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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Label-Free Multiphoton Microscopy in Human Tissue Enabled by an Er:Fiber-Laser Based Tunable Source
摘要： Multiphoton microscopy (MPM) is an important bio-imaging tool. Different modalities can serve as a contrast agent, such as second-/third-harmonic generation (SHG/THG) and two-/three-photon excitation fluorescence (2PEF/3PEF). Ultrafast lasers with flexible wavelength tunability are crucial for driving MPM bio-imaging, and the conventional solution relies on ultrafast Ti:sapphire lasers plus an optical parametric oscillator/amplifier. Recently, we have demonstrated that ultrafast fiber lasers are a potential solution to implementing compact, robust, and wavelength tunable femtosecond sources for driving MPM. To realize wavelength tunability we employ self-phase modulation (SPM) in optical fibers to broaden a narrowband input spectrum of Yb-/Er-doped fiber lasers (YDFLs/EDFLs) up to >400-nm wide with well-isolated spectral lobes; filtering the leftmost/rightmost lobes leads to nearly transform-limited pulses [1–6]. Such a SPM-enabled spectral selection (SESS) allows us to obtain wavelength widely tunable femtosecond pulses for MPM [2,5,6]. In this submission, we representatively demonstrate label-free harmonic generation microscopy (HGM) in human skin and brain tissues. Figure 1(a) depicts a scanning microscope driven by an EDFL-based SESS source. The EDFL operates at 31-MHz repetition rate and generates 290-fs pulses centered at 1550 nm. The narrowband EDFL [blue curve in Fig. 1(a)] is coupled into 9-cm optical fiber (10-μm mode-field diameter and -10 fs2/mm group-velocity dispersion at 1550 nm). The output spectrum is shown as the red curve in Fig. 1(b) under 85-nJ coupled pulse energy. We use optical filters to select the leftmost spectral lobe peaking at 1250 nm, which leads to 11.7-nJ, 47-fs pulses. Then we employ these pulses to drive a scanning microscope and conduct HGM in human skin and brain tissues. Figure 1(c) shows the dermal papilla at the junction of epidermis and upper dermis in human skin. Basal cells are visualized by THG (cyan hot) due to optical inhomogeneity at the interface (e.g., cell membrane); SHG (red hot) originates from the non-centrosymmetric structure of collagen fibers. In Fig. 1(d), neural network and brain vasculature in human brain tissue can be visualized by THG and SHG, respectively [Fig. 1(d)]. THG contrast inside the vasculature shows also red blood cells. In conclusion, we report on MPM deep-tissue imaging enabled by an EDFL-based SESS source. It is noteworthy that besides HGM excited by 1250-nm femtosecond pulses demonstrated here, the SESS source also supports 1300-/1700-nm illumination for 3PEF of green/red fluorescent protein (GFP/RFP) [7,8]. Such a fiber-based solution can be applied to many important applications, such as histopathology, morphology, and neuroscience.
关键词： ultrafast fiber lasers,Multiphoton microscopy,self-phase modulation,bio-imaging,harmonic generation microscopy
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Avoidance of Cross-Phase Modulation in Femtosecond Stimulated Raman Scattering
摘要： Raman scattering has become a powerful method in analytic spectroscopy and label-free imaging to study the chemical composition of a sample. Especially coherent Raman scattering (CRS) techniques such as stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) are developed further and show a great potential, due to their amplified signal in comparison to spontaneous Raman scattering. In CRS the interaction of light pulses with molecular vibrations are investigated, typically resulting in a characteristic molecular spectrum. However, especially CARS spectra are effected by the non-resonant contribution to the Raman scattering signal, resulting in a non-specific background and a distortion of the lineshape. SRS is free of a non-resonant background but is affected by two-photon absorption, cross-phase modulation (XPM) [1], and thermal scattering (temperature-induced refractive index change) [2]. The resulting artefacts in SRS become relevant for high pulses energies and ultra-short pulses. One example, where high pulse energies and ultra-short pulses are needed, is the field of spatial resolution enhancement in CRS. All techniques to overcome the diffraction limit by Abbe are based on depleting one of the scattering resources, as for example the pump or probe photons [3]. High pulse energies in the order of up to 200-300 nJ are needed, leading to also unwanted spectral distortions through XPM [4]. In this paper, we present the avoidance of spectral XPM artefacts in femtosecond stimulated Raman scattering, based on a systematic study varying the pump pulse duration in the high pulse energy regime. To study the effects of different pump pulse durations on the FSRS spectra a setup as shown in Fig. 1 (a) was realized, which consists of a folded Fourier filter, to freely choose the pump pulse duration, and a supercontinuum generation in a gadolinium vanadate crystal (GdVO4) working as broadband probe pulse source. After combining the two beams at the dichroic mirror (DM) measurements with different peak powers of the pump pulse, time delays between pump and probe and varying pump pulse durations were performed. For pump pulses with a duration between 2 ps and 3 ps the Raman spectra are nearly unaffected by XPM, whereas pulses shorter than 1 ps show a significant XPM induced distortion of the stimulated Raman lineshape and a reduction of the Raman resonance maximum, becoming visible as a negative stimulated Raman loss (SRL) signal on the high wavenumber side of the Raman resonances. Thus, it is favourable to choose pump pulses with a longer pulse duration in the regime of high peak power to reduce the impact of cross-phase modulation in stimulated Raman scattering.
关键词： stimulated Raman scattering,Raman scattering,femtosecond pulses,spectral distortions,cross-phase modulation
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Self-Reference Optical Phase Demodulation using Narrow Absorption/Gain Peak
摘要： Detection (or demodulation) of the phase modulation is a critical operation in different photonic applications, in particular, in interferometric fiber sensors, material evaluation via laser induced ultrasound, coherent communications etc. Different techniques were proposed to overcome the main problem of conventional interferometers: necessity of stabilization of the quadrature operation point and compensation of the complex spatial structure of the detected wave. Here one can mention, active operation point stabilization via external opto-electronic loop [1], adaptive dynamic gratings based systems [2], self-reference (i.e. without external reference wave) confocal Fabry-Perot cavities [3], and configuration based on quantum phase memory of an ensemble of two-level atoms/molecules [4]. All these seemingly different approaches have the same common feature – they are based on utilization of a narrow absorption/gain spectral profile, artificial, as in the first two above-mentioned techniques, or natural, as in the two last cases. Here we analyse a general case of passing of the phase modulated wave through a narrow absorption/gain spectral line as a technique for its demodulation (i.e. its partial transformation into the intensity modulation wave) with the following detection by conventional photodetector.
关键词： photodetector,phase modulation,narrow absorption/gain spectral line,demodulation
更新于2025-09-12 10:27:22
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Modulation Effects on Power-loss and Leakage Current in Three-Phase Solar Inverters
摘要： Three-phase solar inverters can be operated with different modulation strategies such as sinusoidal pulse-width modulation (SPWM), space vector modulation (SVM), third-harmonic injection PWM (THIPWM). The selection of the modulation strategy can significantly affect PV leakage currents and power losses and compromise the inverter performance. This paper presents a detailed analysis of traditional and modified modulation effects on power semiconductor losses and photovoltaic (PV) leakage current. The research focuses on a three-phase Three-Level T-type Neutral Point Clamped (3L-TNPC) solar inverter. A detailed exploration of traditional SPWM and SVM is presented and a new modified modulation strategy referred to as two-triplen harmonic injection PWM (2THIPWM) which can substitute the SVM scheme. Analytical expressions are derived for the proposed 2THIPWM strategy which can be used for the SVM power-loss approximation. The comparison between 2THIPWM and SPWM shows a conduction-loss difference greater than the 5% normalized range. The modulation strategies are discussed with respect to the PV common mode current signature and specific design techniques in modulation lead to reduced leakage currents for safety compliance. The theoretical and experimental results obtained using a 3L-TNPC solar inverter are presented to evaluate the modulation strategies and their impact on performance.
关键词： three-phase modulation,PV leakage current,Three-phase three-level (3L) TNPC power semiconductor losses
更新于2025-09-11 14:15:04
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Pulse shaping by a frequency filtering of a sawtooth phase-modulated cw laser
摘要： The spectrum of a cw field the phase of which experiences a periodic sawtooth modulation is analyzed. Two types of the sawtooth phase modulation are considered. One is created by combining many harmonics of the fundamental frequency. The second is produced by an electro-optic modulator fed by the relaxation oscillator, which generates a voltage slowly rising during charging of the energy storage device and dropping fast due to discharge by a short circuit. It is proposed to filter out the main spectral component of the sawtooth phase-modulated field. This filtering produces short pulses from the phase-modulated cw field. The duty cycle of this train is equal to the modulation period, while duration of the pulses depends on the rate of the phase drop. In the case of harmonics, this duration is 4N + 2 shorter than the period of the fundamental frequency, where N is the number of the harmonics. In the case of the charge and discharge, duration of the pulses is close to TD/2, where TD is a drop time of the phase in the discharge period. Depending on the modulation frequency, the proposed method is capable to produce pulses with duration ranging from nanoseconds to a fraction of a picosecond.
关键词： relaxation oscillator,electro-optic modulator,frequency filtering,sawtooth phase modulation,pulse shaping
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Pure-Quartic Solitons from a Dispersion Managed Fibre Laser
摘要： In optical fibre resonators, the balance between anomalous quadratic dispersion and self-phase modulation (SPM) gives rise to optical solitons [1]. These pulses have made a significant impact in a wide range of photonic applications including telecommunications and lasers. However, these conventional soliton-based lasers can only deliver modest pulse energy due to the appearance of Kelly sidebands arising from periodical perturbations in the cavity [2] and a fixed energy-width scaling. Recently, a new class of soliton, arising from the balance of anomalous quartic dispersion and SPM, called pure-quartic soliton (PQS), were observed in a dispersion engineered photonic crystal waveguide [3]. PQSs have huge potential for generating ultrashort pulses with high energy due to their generalized area theorem (E ~ 1/(cid:507)(cid:306)3), however they are yet to be observed in fibre platforms [4]. Here we report on the generation of PQS pulses from a passively mode-locked fibre laser incorporating a programmable spectral pulse-shaper that induces a dominant quartic net cavity dispersion. We find that the spectral profile of the generated pulses are in good agreement with the spectral shape of PQSs [3]. We also observe spectral sidebands in this quartic-dispersion cavity, in analogy to the conventional soliton case [2], and find that their positions are in excellent agreement with analytic predictions. These are strong evidences of a novel type of mode-locked laser, the PQS laser, which has the potential to reach dramatically higher energies at short pulse durations than its conventional soliton counterpart [3,4].
关键词： dispersion managed fibre laser,pure-quartic solitons,optical solitons,photonic applications,self-phase modulation
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Coherent Beam Combining of Fiber Amplifiers Seeded by Commercial Low Cost Diode
摘要： Coherent beam combining (CBC) is one of the methods for power scaling of high power lasers. In the scientific literatures a low power narrow linewidth source is used for CBC, however in this report, a low cost diode laser source (spectral bandwidth <10 MHz) is used to fabricate near diffraction-limited 300 W fiber laser with CBC method. The diode laser is amplified by a preamp fiber amplifier and then divided in to three channels. Each channel is amplified to achieve 120W output with homemade power amplifiers. A 1×3 diffractive optical element is used to combine the output of the amplifiers with ~83% efficiency. Active phase locking is achieved with the stochastic parallel gradient descent (SPGD) method. A major obstacle towards high power narrow-linewidth fiber lasers is stimulated Brillouin scattering (SBS). It has been proven that phase modulation is an effective way to suppress SBS in these kind of lasers. Applying phase modulation to a single frequency laser, splits its spectrum to many spectral lines with the same frequency spacing of the modulation frequency. In this work, we use a sinusoidal signal phase modulation (30 MHz frequency and 10 V amplitude) to suppress SBS effect. The result of this modulation on the spectrum of the laser is illustrated. The maximum achievable output power for each channel without modulation is ~40 W. However, by applying modulation SBS threshold power enhances with a factor of 3. The output power of combined beam versus pump diode current (solid line) is shown. Also the backward output power (dashed line) is illustrated. As it can be seen in the figure the maximum backward power is 110 μW which proves the SBS effect is cancelled.
关键词： stimulated Brillouin scattering,diode laser,phase modulation,Coherent beam combining,fiber amplifiers
更新于2025-09-11 14:15:04
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[IEEE 2018 20th International Conference on Transparent Optical Networks (ICTON) - Bucharest (2018.7.1-2018.7.5)] 2018 20th International Conference on Transparent Optical Networks (ICTON) - Performance Analysis of Radio-over-Fiber Based on Phase-Modulation and Direct-Detection for the Future 5G Network
摘要： Access transport network plays a vital role in actual mobile networks to support the need of high data rates, real time services and spectrum efficiency. The future 5G requires an access network with high performances especially in the mm-wave bands. In this work, we propose a radio-over-fiber architecture based on intensity modulation for the downlink and phase modulation for the uplink. This solution permits to cancel optical sources and reduces electrical consumption in the multiple base stations. We focus our study on the phase modulation and direct detection link by evaluation of the quality of the wireless transmitted signal.
关键词： radio-over-fiber,5G network,optical phase modulation,filtered-OFDM
更新于2025-09-11 14:15:04
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Stable Low-Repetition-Rate Time-Lens Picosecond Seed Source
摘要： The design and performance of a sub-10-picosecond source based on temporal focusing implemented with a lithium niobate phase modulator and a grating compressor are presented. The source is optimized taking into account spectral gain narrowing in subsequent amplifiers. It advantageously delivers low-repetition-rate pulses with tunable pulse duration, and its short-term jitter and long-term drift over extended periods of time are of the order of a few picoseconds.
关键词： Chromatic dispersion,pulse compression methods,phase modulation,optical pulse generation
更新于2025-09-11 14:15:04
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[IEEE 2018 IEEE International Conference on Computational Electromagnetics (ICCEM) - Chengdu, China (2018.3.26-2018.3.28)] 2018 IEEE International Conference on Computational Electromagnetics (ICCEM) - Single Layer Multi-Polarization Transmitarray in C-Band with Beam-Steering Capability
摘要： A single layer multi-polarization transmitarray is presented in this paper, whose structure can be much simpler than the traditional multi-layer frequency selective surface transmitarray. The proposed transmitarray consists of numerous split ring elements and has three working conditions. It is designed, simulated and measured in C-band. The measurement result shows that the transmitarray can switch polarization of the incident wave and set separation beam direction at the same time
关键词： beam separation,phase modulation,transmitarray,polarization conversion
更新于2025-09-10 09:29:36