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.

Common-path multimodal optical microscopy.

We have developed a common-path multimodal optical microscopy system that is capable of using a single optical source and a single camera to image amplitude, phase, and fluorescence features of a biological specimen. This is achieved by varying either contrast enhancement filters at the Fourier plane and/or neutral density/fluorescence filters in front of the CCD camera. The feasibility of the technique is demonstrated by obtaining brightfield, fluorescence, phase-contrast, spatially filtered, brightfield+fluorescence, phase+fluorescence, and edge-enhanced+fluorescence images of the same Drosophila embryo without the need for image registration and fusion. This comprehensive microscope has the capability of providing both structural and functional information and may be used for applications such as studying live-cell dynamics and in high throughput microscopy and automated microscopy.

Coherent population oscillations and superluminal light in a protein complex.

We observed superluminal light in aqueous solution of the protein complex bacteriorhodopsin (bR) at 647.1 nm wavelength where it exhibits reverse saturable behavior, exploiting the technique of coherent population oscillations (CPO). With a modulation frequency of 10 Hz, the signal pulse through a 1 cm path cell is ahead by 3 msec relative to the reference pulse, corresponding to a group velocity of -3.3 m/sec. Following our early work on slow light in the same sample at the saturable wavelength 568.2 nm, we now explicitly observed the narrow spectral hole in the absorption band of the stable B state and further, demonstrated a close correlation between the profile of the hole and the corresponding pulse delay for various modulation frequencies. A similar behavior is observed for superluminal light versus antihole blown in the absorption band.

Optical Fourier techniques for medical image processing and phase contrast imaging.

This paper briefly reviews the basics of optical Fourier techniques (OFT) and applications for medical image processing as well as phase contrast imaging of live biological specimens. Enhancement of microcalcifications in a mammogram for early diagnosis of breast cancer is the main focus. Various spatial filtering techniques such as conventional 4f filtering using a spatial mask, photoinduced polarization rotation in photosensitive materials, Fourier holography, and nonlinear transmission characteristics of optical materials are discussed for processing mammograms. We also reviewed how the intensity dependent refractive index can be exploited as a phase filter for phase contrast imaging with a coherent source. This novel approach represents a significant advance in phase contrast microscopy.

Slow light in bacteriorhodopsin solution using coherent population oscillations.

Slow light is demonstrated in liquid phase in an aqueous bacteriorhodopsin (bR) solution at room temperature. Group velocity as low as 3 m/s (all the way to c) is achieved by exploiting the photoisomerization property of bR for coherent population oscillations. Slow light in the liquid phase offers several advantages over solids or vapors for a variety of applications: (i) shorter lifetimes of the M state facilitate slow light at higher modulation frequencies, (ii) convection makes it possible to obtain large signal delays even at high input powers, and (iii) solution concentration is another convenient parameter to vary the signal delay over a wide range.

All-optical spatial filtering with power limiting materials.

We demonstrated that a power limiting mechanism could potentially be used for self-adaptive, all-optical Fourier image processing. Reverse saturable absorbers like porphyrins are chosen due to their fluence dependent power limiting property, which triggers at relatively low intensities. At low input intensities, below the power-limiting threshold, the 4-f configuration will image the object onto the CCD camera without any spatial frequency filtering. As the input intensity is increased above the threshold level, dc and low spatial frequencies are blocked resulting in edge-enhanced images containing high spatial frequencies. The incident intensity sets the higher limit on the band of frequencies blocked. In addition, the use of the same experimental setup for both power limiting experiments and optical image processing demonstrates that in the case of any bright image bearing laser beam, the sensitive detectors are protected, by blocking the intense low spatial frequencies.

Controllable snail-paced light in biological bacteriorhodopsin thin film.

We observe that the group velocity of light is reduced to an extremely low value of 0.091 mm/s in a biological thin film of bacteriorhodopsin at room temperature. By exploiting unique features of a flexible photoisomerization process for coherent population oscillation, the velocity is all-optically controlled over an enormous span, from snail-paced to normal light speed, with no need of modifying the characteristics of the incident pulse. Because of the large quantum yield for the photoreaction in this biochemical system, the ultraslow light is observed even at low light levels of microwatts, indicating high energy efficiency.

Nonlinear optical Fourier filtering technique for medical image processing.

Real-time nonlinear optical Fourier filtering for medical image processing is demonstrated, exploiting light modulating characteristics of thin films of the biophotonic material bacteriorhodopsin (bR). The nonlinear transmission of bR films for a 442 nm probe beam with a 568 nm control beam and vice versa is experimentally studied in detail. The spatial frequency information carried by the blue probe beam is selectively manipulated in the bR film by changing the position and intensity of the yellow control beam. The feasibility of the technique is first established with different shapes and sizes of phantom objects. The technique is applied to filter out low spatial frequencies corresponding to soft dense breast tissue and displaying only high spatial frequencies corresponding to microcalcifications in clinical screen film mammograms. With the aid of an electrically addressed spatial light modulator (SLM), we successfully adapt the technique for processing digital phantoms and digital mammograms. Unlike conventional optical spatial filtering techniques that use masks, the technique proposed can easily accommodate the changes in size and shape of details in a mammogram.

Spectral phase based medical image processing.

RATIONALE AND OBJECTIVES: To exploit the spectral phase characteristics of digital or digitized mammograms for early detection of microcalcifications, shape, and sizes of suspected lesions and to demonstrate its use for training radiologists to discriminate signal features in different spatially varying backgrounds. MATERIALS AND METHODS: We propose two algorithms: in the phase-only image (POI) reconstruction algorithm the spectral phase of the digital mammogram is extracted from its Fourier spectrum. This is coupled with unit magnitude and inverse Fourier transformed to reconstruct the POI thus enhancing the features of interest such as microcalcifications, shape, and sizes of suspected lesions. In the algorithm for image reconstruction from a priori phase-only information, spectral phase is used to extract signal features of the digital mammogram and then this is combined with spectral magnitude that is extracted and averaged over an ensemble of unrelated digital mammograms. RESULTS: The results for several digital phantoms and mammograms show that POI reconstructs only high spatial frequencies related to the features such as microcalcifications, shape, and size of masses like cysts and tumors. The results on image reconstruction from a priori phase-only information demonstrate the changes in the visibility of signal features when buried in a wide variety of real world mammogram backgrounds with different densities. CONCLUSION: The POI can aid radiologists in early detection of microcalcifications, lesions, and other masses of interest in digital mammograms. This reconstruction method is self-adaptive to changes in the background. The image reconstruction from a priori phase-only information can help the radiologist as a training tool in his decision-making process. Preliminary experiments indicate the potential of the techniques for early diagnosis of breast cancer. Clinical studies on these algorithm procedures are in progress for application as a diagnostic CAD tool in digital mammography. These methods can in general be applied to other medical images such as CT and MRI images.