Download 4PS Talk

Femto-second Measurements of
Semiconductor Laser Diodes
David Baxter
Summary
•What are diode and ultrafast lasers?
•How do we measure with fs resolution?
•Why measure in the fs time scale?
What Are Diode Lasers?
To make a laser you need a gain medium
(semiconductor) with feedback (facet mirrors).
Typical dimensions of the active region are
0.2 x 1.5 x 350 m.
Light output typically
in mW at either
1.3m or 1.5m.
InP (p)
•Amplification by the
•Stimulated
•Emission of
•Radiation
Metal
InP (n)
Trench
InP (p)
Used in telecom,
domestic, medical and
research applications.
•Light
Active region
InP (n)
Intensity
What is an Ultrafast Laser?
100fs
Intensity
Time
12.5ns
To measure with fs resolution, fs
events are required.
Time
Non-linear Optics
Amplitude
Non-linear response
Amplitude
Amplitude
Amplitude
Amplitude
Linear response
Optical polarization
Amplitude
Induced polarizations within a
crystal become non-linear at
high E-field intensities due to
asymmetric electron potentials.
Fundamental polarization
Second-harmonic polarization
Steady dc polarization
Non-linear Optics Cont…
 As a result of non-linear
crystals two photons can be
combined to form a new
photon. This is call frequency
Up conversion
 The beam is produced as
shown to conserve
momentum.
 Second harmonic beam is
ONLY present when incident
beams overlap in time and
space.
1,k1
21,k3
Non-linear crystal
1,k
2
Auto-correlation Set-up
By moving retro-reflector the relative
path difference is also changed.
Beam splitter
Lens
SHG
Non-linear
crystal
 One pulse moves in time
with respect to the other
pulse.
 Signal proportional to the
overlap of the two pulses.
 Resultant trace is a
convolution of the pulse
with itself.
 Can be de-convoluted to
obtain original pulse.
 Only gives intensity
information, no phase
information is gained.
Intensity
Auto-correlation Cont…
E-field
envelopes
Time (ps)
Overlap generates signal
Cross-correlation Set-up
Sample
SFM
 Pump pulse excites a
response from the sample
under study, for example, a
semiconductor laser diode.
 Pump pulse does not
necessarily have to be
same wavelength as probe
pulse.
Intensity
Pump Beam
Response
Time (ps)
 Probe pulse much shorter
in time than the response.
 Using the delay stage to
‘scan in time’ along the
response.
 100 ps in time requires a
path change of 15 mm.
 100 fs resolution requires
a path change of 15 μm.
 Obtain the response
intensity as a function
of time.
Intensity
Probe Beam
Probe
Response
Time (ps)
Frequency Resolved Optical Gating
(FROG)
Can extend the previous technique by replacing the detector with a
spectrometer. Can therefore measure spectrum as a function of time.
This is similar to a music score which dictates the notes (or
frequencies) to be played as a function of time.
PG FROG*
with chirp
Frequency
Frequency
PG FROG*
without chirp
Time
Requires a FROG algorithm
to return intensity AND
phase information.
Time
* FROG traces generated using Femtosoft Technologies Java Applet at http://www.femtosoft.biz/frogapp.shtml
What Can We Measure With Fs
Pulses?
Round-trip loss
Time
Intensity
Intensity
 Pulse trains from a laser – direct measurement of the round trip
gain/loss
 Fast electron decay mechanisms
 Chirp
 Examine responses from new materials e.g. nitride and quantum dot
materials
Chirp
Time
Current Status
I am currently characterising the fs pulse from the
ultrafast laser using my auto-correlation set-up.
Acknowledgements
•Professor Jeremy Allam
•Dr Konstantin Litvinenko