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50W single-mode linearly polarized high peak power pulsed fiber laser with tunable ns-µs pulse durations

and kHz-MHz repetition rates

V. Khitrov*, B. Samson, D. Machewirth, K. Tankala

* Presenting author

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• 10 ~ 100W linearly polarized pulsed fiber lasers have been demonstrated (IPG Photonics, Aculight, Sandia Labs and others)

– 0.1~0.5mJ pulse energies, 1~10ns pulse durations

– Near diffraction limited beam (M2 < 1.5)

– Yb-doped large mode area (LMA) double clad fiber

– Operating wavelengths 1060-1080nm– Limited set of parameters per device, typical sets

• High rep rate ~1MHz, low energy <0.1mJ, high average power 50~100W• Low rep rate <50kHz, high energy ~0.5mJ, low average power ~10W

• We demonstrate 50W single-mode linearly polarized high peak power pulsed fiber laser with tunable ns-µs pulse durations and kHz-MHz repetition rates capable to address a wide range of applications: frequency conversion, LIDAR and others

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LMA core High NA cladding

Glass CladdingPM LMA Yb-doped double clad fiber – key

component

• Silica fiber with high optical damage threshold• 20-40μm core, low NA<0.1 for good beam

quality• 200-400μm cladding, high NA~0.5 for efficient

pump coupling• High Yb concentrations for short fiber lengths• Hi birefringence for single polarization

Challenges for high power pulsed polarized fiber lasers

• Nonlinear effects: SBS, SRS, Self Phase Modulation, Four Wave Mixing – limiting output power, broadening narrow spectral linewidth and destroying integrity of output spectrum

• Polarization control problems – power de-coupling from fundamental linearly polarized mode to parasitic higher order modes

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• Master Oscillator Power Fiber Amplifier (MOPFA) design – flexible all-fiber format

• Low power semiconductor laser diode as seed source

• High power PM LMA fiber based amplifier

Master oscillator

High power isolator

High power amplifier

Low power pre-amp

Principal design

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1.E-09

1.E-07

1.E-05

1.E-03

1.E-01

1.E+01

1.E+03

50 100 150 200

Coil Diameter (mm)

Be

nd

Lo

ss a

t 1

06

0 n

m

(dB

/m)

Slow Pol

Fast Pol

Fiber Design• Yb-doped LMA 30/250μm fiber• Low core NA (0.06) • 0.46 NA cladding• High birefringence (2.5x10-4)

Coil Technique• Utilizes differential bend loss of

fundamental and higher order modes• 8 cm coil maintains single-mode linear

polarization operation

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Choice of seed pulsed lasers

• Pulsed Nd:YAG, Yb:YAG lasers• Microchip lasers• Semiconductor diode seeded multi-stage fiber amplifiers• Q-switch fiber lasers

Semiconductor diode advantages

• Robust PM fiber-coupled output• Flexible pulse repetition rates 10kHz-50MHz• Flexible pulse durations 2ns-10µs• Suitable output for seeding the multi-stage fiber amplifier (~0.5W peak power)

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PM LMA Yb-doped

30/250μm fiber

1064nm pulsed signal

PM isolator

Signal/pump

fiber multiplexer

1064nm pulsed MO

976nm pump 976nm pump 976nm pump

SM PM Yb-doped fiber

PM isoPM iso

Signal/pump

fiber multiplexer

976nm pump

• 0.5W peak power seed laser• 2-stage SM preamplifier• High power amplifier based on 3m of PM-LMA-YDF-30/250 fiber • Up to 92 W of coupled pump power (at 976nm, ~3nm line width)

through signal/pump multiplexers

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• Output power increases linearly with pump power • 51 W (10ns, 350kHz) output achieved with 92 W coupled pump• Slope efficiency 60%, overall optical efficiency 56%• Polarization Extinction Ratio 15dB

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• Pulse duration 10ns, rep rate 350kHz• Near diffraction limited output M2 = 1.24

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• 1064nm central wavelength, 0.8 nm spectral width • No sign of SBS or SRS• Moderate spectral broadening

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• Stable 50W output at 50kHz-50MHz pulse repetition rates • Pulse durations 2.5ns-200ns• Pulse energies up to 1mJ at 50kHz rep rate

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Pulse shapes at 200ns duration and 50kHz rep rate from MO (left) and amplifier output (right)

• Strong pulse distortion (collapsing towards leading edge) at the output at low repetition rates

• Due to Yb inversion depletion• Results in extremely high peak power in the beginning of the pulse causing detrimental non-

linear effects like SBS, SRS, etc.• Pulse engineering applied to seed laser is required to correct pulse distortion *• >1mJ possible to achieve with corrected pulse shapes* - K.T.Vu, A.Malinowski, D.J.Richardson, F.Ghiringhelli, L.M.B.Hickey, M.N.Zervas “Adaptive pulse shape

control in a diode-seeded nanosecond fiber MOPA system”, Optics Express, Vol 14, No 23, 2006, pp. 10996-11001

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0

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50 60

1064nm power (W)

53

2n

m p

ow

er

(W)

34% SHG efficiency

• 3x3x15mm LBO crystal operating at room temperature, single-pass SHG, ~70µm beam size

• 17 W (10ns, 350kHz) output at 532nm achieved with 50W 1064nm input• 34% SHG efficiency close to 40% theoretical efficiency

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• Demonstrated high power pulsed, monolithic fiber laser– Robust all-fiber MOPFA design– 51W average output power– 1064nm wavelength, 0.8nm linewidth– Flexible 2ns-0.2µs pulse durations, 50kHz-50MHz repetition rates, up to 1mJ

pulse energy – PER = 15dB, M2 = 1.24– 56% optical efficiency– 17W at 532nm, 34% frequency doubling efficiency limited by available

crystal

• Practical design to address a number of applications: frequency conversion, LIDAR and other applications

Summary