CLEO 2000: Coiled fiber amplifiers produce high power single-mode pulses
By: Yvonne Carts-Powell
San Francisco, CA—The high intensity in fiber cores allows fiber lasers to produce nonlinear effects at relatively low powers, which is a boon for those who want to exploit these effects, but the bane of users who merely want high-power pulses from the relatively compact fiber lasers. Researchers at the Naval Research Laboratory (Washington, DC) and Sandia National Laboratories (Livermore, CA) have developed a simple method that allows more powerful single-transverse-mode pulses from multimode fiber amplifiers. The work is attractive because it uses a relatively common fiber design and it is both inexpensive and straightforward to implement: bend the amplifier.1
How it works
The work is based on an interesting property of fiber—all the modes propagating in fiber see bending loss, but the fundamental mode is the least sensitive to it. Jeffrey Koplow, and Lew Goldberg at NRL and Dahv Kliner at Sandia calculated 1.58 cm as an optical radius of curvature at which the lowest order mode (LP01) has minimal bending loss but all the higher order modes are high loss. Wrapping the gain fiber around a cylinder of this radius thus provides a form of distributed mode filtering.
One side-benefit of this method is that it can be used alone or combined with another method for increasing the output power of fiber amplifiers: using fibers with larger-than-normal cores and lower-than-normal numerical apertures (NAs). Just like with single-mode fibers, reducing the NA and increasing the dopant concentration in the multimode amplifier increases the stored energy and ability of the fiber to handle high peak power pulses.
"Exploiting bend loss to discriminate against higher-order modes allows the core diameter to be increased significantly beyond the single-mode limit," say the researchers, "allowing fiber lasers and amplifiers to be scaled up to much higher peak and average powers."
Kliner explains that by also using double-clad fiber, the pump absorption coefficient increases quadratically with core diameter. This allows users to either decrease the length of fiber or use larger higher-power pump sources without requiring much longer fibers.
Experimental system
The researchers constructed a coiled amplifier using double-clad fiber with a core diameter of 25 µm, a core NA of about 0.1, and a ytterbium concentration of about 1.5% by weight. The 200-µm-dia. hexagonal inner cladding layer (through which the pump light propagates) had a numerical aperture of 0.47, and the absorption coefficient at 975 nm was about 7.5 dB/m. The device was pumped by imaging the output of a fiber-coupled 974-nm diode array (with a 250-mm diameter and NA of 0.22) onto the polished end face of the fiber. The launched pump power was measured to be about 7.3 W.
According to the group, with the amplifier operated as an amplified spontaneous emission source, the M2 of the output beam reached 1.09 ± 0.09, which is fairly close to diffraction-limited operation (an M2 value of 1). When the coiled amplifier was seeded with 1064-nm light from a single longitudinal mode neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, the slope efficiency was similar to that of an uncoiled amplifier.
The technique allows researchers to scale pulsed fiber lasers and amplifiers to significantly higher pulse energies and peak powers, without trading higher power for lower beam quality. It could also be applied to raising the average power of CW fiber sources.
Reference
1. Jeffrey Koplow, Dahv Kliner, and Lew Goldberg, "Use of bend loss to obtain single-transverse-mode operation of a multimode fiber," Conference on Lasers and Electro-Optics (CLEO 2000), paper # CWJ6, San Francisco, CA.
About the author…
Yvonne Carts-Powell is a freelance science and technology writer based in Belmont, MA.