The abilities of operating and maintaining optical instruments are crucial in modern society. Besides the basic knowledge in optics, the optics courses in the National University of Defense Technology also focus on the training on handling typical optical equipment. As the link between classroom courses on applied optics and the field trips, the integrated design course of applied optics aims to give the students a better understanding on several instantly used optical equipment, such as hand-held telescope and periscope, etc. The basic concepts of optical system design are also emphasized as well. The course is arranged rightly after the classroom course of applied optics and composed of experimental and design tasks. The experimental tasks include the measurements of aberrations and major parameters of a primitive telescope, while in the design parts, the students are asked to design a Keplerian telescope. The whole course gives a deepened understandings on the concepts, assembling, and operating of telescopes. The students are also encouraged to extend their interests on other typical optical instruments.
Precise simulation of transient electrical behaviors of photodetectors under laser irradiation is becoming an increasingly concern. It not only can allow a detailed study and analysis of complex phenomena that cannot be carried out by experiments, but gives valuable information about the physical mechanisms which ultimately determine the response of the photodetectors. Finite difference numerical technique is adopted in the simulation to calculate the current response of photodetectors under pulsed laser irritation in this paper. To simulation the behaviors of photodetectors under pulsed laser irritation, the transport and trapping of carries and external circuit effects, including load resistance, junction capacitance, and parasitic capacitance, are considered. The basic equations governing the carrier behaviors are solved, including Poisson’s equation, the carrier motion equations, and the carrier continuity equations. The simulated transient carrier density and velocities are present, as well as corresponding transient electric field distributions. The behaviors of electrons and holes and its contributions to the external current response are analyzed. Then a general and brief image of the transient progress of photodetectors under pulsed laser irritation is established. How the carrier is induced, transported, and trapped and whether they make any significant contribution to the external current response are discussed. Besides, bias dependent response is also studied. Higher bias will improver the behaviors of photodetectors under pulsed laser irritation. The simulated results and theory analysis will show valuable clue for future research on the behaviors of photodetectors irradiated by pulsed laser.
In the experiments of photovoltaic detectors illuminated by CW lasers, some new mechanism has been
found, such as power saturation of photovoltage, hot carrier effect, as well as thermovoltage effect. To
investigate whether there is similar mechanism with pulsed laser irradiating, an 808nm femtosecond
pulsed laser is adopted. In the experiments, three photovoltaic infrared detectors are used, namely short
wavelength HgCdTe detector, medium wavelength HgCdTe detector and medium wavelength InSb
detector. Actually, the 808nm pulsed laser is spectral related laser for short wavelength HgCdTe
detector while spectral unrelated laser for medium wavelength HgCdTe and InSb detector. Under
various power densities, the detectors have a series of outputs. Power saturation of photovoltage is
observed. However, the characteristics of the outputs of these three detectors are quite different, even
between medium wavelength HgCdTe and InSb detector, which have the same packing method. There
are three major contributions in the paper. Firstly, explain the mechanism of power saturation of
photovoltage, mainly from hot carrier effect and the depressed ability of PN junction to separate
electrons and holes with the higher temperature induced by the laser. Secondly, compare the differences
between medium wavelength HgCdTe and InSb detector and give a qualitative analysis. Finally, the
difference of the outputs between short and medium wavelength HgCdTe detector is compared and
qualitatively analyzed, too, with the different mechanisms of interaction between infrared detectors and
spectral related or spectral unrelated laser. The experimental results and theory analysis will show
valuable clue for future research on photovoltaic detector irradiated by pulsed laser.