Amplitude Spatial Light Modulator Research Articles

A Route to Ultra‐Fast Amplitude‐Only Spatial Light Modulation using Phase‐Change Materials

AbstractA phase‐change material based, thin‐film, amplitude‐only spatial light modulator is presented. The modulator operates in reflection and modulates the amplitude of light incident on its surface with no effect on optical phase when the phase‐change material is switched between its amorphous and crystalline states. This is achieved using a thin‐film device with an embedded, switchable, GeTe phase‐change layer. Test modulation patterns are written to the device using laser scans, and the amplitude and phase response measured, using optical spectroscopy and off‐axis digital holography. Experimental results reveal reflected intensity to be modulated by up to 38%, with an averaged phase difference of less than ≈π/50. Since phase‐change materials such as GeTe can be switched on sub‐microsecond timescales, this approach maps out a route for ultra‐fast amplitude spatial light modulators with widespread applications in fields such as wavefront shaping, communications, sensing, and imaging.

Emulating atmospheric turbulence effects on a micro-mirror array: assessing the DMD for use with free-space-to-fibre optical connections

Emulated atmospheric turbulence effects constructed from a set of 22 Zernike modes have been written upon a DMD micromirror array operating as a binary amplitude spatial light modulator. Sequences of aberrated frames with controlled amounts of turbulence have been produced and can be operated at controlled rates that can exceed 1 kHz rates which can be seen in strong turbulence. In this case 400 Hz was chosen and the scintillation levels observed for the same turbulence sequence with single, few and multi-mode fibres at a receiver. Resulting scintillation levels are consistent with standard turbulence models. Increased received intensity and reduced scintillation was observed with larger core fibres and related to aberration-induced focal spot size. Correlation between the received intensity variation and the amplitude variations for individual Zernike modes demonstrates specifically the effect of atmospheric induced beam wander when focusing into a receive fibre. The DMD is thus shown to be able to generate both the necessary frequency content and range of scintillation required for atmospheric emulation.

Ultrahigh-dynamic-range wavefront sensor based on absolute double-slit interferometry

This Letter reports a new, to the best of our knowledge, technique for the quality testing of steep optical samples by introducing an absolute interferometry method based on a double-slit interference experiment. We determine the quality of the sample with an ultrahigh-dynamic-range wavefront sensor by determining the deformation of the central fringe of the double-slit interferometer recorded for two different separations of the slits. The transmission function of the double slit is implemented on an amplitude spatial light modulator. Therefore, the slits’ location can be easily displaced over the entire area of the sample’s wavefront. We applied the proposed method on two samples: a microscope slide and a conventional ophthalmic lens, and maximum absolute phase variations of 0.33 and 26.7 rad were measured, respectively. Our estimation shows that an absolute phase variation of about 700 rad can be measured with this method.

High-performance beam steering based on liquid crystal on silicon device operating at low bit depths with the maximum efficiency

The bit depth of the state-of-the-art liquid crystal on silicon (LCOS) devices is increasing beyond the standard 8 bit. The extra bit depth is expected to improve the image quality of the information display systems, in which the LCOS devices operate as an amplitude spatial light modulator (SLM). The benefit of the extra depth, however, has not been proven when the LCOS devices operates in the phase-only mode for the beam steering applications. The impact of the extra bit depth on the performance of the hologram optimisation algorithms has not been investigated either. In this paper, we evaluated the beam steering performance of the phase-only LCOS devices operating at different bit depths. We also demonstrated a hologram optimisation technique that was less sensitive to the bit depth of the LCOS device.

Optimized dual spatial light modulators holographic display based on wavefront frequency decomposition

An optimized method of dual spatial light modulators holographic display is proposed in accordance with wavefront frequency decomposition. The wavefront frequency decomposition is conducted on the object light wave to generate two frequency images. The low frequency image is determined by amplitude computer-generated hologram (CGH) and then loaded on an amplitude spatial light modulator (SLM). The high frequency image is decided by phase CGH and loaded on a phase SLM. The mentioned two sub-holograms are reconstructed at the identical location and integrated superposed as the final result, so the pixel-to-pixel alignment between two spatial light modulators is not required to be considered. Moreover, the numerical analysis of image evaluation functions demonstrates that the proposed method is valid. As opposed to the conventional methods, the amplitude and phase information are enhanced. The quality of reconstructed images is optimized effectively (the peak signal-to-noise ratio by 3%, the image structure similarity by 3%, the speckle contrast by 30%, and the Michelson contrast by 15%). Optical experimental results show that the reconstruction details are more vivid.

Spectral-Coding-Based Compressive Single-Pixel NIR Spectroscopy in the Sub-Millisecond Regime.

In this contribution, we present a high-speed, multiplex, grating spectrometer based on a spectral coding approach that is founded on principles of compressive sensing. The spectrometer employs a single-pixel InGaAs detector to measure the signals encoded by an amplitude spatial light modulator (digital micromirror device, DMD). This approach leads to a speed advantage and multiplex sensitivity advantage atypical for standard dispersive systems. Exploiting the 18.2 kHz pattern rate of the DMD, we demonstrated 4.2 ms acquisition times for full spectra with a bandwidth of 450 nm (5250–4300 cm−1; 1.9–2.33 µm). Due to the programmability of the DMD, spectral regions of interest can be chosen freely, thus reducing acquisition times further, down to the sub-millisecond regime. The adjustable resolving power of the system accessed by means of computer simulations is discussed, quantified for different measurement modes, and verified by comparison with a state-of-the-art Fourier-transform infrared spectrometer. We show measurements of characteristic polymer absorption bands in different operation regimes of the spectrometer. The theoretical multiplex advantage of 8 was experimentally verified by comparison of the noise behavior of the spectral coding approach and a standard line-scan approach.

Optical image encryption and authentication scheme based on partial joint power spectrum and phase-retrieval technique

In this paper, a novel optical image encryption and authentication scheme based on partial joint power spectrum (JPS) and phase-retrieval technique has been proposed. In the proposed encryption process, a pure amplitude spatial light modulator (SLM) is utilized to modulate and display the entire input plane comprised with the real-valued plaintext and the amplitude key while the random phase mask (RPM) merged with the plaintext could be provided by the ground glass in the practical setup. The entire input plane is regarded as the input of a joint transform correlator (JTC)-based encryption structure while the sub-block (partial JPS) recorded by the charged-couple device (CCD) is the output (ciphertext). Since only the sub-block is recorded in the encryption process, the size of the plaintext could be larger than that of the CCD in a certain degree, which is a major advantage comparing to the conventional JTC-based cryptosystem. In addition, since the size of the ciphertext is irrelative to the size of the plaintext, the security of location key in the entire plane has been enhanced. In addition, the location keys of the chosen sub-block used as the ciphertext are regarded as additional private keys, which enlarges the key spaces of the conventional JTC-based cryptosystem. In the proposed authentication process, the sub-block (ciphertext) is used as the partial amplitude constraint in the iterative process based on phase-retrieval technique and then the decoded image is authenticated by the nonlinear optical verification operator. It is the first time that the optical image encryption and authentication using partial JPS as the ciphertext has been proposed. Numerical simulation is carried out to validate the feasibility and effectiveness of the proposed scheme.

Parallel encoding in optically multiplexed image camera for temporal resolution improvement

An optically multiplexed image camera often requires a photodetector for image sensing in conjunction with an optical multiplexer, such as a digital micromirror device or an amplitude spatial light modulator. For this type of camera, the advantages of using one photodetector, unfortunately, can be outrun by the fairly large acquisition time, consumed in the image multiplexing process. Therefore, we propose a solution to efficiently improve the image acquisition time span and to relax the computational burden in the reconstruction process. Our camera design incorporates a reflection optical multiplexer, for image encoding, in conjunction with multiple photodetectors. The working principle relies on sequential image encoding using multiple low-resolution sets of binary masks, applied synchronously. The reconstructed data sets form the segments of the final image, which are stitched together with dark boundary pixels.

Relative limitations of increasing the number of modulation levels in computer generated holography

Phase and amplitude spatial light modulators (SLMs) capable of both binary and multi-level modulation are widely available and offer a wide range of technologies to choose from for holographic applications. While the replay fields generated with multi-level phase-only SLMs are of a significantly higher quality than those generated by equivalent binary phase-only SLMs, evidence is presented in this letter that this improvement is not as marked for amplitude SLMs, where multi-level devices offer only a small benefit over their binary counterparts. Heuristic and numerical justifications for this are discussed and conclusions drawn.

Orbital angular momentum modes identification of optical vortices using binaural circular aperture

The identification of orbital angular momentum (OAM) modes is crucial in investigating and applying optical vortices (OVs). Here, we propose a novel diffraction device, a binaural circular aperture (BCA), to detect the OAM modes of OVs. The BCA is well designed and composed of a lemniscate and two semicircles. We first investigate the intensity distribution of the OVs passing through the BCA theoretically. The OVs with different OAM modes will produce different diffraction patterns, and the modes can be directly identified from the diffraction fringes. Building this BCA with an amplitude spatial light modulator, the OAM modes ranging from −6 to +6 are experimentally identified, and the results are well consistent with the simulation. When the BCA is resized in the millimetre level, the influence on the identification of OAM modes is very limited. These results show that the BCA we proposed can provide a feasible and effective way to rapidly identify the OAM modes of OVs, and shows its insensitivity to the dimensional deviation.

Generalized single-sideband three-dimensional computer-generated holography.

The conjugate image and zero-order beam have considerable influences on the optical reconstructions in computer-generated holographic display systems based on the amplitude spatial light modulators. We propose a generalized single-sideband method for suppressing the unwanted terms in computer-generated holography. Computer-generated holograms (CGHs) are calculated based on frequency filtering of the object wave, which redistributes the diffraction wave in spatial frequency domain for spectrum filtering during optical reconstruction. Numerical simulations and optical experiments demonstrate that the proposed method is generally effective for different kinds of CGH algorithms to reconstruct quality three-dimensional scenes that are free of conjugate image and zero-order beam.

Quantitative phase imaging with increased spatial coherence based on Fourier filtering.

This Letter reports a method for tuning the degree of spatial coherence in an optical microscope. The method employs an amplitude spatial light modulator (aSLM) placed in the Fourier plane of the microscope. The aSLM displays a set of binary filters that blocks specific frequencies of the Fourier spectrum of the complex object field. It is shown that numerical processing of these filtered fields provides a final intensity with increased spatial coherence. Coherence tuning is used in a new hybrid phase reconstruction algorithm that employs the transport of intensity equation and an iterative phase retrieval technique. It is validated experimentally that this hybrid approach is able to retrieve the phase information with high resolution, and effective artifact noise suppression when employing at minimum two symmetrical defocused intensities and large sources.

Widefield Confocal Microscope for Surface Wave K-Vector Measurement

Optical surface waves are guided light on surface of optical structures. There are several optical structures that support optical surface waves, such as, nanostructures, gratings, optical waveguides and metamaterials. Optical surface waves have proven themselves very promising candidates for several applications including, biosensing, optical computing and optical circuitry. The characteristics of surface wave can be characterised by the wave vector (k-vector) of the surface wave. In this paper, we will discuss how a modified confocal microscope integrated with a phase spatial light modulator allows us to measure both the real part and the imaginary part (attenuation coefficient) of the surface wave k-vector. Surface plasmon resonance (SPR) excited on a uniform gold surface through Kretschmann configuration is employed as an example in this talk. Note that the system is not limited to the SPR. It is also applicable to other types of optical surface waves. We have demonstrated in our recent publication that the modified confocal not only provides the k-vector measurement both real and imaginary, it also allows us to separate different loss mechanisms in SPR. One limitation of the system was the single point detection. Here we will discuss the current stage of our development in widefield confocal surface plasmon microscope, which allows us to measure multiple points simultaneously. This has been achieved by integrating another amplitude spatial light modulator in the image plane of the objective lens. Use of orthogonal illumination patterns allows the image plane to be sequentially amplitude modulated and post-processed to recover the confocal image.

电寻址振幅型空间光调制器的光束整形能力

电寻址振幅型空间光调制器的光束整形能力

High Resolution Quantitative Angle-Scanning Widefield Surface Plasmon Microscopy.

We describe the construction of a prismless widefield surface plasmon microscope; this has been applied to imaging of the interactions of protein and antibodies in aqueous media. The illumination angle of spatially incoherent diffuse laser illumination was controlled with an amplitude spatial light modulator placed in a conjugate back focal plane to allow dynamic control of the illumination angle. Quantitative surface plasmon microscopy images with high spatial resolution were acquired by post-processing a series of images obtained as a function of illumination angle. Experimental results are presented showing spatially and temporally resolved binding of a protein to a ligand. We also show theoretical results calculated by vector diffraction theory that accurately predict the response of the microscope on a spatially varying sample thus allowing proper quantification and interpretation of the experimental results.

광 스캐닝 홀로그래피를 이용한 양안식 3차원 홀로그래픽 영상 시스템

In this paper we propose a binocular holographic three-dimensional (3D) imaging system using optical scanning holography. To realize a binocular 3D holographic imaging system, we could acquire the complex holograms of a real object after designing a holographic display system based on interpupillary distance and pupil size, and these holograms could be optically reconstructed following numerical signal processing with an amplitude spatial light modulator. The proposed binocular 3D holographic imaging system using optical scanning holography was verified experimentally.

Color wavefront printer with mosaic delivery of primary colors

The paper presents design and implementation of a color wavefront holographic printer which prints white light viewable holograms of three-dimensional (3D) objects from digital contents. Similarly to other holographic printers, the printed hologram is composed as a two-dimensional array of elemental volume holograms. In the proposed wavefront printer, the 3D information was encoded in computer generated holograms displayed in succession on an amplitude spatial light modulator. The light beam diffracted from the modulator was filtered to extract the beam coming from the object and demagnified to be recorded onto the holographic emulsion as a small size elemental hologram that made possible application of mosaic delivery of exposures at primary colors. As a result, each elemental hologram corresponded to a single color channel. A modified phase-added stereogram approach based on a hologram partitioning method was proposed to accelerate computer generation of digital contents. We achieved bright 3D reconstruction with a motion parallax at saturated colors from holograms of test objects that were printed on a silver-halide emulsion. Thus we proved experimentally feasibility of recording analog color volume holograms from digital contents by applying spatially separated exposures at primary colors to the elemental holograms.

Variable optical attenuator and dynamic mode group equalizer for few mode fibers.

Variable optical attenuation (VOA) for three-mode fiber is experimentally presented, utilizing an amplitude spatial light modulator (SLM), achieving up to -28dB uniform attenuation for all modes. Using the ability to spatially vary the attenuation distribution with the SLM, we also achieve up to 10dB differential attenuation between the fiber's two supported mode group (LP₀₁ and LP₁₁). The spatially selective attenuation serves as the basis of a dynamic mode-group equalizer (DME), potentially gain-balancing mode dependent optical amplification. We extend the experimental three mode DME functionality with a performance analysis of a fiber supporting 6 spatial modes in four mode groups. The spatial modes' distribution and overlap limit the available dynamic range and performance of the DME in the higher mode count case.

Anamorphic optical transformation of an amplitude spatial light modulator to a complex spatial light modulator with square pixels [invited

A method is proposed for the construction of a square pixel complex spatial light modulator (SLM) from a commercial oblong full-high-definition (full-HD) amplitude SLM using an anamorphic optical filter. In the proposed scheme, one half-band of the optical Fourier transform of the amplitude-only spatial light field is rejected in the optical Fourier plane and the other half-band is reformatted to be an effective complex SLM with square pixels. This has an advantage in the viewing window plane since the shape of the viewing window becomes square and more ideal for observers who watch the hologram contents through it. For optimal transformation, the amplitude computer generated hologram encoding scheme was developed. Mathematical modeling of the proposed system is described herein, and it was experimentally demonstrated that the effective complex SLM displays complex holographic three-dimensional images with a clear depth discrimination effect.

Holographic memory optical system based on computer-generated Fourier holograms

Holography is known to be a prospective tool for storing large amounts of digital information, providing long lasting safety and high speed data access. In this paper, we present a new approach to holographic memory system design. Our method is based on an application of discrete Fourier-transform calculations to encode two-dimensional binary data pages as computer-generated amplitude Fourier holograms (CGFHs). These CGFHs, represented as grayscale raster images, can be displayed with the use of a high resolution amplitude spatial light modulator (SLM) in an optical projection system and exposed on holographic medium with multiple reduction. The optical scheme required for the technical realization of this method appears significantly simpler compared with known holographic memory recording devices; moreover, it can be built using either coherent or incoherent light sources. Coding of data pages by precise pseudorandom phase masks during CGFH synthesis allows us to achieve about 3% of the recorded microholograms diffraction efficiency. The experimental results of CGFH projection recorded with a 20× reduction on photosensitive holographic medium and its reconstruction are presented.