Transmissive silicon photonic dichroic filters with spectrally selective waveguides

Development and Validation of a New Type of Displacement-Based Miniatured Laser Vibrometers

Developing a miniatured laser vibrometer becomes important for many engineering areas, such as experimental and operational modal analyses, model validation, and structural health monitoring. Due to its compact size and light weight, a miniatured laser vibrometer can be attached to various mobilized platforms, such as an unmanned aerial vehicle and a robotic arm whose payloads can usually not be large, to achieve a flexible vibration measurement capability. However, integrating optics into a miniaturized laser vibrometer presents several challenges. These include signal interference from ghost reflectance signals generated by the sub-components of integrated photonics, polarization effects caused by waveguide structures, wavelength drifting due to the semiconductor laser, and the poorer noise characteristics of an integrated laser chip compared to a non-integrated circuit. This work proposes a novel chip-based high-precision laser vibrometer by incorporating two or more sets of quadrature demodulation networks into its design. An additional set of quadrature demodulation networks with a distinct reference arm delay line length can be used to conduct real-time compensation to mitigate linear interference caused by temperature and environmental variations. A series of vibration measurements with frequencies ranging from 0.1 Hz to 1 MHz were conducted using the proposed laser vibrometer to show its repeatability and accuracy in vibration and ultrasonic vibration measurements, and its robustness to test surface conditions. The proposed laser vibrometer has the advantage of directly measuring the displacement response of a vibrating structure rather than integrating its velocity response to yield the measured displacement with a conventional laser Doppler vibrometer.

Broadband and fabrication-tolerant 3-dB couplers with topological valley edge modes

3-dB couplers, which are commonly used in photonic integrated circuits for on-chip information processing, precision measurement, and quantum computing, face challenges in achieving robust performance due to their limited 3-dB bandwidths and sensitivity to fabrication errors. To address this, we introduce topological physics to nanophotonics, developing a framework for topological 3-dB couplers. These couplers exhibit broad working wavelength range and robustness against fabrication dimensional errors. By leveraging valley-Hall topology and mirror symmetry, the photonic-crystal-slab couplers achieve ideal 3-dB splitting characterized by a wavelength-insensitive scattering matrix. Tolerance analysis confirms the superiority on broad bandwidth of 48 nm and robust splitting against dimensional errors of 20 nm. We further propose a topological interferometer for on-chip distance measurement, which also exhibits robustness against dimensional errors. This extension of topological principles to the fields of interferometers, may open up new possibilities for constructing robust wavelength division multiplexing, temperature-drift-insensitive sensing, and optical coherence tomography applications.

Deep photonic network platform enabling arbitrary and broadband optical functionality

Expanding applications in optical communications, computing, and sensing continue to drive the need for high-performance integrated photonic components. Designing these on-chip systems with arbitrary functionality requires beyond what is possible with physical intuition, for which machine learning-based methods have recently become popular. However, computational demands for physically accurate device simulations present critical challenges, significantly limiting scalability and design flexibility of these methods. Here, we present a highly-scalable, physics-informed design platform for on-chip optical systems with arbitrary functionality, based on deep photonic networks of custom-designed Mach-Zehnder interferometers. Leveraging this platform, we demonstrate ultra-broadband power splitters and a spectral duplexer, each designed within two minutes. The devices exhibit state-of-the-art experimental performance with insertion losses below 0.66 dB, and 1-dB bandwidths exceeding 120 nm. This platform provides a tractable path towards systematic, large-scale photonic system design, enabling custom power, phase, and dispersion profiles for high-throughput communications, quantum information processing, and medical/biological sensing applications.

High-performance and compact integrated photonic dichroic filters and triplexer realized by an efficient inverse design

Integrated optical filters are key components in various photonic integrated circuits for applications of communication, spectroscopy, etc. The dichroic filters can be flexibly cascaded to construct filters with various channel numbers and bandwidths. Therefore, the development of high-performance and compact dichroic filters is crucial. In this work, we develop the dichroic filters with 1.49/1.55-µm channels by an inverse design. Benefiting from a search-space-dimension control strategy and advanced optimization algorithm, our efficient design method results in two high-performance dichroic filters without and with subwavelength gratings (SWGs). The comparison suggests that SWGs in filters can be useful for loss reduction and footprint compression by dispersion engineering. The developed dichroic filter with SWGs exhibits measured bandwidths of 26/29 nm, excess losses of < 0.5 dB, and crosstalks of <-10 dB with a compact footprint of 2.5 × 22.0 µm2. It has advantages in performance or compactness compared to the previously reported counterparts. A triplexer with a footprint of 10.5 × 117 µm2 is developed based on the dichroic filters, also showing decent overall performance and compactness.

Demonstration of polarization-insensitive optical filters on silicon photonics platform

We experimentally demonstrate a polarization-insensitive optical filter (PIOF) using polarization rotator-splitters (PRSs) and microring resonators (MRRs) on the silicon-on-insulator (SOI) platform with complementary metal-oxide-semiconductor (CMOS) compatible fabrication process. The PRS consists of a tapered-rib waveguide and an asymmetrical directional coupler (ADC), which realize the polarization rotation and splitting, to ensure the connected MRRs-based optical filter operating at one desired polarization when light with different polarizations are launched into the device. The measured results show that the optical transmission spectra of the device are identical for TE and TM polarization input. The box-like filtering spectra are also achieved with a 3-dB bandwidth of ∼0.15 nm and a high extinction ratio (ER) over 30 dB.

Integrated photonic high extinction short and long pass filters based on lateral leakage

In this contribution we present a new approach to achieve high extinction short and long pass wavelength filters in the integrated photonic platform of lithium niobate on insulator. The filtering of unwanted wavelengths is achieved by employing lateral leakage and is related to the bound state in the continuum phenomenon. We show that it is possible to control the filter edge wavelength by adjusting the waveguide dimensions and that an extinction of hundreds of dB/cm is readily achievable. This enabled us to design a pump wavelength suppression of more than 100 dB in a 3.5 mm long waveguide, which is essential for on-chip integration of quantum-correlated photon pair sources. These findings pave the way to integrate multi wavelength experiments on chip for the next generation of photonic integrated circuits.

Towards integrated photonic interposers for processing octave-spanning microresonator frequency combs

Microcombs-optical frequency combs generated in microresonators-have advanced tremendously in the past decade, and are advantageous for applications in frequency metrology, navigation, spectroscopy, telecommunications, and microwave photonics. Crucially, microcombs promise fully integrated miniaturized optical systems with unprecedented reductions in cost, size, weight, and power. However, the use of bulk free-space and fiber-optic components to process microcombs has restricted form factors to the table-top. Taking microcomb-based optical frequency synthesis around 1550 nm as our target application, here, we address this challenge by proposing an integrated photonics interposer architecture to replace discrete components by collecting, routing, and interfacing octave-wide microcomb-based optical signals between photonic chiplets and heterogeneously integrated devices. Experimentally, we confirm the requisite performance of the individual passive elements of the proposed interposer-octave-wide dichroics, multimode interferometers, and tunable ring filters, and implement the octave-spanning spectral filtering of a microcomb, central to the interposer, using silicon nitride photonics. Moreover, we show that the thick silicon nitride needed for bright dissipative Kerr soliton generation can be integrated with the comparatively thin silicon nitride interposer layer through octave-bandwidth adiabatic evanescent coupling, indicating a path towards future system-level consolidation. Finally, we numerically confirm the feasibility of operating the proposed interposer synthesizer as a fully assembled system. Our interposer architecture addresses the immediate need for on-chip microcomb processing to successfully miniaturize microcomb systems and can be readily adapted to other metrology-grade applications based on optical atomic clocks and high-precision navigation and spectroscopy.

Ultra-dense dual-polarization waveguide superlattices on silicon

A dual-polarization waveguide superlattice is designed and realized by using 340 nm-thick silicon photonic waveguides. The silicon waveguide superlattices are formed with periodically arranged waveguides. Each period consists of five optical waveguides with core-widths designed optimally for minimizing the crosstalk among the optical waveguides. The optimized core-widths are 390 nm, 320 nm, 260 nm, 360 nm, and 300 nm when the separation between two adjacent waveguides is as small as 0.8 µm. With this design, the silicon waveguide superlattice works with low crosstalk (nearly -18 dB or less) for both polarizations within the range of 1530 nm to 1560 nm, which agrees well with the theoretical analysis.

Ultrawide-bandwidth on-chip spectrometer design using band-pass filters

Here, we present the design and simulation of an ultrawide-bandwidth on-chip spectrometer that can be used in various applications, e.g. spectral tissue sensing. It covers 1200 nm wavelength range (400 nm-1600 nm) with 2 nm spectral resolution. The overall design size is only 3 × 3 cm². The ultra-wide spectral range is made possible by using novel on-chip band-pass filters for the coarse wavelength division. The fine resolution is provided by the arrayed waveguide gratings. The band-pass filter is formed by using bend waveguides and adiabatic full-couplers. The additional loss caused by the band-pass filter is relatively small. The proposed spectrometer covers entire 400 nm-1600 nm range continuously with low crosstalk values. We envision that this design can be used in several different applications including food safety, agriculture, industrial inspection, optical imaging, and biomedical research.

Scaling and cascading compact metamaterial photonic waveguide filter blocks

In this work, we reported the design and fabrication of a compact and scalable metamaterial longpass filter with an ultrasmall footprint of 5.1µm×5.1µm. In the stopband, light transmission can be blocked and reflected with ∼25dB attenuation. In the passband, light can pass through with a low insertion loss around -0.28dB. The transition band can be redshifted or blueshifted by scaling up or down the filter block; i.e., scaling down 1% can produce a transition band blueshift of 11.4 nm. The power roll-off can be enhanced by cascading multiple filter blocks, i.e., 1.34 dB/nm by cascading three filters. These results demonstrate the great potential of the metamaterial-based waveguide devices for scalable photonic filtering applications.

Ultra compact Bragg grating devices with broadband selectivity

Current silicon waveguide Bragg gratings typically introduce perturbation to the optical mode in the form of modulation of the waveguide width or cladding. However, since such a perturbation approach is limited to weak perturbations to avoid intolerable scattering loss and higher-order modal coupling, it is difficult to produce ultra-wide stopbands. In this Letter, we report an ultra-compact Bragg grating device with strong perturbations by etching nanoholes in the waveguide core to enable an ultra-large stopband with apodization achieved by proper location of the nanoholes. With this approach, a 15 µm long device can generate a stopband as wide as 110 nm that covers the entire ${\rm C} + {\rm L}$C+L band with a 40 dB extinction ratio and over a 10 dB sidelobe suppression ratio (SSR). Similar structures can be further optimized to achieve higher SSR of $ \gt {17}\;{\rm dB}$>17dB for a stopband of about 80 nm.

A Silicon Photonic Data Link with a Monolithic Erbium-Doped Laser

To meet the increasing demand for data communication bandwidth and overcome the limits of electrical interconnects, silicon photonic technology has been extensively studied, with various photonics devices and optical links being demonstrated. All of the optical data links previously demonstrated have used either heterogeneously integrated lasers or external laser sources. This work presents the first silicon photonic data link using a monolithic rare-earth-ion-doped laser, a silicon microdisk modulator, and a germanium photodetector integrated on a single chip. The fabrication is CMOS compatible, demonstrating data transmission as a proof-of-concept at kHz speed level, and potential data rate of more than 1 Gbps. This work provides a solution for the monolithic integration of laser sources on the silicon photonic platform, which is fully compatible with the CMOS fabrication line, and has potential applications such as free-space communication and integrated LIDAR.

Coherent poly propagation materials with 3-dimensional photonic control over visible light

The purpose of the present research was to identify and examine materials demonstrating a previously undiscovered property of coherent poly propagation (CPP). The materials were amorphous silicates as natural precious opals. CPP enabled three-dimensional photonic control over mono and polychromatic visible light wavelengths. CPP caused coherent diffraction of incident poly and monochromatic light. Apart from the iconic play-of-color of precious opal, CPP specimens demonstrated diffractive photonic demultiplexing and/or upconversion and/or downconversion of incident light with strong photonic coherence such that the shape of the incident light source was propagated over three dimensions over multiple visible frequencies. CPP events manifested as each specimen was rocked under the incident light. Additionally, the specimens demonstrated atypical control over internally reflected and transmitted light. The specimens applied axial rotational symmetry over the incident light. Amorphous materials would be expected to exert no symmetry control. CPP and rotational properties occurred in isolation from exogenous thermal, photonic and electrical influences. Furthermore, several non-destructive analytical instruments were employed, such as: spectrophotometer, polariscope and refractometer. The analytical methods revealed unusual behaviors of these specimens. The application of materials demonstrating three-dimensional photonic control will have far-reaching implications for many industries, including: photonic wavelength demultiplexing, fiber optics, imaging, microscopy, projections, security, cryptography, computers and communications.

Integrated CMOS-compatible Q-switched mode-locked lasers at 1900nm with an on-chip artificial saturable absorber

We present a CMOS-compatible, Q-switched mode-locked integrated laser operating at 1.9 µm with a compact footprint of 23.6 × 0.6 × 0.78mm. The Q-switching rate is 720 kHz, the mode-locking rate is 1.2 GHz, and the optical bandwidth is 17nm, which is sufficient to support pulses as short as 215 fs. The laser is fabricated using a silicon nitride on silicon dioxide 300-mm wafer platform, with thulium-doped Al₂O₃ glass as a gain material deposited over the silicon photonics chip. An integrated Kerr-nonlinearity-based artificial saturable absorber is implemented in silicon nitride. A broadband (over 100 nm) dispersion-compensating grating in silicon nitride provides sufficient anomalous dispersion to compensate for the normal dispersion of the other laser components, enabling femtosecond-level pulses. The laser has no off-chip components with the exception of the optical pump, allowing for easy co-integration of numerous other photonic devices such as supercontinuum generation and frequency doublers which together potentially enable fully on-chip frequency comb generation.