Phd defense of xin you

Institut Mines-Télécom
EDITE de Paris
Vectorial statistical characterization
of optical signals for very high
speed communication and quantum
communication
Xin You
Advisors: Philippe Gallion & Christophe Gosset
PhD defense

Outline
03/11/2015 PhD defense Xin You2
 Motivations & objectives
 The proposed architecture of pulsed source
 Pulsed source characterization
 Application in optical undersampling
 Conclusion & perspectives

Institut Mines-Télécom03/11/2015 PhD defense Xin You3
Fiber communication technology
Three main technological innovations in the last 30 years
3 K. Kikuchi, Coherent Optical Communications, Springer, 2010
T. Morioka et al., NTT Technical Review 9, 1, 2011.
Time Division
Multiplexing (TDM)
Wavelength Division
Multiplexing (WDM)
IQ digital coherent
communication

TDM
• Several signals
appear on one
channel that is
divided into recurrent
time slots
• Improve time
efficiency

WDM
• Signals are spacing
in frequency
domain to avoid
interference
• Improve spectral
efficiency

Institut Mines-Télécom03/11/20156
Nyquist-shannon
theorem:
Sampling rate at least
2 times larger than the
bit rate
𝑓𝑠 < 2𝑓
𝑓𝑠 = 2𝑓
𝑓𝑠 > 2𝑓Always requires faster
electronics for higher
bit rate
Direct detection

IQ digital coherent communication
Increase the bit rate
per channel while
keeping the channel
spacing
Modulation on multiple
dimensions: amplitude,
phase and polarization

Coherent detection
8
× =
× =
Optical signal
modulated on
both I and Q
Local oscillator In-phase component
Local oscillator phase rotated
by 𝜋/2
Quadrature component
𝐼 = 𝑅𝐸𝐿𝑂 𝐸𝑠 cos 𝜃𝑠 𝑡 + 𝜃 𝑛 𝑡 + 𝑛1(𝑡)
Q= 𝑅𝐸𝐿𝑂 𝐸𝑠 sin 𝜃𝑠 𝑡 + 𝜃 𝑛 𝑡 + 𝑛2(𝑡)

Challenges for the detection technique
The increasing of symbol rate
requires new method for
monitoring signal quality without
increasing the electronics
performance significantly
Sensitiveness to phase and
polarization state of
incoming signal
Post detection Digital Signal
Processing (DSP):
compensation for phase drift
and linear effects

The optical undersampling by employing
ultrashort pulses
10
𝑡 𝑠 = 𝑘𝜏 𝐷 + 𝛿𝑡, 𝑘 ∈ 𝑁, 0 < 𝛿𝑡 < 𝜏 𝐷.
☑Monitoring of high transmission rate
signal
☑Allow low repetition rate sampling
pulses and low performance electronics
☑Bandwidth potentiality limited by the
pulse width and timing jitter
P. Gallion, C. Gosset, X. You, and J. Zhou, Opt Quant Electron
2015
𝜏 𝑠: pulse width;
𝑇𝑠: pulse repetition period;
𝜏 𝐷: bit time;
𝑡 𝑠: moment of sampling
Pulse train repetition rate
smaller and asynchronous with
bit rate
Pulses sample at different
position of symbol
A statistical representation of
signal is obtained (eye diagram)

Quantum key distribution
The quantum key distribution use of single-photon Fock states |𝑛 ,
since Eve can easily steal one photon in a state of multi-photon
The probability of obtaining
n photon for a given average
photon number coherent
state 𝑛 follows Poissonian
distribution
Method for
obtaining single-
photon state
Single photon
source
Not available
Fainted laser
pulses
Balancing between the
rate of key distribution
and the proportion of
multi-photons state Blue curve: probability of
zero photon state;
Red curve: probability of
obtaining multi-photons
state among non-zero
photon state
☑Tunable repetition rate in MHz range
☑High extinction ratio and low intensity
noise

Institut Mines-Télécom PhD defense Xin You12
Object: versatille source for application in optical
undersampling and quantum key distribution
2015/12/2312
An ideal source
Picosecond pulse
width and low timing
jitter for THz
bandwidth potentiality
Tunable repetition
rate in MHz to GHz
range
High extinction ratio
and low intensity noise
Mode-locked laser
Cavity-less source
combing with
nonlinearities in SOA
and fibers

Generation of the native pulse train
Intensity
modulator
Continuous
wave laser
SOA
☑ 40 ps pulse train
obtained at the output
☑ Mechanism of pulse
generation not depend
on the repetition rate
☑ The dynamics of SOA
has a large influence
over the pulse shaping

SOA gain recovery dynamics study
Gain recovery time of a SOA is
defined as the time between 10%
and 90% of saturation during the
recovery of gain
Pump and probe at different
wavelength
The rectangular pump source saturates
the SOA gain and cause the decrease
of small signal gain for probe signal
The probe signal is filtered and the
process of gain recovery is observed
The gain recovery
time determines the
upper limitation of
pulse train repetition
rate to be about 1
GHz

Solitonic pulse compression
16
SSMF
EDFA
𝜕𝐴
𝜕𝑧
+
𝛼
2
𝐴 +
𝑖𝛽2
2
𝜕2
𝐴
𝜕𝑇2
− 𝑖𝛾 𝐴 2 𝐴 = 0
Group Velocity
Dispersion
Self Phase
Modulation
Fiber
loss

Institut Mines-Télécom PhD defense Xin You17 2015/12/2317
Pulse autocorrelation
after 12 km
Pulse width
compressed from
40 ps to 11.6 ps
Pedestal level
0.15
after 4.2 km
Pulse width
compressed from
40 ps to 4.5 ps
Pedestal level
0.19
after 2 km
Pulse width
compressed from
40 ps to 2.1 ps
Pedestal level
0.29

Pedestal suppression and further compression
18
Optical filter: bandwidth set at its maximum value of 13 nm to keep more energy

Simulation
𝜕𝐴
𝜕𝑧
+
𝛼
2
𝐴 +
𝑖𝛽2
2
𝜕2
𝐴
𝜕𝑇2
−
𝛽3
6
𝜕3
𝐴
𝜕𝑇3
= 𝑖𝛾 𝐴 2 𝐴 +
𝑖
𝜔0
𝜕
𝜕𝑇
𝐴 2 𝐴 − 𝑇𝑅 𝐴
𝜕 𝐴 2
𝜕𝑇
Group Velocity
Dispersion
Self Phase
Modulation
Fiber
loss
3rd order
dispersion
Self-
Steepening
Intrapulse
Raman
Scattering
• FWHM of input pulse: 4.49 ps
• Input peak power of 60 W or 491.5 pJ per pulse
• 3rd order dispersion slope at 1540 nm: 0.09 ps·nm-2·km-1
• Intrapulse Raman scattering coefficient: 3 fs
• Dispersion parameter: 20.3 ps·nm-1·km-1
• Nonlinear coefficient: 0.002 W-1·m-1

Institut Mines-Télécom PhD defense Xin You20
Our proposed architecture
20
1: native pulse generation stage
2: pulse compression stage
3: pedestal suppression stage
NRZ Waveform
Generator
Synthesizer
SOA SSMF
Oscilloscope
OSA
EDFA
Autocorrelator
90%
EDFA
10%
SSMF
1 2
3 Polarization
Controller
A polarization controller is added due to the sensitiveness to polarization
state of autocorrelator
2-stages compression
ratio up to 40 times

Pulsed source repetition rate tunability
Repetition
rate up to
790 MHz
The average output
power doubles
when repetition rate
doubles
250 MHz repetition
rate pulse train is
amplified to 7.5
dBm for solitonic
compression in 4.2
km SSMF
1st EDFA output
range: Pout<12.5
dBm (APC mode)
Reptition
rate limited
by the SOA
gain
recovery to
be about 1
GHz
Repetition
rate
determined
by the 2nd
EDFA output
range: <1.1
GHz

Demonstration of different repetition rate
tunablity
100 MHz 250 MHz 500 MHz
Bandwidth of filter in pedestal suppression stage set at its maximum
value of 13 nm

Repetitio
n rate
(MHz)
Pulse
FWHM (ps)
FWHM of
spectrum
(nm)
Time-
bandwidth
product
Extinction ratio of
autocorrelation
trace
Extinction ratio of
pulses under Gaussian
pulse assumption (dB)
100 1.09 13.0 1.79 13 15.8
250 1.00 5.8 0.74 29 19.1
500 1.13 3.5 0.50 39 20.4
☑ Pulse width around 1 ps
☑ Larger spectrum FWHM and smaller extinction
ratio at lower repetition rate
☑ The frequency-shifted spectrum component is
more flat at low repetition rate.

Spectrum at 100 MHz Spectrum at 500 MHz
Spectrum at 100 MHz
before the filter
Spectrum at 500 MHz
before the filter
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
100 MHz repetition rate 500 MHz repetition rate
Ratio of energy contained in the filter bandwidth
Lower
extinction
ratio at low
repetition
rate
More energy
contained in the
frequency-shifted
part at lower
repetition rate
Spectrum is
broader and
more flat at low
repetition rate
The limitation of
the maximum
bandwidth of
optical filter

Characterization for different carrier wavelengths
Carrier wavelength of
1550 nm
1565 nm
1540 nm

Carrier
wavelength
(nm)
Pulse FWHM
(ps)
FWHM of
spectrum
(nm)
Time-
bandwidth
product
Extinction
ratio of
autocorrelatio
n trace
Extinction ratio of
pulses under
Gaussian pulse
assumption (dB)
1530 No data due to the property of 2nd amplifier
1540 1.13 3.5 0.50 39 20.4
1550 1.08 5.3 0.71 47 21.2
1565 1.37 2.6 0.44 20 17.5
1530 nm not covered by the
amplification range of 2nd
EDFA used in experiment
ASE spectrum of the 2nd EDFA
Acceptable performance

RF spectrum analysis for noise characterization
𝐿 𝑛 𝑓 = 10𝑙𝑜𝑔10
𝑃𝑛(𝑓)
𝐵𝑒𝑞 𝑃𝑐𝑛
𝑃n 𝑓 : power spectrum at nth order
harmonics;
𝐵𝑒𝑞: equivalent bandwidth;
𝑃𝑐𝑛: peak power at nth order harmonics.
Single
side
band
noise
density

𝐿𝐽 𝑓 = 10𝑙𝑜𝑔10(
10 𝐿 𝑛 (𝑓)/10
− 10 𝐿1 (𝑓)/10
𝑛2
)
𝜎𝐽 𝑓1, 𝑓𝑢 =
1
2𝜋𝑓𝑟
2
𝑓1
𝑓𝑢
10 𝐿 𝐽(𝑓)/10
𝑑𝑓
1/2
Only timing jitter varies with
the harmonic order in a
quadratic way
RMS timing jitter 𝜎1 calculated for different lower
frequency 𝑓1 and higher frequency 𝑓𝑢
𝑓1 and 𝑓𝑢 are
determined to be 2
kHz and 118 kHz,
respectively

Measurement
#
Lower frequency
f1 (kHz)
Upper frequency
fu (kHz)
15th order
timing jitter (ps)
19th order
timing jitter (ps)
1 1 118 2.27 1.83
2 0.8 118 1.36 2.15
3 2 119 2.04 2.92
4 2.5 145 1.75 3.08
5 2 118 1.17 2.10
6 2 73 1.23 3.04
Average 1.7 115 1.65 2.52
RMS timing jitter not larger than 2.5 ps
It is believed that most of the noise
comes from the electronics
Parameter of pulse pattern generator:

RMS intensity noise
𝐿 𝐴 𝑓 = 10𝑙𝑜𝑔10(10 𝐿1 𝑓 /10
− 10 𝐿 𝐽 𝑓 /10
)
𝜎𝐴 𝑓1, 𝑓𝑢 = 2
𝑓1
𝑓𝑢
10 𝐿 𝐴(𝑓)/10
𝑑𝑓
1/2
Measureme
nt #
Lower
frequency f1
(kHz)
Upper
frequency fu
(kHz)
15th order
intensity
fluctuation
19th order
intensity
fluctuation
1 0.8 118 0.98% 0.83%
2 1 118 0.82% 0.93%
3 2.3 142 1.10% 0.87%
4 2.3 72 0.98% 0.83%
Average 1.6 112 0.97% 0.86%
RMS intensity
noise not larger
than 1%
Evaluation of
𝑓1 and 𝑓𝑢

Relative intensity noise of tunable laser
𝑅𝐼𝑁 𝜔 = 10𝑙𝑜𝑔10
𝑆 𝑝(𝜔)
𝐵𝐺𝐼 𝑝ℎ
2
𝑅
𝑆 𝑝(𝜔): difference between the noise power
spectrum of signal and the dark noise spectrum at
frequency 𝜔
𝐵: resolution bandwidth of ESA
𝐺: gain of electrical amplifier that is integrated in
optical receiver
𝐼 𝑝ℎ: the difference between the photocurrent of
illuminated detector and the dark current
𝑅: impedance of amplifier
Identification of the intensity
noise of pulsed source

𝐼𝐹 𝑓1, 𝑓𝑢 = 2
𝑓1
𝑓𝑢
10 𝑅𝐼𝑁/10
𝑑𝑓
1/2
Measureme
nt #
Lower
frequency f1
(kHz)
Upper
frequency fu
(MHz)
Intensity
fluctuation in
percentage
1 5 17.1 0.88%
2 5 17.1 0.89%
3 5 17.1 0.89%
4 5 17.1 0.89%
Average 5 17.1 0.89%
Evaluation
of 𝑓1 and 𝑓𝑢
The intensity noise of
tunable laser is the main
source of intensity noise
of pulse train

03/11/2015
35
Pulsed local oscillator mixing with optical signal
for undersampling
𝑇𝑠 = 𝑘𝜏 𝐷 + 𝛿𝑡, 𝑘 ∈ 𝑁, 0 < 𝛿𝑡 < 𝜏 𝐷.
∆𝜑 𝑡 = 𝜑 𝐷 𝑡 − 𝜑𝑠(𝑡)
Signal under test:
𝜀 𝐷 𝑡 = 𝐸 𝐷 𝑡 exp(−𝑖𝜔0 𝑡 + 𝑖𝜑 𝐷(𝑡))
Pulsed local oscillator:
𝜀 𝑠 𝑡 =
𝑁
𝐸𝑠 𝑡 − 𝑁𝑇 exp[−𝑖𝜔0 𝑡 + 𝑖𝑁𝜑 + 𝑖𝜑𝑠(𝑡)] At the output of a coherent receiver:
𝜀 𝐷 𝑡 ∙ 𝜀 𝑠
∗
𝑡 =
𝑁
𝐸 𝐷 𝑡 ∙ 𝐸𝑠
∗
𝑡 − 𝑁𝑇 exp 𝑖 −𝑁𝜑 + ∆𝜑 𝑡
= 𝐼 𝐵𝐻1 + 𝑖𝐼 𝐵𝐻2
Phase level detected:
𝜙 = arg(𝐼 𝐵𝐻1 + 𝑖𝐼 𝐵𝐻2)

Pulsed local oscillator mixing with Binary Phase
Shift Keying (BPSK) signal (experiment)
• Eye diagram of -10
dBm 10 Gb/s BPSK
signal
• Modulation on phase
• Q factor: 15.6
𝑇𝑠: 2.006 ns
𝜏 𝑠: ~1 ps
Average power: -12 dBm
Phaseeyediagram

Results of coherent detection
 The center position
correponds to the phase
results of coherent detection
 Only one phase level is
detected
 Probably due to the intensity
noise of pulses
☑Samples are superposed
along the period of the pulses
☑Red arrows indicate the
position of pulses
☑The phase of noise is of none
sense
Phaseeyediagram

Conclusion
☑ The proposed architecture has been demonstrated for effective
picosecond pulse train generation
☑ Up to 40 times compression ratio using only EDFAs and SSMFs
☑ The repetition rate tunability is experimentally demonstrated for
100, 250 and 500 MHz, and wavelength tunability for 1540, 1550
and 1565 nm in C band
☑ RMS timing jitter inferior to 2.5 ps
☑ Intensity fluctuation less than 1%, and it mainly comes from the
tunable laser
☑Undersampling of BPSK signal has not succeeded

Perspectives
For quantum key distribution
(Integrated Laser electro-
absorption Modulator)
Spectral linewidth of laser
source 0.05 nm
Extinction ratio 18 dB
Pulse width 5 ps
Repetition rate 4 MHz*
For optical undersampling
Sampling pulse duration: 1 ps
Sampling frequency: 250 MHz
Sampling laser power: 0 dBm
Timing jitter: <0.2 ps**
Typicalvalues
inapplication
☑ Change the tunable laser for
lower intensity noise
☑ The repetition rate can be
easily solved by changing
amplifiers of different power
range
☑ Changing for better
electronics (?)
Solutions
*PhD Thesis Qing Xu 2009
**P. Gallion, C. Gosset, X. You, and J. Zhou, Opt Quant
Electron 2015

Phd defense of xin you

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