We demonstrate a photonic crystal (PhC) waveguide modulator based on a Mach-Zehnder Interferometer (MZI) configuration on a silicon-on-insulator (SOI) substrate. A p-n junction is embedded in the PhC waveguide such that the carrier concentrations in the waveguide can be changed with applied voltage, which changes the refractive index of the silicon material forming the waveguide. Our PhC waveguide is designed to work at 1550 nm. The slow light effect in the photonic crystal waveguide can enhance light-matter interaction, which helps to achieve sufficient modulation in a short interaction length around 100 micron, which may also help reduce the power consumption. Modulation speed of 8.5Gb/s has been measured.
We investigate the response time of silicon-based thermo-optic switches under different device configurations. We design two tunable thermo-optic switches on a silicon-on-insulator (SOI) chip. One uses a waveguide embedded phase shifter based on direct heating due to electric current flow through waveguide. The other traditional switch structure has a metallic heater on top of the waveguide. Owing to direct current injection to heat the waveguide, which avoids the heat conduction from heater to waveguide, the switching time would reduce significantly. The experimental result shows that the direct heating device realizes a fast response time close to 1.5μs. As a comparison, the traditional heater-on-top device’s response time is over 10μs. That is to say, switching time of the direct-current-injection device is over ten times less. The insertion loss of both devices are reasonable. The fast heating device shows a potential for applications in the future optica interconnects.
In a silicon Mach-Zehnder modulator with on-chip termination, the reflection of the driving signal in the traveling wave electrode often occurs due to non-ideal factors. This may influence the modulating signal characteristics. To study such influence, the S11 response of the electrode is averaged over different frequency ranges to characterize the reflection of the driving signal in the electrode. We find that the signal-to-noise ratio and the jitter can be substantially affected by the reflection. In addition, for the same characteristic, when the averaging frequency range for S11 varies, the relations between mean S11 and the output signal characteristics may also vary to some extent. Understanding of these phenomena are potentially useful to the applications of the modulators in the optical communication systems.
This work introduces a 2×2 silicon thermo-optical switch based on photonic crystal waveguides. Photonic crystal waveguides (PCWs) were used to replace conventional channel waveguides in the phase shifters of a Mach-Zehnder interferometer (MZI). This device was fabricated on silicon-on-insulator (SOI) platform. The active length of the PCWs-based phase shifter was only 50μm. Because of the slow light effect of PCWs, the heat needed to generate a phase shift of π reduced dramatically. The experiment results showed that the power needed for phase shifter of π is 6 times less than that of conventional MZI based optical switches.
In this paper, a method that adds an inverse taper at the endface of the waveguide is analyzed to reduce the silicon waveguide endface reflection (SWER). A high index-contrast optical waveguide on a silicon-on-insulator (SOI) wafer allows for the strong optical confinement, while it also brings in a substantial endface reflection. Most parameters of the taper, such as the length and tip width of the taper, and wavelength of the guided light, even the shape of the taper, have been studied in detail using the three-dimensional finite-difference time-domain (3D-FDTD) method to reduce the SWER. In addition, we have also proposed a new structure that adds the special taper to the straight-through port of the 3-dB directional coupler (DC) to measure the SWER in experiments. The experimental results show good agreement with our simulation results. This taper is useful and small enough that can be applied to many silicon photonic devices and large scale photonic integration circuits (PICs).
In this work, we design and fabricate a highly compact third-order racetrack add-drop filter consisting of silicon waveguides with modified widths on a silicon-on-insulator (SOI) wafer. Compared to the previous approach that requires an exceedingly narrow coupling gap less than 100nm, we propose a new approach that enlarges the minimum feature size of the whole device to be 300 nm to reduce the process requirement. The three-dimensional finite-difference time-domain (3D-FDTD) method is used for simulation. Experiment results show good agreement with simulation results in property. In the experiment, the filter shows a nearly box-like channel dropping response, which has a large flat 3-dB bandwidth (~3 nm), relatively large FSR (~13.3 nm) and out-of-band rejection larger than 14 dB at the drop port with a footprint of 0.0006 mm2 . The device is small and simple enough to have a wide range of applications in large scale on-chip photonic integration circuits.
Parallel-coupled dual racetrack micro-resonator structures have potential applications for quadrature amplitude
modulation. Fabrication of parallel-coupled dual racetrack silicon micro-resonators was conducted, while overcoming
for some barriers to fabrication. Fabrication process limitations and design considerations are discussed. Fabrication
results are presented. Some barriers to fabrication include stitching and overdosing in electron beam lithography. A
multi-input and output test bed with optical and electrical control was necessary for device characterization. The
characterization of the fabricated devices is presented, along with the related procedures. Some of the tests performed are
wavelength scans and top surface scans.
In this paper, we report the theoretical study of polymer-based photonic crystals for laser beam steering
which is based on the superprism effect as well as the experiment fabrication of the two dimensional
photonic crystals for the laser beam steering. Superprism effect, the principle for beam steering, was
separately studied in details through EFC (Equifrequency Contour) analysis. Polymer based photonic
crystals were fabricated through double exposure holographic interference method using SU8-2007. The
experiment results were also reported.
In this paper, we review some of our works on tuning the slow-light effect and superprism effect in photonic crystals from a synergistic perspective. The dispersive effects in a generic photonic crystal structure are classified into the longitudinal dispersion and angular dispersion according to their orientations with respect to the direction of light propagation. The slow-light effect originates from the longitudinal dispersion whereas the superprism effect originates primarily from the angular dispersion. The ability to tune these two categories of dispersive effects leads to several interesting topics of photonic crystal research, including slow-light photonic crystal modulators and superprism demultiplexers and sensors. We will discuss commonality and difference between the tunings of these two effects.
We experimentally demonstrate a novel slot photonic crystal waveguide for guiding light with low group velocity in a
100-nm-wide low-index region. The unique optical property and structural features of the slotted photonic crystals best
match the requirements for active material-based silicon devices. We integrate the novel photonic crystal waveguide with
a multimode interference-based coupling structure and measure a 20dB efficiency enhancement compared with direct
coupling configuration. The measured transmission spectra are in good agreement with simulated band diagram.
An analysis of superprism effect in low index contrast polymer photonic crystal is presented. It
shows extremely sensitivity to the wavelength and angle of the incident light due to the strong
anisotropy of photonic band structures. Two-dimensional (2-D) polymer photonic crystals with
triangular lattice structure were fabricated by soft lithography using elastomeric
polydimethylsiloxane (PDMS) templates. Dense two dimensional photonic crystal superprism
structures with feature sizes of 150-500nm and aspect ratios of up to 1.25 were successfully
replicated by soft lithography. Large field size and easy fabrication are two major advantages when
compared with other imprint technology. Atomic Force Microscopy images showed that the molded
structures had high fidelity to the masters. Such an effective, low cost, and high throughput soft
lithography technique could find wide use in making photonic crystal based nanostructures.
A 3-slot optical backplane bus demonstrator based on glass substrate with photopolymer volume gratings array (PVGA)
on top surface is built to allow 16 channels of data to be broadcast from central slot to two daughter slots or uploaded from
any daughter slot to central slot. VCSELs and photodetectors packaged in the form of TO-46 can are assembled on top of
each PVG and interleaved to reduce the crosstalk to below noise level. By carefully aligning the fabrication system, the
incident angle deviation from Bragg condition is reduced to below 0.1° to maximize optical power delivery. The
orientation and period of hologram fringes are uniform in the active area by collimating recording beams.
Above 4.8Gbps aggregated data transmission is successfully demonstrated using the multi-channel system. Three
computer mother boards using FPGA are made to verify the data transmission among the slots. Interface boards between
the FPGA boards and optical transceivers are designed and fabricated to separate the implementation of digital layer and
optical layer. Single channel transmissions with 3.2Gbps and even 10Gbps data rate are also tested with above 100uW
input power, showing the potential to improve the total two-way bandwidth to above 102.4Gbps. Alignment tolerance of
the optical interconnect system is investigated theoretically and experimentally. By analyzing the diffractive
characteristics, the bandwidth limit of the optical layer is determined to be in the order of Terahertz. Design and
fabrication issues are discussed for future optical backplane bus to make terahertz bandwidth into reality. Based on the
experiments for Bit-interleaved Optical Backplane bus and Multi-channel optical backplane bus demonstrators,
theoretical analysis of the bandwidth limit of the optical backplane bus using photopolymer volume gratings has been
carried out.
An ultra-compact silicon Mach-Zehnder interferometer (MZI) modulator featuring p-i-n-diode-embedded photonic crystal
waveguides has been fabricated. As carrier injection is the only practical option for optical modulation in silicon, a lower limit of
current density (~104A/cm2) exists for achieving gigahertz modulation in the widely employed p-i-n diode configuration. Electrical
simulations have been performed to design and analyze the device. The device interaction length was reduced by one order of
magnitude compared to the conventional waveguide based MZI modulators by taking advantage of the slow group velocity exhibited
by photonic crystal waveguides (PCWs). A maximum modulation depth of 93% has been obtained under an injected current of 7.1
mA. High-speed optical modulation at 1 Gbit s-1 in the 1.55 micron wavelength region was experimentally demonstrated. To our
knowledge, this is the fastest speed ever achieved for a p-i-n diode based integrated silicon MZI modulator.
Photonic crystal based superprism offers a new way to design new optical components for beam steering and DWDM application. 3D photonic crystals are especially attractive as they could offer more control of the light beam based on the needs. A polygonal prism based holographic fabrication method has been demonstrated for a three-dimensional face-centered-cubic (FCC)-type submicron polymer photonic crystal using SU8 as the photo-sensitive material. Therefore antivibration equipment and complicated optical alignment system are not needed and the requirement for the coherence of the laser source is relaxed compared with the traditional holographic setup. By changing the top-cut prism structure, the polarization of the laser beam, the exposure and development conditions we can achieve different kinds of triclinic or orthorhombic photonic crystals on demand. Special fabrication treatments have been introduced to ensure the survivability of the fabricated large area (cm2) nano-structures. Scanning electron microscopy and diffraction results proved the good uniformity of the fabricated structures. With the proper design of the refraction prism we have achieved a partial bandgap for S+C band (1460-1565nm) in the [111] direction. The transmission and reflection spectra obtained by Fourier transform infrared spectroscopy (FTIR) are in good agreement with simulated band structure. The superprism effects around 1550nm wavelength for the fabricated 3D polymer photonic crystal have been theoretically calculated and such effects can be used for beam steering purpose.
Silicon nanophotonics has recently attracted great attention since it offers an opportunity for low cost opto-electronic
solutions based on silicon complementary metal oxide semiconductor (MOS) technology. Photonic crystal (PhC)
structures with slow photon effect are expected to play a key role in future large-scale ultra-compact photonic
integrated circuits. A novel vertical-MOS-capacitor-based silicon PhC waveguide structure was proposed to achieve
active transmission control via the free carrier plasma dispersion effect. We designed and fabricated a single-arm
PhC waveguide with MOS gate defect using silicon-on-insulator (SOI) substrate and demonstrated that a defect
mode was present in the infrared region. Plane wave expansion (PWE) method based simulation indicated that high
group index of the fabricated PhC waveguide could be achieved near the transmission band edge. Further
investigation demonstrated that such PhC MOS capacitor would be a good candidate to realize ultra-compact
transmission control.
Ultra-compact silicon-photonic-crystal-waveguide-based thermo-optic and electro-optical Mach-Zehnder interferometers
have been proposed and fabricated. Thermal and electrical simulations and optical characterizations have been performed.
Experimental results were in good agreement with the theoretical predictions.
Photonic crystals (PhCs) provide a promising nanophotonic platform for developing novel optoelectronic devices with significantly reduced device size and power consumption. Silicon nanophotonics is anticipated to play a pivotal role in the future nano-system integration owing to the maturity of sub-micron silicon complementary metal oxide semiconductor (CMOS) technology. An ultra-compact silicon modulator was experimentally demonstrated based on silicon photonic crystal waveguides. Modulation operation was achieved by carrier injection into an 80-micron-long silicon PhC waveguide of a Mach-Zehnder interferometer (MZI) structure. The driving current to obtain a phase shift of pi across the active region was as low as 0.15 mA, owing to slow light group velocity in PhC waveguides. The modulation depth was 92%. The electrode between the two waveguide arms of the MZI structure was routed to the space outside the MZI. In real devices, this planarized routing design would be essential to integrating the silicon modulator with electrical driving circuitry on a single silicon chip. For laboratory test, this routing scheme also eliminated the need of placing a bulky pad between the two arms and gave our modulator a distinctive slim profile and a much smaller footprint. Polymeric photonic crystals were designed for superprism based laser beam steering applications, and were fabricated by nano-imprint and other techniques.
Si nanophotonics is anticipated to play a critical role in the future ultra-compact system integration due to the maturity of sub-micron silicon complementary metal oxide semiconductor (CMOS) technology. Photonic crystals (PhCs) provide a promising platform for developing novel optoelectronic devices with significantly reduced device size and power consumption. The active control of photonic crystal waveguides (PCWs) incorporated in Mach-Zehnder interferometers has been investigated in this paper. We designed and fabricated a PCW based silicon thermo-optic (TO) switch operating at 1.55 μm. A novel device structure was proposed to enhance the heat exchange efficiency between the source and the active PCW region, which resulted in a faster switching time (< 20μs) compared with the conventional structure. The required π phase shift between the two arms of the MZI has been successfully achieved within an 80 μm interaction distance. The maximum modulation depth of 84% was demonstrated for switching power of 78mW. For high-speed applications, a p-i-n structure based PCW electro-optical (EO) MZI modulator was proposed. The transient performance of such a device was evaluated using a two-dimensional semiconductor device simulator MEDICI. The simulated structure demonstrated a great potential to realize high-speed ultra-compact Si modulators in the GHz region.
An ultra-compact silicon electro-optic modulator was experimentally demonstrated based on highly dispersive silicon photonic crystal (PhC) waveguides. Modulation operation was demonstrated by carrier injection into an 80 μm-long silicon PhC waveguide of a Mach-Zehnder interferometer (MZI) structure. The π phase shift driving current, Iπ, across the active region is as low as 0.15 mA, which is equivalent to a Vπ of 7.5 mV when a 50 Ω impedance-matched structure is applied. The modulation depth is 92%. Highly dispersive PhC fibers were previously proposed to reduce the payload of true-time delay (TTD) modules for phased-array antenna (PAA) systems. The payload reduction factor is proportional to the enhanced dispersion of highly dispersive PhC fibers. An ultra-large dispersion of -1.1×104 ps/nm•km with the full width at half maximum (FWHM) of 40 nm was numerically simulated from a dual core PhC fibers. The payload reduction factor of the TTD module is as high as 110 compared to that using conventional dispersion compensation fibers (D = -100 ps/nm•km).
The primary technical challenge for optical backplanes involve the alignment and optical isolation of multiple data channels. Since most backplanes require data transfer rates greater than a single optical channel can costeffectively provide, multiple data channels is the common solution for higher aggregate transfer rates. Established optical alignment and isolation techniques include spatial separation of optical channels, use of lensing elements to focus specific transmitter outputs to specific receiver areas, use of differing wavelengths for adjacent channels with appropriate frequency filtering on receivers, and the use of "light guide tubes" for each channel. This presentation will examine another promising option, the use of "matched" Holographic Optical Elements (HOEs) to provide both cross channel optical isolation and to significantly relax traditional optical alignment requirements. Matched HOEs can both induce upon a transmitted optical stream, and then filter upon a received optical stream, a number of distinguishing characteristics such as wavelength, polarization, phase, and amplitude. Thus the use of a unique "matched HOE" pair with each transmitterreceiver pair of multiple optical data channels can provide an efficient mechanism to isolate individual data streams even when they may be physically coincident, such as in a length of fibre optic or when multiple free space data transmitters illuminate several channel's receiver elements. Thus, the alignment issue is relaxed from the usual constraint of attempting to physically separate channels to one where, as long as the receiver is within the optical cone of it's matched transmitting element, cross channel interference can be effectively eliminated.
Optical backplane bus based on glass substrate with volume holographic gratings on top surface possesses a great ability to broadcast information. This feature is utilized to accomplish a bit-interleaved optical interconnect system. In this system, each daughter board sends only one bit per round and the bit pulses from different boards can cascade in a designed series when the transmitters are distributed in an appropriate manner. In this way, even slow electronic chips can be coordinated to generate an aggregate bandwidth up to 10Gbps, which is impossible to achieve with a multi-drop electrical bus. Besides the benefits of high data rate and low crosstalk, such a bit-interleaved architecture provides a secure data storage method. Each daughter board only stores a quarter bits of any byte, so that no single board has the entire information and security is enhanced. Alignment tolerance and power budget of the proposed optical interconnect system is theoretically calculated and experimentally verified. With collimating lenses, the packing density of transceivers is more than 4/cm2, and thus the signal density can be above 40Gbps/cm2/board. The insertion loss due to misalignment and beam divergence is measured to be approximately 3dB. The bit error rate (BER) of 10Gbps receivers with -12dBm sensitivity is estimated to be below 10-12.
This paper reviews photonic crystal (PC) based demultiplexers, and briefly reports our latest experimental achievement in ultra-compact, power-efficient silicon photonic crystal waveguide (PCW) modulators. We review the modeling techniques for photonic crystal superprism devices, which utilize anomalous refraction on a photonic crystal surface for wavelength demultiplexing. The finite difference time domain method tends to be time consuming for the superprism devices as such devices demand fine spatial grids to resolve fine wavelength difference. Other theoretical methods suffer a variety of other drawbacks. A general, efficient PC refraction theory that can handle any surface orientation is needed for scientific research and device design. We review a rigorous PC refraction theory that we recently developed for these needs. Essentially, the refraction problem can be rigorously solved by computing the electromagnetic field in only a single cell on the surface. A new concept, surface-orientation-dependent eigenmode degeneracy, is introduced to explain certain subtle effect that occurs when the surface orientation undergoes a slight change. In addition, the transmission of a Gaussian beam or other realistic beam profiles is discussed. A complete theoretical framework of the photonic crystal refraction and transmission has thus been established. The theory has been applied to design a high channel-count dense WDM demultiplexer with 3dB or lower losses. Lastly, a silicon PCW Mach-Zehnder modulator with an 80-micron interaction length is reported. The slow group velocity in PCWs is exploited to enhance the modulation efficiency and reduce the peak drive current to 0.15mA at a modulation depth over 90%.
Photonic crystal based devices received attention in recent years. Based on the superprism effect in photonic crystals, beam steering devices can be made with properties sensitively dependent on the wavelength and incident angle of light. One stumbling block for designing superprism-based demultiplexers is that current numerical methods have difficulties in simulating a practical superprism device with commonly available computational facilities. Examining the superprism effect in a more general perspective, we previously developed a rigorous theory to solve the photonic crystal refraction problem for any surface orientation and any lattice type. This paper will compare our theory with other methods with regard to computational workload to demonstrate the advantages of our theory. Excellent agreement of numerical results with the transfer matrix method is also demonstrated. Heuristic discussions on the beam width variation and energy conservation are presented. A technique for direct computation of the dispersion surface is compared with the methods that combine a photonic band solver with certain interpolation or 1D-searching techniques.
Nanophotonics including photonic crystals promises to have a revolutionary impact on the landscape of photonics technology. Photonic crystal line defect waveguides show high group velocity dispersion and slow photon effect near transmission band edge. By using photonic crystal waveguides to build true time delay based phased array antenna or other optical signal processing systems, the length of the tunable true time delay lines can be dramatically reduced inversely proportional to group velocity dispersion in dispersion enhanced system architecture or reduced inversely proportional to group index in slow photon enhanced system architecture. The group index of the fabricated silicon photonic crystal line defect waveguide is experimentally demonstrated as high as 40 at optical wavelength around 1569 nm. The group velocity dispersion of the fabricated silicon photonic crystal line defect waveguide is as high as 50 ps/nm∙mm at wavelength around 1569 nm, which is more than 107 times the dispersion of the standard telecom fiber (D = 3 ps/nm∙km). Due to the integration nature of photonic crystals, system-on-chip integration of the true time delay modules can be easily achieved.
Photonic crystal based structures have been considered for optical communication applications. A class of novel symmetric structures consisting of cavities and waveguides have been proposed to serve as optical add-drop multiplexers. Light transfer processes in these structures are analyzed briefly. The problem of deviating from the perfect accidental degeneracy is addressed for practical designs, and the backscattering intensities are shown low for the slight deviations. Anomalous light refraction at a surface of a photonic crystal has also been studied. The limitations of prior theoretical methods for the transmission problem are discussed. An outline of a new analytic theory that overcomes these limitations is presented. Photonic crystals are fabricated on polymer multi-layer films and integrated with conventional channel waveguides.
We analyze the abnormal refraction and propagation when a light
beam of finite and practical width enters a photonic crystal from a
uniform medium. The beam propagation in the photonic crystal is
very complex, and in many cases, is beyond the realm of refraction
(even with a renormalized refractive index given by photonic band
calculation). Generally, light propagation is restricted to a
triangular region (or a fan), although the light may not fill the
whole triangle, nor is the light intensity uniform in the triangle. It is found beam divergence does not have a definite connection with
the fan shape of the region of light propagation, in contrast to
dynamic X-ray theory. A new origin of the fan shape is suggested.
Also simulations indicate that at microscale, a narrow light beam
may zigzag in a photonic crystal with sufficiently high index contrast. An application of this phenomenon is to make a wide angle bend for waveguides. The designed bending structure has low loss and matches the mode size of a typical single-mode waveguide for fiber-optic communications. Our simulations are based on two-dimensional photonic crystals.
A variety of ball-lens based optical add/drop multiplexers (OADMs) are designed and implemented. Insertion losses as low as 0.5 to 0.6 dB for the reflection light-path, and 1.2 to 1.5 dB for the transmission light-path are demonstrated. The 0.5-dB passband and -30 dB stopband for 100-GHz OADM are 0.35 nm and 1.15 nm, respectively. The reflection path has an isolation 15 dB. In addition to the distinct cost advantage of ball lenses over the GRIN lenses, the ball-lens based OADMs also offer a significant simplification in packaging due to the intrinsic spherical symmetry of ball lenses. Optical designs and optics-related packaging issues are discussed in detail.
Communication between computing systems is recognized as the main limitation to increasing the speed of all-electronic systems beyond levels currently achieved in existing supercomputers. Optical interconnects hold great promise in eliminating current communication bottlenecks because of properties that stem from optics inherent parallelism. Wavelength-division multiplexing (WDM) technology, by which multiple optical channels can be simultaneously transmitted at different wavelengths through a single optical transmission medium is a useful means of making full use of optics parallelism over a wide wavelength region. In this talk, we review the working principles of wavelength division (de) multiplexers (WD(D)M) for optoelectronic interconnection in high-throughput optical links and address the optical design issues of Wd9D)Ms. Several grating-based WD9D)M structures are analyzed. We report experimental data for several versions of WD(D)Ms which exhibit low insertion loss, high reliability, and low cost.
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