1. Optical communication Circuits & waveguides
Lecture 4th

2. Contents Integrated Optical Circuits (OIC) Optical signal processing
Optical filters
Optical Couplers
Optical Modulators
Optical Wavelength Converters

3. Optical networks I
1) First Generation of Optical networks
Optical Signals routing through Electrical Circuits
2) Second Generation of Optical networks
Optical Signals routing through Optical Circuits
Multiplexing Time Division Multiplexing – Optical Time Division Multiplexing ( TDM – OTDM )
Multiplexing Wavelength Division Multiplexing (WDM) (n=number of wavelengths)

4. Optical networks II Node 3 Node 2 Node 3
The routing of Optical Signal is difficult. The space switching, which is an easy and classical way in conventional circuits, can NOT be used in pure optical networks.
Wavelength Multiplexing gives the ability of choοsing a specific wavelength and give an extra degree of freedom in the system.
The chosen wavelength can be used for the routing of data.
Two categories of 2nd generation of networks, which use multiplexing for signal routing through the nodes
Broadcast and Select Networks (BS)
Wavelength Routing (WR)
Node Node 3
Node 3
Star coupler
LASER
LASER
RECEIVER
RECEIVER

5. Basic components of optical circuits
Optical Filters
Space Switching
Wavelength Converters
Modulators
Optical Sources (LED & Lasers)
Optical Receivers Φωτοδέκτες
Optical Amplifiers

6. Basic components of optical circuits
The dimensions and materials of the circuit’s waveguides is of crucial importance for the correct function of the network
Heterojunctions
Homojunctions

7. Optical Filters and Switches
Each node of the network must combine ALL the wavelengths in one signal and routes the optical signal in an optical fiber (multiplexing). Moreover, each node must separate the wavelengths which end up at the node, through the optical fiber (Demultiplexing).
Diffraction gratings
Bragg gratings
Mach Zender Interferometer
Thin layer filters
Fabry-Perot filters
Arrayed Waveguide Grating (AWG)

8. Basic features of optical filters
The basic feature of the filters is
the Free Spectral Range (FSR) which corresponds to the frequency distance between two neighbouring resonant frequencies of the filter.
The Full Width Half Maximum (FWHM) which correspond to width of half power (-3dB) of each peak.
The Finess (F). It is the ratio the FSR to FWHM and it measures the filter’s selectivity of the repletion period.

9. Optical Gratings
The term grating is used to describe each device which function is based on the interference effect of two or more signals originating from the same light source and have phase difference between them. The phase difference is affected from the optical wavelength λ. The wavelengths which have multiples of 2π phase difference interfere positively and their intensity is increased while those who have an odd multiple of π interfere negatively, cancelling each other. Consequently, the intensity of the optical field depends on its wavelength while the grating is used as an optical filter

10. Diffraction gratings Two types of diffraction gratings
Transmission grating (a)
Reflection grating (b)
Their function is based on the dependence of the refractive index on the wavelength. Specifically, the transmission (reflection) angle is determined by the Snell Law. So different wavelength, have different refractive indexes and finally different transmission (reclection) angles.
(a)
(b)

11. Diffraction gratings
The phase difference Δφ between two signals originating from two neighboring slots is
The signal has its maximum amplitude when Δφ=mλ
Towards Image Level

12. Diffraction Grating m: Grating ‘s Class
Placing an optical fiber in the direction of the diffraction angle θd(λ1) then the incident light will be concentrated around the λ1 . So the diffraction grating acts as a Wavelength Demultiplexor. On the Working in a same way, we can show that if wavelengths λ1, λ2 ,...., λN fall on the grating with angle θd (λ1), θd(λ2),...., θd(λN) then on the θi direction the wavelengths will combine into one. So the grating acts as Wavelength Mulltiplexor.

13. Bragg diffraction
Any periodic disorder in the transmission medium is called a Bragg grating.
The periodic disorder in the transmission medium is made by the change of the refractive index of the medium
Due to the periodic disorder, a coupling between the transmitted modes of the medium takes place
The exchange of energy is maximized if the Bragg condition applies

14. Diffraction grating
In Bragg Gratings used for wavelength multiplexing and demultiplexing, the modes in which the energy exchange takes place, have the same transmission constant but opposite transmission direction
The wavelength in which the energy exchange is maximized is λ0 and is given by
Near λ0, light is reflected by the grating because the power carried by the mode transmitted in the +z direction, is exchanged with the mode transmitted in the –z direction. Consequently, the Bragg Grating can be used as a wavelength selector (λ=λ0).

15. Energy bands Portion of the light is reflected at each interface.
Only for one wavelength can a coherent addition be performed.
Wavelength satisfying the Bragg Condition, λB:
neff: is the effective index of the waveguide core.
Very narrow bandwidths are possible.

16. Bragg diffraction
κ = coupling constant
L = Bragg’s grating length

17. Add & drop multiplexer
The recirculator takes a signal as an input in one port and extracts it from another according to the following rules ( 1->2 , 2->3 , 3->1 )
All the wavelengths come in from port one, come out from port 2
Bragg Gratting reflects λ2 and let λ1 , λ3 , λ4.
λ2 comes in port 2, comes out from port 3 and then is dropped.
The 5-95 coupler is used so the λ2 is added to the signal.

18. Optical Couplers
The directional coupler is a simple flexible structure. It can be used as
Optical power splitter
Wavelength filter
Selector or Converters of polarization
Switcher for the routing of the optical signals
The directional coupler is consisted of parallel, single-mode waveguides, placed in close distance.
The optical power inserted in one waveguide, is transferred in the other waveguide, as a consequence of the overlapping of the damped optical Fields of the two single-moded waveguides.

19. Optical couplers

20. Optical couplers
The function of optical cuplers is described in approximation from the theory of the supermodes. Considering that the waveguides of the coupler allow single mode transmission, the zero-class or fundamental supermode is approximated by the sum of the two separate optical fields while the first- class supermode is approximated by the subtraction of them.
These supermodes have even and odd symmetry as to the direction of transmission z and have transmission constants βe and βo..
The interference of the two modes in every point of the z axis describes the transmission along the length of the coupler and is a function of the phase difference of the two supermodes Δφ=( βe - βo.)z. Total optical power transfer is achieved for lengths satisfying the following equation

21. Optical Couplers
Odd Symmetry mode
Even Symmetry mode

22. Optical couplers

23. Mach zehder interferometer
In Mach-Zehder interferometer, the optical signal interferes with a delayed version of itself. In the diffraction grating, the amplitude of the field depends on the wavelength. The same principle applies in the Mach-Zehder interferometer.
The Mach-Zehder interferometer is made by using optical integrated techniques. It is consisted of two couplers. The firtst coupler splits the optical power between two waveguides, which have different length.
inputs
Route Difference=ΔL
outputs

24. Mach zehder interferometer
The Mach-Zehder Interferometer can be used as an optical filter

25. Mach zehder interferometer
Periodic Filter Response
For max output power
For min output power
With cascaded MZ interferometers can also be used
add/drop filter
Heaters for tuning

26. Mach zehder interferometer
Multiple MZ interferometers can be used as a multiple filter
Mach Zehder 1 (ΔL)
Mach Zehder 2 (2ΔL)
Mach Zehder 3 (3ΔL)
iNPUTS
OUTPUTS

27. THIN FILMS
Multiple thin films filter function is based on the reflection properties of light on a cascaded multiple thin films struct. Each film is made of a different dielectric material
As the optical signal is transmitted throufh the first thin film, a part of the optical power is reflected on the struct while the other is transmitted through the struct.
The portion of the reflected and transmitted power dependes on the wavelength of the optical signal. Consequently, the struct act as aoptical filter
Transmitted wave
Reflected wave

28. Consecutive Reflections
Fabry perot
The function of Fabry-Perot filter is based on the multiple transits of an optical signal through a cavity, which is placed between two reclectors
The transmitted waves interfere with one another. Their intensity depends on the wavelength of the optical signal
Fabry-Perot Cavity
Output signals
Input signal
Consecutive Reflections

29. Fabry perot ιι
Filter Parameters
Response
Max
FWHM
Finesse with

30. Fabry perot response
Typical Lossless response
Max
Min

31. Anti reflection coatings
T01 = 1-R01 και T1S and
The use of an intermediate layer to form an antireflection coating can be thought of as analoguous to the technique of impedance matching of electrical signals

32. Arrayed waveguide gratings (awg)
The Arrayed Waveguide Gratings (AWG) are constructed on a substrate of Silica (Si) or Indium Phosphide (InP) and are used extensively on WDM networks as multiplexors, demultiplexors and routing wavelength devices.
One ADM can be constructed by integrating one AWG with semiceonductor amplifiers on the same substrate
Arrayed waveguides (AW) can act as an optical filter, in a similar way as the diffraction grating does.
The WDM signal enters the waveguides through the star coupler, which is consisted of a planar dielectric waveguide.
Inside the star coupler the optical beam is widen and incidents on the AW
Το μήκος του κάθε κυματοδηγού διαφέρει κατά μία σταθερή ποσότητα ΔL από το μήκος του γειτονικού του κυματοδηγού.
Τα οπτικά κύματα που διαδίδονται μέσα στους κυματοδηγούς της συστοιχίας φτάνουν στο συζεύκτη αστέρα εξόδου, με μία διαφορά φάσης που εξαρτάται και από το μήκος κύματος
Ο συζεύκτης εξόδου εστιάζει την οπτική δέσμη στους κυματοδηγούς εξόδου ανάλογα με τη διαφορά φάσης που έχουν μεταξύ τους τα οπτικά κύματα και η οποία είναι συνάρτηση του μήκους κύματος
Η εστίαση είναι διαφορετική για κάθε συνιστώσα του WDM σήματος, με αποτέλεσμα οι διάφορες συνιστώσες να διαχωρίζονται στην έξοδο της διάταξης
Different Lengths Waveguides
Output
Star
Coupler
Input
Star
Coupler
Arrayed Waveguides Grating

33. Arrayed waveguide gratings (awg)
The length of each waveguide differs from the adjacent waveguides by ΔL.
The optical waves transmitted through the AWG arrive at the Output Star Coupler, having a phase difference Δφ. Δφ depends on the wavelength
The Output Star Coupler focuses the optical beam. The direction of focus depends on the phase difference of the signals which depends on the wavelength.
The angle of focus is different for each signal of the optical beam. Using this effect the signals are separated on the output of the AWG.
Different Lengths Waveguides
Output
Star
Coupler
Input
Star
Coupler
Arrayed Waveguides Grating

34. Control methods of optical signals
Amplitude modulation
Phase modulation
Change of polarization
Change direction of the optical wave
Filter wavelength
External Signal
Optical Input Signal
Optical Output Signal
Active region of interactibetween transmitted waves and external signal

35. Control methods of optical signals ι
Opticoacoustical control. By using elastical surface waves (acoustic), the refractive index of a proper crystal changes (ZnO, LiNbO3). The source of the control waves is a piezoelectric material, which is stimulated by electric signals. Because the longitudinal waves, as the acoustic waves, show periodic compression and rarefaction, the change of the refractive index of the crystal exhibits the same periodicity. This method is used to produce diffraction gratings with many applications.
Thermoptical control. The dependence of the refractive index of a specific group of materials is very strong. For example, the refractive index of LiNbO3 increases 5 to 10 when the temperature increases from 25 to 100 degree C. This effect, is known as thermoptical effect and is observed in all the transparent dielectrics and all materials used for optical integration. In semiconductor devices of III-V this effect is used in the construction of arrayed switching. But the speed of switching was constricted in 1ms because of the non zero thermocapacitance of these devices.

36. Control methods of optical signals ιι
Opticomagnetic control is based on the opticomagnetic effect, which affects the crystal ’s refractive index under the effect of α magnetic field. In this case, the crystal ‘s dielectric tensor loses its symmetry and the principle of reciprocity of the course of light, no longer applies. The devices based on this type of control are used mainly as optical isolators.
Electrooptical control. In this case, the refractive ondex of the crystal is changed by the fluctuations of an external electric field. The electroptical effect is used on hybrid structures of LiNbO3 and on similar monolithic integrated devices. The electroptical effect in LiNbO3 devices is extremely strong.
Electrooptical absorption. The effect of an external electric field on the interband absorption and for photon energy very close to the Energy Gap of the semiconductor is expressed thoufh the Franz- Keldysh and Stark effect.

37. Optical modulation Direct modulation on semiconductor lasers:
Output frequency shifts with drive signal
Carrier induced (chirp)
temperature variation due to carrier modulation
Limited extinction ratio à because we don’t want to turn off laser at 0-bits
Effect on the product Distance * Bit-Rate
External modulation
Electro-optical modulation
Electroabsorption (EA) modulation
Chirp can still exist (but is NOT so strong)
Facilitates integration
Always incur 6-7 dB insertion loss

38. Advantages – Disadvantages
Direct
Simple
Cost-effective
Compact
External
Additional Component
Additional Loss
Higher Speed
Large extinction ratio
Low chirp
Low modulation distortion

39. ElectrOptical Modulators (eo)
Input light
beam
Electrodes
LiNbO3, electroptical materials
Waveguide
Modulated Light
Cross Section
Electrodes
Input Light
Amplitude Modulated Light

40. Function of IM modulator

41. Transfer function of the Mach-zehnder modulator
The transfer function of Mach-Zehnder is
I(t): transmitted Intensity
A: insertion loss
Iθ : Input Intensity from LD
V(t): applied voltage
It is necessary to set the static bias on the transmission curvethrough Bias electrode. It is a common practice setbias point at 50% transmission point, Quadrature Bias point. As shown here, electrical digital signals are transformed into optical digital signal by switching voltage to both end from quadrature point.

42. Functional parameters
Sample Spec.
Comments
Modulation speed
10Gbits/s
Capapability to transmit digital signals (e.g. billion times per second
Insertion Loss
Max 5dB
Defined as the optical power loss within the modulator
Driving Voltage
Max 4V
The RF voltage required to have a full modulation
Optical Bandwith
Min 8GHz
3dB roll-off in efficiency at the highest frequency in the modulated signal fs spectrum
ON/OFF
Extinction Rate
Min 20dB
The ratio of max optical power (ON) and min optical power (OFF)
Polarization
The ratio of two polarization states (TM mode and the TE mode) at output

43. Modulator basics Insertion loss (dB) = 10 log10 (Imax/I0)
Extinction ratio (dB) = -10 log10 (Imin/Imax)

44. Typical electroptic modulator

45. Electroabsorption (EA) Modulator
EA modulator is a semiconductor device
Typically has the same structure as the laser diode.
Use an applied electric field (reverse bias) to change the absorption spectrum.
Absorption spectrum changes with applied electric field Franz -Keldysh Effect
Extinction ratio enhancement in quantum well structures
Quantum confined Stark effect

46. Schematics of an EA modulator
External electric field effects the attenuation coefficient

47. EA absorber advantages
Zero biasing voltage
Low driving voltage
Low/negative chirp
High speed
Integrated with DFB

48. Wavelength converters
Opticoelectric conversion. In optoelectric converters, the optic signal is converted in an electrical signal, which modulates a LASER, which is tuned to a specific wavelength
Gain Modulation in SOA. The gain of a SOA depends on the power of the signal which is inserted in the SOA. Due to the dependence of the amplifier ‘s gain on the wavelength of the signal, a monochromatic signal of wavelength λ1 has its amplitude modulated from another signal on another wavelength λ2, so the information on the λ2 signal wavelength is transferred on the of λ1 signal.
Phase Modulation in SOA. A wave which is transmitted through the SOA on λ2 wavelength, the affects SOA ‘s refractive index, which affects the phase of another monochromatic signal on λ1 wavelength. With proper connections of two SOAs, the changes in the phase are converted in amplitude changes, so the monochromatic signal on λ1 is modulated in amplitude from the λ2 signal.
Wave mixing in SOA. With this method, two optical waves transmitted through the SOA, produce another wave because of the four-wave mixing with wavelength, which depends on the other two original signals. If one of the orgianl signals is modulated, then the modulation is transferred to the produced signal, resulting in wavelength conversion.

49. Mems for optical switching
Advantage: Simple digital control
Disadvantage: Optical beam traveling limited to 2-D plane, need N2 mirrors for N port. 32-port is probably the limit for single stage
Advantage: Optical beam traveling in 3-D space, need 2N mirrors for N port, potentially scalable to 4000-port in single stage
Disadvantage: Needs precise analog control - servo feedback required in general

50. 2N – 3 Dimensional Optical Switch
Using tiny electromechanical mirrors light steering is possible.
Higher volumes of data and voice traffic are possible
Enables any one of 256 optical inputs fibers to be routed to any one of 256 output fibers.
Principle of operation
Light coming in through an optical fiber at a MEMs mirror. Tipped at a slight angle, the mirror redirects the light to another MEMs mirror an out a different fiber
By changing the angle of the MEMs mirrors, the light is sent in a different direction and out a different fiber
Advantages
Electroptical conversions are NOT necessary
Lower power consumption
Possible expansion without replacing existing equipment
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