Wavefront coding, a technique of optical-digital hybrid image, can be used to extend the depth of the field. However, it sacrifices the signal-to-noise ratio (SNR) of system at a certain degree, especially on focus situation. The on-focus modulation transfer function (MTF) of wavefront coding system is much lower than that of generally traditional optical system. And the noise will be amplified in the digital image processing. This paper analyzes characteristics of the SNR of the wavefront coding system in the frequency domain and calculates the rate of noise amplification in the digital processing. It also explains the influence of the image detector noise severely reducing the restored quality of images. In order to reduce noise amplification in the process of image restoration, we propose a modified wiener filter which is more suitable for restoration in consideration of noise suppression. The simulation experiment demonstrates that the modified wiener filter, compared with traditional wiener filter, has much better performance for wavefront coding system and the restored images having much higher SNR in the whole depth of the field.
In this paper we demonstrated a coherent raster-scan imaging system that can acquire phase information based on continuous terahertz imaging. It mixes the terahertz with a Fs-laser by a electro-optic crystal of ZnTe to make a hybrid modulation on the crystal to achieve continuous terahertz detection. In this way, it can not only propagate for a long distance but also achieve phase detection for continuous terahertz imaging. The surface images of objects that are under test can be obtained by the Backward-Wave Oscillator, which the output power is 10mW at 205.994GHz. With the repetition frequency of 80MHz, the output power of the MaiTai is 1.65W and 100fs pulse light at 800nm. The images can achieve diffraction-limited resolution approximately. And the simulated results show that the system can obtain phase imaging of test objects based on continuous terahertz source. The way to get the phase of the signal has significant meaning for coherent detection of continuous terahertz source.
Intense Terahertz waves generated from air-induced plasma and serving as broadband THz source provide a promising broadband source for innovative technology. Terahertz generation in selected gases has attracted more and more researchers’ interests in recent years. In this research, the THz emission from different atoms is described, such as nitrogen, argon and helium in Michelson. The THz radiation is detected by a Golay Cell equipped with a 6-mm-diameter diamond-inputting window. It can be seen in the first time that when the pump power lies at a stable level, the THz generation created by the femtosecond laser focusing on the nitrogen is higher than which focusing on the helium, and lower than that produced in the argon gas environment. We believe that the THz intensity is Ar > N > Ne because of its atomic mass, which is Ar > N > Ne as well. It is clear that the Gas molecular decides the release of free electrons ionized from ultra short femtosecond laser through the electronic dynamic analysis. The higher the gas mass is, the stronger the terahertz emission will be. We further explore the THz emission at the different laser power levels, and the experimental results can be commendably quadratic fitted. It can be inferred that THz emission under different gas medium environment still complies with the law of four-wave mixing (FWM) process and has nothing to do with the gas environment: the radiation energy is proportional to the quadratic of incident laser power.
Terahertz wave generation from air plasma induced by ultrashort laser pulses has been widely studied in the past decade. We report the study of terahertz wave generation from the laser induced plasma where there is a preformed air plasma. We found that the power of the terahertz wave generated by the main pump pulse decreases in the presence of the preformed plasma. The amount of the power drop increases with the power of pulse that generates preformed plasma. The result confirms the key role of tunneling ionization in the terahertz generation mechanism.
THz wave generation in laser-included plasma has attracted considerable attention recently and the intense THz waves generated from air-induced plasma, serving as broadband THz source for sensing and imaging applications, has attracted more and more researchers’ interests in recent years. In our experiment, terahertz is detected using THz Air-Biased Coherent Detection (THz-ABCD) method. This method can achieve the third-order nonlinear susceptibility tensor to produce field-induced optical second harmonic photons. In the THz-ABCD system, red-shift is observed in frequency spectra with enhanced pump power and decreased bandwidth. After changing the probe power, the red-shift in frequency spectral can also observed with enhanced probe power, but the bandwidth is broadening as the prober power increasing. We further explore of these phenomena through intense self-phase modulation of the optical pulse in the plasma and the collision behavior. This study reveals that we can control THz intensity and bandwidth by changing pump power and probe power in the ABCD system.
We report intense (~10 mW), ultra-broadband (~150 THz wide), terahertz-to-infrared, Gaussian-wavefront emission from nanopore-structured metallic thin films under femtosecond laser pulse irradiation. The proposed underlying mechanism is thermal radiation. The nanostructures of the metal film are produced by random holes in the substrate. Under pulse-train femtosecond laser irradiation, we found dramatically enhanced optical absorption, with an absorptivity that was equal to as much as 95% of the metallic surface nanostructure, due to both an antireflection mechanism and dissipation of excited surface plasmon polaritons into the metal surface.
Terahertz (THz) emission from laser-induced air plasma is a well known and widely used phenomenon. We report that when two laser beams from the laser creating two plasma filaments interact with each other, THz absorption is observed. We believe that a change in the refractive index of the plasma causes the THz-wave absorption. The following experimental results reveal that the THz absorption becomes more pronounced with increasing pump power and that the gas species surrounding the femtosecond laser filament can also influence the THz absorption rate.
Terahertz wave which can provide innovative sensing and imaging techniques can obtain spectroscopic
information unavailable at other wavelengths. The terahertz air-biased-coherent-detection (ABCD) method can achieve
the third-order nonlinear susceptibility tensor to produce field-induced optical second harmonic photons. Therefore, the intense terahertz wave generated and detected by the laser-induced air plasma provides a promising ultra-broadband terahertz source and sensor for spectroscopy and imaging technique. Aiming at that purpose, an understanding of the
frequency spectrum characterization of terahertz pulse is crucial. In this work, we investigated the variation of the THz
pulse bandwidth measured through the third harmonic generation using the coherent detection scheme, by increasing the
optical probe pulse power and biased electric field. A bandwidth broadening of the measured THz pulse is observed by
increasing either the probe pulse power or the bias voltage strength. We speculate that a pulse shape change of the probe beam and a saturation effect during the second-harmonic generation might cause the bandwidth broaden with probe
power. To further investigate the mechanism, we fixed the power of probe laser at 150mW and changed the bias voltage.
The results show that the frequency spectrum width becomes wider gradually with the increasing of the bias voltage. A theoretical explaination shows that the bandwidth broadening with bias field might be introduced by a pulse shape
change of the bias field induced second harmonic wave. This study reveals that we can control THz intensity and
bandwidth by changing probe power and bias voltage in the ABCD system.
The characterization of the emission of terahertz (THz) waves generated by four-wave mixing in the presence of laser-induced air plasma under different pump power is presented. In our experiment, terahertz is detected using THz Air-Biased Coherent Detection (THz-ABCD) method. Red-shift is observed in frequency spectra with enhanced pump power and the bandwidth is narrowing down. Then the localized terahertz radiation along the plasma was studied, by constricting the emission area with a pinhole. The spatio-frequency dependent of THz emission from laser-induced air plasma is observed. These phenomena are explained by intense self-phase modulation of the optical pulse in the plasma. This study reveals that terahertz spectrum can be controlled by changing the pump power to get useful frequency range.