Abstract
Torsional ultrasound modes can couple the optical polarization states in a birefringent fiber. Polarization coupling produced by interaction with a higher-order torsional mode slightly above its cutoff may provide a route to producing an in-fiber isolator suitable for use in narrow-band high-power fiber amplifiers. This paper describes a model of a transducer generating torsional modes in a cylindrical fiber. This model predicts that almost all of the power applied to the transducer is radiated into the desired mode. The paper also discusses effects produced by acoustic absorption and the dependence of the acoustic velocity on temperature.
Highlights
By matching the optical polarization beat length LB in a birefringent silica optical fiber to the axial wavelength of a traveling torsional wave of angular frequency Ω propagating in the fiber, it is possible to couple light from one polarization to the other [1, 2]
When light is coupled from the slow polarization to the fast polarization, it is down-shifted in frequency by Ω if the light and sound are copropagating, but up-shifted if the light and sound are counter-propagating
The relative acoustic phase of the two transducers is adjusted such that in the forward direction all of the light at the wavelength of interest returns to the slow polarization, whereas in the backward direction all of the light at this wavelength goes into the fast polarization
Summary
By matching the optical polarization beat length LB in a birefringent silica optical fiber to the axial wavelength of a traveling torsional wave of angular frequency Ω propagating in the fiber, it is possible to couple light from one polarization to the other [1, 2]. Wavelength (microns) Figure 2: Calculated forward (red) and backward (green) transmission through an isolator using fiber with beat length LB = 3 mm and transducer separation D = 105 cm The position of these combs depends on the relative acoustic phase of the two transducers. The isolator can be used for multiple wavelengths commensurate with the comb period, such asthose that might be produced by spectral beam combination It cannot protect against inadvertent lasing at other wavelengths within the gain bandwidth of Yb. it cannot protect against inadvertent lasing at other wavelengths within the gain bandwidth of Yb This type of isolator can be tuned for use with other types of fiber amplifier besides Yb, such as Er or Tm. A transducer spacing of 1 m imposes a power limitation on operation of an AO isolator, since undesirable nonlinear effects can build up over this distance. This can filter out undesired wavelengths, including wavelengths within the gain bandwidth of the amplifying medium
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