January 15, 1992 / Vol. 17, No. 2 / OPTICS LETTERS

High-speed optical coherence domain reflectometry

E. A. Swanson

Lincoln Laboratory, Massachusetts Institute of Technology, 244 Wood Street, Lexington, Massachusetts 02173-9108

D. Huang, M. R. Hee, and J. G. Fujimoto

Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

C. P. Lin and C. A. Puliafito

Laser Research Laboratory, New England Eye Center, Tufts University School of Medicine, Boston, Massachusetts 02111

Received September 26, 1991

We describe a high-speed optical coherence domain reflectometer. Scan speeds of 40 mm/s are achieved with

a dynamic range of >90 dB and a spatial resolution of 17 gm. Two applications are presented: the noninva-

sive measurement of anterior eye structure in a rabbitinterelement spacing in a multielement lens.

Optical coherence domain reflectometry (OCDR)has emerged as an attractive method for obtaininghigh-spatial-resolution (<10 Am) reflectance mea-surements of fiber-optic, integrated-optic, and bio-logical structures.` 8 Most previous OCDR systemshave achieved high sensitivity and dynamic rangeby using narrow-band heterodyne detection bypiezoelectric path-length modulation and lock-inamplifier techniques. For many applications,present techniques are unacceptably slow. Highspeed is essential for applications to biological andmedical diagnostics, where measurements must beperformed rapidly compared with movement of aliving subject. Speed is also important for processor assembly line diagnostics, as well as in any appli-cation that requires high data acquisition rates. Byusing a high-speed linear translation stage, OCDRmeasurements can be performed at high speedswith significant simplifications in system design.In this Letter we demonstrate a new OCDR tech-nique for high-speed optical ranging measurements.Speeds approaching 40 mm/s with a dynamic rangeof >90 dB and a resolution of -17 ,4m are achieved.

The basic schematic of the OCDR setup is shownin Fig. 1. A broadband light source such as a super-luminescent diode (SLD) is coupled to a fiber-opticMichelson interferometer. One arm of the inter-ferometer leads to the sample of interest, and theother leads to a reference mirror. Fiber-optic andintegrated-optic samples can be directly attached tothe sample arm fiber. For bulk-optical or biologicalsamples, a probe module is used to direct the beamonto the sample and to collect the reflected signal.To aid in sample alignment, we use a fiber-coupledvisible aiming laser. The reflected light beams aredetected at the photodetector. They coherently in-terfere only when the sample and the reference pathlengths are equal to within the source coherencelength. Heterodyne detection is performed by tak-

in vivo and the characterization of reflections and

ing advantage of the direct Doppler frequency shiftthat results from the uniform high-speed scan of thereference path length. Recording the interferencesignal magnitude as a function of the reference mir-ror position profiles the reflectance of the sample.

The spatial resolution, scan speed, and dynamicrange of the OCDR system can be designed and opti-mized according to the desired application. Ifwe assume a Gaussian line shape, the round-tripFWHM of the source intensity coherence envelope(AL) is related to the FWHM wavelength bandwidth(AA) of the source by

AL = ln(2) -A 2(1)

where A is the optical wavelength. The Doppler fre-quency shift (fD) that results from a uniform velocityscan (v) and the FWHM power bandwidth of the sig-nal (Af ) are given by

fD = 2 (2)A

Af = ln(4) 4 v- (3)

The optimum bandwidth for the bandpass filter isapproximately 2Af Wider bandwidths will decreasesensitivity, whereas smaller bandwidths will de-crease resolution.

Figure 2 shows the measured power spectral den-sity of the light source (Laser Diode Model LDT-3201-SMF) and its FWHM value of 17.4 nm.Figure 3 show the measured interference signal atthe photodetector and the demodulated envelopeobtained by scanning the reference mirror at37.5 mm/s. This speed was used in all the mea-surements reported here. Good agreement betweenthe measured coherence envelope FWHM of 17.2 Atmand the theoretical value of 16.7 ,m [Eq. (1)] was

0146-9592/92/020151-03$5.00/0 © 1992 Optical Society of America

151

152 OPTICS LETTERS / Vol. 17, No. 2 / January 15, 1992

REFERENCE

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Fig. 1. Schematic of the high-speed OCDR system.analog-to-digital converter.

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obtained. The Doppler frequency shift was mea-sured at -93 kHz, also in good agreement withEq. (2).

The approximate signal-to-noise ratio (SNR) inthe bandpass filter if we assume shot-noise-limiteddetection is given by

surements of anterior chamber depth are useful indetermining the refractive power of intraocular lensimplants'" as well as in the diagnosis of angle-closure glaucoma." Although ultrasound ranginghas been used extensively in ophthalmology,'2 it re-quires direct physical contact of the eye by an ultra-sound transducer or saline immersion of the eye inorder to facilitate the transmission of acoustic wavesinto the eye. In contrast, OCDR is a noncontactdiagnostic.

In order to demonstrate the clinical potential ofhigh-speed OCDR, measurements of eye structurewere performed in an anesthetized Dutch pigmentedrabbit in vivo. Figures 5(a) and 5(b) show an OCDRmeasurement of the anterior eye along with a sketchof rabbit eye structure for comparison.' The out-put from the sample fiber was collimated with a20-mm focal-length lens and subsequently focusedinto the anterior eye by using a 150-mm focal-lengthlens. The depth of field was measured to be -3 mmFWHM. The scan speed was 37.5 mm/s, and thescan range was 7.5 mm. Reflection peaks at theair-cornea, cornea-aqueous, and aqueous-lens inter-faces as well as scattering from within the corneacan be seen. An echo of the air-cornea interface isalso visible. It results from the imperfect SLD facetcoating mentioned above. The refractive indices ofthe cornea (n = 1.376) and the aqueous (n = 1.336)(Ref. 13) were used to determine the measuredcornea thickness of 396 4m and the anterior cham-ber depth of 2.40 mm. The optical power incidenton the sample eye was 29.4 MW far below the Ameri-

SNR= 1 lqPs R42 hp' NEB (4)

where f7 is the detector quantum efficiency, P, is thesample signal power, h is Planck's constant, v is theoptical frequency, R, is the sample reflectivity, andNEB is the noise-equivalent bandwidth of the band-pass filter. As stated above, NEB 2Af UsingEq. (4) and assuming that a SNR = 2 is the limit ofsensitivity, we can calculate the minimum resolv-able reflectivity. Figure 4 shows the measuredsystem sensitivity. A neutral-density 2.2 filter fol-lowed by a mirror was used to create a sample with areflectivity of -44 dB. More than 90 dB of dynamicrange is obtained at a power of 29.4 ,W. The side-lobes are echoes that are due to imperfect antireflec-tion coating in the SLD. Their location correlateswell with the optical path length of the SLD. Thefilter NEB was 9.1 kHz, which is 2.3 times the sig-nal FWHM of Af = 3.9 kHz. With the above dataand r7 = 0.8, the theoretical dynamic range is calcu-lated to be 91 dB, indicating that the system hasshot-noise-limited sensitivity.

OCDR provides a noncontact, high-sensitivity,high-resolution technique for optical ranging. Highscan speeds are especially relevant for medical andbiological diagnostic applications. One ophthalmo-logic problem of special interest is the noninvasivemeasurement of anterior eye dimensions. Measure-ments of corneal thickness or incision depth areuseful for keratorefractive surgeries 9, while mea-

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January 15, 1992 / Vol. 17, No. 2 / OPTICS LETTERS 153

(a)

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-30

-40

-50

-60

-70

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Fig. 5. (a) Measurement of the anterior of a rabbiteye performed in vivo. (b) Schematic of eye structure.

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Fig. 6. (a) Measurement of multielement laser-diode colli-mating lens. (b) An x ray of the lens.

can National Standards Institute safe ocular expo-sure standard of 200 4ttW at 830 nm.

In addition to diagnostics in biological systems,high-speed OCDR has numerous applications fornoncontact diagnostics of precision mechanical andoptical systems as well as for process control andmonitoring in manufacturing. One example is thetesting of optical components. We investigated thefeedback characteristics of a commercially availablemultielement laser-diode collimating lens (FujinonLSR-35B) using high-speed OCDR scanning. Diag-nostics of multielement collimating lenses are impor-tant because semiconductor laser diodes are knownto be sensitive to feedback. In addition, OCDR pro-vides a direct measurement of interelement dis-

tances in the lens assembly. The Fujinon lens wasused to collimate the light output from the samplearm. Figures 6(a) and 6(b) show the measured re-flectance profile along with an x ray of the lens.The lens has five elements and an optical path lengthof approximately 23 mm. The maximum scanlength of the reference mirror is 20 mm. Figure 6(a)was constructed by using a composite of two scans.The spacing between lenses, the lens thickness, andthe reflection coefficients of the various interfacescan be seen. Although the reflection amplitudesare sensitive to alignment, a maximum feedback ofapproximately -70 dB arises from the last surfaceof the last lens.

In summary, we have demonstrated an OCDR sys-tem that achieves high-speed, high-sensitivity, andhigh-resolution measurements by performing hetero-dyne detection at the Doppler frequency shift thatresults from the high-speed scan. This new tech-nique makes possible a variety of new OCDR appli-cations, including diagnostics in medicine andbiological systems and the precision monitoringof mechanical systems in process control andmanufacturing.

We thank J. S. Schuman, W G. Stinson, T. F.Deutsch, and R. Birngruber for helpful commentsand suggestions. This research was supported byNational Institutes of Health grant RO1-GM35459-06, U.S. Office of Naval Research Medical Free Elec-tron Laser Program grant N00014-91-C-0084, theJohnson and Johnson Foundation, and the U.S. De-partment of the Air Force.

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(Mosby, St. Louis, Mo., 1980).

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