An efficient, compact 970 nm laser diode end-pumped Er:YSGG laser operating in continuous and pulsed modes is reported. In the continuous-wave regime, an output power of ∼1.6 W mid-infrared laser with 2.79 μm wavelength is obtained, corresponding to an optical efficiency of 19.6% and slope efficiency of 25.3%. In the pulsed regime, an energy of 144 mJ and a slope efficiency of 43.5% are achieved with a 30 at. % Er:YSGG crystal in 3×3×5 mm3 dimensions. The high-pulse energy and the high efficiency indicate that Er:YSGG lasers can potentially be used for blood sampling.
To enhance the complexity of the output sequence of the optical injection structure and improve the security of the bit plane encryption, this work proposes an image encryption algorithm based on optical chaos and quadrature amplitude modulation (QAM)-like. The external disturbance generated by the mutual injection of the dual-drive laser improves the complexity of the optical chaos output from the laser in the optical injection structure. In the image encryption algorithm, this work adopts a QAM-like coding mode, which encrypts the pixel value of the plaintext image by transforming the coordinates of the QAM constellation diagram and the coding. In the coding stage, the different coding modes of QAM-like are selected using the encryption sequence composed of optical chaotic sequence and key to transform the color plaintext image into the coded image, and the pixel value of the plaintext image is changed. In the scrambling stage, the encrypted sequence is used to scramble the multi-dimensional rows and columns of the three-dimensional coded image, and the position information of the pixel points is encrypted. In the diffusion stage, the scrambled images of each color component are diffused by GF(17) multiplication and finally merged to generate ciphertext images. Simulation results show that the proposed image encryption algorithm has sufficient key space and information entropy is close to the theoretical maximum, which can effectively resist known plaintext attacks, selective plaintext attacks, and statistical feature attacks.
A tunable and switchable multi-wavelength random fiber laser (MW-RFL) is proposed and experimentally demonstrated in this work, which consists of a random distributed feedback mechanism, an improved Lyot filter, and a nonlinear optical loop mirror (NOLM). The theoretical derivation and simulation analysis of the improved Lyot filter are carried out by using the transfer matrix theory. The results show that by adjusting the polarization controller (PC) in the filter, not only the channel interval can be switched between 0.9 and 0.45 nm, but also the extinction ratio (ER) of the transmission spectrum can be adjusted and the wave peak position can be moved. The feedback mechanism of the proposed laser is provided by the randomly distributed backward Rayleigh scattering generated in the single-mode fiber. By changing the pump laser power, the number of output laser channels will be linearly increased. When the channel interval is kept constant, the wavelength can be tuned within a certain range by carefully adjusting the PC3 in the NOLM. When the channel interval of the spectrum is 0.9 nm, the wavelength tuning ranges of the three and four laser channels are 3.6 and 3.5 nm, respectively. When the channel interval of the spectrum is 0.45 nm, the wavelength tuning ranges of the three and four laser channels are 2.8 and 2.77 nm, respectively. Then the variations of the output spectrum when using 25 km single-mode fiber (SMF) are studied. It is observed that the linewidth and side mode suppression ratio (SMSR) of the laser channel are significantly increased. Finally, the stability of the random laser is discussed, and the maximum central wavelength drift and peak power fluctuation measured at room temperature are 0.05 nm and 0.34 dB, respectively. The average peak power and SMSR are maintained at −4.47 dBm and 34.6 dB, respectively, which confirms that the laser has good stability.
The longitudinal mode distribution in the self-mode-locked (SML) laser with flexible reflectivity and a tilted mirror is investigated. First, the longitudinal mode characteristics of the SML laser are analyzed theoretically. Then by employing an intracavity etalon, the output mirror with different parameters is used for longitudinal mode manipulation in experimental research. By varying the reflectivity and tilting the output mirror, the longitudinal mode grouping and spectral shifting are observed. The experimental results show that, when the output mirror reflectivity decreases from 95% to 60%, the transmission peak linewidth of the intracavity etalon becomes narrower, and the number of the longitudinal mode selected by each transmission peak decreases from a maximum of 7 to a minimum of 1. When output mirror is tilted by +0.03 deg and −0.03 deg, the longitudinal mode wavelengths redshift and blueshift by 0.02 nm. The experimental results are in good agreement with the theory.
A bidirectional high-speed chaotic optical communication scheme with physical layer encryption is proposed and studied theoretically. The external cavity optical feedback method is analyzed. An erbium-doped fiber amplifier is used to represent the all-optical delay feedback loop of the traditional external cavity; it can overcome the shortcomings of the traditional method such as lower precision and high equipment volume requirements. The bidirectional communication scheme is discussed; it sets the encryption device at the transmitter, uses the correlation between the two encryption signals to decrypt, and cancels the decryption device at the receiver, which not only simplifies the experimental equipment but also solves the problem that the receiver cannot decrypt synchronously due to the channel damage in remote communication. Further, it is easily applied in production and life. Finally, the simulation results show that the bit error rate is <10 − 5, and the bidirectional transmission of 10-Gb / s information over an 85-km single-mode fiber is successfully realized.
A high-repetition pulsed Nd : YVO4 laser based on the multi-longitudinal-mode beat note is presented. The mechanism of multi-mode oscillating in the Nd : YVO4 laser with FP cavity is analyzed. With the multi-mode Nd : YVO4 laser in experiments, it is found that the more longitudinal modes beating in the laser cavity, the narrower time duration of the laser pulse is obtained. An adjustable aperture is employed in the laser cavity to weaken the transverse mode interference and improve the laser pulse train stability. At last, when the FP laser cavity length is 88 mm, by slight tuning the Nd : YVO4 crystal temperature, a 1.56 GHz high-repetition laser pulse with 0.18-ns time duration is achieved. The average output power of the pulse train is 1605 mW with the pump power at 6788 mW. The slope efficiency and the optical-to-optical efficiency are calculated as 31% and 24%, respectively.
We present an intensity-balanced dual-frequency laser (DFL) based on an Nd: GdVO4 microchip crystal. The intensity balance ratio tuning mechanism of the DFL, which is governed by the heat sink temperature Tc of the laser crystal, is experimentally studied. The experimental results indicate that keeping a balanced intensity condition, when the pumping power of the DFL increases, the heat sink temperature Tc of the laser crystal must be lowered to rebalance the DFL signal intensities. The Tc versus pumping power slope is experimentally measured to be −17.95 ° C / W. By fixing the pumping power at 5.6 W, an intensity-balanced microchip DFL with output power up to 103 mW and frequency separation up to 61 GHz is achieved, whose slope efficiency is 9%.
A stable multiwavelength semiconductor amplifier (SOA) fiber laser using a nonlinear fiber loop mirror (NOLM) and a Lyot–Sagnac loop mirror is presented and experimentally demonstrated. The Lyot–Sagnac loop mirror serves as a wavelength selection device, and the wavelength spacing is controlled by the length of polarization-maintained fiber (PMF). The NOLM serves as a power equalizer to achieve power equalization and improve stability. Moreover, the NOLM can also suppress the mode competition effect within homogenous broadening linewidth of SOA. Multiwavelength output with power equality has been achieved at room temperature, with no obvious power fluctuation. The number of lasing lines is 31 within the 10-dB bandwidth, and the wavelength spacing is 0.63 nm. In addition, the average optical signal-to-noise ratio is up to 25.15 dB. Finally, narrower wavelength spacing is obtained by increasing the length of the PMF.
The intensity balance ratio (IBR) tuning mechanism of Nd:GdVO4 monolithic microchip dual-frequency laser (DFL) is presented. The intensity balanced DFL signals are obtained by precisely controlling the heat sink temperature of the Nd:GdVO4 crystal. In experiments, the DFL signal with frequency separation at 64 GHz and IBR above 0.99 is realized with the temperature at 47.6 °C. The other balanced intensity distribution can be reached at -0.9 °C before mode hopping. Moreover, utilizing the fluorescence spectrum and the intensity balance points of Nd:GdVO4 DFL, we obtain the temperature difference between internal and external of Nd:GdVO4 crystal ΔT = 24.0 °C.
KEYWORDS: Orthogonal frequency division multiplexing, Digital signal processing, Computer security, Single mode fibers, Passive optical networks, Chaos, Quadrature amplitude modulation, Legal, Receivers, Complex systems
To effectively resist malicious eavesdropping and performance deterioration, a novel chaos-based secure transmission scheme is proposed to enhance the physical layer security and reduce peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing passive optical network (OFDM-PON). By the randomly extracting operation of common CAZAC values, the specially-designed constant amplitude zero autocorrelation (CAZAC) is created for system encryption and PAPR reduction enhancing the transmission security. This method is verified in ~10-Gb/s encrypted OFDM-PON with 20-km fiber transmission. Results show that, compared to common OFDM-PON, our scheme achieves ~3-dB PAPR reduction and ~1-dB receiver sensitivity improvement.
A new theory for describing the ultraviolet (UV) laser generation mechanism by frequency doubling of focused Gaussian beam in nonlinear crystal was presented. The nonlinear crystal used for UV laser generation, such as β-barium borate (BBO), has dramatically small phase matching acceptance angles Δθx and large walk-off angle ρ. Numerical simulation shows the variation of the waist of input Gaussian beam brings change in conversion efficiency and far-field transverse intensity distribution. For validating the theory, we investigated the single-pass conversion efficiency in BBO crystal by using acoustic-optic Q switched 532 nm laser; and took a photograph of fringe structure far-field intensity distribution of the UV beam. Comparison between the numerical simulation and experimental results support our theory and analysis. Our theory provides a useful tool for estimating ultraviolet laser generation processes for low power nanosecond pulsed laser and cw laser.
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