Paper review 1. Introduction 2. Basic technique and fabrication 3. - - PowerPoint PPT Presentation

Special Topics in Optical Engineering II (15/1) 유병민

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

Special Topics in Optical Engineering II (15/1) 유병민

Introduction

• Demand of modulation bandwidth
• Fiber dispersion

 Lithium niobate (LiNbO3) external modulator

• Advantage of LiNbO3 modulator
• Zero-chirp or adjustable-chirp operation
• Stable operation over temperature
• Very low voltage operation

 LiNbO3 modulator has strong demand for use in fiber-optic communication systems.

Special Topics in Optical Engineering II (15/1) 유병민

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 LiNbO3 MZM

Special Topics in Optical Engineering II (15/1) 유병민

Device Fabrication

• A. LiNbO3 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 1μm wavelength
• Different crystal cut  different characteristics
• x- and y-cut: simple method
• z-cut: special handling procedure

(piezo and pyro electric)

Structure of LiNbO3 and Cut type

Special Topics in Optical Engineering II (15/1) 유병민

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

Special Topics in Optical Engineering II (15/1) 유병민

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 μm to >15 μm (for high speed modulator)
• Electrode metal: gold is generally used
• Small grain size, minimum feature distortion, high purity metal

SEM picture of gold electrode

Special Topics in Optical Engineering II (15/1) 유병민

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

Special Topics in Optical Engineering II (15/1) 유병민

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

Special Topics in Optical Engineering II (15/1) 유병민

Device Design

Four most common electrode

• 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

Special Topics in Optical Engineering II (15/1) 유병민

Device Design

E/O response in various application E/O response in same electrode length Normalized electrode length for several application

Special Topics in Optical Engineering II (15/1) 유병민

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

Special Topics in Optical Engineering II (15/1) 유병민

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

Special Topics in Optical Engineering II (15/1) 유병민

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
• 10 ∙ log

1 1

• 20 dB extinction ratio  > 90% eye opening

Special Topics in Optical Engineering II (15/1) 유병민

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

Special Topics in Optical Engineering II (15/1) 유병민

Systems Requirements and Digital Performance

Acquired waveforms of 10 Gb/s NRZ and RZ data

Special Topics in Optical Engineering II (15/1) 유병민

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

Special Topics in Optical Engineering II (15/1) 유병민

Product Development and Manufacturability

LiNbO3 modulator produce step

• Tradeoff in manufacturing
• Performance
• Standard manufacturing process

capability

• Development time
• Standard design elements and process

 reduction cycle of manufacture

• ptical circuit fabrication
• Device packaging
• Testing

Special Topics in Optical Engineering II (15/1) 유병민

Reliability

• One of important performance parameter

Bias voltage drift 2.5 Gb/s Ti modulator x-cut 85 and 100 ℃ Bias voltage drift 2.5 Gb/s APE modulator x-cut 85 and 100 ℃

Special Topics in Optical Engineering II (15/1) 유병민

Reliability

Bias voltage drift 10 Gb/s Ti modulator x-cut various temperature

Bias voltage drift 10 Gb/s Ti modulator x- cut 85 ℃ Insertion loss change 85℃, 2.5Gb/s, x-cut Ti modulator

Special Topics in Optical Engineering II (15/1) 유병민

Conclusion

• LiNbO3 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 LiNbO3 modulator