Wavelength-stabilized lasers with surface Bragg grating

FBH develops diode lasers with wavelengths in the visible and infrared range. With frequency-selective Bragg gratings (Distributed Bragg Reflector) a low temperature dependence and a very narrow line width of the emitted laser light can be achieved. The fabrication of such Bragg gratings by etching the semiconductor surface is a major challenge regarding the technological process. In addition to a qualified photolithography for preparation of very small lattice structures in a polymer mask (photo resist or electron beam resist), especially the dry chemical transfer of the pattern into the underlying semiconductor material is particularly demanding in the technological process of grid formation. Depending on the desired grating order, the patterning can be performed by optical projection lithography using a wafer stepper (for high grating orders with grating periods of 800 to 1200 nm), by optical interference lithography (holography) or electron beam lithography (for lower grating orders with grating periods of 200 to 500 nm). While lattice structures of low order require a high aspect ratio (ratio between horizontal and vertical structural dimensions), for lattice structures of high order a V-shaped grating profile is desirable.

Scientists at FBH have developed a dry-chemical etch process controlling the tilt angle of the lattice structure and thus precisely adjusting the profile of the grid near the active layer. The technique uses sidewall passivation during dry etching to adjust the angle of the structures to be etched by controlling the gap between the grid bars. The dry etch process runs on a Sentech Instruments RIE plasma etch system (RIE = reactive ion etching) type "SI 500 RIE" that is specifically designed for such requirements. The etching depth is monitored in-situ and in real time by a Michelson interferometer. Besides gas mixture, parameters such as plasma power, process pressure and wafer temperature play a crucial role in the formation of the desired etch profile. By appropriate variation of these etching parameters it is possible to adjust the geometry of the Bragg grating and thus its reflectivity. Figures 1 and 2 show Bragg gratings of various grating orders of wavelength-stabilized diode lasers with low line width fabricated at FBH.

Publication:

J. Fricke, F. Bugge, A. Ginolas, W. John, A. Klehr, M. Matalla, P. Ressel, H. Wenzel, and G. Erbert, "<link /fileadmin/fbh-berlin/english/ver10/pub10.htm _blank internal-url-new-window>High-Power 980-nm Broad-Area Lasers Spectrally Stabilized by Surface Bragg Gratings", IEEE Photonics Technol. Lett., vol. 22, no. 5, pp. 284-286 (2010).

FBH research: 28.03.2011