Hanbury Brown-Twiss approach for imaging through a dynamic scattering medium.

The Hanbury Brown-Twiss approach, associated with the correlation of intensity fluctuations at two different points in a wave field, unveils fundamental aspects of light. Here, we propose and experimentally demonstrate an imaging and phase recovery technique through a dynamic scattering medium using the Hanbury Brown-Twiss approach. A detailed theoretical basis is presented and verified by experimental demonstrations. To validate the application of the proposed technique, the randomness of the dynamically scattered light is exploited using temporal ergodicity for evaluating the correlation of intensity fluctuations and consequently applying it in the reconstruction of the object hidden behind the dynamic diffuser.

On-axis phase-shifting correlation holography with un-polarized light.

We propose and experimentally demonstrate an on-axis phase-shifting correlation holography technique with un-polarized light. The randomness of the un-polarized light is exploited to evaluate the higher-order polarization correlation and to consequently apply it in the reconstruction of the hologram from the randomness. A detailed theoretical analysis is developed and verified by numerical simulations and followed by experimental demonstrations. To validate the application of the proposed technique, imaging of different helical phase objects with randomness is presented. A good affinity is found between simulation and experimental results, which validates the accuracy of the proposed technique.

Label-free detection of Escherichia coli bacteria by cascaded chirped long period gratings immunosensor.

An optical fiber based immunosensing platform formed by two identical chirped long period gratings (CLPGs) for sensitive and label-free detection of Escherichia coli (E. coli) is presented. The proposed immunosensor having two CLPGs with an inter-grating space (IGS) works like a Mach-Zehnder interferometer. The important feature of this sensor is that by changing the refractive index (RI) induced phase at the IGS region, a corresponding shift in resonance wavelength can be realized without affecting the actual grating region. Additionally, the confining bandwidth of the interference fringes promotes greater resolution in the resonance wavelength analysis of the transmission spectrum. The main aim of this study was the specific and rapid detection of the E. coli bacteria in phosphate buffer saline (detection range: 10 cfu/ml to 60 cfu/ml) by using the bio-functionalized IGS region as a sensing probe of the dual CLPG structure. The observed detection limit was 7 cfu/ml. For specificity analysis, Salmonella typhimurium and Staphylococcus aureus were tested and no significant shift in resonance wavelength was observed. In addition, the proposed immunosensor has the ability to selectively detect E. coli in real samples including lake water. The high specificity, good sensitivity to ambient RI, and robustness of the developed sensing platform will open a new avenue in optical immunosensing technology, and cascaded long CLPG sensors can contribute significantly to the detection of pathogenic bacteria in water and food samples.

Lensless complex amplitude image retrieval through a visually opaque scattering medium.

We propose and experimentally demonstrate lensless complex amplitude image retrieval through a visually opaque scattering medium from spatially fluctuating fields using intensity measurement and a phase-retrieval algorithm. The complex amplitude information of the hidden object is encoded in the form of a real and nonnegative amplitude function represented as an interference pattern. A single charge coupled device (CCD) image of the scattered light collected through a visually opaque optical diffuser contains enough information to digitally regenerate the interference pattern. Furthermore, a lensless configuration is implemented which eliminates any possible aberration effects associated with optical components, and this further has promising applications where the use of imaging optics is not feasible. Experimental results for the recovery of complex fields corresponding to optical vortices of two different topological charges are presented.

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-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.

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.

Digital holographic microscopy discriminates sex differences in medial prefrontal cortex GABA neurons following amphetamine sensitization.

Sex differences have been noted in patterns of drug use and relapse, and in particular with amphetamine abuse, implicating estradiol in mediating female neurobehavioral responses. To investigate the interaction of estradiol with amphetamine-induced hyperactivity, we compared male, intact female (INTACT), ovariectomized (OVX) and ovariectomized estradiol-treated (OVX+EB) female rats receiving repeated amphetamine (AMPH) treatment. All rats received intermittent AMPH injections for three days, and baseline and post-injection locomotor activity as well as fine-motor movements were recorded. Upon completion of behavioral experiments, immunohistochemistry was performed to assess parvalbumin-immunoreactive (PV-IR) GABAergic neurons in the medial prefrontal cortex (mPFC). Results indicate that AMPH induced greater behavioral response during habituation among the INTACT animals, and post-injection hyperactivity was apparent on days 2 and 3, among INTACT and OVX+EB females. For INTACT animals, the hyperactivity was most pronounced when estrogen levels were high. Immunohistochemical analysis using digital holographic microscopy revealed INTACT and OVX+EB females had less expression and smaller somatic area of PV-IR neurons in the mPFC. These data provide evidence for rapid development of sex differences in response to AMPH that correlates with sexually dimorphic alterations in a subset of mPFC GABAergic neurons implicated in modulating forebrain dopamine projections.

Parallel-quadrature phase-shifting digital holographic microscopy using polarization beam splitter.

We present a digital holography microscopy technique based on parallel-quadrature phase-shifting method. Two π/2 phase-shifted holograms are recorded simultaneously using polarization phase-shifting principle, slightly off-axis recording geometry, and two identical CCD sensors. The parallel phase-shifting is realized by combining circularly polarized object beam with a 45° degree polarized reference beam through a polarizing beam splitter. DC term is eliminated by subtracting the two holograms from each other and the object information is reconstructed after selecting the frequency spectrum of the real image. Both amplitude and phase object reconstruction results are presented. Simultaneous recording eliminates phase errors caused by mechanical vibrations and air turbulences. The slightly off-axis recording geometry with phase-shifting allows a much larger dimension of the spatial filter for reconstruction of the object information. This leads to better reconstruction capability than traditional off-axis holography.

Quantitative phase microscopy using dual-plane in-line digital holography.

We present detailed theoretical evaluation and thorough experimental investigation of quantitative phase imaging using our previously demonstrated dual-plane in-line digital holographic microscopy technique [Opt. Lett. 35, 3426 (2010)]. This evaluation is based on the recording of two interferograms at slightly different planes and numerically reconstructing the object information. The zero-order diffracted wave is eliminated by using the method of subtraction of average intensity of the entire hologram, and the twin-image diffracted wave is removed by Fourier domain processing of the two recorded holograms. Experiments are performed using controlled amplitude and phase objects and human muscle cells to demonstrate the potential of this technique.

Dual-channel in-line digital holographic double random phase encryption.

We present a robust encryption method for the encoding of 2D/3D objects using digital holography and virtual optics. Using our recently developed dual-plane in-line digital holography technique, two in-line digital holograms are recorded at two different planes and are encrypted using two different double random phase encryption configurations, independently. The process of using two mutually exclusive encryption channels makes the system more robust against attacks since both the channels should be decrypted accurately in order to get a recognizable reconstruction. Results show that the reconstructed object is unrecognizable even when the portion of the correct phase keys used during decryption is close to 75%. The system is verified against blind decryptions by evaluating the SNR and MSE. Validation of the proposed method and sensitivities of the associated parameters are quantitatively analyzed and illustrated.

Dual plane in-line digital holographic microscopy.

We report a dual plane in-line digital holographic microscopy technique that exploits the method of subtraction of average intensity of the entire hologram to suppress the zero-order diffracted wave. Two interferograms are recorded at different planes to eliminate the conjugate image. The experimental results demonstrate successful reconstruction of phase objects as well as of amplitude objects. The two interferograms can be recorded simultaneously, using two CCD or CMOS sensors, in order to increase the acquisition rate. This enhanced acquisition rate, together with the improved reconstruction capability of the proposed method, may find applications in biomedical research for visualization of rapid dynamic processes at the cellular level.

Reliability of content-addressable data search in a defocused volume holographic data storage system.

We investigate for suitable methods that enable reliable content-addressable data search in a defocused volume holographic data storage system. Two techniques have been introduced and are shown to overcome the shortcomings of the known methods used to perform content searching in defocused holographic recording geometry. In effect, we remove the deterministic errors that result because of the presence of nonmatching database records, producing almost the same correlation scores as the true targeted correlation scores. Such deterministic errors give rise to erroneous search outcomes and reduce the speed advantage of the parallel holographic data search. We present experimental results and discuss the improvements offered by the two introduced methods in terms of storage density and measured correlation scores. Both the methods of modified-balanced and 25% sparse modulation coded data pages are found to produce good results, negating the undesired correlation characteristics.

Phase-image-based sparse-gray-level data pages for holographic data storage.

We propose a method for implementation of gray-scale sparse block modulation codes with a single spatial light modulator in phase mode for holographic data storage. Sparse data pages promise higher recording densities with reduced consumption of the dynamic range of the recording material and reduced interpixel cross talk. A balanced sparse-gray-level phase data page gives a homogenized Fourier spectrum that improves the interference efficiency between the signal and the reference beams. Construction rules for sparse three-gray-level phase data pages, readout methods, and interpixel cross talk are discussed extensively. We also explore theoretically the potential storage density improvement while using low-pass filtering and sparse-gray-level phase data pages for holographic storage, and demonstrate the trade-off between code rate, block length, and estimated capacity gain.

Improved data search by zero-order (dc) peak filtering in a defocused volume holographic content-addressable memory.

Defocusing the recording material away from the Fourier plane is necessary to reduce the strong dc component and produce a more homogeneous object beam distribution in the hologram plane in volume holographic digital data-storage systems with amplitude-modulated data pages. However, content-addressable searching with defocused recording results in higher cross-correlation peak intensities. We present a method for performing a faithful content-addressable search in a defocused volume holographic data-storage system. A new dc-filtered content-addressable search method for defocused volume holographic data-storage systems with binary data pages is demonstrated. Both simulation and experimental results are presented. The experimental results show good discrimination capability and confirm the feasibility of the proposed technique.

Performance analysis of content-addressable search and bit-error rate characteristics of a defocused volume holographic data storage system.

One of the methods for smoothing the high intensity dc peak in the Fourier spectrum for reducing the reconstruction error in a Fourier transform volume holographic data storage system is to record holograms some distance away from or in front of the Fourier plane. We present the results of our investigation on the performance of such a defocused holographic data storage system in terms of bit-error rate and content search capability. We have evaluated the relevant recording geometry through numerical simulation, by obtaining the intensity distribution at the output detector plane. This has been done by studying the bit-error rate and the content search capability as a function of the aperture size and position of the recording material away from the Fourier plane.