Laser oscillation

Lick Observatory. The success of the LLNL/AVLIS demonstration led to the deployment of a pulsed dye laser / AO system on the Lick Observatory 3-m telescope (Friedman et al., 1995). LGS system (Fig. 14). The dye cells are pumped by 4 70 W, frequency-doubled, flashlamp-pumped, solid-state Nd YAG lasers. Each laser dissipates 8 kW, which is removed by watercooling. The YAG lasers, oscillator, dye pumps and control system are located in a room in the Observatory basement to isolate heat production and vibrations from the telescope. A grazing incidence dye master oscillator (DMO) provides a single frequency 589.2 nm pulse, 100-150 ns in length at an 11 kHz repetition rate. The pulse width is a compromise between the requirements for Na excitation and the need for efficient conversion in the dye, for which shorter pulses are optimum. The laser utilizes a custom designed laser dye, R-2 perchlorate, that lasts for 1-2 years of use before replacement is required. 

Figure 2. Mixing of a Laser oscillator with a spread spectrum Optical source...

The fluorescence and laser properties of symmetrically and unsymmetrically substituted 2,5-diaryl-l,3,4-oxadiazoles were experimentally studied. It has been found that symmetrically substituted molecules (e.g., 2,5-di(2-naphthyl)-l, 3,4-oxadiazole) give laser oscillation at room temperatures, while unsymmetrical 2-(2-naphthyl)-5-phenyl-l,3,4-oxadiazole does not give laser action under any conditions, even at low temperatures <2000SAA2157>. 

The representative example of this type of laser is the CO2 laser. Laser oscillation is achieved on many rotational lines within two vibrational transitions of the molecule. Without any line selection, the system oscillates at only around 10.6 /u.m. This transition is found to give a continuous wave (cw) output power of several kilowatts with an efficiency of around 30 %, which is quite exceptional for a gas laser. 

Tunable coherent light sources can be realized in several ways. One possibility is to make use of lasers that offer a large spectral gain profile. In this case, wavelength-selecting elements inside the laser resonator restrict the laser oscillation to a narrow spectral interval and the laser wavelength may be continuously tuned across the gain profile. Examples of this type of tunable laser are the dye lasers were treated in the previous section. 

Yokoyama, H. and Ujihara, K. (eds), 1995, Spontaneous emission and laser oscillation in microcavities, CRC Press, New York. 

Section IV explains a new approach to high resolution spectroscopy based on various kinds of saturation effects. Some of the experiments are performed inside the laser resonator, which implies the presence of coupling phenomena between the absorbing molecules under investigation and the laser oscillation itself. These feedback effects can be used for high-precision frequency stabilization and to measure frequency shifts and line profiles with an accuracy never... 

The wavelength of a laser line, however, is determined by two factors the fluorescence profile of the corresponding transition in the laser medium and the eigenfrequencies of the laser resonator modes. At normal multimode operation of a laser, where many axial and transverse modes participate in laser oscillation, these eigenfrequencies cover the whole spontaneous line profile nearly uniformly. 

The last method has been pushed to an impressive sensitivity by putting the probe inside the cavity of a cw dye laser oscillating on several modes close above threshold. The sensitivity of such a broad-band dye laser to selective intracavity absorption on a single mode is proportional to the number of oscillating modes due to... 

In the case of a common lower level, the second absorption transition would show this narrowing effect when probed with a tunable monochromatic laser line. This example can be realized if atoms or molecules in a magnetic field are pumped by a laser, oscillating simultaneously on two cavity modes 324). if the Zeeman splitting of the probe equals the mode spacing of the laser, both transitions are pumped simultaneously and each laser mode selectively eats... 

Let us consider a laser oscillating at a single frequency (single-mode operation) and gas molecules inside the laser resonator which have absorption transitions at this frequency. Some of the molecules will be pumped by the laser-light into an excited state. If the relaxation processes (spontaneous emission and collisional relaxation) are slower than the excitation rate, the ground state will be partly depleted and the absorption therefore decreases with increasing laser intensity. 

The idea of using the same medium as absorber and active material has been proposed and realized by several authors 340-343) Leg and Skolnick 40) used a neon gas discharge at low current and low pressure as saturable absorber inside the cavity of a He-Ne laser oscillating at X = 6328 A. The Lamb-dip halfwidth obtained was 30 Mc/sec compared to 1500 Mc/sec for the doppler line. The disadvantage of this arrangement is that the frequency of the neon transitions depends upon pressure and current 341) in the absorption cell, and this limits the stability and reproducibility of the Lamb dip center frequency. 

Similar results were obtained by De Shazer using a different detection technique, where laser oscillations in the sample were forced to develop from the narrow-band radiation, injected from a second small aperture laser into the sample laser cavity. The interionic transfer allowed the feeding of this narrow-band radiation by ions having frequencies outside this interval. The effeciency of energy extraction within the narrow bandwidth and the degree of depolarization of the laser oscillations parametrize the cross relaxation effects. 

A chemically pumped CO2 laser oscillating at 10 p was reported by GrossIn this system vibrationally excited COj molecules are produced by inelastic collisions with vibrationally excited DF which was formed by ultraviolet photolysis of a F2O-D2 mixture with a Xe flashlamp, producing free fluorine atoms which could react with Dj... 

Major, A., Cisek, R., and Barzda, V. 2006. Femtosecond Yb KGd(W04)(2) laser oscillator pumped by a high power fiber-coupled diode laser module. Opt. Exp. 14 12163-68. 

Initial laser pulse generation is achieved with the use of a twin-tube excimer laser in which one channel is a XeCl laser oscillator dehvering 15-ns, 308-nm, 80-mJ pulses for the driving of two dye lasers needed for difference frequency generation. The second channel is used for amphfication of subpicosecond 308-nm pulses that become pump pulses. 

Laser oscillation will start when V (t) has increased to such a value that the overall amplification in one round trip of a photon through the laser resonator is greater than unity, resulting in the oscillation condition... 

The necessary pump powers can be achieved either by other lasers (e.g. nitrogen lasers, solid-state lasers or even focussed He-Ne- or Ar+-gas lasers) or by flash-lamps. The simplest practical arrangement is a square spectrophotometer cell, polished on all sides, containing the dye solution which is pumped by a nitrogen laser whose beam is focussed into a line parallel to and directly behind one of the cell windows. Then the Fresnel reflection from the two adjacent windows gives enough feedback in most cases, so that no additional resonator mirrors are needed and the dye laser oscillation starts. 

Contents Population Inversion and Molecular Amplification. Energy-partitioning in Elementary Chemical Reactions Vibrational Relaxation. Requirements for Laser Oscillation. Design Parameters of Pulsed Chemical Lasers. Specific Chemical Laser Systems. Future Chemical Lasers. Present Perspectives of High-Power Chemical Lasers. Kinetic Information through Chemical Laser Studies. 

Indeed, the laser oscillation itself frequently is a transient emission that falls into the broad definition of fluorescent lifetimes. In particular, spiking phenomena (5) and Q Switching (4) clearly bear a close relationship to relaxation times and processes. One could say that laser phenomena are associated with the study of fluorescent decays of inverted populations. 

Johnson et al. (55) have reported a phonon-assisted energy exchange from trivalent erbium to trivalent thulium or to trivalent holmium. In this case, these authors were able to rule out resonance exchange completely Of some importance is that these systems are useful for laser oscillators, and the energy exchange results in a substantial decrease in threshold. 

A highly unusual technique for the measurement of mean life was introduced by Yariv and co-workers (74). They took advantage of the intimate connection between lifetime and laser-oscillation threshold. This interesting method warrants some additional discussion. 

Laser oscillation will start if N can be increased to a certain value Nc> called the critical inversion. Nc depends upon the spontaneous emission coefficient and the losses in the laser cavity. 

Huffman (87) studied the transient emissions from terbium in a vinylic resin matrix. His compound was Tb tris-[4,4,4-trifluoro-l-(2-thienyl)-1,3-butaneodione] in polymethylmethacrylate. This may be conveniently abbreviated as TbTTA in PMMA. The compound EuTTA in PMM A had previously been reported by Wolff and Pressley (99) to give laser oscillation. Working with small fibers at 77°K, Huffman found distortions from the normal fluorescent decay curves when the optical pumping was large. He interprets this as evidence for stimulated emission. A comparison of these distorted decays with EuTTA in PMMA indicated a similar behavior, thus tending to substantiate his hypothesis. 

Figure 31(a) shows these data and 31(6) the thresholds for pulsed-laser oscillation. Nassau concludes that the rise in oscillation threshold above 5 per cent is at least in part ascribable to the fall in lifetime. This seems very reasonable, since the decrease in lifetime probably reflects the quantum efficiency. 

Since the chromium-neodymium-exchange time is much longer than the neodymium-decay time, the threshold for pulsed-laser oscillation will not be significantly improved. However, for continuous operation the threshold should drop. Such was found to be the case. 

It is of historical interest to note that, following Snitzer s work, Maurer (126) showed that laser oscillation was possible at 0.9180 /x in a silicate glass. 

Pearson and colleagues (135) studied laser oscillation in glasses from the system calcium oxide, lithium oxide, boron oxide, doped with neodymium oxide. These glasses have been called the Calibo glasses. 

Melamed et al. (136) describe laser oscillation in uranyl-sensitized neodymium-doped barium crown glass. They believe that the energy migrates from the uranyl to the neodymium by an electric dipole-dipole process. The results of their measurements on the uranyl lifetime, uranyl-neodymium-transfer time, and the quantum efficiency are summarized in Table VII. 

Lempicki and Samelson (160) in their first paper reporting laser oscillation in europium made use of europium benzoylacetonate (EuB3) dissolved in a mixture of ethyl and methyl alcohol. The ratio was 3 to 1 and the concentration of EuB3 was 5.2 x 1018 molecules/cm3. These solutions can be frozen into a glass uniformly at temperatures between —120° and — 170°C. In this range they report a fluorescent lifetime of about 5 x 10-4 sec. Additional information on laser action, fluorescent lifetimes, and spectral studies in chelates may be found in Ref. (138). 

Johnson et al. (55) observed energy transfer from erbium to thulium and from erbium to holmium ions in crystals. They were able to obtain substantial decreases in laser thresholds because of this energy migration. The fluorescent lifetime of the 3//4 state of thulium in CaMo04 containing 0.75 atomic per cent erbium and 0.5 atomic per cent thulium as inferred from the time delay before the onset of laser oscillation is 900 /xsec at both IT and 20°K. 

Experiments were performed using a titanium sapphire laser oscillator capable of producing pulses with bandwidths up to 80 nm FWHM. The output of the oscillator was evaluated to make sure there were no changes in the spectrum across the beam and was compressed with a double prism pair arrangement. The pulse shaper uses prisms as the dispersive elements, two cylindrical concave mirrors, and a spatial light modulator (CRI Inc. SLM-256), composed of two 128-pixel liquid crystal masks in series. The SLM was placed at the Fourier plane [5]. After compression and pulse shaping, 200 pJ pulses were used to interrogate the samples. 

The laser system consisted of a home-built Ti sapphire fs laser oscillator and regenerative amplifier (RGA). The pulse duration was 50 fs at 800 nm and 1 kHz repetition rate. The output of the RGA was split into two parts. One part was used as pump pulse. The other part served as a source for the generation of probe pulses with the help of a non-collinear optical parametric amplifier (NOPA, Clark). The sample preparation was explained elsewhere [7]. Briefly, sodium (Alfa Aesar) was used as received and sodium bromide (Alfa Aesar) was dried and re-crystallized under vacuum. The preparation of the samples was carried out in a glovebox under argon atmosphere. Localized electrons were generated by heating the metal-salt mixture to 800 °C, i.e. well above the melting point of the salt. 

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