As a potential high-energy laser, diode pumped metastable rare gas laser (DPRGL) and diode pumped alkali metal vapor laser (DPAL) have drawn attention worldwide. By using chemically inert metastable rare gas instead of aggressive alkali metal vapor as the active medium, DPRGL has overcome a major problem in DPAL that active medium would react with hydrocarbons and chamber windows. Though one of the advantages of DPRGL is that it can convert diode laser (pumping laser) into laser with high output power and excellent beam quality, efficient pumping of DPRGL requires high pumping intensity and narrow linewidth. By applying narrow bandwidth filters and external optical reflectors, the broad spectra linewidth of diode lasers can be narrowed. Development of volume Bragg grating provides a more convenient and efficient way of linewidth narrowing where VBG is applied as both optical filter and output coupler. In this program, an external cavity diode laser stack (ECDL) coupled with a reflective volume Bragg grating for metastable rare gas laser pumping is presented. The maximum output power of 130 W with 78 pm (FWHM) linewidth was realized. Higher output power can be achieved by vertically increasing the number of laser bars in the diode laser stack. A tuning range of 230 pm was measured which is sufficient for DPRGL pumping. The stability of output wavelength in long-time operation was evaluated by a 5-h test run where wavelength shifted ±7 nm. Total losses caused by passive optical elements, absorptions, etc. do not exceed 14 %. Performance of this narrow-linewidth diode laser stack as pumping source of DPRGL was furtherly examined by transverse pumping of metastable argon laser where the oscillation of metastable argon laser and absorption of pumping laser were detected by photon diode and spectrometer respectively
CW (continuous wave) coherent anti-Stoke Raman spectroscopy (CARS) of gases using a hollow-core photonic crystal fiber (HC-PCF) is demonstrated with gaseous CO2 as an example. Raman transition is pumped by a tunable dye laser. The seeding stokes light is provided by a home-made ECDL. Both lasers operate in single longitudinal mode. The output radiation from the fiber is filtered and coupled into an imaging spectrometer. Anti-stokes signal is observed with 10 mW pump power and 1 mW stokes power. The peak anti-Stokes signal of ν1 band of 6 atm CO2 is 59000 CCD counts with the exposure time set to 5 ms. The v1 and 2v2 components of the fermi dyad of CO2 is resolved.
Optically pumped rare gas lasers have the potential to provide high-power output with excellent beam quality. They lase at wavelengths that are readily transmitted by the atmosphere, and their optical characteristics are closely similar to those of diode-pumped alkali vapor lasers (DPAL’s). As compared to DPAL’s they present two significant advantages. The first is that they use entirely inert reagents that are gases at ambient temperature (Helium plus a heavier rare gas). As there is no chemistry involved, these devices can be operated in a completely closed-cycle mode. A second advantage is derived from the energy level structures of metastable rare gas atoms. While the sp transitions of alkali metals will provide just one laser wavelength for a given element, each rare gas atom can offer a range of different output wavelengths from the first manifold of sp transitions. The primary technical challenge for the optically pumped rare gas laser is the requirement to generate the lasing medium (metastable Rg(3P2) where Rg=Ne, Ar, Kr or Xe) at a density of approximately 1e13 cm-3 in the presence of helium at total pressures in the range 0.5 – 1.0 atm. For this application we have developed a quasi-CW discharge driven by a high repetition rate power supply. The frequency and time duration of the pulses are tailored to exploit the non-steady high field characteristics of the pulsed breakdown, while sustaining a temporally steady argon metastable concentration in the gain medium. This system has been scaled to a 912.3 nm output of ~4 W when pumped by a 20 W diode laser. Recently, the pulsed discharge system has been improved by increasing the upper limit for the voltage available from the power supply. The present system operates at voltages up to 2200 V with a nominal pulse duration of 50 ns and pulse repetition frequency of 100 kHz. This has been used to sustain a discharge in 1 atm of a He/Ar mixture that produces an Ar* metastable density of 2e13 cm-3, a path length of 3 cm and a total volume of 1.2 cm3 (0.64x0.64 cm2 cross section). A diode pumped Ar* laser that employed this discharge has been operated for extended periods with no sign of performance degradation. As expected for a system that uses only inert gases, there was no indication of window damage or chemical activity. Computational models indicate that scaling to the 100 kW level is feasible with a discharge volume of 10 cm3 and discharge power of 200 W.
Graphene has attracted much attention due to its unique optical properties as a new kind of plasmonic metamaterial in the terahertz regime. Here, we theoretically investigated a wavelength tunable plasmon induced transparency (PIT) device based on graphene metamaterials which is composed of periodically patterned graphene nanostructures. The interactions and coupling between plasmonic modes are investigated in detail by analyzing the field distributions and spectral responses. The coupled Lorentz oscillator models are used to explain the physical mechanism of the PIT. The finite-difference-time-domain (FDTD) method is used to investigate the tunable properties of the structure. It is shown that the coupling strength between the bright mode and dark mode is tuned by the coupling distance between the elements of the proposed structure. By varying the Fermi level of graphene, the PIT resonant frequency can be dynamically tuned. Furthermore, we demonstrate numerically that tunable slow light can be realized in our patterned graphene metamaterials.
Studies on the application of a parabolic reflector in spontaneous Raman scattering for low background Raman analysis of gas samples are reported. As an effective signal enhancing sample cell, photonic bandgap fiber (HC-PBF) or metallined capillary normally result in a strong continuous background in spectra caused by the strong Raman/fluorescence signal from the silica wall and the polymer protective film. In order to obtain enhanced signal with low background, a specially designed sample cell with double-pass and large collecting solid angle constructed by a parabolic reflector and a planar reflector was applied, of which the optical surfaces had been processed by diamond turning and coated by silver film and protective film of high-purity alumina. The influences of optical structure, polarization characteristic, collecting solid-angle and collecting efficiency of the sample cell on light propagation and signal enhancement were studied. A Raman spectrum of ambient air with signal to background ratio of 94 was acquired with an exposure time of 1 sec by an imaging spectrograph. Besides, the 3σ limits of detection (LOD) of 7 ppm for H2, 8 ppm for CO2 and 12 ppm for CO were also obtained. The sample cell mainly based on parabolic reflector will be helpful for compact and high-sensitive Raman system.
Diode laser pump source with narrow emitting spectrum for optically pumped metastable rare gas laser (OPRGL) of argon was achieved by employing a complex external cavity coupled with volume Bragg grating (VBG). A commercially available c-mount laser diode with rated power of 6 W was used and studied in both the free running mode and VBG external cavity. The maximum output power of 3.9 W with FWHM less than 25 pm and peak wavelength locked around 811.53 nm was obtained from the VBG external cavity laser diode. Precise control of VBG temperature enabled fine tuning of the emission wavelength over a range of 450 pm. Future researches on OPRGL of argon will benefit from it.
Spectroscopy applications of free-running laser diodes (LD) are greatly restricted as its broad band spectral emission.
And the power of a single blue-violet LD is around several hundred milliwatts by far, it is of great importance to obtain
stable and narrow line-width laser diodes with high efficiency. In this paper, a high efficiency external cavity diode laser
(ECDL) with high output power and narrow band emission at 405 nm is presented. The ECDL is based on a
commercially available LD with nominal output power of 110 mW at an injection current of 100 mA. The spectral width
of the free-running LD is about 1 nm (FWHM). A reflective holographic grating which is installed on a home-made
compact adjustable stage is utilized for optical feedback in Littrow configuration. In this configuration, narrow line-width
operation is realized and the effects of grating groove density as well as the groove direction related to the beam
polarization on the performances of the ECDL are experimentally investigated. In the case of grating with groove density
of 3600 g/mm, the threshold is reduced from 21 mA to 18.3 mA or 15.6 mA and the tuning range is 3.95 nm or 6.01 nm
respectively when the grating is orientated in TE or TM polarization. In addition, an output beam with a line-width of 30
pm and output power of 92.7 mW is achieved in TE polarization. With these narrow line-width and high efficiency, the
ECDL is capable to serve as a light source for spectroscopy application such as Raman scattering and laser induced
fluorescence.
Spontaneous Raman scattering is an effective technique in gas analysis, but the detection of minor constituents is difficult because of the low signal level and the usually existed background. Imaging spectrometer can provide highly spatial resolved spectra, so it should be much easier to pick up Raman signal of minor constituents from the Raman/fluorescence background of the sample cell and transporting optics compared with the widely used fiber-coupled spectrometers. For this reason, an imaging spectrometer was constructed from transmitting volume phase holographic grating, camera lenses and CCD detector. When it was used to analyze the gas sample in metal-lined capillary, which is a sample cell believed with great enhancement of Raman signal, the background was compressed obviously. When it was used to analyze the gas in a sample cell including a parabolic reflector, only weak background signal was observed, as the wide separation between the collecting zone (the focus point of the parabolic surface) and the wall of sample cell benefitted to the analysis by imaging spectrometer. By using the last sample cell, the signal from CO2 in ambient air was able to be found by an exposure time about 20 sec, and limits of detection for H2, CO2 and CO were estimated as 60 ppm, 100 ppm and 300 ppm respectively by the results of a longer exposure time. These results show that an imaging spectrometer paired with a well-arranged sample cell will lower the detecting limit effectively.
Laser produced plasma (LPP) light sources for extreme ultraviolet (EUV) lithography currently has been extensively studied. Most of the studies are based on CO2 laser induced plasma from mass limited tin targets. In this work, a droplet dispenser that produces uniform droplets size of about 150μm was established. A pulsed TEA-CO2 laser and a Nd: YAG laser irradiated the droplets producing plasma respectively to get EUV emission. An X-ray Spectrometer and EUV photodiodes were used to collect the spectra and EUV radiation. The different EUV spectral composition and angular distribution of EUV emission from plasmas induced by the CO2 and Nd: YAG laser were studied.
The single-filament schlieren method was based on the beam deflection in non-uniform medium. In this paper, a fourelement photodiode was used to acquire the deflection of the probing beam. The effects of electromagnetic interference (EMI) and the vibration of the blower on the output of the photodiode were investigated in detail and they have little impact on the measurements of the flowing characteristic after discharge. Then the perturbation in the discharge region was investigated. The heated gas in the discharge region can be easily detected and the gas velocity can be calculated by tracing the drift of the heated gas. This method also showed a high sensitivity and convenience to observe the acoustic waves originated from fast energy deposition. The results showed that the reflective acoustic wave existed for about 4 ms after discharge and it had a major effect on the non-uniformity of gas medium before the subsequent pulsed discharge.
Recent studies on signal enhancement of spontaneous Raman scattering for developing of sensitive Raman gas detectors
are reported. Raman scattering is a gas detecting method with high feasibility, but usually its signal is very low. To
improve the level and the quality of the Raman signal, the effects of pumping laser source, sample cell, and optical
arrangement are studied in detail. It is found that not only the wall of sample cell will give a wide Raman or fluorescence
background which will decrease the sensitivity, but the dichroic beam splitter will also contribute considerable
background if it is not aligned properly. The sample cell of hollow fiber is characterized by its high responsibility as well
as its high background and low signal contrast. When the hollow fiber is replaced by a free-space sample cell consisted
of metal-coated parabolic reflector, the wide background is largely suppressed. If there is no common optical elements
between the pumping and collecting optical systems, the wide background will be cut down obviously, which is proved
by the intracavity-enhanced Raman scattering in a He-Ne laser. These experimental results will be helpful for the
research and developing of highly sensitive Raman gas detectors.
An coherent terahertz (THz) source was reported by quasi-phase matched different frequency generation in a stacked GaAs wafers pumped by one CO2 laser with dual-wavelength output. The THz generation was increased with the increase of the number of GaAs wafers. The maximum single pulse energy of 12 nJ was generated at a frequency of 0.94 THz (319 μm) by using ten GaAs wafers, corresponding to a peak output power 200 mW
Experiments of pulse CO2 laser produced tin plasma had been carried out. Plasma parameters of electron temperature and density measurements both in axial and radial direction had been performed from a two-dimensional time and space resolved image spectra analysis. Debris speed of laser produced plasma in various buffer gas was quantitatively estimated by means of a fast gated intensified charge coupled device imaging system. The stopping power of the hydrogen buffer gas was assessed under ambient pressure ranging from 30 to 104 Pa.
Triethylamine (C6H15N) is widely used to stabilize the discharges of wavelength-nonselective transversely excited atmospheric (TEA) CO2 lasers. In this paper, the influences of triethylamine on the discharge stability and line characteristics of a high-energy wavelength-selective TEA CO2 laser are experimentally studied. The laser has 49 lasing lines ranging from 9.23 μm to 10.74 μm, and the energy of the strongest lines is above 50 J. However, the energy of the laser pulses is very unstable. The discharge stability is improved considerably by adding small amounts of triethylamine in the working gas mixture. In addition, lines in the 9 μm band are weakened greatly, and many of them even can no longer oscillate. In contrast, there are no distinct influences on the 10 μm lines. The results obtained in this study demonstrate the applied range of triethylamine in stabilizing the discharges of wavelength-selective CO2 lasers.
The evolution of shock wave generated by discharge in laser chamber is one of the key factors which affect laser beam
quality, discharge stability, and repetition rate of TEA gas laser. In this paper, Mach-Zehnder interferometer is applied to
observe both the longitudinal and transversal shock waves between electrodes as well as the acoustic waves originated
by preionization in the discharge pumping zone of TEA gas laser. By changing the discharge voltage, gas pressure and
gas composition concentration, the developing processes in different conditions are compared and analyzed. It is
observed that the shock waves originating from cathode is different from the anode's ones even in the symmetric
electrode construction. And the carbon dioxide concentration in helium-buffered working gas can affect the speed of the
wave obviously. However, the increasing trend of shock wave speed, when increasing discharge voltage or reducing
discharge gas pressure, is inconspicuous.
The efficiency of a simply designed optically pumped terahertz laser is studied experimentally. The terahertz laser cavity
only consists of a quartz glass tube, an antireflection-coated Ge window and a SiO2 window. The Ge crystal acts as the
high-reflectivity mirror of terahertz radiation and the input coupler of pump laser instead of complicated metal-mesh
mirrors. The Ge crystal is near 3 mm thick, whose exact thickness is designed according to etalon effects to maximize
terahertz reflectivity. NH3 gas is filled in the cavity as the active medium and pumped by a TEA CO2 laser. As high as
25.9 mJ terahertz radiation at the wavelength of 151.5 μm is extracted from 1.76 J pump energy. The corresponding
photon conversion efficiency of this terahertz laser reaches 41.5%. A 4.7-mm-thick GaAs crystal and a 6-mm-thick ZnSe
crystal are also chosen to be the input coupler. The experimental results show that the efficiency of the Ge window is
54% and 66% higher than that of the GaAs and ZnSe windows, respectively. The reason of the higher efficiency of the
Ge window is demonstrated experimentally and theoretically in this paper.
The recent results on pulsed CO2 laser, including the effects of preionization structure, electrode profile, and discharge
circuit are reported. Two sizes of discharge gap, 70 mm and 100 mm were selected. For the 70 mm gap, lateral
preionization was used; for the 100 mm gap, preionization beneath the meshed electrode, and mixed preionization
beneath and beside the electrodes were all tested. Strong lateral discharge was found in the case of near-Rogowski
profile or Stappaerts profile, and strong central discharge was found in the case of Ernst profile. When the near-
Rogowski profiled electrode was paired with an Ernst profiled electrode, better performance was obtained. Discharge
distribution corresponding to the spark pins under the mesh grid was found obviously, which means that the uniformity
of preionization should be further improved. Small signal gain coefficient was measured to evaluate the discharge. For
the experimental setup with 70 mm discharge gap and 4 liters discharge volume, the highest small signal gain coefficient
was 2.94% cm-1. When a laser cavity was installed, the highest pulsed energy obtained from this setup was 169 J. When
the discharge volume was enlarged to 16 liters, pulsed laser energy up to 580 J was obtained.
This paper presents some results on the air-breathing propulsion experiments with a parabolic light craft and a self-made
UV-preionized TEA CO2 laser with 115 J maximum pulse energy. In wire-guided vertical flight experiment, impulse-coupling
coefficient 390 N/MW is obtained at the pulse energy 60 J. The influence of the shape of the lightcraft and the
laser pulse profile on the coupling coefficient was discussed. The spectrum of the plasma was detected using
monochromator, and the time evolvement process of line spectrum of the plasma was analyzed.
The studies on large aperture volume discharge of TEA (transversely excited atmospheric) CO2 laser are reported. To
obtain stable and uniform large aperture volume discharge, the effects of electrode profiles, preionization methods and
parameters of discharge circuit have been studied experimentally. For a discharge construction with 70 mm gap and
pumped by a 3-stage Marx high voltage pulse generator circuit isolated by resistance with 0.5 μF capacitance each stage,
150 J pulse energy was obtained from a discharge volume about 4 l.
The studies on a 1 00-Joule-class UV-preionized TEA (transversely excited atmospheric) CO2 laser are reported. A maximum pulse energy 1 15 J was obtained by using an energy-optimized gas mixture of C02/N2/H2 =3/15/2 and a Marx pulse generator discharge circuit. The temporal waveform of the laser pulse with the optimized gas mixture consists of a sharp spike of 250 ns and a tail of 4 µs in full width at half maximum (FWHM). Simple air breathing laser propulsion was demonstrated, a high impulse coupling coefficient 390 N/MW was obtained at the pulse energy 60 J, which shows the attractive point ofthe UV-preionized TEA CO2 laser.
The effects of chopper on laser-texturing of rolls by CO2 laser are reported in the paper. The propagation of laser radiation through chopper can be described as four states: (1) chopping state; (2) transition state from chopping to transmitting; (3) transmitting state; and (4) transition state from transmitting to chopping. The distribution of laser energy flux on roll surface is derived from these descriptions and the assumption of Gauss beam, and the thermal physical process of laser-texturing is numerically simulated. It is true that the reflected laser radiation from state (1) and the direct laser radiation from state (3) can be superimposed to result in efficient texturing, but these effects are not so obvious as stated in published literatures. Shorter focal length will benefit in more efficient laser texturing because laser power density increase quickly with the decrease of focal length. In the case of short focal length, the superimposition effect is almost invisible, and the position of chopper because important, it should be near to the roll surface for a clear pattern of separated texturing points.
This paper studies the influence of the cooling gas on the thermal lens effect (TLE) theoretically. It provides a method to calculate the influence of the cooling gas on the temperature of the focusing lens and presents a parameter called relax time of the temperature change (RTTC) of the focusing lens to characterize the temperature change. It studies the change of the laser power density on the surface of workpiece with the time or the cooling gas rate. Cooling gas can reduce the temperature of the lens and make it stable more rapidly. Increasing the flow rate of cooling gas can reduce the TLE greatly, however, it has a limitation. Beyond this limitation, increasing the rate leads to less reducing of the TLE and the RTTC. Double sides cooling is much more efficient than the single side one. Besides, increase of the diameter of the lens can also reduce the RTTC. It also shows that TLE is caused mainly by the change of refractive index of the lens with temperature.
The characteristics and mechanism of superpulse discharge for high power FAF CW CO2 lasers are based on the rate equation theory. The mathematical modeling analysis of superpulse discharge especially considering the influence of turbulence and convection is simulated on the computer. Detailed mathematical modeling curves which demonstrate how superpulse operation is employed around population inversion, gain and injection power density are obtained. The results are relevant for superpulse CO2 lasers.
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