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Measurement of Waveguide Birefringence Using a Ring Resonator James T. A. Carriere, Jesse A. Frantz, Bruce R. Youmans, Seppo Honkanen, and Raymond K. Kostuk
Abstract—A simple method for accurately measuring the birefringence of a ring resonator waveguide using its resonant characteristics is presented. A measurement accuracy of 1 10 6 is observed. This approach is then used to measure the effect of thermal annealing on the level of birefringence in an ion-exchanged waveguide. Index Terms—Birefringence, integrated optics, optical resonators, optical waveguides.
I. INTRODUCTION
B
IREFRINGENCE is an important parameter that must be considered in the design of optical waveguide devices. For many sensor and communications applications, it is necessary to create low birefringence waveguides to reduce factors such as polarization-dependent loss. Accurate measurements of low can be difficult to make, espebirefringence values cially for channel waveguides. Prism coupling [1], grating coupling [2], and double Lloyd’s interferometer [3] methods are and are difficult to implement for limited to an accuracy of channel waveguides. Other methods [4] can improve the accuand even to [5] but are destructive racy to better than tests requiring the sample to be cut, as well as being much more complicated and difficult to implement. An alternative approach to making this measurement is to fabricate a ring resonator and monitor its resonant transmission behavior. Ring resonators are routinely used to measure propagation losses in low loss waveguides. In this letter, we describe a technique for using ring resonators to determine low levels of birefringence. II. THEORY A ring resonator consists of a straight waveguide section that is evanescently coupled to a low loss ring, as seen in Fig. 1 [6]. Resonance occurs when light coupled into the ring interferes with light passing through the straight waveguide. When the phase delay between the two paths is an integer number of half wavelengths, destructive interference causes a dip in the transmittance measured through the coupling waveguide. In any given ring, the wavelength dependence of the phase matching
Manuscript received August 24, 2003; revised December 16, 2003. This work was supported in part by BAE SYSTEMS. J. T. A. Carriere, J. A. Frantz, and S. Honkanen are with the Optical Sciences Center, University of Arizona, Tucson, AZ 85721-0094 USA (e-mail:
[email protected]). B. R. Youmans is with BAE SYSTEMS, Sierra Vista, AZ 85635-9263 USA. R. K. Kostuk is with the Department of Electrical and Computer Engineering, University of Arizona, Tucson, AZ 85721-0104 USA. Digital Object Identifier 10.1109/LPT.2004.824929
Fig. 1. Single-arm ring resonator composed of a straight waveguide evanescently coupled to a ring of radius R.
condition results in a series of dips in the transmittance as the wavelength of the incident light is tuned. For a ring of radius , effective index and group index at wavelength , the separation between adjacent dips is called the free spectral range (FSR) [7] of the resonator, and is given by FSR
(1)
where (2) The waveguide birefringence is defined as the difference between the effective refractive indexes of transverse electric and transverse magnetic modes. In optical materials far from any dispersion resonances, the dispersion is a slowly varying function, so for low levels of birefringence where this method is most useful, the difference in dispersion between TE and TM modes is small (3) For a typical optical glass such as BK7, the dispersion over a 10-nm range around 1550 nm is approximately m . Given a birefringence of , this assumption produces an error of only 0.013% in the measured birefringence. If birefringence is present in the ring resonator waveguide, the phase matching condition required for resonance will be different for TE and TM polarized light. As a
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CARRIERE et al.: MEASUREMENT OF WAVEGUIDE BIREFRINGENCE USING A RING RESONATOR
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result, there is a corresponding shift in the FSR and resonant wavelength positions between the two polarizations (4) Substituting (1) and (2) into (4) and rearranging terms, the is given by birefringence
(5) where (6) Therefore, a simple measurement of the FSR and the change in separation between TE and TM resonant dips as a function of wavelength is sufficient to determine the magnitude of the birefringence in a ring resonator. Additionally, if the polarization of each resonance is measured, the sign of the birefringence can be determined by simply knowing that the polarization with larger FSR must have a smaller effective index according to (1). For a given pair of resonant dips, if the separation between the dips is decreasing as a function of wavelength, the FSR of the left dip must be larger than the FSR of the right dip. Conversely, if the separation between dips is increasing as a function of wavelength, the FSR of the right dip must be larger than the FSR of the left dip. Knowing the sign of the birefringence can be important. In the case of ion-exchanged glass waveguides, the form birefringence induced by the ion exchange and burial process typically favors a larger TE effective index, while compressive stresses induced in the glass by the Ag ions favor TM [8]. By balancing these two competing factors, it is possible to create zero birefringence waveguides. It is also possible to extract the effect of bend-induced birefringence in the ring with a race track type resonator [6]. If a ring and race track resonator are fabricated on the same chip, with the same bend radius, the difference in measured birefringence between the two resonators will be entirely due to the additional straight sections in the race track design. This would allow the contribution of the curved sections to the birefringence of the waveguide to be isolated from the straight sections. III. EXPERIMENT A ring resonator was constructed using Ag Na ion exchange in Schott IOG-10 silicate glass, followed by a field-assisted burial [9]. Due to the nature of the diffusion process, these waveguides have a low characteristic birefringence and may be modified by a thermal annealing process [10]. The ring resonator was designed to be single mode at 1550 nm, with a radius of 14 mm. A fiber-based polarization controller was used to launch TE, TM, or a mixture of TE and TM polarizations into the ring. This permitted the attenuation of each polarization dip in the resonance to determine the sign of the birefringence, as well as the simultaneous measurement of both resonance positions to minimize environmental effects. The FSR was deterpm. Fig. 2 shows a typical resonance diamined to be gram for the device with both TE and TM polarizations present.
Fig. 2. Typical resonance diagram for a birefringent ring resonator. The fine splitting of the resonance indicates a different resonance condition for TE and TM polarizations.
Fig. 3. Separation between TE and TM polarization dip positions as a function of wavelength. The slope of this line is determined by the difference in FSR between the two polarizations.
A difference in FSR due to birefringence in the sample causes the separation between the TE and TM resonant dips to change as the wavelength is tuned. A plot of this separation as a function of wavelength is shown in Fig. 3. The slope of the best-fit line is simply the variable in (4). The slope is negative in this case, implying that the FSR for TE is larger than TM and that the birefringence has negative sign. For this sample, the birefringence . The largest factor contributing to is the uncertainty in this measurement is the measurement of the FSR. If a light source with greater wavelength tuning accuracy is used, the measurement uncertainty could be further improved. The effect of thermal annealing on the birefringence of the ring resonator waveguide was measured by placing the sample at room temperature in a 280 C oven for 30 min for each anneal step. The observed change in birefringence during this annealing process is shown in Fig. 4. For each anneal step, five independent measurements of the birefringence were taken. Each individual measurement had an accuracy of approximately , however, the sample was not environmentally isolated
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 4, APRIL 2004
. An accurate method for controlwith an accuracy of ling net waveguide birefringence was also demonstrated using thermal annealing. ACKNOWLEDGMENT The authors would like to thank E. A. J. Vikjaer and A. Sato for their assistance in processing samples and obtaining experimental data. REFERENCES
Fig. 4. Effect of thermal annealing on the measured birefringence of the ring resonator waveguide. A positive value indicates form birefringence is dominant. A negative value indicates stress birefringence dominates.
so slight shifts in the measured birefringence were present. The standard deviation among the five individual measurements was for each anneal step. on the order of The device originally had a positive birefringence, indicating that form birefringence in the waveguide was the dominant effect. As the sample was annealed, thermal diffusion in the waveguide quickly reduced this form birefringence, leaving stress-induced birefringence as the dominant effect and an overall negative birefringence. IV. CONCLUSION We have proposed and demonstrated a simple new technique for measuring waveguide birefringence. The waveguide birefringence for an ion-exchanged ring resonator was measured
[1] H. Nishihara, M. Haruna, and T. Suhara, Optical Integrated Circuits. New York: McGraw-Hill, 1985. [2] M. J. Li, S. I. Najafi, W. J. Wang, J. R. Simard, J. Albert, K. O. Hill, and A. Leung, “Fabrication and characterization of ion-exchanged glass channel waveguides with etched and diffused grating taps,” presented at the SPIE Current Developments in Optical Engineering IV, San Diego, CA, 1990. [3] E. Fazio, W. A. Ramadan, M. Bertolotti, and G. C. Righini, “Direct measurement of birefringence in ion-exchanged planar waveguides,” Opt. Lett., vol. 21, pp. 1238–1240, 1996. [4] P. Äyräs, G. N. Conti, S. Honkanen, and N. Peyghambarian, “Birefringence control for ion-exchanged channel glass waveguides,” Appl. Opt., vol. 37, pp. 8400–8405, 1998. [5] V. Minier, D. Persegol, J. L. Lovato, and A. Kevorkian, “Integrated optical current sensor with low-birefringence optical waveguides,” presented at the 12th Int. Conf. Optical Fiber Sensors, Williamsburg, VA, 1997. [6] R. G. Walker and C. D. W. Wilkinson, “Integrated optical ring resonators made by silver ion-exchange in glass,” Appl. Opt., vol. 22, pp. 1029–1034, 1983. [7] M. Born and E. Wolf, Principles of Optics, 6th ed. Cambridge, U.K.: Cambridge Univ. Press, 1959. [8] A. Brandenburg, “Stress in ion-exchanged glass waveguides,” J. Lightwave Technol., vol. LT-4, pp. 1580–1593, Oct. 1986. [9] R. V. Ramaswamy and S. I. Najafi, “Planar, buried, ion-exchanged glass waveguides: Diffusion characteristics,” IEEE J. Quantum Electron., vol. 22, pp. 883–891, June 1986. [10] J. Saarinen, S. Honkanen, S. I. Najafi, and J. Huttunen, “Double-ion-exchange process in glass for the fabrication of computer-generated waveguide holograms,” Appl. Opt., vol. 33, pp. 3353–3359, 1994.
