Slab Coupled Fibers
Overview - What is a Slab Coupled Fiber?
Slab Coupled Fibers perform as wavelength selective couplers with a wide variety of applications. The BYU Photonics lab's technique for exposing the
evanescent field of an optical D-fiber serves as a unique platform for creating Slab Coupled Fibers. Coating the exposed evanescent field of
a fiber with a polymer slab waveguide offers wavelength selective coupling between the optical fiber and polymer slab. Coupled Mode Theory indicates
that this coupling occurs at resonant frequencies based on slab dimensions and index. Our results show wavelength
selective coupling with narrow resonances and extreme sensitivity to index variation of the polymer slab. While very similar in operation to Side Polished
Fibers with a slab overlay, Slab Coupled Fibers offer new freedoms in the fabrication process that allow further control in the effectiveness of the device.
The presence of the slab waveguide in the evanescent region of the etched D-fiber above acts as a mode sink for coupled modes. The coupling spectrum on the right shows
the transmission spectrum for the D-fiber with a power drop at the location of the mode coupled into the slab waveguide. Using environmentally sensitive materials for the
slab waveguide yields shifting of the coupled mode based on changes in environmentally sensitive variables such as temperature, electric field, and chemical presence.
Navigate the following expandable list for further information on Slab Coupled Fibers.
- Benefits of D-Fiber
- Optical D-fiber, supplied by KVH industries, offers beneficial geometry for creating in-fiber devices. The core resides within ~13 um of
the flat side of the fiber. By means of a simple chemical wet etch, the upper cladding of a section of D-fiber can be removed, exposing the
evanescent portion of guided modes and leaving the rest of the fiber in tact. Many in-fiber devices, including
surface-relief fiber Bragg gratings, are based upon interaction with the evanescent field of the fiber core.
- Coupled Mode Theory
- Coupled-Mode Theory describes how a field in one mode/waveguide is coupled into another mode/waveguide. The coupling strength depends on
the degree of field overlap between waveguides, phase-matching of modes, and the length of overlap. Given proper conditions, complete coupling from
one waveguide to another waveguide can occur.
- The above diagram shows two arbitrary waveguides in far proximity and close proximity to each other. The waveguides within close proximity have
mode overlap in their evanescent fields and can couple power from one to another. For an optical fiber coupled to a slab waveguide, phase matching requirements
indicate that modes will couple at resonant frequencies based on the following equation:
- where t is the thickness of the slab waveguide and n0 is the index, while nef is the effective index of the fiber mode. The coupled wavelength,
lambda, depends on the mode number, m.
- Fabrication of Slab Coupled Fibers
- 1a) A small section of jacket is removed from the D-fiber to expose the glass fiber for the fabrication process.
- 1b) The section of bare fiber is etched in a solution of hydrofluoric acid to expose the evanescent portion of guided modes. This will allow interaction
with the evanescent field of the D-fiber through the flat portion of the etched section.
- 2) The fiber is embedded into a planarized substrate.
- 3) A slab waveguide can be applied through traditional spin coating or other material growth processes.
- Slab-coupled fibers have the ability of detecting environmental changes by observing shifts in their coupling spectrum with a standard interrogator or optical monitoring system.
Spectral shifting comes as a result of change in the optical index of the polymer slab waveguide:
- Chemical Sensing - Slab coupled fibers are capable of detecting the presence of chemicals and chemical vapor. By using a polymer slab waveguide with an optical
index sensitive to chemical presence (such as PDMS' sensitivity to organic solvents), a shifted coupling spectrum will indicate exposure to specific chemicals.
- Electric Field Sensing - Non-linear optical polymers can be made with sensitivity to externally applied electrical fields. A common combination, DR1 chromophore side chains
in a PMMA polymer host, can be poled under high fields and elevated temperatures such that the DR1 molecules align to a fixed orientation and then contribute a
net change in optical index under the presence of electric fields. Non-linear opticalpolymers can supply effective electric-field monitoring when used as slab waveguides
in slab-coupled fibers.
- Temperature and pressure sensing - All polymers exhibit a certain degree of index change relative to ambient temperature and pressure. Depending on the range and sensitivity of
temperature or pressure monitoring desired, polymers can be chosen for the slab waveguide to effectively monitor temperature or pressure variations in the environment of interest.
- Current Results
- Specifications: Current fabrication techniques allow the coupling efficiency of slab-coupled fibers to reach ~20 dB coupling strength with a linewidth
of 1-3 nm and minimal to mid insertion loss.
- Temperature testing PMMA demonstrates extreme sensitivity of the coupled wavelength to changes in temperature. The following movie illustrates the shift in coupling with
increase in temperature: Temperature Shift (Quicktime Movie).
For inquiries and suggestions, please email: Rich Gibson