Fiber Laser System Produces Femtosecond Pulses at 25 W of Average Power
There are several approaches to generating femtosecond pulses, but few of them are capable of producing the tens of watts of average power required for micro- and nanometer-scale materials processing. Recently, scientists at the University of Southampton in the UK designed and demonstrated an Yb-doped fiber oscillator-amplifier system capable of generating ultrashort, 410-nJ pulses at a 62-MHz repetition rate -- that is, an average power in excess of 25 W.
Twenty-five watts is not the highest average power reported from a femtosecond fiber laser system. For example, researchers at Friedrich Schiller Universität in Jena, Germany, reported up to 75 W of average power by chirping and stretching pulses to nanosecond durations before amplifying and subsequently recompressing them. But that course is more complex and less compact than the technique developed at Southampton, and it produces longer pulses, typically on the order of 400 fs, compared with the ~100-fs pulses obtained after recompression at Southampton.
Nonlinear effects, such as self-phase modulation and Raman scattering, are the ultimate limitation to the power obtainable with a fiber laser. These nonlinearities are inevitable in a high-power fiber laser because of the tight confinement of the light over long distances in the fiber, but they need not always degrade the quality of the laser's output pulses. In particular, under certain conditions, pulses with parabolic intensity profiles can experience significant self-phase modulation and pulse stretching in an amplifier, but they maintain a linear chirp and therefore can be recompressed to ~100-fs duration after they emerge from the amplifier. The Southampton scientists utilized this mechanism in the design of their laser.
Figure 1. The master oscillator power amplifier system generates high-quality, high-average-power femtosecond pulses, despite strong self-phase modulation in the second amplifier.
The laser comprised an Yb-doped fiber oscillator and two Yb-doped fiber amplifiers (Figure 1). The oscillator was mode-locked at 62 MHz and produced an output pulse train of 1.8-ps pulses containing ~30 pJ at a center wavelength of 1055 nm.
The first amplifier was forward-pumped with up to 3 W of 975-nm light from a diode laser, and the second amplifier was backward-pumped with 60 W of 972-nm diode laser light. Although the 40-µm core of the second amplifier was capable of supporting about seven transverse modes, the increased loss to high-order modes induced by loosely coiling the fiber -- as well as the center-heavy doping of the fiber -- resulted in a nearly single-transverse-mode output.
The ~400-nJ pulses emerging from the second amplifier had a pulse duration of ~6 ps. To avoid optical damage, the scientists passed only a fraction of this energy through a conventional grating pair, which compressed the pulses to ~100 fs. Had appropriate gratings with greater power-handling capability been available, the scientists believe that they could have compressed approximately 70 percent of the pulse energy.
Figure 2. The increase in pulse bandwidth and the decrease in optimal grating spacing indicate a strong nonlinear interaction within the fiber amplifier. Nonetheless, the laser produced high-quality femtosecond pulses.
The bandwidth of the output pulses increased from 16 to 26 nm as the output pulse energy increased from the lowest values to more than 400 nJ (Figure 2). The 16-nm bandwidth was less than the input bandwidth from the mode-locked oscillator. The researchers attribute this reduction to gain-narrowing at the front end of the second amplifier, where the population inversion is minimal because most of the pump light has been absorbed before it gets there. They also believe that the upper limit on pulse bandwidth is 26 nm, because that is the bandwidth of the amplifier.
The required separation between the gratings decreased with pulse energy (Figure 2), indicating a greater chirp at higher energies. The fact that both chirp and bandwidth increased with energy strongly indicates greater self-phase modulation with rising output energy. Despite the strength of this nonlinear interaction, the scientists observed high-quality compressed pulses of ~100-fs duration.
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