Laser diodes are the hidden champions of contemporary laser technology. From simple laser pointers to complex quantum communication satellites, laser diodes are everywhere. It has excellent performance, compact construction, numerous types, and most importantly, it is becoming more and more affordable.
Many people are already thinking about using laser diodes in their products, sometimes it may be a completely new system, sometimes it may be to replace an old laser. Faced with a wide variety of laser diodes, how should engineers choose?
The purpose of this article is to answer: How to choose the right laser diode?Which parameters are critical and which can be ignored?
Here are four steps to help you determine the size of your desired laser diode.
Step 1: Convert the application requirements into laser parameters.
In order to find the right laser diode for your application or product, you may first determine a set of parameters based on the application. Let's say we want to build a laser interferometer for surface profile analysis or velocity measurements.
To build this device, we need a laser diode with a coherent length of 1 to 10 m, and the interference pattern should be at a temperature change (01 nmk). We need a collimated Gaussian beam with a power of 80mW. The detector we use is based on silicon (Si) and is only available at 1100 nm wavelengths. In this case, the center wavelength itself and polarization are less important. At the moment, we don't know what type of package the laser diode is.
Table 1Application requirements and laser diode parameters (example data shown in red).
The application or product requirements are listed on the left side of Figure 1 above, and the laser parameters are listed on the right. From the coherent length, it can be calculated that δ use line width = c l= 96-95.5 MHz.
For those who are new to the field, the first thing you need to know is what these parameters mean.
The coherence length is the distance at which the coherence is significantly attenuated. Please refer to the following formula:
c l where δ is the bandwidth (or line width), c the speed of light, and l the coherent length.
Spectral resolution, which represents the relationship between bandwidth (nanometers) and wavelength: r = δ In the case of a spectrometer or more general spectrum, a measure of the laser's ability to resolve features of the electromagnetic spectrum.
If you want to calculate the bandwidth (MHz) using nanometer (nm) values, you can use δ = δ C2
Bandpass, sensors that detect laser signals typically use interference filters to block interfering ambient light. Therefore, the wavelength of the laser source must be kept within the transmission range of the filter. In this case, we can usually ignore the limited center wavelength tolerance.
Beam quality, which can be defined in several ways. One is the m2 factor, which indicates the proximity of the beam to the ideal Gaussian shape. Therefore, 10 indicates a perfect Gaussian beam. The other is the beam parameter product (BPP), for which we have to multiply the focused beam waist by the far-field divergence.
Intensity, which indicates the laser power in the beam region (preferably the focal point). Therefore, its unit is W cm 2.
Beam profile, which refers to the intensity distribution of the laser beam. It may be a flat-top (rectangular distribution) or a Gaussian distribution. A single-mode beam is usually (close to) a Gaussian beam, while a multimode beam is usually not a Gaussian beam. Depending on the number and intensity distribution of blending modes, it may have a variety of shapes.
The brightness of the laser source, which measures its output power and beam quality. Essentially, it is laser power divided by BPP. The unit is W cm2*SR.
Step 2: Select the laser type.
In the second step, we will describe the laser type in more detail. We are faced with many choices. The right way to solve this problem is to weigh the options. The shaded gray section identifies the different options commonly used for single-mode laser diodes.
Table 2Parameter selection and weighting.
For some types of laser diodes, higher beam quality is usually accompanied by lower output power.
We label the parameters that are appropriate for the application (using the example of building a laser interferometer). For wavelength tolerances, there are no restrictions. Hence the weight is zero. For line widths, the calculation range is between 10 and 100 MHz, so stabilizing the 50 MHz in the ridged waveguide column sounds reasonable. Since this is a key parameter, it has a weight of 2.
We handle the other parameters in the same way. In the last row, we multiply all the markers and multiply them by the weights. It turns out that the column "Single Frequency Lasers Stabilized Ridge Waveguide Lasers" has a maximum weight of 9. So, this is the type of laser diode we're looking for.
Step 3: Select the laser material.
Wavelength is often very important for an application.
Table 3Selection of laser diode material.
Table 3 provides an overview of specific materials and their wavelength ranges. In the example, the detector is SI based and the laser emission wavelength is limited to 1100 nm. This means that a laser diode of gallium nitride (GaN) or gallium arsenide (GaAs) may be suitable for us. In general, in the visible light (VIS) or near-infrared (NIR), ultraviolet (UV) solutions are more expensive than laser diodes, hence the Vis-to-NIR material is marked.
Step 4: Make the final chart and start looking for quotients.
Now we have all the parameters we need for the right laser diode. Table 4 shows a set of parameters derived from the previous chart, and we will discuss the others below:
Mode of operation (CW, pulsed or modulated). This can have a huge impact on thermal management as well as package style. For pulsed or pulse-modulated laser diodes with low duty cycles, there may be less waste heat and therefore a smaller package size.
Beam collimation (free space, integrated optics or fiber pigtail). It depends a lot on your application. In general, standardized optical connector interfaces, such as ferrule connectors (FC) or standard connectors (SC), are useful.
Encapsulation. Planar package or to package. Overall size, compatibility of existing solutions, pin configuration. These are all considerations.
*。In the industrial sector, some common wavelength laser diodes are much cheaper than others.
On the other hand, if your demand is in the tens of thousands or hundreds of thousands of laser diodes per year batch. You can consider becoming a strategic partner of laser diode manufacturers, because customized laser diodes can make mass production more important because, until now, this order of magnitude has not been noticed by large manufacturers.
Table 4Laser diode parameters.
With the data in Table 4, you can start looking for laser diode vendors who can understand your needs and provide possible solutions as soon as possible.