Superluminescent diodes (SLD/SLED) applications
In one of our last articles (see article) we explained the basic principles of superluminescent diodes (SLD) operation. After understanding what means that the SLDs have a high spatial coherence and a low temporal coherence (see article), we can study their applications. Nowadays, the most important ones are the following:
- Optical coherence tomography (OCT):
Is a high-resolution (µm or even lower) optical imaging technique used to create 2D or 3D images of transparent or semi-transparent objects of limited depth, such as biological tissues.
Right now, OCT is being used in a wide range of fields in biomedics like ophthalmology (diagnosis of ocular diseases, for instance; glaucoma, macular degeneration), cardiology (coronary artery imaging), oncology (detection of cancer and precancerous lesions) or dermatology (diagnosis of carcinomas).
The operation is the following: a tightly focused laser beam scans a sample and the backscattered light is measured using an interferometer to reconstruct an image of the sample. In order to obtain a 2D image the light source is displaced across the sample. The third dimension is added by recording the interference pattern as a depth profile for each scan point.
Why a SLD? Because we need a light source that efficiently directs the light towards a single point in the sample (strong directionality of the light beam, that is, high spatial coherence) and because a broadband light source enables a higher resolution, as we don’t have the limitation caused by speckle (therefore, low temporal coherence is required).
- Fiber sensors: we introduce 2 types of sensors:
- Fiber Bragg Gratings (FBG): is a periodic structure done along a small length (around 0.5 µm) of the core of an optical fiber that consists of a series of transitions between two values of the refractive index.
This structure is characterized by its resonance Bragg wavelength λB = 2 ∙ neff ∙ Λ, where neff is the effective refractive index of the structure and Λ is its period. At this wavelength the reflection reaches a maximum and the transmission a minimum.
The interesting properties about this structure are that λB changes linearly with temperature and strain (sensors for one of these parameters can be developed when the sensitivity to the other is supressed), it is not affected by the variation of the light intensity and it allows sensor multiplexing (several sensors along the same optical fiber).
Figure 1. Fiber Bragg Gratings (FBG).
- Long Period Gratings (LPG): it follows the same periodic structure that the FBG but the period Λ is longer (around 103 greater than in the FBG). In this case, there is no reflection and part of the light is coupled into the cladding of the fiber, where it suffers from a high attenuation, producing a minimum in the transmission.
Due to their larger period, the LPG fabrication method is easier. Furthermore, LPG are not only sensitive to temperature and strain, but also to bending that causes a curvature, to hydrostatic pressure, to torsion and to ambient refractive index changes. This fact allows the development of a wide range of sensors.
Why a SLD? In order to obtain information from sensors based on FBG or LPG we have to interrogate them, that is, send light using a light source and employ an optical spectrum analyser (OSA) to know which part of the light has been reflected (FBG) or absorbed (LPG). Regarding the light source, we choose a SLD because a broadband light source is needed and it is preferable to use a high power source, as we can work with sensors over a great length of optical fiber.
- Fiber Optic Gyroscope (FOG): is a device that measures changes in orientation thanks to the Sagnac effect. The beam from a light source is split in two beams and they are injected in a looped fiber optic coil (with a great number of loops) in opposite directions, that is, one clockwise (CW) and the other one counter clockwise (CCW).
Figure 2. Schematic diagram of a fiber optic gyroscope (FOG).
If the loop is still, the two beams will travel the same distance. Nevertheless, if the loop is rotating, the two beams will travel different distances. The path difference will cause a phase shift between the two beams that can be determined through interferometry, and thus, the orientation of the loop. FOGs are used in inertial navigation systems (INS).
Why a SLED? In the first place, we require a high power light source, as the looped fiber optic coil used can have lengths of several kilometres. In the second place, it’s better to use a broadband light source because if we interrogate the sensor with it, we reduce the problems caused by Rayleigh backscattering (scattering induced by particles much smaller that the wavelength of light) and Kerr effect (nonlinear effect caused by the propagation of intense light), undesired phenomena that behave as noise sources. That’s why SLDs are also used in this application.
We hope that this article has helped you reach a better insight of SLDs applications.
Armenise, M.N. (2010). “Fiber Optic Gyroscopes”, chapter from “Advances in Gyroscope Technologies” (pp.29-50), Springer, Berlin, Heidelberg doi:10.1007/978-3-642-15494-2_4
Eftimov, Tinko. (2007). “Sensor Applications of Fiber Bragg and Long Period Gratings”, chapter from “Optical Waveguide Sensing and Imaging” (pp.1-23)doi: 10.1007/978-1-4020-6952-9_1.