Sensor properties of planar waveguide structures with grating couplers

Abstract

The paper presents the results of theoretical analysis as well as the results of experimental research involving planar sensor structures with input grating couplers of the period Λ = 800 nm. In the theoretical part of the paper we discussed the influence of the parameters of a sensor structure on it sensitivities. The experimental part of the work presents the results of experimental research involving the influence of refractive index of the cover on the coupling characteristics of sensor structures with grating couplers. The full widths at half maximum (FWHM) were from 0.023° to 0.029°. For the investigated structures we estimated detection thresholds for the changes of refractive index of the cover and the changes of sensitive film thickness. It has been demonstrated that by the application of the elaborated structures we can detect minimal changes of the refractive index (Δnc)min = 2.1×10−6 when the refractive index of the cover nc = 1.333 and (Δnc)min = 1.0×10−6 when nc = 1515. For sensitive films of the thickness w < 100 nm, by using the elaborated structures, we can detect mean changes of the thickness along the values lower than 10−3 nm.

Optical uniform/gradient waveguide sensor structure - characterization

“…In Ref. 21, for the input grating cou− pler of the period L = 800 nm, the author estimated the detection limit of (Dn c ) min = 2.1´10 -6 [21]. In Ref.…”

“…The fundamentals of this method are widely discussed in Refs. 29 and 30 and the equations characteristic for the asymmetric structure were provided by the author in the earlier work [21]. It was demonstrated in Refs.…”

“…The influence of the parameters of a uniform sensor structure on homogeneous sensitivity is extensively discussed in Ref. 21. Figure 4 presents the calculated characteristics of homo− geneous sensitivity, corresponding to different cases of dif− ference interference.…”

“…The said effects bring about the change of effective refrac− tive indexes of modes propagating in the structure. The changes of effective refractive indexes can be measured by the application of the interferometers of Mach−Zehnder [5][6][7], Young [8,9], Michelson [10] and with a polarimeter [2,11,12] or grating couplers [3,[13][14][15][16][17][18][19][20][21][22]. Due to technologi− cal constraints (the interaction length is limited to single centimetres), the operating range in PEWS is most fre− quently limited to a single interference fringe.…”

Embossable grating couplers for planar evanescent wave sensors

“…Consequently, a part of the power of the guided mode is rising which is propagating in the cover, and the mode is becoming more sensitive to the changes of the refractive index n c and the homogeneous sensitivity ( ¶N/ ¶n c ) is rising. When n c > n b , then the power propagating in the waveguide's cover is higher than the power propagating in the substrate [31]. Such waveguides are referred to as reverse−symmetry wave− guides [32].…”