The attached narrated power point presentation attempts to explain the different types of tunable filters for WDM Systems. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.

1. 1
Tunable Filters
MEC

2. 2
Contents
• Basic Working Principle.
• Properties and Features.
• System Parameters.
• Types of Tunable Filters.
• Working and Block Schematics.

3. 3
Tunable Filters
• Flexibility of WDM increases with tunable
optical filters.
• Similar principle as passive devices.
• Operates over a range of frequencies.
• Can be tuned for only one optical
frequency to pass through it.
• Filters with fixed frequency spacings and
channel separations of multiples of 100
GHz used in WDM.

4. 4
Tunable Filters
Optical interference or diffraction as
the underlying physical mechanism.

5. 5
Desirable Properties
• Wide tuning range to maximize the number of
channels that can be selected.
• Negligible crosstalk to avoid interference from
adjacent channels.
• Fast tuning speed to minimize access time.
• Small insertion loss.
• Polarization insensitivity.
• Stability against environmental changes
(humidity, temperature, vibrations, etc.).
• Low cost/Economy.

6. 6
System Parameters
• Tuning Range.
• Channel Spacing.
• Maximum Number of Channels.
• Tuning Speed.

7. 7
Tuning Range
• Range over which the filter can be tuned.
• Most common wavelength transmission
window is 1300 nm to 1500 nm.
• 25 THz is the reasonable tuning range.
• 4.4 THz centered at 193.1 THz adequate
in networks using fiber based optical
amplifiers.
• Gain flattened amplifiers need wider
ranges.

8. 8
Channel Spacing
• Minimum frequency separation between
channels for minimum cross talk.
• Cross talk from adjacent channel to be 30
dB for desirable performance.

9. 9
Maximum Number of Channels
• Maximum number of adequately spaced
channels that can be packed into the
tuning range for low level of adjacent
channel crosstalk.
• Ratio of tuning range (Δv) to channel
spacing (δv).

10. 10
Tuning Speed
• How quickly the filter can be reset from
one frequency and tuned to another.
• Milliseconds for applications where a
channel is left setup for a relatively long
time (minutes to hours).
• Submicroseconds when information
packets switch rapidly.

11. 11
Channel Selection
• Wavelength selective mechanism of filters
based either on interference or diffraction.
• Mode spacing of the optical filter narrow
enough to transmit one of the signal
frequencies without passing adjacent
channel frequencies.
• Channel spacing to be greater than the
bandwidth of the individual channels.

12. 12
Channel Selection
Filter bandwidth large enough to transmit the desired channel, small
enough to block neighboring channels.
Mode Spacing

13. 13
Tunable Filter Types
• Tunable 2 x 2 Directional Couplers.
• Tunable Mach-Zehnder Interferometers.
• Fiber Fabry-Perot Filters.
• Tunable Waveguide Arrays.
• Liquid Crystal Fabry-Perot Filters.
• Tunable Multigrating Filters.
• Acousto-Optic Tunable Filters

14. 14
Tunable Filter Types

15. 15
Tunable 2 x 2 Directional Couplers
• Multielectrode asymmetric, 4 port device
fabricated on a LiNbO3 crystal, one arm
thinner than the other.
• Specific voltage applied to the electrodes
to change the refractive index of the
waveguides to select the wavelength to be
coupled to the second waveguide so that it
enters port 4.

16. 16
Tunable 2 x 2 Directional Couplers
• Remaining M-1 wavelengths pass through
the device and leave from port 3.
• To insert a wavelength combine it with an
input stream entering port 1 through port 2
so that it couples across to the top
waveguide and exits port 3 along with
other wavelengths.

17. 17
Tunable 2 x 2 Directional Couplers
λ1….λi-1, λi+1….λM
Add
Wavelength
Drop
Wavelength
1 2
3 4

18. 18
Interferometer
• Superimpose two or more sources of light
to create an interference pattern.
• Pattern measured and analyzed.
• Interferogram - superposition of spatial
components proportional in frequency and
amplitude to the corresponding spectral
components.

19. 19
Optical Schematic of a Tunable
Filter
• Spatial component of
interferogram whose
spatial frequency
matches the spatial
frequency of the grill is
modulated.
• Amplitude of detector
voltage modulation
proportional to the
amplitude of spatial
component and hence
to the intensity of the
corresponding spectral
component.
Shears incident ray.

20. 20
Mach-Zehnder Interferometer
• A 2 x 2 Mach-Zehnder Interferometer (MZI)
comprises of:
- initial 3-dB directional coupler which splits
input signals.
- a central section with one of the waveguides
longer by ΔL to give wavelength dependent
phase shift between two arms.
- another 3-dB coupler which recombines the
signal at the output.

21. Mach-Zehnder Interferometer
Phase shift of the propagating wave increases with L,
Constructive or destructive interference depending on L.

22. 22
Mach Zehnder Filters
• Two 3-dB couplers connected forming an
interferometer.
• Incident beam split into two fiber paths and
then recombined with the second 3-dB
coupler.
• Phase shifting device placed in one arm of
the interferometer.
• Shift accomplished by changing the optical
path length (ΔnL) in one interferometer arm.

23. 23
Mach Zehnder Filters
• Use of thermo-optic or electro-optic
coupling mechanisms to change the length
of interferometer arms.
• Time delay (τ) between the phase of the
light propagating in each arm fiber
interferometer τ = ΔnL /c where ΔnL is the
optical path length.
• Coherent addition of the two beams
results in a change in the intensity of the
signal transmitted through an arm of the
interferometer.

24. 24
Mach Zehnder Filters
• Cascaded MZIs, transmittance of a chain
of M interferometers (τm is the delay for the
mth interferometer):
• Required delay times for a cascaded
interferometer system with channel
spacing of Δνch are

25. 25
Temperature Tunable Mach
Zehnder

26. 26
Three Stage Tunable MZI Filter

27. 27
Fiber Fabry-Perot Filters
• Principle of partial interference of the incident
beam with itself in a mirrored resonant cavity
to produce transmission peaks and nulls in
frequency domain.
• Frequency spacing between two successive
transmission peaks called the free spectral
range.
ng - group index of intracavity material for a
filter of length L.

28. 28
Fiber Fabry-Perot Filters
• Ends of two single mode fibers polished,
coated with a reflecting dielectric material.
• Fibers mounted between two piezoelectric
crystals, spaced apart to form Fabry-Perot
cavity.
• Apply voltage to piezoelectric crystals to
expand slightly and change the spacing
between dielectric mirrors.

29. 29
Fiber Fabry-Perot Filters
• Resonant frequency changed by adjusting
the spacing in the cavity.
• Narrow band tuning capability, wide
spectral range, relatively slow switching
speeds.

30. 30
Fiber Fabry-Perot Filters

31. 31
Liquid Crystal Fabry Perot Filters
• Use of high speed electroclinic liquid
crystals inside a Fabry-Perot cavity.
• Liquid Crystal positioned between two
fiber end faces becomes part of Fabry
Perot cavity.
• Tuned by applying voltage across the
crystal, change in refractive index and
hence optical path length in the cavity
material.

32. 32
Liquid Crystal Fabry Perot Filters
• Switching times less than 10 μs, tuning
range of 13 nm, channel spacing of 0.7 nm
@ 1550nm operating wavelength.

33. 33
Grating Based Filters
• Wavelength selective devices fabricated in
InGaAsP/InP materials consisting of a
planar waveguide and a section with an
etched DBR or DFB grating.
• Wavelength selectivity of the grating
section electrically tuned by applying a
voltage to electrodes fabricated over the
grating section.
• Voltage induces electrorefraction that
changes the Bragg wavelength.

34. 34
Tunable Multigrating Filters
• Can be used to add/drop any number of
different wavelengths.
• Use of three port circulators with a series
of electrically tunable fiber based reflection
gratings placed between them.
• One grating used for each wavelength.
• Demultiplexer separates dropped
wavelength into individual channels.

35. 35
Tunable Multigrating Filters
• Multiplexer combines wavelengths for
transmission over trunk line.
Tunable
Fiber
Gratings
Demux Mux
11 2 2
3 3
CirculatorCirculator
Drop Add
ReflectionReflection
Optical Circulator directs signal
sequentially from one port to next.
… …….

36. 36
Tunable Multigrating Filters
• Tuned gratings reflect wavelengths to be
dropped or added.
• Light reflected back re-enter the left hand
circulator through port 2 and exit from port 3.
• To add a wavelength, inject it into port 3 of
the right hand circulator, will come out of port
1, get reflected, head back to the circulator to
pass through to port 2.
• Wavelengths not reflected pass through.

37. 37
Acousto-Optic Filters
• Tuning range achieved with etched gratings,
voltage induced electrorefraction changes
relatively small.
• Larger tuning ranges (>100 nm) achieved
using acousto-optic filters.
• Only filter to select several wavelengths
simultaneously.
• Operate through interaction of photons and
acoustic waves in solids such as LiNbO3.

38. 38
Acousto-Optic Filters
• Acoustic Transducer modulated by a
nominal 175 MHz RF signal produce
surface acoustic waves in LiNbO3.
• Grating period determined by the
frequency of the driving RF crystal.
• More than one grating produced using a
number of different driving frequencies.
• Switching speeds of the order of 10 μs.

39. 39
Acousto-Optic Filters
• Bragg matched wavelengths coupled to
second branch, others continue.
• When input light entirely TE polarized, at
the output end a polarizer placed that
selects only TM polarized light.
• Acousto-optic (AO) device changes the
polarization of a narrow spectral band of
light from TE to TM.
• Light then pass out of the device.

40. 40
Acousto-Optic Filters
(Passed)
(Dropped)

41. 41
Thank You