Laser Cable Selection Guide
To achieve the best performance with your laser diode, you need to make sure you are using the proper cable. Choosing the right cable to connect your laser diode depends on a few factors: one, managing the voltage loss in the cable; two, do you need any additional signal lines (such as photodiode or remote voltage sense connections) or shielding; and three, will you be modulating the laser or operating in quasi-CW (QCW) mode? We’ll cover each of these below. If you’re looking for a pre-made cable, we offer several different types, depending on your application. See the chart below for more details.
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Arroyo Instruments offers the following cable styles for use with our instruments: 1220B / 1221B 1224A / 1224B / 1224C 1228C / 1229C 1230 / 1231 1232 / 1233 The table below lists the current ranges, and suggested cable part numbers. |
| Current Rating | Connector Type | Connectorized | Pigtailed |
|---|---|---|---|
| Up to 8A | DB9 | 1220B | 1221B |
| Up to 20A | 9W4 | 1228C | 1229C |
| Up to 20A | 13W3 | 1230 | 1231 |
| Up to 40A | 13W3 | 1232 | 1233 |
Voltage Loss
All wire has some amount of internal resistance, although it is typically very small. Connectors also add to the resistance of the cable. When choosing wire and connectors for your laser application, it is important to select a wire and connector that is large enough to keep the voltage loss through the cable to a minimum. In addition, smaller gauge wires typically have higher capacitance, causing waveform distortion when modulating the laser (more on that below).
The maximum allowable voltage loss is the difference between what the instrument is capable of delivering (for example, 5V on the 4300 Series LaserSource) and the maximum voltage required by your laser. For example, if your laser’s maximum voltage was 2V, and your driver could provide up to 5V, then you could lose up to 3V before you ran into problems driving the laser. However, large voltage losses may overheat and damage the cable, or even pose a safety threat. Typically, you want to keep the voltage loss to under a volt, and preferably to less than half a volt. Overall cable length is also important to consider, as resistance adds with length, e.g., a 2 meter cable has twice the resistance of a 1 meter cable.
Wire resistance is usually specified by the cable manufacturer in ohms per foot or meter. Most manufacturers specify their cable resistance in ohms per 1000 feet. The table below lists typical wire resistance for a 2m cable:
Gauge |
Resistance (2m) |
|---|---|
24 |
0.168 |
22 |
0.106 |
20 |
0.067 |
18 |
0.042 |
16 |
0.026 |
14 |
0.017 |
12 |
0.010 |
If you have multiple conductors available, you can use half for the anode connection and half for the cathode connection, lowering the resistance and increasing the current capacity of the cable. If your cable is twisted pair, make sure to use one side of each pair for the anode, and the other side of each pair for the cathode. In other words, in each twisted pair, one wire should be anode and one wire should be cathode.
You also need to consider that the connectors or terminal blocks used on either end of the cable also add resistance. If you choose to solder the wires, this will eliminate most of the connection resistance as long as the solder connection is well made. For most DB9/DB15 connectors (including those used by Arroyo), the maximum contact resistance is 15 milliohms, while the 9W4 connector used by Arroyo for high power applications has a maximum contact resistance of < 1 milliohm.
To calculate total resistance, add twice the wire resistance (the current travels to the laser and back), and four times the contact resistance of the connector (current travels through a total of four connector interfaces). The resulting value is the total cable resistance, and also the voltage drop per amp of current.
For example, if you calculated a total of 0.144 ohms of resistance, this also means that there will be 0.144V of loss for each amp of current. At 4A, the voltage drop would be 0.576V (i.e., 4 x 0.144).
While calculated values will give you a indication of the cable performance, they will typically be higher than the actual performance, and should only be used as a general starting point. For example, the Arroyo 1220 LaserSource cable, which is constructed with 18GA wire and uses DB-9 connectors, would have a calculated maximum resistance of 144 milliohms for the full loop. However, actual measurements show that the actual resistance is only 80 milliohms. Also, you should always measure cable resistance using a 4-wire resistance measurement, as 2-wire measurements will not give you correct results.
Additional Signals & Shielding
You will often need to carry additional signals from your laser to the laser driver. The most common signals are photo diode feedback and remote voltage sense, but some lasers may have a built-in thermistor, and you may also need to carry the interlock signals for remote safety devices.
At lower currents, you can often use a multi-conductor cable to carry both the laser drive current as well as the signal wires. However, at higher currents, it is difficult or expensive (or both) to use a multi-conductor cable that has wires of sufficient size to carry the current and smaller wires for the auxiliary signals. In these cases, it is often better to use two cables: one that has only the two conductors needed for the laser current, and a second small gauge multi-conductor cable for the signals. For the signal cable in particular, you can often use off-the-shelf serial cables, which have 9 signal lines and DB-9 connectors on either end.
For most applications, shielding is not a critical factor, as DC current for the laser (or photo diode) is fairly immune to coupled noise. However, if you are using remote voltage sense lines or temperature sensors, those are high impedance signals which are susceptible to noise, and should be shielded when possible.
Modulation or Quasi-CW Operation
When operating the laser in DC mode (no modulation), you only need to be concerned with the shielding suggestions above. However, if you will be modulating the laser above a few kilohertz (lower for higher currents), then the impedance of the laser cable becomes an important factor. High impedance cables will distort the output waveform. For pulse/square wave operation, this will lead to excessive overshoot, while sine wave modulation will skew the waveform, and have different rise and fall times.
While it may not be possible to determine the impedance of your cable, typically heavier gauge wire has lower impedance, so simply moving to a heavier wire can minimize waveform distortion. Specially designed low impedance cable can also be used, but can be hard to find. Using cables constructed with twisted pair wires will also help reduce impedance. Total cable length should also be minimized.
To fully understand the changes in waveforms based on cable selections, you should use a current probe connected to a high bandwidth (100MHz or higher) oscilloscope, as this will have a little impact on actual system performance.
