1

T. Kataoka, S. E. Day, D. R. Selviah, A. FernándezDepartment of Electronic and Electrical EngineeringUniversity 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 ( = 21.23 – 43.70 GHz) (K. Hirabayashi., et al, NTT, 1991)

Advantage

Problem

3

0

0.5

1

1.5

2

2.5

3

1500 1510 1520 1530 1540 1550 1560 1570 1580

Wavelength (nm)

To

tal

tra

ns

mit

ted

in

ten

sit

y

1.70

1.69

1.68

1.67

1.66

1.65

1.64

1.63

1.62

1.61

1.60

Unpolarized incident light, 9 dielectric layer mirror, LC layer (HWP, no=1.50, ne=1.70, second-order = 19.38 μm) design λ = 1550 nm

Transmission Spectra of a Conventional LC-FPF with Variable Birefringence of LC

S P P Sne

no ne none

4

Aim

LC-FPF Insensitive

to Input Polarization

Design & Optimization

Coinciding Two Sets of Orthogonal

Polarization Peaks

Attempt

Tuning PeaksAt the Same Rate

LC-FPF with Extra

BirefringentLayers

Solution

5

Two Types of LC-FPFs with Extra Birefringent Layers

Transmitted Intensity

Output PolarizationProperties

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 parallelto the optic axis of LC

Polarization normalto the optic axis of LC

Quarter-wave plate

The optic axis of LC

LC layer

Dielectric mirrors

ITOITO

Alignment layer

AR coatSubstrate

Y

XZ

~A.C

7

Transmission Spectra of LC-FPF with a Single QWP in the Cavity

0

0.5

1

1.5

2

2.5

3

1500 1510 1520 1530 1540 1550 1560 1570 1580

Wavelength (nm)

Tota

l tra

nsm

itted

inte

nsity

1.70

1.69

1.68

1.67

1.66

1.65

1.64

1.63

1.62

1.61

1.60

Unpolarized incident light, 9 dielectric layer mirror QWP (no=1.50, ne=1.70, first-order = 9.69 μm), design λ = 1550 nmLC layer (HWP, no=1.50, ne=1.70, second-order = 19.38 μm)

ne12 nm

8

0

0.5

1

1.5

2

2.5

3

1500 1510 1520 1530 1540 1550 1560 1570 1580

Wavelength (nm)

Tra

ns

mit

ted

in

ten

sit

y

1.70(45)1.70(-45)1.69(45)1.69(-45)1.68(45)1.68(-45)1.67(45)1.67(-45)1.66(45)1.66(-45)1.65(45)1.65(-45)1.64(45)1.64(-45)1.63(45)1.63(-45)1.62(45)1.62(-45)1.61(45)1.61(-45)1.60(45)1.60(-45)

Polarized Transmission Spectra of LC-FPF with a Single QWP with a +45º / -45º Polariser

Unpolarized incident light, 9 dielectric layer mirrorQWP (no=1.50, ne=1.70, first-order), LC layer (HWP, no=1.50, ne=1.70, second-order)

+45º -45º +45º -45º +45º -45º

0

0.5

1

1.5

2

2.5

3

1500 1510 1520 1530 1540 1550 1560 1570 1580

1.70

1.69

1.68

1.67

1.66

1.65

1.64

1.63

1.62

1.61

1.60

ne

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

0

0.5

1

1.5

2

2.5

3

1500 1510 1520 1530 1540 1550 1560 1570 1580

Wavelength (nm)

Tota

l tra

nsm

itted

inte

nsity

1.70

1.69

1.68

1.67

1.66

1.65

1.64

1.63

1.62

1.61

1.60

Unpolarized incident light, 9 dielectric layer mirror, QWPs (no=1.50, ne=1.70, first-order = 9.69 μm), design λ = 1550 nmLC layer (HWP, no=1.50, ne=1.70, second-order = 19.38 μm)

ne

11

0

0.5

1

1.5

2

2.5

3

1500 1510 1520 1530 1540 1550 1560 1570 1580

Wavelength (nm)

Tra

ns

mit

ted

in

ten

isty 1.70(45)

1.70(-45)1.69(45)1.69(-45)1.68(45)1.68(-45)1.67(45)1.67(-45)1.66(45)1.66(-45)1.65(45)1.65(-45)1.64(45)1.64(-45)1.63(45)1.63(-45)1.62(45)1.62(-45)1.61(45)1.61(-45)1.60(45)1.60(-45)

Polarized Transmission Spectra of LC-FPF with a Pair of QWPs with a +45º / -45º Polariser

Unpolarized incident light, 9 dielectric layer mirror, QWPs (no=1.50, ne=1.70, first-order), LC layer (HWP, no=1.50, ne=1.70, second-order)

+45º -45º +45º -45º -45º +45º+45º -45º ne

0

0.5

1

1.5

2

2.5

3

1500 1510 1520 1530 1540 1550 1560 1570 1580

1.70

1.69

1.68

1.67

1.66

1.65

1.64

1.63

1.62

1.61

1.60

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Transmitted light

Incident light

Reflected light Crossed Quarter-wave plates

The optic axis of first QWP

+45ºY

+45ºXZ

+45º / -45º Linearly Polarized Incident Beam Traveling Through LC-FPF Filter

LC layer (HWP)

The optic axis of second QWP

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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 differencebetween the polarizations

the integral multiple to the incident wavelength

No splitting peakAt 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

0

0.5

1

1.5

2

2.5

3

1500 1510 1520 1530 1540 1550 1560 1570 1580

Wavelength (nm)

Tota

l tra

nsm

itted

inte

nsity

1.70

1.69

1.68

1.67

1.66

1.65

1.64

1.63

1.62

1.61

1.60

Unpolarized incident light, 9 dielectric layer mirror, QWPs (no=1.50, ne=1.70, first-order = 9.69 μm), design λ = 1550 nmLC layer (HWP, no=1.50, ne=1.70, second-order = 19.38 μm)

ne

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 differencebetween the polarizations

the integral multiple to the incident wavelength

No splitting peakAt 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.