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Waveguide modulators Modulator of light - definition A device that imposes signal on a carrier telecommunications In general, changes in one wave train caused by another wave, such as amplitude or frequency modulation in radio. In contemporary fiber optics modulation usually means transferring information from electrical to optical domain. In optics the term generally is used as a synonym for contrast, particularly when applied to a series of parallel lines and spaces imaged by a lens (e.g. SLM) - optics (C) Sergiusz Patela 1997-2004 Waveguide modulators 2 Light modulators in photonics 1. Direct modulation of a light source (e.g. current of LD) 2. External modulators (for CW light sources) (C) Sergiusz Patela 1997-2004 Waveguide modulators 3 Why do we need external light modulators? • For some light sources direct modulation is impossible (e.g. fiber lasers) • Semiconductor light sources chirp (change wavelength) when modulated • Modulation speed is limited by the electrical capacitance of the source and the speed of migration of the charge carriers (C) Sergiusz Patela 1997-2004 Waveguide modulators 4 Parameters of electromagnetic wave r r ∂ E 2 ∇ E − µε 2 = 0 ∂t r E = E0 ( x, y, z) exp[i(ω t − β z )] 2 Eo - amplitude (intensity) Φ - phase P - polarization λ (ω) - wavelength (frequency) (C) Sergiusz Patela 1997-2004 Waveguide modulators 5 Classification of effects utilized in waveguide modulators 1. Absorptive effects: modifications of absorption coefficient (change of beam intensity). 2. Refractive effects: modifications of refractive index (resulting in changes of phase or direction of the beam, change of critical angle in total internal reflection). 3. (Micro)mechanical modulation (C) Sergiusz Patela 1997-2004 Waveguide modulators 6 The physical effects of light modulation 1. Absorptive effects a. Franz-Keldysh effect b. Quantum Confined Stark Effect c. Band filling with free carriers d. Stimulated emission 2. Refractive effects a. Electro-optic b. Magneto-optic c. Elasto-optic d. Acousto-optic e. Thermo-optic f. Free carriers depletion g. Polarization control in liquid crystals h. All absorptive effects through Kronig-Kramers relations 3. (Micro)mechanical modulation a. simple mechanical choppers b. optical scanners c. MEMS (micro-electro-mechanical systems), MOEMS (C) Sergiusz Patela 1997-2004 Waveguide modulators 7 Four types of light modulators • Electrooptic and magnetooptic modulators. Materials change refractive index under electric or magnetic fields. Special devices (e.g. a Mach-Zehnder interferometer) required to convert phase modulation into amplitude modulation • Electro-absorptive modulators. Material or structure changes absorption under applied electric field (e.g. reverse biased p-n junction). EA modulators are usually integrated with LDs. • Acoustooptic modulators. High frequency sound traveling inside material or structure diffracts light. • MOEMS modulators. Micromechanical beam deflectors or shutters change light intensity. (C) Sergiusz Patela 1997-2004 Waveguide modulators 8 Complex coefficient of refraction n = n’ - j n” n’ - real part, in colloquial language “refractive index” (responsible for phase changes, beam refraction, propagation speed) n” - imaginary part, sometimes presented as extinction k (beam attenuation) (C) Sergiusz Patela 1997-2004 Waveguide modulators 9 Physical effects utilized in light modulators - (1) absorptive effects Absorption (amplification) α = 4 πn”/λ n'’ Physical effects responsible for attenuation • Franz-Keldysh effect • QCSE (quantum confined Stark effect) - shift of quantum well exciton line • Band filling with free carriers • Stimulated emission (C) Sergiusz Patela 1997-2004 Waveguide modulators 10 Physical effects utilized in light modulators - (2) refraction effects Refractive index (n’) • Electro-optic effect • elastooptical • acoustooptical • magnetooptical, Faraday effect • free carrier injection (e.g. free electron plasma: free carrier absorption, band filling) • Free carriers depletion • QCSE • Polarization control in liquid crystals (C) Sergiusz Patela 1997-2004 Waveguide modulators 11 Physical effects utilized in light modulators - (3) (micro)mechanical devices (Micro)mechanical modulation a. simple mechanical choppers b. optical scanners c. MEMS, MOEMS (C) Sergiusz Patela 1997-2004 Waveguide modulators 12 Advantages and applications of optical modulators Advantages of waveguide modulators: • increase modulation speed and transmission bandwidth, • improve modulation quality (lower dispersion and distortion, eliminate chirp and crosstalk) • make optoelectronic converters obsolete. Applications: • Telecommunications: multimedia transmission (voice, video, data), ISDN (Integrated Services Digital Network), B-ISDN (Broad band ISDN) • Aerial terminals • Fiber optic gyroscopes • Laser pulse forming (C) Sergiusz Patela 1997-2004 Waveguide modulators 13 Laser chirp Every time a laser diode emits a pulse of light, free carrier concentration in active area is changed -> which results in refraction index change-> which changes wavelength of emitted light. The effect is called laser wavelength chirp. The result is wider spectral linewidth and bigger fiber dispersion . In fast optical telecommunications transmission systems (>10 Gbit/s, > 100 km inter-repeater distance) chirpfree modulation is necessary. One noteworthy exception is predistortion, intentionally introduced chirp that cancels dispersion. (C) Sergiusz Patela 1997-2004 Waveguide modulators 14 Bandwidth requirements Bandwidth requirements for modulators: • digital stereo sound 106 bit/s • digital TV 108 bit/s (100 Mbit/s) • high resolution TV ~1Gbit/s • 3D TV, teleconferencing 100 Gbit/s Speed requirements for switches: • speed 10 kbit/s -> 100 Gbit/s • multitude of link possibilities: point-point, point-multi point (splitter, multiplexer), unidirectional, bi-directional. (C) Sergiusz Patela 1997-2004 Waveguide modulators 15 Switching systems (multiplexer types) Multiplexing domains • Time division • Space division • Wavelength division multiplexing (WDM) (C) Sergiusz Patela 1997-2004 Waveguide modulators 16 Materials for fabrication of modulators and switches M aterial dielectric semiconductor (C) Sergiusz Patela 1997-2004 Physical effect refractive index change refractivei index or absorption change Waveguide modulators 17 Effect e-o: phase change Phase: φ= 2⋅π n⋅L λ 1 n = n0 − n03 ⋅ r ⋅ E 2 For GaAs modulator (100) when electric field is applied in <011> direction: φ 011 V - voltage Γ - overlap integral d - inter-electrode distance (C) Sergiusz Patela 1997-2004 2π L 3 = n r41VΓ λ d Waveguide modulators 19 Modulators – basic structures. Solid state electroabsorption modulator or phase modulator Mach-Zehnder modulator signal signal X coupler signal signal acoustooptic (diffraction) modulator Mode transformer (digital optical switch) signal signal Light beam in a planar waveguide (C) Sergiusz Patela 1997-2004 directional coupler Waveguide modulators few mrad 20 Modulators - basic structures. Micromechanical micromechanical modulator signal (C) Sergiusz Patela 1997-2004 Waveguide modulators 21 Strip waveguide structures (a ) (c) (b) (d) a) strip waveguide (elevated), b) built-in strip waveguide, (C) Sergiusz Patela 1997-2004 c) ridge waveguide, d) strip loaded waveguide Waveguide modulators 22 Electro absorption modulator Distance necessary to obtain asumed extinction coefficient Ξ [dB] (C) Sergiusz Patela 1997-2004 Waveguide modulators Ξ l= 4. 34 ⋅ ∆α 23 Mach-Zehndera modulator I out Distance to obtain phase shift of ∆β l = π l= I in (1 + cos Φ ) = 2 λ 2 ⋅ neff ⋅ ∆ eq Example characteristic length: ∆eq = 10-3 ÷ 10-8. For ∆eq ~ 10-5, L ~ 1cm (C) Sergiusz Patela 1997-2004 Waveguide modulators 24 Directional coupler Characteristic lenght (minimal coupling distance) (C) Sergiusz Patela 1997-2004 Waveguide modulators l= 3 ⋅λ 2 ⋅ n eff ⋅ ∆ eq 25 X coupler Electrode length is determined by an angle and strip width. l= 2 w ⋅ m 2 ⋅ ∆ eq w = strip width θ m= θc (C) Sergiusz Patela 1997-2004 Waveguide modulators 26 Directional coupler - basic structure (C) Sergiusz Patela 1997-2004 Waveguide modulators Schotky barrier Au-Pt Epitaxial layer GaAs Substrate GaAs Contact Au 27 Details of modulator structure (1) a) Ti/Au/Au - electrodes 2 um Al 0.032 Ga 0.968 As 0.968 As 1,6 um GaAs 5 um Al 0.032 Ga light beam 1,3 um substrate GaAs (C) Sergiusz Patela 1997-2004 Waveguide modulators 28 Details of modulator structure (2) b) Ti/Au/Au - electrodes 3 um GaAs 4 um Al 0.032 Ga 0.968 As light beam 1,3 um substrate GaAs (C) Sergiusz Patela 1997-2004 Waveguide modulators 29 Semiconductor electrooptic modulator Heterostructure Waveguide Microwave package U-groove Conductive epoxy Alundum substrate Microwave connector SMA Fiber Microwave microstrip line M.-Z waveguide modulator Modulator design (C) Sergiusz Patela 1997-2004 Waveguide modulators 30 Planar waveguide 2 µm Al0.03Ga0.97As separator 1 µm GaAs, Plan. waveguide light 1,3 µm GaAs, substrate Heterostructure (C) Sergiusz Patela 1997-2004 Waveguide modulators 31 Semiconductor electrooptic modulator Rib waveguide 4 µm A ridge waveguide structure (C) Sergiusz Patela 1997-2004 Waveguide modulators 32 Simple Mach-Zehnder interferometer Modulated output signal Y splitter Strip waveguide GaAs AlxGa1-xAs Input sinal λ=1.3 µm (C) Sergiusz Patela 1997-2004 GaAs Substrate Waveguide modulators 33 Details of modulator structure (2) b) Ti/Au/Au - electrodes 3 um GaAs 4 um Al 0.032 Ga 0.968 As light beam 1,3 um substrate GaAs (C) Sergiusz Patela 1997-2004 Waveguide modulators 34 Optoelectronic packaging of advanced modules Microwave package U-grove Conducting glue Alundum substrate Fiber waveguide Microwave SMA connect. microstrip line Optoelectronic modulator in a microwave package. Package contains modulator chip, microwave preamplifier, impedance matching circuit. (C) Sergiusz Patela 1997-2004 Waveguide modulators 35 Semiconductor electrooptic modulator M.-Z waveguide modulator (C) Sergiusz Patela 1997-2004 Waveguide modulators 36 M.-Z modulator speed evaluation 3.00 12.0 C/L [pF/cm] ∆f=1/ πRC Inter-electrode distance d = 10 µm electrode width W = 100 µm d/W = 0,1 ⇒ BL ~ 4 for L = 1 cm we obtain B = 4 GHz BL [GHz cm] 2.00 8.00 1.00 4.00 0.00 0.01 0.10 1.00 0.00 10.00 d/W Capacitance per unit length for coplanar electrodes structure in GaAs as a function of electrode length /width ratio. Also shown is bandwidth-length parameter (BL) for R=50Ω. (C) Sergiusz Patela 1997-2004 Waveguide modulators 37 Typical parameters of photonics modulators Parameter Bandwidth working wavelength Losses optical return loss maximal accepted optical power Extinction phase modulation efficiency Fiber waveguides working conditions * Value ** 2,5 (20) Selected telecommunications window (1300, 1500) 5 >40 <100 >20 ≤1 standard singlemode or PM standard or “typical laboratory” Unit GHz nm dB dB mW dB Rad/V 1 + ∆Φ 2 1 − ∆Φ 2 Mach-Zehnder interferometer (C) Sergiusz Patela 1997-2004 Waveguide modulators 38