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4 PAX1000 Settings

Prior to explain PAX1000 Setting, lets shortly recapitulate the

measurement procedure that is discussed in detail in section

Rotating Waveplate Technique :

The PAX1000 uses a rotating quarter waveplate for polariza-

tion analysis.

A half turn of the waveplate is sufficient to uniquely determine

the state of polarization. The parameter Basic Sample rate is

always related to this half-turn.

The photo current measured during this half turn is analyzed

by a Fast Fourier Transformation (FFT). By default, the FFT

"splits" the data into 512 measurement points (ADC values).

In order to increase the measurement accuracy:

· the number of FFT points can be set to 1024 or 2048;

· the FFT can be carried out over a full turn or over 2 full turns

of the waveplate.

The parameter Operating Mode reflects that:

For example, the operating mode "1/2 (512 pts FFT)" means, that the data for calculation are

collected per 1/2 revolution of the waveplate (number of basic periods =

), and the FFT is

carried out on 512 points (number of AD converter values) within this half turn. So, the resulting

sample rate or measurement speed can be written

Mathematically, the number of points used for FFT does not impact the measurement speed.

The more FFT points are used, the higher the measurement accuracy, but the higher is also

the workload of the microcontroller. Particularly, at high measurement speed and high FFT res-

olution, measurement data might be not captured because the microcontroller is busy with FFT

calculation.

Recommendations

For fast measurements use a high basic sample rate, a small operating mode value and fewer

FFT calculation points. At this setting, measurement accuracy might be impacted.

When measuring an optical signal at low power power or with significant noise, decrease the

basic sample rate, increase the operating mode and use more FFT points.

Note

When the PAX1000 is powered via USB only, the Basic Sample Rate can be set between 50

and 100 Hz. With external power supply, the maximum Basic Sample Rate is 400 Hz.

THORLABS PAX1000 User Manual

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