Optimization and realignment of OAM mode excitation in ring-core optical fibers using machine learning.

Light beams carrying orbital angular momentum (OAM) in free space or within optical fibers have a wide range of applications in optics; however, exciting these modes with both high purity and low loss generally requires demanding optimization of excitation conditions in a high dimensional space. Furthermore, mechanical drift can significantly degrade the mode purity over time, which may limit practical deployment of OAM modes in concrete applications. Here, combining an iterative wavefront matching approach and a genetic algorithm, we demonstrate rapid and automated excitation of OAM modes with optimized purity and reduced loss. Our approach allows for systematic computational realignment of the system enabling drift compensation over extended durations. Our experimental results indicate that OAM purity can be optimized and maintained over periods exceeding 24 h, paving the way for the applications of stable OAM beams in optics.

Vibration-insensitive polarimetric fiber optic current sensor based on orbital angular momentum modes in an air-core optical fiber.

Current or magnetic field sensing is usually achieved by exploiting the Faraday effect of an optical material combined with an interferometric probe that provides the sensitivity. Being interferometric in nature, such sensors are typically sensitive to several other environmental parameters such as vibrations and mechanical disturbances, which, however, inevitably impose the inaccuracy and instability of the detection. Here we demonstrate a polarimetric fiber optic current sensor based on orbital angular momentum modes of an air-core optical fiber. In the fiber, spin-orbit interactions imply that the circular birefringence, which is sensitive to applied currents or resultant magnetic fields, is naturally resilient to mechanical vibrations. The sensor, which effectively measures polarization rotation at the output of a fiber in a magnetic field, exhibits high linearity in the measured signal versus the applied current that induces the magnetic field, with a sensitivity of 0.00128 rad/A and a noise limit of 1×10-5/H z. The measured polarization varies within only ±0.1% under mechanical vibrations with the frequency of up to 800 Hz, validating the robust environmental performance of the sensor.

Scaling information pathways in optical fibers by topological confinement.

Spatial mode-count scalability in optical fibers is of paramount importance for addressing the upcoming information-capacity crunch, reducing energy consumption per bit, and for enabling advanced quantum computing networks, but this scalability is severely limited by perturbative mode mixing. We show an alternative means of light guidance, in which light's orbital angular momentum creates a centrifugal barrier for itself, thereby enabling low-loss transmission of light in a conventionally forbidden regime wherein the mode mixing can be naturally curtailed. This enables kilometer-length-scale transmission of a record ~50 low-loss modes with cross-talk as low as -45 decibels/kilometer and mode areas of ~800 square micrometers over a 130-nanometer telecommunications spectral window. This distinctive light-guidance regime promises to substantially increase the information content per photon for quantum or classical networks.

Angular momentum driven dynamics of stimulated Brillouin scattering in multimode fibers.

The strength of stimulated Brillouin scattering (SBS) in optical fibers is largely governed by the spatial overlap between supported optical and acoustic modes, leading to a complicated amalgamation of photon-phonon interactions in multimode fibers. Here, we study SBS dynamics in ring-core fibers that support modes carrying orbital angular momentum (OAM), which result in distinctive characteristics. We find that the OAM SBS response, as well as modal content, strongly depends on the polarization state of the pump, as OAM modes in fiber have distinct propagation dynamics depending on whether the input is circularly or linearly polarized. This is in contrast to conventionally posited wisdom that SBS strength is independent of the pump's input polarization state in an isotropic material. This increased specificity can lead to interesting effects such as spatial phase conjugation even in the presence of stably transmitted, i.e. non-aberrated, spatial pump modes. More generally, we show that using OAM modes yields additional degrees of control over SBS interactions beyond more conventional parameters, such as effective area, acousto-optic spatial overlaps, and material composition.

Raman gain control in optical fibers with orbital-angular-momentum-induced chirality of light.

Stimulated Raman scattering is a particularly robust nonlinearity, occurring in virtually every material because its spectral linewidth and associated frequency shift do not typically depend on phases or directions (i.e. wavevectors) of the interacting light beams. In amorphous materials such as glass fibers, Raman bandwidths are large, enabling its use as a broadband gain element. This ubiquity makes it a versatile means for achieving optical amplification or realizing lasers over a large range of pulsewidths at user-defined colors. However, this ease of deploying the effect also presents itself as a stubborn source of noise in fiber-based quantum sources or parasitic emission in fiber lasers. Here, we show that orbital angular momentum carrying light beams experiencing spin-orbit interactions yield novel phase-matching criteria for Raman scattering. This enables tailoring its spectral shape (by over half the Raman shift in a given material) as well as strength (by ∼ 100×) simply by controlling light's topological charge - a capability of utility across the multitude of applications where modulating Raman scattering is desired.

Remote Measurement of the Angular Velocity Vector Based on Vectorial Doppler Effect Using Air-Core Optical Fiber.

Rotational Doppler effect has made tremendous development in both theoretical and applied research over the last decade. Different from the inertial thinking of focusing on the scalar field dominated by helical phase light, we have revealed a vectorial Doppler effect in our previous work, which is based on the spatially variant polarized light fields to simultaneously acquire the speed and direction of a target. Here, further, we propose a method to construct a flexible and robust velocimeter based on that novel effect by employing an air-core fiber with kilometer-length scale for remotely measuring the vectorial information of angular velocity in situ. In addition, we experimentally substantiate that the measurement system still has commendable accuracy in determining the direction of movement even when the air-core fiber is interfered by the external environment. The demonstrations prove the potential of vectorial Doppler effect in practical scenarios and remote measurements.

Multiphoton microscopy for label-free multicolor imaging of peripheral nerve.

SIGNIFICANCE: Means for quantitation of myelinated fibers in peripheral nerve may guide diagnosis and clinical decision making in management of peripheral nerve disorders. Multiphoton microscopy techniques such as the third-harmonic generation enable label-free in vivo imaging of peripheral nerves. AIM: Develop a multiphoton microscope based on a custom high-power infrared fiber laser for label-free imaging of peripheral nerve. APPROACH: A cost-effective multiphoton microscope employing a single fiber laser source at 1300 nm was designed and used for stain-free multicolor imaging of murine and human peripheral nerve. RESULTS: Second-harmonic generation signal from collagen centered about 650-nm delineated neural connective tissue, whereas third-harmonic general signal centered about 433-nm delineated myelin and other lipids. In sciatic nerve from transgenic reporter mice expressing yellow fluorescent protein within peripheral neurons, three-photon-excitation with emission peak at 527-nm delineated axoplasm. The signal obtained from unlabeled axially sectioned samples was adequate for segmentation of myelinated fibers using commercial image processing software. In unlabeled whole mount specimens, imaging depths over 100-µm were achieved. CONCLUSIONS: A multiphoton microscope powered by a fiber laser enables stain-free histomorphometry of mammalian peripheral nerve. The simplicity of the microscope design carries potential for clinical translation to inform decision making in peripheral nerve disorders.

High resolution spectral metrology leveraging topologically enhanced optical activity in fibers.

Optical rotation, a form of optical activity, is a phenomenon employed in various metrological applications and industries including chemical, food, and pharmaceutical. In naturally-occurring, as well as structured media, the integrated effect is, however, typically small. Here, we demonstrate that, by exploiting the inherent and stable spin-orbit interaction of orbital angular momentum fiber modes, giant, scalable optical activity can be obtained, and that we can use this effect to realize a new type of wavemeter by exploiting its optical rotary dispersion. The device we construct provides for an instantaneous wavelength-measurement technique with high resolving power R = 3.4 × 106 (i.e., resolution < 0.3 pm at 1-µm wavelengths) and can also detect spectral bandwidths of known lineshapes with high sensitivity.

Jitter-free, dual-wavelength, ultrashort-pulse, energetic fiber sources using soliton self-mode conversion.

We demonstrate an energetic dual-wavelength ultrashort pulsed source by exploiting the inherent features of the newly discovered process of soliton self-mode conversion (SSMC) in a multimode fiber. The generated pulses are at wavelengths of 1205 nm and 1273 nm, respectively, and the pulse energies are approximately 30 nJ. The natural group-velocity-locking feature of SSMC ensures minimal relative timing jitter, hence highlighting the utility of exploiting the new degrees of freedom afforded by field of multimode nonlinear fiber optics. The relative timing jitter is evaluated by measuring the power fluctuations of generated sum-frequency signals. When compared to a conventional fiber based dual-wavelength source based on traditional frequency-shifted solitons, the relative timing jitter is found to be reduced by greater than 11 dB. Since this process is wavelength-agnostic within the transparency window of optical fibers, our source provides an attractive means of achieving integrated multi-color ultrashort pulse sources for a variety of applications.

High-power, cascaded random Raman fiber laser with near complete conversion over wide wavelength and power tuning.

Cascaded Raman fiber lasers based on random distributed feedback (RDFB) are proven to be wavelength agile, enabling high powers outside rare-earth doped emission windows. In these systems, by simply adjusting the input pump power and wavelength, high-power lasers can be achieved at any wavelength within the transmission window of optical fibers. However, there are two primary limitations associated with these systems, which in turn limits further power scaling and applicability. Firstly, the degree of wavelength conversion or spectral purity (percentage of output power in the desired wavelength band) that can be achieved is limited. This is attributed to intensity noise transfer of input pump source to Raman Stokes orders, which causes incomplete power transfer reducing the spectral purity. Secondly, the output power range over which the high degree of wavelength conversion is maintained is limited. This is due to unwanted Raman conversion to the next Stokes order with increasing power. Here, we demonstrate a high-power, cascaded Raman fiber laser with near complete wavelength conversion over a wide wavelength and power range. We achieve this by culmination of two recent developments in this field. We utilize our recently proposed filtered feedback mechanism to terminate Raman conversion at arbitrary wavelengths, and we use the recently demonstrated technique (by J Dong and associates) of low-intensity noise pump sources (Fiber ASE sources) to achieve high-purity Raman conversion. Pump-limited output powers >34W and wavelength conversions >97% (highest till date) were achieved over a broad - 1.1µm to 1.5µm tuning range. In addition, high spectral purity (>90%) was maintained over a broad output power range (>15%), indicating the robustness of this laser against input power variations.

A light-in-flight single-pixel camera for use in the visible and short-wave infrared.

Single-pixel cameras reconstruct images from a stream of spatial projection measurements recorded with a single-element detector, which itself has no spatial resolution. This enables the creation of imaging systems that can take advantage of the ultra-fast response times of single-element detectors. Here we present a single-pixel camera with a temporal resolution of 200 ps in the visible and short-wave infrared wavelengths, used here to study the transit time of distinct spatial modes transmitted through few-mode and orbital angular momentum mode conserving optical fiber. Our technique represents a way to study the spatial and temporal characteristics of light propagation in multimode optical fibers, which may find use in optical fiber design and communications.

High-power, widely wavelength tunable, grating-free Raman fiber laser based on filtered feedback.

The cascaded Raman fiber laser is a proven technology that provides wavelength agile high-power fiber lasers outside the rare-earth emission windows. However, conventional cascaded Raman fiber lasers lack wavelength agility due to the use of fixed wavelength fiber Bragg gratings. Recently, proposed cascaded Raman fiber lasers based on random distributed feedback have provided a grating-free solution enabling wavelength agility. With these lasers, wide wavelength tunability has been achieved. However, there are still limitations in scaling output power while maintaining high spectral purity of wavelength conversion. Spectral purity is characterized by the in-band power ratio, which is the ratio of the output power in the required wavelength to the total power. The origin of this limitation arises from the inability to efficiently terminate the Raman cascade at a specific wavelength with increasing power. In this Letter, we propose a novel filtered distributed feedback mechanism to terminate the Raman cascade at any desired wavelength, enabling power scaling with high spectral purity. Output power up to 28 W has been achieved with >85% in-band power ratio and >400 nm tuning range from 1118 to 1535 nm.

Identification of Histophilus somni by a nanomaterial optical fiber biosensor assay.

Histophilus somni is an opportunistic pathogen responsible for respiratory and systemic diseases of cattle and sheep. Rapid and accurate detection of H. somni is essential to distinguish H. somni from other potential pathogens for proper control and treatment of infections. Nanomaterial optical fiber biosensors (NOFS) recognize analyte interactions, such as DNA hybridization, with high specificity and sensitivity, and were applied to detect H. somni DNA in culture and clinical samples. An ionic self-assembled multilayer (ISAM) film was fabricated on a long-period grating optical fiber, and a biotinylated, nucleotide probe complementary to the H. somni 16S rDNA gene was coupled to the ISAM film. Exposure of the ISAM::probe to ⩾100 killed cells of H. somni strain 2336 without DNA amplification resulted in attenuation of light transmission of ⩾9.4%. Exposure of the complexed fiber to Escherichia coli or non- H. somni species of Pasteurellaceae reduced light transmission by ⩽3.4%. Exposure of the ISAM::probe to blood, bronchoalveolar fluid, or spleen from mice or calves infected with H. somni resulted in ⩾24.3% transmission attenuation. The assay correctly detected all 6 strains of H. somni tested from culture, or tissues from 3 separate mice and calves tested in duplicate. Six heterologous strains (representing 6 genera) reacted at below the cutoff value of 4.87% attenuation of light transmission. NOFS detected at least 100 H. somni cells without DNA amplification within 45 min with high specificity. Although different fibers could vary in signal sensitivity, this did not affect the sensitivity or specificity of the assay.

Multimode-pumped Raman amplification of a higher order mode in a large mode area fiber.

We report the first demonstration of Raman amplification in a fiber of a single Bessel-like higher order mode using a multimode pump source. We amplify the LP08-mode with a 559-µm2 effective mode area at a signal wavelength of 1115 nm in a pure-silica-core step-index fiber. A maximum of 18 dB average power gain is achieved in a 9-m long gain fiber, with output pulse energy of 115 µJ. The Raman pump source comprises a pulsed 1060 nm ytterbium-doped fiber amplifier with V-value ~30, which is matched to the Raman gain fiber. The pump depletion as averaged over the signal pulses reaches 36.7%. The conversion of power from the multimode pump into the signal mode demonstrates the potential for efficient brightness enhancement with low amplification-induced signal mode purity degradation.

Quantum cryptography with structured photons through a vortex fiber.

Optical fiber links and networks are integral components within and between cities' communication infrastructures. Implementing quantum cryptographic protocols on either existing or new fiber links will provide information-theoretical security to fiber data transmissions. However, there is a need for ways to increase the channel bandwidth. Using the transverse spatial degree of freedom is one way to transmit more information and increase tolerable error thresholds by extending the common qubit protocols to high-dimensional quantum key distribution (QKD) schemes. Here we use one type of vortex fiber where the transverse spatial modes serves as an additional channel to encode quantum information by structuring the spin and orbital angular momentum of light. In this proof-of-principle experiment, we show that two-dimensional structured photons can be used in such vortex fibers in addition to the common two-dimensional polarization encryption, thereby paving the path to QKD multiplexing schemes.

12 mode, WDM, MIMO-free orbital angular momentum transmission.

Simultaneous MIMO-free transmission of 12 orbital angular momentum (OAM) modes over a 1.2 km air-core fiber is demonstrated. WDM compatibility of the system is shown by using 60, 25 GHz spaced WDM channels with 10 GBaud QPSK signals. System performance is evaluated by measuring bit error rates, which are found to be below the soft FEC limit, and limited by inter-modal crosstalk. The crosstalk in the system is analyzed, and it is concluded that it can be significantly reduced with an improved multiplexer and de-multiplexer.

Ultra-low loss dispersion control with chirped transmissive fiber gratings.

Dispersion control is a critical functionality required in systems involving ultra-short (∼100 fs) pulses, and we demonstrate the use of chirped long-period fiber gratings for this purpose. The operation principles of this device share many attributes with the more established fiber Bragg grating technology for dispersion compensation, but with the added benefit of record low loss (0.2 dB) and the potential of being free from group-delay ripple distortions. The bandwidth of the transmissive grating device we demonstrate exceeds 12 nm, and it provides +52 fs/nm of dispersion in the 1 µm wavelength range. This corresponds to the capability of compensating the dispersion of ∼100 fs pulses in approximately meter-long single mode fibers.

Observation of Interaction of Spin and Intrinsic Orbital Angular Momentum of Light.

The interaction of spin and intrinsic orbital angular momentum of light is observed, as evidenced by length-dependent rotations of both spatial patterns and optical polarization in a cylindrically symmetric isotropic optical fiber. Such rotations occur in a straight few-mode fiber when superpositions of two modes with parallel and antiparallel orientation of spin and intrinsic orbital angular momentum (IOAM=2ℏ) are excited, resulting from a degeneracy splitting of the propagation constants of the modes.

13.4km OAM state propagation by recirculating fiber loop.

Enabled by an enhanced effective index separation (Δneff = 1.7 × 10-4) and low transmission loss (0.8dB/km), OAM states are propagated over 13.4km in an air core fiber using a recirculating fiber loop. We observe that intermodal crosstalk decreases rapidly with increasing effective index separation, Δneff, and an order of magnitude lower crosstalk may be achieved just by doubling Δneff. We find that, in agreement with coupled power theory, our fiber has mode coupling properties analogous to elliptical core PM fibers, which yield ~10 × or more lower crosstalk than for conventional LP fiber mode orders with the same Δneff. This confirms that, for OAM modes, birefringent perturbations rather than shape perturbations matter most. In the process of performing the loop experiment, we demonstrate that OAM states in these fibers can be preserved with low loss (≤ 0.2dB) and low crosstalk (-15dB) while splicing distinct segments of the air-core fiber. For well-designed fibers, we demonstrate that OAM modes can travel distances relevant for large-scale data centers.

Amplification of 12 OAM Modes in an air-core erbium doped fiber.

We theoretically propose an air-core erbium doped fiber amplifier capable of providing relatively uniform gain for 12 orbital angular momentum (OAM) modes (|L| = 5, 6 and 7, where |L| is the OAM mode order) over the C-band. Amplifier performance under core pumping conditions for a uniformly doped core for each of the supported pump modes (110 in total) was separately assessed. The differential modal gain (DMG) was found to vary significantly depending on the pump mode used, and the minimum DMG was found to be 0.25 dB at 1550 nm provided by the OAM (8,1) pump mode. A tailored confined doping profile can help to reduce the pump mode dependency for core pumped operation and help to increase the number of pump modes that can support a DMG below 1 dB. For the more practical case of cladding-pumped operation, where the pump mode dependency is almost removed, a DMG of 0.25 dB and a small signal gain of >20 dB can be achieved for the 12 OAM modes across the full C-band.