1 T. Kataoka, S. E. Day, D. R. Selviah, A. Fernández Department of Electronic and Electrical Engineering University College London, U.K. Polarization-Insensitive Liquid-Crystal Fabry-Perot Tunable Optical Filter Content 1. Motivation and Aim 2. LC-FPF with Extra Birefringent layers A Single Quarter-Wave Plate (QWP) A Pair of Quarter-Wave Plates (QWPs) 3. Conclusion

2 Dependence on Polarization States LC-FPF for Optical Fiber Communication Systems Electrically tunable filter with low voltage Wide Tuning Range 60 nm ( = Free Spectral Range, FSR) (M. W. Maeda., et al, Bellcore, 1990) Narrow Band (Full Width of Half Maximum, FWHM) 0.17 – 0.35 nm ( = – GHz) (K. Hirabayashi., et al, NTT, 1991) Advantage Problem

3 Unpolarized incident light, 9 dielectric layer mirror, LC layer (HWP, n o =1.50, n e =1.70, second-order = μm) design λ = 1550 nm Transmission Spectra of a Conventional LC-FPF with Variable Birefringence of LC SPPS nene nono nene nono nene

4 Aim LC-FPF Insensitive to Input Polarization Design & Optimization Coinciding Two Sets of Orthogonal Polarization Peaks Attempt Tuning Peaks At the Same Rate LC-FPF with Extra Birefringent Layers Solution

5 Two Types of LC-FPFs with Extra Birefringent Layers Transmitted Intensity Output Polarization Properties Calculation and Analysis LC-FPF with A single QWP LC-FPF with A pair of QWPs Structure LC is homogeneously aligned nematic

6 Structure of LC-FPF with a Single QWP in the Cavity Incident light Reflected light Transmitted light Polarization parallel to the optic axis of LC Polarization normal to the optic axis of LC Quarter-wave plate The optic axis of LC LC layer Dielectric mirrors ITO Alignment layer AR coat Substrate Y X Z ~ A.C

7 Transmission Spectra of LC-FPF with a Single QWP in the Cavity Unpolarized incident light, 9 dielectric layer mirror QWP (n o =1.50, n e =1.70, first-order = 9.69 μm), design λ = 1550 nm LC layer (HWP, n o =1.50, n e =1.70, second-order = μm) nene 12 nm

8 Polarized Transmission Spectra of LC-FPF with a Single QWP with a +45º / -45º Polariser Unpolarized incident light, 9 dielectric layer mirror QWP (n o =1.50, n e =1.70, first-order), LC layer (HWP, n o =1.50, n e =1.70, second-order) +45º-45º+45º-45º+45º-45ºnene

9 Polarization parallel to the optic axis of LC Polarization normal to the optic axis of LC Structure of LC-FPF with a Pair of QWPs in the Cavity Transmitted light Incident light Reflected light Crossed Quarter-wave plates The optic axis of LC Y X Z

10 Transmission Spectra of LC-FPF with Two QWPs in the Cavity Unpolarized incident light, 9 dielectric layer mirror, QWPs (n o =1.50, n e =1.70, first-order = 9.69 μm), design λ = 1550 nm LC layer (HWP, n o =1.50, n e =1.70, second-order = μm) nene

11 Polarized Transmission Spectra of LC-FPF with a Pair of QWPs with a +45º / -45º Polariser Unpolarized incident light, 9 dielectric layer mirror, QWPs (n o =1.50, n e =1.70, first-order), LC layer (HWP, n o =1.50, n e =1.70, second-order) +45º-45º+45º-45º +45º -45º nene

12 Transmitted light Incident light Reflected light Crossed Quarter-wave plates The optic axis of first QWP +45ºY +45ºX Z +45º / -45º Linearly Polarized Incident Beam Traveling Through LC-FPF Filter LC layer (HWP) The optic axis of second QWP

13 Cause for Splitting of Peaks of LC-FPF with a Pair of QWPs +45º / -45º polarization encounters only either the fast or the slow axis of the QWPs Two different sets of peaks The optical path difference between the polarizations the integral multiple to the incident wavelength No splitting peak At 1550 nm No longer the integral multiple when the birefringence of LC is changed Started splitting

14 Transmission Spectra of LC-FPF with Two QWPs in the Cavity Unpolarized incident light, 9 dielectric layer mirror, QWPs (n o =1.50, n e =1.70, first-order = 9.69 μm), design λ = 1550 nm LC layer (HWP, n o =1.50, n e =1.70, second-order = μm) nene

15 Cause for Splitting of Peaks of LC-FPF with a Pair of QWPs +45º / -45º polarization encounters only either the fast or the slow axis of the QWPs Two different sets of peaks The optical path difference between the polarizations the integral multiple to the incident wavelength No splitting peak At 1550 nm No longer the integral multiple when the birefringence of LC is changed Started splitting

16 Conclusion Acknowledgements LC-FPF with a single QWP All peaks tuned by external electric field with unpolarized incident light Spectrometer for Gas sensing systems. LC-FPF with a pair of QWPs Polarization-insensitive operation A tuning range of 7 nm at near 1550 nm. A Splitting of peaks caused by the birefringence of QWPs. K. Wang in the Optical Devices and Systems Group, UCL. SID, IEE, EPSRC, UCL Graduate School for Travel Grants.