Search

Phase imaging techniques are pivotal for achieving high-contrast visualization of unstained biological specimens in vitro, which is typically not applicable in narrow spaces. Recently, multimode fiber (MMF) has shown promise in enabling high-resolution in vivo endoscopy in biological research. Herein, we introduce a novel, to the best of our knowledge, phase imaging microscopy technique employing a single multimode fiber, showcasing remarkable capabilities in high-contrast imaging and quantitative shape reconstruction through frequency-domain modulation. Our method, validated through comparisons with reflection and phase-contrast results, demonstrates exceptional ability in imaging diverse samples, including microspheres, semiconductor chips, and oral epithelial cells. Notably, the quantitative reconstruction of surface shape achieves a sensitivity of less than 100 nm, enabling the extraction of three-dimensional information from single focal plane images. Moreover, our technique excels in contrast enhancement and defocused background suppression, presenting a promising avenue for minimally invasive, high-contrast, label-free in vivo phase imaging.

The recent flourishing development of two-dimensional (2D) graphene has sparked considerable interest and extensive research on graphene-based optical fiber polarizers. However, studies on graphene-optical fiber polarizers focused on the structure with graphene films attached to side-polished fibers, which face challenges such as low birefringence of 10 -6 , low polarization extinction ratio (PER), and narrow polarizing window of tens of nanometers. Here, a fiber polarizer based on a graphene-photonic crystal fiber (Gr-PCF) is proposed firstly, which exhibits high birefringence of ∼2.5 × 10 -3 , high PER of ∼111 dB/mm, broad polarizing window of >400 nm, and tunable polarization states. Graphene or graphene/hBN/graphene (Gr/hBN/Gr) heterojunctions are attached to the surface of two square holes in the PCF to make one of the polarizing modes attenuate significantly. The tunability of the Fermi level (E F ) in Gr/hBN/Gr enables the proposed device to function as a polarizer or a polarization-maintaining fiber. The combination of PCF's endless single-mode feature and graphene's broadband optical response feature enables the fiber polarizer to exhibit a wide spectrum range with single-mode transmission characteristics.

Recent advancements have brought significant attention to photonic terahertz (THz)-integrated sensing and communication (ISAC) systems. In this work, we present an adaptive frequency offset (FO) compensation method for dual-chirp-based ISAC waveforms, using the fractional Fourier transform (FrFT) method. The proposed scheme can enable frequency synchronization without a need for training preambles and exhibit robustness against system noise. We validate this approach through an experimental demonstration in a 300 GHz photonic THz-ISAC system with 20 Gbps quadrature-phase shift keying (QPSK) data transmission and 1.5 cm range resolution. The experiment successfully compensates for frequency offsets ranging from -5 to 5 GHz, achieving an estimation error of less than 0.08% and a chirp-pilot power overhead of 0.5%.

A single multimode fiber has been applied in minimally invasive endoscopy with wavefront shaping for biological research such as brain imaging. Most of the fibers, such as step-index and graded-index multimode fibers, give rise to spatially variant blur due to limits on the numerical aperture and collection efficiency. Routines to solve this problem are based on iterative algorithms, which are often slow and computer-intense. We developed a method to synthesize datasets for driving a deep learning network to deblur and denoise the spatially variant degraded image. This approach is fast (5 ms), up to three orders of magnitude faster than the iterative way. Furthermore, our method can be applied to different types of fiber endoscopy, and two types of fiber are tested here. The performance is verified on fluorescence beads and three kinds of biological tissue sections in the experiment, demonstrating effectiveness in image enhancement.

Time-gated luminescence spectra are usually measured by laboratory instruments equipped with high-speed excitation sources and spectrometers, which are always bulky and expensive. To reduce the reliance on expensive laboratory instruments, we demonstrate the first, to the best of our knowledge, use of a smartphone for the detection of time-gated luminescence spectra. A mechanical chopper is used as the detection shutter and an optical switch is placed at the edge of the wheel to convert the chopping signal into a transistor-transistor logic (TTL) signal which is used to control the excitation source and achieve synchronization. The time-gated luminescence spectra at different delay times of Eu(TTA) 3 powder and the solutions of Eu-tetracycline complex are successfully detected with a temporal resolution of tens of microseconds by the proposed approach. We believe our approach offers a route toward portable instruments for the measurement of luminescence spectra and lifetimes.

In this Letter, a contact polarization holographic photoalignment method is proposed. In the holographic recording, a phase mask is contacted with a photoalignment film, making light carrying wavefront information interfere with reference light in the near-field region to realize polarization holographic pattern recording with a sub-micrometer feature size. The relevant theoretical derivation is given, and holographic recording of a 0.4 µm feature-size phase mask is realized. The proposed method can conveniently realize liquid-crystal binary diffractive optical elements with a sub-micrometer feature size. Off-axis diffraction can also be realized by superimposing the grating information by changing the angle between the substrate and the interference light.

Real-time 3-D tracking of a fast-moving object has found important applications in industry, traffic control, sports, biomedicine, defense, etc. However, it is difficult to adopt typical image-based object tracking systems in a fast-moving object tracking in real time and for a long duration, because reliable and robust image processing and analysis algorithms are often computationally exhausted, and limited storage and bandwidth can hardly fulfill the great demand of high-speed photography. Here we report an image-free 3-D tracking approach. The approach uses only two single-pixel detectors and a high-speed spatial light modulator for data acquisition. By illuminating the target moving object with six single-period Fourier basis patterns, the approach is able to analytically calculate the position of the object with the corresponding single-pixel measurements. The approach is low-cost, and data- and computation-efficient. We experimentally demonstrate that the proposed approach can detect and track a fast-moving object at a frame rate of 1666 frames per second by using a 10,000 Hz digital micromirror device. Benefiting from the wide working spectrum of single-pixel detectors, the reported approach might be applicable for hidden fast-moving object tracking.

A polarization beam splitter assisted by a subwavelength grating (SWG) is proposed. The SWG enables nearly 20-fold beat length reduction for TE, which makes the high extinction ratio (ER) possible. On the other hand, the embedded SWG preferably affects the refractive index of the even mode in the coupling region and broadens the bandwidth of the splitter. As a result, the ER of 28.7 dB (24.8 dB) for TE (TM) is obtained, while the insertion loss is only 0.10 dB (0.11 dB) at the wavelength of 1550 nm. The ER is more than 10 dB in the wavelength range of 1450-1625 nm for TE and 1495-1610 nm for TM.

This Letter proposes and demonstrates a linear-regression-based loss-extraction model that is applicable for all-pass rings and symmetrically and asymmetrically coupled add-drop rings. The model is derived by transforming the transmission spectra of a ring into linear relationships with no approximation. An all-pass ring resonator is fabricated to verify the model, and the experimental results indicate that the proposed model is more reliable than previously reported models [Opt. Express14, 3872 (2006)OPEXFF1094-408710.1364/OE.14.003872; Opt. Express15, 10553 (2007)OPEXFF1094-408710.1364/OE.15.010553; and J. Lightwave Technol.16, 1308 (1998)JLTEDG0733-872410.1109/50.701410].

This Letter presents both numerical and experimental results of a polarization-independent directional coupler based on slot waveguides with a subwavelength grating. The measured coupling efficiency is 97.4% for TE and 96.7% for TM polarization at a wavelength of 1550 nm. Further analysis shows that the proposed subwavelength grating directional coupler has a fabrication tolerance of ±20  nm for the grating structure and that the coupling efficiencies for the two polarizations are both higher than -0.5  dB (∼89%), exceeding the entire C-band (1525-1570 nm) experimentally.

Established diffractive optical elements (DOEs), such as Dammann gratings, whose phase profile is controlled by etching different depths into a transparent dielectric substrate, suffer from a contradiction between the complexity of fabrication procedures and the performance of such gratings. In this Letter, we combine the concept of geometric phase and phase modulation in depth, and prove by theoretical analysis and numerical simulation that nanorod arrays etched on a silicon substrate have a characteristic of strong polarization conversion between two circularly polarized states and can act as a highly efficient half-wave plate. More importantly, only by changing the orientation angles of each nanorod can the arrays control the phase of a circularly polarized light, cell by cell. With the above principle, we report the realization of nanorod-based Dammann gratings reaching diffraction efficiencies of 50%-52% in the C-band fiber telecommunications window (1530-1565 nm). In this design, uniform 4×4 spot arrays with an extending angle of 59°×59° can be obtained in the far field. Because of these advantages of the single-step fabrication procedure, accurate phase controlling, and strong polarization conversion, nanorod-based Dammann gratings could be utilized for various practical applications in a range of fields.

Free choice of splitting ratio is one of the main properties of a power splitter required in integrated photonics, but conventional multimode interference (MMI) power splitters can only obtain a few discrete ratios. This Letter presents both numerical and experimental results of an arbitrary-ratio 1×2 MMI power splitter, which is constructed by simply breaking the symmetry of the multimode region. In the new device, the power splitting ratio can be adjusted continuously from 100:0 to 50:50, while the dimension of the multimode section stays in the range of 1.5×(1.8-2.8)  μm. The experimental data also indicate that the proposed arbitrary-ratio splitter keeps the original advantages of MMI devices, such as low excess loss, weak wavelength dependence, and large fabrication tolerance.

Optical mode mismatch makes coupling between strip and slot waveguides a tough issue in integrated photonics. This Letter presents both numerical and experimental results of a strip-slot mode converter based on symmetric multimode interference (MMI). Distinct from previous reported converters which gradually convert the mode through sharp tips, the proposed solution makes full use of the symmetry of the two-fold image of MMI, and its field distribution similarity with a slot waveguide to convert the mode. A converter based on this mechanism is able to convert light from a TE-polarized fundamental mode of a strip waveguide to that of a slot waveguide, and vice versa. Strip-slot waveguide coupling though this mode converter has a measured efficiency of 97% (-0.13  dB), and the dimensions are as small as 1.24×6  μm. Further analysis shows that the proposed converter is highly tolerant to fabrication imperfections, and is wavelength-insensitive.

Extraordinary gain-assisted transmission in telecommunication regimes through circular nanohole arrays drilled on a metallic film is investigated theoretically. Silicon-compatible Er-Yb silicate, which has a photoluminescence peak in the telecommunication regime, was selected for optical amplification purposes. Geometrical parameters were optimized analytically in order to present transmission resonances at telecommunication regions. The required gain value for lossless propagation was determined by considering the surface-plasmon dispersion relation. Simulation results show that the predicted gain for lossless propagation cannot completely compensate the loss. By increasing gain value, absorption becomes zero and transmission approaches unit through a laser with a pumping power of 372 mW at 1480 nm.

This work proposes a new type of resonator: a single-mode half-disk resonator. Half of the resonator is solid, allowing large electrical or mechanical contacts, and a single-mode operation can be retained. A systematic design method is demonstrated and analyzed. For a 3 μm radius, the simulation predicts an internal Q-factor as high as 2.4×10⁵, and a loaded Q-factor of ~9000 is measured in experiments, comparable even with uncontacted microrings of bigger radii. The large contacts will improve the performance of a wide range of active devices. We present the contact resistance of a vertical PN junction modulator based on this structure, which can be reduced to nearly an order of magnitude, enabling a much faster modulation speed.

We propose a photolithographic method to fabricate nanostructures by employing a localized surface plasmon (LSP) mask generated by a soft mold on a thin metal film. The soft mold can be formed by transparent materials, such as polydimethylsiloxane, contacting firmly to the metal film. The pattern edges of the mold, serving as the fine tapers, can be used to excite LSPs and accumulate a large amount of localized energy from the incident light field, providing a modulated optical field in the resist with nanometer feature size. Nanolithographic results with a minimum feature size of 30 nm are demonstrated.