Apr 15, 1995 - Received January 4, 1995. We describe a method for measuring modal birefringence in optical fibers. It combines an interferometric technique ...
April 15, 1995 / Vol. 20, No. 8 / OPTICS LETTERS
869
Birefringence dispersion measurement in optical fibers by wavelength scanning M. G. Shlyagin, A. V. Khomenko, and D. Tentori Centro de Investigacion ´ Cient´ıfica y de Educacion ´ Superior de Ensenada, Km. 107 Carretera, Tijuana-Ensenada, Aportado Postal 2732, Ensenada, B.C., Mexico Received January 4, 1995 We describe a method for measuring modal birefringence in optical fibers. It combines an interferometric technique with wavelength scanning and permits a precise nondestructive measurement of the birefringence along different sections of a long optical fiber. The experimental results for high-birefringence fibers, 10 and 100 m long, are presented. An accuracy of approximately 0.1% is achieved in the spectral range of 600 – 850 nm.
Polarization-maintaining fibers are used in many areas related to fiber optics, for example, coherent optical communications, integrated-optic devices, and optical sensors based on interferometric techniques. For these applications it is important to know the modal birefringence of the fiber, and for some applications it is even necessary to know how the dispersion of the modal birefringence varies along the fiber. Several methods have been developed to measure the modal birefringence of polarization-maintaining fibers. A modulation method seems to be the most advanced and widely used1 – 5 and was demonstrated with electro-optic,1 magneto-optic,1,2 and acoustooptic modulators,3 the optical Kerr effect,4 and a lateral mechanical stress.5 Recently a modification of the modulation method was reported6 in which a broadband light source was used; however, the modal birefringence was measured only for the central wavelength of the source spectrum range. A measurement accuracy of 0.7% was reported for this method. A wavelength scanning technique was also proposed for measurement of the dispersion of fiber birefringence,5,7 but this method can be applied only to short fibers. The maximum length estimated for a fiber with a 2-mm beat length5 was 20 cm. Here we present a technique that simultaneously gives (1) the high-accuracy data, (2) a broad spectral range, and (3) a local evaluation along any part of a long optical fiber. We demonstrate a measurement of the birefringence dispersion of two highbirefringence fibers of different lengths. The experimental configuration is shown in Fig. 1(a). White light emitted from a 2-W tungsten lamp passes through collimation lens L1 and polarizer P and is launched into the polarizationmaintaining fiber by microscope objective MO1. A linear polarizer is used to excite only one polarization eigenmode. Light emitted from the output end of the fiber passes microscope objective MO2 and linear polarization analyzer A and is launched into a CCD spectrograph by lens L2. A computer-controlled CCD spectrograph gives us the ability to observe an output light spectrum in real time. The CCD array detector has 1024 sensitive elements, and spectra approximately 350 nm wide can be observed. 0146-9592/95/080869-03$6.00/0
Polarization mode coupling is induced at two points along the fiber by a lateral mechanical force. At each point the fiber is placed between a polished glass plate and a polished metallic rod with a diameter of 1 mm. One of the two rods is attached to a linear translation stage to change the distance between coupling points with an accuracy of 10 mm. To maximize mode coupling, we apply a lateral force at an angle of approximately 45± to the fiber polarization axes. In our experiments we have measured the modal birefringence of the two fibers with stressinduced birefringence. The fibers, F-SPV supplied by Newport Corporation, are of the same type and have lengths of 10 and 100 m. The manufacturer specifies an operating wavelength of 633 nm and a beat length of less than 5 mm. The portion of the fiber between the coupling points can be considered as the fiber interferometer shown schematically in Fig. 1(b). Two squeezers act as beam splitters, and the light is split between the po-
Fig. 1. (a) Experimental setup: PC, personal computer; Hi-Bi, high-birefringence. (b) Schematic of the modes’ propagation. 1995 Optical Society of America
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OPTICS LETTERS / Vol. 20, No. 8 / April 15, 1995
larization eigenmodes, just as light is split in the two arms of a conventional interferometer. The light is launched into one fiber eigenmode, and an analyzer selects the light from the other eigenmode. Let us assume that the coupling coefficient is the same for both points, k sI2yI1 d1/2 , where I1 and I2 are the intensities of the launched eigenmode and the coupled polarization mode. The output spectrum can be written as ∏æ Ω ∑ 2pDnsld x , (1) I sl, xd 2ks1 2 kdI0 sld 1 1 cos l where I0 sld is the intensity spectrum of the light launched into the fiber and x is the distance between the coupling points. According to Eq. (1), the recorded spectrum is modulated owing to mode interference. The frequency of modulation is proportional to the distance x between the coupling points and does not depend on the distance from the output end of the fiber. That gives us an opportunity to perform measurements for any part of a long optical fiber. To resolve spectral oscillations, we see that the distance between coupling points must satisfy the condition
x#
lmin 2 , Dnslmin ddl
(2)
where lmin is the minimum wavelength in the scanned spectral range and dl is the spectral resolution of the spectrograph. We record three signals to calculate the modal birefringence. The spectrum I sl, x1 d is recorded first. For maximum accuracy of the modal birefringence measurement an initial distance x1 between coupling points is chosen as the maximum value that satisfies relation (2), and therefore I sl, x1 d has the maximum possible number of modulation periods. The second signal is the intensity I sl0 , xd of the spectral component for a fixed wavelength l0 from a scanning range as a function of the coupling points’ separation x. This signal is recorded as one coupling point is moved toward the second point between positions x1 and x2 . We complete the measurement procedure by recording the second spectrum I sl, x2 d with x2 . The recorded signals are then processed in a computer. The signal modulation is represented by the cosine term in Eq. (1), and the spectra I sl, x1 d and I sl, x2 d are represented in the scale of wave number, k 2pyl, as has been described previously.8 Then the phases of the signal modulation of the signals are calculated, and to increase the accuracy of calculation we include digital filtering in the signal processing after Fourier decomposition of the signals. The phases of I sl, x1 d and I sl, x2 d can be written as w1 sld
2pDnsld x1 1 2pN1 , l
(3a)
w2 sld
2pDnsld x2 1 2pN2 , l
(3b)
where N1 and N2 are unknown numbers. The value DN sN1 2 N2 d can be written as DN
fw1 sl0 d 2 w2 sl0 dg Dnsl0 d sx1 2 x2 d , 2 2p l0
(4)
where the second term on the right-hand side is the number of beat lengths of the fiber passed by the coupling point when it moves from the first position to the second. The value of this term is the period number of the recorded signal I sl0 , xd. From Eqs. (3) and (4) the fiber birefringence is
Dnsld
l sx1 2 x2 d
∑
∏ w1 sld 2 w2 sld 2 DN . 2p
(5)
According to our estimates, the accuracy of the birefringence measurement for our experimental setup is restricted mainly by the spectrograph resolution, which has an uncertainty of 0.1%. To verify this value we calculated the group refractive index as a function of the wavelength, using our experimental data for modal birefringence. The result of the calculation was compared with the experimental data for the group refractive-index difference obtained with a high-accuracy interferometric method,9 and this comparison confirmed the estimated accuracy of the method. To produce mode coupling we use mechanical squeezers that affect the fiber core through the protective plastic jacket and cladding. The diameter of the plastic jacket is 250 mm, and the length of the fiber in which the coupling is produced has the same order of magnitude. Therefore we estimate that the minimum beat length can be measured as 0.5 mm. This value is close to the minimum beat length that can be practically achieved with birefringent fibers.10 The maximum beat length that can be measured depends on the travel range of the translation stage
Fig. 2. Modal birefringence of the 100-m fiber measured three times at the same position along the fiber.
April 15, 1995 / Vol. 20, No. 8 / OPTICS LETTERS
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which was not controlled in our experiments, and thus the measured birefringence variation could be attributed partially to the temperature instability. The modal birefringence of two different fibers of the same type differs by approximately 14%. We assume that these two fibers were pulled from different preforms. In conclusion, we have presented a method for measuring modal birefringence in fibers with polarization beat lengths in the range of 0.5 –50 mm. The accuracy of the measurement is 0.1% and is currently limited by the resolution of our spectrograph. The method permits the testing of long birefringent fibers, and the modal birefringence can be measured at any position within the sample. We thank S. Miridonov for fruitful discussions. The research of M. Shlyagin and A. Khomenko was supported by Direcci´on Adjunta de Investigaci´on Cient´ıfica /Consejo Nacional de Ciencia y Tecnologia. Fig. 3. Modal birefringence of the 100-m fiber measured at different points along the fiber and the modal birefringence of the 10-m fiber (filled diamonds).
and is currently limited to 5 cm for our experimental setup. The modal birefringence of the fibers has been measured at different positions along two fibers with lengths of 100 and 10 m. The resolution of the spectrometer is ,0.7 nm, the initial distance between the coupling points is ,35 cm, and the maximum travel range of a precision linear translation stage used to move the coupling point is 50 mm. To verify the high reproducibility of the method, we measured the birefringence three times at a distance of 15 m from the output end of the 100-m fiber. These data are shown in Fig. 2. The birefringence data versus wavelength measured at different positions along the 100-m fiber and the 10-m fiber are shown in Fig. 3. The birefringence of the 100-m fiber changes within 0.5% at different points along the fiber length. The fiber birefringence depends on the temperature,11
References 1. A. Simon and R. Ulrich, Appl. Phys. Lett. 31, 517 (1977). 2. S. C. Rashleigh, Opt. Lett. 8, 336 (1983). 3. V. N. Filippov, O. I. Kotov, and V. M. Nikolayev, Electron. Lett. 26, 658 (1990). 4. F. Parvaneh, V. A. Handerek, and A. J. Rogers, Opt. Lett. 17, 1346 (1992). 5. Ch. Chojetzki, J. Vobian, and W. Dultz, J. Opt. Commun. 13, 140 (1992). 6. W. J. Bock and W. Urbanczyk, Appl. Opt. 32, 5841 (1993). 7. S. C. Rashleigh, Opt. Lett. 7, 294 (1982). 8. A. Khomenko, M. Shlyagin, S. Miridonov, and D. Tentori, Opt. Lett. 18, 2065 (1993). 9. A. Khomenko, M. Shlyagin, S. Miridonov, and D. Tentori, Proc. Soc. Photo-Opt. Instrum. Eng. 2003, 49 (1993). 10. L. B. Jeunhomme, Single-Mode Fiber Optics (Dekker, New York, 1983), p. 275. 11. A. Ourmazd, M. P. Varnham, R. D. Birch, and D. N. Payne, Appl. Opt. 22, 2374 (1983).
