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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 1, JANUARY 2001
Spectral Domain Interferometry for OCDR Using Non-Gaussian Broad-Band Sources E. D. J. Smith, S. C. Moore, N. Wada, Member, IEEE, W. Chujo, Member, IEEE, and D. D. Sampson
Abstract—A static Michelson interferometer coupled with optical spectral analysis has been used to perform optical coherencedomain reflectometry (OCDR). Compensation of both the coherence sidelobes caused by a non-Gaussian source spectrum and the coherence broadening caused by dispersion is shown. Reflections indistinguishable using an EDFA source and conventional scanning OCDR are clearly distinguished with this method. Index Terms—Interferometry, optical coherence domain reflectometry, reflectometry, spectral domain analysis, spectral domain interferometry.
I. INTRODUCTION
O
PTICAL coherence-domain reflectometry (OCDR) systems using a scanning Michelson interferometer and a broad-band source are well understood, and commercially available. The closely related technique of optical coherence tomography (OCT) [1] is an application of OCDR to measurement and imaging within three-dimensional objects. In OCDR and OCT, the time-domain response due to an isolated broad-band reflector is given by the autocorrelation, or coherence function, of the broad-band light, which, in the absence of dispersion, is the Fourier transform of the power spectral density. The shape of the source spectrum thus determines the shape of the time-domain response, and the spectral width determines the time-domain resolution. Hence, OCDR and OCT systems generally use sources with near-Gaussian lineshapes such as light-emitting diodes and superluminescent diodes, although these still may have significant coherence function sidelobes [2], and it is desirable to have as wide a source bandwidth as possible. In addition, any dispersion imbalance within the interferometer produces a broadening of the time-domain response. The use of ultrabroad-band, superfluorescent sources in OCDR systems is very attractive in order to achieve either ultrahigh spatial resolution or the broad-band coverage required for components used in current and future-generation wavelength-division multiplexing systems. Compensation for or determination of dispersion over such broad bandwidths is also highly desirable. However, the large and highly structured sidelobes of the coherence functions of such sources
preclude their use in conventional OCDR systems. The use of superfluorescent sources with non-Gaussian lineshapes in standard OCT systems is also very attractive due to the high optical powers typically available, but significant degradation in image quality is caused by the high sidelobe levels of their coherence functions [3]. Most OCDR systems only record the envelope of the interferometer output, but if the interference fringes are accurately recorded, then it is possible to perform a Fourier transform to measure and/or numerically correct for both source shape and dispersion in the spectral domain [4], [5]. A recent work has used this method to calculate interferograms corresponding to a narrow tunable Gaussian source [6]. Such phase-sensitive interferogram measurements and Fourier transforms are the basis of Fourier-transform spectrometers, and accurate dispersion measurements of many devices have been performed in this manner, as reported in [7], for example. However, the mirror position in such a system needs to be known very accurately, and a second interferometer using a laser is generally required for this. In this letter, we describe a novel scheme to perform OCDR without interferometer scanning, and we demonstrate numerical tailoring of the effective source spectral shape, significantly reducing the corresponding time-domain sidelobes. This is done by directly recording the spectral components of the interferogram in the optical frequency domain, by employing an optical spectrum analyzer at the output of a static interferometer. Such measurement is similar to the recording of spectral fringes that occurs in spectral holography, as demonstrated with ultrashort pulses in [8]. Our method is insensitive to reflections beyond the scan range of the system, unlike the work of [9], and also the scan range limit is four times larger than that in [9]. We demonstrate that an otherwise obscured small reflection next to a large reflection can be revealed by appropriately weighting the data to reshape the source spectra. We also correct numerically for the unbalanced dispersion in the interferometer. Such sidelobe reduction addresses the main disadvantage of using broad-band, high-power erbium-doped fiber amplifiers (EDFAs) and other superfluorescent sources in OCDR and OCT systems. II. EXPERIMENTAL SETUP
Manuscript received June 23, 2000; revised September 19, 2000. The work of E. D. J. Smith was supported by the COE visiting researcher program for his stay at CRL, and by the ARC Large Grant scheme. E. D. J. Smith, S. C. Moore, and D. D. Sampson are with the Optical and Biomedical Engineering Laboratory, Department of Electrical and Electronic Engineering, The University of Western Australia, Nedlands WA 6907, Australia (e-mail:
[email protected]). N. Wada and W. Chujo are with the Communications Research Laboratory (CRL), Ministry of Posts and Telecommunications, 4-2-1, Nukui-kitamachi, Koganei-shi, Tokyo 184-8795, Japan. Publisher Item Identifier S 1041-1135(01)00503-1.
The experimental configuration is shown in Fig. 1. The optical fiber interferometer used dispersion-shifted fiber (DSF) throughout. A broad-band 3-dB coupler was used in the interferometer, and the reference arm consisted of a translation stage and a piezoelectrically adjustable reference mirror. The sample arm contained a polarization controller, and both the reference and sample arms had FC/APC connections where patch cords could be inserted to match the path lengths as necessary. The
1041–1135/01$10.00 © 2001 IEEE
SMITH et al.: SPECTRAL DOMAIN INTERFEROMETRY FOR OCDR
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Fig. 1. Interferometer setup. BBS: broad-band source; DFB: monitor laser; TXS: translation stage; PZM: mirror with piezoelectric translation; DUT: device under test; OSA: optical spectrum analyzer.
broad-band source and the optical spectrum analyzer were connected via the 90% ports of the 90 : 10 couplers, and the 10% ports were used in monitoring the interferometer phase drift. A single-mode DFB laser at 1572.3 nm was used for this purpose, together with cascaded bandpass filters to exclude the broad-band light before detection. The broad-band source was the amplified spontaneous emission (ASE) from an 80-nm bandwidth tellurite-based EDFA (NEL, Model No. FA1500QLT). A computer was used to control and/or capture data from the translation stage, the piezoelectric reference mirror, the spectrum analyzer, and the photodetector. The translation stage allowed basic positioning and testing, and comparison with the normal scanning OCDR trace. The effective fiber length that could be analyzed at one time was 3.3 mm, which was limited by the coarse 0.5-nm spectral resolution employed, and assuming a fiber group refractive index of 1.45. The piezoelectric reference mirror (PZM) allowed fine modulation of the interferometer path length difference. III. PROCEDURE If a sample under test is known to produce no reflections until or beyond a certain depth, then it is possible to obtain an OCDR trace directly from a spectral measurement of the interferometer output [9]. However, the effective scan depth is reduced by a factor of four from that determined by the spectral resolution, the reference mirror position with respect to the sample is prescribed [9], and the required spectral resolution may be impractical for long samples. In order to calculate the time-domain response unambiguously over the full depth range determined by the spectral resolution, without any restrictions on the sample extent, two spectral measurements of the interferometer output are required. These measurements must be in quadrature, and it is also necessary to have a measurement of the power spectrum when no interference occurs. This may be obtained by separately measuring the power spectra from only the sample arm and only the reference arm. As a first step toward the ideal quadrature system, the measurement procedure consisted of first moving the PZM over two fringes and measuring the maximum and minimum power level at each wavelength. The PZM was then fixed and a single measurement of the power spectrum was made, whilst monitoring the photodetector for any phase drift. By comparing the single measurement of the power spectrum with the maximum and minimum spectra, the relative phase (modulo ) between the light from the reference and sample arms at each wavelength can be estimated. The phase ambiguities were resolved by assuming that the phase direction, as
Fig. 2. Reconstructed OCDR traces for a strong and weak reflector, using broad-band EDFA source. Strong reflector at 0 m, weak reflector at 90 m.
0
a function of wavelength, did not change abruptly. This phase , where is the optical frequency, estimate, denoted by was then corrected for any phase drift during the spectral scan. Using the measured power spectra from the reference and , respectively, along and sample arms, , the time-domain response is with the phase estimate . given by the Fourier transform of Compensation for source spectral shape is performed by multiplication with the ratio of the ideal Gaussian power spectrum , when the sample is a mirror. to The unbalanced dispersion within the interferometer was with a mirror as the sample and calculated by measuring fitting it to a cubic polynomial about the center frequency. This function was subtracted from later measurements of when compensating for this dispersion. Any dispersion within the sample was not accounted for here, although we note that it is straightforward to do so with this spectral data [4]. IV. RESULTS Shown in Fig. 2 is an enlargement of the time-domain reconstruction for a bulk-optic sample consisting of a strong and a weak reflector. The axis has been labeled in terms of the distance within the device under test, assuming unity refractive index. The upper two traces show a broad response due to the non-Gaussian ASE spectrum of the 80-nm bandwidth EDFA, which is inset in Fig. 2. The lower two traces show that reshaping the source spectrum to a Gaussian (6-THz FWHM) successfully narrows this response, thus clearly revealing the presence of the weak reflector. The effect of the unbalanced dispersion in the system can also be seen. The width of the peak is 22 m (FWHM) after balancing dispersion, or approximately 15 m in fiber. The uppermost trace, using the original spectrum and including dispersion, accurately corresponds to that measured by typical interferometer scanning and envelope detection (not shown). Fig. 3 shows two of the traces from Fig. 2 on a logarithmic scale over the 5-mm range in air corresponding to the spectral resolution. After reshaping the source spectrum and balancing the interferometer dispersion, Fig. 3 shows that the reconstructed trace has a much better dynamic range than the uncompensated OCDR trace near a strong reflector, and a similar dynamic range toward the limits of the response. The
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 1, JANUARY 2001
V. DISCUSSION AND CONCLUSION
Fig. 3. Reconstructed OCDR traces for a strong and weak reflector, logarithmic scale. Strong reflector at 0 mm, weak reflector at 0.09 mm.
0
Several issues relating to the experiment are noted here. The effective scanning distance of 5 mm in air was limited by the 0.5-nm spectral resolution used in these preliminary experiments. Using a spectral resolution of 0.05 or 0.015 nm, which are available with commercial optical spectrum analyzers, would allow effective scanning distances of the order of 50 or 170 mm, respectively. The laser wavelength used in these experiments was restricted by the available equipment to be within the broad-band source range, and its effect on the spectral data was removed by a 0.7-nm notch filter. The ideal laser wavelength would have been just outside the broad-band source range, avoiding the need for such filtering. Finally, the ambiguities in the relative spectral phase estimate could be resolved by quadrature phase measurement. We have demonstrated that OCDR analysis of a device under test can be performed by optical spectral measurements combined with a static Michelson interferometer, eliminating the need for accurate interferometer scanning. Nonideal spectral shape of a broad-band EDFA source and dispersion have been compensated for, revealing features that would otherwise be obscured, and giving an effective resolution in fiber of 15 m (FWHM). Compensation for nonideal source spectra potentially allows the use of very broad-band high-power sources both in OCDR and OCT systems, with the potential to increase the resolution and wavelength span available over typical OCDR systems, and improve OCT image quality. REFERENCES
Fig. 4. Reconstructed OCDR traces for a Sagnac mirror, using broad-band EDFA source.
primary reason for the limit on dynamic range after spectral reshaping to an ideal Gaussian lineshape is the noise in the esti, which arises from both the spectral measurements mate of and the photodetector monitoring the phase drift. It is expected that with appropriate modifications to the interferometer and spectrometer design, substantial improvements could be made in these respects. For example, simply minimizing the number of components and the fiber lengths in the interferometer, together with suitable thermal and mechanical isolation, would reduce the phase drift significantly. Also, reduced phase drift together with a modified spectrometer would have permitted more accurate quadrature phase measurement. Fig. 4 shows the reconstructed OCDR traces when a broad-band Sagnac mirror is used as the sample, both with and without compensation of the source lineshape and dispersion. Also shown in Fig. 4 is the limit expected if the relative spectral was ideal. The floor of this function is phase estimate determined by the extent of the reshaped Gaussian spectrum, which had approximately 40-nm FWHM, centered at 1575 nm.
[1] D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science, vol. 254, pp. 1178–1181, Nov. 1991. [2] S. R. Chinn and E. A. Swanson, “Blindness limitations in optical coherence domain reflectometry,” Electron. Lett., vol. 29, pp. 2025–2027, Nov. 1993. [3] B. E. Bouma, L. E. Nelson, G. J. Tearney, D. J. Jones, M. E. Brezinski, and J. G. Fujimoto, “Optical coherence tomographic imaging of human tissue at 1.55 m and 1.81 m using Er- and Tm-doped fiber sources,” J. Biomed. Opt., vol. 3, pp. 76–79, Jan. 1998. [4] E. Brinkmeyer and R. Ulrich, “High resolution OCDR in dispersive waveguides,” Electron. Lett., vol. 26, pp. 413–414, Mar. 1990. [5] A. Kohlhaas, C. Frömchen, and E. Brinkmeyer, “High-resolution OCDR for testing integrated-optical waveguides: Dispersion-corrupted experimental data corrected by a numerical algorithm,” J. Lightwave Technol., vol. 9, pp. 1493–1502, Nov. 1991. [6] S. Mechels, K. Takada, and K. Okamoto, “Optical low-coherence reflectometer for measuring WDM components,” IEEE Photon. Technol. Lett., vol. 11, pp. 857–859, July 1999. [7] K. Takada, T. Kitagawa, K. Hattori, M. Yamada, M. Horiguchi, and R. K. Hickernell, “Direct dispersion measurement of highly-erbium-doped optical amplifiers using a low coherence reflectometer coupled with dispersive spectroscopy,” Electron. Lett., vol. 28, pp. 1889–1891, Sept. 1992. [8] A. M. Weiner, D. E. Leaird, D. H. Reitze, and E. G. Paek, “Spectral holography of shaped femtosecond pulses,” Opt. Lett., vol. 17, pp. 224–226, Feb. 1992. [9] A. F. Fercher, C. K. Hitzenberger, and S. Y. El-Zaiat, “Measurement of intraocular distances by backscattering spectral interferometry,” Opt. Commun., vol. 117, pp. 43–48, May 1995.
