Silicon Photonic Integrated Circuits (PICs) offer multiple advantages, including high-density integration scalability, cost-effectiveness in large volume production, and high yield manufacturing due to mature CMOS microelectronics processes. They find applications in optical field-programmable gate arrays, energy-efficient optical signal processors, and handheld optical sensing systems. This presentation reviews recent progress in integrated coherent networks on the silicon photonics platform, exploring their applications in reconfigurable processors, optical communications, and optical sensing. Innovative computation units based on Micro-ring resonators are introduced, enabling efficient optical delay lines and pulse shaping. Coherent networks also facilitate mode unscrambling in high-capacity optical communication systems and improve resolution and bandwidth in on-chip speckle spectrometers.
We present our recent results on the use of two quite different approaches for photonic integration. First we shall describe how we used the concept of bound states in the continuum (BiC) to make channel guided devices without the need for any dry etching. The BiC channel waveguide employs a substrate that is completely flat. The completely flat structure is attractive for hybrid integration of 2D materials because it does not introduce sharp corners which can reduce the electrical mobility of the 2D material. Channel guiding of light can nonetheless be achieved by spin coating a lower-refractive-index polymer/photoresist on the 2D material and developing it to form a channel. This approach for integrating 2D materials also increases the optical overlap with the 2D material. We used this approach for the hybrid integration of graphene on lithium niobate for making 40-GHz-bandwidth channel-guided photodetectors and electroabsorption modulators on lithium niobate. The BiC concept facilitates the hybrid integration of 2D materials on different substrates and may also be used to increase the effective optical nonlinearity of the underlying substrate by hybrid integration of the appropriate 2D material. Second we shall discuss the InP membrane waveguide platform for nonlinear applications. InP has a third order nonlinearity that is over an order of magnitude larger than silicon, and is therefore of potential interest for spontaneous four wave mixing to produce entangled photons. The use of InP membranes can potentially facilitate the integration of active III-V lasers, and Geiger mode avalanche photodiodes for single photon detection and nonlinear devices on large scale silicon wafers which can integrate the large delay interferometers and filters needed for quantum information processing. We discuss the advantages and disadvantages of InP for SFWM and present recent results on the use of InP membranes for generating heralded single photons.
Mid-infrared (MIR) resonators with high quality (Q) factors play crucial roles in a variety of applications in nonlinear optics, lasing, biochemical sensing, and spectroscopy by virtue of their features of long photon lifetime as well as strong field confinement and enhancement. Previously, such devices have been mainly studied on silicon integration platforms while the development of high-Q germanium resonators is still in its infancy due to quality limitations of current germanium integration platforms. Compared with silicon, germanium possesses a number of advantages for MIR applications, such as a wider transparency window (2 - 15 µm), a higher refractive index (~4), and a higher third-order nonlinear susceptibility. Here we present our experimental demonstration of two types of MIR high-Q germanium resonators, namely, a microring resonator and a photonic crystal nanobeam cavity. A maximum Q factor of ~57,000 is experimentally realized, which is the highest to date on germanium platforms. Moreover, we demonstrate a monolithic integration of the high-Q germanium resonators with suspended-membrane waveguides and focusing subwavelength grating couplers. Our resonators pave a new avenue for the study of on-chip light-germanium interactions and development of on-chip MIR applications in sensing and spectroscopy.
Optomechanical crystals (also referred to as photonic–phononic crystals or phoxonic crystals) exploit the simultaneous photonic and phononic bandgaps in periodic nanostructures. They have been utilized to colocalize, couple, and transduce optical and mechanical (acoustic) waves for nonlinear interactions and precision measurements. Devices that involve standing or traveling acoustic waves of high frequencies usually have advantages in many applications. Here, we review recent progress in nano-optomechanical devices where the acoustic wave oscillates at microwave frequencies. We focus on our development of an optomechanical crystal cavity and a phoxonic crystal waveguide with special features. The development of near-infrared optomechanical crystal cavities has reached a bottleneck in reducing the mechanical modal mass. This is because the reduction of the spatial overlap between the optical and mechanical modes results in a reduced optomechanical coupling rate. With a novel optimization strategy, we have successfully designed an optomechanical crystal cavity in gallium nitride with the optical mode at the wavelength of 393.03 nm, the mechanical mode at 14.97 GHz, the mechanical modal mass of 22.83 fg, and the optomechanical coupling rate of 1.26 MHz. Stimulated Brillouin scattering (SBS) has been widely exploited for applications of optical communication, sensing, and signal processing. A recent challenge of its implementation in silicon waveguides is the weak per-unit-length SBS gain. Taking advantage of the strong optomechanical interaction, we have successfully engineered a phoxonic crystal waveguide structure, where the SBS gain coefficient is greater than 3×104 W−1 m−1 in the entire C band with the highest value beyond 106W−1 m−1, which is at least an order of magnitude higher than the existing demonstrations.
We simulate and test a new structure for light coupling from silicon strip waveguide to plasmonic slot waveguide. The conventional approach of simply using a taper-funnel structure for the mode matching between two types of waveguides is typically insufficient for high coupling efficiency. Here we propose the use of an additional silicon strip-to-slot mode converter, which has a low insertion loss itself and achieves better mode matching. The experimental results show the new design, with slot width fixed at 200nm, achieves a higher coupling efficiency than conventional one. The newly implemented design has 1.5 dB less loss than the conventional taper-funnel coupler, with a theoretical coupling loss of 2.18 dB/coupler and an experimentally measured loss of 3 dB/coupler at 1640nm wavelength.
We propose and demonstrate a wavelength-tunable and narrow-linewidth erbium-doped fiber (EDF) laser using siliconon- insulator (SOI) based micro-ring. We discuss the wavelength selection and wavelength-tunable operation of the proposed fiber laser. The SOI based micro-ring is fabricated on a SOI wafer with a 0.22 um thick top silicon layer and a 2 um thick burial oxide (BOX) layer. In order to enhance the coupling efficiency between the SOI based micro-ring and the EDF, a pair of uniform period grating couplers are used. In the experiment, the lasing wavelengths can be tuned in the wavelengths range from 1532 nm to 1567.2 nm with a tuning step of 2 nm. The wavelength range and the tuning step are determined by the EDFA gain-bandwidth and the FSR of the SOI based micro-ring respectively. The OSNR of each lasing wavelength is > 42 dB. By using a double-ring configuration, a narrow laser linewidth of 50 kHz can be achieved.
We review our recent work on waveguide grating couplers, including an apodized grating coupler with engineered
coupling strength to achieve Gaussian-like output profile, which greatly improves the fiber-chip coupling efficiency. We
will also discuss a new class of grating couplers involving the use of sub-wavelength nanostructures to engineer the
optical properties. Effective medium theory can be used in the design of sub-wavelength structures, which, when
properly engineered, can offer broadband coupling and polarization independence. Other applications of waveguide
gratings, for example bi-wavelength two dimensional gratings coupler for (de-)multiplexing two different wavelengths,
fiber-waveguide hybrid lasers and mid-infrared grating couplers on silicon-on-sapphire wafer will also be briefly
discussed.
We review our recent work on chirped waveguide gratings for efficient coupling between standard single mode optical
fibers and silicon photonic wire waveguides. The use of a linear chirp in grating period reduces the second order Bragg
reflection from the waveguide gratings and increases the coupling efficiency for perfectly vertical optical fibers.
Measurement results obtained from devices fabricated using deep UV lithography yielded coupling efficiencies of over
34%. Techniques to further improve the coupling efficiency will be discussed. The use of chirped waveguide gratings
for low cost photonic packaging and the application of waveguide gratings for splitting/combining light will also be
presented.
We review recent work on low-dose high-energy helium ion implantation into silicon-on-insulator (SOI) optical
waveguides. The important role of free carriers generated by two-photon absorption (TPA) in nonlinear silicon
waveguide devices is discussed and a generalized definition of the nonlinear effective length which takes into account
the presence of nonlinear losses is proposed. We describe experimental studies and simulations of helium ion
implantation for carrier lifetime reduction to increase the nonlinear effective lengths of silicon waveguides. Helium ion
implantation can also enhance the photodetection responsivity of silicon at below-bandgap wavelengths. We review our
work on a possible application of the helium ion implanted waveguides for in-line optical power monitors (ICPM) which
monitor the output from erbium doped fiber amplifiers (EDFA) and, when combined with silicon variable optical
attenuators, can perform EDFA gain tilt and gain transient compensation.
Recent results on the nonlinear optical properties of silicon-on-insulator waveguides are reviewed. Interest on two photon absorption (TPA) in silicon has been raised by its potential applications for square law detection in autocorrelators for measuring pulsewidths and cross correlators for high speed all-optical clock recovery, high speed optical switches and optical wavelength conversion. The results reported by different groups on two photon absorption, optical Kerr effect, and Raman gain are compared and the range of possible values for the nonlinear figure of merit are calculated. The nonlinear figure of merit obtained from the average values of the various published experimental values suggests that silicon is suitable for all-optical nonlinear refractive switches which need only pi phase change but is not suitable for switches which need several pi of nonlinear phase change. The recent developments on obtaining optical gain using stimulated Raman scattering in silicon are also discussed.
We report some of our recent results on all-optical wavelength conversion including result on broadband orthogonal pumped four-wave-mixing in semiconductor optical amplifiers and dual-wavelength injection-locking of a Fabry- Perot laser diode. The high speed performance of wavelength conversion buy dual-wavelength injection locking of a Fabry- Perot laser was investigated experimentally and we present for the first time results on wavelength conversion at 10Gbit/s using dual wavelength injection-locking of a Fabry- Perot laser diode. The experimental results for all-optical wavelength conversion using broadband orthogonal pumping in a fiber ring containing a semiconductor optical amplifier is described. We also describe the latest result on a 40Gbit/s polarization independent all-optical wavelength converter based on polarization diversity and four-wave-mixing in a single semiconductor optical amplifier.
The all-optical nonlinearity of a quantum well waveguide is studied by measuring the intensity
dependent transmission through a Fabry-Perot cavity formed around the guide. Values for the
nonlinear refractive index coefficient, n2 ,at a wavelength of 1.O6mare obtained for light whose
polarisation is either parallel or perpendicular to the quantum well layers. A simple measurement
to estimate the two photon absorption coefficient, 13 2, using relatively low optical power levels is
also described.
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