Coherent Optical Techniques Research Articles

Robust and adjustable dynamic scattering compensation for high-precision deep tissue optogenetics

The development of high-precision optogenetics in deep tissue is limited due to the strong optical scattering induced by biological tissue. Although various wavefront shaping techniques have been developed to compensate the scattering, it is still a challenge to non-invasively characterize the dynamic scattered optical wavefront inside the living tissue. Here, we present a non-invasive scattering compensation system with fast multidither coherent optical adaptive technique (fCOAT), which allows the rapid wavefront correction and stable focusing in dynamic scattering medium. We achieve subcellular-resolution focusing through 500-μm-thickness brain slices, or even three pieces overlapped mouse skulls after just one iteration with a 589 nm CW laser. Further, focusing through dynamic scattering medium such as live rat ear is also successfully achieved. The formed focus can maintain longer than 60 s, which satisfies the requirements of stable optogenetics manipulation. Moreover, the focus size is adjustable from subcellular level to tens of microns to freely match the various manipulation targets. With the specially designed fCOAT system, we successfully achieve single-cellular optogenetic manipulation through the brain tissue, with a stimulation efficiency enhancement up to 300% compared with that of the speckle.

Quantum-Controlled Collisions of H2 Molecules.

The amount of information that can be obtained from a scattering experiment depends upon the precision with which the quantum states are defined in the incoming channel. By precisely defining the incoming states and measuring the outgoing states in a scattering experiment, we set up the boundary condition for experimentally solving the Schrödinger equation. In this Perspective we discuss cold inelastic scattering experiments using the most theoretically tractable H2 and its isotopologues as the target. We prepare the target in a precisely defined rovibrational (v, j, m) quantum state using a special coherent optical technique called the Stark-induced adiabatic Raman passage (SARP). v and j represent the quantum numbers of the vibrational and rotational energy levels, and m refers to the projection of the rotational angular momentum vector j on a suitable quantization axis in the laboratory frame. Selection of the m quantum numbers defines the alignment of the molecular frame, which is necessary to probe the anisotropic interactions. For us to achieve the collision temperature in the range of a few degrees Kelvin, we co-expand the colliding partners in a mixed supersonic beam that is collimated to define a direction for the collision velocity. When the bond axis is aligned with respect to a well-defined collision velocity, SARP achieves stereodynamic control at the quantum scale. Through various examples of rotationally inelastic cold scattering experiments, we show how SARP coherently controls the dynamics of anisotropic interactions by preparing quantum superpositions of the orientational m states within a single rovibrational (v, j) energy state. A partial wave analysis, which has been developed for the cold scattering experiments, shows dominance of a resonant orbital that leaves its mark in the scattering angular distribution. These highly controlled cold collision experiments at the single partial wave limit allow the most direct comparison with the results of theoretical computations, necessary for accurate modeling of the molecular interaction potential.

Design and development of coherent-OOFDM techniques in wavelength division multiplexing optical network for long-haul communication

Abstract Today, communication is a crucial factor and requires a high data rate for transmission with in an error-free environment, so optical communication has emerged as a best solution to high-speed transmission systems due to its higher bandwidth and higher data rates. Fiber optic transmission integrating with some modern processing and modulator techniques are applied in various areas that need transfer of data from one point to another through reducing an optical affect. Coherent optical orthogonal frequency division multiplexing is a popular technique in optical communication today because of the excessive spectral efficiency, immune to interference, tolerance of the fiber dispersion and it is long-haul application. It employs the OFDM modulation method, which is a type of multicarrier modulation in which data statistics are distributed over a large number of low-cost subcarriers and is a solution to inter-symbol interference. Inter-symbol interference is generated by chromatic dispersion in high-speed optical transmission systems, and it is a severe issue in long-haul systems in the case of higher data rates transmission. The use of standard dispersion compensating devices, such as dispersion compensating fiber, does not completely compensate for high order dispersion, which includes third-order and high dispersion effects and also increase the loses of the system because it is a fiber in itself. This study proposes introducing the concept of coherent optical orthogonal frequency division multiplexing techniques in to the wave division optical network in the case of multiple users, high data rate transmission and long-haul and ultra-long-haul optical link to reduce the optical transmission on related issues and to improve the performance of optical network. The performance of the proposed network can be evaluated by using different parameters such as signal to noise ratio, bit error rate, optical signal to noise ratio, Quality factor and eye-diagram at different distance transmission up to 8000 km by using Optisystem software.

QCOB: a novel QPSK coherent optical OFDM modulation technique for mitigation of fiber impairments

Abstract Modern optical communication heavily relies on coherent optical orthogonal frequency division multiplexing (CO-OFDM) modulation. But the usage of optical OFDM for fiber optic communication is still constrained in comparison to other modes of communication like visible light communication (VLC) and wireless communication due to a number of issues such as various sorts of fiber impairments including dispersion and attenuation. In this research paper, a novel optical OFDM modulation technique has been proposed. It is quadrature phase shift keying (QPSK) CO-OFDM modulation with Bessel optical filter in fiber optical link and is named as QCOB. This modulation technique works for optical fiber transmission. Its testing has been carried out for parameters like bit error rate (BER) and quality-factor (Q-factor) over a range of fiber length, fiber dispersion coefficient and input power. This modulation employs coherent optical OFDM with QPSK for subcarrier modulation. The optical link utilizes a Bessel optical filter. Improved results have been achieved over conventional QPSK CO-OFDM (QCO). The obtained results demonstrate the superiority of the QCOB technique in terms of performance enhancement and fiber impairment reduction. In comparison to the QCO technique, the BER performance (reduction in BER) of the QCOB technique shows an improvement of around 7.4, 100, 14.4, 46.4 and 3.4% for dispersion coefficient values of 10, 20, 30, 50 and 100 (in ps/nm-km), respectively.

Mitigation of fiber impairments by developing a novel coherent optical 16-QAM OFDM modulation technique

Mitigation of fiber impairments by developing a novel coherent optical 16-QAM OFDM modulation technique

Review of Shearography for Dual-Directional Measurement

Shearography is a coherent optical technique that allows the identification of the first derivative of deformation in the shearing direction. Due to direct measuring strain information, shearography is suited for non-destructive testing and evaluation (NDT/NDE). However, if there is a small defect parallel to the shearing direction, the first derivative of deformation in the direction has no noticeable change, and the defect is not visible. Therefore, the development of a shearography system with dual-directional simultaneous measurement of the first derivatives of deformation both in x- and y-directions is highly demanded in the field of NDT/NDE. It is suited to inspect complicated defects, such as long and narrow slots, microcracks, etc. This paper presents a review of shearography for different dual-directional systems developed in the last two decades. After a brief overview of shearography, the paper will display two dual-directional shearographic techniques—temporal phase-shift (TPS) and spatial phase-shift (SPS) methods. TPS dual-shearing systems are suited for static measurements, while the SPS dual-shearing systems are useful for dynamic measurements. The basic theories, optical layouts, and comparisons are presented. The advantages and disadvantages of practical applications are discussed.

Self-Homodyne Coherent Optical Transmission Techniques

Self-Homodyne Coherent Optical Transmission Techniques

Reliability of wavefront shaping based on coherent optical adaptive technique in deep tissue focusing.

Wavefront shaping can compensate the wavefront distortions in deep tissue focusing, leading to an improved penetration depth. However, when using the backscattered signals as the feedback, unexpected compensation bias may be introduced, resulting in focusing position deviations or even no focus in the illumination focal plane. Here we investigated the reliability of wavefront shaping based on coherent optical adaptive technique in deep tissue focusing by measuring the position deviations between the foci in the illumination focal plane and the epi-detection plane. The experimental results show that when the penetration depth reaches 150 μm in mouse brain tissue (with scattering coefficient ~22.42 mm-1 ) using a 488 nm laser and an objective lens with 0.75 numerical aperture, the center of the real focus will deviate out of one radius range of the Airy disk while the optimized focus in the epi-detection plane maintained basically at the center. With the penetration depth increases, the peak to background ratio of the focus in the illumination focal plane decreases faster than that in the epi-detection plane. The results indicate that when the penetration depth reaches 150 μm, feedback based on backscattered signals will make wavefront shaping lose its reliability, which may provide a guidance for applications of non-invasive precise optogenetics or deep tissue optical stimulation using wavefront shaping methods. A, Intensity distribution in the epi-detection plane and the illumination focal plane before and after correction, corresponding to brain sections with 250 and 300 μm thickness, respectively. Scale bar is 2 μm. B, Averaged focusing deviations in the epi-detection plane (optimized) and the illumination focal plane (monitored) after compensation. The unit of the ordinate is one Airy disk diameter. Black dashed line represents one Airy disk radius. Bars represent the SE of each measurement set.

Aberration corrections of doughnut beam by adaptive optics in the turbid medium.

The doughnut beam is a spatially structured beam which has been widely used in super-resolution microscopy, laser trapping and so on. However, when it passes through thick scattering medium, aberrations will seriously affect its performance. Currently, adaptive optics (AO) has become one of the most powerful tools to compensate aberrations. However, conventional AO always suffers from limited corrected field of view (FOV). Here, we propose a method with conjugate AO system based on coherent optical adaptive technique. The results show that the corrected FOV can be improved effectively. For a wide range of the optical applications with doughnut beam, our method has potentials in correcting aberrations with high speed in turbid media.(A) Mouse brain slice, (B) the distribution of r PAO , (C) the distribution of r CAO . The vortex beam focus of the blue point in (B) and (C) among a 137.5 × 137.5 μm FOV (D1) ideally, (D2) with scattering, (D3) in pupil AO system and (D4) in conjugate AO system.

Multidither coherent optical adaptive technique for deep tissue two-photon microscopy

Two-photon microscopy normally suffers from the scattering of the tissue in biological imaging. Multidither coherent optical adaptive technique (COAT) can correct the scattered wavefront in parallel. However, the determination of the corrective phases may not be completely accurate using conventional method, which undermines the performance of this technique. In this paper, we theoretically demonstrate a method that can obtain more accurate corrective phases by determining the phase values from the square root of the fluorescence signal. A numerical simulation model is established to study the performance of adaptive optics in two-photon microscopy by combining scalar diffraction theory with vector diffraction theory. The results show that the distortion of the wavefront can be corrected more thoroughly with our method in two-photon imaging. In our simulation, with the scattering from a 450-[Formula: see text]m-thick mouse brain tissue, excitation focal spots with higher peak-to-background ratio (PBR) and images with higher contrast can be obtained. Hence, further enhancement of the multidither COAT correction performance in two-photon imaging can be expected.

Multi-point fiber refractometer using Fresnel reflection and a coherent optical frequency-domain multiplexing technique.

We demonstrate a simple multi-point refractometer based on the coherent optical frequency-domain multiplexing technique. As local refractive index sensors, interferometers of different lengths formed between cleaved fiber tips and reference weak reflectors are employed. Referent reflectors were fabricated by splicing into lead SMF-28 fiber a very short section of hollow optical fiber using a standard fiber splicer and a cleaver with an optical microscope. Despite the very simple configuration and manufacturing of the sensor, the refractive index resolution of 6×10-4 was demonstrated during proof-of-concept experiments.

Compressive <italic>k</italic>-Space Tomography

The coherent optical processing technique, referred to as k -space tomography, enables simultaneous detection of radio waves’ frequency and angle of arrival. As with the majority of signals, the spatial and spectral distribution of RF signals in the electromagnetic environment is sparse in nature. This inherent characteristic provides an ideal opportunity to merge this imaging modality with compressive sensing techniques. Compressive sensing has set a foundation for data acquisition of sparse signals from a small amount of data. By adapting specific aspects from this methodology, and applying them to k -space tomography, compressive k -space tomography is developed. Fundamental performance limits of this compressive sensing application are defined through numerical simulations, and these responses are subsequently utilized in experimental spatial-spectrum utilization mapping for impending 5G bands between 28 and 40 GHz. As a result, reconstruction data size and processing time are reduced consistently by a factor of over 100, without loss of information, for sufficiently sparse RF scenes.

Large field of view correction by using conjugate adaptive optics with multiple guide stars.

Adaptive optics has been widely used in the optical microscopy to recover high-resolution images deep into the sample. However, the corrected field of view (FOV) with a single correction is generally limited, which seriously restricts the imaging speed. In this article, we demonstrate a high-speed wavefront correction method by using the conjugate adaptive optical correction with multiple guide stars (CAOMG) based on the coherent optical adaptive technique. The results show that the CAOMG method can greatly improve the corrected FOV. For 120-μm-thick mouse brain tissue, the corrected FOV can be improved up to ~243 times of the conventional pupil adaptive optics (PAO) without additional time consumption. Therefore, this study shows the potential of high-speed imaging through scattering medium in biological science.

Micro-structured fiber distributed acoustic sensing system for borehole seismic survey

Distributed fiber optic sensing is increasingly recognized as a viable alternative to geophone arrays for the acquisition of borehole seismic data. The ability to acquire borehole seismic data in a producing well without the need to disrupt production also offers significant benefits to the operator. A novel fiber distributed acoustic sensor (DAS) with ultra-high sensitivity is presented. Through designing and fabricating the distributed micro-structured optical fiber (DMOF) with successive longitudinal microstructures to enhance the signal-to-noise ratio (SNR) of the backscattered light, and employing the coherent optical time domain reflectometer technique (C-OTDR), the sensing performance of acoustic signal can be greatly improved. Wide frequency band from 0.1Hz to 45 kHz along the 500m long sensing fiber with ultra-high strain resolution of 0.16nε is experimentally demonstrated on the ground surface and in a shallow borehole.

Aero-optical effects in free and wall-bounded turbulent compressible flows

The optical aberrations induced by turbulent compressible flows are a serious concern in airborne communication, imaging and optical systems because the quality of the laser beam degrades due to variations of the index of refraction along its path. The aero-optical aberrations produced by compressible flows are a direct consequence of the detailed physics of the flow. Time-accurate computational study of aero-optical distortions is performed based on large-eddy simulation technique because of its ability to account for the optical phase errors due to anisotropy of the flowfield and to resolve large-scale coherent structures at a reasonable computational cost. The results of numerical simulation of turbulent compressible flows in a flat plate boundary layer, free mixing layer and free jet provide general statistics of optical distortions by examining the time-averaged intensity pattern and instantaneous flowfield. The developed computational tools and results are beneficial in design and optimization of optical systems, where the control of laser beam propagation is an important problem, and coherent optical adaptive equipment and techniques based on the optical detection of light-scattering properties of the flow.

Label-free quantitative 3D tomographic imaging for partially coherent light microscopy.

Crucial benefits provided by partially coherent light microscopy such as improved spatial resolution, optical sectioning and speckle-noise suppression are exploited here to achieve 3D quantitative imaging: reconstruction of the object refractive index (RI). We present a partially coherent optical diffraction tomography technique (PC-ODT) that can be easily implemented in commercially available bright-field microscopes. We show that the high numerical apertures of the objective and condenser lenses, together with optical refocusing, are main issues for achieving fast and successful 3D RI reconstruction of weak objects. In particular, the optical refocusing is performed by a high-speed focus tunable lens mounted in front of the digital camera enabling compatibility with commercial microscopes. The technique is experimentally demonstrated on different examples: diatom cells (biosilica shells), polystyrene micro-spheres and blood cells. The results confirm the straightforward 3D-RI reconstruction of the samples providing valuable quantitative information for their analysis. Thus, the PC-ODT can be considered as an efficient and affordable alternative to coherent ODT which requires specially designed holographic microscopes.

Application of functional link artificial neural network for mitigating nonlinear effects in coherent optical OFDM

Artificial Neural Networks are developed as an important technique for equalization and have been widely used to mitigate the nonlinear effects in coherent optical systems. For the compensation of nonlinearities in coherent optical orthogonal frequency division multiplexing technique, the most popular artificial neural network model is a multilayer perceptron (MLP), as it is able to perform complex mapping between input and output spaces with significant success. However due to the complexity of multilayer perceptron nonlinear equalizer (MLP-NLE) model training of neural network is difficult. To overcome computational complexity issues of MLP-NLE, a single neuron based functional link artificial neural network nonlinear equalizer (FLANN-NLE) has been developed in this paper. Better performance of an equalizer is attributed to the usage of aPSO-BP algorithm for training the FLANN-NLE. The proposed FLANN-NLE surpasses the existing works both in terms of Q-Factor and computational complexity. For a fiber length of 1000 km and at launch power of −6 dBm, the improvement in Q-Factor is approximately equal to 3.3 and 1 dB in contrast to the previously reported values of approximately 3 and 0.7 dB at bit rate of 40 and 80 Gbps respectively.

Coherent optical adaptive technique improves the spatial resolution of STED microscopy in thick samples.

Stimulated emission depletion microscopy (STED) is one of far-field optical microscopy techniques that can provide sub-diffraction spatial resolution. The spatial resolution of the STED microscopy is determined by the specially engineered beam profile of the depletion beam and its power. However, the beam profile of the depletion beam may be distorted due to aberrations of optical systems and inhomogeneity of specimens' optical properties, resulting in a compromised spatial resolution. The situation gets deteriorated when thick samples are imaged. In the worst case, the sever distortion of the depletion beam profile may cause complete loss of the super resolution effect no matter how much depletion power is applied to specimens. Previously several adaptive optics approaches have been explored to compensate aberrations of systems and specimens. However, it is hard to correct the complicated high-order optical aberrations of specimens. In this report, we demonstrate that the complicated distorted wavefront from a thick phantom sample can be measured by using the coherent optical adaptive technique (COAT). The full correction can effectively maintain and improve the spatial resolution in imaging thick samples.

Plasmon Injection to Compensate and Control Losses in Negative Index Metamaterials.

Metamaterials have introduced a whole new world of unusual materials with functionalities that cannot be attained in naturally occurring material systems by mimicking and controlling the natural phenomena at subwavelength scales. However, the inherent absorption losses pose a fundamental challenge to the most fascinating applications of metamaterials. Based on a novel plasmon injection (PI or Π) scheme, we propose a coherent optical amplification technique to compensate losses in metamaterials. Although the proof of concept device here operates under normal incidence only, our proposed scheme can be generalized to an arbitrary form of incident waves. The Π scheme is fundamentally different from major optical amplification schemes. It does not require a gain medium, interaction with phonons, or any nonlinear medium. The Π scheme allows for loss-free metamaterials. It is ideally suited for mitigating losses in metamaterials operating in the visible spectrum and is scalable to other optical frequencies. These findings open the possibility of reviving the early dreams of making "magical" metamaterials from scratch.

A two-level detection algorithm for optical fiber vibration

Optical fiber vibration is detected by the coherent optical time domain reflection technique. In addition to the vibration signals, the reflected signals include clutters and noises, which lead to a high false alarm rate. The “cell averaging” constant false alarm rate algorithm has a high computing speed, but its detection performance will be declined in nonhomogeneous environments such as multiple targets. The “order statistics” constant false alarm rate algorithm has a distinct advantage in multiple target environments, but it has a lower computing speed. An intelligent two-level detection algorithm is presented based on “cell averaging” constant false alarm rate and “order statistics” constant false alarm rate which work in serial way, and the detection speed of “cell averaging” constant false alarm rate and performance of “order statistics” constant false alarm rate are conserved, respectively. Through the adaptive selection, the “cell averaging” is applied in homogeneous environments, and the two-level detection algorithm is employed in nonhomogeneous environments. Our Monte Carlo simulation results demonstrate that considering different signal noise ratios, the proposed algorithm gives better detection probability than that of “order statistics”.