Dual FSI 1

Halfway PresentationDual Laser FSI: Distinguishing lasers using

AM tuning speed differences

4th August 2009Alice Lighton

Dual FSI 2

Aim of project• Want to use two lasers, scanning in opposite directions,

to measure one distance.• Need a way of distinguishing the laser signals from each

other• Amplitude modulate the signals!

Dual FSI 3

Equipment set-up • Have a lot of equipment nobody has ever used before.• Most of it wasn’t working.• Most of it now is.

Dual FSI 4

What do I have?• New DAQ can take data at 50MHz• New photodiodes from telecommunications devices• Photodiodes and ADCs are both in boxes with amplifiers

(the “p-amp” and the “adc-amp”• New amplifier controller gadget

Dual FSI 5

Gadget for controlling amplifiers

This is a USB device plugged into mealicar1

I made a LabView program to set the amplifier gains

Unfortunately this got lost due to problems with car1 and the network so I will need to do it again.

Dual FSI 6

ADC + Amplifiers• Using a signal generator,

worked out which inputs were upside down

• Turns out the signal is AC coupled so it doesn’t really matter

• Obtained a gain curve• Tested frequency response

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Amplitude Out/.1mV

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Dual FSI 7

Photodiode box• Half the photodiodes don’t work –

they are static sensitive so probably damaged during installation

• Signal is noisy – wide bandwidth• There is apparently some in-built

filtration, before the amplification stage, but this isn’t very good

• Hopefully the java will take care of filtering sufficiently

• Due to their origin (telecomms) the diodes work from very little signal – easy to saturate them

RMS Intensity vs Frequency

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Frequency/MHz

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nsity

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-2 V

Dual FSI 8

AM modulatingTwo possible methods:• Use external modulation on laser• Use little gadget – the JDSU X5

Dual FSI 9

Externally modulating the laser

Works!!Signals look like sine waves – used low power as diodes

very sensitive.The spectrum we expect when we Fourier transform AM

signal is central peak with two sidebands.

Dual FSI 10

Central peak at modulating frequency

Sidebands at modulating frequency ± FSI frequency

Filter

Dual FSI 11

The length analysis programme can find the sidebands, even though they are small.

Haven’t put them through the dual laser analysis yet because it wasn’t working.

Possible problem: the laser frequency might be affected by external modulation. Need to compare laser modulation with AM modulator.

Dual FSI 12

JDSU X5Setbacks with this – needed a connector from America – so

only just got it working.

It also works!!! (but haven’t actually made length measurements yet)

Dual FSI 13

Signal isn’t perfect because laser isn’t scanning, but the device works – get a peak at 1MHz

Dual FSI 14

Other problemsThe signal from the reference interferometer is very noisy -

the length analysis finds ~ 100 peaks in the spectrum of the reference interferometer when it finds ~10 for one on the table.

May be able to get rid of this by playing with computer filtering, making sure the vacuum is good.

Dual FSI 15

What I’ve done• Tested the equipment, found out what works, and had

the parts which don’t work fixed at least to the extent that they are operational (have 4 channels working)

• Have played with equipment and found a set-up which gives good (analysable) data

• Learnt how to use the java analysis core and made a couple of bits of programs (eg finding RMS of data, inverting data)

Dual FSI 16

What I now need to do• Compare the X5 and the external laser modulation

device – idea situation would be to use both, one for the Agilent, one for the Santech

• May need to get the other X5 set up• Take dual laser readings• Use Java to analyse it• Write up a guide to getting useful data from the

equipment (already in progress)