1. Integrated Semiconductor Modelocked Lasers
Hazara University
Mansehra, KP, Pakistan
Integrated Semiconductor Modelocked Lasers
Dr Jehan Akbar

2. Outline Introduction Introduction to Semiconductor lasers
Modes of a Laser
Semiconductor mode locked lasers
Wafer structure
Modelocked lasers features & fabrication
Devices structure
Devices characterization
High power modelocked lasers

3. Semiconductor lasers
The Semiconductor Laser was Invented almost simultaneously by four groups in 1962.
In 1972: Charles H. Henry invents the quantum well laser, which requires much less current to reach lasing threshold than conventional diode lasers and which is exceedingly more efficient
Comparing to the other types of lasers, semiconductor lasers are attractive due to their compact size, direct electrical pumping, high efficiency and low cost.
Semiconductor technology is easy to make and compatible with other electronic devices
In September 1962 researchers from IBM, independently and almost simultaneously with researchers fromGeneral Electric and MIT's Lincoln Laboratory, demonstrated laser action in the semiconductor gallium arsenide
Semiconductor lasers can emit light in a wide spectral range spanning from the near ultraviolet to the far infrared
The most commonly used semiconductor laser material systems include GaAs/AlGaAs, InGaAsP/GaInAs/InP and InGaAs/AlGaInAs/InP

4. Semiconductor Lasers: Basics
In semiconductor lasers, electrons and holes are injected into the active region
through electrical pumping, which introduces population inversion and produces
optical gain via stimulated emission. If the injected carrier density is large enough,
the stimulated emission of the photons overcomes the losses and the laser achieves
gain.
Electrical Pumping
Population inversion
Stimulated emission
Lasing action (Laser)

5. Mode locking
Mode-locking is a technique used to generate coherent, high repetition rate and ultra short pulses by virtue of phase locking of the longitudinal modes inside a laser cavity λ
Intensity λ
Laser Output Spectrum
Intensity

6. Schematic of a Modelocked Laser
Frequency of the laser corresponds to the total length of the cavity:
Practical constraints limit stable mode locked operation to 640 GHz
Higher repetition frequencies are obtained by using Harmonic mode locking

7. Device Features Single mode operation:
The ridge waveguide of the laser was optimized by beam propagation simulations for single mode Operation of the device.
n-InP substrate
MQW-GRINSCH
Ti/Pt/Au
SiO2
AlGaInAs dry etch stop layer

8. Experimental setup for output power measurementsGe PD
Device
Temperature controlled Copper
mount

9. Output Power Measurements
Current
Voltage
Average output power is more than 50 mW

10. Experimental setup for mode-locking characterisation

11. Mode Locking results SA 3V, Gain current 60mA AC Pulse train
25.3ps
∆t = 0.9ps
AC Pulse train
Isolated Pulse

12. Mode locking results: Cont;
3 dB BW 9.2 nm
Optical spectrum
The pulse width increases as the gain current is increased. This is due
to the increase in the non-linear effects such as self phase modulations

13. Radio Frequency (RF) Measurements
SA 3V, Gain current fixed at 60mA
∆ʋ = 130kHz
RF spectrum (full span)
RF spectrum (zoomed)

14. Far-field Measurement Results
Farfield-3D view
Farfield-2D view
3 QW Laser

15. Problems in MLLS Mode locking – optical pulse generation
Noise in semiconductor mode locked lasers
Simplified & inexpensive method for reducing phase noise in PMLLDs
Pulse stabilisation and sub-picosecond jitter in a 40 GHz PMLLD
Solution
All-optical regenerative mode locking

16. Passively operating mode locked laser at 40 GHz
Pulse width = 2.1ps

17. Noise in mode locked lasers
Changes in amplitude and phase in the circulating field due to:
Spontaneous emissions
Thermal and other technical noise
Resonator losses
Phase fluctuations – random walk
Linewidth enhancement factor – differential gain
Schawlow–Townes equation for linewidth of laser is :
where Toc denotes the output coupler transmission, ltot the total resonator losses (which may be larger than Toc), Trt the resonator round-trip time

18. Optical regenerative mode locking

19. 40 GHz laser – jitter and linewidth reduction

20. Supermode noise

21. Supermode noise suppression technique

22. 20 GHz Passively mode locked laser

23. Supermode noise suppression - results

24. Linewidth and phase noise reductions

25. Optical spectra and pulse width
3dB Bandwidth = 5 nm
Δpw = 2 ps

26. Conclusions AlGaInAs/InP Mode-Locked Lasers operating at 40 GHz:
Stable single mode output, Lower pulse widths and RF line-widths
Wider range of stable mode locking
Increased coupling efficiency with optical fibers due to lower divergence angles
Regenerative Optical Mode-locking:
Simplified & inexpensive method for reducing phase noise
Pulse stabilisation and sub-picosecond jitter in a 40 GHz MLL
Super-mode noise suppression > 40 dB using composite cavity loop
Not limited by high frequency driving electronics (i.e. low noise terahertz lasers)

27. Thanks a lot for your attention !
Hazara University,
Mansehra, KP,
Pakistan
Thanks a lot for your attention !