Influence of Optical Feedback on Frequency Response of GaAs / AlGaAs Quantum Cascade Laser

A theoretical analysis of the optical feedback from external reflector influence on the Frequency response of GaAs/Al0.45Ga0.55As triple quantum well Quantum Cascade Laser emitting at (λ=9 μm) is presented. The employed theoretical model was based on Schrodinger equation. To demonstrate the effects of external cavity, many of the external cavity parameters were controlled such as the length of the external cavity, the proportion of optical feedback and the relative phase difference. 1Introduction Optical feedback is introduced into a diode laser by returning some portion of the optical output back into the device. The introduction of such feedback has been found to have dramatic and varied effects on the Influence of Optical Feedback on Frequency Response of GaAs/ AlGaAs ... ٥٧ operating characteristics of the solitary diode laser. Feedback can be disadvantageous, as it may cause unwanted instabilities in the laser output, or advantageous, as under certain conditions it can improve various features of the solitary laser, such as increasing the sidemode suppression ratio and narrowing the linewidth. As for the modulation characteristics of external cavity semiconductor laser (ECSL), many contributions, such as the modulation response of semiconductor laser with an external cavity. The influence of current modulation on the dynamics of semiconductor lasers with optical feedback is of interest for many applications. For example, high-frequency modulation of the injection current is often employed to reduce the relative intensity noise induced by optical feedback. It is well known that under current modulation, optical feedback, or optical injection, semiconductor lasers exhibit a rich variety of nonlinear behavior. The effects of optical feedback on the operating characteristics of a diode laser depend on several parameters. These include the level of the feedback in comparison to the diode laser output power, the relative phase of this feedback, the length of the external cavity, and the injection current of the solitary diode laser. It is found that there are five distinct regimes that are defined by the level of the feedback power ratio. In general, the boundaries between the regimes also depend on the internal parameters of the solitary diode laser, such as the linewidth enhancement factor, the diode dimensions, and the facet coatings. The quantum cascade laser (QCL) is an excellent example of how quantum engineering can be used to design new laser materials and related light sources. It is a unipolar semiconductor laser and based on intersubband transitions between excited states of coupled quantum wells and on resonant tunneling as the pumping mechanism. The population inversion between the states of the laser transition is achieved by engineering the electron intersubband scattering times. In contrast to all other laser sources, the wavelength is entirely determined by quantum confinement in coupled quantum wells rather than by the chemical properties of the material. Therefore, the wavelength can be tailored over a very wide spectral range using the same heterostructure material. As a result, the emitted photon energy is determined by the thicknesses of the wells and barriers and can be tailored by band-gap engineering. Laser wavelengths ranging from 4 to 11 μm have been demonstrated in the two material systems GaAs/AlGaAs and InGaAs/InAlAs. This important spectral range has so far been accessible mainly with relatively unreliable and expensive lead salt based diode lasers. Applications include gas sensors for pollution monitoring and industrial process control for environmentally safe manufacturing because many hazardous and toxic chemicals have optical absorption fingerprints at these Dr. Ragheed M. Ibrahim & Dr. Erada A. Al-Dabbagh ٥٨ wavelengths. There are also several transmittance windows of the atmosphere in this spectral range which allow for laser communication between earth and satellites as well as for distance measurements in avionic. In this article we will study the modulation response in GaAs/Al0.45Ga0.55 As material quantum cascade laser with wavelength equal (9 μm), with optical feedback from external reflector, and taken into account the influence of many parameters such as external cavity length, strength of optical feedback and relative phase of this feedback. 2Theory Under lasing conditions, the diode cavity is filled with gain medium, which, to a large extent, compensates for the diode cavity loss. It, therefore, has substantially greater effective quality factor, and consequently, greater influence on the laser behaviors, than the passive external cavity. For this reason, the following form of field equation has been adopted for a compound cavity laser configuration, obtained by adding an external feedback term to a standard laser equation in complex form; that is, ) 1 ( e τ) E(t k e E(t) ) Γ (G(n) 2 1 (n) iω e E(t) dt d τ) (t Ω i A t Ω i 0 N t Ω i − + ⎭ ⎬ ⎫