Photonic crystal fibers with high and flattened dispersion

We investigate the dispersion property of space filling mode of photonic crystal structures and find a new type of dispersion—structure induced dispersion. By incorporating this new source of dispersion we designed PCF with large normal dispersion ~ 350 ps²/km. Our simulation indicates the dispersion of such fiber changes less than 3% in 1.4-1.7 μm wavelength range and we also show that our design is insensitive to the structure changes.     

Design of high duty ratio nonlinear photonic crystal fiber with near-zero flattened dispersion after 1.205 μm

The paper presents a high nonlinear photonic crystal fiber (HN-PCF) with highly duty ratio for time-stretch analog-to-digital conversion (TSADC). The simulation results show that a nonlinear of 42.26 W⁻¹ km⁻¹ and the flattened dispersion of less than 1.0 ps/(nm km) are obtained in more than 495 nm waveband (1205–1700 nm). Owing to its high nonlinear coefficient and flattened dispersion, the high nonlinear PCF is expected to be suitable for supercontinuum (SC) generation. The numerical simulation results demonstrate that the proposed high duty ratio HN-PCF can generate wideband SC.     

~ 1.2 μm near-infrared emission and gain anticipation in Ho doped heavy-metal gallate glasses

Ho³⁺-doped low-phonon-energy heavy-metal gallate glasses (LKBPBG) have been prepared and efficient 1.199 μm emission originating from the ⁵I₆ → ⁵I₈ radiative transition has been observed under 900 nm excitation. The spontaneous emission probability and the maximum stimulated emission cross-section were derived to be 294.31 s^− 1 and 3.46 × 10^− 21 cm², respectively. The ratio of quantum yields between ~ 1.2 and ~ 2.0 μm emissions was identified to be 16%, demonstrating that the ⁵I₆ → ⁵I₈ transition is favorable for optical amplification. The maximum gain coefficient of 1.84 dB/cm at 1.199 μm wavelength was anticipated in the ideal status. These results indicate that the Ho³⁺-doped LKBPBG glasses have a promising potential for the development of ~ 1.2 μm signal amplifier devices.     

Dispersion-ultra-flattened square-lattice photonic crystal fiber with small effective mode area and low confinement loss

Proposed in this paper is a simple square-lattice photonic crystal fiber (PCF) with nearly zero flattened dispersion (NZFD) over a wide wavelength span. We make a trade-off between the coupling efficiency and the effective mode area in order to obtain relatively low confinement loss and high nonlinearity. Via full-vector finite element method with hybrid edge/node elements, over 1137–1710 nm, the dispersion coefficient is 0.3 ± 0.3 ps/(km nm), the confinement loss is relatively low, in level of 10⁻⁷–10⁻⁴ dB/km and the effective mode area remains 5.88–6.59 μm².     

Supercontinuum generation in square photonic crystal fiber with nearly zero ultra-flattened chromatic dispersion and fabrication tolerance analysis

This paper presents a simple index-guiding square photonic crystal fiber (SPCF) where the core is surrounded by air holes with two different diameters. The proposed design is simulated through an efficient full-vector modal solver based on the finite difference method with anisotropic perfectly matched layers absorbing boundary condition. The nearly zero ultra-flattened dispersion SPCF with low confinement loss, small effective area as well as broadband supercontinuum (SC) spectra is targeted. Numerical results show that the designed SPCF has been achieved at a nearly zero ultra-flattened dispersion of 0 ± 0.25 ps/(nm·km) in a wavelength range of 1.38 μm to 1.89 μm (510 nm band) which covers E, S, C, L and U communication bands, a low confinement loss of less than 10⁻⁷ dB/m in a wavelength range of 1.3 μm to 2.0 μm and a wide SC spectrum (FWHM = 450 nm) by using picosecond pulses at a center wavelength of 1.55 μm. We then analyze the sensitivity of chromatic dispersion to small variations from the optimum value of specific structural parameters. The proposed index-guiding SPCF can be applicable in supercontinuum generation (SCG) covering such diverse fields as spectroscopy applications and telecommunication dense wavelength division multiplexing (DWDM) sources.     

Tuning visible light generation based on multi-channel quasi-periodically poled LiTaO3

We demonstrate broadband tuning visible light generation based on a multi-channel quasi-periodically poled LiTaO₃ crystal, in which a quasi-phase matched optical parametric generation process and a quasi-phase matched sum-frequency mixing process were achieved simultaneously. The conversion characters on spectrum and energy were studied by using a nanosecond pulse laser at 1.064 μm as pump light. We could tune the visible light over ~ 26 nm by means of changing the crystal's channel and temperature. The ratio of the output wavelength variation to that of temperature was ~ 0.07 nm/°C. The single-pass slope efficiency was 6.3% with the maximum output energy of 25 μJ.     

The study of a Tm:YLF laser pumped by a Raman shifted Erbium fiber laser at 1678 nm

A Tm:YLF laser pumped by a Raman shifted Er-fiber laser at 1.678 μm was studied at two Tm³⁺ ion concentrations equal to 1.5% and 5%. At output powers up to 460 mW the measured lasing efficiency at a wavelength of ~ 1.93 μm was as high as ~ 50%. The lasing performance was compared with that obtained under pumping by a 792-nm laser diode. The temporal structure of the laser pulse was recorded and the beam propagation factor M² was measured for all pumping conditions.     

New nonlinear and dispersion flattened photonic crystal fiber with low confinement loss

A new nonlinear dispersion flattened photonic crystal fiber with low confinement loss is proposed. This fiber has threefold symmetry core. The doped region in the core and the big air-holes in the 1st ring can make high nonlinearity in the PCF. And the small air-holes in the 1st ring and the radial increasing diameters air-holes rings in cladding can be used to achieve the dispersion properties of the PCF. We can achieve the optimized optical properties by carefully selecting the PCFs structure parameters. A PCF with flattened dispersion is obtained. The dispersion is less than 0.8 ps/(nm km) and is larger than −0.7 ps/(nm km) from 1.515 μm to 1.622 μm. The nonlinear coefficient is about 12.6456 W⁻¹ km⁻¹, the fundamental mode area is about 10.2579 μm². The confinement loss is 0.30641 dB/km. This work may be useful for effective design and fabrication of dispersion flattened photonic crystal fibers with high nonlinearities.     

Optical sensing characteristics of the photonic crystal fiber filled with magnetic fluid

A novel photonic crystal fiber sensing theory filled with magnetic fluid is proposed based on the change of the MF refractive index under varied magnetic field. The magnetically induced tuning of the magnetic fluid filled PCF propagation properties were investigated by the full-vector finite element method with a perfectly matched layer. Theoretical calculations show that both the effective refractive index and the effective mode area increase vs. the increased magnetic field, and the PCF filled MF with larger d/Λ is more sensitive to magnetic field. When the wavelength λ = 1550 nm, the duty ratio d/Λ = 0.9, d/Λ = 0.6, the effective refractive indexes increase respectively from 1.598279 to 1.617572, from 1.61948 to 1.632484, and the effective mode areas increase respectively from 3.561115 μm² to 7.052360 μm², from 6.167494 μm² to 37.221998 μm² as the magnetic field changes from 25 Oe to 175 Oe. This scheme provides theoretical foundation to use magnetic field to control light in photonic crystal fiber and also offers a potential method for magnetic field sensing based on the TIR-PCF.     

Design and analysis of a photonic crystal fiber with four-hole unit

A novel photonic crystal fiber (PCF) based on a four-hole unit is proposed in order to meet the requirements of high birefringence, negative dispersion and confinement loss in fiber-optic communication. The proposed design has been simulated based on the full vector finite element method (FVFEM) and anisotropic perfectly matched layers (APML). Analysis results show that the proposed PCF can achieve a high birefringence to the order of 10⁻² at the wavelength of 1.55 μm, a large negative dispersion over a wide wavelength range and confinement losses lower than 10⁻⁹ dB/m simultaneously, which has important applications in polarization-maintaining (PM) fibers, single-polarization single-mode (SPSM) fibers, dispersion compensation fibers and so on.     

Highly birefrigent photonic crystal fiber with high negative dispersion for broadband dispersion compensation

In this paper, we proposed a dual-enhanced core photonic crystal fiber (PCF) with high birefringence and ultra-high negative dispersion for dispersion compensation in a polarization maintained optical system. Using finite difference time domain (FDTD) method, we presented dispersion compensating PCF (DC-PCF) with negative dispersion between −1650 ps nm⁻¹ km⁻¹ and −2305 ps nm⁻¹ km⁻¹ in C-band and particularly −2108 ps nm⁻¹ km⁻¹ in λ = 1.55 μm wavelength. By this method, we can compensate dispersion in 124 km long span of a conventional single mode fiber (SMF) by 1 km-long of the DC-PCF at λ = 1.55 μm wavelength. Moreover, fundamental mode of the proposed PCF can induce birefringence about 3.5 × 10⁻³ at 1.55 μm wavelength.     

Highly efficient optical parametric chirped-pulse amplification based on BiB3O6 and β-BaB2O4 crystals at low pump intensity

We report a very high signal gain of 1.13 × 10⁷ at a low pump intensity of 260 MW/cm² in a two-stage optical parametric chirped-pulse amplifier (OPCPA), which is used as a pre-amplifier for a short-pulse front-end Nd: glass high-energy laser system. A signal energy of 0.17 nJ was amplified to 2 mJ with a central wavelength of 1053 nm and a repetition rate of 10 Hz using the OPCPA with a 15 mm-long BBO crystal at optical parametric amplifier (OPA) stage 1 and a 12 mm-long BiBO crystal at OPA stage 2.     

Polarization maintaining large nonlinear coefficient photonic crystal fibers using rotational hybrid cladding

In this paper, we present and explore a new hybrid cladding design for improved birefringence and highly nonlinear photonic crystal fibers (PCFs) in a broad range of wavelength bands. The birefringence of the fundamental mode in such a PCF is numerically analyzed using the finite element method (FEM). It is demonstrated that it is possible to design a simple highly nonlinear hybrid PCF (HyPCF) with a nonlinear coefficient of the about 46 W⁻¹ km⁻¹ at a 1.55 μm wavelength. According to simulation, the highest modal birefringence and lowest confinement loss of our proposed structure at the excitation wavelength of λ = 1.55 μm can be achieved at a magnitude of 1.77 × 10⁻² and of the order less than 10² dB/km with only five rings of air-holes in the fiber cladding.     

Characteristics analysis of extruded elliptical hole photonic crystal fibers with square air-core

An extruded elliptical hole photonic crystal fibers PCF with square air-core is proposed. By using a full vector finite-element method FV-FEM and anisotropic perfectly matched layers APML, the structure and optical properties of the proposed PCF are analyzed. Simulation results show that the birefringence of the proposed photonic crystal fiber can be up to the order of 10⁻², and has a flattened dispersion from 1.20 μm to 1.80 μm. The proposed PCF may have important application in super-continuum SC generation, dispersion compensation, fiber-optic sensing systems and other aspects.     

Dispersion characteristic of hexagonal and square lattice chalcogenide As2Se3 glass photonic crystal fiber

In this paper, we report a chalcogenide As₂Se₃ glass photonic crystal fiber (PCF) for dispersion compensating application. We have used the improved fully vectorial effective index method (IFVEIM) for comparing the dispersion properties (negative and zero dispersion) and effective area in hexagonal and square lattice of As₂Se₃ glass PCF using different wavelength windows. It has been demonstrated that due to their negative dispersion parameter and negative dispersion slope in wavelength range 1.2-2.5 μm, both lattice structures of As₂Se₃ glass PCFs, with pitch (Λ = 2 μm), can be used as dispersion compensating fibers. Further, design parameters have been obtained to achieve zero dispersion in these fibers. It is also shown that As₂Se₃ glass PCF provides much higher negative dispersion compared to silica PCF of the same structure, in wavelength range 1.25-1.6 μm and hence such PCF have high potential to be used as a dispersion compensating fiber in optical communication systems.     

Writing low-loss waveguides in borosilicate (BK7) glass with a low-repetition-rate femtosecond laser

Borosilicate glass (BK7) is a widely-used material in integrated optics devices and in the optical communications industry. We report on laser-written waveguiding in BK7 glass using a low-repetition-rate (1 kHz) laser producing 40 fs pulses of 800 nm light. A 500 μm slit is used to write structures 100 μm below the glass surface. These waveguides show strong guidance at 635 nm, with an index contrast of 3 × 10^− 4 and a propagation loss of ~ 0.5 dB/cm. We measured the change in refractive index for a range of writing conditions as quantified in terms of energy dose; there is an energy dose window (> 0.6 μJ μm^− 3 and < 1.5 μJ μm^− 3) within which the written structures show guidance.     

High birefringence and low loss circular air-holes photonic crystal fiber using complex unit cells in cladding

We propose a high birefringence and low loss index-guiding photonic crystal fiber (PCF) using the complex unit cells in cladding by the finite-element method. Results show that the birefringence and confinement loss in such PCF fiber is determined not only by the whole cladding asymmetry but also the shape of the PCF core. The maximal modal birefringence and lowest confinement loss of our proposed structures at the excitation wavelength of λ = 1550 nm can be achieved at 8.7 × 10⁻³ and 5.27 × 10⁻⁵ dB/km, respectively.     

Specialty Yb fiber amplifier for microchip Nd laser: Towards ∼1-mJ/1-ns output at kHz-range repetition rate

We demonstrate and optimize, for a mJ/ns release at the wavelength 1.064 μm, the operation of a compact laser system designed in the form of a hybrid, active-passive, Q-switched Nd³⁺:YAG/Cr⁴⁺:YAG microchip laser seeding an Yb-doped specialty multi-port fiber amplifier. As the result of the amplifier optimization, ∼1 mJ, ∼1 ns, almost single-mode pulses at a 1-10-kHz repetition rate are achieved, given by a gain factor of ∼19 dB for an 11-μJ input from the microchip laser. Meanwhile, a lower pulse energy, ∼120 μJ, but a much higher gain (∼25 dB) are eligible for the less powerful (0.35 μJ) input pulses.     

A novel polarization splitter based on dual-core hybrid photonic crystal fibers

Highly birefringent dual-core photonic crystal fibers (PCFs) can be used as a polarization splitter because the orthogonal polarization modes with dissimilar coupling lengths are easily separated from each other. Different from the traditional methods achieving high birefringence, a new highly birefringent hybrid PCF that guides light by both index guiding and bandgap guiding is proposed. Firstly, a novel polarization splitter based on this kind of dual-core hybrid PCF is designed. The transmission modes, coupling lengths for the two orthogonal polarizations and performance of the proposed polarization splitter are investigated and numerically analyzed. The results demonstrate that it is possible to obtain a 4.72-mm-long polarization splitter. The splitting ratio is better than −20 dB in a large wavelength range of 1.53-1.72 μm. Its bandwidth is about 190 nm.     

Designing of endlessly single mode polarization maintaining highly birefringent nonlinear micro-structure fiber at telecommunication window by FV-FEM

An endlessly single mode highly polarization maintaining nonlinear microstructure fiber at telecommunication window is reported via full-vector finite element method. By taking three ring hexagonal PCF with suitable fiber parameter such as air hole diameter in cladding region d = 0.8 μm, pitch 2.3 μm and introducing four symmetrical large air holes near core region d′ = 2 μm, single mode (V_eff ≤ π), small effective mode area 2.7 μm², nonlinear co-efficient 44.39 W⁻¹ km⁻¹, high phase birefringence of the order of 10⁻³ and group birefringence of the order of 10⁻⁴ with beat length 0.3 μm at wavelength 1.55 μm are achieved.