Fiber Optic Tutorials
Light-Emitting Diodes (LEDs)
This is a continuation from the previous tutorial - lateral structures of semiconductor junctions. LEDs are simple, but important, solid-state light sources that have a wide range of applications. LEDs that emit light in the visible spectral region are widely used in displays and in fiber-optic illumination. Infrared LEDs are useful for fiber-optic communications in those systems where the coherence, high power, and high speed of semiconductor lasers are not needed. Recent breakthroughs have resulted in LEDs of very high performance, in terms of efficiency and brightness, and have extended the spectral range of these high-brightness LEDs to the...
Lateral Structures of Semiconductor Junctions
This is a continuation from the previous tutorial - semiconductor junction structures. The junction structure of a device determines the carrier and optical field distributions in the vertical direction perpendicular to the junction plane of a device. The carrier and optical field distributions in the transverse directions parallel to the junction plane are determined by the lateral structure. The lateral structure of a surface-emitting device can have either a broad active area, formed with little or no lateral restrictions on the injected current, or a small active area, formed by restricting the current flow into a confined area, as...
Semiconductor Junction Structures
This is a continuation from the previous tutorial - spontaneous emission in semiconductors. A semiconductor junction device can have either a homostructure or a heterostructure. A basic homostructure simply consists of a p-n homojunction. There are a number of different heterostructures, but the two basic concepts are the single heterostructure (SH), which consists of a single heterojunction, and the double heterostructure (DH), which consists of two heterojunctions. When the layer between the junctions of a DH is thin enough, the structure becomes a quantum well (QW) because of the quantum size effect in the thin layer. An electrically pumped...
Spontaneous Emission in Semiconductors
This is a continuation from the previous tutorial - optical gain in semiconductors. The spontaneous emission spectrum of a semiconductor can be explicitly related to the absorption and gain spectra of the semiconductor. By using (13-32) [refer to the optical gain in semiconductors tutorial] to eliminate \(\rho(\nu)\) in (13-28) [refer to the band-to-band optical transitions in semiconductors tutorial], we find \[\tag{13-41}R_\text{sp}(\nu)=\frac{8\pi{n}^2\nu^2}{c^2}\alpha_0(\nu)f_\text{c}(E_2)[1-f_\text{v}(E_1)]\] Using (13-33), (13-34) [refer to the optical gain in semiconductors tutorial], and (13-41), we can express the spontaneous emission spectrum \(R_\text{sp}(\nu)\) in terms of the absorption spectrum \(\alpha(\nu)\) and the gain spectrum \(g(\nu)\) as follows: \[\tag{13-42}R_\text{sp}(\nu)=\frac{8\pi{n}^2\nu^2}{c^2}\frac{\alpha(\nu)}{\text{e}^{(h\nu-\Delta{E}_\text{F})/k_\text{B}T}-1}=\frac{8\pi{n}^2\nu^2}{c^2}\frac{g(\nu)}{1-\text{e}^{(h\nu-\Delta{E}_\text{F})/k_\text{B}T}}\] In the...
Optical Gain in Semiconductors
This is a continuation from the previous tutorial - band-to-band optical transitions in semiconductors. By following a line of reasoning similar to that used in the optical absorption and amplification tutorial while associating \(R_\text{a}(\nu)\) with \(N_1W_{12}(\nu)\) and \(R_\text{e}(\nu)\) with \(N_2W_{21}(\nu)\), we can write down the absorption and gain coefficients contributed by direct band-to-band transitions in a semiconductor as \[\tag{13-30}\alpha(\nu)=\frac{h\nu}{I(\nu)}[R_\text{a}(\nu)-R_\text{e}(\nu)]=\frac{c^2}{8\pi{n}^2\nu^2\tau_\text{sp}}[f_\text{v}(E_1)-f_\text{c}(E_2)]\rho(\nu)\] and \[\tag{13-31}g(\nu)=\frac{h\nu}{I(\nu)}[R_\text{e}(\nu)-R_\text{a}(\nu)]=\frac{c^2}{8\pi{n}^2\nu^2\tau_\text{sp}}[f_\text{c}(E_2)-f_\text{v}(E_1)]\rho(\nu)\] respectively. By definition, \(g(\nu)=-\alpha(\nu)\). The relations in (13-30) and (13-31) are valid for carriers in either an equilibrium state or a quasi-equilibrium state because their validity follows from that of the relations in (13-26) and (13-27) [refer to...