Enhanced temperature sensing with liquid-crystal filling

The wide use of automated control and actuation systems, the adoption of new technological processes, and conversion to flexible manufacturing systems have stimulated the development of sensing and measurement devices. Sensors are also important elements of fire-alarm and disaster-protection systems. As well as high metrological (measurement) performance, sensors must offer reliability, durability, stability, light weight, compact size, low power consumption, and compatibility with microelectronic information processing devices. Fiber and integrated optical devices are those best able to fulfill all these requirements.¹ Waveguide devices, in particular, have the capability to measure small changes in refractive index, and hence the physical quantities that cause those changes.

An interesting design for sensing purposes is the slot waveguide, which consists of two strips of a high-refractive-index material separated by a subwavelength-scale region of low-refractive-index material (the slot). The electric field discontinuity that arises at the interface between high-index-contrast materials results in high optical intensity concentration in the low-index slot region.² This ability of a slot waveguide to strongly confine light, combined with the properties of a micro-ring resonator (an optical waveguide in a closed loop having micron-scale radius), has shown great potential for optical sensing applications.

As an improvement on this design, waveguides with a double-slot arrangement have also been proposed in recent years. Micro-ring resonators based on such double-slot waveguides have been shown to tolerate larger fabrication errors than their single-slot counterparts.³ Moreover, sensitivity can be enhanced by filling the waveguide slot with a substance whose refractive index is responsive to the parameter being measured. In the case of temperature sensing, we have found liquid crystal (LC) to be a useful and interesting filling material because—as well as depending on the molecular constituents and wavelength—its refractive index varies considerably in response to operating temperature changes.⁴

To pursue this line of work we theoretically investigated the double-slot waveguide micro-ring resonator illustrated in Figure 1 for temperature sensing.⁵ The arrangement features a resonator ring with a bend radius in the order of tens of microns. The slots of the ring waveguide are filled with LC, which is also used as the surrounding cladding material. Optical signals are input and output via straight optical waveguides coupled with the ring resonator waveguide through evanescent fields.

Figure 1. Structure of a double-slot waveguide micro-ring resonator showing the waveguide mode transverse electric field distribution (black curve). Liquid crystal (LC) fills the ring waveguide slots.

The micro-ring resonator is characterized by a set of resonance wavelengths. An input signal whose wavelength coincides with one of the resonance wavelengths couples into the ring waveguide and passes through it to the output. Any variation in the resonator's optical length L·n_eff, where L is the geometric resonator length and n_eff is the waveguide mode effective index, leads to a shift in its resonance wavelength. This in turn alters the intensity of the output signal on the carrier wavelength coinciding with the resonance wavelength of the unperturbed resonator.

The sensing principle of operation is that temperature-induced changes in the refractive index of the LC filling will alter the properties of the slot waveguide and hence the resonance conditions of the microresonator, yielding an optical output highly sensitive to temperature. Figure 2 compares the temperature sensitivity of a micro-ring resonator based on a strip waveguide (without LC filling) against that of micro-ring resonators based on slot and double-slot waveguides (with LC filling). These curves show that the sensitivity of a sensor based on an LC-filled double-slot waveguide is about five times higher than that of one based on a conventional strip waveguide without LC filling. In addition, the proposed sensor could employ two orthogonally polarized waves to simultaneously obtain two separate temperature-dependent outputs. Averaging those two outputs would increase the measurement accuracy. Finally, the operating speed of such a sensor is determined by the time needed to establish a steady-state regime in the micro-ring resonator, which is in the order of tens of nanoseconds.

Figure 2. Variations with temperature of the electrical current output (I_out) of a photodetector on the output of micro-ring resonators with LC cladding and 32μm bend radius based on conventional strip waveguide without LC filling (curve 1), LC-filled slot waveguide (curve 2), and LC-filled double-slot waveguide (curve 3).

In summary, our work shows that a temperature sensor based on a slot-waveguide micro-ring resonator with LC filling can achieve fast response, high accuracy, and greatly improved sensitivity. Moving forward, we envisage constructing a measuring device composed of multiple such sensing elements, capable of simultaneously monitoring the temperature at different points within a fluid flow or substance volume. We also propose to develop an implementation of this sensing principle for non-contact measurement of electric fields.

The authors wish to thank the European Cooperation in Science and Technology (COST) Action MP0702 project Toward Functional Sub-Wavelength Photonic Structures for the stimulating exchanges of ideas.