Orbital angular momentum (OAM) holds significant potential for achieving extremely high communication capacity, attributed to its orthogonality and infinite modes. Employing convolutional neural networks (CNN) for OAM mode recognition is an effective strategy to mitigate the effects of turbulence. However, recognition accuracy can be compromised when the training dataset is limited. To address this, we leveraged a conditional generative adversarial network (cGAN) for data augmentation (DA). The well-trained cGAN generated abundant augmented data with mode information, thereby enhancing the performance of the CNN. Experimental results clearly demonstrate that cGAN-based DA is an effective method for boosting recognition accuracy, resulting in a significant increase in recognition accuracy, rising from 24% to more than 99%. In addition, analyzing the relationship between the degree of DA and accuracy was instrumental in finding a balance between generation time cost and accuracy improvement. In addition, the application of cGAN-based DA to decomposed OAMs from the vortex array further validates its applicability in enhancing recognition performance.
Currently, most space-borne optical cameras have fixed focal length and depth of focus. In this case, the range within which the target can be clearly imaged has been pre-determined before launch. However, the distance of the target to the optical camera might be unknown or change very fast and therefore focus adjustment has to be carried out to obtain clear images. However, no matter which refocusing technique is used, focus adjustment might lag behind the object distance variation and depth of focus extension is a better way. Wave-front coding can be used to extend the depth of focus of incoherent imaging system but the surface profile of the phase mask could not be changed dynamically, which is not flexible for application. In this manuscript, by combing the variable curvature mirror (VCM) and coded imaging technique together, a new depth of focus extension technique is proposed. According to our previous studies, the focal plane could be quickly adjusted by changing the curvature radius of VCM. Compared with the curvature variation speed, the exposure time of the camera is quite long. Therefore, by adjusting the focal plane very fast in a wide range during the exposure through VCM, an equivalent coded optical transfer function having no null frequency points within bandwidth is generated and the image captured is uniformly blurred. After that, with the help of digital restoration, the clear image could be obtained. Because the focal plane could be adjusted through variable curvature mirror in the range of millimeter, the proposed method could be used to obtain clear images with greatly extended depth of focus.
With the rapid development of the times, traditional communication methods such as microwave communication have been difficult to meet the needs of high-speed and high-capacity communication. People begin to look for new communication technologies to replace the existing communication methods. Laser communication has the advantages of high bandwidth, small terminal volume and good security. In the process of space laser communication, the coupling of space light to single-mode fiber is one of the difficulties. In the process of practical application, external jitter (atmospheric turbulence, platform vibration, etc.) makes the focus spot offset from the end face of the optical fiber, greatly reduces the energy coupled into the single-mode optical fiber, and seriously affects the communication quality. Applying the mature optical fiber communication technology to space laser communication can not only improve the transmission rate of the link, but also improve the communication quality and simplify the system composition. Based on laser nutation, aiming at the problem of space optical coupling into single-mode fiber in space laser communication, this paper analyzes the influence of alignment error on coupling efficiency. The lateral deviation has the greatest influence. In this paper, a laser nutation coupling system model based on the fast mirror is built. The model is based on the principle of mode field matching. The nutation signal of the nutation fast mirror is used as the modulation signal, and the optical power output by the photodetector is used as the feedback for calculation. The fast mirror is controlled in real time to compensate the transverse deviation. The algorithm flow of laser nutation is described, the simulation model is built, and the feasibility of the algorithm is verified. Based on different algorithm parameters such as nutation radius and convergence step, the relationship between parameter value and algorithm performance is obtained.
KEYWORDS: Interference (communication), Signal detection, Signal to noise ratio, Modulation, Laser communications, Spectrum analysis, Signal processing, Sensors, Satellites, Fourier transforms
With the development of the information age, the transmission rate and transmission capacity of information continue to increase. As a wireless communication method to achieve high bit rate communication, space laser communication plays an increasingly obvious role in global communications. In the current space laser communication system, the optical communication terminal uses a detector to detect the position of the received optical signal, and then the servo mechanism on it performs facula tracking. However, when a four-quadrant detector (4QD) is used to detect optical signal, the received optical signal generally contains three main types of noise, which are background radiation noise, shot noise and thermal noise. These three kinds of noise can be equivalent to Gaussian white noise. In this paper, a single-frequency cosine signal is used to modulate the intensity of the optical signal received by the 4QD, and then the modulated optical signal is detected by a spectrum analysis method based on the cross-correlation algorithm. This method reduces the relative error of the spectral line amplitude when the SNR is -5dB from 2.23% to 0.88%, and reduces the relative error of the spectral line amplitude when the SNR is -20dB from 17.6% to 5.49%. Therefore, this method can well suppress Gaussian white noise and improve the detection accuracy of modulated optical signal under extremely low SNR conditions.
We report on the dissipative soliton operation of a diode-pumped single-crystal bulk Yb:KGW laser oscillator in the all-positive-dispersion regime. Stable passively mode-locked pulses with strong positive chirp and steep spectral edges are obtained. The spectral centering at 1038.6 nm has a bandwidth of about 6.9 nm, and the chirped pulses have a pulse duration of 4.317 ps. The maximum average power can be up to 2.07 W when pumped by absorbed pump power of 5.3 W. The mode-locked slope efficiency and optical–optical conversion efficiency are shown to be 62% and 39%, respectively. Considering the pulse repetition rate with a value of 52 MHz, the corresponding pulse energy is estimated to be 39.8 nJ.
KEYWORDS: Crystals, Modulation, Monte Carlo methods, Laser crystals, Modulators, Laser communications, Electro optics, Signal attenuation, Ocean optics, Refractive index
Indirect modulation with electro-optical crystal is a useful way to generate optical signal for underwater blue-green laser communication. However, as crystal surface is not strictly flat in practical application, light intensity distribution in the cross section is non-uniform, which would affect extinction ratio of modulated signal and system performance. In this letter, we study this issue with Monte Carlo method. The result shows that with the increase of crystal flatness, extinction ratio is decreasing dramatically, and it should be smaller than 0.78μm in order to make the extinction ratio greater than 10dB while 0.25μm for 20dB, 0.08μm for 30dB, and 0.025μm for 40dB.
In underwater wireless optical communication (UWOC), one of the key technologies is to generate high-speed communication signal for transmitter. In this paper, we designed such transmitter based on laser diode (LD) arrays, which is composed of three LDs with central wavelength 450nm. The modulation format is non-return-to-zero on-off keying (NRZ-OOK) with data rate up to 50Mbps. Using such transmitter, we established a point-to-point underwater wireless optical communication link in an experimental tank with 20m length, 20m width and 14 depth. The experimental results show that the maximum error-free data rate of the system can reach 50Mbps with 10.7m transmission distance, while the maximum error-free transmission rate is 30Mbps with 14.7m transmission distance. These results verify the feasibility of the LD-based modulation scheme for high-speed UWOC applications.
Lots of researches have simulated performance of optical system based on plane wave or spherical wave model, but
optical field is Gauss distribution in real optical communication system. So we derive the relationships between the
signal-to-noise ratio (SNR), bit error rate (BER), channel capacity (C) and the transmission distance of the space laser
communication system, which is obtained by the Gaussian beam propagation model for on-off keying (OOK) modulated
signal under horizontal link. We numerically study the influence of atmospheric turbulence on system performance. we
get the conclusions: Under weak turbulence intensity, as the turbulence intensity increases, the C and SNR for OOK
modulated space laser communication system decrease while the BER increase; In the case of strong turbulence intensity,
the intensity scintillation and the BER dramatically increase with the increase of transmission distance, and then become
saturated; In the condition of turbulence intensity and transmission distance are both constant, the longer the wavelength
results in greater C, higher SNR and lower BER. Selecting longer communication wavelength can mitigate the impact of
the atmosphere turbulence on the communication system in some extent.
We present simulation results for a 5 Gb/s optically pre-amplified, differential phase shift keying communication
system achieving -48.1 dBm (about 24.7 photons/bit) receiver sensitivity at 10-9 bit-error-rate and using an optical delay
line interferometer made by ourselves with 1 bit delay as the demodulator. The system is also experimental tested and
sensitivity of the receiver is -36.5 dBm (about 106.6 photons/bit). The experimental results have some penalty in
sensitivity comparing to the results in the simulation because the 1 bit delay interferometer is sensitive to the
environment such as temperature, tremble and so on. To our knowledge if the interferometer is well designed and
optimized, the experimental results can correspond with the simulative results and an optically pre-amplified direct
detection DPSK receiver with high sensitivity can be realized.
In order to realize the all-optical encryption for multi-wave and ultra-high speed, an all-optical encryption scheme is
proposed, which is based on pockels effect of LiNbO3 waveguide and optical-time-division-multiplexing (OTDM) in
LibNO3 waveguide. The coherently ultra-high speed plaintext and key are generated by OTDM in LibNO3 waveguide
and the XOR operation of optical plaintext and key are achieved by direct light interference. The feasibility and validity
of this scheme is verified by simulative experiment at 8 x 80Gbps with wavelength interval of 2nm.
An all-optical encryption system which can be used in fiber WDM system and directly encrypt multi-wave optical
signals is proposed. This scheme achieves multi-wave signals encrypting by the technology of generating coherently
multi-wave optical plain text, Key text and the technology of phase shift interference. The feasibility and validity of this
encrypting scheme are verified by simulative experiment for multi-wave signals at 8x10Gbps with wavelength interval
of 0.8nm, the simulative result shows that when the input Pi and Ki is Gauss pulse with duty cycle of 0.3, the bit error
rate and Q factor of encryption output are 0 and 164 respectively. Theory analysis and simulative experiment shows the
scheme is feasible and efficient.
The impact of phase noise induced by amplified spontaneous emission and nonlinear Kerr effect is analyzed in
investigating the performance of BPSK homodyne systems based OPLL in long-haul optical fiber communications.
Under the Gaussian probability density function assumption the equivalent power spectral density of linear and nonlinear
phase noise are determined, respectively. Using the power spectral density function the expressions of phase error
variance taking into account linear phase noise, nonlinear phase noise and shot noise of corresponding photodiodes are
determined. The impact of the number of amplifiers, bit rate and signal power on the phase error variance is analyzed. Using the minimum phase error variance and corresponding BER expression the receiver sensitivity and BER curves are given, respectively. Under the required BER, it is shown that there is a limitation on the maximum transmission distance of the BPSK homodyne system based OPLL. The theory expressions and numerical results obtained in this paper are helpful for designing OPLL and evaluating the impact of phase noise on the receivers based OPLL in long-haul optical fiber communications.
Considering the state of the art of wavelength conversion technology, it is likely to dictate a more limited and sparse deployment of wavelength converters in optical burst switching (OBS) networks. Without wavelength conversion capabilities at optical switches, how to avoid burst contention becomes an important issue. This paper proposes an approach to controlling wavelength-division-multiplexing passive optical stars, consisting in queuing arrivals for delayed reception, to be used in OBS networks. This solution brings optical star networks a step closer to all-optical realization. In addition, it allows almost total utilization of the channels, obtainable until now only by time-division multiplexing control. The proposed solution does not suffer performance degradation under heterogeneous traffic conditions and increasing number of nodes. It is thus unique in offering the potential for an all-optical solution and providing at the same time high throughput, low delay, small buffer requirements, and robustness under all traffic conditions.
We experimentally demonstrate an Er3+-doped femtosecond mode-locked optical fiber laser which operates at the 1550-nm wavelength and generates ultrashort optical pulses with width of 270 fs, repetition frequency of 20 MHz, and average output power of 146μW. We find that the pulse width keeps stable increasing when the pump source optical power is enhanced in the range from 25 mW to 60 mW. However, if the pump source optical power exceeds 60 mW, the oscillation phenomenon would occur in pulses width. To be used as an optical source in fiber-optic communications system, the output femtosecond pulses from the mode-locked laser are also amplified specially through an Er3+-doped optical fiber amplifier. The relationships between pump optical power and amplified characteristics including amplified optical pulses width, amplified average optical power, amplified single optical pulse energy, amplified gain and spectrum shape are studied experimentally in the amplified experiment.
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