Short wave infrared (SWIR) imaging systems have several advantages due to the
spectral content of the nightglow and better discrimination against camouflage.
Achieving single photon detection sensitivity can significantly improve the image quality
of these systems. However, the internal noise of the detector and readout circuits are
significant barriers to achieve this goal. One can prove that the noise limitations of the
readout can be alleviated, if the detector exhibits sufficiently high internal gain.
Unfortunately, the existing detectors with internal gain have a very high noise as well.
Here we present the recent results from our novel FOcalized Carrier aUgmented Sensor
(FOCUS). It utilizes very high charge compression into a nano-injector, and subsequent
carrier injection to achieve high quantum efficiency and high sensitivity at short infrared
at room temperature. We obtain internal gain values exceeding several thousand at bias
values of less than 1 volt. The current responsivity at 1.55 μm is more than 1500 A/W,
and the noise equivalent power (NEP) is less that 0.5 x10-15 W/Hz1/2 at room temperature.
These are significantly better than the performance of the existing room temperature
devices with internal gain. Also, unlike avalanche-based photodiodes, the measured
excess noise factor for our device is near unity, even at very high gain values. The stable
gain of the device combined with the low operating voltage are unique advantages of this
technology for high-performance SWIR imaging arrays.
Bandwidth of a traveling-wave photodetector (TWPD) is limited by the optical absorption coefficient, velocity and impedance mismatches, and the drift time of photo-generated carriers in the intrinsic region. In these parameters, velocity and impedance mismatches have much influence on the bandwidth of TWPD. In this paper, we focus on mismatches, and propose a novel design to enhance the bandwidth. In the new structure, the thickness of a ground electrode increases as much as the ridge thickness. It forces the structure to have characteristics similar to a coplanar waveguide. We simulate this structure by finite different time domain Method in three dimensions and look-over frequency dependent parameters by Fourier transform for the detailed analysis of microwave characteristics such as characteristic impedance, microwave effective index, and microwave attenuation of TWPD. As a result, we obtain 50 (Omega) impedance matching and 89.7 % velocity matching using our novel structure.
We present mushroom-type TWEAM, which has improved velocity mismatch, with optimized impedance match compared to conventional modulators by reducing the distance between signal and ground metal line. In this paper, the layer structure of mushroom-type TW MQW EAM is designed for the operation of 1.55 um and optical index of active layer is designed to be 3.6. Also, we simulate an 1.55 um InGaAs/InGaAsP traveling-wave multiple quantum well electro- absorption modulator using 3D Finite Difference Time Domain method. Also, we investigate microwave characteristics in detail.
We present numerical analysis of traveling-wave (TW) multiple quantum well (MQW) electro-absorption modulator which can be used for wide-band applications, covering DC to 30 GHz or higher frequencies. Considerations in design of TW modulators are microwave characteristics such as, waveguide attenuation and phase velocity matching between guided lightwave and microwave. In this study, we simulate a 1.3 micrometer InGaAs/InGaAsP TW MQW EAM using the 3D Finite Difference-Time Domain method, and investigate frequency dependent parameters by using the Fourier transform for analysis of microwave characteristics in detail. We identify that as the distance between signal and ground electrode increase, the characteristic ridge-type TW EAM change from planar CPW to that of microstrip structure. It is believed that our calculated data provide useful information to optimize and fabricate ridge-type TW CPW EAM.
We study the effects of microwave loss and device length on the nonlinear characteristics of intensity modulation response in 1.55 micrometer traveling-wave coplanar waveguide InGaAsP bulk electro-absorption modulator. By using device length segmentation scheme optical transmission curve reflecting the change of the electro-absorption effects at each segmented position due to the microwave loss is obtained, and then the intermodulation distortion and spurious free dynamic range characteristics of RF signal are analyzed. Device length decreases Vb3, which is the bias voltage minimizing the third order and increases the third order intermodulation distortion (IMD3). On the other hand, microwave loss increases Vb3, and reduces IMD3.
Bandwidth of traveling-wave photodetectors is limited not only by the optical absorption coefficient and the velocity mismatches, but also by the drift time of photo-generated carriers in the i-layer. The bandwidth limitation effect of the optical absorption coefficient and the velocity mismatches have already been considered and analyzed by presenting the velocity-mismatch impulse response. In this paper, we present a modified velocity-mismatch impulse response considering the effect of the transit time. The modified impulse response is modeled by the optical waveguide analysis and the segmentation of i-layer. The frequency impulse response is also obtained by the FFT of the modified impulse response. It is found that bandwidth limitation effect of the transit time is more serious than the effect of velocity mismatch, especially at high frequency.
We investigate the microwave characteristics of traveling- wave photodetector using the finite-difference time-domain method. This method enables us to consider the full configuration of the device to be studied with no assumption. Therefore we obtain accurate simulation result, though the geometry of ridge-type coplanar waveguide photodetector is complicated. Physical phenomena such as attenuation and dispersion are shown in time domain. We offer two design parameters, the width of PIN region and the thickness of i-layer, and analyze TWPDs property in frequency domain. Microwave loss, characteristic impedance, and RF refractive index are shown and we find out that there exists a tradeoff condition for design.
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