Hi
Has anybody any experiences of working with laser diodes below the lasing threshold?
I need a broad band (at least 50 nm band width) light source for OCT applications, but I cannot use SLDs, because they are expensive! I have been recommended to use laser diodes below the lasing threshold to achieve a broader band width. Is that possible?
Thanks for your time in advance.
Hi,
If you operate the laser diode below threshold but above transparency regime yes that's possible but in commercial lasers transparency and threshold are so close that it's almost impossible to operate the laser diode in between. If you find a way to do that like controlling the bias current precisely and monitoring the spectra on a spectrometer you might be able to do it. An SLD is basically a laser that operates in between transparency and threshold.
Arman Rashidi Thanks for your answer. It was very helpful.
Vincent Lecoeuche The central wavelength should be in the range of 800 nm to 900 nm, the spectral width should be at least 50 nm, and I also need a fiber pigtail (simgle mode fiber with the core diameter of about 5 um), and also need about 1 mW power at the fiber output
I tried to use LED, but it was almost impossible to couple any thing to the fiber! do you have any idea that how can I couple light from LED to a single mode fiber with the core diameter of 5 um?
Hi Sara,
You can use microscope objective to couple into optical fiber.
Vincent Lecoeuche
Thanks for your suggestion, but, unfortunately, this one is multi-mode fiber pigtail and if one ask for a single mode fiber, the price dramatically increases.
Hi,
Technically it is possible and indeed it works. However, there is a need for a series of parameterizations depending on the type and model under consideration. If you want to work with OCT with reduced costs, the best option is to use LEDs. LEDs can emit broad bandwidth and exhibit reduced coherence length. You can get devices around 6 to 10 microns very easily. One drawback of LEDs over Lasers and SLDs is the fact that LEDs are not point sources. However you can build a very cheap device with an LED and a piece of optical fiber about 20 mm. The optical fiber segment should be polished at both ends. You should make a hole in the LED and insert the optical fiber segment into the LED. You should make a transparent epoxy bonding at the desired wavelength. After drying, you should paint the LED with black paint. This isolates all light coming from the LED keeping only the light output from the end of the optical fiber. This simple and inexpensive device reproduces the broadband characteristics and low coherence of the SLD. If you fit, the diameter of the fiber optic, the position of the hole in the LED and the bonding process, you can achieve reasonable powers to work with multiple reflections of the order of 1%, with detectors amplifiers operating around 80dbs.
It is possible to use of multimode lasers. The negative is that will be multimods with different polarisations. You must have precision current source and spectrometer for control of laser beam. We performed the test of 1W multimode laser with a radiation length of about 800 nm. The detales is in attached file of paper.
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