Because of its high range and resolution, light detection and ranging (LIDAR) is a significant technology for numerous applications, such as autonomous vehicles, robotics, aerial or terrestrial mapping, and atmospheric research. Current lidar market is mainly occupied by conventional pulsed time of flight lidars. However, recently emerging companies are utilizing frequency modulated continuous wave lidars for improved and robust range resolution, dynamic range, sensitivity and simultaneous velocity measurement. Here, we propose and demonstrate multi-tone modulated continuous wave (MTCW) lidar system made of a CW laser with multiple fixed RF tones for a high precision range finding and velocimetry. In the proposed approach, the interference of the scattered light with the reference is detected by a PIN photodiode to extract the modulation information. Since, the acquired light is traveled all the way to the target and back to the beam splitter, it carries the range and velocity information about the target as phase and frequency shift, respectively, on the RF modulation tones. We use 1550nm light source and multiple RF tone modulations ranging from 50 MHz to 6 GHz to demonstrate proof of principle for range finding. We also provide sine fitting algorithms on the measured RF tones to extract the range and velocity information in a single shot RF measurement. We show that the precision and range information are scaled by the selection of RF tones. By an engineered selection of RF tones and a laser source, the measurement precision can be increased without compromising the range.
High speed (≥1Ghz) and long distance (≥100km) data communication among CubeSats and NanoSats can accelerate the technology advancement and paves the way for critical applications such as formation flying and remote sensing. Design of a simple, lightweight optical transceiver with full duplex capability, fast-tracking speed and 360° field of regard for CubeSat is crucial due to extreme SWaP-C limitations. In this paper, we describe the design tradeoff between the field of view and collection efficiency in receiver design using Commercial off the Shelf (COTS) optics and detectors. We also briefly discussed the design tradeoffs in transmitter design for optimum performance. We show that to achieve maximum SNR at long distance(≥100km), the laser beam diameter needs to be 80%-90% of the scanning mirror diameter. In addition to that, we show that the intrinsic Field of View (FOV) of high speed(≥600MHz) Avalanche Photodiodes (APD) can be increased to ≥3° by incorporating optimized optics considering form factor of the CubeSat system. In addition, we present a scalable detector array design method using COTS components to achieve a wide full FOV(≥12°) with a uniform collection efficiency around 30%-60%. Furthermore, we demonstrated a multi-wavelength full duplex communication system based on dichroic filters as duplexer that shows significantly low crosstalk. The system also exhibits low transmission power loss(≤4%) as opposed to around 40% that of the conventional beam splitter based system.
We propose a plasmo-thermomechanical mid-infrared detector operating at 4.3 μm wavelength. The design utilizes an array of the bimetallic fishbone nanowires that are suspended 50 nm above a 1.5 μm × 0.3 μm silicon nitride waveguide to create a leaky wave radiation. Moreover, the thermo-mechanically actuated nanowire will induce evanescent wave modulation that can be detected by the leaky wave or transmitted power of the waveguide. The antenna has a strip length of 1.77 μm and can yield an absorption coefficient of 42.4% with a period of 3.1 μm. Six unit cells are connected by a nanowire, and the fishbone-like nanowires are clamped at the two ends, leaving the center free to bend. The mid-infrared energy is absorbed by the resonant metallic antennas, resulting in a temperature increment. The mismatch of the thermal expansion coefficients of the bimetallic materials, gold and nickel, actuates the nanowire, and thus changes the gap between the nanowire and the waveguide. The deformation of the nanowire modulates the waveguide evanescent field, and hence alternates the transmitted power as well as the leak wave power. With a normal incident power of 4 μW/μm<sup>2</sup> , the temperature in the center of the nanobridge can be increased over 135 K above the ambient temperature, leading to an elevation of 23.5 nm in the center and thus weakening the evanescent modulation strength. The difference of S<sub>21</sub> caused by the gap change is 0.106. This methodology can be applied in other spectrums and the fabrication progress will be reported later.
We numerically investigated optical properties, including evanescent intensity ratio (EIR), effective refractive index (<i>N<sub>eff</sub></i>), dispersion coefficient (<i>D</i>), and mode area (<i>A<sub>eff</sub></i>) of the silicon nitride trench waveguides fabricated by using conventional lithography. The waveguides are etched 3 μm deep with potassium hydroxide for triangle and trapezoidal waveguides, which is then followed by 3 μm thermal oxidation and 725 nm silicon nitride deposition. The waveguide with 725 nm thickness has an <i>EIR</i> peak of 0.025 when its bottom width<i> W<sub>btm</sub></i> equals 0.65 μm. A thinner waveguide has higher evanescent intensity ratio, which can be used in sensing applications. The locations of EIR peaks correspond to the quasi-TM and TE mode boundary. Narrower waveguides mainly support quasi-TM modes, whereas wider waveguides can support only TE modes. As the waveguide width increases, higher orders of TE modes emerge. In addition, a boundary of TE single mode and multimode can also be linearly curve fitted, according to the starting points of TE higher modes, in order to provide the single mode condition of the waveguide. The waveguide dispersion can be engineered to be in the anomalous region while at the same time remain close to zero. The waveguide with 725 nm thickness and 0.2 μm bottom width has its anomalous dispersion region between the wavelength of 1356 nm and 1462 nm. The mode area decreases with increasing waveguide width. This is the first time we have studied the mode properties of trench waveguides systematically. The waveguide will find more applications in sensing and nonlinear fields with the help of this mode analysis.
We propose electronically controlled optical tweezing based on space-time-wavelength mapping technology. By using time-domain modulation, the location and the polarity of force hot-spots created by Lorentz force (gradient force) can be controlled. In this preliminary study we use 150 fs optical pulses that are dispersed in time and space to achieve a focused elliptical beam that is ~20 μm long and ~2 μm wide. We use an electro-optic modulator to modulate power spectral distribution of the femtosecond beam after temporal dispersion and hence change the intensity gradient along the beam at the focal spot. We present a theoretical model, and simulation results from a proposed experimental setup. The results show that we can achieve ±200 pN forces on nano objects (~100 nm) without mechanical beam steering. The intensity of wavelengths along the spectrum can be manipulated by using different RF waveforms to create a desired intensity gradient profile at the focal plane. By choosing the appropriate RF waveform it is possible to create force fields for cell stretching and compression as well as multiple hot spots for attractive or repulsive forces. 2D space-time-wavelength mapping can also be utilized to create tunable 2D force field distribution.