50W single-mode linearly polarized high peak power pulsed fiber laser
description
Transcript of 50W single-mode linearly polarized high peak power pulsed fiber laser
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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
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Coil Diameter (mm)
Be
nd
Lo
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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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1064nm power (W)
53
2n
m p
ow
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(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