J. Benitez

High-Performance Low Coherence Interferometry Using SSB Modulation

Low coherence interferometry (LCI) is an optical measurement technique that has attracted the interest for relevant fields like medicine or sensing. With the objective of improving LCI capabilities, microwave photonics (MWP) arises as an innovative technology to enhance LCI possibilities. In this letter, a novel MWP-LCI approach is proposed and experimentally demonstrated to measure the optical path difference (OPD) of a sample. The operation principle of the technique is based on the analysis of the interference pattern through a dispersive element to retrieve its visibility using a vector network analyzer. Different capabilities of the system in terms of sensitivity, resolution, and SNR have been proved. In this case, the proposal is able to avoid carrier-suppression effect leading to a sensitivity improvement of 20 dB in comparison with previous structures for certain values of the OPD. Moreover, the OPD range has been extended up to 10 mm achieving an invariant resolution over all operation range. Finally, the improvement of the SNR of the system has been experimentally demonstrated by controlling properly the RF resonance profile through the adjustment of the optical source power distribution. We have observed an improvement of the dynamic range close to 40 dB for a Gaussian profile.

Demonstration of multiplexed sensor system combining low coherence interferometry and microwave photonics

In this work, a multiplexed sensor system is proposed by means of the combination of low coherence interferometry (LCI) and microwave photonics (MWP). Variations of physical magnitudes can be measured in an array of head sensors by monitoring the optical path difference generated by each sensor. In this case, the characterization of the multiplexed sensor system is done through the electrical transfer function corresponding to the MWP-LCI system. Moreover, the effect produced by the mutual interaction among head sensors is analyzed in this work. Experimental and theoretical demonstration of the system is provided comparing single detection and balanced detection approaches.

Real-time Microwave Photonic technique for Low-Coherence Interferometry applications

Low-Coherence Interferometry (LCI) is a highly relevant optical measurement technique used in a wide variety of applications from sensing to medical imaging among others due to its non-invasive performance with high axial precision. In this work, a novel structure is proposed with the application of Microwave Photonics (MWP) as key enabling technology for real-time LCI applications. The proposed MWP-LCI technique permits the measurement of the optical path difference in the time domain by processing the incoherent optical signal modulated by an electrical RF pulse. The possibility of retrieving the interferogram information by an oscilloscope is experimentally demonstrated achieving a scanning speed of tens of MHz required by real-time LCI applications with moderate signal-to-noise ratio (SNR). We have achieved an acquisition rate of 20 MHz increasing in more than 5 magnitude orders compared to previous techniques operating in frequency domain with similar SNR performance.

Analysis of key parameters in MWP-LCI systems

Low coherence interferometry (LCI) is a well-known measurement technique able to provide an axial precision in the order of the microns. Its application to a wide variety of fields like medicine has attracted the interest of many authors mainly due to its non-invasive characteristic. By combining LCI and microwave photonics (MWP), we take profit from the inherent stability that the RF domain offers to improve the general performance of key parameters. In this paper, an analysis of key parameters in a MWP-LCI structure is carried out. The main target is to demonstrate the feasibility of a LCI system by means of MWP technology featuring unique properties beyond the current state of the art. The analysed scheme is able to obtain the optical path difference (OPD) related to a sample by retrieving the low-coherence interferogram in the RF domain. Moreover, we provide an accurate definition of resolution for MWP-LCI based system, which is closely related to the shape of the optical source profile employed. Finally, experimental demonstration of the analysis carried out in this work is also attached.

SCM Adaptation to Improve Scanning Rate in RF Interferometry Applications

In this letter, we present a novel structure for performing subcarrier multiplexing (SCM) to improve the scanning rate in low coherence interferometry (LCI) systems combined with the microwave photonics (MWP) technology. In this MWP-LCI proposal, the optical path differences (OPDs) produced by different samples between the arms of an interferometer are closely related to the central frequency of different RF resonances generated in the RF domain. By the proposed adaptation of the SCM technique, “M” subcarriers are multiplexed in the modulation stage and each subcarrier is set to sweep simultaneously a concrete part of the spectrum. The complete electrical transfer function of the structure is obtained combining each individual sweep. Therefore, a considerable reduction of the sweep time is provided to collect the complete electrical transfer function. Therefore, the scanning rate is reduced according to the number of subcarriers (M) employed in the multiplexing stage. An OPD range of 8 mm is achieved with a constant resolution of

Sensitivity Enhancement for Low-Coherence Interferometry

120~\mu \text{m}

Third-Order Dispersion Compensation for Resolution Enhancement in RF Interferometry

in the whole range. Finally, a maximum sensitivity of 60 dB is also reached for that operation range.

Low-Coherence Interferometry Using Microwave Photonics for Multilayered Samples

In this letter, a sensitivity improvement for systems combining low-coherence interferometry (LCI) and microwave photonics (MWP) is demonstrated. This improvement is due to the introduction of a different modulation format and an exhaustive control of the optical source profile compared with previous MWP-LCI schemes. Our proposal allows to retrieve the visibility of low-coherence interferograms through the analysis of the interference pattern using a dispersive element. We demonstrate that the use of a phase modulator offers better stability and lower insertion loss since a bias point configuration is not needed compared with the intensity modulators typically used in these schemes. The process for controlling the optical source profile permits a comparison between uniform and Gaussian profiles. In this way, the limiting effects of the sidelobes over the achieved sensitivity level are analyzed. The proposed MWP-LCI structure is experimentally demonstrated through the characterization of the electrical transfer function. In this case, a maximum sensitivity of 65 dB is achieved in our MWP-LCI structure showing a 30-dB improvement compared with current proposals.