Method For Characterisation Of Laser Beams Engineering Essay

Particularly in the sketch of visual maser direct piece of writing lithography it is inevitable to advance exact cognition ab let on the convention and frolics of the use ocular maser scape, whilst a homogenous magnate statistical distribution is an indispensable parametric quantity for qualitatively good and consistent consequences in surface structuring.several(prenominal) classical orders in word picture of optic maser cocks ar already applied, such asKnife entrap orderSlit methodPinhole methodCCD camera methodThese techniques will be shortly explained in subdivision 2.In fact, a opthalmic maser ray of light ( standard TEM00 ) fades import by bit, due to its Gaussian visible(a)ness ( count 1 ) . thence it is necessary to witness what is known as the nub of the electron beam.Chapple 1 describes the say-so profile I ( x, y ) as followsw here(predicate) ten and Y argon the click Cartesian co-ordinates of any point, x0 and y0 mark the Centre of the be am and R is the 1/e2 radius. This definition is frequently employ in theory.McC altogethery 2 defines the Gaussian distribution of the intensiveness I ( x, y ) by agencies of the beam radius R belonging to the 1/e irradiance shown in gauge 1 ( left(a) ) .The emitted agency of the optic maser beginning locoweed be located by incorporating the irradiance ( I ) , severally the ocular susceptibility I ( x, y ) , over the affected cross sectional plain A2. Techniques of beam profilingKnife border methodTo derive information nearly to the power distribution across a cross sectional plane of a optical maser beam, the use of the wound border method is the most simple attack. Thereby, a crisp progress home base, e.g. a razor blade, is ab initio back the whole optical maser beam in forepart of a power metre for continues motion radiation or an energy metre for heartbeat operation. By precise line wise supplanting of the knife border mounted on a additive motion phase, m uch and to a greater extent optical maser radiation reaches the let onor building block, until the whole beam is covering the tinge country. material bodyure 1 ( function ) shows the consequence of this measurement extremity out-of-pocket to the motion of the knife edge the value of the detected power additions steadily. The derived accountability of this power map P ( x, Y ) yields the two dimensional Gaussian profile of the power distribution across the optical maser beam ( regard 1, left ) .Slit methodA 2nd possibility to find the power distribution deep down a optical maser beam is equivalent to the knife border method Alternatively of a crisp edged home base, a really narrow slit is step by step moved across the optical maser beam. As a presumption for valid measurings the pause of the slit has to be smaller than the diameter of the considered beam is. Typically this gap is in the scope of some micrometers.Drawbacks of traditional methods The knife edge- both bit good as the slit method vortex merely a limited measure of beam features topical anaesthetic hot-spots , i.e. countries of fictitious character power, be undetectable due to the incorporating measurement rule. save this method is accommodate for speedy measurings to find the diameter and the class bearing profile of the considered optical maser beam.Pinhole methodThis method measures the strength of irradiance through a really minor hole in a home base ( pinhole ) confronting the optical maser beam. The strength of light go throughing through this pinhole is instead low, so that a photomul peaklier tube-shaped structure ( PMT ) has to be employ to observe the irradiance.Due to the feature of a PMT, e.g. its comparatively low signal to resound ration ( SNR ) , the collected informations are corrugated. However the strength distribution plenty be achieved within a local declaration of about 3micron by using a Gaussian tantrum. indeed by transversal s elicitning across a optical maser beam, a high declaration image of the power distribution within the beam can be gained. Hot musca volitanss are noticeable and to a greater extent elaborate characteristics can be located.CCD Camera methodDiffering from the scanning methods expound above, the use of CCD ( Charge-coupled Device ) cameras for the review of optical maser musca volitanss will lend championself a really speedy feeling of the power distribution within the beam by merely a individual shooting .The feeling of the image, e.g. the declaration, is straight person depending on size and figure of picture elements on the CCD-Sensor-chip. Typically the pel size is in the scope of 6 microns2, linked to a stainless figure of pels in the scope of 1 to 2 mega pels. By utilizing this method it must be noted, that dust and other drosss on the CCD bit can ensue in image deformation and/or misinterpretable beam characteristics. Furthermore, forethought must be taken non to destruct the CCD bit by the resultant optical maser beam.3. Beam word picture by agencies of optical roles windsTo cash in unitarys chips the better of the limited declaration of the CCD- and pinhole method, severally, a new strength profile measuring technique has been developed.Similar to the pinhole method described above, an optical part efflorescence with an aperture of a few 100 nanometer is applied to tump over up light by scanning across a optical maser beam transversely. A photomul ledelier tubing ( PMT ) attached at the out-coupling terminal of the roughage detects the gathered visible radiation during the scanning occasion so that a two-dimensional image of the strength distribution can be evaluated.By altering the sidelong place of the case tip, multiple cross sectional planes can be scanned, taking to a three-dimensional image of the strength distribution e.g. within a focussed optical maser beam.3.1 Near-field matching into the optical case Classical attacks to picture the incoup ling of visible radiation into the fibre tip fail, due to the really bantam gap of the fibre tip ( aperture ) , in the scope of some hundred nanometers, which is considerable smaller than the wavelength of the optical maser visible radiation within the focal point. Due to this really little gap, one can speak about the so called close field To derive information arising from the close field and to transport this information into the far field, it is necessary to transform the evanescent Fieldss into propagating Fieldss. This could be done by seting a Centre of dispersing, in this instance the really narrow tip, into the close field. Thereby dipole- rounds are generated at the boundary of this tip by agencies of the evanescent field ( invention 2 ) .With I0= vacuity permittivity =unit sender = wave transmitter 2/ I I= angular frequence of the oscillation and utilizing the dipole-momentWith a=diameter of the gap Ir=relative permittivityThe succeeding(prenominal) footings for the far field ( R & lt & lt I/2 ) and the close field ( R & gt & gt I/2 ) can be derived, and one gets not-propagating every bit good as propagating field constituentsRespectivelyThe propagating field is straight linked to the close field and can be detected afar. The simulation of the electrical field generated by a plane moving continue of 1 V/m, heading to a fibre tip, is shown in Fig 3. Based on an optical power of 10mW dispersed on an country of 1 square micrometer, ( which is a instead large country ) , one get by utilizingwithI= ocular Intensity S=Pointing vector v=velocity E0=electrical field D0= electric initiation parsimoniousness n= index of refraction ( here air 1.0 ) I0= vacuity permittivity ( rounded 10-11 F/m )= 1010 V/mIt has to be noted that the field within the tip is exponentially diminishing ( preserve to formula N ) and by this merely a really little portion of the generated electrical field, in this illustration 5*1010 V/m straight at the tip, c an be detected at the out matching terminal of the fiber by the PMT.3.2 ApparatusThe PI P-854.00 piezoelectric actuator used in this apparatus has a displacement scope of 25Am for all unspoiled placement and can besides be pre-positioned by a micrometer-screw more approximately. The sensing of gathered visible radiation is done by a Hamamatsu H5784 type photomultiplier tubing ( PMT ) with an E5776 FC type adapter attached ( Figure 4 )A fibre holder mounted to the piezoelectric actuator is used to take and protect the optical fiber tip during the scanning procedure. later on elaboration ( x10 by transimpedance amplifier phase ) and filtering of the gathered strength informations it is converted into digital informations by agencies of a National Instruments 6025e PCI interface card.3.3 type tip productionFor this activity the optical fiber type used to bring forth the tips is a individual manner fiber manufactured by Newport. Its cut-off wavelength is optimised for the usage of o ptical masers in the wavelength scope of 400nm -500nm.There are two possibilities to develop the conic tip of an optical fiber harmonizing to the petitions of a Scaning close field optical lithography ( SNOL ) fibre tipThe optical fiber tip can be realised byPulling methodsChemical etchingDuring the pulling process the optical fiber is preloaded before it is heated up locally with a CO2-laser or a heating-film and forced to run. As the thaw procedure begins, the dragger stretches the optical fiber with extra force apart, so that the fiber is tapert branch and eventually cryings itself bring forthing two tips.The belongingss of the optical fiber tips produced this manner go on the different parametric quantities of the puller as for illustration the brief force.Normally, all pulled SNOL-tips show a really thin geometry ( e.g. Figure 4, left ) .This is particularly disadvantageous for the considered application, because this thin construction tends to flex and hover. More stri ct fibers, which have tips with higher cone shift, are more suited Furthermore, these fibers tips feature a really little transmittal of 10-5 to 10-6 due to the long form of the tip, in which the visible radiation is evanescent ( exponentially diminishing ) . larger cone angles and correspondingly a higher transmittal up to 10-2 is reached by engraved SNOL- fiber tips as used in this work.Several etching methods exist to bring forth fibre tips. The process used here is the so called tube-etching . After cleansing of the optical fiber by agencies of ethyl group alcohol, the fiber is immersed into hydrofluoric window glass. ( Figure 5 )The undermentioned chemical reactions take topographic point to the Si contained in the fiber and/or with the Ge at doped fibersTube-Etching is a farther development of the standard etching-method, invented by turner in 1984, published by Stoeckel et Al. and Lambelet et Al. 4, 5, 6 This process improves the production of swell up smoother tip surfaces. At the tube-Etching process the coating of the optical fiber is non removed before the etching procedure. Due to gravity drawn degradation of the glass merchandises, a concentration incline arises in the hydrofluoric acid, which causes convection. The convection carries the hydrofluoric acid into the pep pill zone of the tip that develops therefore conically.To forestall corrosion by hydrofluoric acid exhausts at the upper parts the fiber, a light mineral anoint was used as a dissolver to gain a better stage above the hydrofluoric acid. The temperature during the production of the tips corresponded to room temperature. By completion of the tube-etching procedure the coating of the optical fiber is removed by dichlorine methane and the tip can be metallized, go forthing a little aperture.4. ConsequencesA trial rig has been make utilizing optical fiber tips for automized three-dimensional strength profile scanning ( Figure 3 ) . With this it is mathematical to derive i nformation about the spacial power distribution within optical maser beams. Due to the little aperture of the fiber tip even really little focal parts can be profiled with high local declaration. Corresponding tract was developed to command the change of location x-y phase, every bit good as the omega actuator automatically utilizing LabviewA . Therefore, it is possible to scan across the optical maser beam tomographically, while roll uping strength informations. The information can either be displayed or visualised by agencies of the scanning package itself. Using a new developed package based on OpenGlA ,the strength informations can be displayed in a 3 dimensional image ( Figure 7 ) with the chance to bypass the way of position in all grades of freedom. Furthermore, different mathematical filters can be applied to better image quality.By scanning more transverse sectional planes, individual pieces can be shown in an overlayed manner, to detect the focal point traveling due to the sidelong alteration of the fibre tip ( Figure 8 )3. DecisionA fresh method for high decision making beam word picture particularly for little aperture optical masers has been described. With this automized 3- dimensional series scanning method it is possible to derive information about the spacial power distribution within a optical maser topographic point.4. Figures / ArtworkFigure 1 left Theoretical power distribution within a TEM 00 optical maser beam can be described by a Gaussian profile right Integrated optical maser power heedful with the knife border method as a map of the cross sectional co-ordinate ten.Figure 2 left E-field in the close field of the tip when illuminated by plane moving ridgeline with field strength of 1 V/m, polarised along the axes of the tip right Cross-sectional position of an optical fiber tip with an cone angle I? and an radius from the aperture rim R .The associated electric field lines are plotted in logarithmic mode 3 Figure 3 Principle and image of the apparatus for high deciding optical maser beam profilingFigure 4 Comparison of a fibre tip produced by the drawing method left and a fibre tip produced by etching right Figure 5 Conventional agency of the aztube- etching method Figure 6 left fictitious character tip generated by the etching method after 60min etching flash back ( delight mention to Figure 4 B ) right Fibre tip generated by the etching method after 120min etching break short ( delight mention to Figure 4 degree Celsiuss )Figure 7 Beam profile of a DVD optical pickup unit ( OPU ) gained with the fiber tip based system ( declaration 150nm )Figure 8 left 12- measure sidelong scan through a ( TEM00 ) optical maser focal point of a DVD optical pickup unit ( OPU ) gained with the fiber tip based system ( cross position ) right ( sidelong position )5 Literatures 1 Chapple, P.B. , ( 1994 ) , Beam waist and M2 measuring utilizing a finite slit, Opt. Eng. ,33, 2461-2466 2 McCal ly, R.L. , ( 1984 ) Measurement of Gaussian beam parametric quantities, Appl. Opt. , 23, 2227 3 Drezet, A. , Nasse, M.J. , Huant, S. , Woehl, J.C. , ( 2004 ) , The optical near-field of an aperture tip Europhys. Lett. , 66 ( 1 ) , 41-47 4 Stockle, R. , Fokas, C. , Deckert, V. , Zenobi, R. , Sick, B. , Hecht, B. , Wild, U.P. , ( 1999 ) , High quality near field optical investigations by tubing etching, apply physics Letters, 75 ( 2 ) , 160-2 5 Lambelet, P. , Sayah, A. , Pfeffer, M. , Philipona, C. , Marquis Weible, F. , ( 1998 ) , Chemically etched fiber tips for close field optical microscopy a procedure for smoother tips, Applied Optics, 37 ( 31 ) , 7289-7292 6 Suh, Y. , Zenobi, R. , ( 2000 ) , Improved Probes for Scaning Near Field Optical Microscopy, Advanced Materials, 12 ( 15 ) , 1139-11426.0 Keywords Near field, far field, optical maser beam, focal point, focal plane, fibre tip, pinhole, knife border7.0 TerminologyI= Intensity W/m P=Power W A=Area m2 I=Wavelength m E =Energy W/s I= Vacuum permittivity A2As4Akg-1Am-3 Ir=Relative permittivity A2As4Akg-1Am-3 =Unit vector=Wave vector 2/ It=Time s I= Angular frequence of the oscillation s-1 =Dipole-moment AAm2 a=Diameter of the gap m