Laser Damage Threshold System For Final Optics Testing

Transcription

1 University of California, San Diego UCSD-LPLM Laser Damage Threshold System For Final Optics Testing Lane Carlson June 5, 2006 Laser Plasma and Laser Matter Interactions Center for Energy Research University of California, San Diego 9500 Gilman Drive La Jolla, CA

2 Laser Damage Threshold System for Final Optics Testing Revised: 5 June 2006 by Lane Carlson Overview: The Laser Damage Threshold System (LDTS) consists of the National Instruments Compact Vision System (CVS) and Basler A602F 100 fps digital CMOS camera. The camera looks into the chamber with a Toyo Optics TV zoom lens ( mm, f 1.8) at the mirror being tested. The vision system inspects the acquired image for a certain percentage of pixels that are above a certain threshold value. If none are found, the CVS outputs a 5V TTL signal to the LPX Pro Excimer laser to trigger it to fire. This sequence continues for every frame captured, with speeds up to 100 Hz. If the algorithm's criterion fails, then the loop terminates and the trigger signal is no longer sent to the laser. The adjacent Dell Dimension 5150 computer communicates with the CVS via the Netgear ethernet card with the IP address and acts as the host computer for programming and controlling the CVS. Vision Builder for Automated Inspection 2.6 (VBAI) is used to create the algorithm for the target CVS and is the communicating interface. The LDTS has user-programmable options depending on the specific characteristics of the test. If the mirror is at the proper grazing incidence angle, then any mirror movement should not affect the image capture. However, if the camera is moved for any reason, then the settings in VBAI need to be adjusted. Recommended settings will be given as well as a tutorial on how these settings can be modified. Operating Procedure: 1. Ensure that the CVS is powered on and the ethernet, firewire camera, digital I/O plug with jumper wire, and monitor are all plugged in. Also, the SMB cable must be plugged into "TRIG 2." This BNC cable should then split to an oscilloscope and to the Ext. Trig. In on the back of the laser. 2. Start up the LPX Pro and set it via its keypad to fire on an external trigger signal. 3. On the PC, open VBAI 2.6 software. 4. Select the CVS1454 on the "Execution Target" pull-down menu, then "Configure Inspection." 5. Open the "Laser Damage Threshold 04" file. The CVS is now ready to run the algorithm and output TTL signals. 6. Press the button "Run until failure" which shows an arrow pointing to an "x." 7. The CVS will now begin inspecting images and outputting the TTL at a rate of ~10 Hz. The monitor will show live images from the camera. Pressing the red stop button will forcibly stop the image capture and TTL signals, as will an image that fails the algorithm's criterion.

3 Algorithm Settings: The software algorithm consists of an "acquire image" block, a "count pixels" block, and various I/O signals for triggering. Acquire Image (IEEE 1394): This block captures the image from the Basler firewire camera. The video mode must be "format 7, mode 0, 656x491." Under the features tab, the brightness is ~780, shutter ~490, and gain ~80. The shutter speed is a critical number - if it is much longer than 490, then the frame rate will decrease from 100 fps. These parameters can be slightly adjusted, as well as the camera's zoom lens, in order to maximize the contrast/light ratio and make it easier to pick out only the failed bright spot on the mirror. Under the format 7 features tab, the region of interest (ROI) is set at Left 290, Width 120, Top 230, and Height 50. Packet size is the maximum By reducing the captured ROI of the camera, the frame rate increases and the algorithm can more quickly search the smaller region for the pixels above threshold. The reduced ROI and simple algorithm is key to reaching the 100 Hz mark. Note that if the camera is moved for any reason, the ROI must be revised to accommodate the beam s new location. A ROI of 120 pixels wide (1.2 mm) and 50 pixels high (0.5 mm) has been found to work well. The ROI is elongated a bit lengthwise in order to accommodate the similarly sized beam footprint. The diode laser beam must be centered in the middle of this ROI, as shown below i.e the Excimer beam s footprint must fall within this ROI. Camera s ROI with diode laser spot (1.2 mm wide, 0.5 mm high) left top height width Camera s ROI specification and location terms Count Pixels: This block counts the number of pixels above a certain threshold within a ROI. Under the settings tab, look for bright objects with a manual threshold. A lower value of 125 works well for filtering out low intensity light and scattered laser light in the chamber. Under the limits tab, the maximum percentage should be checked and indicate 2%. Under normal mirror testing operation, even with the HeNe alignment beam on, the percentage of pixels in intensity range should be zero. Only when a bright flash of light is detected will the pixel intensities exceed the maximum percentage limit. A ROI can be drawn and resized around the desired area to be searched. The ROI shown below is

4 adequate for capturing the entire Excimer beam footprint on the mirror. Note that a smaller ROI with the same spot size will yield a pixel percentage that is higher than the anticipated 2% limit. In the picture below showing the diode laser shining on the mirror, the pixel percentage above the 125 threshold in that ROI is about 3%, close to the limit of 2% and not allowing excessive failure of the mirror. Count pixels ROI within camera s ROI, showing 3% of pixels in ROI above the 125 threshold Zoom lens: The zoom lens is currently zoomed in all the way to maximize the spot size for the camera. It has been focused on the red alignment diode laser, which has been used to simulate the light intensity levels that the camera will see when the mirror fails. The aperture is 4 clicks open from being fully closed in order to filter out all extraneous light and laser reflections in the chamber. The system in not affected by the room lights being either on or off, neither by a flashlight being shown into the chamber. Input/Output: Under the tools menu on top of the screen in VBAI, select configure inputs/outputs. Under the background tasks, pulse generation should be selected. Under setup, doubleclick the pulse train. The line is TRIG 2, dive high, trigger line TTL input 0, rising edge. The delay should be 90,000 microseconds for a 10 Hz pulse train, and a pulse width of 2,000 microseconds is adequate. This will give a pulse width of 2 ms and amplitude 5V (with 1MΩ impedance on the oscilloscope.) (The LPX Pro requires a >15 µs, 3.3 5V, > 5kΩ TTL pulse, and it triggers on the positive slope.) Before connecting the BNC cable from TRIG 2 to the laser, it is a good idea to run the algorithm and verify the pulse train is running at the correct frequency on an oscilloscope. The delay may need to be tweaked a bit if another frequency is desired. For example, a delay of 5,000 microseconds is needed to give a 100 Hz pulse train. Going back to the configure I/O screen, setup static digital output under inspection output. TTL OUT (5) should drive high for an inspection PASS signal. This pass signal from the image algorithm is what triggers the pulse generation. Going back to the configure I/O screen, setup image logging under inspection output. Select log images only when inspection fails and limit the number of logged images to about 20 any more and the CVS s small memory will be overloaded. Setup the image logging and select the folder path to be C:\LASER DAMAGE THRESHHOLD IMAGES with a prefix of LDTfail and a JPEG image quality of 900 (out of 1000). This is the directory on the CVS where the last frame at failure is saved to the memory. After failure, this picture can then be downloaded from the CVS by opening the ftp link CVS on the PC desktop located at ftp:// / and opening the laser damage threshold images folder. These

5 can then be appropriately named with the optic and location number and transferred to the link on the PC desktop called LDT Failure Pics on CVS. An actual mirror failure spot captured by the CVS immediately before the laser stopped after 10 Hz operation. This frame terminated the algorithm after the CVS determined there were more than 2% of the pixels over the threshold limit than allowable. video camera CVS camera w/ zoom lens laser diode Optical table setup showing video camera, CVS digital CMOS camera, and laser diode all looking into the chamber

6 NI CVS 1454 with ethernet, trigger SMB/BNC, monitor, firewire camera, digital i/o plug, and power connected