Fiber-optic intensity sensors (FOS) based on multimode1 (MM) and single mode2 (SM) fibers need a self-referencing method to minimize the influences of long-term aging of source characteristics, as well as short-term fluctuations in optical power loss in the leads to and from the transducer. Time division, wavelength normalization,1 3 and frequency-based self-referencing methods4 5 based on MM fibers, and a Michelson topology with SM fibers6 have been reported.
In this letter, we propose a novel frequency-based approach using a ring resonator (RR), with an improved sensitivity. Its principle and properties are discussed and tested.
The new sensing scheme is a RR and a FOS (see Fig. 1). The RR operates under an incoherent regime, so τ≫Tc, where Tc is the source coherence time and τ is the loop transit time. The RR relative output power, P3/P1 , is given by:g=(1−γ), m is the measurand, F(m) is the FOS calibration curve, γ and K are the coupler excess loss and coupling coefficient ω is the modulating signal, pulsation α is the fiber attenuation coefficient in dB/km, A is an attenuation, and L is the loop length. The FOS modulates the RR loss, H, and the output power frequency P3/P1 , see inset of Fig. 1 for K∈(0−0.5); there is a constant maximum if cos(ωτ)=+l , and a dependent on H minimum if cos(ωτ)=−l . The frequency normalization method is based on the sinusoidal modulation of the optical power source at two frequencies f1 and f2, as seen in Figs. 1 and 2. In this method, the measurement parameter is RM1 : (f1), a coupler inside the RR for measuring P4, and two down leads under identical external conditions; and the measurement parameter is RM2 : SF=δF/δm the FOS sensitivity, k1 is a constant and i=1, 2 for the frequency and two ports normalization method, respectively. This system sensibility is enhanced by SMi . If f1 is the resonance frequency, SM1 is given by: SM1 tends to ∞ if H→H0=K/(1–2K), The presence of noise limits the real value of the sensitivity. SM1 is plotted at Fig. 2, for a f1 frequency of 1,302 MHz, in a RR with a loop length of 1067 m. There is an inflection point for every K at the H0 value. For every quiescent point, a certain K can be selected for achieving high sensitivities. SM2 behaves quite similar to SM1.
The experimental setup is made of a LD of 1.5 μm, with 5 MHz linewidth, internally modulated with a signal coming from the tracking generator of a RF spectrum analyzer. The sensing scheme (see Fig. 1) is made of a polarization maintaining 2×2 variable ratio fiber coupler with pigtails of 1 m, 1067 m of standard SM fiber, and an attenuator simulating the FOS. f1 is 1.302 MHz, f2 is 1.207 MHz, and K=0.22. The calibration curves, for both self-referencing methods, are reported in Fig. 3. There is a great agreement between theory and measurements, and the system reveals good sensitivity compared to other topologies;5 even though f1 is not in the resonance frequency. Measurements variations, around 4, could be improved using a low coherence source in order to decrease the source induced noise.
Two different self-referencing methods for intensity fiber-optic sensors are described and their sensitivities are theoretically analyzed. The proposed scheme, using RR operating under incoherent regime, is flexible because the operation point and sensitivity is controlled by a coupling coefficient. Experimental calibration curves are reported validating the utility of the model developed. This configuration has a better sensitivity to other topologies.
We wish to thank J. M. Sa´nchez-Pena and S. Vargas. This work was supported by Comision Interministerial de Ciencia y Tecnologı´a (TIC2003-03783).