# Koch Lab:Research/How to build your own laser diode

#### Introduction

This article describes our build of the OEM laser diode system offered from Thorlabs that is to be incorporated into our laser tweezers system. Our laser tweezers will be tweezing dielectric beads attached to chromatin and not the more traditional tweezing of live cells. This means that we can use any wavelength for our tweezers as opposed to the infrared wavelengths used for live cellular manipulation. To ensure cell viability and tweezing capabilities researchers choose high powered infrared laser systems like Nd:YAG lasers for tweezing.

The choice for using infrared lasers is twofold: power and wavelength. Solid state lasers can output a tremendous amount of continuous power. This high power is needed to ensure that enough power is sent to the sample through a microscope. Since light microscopes are typically used for visible wavelengths, their optics are not coated for antireflection in the infrared. This leads to a lot of power loss if using infrared light and hence the need for a lot of initial power. Also, the wavelength of infrared lasers is ideal for cellular manipulation because viability is increased and the absorption of water at around 1060 nm has a local minimum.

The downside to using a solid state laser is cost. They can start around $25k and can sky rocket in price dependent on how many features the user chooses. This is why researchers have been choosing laser diode systems recently. Typically, a lab will choose a complete laser diode system around$10k that is specific to one wavelength and one power output. Our OEM laser diode build is not restricted by wavelength and power in the same way commercial systems are and will ultimately cost about half that of a commercially available laser diode system.

The first step in building the OEM laser diode system is to build an enclosure that protects the circuits from extraneous signals and enables forced air cooling. Below, we will describe all the necessary parts, tools, and procedures used in making an enclosure for our laser diode system.

#### Building the enclosure

Ultimately we want to force air cooling on the circuits to maintain optimal performance of the system. To ensure proper air flow, we made an enclosure that suited our needs. Another reason for building an enclosure is to prevent extraneous signals from interfering with the circuitry. As is stated on page 2 of the manual "handy phones", i.e. cell phones, are not to be used within 3 meters of the circuits. Building a box around the circuits will help prevent any problems that may arise due to cell phone transmissions.

##### Front plate

We started by using this box

which comes in three pieces, 2 sides and one major enclosure. Our first step was to use a 400 Series XPR Dremel equipped with a cut-off wheel to place the front plate from Thorlabs on the box. We found that centering the front plate on the side of the major enclosure allows for proper placement of the current controller in the box. Below is a diagram showing the dimensions we used to cut out the hole for the front plate.

Included in the above picture is the WorkStation for the Dremel. This is an attachment for the 400 XPR series Dremel that is indispensable for cutting out holes in any material.

After cutting the hole for the front plate, we used a set of needle files to get rid of the burrs left behind from cutting. One can also use either sandpaper or emery paper to clean up the edges so that you do not cut yourself. The finished hole looks like this:

After cutting the major hole for the front plate, we needed to drill holes in the enclosure such that the front plate mounted onto it. In order to do this we used self tapping screws that we scavenged from an old computer and the computer power supply. We were lucky to find these, however, if you cannot find self tapping screws, you can purchase them here. They are #4 screws that will just barely fit in the holes of the front plate. Actually, they will quasi tap themselves into the front plate but do not worry about it. If you can find #3 self tapping screws, you can use these which will clear the front plate holes entirely so that they tap into the enclosure only. Make sure to get the 1/4" length screws with a flat head.

We used the drill press capability of the WorkStation to make the holes for the screws. To do this we marked where the holes should be using the front plate as a guide. Remember to under size your holes so that the self tapping capability of the screws will create threads in the sheet metal of the enclosure. We used a #43 drill bit to ensure that the screws would make a threaded hole.

Of course when using a drill bit, one must use a center bit to start the hole such that the drill does not walk across the sheet metal. You can use what is called a center punch to align in the holes of the front plate or you can use what we used which was a hand punch. We utilized the 3/32" punch (below figure) as a center punch by just banging it into the sheet metal and then using the Dremel to drill the holes. As you can see in the figure, the punch has a small nipple at the end that creates an indentation in the sheet metal when impressed upon it. Using the punch from your hand punch keeps you from purchasing another tool but if you have access to center punches (also known as pin punches or pilot punches), it is recommended to use them and not your hand punch punches.

go ahead and tap your screws into the sheet metal with the frontplate in place.

As you can see, we did not center one screw properly on the enclosure which made tapping a hole impossible in the upper left hand corner of the frontplate. This is why if you have access to center punches, you should use them.

##### Forced air cooling

In the manual for the ITC133 current controller, it states on page 25 that forced air cooling would be beneficial and one should ensure that the unit is mounted vertically. This enclosure allows us to position the current controller vertically and has enough space to place a fan in it. Again, we used a scavenged computer fan for forced air cooling, however, you can purchase this fan and this cover. Our cover is made of metal, but it does not matter as long as the fan blades are covered so as to prevent finger chopping. Do make sure that the fan you use is rated for 12 V because the ITC133 runs off of 12 V and hooking up a 12 V requires no additional steps for operation. If you cannot find a scavenged fan that is rated for 12 V, you can always build a voltage divider to step down the voltage from the power supply to supply to the fan if it is rated for a lower voltage. If you want to use a higher voltage fan, then you will have to make additional circuitry.

Again, use the Dremel and files to make a hole large enough so that the fan sucks in air to the enclosure. We used one of the sides of the box for this. Also, make holes in the sheet metal so that you can fit screws through it (a clearance hole) and the guard to tap into the fan. Since we used an old computer fan, the screws were already available. However, if you need screws, you can use these which are #10 screws. Do make sure that this is the proper size screw for the fan you purchased if it does not come with screws. They are self tapping so that they tap into the fan. This is why the holes made in the side of the enclosure are clearance holes. The easiest way to do this is to use the hand punch. However, if you do not have a hand punch, just drill the holes in the sheet metal.

Since we are blowing in air to the enclosure we decided to also make a hole for air to escape. We did so by cutting out a square hole on the top of the enclosure and using another fan guard to prevent large objects from falling in to the circuitry. Follow the same procedure in making this hole as you did for making the large front plate and fan holes.

##### Power connections

To supply power (±12 V and COM) to the ITC133 we purchased panel mount banana connectors. You can color code the banana jacks as blue for -12 V, green for COM, and yellow for +12 V as is the standard color coding or just buy a 10 pack of black which is what we did. To help prevent shorting, make sure that you get banana connectors with insulated solder posts. We marked out where we wanted the jacks to be on the other end of the major enclosure opposite to the front plate and drilled 5/16" holes for the jacks to clear the sheet metal. The Dremel will not hold 5/16" drill bits so we had to do this on the machine shop's drill press. Remember to center drill your holes lest your drill bit walk across the sheet metal and marring your work.

The jacks come with a nut that secures them to the enclosure. As you can see in the below picture, the solder posts are insulated.

To make the wires that connect the banana panel mounted power connectors to the back plane, see the section on making wires below. To connect to the banana plugs, just solder the other end of your wire to the post. The manual states that both pins A30 and C30 need to be connected to +12 V and similarly for -12 V and COM. Make sure you do this. We did it by just making two wires for each power connection, i.e. 2 wires for +12 V one for A30 and another for C30 and so forth.

##### Operating switches

The ITC133 is an OEM product. This means that it has an incredible amount of functions that the user can make accessible by wiring the correct pins on the ITC133 back plane (ST1 in the manual). The system requires the user to wire on/off switches in order to send signals to the laser diode holder (TCLDM9) to either turn on or off the temperature controller or the laser diode. We opted for panel mount switches (SPDT) and panel mount LEDs, one yellow and one green to control the on/off functionality.

To mount these items, we marked their placement next to the fan and used the hand punch to punch 1/4" holes in the sheet metal.

We used the green LED for the temperature controller on/off and the yellow LED for the laser diode on/off.

#### Switch circuit

The back plane is where one connects the switches. Below is a picture of the back plane on the ITC133 with its corresponding pin numberings.

It is easier to mount circuits and jumps on a board than it is to try and wire things directly from the back plane which then goes to a switch and then to a resistor and then to an LED. So, we decided to make our own break away circuit board that connects the LEDs, their resistors, and the switches to the back plane. To do this, we used an antiquated method of chemical etching since we had some ferric chloride in the lab. NOTE: Using ferric chloride for chemical etching of circuit boards is dangerous to the environment and your health. Do read the MSDS for the chemical and don't hurt yourself! You can purchase the etching solution from RadioShack but they don't tell you the concentration of ferric chloride so if you have the anhydrous version of the chemical sitting around, you are on your own about finding the correct concentration to use and how to mix it. There is an alternative to using ferric chloride, and it can be found here but we are not sure if this is any better. Either way, DO NOT throw away the used liquid down the drain. Remember, every thing that goes down the drain is recycled and you will ultimately be drinking what you put down it later in life.

With that said, etching a circuit board is not complex and a detailed description can be found here. Briefly, you will want to do the following:

1. Make your artwork, i.e. the circuit you want.
2. Print using a laser printer on either transparencies or a plastic coated paper.
3. Prepare the copper clad board.
1. Scrub the board with a scouring pad.
2. Rinse with acetone.
4. Use either a hot plate or an iron to transfer the toner from your artwork to the copper proto-board.
5. Chemically etch away all the copper that is not protected by the toner.

The process is simple, but time consuming. Below is an expanded version of the circuit we built to house the various break away components.

The color coding shows how each component will break away from this board and how they are connected. The real circuit board can be downloaded here. Below is a diagram of how the break away circuit works.

Each jump (tiny circular component) is spaced "1 mil" (0.100") apart. This is standard spacing for DIPs. To build the circuit, you will need:

##### Toner transfer

Your printed artwork should look like the below picture if you printed on a transparency.

The next stage is to clean the circuit board and transfer the image onto it. If you do not scrub the circuit board with a scouring pad and clean it with acetone, the image will not adhere. The below picture shows a before cleaning and after cleaning the circuit board picture.

As you can see, the board looks much brighter after cleaning it. The next stage is to transfer the image to the board. We did this with a hot plate in the below image.

Once the image is transferred onto the copper, just peel away the transparency. The image will not be perfect and hence the need for a permanent marker. If you see any spots on the transferred image where there should be black toner but there isn't, then fill it in with the pen. The etchant will not etch away permanent marker so feel free to slather it on if you like.

Once you are finished with the artwork and filling in any spots missed with the Sharpie, it's time to remove only the portion of the copper clad proto-board that we need. We used what is called a nibbler to basically "nibble" out the circuit board from the copper.

##### Etching

The above image show the process of etching away the copper that is not protected by the toner. We used a large weigh dish to put the board and etchant in making sure that the copper side is facing down. Once the board hits the chemical, you will want to agitate it till the process is done, about 20 minutes. Agitating the system speeds up the kinetics which in turn helps you by not letting the board soak so long that the chemical eats away the copper under the toner. The second picture shows the process almost completed and the third shows the board completed.

##### Board population

Now you need to drill holes. A #64 drill bit works for drilling out the middle of the jumps (circular holes). If you have access to a circuit board drill bit (like this one), it is best to use that since you will not have to center drill. You will also want to drill clearance holes for the 4-40 screws which are marked in the four corners of the circuit board as the larger circles (#40 drill bit).

After you have all your holes drilled you can start inserting the components to the board. As a general rule of thumb, you will want to solder items that are the shortest first. In our case, the shortest items are the resistors so we shall start with those. Look at the blow up diagram of the circuit to ensure that you put the resistors in the correct spot.

Once the resistors are in place, the next item to be installed is the right angle headers. Break off two individual headers from the break away row. The reason why we used right angle headers is to differentiate them from the rest of the connectors as being power and ground connections. This ensures that you don't put power to the wrong pin.

We chose to position the power headers opposite to each other on the board and facing in different directions to help ensure their differentiation from the rest of the connectors. Also, it is a good idea to go ahead and start labeling the pins on your board. Use an ultra fine point permanent marker for this.

The last step is to add the straight headers on the board. You will need 4 with 3 connectors next to each other. Just solder them into the places where there are holes left. You can see that in the middle rows where the wires from the switches go, we have snipped the middle pin. This is because the pin is not used and thus not necessary to remain on the board.

One should not have to say this but you should have been using a multimeter with a continuity check on it to ensure that all the connections on the board you have made work properly.

#### Making wires

The next step is to make the wires that jump from our circuit board to the back plane of the ITC133. The back plane connector has a lot of pins that we will not need to use for the simple turning on/off of the laser. We chose to remove all the pins and only use the ones we needed. Later, if more functionality is required of the board, you can add the pins back to the plane and make the necessary connections to the outside of the enclosure to use the pin.

The pins can be directly soldered on, just make sure that you don't solder past the last fins on the pin otherwise it won't fit back into the connector. You may also want to use some heat shrink tubing to prevent any shorts from the connected wires once they return to the back plane because the spacing is very close. Also make sure that you remember the orientation of how the pin came out because you will want to return it back to the connector in the same fashion. The end result looks like the below picture.

The next step is to make connectors that connect to the circuit board. To do this, you will need some header housings and some header crimp connectors. Make sure that the crimp connectors you purchase and the housings will fit into each other. Many companies make these types of connections and while the form factor is standard, the way in which the connector fits into the housing is not. To make the header connectors, just follow the below steps.

Now that you know how to make these wires, it is just a matter of connecting the correct pins on the circuit board to the correct pins on the switches and the back plane. You will want to make header connectors for the LEDs as well since they fit on our circuit board with header connections.

Note:
During this process you will find that you will mess up a lot with making the wires unless you are supremely skilled at it. It is thus our suggestion that you purchase more than you think you will need, we typically purchase twice what we need so that we don't have to waist our time waiting for more parts to arrive.

#### Assembly

Once all of your ducks are in a row, it is time for assembly. Before we assemble, we need to discuss the various components on the ITC133 and what they are used for.

The above figure is a blow up of the figure given in the manual that shows the placements of the various switches, jumpers, connectors and potentiometers on the ITC133. You will need to adjust the switches dependent upon what type of diode you purchased. Below is a description of each adjustable component on the board.

##### ITC133 Switches

Constant current / Constant power mode

 S1 Left $\displaystyle \rightarrow$ Constant Power (CP) Right $\displaystyle \rightarrow$ Constant Current (CC)

We chose constant current mode for various reasons.

Grounding for the laser diode

 S2 Left $\displaystyle \rightarrow$ Cathode Grounded (CG) Right $\displaystyle \rightarrow$ Anode Grounded (AG)

The selection of AG or CG is determined from the specs for the LD. In our case for the laser diode we purchased, it requires the switch to be placed in AG.

Temperature control

 S3 Left $\displaystyle \rightarrow$ Thermistor (TH) Right $\displaystyle \rightarrow$ Transducer (TD)

For correct operation with the TCLDM9 S3 needs to be in the right (TD) position on the ITC133.

Current sharing (more on this later)

 S4 Up $\displaystyle \rightarrow$ Current sharing off Down $\displaystyle \rightarrow$ Current sharing on

##### ITC133 potentiometers

LD current limit

This sets the current limit for the LD. If something goes wrong and the current goes over this limit, the system shuts down thus saving our diode.

 Potentiometer Rotary Switch Placement Frontplate label P1 ILIM ILDLIM

LD set current

This sets the operating current for the LD.

 Potentiometer Rotary Switch Placement Frontplate label P2 ILD ILD/IPD

TEC current limit

This sets the current limit for the TCLDM9 heat sink. If the current goes over this, then it will shut down the system.

 Potentiometer Rotary Switch Placement Frontplate label P3 ITELIM ITELIM

TEC set

This will actually set the temperature that the user wants to operate the LD at.

 Potentiometer Rotary Switch Placement Frontplate label P4 TSET TSET/RSET
##### ITC133 Jumpers
 Jumper Use JP1 Positive bias JP2 Negative bias JP3 PID temperature window

If you don't have the switches in the correct spot, you may cause damage to the system so take care when you are initially setting up.

We used standoffs (the same we used for the circuit board) to protect the ITC133 from shorting when it is attached to the enclosure.

You will just have to fuss around with the placement of the board with its standoffs and make the appropriate holes so that you can mount to the board properly. Be sure that the board fits properly into the front plate as well. Once the board is secure, attach the back plane connector with all its proper connections to the board.

The next step is to attach the circuit board. As you can see, we tried to attach it to the enclosure using double stick tape. This was a bad idea and you should mount it directly using screws and the standoffs.

#### Finished Product

Once you have everything in the box, close it up and attach the cable to the TCLDM9.

On the TCLDM9 you will need to tell it how your laser diode is grounded as well. This information should come in the spec sheet for the diode. Also when you attach the diode into TCLDM9, make sure you put it in correctly. Improper installation may damage your diode so be careful.

When putting the diode into the TCLDM9, make sure you follow the instructions outlined in the Manual. If you don't, the diode will overheat and burn out quickly.

PID control is setup for controlling temperature. The only advice we can give for this is to be patient and don't follow the manual for this. When you are ready to start PID control, turn switch 4 (current sharing) on and then try to follow this tutorial on how to do it. Make sure the diode is on when you do setup PID temperature control. Eventually you will find the correct settings.

Once the project is done, you may want to think about putting racing stripes on it because it is a super functional and is a highly modular laser diode system. And, the best part is you can exchange diodes when ever you need to.