1. 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?

2. What Are Diode Lasers?
To make a laser you need a gain medium (semiconductor) with feedback (facet mirrors).
Light
Amplification by the
Stimulated
Emission of
Radiation
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 (n)
InP (p)
Metal
Active region
Trench
Used in telecom, domestic, medical and research applications.

3. What is an Ultrafast Laser?
Intensity
100fs
Time
12.5ns
Intensity
To measure with fs resolution, fs events are required.
Time

4. Non-linear Optics Optical polarization
Induced polarizations within a crystal become non-linear at high E-field intensities due to asymmetric electron potentials.
Amplitude
Fundamental polarization
Linear response
Amplitude
Amplitude
Second-harmonic polarization
Non-linear response
Amplitude
Amplitude
Steady dc polarization
Amplitude

5. Non-linear Optics Cont…
1,k1
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.
21,k3
Non-linear crystal
1,k2

6. Auto-correlation Set-up
By moving retro-reflector the relative path difference is also changed.
Beam splitter
Lens
SHG
Non-linear crystal

7. Auto-correlation Cont…
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
E-field envelopes
Time (ps)
Overlap generates signal

8. Cross-correlation Set-up
Sample
SFM

9. Pump Beam Intensity Response Time (ps)
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
Response
Time (ps)

10. Probe Beam Probe Intensity 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
Response
Time (ps)

11. 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* without chirp
PG FROG* with chirp
Frequency
Frequency
Requires a FROG algorithm to return intensity AND phase information.
Time
Time
* FROG traces generated using Femtosoft Technologies Java Applet at

12. What Can We Measure With Fs Pulses?
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
Intensity
Round-trip loss
Intensity
Time
Time

13. Current Status Acknowledgements
I am currently characterising the fs pulse from the ultrafast laser using my auto-correlation set-up.
Acknowledgements
Professor Jeremy Allam
Dr Konstantin Litvinenko