Hysteresis in quantum-dot mode-locked lasers with optical injection

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Hysteresis in quantum-dot mode-locked lasers with
optical injection

• Tatiana Habruseva, Natalia Rebrova, Stephen P.
  Hegarty, and Guillaume Huyet

Tyndall National Institute and Cork Institute of
Technology, Cork, Ireland
Collaborators Dmitrii Rachinskii (University
College Cork), Evgeny Viktorov (Université Libre
de Bruxelles) Alexander Pimenov, Andrei
Vladimirov (Weierstrass Institute)
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Outline

• Passively mode-locked quantum-dot lasers (QD-MLL)
• Mode-locked laser with external CW optical
  injection
• Laser characteristics with optical injection
• Optical spectrum narrowing, red shift, modal
  optical linewidth
• Regimes of the optically injected laser
• Bi-stabilities and hysteresis of
  optically-injected mode-locked lasers
• Bi-stabilities observed in optically injected MLL
• Hysteresis

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Passively mode-locked laser

• Two section MLL

Absorber saturates faster than gain ? opens a
short net gain window ? when the pulse arises

Saturable absorber
I
Gain
Forward bias

-V
Reverse bias
Short pulses
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Passively mode-locked laser

• Two section MLL

• emit short pulses of a few ps duration with high
  repetition rate

Saturable absorber
I
Gain
Forward bias

-V

• optical spectrum ? equally spaced phase locked
  modes

Reverse bias
Power spectrum
Modal linewidth
RF linewidth
RF Power
3Frep
2Frep
Frep
Each mode has its line width (FWHM)
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Quantum-dot mode-locked laser

• Monolithic Quantum-Dot Lasers
• wide frequency comb (inhomogeneous broadening)
• fast absorber recovery time
• short pulse widths
• high repetition rates
• thermal stability
• low coupling of ASE
• low phase noise
• (low confinement factor)

• monolithic two-section InAs/GaAs quantum-dot
  laser
• emitting at 1.3µm wavelength
• repetition rate around 10 GHz

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Ring cavity model

A(t) field
Gain bandwidth
T roundtrip time

• Simplification of traveling wave model
• Allows the use of continuation software
• We use pump-probe measurements to define
  realistic parameters

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Pump-probe measurements
Gain dynamics
Recovery times for gain and absorber
Th. Erneux et. al., Appl. Phys. Lett. 94, 113501
2009 T. Piwonski et.al., Appl. Phys. Lett. 94,
123504 2009
Phase dynamics
Alpha Factor
Different alpha Factors in gain and absorber
regimes
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Model equations
-field
-gain
-absorber
A.Vladimirov and D. Turaev, Model for passive
mode locking in semiconductor lasers, Phys.
Rev. A (2005)
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External optical injection
Master Laser
Slave Laser

• CW optical injection ? stabilize and control
  laser emission
• Master laser commercial source with narrow
  linewidth ( 100 kHz)

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Outputs of optical injection
Injection locking affects laser characteristics
optical spectrum, repetition rate, power, noise
properties and modal linewidth of the slave laser.

• Optical spectrum narrowing

• Optical spectrum tuning

• Red shift

black line free-running red line injected
Black line free-running Colored lines injected
MLL
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Optical injection

• Optical injection from a CW laser

? amplitude, ? detuning from central
frequency of the slave laser.
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Optical injection

• Optical injection from a CW laser

? amplitude, ? detuning from central
frequency of the slave laser.

• Spectral narrowing

• Red shift phenomenon

injection

• during the pulse

• the choice

• results in

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Modal optical linewidth

• Free-running regime (black)
• linewidth 20 MHz
• red line parabolic fit

• Single-tone injection (red)
• modes at the injection frequency are phase
  locked to the master laser and take narrow
  linewidth of the master source
• Far from the injection frequency slave laser
  linewidth increases

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Sweep directions
Master frequency is fixed
Power
?
Slave frequency decrease ? lock mode from the red
side of the mode
Slave frequency increase ? lock mode from the
blue side of the mode
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Hysteresis in the optically-injected lasers
Injection locking affects optical spectrum,
repetition rate, power, noise properties and
modal linewidth of the slave laser.

• With injection
• Osa narrowing
• Injected mode optical linewidth narrowing
• Frep pulling
• Power change

Slave Frep change with injection locking for both
directions of the detuning
locking from the blue side
locking from the red side
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Regimes of injected laser
Master Laser
Slave Laser

• Unlocked ? slave ignored injection from the
  master

• Locked ? slave output affected by injection
  optical linewidth of the injected mode takes
  value of narrow master linewidth

• Single mode ? slave optical spectrum gradually
  becomes narrower with fewer modes finally there
  is only one mode at the injection wavelength

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Schematic diagram of observed regimes
Main regimes U ML and unlocked from the
master L ML phase locked to the master SM
single-mode, phase-locked to the master
1. Bi-stability between unlocked (when negative
detuning) and locked (positive) 2. Bi-stability
between U and SM (pink) 3. Bi-stability between L
(negative) and SM (positive detuning) 4.
Bi-stability between SM(blue) and SM (pink)

• - decrease current (positive detuning)
• - increase (negative detuning)
• - both together

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Bistabilities
L or U
SM or U
SM or L
SMn or SMn-1
Detuning
Diagram for 4 consecutive modes, taking into
account power increase with current
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Experimental map
SM regimes with the same wavelength, but
different power (blue region)
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Hysteresis
Hysteresis area increases with external injection
strength at low optical injection power at high
optical injection powers the hysteresis area
saturates the hysteresis disappear at ultra-high
injection powers (when Pinj gtgt PMLL)
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Conclusion

• Single-tone injection ? optical spectrum
  narrowing, tuning, repetition rate frequency
  pulling, power change, modal loptical inewidth
  narrowing

• Slave spectrum is red shifted due to unequal
  alpha factors for gain and absorber

• QD-MLLs represent bi-stable behavior with
  external optical injection depending on the
  direction of master-slave detuning ? hysteresis
  experimentally observed at lower locking boundary

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Thank you for your attention
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Modal optical linewidth
The linewidth was measured by beating of
different laser modes with a tunable laser source
(TLS linewidth was 100 kHz).
The beating signal ? Lorentzian.
Kärtner et al., Topic Appl. Phys. (2004)
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Modal optical linewidth
Optical spectrum of passively mode-locked laser
Power
FWHM
Frequency
What is the linewidth of each laser mode?
We beat different laser modes with a tunable
laser source (TLS linewidth was 100 kHz).
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Modal optical linewidth

• linewidths depends parabolically on the mode
  number
• red line parabolic fit
• blue line formula 4

• ??RF,1 is the RF linewidth of the 1st harmonic

Opt. Lett., vol. 34, pp. 3307-3309 (2009)
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Passively mode-locked lasers
Reverse bias
Saturable absorber
Power
Frequency

• Pulses of a few ps duration
• High repetition rate (tens of GHz)
• Frequency combs optical spectrum is composed of
  a comb of phase locked equally spaced modes (up
  to 200 modes)