This document discusses advances in optical fiber transmission capacity and techniques to increase capacity. It examines using higher order quadrature amplitude modulation (QAM) formats but these have reduced reach due to nonlinearity. Using both erbium-doped fiber amplifiers and Raman amplifiers can help. The document also tests transmitting 1 Tb/s superchannels using different QAM formats over 762 km of fiber. It achieves the longest distance to date for PM-64QAM transmission at this data rate and symbol rate. Key results include transmitting PM-16QAM over 1571 km, PM-32QAM over 1065 km, and PM-64QAM at the highest symbol rate over 750 km.

1. Stefano Calabr`o, Erik de Man, Uwe Feiste, Antonio Napoli, Marc Bohn, Ginni Khanna, Norbert Hanik, Claude Le
Bou¨ett´e, J´er´emie Jauffrit, Sylvain Bordais, Celine Andr´e, Christian Dourthe, Bruno Ragu´en`es, Chigo M.
Okonkwo, and A. M. J. Koonen.
MADE BY : Ammar Abdul Jalil & Zainab Jassim

2. THE recent advances in social networking, cloud services and media content
quality (4k/8k video) is resulting in exponential growth of data traffic.
capacity of optical fibers is rapidly approaching its limits expected over next five
years.
In order to economically enhance the capacity of deployed fiber networks, high-
order quadrature amplitude modulation (QAM) formats are considered.
Achievable transmission reach of high capacity QAM formats is however limited
due to reduced nonlinear tolerance and higher required optical signal to noise ratio
(OSNR)
So these days employing both erbium doped fiber amplifier (EDFA) and Raman
amplifier (RA) have gained significant interest.

3. Some needed expressions:
forward error correction (FEC):or channel coding is a technique used for controlling
errors in data transmission over unreliable or noisy communication channels.
Bit error rate (BER):is the number of bit errors per unit time.
pre-FEC BER: are the bit errors caused by attenuation, ageing, temperature changes of
the optical fiber.
PRE-FEC: indicates that the signal on the optical fiber is FEC encoded.
long-haul transmission can be achieved by utilizing a fraction of QAM formats’
capacity for stronger FEC codes with overhead (OH) as high as 28%.
In this context, soft decision (SD-) low density parity check (LDPC) FEC codes
requiring pre-FEC BER as high as 9.88 × 10−2 have been experimentally
demonstrated.

4. Flex-grid networks are specially suited for future
generation transponders aiming for data rates as high as
1 Tb/s achievable through superchannels structures.
In order to reduce cost per transmitted bit, reduction in
number of subcarriers per terabit is desired which
necessitates higher symbol rates and higher order QAM
format.

5. G.652 standard single mode fiber used with total length of fiber between Lyon and
Marseille was 381 km consisting of five spans, Both transmitter and receiver were set
up in Lyon and an optical loopback, without regeneration , was established in
Marseille . Consequently, the transmitted signal passed through a total of 762 km of
dispersion uncompensated link with a total of ten spans. The span attenuation was
compensated completely by hybrid EDFA-Raman amplification for eight spans. The
two shortest spans of 60 km (Link 3) were purely EDFA amplified. The total
accumulated group velocity dispersion (GVD) at 1550 nm was ∼13 000 ps/nm.
The combined pump power was ∼500 mW with an on/off gain of 12 dB.
in order to comply with the required gain range of 10–25 dB and maximum output
power of 21 dBm , two fixed gain amplifiers combined with variable optical
attenuators where used.

6. The subcarriers under test and rest of the WDM channels were generated by two separate
setups.
DAC 1, operating at 64 Gsamples/s, was used for neighboring channels while DAC 2,
operating at 88 Gsamples/s, was used to generate the subcarrier under test. In order to
generate the even and odd neighboring optical m QAM signals, two single polarization
LiNbO3 IQ- modulators were used, which were driven by the four amplified electrical
signals from DAC 1. DAC 1 outputs independent waveforms on each channel modulating
even and odd lasers
by independent data. Each modulator
was fed in by either sixteen even or
sixteen odd continuous wave, 3-dB
coupler followed by a split-delay-
combine polarization multiplexing
emulation stage resulting in a dual
polarization signal with 32 channels.

7. The symbol rate per subcarrier and the inter subcarrier spectral spacing are
different. However, the total number of subcarriers required per terabit is always
kept at four. Net data-rate of each subcarrier for PM-16QAM and PM-32QAM
configuration was 250 Gb/s and that for PM-64QAM configuration was 300 Gb/s.
𝑹𝒔 =
𝑹𝑩
𝑪
Symble rate =
Net data rate
constellation constrained capacity
The minimum symbol-rate required to achieve a given data-rate is constrained by
the given PM-mQAM
format.

8. A. Quad Subcarrier PM-
16QAM 1.0 Tb/s
Superchannel
• based superchannel was modulated at
41.2 GBd
• Limiting the occupied bandwidth of each
subcarrier to 49.4 GHz. Spectral spacing
between subcarriers was set to 50 GHz
• The terabit superchannel occupied a
spectral width of 200 GHz
• potential C-band capacity of 24 Tb/s.
• BER floor of 2 × 10−5
• pre-FEC limit of ∼3.8 × 10−2 , achievable
at an OSNR0.1 nm of ∼18.4 dB
B. Quad Subcarrier PM-
32QAM 1.0 Tb/s Superchannel
• based superchannel was 33 GBd.
• limit the bandwidth within ≤37.5 GHz
• four flexi-grid slots of 37.5 GHz
• Each superchannel occupied 150 GHz
optical bandwidth
• potentially allowing in total 32 × 1 Tb/s
superchannels over the C-band
• BER floor at 3.5×10−4
• The FEC threshold BER is 3.8 × 10−2 at
OSNR0.1 nm of ∼21.2 dB
C. Quad Subcarrier PM-
64QAM 1.2 Tb/s Superchannel
• based superchannel was 34 GBd
• occupied an optical bandwidth of 37.4
GHz
• spacing between subcarriers was adjusted
to 37.5 GHz
• The superchannel occupied a bandwidth
of 150 GHz
• potential C-band capacity was 38.4 Tb/s
• BER floor of 6 × 10−3
• pre-FEC limit of 5.7 × 10−2 at OSNR0.1
nm of ∼22.4 dB

9. As the symbol-rate of each subcarrier is increased, self phase modulation (SPM) effects become more
significant independent of the order of modulation. Considering cross phase modulation (XPM)
effects, The WDM signal received from the link passed through a waveshaper WS (was used to
multiplex the test channel with the neighboring channels and to balance the optical power),
The filtered signal and local oscillator (LO) then went into an integrated coherent receiver
(ICR))lieaner optical demodulation( which consisted of 2 × 4 90◦ optical hybrids followed by balanced
photodiodes that performed optical to electrical conversion.
receiver processing consisted of resampling the data to a rate of 2 samples per symbol, then equalizer
then located in sample stream then periodically inserted pilot symbols. Finally, the resulting data went
into a soft de-mapper and the FEC decoder, with overhead of 24% for FEC.

11. -For the first time WDM transmission of PM-64QAM over a field deployed G.652 fiber with the highest
symbol-rate over >750 km.
- Maximum transmission distance achieved for PM-16QAM case was ∼1571 km and that for PM-32QAM
case was ∼1065 km.