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The performance of a cost-effective optical comb source using commercial off the shelf (COTS) components in a WDM passive optical network is demonstrated. Eight comb modes are individually modulated at 10.7Gb/s and transmitted over 50km of single mode fiber for downlink transmission. Error free performance is obtained for each comb line and a maximum performance difference of 1.4dB is experienced between the eight channels. Colorless operation of the optical network unit is achieved by utilizing an integrated module consisting of a tunable laser and an electro-absorption modulator as an uplink transmitter. Finally the predicted downstream performance of the system, when all the channels are transmitted simultaneously, is numerically simulated.
©2010 Optical Society of America
1. Introduction
Wavelength division multiplexed passive optical networks (WDM-PON) have been extensively investigated in recent years as such systems promise extremely high dedicated bandwidth to the home, protocol agnosticism and network simplification. However one of the key drawbacks associated with a true WDM-PON is the extremely high cost of the wavelength dependant transmitter and receiver subsystems in both the optical line termination (OLT) and the optical network units (ONU). The extensive capital expenditure required for such subsystems, combined with the expense of wavelength selective optical components has significantly reduced the competiveness of WDM-PONs [1]. Therefore novel and innovative cost efficient solutions are a fundamental requirement for widespread market adoption of WDM passive optical networks.
The cost of a WDM based passive optical network can be substantially reduced by employing novel OLT transmitters and colorless ONUs. In a true WDM-PON, where each user is assigned a specific wavelength, the OLT typically comprises of a bank of fixed wavelength laser sources (coupled with wavelength lockers) or a bank of externally injected Fabry-Perot (FP) laser diodes. Both temperature stabilization and large inventory of such devices pose as a major stumbling block for WDM-PONs [2]. A number of alternative techniques have been implemented to overcome the wavelength dependence and stability of both the OLT and ONU transmitters. Such schemes include automatic injection locking of FP lasers using spectrally sliced seed light for both downstream and upstream transmission or reflective modulator arrays coupled with centralized comb sources [3,4].
Alternatively optical comb generation is a promising technique to generate a number of frequency tones from a single device, which can be subsequently individually modulated for independent transmission from the OLT in a passive WDM network. The primary advantage of this technique is the inherent reduction of laser transmitters in the OLT and also the reduced cost of wavelength stabilization, as only one temperature control mechanism is required for the stabilization of multiple (8-10) channels. There have been many techniques proposed to generate frequency combs including fiber lasers, fiber non-linearity, amplitude modulation or hybrid amplitude-phase modulation [5]. Although these methods have the capacity to generate a stable frequency comb, they all require complex arrangements or expensive optical components. Recently Akrout et al. reported error free transmission of eight 10Gb/s channels spaced by 100GHz, which were generated from a mode-locked laser [6]. A similar mode locked quantum dash device has also been reported by Nquyen et al. [7] which demonstrated very efficient comb generation (42.7GHz channel spacing) for use as seed sources for both the upstream and downstream transmission. Although efficient comb generation was achieved from a single source, the laser device required was a sophisticated quantum-dash Fabry-Perot structure and such laser devices are currently not commercially available.
Conversely, in this paper we employ a simple frequency comb source that is based on a commercial off the shelf (COTS) gain-switched discrete mode laser [8], in a 10.7Gb/s WDM passive optical access network. The comb source generates eight tones within a power variation of 1.3dB, which are individually filtered and modulated prior to transmission over 50km of standard single mode fiber (SSMF). Error free performance is obtained for each of the eight modulated comb lines with a maximum variation of 1.4dB in receiver sensitivity between the channels. Distribution of the data signals is achieved by utilizing arrayed waveguide gratings (AWG) in the remote distribution node (RN) and colorless ONU operation is maintained by employing wavelength tunable electro-modulated sources for upstream transmission. Finally the predicted performance of the gain-switched comb source, when all the channels are transmitted simultaneously, is numerically simulated using the Virtual Photonics Incorporated software package (VPI TransmissionMaker 8.0).
2. Cost-effective optical comb source using COTS components
The optical comb source was previously reported in [8] and is based on gain-switching a COTS discrete mode (DM) laser diode. Gain-switching was achieved by amplifying a 10.7GHz sinusoidal signal from a radio frequency (RF) synthesizer (RF power: 20dBm) and applying it directly to the DM laser in conjunction with a bias current (60.43mA), as depicted Fig. 1 . The DM laser was a ridge waveguide FP laser diode constrained to lase within a single mode of the FP cavity and its operation is fully explained in [9]. It was packaged in an optically isolated temperature controlled butterfly package with a room temperature bandwidth of approximately 10GHz and a threshold current of 10mA. The continuous wave (CW) central wavelength was 1540nm at 20°C and a bias current of 60.43mA. The modulated spectrum of the DM laser is illustrated in Fig. 1(a) and was recorded using a high resolution (20MHz) optical spectrum analyzer (OSA). The modulated laser demonstrates efficient sideband generation in the lasing mode, with eight clearly resolved 10.7GHz coherent tones generated within 1.3dB of the spectral envelope peak.
Fig. 1 Experimental setup of cost-effective optical comb source, (a) optical comb at output of laser diode and (b) filtered optical comb mode with a 32dB rejection ratio.
The cost-efficiency of this OLT transmitter arises from the simplistic and inexpensive DM laser source. The ridge waveguide FP device achieves single wavelength operation through the introduction of index perturbations in the form of etched features positioned at a small number of sites distributed along the ridge of the laser cavity. These ridge and index perturbations are realized using surface processing techniques, which do not involve epitaxial regrowth and only requires standard optical lithography. DM lasers are subsequently cheaper to manufacture than the more commonly used distributed feedback laser [9].
In order to exploit the commercially available DM laser based comb source in a WDM-PON scenario it is imperative to filter each of the comb tones prior to impressing the signal modulation. This task would normally be performed by an AWG, that exhibits a free spectral range (FSR) equal to the modulation frequency (10.7GHz), employed directly after the laser. Such a component has been experimentally demonstrated [10] but was not commercially available at the time of this work. Therefore a single comb tone was filtered using a tunable fiber Bragg grating (FBG) that had a 3dB optical bandwidth of 10.4GHz. A rejection ratio of 25dB was the minimum achieved for all eight comb lines after optical filtering. One of the filtered modes, with a rejection ratio of 32dB is displayed in Fig. 1(b). Each of the eight filtered comb tones are superimposed in Fig. 2(a) , with the optical rejection ratio ranging from 25 to 32dB. The linewidth of the individual modes were also measured using the delayed self-heterodyne technique [11]. Each tone had a full width at half the maximum (FWHM) linewidth of approximately 5MHz and a single tone is illustrated in Fig. 2(b). This value compares favorably to more expensive commercially available distributed feedback lasers (as presented in [7]) and is another primary advantage of this cost-effective optical comb source, especially when utilizing phase sensitive modulation formats such as differential phase shift keying (DPSK). Such systems are tolerant to a modest variation in optical phase as the symbols are separated by a π phase shift. However it is still imperative to maintain a low linewidth to stay within this margin and this requirement becomes more critical when considering higher order modulation formats such as differential quadrature PSK (DQPSK).
Fig. 2 (a) Eight filtered comb lines and (b) measured self heterodyne spectrum of a single filtered comb line with an inferred FWHM linewidth of 5MHz.
3. WDM-PON test-bed
The experimental test-bed used to demonstrate the operation of our comb source in a WDM-PON system is shown in Fig. 3 . The optical comb source is identical to that explained in section two. The filtered comb line (λ1) was passed through a Mach-Zehnder modulator (MZM), biased at its null point, thus generating a DPSK signal. A 10.7Gb/s amplified pseudorandom bit sequence of length 231-1 was applied to the modulator and the data amplitude was adjusted to 2Vπ to ensure optimum phase modulation. The DPSK signal was then passed through a Gemfire AWG that had a 100GHz free spectral range with a wideband filter profile and a 5dB insertion loss. The inclusion of 100GHz spaced AWGs provided a more realistic system architecture and power budget. However as previously stated, in a practical PON these passive components would be replaced by AWGs that exhibit a FSR of 10.7GHz. The signal was subsequently amplified using an erbium doped fiber amplifier (EDFA) that exhibited a noise figure (NF) of 5dB, saturation power (Psat) of 17dBm and a gain of 32dB. This provided a launch power of 3dBm, measured after the circulator. The phase modulated signal traversed a 50km span of SSMF which exhibited a dispersion parameter of 17.5ps/nm.km and a dispersion slope of 0.06ps/km/nm2.
The remote node consisted of a second AWG which passed the filtered signal through a red-blue optical diplexer, which was used to passively separate the downstream and upstream channels into different spectral windows. The output of the filter was passed into the ONU, where an optical circulator allowed the downstream DPSK signal to be demodulated using a MZ delay interferometer through port 3, before being detected with an EDFA pre-amplified receiver. The pre-amplified receiver consisted of a low saturation power (10dBm) EDFA with high gain (~40dB) and a noise figure of 3.5dB, followed by a 2nm band pass filter and a booster EDFA (NF: 4.5dB, Psat: 17dBm, gain: 30dB) in conjunction with a second 2nm filter. The signal was detected (single ended) using a pin photodiode. BER analysis was carried out using a 12.5Gb/s error detector and the received eyes were recorded with a high speed digital sampling oscilloscope. It is important to note that the EDFA pre-amplified receiver was employed specifically to demonstrate the BER performance of the downstream transmitter. However, in a practical system the ONU receiver would typically consist of an avalanche photodiode (APD) based receiver.
The upstream transmitter consisted of a sampled grating distributed Bragg reflector (SG DBR) tunable laser (TL) and an electro-absorption modulator (EAM). The TL was switched to a set-and-forget channel with a wavelength of 1551.36 nm (λ9) and an output power of 8dBm. The upstream channel therefore operated in a separate spectral window to that of the downstream data traffic, thus negating the effects of coherent Raleigh backscattering [12]. A 2.625Gb/s PRBS of length 231-1 was electrically amplified and applied directly to the EAM, thus generating an on-off keyed (OOK) data signal. The upstream channel was passively multiplexed using the red-blue filter and the AWG, providing a launch power of −13dBm. After traversing the 50km span of SSMF the upstream channel was fed through a third AWG before detection. BER analysis was also carried out on the single ONU upstream channel using an EDFA pre-amplified receiver and an error detector.
4. Results and discussion
4.1. 10.7Gb/s DPSK downstream
Each of the eight filtered comb lines were implemented in the bidirectional transmission system and BER analysis of each channel was carried out. Figure 4 illustrates the BER performance for each of the generated comb lines for both a back-to-back scenario and also after propagating over 50km of SSMF. Channel 1 refers to the left most comb line (lowest wavelength), as displayed in Fig. 2(a) with channel 8 therefore being the right most comb line (highest wavelength). An error rate of 10−9 was achieved at a received power of −39.5dBm for the first frequency tone (Ch.1) for the back-to-back scenario [Fig. 4(a)]. This received sensitivity could have been improved by 3dB if a balanced detector was used. After propagating over 50km of SSMF, the filtered comb mode incurred a power penalty of 3.2dB, mainly due to the effect of fiber dispersion. The clear eye opening, inset of Fig. 4(a), illustrates the excellent performance of the comb source in a passive optical network, without inline amplification or dispersion compensation.
Fig. 4 (a) BER as a function of received power for channels 1 and 2 for both the back-to-back scenario and after 50km transmission, (b) channels 3 and 4, (c) channels 5 and 6, (d) channels 7 and 8.
Consistent system performance is achieved for each of the eight tones across the comb after propagating over the 50km span of SSMF. A small variation in system performance of 1.4dB is experienced across the entire 8-channel comb spectrum, which is comfortably within an acceptable power budget of the passive network. This variation in received power can be attributed to the non-ideal uniformity of the mode comb envelope and also the slight variation in the rejection ratio of each channel, which was determined by the stability of the FBG. The excellent transmission performance of this COTS gain-switched DM laser comb source further illustrates its applicability to future WDM-PON systems. However, in this WDM-PON scenario, only one filtered mode could be tested at any given time, resulting primarily from the absence of a periodic filter with a 10.7GHz FSR. If all eight channels were modulated individually with 10.7Gb/s data and simultaneously multiplexed onto a single fiber, then it could be expected that there would be coherent crosstalk interference between the optical carriers and sidebands from adjacent channels.
However a major advantage of this comb source is the coherence of the generated frequency carriers. It has been demonstrated that the inter-channel crosstalk between the sub-carriers and the modulation sidebands depends on the phases of the sub-carriers. Therefore by controlling the phase of each sub-channel to achieve orthogonality (by incrementing the phase by π/2 for each sub-channel with respect to its neighbor), the signal-crosstalk beating at the decision point can be eliminated [13]. This has been verified by analyzing the predicted performance of the gain-switched pulse source via numerical simulations carried out using VPI TransmissionMaker 8.0 as seen in section 5.
4.2. 2.625Gb/s OOK Upstream
A key aspect of WDM-PONs is to reduce the amount of inventory and sparing of wavelength specific line cards for both the OLT and the ONUs. The simplest scheme which alleviates forecasting of optical components for network vendors is to maintain colorless operation. This is extremely important for the ONU considering the number of units that would be in operation in a practical system. Therefore the ONU in our proposed network architecture (Fig. 3) consists of a widely tunable laser and an electro-absorption modulator. In this experiment an SG DBR tunable laser was used to generate a set-and-forget CW light source at a wavelength residing in a separate spectral window to that of the downstream traffic. Figure 5 illustrates the BER performance of the uplink channel for both the back-to-back scenario and after propagating through the entire WDM-PON. An error rate of 10−9 was recorded at a received power of −40.61dBm for the back-to-back case and a small power penalty of 0.9dB was experienced after propagating over 50km of SSMF.
Fig. 5 BER as a function of received power for the 2.625Gb/s upstream channel for both the back-to-back scenario and after traversing a 50km span of SMF.
To further reduce capital expenditure, a cost-effective solution for the tunable ONU transmitter could be implemented. This upstream transmitter could consist of a TL device based on similar discrete mode technology [14], which is integrated with an external modulator or indeed a more cost efficient direct modulation scheme [15].
5. Numerical Simulation: DPSK Downstream Transmission
As discussed in section 4, when the data rate of each WDM channel is equal to the channel spacing, coherent crosstalk interference occurs as a result of beat signals arising from the spectral overlap of adjacent channels. This is a noise like process if the relative phase difference between the adjacent channels is random, which is the case when independent lasers are used for each channel. However, as mentioned earlier, one of the key advantages associated with our gain-switched comb source is the coherence of the generated comb lines. Therefore the relative optical phase difference between successive sub-channels can be easily controlled, which can be subsequently utilized to generate a deterministic interference signal, thus increasing the opening of the received eye [16]. This technique is commonly referred to as coherent WDM (CoWDM) and can be used to achieve high information spectral densities.
The predicted performance of the gain-switched comb source, when seven channels are transmitted simultaneously, was numerically simulated using the VPI TransmissionMaker 8.0. The simulation model is depicted in Fig. 6 . The gain-switched source was realized by applying an amplified 10.7GHz sinusoidal signal to a distributed feedback laser (DFB), whose output was passed through an optical filter (70GHz) to reject unwanted side modes. The filtered comb was subsequently passed through a set of cascaded dis-inteleavers based on asymmetric Mach-Zehnder interferometers (AMZI) with a free spectral range (FSR) of 21.4GHz. The dis-inteleavers separated the seven sub-carrier optical comb into four odd and three even sub-channels. Figures 7(a) –7(c) illustrate the generated comb, the four odd (−3, −1, 1 and 3) and the three even (−2, 0 and 2) sub-carriers respectfully. It is shown that the odd and even channels exhibit an extinction ratio of about 60dB. Both the odd and even channels passed through separate phase shifters, DPSK modulators and polarization controllers before being passively coupled with the aid of a power combiner. Two decorrelated 10.7Gb/s PRBS electrical signals were applied to the DPSK modulators, which resulted in a 74.9Gb/s (7x10.7Gb/s) DPSK CoWDM signal. The relative phase between the odd and even sub-carriers was controlled using the phase shifters and by setting this difference to π/2 the signal crosstalk beating at the decision point was eliminated [13].
Fig. 6 VPI TransmissionMaker simulation model of 7-channel CoWDM system using a gain-switched comb source.
Fig. 7 (a) Simulated 10.7GHz gain-switched comb, (b) filtered odd channels (−3, −1, 1, 3) and (c) filtered even channels (−2, 0, 2).
At the receiver side a combination of an AMZI with an FSR of 21.4GHz and a 40GHz optical bandpass filter was used to select out a single sub-carrier. A variable optical attenuator (VOA) was used to vary the input power falling on the optically pre-amplified receiver. The receiver consisted of a high gain (40dB), low saturation power (5dBm) EDFA (NF: 4dB), a 2nm OBPF and a 10.7 DPDK demodulator. The demodulated signal was detected using a balanced photodiode and amplified using a limiting differential amplifier. The BER and received eyes of the detected signal were subsequently numerically calculated. Typical eye diagrams before and after the balanced detector for one of the demodulated sub-carriers (3) is shown in Fig. 6 (left).
The performance of the system was evaluated by numerically simulating the BER as a function of received power when the phases were optimized and is shown in Fig. 8 . The received powers at a log BER of −9 for the odd (−3, −1, 1, 3) CoWDM sub-carriers were −39.5, −34.5, −37.8 and −39dBm respectfully and for the even (−2, 0, 2) CoWDM sub-carriers was −35.8, −36.7 and −36.5dBm respectively. The best performance was achieved by the extreme odd channels (−3 and 3) and this can be attributed to the ideal optical filtering at the transmitter which completely rejected the unwanted side tones. Therefore only one adjacent channel spectrally overlapped with each extreme sub-carrier, thus the extent of the interference is reduced. The power penalty between the seven channels could be largely attributed to the uneven comb mode spectrum, which exhibited a power variation of approximately 3dB. This could be further improved by optimizing the comb flatness. The BER performance of a single filtered tone line is also shown in Fig. 8. The filtered comb line achieved a log BER of −9 at a received power of −40.6dBm. Therefore the power penalties relative to this back-to-back scenario, experienced when all seven channels were transmitted simultaneously, varied from 1.1 to 6.1dB.
Fig. 8 Numerically simulated BER against received power for the 74.9Gb/s CoWDM system when the relative phase was optimized.
The performance of the gain-switched comb source in this CoWDM simulation demonstrates that the cost-efficient discrete mode laser based comb source described in the preceding sections could be successfully employed in a high information spectral density WDM transmission system. As depicted in Fig. 7(a), if a comb with a greater number of tones is achieved experimentally a greater CoWDM capacity could be employed, making the gain-switched comb source an ideal candidate for high-speed multi-carrier transmission systems. This would also further decrease the required number of discrete laser sources at the optical line termination. Although the implementation of coherent WDM would add cost to the proposed system, it provides a higher information spectral density. Consequently there exists an inevitable trade-off between the information spectral density of the system and the cost of the system.
6. Conclusion
The authors have implemented a coherent optical frequency comb source using COTS components in a bidirectional WDM passive optical network. The comb source consisted of a gain-switched discrete mode laser and generated eight comb lines within 1.3dB of the spectral envelope peak. The comb source was implemented in the OLT and the performance of each frequency tone was investigated in a passive WDM access system. Each mode was modulated with 10.7Gb/s data and the DPSK modulation format was used. Error free performance was achieved for each of the comb modes with a small deviation in received power of 1.4dB over the eight sub-channels. Colorless operation of the ONU was preserved by using a widely tunable laser and an electro-absorption modulator. The excellent performance of this comb source further illustrates its potential for future cost-effective WDM-PON based access networks. However to enhance the cost efficiency of the customer premises equipment, more cost-effective tunable lasers or directly modulated tunable sources must be considered. Finally the performance of a gain-switched comb source, when all the sub-carriers were simultaneously transmitted, was predicted using the VPI TransmissionMaker software package. By optimizing the relative phase difference between the coherent comb modes, the interferometric beat noise at the decision point was eliminated. Error free performance was achieved for a seven channel system. This simulation indicates that the discrete mode laser based comb source would perform adequately in a high information spectral density transmission system.
Acknowledgement
The authors would like to thank Dr. Selwan Ibrahim for his intuitive discussions on coherent WDM. This work is supported in part by the Enterprise Ireland Commercialization Fund Technology Development Phase (EI CFTD/2008/324), Science Foundation Ireland (SFI) principal investigator grant and also by the Higher Education Authority Program for Research in Third Level Institutions (2007-2011) via the INSPIRE programme.
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