Wavelength-multiplexed digital holography for quantitative phase measurement using quantum dot film

Snapshot dual-wavelength digital holography with LED and laser hybrid illumination

To address the problem of the time-sharing recording of dual-wavelength low-coherence holograms while avoiding the use of customized achromatic optical elements, a snapshot dual-wavelength digital holography with LED and laser hybrid illumination is proposed. In this method, the parallel phase-shifting method is firstly employed to suppress zero-order and twin-image noise, and to record a LED hologram with low speckle noise and full field of view. Secondly, another laser hologram with a different center wavelength affected by speckle noise is recorded simultaneously using the spatial multiplexing technique. Finally, dual-wavelength wrapped phase images are reconstructed from a spatial multiplexing hologram, and then are combined to achieve low-noise phase unwrapping utilizing the iterative algorithm. Simulation and optical experiments on a reflective step with a depth of 1.38µm demonstrate that the proposed method can achieve single-shot and large-range height measurements while maintaining low-noise and full-field imaging.

Extended autofocusing in dual-wavelength digital holography

In single-wavelength digital holography (DH), the phase wrapping phenomenon limits the total object depth that can be measured due to the requirement for well-resolved phase fringes. To address this limitation, dual-wavelength DH is proposed, enabling measurement of much deeper objects. In single-wavelength DH, because the object depth is limited, the depth of focus (DOF) of DH's optical system at a reconstruction distance is sufficient to cover the object depth. To date, many autofocusing algorithms have been proposed to obtain a correct reconstruction distance. However, in dual-wavelength DH, because the object depth is extended, the DOF at a reconstruction distance cannot cover the extended object depth. The extended object depth can span multiple DOFs, causing partially out of focus object depth. Therefore, in dual-wavelength DH, relying solely on autofocusing algorithms for a single distance is insufficient. But extended autofocusing algorithms, which can autofocus objects through multiple DOFs, are demanded. However, there are no such extended autofocusing algorithms in dual-wavelength DH. Therefore, we propose an extended autofocusing algorithm for dual-wavelength DH based on a correlation coefficient. The proposed algorithm is able to focus the whole object depth when the depth spans multiple DOFs. Through theoretical analysis, simulations, and experiments, the necessity and effectiveness of the proposed algorithm are verified.

Enhancement of image sharpness and height measurement using a low-speckle light source based on a patterned quantum dot film in dual-wavelength digital holography

A dual-wavelength single light source based on a patterned quantum dot (QD) film was developed with a 405nm LED and bandpass filters to increase color conversion efficiency as well as to decouple the two peaks of dual-wavelength emitted from the QD film. A QD film was patterned laterally with two different sizes of QDs and was combined with bandpass filters to produce a high efficiency and low-speckle dual-wavelength light source. The experimental results showed that the developed dual-wavelength light source can decrease speckle noise to improve the reconstructed image sharpness and the accuracy on height measurement in dual-wavelength digital holography.

Incoherent color digital holography with computational coherent superposition for fluorescence imaging [Invited]

We present color fluorescence imaging using an incoherent digital holographic technique in which holographic multiplexing of multiple wavelengths is exploited. Self-interference incoherent digital holography with a single-path in-line configuration and the computational coherent superposition scheme are adopted to obtain color holographic three-dimensional information of self-luminous objects with a monochrome image sensor and no mechanical scanning. We perform not only simultaneous color three-dimensional sensing of multiple self-luminous objects but also color fluorescence imaging of stained biological samples. Color fluorescence imaging with an improved point spread function is also demonstrated experimentally by adopting a Fresnel incoherent correlation holography system.

Multiwavelength-multiplexed phase-shifting incoherent color digital holography

We propose multiwavelength-multiplexed phase-shifting incoherent color digital holography. In this technique, a monochrome image sensor records wavelength-multiplexed, phase-shifted, and incoherent holograms, and a phase-shifting interferometry technique selectively extracts object waves at multiple wavelengths from the several recorded holograms. Spatially incoherent light that contains multiple wavelengths illuminates objects, and multiwavelength-incoherent object waves are simultaneously obtained without using any wavelength filters. Its effectiveness is experimentally demonstrated for transparent and reflective objects.

Multiwavelength-selective phase-shifting digital holography without mechanical scanning

We propose multiwavelength in-line phase-shifting digital holography with a monochrome image sensor, single reference arm, and no mechanical scanning. We use phase-shifting interferometry selectively extracting wavelength information as a method to obtain multiwavelength object waves separately from wavelength-multiplexed phase-shifted holograms. By exploiting a liquid crystal on silicon spatial light modulator with a wide phase-modulation range as an electrically driven phase shifter in this interferometry, we remove mechanically moving parts during phase shifting. Its effectiveness is experimentally demonstrated.

Digital holography free of 2π ambiguity, using coherence modulation

In this Letter, a quantitative measurement method with an extended axial range in low-coherence light digital holography is presented. Based on the characteristics of the light source, the degree of coherence and phase values are obtained. Because the degree of coherence is modulated with respect to the optical path difference, it can be used to remove the 2π ambiguity of the phase, without the use of numerical or dual-wavelength methods. The mathematical procedures from three phase-shifting holograms are numerically described. From experimental results, the accurate measurements of a sample with high step are presented to confirm the effectiveness.

Wavelength-Selective Phase-Shifting Digital Holography: Color Three-Dimensional Imaging Ability in Relation to Bit Depth of Wavelength-Multiplexed Holograms

The quality of reconstructed images in relation to the bit depth of holograms formed by wavelength-selective phase-shifting digital holography was investigated. Wavelength-selective phase-shifting digital holography is a technique to obtain multiwavelength three-dimensional (3D) images with a full space-bandwidth product of an image sensor from wavelength-multiplexed phase-shifted holograms and has been proposed since 2013. The bit resolution required to obtain a multiwavelength holographic image was quantitatively and experimentally evaluated, and the relationship between wavelength resolution and dynamic range of an image sensor was numerically simulated. The results indicate that two-bit resolution per wavelength is required to conduct color 3D imaging.