Notes:
Infrared LED (to fall in range of 1300nm)
Optical fibers transmit infrared wavelengths with less attenuation and dispersion
Intensity modulation
LED wavelength of around 1310 nm
An LED has a spectral width of about 25-80nm
Rise time average of 2-10ns
3-dB modulation of 30-180MHz
References for my early LED research:
Band | Descriptor | Wavelength range |
O band | Original | 1260–1360 nm |
E band | Extended | 1360–1460 nm |
S band | Short wavelength | 1460–1530 nm |
C band | Conventional | 1530–1565 nm |
L band | Long wavelength | 1565–1625 nm |
U band | Ultralong wavelength | 1625–1675 nm |
LED vs. Laser characteristics:
Characteristics | LEDs | Lasers |
Output Power | Linearly proportional to drive current | Proportional to current above the threshold |
Current | Drive Current: 50 to 100 mA Peak | Threshold Current: 5 to 40 mA |
Coupled Power | Moderate | High |
Speed | Slower | Faster |
Output Pattern | Higher | Lower |
Bandwidth | Moderate | High |
Wavelengths Available | 0.66 to 1.65 µm | 0.78 to 1.65 µm |
Spectral Width | Wider (40-190 nm FWHM) | Narrower (0.00001 nm to 10 nm FWHM) |
Fiber Type | Multimode Only | SM, MM |
Ease of Use | Easier | Harder |
Lifetime | Longer | Long |
Cost | Low ($5-$300) | High ($100-$10,000) |
Intensity Wavelength diagram:
From here examples were taken of potential LEDs from manufacturers in hopes of finding one to fall in with our required specs:
- 1300nm wavelength
- 12.5MHz frequency
- Output power above 0.5W
- Most efficient rise time
- Lowest possible spectral width
LED Product Examples
This is an initial Infrared LED, but the power output is much lower than needed.
Offers only 850nm, but does give a good frequency of 75MHz.
Only offers a 935nm wavelength range.
This fell in line with the required wavelength, but as with the majority of these LEDs, the power requirements maxed out at 75microwatts.
This was added purely out of interest. It is a 29 watt LED, showing the possibility of producing the extremely high powers needed for our fiber optic device, but unfortunately it fell short of our wavelength. Ideally this would have been a good guideline for adapting a custom LED to give similarly high power output, but falling in range of 1300nm..
From here I became interested in the possibility of customising an LED for our system with a high drive current, therefore allowing us to produce extremely high output power
This example of boosting current into the system led me onto a device which can produce a 2.8A drive current with an 18V drive voltage. Using this in a suitable LED would produce 50.4W of power for our system.
From here I knew that our spectral width would most likely be too high for the system, and we would need a way to lower it in our system. I researched novel ways of reducing it, including this article.
It became clear that a simple LED would not be enough to provide the power we were looking for, so I researched super luminescent LEDs, with encouraging results.
These diodes are able to emulate the high power levels of laser diodes, without actually being a laser. I looked into several of these product types in hopes of finding a suitable example.
This offered a good example of SLDs, offering 1310nm wavelength, 38nm spectral width, but the power output was still at a very low 50mW.
With further research I was able to find varying specifications for SLDs, and on average:
Wavelength: 400nm-1700nm
Spectral width: 5nm-100nm (5nm-50nm should be achievable)
Lowering frequency range to 12.5MHz optimise power for LED
1mW-60mW range ordinarily
60mA-1500mA (1.5A) ordinarily
Allow for optimal beam quality (minimal beam divergence)
The lowest value (around 127 dB/Hz) is attained by the most powerful SLEDs in the 1310 nm window and in the frequency range limited to values less than 500 MHz
They are similar to laser diodes, containing an electrically driven p–n junction and an optical waveguide, but SLDs lack optical feedback, so that no laser action can occur
Unlike standard power supplies, High Output Wide Range LED Power Modules deliver a fixed current to the output. The output voltage will vary as required to maintain the specified output current with differing forward drop voltages of LED junctions
Blending this with an LED driver would have been preferable. An LED driver is a self-contained power supply that has outputs matched to the electrical characteristics of an LED or array of LEDs. There are currently no industry standards, so they can be customised to fit any need.
Using 2.8A high driving current technique (LED driver), with a range of 4-18v, offers a power of 11.2-50.4 watts, which would have been brilliant!
Final Super Luminescent Diode
Wavelength - 1310 nm (Range: 400nm-1700nm)
Spectral width - 38nm (Range: 5nm-50nm)
Frequency - 12.5MHz (Range: >500MHz)
Current Drive - 1.5A (Range: 60mA-1500mA)
Voltage Drive - Based on other models, we should expect this to be 1V
Output Power - 1.5W (Possibility of 50.4W using LED Driver of 2.8A/18V)
Rise time - 2-10ns
*These results are based on an SLD system I discovered with no mention of power output. I used the current drive and power output of the average SLD system (being 60mA and 60mW) to get an average voltage drive of 1V. From here I used the upper availability of 1500mA (1.5A) in the SLD in question, with this 1V average, to produce a power output of 1.5W. Dependant on the diode, this could be a wrong assumption, but it is the closest SLD I could find to producing anywhere near the amount of power we need*
From here I looked into surface emitting diodes, in the hopes of finding a suitable power/wavelength blend, as SLDs offer either a very good power output, or a very good wavelength, but never both at the same time. I summed up the information as:
- Novel surface emitting long-wavelength LED structure with narrower output spectrum
- The LED power spectrum is narrowed by growing an integral filtering layer which absorbs the power emitted
- Resulting LED spectra can be as narrow as edge-emitting diodes
Some good examples have cropped up of ultra high power IR LEDs, which again suffer from the power/wavelength trade-off. If this LED had a 1300nm wavelength, we could have scrapped the SLD idea
(850nm, 40nm, 3.6W, 1A DC, 10ns)
An article by L. Goldberg claims that 470mW of power can be output when 165mW LED is seeded with low power SLD. I will look into this as an option for our fiber optic source.
Finally, the average cost of a 1300nm multi-mode LED comes to $695.00
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