Short-reach 200 Gb/s SDM Network Employing Direct-detection and

Optical SSBI Mitigation

Tiago D. Freitas M.

1 a

, Tiago M. F. Alves

1,2 b

and Adolfo V. T. Cartaxo

1,2 c

ISCTE - Instituto Universit

ario de Lisboa, Lisbon, Portugal

Instituto de Telecomunicac¸

oes, Lisbon, Portugal

Keywords:

Signal-signal Beat Interference, Direct-detection, Multicore Fibre, Optical Fibre Communications, Space

Division Multiplexing, Short-reach Networks.

Abstract:

We propose a new transmission scheme for direct-detection (DD) short-reach networks based on transmitting

the carriers and the data signals in separated cores of a multicore-ﬁbre (MCF). With this scheme, a low com-

plexity signal-signal beat interference (SSBI) mitigation approach is proposed at the receiver side, which may

be required to compensate electronically the chromatic dispersion of the singlemode ﬁbre. The performance

of a 200 Gb/s binary NRZ signal in a MCF short-reach network employing the proposed transmission scheme

is assessed by numerical simulation. The combined effect of the skew and the laser phase noise on the system

performance is evaluated. It is shown that the SSBI mitigation technique enables distances up to 180 m when

dispersion is not compensated, showing the potential to be implemented in intra data-centre (DC) networks,

when the signal mean optical power is much higher than the carrier mean optical power, and when the SSBI

estimation is not corrupted by electrical noise. The results also show that in systems with full dispersion

compensation, a signiﬁcant performance improvement is achieved by the proposed SSBI mitigation approach,

enabling higher connection lengths.

1 INTRODUCTION

Optical ﬁbre networks are facing an exponential

growth on the capacity demands (Cisco, 2018). Space

division multiplexing (SDM) technology is pointed

out as a solution to overcome the so-called capac-

ity crunch (around 100 Tb/s per ﬁbre) (Butler et al.,

2017). Short-reach connections, such as intra data-

centre (DC) connections, are experiencing continuous

data trafﬁc increase, and this phenomenon brings con-

strains such as space limitations and the need to max-

imize the throughput of each connection (Kachris and

Tomkos, 2012). Multicore ﬁbres (MCFs) are one of

the new technologies brought by SDM. This technol-

ogy can enhance the network capacity by transmit-

ting data simultaneously on a high number of ﬁbre

cores. Recent experiments on MCFs achieved capaci-

ties of up to 2.05 Pb/s using a few-mode MCF (Soma

et al., 2015) and 2.15 Pb/s using a 22 core homoge-

neous single-mode (SM) MCF (Puttnam et al., 2015),

https://orcid.org/0000-0002-6841-4061

https://orcid.org/0000-0001-7382-0737

https://orcid.org/0000-0002-4514-2737

well beyond the fundamental limit of SM single core

ﬁbres. In MCFs, the ﬁbre capacity can be theoreti-

cally increased by N times, where N represents the

number of independent cores that are incorporated in

the same ﬁbre cladding. The number of cores, and

the core-to-core distance, will determine the inter-

core crosstalk (ICXT) levels. In the particular case of

weakly-coupled homogeneous MCFs, different cores

of the same ﬁbre present identical core properties and,

thus, the ICXT arises due to the coupling between

cores. Nevertheless, in short-reach networks, the im-

pact of the crosstalk on the system is reduced due to

the small accumulated ICXT. Therefore, this type of

MCF presents high potential to be employed in DC

connections (Hayashi et al., 2019).

In short-reach networks, low cost and complexity

are crucial requirements. Thus, direct-detection (DD)

receivers are preferable for these networks since they

are cheaper, smaller and less power hungry than co-

herent detection systems (Cartledge and Karar, 2014).

Still, these receivers cause, due to the square law de-

tection of a single photodiode, performance degrada-

tion induced by signal-signal beat interference (SSBI)

(Ishimura et al., 2019). The SSBI can be mitigated

122

M., T., Alves, T. and Cartaxo, A.

Short-reach 200 Gb/s SDM Network Employing Direct-detection and Optical SSBI Mitigation.

DOI: 10.5220/0010343101220130

In Proceedings of the 9th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2021), pages 122-130

ISBN: 978-989-758-492-3

using complex digital signal processing (DSP) tech-

niques (Ju et al., 2015), (Nezamalhosseini et al.,

2013), which are highly undesired in short-reach net-

works due to the high costs associated.

In this work, a new transmission scheme based on

MCFs and with high potential to mitigate the SSBI

with reduced complexity is proposed. This scheme is

based on transmitting the virtual carriers, that are used

to assist the detection, and the data signals in sep-

arated cores of the weakly-coupled MCF. With this,

estimation and mitigation of the SSBI at the receiver

without resorting to complex DSPs can be achieved.

This low complexity SSBI mitigation technique may

be of particular interest for DD systems where SSBI

mitigation is required at the receiver side prior to

employ digital techniques to mitigate the impact of

the dispersion impairments on the detected data sig-

nal. Also, with the proposed transmission scheme,

in which the carriers are transmitted independently

from the data signals, additional cost savings may be

achieved in bidirectional networks. This occurs be-

cause local lasers can be replaced by a single optical

comb with carriers being distributed as optical seeds

in a single MCF core along the whole network.

The performance of the proposed transmission

scheme is assessed by evaluating the impact of the

combined effect of the skew and the laser phase noise

on the performance of MCF-based short-reach net-

works. Also, the improvement of the performance

due to the SSBI mitigation technique is studied, which

can afterwards potentiate the use of simple DSP tech-

niques to electronically remove the dispersion effects

on the signal.

2 PROPOSED TRANSMISSION

SCHEME

2.1 Concept Description

Figure 1: Equivalent model of the MCF-based transmission

scheme with DD and SSBI removal.

Figure 1 presents the proposed MCF-based transmis-

sion scheme with one core (core 1) dedicated to sup-

port only the carriers transmission. The optical trans-

mitter (TX) consists in two branches. One branch

generates the optical carrier to be transmitted in core

1. This optical carrier is used at the receiver (RX)

side to assist the detection. The other branch gener-

ates the optical data signal with carrier suppression.

The outputs of the TX are transmitted into two differ-

ent cores of the MCF. When transmitting data in two

different cores, the signals (carrier and data signal)

suffer different delays. The relative delay between

the signals at the cores output, i.e., the skew, is cru-

cial when adding up the signals. This occurs because

the combined effect of the skew and the laser phase

noise may lead to destructive interference at the re-

ceiver side causing performance degradation.

After ﬁbre propagation, the DD-based RX pho-

todetects the signals performing the optical to elec-

trical conversion. In the positive-intrinsic-negative

(PIN) photodiode A, after adding the carrier and the

data signal in the RX input, the photodetection of the

resulting optical signal is performed. As a result of

the photodetection process, an unwanted SSBI term

is originated along with the wanted data signal. In

PIN B, the carrier suppressed data signal, that the pro-

posed transmission scheme allows to obtain, is pho-

todetected. This allows to estimate separately the

SSBI component. Then, this SSBI term is used to

subtract the SSBI term originated in PIN A (where the

desired signal is located), obtaining, ideally, a SSBI

free signal.

2.2 Transmission Scheme Limitations

The performance of the proposed transmission

scheme strongly depends on the delay suffered by

the signals in different ﬁbre cores, and in particular

the skew between the two cores. The relation be-

tween the skew and the laser phase noise coherence

time, t

, can provide a solid estimation about how

the skew may impact the system performance due to

phase noise. The laser phase noise coherence time is

given by (Goodman, 2015)

π · ∆ν

(1)

where ∆ν

is the laser linewidth. If the laser elec-

trical ﬁeld at two different time instants, t and t + T,

is considered, the coherence time can be deﬁned as

the maximum T value for which the phase difference

between the electric ﬁeld at the two instants remains

predictable. If t

is much longer than the skew, the

degradation caused by the phase noise is small since

the relative delay time between the two cores leads to

a situation where, at a given time instant, the phase

noise at the output of the two cores has similar am-

plitudes. In this case, when the carrier is added to the

Short-reach 200 Gb/s SDM Network Employing Direct-detection and Optical SSBI Mitigation

123

signal, no destructive interference occurs. Contrarily,

if t

is not much longer than the skew, the degradation

caused by the laser phase noise can be signiﬁcant.

Since the DD receiver photodetects each signal

through a PIN photodiode, the current associated with

the carrier-added data signal (output of PIN A), con-

sidering the skew and the laser phase noise, can be

written as (for a PIN responsivity of 1 A/W):

(t) = |A

jφ

(t)

+ s(t − T )e

jφ

(t−T)

(2)

where A

is the carrier amplitude, s(t) is the data sig-

nal at the MCF output, T is the skew between cores

and φ

(t) is the laser phase noise.

Based on equation 2, the aforementioned relation

between t

and the skew can be mathematically ex-

pressed. If the skew is much shorter than t

, then

jφ

(t−T)

≈ e

jφ

(t)

(3)

resulting for the current at the output of PIN A:

(t) ≈





+ s(t − T )



· e

jφ

(t)



+ s(t − T )



(4)

In this case, the received signal is not impaired

by the laser phase noise. As opposed, in the sce-

nario where t

is not sufﬁciently longer than the skew,

equation 3 is not veriﬁed and so the phase noise is

not eliminated when the signal is photodetected, caus-

ing phase-to-intensity noise conversion, which will be

analysed further on.

One of the main goals of this work is to identify

the conditions under which a negligible degradation

due to the combined effect of the skew and the laser

phase noise is obtained. When the degradation due to

the skew and the laser phase is negligible, the current

at the output of PIN A is given by (from equation 4,

for a PIN responsivity of 1 A/W):

(t) = A

+ 2 · A

· ℜ{s(t − T )} + |s(t − T )|

(5)

In this scenario, there are two major impairments that

can cause performance degradation: the SSBI (last

term) and the chromatic dispersion. The delay, T , is

due to the propagation and can be compensated with-

out causing distortion. The dispersion effect is repre-

sented in the term s(t) as follows

s(t) = s

(t) ∗ h(t) (6)

where s

(t) is the data signal at the MCF input, ”∗”

is the convolution operator and h(t) is the impulse

response of the SM ﬁbre, which contains the atten-

uation and dispersion effects of the MCF. It is pos-

sible to electronically remove the dispersion effects

(from Equation 5) through DSP techniques. How-

ever, this can be performed only after the SSBI term

is removed. This SSBI removal process is analysed in

Section 4.

3 SYSTEM SETUP

In this work, a 200 Gb/s polar nonreturn-to-zero

(NRZ) signal is considered. A continuous wave (CW)

laser generates the optical carrier, affected by the

phase noise. The model used to describe the laser

phase noise is a Wiener process (Peng, 2010) charac-

terized by the laser linewidth (∆ν

). The NRZ signal

is converted into the optical domain using a single-

arm chirpless Mach-Zehnder modulator (MZM), bi-

ased at the minimum bias point in order to generate

the optical signal with suppressed carrier. For the

optical transmission, a two core MCF is considered.

One core is used to transmit the 200 Gb/s NRZ signal

and the other core is used to transmit the carrier. In the

case of a wavelength division multiplexing (WDM)

system, the optical carriers used to assist the detec-

tion of the different NRZ signals would be all trans-

mitted in the same core. The selection of the proper

carrier at the RX side is performed after carrier de-

multiplexing. Linear propagation along each SM core

is assumed, with a dispersion parameter of approx-

imately 18 ps/nm/km and a ﬁbre attenuation coefﬁ-

cient of 0.21 dB/km.

Since the RX is based on DD, a single photodetec-

tor is considered for each branch. The photodetection

is performed by PIN photodetectors with a responsiv-

ity, R

, of 1 A/W, followed by an electrical ﬁltering

using a third-order Bessel ﬁlter with a -3 dB cutoff

frequency of 160 GHz.

In this IM-DD system, the evaluation of the degra-

dation induced by the system impairments is ﬁrstly

done by assessing the eye-opening penalty (EOP),

which gives a fast estimation of the signal quality, that

can reﬂect the effects of signal distortion and eye clo-

sure due to the noise. The EOP is given by

EOP = −10 · log



− I



[dB] (7)

where I

represents the lowest current level associ-

ated with bits 1 and I

represents the highest current

level associated with bits 0, both values being taken at

the time instant for which the eye-diagram opening is

maximum. I

is the detected current corresponding

to the average power given by

= (P

+ P

) · R

(8)

where P

is the NRZ signal mean optical power and P

is the carrier mean optical power, both powers being

calculated at the RX input. As 2 I

is the greatest eye-

opening that can be obtained, when I

− I

is equal to

the EOP is 0 dB.

To further assess the system performance, by

quantifying the impact of the skew, the laser phase

noise and the dispersion, the bit error ratio (BER)

PHOTOPTICS 2021 - 9th International Conference on Photonics, Optics and Laser Technology

124

- calculated through the direct-error counting (DEC)

method - is used as ﬁgure of merit. The BER is eval-

uated before and after SSBI removal results, in order

to identify the scenarios in which the proposed SSBI

mitigation technique is of particular interest, i.e, when

the technique provides system performance improve-

ment.

4 RESULTS AND DISCUSSION

In this section, the impact of the skew, the phase-to-

intensity noise conversion and the chromatic disper-

sion is assessed. This is accomplished by analysing

EOP and BER results, evaluating the system perfor-

mance and the improvement obtained by employing

the proposed transmission scheme and SSBI mitiga-

tion approach.

4.1 Skew Impact on the

Phase-to-intensity Noise Conversion

Figure 2 shows the impact of the skew on the EOP

of the detected NRZ signal and how the results are

affected by the laser linewidth. The results of Figure 2

consider only the skew and phase noise impairments.

A MCF length of 2 km is considered.

Figure 2: EOP as a function of the skew, for different laser

linewidths.

The null linewidth results offer a baseline com-

parison, allowing to quantify in terms of EOP the im-

pact of the skew compared to the ideal case, where the

skew does not impair the system. Different linewidths

impact the system performance differently for the

same skew. For a skew of 30 ns, linewidths of 500

kHz and 1 MHz show a EOP above 1.5 dB, while the

EOP for a linewidth of 100 kHz is not signiﬁcantly

affected by any of the tested skew values.

4.2 Assessment of Beneﬁcial Conditions

for SSBI Removal

First and foremost, it is needed to determine un-

der which conditions the proposed SSBI removal ap-

proach is effective. The SSBI component is gener-

ated from the NRZ signal after photodetection. When

the carrier power is much higher than the NRZ sig-

nal power, the performance is impaired mainly by the

noise levels and the SSBI term is negligible. Other-

wise, the system performance is dominantly impaired

by the SSBI. From Equation 5, we conclude that the

impact of the SSBI term depends on the ration be-

tween the carrier and the signal mean optical power.

Figure 3 shows the EOP as a function of the rela-

tive power, P

, deﬁned as the ratio between the signal

mean optical power (P

) and the carrier mean opti-

cal power (P

). The results were obtained with and

without considering electrical noise, and for a P

of 0

dBm.

Figure 3: EOP as a function of the relative power, before

and after SSBI removal, with and without electrical noise.

When the relative power is lower than 0 dB, the

EOP reﬂects the eye-closure due to the lowering of

values. It is also seen that under these conditions,

removing the SSBI can worsen the results, in the pres-

ence of electrical noise. This happens because when

subtracting the output of PIN A with the output of PIN

B, in fact two random and independent noise compo-

nents with the same power are being added. On the

other side, for P

higher than 0 dB, the SSBI removal

may show improvements. In this study, for these P

values, the SSBI term is dominant over the electri-

cal noise power considered, and so the electrical noise

does not affect the retrieved signal in terms of the EOP

in a visible manner.

Other study is conducted to understand the im-

pact of the dispersion effects on the performance of

the system operating at the identiﬁed ”ideal point”,

where the signal and carrier mean optical powers are

Short-reach 200 Gb/s SDM Network Employing Direct-detection and Optical SSBI Mitigation

125

the same. Figure 4 shows the EOP as a function of

the MCF length, for different laser linewidths, with

and without dispersion considered in the ﬁbre and for

a P

of 0 dB. The walk-off is given as a input param-

eter to the system instead of the skew.

(a)

(b)

Figure 4: EOP as a function of the MCF length for differ-

ent laser linewidths, with and without dispersion, for: (a) a

walk-off of 1 ns/20 km and (b) a walk-off of 10 ns/20 km.

Results of Figure 4 show that the dispersion

clearly constrains the system even if working in the

ideal situation (where P

is 0 dB), showing a mini-

mum of 2.5 dB penalty for all laser linewidths for dis-

tances higher than 200 m. For both walk-off values

studied, the dispersion is the main impairment respon-

sible for the degradation, but it is noticeable that in-

creasing the walk-off increases slightly the EOP. This

happens because increasing the walk-off originates a

higher skew for a given distance, and then the com-

bined effect of the skew and the increasing laser phase

noise (broader linewidths) leads to an additional per-

formance penalty due to phase-to-intensity noise con-

version. From the results in Figure 4, it is possible to

conclude that even if considering the P

that gives the

best performance (based on Figure 3), the system is

impaired by the dispersion, and this effect overcomes

all other effects considered, thus being the most sig-

niﬁcant. For this reason, if system performance im-

provement is required, dispersion needs to be fully

compensated.

As shown in equation 5, for the dispersion to be

compensated electronically at the RX side, the SSBI

term needs to be successfully mitigated. As it was

seen in Figure 3, an efﬁcient SSBI mitigation only

occurs for high P

. A similar study to the one pre-

sented in Figure 3, now considering the BER as the

performance metric, is carried out. Electrical noise

is again considered, but now it is categorised by the

signal-to-noise ratio (SNR), deﬁned as:

SNR =

tot

noise

(9)

where P

tot

is the carrier-added NRZ signal mean

power at the output of PIN A and P

noise

is the noise

power. From equation 9, given a certain SNR value

under test and the mean power of the received signal

imposed by P

, P

noise

is determined and used to sim-

ulate the noise component added after photodetection

on both branches (PIN A and B).

Figure 5 shows the BER as a function of the P

before and after SSBI removal, for different SNR val-

ues. These results were obtained without consider-

ing skew, laser phase noise and dispersion, in order to

ﬁrstly assess the SSBI mitigation technique effective-

ness in a more favorable scenario, where only electri-

cal noise is considered. Under these conditions, the

SSBI mitigation is not impaired by the phase noise

nor the dispersion. Figure 5 shows that the SSBI mit-

igation for values of P

under 15 dB only degrades

the BER, meaning that, for the SNR values consid-

ered, the SSBI estimation is corrupted by the elec-

trical noise and, thus, no effective SSBI mitigation

is achieved. In accordance with the results shown in

Figure 3, the SSBI removal is not effective for these

tested values.

Figure 5: BER as a function of the relative power before

(continuous line) and after (dashed line) SSBI removal for

a SNR of: 12 (blue), 13 (red), 14 (yellow), 15 (purple) and

16 (green) dB.

In Figure 6, an extension of Figure 5 to relative

powers between 15 and 25 dB is shown. With this,

system situations in which the SSBI is more powerful

are tested. The sub-ﬁgures of Figure 6 consider two

different sets of SNR values that were tested. When

PHOTOPTICS 2021 - 9th International Conference on Photonics, Optics and Laser Technology

126

the SNR is increased, converging to a ”no-electrical

noise” scenario, a BER improvement is obtained af-

ter SSBI mitigation. This indicates that, under these

conditions (SSBI mitigation not corrupted by electri-

cal noise), it is possible to effectively estimate and

mitigate the SSBI. Given that, and bearing in mind

that further on it may be wanted to compensate the

dispersion effect on the signal, we need to operate in

the referred P

and SNR values, since the dispersion

compensation process directly depends on a success-

ful SSBI mitigation.

(a)

(b)

Figure 6: BER as a function of the relative power before

(continuous line) and after (dashed line) SSBI removal, for

two different sets of SNR values.

Following the results obtained in Figure 6, the

evaluation on the SNR improvement before and af-

ter SSBI removal may be done. This is performed for

a targeted BER of 10

−3

, and only considering the ef-

fect of electrical noise (no skew nor dispersion effects

are considered). The SNR improvement required is

deﬁned as the difference, in dB, between the SNR

needed to obtain a BER of 10

−3

before SSBI removal

and after SSBI removal.

Figure 7 shows the SNR improvement required as

a function of the signal mean optical power, for differ-

ent carrier mean optical powers. Figure 7 shows once

more that the SSBI removal is only effective when the

NRZ signal mean optical power is much higher than

the carrier mean optical power. Additionally, results

of Figure 7 show also that the key factor to obtain a

certain SNR improvement is the power relation (P

)

between the data signal and the carrier, and not the

absolute mean optical power values of these signals.

As an example, 6 dB improvement on the required

SNR to achieve a BER of 10

−3

may be achieved with

= 0 dBm and P

= 18 dBm, or with P

= -10 dBm

and P

= 8 dBm. For this reason, the studies presented

in the following subsections are realized considering

= 18 dBm and P

= 0 dBm.

Figure 7: SNR improvement required as a function of the

NRZ signal mean optical power, for different carrier mean

optical power levels: 0 dBm (blue), -5 dBm (red), -10 dBm

(yellow) and -15 dBm (purple).

4.3 Skew Impact on the System

Performance

In this subsection, the degradation induced in the BER

by the combined effect of the skew and the laser phase

noise is studied for different laser linewidths. As a ref-

erence, Table 1 indicates the coherence time of each

laser linewidth considered in this subsection.

Table 1: Coherence time for different laser linewidths.

Laser linewidth [MHz] t

[ns]

0.1 3183

0.5 636.6

1 318.3

5 63.66

Figure 8 shows the BER before and after SSBI re-

moval as a function of the SNR with P

= 18 dB, for

different skew values and for the laser linewidths in-

dicated in Table 1.

Figure 8 (a) shows that, for a laser linewidth of

100 kHz, the impact of the skew on the system per-

formance is negligible. By analysing the t

(see Table

1) obtained for a 100 kHz linewidth, it can be seen that

the coherence time is much higher than the skew val-

ues under test (approximately 100 times higher than

30 ns). So, the skew effect on the phase-to-intensity

noise conversion is low. Figure 8 (a) enables also to

conclude that for the 100 kHz laser linewidth, a BER

Short-reach 200 Gb/s SDM Network Employing Direct-detection and Optical SSBI Mitigation

127

of 10

−3

is reached for all skew values considered, for

a SNR of 25 dB, when SSBI mitigation is employed.

When increasing the laser linewidth and thus de-

creasing the respective t

, the system performance be-

comes more sensitive to the skew. As it can be seen

in Figure 8 (b) (linewidth of 500 kHz), the curves cor-

responding to the SSBI removal results start to show

the effect of the laser phase noise. When increasing

the skew, it can be seen that the BER starts to increase

as well. Still, the coherence time corresponding to the

500 kHz linewidth is 21 times higher than the longest

skew considered. So, although the effects are felt,

they are not excessively degrading the system perfor-

mance. For this laser linewidth, a SNR of at least 26

dB guarantees a BER lower than 10

−3

for all skew

values tested.

Figure 8 (c) shows that the skew effect on the

phase-to-intensity noise conversion starts to become

signiﬁcant for a linewidth of 1 MHz, degrading the

system performance. For a skew of 30 ns, the coher-

ence time corresponding to 1 MHz laser linewidth is

only 10 times longer than the skew. So, it is expected

(and proven by the results), that the degradation due

to the phase noise severely impacts the system. For

a skew of 30 ns, it is not possible to reach a BER

lower than 10

−3

after SSBI mitigation. In contrast,

the laser linewidth of 1 MHz can provide a BER lower

than 10

−3

after SSBI removal, for SNR values of or

above 26 dB, up to a skew of 15 ns. It is worth noting

that for a skew of 15 ns, the results are identical to

the results obtained for a skew of 30 ns and the 500

kHz linewidth. For both these cases, the relation be-

tween the skew and the laser’s coherence time is the

same, being the coherence time 21 times higher than

the skew.

Figure 8 (d) shows the BER results for a laser

linewidth of 5 MHz. Results of Figure 8 (d) show that

the 5 MHz linewidth has very low tolerance to the

increasing skew values. For the results presented in

Figure 8 (d), the beneﬁts of employing the proposed

SSBI mitigation approach occur only for a skew of 1

ns. For 15 and 30 ns skew, the SSBI mitigation tech-

nique is not effective due to high levels of phase-to-

intensity noise conversion. Bearing in mind the co-

herence time corresponding to 5 MHz linewidth, and

taking into account the results described throughout

this subsection, it is possible to deduce that a laser

with a linewidth of 5 MHz only tolerates a degrada-

tion caused by phase-to-intensity noise conversion for

a maximum skew of 3 ns. In this case, the coher-

ence time corresponding to the 5 MHz linewidth is 21

times higher than the 3 ns skew, which, as seen be-

fore, guarantees a BER under 10

−3

for SNR values

exceeding 25 dB.

(a)

(b)

(c)

(d)

Figure 8: BER as a function of the SNR before (continuous

line) and after (dashed line) SSBI removal considering dif-

ferent skew values, for a laser linewidth of: (a) 100 kHz, (b)

500 kHz, (c) 1 MHz and (d) 5 MHz.

4.4 Dispersion Impact on the System

Performance

In this subsection, the impact of the chromatic disper-

sion on the system performance is evaluated, in the

PHOTOPTICS 2021 - 9th International Conference on Photonics, Optics and Laser Technology

128

presence of skew and laser phase noise. From the

study of subsection 4.3, the SNR values required for a

BER lower than 10

−3

were identiﬁed. So, this study is

performed for a SNR of 26, 27 and 28 dB; for lower

SNR values, the BER after SSBI removal is higher

than 10

−3

, showing no interest for the scenarios un-

der evaluation. The laser linewidths chosen for this

study are 100 kHz and 5 MHz.

Figure 9 demonstrates the dispersion and skew im-

pact on the BER. Each set of curves are composed by

3 lines, which represents a SNR of 26 dB (higher BER

of each set), 27 dB and 28 dB (lowest BER of each

set). The results were obtained in the following con-

ditions: before SSBI removal with dispersion (con-

tinuous line) and without dispersion (dashed and dot-

ted line); after SSBI removal with dispersion (dashed

line) and without dispersion (dotted line), in order to

evaluate how differently each impairment impacts the

system. A walk-off of 10 ns/20 km was considered.

Figure 9 (a) shows the BER as a function of the

MCF length for a laser linewidth of 100 kHz. Com-

paring with Figure 9 (b), which shows the 5 MHz

linewidth results, it is possible to conclude that very

similar results are obtained. In presence of disper-

sion, both linewidths surpass a BER of 10

−3

for ﬁ-

bre lengths longer than 180 m after SSBI removal

for the most favorable case (SNR of 28 dB). In con-

trast, the results that emulate an ideally compensated

dispersion environment (dotted and dashed and dot-

ted lines - null total dispersion) show that the BER

remains practically unchanged for the tested length.

Under these conditions, the phase-to-intensity noise

conversion is the only effect impacting the system

performance, and the results in Figure 9 (a) and (b)

show that, for the tested walk-off and ﬁbre lengths,

the system performance is weakly affected, regardless

the analysed laser linewidth. As an example, for 300

m of MCF length, we have a skew of 0.15 ns, which,

as has been shown in subsection 4.3, is not enough

to degrade the system performance due to phase-to-

intensity noise conversion. These results show that,

in presence of dispersion, the results before and after

employing SSBI mitigation rapidly converge, as the

ﬁbre chromatic dispersion signiﬁcantly degrades the

system performance. However, when the dispersion is

compensated, promising results can be achieved when

SSBI removal is performed. In order to compensate

for the dispersion, the SSBI component needs to be

suppressed. This can successfully occur when a high

is used, since a BER performance improvement be-

fore and after SSBI removal is only achieved under

those conditions.

Figure 9 results help to emulate a real-use sce-

nario, where dispersion and walk-off are inherent ef-

(a)

(b)

Figure 9: BER as a function of the MCF length, before

SSBI removal with dispersion (continuous line) and without

dispersion (dashed and dotted line), and after SSBI removal

with dispersion (dashed line) and without dispersion (dotted

line), for a laser linewidth of: (a) 100 kHz and (b) 5 MHz.

fects of the ﬁbre. When dispersion compensation is

not employed, the maximum link length reached is

between 100 and 180 m (for the considered SNR val-

ues), hence being suitable for intra DC connections.

Longer connections may be achieved, reaching inter

DC lengths, by utilising dispersion mitigation tech-

niques. However, that study is out of the scope of this

work.

5 CONCLUSIONS

An innovative transmission scheme has been pro-

posed, where the 200 Gb/s NRZ signal and the vir-

tual carrier are transmitted in separate cores of a MCF,

targeting a low complexity SSBI mitigation approach

at the RX side to improve the system performance

of short-reach MCF-based networks employing DD

receivers. It has been shown that the combined ef-

fect of the skew and the laser phase noise affects the

signal quality. Systems employing lasers with higher

linewidths become more sensitive to the skew, limit-

ing further the system performance due to phase-to-

intensity conversion. It has been also shown that the

proposed SSBI mitigation technique is effective only

Short-reach 200 Gb/s SDM Network Employing Direct-detection and Optical SSBI Mitigation

129

when a high P

is considered, and for a low electrical

noise environment.

For systems not impaired by the dispersion, a BER

improvement (to values lower than 10

−3

) after SSBI

removal is obtained, for a P

of 18 dB and a skew of

1 ns, and for a laser linewidth up to 5 MHz. Also, a

BER lower than 10

−3

is achieved for a skew up to 15

ns and a laser linewidth of 1 MHz. It has been shown

that the dispersion rapidly degrades the received sig-

nal quality, exceeding a BER of 10

−3

for a MCF

length longer than approximately 180 m. The re-

sults indicate that, in absence of dispersion compensa-

tion, the proposed transmission scheme show poten-

tial to be employed in intra DC connections. Never-

theless, the higher potential of the proposed transmis-

sion technique is achieved for systems in which the

dispersion effect is compensated electronically at the

RX side. For systems with full dispersion compensa-

tions, the results show a signiﬁcant performance im-

provement achieved by the SSBI mitigation approach

employed, showing great interest for high capacity

short-reach SDM networks.

ACKNOWLEDGEMENTS

This work was partially funded by FCT/MCTES

through national funds and when applica-

ble co-funded EU funds under the project

UIDB/EEA/50008/2020 and project Dig-

Core/UIDP/50008/2020.

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