1. Sensitivity of quantum dot semiconductor lasers to optical feedback
Guillaume Huyet
Physics Department,
University College, Cork, Ireland

2. People Cork Berlin Ioffe Institute, St Peterburg David O’Brien
Stephen Hegarty
Guillaume Huyet
Sasha Uskov (NMRC)
Berlin
Christian Ribbat & Dieter Bimberg
Ioffe Institute, St Peterburg
V.M. Ustinov, A.E. Zhukov, S.S. Mikhrin and A.R.Kovsh

3. Outline Quantum dot semiconductor lasers
Measurement of the alpha parameter
Relative Intensity noise spectra
Sensitivity to optical feedback
Model

4. Quantum dot semiconductor lasers
Idea: Quantum confinement to increase the density of states
Artificial atoms,
Density of states => more gain
No line-width enhancement factor

5. Mesurement of the  factor
Hakki-Paoli method applied to single mode 1.3 micron QDL.
After correction for Joule heating, a value of a = 1.6 was obtained.
Value is superior to that of similar QWL, (similar laser length and stripe width), where a = 5.

6. Hakki-Paoli Set-up single mode QD laser SMF OSA, 10 pm resolution
AR GRIN fibre
assembly
lens
Pulsed voltage
source
~100 ns pulses PC

7. Subthreshold spectra Finding lasing point of
devices - look at subthreshold
optical spectra at this wavelength
Change the current levels,
compare spectra

8. Correction for Joules heating
heating density very high, cannot run very low duty cycle as trough power low
need to correct for thermal drift…
reducing duty cycle in steps to 0.5% and extrapolating for 0% duty cycle
Result: alpha ~ 1.6
Similar experiment for QWL yielded alpha ~ 5

9. Are these lasers sensitive to optical feedback???
From the measurement of the -factor, we expect behaviour similar to that of quantum well ??
If not, some other factors are different??

10. The experimental set-up
Current source
beamsplitter
Variable ND
lens
To photodiode and ESA, OSC etc

11. Results
Stability (or otherwise) inferred from ESA spectrum. Absence of RF
modulation of intensity, in particular at round-trip frequency,criterion used.
WE DID NOT OBSERVE LFF

12. Comparisons with Quantum well lasers
Quantum dot semiconductor lasers have a reduced sensitivity to optical feedback but have similar -factor.
Measurement of the Relative Intensity noise
QW => Relaxation oscillation peak
QD => NO relaxation oscillation peak

13. Turn-on transient
Our set-up has an electrical bandwidth of 1GHz -need to improve but similar results obtained at TUB
Top QW laser
Bottom QD

14. Sensitivity to optical feedback
Depend on the alpha factor
But also on the damping rate of the relaxation oscillations
Would we see LFF in QD laser with low damping?
Mechanism for strong relaxation oscillations damping?

15. Sensitivity of quantum dot lasers to optical feedback
Lasers at 1.1m
weekly damped RO
Sensitivity to OF
Observed in RIN and turn-on transient

16. Relaxation oscillations
The electrons/holes are not injected directly into the dot
Injection => Bulk => QW=> QD
Characteristic times, capture escape and process are important parameters

17. Rate equations approach
The capture time can strongly affect the turn-on transient.
Phonon captures, Auger capture
Escape of carriers can also be included.

18. Model to describe sensitivity to optical feedback
The equations for the densities in the well and in the dots remain the same.
The equation for the complex amplitude of the electric field replaces the equation for the Photon number
The refractive index depends on the carrier density in the well and the dot
Dots = two level atoms almost in resonance. We assume that n does not depend on the density of carrier within a dot

19. The model First results show that damping of the relaxation
Oscillations supress the LFF regime…
but more work is required

20. Future
Provide a full description to describe the sensitivity of QD lasers to optical feedback
Add-on other effects such as inhomogeneous broadening, multimode dynamics and so on
Study these effects with Quantum Dot Vcsels.

21. Conclusion
Measured the line-width enhancement factor of quantum dot lasers
Observed a reduce sensitivity to optical feedback
Showed the importance of the daming rate of the relaxation oscillations
Introduced a model to study these effects

22. Acknowledgements DOTCOM Project Science Foundation Ireland
VISTA Network
Kenton White (Bookham) for providing Quantum well and quantum dot devices