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May 4, 2007 F I N I S A R C O R P O R A T I O N
Chirp Managed Laser (CML) and
Applications
Daniel Mahgerefteh
March 23, 2011
Outline of Talk What is Chirp Managed Laser (CML)? How CML works? Applications of CML
> 360 km reach 10 Gb/s NRZ with pluggable optics RZ-Alternate Mark Inversion RZ-DPSK, DPSK, DQPSK generation
CML Device Technology C band Tunable MGY DBR for 10 Gb/s CML 4 x 25 Gb/s FM DBR Array PLC based CML
3
What is Chirp Managed Laser (CML™)?
Methods for modulation of light: External Modulation (e.g., EML):
+ Dispersion Limit (80km @ 10G) - Requires modulator (e.g. EA, LiNbO3) - Lower Output Power - Power hungry Driver, higher cost
Direct Laser Modulation (e.g., DFB): + Compact, Low consumption, low cost - Dispersion Limit (5 km @ 10 G)
Chirp Managed Laser: + Compact, Low consumption, low cost + Dispersion Limit (> 200 km @ 10 G) + Tolerance to negative dispersion + Phase-Amplitude coding possible
Laser Modulator
Electrical DATA Optical DATA
Laser
Electrical DATA Optical DATA
Laser
Electrical DATA Optical DATA
Filter
1. Externally Modulated Laser
2. Directly Modulated Laser (DML)
Chirp Managed Laser (CML)
4
Value Proposition: Use Directly Modulated Laser as a low-power consumption, compact, high performance alternative to externally modulated sources for metro and long-haul.
EML InP-MZ
CMLTM
CML: Better performance at Lower cost without external Modulators
LiNbO3-MZ
CMLTM
MZ + Dispersion Comp. DFB/EML EML/MZ 2- 40 km 40 - 8km 80km-320km >2400km
5
What is Chirp? Chirp is change in instantaneous frequency
(or wavelength) of an optical pulse with time EMLs use negative transient chirp to
increase reach to 80 km @ 10 Gb/s Red shifted chirp at rising edge Blue shifted chirp at trailing edge Blue is faster than red @ 1550 in SMF Trailing edge catches up to rising edge Pulse compresses after fiber Bit Error Rate is improved up to a certain
distance before it gets progressively worse DMLs have positive transient chirp, which
limits reach to 5 km at 10 Gb/s Blue shifted chirp at rising edge Red shifted chirp at falling edge
DMLs also have adiabatic chirp CML chirp and its use is a different story
El
ectr
ic F
ield
Time (fs)
Intensity Envelope Optical Carrier
Red Shift Blue Shift
Gaussian Pulse with Chirp
Vblue Vred
6
How CML works? Ingredients for CML:
1. High speed FM (Freq. Mod. source; DFB) 2. Passive Optical Spectrum Reshaper (OSR) Filter 3. Supporting Optics
Main Recipe for CML: 1. Reduce transient chirp 2. Reshape amplitude profile 3. Set adiabatic chirp appropriately 4. Reshape frequency profile using OSR 5. Lock DFB wavelength to OSR 6. Compensate for Thermal Chirp
Key characteristics of CML: 1. CML is not an external cavity laser 2. CML uses an isolated single mode DFB 3. Mode stability, wavelength tuning determined by
DFB characteristics 4. DFB wavelength is locked to OSR filter
DFB
Optical Filter
CML in TOSA package
Isolator
Multi-cavity Filter
PD1
PD2
10 Gb/s
DML
10 Gb/s
DML Multi-cavity Filter
PD1
PD2
Isolator
Multi-cavity Filter
PD1
PD2
10 Gb/s
DML
10 Gb/s
DML
10 Gb/s
DML
10 Gb/s
DML Multi-cavity Filter
PD1
PD2
Isolator
7
-6
-4
-2
0
2
4
0 200 400 600 800 1000Time (ps)
Transient chirp
Adiabatic chirp
Transient chirp
Not All Chirp in A Direct Mod Laser is Bad Chirp!
How a standard Direct Mod is used: Biased close to threshold, Ith High transient chirp limited fiber
transmission How CML Direct Mod is used:
Bias high above laser threshold ~ 5 x Ith Drive DFB to get 1-2 dB Extinction Ratio Higher damping in DFB Transient chirp reduced High Extinction Ratio will be achieved by
Passing this signal through OSR filter
Determines Adiabatic Chirp
In the rest of this presentation when we say Chirp we mean Adiabatic chirp
8
• Extinction Ratio: AMER = 1-2 dB
Dielectric Coatings Cavity
• Extinction Ratio: ER = AMER + S x FM • ER ~ 10-12 dB
AM ER ER
Filter increases Extinction Ratio by FM to AM conversion
Optical frequency Time
Before OSR After OSR
Inte
nsity
0 bits 1 bits
FM
OSR filter
Time
Inte
nsity
Adiabatic chirp makes 1 bits blue shifted relative to 0 bits Difference between optical frequency of 1 bits and 0 bits is FM ~ 3-8 GHz Output of Direct Mod Laser is aligned with transmission edge of OSR filter OSR filter discriminates 1s and 0s higher ER; filter edge slope ~ 1-2 dB/GHz
1 bits
0 bits Tr
ansm
issi
on
9
Key to Dispersion Tolerance of CML: Destructive Inter-symbol Interference
Standard NRZ
Constructive interference Pulses in phase
CML
Φ=0 Φ=0
Φ=0 Φ=π Destructive interference
Φ=0 Φ=π Pulses π out of phase
Dispersion
Dispersion
Adiabatic Chirp to ~ ½ the bit rate 5 GHz for 10 Gb/s CML Phase Rule: 1 bits separated by odd # of 0 bits are π out of phase Destructive interference of 1 bits in the middle 0 bit slot keeps eye
open after fiber dispersion 0 km
100 km
150 km 200 km
100 km
250 km
0 km100 km
150 km 200 km
100 km 0 km100 km
150 km 200 km
100 km
250 km
CML @ 200 km
EML @ 80 km
10
Chirp ~ ½ Bit Rate makes carrier phase to slip by π every 0 bit
Set adiabatic Chirp ΔfA ~ 5 GHz at 10 Gb/s
Continuous π phase slip for each 0 bit 1 bits separated by odd # of 0s are π out of phase Same as Optical Duobinary phase rule
ππφ =××=Δ psGHz 10052
Aligned with peaks
Time
Intensity Envelope
Optical Carrier
Red Shifted 0 bit
Φ = 0 Φ = π Time Ticks
1 bit
Aligned with troughs
Phase slipping…
1 bit
CML has no carrier and ½ the BW Standard NRZ
Φ=0 Φ=π
=
Φ=π
Φ=0
+
10 Gb/s
5 Gb/s
20 dB BW: 15 GHz
CML Laser
No Carrier
Standard NRZ
Carrier
20 dB effective BW: 33 GHz
Effective BW
½ of bits have 0 phase, ½ have π phase Power in optical carrier is:
Spectrum is ½ size of NRZ- 2 ways to see it: 1) Correlation between bits (e.g. 1 0 1 and 1
0 0 -1 cannot occur only 1 0 -1, and 1 0 0 1) 2) Random bit sequence @ 10 Gb/s is sum of
two sequences @ 5 Gb/s: e.g.
0~)()0(2
∫+∞
∞−== dttEP ω
But… Chirp = ½ bit rate is not enough!
Simulations and experiments show that having chirp = ½ the bit rate (5 GHz for 10 Gb/s)…
for example, by using a DFB for FM generation and EA for get high Extinction Ratio…
can only get us to 120 km…
leaving 5 dB penalty at 200 km!
What is missing?
What’s Missing?
Need to Convert Adiabatic Chirp to Flat Top Chirp with abrupt phase shifts
Freq
uenc
y (G
Hz)
0
5
10
15
20
-10
-5
0
5
10
1 0 1 1 0 0 1 0 0
Inte
nsity
Adiabatic Chirp
0
5
10
15
20
-5
-4
-3
-2
-1
0 Ph
ase
(x π
radi
ans)
Inte
nsity
Adiabatic Phase Change 0
5
10
15
20
-50 -40 -30 -20 -10 0 10
Freq
uenc
y (G
Hz)
Flat Top Chirp
Inte
nsity
In
tens
ity
0
5
10
15
20
-5
-4
-3
-2
-1
0 π π
2π
Phas
e (x
π ra
dian
s) Abrupt Phase Change
Adiabatic Chirp vs Flat Top Chirp
Nonlinear phase across pulse Perfect cancellation at one point Incomplete cancellation in 0 bits 5 dB penalty at 200 km
Constant phase across pulse Cancellation across pulse Better destructive interference in 0 bits 1 dB penalty at 200 km
Filter Convert Adiabatic to Flat Top Chirp by AM to FM conversion
Pass signal through a filter edge Ignore phase of filter for now…
OSR adds weighted 2nd derivative of AM to frequency profile
Δ+
′−Δ
=Δ
−
))()(()(tan)(
)(
1
tbatAtAb
dtdt
tAfter
ωω
ω
ω
)()()( inputinput tEdtdibtaEtEAfter +=
ωω baT +=)(OSR
Input signal
The i in this term converts AM to FM
-100 -80 -60 -40 -20 0 20 40 60 80 100 -0.02
0
0.02 Fr
eque
ncy
(GHz
) -100 -80 -60 -40 -20 0 20 40 60 80 100 0
0.5
1
Inte
nsity
Time
)(tA is envelope of input signal
)()( tAt ∝ω is input adiabatic chirp
b is OSR slope
Flat Top Chirp
Spike Chirp
After OSR
Flat Top Chirp and Abrupt Phase Change Require High OSR Slope
0
1
2
3
4
5
6
7
-400
-300
-200
-100
0
100
1.1 1.2 1.3 1.4 1.5 1.6Time (ns)
Low Slope OSR 1 dB/GHz
High Slope OSR 1.8 dB/GHz
High resolution measurements courtesy of APEX
Outline of Talk What is Chirp Managed Laser (CML)? How CML works? Applications of CML
200 km reach at 10 Gb/s pluggable XFP transceiver product > 360 km reach 10 Gb/s NRZ with driver pre-emphasis RZ-Alternate Mark Inversion RZ-DPSK, DPSK, DQPSK generation
CML Device Technology C band Tunable MGY DBR for 10 Gb/s CML 4 x 25 Gb/s FM DBR Array PLC based CML
18
EDC ON
111213141516171819202122
-3000 -2000 -1000 0 1000 2000 3000 4000 5000 6000
DISPERSION (ps/nm)
Req
uire
d O
SN
R fo
r B
ER
= 1
x10-3
(dB
)
1-dB window: -2150ps/nm to +4000ps/nm
111213141516171819202122
-3000 -2000 -1000 0 1000 2000 3000 4000 5000 6000
DISPERSION (ps/nm)
Req
uire
d O
SN
R fo
r B
ER
= 1
x10-3
(dB
)
EDC OFF
1-dB window: -1900ps/nm to +3600ps/nm
-1700ps/nm +3400ps/nm 0ps/nm
S. Chandrasekhar et al. OFC 05 Post Deadline paper PD30
DFB
OSR
DM200 CML™ reaches over 5 x dispersion limit
Standard externally modulated transmitter is limited to ~ 1600 ps/nm
Detectors
18.3 mm x 78 mm x 8.5 mm
XFP
Demonstration of 360 km Reach using pre-emphasis CML
9© 2008 Finisar Corporation, Confidential
Fig.5
Fig.5 Performance comparison of CML with Uni-polar chirp and EDC.Fig.5 Performance comparison of CML with Uni-polar chirp and EDC.
10111213141516171819
0 100 200 300 400
Transmission Distance (km)
Req
uire
d O
SNR
(dB
/0.1
nm)
At 250 km the reach of CML is limited by distortion of the 1 0 sequence
Pre-emphasis on 1 0 transitions (uni-polar transient chirp) mitigates distortion of isolated 1 bits after fiber
Uni-polar chirp red shifts trailing edge of isolated 1s and cause destructive interference with adjacent 0 bit
Used Finisar driver with 1 bit Digital Signal Processing (DSP) pre-emphasis
Power consumption of DSP = 30 mW SFP+ Transceiver Power consumption,
including Tx, and Rx < 1.2 W Longest Reach/Power Consumption solution
for > 300 km reach
Xueyan Zheng et al. OFC 2009
300 km BB
© 2008 Finisar Corporation, Confidential
Comparison of the key bits with and w/o uni-polar chirp
Proprietary and Confidential Information of Azna LLC
t t
t t
VB > VR
VR
VB
VR
VB > VR
VR
VT
VR>VT
Before Fiber After Fiber
With Uni-Polar Chirp
Without Uni-Polar Chirp
Destructive Interference
No Interference
RF with uni-polar function
RF without uni-polar function
Drive CML with standard RZ signal Adiabatic chirp ~ 10 GHz Adiabatic chirp generates π shift
between adjacent 1 bits (RZ-AMI):
Narrower spectrum than RZ-OOK, and suppressed carrier
Higher Tolerance to dispersion, nonlinearity:
1 dB penalty dispersion window: -450 ps/nm to 950 ps/nm (compared with +/-550 ps/nm for RZ-OOK)
Replaces DFB + MZ + 1 bit delay line interferometer
9000 km transmission with CML RZ-AMI
CML can generate RZ-Alternate Mark Inversion (AMI)
ER ~ 15 dB
+1 -1 0 +1 -1 0 0 +1
Example of an AMI bit sequence
50 ps
~10 GHz
AM
FMππ =×× ps5GHz 102 0
S. Chandrasekhar et al. ECOC Post Deadline session 2, (2005)
21
CML can generate RZ-DPSK signal
Patents Pending
Drive CML with a 3 level RZ driver at bit rate B: Apply ΔI during 1st ½ bit period to get π shift chirp = bit rate Apply 2ΔI during 1st ½ bit period to get 2π shift chirp = 2 x bit rate OSR filter equalizes amplitudes and produces abrupt phase transitions No differential coding needed! (since phase shift is integral of chirp)
FINISAR CONFIDENTIAL 27
Frequency
CML Output Power
f2
Time
DFB Current
I0
f1
f0
I1 I2
23
CML can generate RZ-DPSK
• Drive current change 2ΔI causes adiabatic chirp 2Δf for a 2π phase change representing a Zero bit
• Drive current change ΔI causes adiabatic chirp Δffor a π phase change representing a One bit
ZERO
ONE
0 0 1 0 1Input
Cur
rent
Pha
se
0 π
Chi
rp (G
Hz)
Tππ2π3π4π5
0
0
f2f1f0
Driver Output
Output after OSR
Laser OutputBefore OSR
Time
0
π
0
0
π
0 π
I2Δ IΔ
f2Δ fΔ
Demonstrations of RZ-DPSK using CML • RZ-DPSK at 2.67 Gb/s (J. Franklin et al.)
• Tri-level swings 13 mA pp and ~26 mA pp • Chirp levels = 3 GHz and 6 GHz • 1550 nm DFB FM efficiency 0.24 GHz/mA • Output power of CML 3.5 dBm • 0.7 dB penalty compared to RZ-DPSK
generated using 2 x LiNbO3 modulators
• RZ-DPSK CML at 10 Gb/s requires 20 GHz chirp Not sufficient FM efficiency in current 10 G DFB (1550 nm BH)
• DPSK- CML at 10 Gb/s requires 10 GHz chirp: use 0 phase instead of 2π
• Current DFB has sufficient FM efficiency
• W. Jia et al. demonstrated DPSK CML as well as DQPSK CML (OFC 2011) using standard 10 G CML in XFP
• Performance of at 10 G limited by bandwidth, of DFB laser used
-56 -55 -54 -53 -52 -51 -50 -49 -48 -47 -46-10-9
-8
-7
-6
-5
-4
-3
-2
Received Power (dBm)
Log(
BER
)
Lithium NiobateCML-RZ-DPSK
J. Franklin et al. OFC 2008
Wei Jia et al. PTL 23, p. 173 (2011)
RZ-DPSK @ 2.67 Gb/s
DPSK CML at 10 Gb/s
Outline of Talk What is Chirp Managed Laser (CML)? How CML works? Applications of CML
> 360 km reach 10 Gb/s NRZ pluggable optics RZ-Alternate Mark Inversion RZ-DPSK, DPSK, DQPSK generation
CML Device Technology C band Tunable MGY DBR for 10 Gb/s CML 4 x 25 Gb/s FM DBR Array PLC based CML
Gain
FilterBS
PD
PD
BS
PD
OSR Filter10-Gb/s MG-Y laser
Gain
MMI (phase)DBR
PDS bendMMI
DBR
C band Tunable 10 Gb/s Modulated Grating Y branch (MGY) DBR CML
Active
MG DBR Right
MG DBR Right
12µm gap12µm gap
50µm
50µm
200um penetration
200um penetration
350µm350µmPassivePassive
GainGainGainMMI (Phase)MMI (Phase)MMI (Phase)
S bend (thermal chirp compensation)S bend (thermal chirp compensation)S bend (thermal chirp compensation)MG DBR Left
MG DBR Left50µm
50µm
1.38Q, 350nm thick1.38Q, 350nm thick1.38Q, 350nm thick
MG DBR Right
MG DBR Right
12µm gap12µm gap
50µm
50µm
200um penetration
200um penetration
350µm350µmPassivePassive
GainGainGainMMI (Phase)MMI (Phase)MMI (Phase)
S bend (thermal chirp compensation)S bend (thermal chirp compensation)S bend (thermal chirp compensation)MG DBR Left
MG DBR Left50µm
50µm
1.38Q, 350nm thick1.38Q, 350nm thick1.38Q, 350nm thick
Injection tune DBR L,R with Vernier effect Full C band tuning
FM efficiency (FM) and relaxation oscillation frequency, fr are diluted by the passive section
Key Elements of Design: Reduce passive length Very high speed, high FM Active section
PD = photodiode BS = beam splitter
%54≈+
∝PassiveActive
Active
LLLFM
%74≈+
∝PassiveActive
Activer LL
Lf
Yasuhiro Matsui et al. ECOC Post Deadline paper p1-2 (2009)
Tunable high speed, high FM MGY DBR
• Laser Design parameters: • Y branch gap reduced from 30 um for CW laser design to 12 um
reduce S bend increase Lactive/Ltot
• Gain: 8 strained MQWs High speed: fr = 11-15 GHz • Gain: 100 nm SCH layers High FM efficiency: 0.13-0.18 GHz/mA • fr varied with wavelength due to detuning and with DBR bias due to loss
•
-6
-3
0
3
6
0 5 10 15Frequency (GHz)
FM
res
po
nse
(d
B)
35°C - 1536nm35°C - 1567nm50°C - 1538nm50°C - 1570nm
3-dB BW: 18 GHz at 35°C
FM frequency response
© 2008 Finisar Corporation, Confidential
Resonant Frequency of MGY laser, fr > 10 GHz
10
11
12
13
14
15
16
1520 1530 1540 1550 1560 1570 1580Wavelength (nm)
Res
onan
t fre
quen
cy (G
Hz) Injection to DBR
Fr: 11 GHz – 15 GHz
Injection to DBR increases loss in DBR:• Reduces differential gain and therefore Fr
Fr : 11 GHz – 15 GHz• Fr at longer wavelength is smaller due to detuning from gain peak• Dilution of Fr compared to FP laser is ~ 75% near gain peak (expected)• Detune loading effect slightly favors Fr at the expense of FM
High
LowFP for 300µm cavityFr = 17 GHzK factor = 0.17 ns
24 1
g tot z
Kdg
π ευ α
= +
Γ ⋅
K factor: 0.3ns – 0.5ns
H
Low
Relaxation oscillation frequency
© 2008 Finisar Corporation, Confidential
24% mask margin
Tunable MGY CML using Delayed-Line Interferometer filter for < 100 km
FSR=21 GHz DLI filter shape
-60
-50
-40
-30
-20
-10
0
1538.8 1538.9 1539 1539.1 1539.2 1539.3 1539.4Wavelength (nm)
Tran
smis
sion
(dB
)
Output SpectrumInput Spectrumfilter with 21 GHz FSR
Before DLIBefore DLIBefore DLI
After DLIAfter DLIAfter DLI
-60
-50
-40
-30
-20
-10
0
1538.8 1538.9 1539 1539.1 1539.2 1539.3 1539.4Wavelength (nm)
Tran
smis
sion
(dB
)
Output SpectrumInput Spectrumfilter with 21 GHz FSR
Before DLIBefore DLIBefore DLI
After DLIAfter DLIAfter DLI
Use Delay Line Interferometer (DLI) OSR filter instead of 3 cavity band-pass used before
DLI filter is not bandwidth limiting Rise-fall times after DLI filter are shorter than at
output of laser due to FM/AM conversion Improved mask margin for CML Dispersion tolerance limited to 100 km
Higher harmonics passed throughHigher harmonics Higher harmonics passed throughpassed through
1.E-121.E-111.E-101.E-091.E-081.E-071.E-061.E-051.E-041.E-03
-32 -30 -28 -26 -24 -22 -20
Received Power (dBm)
Bit
Erro
r Rat
e
1565-L
1559-H
1553-L
1533-L
1529-H
1553 L
1535 H
1553 H
1559 H
1565 H
OSNR = 26 dB/0.1 nm1600 ps/nm@ 10.5 Gb/sPRBS 231-1
OSNR = 26 dB/0.1 nm1600 ps/nm@ 10.5 Gb/sPRBS 231-1
4 x 25 Gb/s DBR Laser Array CML (S. Matsuo et al. NTT) Key design feature: FM generated by in-
cavity phase modulation Speed of FM limited only by RC time constant
and cavity FSR speed not limited by gain dynamics Loss modulation of phase section has to be
minimized (high α modulator) Using SOA to compensate loss of MMI OSR filter BW = 20 GHz, slope = 0.8 dB/GHz 2 Vpp applied to phase 10 GHz chirp Demonstrated Performance: Extinction Ratio = 5 dB 40 km reach at 25 Gb/s Potential Application to DWDM 100 G
S. Matsuo et al., PTL , vol. 20 p. 1494 (2008)
Phase section
Data modulation
gain MMI combiner
Photonics Integrated Circuit (PLC) CML (Y. Yokoyama NEC)
Feature of device: 10 G DFB flip-chipped onto Si platform SiON waveguide on Si ring resonator OSR
filter no optical isolator needed Single ring equivalent to single cavity filter
BW and slope are constrained Metal heater on ring adjusts wavelength Monitor diodes used to lock laser
wavelength to ring filter by tuning temperature of PLC
Size: 2 mm x 2.9 mm Performance: ER = 7 dB 300 km reach @ 10 Gb/s ~ 15 dB OSNR Power consumption for XFP < 3 W
PLC based CML
Conclusion
Laser Chirp is a Good Thing!
May 4, 2007 F I N I S A R C O R P O R A T I O N
Backup Slides
© 2008 Finisar Corporation, Confidential
Digital pre-emphasis of CML: Towards 500 km Reach at 10 Gb/s Digital pre-emphasis concept: At each distance a particular sequence of ‘problem bits’ limit the BER Problem bits up to 200 km: 101 Problem bits for 300-400 km: 010, 011, 110
Pre-emphasis of ‘problem’ bit sequence by driver DSP to clean up distortion after fiber
Longer distance requires more bits Simulation: Reach > 450 km with 6 bit DSP Eye remains open 0-450 km Back-to-back eye suffers some penalty Driver DSP power consumption 200mW
assuming Bi-CMOS technology Work needed to further optimize reach EDC at receiver can extend reach further
Bit time slots
5
7
9
11
13
15
17
19
21
23
25
0 100 200 300 400 500
Transmission Distance (km)
Requ
ired
OSNR
for 1
0-3
BER 6 Bit DSP
1 Bit DSP
Simulation courtesy of Bob Drost, Finisar
Other features needed to make CML work
1. OSR filter limits band width of resulting spectrum Increases rise-time/fall times Less dispersion
– OSR bandwidth chosen to be ~ 8 GHz for 10 Gbps 2. Lock DFB wavelength to OSR
– OSR edge doubles up as in-line wavelength locker 3. Compensate for Thermal Chirp
– Heating of laser by injection generates unwanted red chirp at low frequencies base line wonder of chirp
– Thermal chirp compensated using low frequency bias circuit – Integrate incoming data with 20 MHz low pass filter – Use this signal to pull laser DC bias to generate proportionate blue chirp
– Requires DFB to have sufficiently high FM efficiency – For DBR laser, phase section or DBR section provide independent knob
for better compensation
Compensate for Thermal Chirp using Low Frequency Circuit
Thermal chirp shifts to the red for long series of 1 bits
Adiabatic chirp blue shifts Compensation circuit pulls bias
up for long series of 1s Additional blue shift cancels
thermal red shift Circuit BW < 100 MHz
Time
Laser Current
Laser Temperature
Laser Frequency drift due to Thermal Chirp
High Density of 1s High Density of 0s
Intensity Output of OSR without Thermal Compensation
Time
Laser Frequency with Thermal Chirp Compensation
Laser Package
CF
RF
RF
50Ω
R1
R1 R2
RL
Driver
DATA
DATADC
Block
C1
CF
+-
-5V
-+
-5V
-5V
5V
R3
Laser Package
CF
RF
RF
50Ω
R1
R1 R2
RL
Driver
DATA
DATA
Driver
DATA
DATADC
Block
C1
CF
+-
-5V
-+
-5V
-5V
5V
R3
Compensation circuit
CML Chirp Tolerance for 200 km
There is only ~ 1 dB sensitivity penalty for +/-15% change in chirp Aging of DFBs show < 3% change in FM over 110 yrs (limited by measurement
error 3σ +/5%) Interference power scales as cos2(Δφ), which is slow near 0 good
tolerance
-28 -27 -26 -25 -24 -23 -22 -21 -20 -19 -18
-40 -30 -20 -10 0 10 20 30 Change in Chirp (%)
Sens
itivi
ty a
t BER
=1e-
12 (d
Bm
) BB 200km
Clean zero Bump does not close the eye
Intensity
Flat Top Chirp
Isolated 1 Bits Compress by Destructive Interference
blue
red red
Back-back 200 km
Isolated 1 bit interferes with 0 bit in leading edge Oscillations open the eye around isolated 1 bit After a certain distance bump in leading edge generates 01 errors
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