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Dual-Mode Semiconductor Lasers and Their Applications in Optical Clock Recovery and Photonic Microwave Generation
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 140 (2016) 43 – 48
MRS Singapore – ICMAT Symposia Proceedings 8th International Conference on Materials for Advanced Technologies
Dual-Mode Semiconductor Lasers and Their Applications in Optical Clock Recovery and Photonic Microwave Generation Dan Lu, Lingjuan Zhao*, Key Laboratory of Semiconductor Material Sciences, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
Abstract As a compact multi-functional device, monolithic integrated multi-section semiconductor laser has a wide range of applications in optical communication systems and microwave photonics with its advantages in cost, size, power consumption and integration possibilities with other components. In this paper, we will review the design and characteristics of monolithically integrated dual mode semiconductor laser, with special emphasis on amplified feedback lasers (AFL). Then, their application in optical clock recovery and high frequency photonic microwave generation will be discussed © 2016 2015The TheAuthors. Authors. Published Elsevier © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT Keywords: Photonics integrated device; amplified feedback laser; clock recovery; photonic microwave
1. Introduction Monolithically integrated multi-section semiconductor lasers (MISL) have attracted much interest from researchers due to their rich dynamics and wide applications in optical communication, microwave photonics and metrology. As a simple form of multi-section MISL, distributed feedback (DFB) laser or distributed Bragg reflector (DBR) laser with an integrated optical feedback cavity are of both fundamental and practical importance. The integrated feedback cavity can be either a simple passive section providing the phase tuning function, or a combination of a passive phase section and an amplifier section providing the phase and feedback strength controlling function. Based on the function of the
* Corresponding author. Tel.: +86-10-82304437; fax: +83-10-82304437. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT
doi:10.1016/j.proeng.2015.07.357
44
Dan Lu and Lingjuan Zhao / Procedia Engineering 140 (2016) 43 – 48
integrated feedback cavity, such lasers are called passive feedback laser (PFL) or amplified feedback laser (AFL). With the additional flexibility to control the feedback strength, the AFL has a larger operation regimes in terms of dual-mode frequency tuning range [1]. By taking advantages of the dual-mode state of such lasers, all-optical clock recovery [2–7] and photonic microwave generation [8–11] have been realized to meet the demands in various applications . In this paper, we will first review the design, fabrication and characteristics of the AFL. Then, the application of AFL in all optical recovery and photonic microwave generation will be discussed. 2. Design, manufacture and characteristics Fig.1(a) shows the schematic structure of the AFL. The DFB laser section servers as the master laser in this integrated device, while the phase section and amplifier section serve as the feedback cavity. The three sections can be independently driven by separate current sources, denoted as IDFB, IP and IA, respectively. Such a structure is equivalently an external cavity laser, which is capable of supporting single mode, dual mode, periodic oscillation mode and chaotic mode [12], depending on the feedback strength and phase, which can be adjusted via the control of IP and IA. For dual-mode applications, the mode separation scales with the feedback coefficient and reciprocal of the cavity roundtrip time [13]. To get a high mode beating frequency from the dual modes, a shorter cavity is required. When fabricating the AFL, the device material is grown on an S-doped n-type InP substrate by the metal-organic chemical vapor deposition (MOCVD). The epitaxial structure consists of five pairs of compress strained InGaAsP multi-quantum wells (MQWs) sandwiched between two separate confinement heterostructure (SCH) layers. The grating of the DFB laser is of complex-coupled type to guarantee the single mode lasing of this section. Quantum Well Intermixing (QWI) technique is used to realize a band-gap wavelength shift of 100~120 nm between the amplifier sections and the phase region. The isolation between the three sections are accomplished by etching the p-InGaAs layer off and He+ implantation. Contact window opening, metal definitions and annealing follow the standard semiconductor laser fabrication process. The characterization of the AFL is performed by monitoring the optical spectrum of the laser output and the RF spectrum converted by a high speed photodiode. Fig.1(b) shows the mapping of the dynamic states of an AFL when scanning the phase and amplifier section currents, while current applied on the DFB lasers section is fixed at IDFB=70 mA [12]. Regions of steady single mode, mode beating (M-B) resulted from dual-mode state, periodical oscillation including period one (P1) and period doubling (P2), and chaos can be clearly recognized in this mapping at different combination of IP and IA. For dual-mode lasing state, the mode spacing of the AFL can also be adjusted through I P and IA , which provides flexible means to generate tunable photonics microwave or to extract the optical clocks from signals with varied data rate. Detailed simulation and analysis can be referred to [12].
IDFB
Output
Phase
DFB
IA
Ip
Amplifier
Feedback Cavity
(a)
(b)
Fig. 1. (a) Schematic structure of the AFL ; (b) mapping of the dynamic states in the IA-IP plane (after Ref [12])
45
Dan Lu and Lingjuan Zhao / Procedia Engineering 140 (2016) 43 – 48
3. Applications As a slave laser, the AFL can be injection locked by a master injecting signal, thus realizing the clock recovery function. As a dual-mode laser source, the beating of the two laser modes can be converted to microwave outputs, functioning as a photonic microwave generator. In the following parts, we will discuss the applications of AFL in all optical clock recovery and high frequency photonics microwave generation. 3.1. All optical clock recovery The basic principle for all optical clock recovery is the injection locking of a dual-mode AFL by the clock components presenting in the data signal. When the optical clock components are aligned with the lasing modes of the dual-mode AFL, the two modes of the AFL will be injection locked, and in synchronization with the incoming clock. The beating of the two modes from the injection locked AFL will be a pure optical clock tone. By using the AFLs, we have demonstrated all optical recovery from 20-Gb/s return-to-zero (RZ) [4] and 40-Gb/s RZ [3,5,6] signal. Fig. 2 shows an experimental setup for 40-Gb/s all optical clock recovery [6]. The incoming signal was intentionally degraded by an ASE source.. After injecting into the AFL, clean optical clock could be obtained, with timing jitter ranging from 200 fs to 600 fs when the optical signal-to-noise ratio (OSNR) was varied from 30 dB to 7 dB. Fig.2(b) shows the seriously degraed incoming signal at an OSNR or 7 dB, clear clock trace could still obtained, as shown in Fig.3(c).
(a)
(b)
(c)
Fig.2 All optical clock recovery from degraded 40-Gb/s RZ signal ( [6]). (a) experimental setup;(b) degraded injecting data stream; (c) recovery optical clock
Beside intensity modulated RZ signal which contains strong clock components, AFL can also be used to extract the clocks from signals that do not have obvious clock components such as non-return-to-zero (NRZ) and phase modulated signals. The reason is that most NRZ and phase modulated signals are generated from intensity modulators, which inevitably induce weak clock components in the data spectrum. It is still possible to align the clock components with the two modes of the AFL. We have demonstrated the all optical clock recovery from 40-Gbaud NRZ-QPSK signal with the AFL [7]. Although with a common experiment setup like previous one, it is possible to extract the clock, the clock quality is usually not as good, due to the disturbance of the the phase information presented in the data spectrum. A preprocessing stage based on the cross gain modualtion (XGM) effect in a SOA is adopted to eliminate the phase information in the incoming signal. After preprocessing, a high-quality recovered clock signal with an RMS timing jitter of 363 fs is obtained. Fig.3(a) shows the experimental setup. Fig3.(b) is the waveform of the 40-Gbaud NRZ-QPSK signal, and Fig.3(c) is the recovered clock in the optical and RF domain.
46
Dan Lu and Lingjuan Zhao / Procedia Engineering 140 (2016) 43 – 48
(a)
(b)
(c)
Fig.3 All optical clock recovery from 40-Gbaud NRZ-QPSK signal [7]. (a) the experiment setup (b) the waveform of the NRZ-QPSK signal, (c) recovered optical clock in the time domain (upper) and RF domain (lower)
3.2. Photonic microwave generation Photonic microwave generation is another important function that the AFL can provide. With a convenient adjustment of the injection currents on the phase and amplifier sections, the mode spacing between the two modes of the AFL can be tuned continuously in a broad range. Since the two laser modes are generated from the same chip, they are more coherent than independent beating laser sources. The typical beating linewidth is usually around 3~5 MHz for the AFL. We have fabricated AFLs with different dual-mode spacing tunable in the frequency ranges of (19 GHz~26 GHz) [14], (30 GHz~38 GHz) [10], (46 GHz~72 GHz) [15] and (87 GHz~103 GHz) [16]. Fig.4 shows the AFL working in the 60-GHz band, which can be used in RoF system. Fig.3(a) shows the frequency tuning map of the AFL. By tuning the phase and amplifier currents, the AFL generated a tunable beating signal from 46 GHz to 72 GHz. The optical spectra is shown in Fig.4(b). To measure the RF spectra of the beating frequency, a down conversion method was performed. Fig.4(c) shows the down-converted RF signal.
(a)
(b)
(c)
Fig.4 The performance of a dual-mode AFL working in the range of (46 GHz~72 GHz) [15]. (a) the frequency tuning map of the AFL;(b) the optical spectra of the frequency tunable AFL; (c) the RF spectra after photodetection and frequency down conversion
(a)
(b)
(c)
Fig.5 Optical microwave generation using dual-mode AFL with self-injection scheme [10].. (a) experimental setup. (b) the linewidth comparison between the free running spectrum and self-injection locked spectrum (c) the overlapped RF spectra;
47
Dan Lu and Lingjuan Zhao / Procedia Engineering 140 (2016) 43 – 48
Although the beating linewidth of the dual modes from the AFL is small comparing with that of independent beating laser sources which are usually around 10-20 MHz, it is still desirable to further narrow down the beating linewidth. A delayed self-injection scheme was adopted to narrow the optical linewidth of the each mode of the AFL [10]. In the experiment, a dual-loop structure were used to suppress the densely spaced fiber modes, as shown in Fig.5(a). After self-injection locking, the RF spectrum of the beating signal was considerably narrowed from 5 MHz to 2 kHz, as shown in Fig.5(b). The frequency of the beating mode could be tuned from 30 GHz to 38 GHz, as shown in Fig.5 (c). Beside optical injection, an electrical injection was also performed. The purpose of electrical injection is to lock the two laser mode in phase. But for microwave generation purpose, it is of little importance to lock the two laser modes with the fundamental frequency locking. We adopted a subhamonic injection locking scheme [11]. By using an RF signal at the subharmonic frequency of the dual-mode separation, it is possible to generate higher frequency RF signals with a weak RF injection. When the DFB laser section is modulated by an RF signal, as shown in Fig.6(a), at its subharmonic frequency, the modulation sidebands will coincide with the two modes, thus realizing an sidemode injection locking, as shown in Fig.6(b). Using this scheme, with a low RF driven power of 2.8 dBm, tunable microwave outputs ranging from 15 GHz to 33 GHz has been demonstrated at a 1/2 subharmonic RF driven source, as shown in Fig.6(c). This scheme requires much lower RF driven power than tradional frequency doubling method. Optical sepctrum
λ1 -1st
-2nd
nd
-2
λ2 +1st +1st -1st +2nd +2nd
Modulation sidebands
Frequency
(a)
(b)
Power (dBm)
-20
Optical carrier
-40 50 dB
-60 -80 24.6 25.2 25.8 26.4 27.0 27.6 Frequency (GHz)
(c)
Fig.6 The subharmonic RF injection locking of an AFL [11]. (a)the schematic plot of the scheme (b) the working principle; (c) the obtained RF spectrum
4. Conclusion By using the dual mode lasing property of integrated semiconductor lasers, we have demonstrated its application in the areas of all optical clock recovery and photonics microwave generation. All optical clock recovery experiments for RZ signal and NRZ-QPSK signal at different data rate are reviewed. Direct photonics microwave generation method as well as injection locked generation methods based on dual-mode lasers are discussed.
Acknowledgements This work is supported by the National 973 project (Grant No. 2011CB301702) and the National Natural Science Foundation of China (Grant No. 61201103, 61335009, 61474111). References 1. S. Bauer, O. Brox, J. Sieber, and M. Wolfrum, "Novel concept for a tunable optical microwave source," in Optical Fiber Communication Conference and Exhibit (2002), pp. 478–479. 2. Y. Sun, J. Q. Pan, L. J. Zhao, W. X. Chen, W. Wang, L. Wang, X. F. Zhao, and C. Y. Lou, "All-optical clock recovery for 40Gbs using an amplified feedback DFB laser," in Asia Communications and Photonics Conference (2009), Vol. 7631, p. 76310E–76310E–7. 3. L. Wang, X. Zhao, C. Lou, D. Lu, Y. Sun, L. Zhao, and W. Wang, "40 Gbits/s all-optical clock recovery for degraded signals using an amplified feedback laser.," Appl. Opt. 49, 6577–81 (2010). 4. Y. Sun, J. Q. Pan, L. J. Zhao, W. Chen, W. Wang, L. Wang, X. F. Zhao, and C. Y. Lou, "All-Optical Clock Recovery for 20 Gb/s Using an Amplified Feedback DFB Laser," J. Light. Technol. 28, 2521–2525 (2010). 5. C. Chen, Y. Sun, L. Zhao, J. Pan, J. Qiu, S. Liang, W. Wang, and C. Lou, "Amplified feedback DFB laser for 40Gb/s all-optical clock recovery," Opt. Commun. 284, 5613–5617 (2011).
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Dan Lu and Lingjuan Zhao / Procedia Engineering 140 (2016) 43 – 48 6. J. Qiu, C. Chen, L. Zhao, Y. Sun, and D. Lu, "Detailed analysis of a 40 GHz all-optical synchronization based on an amplified-feedback distributed feedback laser," Appl. Opt. 51, 2894–2901 (2012). 7. L. Yu, Y. Li, D. Lu, L. Zhao, J. Zang, D. Kong, J. Pan, J. Qiu, and S. Wei, "All-optical clock recovery for 40 Gb/s NRZ-QPSK signals using an amplified feedback DFB laser," in CLEO: 2013 (OSA, 2013), p. CTh4L.2. 8. Y. Sun, Y. B. Cheng, and Y. Wang, "Widely Frequency-Tunable Optical Microwave Source based on Amplified Feedback laser," in IEEE PhotonicsGlobal@Singapore (2008), p. Vols 1 and 2: 524–527. 9. D.-S. Yee, Y. A. Lee, S.-B. Kim, E. . Sim, H.-G. Yun, D. C. Kim, and K. H. Park, "Widely frequency-tunable amplified feedback lasers for 10GHz optical pulsation," IEEE Photonics Technol. Lett. 17, 1151–1153 (2005). 10. B. Pan, D. Lu, Y. Sun, L. Yu, L. Zhang, and L. Zhao, "Tunable optical microwave generation using self-injection locked monolithic dualwavelength amplified feedback laser.," Opt. Lett. 39, 6395–8 (2014). 11. L. Yu, D. Lu, Y. Sun, and L. Zhao, "Tunable photonic microwave generation by directly modulating a dual-wavelength amplified feedback laser," Opt. Commun. 345, 57–61 (2015). 12. L. Yu, D. Lu, B. Pan, L. Zhao, J. Wu, G. Xia, Z. Wu, and W. Wang, "Monolithically Integrated Amplified Feedback Lasers for High-Quality Microwave and Broadband Chaos Generation," J. Light. Technol. 32, 3595–3601 (2014). 13. A. A. Tager, K. Petermann, and S. Member, "High-Frequency Oscillations and Self-Mode Locking in Short External-Cavity Laser Diodes," Quantum 30, 1553–1561 (1994). 14. Y. Sun, Y. B. Chen, Y. Wang, J. Q. Pan, L. J. Zhao, W. X. Chen, and W. Wang, "Widely Frequency-Tunable Optical Microwave Source Based on Amplified Feedback Laser," in 2008 IEEE PhotonicsGlobal@Singapore (IEEE, 2008), pp. 1–4. 15. B. Pan, D. Lu, L. Zhang, and L. Zhao, "Widely Tunable Amplified Feedback Laser with Beating-Frequency Covering 60-GHz Band," Photonics Technol. Lett. IEEE to be published (n.d.). 16. B. Pan, L. Yu, D. Lu, L. Zhang, and L. Zhao, "Simulation and experimental characterization of a dual-mode two-section amplified feedback laser with mode separation over 100 GHz," Chinese Opt. Lett. 9267, 92670Z (2014).
ScienceDirect Procedia Engineering 140 (2016) 43 – 48
MRS Singapore – ICMAT Symposia Proceedings 8th International Conference on Materials for Advanced Technologies
Dual-Mode Semiconductor Lasers and Their Applications in Optical Clock Recovery and Photonic Microwave Generation Dan Lu, Lingjuan Zhao*, Key Laboratory of Semiconductor Material Sciences, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
Abstract As a compact multi-functional device, monolithic integrated multi-section semiconductor laser has a wide range of applications in optical communication systems and microwave photonics with its advantages in cost, size, power consumption and integration possibilities with other components. In this paper, we will review the design and characteristics of monolithically integrated dual mode semiconductor laser, with special emphasis on amplified feedback lasers (AFL). Then, their application in optical clock recovery and high frequency photonic microwave generation will be discussed © 2016 2015The TheAuthors. Authors. Published Elsevier © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT Keywords: Photonics integrated device; amplified feedback laser; clock recovery; photonic microwave
1. Introduction Monolithically integrated multi-section semiconductor lasers (MISL) have attracted much interest from researchers due to their rich dynamics and wide applications in optical communication, microwave photonics and metrology. As a simple form of multi-section MISL, distributed feedback (DFB) laser or distributed Bragg reflector (DBR) laser with an integrated optical feedback cavity are of both fundamental and practical importance. The integrated feedback cavity can be either a simple passive section providing the phase tuning function, or a combination of a passive phase section and an amplifier section providing the phase and feedback strength controlling function. Based on the function of the
* Corresponding author. Tel.: +86-10-82304437; fax: +83-10-82304437. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT
doi:10.1016/j.proeng.2015.07.357
44
Dan Lu and Lingjuan Zhao / Procedia Engineering 140 (2016) 43 – 48
integrated feedback cavity, such lasers are called passive feedback laser (PFL) or amplified feedback laser (AFL). With the additional flexibility to control the feedback strength, the AFL has a larger operation regimes in terms of dual-mode frequency tuning range [1]. By taking advantages of the dual-mode state of such lasers, all-optical clock recovery [2–7] and photonic microwave generation [8–11] have been realized to meet the demands in various applications . In this paper, we will first review the design, fabrication and characteristics of the AFL. Then, the application of AFL in all optical recovery and photonic microwave generation will be discussed. 2. Design, manufacture and characteristics Fig.1(a) shows the schematic structure of the AFL. The DFB laser section servers as the master laser in this integrated device, while the phase section and amplifier section serve as the feedback cavity. The three sections can be independently driven by separate current sources, denoted as IDFB, IP and IA, respectively. Such a structure is equivalently an external cavity laser, which is capable of supporting single mode, dual mode, periodic oscillation mode and chaotic mode [12], depending on the feedback strength and phase, which can be adjusted via the control of IP and IA. For dual-mode applications, the mode separation scales with the feedback coefficient and reciprocal of the cavity roundtrip time [13]. To get a high mode beating frequency from the dual modes, a shorter cavity is required. When fabricating the AFL, the device material is grown on an S-doped n-type InP substrate by the metal-organic chemical vapor deposition (MOCVD). The epitaxial structure consists of five pairs of compress strained InGaAsP multi-quantum wells (MQWs) sandwiched between two separate confinement heterostructure (SCH) layers. The grating of the DFB laser is of complex-coupled type to guarantee the single mode lasing of this section. Quantum Well Intermixing (QWI) technique is used to realize a band-gap wavelength shift of 100~120 nm between the amplifier sections and the phase region. The isolation between the three sections are accomplished by etching the p-InGaAs layer off and He+ implantation. Contact window opening, metal definitions and annealing follow the standard semiconductor laser fabrication process. The characterization of the AFL is performed by monitoring the optical spectrum of the laser output and the RF spectrum converted by a high speed photodiode. Fig.1(b) shows the mapping of the dynamic states of an AFL when scanning the phase and amplifier section currents, while current applied on the DFB lasers section is fixed at IDFB=70 mA [12]. Regions of steady single mode, mode beating (M-B) resulted from dual-mode state, periodical oscillation including period one (P1) and period doubling (P2), and chaos can be clearly recognized in this mapping at different combination of IP and IA. For dual-mode lasing state, the mode spacing of the AFL can also be adjusted through I P and IA , which provides flexible means to generate tunable photonics microwave or to extract the optical clocks from signals with varied data rate. Detailed simulation and analysis can be referred to [12].
IDFB
Output
Phase
DFB
IA
Ip
Amplifier
Feedback Cavity
(a)
(b)
Fig. 1. (a) Schematic structure of the AFL ; (b) mapping of the dynamic states in the IA-IP plane (after Ref [12])
45
Dan Lu and Lingjuan Zhao / Procedia Engineering 140 (2016) 43 – 48
3. Applications As a slave laser, the AFL can be injection locked by a master injecting signal, thus realizing the clock recovery function. As a dual-mode laser source, the beating of the two laser modes can be converted to microwave outputs, functioning as a photonic microwave generator. In the following parts, we will discuss the applications of AFL in all optical clock recovery and high frequency photonics microwave generation. 3.1. All optical clock recovery The basic principle for all optical clock recovery is the injection locking of a dual-mode AFL by the clock components presenting in the data signal. When the optical clock components are aligned with the lasing modes of the dual-mode AFL, the two modes of the AFL will be injection locked, and in synchronization with the incoming clock. The beating of the two modes from the injection locked AFL will be a pure optical clock tone. By using the AFLs, we have demonstrated all optical recovery from 20-Gb/s return-to-zero (RZ) [4] and 40-Gb/s RZ [3,5,6] signal. Fig. 2 shows an experimental setup for 40-Gb/s all optical clock recovery [6]. The incoming signal was intentionally degraded by an ASE source.. After injecting into the AFL, clean optical clock could be obtained, with timing jitter ranging from 200 fs to 600 fs when the optical signal-to-noise ratio (OSNR) was varied from 30 dB to 7 dB. Fig.2(b) shows the seriously degraed incoming signal at an OSNR or 7 dB, clear clock trace could still obtained, as shown in Fig.3(c).
(a)
(b)
(c)
Fig.2 All optical clock recovery from degraded 40-Gb/s RZ signal ( [6]). (a) experimental setup;(b) degraded injecting data stream; (c) recovery optical clock
Beside intensity modulated RZ signal which contains strong clock components, AFL can also be used to extract the clocks from signals that do not have obvious clock components such as non-return-to-zero (NRZ) and phase modulated signals. The reason is that most NRZ and phase modulated signals are generated from intensity modulators, which inevitably induce weak clock components in the data spectrum. It is still possible to align the clock components with the two modes of the AFL. We have demonstrated the all optical clock recovery from 40-Gbaud NRZ-QPSK signal with the AFL [7]. Although with a common experiment setup like previous one, it is possible to extract the clock, the clock quality is usually not as good, due to the disturbance of the the phase information presented in the data spectrum. A preprocessing stage based on the cross gain modualtion (XGM) effect in a SOA is adopted to eliminate the phase information in the incoming signal. After preprocessing, a high-quality recovered clock signal with an RMS timing jitter of 363 fs is obtained. Fig.3(a) shows the experimental setup. Fig3.(b) is the waveform of the 40-Gbaud NRZ-QPSK signal, and Fig.3(c) is the recovered clock in the optical and RF domain.
46
Dan Lu and Lingjuan Zhao / Procedia Engineering 140 (2016) 43 – 48
(a)
(b)
(c)
Fig.3 All optical clock recovery from 40-Gbaud NRZ-QPSK signal [7]. (a) the experiment setup (b) the waveform of the NRZ-QPSK signal, (c) recovered optical clock in the time domain (upper) and RF domain (lower)
3.2. Photonic microwave generation Photonic microwave generation is another important function that the AFL can provide. With a convenient adjustment of the injection currents on the phase and amplifier sections, the mode spacing between the two modes of the AFL can be tuned continuously in a broad range. Since the two laser modes are generated from the same chip, they are more coherent than independent beating laser sources. The typical beating linewidth is usually around 3~5 MHz for the AFL. We have fabricated AFLs with different dual-mode spacing tunable in the frequency ranges of (19 GHz~26 GHz) [14], (30 GHz~38 GHz) [10], (46 GHz~72 GHz) [15] and (87 GHz~103 GHz) [16]. Fig.4 shows the AFL working in the 60-GHz band, which can be used in RoF system. Fig.3(a) shows the frequency tuning map of the AFL. By tuning the phase and amplifier currents, the AFL generated a tunable beating signal from 46 GHz to 72 GHz. The optical spectra is shown in Fig.4(b). To measure the RF spectra of the beating frequency, a down conversion method was performed. Fig.4(c) shows the down-converted RF signal.
(a)
(b)
(c)
Fig.4 The performance of a dual-mode AFL working in the range of (46 GHz~72 GHz) [15]. (a) the frequency tuning map of the AFL;(b) the optical spectra of the frequency tunable AFL; (c) the RF spectra after photodetection and frequency down conversion
(a)
(b)
(c)
Fig.5 Optical microwave generation using dual-mode AFL with self-injection scheme [10].. (a) experimental setup. (b) the linewidth comparison between the free running spectrum and self-injection locked spectrum (c) the overlapped RF spectra;
47
Dan Lu and Lingjuan Zhao / Procedia Engineering 140 (2016) 43 – 48
Although the beating linewidth of the dual modes from the AFL is small comparing with that of independent beating laser sources which are usually around 10-20 MHz, it is still desirable to further narrow down the beating linewidth. A delayed self-injection scheme was adopted to narrow the optical linewidth of the each mode of the AFL [10]. In the experiment, a dual-loop structure were used to suppress the densely spaced fiber modes, as shown in Fig.5(a). After self-injection locking, the RF spectrum of the beating signal was considerably narrowed from 5 MHz to 2 kHz, as shown in Fig.5(b). The frequency of the beating mode could be tuned from 30 GHz to 38 GHz, as shown in Fig.5 (c). Beside optical injection, an electrical injection was also performed. The purpose of electrical injection is to lock the two laser mode in phase. But for microwave generation purpose, it is of little importance to lock the two laser modes with the fundamental frequency locking. We adopted a subhamonic injection locking scheme [11]. By using an RF signal at the subharmonic frequency of the dual-mode separation, it is possible to generate higher frequency RF signals with a weak RF injection. When the DFB laser section is modulated by an RF signal, as shown in Fig.6(a), at its subharmonic frequency, the modulation sidebands will coincide with the two modes, thus realizing an sidemode injection locking, as shown in Fig.6(b). Using this scheme, with a low RF driven power of 2.8 dBm, tunable microwave outputs ranging from 15 GHz to 33 GHz has been demonstrated at a 1/2 subharmonic RF driven source, as shown in Fig.6(c). This scheme requires much lower RF driven power than tradional frequency doubling method. Optical sepctrum
λ1 -1st
-2nd
nd
-2
λ2 +1st +1st -1st +2nd +2nd
Modulation sidebands
Frequency
(a)
(b)
Power (dBm)
-20
Optical carrier
-40 50 dB
-60 -80 24.6 25.2 25.8 26.4 27.0 27.6 Frequency (GHz)
(c)
Fig.6 The subharmonic RF injection locking of an AFL [11]. (a)the schematic plot of the scheme (b) the working principle; (c) the obtained RF spectrum
4. Conclusion By using the dual mode lasing property of integrated semiconductor lasers, we have demonstrated its application in the areas of all optical clock recovery and photonics microwave generation. All optical clock recovery experiments for RZ signal and NRZ-QPSK signal at different data rate are reviewed. Direct photonics microwave generation method as well as injection locked generation methods based on dual-mode lasers are discussed.
Acknowledgements This work is supported by the National 973 project (Grant No. 2011CB301702) and the National Natural Science Foundation of China (Grant No. 61201103, 61335009, 61474111). References 1. S. Bauer, O. Brox, J. Sieber, and M. Wolfrum, "Novel concept for a tunable optical microwave source," in Optical Fiber Communication Conference and Exhibit (2002), pp. 478–479. 2. Y. Sun, J. Q. Pan, L. J. Zhao, W. X. Chen, W. Wang, L. Wang, X. F. Zhao, and C. Y. Lou, "All-optical clock recovery for 40Gbs using an amplified feedback DFB laser," in Asia Communications and Photonics Conference (2009), Vol. 7631, p. 76310E–76310E–7. 3. L. Wang, X. Zhao, C. Lou, D. Lu, Y. Sun, L. Zhao, and W. Wang, "40 Gbits/s all-optical clock recovery for degraded signals using an amplified feedback laser.," Appl. Opt. 49, 6577–81 (2010). 4. Y. Sun, J. Q. Pan, L. J. Zhao, W. Chen, W. Wang, L. Wang, X. F. Zhao, and C. Y. Lou, "All-Optical Clock Recovery for 20 Gb/s Using an Amplified Feedback DFB Laser," J. Light. Technol. 28, 2521–2525 (2010). 5. C. Chen, Y. Sun, L. Zhao, J. Pan, J. Qiu, S. Liang, W. Wang, and C. Lou, "Amplified feedback DFB laser for 40Gb/s all-optical clock recovery," Opt. Commun. 284, 5613–5617 (2011).
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