Method For Characterisation Of Laser Beams Engineering Essay
Particularly in the field of optical maser direct authorship lithography it is necessary to derive exact cognition about the form and features of the used optical maser beam, whilst a homogenous power distribution is an indispensable parametric quantity for qualitatively good and consistent consequences in surface structuring.
Several “ classical ” methods in word picture of optical maser beams are already applied, such as:
Knife border method
CCD camera method
These techniques will be shortly explained in subdivision 2.
In fact, a optical maser beam ( “ standard TEM00 ” ) fades bit by bit, due to its Gaussian profile ( Figure 1 ) . Therefore it is necessary to find what is known as the Centre of the beam.
Chapple [ 1 ] describes the strength profile I ( x, y ) as follows:
where ten and Y are the cross Cartesian co-ordinates of any point, x0 and y0 mark the Centre of the beam and R is the 1/e2 radius. This definition is frequently used in theory.
McCally [ 2 ] defines the Gaussian distribution of the strength I ( x, y ) by agencies of the beam radius R belonging to the 1/e irradiance shown in Figure 1 ( left ) .
The emitted power of the optical maser beginning can be determined by incorporating the irradiance ( I ) , severally the optical strength I ( x, y ) , over the affected cross sectional country A:
2. Techniques of beam profiling
Knife border method
To derive information about the power distribution across a cross sectional plane of a optical maser beam, the use of the knife border method is the most simple attack. Thereby, a crisp edged home base, e.g. a razor blade, is ab initio covering the whole optical maser beam in forepart of a power metre for continues wave radiation or an energy metre for pulse operation. By precise line wise supplanting of the knife border mounted on a additive motion phase, more and more optical maser radiation reaches the detector unit, until the whole beam is covering the feeling country.
Figure 1 ( right ) shows the consequence of this measurement process: Due to the motion of the knife edge the value of the detected power additions steadily. The derived function of this power map P ( x, Y ) yields the two dimensional Gaussian profile of the power distribution across the optical maser beam ( Figure 1, left ) .
A 2nd possibility to find the power distribution within a optical maser beam is similar 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 stipulation for valid measurings the gap 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- every bit good as the slit method offer merely a limited measure of beam features: local “ hot-spots ” , i.e. countries of superior power, are undetectable due to the incorporating measurement rule. However this method is suited for speedy measurings to find the diameter and the class strength profile of the considered optical maser beam.
This method measures the strength of irradiance through a really little 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 photomultiplier tubing ( PMT ) has to be used 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 can be achieved within a local declaration of about 3micron by using a Gaussian tantrum. Therefore by transversal scanning 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 more elaborate characteristics can be located.
CCD Camera method
Differing from the scanning methods described above, the use of CCD ( Charge-coupled Device ) cameras for the review of optical maser musca volitanss will give a really speedy feeling of the power distribution within the beam by merely a “ individual shooting ” .
The quality of the image, e.g. the declaration, is straight depending on size and figure of pels on the CCD-Sensor-chip. Typically the pel size is in the scope of 6 microns2, linked to a entire 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, attention must be taken non to destruct the CCD bit by the incident optical maser beam.
3. Beam word picture by agencies of optical fibers tips
To get 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 fiber tip with an aperture of a few 100 nanometer is applied to roll up light by scanning across a optical maser beam transversely. A photomultiplier tubing ( PMT ) attached at the out-coupling terminal of the fiber detects the gathered visible radiation during the scanning procedure so that a two-dimensional image of the strength distribution can be evaluated.
By altering the sidelong place of the fibre 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 fiber
“ Classical ” attacks to picture the incoupling of visible radiation into the fibre tip fail, due to the really little 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- oscillations are generated at the boundary of this tip by agencies of the evanescent field ( Figure 2 ) .
With: Iµ0= vacuity permittivity ; =unit vector ; = wave vector: 2/ I» ; I‰= angular frequence of the oscillation and utilizing the dipole-moment:
With: a=diameter of the gap ; Iµr=relative permittivity
The undermentioned 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 constituents:
The 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 ridge 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 utilizing:
I= Optical Intensity ; S=Pointing vector ; v=velocity ; E0=electrical field ; D0= electric initiation denseness ; n= index of refraction ( here air: 1.0 ) ; Iµ0= vacuity permittivity ( rounded: 10-11 F/m )
= 1010 V/m
It has to be noted that the field within the tip is exponentially diminishing ( refer 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, can be detected at the out matching terminal of the fiber by the PMT.
The PI P-854.00 piezoelectric actuator used in this apparatus has a displacement scope of 25Aµm for all right 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. After 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 Fibre tip production
For this application 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 optical masers in the wavelength scope of 400nm -500nm.
There are two possibilities to develop the conelike tip of an optical fiber harmonizing to the petitions of a Scaning close field optical lithography ( SNOL ) – fibre tip:
The optical fiber tip can be realised by:
During 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 puller stretches the optical fiber with extra force apart, so that the fiber is tapert first and eventually cryings itself bring forthing two tips.
The belongingss of the optical fiber tips produced this manner vary on the different parametric quantities of the puller – as for illustration the drawing 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 stiff fibers, which have tips with higher cone angle, 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 alcohol, the fiber is immersed into hydrofluoric acid. ( Figure 5 )
The undermentioned chemical reactions take topographic point to the Si contained in the fiber – and/or with the Ge at doped fibers:
Tube-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 well 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 debasement of the glass merchandises, a concentration incline arises in the hydrofluoric acid, which causes convection. The convection carries the hydrofluoric acid into the upper zone of the tip that develops therefore conically.
To forestall corrosion by hydrofluoric acid exhausts at the upper parts the fiber, a light mineral oil was used as a dissolver to gain a separate 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.
A trial rig has been build utilizing optical fiber tips for automized three-dimensional strength profile scanning ( Figure 3 ) . With this it is possible to derive information 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 package was developed to command the traveling 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 revolve 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 )
A fresh method for high deciding 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 / Artwork
Figure 1: [ left ] Theoretical power distribution within a TEM 00 optical maser beam can be described by a Gaussian profile [ right ] Integrated optical maser power measured 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 ridge 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 profiling
Figure 4: Comparison of a fibre tip produced by the drawing method [ left ] and a fibre tip produced by etching [ right ]
Figure 5: Conventional representation of the aˆztube- etching method ”
Figure 6: [ left ] Fibre tip generated by the etching method after 60min etching clip ( delight mention to Figure 4 “ B ” ) [ right ] Fibre tip generated by the etching method after 120min etching clip ( 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 )
[ 1 ] Chapple, P.B. , ( 1994 ) , Beam waist and M2 measuring utilizing a finite slit, Opt. Eng. ,
[ 2 ] McCally, 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, Applied 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 ) , 1139-1142
6.0 Keywords: Near field, far field, optical maser beam, focal point, focal plane, fibre tip, pinhole, knife border
I= Intensity [ W/m ]
P=Power [ W ]
A=Area [ m^2 ]
I»=Wavelength [ m ]
E=Energy [ W/s ]
Iµ= Vacuum permittivity [ A2A·s4A·kg-1A·m-3 ]
Iµr=Relative permittivity [ A2A·s4A·kg-1A·m-3 ]
=Wave vector: 2/ I»
t=Time [ s ]
I‰= Angular frequence of the oscillation [ s-1 ]
=Dipole-moment [ AA·m2 ]
a=Diameter of the gap [ m ]