Mode-locked Fiber Lasers

Erbium-doped fibers, as otherwise used mainly for erbium-doped fiber amplifiers, exhibit a broad gain bandwidth, with a maximum either at 1535 nm or at some longer wavelength such as 1550 nm – depending on the composition of the fiber core and on the inversion level, which itself is determined by fiber length, doping concentration and resonator losses.

A particularly simple type of erbium-doped fiber laser [4] is shown in Figure 1. The linear laser resonator is terminated by a semiconductor saturable absorber mirror (SESAM) as a passive mode locker on the left-hand side and a bare fiber end (with roughly 4% Fresnel reflection) on the right-hand side. The erbium-doped fiber is pumped with a low-power laser diode, the output of which is introduced into the resonator with a dichroic fiber coupler. The fiber length determines the pulse repetition rate, and the pulse duration of e.g. some hundred femtoseconds is determined by the interplay of dispersion, nonlinearity and the gain.

Shorter pulses can be obtained from more sophisticated resonators. An example is the “figure-of-eight” fiber laser setup [2] as shown in Figure 2. The ring on the right-hand side is a nonlinear amplifying loop mirror. Light coming from the main resonator (the left ring) is split into two counterpropagating components in the loop mirror. As the light propagating in clock-wise direction first passes a long nonlinear fiber with low power and is then amplified in the erbium-doped fiber, whereas light propagating in the other direction is amplified first, the nonlinear phase shift is larger for the latter component. If the difference in nonlinear phase shifts is π (the ideal case), the two components will interfere in such a way at the coupler in the middle that all the light is sent towards the bottom of the main resonator; light sent in the other direction will be eliminated by the Faraday isolator. The round-trip gain is thus very small for low powers, but much larger in a certain range of powers (ideally arranged to be near the peak power of the pulses). Effectively this setup acts like the combination of some laser gain with a saturable absorber, favoring the peak of a circulating pulse against the low-power background light. This artificial saturable absorber generates a single pulse circulating in the resonator and thus a pulse train emitted at the lower left port.

In the simpler cases (e.g. in the figure-of-eight laser described above), the intracavity chromatic dispersion is adjusted for soliton mode locking, i.e., the circulating pulse is a quasi-soliton pulse, with soliton parameters according to the average dispersion and nonlinear phase shift. Although soliton operation can provide clean pulse shapes and is comparatively easy to describe with some basic quantitative guidelines, it has severe limitations concerning pulse energy and peak power, essentially due to nonlinear effects which become too strong at short pulse durations (see below). Typically, pulse durations do not get below several hundred femtoseconds, pulse energies are in the picojoule region, and the average powers are only a few milliwatts. Higher pulse energies (well above 1 nJ) in picosecond pulses have been obtained by incorporating a fiber Bragg grating for increasing the anomalous dispersion [10], but this is actually no more true soliton propagation in the fiber. Other mode-locking techniques (see below) allow for higher pulse energies and shorter pulse durations, but at the cost of significantly more complicated physics of pulse shaping, and often a reduced pulse quality. With careful optimization, erbium-doped femtosecond fiber lasers can reach pulse durations below 100 fs, sometimes even below 50 fs [29]. Typical output powers are of the order of some tens of milliwatts. Higher pulse energies are possible with different kinds of dispersion management, as discussed below.

In addition to nonlinear amplifying loop mirrors, there are other kinds of artificial saturable absorbers. A popular choice is to exploit nonlinear polarization rotation (polarization additive pulse mode locking), where a power-dependent polarization change is converted into a power-dependent transmission through a polarizing optical element. Figure 3 shows a ring laser setup based on this principle, which can be applied also to linear fiber laser resonators. The technique is similarly versatile as Kerr lens mode locking of bulk lasers, but has the disadvantage that it normally suffers from polarization changes which can be introduced by, e.g., temperature variations. That problem can be avoided by using a modified approach based on polarization-maintaining fibers in combination with Faraday rotators [6]. That approach leads to an environmentally stable laser in the sense the temperature-induced polarization changes are eliminated. However, the technique is less easily implemented in the form of an all-fiber setup, and is thus also not ideal for stable devices.

There are also actively mode-locked fiber lasers, using an intracavity modulator, e.g. of Mach–Zehnder type. This technique is important particularly in the area of optical fiber communications, where synchronization of different sources is required.

Mode-locked fiber lasers can also be based on ytterbium-doped fiber for emission somewhere in the range of roughly 1000–1100 nm. An important difference for this technology, as compared with erbium-doped lasers, is that the dispersion of silica fibers is normally well in the normal dispersion regime. The generation of femtosecond pulses often requires additional anomalous dispersion, which can be generated with, e.g., a fiber Bragg grating or a bulk grating pair. Anomalous dispersion in the 1-μm wavelength region can also occur in photonic crystal fibers. Such fibers can be used as passive fibers for intracavity dispersion compensation or as active fibers. They allow one to apply basically all the techniques originally developed for erbium fiber lasers.

Ytterbium-based femtosecond fiber lasers achieve similar pulse durations as their erbium-doped cousins. The output powers achievable are also similar, or somewhat higher. Such lasers may thus be seen as competitors for some of the mode-locked bulk lasers in the regime of low pulse energies.

In various applications areas, such as telecommunications, pulse trains with multi-gigahertz pulse repetition rates are required. Fiber laser resonators are usually too long to achieve such repetition rates with fundamental mode locking, i.e. with a single pulse circulating in the resonator. Therefore, harmonic mode locking is often required, where multiple (sometimes hundreds or even thousands) of pulses circulate with a well-defined spacing. Various techniques have been developed to guarantee the equal spacing and suppress supermode noise, but these tend to increase the complexity of the setup and often do not allow very robust operation of the laser.

Compared with a bulk laser, a fiber laser has much stronger dispersive and nonlinear effects (particularly via the Kerr effect) in its resonator. Strong dispersive effects arise from the long path lengths in the glass (in most cases well above 1 m per resonator round trip, compared with a few millimeters in a bulk laser), and nonlinear effects are in addition strengthened by the small mode areas of the single-mode fibers used, which leads to high optical intensities. Particularly the strong nonlinear effects introduce strong performance limitations, as discussed in the following sections.

At the least the fiber dispersion in the laser resonator can be compensated (→ dispersion compensation) e.g. by combining fibers with dispersion of different signs. However, it is not always easy to compensate both second-order and higher-order dispersion, as is necessary for the generation of very short pulses.

In some cases, fairly large amounts of dispersion are required in laser resonators, which cannot be generated with dispersive fibers. Chirped fiber Bragg gratings in combination with optical circulators can then be a solution. Bulk grating pairs are also sometimes used, but the required free-space optics introduce alignment issues, dust sensitivity, etc., and thus destroy some of the inherent advantages of fiber lasers.

In the case of a soliton mode-locked fiber laser, the quasi-soliton pulses become unstable if the nonlinear effects per resonator round trip become too strong. Below that threshold, characteristic effects such as Kelly sidebands can be seen. (Mode-locked bulk lasers nearly never exhibit this effect.) This is particularly the case for short pulse durations, because these lead to a short soliton period or, in other words, to a strong nonlinear phase shift per unit length. On the other hand, longer fundamental soliton pulses have lower pulse energies. One therefore often has to limit the pulse energy to values far below what would be possible in terms of average pump and output power.

To some extent, the nonlinearity can be decreased by using large mode area fibers. However, the nonlinearity still remains very strong compared with that in bulk lasers, so that the pulse energies achievable are far lower and often do not reach the limit set by the gain bandwidth of the active fiber.

A technique for raising the possible pulse energy and/or for lowering the pulse duration of a mode-locked fiber laser is used in stretched-pulse fiber lasers [5, 8, 9, 13, 19, 22], also sometimes called dispersion-managed fiber lasers. The basic idea is to use dispersion management with fiber spans of alternating dispersion, so that the pulses are periodically stretched and recompressed in every resonator round trip. As the average pulse duration in the resonator can be strongly increased, the pulse energy can be accordingly increased without obtaining excessive nonlinear phase shifts. Nanojoule energy femtosecond pulses can be generated in this way. For generating bandwidth-limited pulses, the output coupling needs to be arranged at an appropriate location in the fiber laser resonator, or external dispersive pulse compression must be applied. Figure 4 shows a ring laser setup based on the stretched-pulse principle, but linear resonators and so-called sigma (σ) resonators [13] are also used.

For picosecond fiber lasers, the stretched-pulse technique can also be used. However, it is then hardly practical for obtaining transform-limited pulses, since the large stretching and compression would then require very large amounts of dispersion.

Another approach is the application of parabolic pulse amplification within a fiber laser resonator with normal dispersion. After amplification and output coupling, the pulse bandwidth is then reset with an optical filter. As the pulses undergo a self-similar evolution in the amplifier fiber, such lasers are called similariton fiber lasers. They often emit strongly chirped pulses, which can be dispersively compressed outside the laser resonator, resulting in pulse durations well below 100 fs.

Such lasers can reach good performance parameters. However, it is difficult to realize them in all-fiber technology.

In addition to similariton lasers, there is a wider class of all-normal-dispersion femtosecond fiber lasers, where anomalous dispersion is also avoided, but the pulse evolution is generally not self-similar [31]. In typical cases, the action of a spectral filter plays an essential role in pulse formation. The pulses generated are strongly chirped, but can often be well compressed outside the laser resonator.

A special form is the Mamyshev oscillator [47]. Such a device contains two different bandpass filters with non-overlapping pass bands. Without the strong spectral broadening before those filters, a circulating pulse with any center wavelength would necessarily have a very large round-trip loss. Thus, the combination of those filters effectively leads to a kind of artificial saturable absorber, which forces the laser to operate in a pulse generation regime.

The limits of such techniques have so far not been fully explored, but pulse energies of tens of nanojoules are certainly possible [26], even using fibers with moderate effective mode areas. With large mode area fibers, pulse energies above 100 nJ [39] or even of the order of 1 μJ [43] can be obtained. With a Mamyshev oscillator, approximately 1 MW peak power has been reached [47]. However, all-fiber realizations are difficult to achieve.

Another possibility for obtaining higher pulse energies is to use a relatively low-power fiber seed laser and amplify its output in a fiber amplifier (→ master oscillator power amplifier, master oscillator fiber amplifier). The management of nonlinearity is easier in an amplifier, as nonlinear phase shifts are less critical, and techniques are available to deal with them. For example, chirped-pulse amplification is suitable for very high pulse energies.

Note that many commercial ultrafast fiber lasers actually contain a mode-locked laser and a fiber amplifier.

Ultrafast fiber lasers often do not exhibit self-starting mode locking. The main reasons are parasitic reflections within the laser resonator, which are difficult to suppress, the use of fast (artificial) saturable absorbers for mode locking, and the often long laser resonator, leading to a low ratio of pulse duration to round-trip time.

As mentioned above, semiconductor saturable absorber mirrors (SESAMs) can be used for passive mode locking of fiber lasers. However, the parameters of SESAMs for fiber lasers are often different from those required for bulk lasers. In particular, the modulation depth has to be substantially higher in most cases, essentially since various nonlinear and dispersive effects are stronger in a fiber laser, and sometimes also because of parasitic reflections. The higher modulation depth is usually obtained by employing multiple quantum well absorbers or multiple thicker absorber layers. Unfortunately, this often leads to excessive mechanical strain within the device, as the absorber layers are often not exactly lattice-matched to the Bragg mirror structure. This can lead to problems with absorber damage or limited device lifetime.

Although fibers have a number of beneficial properties for short pulse generation, the dispersion and particularly the high nonlinearity of fibers severely limits the performance of mode-locked fiber lasers particularly in terms of pulse energy, pulse duration, and often also pulse quality. Various methods are used to deal with these issues. Often, these involve relatively complicated physics (e.g. arising from highly nonlinear pulse propagation), which can make it substantially more difficult than for bulk lasers to find well working or even optimized laser designs.

For lower pulse energies, mode-locked fiber laser products can be attractive due to their potentially low fabrication cost. Their setups can be fairly compact and robust, provided that an all-fiber setup containing only polarization-maintaining fibers can be used. Maximum performance is often achievable only by incorporating bulk-optical components, but the specific advantages of fiber optics are then often lost. In particular, it is problematic to require coupling of free-space laser beams into single-mode fibers, where micrometer-precise alignment is required and a single dust particle may seriously affect the performance.

Fiber lasers are in many respects very different from their bulk laser counterparts. These are not only quantitative differences; partly there are entirely new mechanisms forming and influencing the generated pulses, and qualitatively different operation regimes which are not required for bulk lasers and more difficult to analyze and optimize. A good understanding of such mechanisms, complemented with numerical modeling, is required in order to realize mode-locked fiber lasers with high performance and good stability as required for an industrial product. A first step towards the realization of a mode-locked fiber laser meeting certain specifications is to identify the most appropriate principle of operation for given output specifications.

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