New camera on Subaru Telescope may directly observe exoplanets
The Subaru Telescope, located on the summit of Mauna Kea, is dedicated to exploring the cosmos, gaining a deeper and more thorough understanding of everything that surrounds us. With an 8.2-meter mirror and a suite of sophisticated instruments, astronomers at the Subaru Telescope explore nearby stars looking for planetary systems. A giant step towards this goal was made recently with the "first-light" inauguration of a new state-of-the-art camera.
Subaru uses eight innovative cameras and spectrographs optimized for various astronomical investigations in optical and near-infrared wavelengths. On the night of December 3, 2007, the High Contrast Instrument for Adaptive Optics (HiCIAO) camera was brought to life. The HiCIAO is a technologically adaptable system that will replace the infrared Coronagraphic Imager with Adaptive Optics (CIAO) unit in operation since April 2000. Both systems are designed to block out the harsh direct light from a star, so that nearby faint objects such as planets can be viewed. The new system benefits from a contrast improvement of ten to 100 times, allowing astronomers glimpses into regions never explored.
A further advantage of the HiCIAO camera is that it will be used in concert with an adaptive optics (AO) system that was recently significantly upgraded, which, in turn, increased the clarity of Subaru's vision by a factor of ten, opening up more of the night sky to observing. The new AO system uses 188 actuators behind a deformable mirror to remove atmospheric distortion, allowing the Subaru Telescope to observe close to its theoretical performance limits. In addition, a laser guide-star system was installed to enable observations of tiny regions of sky without bright stars to steady the AO system on.
The HiCIAO system, initiated in 2004, was developed by a team of scientists and engineers from the Subaru Telescope, National Astronomical Observatory of Japan, and the University of Hawaii's Institute for Astronomy. Dr. Ryuji Suzuki, a Subaru astronomer leading the HiCIAO project, says "the unique instrument was primarily designed for the direct detection of extrasolar planets and disks." The system's design allows for high-contrast coronagraphic techniques in three observing modes: direct imaging, polarization differential imaging, and spectral differential imaging. HiCIAO directly detects and characterizes young extrasolar planets and brown dwarfs, sub-stellar objects that occupy the mass range between that of large gas giant planets (e.g. Jupiter), and the lowest mass stars. With the aid of the laser guide-star AO system, HiCIAO targets dim objects including young stars, protostars, and star-forming regions.
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HiCIAO is also extremely useful for detecting faint dust disks around nearby stars, and for studying small-scale and inner disk structures and dust grain properties, both of which lead to a clearer understanding of extra-solar planetary systems and their evolutionary processes. Dr. Suzuki reports that "although we already know of more than 250 extrasolar planets, they have all proven their existence indirectly by the Doppler or transit method. Because the direct imaging of an extrasolar planet has never been done, if it happens, that will be exciting." Subaru Telescope may be the first to directly observe a planet outside our solar system. (http://www.laserfocusworld.com)
| MODULE 6 PROPERTIES OF LASERS Texts: A. Properties of Some Important Lasers B. Soldiers in Lockstep C. Average Power Scaling |
Terminology
1) ground state – основное состояние системы; steady state – стационарный режим;
2) self - terminated operation – пичковый режим (в отличие от стационарного);
3) relaxation time – время релаксации (жизни);
4) mode – мода, тип колебаний; mode-locking –синхронизация мод;
5) to store – хранить, запасать, накапливать; storage – память, накопление;
6) Q- switching – модуляция добротности;
7) cavity dumping – затухающие колебания ;
8) curve –кривая линия; gain curve – контур усиления;
9) performance – работа, интенсивность работы, рабочие характеристики;
10) to saturate – насыщать; saturation flux – поток насыщения,
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Preliminary exercises:
1. Read and translate without a dictionary:
thermal, system, stimulate, integrate, intense, alternately, nominal, radioactivity, signal, combination, diode, orange, rhodamine, collectively.
2. Translate the word-combinations that follow:
medium - laser medium, gain medium, pulse-pumped laser medium, energy-storage medium; level - laser level, ground level, lower laser level, upper laser level, three-level laser system, upper laser level relaxation time, higher-lying pump level; density - average power density, population inversion density, input (output) power density.
3. Find equivalent phrases either in Text 6A or in the right-hand column:
| 1.тип уширения линии (насыщения) | a.spectral gain bandwidth |
| 2.свойства усиливающей среды | b. saturation flux |
| 3.источник накачки | c. stimulated emission cross-section |
| 4.состояние индуцированного излучения | d. properties of gain medium |
| 5.накопленная энергия | e. weak pulse |
| 6.импульсное излучение | f. colour center laser |
| 7.режим действия | g. pumping source |
| 8.слабый импульс | h. stored energy |
| 9. лазер с окрашенными центрами | i. type of saturation |
| 10.поток насыщения | j. release in a pulse |
| 11. ширина полосы спектрального усиления | k. mode of operation |
4. Read Text 6A and answer the following questions:
1) От чего зависит режим действия лазера?
2) При каких условиях лазер работает в стационарном режиме?
TEXT 6A PROPERTIES OF SOME IMPORTANT LASERS
The output energy and/or power obtainable from a given laser medium are determined both by the microscopic properties of the gain medium and by its associated “scaling laws”[17].
In general terms, a laser medium is said to be a "three-level laser system" when the lower laser level is the ground state of the system, the other two levels being the upper laser level and a higher-lying pump level; it is said to be a "four-level system" when the lower laser level is a level lying above the ground level of the system (usually with sufficient energy, so that it is thermally unoccupied).
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The relaxation times of the upper and lower laser levels determine the basic modes of operation possible for the laser itself. If the relaxation time of the lower laser level is much
shorter than the upper laser level relaxation time (due to stimulated as well as spontaneous processes) then the laser may be operated in the steady state with a cw output. When the inverse relation between level relaxation times is obtained, cw operation is precluded and self-terminated pulsed operation may occur.
A pulse-pumped laser medium is said to be an energy-storage medium when the lifetime of the upper laser level is much longer than the desired pulse duration of the output pulse. In this situation the upper laser level is able to integrate the power supplied by the pumping source. Stored energy can then be released in an output pulse using mode-locking, Q-switching, or cavity dumping techniques described above; alternatively, pump energy stored in a laser power amplifier can be released in an intense short pulse upon passing a weak short pulse from a master-oscillator through the power amplifier (MOPA).
The key microscopic (intrinsic) laser parameters of the gain medium are: nominal wavelength; stimulated emission cross-section; spectral gain-bandwidth and type of saturation (homogeneous/inhomogeneous); saturation fluence or flux; radiative and kinetic lifetimes of upper and lower laser levels; and the characteristic specific excitation parameters are population inversion density; small signal gain coefficient; input and output power (energy) densities.
Spectral tunability is a particularly useful property of many laser sources. Semiconductor diode lasers, organic dye lasers and colour center lasers are particularly known for this property. The nominal spectral regions these types of lasers operate in are shown below. Using several different dye types and various solvents, the spectral region from 350 to 1000 nm can be spanned with tunable dye lasers. A single dye-solvent combination typically can be tuned several hundred wave numbers (cm-1) away from the spectral peak of the gain curve. Best dye laser performance is currently achieved with the yellow – orange rhodamine dye. Power and energy availability tend to roll-off to[18] the blue and to the red, also useful amounts of energy and power can be achieved in these spectral regions.
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Semiconductor lasers of various types collectively span the spectral region from 330 nm to beyond 15nm. Depending on the type of diode tuning can be accomplished using an applied magnetic field, by changing the current passing through the diode, or by applying pressure to the diode .
3400 п.зн.
Words to be learnt:
to preclude – устранять, предотвращать;
to occur – иметь место, случаться;
to release – выпускать освобождать;
to span – охватывать; покрывать (пространство, промежуток времени).
Exercises
1. In each group find the word that doesn’t belong:
a) lower, upper, smaller, power, bigger, hotter, shorter, higher;
b) associated, terminated, stored, precluded, released, unoccupied, described, tuned;
c) sufficient, different, coefficient, magnificent, fluorescent, efficient.
2. Find an antonym for each verb below in Text 6A:
lower level, longer than, output power, direct relation, occupied level, released energy, weak pulse, homogeneous saturation, above.
3. Complete the sentences below with the appropriate word or word-combination from Text 6A:
1) When the lower level is above the ground level of the system, a laser medium is said to be...
2) When the lifetime of the upper laser is much longer than the desired pulse duration of the output pulse, a laser medium is said to be...
3) When the relaxation time of the lower laser level is much longer than that of the upper level...
4) Tuning of semiconductor lasers can be accomplished by...
5) Semiconductor lasers, organic dye lasers and colour center lasers are famous for the property of...
4. Translate the sentences below focusing on the underlined words:
1) The term laser stands for light amplification stimulated emission of radiation. 2) There are many phenomena of the interaction of light with matter, which are readily described in terms of photon. 3) Modeling of continuous systems should be analyzed in terms of modified curves. 4) In broad terms it is found that optical threshold depends on the wavelength of theincident radiation. 5) Semiconductor lasers are usually differentiated in terms of the means by which the hole-electron pair population inversion is produced. 6) Laser sources are commonly classified in terms of the state of matter of the active medium. 7) Laser oscillation is marked by dramatic narrowing of the spectral and angular distribution of the spontaneous emission radiation. This statement was first made by Maiman in 1960. 8) The United States of America is a Federal Republic of 50 states done together by the pact of 1787. 9) As the engine has a mechanical compression it is capable of operating under static conditions.
5. Answer the following questions:
1) What determines the output energy? 2) What is the difference between a three-level laser system and a four-level laser one? 3) How does laser operation depend on the relaxation time? 4) In what case can self-terminated pulsed operation occur? 5) What laser property allows spanning the spectral region? 6) What types of lasers are known for this property? 7) What can a semiconductor laser be tuned by?
6. Write an abstract of Text 6 A
7. Read Text 6 В (time limit 3-4 min.) and answer the following questions: Почему излучение лазера имеет высокую направленность?
TEXT 6B SOLDIERS IN LOCKSTEP[19]
То understand why light from the laser is so concentrated, you must recall that light travels in waves, like ripples on a pond. The distance from the crest of one wave to the crest of the next is the wavelength. Ordinary white light is made up of many wavelengths travelling in every direction. This is known as incoherent light. Laser light, on the other hand, is coherent. It is essentially of one wavelength, with all the waves moving in one direction. Because the laser wavelengths reinforce each other, like soldiers marching in lockstep, they can remain in an unbelievably straight narrow beam for long distances instead of fanning out like a flashlight beam. Almost any substance can be forced to “lase” if you work hard with it. Gas lasers give off continuous beams of laser light, in contrast to the sharp pulses of the ruby laser. Tiny semiconductor lasers made of bits of such materials as gallium arsenide work best at ultra-cold temperatures. Many lasers give off invisible radiation, either infrared or ultraviolet. The carbon-dioxide laser, one of the most powerful yet invented, shoots a continuous beam of intensely hot but invisible infrared light.
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8. Translate Text 6C in writing using a dictionary ( time limit 50 min.):
TEXT 6C AVERAGE POWER SCALING
Increasing the average power output of a laser as it is made bigger is determined primarily by the rate at which waste heat generated in the laser process can be removed from the laser medium and/or the active volume enclosed by the optical resonator. In average power producing lasers of practical interest, removal of waste heat is accomplished by either convection or conduction, the choice depending on the class of laser medium involved. For both gaseous and liquid laser media, scaling to high average power is achieved using convective flow of the waste heat (and spent laser medium) out of the active volume defined by the laser resonator. In the case of gas lasers, the flow may be supersonic (as in the C02 gasdynamic laser which has resulted in the highest average output power yet achieved) or it may be subsonic. In the case of liquid dye lasers, significant average power has been obtained using a confined transverse flow of the organic dye laser medium through the optically pumped laser volume, as well as by using a free - flowing transverse jet stream.
In the case of solid state lasers, the laser medium itself cannot be rapidly and continuously moved through the volume of space defined by the laser resonator and the cooling of the laser medium must be accomplished by conduction of waste heat to an exterior surface. This surface can then be cooled using a gaseous or liquid cooling fluid flowing across it. Crystalline materials generally exhibit relatively high thermal conductivities which are strongly temperature dependent compared to those of amorphous glasses which are essentially temperature independent.
1700 п.зн.
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