A distributing arranged waveguide Bragg gratings (WBGs) in PLC splitter chip based remote coding scheme is proposed and analyzed for passive optical network (PON) monitoring, by which the management system can identify each drop fiber link through the same reflector in the terminal of each optical network unit, even though there exist several equidistant users. The corresponding coding and capacity models are respectively established and investigated so that we can obtain a minimum number of the WBGs needed under the condition of the distributed structure. Signal-to-noise ratio (SNR) model related to the number of equidistant users is also developed to extend the analyses for the overall performance of the system. Simulation results show the proposed scheme is feasible and allow the monitoring of a 64 users PON with SNR range of 7.5~10.6dB. The scheme can solve some of difficulties of construction site at the lower user cost for PON system.
© 2016 Optical Society of America
Optical fiber technology offers the best solution for obtaining higher bandwidth and speed services over longer reaches, supporting various broadband services expected in the coming years. The fiber-to the-x technology (FTTX, X = B for building, H for home and D for desk) is regarded as the best infrastructure candidate for the broadband access. Passive optical networks (PONs) are very promising and cost-effective architecture for fiber access systems in the contemporary world. Broadband PON based FTTX system is suitable to provide triple-play services at a high speed level, which is gradually becoming reality while commercial deployments are implemented around the world [1,2]. Important smart city deployments have also been carried out worldwide recently, especially in China .
The introduction of PON allows the network to transport huge amounts of data and provide communication services that are indispensable to many of our daily social and economic activities. Network reliability is a key issue of deep concern to network operators being eager to deploy high rate and large capacity fiber networks. The service interruption caused by a failure on the physical link may lead to a lot of complaints and operators’ loss of revenue, as the failure is not detected and recovered timely. Therefore, the issue related to monitoring of PON is becoming more and more essential. Monitoring of PONs can significantly omit or shorten the down-time caused by such failures, and so plays a critical role in the deployment of future access networks.
As far as we know, many exiting approaches have been devised for fault detection and localization. Unfortunately, most of them are usually limited by the network capacity or the topology of PON as well as the severe cost issue [4,5]. As an indispensable test and measurement tool, optical time-domain reflectometry (OTDR) with continued performance improvement is widely used in PON monitoring . However, it is not effective for point-to-multipoint (P2MP) network due to the superposed backward signals. Periodic coding scheme using fiber Bragg gratings (FBGs) for encoder embodies the characteristics of non-invasion and simple fabrication [7,8]. However, the poor correlation characteristic increases the difficulty for the network recognition process. The frequency-hopping/period code (FH/PC) scheme improves the correlation characteristic and lowers the multiple-customer interference (MCI) probability while increasing complexity at each terminal . Also, in these existing schemes based on optical code division multiplexing (OCDM), the optical network units (ONUs) are required to configure different encoders to ensure a unique optical code for each terminal , which increases user cost and installation complexity.
In this paper, we propose a remote coding scheme for PON monitoring by using waveguide Bragg gratings (WBGs) in planar lightwave circuit (PLC) chip. That is, various arrays of WBGs with different center-reflecting wavelengths are integrated into respective branches of the PLC splitter chip at the remote node (RN) . The PLC splitter with WBGs based on single silica-on-silicon (SoS) platform can realize power splitting and optical coding simultaneously. For the fabrication, arbitrary Bragg grating devices can be made for any desired central wavelength via direct UV grating writing with electro-optic phase modulation . For a limited size PLC splitter chip, we develop a simple algorithm to guarantee the total number of gratings minimum without impacting on the capacity and unique output. Thus, multiple cascaded gratings written on the branches of different stages can be used to generate the corresponding optical codes. Note that the coding operation is realized at the RN in a centralized way, rather than each terminal located at the ONU. The states of each drop fiber (DF) and the feeder fiber (FF) can be assessed by the features of the reflected detecting signals in the receiver module, which is physically close to optical line terminal (OLT). Meanwhile, it is favorable to just deploy low-cost identical reflector at each ONU, embedded in the fiber adapter can reduce PON capital expenditures (CAPEX) and difficulty of construction site. The integrative chip system locating at the RN allows the CAPEX to be shared by all users. The identical reflector may release pressure of constructors and be more suitable for mass production. Considering all the above benefits, it is reasonable to believe our proposed scheme can be an attractive solution for fault detection in the access network.
2. Principle and models for PON monitoring scheme with remote coding
Figure 1 illustrates the principle of the proposed PON monitoring scheme using the PLC splitter chip with WBGs at the RN. The inset shows a sketch of optical coding signals on the PLC splitter chip usingwavelength chips forONUs. The identical U-band reflector is embedded in the front of each ONU. The tunable source with direct modulation generates the detecting pulse that containswavelength chips. Note that the broadband light source (BLS) with direct modulation can be used to generate detecting pulses,wavelength chips of which are filtered via tandem FBGs. Also, the transmitter module can be replaced bydistributed feedback (DFB) lasers to generate a higher quality of detecting signals, i.e., the relative intensive noise (RIN) can be effectively eliminated. The detecting pulse in U-band is coupled with normal data traffic in C-band via WDM coupler and then encoded by the PLC splitter with WBGs at the RN. Unique optical coding signal is assigned to each DF that can be obtained by different arrays of WBGs written on branches of PLC splitter chip, and then reflected back to the receiver module by the identical U-band reflector at each ONU. The reflected optical coding signals are passed into the receiver module via an optical circulator, and then de-multiplexed towavelength channels by an arrayed waveguide grating (AWG), which is followed byphotoelectric detectors (PDs). After the photoelectric conversion, the reflected coding signals are sent to network recognition module to perform electronic signal identification. The corresponding signals are the unique combination of wavelength chips, features of which can be used to estimate state of the individual DF link. For the monitoring source, the wavelength chips included in detecting pulses are transmitted and received by the transmitter and receiver module, respectively. For example,wavelength chips separated in the time domain are transmitted from the transmitter module. That is, the detecting pulses containingwavelength components can be freely distributed intime slots, shown in Fig. 2(a). In Fig. 2(b), the received reflected signals are separated in the frequency domain by wavelength-division demultiplexer, and finally superimpose at some single wavelength chips. For the transmitter module, the wavelength chips of the monitoring source can also superimpose in the time domain, provided that no fiber nonlinearity occurs. In this case, the received reflected signals are easier to process, which may be benefit for the network recognition algorithm. Thus, in the rest of the paper, we mainly consider the superimposed case for the detecting source.
In Fig. 2(b), time differencebetween ONUi and ONUj, both of which contain the same wavelength chip can be calculated as:Fig. 2(b), can be considered as the reference point. And the user location information is easily obtained by measuring the distance between the reference point and the corresponding coding pulse signals.
2.2.1 Remote coding
For PLC splitter chip with a limited size, it is difficult for multiple WBGs to be written on a single sub-branch, especially for large number. Moreover, longer single grating may make it much worse. Fortunately, binary tree structure of the PLC splitter relieves stress of size while also significantly reduces quantity of required gratings. Here, all the gratings intensively locate on one sub-branch, i.e., the end of branches, referred to as the centralized structure (CS). Similarly, at most one grating is written on each sub-branch, which is so-called distributed structure (DS).
According to the above definition, the total number of gratings between CS and DS is given in Table 1. To ensure unique output of the coding signal for each branch port, i.e., 64 branch ports, the CS requires 7 different wavelength chips and 154 gratings. However, the DS only needs 63 gratings with the same wavelength chips. That is, the DS can further reduce the production cost and difficulty.
From the Table 1, we clearly observe that the difference between total gratings and branch ports is one. Theoretically, it is easy to prove that the number of gratings in the DS is minimum by using the induction. Based on this, we develop a simple algorithm to guarantee the fewest gratings and unique output:
- 1) wavelength chips are chose to supportbranches;
- 2) The PLC splitter withbranch ports is divided intostages, and the number of gratings in each stage is equal to;
- 3) From thestage, one of twostage branches stemming from thestage branch is employed to write grating, and the center-reflecting wavelength is.
As is shown in Fig. 1(b), numbered branch ports of the PLC splitter is divided into stages, i.e., 1st, 2nd,…, nth. WBGs are written on respective sub-branches according to the above algorithm. For convenience, we assume that each grating is written on the upper bifurcation uniformly, the output signal corresponding to each branch port can be calculated as Eq. (2). Firstly, the weightof the power expansion satisfies:
As an integrative chip system, WBGs written on the PLC splitter chip inherits some excellent properties of the PLC device: broad operating wavelength range, low insertion loss and uniformity, low temperature dependent loss, small dimension and long term reliability . Furthermore, using a series of Bragg grating written within a PLC chip make it possible to nondestructively determine its precise optical characteristics, such as propagation loss and dispersion [14,15].
In addition, the PLC splitter as the carrier of WBGs is thin film photonics device that is usually fabricated on silicon substrate by using the conventional PLC technology. For example, flame hydrolysis deposition (FHD) and reactive ion etching (RIE) are employed to fabricate silica-based optical waveguides on silicon substrates . More specifically, the processes can be basically divided into three steps. Firstly, FHD is used to deposit two successive glass particle layers (Silica and Ge-doped silica), which form the under cladding and core, respectively. Secondly, the substrate with these two porous glass layers is heated to about 1300 for consolidation after the deposition. The waveguide core ridges are then defined by photolithography, as the unwanted parts of the core region are removed by RIE. Thirdly, FHD is used again to cover the core ridges with an overcladding. After above steps, a waveguide with stable performance and low propagation loss is generated. Note that germanium (Ge) doping is usually used in the core waveguide, which can increase the refractive index difference between the core and the surrounding cladding . Meanwhile, hydrogen loaded is necessary to increase the photosensitivity of the optical waveguide with Ge-doped. For the PLC splitter, a narrow core is inserted before and after the Y branch as a mode filter, which reflect the BPM simulation on cascaded Y-branching structures . Technology such as phase-mask can be used to write Bragg grating on the branch waveguides.
For direct UV grating writing , the photosensitive platform is a triple-layer SoS composite, including a thermal oxide under cladding, a photosensitive core and an upper cladding, which is also fabricated by using FHD. Similarly, hydrogen loaded is used to increase the photosensitivity of the core with Ge-doped. Obviously, direct writing can greatly simplify the fabrication of thin film photonics devices, since some complicated processing steps including photolithography and RIE may be omitted .
When various gratings are simultaneously written on the branches of the waveguide, wavelength tolerance may exist between the gratings with the same center-reflecting wavelength, i.e., change of external environmental forces cause the wavelength shift. Then, the wavelength between the monitoring source and WBG does not completely coincide that leads to partial reflection of high reflectivity gratings. The coding signals are mixed into unwanted coding wavelengths. Fortunately, a combination of narrow linewidth monitoring source and large bandwidth grating can ease the situation. However, coexistence between high reflectivity and large bandwidth may be difficult for a common grating. Meanwhile, the flatness of reflection spectrum for the grating is conducive to improve the quality of the coding signals.
2.2.2 Monitoring capacity
The monitoring capacity of PON largely depends on the number of wavelength chips included in the detecting pulse. Assume that the number of wavelength chips is. As illustrated in the inset of Fig. 1, each branch port of the PLC splitter connected to an ONU is assigned to a unique optical coding signal. When only a grating with center-reflecting wavelengthis written on the branch of the PLC splitter chip, the wavelength chips are sent to one ONU andis reflected back to the receiver module at the same time. Thus, the optical coding signal of the corresponding DF link is viewed as. Similarly, when no grating is written along sub-branch of stages on the chip, all wavelength chipsas the optical coding signal are sent to the corresponding ONU. Note thatgratings corresponding towavelength chips cannot be fabricated simultaneously in single or cascaded branches of the PLC splitter. In that case, no optical coding signal is allocated to the corresponding ONU for link monitoring. The maximum monitoring capacityof PON can be expressed as Eq. (3) in accordance with the principle of permutation and combination,Eq. (3), an additional wavelength chip is also needed forbranch ports to ensure unique output at each port. As a result,coding signals are useless as wavelength chips are selected to support users. However, few wavelength chips can also support big users. Similarly, wavelength chips are required for users in the DS, however, the number of coding gratings is minimum. In principle, any coding signal included in the combination of wavelength chips can be extracted by the arrangement of WBGs on the branches of PLC splitter. For the monitoring wavelength, the ITU-T L.66 (2007) Recommendation reserves the U-band (1625-1675nm) for maintenance and lists several methods to implement PON in-service maintenance function. In the proposed scheme, the monitoring wavelength chips can choose to separate from each other by arbitrary spacing in 50nm-wide U band.
2.2.3 Transmission impairment of the detecting pulse
The arrangements of WBGs on the branches determine the coding signals corresponding to each ONU. However, the detecting pulse contain different coding wavelength chips, propagating in the system may change in the waveform and amplitude. It is necessary to consider the influence for accurate detection and recognition. For pulses of initial width longer than, one can obtain the simplified equation that pulses propagate inside single-mode optical fibers with passive components .Eq. (4).
In addition, we also investigate signal noise ratio (SNR) that influence the signal detection when performing the electrical operation. Recall that the transmitter module can be realized by lasers (coherent sources) in the proposed scheme, thus no RIN incur but beat noise (BN). However, the spectrum slicing of the BLS (incoherent sources) beat less but introduces RIN, resulting from the beat effect of Fourier components. Major noise sources in a photodiode can be categorized as thermal noise (TN), shot noise (SN), and dark current noise (DN). In order to assess the performance of the proposed scheme, we consider the following expression for the SNR:21,22]. The SNR of fiber fault PON monitoring based on optical coding has been investigated in . In the proposed scheme, multiple PDs are used to receive the respective wavelength chip of coding signals. The coding signals can be viewed as a special 2D scheme with the same tapped delay lines before reaching AWG. The reflected coding signals, demultiplexed by AWG, containing identical wavelength that can be regard as 1D scheme with code weight. The value of the desired signal depends on the superimposed amplitudes with identical wavelength at the same location. The terms in Eq. (5) can be written as the following equations:
3. Simulation and analysis
3.1 Coding for DS
In the sub section 2.1, the arrangement based on the above algorithm guarantees the number of the gratings minimum. However, by the numbers, the arrangement may be not unique, as is shown in Fig. 3. The arrangement on PLC splitter chip with 16-ports shown in Fig. 3(a) also complies with the definition of DS. The irregular arrangement, compared with Fig. 3(b), reaches the minimum by extracting the common gratings stepwise. For the two arrangements in the DS, 5 output signals are different after removing the common coding signals. The different coding signals corresponding to Fig. 3(a) and Fig. 3(b) are andrespectively. From the different output signals in the 16-ports PLC splitter, the output coding signals is simpler based on the above algorithm. Fewer wavelengths included in the coding signal may be more suitable for the electrical domain detection and then further simplify network recognition algorithm. Note, however, that the similar but time consuming arrangement is also present in the case of 8, 32 and 64 branch ports. Aside from the complexity of the coding signals, the irregular arrangement also needs more variety of gratings and may be not conducive to fabrication.
As is also illustrated in the sub section 2.1, the output signals corresponding to each branch port is given in Eq. (2). Each ONU connected to the numbered output branch port is assigned to unique coding signal. In Eq. (2), subscriptdescribes the number of the coding wavelength chips included in a coding signal. The number of coding wavelengths corresponding to each output port of the PLC splitter chip at the RN is shown in Fig. 4. Here, we select. The coding wavelength chips in a detecting pulse are allocated to each ONU that superimpose together before reaching AWG. For the receiver module,is the number of the reflected wavelength chips that arrives at the PDs simultaneously.
3.2 Transmission and reception
3.2.1 Amplitude variation
In a real scenario, the peak power of the incident pulse, being lower than 10dBm insufficiently induces nonlinear impairment . Then, the forth term of Eq. (4) can be neglected. For the common fibers,fiber loss anddispersion is considered for U-band signals. Detecting pulse evolves in the light path of the proposed scheme can be calculate by Eq. (4). In general, the initial width of input pulses is inversely proportional to the pulse broadening ratio. Not considering the dispersion induced by passive components, we obtain that the pulse with incident power less than 4dBm and duration 1ns transferring 50km is almost no broadening. That is, the dispersion effect affecting the amplitude variation can also be neglected. However, the loss induced by fibers and passive components significantly decrease amplitude of the detecting pulse. Figure 5 indicates the amplitude variation of the detecting pulse containing required wavelength chips varies with 50-km propagation distance in the monitoring system. In Fig. 5, the amplitude as ordinate is processed by normalization firstly, and then expressed in dB. We assume that the detecting pulse generated by the transmitter module is sent to the network using a circulator with 1dB-insertion loss; a FF is 20km and a 1:64 PLC splitter with WBGs serves at the RN. All length of DF links is 5km and the U-band reflectors are FBGs with reflectivity of roughly 100%. Other losses, i.e., the connectors and splicings, are considered for 4 dB and evenly spilt among the passive components for the simplified calculation. Clearly, the insertion loss of passive elements is doubled since the round-trip path of the detecting pulses. The break point in the curve indicates the loss caused by passive components. The amplitude variation of coding signals corresponding to 1st, 34th, 64th port of PLC splitter passing through the chip has been shown in the detail view of Fig. 5. 1:64 PLC splitter is divided into 6 stages according to the proposed algorithm, i.e., 1st, 2nd, 3rd, 4th, 5th, 6th stage. The coding signals pass through each stage that incur amplitude loss by the reflection of WBGs and binary split, i.e., 1st and 64th branch port suffer the most and least severe loss, respectively. Note that the severe loss of the detecting pulse limits the network capacity. However, it can be mainly improved by employing high-gain avalanche photodetectors (APD) due to no U-band amplifier .
For two available detecting sources in the proposed scheme, we just evaluate the SNR of coherent source (lasers), and then the RIN can be removed. BN occurs when various optical pulses with identical wavelengths are incident simultaneously on the same detector . The beating between pulses with different center wavelengths is eliminated by AWG. Obviously, the beating between pulses with the same wavelength depends on geographical distribution. In the proposed scheme, each set of coding signals contains, which may suffer from the severest noise. That is, more equidistant users lead to severer noise, and totally different DF links introduces no BN. Assume that the geographical distribution conforms to the uniform radial (UR) model with a fixed coverage area (5km2) of the users . This corresponds to a maximum separation length of 1.26km for the UR distribution model. The duration and power of the single detecting pulse are 1ns and 1.55dBm, respectively. And, the stack power is rough 10dBm, which cause no nonlinear impairment . The component parameters for the simulation in Eq. (6) are given in Table 2.
We assume that the distance of user 1 from the RN to ONU upon UR model is. Obviously, the distance between users being equal tointroduces interference and noise. In Fig. 6, the blue curve shows that the SNR varies the equidistant users with user1 in a PON with 64 users. In two extreme cases, no and all equidistant users are 10.6 dB and 7.5dB, respectively. Note that higher power detecting pulse may be necessary due to the huger loss in the high capacity network. The superimposed pulses can be replaced with pulse train to avoid fiber nonlinearity. That is, the detecting pulses are not allowed to be simultaneously transmitted since their total power is over a certain value, as is shown in Fig. 2(a). For single pulse in the pulse train, the transmitted power is considered to be less than 4dBm in order not to induce any nonlinear effect . Considering a transmitted power of 4dBm with the pulse width of 1ns, the range of SNR is 8.5~13.1dB shown with the red curve in Fig. 6. Note that the stack power is divided into 7 single pulse power at the RN. Theoretically, higher transmitted power leads to better SNR within the range of the fiber nonlinearity.
According to the ITU-T recommendation, we can evaluate the loss budget under the realistic conditions. For example, in a PON with a 20-km FF and 64 users, the link budget required by standards is not less than 30 dB and the dynamic range requirement (one-way transmission) is about 15 dB. The maximum total loss experienced by optical coding pulses exceeds 60dB and the received power can be as small as −58.5dBm. The sensitivity and dynamic range of the photoreciver should be not inferior toscale and 30dB, respectively.
For a PON with 64 users, 7 different coding wavelengths are required and all coding signals contain an identical coding wavelength. For ONU1, the coding signal only contains the identical coding wavelength. In the worst case, each coding wavelength overlaps each other after demultiplexing. If a break occurs in the DF corresponding to ONU1, the pulse amplitude corresponding to the identical coding wavelength roughly reduces by 1/64 without considering the dynamic range requirement. However, difference of the received pulse power between the maximum and minimum is about 30dB, resulting from the dynamic range requirement that makes the detection more difficult. In principle, the receiver module should detect the variation of the weakest pulse amplitude when it overlaps other stronger pulses. However, it may just work in the worst case and thus make rigorous demand on the receiver module.
In addition, a high distance resolution is associated with a low sensitivity and vice versa in the proposed scheme. Thus, a tradeoff between temporal resolution and sensitivity can be made. For example, if the detecting pulsewidth is 1ns and the difference of fiber length between two DF links exceed 0.1m, the reflected pulses with the same coding wavelength separate from each other. That is, the distance difference between two DF links can be clearly distinguished. However, the pulse of width longer than ~1ns with the same difference may cause overlap. Consequently, the distance resolution is reduced but sensitivity is improved due to the stack power. In general, longer pulsewidth leads to lower distance resolution. For the weak received pulse power, higher sensitivity makes it easier to be detected at the expense of resolution reduction.
3.3 Simulation result
In order to elaborate the principle of the proposed scheme, we perform a simulation that is shown in Fig. 7. Four wavelength chips of detecting pulse launch into a FF via an optical circulator to achieve the RN. At the RN, the detecting pulse is power splitted and optical encoded by the 1:8 PLC splitter with WBGs, the corresponding coding signals are assigned to each OUN and reflected back to receiver module by the reflector embedded at each ONU. Based on the above algorithm, the coding signals for 8 ONUs are, in order,,,,,,,,. The corresponding distance from the RN to ONUs are respectively, 0.5km,1km, 1km, 1.2km, 1.4km,1.6km, 1.8km,2km. In the case that all links are healthy, Fig. 7(a) illustrates the total received coding signals from 8 ONUs with specified lengths. Figure 7(b) shows the states of 8 DF links in the healthy case. In Fig. 7(c), reduced pulse waveforms ofandindicate a break in a DF link, corresponding to ONU2, compared with the waveforms in Fig. 7(b). Figure 7(d) indicates that the DF link of ONU3 occurs a fault, which is equal to that of ONU2. Note that if the disruption occurs in the FF link, no pulse waveform can be observed.
Moreover, a similar simulation with 6 wavelengths is also presented to illustrate the challenges of the proposed scheme because a real PON system typically has splits of 32 or 64.That is, the simulation is carried out under more realistic conditions. The coding signals corresponding to each ONU can be obtained from Eq. (2) with. In the simulation, 10 equidistant DF links are assigned to some of 32 ONUs. At the same time, dynamic range requirement is also considered and randomly allocated. Here, ONU1 is intentionally included in the set of the equidistance. For the DF link corresponding to ONU1, only one coding wavelength is included in the coding signal, which is used to verify the previous discussion. ONUs with the same DF link are ONU1, ONU2, ONU5, ONU7, ONU8, ONU16, ONU20, ONU25, ONU26, ONU30, respectively. The simulation results are shown in Fig. 8, and the corresponding link states can be observed. The position of equidistance is roughly at 60ns. Note that the received pulse amplitudes are different due to the dynamic range requirement. For example, the difference of received power reflected from ONU16 and ONU32 is about 11dB. In Fig. 8(a), 6 different wavelength chips with 32 users are in the healthy state. Figure 8(b) shows that a break occurs in the DF link corresponding to ONU1. The wavelength chip of coding signal only containsand the pulse amplitude ofroughly reduces by 0.031V, compared with the healthy case. Figure 8(c) simulates a break in DF link corresponding to ONU16, the pulse amplitudes of andsimultaneously decrease with the same slight change of 0.025V, except that of. The changes of pulse amplitudes corresponding to different faulty cases are marked by the dotted circles in Fig. 8(b) and 8(c) at about 60ns of time, respectively. The partial enlarged graphics of pulses at about 60ns of time at and in Fig. 8(a) and 8(c) are given, arrowed by the dash lines, which can show the slight changes between the healthy and faulty cases.
A remote coding scheme for PON monitoring using WBGs has been proposed and analyzed in this paper. The optical coding is completed at the RN by using the realizable PLC splitter chip with WBGs and each ONU configure the identical reflector, greatly reducing the cost and system complexity on the terminals. The DS corresponding to the coding model, as an optimal design, is presented for simpler structure and lower production cost in the process of WBGs writing. The transmission impairment and SNR of the detecting pulse affecting the signal recognition in the electrical domain is investigated. The SNR of the proposed scheme with coherent source for 64 user networks is up to 7.5dB in the worst scenario. Multiple cost-cutting application and excellent performance make the proposed scheme provide a promising way to PON monitoring for the construction of smart city.
This work is supported by the National Natural Science Foundation of China (NSFC) (No. 61271206) and the Advanced Science Foundation of Jiangsu Province, China (No. BY20140127-04).
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