Tunable integrated module for the optical dispersion compensation based on Bragg gratings in silicon

Chromatic dispersion strongly limits high speed data transmission in optical networks . A correct detection of the pulse sequence requires a dispersion compensation. In particular the realization of an integrated device for the dispersion compensation at the receiver is necessary for achieving a monolithic receiver module with combination of optics and electronics. Silicon is an ideal material for the realization of optical components, since it is transparent for the wavelengths used in telecommunications. Furthermore the standard fabrication processes of microelectronics can be applied too. Aim of the work was the realization of an integrated dispersion compensator based on Bragg gratings in silicon. The theoretical background of Bragg gratings has been analyzed and discussed. Their complete description requires precise computation methods which are able to consider the coupling between the waveguide modes for all grating strengths. The bidirectional eigenmode expansion proved to provide reliable results also for strong gratings. Therefore it was applied for the design of the desired grating structures. The influence of the different geometric parameters has been systematically studied, in order to obtain gratings reflecting at 1550 nm over a bandwidth of 1 nm and with a loss of 0.2 dB. The fabrication of the designed structures has been performed with standard bi-CMOS processes based on DUV-lithography. It has been shown that the obtained results are comparable with the ones achieved with the state of the art (gratings patterned with electron beam lithography). The new approach permits a higher throughput and the possibility to realize „stitching“-free gratings without length limitations. A stable method for the implementation of the chirp in gratings for dispersion compensation has been presented, linearly increasing the waveguide width (taper). It has been demonstrated that a grating patterned on a tapered waveguide with a taper width of 150 nm and a length of 5 mm can compensate a dispersion of about 100 ps/nm over a bandwidth of 1 nm. However such devices cannot be directly applied for the dispersion compensation since they exhibit a strong group delay ripple (GDR). This can be canceled with an apodization. The grating strength is reduced at the ends in order to suppress undesired reflections. It can be achieved varying the width of the grating openings. Apodized gratings permit a dispersion compensation of about 250 ps/nm for one 100 GHz WDM channel and a correct signal detection for a data rate up to 40 Gbaud. The designed gratings have been placed in filter structures consisting of two cascaded grating pairs. This permits to double the achievable dispersion and to separate the compensated channel without the use of an optical circulator. The structure exhibits an overall length of 2.5 cm, a width of few tens of microns and permits hence a high integration level. Thanks to the thermo-optic effect and the thermal expansion of the material, a temperature variation in the grating produces a shift of the reflected wavelength of 80 pm/K. This property has been exploited to tune the device dispersion with temperature gradients. The gradients induce a further chirp, with a consequent variation of the slope of the group delay curve. The thermal gradients has been produced placing metallic heaters along the gratings and driving them with external currents. A dispersion tuning between 100 ps/nm and 600 ps/nm with a gradient amplitude of 30 K has been reached. Reversing the gradient amplitude is has been also possible to reverse the sign of the dispersion. For each gradient configuration a power consumption of 4.74 W for the whole dispersion compensator has been measured. The proposed device permits a dispersion compensation of a 100 GHz WDM channel between -500 and +600 ps/nm. This values can be increased using longer gratings. They are comparable with the results obtained with serial FIR filters and AWGs from the literature. However gratings offer a more compact solution and a simpler operation, since the dispersion is intrinsic. Furthermore the dispersion tuning can be simply performed applying linear temperature gradients.