Paper review 1. Introduction 2. Basic technique and fabrication 3. Design, structures, and functions 4. Application (digital & nondigital) 5. Product development cycle and manufacturability 6. Conclusion 유병민 Special Topics in Optical Engineering II (15/1)
Introduction • Demand of modulation bandwidth Lithium niobate (LiNbO 3 ) external modulator • Fiber dispersion • Advantage of LiNbO3 modulator - Zero-chirp or adjustable-chirp operation - Stable operation over temperature - Very low voltage operation LiNbO 3 modulator has strong demand for use in fiber-optic communication systems. 유병민 Special Topics in Optical Engineering II (15/1)
Device Fabrication • Bandwidth constraints by physical and switching voltage tradeoff • Travelling wave electrodes - Much higher bandwidth - Electrical attenuation - Velocity mismatch (optical & electrical) • Required materials - Electro-optic substrate, buffer layer, electrode metal, electrode adhesion layer, dopant Cross section of x-cut LiNbO 3 MZM 유병민 Special Topics in Optical Engineering II (15/1)
Device Fabrication A. LiNbO 3 wafers • Reason why LiNbO3 is used for optical modulator - High electro-optic coefficients - High optical transparency in near infrared - Thermally, chemically, and mechanically stable materials • Susceptible to optical damage under 1 μ m wavelength • Different crystal cut different characteristics - x- and y-cut: simple method - z-cut: special handling procedure (piezo and pyro electric) Structure of LiNbO 3 and Cut type 유병민 Special Topics in Optical Engineering II (15/1)
Device Fabrication B. Waveguide fabrication • Indiffusion of Ti: simple method - Precaution is need (out-diffusion of Li) - Unwanted z-polarized & modulation performance adverse effect • Annealed proton exchange(APE)process - low-temperature process from acid bath (not applied y-cut) - Annealing process: index instability and deterioration of electro-optic effect 유병민 Special Topics in Optical Engineering II (15/1)
Device Fabrication C. Electrode fabrication • Method for electrode - Directly on surface - Optical transparent buffer layer (for reduce optical loss & velocity matching) • Electrode thickness: a few μ m to >15 μ m (for high speed modulator) • Electrode metal: gold is generally used - Small grain size, minimum feature distortion, high purity metal SEM picture of gold electrode 유병민 Special Topics in Optical Engineering II (15/1)
Device Fabrication D. Dicing and polishing • Substrate end faces are cut at and angle & polished to an optical finish - For reflect elimination - For good fiber-to-waveguide coupling E. Pigtailing, packaging, and test • Pigtailing, packaging optical and electrical signals can be efficiently and effectively directed into and away from the device. • Three principle subcomponents - Integrated-optic chip - Optical-fiber assemblies - Electrical interconnect and housing 유병민 Special Topics in Optical Engineering II (15/1)
Device Design • Types of electro-optic modulator: MZI based modulator, directional coupler type • MZI - High-bandwidth electrode structures - Long electrode are needed to reduce driving voltage (< 6V) • Directional coupler type - Lower speed switching applications (small size and polarization diversity) - Tight electrode gap easily accommodated Mach-Zehnder modulator Directional coupler type modulator 유병민 Special Topics in Optical Engineering II (15/1)
Device Design • x-cut - Unnecessary buffer layer - But for high speed, buffer layer is needed for velocity matching - Relatively lower overlap (e-field & waveguide) • z-cut - Conductive buffer layer: dc drift ↓ , optical loss ↓ - Charge bleed layer: pyroelectric charge build up ↓ - High overlap (e-field & waveguide) • Dual-drive topology - Factor of two modulation efficiency improvement - Zero-chirp operation Four most common electrode 유병민 Special Topics in Optical Engineering II (15/1)
Device Design E/O response in various application E/O response in same electrode length Normalized electrode length for several application 유병민 Special Topics in Optical Engineering II (15/1)
Systems Requirements and Digital Performance • Predominant technique for optical fiber: on-off keying (OOK) - Minimizing the fiber link degradation due to group velocity dispersion - 2.5Gb/s (1000 km), 10 Gb/s (500 km) • Data encoding: nonreturn-to-zero (NRZ) • Eye diagram: digital data is typically presented Data modulator topology for NRZ transmission 유병민 Special Topics in Optical Engineering II (15/1)
Systems Requirements and Digital Performance 2.5(left) and10(right) Gb/s data transmission • Eye opening: accuracy of the receiver and lower BER • Stochastic process in nature: long transmission time accuracy quantify system’s BER • Key performance parameter of modulator - Switching voltage, S21, S11, extinction ratio, excess optical loss, optical chirp 유병민 Special Topics in Optical Engineering II (15/1)
Systems Requirements and Digital Performance • S21: electro optic response - In digital system, 3 dB rolloff at high frequency - Ex) for 2.5 Gb/s operation, 2.2 GHz 3 dB bandwidth is needed • S11: electrical return loss - Typically less than -10 dB < 5% reduction in electro optic efficiency • Extinction ratio: ratio between on state and off state - In digital application > 20 dB is needed - Link power penalty �� � 1 � ��� � � �10 ∙ log � � 1� - 20 dB extinction ratio > 90% eye opening 유병민 Special Topics in Optical Engineering II (15/1)
Systems Requirements and Digital Performance • Limiting NRZ data encoding - Very high DWDM channel loading - High bit-rate requirement - Wavelength intelligent network • RZ data is employed for high data rate terrestrial and submarine systems 유병민 Special Topics in Optical Engineering II (15/1)
Systems Requirements and Digital Performance Acquired waveforms of 10 Gb/s NRZ and RZ data 유병민 Special Topics in Optical Engineering II (15/1)
Nondigital Applications • Nondigital applications - CATV signal distribution - Wide-band microwave signal distribution - Antenna remoting • CATV: first successful commercial application • Modulator is specialized in CATV signal distribution - Amplitude and phase modulation Brillouin scattering ↓ - Separate bias port - Two complimentary optical output • LiNbO3 modulator in nondigital application - Military and aerospace application - Rader delay line, towed aircraft decoys and wide band antenna remoting 유병민 Special Topics in Optical Engineering II (15/1)
Product Development and Manufacturability • Tradeoff in manufacturing - Performance - Standard manufacturing process capability - Development time • Standard design elements and process reduction cycle of manufacture - optical circuit fabrication - Device packaging - Testing LiNbO 3 modulator produce step 유병민 Special Topics in Optical Engineering II (15/1)
Reliability • One of important performance parameter Bias voltage drift 2.5 Gb/s Ti Bias voltage drift 2.5 Gb/s APE modulator x-cut 85 and 100 ℃ modulator x-cut 85 and 100 ℃ 유병민 Special Topics in Optical Engineering II (15/1)
Reliability Bias voltage drift 10 Gb/s Ti modulator x- cut 85 ℃ Bias voltage drift 10 Gb/s Ti modulator x-cut various temperature Insertion loss change 85 ℃, 2.5Gb/s, x-cut Ti modulator 유병민 Special Topics in Optical Engineering II (15/1)
Conclusion • LiNbO 3 modulator is widely used in fiber-optic systems • Chirped and zero-chirp NRZ and RZ digital transmission formats • Extremely reliable digital and analog communication systems • Growing complexity of fiber-optic communication systems and networks development of LiNbO 3 modulator 유병민 Special Topics in Optical Engineering II (15/1)
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