We demonstrate optical signal amplification in a solid-state dye-doped polymer with a rib waveguide structure. The device consists of a 1μm x 120μm poly(methyl methacrylate) (PMMA) waveguide, doped with 1% by weight Rhodamine 640 dye, spin coated onto a silica substrate. A 625nm pulsed signal and a co-linear 575nm pump are facet coupled into the waveguide and optical amplification of the signal is demonstrated.
Depending on the signal intensity, a maximum internal gain in the 21-26dB range at 625nm is achieved using a 1.2cm long waveguide. The device exhibits a promising signal-to-noise ratio in the 9-16dB range and has the potential for tuning in a 40-50nm wavelength window with the same dye, and throughout the visible spectrum using other dyes. The wavelength operating range of this device is also analyzed.
We therefore present a compact, easy to fabricate, high gain block suitable for use in conjunction with plastic optical fibers, which have a low-loss window at around 640nm. Since most of the optical amplification takes place in a short region (<10μm), an even more compact device geometry can be envisaged using a shorter waveguide.
Extensive mode-locking investigations are performed in InGaAs/InAs/GaAs quantum dot (QD) lasers. Monolithic mode-locked lasers are fabricated using QD material systems grown by MOCVD and MBE techniques and emitting at 1.1μm and 1.3μm, respectively. The mode-locking performance is evaluated using a variety of laser designs, with various ridge waveguide geometries, cavity and absorber lengths. Passive and hybrid mode-locking are studied and compared in 3.9mm long devices emitting at 1.1μm and operating at a repetition rate of 10GHz. Using 2.1mm long devices emitting at 1.3μm, 18GHz passive mode locking with 10ps Fourier transform limited pulses is demonstrated. This confirms the potential of quantum dot laser for low chirp, short optical pulse generation. Preliminary investigation of the timing jitter of QD passively mode-locked lasers and the behaviour of the QD absorber are also presented. Finally, we report 36GHz passive mode-locking with 6ps optical pulse obtained using 1.1mm long QD lasers emitting at 1.3μm.
In this work we present a detailed study of picosecond optical pulse generation using high-repetition rate mode-locked quantum dot lasers. MOCVD-grown quantum dot lasers emitting at 1.1μm and MBE-grown quantum dot lasers emitting at 1.3μm are investigated. Passive mode-locking at 10GHz, 18GHz and 36GHz with pulse widths in the 6-12ps range are reported. Hybrid mode-locking is demonstrated at 10GHz, showing a significant improvement in the RF spectral characteristics when compared with passive mode-locking. A timing jitter of 600fs (2.5MHz to 50MHz) is measured in the 18GHz passively mode-locked laser. Autocorrelation techniques are used to characterise the high repetition rate mode-locked lasers as well as the time-bandwidth product of the optical pulses. Fourier-transform
limited pulses are obtained from passively mode-locked QD lasers.
Universal self-organisation on surfaces of semiconductors upon deposition of a few non-lattice-matched monolayers using MOCVD or MBE lead to the formation of quantum dots. Their electronic and optical properties are closer to those of atoms than of solids.
We have demonstrated for QD-lasers a record low transparency current density of 6A/cm<sup>2</sup> per dot layer at 1.16 μm, high-power of 12W, an internal quantum efficiency of 98%, and an internal loss below 1.5 cm-1. Relaxation oscillations indicate the potential for cut-off frequencies larger than 10 GHz.
GaAs-based QD-lasers emitting at 1.3 μm exhibit output power of 5 W and single transverse mode operation up to 300 mW. At 1.5 μm again an output power of 5 W has been obtained for first devices showing a transparency current of 700 A/cm<sup>2</sup>.
Single mode lasers at 1.16 and 1.3 μm show no beam filamentation, reduced M<sup>2</sup>, sensitivity to optical feedback by 30 db and α-parameter as compared to quantum well lasers.
Passive mode locking of 1.3 μm lasers up to 20 GHz is obtained.
Thus GaAs-lasers can now replace InP-based ones at least in the range up to 1.3 µm, probably up to 1.55 μm.
Threshold reduction and enhanced mode selectivity are demonstrated in pulsed GaN-based lasers upon the introduction of 5(lambda) /4 air/nitride Bragg gratings defined by focused ion beam (FIB) etching. A 13% reduction in threshold current is obtained from a laser with a 5 micrometers wide ridge by introducing a deep-etch air/nitride mirror. The presence of a reduced-depth Bragg grating, etched across 4 micrometers wide ridge structure using a lower FIB dose, results in single-peak spectral characteristics for currents up to 1.14(DOT)I<SUB>Th</SUB>. The introduction of the Bragg mirrors always results in a broadening of the near field parallel to the epitaxial planes.
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.
The atom-dielectric surface interaction in presence of light has been experimentally investigated. A high rate atomic desorption effect induced by light is presented and the most recent experimental results with rubidium and cesium on silane coated surfaces are discussed. The realization of an atomic source controlled by light is presented.
Manual design of membership functions and rule bases for fuzzy systems often produces non optimal controllers, both in terms of performance and rule-base complexity. Even algorithms for automatic generation of these two components do generally miss their simultaneous optimal determination, therefore producing fuzzy system with lower performance. This paper addresses the use of a genetic algorithm for the optimization of a working fuzzy controller through the simultaneous tuning of membership functions and fuzzy rules. The parameter coding used by the method does allow the fine tuning of membership functions and, a the same time, the simplification of the rule base by identifying the necessary rules and by selecting the relevant inputs for each of them. Results obtained by applying the method to a fuzzy controller implementing the wall-following task for a real mobile robot are shown and compared, both in terms of performance and rule base complexity, with those provided by the original non-optimized version.
Electronic emission bands of In2 are generated (a) in a microwave discharge in a quartz
cell containing the metal plus a low pressure of Ar or Ne and (b) by radiative combination of In
atoms excited to the 6SS1fl state with In atoms in the ground (52P1) state. In (a) a band
progression to high energies was observed commencing at 25 730 cm1 which can be fit by
moelcular constants of ue 105.3 cm1, COXe 26.2 cm1, and De 212 cm1. In (b) strong,
unresolved, overlaped band features were observed in the 380 nm wavelength region (25 500
cm1 to 27 000 cm ) and a weaker, broad feature at 375 cm1 (28 300 cm1 to 30 000 cm1).
This work illustrates the temporal analysis we have done on the fluorescence signals originating from levels with
pecularly different characteristics populated either via Energy Pooling Collision (EPC) or via Energy Pooling Ionization
(EPI). The fluorescence from the first excited level (S ia) gives informations about the limited radiation trapping due the
fine structure of the ground state l/2 3/2 the ones from the 1 1P and 5D levels allow to discriminate between the
different population mechanism, EPC and EPI respectively. The cross sections for the two different processes may also be
Our most recent experiments on the light-induced drift effect are reviewed and discussed. Results are presented on the
isotope separation of Rubidium, for the first time experimentally demonstrated upon saturated vapor conditions. A new
manifestation of the light-induced drift, known as "white light induced drift", has been experimentally demonstrated by
using a laser with a particular cavity configuration and special-spectral characteristics. Finally the production of vapor
jets by light controlled diffusion is reported and its possible application discussed.