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Title: Wavelength shift by quantum well intermixing for semiconductor lasers used in optical communications
Other Titles: Ji yu liang zi jing hun he de guang tong xin ban dao ti ji guang qi de bo chang tiao xie yan jiu
Authors: Zhan, Li (詹黎)
Department: Dept. of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2004
Publisher: City University of Hong Kong
Subjects: Optical communications
Optoelectronic devices
Quantum wells
Semiconductor lasers
Notes: CityU Call Number: TA1700.Z43 2004
Includes bibliographical references
Thesis (Ph.D.)--City University of Hong Kong, 2004
[10], 182 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: As a cornerstone for the new information times, optical communication has become worldwide telecommunication networks. The semiconductor lasers have been playing a key role on the achievement of optical communications. In particular, monolithically integrated distributed feedback (DFB) laser arrays with different wavelengths are preferable for the applications in present DWDM communication systems, which requires light sources have precise emission wavelengths to match the wavelength channels. The quantum well (QW) intermixing is such a planar technique that can integrate regions of different band-gaps within the same wafer without etch and regrowth processes. However, as a large increase in the threshold current of QW lasers after QW intermixing limits the technique’s applications, it is becoming important for QW laser design to understand the threshold current changes with intermixing. Meanwhile, as a technique used in the fabrication of multiple-wavelength DFB laser arrays, it is of fundamental importance to maximize the wavelength shift range. Following these questions, in this thesis, we concentrate our interests on the analysis of the characteristics related to the wavelength shift and threshold currents in the intermixed QW lasers. After introducing the basic knowledge related to optoelectronic devices, we firstly analyze the effects of various interdiffusion processes on the Auger and radiative currents of InGaAsP/InP QW lasers by using self-consistent method, in which the electrostatic band deformation effect due to low confinement potentials is included. The results show the Auger recombination has a large contribution to the threshold current and the threshold current does not enormously increase with the QW intermixing. Our theoretical values of the threshold current agree with the experimental value obtained in Mckee et al. The result are important, as they indicates that photonic integrated circuits with interdiffused QW lasers do not require any high increase in injected currents. In the coming chapter, we study the temperature effects of the Auger and radiative recombination currents in intermixed InGaAsP/InP QW lasers. The results show the Auger recombination has a larger contribution to the threshold current with the increase of temperature. Especially, its percentage contributed to the threshold current increases more fast for a lower barrier QW structure in the group V interdiffusion. It is clarified that the low barrier is a more important factor to determine the temperature sensitivity of long wavelength lasers rather than the intrinsic sensitivity of the Auger coefficients. The results are important as they indicate that QW intermixed lasers can reduce the temperature sensitivity of the threshold currents using Group III atom interdiffusion. Finally, it is demonstrated that there is a low temperature sensitivity when low injection level in QW structure. This behavior is important for explaining the low temperature sensitivity in multi- quantum well lasers as the injection level is lower than in single QW structures. To understand the wavelength shift behavior in DFB lasers due to QW intermixing, we present a theoretical model for the detailed analysis of the limitation mechanisms of the shift range in the λ/4 shifted index-coupled DFB laser. For a large period grating studied in experiment, the wavelength shift range is limited to 4.2 nm. The predicted value of 0.46nm at 5.9 nm band-gap blue-shift agrees well with the experimental data obtained by T. K. Sodoh et al. The major contribution of our study is a new method proposed for maximizing DFB laser’s wavelength shift range. By using a smaller period grating, a wavelength red shift is obtained and the shift range is substantially increased to 22 nm, which is as high as 50% of the bandgap shift. This result shows that with a properly designed grating period, QW intermixing can be used to tune wavelength across a large number of channels in the fabrication of multi-wavelength DFB laser arrays for WDM communication systems. However, for λ/4-shifted index-coupled DFB lasers, more extra carriers need to be injected into active layer due to the gain decrease with the QW intermixing. It is unfavorable to design the equal threshold current lasers in a multi-wavelength laser array. To overcome this disadvantage, we study the characteristics of wavelength shift due to QW intermixing in a partly gain-coupled DFB laser. A 9.2nm maximum possible wavelength shift range is predicted for such a laser with a shorter period DFB grating. In contrast to the index-coupled λ/4-shifted DFB laser, the threshold carrier density decreases in the red-shift range due to the increase of the coupling coefficient with QW intermixing. No extra carriers need add into the active layer to compensate the decrease of gain with QW intermixing. It is very useful to design the equal threshold current DFB lasers with different wavelengths in the monolithically integrated arrays using this technique. The wavelength shift range by QW intermixing in our new method can be across a large number of channels of DWDM system. With such a large shift range, the intermixing technique not only can be employed to use in the wavelength error correction, but also be suitable for the fabrication of the multiple-wavelength laser arrays. If QW intermixing is used to fix the operation wavelengths in the array after fabrication, the DFB structures can then be fabricated using photolithograthy, which is more efficient and less expensive. Our studies may be useful in the development of multi-wavelength laser arrays used in the DWDM optical communication systems. Index terms: semiconductor laser, quantum well laser, quantum well intermixing, quantum well disordering, distributed feedback laser, wavelength shift, wavelength tuning, photonic integrated circuit, optical communication.
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