Utilization of self-injection Fabry-Perot laser diode for long-reach WDM-PON

Utilization of self-injection Fabry–Perot laser diode for long-reach WDM-PON

Fu Yuan Shih

, Chien Hung Yeh

, Chi Wai Chow

, Chia Hsuan Wang

, Sien Chi

a

Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan

b

Information and Communications Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 31040, Taiwan

c

Department of Photonics Engineering, Yuan Ze University, Chung-Li 32003, Taiwan

a r t i c l e

i n f o

Article history: Received 4 May 2009 Revised 21 August 2009 Available online 31 October 2009 Keywords:

WDM-PON Self-injection Fabry–Perot laser diode

a b s t r a c t

In this investigation we propose and demonstrate a wavelength widely tunable laser source employing a self-injected Fabry–Perot laser diode (FP-LD) for long-reach wavelength-division-multiplexed passive optical network (WDM-PON). By using a tunable bandpass filter and an optical circulator inside the gain cavity, a stable and single-longitudinal-mode (SLM) laser output is achieved. Besides, the proposed laser sources are directly modulated at 2.5 Gb/s for both downlink and uplink transmissions of 85 km single mode fiber (SMF) in PON without dispersion compensation.

Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction

Wavelength-division-multiplexed passive optical network (WDM-PON) is an efficient technique for fiber access networks. It is an emerging and efficient method providing the point-to-point connectivity to multiple remote locations through optical fiber. Therefore, WDM-PON can provide high data rate than present time division multiplexed (TDM)-PON through the use of wavelength domain[1,2]. However, the WDM light sources both in optical line terminal (OLT) and optical network unit (ONU) face challenges for mass deployment nowadays due to their high cost[1].

Several stable and wavelength-tunable laser sources were re-ported and demonstrated in different operating ranges, such as S–, C–, L–bands for the optical communications [3–6]. Previous works employing fiber grating as the tuning element have been proposed[7–12], however, the wavelength tunability maybe lim-ited by the grating, and the wavelength stability of the laser greatly depends on the temperature stability of the grating. Conventional tunable lasers or different fixed wavelength lasers could be used in each ONU; however, a cost-effective solution would ideally use the same components in each ONU[13]. Using injection locked Fabry–Perot laser diode (FP-LD) is a potential low cost candidate for the WDM-PON. Previous study using a FP-LD and an optical fil-ter inside the ring cavity can produce a stable and tunable single-wavelength output[6]. Using external light sources to injection lock the ONUs for the WDM-PON could be attractive; however, this would also increase the cost[14–18]. Besides, if the external light sources [19,20] are sending from the OLT, interferometric beat

noise caused by Rayleigh backscattering[2]will produce impair-ments to the upstream signal. Signal remodulation PON, where the upstream signal is generated by the downstream signal using reflective semiconductor optical amplifier (RSOA), has been re-ported[21,22]. Previous demonstration using fiber reflective mir-ror for self-injection locking of FP-LD can produce stable and single mode laser output[23]; however, the output power of the laser depends on the reflectivity of the fiber mirror, and the tun-ability of the laser may be limited by the coating characteristics of the fiber mirror.

In this work, we propose and experimentally demonstrate a widely wavelength-tunable laser source with single-longitudinal-mode (SLM) output (having high side-single-longitudinal-mode suppression ratio (SMSR) >40.7 dB), employing self-injected FP-LD for long-reach WDM-PON network. Besides, there is no external source or fiber amplifier inside the gain cavity of the proposed laser source, hence, the network could be cost-effective. The output performances of the laser source used in the WDM-PON are also analyzed and dis-cussed. The proposed laser source is directly modulated at 2.5 Gb/s and shows less than 1 dB power penalty at the bit error rate (BER) of 10 9 after 85 km transmission distance without dispersion compensation.

2. Experimental setup

Fig. 1 shows the experimental setup of the proposed laser source in the OLT and the ONU for the long-reach WDM-PON archi-tecture. In the OLT side, the WDM downlink source consists of N multi-mode FP-LDs, a 2 N  1 AWG with insertion loss of 4 dB, a 1  2 and 50/50 optical coupler (CP) with insertion loss of 0.5 dB, an optical circulator (OC) with insertion loss of 1 dB and a

1068-5200/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.yofte.2009.10.001

*Corresponding author.

E-mail address:[email protected](C.H. Yeh).

Optical Fiber Technology 16 (2010) 46–49

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Optical Fiber Technology

polarization controller (PC). The 3-dB bandwidth of the AWG is 1 nm. The AWG can be used to filter and align the corresponding mode of the FP-LD. Then the filtered mode would be reflected by OC and inject into the FP-LD. Hence, the SLM output is generated. It is worth to mention that only one OC loop is required for all the FP-LDs inside the OLT to produce the multiple wavelength light sources. Moreover, the polarization dependence issue of injection locking can be mitigated since the FP-LDs, AWG and the OC loop are located at the same module and polarization maintaining fibers can be used.

In each ONU, the wavelength-tunable optical transmitter is con-structed by a multi-mode FP-LD, a 1  2 and 50/50 CP, an OC, a PC, and a tunable bandpass filter (TBF). In this experiment, the multi-mode FP-LD has 1.32 nm multi-mode spacing (Dk) and 9.5 mA threshold current, respectively. The PC is used to maintain the polarization state of the self-injected light for successful injection locking. The TBF is used to select the output wavelength of the laser source, with the 3-dB bandwidth and insertion loss of 0.8 nm and 3.5 dB, respectively. And its tuning range is from 1524 to 1564 nm. For practical implementation, the TBF can be replaced by low cost ‘‘set and forget” filter, as described in [24]. In order to achieve SLM output, the pass-band of the TBF is tuned to match with the corresponding longitudinal mode of the FP-LD. Hence, the 1.32 nm tuning step is determined by the longitudinal mode of the FP-LD. The side-mode of the FP-LD can be suppressed due to self-injection when the TBF is tuned to the corresponding longitu-dinal mode, with proper bias current and temperature. The output spectrum and optical power are measured by an optical spectrum analyzer (OSA) with resolution of 0.02 nm and power meter, respectively. Experimental results show that 3 dBm average power was observed at the output of the ONU. And this is just enough to achieve the long-reach (LR) transmission by considering that the transmission loss of 85 km SMF fiber (17 dB) and the insertion losses from two AWGs (total 8 dB) and the 3-dB coupler (CP). Opti-cal amplifiers in the transmission link should be needed if higher power margin is required in the proposed long-reach PON. 3. Results and discussion

The operation mechanism of the proposed self-injected laser is the same in both OLT and ONU. For the laser in the OLT, we use a fixed bandpass filter (the AWG). For the laser in ONU, we use a TBF. The experimental characterization of the laser is performed at the ONU side.Fig. 2shows the measured output spectrum of the free-run FP-LD without self-injection (red line) when the bias current and temperature of the FP-LD is 25 mA and 25 °C, respectively. When the self-injection is operated, as also shown inFig. 2, the output wavelength of the FP-LD (blue line) is locked and tuned in the wavelength range of 1535.64–1557.44 nm in this experiment.

Fig. 3shows the output power and SMSR versus different wave-lengths for the proposed self-injected FP-LD. The minimum output optical power and SMSR are 1.76 dBm and 40.7 dB within the tun-ing range from 1535.64 to 1557.44 nm, respectively. When com-pared with the previously proposed laser in [23], the

self-Wavelength (nm)

1530 1540 1550 1560

Power (10 dB/div)

Free-run self-injected

Fig. 2. Output spectra of free-run FP-LD (red, dotted line) and the lasing wavelength of proposed laser source (blue, solid line) between 1547.80 nm and 1556.04 nm when the bias current and temperature is 25 mA and 25 °C, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Wavelength (nm) 1531 1536 1541 1546 1551 1556 1561 Power (dBm) 0 1 2 3 4 5 6 SMSR (dB) 0 20 40 60 80 Power SMSR

Fig. 3. Output Power and SMSR versus different lasing wavelength with 1.32 nm tuning steps.

OLT

ONU

AW

G

_AW

G

AW

G

_AW

G

Fig. 1. Proposed long reach WDM-PON system based on broadly tunable self-injected Fabry–Perot laser diode. Inset: the wavelength plan of uplink and downlink signals. F.Y. Shih et al. / Optical Fiber Technology 16 (2010) 46–49 47

injection locking is easier to achieve in the proposed laser. Besides, as shown inFig. 3, the proposed laser can have a higher output power (>1 dBm) in the operation wavelength 1535–1557 nm, with smaller mode-to-mode power variation (3.2 dB). However, in the laser reported in[23], it has smaller output power ( 8 dBm) in the operation wavelength 1535–1557 nm, with higher mode-to-mode power variation (5 dB).

To investigate the wavelength locking stability, we measured the output power and SMSR as a function of temperature as shown inFig. 4. The locked wavelength increases when the temperature increases. It increases from 1550.02 nm to 1550.78 nm when the temperature increases from 14 to 30 °C. We believe that the power change is mainly due to the mismatch of the temperature coeffi-cients of the FP-LD and the TBF. This shows that non-precise tem-perature control can be used to maintain the wavelength locking and output power (also maintaining the output wavelength within the AWG channel). This may further reduce the transmitter cost. Besides, we also performed the short-term stability test of the pro-posed light source as shown inFig. 5. The lasing wavelength is initially at 1550.53 nm and the observing time is over 30 min.

Fig. 5shows that the power fluctuation and the wavelength varia-tion for the proposed laser source are 0.76 dB and 0.01 nm, respectively.

In order to investigate the transmission performance of the pro-posed laser source, bit error rate (BER) measurement is performed in the LR-PON. The proposed self-injection laser source is directly modulated by a 2.5 Gb/s non-return-to-zero (NRZ) pseudo random

binary sequence (PRBS). In this measurement, we choose two las-ing wavelengths at 1547.80 and 1556.04 nm for the uplink and downlink channels, respectively.Fig. 6shows the measured BER versus the received power at back-to-back and transmissions of 85 km of single mode fiber (SMF), respectively, without dispersion compensation. The corresponding eye diagrams are also illustrated in the inserts ofFig. 6. Under 85 km transmission, the power pen-alties in uplink and downlink traffic are both less than 1 dB at BER of 10 9.

4. Conclusion

We proposed and experimentally demonstrated a widely tun-able laser source by self-injected FP-LD for long-reach WDM-PON. The output power is over 1.76 dBm and the SMSR is large than 40.7 dB in the proposed laser source. Besides, the stability of the output power, wavelength and SMSR are also investigated. The proposed wavelength-tunable laser source has the advantages of simple architecture, good performance of output power stability and wide wavelength tuning range. For practical implementation, the TBF can be replaced by low cost ‘‘set and forget” filter. At the OLT, only one OC loop is required for all the FP-LDs to produce mul-tiple wavelength light sources for the downlink data. Error free 2.5 Gb/s BER measurements at back-to-back and 85 km SMF trans-mission are achieved.

Acknowledgment

This work was supported by the National Science Council, Tai-wan, ROC, under Contract NSC 2218-E-009-025-MY2, NSC 96-2221-E-155-038-MY2-1, NSC 96-2221-E-155-039-MY3-1, and NSC 97-2221-E-009-038-MY3.

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Fu Yuan Shih is currently pursuing his Ph.D. degree in the Institute of Electro-Optical Engineering, National Chiao Tung University, Taiwan. His research interests are optical fiber communications, fiber lasers, fiber amplifiers, WDM transmissions, and fiber access net-work technologies.

Chien Hung Yeh received his Ph.D. degree from the Institute of Electro-Optical Engineering, National Chiao Tung University, Taiwan in 2004. In 2004, he joined the Information and Communications Laboratories (ICL), Industrial Technology Research Institute (ITRI) in Tai-wan, as a Researcher. In 2008, he was promoted as a Senior Researcher in ICL/ITRI. His research interests are optical fiber communication, fiber laser and amplifier, mm-wave generator, and wireless/wire access network technology.

Chi Wai Chow received the B.Eng. (First-Class Hons) and Ph.D. degrees both from the Department of Elec-tronic Engineering, the Chinese University of Hong Kong in 2001 and 2004, respectively. After graduation, he was appointed as a Postdoctoral Fellow at the CUHK, work-ing on hybrid integration of photonic components and silicon waveguides. Between 2005 and 2007, he was a Postdoctoral Research Scientist in the Tyndall National Institute and Department of Physics, University College Cork in Ireland. In 2007, he joined the Department of Photonics, National Chiao Tung University in Taiwan, as an Assistant Professor.

Chia Husan Wang is currently pursuing his Ph.D. degree in the Institute of Electro-Optical Engineering, National Chiao Tung University, Taiwan. His research interests are optical fiber communications, fiber lasers, fiber amplifiers, WDM transmissions, and fiber access net-work technologies.

Sien Chi received his Ph.D. in Electrophysics from the Polytechnic Institute of Brooklyn, New York, in 1971, and joined the faculty of National Chiao Tung University, where he is currently a Professor of Electro-Optical Engineering. From 1993 to 1996 he received the Distin-guished Research Award sponsored by the National Science Council, Taiwan. Since 1996, he has been the chair professor of the Foundation for Advancement of Outstanding Scholarship. His research interests are optical fiber communications, optical solitons, and opti-cal fiber amplifiers. He is a fellow of the Optiopti-cal Society of America and the Photonics Society of Chinese-Americans. F.Y. Shih et al. / Optical Fiber Technology 16 (2010) 46–49 49