Exposure and characterization of superstructure gratings for DBR lasers generated by direct write electron-beam lithography

Abstract

Semiconductor lasers with a wide tuning range can be fabricated with the use of super structure gratings (SSG). In this paper we give a short introduction to SSGs, the technical problem of their realization and the existing techniques to generate SSGs. A method is presented to generate 'true' continuously chirped gratings overcoming the technical restrictions.Furthermore we make a proposal how to characterize SSGs in resist profiles. lithography INTRODUCTIONTunable lasers with a wide wavelength range are key components for high-performance optical telecommunication systems. Unfortunately state-of-the-art lasers with Bragg gratings of a fixed periodicity exhibit a rather narrow tuning range of only a few nanometers due to the limits in the change of the refractive index' achievable by current injection. Simulations and first experimental results have shown that the tuning range of a semiconductor laser can be extended to about 100 nm by the use of super structure gratings (SSG)2'3. Similar to conventional DBR gratings the SSG consists of repeating bars, the pitch however being continuously slightly changed ('chirped'). Usually the pitch of a SSG linearily chirps from a Bragg period AA to a period AB over the distance of the super period As. This arrangement is repeated several times in a regular (see Fig. 1) or irregular manner. Pitch As z A5 AA N NN AA AB a)ABN' b)fl J1J__l:x Fig. 1 Concept of a linearily chirped SSG demonstrated in a) the frequency range and b) the space rangeWhereas a conventional distributed Bragg reflector shows a single reflection peak, the SSG reflector exhibits multiple reflection peaks due to the periodical super structure arrangement5. The oscillation wavelength of a DBR laser can be adjusted by selecting a mode from among these peaks. Several mechanisms for wavelength tuning with SSGs have been decscribed, e. g. for tuning with multiple-phase-shift SSGs5. The main drawback of generating 'true' continuously chirped gratings is the required positioning accuracy of down to a few tenths of Angstroms. Typical values for AA = 245 nm, AB = 230 nm and A = 61.600 pm result in a medium pitch variation of about 0.05 nm which has to be addressed in the electron-beam writer (e-beam). Several simplified versions of SSGs have been fabricated: sampled gratings2 consist of short regions of a conventional grating seperated by non-structured sections, discretely chirped gratings3 use steps of the utmost minimal resolution of the electron-beam writer, multiple-phase-shift SSGs4 comprise additionally inserted phase shifters at appropriate positions in an uniform grating.We present a method to generate SSGs with almost no limitation by the beamwriters resolution and give a proposal to characterize SSGs realized in resist. 204 ISPIE Vol. 2437 0-8194-1785-8/95/$6.00 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/23/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx