Single-mode Fiber Array Research Articles

Next-generation endoscopic probe for detection of esophageal dysplasia using combined OCT and angle-resolved low-coherence interferometry.

Angle-resolved low-coherence interferometry (a/LCI) is an optical technique that enables depth-specific measurements of nuclear morphology, with applications to detecting epithelial cancers in various organs. Previous a/LCI setups have been limited by costly fiber-optic components and large footprints. Here, we present a novel a/LCI instrument incorporating a channel for optical coherence tomography (OCT) to provide real-time image guidance. We showcase the system's capabilities by acquiring imaging data from in vivo Barrett's esophagus patients. The main innovation in this geometry lies in implementing a pathlength-matched single-mode fiber array, offering substantial cost savings while preserving signal fidelity. A further innovation is the introduction of a specialized side-viewing probe tailored for esophageal imaging, featuring miniature optics housed in a custom 3D-printed enclosure attached to the tip of the endoscope. The integration of OCT guidance enhances the precision of tissue targeting by providing real-time morphology imaging. This novel device represents a significant advancement in clinical translation of an enhanced screening approach for esophageal precancer, paving the way for more effective early-stage detection and intervention strategies.

Cryogenic optical packaging using photonic wire bonds

The widespread adaptation of systems relying on optically controlled quantum information will require reliable and efficient multi-channel fiber-to-chip connections that function at cryogenic temperatures. Here we demonstrate low loss (2 dB per channel) connections between a single mode fiber array and tapered silicon waveguides down to 5 K using polymer based photonic wire bonds (PWBs). A method is described for assembling the silicon chip and fiber array such that the PWB connections are robust to temperature cycling and cryostat bakeout. The threshold power handling capability of the PWBs is greater than 4 dBm, sufficient to demonstrate optical bistability in silicon microring resonators coupled to the waveguides at 5 K.

Highly Compact All-Solid-State Beam Steering Module Based on a Metafiber

Optical phased arrays occupy the predominant position in the solid-state light detection and ranging; however, their applications are limited by high insertion loss, complex control, and the need for tunable lasers. Here, by assembling a quadratic silicon metalens onto the end face of a single-mode fiber array to form a metafiber, we propose an all-solid-state beam steering module. With the introduction of the flat metalens, the module is highly compact, and the field of view may be extremely wide. A large field of view up to about 60° is testified experimentally based on a one-dimensional module. The application for parking space monitoring is also demonstrated based on a two-dimensional module. The beam steering module can be switched between the scanning mode and the flash mode compatibly and can also be extended to a larger scale with a higher scanning precision by increasing the size of the fiber array and scaling the metalens. The presented scheme featured with high compactness, high performance, and good compatibility provides a distinctive beam steering candidate for light detection and ranging or optical wireless communication applications.

Optical orbital angular momentum multiplexing communication via inversely-designed multiphase plane light conversion

Multiplexing and demultiplexing of optical orbital angular momentum (OAM) are critical operations in mode-division multiplexing communications. Traditional Dammann gratings, spiral phase planes, and optical geometric transformations are regarded as convenient methods for OAM mode (de)multiplexing. However, crosstalk between the different modes and the difficulty of mode multiplexing greatly limit their application to mode-division multiplexing communications. Here, using a set of inversely-designed phase planes, we demonstrate an OAM (de)multiplexer based on multiphase plane light conversion that can enable perfect OAM multiplexing communication. The sorted patterns are Gaussian-like and can be coupled easily into single-mode fiber arrays. Inputs from the fiber array are turned into coaxial OAM modes after the phase planes. OAM mode crosstalk generated by the multiplexer is less than − 20 dB , with insertion loss of less than 2.6 dB. OAM modes are sorted by the demultiplexer with mode crosstalk below − 10 dB , and the sorting results are coupled to the fiber array. OAM modes carrying 10 Gbit/s on–off keying signals were transmitted in a 5 km few-mode fiber. The measured bit-error-rate curves have power penalties of less than 10 dB. The proposed configuration is highly efficient and convenient and will be beneficial for potential applications in quantum information, information processing, and optical communications.

High-speed multicolor structured illumination microscopy using a hexagonal single mode fiber array.

Structured illumination microscopy (SIM) is a widely used imaging technique that doubles the effective resolution of widefield microscopes. Most current implementations rely on diffractive elements, either gratings or programmable devices, to generate structured light patterns in the sample. These can be limited by spectral efficiency, speed, or both. Here we introduce the concept of fiber SIM that allows for camera frame rate limited pattern generation and manipulation over a broad wavelength range. Illumination patterns are generated by coupling laser beams into radially opposite pairs of fibers in a hexagonal single mode fiber array where the exit beams are relayed to the microscope objective's back focal plane. The phase stepping and rotation of the illumination patterns are controlled by fast electro-optic devices. We achieved a rate of 111 SIM frames per second and imaged with excitation patterns generated by both 488 nm and 532 nm lasers.

The Single-mode Complex Amplitude Refinement (SCAR) coronagraph

We present the monochromatic lab verification of the newly developed SCAR coronagraph that combines a phase plate (PP) in the pupil with a microlens-fed single-mode fiber array in the focal plane. The two SCAR designs that have been measured, create respectively a 360 degree and 180 degree dark region from 0.8–2.4λ∕D around the star. The 360 SCAR has been designed for a clear aperture and the 180 SCAR has been designed for a realistic aperture with central obscuration and spiders. The 360 SCAR creates a measured stellar null of 2–3 × 10−4, and the 180 SCAR reaches a null of 1 × 10−4. Their monochromatic contrast is maintained within a range of ±0.16λ∕D peak-to-valley tip-tilt, which shows the robustness against tip-tilt errors. The small inner working angle and tip-tilt stability makes the SCAR coronagraph a very promising technique for an upgrade of current high-contrast instruments to characterize and detect exoplanets in the solar neighborhood.

The Single-mode Complex Amplitude Refinement (SCAR) coronagraph

Context. The recent discovery of an Earth-mass exoplanet around the nearby star Proxima Centauri provides a prime target for the search for life on planets outside our solar system. Atmospheric characterization of these planets has been proposed by blocking the starlight with a stellar coronagraph and using a high-resolution spectrograph to search for reflected starlight off the planet. Aims. Due to the large flux ratio and small angular separation between Proxima b and its host star (≲10−7 and ≲2.2λ/D respectively; at 750 nm for an 8 m-class telescope) the coronagraph requires high starlight suppression at extremely-low inner working angles. Additionally, it must operate over a broad spectral bandwidth and under residual telescope vibrations. This allows for efficient use of spectroscopic post-processing techniques. We aim to find the global optimum of an integrated coronagraphic integral-field spectrograph. Methods. We present the Single-mode Complex Amplitude Refinement (SCAR) coronagraph that uses a microlens-fed single-mode fiber array in the focal plane downstream from a pupil-plane phase plate. The mode-filtering property of the single-mode fibers allows for the nulling of starlight on the fibers. The phase pattern in the pupil plane is specifically designed to take advantage of this mode-filtering capability. Second-order nulling on the fibers expands the spectral bandwidth and decreases the tip-tilt sensitivity of the coronagraph. Results. The SCAR coronagraph has a low inner working angle (∼1λ/D) at a contrast of < 3 × 10−5 for the six fibers surrounding the star using a sufficiently-good adaptive optics system. It can operate over broad spectral bandwidths (∼20%) and delivers high throughput (> 50% including fiber injection losses). Additionally, it is robust against tip-tilt errors (∼0.1λ/D rms). We present SCAR designs for both an unobstructed and a VLT-like pupil. Conclusions. The SCAR coronagraph is a promising candidate for exoplanet detection and characterization around nearby stars using current high-resolution imaging instruments.

Demonstration of Automated Adjustment and Coupling of Fiber Array to PICs Based on the Detection of Edges

ABSTRACTProgress in integrated photonics enables development of photonics integrated circuits (PIC) with new unique properties and overcomes current limits in information and communication technologies. The effective coupling of optical radiation between telecom optical fiber and photonic integrated circuits is necessary. To address these challenges, we present our concept of photonics integrated circuit (PIC) fully customized packaging with microwave, direct current, and fiber array ports with automated active alignment system. Automated adjustment and coupling of single-mode fiber (SMF) arrays with 8 fibers to PIC based on the detection of edges and markers on PIC and SMF arrays were successfully realized.

Glass Optical Connector Ferrule for Solder-Reflow-Compatible Photonic IC Edge Interconnections

A glass-tube-based MT-style fiber array ferrule is presented that provides a compact interface to PIC (Photonic Integrated Circuit) edge waveguides. The approach aligns and attaches a glass ferrule on the PIC substrate top surface using solder-reflow-resistant UV curable adhesives. A mating fiber array connector ferrule is attached to the PIC after solder reflow. Mate/demate experiments on pairs of matching glass-based connector ferrules demonstrated IL (insertion loss) values across eight single mode fiber array channels of less than 1.5 dB without using index matching fluid between coupled arrays. A compact connector packaging housing compatible with glass tube-based ferrules is also presented.

Single photon extraction from self-assembled quantum dots via stable fiber array coupling

We present a direct fiber output of single photons from self-assembled quantum dots (QDs) realized by a stable fiber array-QD chip coupling. The integration of distributed Bragg reflector cavity and the etching of micropillar arrays isolate QDs and enhance their normal emission. The matched periods and mismatched diameters of the pillar array and the single-mode fiber array with Gaussian-shaped light spots enable a large alignment tolerance and a stable, efficient (i.e., near-field), and chip-effective (i.e., parallel) coupling of single QD emission, as compared to the traditional “point-based” coupling via a confocal microscope, waveguide, or fiber. The single photon counting rate at the fiber end reaches 1.87 M counts per second (cps) with a time correlation g2(0) of 0.3 under a saturated excitation, and 485 K cps with a g2(0) of 0.02 under a weak excitation, demonstrating a nice “all-fiber” single-photon source.

Self-aligned multi-channel superconducting nanowire single-photon detectors.

We describe a micromachining process to allow back-side coupling of an array of single-mode telecommunication fibers to individual superconducting nanowire single photon detectors (SNSPDs). This approach enables a back-illuminated detector structure which separates the optical access and electrical readout on two sides of the chip and thus allows for compact integration of multi-channel detectors. As proof of principle, we show the integration of four detectors on the same silicon chip with two different designs and their performances are compared. In the optimized design, the device shows saturated system detection efficiency of 16% while the dark count rate is less than 20 Hz, all achieved without the use of metal reflectors or distributed Bragg reflectors (DBRs). This back-illumination approach also eliminates the cross-talk between different detection channels.

Large-Tolerant Direct Coupling to an SMF Array With a Lens-Integrated Surface-Emitting Laser

As a solution to provide a small-size and low-cost 100-Gb/s optical module, direct optical coupling between a lens-integrated surface-emitting DFB laser (LISEL) array and a single-mode fiber (SMF) array is demonstrated. A very-small far-field angle ( $\sim 2$ °) was obtained by monolithic integration of an InP lens with a laser chip. As a result, each channel of the LISEL array can directly couple to an SMF without any additional external optical components. The measured misalignment tolerance between the LISEL array and an SMF array was sufficiently large, namely, $\pm 5.5~\mu \text{m}$ , at excess loss of 1 dB over all four channels with the maximum coupling efficiency of 8.4 dB. These measurement results are in good agreement with a theoretical calculation based on Gaussian optics. These results demonstrate that high coupling efficiency (−3 dB) can be achieved while large misalignment tolerance ( $\pm 4.5~\mu \text{m}$ ) at excess loss of only 1 dB is maintained.

Interferometric nulling limits with tip-tilt-piston deformable mirrors and a pinhole spatial filter array

We explore the use of hexagonal segment tip-tilt-piston deformable mirrors alone and paired with pinhole spatial filter arrays for high-order wavefront correction of nulling interferometers used for visible light study of exoplanetary systems at 107 to 1010 contrast within regions extending ∼0.1 to 6 arc s from a parent star. A similar system has been proposed using a single-mode fiber array as an alternative to using multiple deformable mirrors to correct both phase aberrations and balance electric field amplitude, the benefit being drastically reduced component and control complexity. Performance is compared using measured deformable mirror data for hexagonal arrays consisting of a number of rings NR=2 to 18, emphasizing the trade between throughput and the additional contrast gained from suppressing wavefront errors introduced by the deformable mirror at spatial frequencies Λ≥NR that are otherwise present in the image at corresponding field locations. Taking into account effects of loss of throughput and vignetting, the nulled signal-to-noise ratio is shown to improve for filtered systems in the outer portion of the field of view. Modeled performance shows no significant change in signal-to-noise in the inner field of view.

Demonstration of coherent imaging through a phased array of single-mode optical fibers

We demonstrate coherent imaging through a phased array of single-mode optical fibers. Specifically, we show that two laboratory point sources located within the individual-fiber single-mode acceptance angle can be resolved. Such techniques could eventually enable direct imaging with arrays of small collecting telescopes.

HIGH-CONTRAST, NARROW-FIELD EXOPLANET IMAGING WITH A MULTI-APERTURE TELESCOPE PHASED-ARRAY CORONAGRAPH

The coronagraphic imaging of nearby solar systems with a densely packed array of small, inexpensive collector telescopes is considered. A reduced-scale segmented pupil can be assembled downstream of the collectors either by means of an array of delay lines, or more conveniently, by means of an array of single-mode fibers. In either case, the individual pupil elements are completely decoupled from each other. Tuning the intensity and phase of the light in each pupil element then allows complete control over the pupil-plane field, thus enabling arbitrary and tunable complex pupil-plane apodization. Calculations show that such a phased-array coronagraph (PAC) can in principle provide the10–10 image-plane contrast required for terrestrial exoplanet observations near bright stars. A PAC may thus provide a route to coronagraphic observations of faint exoplanets that is both flexible and potentially relatively inexpensive.

Pupil remapping for high contrast astronomy: results from an optical testbed

The direct imaging and characterization of Earth-like planets is among the most sought-after prizes in contemporary astrophysics, however current optical instrumentation delivers insufficient dynamic range to overcome the vast contrast differential between the planet and its host star. New opportunities are offered by coherent single mode fibers, whose technological development has been motivated by the needs of the telecom industry in the near infrared. This paper presents a new vision for an instrument using coherent waveguides to remap the pupil geometry of the telescope. It would (i) inject the full pupil of the telescope into an array of single mode fibers, (ii) rearrange the pupil so fringes can be accurately measured, and (iii) permit image reconstruction so that atmospheric blurring can be totally removed. Here we present a laboratory experiment whose goal was to validate the theoretical concepts underpinning our proposed method. We successfully confirmed that we can retrieve the image of a simulated astrophysical object (in this case a binary star) though a pupil remapping instrument using single mode fibers.

Nonlinear localization of light in disordered optical fiber arrays

Light propagating through optical fiber arrays tends to localize in only a few center cores. The recent experiments in two-dimensional single-mode optical fiber arrays suggest that an interplay of deterministic and random linear and nonlinear effects might be responsible for this localization. We study this phenomenon, both analytically and numerically, in a hexagonal optical fiber array similar to that used in the experiments. Our analysis reveals that Anderson localization is evident in the linear and intermediate regimes, where a larger fraction of initial energy is returned to the center fiber due to the stochastic effects. For very high levels of input energy, the system is strongly nonlinear, with the randomness amplifying the Kerr localization.

Coupling and cross-talk effects in 12-15 μm diameter single-mode fiber arrays for simultaneous transmission and photon collection from scattering media

We examine signal degradation effects in fiber arrays from fiber-to-fiber coupling and from cross talk attributable to backscatter from the sample medium originating from adjacent fibers in the array. An analysis of coupling and cross talk for single-mode fibers (SMFs) operating at 1310 nm with different core diameters, interaction lengths, core center spacing, and numerical apertures (NAs) is evaluated. The coupling was evaluated using beam propagation algorithms and cross talk was analyzed by using Monte Carlo methods. Several multimode fiber types that are currently used in fiber image guides were also evaluated for comparative purposes. The analysis shows that an optimum NA and core diameter can be found for a specific fiber center separation that maximizes the directly backscattered signal relative to the cross talk. The coupling between fibers can be kept less than -35 dB for interaction lengths less than 5 mm. The calculations were compared to an experimentally fabricated SMF array with 15 microm center spacing and showed good agreement. The experimental fiber array without a lens was also used in a coherent detection configuration to measure the position of a mirror. Accurate depth ranging up to a distance of 250 microm from the tip of the fiber was achieved, which was five times the Rayleigh range of the beam emitted from the fiber.

Fluidic Assembly of Optical Components

A self-assembly technique is used to match microscopic components in a desired configuration with a high degree of precision. The basis of this self-assembly technique is capillary forces that are used to assemble the micro components. A liquid solder provides the capillary forces by wetting a 3-D cavity in each of the parts to be assembled. An advantage of the proposed design is that the high degree of precision of the mating feature relative to the optical guides ensures a high degree of concentricity of the optical path in the device. The application presented is the self-assembly of arrays of single-mode optical fibers onto an optical chip for use in optical switches or tunable filters. Optical measurements revealed an insertion loss of less than 0.85 dB, which demonstrates misalignment in the order of 1 mum

Submicron alignment of a two-dimensional array of multiple single-mode fibers

This letter presents a patented method to align single- or multimode optical fibers on top of an optoelectronic emitter array. Electrostatic forces are used to align copper-coated optical fibers within adhesive filled holes. The manufacturing and alignment performance of a microholder with a hole diameter of 1000 μm are fully characterized. The outer diameter of the optical single-mode fiber is 125 μm including the cladding. A submicron alignment accuracy can be inferred from the measurements taken. The sensitivity of the alignment procedure has been found to be between 0.25 and 1 μm per volt depending on the clamping distance. The combination of the electrostatic movement and the optical monitoring of the fiber position in a closed-loop feedback system yield a highly accurate and sensitive active alignment system.