We demonstrate a compact and simple two-stage Yb:YAG single crystal fiber amplifier which delivers 160 W average power, 800 fs pulses without chirped pulse amplification. This is the highest average power of femtosecond laser based on SCF. Additionally, we demonstrate the highest small signal gain of 32.5 dB from the SCF in the first stage and the highest extraction efficiency of 42% in the second stage. The excellent performance of the second stage was obtained using the bidirectional pumping scheme, which is applied to SCF for the first time.
© 2015 Optical Society of America
Owing to the considerable ultrafast laser performance development in the past decade, currently there are increasing requirements on reliability, power, flexible repetition rate and price. Single crystal fibers (SCF) stand out as a promising technology for ultrashort pulse amplification to high average power in a simple and compact architecture. Although known for decades [1–4], SCF found their application in high average power lasers only recently . The concept lies in between crystals and fibers and benefits several advantages from both.
High repetition rates of several megahertz are beneficial for both scientific and industrial applications. In scientific experiments, ultrashort lasers with high average power and high repetition rates improve the measurement signal-to-noise ratio thus reducing measurement time. In micromachining, the ultrashort pulses lead to the high surface quality with precise patterning depth and without grooves, rims or visible melting damage. High throughput is essential in industrial applications to compete with already established techniques such as mechanical or chemical material processing and it can be achieved with high repetition rate lasers. The efficiency of the ablation process reaches its maximum at a certain optimum fluence [6,7]. Thus, for a fixed pulse energy or fluence, working at high repetition rates requires high average powers.
Crystals exhibit high gains and are used in several architectures. Regenerative amplifiers, bulk or thin disk, are shown to amplify pulsed laser radiation to 100s of milijoules [8,9]. However, they are limited to relatively low repetition rates, typically few hundreds of kilohertz, due to the high-voltage-driven Pockels cells.
Amplifiers based on Innoslab technology have shown an impressive performance in terms of average power at high repetition rates. 1.1 kW have been demonstrated with Yb:YAG slab crystal and 615 fs pulses . However, their inherent complexity and large number of signal passes without waveguiding, pose a challenge for its stability and wider industrial applications.
Optical fibers are another approach in ultrashort pulses amplification achieving high average power and high repetition rate . They possess an average power scaling potential owing to their good thermal management due to high surface-to-volume ratio. The main drawback is that the signal is confined in small cross-section core, which induces nonlinear effects such as self-phase modulation and self-focusing, and consequently limits the achievable peak power and pulse energy. When used for ultrashort pulses amplification, a chirped pulse amplification (CPA) technique must be employed. Femtosecond pulses with energy of 2.2 mJ  and 830 W of average power  have been demonstrated, but using large and complex CPA techniques.
SCF exhibits an aspect ratio of a short rod fiber or a thin and long crystal benefiting from both straightforward thermal management of a fiber and the spectroscopic and thermo-mechanical properties of a crystal. Typical diameter is less than 1 mm while a length is several tens of millimeters. They are designed for a pump light guidance and a free-space propagation of a laser signal. This geometry exhibits several advantages in the ultrashort pulse amplification. Owing to the high thermal conductivity of Yb:YAG and high surface-to-volume ratio, SCF provides good thermal management. Relatively short interaction length and large signal beam diameter minimize the nonlinear effects and allow direct amplification of femtosecond pulses without the standard CPA technique. This reduces the complexity and allows more compact foot print, lower cost and increased robustness. The guidance of pump light within SCF increases the overlap between the pump and the signal beam compared to the standard bulk technology . High signal gain can be achieved with single or double pass configurations. Therefore, regenerative amplification approach can be avoided and operation at high repetition rates is achieved.
In a proof-of-principle experiment, 250 fs pulses were directly amplified from 0.4 W to 12 W at 30 MHz repetition rate using Yb:YAG SCF . With CPA technique, operation in the range from 10 kHz to 10 MHz have been demonstrated, with maximum pulse energy of 1 mJ at 10 kHz and maximum average power of 30 W at 10 MHz . SCF were recently also used in a coherent beam combining approach, where from two channels at 6 kHz, maximum compressed pulse energy of 3 mJ with a pulse duration of 695 fs has been achieved .
In this work, we present direct amplification of the femtosecond pulses in a simple and compact two-stage SCF amplifier. The small signal gain of the first stage amounts to remarkable 32.5 dB while the maximum output power is 160 W. This is to the best of our knowledge the highest small signal gain and the highest average power of femtosecond pulses achieved with crystal fibers. In addition, it is the first time that a bidirectional pumping scheme is used.
2. Experimental setup
The experimental setup is shown in Fig. 1. The seed laser is a SESAM mode-locked Yb-based oscillator (YBIX from JDSU), delivering high peak power and ultrashort pulses while maintaining industrial-grade long-term stability. The customized seed laser used here operates at 83.4 MHz with 2.8 W of average power and has beam quality factor M2<1.1 in both directions. The optical spectrum is centered at 1030.5 nm with 2.4 nm full-width at half maximum. The pulse duration was measured to be 380 fs. The oscillator pulse train is directly seeded into the two-stage power amplifier, avoiding the standard chirped pulse amplification technique. The amplifier is based on the Yb:YAG single crystal fibers with 1 at. % doping rate, 1 mm diameter and 40 mm length, and whose facets are anti-reflection coated for both the signal and the pump wavelength. The rod is embedded in a water-cooled heat sink with temperature set to 18°C in this experiment. The first stage is end-pumped by a high brightness 106 µm fiber-coupled laser diode with an NA = 0.22 emitting up to 140 W at 940 nm (Stingray from JDSU). The pump fiber output is imaged into the SCF and the seed beam is focused to a 400 µm diameter spot inside the SCF. The signal is double passed using a retro-reflective mirror and Faraday rotator. The retro-reflective mirror images the signal back into the SCF thus compensating for thermal lensing, while Faraday rotator rotates the polarization by 90° and allows extraction of the amplified pulses by the polarizing cubic beam splitter located at the input.
These pulses are then seeded into the second amplifier stage. This stage is bi-directionally pumped by the 106 µm (Stingray from JDSU) and 200 µm (IS10 Series from DILAS) fiber-coupled laser diodes, emitting 940 nm light up to 140 and 210 W, respectively, with an NA = 0.22. The only reason we used a lower brightness diode as the second pump is because we did not have another higher brightness diode available. The output of both, 100 μm and 200 μm fiber diode, are imaged to approximately 400 μm diameter spot inside the SCF. In this configuration, transmitted light of one pump diode reaches the fiber output of the other one, which in principle can cause some instability or damage. However, it was measured that less than 2% of light is coupled into the facing diodes, which is within the specified tolerance range, and no measurable effect was observed during the operation. In order to compensate for thermal lensing induced by the two pump beams, the seed was injected into the SCF slightly divergently. The signal is single passed and the output is extracted by a dichroic mirror, which transmits pump light and reflects the signal beam. Two optical isolators placed after the oscillator and the first stage amplifier are used to prevent back-reflections.
In the first amplifier stage, high brightness pumping together with the double pass signal configuration allowed us to achieve remarkably high small signal gain value in a simple setup. The gain curve displaying measured gain in dB as a function of output power in W is shown in Fig. 2. The amplifier operates in the small signal gain regime for the output powers below 3 W, with the small signal gain value of 32.5 dB, i.e. almost 2000. This is to the best of our knowledge the highest value reported for the single crystal fibers. On the other hand, maximum output power achieved when seeding 2.5 W is 42 W, resulting in an extraction efficiency of 28%. It is worth mentioning that this system can operate at very low seed powers without disturbance by amplified spontaneous emission. When the seed beam was blocked, the measured output power for the maximum pump power was less than 1 mW.
The output of the first stage is then seeded into the second stage amplifier. Bidirectional pumping provides the total available pump power of more than 300 W. Figure 3 shows the output power of the second stage as a function of the pump power. The maximum output power is 162 W for 300 W of pump power, corresponding to the extraction efficiency of 42%, calculated as the difference between output and input power divided by pump power. This is to the best of our knowledge the highest output power of femtosecond pulses and the highest extraction efficiency achieved with SCF.
The beam quality factor of the second stage amplifier output beam was measured for the output powers higher than 100 W. It was found that the beam quality remains very good up to 100 W, with Mx2 = 1.3 and My2 = 1.3 measured for 102 W beam. With increasing output power due to larger pump-induced thermal effects beam quality degrades. Mx2 = 1.4, My2 = 1.5 and Mx2 = 1.9, My2 = 1.9, are measured for 124 W and 162 W, respectively. Figure 4 shows the beam profiles in the focal plane with about 200 μm diameter obtained during the M2 measurements. The beam profile remains Gaussian-like but at 162 W small deviations start to appear. A way to improve the beam quality at higher power is to optimize the signal and pump spot sizes inside the amplifier and find a good compromise between the extraction efficiency and the beam quality. Namely, by focusing the pump beam into a spot larger than that of the signal beam one could obtain a better beam profile at the expense of the output power. We are currently investigating that possibility.
The autocorrelation measurement of 162 W beam, shown in Fig. 5, reveals the pulse duration of less than 800 fs assuming sech2 temporal shape. This corresponds to a time-bandwidth product of 0.38, as the optical spectrum is centered at 1030.5 nm with 1.7 nm bandwidth.
For industrial applications, such a femtosecond high average power laser system should operate at the repetition rates in the range of 1 to 10 MHz. Although, no external acousto-optic modulator (AOM) was installed at the time being, we estimated the output power of such a configuration by taking into an account the power scaling at various repetition rates and diffraction losses of an AOM of 60%. Measurements of the output power as a function of the input power for the maximum pump power in both amplification stages are shown in Fig. 6. For 5 MHz, the seed power was calculated to be 100 mW, which results in the first stage output of 20 W and the final output power of 119 W. Thus, after an external modulator the average power should remain well above 100 W at 5 MHz pulse repetition rate corresponding to pulse energy of > 20 μJ.
We have demonstrated the amplification of femtosecond pulses up to 160 W of average power using a compact two-stage SCF amplifier without CPA. High brightness pumping and a double-pass signal configuration of the first stage resulted in the small signal gain value of 32.5 dB, i.e. almost 2000. This is to the best of our knowledge the highest value achieved with SCF so far. In the second stage, we implemented for the first time the bidirectional pumping scheme. The total pump power of 300 W allowed us to achieve the highest average power of femtosecond pulses and the highest extraction efficiency reported so far for a SCF amplifier, i.e. 160 W and 42% respectively. The beam quality remains excellent up to 100 W, with M2 = 1.3 measured in both directions. At the maximum output power, M2 = 1.9 was measured in both directions. We believe that the beam quality at 160 W can be improved at the expense of the extraction efficiency by optimizing the pump and signal spot sizes inside the SCF. Finally, we have estimated that with an external modulator, this laser system can achieve >100 W of average power at 5 MHz pulse repetition rate, with > 20 μJ of pulse energy and about 800 fs pulse duration. Such a compact, robust and low-cost system would be very attractive for various applications such as high speed surface texturing and we are setting up new experiments to verify that.
This work was partially financially supported by the European Union’s Seventh Framework Program for Research within Appolo project under grant agreement number 609355.
References and links
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