The pulsed optical output of a 940-nm LD can be amplified using a current driven semiconductor optical amplifier (SOA). The pulsed light from the 940-nm LD has a pulse width of 50 ps and a repetition rate of 50 MHz, and incident on the 500-mA current driven SOA. A CW light from a 905-nm LD incident on the SOA in colinear with the pulsed light. Due to two effects of current driven amplification and optically pumped amplification, the peak power of the pulsed light was increased up to 2W and the pulse contrast was improved by a factor of 20.
We have proposed a MOPA configuration with a stepwise optical frequency pulse train (SOFPT) seed in which the SBS threshold does not decrease even if the duty-cycle of the pulse train is increased up to ~100%. In this paper we present the SBS thresholds of a passive fiber in range of 45ns-500ns pulse duration under the low-duty-cycle condition. Based on the experimental results and theoretical consideration, we estimated the achievable average power of > 8 kW in a MOPA configuration with the SOFPT, in which each pulse has a sub-GHz linewidth.
We report on tunable pulse width and high peak power pulse generation from a nonlinearly compressed monolithic fiber MOPA system. The master seed source employs a Mach-Zehnder intensity modulator (MZIM). This seed source has operational flexibility with respect to pulse width, 90 ps to 2 ns and repetition rate, 200 kHz to 2 MHz. The seed pulses are amplified by a monolithic three-stage amplifier system based on polarization maintain Yb-doped fibers. The maximum output power was 32 W at the shortest pulse condition, the pulse width of 90 ps and the repetition rate of 750 kHz. A spectral width after amplification was broadened to 0.73 nm at RMS width. Both of ASE and SRS are not observed in the spectrum. After amplification, we also demonstrated pulse compression with a small piece of chirped volume Bragg-grating (CVBG) which has the dispersion rate of 81 ps/nm. As a result of pulse compression, the shortest pulse width was reduced from 90 ps to 3.5 ps, which brought an increase of the peak power up to 3.2 MW. The compressed pulses are clean with little structure in their wings. We can expand the operation range of the monolithic fiber MOPA system in pulse width, 3.5 ps to 2 ns.
We demonstrate a single mode fiber based master oscillator power amplifier (MOPA) with a single polarization and a fully monolithic design. We have built a passive mode-locked polarization maintaining Yb doped fiber as the master oscillator contains a semiconductor saturable absorber mirror and a chirped fiber Bragg grating for the dispersion management. The net intracavity dispersion was managed to be slightly anomalous. The oscillator generates the 150 fs (sech2) pulses at the center wavelength of 1065 nm, and the repetition rate of 42 MHz. The oscillator output was amplified to 1.4 W from 80 mW in the single stage fiber amplifier which results in pulse shape distortion. The pulse shaping with a band pass filter and a compressor was applied to the amplified pulses. The shaping pulses have the pulse width of 90 fs and the pulse energy of 16 nJ.
Chirally-Coupled-Core (3C) fiber structure can preserve a single mode quality and even a linear polarization for a large core size. A principal advantage of fiber laser is its compatibility with monolithic integration and robust system. But so far, devices such as a combiner using the 3C fibers have not been reported. Here we report the first demonstration of such monolithic amplifier structure which contains an active fiber and a combiner based on 3C fibers. A single-stage amplifier is seeded by an EO Q-switched micro-laser and pumped by two high power fiber pigtailed 976-nm laser diodes via an in-house fabricated (2 + 1) × 1 pump signal combiner. The active fiber is based on a 3-m-long, 3C Yb-doped fiber (33 μm/250 μm core/cladding diameter with 0.06/0.46 NA). The amplifier demonstrates scaling up to 30W average power and 150 kW peak power in 0.3mJ, 2ns pulses. The beam profiles and beam qualities were characterized as its output power was varied up to 30W. The beam profile was maintained at a high beam quality of around M2=1.2. The spectral properties of the 3C fiber were also characterized as its output peak power was varied.
We investigate a technology to create a high temperature heat source on the tip surface of the glass fiber proposed for medical surgery applications. Using 4 to 6 W power level semiconductor lasers at a wavelength of 980 nm, a laser coupled fiber tip was preprocessed to contain a certain amount of titanium oxide powder with a depth of 100 μm from the tip surface so that the irradiated low laser energy could be perfectly absorbed to be transferred to thermal energy. Thus, the laser treatment can be performed without suffering from any optical characteristic of the material. A semiconductor laser was operated quasi-continuous wave mode pulse time duration of 180 ms and >95% of the laser energy was converted to thermal energy in the fiber tip. Based on two-color thermometry, by using a gated optical multichannel analyzer with a 0.25 m spectrometer in visible wavelength region, the temperature of the fiber tip was analyzed. The temperature of the heat source was measured to be in excess 3100 K.
In most of medical and dental laser treatments, high power pulsed laser have been used as desirable light sources employing with an optical fiber delivery system. The treatment process involves high temperature thermal effect associated with direct laser absorption of the materials such as hard and soft tissues, tooth, bones and so on. Such treatments sometimes face technical difficulties suffering from their optical absorption properties. We investigate a new technology to create high temperature heat source on the tip surface of the glass fiber proposed for the medical surgery applications. Using a low power level (4~6W) semiconductor laser at a wavelength of 980nm, a laser coupled fiber tip was pre-processed to contain certain amount of TiO2 powder with a depth of 400μm from the tip surface so that the irradiated low laser energy could be perfectly absorbed to be transferred to thermal energy. Thus the laser treatment can be performed without suffering from any optical characteristic of the material. Semiconductor laser was operated quasi-CW mode pulse time duration of 180ms and more than 95% of the laser energy was converted to thermal energy in the fiber tip. by Based on twocolor thermometry by using a gated optical multichannel analyzer with 0.25m spectrometer in visible wavelength region, the temperature of the fiber tip was analyzed. The temperature of the heat source was measured to be approximately 3000K. Demonstration of laser processing employing this system was successfully carried out drilling through holes in ceramic materials simulating bone surgery.
A drill for concrete and rocks is being developed using a laser-based hybrid technique. The design locates the outlet hole
of laser beam and drill blades on a common rotational axis. The laser beam weakens the concrete, and the blade breaks
the weakened layer. The target performance is a drilling speed of 30 mm/min at a sound level less than 70 dB using 1-2
kW laser power to produce a φ20 mm × 300 mm hole.
A laser-based hybrid technique is shown to be effective for cutting concrete and drilling rock. Experimental conditions
used included a laser power of 1-10 kW, 10 mm laser beam diameter and 1.25-50 mm/s scanning speed. The results of
surface scanning tests and 100 mm deep cut tests indicated that the shortest operation time was about 3.5 hr/m2 for plain
concrete and 4.5 hr/m2 for heavy concrete. The specific energy for the rocks (the amount of energy required to remove a
unit volume of rock), which were granite and sandstone, was as same as that for heavy concrete. Thermal decomposition
was observed for limestone. The specific energy of limestone was more than 1.5 times higher than that for granite and
sandstone.
Nozzle banks for an ejector-chemical oxygen-iodine laser consisting of two-dimensional slit nozzles with a trip-jet mixing system were tested in the cold- and hot-flow operation regimes. Horizontal Pitot scan experiments demonstrated that the mixing ability of the trips is excellent. The Mach number of the mixed flow was approximately 3. The gain measurements were conducted, and the results of measurements revealed that the maximum gain was around 0.6%/cm. The gain cut-off length was more than 200mm. The lasing experiments were also conducted, and the power was about 3kW at the chemical efficiency of about 20%.
A high peak-power, high repetition rate Master Oscillator Power Amplifier (MOPA) system incorporating an
Yb-doped fiber amplifier and its second harmonic generation were investigated in detail. The oscillator is actively
Q-switch microchip laser at repetition rate of 50 kHz with a pulse width of 2.8ns. The amplifier employing Yb-doped
polarization maintaining fiber, having a large mode area was exited by a laser diode with an optical power of 17W. As
results, the amplified average output power of 10W and the optical conversion efficiency of 59% were achieved. In
this MOPA system, second harmonic generation (SHG) experiments were performed using KTP and LBO crystals. The
conversion efficiency of 21% and 40%, the maximum SHG power of 0.92 W and 3.3W were obtained for KTP crystal and LBO crystal respectively.
Cutting performance of a laser based hybrid technique on concrete was measured under different conditions, which are
laser power (1-20kW), scan speed (1-100mm/s), laser beam diameter (5-10mm), and O2 gas flow rate (0-50L/min).
KEYWORDS: Methane, Chemical oxygen iodine lasers, Fiber lasers, Energy efficiency, Gas lasers, YAG lasers, Laser energy, Optical fibers, High power lasers, Molecules
Methane hydrate is a energy resource distributed even in the resource-less countries such as Japan and India.
The energy is necessary to extract the methane from the methane hydrate and COIL is the unique solution for the extraction tool, because fuels of COIL can be produced from the natural energy without the fossil energy resources. The experimental results of laser light-methane hydrate interaction, multi kW laser light transmission through 1 km optical fiber, and the estimation of the extracted methane by radiation of the multi hundreds kW COIL output on the methane hydrate layer, will be reported.
We have demonstrated the measurements of attenuation constant of a multi-mode fiber (300μm core diameter and 1km
length) at 1070nm. The observed attenuation constant was below 0.7dB/km. The laser power of 5kW was coupled into
the 1km fiber at 1070nm. The overall transmittance was 85 %. We observed the first Raman stokes in the transmitted
laser spectrum.
We demonstrated concrete cutting with a 4kW fiber laser at 1070nm. The demonstrated slab thickness was 100mm.
This technique can be extended to thick concrete slabs more than 1m without laser power increasing.
A new nozzle bank for an ejector-chemical oxygen-iodine laser consisting of two-dimensional slit nozzles with a trip-jet mixing system was designed, fabricated, and tested in the cold flow operation regime. A computational fluid dynamics (CFD) is used to optimize the trip system design. Horizontal Pitot scan experiments demonstrated that the mixing ability of the trips is excellent. The Mach number of the mixed flow was approximately 3. The average Pitot pressure at the laser cavity was more than 100 Torr. Backpressure tests were conducted, and the results of these tests revealed that the static pressure in the cavity remained constant at approximately 10 Torr until the supersonic diffuser breaks back and the cavity unstarts at a backpressure of approximately 80 Torr.
The transmission of chemical oxygen-iodine laser (COIL) emission through several kinds of multimode quartz optical fiber is measured in order to explore the applicability of COILs for the extraction of natural resources. The minimum transmission loss through these fibers is 0.64 dB/km, and high-power transmission for a distance of kilometers is shown to be feasible. Laser emission at an average input power of 1 kW is successfully transmitted through a multimode optical fiber for a distance of 1 km with an efficiency of 80%.
Chemical oxygen-iodine laser (COIL) has a great potential for applications such as decommissioning and dismantlement (D&D) of nuclear reactor, rock destruction and removal and extraction of a natural resource (Methane hydrate) because of the unique characteristics such as power scalability, high optical beam quality and optical fiber beam. Five-kilowatt Chemical oxygen-iodine laser (COIL) test facility has been developed. The chemical efficiency of 27% has been demonstrated with a moderate beam quality for optical fiber coupling. Our research program contains conventional/ejector-COIL scheme, Jet-SOG/Mist-SOG optimization, fiber delivery and long-term operation.
This report describes the experimental and theoretical analysis of a cross-flow jet-type singlet oxygen generator (cross-flow J-SOG) in order to identify the optimal conditions needed to satisfy the ejector-COIL requirement. The optimal conditions had been analyzed under various generator geometries (reaction region length, jet diameter, and chlorine inlet height), gas/BHP flow rates, and gas pressures. The performance was achieved Cl2 utilization of 90% and O2(1Δ) yield of 70% at the plenum pressure of 20 Torr.
A mist singlet oxygen generator is possible to improve the HO2 utilization at the one pass reaction between basic hydrogen peroxide (BHP) and chlorine. In this investigation, BHP was atomized to small droplets by the gas flow. Chlorine, which is required for stoichiometric reaction with HO2 in the BHP, was used for atomization of BHP in order to reduce the buffer flow rate for atomization. We obtained the results that the conversion efficiency from chlorine to singlet oxygen (Ux Y) was 9.7% with purely chlorine atomization and 16% with x0.93 dilution ratio of nitrogen buffer at 18.7 mmol/s input chlorine flow and 5.8 ml/s BHP flow rate in a free space reaction chamber.
KEYWORDS: Methane, Chemical oxygen iodine lasers, Fiber lasers, Wind energy, Energy efficiency, High power lasers, YAG lasers, Optical fibers, Physics, Gas lasers
Possibilities to extract methane from methane hydrate are discussed. COIL is a unique solution from technological and economical stand points and systems for excavation are proposed
Chemical oxygen-iodine laser (COIL) has a great potential for applications such as decommissioning and dismantlement (D&D) of nuclear reactor, rock destruction and removal and extraction of a natural resource (Methane hydrate) because of the unique characteristics such as power scalability, high optical beam quality and optical fiber beam. Five-kilowatt Chemical oxygen-iodine laser (COIL) test facility has been developed. The chemical efficiency of 27% has been demonstrated with a moderate beam quality for optical fiber coupling. Our research program contains conventional/ejector-COIL scheme, Jet-SOG/Mist-SOG optimization, fiber delivery and long-term operation.
Possibilities to extract methane from methane hydrate are discussed. COIL is a unique solution from technological and economical stand points. COIL system suitable for excavation are proposed.
A cross flow jet SOG has been developed in Miki Pulley Co. Ltd. to supply O2(1D) for different types of COILs. Performance testing of the SOG has been conducted through a wide range of gas pressures (5~40 Torr), specific surface areas (4~7 cm-1), gas velocities (5~30 m/s), and gas temperatures to characterize and optimize the device. The inflow and out flow of the reactants and products, including O2(1D), Cl2, H2O were measured using optical and conventional techniques. The gas temperatures in the measurement duct were estimated from stagnation pressures, mass flow rates, and critical cross section at the gas chocking point in order to determine the partial pressures of the gas products at the measurement point. Calibration method of the O2(1D) measurement suggested by Zagidullin is basically employed with a slight correction of upper limit definition of O2(1D) yield associated with the pooling loss, which remains even at the minimum P t condition of our device. Assuming that the gas temperature after passing through the jets is equilibrium with that of the BHP jets (-18 degree Celsius) in our calibration condition, the upper limit yield can be derived from the increase in the gas temperature. The estimated value of the yield limit was 94 %. A wide range of output values (40-95 % of Cl2 utilization, 50-90 % of O2(1D) yield) was obtained and analyzed to characterize the device. As a result of optimization, a 27 % of chemical efficiency was obtained when Cl2 utilization was 95 %, O2(1D) was 90 %, O2 partial pressure was 6.7 Torr, and N2 dilution ratio was 2. Discussion on the validity of the gas temperature estimation method is provided by comparing the results to the heat release based on the pooling model.
Analysis of heat release into operative gas of Chemical Oxygen Iodine Laser (COIL) is discussed. Pooling reaction of oxygen molecules in the excited state, the iodine dissociation process and the interaction of them with water vapor release energy of in the excited state oxygen molecules as heat energy. As results of heat release in the plenum, a rise of the total pressure as a rise of the total temperature is observed, and in the supersonic region a rise of static pressure and a decrease of total pressure as a rise of total temperature are observed. By following our analysis technique regarding pressure data of three different nozzles, the evaluations such as energy loss in a duct from a Singlet delta Oxygen
Generator (SOG) and the number of dissipated oxygen molecules for the iodine dissociation can be estimated.
KEYWORDS: Diffusers, Throat, Iodine, Chemical oxygen iodine lasers, Laser resonators, Chemical analysis, Chemical reactions, Molecules, Laser development, Chemical lasers
Optimization of iodine injection scheme was conducted in Miki Pulley Co., Ltd. using CFD approach. Variation of the mixing speed as a function of an I2 jet penetration depth, nozzle expansion ratio, and I2 injection point are analyzed. It was found that the inherent geometry of our nozzle and I2 injector affects the I2 mixing process. Influence of a backpressure to the cavity flow condition was also investigated in order to estimate the pressure recovery capability of our diffuser. The normal shock based diffuser efficiency for Mach=2.7 flow was 54.5%.
Miki Pulley has pursued the development of a prototype COIL module for field and industrial applications since its transfer from Tokai University in 2000. The test module has already been constructed in our laboratory. The current status of the development is presented. The achieved chemical efficiency was 17.5% at the chlorine flow rate of 13.2mol/min. The corresponding laser power was 3.5kW.
Laser medium parameters of multi-kW grid nozzle supersonic Chemical Oxygen Iodine Laser (COIL) were experimentally studied. Small-signal gain (SSG) diagnostics was done by a narrow line width tunable laser by scanning 1 GHz range around (2P1/2 ) - (2P3/2) spin-orbit transition line of atomic iodine. SSG was investigated as a function of Mach number and gas flow rates. Modeling of gain for different flow conditions was done as well. Multi-kW COIL device was recently developed in Miki Pulley Co., Ltd. (Japan) and has 37.5 cm length of active medium.
A new strategy for pulse oscillation of chemical oxygenÑiodine laser based on a combination of a porous pipe SOG with an instantaneous atomic iodine generation, has been developed to seek the potential of COIL as an amplifier of the nuclear fusion driver. This new scheme allows one to produce a large aperture high pressure laser medium, which is favorable to the laser amplifier, while maintaining a minimum degradation of stored energy by water vapor. The experimental apparatus consists of the porous pipe SOG, an iodine donor (CH3I) injector, a flash lamp for the iodine dissociation, and an optical resonator. Operational characteristics of the apparatus including dependence of output energy on an iodine concentration was studied. As the result, the maximum output energy of 800mJ was obtained. It was also found that the CH3I was dissociated through unidentified chemical reaction associated with the O2(1Æ).
In high power applications of multimode optical fibers such as high power beam delivery and optical phase conjugation, the estimation of critical power of stimulated Brillouin scattering is important. Nevertheless, the estimations have taken no account of mode dispersion effect to date. In this paper we show that the critical power in a multimode fiber depends on the numerical aperture (NA) of fiber, i.e. mode dispersion, and fmd out the analytical solution taking account ofthe effect. The critical power increases as the fiber NA increases.
A study of chemical oxygen-iodine laser (COIL) for the use of decommissioning and dismantlement of nuclear facilities is conducted. A scaled-down model was developed as a prototype. Laser duct and optical cavity were designed so that it can be operated in both supersonic mode and high-pressure subsonic mode for the comparative study. A 1.34kW output with chemical efficiency of 24.6% was obtained in the supersonic mode. In the high-pressure subsonic mode, output power was 1 .12kW with chemical efficiency of 20.6%. A subsonic operation at 12Torr was demonstrated for the first time. A preliminary experiment of thick steel cutting was demonstrated by the developed system. The obtained data was in good agreement with published data.
High-pressure subsonic mode operation of chemical oxygen- iodine laser (COIL) is studied. In this mode, the singlet oxygen generated by the liquid-jet singlet oxygen generator (SOG) is directly utilized in the optical cavity without supersonic expansion. Drastic reduction of the required vacuum pump capacity, and iodine consumption was obtained. We have demonstrated a 25.0 percent of chemical efficiency with a small-scale device. The scale-up version of the COIL is developed and initial tests are conducted. The device is so designed that it will operate for 2 hours at 1kW laser output. Due to the inadequate heat exchanger of basic hydrogen peroxide (BHP), performance of the system was not yet satisfactory. However, a 30-minute continuous operation o the counter-flow type jet SOG with recirculation of BHP was demonstrated for the first time.
A ring type self-pumped phase-conjugate mirror with rhodium- doped barium titanate is used to correct aberrations of an LD- pumped sigzag slab Nd:YAG oscillator-amplifier system. A diffraction limited output of 360 mJ is achieved at the repetition rate of 100 Hz.
Conceptual designs of a chemical oxygen-iodine laser (COIL) facility for decommissioning and dismantlement (DD) of nuclear facility is proposed. The requisite output power and beam quality was determined base don our preliminary experiments of nonmetal material processing. Assuming the laser power of 30kW, it is derived that the beam quality of M2 equals 36 required to cut a biological shield wall of a nuclear power plant at a cutting speed of 10mm/min. Then the requisite specification of an optical fiber to deliver the laser is calculated. It turned to be quite extreme, core diameter of 1.7mm and NA equals 0.018. The mass flow and heat balance of proposed facility is calculated based on our recent COIL studies. With the high-pressure subsonic mode, the vacuum pump size is minimized compared to the supersonic operation. Finally, the size of the facility is estimated assuming tow-hour continuous operation. It is revealed that such a system can be packed in five railway containers.
Laser diode pumped zigzag slab Nd:YAG MOPA (Master Oscillator Power Amplifier) system featuring high pulse energy and high average power has been developed for the pumping of ultra- short pulse laser system. The MOPA system consists of an oscillator, a preamplifier, two postamplifiers and image relay telescopes. The post amplifiers has an angle multiplexed ring type double-pass configuration. A pulse energy of 1.26 J and an average power of 251 W are obtained at the repetition rate of 200 Hz. The frequency doubled power using a LBO (LiB3O5) crystal is 105 W at the repetition rate of 170 Hz. The intensity profiles of the fundamental and the second harmonic are near top hat and is suitable for the pumping. In a preliminary experiment, effectiveness of the MOPA as a pump source of all solid state chirped pulse amplifier (CPA) system is demonstrated.
KEYWORDS: Wavefronts, Control systems, Wavefront sensors, Deformable mirrors, Mirrors, Pulsed laser operation, Wavefront distortions, Signal processing, Electrodes, Digital signal processing
We developed a technique to compensate the wavefront distortion of ultra-short pulse Chirped Pulse Amplification (CPA) laser by applying a closed loop wavefront control system consisting of a Shack-Hartmann type wavefront sensor and Bimorph type deformable mirror. As a result of the experiments both Root Mean Square (RMS) and Peak to Valley (P-V) value of the wavefront distortion were reduced to less than 1/5 of the original value, and it was shown that the brightness of laser beam was improved by about five times.
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