Integrated optical structures that contain ring resonators and Bragg gratings have been proposed as external feedback elements for semiconductor lasers. These feedback elements would constrain the laser spectra to narrow lines (more precisely, narrow spectral bands) centered at desired wavelengths. In the original intended applications, these feedback elements and the associated lasers would be constructed and operated in pairs to generate pairs of spectral lines separated by known wavelength intervals in the approximate range of 1 to 10 nm, as needed to resolve integer-multiple-of-wavelength ambiguities in laser metrology.
A ring resonator according to the proposal (see figure) would include a ring optical waveguide comprising a core of SiON (index of refraction = 1.483) and a cladding of SiO2 (index of refraction = 1.463), formed on a substrate of Si by use of plasma-enhanced chemical vapor deposition. The resonator would also include two straight waveguides - one for input and one for output. The straight waveguides would be laid out in proximity to the ring waveguide, so that light would be coupled between the straight and ring waveguides via the evanescent-wave interaction. The overall operation of the resonator, in terms of transmissivity between ports 1 and 2 as a function of wavelength, would be similar to that of a Fabry-Perot etalon: the transmissivity spectrum would contain multiple sharp resonance peaks reminiscent of a comb.
A Bragg grating would be incorporated into one of the straight waveguides to select one of the ring resonances. To be effective for this purpose, the Bragg grating must be designed and fabricated to have a reflectance peak narrower than twice the free spectral range of the resonator - that is, narrower than double the spectral interval between successive resonance peaks. It would also be necessary to match the transmission pass band of the grating with one of the resonance peaks. An electric-heater element could be deposited on the ring or on the grating for this purpose: selective heating of the ring or grating could vary the resonance peaks or the pass band to effect this match.
For an example involving a wavelength of 1.55 µm, ring radius of 3 mm, and propagation loss of 0.2 dB/cm, the resonance quality factor (Q) was estimated to be 105. Assuming further that the requirements stated in the preceding paragraph were satisfied, that the reflectivity of the Bragg-grating in the vicinity of one of the ring resonances was about 0.97, and that the interface reflectivity was 3 percent, it was estimated that laser gain chip that would ordinarily emit a spectral line 50 MHz wide when operated by itself would emit a spectral line with a width of 10 kHz or less when operated with this feedback structure connected to it. A width of 10 kHz is comparable to the spectral widths of neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers used heretofore in metrology.
This work was done by Alexander Ksendzov of Caltech for NASA's Jet Propulsion Laboratory.
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