson2

1Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS), Centre for Lasers and Applications (CLA),Department of Physics, Macquarie University, North Ryde, New South Wales 2109, Australia

2Optical Fibre Technology Centre, Sydney University, 206 National Innovation Centre, Australian Technology Park,New South Wales 1430, Australia

*Corresponding author: njovanov@ics.mq.edu.au

Received February 13, 2007; revised March 2, 2007; accepted March 2, 2007;posted March 7, 2007 (Doc. ID 80090); published May 4, 2007

We report on a narrowband 5 W cw fiber laser incorporating a point-by-point fiber Bragg grating inscribedinto the core of a Yb3+-doped double-clad fiber. The laser featured excellent long-term wavelength and powerstability (0.3%), as well as a very narrow �15 pm� linewidth, when passive temperature stabilization of thegrating was implemented. © 2007 Optical Society of America

OCIS codes: 060.2320, 140.3510, 230.1480, 350.3390.

High-power fiber lasers operating in the near infra-red have become a topic of great interest due to ad-vances in inexpensive high-power laser diodes andimproved optical-fiber design [1]. High slope efficien-cies, power scalability, broad wavelength tunability,and diffraction-limited beam qualities at high powermake fiber lasers useful for a range of applications.Ultrahigh-power (i.e., kilowatt level) laser systemscan be used for cutting, welding, and drilling andlow-power (i.e., milliwatt level) lasers can be used fortelecommunication applications. At intermediatepower levels, fiber laser systems operating in boththe infrared and visible part of the spectrum can beused for medical procedures, imaging [2], and sens-ing.

Fiber lasers fall into two main categories: thosewith extrafiber feedback and those with intrafiberfeedback. Extrafiber feedback systems refer to sys-tems that use wavelength selective elements externalto the laser fiber such as dichroic mirrors, bulk grat-ings, or master-oscillator power-amplifier (MOPA) ar-rangements. These systems offer the greatest flex-ibility, and systems with hundreds of watts [3] up tothe kilowatt cw level [4] have been demonstratedwith varying spectral bandwidths. The disadvantageof these systems is that they are complex, expensive,and less robust than all-fiber arrangements.

Intrafiber feedback in fiber laser systems refers tothe use of fiber Bragg gratings (FBGs) inscribed inthe core of the laser to act as the frequency selectiveelement. FBGs can offer a narrowband output, buttheir use in high-power fiber lasers has, until now,been limited. The standard fabrication method re-quires the use of photosensitive fiber and is hence notwell suited to rare-earth-doped fibers since they typi-cally use aluminosilicate glass that exhibits a verylow photosensitivity. To circumvent this problem, thegratings can be written into a high Ge content (i.e.,high photosensitivity) single-mode fiber (SMF) that issubsequently spliced onto the active fiber. A fiber la-

ser generating as much as 20.4 W, with a laser line-

0146-9592/07/111486-3/$15.00 ©

width of 0.37 nm, was realized using this technique[5]; however, two splices were introduced, which is anunwanted complication and a potential additionalloss mechanism. In addition, as the writing processstrongly depends on the formation of defects ther-mally related instabilities are an issue, especially inrespect to high-power applications [6].

Recently, a revision of the point-by-point (PBP)technique to inscribe FBGs in standard telecom fi-bers was reported [7]. The approach utilized a femto-second laser pulse to modify the refractive index in asmall area within the core of a fiber [8] and, by syn-chronizing the translation of the fiber with the rep-etition rate of the femtosecond laser, highly reflectingnarrowband gratings with good thermal stability canbe fabricated at various wavelengths [9]. These FBGshave polarization-dependent characteristics [10] andhave been used to produce a distributed feedbackEr3+:Yb3+-codoped fiber laser with a 0.2 mW narrow-band output [11]. More recently, a scanning phasemask technique was used to create a 38 mW Er fiberlaser operating at 1550 nm with a bandwidth of0.15 nm [12].

In this Letter, we present a high-power (multiwatt)fiber laser operating at 1 �m that utilized a FBG in-scribed directly into the core of a Yb3+-doped double-clad fiber using the PBP technique. We demonstratethe high thermal stability of the inscribed grating

Fig. 1. Schematic of the experimental setup.

2007 Optical Society of America

June 1, 2007 / Vol. 32, No. 11 / OPTICS LETTERS 1487

and elucidate the highly narrow linewidth feature ofthe output. We also demonstrate the potential for la-ser wavelength variation using temperature andstrain tuning of the FBG.

The experimental arrangement is shown in Fig. 1.The Yb3+-doped silica fiber (manufactured by the Op-tical Fibre Technology Centre, University of Sydney)had an 8 �m core diameter with a dopant ion concen-tration of 7400±800 ppm. The fiber had a hexago-nally shaped pump core with a 300 �m corner-to-corner separation. In the fabrication of the FBG, thefiber was mounted on a high-precision air-bearingtranslation stage, the outer cladding was stripped,and the femtosecond laser pulses were focusedthrough the pump core into the active core using a0.8 NA, 20� oil-immersion objective lens. The in-scribing pulses had a �120 fs pulse duration, a 1 kHzrepetition rate, and a pulse energy of 220 nJ, pro-vided by a regeneratively amplified Ti:sapphire laseroperating at 800 nm. The FBG, which acted as thehighly reflecting mirror, was 15 mm long and had aperiod of 1.12 �m, which corresponds to a third-ordergrating for 1080 nm light. Fresnel reflection com-pleted the cavity. A dichroic mirror filtered out the re-maining unabsorbed pump. The 20 m long fiber waspumped using a 20 W, 980 nm laser diode connectedto a 400 �m diameter fiber pigtail. The launch effi-ciency was 64%.

The output power measured as a function of thelaunched power is shown in Fig. 2. The fiber lengthwas optimized to 20 m, and the maximum recordedoutput power was 5.05 W with a slope efficiency of46%. There were no observed indications of outputsaturation, which demonstrates the possibility of fur-ther power scaling. Indeed, when the fiber laser waspumped from both ends, 10 W of stable, narrowbandlight was achieved without any signs of saturation.We will, however, focus on the 5 W laser system.When a 100% reflecting dichroic mirror was used asthe high reflector instead of the FBG, slope efficien-cies as high as 70% were obtained for the same fiberlaser length. The relatively low slope efficiency of thelaser with the FBG is attributed to the FBG reflect-ing less than 100% of the laser light.

Fig. 2. Output power and laser linewidth versus inputpower for the fiber laser. The inset shows the very narrowsingle laser line of the fiber laser.

The laser linewidth was 15 pm �3.87 GHz� at anoutput power of 5.00 W, as shown in the inset to Fig.2. The center wavelength of the laser was observed toshift toward longer wavelengths as the pump powerwas increased and is closely predicted by the thermo-optic effect in the grating. At high pump power, thecenter wavelength fluctuated over a 100 pm range,an effect which is believed to be due to fluctuations inthe temperature of the grating. Improved laser wave-length stability was achieved by simply enclosing theFBG between two water-cooled copper blocks. Alsoshown in Fig. 2 is the laser linewidth for each pump-power level averaged over a 10 min period. It can beobserved that the laser linewidth fluctuates less athigh power. The reduced stability at low power is at-tributed to the large transmission losses or a nonop-timized cavity. Above 2 W, the laser linewidth in-creases linearly with pump power (slope of0.485 pm/W with an initial offset of 12.8 pm), an ef-fect that may relate to thermal chirping of the grat-ing.

The stability of the center wavelength, linewidth,and maximum output power was investigated at fullpower over a 4 h period. Figure 3 shows the resultswith and without the use of passive temperature sta-bilization. The mean and standard deviation of themeasurements are summarized in Table 1. The use ofpassive temperature control of the grating region sig-nificantly improved the stability of the output.

Wavelength tunability was explored with bothstrain and temperature tuning. We heated the FBGto temperatures as high as 600 °C without showingany signs of degradation, and we observed a wave-length shift of 9.4 pm/ °C. By applying strain to thegrating region, it was possible to tune the laser wave-length by 38.7 pm/�m. The 15 mm grating could bestretched a total amount of 135 �m (strain of 0.9%)before it broke, giving a tuning range of 5.2 nm. Cur-rently we do not inscribe the FBG through the poly-mer jacket; however, it has been shown in the case of

Fig. 3. Stability of the output power, center wavelength,and laser linewidth over a 4 h period with (thick blackcurve) and without (thin gray curve) a temperature stabi-lizing water-cooled copper block. The optical spectrum ana-lyzer had a maximum resolution of 10 pm and the step sizewas set to 2 pm. The relative center wavelength was calcu-lated by subtracting the operating wavelength from themean center wavelength of 1078.270 nm with the cooling

applied and 1078.470 nm without cooling applied.

1488 OPTICS LETTERS / Vol. 32, No. 11 / June 1, 2007

SMFs that inscribing the FBG into a coated fiber of-fers the advantage of a greater stretching length [13].The maximum measured stretching length of acoated double-clad fiber was �470 �m (strain of3.2%), which, in practice, would correspond to amaximum possible tuning range of 18 nm.

The reflection spectrum of the FBG is shown inFig. 4. It can be seen that despite the fact that thelaser operated on only a single narrow line that cor-responds to the third peak in the reflection spectrum(center wavelength of 1078.15 nm), the FBG wascomposed of four reflection peaks. Since the fourpeaks are equally spaced in frequency, we attributethem to the formation of a sampled grating that wascreated by a low-frequency oscillation in the air-bearing stage during the writing process. A quantita-tive measurement of the grating reflectivity could notbe made due to experimental difficulties in obtainingstable and efficient coupling from the core of the

Fig. 4. Reflection spectrum of the highly reflecting FBG.The inset shows a high-resolution scan of the grating peakin which the laser operated.

Table 1. Mean Values and Standard DeviationsFrom Curves in Fig. 3a

Fiber LaserFeatures

WithStabilization

WithoutStabilization

Center wavelengthdeviation (pm)

2 32

Mean laserlinewidth

[pm (GHz)]

15.08(3.87)

19.2(4.95)

Standard deviationof linewidth[pm (GHz)]

0.8(0.21)

2.8(0.72)

Linewidthfluctuation (%)

5.3 14.6

Mean outputpower

(W)

4.96 4.94

Standard deviationof output power (W)

0.015 0.063

Output powerfluctuation (%)

0.3 1.3

aLinewidth and output power fluctuations are taken as the ratioof the standard deviation to the mean.

double-clad fiber to the optical spectrum analyzer;

however, it is hypothesized that the reflectivity of thegrating is less than 100% due to the relatively lowmeasured slope efficiency. The inset to Fig. 4 shows ahigh-resolution scan of the third peak, which showsthe bandwidth to be 100 pm, which is �5 timesbroader than the actual bandwidth of the fiber laser.A self-narrowing feature in the Yb fiber laser is evi-dent and is currently under investigation.

In conclusion, we have demonstrated power scalingto the 5 W level of Yb3+-doped silica fiber laser thatutilizes a FBG inscribed by the PBP technique. Wehave shown that PBP written FBGs are robust andcan easily withstand the high core light intensitiespresent in high-power fiber lasers. Minimal fluctua-tion of the center wavelength, bandwidth, and outputpower resulted from passive temperature stabiliza-tion of the FBG. We have recently demonstrated fur-ther power scaling to 10 W by double end pumpingthe fiber laser, without any saturation of the outputpower. The PBP approach offers the quick and easyfabrication of FBGs that provide spectrally narrowlinewidth outputs that are well suited to high-powerfiber lasers and their frequency conversion.

This work was produced with the assistance of theAustralian Research Council under the ARC Centresof Excellence and LIEF programs. We thank MattiasÅslund for his insights into probing the FBG in thedouble-clad fiber.

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