Silicon FMCW LiDAR chip integrated with SLG beam scanner and k-clock interferometer for operation with wavelength-swept laser source.

We fabricated a frequency-modulated continuous-wave light detection and ranging (FMCW LiDAR) chip that integrates a slow-light grating (SLG) beam scanner and an optical interferometer for k-clock generation using silicon photonics. Beam scanning and FMCW light generation were performed simultaneously through a wavelength sweep, while the sweep nonlinearity was compensated by resampling the ranging signal using the k-clock. The interferometer incorporated a 24-cm-long Si waveguide delay line, facilitating ranging up to 7.1 m and the capture of point cloud images. The possibility of ranging longer distances by lengthening the waveguide and increasing the interpolation is discussed.

Ambient light immunity of a frequency-modulated continuous-wave (FMCW) LiDAR chip.

The interference between a frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) and other LiDARs or sunlight was theorized, considering the spatial overlap, frequency overlap, and intensity ratio. It has been concluded that the interference probability between LiDARs can be lower than a safety standard value for autonomous vehicles when the number of the resolution points of a single LiDAR is increased sufficiently and that the interference with incoherent sunlight does not occur. Due to the coherent detection of FMCW, such ambient light immunity is much better than time-of-flight LiDAR. The dependence of the interference on the wavelength range, sweep bandwidth, and sweep period was also observed experimentally using a silicon (Si) photonics FMCW LiDAR chip incorporating slow-light grating beam scanners. It was shown that the interference can be suppressed by increasing the number of resolution points and changing their common parameters moderately. Regarding the contamination of sunlight, unwanted beam shift due to heating was observed, although it will be suppressed simply by wavelength filtering.

OFDR analysis of Si photonics FMCW LiDAR chip.

We experimentally analyzed the internal reflection and loss of each component in a Si photonics frequency-modulated continuous-wave light detection and ranging (FMCW LiDAR) device using optical frequency domain reflectometry (OFDR) with a spatial resolution of better than 2.5 µm. Sweeping the incident laser wavelength by 120 nm, the reflections and losses of wire waveguides, widened waveguides, and optical switches on the chip were individually revealed. The slow-light grating (SLG) beam scanner, which has a limited working wavelength range, was evaluated with a spatial resolution of >10 µm by narrowing the wavelength sweep range. Consequently, a strong reflection was observed at the transition between the wire waveguide and the SLG, which can be a noise source in the FMCW LiDAR. Additionally, this study showed that the OFDR can be an important analysis tool for Si photonics integrated circuits. To our knowledge, this is the first demonstration, showing that the OFDR can be an important analysis tool for Si photonic integrated circuits.

High-efficiency upward radiation in a slow-light grating beam scanner.

Slow-light grating (SLG) is used as a solid-state optical beam scanner, but the efficiency of conventional SLGs has been constrained by unwanted downward radiation. In this study, we developed a high-efficiency SLG consisting of through-hole grating and surface grating, which selectively radiates upward. Via the optimization using the covariance matrix adaptation evolution strategy, we designed a structure showing a maximum upward emissivity of 95% as well as moderate radiation rates and beam divergence. Experimentally, the emissivity was enhanced by 2-4 dB and the roundtrip efficiency was improved by 5.4 dB, which is significant in applications to light detection and ranging.

All-optic control using a photo-thermal heater in Si photonics.

We propose and demonstrate a simple all-optic control for Si photonics using a photo-thermal heater. The control light is absorbed in a heavily doped control waveguide and the signal light phase is tuned through thermal diffusion in a signal waveguide adjacent to but not optically coupled with the control waveguide. We designed and fabricated Mach-Zehnder- and microring-type devices requiring 17 (π-phase shift) and 4 (switching between resonance and non-resonance with 6 dB extinction) mW of control power, respectively. We confirmed that the heating efficiency of all-optic control exceeded that of an electrical heater placed above the signal waveguide.

Imaging of electromagnetic waves using a radio over fiber system including an Si photonics microring modulator array.

We propose an electromagnetic wave imaging system using Si photonics microring modulators (MRMs) as radio over fiber components. In this system, radio frequency signals detected at microwave probes are applied to the connected MRMs; an MRM is tuned to the laser wavelength using the thermo-optic effect to read-out through the optical transmission. By repeating this process for all MRMs and mapping the read-out signals in two dimensions, the distribution of electromagnetic waves can be visualized. Such system operation was demonstrated at 3.5 GHz in a 37.5 × 37.5 mm2 probing area, for which we confirmed the agreement with theoretical simulations.

Step-like beam scanning in a slow-light grating beam scanner for a FMCW LIDAR.

We have developed a nonmechanical beam scanner equipped with a Si photonics slow-light grating toward an on-chip frequency-modulated continuous-wave light detection and ranging (FMCW LIDAR) device. An optical beam is scanned thermo-optically, but it is also shifted sensitively to the frequency modulation, which is inconvenient for FMCW LIDAR. In this study, we canceled this shift and obtained step-like beam scanning with synchronized thermo-optic signals, which was confirmed in space-time-domain beam observations. The step-like scanning allows finer angular resolution of the range profile.

Synthetic dimension band structures on a Si CMOS photonic platform.

Synthetic dimensions, which simulate spatial coordinates using nonspatial degrees of freedom, are drawing interest in topological science and other fields for modeling higher-dimensional phenomena on simple structures. We present the first realization of a synthetic frequency dimension on a silicon ring resonator integrated photonic device fabricated using a CMOS process. We confirm that its coupled modes correspond to a one-dimensional tight-binding model through acquisition of up to 280-GHz bandwidth optical frequency comb-like spectra and by measuring synthetic band structures. Furthermore, we realized two types of gauge potentials along the frequency dimension and probed their effects through the associated band structures. An electric field analog was produced via modulation detuning, whereas effective magnetic fields were induced using synchronized nearest- and second nearest-neighbor couplings. Creation of coupled mode lattices and two effective forces on a monolithic Si CMOS device represents a key step toward wider adoption of topological principles.

Doppler velocimeter and vibrometer FMCW LiDAR with Si photonic crystal beam scanner.

In this paper, we propose and demonstrate a frequency-modulated continuous-wave light detection and ranging (LiDAR) with a Si photonic crystal beam scanner, simultaneously enabling scanning laser Doppler measurements. This nonmechanical solid-state device can reduce the size of conventional scanning laser Doppler vibrometers, making LiDAR a multimodal imaging sensor, which can measure the distributions of distance, velocity, and vibration frequency. We fabricated this device using Si photonics process and confirmed the expected operations. Distance and velocity resolutions were less than 15 mm and 19 mm/s, respectively. The detection limit of the vibration amplitude determined by the signal-to-noise ratio was 2.5 nm.

Si photonic crystal slow-light waveguides optimized through informatics technology.

We modeled the photonic bands of SiO2-cladded Si lattice-shifted photonic crystal waveguides via machine learning and found a structure that generates low-dispersion slow light with a group index of approximately 20 in the full C-band at telecom wavelengths. The normalized delay-bandwidth product is as large as 0.45, which is close to the theoretical upper limit. The transition structure between this waveguide and a Si-channel waveguide was designed using an evolutional optimization, and a C-band average loss of 0.116 dB/transition was calculated. These results prove the possibility of further enhancing the versatility of slow light.

Space-time-domain observation of high-speed optical beam scanning in a thermo-optic Si photonic crystal slow-light beam scanner.

We developed a thermo-optically controlled nonmechanical optical beam scanner using a Si photonic crystal slow-light waveguide with a diffraction grating to achieve on-chip light detection and ranging (LIDAR). This Letter applies pre-emphasis signals to the thermo-optic control, and the cutoff frequency increases to 500 kHz. Observing the beam scanning in the space-time domain showed that the turn-on and turn-off times of the scanner for a rectangular drive voltage were 10 µs and reduced to 2.7 µs when the pre-emphasis signals were optimized. This new, to the best of our knowledge, result enables a frame rate of 29 fps for 12,800 resolution points in LIDAR.

Clinical evidence that a dysregulated master neural network modulator may aid in diagnosing schizophrenia.

There are no validated biomarkers for schizophrenia (SCZ), a disorder linked to neural network dysfunction. We demonstrate that collapsin response mediator protein-2 (CRMP2), a master regulator of cytoskeleton and, hence, neural circuitry, may form the basis for a biomarker because its activity is uniquely imbalanced in SCZ patients. CRMP2's activity depends upon its phosphorylation state. While an equilibrium between inactive (phosphorylated) and active (nonphosphorylated) CRMP2 is present in unaffected individuals, we show that SCZ patients are characterized by excess active CRMP2. We examined CRMP2 levels first in postmortem brains (correlated with neuronal morphometrics) and then, because CRMP2 is expressed in lymphocytes as well, in the peripheral blood of SCZ patients versus age-matched unaffected controls. In the brains and, more starkly, in the lymphocytes of SCZ patients <40 y old, we observed that nonphosphorylated CRMP2 was higher than in controls, while phosphorylated CRMP2 remained unchanged from control. In the brain, these changes were associated with dendritic structural abnormalities. The abundance of active CRMP2 with insufficient opposing inactive p-CRMP2 yielded a unique lowering of the p-CRMP2:CRMP2 ratio in SCZ patients, implying a disruption in the normal equilibrium between active and inactive CRMP2. These clinical data suggest that measuring CRMP2 and p-CRMP2 in peripheral blood might reflect intracerebral processes and suggest a rapid, minimally invasive, sensitive, and specific adjunctive diagnostic aid for early SCZ: increased CRMP2 or a decreased p-CRMP2:CRMP2 ratio may help cinch the diagnosis in a newly presenting young patient suspected of SCZ (versus such mimics as mania in bipolar disorder, where the ratio is high).

Particle swarm optimization of silicon photonic crystal waveguide transition.

Slow light generated through silicon (Si) photonic crystal waveguides (PCWs) is useful for improving the performance of Si photonic devices. However, the accumulation of coupling loss between a PCW and Si optical wiring waveguides is a problem when slow-light devices are connected in a series in a photonic integrated circuit. Previously, we reported a tapered transition structure between these waveguides and observed a coupling loss of 0.46 dB per transition. This Letter employed particle swarm optimization to engineer the arrangement of photonic crystal holes to reduce loss and succeeded in demonstrating theoretical loss value of 0.12 dB on average in the wavelength range of 1540-1560 nm and an experimental one of 0.21 dB. Crucially, this structure enhances the versatility of slow light.

pH-sensitive GaInAsP photonic crystal fractal band-edge laser.

The light emission characteristics of GaInAsP semiconductors in aqueous solutions are modified by the surface charge, depending on the balance between radiative and nonradiative recombination. This Letter demonstrates the application of a GaInAsP photonic crystal band-edge laser in sensing a solution pH, which reflects the surface charge. The sensitivity is enhanced by a hybrid of fractal honeycomb and close-packed structures with a large surface-to-volume ratio and a high quality factor (Q), which allows a low threshold carrier density. We experimentally obtained a maximum pH sensitivity of 1.9 dB/pH near the threshold and a signal-to-noise ratio of 52/pH (pH resolution of 0.019) at a pump power 2.4 times the threshold where the intensity noise diminished.

Prism lens for beam collimation in a silicon photonic crystal beam-steering device.

The doubly periodic Si photonic crystal waveguide operates as a nonmechanical beam-steering device that can be applied in light detection and ranging. In this Letter, we develop and describe a prism lens that collimates a fan-shaped beam, emitted from the waveguide independent of the steering angle. Its fundamental profile is investigated using a theoretical analysis for thick lenses, and then, its detailed aspherical design is obtained. In ray tracing, this prism lens suppresses the beam divergence to less than the diffraction limit in most of the targeted beam-steering range. The prism lens is fabricated by acrylic cutting, and its expected characteristics are observed.

Enhanced pH sensitivity in photoluminescence of GaInAsP semiconductor photonic crystal slab.

Semiconductor ion sensors that respond to the surface electric charge in a solution are used for chemical and biological sensing. Photonic sensors exploiting such a response in the photoluminescence intensity enable a simple system consisting only of a photopump source and a photodiode; however, their sensitivity is usually lower than that of electric sensors, such as ion-sensitive field-effect transistors. This study employed a GaInAsP semiconductor honeycomb photonic crystal slab as a photonic sensor structure and obtained a high ion sensitivity. The surface recombination, which is the origin of the ion sensitivity, was enhanced by increasing the surface-to-volume ratio and moderately suppressing the photopump level. Nevertheless, a sufficient signal-to-noise ratio was maintained by improving the light extraction efficiency. Moreover, a high pH sensitivity of 0.27 dB/pH, which is six times that without photonic crystals, was obtained and resulted in a pH resolution of 0.011 at pH ∼7 comparable with that of electric sensors.

64 Gbps Si photonic crystal slow light modulator by electro-optic phase matching.

We demonstrate 64 Gbps operation in a compact Si photonic crystal optical modulator that employs meander line electrodes and compensate for the phase mismatch between slow light and RF signals. Although low dispersion slow light increases the modulation efficiency, maintaining a sufficiently wide working spectrum, the phase mismatch becomes a limiting factor on the operation speed even when the phase shifter length is as short as 200 µm. Meander line electrodes broke this limit and enhanced the cutoff frequency by up to 31 and 38 GHz using 50 Ω and 20 Ω termination resistors, respectively. This allowed to use a group index of slow light higher than 20, and greatly improved the quality of the modulation characteristics at 25 and 32 Gbps. Clear open eye was observed even at 40-64 Gbps.

Wavelength-division multiplexing Si photonic crystal beam steering device for high-throughput parallel sensing.

We proposed and demonstrated a wavelength-division multiplexing (WDM) optical beam-steering device consisting of a thermally controlled doubly periodic Si two-dimensional bulk photonic crystal waveguide and coupled microring multiplexers. Beam forming and steering while maintaining a sharp profile is much easier in this device than with optical phased arrays which need the fine phase control. By dividing the range of beam-steering angles into different wavelength channels, it is possible to cover a wide range of angles, even when each angle is small. In this study, we fabricated a device with four wavelength channels, each of which showed beam steering of 4°-5° as a result of heating, resulting in a total of 16°. Two-dimensional steering is also achieved by loading a collimator lens and selecting one waveguide from those arrayed. We evaluated 112 resolution points with four wavelengths and 448 points in total by switching four waveguides. If this WDM concept is introduced into light detection and ranging and the number of wavelengths is increased, it will be possible to increase the sensing throughput, which is usually constrained by the round-trip time of light, by simultaneous parallel operation.

Experimental simulation of ranging action using Si photonic crystal modulator and optical antenna.

Time of flight light detection and ranging (LiDAR) has been tested and used as a key device for auto-driving of vehicles. Frequency-modulated continuous-wave (FMCW) LiDAR potentially achieves a high sensitivity. In this study, we fabricated and tested two components of FMCW LiDAR based on Si photonics. The ranging action was also experimentally simulated. A Si photonic crystal slow light Mach-Zehnder modulator was driven by linearly frequency-chirped signals to generate quasi-frequency-modulated signal light. Then, the light was inserted into a fiber delay line of 20-320 m. Its output was irradiated to a photonic crystal slow beam steering device that acted as an optical antenna via a free-space transmission. The detected light was mixed with the reference light branched after the modulation in balanced photodiodes. A sufficiently sharp beat spectrum was observed, whose frequency well agreed with that expected for the delay line. The experimental simulation of the FMCW LiDAR, thus, was achieved.

Label-free and spectral-analysis-free detection of neuropsychiatric disease biomarkers using an ion-sensitive GaInAsP nanolaser biosensor.

The emission intensity of GaInAsP semiconductors that show an ion sensitivity is altered by the surface charge. In this study, we propose a biosensing technique using GaInAsP photonic crystal nanolasers based on this principle. Here, simple and rapid detection of collapsin response mediator protein 2 (CRMP2) is demonstrated, which is a promising biomarker candidate for neuropsychiatric diseases existing in peripheral white blood cells. We prepared CRMP2 as a standard protein and introduced sodium dodecyl sulfate (SDS) as an anionic surfactant to enhance the net negative charge of the protein. The nanolaser was modified in advance with an anti-CRMP2 antibody and then photopumped at a constant power. The laser emission intensity was monitored during the antibody-antigen reaction. Consequently, CRMP2 was detected as a decrease in the emission intensity. We achieved a lower limit for detection of 3.8 µg/mL that satisfies the requirement for clinical biomarker testing. Without the requirements of any kind of labels and spectral analyses, this technique allows for simple, rapid, and low-cost biomarker detection.