I have constructed a cost-effective mechanical laser shutter system for our 1064 IR-laser. System contains a mechanical shutter, a control box and a power supply. There are many shutter systems available commercially; mechanical, semi-mechanical, electromagnetic and optical. Most of the systems are expensive and above $1000. These systems are fast (msecs to micro secs) and precise. In our case we needed a shutter which is precise and cost effective but not so fast. So here it is. Some of the specifications of the system:
Type mechanical with electronic control. Can also be controlled by a micro controller.
Open time 1.5 second, .3 second over the beam waist.
Cost $ 30 excluding the power supply.
Operational voltage 3.3, 5 and 12 volts.
Laser intensity upto 10 W.
There might be many ways to make a mechanical shutter; I used eclipsing technique in which a shutter moves over and across an aperture through which the beam passes. To move the shutter I used an electric motor and a gear box. This method is relatively hard because of the motor timing (build all the electronics is not easy). While working on this I got couple of ideas of achieving the same goal in 1/10 of the work and time: A 6 inches audio speaker with/without a micro controller can be converted in to a shutter (I will use this method if I plan to make a shutter for He-Ne) OR simply an electromagnet with a cantilever can be used as a shutter. The advantages of such a systems are, easy to make and similar in speed to commercially available systems.
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Step 1: Components
There are three major parts of the system.
The components used:
Glade automatic air spray: Air spray uses a motor and gearbox to mechanically operate the spray bottle, which i used to move shutter.
Thorlab Cage System Removable Filter Holders CFH1: To make shutter holder and shutter.
Two 3 inches cage assembly rods: To install the shutter holder on the skin of air spray.
Black aluminum foil: To make the shutter.
2 3-pole on/off switch
1 Push on/off button
2 CD4013 4013 IC CMOS DUAL D FLIP FLOP IC
1 Resistor 100K Ohm
2 Resistor 10K Ohm
1 Resistor 100 Ohm trimmer (pot)
1 Resistor 1.5k Ohm
3 Resistor 240 Ohm
1 1N4148 diode
1 0.1uF 50V Ceramic Capacitor
1 222uF Radial Electrolytic Capacitor
1 npn KD-7B C1815 Y transistor
2 14 pin DIP IC Socket Adaptor Solder Type
2 RED LED 3mm
1 Yellow LED 3mm
1 A-5032 Prototyping Board 300x400 mm
1 12V relay
1.5m red, black, green and yellow wires
1 6X8 cm aluminum box.
3 Banana connectors M&F (yellow (+12V), red (+5V) and orange (+3.3V))
1 Powe Jack M&F
I used 300W desktop computer power supply which i salvaged from an old computer.
Step 2: Construction
I made the shutter system inside the spray box. First I removed the electronics which controls the motor. Then I made two holes to install the cage rods directly on the box’s skin, these rods support the cage holder at the gearbox level roughly 5 cm away from it. I used a plastic stick to connect the gear-arm with the shutter (inside the holder), so the shutter moves in and out with arm as motor rotates ten full rotations. I also installed two touch sensors at the ends of the shutter, so they can sense when the shutter is fully open or closed. I made these sensors by carving two sets of conduction strips and directly installed them directly on the skin and then solder wires into them. When the shutter travels to the end it pushes on strip into another. Once the strips are connected the current flows so the touch sensor works as an on/off switch and senses the edge of the shutter. I ran totally 5 wires into the shutter 2 for the motor, 3 for the touch sensors out of 1 is common and 1 of each wire is for red and yellow LEDs. Red LED shines when shutter is fully open and yellow LED shines when the shutter is fully closed. I made ¼ inche hole to install the post directly on the skin, so the box can be directly installed on the optical table with the post holder.
I used IC 4013 based pulse generation circuit. The pulse comes out of pin 13 of 4013B IC. The duration of the pulse (FWHM) depends on the value of the RC circuit (t(sec)=R(ohm)xC(uF)). Pulse duration in seconds is given by the product of resistance and capacitance. The slopes of the pulse looks like a capacitor charge- discharge curve. Since the pulse is week it cannot directly operate relay or the motor. So I used NPN transistor to amplify it. The collector is connected to the relay, so the relay operates over the duration of the pulse, which turns-on the motor. So the time over which the motor operates can be directly controlled by modulating the value of the RC. It takes ten motor rotations to open/close the shutter which is equal to 2 seconds. So I choose the RC value such that the pulse lasts for 2.2 seconds (2.2=1e4x222e-6). I used a POT between the relay and motor, to control the speed of the motor further. It is very important to control the speed of the motor, because the gears generate lots of torque. If this is not controlled than the extra torque can destroy the plastic gears. By tuning the POT I give the right voltage and current to the motor. POT can also be used to control the closing time of the shutter directly. The output of the POT is connected to the central pole of the flip/flop shutter switch. Switch has 2 sets of 2 independent poles, one set on each side of the central pole. There are two poles in the center, one pole is connected to the pot and other is to the ground. The poles on the left and right are cross connected. The poles on the right are then connected to the motor. So what it does; as the switch is flipped from left to right it interchanges the polarity of the motor wires. So the motor runs clockwise when the left side on the switch is conducting, hence open the shutter when the pulse comes and vise versa for right side. Master switch controls all the three voltages.
Step 3: Operation Scheme
Master Switch Shutter Switch open/close Push Button Operation Pulse Motor Operation Shutter Operation
ON (Red LED ON) To open-flip the switch up Push it once Forward pulse generated Open-Red LED ON
ON (Red LED ON) To close-flip the switch down Push it once Backward pulse generated Close-Yellow LED ON
Step 4: Comments
The shutter is fairly simple and the way it is developed, it really does not need any electronics; shutter, +3V power supply, 2 pole switch and a push button is enough. If the optical setup does not need the shutter to operate several times during the experiment, than this is the best scheme. The usage of electronics makes it easy and safe for the shutter to be used several times during the experiment, that’s why I used the electronics.
I have been using this shutter for 2 years now for more than few times a day, and it is working great. My idea behind using the air-freshen was; that air-freshers are built to run many time during an hour for many years which requires to have tough mechanics. This shutter has just proved that. Following link shows the shutter; an important part of the optical tweezers i built. It is not considered good for an expensive laser to turn on and off many times during the experiments. And while experiments i have to look into the microscope many time at the risk of my eyes getting exposed to the harmful infrared radiation of several 100mW. But with the shutter i am confident that i am safe as well as my laser sustains long life.
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