Multiple half-wave filters can be detuned to provide a nonpolarizing edge at a specific angle of incidence. For the case of quarter-wave layers, an analytic expression for the necessary amount of detuning can be derived. For non-quarter-wave layers, a more empirical design approach is used. The resulting two-material producible designs improve current limitations in wavelength-division multiplex transmission over multimode optical fibers.

Nonpolarizing edge filters

Ohmer's algorithm for the complete exact synthesis of three-layer equivalent periods has been generalized for oblique incidence, and a theory was developed for periods with unsplit equivalent admittances, observing simultaneously the corresponding equivalent phase thicknesses. In a parallel development general conditions were deduced for the depolarization of outer media by quarterwave extensions of the central system. In combination, these building blocks enable the design of depolarized partial reflectors. A number of concrete designs are presented on various reflection levels.

Equivalent layers in oblique incidence: the problem of unsplit admittances and depolarization of partial reflectors.

Truly polarization-independent beam splitters for 0.6328- and 10.6-microm (He-Ne and CO(2)) laser radiation are designed using single-layer-coated (Cleartran) ZnS and Ge prisms. These devices are found to be reasonably achromatic, their reflectance (beam-splitting ratio) can be varied over a wide range with little accompanying polarization error, and they are tolerent to small film-thickness and film refractive-index errors.

Variable-reflectance thin-film polarization-independent beam splitters for 0.6328- and 10.6-microm laser light.

A new method of designing dielectric edge filters for wavelength-selective optical tapping elements based on the beam splitter concept relies on the analysis of the envelope of the reflectance for the beam components oscillating normal and parallel to the plane of incidence in the passband of the filter Despite the 45° angle of incidence of the beam, the strong polarizing effects usually encountered are almost entirely avoided for the two wavelengths that are to be demultiplexed A number of multilayer stack designs of the type S(H/2 L H/2) k S are described in which the ideal reflectance Rs = Rp = 0 is realized for the s and p components at the wavelength to be passed, while the desired value Rs≈Rp≈1 is realized for the wavelength to be rejected

Wavelength-selective Beam Splitters with Minimum Polarizing Effects for Wavelength-division Multiplexing in Optical Communication Systems

This paper gives solutions for multilayer dielectrics that yield polarization-independent operation at a given angle of incidence and at a given frequency. An exhaustive analysis is given for symmetric three-layer dielectrics surrounded by the same medium. Special solutions are also given for symmetric multilayer dielectrics with more than three layers surrounded by the same medium and for multilayer dielectrics surrounded by different media. Rules are presented for obtaining new polarization-independent solutions by cascading given solutions. Finally, for the purpose of demonstration and comparison of various solutions, different design examples are carried out for polarization-independent 3-dB beam splitters to operate at millimeter wavelengths.

Polarization-independent, multilayer dielectrics at oblique incidence

The spectral behaviour of the reflected and transmitted light from a dielectric multilayer film at an oblique angle of incidence is discussed. These spectra are different for both states of polarizations p and s even when the incoming light is unpolarized. The difference between both spectra of the two states of polarization is regarded as the polarization effect. This effect is investigated for three cases: all dielectric Fabry-Perot interference filters (ADI), Long and Short-wave Pass Filters (LWPF and SWPF) and multilayer antireflection coatings (AR). The conditions for avoiding or reducing these polarization effects in multilayer dielectric thin films of the above-mentioned cases are found and applied. Typical designs and calculated spectra are given and discussed.

K. Rabinovitch

Papers

Polarization Effects in Multilayer Dielectric Thin Films