This document presents research on using a photorefractive crystal to create an all-optical tunable switch and filter. It describes using a potassium niobate crystal and adjusting the transmission grating period or incident angle to tune the filter. It analyzes signal beam propagation and diffraction efficiency using a simulation. The results show maximum diffraction towards the c-axis is preferable. Adjusting the control beam intensity can vary the diffraction efficiency and bandwidth. Both transmission and reflection grating configurations are investigated, with reflection offering narrower bandwidth below 0.1nm. In conclusion, the crystal allows selecting and controlling signal wavelengths by adjusting the grating properties.

1. By:
Hojjatollah Sarvari
Department of Electrical Engineering
Shiraz University, Shiraz-Iran
January-2009
All-Optical Switch andTunable
Filter with Photorefractive Crystal

2. INTRODUCTION
WDM network systems increase bandwidth & bit rate
Tunable optical filter is a important device in telecommunication
Use KNbO3 as Photorefractive crystal

3. Tunable optical filter with photorefractive waveguide
Using transmission grating
Adjusting the period of the grating by changing the incident angel, wavelength or
phase distribution of the control beams

4. Analytical model for tunable optical filter using
transmision grating
Gaussian beam as inut with:
maximum value of intensity Is 1.0W/cm²
Radius is 0.5mm
The incident angle is θs=5.5°
The index grating induced by the
control beams with spatial range
of z=0mm and z=8mm
Intensity ratio of the control beam to
signal beam as Ic/Is=5
We analyze signal beam propagation in a photorefractive material using a
Fnite Dimensional Beam Propagation Method (FD-BPM)
We investigate diffraction efficiency and bandwidth of an output signal

5. Signal beam propagation in photorefractive material and
its relative permittivity distribution
signal beam is diffracted toward c-axis
signal beam is diffracted toward anti-c-axis

6. Simulation results for Tunable optical filter using
transmision grating
The diffraction efficiency has a maximum value 0.92 and 0.79 at z=3.8mm and 4.6mm
in case of diffraction toward c-axis and anti c-axis
The difference between these maximum diffraction efficiencies or these optimum
interaction lengths decrease as the intensity of the control beam increases
because the index grating induced by the signal beam becomes small
relatively to that induced by the control beam
Considering the D.E & Lopt & I it may be concluded that it is preferable to use the
optical configuration of diffraction toward C-axis

7. Simulation results for Tunable optical filter using
transmision grating
Diffraction Efficiency has maximum value for L=4 & 5 mm
Compare FWHM with D.E (previous picture) for optimum intraction length
The minimum values of FWHM are 2, 1.8, 1.5nm at L=5, 6, 6mm
Intensity ratio of the control beams is adjusted to 1:1, 1:5 and 1:10
The maximum diffraction efficiencies are 0.91, 0.88 and 0.85

8. Using reflection grating
Tunable optical filter with photorefractive waveguide

9. Analytical model for optical filter using reflection grating
φc is the phase difference between the interference pattern and the index
modulation .It is equal to -π / 2 or π / 2

10. Reflection efficiency against interaction length and beam
intensity ratio of control beam to signal beam

11. Reflection efficiency normalized by the maximum value
against wavelength of the signal beam
λ=532nm for signal beam for satisfying phase matching
For satisfying phase matching condition Δκ=2κ-κg=0

12. Experimental result of optical filter using transmision
grating
YAG/SHG laser (λ=532nm)
He : Ne laser (λ=633nm)
BaTiO3 bulk crystal as PR material whose size
is 5mm×5mm
Dofference between theory and exprimental:
1-Energy loss due to absorption and
optical reflection on the crystal surface
2-Non-linear scattering of the
diffracted signal beam toward the c-axis
of the crystal via fanning effect

13. CONCLUSIION
It is able to select the required wavelength components
and control the bandwidth of the signal beam by
adjusting the period and the width of the grating
The diffraction efficiency of the signal beam against the
interaction length using the transmission grating is larger
than that using the reflection grating
FWHM of the filter using the transmission grating is
much broader than that using the reflection grating
Ultra-narrowband optical filter with FWHM under 0.1nm
can be achieved using the reflection grating
configuration

14. Reference
[1] “Tunable optical filter for wavelength division multiplexing using
photorefractive planar waveguide” Satoshi Honma, Hirokazu Ito, Mitsuhiro
Komatsu,Shinzo Muto,Atsushi Okamoto; Proc. of SPIE Vol. 6582,
65821S, (2007)

15. Thanks for your attention