Distributed Bragg reflector

A distributed Bragg reflector (DBR) is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with varying refractive index, or by periodic variation of some characteristic (such as height) of a dielectric waveguide, resulting in periodic variation in the effective refractive index in the guide. Each layer boundary causes a partial reflection of an optical wave. For waves whose vacuum wavelength is close to four times the optical thickness of the layers, the many reflections combine with constructive interference, and the layers act as a high-quality reflector. The range of wavelengths that are reflected is called the photonic stopband. Within this range of wavelengths, light is 'forbidden' to propagate in the structure. A distributed Bragg reflector (DBR) is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with varying refractive index, or by periodic variation of some characteristic (such as height) of a dielectric waveguide, resulting in periodic variation in the effective refractive index in the guide. Each layer boundary causes a partial reflection of an optical wave. For waves whose vacuum wavelength is close to four times the optical thickness of the layers, the many reflections combine with constructive interference, and the layers act as a high-quality reflector. The range of wavelengths that are reflected is called the photonic stopband. Within this range of wavelengths, light is 'forbidden' to propagate in the structure. The DBR's reflectivity, R {displaystyle R} , for intensity is approximately given by where n o ,   n 1 ,   n 2 {displaystyle n_{o}, n_{1}, n_{2}} and n s {displaystyle n_{s},} are the respective refractive indices of the originating medium, the two alternating materials, and the terminating medium (i.e. backing or substrate); and N {displaystyle N} is the number of repeated pairs of low/high refractive index material. The frequency bandwidth Δ f 0 {displaystyle Delta f_{0}} of the photonic stopband can be calculated by where f o {displaystyle f_{o}} is the central frequency of the band. This configuration gives the largest possible ratio Δ f 0 f 0 {displaystyle {frac {Delta f_{0}}{f_{0}}}} that can be achieved with these two values of the refractive index. Increasing the number of pairs in a DBR increases the mirror reflectivity and increasing the refractive index contrast between the materials in the Bragg pairs increases both the reflectivity and the bandwidth. A common choice of materials for the stack is titanium dioxide (n≈2.5) and silica (n≈1.5). Substituting into the formula above gives a bandwidth of about 200 nm for 630 nm light. Distributed Bragg reflectors are critical components in vertical cavity surface emitting lasers and other types of narrow-linewidth laser diodes such as distributed feedback (DFB) lasers and distributed bragg reflector (DBR) lasers. They are also used to form the cavity resonator (or optical cavity) in fiber lasers and free electron lasers. This section discusses the interaction of transverse electric (TE)and transverse magnetic (TM) polarized light with the DBR structure, over severalwavelengths and incidence angles. This reflectivity of the DBR structure (described below)was calculated using the transfer-matrix method (TMM), wherethe TE mode alone is highly reflected by this stack, while the TM modes are passed through. This also shows the DBR acting as a polarizer.