Christian Otto. This thesis deals with the dynamics of state-of-the-art nanophotonic semiconductor structures, providing essential information on fundamental aspects of nonlinear dynamical systems on the one hand, and technological applications in modern telecommunication on the other. Three different complex laser structures are considered in detail: i a quantum-dot-based semiconductor laser under optical injection from a master laser, ii a quantum-dot laser with optical feedback from an external resonator, and iii a passively mode-locked quantum-well semiconductor laser with saturable absorber under optical feedback from an external resonator.
It seems that you're in Germany. We have a dedicated site for Germany. This thesis deals with the dynamics of state-of-the-art nanophotonic semiconductor structures, providing essential information on fundamental aspects of nonlinear dynamical systems on the one hand, and technological applications in modern telecommunication on the other.
In many different systems, by using intrinsic or hybrid optical circuits, the optical bistability was observed. If a system has two output states for the same value of input over some range of input values, the system is considered optically bistable. Under some operating conditions, the two optical cases originate from the stable-state and transient characteristics of the nonlinear optical system. That input and output relation is described by a multivalued function and has many stable and transient states in a nonlinear system which known as multistability [ 29 ].
In our experiment of chaotic instability, by using the configuration setup in Figure 2 , the result appears that the chaotic instability of optical feedback by optical feedback attenuation as a control parameter is monostability, as shown in Figure 9. There are three primary message encryption methods utilizing optical communications chaos [ 30 ], which will be explained in a next section.
Then, the message is directly added to the chaotic carrier, see Figure 10a. Chaotic shift keying CSK : the injection current of the transmitter laser is used to modulate the message directly.
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Therefore, the transmitter laser produces the chaotic carrier for a message concealed in it, as shown in Figure 10b. Chaotic modulation CMO : it involves adding the output power of the transmitter laser to the message. Therefore, this mixer of the signal and message is sent back to the transmitter laser through a feedback loop as a modulation to produce a chaotic carrier as shown in Figure 10c [ 30 ]. There are two kinds of chaos modulation such as direct current modulation and external modulation of the semiconductor laser [ 32 ]; in our experiment, the external modulation was used.
The external modulation in optical communication divides into two types.
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A first kind is dependent on the absorption modification of a semiconductor material when the external electric field is applied, which is known as an electro-absorption modulator, while the second kind relies on the refractive index variation observed in several crystals under an external electric field that is called an electro-optic modulator. Additionally, with the interferometry structure, like a Mach-Zehnder structure, there can be a modulation of the intensity of a light wave because a change in the refractive index itself does not allow modulation of the intensity of a light wave.
A Mach-Zehnder structure enables to convert the induced phase modulation into the desired intensity modulation. The effective band gap Eg of a semiconductor material decreases when an external voltage is applied; hence, this fact is important in this kind of modulation. Through duly selecting the wavelength signal so that it expertise a significant variation in the absorption when the voltage is applied, consequently it becomes possible to perfect optical modulation controlled via an electrical signal.
An ideal absorption against the function of applied voltage transfer for an electro-absorption modulator is shown in Figure Since the refractive index of a semiconductor material and the absorption is linked by Kramers-Kronig relations of the kind. Hence, via an electro-absorption modulator, some amount of frequency chirping will be introduced. The produced frequency chirp will usually be lower than when a semiconductor laser direct current modulation is used, as shown Figure 11 [ 32 ].
In this chapter, the experiment setup of semiconductor lasers and signal generators in order to satisfy chaos modulation and study the resonance phenomena is shown in Figure Then the results are analyzed by a personal computer with an origin program. The intrinsic dynamic resonance in the photon-carrier interaction determines the modulation response of semiconductor lasers.
We see that the external frequency in the pumping current is able to enhance the regularity of the drop-out time series emitted by the laser. Frequency in this experimental setup can also enhance the response of the laser to an external periodic drive as shown in Figures 13 — 16 which represent the chaos modulation.
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Coherence resonance is a manner in the chaotic dynamic, that is, regularity of the time between power dropouts for a given value of the feedback strength [ 33 ]. This phenomenon occurs by applying an external noise signal to the chaotic system [ 34 ], while the laser with optical feedback may display chaotic dynamics with fast pulses at the time scale of the time delay and much slower power drops [ 33 ]. In our experiment the coherence resonance phenomena appear when applied different frequencies.
The LFF was investigated for high-frequency injection-current modulation in a semiconductor laser with optical feedback. However, LFF appeared for the detained modulation frequency from an external cavity mode. This result agrees with [ 16 ]. The power spectrum of different frequencies is analyzed, observing a sharp peak, low spiking peak, and hidden frequency peak. Figure 17 shows the changes in applied power with increased frequency. Figure 18 shows the bifurcation diagram of the laser intensity as a function of the external modulation frequency, in accordance with [ 35 ].
In this figure, one observed that the dynamic is chaotic behavior when applied to different frequencies. Figure 19 shows the distribution of the bifurcation diagram.
Dynamics of quantum dot lasers subject to optical feedback and external optical injection
These regions are important in satisfying an encryption message in the investigations of secure communication. A laser is described theoretically by three variables: the electric field in the laser, the polarization of the laser medium, and the population inversion to induce the laser oscillation [ 25 ].
However, the dynamic characteristics must be described with time-dependent equations of the systems.
A laser is essentially a chaotic system; however, every laser does not show chaotic oscillations. According to the scales of the decay rates in the differential equations, lasers are classified into three categories. When we need all of three rate equations to describe a laser, the laser is a chaotic system and it is called a class C laser. Actually, infrared oscillating gas lasers like NH3 and Ne-Xe lasers exhibit chaotic oscillations in their output powers [ 25 ].
Pattern Dynamics in Nonlinear Optical Cavities
The second one is a class B laser, in which the time constant of the polarization is very fast and the polarization equation is adiabatically eliminated. Then, the laser is described by the two equations of field and population inversion, and it is a stable laser if there is no external perturbation. The third one is a class A laser, whose field equation is enough to describe the system, and it is the most stable class of lasers. The polarization term is adiabatically eliminated for class B semiconductor laser; this effect is replaced by a linear relation between polarization and the field.
Semiconductor lasers population inversion is replaced by density carriers N produced by a recombination of the electron hole. The absolute square of the field amplitude which is equivalent to the photon number and the density carriers are frequently used as the variant of an equation rate for semiconductor lasers [ 25 ].
The Lang-Kobayashi equations for the complex amplitude electric field E t and the carrier number N t are written as follows [ 38 ]:. The spontaneous emission processes are considered by introducing independent Gaussian noise sources X with zero mean and unity variance into the rate equation.
Related Dynamics of Quantum Dot Lasers: Effects of Optical Feedback and External Optical Injection
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