Dispersion tailoring and compensation by modal interactions in OmniGuide fibers

Non-Invasive Laser Surgery With Deep Operating Depth Using Multibeam Interference

Purpose

Laser surgery can use photo-chemical, photo-thermal, photo-ablative, and photo-mechanical effects to treat various tissues in the human body, and has unique advantages of extremely high precision, non-invasive penetration, and fast operation speed. However, at present, the effective penetration depth of directly illuminating light in the body is only several millimeters. Therefore, increasing the safe operating depth for non-invasive laser surgery will have important, widespread, and irreplaceable applications in the future.

Methods

The method is based on improving a recently emerged technique. Its principle involves using a negative dispersion device to broaden the width of the short light pulse first. Then, after the pulse enters the body, as its peak intensity is reduced, the skin and healthy tissues in the laser propagation path cannot be injured. Meanwhile, since body tissues have positive dispersion, the broadened width of the laser pulse will be shortened back. When the broadened pulse is completely shortened, a thin inner light layer with high intensity will be formed in the body and used as a scalpel to treat target tissue.

Results

The theoretical calculation results have shown that the designed apparatus has excellent performance. Its safe non-invasive operating depth can be more than 70 millimeters with the possibility of up to 130 millimeters. Surgery precisions are around 1 micron transversely and about 1 millimeter longitudinally in theory.

Conclusion

An improved method of non-invasive laser surgery with deep operation depth has been investigated theoretically. The calculations show that the designed apparatus has excellent performance. The proposed method depends on two well-known physical phenomena: light pulse broadening and shortening caused by optical negative and positive dispersions, and thus has solid basis. The developed method will have important, widespread and irreplaceable applications in the medical surgery field.

Experimental demonstration of passive microwave pulse amplification via temporal Talbot effect

The temporal Talbot effect is a passive phenomenon that occurs when a periodic signal propagates through a dispersive medium with a quadratic phase response that modulates the output pulse repetition rate based on the input period. As previously proposed, this effect enables innovative applications such as passive amplification. However, its observation in the microwave regime has been impractical due to the requirement for controlled propagation through a highly dispersive waveguide. To overcome this challenge, we employed an ultra-wide band linearly chirped Bragg grating within a standard microwave X-Band waveguide. By utilizing backwards Talbot array illuminators aided by particle swarm optimization, we achieved passive amplification with a gain of 3.45 dB and 4.03 dB for gaussian and raised cosine pulses, respectively. Furthermore, we numerically verified that with higher quality substrates this gain can be theoretically increased to over 8 dB. Our work paves the way for numerous applications of the Talbot effect in the microwave regime, such as temporal cloaking, sub-noise microwave signal detection, microwave pulse shaping, and microwave noise reduction.

Combining Hollow Core Photonic Crystal Fibers with Multimode, Solid Core Fiber Couplers through Arc Fusion Splicing for the Miniaturization of Nonlinear Spectroscopy Sensing Devices

The presence of fiber optic devices, such as couplers or wavelength division multiplexers, based on hollow-core fibers (HCFs) is still rather uncommon, while such devices can be imagined to greatly increase the potential of HCFs for different applications, such as sensing, nonlinear optics, etc. In this paper, we present a combination of a standard, multimode fiber (MMF) optic coupler with a hollow core photonic bandgap fiber through arc fusion splicing and its application for the purpose of multiphoton spectroscopy. The presented splicing method is of high affordability due to the low cost of arc fusion splicers, and the measured splicing loss (SL) of the HCF-MMF splice is as low as (0.32 ± 0.1) dB, while the splice itself is durable enough to withstand a bending radius (rbend) of 1.8 cm. This resulted in a hybrid between the hollow core photonic bandgap fiber (HCPBF) and MMF coupler, delivering 20 mW of average power and 250-fs short laser pulses to the sample, which was good enough to test the proposed sensor setup in a simple, proof-of-concept multiphoton fluorescence excitation-detection experiment, allowing the successful measurement of the fluorescence emission spectrum of 10−5 M fluorescein solution. In our opinion, the presented results indicate the possibility of creating multi-purpose HCF setups, which would excel in various types of sensing applications.

3D printed hollow core terahertz Bragg waveguides with defect layers for surface sensing applications

We study a 3D-printed hollow core terahertz (THz) Bragg waveguide for resonant surface sensing applications. We demonstrate theoretically and confirm experimentally that by introducing a defect in the first layer of the Bragg reflector, thereby causing anticrossing between the dispersion relations of the core-guided mode and the defect mode, we can create a sharp transmission dip in the waveguide transmission spectrum. By tracking changes in the spectral position of the narrow transmission dip, one can build a sensor, which is highly sensitive to the optical properties of the defect layer. To calibrate our sensor, we use PMMA layers of various thicknesses deposited onto the waveguide core surface. The measured sensitivity to changes in the defect layer thickness is found to be 0.1 GHz/μm. Then, we explore THz resonant surface sensing using α-lactose monohydrate powder as an analyte. We employ a rotating THz Bragg fiber and a semi-automatic powder feeder to explore the limit of the analyte thickness detection using a surface modality. We demonstrate experimentally that powder layer thickness variations as small as 3μm can be reliably detected with our sensor. Finally, we present a comparative study of the time-domain spectroscopy versus continuous wave THz systems supplemented with THz imaging for resonant surface sensing applications.

Large-effective-area dispersion-compensating fiber design based on dual-core microstructure

We present a microstructure-based dual-core dispersion-compensating fiber (DCF) design for dispersion compensation in long-haul optical communication links. The design has been conceptualized by combining the all-solid dual-core DCF and dispersion-compensating photonic crystal fiber. The fiber design has been analyzed numerically by using a full vectorial finite difference time domain method. We propose a fiber design for narrowband as well as broadband dispersion compensation. In the narrowband DCF design, the fiber exhibits very large negative dispersion of around -42,000 ps nm(-1) km(-1) and a large mode area of 67 μm(2). The effects of varying different structural parameters on the dispersion characteristics as well as on the trade-off between full width at half-maximum and dispersion have been investigated. For broadband DCF design, a dispersion value between -860 ps nm(-1)km(-1) and -200 ps nm(-1) km(-1) is obtained for the entire spectral range of the C band.

Wideband and low dispersion slow-light waveguide based on a photonic crystal with crescent-shaped air holes

We present a procedure to generate slow light with a large group index, wideband, and low dispersion in our suggested photonic crystal waveguide. By modulation of the declinations in the first two rows of air holes, the group index, the bandwidth, and the dispersion can be tuned effectively. Utilizing the two-dimensional plane wave expansion method (PWE) and the finite-difference time-domain method (FDTD), we demonstrate slow light with the group indices of 23, 35, and 45, respectively, while restricting the group-index variation within a 10% range. We accordingly attain an available bandwidth of 40.7, 23.7, and 5.1 nm, respectively. Meanwhile, the normalized delay-bandwidth product stays around 0.45, with minimal dispersion less than 0.2 (ps2/m) for all the cases.

Preparation and transmission of low-loss azimuthally polarized pure single mode in multimode photonic band gap fibers

We demonstrate the preparation and transmission of the lowest loss azimuthally polarized TE₀₁₋ like mode in a photonic band gap (PBG) fiber. Using the nature of the mode and the properties of the band gap structure we construct a novel coupler that operates away from the band gap's center to enhance the differential losses and facilitate the radiative loss of hybrid fundamental fiber modes. Remarkably, even though the coupler is highly multimoded, a pure azimuthally polarized mode is generated after only 17 cm. Theoretical calculations verify the validity of this technique and accurately predict the coupling efficiency. The generation and single mode propagation of this unique azimuthally polarized, doughnut shaped mode in a large hollow-core fiber can find numerous applications including in optical microscopy, optical tweezers, and guiding particles along the fiber.

Polymer-composite fibers for transmitting high peak power pulses at 1.55 microns

Hollow-core photonic bandgap fibers (PBG) offer the opportunity to suppress highly the optical absorption and nonlinearities of their constituent materials, which makes them viable candidates for transmitting high-peak power pulses. We report the fabrication and characterization of polymer-composite PBG fibers in a novel materials system, polycarbonate and arsenic sulfide glass. Propagation losses for the 60 microm-core fibers are less than 2dB/m, a 52x improvement over previous 1D-PBG fibers at this wavelength. Through preferential coupling the fiber is capable of operating with over 97% the fiber's power output in the fundamental (HE(11)) mode. The fiber transmitted pulses with peak powers of 11.4 MW before failure.

Highly nonlinear hybrid AsSe-PMMA microtapers

We report the fabrication and characterization of an AsSe microtaper with a protective cladding made of PolyMethyl MethAcrylate (PMMA). The AsSe core of the microtaper provides an ultrahigh nonlinearity up to gamma = 133 W(-1)m(-1) whereas the polymer cladding provides mechanical strength for normal handling of the device and reduces sensitivity to the surrounding environment.

Enabling coherent superpositions of iso-frequency optical states in multimode fibers

The ability to precisely and selectively excite superpositions of specific fiber eigenmodes allows one in principle to control the three dimensional field distribution along the length of a fiber. Here we demonstrate the dynamic synthesis and controlled transmission of vectorial eigenstates in a hollow core cylindrical photonic bandgap fiber, including a coherent superposition of two different angular momentum states. The results are verified using a modal decomposition algorithm that yields the unique complex superposition coefficients of the eigenstate space.

Photonic bandgap fibers with resonant structures for tailoring the dispersion

Numerical simulations on different kinds of realistic photonic bandgap fibers exhibiting reversed dispersion slope for the propagating fundamental mode are reported. We show that reversed or flat dispersion functions in a wide wavelength range using hollow-core, air-silica photonic bandgap fibers and solid core Bragg fibers with step-index profile can be obtained by introducing resonant structures in the fiber cladding. We evaluate the dispersion and confinement loss profiles of these fibers from the Helmholtz eigenvalue equation and the calculated fiber properties are used to investigate the propagation of chirped femtosecond pulses through serially connected hollow core fiber compressors.

Surface plasmon resonance-like integrated sensor at terahertz frequencies for gaseous analytes

Plasmon-like excitation at the interface between fully polymeric fiber sensor and gaseous analyte is demonstrated theoretically in terahertz regime. Such plasmonic excitation occurs on top of a approximately 30 microm ferroelectric PVDF layer wrapped around a subwavelength porous polymer fiber. In a view of designing a fiber-based sensor of analyte refractive index, phase matching of a plasmon-like mode with the fundamental core guided mode of a low loss porous fiber is then demonstrated for the challenging case of a gaseous analyte. We then demonstrate the possibility of designing high sensitivity sensors with amplitude resolution of 3.4 x 10(-4) RIU, and spectral resolution of 1.3 x 10(-4) RIU in THz regime. Finally, novel sensing methodology based on detection of changes in the core mode dispersion is proposed.

High-speed fiber-optic spectrometer for signal demodulation of inteferometric fiber-optic sensors

We demonstrated a spectrometer that is capable of acquiring the spectra from interferometric fiber-optic sensors at high speed. The high spectrum acquisition rate is enabled by transforming the spectral information from frequency domain to time domain using a dispersive element and high-speed data acquisition devices. Preliminary results show that the prototype system can record 10,000 frames of spectra per second and achieve a spectrum measurement resolution of 15 nm. Better performance could be realized by using a Raman amplifier and by optimizing the parameters of the system and data processing methods.

Design of dispersion-compensating Bragg fiber with an ultrahigh figure of merit

We report a novel idea for achieving highly efficient dispersion-compensating Bragg fiber by exploiting a modified quarter-wave stack condition. Our Bragg fiber yielded an average dispersion of approximately -1800 ps/(nm km) across the C band for the fundamental TE mode and an ultrahigh figure of merit of approximately 180,000 ps/(nm dB), which is at least 2 orders of magnitude higher than that of conventional dispersion-compensating fibers. The proposed methodology could be adopted for the design of a dispersion compensator across any desired wavelength range.

Dynamic all-optical tuning of transverse resonant cavity modes in photonic bandgap fibers

Photonic bandgap fibers for transverse illumination containing half-wavelength microcavities have recently been designed and fabricated. We report on the fabrication and characterization of an all-optical tunable microcavity fiber. The fiber is made by incorporating a photorefractive material inside a Fabry-Perot cavity structure with a quality factor Q >200 operating at 1.5 microm. Under short-wavelength transverse external illumination, a 2 nm reversible shift of the cavity resonant mode is achieved. Dynamic all-optical tuning is reported at frequencies up to 400 Hz. Experimental results are compared with simulations based on the amplitude and kinetics of the transient photodarkening effect measured in situ in thin films.

Remote coupling in Bragg fibers

We propose a remote directional coupler in Bragg fibers that allows for efficient conversion of the fundamental core mode into an annular mode structure over remote distances. We give design guidelines and confirm the results by finite-difference time-domain calculations.

Photonic crystal fibres

Photonic crystal fibres have wavelength-scale morphological microstructure running down their length. This structure enables light to be controlled within the fibre in ways not previously possible or even imaginable. Our understanding of what an optical fibre is and what it does is changing because of the development of this new technology, and a broad range of applications based on these principles is being developed.