Single-mode Light Research Articles

Experimental Research on the Correction of Vortex Light Wavefront Distortion

Wavefront distortion occurs when vortex beams are transmitted in the atmosphere. The turbulence effect greatly affects the transmission of information, so it is necessary to use adaptive optical correction technology to correct the wavefront distortion of the vortex beam at the receiving end. In this paper, a method of vortex wavefront distortion correction based on the deep deterministic policy gradient algorithm is proposed; this is a new correction method that can effectively handle high-dimensional state and action spaces and is especially suitable for correction problems in continuous action spaces. The entire system uses adaptive wavefront correction technology without a wavefront sensor. The simulation results show that the deep deterministic policy gradient algorithm can effectively correct the distorted vortex beams and improve the mode purity, and the intensity correlation coefficient of single-mode vortex light can be increased to about 0.88 and 0.69, respectively, under weak turbulence and strong turbulence, and the intensity coefficient of weak-turbulence multi-mode vortex light can be increased to about 0.96. The experimental results also show that the adaptive correction technology based on the deep deterministic policy gradient algorithm can effectively correct the wavefront distortion of vortex light.

Optimal quantum metrology of two-photon absorption

Two-photon absorption (TPA) is a nonlinear optical process with wide-ranging applications from spectroscopy to super-resolution imaging. Despite this, the precise measurement and characterisation of TPA parameters are challenging due to their inherently weak nature. We study the potential of single-mode quantum light to enhance TPA parameter estimation through the quantum Fisher information (QFI). Discrete variable quantum states (defined to be a finite superposition of Fock states) are optimised to maximise the QFI for given absorption, revealing a quantum advantage compared to both the coherent state (classical) benchmark and the single-mode squeezed vacuum state. For fixed average energy n¯∈2N , the Fock state is shown to be optimal for large TPA parameters, while a superposition of vacuum and a particular Fock state is optimal for small absorption for all n¯ . This differs from single-photon absorption where the Fock state is always optimal. Notably, photon counting is demonstrated to offer optimal or nearly optimal performance compared to the QFI bound for all levels of TPA parameters for the optimised quantum probes, and their quantum advantage is shown to be robust to single-photon loss. Our findings provide insight into known limiting behaviours of Gaussian probes and their different FI scalings under photon counting ( ∝n¯2 for squeezed vacuum states versus n¯3 for coherent states). The squeezed state outperforms coherent states for small TPA parameters but underperforms in the intermediate regime, becoming comparable in the large absorption limit. This can be explained through fundamental differences between behaviours of even and odd number Fock states: the former’s QFI diverges in both large and small absorption limits, while the latter diverges only in the small absorption limit, dominating at intermediate scales.

OAM-mode coupling by segmented helical-ring-core waveguides inscribed with a femtosecond laser beam.

Azimuthally segmented helical-ring-core waveguides (HRCWs) were fabricated in silica glass by direct laser writing. Triple-segmented waveguides provided single-mode or few-mode light guidance depending on the core diameter. Bragg resonance reflection of the light carrying orbital angular momentum (OAM) was registered confirming the generalized angular phase-matching condition, which considers the azimuthally segmented structure of the waveguides.

Bragg gratings with novel waveguide models fabricated in bulk glass via fs-laser writing and their slow-light effects.

We present the experimental realization of an innovative parallel partially overlapping waveguides (PO-WGs) model grounded in the thermal accumulated regime and fabricated using femtosecond (fs) laser direct-writing within low-iron bulk glass. The 75mm long novel PO-WGs model was made by partially overlapping the shell parts of two core-shell types of waveguides via a back-and-forth single pass fs-laser inscription. The detailed evolution of the PO-WGs model from inception to completion was offered, accompanying by a thorough characterization, which unveils a substantial refractive index (RI) change, on the order of 10-3, alongside low propagation loss (0.2 dB/cm) and distinctive features associated with the single mode and shell-guided light. Notably, the unsaturated performance of PO-WGs model after the primary inscription paves the way for potential applications in the successful creation of two distinctive types of Bragg gratings: first-order dot-Bragg grating and second-order line-Bragg grating. The 75 mm long dot-Bragg grating was written by a periodic dot array with a height of 6 µm atop the PO-WGs, and the birefringence was measured of 1.5 × 10-5 with a 16 pm birefringence-induced wavelength difference. The line-Bragg grating, which was inscribed with dual PO-WGs extending the line grating part to 40 mm in length along its period for increasing the transmission dip, exhibits a pronounced polarization dependence showcasing an effective birefringence of 4.2 × 10-4 at the birefringence-induced wavelength difference of 0.45 nm. We delved into the slow-light effects of the two Bragg gratings thoroughly, which the theoretical analysis revealed an effective group delay of 0.58 ns (group index 2.3) for the dot-Bragg grating. Similarly, the line-Bragg grating exhibited an effective group delay of 0.3 ns (group index 2.3), in good agreement with experimental measurements. These findings underscore the exciting potential of our gratings for creating optical slow-wave structures, particularly for future on-chip applications.

Exploring Statistical and Nonclassical Properties in Cavity Optomechanical Systems Applying Arbitrary Single-mode Light States

Exploring Statistical and Nonclassical Properties in Cavity Optomechanical Systems Applying Arbitrary Single-mode Light States

Delta interferometer: A polarization sensitive interferometer

Abstract A new type of polarization sensitive interferometer is proposed, which is named as Delta interferometer due to the geometry of the simplest interferometer of this type looks like Greek letter, Delta. The main difference between Delta interferometer and other existed interferometer, such as Michelson interferometer, Mach-Zehnder interferometer, Young’s double-slit interferometer, is that the two interfering paths are asymmetrical in Delta interferometer. The visibility of the first-order interference pattern observed in Delta interferometer is dependent on the polarization of the incidental light. Optical coherence theory is employed to interpret the phenomenon and single-mode continuous-wave laser light is employed to verify the theoretical predictions. The theoretical and experimental results are consistent. Delta interferometer provides a perfect tool to study the reflection of electromagnetic field in different polarizations and may find applications in polarization sensitive scenarios.

Single-mode waveguide-coupled light emitting diodes in unmodified silicon photonics fabrication processes

We realize single-mode, waveguide-coupled, electrically driven silicon light emitting diodes in commercial, unmodified silicon photonics foundry processes and develop a model of both the electrical and optical behavior to understand the performance limitations. We measure a center wavelength of 1130 nm, a 90 nm 3 dB optical bandwidth, and 200 pW of optical power propagating in each direction. We show on-chip modulation and detection of the generated light using native resonant photodetectors integrated in the same chip. Our work unveils a new native light source available in silicon photonics processes, which can find applications ranging from device screening and fabrication quality assessment to imaging, refractive index sensing, or intra-chip communication.

Highly sensitive SPR curvature sensor based on graded-index fiber.

At present, fiber curvature sensors based on surface plasmon resonance (SPR) are mostly of the multimode fiber core type or cladding type. These types have many SPR modes, resulting that the sensitivity cannot be adjusted and is difficult to improve. In this Letter, a highly sensitive SPR curvature sensor based on graded-index fiber is proposed. The light-injecting fiber is eccentrically connected with the graded-index fiber to inject single-mode light. Due to the self-focusing effect, the light beam propagates in the graded-index multimode fiber with a cosine trajectory, and the cosine beam contacts the flat grooved sensing region fabricated on the graded-index fiber to generate SPR. Due to the single transmission mode of the proposed fiber SPR sensor, the curvature sensing sensitivity is greatly improved. By changing the light injection position of the graded-index multimode fiber, the sensitivity can be adjusted. The proposed curvature sensing probe has a high sensitivity and can identify the bending direction. When bending in the X direction, the sensitivity reaches 5.62 nm/m-1, and when bending in the - X direction, the sensitivity reaches 4.75 nm/m-1, which provides a new scheme for highly sensitive and directionally identifiable curvature measurement.

Mid-infrared negative curvature hollow-core fiber with elliptically nested tubular structure

Arsenic triselenide chalcogenide negative curvature hollow-core fiber with ellipse-nested tubular cladding elements was numerically presented for the low-loss light guidance in the mid-infrared region. Confinement and total losses were calculated for the fiber with a five-tube design as low as 0.34 dB/km and 4.55 dB/km at 10.6μm, respectively. The single-mode light guidance characteristic of the proposed design was investigated, and a strong suppression on the higher-order modes shows that the fiber design favors the light guidance with the fundamental mode. A dispersion control study was carried out to optimize the primary design parameters to shift the zero-dispersion wavelength close to 10.6μm. The bending loss performance of the proposed design was analyzed in the targeted spectrum. The proposed negative curvature hollow-core fiber could be potentially used in low-loss guidance for high-power lasers and could also be utilized for several other applications, such as surgery and drug delivery in the mid-IR spectrum.

Experimental study on a light–heavy interface evolution induced by two successive shock waves

Richtmyer–Meshkov instability induced by two successive shock waves is experimentally studied in a specific shock tube. To create two successive shock waves synchronously, a driver section is added between the driver and driven sections of the standard shock tube, and an electronically controlled membrane rupture equipment is adopted. The shock-tube flow after the membranes rupture is well described by combining the shock relations, isentropic wave relations with compatibility relations across the contact surface (region). The new shock tube is capable of generating two successive shock waves with controllable strengths and time interval, and provides a relatively ‘clean’ wave system. Then the developments of single-mode light–heavy interfaces with different initial conditions induced by two successive shock waves are investigated. The initial amplitudes are all small enough such that the first-shocked interface is within the linear growth regime at the arrival of the second shock. The results show that if the pre-second-shock perturbation amplitude is small, the linear, nonlinear and modal evolutions of the double-shocked interface can be reasonably predicted by the existing models proposed for predicting the perturbation growth induced by a single shock. For the double-shocked interface, the second shock provides an additional perturbation velocity field to the original one introduced by the first shock impact. The validity of the linear superposition model indicates that the linear superposition of these two perturbation velocity fields is satisfied. Therefore, a double-shocked interface evolves similarly to a single-shocked interface provided that their postshock amplitudes and linear growth rates are the same.

Focus on lasers, imaging, microscopy, and photonics

Focus on lasers, imaging, microscopy, and photonics

Nanoscale Valley Modulation by Surface Plasmon Interference.

Excitons in two-dimensional (2D) materials have attracted the attention of the community to develop improved photoelectronic devices. Previous reports are based on direct excitation where the out-of-plane illumination projects a uniform single-mode light spot. However, because of the optical diffraction limit, the minimal spot size is a few micrometers, inhibiting the precise manipulation and control of excitons at the nanoscale level. Herein, we introduced the in-plane coherent surface plasmonic interference (SPI) field to excite and modulate excitons remotely. Compared to the out-of-plane light, a uniform in-plane SPI suggests a more compact spatial volume and an abundance of mode selections for a single or an array of device modulation. Our results not only build up a fundamental platform for operating and encoding the exciton states at the nanoscale level but also provide a new avenue toward all-optical integrated valleytronic chips for future quantum computation and information applications.

Scalable single-mode surface-emitting laser via open-Dirac singularities.

Single-aperture cavities are a key component of lasers that are instrumental for the amplification and emission of a single light mode. However, the appearance of high-order transverse modes as the size of the cavities increases has frustrated efforts to scale-up cavities while preserving single-mode operation since the invention of the laser six decades ago1-8. A suitable physical mechanism that allows single-mode lasing irrespective of the cavity size-a 'scale invariant' cavity or laser-has not been identified yet. Here we propose and demonstrate experimentally that open-Dirac electromagnetic cavities with linear dispersion-which in our devices are realized by a truncated photonic crystal arranged in a hexagonal pattern-exhibit unconventional scaling of losses in reciprocal space, leading to single-mode lasing that is maintained as the cavity is scaled up in size. The physical origin of this phenomenon lies in the convergence of the complex part of the free spectral range in open-Dirac cavities towards a constant governed by the loss rates of distinct Bloch bands, whereas for common cavities it converges to zero as the size grows, leading to inevitable multimode emission. An unconventional flat-envelope fundamental mode locks all unit cells in the cavity in phase, leading to single-mode lasing. We name such sources Berkeley surface-emitting lasers (BerkSELs) and demonstrate that their far-field corresponds to a topological singularity of charge two, in agreement with our theory. Open-Dirac cavities unlock avenues for light-matter interaction and cavity quantum electrodynamics.

High-resolution sub-millimetre diameter side-viewing all-optical ultrasound transducer based on a single dual-clad optical fibre.

All-optical ultrasound (OpUS), where ultrasound is both generated and received using light, has emerged as a modality well-suited to highly miniaturised applications. In this work we present a proof-of-concept OpUS transducer built onto a single optical fibre with a highly miniaturised lateral dimension (<0.8 mm). A key innovation was to use a dual-clad optical fibre (DCF) to provide multimode light for ultrasound generation and single mode light for ultrasound reception. The transducer comprised a proximal section of DCF spliced to a short section of single mode fibre (SMF). Multimode light was outcoupled at the splice joint and guided within a square capillary to provide excitation for ultrasound generation. Whilst single mode light was guided to the distal tip of the SMF to a plano-concave microresonator for ultrasound reception. The device was capable of generating ultrasound with pressures >0.4 MPa and a corresponding bandwidth >27 MHz. Concurrent ultrasound generation and reception from the transducer enabled imaging via motorised pull-back allowing image acquisition times of 4 s for an aperture of 20 mm. Image resolution was as low as ~50 µm and 190 µm in the axial and lateral extents, respectively, without the need for image reconstruction. Porcine aorta was imaged ex vivo demonstrating detailed ultrasound images. The unprecedented level of miniaturisation along with the high image quality produced by this device represents a radical new paradigm for minimally invasive imaging.

Squeezing Light via Levitated Cavity Optomechanics

Squeezing light is a critical resource in both fundamental physics and precision measurement. Squeezing light has been generated through optical-parametric amplification inside an optical resonator. However, preparing the squeezing light in an optomechanical system is still a challenge for the thermal noise inevitably coupled to the system. We consider an optically levitated nano-particle in a bichromatic cavity, in which two cavity modes could be excited by the scattering photons of the dual tweezers, respectively. Based on the coherent scattering mechanism, the ultra-strong coupling between the cavity field and the torsional motion of nano-particle could be achieved for the current experimental conditions. With the back-action of the optically levitated nano-particle, the broad single-mode squeezing light can be realized in the bad cavity regime. Even at room temperature, the single-mode light can be squeezed for more than 17 dB, which is far beyond the 3 dB limit. The two-mode squeezing light can also be generated, if the optical tweezers contain two frequencies, one is on the red sideband of the cavity mode, the other is on the blue sideband. The two-mode squeezing can be maximized near the boundary of the system stable regime and is sensitive to both the cavity decay rate and the power of the optical tweezers.

Influence of top mirror on performance of GaN-based resonant cavity light-emitting diode

In this paper, two kinds of distributed Bragg reflectors (DBRs) with high-reflective-film structure and filter structure are designed and evaporated on the top of GaN-based resonant cavity light emitting diode (RCLED), respectively. Firstly, the reflectivity spectra of the two kinds of DBRs are simulated. Then, the differences in performance including optical longitudinal modes, spectral linewidth, and output light intensity between the two kinds of RCLED devices with different top mirrors, are compared and analyzed. Finally, the influence of the top mirror reflection characteristics on the output spectrum of the RCLED is studied in detail. The results show that the top mirror is an important part of RCLED, and its reflection characteristics determine the optical performance of the device. For the conventional DBR with high-reflective-film structure, its reflectivity spectrum has a wide high-reflection band. Accordingly, the spectral linewidth of the RCLED can be effectively narrowed by using the conventional DBR as the top mirror. However, the spectrum still consists of multi-longitudinal modes. For the DBR with filter structure, its reflectivity spectrum has a narrow high-transmittance band at the central wavelength. Depending on the modulation effect of the high-transmittance band to the output light, single longitudinal mode light emission is realized for the RCLED with the specially designed DBR as the top mirror, which shows a broad application prospect in optical communication and optical fiber sensing. Moreover, the spectral characteristics of the RCLED can be further optimized to meet its application requirements in much more fields, by designing the top mirror structure and changing its reflectivity spectrum characteristics.

Design of a half-ring plasmonic tweezers for environmental monitoring

An integrated near-field plasmonic tweezers, based on two gold half-ring elements, is proposed. The device consists of a silicon-on-insulator (SOI) waveguide on which two symmetric half-ring-shaped elements are deposited. The SOI waveguide, having dimensions 500 × 220 nm, enables single mode light propagation at 1.55 μm. The structure of the half-ring plasmonic tweezers exhibits up to five design parameters: inner radius ri, outer radius re, gap g, height h and angle ϑ. Gold permittivity is modeled by means of a Drude equation. The trapping performance is investigated by evaluating the optical force exerted on a nanoparticle having a radius of 25 nm, together with the stiffness and the trapping potential. Simulation results show that stable trapping is achieved for an input power as low as 27 mW at 1.55 μm, with a stiffness of −4.06 fN mW−1 nm−1.

Comparison of the semiclassical and quantum optical field dynamics in a pulse-excited optical cavity with a finite number of quantum emitters

The spectral and temporal response of a set of $N$ quantum emitters embedded in a photonic cavity is studied. Quantum mechanically, such systems can be described by the Tavis-Cummings (TC) model of $N$ two-level systems coupled to a single light mode. Here we compare the full quantum solution of the TC model for different numbers of quantum emitters with its semiclassical limit after a pulsed excitation of the cavity mode. Considering different pulse amplitudes, we find that the spectra obtained from the TC model approach the semiclassical one for an increasing number of emitters $N$. Furthermore they match very well for small pulse amplitudes. While we observe a very good agreement in the temporal dynamics for photon numbers much smaller than $N$, considerable deviations occur in the regime of photon numbers similar to or larger than $N$, which are linked to collapse and revival phenomena. Wigner functions of the light mode are calculated for different scenarios to analyze the quantum state of the light field. We find strong deviations from a coherent state even if the dynamics of the expectation values are still well described by the semiclassical limit. For higher pulse amplitudes Wigner functions similar to those of Schr\"odinger cat states between two or more quasi-coherent contributions build up.

High-power operations of single-mode surface grating long oxide aperture VCSELs

A high-power single-mode vertical cavity surface emitting laser (VCSEL) based on a slow light mode (SLM) concept is demonstrated. Shallow surface gratings were implemented to select a single slow light mode in mm long oxide aperture VCSELs. The vertical structure and well-established fabrication processes are unchanged from those of conventional 850 nm VCSELs. An electron-beam lithography is used to form a 1st-order shallow grating on the surface. Thanks to the low effective index and the intensity penetration of the SLM to the surface, the grating fabrication process is much easier than conventional guided mode distributed feedback lasers. A stable single mode with a high side suppression ration of more than 30 dB is obtained for the entire current range under CW operations. A narrow divergence of 0.038° in far field patterns is achieved for a 6 mm long device. A record single-mode power of more than 5 W is realized before saturation under 50 ns pulse operations. Unlike long cavity in-plane edge emitting lasers, a SLM-based VCSEL can provide higher output powers without limitation of catastrophic optical mirror damage. The high power, superior mode stability, and good beam quality of our unique design paves a way for VCSELs to be used for wide ranges of high-power applications.

(Invited) Selective Epitaxy of Submicron Ge Wire Structures for Photodetectors and Optical Modulators in Si Photonics

Ge epitaxial layer on a Si-on-insulator substrate has been applied to waveguide photodetectors (PDs) in Si photonics, which operate at the optical communication wavelengths (1.3–1.6 µm). For enhancing the operating frequency over 50 GHz with keeping a high photodetection efficiency, a sub-micron-wide Ge wire structure with a lateral pin junction is effective because of the RC delay smaller than that for conventional vertical pin PDs. Such a Ge wire structure allows a single-mode light propagation, being also applicable to electro-absorption optical modulators (EA-MODs) based on the Franz-Keldysh effect. Here, in terms of higher-performance PDs and EA-MODs, sub-micron Ge wire structures by selective epitaxial growth on Si are presented. Bandgap engineering is discussed utilizing a control of grown-in tensile strain in sub-micron Ge.Figure 1(a) shows a typical cross-sectional scanning electron microscope image of a Ge wire structure with the width as narrow as 0.6 µm. The Ge wire in the [110] direction was selectively grown on an exposed Si (001) surface in a SiO2-covered Si substrate by ultrahigh-vacuum chemical vapor deposition at 700°C. The wire surface was surrounded mainly with inclined {113} and {111} facet planes, remaining a narrow (001) top plane. For protecting the fragile Ge surface, a Si cap layer was subsequently grown at 600°C, whereas a surface roughening was unfavorably induced on the {113} planes near the boundary with the {111} plane, as in Fig. 1(b). Raman spectra show that a SiGe alloy was accompanied on the wire surfaces, although no such SiGe was detected on the (001) plane of wide mesa top [1]. Based on theoretical analyses, a mass transport together with a Si-Ge interdiffusion would be induced by a stress concentration at the {113}/{111} facet boundaries. It is important that the roughening and alloying were significantly suppressed by decreasing the growth temperature for the Si cap layer to 530°C, as in Fig. 1(c).Typical photoluminescence (PL) spectra are shown in Fig. 1(d). For the Ge wire structure, the PL peak due to the direct transitions was located at the wavelength of 1.55 µm, which is shorter than 1.57 µm for the blanket Ge film on Si. The peak position is similar to that for a bulk Ge wafer. The blue shift for the wire corresponds to a widening of direct bandgap, which results from an elastic relaxation [2] of grown-in tensile strain generated by the thermal expansion mismatch with the Si substrate. The blue shift plays a significant role for Ge EA-MODs. As reported recently [3], in contrast to the operating wavelength range of > 1.60 µm in previous studies, EA-MODs of a sub-micron Ge wire favorably operates in the 1.55 µm range of C band (1.530–1.565 µm).[1] Y. Ishikawa et al., Proc. SPIE 11193, 1119309 (2019).[2] M. Nishimura et al., ECS Trans. 86, 3 (2018).[3] J. Fujikata et al., Opt. Express 28, 33123 (2020). Figure 1