ISOMET. Compensation look-up-table (LUT) and Scan Uniformity

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1 Compensation look-up-table (LUT) and Scan Uniformity The compensation look-up-table (LUT) contains both phase and amplitude data. This is automatically applied to the Image data to maximize diffraction efficiency and improve the uniformity across the scan according to the output frequency. The LUT is a fixed 2048 points covering the full frequency range of the ims4-. (12.5MHz - 200MHz). [The LUT can also be programmed with a user defined 12-bit word and is output when the specified frequency(s) are called. It is an alternative to the Image point defined Synchronous data field programmed in the Image data. One or other is selected as the source for the synchronous output on connector J7 of the ims4- ]. A little background theory to explain: Maximum diffraction efficiency exists when laser beam and acoustic column in the AO device are at the Bragg angle. This angle is a function of the RF drive frequency and laser wavelength. In an AO deflector, the frequency defines the output scan angle and so the precise Bragg angle is continuously changing. For optimum performance the Bragg angle needs to be readjusted according to the RF drive frequency (= scan angle). A fast and dynamic method to achieve this angle correction is to steer the acoustic beam in the crystal using a phased array transducer. Such transducers feature multiple RF inputs, driven with a progressive phase shift. The magnitude of this phase shift depends on the transducer geometry (constant) and the applied RF frequency (variable). At the center frequency, the phase difference between the two AOD inputs is zero. At all other frequencies, there is a positive (+ph) or negative phase (-ph) difference. At any given moment, the frequency and amplitude applied to each AOD has the same value. Only the phase is different between the two or more inputs of the AOD 0th 0th 0th IN -1st IN -1st IN J1(0), J2(+ph) J1(0), J2(0) J1(0), J2(-ph) -1st Low Freq Centre Freq High Freq To summarize: Controlling the phase as described above corrects for the Bragg angle error. These are the Phase values in the Compensation Look-Up-Table (LUT). These are appended to the Image point frequency and amplitude data according to the Image point frequency. A second function of the LUT is to correct for frequency dependent amplitude variations. e.g. conversion losses in the transducer gain variations in the RF amplifier 1

2 These are the Amplitude values in the Compensation Look-Up-Table (LUT) These are multiplied with the Image point amplitude data according to the Image point frequency. The LUT is downloaded into the ims4-p at start up. Once configured, the LUT can be loaded and stored in non-volatile memory within the ims4- Subsequently the LUT is automatically applied on power on of the IMS4 (see SDK) Creating the LUT. The LUT used during testing at Isomet will be provided. THIS SHOULD BE ADEQUATE FOR MOST APPLICATIONS For optimal performance the LUT amplitude programming can be repeated during system integration. E.g. the laser input angle and position in the AOD is unlikely to be identical to the Isomet test set-up. Default LUT s. Phase is calculated. The calculated values assume that the Bragg angle is adjusted at the AO mean frequency. This is a near mid-frequency that will give a balanced positive and negative phase shift value at the min/max scan limits. Phase calculation: G o 10 3 ( f ) 180 no f 2 1 f 1 f where: = Device specific Geometric Constant = frequency (MHz) = AO device center frequency = mean frequency for balanced +/- phase shift = fc fc 2 [fmax fmin] = free space wavelength (nm) = refractive index When the phase value is negative, add 360 degrees. The LUT amplitude data is not easy to estimate in advance and is determined during AOD testing. A proficient C++ programmer will be able to develop an automatic calibration routine employing the ADC inputs of the ims4-. Alternatively, two methods using the Isomet GUI and Excel spreadsheet functions are described below 2

3 Both will need optical power meter. LUT Amplitude Determination Typical set-up. Align the laser beam centrally in the AO active aperture. (refer app note: Optimizing Effiency.pdf) - 1st order scan (defined by connection from ims4 to RFA inputs) D1372(X)-aQ110 (TOP VIEW) (Coolant) Aperture/ Block Laser Input ims4-p J5 J4 J3 J2 J1 INT. J1 J2 J3 J4-1st scan RFA0110-2x Host PC In1 RF1 RF2 OPTICS BED 15wD INT As shown, AOD orientated for -1st order In2 RF3 RF4 (For clarity, DC power and coolant connections not shown) Please refer to the ims4-p- manual and Isomet GUI Software Guides; Method 1: Using the Calibration mode, Isomet Studio GUI 1: Select Signal Path panel Amplitude Control Source: Wiper 1 Wiper 1 Power: 100% DDS Power: 50% Amplifier Enable Button: Green (On) 2: Select Calibration panel tab Frequency slider: mean frequency, f1 Amplitude slider: ~50% (starting value) Phase slider: zero degrees 3

4 3: Adjust AO device Bragg angle to find the peak efficiency. Adjust the Amplitude slider, (Calibration panel) to maximize efficiency. If necessary select Signal Path panel and readjust the DDS Power Take care not to apply excessive RF 4: RECORD the efficiency, amplitude and frequency values 5: Repeat the process for a (say) 10 frequency points across the desired scan Select Calibration panel tab Frequency slider: desired frequency, Amplitude slider: ~50% (starting value) Phase slider: calculated degrees (starting value) FIRSTLY Peak the efficiency by adjusting the phase slider value. (In most cases the calculated value is appropriate and readjustment gives minimal improvement). SECONDLY Match efficiency (step 4): at Bragg by adjusting amplitude slider value 6: RECORD the efficiency, amplitude, frequency and phase values 7: Repeat for next frequency Record all values. The LUT table is a defined format. Enter your values to the nearest LUT frequency point into the Isomet LUT generator routine. (under development) 4

5 Method 2: Using the Image mode, Isomet Studio GUI Follow steps 1 to 3 of Method 1 above to set the correct Bragg angle 1: Load an Image file that generates a linear frequency sweep. Data for the Image file can be generated in an Excel Spread sheet e.g. 120pt_40M_Swp.xls and copied into the Isomet GUI or be created directly in the GUI input table. 2: Import and Download the basic compensation table that applies to the AOD. e.g. 100%_ampl_LUT_Dxxxx. This contains calculated phase compensation and a constant amplitude weighting of 100%. e.g. Plot of LUT phase and amplitude weighting values: 3: Select Signal Path panel Amplitude Control Source: Wiper 1 Wiper 1 Power: 100% DDS Power: See below Amplifier Enable Button: Green (On) Set the DDS Power (and is required Wiper 1 Power) to give approximately 50% average efficiency level. This will determine the unsaturated non-(amplitude) compensated sweep response of AOD. Example: Trace 4: = 100% efficiency level Trace 2: = 0% efficiency level (and sync pulse) Trace 1: = detector output 5

6 4: Record 5 (or more) data points from the photodetector output. e.g. start value, stop value, all peak and trough values. 5: Use a cubic-spline software* routine to fit a normalized curve function to the detector data points. 6: Invert this data and scale. Apply the result to the amplitude LUT value for each frequency point in the image 7: Generate the LUT file with phase and amplitude programmed values. As of , data for the Compensation LUT file must be generated in an Excel Spread sheet e.g. CPEN_ampl_LUT_Dxxxx.xls and converted into the correct format using the Isomet routine. ims4_lut.exe. In the Isomet spreadsheets, shaded areas indicate user input. Phase calculation is automatic and depends on the frequency and AO device parameters. Drag the xls file over the ims4_lut.exe to run (or type file name after the.exe) 8: Import and Download the new compensation table that applies calculated phase compensation but now with measured amplitude weightings Trace 4: = 100% efficiency level Trace 2: = 0% efficiency level (and sync pulse) Trace 1: = detector output 6

7 9: Adjust the global power level to reach the desired efficiency. i.e. Select Signal Path panel and readjust the DDS Power. Take care not to apply excessive RF Trace 4: = 100% efficiency level Trace 2: = 0% efficiency level (and sync pulse) Trace 1: = detector output * for Excel, spline fit functions can be downloaded free from SRS1CubicSplineForExcel Detector output spline fit * Inverted, normaliz ed detector output Compensated LUT Ampl' data % Measured data points 7 Freq. MHz Freq sweep start MHz Freq sweep stop MHz (copy paste active freq values from LUT worksheet into Col H:) ** Detector Output Freq Normalized Data points Scope div Y-scope MHz Output pre pre-point measured measured measured measured measured measured measured post 8 3 post-point ** Enter additional pre and post data points for a smoother spline fit. Enter points from peaks and troughs of oscilliscope trace of photo detector output