The focusing characteristics of Fresnel zone plate (FZP) on vortex beams at 1550 nm are investigated. Employing the Fresnel diffraction integral, the diffraction characteristics on different circular structures are calculated. Many calculations and measurements of the transmission light field-phase distribution demonstrate that the focusing characteristics of the vortex beam through FZP are similar to Gaussian beam. The effect of radius-dependent phase delay on the focus position is verified. At the same time, it is proved that the topological charge has nothing to do with the major focal position and the FZP will not change the topological charge of the incident vortex beam.
Due to the epsilon near zero (ENZ) effect, indium tin oxide (ITO) can be used in optical modulators and reduce the modulator’s size dramatically. The tunability of optical properties and the CMOS compatible capability make ITO more attractive. To study the properties of ITO thin films, several works have been done. Firstly, thin ITO thin films were obtained by magnetron sputtering with different oxygen flow rates ranging from 0 to 50sccm. Secondly, EDS was carried out to investigate the elements' content. It can be found that increasing oxygen flow rate increases the percentage of oxygen atom and Sn atom of ITO thin films. Thirdly, surface profiler was used to measure the stress value of the ITO thin films. We find that the tensile stress of ITO thin films tends to transform into compressive stress when the oxygen flow rate rises, which is worth considering in the design of devices. Fourthly, spectrometer and Hall effect measurement were applied to measure the normal incidence transmittance and electrical properties of the ITO thin films. Larger oxygen flow rate leads to the normal incidence transmittance of ITO thin films becoming larger. Hall effect measurement contributes to the conclusion that the carrier concentration of ITO thin films is able to range from 1019 to 1021 cm-3, and that when the oxygen flow rate is not too large, as the environment oxygen increases, the carrier concentration decreases and the mobility increases. This research can contribute to the design of compact ITO based optical modulators so as to achieve a better performance, which can further the integration of optical modulators.
A compact polarization demultiplexer (P-DeMux) is proposed and characterized to enable wavelength-divisionmultiplexing and mode-division-multiplexing simultaneously. The proposed structure is composed of a microring resonator in ultrathin waveguide and two bus channels in the novel silicon nitride silica silicon horizontal slot waveguides. In the slot waveguide, the transverse electric (TE) mode propagates through the silicon layer, while the transverse magnetic (TM) mode is confined in the slot region. In the designed ultra-thin waveguide, the TM mode is cutoff. The effective index of the TE modes for ultrathin and slot waveguides have comparable values. Thanks for these distinguishing features, the input TE mode can be efficiently filtered through the ultra-thin microring at the resonant wavelength, while the TM mode can directly output from the through port. Simulation Results show that the extinction ratio of the proposed P-DEMUX for TE and TM modes are ∼36.5 and 31.27 dB, and the insertion losses are ∼0.22 and 0.249 dB respectively.
An interpretation of optical unitary transformation is proposed for general non-overlapping-image multimode interference (MMI) couplers with any input and output ports based on the matrix mechanics. The light transformation in the MMI couplers can be considered as the optical field matrix acting on the input light column vector. We investigate the general phase principles of output light images. The complete proof of nonoverlapping-image MMI coupler’s optical unitarity is provided along with the phase analysis of matrix element. Based on a two-dimensional finite-difference time-domain simulation, the unitary transformation is obtained for a 4×4 non-overlapping-image MMI coupler within the deviation of 4×10-2 for orthogonal invariance among the C-band spectral range.
Based on the epsilon-near-zero (ENZ) effect of indium tin oxide (ITO), we numerically demonstrate a high efficiency ITO phase/intensity modulator by exploiting ultra-thin silicon strip waveguide configuration. Heavily n-doped indium tin oxide is used as the semiconductor together with p-doped silicon and hafnium oxide (HfO2) to form a MOS waveguide. Due to the special feature of the ultra-thin silicon waveguide structure, the propagating transverse electric (TE) mode is less confined to the silicon core and penetrates deeper into the cladding layer, which will enhance the interaction between the active material and the optical mode. The combination of the ultra-thin silicon strip waveguide and ITO material exhibits high modulation efficiency together with broad optical bandwidth. When the modulator operates as a phase modulator, the effective refractive index change can reach the value 8:95x10-3 for the light wavelength λ = 1550 nm when the applied voltage is 6 V. Thus, the phase shifter length which can induce a π phase shift is supposed to be only about 97 µm, giving a corresponding VπL of 0.58 V∙mm. The effective index change even keeps > 7:32 x 10-3 with the wavelength increasing from 1300 nm to 1800 nm, indicating the broad modulation bandwidth. Meanwhile, the modulator can also operate as a variable optical attenuator or an intensity modulator. The modulation depth (MD) is about 0.074 dB/µm at 9 V when the wavelength is 1550 nm. This device confirms electrical phase shifting in ITO enabling its use in applications such as compact phase shifters, sensing, and phased array applications for LiDAR.
We propose and numerically simulate a polarization-independent 1×3 broadband beam splitter based on silicon-on-insulator (SOI) technology with adiabatic coupling. The designed structure is simulated by beam-propagation-method (BPM) and gets simulated transmission uniformity of three outputs better than 0.3dB for TE-polarization and 0.8dB for TM-polarization in a broadband of 180nm.
Photonic integrated circuits on silicon substrates are fully compatible with CMOS processes. The measurements show that light coupling with insertion loss of ~2 dB between grating waveguides and optical fibers, 60 Gbps optical modulation with Vpp of less than 4 V, and 4-port optical switches with cross-talk of < -12 dB and extinction ratio of > 15 dB. Another work shows that a depletion mode PN junction can be operated as a 40 Gbps photodetector under a special reverse bias of 7 - 8 V.
We present a kind of depletion-mode silicon modulators based on cascade interleaved PN junctions, which
simultaneously provide high modulation efficiency and large modulation bandwidth. The interfaces of the PN junctions
are vertical to the waveguide's propagation direction and tolerant with ± 150nm junction misalignment on the cost of
little degradation on the modulation efficiency. The device was fabricated with standard 0.18μm CMOS process, and
provides a VπLπ < 1V • cm and an intrinsic bandwidth 39GHz. Over 10GHz electro-optical modulation bandwidth of the
device was experimentally obtained. High speed non-return-zero modulation with a bit rate up to 25Gbit/s was finally
demonstrated.
Improved Extinction Ratio of 25 dB was demonstrated in silicon based optical modulators on CMOS platform in China.
The measurement results agree with the simulation, followed by a discussion about the effects of both propagation loss
in Mach-Zehnder arms and power ratio at beam splitters and combiners. The analyses indicate that many considerations
have to be taken into design and development of the compatible fabrication of these integrated silicon photonics,
especially for the improved extinction ratio of optical modulators. In this summary, we propose the integrated optical
modulators in SOI by use of the compatible CMOS processes under the modern CMOS foundry in Chinese homeland.
And the measured results were shown, the fast response modulator with the data transmission rate of 10 Gbps.
A silicon p-i-n diode Mach-Zehnder optical modulator integrated with grating couplers is fabricated in 0.18-μm complementary metal oxide semiconductor technology. The device has an ultracompact length of 200 μm. High modulation efficiency with a figure of merit of VπL = 0.22 V mm is demonstrated. A novel pre-emphasis technique is introduced to achieve high-speed modulation, and a data transmission rate of 3 Gbps is present.
SOI (silicon-on-insulator)-based micro-resonator is the key building block of silicon photonics, which is considered as a
promising solution to alleviate the bandwidth bottleneck of on-chip interconnects. Silicon-based sub-micron waveguide,
microring and microdisk devices are investigated in Institute of Semiconductors, Chinese Academy of Sciences. The
main progress in recent years is presented in this talk, such as high Q factor single mode microdisk filters, compact thirdorder
microring filters with the through/drop port extinctions to be ~ 30/40 dB, fast microring electro-optical switches
with the switch time of < 400 ps and crosstalk < -23 dB, and > 10 Gbit/s high speed microring modulators.
The novel design of a silicon optical switch on the mechanism of a reverse p-n junction is proposed. The figuration of
contact regions at slab waveguides and the ion implantation technology for creation of junctions are employed in the new
design. The two-layer rib structure is helpful for reduction of optical absorption losses induced by metal and
heavily-doped contact. And more, simulation results show that the index modulation efficiency of Mach-Zehnder
interferometer enhances as the concentrations of dopants in junctions increase, while the trade-off of absorption loss is
less than 3dB/μm. The phase shift reaches about 5×10-4 π/μm at a reverse bias of 10V with the response time of about
0.2ns. The preliminary experimental results are presented. The frequency bandwidth of modulation operation can arrive
in the range of GHz. However, heavily-doped contacts have an important effect on pulse response of these switches.
While the contact region is not heavily-doped, that means metal electrodes have schottky contacts with p-n junctions, the
operation bandwidth of the switch is limited to about 1GHz. For faster response, the heavily-doped contacts must be
considered in the design.
SOI (Silicon on Insulator) based photonic devices, including stimulated emission from Si diode, RCE (Resonant Cavity Enhanced) photodiode with quantum structure, MOS (Metal Oxide Semiconductor) optical modulator with high frequency, SOI optical matrix switch and wavelength tunable filter are reviewed in the paper. The emphasis will be played on our recent results of SOI-based thermo-optic waveguide matrix switch with low insertion loss and fast response. A folding re-arrangeable non-blocking 4×4 matrix switch with total internal reflection (TIR) mirrors and a first blocking 16×16 matrix were fabricated on SOI wafer. The extinction ratio and the crosstalk are better. The insertion loss and the polarization dependent loss (PDL) at 1.55 μm increase slightly with longer device length and more bend and intersecting waveguides. The insertion losses are expected to decrease 2~3 dB when anti-reflection films are added in the ends of the devices. The rise and fall times of the devices are 2.1 μs and 2.3 μs, respectively.
A folding nonblocking 4×4 optical matrix switch in simplified-tree architecture was designed and fabricated on a silicon-on-insulator wafer. To compress chip size, switch elements (SEs) were connected by total internal reflection mirrors instead of conventional S-bends. For obtaining smooth interfaces, potassium hydroxide (KOH) anisotropic chemical etching of silicon was employed. The device has a compact size of 20×3.2 mm2 and a fast response of 8±1 µs. Power consumption of 2×2 SE and excess loss per mirror were 145 mW and –1.1 dB, respectively.
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