Novel photonic approach to microwave frequency measurement using tunable group delay line

A photonic approach for measuring microwave frequency over a wide bandwidth is proposed.An optic group delay line composed of several magneto-optical switches and a 1.6-km single-mode fiber is used as a tunable dispersive medium in the measurement setup.A minimum frequency accuracy of 80 MHz in the range of 1-20 GHz is achieved experimentally.     

High-Q, tunable, photonic microwave single passband filter based on stimulated Brillouin scattering and fiber Bragg grating filtering

We experimentally demonstrate a single passband, photonic microwave bandpass filter scheme based on stimulated Brillouin scattering (SBS) in optical fiber. A single side band (SSB)-modulated probe and a single frequency, tunable pump beam that have been produced by the combination of conventional Mach-Zehnder modulators and fiber Bragg grating filters, are used for the generation of SBS phenomenon in a 1-km long highly nonlinear dispersion-shifted optical fiber. Using the proposed scheme a single microwave passband with a ∼26 MHz bandwidth is readily achieved over a radio frequency tuning range from 1.3 to 9.3 GHz. The maximum Q-factor is measured to be ∼360.     

Observation of inhomogeneous spectral broadening of stimulated brillouin scattering in an optical fiber

We report experimental results which show that the stimulated Brillouin scattering (SBS) spectrum in an optical fiber is inhomogeneous, exhibiting spectral broadening and hole burning under cw monochromatic laser excitation. This phenomenon arises from the waveguide interaction of the pump and Stokes signals and is a fundamental property of SBS in waveguiding systems due to their ability to confine a fan of radiation wave vector directions.     

一种与标准单模光纤高适配的低弯曲损耗光子晶体光纤

设计并研制出一种与普通单模光纤高适配的低弯曲损耗光子晶体光纤. 结构采用光纤预制棒制作工艺上易于实现的掺锗芯六孔结构. 应用间接测量方法, 对其模式、弯曲及色散特性进行了系统的评估. 在波长1550 nm处研制光纤的模场面积为79.26 μm², 色散为21.7 ps·km^-1·nm^-1, 模场面积和色散特性与标准单模光纤具有高的适配性. 在光纤弯曲半径为5 mm时, 在波长1550 nm处的弯曲损耗为0.0365 dB/圈, 小于G.657B的弯曲损耗0.5 dB/圈. 研究成果为光纤到户用低弯曲损耗光纤的实用化奠定了良好的基础. 关键词： 光子晶体光纤 低弯曲损耗 光纤到户 高适配性     

基于单模光纤的交叉相位调制型频率分辨光学开关超短脉冲测量

利用单模光纤中的光弹效应和交叉相位调制(XPM)效应,提出了一种频率分辨光学开关法测量超短脉冲的新方案.在本方案中,单模光纤的前一部分产生可变延迟,后一部分作为非线性介质产生非线性效应.该方案只需一根单模光纤,无须复杂的光路校准,结构简单,损耗低;光纤中的XPM效应易发生,无须相位匹配.对提出的方案进行了数值模拟,采用基于矩阵的主元素广义投影算法,恢复出待测脉冲的幅度和相位信息,并研究了光纤长度和待测超短脉冲的脉冲宽度对测量结果的影响.结果表明:测量准确度随着光纤长度的增加而提高,选取长度为2 km的光纤,就可以实现对超短脉冲的准确测量;本文方案适用于脉冲宽度不小于80 fs的超短光脉冲的测量.     

Threshold reduction of stimulated Brillouin scattering in photonic crystal fiber

A simple configuration for the reduction of stimulated Brillouin scattering threshold in photonic crystal fiber is presented. The threshold reduction is contributed by the bidirectional pumping scheme through the reflection of transmitted Brillouin pump back to the gain medium. The pump recycling scheme has greatly reduced the Brillouin threshold of a 200-m long of photonic crystal fiber from 50 to 30 mW of Brillouin pump power.     

Photonic instantaneous measurement of microwave frequency using fiber Bragg grating

A photonic approach to realizing instantaneous measurement of microwave frequency based on optical monitoring using a fiber Bragg grating (FBG) is proposed and demonstrated. In the approach, a frequency-unknown microwave signal is modulated on an optical carrier in a Mach-Zehnder modulator biased at the minimum transmission point. After detecting the transmission and reflection optical powers at the output of the FBG, the microwave frequency can be determined according to the value of transmission-to-reflection power ratio, due to the fixed relationship between the microwave frequency and the power ratio. A proof-of-concept experiment has been performed, which demonstrates that a measurement resolution of ±0.08 GHz over a 10 GHz measurement bandwidth is achieved. The measurement performance in terms of resolution is better than previously reported results.     

Dependence of the brillouin frequency shift on strain and temperature in a photonic crystal fiber

The dependence of the Brillouin frequency shift on strain in a photonic crystal fiber (PCF) was measured at a wavelength of 1320 nm for the first time to the authors' knowledge. Together with measurements of the dependence of the Brillouin frequency shift on temperature in the PCF, we demonstrate the feasibility of the highly precise simultaneous measurement of temperature and strain by use of the PCF in a distributed Brillouin sensing system with a spatial resolution of 15 cm.     

Passive mode-locking of a ruby laser using low-frequency stimulated brillouin scattering in birefingent crystals

A new technique of mode-locking Q-switched lasers using a birefringent crystal as an intracavity element is reported. Intense elastic waves are generated inside a birefringent crystal through the low-frequency, stimulated Brillouin scattering process and these in turn modulate the refractive index of the crystal and thus provide the necessary phase synchronization for mode-locking. Strong coupling between several neighboring modes of the laser is evidenced by the observation of high pulse repetition rates (several pulses circulating simultaneously within the cavity).     

Investigation on threshold power of stimulated Brillouin scattering in photonic crystal fiber

We have developed a theoretical approach to simulate stimulated Brillouin scattering (SBS) generation in photonic crystal fiber (PCF). The threshold condition for the SBS occurrence has been derived as a function of fiber parameters and input pump power. A particular emphasis is given to the influence of the input pump power and fiber length on the Brillouin gain in the PCF. To assess threshold pump power accurately, the pump depletion effect has been included by employing the 1% criterion. This simulation can anticipate the Brillouin threshold gain value precisely. The threshold gain varies from 14 to 18 depending on the PCF length.     

A MZI-based photonic instantaneous microwave frequency measurement simplified by an optical polarizer

A novel photonic scheme of microwave signal frequency measurement with adjustable measurement range and resolution is proposed and experimentally demonstrated. The proposed scheme is based on simultaneous optical phase modulation and intensity modulation with interferometric detection. A low-pass frequency response is achieved by a Mach–Zehnder interferometer (MZI) while a bandpass frequency response is produced by a polarizer placed on the back instead of in front of the MZI. The microwave frequency can be estimated by the measured amplitude comparison function (ACF) obtained from the ratio of the two frequency responses. This scheme is simple, cost-effective as it requires no extra laser sources or modulators in the basic analog modulation link. The measurement errors as shown in experimental results can be kept in 0.1 GHz over a frequency range of 0.1–8.5 GHz.     

Novel method for fiber chromatic dispersion measurement based on microwave photonic technique

In this letter,we propose and demonstrate a simple and novel method for fiber chromatic dispersion(CD) measurement based on microwave photonic technique.The radio frequency(RF) signal is modulated simultaneously on two light-waves with different wavelengths,and the light-wave carrying RF signals transmit through the dispersive medium under test.CD can be obtained by monitoring the power changing of the interference RF signals after photo detector.The CD values of the single-mode and dispersion compensation fibers are both measured within the wavelength range from 1 525 to 1 605 nm,which verifies the feasibility of this method.     

Frequency locking to the center of a 532 nm iodine absorption line by using stimulated brillouin scattering from a single-mode fiber

A simple method for frequency locking a frequency-doubled Nd:YAG laser to the center of line 1109 of the iodine absorption spectrum is described. The 31.6 GHz frequency shift provided by stimulated-Brillouin scattering from a single-mode silica fiber provides a probe signal that lies on the edge of line 1105 of the iodine spectrum. We adjust the frequency of the laser to maintain the transmission of a 5 cm iodine absorption cell at a value that places the unshifted laser line in the center of line 1109.     

光子晶体光纤中布里渊散射声波模式特性的分析

推导了光子晶体光纤中声波微小位移波动方程; 研究了泵浦波长以及纤芯折射率对声波模式的影响; 应用石英圆柱模型研究了小芯径光子晶体光纤中纤芯直径对布里渊声波模式色散的影响. 结果表明在光子晶体光纤中, 纵向声波和横向声波共同作用产生质点声场, 两者相互耦合将产生混合声波模式; 可以通过改变泵浦波长或光子晶体光纤纤芯折射率来改变参与布里渊散射(BS) 过程的声波模式的传播常数; 随着光子晶体光纤(PCF) 纤芯直径的增大, 声波模式耦合程度得到加强, 相速度呈减小趋势, 且同一传播常数下, 声波模式数呈增多趋势; 随着泵浦波频率的增大, 声波相速度减小.     

Variable single-passband narrowband optical filter based on forward stimulated interpolarization scattering in photonic crystal fiber

A variable transmission spectrum single-passband narrowband optical filter is proposed and experimentally demonstrated. It is based on forward stimulated interpolarization scattering (SIPS) in a photonic crystal fiber by applying a differential quadrature phase-shift keying modulation to the pump wave to broaden and shape the SIPS gain spectrum. By choosing the bit rate of the modulation data pattern, a flat-top steep-cutoff optical bandpass filter with a 3?dB bandwidth of 70?MHz and a 10?dB bandwidth of 90?MHz is realized. In addition, a variable narrowband optical notch filter is also realized by attenuation of the pump wave.     

Design and characterization of single mode circular photonic crystal fiber for broadband dispersion compensation

In this paper, we present a single mode circular photonic crystal fiber (C-PCF) for broadband dispersion compensation covering 1400 to 1610 nm wavelength band over the telecommunication windows. Investigations of guiding properties are carried out using finite element method (FEM) with circular perfectly matched layer boundary condition. Numerical study reveals that a negative dispersion coefficient of about −386.57 to −971.44 ps/(nm km) is possible to obtain over the wavelength ranging from 1400 to 1610 nm with a relative dispersion slope (RDS) of about 0.0036 nm⁻¹ at 1550 nm wavelength. In addition, the single mode behaviour of C-PCF is demonstrated by employing V parameter. According to simulation, it is found that the proposed C-PCF acts as a single mode fiber within 1340 to 1640 nm wavelength. Moreover, effective dispersion, relative dispersion slope, birefringence and confinement loss are also presented and discussed.     

A mirror based on stimulated brillouin scattering with a double feedback loop in a laser cavity

The possibility of switching an external mirror based on stimulated Brillouin scattering (SBS) with a double feedback loop to a laser cavity is shown experimentally. A significant increase in the output laser power is obtained with conservation of the diffraction quality of the initial radiation. Experimental comparison with an ordinary SBS mirror, excited in a single pass through a nonlinear medium, is performed and the advantage in the energy parameters of the scheme with a double feedback loop is shown. The threshold characteristics of the SBS mirror under study are calculated and good agreement with the experimental data is obtained.     

Experimental verification of improved effective index method for endlessly single mode photonic crystal fiber

We report the experimental verification of Improved Effective Index Method (IEIM) for Endlessly Single Mode Photonic Crystal Fibers (ESM PCFs) from far field intensity measurements. In this paper all the key parameters i.e. V-Number, effective refractive index of the cladding, radius of the core and numerical aperture have been obtained from the experimentally measured far-field intensity pattern of ESM-PCFs. It is shown that the transmission characteristics of ESM-PCFs match with the simulation result obtained using IEIM and also with the core diameter of PCF obtained from scanning electron microscope (SEM) within the experimental limit.     

Refractive index measurement based on core-cladding mode interferometry in endlessly single mode fiber

We propose a simple almost “all-fiber” sensor based on intermodal interference of core and cladding mode in an endlessly single mode photonic crystal fiber. Sensitivity to an external refractive index is studied experimentally and theoretically.