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A Tale of Two Metrics:
Useful Parameters for Diode Comparison and the Need for High Quality Data
It's typical to choose a diode for a build based on uniqueness of wavelength in our current collection or maximum output power. Another feature we might consider is beam characteristics. Those of us that are after aesthetics over power may want to opt for the single-mode varieties. What do we do when there are two or more comparable diodes with similar specifications?
What do we do if the build is constrained by size limtations or if we want to maximize efficiency?
We can choose a diode on the basis of efficiency.
The typical data we use to determine how hard we can or should drive a laser diode includes PIV information. That is, we use preliminary tests (thanks to those who do these) to obtain information about the power output of a laser diode at a range of current settings. This information tells us where diodes are likely to fail, where they produce maximum power, and what the lasing threshold is. Often, the output power per unit input current (W/A; we can call this current efficiency) is used as a measure of efficiency. Of course, we could refer to manufacturer datasheets for diode specifications but sometimes they are not available and we typically operate diodes in very different conditions than whay they were intended for. I wanted to investigate another way to characterize diodes: "power efficiency."
I am defining this as the ratio of output power to total power (Po/Pt) where Po is the measured output power of a laser diode and the total power, Pt, is simply the product of forward Voltage (Vf) and input current (Ii). This value of power efficiency (Ep) tells us what proportion of the output power is light and, relatively, how much is heat. It also gives us a basis to compare laser diode behavior across diode types - different diodes often have different forward Voltage which makes the output power to input current measure less informative. When considering how to build a portable laser, we can also consider heat management (i.e. what kind of heatsink to use). It is well known that temperature has an effect on laser diode output power and the peak emission wavelength. Portable lasers are always constrained by mass so it is useful to have some basis for comparing efficiency across diodes to make better informed decisions about which diode is best suited to a particular build. This may help increase the lifetime of our diodes or allow us to design a better system to, for example, keep a diode within operating conditions for a desired center wavelength or avoid other problems caused by overheating.
Here, I plotted the PIV values from some diodes with test results on DTR's shop.
The plot gives us a reasonable idea of how a diode behaves at a given current.
In this next series of plots, you can see how differently these diodes behave as we set them for higher output power. These plots show power efficiency vs current input.
Plotting the current efficiency (Po/Ii) gives a very similar result.
Note: The sampling resolution will have a notable impact on the shape of these curves as you can see from the plot of the Oclaro 638nm diode. The Voltage reading has only two significant digits so between some of the points, there is an artificially abrupt change in the efficiency (the problematic points have been removed for the last plot). A higher resolution sampling of the Voltage should provide a more gradual, more realistic change in Ep.
Although the shapes of the curves are similar, the power efficiency parameter is more informative than current efficiency and gives us a better basis for comparing diodes.
Ep will always be some value between 0 and 1 because it is the proportion of total output power that is optical output power. Ei, on the other hand, can be any positive value. Because Ep is incorporates information about forward Voltage, it provides a more comprehensive summary of the performance characteristics of a laser diode. This allows us to determine the optimal drive current, assess how much heat the diode will produce at a given setting (Heat produced in proportion to Pt is 1 - Ep), and gives us a basis for comparing across diodes and diode types. A real world example will help explain what I mean:
I recently have been researching diodes for some pen builds. I knew that I wanted a blue laser in the 0.3-0.4W power range with a preference for longer wavelengths. I also wanted a 638nm laser from 0.7-1.0W. Intuitively, I would have chosen the PLTB 450B diode for the blue (better beam characteristics and can reach the desired output and longer wavelength if pushed hard) and the Oclaro 638 700mW diodes. However, I knew that these builds will be compact and so limited mass for heatsinking will be a factor for which diode I should choose. This is what led me to collect test data and summarize it in these various plots.
This last graph shows what I think is the most useful part of this essay.
In the above plot, you can see that for blues, the high power M462 is more efficient in the power range I want than the PLT5 450B. This means it will run cooler at the same output power. This could not be determined using Po/Ii alone. Notice also that the Mitsubishi 638nm 500mW diode is more efficient between 0.7-1.0W than the Oclaro, despite the latter being designed for that output range.
Now is a good time to throw in a few caveats and make a suggestion for keeping data on the forum.
The data I am using here is not my own, so I could not standardize the test conditions. I don't know what lenses were used, what power meters, or how heat during the tests might have had an effect on the results. Additionally, my sample size here is 1 for each diode. Sample to sample variation will have some impact on the results and of course it would be much more useful to have much more data. However, I do think that these data should be at least roughly representative and for this being a hobby, I'm not too worried about statistical legitimacy.
My second point with this essay is to demonstrate that summarizing data this way is a great way to, at a glance, understand the characteristics of a diode. Take, for example, the 505nm diode in the last plot. You can clearly see at what point the diode drops in power - this is also clearly evident in the PIV plot. Visual representations like this make data easier to understand, patterns easier to see, and all of this information quicker to share and access. My suggestion is that we can start organizing data more effectively for the benefit of the community. I'd like to see what input you all have to offer on this.
@DTR, if you would like a copy of the PIV plots I made from the diodes I've used in this post, let me know and I can send them.
Useful Parameters for Diode Comparison and the Need for High Quality Data
It's typical to choose a diode for a build based on uniqueness of wavelength in our current collection or maximum output power. Another feature we might consider is beam characteristics. Those of us that are after aesthetics over power may want to opt for the single-mode varieties. What do we do when there are two or more comparable diodes with similar specifications?
What do we do if the build is constrained by size limtations or if we want to maximize efficiency?
We can choose a diode on the basis of efficiency.
The typical data we use to determine how hard we can or should drive a laser diode includes PIV information. That is, we use preliminary tests (thanks to those who do these) to obtain information about the power output of a laser diode at a range of current settings. This information tells us where diodes are likely to fail, where they produce maximum power, and what the lasing threshold is. Often, the output power per unit input current (W/A; we can call this current efficiency) is used as a measure of efficiency. Of course, we could refer to manufacturer datasheets for diode specifications but sometimes they are not available and we typically operate diodes in very different conditions than whay they were intended for. I wanted to investigate another way to characterize diodes: "power efficiency."
I am defining this as the ratio of output power to total power (Po/Pt) where Po is the measured output power of a laser diode and the total power, Pt, is simply the product of forward Voltage (Vf) and input current (Ii). This value of power efficiency (Ep) tells us what proportion of the output power is light and, relatively, how much is heat. It also gives us a basis to compare laser diode behavior across diode types - different diodes often have different forward Voltage which makes the output power to input current measure less informative. When considering how to build a portable laser, we can also consider heat management (i.e. what kind of heatsink to use). It is well known that temperature has an effect on laser diode output power and the peak emission wavelength. Portable lasers are always constrained by mass so it is useful to have some basis for comparing efficiency across diodes to make better informed decisions about which diode is best suited to a particular build. This may help increase the lifetime of our diodes or allow us to design a better system to, for example, keep a diode within operating conditions for a desired center wavelength or avoid other problems caused by overheating.
Here, I plotted the PIV values from some diodes with test results on DTR's shop.
The plot gives us a reasonable idea of how a diode behaves at a given current.
In this next series of plots, you can see how differently these diodes behave as we set them for higher output power. These plots show power efficiency vs current input.
Plotting the current efficiency (Po/Ii) gives a very similar result.
Note: The sampling resolution will have a notable impact on the shape of these curves as you can see from the plot of the Oclaro 638nm diode. The Voltage reading has only two significant digits so between some of the points, there is an artificially abrupt change in the efficiency (the problematic points have been removed for the last plot). A higher resolution sampling of the Voltage should provide a more gradual, more realistic change in Ep.
Although the shapes of the curves are similar, the power efficiency parameter is more informative than current efficiency and gives us a better basis for comparing diodes.
Ep will always be some value between 0 and 1 because it is the proportion of total output power that is optical output power. Ei, on the other hand, can be any positive value. Because Ep is incorporates information about forward Voltage, it provides a more comprehensive summary of the performance characteristics of a laser diode. This allows us to determine the optimal drive current, assess how much heat the diode will produce at a given setting (Heat produced in proportion to Pt is 1 - Ep), and gives us a basis for comparing across diodes and diode types. A real world example will help explain what I mean:
I recently have been researching diodes for some pen builds. I knew that I wanted a blue laser in the 0.3-0.4W power range with a preference for longer wavelengths. I also wanted a 638nm laser from 0.7-1.0W. Intuitively, I would have chosen the PLTB 450B diode for the blue (better beam characteristics and can reach the desired output and longer wavelength if pushed hard) and the Oclaro 638 700mW diodes. However, I knew that these builds will be compact and so limited mass for heatsinking will be a factor for which diode I should choose. This is what led me to collect test data and summarize it in these various plots.
This last graph shows what I think is the most useful part of this essay.
In the above plot, you can see that for blues, the high power M462 is more efficient in the power range I want than the PLT5 450B. This means it will run cooler at the same output power. This could not be determined using Po/Ii alone. Notice also that the Mitsubishi 638nm 500mW diode is more efficient between 0.7-1.0W than the Oclaro, despite the latter being designed for that output range.
Now is a good time to throw in a few caveats and make a suggestion for keeping data on the forum.
The data I am using here is not my own, so I could not standardize the test conditions. I don't know what lenses were used, what power meters, or how heat during the tests might have had an effect on the results. Additionally, my sample size here is 1 for each diode. Sample to sample variation will have some impact on the results and of course it would be much more useful to have much more data. However, I do think that these data should be at least roughly representative and for this being a hobby, I'm not too worried about statistical legitimacy.
My second point with this essay is to demonstrate that summarizing data this way is a great way to, at a glance, understand the characteristics of a diode. Take, for example, the 505nm diode in the last plot. You can clearly see at what point the diode drops in power - this is also clearly evident in the PIV plot. Visual representations like this make data easier to understand, patterns easier to see, and all of this information quicker to share and access. My suggestion is that we can start organizing data more effectively for the benefit of the community. I'd like to see what input you all have to offer on this.
@DTR, if you would like a copy of the PIV plots I made from the diodes I've used in this post, let me know and I can send them.
