Figure 1. A JAC fiber has a 10-µm core centered within a 28-µm Yb-ion-doped cladding layer, which is in turn surrounded by three layers of "holey" silica and then a silica outer shielding. (UNIVERSITY OF SOUTHAMPTON)
By doping fibers with neodymium (Nd), ytterbium (Yb), and other rare-earth materials, engineers can create extremely long-lasing cavities with very controlled optical properties for a wide variety of applications, including telecom, lighting, and laser marking. Although fiber lasers have not been known for their brightness, due in part to nonlinear effects created by the high length-to-width ratio of the lasing cavity, new fiber designs have boosted the output of these devices to a level that challenges that of traditional solid-state lasers for a fraction of the cost.
"Power scaling hasn't been a big part of fiber-laser research in the past," says Greg Quarles, director of research and development for VLOC Inc. (New Port Richey, FL) and technical program committee member for the Advanced Solid-State Photonics Topical Meeting (San Antonio, TX). "In the past, 50- to 100-W fiber lasers were out there, but there hadn't been a lot of reports about people who were starting to encroach on the half-kilowatt regimeand then boom, this paper comes out. The output power is very good, and there don't seem to be any real major restrictions."
Researchers at the Institute of Applied Physics (Jena, Germany) demonstrated an ultra-high-brightness, continuous-wave (CW) fiber laser capable of delivering 500 W of output power from a single fiber co-doped with neodymium oxide and ytterbirum oxide, an achievement Quarles attributes to engineering and fiber core issues. Fiber-laser output has been limited in the past by inelastic processes such as stimulated Raman scattering in the forward direction and stimulated Brillouin scattering (SBS) in the reverse direction. Because of the high density of the optical pump power in the relatively small-diameter core (on the order of 10 µm) plus the long interaction length of the cavity (from meters to tens of meters), scattering leads to nonlinear interactions that limit CW output power.
To reduce these effects, the Jena group chose a large-mode-aperture (LMA) fiber with a 30-µm core and 0.06 numerical aperture. The LMA core disperses the pump power over an area roughly nine times the size of a normal fiber. When combined with three multiplexed pump lasers (350 W at 976 nm, 175 W at 940 nm, and 175 W at 808 nm), the fiber produced 485 W of near-diffraction-limited output from 700 W of pump power for a conversion slope efficiency of 72%.
In a subsequent experiment, the group achieved 100 W of CW, narrow-linewidth, single-frequency output from a master-oscillator fiber power amplifier at 976 nm. Higher output powers were limited by SBS interactions that increased substantially at 108 W, although researchers estimate that the power could be boosted in excess of 200 Wfar beyond the current record of 135 W from a 60-m Yb-doped fiber if the fiber length were reduced from 9.4 m to 5 m to limit SBS scattering.
In separate work from the University of Southampton's Optoelectronics Research Centre and Southampton Photonics (Southampton, UK), researchers used a jacketed-air-clad (JAC) fiber to generate 3.5-W CW single-mode output at 977 nm with a 0.2-nm linewidth, which is about 2.5 W more than the maximum output of single-mode diode lasers used today to pump Yb-amplifiers for telecom.
The Jena group used a neodymiu,-doped yttrium alumunim garnet nonplanar ring oscillator to create the narrow linewidth signal and then amplified that signal using an array of multimode 250-W diode-pump lasers on the double-clad Yb-doped LMA fiber, while the Southampton group's setup uses only 18 W of diode-pump power on a special JAC fiber. The JAC fiber is essentially a double-clad fiber with a 10-µm core centered in a 28-µm Yb-doped cladding (the pump layer), surrounded by layers of silica holes in the fiber (air cladding). A solid silica shell encapsulates the entire fiber (see figure on page 7). This structure helps to limit re-absorption of the 980-nm light by the Yb ions, which can lead to undesirable emissions around 1030 nm.
Like that of the Jena group, the extremely narrow linewidth of the Southampton fiber laser lends itself to frequency doubling in periodically poled materials for applications involving high-brightness blue lasers.
"What made people really take notice of these findings was that these sources are getting to a level where they can compete with solid-state lasers, which are significantly more expensive. So, you're looking at lower-cost, high-brightness alternatives," Quarles says.