TAILIEUCHUNG - Advances in optical and photonic devices Part 5

Tham khảo tài liệu 'advances in optical and photonic devices part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Optical Injection-Locking of VCSELs 71 consequently encountered concerns thermal properties of InP-based materials that intervene to affect the process in following ways Shau et al. 2004 Piprek 2003 For the fabrication of long wavelength VCSELs there are mainly In1-xGaxAsyP1-y alloys available which have to be grown on InP substrates. Due to the effects of non negligible Auger s recombination effects and intra-valence band absorption these materials suffer from temperature-dependent losses. The thermal conductivity is greatly reduced due to alloy disorders which causes phonon scattering. This reduction in thermal conductivity is particularly adverse for effective heat sinking through the VCSELs DBRs usually having a thickness of several pms. AlAs-AlxGa1-xAs DBRs have a good thermal conductivity and could be thinner but due to lattice mismatch could not be grown on the InP substrate. DBR growth has been one of the fundamental problems regarding the fabrication of long wavelength VCSELs that has hampered the entry of VCSELs in high-speed data command and telecommunications domain. Optical and electrical confinement Growing stacks of DBRs was not the only problem encountered by VCSEL manufacturers. One of the primary objectives of VCSEL design was to fabricate short cavity single mode devices. The short cavity did eliminate the undesirable longitudinal modes but it gave birth to another unforeseen problem. Initial VCSEL designs suggested that the carriers and the photons share a common path traversing the DBRs. This led to the heating of certain zones of the DBRs due to carrier flow and resulted in a variable refractive index distribution inside the VCSEL optical cavity. This phenomenon is known as Thermal Lensing . Instead of being concentrated in the centre in the form of a single transverse mode the optical energy is repartitioned azimuthally inside the optical cavity. This particular optical energy distribution is observed in the form of transverse modes. .

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