Optimisation of distributed feedback laser biosensors

Abstract

A new integrated optical sensor chip is proposed, based on a modified distributedfeedback (DFB) semiconductor laser. The semiconductor layers of different refractive indices that comprise a laser form the basis of a waveguide sensor, where changes in the refractive index of material at the surface are sensed via changes in the evanescent field of the lasing mode. In DFB lasers, laser oscillation occurs at the Bragg wavelength. Since this is sensitive to the effective refractive index of the optical mode, the emission wavelength is sensitive to the index of a sample on the waveguide surface. Hence, lasers are modelled as planar waveguides and the effective index of the fundamental transverse electric mode is calculated as a function of index and thickness of a thin surface layer using the beam propagation method. We find that an optimised structure has a thin upper cladding layer of 0.15 mm, which according to this model gives detection limits on test layer index and thickness resolution of 0.1 and 1.57 nm, respectively, a figure which may be further improved using two lasers in an interferometer-type configuration.

The development of a compact free spectral range semiconductor laser biosensor

“…To investigate which modifications would be most beneficial, we created simplified 2-D models of laser waveguide structures and used a commercial mode solver (BeamPROP) to calculate the effective index of the guided modes when the refractive index of a thin surface test layer is varied. Details of this work are described in a previous paper [4]. Simulations showed that thinning the p-cladding layers above the laser active region greatly enhanced sensitivity by increasing the penetration depth of the evanescent field.…”

“…According to the modelling, a laser device with an optimised structure for this application would have a sensitivity of 1.97 × 10 −4 [4]. Taking µ e f f to be 3.52 (see Section 3), we can obtain a predicted shift in FSR for a change in µ t of 0.4:…”