T. Kataoka, S. E. Day, D. R. Selviah, A. Fern

1
Polarization-Insensitive
Liquid-Crystal Fabry-Perot
Tunable Optical Filter
T. Kataoka, S. E. Day, D. R. Selviah, A.
Fernández Department of Electronic and Electrical
Engineering University College London, U.K.

• Content
• Motivation and Aim
• LC-FPF with Extra Birefringent layers
• ? A Single Quarter-Wave Plate (QWP)
• ? A Pair of Quarter-Wave Plates (QWPs)
• 3. Conclusion

2
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

• Dependence on Polarization States

3
Transmission Spectra of a Conventional
LC-FPF with Variable Birefringence of LC
Unpolarized incident light, 9 dielectric layer
mirror, LC layer (HWP, no1.50, ne1.70,
second-order 19.38 µm) design ? 1550 nm
no
ne
no
ne
ne
S
P
P
S
4
Aim
5
Two Types of LC-FPFs with
Extra Birefringent Layers
6
Structure of LC-FPF with a
Single QWP in the Cavity
Dielectric mirrors
ITO
ITO
The optic axis of LC
Substrate
AR coat
Incident light
Transmitted light
X
Z
Y
Reflected light
LC layer
Alignment layer
Quarter-wave plate
A.C
7
Transmission Spectra of LC-FPF
with a Single QWP in the Cavity
Unpolarized incident light, 9 dielectric layer
mirror QWP (no1.50, ne1.70, first-order 9.69
µm), design ? 1550 nm LC layer (HWP, no1.50,
ne1.70, second-order 19.38 µm)
ne
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 (no1.50, ne1.70, first-order), LC
layer (HWP, no1.50, ne1.70, second-order)
45º
-45º
45º
-45º
45º
-45º
ne
9
Structure of LC-FPF with a
Pair of QWPs in the Cavity
The optic axis of LC
Incident light
Transmitted light
X
Z
Y
Reflected light
Crossed Quarter-wave plates
10
Transmission Spectra of LC-FPF
with Two QWPs in the Cavity
Unpolarized incident light, 9 dielectric layer
mirror, QWPs (no1.50, ne1.70, first-order
9.69 µm), design ? 1550 nm LC layer (HWP,
no1.50, ne1.70, second-order 19.38 µm)
ne
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 (no1.50, ne1.70, first-order),
LC layer (HWP, no1.50, ne1.70, second-order)
ne
45º
-45º
45º
-45º
-45º
45º
45º
-45º
12
45º / -45º Linearly Polarized Incident
Beam Traveling Through LC-FPF Filter
The optic axis of first QWP
The optic axis of second QWP
Incident light
Transmitted light
45ºX
Z
LC layer (HWP)
45ºY
Reflected light
Crossed Quarter-wave plates
13
Cause for Splitting of Peaks of
LC-FPF with a Pair of QWPs
14
Transmission Spectra of LC-FPF
with Two QWPs in the Cavity
Unpolarized incident light, 9 dielectric layer
mirror, QWPs (no1.50, ne1.70, first-order
9.69 µm), design ? 1550 nm LC layer (HWP,
no1.50, ne1.70, second-order 19.38 µm)
ne
15
Cause for Splitting of Peaks of
LC-FPF with a Pair of QWPs
16
Conclusion

• 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.

Acknowledgements

• K. Wang in the Optical Devices and Systems
  Group, UCL.
• SID, IEE, EPSRC, UCL Graduate School for Travel
  Grants.