Tunable double micro-ring ( DMR ) filters for widely tunable lasers

Widely tunable double micro-ring filters by thermo-optic effect are demonstrated. The tuning range is more than 40nm. The required electrical power to achieve 40 nm using thermo-optic effect is less than 20 mW. A widely tunable laser using this filter and EDFA gain is demonstrated. The side mode suppression ratio is greater than 30dB and the tuning range is 40nm. Keyword: Tunable laser, Micro-resonator, Electro-optic polymer, Thermo-optic effect Introduction Widely tunable filter and lasers are highly required for the wavelength division multiplexing (WDM) systems. Previously widely tunable lasers were demonstrated using vernier effect and Bragg gratings [1][2]. These structure use semiconductor Bragg gratings and tune by injecting current to the Bragg section of the device. Sampled grating DBR is used to achieve periodic reflections. Widely tunable lasers are demonstrated using this technology. Recently Bin Liu et.al [3] introduced the idea of using micro-resonator (MR) instead of Bragg gratings to fabricate widely tunable lasers. There are many advantages to DMR devices. First the frequency response of MR is periodic and the transmission is equal for all the wavelengths. However the SG frequency response is not constant and the peak reflection decreases far from the Bragg condition. This limits the enhancement factor practically to 10 for SG-DBR structure [3]. However in a MR structure provided that Q is high enough the enhancement factor can be very large. Also for the tuning mechanism previously carrier injection was demonstrated. However carrier injection changes the absorption and changes the threshold of the laser. Using electro-optic effect not only the absorption remains the same but also it is possible to change the lasing wavelength very rapidly. However since the change in the refractive index is very small only DMR structure with very high M can be used to achieve a widely tunable laser using electro-optic effect. The DMR device has narrow linewidth due to the long cavity formed by MR [4]. Since electro-optic effect is used it might be possible to make ultra-fast tunable devices, which might open the door to optical frequency shift keying (OFSK) or OCDMA systems. Polymers can be used to make micro-resonator with very high Q [5]. Also electro-optic polymers with very large electro-optic coefficient can be used to make micro-resonators [5]. In this paper we have demonstrated for the first time the tunable DMR. For tuning thermo-optic effect is used. However electro-optic effect can also be used. Finally using an EDFA gain medium we have demonstrated a widely tunable laser. Thermo-optic device fabrication The fabrication of the device is very similar to previous structures we have fabricated [5]. The cross section and top view of the device is shown in Fig 1. The fabrication of thermo-optic devices starts with spin coating a lower cladding of UV15 epoxy. The lower cladding is chosen to be 15μm. This layer is a thermal isolator between the thin film heater and the bottom substrate. Two micro-resonators with 240μm and 246μm radius are fabricated. The FSR of MR is approximately 120GHz. The tuning enhancement factor is 40. The distance between the waveguide and the MR is chosen so that 10% coupling is achieved. Based on this design one would get discrete tuning with 120GHz spacing. Notice that one can easily modify these dimensions to achieve ITU standards.