Time and Wavelength Division Multiplexing of Fiber Bragg Gratings with Bidirectional Electro-Optical Frequency Conversion

Incoherent Optical Frequency-Domain Reflectometry Based on Homodyne Electro-Optic Downconversion for Fiber-Optic Sensor Interrogation

“…Globally, the performance of this I-OFDR reflectometer for RBS characterization is similar to that of conventional I-OFDR systems based on VNA for sensor interrogation [17,42] or in-line monitoring [31], but with the identified limitations of polarization-dependent detection and echoes induced by harmonic distortion. The first can be solved by incorporating polarization switching [43,44] or scrambling [23]; the second represents a deterministic impairment that can be avoided, if necessary, by a previous calibration of the distortion and subsequent equalization.…”

“…Polarization control at the receiver’s side mitigates this problem, but this option is only viable for low ranges. The standard approaches to overcome this issue include the use of polarization-maintaining fiber [43]; the use of depolarizers or polarization scramblers to average the response over polarization at the price of an additional 3 dB power penalty [23]; or the use of polarization switching [43,44]. In our measurements of point reflectors, we implemented polarization switching, as described below.…”

“…It has also been enriched with concepts from Microwave Photonics [10,21], and much effort has been dedicated to the incorporation of additional functionalities to the technique. Wavelength-resolved reflectograms have been obtained by sweeping the wavelength of a narrowband optical probe wave [11] or filtering the backreflected light under broadband illumination [22,23], and the addition of dispersive elements has been used to map Bragg wavelength shifts into RF time delays directly measurable by the VNA [24,25,26]. In addition, specific techniques for discrete narrowband reflectors, typically FBG, have been adapted to I-OFDR systems, such as slope-assisted dual-wavelength [27] or dual-filter [28] interrogation, or fast techniques relying on model-based search [29] or equally-spaced arrays of reflectors [30].…”

“…Due to the limited bandwidth of these devices, extension to GHz or multi-GHz bandwidths requires the alternative use of electro-optic amplitude or phase modulators. At moderate bandwidths (<GHz), homodyne downconversion of the reflected light can be performed by operating the modulator only in the forward direction [23,24,33]. In multi-GHz systems, however, modulation is only efficient if the RF modulation tone travels through the electrode in the same direction as light, and so it requires the use of specific driving stages to operate the electrode bidirectionally [34,35], the change of the modulator’s design [36], or the use of two different modulators as in the original heterodyne experiment [6].…”