Seiji Kurashina

Detection of terahertz radiation from quantum cascade laser, using vanadium oxide microbolometer focal plane arrays

The authors tried real-time imaging of THz radiation from Quantum Cascade Laser (QCL), using vanadium oxide (VOx) microbolometer focal plane arrays (FPAs) of 320x240 with pitches of 37 μm and 23.5 μm as well as 640x480 with 23.5μm pitch. The QCL has such parameters as 3.1 THz emission frequency (97μm in wavelength), 300-400 nsec pulse width, 1.07 msec repetition period, 30 mW peak intensity, 15K operation temperature. The THz radiation from QCL is collimated by off-axis parabola (OAP) and focused on FPA by another OAP. The 10 μm range infrared radiation from scene is blocked by sapphire disk or metal mesh filter. Noise Equivalent Power (NEP) at 3.1 THz is estimated to be 200~400 pW.

Development of Terahertz focal plane arrays and handy camera

Uncooled Terahertz (THz) focal plane array (FPA), 320x240 format-23.5 μm pitch, and THz imager were developed. There are two types of THz-FPAs, i.e., broad-band type and narrow-band type. Since broad-band type THz-FPA was developed, a couple of modifications have been made to improve Noise Equivalent Power. The narrow-band type THz-FPA has such a new structure that Si cover is put above thermal isolation structure of broad-band type THz-FPA at a distance of half of wavelength of interest. Measurements on responsivities of narrow-band type FPAs show enhancement by a factor of ca. 3. Lock-in imaging technique has been developed, which increases signal-to-noise ratio as a function of square root of the number of frames of integration. Both passive and active THz imaging experiments were finally described.

Palm-size and real-time terahertz imager, and its application to development of terahertz sources

This paper describes features of uncooled palm-size and real-time Terahertz (THz) imager. The THz imager and powerful THz quantum cascade laser were assembled into THz microscope with which THz images of narrow string were obtained at 4.3 and 2.0 THz. The analyses on these images show that spatial resolutions evaluated at two frequencies are consistent with Fraunhofer diffraction limit. THz imager has been applied to investigate beam patterns for a variety of THz sources. The experimental results on beam patterns show that THz imager plays an important role in developing THz sources. A method for reducing non-uniformity due to strong coherency of THz sources is finally presented.

New thermally isolated pixel structure for high-resolution (640×480) uncooled infrared focal plane arrays

A new pixel structure with twice-bent beams and eaves structure, suitable for high-resolution uncooled infrared (IR) focal plane arrays (FPAs), is proposed. In comparison with previous results (FPA of 37-µm pixel pitch), the thermal conductance of the test device with the proposed pixel structure of 23.5-µm pitch is reduced about 2.5 times. The eaves structure, which is adopted to increase the fill factor of pixels, improves the responsivity by a factor of 1.3. A 640×480 bolometer-type uncooled IRFPA is demonstrated by utilizing the new pixel structure, with supplementary modification to improve thermal conductance and thermal time constant. It shows a noise equivalent temperature difference (NETD) of 50 mK for F/1.0 optics at 30 frames/sec, a thermal conductance of 0.03 µW/K, and a thermal time constant of 16 msec.

Performance of 320x240 uncooled bolometer-type infrared focal plane arrays

The performance of a 320 x 240 bolometer-type uncooled infrared (IR) Focal Plane Array (FPA) is described. Vanadium oxide thin film is adopted for the bolometer material, having a sheet resistance of approximately 10 kohms/square. It is patterned such that the bolometer resistance is by a factor of 10 larger than the sheet resistance. On-chip readout integrated circuit (ROIC) is designed to reduce signal drift, extend dynamic range for object temperature and extend ambient temperature range in which operates non-uniformity correction is carried out with about 1/10 fewer frequency than the former ROIC.The 320 x 240 FPA consists of pixels sensitive to IR radiation and optical black (OB) pixels covered with plate which shuts out IR radiation. Drift is reduced by current mirror circuit, using the OB pixels and digital compensation circuit based on voltage change of OB pixels resulting from change in operation temperature. Both the dynamic range and the ambient temperature range are extended by decreasing integration gain and developing low-noise, low-power and large swing operational amplifier(OP-AMP). Since decrease in integration gain degrades noise equivalent temperature difference (NETD), bias voltage for bolometer is increased by factor of 2 and bandwidth is reduced by route half. Finally, IR image was obtained with prototype camera and NETD value was found to be smaller than 0.1K for F/1 optics at 60Hz frame rate and thermal time constant was measured to be 12 msec.

New thermally isolated pixel structure for high-resolution uncooled infrared FPAs

This paper proposes a new thermally isolated pixel structure, having a twice-bent beam structure and eaves structure, suitable for high-resolution uncooled infrared (IR) focal-plane arrays (FPAs). It also describes the properties of test devices, fabricated to verify the effect of the new pixel structure. Although the pixel size of the test devices is 23.5 μm × 23.5 μm, which represents a smaller area by a factor of about 2.5 than the 37 μm × 37 μm pixel size for the 320 × 240 bolometer-type uncooled IRFPA, previously developed by the authors, the test devices have beams with almost the same length as in the previous IRFPA by utilizing the new beam structure. In addition, the cross-sectional area of the beam is reduced. Accordingly, the thermal conductance of the test devices can be reduced by a factor of about 2.5. The eaves structure, which is adopted to increase the fill factor of pixels, improves the responsivity by a factor of 1.3, which is consistent with our calculations. By utilizing the new thermally isolated pixel structure, the test devices with 23.5 μm pixels enable us to achieve thermal sensitivity equivalent to the previous 37 μm pixels.

High-Sensitivity and Broadband, Real-Time Terahertz Camera Incorporating a Micro-Bolometer Array With Resonant Cavity Structure

Terahertz (THz) cameras comprising an uncooled micro-bolometer array have been developed for simple THz imaging, and the improvement of their sensitivity is one of the important issues. We fabricated a new micro-bolometer array with a resonant cavity structure for a real-time THz camera, alongside a new method for evaluating the sensitivity across a wide range of the THz frequency region. The frequency dependence of the sensitivity of the THz camera is measured in the 0.5-2.0-THz frequency range taking the polarization dependence into account. It was found that the resonant cavity structure effectively increased the sensitivity of the THz camera, and, actually, the improvement by one order of magnitude was achieved in the frequency range below 1 THz. The THz camera with much enhanced sensitivity will expand the frontiers of real-time THz imaging such as molecular imaging and nondestructive inspection.

Uncooled infrared detectors toward smaller pixel pitch with newly proposed pixel structure

Abstract. An uncooled infrared (IR) focal plane array (FPA) with 23.5 μm pixel pitch has been successfully demonstrated and has found wide commercial applications in the areas of thermography, security cameras, and other applications. One of the key issues for uncooled IRFPA technology is to shrink the pixel pitch because the size of the pixel pitch determines the overall size of the FPA, which, in turn, determines the cost of the IR camera products. This paper proposes an innovative pixel structure with a diaphragm and beams placed in different levels to realize an uncooled IRFPA with smaller pixel pitch (≦17  μm). The upper level consists of a diaphragm with VOx bolometer and IR absorber layers, while the lower level consists of the two beams, which are designed to be placed on the adjacent pixels. The test devices of this pixel design with 12, 15, and 17 μm pitch have been fabricated on the Si read-out integrated circuit (ROIC) of quarter video graphics array (QVGA) (320×240) with 23.5 μm pitch. Their performances are nearly equal to those of the IRFPA with 23.5 μm pitch. For example, a noise equivalent temperature difference of 12 μm pixel is 63.1 mK for F/1 optics with the thermal time constant of 14.5 ms. Then, the proposed structure is shown to be effective for the existing IRFPA with 23.5 μm pitch because of the improvements in IR sensitivity. Furthermore, the advanced pixel structure that has the beams composed of two levels are demonstrated to be realizable.

A real-time terahertz imaging system consisting of terahertz quantum cascade laser and uncooled microbolometer array detector

Terahertz imaging has attracted much attention in recent years, because the technique can be applied to many application fields such as nondestructive analysis and imaging method through opaque materials. A terahertz real-time imaging technique (Terahertz Camera) considered increasingly important in the future has been developed. The terahertz camera consists of a light source (Terahertz quantum cascade laser) and an un-cooled micro-bolometer array, which can easily get real-time terahertz-image. As an application of the terahertz camera, a stand-off imaging system that could be useful in a fire disaster relief and a label-free bio-materials detection system have developed and demonstrated.

Uncooled infrared detector with 12μm pixel pitch video graphics array

Uncooled infrared detectors with 12μm pixel pitch video graphics array (VGA) have been developed. To improve the signal to noise ratio (SNR) for 12μm pixel pitch, a highly sensitive bolometer material, an advanced pixel structure for thermal isolation and a newly designed read-out IC (ROIC) have been also developed. The bolometer material has been improved by using vanadium niobate. Over a wide range of temperature, temperature coefficient of resistance (TCR) is achieved higher level than -3.6%/K, which is 2 times higher than that for the conventional bolometer material. For thermal isolation, thermal conductance (Gth) value for the new pixel structure, fabricated by using triple level sacrificial layer process, is estimated to be 5nW/K, which is 1/5 times lower than that for the conventional pixel structure. On the other hand, since the imaging area is reduced by the pixel pitch, the uniformity of pixel can be improved. This enables to remove the non-uniformity correction (NUC) circuit in the ROIC. Removal of this circuit is effective for low power and low noise. This 12μm pixel pitch VGA detector is packaged in a compact (24 × 24 × 6.5 mm) and lightweight (11g) ceramic package. In addition, it has been incorporated in a newly developed prototype miniature imager. The miniature imager has dimension of 25(H) ×25(W) ×28(L) mm and weight of 30g. This imager is compact and small enough to fit in your hand. Hereafter, this imager is greatly expected to be applied to mobile systems.