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Optical Design & Engineering

Getting together

By integrating mechanical, thermal, structural, and optical aspects of design, engineers can better meet overall performance objectives.

From oemagazine January 2003
31 January 2003, SPIE Newsroom. DOI: 10.1117/2.5200301.0006

The integration of optical-system design tools, including thermal, structural, and optical analyses, enables high-performance optical systems to be designed and built for optimum performance. Structural, thermal, and optical engineers typically work independently of each other and use unrelated tools, models, and methods. Integrated modeling has been enhanced by the development of interfacing software to achieve smarter and more efficient pathways between industry-standard tools. This allows engineers trained in their respective fields to continue operating with their preferred software tools and exchange data while making detailed design tradeoffs; for example, computing optical performance as a function of mechanical-mounting configurations and operational environments.

Figure 1. The plot of cross talk as a function of wavelength and temperature shows improved performance due to the optomechanical design.

The design of a lens element for a telecommunications wavelength selective switch (based on U.S. Patent No. 6,285,500) incorporates integrated modeling techniques. The device features two input fibers carrying 80 channels of data over the telecom C-band (1530 to 1561 nm) and two output fibers with a symmetric optical system on each side of a centered LCD. The data from each input fiber enters the optical system and is converted into s- and p-polarizations, respectively; then the wavelengths are superimposed by a series of optical components. A lens focuses each wavelength channel onto a given pixel of an LCD array. The LCD array dictates which output fiber a given wavelength exits by either converting the incident polarization into an orthogonal state (on-state) or allowing it to pass unchanged (off-state). The lens following the LCD recollimates the light, which passes through a series of optical elements and then into either one of two output fibers.

Functionality of the switch is dependent upon maintaining the intended polarization state throughout the optical system. Mechanical stress acting on the lens elements, caused by thermal expansion coefficient mismatch between the glasses and mounting materials, affects the state of polarization by creating birefringence, which causes cross-talk between the output optical fibers. Cross-talk design limits were set to -45 dB due to stress in the focusing and collimating lens elements over the operational temperature range of 0 to 70°C.

Methods to mount the lens elements within a mechanical cell include rigid epoxy, RTV, and tangential flexures. Cell materials considered were stainless steel, titanium, and aluminum. Three doublets were designed using Schott glass types to provide achromatized performance over the C-band, to minimize variations in focus position and effective focal length with temperature, and to have good physical properties (N-BaK1/N-SF4, N-BaK4/N-F2, and N-BaF51/SFL57). In each case, the composite rms wavefront error is less than 0.04 waves, giving diffraction-limited performance.

Integrating the design tools allowed an efficient means to compare the cross-talk as a function of the proposed cell materials, mounting configurations, and lens designs. Finite element analysis predicted the stress distribution in the lens elements as a function of temperature. This data was imported into optical design software using an optomechanical interface program. The best performing design was the N-BaF51/SFL57 doublet mounted in an aluminum cell using three discrete RTV pads (see figure).

In general, as in the above example, using interface software to integrate the thermal and structural models with the optical-design software allows optical performance metrics to be computed as a function of mechanical design variables, which allows design concepts to be chosen quickly and efficiently. In addition, integrating the analyses reduces the often-redundant safety margins by the individual disciplines, resulting in improved designs. oe

Keith Doyle
Keith Doyle is vice president of Sigmadyne Inc., Rochester, NY, and a senior systems engineer at Optical Research Associates, Westborough, MA. Doyle is the author of SPIE Tutorial Text TT58 - Integrated Optomechanical Analysis; he also teaches a short course by the same title (#SC254).