Self-assembled lateral aligned InAs quantum dot molecules (QDMs) with InxGa1-xAs strain-reducing layer are grown on
GaAs substrate by metal-organic chemical vapor deposition. The effects of growth temperature and In content of InxGa1-xAs on the structural and optical properties of QDMs are investigated by using atomic force microscopy and
photoluminescence. It is found that through appropriately selecting growth parameters, QDMs composed of two closely
spaced InAs QDs are formed, and a redshift of emission wavelength and wideband photoluminescence spectra of QDMs
are observed, which make QDM a potential candidate for broadband optical devices.
The optical gain of quantum-dot (QD) semiconductor optical amplifier (SOA) is usually seriously dependent on
polarization, we propose QD coupled to tensile-strained quantum-well (QW) structure to obtain polarization
insensitive QD SOA. A polarization-dependent coupled carrier rate equation model and multi-section SOA
model are used to study the performance of QD-coupled QW SOA. Gain dependence on polarization is studied,
TE gain contribution by QWs and QDs is compared, and carrier distribution competition among QWs and QDs
is analyzed. It is shown that polarization insensitive gain can be realized in a relatively wide range of
wavelength if QD-coupled tensile-strained QW SOA is properly designed.
Two-segment semiconductor optical amplifier (SOA) is proposed and studied to manipulate the gain and saturation
characteristics of SOA; this kind of SOA is separated into two segments that are electrically insulated from each other by
two split electrodes; the two segments share the same active region and other internal structure. By changing the
injection current density of the front segment or back segment, the saturation output power, bandwidth and gain
compression can be easily controlled. Therefore it is convenient for SOA to meet various application requirements in
optics communication.
We investigate the optical gain properties of InGaN quantum well with different symmetry barriers and asymmetry
barriers based on a self-consistent calculation which solves the Schrodinger equations and Poisson equations
simultaneously. It is found that the AlxInyGa1-x-yN barriers which can eliminate the internal polarized field by adjusting
the component x and y can improve the emission intensity in a large extent compared with other barriers. The internal
polarized field is an important but not the only one factor to affect the emission power, the barrier confinement, the
energy band are all have to be taken into considered. Otherwise, a quantum well which has proper asymmetry barriers
also can obtain better emission efficiency than the well with symmetry barriers.
We propose a new scheme of resonant-cavity (RC) based monolithic white LED, it relaxes the hard requirement of
high internal quantum efficiency of yellow multi-quantum (MQW) and offers an easy way to obtain high luminous
efficacy white light emission. In the proposed white LED, the blue MQW and yellow MQW active layer are embedded in
a resonant-cavity defined by the bottom distributed Bragg reflector(DBR) and top DBR. For a optimal design of
RC-based white LED, the extraction efficiency for yellow light is enhanced, while that for blue light is suppressed, thus
intensity ratio of yellow light in the emitting light is increased, which not only helps to obtain white emission in spite of
the low internal quantum efficiency of yellow light, but also doubles luminous efficacy. The color coordinates and
luminous flux of the emitting light from RC-based white LED are calculated and the performance dependence on
directionality is investigated.
Apart from gain dependence on polarization state, phase modulation in semiconductor optical amplifier (SOA) is usually polarization dependent as a result of birefringence. This paper analyzes the factors that determine the polarization dependence of phase modulation and proposes the necessity of fabricating SOA with polarization-insensitive gain and polarization-insensitive phase modulation. Based on energy band theory, the strain effects on polarization dependences of gain and differential refractive index in multiple quantum-well (MQW) SOA are studied, it is demonstrated that SOA with polarization insensitive gain and polarization insensitive phase modulation can be achieved by optimal design of active region and waveguide structure.
Cross modulations such as cross polarization modulation, cross gain modulation, cross phase modulation are import nonlinear effects in semiconductor optical amplifier (SOA). In this paper, the relationship among them is explored systematically; theoretical simulation is made based on a multi-section SOA model which takes into birefringence effect account, and the results agree well with experimental results. The linear relationship among cross gain modulation (in dB unit), cross phase modulation and cross polarization modulation is indicated. A more significant cross modulation effect is observed when pump wavelength is nearer to peak-gain wavelength region, while it becomes smaller when pump wavelength is more away from peak-gain region. No perceptible polarization state change of output probe beam is observed when the input polarization of pump beam is varied due to that the gain of the SOA is polarization insensitive.
Semiconductor white sources used for illumination have attracted much attention because of the theoretical high electro-optical efficiency and potential huge application market. However the practical electro-optical efficiency for the white light-emitting diode (LED) is far from the theoretical predict. In this paper we propose a novel white light superluminescent diode (WSLD), which has two active layers, and double-wavelength reflection filter films are coated on the facets of the chip. With this design we can get superluminescent light emission at two peak-gain wavelengths, that is, blue light and green-yellow light, thus producing white light. It is shown that WSLD has much higher electro-optical efficiency and better performance than ordinary white LEDs. This novel WSLD will be a new way to realize high-flux and high-efficiency semiconductor white light source.
In this paper, a comprehensive broad-band model of tunable wavelength converter based on four-wave mixing (FWM) in semiconductor fiber ring laser (SFRL) is presented. Critical factors, eg., the material gain profile, the longitudinal variation of the optical field, the carrier density and the broad-band spontaneous noise emission are considered in the model. By numerical simulation, the effects of the input signal power, injection current, the coupling of the output couplers and the lasing wavelength on the performances of the wavelength converter, such as the conversion efficiency and the SBR, are investigated. Simulation results are in agreement with the experimental results in literatures.
A novel technique is presented in obtaining the performance characteristics of traveling-wave semiconductor optical amplifier (TWA) with tensile-strained-barrier multiple-quantum-well structure. In-out fiber ends of TWA are used to construct an external cavity resonator to produce big ripple on amplified spontaneous emission (ASE) spectrum. By this means, Hakki-Paoli method is adopted to obtain gain and differential gain spectra over a wide spectral range. From measured longitudinal mode spacing and peak wavelength shift due to increased bias current, we further calculate the effective refractive index, carrier-induced refractive index change and linewidth enhancement factor. Some special features about ASE mode spectrum shift and refractive index change above lasing threshold are revealed and explained.
The longitudinal spatial hole burning (LSHB) in semiconductor optical amplifiers (SOA) is investigated using an improved wideband numerical model. The main new feature of the model is that it takes into account the current self-distribution effect, which induced by the axial variations of the separation between quise-Fermi levels in active region. The current self-distribution effect leads to the nonuniform current injection, and it tends to smoothen the carrier density distribution over the active region and reduces the strength of LSHB. It is found that the internal series resistance of SOA, which comes from mental-semiconductor Ohmic contacts, heterointerface and semiconductor bulk resist, significantly influences the strength of current self-distribution effect. The assumption of current injected uniformly gives rise to an overestimation of the strength of LSHB in SOA. The simulation results also show that the series resistance influences the gain and noise figure of SOA greatly. It is proposed that reducing the series resistance can enhance the small signal gain and reduced the noise figure of SOA efficiently.
Birefringence in semiconductor optical amplifier (SOA) has begun to receive increasing attentions in recent years, in this paper we investigate the effect of birefringence on spectral broadening of ultrashort optical pulse, based on Jones Matrix method, Fourier transform method and a multi-section model, which takes into account the polarization dependency of phase modulation. The time-dependent carrier distribution, pulse amplification process and pulse spectrum evolution along SOA is simulated. It is found that a considerable part of spectral broadening comes from birefringence, this kind of additional spectral broadening depends on bias current, optical power and pulse shape of input pulse.
The effects of birefringence on performances of SOA-based interferometer devices are analyzed theoretically, the necessity of fabricating SOA whose gain and induced phase shift are both independent on polarization state is proposed. Birefringence in SOA will degrade the performances of SOA-based devices due to polarization walkoff of the two split probe light. The calculations for a SOA-based Mach-Zehnder interferometer wavelength-converter
indicate that the extinction ratio will decrease and become very sensitive to the polarization state of input light if there is birefringence in SOAs. A scheme is proposed to eliminate these effects and the possibility of making a SOA polarization insensitive in gain and induce phase shift is analyzed theoretically.
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