Since the elucidation of total internal reflection by Colladon, Babinet and Tyndall over 150 years ago, bound states of light have been the primary means to transport light, whether on a chip or in optical fibers. We show that light carrying sufficiently high orbital angular momentum can create a centrifugal barrier for itself, thereby enabling guidance even in a regime where a mode is normally considered “cutoff.” We will discuss how this discovery, which has parallels with why binary stars don’t collapse into each other due to gravity, has applications in diverse areas, including classical and quantum communications and computing with high dimensionality, power scaling of fiber lasers by mitigating nonlinearities and nonlinear optics with greater degrees of freedom.
Entangling photons in the orbital angular momentum (OAM) degree of freedom, demonstrated over 20 years ago, yields a promising pathway for increasing the dimensionality of quantum sources, of utility in high-dimensional communications as well as in efficient quantum computing algorithms in higher-dimensional Hilbert spaces. An efficient integrated-optic means of achieving this has, however, eluded the community thus far.
One potential solution to this problem may be found by exploiting the newly discovered phenomenon of topological confinement, which has yielded a multimode platform with as many as 60 stable OAM modes in optical fibers. This provides for a plethora of spontaneous four-wave mixing phase matching possibilities enabling control not only over the wavelengths at which the photon pairs can be generated, but also over the shapes of their joint spectral densities, ranging from correlated, uncorrelated, to anti-correlated.
This talk will describe recent experiments aimed at the development of compact, single-aperture devices whose biphoton emissions can be engineered to exploit the spatial dimension of light.
Diverse spatial mode bases can be exploited in mode-division multiplexing (MDM) to sustain the capacity growth in fiber-optic communications, such as linearly polarized (LP) modes, vector modes, LP orbital angular momentum (LP-OAM) modes, and circularly polarized OAM (CP-OAM) modes. Nevertheless, which kind of mode bases is more appropriate to be utilized in fiber still remains unclear. Here, we aim to find the superior mode basis in MDM fiber-optic communications via a system-level comparison in air-core fiber (ACF). We first investigate the walk-off effect of four spatial mode bases over 1-km ACF, where LP and LP-OAM modes show intrinsic mode walk-off, while it is negligible for vector and CP-OAM modes. We then study the mode coupling effect of degenerate vector and CP-OAM modes over 1-km ACF under fiber perturbations, where degenerate even and odd vector modes suffer severe mode cross talk, while negligible for high-order degenerate CP-OAM modes based on the laws of angular momentum conservation. Moreover, we comprehensively evaluate the system-level performance for data-carrying single-channel and two-channel MDM transmission with different spatial mode bases under various kinds of fiber perturbations (bending, twisting, pressing, winding, and out-of-plane moving). The obtained results indicate that the CP-OAM mode basis shows superiority compared to other mode bases in MDM fiber-optic communications without using multiple-input multiple-output digital signal processing. Our findings may pave the way for robust short-reach MDM optical interconnects for data centers and high-performance computing.
Recently, structured light beams have attracted substantial attention in many applications, including optical communications, imaging, optical tweezers, and quantum optics. We propose and experimentally demonstrate a reconfigurable structured light beam generator in order to generate diverse structured light beams with adjustable beam types, beam orders, and beam sizes. By controlling the sizes of generated free-space structured light beams, free-space orbital angular momentum (OAM) beams and vector beams are coupled into an air–core fiber. To verify that our structured light generator enables generating structured light with high beam quality, polarization distributions and mode purity of generated OAM beams and vector beams in both free space and air–core fiber are characterized. Such a structured light generator may pave the way for future applications based on higher-order structured light beams.
Recently, structured light beams have attracted substantial attention in many applications, including optical communications, imaging, optical tweezers, and quantum optics. We propose and experimentally demonstrate a reconfigurable structured light beam generator in order to generate diverse structured light beams with adjustable beam types, beam orders, and beam sizes. By controlling the sizes of generated free-space structured light beams, free-space orbital angular momentum (OAM) beams and vector beams are coupled into an air–core fiber. To verify that our structured light generator enables generating structured light with high beam quality, polarization distributions and mode purity of generated OAM beams and vector beams in both free space and air–core fiber are characterized. Such a structured light generator may pave the way for future applications based on higher-order structured light beams.
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.
We describe the recent discovery of an ultrafast Raman scattering mechanism, soliton-self mode conversion (SSMC). It features power-scalable frequency conversion of ultrashort pulses with near-100% photon-conversion efficiency, and dominates over even concurrent conventional seeded processes in spite of being noise initiated. Realising spatially coherent, MW peak-power-level, transform-limited, sub-100-fs pulses at non-traditional wavelengths directly out of fibers facilitates applications that are currently accessible primarily by bulk optical parametric oscillators. In addition, this mechanism enables endoscopic or flexible delivery of ultrashort energetic pulses at user-defined colours. We show representative applications of SSMC in multiphoton microscopy at the biologically crucial 1300-nm spectral range.
We report stimulated Brillouin scattering (SBS) measurements in orbital angular momentum (OAM) carrying modes of an optical fiber. The additional degree of freedom afforded by angular momentum selectivity leads to new possibilities to control SBS gain. We find that, depending on the OAM value in relation to the polarization state of the launched pump light, the SBS threshold can vary by as much as 2.6 dB, even when normalized for mode area. Combined with their inherently large mode areas and naturally polarization maintaining behavior, OAM fiber modes may offer an alternative means for scaling power in narrow-linewidth fiber lasers.
Cascaded Raman fiber lasers is the only proven technology which enables high power continuous wave (CW) fiber laser sources outside rare-earth emission bandwidths. Among these systems, recently demonstrated cascaded Raman fiber lasers based on Random distributed feedback (RDFB) provide wavelength agility enabling high-power, ultra-broad band wavelength tunable fiber lasers. However, these systems are limited in terms of power scaling due to degrading spectral purity with increasing input pump power which in turn limits the applicability of these systems. This is due to conversion of desired wavelength into next higher order Raman stokes with increase in pump power. In this work, we demonstrate a high power, ultra-high spectral purity, broadly wavelength tunable cascaded Raman fiber laser. This was enabled by culmination of two significant advances over the last year. To terminate the Raman cascade at the required output wavelength, we utilized our recently proposed distributed filtered feedback mechanism. To achieve high spectral purity, we used a recently demonstrated technique of using high power fiber based Amplified Spontaneous emission (ASE) sources as the input pump. A maximum output power of ~33W at 1.5μm and ~27W at 1.4μm was achieved. High spectral purity of <97% at the final wavelength was achieved over a wide-range of output powers.
Mid-infrared (IR) fibers have been extensively investigated due to their applicability in chemical sensing and remote laser delivery, among others. Materials such as chalcogenides and fluoride glasses transmit mid-IR wavelengths with low practical losses. However, their low glass transition temperatures make them chemically unstable, even at room temperatures, resulting in performance degradation over time. Semiconductors, such as germanium, have a wide transmission window in the mid-IR region, and offer significantly improved chemical stability. In this research, germanium-core, borosilicate-cladded fibers were drawn by a ‘rod in tube’ method using a mini draw tower assembled in-house at 1000°C, which is significantly lower than the drawing temperatures of 2000-2200°C for conventional silica fibers. Typical drawn fibers had a 40 μm core diameter and 177 μm cladding diameter. Transmission electron microscopy (TEM) studies showed that diffusion of oxygen and silicon from the cladding to the core during the drawing process was minimal, with diffusion distances of the order of 10s of nm. This is encouraging for mid-IR transmission, since the presence of oxygen in the fiber core is known to increase transmission losses in the mid-IR spectrum. This low diffusivity is presumably due to the relatively low drawing temperature. Transmission losses through the fibers were measured with a quantum cascade laser (QCL) and the losses were found to be in the 3-9 dB/cm range in the spectral range of 5.75-6.3 μm.
We analyze the modal properties of an 85μm core distributed mode filtering rod fiber using cross-correlated (C2) imaging. We evaluate suppression of higher-order modes (HOMs) under severely misaligned mode excitation and identify a single-mode regime where HOMs are suppressed by more than 20dB.
Femtosecond fiber lasers are currently of great interest due to their small size, stable operation, long lifetime and low
cost compared to bulk lasers. However, for operation in the 1 µm wavelength range of Yb lasers, a major obstacle has
been the lack of suitable fibers with anomalous dispersion that can compensate for the normal dispersion of the
conventional active and passive fibers used. However, a new promising fiber device using a higher order mode (HOM)
with anomalous dispersion in the 1 μm range has recently been demonstrated. The device comprises integrated all fiber
mode converters based on long period gratings (LPG), and hence has the potential to be low loss and easy to splice,
while offering a large effective area, and the possibility of third order dispersion compensation.
In this paper, optimization of HOM fibers with anomalous dispersion in the 1 μm range has been investigated
theoretically and experimentally. Fibers with dispersion coefficients ranging from +50 to +300 ps/(nm·km) at 1060 nm
have been fabricated and devices including integrated LPG mode converters have been characterized. Modeled and
measured properties of the modules, such as dispersion, grating bandwidth etc., are found to correlate well. It is shown
that there is a tradeoff between a high dispersion coefficient and the bandwidth of LPG mode converters.
The characteristics of such HOM devices have been studied in a linear, passively mode-locked laser-cavity using
SESAM as saturable absorber.
Ionic self-assembled multilayers (ISAMs) adsorbed on long period fiber gratings (LPGs) can serve as an inexpensive,
robust, portable, biosensor platform. The ISAM technique is a layer-by-layer deposition technique that creates thin films
on the nanoscale level. The combination of ISAMs with LPGs yields exceptional sensitivity of the optical fiber
transmission spectrum. We have shown theoretically that the resonant wavelength shift for a thin-film coated LPG can
be caused by the variation of the film's refractive index and/or the variation of the thickness of the film. We have
experimentally demonstrated that the deposition of nm-thick ISAM films on LPGs induces shifts in the resonant
wavelength of > 1.6 nm per nm of thin film. It has also been shown that the sensitivity of the LPG to the thickness of the
ISAM film increases with increased film thickness. We have further demonstrated that ISAM-coated LPGs can function
effectively as biosensors by using the biotin-streptavidin system and by using the Bacillus anthracis (Anthrax) antibody-
PA (Protective Antigen) system. Experiments have been successfully performed in both air and solution, which
illustrates the versatility of the biosensor. The results confirm that ISAM-LPGs yield a reusable, thermally-stable, and
robust platform for designing and building efficient optical biosensors.
We have shown that ionic self-assembled multilayers (ISAMs) deposited on optical fiber long period gratings (LPGs) yield dramatic resonant-wavelength shifts, even with nanometer-thick films. Precise control of the refractive index and the thickness of these films was achieved by altering the relative fraction of the anionic and cationic materials combined with layer-by-layer deposition. We demonstrate the feasibility of this highly controllable deposition-technique for fine-tuning grating properties for grating applications. In addition, we confirm theoretically that the resonant wavelength shift can result from either the variation of the thickness of the film and/or the variation of its refractive index. Finally, we demonstrate that ISAMs adsorbed on LPGs function effectively as biosensors. These simulations and experimental results confirm that ISAM-coated-LPGs provide a thermally-stable, reusable, robust, and attractive platform for building efficient fiber optic sensors and devices.
The amount of uncompensated dispersion that is tolerated in optical communications links decreases sharply with increase in channel line-rates. Since this holds for each channel individually, multi-wavelength WDM transmission at high bit-rates requires simultaneous dispersion compensation over the entire spectral range of interest. This entails compensating for the dispersion slope as well as the dispersion of the transmission fibers used in the link. The class of non-zero dispersion-shifted fibers (NZDSF), which are widely deployed today, exhibit high relative dispersion slopes. Thus, dispersion management of 40 Gb/sec links comprised of NZDSF poses a significant challenge. This talk will introduce a novel dispersion compensation scheme that utilizes propagation in a higher-order mode of a few-moded fiber. The primary advantage of higher order modes is that they exhibit larger dispersions as well as dispersion slopes. This facilitates broadband compensation for any transmission fiber among the class of NZDSF. In addition, these modes have larger effective areas, and hence reduce non-linear distortions in the transmitted signal. Consequently, this compensation technique provides the additional benefit of maintaining higher signal powers throughout the span, which in turn leads to longer transmission distances. Recent breakthroughs that allow for low loss, broadband mode-conversion have made this technology viable for practical applications. The physics and characteristics of this technology will be described, and its attendant systems advancements will be compared with that of alternate schemes.
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