A high performance imaging system for diffuse optical tomography is disclosed. A dense grid utilizing sources, e.g., light emitting diodes ('LEDs'), that achieve high performance at high speed with a high dynamic range and low inter-channel crosstalk are complemented by a system of discrete, isolated receivers, e.g., avalanche photodiodes ('APDs'). The source channels have dedicated reconfigurable encoding control signals, and the detector channels have reconfigurable decoding, allowing maximum flexibility and optimal mixtures of frequency and time encoding and decoding. Each detector channel is analyzed by dedicated, isolated, high-bandwidth receiver circuitry so that no channel gain switching is necessary. The resulting improvements to DOT system performance, e.g., increased dynamic range and decreased crosstalk, enable higher density imaging arrays and provide significantly enhanced DOT image quality. A processor can be utilized to provide sophisticated three dimensional modeling as well as noise reduction.

High performance imaging system for diffuse optical tomography and associated method of use

A photon-counting Geiger-mode avalanche photodiode intensity imaging array includes an array of pixels (200), each having an avalanche photodiode (250). A pixel senses an avalanche event and stores, in response to the sensed avalanche event, a single bit digital value therein. An array of accumulators (320) are provided such that each accumulator is associated with a pixel. A row decoder circuit (310) addresses a pixel row within the array of pixels. A bit sensing circuit (300) converts a precharged capacitance into a digital value during read operations.

Digital photon-counting geiger-mode avalanche photodiode solid-state monolithic intensity imaging focal-plane with scalable readout circuitry

A low light level system is described wherein electrons, created by images directed to a photocathode (31), directly bombard active pixel sensors. Both the active pixel sensors and the output end of the photocathode are positioned in a vacuum envelop. The electron released from the photocathode (31), are directed to and at the active pixel sensors (32). The output of the active pixel sensor (32) detector may be stored, viewed or displayed at a remote location. Various applications are described including a novel camera.

Electron bombarded active pixel sensor

Hybrid Photodiodes (HPD) represent one of the most promising options for high granularity single photon detection. HPD's are photodetectors consisting of a photocathode, which is deposited on the inner side of the entrance window, and a solid state sensor encapsulated in a vacuum envelope. HPD's combine the high sensitivity of photomultiplier tubes with the excellent space and energy resolution of solid state detectors. After reviewing the physical principles of HPD's the article gives a short overview of the history of this detector. A number of commercially available devices will be discussed. Current and future applications in high energy physics lie in the fields of scintillator readout (calorimetry, fibre tracking) and Cherenkov light detection. It is for the latter application that various large area Hybrid Photodiodes are currently under development.

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Large area hybrid photodiodes

We report benchmark tests of a new single-photon counting detector based on a GaAsP photocathode and an electron-bombarded avalanche photodiode developed by Hamamatsu Photonics. We compare its performance with those of standard Geiger-mode avalanche photodiodes. We show its advantages for FCS due to the absence of after-pulsing and for fluorescence lifetime measurements due to its excellent time resolution. Its large sensitive area also greatly simplifies setup alignment. Its spectral sensitivity being similar to that of recently introduced CMOS SPADs, this new detector could become a valuable tool for single-molecule fluorescence measurements, as well as for many other applications.

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Hybrid photodetector for single-molecule spectroscopy and microscopy.

In order to be utilized in such application fields as high energy physics or medical imaging, where a huge number of photodetectors are assembled in designated small area, the world's smallest hybrid photodetector (HPD), the compact HPD, has been developed. The overall diameter and the length of the tube are 16 mm and 15 mm, respectively. The effective photocathode area is 8 mm in diameter. At applied voltage of -8 kV to the photocathode, the electron multiplication gain of a PD incorporated HPD (PD-HPD) is 1,600, and that of an APD (APD-HPD) is 65,000. In the pulse height distribution measurement, photoelectron peaks up to 6 photoelectrons are clearly distinguishable with the APD-HPD. Experiments established that there was no degradation of gain in magnetic fields up to 1.5 T, an important performance characteristic of the compact HPD for application in high energy physics.

A compact hybrid photodetector (HPD)

An optical measurement apparatus (10) comprises a photon detection unit (12) that detects incident photons, a time signal output unit (14) that outputs a time signal, and a storage unit (16) where the time signal outputted from the time signal output unit (14) is stored when photons are detected by the photon detection unit (12). The photon detection unit (12) comprises an HPD (24) having a photo-cathode (24a) and an APD (24b), a TZ amplifier (26), a peak-holding circuit (28), and an A/D converter (30). The time signal output unit (14) comprises a timer (32) and a counter (34). The storage unit (16) comprises a comparator (36) and a memory (38). When photons impinge on the HPD (24), a trigger signal is outputted from the comparator (36), and the photon count outputted from the A/D converter (30) and the time information outputted from the counter (34) are stored in the memory (38).

Optical measurement apparatus and method for optical measurement

This invention relates to an electron tube having a structure for enabling a stable operation for a long time In the electron tube, at least a confining mechanism is arranged between a photocathode and the electron incident surface of a semiconductor device, which are arranged to oppose each other In the arrangement, the area of the opening of the confining mechanism is at least equal to or smaller than that of the electron incident surface, thereby confining the orbits of photoelectrons from the photocathode This structure avoids bombardment of electrons arriving at portions other than the electron incident surface of the semiconductor device and prevents the semiconductor device from being unnecessarily charged

Electron tube having a photoelectron confining mechanism

A hybrid photodetector (HPD) combined with a III-V photocathode is considered to be an ideal photodetector, because the quantum efficiency (QE) is high, the collection efficiency is 100%, and the excess noise factor is negligible. As the first step, an HPD combined with a GaAs photocathode or a blue-enhanced GaAs (BE-GaAs) photocathode has been developed. Presently, the QE is approximately 20% from 550 nm to 850 nm, which is much higher than that of a conventional multi-alkali (S-20) photocathode. The 100% collection efficiency calculated by the computer simulation is supported by the test result of the flat uniformity. Considering the application in high energy physics using a wavelength shifter, energy resolutions of both the BE-GaAs-HPD and the conventional S20-HPD were measured using 650 nm LED light pulse. The BE-GaAs-HPD shows a resolution of 17%, finer than 34% of the S20-HPD. The experimental results are consistent with the theoretical estimation. The QE of the HPD is especially important, because the electron multiplication process is noiseless.

A hybrid photodetector (HPD) with a III-V photocathode

As a fundamental study of photodiodes (PDs) for electron bombardment, two types of PDs have been experimentally investigated to be applied in electron tubes. A PD bombarded from the front surface (FB-PD), where a pn junction of planar structure existed, was evaluated to measure the fast response characteristics such as a 2.1 ns rise time. However, more than an order of magnitude increase of dark current was also confirmed after a long term stability test of 1000 hours. On the other hand, a PD bombarded by electrons from the rear surface (RB-PD) showed no increase of dark current after the stability test and a fast rise time of 2.7 ns. However, it was clarified that the rise time of RB-PD depended on the applied voltage to the PD, and an applied voltage of 200 V was necessary to achieve such a fast response. Since it was a much higher voltage than expected, some modifications may be necessary to achieve a fast response with a lower applied voltage, considering the yield of the PDs. Comparisons of the two types of PDs for some other characteristics are discussed. Preliminary test results of an electron bombardment APD are also discussed. >

Tetsuya Morita

Papers

A compact hybrid photodetector (HPD)

Optical measurement apparatus and method for optical measurement

Electron tube having a photoelectron confining mechanism

A hybrid photodetector (HPD) with a III-V photocathode

Fundamental investigation of vacuum PD tube [photodiode]