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1 White Paper WDM-PON Technologies Contents 1- Introduction 2- WDM-PON Architectures -Type 1 : Tuneable Laser -Type 2 : Sliced Broadband Source -Type 3 : Reflective Architectures a) Injection locked Fabry-Perot Laser b) Single polarisation RSOA c) Polarisation independent RSOA d) Reflective EAM 3- Experimental evaluation of a Single Polarisation RSOA in WDM-PON 4-Conclusion 5- References 6- Appendix ( RF and bias settings)

WDM-PON Technologies

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Page 1: WDM-PON Technologies

1

White Paper

WDM-PON Technologies

Contents

1- Introduction

2- WDM-PON Architectures -Type 1 : Tuneable Laser -Type 2 : Sliced Broadband Source -Type 3 : Reflective Architectures a) Injection locked Fabry-Perot Laser b) Single polarisation RSOA c) Polarisation independent RSOA d) Reflective EAM

3- Experimental evaluation of a Single Polarisation RSOA in WDM-PON

4-Conclusion 5- References 6- Appendix ( RF and bias settings)

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1. Introduction Passive optical networks (PONs) were originally developed in the 1980’s [1] as a cost effective method of sharing fibre infrastructure for narrowband telephony (TPON) to business premises. Since those early days the application of PONs has moved on to interactive broadband networks implemented as either BPON (Broadband PON) or Ethernet PON (EPON) and now GPON (Gigabit PON) [2].

Figure 1: Basic PON diagram

All these systems are based on the same idea of time sharing the optical medium by TDMA (time division multiplexed access). However, it has long been realised that using wavelength division multiplexing (WDM) offers an alternative method of sharing the capacity of a PON between multiple users and would offer advantages in terms of capacity, low latency and service transparency. Whilst the concept of using WDM within a PON has been widely demonstrated within research projects it is only recently that the enabling technology has become sufficiently mature for commercial consideration.

Recently the interest in WDM -PONs has grown significantly, especially in parts of Asia, and it is widely believed to be the route towards the next generation of PONs. The technology challenge for WDM-PON has been to avoid the need for expensive wavelength selective optical components in each end-users optical network unit(ONU). In practice this means that it is cost prohibitive to use the type of lasers currently available for long haul dense WDM (DWDM) transmission within a WDM-PON. Moreover, it would be impractical for each customer’s terminal to be built with a fixed single wavelength laser because managing the inventory of lasers would be complex and costly for the network operator. For the customers ONU to be ‘colourless’ either a tuneable laser is required or an alternative WDM-PON network design based on a reflective architecture must be used. In the

Central Office (CO)

Passive Optical Power Splitter

Optical Network Unit #1

ONU #2

ONU #N

ONU #4

ONU #3

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longer term the tuneable laser approach probably offers the highest performance WDM-PON with the greatest potential number of wavelength channels but at present the cost of tuneable lasers is still far too high. Moreover, a tuneable laser solution may require additional network control and management to set and maintain wavelengths. The reflective architecture takes a different approach since all of the individual wavelengths are provided by a shared network resource, such as shown in fig 2. In this scheme the upstream transmitter within the ONU only requires a reflective optical modulator.

Figure 2: Reflective PON diagram

Over the past 5 years there have been a wide range of reflective WDM-PON architectures reported in the research literature and more recently extensive network trials have been announced. This white paper first reviews the alternative approaches to the reflective WDM-PON architecture and then goes onto to discuss the attributes required from the key optical component within the ONU .

2. WDM-PON Architectures Within this paper only the upstream path is considered because this is the most demanding as far as the cost critical customer’s ONU is concerned. In principle the same transmission architecture can be used for both upstream and downstream as might be the case when a symmetric service in terms of bit rate is required. However, this may not always be the case as higher data rates are often required for the downstream. A further variation of the overall architecture is when it is necessary to both wavelength-share and time-share the PON [3]. In this situation a wavelength multiplex is used to

Optical Line Terminal

(OLT) ONU #1

ONU #N

ONU X

Upstream Shared Network

Source (e.g. ASE source)

AWG

λ1, λ2, ,λN RX

R-MOD

Downlink

Uplink

RX X

Downstream Signal

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increase the capacity of the primary feeders in the access network and the distribution PON capacity is then further shared using optical power splitters in a classical TDMA approach. Where combinations of both time sharing and WDM are used a greater power budget is required which can be stretching for some types of reflective architecture PON. In the schemes outlined below only wavelength sharing of the PON is considered for simplicity.

Type 1 Tuneable Laser Scheme This uses a tuneable laser within the ONU and is a scheme that offers the ultimate in terms of optical performance and flexibility. If the splitters in the network are solely WDM devices, such as planar array waveguide grating (AWG) devices, the number of ONUs supported will be determined by the channel spacing of the AWGs and the tuning range of the laser. Use of broadband splitter/combiners at the distribution point and more wavelength selective filters at the central office would allow more channels to be accessed, although available power budget might then be a consideration. To design a system which used the available spectrum most efficiently and make good use of the available power budget a combination of both WDM splitter/combiners and broadband splitter/combiners would be used in the distribution network.

Modulator TuneableLaser

Customers ONU

Splitters (or AWG)

AWG SplitterOptical Receiver

Central Office

Data Out

Data In

1530nm-1565nmC band

Upstream Spectrum

˜ 0.1nm

Tx Spectrum

A

A

A

Modulator TuneableLaser

Customers ONU

Splitters (or AWG)

AWG SplitterOptical Receiver

Central Office

Data Out

Data In

1530nm-1565nmC band

Upstream Spectrum

1530nm-1565nmC band1530nm-1565nmC band1530nm-1565nmC band

Upstream Spectrum

˜ 0.1nm

Tx Spectrum

˜ 0.1nm

Tx Spectrum

A

A

A

Figure 3: Type 1 architecture: WDM-PON with tunable laser at the ONU.

The problem with the tuneable approach is that a much more sophisticated laser is required within the customer’s ONU compared to conventional EPON and GPON systems. Tuneable lasers also usually require internal wavelength lockers or an external network wavelength reference to ensure they operate at the correct wavelength channel. Furthermore, to maintain a stable laser operating regime, external modulation rather than simple direct laser modulation is the norm. When the PON uses WDM devices for combining signals, the constraints on wavelength control can be slightly eased because

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only when the laser is operating at the correct wavelength can the signal pass through the WDM, so the demands of wavelength control and blanking during laser start up are eased.

Type 2 Sliced Broadband Source In this case each ONU contains a broad optical spectrum source within the transmitter, such as a superliminescent light emitting diode (SLED). The broad spectral output of the customer’s ONU is connected to one port on a WDM device, which could be thin film filter or AWG based. Only the optical spectral components from the LED which can pass through the WDM channel are transmitted through to the central office and the remaining power is wasted. Although all the customers ONUs have identical SLEDs, because each is connected to a different port on the WDM combiner, it is possible to slice a different part of the available optical spectrum for each ONU.

SLEDs would normally be used as the transmitter device in the customers ONU although, alternatively, the self-amplified spontaneous emission from a reflective semiconductor optical amplifier (SOA) can be used instead. The latter has the advantage of producing a greater optical output power for a lower drive current but has the disadvantage that it can be sensitive to optical back reflections resulting in an amplitude ripple in the output spectra and in the worst case laser action.

When a simple LED is used as the transmitter it is only practical to have a few upstream data channels of 155Mbit/s from this scheme. With a RSOA as the slicing source up to 32 X 155Mbit/s has been reported [4]. The number of channels and the data rate for each channel is determined by the excess optical intensity noise produced by the slicing process which is itself a function of the ratio of the bit rate to the optical bandwidth of the sliced source.

Super Luminescent Diode (SLD)

Customers ONUWDM SplitterOptical Receiver

Central Office

WDM(Slicer)

Data In

Data Out

1530nm-1565nmC band

AB

AB

˜ 10nm

Upstream Spectrum

Spectrum from SLD

C

C

Super Luminescent Diode (SLD)

Customers ONUWDM SplitterOptical Receiver

Central Office

WDM(Slicer)

Data In

Data Out

1530nm-1565nmC band

AB

AB

˜ 10nm

Upstream Spectrum

AB

˜ 10nm

Upstream Spectrum

Spectrum from SLD

C

C

Figure 4: Type 2 architecture: WDM-PON with broad spectrum optical source for the

uplink at the ONU.

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Type 3 Reflective Schemes In this class of schemes a separate sliced source is used to seed the return path modulators within the customer’s ONU. This has the advantage over the previous non-seeded scheme because the optical power produced by the ONU transmitter is now all within the required spectral channel and so none is wasted. A second advantage with some schemes is that the excess intensity noise produced by the slicing process can be reduced by gain saturation effects.

Four types of reflective WDM-PON are outlined below.

Type 3A Reflective : Spectral Slicing with Injection locked Fabry-Perot Laser

Customers ONUAWG SplitterOptical Receiver

Central Office

AWGCombiner

Laser with Asymmetric facetreflectivity

SLD or EDFABroadband Source

Seed

Data In

Data Out 1530nm-1565nmC band1530nm-1565nmC band

Seed Spectra

˜ 1nm˜ 1nm

Return Spectrum(with FP modes)

˜ 1nm˜ 1nm

Return Spectrum(with FP modes)

A

1530nm-1565nmC band1530nm-1565nmC band1530nm-1565nmC band

Upstream Spectrum

Customers ONUAWG SplitterOptical Receiver

Central Office

AWGCombiner

Laser with Asymmetric facetreflectivity

SLD or EDFABroadband Source

Seed

Data In

Data Out 1530nm-1565nmC band1530nm-1565nmC band

Seed Spectra

1530nm-1565nmC band1530nm-1565nmC band

Seed Spectra

˜ 1nm˜ 1nm

Return Spectrum(with FP modes)

˜ 1nm˜ 1nm

Return Spectrum(with FP modes)

A

1530nm-1565nmC band1530nm-1565nmC band1530nm-1565nmC band

Upstream Spectrum

1530nm-1565nmC band1530nm-1565nmC band1530nm-1565nmC band

Upstream Spectrum

Figure 5: Type 3A architecture: WDM-PON with Injection locked FP as ONU transmitter

In this scheme [5] a central broadband seeding source is used, this is typically the amplified spontaneous emission of an erbium doped fibre amplifier (EDFA), reflective SOA (RSOA) or SLED. The broadband seed signal is sliced by the AWG splitter/combiner located within the network. The sliced continuous wave (cw) seed light is used to “injection lock” a Fabry-Perot (FP) laser transmitter within the customer’s ONU. This is not the same as coherent injection locking of two lasers as the ONU laser is effectively operating as a saturated resonant amplifier. In this scheme the Fabry-Perot cavity modes which fall within the spectral passband of the AWG will be amplified and modulated by the ONU laser and returned back through the AWG. Resonant operation of the reflective transmitter reduces the required drive current and can reduce the slicing noise through gain saturation. The laser required is a little different from normal FP transmission lasers because the laser chip should have lower than normal reflectivity on the front facet and enhanced reflectivity on the back facet. The chip should

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also be longer than a conventional FP laser to reduce the cavity mode spacing to enable more laser longitudinal modes to fall within the spectral passband of the AWG. This effect can reduce the impact of mode competition noise. A laser of this type developed at CIP was used in the demonstration of spectral slicing directed at very-high digital subscriber line (VDSL) transmission over optical fibre [6]. To optimise the performance of the injection locked scheme requires control of both the laser parameters and the injected optical power. The optimisation of the operating point will also be dependent on the position of the natural gain peak of the laser compared to the injected spectral slice so is likely to vary between ONU’s.

Type 3B Reflective : Spectral Slicing with single polarisation RSOA

The reflective SOA (RSOA) is in some ways related to both the asymmetric FP laser and the SLED, however, it also has important differences. By reducing the front facet reflectivity close to zero it is possible to suppress all of the natural cavity modes, but unlike the SLED it is necessary to also ensure that there is an efficient reflection from the back facet. The basic concept of the RSOA is not new [7] but designs have been refined since by the use of angled stripes and mode expansion to reduce output facet reflectivity and improve fibre coupling. RSOAs can be designed to be either input signal polarisation dependent or polarisation independent. Generally the polarisation dependent devices have the best temperature performance, up to 70deg C and by trading polarisation sensitivity offer greater design freedom in most other parameters.

The single polarisation RSOA can be used in essentially the same system architecture as the asymmetric Fabry-Perot laser scheme 3a, since the system uses an un-polarised seed source.

Customers ONUAWG SplitterOptical Receiver

Central Office

AWGCombiner

Reflective SemiconductorOptical Amplifier (RSOA)

SLD or EDFABroadband Source

Data out

Data In

1530nm-1565nmC band1530nm-1565nmC band

Seed Spectra

˜ 1nm

Return Spectra

˜ 1nm

Return Spectra

1530nm-1565nmC band1530nm-1565nmC band1530nm-1565nmC band

Upstream Spectra

1530nm-1565nmC band1530nm-1565nmC band1530nm-1565nmC band

Upstream Spectra

Customers ONUAWG SplitterOptical Receiver

Central Office

AWGCombiner

Reflective SemiconductorOptical Amplifier (RSOA)

SLD or EDFABroadband Source

Data out

Data In

1530nm-1565nmC band1530nm-1565nmC band

Seed Spectra

1530nm-1565nmC band1530nm-1565nmC band

Seed Spectra

˜ 1nm

Return Spectra

˜ 1nm

Return Spectra

1530nm-1565nmC band1530nm-1565nmC band1530nm-1565nmC band

Upstream Spectra

1530nm-1565nmC band1530nm-1565nmC band1530nm-1565nmC band

Upstream Spectra

Figure 6: Type 3B architecture: WDM-PON with single polarisation RSOA as ONU

transmitter .

Page 8: WDM-PON Technologies

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The seeded RSOA approach has an advantage over the un-seeded SLED approach (type2) because the optical power will be higher in the selected spectral slice due to the optical gain of the RSOA. Moreover, the use of gain saturation can be used to “squeeze” the excess noise produced by slicing [8]. This will allow higher figures of merit ( channels X bit rate) to be achieved compared to un-seeded slicing and in practice 32 channels of Gigabit Ethernet (GbE) is achievable over a 20km reach PON. The maximum speed that the RSOA can be modulated at is determined by carrier lifetime and the ability to provide up to 100mA of electrical drive current. When the device is operated under gain saturation the effective lifetime is reduced by clamping of the carrier level and in practice modulation speeds of 2.5Gbit/s are possible.

For a longer reach PONs operating over longer distances from the central office impairments from chromatic dispersion can result when using a filtered broadband seed source. For these longer reach systems [3] it is better to use a comb of wavelengths generated by a bank of single-wave lasers as the seed source, but this usually requires the use of a polarisation independent RSOA within the customers ONU.

Type 3C Reflective: Spectral Slicing with polarisation independent RSOA

In this scheme the RSOA within the customer’s ONU has polarisation independent gain (SOA-RL-OEC-1550) [9] . This means that the seeding source can now be polarised and derived from a bank of single wavelength lasers. The use of a spectrally pure seed reduces the impact of chromatic dispersion for the return channel in long reach applications and avoids the excess noise produced by the slicing process. In principle more wavelength channels and at higher bit rates are possible. However, in practice another noise source, coherent Rayleigh backscatter, [10] must also be considered in situations where a single shared feed fibre is used for both the seed wavelength and modulated return signal. This problem can be reduced by using a separate fibre for the seed and return, as is shown below.

Customers ONUAWG SplitterOptical Receiver

Central Office

AWGCombiner

Reflective SemiconductorOptical Amplifier (RSOA)

SeedData out

Data In

AWG

Seed lasers

1530nm-1565nmC band

Seed Laser Bank

1530nm-1565nmC band

Seed Laser Bank

1530nm-1565nmC band

Upstream Spectrum

1530nm-1565nmC band

Upstream Spectrum

<0.1nm

Return Spectra

<0.1nm

Return Spectra

Customers ONUAWG SplitterOptical Receiver

Central Office

AWGCombiner

Reflective SemiconductorOptical Amplifier (RSOA)

SeedData out

Data In

AWG

Seed lasers

1530nm-1565nmC band

Seed Laser Bank

1530nm-1565nmC band

Seed Laser Bank

1530nm-1565nmC band

Upstream Spectrum

1530nm-1565nmC band

Upstream Spectrum

<0.1nm

Return Spectra

<0.1nm

Return Spectra

Figure 7: Type 3C architecture: WDM-PON with polarisation independent RSOA as ONU

transmitter.

Page 9: WDM-PON Technologies

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Type 3D Reflective Architecture: Reflective EAM

The carrier lifetime modulation speed limitation when using a RSOA as the transmitter in the reflective architecture can be eliminated by using a reflective electro-absorption modulator (REAM) (R-EAM-1550) [9]. With this reflective device, data modulation rates of 10Gb/s or higher (>40Gb/s) are possible. Obviously by using a REAM rather than the RSOA configuration there is no optical amplification at the ONU so all of the optical power needed to overcome the loss budget must be available from the central office seed lasers. For high bit rate systems it is most likely that lasers would be used as the seed sources and therefore for some applications dual fibre feeding may be necessary to reduce coherent Rayleigh back scatter noise. For lower data rates the REAM scheme is attractive when it is necessary to minimise the electrical power within remote equipment because the EAM requires a low drive voltage (<2V) and has intrinsically high electrical impedance. Low power consumption is of particular interest where EAM is used for VADSL line extension using WDM-PON [6] and radio-on-fibre applications [11].

Customers ONUAWG SplitterOptical Receiver

Central Office

AWGCombiner

Reflective Electro-Absorption Modulator (REAM)

SeedData out

Data In

AWG

Seed lasers

Customers ONUAWG SplitterOptical Receiver

Central Office

AWGCombiner

Reflective Electro-Absorption Modulator (REAM)

SeedData out

Data In

AWG

Seed lasers

Figure 8: Type 3D architecture: WDM-PON with reflective EAM as ONU transmitter.

For the longer term, new types of devices are likely to be available for long reach high split architectures [3]. These devices will combine the higher modulation speed of the REAM with the gain and output power of the RSOA.

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Table 1: Comparison of Upstream WDM-PON Architectures

Scheme Description Channel

Bit rate

Number of λ

Channels

Positives Negatives

1 Tuneable laser ONU

High

>10Gb/s

High

>32

.Power budget

.No seed source

.λ Scalability

.ONU complexity

.Additional network control

2 Spectrum slicing with LED based ONU

Low

<155Mb/s

Low

<32

.Cheap ONU

.No seed source

.Poor scalability

3A Reflective Architecture: Spectral Sliced ILD

Medium

<2.4Gb/s

Medium

32

.No cooler in ONU

.long term stability?

.Limited scalability

.Seed source

3B Reflective Architecture :Spectral Sliced with RSOA based ONU.

Medium

<2.4Gb/s

Medium

32

.No Cooler in ONU

.Limited scalability

.Seed source

3C Reflective Architecture : Central laser bank with RSOA based ONU.

Medium

<2.4Gb/s

High

>32

. λ Scalability

.Cooler required

.Rayleigh noise

.Seed source

3D Reflective Architecture: Central laser bank with REAM based ONU.

High

>10Gb/s

Low

<32

.High bit rates

.Low electrical power

.Poor scalability

.Rayleigh noise

.Seed source

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3. Experimental Evaluation of RSOA in WDM-PON

A CIP single polarisation RSOA [9], SOA-R-OEC-3169 was tested in a type 3B architecture. The device offers extended temperature operation as shown in figure 9. A stable gain at 1550nm, ~27dB, can be obtained between 10 and 70˚C, when the device bias current is varied between 15 and 80mA. The small signal bandwidth (measured for PinSOA=-10dBm and DC bias between 20 and 70mA) was measured to be around 1GHz between 20 and 80˚C.

15

17

19

21

23

25

27

29

0 10 20 30 40 50 60 70 80 90

Temp (C)

Gai

n (d

B)

Figure 9: Continuous Wave (CW) Gain at 1550nm of RSOA versus Temperature. The

RSOA bias is adjusted at each temperature to keep the gain constant.

Figure 10 shows the Amplified spontaneous emission (ASE) at each temperature and bias current settings. It can be observed that, as the temperature increases, the ASE broadens and its peak shifts to longer wavelengths.

-50-47-44-41-38-35-32-29-26-23-20

1460

1470

1480

1490

1500

1510

1520

1530

1540

1550

1560

1570

1580

1590

1600

1610

1620

1630

1640

wavl(nm)

ASE

(dB

m)

80mA_80C 80ma_70C 45mA_60C 35mA_50C30mA_40C 25mA_30C 19mA_20C 15mA_10C

Figure 10: ASE of RSOA at different temperature and drive settings.

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As a consequence of the different operating condition of the RSOA when the temperature is varied, the saturated input and output powers of the device change as well. The 1550nm results are shown in figure 11.

-30

-25

-20

-15

-10

-5

0

0 10 20 30 40 50 60 70 80 90Temp (C)

Psat

_in

(dBm

)

-5-3-113579111315

Psat

_out

(dB

m)

In Out

Figure 11: CW Saturated Input and Output power versus Temperature. The device bias

varies at each temperature.

The system architecture is shown in figure 12. At the central office, the un-polarised broadband ASE source for the upstream signal, is provided by a 2 stage EDFA amplification. The 5nm bandwidth filter in between, centred at 1550nm, is needed to concentrate the second EDFA output power at 1550nm. The attenuator is used to vary the power reaching the ONU which is monitored via the 10% tap. Between the CO and the ONU, a 100GHz AWG slices the EDFA spectrum. Different lengths of SMF-28 fibres can be inserted after the AWG. At the ONU, the RSOA is DC and RF biased via a Bias Tee. The device drive was varied depending on the temperature setting of the device. During the experiment the DC drive was set in order to minimize the Bit error rate at the Receiver and optimise the sensitivity. The RF voltage (AC coupled) was set to optimise the extinction ratio with the eye "0 level" down to real zero (i.e. with the RSOA switched off). The upstream signal, after passing through the AWG, is sent to the pre-amplified receiver. The system sensitivity is measured with the attenuator A. The signal is then amplified and filtered and detected using a Lightwave converter which output is filtered using a low pass 3GHz filter. The electrical signal is evaluated using an oscilloscope for eye quality and extinction ratio and analysed for Q and BER via a BER detector.

Page 13: WDM-PON Technologies

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Figure 12: Reflective WDM-PON experimental setup with RSOA at the the ONU.

The system performance in a back to back configuration at 1.25 Gb/sec was measured. Sensitivity, Q and BER were assessed versus RSOA operating temperature and RSOA Optical Input power. Measurements were carried out with both 10km and 20km of SMF-28 fibre between the CO and ONU.

For the back to back experiment at 1.25Gb/s, the RSOA RF drive was a PRBS, 231-1 NRZ pattern (detailed voltage and bias settings can be found in the appendix). System performance was assessed for 2 possible configurations: with the circulator and receiver as in figure 12, at the CO, or with the circulator placed between the AWG and the ONU. In the later scenario we investigated any possible penalty coming from the fact that the signal that reaches the receiver, which includes the RSOA ASE, doesn’t get filtered at the AWG. Results are shown in figure 13. The two plots are very similar and show the beneficial effect of noise “squeezing” at high input optical power. For example at 20˚C, the sensitivity and Q value are stable for Pin_SOA>-9dBm (at this power the SOA is well into saturation, figure 11). In fact, at low input powers, when the RSOA is not into deep saturation, the system is limited by the noise from the broadband ASE source that limits the BER and, hence the sensitivity. The benefits of intensity noise suppression by the saturated RSOA are also visible in the BER curves of figure 15. Only for Pin_SOA≥-17dBm the BER curves are not limited by a noise floor and low BER (1*10-11) is achievable: the BER curve gradient increases with increased Pin_SOA. While 20 and 40˚C results are comparable, the system performance is slightly poorer at higher RSOA temperatures. Nevertheless the system was still error free at 80˚C. As the RSOA unsaturated gain is over 30dB at room temperature, the device has still a reasonable gain when in the “noise-squeezing” regime, as shown in figure 14.

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-44-42-40-38-36-34-32-30-28-26-24-22-20

-27 -24 -21 -18 -15 -12 -9 -6 -3 0 3 6

Pin_SOA (dBm)

S (d

Bm

)

024681012141618202224

Q(d

B)

Sens_20C Sens_40C Sens_60C Sens_70CSens_80C Q_20C Q_40C Q_60CQ_70C Q_80C

-44-42-40-38-36-34-32-30-28-26-24-22-20

-27 -24 -21 -18 -15 -12 -9 -6 -3 0 3 6

Pin_SOA (dBm)

S (d

Bm

)

024681012141618202224

Q(d

B)

Sens_20C Sens_40C Sens_70C Sens_80CQ_20C Q_40C Q_70C Q_80C

Figure 13: Sensitivity and Q at 1.25Gb/s for the back to back system. The top plot is for

the system with the circulator near the RSOA , while the bottom plot is the for the scheme with the circulator at the CO.

-226

101418222630

-27 -24 -21 -18 -15 -12 -9 -6 -3 0 3 6Pin_SOA_TOTAL(dBm)

Gai

n (d

B)

20C_50mA 40C_50mA 60C_60mA70C 70mA 80C 70mA

Figure 14: Gain (under 1.25Gb/s modulation) of the RSOA when inserted in the WDM-

PON 1.25Gb/s system at different temperatures and bias settings.

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-4

-5

-6

-7

-8

-9

-10

-11

-12

-44

-42

-40

-38

-36

-34

-32

-30

-28

-26

Received Power (dBm)

Log(

Log(

BER

))

"" B2B_-25dBm_50mA_20CB2B_-17dBm_50mA_20C B2B_-15dBm_50mA_20CB2B_-7dBm_50mA_20C B2B_-13dBm_50mA_20CB2B_-11dBm_50mA_20C B2B_-9dBm_50mA_20CB2B_-7dBm_50mA_20C B2B_-5dBm_50mA_20CB2B_-2dBm_50mA_20C B2B_0dBm_50mA_20CB2B_2dBm_50mA_20C B2B_3-7dBm_50mA_20CB2B_-19dBm_50mA_20C

Figure 15: BER curves (at 1.25Gb/s) versus Received power at different RSOA input

powers. RSOA at 20˚C.

Figure 16: 1.25Gb/s eye diagrams. Top row, from left to right: Pin_SOA=-19, -15 and +1.4dBm and RSOA kept at 20˚C and DC current of 40mA (RF drive of 4.7V). Middle row,

from left to right, Pin=-15 and +1.4dBm and RSOA kept at 70˚C, and DC current of 65mA (RF drive of 6.5V). Bottom row, Pin=0dBm, RSOA kept at 80˚C, DC drive of 70mA

and RF drive of 6.5V.

Page 16: WDM-PON Technologies

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This noise filtering is also visible in the eye diagrams of figure 16: while at low input power the eye shows a significant amount of noise in the “1 level” of the eye, the eye opens up at higher injected powers. Extinction ratios (XR) were all pretty good, between 10.3 and 12dB at the different temperatures. The improvement in performance at high injected power comes also from the enhanced electrical to optical bandwidth of the device when in saturation.

For the transmission experiment (over 20km of SMF-28) at 1.25Gb/s, the RSOA RF drive was a PRBS, 27-1 NRZ pattern. Results are shown in figure 17. The figure shows as well the back to back results for an easy comparison of the data. Due to the 5dB insertion loss of the fibre, there is less optical power available to saturate the RSOA. Therefore a reduced “noise squeezing” beneficial effect will be available. Nevertheless, the results are good and low BER (1*10-11) were obtained as shown in the BER plots of figure 19. The eye diagrams (fig. 18) show a significant amount of noise at low injected power on the “1” level but the eyes open as the SOA gets more into saturation. Eye XRs varied between 10.5dB and 12.3dB.

The plot of figure 20 shows the minimum amount of RSOA input power required at the different temperatures to get a BER of 1*10-9 for the back to back and transmission cases. The two cases’s required powers are very comparable. The discrepancy at 70 degree could be due to a better DC and RF driving of the RSOA in the transmission case.

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-21 -18 -15 -12 -9 -6 -3 0 3

Pin_SOA_TOTAL(dBm)

S (d

Bm

)

024681012141618202224

Q(d

B)

Sens_20C Sens_40C Sens_70C Sens_80CQ_20C Q_40C Q_70C Q_80C

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-21 -18 -15 -12 -9 -6 -3 0 3

Pin_SOA (dBm)

S (d

Bm

)

024681012141618202224

Q(d

B)

Sens_20C Sens_40C Sens_70C Sens_80CQ_20C Q_40C Q_70C Q_80C

Figure 17: Sensitivity and Q at 1.25Gb/s for the back to back (top plot) and 20km

transmission (bottom plot) systems

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.

Figure 18: 1.25Gb/s eye diagrams for the transmission system. From left to right: T=20˚C & Pin=-19dBm, T=20˚C & Pin=-3.7dB, T=70˚C & Pin=-20dBm, T=70˚C & Pin=-

3.7dBm

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Received Power (dBm)

Log(

Log(

BER

))

"" B2B_-10dBm_42mA_20C_20kmB2B_-5dBm_42mA_20C_20km B2B_-3.7dBm_41mA_20C_20kmB2B_-15dBm_42mA_40C_20km B2B_-10dBm_42mA_40C_20kmB2B_-15dBm_42mA_20C_20km B2B_-5dBm_42mA_40C_20kmB2B_-3.7dBm_42mA_40C_20km B2B_-15dBm_66mA_70C_20kmB2B_-10dBm_66mA_70C_20km B2B_-3.7dBm_66mA_70C_20kmB2B_-20dBm_66mA_70C_20km B2B_-5dBm_66mA_80C_20kmB2B_-19dBm_42mA_20C_20km

Figure 19: Transmission over 20km of SMF-28. BER curves (at 1.25Gb/s) versus Received

power at different RSOA input powers and Temperature settings.

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0

0 20 40 60 80 100Temp (C)

Pin

for

BER

=1*1

0-9

(dBm

)

20km Back to Back

Figure 20: Minimum RSOA input power to have BER of 1*10-9 in the back to back and

transmission case at 1.25Gb/s versus RSOA temperature.

4. Conclusion

WDM-PON is becoming of great interest to operators as a means of increasing the capacity of optical access networks. WDM-PON also offers the possibility of both sharing fibre plant and at the same time providing a virtual point-to-point link from the central office to the end customer which gives both flexibility and service transparency not possible with a classical TDMA PON. The reflective WDM-PON architectures avoid the need for wavelength specific components within the customers ONU and gets round the usual inventory problem of WDM systems. There have been significant advances in the reflective components required to enable the colourless ONU and there are now RSOA and REAM devices available for a wide range of possible architectures .

5. REFERENCES

1. TPON-a passive optical network for telephony, J R Stern et-al, ECOC 1988

2. ITU-T Recommendation G.984.1: “General characteristics for Gigabit- capable Passive Optical Networks”

3. Progress in PON research in PIEMAN and MUSE, Davey R, NOC2006

4. Demonstration of DWDM-PON Employing Spectrum-sliced RSOA, S B park, et-al, ECOC 2006

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5. Fiber-to-the-home services based on wavelength division multiplexing passive optical network, S J Park, Journal of Lightwave Technology, Vol22, no 11, November 2004

6. WDM-based optical feeder for VDSL with electrical powering from the customer premises”, A. Borghesani et-al, ECOC2006.

7. US Patent US5015964 published 1987

8. Spectral slicing WDM-PON using wavelength-seeded reflective SOAs”, C Ford et-al, Electron. Lett., Vol 37, No 19, 2001

9. see data sheets www.ciphotonics.com

10. Rayleigh backscattering impairments in access networks with centralised light source, G. Talli et-al, Electronics Letters, vol. 42, no. 15, 2006

11. Passive picocell:a new concept in wireless infrastructure, D Wake et-al, Electronics Letters, 33 no5, 1997.

6. Appendix ( RF & Bias Settings)

RF and DC bias settings for 1.25Gb/s experiment (back to back configuration).

Temperature ˚C DC Bias Current (mA) RF Voltage (peak to peak) (V)

20 40 4.7

40 40 4.7

60 50 5.6

70 65 6.5

80 70 6.5

RF and DC bias settings for 1.25Gb/s experiment (transmission over 20km of SMF-28).

Temperature ˚C DC Bias Current (mA) RF Voltage (peak to peak) (V)

20 42 4.7

40 50 4.7

70 66 7.2

80 66 7.2