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Journal of the Optical Society of America B

Journal of the Optical Society of America B

| OPTICAL PHYSICS

  • Editor: Henry van Driel
  • Vol. 29, Iss. 5 — May. 1, 2012
  • pp: 916–923

Frequency modulation background signals from fiber-based electro optic modulators are caused by crosstalk

Isak Silander, Patrick Ehlers, Junyang Wang, and Ove Axner  »View Author Affiliations


JOSA B, Vol. 29, Issue 5, pp. 916-923 (2012)
http://dx.doi.org/10.1364/JOSAB.29.000916


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Abstract

Frequency modulated spectroscopy (FMS) performed by the use of fiber-coupled electro optic modulators (FC-EOMs) is often plagued by background signals that bring in noise and, by their temperature dependence, cause severe drifts. These signals cannot be zeroed out by the conventional technique of using a carefully adjusted polarizer that can be applied to free space electro optic modulators (EOMs). This can limit the use of FC-EOMs in high performance detection techniques. Here we provide an explanation to these background signals that is based upon crosstalk between various polarization directions of light in the fixed mounted polarization-maintaining fibers and the electro optic crystal. The description provides a basis for the previously demonstrated technique that utilizes an EOM regulated simultaneously by temperature and DC voltage to eliminate background signals from systems encompassing FC-EOMs.

© 2012 Optical Society of America

OCIS Codes
(060.2340) Fiber optics and optical communications : Fiber optics components
(060.2630) Fiber optics and optical communications : Frequency modulation
(130.3120) Integrated optics : Integrated optics devices
(300.6310) Spectroscopy : Spectroscopy, heterodyne
(300.6380) Spectroscopy : Spectroscopy, modulation
(140.3518) Lasers and laser optics : Lasers, frequency modulated

ToC Category:
Integrated Optics

History
Original Manuscript: October 21, 2011
Revised Manuscript: January 10, 2012
Manuscript Accepted: January 18, 2012
Published: April 6, 2012

Citation
Isak Silander, Patrick Ehlers, Junyang Wang, and Ove Axner, "Frequency modulation background signals from fiber-based electro optic modulators are caused by crosstalk," J. Opt. Soc. Am. B 29, 916-923 (2012)
http://www.opticsinfobase.org/josab/abstract.cfm?URI=josab-29-5-916


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  31. The various parts of the first term in Eq. (15) can be understood as follows. cos(2θ10), which can be interpreted as cos2(θ10)−sin2(θ10), represents the part of the light that propagates along the primary axis of the first fiber (element 1). This light is then cross coupled into both the primary and secondary axes of the EOM (element 2), with transmission factors of cos(θ21) and sin(θ21), respectively, whose differences in paths provide a phase shift of Δϕ2. Both of these components then again couple into the primary axis of the second fiber (element 3), with transmission factors of cos(θ32) and sin(θ32), respectively, where they interfere, producing an intensity at the beat frequency that is given by the products of the two interfering components. Since the product of a sinus and a cosine function can be written in terms of the double angle, e.g., 2 sin(θ21)cos(θ21)=sin(2θ21), this interference between the two components can be written in terms of the product of two sinus functions of the double angles, i.e., sin(2θ21)sin(2θ32). This thus explains the middle part of the first of the nine terms. Since the output from the second pm-fiber (produced by the first term) is fully polarized along the principal axis of the secondary pm-fiber, it produces an output after the final polarizer (element 4) that is proportional to cos(2θ43).
  32. Since a similar set of curves can be obtained by several sets of parameters, the actual parameter values for the prediction are not assumed to necessarily correspond to the prevailing situation; they rather serve the purpose of illustrating that the actual form of the measured set of curves indeed can result from the model of FM background signals in an FC-EOM presented in this work. A more detailed analysis of the prevailing parameter values for the EOM investigated is therefore outside the scope of this work.
  33. The dissimilarities between the measured and predicted plots, which are considered to be of less importance in this work, can be attributed to higher order effects, hysteresis in the EOM’s voltage response, multiple reflections between the crystal surfaces, and a misalignment between the crystal orientation and the electrodes in the EOM, resulting in an additional effect originating from the electro optic coefficient r51.

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