A detailed analysis of the efficiency of four-wave mixing (FWM) is performed for a closely spaced energy state quantum dash semiconductor optical amplifier (SOA). The analysis takes into account the effect of all discrete energy states, the gain dispersion of the quantum dash, and the effect of doping on the FWM characteristics. Our analysis reveals that large FWM efficiency can be obtained when the energy of the pump signal is equal to the first excited state. We find that at low-applied current doping the dashes by p-type concentration enhances the FWM efficiency. While at high applied current, we find that doping the SOA with p-type concentration slightly enhances the FWM efficiency of the ground state and the first excited state and lowers the FWM efficiency of the higher excited states. Also we find that doping the dashes by n-type concentration slightly modifies the FWM efficiency spectrum.
Since image watermarking has become an important tool for intellectual property protection and authentication, a wavelet-based watermarking scheme for color images is proposed. The watermarking scheme is based on implementing the generalized squared interpoint distance (GSID) in designing a weighting function to be used in a spread-spectrum fashion watermarking (SSW) and is denoted as SSW-GSID. Watermark embedding process is carried out by transforming the host image into the wavelet domain. For highly robust and imperceptible embedding, watermark bits are added to the discrete wavelet transform (DWT) coefficients of all subbands after being weighted using the product of the calculated GSID values (of the host image) and the basis function amplitudes of the DWT decomposition. Experimental results have shown that SSW-GSID exhibits highly reliable extraction of the watermark from attacked images. Based on the provided experimental results, it can be observed that the proposed SSW-GSID method is robust against a wide variety of attacks. Comparison with other existing methods shows the superiority of the proposed SSW-GSID method.
The small-signal parameters of quantum dash (QDash) lasers with multiple coupled energy states have been derived using a rate equation model. Analytical expressions for the small-signal differential gain, resonance frequency, and recombination lifetime have been derived. The linewidth enhancement factor of QDash lasers with multiple coupled energy states is studied using our derived model. With the help of our model, we find that introducing p-type doping in the active region of the QDash layer does not enhance the small-signal resonant frequency, but reduces the small-signal recombination lifetime and the linewidth enhancement factor. Also, we find that increasing the n-type doping concentration decreases slightly the small-signal recombination lifetime and the resonant frequency, and increases slightly the linewidth enhancement factor. Our analysis reveals that an undoped QDash laser can be designed to operate at the second excited state energy and to yield high modulation bandwidth and low linewidth enhancement factor.
A detailed theoretical analysis of stability in a quantum dash distributed Bragg reflector (DBR) laser is presented under the small-signal condition. The influence of p-type doping and inhomogeneous line broadening on the hysteresis width of the quantum dash DBR laser is studied using a rate equation model that includes all of the multidiscrete energy levels in the valence and conduction bands. Our calculations show that a large hysteresis width is obtained by detuning the laser by ~10 meV above the ground state energy and doping the dashes by acceptor concentration NA=3.7×1017 cm−3. Also we find that a large self-pulsation frequency is obtained by detuning the laser by −15 meV from the ground state energy and doping the dashes by NA=2.5×1017 cm−3. The laser hysteresis width can be greatly reduced by doping the dashes with NA>1×1018 cm−3.
Omar Qasaimeh, Ferous Ganikhanov, E. Dean, Jong Yi, Lars Eng, R. Juluri, J. Stayt, S. Broutin, J. Johanson, Richard Bylsma, Leonard Ketelsen, John Johnson, David Ackerman, Mark Hybertsen, Scott Roycroft, G. Rao, T. Pinnington, Ciaran O'Cochlain
High power buried heterostructure 1.55micrometers tunable DBR lasers have been designed, fabricated and characterized. The laser consists of a gain section, a distributed Bragg reflector, a semiconductor optical amplifier and front photodetector for automated power control. The heterostructures were grown by MOCVD with the help of selective area growth techniques and dual waveguide heterostructure. Several advantages stem from this integration scheme which include simplicity of design and fabrication, increased reliability and low cost. The laser exhibits output power of 13dBm in the fiber and is tunable over 30(50GHz) ITU channels. The laser exhibits excellent performance and long-term control and reliability. The laser/transmitter also demonstrates significant increase of its functionality while its size remains small.
InP-based 1.6 micrometer microcavity light emitting diodes (MCLEDs) have been designed, fabricated and characterized. Oxide-confined MCLEDs with lateral aperture size down to 1 micrometer have been realized with enhanced output slope efficiency and excellent spectral and spatial properties. Both mirror-free cylindrical MCLEDs with oxide-aperture and 3D MCLEDs have been fabricated and a maximum output power of 0.03 mW at 30 mA for a 30 micrometer device has been realized. The effect of the aperture size on the slope efficiency and spatial properties have been systematically investigated, which shows an enhanced slope efficiency and narrower emission angle for small aperture size devices. The temperature dependent cavity properties have also been carefully examined for both the active region emission peak and cavity resonance peak.
It is possible to grow defect-free strained layers on patterned substrates (mesas or grooves) up to thicknesses far exceeding the critical thickness. Defect nucleation and propagation are inhibited in such growth. We have exploited this property to design and fabricate InP-based 1.55 micrometers vertical cavity surface emitting lasers. Careful photoluminescence and TEM studies have confirmed that there are no propagating defects in the GaAs/AlxGa1-xAs DBR grown on the patterned active region, or the MQW region. Lasers have been made with InP/InGaAsP bottom mirrors, laterally oxidized InAlAs current confining layers and GaAs/AlxOy top DBR mirrors. Lasers with 8 - 40 micrometers diameter have been characterized. A threshold current of 5 mA is observed at 15 degree(s)C for a 8 micrometers diameter device; and up to 60 (mu) W of light output is recorded.
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