Characteristic Of Slow Light In Vertical-Cavity Surface-Emitting Lasers

Recently, slow light has attracted lots of interest for its significant applications ranging from optical communication, signal processing, to phase-array antenna systems. Varieties of physical mechanisms for slow light have been demonstrated including electromagnetically induced transparency (EIT), coherent population oscillations (CPO), stimulated Brillouin scattering (SBS), vertical-cavity surface-emitting laser (VCSEL), and photonic-crystal waveguide. In particular, tunable slow light using the VCSEL is very attractive since it has the advantages of low power consumption, effective integration, working at room temperature, and low cost in production. We present a detailed analysis for the slow light based on the VCSELs in the thesis. Our caiculations and experimental measurements show that VCSELs are promising to achieve a controllable optical memory that is reasonably useful for future optical buffers and all-optial information processing systems.The thesis is structured as fllows. Section I discusses the physical mechanisms and appliciations for slow light in different materials, especially in VCSELs, and considers their fundamental physical limitations for group delay. Section II concentrates on the models of slow light in VCSELs which are developed using F-P cavity analytical method and transfer matrix method. Section Ⅲ considers the group delay, bandwidth and delay-bandwidth product of slow light inVCSELs. Section IV is devoted to the analysis of the saturation effect on the slow light using VCSELs. Section V analyses the new schemes of cascaded VCSELs to improve the bandwidth for slow light. Section VI presents a new coupled-cavity structure to realize slow light in VCSELs. Section Ⅶ summarizes the main conlusions of the thesis.The main contributions of this dissertation are listed as follows:(1) The models for slow light in VCSEL are developed by F-P cavity method and transfer matrix method. Based on the F-P cavity analysis, by combining the boundary conditions and the optical field distribution in VCSELs, the delay expression is derived. Then on the basis of the multilayer dielectric films theory, a model of VCSEL using transfer matrix equation is established. The response function of the VCSEL is obtained. The theoretical results show that the group delay is dependent on the DBR reflectivities and single-pass gain. To verify our calculations, a tunable slow light of2.5-Gbit/s Pseudo-Random Binary Sequence (PRBS) signal is demonstrated using a1550nm VCSEL. By tuning the bias current to the threshold, a tunable delay as large as98ps has been experimentally achieved.(2) It is theoretically and experimentally investigated the capability and limitations for slow light in VCSELs. An analytical expression for the delay-bandwidth product of slow light is derived, which reflects the dependence of the delay-bandwidth product on the DBR reflectivities and single-pass gain. The theoretical calculations show that the delay-bandwidth product has a maximum, which is only dependent on the front facet reflectivity. For a practical VCSEL with required front facet reflectivity exceeding90%, the resulting maximum DBP is close to a fixed value of0.64. This result has also been verified in the experiment. With increased biased current, the delay-bandwidth product rolls down quickly. The maximum delay-bandwidth product of0.62is obtained, which agrees well with the theoretical results.(3) The dependence of slow light on the saturation effect of VCSELs is theoretically and experimentally demonstrated. Conbining the transfer matrix method with the rate equation, the relationship between the signal power and the group delay is developed. After that, we experimentally show the variations of group delay and the eye diagrams of slow light with the increased signal power. The results indicate that the gain saturation leads to the decreased group delay, enhanced bandwidth, and the peak gain wavelength variation. By tuning the input signal to track the peak gain wavelength of the VCSEL, slow light of a power penalty as low as1dB is achieved.(4) A novel scheme of bandwidth improvement for slow light using cascade VCSELs is proposed and experimentally demonstrated. In such scheme, a proper adjustment to the gain peaks of VCSELs enables the generation of the desired composite gain spectrum which has flat-top gain and delay profiles with enhanced peak values. By using double cascade VCSELs in the experiment, a tunable group delay up to135ps for5-Gbit/s PRBS signal is demonstrated. Compared with the single VCSEL, the time delay is enlarged by35%, and the signal distortion is relatively lower.(5) A new structure of coupled-cavity VCSEL is proposed to broaden the bandwidths of gain and delay spectra. In this structure, the couple cavity is formed by constructing a passive cavity coupled with the active cavity of VCSEL. By rendering the strength of these two resonant cavities, the gain and delay bandwidth are largly increased by340%and by800%as compared with the signal-cavity VCSEL case. Measnwhile, the achieved spectra present more square-like profiles which are highly expected in slow light system. Utilizing this configurtation for slow light performance, a tuable delay about13ps for20-Gbit/s super Gaussian signal is achieved with very desirable signal quality. After that, the dependence of gain and delay on the coupled-cavity structures is further investigated. A new expression is proposed to evaluate the performances of slow light in different structures of coupled-cavity VCSELs.