Spectrum-Splitting Diffractive Optical Element of High Concentration Factor and High Optical Efficiency for Three-Junction Photovoltaics

A spectrum-splitting and beam-concentrating （SSBC） diffractive optical element （DOE） for three-junction pho- tovoltaics （PV） system is designed and fabricated by five-circ/e micro-fabrication. The incident solar light is efficiently split into three sub-spectrum ranges and strongly concentrated on the focal plane, which can be di- rectly utilized by suitable spectrum-matching solar cells. The system concentration factor reaches 12x. Moreover, the designed wavelengths （450nm, 550nm and 65Onto） are spatially distributed on the focal plane, in good agree- ment with the theoretical results. The average optical effic/ency of all the cells over the three designed wavelengths is 60.07%. The SSBC DOE with a high concentration factor and a high optical efficiency provides a cost-effective approach to achieve higher PV conversion efficieneies.

Solar Flux Measuring and Optical Efficiency Forecasting of the Linear Fresnel Reflector Concentrator after Dust Accumulation

The linear Fresnel reflector concentrator(LFRC)is widely used in the field of solar energy utilization due to its simple structure,low cost,and excellent wind resistance.Nevertheless,the LFRC operates outdoors all year round,and the dust accumulation on the mirror will reduce the optical efficiency of the system,so it needs to be perfected and improved.In this paper,a focal plane energy flux experimental device was designed to test the energy flux of the system under different dust accumulation times.The results indicate that,the dust density on the mirror increased and the energy flux on the focal plane decreased with increase of dust accumulation time.After undergoing dust accumulation for 35 days,the dust density on the mirror reached 4.33 g/m^(2)and the average energy flux on the focal plane decreased to 1.78 kW/m^(2).Additionally,the variation of reflectivity caused by dust accumulation on mirror was taken as the quantitative index,and a prediction model for the impact of dust on the optical efficiency of the system was proposed.The results will provide guidance for improving the optical efficiency of the LFRC.

Simple and universal method in designs of high-efficiency diffractive optical elements for spectrum separation and beam concentration

Diffractive optical elements（DOEs） with spectrum separation and beam concentration（SSBC） functions have important applications in solar cell systems. With the SSBC DOEs, the sunlight radiation is divided into several wave bands so as to be effectively absorbed by photovoltaic materials with different band gaps. A new method is proposed for designing high-efficiency SSBC DOEs, which is physically simple, numerically fast, and universally applicable. The SSBC DOEs are designed by the new design method, and their performances are analyzed by the Fresnel diffraction integral method.The new design method takes two advantages over the previous design method. Firstly, the optical focusing efficiency is heightened by up to 10%. Secondly, focal positions of all the designed wavelengths can be designated arbitrarily and independently. It is believed that the designed SSBC DOEs should have practical applications to solar cell systems.

Capacity Scaling Limits and New Advancements in Optical Transmission Systems

Optical transmission technologies have gone through several generations of development.Spectral efficiency has significant ly improved,and industry has begun to search for an answer to a basic question：What are the fundamental linear and nonlin ear signal channel limitations of the Shannon theory when there is no compensation in an optical fiber transmission system？Next-generation technologies should exceed the 100G transmis sion capability of coherent systems in order to approach the Shannon limit.Spectral efficiency first needs to be improved be fore overall transmission capability can be improved.The means to improve spectral efficiency include more complex modulation formats and channel encoding/decoding algorithms,prefiltering with multisymbol detection,optical OFDM and Ny quist WDM multicarrier technologies,and nonlinearity compen sation.With further optimization,these technologies will most likely be incorporated into beyond-100G optical transport sys tems to meet bandwidth demand.

Improving the intensity and efficiency of compressed echo in rare-earth-ion-doped crystal

We investigate the intensity and efficiency of a compressed echo, which is important in arbitrary waveform generation（AWG）. A new model of compressed echo is proposed based on the optical Bloch equations, which exposes much more detailed parameters than the conventional model, such as the time delay of the chirp lasers, the nature of the rare-earth-iondoped crystal, etc. According to the novel model of compressed echo, we find that reducing the time delay of the chirp lasers and scanning the lasers around the center frequency of the inhomogeneously broadened spectrum, while utilizing a crystal with larger coherence time and excitation lifetime can improve the compressed echo＇s intensity and efficiency. The theoretical analysis is validated by numerical simulations.

Low-cost method of fabricating large-aperture,high efficiency,Fresnel diffractive membrane optic using a modified moir technique

To reduce the cost and achieve high diffraction efficiency, a modified moir@ technique for fabricating a large- aperture multi-level Fresnel membrane optic by a novel design of alignment marks is proposed. The modified moire fringes vary more sensitively with the actual misalignment. Hence, the alignment accuracy is significantly improved. Using the proposed method, a 20 μm thick, four-level Fresnel diffractive polyimide membrane optic with a 200 mm diameter is made, which exhibits over 62% diffraction efficiency into the ＋1 order, and an efficiency root mean square of 0.051.

Flexible,robust and highly efficient broadband nonlinear optical materials based on graphene oxide impregnated polymer sheets

We report a simple solution-processed method for the fabrication of low-cost,flexible optical limiting materials based on graphene oxide(GO) impregnated polyvinyl alcohol(PVA) sheets.Such GO–PVA composite sheets display highly efficient broadband optical limiting activities for femtosecond laser pulses at 400,800,and 1400 nm with very low limiting thresholds.Femtosecond pump–probe measurement results revealed that nonlinear absorption played an important role for the observed optical limiting activities.High flexibility and efficient optical limiting activities of these materials allow these composite sheets to be attached to nonplanar optical sensors in order to protect them from light-induced damage.

An efficient intracavity-pumped KTP optical parametric oscillator at 1572 nm

We demonstrate an efficient and eye-safe wavelength intracavity optical parametric oscillator （OPO）, based on a KTP crystal inside a Q-switched Nd：YVO4 laser end pumped by a fiber-coupled diode laser. In the acousto-optic Q-switched operation with the pulse repetition rate of 10 kHz, a 1572-nm eye-safe laser with the average power of 237 mW at the incident pump power of 5.64 W is obtained. Under the pulse repetition rate of 5 kHz, the signal light with pulse width of 2 ns and peak power of 18.5 kW is achieved. The conversion efficiency of the average power is 4.2~0 from pump diode to OPO signal output and the signal pulse duration is about 13 times shorter than that of the depleted pump light. OCIS codes： 140.3480, 140.3580, 190.4410, 190.2620.

Performance evaluation of a WDM/OCDM based hybrid optical switch utilizing efficient resource allocation

A hybrid optical switch （HOS） with physical layer of wavelength division multiplexing and optical code division multiplexing （WDM/OCDM） scheme is proposed. An additional feature to the HOS than optical cross connect （OXC） is that the controller can process requests for both circuit establishment and burst scheduling. In our study, the measurement criteria of HOS are the blocking probability, probability of error, and probability of outage. To simplify the analysis, no distinction is made between a circuit in progress and a burst in progress. Moreover, a minimum fit （MinF） resource allocation strategy is applied in order to increase the bandwidth efficiency and control the multiplexing interference of the OCDM. A 2D Markov model for the HOS is presented using the MinF strategy. Numerical results reveal that the code parameters and the resource allocation strategy greatly affect the performance. Certain periority can be achieved by assigning shorter codes to high periority users and longer codes to low periority users. Also, the probability of error and outage are reduced bv aonling the MinF strategy.

Smart luminescent solar concentrator as a BICPV window

Building integrated concentrating photovoltaic(BICPV)windows have attracted numerous studies in recent years.However,there is a tradeoff between the light transmittance and power generation efficiency in the design of BICPV window.In this paper,a smart luminescent solar concentrator(LSC)is introduced as the BICPV window.The proposed smart LSC system features on the combination of fluorescent dyes with thermochromic materials to enhance photoelectric conversion efficiency as well as form a dynamic response mechanism to ambient solar radiation and environmental temperature.In this study,a BICPV smart window system consists of the waveguide doped with organic dye Lumogen F Red-305(BASF)and the thermochromic hydrogel membrane has been developed.The research on analytic design parameters is executed through optical simulation by ray tracing technology along with outdoor comparative experiments.From simulations for a smart LSC of 100 mm×100 mm×3 mm with a bottom-mounted solar cell of 100 mm×10 mm,the optical effective concentration is found to be with the range of 1.23 to 1.31 when a highest gain of 1.26 in power over the bare solar cell is obtained from experiments.

Numerical simulation of the performance of the a-Si:H/a-SiGe:H/a-SiGe:H tandem solar cell

The computer program AMPS-1D（analysis of microelectronic and photonic structures） has been employed to simulate the performance of the a-Si：H/a-SiGe：H/a-SiGe：H triple-junction solar cell at the radiation of AM1.5G（100 mW/cm2/ and room temperature. Firstly, three sub-cells with band gaps of 1.8, 1.6 and 1.4 eV are simulated, respectively. The simulation results indicate that the density of defect states is an important factor, which affects the open circuit voltage and the filling factor of the solar cell. The two-step current matching method and the control variate method are employed in the simulation. The results show that the best solar cell performance would be achieved when the intrinsic layer thickness from top to bottom is set to be 70, 180 and 220 nm, respectively. We also optimize the tunnel-junction structure of the solar cell reasonably, the simulation results show that the open circuit voltage, filling factor and conversion efficiency are all improved and the S-shape current density–voltage curve disappears during optimizing the tunnel-junction structure. Besides, the diagram of the energy band and the carrier recombination rate are also analyzed. Finally, our simulation data are compared to the experimental data published in other literature. It is demonstrated that the numerical results agree with the experimental ones very well.