Mechanical means for temperature compensation of planar diffractive optical interconnects: feasibility study

Abstract

Abstract. Planar diffractive optical interconnects have many advantages, however, their inherent chromaticity leads to temperature instability due to the wavelength shift of laser diodes' radiation. This shift can be compensated if the optical interconnect is bended in an appropriate direction with curvature proportional to the relative wavelength shift. The bending can be performed by attaching an additional plate to the element with a different thermal expansion coefficient. Theoretical analysis and ray tracing are reported. In the semiconductor industry, there is a continually aggravating problem of increasing communication volume. Optical interconnection technology is seen as a prime option for solving this problem.1,2 In planar optical interconnects ͑POI͒ light is injected ͑by an in-coupling optical element͒ into a transparent slab ͑light guide͒ at a total internal reflection angle ͑ in Fig. 1͒ and propagates along it until it meets an out-coupling optical element and goes to the detector. Diffractive lenses as coupling optical elements have many advantages [3][4][5] and have been suggested also in the context of quasi-optics 6 for rapidly expanding terahertz technology.7 However, their intrinsic dependence of optical properties on the wavelength-chromaticity-poses difficult problems.8 Chromaticity leads also to temperature instability. Namely, the wavelength of semiconductor lasers is temperature-dependent. Temperature change causes wavelength shift, therefore the laser beam as deflected by the diffractive in-coupling optical element deviates from its desired path and can miss the out-coupling element. For example, for a typical 850-nm vertical-cavity surfaceemitting lasers ͑VCSEL͒ source the wavelength shift is d / dT = 0.06 nm/ K, 9 i.e., relative wavelength shift is ͑1/͒͑d / dT͒ϳ7 ϫ 10 −5 K −1 . If POI was assembeled at 20°C, the working temperature is 140°C ͑Max3905 from Dallas Semiconductors, e.g.͒, and the POI length is 10 cm, the resulting beam deviation is about 1.7 mm ͑see below the calculation͒. This temperature instability is specific for diffracive POI since other thermal effects are much less: e.g., for fused silica the explicit refraction index temperature dependence is −3 ϫ 10 −6 K −1 . 10 The implicit temperature dependence ͑due to the refraction index wavelength dependence dn / d and the laser wavelength shift d / dT͒ is even smaller: dn / d =4ϫ 10 −5 nm −1 , 10 leading to dn / dT = ͑dn / d͒͑d / dT͒ = 2.4ϫ 10 −7 K −1 . We propose to fix this problem by bending POI in accordance to the temperature change. We suppose that POI and the laser have the same temperature since they are close to each other. A simple bending scheme is absolutely passive and implies attaching to POI a second plate, having different thermal expansion coefficient ͑in mass production, the attachment may be done by lamination technology 11 ͒. The curvature of this bending is proportional to the temperature change ͑as long as the thermal expansion can be considered as linear with temperature͒. As long as the laser wavelen...