Oliver W.Fa¨hnle,Hedser van Brug,and Hans J.Frankena
We present a newfinishing process that is capable of locally shaping and polishing optical surfaces of
complex shapes.Afluid jet system is used to guide a premixed slurry at pressures less than6bars to
the optical surface.We used a slurry comprising water and10%#800SiC abrasives͑21.8m͒to reduce
the surface roughness of a BK7sample from350to25nm rms and to vary the shape of a polished sample
BK7,maintaining its surface roughness of1.6nm rms,thereby proving both the shaping and polishing
possibilities of the presented method.©1998Optical Society of America
OCIS codes:240.5450,220.1250,240.6700.
1.Introduction
Increasing requirements for aspherical optical com-
ponents͑e.g.,for lithography͒together with grow-
ingfields of application͑e.g.,conformal optics͒
result in a strong need for opticalfinishing methods
that can be applied locally to polish complex shaped
aspheres in brittle materials͑e.g.,glass͒.Today
aspherical optical surfaces are usuallyfinished by
computer-controlled polishing with subaperture
pads.1Computer-controlled polishing applies the
traditionalfinishing process of loose abrasive load-
controlled polishing.2This is a three-body process
in which abrasive particles͑suspended in afluid͒
are pressed against the optical surface by use of a
deformable polishing tool͑pad͒,and material is re-
moved by a chemomechanical process.Alterna-
tively,minimizing the contact area between pad
and surface,elastic emission machining3͑EEM͒is a float polishing process4in which the tool isfloating
on the liquid layer containing the abrasive parti-
cles.The thickness of this liquid layer amounts to
a multiple of the diameter of the abrasive particles,5
and the process’s determining parameters include
the hydraulic pressure generated by the tool and
the kinetic energy of the abrasives.Whereas in
EEM the tool is not pressing the abrasives directly onto the surface,the tool is abandoned entirely in magnetorheologicalfinishing.6Afluid containing magnetic-sensitive particles mixed with polishing compound is locally stiffened by a magneticfield and in this way is used for local polishing and shap-ing.On the other hand,it is possible to cut glass by use of abrasive slurry jets͑ASJ’s͒,in which a stream of premixed slurry is entrained and guided to the surface by a nozzle.ASJ systems can be divided into low-pressure͑70–500bar͒and high-pressure͑500–5400bar͒systems.7
2.Fluid Jet Polishing
In this paper we report a novel subaperture polishing and shaping process,fluid jet polishing͑FJP,patent pending͒,that resembles the two-body process em-ployed in an ASJ system to guide a premixed stream of slurry to the surface at pressures comparable with those of EEM orfloat polishing͑0.5–6bars͒.Exper-iments were performed to demonstrate the process of shaping and polishing of optical components by FJP. Figure1shows a sketch of the experimental setup.
A cylindrical nozzle wasfixed within a lapping ma-chine above aflat piece of glass at a certain radial distance r with respect to the vertical axis of rotation f of the workpiece.The removal rate and polishing process depend on the concentration,the size and kind of abrasive particles,the kind offluid,the pres-sure of the premixed slurry,the machining time,the kind of workpiece material,and the geometry,rela-tive position,and orientation of the nozzle with re-spect to the surface.
For the experiments the following parameters were chosen.The premixed slurry was water con-taining10%#800SiC grinding compound͑21.8m͒. The cylindrical nozzles had a length of1in.͑1in.ϭ2.54cm͒and a diameter ofϭ0.84mm and were
O.W.Fa¨hnle is with the TNO Institute of Applied Physics,P.O.
Box155,NL-2600AD Delft,The Netherlands͑e-mail,fahnle@
optica.tn.tudelft.nl͒.H.van Brug and H.J.Frankena are with
the Delft University of Technology,Laboratory of Applied Physics,
Optics Research Group,Lorentzweg1,NL-2628CJ Delft,The
Netherlands͑email for H.van Brug,brug@optica.tn.tudelft.nl͒.
Received23February1998;revised manuscript received3June
1998.
0003-6935͞98͞286771-03$15.00͞0
©1998Optical Society of America
1October1998͞Vol.37,No.28͞APPLIED OPTICS6771
positioned vertically ͑ϭ0°͒above the workpiece at different radial positions.If the workpiece was not rotating ͑ϭ0Hz ͒during machining,this resulted in circular pits in the surface.Two flat samples of glass ͑BK7,diameter 100mm ͒were processed.One was prepared in advance by loose abrasive grinding with #800SiC abrasives,resulting in a surface roughness of 350nm rms,and another was polished to a surface roughness of 1.6nm rms.For both samples we investigated the dependence of the depths and the widths of the circular pits on the pressure p of the slurry ͑Figs.2and 3͒and on the machining time t m ͑Figs.4and 5͒.Surface rough-ness was measured at the bottom of the generated pits with a commercial Alpha Step Stylus Pro-filometer for the ground sample and a commercial phase-stepping interferometer for the polished sample.For both the machining process showed an offset pressure of 1bar,below which the surface was unaltered.For p Ͼ1bar,the depth of the pits increased with p and above 3bars reached an ap-proximately linear inclination of 6m ͞bar in 15min for the ground sample ͑Fig.2͒and 1.2m ͞bar in 10min for the polished sample ͑Fig.3͒while ,for both samples,was approximately constant.The ratio ͞between the machined width of the pit and the used nozzle diameter was measured for ϭ0Hz,ϭ2mm,p ϭ6bars,ϭ0°,and t m ϭ5min and was found to be approximately 3for the ground sample ͑ϭ0.84mm ͒.For the polished sample,͞was measured for the same parameters for
three different nozzle diameters of 0.41,0.84,and 1.54mm and was found to be constant within the measuring accuracy of Ϯ0.1,yielding 3.1,3.2,and 3.1,respectively.The depth for both samples increased linearly with t m ͑with ϳ1m ͞min for the ground and ϳ0.3m ͞min for the polished sample ͒.The lower material-removal rates for the polished sample can be explained by its lower surface rough-ness.For the polished sample the dependence of the width on the standoff distance was measured and found to be constant,ϭ2.7mm Ϯ0.1
mm,
Fig.1.Schematic diagram of the setup and definition of param-
eters.
Fig.2.Dependence on the applied pressure p of the width and the depth of the circular depressions that were machined into the ground BK7sample at ϭ0Hz,ϭ2mm,ϭ0°,and t m ϭ15
min.Fig.3.Dependence on the applied pressure p of the width and the depth of the circular depressions that were machined into the polished BK7sample at ϭ0Hz,ϭ1mm,ϭ0°,and t m ϭ10
min.
Fig.4.Dependence on the machining time t m of the width and the depth of the circular depressions that were machined into the polished BK7sample at ϭ0Hz,ϭ2mm,ϭ0°,and p ϭ5
bars.
Fig.5.Dependence on the machining time t m of the width and the depth of the circular depressions that were machined into the polished BK7sample at ϭ0Hz,ϭ1mm,ϭ0°,and p ϭ5.5bars.At t m ϭ3min,the stream was accidentally mixed with air resulting in an increased wear and a dull surface of 160nm rms.
6772
APPLIED OPTICS ͞Vol.37,No.28͞1October 1998
within the range 1mm ϽϽ10mm ͑with p ϭ5.5bars,ϭ0Hz,ϭ0°,and t m ϭ5.5min ͒.On the ground sample the surface roughness within the machined pits ranged between 80and 25nm rms ͑ϭ0Hz,ϭ2mm,ϭ0°,and p ϭ5bars,t m ϭ15min ͒,whereas the polished sample always maintained its specular appearance with a surface roughness of 1.6nm rms.Finally,the polished sample of BK7was rotated with 1Hz and was machined at three radial positions at three different pressures of 5,3,and 2bars.The parameters used were ϭ2mm,ϭ0°,and t m ϭ60min.Figure 6shows a cross section of the machined surface,and Table 1shows the corresponding measured values.The surface roughness ranged between 1.8nm rms at 2bars and 1.5nm rms at 5bars,and the width of the machined grooves was constant at ϭ3mm.Thanks to the low pressures applied,the inside of the nozzles were hardly worn.͑The diameter of one nozzle increased after 20h of machining from 840to 845m.͒
3.Conclusions
In conclusion,we have presented a new polishing pro-cess,fluid jet polishing ͑FJP ͒,for optical surfaces in brittle materials;FJP resembles the two-body process employed in an ASJ system to guide a premixed
stream of slurry to the surface at pressures compara-ble with those in EEM or float polishing ͑0.5–6bars ͒.We applied relatively coarse abrasive particles ͑SiC,21.8m ͒and were able to reduce the surface rough-ness of a previously ground glass surface from 350to 25nm rms and to shape a previously polished glass surface without increasing its surface roughness of 1.6nm rms.In the experiments presented the machin-ing was stopped when the generated pits had a depth of 30m for the ground sample and 3m for the polished sample.There was no indication of a limit to removal that could be achieved without degrading the polish.Material removal is caused by the abrasives used ͑the surfaces remained unaltered when only wa-ter was used ͒.In addition,we found an offset pres-sure below which the surfaces stayed unaltered.Taking into account the fact that when coarse grinding abrasives were applied it was possible to decrease the surface roughness of the ground sample and to shape the polished sample without increasing its surface roughness,we infer that the material is removed in a kinetic ductile manner.Since FJP employs a fluid for machining,no tool wear occurs,and the tool is cooling and removing debris in process.Thanks to the low pressures applied,FJP does not make high demands on the necessary ASJ system.As the dimension of the polishing area depends on the nozzle diameter and the process is not sensitive to variations in the standoff distance,FJP can be applied for locally polishing and shaping complex shaped surfaces as required,e.g.,for conformal optics.Therefore FJP is a consequent step further from the mechanical contact between tool and surface usually entailed by optical fabrication tech-niques.Thanks to the low pressures and the control-lable removal process,FJP is a promising finishing method.Further investigations concerning influ-ences of the kind,size,and concentration of the abra-sives used together with a theoretical process analysis are currently being carried out.
We are indebted to R.Partosoebroto for his valu-able assistance in setting up and performing the ex-periments.
References
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.Fig.6.Surface profile of the polished sample,containing three grooves machined at 5,3,and 2bars with ϭ1Hz,ϭ0°,ϭ2mm,and t m ϭ60min.
Table 1.Depth and Surface Roughness of the Machined Grooves in
BK7͑Starting Roughness,1.6nm rms ͒Shown in Fig.6
p ͑bars ͒r ͑mm ͒͑nm ͒Rms Roughness
͑nm ͒
526133 1.531416 1.62
8
11
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1October 1998͞Vol.37,No.28͞APPLIED OPTICS
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