User:Pranav Rathi/Notebook/OT/2010/05/12/Characterization of AOM module
I am going to use AOM as power modulator for the tweezers: AOM (Gooch & Housego R23080-2-1.06-LTD; 138252) operates in CW and normal mode.I am using 1st order diffraction beam. So there is need to characterize 1st order in CW and normal mode. In cw mode-operation characterization is done; power output in 1st order diffraction beam Vs input laser power in ascending order. In normal mode-operation Characterization is done; analog RF-input voltage Vs output power in 1st order diffraction beam, for several input laser powers. I control laser power in 1st order diffraction beam through NI-DAQ (which applies an analog voltage signal from 1 to 5 volts in any increments) controlled by "feedback control 96main" program written in Labview V7.1.
Relatively simple setup ThorLabs power meter is place infront of the diffracted beam from AOM. An aperture is used infront of the power meter to keep off all the stray beams.
CW mode operation characterization:
The AOM is operating in CW mode. Data is recorded; laser power (.25W to 4W in .25W increments) Vs lase power in 1st order diffraction beam. The laser power in 1st order diffraction beam is the usable power for optical tweezers. The data is presented below:
The relationship is linear. AOM gives an average output power of 70% in 1st order diffraction beam . This help us calculating the usable power of any input power without measurement; for example if I set the laser on .10W, then the usable power at the tweezers is 70% of it, which is .07W or 70mW. The maximum usable power for the tweezers is 2.66W. If we want to stay in the single mode operation regime, it is 1.4W (at room temperature). When the laser is cooled down to 60F, it is stretched to 1.9W (2.75W input).
Note: This study was done with 1064nm 4W coherent compass laser. My study showed that this particular laser trips to higher transverse modes at the power above 1.7W.
Later on we replaced this laser with 1064nm 2W crysta laser which does not have any of the problems.
Normal mode operation characterization
In normal mode, AOM is controlled through NI-DAQ by LabView program. Voltage input for RF-input is from 1 Volt to 5 Volts in any Volt increment. 1st order diffraction beam power is recorded for every ascending voltage increment of .1Volt for .5W, 1.5W, 2W and 2.5W input laser powers. Different laser powers are used to check: If the characteristic of AOM is input laser power dependent (which it should not be). RF-input voltage Vs 1st order diffracted beam power will help us in obtaining the best workable range of voltages over which power increases linearly. It will also help us in characterizing the relationship between the voltage and diffracted power. This is very important in data analysis. While data acquisition we can not measure the laser power in trap, it is unpractical but if know the characteristic RF-input voltage Vs laser power in trap we can calculate the laser power in trap from the voltage using the characteristic curve. The data is presented below:
- Data Sheet 1 and Chart 1: By looking the chart it’s clear; diffracted beam power is not linearly related with voltage. It has strange characteristic, which is same for all laser powers: So there is no laser power dependence and any laser power can be chosen to define the AOM characteristic. Characteristic: It runs somewhat exponentially in the beginning from 1 to 1.4 volts, somewhat linear between 1.5 to 3.1 volts (the two straight vertical lines on the chart represents that) and second degree polynomial in the end from 3.2 to 5 volts. So the most appropriate workable range for linear power modulation (in relatively same size of steps) is from 1.5 to 3.1 Volts at any input laser power.
Now since i have the characteristic curve i can calculate the laser power in the trap through the applied RF-input voltage, for data analysis.
- Data Chart3: Shows how power increases in % over 1 to 5 Volts at 2W of laser power. By joining the head of each histogram the characteristic curve can be traced out. Between 1.5 and 3.1 Volt the power increases almost in same step size of avg 3.45% (.069W) /step in comparison to .8% (not linear; over range of 1 to 1.4 Volts) and .42% (not linear; over range of 3.1 to 5 Volt). 3.45% step size is more convenient for power modulation and power is also linear over the steps.
- Data Chart 2: Shows power step in %; the difference between the two consecutive power-increments over the difference in the two consecutive voltage increments (.1 Volt; steps). It gives us the better idea of over what range of voltage, the steps are over all large with somewhat of same histogram height difference. The region between 1.5 to 3.21 Volts contains that.