1. Introduction

High-average power, high repetition rate infrared (IR) and green picosecond (ps) pulses are important sources for laser machining [1], optical parametric oscillators [2], and future particle accelerators [3]. Fiber lasers are an excellent choice as they can deliver high average power with high slope efficiency and diffraction-limited beam quality [4, 5]. High-power pump light can be efficiently coupled into the large-mode area, double-clad gain fibers, delivering IR ps pulses with more than 100 W average power [6–15], and green ps pulses through second harmonic generation (SHG) [10–12, 15]. Also, the diffraction-limited beam quality can be reliably maintained in the very high-power regime due to the outstanding thermo-optical properties and the waveguide structure in the double-clad fibers.

The mode field area of the double-clad active fibers is mostly limited to about 700 μm². Above a certain power level, the peak power in the fiber core becomes very high, and the nonlinear phase shift accumulated in the amplifier becomes significant, broadening the optical spectrum due to the self-phase modulation (SPM) and even distorting the temporal profile. One way to curb the nonlinear effect in the amplifier is to modestly chirp the input pulses and thus reduce the peak power in the fiber core [12]. By this means, the nonlinear effect can be efficiently removed, and the amplified pulses then de-chirped through a pair of gratings. The grating compressor, however, adds alignment complexity and diffraction losses.

Another way to mitigate the nonlinear effect is to employ a larger effective area rod-type photonic crystal fiber (PCF)s and also to shorten the fiber length [15]. Rod-type fibers with very large mode field area of 4500 μm² have been used to generate megawatt peak powers. However, the beam quality could degrade with more than 100 W or even be unstable with more than 130 W average power [15], and the thermal-induced effect limits the average output power up to 150 W [16]. Recently, efforts had been made to improve the design of photonic bandgap rod-type fibers [17], and rod fibers with a mode field area of 2800 μm² have been demonstrated in the high-power operation of Q-switched nanosecond [18] and 30-ps pulse lasers [19]. In spite of smaller mode-field area, the threshold power of the mode instability is improved, which makes it more suitable for high average-power applications.

Here we demonstrate the direct amplification of 1-ps pulse at 1.3 GHz repetition rate using a rod-type fiber with 3300 μm² mode-field area. An average power of 167 W at 1040 nm is generated with nearly transform-limited duration, single polarization, and diffraction-limited beam quality. Frequency doubling yields 124 W at 520 nm with diffraction-limited beam quality. While the average green power is comparable to the highest reported for a fiber laser [20], we have achieved the frequency doubling efficiency of 75% and the pump-to-green efficiency of 45%, which are to our knowledge the highest among fiber-based laser systems.

2. Experimental setup

A schematic of the amplifier system is shown in Fig. 1. The Yb-doped rod fiber master oscillator power amplifier (MOPA) consists of a seed source, preamp I, a grating compressor, preamp II, and a main rod fiber amplifier. The seed source is a commercial Yb-doped fiber oscillator (Pritel, Inc.) actively mode-locked to an external radio frequency (RF) source at 1.3 GHz. The oscillator generates chirped pulses with 8 pJ energy, 10 ps (FWHM) duration, and 2 nm (FWHM) spectral bandwidth at 1040 nm (Fig. 2(b)).

 

Fig. 1 Schematic of the rod fiber amplifier: YDF, Yb-doped fiber; SC, single-clad; LMA, large mode area; WDM, wavelength division multiplexer; ISO, optical isolator; DM, dichroic mirror; SHG, second harmonic generation.

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Fig. 2 IR output versus pump power (a); optical spectrum from oscillator, preamp II, and the main amplifier, respectively (b); autocorrelation signal of the IR pulses (c); and M² measurement at 150 W (d).

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The pulses from the oscillator are first amplified in the preamp I stage. Preamp I is a 30-cm single-mode Yb-doped fiber (Yb 406, Coractive). 700 mW of pump power yields 330 mW output, corresponding to a pulse energy of 250 pJ. Through a pair of transmission gratings (1250 lines per mm), the optical pulses are compressed to 1 ps, close to the transform-limited duration of 0.9 ps, and no spectral change is observed. The grating compressor can be put before preamp I, but the low power from the oscillator makes it difficult to align.

The compressed pulses are then amplified in a 1.5-meter single mode, polarizing double-clad (DC) Yb-doped fiber with 700 μm² mode area (DC-200/40-PZ-Yb, NKT Photonics). The gain fiber is coiled with a diameter of 30 cm and placed on a water-cooled metal plate for heat dissipation. The IR output is 35 W with 55 W of pump power and the slope efficiency is 63% from the pump to IR output. The polarization extinction ratio (PER) is 15 dB, and the laser beam exhibits M² values of less than 1.1 in both directions. However, the optical spectrum begins to narrow with more than 12 W output power, and we chose to operate this stage at 19.8 W IR output with 33.5 W pump power. The bandwidth is about 1.66 nm (Fig. 2(b)), and the nonlinear phase shift accumulated in the preamp stage is about 1 radian at this output.

After an isolator, 16 W IR from preamp II seeds the main rod fiber amplifier. The 80-cm Yb-doped rod fiber with 3300 μm² mode field area (aeroGAIN-ROD-PM85, NKT photonics), is counter-pumped with up to 250 W laser diode at 976 nm. The rod fiber is placed in an aluminum V-groove, and cooled down to 12 °C in order to dissipate the heat.

3. Characterization of amplified pulses

The maximum average power of 167 W is obtained with 244 W pump power [Fig. 2(a)], corresponding to a pulse energy of 128 nJ. The slope efficiency is 66%, excluding the seed power, and diminishes slightly due to the increasing heat dissipation and the drift of the pump wavelength above 220 W pump power. As shown in Fig. 2(b), the spectral bandwidth is 1.6 nm (FWHM), and the optical spectrum exhibits slight red-shift due to the thermally-induced index variation [19]. The nonlinear phase shift accumulated during the amplification is calculated to be about 1.8 radians, and the amplified spontaneous emission is suppressed more than 30 dB below the amplified signal. No stimulated Raman scattering (SRS) or other nonlinear noise is observed. As shown in Fig. 2(c), the optical pulses at 160 W are measured to be 1.55 ps (FWHM), corresponding to 1.1 ps duration if a Gaussian pulse shape is assumed. The time bandwidth product is 0.50, and the output pulses are nearly transform-limited.

The polarization extinction ratio (PER) is 16 dB at 40 W IR output, and improves to 20 dB in the high-power regime (>110 W). The un-polarized light is the leaky component from the air holes of the crystal rod fiber. The beam diameters in the horizontal and vertical directions are almost identical in the near and far fields, and the M² values in both directions are 1.02 at 150 W IR output [Fig. 2(d)], based on D4σ method. In contrast to the results of [15], no degradation of the spatial mode is observed in the high-power output regime.

The IR output power is limited by the available pump. With increased pump power, the optical pulses could be amplified to about 250 nJ, and the corresponding average power of 325 W is within the mode instability threshold [21]. Since the peak power would be far below the threshold for SRS, and the nonlinear phase shift would be less than 3 radians, the spectral distortion should be small enough to avoid major impact on the harmonic generation efficiency. With the constraint of the average and peak power, the present rod amplifier could be readily employed to amplify optical pulses with higher repetition rate or longer duration, and produce average powers of 300 W with single-mode, transform limited pulses.

4. Green light generation through efficient frequency doubling

High average green power, high-repetition rate lasers are important sources for optical parametric oscillators, laser machining, and as drivers for photocathode electron sources. Second harmonic generation (SHG) is carried out through a type I noncritical phase matched lithium triborate (LBO) crystal. The 5 × 5 × 15-mm³ crystal is held at 178 °C. The phase matching bandwidth is 2 nm. A lens with focal length of 12.5 cm is employed to focus the beam to 80 μm such that the Rayleigh length is slightly longer than the crystal length. Behind the SHG crystal, three dichroic mirrors are used to remove the fundamental-frequency light.

The green power and the conversion efficiency are shown in Fig. 3(a). A maximum power of 124 W green pulses at 520 nm is obtained with 167 W IR input, and a conversion efficiency of 75% is achieved in the high-power regime, thanks to the excellent spatial mode in the IR pulses. The excellent conversion efficiency is evidence of the high pulse quality; the peak power is as estimated from the pulse energy and duration, and there cannot be significant energy in any pedestal or wings. In [21] the production of 135 W green power was reported, through 66% conversion efficiency. Here we achieve slightly lower average power, but improve on the excellent conversion efficiency. With respect to the lumped pump power including the preamplifiers and the main amplifier, the pump-to-green conversion efficiency is 45% [22]. To our knowledge, both the frequency doubling efficiency and the pump-to-green efficiency are the highest among fiber laser systems.

 

Fig. 3 Green power generation and conversion efficiency versus IR input (a); Optical spectrum at various power (b); autocorrelation signal (c) and M² values (d) at 120 W green output power. The inset is the far-field spatial mode of the green light at 120 W.

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The optical spectra at 35, 70 and 120 W green power are shown in Fig. 3(b). The bandwidth is 0.48 nm in the low power regime, and it broadens to 1.1 nm in the high-power regime, due to the SPM and the cross-phase modulation [15]. The intensity autocorrelation at 120 W is shown in Fig. 3(c), and the full-width at half-maximum (FWHM) is 1.50 ps, which corresponds to about 1.05 ps duration. The time-bandwidth product is 1.3, so the green pulses are somewhat chirped.

The second-harmonic pulses exhibit excellent beam quality at all powers, and the beam profile is highly symmetric in the horizontal and vertical directions. The M² values are very close to 1 at low power, and increase to 1.14 at 120 W [Fig. 3(d)]. Thus the beam quality of green pulses is diffraction-limited at any high-power output.

5. Summary

In summary, we have demonstrated a direct amplification of 1-ps pulses at 1.3 GHz repetition rate by using a large mode area rod fiber. 167 W IR power with nearly transform-limited duration, single-polarization, and diffraction-limited beam quality is generated, and frequency doubling yields 124 W green power, also with diffraction-limited beam quality. We have achieved to our knowledge the highest frequency doubling and the highest pump-to-green conversion efficiency in the fiber-based laser amplifiers. Such a high-power fiber laser system should be important for a number of scientific applications.