1. 1

2. 23/03/2018 2 Integrated Optical Modulators Satya Prasanna mallick Regd.no-17708

3. Phase and Polarisation Modulators 23/03/2018 3

4. 4

5. Integrated Optical Modulators The electro-optic effect takes place over the spatial overlap region between the applied field and the optical fields. The spatial overlap efficiency is represented by a coefficient  The phase shift is  and depends on the voltage V through the Pockels effect 23/03/2018 5

6. Integrated Optical Modulators Spatial overlap efficiency = 0.5 – 0.7 Pockels coefficient Different for different crystal orientations Electrode separation Length of electrodes Induced phase change Applied voltage 23/03/2018 6

7. Integrated Optical Modulators: An Example Consider  an x-cut LiNbO 3 modulator with d  10  m, operating at  = 1.5  m This will have V /2 L  35 V  cm A modulator with L = 2 cm has V /2 = 17.5 V By comparison, for a z-cut LiNbO 3 plate, that is for light propagation along the y-direction and E a along z, the relevant Pockels coefficients (r 13 and r 33 ) are much greater than r 22 so that V  /2 L  5 V  cm  depends on the product V × L When  = , then V × L = V /2 L 23/03/2018 7

8. A LiNbO 3 Phase Modulator A LiNbO 3 based phase modulator for use from the visible spectrum to telecom wavelngths, with modulation speeds up to 5 GHz. This particular model has V /2 = 10 V at 1550 nm. ( © JENOPTIK Optical System GmbH.) 23/03/2018 8

9. Polarisation Modulator 23/03/2018 9  If Δφ=π/2 is created due to the Pockel cell then it acts like a QWP  If Δφ= π is created due to the Pockel cell then it acts like a HWP Hence if we adjust the voltage such that Δφ=π/2(or) π then we have created change in the polarisation ====> Pockel Cell acts as an Polarisation Modulator

10. Machzender Modulators 23/03/2018 10

11. Integrated Mach-Zehnder Modulators An integrated Mach-Zehnder optical intensity modulator. The input light is split into two coherent waves A and B, which are phase shifted by the applied voltage, and then the two are combined again at the output. 23/03/2018 11

12. Integrated Mach-Zehnder Modulators E out  Acos(  t +  ) + Acos(  t   ) = 2Acos  cos(  t) Approximate analysis Input C breaks into A and B A and B experience opposite phase changes arising from the Pockels effect A and B interfere at D. Assume they have the same amplitude A But, they have opposite phases Output power P out  E out 2 Amplitude 23/03/2018 12

13. Mach-Zehnder Modulator 23/03/2018 13

14. A LiNbO 3 Mach-Zehnder Modulator A LiNbO 3 based Mach-Zehnder amplitude modulator for use from the visible spectrum to telecom wavelengths, with modulation frequencies up to 5 GHz. This particular model has V /2 = 5 V at 1550 nm. (© JENOPTIK Optical System GmbH.) 23/03/2018 14

15. Coupled wave guide Modulators 23/03/2018 15

16. Coupled Waveguide Modulators (a)Cross section of two closely spaced waveguides A and B (separated by d) embedded in a substrate. The evanescent field from A extends into B and vice versa. Note: n A and n B > n s (= substrate index). (b)(b) Top view of the two guides A and B that are coupled along the z-direction. Light is fed into A at z = 0, and it is gradually transferred to B along z. At z = L o, all the light has been transferred to B. Beyond this point, light begins to be transferred back to A in the same way. 23/03/2018 16

17. Coupled Waveguide Modulators L o = Transfer distance 23/03/2018 17 If A and B are identical, full transfer of power from A to B occur over a coupling distance L o, called the transfer distance

18. Coupling Efficiency  =  3/L o  =  A   B = Mismatch between propagation constants AA BB When the mismatch then, power transfer is prevented We can induce this mismatch by applying a voltage (Pockels effect) 23/03/2018 18

19. Coupled Waveguide Modulator Voltage needed to switch the light off in B 23/03/2018 19 Voltage induced mismatch

20. Modulated Directional Coupler An integrated directional coupler. The applied field E a alters the refractive indices of the two guides (A and B) and therefore changes the strength of coupling. 23/03/2018 20

21. Modulated Directional Coupler: Example Example: Modulated Directional Coupler Suppose that two optical guides embedded in a substrate such as LiNbO 3 are coupled as in Figure 6.31 to form a directional coupler, and the transmission length L o = 10 mm. The coupling separation d is ~10  m,   0.7, the operating wavelength is 1.3  m where Pockels coefficient r  10  10  m/V and n  2.20. What is the switching voltage for this directional coupler? Solution 23/03/2018 21

22. Recent Developments 23/03/2018 22

23. 23/03/2018 23

24.  on-off keying (OOK), that is, a simple intensity modulation  All the Modulation Schemes that are used in Digital communication systems can be paractically implemented.  Nowadays not only silicon based Modulators but Graphene based Modulators are used as opticakl mopdulators 23/03/2018 24

25. Summary  Pockel and Kerr’s effects are the electric optic effects used as basis for all the optic modulators.  Electro optic effects are very necessary in the world that’s struggling with the losses due to Electric and unable to access the advantages and enormous gain of Optical sysytems.  The optical modulators play a very important role in the Digital communication systems which are the order of the day. 23/03/2018 25

26. REFERENCES  Optoelectronics and Photonoics,3 rd edition by S.O.Kasap  Wikipedia  http://users.encs.concordia.ca/~mojtaba/Light%20polarization.pdf http://users.encs.concordia.ca/~mojtaba/Light%20polarization.pdf  Fundamentals of Photonics Bahaa E. A. Saleh, Malvin Carl Teich  http://home.earthlink.net/~jimlux/hv/eotheory.htm#pockels http://home.earthlink.net/~jimlux/hv/eotheory.htm#pockels  Recent Developments in Optical Modulators  http://www.doitpoms.ac.uk/tlplib/dielectrics/dielectric_refractive_in dex.php http://www.doitpoms.ac.uk/tlplib/dielectrics/dielectric_refractive_in dex.php 23/03/2018 26

27. 23/03/2018 27