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Wavelength Division Multiplexing Based Photonic Integrated Circuits on Silicon-on-Insulator Platform
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We review recent advances in the development of silicon photonic integrated circuits for high-speed and high-capacity interconnect applications. We present detailed design, fabrication, and characterization of a silicon integrated chip based on wavelength division multiplexing. In such a chip, an array of eight high-speed silicon optical modulators is monolithically integrated with a silicon-based demultiplexer and a multiplexer. We demonstrate that each optical channel operates at 25 Gb/s. Our measurements suggest the integrated chip is capable of transmitting data at an aggregate rate of 200 Gb/s. This represents a key milestone on the way for fabricating terabit per second transceiver chips to meet the demand of future terascale computing.
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... Four-channel coarse wavelength division multiplexing (CWDM) in the O-band spectral range is widely used in datacenter applications owing to its cost-effectiveness [1][2][3][4]. In optical transceivers, optical demultiplexers are essential for filtering WDM optical signals and spatially separating them into output ports at the receiver end. ...
... In optical transceivers, optical demultiplexers are essential for filtering WDM optical signals and spatially separating them into output ports at the receiver end. These optical demultiplexers are typically fabricated on silicon-on-insulator (SOI) platforms, which provide technical advantages such as small footprint, cost-effectiveness, and monolithic integration, along with other functional devices such as optical modulators and detectors [2,3,5,6]. The operation of optical demultiplexers is commonly based on lightwave interferences in microring resonators [7][8][9], multistage delayed Mach-Zehnder interferometers (DMZI) [10][11][12][13][14][15][16][17], and arrayed waveguide gratings (AWGs) [18][19][20][21]. ...
... 6 Good Good Good Bad Bad 1 Proposed device scheme without TM 00 -mode rejection filters. 2 Proposed device scheme integrated with TM 00mode rejection filters. 3 Wavelength crosstalk from neighboring channels. ...
Coarse wavelength division multiplexing (CWDM)-targeted novel silicon (Si)-nanowire-type polarization-diversified optical demultiplexers were numerically analyzed and experimentally verified. The optical demultiplexer comprised a hybrid mode conversion-type polarization splitter rotator (PSR) and a delayed Mach–Zehnder interferometric demultiplexer. Si-nanowire-based devices were fabricated using a commercially available Si photonics foundry process, exhibiting nearly identical spectral responses regardless of the polarization states of the input signals under the PSR. The experiment demonstrated a low insertion loss of 1.0 dB and a polarization-dependent loss of 1.0 dB, effectively suppressing spectral crosstalk from other channels by less than −15 dB. Furthermore, a TM-mode rejection-filter-integrated optical demultiplexer was designed and experimentally validated to mitigate unwanted TM-mode-related polarization crosstalk that arose from the PSR. It exhibited an improved polarization crosstalk rejection efficiency of −25 dB to −50 dB within the whole CWDM spectral range.
... In light of recent increases in data traffic spurred by digital transformation and the advent of 6G mobile communication networks, there is a growing demand for more advanced optical multiplexing technologies like wavelength division multiplexing (WDM) or coherent phase modulation techniques, particularly in datacenter applications [1][2][3][4][5][6][7][8][9]. Silicon photonics stands out as a highly promising technological platform for the development of photonic integrated circuits (PICs) characterized by high bandwidth and low energy consumption. ...
... CWDM provides many advantages over DWDM due to its less strict prerequisites for the accurate control of diode laser oscillation frequency and broader available operating range in the wavelength domain. Additionally, CWDM devices must properly work within a wavelength range exceeding 60 nm, as the operating point is spaced by 20 nm [5][6][7]11]. Furthermore, the optical DeMUX at the receiver's end must possess the capability to handle arbitrary polarization input signals [28,30,35,36,40,42,47,49]. To date, various methods have been investigated to manage randomly polarized CWDM signals for use in the optical receiver, which can be categorized into three groups: the zero-birefringence scheme [28], the polarization compensation scheme [30,40], or the polarization diversity schemes [35,36,47,49], applied to silicon nitride (SiN) [28,40,47] and silicon (Si) [30,35,36,49] materials. ...
Several types of silicon-nanowire-based optical demultiplexers (DeMUXs) for use in short-reach targeted datacenter applications were proposed and their spectral responses were experimentally verified. First, a novel 100-GHz-spaced 16λ polarization-diversified optical DeMUX consisting of 2λ delayed interferometer (DI) type interleaver and 8λ arrayed waveguide gratings will be discussed in the spectral regimes of C-band, together with experimental characterizations showing static and dynamic spectral properties. Second, a novel 800-GHz-spaced 8λ optical DeMUX was targeted for use in LR (long reach) 400 Gbps Ethernet applications. Based on multiple cascade-connected DIs, by integrating the extra band elimination cutting area, discontinuous filtering response was analytically identified with a flat-topped spectral window and a low spectral noise of <−20 dB within an entire LR-8 operating wavelength range. Finally, a 20-nm-spaced 4λ coarse wavelength division multiplexing (CWDM)-targeted optical DeMUX based on polarization diversity was experimentally verified. The measurement results showed a low excessive loss of 1.0 dB and a polarization-dependent loss of 1.0 dB, prominently reducing spectral noises from neighboring channels by less than −15 dB. Moreover, TM-mode elimination filters were theoretically analyzed and experimentally confirmed to minimize unwanted TM-mode-oriented polarization noises that were generated from the polarization-handling device. The TM-mode elimination filters functioned to reduce polarization noises to much lower than −20 dB across the entire CWDM operating window.
... Considerable progress has been made toward developing lattice (de)multiplexers on commonly used silicon-on-insulator (SOI) platforms [15][16][17][18][19][20][21]. However, fabricated devices either suffer from high crosstalk or do not show a perfectly flat passband, even if simulation models predict an ideal response. ...
We present the design and experimental characterization of the first multistage ring-assisted Mach-Zehnder interferometer (RAMZI) lattice (de)multiplexer implemented with silicon nitride optimized for four channels with a spacing of 100 GHz in the L-band. The device comprises two RAMZI stages to provide a sharp box-like response characterized by a shape factor of 0.9, a flat passband over the entire channel, and a crosstalk level better than -14 dB. The maximally flat passband of the demultiplexer enables a passband width twice that of the maximum spectral excursion defined in the NG-PON2 standard.
... Silicon photonics has become one of the most suitable integrated optical platforms due to the ability to use complementary metal oxide semiconductor (CMOS)-compatible facilities. In the platform of silicon photonics, various photonic components have been demonstrated, such as modulators [2][3][4], (de)multiplexers [5][6][7][8] and photodetectors [9,10]. The lack of a reliable silicon laser has been the major impediment due to silicon's indirect bandgap. ...
A highly strained InGaAs quantum well (QW) vertical-cavity surface-emitting laser (VCSEL) with low threshold current density, high efficiency and output power emissions around 1130 nm was grown by MOCVD. Its static characteristics at room temperature and high operation temperature were studied in detail. The 7 μm oxide aperture device exhibits a threshold current of 0.68 mA, corresponding to a threshold current density of 1.7 kA/cm2. The slope efficiency is 0.43 W/A and the maximum output power is 3.3 mW. Continuous-wave (CW) operation in the 10–80 °C temperature range is observed. The slope efficiency is almost constant at 10–80 °C. The threshold current becomes lower at high temperatures thanks to the alignment between gain peak and cavity mode. The 3 μm oxide aperture device’s lasing in single mode with the RMS spectral width of 0.163 nm and orthogonal polarization suppression ratio (OPSR) is ~15 dB at 25 °C. The small-signal response analysis indicates that reducing the parasitics of the device and refining the fabrication process will improve the dynamics response characteristics. These results indicate that the 1130 nm GaAs-based VCSEL with highly strained InGaAs QWs is expected to be used as source for silicon photonics.
... Diverse applications have been demonstrated with PICs, including high-speed optical communication, signal processing, computing, and emerging technologies in quantum, biomedicine, and sensing. [23][24][25][26][27][28] Particularly, the advent of lithium niobate on insulator (LNOI) has propelled PICs as a promising platform for future high-speed electro-optic (EO) integrated devices, 29 such as high-performance modulators, 30,31 frequency combs, 32,33 polarization controllers, 34,35 and quantum optics circuits. 36,37 However, the full utilization of the DOFs of light in traditional two-dimensional PICs has not been realized, thereby limiting their application in optical information technologies. ...
Multidimensional multiplexing technologies have been proven to be a promising scheme to meet the demands of high‐capacity optical interconnects and optical processing networks. However, challenges remain to realize multidimensional hybrid multiplexing data transmission and signal processing between few‐mode fiber transmission links and on‐chip optical processing networks, since the optical coupling between fiber and chip is almost exclusively in the single‐mode regime. Here, a multidimensional fiber‐to‐chip optical processing system is proposed and demonstrated for hybrid wavelength‐, mode‐, and polarization‐division multiplexing signals, where multidimensional coupling between few‐mode fiber transmission link and on‐chip multi‐mode processing network is realized by using a 3D photonic integrated (de)multiplexers on a glass chip as well as 2D photonic integrated (de)multiplexers on a silicon chip, and the on‐chip multidimensional optical processing network composed by parallel cascaded micro‐ring resonator array performs a function of reconfigurable optical add/drop multiplexer. By operating wavelength‐division multiplexing in conjunction with mode‐division multiplexing and polarization‐division multiplexing, the communication bandwidth of fiber‐to‐chip optical processing systems can be further scaled.
Si-wire-based polarization independent CWDM demultiplexing was experimentally verified. TM-mode rejection filters integrated with mode conversion-type polarization splitter-rotator and 4-channel optical DeMUXs made polarization crosstalk much lower than −25 dB, maintaining the performance of optical DeMUX.
We report the demonstration of 11dB fiber-to-fiber optical gain in a silicon Raman amplifier. Pulsed pumping is employed to reduce the TPA induced free carrier losses resulting in net signal amplification. The influence of free carriers is elucidated by observing the dependence of gain on pulse energy.
We report on evanescently coupled Ge waveguide photodetectors that are grown on top of Si rib waveguides. A Ge waveguide detector with a width of 7.4mum and length of 50 mum demonstrated an optical bandwidth of 31.3 GHz at -2V for 1550nm. In addition, a responsivity of 0.89 A/W at 1550 nm and dark current of 169 nA were measured from this detector at -2V. A higher responsivity of 1.16 A/W was also measured from a longer Ge waveguide detector (4.4 x 100 mum2), with a corresponding bandwidth of 29.4 GHz at -2V. An open eye diagram at 40 Gb/s is also shown.
Optical Phase Modulators and Variable Optical AttenuatorsThe Mach–Zehnder InterferometerThe Waveguide BendThe Waveguide-to-waveguide CouplerThe Arrayed Waveguide Grating (AWG)Waveguide Couplers for Small-dimension WaveguidesReferences
Silicon-on-insulator (SOI)Fabrication of Surface Etched FeaturesOxidationFormation of Submicron Silicon WaveguidesSilicon DopingMetallisationSummaryReferences
The following corrections should be made to the above paper. In (3), n² should be n³.
We demonstrate 0.8-dB/cm transmission loss for a single-mode, strip Si/SiO2 waveguide with submicrometer cross-sectional dimensions. We compare the conventional waveguide-fabrication method with two smoothing technologies that we have developed, oxidation smoothing and anisotropic etching. We observe significant reduction of sidewall roughness with our smoothing technologies, which directly results in reduced scattering losses. The rapid increase in the scattering losses as the waveguide dimension is miniaturized, as seen in conventionally fabricated waveguides, is effectively suppressed in the waveguides made with our smoothing technologies. In the oxidation smoothing case, the loss is reduced from 32 dB/cm for the conventional fabrication method to 0.8 dB/cm for the single-mode waveguide width of 0.5 mum. This is to our knowledge the smallest reported loss for a high-index-difference system such as a Si/SiO2 strip waveguide.
The authors present a germanium on silicon p-i-n photodiode for vertical light incidence. For a Ge p-i-n photodetector with a radius of 5 μm a 3 dB bandwidth of 25 GHz is measured at an incident wavelength of 1.55 μm and zero external bias. For a modest reverse bias of 2 V, the 3 dB bandwidth increases to 39 GHz. The monolithically integrated devices are grown on Si with solid source molecular beam epitaxy. The complete detector structure consisting of a highly p-doped Ge buried layer, an intrinsic absorption region, and a highly n-doped top contact layer of Ge/Si is grown in one continuous epitaxial run. A low growth temperature sequence was needed to obtain abrupt doping transitions between the highly doped regions surrounding the intrinsic layer. A theoretical consideration of the 3 dB bandwidth of the Ge detector was used to optimize the layer structure. For a photodiode with 5 μm mesa radius the maximum theoretical 3 dB frequency is 62 GHz with an intrinsic region thickness of 307 nm.
Research on waveguide modulators is now in a rather advanced stage, and
the evolution to application in such areas as optical communication and
high-speed signal processing appears to be imminent. A review is
conducted of the current status of the field with emphasis on design
considerations for optimizing overall device performance. In connection
with the rapid reduction of loss in glass fibers in the late 1960's,
waveguide modulators were investigated primarily because of their
compatibility with fibers and because of the ultimate goal of
integration. However, in addition, the drive power advantage of
waveguide modulators over their bulk counterparts was quickly realized.
Dielectric waveguides and their fabrication are considered along with
the electrooptic effect, the modulator voltage, the lumped electrode,
the traveling wave, semiconductor waveguide modulators, and intensity
modulators. A summary of reported modulator results is provided, taking
into account design considerations, and applications.
This chapter introduces the reader to the technology of silicon photonics as well as provide some insight to waveguides and devices in general. The chapter leads the reader through the fundamentals of modes of an optical waveguide, propagation constants, reflection coefficients and electric field profiles. Both planar and rib waveguides are discussed in the context of modal properties, and the discussion of propagation constants leads on to the concept of effective index. The contributions of loss to optical waveguides are discussed together with ways to measure optical loss. Finally a series of fundamental devices that make up many optical circuits are described together with their operational characteristics. These notes have been adapted from [G.T. Reed, A.P. Knights, Silicon Photonics: An Introduction, Wiley, UK, January 2004, ISBN 0-470-87034-6] with the permission of John Wiley and Sons, UK.
Silicon p(+)-i-n(+) diode Mach-Zehnder electrooptic modulators having an ultra-compact length of 100 to 200 mum are presented. These devices exhibit high modulation efficiency, with a V(pi)L figure of merit of 0.36 V-mm. Optical modulation at data rates up to 10 Gb/s is demonstrated with low RF power consumption of only 5 pJ/bit.