Lecture: 10 New Trends in Optical Networks

Ajmal Muhammad, Robert ForchheimerInformation Coding Group

ISY Department

Outline

Challenges Multiplexing Techniques Routes to Longer Reach

Distributed amplification

Hollow core fibers

Routes to Higher Transmission Capacity

Space division multiplexing (SDM)

The Challenge

Traffic grows exponentially at approximately 40% per year Optical system capacity growth has been approximately 20%

per year In less than 10 years, current approaches to keep up will not

be sufficient

Main physical barriers:

Channel capacity (Shannon) + available optical bandwidthTransmission fiber nonlinearities (Kerr)

Capacity Limits

Signal launch power [dBm]

Ref:IEEE, vol.100, No.5May 2012

Noise

Fiber nonlinearity

… Moore’s Law for Ever… ?

Courtesy ofPer O. Andersson

Multiplexing Techniques

100G Fiber Optic Transmission :: DP-QPSK

DP-QPSK: Dual Polarization Quadrature Phase Shift Keying

DP-QPSK is a digital modulation technique which uses two orthogonal polarization of a laser beam, with QPSK digital modulation on each polarization

QPSK can transmit 2 bits of data per symbol rate, DP-QPSK doubles that capacity

For 100Gbps, DP-QPSK needs 25G to 28G symbols per second. Electronics have to work at 25 to 28 GHz

BPSK- Binary Phase Shift Keying

BPSK transmits 1 bit of data per symbol rate, either 1 or 0

QPSK- Quadrature Phase Shift Keying

Use quadrature concept, i.e., both sine and cosine waves to represent digital data

Two BPSK used in parallel

Cosine wave

DP-QPSK in Fiber Optic Transmission

DP-QPSK transmits 4-bits of data per symbol rate

Laser source is linearly polarized

Cosine wave

Sine wave

Vertical polarized

Horizontal polarized

Assume horizontal polarized laser source

Data stream

Outline

Challenges Multiplexing Techniques Routes to Longer Reach

Distributed Amplification

Hollow Core Fibers

Routes to Higher Transmission Capacity

Space Division Multiplexing (SDM)

Routes to Longer Reach

Deal with low SNR Advance FEC More power efficient modulations format

Maintain a high SNR Ultralow noise amplifiers Distributed amplification

Deal with more nonlinearities Digital back-propagation

Reduce the nonlinearity Install new large-area or hollow-core fibers

Distributed Amplification

Raman pump power= 700 mWEDFA gain=20 dB, NF=3 dB

High SNR but will excite nonlinearities

SNR degrades due to shot noiseno issues of nonlinearity

Ideal distributed amplification (constant average signal power in the entire span)

PSA: Phase sensitive amplifierwith noise free gain medium

Courtesy:Peter Andrekson, Chalmers Uni.

New Telecom Window at 2000 nmHollow-Core Fibers

Guiding by Photonic Bandgap Effect

Key potential attributes:Ultra-low loss predicted near 2000nm (not single mode operation) (~ 0.05 dB/km predicted opt. Express, Vol.13, page 236, 2005)Very wide operating wavelength range (700 nm)Very small non-linearity: 0.001 x standard SMFLowest possible latencyDistributed Raman amplification may be challenging, however.

Hollow-Core Fiber :: SNR

Comparison of ultralow loss (0.05 dB/km) hollow-core fiber and EDFAIn conventional fiber (0.2 dB/km)

Courtesy:Peter Andrekson, Chalmers Uni.

Hollow-Core Fiber :: SNR

Comparison of ultralow loss (0.05 dB/km) hollow-core fiber, EDFA and distributed Raman amplification in conventional fiber (0.2 dB/km)

Span loss: 20 dB Backward Raman (100 km)Bidirectional Raman (100 km) (10 + 10 dB)

A low-loss hollow core fiber with EDFA spacing of 400 km performs similar to backward pumped Raman system with 100 km pump spacing

Courtesy:Peter Andrekson, Chalmers Uni.

Spectral Efficiency Impact of Nonlinear Coefficient

+ 2.2 b/s/HZ for each X 10Gamma reduction

Ref: R-J. Essiambre proc. IEEEvol. 100, p. 1035, 2012

Thulium-Doped Silica Fiber Amplifiers (TDFA)at 1800-2050 nm

• Suitable with low-loss hollow core transmission fiber• Very wide operation range (> 200nm)• Noise figure ~ 5 dB• Laser diode pumping at 1550 nm• 100 mW saturated output signal power

ECOC 2013 Paper Tu.1.A.2

Outline

Challenges Multiplexing Techniques Routes to Longer Reach

Distributed Amplification

Hollow Core Fibers

Routes to Higher Transmission Capacity

Space Division Multiplexing (SDM)

Routes to Higher Transmission Capacity

CLB= N * B * log2(1+SNR)

Overall transmission capacity:

Available optical bandwidth (B) New amplifiers Extend low-loss window

X

Spectral efficiency (bit/sec/Hertz) Electronics signal processing Low nonlinearity

X

Number of channels (N) Install new multi-core/multi- mode fibers

Typical Attenuation Spectrum for Silica Fiber

Only 8-10 % is utilized in C bandWith SE of 10 per polarization a fiber can support well over a Pb/s

Space Division Multiplexing (SDM)

Inter-Core Crosstalk (XT)

Inter-Core Crosstalk (XT)

From WDM Systems to SDM & WDM Systems

Flexible upgrade:Add transponder in lambda and M

State of the Art Systems