Imaging in the Terahertz (THz) and millimeter-wave (mm-wave) bands offer advantages over doing do in other conventional bands, such as the visible, infrared (IR). The THz band ranges from 300 GHz to 3 THz or in wavelength, 1 mm to 100 μm. These longer wavelengths allows THz radiation to pass unobscured through some materials allowing for imaging hidden threats or defects within such materials. Going further, millimeter-waves cover the spectral band of 30 – 300 GHz, or 10 cm to 1 mm. In addition to passing through denser materials, they also have much less atmospheric absorption, thus are ideal for imaging in adverse weather conditions.
In the THz/mm-wave, the greatest challenge to real-time active imaging was previously the lack of compact sensor arrays. INO has overcome this by optimizing its microbolometer focal plane array (originally developed for the infrared) for the longer wavelengths, covering both the THz and mm-wave bands. The remaining challenge for active imaging is how to obtain useful imagery using coherent sources. INO has been working on improving the quality of the illumination beam over the past few years, as well as designing high quality fast imaging optics. This paper will focus on the different techniques that have been tested across the THz and into the mm-wave bands in both transmission and reflection imaging modes. The impact on image quality will be demonstrated, and their implications to developing useful systems for different applications will be discussed.
This paper reviews recent developments in customized packaged bolometers at INO with an emphasis on their applications. The evolution of INOs bolometer packages is also presented. Fully packaged focal plane arrays of broadband microbolometers with expanded absorbing range are shown, for applications in spectroscopic and THz imaging. This paper also reports on the development of customized packaged bolometer focal plane arrays (FPAs) for space applications such as a multispectral imaging radiometer for fire diagnosis, a far infrared radiometer for in-situ measurements of ice clouds and a net flux radiometer for Mars exploration.
Subwavelength imaging has recently seen increased interest in multiple fields. There are various applications and distinct contexts for performing subwavelength imaging. The technological ways to proceed as well as the benefits obtained are as various as the applications foreseen. To benefit from subwavelength imaging a way around standard imaging procedure is often required.
INO is also involved in this activity mainly for the infrared and the THz wavebands. In the infrared band a detector with 17 um pixel pitch, larger than the pixel, was used in conjunction with a microscanning device to oversample the image at a pitch much smaller than the wavelength. In this case the pixel size is in the order of the wavelength but the sampling is at subwavelength level. In the THz band a 35 um pixel pitch is used at wavelength ranging from 70 um to 1,063 mm to perform imaging through various objects. In this case, the pixel itself is smaller than the wavelength.
Subwavelength imaging is not without its challenges, though. For instance, while the use of ultra-fast optics provides better definition, their design becomes more challenging as the models used are at their very limits. Questions about information content of images can be raised as well. New research avenues are being investigated to help address the challenges of subwavelength imaging with the goal of achieving higher imaging system performance. This paper discusses aspects to be considered, review some results obtained and identify some of the key issues to be further addressed.
This paper reports on the development of a fully packaged focal plane array of broadband microbolometers. The detector makes use of a gold black thin film to expand its absorption range from 3 to 14 μm. A low temperature packaging process was developed to minimize sintering of the gold black absorber during vacuum sealing of the bolometer array package. The gold black absorber was also laser trimmed to prevent lateral diffusion of heat and promote a better MTF. The resulting FPAs show a NETD lower than 25 mK at a frame rate of 50 Hz
We report on the design and instrumentation of an aircraft-certified far infrared radiometer (FIRR) and the resulting instrument characteristics. FIRR was designed to perform unattended airborne measurements of ice clouds in the arctic in support of a microsatellite payload study. It provides radiometrically calibrated data in nine spectral channels in the range of 8-50 μm with the use of a rotating wheel of bandpass filters and reference blackbodies. Measurements in this spectral range are enabled with the use of a far infrared detector based on microbolometers of 104-μm pitch. The microbolometers have a new design because of the large structure and are coated with gold black to maintain uniform responsivity over the working spectral range. The vacuum sealed detector package is placed at the focal plane of a reflective telescope based on a Schwarschild configuration with two on-axis spherical mirrors. The telescope field-of-view is of ~6° and illuminates an area of ~2.1-mm diameter at the focal plane. In operation, FIRR was used as a nonimaging radiometer and exhibited a noise equivalent radiance in the range of 10-20 mW/m2-sr. The dynamic range and the detector vacuum integrity of FIRR were found to be suited for the conditions of the airborne experiments.
We have designed and numerically simulated a novel spot size converter for coupling standard single mode fibers with 10.4μm mode field diameter to 500nm × 220nm SOI waveguides. Simulations based on the eigenmode expansion method show a coupling loss of 0.4dB at 1550nm for the TE mode at perfect alignment. The alignment tolerance on the plane normal to the fiber axis is evaluated at ±2.2μm for ≤1dB excess loss, which is comparable to the alignment tolerance between two butt-coupled standard single mode fibers. The converter is based on a cross-like arrangement of SiOxNy waveguides immersed in a 12μm-thick SiO2 cladding region deposited on top of the SOI chip. The waveguides are designed to collectively support a single degenerate mode for TE and TM polarizations. This guided mode features a large overlap to the LP01 mode of standard telecom fibers. Along the spot size converter length (450μm), the mode is first gradually confined in a single SiOxNy waveguide by tapering its width. Then, the mode is adiabatically coupled to a SOI waveguide underneath the structure through a SOI inverted taper. The shapes of SiOxNy and SOI tapers are optimized to minimize coupling loss and structure length, and to ensure adiabatic mode evolution along the structure, thus improving the design robustness to fabrication process errors. A tolerance analysis based on conservative microfabrication capabilities suggests that coupling loss penalty from fabrication errors can be maintained below 0.3dB. The proposed spot size converter is fully compliant to industry standard microfabrication processes available at INO.
In recent years, smart phone applications have both raised the pressure for cost and time to market reduction, and the
need for high performance MEMS devices. This trend has led the MEMS community to develop multi-die packaging of
different functionalities or multi-technology (i.e. wafer) approaches to fabricate and assemble devices respectively. This
paper reports on the fabrication, assembly and packaging at INO of various MEMS devices using heterogeneous
assembly at chip and package-level. First, the performance of a giant (e.g. about 3 mm in diameter), electrostatically
actuated beam steering mirror is presented. It can be rotated about two perpendicular axes to steer an optical beam within
an angular cone of up to 60° in vector scan mode with an angular resolution of 1 mrad and a response time of 300 ms. To
achieve such angular performance relative to mirror size, the microassembly was performed from sub-components
fabricated from 4 different wafers. To combine infrared detection with inertial sensing, an electroplated proof mass was
flip-chipped onto a 256×1 pixel uncooled bolometric FPA and released using laser ablation. In addition to the microassembly
technology, performance results of packaged devices are presented. Finally, to simulate a 3072×3 pixel
uncooled detector for cloud and fire imaging in mid and long-wave IR, the staggered assembly of six 512×3 pixel FPAs
with a less than 50 micron pixel co-registration is reported.
Femtosecond laser is used to form three-dimensional (3D) microstructures embedded in Foturan, a photosensitive glass. The microstructures are realized using a three steps process including infrared femtosecond exposure, heating process and etching in an ultrasonic solution of hydrofluoric acid in water. The experiments were carried out using a specially designed ultrafast laser micromachining station, which included a femtosecond laser (Spectra Physics, 110fs, 800nm, 1 mJ/pulse at repetition rate of 1kHz), systems for the delivery, high-precision focusing and spatial-temporal control of the laser beam, and a fully automated and programmed system for the precise target positioning over a prescribed 3D trajectory. Microstructures were compared to those obtained with excimer laser micromachining. Efficiency of the fabrication process will be discussed in terms of the various laser and etching fabrication parameters. This process has some potential interest for the fabrication of 3D microfluidic systems.
A microfabrication system with the use of a femtosecond laser was designed for 3D processing of industrially important materials. The system includes a 120 fs, 1 kHz laser; beam delivery and focusing system, systems for automated 3D target motion and real-time imaging of the sample placed in a vacuum chamber. The first tests of the system on the processing of stainless steel and silicon are presented. We established thresholds and regimes of ablation for both materials. It was found that at relatively low laser fluences I < 3-5 J/cm2 the regime of “gentle” ablation takes place, which is characterized by exceptional quality of the ablated surface, but slow ablation rate (< 25 nm/pulse). This regime is especially efficient for the patterning of markers on steel or silicon surfaces. The “fast” ablation regime at I > 10 J/cm2 provides much higher ablation rate of 30-100 nm/pulse, giving an opportunity of fast high-quality processing of materials. This regime is well suited for drilling of through holes and fast cutting of materials. However, it was found that fast ablation regime imposes additional requirements on the quality of delivery and focusing of the laser beam because of the presence of parasitic ablation around the main spot on the tail of the radiation intensity distribution. As industrial machining examples, we demonstrate heat-affected-zone free drilling of through holes in a 50 μm thick stainless steel foil and cutting of a 50 μm thick Si wafer with a net cutting speed of 8 μm/sec.