Bhargab Das

Quantitative phase-contrast imaging through a scattering media

We report a new technique for the recovery of quantitative phase and amplitude information of an object hidden behind a scattering medium. Two-point intensity correlation measurement together with digital holography principles are utilized for this purpose. The hologram information of the object and a reference beam is scrambled by the presence of a scattering medium in its path. A direct digital holographic recording of this scattered light does not lead to the reconstruction of actual object information. We propose the idea of recovering this hologram information from the spatially fluctuating field of a laser speckle pattern using the intensity correlation, and subsequently apply digital reconstruction of the hologram for recovery of quantitative phase and amplitude information of objects hidden by a random diffuser.

Elimination of Light Intensity Noise Using Dual-Channel Scheme for Fiber MZI-Based FBG Sensor Interrogation

Fiber Mach-Zehnder interferometers (F-MZI) are widely used for fiber Bragg grating (FBG) sensor interrogation purpose due to their advantages in terms of wide bandwidth, high resolution, and tunable sensitivity. They are appropriate for dynamic strain measurement applications in the areas of vibration analysis, hydrophones, and acoustic emission studies. The interferometer converts the Bragg wavelength shift of an FBG sensor into a corresponding phase shift which is finally recorded as intensity variations at the output. Power fluctuations of the light source as well as disturbances in the light guiding path and/or dependence of light source intensity on wavelength introduce fictitious output intensity variations resulting into significant measurement errors. We propose and experimentally demonstrate a dual-channel F-MZI approach that eliminates the light intensity disturbance noise. Furthermore, the proposed method also improves the demodulated signal quality by a factor of ~3 dB.

Holographic imaging through a scattering layer using speckle interferometry

Optical imaging through complex scattering media is one of the major technical challenges with important applications in many research fields, ranging from biomedical imaging to astronomical telescopy to spatially multiplexed optical communications. Various approaches for imaging through a turbid layer have been recently proposed that exploit the advantage of object information encoded in correlations of the random optical fields. Here we propose and experimentally demonstrate an alternative approach for single-shot imaging of objects hidden behind an opaque scattering layer. The proposed technique relies on retrieving the interference fringes projected behind the scattering medium, which leads to complex field reconstruction, from far-field laser speckle interferometry with two-point intensity correlation measurement. We demonstrate that under suitable conditions, it is possible to perform imaging to reconstruct the complex amplitude of objects situated at different depths.

Fiber Bragg Grating based Acoustic Sensing with Interferometric Interrogation

We report our experimental studies on sensing of acoustic signal based on interferometric interrogation. Acoustical signals up to a frequency of 1 kHz has been detected using a fiber Bragg grating and a Mach-Zehnder interferometer.

Fiber-MZI-based FBG sensor interrogation: comparative study with a CCD spectrometer

We present an experimental comparative study of the two most commonly used fiber Bragg grating (FBG) sensor interrogation techniques: a charge-coupled device (CCD) spectrometer and a fiber Mach-Zehnder interferometer (F-MZI). Although the interferometric interrogation technique is historically known to offer the highest sensitivity measurements, very little information exists regarding how it compares with the current commercially available spectral-characteristics-based interrogation systems. It is experimentally established here that the performance of a modern-day CCD spectrometer interrogator is very close to a F-MZI interrogator with the capability of measuring Bragg wavelength shifts with sub-picometer-level accuracy. The results presented in this research study can further be used as a guideline for choosing between the two FBG sensor interrogator types for small-amplitude dynamic perturbation measurements down to nano-level strain.

Dynamic signal sensing using fiber Bragg grating sensor and interferometric interrogation

We report our recent experimental studies on phase generated carrier (PGC) based fiber Mach-Zehnder interferometer (F-MZI) interrogation technique for fiber Bragg grating (FBG) sensors. Interferometric techniques such as F-MZI have emerged as most sensitive and accurate technique for demodulation of signal generated by FBGs. F-MZIs are widely used for dynamic strain measurement applications such as vibration analysis, hydrophones, and acoustic emission studies. But, a fiber interferometer is inherently very sensitive to surrounding parameters of temperatures, pressure, and air currents which makes it unsuitable to be used directly as FBG wavelength shift demodulator. So, a PGC is incorporated in MZI to remove these influences. This paper shows the experimental results which supports PGC based demodulation scheme for dynamic signal sensing. FFT based noise elimination technique has also been used here to remove unwanted surrounding and system noises. We have also compared this PGC based F-MZI with CCD spectrometer for demodulation of dynamic signals generated by FBG sensor.

Strain Monitoring of Concrete Bridge using Fiber Bragg Grating Sensors

The distributed strain monitoring of a concrete bridge using packaged FBG sensors is presented. The responses of FBG sensors and conventional sensors were recorded under bridge loading conditions and shown to be in good agreements.

Reliable Data Search in a Holographic Search Engine with Defocused Recording

The holographic search engine based on defocused recording is investigated under rational conditions. New data page modulation coding schemes are introduced for removing the ambiguous correlation characteristics and performing a reliable data search.

Reliability of Content-Addressable Search in a Defocused Volume Holographic Data Storage System

The characteristics of correlation signals in a defocused holographic storage system depend on the similarities among the stored data sets. We achieve reliable performance by using data pages with sparseness values of 0.25 or less.