Lens-assisted beam steering (LABS) has emerged as a promising solution for compact chip-based optical beam steering for light detection and ranging (LiDAR) applications. In a LABS system, light is steered within an integrated optical chip and emitted at a desired location. This emitted light is focused out into the scene with a lens, analogous to a camera operating in reverse. LABS systems offer many advantages compared to competing technologies such as solid-state reliability, simple control, compactness, and fast random access scanning. Different methods for LABS systems are described and compared. Most LABS systems demonstrated thus far have small arrays, and therefore, only offer a relatively small number of possible beam locations. It is important to understand how these systems will scale to the much larger arrays needed for a practical LiDAR system.
Compressive sensing has been used to demonstrate scene reconstruction and source localization in a wide variety of devices. To date, optical compressive sensors have not been able to achieve significant volume reduction relative to conventional optics of equivalent angular resolution. Here, we adapt silicon-photonic optical phased array technology to demonstrate, to our knowledge, the first application of compressive imaging in a photonicintegrated device. Our novel sensor consists of an 8 × 8 grid of grating couplers with a spacing of 100 μm. Path-matched waveguides route to a single multimode interferometer (MMI), which mixes and randomizes the signals into 64 outputs to be used for compressed sensing. Our device is fully passive, having no need for phase shifters, as measurement matrix calibration makes the measurements robust to phase errors. For testing, we use an Amplified Spontaneous Emission (ASE) source with a bandwidth of 40 nm, centered at 1545 nm. We demonstrate simultaneous multi-point (2 sources demonstrated in this work) brightness recovery and localization with better than 10 arcsecond precision in a sub-millimeter thick form-factor. We achieve a single source recovery rate higher than 99.9% using 10 of the 64 outputs, and a 90% recovery rate with only 6 outputs, 10 times fewer than the 64 needed for conventional imaging. This planar optical phased array compressive sensor is well-suited for imaging sparse scenes in applications constrained by form factor, volume, or high-cost detectors, with the potential to revolutionize endoscopy, beam locators, and LIDAR.
Recent advances in silicon photonics have enabled large-scale optical phased arrays for applications such as beam steering and directional light detection. However, to date, these results have only been applied to coherent light. Many applications, including passive imaging with natural illumination, require operation using incoherent and/or broadband light. Here we implement an optical phased array designed for these applications using a planar, fractal, pathlength-matching architecture known as an “H-tree”. We demonstrate electronic beamsteering and natural light imaging using this flat, broadband, photonic-integrated device.
Publisher’s Note: This paper, originally published on 4/1/2016, was replaced with a corrected/revised version on 4/14/2016. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
We describe the performance of versatile high-performance multi-rate WDM laser transmitters using next-generation compact high-extinction-ratio power-efficient (CHERPe) transmitter designs. These leverage periodic time-frequency windowing of directly modulated laser signals to efficiently generate nearly ideal WDM waveforms with only mW-class drive power. facilitating WDM-channelization and providing straightforward access to many THz of available optical spectrum with low-bandwidth electronics. Furthermore, this approach can support scalable multi-rate operation with good power- and photon-efficiency and enable new architectural options. This approach is attractive for numerous applications and systems ranging from small airborne or CubeSAT-sized communication payloads to larger interplanetary lasercom platforms.
Sampling rates of high-performance electronic analog-to-digital converters (ADC) are fundamentally limited by the timing jitter of the electronic clock. This limit is overcome in photonic ADC's by taking advantage of the ultra-low timing jitter of femtosecond lasers. We have developed designs and strategies for a photonic ADC that is capable of 40 GSa/s at a resolution of 8 bits. This system requires a femtosecond laser with a repetition rate of 2 GHz and timing jitter less than 20 fs. In addition to a femtosecond laser this system calls for the integration of a number of photonic components including: a broadband modulator, optical filter banks, and photodetectors. Using silicon-on-insulator (SOI) as the platform we have fabricated these individual components. The silicon optical modulator is based on a Mach-Zehnder interferometer architecture and achieves a VπL of 2 Vcm. The filter banks comprise 40 second-order microring-resonator filters with a channel spacing of 80 GHz. For the photodetectors we are exploring ion-bombarded silicon waveguide detectors and germanium films epitaxially grown on silicon utilizing a process that minimizes the defect density.
Photonic Analog-to-Digital Conversion (ADC) has a long history. The premise is that the superior noise performance of
femtosecond lasers working at optical frequencies enables us to overcome the bottleneck set by jitter and bandwidth of
electronic systems and components. We discuss and demonstrate strategies and devices that enable the implementation
of photonic ADC systems with emerging electronic-photonic integrated circuits based on silicon photonics. Devices
include 2-GHz repetition rate low noise femtosecond fiber lasers, Si-Modulators with up to 20 GHz modulation speed,
20 channel SiN-filter banks, and Ge-photodetectors. Results towards a 40GSa/sec sampling system with 8bits resolution
are presented.
Combining optical and electron beam exposures on the same wafer level is an attractive approach for extending
the usefulness of current generation optical tools. This technique requires high-performance hybrid resists that perform
equally well with optical and e-beam tools. In this paper Rohm and Haas EPICTM 2340, a 193-nm chemically amplified
photoresist, is used in a hybrid exposure role. The e-beam tool was used to pattern 45 nm half-pitch features and a 193-
nm immersion stepper was used to pattern 60-nm half-pitch features in the same resist layer. The effects of processing
parameters and delay times were investigated.
Advances in femtosecond lasers and laser stabilization have led to the development of sources of ultrafast optical pulse
trains that show jitter on the level of a few femtoseconds over tens of milliseconds and over seconds if referenced to
atomic frequency standards. These low jitter sources can be used to perform opto-electronic analog to digital conversion
that overcomes the bottleneck set by electronic jitter when using purely electronic sampling circuits and techniques.
Electronic Photonic Integrated Circuits (EPICs) may enable in the near future to integrate such an opto-electronic
analog-to-digital converters (ADCs) completely. This presentation will give an overview of integrated optical devices
such as low jitter lasers, electro-optical modulators, Si-based filter banks, and high-speed Si-photodetectors that are
compatible with standard CMOS processing and which are necessary for the implementation of EPIC-chips for advanced
opto-electronic ADCs.
Lincoln Laboratory has designed and fabricated a charge-coupled device (CCD) array capable of imaging both polarization and illumination uniformity. The device consists of an 1107-element linear array of UV-optimized silicon photodiodes readout by a three-stage CCD through a single ~1-MHz output amplifier. This yields an effective clock rate for the whole array of ~1 kHz. Each of the active diode surfaces within the 1107-element array is covered by a UV-opaque layer of polysilicon into which are patterned 140-nm, transmissive sampling slits. The orientation and location of the slits enables simultaneous determination of illumination uniformity, degree and direction of polarization, and polarization uniformity. The device is tested with a 193-nm excimer laser equipped with variably polarized illumination and the theoretical performance of the device was supported by finite-difference time domain optical simulations.
Lincoln Laboratory has designed and fabricated a charge-coupled device (CCD) array capable of imaging both polarization and illumination uniformity. The device consists of an 1107-element linear array of UV-optimized silicon photodiodes read out by a three-stage CCD through a single ~1-MHz output amplifier. This yields an effective clock rate for the whole array of ~1 kHz. Each of the active diode surfaces within the 1107-element array is covered by a UV-opaque layer of polysilicon into which are patterned 140-nm, transmissive sampling slits. The orientation and location of the slits allows simultaneous determination of illumination uniformity, degree and direction of polarization, and polarization uniformity. The device was tested with a 193-nm excimer laser equipped with variably polarized illumination and the theoretical performance of the device was supported by finite-difference time domain optical simulations.
A monolithically integrated multi-element photodiode array with 7749 discrete detectors has been fabricated where each sensing element is equipped with a sampling aperture to allow for aerial image measurements with high spatial precision in the focal plane of lithographic lenses. As currently configured, any one of seven 1107-element linear arrays can be used at a given time to allow sampling across the long axis of a scanner lens. The individual elements are located 24 microns apart and are broken down into 27 sets of 41 distinctly different aperture types, with each set spaced 1 mm apart. In addition, the sampling apertures on the device are small enough to allow the device to act as a polarization sensor with high (<50 microns) spatial resolution. The high speed analog output amplifier allows for complete 1107-element images to be obtained at the full repetition rate of lithographic lasers (2KHz).
An attenuating phase shifting mask has been designed, fabricated, and tested at 157 nm. It consists of two layers, a metal attenuator and a transparent phase shifter. The metal, platinum, was chosen for its chemical and radiation stability. The phase shifter was a commercial spin-on glass. A single step of pattern transfer has been implemented, which significantly simplifies the fabrication process of the mask. The lithographic advantage in increased depth of focus was demonstrated for 130-nm spaces and contacts, and it was found to agree with numerical simulations.
The aggressive scaling in critical dimensions, coupled with the increasing use of subresolution features in optical proximity correction (OPC), dictates that maskwriters should have at their disposal electron beam resists capable of printing 100-nm OPC features on 280-nm design rule masks (70-nm features on the wafer). There is a need to survey commercial chemically amplified resists for use as mask making resists, and for completeness, such a survey would require that each resist be compared with an optimized resist process. To accomplish this task in a acceptable time period we have chosen to perform electron beam lithography modeling to quickly identify the resist process combinations that will lead to superior resist performance. We have used the technique of combining electron beam resist modeling with lithography to screen chemically amplified resists for use in electron beam mask making. This was accomplished by comparing experimentally determined resist sensitivities and profiles with those predicted from ProBeam /3D lithography modeling software.
A new resolution enhancement technique for photolithography that makes use of dark-field (DF) illumination was recently reported. In a DF projector, light from the condenser illuminates the reticle at such a steep angle that zero order light is lost from the system. The mask for a DF projector contains a series of sub-resolution gratings that diffract light into the lens and define features so be printed on the wafer. These sub-resolution gratings can be employed to precisely control the amplitude, phase and direction of light diffracted from each point on the mask. Given such precise control, many effect is can be produced with DF lithography that are not possible with enhanced optical lithography, e.g., terminating lines and resolving phase conflicts.
We have dramatically increased the sensitivity of a technique for the rapid inspection of EUV multilayer-coated mask blanks. In this technique an EUV sensitive resist is applied directly to a mask blank which is then flood exposed with EUV light and partially developed. Reflectivity defects in the mask blank results in mounds in a partially developed positive resists that appear as high contrast objects in a standard Nomarski microscope. The use of a higher contrast resist is shown experimentally to result in the creation of dramatically taller mounds. A simple model for the exposure and development of the resists has been developed and the predictions of the model compare well with the experimental results.
We have developed a new technique for at-wavelength inspection of EUV mask blanks. In this technique a thin layer of EUV photoresist is applied directly to a mask blank which is then flood exposed with EUV light and partially developed. We have demonstrated using atomic force microscopy (AFM) that a change in reflectance of only 12% can cause an approximately 50 Angstrom mound in ZEP photoresist. We propose that such mounds could be detected by existing optical inspection tools, and have demonstrated the detection of similar features in ZEP (created using electron beam lithography) by an automated darkfield optical scanning instrument. The greatest advantage of the technique is speed, since both steps (EUV flood exposure and optical inspection) can be done rapidly. Difficulties, such as contamination and resist roughness, are discussed along with possible solutions.
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