We present recent results on experimental micro-fabrication and numerical modeling of advanced photonic devices by means of direct writing by femtosecond laser. Transverse inscription geometry was routinely used to inscribe and modify photonic devices based on waveguiding structures. Typically, standard commercially available fibers were used as a template with a pre-fabricated waveguide. Using a direct, point-by-point inscription by infrared
femtosecond laser, a range of fiber-based photonic devices was fabricated including Fiber Bragg Gratings (FBG) and Long Period Gratings (LPG). Waveguides with a core of a couple of microns, periodic structures, and couplers have been also fabricated in planar geometry using the same method.
The inscription of fibre Bragg gratings using infrared femtosecond laser offers a number of advantages over conventional methods based on UV inscription. The refractive index modification in femtosecond inscription is independent on defect formation and therefore should not experiment the defect-related thermal decay of UV inscribed gratings. In this paper, the response to thermal annealing of fiber Bragg gratings inscribed using a tightly focussed femtosecond laser is investigated. Experimental results reveal a vastly improve thermal stability compared to gratings inscribed using conventional methods based on UV light. Erasure was not observed until temperatures in the range between 900°C and 1000°C. These devices are therefore particularly suited to work in hostile environments and as high temperature sensors.
Long period gratings written into a standard optical fibre were modified by a femtosecond laser, which produced an asymmetric change to the cladding's refractive index resulting in a directional bend sensor.
A technique of direct writing of depressed cladding waveguides by a tightly focused, femtosecond laser beam in laser crystals has been developed. A laser based on a depressed cladding waveguide in a Neodimium doped YAG crystal has been demonstrated for the first time.
Self-seeded, gain-switched operation of an InGaN multi-quantum-well laser diode has been demonstrated for the first time. An external cavity comprising Littrow geometry was implemented for spectral control of pulsed operation. The feedback was optimized by adjusting the external cavity length and the driving frequency of the laser. The generated pulses had a peak power in excess of 400mW, a pulse duration of 60ps, a spectral linewidth of 0.14nm and maximum side band suppression ratio of 20dB. It was tunable within the range of 3.6nm centered at a wavelength of 403nm.
Widely tunable gain switching of a grating-coupled surface-emitting laser (GCSEL) has been demonstrated in a simple external cavity configuration for the first time. Pulse duration in range of 40-100ps and wavelength tuning over 100nm have been achieved. High power, tail-free optical pulses have been observed at 980nm.
A study of the gain-switching process in GaInN MQW laser diodes is reported. Single peak gain-switched optical pulses with pulse widths less than or equal to 40 ps and optical powers equal to 100 mW are observed when electrical pulses with duration of 800 ps are applied. Sub-nanosecond optical pulses with peak powers in excess of 450 mW are also obtained and degradation mechanisms are analyzed. The transient response characteristics of the laser diodes are studied in both the time and spectral domains.