Multiwavelength grating reflectors for widely tunable laser

Design of widely tunable semiconductor lasers and the concept of binary superimposed gratings (BSG's)

“…As already reflected in the preceding sections, an FP laser is made tunable by the addition of tunable DBR grating mirrors with a comb-like reflection spectrum. Direct superposition of holographic exposure would be, perhaps, the simplest way to create the multiwavelength reflectors [18]- [21], and such gratings could be used to demonstrate the Vernier tuning [22]. However, an analog superposition does not seem to be a promising approach in the case of semiconductor waveguides due to high nonlinearity of the exposure, resist development, and etching of semiconductor layers.…”

“…The magnitude and phase of the reflection spectra of both gratings were obtained using the transfer matrix method accounting for dispersion and additional internal loss variation in the BSG's with current. Longitudinal mode frequencies in the neighborhood of the various reflectivity product peaks were found from the phase condition (22) where are the propagation constants in the active and phase regions and and are the phases of the reflectivities of the back and front BSG's. The second term is adjusted such that the principle mode is always at a reflectivity maximum, which in the actual device is accomplished by varying the current through the phase tuning section.…”

Integration of active and passive photonic-crystal-based optoelectronic components

“…A common concept to implement a wide tuning range of a tunable laser diode is based on the contra-directional coupling of two or more mode combs that are defined in separately contacted segments. An often used approach is to define sampled gratings [1][2][3][4] or superstructure gratings [5][6][7] with distributed Bragg reflectors that are placed adjacent to an active gain region. Index modulation here is entirely due to changes of the carrier density resulting in a blue shift of the respective mode comb.…”

Optimization of multiple exposure gratings for widely tunable lasers

“…To fabricate a MWG, a thin photoresist layer is sequentially exposed by two-beam interference patterns with different pitches [4], [6]. Because of the thresholding effect of the photoresist development and the characteristics of the etching process, the transfer of the superimposed fringe patterns into semiconductor material is highly nonlinear.…”

“…These can be seen as an extension of ordinary Bragg gratings toward multiwavelength reflectors, however still relying on holographic exposure for the fabrication. A. Talneau et al [4], [6] first presented these, but until now no theoretical calculations of reflection spectra have been presented. We will show that these multiwavelength gratings (MWG) can generate uniform reflection combs, if the relative phases of the superimposed Bragg gratings are suitably chosen.…”