Correlation-based multiplexing of spectral channels and fiber-optic sensors using unmodulated continuous-wave distributed feedback diode lasers.

In this Letter, we demonstrate a method for the multiplexing of spectral channels and fiber-optic sensors. The method makes use of the correlation-based demultiplexing technique. In our approach, each light source has an inherent phase noise which we propose to use as a "fingerprint" for correlation-based recognition and demultiplexing of spectral channels as well as signals from different sensors in each spectral channel. The proposed method requires for its implementation an extremely simple and robust scheme and can be attractive for budget-sensitive applications. As active elements, it uses only two standard free-running CW distributed feedback (DFB) diode lasers. No modulators or optical filters are required for operation. Results of proof-of-concept experiments are presented for the interrogation of several ultra-weak fiber Bragg gratings (FBGs) with reflectivity of 0.05% in a 4-km-long fiber for temperature measurements with the resolution of 0.1°C.

Passive quadrature demodulation of multiplexed interferometric sensors using a CW correlation reflectometer with a single DFB diode laser.

In this Letter, we report a novel, to the best of our knowledge, and simple approach for passive quadrature-phase demodulation of relatively long multiplexed interferometers based on two-channel coherence correlation reflectometry. Two-wavelength channels are generated using a single unmodulated CW-DFB diode laser and an acousto-optic frequency shifter. The introduced frequency shift determines the optical lengths of the interferometers. In our experiments, all interferometers have the same optical length of 32 cm corresponding to the π/2 phase difference between channel signals. An additional fiber delay line was introduced between channels to destroy coherence between initial and frequency-shifted channels. Demultiplexing of channels and sensors was performed using correlation-based signal processing. Amplitudes of cross correlation peaks obtained for both channels were used to extract the interferometric phase for each interferometer. Phase demodulation of relatively long multiplexed interferometers is experimentally demonstrated. Experimental results prove that the proposed technique is suitable for interrogating a serial array of relatively long interferometers dynamically modulated with phase excursions exceeding 2π. Simultaneous interrogation and phase demodulation were experimentally demonstrated using an in-line array of low-finesse Fabry-Perot interferometric sensors.

Multi-point fiber refractometer using Fresnel reflection and a coherent optical frequency-domain multiplexing technique.

We demonstrate a simple multi-point refractometer based on the coherent optical frequency-domain multiplexing technique. As local refractive index sensors, interferometers of different lengths formed between cleaved fiber tips and reference weak reflectors are employed. Referent reflectors were fabricated by splicing into lead SMF-28 fiber a very short section of hollow optical fiber using a standard fiber splicer and a cleaver with an optical microscope. Despite the very simple configuration and manufacturing of the sensor, the refractive index resolution of 6×10-4 was demonstrated during proof-of-concept experiments.