Special Topics in Optical Engineering Ⅱ C onverged 2015-04-03 Paper Review A dvanced N etwork Soo-Min Kang Tetsuya Kawanishi , Takahide Sakamoto , and Masayuki Izutsu , IEEE YONSEI UNIVERSITY
Introduction Lightwave Modulation : Process of varying properties( A,f, 𝝌 ) of periodic waveform to match the transmission channel Data Signal Channel Optical Modulator High-order PM MZM SSB, FSK Sideband Generation YONSEI 1/11 UNIVERSITY
1. PM Principle : EO(Electro → Optic) effect Electric Voltage → n ↕→ phase of lightwave ↕ Using EO material : Litium niobate(LN), litumn tantalite(LT), gallium arsenide(GaAs) Fig.1. Optical Phase Modulator Optical Output(R) , If f(t) is a sinusoidal signal, 0 Fig.2. 1 st kind of Bessel function 𝐵 𝑀𝑋 : optical transmittance • • V(t) : electric voltage on the electrode 𝐵 𝑆𝐺 : index for optical phase deviation induced by electric signal 𝑔 • • 0 : optical carrier 𝐾 𝑜 𝐵 : 1 st kind n-th order Bessel function • • K : coupling coefficient between E-&O-signal YONSEI 2/11 UNIVERSITY
2. MZM (1) Principle : MZ Intensity Modulator(EO effect) A pair of E-signal → 2-PM, balanced push-pull operation A = 𝒉 𝒖 /2 3-type • A : : induced phase of modulator A = - 𝒉 𝒖 /2 𝑢 : optical phase difference between two arms of MZM • • C – axis : directed at x,z axis in x,z cut LN substrate Fig.3. MZM X-cut MZM Z-cut MZM Z-cut dual-electrode MZM Fig.4. Cross sections of MZM for push-pull operation Output intensity ( lRl ) ON/OFF switching by changing V(t) = Full bias On = Null bias Off Fig.5. Intensity Modulation using MZM Cf) MZM curves YONSEI 3/11 UNIVERSITY
2. MZM (2) In practical, Ref.[31] • ER : Extinction Ratio Problem : Imbalance in 2-arms, High-order radiative wave → ER ↓ Solution : Active trimmer ㅡ w/ trimmer ㅡ w/o ER : 40dB Fig.6. Degradation of ER increases • Bias control with 3 – Electrodes • Trimmer compensates problem → ER ↑ Fig.7. High ER Intensity modulator using active trimmers Fig8. After trimmer Output Application : DSB-SC Modulation Imperfection of MZM fabrication, electric circuits problem Add sinusoidal signals to Electrode C → Null-point at C → generation DSB(USB & LSB) components → Use Trimmer 𝒈 𝟏 + 𝒈 𝒏 𝒈 𝟏 + 𝒈 𝒏 Two-tone lightwave generation 𝒈 𝟏 𝒈 𝟏 - 𝒈 𝒏 𝒈 𝟏 𝒈 𝟏 + 𝒈 𝒏 • DSB-SC : Double-sideband Suppressed Carrier • ∅ 𝐶 : DC bias 𝑔 • 0 : Input optical frequency 𝑔 • 𝑛 : Input RF signal frequency Fig.9. DSB-SC signal w,w/o trimmer Fig.10-11. w/, w/o trimmer • USB(Upper sideband), LSB(Lower sideband) YONSEI 4/11 UNIVERSITY
3. SSB, FSK Modulator (1) SSB Output SSB Principle Ref.[13,14] 2-MZM, RF A,B , DC A,B,C Phase difference of RF signal ↔ Lightwave = ① 90 ° : USB ② - 90 ° : LSB Fig.13. Optical SSB Modulator RF A , RF B : traveling wave electrodes for high speed operation DCc : main MZ has an electrode for dc-bias +𝟐 +𝟐 −𝟐 −𝟐 MZ A P Path 1+3 Key 180 ° RF A Path 1+3 • Path 1 : 𝐝𝐩𝐭(θ + 𝟏 ) / 3 : 𝒅𝒑𝒕(θ + π ) P 90 ° +𝟐 Q −𝟐 MZ B • Path 2 : 𝒅𝒑𝒕(θ + π 𝟑 ) / 4 : 𝒅𝒑𝒕(θ + 3π +𝟐 −𝟐 DCc Q 𝟑 ) Path 2+4 Path 2+4 180 ° RF B Q : 90 ° Delay(assumption) • Path 1,2,3,4 : null-bias → SC • Amplitude of LSB / USB / 1 + 𝑘𝑓 𝑘∅ 𝐺𝑇𝐿 −1 + 𝑘𝑓 𝑘∅ 𝐺𝑇𝐿 2 2 • ∅ 𝐺𝑇𝐿 : Induced phase difference at DCc YONSEI 5/11 UNIVERSITY
Cf) SSB using OHT SSB using Optical Hilbert Transform Ref. Ze Li, Photonics Technology Letters, 1) Make DSB by using 1-MZM Optical Single-Sideband Modulation Using a Fiber-Bragg-Grating-Based Optical Hilbert Transformer, 2011. 𝒈 𝒅 - 𝒈 𝒏 𝒈 𝒅 𝒈 𝒅 + 𝒈 𝒏 2) Power splitter 3) Up-line : pass trough 4) Down-line : Make OPS(Optical Phase-shifter) + OFHT(Optical Fractional Hilbert Transformer) A) 𝜾 = 90 ° : USB B) 𝜾 = -90 ° : LSB + phase - phase YONSEI UNIVERSITY
3. SSB, FSK Modulator (2) FSK Principle Ref.[9,10] FSK Modulator Ref.[11,13] FSK Concept Principle Fig.14. Optical FSK Modulator Why not DCc but RFc ? : RFc is faster than DCc because DCc’s • RF A , RF B : null-point, dc-bias is controlled by switching time is limited by electrode RF A,B response in high speed operation • RFc : make 90 ° phase difference → SSB YONSEI 6/11 UNIVERSITY
3. SSB, FSK Modulator (3) Application (1) : FSK/SSB Modulator(SSB-SC) & 40GHz Optical frequency shift Experiment Ref.[9,13,14,32] Fig.17. Z-cut dual-electrode FSK/SSB Modulator Fig.19. FSK Modulation setup using z-cut FSK/SSB Modulator Modulator 3dB BW : 30GHz • Insertion loss : 4.2dB • Suppress input optical carrier and make SSB-SC Modulation Frequency : 40GHz • Convertible into USB or LSB by changing bias at MZc NRZ PRBS 40Gbps data signal to C1 • 𝒈 𝟏 - 𝒈 𝒏 𝒈 𝟏 + 𝒈 𝒏 80GHz 𝒈 𝟏 Fig.20. Optical spectra of z-cut Fig.21. Overall band specra Fig.22-23. Demodulated FSK Signal & Eye pattern FSK/SSB Modulator YONSEI 7/11 UNIVERSITY
3. SSB, FSK Modulator (4) Application (2) : 80Gbps DQPSK Modulation Ref.[4,19,20,32] DQPSK Concept Optical DQPSK Signal( I ) Electrical Binary NRZ Data Stream( I ) I 1-bit delay Fig.24. DQPSK using z-cut FSK/SSB Modulator • Optical 3dB BW of each electrode : 27GHz Insertion loss : 5.1dB • Modulation Frequency : 40GHz Q • • NRZ PRBS 40Gbps data signal (10Gbps for each channel x 4) Electrical Binary NRZ Data Stream( Q ) Optical DQPSK Signal( Q ) 1- bit delay’s constructive/destructive ports were connected to balanced PDs. 80Gbps transmission success ∆𝜒 : differential optical phase of 1-bit • interferometer B2B : Back – to - Back • Fig.25. Spectrum of Fig.26-27. Eye diagram of DQPSK signal and Optical B2B BER curves 80Gbps optical DQPSK YONSEI 8/11 UNIVERSITY
4. High-order Sideband Generation (1) High-order sideband Ref.[19,20] 4 𝒈 𝒏 Using harmonics : 𝒈 𝟏 + 𝐎𝒈 𝒏 … … • N : N-th order of harmonics 𝒈 𝟏 - 2 𝒈 𝒏 𝒈 𝟏 - 𝒈 𝒏 𝒈 𝟏 𝒈 𝟏 + 𝒈 𝒏 𝒈 𝟏 + 2 𝒈 𝒏 𝒈 𝟏 + 𝐎𝒈 𝒏 𝒈 𝟏 - 𝐎𝒈 𝒏 Why High-order? Upper limit of fm depends on transmission loss in travelling wave electrodes in MZM fm < 50GHz → use harmonics of fm which can make higher frequency components Application (1) : QDSB-SC • QDSB-SC : Quadruple DSB-SC 𝑔𝑛 : modulating frequency, 10.5GHz • Carrier suppression ratio:45.8dB Spurious suppression ratio : 41.8dB Make 42GHz millimeter wave Fig.28. QDSB Modulation Cf) Experiment setup Ref.[20] Advantage of QDSB-SC Phase difference • More Robust in Vibration, between two-modulator Temperature than mode locking , LO-mixing, etc. Disadvantages of QDSB-SC • Small modulation efficiency (use of nonlinearity of Modulator) Fig.29. Optical spectrum of QDSB-SC Fig.30. RF spectrum of QDSB-SC YONSEI 9/11 UNIVERSITY
4. High-order Sideband Generation (2) Application (2) : ROM Ref.[21-24,34-36] Reciprocate : 왕복운동을 하다 • 𝑆𝐺 signal frequency = 39.06GHz Reciprocating Optical Modulation Recycling sideband components → High power(overcome QDSB- SC’s small signal) 2-FBG + 1-PM 𝑔 𝑔 0 0 FBG FBG PM dual port 𝑔 𝑔 0 𝑔 𝑔 𝑔 0 0 −1 1 Fig.31. Schematic of ROM 𝑔 1 𝑔 𝑔 0 𝑔 2 1 𝑔 0 𝑔 1 𝑔 2 𝑔 1 𝑔 𝑔 3 2 𝑔 𝑔 1 𝑔 2 𝑔 1 𝑔 2 𝑔 0 𝑔 3 𝑔 3 4 Cf) Principle of harmonic generation 𝑔 1 𝑔 𝑔 𝑔 0 𝑔 1 𝑔 2 𝑔 𝑔 𝑔 𝑔 𝑔 𝑔 0 𝑔 1 𝑔 2 𝑔 𝑔 2 3 4 5 3 4 3 5 4 Can make upto 8 th order LSB 312.48GHz Spectral components in ROM are stationary phase- locked to each other without PLL Fig.33. Output lightwave spectrum YONSEI 10/11 UNIVERSITY
Conclusion Summary IM, OOK Intensity EO Effect → PM, MZM Phase Application : DSB-SC Modulation SSB, FSK Frequency Application : FSK/SSB Modulation (=SSB-SC) shift High-order Sideband Generation High-order Application : QDSB-SC Modulation, ROM YONSEI 11/11 UNIVERSITY
Special Topics in Optical Engineering Ⅱ C onverged 2015-04-03 Paper Review A dvanced N etwork Soo-Min Kang roemee817@naver.com YONSEI UNIVERSITY
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