Semiconductor ring lasers for all-optical signal processing

Since the late 1980s there has been a strong interest in exploiting optical bistablity for all-optical signal processing. In this scenario, a novel and promising building block is the semiconductor ring laser (SRL) that exhibits bistability between the counter-propagating cavity modes. This thesis reports on the design, fabrication and characterisation of 1550 nm lasing wavelength SRLs that are intended for applications as all-optical flip-flops and logic elements. Substantial optimisation of SRL design and processing technology is carried out in order to promote unidirectional bistable operation and allow high yield. Fabricated, large size, 150 um - 200 um radius SRLs, show robust unidirectional bistable operation with 30 - 35 dB directional extinction ratio (DER) between the counter-propagating modes, from near threshold up to 5 - 6 times threshold current bias. A significant advantage of the optimised technology is that 98% of the devices per chip show continuous wave (cw) and room temperature lasing with an average 2 - 3mA threshold current dispersion. Switch-on and switch-off times as short as 60 ps and 30 ps were measured, respectively, and reliable 10 Gbit/s flip-flop (FF) operation with external triggering optical pulses was achieved with these devices. Temporal measurements and calculations show that the switching speed of the free running SRL is limited by the carrier lifetime. A monostable device consisting of a SRL and an integrated distributed feedback laser (DFB) source is also presented, and this holding beam (HB) configuration is used to demonstrate all-optical NOT operation with data rates up to 2.5 Gbit/s. Dry etch chemistries for realizing 3.2 - 4.5 um deep waveguides, which show minimal bending losses, are developed and evaluated in order to enable dense integration of SRL devices. In addition, compact, milliwatt output power racetrack shaped cavity designs with radii as small as 10 um are presented. These devices exhibit minimal intra-cavity back-reflections by employing bi-level etching couplers and adiabatic straight to curved waveguide convertors. Finally, these developments provide a more than 150 times footprint reduction compared to large radius devices, whilst also preserving the robust unidirectional operation of their relatives with slightly lower, 20 - 30 dB DER

Excitability in optical systems close to Z2-symmetry

We report theoretically and experimentally on excitability in semiconductor ring lasers in order to reveal a mechanism of excitability, general for systems close to Z2-symmetry. The global shapes of the invariant manifolds of a saddle in the vicinity of a homoclinic loop determine the origin of excitability and the fea- tures of the excitable pulses. We show how to experimentally make a semiconductor ring laser excitable by breaking the Z2-symmetry in a controlled way. The experiments confirm the theoretical predictions.Comment: 4 pages, 4 figure

Optical gain in 1.3-μm electrically driven dilute nitride VCSOAs

We report the observation of room-temperature optical gain at 1.3 μm in electrically driven dilute nitride vertical cavity semiconductor optical amplifiers. The gain is calculated with respect to injected power for samples with and without a confinement aperture. At lower injected powers, a gain of almost 10 dB is observed in both samples. At injection powers over 5 nW, the gain is observed to decrease. For nearly all investigated power levels, the sample with confinement aperture gives slightly higher gain

All-optical toggle flip-flop based on monolithic semiconductor ring laser

A novel monolithic all-optical flip-flop of the toggle type with two complementary outputs is demonstrated based on a semiconductor ring laser (SRL), where the intrinsic directional bistability of the SRL device is combined with the saturable absorption effect that takes place in the reverse-biased input waveguides. Toggle flip-flop (TFF) operation is experimentally demonstrated using optical trigger toggle pulses of 500 ps duration. The achieved bit-rate of 500 Mb/s is the highest reported so far for all-optical TFFs. The operational speed is limited by the carrier recovery time of the saturable absorbers; however, the proposed technology has potential for multi-gigabits per second operation

Generation of picosecond pulses over a 40-nm wavelength range using an array of distributed Bragg grating mode-locked lasers

A set of distributed Bragg Reflector mode-locked semiconductor lasers are presented that generate picosecond pulses over a spectral range of 40 nm on a single chip. The same technology is used to realize an integrated laser with two separate Bragg gratings coupled into a common gain and saturable absorption section. This laser is shown to lock together the two spectral slices from each grating in a single pulse train

Revealing the effect of an industrial flash flood on vegetation area: A case study of Khusheh Mehr in Maragheh-Bonab Plain, Iran

Floods are one of the most significant environmental hazards that can have harmful effects on agricultural activities and vegetation. The primary objective of this study is to analyze the de- structive after-effects of an industrial flash flood on the vegetation of the Khusheh Mehr region. This flash flood occurred due to a break in the wastewater ponds of the Kaveh Soda factory on April 25, 2010. Using the Normalized Difference Water Index (NDWI) and the Normalized Differ- ence Vegetation Index (NDVI) derived from time-series Landsat data over the period of 2000–2020, we analyzed the changes in wastewater ponds and regional vegetation before and af- ter the wastewater flood occurred. We also investigate the quantitative and qualitative changes in vegetation during the past 20 years and detect the flood passage through this area. The results show that 538 ha out of the total 2123 ha of Khusheh Mehr were directly affected by the runoff resulting from the 2010 flood, and 1250000 m3 of industrial wastewater was discharged in the study area. The analysis of the vegetation in 2010 showed that during May and July, the effi- ciency and expansion of vegetation in these areas (NDVI >0.3) decreased by 90% and 55%, re- spectively, compared to the same periods in 2009 and 2006. The study is significant because it can help evaluate the effects of the flash flood not only on wastewater ponds and vegetation but also on the contamination of underground and surface waters as chemicals and industrial waste- water held in natural and artificial environments are discharged during a flood and directly affect vegetation. The findings can provide useful information for hazard and land use management and support policymakers, e.g., payment for financial loss of farmers

All-optical directional switching in bistable semiconductor-ring lasers

We investigate the operation of directionally bistable semiconductor-ring lasers as all-optical flip-flops. We demonstrate fast switching between the two lasing directions by injection of optical pulses acting as set and reset control signals with switching times as fast as 20 ps, delay times as short as 60 ps, and switching energy of 150 fJ