Using ultrafast laser light, optical structures can be created within bulk glass either by direct writing with a focused beam, or by interfering two or more light beams. These structures consist of regions in which the refractive index has been altered. Scientists at Osaka University and the National Institute of Advanced Industrial Science and Technology (both of Osaka, Japan) have now come up with a third approach: filamentation of femtosecond laser pulses within glass.1
Under certain conditions, focused ultrafast pulses can form self-trapping filaments within glass that arise from a balance of diffraction and self-focusing from the plasma generated within the filament. These filaments can range from 10 to 500 µm long, with the length of the filament dependent on the numerical aperture (NA) of the focusing lens. The Osaka researchers use these filaments as writing tools, creating either waveguides by translation of the filament along its axis, or sheets of altered index by translation perpendicular to its axis.
Creating waveguides
In one example, 130-fs, 800-nm pulses at a 0.68-µJ pulse energy and 1-kHz repetition rate from a Ti:sapphire laser create a 40-µm-long filament when focused in glass by a 0.30-NA lens. The researchers wrote curved (17-mm radius) and straight waveguides that were close enough to each other (4 µm at their closest) to couple light back and forth. No void-like structures were observed.
Pairs of waveguides were made into directional couplers with, for example, coupling ratios of 1:1 and 1:0.5 for 632.8-nm light. Because directional couplers have coupling ratios that vary as a function of wavelength, these structures can also act as wavelength-division demultiplexers or multiplexers (see Fig. 1).
In a second example, the researchers created volume gratings within bulk glass; one version has the highest diffraction efficiency of any such grating created in bulk glass with ultrafast light (see Fig. 2).2 Two previous fabrication methods—two-beam interference of a single pulse, and direct writing using a focused spot—resulted in efficiencies of 1% and 20% respectively; the grating fabricated by filamentation is 74.8% efficient at its Bragg angle for light at 632.8 nm for TE-polarized light (59.2% for TM-polarized light).
The gratings were created in fused silica (silicon dioxide) glass using lenses with NAs of 0.05, 0.10, or 0.30. Incident energies to form a single filament were 1.5, 1.0, and 0.55 µJ/pulse, respectively. To create a grating one sheet at a time, the filament was translated laterally for each sheet at a speed of 1 µm/s. Gratings with periods of 5, 4, and 3 µm took 5, 6.25, and 8.33 hours to create, respectively, for a grating volume of approximately 300 × 300 µm in size and a thickness ranging from 32.5 to 200 µm. The diffraction efficiencies of gratings created with the 0.05- and 0.10-NA lenses were higher than that with the 0.30-NA lens. Depending on the grating, calculated diffraction efficiencies ranged to 99.8%; the lower experimental efficiencies are attributed to nonuniform refractive-index changes.
Wataru Watanabe, one of the Osaka University researchers, notes that the group would like to fabricate, using filamentation, a compact interferometer with a total length of several microns, as well as a compact optical switch. In addition, a three-dimensional arrayed-waveguide grating (AWG; conventional AWGs are two-dimensional, fabricated by photolithography) is in the works.
REFERENCES
- Wataru Watanabe et al., Optics Letters 28, (Dec. 15, 2003).
- Kazuhiro Yamada et al., Japan J. Appl. Phys. 42, Part 1(11) (November 2003).