We propose and demonstrate an integrated optical filter with variable bandwidth and tunable central wavelength. It consists of two stages of second-order microring resonators with different radiuses. The two stages are serially cascaded, with each stage having a flat-top spectrum. The free spectral range (FSR) of the cascaded structure is expanded via the Vernier effect. The central wavelength of the proposed optical filter is tuned by synchronous tuning of the two stages. The 3dB bandwidth is adjusted via intentional misalignment of the passbands of the two stages. We demonstrate a prototype of such an optical filter on the silicon-on-insulator platform. The radiuses of ring cavities in the two stages are 7 μm and 10 μm, respectively. All the four ring cavities can be thermally tuned independently. The FSR of the fabricated device is around 90 nm. We show the tuning of the central wavelength from 1460 nm to 1550 nm. We adjust the 3dB bandwidth from 37.5 GHz to 100 GHz with a step of 12.5 GHz, with the insertion loss varying from -5.4 dB to -7.9 dB. Keywords: Integrated optics devices, wavelength filtering devices, coupled resonators
Silicon photonics is considered as a promising technology to overcome the difficulties of the existing digital and analog optical communication systems, such as low integration, high cost, and high power consumption. Silicon optical modulator, as a component to transfer data from electronic domain to optical one, has attracted extensive attentions in the past decade. In this paper, I will review our efforts in developing high-speed silicon Mach-Zehnder optical modulators with large optical bandwidths. Firstly, I will introduce how to optimize the modulation efficiency, optical loss, electro-optical bandwith of the silicon optical modulator. The fabricated silicon Mach-Zehnder optical modulator has an electro-optical bandwidth of up to 39.8 GHz. When the device is optically biased at the quadrature pont, it has the dynamic extinction ratios of 6.5 dB, 5.9 dB and 5.2 dB at the speeds of 40 Gbps, 50 Gbps and 64 Gbps for OOK modulation. Secondly, I will introduce two types of silicon PAM-4 optical modulator. One is driven by a PAM-4 electrical signal and the other is driven by two binary electrical signals with different peak-to-peak voltages. Both two devices can generate PAM-4 optical signals at the speed of over 30 Gbaud in the wavelength of 1525-1565 nm. The corresponding bit error rates could reach as low as ~10−6, which is below the hard-decision forward error correction threshold of 3.8×10-3. Finally, I will introduce a silicon 16-QAM optical modulator, which is based on four Mach-Zehnder modulators driven by four binary electrical signals. With the simple electrical driving configuration, the device generates a 16-QAM optical signal at 20 Gbaud with an error vector magnitude of 13.7%.
Silicon photonics is considered as a promising technology to overcome the difficulties of the existing digital and analog optical communication systems, such as low integration, high cost, and high power consumption. Silicon optical modulator, as a component to transfer data from electronic domain to optical one, has attracted extensive attentions in the past decade. In this paper, we review the statuses of the silicon optical modulators for digital and analog optical communications and introduce our efforts on these topics. We analyze the relationship between the performance and the structural parameters of the silicon optical modulator and present how to optimize its performance including electro-optical bandwidth, modulation efficiency, optical bandwidth and insertion loss. The fabricated silicon optical modulator has an electro-optical bandwidth of 30 GHz. Its extinction ratios are 14.0 dB, 11.2 dB and 9.0 dB at the speeds of 40 Gbps, 50 Gbps and 64 Gbps for OOK modulation. The high extinction ratio of the silicon optical modulator at the high speed makes it very appropriate for the application of optical coherent modulation, such as QPSK and 16-QAM. The fabricated silicon optical modulator also can be utilized for analog optical communication. With respect to a noise floor of -165 dBc, the dynamic ranges for the second-order harmonic and the third-order intermodulation distortion are 90.8 dB and 110.5 dB respectively. By adopting a differential driving structure, the dynamic range for the second-order harmonic can be further improved to 100.0 dB while the third-order intermodulation distortion remains the same level.
Matrix-vector multiplication is a fundamental operation in modern digital signal processing. Inspired by the intrinsic
spatial parallelism of optics, many efforts have been made to develop optical apparatuses that can perform such a
parallelizable operation. Here we report the proposal of an on-chip optical signal processor with the function of matrixvector
multiplication, which is composed of laser-modulator array, multiplexer, splitter, microring modulator matrix and
photodetector array. We fabricate a 4×4 microring modulator matrix on silicon-on-insulator (SOI) platform with
complementary metal-oxide-semiconductor (CMOS)-compatible process. 8×107 multiplications and accumulations
(MACs) per second is implemented by such an on-chip microring modulator matrix and off-chip laser-modulator array,
multiplexer and photodetector. The significant progress in integrated optoelectronics makes it possible to integrate all
required functional optical devices and even the driving and controlling circuits on the same chip. Theoretical analysis is
given to estimate the potential performance of such an integrated system, which is several orders of magnitude faster
than current electrical digital signal processors.
Coupled-resonator-induced transparency (CRIT) effect has been widely studied in parallelly coupled double microring
resonators (MRRs), and various applications based on the CRIT effect have been demonstrated. We report the proposal
and demonstration of a directed logic circuit with the functions of XOR and XNOR using CRIT. Two electrical signals
applied to two coupled MRRs represent two operands of the logic operations, and the operation results are represented
by the output optical signal. As proof-of-concept, the thermo-optic modulation scheme is employed with an operating
speed of 10 kbps.
Electro-optic logic is a paradigm which employs the optical switch network to perform the logical operation. The status
of each switch in the optical network is determined by an electrical Boolean signal applied to it. The operation of each
switch is independent of the operations of other switches in the network and the operation result propagates in the
network at the speed of light. The overall latency of the logic circuit is very small and all switches perform their
operations almost simultaneously. Therefore, the electro-optic logic has a very high operation speed. Silicon microring
resonator is an attractive structure owing to its outstanding performances, such as compact size, ultra-low power
consumption and CMOS-compatible process. Therefore, the electro-optic logic based on silicon microring switches is
easy to realize large-scale integration and low-cost manufacture in a high-volume CMOS-photonics foundry. We have
proposed and demonstrated two electro-optic logic circuits based on silicon microring switches including XOR/XNOR
and AND/NAND. We also proposed the architectures to achieve encoder, decoder and half-adder, and demonstrated
them with the thermo-optic effect. In this paper, we will review the recent research on electro-optic logic circuits based
on silicon microring switches and introduce new development in this topic.
We review our recent efforts of optical modulators and routers for photonic networks-on-chip. Through the optimization
of the doping concentration and profile as well as the coplanar waveguide electrodes, we demonstrate a 2-mm-long
carrier-depletion optical modulator which can work at a speed of 40 Gb/s under a differential voltage of 0.36 V with no
reverse bias. We demonstrate a spatially non-blocking five-port optical router based on thermo-optically tuned microring
resonators. The optical router has a footprint of 440×660 μm2, a 3-dB bandwidth of 0.31 nm (38 GHz), an extinction
ratio of 21 dB for through port, and an extinction ratio of 16 dB for drop port at 1551 nm. 12.5 Gbps high-speed data
transmission experiments verify the good routing functionality of the optical router.
We report an eight-channel reconfigurable optical add-drop multiplexers (ROADMs) based on cascaded microring resonators with low power-consumption and compact footprint. Microheaters are fabricated on top of the microring resonators, which can be modulated through the thermo-optic effect to achieve the reconfigurability of the device. We
demonstrate the reconfigurable add-drop multiplexing functionality with the channel spacings of 100GHz and 50GHz,
with the channel centre wavelengths aligned to the International Telecommunication Unit (ITU) grid. The crosstalks for the two channel spacings are less than -22.5dB and -15.5dB, respectively. The tuning efficiency is about 4.854 mw/nm
and the tuning speed is about 12.4 kHz.
KEYWORDS: Waveguides, Channel projecting optics, Eye, Network on a chip, Microrings, Resonators, Silicon, Signal attenuation, Chemical mechanical planarization, Optoelectronics
We experimentally demonstrated four- and five-port non-blocking optical routers for photonic networks-on-chip. The
optical routers are based on cascaded microring resonators. New topology design reduces the number of microring
resonators and crossings, improving the performances in terms of tuning power consumption, optical loss, crosstalk
and channel uniformity of the optical routers. The efficient footprints are 300x340 μm2 and 440x660 μm2 for fourand
five-port optical routers, respectively. Static spectrum tests show that the 3-dB bandwidths are larger than 0.12
nm and 0.31 nm, the extinction ratios are larger than 13 dB and 20 dB for through ports, 30 dB and 16 dB for drop
ports, for four- and five-port optical routers, respectively. Moreover, routing functionality and signal integrity are
verified by 12.5 Gbps high-speed signal transmission experiments using the NRZ 231-1 PRBS pattern.
As a result of the low modulation efficiency of carrier-depletion Mach-Zehnder silicon optical modulator, it always
needs a high voltage around 6 V, which is very difficult to supply in an integrated high-speed CMOS chip. We
demonstrate a carrier-depletion Mach-Zehnder silicon optical modulator which works at a low voltage. Its coplanar
waveguide electrode is carefully designed to make sure the electrical wave loss along the device is low. The device
operates well at a data rate of 12.5 Gb/s, whose phase-shifter length is 2 mm. Voltages with the swinging amplitudes
being 1 V and 2 V are applied to the device with the reverse bias voltages of 0.5 V and 0.8 V. The extinction ratios are
7.67 and 12.79 dB respectively.
Two types of silicon-on-insulator thermo-optic variable optical attenuators (VOAs) based on a Mach-Zehnder interferometer and a multimode-interference coupler are fabricated, one with thermal isolating grooves to improve heating efficiency and the other without. Comparison of optical and electrical properties, such as insertion losses, the maximum attenuation levels and the corresponding power consumptions, and the response times, is carried out between the two types of VOAs. The comparison results indicate that use of thermal isolating grooves leads to better values for most characteristics and is an effective way to improve the performance of Mach-Zehnder interferometer-type thermo-optic devices.
A thermo-optic variable optical attenuator (VOA) based on a Mach-Zehnder interferometer and multimode-interference coupler is fabricated. Not a single-mode but a multimode waveguide is used as the input and output structures of the optical field, which greatly reduces the coupling loss of the VOA with a normal single-mode fiber. The insertion loss of the fabricated VOA is 2.52 to 2.82 dB at the wavelength of 1520 to 1570 nm. The polarization dependent loss is 0.28 to 0.45 dB at the same wavelength range. Its maximum attenuation range is up to 26.3 dB when its power consumption is 369 mW. The response frequency of the fabricated VOA is about 10 kHz.
The internal reflection of the multimode-interference (MMI)-type device is calculated with the bidirectional beam propagation method. The calculated results indicate that the difference of the effective refractive indices between the core region and the surrounding region has a determining effect on the internal reflection of the MMI-type device. The output taper for the MMI-type combiner and splitter has a more evident effect on the internal reflection than the input taper. The internal reflection decreases with increasing the end width of the taper. For the MMI-type device with appropriate tapers, the internal reflection does not show evident degradation with the deviation of the length of the MMI region from its optimal value.
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