Atmospheric turbulence is a crucial factor affecting the performance of space optical communication systems. The mode diversity is considered an effective turbulence compensation method. In the mode diversity system, coherent combining algorithms are needed to combine the received signals from multiple modes to counteract the effects of turbulence. In this paper, we investigate the performance of 3 mode diversity coherent combining based on the selection combining (SC) algorithm in a 2.5 GBaud QPSK real-time back-to-back transmission system. We simulate turbulence with dynamic changes and calculate the bit error rate (BER) periodically. The experimental results show that within 1 second, the SC algorithm can reduce the frame error rate (FER) by 45% compared to using only single mode reception.
This paper presents a two-stage frequency offset estimation (FOE) algorithm to compensate for Doppler frequency shift (DFS) without local oscillator (LO) laser frequency tuning. By increasing the sampling rate to 12 samples per symbol (SPS) and using a small number of pilots, the frequency offset compensation range is increased from ±Rs/8 of the conventional 4-th power FOE to ±12Rs, where Rs is symbol rate. In the offline experiment of a 2.5GBaud QPSK coherent optical communication system, this algorithm compensates simulated DFS, which ranges from -8GHz to 8GHz. The scheme achieves a sensitivity of -50.5dBm at an average bit error rate (BER) of 1E-3. The penalty with or without DFS is less than 1 dB.
KEYWORDS: Adaptive optics, Single mode fibers, Turbulence, Free space optics, Atmospheric turbulence, Telecommunications, Receivers, Optical engineering, Control systems
The performance of mode diversity combined with adaptive optics (AO) based on a multipath stochastic parallel gradient descent (SPGD) algorithm over free-space optical (FSO) links through atmospheric turbulence is presented. The experimental results indicate that, compared with a single-mode fiber (SMF) combined with a single-path SPGD algorithm, three modes diversity combined with a multipath SPGD algorithm effectively reduces the power fluctuation. When the sensitivity of the receiver is assumed, the outage probability for the FSO system based on mode diversity is superior to that of the FSO system based on a SMF. The reduction of outage probability arising from mode diversity is more significant when AO is implemented. To the best of our knowledge, this is the first time that the performance of modes diversity combined with a multipath SPGD algorithm over a turbulence channel has been studied.
We experimentally investigate the bit error rate(BER) performance of soft-decision low density parity check (LDPC) turbo decoding (Turbo-LDPC) in a 56Gbit/s high power sensitivity DQPSK coherent communication system. Power sensitivity of -38.3dBm and -39.3dBm are realized by using LDPC and Turbo-LDPC, respectively. Turbo-LDPC outperforms LDPC by 1dB. Turbo-LDPC successfully decodes signals with 1.8E-2 pre-(forward error correction) FEC BER and traditional LDPC decoder fails to decode error free above 2.6E-2 pre-FEC BER.
Stochastic parallel gradient descent (SPGD) algorithm is a kind of simple and practicable algorithm for adaptive optics (AO) to compensate the effects of turbulence. The relationship between the number of control variables and convergence effects is explored experimentally in this paper. We find that the number of control variables is not always the larger the better. Under the condition of weak turbulence, the algorithm with 6×6 control variables gives better results than 12×12 control variables and 8×8 control variables. With the increase of turbulence strength, the convergence effects of 12×12 control variables are better than 6×6 control variables and 8×8 control variables. In the condition of weak turbulence, SPGD algorithm with 6×6, 8×8, 12×12 control variables can improve coupling efficiency by 3.2dB, 3.1dB and 3.0dB respectively. In the condition of moderate turbulence, SPGD algorithm with 6×6, 8×8, 12×12 control variables can improve coupling efficiency by 6.4dB, 6.6dB, 7.1dB respectively. In the condition of strong turbulence, SPGD algorithm with 6×6, 8×8, 12×12 control variables can improve coupling efficiency by 6.2dB, 7.1dB, 10.2dB respectively
Polarization maintaining fibers (PMFs) can keep linear polarization state against external perturbations by inducing a high effective refractive index difference (Δneff) along one polarization axis. For few mode polarization maintaining fibers (FM-PMFs), Δneff is applicable between both orthogonal linear polarization modes (e.g. LP01x and LP01y) and orthogonal degenerated modes (e.g. LP11a and LP11b), which can enable advanced functionalities in multiple-input multiple- output-free spatial division multiplexing systems and optical fiber sensing systems. Therefore, the measurement of Δneff for polarization modes and degenerated modes is very important for determining the quality of a FM-PMF. However, measurement of the Δneff for FM-PMFs can be complicated due to the requirement for generating and demultiplexing of the higher order modes (HOMs). In this paper, we propose to measure the Δneff of FM-PMFs using Spatially and Spectrally resolved imaging (S2) method for the first time. The presented method is simply by employing a tunable laser and an IR CCD camera, can avoid any mode converter or mode multiplexer/demultiplexer, featuring a rapid testing speed. A proof-of-concept experiment is carried out to measure FM-PMFs with a length of 1.1m and 5m. The Δneff between the orthogonal polarization modes (i.e. LP11ax-11ay, LP11bx-11by, LP21ax-21ay, and LP21bx-21by) are characterized as 7.05×10-4, 6.91×10-4, 1.02×10-3 and 1.04×10-3 respectively. The Δneff of the orthogonal degenerated modes (i.e. LP11ax-11bx, LP11ay-11by, LP21ax-21bx and LP21ay-21by) are also characterized to be 1.39×10-4, 1.24×10-4, 5.61×10-5 and 6.53×10-5 respectively.
In few-mode polarization-maintaining-fiber (FM-PMF), the effective-index splitting exists not only between orthogonally polarization state but also between degenerated modes within a high-order mode group. Hence besides the polarization state evolution, the mode patterns in each LP set are need to be analyzed. In this letter, the completed firstorder mode (LP11 mode) evolution in PM-FMF is analyzed and represented by analogous Jones vector and Poincarésphere respectively. Furthermore, with Jones matrix analysis, the modal dynamics in FM-PMFs is conveniently analyzed. The conclusions are used to propose a PM-FMF based LP11 mode rotator and an PM-FMF based OAM generator. Both simulation and experiments are conducted to investigate performance of the two devices.
A flexible atmospheric turbulence simulator based on spatial light modulator is investigated experimentally. Phase screens generated by Fourier Transform method are used in the proposed scheme. The effect of time-varying atmospheric turbulence with different strength is successfully demonstrated. The far-field intensity and received optical power of Gauss-shaped laser beam under relatively weak, moderate and strong atmospheric turbulence are measured. Experiments show that atmospheric turbulence will cause the distortion of far-field intensity distribution and fluctuation of received optical power.
Polarization-division-multiplexed(PDM) optical coherent systems is considered a promising technique for next generation optical networks. With coherent detection, various impairments in the optical transmission system can be compensated by using digital signal processing (DSP) in the electrical domain. Constant modulus algorithm (CMA), due to its simplicity and immunity to phase noise, has been widely used to demultiplex polarizations and compensate received signals. On the other hand, CMA suffers from the singularity problem which results from Polarization- Dependent-Loss (PDL) and the less sensitivity in phase of CMA. Although many people have researched the singularity problem of CMA both in theory and experiments (modify CMA to avoid singularity problem), their theoretical channel model only contains fiber birefringence and does not consider the situation of multistage channel model which is used in long distance transmission system. Then we analyse the performance of CMA in the channel with multistage channel model through simulation. We change the initial tap of the traditional CMA which can achieve correct polarization demultiplexing without singularity in one stage channel model. But it is not very suitable in multistage channel model. We analysed singularity ratio in long distance transmission system. The simulation results play an active role in following research on singularity problem and the performance of CMA.
A novel mode-beating DBR laser with dual-mode lasing is fabricated. The DBR laser has four parts, a front gain section, a phase section, a DBR grating section, and a rear gain section. When the current of the front gain section is above the threshold, the device is working in single-mode. Dual-mode lasing can be obtained by adjusting the current of the rear gain section. The power difference between the two modes can be less than 1 dB. An optical down-conversion technique was used to measure the beating frequency. The mode-beating frequency of the two modes is about 93 GHz, and the 3- dB linewidth of the mode-beating RF spectrum of the laser when free-running is about 5 MHz. Moreover, the wavelength of the dual-mode can be tuned synchronously when the current injected into the DBR grating section is adjusted. The wavelength tuning range of the device is at least 3 nm.
Star L quadrature amplitude modulation (SLQAM, L representing the number of constellation points) modulated orthogonal frequency division multiplexing (OFDM) is applied to the optical domain for the first time. Numerical simulations are conducted between an S16QAM and a square 16QAM modulated optical OFDM system to compare the tolerance toward various transmission penalties in a 30.2-Gbit/s coherent optical OFDM (CO-OFDM) system over 640, 720, and 800 km. After 640-km transmission, the optimal launch power and corresponding Q2 factor of an S16QAM modulated optical OFDM signal are 2 and 0.6 dB higher, respectively, than the square 16QAM modulated OFDM signal in a 30.2-Gbit/s single-channel single-polarization CO-OFDM system, while in a 5×30.2-Gbit/s wavelength-division multiplexing single-polarization CO-OFDM system, the optimal launch power and corresponding Q2 factor of an S16QAM modulated optical OFDM signal are 1 and 0.7 dB higher than with a square 16QAM modulated OFDM signal, showing increased tolerance toward nonlinearity such as SPM and XPM.
A more simplified photonic microwave frequency measurement configuration based on a loop mirror filter is proposed and experimentally demonstrated. It is implemented by propagating a carrier-suppressed double-sideband modulated optical signal through a loop mirror filter. The ratio between the optical powers of the two output ports provides a direct measurement of the unknown signal frequency. This scheme can solve the main problems that exist in previous work. A proof-of-concept experiment is performed with a measurement range of 6 to 18 GHz and resolution less than 0.35 GHz.
Compared with the optical communication system using intensity modulation with direct detection (IM-DD), the
coherent optical system enhances the sensitivity and enables access to all the optical characteristics. In this paper, a novel
scheme of coherent QAM transmission system based on heterodyne optical detection is experimentally demonstrated. In
the proposed scheme, the intermediate frequency (IF) carrier modulation at double-sideband suppressed carrier (DSB-SC)
mode at the transmitter side enables demodulation of QAM signal with heterodyne detection. Furthermore, the receiver
is simplified by avoiding the use of high frequency broadband microwave devices due to the employment of IF carrier
modulation. The performance of the proposed coherent QAM system is analyzed theoretically based on numerical
simulation. The simulation results show that the improved structure of the proposed system can effectively depress the
phase noise induced by laser. An experiment of 4-QAM transmission exploiting the proposed coherent heterodyne
system is presented to justify the principle.
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