Hazara University
Mansehra, KP, Pakistan
Integrated Semiconductor Modelocked
Lasers
Dr Jehan Akbar
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
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
 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
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)
Mode locking
Mode-locking is a technique used to generate coherent, high repetition rate and ultra
Intensity
short pulses by virtue of phase locking of the longitudinal modes inside a laser cavity
λ
Intensity
Laser Output Spectrum
λ
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
Device Features
Single mode operation:
The ridge waveguide of the laser was optimized by beam propagation
simulations for single mode Operation of the device.
Ti/Pt/Au
SiO2
AlGaInAs dry etch stop layer
MQW-GRINSCH
n-InP substrate
Experimental setup for output power measurements
Device
Temperature controlled Copper
mount
Ge
PD
Output Power Measurements
Current
Voltage
Average output power is more than 50 mW
Experimental setup for mode-locking characterisation
Mode Locking results
SA 3V, Gain current 60mA
25.3ps
∆t = 0.9ps
AC Pulse train
Isolated Pulse
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
Radio Frequency (RF) Measurements
SA 3V, Gain current fixed at 60mA
∆ʋ = 130kHz
RF spectrum (full span)
RF spectrum (zoomed)
Far-field Measurement Results
Farfield-2D view
Farfield-3D view
3 QW Laser
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
Passively operating mode locked laser at 40 GHz
Pulse width = 2.1ps
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
Optical regenerative mode locking
40 GHz laser – jitter and linewidth reduction
Supermode noise
Supermode noise suppression technique
20 GHz Passively mode locked laser
Supermode noise suppression - results
Linewidth and phase noise reductions
Optical spectra and pulse width
3dB Bandwidth = 5 nm
Δpw = 2 ps
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)
Hazara University,
Mansehra, KP,
Pakistan
Thanks a lot for your attention !