YONSEI UNIVERSITY Introduction Lightwave Modulation : Process - - PowerPoint PPT Presentation

YONSEI UNIVERSITY

Converged Advanced Network Special Topics in Optical Engineering Ⅱ 2015-04-03 Paper Review Soo-Min Kang

Tetsuya Kawanishi, Takahide Sakamoto, and Masayuki Izutsu, IEEE

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UNIVERSITY

Introduction

 Lightwave  Modulation : Process of varying properties(A,f,𝝌)

• f periodic waveform to match the

transmission channel Data Signal

Optical Modulator

Channel

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PM MZM SSB, FSK High-order Sideband Generation

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• 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,
• 𝐵𝑀𝑋 : optical transmittance
• 𝑔

0 : optical carrier

• K : coupling coefficient between E-&O-signal
• V(t) : electric voltage on the electrode
• 𝐵𝑆𝐺 : index for optical phase deviation induced by electric signal
• 𝐾𝑜 𝐵 : 1st kind n-th order Bessel function

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Fig.2. 1st kind of Bessel function

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 Principle : MZ Intensity Modulator(EO effect)

• A pair of E-signal → 2-PM, balanced push-pull operation
• 3-type
• ON/OFF switching by changing V(t)

Fig.5. Intensity Modulation using MZM

Cf) MZM curves

 Output intensity ( lRl )

• 2. MZM (1)

Fig.3. MZM

Fig.4. Cross sections of MZM for push-pull operation

• A :

: induced phase of modulator

• 𝑕 𝑢 : optical phase difference between two arms of MZM
• C – axis : directed at x,z axis in x,z cut LN substrate

A = 𝒉 𝒖 /2 A = - 𝒉 𝒖 /2 On = Full bias Off = Null bias

X-cut MZM Z-cut MZM

Z-cut dual-electrode MZM

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• 2. MZM (2)

 In practical, Ref.[31]

• Problem : Imbalance in 2-arms, High-order radiative wave → ER↓
• Solution : Active trimmer
• ER : Extinction Ratio

Fig.6. Degradation of ER

• Fig8. After trimmer

ㅡ w/ trimmer ㅡ w/o

ER : 40dB increases

Fig.7. High ER Intensity modulator using active trimmers

• Bias control with 3 – Electrodes
• Trimmer compensates problem → ER ↑
• DSB-SC : Double-sideband Suppressed Carrier
• ∅𝐶 : DC bias
• 𝑔

0 : Input optical frequency

• 𝑔

𝑛 : Input RF signal frequency

• USB(Upper sideband), LSB(Lower sideband)

 Output  Application : DSB-SC Modulation

𝒈𝟏 𝒈𝟏 + 𝒈𝒏 𝒈𝟏 - 𝒈𝒏

Two-tone lightwave generation

• Add sinusoidal signals to Electrode C → Null-point at C

→ generation DSB(USB & LSB) components

4/11 Fig.10-11. w/, w/o trimmer

• Imperfection of MZM fabrication,

electric circuits problem → Use Trimmer

𝒈𝟏 𝒈𝟏 + 𝒈𝒏 𝒈𝟏 + 𝒈𝒏

Fig.9. DSB-SC signal w,w/o trimmer

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P Q Q : 90 ° Delay(assumption) +𝟐 −𝟐

MZA

Path 1+3

MZB

Path 2+4

P

Path 1+3

Q

Path 2+4

• 3. SSB, FSK Modulator (1)

 SSB Principle Ref.[13,14]

• 2-MZM, RFA,B, DCA,B,C

Fig.13. Optical SSB Modulator

• RFA, RFB : traveling wave electrodes

for high speed operation

• DCc : main MZ has an electrode for dc-bias
• Key
• Path 1 : 𝐝𝐩𝐭(θ + 𝟏) / 3 : 𝒅𝒑𝒕(θ + π)
• Path 2 : 𝒅𝒑𝒕(θ + π

𝟑 ) / 4 : 𝒅𝒑𝒕(θ + 3π 𝟑 )

• Path 1,2,3,4 : null-bias → SC
• Amplitude of LSB / USB

/ 90° DCc 180° RFA 180° RFB

 SSB Output

• Phase difference of RF signal ↔ Lightwave

= ① 90° : USB

② - 90° : LSB −𝟐 +𝟐 +𝟐 −𝟐 +𝟐 −𝟐

1 + 𝑘𝑓𝑘∅𝐺𝑇𝐿 2

• ∅𝐺𝑇𝐿 : Induced phase difference at DCc

−1 + 𝑘𝑓𝑘∅𝐺𝑇𝐿 2 5/11

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 SSB using Optical Hilbert Transform

1) Make DSB by using 1-MZM

Cf) SSB using OHT

• Ref. Ze Li, Photonics Technology Letters,

Optical Single-Sideband Modulation Using a Fiber-Bragg-Grating-Based Optical Hilbert Transformer, 2011.

𝒈𝒅 𝒈𝒅 + 𝒈𝒏 𝒈𝒅 - 𝒈𝒏

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

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• 3. SSB, FSK Modulator (2)

 FSK Principle Ref.[9,10]

• FSK Concept
• Principle

 FSK Modulator Ref.[11,13]

Fig.14. Optical FSK Modulator

• RFA, RFB : null-point, dc-bias is controlled by

RFA,B

• RFc : make 90° phase difference → SSB
• Why not DCc but RFc?

: RFc is faster than DCc because DCc’s switching time is limited by electrode response in high speed operation

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• 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
• Modulation Frequency : 40GHz
• NRZ PRBS 40Gbps data signal to C1

Fig.20. Optical spectra of z-cut FSK/SSB Modulator

• Suppress input optical carrier and make SSB-SC
• Convertible into USB or LSB by changing bias at MZc

𝒈𝟏 𝒈𝟏 + 𝒈𝒏 𝒈𝟏 - 𝒈𝒏

Fig.21. Overall band specra

80GHz

Fig.22-23. Demodulated FSK Signal & Eye pattern

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• 3. SSB, FSK Modulator (4)

 Application (2) : 80Gbps DQPSK Modulation Ref.[4,19,20,32]

• DQPSK Concept

I Q 1-bit delay

• Optical 3dB BW of each electrode : 27GHz
• Insertion loss : 5.1dB
• Modulation Frequency : 40GHz
• NRZ PRBS 40Gbps data signal

(10Gbps for each channel x 4)

Fig.24. DQPSK using z-cut FSK/SSB Modulator Fig.25. Spectrum of 80Gbps optical DQPSK

• 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.26-27. Eye diagram of DQPSK signal and Optical B2B BER curves Electrical Binary NRZ Data Stream( I ) Electrical Binary NRZ Data Stream( Q ) Optical DQPSK Signal( Q ) Optical DQPSK Signal( I ) 8/11

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 Application (1) : QDSB-SC

• Carrier suppression ratio:45.8dB
• Spurious suppression ratio:41.8dB
• Make 42GHz millimeter wave

Cf) Experiment setup Ref.[20]

• QDSB-SC : Quadruple DSB-SC
• 𝑔𝑛 : modulating frequency, 10.5GHz
• 4. High-order Sideband Generation (1)

 High-order sideband Ref.[19,20]

• Using harmonics : 𝒈𝟏 + 𝐎𝒈𝒏

 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

𝒈𝟏 𝒈𝟏 + 𝒈𝒏 𝒈𝟏 - 𝒈𝒏 𝒈𝟏 + 2𝒈𝒏 𝒈𝟏 + 𝐎𝒈𝒏

… …

𝒈𝟏 - 2𝒈𝒏 𝒈𝟏 - 𝐎𝒈𝒏

4𝒈𝒏

Fig.28. QDSB Modulation

• Advantage of QDSB-SC
• More Robust in Vibration,

Temperature than mode locking , LO-mixing, etc.

• Disadvantages of QDSB-SC
• Small modulation efficiency

(use of nonlinearity of Modulator)

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• N : N-th order of harmonics

Fig.29. Optical spectrum of QDSB-SC Fig.30. RF spectrum of QDSB-SC

Phase difference between two-modulator

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𝑔

1 𝑔 2

𝑔

4

𝑔

3

𝑔

4

𝑔

0𝑔 1𝑔 2𝑔 3

𝑔

5

𝑔

1𝑔 2𝑔 3

𝑔

1 𝑔 2

• 4. High-order Sideband Generation (2)

 Application (2) : ROM Ref.[21-24,34-36]

• Reciprocating Optical Modulation
• Recycling sideband components → High power(overcome QDSB-SC’s small signal)
• 2-FBG + 1-PM

Fig.31. Schematic of ROM 𝑔

• Reciprocate : 왕복운동을 하다

𝑆𝐺 signal frequency = 39.06GHz

𝑔

1

𝑔 𝑔

0𝑔 1

𝑔

−1

𝑔

0𝑔 1 𝑔 2

𝑔

0𝑔 1𝑔 2𝑔 3

dual port

𝑔

0 𝑔 1 𝑔 2 𝑔 3𝑔 4

𝑔

4

𝑔

0𝑔 1𝑔 2𝑔 3

𝑔

5

FBG PM FBG

𝑔 𝑔 Fig.33. Output lightwave spectrum Cf) Principle of harmonic generation

• Can make upto 8th order LSB 312.48GHz
• Spectral components in ROM are stationary phase-

locked to each other without PLL

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Conclusion

 Summary

Phase  EO Effect → PM, MZM  Application : DSB-SC Modulation High-order  High-order Sideband Generation  Application : QDSB-SC Modulation, ROM Intensity

 IM, OOK

Frequency shift

 SSB, FSK  Application : FSK/SSB Modulation (=SSB-SC)

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Converged Advanced Network Special Topics in Optical Engineering Ⅱ 2015-04-03 Paper Review Soo-Min Kang

roemee817@naver.com