A novel single-pixel imaging (SPI) technique based on discrete orthogonal Zernike moments is proposed. In this technique, the target object is illuminated by two sets of Zernike basis patterns which satisfy the Zernike polynomials. The Zernike moments of object image are obtained by measuring the reflected light intensities with a single-pixel detector. And the object image is reconstructed by summing the product of Zernike polynomials and detected intensities iteratively. By theoretical and experimental demonstration, an image is successfully retrieved under compressive sampling. As for both gray and binary images with resolution of 128×128 pixels, the images reconstructed by Zernike patterns have better image quality compared with those reconstructed by Fourier patterns when sampling ratio is lower than 10%. This technique yields high efficiency and high imaging quality in single-pixel imaging system.
High power, narrow linewidth all-fiber amplifiers are under intensive investigation in recent years. In this paper, the research status of high power, narrow linewidth all-fiber amplifiers (including those operate at single frequency regime) is briefly summarized. Then the recent progress in our research group is introduced, including more than 500 Watt level single frequency fiber amplifier, more than 4 kW linearly-polarized narrow linewidth all-fiber amplifier and more than 6 kW narrow linewidth all-fiber amplifier. Performance exploring of the operating spectrum property is also discussed.
In this report, by comparing the RIN and beam pointing error of the signal laser at different output powers, the impact of the modal degeneration on both the spacial and temporal noise properties of high-power single-frequency fiber amplifiers will be demonstrated. The new finding reveal that obvious mode-related enhancement of the RIN could occur well below the conventional transverse mode instability (TMI) threshold of the fiber amplifier while the mode-related enhancement of the beam pointing error occurs near the TMI threshold. This work could provide a new insight for obtaining high-power, high spacial stability and low-noise single-frequency fiber lasers.
A novel phase modulation technique, calling multi-phase coded signal (MPCS) modulation, is demonstrated for stimulated Brillouin scattering (SBS) effect managing in high power narrow-linewidth polarization-maintained all-fiber amplifier. Combining with laser gain competition where two signal lasers operating at 1045 nm and 1064 nm are applied, the output power of 1064 nm laser could be boosted to 1023 W with FWHM linewidth of 4.6 GHz. The polarization extinction ratio (PER) is 13.3 dB and the output beam quality is measured to be M2=1.11. This work could provide a feasible method for linewidth controlling in high power narrow linewidth fiber amplifier.
Yb-Raman fiber amplifier is an effective solution to achieving high-power narrow linewidth fiber laser operating at 1100-1200 nm region, in which the selection of the pump source at around 1064 nm has a significant effect on the output power and spectral properties. In this paper, we report an Yb-Raman fiber amplifier pumped by a random fiber laser. Hundred-watt level output power with 3 dB spectral linewidth of only 0.23 nm is obtained by optimizing the linewidth of the random fiber laser. The results could provide a practical reference for the systematic design of the narrow linewidth Yb-Raman fiber amplifier.
Recently, the use of Yb-Raman fiber amplifier has led to a breakthrough in the power of narrow linewidth laser at 1120 nm. In this paper, the effects of seed filtering on the output laser characteristics in a narrow linewidth Yb-Raman fiber amplifier are studied. The experimental results indicate that the seed filtering can not only suppress the ASE noise in the output laser, but also have an influence on the evolution of the sideband noise of the signal spectra. This work can contribute to the understanding of narrow linewidth Yb-Raman fiber amplifiers and can provide a reference for the systematic design.
Stimulated Brillouin scattering (SBS) suppression in high power, narrow linewidth and all polarization-maintained amplifiers with near diffraction limited (NDL) beam quality by rectangular optical spectrum is demonstrated. Rectangular spectrum is generated by using cascaded phase modulations. In the preliminary experiment, output power of 509 W with spectral linewidth of ~ 3 GHz is obtained. At maximal output power, the polarization extinction ratio (PER) is measured to be ~ 14 dB and the beam quality (M2 factor) is M2 ~1.2. The technique presented give useful reference to control the spectral linewidth in high brightness fiber amplifiers. By combining with other linewidth controlling techniques, several kilowatt-level output power with spectral linewidth of < 10 GHz could be expected.
In the field of measuring the laser power, the traditional way is direct measuring based on thermal effects. In this method, the laser power can be measured from the temperature change of the sensor absorbing the laser light. However, it is difficult to operate when the power exceeds 10kW level because of the linearly scaling size of absorbing object and the relatively long recovery (cooling) periods, which results in the proposal and fast development of indirect measuring technique route. Using light pressure to measure laser power is one of the recent research hotspots of indirect measuring. The typical system design is a scale attached with a nearly perfect mirror that had shown the advantages such as high accuracy, fast response, real-time measuring and convenient calibration. Nevertheless, the commercial mirror (with a reflectivity of 0.997 as a typical value) and scale (with a 100nN resolution as a typical value) cannot achieve a stable measuring due to the feeble light pressure. In fact, the measuring uncertainty could be 7%-13% in practical operation. So it is reasonable and feasible to amplify the light pressure. In this paper, we propose a new structure design based on oscillation cavity which can improve the energy utilization and increase the number of reflections. The force measured by our system is expected to be 50 times more than that imparted by single reflection if the reflectivity of the mirrors installed on the oscillation cavity is larger than 0.99. This novel method can not only achieve higher sensitivity but also retain the advantages of indirect measuring such as fast response, real-time measuring. The resolution ratio could be at the level of 1W using the same scale as the sensor. The theoretical principles and system design will be introduced and analyzed in detail.
Accurate measuring the laser power is one of the most important issue to evaluate the performance of high power laser. For the time being, most of the demonstrated technique could be attributed to direct measuring route. Indirect measuring laser power based on light pressure, which has been under intensive investigation, has the advantages such as fast response, real-time measuring and high accuracy, compared with direct measuring route. In this paper, we will review several non-traditional methods based on light pressure to precisely measure the laser power proposed recently. The system setup, measuring principle and scaling methods would be introduced and analyzed in detail. We also compare the benefit and the drawback of these methods and analyze the uncertainties of the measurements.
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