**2. The reflective PON approach**

A very basic schematic view of a WDM-PON architecture is depicted in Figure 4. It makes use of integrated multichannel transmit (TX) and receive (RX) arrays in the OLT, and tunable laser (T-TX) and filters (TOF) in the ONUs. From the OLT, all downstream wavelengths are broadcast via the ODN; therefore, a tunable filter is necessary at each ONU in order to select the correct wavelength. Similarly, each ONU has to be provided with a tunable laser for the correct upstream wavelength selection.

**Figure 4.** WDM-PON architecture

**•** liquid crystal tunable filter [9];

**Figure 3.** TWDM-PON system diagram

Picture taken from [6]

**•** DFB laser with partial TC [12];

**•** thermally tunable FP detector [10].

The related implementation technologies are:

The ONU tunable transmitter can tune its wavelength to any of the upstream wavelengths.

**•** multi-section distributed Bragg reflector laser (electrical control) without cooling [13];

All these solutions are today under consideration for NG-PON2, but none has yet completely demonstrated to be achievable at the (very low) target prices for ONU. Purpose of this Chapter is to describe a solution for the upstream transmission that avoids the need for a tunable laser at the ONU side: it is based on self-coherent reflective PON architectures as a possible

**•** distributed feedback (DFB) laser with temperature control (TC) [11];

370 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

**•** external cavity laser (ECL) with mechanical control without cooling [14];

**•** ECL with thermo/electro/piezo/magneto-optic control without cooling [15, 16].

One of the few physical layer problems over which FSAN is still working today before the final NG-PON2 Recommendation release, is related to these two devices at the ONU. Indeed, they should both have a precision compatible with a 100 GHz wavelength grid, and be able to tune on (at least) four wavelengths, adopting some protection systems during their switch-on time, in order to avoid interferences with other channels (due to the uncontrolled wavelength transmission at the laser power on); moreover, they should operate on a very wide temperature range and they should have a target price compatible with equipment to be installed at the customer premises. Moreover, in the longer term, if more than four wavelengths will be used (for instance for WDM overly), this issue will be particularly critical.

An alternative solution to overcome this problem is represented by reflective PON architec‐ tures, whose key idea is to generate the unmodulated upstream wavelengths at the OLT side, and modulate them in reflection at the ONU side, as schematically depicted in Figure 5. With respect to the setup depicted in Figure 4, the OLT is equipped by an additional multichannel transmit array, for the unmodulated seed signals generation, and the tunable laser at the ONU side is replaced by a reflective transmitter (R-TX). This way, the costs and the technical issues (the wavelength control in particular) related to the tunable lasers are moved from the ONU to the OLT, where they can be more easily managed.

**Figure 5.** Scheme of a possible solution for Reflective-PON ONUs

#### **2.1. Key components for the reflective ONU**

Many variants of WDM reflective PON architectures can be found in literature, all around the common denominator of avoiding expensive tunable lasers at the ONU by means of using a reflective modulator for the upstream transmission that sends back a centrally-generated seed signal. One of the most common approaches adopted in order to obtain this effect at the ONU side is based on the use of Reflective Semiconductor Optical Amplifier (RSOA), which combine the amplification and modulation capabilities in a single device. They are composed by a high reflectivity coating on one facet, along with a curved waveguide and ultra-low reflectivity coating on the other facet, to produce a highly versatile reflective gain medium [17]. Today, few optical devices manufacturers propose commercial RSOA devices (e.g. as CIP, MEL and Kamelian), since they are just used in the reflective PON scenario and at a research level.

Figure 6 schematically depicts a RSOA in a typical low-cost TO-CAN package, while Figure 7 shows the electrical bandwidth measurement result of a typical commercial RSOA (butterflypackaged solutions may have higher bandwidth, but their cost seems too high for application in the extremely cost-sensitive scenario of ONU). From this graph, it is possible to notice that the 3 dB electrical bandwidth of the device under test is about 500 MHz; anyway, the signal obtained by modulating the RSOA bias current *Ib* with a 1 Gbit/s OOK modulation (shown in the inset of Figure 7) is received without inter-symbol interference.

Since 2000, the interest of RSOA as upstream transmitters for WDM-PON applications based on reflective ONU has grown-up. The first results have been proposed in [18], where the uplink data stream was reflected and modulated via the RSOA at 1.25 Gbit/s, as then further devel‐ oped and investigated in several later works (e.g. [19-22]).

Many variants of WDM reflective PON architectures can be found in literature, all around the common denominator of avoiding expensive tunable lasers at the ONU by means of using a reflective modulator for the upstream transmission that sends back a centrally-generated seed signal. One of the most common approaches adopted in order to obtain this effect at the ONU side is based on the use of Reflective Semiconductor Optical Amplifier (RSOA), which combine the amplification and modulation capabilities in a single device. They are composed by a high reflectivity coating on one facet, along with a curved waveguide and ultra-low reflectivity coating on the other facet, to produce a highly versatile reflective gain medium [17]. Today, few optical devices manufacturers propose commercial RSOA devices (e.g. as CIP,

Figure 6 schematically depicts a RSOA in a typical low-cost TO-CAN package, while Figure 7 shows the electrical bandwidth measurement result of a typical commercial RSOA (butterfly-packaged solutions may have higher bandwidth, but their cost seems too high for application in the extremely cost-sensitive scenario of ONU). From this

MEL and Kamelian), since they are just used in the reflective PON scenario and at a research level.

Figure 5. Scheme of a possible solution for Reflective-PON ONUs

**2.1. Key components for the reflective ONU**

is received without inter-symbol interference.

Figure 6. Schematic view of a RSOA **Figure 6.** Schematic view of a RSOA

(the wavelength control in particular) related to the tunable lasers are moved from the ONU

Many variants of WDM reflective PON architectures can be found in literature, all around the common denominator of avoiding expensive tunable lasers at the ONU by means of using a reflective modulator for the upstream transmission that sends back a centrally-generated seed signal. One of the most common approaches adopted in order to obtain this effect at the ONU side is based on the use of Reflective Semiconductor Optical Amplifier (RSOA), which combine the amplification and modulation capabilities in a single device. They are composed by a high reflectivity coating on one facet, along with a curved waveguide and ultra-low reflectivity coating on the other facet, to produce a highly versatile reflective gain medium [17]. Today, few optical devices manufacturers propose commercial RSOA devices (e.g. as CIP, MEL and Kamelian), since they are just used in the reflective PON scenario and at a research level.

Figure 6 schematically depicts a RSOA in a typical low-cost TO-CAN package, while Figure 7 shows the electrical bandwidth measurement result of a typical commercial RSOA (butterflypackaged solutions may have higher bandwidth, but their cost seems too high for application in the extremely cost-sensitive scenario of ONU). From this graph, it is possible to notice that the 3 dB electrical bandwidth of the device under test is about 500 MHz; anyway, the signal obtained by modulating the RSOA bias current *Ib* with a 1 Gbit/s OOK modulation (shown in

Since 2000, the interest of RSOA as upstream transmitters for WDM-PON applications based on reflective ONU has grown-up. The first results have been proposed in [18], where the uplink data stream was reflected and modulated via the RSOA at 1.25 Gbit/s, as then further devel‐

the inset of Figure 7) is received without inter-symbol interference.

oped and investigated in several later works (e.g. [19-22]).

to the OLT, where they can be more easily managed.

372 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

**Figure 5.** Scheme of a possible solution for Reflective-PON ONUs

**2.1. Key components for the reflective ONU**

**Figure 7.** Characterization of a RSOA

In order to increase the upstream data rate up to 10 Gbit/s in such an architecture, using this low-bandwidth devices, different approaches have been proposed by research groups, like the use of optical filtering and electronic equalization [23-25] or the adoption of advanced modulation formats [26]. different approaches have been proposed by research groups, like the use of optical filtering and electronic equalization [23-25] or the adoption of advanced modulation formats [26]. Another device that can be adopted as reflective transmitter at the ONU side is the Reflective Electro-Absorption Modulator (REAM). EAM are semiconductor devices usually made in the form of a waveguide with electrodes for

the RSOA at 1.25 Gbit/s, as then further developed and investigated in several later works (e.g. [19-22]).

In order to increase the upstream data rate up to 10 Gbit/s in such an architecture, using this low-bandwidth devices,

Another device that can be adopted as reflective transmitter at the ONU side is the Reflective Electro-Absorption Modulator (REAM). EAM are semiconductor devices usually made in the form of a waveguide with electrodes for applying an electric field in a direction perpendicular to the modulated light beam. Their principle of operation is based on a change in the absorption spectrum caused by the applied electric field, which changes the band-gap energy without involving the excitation of carriers by the electric field: the so called Franz-Keldysh effect [27]. Reflective EAM include an EAM and a mirror, as schematically depicted in Figure 8, and they can be used to reflect and modulate the incoming light by means of the applied electrical signal (*Vb*). Thanks to EAM, a modulation bandwidth close to 10 GHz can be achieved, as shown in applying an electric field in a direction perpendicular to the modulated light beam. Their principle of operation is based on a change in the absorption spectrum caused by the applied electric field, which changes the band-gap energy without Figure 9, which makes these devices useful for optical fiber communication at 10 Gbit/s and above.

Differently from the RSOA, these devices do not amplify the light but, if coupled in cascade to a Semiconductor Optical Amplifier (SOA), they represent a very interesting solution for the ONU of a reflective PON architecture, since they provide a high-speed modulation capability, combined with the linear amplification of signal provided by the SOA. This solution has been proposed in several works, like for example [28, 29], in order to achieve a 10 Gbit/s upstream transmission in a WDM-PON scenario.

**Figure 8 Schematic view of a REAM** 

**Figure 8.** Schematic view of a REAM

splitter (PBS) looped on itself through the modulator. The MZM provides RF access to both the electrodes independently, allowing modulation to be efficient in both the forward and the backward **Figure 9.** Characterization of a REAM

directions. On one electrode of the MZM, RF drive power is applied in the forward direction while, on the other electrode, the modulation is applied in the backward direction (the same signal is applied to both sides). The two polarizations of the incoming optical signal are split through the PBS and sent to the MZM in counter-propagating directions (the MZM having polarization maintaining fibers on At the time of writing this Chapter, another solution for the reflective transmitter implemen‐ tation is under analysis inside the FP7 EU STREP project titled "FABULOUS" [30]. It consists on a Mach-Zehnder based subsystem allowing polarization independent reflective carrier

back towards the OLT, as schematically depicted in Figure 10.

both the input and the output). After being modulated, they are recombined through the PBS and sent

suppressed optical modulation for application in frequency division multiple access (FDMA) PON. The architecture of this subsystem, presented at first in [31], relies on using a Mach-Zehnder modulator (MZM) simultaneously in both directions within a polarization diversity loop made of a polarization beam splitter (PBS) looped on itself through the modulator. The MZM provides RF access to both the electrodes independently, allowing modulation to be efficient in both the forward and the backward directions. On one electrode of the MZM, RF drive power is applied in the forward direction while, on the other electrode, the modulation is applied in the backward direction (the same signal is applied to both sides). The two polarizations of the incoming optical signal are split through the PBS and sent to the MZM in counter-propagating directions (the MZM having polarization maintaining fibers on both the input and the output). After being modulated, they are recombined through the PBS and sent back towards the OLT, as schematically depicted in Figure 10.

**Figure 10.** Architecture of R-MZM subsystem

Figure 9, which makes these devices useful for optical fiber communication at 10 Gbit/s and

Differently from the RSOA, these devices do not amplify the light but, if coupled in cascade to a Semiconductor Optical Amplifier (SOA), they represent a very interesting solution for the ONU of a reflective PON architecture, since they provide a high-speed modulation capability, combined with the linear amplification of signal provided by the SOA. This solution has been proposed in several works, like for example [28, 29], in order to achieve a 10 Gbit/s upstream

**Figure 8 Schematic view of a REAM** 

EAM

*Vb*

**MIRROR**

PIN

CW

374 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

POUT

**Figure 9 Characterization of a REAM** 

At the time of writing this Chapter, another solution for the reflective transmitter implementation is under analysis inside the FP7 EU STREP project titled "FABULOUS"[30]. It consists on a Mach-Zehnder based subsystem allowing polarization independent reflective carrier suppressed optical modulation for application in frequency division multiple access (FDMA) PON. The architecture of this subsystem, presented at first in [31], relies on using a Mach-Zehnder modulator (MZM) simultaneously in both directions within a polarization diversity loop made of a polarization beam splitter (PBS) looped on itself through the modulator. The MZM provides RF access to both the electrodes independently, allowing modulation to be efficient in both the forward and the backward directions. On one electrode of the MZM, RF drive power is applied in the forward direction while, on the other electrode, the modulation is applied in the backward direction (the same signal is applied to both sides). The two polarizations of the incoming optical signal are split through the PBS and sent to the MZM in counter-propagating directions (the MZM having polarization maintaining fibers on both the input and the output). After being modulated, they are recombined through the PBS and sent

At the time of writing this Chapter, another solution for the reflective transmitter implemen‐ tation is under analysis inside the FP7 EU STREP project titled "FABULOUS" [30]. It consists on a Mach-Zehnder based subsystem allowing polarization independent reflective carrier

back towards the OLT, as schematically depicted in Figure 10.

above.

transmission in a WDM-PON scenario.

**Figure 8.** Schematic view of a REAM

**Figure 9.** Characterization of a REAM

#### **2.2. Transmission issues in reflective PON**

The reflective WDM-PON architectures proposed in literature in the last ten years seem to be completely incompatible with TWDM-PON for a set of different reasons such as:


In a reflective architecture like the one depicted in Figure 5, the upstream signal undergoes the ODN path loss twice (from the OLT to the ONU as CW seed signal and back to the OLT after the ONU reflection). The optical power at the OLT input is thus determined by the following formula:

$$S = \stackrel{\bullet}{P}\_{CW} - 2L \, \_{ODN} + G\_{RONLI} \tag{1}$$

where *P* → *CW* is the optical power of the CW signal at the input of the fiber, *L ODN* is the ODN loss and*GRONU* is the gain of the optical amplifier installed at the ONU. Since the TWDM-PON standard fixed the maximum value for the launched optical power at the ODN input to +11 dBm per wavelength and the gain of the optical amplifier at the ONU is typically of the order of *GRONU* =20 *dB*, the optical power at the OLT input, for the lowest ODN class N1 (*L ODN* =29 *dB*), is of the order of *S* = - 27 *dBm*, which is lower than the sensitivity of the standard direct-detection receivers. Moreover, it is well known that, in such an architecture, the spurious back reflections for a SMF fiber are of the order of 30-35 dB below the forward propagating signal, due to the concentrated reflections on components and the Rayleigh backscattering.

Rayleigh backscattering is an unavoidable phenomenon in optical fiber propagation. It is a fundamental loss mechanism arising from random density fluctuations frozen in the fiber during manufacturing. There is a growing interest in understanding Rayleigh backscattering since it can be a limiting factor in various optical systems. It must be taken into account in the performance computation of bidirectional lightwave system, especially in a wavelength-reuse system, like for example the reflective PON architecture depicted in Figure 11. Rayleigh backscattering is an unavoidable phenomenon in optical fiber propagation. It is a fundamental loss mechanism arising from random density fluctuations frozen in the fiber during manufacturing. There is a growing interest in understanding Rayleigh backscattering since it can be a limiting factor in various optical systems. It must be taken into account in the performance computation of bidirectional lightwave system, especially in a wavelength-reuse system, like for

**Figure11 Rayleigh backscattering effect in reflective PON architecture at the OLT Figure 11.** Rayleigh backscattering effect in reflective PON architecture at the OLT

ቀ ௌ ூ ቁ ௗ

about 30-35 dB below the forward propagating signal.

example the reflective PON architecture depicted inFigure11.

In such a system, the received optical power is composed by the useful signal *S*, as specified above, and the interference signal *I* due to reflections, given by: In such a system, the received optical power is composed by the useful signal *S*, as specified above, and the interference signal *I* due to reflections, given by:

where ܴைே is the power of the reflections. Therefore, the signal to interference ratio is given by:

For a standard direct-detection receiver, even if the best solutions proposed in literature to mitigate the Rayleigh backscattering effect are adopted, the signal to interference ratio should be greater than 5-10 dB. This sets an important limit to the maximum achievable ODN loss. Indeed, the maximum reachable ODN loss is lower than 25 dB, since the optical power of the spurious back reflections is

To improve the performance, one could in principle increase the ܩோைே, but there are technological component issues that limit the maximum reachable gain of optical amplifiers; in addition, another issue can arise from the Rayleigh backscattering that is generated at the ONU side, as depicted in Figure12 and explained in details in [32]. This means that, in order to satisfy the NG-PON2 requirements with a reflective PON architecture, direct-detection at the OLT side is out of question.

$$I = \vec{\dot{P}}\_{CW} - \vec{R}\_{\text{ODN}} \tag{2}$$

ൌ െʹܮைே ܩோைே ܴைே (3)

where *RODN* is the power of the reflections. Therefore, the signal to interference ratio is given by:

$$\left(\frac{\text{s}}{\text{T}}\right)\_{dB} = -2L\_{\text{ODN}} + G\_{\text{RONLI}} + R\_{\text{ODN}}\tag{3}$$

For a standard direct-detection receiver, even if the best solutions proposed in literature to mitigate the Rayleigh backscattering effect are adopted, the signal to interference ratio should be greater than 5-10 dB. This sets an important limit to the maximum achievable ODN loss. Indeed, the maximum reachable ODN loss is lower than 25 dB, since the optical power of the spurious back reflections is about 30-35 dB below the forward propagating signal.

To improve the performance, one could in principle increase the *GRONU* , but there are techno‐ logical component issues that limit the maximum reachable gain of optical amplifiers; in addition, another issue can arise from the Rayleigh backscattering that is generated at the ONU side, as depicted in Figure 12 and explained in details in [32]. This means that, in order to satisfy the NG-PON2 requirements with a reflective PON architecture, direct-detection at the OLT side is out of question.

**Figure12 Rayleigh backscattering effect in reflective PON architecture at the ONU Figure 12.** Rayleigh backscattering effect in reflective PON architecture at the ONU

#### **1.2.3 Self-coherent detection in RPONs 2.3. Self-coherent detection in RPONs**

(b)

converted to baseband.

In a reflective architecture like the one depicted in Figure 5, the upstream signal undergoes the ODN path loss twice (from the OLT to the ONU as CW seed signal and back to the OLT after the ONU reflection). The optical power at the OLT input is thus determined by the

*CW* is the optical power of the CW signal at the input of the fiber, *L ODN* is the ODN

loss and*GRONU* is the gain of the optical amplifier installed at the ONU. Since the TWDM-PON standard fixed the maximum value for the launched optical power at the ODN input to +11 dBm per wavelength and the gain of the optical amplifier at the ONU is typically of the order of *GRONU* =20 *dB*, the optical power at the OLT input, for the lowest ODN class N1 (*L ODN* =29 *dB*), is of the order of *S* = - 27 *dBm*, which is lower than the sensitivity of the standard direct-detection receivers. Moreover, it is well known that, in such an architecture, the spurious back reflections for a SMF fiber are of the order of 30-35 dB below the forward propagating signal, due to the concentrated reflections on components and the Rayleigh backscattering.

Rayleigh backscattering is an unavoidable phenomenon in optical fiber propagation. It is a fundamental loss mechanism arising from random density fluctuations frozen in the fiber during manufacturing. There is a growing interest in understanding Rayleigh backscattering since it can be a limiting factor in various optical systems. It must be taken into account in the performance computation of bidirectional lightwave system, especially in a wavelength-reuse

Rayleigh backscattering is an unavoidable phenomenon in optical fiber propagation. It is a fundamental loss mechanism arising from random density fluctuations frozen in the fiber during manufacturing. There is a growing interest in understanding Rayleigh backscattering since it can be a limiting factor in various optical systems. It must be taken into account in the performance computation of bidirectional lightwave system, especially in a wavelength-reuse system, like for

**Figure11 Rayleigh backscattering effect in reflective PON architecture at the OLT** 

ODN

*RODN*

In such a system, the received optical power is composed by the useful signal *S*, as specified above,

In such a system, the received optical power is composed by the useful signal *S*, as specified

ௐ െ ܴைே (2)

*CW* - *RODN* (2)

Reflective ONU

*PCW LODN GRONU*

*PCW LODN*

ൌ െʹܮைே ܩோைே ܴைே (3)

ܫൌܲሬԦ

*I* =*P* →

where ܴைே is the power of the reflections. Therefore, the signal to interference ratio is given by:

For a standard direct-detection receiver, even if the best solutions proposed in literature to mitigate the Rayleigh backscattering effect are adopted, the signal to interference ratio should be greater than 5-10 dB. This sets an important limit to the maximum achievable ODN loss. Indeed, the maximum reachable ODN loss is lower than 25 dB, since the optical power of the spurious back reflections is

To improve the performance, one could in principle increase the ܩோைே, but there are technological component issues that limit the maximum reachable gain of optical amplifiers; in addition, another issue can arise from the Rayleigh backscattering that is generated at the ONU side, as depicted in Figure12 and explained in details in [32]. This means that, in order to satisfy the NG-PON2 requirements with a reflective PON architecture, direct-detection at the OLT side is out of question.

system, like for example the reflective PON architecture depicted in Figure 11.

example the reflective PON architecture depicted inFigure11.

and the interference signal *I* due to reflections, given by:

about 30-35 dB below the forward propagating signal.

ቀ ௌ ூ ቁ ௗ

**Figure 11.** Rayleigh backscattering effect in reflective PON architecture at the OLT

above, and the interference signal *I* due to reflections, given by:

*CW* - 2*L ODN* + *GRONU* (1)

*S* =*P* →

376 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

following formula:

OLT

*PCW* 

*PRX* 

where *P* →

> seems to be a must in order to satisfy the NG-PON2 requirements, whether a reflective architecture approach is adopted. The fundamental concept behind coherent detection is to use the product of the electrical fields of the modulated signal light (centred at ��) and a CW local oscillator (centred at ���). This produces a Because of the aforementioned problems related to direct-detection, a coherent detection at the OLT seems to be a must in order to satisfy the NG-PON2 requirements, whether a reflective architecture approach is adopted.

> Because of the aforementioned problems related to direct-detection, a coherent detection at the OLT

frequency down-conversion of the signal to the frequency ��� � |�� � ���|, as schematically depicted in Figure13. The fundamental concept behind coherent detection is to use the product of the electrical fields of the modulated signal light (centred at *f <sup>S</sup>* ) and a CW local oscillator (centred at *f LO*). This produces a frequency down-conversion of the signal to the frequency *f IF* =| *f <sup>S</sup>* - *f LO*|, as schematically depicted in Figure 13.

��� �� 0 � (a) A possible implementation of coherent detection in a reflective PON architecture is depicted in Figure 14: in this case, the CW light source placed at the OLT side is used both as a feed to be sent downstream to the reflective ONU and as a local oscillator for the OLT coherent

**Figure13 Spectra of (a) the optical signal and (b) the down-converted IF signal** 

� ��� � |�� � ���| <sup>0</sup>

A possible implementation of coherent detection in a reflective PON architecture is depicted inFigure14: in this case, the CW light source placed at the OLT side is used both as a feed to be sent downstream to the reflective ONU and as a local oscillator for the OLT coherent receiver, executing a self-coherent detection. Therefore, after the optical-to-electrical conversion, the signal is down-

**Figure 13.** Spectra of (a) the optical signal and (b) the down-converted IF signal

receiver, executing a self-coherent detection. Therefore, after the optical-to-electrical conver‐ sion, the signal is down-converted to baseband.

**Figure 14.** Scheme of a possible solution for self-coherent OLT in a reflective-PON architecture

**Figure 15.** Spectral analysis of signal and reflections in reflective PON

As demonstrated by the results available in literature presented in the following, this setup supports higher ODN loss values with respect to the limit of reflective PONs based on directdetection, in particular for the following reasons:


Indeed, using a coherent receiver, the Rayleigh backscattering reflections appear as added close to the "electrical" DC, as depicted in Figure 15, thus they can be easily filtered out by electrical high-pass filters.
