Fiber-lasers for ultrafast optics

The current status of a fiber-based ultrafast technology is reviewed. Pulse generation techniques capable of producing femtosecond pulses are discussed. Here we describe passive and active–passive mode locking techniques as well as linear and nonlinear fiber amplifiers. Thirtyfemtosecond pulses may be generated directly from fiber oscillators or by implementing pulse compression techniques. The use of cladding-pumping and all-fiber chirped-pulse amplification allows the generation of W-level average powers from conceptually simple fiber laser systems. Nonlinear frequency conversion in highly nonlinear crystals allows a significant extension of the accessible wavelength range. Electronically phase-locked fiber lasers with unprecedented timing accuracy may be constructed by exploitation of the low-noise properties of fiber lasers. In turn, the high timing accuracy possible with fiber lasers enables the demonstration of electronic scanning delay lines for all-electronic pump-probe experiments. PACS: 42.60.Da; 42.60.Fc; 42.80 Optical fibers have been viewed as a very attractive medium for the generation and manipulation of ultrafast pulses for a long time. Notably, the realization of fiber based pulse compression [1], and the prediction [2] and demonstration [3] of soliton propagation in optical fibers helped to firmly establish optical fibers in the realm of ultrafast technology [4–6]. However, initially the development of a comprehensive fiberbased ultrafast technology was not possible due to the lack of a fiber-based gain medium. Only with the fabrication of rare-earth-doped fibers [7, 8] have wide-bandwidth fiber gain media become readily available in a large part of the optical spectrum, stretching nearly continuously from 380 nm [9] to 3.9 μm [10]. As part of these efforts, erbium-doped fiber amplifiers (EDFAs) have been developed [11, 12] which are now firmly established in optical telecommunication systems [13] and are one of the most widely used gain media in current laser technology. The high quality and low pump-power requirements of these wide-bandwidth gain media also sparked efforts towards the construction of short-pulse fiber lasers [14], which finally resulted in the demonstration of femtosecond passively mode-locked fiber oscillators by a variety of Kerr-type [15– 18] or semiconductor saturable absorbers [19, 20]. Despite the fact that with rare-earth-doped fibers a simple solid-state femtosecond laser finally became a reality, ultrafast technology continued to advance mainly around ultrafast bulk solidstate lasers, such as the Ti:sapphire laser, which became available at nearly the same time [21]. Initially, Ti:sapphire lasers turned out to be much more versatile in the field of ultrafast optics compared with fiber lasers due to their compatibility with a variety of bulk-optics pulse manipulation techniques and the much higher power levels obtainable. For example, the high extractable average power levels (> 1 W) and pulse energies (> 10 nJ) from Ti:sapphire lasers were employed towards the construction of widely wavelength-tunable ultrafast laser systems by way of direct pumping of optical parametric oscillators [22]. Equally, chirped-pulse amplification (CPA) techniques [23] boosted the power levels of highrepetition-rate compact Ti:sapphire amplifiers into the region of 0.1 TW [24, 25], sufficient to generate X-rays. However, as industrial applications of ultrafast optics proliferate, the wide distribution of these more conventionally constructed systems is affected by their limited potential for integration. Only highly integrated ultrafast lasers can potentially address the needs of industrial systems due to their reduced footprint, their simplified assembly, and their high reliability arising from a compact setup. For any such technology to be widely accepted, it is also imperative that a high degree of flexibility be preserved, i.e. a large range of applications should be possible, just as with bulk-optics lasers. Clearly, fiber lasers are ideally suited in this regard, and due to the great advancements achieved during the last few years, a highly integrated, fiber-based ultrafast technology has indeed become a reality. In this paper we will limit our review to recent work on ultrafast fiber lasers and some of their applications. An early review of ultrafast fiber lasers can be found in [14] and [26]. In Sect. 1 we discuss some of the fundamental physical limitations of optical fibers. In Sect. 2 we describe recent advances in low-peak-power fiber-based pulse sources. Sect. 3