We present an inter-satellite coherent laser communication prototype system, highlighting a low complexity receiver with a simplified two independent real-valued (TIR) equalizer suitable for space laser communication scenarios. An extensive comparison of classical single complex-valued equalizer, 2×2 real-valued equalizer, TIR, and two dependent real-valued (TDR) equalizer is presented on the basis of equalization principles, algorithm performance, and computational complexity. The analysis results show that TIR equalizer has the advantages of low logic resource consumption in implemented on field programmable gate array (FPGA) and zero sensitivity penalty to combat I/Q gain imbalance and I/Q skew caused by device aging. The constant modulus algorithm-based TIR equalizer fits binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), or higher-order QAM modulation format, while the traditional complex-valued equalizer needs to customize the tap coefficient updating algorithm for different modulation formats. In addition, to show the feasibility of the proposed TIR equalizer-based coherent laser receiver as a promising candidate for forthcoming inter-satellite networks, we demonstrate single polarization (SP) on-line 2.5 GBaud data rate, QPSK and BPSK coherent transmissions using an FPGA-based transmitter and receiver prototype. Thanks to our proposed equalizer, the number of hardened multipliers is reduced by 47%, while no receiver sensitivity penalty is observed both in numerical simulation and in real-time FPGA experiment.
Algorithmic and architectural options and trade-offs between performance and complexity/power dissipation are necessary to consider in the transfer of coherent technology from long-haul transmission to short-reach application. An adaptive equalization algorithm cascading polarization demultiplexing and frequency domain equalization is proposed to target passive optical network applications that feature extremely low computational complexity. Three coordinate rotation digital computer-based vector rotators are introduced to realize polarization demultiplexing without performing multiplications. A simplified gradient descent algorithm is proposed to search and trace the state of polarization. A nonbutterfly adaptive equalizer in the frequency domain for each polarization is designed to handle link chromatic dispersion and the imperfection of the transceiver frequency response. The performance of the proposed equalization scheme is experimentally verified by a 32-GBaud dual-polarization Nyquist quadrature phase-shift keying system over 20-km standard single-mode fiber transmission. With the proposed method, the number of hardened multipliers for field-programmable gate array realization is reduced by ∼85 % compared with a conventional 2 × 2 multiple-input multiple-output finite impulse response filter with the same receiver sensitivity.
Receiver in-phase/quadrature (I / Q) imbalance caused by imperfect analog receiver front end can rapidly degrade system performance in a coherent optical transmission system. These imbalances, including I / Q gain and phase imbalance, become increasingly important when higher symbol rate, higher modulation order, and low roll-off pulse shape are used. Here, a low-complexity blind equalization algorithm, cascaded phase, and gain imbalance equalizers, is designed to mitigate these interferences. Furthermore, a field-programmable gate array hardware realization with 128-parallel processing units is performed to demonstrate the proposed algorithm in a 32-GBaud 16-quadrature amplitude modulation off-line transmission experiment. Based on the obtained results, the proposed blind I / Q imbalance compensation algorithm can provide very good compensation performance with low logic element consumption, which is more suitable for high order and high baud rate transmission system.
We adopt hexagonal optofluidic ring scatterers to built two-dimensional photonic crystal waveguide (PCW) with triangular lattice. By studying slow light effects of varieties of optical optofluidic rings, the thickness of optofluidic ring in X and Z direction, and the moving distance of the first row of scatterers near central waveguide, some relatively optimism results have been founded. In addition, in the process of research, we adopt PWE method to simulation calculation. When the thickness of optofluidic ring changes, the optimization results which ng equals 47.2120, bandwidth Δλ is 28.5nm and the group velocity dispersion β2 is 43.3418 ps2/mm. When the moving distance changes, the optimization results we could get that ng equals 15.6569, Δλ is 92.9nm and β2 is 7.8202 ps2/mm. This wideband and low dispersion slow light can be used for storage capacity with certain requirements of the optical buffer, optical sensors, etc.
By logically superimposing three base one dimensional (1D) photonic crystals, the broadband slow light with low dispersion is obtained. The slow light pass band is smoothed by adjusting the period ratio of three base structures, period number of new photonic crystal (PC) and the filling factor. In the optimized structure, the minimum of transmission (T) arrives at 0.5902, the full width half maximum (FWHM) is 55nm, the group velocity is in the range from 0.08326c to 0.2912c, and the group velocity dispersion (GVD) parameter D maximum is 14.65 ps2/nm·mm. Moreover, by material optimization, adjusting the refractive index of na and nb, the slow light properties can be improved further. With suitable materials, the T minimum increases to 0.6485, the group velocity decreases to the range from 0.08014c to 0.2592c, and GVD parameter D maximum decreases to 13.98 ps2/nm·mm with FWHM 52nm.
Based on combining material schemes and structure optimizing of two-dimensional triangular lattice photonic crystal, one methodology to improve slow light properties are proposed to support large NDBP at any group index in a photonic crystal line defect waveguide. Firstly, the effects of first and second row holes reflective indies n1 and n2 has been discussed, respectively. Then, one of the structural parameters, radius of holes in the first row r1, has been adjusted to extend the range of group index with large NDBP. By properly adjusting various combinations of n1 and r1, the value of NDBP higher than 0.319 can be obtained at group index from 50 to 200. The design has shown the feasibility of post-fabrication technology to generate high performance slow light at any group index.
Wide band and low dispersion slow light in optofluidic infiltrated photonic crystal waveguide has been theoretically demonstrated. By adjusting the refractive indices of infiltrated optofluidic in the nearest two rows of air holes beside the defect waveguide and tuning the radius of bulk holes, the group index and bandwidth can be enlarged effectively at the same time. Considering the restricted variation of group index ng within ±10%, the flat band slow light at any group index from 17.80 to 77.55 with large normalized delay bandwidth product staying in the range of 0.32 to 0.36 has been obtained. The most attractive is that there is a stable bulk hole r=0.328a as the refractive index of second row holes n2 changed for optimized slow light performances with large NDBP in large ng range.
A novel structure of photonic crystal coupled resonator optical waveguide (PC-CROW) with elliptical rod around
cavity is proposed to realize compact, high sensitivity modulated and high-performance buffering application. By
adjusting the long axis and short axis of the elliptical rods, the slow light and buffer performance of PC-CROW are
optimized. As ae=0.42a, be=0.20a, the group velocity is below 2.3053×10-4c, simultaneously, the buffer capacity C and delay time Ts reach the optimum value. Then the dynamic modulation of the slow light and buffer performance based on this optimized structure has been discussed systematically. The guided mode shifts linearly to short wavelength and delay time decreases exponentially as the external modulated voltage increases. And the modulation sensitivities are about 3.0nm/mV and 0.467ns/mV, respectively. These results show that the proposed structure has considerable potential for optical buffering application.
hese results show that the proposed structure has considerable potential for optical buffering application.
The significant effect factor of slow light transmission in photonic crystal coupled resonator optical waveguide has been
investigated. The change tendency of guided mode band, group velocity and group delay with the tuning of distance
between neighboring cavities have been discussed in detail. The guided mode is flattened clearly and the group velocity
decreases sharply with the increasing of cavities distance. When the interval between contiguous cavities is 7 lattice
constants of photonic crystal, the ultra small group velocity below 2×10-4c has been obtained. This is roughly two orders
of magnitude smaller than that in line defect PC waveguide. The maximal group delay through CROW of 13.23μm with
supercell 7×7 exceeds 400ps, which are four orders of magnitude larger than that in traditional waveguide with same
length.
We investigate the influence of structure parameters to the band structures of one-dimensional photonic crystals
stacking of alternating positive and negative refractive index materials. The refractive index of these two materials has
the same given absolute value. We find that the layers' thickness of the photonic crystals determines the frequency and
angle of the transmission peaks, and when the thickness is given, the permittivity and permeability of the negative index
material determine the width of frequency peak and angular peak. For realizing ultra-narrow band frequency and sharp
angular filer, we add a defect layer with positive refractive index in the middle of the photonic crystals. Setting its
structure parameters satisfying some special conditions, there will be a narrow peak in the middle of the band gap.
When electromagnetic wave incident on such PC, only those portions with special frequency and special angle can pass
though, and the accuracy can be very high. Such filter will have wide applications in microwave and optical
communications.
Transmission properties of slow light in one dimensional photonic crystal coupled resonator waveguide have been
investigated. By inserting multiple half-wavelength cavities discretely to forming CROW, the slow light band in
photonic crystal bandgap can be broadened effectively. Otherwise cavities distance and refractive difference can flatten
the rough pass band. By modulating these parameters, an optimized broadened flat pass band with 20.99nm has been
obtained. Within the band, the group velocity is in the range of 0.0142c to 0.02148c. The propagation mechanism has
been investigated via analysis of the field distribution in CROW.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.