频率间隔可调的平坦多载波光源结构设计

为了克服密集波分复用系统中多个光源难以同步且成本高等缺点,提出了一种平坦度好、子载波数多、频谱宽度大且频率间隔可调的多载波光源生成结构.系统结构由马赫增德尔调制器、电吸收调制器和相位调制器级联组成,在单一正弦信号驱动下,输出多个低平坦度、频率间隔可调的子载波.在驱动信号频率为9和12.5GHz,系统生成的子载波数量为31个的条件下,平坦度可达到0.09dB.当驱动信号频率在3GHz~16GHz范围变化时,输出的子载波数量基本不变,且在子载波数量为31个的条件下,平坦度均小于0.15dB.当驱动信号频率为16GHz,平坦度小于0.15dB时,系统输出的多载波光源频谱宽度可达480GHz.此外,分析了MZM的上、下臂偏置电压差、EAM的啁啾因子、调制指数对生成多载波光源的平坦度、子载波数以及频谱宽度的影响.     

掺Er光纤飞秒激光器中高信噪比载波包络位相偏移频率获取的实验研究

在实验上分析了掺Er光纤光学频率梳中激光器与一级放大器之间光纤长度、 光纤扰动以及放大器抽运功率对倍频程光谱和载波包络位相偏移频率(f₀)信噪比的影响. 通过系统优化, 实现了40 dB信噪比的f₀输出, 为f₀的长期锁定和光纤光学频率梳的实现提供了技术保障.     

长时间精密锁定的掺Er光纤飞秒光学频率梳

自行研制了基于掺Er光纤飞秒激光器的光学频率梳并实现了重复频率 f_r信号和载波包络偏移频率(Carrier-envelope offset, f_o) 信号的精密锁定, 锁定后的 f_r信号和 f_o 信号的频率抖动量的标准偏差分别为0.515 mHz和93.13 mHz.     

基于谐波拟合产生周期性三角形光脉冲串的实验研究

实验研究了一种基于谐波拟合产生周期性三角形光脉冲串的方法,方案首先利用马赫曾德调制器的载波抑制调制,获得具有周期性起伏的连续光强度信号,然后利用光纤色散所致的射频功率衰落效应,对光强度表达式中四次谐波分量进行抑制,调节调制深度后,光强度表达式将向理想三角形傅里叶展开式的前三项进行逼近,最后以谐波拟合的方式获得重复频率为射频调制频率二倍的周期性三角形光脉冲串.结合实验,在9.862 GHz和7.678 GHz射频调制频率下,获得了脉冲重复频率19.724 Gb/s(脉冲全宽约50.7 ps)和15.356Gb/s(脉冲全宽约65.1 ps)的稳定三角形光脉冲串输出,改变色散量并反向调节调制频率,可进一步改变脉冲的重复频率,所获得的实验结果与理论预期基本符合.     

循环频移方法中射频信号的相位偏移对多载波稳定性及平坦度的影响(英文)

为了得到高性能的多载波光源,研究了射频信号中不同相位差对光多载波的影响.通过理论分析可得,随着相位差从90°降至0°,光多载波性能会逐渐变差,但是相位差偏与90°相差不大时,性能不会下降太多.实验中通过移相器改变相位差,使相位差为90°,45°,0°,得到循环前IQ调制器的输出光谱,并对比循环后光多载波性能.实验结果显示,相位差为90°时,循环前IQ调制器输出为较好的单边带光谱,循环后输出50条平坦稳定光多载波;相位差为45°时,光多载波质量变差;相位差为0°时,得不到光多载波.因此,射频信号相位差为90°时,光多载波的性能最佳,随着相位差变小,性能下降,但是相位差为45°时,产生的光多载波仍可应用于光通信.     

光时分复用(OTDM)系统中基于电吸收调制器(EAM)的短脉冲光源及解复用器的设计考虑

研究了电吸收调制器(EAM)的衰减随外加反向电压增加而指数增加的情形下,短脉冲光源的脉冲输出和解复用器的开关窗口对EAM的消光效率、反向DC偏置电压以及正弦RF驱动信号的幅度等参量的依赖关系.在基于EAM的短脉冲光源中,输出脉冲的消光比等于EAM消光效率η与正弦驱动电压峰峰值V_pp的乘积,输出光脉冲的消光比和脉宽均与EAM的反向偏置电压无关,但输出脉冲的峰值功率与η、V_pp和V_b都有关系.在基于EAM的解复用器中,为了使解复用器的开关窗口近似为矩形,可利用EAM的削波效应,使V_pp/2>V_b.在EAM的消光效率η已知时,通过仔细设计反向DC偏置电压V_b和正弦驱动电压的峰峰值V_pp,达到OTDM解复用器所需要的开关窗口形状、宽度和消光比.     

小世界神经网络的二次超谐波随机共振

随机共振现象是非线性系统中普遍存在的自然现象.主要通过研究小世界生物神经网络中的输出信噪比与反映小世界效应的重连概率p、耦合强度c以及输入信号振幅A之间的关系，来揭示小世界生物神经网络的二次超谐波随机共振的一些规律.发现对于Hodgkin-Huxley小世界神经网络，并不是信号越强，信噪比越大，而是输入信号的振幅A存在一个最优值A_O，此时网络信噪比最大.     

基于射频倍频信号驱动双平行马赫曾德调制器产生光频梳

为克服传统正交频分复用及波分复用系统以激光序列作为多载波光源带来的不易协调同步、成本高等缺点,利用正弦型射频信号与其倍频信号驱动单个双平行马赫曾德调制器,对连续光载波进行调制,在较低的驱动电压下,产生了具有2条、3条、4条、5条、7条及9条谱线的平坦光频梳.其中,具有2、3、5条谱线的光频梳平坦度均为0dB,边模抑制比均在30dB左右;具有4、7、9条谱线的光频梳平坦度在1dB范围内,边模抑制均比高于18dB.该方法结构简单,成本低,易协调,与现有技术相比,产生的光频梳平坦度更高、边模抑制比更大、射频驱动电压更低且条数更为多样.     

计及判决门限与非理想消光比的串扰分析

推导出了在多重串扰情况下,串扰光和系统光噪声共同存在时,接收机输入光信号及其基带输出信号的表达式.计算了在最优判决门限和平均判决门限两种情况下,背景误码率在10^-9条件时,信号光消光比与串扰光消光比对系统误码率的影响.结果表明,对于不同的接收机判决门限,信号光的消光比在10～12dB的范围内,系统的误码特性接近最优.     

全保偏49 GHz高重频多波长光源的产生

频域稳定的高重频多波长光源是信道化和光通讯等领域的理想光源。为了满足高重频多梳齿激光源的应用需求,本文研制了49 GHz梳齿间隔的多波长激光源。通过优化I/Q两端射频信号和三个偏置点,IQ调制器稳定工作在载波抑制单边带模式,杂散频率成分抑制比达到27.5 dB。本文利用循环调制技术产生宽带多波长光频梳,并通过优化射频功率建立环内增益-损耗平衡,有效提升平坦度,获得了梳齿间隔为24.5 GHz、梳齿数目为28根、平坦度仅为3 dB、梳齿信噪比(TNR)达29 dB的多波长光频梳。在此基础上,进一步利用法布罗(FP)滤波技术将频率间隔提升到49 GHz,最终得到梳齿数目达14根,平坦度为2.7 dB,TNR达19 dB,平均光功率为9 dBm的高重频多波长光频梳。由于采用了全保偏光纤器件和集成化技术,本系统具有一键式启动和长时稳定运行的特点,运行半小时的功率抖动标准差仅为0.5%,有望在各微波光子领域中应用。     

Temperature stabilized optical waveguide modulator

A low drive-voltage optical modulator using a Ti-diffused UNbO₃ optical waveguide has been fabricated. Stabilization against ambient temperature change was realized by using a miniature halfwave plate. The halfwave voltage, 3 dB bandwidth, optical insertion loss and extinction ratio were 3·8 V (at 1·06m wavelength), 850 MHz, 10 dB and 13 dB, respectively. A reduction scheme for the optical absorption caused by metallic electrodes, and an analysis of the modulator high frequency response are also reported.     

Electro‐optic polymer/titanium dioxide hybrid core ring resonator modulators

Compact electro‐optic (EO) modulators are desirable for a number of applications. In this study, a ring modulator has been fabricated in the titanium dioxide (TiO₂) core and EO polymer cladding waveguide structure. A 250‐nm thick TiO₂ core was utilized to minimize the ring radius down to 100 μm, to avoid using the top cladding between the EO polymer and the electrode, and to improve the poling efficiency. The resonance obtained by the ring modulator was observed to shift by 0.02 nm/V due to the enhanced in‐device EO coefficient of 105 pm/V. A modulation depth of 3 dB was observed at the frequency response function at 20 kHz using 2‐V V_p–p clock signal.     

Performance evaluation of SCM-WDM communication in the presence of SRS induced crosstalk for different types of fiber

In this paper, the SRS-induced crosstalk has been evaluated in a SCM-WDM communication link at different modulation frequencies for various type of fibers. Results show that SRS-induced crosstalk dominates at low frequency. As the dispersion and effective area of fiber (A_eff) decreases, initially the crosstalk remains high and then it decreases with increase in modulation frequency. The present work shows that out of five different types of fibers, Standard Single Mode Fiber (SMF) has minimum crosstalk (−53 to −64 dB) and True Wave fiber (TW) has maximum crosstalk (−47 to −48 dB).     

A TERAHERTZ PHASE FREQUENCY CHANGER WITH THE CRYSTAL QUARTZ PHASE SECTIONS

A phase frequency changer (PFC) for quasi-optical transmission line with phase sections made of crystal quartz and polarizers executed on the basis of small-period wire gratings has been considered. The PFC is intended for operation in terahertz (THz) frequency region. It has been examined at the frequency f _o= 0.89THz. The influence of the differential phase shift deviation in the sections on the output signal spectrum and the influence of the sections mismatch on the reflection signal spectrum have been considered. In the frequency region ± 10% f _o the levels of the spurious spectral components of output signal are less than −40 dB with regard to the level of the useful signal of the shifted frequency. The levels of all spectral components of the reflection signal are less than −60 dB with regard to the level of the useful component of the PFC output signal.     

Modify SRS-induced crosstalk with variety of fiber in SCM–WDM transmission links

In this paper, the SRS-induced crosstalk has been evaluated in a SCM–WDM communication links at different modulation frequencies and transmission lengths for variety of fiber. Results show that SRS-induced crosstalk dominates at low frequency. As the dispersion and effective area of fiber (A_eff) decreases, initially the crosstalk remains high and then it decreases with increase in modulation frequency. The present work shows that out of five different types of fiber, standard single mode fiber (SMF) has minimum crosstalk (−78 to −38) dB, (−55 to −33) dB and (−46 to −34) dB at modulation frequencies, transmission lengths and optical powers. Dispersion compensation fiber (DCF) has maximum crosstalk (−60 to −12) dB, (−37 to −12) dB and (−27 to −12) dB at modulation frequencies and transmission lengths.     

A novel frequency sextupling scheme for optical mm-wave generation utilizing an integrated dual-parallel Mach-Zehnder modulator

A novel scheme is proposed for frequency sextupling mm-wave generation based on a laser and an integrated dual-parallel Mach-Zehnder modulator (MZM) without optical filter. Theoretical analysis is presented to suppress the undesired optical sidebands for the high quality generation of frequency sextupling mm-wave signal. The performance of the proposed scheme is evaluated by simulations. Utilizing the integrated MZM consisted of two sub-MZMs with extinction ratio of 30 dB, the optical sideband suppression ratio (OSSR) is as high as 29.9 dB and the radio frequency spurious suppression ratio (RFSSR) exceeds 24 dB without any optical or electrical filter. The impact of the nonideal RF driven voltage and phase difference of RF driven signal applied to two sub-MZMs of the integrated MZM on OSSR and RFSSR is discussed and analyzed. After transmission over fiber, the generated optical mm-wave signal demonstrates good performance. Furthermore, the performance of two cases for the proposed scheme is also compared.     

I/Q intensity-demodulation analog photonic link based on polarization modulator

We experimentally investigate a high-linearity analog photonic link based on polarization-modulation in-phase/quadrature (I/Q) intensity-demodulation. In the proposed system, a light wave with its polarization direction oriented at an angle of 45° with respect to the principal axis of the polarization modulator (PolM) is polarization modulated by a drive signal. In the receiver, the polarization-modulated signal is split into two paths using a 50:50 optical coupler (OC). In each path, a polarization beam splitter (PBS) is connected to the PolM via a polarization controller (PC) to realize the in-phase and quadrature signal, respectively. Thanks to the polarization-modulation to intensity-modulation conversion, the fluctuation of the optical phase noise has no impact on the coherent signals. In addition, the use of a single PolM allows the link avoids the matching requirements of dual modulator schemes. The linearization leads to suppression of the third-order intermodulation (IMD3) by more than 40 dB, compared to the quadrature signal. Experimental results show that the link gain is -11.5 dB, noise figure is 26.5 dB, and the third-order limited spurious free dynamic range (SFDR) is 124 dB in a 1 Hz bandwidth.     

Flat optical frequency comb generation using polarization modulator and frequency shifter with double recirculating frequency shifting loops

A scheme to generate ultra-flat and stable optical frequency comb (OFC) based on multi-wavelength optical seed source and polarization-modulator-based complementary frequency shifter (PCFS) has been proposed and demonstrated. The OFC generation scheme includes two stages: 5-carrier light source is generated by utilizing a polarization modulator at the first stage, and the light source is then used as the seed light in the PCFS at the second stage. Compared with the previous flat OFC generator based on a recirculation frequency shifter, the proposed scheme can greatly reduce the recirculation times, and such a property makes the accumulated noise of the system enormously decrease. In order to achieve the frequency comb with the best flatness, the optical amplifier gain, filter stop-band attenuation and modulator drive voltage are optimized. Experiment simulations show that high-quality OFC with hundreds of frequency lines and power fluctuation within 2 dB can be achieved without direct-current bias control and wavelength-selective-switch.     

Reconfigurable photonic generation of microwave hybrid frequency/phase shift keying signals

A photonic approach for the generation of microwave hybrid frequency/phase shift keying (FSK/PSK) signal based on an integrated polarization division multiplexing dual-parallel Mach–Zehnder modulator (PDM-DPMZM) is proposed and demonstrated. In the scheme, the polarization modulator is employed to modulate the linearly polarized lightwave to generate an optical polarization-shift keying (PolSK) signal. Then the PolSK signal is sent to the PDM-DPMZM via a polarization controller to generate optical PSK signal. After photo-detection, a microwave hybrid FSK/PSK signal can be obtained. Simulations are conducted to verify the proposed scheme. As bit rate is set to 1 Gbit/s, a hybrid FSK/PSK signal with frequency of 10/15 GHz and phase shift of\(\pi\)is successfully demonstrated. Hybrid signals with 2-Git/s frequency shift and 1-Git/s phase shift are also implemented. The compression ratio of the generated signal is 25.5 and the main-to-sidelobe ratio is 11.2 dB. The generations of ASK, PSK and FSK signals are discussed, and the impact of the polarization extinction ratio is also analyzed.     

Generation of linearized optical single sideband signal for broadband radio over fiber systems

We propose a new scheme to generate broadband linearized optical single-sideband （OSSB） signal for radio over fiber systems. By using an unbalanced dual parallel Mach-zehnder modulator （DPMZM） followed by optical filtering, a linearized OSSB signal is obtained. With coherent detection, radio frequency （RF） signal can be recovered with simultaneously suppressed second-order distortion and third-order intermodulation. This scheme can be used to realize broadband systems with wide dynamic range.