SANTA CLARA, CA—The feasibility of extending the wavelength of operation of silicon lasers from the near- to the mid-infrared has been given a boost. In a paper published online last month in Nature Photonics, researchers from Intel demonstrated the world’s first cascaded Raman silicon laser, extending the operating wavelength out to 1848 nm, with even longer wavelengths possible. This achievement establishes a pathway to extending the laser wavelength into the mid-IR region for gas sensing and other important spectroscopy applications that are possible today only with complicated, bulky, expensive, or cryogenically cooled lasers.
Cheap and powerful semiconductor lasers that operate in the mid-infrared (2 to 5 micron) region are highly sought for applications such as medical diagnostics and environmental monitoring, but do not exist at present. Haisheng Rong and colleagues at Intel have demonstrated that silicon-chip-based lasers that exploit cascaded Raman lasing may provide the answer. Although silicon Raman lasers have been made before, their wavelength of operation has always been limited to around 1.6 microns.
The Intel team has successfully demonstrated that, by exploiting the Raman effect not once but twice within a silicon waveguide, it is possible to create a silicon-chip laser that emits milliwatt-scale powers at a wavelength of 1.848 microns. This is the longest wavelength reported so far for silicon Raman lasers and is tantalizingly close to the mid-infrared window. The research offers hope that by optimizing the design it should be possible to make lasers that operate at even longer wavelengths.
Silicon is particularly suitable as Raman laser material for the near and mid-infrared (IR) regions due to its high Raman gain and optical transparency in these regions. Through cascaded Raman lasing in silicon, one can convert pump wavelengths in the near IR region for which sources are well-developed and widely available, to wavelengths in the mid-IR region, providing low-cost, compact, and high-performance room-temperature lasers. Such laser sources are highly desirable for many applications ranging from trace-gas sensing, environmental monitoring, and biomedical analysis, to industrial process control, and free-space communications.
Cascaded Raman lasing was previously achieved in glass fibers. However, the high optical losses of glass at longer wavelengths prohibit cascaded lasing into the mid-IR region. In contrast, silicon has a transparency window of up to 6 microns, and low-loss silicon waveguides can be fabricated. Other advantages of using silicon as cascaded Raman laser material include its unique material properties such as high thermal conductivity and optical damage threshold, as well as its extraordinary material purity and great natural abundance.
