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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 26 — Dec. 12, 2011
  • pp: B522–B530
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Self-wavelength initialization method for the Bragg-grating based tunable light source in WDM passive optical network

Jie Hyun Lee, Kwang Ok Kim, Seung-Il Myoung, Sang Soo Lee, and Youn Seon Jang  »View Author Affiliations


Optics Express, Vol. 19, Issue 26, pp. B522-B530 (2011)
http://dx.doi.org/10.1364/OE.19.00B522


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Abstract

We propose a self-wavelength initialization method for the waveguide Bragg-grating based tunable external cavity laser. Reflection characteristics of the Bragg-grating, and the wavelength routing property in the wavelength division multiplexing-passive optical network (WDM-PON) were used in our proposed method. By adding a cost-effective and low- power broadband light source in the central office, the tunable laser can align its output wavelength with the wavelength at the center of the pass band of the WDM multiplexer. This method can be applicable for the Bragg-grating based tunable laser, which is placed either in the central office or in the subscriber region. The operating principle and the feasibility of the proposed method were described by evaluating the transmission performance. By using our initialization method, a tunable laser with a Bragg-grating can be used as a true colorless light source for the WDM-PON.

© 2011 OSA

1. Introduction

Mobile devices, such as smartphones and tablets, have changed personal lives to a great extent. At the same time, these mobile devices had a deep impact on mobile backhaul transport, which brought a bottleneck in the mobile network; this was because the current backhaul infrastructure was not optimized to handle this drastic increase traffic [1

1. M. Milosavljevic, P. Kourtessis, W. Lim, and J. M. Senior, “Next Generation PONs with Wireless Backhauling,” in Proceedings of the ICTON (2011)

-2

2. H. Mickelsson, “WDM-PON in Mobile Backhaul,” in Proceedings of the ICTON (2011)

]. It has been widely considered that the ultimate choice for facilitating large capacity is to use a fiber, owing to its nearly unlimited transmission bandwidth [3

3. L. G. Kazovsky, W. Shaw, D. Gutierrez, N. Cheng, and S. Wong, “Next-generation optical access networks,” J. Lightwave Technol. 25(11), 3428–3442 (2007). [CrossRef]

-4

4. A. Banerjee, Y. Park, F. Clarke, H. Song, S. Yang, G. Kramer, K. Kim, and B. Mukherjee, “Wavelength-division-multiplexed passive optical network technologies for broadband access: a review,” J. Opt. Networking 4(11), 737–758 (2005). [CrossRef]

]. Among the different fiber based solutions available, wavelength division multiplexing-passive optical network (WDM-PON) has been considered to be the most suitable technology for meeting the current demand of high bandwidth. This is because it can provide dedicated user bandwidth up to 10 Gbit/s per subscriber using a designated wavelength regardless of transmission protocols. Furthermore, the reason for the WDM-PON to be considered as one of the most promising candidates as a fiber based backhaul solution is that it can solve the power budget problems and can mitigate the complicated bandwidth allocation or ranging problems in conventional time-shared optical access networks [5

5. C. K. Chan, L. K. Chem, and C. Lin, “WDM PON for next-generation optical broadband access networks,” in Proceedings of the OECC (2006)

].

In order to make WDM-PON cost-effective and practical for widespread deployment, many researchers have been developing colorless light sources for optical network units (ONUs). Colorless light sources can be categorized into tunable lasers and reflective type broadband sources, such as reflective semiconductor optical amplifier (RSOA), and Fabry-Perot laser (FP-LD) [6

6. Y. C. Chung, “Challenges toward practical WDM PON”, in Proceedings of the OECC (2006).

12

12. Y.-O. Noh, H.-J. Lee, J. J. Ju, M.-S. Kim, S. H. Oh, and M.-C. Oh, “Continuously tunable compact lasers based on thermo-optic polymer waveguides with Bragg gratings,” Opt. Express 16(22), 18194–18201 (2008). [CrossRef] [PubMed]

]. Among these light sources, tunable lasers are considered as an attractive solution for use in WDM-PON, owing to their flexibility and scalability [8

8. L. A. Coldren, G. A. Fish, Y. Akulova, J. S. Barton, L. Johansson, and C. W. Coldren, “Tunable semiconductor lasers: a tutorial,” J. Lightwave Technol. 22(1), 193–202 (2004). [CrossRef]

12

12. Y.-O. Noh, H.-J. Lee, J. J. Ju, M.-S. Kim, S. H. Oh, and M.-C. Oh, “Continuously tunable compact lasers based on thermo-optic polymer waveguides with Bragg gratings,” Opt. Express 16(22), 18194–18201 (2008). [CrossRef] [PubMed]

]. However, a wavelength initialization process is necessary before starting communication using tunable lasers; this is because the output wavelength of the tunable lasers is not fixed. A straightforward and simple way to achieve wavelength initialization is to use a lookup table, which is usually predetermined and loaded in the memory of the tunable transceiver. A lookup table has to be generated for each of the lasers because of the manufacturing variations. Moreover, the value of the control parameters in the lookup table need to be adjusted because of either laser aging or temperature changes. The time for generating a lookup table depends on the tuning mechanism of the laser diodes, and although, some methods have been proposed for generating a lookup table in a short time, the overall generating process is exhaustive and requires a time-consuming scanning process [13

13. G. Sarlet, G. Morthier, and R. Baets, “Control of widely tunable SSG-DBR lasers for dense wavelength division multiplexing,” J. Lightwave Technol. 18(8), 1128–1138 (2000). [CrossRef]

]. Because of this, the device packaging cost increases. Furthermore, remote control between the central office and subscriber is required to use the lookup table; this can impair the unique advantage of protocol transparency that the WDM-PON provides. Hence, from a practical point of view, it is strongly preferred to find a wavelength initialization method that keeps operator intervention to a minimum. Several research groups have proposed wavelength initialization methods in the optical domain [14

14. J. H. Lee, H. H. Yoon, M. Y. Oark, and B. W. Kim, “Novel wavelength initialization of the Bragg-grating based tunable external cavity laser for WDM-PON,” in Proceedings of the ECOC (2007), paper P119.

17

17. S. Moon, H. Lee, and C. Lee, “Automatic Wavelength Control Method Using Rayleigh Rackscattering for WDM-PON with Tunable Lasers,” in Proceedings of the CLEO (2011), paper CFH1.

]. One of the group achieved wavelength initialization by alignment of the wavelength by maximizing the back-scattered optical power from the feeder fiber, whereas another group achieved wavelength initialization by maximizing the back-reflected optical power. There was another proposal regarding the use of the beating phenomena between the lasing light output and intentionally added light.

In this paper, we propose a self-wavelength initialization method for Bragg-grating based tunable light sources. The principle and the feasibility of the proposed initialization method are explained in section 2. The effectiveness of the proposed method along with the transmission results is experimentally investigated in section 3. Finally, this paper will be summarized in section 4.

2. Principle and feasibility of self-wavelength initialization method

In the proposed self-wavelength initialization method, we have used the unique characteristics of the Bragg-grating reflector and the WDM-PON. The Bragg-grating, a wavelength selective reflector in the external cavity laser, reflects light not only in the direction toward the inside of the cavity, but also in the direction away from the cavity. Because the optical line terminal (OLT) and the optical network unit (ONU) are logically point-to-point connected in the WDM-PON, the wavelength of the light that arrives at the ONU comes from the OLT and passes through the WDM multiplexer (MUX) and demultiplexer (DeMUX). Therefore, the wavelength that passes through the WDM MUX becomes the target wavelength for the tunable laser in the ONU. As shown in the inset of Fig. 1
Fig. 1 Simplified configuration of the WDM-PON using the Bragg-grating based tunable external cavity laser. Inset is the lasing principle of the Bragg-grating based tunable external cavity laser
, the lasing wavelength of the Bragg-grating based tunable external cavity laser is mainly determined by the peak wavelength of the Bragg reflector. Therefore, by monitoring the amount of the optical power reflected from the Bragg reflector, one can simply check whether the lasing wavelength of the tunable laser is well aligned.

In order to demonstrate the proposed wavelength initialization method, we have used the waveguide-Bragg-grating (WBG) based external cavity laser. The reflection peak of the WBG was tunable in the C-band, and therefore, the laser output also is tunable in the C-band. Thesimplified structure and the operating principle of the tunable external cavity laser are shown in Fig. 1. A laser diode chip was used as a gain medium and a polymer WBG was used as a wavelength selective reflector. The polymer material, which has a large thermo-optic coefficient, was used in order to obtain a wide tuning range. For thermal tuning a Cr/Au thin-film heater was deposited on the top of the polymer WBG. By applying an electric current to the heater, the peak wavelength in the reflection spectrum was tuned, almost linearly proportional to the electrical heat power. Owing to this thermal tuning mechanism, the tuning time would increase by a few seconds. Therefore this type of tunable laser would be not appropriate for a wavelength agile architecture where fast switching is mandatory. However, it can be applied in the WDM-PON because tunable lasers have been used as a substitute for the fixed wavelength transmitter. The details of the structure and the performance of this polymer WBG based tunable external cavity laser have been well described in other papers [10

10. J. H. Lee, M. Y. Park, C. Y. Kim, S.-H. Cho, W. Lee, G. Jeong, and B. W. Kim, “Tunable external cavity laser based on Polymer waveguide platform for WDM Access network,” IEEE Photon. Technol. Lett. 17(9), 1956–1958 (2005). [CrossRef]

12

12. Y.-O. Noh, H.-J. Lee, J. J. Ju, M.-S. Kim, S. H. Oh, and M.-C. Oh, “Continuously tunable compact lasers based on thermo-optic polymer waveguides with Bragg gratings,” Opt. Express 16(22), 18194–18201 (2008). [CrossRef] [PubMed]

].

3. Experiments and discussion

The experimental setup for demonstrating the effectiveness of the proposed method is shown in Fig. 5(a)
Fig. 5 Experimental setup to investigate two different cases: (a) tunable lasers located in the central office and (b) tunable lasers located in the subscriber region.
and Fig. 5(b), which is similar to the setup shown in Fig. 2(a). We have assumed that the wavelength band for the upstream and the downstream signal have been separated by the FSR of the AWG. Therefore, we separately examine their transmission performance, as shown in Fig. 5(a) and Fig. 5(b). When the tunable lasers are located in the central office with the broadband light source, the optical power injected into the laser cavity might be too strong. This is because there is no place to put isolators to adopt the proposed initialization method. Because the refractive index of the semiconductor lasers is strongly dependent on the carrier density, the phase or amplitude of the laser light can be varied by the strong injection power. Therefore, the influence of external injection into the laser cavity has been examined for the downstream transmission with the experimental setup shown in Fig. 5(a). When the tunable lasers are located in the subscriber region, as shown in Fig. 5(b), the upstream transmission performance degrades by back-scattered broadband light source.

We have used an erbium doped fiber amplifier (EDFA) as a reference broadband light. For the WDM MUX and DeMUX, we have used thin-film filters, with a channel spacing of 100 GHz, and 200 GHz. The transmission performance was evaluated using 20 km and 40 km single-mode fibers. A 3 dB power splitter was used in front of the external cavity laser to align the wavelength of the laser. The wavelength of the laser was aligned to 1542.94 nm. The laser was directly modulated at 1.25 Gbit/s NRZ data with a pseudorandom bit sequence (PRBS) pattern length of 27-1. The extinction ratio and the output were about 9 dB,and about 3 dBm, respectively. APD based optical receiver was used to measure the BER plots.

In order to examine the influence of the external optical injection into the cavity, we have controlled the optical variable attenuator to vary the injection power. The extent of performance degradation would be a function of the power ratio between the external injected light and the laser output. However it is difficult to obtain the exact value of this ratio, because of coupling losses and the wavelength dependency of the WBG reflectivity. Moreover the external injection, originating from the reference incoherent light and the backscattered portion of the laser output itself is hard to separate. Therefore, we have overcome these problems by simply defining an injection ratio based on externally measurable parameters. Here, we have defined the external injection power ratio as Pexternal_injected/Poutput. The superposition of the laser output is shown in Fig. 6(a)
Fig. 6 Superposition of the laser output spectrum under different external optical injection ratios. The channel spacing of the MUX/DeMUX were (a) 200 GHz and (b) 100 GHz
and Fig. 6(b). Because the spectral width of the 100 GHz spaced MUX is about half of that of the 200 GHz spaced MUX, the spectral density of the externally injected power is different in both cases, even the external injection power ratio was calculated to be the same. Therefore, the peak wavelength of the laser output has changed with the MUX spacing of 100 GHz.

The BER plots were obtained for different lengths of the fiber to examine the degradation in the transmission performance under such strong injection. Because of the narrow linewidth of the laser output, there was negligible penalty due to the spectral width of the WDM MUX, as shown in Fig. 7(a)
Fig. 7 Downstream bit error rate plots after (a) 0 km transmission and (b) 40 km transmission
. A comparison of Fig. 7(a) and Fig. 7(b), the dispersion penalty after transmission using the single-mode fiber of 40 km is about 0.5 dB when there is no external injection, and when there is a strong external injection, the dispersion penalty after 40 km is about 0.9 dB with the MUX spacing of 200 GHz. When the MUX spacing of 100 GHz is used, the dispersion penalty after 40 km is more than 1.5 dB. The additional power penalty of 0.4 dB and 1 dB were cause by the spectral width of the signal output, as shown in the Fig. 6(b). If the laser is modulated at higher bit rate than 1.25 Gbit/s, the allowable transmission length will be limited owing to the dispersion induced power penalty. Although we examine the degradation effect due to strong optical injection, it may be noted that the high power broadband light source is not necessary in the proposed initialization process. This is because the proposed method only monitors the optical power difference in a dB scale, and not the amount of optical power reflected.

In order to investigate the effectiveness of the proposed method, when the tunable lasers are placed in the subscriber regime, we measured the BER plots using the experimental setup, shown in Fig. 5(b). In such a case, the transmission performance is expected to degrade by the back-scattered broadband light source. The beat noise originates from the mixing or beating of the coherent laser signal with the incoherent broadband light in the same polarization [20

20. N. A. Olsson, “Lightwave systems with optical amplifiers,” J. Lightwave Technol. 7(7), 1071–1082 (1989). [CrossRef]

]. Because the amount of incoherent light, which arrives at the receiver, depends on the optical bandwidth of the WDM, it is assumed the denser spaced MUX would exhibit the larger power penalty. By varying the launching power into the fiber, the BER plots were measured, as shown in Fig. 8(a)
Fig. 8 Upstream bit error rate plots using MUX spacing of (a) 200 GHz and (b) 100 GHz
and Fig. 8(b). There was power penalty lower than 0.5 dB for the MUX with 200 GHz spacing. This penalty of 0.5 dB includes the dispersion penalty after 20 km and the back-scattered induced penalty. For the MUX with 100 GHz spacing, it was difficult to obtain error-free transmission with a launching power of 20 dBm, as shown in the Fig. 8(b). For the upstream application, the maximum allowable launching power of the broadband light would be limited by the beating noise. Similarly, in the downstream case, the minimum allowable launching power would be limited by the sensitivity of the mPD.

4. Summary

We have proposed a self-wavelength initialization method for tunable WBG based external cavity lasers. An additional broadband light source was required in the central office to use the proposed method. We have demonstrated the feasibility of the suggested wavelength initialization method. The transmission performance when the tunable lasers were placed in the central office and in the subscriber region was investigated. The power penalty due to the high power injection was negligible at the back-to-back transmission. However, the additional dispersion penalty after 40 km was measured to be about 0.5 dB and 1 dB, with the MUX channel spacing of 200 GHz, and 100 GHz, respectively. When the tunable lasers are used as a upstream transmitter, the back-scattered deterioration was another reason that the power penalty might increase. The degradations in performance occurred only with the high power broadband light. The minimum allowable output power of the broadband light would be higher than the sum of the detection limit of the mPD and insertion loss between the broadband light source and the mPD. We strongly expect that our novel initialization method will contribute to the cost-effective realization of wavelength tunable WDM-PON in the near future.

Acknowledgements

This work was partly supported by the IT R&D program of MKE/KEIT [KI0018-10039170, Wired and Wireless converged access network based on OFDMA-PON with 10 Gbit/s line rate] and the R&D program supervised by the Korea Communications Agency (KCA-2011-10913-05002).

References and links

1.

M. Milosavljevic, P. Kourtessis, W. Lim, and J. M. Senior, “Next Generation PONs with Wireless Backhauling,” in Proceedings of the ICTON (2011)

2.

H. Mickelsson, “WDM-PON in Mobile Backhaul,” in Proceedings of the ICTON (2011)

3.

L. G. Kazovsky, W. Shaw, D. Gutierrez, N. Cheng, and S. Wong, “Next-generation optical access networks,” J. Lightwave Technol. 25(11), 3428–3442 (2007). [CrossRef]

4.

A. Banerjee, Y. Park, F. Clarke, H. Song, S. Yang, G. Kramer, K. Kim, and B. Mukherjee, “Wavelength-division-multiplexed passive optical network technologies for broadband access: a review,” J. Opt. Networking 4(11), 737–758 (2005). [CrossRef]

5.

C. K. Chan, L. K. Chem, and C. Lin, “WDM PON for next-generation optical broadband access networks,” in Proceedings of the OECC (2006)

6.

Y. C. Chung, “Challenges toward practical WDM PON”, in Proceedings of the OECC (2006).

7.

J. H. Lee, S. Cho, H. Lee, E. Jung, J. Yu, B. Kim, S. Lee, J. Koh, B. Sung, S. King, J. Kim, K. Jeong, and S. S. Lee, “First Commercial Deployment of a Colorless Gigabit WDM/TDM Hybrid PON System Using Remote Protocol Terminator,” J. Lightwave Technol. 28(4), 344–351 (2010). [CrossRef]

8.

L. A. Coldren, G. A. Fish, Y. Akulova, J. S. Barton, L. Johansson, and C. W. Coldren, “Tunable semiconductor lasers: a tutorial,” J. Lightwave Technol. 22(1), 193–202 (2004). [CrossRef]

9.

J. Buus and E. J. Murphy, “Tunable lasers in optical networks,” J. Lightwave Technol. 24(1), 5–11 (2006). [CrossRef]

10.

J. H. Lee, M. Y. Park, C. Y. Kim, S.-H. Cho, W. Lee, G. Jeong, and B. W. Kim, “Tunable external cavity laser based on Polymer waveguide platform for WDM Access network,” IEEE Photon. Technol. Lett. 17(9), 1956–1958 (2005). [CrossRef]

11.

G. Jeong, J. H. Lee, M. Y. Park, C. Y. Kim, S. H. Cho, W. Lee, and B. W. Kim, “Over 26 nm wavelength tunable external cavity laser based on polymer waveguide platforms for WDM access networks,” IEEE Photon. Technol. Lett. 18(20), 2102–2104 (2006). [CrossRef]

12.

Y.-O. Noh, H.-J. Lee, J. J. Ju, M.-S. Kim, S. H. Oh, and M.-C. Oh, “Continuously tunable compact lasers based on thermo-optic polymer waveguides with Bragg gratings,” Opt. Express 16(22), 18194–18201 (2008). [CrossRef] [PubMed]

13.

G. Sarlet, G. Morthier, and R. Baets, “Control of widely tunable SSG-DBR lasers for dense wavelength division multiplexing,” J. Lightwave Technol. 18(8), 1128–1138 (2000). [CrossRef]

14.

J. H. Lee, H. H. Yoon, M. Y. Oark, and B. W. Kim, “Novel wavelength initialization of the Bragg-grating based tunable external cavity laser for WDM-PON,” in Proceedings of the ECOC (2007), paper P119.

15.

J. Moon, K. Choi, S. Mun, and C. Lee, “A self wavelength managed tunable laser for WDM-PONs,” in Proceedings of the ECOC (2008), paper Th.1.F.2.

16.

S. Mun, J. Moon, S. Oh, and C. Lee, “A self wavelength tracking method for a cost effective WDM-PON with tunable lasers,” in Proceeding of the OFC/NFOEC (2010), paper OWG7.

17.

S. Moon, H. Lee, and C. Lee, “Automatic Wavelength Control Method Using Rayleigh Rackscattering for WDM-PON with Tunable Lasers,” in Proceedings of the CLEO (2011), paper CFH1.

18.

J. H. Lee, H. Lee, S. Cho, K. Kim, E. Jung, Y. S. Jang, J. H. Lee, and S. S. Lee, “Self-Wavelength Initialization Method for the Bragg-Grating Based Tunable Light Source in WDM network,” in Proceedings of the ECOC (2011), Th.11.C.1.

19.

J. H. Lee, S. Cho, Y. S. Jang, and S. S. Lee, “Enhancement of Power Budget in RSOA based Loop-back type WDM-PON by using the Cascaded RSOAs,” in Proceedings of the ICTON (2010), paper Tu.B1.5.

20.

N. A. Olsson, “Lightwave systems with optical amplifiers,” J. Lightwave Technol. 7(7), 1071–1082 (1989). [CrossRef]

OCIS Codes
(060.4250) Fiber optics and optical communications : Networks
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Access Networks and LAN

History
Original Manuscript: October 3, 2011
Revised Manuscript: November 11, 2011
Manuscript Accepted: November 22, 2011
Published: November 29, 2011

Virtual Issues
European Conference on Optical Communication 2011 (2011) Optics Express

Citation
Jie Hyun Lee, Kwang Ok Kim, Seung-Il Myoung, Sang Soo Lee, and Youn Seon Jang, "Self-wavelength initialization method for the Bragg-grating based tunable light source in WDM passive optical network," Opt. Express 19, B522-B530 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-26-B522


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References

  1. M. Milosavljevic, P. Kourtessis, W. Lim, and J. M. Senior, “Next Generation PONs with Wireless Backhauling,” in Proceedings of the ICTON (2011)
  2. H. Mickelsson, “WDM-PON in Mobile Backhaul,” in Proceedings of the ICTON (2011)
  3. L. G. Kazovsky, W. Shaw, D. Gutierrez, N. Cheng, and S. Wong, “Next-generation optical access networks,” J. Lightwave Technol.25(11), 3428–3442 (2007). [CrossRef]
  4. A. Banerjee, Y. Park, F. Clarke, H. Song, S. Yang, G. Kramer, K. Kim, and B. Mukherjee, “Wavelength-division-multiplexed passive optical network technologies for broadband access: a review,” J. Opt. Networking4(11), 737–758 (2005). [CrossRef]
  5. C. K. Chan, L. K. Chem, and C. Lin, “WDM PON for next-generation optical broadband access networks,” in Proceedings of the OECC (2006)
  6. Y. C. Chung, “Challenges toward practical WDM PON”, in Proceedings of the OECC (2006).
  7. J. H. Lee, S. Cho, H. Lee, E. Jung, J. Yu, B. Kim, S. Lee, J. Koh, B. Sung, S. King, J. Kim, K. Jeong, and S. S. Lee, “First Commercial Deployment of a Colorless Gigabit WDM/TDM Hybrid PON System Using Remote Protocol Terminator,” J. Lightwave Technol.28(4), 344–351 (2010). [CrossRef]
  8. L. A. Coldren, G. A. Fish, Y. Akulova, J. S. Barton, L. Johansson, and C. W. Coldren, “Tunable semiconductor lasers: a tutorial,” J. Lightwave Technol.22(1), 193–202 (2004). [CrossRef]
  9. J. Buus and E. J. Murphy, “Tunable lasers in optical networks,” J. Lightwave Technol.24(1), 5–11 (2006). [CrossRef]
  10. J. H. Lee, M. Y. Park, C. Y. Kim, S.-H. Cho, W. Lee, G. Jeong, and B. W. Kim, “Tunable external cavity laser based on Polymer waveguide platform for WDM Access network,” IEEE Photon. Technol. Lett.17(9), 1956–1958 (2005). [CrossRef]
  11. G. Jeong, J. H. Lee, M. Y. Park, C. Y. Kim, S. H. Cho, W. Lee, and B. W. Kim, “Over 26 nm wavelength tunable external cavity laser based on polymer waveguide platforms for WDM access networks,” IEEE Photon. Technol. Lett.18(20), 2102–2104 (2006). [CrossRef]
  12. Y.-O. Noh, H.-J. Lee, J. J. Ju, M.-S. Kim, S. H. Oh, and M.-C. Oh, “Continuously tunable compact lasers based on thermo-optic polymer waveguides with Bragg gratings,” Opt. Express16(22), 18194–18201 (2008). [CrossRef] [PubMed]
  13. G. Sarlet, G. Morthier, and R. Baets, “Control of widely tunable SSG-DBR lasers for dense wavelength division multiplexing,” J. Lightwave Technol.18(8), 1128–1138 (2000). [CrossRef]
  14. J. H. Lee, H. H. Yoon, M. Y. Oark, and B. W. Kim, “Novel wavelength initialization of the Bragg-grating based tunable external cavity laser for WDM-PON,” in Proceedings of the ECOC (2007), paper P119.
  15. J. Moon, K. Choi, S. Mun, and C. Lee, “A self wavelength managed tunable laser for WDM-PONs,” in Proceedings of the ECOC (2008), paper Th.1.F.2.
  16. S. Mun, J. Moon, S. Oh, and C. Lee, “A self wavelength tracking method for a cost effective WDM-PON with tunable lasers,” in Proceeding of the OFC/NFOEC (2010), paper OWG7.
  17. S. Moon, H. Lee, and C. Lee, “Automatic Wavelength Control Method Using Rayleigh Rackscattering for WDM-PON with Tunable Lasers,” in Proceedings of the CLEO (2011), paper CFH1.
  18. J. H. Lee, H. Lee, S. Cho, K. Kim, E. Jung, Y. S. Jang, J. H. Lee, and S. S. Lee, “Self-Wavelength Initialization Method for the Bragg-Grating Based Tunable Light Source in WDM network,” in Proceedings of the ECOC (2011), Th.11.C.1.
  19. J. H. Lee, S. Cho, Y. S. Jang, and S. S. Lee, “Enhancement of Power Budget in RSOA based Loop-back type WDM-PON by using the Cascaded RSOAs,” in Proceedings of the ICTON (2010), paper Tu.B1.5.
  20. N. A. Olsson, “Lightwave systems with optical amplifiers,” J. Lightwave Technol.7(7), 1071–1082 (1989). [CrossRef]

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