Hydrogen-filled hollow-core fiber converts wavelength
While some lasers are tunable, and others (for example, laser diodes) can be designed to emit at any wavelength within certain ranges, wavelength conversion through nonlinear optical processes is important for reaching many sought-after wavelength regions.
While some lasers are tunable, and others (for example, laser diodes) can be designed to emit at any wavelength within certain ranges, wavelength conversion through nonlinear optical processes is important for reaching many sought-after wavelength regions. Pumping some gases with laser light at the proper wavelength can result in stimulated Raman scattering (SRS), producing a wavelength shift.
Because SRS occurs only at high optical intensity, bulk laser-pumped gas cells require megawatt-level peak powers to achieve any conversion at all. But with the advent of hollow-core photonic-crystal fibers (PCFs), which can be filled with gas, even low-power laser light produces adequately high peak intensities. Researchers at the University of Bath and Blazephotonics (both of Bath, England) have created a hydrogen-filled PCF that converts wavelength at a 92% quantum efficiency. Another version has a threshold energy of only 3 nJ-a million times lower than for previous efforts.
Purely rotational SRS
Stimulated Raman scattering exists in more than one form. Vibrational SRS in hydrogen has a relatively high Raman gain and a frequency shift of 125 THz, while rotational SRS has a much lower gain and a smaller frequency shift of 18‑THz. The two forms ordinarily compete and reduce the efficiency of conversion to a desired wavelength.
The Bath researchers rely on the PCF not only to produce high intensities within the fiber, but also to select only one form of SRS. Because the PCF has a full photonic bandgap over a relatively narrow region, it acts as a bandpass filter that accepts the 18-THz rotational shift while rejecting the 125-THz vibrational shift.
Based on a PCF with a triangular lattice, the experimental fiber had a core diameter of 7.2 µm, an air-fill fraction of approximately 90%, and a transmission range (with loss less than 100 dB/km) of 1000 to 1150 nm. The hollow core was filled with hydrogen to a pressure of 7 bars and was pumped in a single-pass configuration at 1064 nm.
The low-peak-power requirements allowed a microchip laser to be used as a pump source. For rotational SRS, the laser light was circularly polarized to maximize gain. Tests were done for fiber lengths ranging from 2.9 to 35 m. At a 35-m length, the threshold energy for conversion to the first Stokes order (1135 nm) was on the order of 3 nJ (peak power of 3.75 W) and the conversion efficiency was 35%. At a 2.9-m length, the threshold grew to 20 nJ, but the conversion efficiency reached 86%-corresponding to an exceptionally high 92% photon-conversion efficiency. The researchers concluded that best efficiency can be reached simply by choosing the right fiber length.
Because many gases can be used, the number of available SRS lines is large, according to Jonathan Knight, one of the University of Bath researchers. The group has demonstrated hollow-core fibers guiding through the visible spectrum and approaching 2 µm (see figure).
Green light is guided by a hollow-core photonic-crystal fiber. A hydrogen-filled version that guides near-IR light operates as an efficient, low-threshold stimulated-Raman-conversion light source.
“We have a good coupling efficiency into a reasonable length of low-loss fiber,” he says. “As a result, the volume defined by the spot size multiplied by the interaction length is several orders of magnitude greater than is possible in free space, where diffraction is a limitation. The advantage grows as the fiber attenuation drops, so that with state-of-the-art values for attenuation (of less than 2 dB/km) we would expect to be able to pump such a setup efficiently with a laser diode.”
1. F. Benabid et al., Physical Rev. Lett. 93, 123903 (2004).