Imaging the optical properties of turbid media with single-pixel detection based on the Kubelka-Munk model.

We present a diffuse optical imaging system with structured illumination and integrated detection based on the Kubelka-Munk light propagation model for the spatial characterization of scattering and absorption properties of turbid media. The proposed system is based on the application of single-pixel imaging techniques. Our strategy allows us to retrieve images of the absorption and scattering properties of a turbid media slab by using integrating spheres with photodiodes as bucket detectors. We validate our idea by imaging the absorption and scattering coefficients of a spatially heterogeneous phantom.

Single-pixel imaging with Fourier filtering: application to vision through scattering media.

We present a novel approach for imaging through scattering media that combines the principles of Fourier spatial filtering and single-pixel imaging. We compare the performance of our single-pixel imaging setup with that of a conventional system. First, we show that a single-pixel camera does not reduce the frequency content of the object, when a small pinhole is used as a low-pass filter at the detection side. Second, we show that the introduction of Fourier gating improves the contrast of imaging through scattering media in both optical systems. We conclude that single-pixel imaging fits better than conventional imaging on imaging through scattering media by the Fourier gating.

Signal-to-noise ratio of single-pixel cameras based on photodiodes.

Single-pixel cameras have been successfully used in different imaging applications in the last years. One of the key elements affecting the quality of these cameras is the photodetector. Here, we develop a numerical model of a single-pixel camera, which takes into account not only the characteristics of the incident light but also the physical properties of the detector. In particular, our model considers the photocurrent, the dark current, the photocurrent shot noise, the dark-current shot noise, and the Johnson-Nyquist (thermal) noise of the photodiode used as a light detector. The model establishes a clear relationship between the electric signal and the quality of the final image. This allows us to perform a systematic study of the quality of the image obtained with single-pixel cameras in different contexts. In particular, we study the signal-to-noise ratio as a function of the optical power of the incident light, the wavelength, and the photodiode temperature. The results of the model are compared with those obtained experimentally with a single-pixel camera.

Computational imaging with a balanced detector.

Single-pixel cameras allow to obtain images in a wide range of challenging scenarios, including broad regions of the electromagnetic spectrum and through scattering media. However, there still exist several drawbacks that single-pixel architectures must address, such as acquisition speed and imaging in the presence of ambient light. In this work we introduce balanced detection in combination with simultaneous complementary illumination in a single-pixel camera. This approach enables to acquire information even when the power of the parasite signal is higher than the signal itself. Furthermore, this novel detection scheme increases both the frame rate and the signal-to-noise ratio of the system. By means of a fast digital micromirror device together with a low numerical aperture collecting system, we are able to produce a live-feed video with a resolution of 64 × 64 pixels at 5 Hz. With advanced undersampling techniques, such as compressive sensing, we can acquire information at rates of 25 Hz. By using this strategy, we foresee real-time biological imaging with large area detectors in conditions where array sensors are unable to operate properly, such as infrared imaging and dealing with objects embedded in turbid media.

High-resolution adaptive imaging with a single photodiode.

During the past few years, the emergence of spatial light modulators operating at the tens of kHz has enabled new imaging modalities based on single-pixel photodetectors. The nature of single-pixel imaging enforces a reciprocal relationship between frame rate and image size. Compressive imaging methods allow images to be reconstructed from a number of projections that is only a fraction of the number of pixels. In microscopy, single-pixel imaging is capable of producing images with a moderate size of 128 × 128 pixels at frame rates under one Hz. Recently, there has been considerable interest in the development of advanced techniques for high-resolution real-time operation in applications such as biological microscopy. Here, we introduce an adaptive compressive technique based on wavelet trees within this framework. In our adaptive approach, the resolution of the projecting patterns remains deliberately small, which is crucial to avoid the demanding memory requirements of compressive sensing algorithms. At pattern projection rates of 22.7 kHz, our technique would enable to obtain 128 × 128 pixel images at frame rates around 3 Hz. In our experiments, we have demonstrated a cost-effective solution employing a commercial projection display.

Compressive imaging in scattering media.

One challenge that has long held the attention of scientists is that of clearly seeing objects hidden by turbid media, as smoke, fog or biological tissue, which has major implications in fields such as remote sensing or early diagnosis of diseases. Here, we combine structured incoherent illumination and bucket detection for imaging an absorbing object completely embedded in a scattering medium. A sequence of low-intensity microstructured light patterns is launched onto the object, whose image is accurately reconstructed through the light fluctuations measured by a single-pixel detector. Our technique is noninvasive, does not require coherent sources, raster scanning nor time-gated detection and benefits from the compressive sensing strategy. As a proof of concept, we experimentally retrieve the image of a transilluminated target both sandwiched between two holographic diffusers and embedded in a 6mm-thick sample of chicken breast.

Resolution analysis in computational imaging with patterned illumination and bucket detection.

In computational imaging by pattern projection, a sequence of microstructured light patterns codified onto a programmable spatial light modulator is used to sample an object. The patterns are used as generalized measurement modes where the object information is expressed. In this Letter, we show that the resolution of the recovered image is only limited by the numerical aperture of the projecting optics regardless of the quality of the collection optics. We provide proof-of-principle experiments where the single-pixel detection strategy outperforms the resolution achieved using a conventional optical array detector for optical imaging. It is advantageous in the presence of real-world conditions, such as optical aberrations and optical imperfections in between the sample and the sensor. We provide experimental verification of image retrieval even when an optical diffuser prevents imaging with a megapixel array camera.

Parallel laser micromachining based on diffractive optical elements with dispersion compensated femtosecond pulses.

We experimentally demonstrate multi-beam high spatial resolution laser micromachining with femtosecond pulses. The effects of chromatic aberrations as well as pulse stretching on the material processed due to diffraction were significantly mitigated by using a suited dispersion compensated module (DCM). This permits to increase the area of processing in a factor 3 in comparison with a conventional setup. Specifically, 52 blind holes have been drilled simultaneously onto a stainless steel sample with a 30 fs laser pulse in a parallel processing configuration.

Wavelength tuning of femtosecond pulses generated in nonlinear crystals by using diffractive lenses.

We demonstrate that diffractive lenses (DLs) can be used as a simple method to tune the central wavelength of femtosecond pulses generated from second-order nonlinear optical processes in birefringent crystals. The wavelength tunability is achieved by changing the relative distance between the nonlinear crystal and the DL, which acts in a focusing configuration. Besides the many practical applications of the so-generated pulses, the proposed method might be extended to other wavelength ranges by demonstrated similar effects on other nonlinear processes, such as high-order harmonic generation.

Use of polar decomposition of Mueller matrices for optimizing the phase response of a liquid-crystal-on-silicon display.

We provide experimental measurement of the Mueller matrices corresponding to an on-state liquid-crystal-on-silicon display as a function of the addressed voltage. The polar decomposition of the Mueller matrices determines the polarization properties of the device in terms of a diattenuation, a retardance and a depolarization effect. Although the diattenuation effect is shown to be negligible for the display, the behavior of the degree of polarization as a function of the input polarization state shows a maximum coupling of linearly polarized light into unpolarized light of about 10%. Concerning the retardation effect, we find that the display behaves as a retarder with a fast-axis orientation and a retardance angle that are voltage-dependent. The above decomposition provides a convenient framework to optimize the optical response of the display for achieving a phase-mostly modulation regime. To this end, the display is sandwiched between a polarization state generator and a polarization state analyzer. Laboratory results for a commercial panel show a phase modulation depth of 360 masculine at 633 nm with a residual intensity variation lower than 6 %.

Phase-only modulation with a twisted nematic liquid crystal display by means of equi-azimuth polarization states.

The capability of a twisted nematic liquid crystal display to generate a set of equi-azimuth polarization states is used to achieve a phase-only modulation regime. For this purpose, a liquid crystal display followed by a quarter-wave plate is launched between two polarizers. Theoretical support is provided by means of the Jones matrix calculus and the Poincaré sphere representation. Laboratory results for a commercial liquid crystal display are presented. A phase modulation deep of 270 masculine is obtained at 514 nm with a residual intensity variation which is lower than 2.5 %.

Chromatic compensation in the near-field region: shape and size tunability.

We report a diffractive-lens triplet with which to achieve wavelength compensation in the near field diffracted by any aperture. On the one hand, the all-diffractive triplet allows us to tune, in a sequential way, the Fresnel-irradiance shape to be achromatized by changing the focal length of one diffractive lens. On the other hand, we can adjust the scale of the chromatically compensated Fresnel diffraction field by shifting the aperture along the optical axis. Within this framework, we present an extremely flexible white-light Fresnel-plane array illuminator based on the kinoform sampling filter. A variable compression ratio and continuous selection of the output pitch are the most appealing features of this novel application.

High-contrast white-light Lau fringes.

We present a new optical assembly with which to achieve Lau fringes with totally incoherent illumination. Gratinglike codification of the spatially incoherent source combined with an achromatic Fresnel diffraction setup allows us to achieve Lau fringe-pattern visibility of almost 100% with broadband light. The white-light character to our proposed setup is in stark contrast to previous monochromatic implementations. Potential implications of this fact are identified.

Wavelength-compensated Fourier and Fresnel transformers: a unified approach.

We recognize that one can adapt any dispersion-compensated broadband optical Fourier transformer to achieve wavelength compensation in the Fresnel diffraction region just by inserting a diffractive lens at the input plane and vice versa. This unification procedure is employed in a second stage in the design of a novel hybrid (diffractive-refractive) optical setup that provides, in a sequential way, nearly wavelength-independent Fresnel diffraction patterns in the irradiance of the object transmittance.

Broadband space-variant Fresnel processor.

We present a radically new class of optical setup working with white-light illumination, namely, a chromatically compensated processor operating in the Fresnel domain. The optical configuration is a hybrid (diffractive-refractive) three-lens system that exhibits an intermediate achromatic Fresnel plane and an output image plane without chromatic distortion. As a first application of this optical arrangement we develop a parallel space-variant color pattern-recognition experiment with white light.

Scale-tunable optical correlation with natural light.

We describe two different scale-tunable optical correlators working under totally incoherent light. They behave as spatially incoherent wavelength-independent imaging systems with an achromatic point-spread function (PSF). In both cases it is possible to adapt the scale of the achromatic PSF, i.e., to modify the scaling factor of the PSF and preserve the chromatic compensation, by one's shifting the input along the optical axis. The remarkable properties of these systems allow us to carry out a scale-tunable color pattern-recognition experiment with natural light.

Optical security and encryption with totally incoherent light.

We present a method for securing and encrypting information optically by use of totally incoherent illumination. Encryption is performed with a multichannel optical processor working under natural (both temporal and spatially incoherent) light. In this way, the information that is to be secured can be codified by use of color signals and self-luminous displays. The encryption key is a phase-only mask, providing high security from counterfeiting. Output encrypted information is recorded as an intensity image that can be easily stored and transmitted optically or electrically. Decryption or authentication can also be performed optically or digitally. Experimental results are presented.

All-incoherent dispersion-compensated optical correlator.

We report on a simple, spatially incoherent, wavelength-independent imaging system that, in contrast to the conventional case, exhibits a dispersion-compensated point-spread function. Our hybrid (diffractive-refractive) three-lens imaging configuration thus acts as an all-incoherent dispersion-compensated optical irradiance correlator. So the optical arrangement is well adapted to processing color information (both spatially and temporally incoherent) under natural illumination.

Phase-change visualization in two-dimensional phase objects with a semiderivative real filter.

A method of visualization of phase changes in two-dimensional pure-phase objects by use of two orthogonal Fourier plane filters that realize the half-order differentiation is presented. Real semiderivative filters used in two dimensions and in sequence yield output-image intensity signals proportional to the first derivatives of the input-object phase that appear on a constant background. This nonlinear filtration of spatial frequencies permits the alleviation of the consequences of square-law detection and makes phase changes visible. Phase changes in gradient-index phosphate glass are calculated experimentally. We discuss the accuracy of the proposed method.

Achromatic fourier transforming properties of a separated diffractive lens doublet: theory and experiment.

The strong chromatic distortion associated with diffractive optical elements is fully exploited to achieve an achromatic optical Fourier transformation under broadband point-source illumination by means of an air-spaced diffractive lens doublet. An analysis of the system is carried out by use of the Fresnel diffraction theory, and the residual secondary spectrum (both axial and transversal) is evaluated. We recognize that the proposed optical architecture allows us to tune the scale factor of the achromatic Fraunhofer diffraction pattern of the input by simply moving the diffracting screen along the optical axis of the system. The performance of our proposed optical setup is verified by several laboratory results.