KEYWORDS: Turbulence, Scintillation, Gaussian beams, Signal to noise ratio, Beam splitters, Atmospheric propagation, Global system for mobile communications, Beam propagation method, Atmospheric optics
With the discussion of sixth generation gradually raising the curtain, the requirement to further expand the bandwidth makes the optical wireless communication one of the best alternatives. However, the strong atmospheric turbulence under the sunlight limits the application of the technique in the common environment. Therefore, in this paper, we study a kind of partially coherent beam (PCB), namely Bessel-Gaussian Schell-model (BGSM) beam, in terms of its propagation characteristics in atmosphere, and then construct a spacing division multiplexing (SDM) scheme based on BGSM beam. Based on the results, the BGSM beam has a stable distant annular facula that its radius is independent to the random medium, and it has wonderful atmosphere-resistance that even considering the intensity fluctuation from the PCB source, it shows larger signal-to-noise ratio (SNR) than its coherent counterpart in the strong turbulence. When several coaxial BGSM beams with different coherence distribution propagate through a space, the difference of their ring’s radius makes them illuminating on different circles. Deploying the diversity reception, the source scintillation is mitigated so that every channel could afford stable transmission. Under the different turbulence condition, the SDM scheme at least double the total capacity comparing to the traditional communication scheme.
The acquisition of phase information of light field is the key technology of adaptive optics. Using intensity of the light field to derive the phase distribution of the light field has become a common application technique for phase recovery. Research shows that iterative algorithm is an effective method for phase recovery of light field, but some iterative algorithms have the disadvantages of being sensitive to initial values, easy to fall into local extremum and slow convergence. Here we mainly focus on two iterative optimization algorithms for wavefront distortion correction without wavefront sensing adaptive optics. The first is the Gerchberg-Saxton (GS) algorithm, which combines two complex amplitude distributions on the plane of the optical propagation perpendicular to the optical axis and recovers the phase from the intensity distribution. The second is a genetic algorithm that achieves an optimal solution for the evaluation function through a series of hybridization, mutation, and selection operations. In order to improve its convergence rate, we take Zernike polynomial coefficient required for wavefront reconstruction as the optimization object instead of voltages on corrector traditionally. We numerically simulate the performance of two algorithms, use Zernike polynomial to fit the static aberration, and study a series of parameters, especially single-order aberrations and random multi-order aberrations as the initial phase to the correction effect, and the correction performance of the two algorithms is respectively evaluated using two evaluation functions, Sum-Square Error (SSE) and Strehl Ratio (SR). Time consumption is also mentioned to evaluate the performance of two algorithms.
KEYWORDS: Turbulence, Ocean optics, Atmospheric propagation, Adaptive optics, Global system for mobile communications, Signal to noise ratio, Water, Laser beam propagation, Scintillation, Wave propagation
Reliable communication between aerial and undersea vehicles is a challenging issue because radio frequency signals are attenuated drastically in sea water while acoustic waves are not preferable in terrestrial links. Located in the transmittance windows of both sea water and the atmosphere, blue-green laser based free-space optical communication systems are capable of providing high speed, low latency data links for this very scenario. Apart from the absorbing and scattering attenuations in the air-water channel, another limiting factor impacting efficient laser beam propagation is the turbulence induced intensity fluctuations. Pure attenuation in sea water restricts the laser communication distance to ~100 meters, which will further reduce to ~10 meters in the presence of oceanic turbulence. Meanwhile, atmospheric turbulence can also substantially degrade the beam quality if the aerial vehicle is at high altitude. In this study, we focus our effort on the turbulence effects on beam propagation in the air-water two-stage links, not taking into account media attenuation or water surface distortions. Considering the complexity of the depth dependence of salinity and temperature in sea water and the altitude dependence of air refractive-index structure constant, we use numerical methods to simulate the beam propagation through the two-stage turbulence channel, which is modeled by discrete phase screens generated with parameterized atmospheric and oceanic turbulence power spectrums. On that basis, beam spread, area scintillation and SNR penalty at the receiver end are analyzed for the uplink as well as the downlink transmission.
In this study, a special class of nonuniformly correlated beams with radially symmetric coherence distributions, called
radial partially coherent beams (RPCBs), is numerically studied. By spatially modulating uniformly correlated phase
screens used for generating conventional Gaussian Schell-model beams, RPCBs with arbitrary distributions of degree of
coherence are produced. RPCBs whose degree of coherence decreases from the beam center along the radial direction
were found to self-focus in free-space propagation, leading to augmented optical intensity near the beam center.
Meanwhile, the scintillation mitigation ability of RPCBs remains significant. By means of wave optics simulation,
propagation properties of RPCBs in anisotropic non-Kolmogorov turbulence are analyzed. Simulation results show that,
under certain conditions RPCBs are still able to deliver improved performance in anisotropic non-Kolmogorov
turbulence. Moreover, due to the elliptical far-field irradiance pattern caused by anisotropy, a matched elliptic receiving
aperture can further reduce the turbulence-induced scintillation.
In this paper, the aperture averaged scintillations of the Bessel beams carrying optical vortices propagating in turbulent atmosphere are evaluated. The multistep form of the propagation algorithm and a numerical phase screen simulation method are applied to the calculations of the aperture averaged scintillation. The results show that the Bessel beam with more topological charges delivers the smaller scintillation. The relation between the aperture averaged scintillation and the size of the beams is investigated. The effect of inner and outer scales of turbulence on the scintillations of the Bessel beams is also studied. These results may be useful in long-distance optical communications in free space or in turbulent atmosphere.
We propose a wide field-of-view optical receiver design based on a fisheye lens and an off-axis catadioptric structure for free-space optical communications. The design utilizes a novel fisheye lens group to compress a wide field angle into a narrow field angle and produce the appropriately collimated light that can effectively be coupled into the following aperture of a catadioptric telescope. An off-axis catadioptric telescope with aspheric surface mirrors is designed to compress the incident beam spot size, compensate for the high order optical aberrations and eliminate light loss due to an obstruction. The parallel exit rays are reflected on a double-level tracking mechanism by feeding the position signal from a quadrant detector to correct the pointing error and optimize the coupling efficiency into an optical fiber. The final wide field-of-view optical receiver design is presented along with the evaluation of optical performance results and tracking characteristics. The proposed optical receiver not only can provide a 60-deg wide field-of-view to expand the tracking range, but also mitigates optical aberrations to improve the tracking accuracy for free space optical communication systems in a turbulent atmosphere.
The free space optical communication systems should utilize optical antennas with beam tracking mechanisms. However,
the narrow field of view and optical aberration of antennas degrade the tracking performance of the system. In order to
overcome the problems, we investigate the wide field of view optical antenna technology. The optical antenna consists of
fisheye lens, compensating lens and a catadioptric telescope with off-axis aspheric surface mirrors. The structures and
performances of the optical device elements are numerically analyzed so that their designs can make positive
contribution in enlarging the field of view and reducing the optical aberration. The final optical antenna design is
presented, along with the evaluation of optical performance and tracking characteristics. The proposed optical antenna
could not only provide a wide field of view with approximately 60 degree and expand the range for tracking mechanism,
but also mitigate the optical aberration and improve tracking accuracy of free space optical communication systems in
turbulent atmosphere.
Differential phase shift keying (DPSK) data format is employed to combat noise in the bandwidth limited coherent time spreading (TS) optical code division multiplexing access (OCDMA) system. In addition, the performance of DPSK-OCDMA system with turbo coding is analysed and simulated. Theoretical results show that performance degradation due to bandwidth limitation could be effectively restrained by the DPSK-OCDMA scheme, and further performance improvement could be achieved by incorporating turbo coding into OCDMA system. It is confirmed that the number of active users is increased from K = 11 to K = 25 in bandwidth limited OCDMA system by employing DPSK format and turbo code for a given BER of 10-6.
An innovation way to detect and identify biomolecule encoding is studied and a practical optical-mechanical-electrical integrative sensor system is accomplished, for which, a comprehensive analysis of the spectrum information, grayscale information as well as the location information is conducted. In our system, a LED as a light source, is used to provide a uniform illumination, and a CCD image sensor is used to obtain gray grading information of biomolecule encoding chip. And then, Wavelet analysis technology is used to eliminate noise and smooth the image signals. The location of each encoding dot and its average gray can be realized automatically by means of these methods, the features of the biomolecule encoding can be identified. And all of the characteristics on molecule encoding are displayed on screen in several different ways finally. Compared with NMR and IR technique, our design of the system is small in size, easy to operate and low cost.
Correlation characteristics for phase-coding scheme utilizing superstructured fiber Bragg grating (SSFBG)
as encoder/decoder are evaluated. The impact of transmission impairments and the receiver's bandwidth
limitation in the coherent OCDMA system is investigated, and the performance improvement by using optical
thresholding is also analyzed.
40-Gbit/s optical code-division multiplexing (OCDM)/ wavelength-division multiplexing (WDM) system based
on supercontinuum (SC) and superstructured fiber Bragg grating (SSFBG) is experimentally demonstrated,
simultaneous multi-wavelength optical encoding of a single supercontinuum source using SSFBG is achieved. In
the receiver, a nonlinear amplifying loop mirror (NALM) is introduced to improve the system performance, both
the sidelobe and the pedestal are reduced, the waveform quality of decoded signal is enhanced. The peak power
of the decoded signal before the NALM is ~ 8 mW.
KEYWORDS: Turbulence, Filtering (signal processing), Signal to noise ratio, Receivers, Wireless communications, Adaptive optics, Atmospheric optics, Digital signal processing, Channel projecting optics, Detector development
A model for received signals has been developed for detection of binary data bits in the
presence of atmospheric turbulence noise. The model was used to analyze the likelihood ratio test
(LRT) according to optimal maximum a posterior (MAP) probability criterion. The adaptive
decision thresholding method was developed to maintain the detection threshold near optimum. The
detection threshold that was updated adaptively as the
signal-to-noise ratio (SNR) was changed.
Kalman filter and homomorphic filter were used to predict statistical variable of the bit and
turbulence parameters, respectively. Simulation and the experimental results indicate that bit-error
probability (BEP) performance of the system is greatly improved by an order of magnitude or more.
A novel single-sideband (SSB) modulator using period phase reversal electrode and the theoretical analysis on the
principle is proposed and presented. The results show that the SSB modulator can reach an optical sideband suppression
ratio over 40dB in 60GHz radio-over-fiber (ROF) system, which successfully reduce the power penalty due to the
chromatic dispersion.
A 40 Gbit/s hybrid optical code-division multiplexing (OCDM)/ wavelength-division multiplexing (WDM) system is experimentally demonstrated and simultaneous multiwavelength optical encoding of a single supercontinuum source using superstructured fiber Bragg grating is achieved. The interchannel interference and noise of the OCDM/WDM system is suppressed apparently by incorporating a nonlinear amplifying loop mirror, both the sidelobe and the pedestal are reduced, the waveform quality of decoded signal is improved, and the original 8.2 ps autocorrelation peak of decoded pulse is compressed to 4.8 ps.
Optical wireless communication systems usually adopt on-off keying (OOK), binary pulse position modulation (BPPM), and binary phase-shift keying (BPSK). In this work, the scheme of systems based on a turbo code of OOK, BPPM, and BPSK is proposed, and the three different iterated maximum a posteriori (MAP) algorithms of turbo code are deduced. To use the turbo code based on different modulated formats over optical wireless channels we deduce the formula of an atmospheric channel signal-to-noise ratio versus the atmospheric visibility. Simulations show that the turbo code based on BPSK is well suited for optical wireless communication systems.
A fiber optic imaging sensor is presented that utilizes surface plasmon resonance (SPR) excitation. The configuration of this sensor allows for remote sensing and multiplexing. The sensor is advantageous in that it eliminates the traditional bulk optic prism in favor of a relatively simple and inexpensive design. Two sensor configurations are presented; the principle of one is based on using optical interference, and that of the other is based on using the optical intensity as in traditional bulk-optic SPR imaging systems. Experimental measurements indicate that the fiber optic sensor based on optical interference is compact and immune to noise due to ambient light, and is more powerful and reliable than the common SPR imaging sensor based on optical intensity. Using fiber optics, the theory of SPR, and interference, a new SPR imaging sensor is achieved, which is better fitted to applications.
Based on the chirp-pulse compensation technique, a 40-GHz supercontinuum (SC) source generated in a highly nonlinear fiber (HNLF) with large normal dispersion is investigated. We show numerically and experimentally that the widest SC spectrum can be obtained by setting a two-stage all-fiber pulse compressor consisted of the HNLF and standard single-mode fiber (SMF) in front of the last-section HNLF for SC generation. The third dispersion of the fibers, especially the SMF used as chirp-compensating fiber, is found to greatly degrade the SC spectrum generated in this scheme.
A seven-chip, 280-Gchip/s OCDM system incorporating quaternary phase coding and decoding is experimentally demonstrated. The encoder and decoder, consisting of superstructured fiber Bragg gratings, are fabricated using the equivalent-phase-shift method; only ordinary phase masks and submicrometer precision in control are required. Interchannel interference noise is also considered and evaluated; 40-Gbit/s×2-channel multiplexing is demonstrated. In addition, a nonlinear amplifying loop mirror (NALM) within the receiver is introduced to improve the system performance under two-channel operation. Switching is obtained for peak signal power less than 10 mW. We show that the NALM can act as a nonlinear processing element capable of reducing both the pedestal associated with conventional matched filtering and the width of the associated code recognition pulse.
Atmospheric optical wireless communication is a fading channel because of the effect of atmospheric attenuation. We introduce a novel turbo code named the two-fold turbo code scheme to provide high performance for optical wireless communication. To use the two-fold turbo code over optical wireless channels, first deduce the formula of atmospheric channel signal-noise-ratio versus atmospheric visibility, then analyze the coding and decoding principle of the two-fold turbo code. Simulations show that the proposed two-fold turbo code significantly improves the performance of the system compared to the common turbo code.
A 7-chip, 280-Gchip/s quaternary phase-shift code is assigned to short pulses after reflection from a encoder, The code is then recognized by a decoder. The encoder and decoder used within our experiments are 4-phase shift SSFBGs. Each grating containing 7-chip has a uniform amplitude refractive index level along its length but in which discrete jumps in phase (0,∏/2,∏,or 3∏/2) are written into the grating at the boundaries of adjacent spatial chips. The 4-phase shift SSFBGs were fabricated using "equivalent phase shift" method, wherein the desired phase shift in one channel of Sampled Bragg grating (SBG) could be achieved by simply changing the sampling period. Only ordinary phase masks and submicrometer precision are needed to fabricate the encoding and decoding SSFBGs. The quaternary phase encoding and decoding of short pulses at 40Gb/s is experimentally demonstrated, to the authors' knowledge, this is the highest single channel rate obtained from FBG based OCDM system. Multiple user interference (MUI) is also considered and evaluated, we present an experimental demonstration 2-user 40Gb/s/user (2×40Gb/s) OCDM system employing 4-phase pulse pattern generation and recognition. In addition, a nonlinear optical loop mirror (NOLM) within the receiver is introduced to improve the system performance under multiuser operation, we show that the NOLM can act as a nonlinear processing element capable of reducing both the pedestal associated with conventional matched filtering and the width of the associated code recognition pulse. The system benefits of using the NOLM are experimentally demonstrated under 2-user operation.
Effective utilization of highly nonlinear fiber (HNLF) with relative large normal dispersion to further broaden optical spectrum of 40-GHz, 1.2-ps optical pulse train is investigated, in which an optimized scheme is presented. In the scheme, the pulse is launched into the HNLF of the first section to generate the spectrum broadened linearly up-chirped pulse, and recompressed after propagating through a suitable length of conventional single-mode fiber (SMF) of the second section for the pulse chirp being compensated. At last, the pulse travels through the second HNLF of the third section for further optical spectrum broadening. This new scheme applies the chirp pulse compensation technique and has scalability to the peak power of seed pulse. The experimental results show that ~1.7 nm increment of 3dB spectrum bandwidth is obtained when the new scheme is implemented given the same HNLF length and input seed pluses, and the coherence properties of the original pulses are maintained. This scheme provides a more effective means of pulse spectrum broadening in the HNLF when the peak power of seed pulse is confined by the EDFA output power and the high repetition rate of pulse laser.
Optical wireless communication (OWC) is beginning to emerge in the telecommunications market as a strategy to meet last-mile demand owing to its unique combination of features. Turbo codes have an impressive near Shannon-limit error correcting performance. Twofold turbo codes have been recently introduced as the least complex member of the multifold turbo code family. In this paper, at first, we present the mathematical model of signal and optical wireless channel with fading and bit error rate model with scintillation, then we provide a new turbo code method to use in OWC system, we can obtain a better BER curse of OWC system with twofold turbo code than with common turbo code.
KEYWORDS: Free space optics, Telecommunications, Promethium, Receivers, Free space optical communications, Picosecond phenomena, Signal attenuation, Signal detection, Signal processing, Semiconductor lasers
Commercial and military interest in free-space optical communication (FSO) is growing due to the high bandwidths, portability and high security. FSO systems with proper radio-frequency backup can provide a viable solution for the last mile problem The communication reliability of FSO has been being pay attention to. In this paper, an innovation solution of light power equalization is put forward and some technologies such that enlarging the dynamic range of light power gain and realizing communication for FSO in the condition of all-weather are studied, a method to implement the power equalization is also given in term of system-embedded, some experiments are implemented with a system self-fabricated, too, the experimental results shown that a wide range power equalization system on FSO can effectively reduce the influences from the random variation of
atmospheric medium and other factors on the communication link path.
A Surface Plasmon Resonance(SPR) sensor based on Kretschmann configuration has been setup. In this setup, Ag was applied as supporting metal, and incident angle was fixed. There are two main method, one is Surface Plasmon Microscopy(SPM) based on optical intensity, the other the Surface Plasmon Interference Microscopy (SPIM) based on the theory of Surface Plasmon Resonance and optical interference. SPM and SPIM were analyzed both theoretically and experimentally. Comparing and analyzing the result of theory and experiment, the result shows that SPIM has higher spatial resolution than SPM, and more powerful and immune to noise due to ambient light. So the SPIM is more fitful for sensor applications than SPM.
Optical wireless communication (OWC) systems use the atmosphere as a propagation medium. In optical wireless communication links, atmospheric turbulence causes distortions in the laser beam wavefront, leading to fluctuations in both the intensity and the phase of the received light signal performance, i.e. signal fading. This paper presents a suppression method of turbulence noise on the basis of analyzing the optical signal effect of turbulence.In this paper, according to probability distributions of turbulence-induced intensity, we describe a BEP model. From the model, we develop an adaptive threshold detector technique to decrease the BEP. The BEP increases from 10-11 to 10-7 for a linear increase of normalized standard deviation σx of the log-amplitude from 0 to 0.5 which is shown in fig.1 at a stationary threshold. Finally, we describe the matched receiver to maximize the output signal-to-noise ratio (SNR). We also numerically compute the average bit-error probabilities for the filter case.
The Free Space optical communication (FSO) or wireless optical communication, utilizes the atmospheric medium as transmission channel, where random variety such as fog, atomy and atmosphere flash and the atmospheric turbulence will badly affect the propagation of light, the receiving signal is easily swung and drifted with the change of weather. In this paper, we discussed the attenuation of the atmospheric channel and analyzed the signal characteristics in the condition of the atmospheric overfall, for the OOK modulation, discussed the receiving signal distribution in the atmospheric channel taking account for the noise gain of the light detector, and based on the principle of the Hartman-Shack sensor, we designed a wave-front distortion compensation system with fiber coupler. The signal fading resulted from wave-front phase distortion was compensated effectively by using the compensation system.
The scalar two-dimensional finite difference time domain (FDTD) method is applied to the simulation of light field distribution of waveguide grating couplers. The mode patterns of TE0, TE1, TE2 and TE3 are obtained by using planar optical source as a stimulin. A few important problems are studied carefully, such as: the effect on field distribution with different grating parameters and waveguide structures, the absorbing boundary conditions, etc. The procedure for calculating is compiled in MATLAB. The calculating results are analyzed carefully.
A way to overcome the fading caused by atmosphere is discussed, and the power gain equalization method for the
wireless optical communication system is studied, the results for the system are also given. In the scheme of wide-range power-gain equalization, the power gain of the wireless optical system is controlled automatically in accordance with
visibility change of propagation channel by means of sampling the propagation channel, which has also been shown.
Free space optics (FSO) is attractive for the 'last mile' communication in recent years for many combining advantages of fiber communication and other wireless technologies. FSO can provide high data rate with low power consumption, high immunity to interference, convenient deployment and flexibility. Optical system is an important section in the FSO transceiver terminal. In this paper the design of optical system based on a single Galileo telescope for both transmit and receive is proposed, and a polarization beam splitter is adopted to apart the receiving light from transmitting light. The configuration can avoid interference from the retroreflecting light of the ocular effectively. Some factors that affect the performance of the optical system are analyzed, such as the geometrical spreading loss and the loss increment according to pointing error and telescope maladjustment. Power budget shows that the system can satisfy the need of access for 1km in the light fog, and 2km in the thin fog.
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