By applying a contradirectional surface-mode-coupling (SMC) technique, researchers at the Institut für Festkörperelektronik, the Technische Universität Wien (Vienna, Austria), and the Physikalisches Institut of the Universität Stuttgart (Stuttgart, Germany) have achieved single-longitudinal-mode surface emission from a horizontal-cavity red-emitting gallium indium phosphide/aluminum gallium indium phosphide (GaInP/AlGaInP) laser diode. The scientists also demonstrated the capability to adjust wavelength characteristics by changing the optical thickness of a surface waveguide through hydrogen fluoride (HF) etching. In addition, if the Fabry-Pérot mirrors are etched and not cleaved, the SMC technique allows monitoring and adjusting the emission wavelength of laser groups on the chip, providing a fairly straightforward way to create multiwavelength laser-diode arrays.
According to the researchers, the SMC principle is based on a coupling mechanism between the laser mode and the transverse electrically polarized surface mode that exists in a semitransparent metal/dielectric waveguide structure on the top of the laser diode.1 Phase-matching of the laser and the surface modes is achieved with a surface-relief grating in the top cladding of the waveguide. Either co- or contradirectional coupling is possible, depending on the grating period (see photo).
The grating causes radiation losses of the laser mode (dominated by the emission into the substrate), which are reduced significantly only in a narrow spectral range by the excitation and feedback process of the surface mode. The linewidth of this resonance is comparable to the longitudinal Fabry-Pérot mode spacing of the laser cavity, thus providing an effective mode-selection mechanism that finally leads to single-mode emission.
Additional benefits: As well as offering benefits such as easy wavelength adjustment, narrow beam divergence in one direction, and red emission, Peer Oliver Kellermann of the Institut für Festkörperelektronik reported that the laser diodes also can be fabricated using the established technique for producing a conventional stripe-contact laser with a horizontal cavity. There is no need for complex fabrication processes such as the high-reflection Bragg mirrors required for vertical-cavity surface-emitting laser structures and the epitaxial overgrowing of a grating for distributed-feedback (DFB) lasers.
In addition, the surface grating on a whole wafer can be defined with the help of holography—only one 200-s holographic exposure is necessary. Also, there is no need for e-beam lithography—which would be too slow—and the grating period is larger than for a DFB laser, making it easier to fabricate.
Experimental results so far include the capability to decrease emission wavelength (approximately 679.4 nm) of the laser structures in discrete steps of 0.1 nm (longitudinal-mode spacing) over an interval of 5 nm (with constant temperature). The laser diodes emit via the surface (an area of 5 x 500 µm) with a beam divergence of 0.12° and show single-mode suppression in both ac and dc operation with a minimum spectral linewidth of 0.09 nm. The highest side-mode suppression achieved in dc operation is 26 dB.
Using the SMC technique, the researchers also produced a laser-diode array with a wavelength span of 5 nm and side-mode suppression of 30 dB. The device, which Kellermann reported can facilitate dense wavelength-division multiplexing into multimode plastic fibers, has nine elements, spaced 250 µm apart, that offer seven different wavelengths and two reference lasers. The fabrication process allows etching many arrays of one element type in one step. The device geometry will allow the researchers to image such arrays directly onto the end of a fiber.
Current research efforts include optimizing the red SMC laser diodes for higher surface-emitted power, increasing side-mode suppression ratio, and producing a wider range of wavelength adjustment. These efforts are supported by Volkswagen-Stiftung, Germany, and Gesellschaft für Mikroelektronik, Austria. One application area that could someday benefit from such lasers is optical communications. According to Kellermann, other potential applications include spectroscopy and parallel free-space optical interconnects.
REFERENCE
- P. O. Kellermann et al., Appl. Phys. Lett. 75, 3748 (Dec. 13, 1999).