Fiber MOPAs with high control and high power

Abstract

High power fiber sources have reached several kilowatts of output power, and are now leading contenders for many applications. Important attractions include control, efficiency, manufacturability, and reliability. We will exemplify opportunities and limitations for these revolutionary sources. IntroductionHigh power fiber sources are now leading contenders for many important applications requiring powers from a few watts up to several kilowatts. Beyond raw power, fibers offer advantages such as control, efficiency, manufacturability, and reliability. These derive from the fundamental fiber geometry, as well as advances in fiber design, fabrication, and pump diodes, largely driven by the optical communications industry. Master oscillator -power amplifier (MOPA) configurations build on the unique combination of high power, high efficiency, high gain, and broad gain bandwidth of rare-earth (RE) doped fibers. In MOPAs, the output from highly controlled, low power seed lasers can be amplified to ultrahigh power levels whilst preserving the desired seed characteristics. High control is much simpler at low powers. For example, high-speed lithium niobate modulators can be used to control the amplitude as well as the phase of the seed light. This makes MOPAs revolutionary light sources that can enable and dominate a range of devices and application areas. At the same time, the fiber MOPAs are being pushed ever closer to their physical limits. Here, we will exemplify both opportunities and limits of high-power fiber MOPAs. Fiber Raman amplifier with controllable gain spectrum Raman amplifiers offer the potential to generate gain at any arbitrary wavelength over several Stokes orders with an appropriate pump source. This has proved a very effective and successful way of providing gain at wavelengths not directly available with RE-doped fibers. Most of this success has been achieved using CW pump diodes, but in recent years there has been renewed interest in pulsed pumping of Raman amplifiers. This has been mainly in the telecommunications area through time-division multiplexed pumping schemes [1], but also in other areas due to advances in diode-seeded high power fiber MOPA systems [2]. A MOPA allows for excellent control of the pulse parameters which is not easily available from Q-switched or mode-locked lasers. This opens up new opportunities for controlling the Raman gain spectrum through control of the pulse parameters. We have experimentally investigated this, by pumping a fiber Raman amplifier with step-shaped optical pulses delivered from an ytterbium (Yb) doped fiber MOPA [3], [4]. With the instantaneous power of each step appropriately tailored, different parts of the pulse transfer their energy to different Stokes orders, leading to a controllable gain spectrum covering multiple Stokes orders. This opens up opportunities for an ultra-broadband Raman amplifier with near-instantaneous electronic control of the gain spectrum.