Кузнецова Т.И., Кирсанова Г.В. - Чтение технической литературы на английском языке по оптике (1058939), страница 2
Текст из файла (страница 2)
7. Answer the questions about the text:
1) Who was the first to demonstrate Light Amplification by Stimulated Emission of Radiation? 2) In what substances were population inversions and coherent emission generated? 3) What excitation techniques are used to generate coherent emission? 4) What method of generating coherent radiation resulted in the expanding of the number and types of laser sources? 5) Which is a better way of attaining the extreme power and energy parameters: using laser systems or simple laser oscillators? 6) What determines laser efficiency?
8. Write an abstract of Text 4A.
9. Read Text 4 B without a dictionary and answer the question:
Каковы преимущества лазера на алюмоиттриевом гранате, активированном неодимом (Nd: YAG laser) перед лазером на рубине?
Text 4B Nd: YAG Laser vs. Ruby Laser
The Cr3+ iron-group ion doped (to dope – добавлять) in Al2O3 is the medium in which laser operation was first demonstrated by Maiman in 1960. Cr: Al2O3 or ruby operates as a three-level system and thus, per unit volume, has a comparatively high threshold (порог). Fortunately, the thermal conductivity and mechanical strength of Al2O3 are both high, superior to any other existing laser host (основа, матрица) crystal, and thus successful operation of the ruby laser is possible. For all but (кроме) a few specialized applications the much-lower-threshold, higher-average-power-output Nd: YAG laser has replaced the ruby laser, however. Efficient frequency-doubling (удвоение частоты) techniques for 1.06 ηm radiation have in mаnу cases eliminated the need for 0.69 ηm ruby laser where visible radiation is required.
850 п. зн.
10. Translate Text 4C in writing using a dictionary (time limit 30 min.):
TEXT 4С FREE ELEGTHON LASER
In the Free Electron Laser (FEL) gain is generated by the interaction of photons with an electron beam. A freely propagating electron does not interact with an electromagnetic field. To obtain gain the electrons and photons must interact within a perturbing environment that permits the simultaneous conservation of energy and momentum; spontaneous emission from the electron is then possible. The synchrotron radiation that occurs when the trajectory of a high energy electron is bent by a magnetic field is an example of one such process.
The process that generates gain may be viewed as stimulated scattering, as stimulated “free-free” transitions between continuous states of the perturbed electron-photon system, or as the inverse of the interaction that accelerates electrons in an accelerator. If the velocity distribution of the electrons in the beam is carefully selected, the radiation emitted by each electron adds coherently to the radiation from other electrons in the beam. The wavelength of maximum gain is primarily a function of the energy of the beam. With a minimum of constraints, the operation of an FEL should be possible at any wavelength from millimeter wavelengths into the visible and near ultraviolet.
1300 п. зн.
SUPPLEMENTARY READING TASKS
469nm Fiber Laser Source
With the continued interest in development of solid-state blue laser sources we would like to show that fiber lasers and nonlinear frequency conversion are an attractive approach. Fiber sources are a good choice for nonlinear frequency conversion because of their good beam quality and high brightness. Using non-critical phase matching eliminates the problems of spatial walk off allowing for longer interaction lengths and this leads to higher conversion efficiency.
Our fiber amplifier uses the 4F3/2 - 4I9/2 transition in neodymium and because of the 3-level nature of the transition there is strong competition from the 4F 3/2 - 4I11/2 4-level transition. Optical fiber hosts have the advantage of wavelength selective loss dependent on bend diameter allowing the user to choose a fiber coil diameter to act as a variable short pass filter. In our case we were able to choose a coil diameter that will generate ~10dB of loss for the competing 4 level 1088 nm parasitic transition while generating very little loss at 938 nm.
High power levels have been achieved for this Neodymium transition in crystal hosts; however to our knowledge this is the highest power achieved for this transition in a silica fiber host. The silica host offers a broader absorption spectrum reducing the precision requirements of the pump and a broader emission spectrum (900nm to 950nm) enabling more applications. We have previously reported multi-watt operation on this transition and continue investigating power scalability.
While the idea of quasi-phase matching has been around for a long time engineered nonlinear materials are starting to gain maturity and are commonly used for nonlinear frequency conversion. A lot of progress has been made in both materials and periodic structure fabrication in recent years. Fabricating the short periods required for first order frequency doubling into the blue still remains challenging. Because of its anisotropic lattice structure KTiOPO4 (KTP) exhibits very limited domain wall spreading during the poling process leading to the ability to pole very short domain periods. Also the KTP has a coercive voltage about 10 times lower than congruent LiNbO3 enabling electric field poling of thicker materials.
(Alex Drobshoff, Jay W. Dawson, Deanna M. Pennington, Stephen A. Payne, Raymond Beach,
Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551; Luke Taylor, European Southern Observatory, Karlschwartzchild Strasse 2, 85748 Garching-bei-Muenchen; http://www.osti.gov/energycitations/index.jsp)
Good Fundamentals
Stephen Matthews
For most applications, the size to which a laser beam can be focused is as important a consideration as the laser output power. Frequency doubling, for example, depends on the square of the intensity of the primary laser. The depth of a hole drilled by an industrial laser depends on the laser intensity and the hole diameter is proportional to the spot size.
A beam profile composed entirely of five higher-order modes can look like a TEM00 beam to instruments that measure beam diameter. The figure on the left is the highest order mode in the beam, TEM21. The figure on the right is the apparent profile of the composite beam, which contains no TEM00 component.
Maintaining a consistent beam profile is usually important whether the beam is focused or not. Ophthalmic surgery uses a beam with a flat cross section (a "top hat" profile) that must remain constant during the procedure. All of these applications require a laser designed to produce a consistent and well-characterized beam. To be propagated over a long distance, a laser beam needs to have the lowest divergence possible. Telecommunications combines this requirement with a need to control the spectral content of the beam to ensure data quality. Whenever low divergence or small spot size is required, a laser with TEM00 output is specified.
What is TEM00?
It is useful to think of the light inside of a laser as formed of standing waves with distinct vibrational modes. Only a small number of modes will exist in the transverse direction. The fundamental transverse mode is designated as TEM00, where the "00" indicates no nodes appear in the beam profile. "TEM" stands for "transverse electromagnetic" and refers to the form of the standing waves. The TEM00 mode is mathematically described by the familiar bell-shaped Gaussian curve.
Higher-order modes are formed by multiplying the Gaussian by a polynomial with an exponent that corresponds to the order of the laser mode. These higher-order modes describe the number of nodes that appear in the beam—the TEM11 mode of a rectangular resonator, for example, will appear to have a dark cross in the middle of the profile. Higher-order modes add frequency components to the fundamental mode.
The Gaussian function extends to infinity in the radial direction, leaving open the question of the beam diameter. Measuring a laser beam diameter has been compared to using calipers to measure the width of a cotton ball. The accepted definition is the diameter at which the intensity has fallen to 1/e2 (13.5%) of its peak value in the center.
The 1/e2 definition works well for Gaussian modes, but is not useful for other profiles. In these circumstances the diameter is calculated using the "second moment" algorithm, a combination of integrals similar to a formula for calculating an rms (root-mean-square) value. The second-moment calculation should be used cautiously because it gives heavy weight to the edges of the beam.
Measuring beam size
Early means of determining a profile were essentially visual, such as examining the pattern of a continuous-wave (CW) beam on a lab wall or the burn marks made by a pulsed infrared beam on photographic film. It is an indication of the difficulty in measuring high-power pulses that visual techniques are still used. Instruments that measure beam profiles (profilometers) either use CCD cameras, or else scan a slit or knife-edge through the beam.
A CCD camera is a user-friendly system capable of instantly displaying the entire beam profile. It can be used with both CW and pulsed beams. The intensity distribution of the profile can be displayed as either a two-dimensional (2-D) or three-dimensional (3-D) contour plot. Charge-coupled-device cameras are superior for measuring elliptical beams, and their real-time capability is useful in production control (see Fig. 1).
FIGURE 1. Profilometers based on CCD arrays can provide real-time displays of 3-D beam profiles.
The limitation of this instrument is its resolution, set by the pixel size of the CCD array. Currently this can be as small as 10 µm, but a pixel size closer to 20 µm is more typical. In addition, most beams must be attenuated to avoid saturating the array, and the attenuating element introduces some degree of distortion, although a new CCD array using a diamond substrate appears robust enough to measure short-wavelength pulses directly (see Laser Focus World, May 2000, p. 265). Finally, a CCD camera is not the first choice if second-moment calculations are important—the signal-to-noise ratio of the CCD array decreases at its edges.
Scanning profilometers
These instruments scan a slit or a knife-edge through the beam and correlate the measurement from a detector behind the aperture with the aperture position. Different detectors can be positioned to allow these instruments to work at almost all wavelengths. Resolution, which is limited by diffraction from the scanning edge, is on the order of the wavelength of the beam.
A scanning slit masks most of the beam from the detector, eliminating the need to attenuate the beam. It is important to choose the correct slit size for the beam diameter—a slit too wide will make the measurement appear smaller than the beam itself. The slit should be no wider than one-third of the beam, and preferably narrower.
Knife-edge profilometers have a resolution as fine as 100 nm. As the blade moves across the beam, the detected signal decreases to zero and the measurement is differentiated to obtain the profile. Some systems use the same algorithm as that used in medical tomography for MRI and CAT scans to calculate the profile. However, scanning systems are not useful for pulsed measurements.
Whatever instrument is used, the beam should be measured at a distance from the laser sufficient to allow spontaneous emission and other light noise to diverge and not pollute the measurement. Lasers that produce the profile for which they are designed, free of aberration and the like, are said to be "diffraction limited." This does not mean, however, that their output is TEM00.
Raleigh range and divergence
Gaussian wavefronts start out as planes at a location called the "beam waist" (sometimes located inside of the resonator). The wavefronts become increasingly curved as they propagate from the waist until they reach their smallest radius, after which they flatten out. The distance from the waist to the location at which the wavefront is most curved is called the Rayleigh range.
The region between the beam waist and the Rayleigh range is the near field. In the far field the beam diverges in a cone with (nearly) straight sides. Divergence is always specified in the far field, which is usually chosen to begin around 10 times the Rayleigh range.
The distance from the laser to the far field can be meters, an inconvenient distance for measurement, so a lens is often used to focus the beam, thereby forming a new beam waist. The divergence is then the beam size at the lens divided by the distance from the lens to the focus. It is important that divergence is measured in the far field, or calculations for beam parameters will be incorrect.
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