PHTN1400 Principles of Laser Systems

PHTN1400 Principles of Laser Systems

Laser Light Show Laboratory

An open-shutter photograph of the classic light show Laser U2, as shot by the professor in the MacMillan Planetarium, BC, in 1988.

Introduction

In the high power laser lab (V15), an argon-ion laser with two high-speed Cambridge Technology closed-loop scanners will be used to implement a basic laser light show. As well as beam scanning parameters, blanking techniques (using an AOM) will be investigated.

PreLab

Investigate the basics of laser scanners as used in light shows
Investigate the principles of AOMs
Read the Standard Operating Procedure (SOP) for the Uniphase 2214 air-cooled ion laser

Bring a digital camera to capture images and oscilloscope waveforms

Review the Worksheet so you have an idea of the observations you will be required to make.

Examine the vector format for the scanner for Part D and produce a CSV frame is desired

A few links which might be helpful ...

ILDA Laserist The site of the International Laser Display Association
ELM Homebuilt Laser Projector and excellent page covering ALL of the basics of scanning

The argon used in this lab is a class-IIIB laser producing over 50mW of power. Safety glasses suitable for an argon (i.e. with an OD > 2 at 488nm and 514nm) are required when exposure to the beam is possible. The beam will be aligned at low powers (< 5mW), stray beams blocked, and the actual light show viewed without laser glasses so the utmost care is required to ensure exposure to a direct beam or reflection is not possible before safety glasses are removed.

Experiment Setup

Start the laser by setting the IDLE/RUN switch to the IDLE position, ensure the cooling fan is ON, turn the keyswitch ON, and turn the DISCHARGE switch ON. After a preheat cycle the laser will start and a low-power idle beam will appear. Position the scanner deck so that the beam from the argon laser passes through the entrance apeture in the scanner deck and onto the scanner mirrors. The scanner heads are driven from the driver/power supply module which is connected to the scanner deck via three DB-9 connectors (which connect from under the deck). The signal source for the first part of the experiment is two function generators and the signals are monitored via a connection to the two inputs of a dual-channel TDS-1002 oscilloscope.

Figure 1: Prototype Laser Show setup including signal generators and oscilloscope

The actual scanner deck, pictured below, contains two Cambridge Technologies 6350-136 galvanometer scanners mounted at right angles to each other. The entrance apeture assures proper alignment of the beam. The deck also features tapped holes allowing the addition of an Acousto-Optic Modulator (AOM), used as a blanker, and a PCAOM, used as a colour selector with multi-wavelength lasers.

Figure 2: The scanner deck

Figure 3: Diagram of the setup

Part A: Lissajous Patterns

Start the argon laser and operate at IDLE power. At this power level safety glasses may be removed - do NOT touch the power controls until alignment is complete (i.e. do not increase output power beyond 5mW without safety glasses).

Align the scanner so that the beam strikes the scanner mirrors at right angles exiting to hit the wall where the patterns are to be projected. The beam from the argon laser must strike the mirrors near the centres. Now, block all stray beams which might possibly exit towards the viewing audience.

Wire the scanner amplifier, oscilloscope, and signal generators so that outputs from the signal generators (BNC connectors) are directed to the scanner inputs (also BNC connectors). Each signal from the generator passes through a TEE and is simultaneously monitored by an dual-channel oscilloscope - one channel monitoring the X signal and the other monitoring the Y signal. Set each generator for a 300Hz sine wave, 1 V P-P output. The pattern should form on the wall - do NOT continue unless the pattern is moving. The beam may be positioned by adding a DC offset to the wave (via the OFFSET control on the signal generator).

Block any stray reflections from the scanner then increase the output power of the laser to about 50mW. The pattern can be clearly seen on the wall - safety glasses may be removed to view the pattern however be careful to avoid interception of the beam at any point! At any time when adjustments are to be made to the optical deck, reduce the power to IDLE.

Adjust the frequency of one generator by +/- 0.1Hz to allow the pattern to slowly rotate (the rotational period will be 10 seconds). Vary the frequency slightly and produce a circle as well as a diagonal line by varying the relative phase of the two signals (X and Y). A circle requires that the signals be exactly 90-degrees apart as per figure 4 below which details the X and Y signals and the resulting pattern.

Figure 4: A Lissajous circle and the waveforms used to produce it

Laser Show - In Phase Signals Replicate the in-phase signals as seen on the oscilloscope traces here by varying the frequency of one generator slightly until a diagonal line is seen to be projected. At that point, set both frequencies to be equal to stabilize the pattern. By examining the effects adding a DC-offset from the generator, determine which trace on the scope is X (with the other being Y), as well as which axis corresponds to 'UP', 'DOWN', 'LEFT', and 'RIGHT' (do not assume they are the same as figure 4).

Now, produce several types of lissajous patterns as per the photographs below (which were actually produced using this configuration) by varying the frequency of the X and Y signal generators. Stable patterns will be produced when they are multiples of one another (e.g. when f_X = 2 f_Y, f_X = 3 f_Y, 2 f_X = 3 f_Y, etc.).

Figure 5: Various Lissajous patterns

For each pattern, photograph the pattern (use a slow shutter speed) and describe the signals used to produce it as well. Produce a diagram for each Lissajous pattern (four minimum) which resembles figure 4 above showing (on the same page):

The pattern (photograph)
The signal which produces it
Lines linking various points on the two figures (e.g. where a signal hits a positive peak, negative peak, and zero)

Label the axes on the signals as well (Up, Down, Right, Left).

Part B: Scanner Performance

The biggest limitation on scanner performance is inertia of the galvanometer movement and attached mirror which affects the ability to slow the current motion, rapidly, and change direction. Consider the situation at the corners of a square (produced using two square waves instead of sines):

Use square waves to determine the effect of dampening and PID control (from these closed-loop scanners) in controlling acceleration of the scanners. Starting with a slow frequency, measure the length of on side of the circle and the overshoot and express the overshoot as a percentage. Set the scanning frequency progressively higher and note the effect on the resulting pattern (i.e. a table of 'Scanning frequency' versus 'Overshoot %').

Part C: Blanking (using AOM)

Use of an Acousto-Optic Modulator (AOM) allows the beam to be rapidly blanked - this is required for many patterns such as the production of text (i.e. between letters). Produce a circle pattern with the signal generators set to 300Hz. Install the ISOMET AOM on the scanner deck, inject a TTL level (0 to +5V) signal from a third signal generator into the AOM at a frequency of approximately 3300Hz (+/- 5Hz). Vary the frequency of the blanking signal until a clear pattern is seen (with the circle broken into blanked segments) - this will require adjustment of the angle of the AOM (you must research this as part of the prelab). Photograph the pattern and note the frequency of the blanking signal generator.

Part D: Vector Graphics

Replace the signal generators with the V-30 DSP-based scan engine. Press pushbutton 'S1' on the DSP board and select the 'Radiation' sign graphic. Note how small vectors are used to reduce inertia of the scanner mirrors by preventing high speeds. Monitor the signals on the oscilloscope again and relate the vectors seen to the pattern. Select the ILDA test pattern as well.

Figure 6: A Vector Graphic

In the above example, the scanner is seen producing a vector graphic from the V-30 scan engine. The blanker is clearly not aligned properly in this case since lines are seen between visible segments.

Produce a worksheet (similar to Part A) showing the graphic as well as the signals (on the oscilloscope) used to produce it. Again, annotate the page to show key points. Now, set the scope to "X/Y" mode and observe!

Finally, compare the actual vectors (the format is explained below) to the pattern and identify at least TEN vectors in the file and the corresponding segment on the frame. For example, identify a complete vector such as ...

1,3666,995     ;Start point (X=3666, Y=995)
1,3621,1376    ;Ending point (X=3621, Y=1376)

... and show precisely where this vector is on the photo of the frame. Identify five such vectors.

DSP Scan Engine OPTIONAL: You may upload your own vector-graphics file to the scanner using a terminal emulator such as TeraTerm. First, compose a vector graphics file using a tool such as MKV2 vector tracer (available on the web from the ELM site listed in the pre-lab). This program will produce a CSV (comma separated value) file as follows:

VECTOR,12000,3
Radiation       
0,3666,995
0,3666,995
0,3666,995
0,3666,995
1,3666,995
1,3666,995
1,3621,1376

You may see the entire file RADIATION.CSV here

The first line contains the VECTOR command (which is how the scan engine recognizes an upload). Actual points are found beginning at line 3. The first column is visibility (0=invisible, 1=visible) and the other two X & Y coordinates with values ranging from +32767 to -32768 (i.e. a signed 16-bit number). Once such a file is composed it may be uploaded as follows:

Open a serial connection to the V-30 scan engine and ensure a prompt ("V30>") is received.
From the emulator program SEND the CSV file as an ASCII TEXT file. It will be loaded into the RAM frame buffer (0).
Issue a "RUN 0" command or press the S2 pushbutton on the DSP board to begin scanning from RAM
Preprogrammed frames can be scanned by pressing the S1 pushbutton on the DSP board which cycles through patterns already programmed into flash memory. Alternately one may issue the "RUN x" command where x is a number representing the flash memory location.
Scan speed may be changed using the "SPEED" command. Valid values include "SPEED 4000", "SPEED 6000", "SPEED 8000", "SPEED 10000", and "SPEED 12000" where the numbers represent points-per-second.

PRACTICAL NOTE ON FRAME SIZE: The V-30 scan engine has, in its current form, a frame buffer limited to 1000 points. This is a practical limit since a 1000 point frame, scanned at 10000 pps, has a refresh rate of only 10Hz and so will appear to flicker. A small frame, like 'Radiation' at 235 points, will have a fast refresh rate and be seen as a continuous image. Keep images small for best performance.

Lab Submission

Hand in a minimum of four worksheets (example here) for four different Lissajous patterns.
A table showing the overshoot as a function of scanning frequency and any other interesting observations from that part of the experiment.
One page describing how an AOM works. Be sure to detain things like deflection angles.
The worksheet and observations (including identification of individual vectors) from Part D.