January 1, 2005 / Vol. 30, No. 1 / OPTICS LETTERS 67

60-W green output by frequency doubling of a polarizedYb-doped fiber laser

Anping Liu, Marc A. Norsen, and Roy D. Mead

Aculight Corporation, Suite 113, 11805 North Creek Parkway South, Bothell, Washington 98011

Received July 13, 2004

The frequency doubling of a 110-W linearly polarized diffraction-limited Yb-doped fiber oscillator power am-plifier (FOPA) has generated 60-W near-diffraction-limited �M 2 # 1.33� linearly polarized green output. TheFOPA produces as much as 2.4 kW of peak power and less than 20-pm linewidth at a 10-MHz repetitionrate and 5-ns pulse duration without the onset of nonlinear effects. With two lithium triborate crystals atnoncritical phase matching, a maximum of 54.5% doubling efficiency has been demonstrated. The overallelectrical efficiency to the green portion of the spectrum is 10%. © 2005 Optical Society of America

OCIS codes: 140.3510, 190.2620, 140.4480, 140.3280.

High average power, high brightness, linearly polar-ized green lasers are of great interest for scientific,industrial, medical, and defense applications. Greenlasers that produce several hundred watts of powerhave been demonstrated that are based on Q-switcheddiode-pumped solid-state lasers, but their beam qual-ity is at least 10 times diffraction limited owing tothermal lensing effects in the diode-pumped solid-statelasers.1,2 Yb-doped fiber lasers have demonstrateddiffraction-limited beam quality at powers of kilo-watts,3 as the beam quality is determined by the fiberstructure instead of by thermal lensing effects asin diode-pumped solid-state lasers. Combining thehigh eff iciency, compact size, and high brightness offiber lasers with second-harmonic generation (SHG)is an attractive alternative that will facilitate manyapplications.

There has been success in generating high-powergreen lasers by use of the SHG of f iber lasersources.4 Usually KTP or periodically poled KTP(PPKTP) is used, because these nonlinear crystalsrequire relatively low pulse energies or averagepowers for eff icient conversion. A 6-W averagepower green laser was demonstrated by use of thecombination of a laser-diode-seeded Yb fiber am-plifier and quasi-phase-matched SHG in PPKTP.5

Unfortunately, the beam quality was not reported,and the eff iciency rolled over at higher averagepower owing to local heating in the PPKTP crystal.Creation of color centers, or gray tracks, in KTP hasalso been reported for high-power cw and Q-switchedlasers.6 – 9 Therefore, using PPKTP or KTP to achievehigh conversion efficiency for cw or low-peak-powerfiber lasers will face many obstacles, such as degra-dation in beam quality, power instability, and crystaldamage.

In general, for efficient SHG, narrow linewidth andhigh peak power are desired. However, conventionalYb-doped high-power fiber laser oscillators usuallyoperate with broad linewidths owing to the broad gainbandwidth and the nature of inhomogeneous broad-ening of the rare-earth-doped gain medium. For aconventional fiber laser oscillator without speciallinewidth control, the fiber laser’s linewidth can beas much as several tens of nanometers. In addition,

0146-9592/05/010067-03$15.00/0

high-power f iber lasers may also suffer from the onsetof nonlinear effects, such as stimulated Brillouinscattering (SBS), stimulated Raman scattering (SRS),four-wave mixing, and self-phase modulation, whichwill not only broaden laser linewidth but also affectlaser stability.10

In this Letter we report a 60-W, 10-MHz, linearly po-larized green laser based on a frequency-doubled fiberlaser. To produce narrow linewidth and high peakpower we use a fiber oscillator power amplif ier (FOPA)configuration. With an Yb-doped large-mode-area(LMA) fiber, the FOPA operating at a high repetitionrate with a short pulse duration has generated as muchas 2.4 kW of peak power without the onset of non-linear effects. The diffraction-limited beam qualityfrom the FOPA allows us to replace KTP or PPKTPwith two LBO crystals to increase the interactionlength to achieve eff icient SHG without gray trackingproblems.

The green laser based on frequency doubling of theFOPA consists of a cw fiber oscillator, an amplitudemodulator, a fiber preamplifier, a fiber power am-plifier, and a second-harmonic generator, as shownschematically in Fig. 1. The cw Yb-doped fiberoscillator with two fiber Bragg gratings generates a

Fig. 1. Experimental setup for achieving 60 W of greenpower based on frequency doubling of the fiber oscillatorpower amplifier. G1, G2, fiber Bragg gratings; M, ampli-tude modulator; P, f iber polarizer; LDs, laser diodes; PM,polarization maintaining.

© 2005 Optical Society of America

68 OPTICS LETTERS / Vol. 30, No. 1 / January 1, 2005

narrow linewidth at 1080 nm. The laser linewidthwas measured to be less than 20 pm, and the mea-sured linewidth is limited by the resolution of theoptical spectrum analyzer. The cw laser light is thenmodulated by an amplitude modulator, which can varythe pulse duration and the repetition rate separately.The pulse duration of the seed source can be variedfrom hundreds of microseconds to nanoseconds, andthe repetition rate can be varied from hundreds ofkilohertz to hundreds of megahertz. The averageoutput power after the modulator is determined by themodulation duty factor. At a 10-MHz repetition rate(100-ns repetition period) and 5-ns pulse duration, aduty factor of 0.05 produces approximately 1 mW ofaverage signal power. The signal is then amplifiedby a single-mode Yb-doped fiber preamplifier. Themaximum output power from the preamplif ier is200 mW, and the maximum gain is 23 dB. Fiberisolators are used between the fiber oscillator, thepreamplifier, and the fiber power amplif ier stagesto protect each stage from backref lection, which canaffect the performance of each stage. As the ratioof pulse duration to repetition period varies, theseed source provides a range of peak powers, and thepreamplifier has reached a peak power of tens of wattswithout the onset of nonlinear effects.

The fiber power amplif ier uses an Yb-dopedpolarization-maintaining double-clad LMA fiber witha fundamental mode-field diameter of 18 mm anda numerical aperture of 0.06. Compared with con-ventional single-mode double-clad f ibers, the LMAfiber provides higher absorption and larger mode-f ielddiameter, which thus reduce the f iber length andsignal power intensities that are the key parame-ters for reducing the nonlinear effects. The fiberlength of 10 m absorbs 98% of the pump power at976 nm. The single-mode linearly polarized pulsedsignal from the preamplif ier is launched into theLMA core with a set of specially designed lenses thatachieve mode matching between the single-mode fiberand the LMA fiber to selectively excite the funda-mental mode. To achieve a maximum polarization-extinction ratio, the f iber’s stress-rod plane (or slowaxis) is aligned parallel to the polarization of the seedlight. The pump light is launched into the LMA fiberopposite the pump end, as shown in Fig. 1. The pumpwavelength is set to the strongest absorption peaknear 976 nm to shorten the fiber, thus minimizingpossible nonlinear effects. The dielectric mirror forseparating the pump light and the amplif ied signalprovides .99% ref lectance at 976 nm and ,2%ref lectance at 1080 nm. All lenses are antiref lectioncoated for both pump and signal wavelengths to reduceundesirable backref lection. The maximum powerfrom the fiber-coupled laser diode is 200 W, and thecoupling efficiency between the laser diode and thefiber is estimated to be 85%.

At a 10-MHz repetition rate and a 5-ns pulseduration the f iber power amplif ier produces as muchas 120 W of output power, corresponding to a slopeefficiency of 71% with respect to the launched power,as shown in Fig. 2. The polarization-extinction ratiois better than 95%. The beam quality, measured

with a Model BeamScan-XYGEPREC beam scannerfrom Photon, Inc., is 1.1 times diffraction limited. Nolinewidth broadening beyond the 20-pm instrumentalresolution or nonlinear effects have been observed,even at 120-W output power.

The eff iciency of conversion to green output may beincreased by reduction of the duty factor, which therebyincreases the peak power. This may cause nonlineareffects in the f iber. Among fiber nonlinear effects,SBS has the lowest threshold for a narrow-linewidthsystem, but SBS can be prevented by a simple reduc-tion of the pulse duration to well below 16 ns.11 Oncethe SBS is suppressed, the maximum peak power of thefiber power amplif ier is limited by either SRS or mate-rial damage. For a 10-m-long fiber with a mode-f ielddiameter of 18 mm, the SRS threshold can be esti-mated for a linearly polarized beam by use of Ramangain coeff icient11 gR � 1 3 10213 m�W and assuminga uniform signal distribution along the f iber lengthto be 16�A�gRL � 4 kW. When the fiber power am-plifier is end pumped, the nonuniform signal distribu-tion along the f iber length will make the SRS thresholdmuch higher than 4 kW. To be conservative, we useda 5-ns pulse duration and a 10-MHz repetition rate toenhance the peak power by a factor of 20. As a result,2.4-kW peak power is generated from the fiber poweramplifier at the maximum average power of 120 W.

Lithium triborate (LBO) is used as the frequency-doubling crystal even though it has a relatively lownonlinear coefficient compared with KTP and PPKTP.This is so because multikilowatt peak power is suffi-cient for a long LBO or multiple crystals to achievehigh eff iciency without suffering from gray tracking.As it is difficult to obtain a high-quality long crystaland increasing the number of crystals will affectsystem complexity and may also induce unwantedphase shifts, a system with dual long crystals waschosen. The two antiref lective-coated LBO crystalsare 25 mm long and cut for noncritical phase matchingat the 1080-nm operating wavelength. Based on themanufacturer’s specifications and on the experiments,the two crystals have similar optical characteristics.Each of the crystals is mounted in a temperature-controlled oven with a crystal separation of 5 mm to

Fig. 2. Output power of the f iber power amplifier versuslaunched pump power. Inset, measured M2 for both jj and� polarization at 110 W.

January 1, 2005 / Vol. 30, No. 1 / OPTICS LETTERS 69

Fig. 3. Dependence of the green signal on LBOtemperature.

Fig. 4. Green output power versus fundamental power.Inset, measured M2 of the green power for both jj and �polarization at 50 W.

keep the effective optical path as short as possible.The beam from the fiber power amplifier is focused bya 15-cm focal-length lens, and the beam waist is 56 mm�1�e2� as measured by the high-resolution BeamScanbeam scanner (resolution, ,4 mm). Following thecrystals, two dielectric f ilters are used to separategreen and fundamental beams. The green signal wasmeasured as a function of crystal temperature at lowpower and is shown in Fig. 3. The temperature tol-erance for eff icient phase matching is approximately2 ±C (FWHM). The green output is shown versusfundamental power in Fig. 4. At 110-W fundamentalpower, as much as 60 W of green power is produced,corresponding to maximum conversion efficiency of

54.5%. The peak power of the fiber power amplif ierat 110 W is estimated to be 2.2 kW. The beam qual-ity of the green laser is 1.33 times diffraction limited.Although all crystal temperatures are optimized atlow power, they provide efficient frequency doublingat 60 W of green output power with no temperatureadjustment.

In our experiment the pulse duration is limitedby the speed of the available modulator driver. Themaximum modulation frequency of the modulator canbe as much as 10 GHz; replacing the modulator drivercan easily reduce the pulse duration to 500 ps andincrease the pulse repetition rate to 100 MHz. Withinstruments of the same peak power and configura-tion, we expect to demonstrate similar performance.

In conclusion, we have demonstrated a highly effi-cient, near-diffraction-limited linearly polarized 60-Wgreen laser based on the frequency doubling of a fiberlaser. The overall electrical eff iciency to green is 10%.

We acknowledge useful discussions on frequencydoubling with Mark Bowers and Dave Gerstenbergerand experimental assistance from Tracy Vatter andPaul Hoffman. This research is supported by theU.S. Air Force Research Laboratory under contractAFB F29601-98-D-0190—task order 8. A. Liu’se-mail address is anping.liu@aculight.com.

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