Pump Probe Measurements of Femto-second Pulses

By David Baxter

Femto-second Pulse

• Train of femto-second pulses generated by one of our titanium sapphire mode locked lasers.

• Cannot use conventional diode detectors as they are not fast enough (~0.1ns).

Time

Inte

nsit

y

10 ns

100 fs

Experimental Arrangement

PM tube

SampleUp conversion Crystal

Translational retro-reflector

Femto-second pulse

Beam splitter

Probe →

Pump → Response →

Pump Beam

• Pump pulse excites a response from the sample under study e.g. A semiconductor optical amplifier (SOA).

• 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 response.

• Using the delay stage to ‘scan’ along response.

• 150 ps in time requires a path change of 45 mm.

• 100 fs resolution requires a path change of 30 μm.

Time (ps)

Inte

nsit

y

Response

Probe

Up Conversion

• Non-linear crystal takes two identical photons and creates one larger photon.

• Acts as a gate as will only give a response when probe pulse coincides with response pulse.

ω,k

ω,k

2ω,2k

Experimental Set-up

• Probe pulse is focused onto sample.

• Pulse is injected through a window in the base of the sample.

• Response is then steered towards up-conversion crystal.

Data Analysis

• Rep-rate of pulses is 100 kHz (10 ns).

• At each point perform averaging over many pulses.

• Signal from PM tube is a convolution of response and probe pulse.

• Can fully characterise probe pulse and therefore deduce response.