Planar photonic crystal structure with inherently single-mode waveguides

A planar photonic crystal that allows inherently gap-guided single-mode waveguides is proposed and discussed. This novel structure consists of a two-dimensional lattice of silicon rods embedded on a thin silica slab sandwiched between two silica claddings whose refractive indices are slightly lower than the index of the silica core. The physical parameters of the structure, i.e., rod radius and core thickness, are optimized to maximize the bandgap width for odd modes. Lossless guided modes inside the bandgap and below the claddings' light cone are obtained by reducing the radius of a row of rods. The waveguide bandwidth can be increased by inserting a thin silicon dielectric waveguide instead of the row of rods. The proposed approach may overcome many of the common drawbacks in conventional holes-on-dielectric planar photonic crystal waveguides.     

A kind of single-polarization single-mode photonic crystal fiber

A kind of single-polarization and single-mode totally internal reflection photonic crystal fiber (SPSM TIR-PCF) is proposed in this paper. It is a PCF structure with elliptical air holes in the cladding and four large holes in the first ring. A full-vector plane wave expansion method is employed to analyze this PCF structure. The numerical results show that this PCF structure can realize an ultra-broad SPSM bandwidth of 540?nm with a confinement loss less than 0.1?dB?km^?1, the broadest bandwidth to the best of our knowledge. Moreover, the structure that we proposed can realize a high nonlinear coefficient.     

A single-mode highly birefringent dispersion-compensating photonic crystal fiber using hybrid cladding

Based on the hybrid cladding design, a single-mode photonic crystal fibre (PCF) is proposed to achieve an ultra-high birefringence and large negative dispersion coefficient using finite-element method. Simulation results reveal that with optimal design parameters, it is possible to achieve an ultra-high birefringence of 2.64 × 10^?2 at the excitation wavelength of 1.55 μm. The designed structure also shows large dispersion coefficient about ?242.22 to ?762.6 ps/nm/km over the wavelength ranging from 1.30 to 1.65 μm. Moreover, residual dispersion, effective dispersion, effective area, confinement loss and nonlinear coefficient of the proposed PCF are discussed thoroughly.     

Highly tunable large-core single-mode liquid-crystal photonic bandgap fiber

We demonstrate a highly tunable photonic bandgap fiber, which has a large-core diameter of 25 microm and an effective mode area of 440 microm2. The tunability is achieved by infiltrating the air holes of a photonic crystal fiber with an optimized liquid-crystal mixture having a large temperature gradient of the refractive indices at room temperature. A bandgap tuning sensitivity of 27 nm/degrees C is achieved at room temperature. The insertion loss is estimated to be less than 0.5 dB and caused mainly by coupling loss between the index-guided mode and the bandgap-guided mode.     

Design of a single-polarization single-mode photonic crystal fiber with a near-Gaussian mode field and wide bandwidth

Single-polarization single-mode (SPSM) fiber can efficiently eliminate polarization mode coupling, polarization mode dispersion, and polarization-dependent loss. Up to now, most single-polarization fibers have been designed based on form birefringence, which would result in a non-Gaussian field distribution and a small effective mode field area. In this paper, a novel structure of SPSM photonic crystal fibers based on the resonant coupling phenomena is proposed and analyzed by using a full-vector finite-element method with a second-order transparent boundary condition. From the numerical results it is confirmed that this fiber has a near-Gaussian mode field within the wavelength range from 1.46 to 2.2 μm, where only one polarized mode exists effectively, and the mode field area is about 79 μm(2) at the wavelength of 1.55 μm, matching that of the conventional single-mode fiber.     

Bragg scattering from an obliquely illuminated photonic crystal fiber

Scattering at visible frequencies from a two-dimensional silica/air photonic crystal material in the form of a fine fiber reveals the hexagonal crystal structure of the material. Oblique illumination allows the observation of first-order Bragg conditions even for a crystal structure with a pitch several times the wavelength of light. These scattering measurements demonstrate the feasibility of a low-loss waveguide based on photonic bandgap effects.     

High-stability erbium-doped photonic crystal fiber source

A single-pass backward configuration superfluorescent fiber source (SFS) based on erbium-doped photonic crystal fiber (EDPCF) with a high mean wavelength stability was proposed. The EDPCF was used to improve the intrinsic temperature dependence of the SFS. Using the optimal EDPCF length of 24.2 m and pump power of 204 mW, a 20.7 ppm mean wavelength stability of a prototype SFS was demonstrated with increased temperature from -40 °C to 60 °C. The mean wavelength had an ultra stability of 10.3 ppm with increased temperature from -20 °C to 60 °C.     

Sensitivity analysis of narrowband photonic crystal filters and waveguides to structure variations and inaccuracy

Photonic crystal microcavities, formed by local defects within an otherwise perfectly periodic structure, can be used as narrowband optical resonators and filters. The coupled-cavity waveguide (CCW) is a linear array of equally spaced identical microcavities. Tunneling of light between microcavities forms a guiding effect, with a central frequency and bandwidth controlled by the local defects' parameters and spacing, respectively. We employ cavity perturbation theory to investigate the sensitivity of microcavities and CCWs to random structure inaccuracies. For the microcavity, we predict a frequency shift that is due to random changes in the lattice structure and show an approximate linear dependence between the standard deviation of the structure inaccuracy and that of the resonant frequency. The effect of structural inaccuracy on the CCW devices, however, is different; it has practically no effect on the CCW performance if it is below a certain threshold but may destroy the CCW if this threshold is exceeded.     

A compact optical wavelength splitter in one-dimensional photonic crystal waveguides

A fundamental T-branch in one-dimensional photonic crystal waveguides based on the omnidirectional reflection is constructed. Numerical simulations of this T-branch indicate that without any structural optimization, four high reflectance peaks and three high transmittance peaks appear alternately within a wide enough frequency band. The T-branch with the unique transmission characteristics can be used as a wavelength splitter. Combining the fundamental T-branch with flexible bends of one-dimensional photonic crystal waveguides, we construct simple and compact wavelength splitters with arbitrary branching angles. Those wavelength splitters are expected to be applied to high density photonic integrated circuits.     

Tunable ultrashort electro-optical power divider using coupled photonic crystal waveguides

Photonic crystals (PCs) have many potential applications because of their ability to control lightwave propagation. We have investigated a tunable ultrashort electro-optical power divider in two-dimensional PC structures. The power divider, composed of a dielectric cylinder in air, is studied by solving Maxwell's equations using the plane wave expansion method and finite-difference time-domain method. The power-splitting mechanism is analogous to that of conventional directional couplers, utilizing coupling between guided modes supported by line defect waveguides. To increase the coupling coefficient of the PC coupler, the radius of the rods between two waveguides is reduced. The switching mechanism is a change in the conductance in the coupling region between the waveguides and hence modulating the coupling coefficient, and eventually switching is achieved. Such a mechanism of wavelength multiplexing should open up a new application for designing components in photonic integrated circuits.     

Discretely tunable single-mode lasers on GaSb using two-dimensional photonic crystal intracavity mirrors

We report on single-mode emitting coupled cavity ridge waveguide lasers on the GaSb material system in the 2?μm spectral range using two-dimensional (2D) photonic crystals (PhCs). Eight rows of 2D PhCs lateral to the ridge waveguides act as intermediate mirrors and are used to create two coupled cavities. This leads to preferential emission at one single longitudinal mode in the emission spectrum with side mode suppression ratios of 30-35?dB. Monolithic integration of high reflectivity 2D PhC back mirrors allows the realization of cavity lengths as short as 300?μm with threshold currents as low as 18.5?mA while reaching output powers well above 18?mW. Under variation of driving current the lasers exhibit both discrete and continuous tuning behavior over a wide current range very well explicable by simulation of the sub-threshold spectra, rendering the devices especially interesting for multi-gas sensing by absorption spectroscopy.     

A photonic crystal fiber sensor for pressure measurements

The pressure sensitivity of two photonic crystal fibers (PCFs) was measured. A PCF pressure sensor was then successfully developed with PCF PM-1550-01. The measurement results of the pressure sensor at three different temperatures are presented, and in the working region the maximum deviation is within 1% of the dynamic range of the sensor.     

Modal cutoff properties in germanium-doped photonic crystal fiber

The germanium-doped photonic crystal fiber (PCF) has some characteristics that differentiate it from pure-silica PCF for a germanium element being doped in the core, such as the intensified nonlinearity, the enhanced photosensitivity, and so on. To pave the way for the application of the Ge-doped PCF successfully, it is necessary to study its properties. We investigated the modal cutoff properties of Ge-doped PCF quantitatively by using the beam propagation method. The numerical results show that the effective refractive indices and the normalized frequency V of Ge-doped PCF not only depend on the normalized pitch delta/lambda but also depend on the normalized hole size d/delta, the modal cutoff boundary for the single mode-multimode of the Ge-doped PCF shift to the low d/delta side in contrast to the pure-silica PCF.     

Test of photonic crystal fiber in broadband interferometry

Photonic crystal fibers (PCFs) are microstructured waveguides that are used in metrology, nonlinear optics, and coherent tomography. PCF studies are focused mainly on the improvement of dispersion properties and wide spectral single-mode operating domains. Consequently, in the astronomical context this kind of fiber is a good candidate for use in the design of a fiber-linked version of a stellar interferometer for aperture synthesis. We discuss the potential of these fibers to take advantage of wide spectral single-mode operation. We propose an experimental setup that acts as a two-beam interferometer that uses PCFs to measure fringe contrast at four wavelengths (670, 980, 1328, and 1543 nm), which correspond to the R, I, J, and H astronomical bands, respectively, with the same couple of PCFs. For this purpose we use, for the first time to our knowledge, a piezoelectric PCF optical path modulator.     

Characterization of tunable photonic crystal fiber directional couplers

We present a tunable photonic crystal fiber (PCF) directional coupler fabricated by a side-polishing method. The PCF directional coupler was modeled as a typical single-mode fiber-based directional coupler and analyzed using the improved effective-index method (IEIM). The characteristics of the PCF directional coupler such as the coupling coefficient and the coupling ratio were measured and found to be in good agreement with those predicted by the theoretical model. The PCF directional coupler exhibited an insertion loss of approximately 2 dB for a 3 dB coupler and was able to tune the coupling ratio between 0% and 100% by tilting the angle of the top side-polished quartz block against the fixed-bottom quartz block.     

Bitapered fiber coupling characteristics between single-mode single-core fiber and single-mode multicore fiber

By splicing and tapering at the fusion point of one-core single-mode fiber and three- or four-core single-mode fiber, an effective bitapered fiber coupling technique is implemented. Based on the beam propagation method, the bitapered coupling characteristics between the one-core fiber and the multicore single-mode fiber are simulated and analyzed. The theoretical prediction is confirmed by the experimental results, and the difference between the simulation and the experimental results is also discussed.     

Coupling technique for efficient interfacing between silica waveguides and planar photonic crystal circuits

We describe a two-step-size tapered structure with one defect pair that can markedly enhance the coupling efficiency at the entrance and exit terminals of a planar photonic crystal (PPC) waveguide. PPC waveguides are composed of circular dielectric rods set in two-dimensional square lattices. On the basis of our simulations, we found that the optimized scheme maximizes the power transmission above 90% at a wavelength of 1.55 microm. Besides, one can control the central frequency for optical communications by determining this defect configuration in an optimization procedure. Moreover, by properly adjusting the defect radii in PPC tapers, one can use the PPC circuit as a good reflector.     

Efficient Bragg gratings in phosphosilicate and germanosilicate photonic crystal fiber

We present ArF laser-induced dynamics of Bragg grating (BG) growths in phosphosilicate-doped or germanosilicate-doped core photonic crystal fibers (PCFs). To this end, we have adapted the technique of H2 loading, usually used in conventional fiber, to the case of microstructured fiber, allowing both the concentration of hydrogen in the PCFs to be kept nearly constant for the time of the exposure and the BG spectra to be easily recorded. We compared the characteristics of BG growths in the two types of PCF to those in conventional step-index fibers. We then conducted a study of the thermal stability of the BGs in PCFs through 30 min of isochronal annealing. At the same time we discuss the role played by the microstructuration and the doping with regard to the grating contrast and the Bragg wavelength stability.