Scholarly article on topic 'Channel-reuse bidirectional transmission at 10Gb/s/λ in long-reach DWDM-PON employing self wavelength managed tunable laser'

Channel-reuse bidirectional transmission at 10Gb/s/λ in long-reach DWDM-PON employing self wavelength managed tunable laser Academic research paper on "Electrical engineering, electronic engineering, information engineering"

CC BY-NC-ND
0
0
Share paper
Keywords
{WDM-PON / Channel-reuse / "Tunable laser" / "Automatic wavelength control"}

Abstract of research paper on Electrical engineering, electronic engineering, information engineering, author of scientific article — Zhiguo Zhang, Jiahe Wang, Xu Jiang, Xue Chen, Liqian Wang

Abstract We experimentally demonstrate a channel-reuse, bidirectional, 10Gb/s/λ, long-reach dense wavelength-division-multiplexing passive optical network (DWDM-PON) and an optical beat noise-based automatic wavelength control method for a tunable laser used in a colorless optical network unit (ONU). A 42km reach, channel-reuse, full-duplex, 10Gb/s transmission on a 50GHz DWDM grid is achieved. Transmission performance is also measured with different optical-signal-to-Rayleigh-backscattering-noise ratios (OSRBNRs) and different central wavelength shifts (WSs) between upstream signal and downstream signal in the channel-reuse system.

Academic research paper on topic "Channel-reuse bidirectional transmission at 10Gb/s/λ in long-reach DWDM-PON employing self wavelength managed tunable laser"

Contents lists available at ScienceDirect

Optics Communications

journal homepage: www.elsevier.com/locate/optcom

Channel-reuse bidirectional transmission at 10 Gb/s¡A in long-reach ^ DWDM-PON employing self wavelength managed tunable laser

Zhiguo Zhang *, Jiahe Wang, Xu Jiang, Xue Chen, Liqian Wang

State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), Beijing 100876, China

CrossMark

ARTICLE INFO ABSTRACT

Article history: We experimentally demonstrate a channel-reuse, bidirectional, 10 Gb/s/A, long-reach dense wavelength-

Received 28 September 2014 division-multiplexing passive optical network (DWDM-PON) and an optical beat noise-based automatic

Received in re^ed fom wavelength control method for a tunable laser used in a colorless optical network unit (ONU). A 42 km

7 January 2015 reach, channel-reuse, full-duplex, 10 Gb/s transmission on a 50 GHz DWDM grid is achieved. Transmis-

Avaelabte onHn^SJnuary 2015 sion performance is also measured with different optical-signal-to-Rayleigh-backscattering-noise ratios

(OSRBNRs) and different central wavelength shifts (WSs) between upstream signal and downstream

Keywords: signal in the channel-reuse system.

WDM-pON & 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND

Tunableteer6 license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Automatic wavelength control

1. Introduction

The wavelength-division-multiplexing passive optical network (WDM-PON) has been regarded as a promising solution for next-generation optical access networks requiring high security, easy maintenance, great flexibility, and broad bandwidth [1-4]. Driven by ever-increasing user demands for broad-band services to support high-quality internet protocol television (IPTV), e-learning, interactive games, and future looking peer-to-peer multimedia services, it is expected that the data-rate demand will continuously grow, and that numerous access nodes will be deployed over the next few decades. Owing to the continuous growth of bandwidth-hungry new services, the WDM-PON access networks will migrate to systems with 100 Gb capacities in the near future [5-7]. In addition, developments for supporting longer reach and larger split are also expected [8,9]. As 10 Gb/s per channel will be one of the typical channel rates of WDM-PONs in the near future [10-12], the main challenge when increasing the total system capacity lies in improving the spectral efficiency. Narrow channel spacing and channel-reuse techniques are promising methods for increasing the total system capacity of such WDM-PONs.

One important issue for WDM-PONs is achieving low noise, as well as cost-effective colorless optical sources in the optical network unit (ONU). Various colorless optical sources have been

* Corresponding author. E-mail address: zhangzhiguo@bupt.edu.cn (Z. Zhang).

proposed, including the reflective semiconductor optical amplifier (RSOA) [13,14], the semiconductor optical amplifier-reflective electro-absorption modulator (SOA-REAM) [15], and the tunable laser. The tunable laser (or tunable optical transmitter) is an attractive candidate for a colorless optical source for channel-reuse, long-reach, and high-speed transmission as the wavelength of the upstream optical carrier can be set flexibly in the ONU; this can reduce the Rayleigh backscattering noise existing in loopback mode-based wavelength-reuse system [16,17]. However, an automatic wavelength control method is required by the tunable laser-based colorless optical source to realize true colorless operation with plug-and-play features. Using the Rayleigh backscattering effect is one means of solving this problem [18]. However, the control accuracy will degrade as the length of the drop fiber increases. Accordingly, these solutions either cannot effectively provide the initial wavelength setting, or they increase the system cost.

In this paper, we propose a dense WDM-PON (DWDM-PON) scheme. This scheme is tunable laser-based, capable of channel reuse on 50 GHz DWDM grid, and bidirectional transmission at 10 Gb/s/A. It also exhibits a long reach. Moreover, we propose an optical beat noise-based automatic wavelength control method using a downlink optical signal to manage the wavelength of the tunable laser diode (TLD). A channel-reuse, full-duplex, bidirectional, 10 Gb/s transmission on a 50 GHz DWDM grid is demonstrated using a Mach-Zehnder (MZ) modulator and a direct detection (DD) receiver with a 42 km reach. The transmission performance is also measured with different optical-signal-to-Rayleigh-backscattering-noise ratios

http://dx.doi.org/10.1016/j.optcom.2015.01.027

0030-4018/© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

(OSRBNRs) and different central wavelength shifts (WSs) between upstream signal and downstream signal in the channel-reuse system.

2. System architecture

Fig. 1 shows the proposed tunable laser-based, channel-reuse, bidirectional, 10 Gb/s/A, long-reach DWDM-PON scheme. The optical line terminal (OLT) consists of n optical transceivers constituted by n on-off keying (OOK) intensity modulation (IM) transmitters and DD receivers, n three-port optical circulators (i.e., OCT1, ..., OCTn), and one n x 1 array waveguide grating (AWG). In the remote node (RN), one n x 1 AWG is used to couple n ONUs. In one ONU, the uplink transmitter consists of a TLD and an external optical modulator. The downlink receiver is a DD receiver. An optical beat noise-based automatic wavelength control method using the downlink optical signal to manage the wavelength of the TLD is also designed to initialize the wavelength setting of the TLD. Part of the uplink optical carrier and part of the downlink optical signal are coupled into a photo-detector (PD) using three 2 x 2 optical couplers. At this stage, the optical beat noise between the uplink carrier light and the downlink optical signal is generated at the PD. The beat noise power is measured by a microwave power meter and is sent to a control unit (CU). The wavelength of the uplink optical carrier generated by the TLD is controlled by the CU to match the wavelength of the downlink optical signal.

3. Experiments and results

3.1. Automatic wavelength control method

In order to verify the feasibility of the automatic wavelength control method using the downlink optical signal, we experimentally measured the optical beat noise average powers between the downlink optical signal and the uplink optical carrier generated by a TLD in various central WS cases. Fig. 2 shows the experimental setup. A 10.3125 Gb/s pseudo-random bit sequence (PRBS) OOK optical signal is generated by a 50 GHz DWDM grid tunable optical transmitter (Finisar FTLX4213). The optical signal is then sent to one input port of a 3 dB optical coupler via an AWG, a polarization controller (PC1), and a variable optical attenuator (VOA1). Considering that the polarization state of the downlink optical signal in the WDM-PON system is random and time-varying after transmission over a long length of standard singlemode fiber (SSMF), PC1 is used to generate an optical output with a time-varying polarization state under the artificial control in our experiment. The output of a power-tunable TLD (Souther Photo-nics-TLS150) with a 6-16 dBm output power range is sent to another input port of the 3 dB optical coupler via a PC (i.e., PC2) and a

10 Gb/s data

-1- h.

10 Gb/s DWDM ^

Optical Transmitter

iVÔ^MPÇÏKjLO^USB

►TpdT-»

Microwave Power Meter

J OSA I

Fig. 2. Experimental setup of optical beat noise power test.

VOA (i.e., VOA2). PC2 is used to generate a circularly polarized optical output, which ensures that the optical beat noise is generated in the PD. At this stage, the optical beat noise is generated at a PD (1 GHz optical receiver) and measured by a microwave power meter (Agilent N1911a). The noise power value is sent to a CU, which consists of a computer and control software with a general purpose interface bus (GPIB) interface. The output wavelength of the TLD is controlled by the CU using a universal serial bus (USB) interface to match the wavelength of the downlink optical signal.

In our experiment, the bandwidth of the 10.3125 Gb/s OOK signal generated by the 50 GHz DWDM optical transmitter was about 10 GHz, and the linewidth of the uplink optical carrier generated by the TLD was about 1 MHz. The output optical powers of VOA1 and VOA2 were set at -18 dBm and - 2 dBm, respectively, based on the actual power conditions in the proposed DWDM-PON scheme. The noise power was then measured by the microwave power meter with a frequency setting of 300 MHz. Fig. 3 shows the measured average beat noise powers with different central WSs. The results show that the average noise power was approximately - 36 dBm when the central WS was more than 0.4 nm; this means that the operation wavelengths of the 50 GHz DWDM optical transmitter and the TLD were set at two adjacent 50 GHz DWDM channels. The average noise power was higher than - 33 dBm when the central WS was less than 0.15 nm, and 3 dB higher than the average noise power when the central WS was more than 0.4 nm. Therefore, the CU was able to accurately determine whether the two wavelengths were identical based on the average optical beat noise power measured by the microwave power meter. Moreover, the CU was also able to set or change the central WS in one 50 GHz DWDM channel based on the measured average optical beat noise power. Additionally, considering that it requires less than 2 ms for beat noise power detection and data processing, the tuning period per single adjustment of the TLD was less than 10 ms, the stabilization time of the tunable laser was generally less than 100 ms (for example, 75 ms for the EXFO FLS-2600B tunable laser source; 100 ms for the Souther Photonics tunable laser source), thereby the total time to discover the downlink optical signal wavelength was less than 10 s, even when the number of the DWDM channel was as large as 80.

• Coupler

Feeder Fiber

Fig. 1. Proposed channel-reuse long-reach DWDM-PON architecture.

Fig. 3. Optical beat noise average power versus central wavelength shift (WS).

Considering that the use of the tunable laser with an external modulator and optical beat noise power-based wavelength control device (consisting of a PD, a microwave power meter, a computer, etc.) at the ONU will render this scheme expensive, it is important to simplify the structure and decrease the cost for its practical application. First, a low-cost, tunable directly modulated laser (T-DML) [19] or tunable electro absorption modulated laser (T-EML) [20] can be used to replace the tunable laser and the external modulator; second, the microwave power meter can be replaced by a low-cost microwave power measurement module [21]; and third, the computer can be replaced by a low-cost single-chip microcomputer-based data processing and control module. Furthermore, the T-DML or T-EML, the microwave power measurement module and the single-chip microcomputer-based data processing and control module can be integrated into a circuit board.

We also experimentally verified the feasibility of the optical beat noise-based automatic wavelength control method for a T-EML. In our experiment, the only change from the experimental setup shown in Fig. 2 was that the TLD was replaced by a DWDM grid tunable optical transmitter (Finisar FTLX4213) with an identical optical power output. In this case, the operation wavelength of the uplink optical carrier transmitted by the un-modulated tunable optical transmitter was adjusted channel-by-channel under the control of the computer. The measured result shows that the noise power was about - 36 dBm when the two wavelengths were not in one DWDM channel. It was higher than - 24 dBm when the two wavelengths were identical (i.e., being in one DWDM channel), and about 12 dB higher than the beat noise power when the two wavelengths were not in one DWDM

channel. Therefore, the optical beat noise-based automatic wavelength control method can be used to set the operation channel of the T-EML or the T-DML. Furthermore, the central WS between upstream signal and downstream signal in one 50 GHz DWDM channel can also be accurately set or changed while keeping the T-EML or T-DML un-modulated. This is because the same relationship between optical beat noise power and central WS (as shown in Fig. 3) can be achieved in either a T-EML or T-DML-based WDM-PON.

3.2. Channel-reuse full-duplex 10 Gb/s/A DWDM-PON on a 50 GHz DWDM grid

Fig. 4 shows the experimental setup of the channel-reuse, bidirectional, 10 Gb/s/A DWDM-PON system with a 42 km SSMF and 50 GHz channel spacing. In the downlink direction of channel k, 10.3125 Gb/s downlink OOK signals are generated by a 50 GHz DWDM grid tunable optical transmitter (Finisar FTLX4213) with an operating wavelength between 1528.38 nm and 1560.61 nm and an average output power of 2 dBm. The OOK signal is then sent to a 40 km feeder SSMF via a three-port OC (i.e., OCk) and an 80-channel AWG (with a 0.2 nm bandwidth at - 1 dB), with an overall insertion loss of about 6 dB. After transmission over the 40 km feeder SSMF, the downlink OOK signal is sent to a VOA via another 80-channel AWG, a 2 km branch fiber, an OC (i.e., OCUk), and a 90:10 optical coupler, with overall insertion loss of about 7 dB. The bit-error ratio (BER) is then measured by the error detector. In the uplink direction of channel k, the optical output of a power-tunable TLD (Souther Photonics-TLS150) with a 6-16 dBm output power range is sent to the MZ modulator with 4 dB insertion loss via a PC (i.e., PC1) and an 80:20 optical coupler. A 10.3125 Gb/s OOK optical signal is generated and sent to the 40 km feeder SSMF via an OC (i.e., OCUk), a 2 km branch fiber, and an 80-channel AWG, with an overall insertion loss of about 8 dB. After transmission over the 40 km feeder SSMF, the uplink OOK signal is fed into a VOA via an 80-channel AWG and an OC (i.e., OCk), with an overall insertion loss of 6 dB. At this point, the uplink optical signal in channel k is sent to an error detector. 20% of the TLD output with a circular polarization state after a PC (i.e., PC2) and 10% of the downstream signal are coupled into a PD (1 GHz optical receiver) using a 50:50 optical coupler. The beat noise power is measured by a microwave power meter (Agilent N1911a). After powering on, the operation wavelength of the uplink optical carrier transmitted by the TLD is adjusted channel-by-channel under the control of the CU. Furthermore, the operation wavelength is set at the current channel if the beat noise power of the current channel is 3 dB higher than the beat noise power at the unwanted channel.

For practical applications, the TLD and the MZM in Fig. 4 can be replaced by a T-DML or a T-EML. In this case, the operation wavelength of the uplink optical carrier can be adjusted channel-by-channel under the control of the CU while keeping the T-DML or T-EML un-modulated; the operation wavelength will be set at the

10 Gb/s data [Transmitter

Error Detector

<-[vÔÂ] aP

40 km SSMF

10 Gb/s data OCuk 1

PCTH CTLD1

PC2H20% fan

Microwave Power Meter

I VOA I—>

Error Detector

Fig. 4. Experimental setup for channel-reuse 10 Gb/s/A DWDM-PON with 42 km SSMF transmission.

■30 -29 -28 -27 -26 -25 -24 -23 -22 -21 -20 -19 -18 -17 -30 -29 -28 -27 -26 -25 -24 -23 -22 -21 -20 -19 -18 -17

Received Power (dBm) Received Power (dBm)

Fig. 5. Measured (a) downstream and (b) upstream BERs in channel-40 in back-to-back (B-t-B) after 42 km unidirectional and bidirectional transmissions.

current channel if the beat noise power of the current channel is 3 dB higher than the beat noise power at the unwanted channels. At this stage, the 10.3125 Gb/s OOK signal makes use of the T-EML or T-DML; the T-EML or T-DML is then modulated, and the upstream transmission occurs.

The transmission performances of downstream signal and upstream signal in back-to-back (B-t-B) configuration and after 42 km transmission at channel-40 are shown in Fig. 5(a) and (b). The bidirectional data rates of the non-return-to-zero (NRZ) OOK signal are both 10.3125 Gb/s with a 231-1 PRBS pattern length. In our measurement, the operation wavelength of the TLD was set at 1544.53 nm in the wavelength pass-band of channel-40, which is exactly identical with the operation wavelength of the downlink transmitter in channel-40. The output power of the power-tunable TLD was set at 8 dBm. As a result, the average output power of the MZ modulator was about 2 dBm, thereby matching the average output power of the downlink transmitter in channel-40. The black lines with triangles, circles, and squares in Fig. 5(a) show the measured downstream BERs in B-t-B configuration, after 42 km of unidirectional transmission, and after 42 km of bidirectional simultaneous transmission, respectively. The lines in Fig. 5(b) show the upstream BERs in channel-40 with the same configurations. The lines with triangles and circles in Fig. 5(a) and (b) show that both the downlink and uplink have slight transmission penalties (1 dB) that originate from chromatic dispersion. As shown in Fig. 5 (a) and (b), compared with 42 km unidirectional transmissions, there are power penalties of about 5 dB (at a BER of 1 x 10- 9) in 42 km bidirectional simultaneous transmission. Obviously, the power penalties originate from the Rayleigh backscattering noise as the optical wavelengths in the two directions are exactly identical in one DWDM channel.

In order to check the differences of the transmission performance in different DWDM channels, we measured the BERs separately for 80 DWDM channels in C-band with a 42 km reach. The results are shown in Fig. 6. The results were measured channel-by-channel, keeping -18 dBm of received power unchanged. It can be seen that the BERs (at - 18 dBm of received power) of all the 80 channels are almost completely identical, which indicates the similarity of the transmission performances in all 80 channels. To assess the impact of multichannel operation, a five-channel (the channel number is limited by the number of DWDM optical transmitters we have) bidirectional transmission was implemented and measured based on the experimental setup shown in Fig. 4. In this measurement, the operation wavelengths of the five pairs of transmitters in two directions in channel-38, channel-

Fig. 6. Measured BERs for various channels.

Fig. 7. Measured BERs at channel-40 in single-channel operating mode and in five-channel operating mode. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

39, channel-40, channel-41, and channel-42 were set at 1543.73 nm, 1544.13 nm, 1544.53 nm, 1544.92 nm, and 1545.32 nm, respectively. All of the average output powers of the

■28 -27 -26 -25 -24 -23 -22 -21 -20 -19 -18 0.00 0.02 0.04 0.06 0.08 0.10 0.12

Received Power (dBm) Central Wavelength Shift (nm)

Fig. 8. Impact of crosstalk due to the RB: (a) BERs with different wavelength shift and (b) power penalty versus wavelength shift.

five uplink transmitters and the five downlink transmitters were about 2 dBm. The lines with red circles in Fig. 7 show the measured upstream and downstream BERs in channel-40 in the five-channel operating mode. The lines with black squares in Fig. 7 show the measured upstream and downstream BERs in channel-40 in single-channel operating mode. Compared with the transmission in single-channel operating mode, there is less than 0.5 dB (at a BER of 1 x 10-9) in power penalty in five-channel operating mode. Therefore, the results shown in Fig. 7 indicate that the impact of multichannel operation is slight and can be ignored.

In order to analyze the impact of the Rayleigh backscattering noise, which could worsen OSNR and reduce receiver sensitivity in the channel-reuse system, we measured the BER performances of bidirectional transmission with different central WS values and different OSRBNRs in one DWDM channel. As evidenced in Refs. [22,23], the receiver sensitivity is used to describe the optical signal-to-noise degradation caused by Rayleigh backscattering noise. The experimental setup remains unchanged from that shown in Fig. 4. BER performances with different WS values are shown in Fig. 8(a). The bidirectional power penalties at a BER of 1 x 10-9 for various WSs are shown in Fig. 8(b). In this measurement, the average output powers of the downlink transmitter and uplink MZM in channel-40 both remained at 2 dBm. As a result, both of the OSRBNRs at port 3 of the OCk and at port 3 of the OCUk in channel-40 remained at 21 dB. The uplink and downlink receiver sensitivities were - 26 dBm at a BER of 1 x 10 -4 and - 26.5 dBm at a BER of 1 x 10-4, respectively, when the central WS between optical signals in both directions was 0 nm. Both the uplink and downlink receiver sensitivities rapidly improved when the WS increased. They reached - 28 dBm at a BER of 1 x 10-4 when the central WS was 0.08 nm. The receiver sensitivities did not increase significantly when the central WS was more than 0.08 nm.

Therefore, the impact induced by Rayleigh backscattering noise can be effectively reduced by mismatching the wavelengths of upstream signal and downstream signal in the channel-reuse system: the greater the WS, the smaller the Rayleigh back-scattering noise effect. For the channel-reuse DWDM-PON system proposed in this paper, an intentional central WS can be set to reduce the impact of Rayleigh backscattering noise, based on the measured result between beat noise power and central WS, as shown in Fig. 2. Considering that the bandwidth at - 1 dB of the 50 GHz channel spacing flat top AWG is generally more than 0.2 nm, a central WS of about 0.08 nm is feasible and effective; this

takes into account both the AWG pass-band and the downlink optical signal central wavelength.

The BER performances with different OSRBNRs were also measured to fully assess the impact of Rayleigh backscattering noise. For the measurement in the downlink direction, various OSRBNRs can be generated by a fixed downlink optical signal power and a variable Rayleigh backscattering noise power. The Rayleigh backscattering noise power in the downlink direction can be changed by adjusting the output power of the power-tunable TLD in the uplink direction when the downlink optical signal power at port 3 of the OCUk in channel-40 remains at - 18 dBm. Moreover, the Rayleigh backscattering noise power in the downlink direction can be measured if the downlink optical transmitter is turned off. We measured the BER performances when the OSRBNR was 21 dB, 18 dB, 16 dB, and 13 dB, respectively. The results are shown in Fig. 9(a-d), respectively. As shown in Fig. 9(a), a receiver sensitivity of - 26.5 dBm at a BER of 1 x 10-4 can be achieved when the OSRBNR equals 21 dB and the central WS equals 0 nm; the receiver sensitivity will reach - 28 dBm at a BER of 1 x 10-4 if the central WS increases to 0.08 nm. The receiver sensitivity will drop to - 23 dBm at a BER of 1 x 10-4 when the OSRBNR drops to 18 dB and the central WS equals 0 nm; but it only drops to - 27.5 dBm at a BER of 1 x 10-4 if the central WS is 0.08 nm, as shown in Fig. 9(b). If the OSRBNR drops to 16 dB, the receiver sensitivity will further deteriorate. As shown in Fig. 9(c), - 21 dBm and - 25.5 dBm at a BER of 1 x 10 -4 are achieved when the central WS equals 0 nm and 0.08 nm, respectively. Fig. 9 (d) shows that receiver sensitivity is better than - 18 dB at a BER of 1 x 10-4 only when the central WS is more than 0.02 nm if the OSRBNR drops to 13 dB.

For the measurement in the uplink direction, various OSRBNRs can be generated by a fixed Rayleigh backscattering noise power and a variable uplink optical signal power, where the Rayleigh backscattering noise power in the uplink direction cannot be changed because of the power-fixed (2 dBm output optical power) downlink optical transmitter in the OLT. The uplink optical signal power can be changed by adjusting the output power of the power-tunable TLD in the uplink direction. Moreover, the Rayleigh backscattering noise power in the uplink direction can be measured at port 3 of the OCk if the TLD is turned off. Limited by the uplink optical signal power that can be achieved in the OLT, we only measured the BER performances in the uplink direction with 21 dB, 18 dB and 16 dB OSRBNRs. The results are shown in Fig. 10 (a-c), respectively. As shown in Fig. 10(a), a receiver sensitivity of

Fig. 9. Measured BERs in downlink direction with (a) 21 dB, (b) 18 dB, (c) 16 dB, and (d) 13 dB OSRBNRs.

-26 dBm at a BER of 1 x 10-4 can be achieved when the OSRBNR equals 21 dB and the central WS equals 0 nm; the receiver sensitivity will reach - 28 dBm at a BER of 1 x 10-4 if the central WS increases to 0.08 nm. The receiver sensitivity will drop to - 22 dBm at a BER of 1 x 10-4 when the OSRBNR drops to 18 dB and the central WS equals 0 nm, it drops to - 27 dBm at a BER of 1 x 10-4 if the central WS is 0.08 nm, as shown in Fig. 10(b). If the OSRBNR drops to 16 dB, the receiver sensitivity will further deteriorate. As shown in Fig. 10(c), only - 23 dBm and - 25 dBm at a BER of 1 x 10-4 are achieved when the central WS equals 0.04 nm and 0.08 nm, respectively. Furthermore, the transmission performances are approximately similar between the uplink and downlink directions if the OSRBNRs are equal in the two directions.

Therefore, the receiver sensitivity will be significantly affected by the OSRBNR: the smaller the OSRBNR, the worse the transmission performance. In order to ensure the correct transmission regardless of the central WS, the OSRBNR should be greater than a specific threshold value in one specific transmission system (for example, the OSRBNR in the downlink direction should be greater than 13 dB in this experimental system to ensure the correct transmission with FEC code). To achieve an OSRBNR greater than the threshold value in one specific transmission system, the optical signal powers into the feeder fiber should be as low as possible under the condition that optical power budget in two directions

can be met. Moreover, in order to achieve a large OSRBNR for longer-reach transmissions, the upstream signal can be amplified at the end of the feeder fiber in the OLT, whereas the downstream signal can be amplified at another end of the feeder fiber in RN.

4. Conclusions

We propose and investigate a tunable laser-based, channelreuse, bidirectional, 10 Gb/s/A, long-reach DWDM-PON scheme. An optical beat noise-based automatic wavelength control method is also proposed to manage the wavelength of the tunable laser in a colorless ONU. A channel-reuse, full-duplex, bidirectional 10 Gb/s transmission on a 50 GHz DWDM grid is demonstrated with a 42 km reach. The measurement results show that the impact of the Rayleigh backscattering noise will decrease when the central WS and the OSRBNR increase. Using a 1 x 10- 9 uniform BER standard, the receiver sensitivity improves by 5 dB (maximum) when the central WS increases from 0 nm to 0.08 nm. Regardless of the value of the central WS, correct transmission can be achieved if the OSRBNR is greater than the specific OSRBNR threshold value of the transmission system.

Fig. 10. Measured BERs in the uplink direction with (a) 21 dB, (b) 18 dB, and (c) 16 dB OSRBNRs.

Acknowledgment

This study is supported by National Natural Science Foundation of China (No. 61302079) and Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), PR China.

References

[1] Elaine Wong, "Next-generation broadband access networks and technologies", IEEE J. Ligthwave Technol. 30 (4) (2012) 597-608.

[2] Dirk Breuer, Frank Geilhardt, Ralf Hülsermann, Mario Kind, Christoph Lange, Thomas Monath, Erik Weis, Opportunities for next-generation optical access, IEEE Commun. Mag. 49 (2) (2011) S16-S24.

[3] Frank J. Effenberger, Jun-ichi Kani, "Standardization trends and prospective views on the next generation of broadband optical access systems", iEEE J. Sel. Areas Commun. 28 (6) (2010) 773-77780.

[4] Prince Kamau, Timothy B. Gibbon, Rodes Roberto, et al., "GigaWaM-Next-generation WDM-PON enabling gigabit per-user data bandwidth", J. Ligthwave Technol. 30 (10) (2012) 1444-1454.

[5] Kim Joon-Young, Yoo Sang-Hwa, Moon Sang-Rok, Kim Dong Churl, Lee Chang-Hee, 400 Gb/s (40 x 10 Gb/s) ASE Injection Seeded WDM-PON based on SOA-REAM, in: Proceedings of C2013, OW4D.4, (2013).

[6] Tawade Laxman, "100 Gb/s long-reach WDM-PON implemented by using directly modulated RSOA", Microw. Opt. Tech. Lett. 55 (6) (2013) 1426-1430.

[7] Joonyoung Kim, Sang-Rok Moon, Yoo Sang-Hwa, Lee Chang-Hee, "800 Gb/s (80 x 10 Gb/s) capacity WDM-PON based on ASE injection seeding", Opt. Express 22 (9) (2014) 10359-10365.

[8] Philippe Chanclou, Anna Cui, Frank Geilhardt, Hirotaka Nakamura,

Derek Nesset, "Network operator requirements for the next generation of optical access networks", IEEE Netw. 26 (2) (2012) 8-14.

[9] Lavery Domani, Ionescu Maria, Makovejs Sergejs, Torrengo Enrico, J. Savory Seb, "A long-reach ultra-dense 10 Gbit/s WDM-PON using a digital coherent receiver", Opt. Express 18 (25) (2010) 25855-25860.

[10] Q. Guo, A.V. Tran, C.J. Chae, "10-Gb/s WDM-PON based on low-bandwidth RSOA using partial response equalization", IEEE Photon. Technol. Lett. 23 (20) (2011) 1442-1444.

[11] Ting Su, Min Zhang, Xue Chen, Zhiguo Zhang, Mingtao Liu, Lei Liu, Shanguo Huang, "Improved 10-Gbps uplink transmission in WDM-PON with RSOA-based colorless ONUs and MZI-based equalizers", Opt. Laser Technol. 51 (2013) 90-97.

[12] J. Zhang, J. Yu, F. Li, N. Chi, Z. Dong, X. Li, "11 x 5 x 9.3 Gb/s WDM-CAP-PON based on optical single-side band multi-level multi-band carrier-less amplitude and phase modulation with direct detection", Opt. Express 21 (2013) 18842-18848.

[13] Cho Seung-Hyun, Lee Han Hyub, Lee Jie Hyun, Lee Jong Hyun, Myung Seung II, Lee Sang Soo, "Mitigation of interferometric crosstalk by using a single mode laser with optical feedback in a loop-back WDM-PON based on RSOA", Opt. Fiber Technol. 18 (6) (2012) 523-526.

[14] Zhang Min, Wang Danshi, Cao Zhihui, Chen Xue, Huang Shanguo, "Suppression of pattern dependence in 10 Gbps upstream transmission of WDM-PON with RSOA-based ONUs", Opt. Commun. 308 (2013) 248-252.

[15] Kim Hyun-Soo, Kim Dong Churl, Kim Ki-Soo, Choi Byung-Seok, Kwon O-Kyun, "10.7 Gb/s reflective electroabsorption modulator monolithically integrated with semiconductor optical amplifier for colorless WDM-PON", Opt. Express 18 (22) (2010) 23324-23330.

[16] S. Sekiguchi, K. Takabayashi, A. Hayakawa, S. Tomabechi, A. Uetake, M. Ekawa, and H. Kuwatsuka, 10 Gb/s Wavelength-Tunable EML with Continuous Wavelength Tuning Covering 50 GHz u 8 Channels on ITU Grid, in: Proceedings of OFC' 2011, OMS4, (2011).

[17] Gaurav Pandey, Aditya Goel, "Enhanced colorless and broadcast capable symmetrical 10-Gbps bidirectional transmission in WDM-PON using RSOA and

EAM", Opt.-Int. J. Light Electron Opt. 124 (23) (2013) 6245-6249.

[18] Sang-Rok Moon, Hoon-Keun Lee, Chang-Hee Lee, "Automatic wavelength allocation method using Rayleigh backscattering for a WDM-PON with tunable lasers", J. Opt. Commun. Netw. 5 (3) (2013) 190-197.

[19] Zhengxuan Li, Lilin Yi, Wei Wei, Meihua Bi, Hao He, Shilin Xiao, Weisheng Hu, "Symmetric 40-Gb/s, 100-km Passive Reach TWDM-PON with 53-dB Loss Budget", J. Ligthwave Technol. 32 (21) (2014) 3991-3997.

[20] S. Sekiguchi, K. Takabayashi, A. Hayakawa, S. Tomabechi, A. Uetake, M. Ekawa, and H. Kuwatsuka, 10 Gb/s wavelength-tunable eml with continuous wavelength tuning covering 50 GHz x 8 channels on ITU Grid, in: Proceedings of OFC2007, OMS4, (2007).

[21] De-bo Wang, Xiao-ping Liao, "A terminating-type MEMS microwave power

sensor and its amplification system", J. Micromech. Microeng. 20 (2010) 075021.

[22] Jeongyun Ko, Seongha Kim, Jaehoon Lee, Shinhee Won, Y.S. Kim, Jichai Jeong, "estimation of performance degradation of bidirectional WDM transmission systems due to Rayleigh backscattering and ASE noises using numerical and analytical models", J. Ligthwave Technol. 21 (4) (2007) 938-946.

[23] Shifeng Jiang, Bruno Bristiel, Yves Jaouen, Philippe Gallion, Erwan Pincemin, "Bit-error-rate evaluation of the distributed raman amplified transmission systems in the presence of double Rayleigh backscattering noise", IEEE Photon. Technol. Lett. 19 (7) (2007) 468-470.