Optical Modulation 2024 PDF - KKEE3103 Optoelectronics

Prof. Ir Dr. Ahmad Ashrif A BAKAR, SM.IEEE, SM.OSA, M.IEM Electrical and Electronic Engineering Programme,...

Prof. Ir Dr. Ahmad Ashrif A BAKAR, SM.IEEE, SM.OSA, M.IEM Electrical and Electronic Engineering Programme, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, MALAYSIA. [email protected] Tel: +603-8911 8393 KKEE3103 | Optoelectronics MODULATION OF LIGHT 1 Course Schedule KKEE3103 Physical class Thursday (DK6) : 11AM – 12PM Friday (DK2) : 09AM – 11AM Week Lecturer Topic Dates Comment 1 MHHM Introduction to opto-electronics devices 11-Mar 2 MHHM Wave nature of light 18-Mar Nuzul Quran 28 Mac 3 MHHM Dielectric waveguides and optical fibers 25-Mar (Thursday holiday) 4 MHHM Stimulated Emission Devices: Lasers 1-Apr Online class Raya 10,11 April (No 5 Raya Week 8-Apr class) Mid Sem Break 15-Apr Stimulated Emission Devices: Optical 6 MSDZ 22-Apr Amplifiers 7 MSDZ Concept of Light Detection 29-Apr 8 AAAB Optical Modulation 9 &10 - May 9 AAAB + MHHM Polarization of light + Midsem 15 & 16 - May 10 AAAB WDM concepts and components 1 23 & 24 - May 11 AAAB WDM concepts and components II 30 & 31 - May 12 AAAB Current trend in Optoelectronics 6 & 7 - Jun 13 MSDZ/AAAB Optoelectronics design: simulation 10-Jun Optoelectronics design: simulation + 14 MSDZ/AAAB 17-Jun Assignment Presentation 15 Study Week 24-Jun 16-17 Exam final 1-Jul 2 Course & Programme Outcome HPK1 : Berkebolehan merekabentuk komponen dan sistem aktif berdasarkan pengetahuan teknologi opto-elektronik. CO1: Ability to design an active component and system based on knowledge of optoelectronic technology. Berkebolehan memasang dan mengukur peranti optik dan sistem menggunakan perisian pemodelan dan alat HPK2 : optik. CO2: Ability to assemble and measure optical device and system using modelling software and optical tools. HPK3 : Berkebolehan menyelesaikan masalah berkaitan teknologi asas opto-elektronik. CO3: Ability to solve problems associated with the fundamental optoelectronic. Berkebolehan membentangkan penyelesaian dan menyelesaikan masalah yang diberikan berkenaan applikasi HPK4 : teknologi optoelektronik. CO4: Ability to propose a solution and to solve problems related to optoelectronic application. 3 Overview Modulation of laser diodes Direct modulation External modulation Electro-optic effects Integrated optical modulators Acusto-optic modulator 4 Modulation of laser diode Modulation - Process of putting information onto a lightwave For data rates of less than approximately 10 Gb/s (typically 2.5 Gb/s), the process of imposing information on a laser- emitted light stream can be realized by direct modulation. This involves directly varying the laser drive current with the electrically formatted information stream to produce a correspondingly varying optical output power. For higher data rates one needs to use a device called an external modulator to temporally modify a steady optical power level emitted by the laser. 5 Integrated Optical Devices Also known Photonic Integrated Circuits – PIC Devices used in high-speed coherent optical communication Examples: Modulators (phase, amplitude and polarization), Filters Mainly used material: Lithium Niobate (LiNbO3) Categories of modulation: Direct & External 1) Direct Modulation: Used in the early optical communication system when data rate was slow Modulation data (data signal) is combined with the bias voltage of the laser diode 2) External Modulation: Laser diode operates at a constant bias producing a narrow line width continuous optical carrier The data is imposed on the optical carrier by the modulator, external to the laser cavity. 6 Optical Interference Definition: Interaction of two or more lightwaves yielding a resultant irradiance that deviates from the sum of the component irradiance Superposition of two waves to form a resultant wave of greater or lower amplitude Principle of Superposition: The electric field intensity 𝑬 at a point in space, arising from the separate fields 𝑬𝟏 , 𝑬𝟐 , … of various contributing sources is given by: 𝑬 = 𝑬𝟏 + 𝑬𝟐 ….. Water waves from two in-phase point source in a ripple tank showing the overlapped, partially and completely canceling each How can we mathematically define the optical other. interference phenomenon based on the Principle of Superposition?? 7 Optical Modulation Direct Modulation Data and Bias Modulated LD Voltage of LD optical signal External Modulation Bias Voltage Modulated Modulato of LD (fix) optical signal r Data Quiz 3: What makes the external modulation are more required than direct modulation? Explain the advantages of external modulation against direct modulation. 8 Electro-Optic Effect Definition: Change in dielectric constant of material due to electric field (Pockel effect) Causing the change in the refractive index of the material Most widely used material: Lithium Niobate Example of Devices: Phase modulator, Intensity modulator, Mach Zehnder Interferometer Example on how the electro-optic effect can change the character of propagating light 9 Practical Limitations of Devices pOptical power handling capability: Limited by an effect known as photorefractive damage. The best way to avoid photorefractive damage is to keep the optical intensity below the specified limit. p Piezoelectric: The same electrical signal that produces phase modulation also generates vibrations. Strains induced by these vibrations alter the indices of refraction via the elasto- optic effect. These vibrations can cause unwanted amplitude modulation or beam displacements at the modulation frequency. p Residual amplitude modulation: An ideal phase modulator should not modulate the intensity of an optical beam. Amplitude modulation will be induced by sources of back- reflection placed after the phase modulator. Unwanted amplitude modulation can be minimized by properly aligning the input polarization state to the principal axis of the modulator. 10  A variety of external modulators are available commercially either as a separate device or as an integral part of the laser transmitter package. 11  The basic limitation on the direct modulation rate of laser diodes depends on the spontaneous and stimulated emission carrier lifetimes and on the photon lifetime. The photon lifetime τph, is the average time that the photon resides in the lasing cavity before being lost either by absorption or by emission through the facets. *The carrier lifetime (recombination lifetime) is defined as the average time it takes an excess minority carrier to recombine. 12  If the laser is completely turned off after each pulse, the spontaneous carrier lifetime will limit the modulation rate. 13  Pulse modulation is then carried out by modulating the laser only in the operating region above threshold. In this region, the carrier lifetime is now shortened to the stimulated emission lifetime, so that high modulation rates are possible. 14 Limitation of direct modulation When using a direct modulated laser diode for high-speed transmission systems, the modulation frequency can be no larger than the frequency of the relaxation oscillations of the laser field. The relaxation oscillation depends on both the spontaneous lifetime and the photon lifetime. Theoretically, assuming a linear dependence of the optical gain on carrier density, the relaxation oscillation occurs approximately at 15  An example of a laser that has a relaxation-oscillation peak at 3 GHz is shown in figure Example of the relaxation-oscillation peak of a laser diode 16 Analog modulation Analog modulation of laser diodes is carried out by making the drive current above threshold proportional to the baseband information signal. A requirement for this modulation scheme is that a linear relation exist between the light output & the current input. 17 Signal Degradation However, signal degradation resulting from nonlinearities that are a consequence of the transient response characteristics of laser diodes make the implementation of analog intensity modulation susceptible to Intermodulation cross-modulation effects. The use of pulse code modulation or special compensation techniques can alleviate these nonlinear effects 18 Overview Modulation of laser diodes Direct modulation External modulation Electro-optic effects Integrated optical modulators Acusto-optic modulator 19 Problem with direct modulation When direct modulation is used in a laser transmitter the process of turning the laser on and off with an electrical drive current produces a widening of the laser linewidth. This phenomenon is referred to as chirp and makes directly modulated lasers undesirable for operation at data rates greater than about 2.5 Gb/s. 20 External Modulation For these higher-rate applications - external modulator. In such a configuration, the optical source emits a constant- amplitude light signal, which enters the optical modulator. Instead of varying the amplitude of the light, the electrical driving signal dynamically changes the optical power level. Produces a time-varying optical signal. The external modulator either can be integrated physically in the same package with the light source or it can be a separate device. The two main device types are the electro-optical phase modulator the electro absorption modulator (amplitude). 21  Electro-optic amplitude and phase modulators allow you to control the amplitude, phase, and polarization state of an optical beam electrically There are basically two types of modulators: 1. bulk 2. integrated-optic bulk Bulk modulators are made of discrete integrated-optic pieces of nonlinear optical crystals and are typically used on a lab bench or an optical table. They feature very low insertion losses, and high power-handling capability. Integrated-optic modulators, use waveguide technology to lower the required drive voltages, are wavelength specific. compact 22 Type of Integrated Optical Modulator 23 Optical Phase Modulator The most common bulk phase modulator is the transverse modulator, as shown in figure, which consists of an electro-optic crystal between parallel electrodes. These modulators develop large electric fields between the electrodes while simultaneously providing a long interaction length, l, in which to accumulate phase shift. The optical phase shift, Δφ, obtained from applying a voltage, V, between the electrodes is given, where λ is the free- space wavelength, and d is the electrode separation. 24 Optical Phase Modulator 2𝜋 2𝜋 𝑛! 𝑉 𝑃ℎ𝑎𝑠𝑒 𝑐ℎ𝑎𝑛𝑔𝑒, ∆𝜑 = Δ𝑛𝐿Γ = 𝑟 𝐿Γ 𝜆 𝜆 2 𝑑 Channel waveguide made on a Lithium Niobate substrate Electrodes are used to apply electric field across the waveguide CW light is phase-shifted (PSK light) and taken out from the other end Binary data voltage changes the electric field between the electrodes changing the refractive index of the waveguide The change in refractive index changes the phase of the light The voltage changes between two levels (ON – OFF) making the output phase change between two levels 𝜆: Free-space wavelength of the light, L: Length of the modulator, n: Refractive index of the waveguide material, V: Applied voltage between the electrodes, d: is the separation between the electrodes, r: Electro-optic coefficient, Γ: Spatial overlap between optical intensity profile and the applied electric field 25 Optical Phase Modulator The Phase modulator is a device which changes the "phase" of optical signals by applying voltage. When voltage is not applied to the RF-electrode, n number of waves exist in the certain length. When voltage is applied to the RF-electrode, one more wave is added , which now means n+1 waves exist in the same length. In this case, the phase has been changed by 2π (360degree) and the half voltage of this is called the driving voltage. In case of long distance optical transmission, waveform is susceptible to degradation due to non- linear effect such as self-phase modulation etc. The phase modulator is applied to compensate for this degradation and makes it possible to transmit long distance. 26 Optical Phase Modulator Phase change between the two states is π Required voltage (Vπ) to obtain phase change of π: 𝜆 𝑑 𝑉! = " 𝑛 𝑟 𝐿Γ 27 Optical Phase Modulator A LiNbO3 Phase Modulator A LiNbO3 based phase modulator for use from the visible spectrum to telecom wavelngths, with modulation speeds up to 5 GHz. (© JENOPTIK Optical System GmbH.) 28 Half-voltage Vp. A commonly used figure of merit for electro-optic modulators is the half-wave voltage, Vπ. Vp - The voltage required for inducing a phase change of Π. In the case of an amplitude modulator, this half-wave voltage is the voltage needed to induce a phase difference between the two arms A and B and hence extinguish the output. The electro-optical amplitude modulator Electro-optic modulator is typically made of Lithium Niobate (LiNbO3). In an EO modulator the light bean is split in half and then sent through two separate paths, as shown in Figure 4.32. High-speed electric signal then changes the phase of the light signal in one of the paths. This is done in such a manner that when the two halves of the signal meet again at the device output, they will recombine either constructively or destructively. The constructive recombination produces a bright signal and corresponds to a 1 pulse. 30 * The input light C 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. 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 A LiNbO3 based Mach-Zehnder amplitude modulator, with Eout µ Acos(wt + f) + Acos(wt - f) = 2Acosf cos(wt) modulation frequencies up to 5 GHz. This particular model has Vl/2 = 5 V at 1550 nm. (© JENOPTIK Optical System GmbH.) Pout (f ) = cos 2 f Pout (0) 31 * Eout µ Acos(wt + f) + Acos(wt - f) = 2Acosf cos(wt) The relative phase difference between the two waves A and B is therefore doubled with respect to a phase change Ø in a single arm. Output intensity = adding waves A and B at D. If A is the amplitude of wave A and B (assumed equal power spitting at C), the optical field at the output is Amplitude Output power Pout µ Eout2 ,which is maximum when Ø = 0. At any phase difference Ø, we have 32 * The power transfer is zero when Ø = p/2. In practice, the Y-junction losses and uneven splitting result A and B do not totally cancel out when Ø = p/2. Manufacturers often quote the extinction ratio, that is, the ratio between maximum and minimum power that can be transferred through the modulator by the application of a voltage. 33 Optical Amplitude Modulator Employs INTERFEROMETER principle Important parameters: viii.Polarization dependence i. Switching voltage-current ix. Input/output fiber connectors ii. Extinction ratio x. Insertion loss iii. Frequency response xi. Return loss (bandwidth) xii. Temperature sensitivity iv. Optical frequency chirp xiii.Size v. Distortion vi. Bias voltage stability vii. Operating wavelength range Two commonly used optical amplitude modulator: a) Mach-Zehnder interferometer b) Directional coupler 34 Optical Amplitude Modulator Optical signals from the LD is launched into the LN modulator through the PM fiber, then it is equally split into two waveguide at the first Y-junction on the substrate. When the voltage IS NOT applied to the RF electrode, the two signals are re-combined at the second Y-junction and coupled into a single output as two separated signals are in phase. In this case output signals from the LN modulator is recognized as "ONE“. When voltage is applied to the RF electrode, due to the electro-optic effects of LN substrate, refractive index is Basic structured LN modulator comprises of changed, and the phase of the optical signal in one arm is 1) two waveguides advanced though retarded in the other arm. 2) two Y-junctions When two signals are re-combined at the second Y- 3) RF/DC electrode. junction, they are transformed into higher order mode and lost as a radiation mode. In the case two signals are completely out of phase, all signals are lost into the substrate and the output signal from the LN modulator is recognized as "ZERO". The voltage difference which induces this "ZERO" and "ONE" is called the driving voltage of the modulator, and is one of the important parameters in deciding modulator's performance. 35 *https://www.soc.co.jp/sumitomo_e/business/optoelectronics-business/ln-modulator/application-note-for-ln-modulators/ Optical Amplitude Modulator The transfer function of Mach-Zehnder modulator is expressed as I(t)=aIqcos2(V(t)π/2Vπ), where I(t)=transmitted intensity, a=insertion loss, Iq=Input intensity from LD, V(t)=applied voltage, Vπ=driving voltage. It is necessary to set the static bias on the transmission curve through Bias electrode. It is common practice to set bias 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. DC drift is the phenomena, where this transmission curve gradually shift in the long term. 36 Optical Amplitude Modulator The transfer function of Mach-Zehnder modulator: I(t) = αIθ cos2(V(t)π/2Vπ) where I(t)=transmitted intensity, α=insertion loss, Iθ=Input intensity from LD, V(t)=applied voltage, Vπ=driving voltage. It is necessary to set the static bias on the transmission curve through Bias electrode. It is common to set bias point at 50% transmission point, Quadrature Bias point. Electrical digital signals are transformed into optical digital signal by switching voltage to both end from quadrature point. DC drift is the phenomena, where this transmission curve gradually shift in the long term. 37 Optical Amplitude Modulator Y junction at both input an output splits and combines power equally Let the data voltage is such that the electrodes A introduce a phase shift of in the optical signals in the two branches. We then have: 𝐸! = 𝐸" 𝑒 #$%&/( 𝐸( = 𝐸" 𝑒 )$%&/( The two fields are combined in the output Y- junction to give, out put field 𝐸 = 𝐸! + 𝐸( = 𝐸" 𝑒 #$%&/( +𝐸" 𝑒 )"$%&/( = 2𝐸" 𝑐𝑜𝑠 Δ𝜙/2 The intensity of light: 𝐼 ∝ 𝐸 ( ∝ 𝑐𝑜𝑠 ( Δ𝜙/2 For binary data, the phase changes between 0 and π, and the output intensity changes from 2E0 to 0 38 Optical Amplitude Modulator (Mach- Zehnder Modulator) 39 Acousto-Optic Modulator An acousto-optic modulator (AOM), also called a Bragg cell - uses the acousto-optic effect to diffract and shift the frequency of light using sound waves We can easily generate acoustic or ultrasonic waves in a suitable AO crystal by attaching an ultrasonic transducer, that is, a piezoelectric transducer to one of the crystal faces. The ultrasonic transducer has a piezoelectric crystal with two electrodes, which are driven by an RF source. The piezoelectric crystal vibrates and generates acoustic waves, which are coupled into the AO crystal. 40  The acoustic waves then propagate along the crystal, as visualized in Figure. These acoustic waves in the crystal propagate by rarefactions and compressions of the crystal, lead to a periodic variation in the strain A schematic illustration of the principle of the acousto-optic modulator. Then, lead to a periodic variation in the refractive index in synchronization with the acoustic wave amplitude. 41 Two modes of AOM operation Bragg regime Raman–Nath regime The width of the grating L is sufficiently large that we need to consider the interference of The width L of the diffraction grating is waves flowing from points along the width L so small or along z. The interaction of the incident light and the grating occurs almost along There is a through (transmitted) beam and a line in the x-direction. only one diffracted beam, corresponding to a first-order diffraction 42 Two modes of AOM operation 43  L essentially represents the interaction length of the incident light with the acoustic grating, (equivalent acoustic beam length). The condition for the Bragg regime is that the interaction length L should satisfy In which Λ is the acoustic wavelength given by Figure 6.33: Illustration of (c) Va/f, where Va is the acoustic velocity and f is Definitions of L and H based on the transducer and the AO modulator the acoustic frequency. geometry used. Suppose we take the light wavelength λ to be 1x10-6 m, and generate 100 MHz acoustic waves in a TeO2 crystal (AO modulator crystal) in which Va= 4.2 x 103 ms-1. 44  Figures (a) and (b) illustrate how AO modulators can be used in analog and digital modulation. Note how both the diffracted (I1) and the through beam (I0) are modulated by the voltage applied to the transducer. Analog modulation of an AO modulator. Ii is the input intensity, I0 is the zero-order diffraction, i.e. the transmitted light, and I1 is the first order diffracted (reflected) light. 45 * Bragg Regime Consider two coherent optical waves A and B being reflected from two adjacent acoustic wave fronts to become A1 and B1. These reflected waves can only constitute the diffracted beam if they are in phase. The angle q¢ is exaggerated (typically, this is a few degrees). 46 * 47 Frequency Shift Doppler effect gives rise to a shift in frequency w¢ = w ± Diffracted light frequency W Acoustic frequency Incident light frequency Frequency is w Frequency is w¢ 48 * We can also use photon and phonon interaction Incoming Scattered photon photon Consider energy and momentum conservation Phonon in the w¢ = w ± W crystal 2Lsinq¢ = l/n 49 AO Modulator: Example Example: Suppose that we generate 150 MHz acoustic waves on a TeO2 crystal. The RF transducer has a length (L) of 10 mm and a height (H) of 5 mm. Consider modulating a red-laser beam from a He-Ne laser, l = 632.8 nm. Calculate the acoustic wavelength *acoustic waves in a TeO2,Va= 4.2 x 103 ms-1 L = va/f L >> L2/l L > L2/l L > 1.2 mm, we can assume Bragg regime va (4.2 ´ 103 m s-1 ) -5 L= = 6 -1 = 2.8 ´ 10 m f (150 ´ 10 s ) 51 52