Summary

•What are diode and ultrafast lasers?

•How do we measure with fs resolution?

•Why measure in the fs time scale?

Femto-second Measurements of Semiconductor Laser Diodes

David Baxter

What Are Diode Lasers?

InP (n)

InP (p) Metal

Active region

InP (n)

InP (p) Trench

•Light

•Amplification by the

•Stimulated

•Emission of

•Radiation

To make a laser you need a gain medium (semiconductor) with feedback (facet mirrors).

Light output typically in mW at either 1.3m or 1.5m.

Used in telecom, domestic, medical and research applications.

Typical dimensions of the active region are 0.2 x 1.5 x 350 m.

What is an Ultrafast Laser?

100fs

12.5ns

Time

Inte

nsity

Inte

nsity

Time

To measure with fs resolution, fs events are required.

Non-linear Optics

Linear response

Non-linear response

Optical polarization

Fundamental polarization

Second-harmonic polarization

Steady dc polarization

Am

plit

ude

Am

plit

ude

Am

plit

ude

Am

plit

ude

Am

plit

ude

Am

plit

ude

Induced polarizations within a crystal become non-linear at high E-field intensities due to asymmetric electron potentials.

Non-linear Optics Cont…

1,k1

1,k2

21,k3

Non-linear crystal

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.

Auto-correlation Set-up

Beam splitter

Non-linear crystal

SHG

By moving retro-reflector the relative path difference is also changed.

Lens

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.

Time (ps)

Inte

nsit

y

Overlap generates signal

E-field envelopes

Cross-correlation Set-up

Sample SFM

Pump Beam

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. Time (ps)

Inte

nsit

y

Response

Probe Beam

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.

Time (ps)

Inte

nsit

y

Response

Probe

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.

Requires a FROG algorithm to return intensity AND phase information.

PG FROG* without chirp

PG FROG* with chirp

* FROG traces generated using Femtosoft Technologies Java Applet at http://www.femtosoft.biz/frogapp.shtml

Time Time

Fre

quen

cy

Fre

quen

cy

What Can We Measure With Fs Pulses?

Inte

nsity

Time

Chirp

Inte

nsity

Time

Round-trip loss

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

Current Status

•Professor Jeremy Allam

•Dr Konstantin Litvinenko

Acknowledgements

I am currently characterising the fs pulse from the ultrafast laser using my auto-correlation set-up.