Fourier domain optical coherence tomography with an 800µm diameter axicon lens for long-depth-range probing

Transcription

1 Fourier domin opticl coherence tomogrphy with n 800µm dimeter xicon lens for long-depth-rnge probing Kye-Sung Lee 1, Chuck Koehler, Eric G. Johnson, nd Jnnick P. Rollnd College of Optics nd Photonics: CREOL & FPCE, University of Centrl Florid, 4000 Centrl Florid Blvd., Orlndo FL ABSTRACT Recently, Fourier domin opticl coherence tomogrphy (FDOCT) hs ttrcted much ttention due to the significntly improved sensitivity nd imging speed compred to time domin OCT. The lrge depth of focus is necessry to imge long-depth-rnge smple with constnt trnsverse resolution in FDOCT where dynmic focusing is not considered. Under such imging scheme, n xicon lens cn be used insted of conventionl focusing lens in the smple rm of OCT to chieve both high lterl resolution nd long depth of focus simultneously. In this study, 800µm dimeter xicon lens ws fbricted on silic wfer. We incorported the fbricted xicon lens into the smple rm of our FD OCT system nd investigted the lterl resolution over long depth rnge, compred to the sme FD OCT system using conventionl lens. Keywords: Fourier-domin opticl coherence tomogrphy, opticl coherence tomogrphy, xicon lens 1. Introduction Opticl coherence tomogrphy (OCT) is highly sensitive biomedicl imging technique tht enbles high resolution, cross-sectionl imging in biologicl tissues nd other turbid mterils. 1 High xil resolution of OCT is relized by use of brodbnd light source wheres the lterl resolution is determined by the numericl perture of the focusing lens. Although lrge numericl perture of conventionl focusing lens in the smple rm of OCT enbles high lterl resolution imging, smll numericl perture is required to chieve lrge depth of focus tht llows mking constnt trnsverse resolution imge over long depth rnge. To overcome this limittion, n xicon lens ws recently designed nd incorported into the smple rm of n interferometer to chieve both high lterl resolution nd lrge depth of focus simultneously 2 nd dynmic focusing lenses were used to mintin high trnsverse resolution over long depth rnge. 3 Recently, Fourier domin opticl coherence tomogrphy (FD OCT) hs ttrcted significnt interest becuse of its improved sensitivity nd imging speed when compred to time domin OCT (TD OCT). 4 Axil nd lterl resolutions of FD OCT re lso determined by the source coherence length nd the numericl perture of the focusing lens respectively just like in TD OCT. An xicon lens cn be used s focusing lens in the smple rm of FD OCT to chieve high lterl resolution nd constnt intensity over long depth rnge of imging becuse xicon lenses re opticl elements tht produce long, nrrow focl line long the opticl xis insted of the usul focus point of conventionl lens. 5 In Section 2, we compre theoreticlly the impcts of n xicon lens with tht of conventionl lens to the depth of focus nd the lterl resolution over the depth. In Section 3 we demonstrted the fbriction of 800µm dimeter xicon lens nd FD OCT used in the experiment. In Section 4, we showed the focusing chrcteristics nd OCT imge qulity of n xicon lens nd conventionl lens. 1 kslee@creol.ucf.edu; Phone: ; Fx: Coherence Domin Opticl Methods nd Opticl Coherence Tomogrphy in Biomedicine X edited by Vlery V. Tuchin, Joseph A. Iztt, Jmes G. Fujimoto, Proc. of SPIE Vol , (2006) /06/$15 doi: / Proc. of SPIE Vol

2 2. Theory A schemtic of n xicon lens used in FD OCT s focusing lens is shown in Fig. 1. α β k Fig.1. Schemtic of n xicon lens The depth of focus (DOF) ZD is defined s the distnce from the xicon pex to the geometricl shdow for full-fcet illumintion nd given for smll ngle α of the xicon lens s d Z =, D (1) 2( n 1)α where d denotes the dimeter of the collimted incident bem on the xicon lens nd n is the refrctive index of the xicon lens. The trnsverse intensity distribution creted by collimted incident bem pssing through n xicon lens is described by the first order Bessel function. The centrl lobe size of the first order Bessel is given by ρ 0 = π sin λ β, (2) where is the centrl wvelength of the incident bem nd Gis the bem devition ngle with respect to the opticl xis of the xicon lens, shown in Fig. 1, which cn be clculted s function of the xicon ngle Gs β = α α sin 1 ( n sin ). (3) The centrl lobe size ρ is constnt in the DOF 0 ZD becuse the bem devition ngle is lso constnt within the geometricl shdow s shown in Fig.1. This property yields constnt lterl resolution within DOF in OCT. On the other hnd, conventionl lens such s sphericl lens mkes different bem intensity profile width w ccording to the devition from the nominl focus plne s shown in Fig. 2. Proc. of SPIE Vol

3 kvm ~ Ÿ ~ u ˆ G m Šœš Fig.2. Schemtic of sphericl lens The full width x of the bem profile t nominl focus plne by conventionl lens is given by x = 4λ π f d, (4) where f is the effective focl length of the conventionl lens, nd d is the dimeter of the collimted incident bem. The DOF shown in Fig.2 cn be described s Eq. (5) in cse of lens hving smll numericl perture, w f DOF 2. (5) d Therefore, high lterl resolution cn be chieved round nominl focus plne by conventionl lens however the other lterl resolutions t devited plnes from the focl plne get worse due to the incresed dimeter of the bem. 3. Axicon Fbriction nd Experiment Method We designed nd fbricted n 800µm dimeter xicon lens on silic wfer. An xicon phse msk pttern ws written into Polymethyl-methcrylte (PMMA) on n E-bem mchine, nd then used in stepper s phse msk to crete n nlog xicon profile on silic wfer. It ws then developed nd etched into the silic wfer. The fbricted xicon picture is shown in Fig. 3 (). We lso nlyzed the etched xicon pttern on the silic wfer with the Zygo interferometer to generte polynomil curve-fitting coefficients from the fbricted nlog profile dt. Then the curvefitting dt ws plced into n opticl system design softwre, Zemx, to perform simultions to get bem profiles shown in Fig 3 (b), (c), nd (d) over the DOF. The fbricted xicon lens hs bout 3 degree xicon ngle nd the corresponding DOF Z D nd centrl lobe width ρ of bem profile re given by 15mm nd 13µm with Eq. (1) nd Eq. 0 (2). Proc. of SPIE Vol

4 9. J967E () (b) 9. J95JE E (c) (d) Fig.3. () SEM picture of the fbricted xicon, (b) Bem pttern t the strt of the depth-of-focus (c) Bem pttern t the middle of the depth-of-focus (d) Bem pttern t the end of the depth-of-focus The fbricted xicon lens ws incorported into the smple rm of Fourier domin OCT to test its performnce. The schemtic digrm of the system is shown in Figure 4. The FD-OCT system consists of brod bndwidth (120nm t full-width-t-hlf-mximum centered t 800nm) Titnium:Spphire lser nd commercil spectrometer with CCD rry nd 80/20 fiber coupler which mkes two rms of the interferometer. The 80% bem from the coupler is collimted nd then incident on the xicon lens. So the light is focused on the smple fter propgting through the lens. The other 20% bem is reflected by mirror through the Fourier domin opticl dely line in the reference rm whose min function is to control the overll dispersion in the system. 6 Proc. of SPIE Vol

5 z (ω) k ˆŠ ŒG n ˆ Ž z œ ŠŒ ω j œ Œ j ˆ ŽGsŒ š k š Œ š G j Œ t hÿ Š G sœ š zˆ Œ z ŒŠ Œ Œ jjkgˆ ˆ Fig. 4. Schemtic digrm of Fourier-domin OCT with n xicon s focusing lens in the smple rm. 4. Results We first mesured the intensity profile of the collimted bem before the focusing lens s shown in Fig. 5 nd then mesured the bem profile fter pssing through either the xicon lens or the sphericl lens s function of the distnce from the lens. The collimted bem dimeter ws round 800µm nd its profile ws shped s Gussin s shown in Fig Bem Profile of Femto lser ]. u. [ y t i s n e t I n Bem Size [µm] Fig. 5. Intensity profile of the collimted bem before the xicon lens. The collimted bem ws incident on 800µ dimeter xicon lens nd the bem profiles t different plnes from the xicon pex were mesured s shown in Fig. 6 (). The geometricl depth of focus ZD ws lmost 15mm which ws Proc. of SPIE Vol

6 estimted in section 3, nd the centrl lobe widths were constnt of vlue 13µ over the depth of focus. The pek intensity ws decresed s the mesured plne is moved wy from the xicon lens becuse the incident bem is Gussin in shpe. For comprison with conventionl lens we lso mesured the bem profiles t different distnces from the focl plnes of 8mm focl length sphericl lens s shown in Fig. 6 (b). The full width x of the bem profile t the nominl focus plne by the sphericl lens ws computed to be 10µ Gwith Eq. (4). Although the bem width ws better thn tht of the xicon lens t the focl plne, the bem width t 2mm wy from the focl plne ws found to be round 200µ bsed on Eq. (5) which is highly brodened compred to tht of the xicon lens. Nrn IIz d Intinilty () (b) Fig. 6. Bem intensity profile fter () n xicon lens (b) sphericl lens ccording to the distnce from the lens 15µ { ˆ š Œ šœg zšˆ G Fig. 7. A 15µm slit We imged 15µm slit shown in Fig 7 t three different distnces from the lens for both n xicon lens nd sphericl lens. Fig. 8 (), (c), (e) re the OCT imges of the slit t the distnces of 7mm, 5mm, nd 3mm wy from n xicon lens. Fig. 8 (b), (d), (f) re the OCT imges of the slit t the distnces of 10mm, 8mm, nd 6mm wy from sphericl lens. The 15µm slit ws imged t 7mm, 5mm, nd 3mm s shown in Fig. 8 (), (c), (e) while the sphericl lens hs n bility to imge it only t focl plne s shown in Fig. 8 (d). Results show good correltion between the shrpness of the point spred function nd the bility to resolve the slit hole clerly observed in Figs. 8(), (c), (d), nd (e). Proc. of SPIE Vol

7 ' o!!!!!!!!!!! ' '40 '60 ' '00 '25 ISO Distnce [urn] Distnce [urn] () (b) rn Disnce [urn] (c) (d) 11.0 ' o 2b 4b ob b 'do ido i4o 160 lo Distnce [urn] Disnc [urn] (e) (f) Fig.8. OCT imges of 15µm slit trnsverslly scnned t distnces of () 7mm, (c) 5mm, nd (e) 3mm wy from n xicon lens nd (b) 10mm, (d) 8mm, nd (f) 6mm wy from sphericl lens 5. Conclusion nd Discussion In this pper, we demonstrted the impct of n xicon lens on the lterl resolution in OCT compred to conventionl lens. An xicon lens incorported in OCT cn render constnt high lterl resolution over long depths of focus Proc. of SPIE Vol

8 compred with the conventionl lens which cn imge only short depth rnges with high lterl resolution. We showed in the pper the superior bility of the system with n xicon to resolve 15µm slit cross 4mm DOF compred to sphericl lens. However higher signl to noise rtio imges cn be obtined round the focl plne of sphericl lens in OCT becuse the full bem power is sent to smll re round the focl point while the OCT dpted with n xicon lens yields much of the power being distributed over long depth of focus. Thus n xicon lens coupled with higher power source cn yield both superior resolution nd good signl to noise rtio. Acknowledgements This reserch ws supported in prt by the Florid Photonics Center of Excellence, the NSF IGERT progrm, the UCF Presidentil Instrumenttion Inititive, nd the DARPA & NSF PTAP progrm. References 1. D. Hung, E. A. Swnson, C. P. Lin, J. S. Schumn, W. G. Stinson, W. Chng, M. R. Hee, T. Flotte, K. Gregory, C. A. Pulifito, nd J. G. Fujimoto, Opticl coherence tomogrphy, Science 254, (1991). 2. Zhihu Ding, Hongwu Ren, Yonghu Zho, J. Sturt Nelson, nd Zhongping Chen, High-resolution opticl coherence tomogrphy over lrge depth rnge with n xicon lens, Optics Letters 27, (2002). 3. W. Drexler, U. Morgner, F. X. Kortner, C. Pitris, S. S. Bopprt, X. D. Li, E. P. Ippen, nd J. G. Fujimoto, In vivo ultrhigh-resolution opticl coherence tomogrphy, Optics Letters 24, (1999). 4. R.Leitgeb, C. K. Hitzenberger, nd A. F. Fercher, Performnce of fourier domin vs. time domin opticl coherence tomogrphy, Optics Express 11, (2003), 5. Ann Burvll, Ktrzyn Kolcz, Zbigniew Jroszewicz, nd Ari T. Friberg, Simple lens xicon, Applied Optics 43, (2004). 6. Kye-Sung Lee, A. Ceyhun Akcy, Tony Delemos, Eric Clrkson, nd Jnnick P. Rollnd, Dispersion control with Fourier-domin opticl dely line in fiber-optic imging interferometer, Appl. Opt. 44, (2005). Proc. of SPIE Vol