Jelezin, A

Koltsov, I

Komarov, A

Rybin, V M

CERN Document Server

TUNABLE LASER FOR PLASMA DIAGNOSTICS.A. Jelezin, I. Koltsov, A. Komarov, V. Rybin, MEPhI, Moscow, RussiaWC1P38-160The modern laser employed for thermonuclearplasma diagnostics must provide a wide band radiationspectrum tuning, adjustment of spectrum line width,high speed tuning and precise spectrum forming. Insome cases a laser capable of generating severalwavelength tunable radiation lines is demanded.With such a laser one can scan the object withdiscrete spectrum radiation while simultaneouslyanalyzing several lines of reflected radiation. Thismakes the diagnostics process faster, easier, and lessfrequent tuning of the laser’s spectrum is required.We have developed a prototype utilizing a solid-stateexperimental Ti sapphire laser with an in-resonatorradiation spectrum tuning element (fig.1).ControlcomputerSpectral lineselection elementResonator11215171613111410234895 672 2 2AmplifierSpectrum analysisdevicePumpsourcePower space dispersioncontrol system.Fig.1. A simplified chart of the tunable laser prototype. 1- rear full-reflecting mirror, 2- additional full-reflectingmirror for resonator tuning with half-reflecting mirrors, 3- pump power sensor, 4- wavelength to co-ordinateconvertor, 5, 6 - space modulator and collimator of the beam, 7- object illuminated by laser radiation, 8- laserradiation analysis system photosensor, 9- power space dispersion analysis system photosensor, 10- pump power controlsignal, 11-pump power amplifier readback signal, 12-spectrum control signal, 13- optical valve timing control signal,14- spectral composition data, 15- beam direction control signal, 16- beam focus control signal, 17-laser power spacedispersion data.A TeO2 opto-acoustic filter was employed as themain avelength tuning element. The laser unit consistsof a gas pumping source, gas pumping power control,Z-type resonator with opto-acoustic filter, and anoutput radiation spectrum analysis system. Thespectrum analysis system employs a Fabri-Perotinterferometer, modified spectro-photometer withspectrum sensor, CCD sensors, UHF signal generators,high-frequency mixer, and amplifier. The system alsoincludes a wintel compatible computer with videocapture and UHF signal generator interfaces.During development, the tunable laser prototypediagnostic spectrum control system and associatedspectrum forming algorithms introduced most of thecomplexities. For spectrum forming and stabilization adevice that can determine spectrum composition isrequired. We propose that for spectrum analysis, theFabri-Perot interferometer with a resolution of about R≈2*106 can be used. The analysis of an interferenceimage requires several stages, which are implementedby both the hardware and software of the system. Theinterference image is read from the CCD-sensor,transferred through the interface to the videoInternational Conference on Accelerator and Large Experimental Physics Control Systems121International Conference  Accel rator and L rge Experim ntal Physics Control Systems, 1999, Trieste, Italyprocessing board, converted to a digital form, andplaced into a special memory. The software processesthe image first through a static filter and secondthrough a Fourier filter. Next the set of interferencecircumferences radii is determined, separately for eachcircumference, the set of wavelengths λ0i is calculated,and the mathematical expectation of line wavelengthand its dispersion are computed. The value of themathematical expectation M[λ0] will be used then fordetermining the deviation of the current wavelengthvalue from the set point.The quality of a spectrum control system isdetermined by the control command formulationdelay, and the precision of the line wavelength settingin its width and power. For the different modes of thelaser (a continuous or pulse radiation modes, single- ormulti-frequency modes, narrow- or wide-band modes),different laser powers, different spectral dispersion ofthe different types of CCDs, and different algorithmsof spectrum control are demanded. Thus, it isnecessary to develop a spectrum control system basedon adaptive systems theory and the theory of systemswith alternating structure.We have investigated three different modes ofcontrol that could be applied to our linear CCDsensors during the laser spectrum measurements. Inthe first mode continuous spectrum picture scanning,the charge set resulting from the previous scan is fullyremoved from the sensing area of the CCD during theframe transfer. Next, the charge in the gates of thesensing area will be formed again and the process isrepeated. In this mode the frame sampling frequencydefines the exposure period. To change the exposureindependently, and for example prevent saturation, wehave utilized a second synchronous control mode. Inthis mode, after the scan of the frame, the rest chargecontinuously flows through the shift gate toaccumulate in an analog register which is cleared by aperiodic timing pulse (fig.2).In the third mode,intended for pulsed laser radiation, the CCD controlsystem must start the exposure before the laser pulse.When synchronizing with the laser pulse, a signalfrom the laser pump lamp is required.We propose to compute the regulator elements forCCD-sensors and opto-acoustic devices dynamicallyduring the spectrum control process. This method iscompatible with the above control system structuresynthesis quality criteria and also the crystals of "flex"logic. This reduces the complexity of the controlboard. To develop the system synthesis algorithms wehave applied graph analysis of system conditionmethod. In each condition of the system the previouslydefined control tasks are executed. Optimal design ofthe control system also reduces a system errors anddata flow between processors during parallel dataprocessing.345216 7Photo diodesShift Gate 2Shift Gate 1.Fig.2. The internal structure of a standard linearCCD. 1- clamp amplifier, 2- shift-hold amplifier, 3-light-sensing gates, 4,5- CCD analog shift registers,6- timing pulse generator, 7- gate driver.Another significant problem is the processing of themeasured spectral images. This challenge is quitesimilar to the image recognition problem in controlsystems where precision high sensitivity devices areused. In this task we convert the original informationwhile selecting unique properties which accomplish aprecise classification of the object.Tests of the high-precision radiation sensor modulehave shown that the measured digital image inpractice includes noise components that reduces itsquality. This noise originates from the influence ofseveral external factors. To reduce false information,and to improve system effectiveness, we have tried toapply colour sensors instead of black-and-white ones.Under various measurements conditions the obtainedimages differed. Under conditions of continuouslychanging external illumination, the precision of theessential interference rings radii drops. This arisesfrom the nature of colour and from the influence ofchanging registration modes which leads to the loss ofinformation. But, in the case of colour input that isvarying the external illumination, this results in fewerchanges in the brightness and colours of theinterference picture.Colour, as is well known, is determined by thewavelength of electromagnetic radiation. For theprocessing of the visible part of interference picturespectrum, a two-dimensional CCD matrix with acomposite PAL-encoder was used. The signal from itwas converted to digital form using an additive RGBmodel. A composite signal is usually used in videoequipment (VHS, Video-8) that consists of intensitysignal Y, colour signal U, colour signal V and severaltiming signals. The most common encoding methodfor videodata utilizes a two-dimensional matrix ofintensity, with each element representing the index ofa pixel colour. A doubtless merit of such a datapresentation method is the possibility of efficientlyupdating the picture elements, as necessary, forexample, when a new colour model is required. Theuse of a HLS model (colour tone H, intensity L and122saturation S) allows application of a colour toneconcept to problems of filtration, segmentation, andrecognition, independent of the object illumination.In the standard HLS model of image digitizing it isnecessary to convert the former signal from YUV toRGB (that is provided by hardware), and then toconvert the signal from RGB to HLS (fig. 3). The RGBto HLS transformation is executed by software.YUV RGB HLS.Fig.3. Data conversion sequence.To speed up preliminary data conversion one candirectly convert data from YUV to HLS (see fig.3).Digital processing in HLS form is much easier thanprocessing of RGB signals due to their limitedspectrum. However, in this prototype it is necessary toutilize an RGB-based video capture unit because acommercial YUV-HLS transforming unit hasn't beenfound.Conversion from the YUV model to HLS model canbe implemented at a hardware level with thefollowing formula:H Z= arccos( ), wherecos( ) . .Z V US=⋅ − ⋅⋅0 886 2 5096L Y=S Y U VYU YV UV= ⋅ + ⋅ + ⋅ +⋅ + ⋅ + ⋅( . . .. . . )0 536 1259 10380 415 0 681 01972 2 212Conversion coefficients were calculated taking intoaccount the effective wavelength range of the lasersystem (from 600 nm up to 900 nm). Using theproposed scheme the influence of medium intensityillumination on system efficiency can be eliminated.This had a positive effect on the prototype whenoperating in a conditions of variable illumination. Thereduction in the duration of preliminary imageprocessing was strongly correlated with the magnitudeof the colour image matrix.123

Tunable Laser for Plasma Diagnostics

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