Q-switching


Ultrasonic waves are generated by a Q-switched Nd Yag laser operating at the wavelength of 1.064 /xm with a half-width pulse duration of 15 ns, while a Mach-Zehnder heterodyne interferometer is used for the detection of ultrasound (Fig. 1). The probe, with a large bandwidth 20 kHz - 30 MHz, only measures the out-of-plane displacements with a sensitivity of about 10 run/ /Hz on a mirror-like surface [2]. The laser beam is focused on the surface of the sample by a spherical or a cylindrical lens to form a circular spot or a line source, respectively. The optical power density is adjusted to avoid any damage ensuring a non-destructive testing (thermoelastic regime). Mechanical displacements drive the movement of the sample according to two directions, which is, as the data processing, entirely controlled by computer. The ultrasonic images are visualized (B-scan views) thanks to the softwares developed by our laboratory.  [c.694]

Once a population inversion has been built up, any naturally emitted photon can initiate the lasing action. To improve the overall effect, the laser generator is normally enclosed within a resonance cavity with mirrors at each end (Figure 18.8). The purpose of the mirrors is to cause the lasing action to travel up and down the cavity (less loss of light to the walls of the cavity), increasing the cascade in one direction so that most of the inverted population is stimulated to emit in a very short space of time. However, this action can prevent the excited-state population from building up to a large excess, resulting in a low intensity of laser light output. There are ways of improving the population of excited-state molecules, two of which are Q-switching and mode locking.  [c.126]

Active Q-switching occurs when laser light access to one of the mirrors in the cavity is controlled by an electro-optical cell, which works on the principle of affecting the passage of polarized light (see below Kerr or Pockels cell). These last devices are able to turn on and off the transmission of light by using electric stimuli to alter the optical characteristics of the medium comprising the cell. In effect the cell acts as a very high-speed shutter controlled by a change in voltage. Application of a suitable electric potential prevents the passage of light and its removal allows it (or vice versa). Thus, by placing such a cell in front of one of the mirrors, laser light can be prevented from reaching the mirror until a large inverted population has built up inside the laser cavity. The cell is then switched to allow passage of photons, which pass up and down between the two mirrors and produce a giant pulse of laser light.  [c.127]

Mode locking is similar to the passive Q-switching except that the bleaching is effected by the differing modes of light. For any passage of light up and down the laser cavity, a standing wave must be built up — if d is the distance between the mirrors, then the frequencies of the standing waves are given by Equation 18.3, in which n is the refractive index of the laser material and i is an integer (1,2,3,...).  [c.128]

If d-n matches v, then standing waves are generated. The less good the match, the fewer standing waves are formed. If, as with passive Q-switching, there is an absorbing dye in front of one of the mirrors, the dye will absorb light as before. However, since frequencies v that do not match d-n very well are fewer than those that do match, this small number of frequencies is absorbed totally by the dye, leaving an excess of matching frequencies. Eventually, the dye is bleached by a cascading effect of the selected modes (mode-locking better matched frequencies). The time needed for a photon to make one trip up and down the cavity occurs as the dye becomes completely bleached. The resulting emergent laser pulse is very short and is highly monochromatic since the low population of unmatched frequencies has been largely eliminated. The pulse also lasts only a very short time before the inverted population is lost and the dye returns to its unbleached state. The time taken for a photon to move up and down the cavity is the pulse repetition frequency, not to be confused with the frequency of the emerging laser light.  [c.128]

Liquid solutions can be heated by other means. Microwave irradiation was used very early for T-jump (see Microwave technology). The method giving the fastest heating times has been irradiation using light from a Q-switched, giant pulse laser (3), which can deposit energy in about 10 ns (see Lasers). The laser energy may be absorbed directiy by the solvent. Water absorbs certain wavelengths in the near ir, and reactants are unlikely to absorb there. Thus undesired photochemical side reactions are avoided. Alternatively, an inert species that absorbs the laser radiation but does not interfere with the reaction or the monitoring of concentration changes may be added. Consequentiy, there is no restriction to aqueous solutions. The primary shortcoming is the limited energy available in a laser pulse. Depositing even 1J uniformly throughout the solution requires a very expensive laser system, and 1J produces only a T-jump of 0.25 C/mL of water. Small volumes are needed.  [c.511]

The unique virtue of flash photolysis is that is can be extended another 11 orders of magnitude toward shorter times. Down to times of a few nanoseconds, the most common procedures employ essentially the same principles used for millisecond experiments. The same excitation energy is dehvered in a shorter flash, and faster electronics are used to monitor changes in a continuous probe of concentrations. The probe is modified as necessary to permit faster measurements, using a brighter lamp to maintain an adequate signal-to-noise ratio while measuring faster transmittance changes. In the pre-laser era, flash photolysis technique developed in the direction of generating very energetic excitation flashes capable of making substantial concentration changes throughout a large volume so that kinetic changes could be monitored in a single flash. Signal averaging was rarely employed, except when using certain luminescence methods. It was difficult to make bright excitation flashes shorter than a microsecond, except for luminescence. Once pulsed lasers became available, pulse durations of 10 ns were easily attained. Several technologies, such as Q-switching, pulsed electrical excitation of gas discharge lasers, including the important uv-emitting excimer lasers, cavity-dumping, and even pulsed excitation of semiconductor lasers, conveniently generate pulses having durations near 10 ns. The first two methods can produce pulses with hundreds of millijoiiles of energy at rates of 1 to 1000 Uz the last two generate smaller energy pulses at repetition rates ranging from 1 to 1000 kH2.  [c.512]

Fig. 4. Temporal pulse characteristics of lasers (a) millisecond laser pulse (b) relaxation oscillations (c) Q-switched pulse (d) mode-locked train of pulses, where Fis the distance between mirrors and i is the velocity of light for L = 37.5 cm, 2L j c = 2.5 ns (e) ultrafast (femtosecond or picosecond) pulse. Fig. 4. Temporal pulse characteristics of lasers (a) millisecond laser pulse (b) relaxation oscillations (c) Q-switched pulse (d) mode-locked train of pulses, where Fis the distance between mirrors and i is the velocity of light for L = 37.5 cm, 2L j c = 2.5 ns (e) ultrafast (femtosecond or picosecond) pulse.
In order to produce pulses having smoother temporal characteristics and also to increase the peak power, methods of pulse control were developed. The most common method is called Q-switching. In a Q-switched laser, a switchable shutter is inserted between the laser material and one of the mirrors (14,15). When the laser is first pumped, the shutter is closed. No light can reach the mirror and the process of amplification through stimulated emission caimot build up. The laser medium is excited to the point that the population inversion considerably exceeds the threshold. Then the shutter is rapidly switched to the open position. Because of the large population inversion, the gain can be very high, and the laser pulse develops rapidly. The energy stored in the population inversion can be swept out in a single pulse of high peak power. The pulse duration is much shortened, from the millisecond region to the region of perhaps 30 ns, as illustrated in Figure 4c. The total energy release in the laser pulse is reduced by the Q-switching operation. Because the pulse duration is decreased by many orders of magnitude, the peak power can be much higher than for a normal pulse.  [c.4]

Q-switched mode, ie, pulse duration are on the order of a few tens of nanoseconds and peak power in excess of 10 W.  [c.8]

Second harmonic generation works best in pulsed lasers having high peak power, because the polarization which produces it is proportional the square of the electric field. Thus the most common frequency-doubled lasers are Q-switched devices. Many commercial frequency-doubled lasers have become available. The most popular of these is the frequency-doubled Nd YAG laser, having output in the green at 532-nm wavelength. Frequency-doubled tunable dye lasers, where the original output is in the visible, provide tunable output in the ultraviolet. Frequency doubling is the most common manifestation of nonlinear optics.  [c.13]

Another important appHcation involving material removal is trimming of resistors. Thick-film resistors used in many electronic circuits must be trimmed to the final desired value. This can be done by fabricating the resistor using a low value of resistance and then cutting part way through it with a laser. The resistance can be monitored as cutting proceeds. Trimming is terminated when the desired value is reached. Repetitively Q-switched Nd YAG lasers are commonly used for resistor trimming. Laser trimming has become standard procedure in the electronics industry.  [c.13]

Metal-free, chloroaluminum phthalocyanine [14154-42-8] vanadyl phthalocyanine [13930-88-6], or magnesium phthalocyanines are sufficiently soluble in organic solvents and show enough bleachable absorption at 694.3 nm to serve as repeated Q-switching elements for mby lasers (qv) (180). Phthalocyanines have been used in other lasers as weU (181).  [c.506]

Fortunately, the demand for spectral sensitizers, especially those in the infrared, has extended to systems having somewhat fewer and quite different limitations than for photography. For infrared dyes as examples (1), their photoelectrical effects useful in photography are also important to electrophotography, photopolymerization, and voltage-sensitive biomedical probes. Photochemical reactivity provides especially useful opportunities in photodynamic therapy, at the infrared wavelengths where body tissue has high transparency. Light-induced polarization can be designed to yield infrared nonlinear optical materials. Light absorption and emission, combined with suitable stabiUty and solubiUty properties, gives dyes used in lasers, medical diagnostics, optical data recording (photothermal), laser welding surgery, and Q-switches. Photosensitizers absorbing in the visible region are used in many apphcations, including the industrial scale photooxidation of (—)-citroneUol to (—)-rose oxide (8), photoisomerization of cis- to vitamin A acetate (8), and dye-sensitized neurobiology (9).  [c.429]

A Q-switched, frequency-quadrupled Nd—YAG laser (X, = 266 nm) and its accompanying optical components produce and focus the laser pulse onto the sample surface. The typical laser spot size in this instrument is approximately 2 pm. A He-Ne pilot laser, coaxial with the UV laser, enables the desired area to be located. A calibrated photodiode for the measurement of laser energy levels is also present  [c.588]

The Q in Q/TOF stands for quadrupole (see Chapter 25, Quadrupole Ion Optics ). A Q/TOF instrument — normally used with an electrospray ion inlet — measures mass spectra directly to obtain molecular or quasi-molecular mass information, or it can be switched rapidly to MS/MS mode to examine structural features of ions. The analyzer layout is presented in Figure 20.2.  [c.153]

The films have an area resistance of 100,000—300,000 Q/square. Most appHcations, such as switching on outdoor lights at twilight, require a detector resistance of near 1000 ohms to operate the switching circuit without the need of impedance matching electronics. This is accompHshed by depositing the contacts in an interdigitated geometry as shown in Figure 12. A protective film is deposited over the detector and contacts to provide for long-term stabiHty or the detector stmcture is mounted in a hermetic package as shown. The spectral sensitivity for the thin-fHm CdS detector is shown in Figure 9a. The response shape is similar to that of the human eye (see Table 1).  [c.431]

Assuming, p = 1 for a switching surge (in inductive transference we have to consider long-duration surges only) q = 1.8 for a switching surge  [c.604]

This is too low. But it is possible to make it up by selecting the arrester at the primary with a lower switching if possible, or provide an arrester at the secondary. Moreover, the response factor, q, is considered very high, which may not be true in actual service and an arrester at the secondary may not be necessary in all probability. The design engineer can use a more realistic factor based on his past experience and the data available from similar installations.  [c.623]

For measuring off potentials, thyristors can also be connected between pipeline and overvoltage arrester these operate only if inadmissibly high pipeline potentials arise by making the connection to the arrester. During the rest of the time (low load time), no grounding occurs and therefore there is no problem in making /f -free measurements by the switching method [19], The number of diodes follows approximately from Eq. (23-46) since (/q must not become too positive due to the anodic danger.  [c.530]

Tet us examine how the quasi-resonant switching power supplies employ tank circuits within their topologies. Any energy taken from the tank circuit by the output is a loss of its stored energy to the tank circuit. The problem is to harness this energy within the tank circuit, but not load the tank circuit too much as to ruin its Q. There are two ways to remove energy from an T-C tank circuit by placing relatively high impedance in parallel with the capacitor (parallel loading) or by placing a relatively low impedance in series with the inductor  [c.157]

The particles most likely to cause adverse health effects are the fine particulates, in particular, particles smaller than 10 p and 2.5 mm in aerodynamic diameter, respectively. They are sampled using (a) a high-volume sampler with a size-selective inlet using a quartz filter or (b) a dichotomous sampler that operates at a slower flow rate, separating on a Teflon filter particles smaller than 2.5 mm and sizes between 2.5 mm and 10 mm. No generally accepted conversion method exists between TSP and PM,o, which may constitute between 40% and 70% of TSP. In 1987, the USEPA switched its air quality standards from TSP to PMk,. PM,q standards have also been adopted in, for example, Brazil, Japan, and the Philippines. In light of the emerging evidence on the health impacts of fine particulates, the USEPA has proposed that U.S. ambient standards for airborne particulates be defined in terms of fine particulate matter.  [c.16]

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation.  [c.1281]

One of the ways of making a mirror temporarily nonreflective is to place in front of it a solution of a dye that has suitable absorption bands. As long as the dye absorbs photons, none can get through to the mirror to be reflected. However, at some stage, as the number of photons in the cavity builds up, the dye molecules are all raised to their excited state in a short time and the dye loses its capacity to absorb any more light. It is said to be bleached. At this stage, the laser photons can reach the mirror, which is reflective. The laser photons can now bounce back and forth very quickly between the two mirrors of the cavity, stimulating emission from the large inverted population that has had time to build while the dye was absorbing radiation. There is then a large pulse of intense coherent laser light lasting a few nanoseconds. After this event, most of the dye molecules return to the ground state and the mirror once more becomes nonreflective until just before the next giant pulse is emitted as the process repeats. This sort of Q-switching is said to be passive because it happens without any external intervention.  [c.127]

The output of a Q-switched laser appeared temporally smooth when viewed by phototubes and oscilloscopes in the early 1960s. However by the late 1960s, it was realized that there was substmcture in the Q-switched pulse as well (16,17). Under many conditions, the output of the Q-switched laser could consist of a train of very short pulses, as illustrated in Figure 4d. The individual pulses in the pulse train were separated by the round-trip transit time of the cavity, 2 L/c. The widths of the individual pulses were very short, on the order of tens of picoseconds. This train of pulses is commonly called a mode-locked train, and the individual pulses are called mode-locked or picosecond pulses. This behavior arises because of interference between the different longitudinal modes present in the laser cavity (18,19). The phases of the longitudinal modes lock together to produce the output shown. A temporally smooth Q-switched pulse can be obtained by constraining the laser to operate in a single longitudinal mode. In that case there can be no phase  [c.4]

EO materials are generally used as either bulk optical elements or guided wave devices. Bulk elements are frequendy made of polycrystalline ceramics such as lead lanthanum zirconate titanate (PLZT), (Pb,La)(Zr,Ti)02, whereas guided wave devices are usually fabricated on single crystals of materials such as LiNbO and KTiOPO. Bulk elements are used where significant apertures are required and large modulator bandwidths are less important, and are used in devices such as variable density filters, variable color filters, electrooptic shutters, and as Q-switches for pulsed lasers. The Q-switch takes advantage of the fast response time of the EO material, admitting a single laser pulse by switching between on and off states in less than 1 ns. Guided wave devices are used as signal modulators and as interferometers. A simple example of such a device is a single channel phase modulator, which alters the phase retardation of a beam propagating through the waveguide. A difference in refractive index between the waveguide and the remainder of the crystal keeps the light beam from escaping the channel This refractive index gradient is created by replacing metal ions in the crystal with foreign ions through diffusion, eg Ti for Nb in LiNbO and Tl for K in KTiOPO. Such an element can be used to modulate optical phase at very high frequencies, allowing high bandwidth signal transmission through fiber optic cables.  [c.340]

The activation of solid electrodes has been an important subject in electroanalytical chemistry. The electrodes are frequently contaminated by electrolyzed products, and thereby they cannot stand long-term use as an electrochemical detector. In order to overcome the electrode contamination and moreover activate the electrode, we have developed a technique, in which the electrode surface is renewed by an ablation action of a strong laser pulse. The laser pulse can ablate a top surface of the electrode, and consequently contaminants are removed from the surface. Since the cleaning by laser ablation is available for an electrode remaining in a solution, it is very useful for analyses requiring long-term use of the electrode. In this work, a laser pulse from a Q-switched Nd YAG laser was used for cleaning a Pt electrode of (tp 1 mm). A power of the pulse was adjusted to about 100 mJ/pulse and the pulse width was nominally 5 ns. A voltammogram was recorded, while firing the pulse every 2 seconds. Every current was sampled at 0.75 s after firing the pulse in order to avoid the charging current, and was plotted against the potential. As an example, voltammograms of ascorbic acid (AA) are shown in Fig. 1. All the voltammograms show an S-shaped curve, which suggests that the ablation action can renew not only the electrode surface, but also the diffusion layer growing during the electrolysis. The limiting currents were directly proportional to the concentration down to 0.01 mM, and the slopes obtained from the log-plot analysis were 31.7 2.2 mV in the concentration range from 0.10 to 5.00 mM. These facts indicate that the electro-oxidation of AA is a reversible two-electron process. It has been known that electrolyzed products of AA contaminate the electrode. Taking  [c.79]

The ligand-receptor complex C has changed properties which typically allow it to undergo furtlier, previously inaccessible reactions (e.g. binding to a DNA promoter sequence). The role of L is to switch B from one of its stable confonnational states to anotlier. The approximate equality of tire intramolecular, molecule-solvent and L-B binding energies is an essential feature of such biological switching reactions. An equilibrimn binding constant K q is defined according to tire law of mass action  [c.2824]

EpitaxicaHy oriented single-crystal layers of Fe and Cr have been prepared by MBE on exceptionally clean (001) GaAs single-crystal substrates (60,61). The notation for one individual bilayer is ((001)Fe/Cr(001)). The individual layer thickness ranges from 0.9 to 9 nm and the total number of bilayers in a single film is around 30. Notation for a complete supedattice of 30 bilayers would be (Fe 3 nm /Cr 1.2 nm).Q. Chromium, in bulk form, is antiferromagnetic. At low temperatures and in zero-appHed field, the magnetization behavior of each successive layer is antiparallel and the electdcal resistance of the film is high (Fig. 7a). When a magnetic field is appHed parallel to the layers and is sufficient to overcome the antiparallel arrangement of the layer magnetizations (Fig. 7b), the electdcal resistance of the film begins to decrease. When this occurs, the film is said to exhibit a negative magnetoresistance. The complete switching fields (H ), for a constant negative magnetoresistance, for several Fe/Cr supedattices at 4.2 K, are shown in Figures 8 and 9. JT. for the saturation of magnetoresistance is a function both of composition of the individual layers and the number of bilayers. There is also an anisotropy in the magnetoresistance as illustrated in Figure 9 (60).  [c.394]

These breakers, when used for switching long transmission lines at 420 kV and above, are provided with a pre-insertion resistor across ctich interrupting contact to limit oxervoltages that may occur during a closing or opening sequence, as it restdt of heavy charging currents as noted in Tabic 24.2. The value of the resistor may be around 400 Q for the line parameters, considered in Table 24.2 and may iiry with line parameters. The resistors are connected so that during a closing sequence they shori-cii cuit the making contacts before closing the main contacts for, say. 8-10 milliseconds, and open immediately after the contacts are made. This also happens during an opening sequence.  [c.643]

For consumers, quantity demanded of energy (Q, j) is a function of the price of energy (P), the price of other related goods, disposable income (Y), and other variables (O) such as personal preferences, lifestyle, weather, and demographic variables and, if it is aggregate demand, the number of consumers ( C). Take for example the quantity of electricity demanded by a household. If the price of electricity increases consumers may use less electricity. If the price of natural gas, a substitute for electricity in consumption (P,), decreases, that may cause consumers to shift away from electric water heaters, clothes driers and furnaces to ones that use natural gas, thus increasing the quantity of natural gas demanded. If the price of electric appliances (PJ increases, nr decreases quantity nt electricity demanded, consumers may buy less appliances and, hence, use less electricity. Increasing disposable income is likely to cause consumers to buy larger homes and more appliances increasing the quantity of electricity consumed. Interestingly, the effect of an increase in income does not have to be positive. For example, in the past as income increased, homes that heated with coal switched to cleaner fuels such as fuel oil or gas. In the developing world, kerosene is used for lighting, but as households become richer they switch to electricity. In these contexts coal and kerosene are inferior goods and their ennsumption decreases as income increases. We can tvrite a general consumer energy demand function as follows  [c.1108]


See pages that mention the term Q-switching : [c.127]    [c.342]    [c.834]    [c.14]    [c.885]    [c.1574]    [c.346]    [c.346]    [c.157]    [c.132]    [c.133]    [c.601]    [c.604]    [c.623]    [c.799]   
See chapters in:

Mass Spectrometry Basics  -> Q-switching


Mass Spectrometry Basics (2003) -- [ c.126 ]