Jun 2008, Volume 1 Issue 1-2

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  • ZHOU Bingkun
  • WANG Jian, SUN Junqiang, SUN Qizhen, ZHANG Weiwei, HU Zhefeng, ZHANG Xinliang, HUANG Dexiu
    An all-optical 40 Gbit/s tunable single-to-single channel wavelength conversion is experimentally realized based on cascaded sum- and difference-frequency generation (cSFG/DFG) in periodically poled LiNbO3 (PPLN) waveguides. By employing two tunable filters to effectively suppress the amplified spontaneous emission (ASE) noise, both wavelength down- and up-conversions are simultaneously observed. We also propose and verify a novel cSFG/DFG-based single-to-dual channel wavelength conversion by setting two pumps (pump1, pump2) close to each other or pump2 and the signal close to each other. For the latter, two kinds of cSFG/DFG schemes are both demonstrated. The dependence of the conversion efficiencies of two channel idler waves on pump1 wavelength is discussed. The wavelength relationships between two channel idler waves and the three incident waves are investigated in detail theoretically as well as experimentally.
  • WANG Xue, HE Zhixue, LIU Hongxing, LIU Hong, LI Wei, HUANG Dexiu, CHI Nan
    Orthogonal ASK/DPSK labeling, which encodes label information on optical carriers in a modulation format, is regarded as a competing scheme to sub-carrier multiplexed optical labeling. However, the inherent cross-talk limits the extinction ratio (ER) of the payload and label sensitivity. In this paper, two optical coding methods–Manchester coding and 8B10B coding–which can remarkably improve the system quality to obtain acceptable ER are discussed. A novel optical packet encoding method–mark-insertion coding–is demonstrated to significantly reduce cross-talk between the amplitude shift keying (ASK) payload and the differential phase shift keying (DPSK) label through spectrum shaping. The performance of mark-insertion coding depends on the number of inserted ‘marks’. Finally, the transmission over a 40 km single mode fiber (SMF) is compared with optical label swapping for a 40 Gbit/s ASK payload and 2.5 Gbit/s DPSK label under these coding schemes. The experimental result is consistent with the theoretical analysis.
  • YUAN Jianguo, JIANG Ze, MAO Youju, YE Wenwei
    The three concatenated coding schemes of the inner-outer type, the parallel type and the consecutive type to improve the current forward error correction (FEC) coding technologies are proposed for dense wavelength-division multip
  • WANG Zhi, LIU Yange, KAI Guiyun, LIU Bo, ZHANG Chunshu, JIN Long, FANG Qiang, YUAN Shuzhong, DONG Xiaoyi
    The authors have reviewed some of their recent studies on photonic bandgap fibers (PBGFs). PBGFs that confine light in the core by the photonic bandgap effect of cladding have potential applications in various photonic devices. In this paper, the guided properties and tuned mechanics of anti-resonant PBGFs are theoretically illustrated. The special coupling properties in multi-core PBGFs, such as decoupling and resonant coupling effect, are then introduced. Finally, fiber Bragg grating inscribed in all-solid PBGFs is theoretically and experimentally studied, and special resonant characteristics are also observed.
  • LUO Tianshu, CHEN Jianxin, ZHUO Shuangmu, JIANG Xingshan, Chen Huanglin, Luo Teng, ZOU Qilian
    The layer structures of the esophageal and oral tissues were investigated by using spectral imaging technology based on two-photon excited fluorescence (TPEF) and second-harmonic generation (SHG). Because spectral imaging technology allows a simultaneous record of both the spectra and image, it is capable of identifying the layered structures of the epithelial tissues, including the keratinizing layer, epithelial cell layer and stromal layer in the molecular level, which are strongly correlated to tissue pathology. All this work indicates that this technique has the potential to provide more accurate and comprehensive information for the early pathological diagnosis of tissues with the stratified squamous epithelia.
  • ZUO Tiechuan, QI Heng, YAO Liying, CHEN Tao
    A miniaturized continuous-flow polymerase chain reaction (PCR) microfluidic chip system was developed to perform DNA amplification. This system consists of a 20-cycle continuous-flow PCR microfluidic chip, an electrical heating system and a miniature air pressure-vacuum pump. The chip was ablated with excimer laser direct-writing micromachining technique on a polymethyl methacrylate (PMMA) sheet. The ablated microchannel was inverse trapezoidal with a depth of 70 µm, top width of 200 µm and bottom width of 120 µm. Its surface roughness Ra was 1.42 µm after being treated with excimer laser polishing. The substrate sheet ablated with the microchannel was bonded with other cover sheets using hot-press bonding method to form a closed structure. The electrical heating system consisted of three groups of heating membranes, Pt100 sensors, copper blocks and PID temperature digital controllers. It could provide three distinct maintained temperature zones and a uniform temperature distribution in each zone. PCR amplification of a 170 base pair (bp) DNA fragment was carried out to validate the system’s feasibility. The PCR temperatures were set as 94°C for denaturation, 55°C for primer annealing and 72°C for extension. The flow rate in the microchannel was 40 nL/s and the total time for the completion of a 20-cycle amplification of 20 µL reagent was 15 min.
  • QUAN Guotao, BI Kun, ZENG Shaoqun, LUO Qingming
    In recent decades, diffuse optical tomography (DOT) has drawn more and more interest in molecular imaging because of its advantage of large penetration depth in optical image technology. However, ill-posedness problems have dramatically limited this application technique. In this paper, a new method to remove the ill-posedness of DOT is introduced. With a rotating steady-state domain experiment system, by increasing experimental data that could be obtained from any visual angle, four contrast experiments were simulated. It was proved that when the sum of the experiment data is larger than that of the unknown optical coefficient of phantom, ill-posedness would be reduced and the quality of the reconstructed image could be improved.
  • ZHOU Liwei
    A new theory for temporal aberrations of dynamic optics by applying the direct integral method is put forward in the present paper. A new definition of temporal aberration is given, in which a certain initial energy of electron emission emitted from a photocathode along the axial direction Graphic
    WANG Jian, SUN Junqiang, SUN Qizhen
    A tunable wavelength conversion between picosecond pulses is experimentally demonstrated by using cascaded sum- and difference-frequency generation (cSFG/DFG) in a periodically poled LiNbO3 (PPLN) waveguide. The pulsed signal with 40 GHz repetition rate and 1.57 ps pulse width is adopted. When the input signal and the first control wavelengths are kept at 1554.2 and 1532.5 nm, respectively, the output signal wavelength can be tuned from 1536.0 to 1545.2 nm as the second control wavelength varies from 1550.5 to 1541.0 nm. By varying the first control wavelength to satisfy the quasi-phase matching (QPM) condition for sum-frequency generation (SFG) and simultaneously adjusting the second control wavelength, the tunable output signal wavelength can also be obtained when the input signal wavelength is changed. In the experiment, the amplified spontaneous emission (ASE) noise from the erbium-doped fiber amplifier (EDFA) is effectively suppressed by employing two narrow band tunable filters. Therefore, the wavelength down- and up-conversions are simultaneously observed.
  • FANG Nian, WANG Lutang, HUANG Zhaoming, GUO Shuqin
    To evaluate the security of a chaos optical communication system employing the polarization-shift keying (PolSK) modulation technology, its chaos characteristic needs to be verified. In this paper, an analysis was done for the signal of this system. Three methods were used to judge whether the signal was maintaining chaos characteristics or not: watching the strange attractor in three-dimensional phase space, computing the largest Lyapunov exponent by the equation which meets and Wolf’s method, and evaluating the self-power spectrum density function. As a result, the strange attractor was clearly watched, the largest Lyapunov exponent was positive 0.0364 and 0.0106 respectively, and the self-power spectrum was wide and continuous with the noise background. The evaluation of chaos for the signal transmitted in the system is therefore presented. On the other hand, the minimal embodied dimension of the signal was given by the false nearest neighbors (FNN) method and it reached 6, which showed the higher dimension chaos characteristics of the system. Adding the analysis of the ability of anti-attack for the system, it is concluded that the system has higher security than the normal chaos masking schemes.
  • ZHANG Yejin, SHI Xiaoming, GUO Shuxu, MA Nan, CHEN Minghua, XIE Shizhong
    The paper analyses the effects of intra-channel four-wave mixing (IFWM) on the high-speed optical fiber communication system. A new code format is developed with double 0 and double ? phase separating 1 code; it can decrease the transmission penalty from fiber nonlinearity by counteracting the perturbation terms of IFWM. From the simulation of transmission of two classic code data “11011” and “11100”, we find that our proposed code format has a better suppression effect than the alternative modulation inversion (AMI) or the normal RZ format and supports a wider pulse mode. In a 40 Gbps system, the AMI format is 2 dB less than the RZ format, and our proposed format is 1 dB less than the AMI format when a 1 dB eye open penalty is taken as a metewand. Moreover, it can be realized as easily as the AMI format.
  • CHEN Wei, LI Jinyan, LI Shiyu, JIANG Zuowen, LI Haiqing, PENG Jinggang
    The high nonlinear photonic crystal fiber with pure silica core has been designed and fabricated, and the practical structure parameters of the fabricated fiber sample coincided precisely with the parameters we designed. The core diameter is 1.65 ?m; the air hole diameter is 4.75 ?m; the distance between the center of two holes is 5.35 ?m; the zero dispersion wavelength of the fiber is 1120 nm; the dispersion at 800 nm is -88 ps(nmkm)-1; and the nonlinear coefficient of this photonic crystal fiber is 112 (Wkm)-1. The broadly spanning supercontinuum emission with a smooth spectrum stretching from 450 to 1400 nm was attained by the injection of 30 fs Ti:sapphire laser pulses into 2 m-long high linear photonic crystal fibers, with an energy up to 5 nJ at a pulse repetition rate of 100 MHz and a central wavelength of 800 nm.
  • XU Yongzhao, WEI Yanfen, REN Xiaomin, ZHANG Xia, HUANG Yongqing
    An all-optical regeneration based on self-phase modulation in a highly nonlinear photonic crystal fiber is proposed. The dispersion and nonlinearity properties of a series of photonic crystal fibers are analyzed, and the results show that the nonlinearity coefficient is closely related to the structure of the fiber. In this paper, the nonlinearity coefficient is increased by reducing the effective mode area, and a highly nonlinear photonic crystal fiber with a large air-filling fraction is used as nonlinearity medium in optical regeneration. The numerical results show that good optical regeneration results can be obtained by using a relatively short fiber length due to the high nonlinearity of the fiber. The input peak power launched into the photonic crystal fiber and the parameters of the filter have much influence on optical regeneration. To achieve good optical regeneration, those parameters need to meet certain requirements. Furthermore, the transfer characteristic of the regenerator is also discussed. By adjusting the input peak power and filter parameters, the regenerator can deal with input pulses of different pulse widths.
  • ZHAO Mingfu, LIAO Qiang, CHEN Yan, ZHONG Nianbing
    A new testing method for biomass concentration and sensor design is developed to realize online testing of biomass concentration in the microbial bacterium liquid. The sensor structure theory, optical path, theory analysis and experiment are studied in-depth. First, the optical absorption and optical scattering are related to the optical distance and incidence energy. The change in biomass concentration will cause the change in optical absorption and optical scattering, and then lead to the change in the receiving energy. Based on the physical phenomena and theory, the paper relates the receiving energy to biomass concentration testing, and a new method to measure the biomass concentration is set up. In the experiment, a lamp-house with 760 nm wavelength is chosen. Under the 20°C constant temperature, the experiment is performed on the biomass concentration test. The results indicate that the method is good for testing bacterium liquid concentration, with maximum relative error less than 0.2%. The method, which has merits such as precise online testing, high sensitivity and long life, is a practical new style biomass concentration sensor.
  • LI Libo, LOU Qihong, ZHOU Jun, DONG Jingxing, WEI Yunrong, LI Jinyan
    The output capability of large mode area (LMA) multimode fiber laser depends on the fiber bending. To study the relationship, the output capability of multimode fiber laser was measured under various bending diameters, and it was also theoretically calculated. Measured respectively were the beam quality factor M2, by means of knife edge for the different bending diameters of LMA fiber, and the slope efficiency. When the bending diameters 285, 195 and 130 mm were used, the corresponding beam quality factors were 2.88, 1.82 and 1.67 with the slope efficiency at 39%, 35% and 34%, respectively. In addition, for the LMA multimode fiber used in the experiment, losses of different modes were calculated theoretically under various bending diameters. The experimental results correspond to the calculated results.
  • YE Yunxia, FAN Dianyuan
    Incoherent radiation of amplifying random media was investigated using Monte Carlo simulation and the characteristics of random laser were observed. The entire emission spectra of the random media would become narrow abruptly when pumping energies exceeded certain threshold values, and as the pumping energies further increased, distinct sharp peaks would emerge on the spectrum background. The intensity of a certain spatial point within the scattering media was contributed from emissions of many frequencies. The intensity of a single frequency of the entire spectrum was contributed with emissions from wide spatial location and angle ranges. It has been pointed out that the incoherent radiation of amplifying random media is essentially different from amplified spontaneous emission without any feedback as well as conventional resonant-feedback laser. The explanation on distinct sharp peaks is that rare photons have experienced rather more scattering events and walked longer paths in amplifying random media, which accumulates more gain for these photons.
  • LIU Bo, ZHANG Ruobing, LIU Huagang, MA Jing, ZHU Chen, WANG Qingyue
    Laser sources which have high power, short time duration and are broadly tunable are needed for the application of ultra-fast lasers. Femtosecond optical parametric amplification (OPA) is one of the most important techniques to produce broadly tunable and several femtosecond laser pulses. To obtain an extremely short pulse, the femtosecond OPA should adequately support a large spectral bandwidth. Ultra-broadband type-I phase matching OPA based on BBO was theoretically investigated. The achromatic phase matching (APM) technology was introduced to femtosecond OPA, and a broadband phase matching condition was given when the signal beams were angularly dispersed. The methods were presented to choose the optimized non-collinear angle and angular dispersion. Effects of non-collinear angle and rate of angular dispersion to the parametric bandwidth were also discussed. The results indicate that a proper non-collinear angle of the non-collinear optical parametric amplifier (NOPA) and getting signal beams dispersed at the right rate in near-infrared conditions can increase the parametric bandwidth dramatically.
  • YAO Jianghong, XUE Liangping, YAN Boxia, JIA Guozhi, XU Jingjun, ZHANG Guangyin
    By using a short-pulse field, periodically poled grating (? = 29 ?m) was successfully fabricated in a 1.0 mm-thick MgO:LiNbO3 (mole fraction of doped MgO is 5%). A high-repetition-rate optical parametric generation (OPG) based on periodically poled MgO:LiNbO3 (PPMgLN) was pumped by a 1.064 ?m acousto-optically Q-switched Nd:YVO4 laser. With 3 W of input pump power, 44 mW of output signal power was obtained at a conversion efficiency of 1.5%. Tunable infrared (IR) output from 1.4538–1.4750 ?m was also obtained by tuning the temperature of PPMgLN, which is 45°C–160°C.
  • GUO Tuan, ZHAO Qida, LIU Lihui, HUANG Guiling, XUE Lifang, LI Guoyu, LIU Bo, ZHANG Weigang, KAI Guiyun, YUAN Shuzhong, DONG Xiaoyi
    Temperature-insensitive fiber Bragg grating (FBG) dynamic pressure sensing based on reflection spectrum bandwidth modulation and differential optical power detection is proposed and experimentally demonstrated. A special double-hole cantilever beam is designed to induce linear strain-gradient distribution along the sensing FBG, resulting in FBG reflection spectrum symmetrical broadening and optical power increase. Based on the theory of optical waveguide and material mechanics, the causation of FBG spectrum broadening under the linear strain-gradient is analyzed, and the corresponding force-to-bandwidth broadening relation and force-to-optical power relation are formulized. FBG spectrum bandwidth and reflection optical power linearly change with applied pressure and both of them are insensitive to spatially uniform temperature variations. For a temperature range from -10°C to 80°C, the measured pressure fluctuates less than 1.8% F.S. (120 kPa) without any temperature compensation. The system acquisition time is up to about 80 Hz for dynamic pressure measurement.
  • XI Fengjie, XU Xiaojun, WANG Tiezhi, JIANG Zongfu, GENG Yifeng
    The proposed optimum phase defocus grating for wavefront curvature sensing features an equidistantly quantized two-phase-step and a phase step height of ?. The optimum phase defocus grating suppresses the influence of zero-order diffraction. The high diffractive efficiencies of the ±1 diffraction orders are verified experimentally, showing average values of 38.08% and 40.36% respectively. The phase plate introduces the wavefront deformation, which are measured by wavefront curvature sensing and reconstructed via the Green’s Function algorithm. The reconstructed wavefront is compared with the measurement result of a Veeco interferometer. It is verified that the measurement of the self-designed curvature sensor is correct. The error percent of peak-valley value (PV) is 10%, and the error percent of root mean square (RMS) is 2%.
  • Liu Shiyuan, Wu Xiaojian, Qin Xinyi
    A real-time exposure dose control algorithm for deep ultraviolet (DUV) excimer lasers in a step-and-scan optical lithography is presented. By establishing an abstract scan exposure model and analyzing the pulse-to-pulse energy fluctuation characteristics of DUV excimer lasers, a real-time dose regulation is implemented based on closed-loop feedback control, which especially focuses on reducing the effect of pulse energy overshot and pulse-to-pulse stochastic fluctuation. The experiment conducted on an ArF excimer laser with wavelength of 193 nm, repetition rate of 4 kHz, and pulse energy of 5 mJ confirms that such a real-time dose control algorithm is able to achieve a dose accuracy of above 0.89% even with only 20 pulses. It is fully expected that this algorithm will not only meet increasingly stringent dose accuracy requirements for sub-half-micron lithography, but also be helpful to improve lithography throughput as well as efficiency.
  • LI Xiufang, DENG Zhenbo, ZHANG Yuanyuan, SHI Yumeng, WANG Ruifen
    A new kind of rare earth (RE) complex Tb(o-MBA)3phen was synthesized and used as an emitting material in electroluminescence. The material was doped into poly(N-vinylcarbazole) (PVK) as the emitting
  • JIA Guozhi, YAO Jianghong, SHU Yongchun, WANG Zhanguo
    The InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) are studied as a function of growth temperature at a specific InAs coverage of 2.7 ML. The QDs density is significantly reduced from 8.0 × 1010 to 5.0 × 109 cm-2 as the growth temperature increases from 480°C to 520°C, while the average QDs diameter and height becomes larger. The effects of the growth temperature on the evolution of bimodal QDs are investigated by combining atomic force microscopy (AFM) and photoluminescence (PL). Results show that the formation of the bimodal QDs depends on the growth temperature: at a growth temperature of 480°C, large QDs result from the small QDs coalition; at a growth temperature of 535°C, the indium desorption and InAs segregation result in the formation of small QDs.
  • WANG Baosong, JIANG Haihe, JIA Xiande, ZHANG Qingli, SUN Dunlu, YIN Shaotang
  • LI Baozeng, WANG Anting, GUO Hai, MING Hai, CHEN Xuesheng, JIANG Hua, ZHENG Zhiqiang
    The up-conversion luminescence of Er3+ and Er3+/Yb3+ ions doped in lanthanum-modified lead zirconate titanate (PLZT) under 980 nm excitation at room temperature has been investigated. The green up-conversion luminescence at 540 and 566 nm was observed in Er3+:PLZT; the greater the concentration of Er3+ ions, the stronger the intensity. In Er3+–Yb3+:PLZT, other than the green up-conversion luminescence at 540 and 566 nm, a relatively weak red up-conversion luminescence at 668 nm was also observed. Both green and red up-conversion luminescence intensities presented an approximately quadratic dependence on excitation power, which indicated that two excitation photons are involved in the up-conversion process of Er3+:PLZT and Er3+–Yb3+:PLZT. The characteristics of PLZT ceramic material were also studied by Raman spectroscopy. This work shows a promising future for developing multifunctional up-conversion electro-optical devices using Er3+ and Er3+/Yb3+ ions doped PLZT.
  • YE Zhizhen, WANG Jingrui, WU Yazhen, ZHOU Xincui, CHEN Fugang, XU Weizhong, MIAO Yan, HUANG Jingyun, LÜ Jianguo, ZHU Liping, ZHAO Binghui
    Phosphorus-doped p-type ZnO thin films are prepared on glass substrates by metalorganic chemical vapor deposition (MOCVD). DEZn, O2, and P2O5 powders are used as reactant and dopant sources. The p-type ZnO films are grown at a temperature between 673 K and 723 K. The best p-type sample has a low resistivity of 4.64 ?·cm, a hole concentration of 1.61 × 1018 cm-3, and a Hall mobility of 0.838 cm2·(V·s)-1 at room temperature. A strong emission peak at 3.354 eV corresponding to neutral acceptor bound excitons is observed at 77 K in the photoluminescence spectra, which further verifies the p-type characteristics of the films.
  • Ren Tiexiong, Yu Jian, Sang Mei, Fu Weijia, Ni Wenjun, Kang Yuzhuo, Li Shichen, Hu Yonglan, Shi Ruize
    Compared with other nonlinear optical materials, KTP crystals have prominent advantages. However, they also have a high conductivity and become difficult to efficiently control domain-reverse with the conventional method, due to the existence of ionic current. To conquer the difficulty, it is necessary to monitor the polarization-reversal process of KTP crystals in real time. The real-time monitoring method in the fabrication of PPKTP, short for periodically-poled KTiOPO4, is carried out by utilizing electro-optical effect. The principle is analyzed theoretically and the result demonstrates the validity of the method experimentally. Compared with the result without using the monitoring method, the conversion efficiency of PPKTP crystal increases by many times. It is proved that this method can be used to enhance the quality and repeatability of PPKTP fabrication, and is also effective to examine the quality of PPKTP crystals.
  • LIU Jing, SUN Junqiang, HUANG Dexiu, HUANG Chongqing, WU Ming
    The non-magnetic material closed photonic quantum well (PQW) and magnetic material PQW structures based on the non-magnetic material open PQW are proposed. The transmission spectra and the field distributions of these three PQW structures are calculated by finite-difference time-domain method, the quantized energy states are researched, and the feasibility of enhancing spectral intensity significantly by self-structure is disclosed. It is found that the optical transmittance of the magnetic PQW is close to 1, and the energy loss is less compared to non-magnetic PQW. Compared with the closed PQW structures, the device’s volume can be reduced, the degree of free regulation of the energy band project can be increased, and more photon bound states can be obtained. The results show that the open PQW is the traveling wave well, and its capability of capturing photons is weak. However, the closed PQW and the magnetic PQW are standing wave wells. Their capabilities for capturing photons are strong, while the light field gradient of the material PQW is bigger.
  • JIANG Fagang, YANG Xiaobo, CUI Ling, DAI Nengli, LU Peixiang, LONG Hua
    To explore the possibility of photodisruption in rabbit sclera by femtosecond (fs) laser and seek appropriate incision techniques and relevant parameters, a fs laser (800 nm/50 fs) with different pulse energies was applied to irradiate rabbit sclera in vitro. By moving a computer-controlled three-axis translation stage to which the sample was attached, the laser achieved three types of incisions: transscleral channel, snake pattern and linear cut. The irradiated samples were observed by light microscopy and scanning electron microscopy (SEM). In comparison with fs laser, Nd:YAG was used as control. The experimental results show that through an objective lens with numerical aperture (NA) of 0.2, the fs laser with power intensity larger than 955 TW/cm2 and pulse energy ranging from 37.5–125 ?J, cutting depths from 30–70 ?m may be achieved after linearly scanning in sclera at a translation speed of 0.1 mm/s. However, it failed to make any photodisruption when the power intensity was below 796 TW/cm2 or the pulse energy was less than 31.25 ?J under the same condition. Compared with the Nd:YAG laser, the inner wall of the channel was smoother and the damage to the surrounding tissues was slight with the fs laser. The high precision of intrascleral photodisruption and minimal damage to surrounding tissues by a fs laser display its potential application in the treatment of glaucoma.
  • GONG Yuan, LI Bincheng
    A continuous-wave cavity ring-down (CW-CRD) technique employing a broadband diode laser is developed for high reflectivity measurement. The theory of square-wave modulated CW-CRD is presented. The spectrum of the broadband CW diode laser covers numerous free spectral ranges (FSRs) so that sufficient laser power is coupled into the cavity. Both amplitude and phase-shift of the first harmonic of the CRD signal, measured at an appropriate frequency range, are detected by a lock-in method and fitted to obtain the ring-down time and reflectivity. The measurements are repeated with five different cavity lengths and all the fitted reflectivities are in excellent agreement, indicating a high reliability of the CW-CRD technique. The reflectivity of the cavity mirror is determined statistically to be 99.70%, with an uncertainty of 0.01%.
  • DING Yi, GU Peifu
    The energy from a source was rearranged through reflection or refraction by a freeform optical element in order to get the desired uniform illumination. The numerical results of first-order partial differential equation sets had been investigated to obtain the freeform optical element, and the equations could be used to get the characteristics of the light source and the desired illumination. For example, a light emitting diode (LED) with a Lambertian light emitting surface of 1 mm × 1 mm was applied as the light source. Two kinds of freeform reflectors and one freeform lens were designed for different illuminations, and the simulated uniformity was near to 90%. Considering the size of these freeform optical elements, the illumination system can be very compact and efficient if the freeform optical element is applied in the illumination system of projectors with LED as source.
  • Zheng Zhenrong, Sun Xütao, Gu Peifu, Liu Xü
    An objective lens composed of a spherical Fresnel zone plate mirror and aspheric mirrors is designed. The Fresnel zone plate with a spherical shape is analyzed, and the method to approximately replace the aspherical mirror with spherical Fresnel zone plate is deduced. The objective lens is designed with a single spherical Fresnel zone plate mirror and three aspherical mirrors. Under the condition of 100 × magnification, 2.5 Fresnel number and 120° field angle, the modulation transfer function can reach above 40% at 0.6 line pairs/mm on the magnification side, and the distortion is less than 2.2%. This method can provide a reference for the application of Fresnel zone plate in visible light imaging, and has a bright future with the continuous development of the fabrication technique of Fresnel devices.
  • SHEN Zhaolong, JIANG Bing, CAI Jianwen, XU Min, HUANG Wenhao
    To implement a dual-pickup-head multi-layer data storage system, it is necessary to measure the synchronizing focus error of the two pickup heads to validate the feasibility of the focus servo method. By using the ABCD transforming matrix method of geometric optics, we calculate the relationship between the output energy and the space interval of the two heads. After confirming the detection system working in the linear area, the voice coil motors of the two pickup heads were driven by sine signals with different frequencies and recorded the output signals of the quadrant photo-detector under different movement states respectively, including static, lower pickup head movement and dual-pickup-head synchronizing movement. The synchronizing focus error, which is caused by the characteristic parameter mismatch of the two pickup heads, could be obtained through frequency-domain analysis of these output signals. The experimental result shows that the amplitude of synchronizing focus error under 20 Hz is less than 1 µm, which could adequately satisfy the focus servo requirement of the multi-layer data storage system.
  • HUANG Ziqiang, YANG Wenjun, WANG Jimin
    Based on the analytical expression of the electromagnetic field solution of a helical symmetric dielectric material, the relationship between spectral reflectivity and birefringence of a chiral liquid crystal and the blue shift of the Bragg reflection in the condition of oblique incidence are presented in this paper. The theoretical results indicated that: 1) If birefringence (?n) of the liquid crystal is greater than 0.2 and the thickness of the liquid crystal layer reaches 3?m, the liquid crystal layer will reflect more than 90% of the incident light; 2) To reflect the whole visible spectrum by Bragg reflection, ?n of the liquid crystals in plane alignment state should exceed 0.6; 3) When the incident beam inclines to 60° from the normal direction, the blue shift of the reflective spectrum will reach to 100 nm. On the other hand, since the ?n of the commercial chiral liquid crystals is not larger than 0.2, to get the entire visible reflective spectrum, it needs to introduce a polymer network into the liquid crystals and make a sagging structure on the surface of substrates. The contribution of the network is to establish random anchorage that makes the pitch varied, hence broadening the Bragg reflection spectra. The random distribution of the sagging structure on the surface substrate is used to induce random screw axes of the chiral liquid crystal, which not only causes a blue shift of Bragg reflection but also further stretches the reflection spectra. Experiments demonstrated that the Bragg reflection spectrum could be broadened from 80 nm to more than 120 nm, and the contrast reaches 4:1 by introducing both polymer network and sagging structure on the substrate surface cell.
  • CHEN Feifan, HONG Zhiwei
    The incident angle of a parallel light beam is hard to measure accurately under the requirement of both high precision and large measurement range. A solution based on the optical vernier principle is presented, and a corresponding measuring device is given. Compared with the former method, the suggested apparatus realizes high accuracy in a larger measurement range. The experiments on the designed apparatus show a high precision of better than 0.02° in a measurement range of ±64° and the experimental results are in accordance with analytical results in theory. The invented apparatus can be widely used in many technical areas such as aerospace, precision measurement and automatic control.
  • CHEN Wei, YAO Hanmin, WU Fan, WU Shibin, CHEN Qiang
    To ensure the safe and normal operation of the whole optical system, it is important to test and evaluate the quality of optical components. The article explores the advantages and disadvantages of general parameters used in aspheric surface testing, the composition of the system, the principle of operation and the design of related power spectral density (PSD) software used in testing large aspheric surfaces with Shack-Hartmann and phase shifting interferometer. The results indicate that PSD can give the spatial frequency distribution of the wavefront aberration when large aperture phase shifting interferometer is used as an instrument to test the wavefront, and this can also be applied as an evaluation standard in testing the quality of optical components. In addition, this paper describes the test results of optical components in the size of 64 mm × 64 mm.