Low Group Velocity Dispersion Research Articles

Polarization-independent waveguides in air holes photonic crystals and its slow light

A line-defect waveguide in a triangular lattice photonic crystal (PC) made of air holes in dielectric is demonstrated to support transverse magnetic (TM) as well as transverse electric (TE) guided modes simultaneously. A group of suitable geometric parameters were found to make the guided bands overlapped by means of Genetic Algorithm. The optimized waveguide realizes a polarization-independent single-mode transmission and wide operating bandwidth which reaches 0.012Δωa/(2πc). Moreover, the guided modes are shown to exhibit a wide-bandwidth slow light and an extremely low group velocity dispersion in most frequency range.

Tunable fibre-coupled multiphoton microscopy with a negative curvature fibre.

Negative curvature fibre (NCF) guides light in its core by inhibiting the coupling of core and cladding modes. In this work, an NCF was designed and fabricated to transmit ultrashort optical pulses for multiphoton microscopy with low group velocity dispersion (GVD) at 800nm. Its attenuation was measured to be <0.3dBm(-1) over the range 600-850nm and the GVD was -180±70fs(2) m(-1) at 800nm. Using an average fibre output power of ∼20mW and pulse repetition rate of 80MHz, the NCF enabled pulses with a duration of <200fs to be transmitted through a length of 1.5m of fibre over a tuning range of 180nm without the need for dispersion compensation. In a 4m fibre, temporal and spectral pulse widths were maintained to within 10% of low power values up to the maximum fibre output power achievable with the laser system used of 278mW at 700nm, 808mW at 800nm and 420mW at 860nm. When coupled to a multiphoton microscope, it enabled imaging of ex vivo tissue using excitation wavelengths from 740nm to 860nm without any need for adjustments to the set-up.

Atmospheric window characteristic and channel capacity of THz wave propagation

The increasing demand of unoccupied and unregulated bandwidth for wireless communication systems will inevitably lead to the extension of operation frequencies toward the lower THz frequency range. Since atmospheric transmission windows exist in the lower THz frequency range, it can be realized that carrier frequencies of 300 GHz and beyond will be used for communications once the technology for high bitrate data transmission is available. However, the free-space path-loss and the attenuation due to molecules in the atmosphere can significantly reduce the transmittable data rate in the lower THz frequency range.The main factor affecting the behavior of terahertz band is the absorption by water vapor, which not only attenuates the transmitted signal, but also disperses the signal. A new model of the terahertz wave atmospheric propagation of attenuation and dispersion is developed by using the radiation transmission theory and the empirical continuum absorption based on the HITRAN database. Theoretical aspects of absorption are presented, emphasizing those that deserve special attention as frequency increases. The THz wave atmospheric attenuation experimental results and self- and foreign-continuum coefficients obtained with the improved THz-time domain spectroscopy (THz-TDS) technique are analyzed by this model. The intensities and locations of the observed absorption lines are in good agreement with spectral databases. This model accounts for the group velocity dispersion and the total path loss that a wave in the THz band suffers when propagating 1 km distance. The channel capacity of the THz band is investigated by this model under different conditions including antenna gains, channel bandwidth and transmitter power. In order to keep the considerations as general as possible, the derivations are based on simple assumptions and equations. The special requirement for antenna is also discussed.Three communication channels (340 GHz, 410 GHz and 667 GHz) are obtained in terms of the spectrum. The four parameters of the three channels, i.e., available bandwidth, center frequency, dispersion and transmittable data rate, are summarized and quantized. The signals through the atmosphere for the three communication channels within the corresponding atmospheric windows are not easy to broaden due to the low group velocity dispersion; high data rates of up to 10 Gbps or beyond per 1 GHz bandwidth can be transmitted via these channels if the antennas with high gains are used.

Slow light enabled time and wavelength division demultiplexer in slotted photonic crystal waveguide

We have proposed a design for slow light with ultraflat dispersion in a slotted photonic crystal waveguide consisting of a hexagonal arrangement of elliptical air holes on a silicon-on-insulator substrate. The proposed structure has low group velocity and low group velocity dispersion with a wide normalized bandwidth range of 0.0089. The proposed structure can be used as an optical buffer for the storage of a large amount of data due to the large value of the normalized delay bandwidth product equal to 0.634. An optimized structure for a slotted photonic crystal has been analyzed for its applications as both a time and wavelength division demultiplexer. Furthermore, the time delay between two adjacent wavelengths (1550 and 1555 nm) is more than that reported earlier.

Flat band slow light in silicon photonic crystal waveguide with large delay bandwidth product and low group velocity dispersion

Flat band slow light is achieved in a silicon photonic crystal waveguide with large normalised delay bandwidth product (NDBP) while maintaining smaller values of group velocity dispersion. By altering the waveguide width through shifting the position of two rows of air holes adjacent to the waveguide, it is possible to achieve a high NDBP value. The NDBP values of 0.40, 0.41 and 0.43 over bandwidths of 11, 14 and 18 nm are, respectively, reported. Towards the applicability of the proposed slow light waveguide for buffering applications on silicon, buffering capacity as high as 140 bits can be achieved with the proposed design approach.z

Wideband slow light with low dispersion in lattice-shifted asymmetric photonic crystal waveguides

We propose a different type of asymmetric photonic crystal waveguide to achieve slow light with improved normalized delay-bandwidth product and low group velocity dispersion. The lateral symmetry of the proposed waveguide is broken by periodically shifting the two rows of air holes adjacent to the line defect. Under two simple different procedures of shifting air holes, the group indices of 41, 50, 68, and 114 with bandwidths over 14.9, 11, 7.8, and 4.7 nm, and the group indices of 43, 54, 73, and 124 with bandwidths over 13.9, 10.4, 7.1, and 4.3 nm around 1550 nm are obtained, respectively. Low-dispersion slow light propagation is confirmed by studying the relative temporal pulse-width spreading with the two-dimensional finite-difference time-domain method.

Mid-infrared frequency comb generation in coupled silicon microring resonators

We propose and simulate an on-chip mid-IR frequency comb generator based on the four-wave mixing (FWM) effect in a coupled silicon microring resonator. The resonator is formed by suspended ridge waveguides that are designed to have low group-velocity dispersion. A continuous-wave pump laser coupled into the resonator excites other longitudinal modes of the resonator through FWM-based parametric oscillations. In comparison with the device based on a single-ring resonator, we numerically demonstrate that the pump threshold can be dramatically reduced by introducing an auxiliary ring resonator to compensate the phase mismatch between the pump and the generated comb components. The proposed frequency comb generator has a low pump threshold of ~40mW and a high efficiency of ~25% at the pump power of 100mW.

Near-elliptic Core Triangular-lattice and Square-lattice PCFs: A Comparison of Birefringence, Cut-off and GVD Characteristics Towards Fiber Device Application

In this work, we report detailed numerical analysis of the near-elliptic core index-guiding triangular-lattice and square-lattice photonic crystal fiber (PCFs); where we numerically characterize the birefringence, single mode, cut-off behavior and group velocity dispersion and effective area properties. By varying geometry and examining the modal field profile we find that for the same relative values of <TEX>$d/{\Lambda}$</TEX>, triangular-lattice PCFs show higher birefringence whereas the square-lattice PCFs show a wider range of single-mode operation. Square-lattice PCF was found to be endlessly single-mode for higher air-filling fraction (<TEX>$d/{\Lambda}$</TEX>). Dispersion comparison between the two structures reveal that we need smaller lengths of triangular-lattice PCF for dispersion compensation whereas PCFs with square-lattice with nearer relative dispersion slope (RDS) can better compensate the broadband dispersion. Square-lattice PCFs show zero dispersion wavelength (ZDW) red-shifted, making it preferable for mid-IR supercontinuum generation (SCG) with highly non-linear chalcogenide material. Square-lattice PCFs show higher dispersion slope that leads to compression of the broadband, thus accumulating more power in the pulse. On the other hand, triangular-lattice PCF with flat dispersion profile can generate broader SCG. Square-lattice PCF with low Group Velocity Dispersion (GVD) at the anomalous dispersion corresponds to higher dispersion length (<TEX>$L_D$</TEX>) and higher degree of solitonic interaction. The effective area of square-lattice PCF is always greater than its triangular-lattice counterpart making it better suited for high power applications. We have also performed a comparison of the dispersion properties of between the symmetric-core and asymmetric-core triangular-lattice PCF. While we need smaller length of symmetric-core PCF for dispersion compensation, broadband dispersion compensation can be performed with asymmetric-core PCF. Mid-Infrared (IR) SCG can be better performed with asymmetric core PCF with compressed and high power pulse, while wider range of SCG can be performed with symmetric core PCF. Thus, this study will be extremely useful for designing/realizing fiber towards a custom application around these characteristics.

Slow light in ellipse-hole photonic crystal line-defect waveguide with high normalized delay bandwidth product

In this paper, a novel flatband slow light device with low group velocity dispersion (GVD) is presented in an ellipse-hole photonic crystal (PC) line-defect waveguide. Utilizing dispersion engineering in the proposed structure, normalized delay-bandwidth product (NDBP) under a constant group index criterion is significantly improved. A step-by-step optimization process is done on the adjacent rows to the waveguide, which are filled by silica. For optimum case a high NDBP of 0.461 with a group index of 41.86 and a bandwidth of 17.06 nm is obtained by three-dimensional plane-wave expansion method. To the best of our knowledge, this NDBP is one of the highest values in PC waveguides reported to date, in which the group index value is relatively high. The numerical results show that GVD is negligible over a broad wavelength range. Also, optical pulse propagation through the waveguide is performed based on the finite-difference time-domain method. The results indicate that the shape of output pulse experiences a broadening of 2.1% compared with the incoming pulse after traveling a distance of 30a.

Wideband slow light with low dispersion in asymmetric slotted photonic crystal waveguides

A new procedure of designing slotted photonic crystal waveguides is proposed to achieve slow light with improved normalized delay-bandwidth product and low group velocity dispersion that is suitable for both the W1 defect mode and the slot mode. The lateral symmetry of the waveguide in our study is broken by shifting the air holes periodically along the slot axis. The conversion of the "flat band" from band-up slow light to band-down slow light is achieved for the W1 defect mode. The group index curves of the W1 mode change from U-like to step-like and the group indices of 47, 67 and 130 are obtained with the bandwidth over 7.2, 4.8, and 2.3nm around 1550nm, respectively. We also obtain the group indices of 42, 55, and 108 for the slot mode with the bandwidth over 6.2, 5.6, and 2.2nm, respectively. Then the low dispersion slow light propagation is numerically demonstrated by the finite-difference time-domain method.

Wideband slow light in photonic crystal slab waveguide based on geometry adjustment and optofluidic infiltration

In this paper, a photonic crystal slab waveguide with wideband slow light, large group index (ng), and very low group velocity dispersion (GVD) has been presented. The structure is designed by shifting the first row of the air holes adjacent to the waveguide center in the longitudinal direction, and optofluidic infiltration in the second row. By applying optimized parameters for the two rows, a flexible control of ng(17.5<ng<133) with large bandwidth (2 nm<Δλ<23 nm) is obtained. The GVD decreased at the range of 10(-22) s 2/m. Numerical simulations are performed by the three-dimensional plane-wave expansion method.

Transverse-electric and transverse-magnetic mode slow light propagation in a two-dimensional photonic crystal waveguide

A two-dimensional photonic crystal waveguide structure is designed for both TE- and TM-mode slow light propagation. The minimum group index of the waveguide for TE and TM modes can reach to 137.8 and 126.4, and the two polarizations have the same slow light frequency region. The designed structure can provide a large bandwidth range with very low group velocity dispersion for both TE and TM modes. The transmission property investigation for a suspended two-dimensional slab photonic crystal waveguide (PCW) indicates that such slow light character may be retained when perfect reflectors can be fixed on the horizontal surfaces of the slab. Such high group index for both TE and TM modes in two-dimensional PCWs is, to the best of our knowledge, first reported here, and may provide some useful guides for slow light research in theory.

Self‐phase‐locked degenerate femtosecond optical parametric oscillator based on BiB3O6

AbstractA self‐phase‐locked degenerate femtosecond optical parametric oscillator (OPO) based on the birefringent nonlinear material, bismuth triborate, BiB3O6, synchronously‐pumped by a Kerr‐lens‐mode‐locked Ti:sapphire laser at 800 nm is described. By exploiting versatile phase‐matching properties of BiB3O6, including large spectral and angular acceptance for parametric generation and low group velocity dispersion in the optical xz plane, stable self‐phase‐locked degenerate OPO operation centered at 1600 nm is demonstrated using collinear type I (e → oo) interaction in a 1.5‐mm crystal at room temperature.The degenerate OPO output spectrum extends over 46 nm (∼5.4 THz) with 190 fs pulse duration for input pump pulses of 155 fs with a bandwidth of 7 nm. Phase coherence between the pump and degenerate output is verified using f‐2f interferometry, and discrete frequency beats caused by different carrier‐envelope‐offset frequencies are measured using radio frequency measurements.Photo shows a 1.5‐mm BiB3O6 crystal used as a nonlinear gain medium in a degenerate self‐phase‐locked femtosecond OPO operating at room temperature. The green beam is the result of non‐phase‐matched sum‐frequency mixing between the pump light and the sub‐harmonic OPO field at degeneracy.

Polarization-independent slow light in annular photonic crystals

A photonic-crystal waveguide hosted in an annular photonic crystal is demonstrated theoretically to realize polarization-independent slow light. Through an appropriate set of design parameters, overlapping regions with constant group index and low group-velocity dispersion for both the transverse-electric and transverse-magnetic polarizations can be achieved in the common frequency regime of the waveguide modes. Assuming parameters for silicon and air, an optimal structure with group index of 50 and bandwidth of ∼0.1% is found theoretically for polarization-independent slow light in this structure.

调整圆弓形散射元参数实现低群速和低色散的慢光效应

调整圆弓形散射元参数实现低群速和低色散的慢光效应

Slow light property in ring-shape-hole slotted photonic crystal waveguide

Slow light in photonic crystal waveguide is now being heavily investigated for applications in optical devices. A ring-shape-hole slotted PhCW (RSPhCW) is designed in the paper. Band structures and group indices are calculated through plane wave expansion method. Flat band slow light property is observed, which provides low group velocity dispersion and avoids high speed optics signal from distortion in transmission. A method is proposed by tuning the positions of the first and second rows with the ring-shape-holes adjacent to the slot, which can tune the group index and the bandwidth of the guided band at the same time. This method is used to optimize the slow light property of the RSPhCW. The bandwidths ranging from 1.8nm to 17.3nm for group indices from 15 to 67 are obtained. From the point of view of reality, this method has its advantage of fabricating than tuning the outer and inner radii of the ring-shape-holes.

Optimization of a two-dimensional photonic crystal waveguide for ultraslow light propagation

A two-dimensional photonic crystal waveguide is optimized for ultraslow light propagation. By employing geometry projection method optimization, a flat band photonic crystal waveguide is achieved with the minimum group index around 2335, which is much larger than those reported before. The investigation of the transient propagation of the Gaussian pulse in the waveguide indicated that low group velocity dispersion can cause a small expansion of the pulse. The stability analysis indicates that slightly numerical and angular deviation in fabrication can influence the minimum group index of the waveguide, but the ultraslow light property can still be maintained. The theory simulation in our work can provide some useful guidance for further ultraslow light research.

Slow light based on optical surface modes of two-dimensional photonic crystals

We investigate slow light properties of optical surface modes sustaining at the interface of two-dimensional photonic crystals and uniform medium (air). The manipulation of the structural parameters at the surface governs the modal field distribution of the surface state. The spectral and temporal behaviors of the slow mode are numerically explored by utilizing both plane-wave expansion and finite-difference time-domain methods. We show that the group index and bandwidth can be tuned within a wide range. The distortion free optical pulse propagation is supported by the presence of low group velocity dispersion behavior of the slow light surface mode photonic structure.

Modeling the Dispersion of the Nonlinearity in Slow Mode Photonic Crystal Waveguides

Nanophotonic structures enabling the engineering of linear dispersion are often exploited for nonlinear processing. However, the dispersion of the nonlinear response has been generally overlooked, also because of the difficulty of accurately taking this effect into account. Here, we first show the necessity of considering the nonlinear dispersion and then propose a simplified approach through which the modeling can be performed. As an example, we consider waveguides in which a large group index and low group velocity dispersion are achieved at the same time by modifying a single design parameter, i.e., an asymmetric translation of the hole rows closest to the core. It is shown that the dependence on the wavelength of the normalized Bloch mode overlap integrals for self-phase modulation (SPM) can be interpolated by a four-parameter formula that is similar to the Morse potential. The parameters can be related to specific properties of the nonlinear SPM coefficient, which in the specific case depends on the translation parameter, by simple linear and exponential functions. Cross-phase modulation and four-wave mixing coefficients can then be derived from the self-phase one, through a geometric mean, slightly corrected to account for the interacting wave detuning, and the complete modeling of nonlinear dispersion is achieved.

Wideband slow light with ultralow dispersion in a W1 photonic crystal waveguide

A dispersion tailoring scheme for obtaining slow light in a silicon-on-insulator W1-type photonic crystal waveguide, novel to our knowledge, is proposed in this paper. It is shown that, by simply shifting the first two rows of air holes adjacent to the waveguide to specific directions, slow light with large group-index, wideband, and low group-velocity dispersion can be realized. Defining a criterion of restricting the group-index variation within a ±0.8% range as a flattened region, we obtain the ultraflat slow light with bandwidths over 5.0, 4.0, 2.5, and 1.0 nm when keeping the group index at 38.0, 48.8, 65.2, and 100.4, respectively. Numerical simulations are performed utilizing the three-dimensional (3D) plane-wave expansion method and the 3D finite-difference time-domain method.