Bragg and fiber gratings Mikko Saarinen 27.10.2009
Bragg and fiber gratings
Mikko Saarinen 27.10.2009
Bragg grating
- Bragg gratings are periodic perturbations in the propagating medium,
usually periodic variation of the refractive index
- like diffraction gratings, refractive index variations scatter light
- multiple scatterings can be approximated as two waves propagating in
opposite directions with propagation constant 𝛽0 =2𝜋𝑛𝑒𝑓𝑓
𝜆0
- energy is coupled from one wave to the other if 𝛽0 − −𝛽0 =2𝜋
𝛬 where
Λ is the period of the grating, usually around 0.5 µm
- grating reflects Bragg wavelength λ0=2neffΛ
- reflectance decreases as the incident wavelength differs from Bragg
wavelength -> only one wavelength is reflected
- the high-index regions also scatter light at other wavelengths, but the
scattered waves differ in phase so they cancel each other by destructive
interference
- uniform refractive index pattern change has unwanted side lobes caused
by abrupt start and end of grating
- side lobes can be eliminated with apodized grating, where the refractive
index change is made smaller towards the edges of the grating
- bandwidth is inversely proportional to the length of the grating
Fabrication
- Fiber Bragg gratings are created by "writing" the periodic variation of
refractive index into the core of optical fiber using an ultraviolet source
- typical material is a conventional silica fiber doped with germanium which
makes it extremely photosensitive
- only small refractive index changes needed (Δn≈10-4)
- one manufacturing method is to expose fiber to two interfering UV
beams, which causes the radiation intensity to vary periodically along the
fiber
- requires very high coherence length (amplitude splitting) or spatial
coherence across beam width (Lloyd mirror)
- single – frequency gratings only
- another way is to illuminate phase mask with UV light, which will diffract
light in two directions
- low – coherence UV sources can be used
- arbitrary Λ(z) profiles possible, depends only on the mask
Grating structures - Structures of Fiber Bragg Gratings vary via refractive index (uniform or
apodized) or grating period
- the refractive index profile of the grating may be modified to add linear
variation in the grating period, called a chirp. The reflected wavelength
changes with the grating period, broadening the reflected spectrum
- chirped Bragg gratings can be used to compensate dispersion by making
the grating so that segments which reflect different wavelengths are in
different positions along the length of the grating
Applications - Fiber gratings have low loss (0.1 dB), high wavelength accuracy (0.05 nm),
ease of coupling with other fibers and high adjacent channel crosstalk
suppression (40 dB)
- cheap all-fiber devices with small packing and polarization insensitivity
- typical temperature coefficient 1.25*10-2 nm/○C caused by variation in
fiber length with temperature. This can be compensated with by packing
the grating with a material that nas negative thermal expansion
coefficients
- Bragg wavelength is dependent on strain and temperature
𝛥𝜆𝐵 = 2 𝛬𝜕𝑛𝑒𝑓𝑓
𝜕𝑙+ 𝑛𝑒𝑓𝑓
𝜕𝛬
𝜕𝑙 𝛥𝑙 + 2 𝛬
𝜕𝑛𝑒𝑓𝑓
𝜕𝑇+ 𝑛𝑒𝑓𝑓
𝜕𝛬
𝜕𝑇 𝛥𝑇
- gratings can be used as a sensing element in optical fiber sensors
- Fiber Bragg Gratings have variety of uses in WDM systems: filters, optical
add/drop elements and dispersion compensation
- cascading multiple optical add/drop elements will create a optical
multiplexer/demultiplexer
Long-Period Fiber Gratings - operation is based on energy transfer from the forward propagating
mode in the core onto the forward propagating in the cladding
- coupling occurs between core mode at given wavelengths and pth-order
cladding mode
- phase-matching condition dictates that 𝛽 − 𝛽𝑐𝑙𝑝=
2𝜋
𝛥
- the difference between propagation modes are quite small -> typical
values for Λ are typically from hundred micrometers to few millimeters
- cladding modes are very lossy -> energy will decay along the fiber
- losses are caused by absorption and scattering
- wavelength at which energy will be coupled from can be written as a
function of effective indices 𝜆 = 𝛬 𝑛𝑒𝑓𝑓 − 𝑛𝑒𝑓𝑓𝑝
- wavelength is proportional to grating length -> grating acts as a
wavelength-depending loss element
- loss can be controlled by controlling the UV exposure time during
fabrication
- more complicated transmission spectra can be obtained by cascading
multiple gratings with different wavelengths and exposures
- applications as an efficient band rejection filters and spectral
shaping devices include ASE filtering, removal of undesirable
Stokes’ lines in cascaded Raman lasers and most importantly gain
flattening in EDFA
References
Optical Networks: A Practical Perpective, second edition, R. Ramaswami, K. Sivarajan, 2002, Academic Press
http://en.wikipedia.org/wiki/Fiber_Bragg_grating
http://zone.ni.com/devzone/cda/ph/p/id/90
http://en.wikipedia.org/wiki/Long-period_fiber_grating
Fiber Grating Sensors, A. Kersey, M. Davis, H. Patrick, M. LeBlanc, K. Koo, C. Askins, M. Putnam, E. Friebele,
Journal of Lightwave Technology, 1997
Long period fiber gratings, A. Vengsarkar, Optical Fiber Communications, 1996. OFC '96
Long-period fiber-grating-based gain equalizers, A. Vengsarkar, J. Pedrazzani, J. Judkins, P. Lemaire, Vol. 21,
No. 5, March 1, 1996, Optics Letters