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How to select the right laser diode

 

In a 4-step approach we would like to help you to set the specs of the laser diode you need

Many engineers consider using a laser diode. No matter if you think about an entire new system or just want to replace an older gas laser. The problem is the variety: there are so many different laser diodes available and making the right choice may take days for both, the researcher and the sales expert as well. Our guideline will help to answer the most eminent questions: What is the right laser diode for this application? Which parameters are paramount and which can be neglected?

Our step-by-step guidance will ask a set of questions which directs you towards a table that can be used to find the right laser diode quickly. This may help you as a student just as well as a design engineer in your solution finding process. 

The full tutorial can be found and read at laser Focus World LINK.

Turn application requirements into laser parameter

To find the right laser diode for your case, you probably start with a set of parameters given by your application. We will do this here with the help of an example. Let’s assume we want to build a decent laser interferometer for surface profile or velocity measurement.

For this device we need a laser diode with a coherence length of 1- 10 m, the interferometric pattern should stay stable through temperature change (<0.1 nm/K). I need a collimated Gaussian beam and the power should be higher than 80 mW. The detector I am using is based on Si, which works only for wavelength < 1100 nm. The center wavelength itself and polarization are less important in this case. We have no idea about package or pinning at that point.

Table 1: from application specs to laser parameters. A reference table. Example data in bold

Application requirements Laser parameters
Coherence length         L = 1 – 10 m   

Spectral resolution

Bandpass of the filters etc. 

Linewidth                             Δν = 10 – 100 MHz

Wavelength tolerance

Wavelength stability            < 0.1 nm/K

Wavelength                          λ< 1100 nm

Beam quality, divergence, beam spot profile and size etc.

Gaussian beam

Transverse Mode, M²           M² < 1.1

Intensity, brilliance etc. 

Power                                   P > 80 mW

Table 1 shows you the data we have so far. Pure application requirements are on the left, laser parameters are on the right side. From the coherence length I can calculate the linewidth using Δν =c/πL= 9.6-95.5 MHz.

For those new in the field we will now explain the parameters in more detail. Most of the details are based on Rüdiger Paschottta’s RP Photonics Encyclopedia, which is an excellent resource for all kind of background knowledge.

Coherence length: Distance over which the coherence significantly decays. Actually, it even relates to the temporal coherence length, but for our purposes, the definition above is sufficient. For more details and a calculator, you may refer to www.rp-photonics.com/coherence_length.html . We used the following formula later in this tutorial: Δν =c/πL where Δν is the bandwidth (or linewidth), c the speed of light and L the coherence length

Spectral resolution: The spectral resolution denotes the relation between the bandwidth (in nm) and the wavelength: R= λ/ Δ λ. In case of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its ability to resolve features in the electromagnetic spectrum.

If you want to calculate the bandwidth in MHz from a nm value, you may use the following formula: Δν= Δλ*c/λ² . Or you use one of the internet calculators such as the one at www.photonicsolutions.co.uk/wavelengths.php which even offers the conversion into cm-1 and four other units of bandwidth.

Bandpass Some sensors for the detection of a laser signal use interference filters in order to block disturbing ambient light. Thus the wavelength of the laser source has to be kept within the small transmission range of the filter. For the vendor that’s  important information, but in our example a limited center wavelength tolerance can be neglected.

Beam quality can be defined in several ways. One is the M² factor which says how close a beam is to its ideal Gaussian shape. So 1.0 denotes a perfect Gaussian beam. Another one is the Beam Parameter Product BPP, for which we have to multiply the beam waist at focus with the far field divergence. For more details see: www.rp-photonics.com/beam_quality.html .

Intensity denotes the laser power in the beam area, preferably the focus. Accordingly, its unit is Watt/cm². The question here is what you take as beam area. A detailed discussion can be found at www.rp-photonics.com/optical_intensity.html .

Beam profile is a name for the intensity distribution in the laser beam. Depending on that distribution it might be flat top (rectangular distribution) or Gaussian. A single mode beam is usually (close to) Gaussian, where as a multi-mode beam is usually not Gaussian. It may have a variety of shapes depending on the number and intensity distribution of the mixed modes.

Brilliance or brightness of a laser source gives a measure for its output power and beam quality in one number. Essentially it is the laser power divided by the beam parameter product. Therefore, its unit is Watt/cm²*steradian. A more detailed discussion is here www.rp-photonics.com/brightness.html .

Selecting the laser type

In a second step, we will try to get more specific with the laser type. There, we face the myriad of options. A proper way to get through that is weighting the options and selecting the one with the biggest total weight. This can be done with the help of table 2. The grey shaded fields show the different options that are typically available for single emitter laser diodes.

To get to a decision we first tag all fields with parameters that suit our application (i.e. our example). There is no tag in the line for wavelength tolerance since we did not make any restrictions on that. Accordingly, the weight is zero. For the linewidth we calculated something between 10 and 100 MHz, so < 50 MHz in the RWS column sounds reasonable. Since this is a critical parameter, it gets a weight of 2.

We continue tagging in the other lines and give a weight for the importance in the last column as needed for our application. In the last line under the table we sum up all tags multiplied by their weight. It turns out that the “Single Frequency laser / RWS type” column gets the highest weight of 9. Now, this is the laser type we look for.

You can make this chart even more sophisticated by noting the weight in each cell. This may be clever if the size of one parameter may once be much better than needed (weight: 0.5 nice to have) or just within the specifications (2 critical). You may tweak here if you prefer a cheaper solution on the edge of your parameter space of if you want to be on the safe side for this parameter.

All the abbreviations are explained in a separate Glossar tab.

Table 2: Parameter selection and weighing chart - a reference table -  example in bold (download as pdf here)

Laser Type⇒

 

 

 

Parameters⇓

Multimode Laser

(Typical name: BAL)

Single Mode Laser

(Typical name: RWL,

Fabry Perot Laser)

Single Frequency Laser

 

FBG, VBG

Single Frequency Laser

 

RWS

Single Frequency Laser

 

DFB, DBR

Parameter Importance         

2    = critical

1    = important           

0.5 = nice to have

0   = not important

Wavelength

tolerance

Ca. 10 to 20 nmCa. 5 to 10 nmCa. 2 nmCa. 3 nmCa. 0.5 to 2 nm0

Linewidth

 

Ca. 3 to 5 nmCa. 1 nmCa. 300 GHz< 50 MHz < 2 MHz2

Wavelength

stability

Ca. 0.3 nm/KCa. 0.3 nm/KCa. 0.01 nm/KCa. 0.06 nm/KCa. 0.06 nm/K2

Transverse      

Mode

Multimode

(non Gaussian)

TEM 00

(Gaussian beam)

TEM 00

(Gaussian beam)

TEM 00

(Gaussian beam)

TEM 00

(Gaussian beam)

2

M2

 

>> 2ca. 1.1ca. 1.1ca. 1.1ca. 1.12

Power

(SingleEmitter)

cw: 1-18 W

pulsed: 5-100 W

cw: 50-1000 mW

pulsed: < 3 W

cw: up to 600 mW

pulsed: < 1.5 W

cw: 5-400 mW

pulsed: < 1 W

cw: 5-400 mW

pulsed: < 1 W

1

Accumulated 

weighting result

15797



 

Selecting the laser material

The wavelength is often very important for the application, but do you know why a certain company covers only a particular wavelength range? It’s the material they use that allows or prohibits particular wavelength ranges. Table 3 gives an overview of specific materials and their wavelength ranges. In our example the detector is based on Si, which limits our laser emission wavelength to less than 1100 nm. This means that diodes from Gallium nitride (GaN) or Gallium arsenide (GaAs) could be fine for us. Usually, UV solutions are more expensive than diodes in the visible or NIR, eagleyard offers wavelengths from 630 nm to 1120 nm.

Table 3: laser material selection chart

Emission wavelength SpectrumLaser diode material (substrate) or structure
380 - 470 nmUVGaN

630 - 1120 nm

VIS to NIR

GaAs

1120 - 1650 nm NIRInP
2 - 10 µm IRQCL (Quantum Cascade Laser

 

Make your final chart and go searching

Now we actually have all parameters needed for the proper selection of a diode. Table 4 shows a set of parameters as derived in the previous charts plus some remaining parameters that we will discuss in the following.

Operational mode (cw, pulsed or modulated): This can have huge impact on the thermal management and hence, the package style. For pulsed or pulse modulated diodes with low duty cycle there might be less waste heat and so the package size can be smaller.

Beam collimation (free space, with integrated optics or fiber pigtailed): This depends strongly on your application.

Packaging: Do I have a planar (e.g. PCB, heat sink) or a circular (tube) environment? The first leads to butterfly or flat pack solutions, the latter to a TO can. Are there any overall size limitations? Do I need drop in compatibility with existing solutions? For TO-based packages the pin configuration must be determined (M-/N-P-type) as well as the size (5.6mm vs. 9mm). For 14-pin butterfly packages telecom and pump pinning must be distinguished.

PRICING: Above all, there is one important rule for the pricing of diode lasers. If your absolute center wavelength is not fixed rigidly, look for commodity lasers. Some of the large providers (such as Lumentum or Sony) provide certain diodes for consumer applications such as  game consoles or smart phones. In the industrial area some popular wavelengths exist, such as 852nm or 980nm for fiber laser pumping or spectroscopy. Those diodes are much cheaper than others.

On the other hand, a customized diode has the benefit of more safety for serial manufacturing: You can become a strategic partner of the diode supplier and he would not cancel the production without notice if you order them regularly. This is important if you go from single devices to batches of up to hundreds or tens of thousands of laser diodes per year, a volume still by far under the radar of mass market vendors.

With the example data in table 4 you might either contact a laser diode provider or start a google search on your own. If you choose the direct search, you may take the first four keywords (RWS, single frequency laser, 630-1120 nm / GaAs , cw, > 80 mW) and you get already a decent number of reasonable contacts.

If you go for a diode laser vendor this chart may save you a lot of time. The vendor understands your need  immediately and you can shortcut lengthy discussions on possible solutions that are actually not yours.

Table 4: laser diode parameter chart - a reference table - example in bold (download as pdf here

Parameter Keywords to provide (example)
Power> 80 mW

Beam quality

M2 = 1.1

Laser type (result of step 2) RWS, single frequency laser
Wavelength/Material (result of step 3) 630 - 1120 nm / GaAs
Beam collimation collimated free space optics
Operational mode cw
Polarization TM or TE
Package type /Pinning TO can / M-type

 

Abbreviation list

 

BALbroad area laser

DBR

distributed bragg reflector
DFB distributed feedback
ECDL external cavity diode laser
FBG fiber bragg grating
MOPA master oscillator power amplifier
RWL ridge waveguide laser
RWS stabilized ridge waveguide lasers
SHG  second harmonic generation (doubles the laser frequency)
TEM transverse electro-magnetic mode
VBG volume bragg grating
VCSEL vertical-cavity surface-emitting laser

Figure 1: Among the different laser diode types a higher beam quality usually comes with lower output power.

 

 

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