Various laser sources exist for producing radiation in the mid-infrared (mid-IR) region. The earliest mid-IR semiconductor lasers, for example, used lead salts operating at cryogenic temperatures to produce output wavelengths ranging from 3 to 30 μm. These lasers were limited to relatively low output power levels and exhibited wide variations in emission characteristics, particularly from thermal cycling. As a result, researchers began focusing on Antimonide based laser sources, quantum cascade (QC) laser sources, and optical parametric oscillators (OPOs) as more viable alternatives.
Although there has been significant mid-IR laser development in the past few years, particularly in QC lasers and OPOs, a number of problems persist. For instance, OPO's are bulky and require a large pump laser and moving parts. QC lasers have relatively low efficiency, requiring strict temperature control and in some instances cryogenic temperatures for high power operation. QC lasers also operate at wavelengths longer than 4.2 μm near room temperature, and thus do not sufficiently cover the entire mid-IR wavelength range.
In contrast, mid-IR sources based on the cascaded Raman shifting process exhibit some distinct advantages, including the maturity of underlying technology, simple modulation and wavelength tuning, and excellent beam quality with M2<1.4 (using single spatial mode in fiber), where the M2 factor is the beam quality factor, or beam propagation factor, defined as the beam parameter product divided by λ/π These mid-IR sources have been implemented in fiber-based systems that offer additional advantages such as robustness, compactness, lightweight, ease of use, room temperature operation over a flexible repetition rate, a wide wavelength range from 1-6.5 μm or longer, and very high efficiency (close to quantum limit). Compared with QC lasers, cascaded Raman shifting sources offer a larger tuning range, better beam quality, room temperature operation with flexible repetition rate, scalability to high powers, and higher efficiency. Compared with OPO's, cascaded Raman shifting sources offer a more mature underlying technology, better beam quality, the advantages of fiber-based systems, and a wider wavelength range with better efficiency.
While cascaded Raman shifting sources have been proposed, the state of the art sources are limited in operation and commercial application. Some have proposed cascaded Raman shifting apparatuses designed to shift a single pump signal from one mid-IR wavelength to a different mid-IR wavelength. Yet, because of the varying attenuation characteristics of Raman shifting materials, no single material can be used to cover the entire mid-IR spectral range, which limits the reach of Raman sources. Furthermore, even where techniques have been developed to provide numerous Raman orders in a single cascaded Raman stage, the output is limited to the fully converted, final Raman order. That is, effective techniques for taping different Raman shifted orders during the entire conversation process have not been attained. Some have proposed using a coupler with a single Raman process stage, but such configurations nonetheless limit the output from a mid-IR laser source and they limit the applications that might otherwise benefit from such a source. This limitation is particular noticeable in applications like countermeasures where a supercontinuum produced laser source may be undesirable over a discrete characteristic spectrum.