Temperature sensitivity is an issue that severely affects many integrated silicon photonic devices. Proper circuit functionality is normally ensured by active thermal control at the expense of energy consumption. In some cases, athermal behavior can be achieved exploiting cladding materials with a negative thermo-optic coefficient to counterbalance the positive coefficients of silicon and silica. On the other hand, in echelle grating filters this method is not effective because in the slab free-propagation region the modal overlap with the cladding is small, especially for TEpolarized light. Moreover the need to add non-standard materials to the established silicon-on-insulator (SOI) fabrication process could make these solutions impractical. Here we present the design of a temperature-insensitive echelle grating demultiplexer with four channels operating in the TE polarization that does not use any materials with negative thermooptic coefficient and relies exclusively on standard processes for SOI photonics. The design exploits a temperaturesynchronized Mach-Zehnder interferometer as input to the echelle to compensate the shift of the imaged field with temperature. The device achieves a significant reduction in the temperature dependence of the overall transmission with a residual channel wavelength fluctuation smaller than 45 pm over a temperature range of 20 K, compared to a 1.6-nm shift for the same grating with a conventional waveguide input. The excess loss due to the use of the Mach-Zehnder input is no more than 0.7 dB for all four channels. Furthermore, the proposed design shows a very good tolerance to fabrication uncertainty, with minimum degradation of the performance for waveguide width variations of 10 nm.
A Fourier frequency filtering approach is applied to thin film design in situations well exceeding the theoretical limits of
the conventional Fourier transform method. Previous work is generalized to oblique incidence. Examples of a high-reflectance
filter and a broadband, wide-angle antireflection coating are given.
A parallel was recently established between an empirical procedure for the estimation of reflecting thin film thickness and new results derived from a Fourier Transform (FT) thin film synthesis technique. For simplicity the proposed FT approach was limited to a particular case. The approach is generalized in the present work and practical considerations are discussed. It is shown that good results are possible although the generalized problem is more complex from the FT point of view.
Narrow bandpass optical interference filters that precisely meet arbitrary Chebychev (equiripple) or maximally flat specifications are designed by refractive index refinement of standard multicavity filters, combined with straightforward manipulations of the solutions. The filters are composed mostly of quarter-wave (QW) layers of two materials. A few non-QW tuning layers are introduced in regions of the multilayer stacks that have a low sensitivity to deposition errors in order to control the passband ripple and halfwidth. Multiple solutions with exactly the same passbands are easily generated, from which those that have the best chance of being successfully fabricated can be selected.
Equiripple bandpass interference filters are designed by the refinement of a few layers of standard QW multicavity designs, and some simple manipulations of the solutions. Ways to control the intermediate refractive indices of the running layers, and/or their non-QW optical thickness, are described. Some limitations of earlier approaches are avoided.
A novel optical approach to predicting chemical and physical properties based on principal component analysis (PCA) is proposed and evaluated using a data set from earlier work. In our approach, a regression vector produced by PCA is designed into the structure of a set of paired optical filters. Light passing through the paired filters produces an analog detector signal directly proportional to the chemical/physical property for which the regression vector was designed. This simple optical computational method for predictive spectroscopy is evaluated in several ways, using the example data for numeric simulation. First, we evaluate the sensitivity of the method to various types of spectroscopy errors commonly encountered, and find the method to have the same susceptibilities toward error as standard methods. Second, we use propagation of errors to determine the effects of detector noise on the predictive power of the method, finding the optical computation approach to have a large multiplex advantage over conventional methods. Third, we use two different design approaches to the construction of the paired filter set for the example measurement to evaluate manufacturability, finding that adequate methods exist to design appropriate optical devices. Fourth, we numerically simulate the predictive errors introduced by design errors in the paired filters, finding that predictive errors are not increased over conventional methods. Fifth, we consider how the performance of the method is affected by light intensities that are not linearly related to chemical composition, and find that the method is only marginally affected. In summary, we conclude that many types of predictive measurements based upon use of regression vectors and linear mathematics can be performed more rapidly, more effectively, and at considerably lower cost by the proposed optical computation method than by traditional dispersive or interferometric instrumentation. Although our simulations have used Raman experimental data, the method is equally applicable to NIR, UV-Vis, IR, fluorescence and other spectroscopies.
A thin film synthesis technique based on the refinement of inhomogeneous systems is improved by the implementation of simultaneous refractive index and thickness optimization. Severe artifacts generated at oblique incidence by an earlier version of the method are eliminated. Complex designs comparable to published solutions synthesized with the Needle Method are demonstrated.
A new optical computation method for the monitoring of chemical reactions requires filters with spectral transmittance curves that vary in a complicated way with wavelength. In this paper we consider the design of two different sets of filters, one of which could be used to predict the degree of curing of a polymer from an analysis of its Raman spectra. The problem is not easy because the required filters have sharp spectral features in a narrow spectral region. Two different design methods are used. The performance of one set designed by conventional means is very close to the specifications. However, current thin film deposition methods are probably incapable of producing filters of such thickness. The second solution is based on the use of several filters placed in series. It should be possible to implement this particular solution, but its performance is not nearly as good. Nevertheless, calculations indicate that this filter pair should also result in a satisfactory control of the curing process.
A new, very simple AR coating synthesis method is further developed and compared to a recent `quasi-optimal' technique. A remarkable resemblance is found in the results. The reason for this similarity is given and the performance of both approaches is discussed.
A set of equivalent solutions to the synthesis of graded index optical coatings is obtained by a combustion of Fourier Transform techniques and special properties of the reflection coefficients in the plane of complex wavenumbers. A mathematical transformation changes the phase shift in reflection without affecting the reflectance, and the resulting refractive index profiles are calculated. Disposing of a set of solutions, one has then the possibility of selecting the most attractive for a practical implementation.
A powerful Fourier transform technique used at the NRC for the design of inhomogeneous and quasi-inhomogeneous dielectric optical coatings is reviewed. Its characteristic features and limitations are pointed out and illustrated by numerical examples. A challenging design and fabrication problem is presented with experimental results.
The NRCC Fourier transform synthesis method has been applied for the first
time to an optical thin film design problem in which the external media are different.
Graded index AR coatings for germanium substrates have been synthesized and
compared to multilayer solutions. No limits have been imposed on the refractive
indices. Although such systems cannot be put into practice, they provide basic
insight into the general nature of broadband AR coatings. Admittance diagrams have
been calculated for some of the systems to gain further understanding of their