In a paper entitled “Cascaded Silicon Raman laser” published today in Nature Photonics (by Haisheng Rong, Shengbo Xu, Oded Cohen, Omri Raday, Mindy Lee, Vanessa Sih, and Mario Paniccia), we report the first experimental demonstration of a cascaded Raman laser produced in silicon and some potential applications, including sensing greenhouse gasses. In this blog, I attempt to explain for general audience what this technology breakthrough is about and how we achieved it.Since the invention of the laser in the early 60s, countless scientific, industry, military, medical and commercial laser applications have been developed. Each of these applications relies on one or more of the special properties of the laser, such as high coherency, high monochromaticity, or the ability to reach extremely high powers. While most of the lasers are based on light amplification by stimulated emission of radiation predicted by Einstein, a special category of laser is based on stimulated Raman scattering and, therefore, is referred to as Raman laser. A major attraction of Raman lasers is their ability to use light from an optical “pump” to generate coherent light emission in wavelength regions that are hard to reach with other conventional types of lasers. In addition, Raman lasers can be made from materials such as silicon that do not possess suitable energy band structures to produce laser light by stimulated emission. Three years ago, researchers from Intel’s photonics technology lab demonstrated the first continuous-wave (CW) silicon Raman laser. This result was published in the journal Nature 433, 725-728 (2005). Since then, we have made several major improvements. Instead of applying optical coatings on the chip facets to form a laser cavity, we designed a “mirror-less” ring cavity laser which is monolithically integrated on a chip and fabricated entirely in a CMOS fab. We have achieved 10 times reduction in lasing threshold to below 20 mW and 5 times increase in output power to more than 50 mW. This cavity design is scalable to enable size reduction and can be integrated with other silicon photonic components on the same chip. Another significant characteristic of the silicon Raman laser is its extraordinary spectral purity. This is very important for many laser applications such as high-resolution spectroscopy, as we will discuss later. These improvements led to the current breakthrough in achieving cascaded Raman lasing in silicon. Cascaded lasing is a unique attribute of Raman lasers. A Raman laser’s output beam (which is always at a longer wavelength than the pump) can itself act as a pump to generate Raman lasing at an even longer wavelength. This so-called cascaded process can continue to generate longer wavelengths, as illustrated here. A cascaded Raman laser can be realized by properly designing the laser cavity to take advantage of this unique property. Cascaded lasing enables Raman laser to have greater wavelength coverage, and one can use a low-cost and efficient laser as a pump to generate coherent light in longer wavelength region where it is difficult to achieve lasing by other methods. 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 µm, 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. In the paper published in Nature Photonics, we describe the laser design and fabrication, and the laser performance characteristics. We show that when using a pump laser of 1550 nm which is well developed for optical communications, we achieved stable, single-mode 1st and 2nd order CW lasing at 1686 nm and 1848 nm, respectively. The Raman laser output power exceeded 5 mW and its wavelength could be continuously tuned over a 25 GHz range. The realization of 2nd order silicon Raman lasing paves the way toward higher order cascaded Raman lasing, and opens a new path to producing low-cost, compact, room temperature, high-performance mid-IR lasers. One of the important applications of near and mid-IR lasers is gas sensing and analysis for environmental monitoring, pollution control, agriculture and life sciences, and non-invasive disease diagnosis through breath analysis, as most gas molecules have their IR-active rotational-vibrational transitions in these regions and, thus provide unique “fingerprint” absorption patterns which can be detected and identified using laser spectroscopy techniques. To demonstrate the performance and capability of the cascaded silicon Raman laser in such applications, we performed direct absorption spectroscopy on methane, one of the major greenhouse gases, and water vapor, which needs to be controlled tightly in high-yield semiconductor manufacturing processes. These molecules have characteristic absorption patterns in the regions covered by the cascaded silicon Raman laser. In the experiment, we pass the laser beam through a sample cell containing methane or water vapor. We set the laser wavelength close to their respective absorption regions. For methane we use the first order laser output at 1687 nm, and for water vapor, the second order output at 1847 nm. Thanks to cascaded lasing, these two widely separated wavelengths can be generated from the same silicon Raman laser. We recorded the transmission of the laser beam through the sample cells as a function of the laser wavelength. The resultant curves are referred to as direct absorption spectra (molecular fingerprints). By comparing the measured absorption spectra with those in a database that contains the fingerprints of all known gas molecules, one can identify the gas molecule, its concentration, and other characteristics. The charts here show the measured absorption spectrum of methane and water vapor, respectively. Due to the high spectral purity of the laser, these spectra are well resolved and the width of the absorption lines is only limited by the thermal motion of the molecules (Doppler Effect). As can be seen, our measured absorption profiles agree very well with calculations based on the HITRAN (High-resolution Transmission Molecular Absorption) database. These results demonstrate that the performance of the cascaded silicon Raman laser, in terms of power, stability, and wavelength tunability, is suitable for practical applications even in its current development phase. In addition to its low-cost material and CMOS compatible fabrication aspects, the cascaded silicon Raman laser offers unique advantages such as wavelength selectivity and spectral purity that can compete with complex and bulky solid state laser systems, but in the small form factor of semiconductor lasers. Single-mode, room temperature operation is obtained from a monolithic chip without the extra cost associated with additional cavity components and their assembly and alignment. The spectral coverage of silicon Raman lasers could be further extended by higher orders of cascaded lasing. A theoretical study has shown that up to 6th order cascaded lasing may be possible. The unique features of this new type of laser, combined with its low-cost fabrication and optoelectronics integration potential, could enable an entirely new suite of devices and applications in the near and mid IR spectral region. Haisheng Rong is a Senior Researcher in the Photonics Technology Lab at Intel. Haisheng leads Intel’s research into Raman-based lasers and amplifiers.
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