Single-crystal Fibers

Pedestal Growth Methods

Edge-defined Film-fed Growth

Micro-Pulling-Down Method

Most single-crystal fibers are made from some oxides crystal materials, being chemically and mechanically quite stable. The most common materials are:

• Sapphire (Al₂O₃) is a particularly hard material with high thermal conductivity, exhibiting a wide transparency region ranging up to 3.2 μm (i.e., far more into the infrared than silica). It can be doped with Ti³⁺ ions to obtain very broadband laser crystals (→ titanium–sapphire lasers).
• YAG (Y₃Al₅O₁₂) is also quite hard and can well be doped with many rare-earth gain materials. Due to its cubic crystal structure, its crystal orientation does not matter for most purposes.
• There are various other oxides, e.g. other garnet crystal materials such as GGG (Gd₃Ga₅O₁₂), having somewhat similar properties as YAG, also spinel (MgAl₂O₄).

Some other materials can also be used, but their application is at a more experimental stage:

• Some fluoride crystals (e.g. LiF) have a very wide spectral transmission range.
• Nonlinear crystal materials like lithium niobate (LiNbO₃) can also be used, e.g. for optical parametric amplifiers.
• Semiconductors like silicon (Si), germanium (Ge) or silicon–germanium (SiGe) exhibit transmission in the infrared with strong optical χ⁽³⁾ nonlinearities.

As mentioned above, many single-crystal fibers can be used as laser-active (amplifying) devices when they are doped e.g. with rare-earth ions like Yb³⁺, Nd³⁺, Er³⁺ or Tm³⁺, or with transition metal ions like Ti³⁺.

Single-crystal fibers are similar to common optical glass fibers in terms of their geometrical shape (long, thin and cylindrical) and to some extent concerning their fabrication (pulling from a thicker source block, a preform).

On the other hand, in contrast to real fibers they usually do not contain a fiber core; multimode waveguiding can be achieved only based on the strong refractive index contrast at the crystal–air interface. A cladding layer with a slightly lower refractive index around a core glass would help to reduce the propagation losses and possibly to allow for single-mode guidance, but is difficult to fabricate [4]; some dip coating techniques and other techniques are applied only in rare cases [13, 18]. For high-power operation, there is also a contribution of thermal lensing, which can substantially affect the mode structure. Furthermore, a substantial approximately parabolic index profile can occur e.g. due to inhomogeneity of the doping profile and possibly thermal stress during fabrication [10].

Compared with optical glass fibers, single-crystal fibers allow only very limited bending.

They are operated with a far larger effective mode area, so that the optical intensities for given optical power level are much lower, and material nonlinearities have a much lower effect. Also, the optical damage threshold is correspondingly higher.

For these reasons, single-crystal fibers are suitable for far higher power levels, particularly in the domain of ultrashort pulse amplification. On the other hand, it is tentatively more difficult to obtain a high output beam quality due to the inherent multimode nature.

The other essential difference is the use of monocrystalline rather than amorphous material. Consequences of that are that one can often achieve much higher doping concentrations of laser-active ions, and the absorption and emission spectra of rare-earth ions normally exhibit a substantially smaller bandwidth in crystalline materials. The small bandwidth also implies higher transition cross sections, resulting in stronger pump absorption, higher gain and lower gain saturation fluences and intensities.

Propagation losses in single-crystal fibers are usually substantially higher than in the corresponding bulk materials (or in glass fibers), mostly due to scattering losses at the surface, which exhibits surface defects like growth stripes and has a very strong refractive index contrast (far higher than for the core–cladding interface of a regular fiber). However, they can be well below 1 dB/m or even below 0.1 dB/m, so that they do not seriously reduce the power efficiency of the devices.

The used materials often have a good infrared transmission range, so that sometimes they can be an alternative to mid-infrared fibers based on glasses or polycrystalline material – for example, for transmitting the outputs of Er:YAG lasers at 2.9 μm. Compared to mid-IR fibers, they may allow to transmit much higher optical powers with lower optical power losses. However, they usually cannot be used at similarly long wavelengths as some infrared fibers.

Compared with usual laser crystals, single-crystal fibers offer a far higher propagation length, so that correspondingly higher optical gain can be achieved – e.g. 20 dB or even more. Despite the long length and relatively small beam diameter, no extraordinarily high pump beam quality is required, since the waveguiding effect from the crystal–air interface can be exploited (although at the cost of increased scattering losses).

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