Gain Improvement of Er-doped Amplifiers for the Feedback Filters

The combination of the arsenic trisulfide (As2S3) waveguide and titanium diffused lithium niobate (Ti:LiNbO3) waveguide provide us compact and versatile means for transmitting and processing optical signals, which benefits from the high index contrast between these two materials and the electro-optical properties of Ti: LiNbO3. Furthermore, waveguide gain is introduced through selective surface erbium (Er) doping which yields high quality loss-compensated or even amplifying waveguides without disturbing the excellent electrooptical, acoustooptical and nonlinear properties of the waveguide substrate LiNbO3. The integration of these waveguides allows the development of a whole class of new waveguide devices of higher functionality and complexity. As one kind of the hybrid waveguide devices, a new configuration consisting of an As2S3 channel waveguide on top of an Er doped titanium diffused x-cut lithium niobate waveguide has been investigated by simultaneous analytical expressions, numerical simulations, and experimentation. Both simulation and experimental results have shown that this structure can enhance the optical gain, as predicted by the analytical expressions. An As2S3 channel waveguide has been fabricated on top of a conventional Er:Ti:LiNbO3 waveguide, where the higher refractive index As2S3 waveguide is used to pull the optical mode towards the substrate surface where the higher Er concentration yields an improved propagation gain. The relationship between the gain and As2S3 layer thickness has been evaluated and the optimal As2S3 thickness was found by simulation and experimentation. Side integration was applied to reduce the extra propagation loss caused by the titanium diffusion bump. The propagation gain (dB/cm) has been improved from 1.1 to 2 dB/cm. Another hybrid device which combines the As2S3 and LiNbO3 is to make an As2S3 racetrack ring resonator on top of an x-cut y-propagation Er:Ti:LiNbO3 waveguide which is the potential structure for integrated lossless all-path filter. The ring was side-coupled with the Ti:LiNbO3 waveguide and the optical gain was achieved when the 5mm long coupling region where has been diffused with Er in advance pumped by 144mW pump laser. The free spectral range (FSR) of the measured ring response for TM mode is 0.0587nm (7.33GHz) at 1550nm. The roundtrip loss are 4.4dB (2.60dB/cm) when pump on and 5.8dB (3.44dB/cm) when pump off. The optical gain in the Er diffused area is 0.72dB/cm.