飞秒激光刻蚀非平行壁光纤微腔Mach-Zehnder干涉仪特性及其流体传感研究

实验发现,飞秒激光微加工光纤微腔时,两个侧壁与纤芯轴向并不完全垂直, 刻蚀的非平行壁光纤微腔Mach-Zehnder干涉仪出现光程差随波长增大而线性减小、 微腔总损耗随波长增大呈递减变化等反常现象.对此,提出非平行壁光纤微腔Mach-Zehnder 干涉仪新模型并初步建立了分析理论,采用新模型及分析理论对新型微腔干涉仪特性进行了研究. 数值分析了微腔底角、深度等参数对谱峰波长位置的影响,理论研究了微腔的光波传输损耗、 吸收损耗、插入损耗、材料红外吸收损耗以及对干涉条纹对比度的影响, 理论分析与实验结果相符.实验获得水溶液干涉条纹对比度高达35 dB的非平行壁光纤微腔Mach-Zehnder干涉仪, 将新型光纤微腔干涉仪用于流体传感,其蔗糖水溶液折射率传感灵敏度高达-12937.31 nm/RIU.     

Femtosecond pulse generation using stimulated Raman scattering in integrated silicon optical waveguide device

To generate ultrafast femtosecond optical pulses, we propose a model of an integrated device consisting of a Mach-Zehnder interferometer (MZI) with two symmetric 3 dB directional couplers and a straight waveguide based on the single-mode silicon-on-insulator (SOI) optical waveguide. The principle of pulse generation in the presented device is based on the strong stimulated Raman scattering (SRS) in silicon; the center wavelength of the pulse generated is tunable by changing the center wavelength of the co-propagating pump pulse. Numerical results show that, when a continuous wave (CW) with a weak power at 1670 nm wavelength and a pump pulse with a high peak power at 1550 nm wavelength are co-propagating, a narrow femtosecond pulse with a pulse width (full width at half maximum, FWHM) of ∼60 fs (FWHM of the pump pulse is 166.5 fs) can be achieved in the device proposed. In addition, when the waveguide length, pump peak power, and pump-pulse width are fixed, the properties of generated femtosecond pulse depend strongly on the incident chirp of the pump pulse and the CW power.     

Mach-Zehnder interferometer with a uniform wavelength period

We have realized a first optical filter composed of optical delay lines and couplers that has a periodic response with respect to wavelength, whereas a conventional optical filter is known to have a periodic response to relative optical frequency. This new filter can be constructed by using a phase-generating coupler (PGC) that supplies the optical delay lines with a wavelength-dependent nonlinear phase. The PGC, which is also composed of optical delay lines and couplers, can simultaneously provide a desired phase and an arbitrary amplitude coupling ratio. We applied our transformation method to a conventional Mach-Zehnder interferometer (MZI) as an example of an optical frequency filter and converted it to an optical wavelength filter. We fabricated the designed MZI on a silica-based planar lightwave circuit and successfully realized what is believed to be the first MZI with a uniform wavelength period.     

Flattop all-fiber wavelength interleaver for DWDM transmission: Design analysis, parameter optimization, fabrication and characterization recipe

Flattop response within the passband is an important desirable feature of any wavelength selective component that goes into a DWDM optical network. In this paper, we report design and fabrication recipe for realizing wavelength interleavers/slicers with flattop wavelength response, configured around unbalanced all-fiber 2-stage Mach-Zehnder interferometers (MZI). Based on a comprehensive analysis, we have developed a design process/algorithm to determine the optimum splitting ratios as well as the relationship between the splitting ratios of the couplers that constitute the 2-stage MZI, to achieve a flattop wavelength response. The implication of the second delay line with respect to its presence in the upper/lower arm of the second stage of the MZI is highlighted through an algebraic analysis. This revealed that there would be corresponding changes in the optimum splitting ratios of the couplers. These features of our design analysis were then exploited to fabricate a wavelength interleaver with a near box-like response, which validated our algorithm. The polarization dependent losses of the device were measured to be ≈ 0.4 dB. Our analysis and fabrication recipe should be useful in the realization of flattop all-fiber wavelength interleavers for deployment in DWDM networks of high spectral efficiency.     

Optical sensing of nanoparticles in the infrared by use of silica nanowires

We numerically investigate an optical sensor in the infrared based on a Mach?CZehnder interferometer (MZI) assembled with two single-mode silica nanowires immersed in acetonitrile. We propose to use acetonitrile as the detecting solution because, in contrast to water which has very high losses in the infrared, it has negligible losses at important wavelengths of 1,300 and 1550?nm. By solving for the fundamental mode of a three-layer nanowire, we calculate the propagation constant difference between the sensing and reference arms at the output of the MZI optical sensor. For nanoparticles with a size of 12?nm and an index of refraction of 1.4, the sensitivity of the optical sensor becomes a maximum for a wire diameter of 1.23???m. An optical sensor operating at a wavelength of 325?nm and using water as the detecting solution requires nanowires with a diameter of 240?nm, which is much more difficult to implement.     

Experimental generation of high-contrast Talbot images with an ultrashort laser pulse

A femtosecond Ti:sapphire laser oscillator emitting pulses with 800 nm central wavelength, 10.9 fs pulse width, and 75 MHz repetition rate, combined with a dispersion-compensated diffractive system, was used to implement a large-area, high-contrast, broadband optical interference technique based on the Talbot effect. Chromatic artifacts associated with the huge spectrum of the optical source (approximately 150 nm) are compensated for with an air-separated hybrid diffractive-refractive lens doublet. The spatial resolution of the chromatically compensated Talbot images under femtosecond illumination is nearly identical to that achieved under continuous wave monochromatic illumination. Furthermore, the temporal width of the signal at the Talbot planes is limited by the group-delay-dispersion coefficient which is shown to be small. High-contrast one-dimensional periodic structures of 96.1 μm spacing generated by Talbot diffractometry are experimentally demonstrated.     

82-channel multi-wavelength comb generation in a SOA fiber ring laser

We demonstrate a multi-wavelength semiconductor optical amplifier (SOA) fiber ring laser with a dual-pass Mach-Zehnder interferometer (MZI) filter. Two SOAs with different gain spectra provide sufficient gain and a wider gain spectrum to facilitate multi-wavelength lasing. The dual-pass MZI, configured by adding an optical isolator to the two outputs of the conventional MZI, serves as comb filter for multi-wavelength operation, and its extinction ratio can be enhanced to twofold as that of the conventional MZI in the same parameters. To investigate the influences of a dual-pass MZI filter and a conventional MZI filter on multi-wavelength operation, two different cavity configurations are presented and compared, including a single-SOA ring cavity and a double-SOA ring cavity. Stable simultaneous operation at 82 wavelengths, with a wavelength spacing of 40 GHz and a power deviation of 5 dB, and with a minimum optical signal-to-noise ratio (OSNR) of 28 dB, is observed from the double-SOA ring cavity using a dual-pass MZI filter.     

All-optical 40 Gb/s wavelength conversion using hybrid SOI-based MZI and SOAs configuration

In this paper, we present a compact combination of silicon-based Mach–Zehnder interferometer (MZI) and semiconductor optical amplifiers (SOAs) to perform all-optical return to zero (RZ)-to-RZ and non-return to zero (NRZ) wavelength conversions at a bit rate of 40 Gb/s. Using the proposed theoretical project, the attractive issue is that converted signals with inverted and noninverted formats can be simultaneously achieved at the same target wavelength. Moreover, both extinction ratio and eye-opening ratio of the converted RZ and NRZ signals can be significantly optimized by judiciously adjusting the system parameters and initial conditions.     

Fabrication of an integrated optical Mach-Zehnder interferometer based on refractive index modification of polymethylmethacrylate by krypton fluoride excimer laser radiation

It is known that deep ultraviolet (UV) radiation induces a refractive index increase in the surface layer of polymethylmethacrylate (PMMA) samples. This effect can be used for the fabrication of integrated optical waveguides. PMMA is of considerable interest for bio and chemical sensing applications because it is biocompatible and can be micromachined by several methods, e.g. structuring by photolithography, ablation and hot embossing. In the presented work direct UV irradiation of a common PMMA substrate by a krypton fluoride excimer laser beam through a contact mask has been used to write integrated optical Mach-Zehnder interferometers (MZI). MZI are used as sensitive bio and chemical sensors. The aim was to determine contact mask design and laser irradiation parameters for fabricating single-mode MZI for the infrared region from 1.30 μm to 1.62 μm. Straight and curved waveguides have been generated and characterized to determine the optical losses. The generation of channel waveguide structures has been optimized by a two step irradiation process to minimize the lithographic writing time and optical loss. By flood exposure to UV laser radiation in the first step the optical absorption of PMMA can be increased in the irradiated region. The required refractive index profile is then achieved with a second lithographic irradiation. The spectral behaviour of an unbalanced, integrated optical MZI fabricated by this excimer laser based contact mask method is shown for the first time. Further the optical intensity at the output port of a MZI has been measured while the optical path length difference was tuned by creating a temperature difference between the two arms of the MZI.     

A simple humidity sensor utilizing air-gap as sensing part of the Mach–Zehnder interferometer

A simple high-resolution refractive index (RI) and phase sensor has been demonstrated and the results numerically verified. A free space gap is employed in one arm of a Mach–Zehnder interferometer (MZI) to serve as the sensing mechanism with a physical spacing of 1.4 mm. The propagation constant of transmitted light in the MZI’s gap changes due to the small variation in the ambient RI that will further shift the optical phase of the signal. A free space optical delay line is embedded within the MZI’s other arm to set the phase reference point and compensate for variations in the optical phase difference. The ambient RI is computed by measuring the phase shift in the transmission spectrum A high-resolution sensing of 0.8 pm/%RH corresponds to phase change of 0.012°/%RH has been achieved in 1520 nm.     

The wavelength dependence of gold nanorod‐mediated optical breakdown during infrared ultrashort pulses

This paper investigates the wavelength dependence of the threshold of gold nanorod‐mediated optical breakdown during picosecond and femtosecond near infrared optical pulses. It was found that the wavelength dependence in the picosecond regime is governed solely by the changes of a nanorod's optical properties. On the other hand, the optical breakdown threshold during femtosecond pulse exposure falls within one of two regimes. When the ratio of the maximum electric field from the outside to the inside of the nanorod is less then 7 (the absorption regime) the seed electrons are initiated by photo‐thermal emission, and the wavelength dependence in the threshold of optical breakdown is the result of optical properties of the nanoparticle. When the ratio is greater than 7 (the near‐field regime) more seed electrons are initiated by multiphoton ionization, and the wavelength dependence of the threshold of optical breakdown results from a combination of nanorod's optical properties and transitions in the order of multiphoton ionization. The findings of this study can guide the design of nanoparticle based optical breakdown applications. This analysis also deepens the understanding of nanoparticle‐mediated laser induced breakdown for picosecond and femtosecond pulses at near infrared wavelengths.

     

Mach-Zehnder interferometer switch with a high extinction ratio over a wide wavelength range

We have realized a novel Mach-Zehnder interferometer (MZI) thermo-optic switch that can operate over a much wider wavelength range than a conventional MZI switch. We constructed the novel MZI switch by incorporating a phase-generating coupler (PGC) as an optical coupler. The PGC generates a nonlinear wavelength-dependent phase, which functions as a virtual wavelength-dependent delay part that cancels out the wavelength dependence of a MZI. We fabricated the novel MZI switch on a silica-based planar light-wave circuit and achieved a low insertion loss of less than 1.6 dB and a high extinction ratio of 30 dB over a wide wavelength range of 160 nm.     

Design of a spectrum-expanded polymer Mach�CZehnder interferometer electro-optic switch using two-phase generating couplers

A novel structure, thorough analysis and design technique are presented for a kind of polymer Mach?CZehnder Interferometer (MZI) electro-optic (EO) switch consisting of two-phase generating couplers (PGCs). By optimizing the PGCs structure, the output spectrum of the two PGCs based MZI EO switch is greatly expanded to almost six times of that of the traditional MZI EO switch with two 3?dB directional couplers. Under a central wavelength of 1550 nm, the driving voltages of the designed switch are as low as about 0 (ON state) and ±0.925?V (OFF state) with an EO region length of 5000???m. A wide spectrum more than 350 nm (1380??1730?nm) is achieved, and within this wavelength range, the crosstalk is less than ?30?dB, and the insertion loss is less than 5.91?dB which is larger than that of the traditional MZI EO switch due to the longer waveguide length by introducing two PGCs. The design technique on the two PGCs based MZI EO switch is supported to be reasonable compared with the beam propagation method (BPM). The proposed structure shows potential wide applications especially in optical cross-connect (OXC) and optical add-drop multiplexing (OADM) systems.     

基于多芯光纤级联布喇格光纤光栅的横向压力与温度同时测量

提出并制作了一种基于多芯光纤与单模光纤错位构成的马赫-曾德尔干涉仪,将其与光纤布喇格光栅级联,形成的全光纤传感系统可实现横向压力和温度双参量同时测量.马赫-曾德尔干涉仪是利用多芯光纤和单模光纤的模场不匹配而发生模间干涉,当外界横向压力直接作用在多芯光纤内部光场,干涉仪具有较高的灵敏度.实验结果表明:马赫-曾德尔干涉仪压力灵敏度为28.57nm/(N·mm~(-1)),线性度为0.997,而光纤布喇格光栅在一定范围内对压力变化不敏感;马赫-曾德干涉仪和光纤布喇格光栅对温度变化都具有较高的线性度,温度灵敏度分别为56.1pm/℃和11.3pm/℃.对于分辨率为0.02nm的光谱仪,传感器可实现的压力和温度测量分辨率分别为7.0×10~(-4)N/mm和0.03℃.马赫-曾德尔干涉仪的透射谱和光纤布拉光栅的谐振峰对横向压力和温度的变化有不同的光谱响应,利用光谱仪对传感器的透射谱实时监测,方便地实现了压力与温度双参量的测量.该传感器结构简单,灵敏度高,可用于不同领域的压力传感.     

Controllable photoinduced optical attenuation in a single-mode optical fiber by irradiation of a femtosecond pulse laser

Novel optical attenuation fibers were fabricated by the irradiation of a focused infrared femtosecond pulsed laser onto the core of a silica glass single-mode optical fiber. Optical attenuation at a wavelength of 1.55 microm proportionally increased with increasing numbers of irradiation points and was controllable under laser irradiation conditions. The single-mode property of the waveguide and the mode-field diameter of the optical fiber were maintained after irradiation of the femtosecond laser. It is suggested that the attenuation results from optical scattering at photoinduced spots formed inside the fiber core.     

The integrated prism interpretation of multileg Mach-Zehnder interferometers based on multimode interference couplers

To simplify the driving conditions as well as to improve the wavelength response of multileg Mach-Zehnder interferometers (MZI) based on multimode interference (MMI) couplers we develop a new interpretation of these devices. Used for optical switching and for wavelength filtering, they are shown to work as active or passive integrated prisms. This analogy greatly facilitates the design of such components and simplifies the understanding of their functionality. The driving conditions of optical switches based on such multileg MZI are easility studied and will be reduced to basic active phase shifts. For wavelength filtering applications using such elements, a new design of the phased waveguide array is proposed which improves the characteristics of the filters.     

Analytical model, analysis and parameter optimization of a super linear electro-optic modulator (SFDR > 130 dB)

An analytical model of a super linear optical modulator with high spurious-free-dynamic-range (SFDR > 130 dB) is presented and analyzed. The linear modulator is referred to as IMPACC which stands for Interferometric Modulator with Phase-modulating And Cavity-modulating Components. The modulator is based on a unique combination of a RF-driven phase-modulator (PM) and a ring resonator (RR) within a Mach-Zehnder interferometer (MZI) configuration. Our analysis shows that our design can achieve SFDR values which are ~ 20 dB higher than the standard MZI modulator and 3-5 dB from the Ring Assisted Mach-Zehnder Interferometer (RAMZI) modulator. Both PM and RR in the IMPACC are simultaneously driven by a RF signal of the same frequency, but not necessarily the same amplitudes. The analytical model shows that the combination of these two optical elements, with the proper choice of RF-driving and device parameters, can lead to four important and compelling consequences. First, it offers a wholistic and elegant model in which the standard MZI modulator and the RAMZI modulator are just special cases of IMPACC. Second, the model offers an excellent parameter optimization methodology for fast parameter (internal and/or external) selection and performance evaluation. Third, it provides additional degree of control through the introduction of an external control parameter, the RF power split ratio (F). Lastly, it demonstrates one unique feature of IMPACC such as adaptive SFDR characteristics, where manufacturing tolerances in the transmission coefficient (τ) of the RR can be compensated with proper adjustments of the external parameter of the power split ratio (F).     

Wavelength-spacing continuously tunable multi-wavelength SOA-fiber ring laser based on Mach–Zehnder interferometer

A Mach–Zehnder interferometer (MZI) which is used as a wavelength-spacing tunable comb filter in a fiber ring laser is built by employing an optical variable delay line (OVDL). Stable multi-wavelength semiconductor optical amplifier (SOA)-fiber ring laser based on an SOA and the MZI comb filter is achieved. Wavelength spacing can be continuously tuned by adjusting the OVDL and, as an example, multi-wavelength lasing with the wavelength spacing of 0.4, 0.8, or 1.6 nm is demonstrated. The output of the proposed multi-wavelength SOA-fiber ring laser is quite stable at room temperature and the output spectrum can be adjusted by controlling the bias current of the SOA.     

Multi-color ultrafast laser platform for nonlinear optical imaging based on independently tunable optical parametric oscillators

We report on design of a multi-color laser set up that allows for high spectral, time and spatial resolution imaging based on second- and third-order optical nonlinearities in soft condensed matter. Two femtosecond optical parametric oscillators (OPOs) are pumped simultaneously to provide intrinsically synchronized pulses at more than a dozen tunable colors across visible and infrared wavelengths. We demonstrate the use of independently tunable OPOs in a variety of imaging modalities. In one useful application, we explore brain tissue in a two-photon absorption fluorescence imaging experiment with near infrared optical pulses (λ ~ 1,070 nm). We also demonstrate second and sum-frequency generation microscopies in different tissues. Results from application of time-resolved, three-color coherent anti-stokes Raman scattering in tissue are presented to demonstrate feasibility of quantitative spectroscopic imaging.     

A tunable and reconfigurable microwave photonic filter based on a Raman fiber laser

A tunable and reconfigurable microwave photonic filter is demonstrated by employing a tunable Mach-Zehnder interferometer (MZI) and a wavelength-controllable Raman fiber laser. The free-spectral-range and spectral response of the microwave photonic filter are tuned and reconfigured by controlling the optical variable delay line in one arm of the MZI and the powers of the pump LDs. Experimental results are given to show the enhanced flexibility of the present microwave photonic filter.