A WDM light source must both have a narrow spectral emission and be spectrally stable. Diffraction gratings, either in the light source itself or in the associated optical fiber, are what traditionally provide this control. Researchers at Stanford University (Palo Alto, CA), have devised a narrowband WDM light source that does not rely on gratings, and in the process have eliminated the necessity of butt-coupling the light source to a fiber.
Simple in concept, the device is nothing more than a coupling of two waveguides--one an optical fiber, the other a semiconductor anti-resonant reflective optical waveguide (ARROW). The ARROW contains a light-emitting AlAs/AlGaAs slab core sandwiched between two distributed Bragg reflector mirrors, one of them only partially reflective but bounded by air, producing complete reflection.
The fiber is epoxied into a curved groove in a glass substrate and one side polished to within a few microns of its core. The two waveguides have very different dispersion characteristics, producing phase-matching at only specific wavelengths. When the fiber is contacted with the ARROW using index-matching fluid, its emission is coupled into the fiber at two discrete wavelengths corresponding to transverse electric (TE) and transverse magnetic (TM) resonances.
The 0.819-nm TE resonance has a narrower linewidth than the 0.837-nm TM resonance. When the researchers increased the injection current, they determined that although the TE spectrum shifted to longer wavelengths, the TM spectrum did not. Thus, for TM-polarized light, the device is truly wavelength stabilized. There was no linewidth narrowing with increasing input current.
The light output of the device at an injection current of 100 mA was 7 nW, for an efficiency of as low as 3 x 10-8. The ARROW has no lateral optical confinement, which means that many emitted photons take lateral paths that preclude coupling into the fiber. By producing a one-dimensional waveguide using lithographic techniques, all photons would be emitted collinear with the fiber, greatly boosting efficiency, say the researchers. Introducing optical feedback with the help of fiber Bragg gratings could result in a directly fiber-coupled wavelength-stabilized semiconductor laser. Useful as a WDM source, such a device could also be made into a fiber amplifier pump. Contact Erji Mao at [email protected].
