利用弱共振腔費比布洛雷射二極體於被動式分波多工光纖網路之研究

國

立

交

通

大

學

光電工程研究所

博 士 論 文

利用弱共振腔費比布洛雷射二極體於

被動式分波多工光纖網路之研究

Studies on Weak-Resonant-Cavity-Fabry-Perot-Laser-Diode

-based Wavelength Division Multiplexer Passive Optical Network

研 究 生：廖育聖

指導教授：林恭如 教授

郭浩中 教授

利用弱共振腔費比布洛雷射二極體於

被動式分波多工光纖網路之研究

Studies on Weak-Resonant-Cavity-Fabry-Perot

-Laser-Diode-based Wavelength Division Multiplexer

Passive Optical Network

研 究 生：廖育聖 Student：Yu-Sheng Liao

指導教授：林恭如 Advisor：Gong-Ru Lin

郭浩中

Hao-Chung Kuo

國立交通大學 電機學院

光電工程研究所

博 士 論 文

A Dissertation

Submitted to Department of Photonics & Institute of Electro-Optical Engineering College of Electrical Engineering

National Chiao Tung University in partial Fulfillment of the Requirements

for the Degree of Doctor

in January 2011

Hsinchu, Taiwan, Republic of China

中華民國一百年一月

利用弱共振腔費比布洛雷射二極體於

被動式分波多工光纖網路之研究

研究生：廖育聖

指導教授：林恭如

郭浩中

國立交通大學 電機學院

光電工程研究所 博士班

摘

要

本論文研究之弱共振腔費比布洛雷射二極體於被動式分波多工光纖網

路系統，首先在可選擇的多通道傳輸上我們分析與展示側模注入鎖定費費

比布洛雷射二極體，此外，我們使用直接調變方法的 1%反射率的弱共振腔

費比布洛雷射二極體在注入鎖定效能於 2.5 千兆位元率與 25 公里傳輸討論

與模擬了增強型注入鎖定頻寬的特性可達 0.48 奈米，此弱共振腔費比布洛

雷射二極體可展示具有 25 個通道側模注入鎖定能力、鎖定範圍達 30 個奈

米、最小注入光功率為-7 分貝毫瓦、以及高於 7 分貝的增益消滅比。同時，

藉由使用增益切換式同調脈衝串列以及閥值電流降低的方式，我們展示了

寬增益頻譜的近似無色特性的被動式分波多工光纖網路光源，在溫度 8 度

的改變與誤碼率低於 10

-10

_{下接收靈敏度最多僅只差異 1 分貝，短線對傳的}

接收靈敏度可達-25.6 分貝毫瓦，傳輸 25 公里光纖的接收靈敏度在全 16 個

通道最高僅有 2 分貝的光率靈敏度損失。最後，藉由使用已整合的監控感

光二極體，我們展示一種嶄新的注入鎖定費比布洛雷射二極體自動回復機

制，這樣的架構可以有效地植入於網路備援系統中，於 2.5 千兆位元率可

達成的 50 秒內回覆 Q 值大於 8.2 以及側模抑制比達 35 分貝。這些研究將

有助於未來的被動式分波多工光纖網路系統發展。

Studies on Weak-Resonant-Cavity-Fabry-Perot

-Laser-Diode-based Wavelength Division

Multiplexer Passive Optical Network

Student：Yu-Sheng Liao

Advisors：Dr. Gong-Ru Lin

Dr.

Hao-Chung Kuo

Department of Photonics & Institute of Electro-Optical Engineering

College of Electrical Engineering

National Chiao Tung University

ABSTRACT

In this dissertation, we investigated the injection-locking

wavelength-division-multiplexer passive optical network (WDM-PON) system.

First, we introduced the side-mode injection-locked Fabry-Perot Laser Diode

(FPLD) transmission diagnosis of a multi-channel selectable

weak-resonant-cavity Fabry-Perot Laser Diode (WRC-FPLD). Moreover, the

injection-locking performance of a 1% WRC-FPLD and demonstrate the

2.5-Gbit/s & 25-km WDM-PON application with the directly modulated

WRC-FPLD based transmitter with enhanced injection-locking bandwidth of

0.48 nm was we discussed and simulated. A 25-channel locking capacity is

reported for such a side-mode injection-locked WRC-FPLD with corresponding

wavelength locking range of 30 nm, the minimal requested power of -7dBm and

gain extinction ratio of <7 dB was demonstrated. Furthermore, we investigate

quasi-color-free the WDM-PON transmitters with comparable broadband gain

spectrum by using an optically gain-switching coherent pulse-train and threshold

reduction of WRC-FPLDs. Nevertheless, such a degradation only induces a

power penalty of < 1dB at BER<10

-10

over changing temperature of 9 degree.

A receiving sensitivity back-to-back transmission can be -25.6 dBm, and 25-km

transmission power penalties is up to 2 dB with 16 channels. At last, we

demonstrated a novel in situ self-restoration scheme to track real time the

injection locking of a FPLD by monitoring a built-in integrated photodiode.

Such a scheme can be effectively implemented with a minimum amount of

redundant network resources to achieve self-restoration within 50 seconds with

Q factor >8.2 and side-mode suppression ratio (SMSR) >35 dB at 2.5 Gbit/s.

These investigations would be useful in the next generation injection-locking

based WDM-PON systems.

Acknowledgements

致謝

在博士班研究生的生涯中得到許多人的協助與支持，論文與學位方能順

利完成。要特別感謝我的指導教授 林恭如老師在研究的過程中不斷的給予耐

心的指導，在我低潮的時候給予我無私的協助支持與關懷，使我無論是在求

學態度或是做人處世上都受益良多，您淵博的學識與為人師表的風範，將是

我今後立身處世的標竿。除此之外，也感謝 郭浩中老師在學校時提供寶貴意

見與協助，使我能夠順利完成我的博士學位，在此表達內心最誠摯的謝意。

感謝口試委員 賴賴暎杰教授、陳智弘教授、李柏璁教授、鄭木海教授、

呂海涵教授、黃振發教授撥空來指導我的口試，提供許多寶貴的意見。實驗

室博士班學長張詠誠、林俊榮對我教導與幫忙，無論在學識上、實驗上以及

論文寫作上，都給我極大的幫助。 感謝中華電信研究所前瞻研究室廖虹惠、

林恭政，因為你們的協助，才能讓我的實驗得以順利進行。還有學弟妹們邱

奕祥、吳銘忠、林齊冠、陳家揚、張峻源、林螢聰、游昆潔、康榮瑞、紀裕

傑、程子剛、彭國璿、林嘉琪、林奕宏、林俊儒、李宜錚與所有幫助過我的

學弟妹們，在實驗上與生活上的協助，讓我在台北與新竹兩邊跑的情況下，

能夠順順利利完成學業。

最後我要感謝我最摯愛的爸爸媽媽，給予我一個良好的學習環境，野果

許多鼓勵與關懷，讓我可以順利完成我的博士學業。我哥哥、我姐姐的關心，

以及其他好朋友，謝謝你們默默的給我全力的支持與關愛。在此與你們分享

這份喜悅。謝謝！

CONTENTS

Page

Chinese Abstract

i

English Abstract

ii

Acknowledgements

iv

Contents

v

List of Figures

vii

Chapter 1

Introduction

1

1.1 Introduction of Fiber-to-the-Home (FTTH) 1 1.2 Introduction of Passive Optical Network (PON) 2 1.3 Introduction of Wavelength-Division-Multiplexer PON

(WDM-PON) 3 1.4 Research Motivation 5 1.5 Organization of Dissertation 7 References 8

Chapter 2

Side-mode injection-locked FPLD transmission diagnosis

10

2.1 Introduction and motivation 10 2.2 Side-mode suppressing ratio analysis 12 2.3 Degradation of linewidth enhancement factor on

injection-locked side-mode

17

2.4 Experimental Setup 20

2.5 Data transmission diagnosis of side-mode injection-locked FPLD

22

2.6 Summary 25

References 27

Chapter 3

Weak-resonant-cavity Fabry-Perot laser diode with enhanced

injection-locking bandwidth

29

3.1 Introduction and motivation 29

3.2 System structure 30

3.3 Enhanced injection-locking bandwidth 32 3.4 Modeling and experimental results of WRC-FPLD 38

3.5 Data transmission performance of WRC-FPLD 45

3.6 Summary 50

References 52

Chapter 4

54

Pulsating master and injected slave weak-resonant-cavity laser

diodes based quasi-color-free 2.5Gb/s WDM-PON

4.1 Introduction and motivation 54 4.2 Concept of coherent injection light source and

quasi-color-free injection locking

56

4.3 Performances and discussions of quasi-color-free 2.5Gb/s RZ WDM-PON

58

4.4 Summary 68

References 70

Chapter 5

72

Self-restorable injection-locking monitor by integrated

photodiode

5.1 Motivation and configuration 72 5.2 Modeling and experimental results of MPD-MCU based

auto-restorable FPLD transmitter

75

5.3 Concept and Circuit Design 83 5.4 Injection-locking monitor of self-restoration 87 5.5 System performances of the MCU-based auto-restorable

injection-locking transmitter 90 5.6 Summary 96 References 98

Chapter 6

Conclusions

100 6.1 Summary 100

6.2 Suggestions for Future Work 104 Curriculum Vitae

Figure List

Fig. 1.1 Poin-to-point fiber networks access. 2 Fig. 1.2 Time-division-multiplexer PON fiber networks access. 3 Fig. 2.1 Wavelength locking range of the injection-locked mode in slave FPLD versus injection

power. 12

Fig. 2.2 Measured SMSR curve on adjacent injected longitudinal mode and external optical

power. 13

Fig. 2.3(a) Theoretically simulated SMSR of the side-mode injection-locked FPLD as function of the reflectivity change (R) and the ratio of loss coefficient (m0). 15

Fig. 2.3(b) Comparison on the theoretical and experimental results of the SMSR for one specific

FPLD injection-locked mode. 17

Fig. 2.4 The simulated linewidth of the injection-locked FPLD as a function of the reflectivity

change (R). 18

Fig. 2.5 Spectral linewidths of the injection-locked FPLD in principle mode with (red line), without (blue line) modulation, and a reference laser source (black). 19 Fig. 2.6 The configuration of an experimental system with slave FPLD is side-mode

injection-locked by a wavelength-sliced master FPLD. 21 Fig. 2.7 Measured injected power (hollow markers) and measured Q (solid markers) of the

driving current at the principle longitudinal mode. 23 Fig. 2.8 BER analysis of wavelength injection-locked FPLD at different longitudinal modes and

measured eye diagrams (inset) with and without injection. 25 Fig. 3.1 A DWDM-PON system with a WRC-FPLD based transmitter at the ONU end that is

side-mode injection-locked by a wavelength-sliced master FPLD. 31 Fig. 3.2 A DWDM-PON system with a WRC-FPLD based transmitter at the ONU end that is

side-mode injection-locked by a wavelength-sliced master FPLD. 32 Fig. 3.3 The output optical spectra of the free-running and injection-locking WRC-FPLD at

different biased conditions. 33

Fig. 3.4 Injection-locking power dependent wavelength lock-in range of one longitudinal mode in the slave WRC-FPLD transmitter at the ONU end. 34 Fig. 3.5 Simulation of the normalized free-running spectra of WRC-FPLD with front-facet

reflectivity of 30%, 10%, 1%, 0.1% and 0.01%. 36 Fig. 3.6 The gain spectral linewidth (solid squares) and the gain extinction (hollow squares) vs.

the front-facet reflectivity of WRC-FPLD. 39 Fig. 3.7 The calculated Q factor and locking range of the injection locking WRC-FPLD with

different reflectivity and injection power. 40 Fig. 3.8 The Q-factors of 1-% reflectivity WRC-FPLD and 30-% reflectivity FPLD with

injection power of +3dBm, -3dBm, -9 dBm. 42 Fig. 3.9 The requested injecting power (red solid square), corresponding the measured best

Q-factor (red hollow square), and the minimal requested injecting power for Q=7.2 (blue solid square) at different driving currents. 43 Fig. 3.10 BER analysis of wavelength injection locked 1% WRC-FPLD at different longitudinal

modes and measured eye diagrams (inset) with and without injection. 46 Fig. 3.11 P-I curve of the WRC-FPLD with the different optical power injection of -3dBm,

-12dBm, and free-running. The inset shows the free-running optical spectrum. 48 Fig. 3.12 The numerically calculated small signal frequency response of laser with different

Ibias/Ith of 1.5, 3.5, and 5. 49

Fig. 4.1 The WRC-FPLD based bi-directional quasi-color-free 2.5-Gb/s RZ WDM-PON with a pulsed WRC-FPLD coherent injection-locker. 57 Fig. 4.2. Configuration and band structure of the 1% front-facet AR-coated WRC FPLD. 58 Fig. 4.3(a) The P-I curve of the slave WRC-FPLD under master WRC-FPLD injection-locking

with different power levels. 59

Fig. 4.3(b) Principle of the slave WRC-FPLD RZ transmitter triggered by externally injection from a pulsated master WRC-FPLD and directly modulated by a electrically PRBS

data-stream. 60

Fig. 4.4 Left: optical spectra of the master WRC-FPLD operated at free-running (gray) and gain-switching (red) condition. Upper right: the normalized mode spectra at free-runing (gray) and gain-switching (red) conditions. Lower right: the linewidth and pulsewidth of the master WRC-FPLD with different RF modulation powers. 61 Fig. 4.5 Injection-locking power dependent wavelength lock-in range and corresponding SMSR

of the slave WRC-FPLD transmitter. 62

Fig. 4.6 (a) Free-running AWG sliced WRC-FPLD. (b) Gain-switched AWG sliced WRC-FPLD. (c) Free-running slave WRC-FPLD injected by gain-switched WRC-FPLD. (d) NRZ modulated slave WRC-FPLD injected by gain-switched WRC-FPLD. 63 Fig. 4.7 the diagram of single-wavelength injection and gain-switched injection. 64 Fig. 4.8(a) Optical spectrum of injected WRC-FPLD at the temperature from 21oC to 29oC. 65 Fig. 4.8(b) BER of temperature-controlled WRC-FPLD with different injection locked mode

number. 66

Fig. 4.9(a) Injection-locking power dependent wavelength lock-in range of one longitudinal mode in the slave WRC-FPLD transmitter at the ONU end. 67 Fig. 4.9(b) BER analysis of wavelength injection locked WRC-FPLD at different channels and

measured pulsed RZ eye diagrams (inset). 68 Fig. 5.1 A self-restorable system for injection-locked FPLD with monitor photodiode. The

MCU detects the MPD photocurrent and controls the TEC and the polarization controller. 74 Fig. 5.2 The frequency response of the directly modulated FPLD with 3-dB bandwidth of

4.2GHz 75

Fig. 5.3(a) Spectrum evolution of FPLD by detuning per 0.1-nm wavelength. 76 Fig. 5.3(b) Wavelength locking range of a side mode in the slave FPLD as a function of

injection-locked power. 76

Fig. 5.4 (a) The photocurrent of MPD at the FPLD driving currents of 25, 30, and 35 mA as detuning wavelength. (b) The photocurrent of MPD at the injection power of + 3, 0, -3, -6,

and -9 dBm as detuning wavelength 78

Fig. 5.5 The conceptual flow chart for designing the PD-MCU link based self-restoration. 84 Fig. 5.6 The circuit block for building a PD-MCU link based self-restoration unit. 85 Fig. 5.7 (a) The monitored PD current (upper, square-dotted curve) and the free-running FPLD

spectrum (lower, block lines); (b) the spectra of the FPLD transmitter injection locked at different longitudinal modes (color lines at right part). 88 Fig. 5.8 The recovery experience of the self-restorable unlocked FPLD and corresponding MPD

current controlled by MCU. 90

Fig. 5.9 The BER of the FPLD transmitter measured at three different conditions. 91 Fig. 5.10 Left: (a) Threshold current of FPLD before and after self-restoration. Right: The

Optical eye-diagrams (b) before and (c) after auto-restoration. 92 Fig. 5.11 The BER of the FPLD transmitter before and after self-restoration at different

wavelength-deviation conditions. 93

ճҔ১Ӆਁ๚຤КѲࢶႜ৔Βཱུᡏܭ

೏୏ԄϩݢӭπӀᠼᆛၡϐࣴز

ࣴزғǺᄃػဃ

ࡰᏤ௲௤Ǻ݅ৰӵ

೾੏ύ

୯ҥҬ೯εᏢ ႝᐒᏢଣ

Ӏႝπำࣴز܌ റγ੤

ᄔ

ा

! ! ! ! ! !

ҁፕЎࣴزϐ১Ӆਁ๚຤КѲࢶႜ৔Βཱུᡏܭ೏୏ԄϩݢӭπӀᠼᆛ

ၡس಍Ǵ२Ӄӧёᒧ᏷ޑӭ೯ၰ໺ᒡ΢ךॺϩ݋ᆶ৖ҢୁኳݙΕᙹۓ຤຤

КѲࢶႜ৔ΒཱུᡏǴԜѦǴךॺ٬ҔޔௗፓᡂБݤޑ 1%ϸ৔౗ޑ১Ӆਁ๚

຤КѲࢶႜ৔ΒཱུᡏӧݙΕᙹۓਏૈܭ 2.5 ίӂՏϡ౗ᆶ 25 Ϧٚ໺ᒡ૸ፕ

ᆶኳᔕΑቚமࠠݙΕᙹۓᓎቨޑ੝܄ёၲ 0.48 ڼԯǴԜ১Ӆਁ๚຤КѲࢶ

ႜ৔Βཱུᡏё৖ҢڀԖ 25 ঁ೯ၰୁኳݙΕᙹۓૈΚǵᙹۓጄൎၲ 30 ঁڼ

ԯǵനλݙΕӀф౗ࣁ-7 ϩنడґǵаϷଯܭ 7 ϩنޑቚ੻੃ྐКǶӕਔǴ

ᙖҗ٬Ҕቚ੻ϪඤԄӕፓેፂՍӈаϷሚॶႝࢬफ़եޑБԄǴךॺ৖ҢΑ

ቨቚ੻ᓎ᛼ޑ߈՟คՅ੝܄ޑ೏୏ԄϩݢӭπӀᠼᆛၡӀྍǴӧྕࡋ 8 ࡋ

ޑׯᡂᆶᇤዸ౗եܭ 10

-10

Πௗԏᡫ௵ࡋനӭ໻ѝৡ౦ 1 ϩنǴอጕჹ໺ޑ

ௗԏᡫ௵ࡋёၲ-25.6 ϩنడґǴ໺ᒡ 25 ϦٚӀᠼޑௗԏᡫ௵ࡋӧӄ 16 ঁ

೯ၰനଯ໻Ԗ 2 ϩنޑӀ౗ᡫ௵ࡋཞѨǶനࡕǴᙖҗ٬Ҕς᏾ӝޑᅱ௓ག

ӀΒཱུᡏǴךॺ৖Ң΋ᅿ჻ཥޑݙΕᙹۓ຤КѲࢶႜ৔ΒཱུᡏԾ୏ӣൺᐒ

ڋǴ೭ኬޑࢎᄬёаԖਏӦ෌Εܭᆛၡഢජس಍ύǴܭ 2.5 ίӂՏϡ౗ё

ၲԋޑ 50 ࣾϣӣᙟ Q ॶεܭ 8.2 аϷୁኳ׭ڋКၲ 35 ϩنǶ೭٤ࣴزஒ

Ԗշܭ҂ٰޑ೏୏ԄϩݢӭπӀᠼᆛၡس಍ว৖Ƕ

Studies on Weak-Resonant-Cavity-Fabry-Perot

-Laser-Diode-based Wavelength Division

Multiplexer Passive Optical Network

StudentǺYu-Sheng Liao

!

AdvisorsǺDr. Gong-Ru Lin

Dr.

!

Hao-Chung Kuo

!

Department of Photonics & Institute of Electro-Optical Engineering

College of Electrical Engineering

National Chiao Tung University

ABSTRACT

In this dissertation, we investigated the injection-locking

wavelength-division-multiplexer passive optical network (WDM-PON) system.

First, we introduced the side-mode injection-locked Fabry-Perot Laser Diode

(FPLD) transmission diagnosis of a multi-channel selectable

weak-resonant-cavity Fabry-Perot Laser Diode (WRC-FPLD). Moreover, the

injection-locking performance of a 1% WRC-FPLD and demonstrate the

2.5-Gbit/s & 25-km WDM-PON application with the directly modulated

WRC-FPLD based transmitter with enhanced injection-locking bandwidth of

0.48 nm was we discussed and simulated. A 25-channel locking capacity is

reported for such a side-mode injection-locked WRC-FPLD with corresponding

wavelength locking range of 30 nm, the minimal requested power of -7dBm and

gain extinction ratio of <7 dB was demonstrated. Furthermore, we investigate

quasi-color-free the WDM-PON transmitters with comparable broadband gain

spectrum by using an optically gain-switching coherent pulse-train and threshold

reduction of WRC-FPLDs. Nevertheless, such a degradation only induces a

power penalty of < 1dB at BER<10

-10

over changing temperature of 9 degree.

A receiving sensitivity back-to-back transmission can be -25.6 dBm, and 25-km

transmission power penalties is up to 2 dB with 16 channels. At last, we

demonstrated a novel in situ self-restoration scheme to track real time the

injection locking of a FPLD by monitoring a built-in integrated photodiode.

Such a scheme can be effectively implemented with a minimum amount of

redundant network resources to achieve self-restoration within 50 seconds with

Q factor >8.2 and side-mode suppression ratio (SMSR) >35 dB at 2.5 Gbit/s.

These investigations would be useful in the next generation injection-locking

based WDM-PON systems.

Acknowledgements

ठᖴ

ӧറγ੤ࣴزғޑғఱύளډ೚ӭΓޑڐշᆶЍ࡭ǴፕЎᆶᏢՏБૈ

໩ճֹԋǶा੝ձགᖴךޑࡰᏤ௲௤!݅ৰӵԴৣӧࣴزޑၸำύόᘐޑ๏

ϒऐЈޑࡰᏤǴӧךեዊޑਔং๏ϒךคدޑڐշЍ࡭ᆶᜢᚶǴ٬ךคፕ

ࢂӧ؃Ꮲᄊࡋ܈ࢂ଺ΓೀШ΢೿ڙ੻ؼӭǴாసറޑᏢ᛽ᆶࣁΓৣ߄ޑ॥

ጄǴஒࢂךϞࡕҥيೀШޑ኱ंǶନԜϐѦǴΨགᖴ!೾੏ύԴৣӧᏢਠਔ

ගٮᝊ຦ཀـᆶڐշǴ٬ךૈ୼໩ճֹԋךޑറγᏢՏǴӧԜ߄ၲϣЈന

၈ኑޑᖴཀǶ!

གᖴα၂ہ঩!ᒘᒘ㹦݇௲௤ǵഋඵѶ௲௤ǵ׵࢙⪱௲௤ǵᎄЕੇ௲௤ǵ

ֈੇ఼௲௤ǵ໳ਁว௲௤ኘޜٰࡰᏤךޑα၂Ǵගٮ೚ӭᝊ຦ޑཀـǶჴ

ᡍ࠻റγ੤Ꮲߏ஭ຐ၈ǵ݅ߪᄪჹך௲ᏤᆶᔅԆǴคፕӧᏢ᛽΢ǵჴᡍ΢

аϷፕЎቪբ΢Ǵ೿๏ךཱུεޑᔅշǶ!གᖴύ๮ႝߞࣴز܌߻ᘳࣴز࠻ᄃ

हඁǵ݅ৰࡹǴӢࣁգॺޑڐշǴωૈᡣךޑჴᡍளа໩ճ຾ՉǶᗋԖᏢ

׌ۂॺߋࠧ౺ǵֆሎ۸ǵ݅ሸ߷ǵഋৎඦǵ஭৚ྍǵ݅ᑻᖃǵෞܲዅǵந

ᄪྷǵइျണǵำηখǵ൹୯ᘬǵ݅჏ฐǵֻ݅ࠧǵ݅ߪᏂǵ׵ەᒶᆶ܌

ԖᔅշၸךޑᏢ׌ۂॺǴӧჴᡍ΢ᆶғࢲ΢ޑڐշǴᡣךӧѠчᆶཥԮٿ

ᜐາޑ௃ݩΠǴૈ୼໩໩ճճֹԋᏢ཰Ƕ!!

നࡕךाགᖴךനኑངޑݿݿ༰༰Ǵ๏ϒך΋ঁؼӳޑᏢಞᕉნǴഁ

݀೚ӭႴᓰᆶᜢᚶǴᡣךёа໩ճֹԋךޑറγᏢ཰Ƕךঢঢǵךۆۆޑ

ᜢЈǴаϷځдӳܻ϶Ǵᖴᖴգॺᓨᓨޑ๏ךӄΚޑЍ࡭ᆶᜢངǶӧԜᆶ

գॺϩ٦೭ҽ഻৹ǶᖴᖴǼ

CONTENTS

Page

Chinese Abstract

i

English Abstract

ii

Acknowledgements

iv

Contents

v

List of Figures

vii

Chapter 1

Introduction

1 1.1 Introduction of Fiber-to-the-Home (FTTH) 1 1.2 Introduction of Passive Optical Network (PON) 2 1.3 Introduction of Wavelength-Division-Multiplexer PON

(WDM-PON) 3 1.4 Research Motivation 5 1.5 Organization of Dissertation 7 References 8

Chapter 2

Side-mode injection-locked FPLD transmission diagnosis

10 2.1 Introduction and motivation 10 2.2 Side-mode suppressing ratio analysis 12 2.3 Degradation of linewidth enhancement factor on

injection-locked side-mode

17

2.4 Experimental Setup 20

2.5 Data transmission diagnosis of side-mode injection-locked FPLD

22

2.6 Summary 25

References 27

Chapter 3

Weak-resonant-cavity Fabry-Perot laser diode with enhanced

injection-locking bandwidth

29

3.1 Introduction and motivation 29

3.2 System structure 30

3.3 Enhanced injection-locking bandwidth 32 3.4 Modeling and experimental results of WRC-FPLD 38

3.5 Data transmission performance of WRC-FPLD 45

3.6 Summary 50

References 52

Chapter 4

54

Pulsating master and injected slave weak-resonant-cavity laser

diodes based quasi-color-free 2.5Gb/s WDM-PON

4.1 Introduction and motivation 54 4.2 Concept of coherent injection light source and

quasi-color-free injection locking

55 4.3 Performances and discussions of quasi-color-free 2.5Gb/s

RZ WDM-PON

58

4.4 Summary 68

References 70

Chapter 5

72

Self-restorable injection-locking monitor by integrated

photodiode

5.1 Motivation and configuration 72 5.2 Modeling and experimental results of MPD-MCU based

auto-restorable FPLD transmitter

75 5.3 Concept and Circuit Design 83 5.4 Injection-locking monitor of self-restoration 87 5.5 System performances of the MCU-based auto-restorable

injection-locking transmitter 90 5.6 Summary 96 References 98

Chapter 6

Conclusions

100 6.1 Summary 100

6.2 Suggestions for Future Work 104 Curriculum Vitae

Figure List

Fig. 1.1 Poin-to-point fiber networks access.

Fig. 1.2 Time-division-multiplexer PON fiber networks access.

Fig. 2.1 Wavelength locking range of the injection-locked mode in slave FPLD versus

injection power.

Fig. 2.2 Measured SMSR curve on adjacent injected longitudinal mode and external optical

power.

Fig. 2.3(a) Theoretically simulated SMSR of the side-mode injection-locked FPLD as

function of the reflectivity change ('R) and the ratio of loss coefficient (*m*0).

Fig. 2.3(b) Comparison on the theoretical and experimental results of the SMSR for one

specific FPLD injection-locked mode.

Fig. 2.4 The simulated linewidth of the injection-locked FPLD as a function of the reflectivity

change ('R).

Fig. 2.5 Spectral linewidths of the injection-locked FPLD in principle mode with (red line),

without (blue line) modulation, and a reference laser source (black).

Fig. 2.6 The configuration of an experimental system with slave FPLD is side-mode

injection-locked by a wavelength-sliced master FPLD.

Fig. 2.7 Measured injected power (hollow markers) and measured Q (solid markers) of the

driving current at the principle longitudinal mode.

Fig. 2.8 BER analysis of wavelength injection-locked FPLD at different longitudinal modes

and measured eye diagrams (inset) with and without injection.

Fig. 3.1 A DWDM-PON system with a WRC-FPLD based transmitter at the ONU end that is

side-mode injection-locked by a wavelength-sliced master FPLD.

Fig. 3.2 A DWDM-PON system with a WRC-FPLD based transmitter at the ONU end that is

side-mode injection-locked by a wavelength-sliced master FPLD.

Fig. 3.3 The output optical spectra of the free-running and injection-locking WRC-FPLD at

different biased conditions.

Fig. 3.4 Injection-locking power dependent wavelength lock-in range of one longitudinal

mode in the slave WRC-FPLD transmitter at the ONU end.

Fig. 3.5 Simulation of the normalized free-running spectra of WRC-FPLD with front-facet

reflectivity of 30%, 10%, 1%, 0.1% and 0.01%.

Fig. 3.6 The gain spectral linewidth (solid squares) and the gain extinction (hollow squares)

vs. the front-facet reflectivity of WRC-FPLD.

Fig. 3.7 The calculated Q factor and locking range of the injection locking WRC-FPLD with

different reflectivity and injection power.

injection power of +3dBm, -3dBm, -9 dBm.

Fig. 3.9 The requested injecting power (red solid square), corresponding the measured best

Q-factor (red hollow square), and the minimal requested injecting power for Q=7.2 (blue solid square) at different driving currents.

Fig. 3.10 BER analysis of wavelength injection locked 1% WRC-FPLD at different

longitudinal modes and measured eye diagrams (inset) with and without injection.

Fig. 3.11 P-I curve of the WRC-FPLD with the different optical power injection of -3dBm,

-12dBm, and free-running. The inset shows the free-running optical spectrum.

Fig. 3.12 The numerically calculated small signal frequency response of laser with different

Ibias/Ith of 1.5, 3.5, and 5.

Fig. 4.1 The WRC-FPLD based bi-directional quasi-color-free 2.5-Gb/s RZ WDM-PON with

a pulsed WRC-FPLD coherent injection-locker.

Fig. 4.2. Configuration and band structure of the 1% front-facet AR-coated WRC FPLD.

Fig. 4.3(a) The P-I curve of the slave WRC-FPLD under master WRC-FPLD

injection-locking with different power levels.

Fig. 4.3(b) Principle of the slave WRC-FPLD RZ transmitter triggered by externally injection

from a pulsated master WRC-FPLD and directly modulated by a electrically PRBS data-stream.

Fig. 4.4 Left: optical spectra of the master WRC-FPLD operated at free-running (gray) and

gain-switching (red) condition. Upper right: the normalized mode spectra at free-runing (gray) and gain-switching (red) conditions. Lower right: the linewidth and pulsewidth of the master WRC-FPLD with different RF modulation powers.

Fig. 4.5 Injection-locking power dependent wavelength lock-in range and corresponding

SMSR of the slave WRC-FPLD transmitter.

Fig. 4.6 (a) Free-running AWG sliced WRC-FPLD. (b) Gain-switched AWG sliced

WRC-FPLD. (c) Free-running slave WRC-FPLD injected by gain-switched WRC-FPLD. (d) NRZ modulated slave WRC-FPLD injected by gain-switched WRC-FPLD.

Fig. 4.7 the diagram of single-wavelength injection and gain-switched injection.

Fig. 4.8(a) Optical spectrum of injected WRC-FPLD at the temperature from 21oC to 29oC. Fig. 4.8(b) BER of temperature-controlled WRC-FPLD with different injection locked mode

number.

Fig. 4.9(a) Injection-locking power dependent wavelength lock-in range of one longitudinal

mode in the slave WRC-FPLD transmitter at the ONU end.

Fig. 4.9(b) BER analysis of wavelength injection locked WRC-FPLD at different channels

and measured pulsed RZ eye diagrams (inset).

Fig. 5.1 A self-restorable system for injection-locked FPLD with monitor photodiode. The

MCU detects the MPD photocurrent and controls the TEC and the polarization controller.

Fig. 5.3(b) Wavelength locking range of a side mode in the slave FPLD as a function of

injection-locked power.

Fig. 5.4 (a) The photocurrent of MPD at the FPLD driving currents of 25, 30, and 35 mA as

detuning wavelength. (b) The photocurrent of MPD at the injection power of + 3, 0, -3, -6, and -9 dBm as detuning wavelength

Fig. 5.5 The conceptual flow chart for designing the PD-MCU link based self-restoration unit. Fig. 5.6 The circuit block for building a PD-MCU link based self-restoration unit.

Fig. 5.7 (a) The monitored PD current (upper, square-dotted curve) and the free-running

FPLD spectrum (lower, block lines); (b) the spectra of the FPLD transmitter injection locked at different longitudinal modes (color lines at right part).

Fig. 5.8 The recovery experience of the self-restorable unlocked FPLD and corresponding

MPD current controlled by MCU.

Fig. 5.9 The BER of the FPLD transmitter measured at three different conditions.

Fig. 5.10 Left: (a) Threshold current of FPLD before and after self-restoration. Right: The

Optical eye-diagrams (b) before and (c) after auto-restoration.

Fig. 5.11 The BER of the FPLD transmitter before and after self-restoration at different

wavelength-deviation conditions.

Chapter 1

Introduction

1.1 Introduction of Fiber-to-the-Home (FTTH)

The channel capacity requirement of high-speed access network inevitably accelerates the necessity in developing fiber-optics communication with narrower channel spacing and/or higher channel data rate. To accommodate upcoming demands due to the growing population in future broadband optical access networks, fiber-to-the-home (FTTH) [1.1-1.2] networks have been developed for business, education, communication, and entertainments services. Based on these applications, the requests of high definition (HD) image and video stream are greatly growing. Nowadays, many countries (such as United States, Japan, Korea, and China) define the broadband network access to be an important deployed point. In United States, over 6 million home have enjoyed the FTTH services, and grows quickly. Currently in Taiwan (2010), the dominated broadband service by internet service provider (ISP) is point-to-point (P2P) Ethernet-access network, so-called fiber-to-the-building (FTTB), system as following Fig.1.1. Between the central office and network end node, a power-supplied Ethernet switch is necessary equipment for such P2P structure [1.3-1.4]. The multi-service P2P structure uses the wavelength of 1550 nm for downstream service, and the wavelength of 1310 nm for upstream service. Most of P2P equipments are mature and related low cost which is the main reason chosen by ISP. Recently, the technology of passive optical network (PON) provides another option of broadband service for ISP. Without active components between central office and network end node is the premise of PON. By replacing the active devices, the maintaining cost of ISP can be significant reduced.

Fig. 1.1 Poin-to-point fiber networks access

1.2 Introduction of Passive Optical Network (PON)

The most popular multiplex technology of PON application is time division multiple (TDM) access. The principle of TDM-PON is that each of multiple subscribers used for downstream and upstream data transmission is assigned by individual time slot as illustrated in Fig. 1.2. One or two optical power splitter plays the role of separating and combing the connection of many subscribers. Recently, several alternative PON implementation scheme have been devised such as Ethernet passive optical network (EPON) and gigabit passive optical network (GPON) [1.5-1.6]. Fixed time slot for each subscriber can let central office differentiate the source and destination of data signal. However, due to this configuration, the subscriber cannot use the surplus bandwidth even if the other subscriber does not use. Burst-mode transmitter and receiver in optical network units (ONUs) and optical line terminals (OLTs), respectively, can turn-on and turn-off quickly if the time slot does not be possessed by the subscriber. Another issue of TDM-PON is caused by different transmission distance for each subscriber. For each incoming burst data signal, the time delay, synchronization, and a short guard time are necessary in this TDM-PON system. The management and control function of EPON/GPON application have been developed, and embedded into a high-density-silicon-based application-specified integrated circuit (ASIC)

chip which are namely OLT media access controller (MAC) and ONU MAC. Central Office Fiber ONUs Remote Node (Splitter) 20 km

Fig. 1.2 Time-division-multiplexer PON fiber networks access

Table 1.1. FTTH access Technology Comparison

Attribute Active PON

Active Ethernet EPON GPON

Type of ODN Active Passive Passive

Standardized IEEE 802.3u IEEE 802.3ah ITU-T G.984

Capacity 1 user

per passive tree

16 user per passive tree

32 user per passive tree

Bandwidth of

upstream/downstream

100M/100M 1.25Gbps/1.25Gbps 1.25Gbps/2.5Gbps

Reach 10~80km 20km 20km

1.3 Introduction of Wavelength-Division-Multiplexer PON

Wavelength division multiplexed passive optical network (WDM-PON) is a popular technology for future broadband access networks because it provides large bandwidth, easy upgradability, high security and virtual point-to-point connection to end-users [1.7-1.8]. Each subscriber is assigned one or a pair wavelength, and the transmission data is continuous

at whole time. The multiplex and de-multiplex can be realized using arrayed waveguide gratings (AWGs) or thin-film dielectric filters. The channel spacing can reach as narrow as 50 or 100 GHz (0.4 or 0.8 nm), which is similar as the standard channel spacing of dense wavelength division multiplexer (DWDM) which defined by international telecommunication

union (ITU-T) recommendation. A WDM component is used at the remote node (RN)

instead of a power splitter, and an additional WDM multiplexer is located at central office to separate the wavelength from each subscriber. However, many challenges need to overcome in WDM-PON application. The specific wavelength of transmitter must be exactly the same as wavelength window of AWG. For such DWDM-PON application, retaining specific wavelength (such as thermal electric cooler (TEC) equipment) relatively cost too much, especially in sensitive FTTH market. For the deployment of practical WDM-PONs, the most critical issue is to develop the low cost WDM light sources for the optical network unit. To construct the low-cost WDM-PON, Lin et. al. demonstrate a bidirectional wavelength-division-multiplexed passive optical network with 1.25-Gb/s upstream and 2.5-Gb/s downstream over 20-km transmission distance by employing gain-saturated reflective semiconductor optical amplifiers (RSOAs) for wavelength- independent optical network terminals [1.9]. The upstream signals are generated by remodulating the downstream signals whose modulation amplitude is squeezed through gain-saturated RSOAs. On the other hand, Choi et. al. demonstrate color-free operation of a dense wavelength-division-multiplexing passive optical network based on the wavelength-locked Fabry–Pérot laser diodes with injection of a low-noise broadband light source (BLS) at Manchester coded data rate of 100-Mb/s [1.10]. The color-free operation, i.e., wavelength-independent operation, was obtained at 50-GHz channel spacing with the help of a narrow injection bandwidth of the low-noise BLS. Such injection-locked optical sources (Fabry-Perot laser diodes (FPLDs) and reflective semiconductor optical amplifiers (RSOAs)) have recently become one of the practical solutions for the next-generation WDM-PON

system due to their promising features of broadband wavelength tenability, enhanced side-mode suppressing ratio, and improved modulation bandwidth performances.

1.4 Research Motivation

Most of previous works established the WDM-PON system with broadband injecting source (Amplified spontaneous emission (ASE)) at central office, and such a WDM-PON induces the intra-band crosstalk. The ASE source inherently suffers from large intensity noise (IN) caused by spontaneous-spontaneous beat noise, such that the spontaneous–spontaneous beating noise injects into the WRC-FPLD, which degrades the signal-to-noise ratio (SNR) and causes the penalty in receiving power for obtaining error-free data. However, the spectrally sliced incoherent ASE suffers from large intensity noise to limit the transmission bit-rate at 2.5 Gb/s. With over 2.5-Gbps WDM-PON, the coherence injection-locked sources is inevitable. Nonetheless, it was seldom addressed that the FPLD under side-mode injection-locking condition can also be approached as a high-quality optical light source in particular conditions. Although most of the transmission performances have been comprehensively investigated, some of the important parameters such as the SMSR, the spectral linewidth and its enhancement factor, and the bit-error-rate (BER) power penalty of the optical carrier based on the injection-locked side modes were never discussed. In order to increase the injection efficiency, reducing the front-face reflectivity could let FPLDs be close to ROSA. However, the injection-locked RSOAs of previous works suffer a drawback of serious intensity noise from high bit-rate operation. Most of the proposed WDM-PON works on a FPLD injection-locked scheme, which still meets the problems of wavelength discontinuity and finite injection-locking wavelength range due to the limitation on lasing mode selected by the resonant cavity of the FPLD. To overcome the problems, making the RSOA like a FPLD has thus emerged to deal the trade-off between noise reduction and color-free operation. Therefore, the weak-resonant-cavity FPLD placed at the ONUs enable

a more cost-effective and high-performance infrastructure for WDM-PONS.

All these coherence-injection applications require precisely controlling the wavelength matching between the master and the slave laser, and the shortcoming of maintenance and stability is accompanied in practical WDM-PON systems. Currently, the proposed approaches are limited by colorless issue, low operation bandwidth, power budget and high intensity noise when applying to the optical link. The coherent master BLS injection-locked slave laser diode has oriented new solution towards high-bit-rate WDM-PONs. By the threshold current reduction of injection locked WRC-FPLD, the return-to-zero (RZ) data-format at 2.5 Gb/s using the slave WRC-FPLDs coherently injection-locked by a pulsated WRC-FPLD based quasi-colorless master source is interesting. Without using data-format transformer circuit, both the down- and up-stream slave WRC-FPLDs are directly modulated by the PRBS NRZ data, and coherently injection-locked by the gain-switched master WRC-FPLD after 200-GHz AWG channelization.

At last, even though, all these coherence-injection applications require precisely controlling the wavelength matching between the master and the slave laser, and the shortcoming of maintenance and stability is accompanied in practical WDM-PON systems. The relatively high cost of the transmitters with specified wavelengths has hindered market acceptance. In principle, the injection-locking lasers strictly rely on external seeding or self-feedback injecting a continuous-wave (CW) laser to achieve single-mode pulsed generation. Many versatile injection-locking techniques, such as the clock frequency division [1.11], the 10-Gbps WDM passive optical network (PON) [1.12], the parallel transmission and wavelength routing network (Para-Wave NET) [1.13], and the all-optical non-return-to-zero (NRZ) to pseudo-return-to-zero (PRZ) format transformation [1.14], have been demonstrated. All these applications require precisely controlled injection-locked FPLD, but the maintenance as regards its stability and reliability usually requires complicated modules. Therefore, fault management is one of the crucial aspects in network management

to enhance the network reliability. Of late, many efforts have been focused on the fault-monitoring methods [1.15] and the self-restorable networks [1.16] to achieve network protection. We are interested in investigating a novel injection-locking monitor for real-time and self-restorable tracking the FPLD-based WDM-PON transmitter. Without employing high-speed electronics and instruments, the proposed in situ monitoring and self-restorable architecture uses the integrated MPD, which is usually employed to monitor the optical power illuminated by the FPLD.

1.5 Organization of this Dissertation

The dissertation is organized into six chapters. The present chapter, being the first, introduces the research history of the WDM-PON and motivation of the dissertation. Chapter 2 introduces the side-mode injection-locked FPLD transmission diagnosis of a multi-channel selectable injection-locked Fabry-Perot Laser Diode with anti-reflection coated front facet. The Chapter 3 discusses and simulates the injection-locking performance of a 1% WRC-FPLD and demonstrates the 2.5-Gbit/s WDM-PON application with the directly modulated WRC-FPLD based ONU transmitter with enhanced injection-locking bandwidth. The Chapter 4 demonstrates quasi-color-free the WDM-PON transmitters with comparable broadband gain spectrum by using an optically gain-switching coherent pulse-train and threshold reduction of WRC-FPLDs. Without using data-format transformer circuit, both the down- and up-stream slave WRC-FPLDs are directly modulated by the PRBS NRZ data, and coherently injection-locked by the gain-switched master WRC-FPLD after 200-GHz AWG channelization to perform bi-directional RZ transmission at 2.5 Gb/s over 25 km. The Chapter 5 shows a novel in situ self-restoration scheme to track real time the injection locking of a FPLD by monitoring a built-in integrated photodiode. Finally a brief conclusion for these researched is given in the sixth chapter.

References

[1.1] G. Kesier, FTTX concepts and Applications, Wiley-Interscience, (2006).

[1.2] J. George, “Application compelling fiber to the home,” The Prism FTTH, 3, 47, (2006).

[1.3] D. Meis, “Fiber Terminal Distribution Systems Cut Develop Cost and Risk,”

Broadband Properties, 64, (2006).

[1.4] G. P. Agrawal, “Fiber-optic Communication Systems”, 2nd Ed., John Wiley & Sons, New York, 3, 121, (1997).

[1.5] E. Shraga, “GPON and EPON Economical Comparison,” Flexlight Network White

Paper, (2004).

[1.6] D. Parsons, “GPON vs. EPON Cost Comparison,” Broadlight White Paper, (2005).

[1.7] H. D. Kim, S.-G. Kang, and C.-H. Lee, “A low-cost WDM source with an ASE injected Fabry-Perot semiconductor laser,” IEEE Photon. Technol. Lett., 12, 1067 (2000).

[1.8] S. J. Park, C. H. Lee, K. T. Jeong, H. J. Park, J. G. Ahn, and K. H. Song, “Fiber-to-the-home services based on wavelength-division-multiplexing passive optical network,” IEEE J. Lightw. Technol., 22, 2582, (2004).

[1.9] S.-C. Lin, S.-L. Lee, and C.-K. Liu, “Simple approach for bidirectional performance enhancement on WDM-PONs with directmodulation lasers and RSOAs,” Opt.

Express, 16, 3636, (2008).

[1.10] K.-M. Choi, J.-S. Baik, and C.-H. Lee, “Broad-band light source using mutually

injected Fabry–Pérot laser diodes for WDM-PON,” IEEE Photon. Technol. Lett., 17, 2529, (2005).

[1.11] Y. Matsui, S. Kutsuzawa, S. Arahira, Y. Ogawa, and A. Suzuki, “Bifurcation in

20-GHz gain-switched 1.55-m MQW lasers and its control by CW injection seeding,” IEEE J. Quantum Electron., 34, 1213, (1998).

[1.12] Z. Xu, Y.-J. Wen, W.-D. Zhong, C.-J. Chae, X.-F. Cheng, Y. Wang, C. Lu, and J.

Shankar, “High-speed WDM-PON using CW injectionlocked Fabry-Pérot laser diodes,” Opt. Express., 15, 2953, (2007).

[1.13] N. Kashima, S. Yamaguchi, and S. Ishii, “Optical transmitter using side-mode

injection locking for high-speed photonic LANs,” IEEE J. Lightwave Technol., 22, 550, (2004).

[1.14] Y.-C. Chang, Y.-H. Lin, J. H. Chen, and G.-R. Lin, "All-optical NRZ-to-PRZ format

transformer with an injection-locked Fabry-Perot laser diode at unlasing condition",

Opt. Express, 12, 4449, (2004).

branched access networks using a wavelength-sweeping monitoring source,” IEEE

Photon. Technol. Lett., 5, 614, (1999).

[1.16] K. Lee, S. B. Lee, J. H. Lee, Y. -G. Han, S. -G. Mun, S. -M. Lee, and C. -H. Lee, "A

self-restorable architecture for bidirectional wavelength-division-multiplexed passive optical network with colorless ONUs," Opt. Express, 15, 4863, (2007).

Chapter 2

Side-mode injection-locked FPLD

transmission diagnosis

2.1 Introduction and motivation

To meet increasing demand, the single-longitudinal-mode and low-noise operation of the laser sources with selectable channel wavelengths [2.1-2.6] are critical issues for such a wavelength-division-multiplexing (WDM) optical access system. Some of the previous works focused on developing the specific broadcasting architectures to release the cost issue of the rather expensive DFB laser transmitters by sharing over a large customer base. One solution is the use of 1.5-Pm DFB in conjunction with optical amplifiers to achieve larger link budgets [2.1]. Alternatively, a high-power diode-pumped erbium-doped fiber amplifier (EDFA) based ring laser using an intra-cavity liquid-crystal fiber etalon filter was proposed with its output wavelength electrically tunable from 1525 to 1586 nm [2.2]. In particular, a transmission experiment at 2.5 Gbit/s over 30 km using a wavelength-locked Fabry–Perot laser diode (FPLD) externally controlled by another spectrally sliced Fabry–Perot laser diode was also performed [2.3]. Under such kind of injection-locking, the suppression on second/third harmonic distortion and third-order inter-modulation distortion was demonstrated [2.4]. Moreover, a distinguished and cost-effective method for generation a channel-selectable single-mode FPLD by self-seeding it with low-level injection power was ever reported [2.5]. Nearly single-mode source with side-mode suppressing ratio (SMSR) of higher than 40 dB over all selectable channels with a wavelength tuning range covering 11.5 nm was demonstrated with such a self-seeding FPLD [2.6]. Typically, the aforementioned technology is achieved by use of a tunable linearly-chirped fiber Bragg grating or an active

Fabry–Perot filter to provide wavelength-selective injection and output filtering function. Alternatively, the other approaches using an FPLD injection-locked with a coherent optical source have also been presented in previous works. For example, a spectrally sliced amplified-spontaneous–emission (ASE) light source and a spectrally sliced FPLD have also been proposed as the WDM optical sources, [2.7] which were in connection with the add–drop modules that are composed of “4skip0” and add-drop filters. The experiments in a novel optical distribution network for multistage access with multiple remote nodes (RNs) have shown error-free transmission with simultaneous bidirectional 1.25 Gbit/s per channel up to 20 km. Not long ago, we have also demonstrated a single-longitudinal-mode optical source generated using a mode-beating noise-suppressed FPLD-EDFA link under mutually injection–locking condition [2.8]. Similar FPLD-FPLD injection-locked sources were emerged as the WDM passive optical network (PON) transmitters [2.9], and the high-speed-uplink WDM-PON architecture at 10 Gbit/s with 15-km transmission capability has been demonstrated using the injection-locked FPLDs [2.10]. Up to now, most researching efforts are focused on the injecting architectures of the FPLD at its principle longitudinal mode under high-gain competition. Nonetheless, it was seldom addressed that the FPLD under side-mode injection-locking condition can also be approached as a high-quality optical light source in particular conditions. Although most of the transmission performances have been comprehensively investigated, some of the important parameters such as the SMSR, the spectral linewidth and its enhancement factor, and the bit-error-rate (BER) power penalty of the optical carrier based on the injection-locked side modes were never discussed. In this chapter, we analyze the performances of a side-mode injection-locked FPLD by externally injecting with another spectrally sliced FPLD. The effects of the biased current and the external injection power on the optimization of a side-mode injection-locked FPLD at different longitudinal modes are discussed. The transmission performances such as extinction ratio, Q factor, and BER at 2.5 Gbit/s over 25

km are also characterized.

2.2 Side-mode suppressing ratio analysis

The wavelength locking range of the slave FPLD measured by using a modified delayed-self-homodyne (MDSH) scheme is shown in Fig. 2.1, which is defined as the wavelength injection-locking range for one specific longitudinal mode of the slave FPLD with its SMSR >35 dB. -6 -3 0 3 6 9 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 'O (n m) Injection Power (dBm) Wavelength locking range

Fig. 2.1 Wavelength locking range of the injection-locked mode in slave FPLD versus injection power.

The detuning wavelength is defined as the wavelength shift on the longitudinal mode of the master FPLD with respect to that of the slave FPLD. The stable injection-locking region is bounded by two solid curves and the slave FPLD is operated at higher injection ratio to reach larger injection-locking range under such conditions, where the injection ratio is defined as the power ratio of the injected signal to the free-running optical signal inside FPLD cavity. In addition, a relatively weak signal with considerable noise has also been observed as the injection wavelength is detuned away from the slave FPLD’s longitudinal mode by 0.15 nm. After injection-locking with the master FPLD, the SMSR of the slave FPLD as a function of the detuning external injection power and the order of the side longitudinal mode is shown in Fig. 2.2. The measured SMSR of injection-locked FPLD is at without-modulation condition.

For modulated FPLD, the SMSR will decay up to 3 dB. This figure provides the minimum optical power at all longitudinal modes to initiate the wavelength injection-locking. Note that a higher ordered longitudinal mode acquires larger injecting power to achieve a SMSR > 35 dB. In more detail, the SMSR of the slave FPLD with the seeding from the master FPLD is theoretically discussed as below.

10 20 30 40 50 8 ₄ 0 4 8 -12 -2 -4 -6 -8 10 8 6 4 2 0 SM SR (d B) In jecte d Po w e r (dB m ) L_o n_g itu_d in al _M o_d e

Fig. 2.2 Measured SMSR curve on adjacent injected longitudinal mode and external optical power.

Typically, the SMSR of the FPLD longitudinal mode under external injection-locking condition can be expressed by [2.11]

0 0 0 0 0 0 0 0 0 (1 ) (1 ) ( ) ( ) (0) ( ) (1 ) (1 ) ( ) (0) m m m m S S m m m m m S S g g I I I g g I m I SMSR g g I g g I I I m I c *    c *  u u c *  _{* c}   , (1)

where the optical intensity of the fundamental (the largest) mode and the desired side-mode to be injection-locked in the slave FPLD are denoted as I0 and Im, the index j defines each

parameter, gj denotes the gain coefficient, *j denotes the loss coefficient, and IS denotes the

central wavelength of principle mode) and Om (the injection-locked side-mode) can contribute

to the slave FPLD. In our case, a new parameter 'm is employed to describe the wavelength difference between the injection-locked mth side-mode and the principle mode ('m=0) naturally lasing at the gain peak of the slave FPLD. The material gain spectrum is approximated by a Lorentzian shape function to model the spectral roll-off of the slave FPLD gain profile. The gain of each mth side-mode can thus be described by gm=g0/[2.1+('m/M2)],

where M denotes the total mode number related to the full-width-at-half-maximum of the slave FPLD gain spectrum. If we assume that the saturation conditions of the principle and side modes are equivalent (i.e. Is(0) = Is(m)), the SMSR, I0/Im, can thus be described as a

function of the ratio of loss coefficients.

2 2 1 0 2 1 2 0 2 0 0

(

/(1

/

)

)

(1

(1

/

)

)

(1

/

)

m m m m m

I

C

m M

C

C

C

m M

SMSR

I

m M

c

c

c

c

*

 '

 *



*

 '

*

c

c

c

*

*

 '

*

















2 1 2 , , 0 2 , , 0 1 ln (1 / ) / ln ln (1 / ) / ln e ff m e ff e ff m e ff C C R m M R R m M R ª  c  ' c º ¬ ¼ c  ' c , (2)

where C1 and C2 are constants. In addition, the relationship between the loss coefficient *

and the reflectivity R can be correlated each other by writing the following formula *j

=-ln(R'eff,j)/2L, where R'eff, denotes the effective reflectivity of the slave FPLD cavity under

external injection. Hereafter, we define the reflectivity change ('R) in term of external injection power as

R

I

I

R

o F ext



*

'

1

, (3)

where Iext is the optical intensity of the external injection, and the *F is the coupling loss

between the slave FPLD and the coupled SMF. Thus, the change of the loss coefficient for the slave FPLD can be described as

 

ln

2

2

eff eff

R

_R

R

R

L

LR

ª

c

º

w

'

'* 

«

»

˜' 

c

w «

_¬

»

_¼

, (4)

Consequently, the SMSR of the slave FPLD under external injection-locking can be re-written as a function of the reflectivity change. The ratio of loss coefficients *0'/*m' can be

represented as , , , ,0 0 0 0 0 ,0 ,0 ln( ) 2 2 2 ' ln( ) ' 2 2 2 eff m m eff m eff m m m m eff eff eff R R R LR L LR R R R LR L LR ' ' *    * _Ÿ*  '* ' _' * *  '* _{*   } , , , , ,0

[ln(

)]

[ln(

)]

eff m eff m eff m eff

R

R

R

R

'



#

, (5)

in which the effective reflectivity of the principle mode is assumed to be equivalent to the largest side mode, and the reflectivity change of the side mode is far stronger than that of the principle mode ('Reff,0 >> 'Reff,m# 0 ).

0.0 0.2 0.4 0.6 0.8 1.0 20 30 40 50 0.00 0.05 0.10 0.15 0.20 SMSR (dB)

*

0

*

m 'm=0 'm=1 'm=2 'm=4 'm=8 'm=16 Reflectivity Change

ĩ

'R

Ī

FPLD as function of the reflectivity change ('R) and the ratio of loss coefficient (*m*0).

By setting the output power of the slave FPLD as 0.1 mW under external injection, the cavity length (L) as 250 m, the refractive index (n) as 3.5, and the photon lifetime (TR) as 5.8 ps,

the SMSR of the slave FPLD under external injection-locking is simulated and shown in Fig. 2.4. The SMSR of the slave FPLD as a function of the reflectivity change can also be obtained as shown in Fig. 2.3(a). Obviously, the SMSR of the slave FPLD can be up to 50 dB as the loss of the principle mode is far smaller than that of the injection-locked side-mode (i.e., the *m/*0 is infinitely small). To elucidate the injection-locking performance, we

further compare the experimentally obtained and theoretical simulated SMSRs for one side-mode of the slave FPLD under external injection, as shown in Fig. 2.3(b). As illustrated in Fig. 2.2, we have already shown that the experimentally measured SMSR is well proportional to the externally injection-locking power. Since there is a linear relationship between the effective reflectivity change ('R) and the external injection power (Iext), a

relatively high injection could result in an increasing reflectivity change as well as an enhanced SMSR. The obtained reflectivity changing ('R) is caused by an external injection into the slave FPLD. Between the 'R from 0 to 0.16, the experimental result meets with our simulation. Beyond 'R of 0.16, the injection power should be higher than 8 mW to induce the reflectivity changing, and the experimental result does not meet with simulation result. The reflectivity changing distinctly is dominated by another effect with the high optical injection.

0.00 0.05 0.10 0.15 0.20 0.25 10 20 30 40 50 0 2 4 6 8 10 Simulation Data SMSR (dB) Reflectivity Change ĩ'RĪ Measured Data Injection Power (mW)

Fig. 2.3(b) Comparison on the theoretical and experimental results of the SMSR for one specific FPLD injection-locked mode.

2.3 Degradation of linewidth enhancement factor on

injection-locked side-mode

If we consider the Fabry-Perot etalon effect of the slave FPLD, the 3-dB linewidth of the lasing side-mode from the slave FPLD under external injection can thus be described as





eff eff eff eff m

G

R

G

R

nL



'

1

2

2

S

O

O

, (6)

when the effective reflectivity of the slave FPLD is slightly changed due to the external injection-locking, this may give rise to a change in the longitudinal-mode linewidth of the slave FPLD. That is

>

@

>









@









2





1 2 1 2 ' , ' , 2

/

1

/

/

1

/

)

(

1

2

1

2

_R

_R

_G

_m

_M

M

m

G

R

R

nL

G

R

G

R

nL

m m m eff m eff eff m eff m

'



'



'



'







'

S

O

S

O

O

, (7) Thus, the simulated linewidth of the FPLD can also be plotted as a function of the change in

reflectivity for the slave FPLD due to the side-mode injection, as shown in Fig. 2.4. -0.2 0.0 0.2 0.4 0.6 1E-3 0.01 0.1 10.0 0.2 0.4 0.6 0.8 1.0 'O=0.04nm 'm=25 'm=15 'm=5 Linewi dt h ( n m) Reflectivity Change ('R) 'm=0

Effective Reflectivity (Reff)

Fig. 2.4 The simulated linewidth of the injection-locked FPLD as a function of the reflectivity change ('R).

Therefore, the linewidth reduction and side-mode suppression of the slave FPLD can be understood through the theoretical modeling shown above. The measured 3-dB spectral linewidth for one longitudinal mode of the CW free-running and the directly modulated FPLD are 0.024 and 0.04 nm, respectively. These results correlate well with the theory since that the transient variation in carrier density simultaneously affects the refractive index and the linewidth of the slave FPLD. However, the linewidth of the directly modulated FPLD under external injection-locking are reduced from 0.04 to 0.018 nm, respectively, as shown in Fig. 2.5.

1533.6 1533.7 1533.8 1533.9 -50 -40 -30 -20 -10 0 0.018nm

Modulated Free Running FPLD After Injection Locking

Power ( d Bm ) Wavelength (nm) 0.04nm

Fig. 2.5 Spectral linewidths of the injection-locked FPLD in principle mode with (red line), without (blue line) modulation, and a reference laser source (black).

In particular, the linewidth reduction effect of the injection-locked side modes is significantly degraded with increasing side-mode order ('m). That is, the side-mode exhibits greatly broadened spectrum as compared to the principle mode of the slave FPLD even under injection-locked condition. In digital communication systems, the product of the bit rate of B and the full-width-at-half-maximum of the propagated data bit of

'

T must be

under 1. If we consider the dispersion effect in fiber, the limitation on the side-mode linewidth of the injection-locked slave FPLD can be derived by using the equation of B

'

T<1

=> BL|D|

'O

<1, where L is the fiber length, D is the dispersion, and

'O

is the spectral linewidth. Under a mode-linewidth of 0.025 nm, the transmission limitation of the injection-locked FPLD at the data rate of 2.5 Gbit/s can be over 1000 km. Nevertheless, even the side-mode with 'm up to 25, the degraded linewidth can still support the OC192 transmission over 80 km or longer. In our case, the obtained linewidth of the side-mode injection-locked FPLD is 0.022 nm, which completely meets the requirement for the middle-short WDM optical access system even though the injection-locked side-mode is far

from the principle mode of the FPLD (for example, 'm > 15).

By adopting the linewidth formula, the injection-locked side mode linewidth of the slave FPLD with side-mode order of 'm can thus be modified as [2.12]









2 2 ,0 2 2 , ₂

(1

)

(1

) /(4

)

(4

) 1

/

sp sp m

C

R

C

R

P

P

m M

O

D

O

O

D

S

S

c



'



 '

, (8)

where 'Q denotes the linewidth of the slave FPLD under side-mode injection-locking condition, Rsp is the rate of the spontaneous emission coupled into the lasing mode, D is the

linewidth enhancement factor, P is the average power, 'm is the order of the side-mode away from the central wavelength, M is the total mode number where the gain has fallen to half of its peak value. Note that Rsp is much smaller than the total spontaneous emission rate, since

only a little part of the spontaneous emission is contributed to the injection-locked side-mode. The Rsp can be modified as R'sp,0 /(1+('m/M2)) by using the gain-profile approximation, and

the linewidth is enhanced by 1+D2 with a decreasing linewidth enhancement factordue to the amplitude-phase coupling. If the side mode with an increasing mode number 'm away from the principle mode is considered in our case, the rate of the spontaneous emission (Rsp)

corresponding to the injection-locked side mode is gradually reduced due to the shift of the material gain profile. That is, the narrowest linewidth should be located at the peak of the material gain profile under injection-locking. By assuming the parameters of the slave FPLD as the optical power of 5 mW, the wavelength of 1550 nm, the spontaneous emission (Rsp) of 108, the linewidth ('O) of 0.04 nm, we obtain the linewidth enhancement factor of 1.5

for the principle mode of the slave FPLD under injection-locking. In contrast, the linewidth enhancement factor for the injection-locked side mode of the slave FPLD in same condition is inevitably increasing up to 2.1.

Figure 2.6 schematically illustrates the WDM-PON system based on the side-mode injection-locked FPLDs. A FPLD with an integrated isolator is employed as the master laser for injection-locking the other slave FPLDs used as WM-PON transmitters. The master FPLD is a commercially available one with mode spacing of 1.1~1.2 nm (corresponding to 150 GHz), which could be replaced by a specially designed long-cavity one for obtaining the 50-GHz longitudinal mode spacing. The relative intensity noise (RIN) of the master FPLD as low as -140 dBm/Hz at biased current of 40 mA was measured with a lightwave signal analyzer (Agilent 71401C). The longitudinal mode of the master FPLD is detuned to match the ITU-T DWDM channel by adjusting its temperature, which is further amplified by an EDFA at the central office for obtaining higher modal power. The master FPLD output is filtered by arrayed waveguide grating (AWG) multiplexer with the channel spacing of 100 GHz.

Fig. 2.6 The configuration of an experimental system with slave FPLD is side-mode injection-locked by a wavelength-sliced master FPLD.

The side-mode injection-locking transmissions of the multi-channel selectable FPLD up to 22 wavelengths from 1553.48 nm (CH 1) to 1571.08 nm (CH 22) are demonstrated. The slave FPLD exhibits a threshold current of 8.5 mA, a longitudinal mode spacing of 1.1 nm, and a cavity length of 250 Pm. The temperature of all FPLDs are controlled at 25oC with a fluctuation of <0.1oC to prevent any wavelength drift on the longitudinal modes. The total