Random distributed feedback fibre lasers

The concept of random lasers exploiting multiple scattering of photons in an amplifying disordered medium in order to generate coherent light without a traditional laser resonator has attracted a great deal of attention in recent years. This research area lies at the interface of the fundamental theory of disordered systems and laser science. The idea was originally proposed in the context of astrophysics in the 1960s by V.S. Letokhov, who studied scattering with “negative absorption” of the interstellar molecular clouds. Research on random lasers has since developed into a mature experimental and theoretical field. A simple design of such lasers would be promising for potential applications. However, in traditional random lasers the properties of the output radiation are typically characterized by complex features in the spatial, spectral and time domains, making them less attractive than standard laser systems in terms of practical applications. Recently, an interesting and novel type of one-dimensional random laser that operates in a conventional telecommunication fibre without any pre-designed resonator mirrors–random distributed feedback fibre laser–was demonstrated. The positive feedback required for laser generation in random fibre lasers is provided by the Rayleigh scattering from the inhomogeneities of the refractive index that are naturally present in silica glass. In the proposed laser concept, the randomly backscattered light is amplified through the Raman effect, providing distributed gain over distances up to 100 km. Although an effective reflection due to the Rayleigh scattering is extremely small (∼0.1%), the lasing threshold may be exceeded when a sufficiently large distributed Raman gain is provided. Such a random distributed feedback fibre laser has a number of interesting and attractive features. The fibre waveguide geometry provides transverse confinement, and effectively one-dimensional random distributed feedback leads to the generation of a stationary near-Gaussian beam with a narrow spectrum. A random distributed feedback fibre laser has efficiency and performance that are comparable to and even exceed those of similar conventional fibre lasers. The key features of the generated radiation of random distributed feedback fibre lasers include: a stationary narrow-band continuous modeless spectrum that is free of mode competition, nonlinear power broadening, and an output beam with a Gaussian profile in the fundamental transverse mode (generated both in single mode and multi-mode fibres).     

Low‐wavelength emission of Nd‐doped lasers

This paper gives an overview of the results obtained with diode‐pumped Neodymium‐doped crystals operating below 900 nm. Operation at such low wavelengths requires considering the strong thermal population of the lower level of the laser transition. Based on a theoretical study and simulations, the paper presents the challenges related to the design of these three‐level lasers. Experimental results are given with Nd:YAG and Nd:vanadate crystals. It is explained how to deal with the line competition with emission at 946 nm or 912 nm. Finally, intracavity second‐harmonic generation is presented. The output powers reach a few hundred mW at wavelengths below 450 nm. Hence, the paper demonstrates the potential of Nd‐doped diode‐pumped solid‐state lasers for applications in the blue range, in replacement of gas lasers such as helium‐cadmium lasers.     

Complexity and synchronization in chaotic fiber-optic systems

In this work we investigate experimentally the complexity of chaotic attractors generated by a semiconductor laser subjected to optical feedback and associate their dimensionality with the synchronization efficiency of the corresponding chaotic transmitter-receiver configuration. The complexity is characterized by calculating the correlation dimension D₂ of experimental chaotic time series for different values of the optical feedback η. We present the effect of D₂ on the synchronization efficiency and determine the optimal operating condition that leads to the most complex chaotic carrier and, simultaneously, to the most successful synchronization. Lastly, we associate and explain our experimental results with theoretical predictions in the research literature.     

医用3μm与2μm级联振荡钬光纤激光器的工作原理与初步设计

分析了医用3μm与2μm级联振荡Ho³⁺:ZBLAN光纤激光器的工作原理,首次给出了1.1μm带泵浦下级联振荡Ho³⁺:ZBLAN光纤激光器的动态及稳态速率方程,最后给出了级联振荡Ho³⁺:ZBLAN光纤激光器的初步设计.     

New CW FIR laser lines obtained in ammonia pumped by a CO2 laser, using the Stark tuning method

We report fifty seven CW FIR emissions observed in NH₃, by resonant pumping with a CO₂ laser. Exact coincidences between IR absorption lines of the gas and emission lines of the CO₂ laser have been carried out by Stark tuning. IR frequency shifts, up to 30 GHz, have allowed the pumping of forty three NH₃ transitions.These FIR emissions correspond to thirty one different wavelengths in the 50–400 m range; eighteen ones of them are new emitted wavelengths by pumping with the CO₂ laser.     

Two-wavelength mid-IR absorption diagnostic for simultaneous measurement of temperature and hydrocarbon fuel concentration

A two-wavelength mid-IR laser is used for time-resolved absorption-based measurements of temperature and n-heptane concentration in shock-heated gases. The novel difference-frequency-generation laser provides tunable mid-IR light from nonlinear conversion of near-IR light, enabling access to the strong hydrocarbon absorption bands between 3.3 and 3.5 μm associated with the CH stretching vibration. This laser was modified to alternate between two mid-IR wavelengths at 200 kHz, providing 5 μs time resolution for simultaneous monitoring of temperature and concentration in reactive flows and combustion systems. Temperature-dependent absorption spectra of n-heptane are first measured in a cell from 298 to 773 K using an FTIR spectrometer. These spectra are used to select candidate pairs of wavelengths with good sensitivity to temperature and concentration and to provide accurate temperature-dependent absorption cross-sections at the selected wavelengths. Laser absorption measurements of shock-heated n-heptane are then used to extend the cross-section data to 1300 K and to investigate the sensor accuracy and noise characteristics. The temperature and concentration inferred from the measurements are compared to known post-shock conditions, with a 4.5% RMS deviation from the calculated temperature and 1.7% RMS deviation from the calculated concentration. Finally, at high temperatures, the sensor is used to monitor decomposition of n-heptane, illustrating the potential of this diagnostic for hydrocarbon kinetics experiments in shock tubes. This new sensor concept should prove useful for simultaneous, time-resolved temperature and hydrocarbon concentration measurements in a variety of combustion and propulsion applications.     

Propagation of high-power partially coherent fibre laser beams in a real environment

The propagation performance of high-power partially coherent fibre laser beams in a real environment is investigated and the theoretical model of a high-power fibre laser propagating in a real environment is established. The influence of a collimating system and thermal blooming is considered together with atmospheric turbulence and mechanical jitter. The laser energy concentration of partially coherent beams in the far field is calculated and analysed based on the theoretical model. It is shown that the propagation performance of partially coherent beams depends on the collimating system, atmospheric turbulence, mechanical jitter and thermal blooming. The propagation performance of partially coherent beams and fully coherent beams is studied and the results show that partially coherent beams are less sensitive to the influence of thermal blooming, which results in that the energy degeneration for partially coherent beams is only 50% of that for fully coherent beams. Both partially coherent beams and fully coherent beams become less sensitive to thermal blooming when the average structural constant of the refraction index fluctuations increases to 1.7 × 10^-14 m^-2/3. The investigation presents a reference for applications of a high-power fibre laser system.     

Investigation of the pump wavelength influence on pulsed laser pumped Alexandrite lasers

Recent theoretical modelling and experimental results have shown that excess lattice phonon energy created dur ing the non-radiative energy transfer from the ⁴T₂ pump manifold to the ²E storage level in Alexandrite when pumped with wavelengths shorter than ∼645 nm causes chaotic lasing output. Shorter pump wavelengths have also been associated with increased non-radiative energy decay and reduced laser efficiency. We report studies of fluorescence emission spectra of Alexandrite illuminated at a range of wavelengths from green to red, which demonstrate reduced fluorescence yield for shorter pump wavelengths at elevated crystal temperatures. Investigations of pulsed laser pumping of Alexandrite over the same spectral range demonstrated reduced pump threshold energy for longer pump wavelengths. High repetition rate pulsed pumping of Alexandrite at 532, 578 and 671 nm showed stable and efficient laser performance was only achieved for red pumping at 671 nm. These results support the theoretical model and demonstrate the potential for scalable, red laser pumped, all-solid-state Alexandrite lasers.PACS 42.60.Lh; 42.60.Mi     

Dispersion-tuned multiwavelength actively mode-locked fiber laser using a hybrid gain medium

We demonstrate a multiwavelength 10 GHz pulse source using a dispersion-tuned actively mode-locked fiber ring laser incorporated with a semiconductor optical amplifier and an erbium-doped fiber amplifier. Simultaneous seven-wavelength operation of the laser is obtained. The side-mode suppression of all wavelengths is above 30 dB. Smooth wavelength tuning is achieved over more than 12 nm by changing the modulation frequency or the length of the optical delay line. Pulse characteristics are almost constant over the entire tuning span. Wavelength spacing can also be varied from 0.9 to 10 nm by adjusting the dispersion of the cavity. These experimental observations agree well with theoretical analyses.     

Eleven-wavelength switchable fiber ring laser with a dispersion compensation fiber and a delayed interferometer

We demonstrate a fiber ring laser with a dispersion compensation fiber (DCF) and a delayed interferometer (DI) with temperature control, which is able to switch eleven wavelengths one by one. In ring cavity, DCF supplies different effective cavity lengths for different wavelengths, DI generates a wavelength comb corresponding to the ITU grid, a flat-gain erbium-doped fiber amplifier (EDFA) provides uniform gain for each lasting wavelength, and a semiconductor optical amplifier (SOA) not only acts as active modulator, but also alleviates homogeneous broadening effect of EDFA. Stable pulse trains with a pulsewidth about 40 ps at 10 GHz have been obtained by injecting external optical control signals into the laser. Wavelength switching process among eleven wavelengths is achieved by merely tuning an intracavity optical delay line.     

Measurement of aerosol size distribution functions by wavelength-multiplexed laser extinction

Previous work on the measurement of aerosol size distribution functions (SDFs) by laser extinction mainly relied on light sources from a relatively narrow wavelength range. This paper investigates the potential advantages of extending the extinction method to a general wavelength-multiplexed laser extinction (WMLE) concept by incorporating an arbitrary number of laser sources from a wider wavelength range. This extension improves the sensitivity of SDF measurements over wider aerosol diameter ranges and enables a stable algorithm to invert the extinction data to obtain SDFs. These advantages are illustrated by an example WMLE scheme employing wavelengths in the spectral range from 0.25 to 10μm to measure SDFs of water aerosols. Application of this approach to other aerosol systems is also considered. The WMLE scheme was found to provide stable determination of a variety of SDFs with Sauter mean diameters ranging from sub-micron to about 10μm. The sensitivity of such determinations was evaluated to reveal the optimum applicable range of the wavelengths employed. The analyses performed here provide theoretical background and motivation for practical applications of the WMLE concept.     

Sol–gel combustion synthesis of magnetic MnFe2O4 oxide and FeNi alloy: product dependence on the reduction ability

We report on fabrication of on-chip calcium fluoride (CaF₂) microdisk resonators using water-assisted femtosecond laser micromachining. Focused ion beam (FIB) milling is used to create ultra-smooth sidewalls. The quality (Q) factors of the fabricated microresonators are measured to be 4.2 × 10⁴ at wavelengths near 1,550 nm. The Q factor is mainly limited by the scattering from the bottom surface of the disk whose roughness remains high due to the femtosecond laser micromachining process. This technique facilitates the formation of on-chip microresonators on various kinds of bulk crystalline materials, which can benefit a wide range of applications such as nonlinear optics, quantum optics, and chip-level integration of photonic devices.     

Review of collisional excitation neon-like and nickel-like soft X-ray lasers pumped by multiple infrared laser pulses at the Institute of Laser Engineering,Osaka University

We report on the soft X-ray laser developments performed at the Institute of Laser Engineering, Osaka University especially for the collisional excitation X-ray lasers at wavelengths between 25 nm and 4 nm. The performance of neon-like and nickel-like ion X-ray lasing pumped by a train of short laser pulses has been investigated. We have also demonstrated double pass amplification using a soft X-ray multi-layer mirror at wavelengths of 19.6 nm and 7.9 nm. Based on these results and experimental technique, two targets were placed in series to double the gain length. Two opposing laser beams irradiated the double targets with a suitable time difference for quasi-traveling wave pumping. The double target amplification was successfully demonstrated with two beam irradiation for Ag, Nd, Yb, Hf and Ta lasing. The estimated absolute photon flux from the saturated amplification Ag X-ray laser was ∼300 μJ. Based on the experimental results, we also discuss the applications of an X-ray laser as a super-high brightness soft X-ray source.     

NaLi分子飞秒含时光电子能谱的理论研究

运用三态模型和含时波包方法,研究了NaLi分子处于强飞秒抽运-探测激光场中的波包动力学过程和时间分辨光电子能谱,并且揭示了飞秒激光脉冲参数与NaLi分子光电子能谱之间的关系.研究发现:对于不同的激光波长,波包的振动周期是不同的,而且随着抽运-探测脉冲延迟时间的不同,NaLi分子光电子能谱的谱峰高度和位置发生变化;当λ1=352 nm并且?t=400 fs时,外阱中相应的光电离信号(0.5 eV处)明显强于内阱中相应的光电离信号(1.35 eV处).计算结果表明,NaLi分子激发态41Σ+上波包动力学的一些信息能够通过其光电子能谱反映出来.这些结果可以为实验上实现分子的光控制以及量子操控过程提供一些有价值的参考信息,并为进一步的理论研究提供重要依据.     

High‐brightness long‐wavelength semiconductor disk lasers

A review on the recent developments in the field of long‐wavelength (λ >1.2μm) high‐brightness optically‐pumped semiconductor disk lasers (OPSDLs) is presented. As thermal effects have such a crucial impact on the laser performance particular emphasis is given to modelling the thermal behaviour and optimisation of the heat‐sinking. Selected OPSDL devices, realized in different III‐V and IV‐VI semiconductor material systems, with corresponding emission wavelengths between 1.2 μm and 5.3 μm are presented. Specific applications in this broad spectral range are addressed and methods to obtain high output power are discussed in terms of the underlying material properties and device operating principles.     

Variable high-energy-laser attenuator based on the interference on a transparent plate

The transmittance of a transparent plate is theoretically and experimentally investigated, taking into account Fabry–Perot effects due to Fresnel reflections of a Gaussian beam on the boundaries of a plate. On the basis of these theoretical and experimental predictions, we present the application of a variable laser attenuator based on a thin transparent plate and a temperature regulation. Here, the absorption of the laser energy in the plate should be as low as possible, and its transmittance is changed by the interference due to the different thicknesses and refractive indices for the different temperatures of the plate. Therefore, such an attenuator can be used for a broad range of wavelengths and high-energy laser applications.     

Femtosecond coherent pulses in the keV range from inner-shell transitions pumped by a betatron source

We present a new method to generate ultra-short X-ray laser pulses by using the recently demonstrated laser-driven betatron source to photo-pump inner-shell transitions. The proposed compact set-up will then open the route to a wide range of applications. The betatron spectrum and ion-population kinetics are modeled and the temporal evolution of the gain coefficient for the K-α transitions is assessed. Using measured values of divergence, duration, and number of photons per pulse of the betatron source as input parameters, local gain values close to 60 cm^?1 are calculated for nitrogen at 3.2 nm. Significant gain values are also numerically obtained at shorter wavelengths (for neon at 1.5 nm) when the betatron energy distribution is optimized as suggested by recent laser wakefield electron acceleration experiments.     

Control of the pulse duration in one- and two-axis passively Q-switched solid-state lasers

We study theoretically and experimentally different methods to control the pulses emitted by solid-state lasers passively Q-switched by a saturable absorber. We explore one- and two-axis laser schemes allowing to control the pulse duration, which is ruled by the saturation powers of the transitions in the absorber and in the gain medium. In one-axis lasers, it is shown that the adjustment of the pump and laser beam sizes in the active medium and in the absorber provides an efficient means to control the pulse temporal shape and duration. Furthermore, a two-axis laser cavity supporting so-called forked-eigenstate operation permits to freely adjust the parts of the mode power which circulate in the gain medium and in the absorber. In this case, a lengthening of the pulse duration up to 500 ns is obtained with an increase of the average output power. The theoretical results obtained by using rate equations adapted to each cavity geometry are in close agreement with experiments performed on a diode-pumped Nd³⁺:YAG laser Q-switched by a Cr⁴⁺:YAG saturable absorber. The relevance of the different techniques to control the pulse durations in the framework of potential applications is discussed. Received 3 December 2001     

Out of the blue: semiconductor laser pumped visible rare‐earth doped lasers

The rise of semiconductor‐based pump sources such as In_xGa_1‐xN‐laser diodes or frequency‐doubled optically pumped semiconductor lasers with emission wavelengths in the blue encourages a revisitation of the rare‐earth ions Pr³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Ho³⁺ and Er³⁺ with respect to their properties as active ions in crystalline solid‐state laser materials with direct emission in the visible spectral range. Nowadays, some of these blue‐pumped visible lasers compete with Nd³⁺‐lasers in terms of efficiency and direct lasing at various colors from the cyan‐blue to the deep red can be addressed in very simple and compact laser setups. This paper highlights the spectroscopic properties of suitable rare‐earth ions for visible lasing and reviews the latest progress in the field of blue‐pumped visible rare‐earth doped solid‐state lasers.

     

Dual-wavelength passively mode-locked Nd: GdVO4?laser?with?orthogonal polarizations

A passively mode-locked dual-wavelength Nd: GdVO₄ laser with orthogonal polarizations is experimentally demonstrated. By changing the absorbed pump power, the laser can mode lock either at 1063?nm (with ?? polarization) or 1065 nm (with ?? polarization), or simultaneously at both wavelengths with orthogonal polarizations. When mode locked at 1063 nm and 1065?nm, the laser produced pulses of 10?ps and 5.9?ps, respectively, with repetition rates of 143.7?MHz and 143.9?MHz. When simultaneously mode locked at both wavelengths, the pulsewidth was about?7?ps. The laser was demonstrated by exploiting the thermal lens and birefringence of the Nd: GdVO₄ crystal. The laser uses one cavity and does not require the insertion of additional optical components.