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SRAS: Detection

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SRAS signal chain
SRAS uses a spilt photodiode circuit to capture the light from the sample.

It is based on the Hammamatsu quad Si-PIN, model S6058 (only 2 of the PDs used)

Recall the SAW causes the light beam to ‘wobble’ as it passes underneath the laser spot. This wobble is converted to an electrical signal by this circuit.

Lets look a one ‘arm’ first

Image from: , All optical scanning acoustic microscope, thesis, 2003

SRAS signal chain
We have a reverse biased photodiode
with a total series resistance of 610 ohms
This limits the current through the photodiode to 25mA (for 15V Vs).
This stops it being damaged if a much brighter laser is used.

The generated photocurrent flows to ground through the centre tap of the RF transformer.

The voltage drop across the 100R resistor is proportional to the photocurrent and is sampled by a differential amplifier circuit as the DC for this channel.

SRAS signal chain
We have the same on the other side

SRAS signal chain
The secondary side of the transformer only sees the effect of the ac part of the current (as it requires a changing magnetic field to transfer energy where N is the turns ratio and is the magnetic flux)

When there is no sound wave modulating the position of the laser spot there are only dc currents flowing through the arms of the circuit.

When balanced the net current flowing through the primary winding of the rf transformer is zero.

SRAS signal chain
As the laser beam oscillates between the photodiodes we get an oscillating current, creating an oscillating magnetic field in the primary side and this leads to a current flow in the secondary side.

The turns ratio is 1:2 giving an impedance ratio of 1:4. The input side impedance is 200Ohms giving an output impedance of 50Ohms which is needed to impedance match to most rf amplifiers.

Notice that the output doesn’t see the DC current due to the transformer so we only see the difference current between the two photodiodes.

This is very useful as it cancels a lot of common mode noise carried by the laser beam and we only need to amplify the small signal not the large DC.

SRAS signal chain
How big would a typical signal be?

What do we need to know?

SRAS signal chain
We need to know:
How much light is on the photodiode
The responsivity of the photodiodes
The change in angle of the light (how far it moves on the photodiodes) for a given SAW amplitude
The Gain of the RF amplifier.

SRAS signal chain
P= 5mW per photodiode ,
R = 0.4A/W

The change in I due to the SAW () on the primary side can be shown to be:
F is the focal length of the final lens
r is the radius of the final lens
A is the amplitude of the SAW wave
I is the photocurrent
is the wavelength of the acoustic wave

At the transformer, when the beam moves we get an increase of so the change in current on the secondary side will be winding ratio

SRAS signal chain
The output voltage will be:

If we use typical values from the SRAS instrument of
A = 0.1nm*,
L =λ= 24um,
Recall N=0.5

The modulation depth (V/V) is ~ 0.001 or 1 part in 1000. so the ac signal is very small

This assumes no gain of output rf amplifier (typically x10) and no dc gain off the circuit (also typically x10) so actual signals off the circuit will be slightly bigger. The AC signal is still too small to digitise for most DAQs so additional amplification will be needed.

*this is dependant on acoustic frequency, generation laser power and material so is a ball park figure.

Actual Detector

SRAS signal chain
SAW causes the beam to change direction due to the changing angle of the surface.
The SAW has an amplitude of A at any given point Z of

The rate of change of the amplitude is

The maximum change in surface angle is:

The beam reflected angle is twice this:

Spot moves d due to SAW

Surface at angle
For completeness not examinable

SRAS signal chain
The reflected beam is collimated by a lens with focal length F, and the beam diameter is D. The angular deviation of the reflected beam is transposed to a lateral deviation d behind the lens of :

Because the angles are really small ( is small compared to ) then:

Surface at angle
For completeness not examinable

SRAS signal chain
The area of the beam on the photodiode is

This generates the photocurrent I
If the beam moves by d due to the SAW then the change in area is (assume a rectangle as d is small) and so the change in current is:

Adding d from above we get

Surface at angle
For completeness not examinable

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