1μm全光纤MOPA结构激光器输出特性研究

主振荡功率放大（main oscillation power amplification, MOPA）结构由于其光束质量良好和参数可调的优点,已成为高功率光纤激光器的主流设计之一。为了改善高功率掺镱光纤激光器（ytterbium-doped fiber laser, YDFL）的输出性能,提高系统的光-光转换效率,文中报道了一台基于915 nm泵浦激光器和双包层掺镱光纤（ytterbium-doped fiber, YDF）的MOPA结构全光纤高功率激光器。该高功率光纤激光器由电调制激光二极管（laser diode, LD）泵浦的种子激光器和掺镱光纤放大器（ytterbium-doped fiber amplifier, YDFA）组成。连续光（continuous wave, CW）工作模式下,激光种子源经过YDFA后,实现了中心波长为1 069.96 nm的激光输出,最大平均输出功率可达945.9 W,MOPA激光器整机的斜率效率高达74.12%,具有良好的稳健性。该研究方案对研制高功率MOPA光纤激光器具有参考意义。     

A high-power electro-absorption modulated 2d-array laser source for FM/cw scannerless lidar

A scannerless amplitude-modulated (AM) semiconductor lidar (or laser radar) using frequency-modulation continuous wave ranging (FM/cw) techniques is one of the most promising research directions at present. In this lidar system, the high-power AM laser source is a key component of the lidar transmitter. First of all, we present here the architecture of an FM/cw scannerless AM ladar and summarize and analyze various schemes of high-power laser modulation. Then we put forward a new method for obtaining a high-power AM laser output, that is, to compose a two-dimensional (2D) array light source with laser diodes (LDs) and electro-absorption modulation modules (EAMs). Finally, the optical properties of electro-absorption modulated laser arrays are briefly analyzed. The analysis shows that the modulated laser arrays have the features of integration and miniaturization and can realize incoherent light illumination in the far field. This laser array is potentially a more ideal high-power AM laser source applied to an FM/cw scannerless imaging lidar.     

Laser dye mixtures

Properties necessary to obtain efficient laser action from laser dyes are reviewed. It is concluded that the dye added (booster dye) to a laser dye (active dye) solution to increase its efficiency need not necessarily be a laser dye. This is illustrated by considering different spectral locations of a triplet-triplet absorption band present in the booster dye relative to the location of the laser action region (part of the fluorescence spectrum) of the active laser dye. Conditions required for booster dyes to form efficient dye mixtures are given. These observations are used to explain why specific well known laser dye mixtures show efficient, little, or no laser action at all.     

Viscous liquid expulsion in nanosecond UV laser ablation: From “clean” ablation to nanostructures

This paper reviews recent results on modeling UV nanosecond laser ablation. Particular attention is given to a viscous liquid flow driven by ablation pressure. Based on the analysis of the Navier-Stokes equation, various strongly different manifestations of this phenomenon are explained. These are (i) a “clean” laser ablation, when the laser spot has a clean sharp spot border free of resolidified melt dross; (ii) a new form of material removal in laser ablation, expulsion on a poly(methyl methacrylate) target of long (up to 1 mm) nanofibers with a radius about 150–200 nm to the exterior of the spot under the action of a single pulse of a KrF excimer laser; and (iii) a new method of laser surface nanostructuring, the formation of a surface foam having a structure of micropores interconnected by nanofilaments with diameters of about 100 nm as a result of single-pulse KrF laser ablation of biopolymer films.     

High-power GaN diode-pumped continuous wave Pr3+-doped LiYF4 laser

A cw Pr(3+):LiYF(4) laser at 639 nm pumped by a high-power GaN laser diode (444 nm) is demonstrated. The highest laser power of 112 mW is achieved with an optical-optical conversion efficiency of 33.5%. Characteristics of this laser at elevated temperatures are also investigated for practical applications such as a laser projector.     

Laser-induced sputtering of ZnO,TiO2, CdSe and GaP near threshold laser fluence

Sputtering of ZnO, TiO₂, CdSe and GaP induced by 10 ns laser pulses from a tunable dye laser has been investigated. It is shown that all of these materials exhibit the following characteristics: (1) non-linear dependence of sputtering yield on the excitation laser fluence, (2) existence of the threshold laser fluence, (3) dependence of the threshold laser fluence on laser wavelength, and (4) non-Maxwellian velocity distribution of sputtered particles. These results indicate that the laser-induced sputtering near threshold fluences is not a thermal effect but an effect of dense electron-hole plasma.     

Excimer laser ablation of gold-loaded inverse polystyrene-block-poly (2-vinylpyridine) micelles

Diblock-copolymers (PS(1700)-b-P2VP(450) or PS(1350)-b-P2VP(400)) forming spherical micelles, can be loaded with a Au-salt and deposited on top of various substrates. Such polymer films have been exposed to a pulsed ArF excimer laser in order to remove the polymer matrix and, in parallel, to chemically reduce the salt into metallic Au nanodots. To analyze this process in detail, it was subdivided into three steps: (a) laser ablation of thick and thin diblock-copolymer films; (b) laser irradiation of Au-salt loaded diblock-copolymer films; and (c) laser irradiation of arrays of metallic Au nanodots. In (a) it was found that a complete removal of the polymer by laser ablation is only possible in air under ambient conditions while identical laser irradiations under vacuum result in a residual layer of approximately 14 nm. Substep (b) revealed a nucleation process of the resulting metallic Au within the micellar core leading to clusters of small Au dots. Furthermore, this substep provided evidence for an asymmetric interplay between the macroscopic temperature of a polymer film during laser treatment and the energy density per laser pulse. In (c) it could be demonstrated that metallic Au nanodots on mica are stable against laser irradiation under conditions leading to a polymer removal. Received: 7 August 2000 / Accepted: 2 November 2000 / Published online: 3 April 2001     

Rydberg excitation of neutral nitric oxide molecules in strong UV and near-IR laser fields

Rydberg state excitations of neutral nitric oxide molecules are studied in strong ultraviolet(UV) and near-infra-red(IR) laser fields using a linear time-of-flight(TOF) mass spectrometer with the pulsed electronic field ionization method.The yield of Rydberg molecules is measured as a function of laser intensity and ellipticity,and the results in UV laser fields are compared with those in near-IR laser fields.The present study provides the first experimental evidence of neutral Rydberg molecules surviving in a strong laser field.The results indicate that a rescattering-after-tunneling process is the main contribution to the formation of Rydberg molecules in strong near-IR laser fields,while multi-photon excitation may play an important role in the strong UV laser fields.     

A comparative study of approaches for spectral control of Ti:sapphire pulsed lasers

The results of a comparative study (theoretical and experimental) of two original approaches for spectral control of two-wavelength pulsed Ti:sapphire (Ti:S) lasers and the most popular conventional methods are presented. Single line operation as a particular case is considered. We compare our own methods: (i) combined dual pulse pumping–passive self-injection locking (DPP–PSIL) and (ii) coupled cavity dye laser injection locking (CCLIL) with the conventional methods: (i) intracavity selection (IS), (ii) PSIL and (iii) traditional injection locking using an external dye laser. The most important criteria—laser efficiency, emission line width, tuning range, spectral purity and buildup time are compared under optimized conditions. The comparison is made using the same pump source with constant output energy in each case. The results serve as a basis for choosing correctly the most suitable spectral control technique for a particular Ti:S laser application. It is shown that the DPP–PSIL method guarantees a remarkable improvement of the Ti:S laser performance (nearly doubling the laser efficiency with the same line width and comparable laser tuning range as IS, combined with a protection of the selectors). The CCLIL approach assures a maximum laser efficiency in narrow line emission in addition to a minimal and reproducible buildup time. The IS method provides a maximal tuning range.     

Study of optimal laser parameters for cutting QFN packages by Taguchi's matrix method

This paper reports the study of optimal laser parameters for cutting QFN (Quad Flat No-lead) packages by using a diode pumped solid-state laser system (DPSSL). The QFN cutting path includes two different materials, which are the encapsulated epoxy and a copper lead frame substrate. The Taguchi's experimental method with orthogonal array of L₉(3⁴) is employed to obtain optimal combinatorial parameters. A quantified mechanism was proposed for examining the laser cutting quality of a QFN package. The influences of the various factors such as laser current, laser frequency, and cutting speed on the laser cutting quality is also examined. From the experimental results, the factors on the cutting quality in the order of decreasing significance are found to be (a) laser frequency, (b) cutting speed, and (c) laser driving current. The optimal parameters were obtained at the laser frequency of 2 kHz, the cutting speed of 2 mm/s, and the driving current of 29 A. Besides identifying this sequence of dominance, matrix experiment also determines the best level for each control factor. The verification experiment confirms that the application of laser cutting technology to QFN is very successfully by using the optimal laser parameters predicted from matrix experiments.     

Formation of periodic structures by laser ablation of metals in liquids

Experimental results are presented on ablation of metals (W, Cu, brass and bronze) in a liquid environment (e.g., ethanol or water) by irradiation with either a pulsed copper vapor laser (0.51 μm) or a pulsed Nd:YAG laser (1.06 μm). The target material is ejected into surrounding liquid in the form of nanoparticles. In a certain range of laser parameters (fluence and number of laser shots) the surface of the solid target is composed of micro-cones having a regular structure. The distance between neighboring micro-cones in the structure depends on the laser spot size. The structures allow the observation of up-conversion of the laser frequency due to generation of the second harmonics in the eye retina.     

Cs 5D_(5/2)-6F 728 nm Laser Spectroscopy with Single Pumping Laser

The sub-Doppler absorption laser spectroscopy at 728 nm transition from the 5D5/2 state to the 6 F state of cesium with linewidth near 10 MHz is first experimentally performed with indirect pumping from the ground state 6S_(1/2)to the state 7P_(3/2)by a 455.5 nm diode laser.Using a 455.5 nm diode laser as an indirect pump laser,several excited states will be populated due to spontaneous decay from the 7P state.We first implement the sub-Doppler absorption laser spectroscopy at 728 nm from the 5D_(5/2)state to the 6F state when Cs atoms within thermal glass cell decay to the 5D_(5/2) state.Due to velocity transfer effect,the hyperfine structure of 5D_(5/2)shows a mixed and complicated pattern but very clear structure when the 455.5 nm pumping laser is counter-propagating(or co-propagating)with the 728 nm probing laser.     

Continuous-wave rotational Raman laser in H(2)

A diode-pumped, far-off-resonance cw Raman laser in H(2) with rotational Stokes emission is reported for the first time to our knowledge. The Raman laser can produce single-wavelength emission at either 830 nm (rotational Stokes) or 1180 nm (vibrational Stokes) depending on the frequency tuning of the pump laser. The mirrors for the rotational cw Raman laser are easier to produce; the laser also exhibits a wider continuous tuning range and is less sensitive to thermal effects than the previously studied vibrational Raman laser [Opt. Lett. 26, 426 (2001) and references therein].     

Theoretical Study on Intra-Cavity Distributed-Bragg-Reflection Quasi-Phase-Matched Second-Harmonic Lasers

The characteristics of intra-cavity distributed Bragg reflector (DBR) quasi-phase-matched (IDQPM) second-harmonic-generation (SHG) lasers are theoretically studied. In the IDQPM-SHG laser, a QPM device and a DBR for feedback are separately fabricated on the same substrate with the QPM device placed between the DBR and a semiconductor laser. The threshold current of the IDQPM-SHG laser depends on the coupling efficiency between the laser diode and the QPM waveguide and the reflectivity of the DBR. The SH output of the IDQPM-SHG laser is strongly dependent on the generalized SHG conversion efficiency, x. This laser has the potential to attain an SH output over a 30-mW using a currently available 50-mW semiconductor laser for the fundamental light source, when highly efficient QPM device (x=2.2 W⁻¹) is used. Its tolerance for various deviations from the initial design and the problems to develop a commercially available IDQPM-SHG laser are also discussed.     

Dynamic simulation on the temperature field of laser micro-patterning metal in the liquid

To further understand the mechanism of laser electrochemical etching metal, it is necessary to describe the temperature field induced by laser heating metal at the liquid–solid interfaces. For solving the complex problem of laser heating a metal immersed in a liquid, the thermal phenomena adjacent to the metal–liquid interface, which was induced by means of irradiating a stainless steel sample immersed in a liquid with an 808 nm semiconductor laser beam, were numerically investigated. Based on a simplified method to solve the transient explosive boiling when a continuous wave (CW) laser heating a material in a liquid, a commercial finite element analysis (FEA) code (ABAQUS) was used to directly solve and model the transient temperature fields of laser micro-patterning metal in a liquid. As known from simulation results, the simulation of laser scanning indicates that it realizes the effect of pulse laser heating owing to laser moving and liquid cooling. Moreover, laser scanning achieves the pattern at a high resolution. At the same time, the experiment phenomena also proved that the simulation results were reasonable.     

Absolute frequency measurement of an acetylene-stabilized laser at 1542 nm

The absolute frequency of an acetylene-stabilized laser at 1542 nm is measured at its second harmonic (771 nm) by use of a femtosecond optical comb based on a mode-locked Ti:sapphire laser. Frequency stability and reproducibility of the acetylene-stabilized laser are evaluated by the femtosecond comb with a H maser as a frequency reference. The absolute frequency of a laser diode stabilized on the P(16) transition of 13C2H2 is determined to be 194 369 569 383.6(1.3) kHz. The acetylene-stabilized laser serves as an important optical frequency standard for telecommunication applications.     

Infrared and far-infrared laser emissions from a TE CO2 laser pumped NH3 gas

In this paper, infrared (IR) and far-infrared (FIR) laser emissions from a TE CO₂ laser pumped NH₃ gas are reported. 8 IR laser emissions near the wavelength of 12 μm were observed by using 4 different CO₂ laser lines for the pumping. 3 IR laser emissions in P-branch of vibrational-rotational band (ν₂ → G) oscillated simultaneously in two pumping cases, i.e. pumping with the R(30) or R(16) line of 9.4 μm band from the CO₂ laser. 26 FIR laser emissions (26.45 μm ～ 281.0 μm) were observed by using 12 different CO₂ laser lines, and the 10 FIR emissions of them may be new laser emissions as far as we know.     

Evaluation of probe lasers employed in optical diagnostics for phase transformation of thin films during excimer laser crystallization

The stability and reliability of probe laser is an important factor affecting the inspection of the phase transformation process of Si thin films during excimer laser crystallization using in-situ time-resolved optical measurements. The changes in 2D intensity profile, peak power density, and beam wander of the commonly used helium–neon (He–Ne) and diode laser are investigated experimentally. It is found that the peak power density of He–Ne laser is higher than that of diode laser, while the total power of He–Ne laser is lower than that of diode laser. Although the instability in the peak power density of He–Ne laser will increase with increasing the operation time, the beam stability of He–Ne laser is better than that of diode laser. For long-time operation (>24 h) of optical measurements, the diode laser is a good candidate of probe laser. Conversely, the diode laser is suitable for the short-time operation (<24 h) of optical measurements because the beam-wander is higher than that of He–Ne laser.     

Laser ablation of polymers using 395 nm and 790 nm femtosecond lasers

We compared a Ti:sapphire fs laser (790 nm) with a second harmonics (395 nm) fs laser, and then mixed them for ablating polyethylene (PE). Compared to the 790 nm fs laser, the 395 nm fs laser harmonics could etch PE faster. However, isolated carbon was formed on the ablated surface, in addition to C=O and C=C-H bonds. When we mixed a faint beam of the 395 nm fs laser harmonics with the 790 nm fs laser, the etching depth became even deeper. Moreover, the chemical composition of the ablated surface remained unchanged. At a total laser fluence of 80 mJ/cm², the most suitable laser fluences for the 395 nm fs laser harmonics and the 790 nm fs laser were found to be approximately 2 and 78 mJ/cm² respectively. PACS 81.65.Cf     

Amplified spontaneous emission and laser emission from a high optical-gain medium of dye-doped dendrimer

We measured the amplified spontaneous emission and laser emission from high-gain media of laser-dye encapsulated dendrimers. A highly branched poly(amidoamine) (PAMAM-OH) dendrimer formed a guest-host complex with a conventional laser-dye (DCM), resulting in a high optical-gain. Of particular note was the appearance of a laser threshold, above which a super-narrowed laser spectrum was observed, although laser feedback was caused without any mirror cavity devices. The optical feedback was attributed to spatial confinement of the light due to gain guiding under optical excitation. The laser spectrum clearly indicated a resonant laser-mode with a spectrum linewidth of less than 0.1 nm. This order of spectrum narrowing is comparable to that seen in the laser emission from ordinary laser devices.