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Optics Communications 000 (2000) 000±000 www.elsevier.com/locate/optcom

A thermal light source technique for optical coherence tomography

A.F. Fercher a,*, C.K. Hitzenberger a, M. Sticker a, E. Moreno-Barriuso b, R. Leitgeb a, W. Drexler a, H. Sattmann a

a Institute of Medical Physics, University of Vienna, Waehringer Strasse 13, A-1090 Vienna, Austria b Instituto de Optica ``Daza de Valdes'' (CSIC), Serrano 121, E-28006 Madrid, Spain Received 24 January 2000; received in revised form 10 August 2000; accepted 9 September 2000

Abstract A new technique for optical coherence tomography imaging with spatially low-coherent light sources is presented. In this technique the low coherence interferometry (LCI) depth-scan is performed by the image of the light source, and, therefore, simultaneously by a multitude of mutually incoherent LCI channels, to increase the probe beam power. Thermal light sources have the advantage of extremely low time-coherence with coherence lengths in the 1 lm range. The performance of a tungsten halogen lamp with a thermal spectrum and a xenon arc lamp with broadened spectral lines superimposed on a thermal continuum are compared. Ó 2000 Elsevier Science B.V. All rights reserved.

PACS: 42.25.Hz; 42.25.Kb; 42.30-d; 42.72.Bj Keywords: Optical coherence tomography; Optical tomography; Low space coherence interferometry; Thermal light sources

1. Introduction and high space-coherence. But in general light sources can also oscillate in the transverse multi- The criteria for the use of a particular light mode. The multimode laser and spontaneously source in optical coherence tomography (OCT) emitting light sources like the thermal cavity, the are: coherence properties, power, and wavelength ®lament lamp, the halogen lamp, the electric arc of the emitted radiation. On this background two lamp, and the gas discharge lamp are examples for types of light sources can be distinguished. The the latter ones. These light sources have low space- light sources commonly used in OCT are oscillat- coherence and can have extremely low time-co- ing in their transverse monomode. Corresponding herence. In this work we present a technique which light sources like superluminescent diodes (SLDs) uses besides low time-coherence also low space- and femtosecond lasers have low time-coherence coherence and compare the performance of a tungsten halogen lamp and a xenon arc lamp in OCT. Standard OCT images are synthesized from low * Corresponding author. Tel.: +43-1-4277-60702; fax: +43-1- (time-) coherence interferometry (LCI) signals 4277-9607. obtained by the so-called depth- or z-scan [1]. E-mail address: [email protected] (A.F. Fercher). During the depth-scan the interferometer probe

0030-4018/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S0030-4018(00)00986-X 2 A.F. Fercher et al. / Optics Communications 000 (2000) 000±000 beam is directed onto the object and the back- OCT depth resolution has also been enhanced scattered light interferes with the interferometer by using multiple sources. For example, Schmitt reference beam. The complex degree of the time- et al. used two light emitting diodes at peak coherence plays the role of a longitudinal or depth wavelengths of 1240 and 1300 nm to synthesize a point spread function, its full width at half maxi- source with a short coherence length [9]. The mum (FWHM) de®nes depth resolution in OCT outputs of the two diodes were combined in a [2]. For Gaussian light, OCT depth resolution is single-mode coupler and yielded a depth resolution of 14.4 lm compared to 20.8 lm with only one of cs 2ln2 k2 l the sources. In a similar technique, Baumgartner et Dz ˆ c ˆ ˆ c : 1† 2 p Dk 2 al. used a synthesized light source made up of two SLDs with mean wavelengths at k ˆ 830 nm and sc is the FWHM coherence time, lc is the FWHM k ˆ 855 nm and obtained a depth resolution of 7 coherence length, k is the mean wavelength, and lm [10]. Dk is the spectral wavelength width. Bouma et al. [11] used a Kerr-lens mode-locked

High depth resolution in OCT demands for Ti:Al2O3 oscillator at a mean wavelength of large bandwidth of the light source. First imple- k ˆ 820 nm and obtained a depth resolution of 3.7 mentations of the OCT principle used SLDs at lm at an output power of 400 mW. Later, Bouma k ˆ 830 nm. These diodes yield coherence lengths et al. [12] obtained a resolution of 6 lm using an in the 10-lm range and a beam power in the 10- all-solid-state Kerr-lens mode-locked Cr:forsterite mW range. A depth resolution Dz of 17 lminair laser operating at k ˆ 1:28 lm. More recently, has been reported by Huang et al. [1]. Their small Drexler et al. [13] presented an ultrahigh resolution size and ease of operation made SLDs to the most OCT system using a Kerr-lens modulated femto- popular light sources in OCT. To improve depth second Ti:sapphire laser emitting pulses with a resolution in OCT as well as in optical low co- depth resolution of approximately 1 lmatk ˆ 800 herence re¯ectometry, a technique closely related nm and 1 mW power of the probe beam. to OCT, single quantum well (AlGa)As SLDs [3] with a tandem-type structure with two sections have been developed. Using these SLDs a depth 2. Thermal light coherence properties and optical resolution of 4.5 lm has been obtained in OCT [4]. coherence tomography technique In another approach to high depth resolution, spontaneously emitting light sources were used. Thermal light sources emit blackbody radiation For example, Youngquist et al. [5] use a laser di- with a spectral distribution given by PlanckÕs law ode operating at k ˆ 830 nm and biased well below for cavity radiation. The complex degree of time- threshold. A resolution of 10 lm has been ob- coherence of blackbody radiation is obtained by a tained at a power of 20 lW. Using the same Fourier transform of PlanckÕs spectrum as technique we have carried out ®rst measurements X1 of the axial eye length using LCI and have ob- c t†ˆ90p4 n ‡ ib†4; tained a resolution in the 10-lm range [6]. Clivaz nˆ1 et al. [7], for example, used the ¯uorescence light where b ˆ t kBT =h† with kB denoting BoltzmannÕs from a Ti±sapphire crystal pumped by an argon constant, T the temperature, and h denoting laser. At a mean wavelength of k ˆ 780 nm a beam PlanckÕs constant divided by 2p. Here the degree power of 4.8 lW has been obtained with a depth of time-coherence jjc s† decreases with a relaxation resolution of 1.9 lm. To improve depth resolution time of the order of sc ˆ h= kBT † [14]. The corre- in high-resolution re¯ectometry and range-gating sponding coherence length can be estimated by imaging Liu et al. [8] used a 4-W multiline Ar-ion laser pumped Rhodamine 590 dye jet and obtained ch csc ˆ : 2† a depth resolution of 2.9 lm at 9 mW beam power. kBT A.F. Fercher et al. / Optics Communications 000 (2000) 000±000 3

In case of tungsten halogen lamps with typical plex degree of space-coherence at the interferom- temperatures of T ˆ 3300 K the coherence time is eter entrance pupil of 15 sc ˆ 2:3  10 s, and the corresponding coher- 2J1 k DrNA† ence length is csc ˆ 0:69 lm. This extremely short cLS Dr†ˆ ; 3† coherence length of thermal light is of great in- k DrNA terest for OCT. Many thermal light sources, where J1 is the ®rst-order Bessel function, Dr is the however, do not emit true blackbody radiation. distance between the two points in the source im-

Their radiation may be ®ltered by the surrounding age. NA ˆ q=D is the paraxial numerical aperture media and by optical components transmitting the of the interferometer, q is the radius of the inter- emitted light. Furthermore, the responsivity of the ferometer aperture stop and D its distance from photodetector is decisive for the spectrum e€ec- the entrance pupil. Obviously, Eq. (3) corresponds tively used in OCT. to the magnitude of the Airy pattern associated In addition, to provide interference between with the interferometer. OCT reference beam and OCT probe beam space- As a consequence an entrance pupil diameter d coherence is important. The complex degree of larger than the central part of the Airy pattern spectral space-coherence of blackbody radiation associated with the system encloses a multitude of varies with sin k Dr†= k Dr† [14], where Dr is the coherently illuminated source image spots. Points distance between the two points on the source separated by more than the radius of the ®rst dark surface; k ˆ 2p=k is the wave number. This poor ring of the associated Airy pattern apart can be space-coherence is the main obstacle for the use of considered mutually incoherent. This distance can thermal light in coherent optics. be de®ned as transverse coherence-distance dc ±we In the interferometric scheme used in this work use the term ``transverse coherence-distance'' in the source is imaged by the condenser on to the preference to ``transverse coherence-length'' to entrance pupil of the interferometer, see Fig. 1. In avoid possible uncertainties between the expres- this case the illumination of the exit pupil of the sions ``coherence-length'' and ``transverse coher- condenser is e€ectively spatially incoherent. The ence-length:'' van Cittert±Zernike theorem [15] predicts a com- dc ˆ 3:832=kNA: 4† These coherent source image spots of diameter

dc can be assumed mutually non-correlated. Note that dc equals RayleighÕs de®nition of the resolving power of an image-forming instrument [15]. In the technique described in this communica- tion the primary light source is imaged onto the interferometer entrance pupil. The interferometer entrance pupil represents the secondary light source of the interferometer. The LCI depth-scan is performed by the image of the secondary light source, formed by the focusing lens L at the object, see Fig. 1. This image consists of a multitude of coherently illuminated but mutually incoherent source image spots. Therefore, the OCT depth- scan is simultaneously performed by many mutu- Fig. 1. OCT device with a thermal light source. The LCI scan is ally incoherent ``LCI channels.'' The light of these performed by moving the object. AS, aperture stop; BS, beam channels overlaps during propagation but sepa- splitter; CO, condenser; EP, entrance pupil of the interferom- eter; L, focusing lens; MF, multimode ®ber; OB, object; PD, rates at planes optically conjugate to the interfer- photodetector; PO, photodetector optics; RM, reference mir- ometer entrance pupil. This technique has the ror; XL, xenon arc lamp. potential of multiplexing many LCI depth-scans. 4 A.F. Fercher et al. / Optics Communications 000 (2000) 000±000

Each LCI channel can be detected by a separate 6000=3300†2  3:3 will be obtained. Including the photodetector positioned at a plane conjugate to energy ¯ux of the spectral lines might increase the the interferometer entrance pupil by using, for coherently emitted energy ¯ux by a factor of ap- example, a photodetector array. This property also proximately 2. Nevertheless, this low coherent indicates a close relationship between the OCT energy ¯ux demonstrates the dramatic lack of technique presented here and the ``coherence ra- space-coherence of thermal radiation. Therefore, dar'' technique. However, the coherence radar we combine several LCI channels to increase the technique does not allow the application of het- probe beam power. erodyne techniques [16]. Finally, in OCT a beam of reasonable power, for medical applications in the range of some lW 3. Methods and results to some mW, must be provided. In this respect a light emitting source can be characterized by its In this work the LCI depth-scan is performed radiance, L, the energy ¯ux per area A and the by the image of the secondary light source. Hence, solid angle X of the radiation collected by the all mutually incoherent LCI channels are com- condenser. In air the energy ¯ux of a Lambertian bined to one LCI depth-scan beam thus increasing source with surface area A is U ˆ LAX [17]. For a the probe beam power. As explained above the plane circular Lambertian thermal light source of secondary light source consists of image spots of radius d=2 emitting radiation isotropically the van diameter dc which are mutually non-correlated. Cittert±Zernike theorem predicts a degree of co- Hence, this technique is equivalent to simulta- herence larger than 0.5 within an angle of neously scanning the object by mutually non-co- w 6 2:215= kd=2† or within the solid coherence herent foci. Each coherent source image spot 2 angle XC ˆ w p=4; k is the mean wave number. represents a separate LCI channel which contrib- Hence, the light energy ¯ux emitted into the solid utes to the detector signal. As long as these con- coherence angle is: tributions have the same phase they increase the detector signal. Phase variations can be caused by 2 UC ˆ LAXC ˆ 0:307Lk : 5† a tilted light remitting interface. Regularly re- ¯ected light will then miss the photodetector optics The tungsten halogen lamp used in this work aperture and reduce the corresponding depth-scan operates at a distribution temperature of about signal. In case of scattering objects, where scatterer T ˆ 3242 K. From the Stefan±Boltzmann law the from di€erent depths contribute to the signal, total radiated intensity emitted from a body at speckle might occur and reduce the signal. Due to 4 temperature T is Itotal ˆ grT , where g is the the extremely short coherence length of the light emissivity, that varies for di€erent materials be- used here, however, this e€ect may not play a tween zero and unity, and r ˆ 5:67  108 dominant role. Wm2 K4. Hence, the maximal radiance is Besides demonstrating the increase of coherent Lmax ˆ Itotal=2p and the maximal energy ¯ux co- ¯ux we compare the performance of a tungsten herently emitted at k ˆ 0:85 lm is approximately halogen low voltage lamp (OSRAM 64625) and a 0.22 lW. The xenon arc lamp is a plasma lamp. It xenon short arc lamp (OSRAM XBO 75) as light has broadened spectral lines superimposed to a sources in OCT. The nominal size of the tungsten thermal spectrum. The lamp used in this work ®lament is 4:2  2:3mm2. The xenon arc has a size operates at a temperature of about T ˆ 6000 K. of 0:25  0:5mm2; its maximal intensity is emitted Hence, even if we neglect the energy ¯ux of the near the cathode. The object is a pellicle beam spectral lines, the xenon arc lamp is superior to the splitter with a geometrical thickness of approxi- tungsten halogen lamp in terms of the maximal mately 2 lm. Fig. 1 depicts the optical scheme of energy ¯ux. Because of WienÕs displacement law the OCT interferometer. The depth-scan is per- and Eq. (5) an improvement in the coherently formed by moving the object with the help of a emitted energy ¯ux of the thermal spectrum of stepper motor driven stage at 10 mm s1. Con- A.F. Fercher et al. / Optics Communications 000 (2000) 000±000 5 denser CO (focal length 30 mm) forms the sec- Mod. 2001; gain set at 3  104; Doppler frequency ondary image of the light source at the entrance 22 kHz). The electric photodetector signal is re- pupil EP (diameter d ˆ 0:1 mm) of the interfer- corded with a personal computer. From the signals ometer. Lens L (focal length 50 mm) forms the recorded the envelope has been computed to gen- probe beam with the image of the entrance pupil at erate the OCT images. Pure data acquisition time unity magni®cation. This image is used to perform of a 1 mm  10 lm image was approximately 2.5 the LCI depth-scan. The aperture stop diameter is s. As the x-scan had to be started manually, total 2q ˆ 3 mm; its distance from the entrance pupil is acquisition time for one tomogram scan was ap- D ˆ 100 mm; hence, the numerical aperture of the proximately 15 min. probe beam is NA ˆ 0:015. Due to the small nu- It might be noticed, that the scheme shown in merical aperture only approximately eight mutu- Fig. 1 resembles closely to LinnikÕs interference ally non-correlated source image spots were used. microscope [17]. However, the Linnik interference Probe beam powers were 1 lW with the halogen microscope uses fringes at the object surface image lamp and 3 lW with the xenon lamp. Photode- obtained at the interferometer exit to indicate the tector optics PO forms an ``object image'' of the path di€erences between the surface of the object secondary image of the entrance pupil via the in- and the surface of the reference mirror. In the terferometer object arm at the photodetector. OCT technique described here the object depth is Similarly, a ``reference image'' of the entrance scanned in transversely adjacent positions with the pupil EP of the interferometer is formed in two image of the light source and the signal of the imaging stages via the interferometer reference photodetector at the interferometer exit is used to arm at the photodetector. Interference occurs, if synthesize an OCT image. both, the ``object image'' and the ``reference im- Fig. 2a and b depict the spectra of the two age'' are spatially matched in size and orientation lamps used in the experiments. The responsivity of within the corresponding transverse coherence- the photodetector extends from approximately 300 distance dc and the path lengths are matched to 1100 nm with a maximum at approximately 875 within the coherence-length lc. Then all coherent nm. The spectrum of the tungsten halogen lamp source image spots imaged via the object and the has been obtained from a comparison with a 1000 reference mirror ®nally overlap at the photode- W normal lamp. An Ulbricht-sphere technique tector (multimode ®ber with 50 lm core diameter was used with a double-prism monochromator coupled to silicon PIN photoreceiver New Focus, and a photomultiplier as photodetector. The lamp

Fig. 2. (a) Spectrum of the tungsten halogen low voltage lamp (OSRAM 64625), and (b) of the xenon short arc lamp (OSRAM XBO 75). 6 A.F. Fercher et al. / Optics Communications 000 (2000) 000±000 used in this work has a wavelength of maximal depth spread function has a relatively narrow peak emission at 880 nm with a FWHM bandwidth of caused by the continuum of the spectrum super- 510 nm. E€ective spectra for the photodetector imposed on a broad background generated by the signal can be obtained by multiplying the lamp line spectrum of the xenon lamp. Hence, the spectra with the spectral photodetector respon- FWHM concept to de®ne the coherence length of sivity. The e€ective mean wavelength of the halo- the xenon light cannot directly be applied and, gen lamp appears shifted towards approximately therefore, the depth resolution of 1.1 lm indicated 887 nm and the e€ective bandwidth is reduced to in Fig. 3b is somewhat questionable. about 320 nm. Hence, a depth resolution of ap- OCT images of the pellicle beam splitter are proximately 1.1 lm has been obtained. shown in Fig. 4. As described above, the depth- The spectrum of the xenon lamp is the nominal scan has been performed with the image of the spectrum as supplied by the producer of the lamp. interferometer entrance pupil. The depth coordi- It has large peaks in the wavelength range between nate is geometrical depth times group refractive 800 and 1000 nm where detector responsivity is index. However, the group refractive index is not maximal. known. There are ¯uctuations in the pellicle sur- Fig. 3 shows the real part of the mutual co- face contour which we attribute to mechanical herence function of the probe waves re¯ected at oscillations caused by infrasound. the pellicle and the reference mirror. The left part The performance of this technique can be of each signal corresponds to the wave re¯ected at characterized by its dynamic range. Dynamic the anterior surface of the pellicle. It is the auto- range DR can be de®ned by the ratio of the mean correlation of the source light and de®nes the OCT electrical signal power and the total electrical 2 2 depth spread function of the corresponding light noise power DR‰dBŠˆ10log ISignal=INoise†ˆ source. The right part of the signals shown in Fig. 10log SNR†;hereI is electric current. The theo- 3a and b represent the cross-correlations between retical signal-to-noise ratio (SNR) for shot-noise the source light and source light transmitted twice limited detection is SNR ˆ 1=4†gPR= E Df †; g is through the pellicle. Both envelopes of the halogen the detector quantum eciency, P is the probe lamp signal (Fig. 3a) are approximately Gaussian beam power, R is the sample re¯ectivity, E is the due to the approximate Gaussian spectrum of this (mean) photon energy, and Df is the noise equiv- lamp. The envelopes of the xenon lamp signal (Fig. alent bandwidth of the bandpass ®lter [18]. With a 3b) deviate from this monotonic form. Their OCT single mode of the halogen lamp we have P  0:11

Fig. 3. Real part of the mutual coherence functions of the probe waves re¯ected at the pellicle and at the reference mirror. (a) Halogen lamp, (b) xenon lamp. Dashed lines: envelopes. The left part of each envelope is the OCT depth spread function. A.F. Fercher et al. / Optics Communications 000 (2000) 000±000 7

Fig. 4. OCT images (80 depth  108 transverse pixels) of a 2 Æ 0:5 lm thick nitrocellulose membrane with a nominal phase index of refraction of 1.5. Logarithmic gray scale. Front surface on top, back surface below. (a) Halogen lamp OCT; (b) xenon lamp OCT. Note: this ®gure demonstrates the high depth resolution of thermal light OCT. The true thickness of the membranes remains uncertain within the coherence length. lW because of transmission losses in the device electrical signal has been digitally ®ltered by a (we used o€-the-shelf optics without broad-band second order Butterworth ®lter with a bandwidth anti-re¯ection coatings). The theoretical dynamic of 4 kHz. As can be seen from Eq. (4) the coherent range for shot-noise limited detection at an elec- source image spot diameter dc is inversely pro- tronic bandwidth of 100 kHz in this case is 60 dB. portional to the numerical aperture. Hence, the With eight modes we have P  1 lW and a theo- number of modes, as well as the probe beam power retical dynamic range of DR  70 dB. Using a is proportional to the square of the numerical plane mirror as sample we measured 52 dB with aperture. We obtained a dynamic range of the tungsten halogen lamp. The gap between the DR ˆ 49 dB with NA ˆ 0:016, DR ˆ 68 dB with experimental and the theoretical dynamic range is NA ˆ 0:052, and DR ˆ 73 dB with NA ˆ 0:138. primarily caused by incomplete modulation of the These data clearly demonstrate an increase of the data acquisition card (10 dB) and furthermore by dynamic range with increasing numerical aperture. dispersion and polarization problems. For exam- This con®rms the concept of the technique pre- ple, the beam splitter is not symmetric in the two sented in this paper. At larger numerical apertures, beams and generates dispersion problems. In ad- however, asymmetries in the optical components dition, polarization control is dicult in the con- become more important and hamper a corre- verging probe and reference beams and has not sponding increase of the dynamic range. been carried out. Therefore, to demonstrate the gain from the use of a multitude of spatial modes we slightly chan- ged the illumination scheme of the interferometer 4. Conclusions to enable larger numerical apertures and replaced the ®xed aperture stop (AS in Fig. 1) by an iris A technique is presented which uses spatially stop with variable diameter 2q† to adjust the incoherent light sources in the OCT depth-scan. numerical aperture. Also, to demonstrate the dy- The probe beam power is increased at the expense namics of this technique, we attenuated the light of transversal resolution. To increase the power of beam illuminating the interferometer by a 103 the probe beam the object is scanned by the image neutral density ®lter at the interferometer entrance of the entrance pupil of the low-coherence inter- pupil (EP in Fig. 1; 100 lm diameter) and used a ferometer thus combining approximately eight second low-noise ampli®er (Stanford Research mutually incoherent light beams to one LCI depth- Systems, Mod. SR 560) following the New Focus scan beam. Probe beam powers in the 1 lW range receiver, Mod. 2001. Furthermore, the ampli®ed were obtained at a transversal resolution of 100 lm. Higher probe beam powers can be obtained 8 A.F. Fercher et al. / Optics Communications 000 (2000) 000±000 with larger numerical apertures of the probing Acknowledgements beam. For example, numerical apertures of

NA ˆ 0:5 would yield probe beam powers in the 1 We gratefully acknowledge ®nancial support mW range at a transversal resolution of 100 lm from the Austrian Fonds zur Forderung der Wis- together with a corresponding improvement of the senschaftlichen Forschung (project no. 10316) and dynamic range. Improving transverse resolution, from the Jubilaumsfonds (project no. 7428) of the however, would decrease the available probing Austrian National Bank. beam power. For example, a transversal resolution of 10 lm would reduce the available probe beam power of a standard halogen lamp at N ˆ 0:5to A References about 20 lW. The technique presented here might be useful in [1] D. Huang, E.A. Swanson, C.P. Lin, J.S. Schuman, W.G. cases with reduced demands on transversal reso- Stinson, W. Chang, M.R. Hee, T. Flotte, K. Gregory, C.A. lution, like in the measurement of the thickness of Pulia®to, J.G. Fujimoto, Science 254 (1991) 1178. tissue layers and in topographic measurements. [2] A.F. Fercher, C.K. Hitzenberger, in: T. Asakura (Ed.), Due to the large available bandwidth spectral Springer Series in Optical Sciences, vol. 4, Springer, Berlin, 1999, pp. 359±389. OCT might also be a ®eld of application if reduced [3] A.T. Semenov, V.K. Batovrin, I.A. Garmash, V.R. Shid- transversal resolution can be tolerated. Another lovsky, M.V. Shramenko, S.D. Yakubovich, Electron. possible application of the technique presented Lett. 31 (1995) 314. here is parallel OCT detection with smart one- and [4] F. Lexer, C.K. Hitzenberger, W. Drexler, S. Molebny, H. two-dimensional photodetector arrays [19]. Here Sattmann, M. Sticker, A.F. Fercher, J. Mod. Opt. 46 (1999) 541. each LCI channel can be associated to one pho- [5] R.C. Youngquist, S. Carr, D.E.N. Davies, Opt. Lett. 12 todiode. The attractive feature of the technique (1987) 158. described in this communication is the fact that [6] A.F. Fercher, E. Roth, Proc. SPIE 658 (1986) 48. there is no loss of power when using many pho- [7] X. Clivaz, F. Marqis-Weible, R.P. Salathe, Electron. Lett. todiodes because each photodiode can be associ- 28 (1992) 1553. [8] H.-H. Liu, P.-H. Cheng, J. Wang, Opt. Lett. 18 (1993) 678. ated with a separate light source point. [9] J.M. Schmitt, S.L. Lee, K.M. Yung, Opt. Commun. 142 Furthermore, the limited space coherence provides (1997) 203. reduced cross-talk between the channels. Finally, [10] A. Baumgartner, C.K. Hitzenberger, H. Sattmann, W. this technique can also be used to obtain OCT Drexler, A.F. Fercher, J. Biomed. Opt. 3 (1998) 45. images with (high-power) transversal multi-mode [11] B. Bouma, G.J. Tearney, S.S. Boppart, M.R. Hee, M.E. Brezinski, J.G. Fujimoto, Opt. Lett. 20 (1995) 1486. lasers. [12] B.E. Bouma, G.J. Tearney, I.P. Bilinski, B. Golubovic, The performance of a tungsten halogen low J.G. Fujimoto, Opt. Lett. 21 (1996) 1839. voltage lamp (OSRAM 64625) and of a xenon [13] W. Drexler, U. Morgner, F.X. Kartner, C. Pitris, S.A. short arc lamp (OSRAM XBO 75) as light sources Boppart, X.D. Li, E.P. Ippen, J.G. Fujimoto, Opt. Lett. 24 in OCT have been compared in this work. A depth (1999) 1221. [14] H.P. Baltes, in: H.P. Baltes (Ed.), Inverse Source Problems resolution of 1.1 lm was obtained in both cases. in Optics, Springer, Berlin, 1978, pp. 119±154. Whereas the xenon lamp provides a higher radi- [15] M. Born, E. Wolf, Principles of Optics, Cambridge ance than the halogen lamp the broad basis of its University Press, Cambridge, 1998. depth spread function will reduce contrast in OCT [16] G. Hausler, M.W. Lindner, J. Biomed. Opt. 3 (1998) 21. images. Furthermore, its irregular shape can pro- [17] W.H. Steel, Interferometry, Cambridge University Press, Cambridge, 1983, pp. 30±31. duce artefacts in the OCT image. [18] R. Muller, Rauschen, Springer, Berlin, 1990, p. 198. [19] S. Bourquin, V. Monterosso, P. Seitz, R.P. Salathe, Opt. Lett. 25 (2000) 102.