- Email: [email protected]
WDM-PON upstream transmission using Fabry–Perot laser diodes externally injected by polarization-insensitive spectrum-sliced supercontinuum pulses
Optics Communications 260 (2006) 691–695 www.elsevier.com/locate/optcom
WDM-PON upstream transmission using Fabry–Perot laser diodes externally injected by polarization-insensitive spectrum-sliced supercontinuum pulses Yang Jing Wen a b
a,*
, Chang-Joon Chae
b
Lightwave Department, Institute for Infocomm Research, A*STAR, 21 Heng Mui Keng Terrace, Singapore 119613, Singapore National ICT Australia, Ltd., (NICTA), Victoria Research Laboratory, Department of Electrical and Electronic Engineering, The University of Melbourne, Victoria 3010, Australia Received 8 September 2005; received in revised form 14 November 2005; accepted 18 November 2005
Abstract We propose, a novel upstream transmission scheme for high speed wavelength division multiplexed passive optical networks. Upstream transmission at bit rate of 2.5 Gb/s was demonstrated using a Fabry–Perot (FP) laser diode (LD) externally injected by a spectrum-sliced polarization-insensitive supercontinuum pulse source, located at central office. The impact of Rayleigh backscattering on transmission performance is also investigated. The proposed scheme is expected to be cost-effective since low-cost FP LDs are used for light sources. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Optical communications; Wavelength division multiplexing; Passive optical network; External injection; Semiconductor laser
1. Introduction The wavelength division multiplexed passive optical network (WDM-PON) has been considered an ultimate broadband access solution since two dedicated wavelengths are allocated to establish an ultra-wideband bidirectional link between the central office (CO) and each customer [1,2]. It is cost-effective in the sense that the long feeder fiber is shared by a large number of customers and offers additional features such as channel independence and per-customer based flexible upgrade. In most WDM-PON systems, two sets of wavelengths carry upstream and downstream data and the two wavelengths allocated for each customer are separated by more than a free spectral range of the arrayed waveguide grating (AWG) to combine and separate them using low-cost coarse wavelength division multiplexers. Stabilized WDM *
Corresponding author. Tel.: +65 6874 6812; fax: +65 6779 8841. E-mail address: [email protected] (Y.J. Wen).
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.11.029
light sources are required at the CO and customer premises to deliver data and other services. Since the environment of customer premises is not controlled in most cases and the cost is a real big issue for customers, low-cost and rugged light sources or upstream optical transmitters are one of key success elements for practical implementation in real world. Light sources considered so far include spectrum-sliced light-emitting diodes (LEDs) [3], wavelength-seeded reflective semiconductor optical amplifiers (SOAs) [4], spectrumsliced free-running Fabry–Perot laser diodes (FP-LDs) [5], and received downstream signals for re-modulating [6]. However, methods using LEDs and SOAs suffer from low power budget and high packaging cost, respectively. Spectrum slicing of a free-running FP-LD suffers from strong intensity noise, while the re-modulation scheme needs further development to suppress cross-talk from the residual downlink data and alleviate the dependence of polarization state of downlink data [6]. Recently, injection-locking low-cost FP-LDs using amplified spontaneous
692
Y.J. Wen, C.-J. Chae / Optics Communications 260 (2006) 691–695
emission noise has been demonstrated with a good performance at bit rate up to 1.25 Gb/s [7–11]. Yet, there has been no report on higher-speed upstream transmission, which may be required for business customers. In this paper, we propose a polarization-insensitive upstream transmission scheme using low-cost FP-LDs that are injection-locked by supercontinuum pulses and experimentally demonstrate its feasibility at 2.5 Gb/s. 2. Experimental setup and operational principle The experimental setup of the proposed scheme is shown in Fig. 1, which involves a polarization-insensitive broadband pulse source (PIBPS), a feeder fiber, two arrayed waveguide gratings (AWGs), FP-LDs, and a detection part. The pulse source for PIBPS was generated by a distributed feedback (DFB) LD gain-switched at 6 GHz and was linearly compressed by a 1 km long dispersion compensation fiber (DCF). After being boosted by an erbium doped fiber amplifier (EDFA), the pulses were input into a supercontinuum generation fiber (SCF) to obtain a broadband pulse source. Here the SCF was 1 km long dispersion shifted fiber with a zero dispersion wavelength of 1526.4 nm and non-linear coefficient around 2 l/W/km, and the optical power into the SCF was around 26 dBm to generate around 8 nm wide supercontinuum spectrum. Wider spectrum can be achieved by using better SCF like better dispersion flatness and higher non-linear coefficient, and higher launch power. Then the pulse signal was divided into two channels by a 50/50 fiber coupler, with one channel delayed by half a repetition period via an optical delay line. The polarization states of the two channels were adjusted to be orthogonal using two polarization controllers (PC1 and PC2). The two channels were combined together using a polarization beam combiner (PBC) to generate the PIBPS. The polarization-insensitive broadband pulse source was divided by a power splitter to make the pulse source be shared by many WDM-PONs. One of the streams was input into a 10 km single mode fiber (SMF) via an optical circulator and spectrum-sliced by an 8-channel AWG (AWG1) with a bandwidth of 0.3 nm. Then the sliced pulse source was injected into a FP-LD to injection-lock one of the multiple cavity modes, which was directly modulated by 2.5 Gb/s (223 1) pseudo-random data provided by a pulse pattern generator (PPG). The output of the FP-LD then went through AWG1 to be comSCF 50/50 PC1
PBC Splitter
SMF 10 km AWG1
DCF
DFB LD EDFA Synthesizer 6 GHz
τ Delay Line
PC2
AWG2 Rx
PIBPS
A
bined with other channels and transmitted upstream through the SMF. Then the upstream signals were demultipelxed by another AWG (AWG2) and detected by a PIN photo detector with a bandwidth of 2.5 GHz, followed by a communication signal analyzer (CSA), an RF spectrum analyzer (RFSA) and a bit error rate tester (BERT). Polarization insensitivity of the external injection was achieved by polarization-orthogonal optical time division multiplexing as shown in Fig. 1. This scheme has also been used to achieve polarization insensitivity for optical clock recovery [12]. The output of the PBC has a combined polarization state, with optical field expressed as ~ EðtÞ ¼ ^xf ðtÞj/1 þ ^y f ðt T b =2Þej/2 , as shown in Fig. 2, where f(t) is the optical field envelope of the pulses, Tb is the pulse repetition period, and /1 and /2 are optical phase of each channel. If the polarization state of the FP-LD is at an angle of h(t) with respect to the x state, the effective optical power injected into the FP-LD becomes 2
jEeff ðtÞj ¼ f 2 ðtÞcos2 hðtÞ þ f 2 ðt T b =2Þsin2 hðtÞ
ð1Þ
assuming f(t)f(t Tb/2) = 0. Here h is time dependent due to polarization state change of injected pulses, which may be caused by environment temperature variation or unexpected fiber movement. However, the change of h(t) with time is much slower (large time scale) in speed compared with that of the repetition rate of the pulse source, we can average the effective optical injection power in a proper time scale (much smaller than the polarization variation time while much larger than pulse repetition period), which gives P eff ¼ ðcos2 hðtÞ þ sin2 hðtÞÞP 0 =2 ¼ P 0 =2, where P0 is the average power of the spectrum-sliced pulse source and we have assumed the two tributaries have the same average optical power. This shows that the effective power is maintained at a constant level for any values of angle h(t). In [11], the wavelength separation between the incoming data and the clock recovery laser is around 4 nm, so the optical injection is incoherent and only power is involved in the interaction between the optical fields of the incoming data signal and modes inside the FP laser. However, in this investigation, the spectrum-sliced pulse source will be used to injection-lock one of the cavity modes of the FP laser, so it is coherent injection, and optical phase together with power will be involved in the interaction between the optical fields of the injected pulses and the FP-LD. Fortunately, the interaction is still dominated by the effective optical power, leading to relative polarization insensitivity, and the phase contribution will introduce slight polarization dependence, which has been verified by the following experiments.
FP-LD 2.5 Gb/s
x
PPG
CSA RFSA BERT
Fig. 1. Experimental setup for upstream WDM-PON transmission with proposed scheme.
y
t
Fig. 2. Polarization states of the polarization orthogonal optical time division multiplexed pulse source.
Y.J. Wen, C.-J. Chae / Optics Communications 260 (2006) 691–695
3. Results and discussion
693
The FP-LD used in our investigation has a threshold current of 18 mA and was biased at 26 mA in the experiment to get enough modulation bandwidth. Fig. 3 shows the optical spectra of the spectrum-sliced pulses and the FP-LD outputs before and after injection-locking measured at point A by inserting an additional circulator. The spectrum-sliced pulse source had multiple modes with a mode spacing of 6 GHz, due to the broad bandwidth of the AWG (Fig. 3(a)). Without optical injection, the FPLD exhibited multiple mode oscillation, with the width of each cavity mode being broadened due to the effect of direct data modulation, as shown by the dashed curve of Fig. 3(b). Since the semiconductor laser has a locking range of a couple of GHz under external injection condition, when one of the cavity modes is within the optical spectral range of the incoming light, the cavity mode can be locked to one of its incoming light modes, with other cavity modes being greatly suppressed. The solid curve in Fig. 3(b) shows the optical spectrum of the FP-LD externally injected with 0 dBm injection power, which shows a side mode suppression ratio of greater than 40 dB. If we consider all the loss the components involved, including 1 dB circulator loss, 3 dB feeder fiber loss, 5 dB AWG insertion loss, and 4 dB spectrum-slicing loss (for 0.3 nm bandwidth and 100 GHz channel spacing), the total loss will be around 13 dB. For 16 channels, the required PIBPS output power would be
BER (log)
-5
-6 -7 -8 -9 -10 -28
-27.5 -27 -26.5 -26 -25.5 -25 -24.5 -24 Received Optical Power (dBm)
Fig. 4. BERs vs. received optical power for different polarization states of the injected pulses.
25 dBm for 0 dBm injection power, and for 40 channels, the required power would be 29 dBm, which is achievable using high power EDFA. The performance of the upstream transmitter was evaluated by measuring bit error rates (BERs) for the transmitted signal with the same conditions of 26 mA of bias current for the FP-LD and 0 dBm of injection power. In order to confirm the polarization insensitivity, we inserted an additional polarization controller between the PBC and the optical circulator. The polarization state of the injected light was randomly adjusted. Fig. 4 shows the mea-
10 6 GHz
0
Intensity (dBm)
-10 -20 -30 -40 -50 -60 -70 1548.0 1548.5 1549.0 1549.5 1550.0 1550.5 a Wavelength (nm) 10 0
Before Injection After Injection
Intensity (dBm)
-10 -20 -30 -40 -50 -60 -70 1544 b
1546
1548 1550 1552 Wavelength (nm)
1554
Fig. 3. Optical spectra of (a) spectrum-sliced pulse source and (b) FP-LD outputs before and after external injection.
Fig. 5. RF spectrum and data eye pattern at 109 BER after uplink transmission.
Y.J. Wen, C.-J. Chae / Optics Communications 260 (2006) 691–695
sured BER curves of the upstream transmitted data for different polarization states of injected pulses into the FP-LD. For all the random polarizations evaluated, BER at 109 could be achieved without any sign of BER floor. Slightly different receiver sensitivities were observed among the four evaluated cases, which we believe might be attributed to the involvement of the optical phase of the injected light in the injection process, as we discussed in Section 2. As the received optical power is larger than 24 dBm, error-free operation was observed for all polarization states, which verifies the polarization insensitivity of our proposed upstream transmission scheme. Fig. 5 shows the eye diagram and RF spectrum at 109 BER. The RF spectrum shows that the component at 6 GHz arising from the injected pulses is much lower in intensity than that of the data spectrum, which proves that it has no effect on the performance of upstream transmitter. 4. Impact of Rayleigh backscattering The system configuration in Fig. 1 is bidirectional, where the seeding pulse source is first downstream transmitted, and then the modulated data signal is upstream transmitted. Due to Rayleigh backscattering, part of the downstream pulse signal is reflected back and mixed with upstream data signal. The back reflected pulse has the same wavelength with that of the uplink data signal which cannot be separated by using an optical filter, and contributes to in-band crosstalk. This section will investigate the impact of Rayleigh backscattering on the uplink transmission performance. Different from the configuration in Fig. 1, here we use two separate fibers for downlink and uplink transmission, as shown in Fig. 6. Again we inserted an additional polarization controller just before the optical circulator in Fig. 6, and the polarization state of the injected light was randomly adjusted. Fig. 7 shows the measured BER curves of the upstream transmitted data for different polarization states of injected pulses into the FP-LD. For all the random polarizations evaluated, BER at 109 could be achieved without any sign of BER floor. Slightly different receiver sensitivities were also observed among the evaluated four cases. Compared with Fig. 4, all the BER curves under different polarization states in Fig. 7 exhibit
-5
-6
BER (log)
694
-7 -8 -9 -10 -28.5 -28 -27.5 -27 -26.5 -26 -25.5 -25 -24.5
Received Optical Power (dBm) Fig. 7. BER curves with different polarization states of injected pulses using two fibers.
around 1 dB receiver sensitivity improvement. Since all the other operation conditions are the same for the two cases, we believe the sensitivity improvement was attributed to the removal of Rayleigh backscattering component. If the feeder fiber length is increased to 20 km or longer, the Rayleigh backscattered noise will be increased [13]. 5. Conclusions We proposed a new WDM-PON upstream transmission scheme based on polarization-insensitive injection-locking technique and the use of low-cost FP-LDs. 2.5 Gb/s upstream transmission was experimentally demonstrated using a FP-LD externally injected by spectrum-sliced broadband supercontinuum pulses. The polarization insensitivity was achieved by polarization-orthogonal multiplexing of broadband pulse source. Impact of Rayleigh backscattering on the uplink transmission performance is also investigated. The proposed scheme is expected to be cost-effective from the fact that the polarization-insensitive broadband pulse source located at the central office can be shared among many WDM-PONs and low-cost FP-LDs are used as light sources for 2.5 Gb/s upstream transmission. It should be noted that although we only demonstrated the scheme for upstream transmission, the externally injected FP-LDs can be also utilized for downstream transmission. Acknowledgments
SCF 50/50 PC1
PBC
DFB LD EDFA Synthesizer 6 GHz PIBPS
Splitter
τ Delay Line
SMF 10 km AWG1
DCF
PC2
SMF 10 km
FP-LD 2.5 Gb/s
The authors are grateful to Dr. Yasuhiro Matsui for providing the FP-LD and Dr. Hai-Feng Liu for his support in this investigation. References
PPG AWG2 Rx CSA RFSA BERT
Fig. 6. Experimental setup for uplink transmission using two separate fibers.
[1] J.B. Stark, M.C. Nuss, W.H. Knox, S.T. Cundiff, L. Boivin, S.G. Grubb, D. Tipton, D. DiGiovanni, U. Koren, K. Dreyer, IEEE Photon. Technol. Lett. 9 (1997) 1170. [2] G. Maier, M. Martinelli, A. Pattavina, E. Salvadori, J. Lightwave Technol. 18 (2000) 125. [3] D.K. Jung, H. Kim, K.H. Han, Y.C. Chung, Electron. Lett. 37 (2001) 308.
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[9] D.J. Shin, D.K. Jung, J.K. Lee, J.H. Lee, Y.H. Choi, Y.C. Bang, H.S. Shin, J. Lee, S.T. Hwang, Y.J. Oh, Electron. Lett. 39 (2003) 1331. [10] S.-M. Lee, K.-M. Choi, S.-G. Mun, J.-H. Moon, C.-H. Lee, IEEE Photon. Technol. Lett. 17 (2005) 1579. [11] H.D. Kim, C.H. Lee, OECC 2002, 10P-15, 2002. [12] Y.J. Wen, C.-J. Chae, H.F. Liu, IEEE Photon. Technol. Lett. 17 (2005) 1304. [13] H.-C. Kwon, W.-S. Jang, S.-K. Han, IEE Proc. – Optoelectron. 152 (2005) 247.
WDM-PON upstream transmission using Fabry–Perot laser diodes externally injected by polarization-insensitive spectrum-sliced supercontinuum pulses Yang Jing Wen a b
a,*
, Chang-Joon Chae
b
Lightwave Department, Institute for Infocomm Research, A*STAR, 21 Heng Mui Keng Terrace, Singapore 119613, Singapore National ICT Australia, Ltd., (NICTA), Victoria Research Laboratory, Department of Electrical and Electronic Engineering, The University of Melbourne, Victoria 3010, Australia Received 8 September 2005; received in revised form 14 November 2005; accepted 18 November 2005
Abstract We propose, a novel upstream transmission scheme for high speed wavelength division multiplexed passive optical networks. Upstream transmission at bit rate of 2.5 Gb/s was demonstrated using a Fabry–Perot (FP) laser diode (LD) externally injected by a spectrum-sliced polarization-insensitive supercontinuum pulse source, located at central office. The impact of Rayleigh backscattering on transmission performance is also investigated. The proposed scheme is expected to be cost-effective since low-cost FP LDs are used for light sources. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Optical communications; Wavelength division multiplexing; Passive optical network; External injection; Semiconductor laser
1. Introduction The wavelength division multiplexed passive optical network (WDM-PON) has been considered an ultimate broadband access solution since two dedicated wavelengths are allocated to establish an ultra-wideband bidirectional link between the central office (CO) and each customer [1,2]. It is cost-effective in the sense that the long feeder fiber is shared by a large number of customers and offers additional features such as channel independence and per-customer based flexible upgrade. In most WDM-PON systems, two sets of wavelengths carry upstream and downstream data and the two wavelengths allocated for each customer are separated by more than a free spectral range of the arrayed waveguide grating (AWG) to combine and separate them using low-cost coarse wavelength division multiplexers. Stabilized WDM *
Corresponding author. Tel.: +65 6874 6812; fax: +65 6779 8841. E-mail address: [email protected] (Y.J. Wen).
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.11.029
light sources are required at the CO and customer premises to deliver data and other services. Since the environment of customer premises is not controlled in most cases and the cost is a real big issue for customers, low-cost and rugged light sources or upstream optical transmitters are one of key success elements for practical implementation in real world. Light sources considered so far include spectrum-sliced light-emitting diodes (LEDs) [3], wavelength-seeded reflective semiconductor optical amplifiers (SOAs) [4], spectrumsliced free-running Fabry–Perot laser diodes (FP-LDs) [5], and received downstream signals for re-modulating [6]. However, methods using LEDs and SOAs suffer from low power budget and high packaging cost, respectively. Spectrum slicing of a free-running FP-LD suffers from strong intensity noise, while the re-modulation scheme needs further development to suppress cross-talk from the residual downlink data and alleviate the dependence of polarization state of downlink data [6]. Recently, injection-locking low-cost FP-LDs using amplified spontaneous
692
Y.J. Wen, C.-J. Chae / Optics Communications 260 (2006) 691–695
emission noise has been demonstrated with a good performance at bit rate up to 1.25 Gb/s [7–11]. Yet, there has been no report on higher-speed upstream transmission, which may be required for business customers. In this paper, we propose a polarization-insensitive upstream transmission scheme using low-cost FP-LDs that are injection-locked by supercontinuum pulses and experimentally demonstrate its feasibility at 2.5 Gb/s. 2. Experimental setup and operational principle The experimental setup of the proposed scheme is shown in Fig. 1, which involves a polarization-insensitive broadband pulse source (PIBPS), a feeder fiber, two arrayed waveguide gratings (AWGs), FP-LDs, and a detection part. The pulse source for PIBPS was generated by a distributed feedback (DFB) LD gain-switched at 6 GHz and was linearly compressed by a 1 km long dispersion compensation fiber (DCF). After being boosted by an erbium doped fiber amplifier (EDFA), the pulses were input into a supercontinuum generation fiber (SCF) to obtain a broadband pulse source. Here the SCF was 1 km long dispersion shifted fiber with a zero dispersion wavelength of 1526.4 nm and non-linear coefficient around 2 l/W/km, and the optical power into the SCF was around 26 dBm to generate around 8 nm wide supercontinuum spectrum. Wider spectrum can be achieved by using better SCF like better dispersion flatness and higher non-linear coefficient, and higher launch power. Then the pulse signal was divided into two channels by a 50/50 fiber coupler, with one channel delayed by half a repetition period via an optical delay line. The polarization states of the two channels were adjusted to be orthogonal using two polarization controllers (PC1 and PC2). The two channels were combined together using a polarization beam combiner (PBC) to generate the PIBPS. The polarization-insensitive broadband pulse source was divided by a power splitter to make the pulse source be shared by many WDM-PONs. One of the streams was input into a 10 km single mode fiber (SMF) via an optical circulator and spectrum-sliced by an 8-channel AWG (AWG1) with a bandwidth of 0.3 nm. Then the sliced pulse source was injected into a FP-LD to injection-lock one of the multiple cavity modes, which was directly modulated by 2.5 Gb/s (223 1) pseudo-random data provided by a pulse pattern generator (PPG). The output of the FP-LD then went through AWG1 to be comSCF 50/50 PC1
PBC Splitter
SMF 10 km AWG1
DCF
DFB LD EDFA Synthesizer 6 GHz
τ Delay Line
PC2
AWG2 Rx
PIBPS
A
bined with other channels and transmitted upstream through the SMF. Then the upstream signals were demultipelxed by another AWG (AWG2) and detected by a PIN photo detector with a bandwidth of 2.5 GHz, followed by a communication signal analyzer (CSA), an RF spectrum analyzer (RFSA) and a bit error rate tester (BERT). Polarization insensitivity of the external injection was achieved by polarization-orthogonal optical time division multiplexing as shown in Fig. 1. This scheme has also been used to achieve polarization insensitivity for optical clock recovery [12]. The output of the PBC has a combined polarization state, with optical field expressed as ~ EðtÞ ¼ ^xf ðtÞj/1 þ ^y f ðt T b =2Þej/2 , as shown in Fig. 2, where f(t) is the optical field envelope of the pulses, Tb is the pulse repetition period, and /1 and /2 are optical phase of each channel. If the polarization state of the FP-LD is at an angle of h(t) with respect to the x state, the effective optical power injected into the FP-LD becomes 2
jEeff ðtÞj ¼ f 2 ðtÞcos2 hðtÞ þ f 2 ðt T b =2Þsin2 hðtÞ
ð1Þ
assuming f(t)f(t Tb/2) = 0. Here h is time dependent due to polarization state change of injected pulses, which may be caused by environment temperature variation or unexpected fiber movement. However, the change of h(t) with time is much slower (large time scale) in speed compared with that of the repetition rate of the pulse source, we can average the effective optical injection power in a proper time scale (much smaller than the polarization variation time while much larger than pulse repetition period), which gives P eff ¼ ðcos2 hðtÞ þ sin2 hðtÞÞP 0 =2 ¼ P 0 =2, where P0 is the average power of the spectrum-sliced pulse source and we have assumed the two tributaries have the same average optical power. This shows that the effective power is maintained at a constant level for any values of angle h(t). In [11], the wavelength separation between the incoming data and the clock recovery laser is around 4 nm, so the optical injection is incoherent and only power is involved in the interaction between the optical fields of the incoming data signal and modes inside the FP laser. However, in this investigation, the spectrum-sliced pulse source will be used to injection-lock one of the cavity modes of the FP laser, so it is coherent injection, and optical phase together with power will be involved in the interaction between the optical fields of the injected pulses and the FP-LD. Fortunately, the interaction is still dominated by the effective optical power, leading to relative polarization insensitivity, and the phase contribution will introduce slight polarization dependence, which has been verified by the following experiments.
FP-LD 2.5 Gb/s
x
PPG
CSA RFSA BERT
Fig. 1. Experimental setup for upstream WDM-PON transmission with proposed scheme.
y
t
Fig. 2. Polarization states of the polarization orthogonal optical time division multiplexed pulse source.
Y.J. Wen, C.-J. Chae / Optics Communications 260 (2006) 691–695
3. Results and discussion
693
The FP-LD used in our investigation has a threshold current of 18 mA and was biased at 26 mA in the experiment to get enough modulation bandwidth. Fig. 3 shows the optical spectra of the spectrum-sliced pulses and the FP-LD outputs before and after injection-locking measured at point A by inserting an additional circulator. The spectrum-sliced pulse source had multiple modes with a mode spacing of 6 GHz, due to the broad bandwidth of the AWG (Fig. 3(a)). Without optical injection, the FPLD exhibited multiple mode oscillation, with the width of each cavity mode being broadened due to the effect of direct data modulation, as shown by the dashed curve of Fig. 3(b). Since the semiconductor laser has a locking range of a couple of GHz under external injection condition, when one of the cavity modes is within the optical spectral range of the incoming light, the cavity mode can be locked to one of its incoming light modes, with other cavity modes being greatly suppressed. The solid curve in Fig. 3(b) shows the optical spectrum of the FP-LD externally injected with 0 dBm injection power, which shows a side mode suppression ratio of greater than 40 dB. If we consider all the loss the components involved, including 1 dB circulator loss, 3 dB feeder fiber loss, 5 dB AWG insertion loss, and 4 dB spectrum-slicing loss (for 0.3 nm bandwidth and 100 GHz channel spacing), the total loss will be around 13 dB. For 16 channels, the required PIBPS output power would be
BER (log)
-5
-6 -7 -8 -9 -10 -28
-27.5 -27 -26.5 -26 -25.5 -25 -24.5 -24 Received Optical Power (dBm)
Fig. 4. BERs vs. received optical power for different polarization states of the injected pulses.
25 dBm for 0 dBm injection power, and for 40 channels, the required power would be 29 dBm, which is achievable using high power EDFA. The performance of the upstream transmitter was evaluated by measuring bit error rates (BERs) for the transmitted signal with the same conditions of 26 mA of bias current for the FP-LD and 0 dBm of injection power. In order to confirm the polarization insensitivity, we inserted an additional polarization controller between the PBC and the optical circulator. The polarization state of the injected light was randomly adjusted. Fig. 4 shows the mea-
10 6 GHz
0
Intensity (dBm)
-10 -20 -30 -40 -50 -60 -70 1548.0 1548.5 1549.0 1549.5 1550.0 1550.5 a Wavelength (nm) 10 0
Before Injection After Injection
Intensity (dBm)
-10 -20 -30 -40 -50 -60 -70 1544 b
1546
1548 1550 1552 Wavelength (nm)
1554
Fig. 3. Optical spectra of (a) spectrum-sliced pulse source and (b) FP-LD outputs before and after external injection.
Fig. 5. RF spectrum and data eye pattern at 109 BER after uplink transmission.
Y.J. Wen, C.-J. Chae / Optics Communications 260 (2006) 691–695
sured BER curves of the upstream transmitted data for different polarization states of injected pulses into the FP-LD. For all the random polarizations evaluated, BER at 109 could be achieved without any sign of BER floor. Slightly different receiver sensitivities were observed among the four evaluated cases, which we believe might be attributed to the involvement of the optical phase of the injected light in the injection process, as we discussed in Section 2. As the received optical power is larger than 24 dBm, error-free operation was observed for all polarization states, which verifies the polarization insensitivity of our proposed upstream transmission scheme. Fig. 5 shows the eye diagram and RF spectrum at 109 BER. The RF spectrum shows that the component at 6 GHz arising from the injected pulses is much lower in intensity than that of the data spectrum, which proves that it has no effect on the performance of upstream transmitter. 4. Impact of Rayleigh backscattering The system configuration in Fig. 1 is bidirectional, where the seeding pulse source is first downstream transmitted, and then the modulated data signal is upstream transmitted. Due to Rayleigh backscattering, part of the downstream pulse signal is reflected back and mixed with upstream data signal. The back reflected pulse has the same wavelength with that of the uplink data signal which cannot be separated by using an optical filter, and contributes to in-band crosstalk. This section will investigate the impact of Rayleigh backscattering on the uplink transmission performance. Different from the configuration in Fig. 1, here we use two separate fibers for downlink and uplink transmission, as shown in Fig. 6. Again we inserted an additional polarization controller just before the optical circulator in Fig. 6, and the polarization state of the injected light was randomly adjusted. Fig. 7 shows the measured BER curves of the upstream transmitted data for different polarization states of injected pulses into the FP-LD. For all the random polarizations evaluated, BER at 109 could be achieved without any sign of BER floor. Slightly different receiver sensitivities were also observed among the evaluated four cases. Compared with Fig. 4, all the BER curves under different polarization states in Fig. 7 exhibit
-5
-6
BER (log)
694
-7 -8 -9 -10 -28.5 -28 -27.5 -27 -26.5 -26 -25.5 -25 -24.5
Received Optical Power (dBm) Fig. 7. BER curves with different polarization states of injected pulses using two fibers.
around 1 dB receiver sensitivity improvement. Since all the other operation conditions are the same for the two cases, we believe the sensitivity improvement was attributed to the removal of Rayleigh backscattering component. If the feeder fiber length is increased to 20 km or longer, the Rayleigh backscattered noise will be increased [13]. 5. Conclusions We proposed a new WDM-PON upstream transmission scheme based on polarization-insensitive injection-locking technique and the use of low-cost FP-LDs. 2.5 Gb/s upstream transmission was experimentally demonstrated using a FP-LD externally injected by spectrum-sliced broadband supercontinuum pulses. The polarization insensitivity was achieved by polarization-orthogonal multiplexing of broadband pulse source. Impact of Rayleigh backscattering on the uplink transmission performance is also investigated. The proposed scheme is expected to be cost-effective from the fact that the polarization-insensitive broadband pulse source located at the central office can be shared among many WDM-PONs and low-cost FP-LDs are used as light sources for 2.5 Gb/s upstream transmission. It should be noted that although we only demonstrated the scheme for upstream transmission, the externally injected FP-LDs can be also utilized for downstream transmission. Acknowledgments
SCF 50/50 PC1
PBC
DFB LD EDFA Synthesizer 6 GHz PIBPS
Splitter
τ Delay Line
SMF 10 km AWG1
DCF
PC2
SMF 10 km
FP-LD 2.5 Gb/s
The authors are grateful to Dr. Yasuhiro Matsui for providing the FP-LD and Dr. Hai-Feng Liu for his support in this investigation. References
PPG AWG2 Rx CSA RFSA BERT
Fig. 6. Experimental setup for uplink transmission using two separate fibers.
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