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A new negative curvature hollow-core fiber layout with optimally positioned nested elements is presented. Confinement losses of 0.003 dB/km at 1.55 µm are achieved, corresponding to two orders of magnitude improvement compared to conventional designs.

Nested negative curvature hollow-core fibers (NCFs) represent state-of-art optical guidance in the near-infrared (near-IR) region. In this Letter, we propose a unique design approach for these types of fibers in order to further improve optical transmission via the optimal positioning of the nested elements. The nested elements in the proposed design are located at the center of the cladding tubes and are supported by bar-type structures. The topological optimization for the nested elements results in improved light guidance by two orders of magnitude with confinement losses as low as 0.003 dB/km within the targeted wavelength range of 1450 nm to 1600 nm. This bar-supported design features strong single-mode operation and low bending sensitivity in a wide range of bending radii.

A frequency selective negative curvature fiber design is proposed for potential optical sensing applications. Loss modulation depth of 11.78 dB and differential loss ratio of 51 between the baseline and filtered wavelengths were achieved.

We report a study investigating the effects of thermal annealing on the optical properties of Si-Ge alloy-core silica-cladded fibers. Low temperature fiber draw was performed with a laboratory-made draw tower at 1760 °C that minimizes impurity diffusion from cladding to the core. As a post-drawing process, Si–Ge core fibers were annealed in a box furnace to alter the core structure. Microstructural and optical properties of fibers were investigated, and transmission losses were measured as 28 dB/cm at 6.1 µm. Numerical studies were performed to analyze the experimental results and to find the optimum structure for low loss semiconductor-core glass-cladded fibers.

A broadband supercontinuum is generated from a thulium ultrafast soliton fiber laser in a high numerical aperture silica fiber with low nJ seed pulse energies. Amplification of a thulium based ultrafast soliton ring fiber laser in two customized thulium doped fiber amplifiers provides pulses varying in energy between 1.8 nJ and 13.5 nJ. Coupling these seed pulses into a high numerical aperture silica fiber UHNA 7 of varying length, a systematic study of the impact of pulse energy, pulse duration and nonlinear fiber length is conducted. Based on the normal dispersion, self-phase modulation dominates for lower pump power values and results in a symmetric spectral broadening process. For higher pulse energies, four-wave mixing and Raman scattering contribute to a strong red-shift of the spectrum beyond 2.3 μm. For a fiber length of 20 m of UHNA 7, a broadband supercontinuum spanning from 1.7 μm to 2.33 μm is generated with a 20 dB spectral bandwidth of 502 nm with an output pulse energy of 4.4 nJ, corresponding to an output power of 92 mW. This represents a low seed energy threshold for efficient flat supercontinuum generation at infrared wavelengths.

Conference Name: 2021 IEEE Research and Applications of Photonics in Defense Conference (RAPID) Date of Conference: 27 August 2021 The sources of the confinement losses in semiconductor-core optical fibers are numerically investigated by introducing fabrication-inspired imperfections such as micro-cracks and core ellipticity. The imperfections increase the losses up to 200 dB/m at the targeted spectrum that is comparable to experimental findings.

In this study, a novel tubular hollow-core fiber design with extended cladding structures aiming low transmission losses and dominant single-mode guidance in the infrared region is proposed. The fiber parameters are optimized to achieve low-loss operation at the target wavelength of 2 μ m , at which the emission spectrum of thulium-doped fiber lasers are centered. The confinement loss of the fundamental mode is found to be less than 0.02 dB/km in the near- and mid-infrared region, which makes the proposed design a strong alternative for conventional step-index silica optical fibers. Strong suppression of the higher order modes, and low bending sensitivity are also observed in the proposed design. The proposed fiber shows promising results to achieve low-loss transmission in the infrared spectrum.

Date of Conference: 26–29 July 2021 Conference name: OSA Advanced Photonics Congress 2021 A new negative curvature hollow-core fiber with a compound cladding structure is proposed. Compared to conventional nested structures, the proposed design features one order of magnitude improvement in confinement losses as low as 0.03 dB/km

Date of Conference: 10-12 August 2020 Conference name: 3rd IEEE Research and Applications of Photonics in Defense Conference, RAPID 2020 Si-Ge alloy-core silica-cladded fibers were drawn at a low temperature to minimize impurity diffusion. The elemental segregation in the as-drawn fibers was overcome by a thermal treatment. The transmission losses of the fibers were calculated as 28 dB/cm at 6.1 μm wavelength.

Polarization rotation dynamics for group velocity locked vector solitons in higher order harmonically mode-locked soliton fiber lasers are studied for the first time, in a thulium/holmium fiber laser. In a linear ultrafast fiber laser with net anomalous dispersion, harmonically mode-locked states are generated continuously by increasing the pump power for specific net cavity birefringence settings. The polarization evolution frequency remains the same for all harmonically mode-locked pulse trains, converging toward a singular solution close to the maximum pulse intensity modulation depth for each respective harmonically mode-locked state. With a polarization resolved output, an intensity modulation of the ultrafast pulse train with varying pulse binning is recorded, corresponding to the polarization rotation of the vector solitons. In addition, a polarization rotation vector soliton state with two different sets of RF sidebands symmetric to the repetition rate is observed, which leads to a more complex modulated pulse train. The experimental results of the polarization evolution also matche well with the presented theoretical modeling. Similar relative intensity noise

We demonstrate a single-mode Tm/Ho co-doped mode-locked fiber laser with a repetition rate of 253 MHz that is directly core pumped at a wavelength of 790 nm. In a soliton mode-locked linear fiber laser cavity, femtosecond pulses with transform-limited pulse durations of 315 fs are generated. With single-mode core pumping, the high peak absorption of the gain medium is utilized to obtain an optical spectrum with suppressed ripples and high signal-to-background levels in the RF spectrum above 70 dB. Good noise performance with a rms relative intensity noise of 0.11% [10 Hz, 2 MHz] and a timing jitter of 20 fs [100 Hz, 2 MHz] is demonstrated. For a single pulsing mode-locking regime, a reproducible and stable peak wavelength tunability of the optical spectrum is achieved over 11 nm by pump power tuning.

Pathways towards the generation of extreme optical pulsation in a chaotic transition regime in a linear fibre laser cavity configuration are presented. In a thulium mode-locked fibre laser, extreme events that can be controllably induced by manipulating the cavity birefringence for pulse energies exceeding the single soliton pulse operating regime are studied in detail for the first time. While a solitonic pulsation structure at the fundamental repetition rate is maintained, additional energy is shed in a chaotic manner, leading to broader spectral generation and shorter pulse durations whose behaviour deviates significantly from a classical statistical distribution. These pulses display markedly different characteristics from any previously reported extreme events in fibre lasers associated with multiple solitons and pulse bunching, thus presenting a novel observation of extreme pulsation. Detailed noise studies indicate that significant enhancement of relaxation oscillations, modulation instability and the interplay with reabsorption mechanisms contribute in this transient chaotic regime. The extreme pulsation generated in a compact fibre laser without any additional nonlinear attractors can provide an attractive platform to accelerate the exploration of the underlying physics of the chaos observed in mode-locked laser systems and can lead to novel fibre laser cavity designs.

Scaling of a compact linear soliton mode-locked Tm-doped fiber lasers with repetition rates of 531.9 MHz, 704.3 MHz, and 969.0 MHz with transform limited pulse durations down to 358 fs is demonstrated.

Polarization dynamics in soliton mode-locked femtosecond thulium fiber lasers are presented that can lead to pulse modulation. A dual output pulse train with orthogonally polarized interlaced pulses is discussed.

In a Tm/Ho-doped ultrafast linear cavity fiber laser, extreme optical pulsation is demonstrated. Single-pulsing behavior at the fundamental repetition rate is maintained while the level of chaotic pulsation is adjusted by an intracavity polarization controller.

Ultra-high precision dual-comb spectroscopy traditionally requires two mode-locked, fully stabilized lasers with complex feedback electronics. We present a novel mode-locked operation regime in a thulium-holmium co-doped fiber laser, a frequency-halved state with orthogonally polarized interlaced pulses, for dual comb generation from a single source. In a linear fiber laser cavity, an ultrafast pulse train composed of co-generated, equal intensity and orthogonally polarized consecutive pulses at half of the fundamental repetition rate is demonstrated based on vector solitons. Upon optical interference of the orthogonally polarized pulse trains, two stable microwave RF beat combs are formed, effectively down-converting the optical properties into the microwave regime. These co-generated, dual polarization interlaced pulse trains, from one all-fiber laser configuration with common mode suppression, thus provide an attractive compact source for dual-comb spectroscopy, optical metrology and polarization entanglement measurements.

The impact of core-pumping at pump wavelengths of 790 nm and 1565 nm on the optical performance, relative intensity and phase noise characteristics of a linear cavity thulium soliton mode-locked fiber lasers are presented.

Vector solitons and the evolution of their polarization evolution frequency are studied in harmonically mode-locked Tm/Ho co-doped fiber laser states.

An ultrafast pulse train composed of co-generated, consecutive, equal intensity and orthogonally polarized pulses is experimentally shown for the first time in a vector soliton mode-locked fiber laser.

A Tm/Ho co-doped mode-locked soliton fiber laser design is presented with stable and low noise single-pulsing operation at a repetition rate of 135.2 MHz and a transform limited pulse duration of 375 fs. The fiber laser is directly core pumped at a wavelength of 790 nm. In single-pulsing operation, the fiber laser is centered at a wavelength of 1983 nm and can be continuously tuned over an 8 nm bandwidth. The fiber laser consists of a linear cavity which allows scaling of the repetition rate further by reducing the cavity length and utilizing the high pump absorption at 790 nm and efficient absorption/emission dynamics without photodarkening. In addition, co-doping with Tm/Ho increases the efficiency of the lasing with enhanced cross-relaxation rates. Stable mode-locked operation with reduced ripples in the optical spectrum and high signal-to-background ratios in the RF spectrum is observed. A low relative intensity noise with an rms fluctuation level of 0.13 % (frequency interval of 10 Hz to 1 MHz) and a low phase noise with a timing jitter of 20 fs (frequency range of 100 Hz to 1 MHz) characterizes the mode-locked laser.

Conference name: Proceedings of the conference - Wave Propagation: From Electrons to Photonic Crystals and Metamaterials in honor of Prof. C. M. Soukoulis’ 60th birthday Date of Conference: 8-11 June 2011 A dual-frequency division de-multiplexing mechanism is demonstrated using cascaded photonic crystal waveguides with unequal waveguide widths. The de-multiplexing mechanism is based on the frequency shift of the waveguide bands for the unequal widths of the photonic crystal waveguides. The modulation in the waveguide bands is used for providing frequency selectivity to the system. The slow light regime of the waveguide bands is utilized for extracting the desired frequency bands from a wider photonic crystal waveguide that has a relatively larger group velocity than the main waveguide for the de-multiplexed frequencies. In other words, the wider spatial distribution of the electric fields in the transverse direction of the waveguide for slow light modes is utilized in order to achieve the dropping of the modes to the output channels. The spectral and spatial de-multiplexing features are numerically verified. It can be stated that the presented mechanism can be used to de-multiplex more than two frequency intervals by cascading new photonic crystal waveguides with properly selected widths.

GHz high repetition rate compact sources with femtosecond pulse durations and stable performance can enable a wide range of applications. In this paper, several high repetition rate ultrafast thulium fiber lasers with repetition rates varying between 532 MHz to 1.25 GHz are demonstrated with femtosecond pulse durations down to 426 fs. An approach of maintaining comparable pulse energies while scaling the repetition rates allows high-quality femtosecond mode-locking performance with low noise performance in thulium soliton lasers for the first time.

Passively harmonic mode-locking in a linear Tm/Ho co-doped fiber laser is demonstrated with self-organized, temporally equally spaced solitons at repetition rates over 400 MHz with high side-mode suppression.

A single-mode Tm/Ho co-doped mode-locked fiber laser that is directly core-pumped with 790 nm pump light is presented. In a short gain fiber cavity, femtosecond pulses at a repetition rate of 135 MHz are demonstrated.

An asymmetric, reciprocal, diffraction-free transmission of linearly polarized waves in a new diodelike, three-layer, ultrathin, chiral structure is studied theoretically and experimentally. The exploited physical mechanism is based on the maximization of the cross-polarized transmission in one direction due to the polarization selectivity dictated by the peculiar eigenstate combination, which is efficiently controlled by the electromagnetic tunneling through the metallic subwavelength mesh sandwiched between these layers. Simulation and microwave experiment results demonstrate a nearly total intensity transmission at normal incidence in one direction and a small intensity transmission in the opposite direction.

A circular polarizer, which is composed of periodic and two-dimensional dielectric high-contrast gratings, is designed theoretically such that a unity conversion efficiency is achieved at lambda(0) = 1.55 mu m. The operation is obtained by the achievement of the simultaneous unity transmission of transverse magnetic and transverse electric waves with a phase difference of pi/2, meaning that an optimized geometrical anisotropy is accomplished. By the utilization of the rigorous coupled-wave analysis and finite-difference time-domain methods, it is shown that a percent bandwidth of similar to 50% can be achieved when the operation bandwidth is defined as the wavelengths for which the conversion efficiency exceeds 0.9. (c) 2012 Optical Society of America

Cataloged from PDF version of article. We report numerical and experimental investigations of asymmetric light propagation in a newly designed photonic structure that is formed by creating a chirped photonic crystal (PC) waveguide. The use of a non-symmetric distribution of unit cells of PC ensures the obtaining of asymmetric light propagation. Properly designing the spatial modulation of a PC waveguide inherently modifies the band structure. That in turn induces asymmetry for the light's followed path. The investigation of the transmission characteristics of this structure reveals optical diode like transmission behavior. The amount of power collected at the output of the waveguide centerline is different for the forward and backward propagation directions in the designed configuration. The advantageous properties of the proposed approach are the linear optic concept, compact configuration and compatibility with the integrated photonics. These features are expected to hold great potential for implementing practical optical rectifier-type devices. (C) 2012 Optical Society of America.

Cataloged from PDF version of article. We demonstrate the operation of a compact wavelength de-multiplexer using cascaded single-mode photonic crystal waveguides utilizing the slow light regime. By altering the dielectric filling factors of each waveguide segment, we numerically and experimentally show that different frequencies are separated at different locations along the waveguide. In other words, the beams of different wavelengths are spatially dropped along the transverse to the propagation direction. We numerically verified the spatial shifts of certain wavelengths by using the two-dimensional finite-difference time-domain method. The presented design can be extended to de-multiplex more wavelengths by concatenating additional photonic crystal waveguides with different filling factors. (C) 2011 Optical Society of America

An electrically thin chiral metamaterial structure composed of four U-shaped split ring resonator pairs is utilized in order to realize polarization rotation that is dependent on the polarization of the incident wave at 6.2 GHz. The structure is optimized such that a plane wave that is linearly polarized at an arbitrary angle is an eigenwave of the system at this frequency. The analytical relation between the incident polarization and the polarization rotation is derived using transmission matrices. Furthermore, the proposed structure exhibits an asymmetric transmission of linearly polarized waves at 6.2 GHz. Plane waves traveling in opposite but perpendicular directions to the material plane are rotated by different angles. On the other hand, four incident polarization angles have been found for the same structure, at which the transmission is symmetric. The experiment results are in good agreement with the numerical results. © 2011 Optical Society of America.

Date of Conference: 8-11 Aug. 2011 In this study, tight-binding formalism is applied to a photonic crystal coupled cavity structure in order to investigate the characteristics of ultra slow light modes. Eigen-mode splitting is observed and resulting group indices obtained from the tight-binding formalism and numerical results are compared. © 2011 IEEE.

Date of Conference: 8-11 Aug. 2011 An optical race-track has been investigated in order to obtain a multi resonant structure with high-Q factors. Photonic crystal based structure provides strong field confinement and scalability in the dimensions of the structure. The average value of the quality factors at the resonances have been calculated to be on the order of ∼105. © 2011 IEEE.

Cataloged from PDF version of article. An asymmetric chiral metamaterial structure is constructed by using four double-layered U-shaped split ring resonators, which are each rotated by 90 degrees with respect to their neighbors. The peculiarity of the suggested design is that the sizes of the electrically and magnetically excited rings are different, which allows for equalizing the orthogonal components of the electric field at the output interface with a 90 degrees phase difference when the periodic structure is illuminated by an x-polarized wave. As a result, left-hand circular polarization and right-hand circular polarization are obtained in transmission at 5: 1 GHz and 6: 4 GHz, respectively. The experiment results are in good agreement with the numerical results. (C) 2011 Optical Society of America

Fabrication and experimental characterization of a broadband quarter-wave plate, which is based on two-dimensional and binary silicon high-contrast gratings, are reported. The quarter-wave plate feature is achieved by the utilization of a regime, in which the proposed grating structure exhibits nearly total and approximately equal transmission of transverse electric and transverse magnetic waves with a phase difference of approximately pi/2. The numerical and experimental results suggest a percent bandwidth of 42% and 33%, respectively, if the operation regime is defined as the range for which the conversion efficiency is higher than 0.9. A compact circular polarizer can be implemented by combining the grating with a linear polarizer. (C) 2012 Optical Society of America