Optical phase shifting with acousto-optic devices

Quantitative phase imaging based on holography: trends and new perspectives

In 1948, Dennis Gabor proposed the concept of holography, providing a pioneering solution to a quantitative description of the optical wavefront. After 75 years of development, holographic imaging has become a powerful tool for optical wavefront measurement and quantitative phase imaging. The emergence of this technology has given fresh energy to physics, biology, and materials science. Digital holography (DH) possesses the quantitative advantages of wide-field, non-contact, precise, and dynamic measurement capability for complex-waves. DH has unique capabilities for the propagation of optical fields by measuring light scattering with phase information. It offers quantitative visualization of the refractive index and thickness distribution of weak absorption samples, which plays a vital role in the pathophysiology of various diseases and the characterization of various materials. It provides a possibility to bridge the gap between the imaging and scattering disciplines. The propagation of wavefront is described by the complex amplitude. The complex-value in the complex-domain is reconstructed from the intensity-value measurement by camera in the real-domain. Here, we regard the process of holographic recording and reconstruction as a transformation between complex-domain and real-domain, and discuss the mathematics and physical principles of reconstruction. We review the DH in underlying principles, technical approaches, and the breadth of applications. We conclude with emerging challenges and opportunities based on combining holographic imaging with other methodologies that expand the scope and utility of holographic imaging even further. The multidisciplinary nature brings technology and application experts together in label-free cell biology, analytical chemistry, clinical sciences, wavefront sensing, and semiconductor production.

High-precision frequency-controlled optical phase shifter with acousto-optic devices

A fundamental parameter to determine how electromagnetic waves interfere is their relative phase, and achieving a fine control over it enables a wide range of interferometric applications. Existing phase control methods rely on modifying the optical path length either by changing the path followed by the light or by altering the thickness or index of refraction of an optical element in the setup. In this Letter, we present a novel, to the best of our knowledge, method, based on acousto-optic modulators (AOMs), which allows adjusting the phase by shifting the frequency of the light in a segment of its path. Since the amount of phase shift depends on the length of the segment, an optical fiber is used to realize a 2π shift. Two experimental implementations are described which deal with different sources of phase fluctuations. The first addresses fluctuations resulting from the optical fiber, while the second tackles unwanted variations originating from the AOMs.

Cascaded optically injection-locked semiconductor laser, rate equations analysis, frequency response, and its application for complex optical signal generation

In this paper, we introduce the concept of the cascaded optically injection-locked (OIL) semiconductor laser and present its novel rate equations. Then, the new locking range for this configuration has been obtained by mathematical demonstration. Subsequently, we modified a new adjustment for the detuning frequencies (Δ f i n j ) of the cascaded OIL system as well as the linewidth enhancement factors (α) values. Utilizing these tunings, improvements in the steady-state photon number and the phase modulation (PM) range become possible. Afterward, we define the generation of the complex optical signal area and extract the transfer function for investigating the frequency response of the cascaded system. The simulations have been performed once with identical α values and once with the various α values in the slave laser (SL) stages. We conclude that these novel proposed adjustments, combined with a strong injection ratio (Rinj) of 15 dB and a high bias current, can significantly broaden the bandwidth near 700 GHz while maintaining the fair gain available up to 180 GHz. Further, the generation of complex optical signal areas has been boosted for high-quality complex modulation applications. Eventually, we exhibit a novel approach for generating different α values in the SL stages by applying managed temperature variations in the experimental setup of the cascaded system, regardless of employing similar SLs.

Liquid crystal on silicon-based real time phase stepping interferometry by spatial multiplexing

In this paper, we present a method to perform phase-shifting interferometry in real time. The technique is based on the use of a parallel aligned liquid crystal on a silicon display as a customized reference mirror. In order to implement the four-step algorithm, a set of macropixels is programmed onto the display, and these are divided into four zones with the appropriate phase shifts. This way, by spatial multiplexing, it is possible to obtain the phase of the wavefront at a rate limited only by the integration time of the employed detector. The customized mirror is able to both compensate the initial curvature of the object under study and introduce the necessary phase shifts to perform phase calculation. Examples of the phase reconstruction of static and dynamic objects are shown.

A Cost-Effective Distributed Acoustic Sensor for Engineering Geology

A simple and cost-effective architecture of a distributed acoustic sensor (DAS) or a phase-OTDR for engineering geology is proposed. The architecture is based on the dual-pulse acquisition principle, where the dual probing pulse is formed via an unbalanced Michelson interferometer (MI). The necessary phase shifts between the sub-pulses of the dual-pulse are introduced using a 3 × 3 coupler built into the MI. Laser pulses are generated by direct modulation of the injection current, which obtains optical pulses with a duration of 7 ns. The use of an unbalanced MI for the formation of a dual-pulse reduces the requirements for the coherence of the laser source, as the introduced delay between sub-pulses is compensated in the fiber under test (FUT). Therefore, a laser with a relatively broad spectral linewidth of about 1 GHz can be used. To overcome the fading problem, as well as to ensure the linearity of the DAS response, the averaging of over 16 optical frequencies is used. The performance of the DAS was tested by recording a strong vibration impact on a horizontally buried cable and by the recording of seismic waves in a borehole in the seabed.

The Phase Modulating Micro-Mover Based on the MHD/MET System in the Reference Arm of the Scanning Interferometer

The possibility of using a magnetohydrodynamic drive (MHD) and amolecular-electronic transfer (MET) sensor as a single device for moving and precise control of the displacement of a movable mirror, which is part of a scanning interferometer, is considered. A prototype of such a device was developed and experimentally studied. A digital holographic image of the test object was obtained using an optical scheme containing a scanning interferometer with an MHD drive. The important advantages of the MHD drive in the problems of digital recording of hyperspectral holographic images have been discussed.

Dual spectral phase and diffraction angle compensation of a broadband AOM 4-f pulse-shaper for ultrafast spectroscopy

AOM-based pulse shaping as a method has been shown to provide many advantages in the field of ultrafast spectroscopy, in particular for the creation of phase matched pulse pairs for two-dimensional IR and electronic spectroscopy. In this paper we demonstrate the capabilities of a quartz-based AOM pulse-shaper to provide fine control over the phase and spatial dispersion of ultrafast supercontinuum pulses. We show that by using the Bragg condition, we can define a mask function for our AOM such that the angle of diffraction is constant for all frequencies. By summing all the contributions to spectral phase due to normal and anomalous dispersion of our optical components, and taking into account the intrinsic frequency dependent phase as a result of the acoustic sine wave propagating through the AOM, we can determine an optimal mask function that meets the Bragg condition for all frequencies, and generates compressed (∼50 fs) supercontinuum pulses.

Microfiber interferometer integrated with Au nanorods for an all-fiber phase shifter and switch

All-fiber integrated phase shifters and optical switches have important applications in photonic devices, such as optical controlling, optical fiber sensing, and signal processing. In this Letter, for the first time to the best of our knowledge, we integrated the photothermal effect of a nanomaterial based on gold nanorods (GNRs) and a microfiber interferometer to realize a compact all-optical fiber phase shifter. GNRs surrounding the microfiber were excited by near-infrared light via the evanescent interaction, subsequently releasing the heat through the photothermal effect. Then, the refractive index around the microfiber was varied to shift the interference dips in a reversible manner. Experimentally, a spectral shift efficiency of 0.16 nm/mW near the wavelength of 1550 nm was obtained using an excitation laser at the wavelength of 808 nm. The device also provided an all-optical switching with the modulation depth of 76.4%. The proposed GNR-based all-fiber device can provide high potentials in all-optical signal control applications.

Scalable Fourier transform system for instantly structured illumination in lithography

We report the development of a unique scalable Fourier transform 4-f system for instantly structured illumination in lithography. In the 4-f system, coupled with a 1-D grating and a phase retarder, the ±1st order of diffracted light from the grating serve as coherent incident sources for creating interference patterns on the image plane. By adjusting the grating and the phase retarder, the interference fringes with consecutive frequencies, as well as their orientations and phase shifts, can be generated instantly within a constant interference area. We demonstrate that by adapting this scalable Fourier transform system into lithography, the pixelated nano-fringe arrays with arbitrary frequencies and orientations can be dynamically produced in the photoresist with high variation resolution, suggesting its promising application for large-area functional materials based on space-variant nanostructures in lithography.

Effect of input phase modulation to a phase-sensitive optical amplifier

Many optical applications depend on amplitude modulating optical beams using devices such as acousto-optical modulators (AOMs) or optical choppers. Methods to add amplitude modulation (AM) often inadvertently impart phase modulation (PM) onto the light as well. While this PM is of no consequence to many phase-insensitive applications, phase-sensitive processes can be affected. Here we study the effects of input phase and amplitude modulation on the output of a quantum-noise limited phase-sensitive optical amplifier (PSA) realized in hot ⁸⁵Rb vapor. We investigate the dependence of PM on AOM alignment and demonstrate a novel approach to quantifying PM by using the PSA as a diagnostic tool. We then use this method to measure the alignment-dependent PM of an optical chopper which arises due to diffraction effects as the chopper blade passes through the optical beam.

Photonic Aharonov-Bohm effect in photon-phonon interactions

The Aharonov–Bohm effect is one of the most intriguing phenomena in both classical and quantum physics, and associates with a number of important and fundamental issues in quantum mechanics. The Aharonov–Bohm effects of charged particles have been experimentally demonstrated and found applications in various fields. Recently, attention has also focused on the Aharonov–Bohm effect for neutral particles, such as photons. Here we propose to utilize the photon–phonon interactions to demonstrate that photonic Aharonov–Bohm effects do exist for photons. By introducing nonreciprocal phases for photons, we observe experimentally a gauge potential for photons in the visible range based on the photon–phonon interactions in acousto-optic crystals, and demonstrate the photonic Aharonov–Bohm effect. The results presented here point to new possibilities to control and manipulate photons by designing an effective gauge potential.

The Aharonov–Bohm effect describes the influence of an electromagnetic vector potential on the phase of a charged particle. Here, Li et al. demonstrate that photon–phonon interactions can lead to the Aharonov–Bohm effect also for the electrically neutral photons.

Optical phase noise engineering via acousto-optic interaction and its interferometric applications

Rapid and fine control over the phase of light is demonstrated by transferring digitally generated phase jumps from radio-frequency electrical signals onto light by means of acousto-optic interaction, and the underlying mechanism elucidated. This technique was used to engineer optical phase noise by tailoring the statistics of phase jumps in the electrical signal, which was then quantified using visibility measurements of the interference fringes. Such controlled dephasing finds applications in modern experiments involving the spread or diffusion of light in optical networks. In addition, the zero-delay intensity-intensity correlation [G2(0)] values of light emerging from different ports of a well-stabilized Mach-Zehnder interferometer in the presence of engineered partial phase noise are calculated, and it is shown analytically how the dark port of the interferometer nontrivially becomes a weak source of highly correlated or bunched photons.

Fast, externally triggered, digital phase controller for an optical lattice

We present a method to control the phase of an optical lattice according to an external trigger signal. The method has a latency of less than 30 μs. Two phase locked digital synthesizers provide the driving signal for two acousto-optic modulators which control the frequency and phase of the counter-propagating beams which form a standing wave (optical lattice). A micro-controller with an external interrupt function is connected to the desired external signal, and updates the phase register of one of the synthesizers when the external signal changes. The standing wave (period λ/2 = 390 nm) can be moved by units of 49 nm with a mean jitter of 28 nm. The phase change is well known due to the digital nature of the synthesizer, and does not need calibration. The uses of the scheme include coherent control of atomic matter-wave dynamics.

High-speed three-dimensional shape measurements of objects with laser speckles and acousto-optical deflection

Many three-dimensional (3D) shape measurement techniques in stereophotogrammetry with temporal coded structured illumination are limited to static scenes because the time for measurement is too long in comparison to the object speed. The measurement of moving objects result in erroneous reconstructions. This is apparent to reduce measurement time to overcome this limitation, which is often done by increasing the projection rate for illumination while shrinking the amount of images taken for reconstruction. The projection rate limits most applications in its speed because digital light processing (DLP) projectors, which are widely used, bring a limited projection rate along. Our approach, in contrast, does not take a DLP. Instead we use laser speckles as projected patterns which are switched using an acousto-optical deflector. The projection rate is 10× higher than what the fastest stripe projection systems to our knowledge achieve. Hence, we present this uncommon but potential approach for highspeed (≈250 3Dfps= [3D measurements per second]), dense, and accurate 3D measurements of spatially separated objects and show the media that emphasizes the ability of accurate measurements while the objects under testing move.

Variable phase retarder made of a dielectric elastomer actuator

We present a polymeric optical phase retarder that is electrically tunable by a dielectric elastomer actuator. The soft material device affords a large tuning range (14pi at lambda=488 nm) combined with high accuracy in optical path length and low drift rate (8.3 nm/min). Furthermore, the phase retarder is not sensitive to polarization, introduces a wavefront distortion<lambda/30, and tolerates high power densities (>141 kW/cm2). We show the dynamics for periodic phase modulation and demonstrate a simple drive technique for fast phase stepping. The polymer-based device is inexpensive, easy to fabricate, and its design can be adapted to specific applications.