Near-zero Dispersion Research Articles

Ultra-low-noise supercontinuum generation with a flat near-zero normal dispersion fiber.

A pure silica photonic crystal fiber with a group velocity dispersion (β2) of 4 ps2/km at 1.55μm and less than 7 ps2/km from 1.32μm to the zero dispersion wavelength (ZDW) 1.80μm was designed and fabricated. The dispersion of the fiber was measured experimentally and found to agree with the fiber design, which also provides low loss below 1.83μm due to eight outer rings with increased hole diameters. The fiber was pumped with a 1.55μm, 125fs laser and, at the maximum in-coupled peak power (P0) of 9kW, a 1.34-1.82μm low-noise spectrum with a relative intensity noise below 2.2% was measured. The numerical modeling agreed very well with the experiments and showed that P0 could be increased to 26kW before noise from solitons above the ZDW started to influence the spectrum by pushing high-noise dispersive waves through the spectrum.

Design of low-loss and near-zero ultraflattened dispersion PCF for broadband optical communication

A 2D finite difference time domain method is used to design a single-mode photonic crystal fiber (PCF) with engineered parameters that is studied with the aim of simultaneous achievement of low loss (below 10−14dB/km) and low chromatic dispersion over a broad band for future optical communication. The superiority of the proposed structure over recently reported structures is shown by the near-zero ultraflattened chromatic dispersion of 1.69±0.08 ps/(km nm) obtained over a broad wavelength range from 1 to 2 μm, covering the entire optical communication bands (O, E, S, C, L, and U). Also, the large effective area of more than 130 μm2 and as a result the low nonlinearity in addition to the near-zero ultraflattened chromatic dispersion and ultralow loss achieved for the proposed PCF make the structure a good candidate for high-capacity optical communication with a high bitrate over long distances for future ultradense wavelength-division multiplexing.

Revelation of the birth and extinction dynamics of solitons in SWNT-mode-locked fiber lasers

The dispersive Fourier transform (DFT) technique opens a fascinating pathway to explore ultrafast non-repetitive events and has been employed to study the build-up process of mode-locked lasers. However, the shutting process for the mode-locked fiber laser seems to be beyond the scope of researchers, and the starting dynamics under near-zero dispersion remains unclear. Here, the complete evolution dynamics (from birth to extinction) of the conventional soliton (CS), stretched pulse (SP), and dissipative soliton (DS) are investigated by using the DFT technique. CS, SP, and DS fiber lasers mode locked by single-walled carbon nanotubes (SWNTs) are implemented via engineering the intracavity dispersion map. The relaxation oscillation can always be observed before the formation of stable pulse operation due to the inherent advantage of SWNT, but it exhibits distinct evolution dynamics in the starting and shutting processes. The shutting processes are dependent on the dispersion condition and turn-off time, which is against common sense. Some critical phenomena are also observed, including transient complex spectrum broadening and frequency-shift interaction of SPs and picosecond pulses. These results will further deepen understanding of the mode-locked fiber laser from a real-time point of view and are helpful for laser design and applications.

Extremely nonlinear carbon-disulfide-filled photonic crystal fiber with controllable dispersion

In this paper, we present a new design of carbon-disulfide-filled photonic crystal fibers (CS2-PCFs) by combining two unrelated optical properties, ultra high nonlinearity and tunable nearly-zero flattened disperion profile. Considering the fabrication challenges and using realistic hole dimensions, we design a CS2-PCF with nonlinear coefficient of 7940 W-1km-1, confinement loss of 1×10−3 dB/km, total loss lower than 0.3 dB/m, nearly-zero dispersion of 0.00007 ps/(nm km) and a dispersion slope of 0.0000018 near 1550 nm. We demonstrate that the small changes due to fabrication imperfections does not have a significant effect on the dispersion characteristics or nonlinearity of the proposed PCF. In addition, we show that one can easily tune the dispersion profile of the proposed PCF by simply changing the core diameter. Therefore, the core diameter can be served as a controlling parameter to change the spectral range of flattened-dispersion profile of the CS2-PCF. The proposed PCF is a promising candidate for numerous nonlinear applications which require high nonlinearity and/or specific dispersion characteristics including applications based on four-wave mixing.

Electrically Tunable Propagation Properties of the Liquid Crystal-Filled Terahertz Fiber

A bandgap-guiding microstructured fiber for terahertz (THz) radiation was designed by infiltrating the cladding air holes with nematic liquid crystal. Structural parameter dependence of the photonic bandgaps, polarization-dependent bandgap splitting, and electrically tunable propagation properties of the designed fiber were investigated theoretically by using the finite-element method. An external electric field applied across the designed fiber can broaden the effective transmission bandwidth and achieve single-mode single-polarization guidance. Flattened near-zero group-velocity dispersion of 0 ± 1 ps/THz/cm was obtained for the y-polarized fundamental mode within a broad frequency range. Our results provide theoretical references for applications of liquid crystal-filled microstructured fiber for dynamic polarization control and tunable fiber devices in THz frequency.

Timing jitter of high-repetition-rate mode-locked fiber lasers.

We characterize the timing jitter of the pulse trains from 880MHz Yb-doped nonlinear polarization rotation mode-locked fiber lasers based on a balanced optical cross-correlation method. Jitter spectral density at different net-cavity dispersions has been characterized, and the near-zero dispersion shows the lowest rms timing jitter (10fs rms, integrated from 30kHz to 5MHz). The measurements have been compared with analytical models. The comparison shows that the RIN-coupled timing jitter by nonlinearity is the dominated origin of the measured timing jitter below ∼100 kHz, while amplified spontaneous emission noise makes a major contribution in the high frequency range above hundreds of kilohertz. To the best of our knowledge, this is the first high-precision timing jitter characterization for the ∼gigahertz level repetition rate mode-locked fiber lasers. The results will be of great importance for further improving the laser performance for many applications.

Dispersion-engineered microstructured optical fiber for mid-infrared supercontinuum generation.

Due to the large scientific and technical interest in the mid-infrared (MIR) spectral region, and the limitations of MIR light sources, we focus on the generation of a broad supercontinuum inside a short piece of As2Se3 microstructured optical fiber (MOF) with a square lattice. This is accomplished by filling the holes in the innermost ring of the proposed MOF with Ge33As12Se55 to produce ultraflat and near-zero dispersion. Simulations reveal that, by launching 100fs input pulses centered at λ0=6.2 μm with a peak power of 2kW into the MOF, an optical spectrum as wide as 9.5μm will be achieved. This spectrum is a suitable source for MIR applications such as spectroscopy, food quality control, and gas sensing.

75-km Long Reach Dispersion Managed OFDM-PON at 60 Gbit/s With Quasi-Color-Free LD

A quasi-color-free laser diode (QCFLD)-based 60-Gbit/s long-reach (LR) OFDM-PON with dispersion management is demonstrated. After injection locking, the QCFLD achieves an ultimate back-to-back transmission capacity of 60 Gbit/s. After propagating through 25-, 50-, and 75-km single-mode fiber (SMF)-based LR-OFDM-PON, the RF power fading effect induces signal deterioration to inevitably degrade the transmission capacities of the QCFLD at 45, 27, and 21 Gbit/s, respectively. To avoid the RF fading effect, dispersion management is introduced by using different segments of nonzero dispersion-shifted fiber (NZ-DSF) and dispersion compensating fiber (DCF) with corresponding dispersion constants of +5.6 and −5 ps/nm/km, respectively. Using either 25-km NZ-DSF or 25-km DCF enables the OFDM-PON to operate at up to 57 Gbit/s. Combining the 25-km NZ-DSF with the 25-km DCF establishes the near-zero dispersion to effectively support the 50-km LR OFDM-PON up to 51 Gbit/s, which is nearly twice data rate as compared to the 50-km SMF case. For implementing the 75-km LR OFDM-PON, the scenario of 25-km NZ-DSF in connection with 50-km DCF is employed to achieve 48-Gbit/s transmission capacity. In comparison with the same OFDM-PON constructed with 75-km SMF, the achievable data rate is doubled through the synergy of the LR OFDM-PON technology with dispersion management.

Broadband ultra-flattened dispersion, ultra-low confinement loss and large effective mode area in an octagonal photonic quasi-crystal fiber.

In this work, an octagonal Penrose-type photonic quasi-crystal fiber (PQF) with dual-cladding is proposed. By optimizing three geometric degrees of freedom, the PQF exhibits ultra-flattened near-zero dispersion of 0.014±0.293 ps/nm/km, ultra-low order confinement loss of 10-4 dB/km, and large effective mode area of over 16.2 μm2 in a broadband of wavelength from 1.27 to 1.67μm, covering almost all optical communication bands. At the common communication wavelength 1.55μm, completely opposite trends of the dispersion and the confinement loss varying with the air-filling factor in the inner cladding are demonstrated. In addition, the robustness of optical properties including dispersion, confinement loss, and effective mode area in this PQF is discussed, assuming a deviation ±3% of all air holes.

Experimentally Validated Dispersion Tailoring in a Silicon Strip Waveguide With Alumina Thin-Film Coating

We propose a silicon strip waveguide structure with alumina thin-film coating in-between the core and the cladding for group-velocity dispersion tailoring. By carefully designing the core dimension and the coating thickness, a spectrally-flattened near-zero anomalous group-velocity dispersion within the telecom spectral range is obtained, which is predicted to significantly broaden the bandwidth of four-wave mixing. We validate this by characterizing the wavelength conversion in a waveguide sample by atomic layer deposition technology, which to our best knowledge is the first experimental demonstration of the proposed structure. Due to the alumina thin-film coating, the wavelength conversion bandwidth reaches $58\,\mathrm{nm}$, an increase by a factor of 1.3 compared to the corresponding structure without coating. This method can also be applied to other material platforms and applications requiring accurate group-velocity dispersion control.

Zeonex-based asymmetrical terahertz photonic crystal fiber for multichannel communication and polarization maintaining applications.

We report on the design, in-depth analysis, and characterization of a novel elliptical array shaped core rectangular shaped cladded photonic crystal fiber (PCF) for multichannel communication and polarization maintaining applications of terahertz waves. The asymmetrical structure of air holes in both core and cladding results in increased birefringence, while a compact geometry and different cladding air hole size makes the dispersion characteristic flat. The modal characteristics of the PCF are calculated using a finite element method. The simulated results show a near-zero dispersion flattened property of ±0.02 ps/THz/cm, high birefringence of 0.063, low effective material loss of 0.06 cm-1, and negligible confinement loss of 5.45×10-13 cm-1 in the terahertz frequency range. Additionally, the core power fraction, effective area, physical attributes, and potential fabrication possibilities of the fiber are discussed.

Guiding characteristics of sunflower-type fiber

In this paper, the guiding characteristics of sunflower-type fiber (SFF) with 6-fold rotational symmetry are investigated theoretically using finite element method (FEM). The behavior of single-mode propagation in SFF is verified. Numerical results reveal that, the cutoff ratio for endlessly single-mode propagation in SFF is 0.575 which is larger than that of photonic crystal fiber (PCF) and photonic quasi-crystal fiber (PQF). Moreover, SFF can present ultra-flattened near-zero chromatic dispersion, 0.249 ± 1.146 ps/nm/km, in a broadband of wavelength covering 1.20–1.84 μm over all the telecommunication wavelengths. In term of chromatic dispersion and confinement loss in the wavelength range from 1.00 to 2.00 μm, a comparison between SFF, PCF and PQF with same structure parameters is carried out. Importantly, the rotational symmetry, as a new manageable structure parameter beyond common air hole diameter and lattice constant, can be employed to manipulate the chromatic dispersion, confinement loss, effective mode area and non-linear coefficient and it dependences on these guiding characteristics are discussed in detail.

Broadband wavelength conversion in a silicon vertical-dual-slot waveguide

We propose a silicon waveguide structure employing silica-filled vertical-dual slots for broadband wavelength conversion, which can be fabricated using simple silicon-on-insulator technology. We demonstrate group-velocity dispersion tailoring by varying the width of the core, the slots and the side strips, and put forward a method to achieve spectrally-flattened near-zero anomalous group-velocity dispersion at telecom wavelengths. A proposed structure provides a group-velocity dispersion parameter β2 of −60 ps2/km with an effective mode area Aeff of 0.075 µm2 at 1550 nm. This structure is predicted to significantly broaden the bandwidth of wavelength conversion via four-wave mixing, which is validated with experimentally measured 3 dB bandwidth of 76 nm.

Silica-coated gold nanorods as saturable absorber for bound-state pulse generation in a fiber laser with near-zero dispersion

We presented a bound-state operation in a fiber laser with near-zero anomalous dispersion based on a silica-coated gold nanorods (GNRs@SiO2) saturable absorber (SA). Using a balanced twin detector measurement technique, the modulation depth and nonsaturable loss of the GNRs@SiO2 SA were measured to be approximately 3.5% and 39.3%, respectively. By virtue of the highly nonlinear effect of the GNRs@SiO2 SA, the bound-state pulses could be easily observed. Besides the lower-order bound-state pulses with two, three, and four solitons, the higher-order bound states with up to 12 solitons were also obtained in the laser cavity. The pulse profiles of the higher-order bound states were further reconstructed theoretically. The experimental results would give further insight towards understanding the complex nonlinear dynamics of bound-state pulses in fiber lasers.

Near-zero dispersion flattened, low-loss porous-core waveguide design for terahertz signal transmission

We demonstrate a photonic crystal fiber with near-zero flattened dispersion, ultralower effective material loss (EML), and negligible confinement loss for a broad spectrum range. The use of cyclic olefin copolymer Topas with improved core confinement significantly reduces the loss characteristics and the use of higher air filling fraction results in flat dispersion characteristics. The properties such as dispersion, EML, confinement loss, modal effective area, and single-mode operation of the fiber have been investigated using the full-vector finite element method with the perfectly matched layer absorbing boundary conditions. The practical implementation of the proposed fiber is achievable with existing fabrication techniques as only circular-shaped air holes have been used to design the waveguide. Thus, it is expected that the proposed terahertz waveguide can potentially be used for flexible and efficient transmission of terahertz waves.

Extremely low‐loss hollow‐core bandgap photonic crystal fibre for broadband terahertz wave guiding

We demonstrate a simple polymer Topas-based low-loss hollow-core photonic crystal fibre for efficient guiding of terahertz radiation. The guiding properties are numerically investigated using finite-element method. It is shown that the proposed fibre exhibits extremely low average effective material loss of 0.017 cm−1, very small near-zero average group velocity dispersion of 0.263 ps/THz/cm with low total loss over 2–2.20 THz bands. Moreover, high air-core power fraction ∼94% having single-mode guiding is also reported for the optimum design parameters.

Ultra-flattened nearly-zero dispersion and ultrahigh nonlinear slot silicon photonic crystal fibers with ultrahigh birefringence

A slot silicon photonic crystal fiber (PCF) is proposed to simultaneously achieve ultrahigh birefringence, large nonlinearity and ultra-flattened nearly-zero dispersion over a wide wavelength range. By taking advantage on the slot effect, ultrahigh birefringence up to 0.0736 and ultrahigh nonlinear coefficient up to 211.48W−1m−1 for quasi-TE mode can be obtained at the wavelength of 1.55μm. Moreover, ultra-flattened dispersion of 0.49ps/(nmkm) for quasi-TE mode can be achieved over a 180nm wavelength range with low dispersion slope of 1.85×10−3ps/(nm2km) at 1.55μm. Leveraging on these advantages, the proposed slot PCF has great potential for efficient all-optical signal processing applications.

Design of ultra-flattened nearly zero dispersion and ultrahigh non-linear slot microfibres with ultrahigh birefringence

We proposed a new simple design of microfibre employing an elliptical silica rod in the centre of fibre core region as a slot core for the purpose of controlling the chromatic dispersion properties of the microfibre and enhancing the performance of non-linearity and birefringence. The simulation results show that the proposed slot microfibre has ultra-flattened near-zero dispersion of 0.94 ps/(nm km) for quasi-TE mode over a 50-nm wavelength range, ultrahigh birefringence up to the order of 10−1, and ultrahigh non-linear coefficients of 38.35 and 37.92 W−1 m−1 for the fundamental quasi-TE mode and quasi-TM mode at the wavelength of 1.55 μm. The outstanding advantage of this new design is that nearly zero ultraflattened dispersion, ultrahigh modal birefringence and ultrahigh non-linearity can be realized simultaneously simply using a slot fibre core. Benefiting from its excellent performance, the proposed slot microfibre will have great potential for all-optical signal processing applications.

Versatile dispersion characteristics of water solution of glycerine in selective filling of holes in photonic crystal fibers.

Using a glycerine-water solution with various concentrations, we investigate the dispersion characteristics of photonic crystal fibers by selective filling of holes. Our analysis is based on a simple but accurate semi-vectorial solution of Helmholtz's equation by the finite difference method devised with a mode-field convergence technique and crosschecked by results with those from a deeply involved multipole method. Significantly, a better ultra-flatness but near-zero group velocity dispersion is revealed with a 20% glycerine-water solution that is superior to pure water of a very recent case when the holes of the first ring of the fiber are filled. This versatile effect in management of holes of identical diameter with liquid is expected to play a guiding role in studies of supercontinuum generation.

Supercontinuum generation in the normal dispersion regime using chalcogenide double-clad fiber

We experimentally demonstrate mid-infrared (MIR) supercontinuum (SC) generation in a 2.8-cm-long chalcogenide double-clad fiber (Ch-DCF). The Ch-DCF made of As2Se3, AsSe2, and As2S5 glasses has near-zero chromatic dispersion in the normal dispersion regime. We pump the Ch-DCF using a 200 fs pulse laser at 10 µm, and obtain MIR SC extending from 2 to 14 µm. Numerical results show that the Ch-DCF has the potential of generating a highly coherent MIR SC light source.