Semiconductor Optical Communication Components and Devices Lecture 39: Optical Modulators Prof. Utpal Das Professor, Department of Electrical Engineering, Laser Technology Program, Indian Institute of Technology, Kanpur http://www.iitk.ac.in/ee/faculty/det_resume/utpal.html
Modulation of light Diode Lasers can be directly modulated, however, for high-speed modulation there is line broadening due to Chirping through the Henry factor (a H ). Hence a separate external modulator is preferred. Here we would discuss only semiconductor based modulators as they are only suitable for integration with the lasers and photodetectors. The external modulators are essentially based on three factors: 1. Electro-optic Modulator (EOM): based on refractive index change due to applied electric field, 2. Electro-absorption Modulator (EAM): based on the change in the absorption coefficient due to the application of electric field, 3. All optical switch/modulator: based on non-linear refractive index change due to application of a large optical field. It is essential that the modulation occurs in low insertion loss conditions and hence those phenomena that employ free carrier generation are avoided. In bulk semiconductors the electro-optic effect used is the Frantz-Keldysh Effect (FKE) to lower the band gap on application of electric field. This is typically used for EAM. As the operating wavelength is close to the band edge, the insertion loss is generally high due to band tails and chirping is also quite high. For EOM devices on bulk semiconductors both Pockel s Effect and Kerr effect are applied together. However, the electro-optic coefficients are quite low for bulk semiconductors as compared to other material such a Lithium Niobate. Hence quantum well devices are used where the Quantum Confined Stark Effect (QCSE) is much stronger. This could be used for both EOM and EAM.
Optical Modulators in Integrated form For EOM the most popular is the Mach-Zehnder interferometric modulator, where the phase is modulated in one or both the arms of the interferometer resulting in a change in the intensity. The trick is to choose the detuning from the band-edge such that required voltage drive is not large but at the same time insertion loss and chirping is kept to a minimum. The speed of the actual external modulators are today technology limited due to their parasitic resistance-capacitance (RC) frequency roll-off. External modulators have a special economic interest when integrated with lasers in simple Photonic Integrated Circuits (PICs). PICs and optoelectronic integrated circuits (OEICs) have been explored for more than 20 years and have now emerged as a key driver in extending speed, reliability, and cost. With > 60 GHz bandwidth and 86 100 Gb/s data rate demonstrations for only 1 V(peak to paek) drive opens a way to more complex PICs that are able to handle much larger data rates due to the very small size of Electro-Absorption/Optic Modulators. Examples of this using wavelength tunable chirped distributed Bragg reflector lasers, electro-absorption modulated lasers, and phase adjusters is given in Lecture 41.
Power after interference Optical Modulators Single mode waveguides Machzehnder Interferometric Modulator Higher order mode does not propagate through single mode waveguide 40Gbps push pull operation shown in IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 18, SEPTEMBER 15, pp1922-24, 2006 V p L No putput Single mode waveguides Bias for optimum Linearity Vp/2 P out(t) η v in(t) = 1+Cos π 0 V Pin 2 V π h accounts for the insertion loss of the modulator. Instead of using an electrode in one arm only high speeds can be obtained when the modulator is used in Push-Pull mode. i.e. both arms are DC biased at V p/2 and +/- v in /2 is applied on either arm. P out( ) = H( ) freq _response, P out(0) 1 f3db,opt. 3.f3dB,elect. L Reducing the length is no option as the switching voltage V p increases. The efficiency-bandwidth tradeoff can be significantly improved by using a distributed interaction of the applied electric field with the optical wave in a travelling-wave modulator. Vin(t)
EAM: using QCSE The Quantum Confined Stark Effect for a semiconductor Quantum Well is schematically shown below. RF out R L Ground Plane Optical power in Ground Plane Modulated power out RF in R G V S A travelling wave Coplanar Waveguide Electro-absorption Modulator (EAM) is shown here.
S 11 -S 12 (x10-20 m 2 /V 2 ) r 41 (x10-12 m/v) Electro optic constants for III-V cubic materials Linear (Pockel s) EO effect in Zinc-Blende crystals 0 0 0 0 0 0 0 0 0 r41 0 0 0 r 0 41 0 0 r 41 Quadratic (Kerr) EO effect in Zinc-Blende crystals (43m) s11 s12 s12 0 0 0 s12 s11 s12 0 0 0 s12 s12 s11 0 0 0 0 0 0 s44 0 0 0 0 0 0 s 0 44 0 0 0 0 0 s 44 2 0-2 -4-6 -8-10 EO coefficients are shown here are for In 1-x Ga x As y P 1-y y=0.34. -2-4 -6-8 -10 l 1.2 1.6 2.0 2.4 2.8 g Wavelength (mm) 2 0
Change in Phase (degree/cm) Electro optic constants for III-V cubic materials pnr 2. K r E K S S E [110] 1 41 y 2 11 12 y o l [110] S 11 -S 12 A Waveguide Electro-Optic Phase Modulator which uses Pockels, Kerr, and QCSE is shown here. [100] E 2.1 [110] [110] E 2.0 [110] In the Franz-Keldysh effect an electron can complete a transition to the (electric field tilted) conduction band by tunneling. 1.9 [110] 2 0-2 -4-6 -8 Applied reverse bias (V)
l signal All optical modulator This is based on the same principle of change of refractive index in one arm of a Mach-Zehnder Interferometer, but the refractive index change is due to nonlinearity of the material of the arm dependent on an intensity of a non absorbing wavelength other than that of the signal. Output dependent on the Switching signal intensity Single mode waveguides l switch Directional Coupler l signal n n n I tot 1 2 switch Coupling Length (L c ) Bar Cross l signal The coupled mode da 1(z) a jk T za 1(z) a 1(z) j12a 1(z) equations are: dz 2 da 2(z) a jk T za 2(z) a 2(z) j21a 2(z) waveguide loss coefficient. dz 2 jk Where zz a 1(z) A.Cos( z).e P jkzz Bar (z) A.Cos 2 ( z).e a a 2(z) C.Sin( z).e P 2 With the boundary conditions: a (0) A,and,a (0) 0 Cross (z) C.Sin ( z).e a 1 2 For symmetrical waveguides 12 = 21 = and a T is the T z T z
Directional Coupler As the phase relation is changed by the electrooptic effect one of the coupler waveguides on application of Voltage on the structure, the effective length of the coupling length could be doubled such that all power comes out through the Bar port (Yellow profile). l signal Double the Coupling Length Bar l signal Cross 3dB Coupler 3dB Coupler Splits the intensity in half. The input power is split into equal halves to the two output ports. The figure shows a codirectional coupler. Half the Coupling Length (L c )
Relative output power 3dB Coupler Modulator Push-Pull type The input power is split into equal halves to the two output ports on application of bias V. Coupling Length with bias for QCSE l signal Single mode waveguides V+v Bar V-v Cross 1 Bar Push-pull used to operate in the linear region Cross 0.5 2mm ridge Strained MQW waveguides with 1.5mm separation for coupling. 0 0 2 4 6 Applied Voltage 2v
Response (db) Push Pull Mach-Zehnder Switch with 3dB coupler Hot (Plus-d.c.) Regions etched through to substrate R. G. Walker, Appl. Phys. Lett., vol 54(17), 1613-1615 (1989) +d.c. Ground (Plus-d.c.) 0-2 -4-6 -8 0 2 4 6 8 10 Frequency (GHz)
Embedded ring resonator Filters/Modulators - I Embedded ring resonator filters made of waveguides has separate resonances for each ring in the resonator, which when aligned by design or tuning can resonate at wavelengths close to one another. This alignment of resonances gives rise to steep roll-off features in the spectrum of the device due to an increase of the order of the filter and/or because of formation of alternative light paths through the inner rings. l1, l2,..., ln l1, l3,..., ln l 2
Embedded ring resonator Filters/Modulators - II When the ring waveguides have quantum structure imbeded or if the material is Electro-optic, whose refractive index is tuned by a method other than electrically generated free carrier plasma makes, a Low Loss and Fast Electro-optically tuned embedded ring modulator or filter can be fabricated. The device constraints are the low dimensions of ~100s of nm widths of the waveguides and gaps involved. V 1 V 2 V 1
Embedded ring resonator Filters/Modulators - III Note the curves for with 1V bias and no bias cases, where the curves shift with bias. So if one takes for example a wavelength of 1530nm, at no bias the throughput power is low whereas with 1V bias the throughput power is high. Up 80GHz modulation has been theoretically predicted for InGaAsP/InP QW devices with a double resonator design.
Review Questions 1. What are the disadvantages of direct modulation of diode lasers? 2. What are the main phenomena on which an external modulator works? 3. Which devices are suitable for digital and which for analog external modulation of a laser? 4. What limits the operation frequency of external modulators? 5. What are the advantages and disadvantages of resonant ring modulators? 6. Why are embeded ring modulators becoming important? What are its limitations?