Holey fiber overcomes limitations of conventional fiber for applications in lasers and amplifiers
Kim P. Hansen and Jes Broeng, Crystal Fibre A/S
The use of optical fiber has expanded dramatically in recent years. Arising from a technology strongly focused on telecommunications, fiber now may be found in such diverse fields as biotechnology, astronomy and medicine.
One of the fastest growing and most promising applications of optical fiber is in high-power fiber lasers. In the past few years, fiber lasers have evolved from low-power systems for niche applications into challengers to the traditional high-power industrial lasers for materials processing.1 Fiber lasers are gaining market share at a strong pace and are projected to be the industry standard within a few years for marking, printing, cutting and welding.
Their advantages are low operating costs, high beam quality and high efficiency in a maintenance-free format with a small footprint and low weight. In particular, they offer a superior beam quality compared with other laser systems for power levels in the kilowatt range. This improves precision and processing speeds for industrial materials processing systems and introduces the possibility for achieving extreme power levels by combining multiple lasers.
The highest reported power level for a single-mode fiber laser is approximately 3 kW, demonstrated by IPG Photonics Corp. of Oxford, Mass. It is worth noting that the record power level has increased dramatically over a very short time — and further increases no doubt lie ahead.
Fiber laser concepts
Photonic crystal fiber is perhaps the most promising technology to significantly lift the current power levels. This is because of its increased flexibility in single-mode core sizes, the increased numerical aperture of pump cores in double-clad fiber configurations and the high thermal stability of low-loss all-glass structures.
Figure 1. A double-clad fiber laser converts low-beam-quality pump light into laser light with high beam quality.
A fiber laser is based on a rare-earth-doped fiber that serves as a gain medium. An optical pump source — typically, for high-power applications, in the form of multimode laser diodes — provides energy for the gain medium. The active fiber converts the low-beam-quality pump light into signal light with high beam quality (Figure 1).
Typically, double-clad fiber designs are employed, where light from the laser diodes is coupled into a multimode cladding (also referred to as the pump core) surrounding the active single-mode signal core. Traditional active fiber, such as that used for amplifiers in telecommunications systems, basically is a standard transmission fiber in which the core region has been doped with rare-earth ions. Because the pump light must be coupled into the small signal core, this type of fiber preferably is pumped with single-mode pump lasers. The power limitations of single-mode pump sources make the fiber unsuitable for high-power applications. The primary advantage of the double-clad design is the result of the large pump area and high numerical aperture, enabling pumping with relatively low-cost multimode diode stacks.2
The pump light can be coupled by free-space optics such as lenses and mirrors or can be delivered in one or more multimode fibers that are fused onto the active fiber. The preferred gain medium for high-power lasers is ytterbium, a result of its high efficiency: Approximately 80 percent of the pump light can be converted to the signal light. The laser cavity is formed using free-space mirrors, fiber Bragg gratings, the fiber’s facets or a combination of these (Figure 2a).
Figure 2. Fiber lasers can be simple oscillators in which Bragg reflectors (shown here), fiber facets or external mirrors define the resonator (a). Alternatively, they may be configured as master oscillator power amplifiers in which a signal from a seed laser is amplified in a power amplifier (b).
Besides configurations that feature a single gain medium, fiber lasers also may be built in the so-called master oscillator power amplifier configuration (Figure 2b). Typically, the master oscillator is a single-mode laser, such as a fiber or other type of laser, running at relatively low power. It serves as a seed laser for the power amplifier, which boosts the signal. Master oscillator power amplifier configurations often are preferred for lasers with a narrow linewidth because it is easier to control the linewidth via the low-power master oscillator than for a single, high-power laser cavity.
When scaling fiber lasers to higher powers, it is important to optimize the fiber for the efficient coupling of the pump light, reduction of nonlinear effects, high conversion of pump light and good thermal properties. In short, desirable features of the fiber are a high numerical aperture for the pump light, a large core size, a short fiber length, a low thermal load and high temperature resistance, and a low residual pump power.
The high numerical aperture enables a reduction in the size of the pump core and thereby an increase in pump absorption. High pump absorption is important to ensure short fiber lengths, which are desirable to minimize the length-dependent nonlinear effects as well as to reduce residual pumping and to increase the efficiency of the system. Moving to larger signal cores further increases the pump absorption and lowers the intensity in the core, suppressing nonlinear effects.
Standard fibers are limited in both single-mode core size and pump-core numerical aperture, however, which ultimately limits their use in high-power applications. Photonic crystal fiber technology provides a means to bypass these limitations and to realize fibers optimized for high-power operation.
Photonic crystal fibers
The term photonic crystal fiber is inspired by the unique cladding structure of this fiber class. As with standard fibers, most types of photonic crystal fiber guide light by total internal reflection between a core with a high refractive index and a cladding with a lower index, but the index differences in photonic crystal fiber are obtained by forming a matrix of various materials with high and low refractive indices — typically, airholes in silica. This results in the creation of a hybrid material with properties unobtainable in solid materials, such as a very low effective index or novel dispersion.
The hybrid material regions typically are the result of the introduction of a number of airholes in a silica background. The air-silica matrix can be used as a low-index cladding if the holes are sufficiently small. The field, then, cannot resolve the structure, and the result is an effective refractive index that is determined by the fraction of air in the silica.
This air-silica matrix structure has given rise to several other names, including microstructured and holey fiber, but despite the differences in terminology, they all refer to the same class of fiber. A special subclass of photonic crystal fiber is photonic bandgap fiber, which is not the subject of this discussion. Light in photonic bandgap fiber is guided not by total internal reflection but by a photonic bandgap analogous to the electronic bandgap in semiconductors.
Figure 3. In a typical triangular-cladding, single-core (i.e., not double-clad) photonic crystal fiber, light is guided in a solid core embedded in a triangular lattice of airholes. As with standard fiber, photonic crystal fiber is coated with a high-index polymer for protection and to strip off cladding modes.
The classical triangular-cladding, single-core photonic crystal fiber forms the basis of many fiber types (Figure 3). Because of the wavelength-dependent refractive index of the cladding region, such fibers can be designed to be “endlessly single mode”; i.e., to support only the fundamental mode, regardless of wavelength or core size. However, for active fibers used in lasers and amplifiers, one often does not need the bandwidth of endlessly single-mode operation. Fiber for these applications thus typically is designed to be single-mode at the operation wavelength and to have a higher-order-mode cutoff at shorter wavelengths.
The microstructuring of photonic crystal fiber provides a means of controlling the refractive index profile more precisely than any other fiber technology. In particular, microstructures allow ultralow numerical apertures for fabricating very large, strictly single-mode cores — and very high numerical apertures for making high-brightness pump cores.
A typical photonic crystal fiber for high-power applications comprises an active core, an air-silica microstructured inner cladding, a ring of large airholes (referred to as air cladding) to confine light to the pump core and a solid silica overcladding. The benefits of the active air-clad design are:
• Large, low-numerical-aperture, single-mode core.
• High numerical aperture for the pump core.
• All-glass design.
• Superior core design flexibility.
Large single-mode core
A major advantage of photonic crystal fiber technology is the high precision afforded in the manipulation of the core and cladding indices. The cladding index can be controlled to within approximately 10–5 by the dimensions and arrangement of the holes in the cladding, enabling ultraflat index profiles of arbitrary index level. Such precision in the index profile is one or two orders of magnitude higher than for conventional fiber, and it enables the fabrication of strictly single-mode fiber with mode areas more than 10 times larger than can be obtained in standard fiber.
Conventional single-mode fiber has a maximum mode field diameter of approximately 15 μm. Recent experimental results have demonstrated single-mode photonic crystal fiber with a mode field diameter of 60 μm, and further progress is expected. However, at such large core dimensions, the core numerical aperture is so low that the fibers cannot be bent without inducing macro-bending loss, and the fibers are developed as a short glass rod, allowing for compact packaging — the so-called rod-type fiber.
For photonic crystal fiber that can be bent without macro-bending loss, the mode field diameter can be as large as 40 μm, with robust, single-mode operation ensured. This may be obtained through strictly single-mode designs with bending radii of approximately 40 cm. Alternatively, slightly multimode designs may be employed in which the fiber is coiled to smaller diameters, whereby the higher-order modes are stripped off.
Compared with conventional large-mode-area fiber (in which tight coiling techniques are a necessity, as the fiber otherwise would be multimode), photonic crystal fiber has the advantage of more-uniform index profiles, both longitudinally and in cross section. This provides lower bending losses at a given bending diameter and simpler systems engineering via a higher reproducibility of coiling diameter from fiber to fiber. However, tight coiling comes at the price of a reduced efficiency resulting from bending losses for the fundamental mode, a risk of mechanical weakness and a sensitivity to environmental changes such as temperature. Hence, strictly single-mode fiber is considered the most robust and stable configuration for high-power lasers.
High numerical aperture
Because of the large index contrast, the air-clad design enables higher numerical apertures for the coupling of pump light into the pump core than can be obtained with conventional optical fiber. A high numerical aperture is desired to launch pump light into the smallest possible pump core, as this provides the highest overlap between the pump light and active ions in the signal core —and therefore the highest pump absorption values and shortest fiber lengths.
Figure 4. In conventional double-clad fiber, the outer cladding is a polymer or low-index glass, resulting in a relatively low numerical aperture for the pump core (a). The refractive index difference between the outer cladding and the pump core can be much greater in photonic crystal fiber, resulting in a much higher numerical aperture for the pump core (b). The pump core of photonic crystal fiber typically has very low loss for the pump light (~0.005 dB/m).
Consequently, there is a power-scaling advantage of air-clad fiber compared with conventional fiber. Conventional fiber that employs polymer-based outer claddings (Figure 4a) can achieve pump-core numerical aperture values of up to approximately 0.46. In contrast, air-clad fiber (Figure 4b) is routinely made with a pump-core numerical aperture of 0.70, and values of around 0.90 are feasible. By itself, this advantage translates into a power-scaling factor of between two and four.
Air-clad fiber has a further advantage in terms of utilizing the pump light to its full extent. As the result of chaotic scattering of the pump light from the airholes in the photonic crystal fiber, skew pump rays (spiral or helical modes that may not intersect the core, causing reduced efficiency) are eliminated. Therefore, residual pump power and “hot spots” at fiber ends are reduced, providing simpler thermal management. This chaotic scattering eliminates the need for the asymmetric, or D-shaped, pump cores often found in conventional fiber.
All-glass solutions
Conventional double-clad fiber uses polymer claddings to confine the pump light. Polymers have intrinsically lower thermal damage thresholds than glass, lending all-glass photonic crystal fiber a natural benefit in high-power systems with a high thermal load.
Figure 5. Collapsing the airholes at the end of a photonic crystal fiber provides an integrated, homogeneous end cap that expands the beam to avoid optical damage to the facet. The end cap also seals the fiber from environmental contaminants.
The all-glass design offers further advantages when one needs beam expansion to reduce the intensity at the end facets and thereby to avoid facet damage. By collapsing the airholes, an integrated homogeneous end cap can be created easily (Figure 5). Because this is done without the addition of other materials, “hot spots” and the risk of contamination at the high-intensity core region are reduced significantly.
Cleaving the sealed fiber end at precisely the correct position controls beam expansion, and cleaving at an oblique angle to avoid backreflection is routine. Finally, the end-facet sealing ensures that the waveguiding parts of the crystal fiber are protected completely from the environment.
Polarization-maintaining output
Photonic crystal fiber can be made to exhibit conventional polarization-maintaining properties or to be polarizing — meaning that the fiber supports only a single polarization state, the other being antiguided. It enables this through full freedom in design of the fiber cross section with submicron precision.
Figure 6. The structural asymmetry in the air-clad photonic crystal fiber introduces a birefringence that results in one polarization being guided and the orthogonal polarization not guided (a). Courtesy of Institut für Angewandte Physik, Friedrich Schiller Universität Jena. The photonic crystal fiber supports only a single polarization over a very broad spectral range (b). The polarization properties of the fiber are stable over more than 170 K and are tolerant of relatively tight bending.
Recently, the first single-mode, active, polarizing photonic crystal fiber with a large mode field diameter of approximately 33 μm was demonstrated (Figure 6a).3 Lasing with optical efficiency of 75 percent and a polarization extinction ratio of more than 15 dB was achieved without the use of any additional polarization-controlling units. The polarization is determined by the geometry of the fiber, and therefore, polarization orientation of the laser output can be controlled by simple rotation of the fiber.
This fiber’s polarization capability covers a wavelength range of approximately 100 nm (Figure 6b). Highly birefringent designs could increase this to 400 nm and beyond. The very large polarization bandwidth increases system flexibility and affords a robustness to external perturbations.
Nonlinear effects are the limiting factor for the achievable power levels in a fiber amplifier or laser. Because these effects are length-dependent, it is critical to minimize the length of the fiber.
Shorter fibers require higher pump light absorption per meter, which can be realized by reducing the ratio of the area of the pump and signal cores. This is achieved through a combination of smaller pump cores with higher numerical apertures and larger signal cores.
Extracting high power from short lengths of fiber requires very good thermal properties. Optical fiber normally features a protective coating that can be damaged at very high temperatures. This problem can be overcome by removing the protective coating and making the fiber so thick that it has the necessary mechanical stability itself. Such a rod-type fiber features an outer diameter millimeters in size, a very large single-mode core and a small pump-core diameter, resulting in extremely high pump absorption per unit length (Figure 7).
Figure 7. Rod-type fiber has an increased outer diameter, a larger single-mode core and a smaller inner diameter than normal photonic crystal fiber. This type of fiber also has no outer protective coating, which can limit its thermal performance. The typical outer diameter of rod-type fiber is 1 to 2 mm.
Rod-type fiber has been used in CW oscillator-only fiber lasers and in pulsed master oscillator power amplifier setups. Recently, Aculight Corp. of Bothell, Wash., demonstrated a pulsed amplifier made from a 90-cm-long rod-type fiber featuring a 70-μm single-mode core and a 200-μm high-numerical-aperture pump core. The outer diameter was 1.7 mm, and the pump absorption was 30 dB/m at 976 nm. It was possible to extract pulses with more than 2 mJ of energy with a peak power of 2 MW while keeping the system single-mode (M2 of 1.1). These values were limited only by available pump power. No roll-off at high pump-power was observed in the data, suggesting that the system can produce even higher powers.4
Commercial holey fiber laser
By combining the active air-clad fiber with specially designed pump combiners, laser and amplifier subassemblies can be created based on photonic crystal fiber that can be readily pumped with standard industry diode lasers. The first such commercial laser, intended for applications such as marking and low-power welding, is the aeroLase-20 from Crystal Fibre A/S of Birkerød, Denmark (Figure 8).
Figure 8. The aeroLase-20, the first commercial all-fiber laser based on photonic crystal fiber, is a simple fiber oscillator with Bragg gratings.
The 10 × 3 × 22-cm instrument comprises a single-mode ytterbium-doped air-clad fiber with a complete Bragg cavity, a standard 7:1 pump combiner and a step-index delivery fiber. The system is designed to be pumped with seven fiber-coupled single-emitter diodes in 100-μm 0.22-NA fibers.
When pumped with seven 915-nm, 5-W diodes, the output from the aeroLase-20 is more than 22 W, delivered in a conventional single-mode fiber. The slope efficiency is 74 percent, and the total optical-to-optical efficiency is more than 64 percent, resulting in a wall-plug efficiency of more than 30 percent. Testing shows a drop in the output power of less than 10 percent over 1000 hours, including pump diode degradation. The output wavelength is 1075 nm, and the linewidth at full power is less than 200 pm.
Meet the authors
Kim P. Hansen is a project manager and fiber designer at Crystal Fibre A/S in Birkerød, Denmark; e-mail: [email protected].
Jes Broeng is a project manager and co-founder of Crystal Fibre.
References
1. A. Tünnermann et al (May 14, 2005). The renaissance and bright future of fibre lasers. J PHYS B-AT MOL & OPT PHYS, S681-S693.
2. E. Snitzer et al (1988). Double clad, offset core Nd fiber laser. Optical Fiber Sensors, 1988 Technical Digest Series, Vol. 2, postdeadline paper PD5.
3. T. Schreiber et al (Sept. 19, 2005). Stress-induced single-polarization single-transverse mode photonic crystal fiber with low nonlinearity. OPT EXPRESS, pp. 7621-7630.
4. F. Di Teodoro and C.D. Brooks (February 2006). Very large-core, single-mode, Yb-doped photonic crystal rod for multi-MW peak power generation. Proc. SPIE, Vol. 6102, Fiber lasers III: technology, systems, and applications, pp. 89-93.