Multimode Photonic Crystal Waveguides Research Articles

Ultra-compact, broadband tunable optical bandstop filters based on a multimode one-dimensional photonic crystal waveguide.

In this paper, ultra-compact, broadband tunable optical bandstop filters (OBSFs) based on a multimode one-dimensional photonic crystal waveguide (PhCW) are proposed and systematically investigated. For the wavelengths in the mini-stopband, the input mode is coupled to a contra-propagating higher order mode by the PhCW and then radiates in a taper, resulting in a stopband at the output with low backreflection at the input. Three-dimensional finite-difference time-domain method is employed to study the OBSFs. The influence of main structural parameters is analyzed, and the design is optimized to reduce the back-reflection and band sidelobes. Using localized heating, we can shift the stopband and tune the bandwidth continuously by cascading the proposed structures. Due to the strong grating strength, our device provides a more compact footprint (40 μm × 1 μm) and much broader stopband (bandwidth of up to 84 nm), compared to the counterparts based on microrings, long-period waveguide gratings, and multimode two-dimensional PhCWs.

Compact bends for multi-mode photonic crystal waveguides with high transmission and suppressed modal crosstalk

We demonstrate an extremely compact bend for a photonic crystal waveguide supporting three spatial modes. The bend exhibits nearly 100% transmission over a relative bandwidth of 1% with less than 1% crosstalk. We show that our design is robust with respect to fabrication errors. Our design method is applied to create a structure consisting of dielectric rods, as well as a structure consisting of air holes in a dielectric background.

Based on total internal refection multi-mode interference photonic crystal waveguide

This paper studies the role of total internal refection in photonic crystals multimode waveguides, discussed and analyzed the multimode interference situation and self-image using conventional coupled-mode theory. Research shows that the input light still can be propagated with high efficiency down to multimode optical area, and resemble index-guided modes owing to the combination of total internal reflection and distribution Bragg reflection. Designing a total internal reflection multi-mode interference photonic crystal waveguide polarization-insensitive power splitter, this power splitter has great potential value in the integrated optics.

Tunable slow-light multi-mode photonic crystal waveguides based on the coupling of square cavities

The light transmission properties through two-dimensional photonic crystal waveguides based on coupling of square cavities are studied by the finite-difference time-domain technique. Through interlacing the adjacent cavities along the direction vertical to the waveguide, the coupling distance between the adjacent cavities is extended, and the group velocity of the guiding modes can be slowed by five-fold compared with that in vacuum. Because of the different spatial field distributions of various resonant modes, the corresponding group velocities are also different for the same CROW structure.

Detailed investigation of self-imaging in multimode photonic crystal waveguides for applications in power and polarization beam splitters

Properties of the self-imaging effect based on multimode interference (MMI) in multimode photonic crystal waveguides (PCWs) are investigated and analyzed in detail. By combining photonic band gap (PBG) and total internal reflection (TIR) effects in PCWs, self-imaging phenomena are achieved for both TE and TM polarizations. To observe the images reproduced by this self-imaging phenomenon, finite-difference time-domain (FDTD) simulations are performed on a multi-mode PCW. From these numerical simulations the reproduced images are identified and their positions along the propagation axis are described. We also, present the design and simulation of novel polarization-insensitive power splitter and polarization splitter. The simulation results show that the optimized devices have excellent transmission efficiencies as well as wide operating frequency bandwidths and small structure size. So, the proposed devices are promising and may play an important role in high-density photonic integrated circuits in the future.

Ultracompact 2$\,\times\,$2 Photonic Crystal Waveguide Power Splitter Based on Self-Imaging Effect Realized by Asymmetric Interference

The properties of two-fold images, based on asym metric interference in multimode interference-based photonic crystal waveguides, are numerically studied with finite-difference time-domain simulations. With these image properties, 2 × 2 power splitters (PSs) with an ultracompact size are designed. Ar bitrary splitting ratio can be achieved by modifying the sizes (the effective refractive indices) of one pair of rods in the multimode region. As an example, a 3-dB 2 × 2 PS with a size of 24 × 8 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is presented with an output efficiency as high as 98%. The method can be extended for the design of M × N PSs for photonic integrated circuits applications.

Ultrasharp ministop-band edge for subnanometer tuning resolution

We propose and demonstrate a method that enables spectral tuning with subnanometer accuracy, and is based on the transmission ministop-band (MSB) in line-defect multimode photonic crystal (PhC) waveguides. The fabricated MSB filter has ultrasharp edges which show a 30 dB drop in transmission in a 4 nm wavelength span. The use of the ultrasharp MSB edge to (optically) determine PhC fabrication accuracy is demonstrated. The wavelength position of the MSB could be tuned by temperature, with a coefficient of 0.1 nm/°C. The spectral characteristics of the MSB realized in this work are promising for sensing, tuning, and modulation applications.

Propagation properties of light in multimode photonic crystal waveguides with triangular lattices based on total internal reflection

The properties of the propagating field in multimode photonic crystal waveguides exhibiting no photonic band gaps with triangular lattices are investigated. By the effective refractive index method, these structures are regarded as three layer planar dielectric waveguides. And the multimode interference effect is numerically analyzed and simulated based on the self-imaging principle. The results show that the field can propagates effectively and resembles index-guided mode due to the combination of total internal reflection and distribution Bragg reflection. High transmission region shifts toward the shorter wavelength region, the bandwidth and the transmissivity increase with the increase of filling ratio, which provides a theoretical reference for selecting an appropriate filling ratio in practical applications.

Selective modes excitation in multimode photonic crystal waveguides

We investigate modes excitation with the input field of different positions in two-dimensional multimode photonic crystal waveguides. Odd modes can be selectively excited by the input field of odd symmetry. The input field with different positions can excite different modes due to the field intensity distribution of modes. When the input field locates at the position of the zero field, intensity of waveguide modes is zero and the modes are not excited. The finite-difference time-domain method is used to obtain the excited field distributions.

Power splitter based on cascaded multimode photonic crystal waveguides with triangular lattice of air holes

We propose a power splitter based on two-dimensional photonic crystals with triangular lattice of air holes. The structure can be divided into three sections consisting of the input waveguide, area-defect regions, and output waveguides. The operational principle of the splitter is that an incident field in the frequency range of interest is split into a twofold mode pattern with small distance in the first area-defect and then one of which is shifted by the secondary area-defect regions to output port. We optimize the size of area-defects for the mode-splitting and the shifting from the numerical analysis of distribution of time-averaged Poynting vectors by a finite-difference time domain method. And the transmittance over 45% per each output with a bandwidth about 0.06 a/ λ is achieved.

Analysis and Optimization of Compact Demultiplexer Monitor Based on Photonic-Crystal Waveguide

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A wavelength selective lateral outcoupler based on multimode photonic-crystal waveguides (WGs) is presented. Its fundamental advantages are a small footprint, an inherently flexible and versatile design, and a natural WG integration. Devices can serve demultiplexing or monitoring purposes. We numerically discuss design and optimization issues and corresponding details of experimental investigations. </para>

Self-imaging effect of TM modes in photonic crystal multimode waveguides only exhibiting band gaps for TE modes

This Letter presents the properties of transverse-magnetic (TM) modes in multimode photonic crystal waveguides (PCWs), which only exhibit photonic band gaps for transverse-electric (TE) modes. A good equivalent model is applied to analysis the designed structures on the basis of multimode interference effect and self-imaging principle. The performance shows that the TM modes can also be propagated with high efficiency, and resemble index-guided modes owing to the combination of total internal reflection (TIR) and distribution Bragg reflection. It provides a novel way to realize the components for both TM and TE polarizations by combining PBG and TIR effect in PCWs. As one of potential applications, polarization-insensitive power splitter based on the proposed structures can be designed.

Imaging Properties of Multimode Photonic Crystal Waveguides and Waveguide Arrays

Light propagation in multimode photonic crystal (PC) waveguides and in arrays of single-mode PC waveguides is analyzed. The dispersion relations of the modes traveling along these devices explain the numerically observed formation of single and multiple copies of the input beam. As the analogous devices constructed in a standard planar geometry, the proposed PC devices can be employed as filters and demultiplexers. The use of PC structures, where the light is truly confined by the stopband, adds the valuable benefit of a dramatic reduction of the overall geometrical dimensions

Mapping of complex optical field patterns in multimode photonic crystal waveguides by near-field scanning optical microscopy

We use near-field scanning optical microscopy (NSOM) operating in collection mode to map the optical field distribution of continuous wave infrared light propagating along an air-bridged W3 silicon photonic crystal (PC) slab multimode waveguide in a subwavelength resolution. The detected near-field optical intensity distribution patterns show very different longitudinal propagation features and transverse field profiles at different wavelength ranges. We have analyzed the dispersion of the guided modes in the PC waveguide and the corresponding eigenfield profile for each guided mode by means of the three-dimensional plane-wave expansion simulations. It is found that the experimentally observed complex while interesting near-field pattern features can be well explained by the field profiles of the multiple waveguide modes and their superposition at different excitation wavelengths. The detection and analysis of this multimode PC slab waveguide via NSOM could help us to understand complex wave propagation behavior and to further design novel PC devices.

Broadband Notch Filters Based on Quasi-2-D Photonic Crystal Waveguides for InP-Based Monolithic Photonic-Integrated Circuits

Asymmetric (type B) multimode photonic crystal line-defect waveguides with varying channel widths have been fabricated and analyzed experimentally. The waveguide channel widths influence the frequencies, bandwidths, and extinction of the resulting stopbands. Transmission stopbands were obtained with bandwidths greater than 20 nm and extinction equal to 20 dB. A scheme for reducing the distributed-feedback reflections from the waveguides was also devised, and a suppression of at least 10 dB was achieved. These waveguides show good prospects for the realization of very compact broadband notch filters for monolithic photonic-integrated circuits

Terahertz photonic crystal switch in silicon based on self-imaging principle

A compact and high quality terahertz photonic crystal switch is proposed in silicon. The switch operates based on the self-imaging principle in multi-mode photonic crystal waveguide of triangular lattice. The finite-difference time-domain method and the plane wave expansion method are used to verify and analyze the characteristics of the proposed switch. Numerical simulation results agree well with the theoretical expectation. This kind of device is potentially important for terahertz application and might be a new breakthrough to design other kinds of photonic crystal switches.

Spontaneous emission enhancement at a photonic wire miniband edge

In a multimode photonic-crystal waveguide, we observe strong enhancement of the photoluminescence of embedded quantum dots at the edges of the so-called mini-stopband that were opened by Bragg diffraction between two guided modes. Taking into account light collection, we relate this observation to the singular photon density of states that is characteristic of a one-dimensional photon system. Furthermore, we quantify by how much the radiation losses smooth the divergence. For the first time to our knowledge, a clear account of the control of spontaneous emission in a one-dimensional system is thus demonstrated.

Compact wavelength monitoring by lateral outcoupling in wedged photonic crystal multimode waveguides

A device concept for laterally extracting selected wavelengths from an optical signal traveling along a waveguide, for operation in metropolitan area networks, is presented. The signal on the fundamental mode of a multimode photonic crystal waveguide is coupled to a higher-order mode, at a center frequency that spatially depends on the slowly varying guide parameters. The device is compact, intrinsically fault tolerant, and can split any desired fraction of the signal for monitoring purpose. Characterizations by the internal light source technique validate the optical concept whereas an integrated device with four photodiodes qualifies its potential with respect to real-world applications.

Light coupling with multimode photonic crystal waveguides

We use a transfer-matrix method (TMM) to investigate light coupling into and out of single-end multimode photonic crystal waveguides. Without multiple-reflection complexity, this approach allows for unambiguous quantitative determination of the coupling efficiency of external light into each guided mode and transition among various guided modes. The TMM can provide a powerful analytical tool to understand and design complex multimode photonic crystal waveguides.

High transmission and single-mode operation in low-index-contrast photonic crystal waveguide devices

We investigate photonic crystal films made of low-index glass materials. Polarization-sensitive waveguiding and single-mode operation (with losses as low as 1.7 dB/mm) have been observed in wide multimode photonic crystal waveguides after propagation of some millimeters. Furthermore, single-mode operation and transmission as high as about 50% per bend has been observed experimentally in low-index photonic crystal waveguide bends.