Efficient single-pixel imaging based on a compact fiber laser array and untrained neural network.

This paper presents an efficient scheme for single-pixel imaging (SPI) utilizing a phase-controlled fiber laser array and an untrained deep neural network. The fiber lasers are arranged in a compact hexagonal structure and coherently combined to generate illuminating light fields. Through the utilization of high-speed electro-optic modulators in each individual fiber laser module, the randomly modulated fiber laser array enables rapid speckle projection onto the object of interest. Furthermore, the untrained deep neural network is incorporated into the image reconstructing process to enhance the quality of the reconstructed images. Through simulations and experiments, we validate the feasibility of the proposed method and successfully achieve high-quality SPI utilizing the coherent fiber laser array at a sampling ratio of 1.6%. Given its potential for high emitting power and rapid modulation, the SPI scheme based on the fiber laser array holds promise for broad applications in remote sensing and other applicable fields.

Efficient and noise-resistant single-pixel imaging based on Pseudo-Zernike moments.

An efficient and noise-resistant single-pixel imaging (SPI) technique based on Pseudo-Zernike moments (PZ-SPI) is proposed. In this technique, the illumination light fields are modulated to satisfy the Pseudo-Zernike polynomials. Then the modulated light fields are projected onto the object. And the single-pixel detector is used to measure the reflected light intensities to calculate the Pseudo-Zernike moments. Finally, the object image is reconstructed by iterative summation of the product of the Pseudo-Zernike polynomials and the Pseudo-Zernike moments. Through the numerical simulation and experimental demonstration, PZ-SPI can effectively reconstruct image at low sampling ratios. Besides, comparing with the Fourier-SPI and Zernike-SPI, PZ-SPI has good robustness to background noise in SPI system. These advantages expand the application of PZ-SPI in complex environments.

Ghost imaging based on Fermat spiral laser array designed for remote sensing.

We propose a Fermat spiral laser array as illumination source in ghost imaging. Due to the aperiodic structure, the Fermat spiral laser array generates illuminating light field without spatial periodicity on the normalized second-order intensity correlation function. A single-pixel detector is used to receive the signal light from object for image reconstruction. The effects of laser array parameters on the quality of ghost imaging are analyzed comprehensively. Through experimental demonstration, the Fermat spiral laser array successfully achieves ghost imaging with high quality by combining with the compressive sensing reconstruction algorithm. This method is expected to be applied in remote sensing by combining with phased and collimated fiber laser array equipped with the high emitting power and high-speed modulation frequency.

Fast object imaging and classification based on circular harmonic Fourier moment detection.

Limited by the number of illumination fields and the speed of a spatial light modulator, single-pixel imaging (SPI) cannot realize real-time imaging and fast classification of an object. In this paper, we proposed the circular harmonic Fourier single-pixel imaging (CHF-SPI) for the first time to realize fast imaging and classification of objects. The light field distribution satisfies the circular harmonic Fourier formula, and the light intensity values of the single-pixel detector are equivalent to the circular harmonic Fourier moments. Then the target can be reconstructed under low sampling ratio by inverse transformation. Through simulation and experimental verification, clear imaging can be performed at a sampling ratio of 0.9%. In addition, circular harmonic Fourier moments are used to construct multi-distortion invariant to classify objects with rotation and scale change. The scale change multiples of objects can be calculated and the objects can be classified by using 10 light fields. It is of great significance to classify objects quickly without imaging.

Single-pixel imaging using discrete Zernike moments.

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 through 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 demonstrations, an image with high quality is retrieved under compressive sampling. Moreover, the Zernike illuminating patterns are used for object classification due to the rotation invariant of Zernike moments. By measuring the amplitudes of a few specific Zernike moments through the SPI system, the rotated images with different angles and the same content are classified into the same class on experiment. This classification technique has the advantages of high efficiency and high accuracy due to the high modulation speed and high sensitivity of SPI system.

694 W sub-GHz polarization-maintained tapered fiber amplifier based on spectral and pump wavelength optimization.

The comprehensive suppression of the stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) is a critical issue for the power scaling of fiber laser with sub-GHz spectral linewidth. In this manuscript, a narrow linewidth and polarization-maintained (PM) fiber amplifier based on tapered Yb-doped fiber (T-YDF) is established, and the effects of spectral linewidth, spectral shape and pump wavelength on the SBS and/or TMI thresholds are investigated. Up to 694 W polarization-maintained fiber laser with just ∼790 MHz linewidth is obtained by combining the advantages of tapered Yb-doped fiber, near-rectangular spectral injection and 915 nm pump manner. This work could provide a well reference solution for the realization of high-power ultra-narrow linewidth fiber lasers.

Experimental study on the impact of signal bandwidth on the transverse mode instability threshold of fiber amplifiers.

In this work, we conduct a detailed experimental study on the impact of signal bandwidth on the TMI threshold of fiber amplifiers. Both the filtered superfluorescent fiber sources and the phase-modulated single-frequency lasers are employed to construct seed lasers with different 3 dB spectral linewidths ranging from 0.19 nm to 7.97 nm. The TMI threshold of the fiber amplifier employing those seed lasers are estimated through the intensity evolution of the signal laser, and different criteria have been utilized to characterize the spectral linewidth of the seed lasers. Notably, the experimental results reveal that the TMI threshold of fiber amplifiers grows, keeps constant, and further grows as a function of spectral linewidth of seed lasers. Our experimental results could provide a well reference to understand the mechanism of the TMI effect and optimize the TMI effect in high-power fiber amplifiers.

Compact and low-cost superfluorescent fiber source assisted narrow linewidth Yb-Raman fiber amplifier.

Recent work has shown that temporally stable optical sources are required in a narrow linewidth Yb-Raman fiber amplifier to suppress the spectral broadening phenomenon. Superfluorescent fiber sources (SFSs) with different spectral widths are used as the Raman-pumped lasers in a 200-watt level narrow linewidth Yb-Raman fiber amplifier for the first time to the best of our knowledge. The experimental results reveal that the spectral broadening phenomenon could be well controlled by using the broadband SFS. Therefore, the narrow linewidth operation could be well maintained during the power scaling process. Moreover, the suppression of the spectral broadening phenomenon would deteriorate when the spectral width of the SFS decreases. This work could provide a compact, low-cost choice for the Raman-pumped laser in narrow linewidth Yb-Raman fiber amplifiers.

Comprehensive investigation on the power scaling of a tapered Yb-doped fiber-based monolithic linearly polarized high-peak-power near-transform-limited nanosecond fiber laser.

An all-fiberized linearly polarized nanosecond master oscillator power amplifier based on polarization-maintaining large-mode-area Yb-doped tapered double cladding fiber (T-DCF) is comprehensively investigated. Firstly, excellent performance of the Yb-doped T-DCF for suppressing nonlinear effects, including stimulated Brillouin scattering (SBS) effect and spectral broadening effects, is experimentally demonstrated and qualitatively analyzed. An SBS-free average output power of 8.8 W is obtained under pulse duration of 3.8 ns and repetition frequency of 80 kHz, with peak power of ∼30 kW, pulse energy of 110 µJ and nearly transform-limited linewidth of < 283.8 MHz respectively. The polarization extinction ratio is > 16 dB and near-diffraction-limited beam quality with M2 factor of 1.2 is maintained at the maximal output power. Moreover, the discussion on the optimization of the system for further power scaling is carried out based a nonlinear dynamic model that is capable of simultaneously evaluating the time-domain and frequency-domain evolution properties of the narrow-linewidth linearly-polarized pulsed laser, and meaningful conclusion is obtained.

Tapered Yb-doped fiber enabled monolithic high-power linearly polarized single-frequency laser.

The all-fiber high-power linearly polarized single-frequency fiber laser based on the polarization-maintaining tapered Yb-doped fiber (T-YDF) is systematically studied. As a result, a 300 W-level stable output with linear polarization and nearly diffraction-limited beam quality is demonstrated. In particular, the overall properties of the transverse mode instability (MI) effect in such a single-frequency laser system are discussed in detail for the first time, to the best of our knowledge, including temporal, frequency, polarization, and spatial domains. Furthermore, the beam pointing error taking the MI effect into account is investigated. Theoretical analyses covering both stimulated Brillouin scattering and the MI effects reveal the great potential of the T-YDF for further power scaling as well.

550 W single frequency fiber amplifiers emitting at 1030†nm based on a tapered Yb-doped fiber.

In this paper, we report a high power single frequency 1030 nm fiber laser with near-diffraction-limited beam quality based on a polarization-maintaining tapered Yb-doped fiber (T-YDF). The T-YDF has advantages of effectively suppressing stimulated Brillouin scattering (SBS) while maintaining good beam quality. As a result, a record output power of 379 W single frequency, linearly polarized, nearly single-mode fiber amplifier operating at 1030 nm is demonstrated. The polarization extinction ratio is as high as 16.3 dB, and the M2 is measured to be 1.12. Further, the dependence of the thermal-induced mode instability (TMI) threshold on the polarization state of an input signal laser is investigated for the first time. By changing the polarization state of the injected seed laser, the output power can increase to 550 W while the beam quality can be maintained well (M2=1.47). The slope efficiency of the whole amplifier is about 80%. No sign of SBS appears even at the highest output power and the further brightness scaling of both situations is limited by the TMI effect. To the best of our knowledge, this result is the highest output power of all-fiberized single frequency fiber amplifiers.