Suppression of Laser Output Fluctuation in THz Scanning Transmission Imaging
THz laser can penetrate a wide range of materials which are opaque to visible and near infrared light. So THz imaging has been an important research direction nowadays. When using the THz imaging methods of point-by-point scanning transmission, the laser output power's fluctuation has a big influence on the imaging quality, which is the main source of the imaging noise. In this paper, the influence of THz laser output power's fluctuation on the imaging quality is analyzed. The homomorphic filtering algorithm is introduced to the transmission imaging model, which is used to estimate and suppress the noise. Also the average power in each row and the imaging model are used to reconstruct the image. The two methods above can achieve good results in the real THz image processing.
Intensity Fluctuations in the Output of cw Laser Oscillators.
Using linearized rate equations, we have calculated the intensity fluctuations expected in the output of four-level cw laser oscillators. Intrinsic quantum effects have been included by restricting the eigenvalues of photon and atomic population operators to discrete integral values and by using equivalent noise sources. The total output fluctuations increase monotonically with output power; the relative fluctuations decrease monotonically, and at high output levels inversely as the output power squared. The low-frequency fluctuations differ in that the relative fluctuations increase with output below threshold (noise amplifier region) but decrease sharply above threshold (dynamic saturation region). Curves typical of gas, solid, and diode lasers are illustrated. The theory agrees well with preliminary experimental results.