EVALUACIÓN DE LA PROPUESTA PEDAGÓGICA ALTERNATIVA

The intensity of light detected by the two photodiodes in the balanced detector

system is determined by the phase between the probe and reference pulses, with a Cos function dependency shown in Figure 8.

Signal 0.5

-0 .5

- 1 _2n

0 71/2

Phase

Figure 8. Normalised signal from balanced detector system

If the probe and reference pulses are in phase, no signal will be detected by the photodiode in the TM arm of the detector system, and a maximum signal will be

detected in the TE arm. Equivalently, a phase difference of 180 degrees will result in an entirely TM polarised signal. Any phase difference between these two extremes will produce a signal on each of the two detectors, as indicated in Figure 8.

The phase difference between the probe and reference pulses is stabilised at %!2

so that small changes in the phase of the probe will produce a linear change of intensity as detected by the two photodiodes.

The intensity of the pump pulse in all of the experiments described in this chapter was kept below that which gave a phase shift of %/6, so that the measured

signal was always within the linear region shown in Figure 8.

The phase shift detected by the interferometer was calibrated before results in any operating regime were recorded. The calibration was performed by blocking the pump pulse and switching off the stabilisation box, and then translating one of the arms of the interferometer so that it passed through several interference fringes. The

magnitude of the fringes, measured as a voltage by the balanced detector system, was recorded using a storage oscilloscope. The voltage corresponding to the height of an

interference fringe relative to zero volts was a phase shift of nil. All measurements recorded in that operating regime were then cahbrated against this voltage.

3,3.4 Using the Time^Division Interferometer

As has previously been described, care was taken to ensure that the probe, reference and pump beams were all collinear on the input side of the amplifier under investigation. It was also required that the probe and reference beams be collinear upon recombination afi;er travelling in their respective delay arms on the output side of

the interferometer, so that the probe and reference would be overlapped and would produce interference fiinges. This was done by introducing a vibration to one of the arms of the interferometer to produce interference fringes, and then actuating the comer-cube and two-mirror retro-reflectors until the interference pattern produced was maximised.

The intensity of the reference pulse was adjusted as necessary on the input side of the experiment, such that it was equal to the probe pulse on the output side of the experiment. This provided an interference pattern that was symmetric above and below zero, as measured by the balanced detector system (see Figure 8).

It was desirable that attenuation of the probe pulse by the pump was kept to a

minimum to prevent distortion of the interference pattern produced by probe and reference pulses, and thereby avoid distortion of the phase shift measurements. To this end, the intensity of the pump beam was kept to a minimum, and phase shifts measured

using the TDI are consequently small (less than tc/6). This limitation of the size of

measured phase shifts also ensured the linearity of phase shift measurements as

described in section 3.3.3.

The time delay between the pump and probe pulses was adjusted via a stepper motor, which was adjustable in micron steps using a personal computer. The computer program used for the experiment was adapted from one previously used to perform pump-probe experiments. When taking measurements, the computer actuated the stepper motor and then paused for three tenths of a second to allow the stabilisation

circuit to re-establish a %H phase difference between the probe and reference beams, A lock-in amplifier then measured the signal produced from the balanced detector scheme which was recorded and plotted on a monitor screen in real time.

In theory, the effect of the pump beam on the measured signal was neutralised

by virtue of its intensity being equally detected by the two detectors in the balanced detector system. However, in practice, the polarisation rotation elements used in the experiment were not perfect, and the balanced detector system picked up a signal

derived fi*om the pump beam. This signal interfered with the reference beam around the zero time delay point and obscured the detected phase shift:. The problem was reduced

by mounting one of the mirrors which was used to steer the pump beam on a piezo­ electric transducer, and modulating the transducer so that interference caused by the pump beam was averaged to zero.

A further improvement to the signal-to-noise ratio obtained from the experiment was realised by performing each measurement three times and averaging

the results.

3.4 Results