Optical waveguides – Directional optical modulation within an optical waveguide – Acousto-optic
Patent
1996-12-30
1999-06-22
Ngo, Hung N.
Optical waveguides
Directional optical modulation within an optical waveguide
Acousto-optic
385 43, 385 96, 385 42, G02B 610
Patent
active
059150505
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to optical devices.
DESCRIPTION OF THE RELATED ART
Acousto-optic devices have been proposed for use as optical frequency shifters, tunable filters, optical switches and optical modulators. These devices make use of an interaction between light passing through a transmission medium and acoustic vibrations set up within the transmission medium.
An example of a known acousto-optic device is the Bragg cell, which comprises a light transmitting cell in which acoustic vibrations (in the form of travelling waves) are set up by a transducer attached to one face of the cell.
The acoustic vibrations cause a periodic perturbation in the refractive index of the cell material, as the material is successively compressed and rarefied. These periodic perturbations act as a travelling diffraction grating within the cell. In operation, light incident on the cell at the so-called Bragg angle (a known function of the light's wavelength and the period of the diffraction grating) is diffracted by the travelling grating. This changes the direction of propagation of the light. In addition, because the grating is travelling through the cell (with the travelling acoustic vibrations), the diffracted light is also Doppler shifted, so that the frequency of the light is altered by an amount equal to the frequency of the acoustic vibrations. When the acoustic vibrations are removed, the light is not diffracted and emerges from the cell substantially unchanged.
In this way, the Bragg cell can operate to shift the frequency of incident light, to change the direction of propagation of the incident light (thereby acting as a switch) or to provide frequency or amplitude modulation of the incident light. However, the Bragg cell has the significant disadvantage that it is an expensive and complex bulk optical device. It is difficult to employ a Bragg cell in an optical fibre system, since the optical fibres have to be mechanically "pig-tailed" to the cell. This can lead to an insertion loss of 5 to 6 dB (decibels), which can be a major problem in, for example, an optical fibre communication system where the distance and speed of communication can depend on the optical power available at the communication receiver. A further disadvantageous feature of the Bragg cell is that a high electrical power (for example, of the order of 1 Watt), is required to drive the acoustic transducer. This is because a large proportion of the cell must be driven with the acoustic vibrations, although the actual overlap (volume of interaction) between the light and the acoustic vibrations occupies only a small proportion of the cell.
As an alternative to the Bragg cell, it has been proposed to propagate light along an optical fibre having two light transmission modes, and to induce flexural acoustic vibrations in the fibre to couple light from one of the fibre modes to the other. The two modes may be two different spatial modes of a dual-mode fibre (for example the so-called LP.sub.11 and LP.sub.01 modes), the two orthogonal polarisation modes of a birefringent fibre, or the so-called supermodes of a dual-core fibre.
These optical fibre acousto-optic devices make use of the fact that the two modes propagate at different velocities along the optical fibre, setting up a spatial beating between the two modes along the fibre length. When the "beat length" matches the wavelength of a flexural vibration applied to the fibre, there is resonant coupling of light from one of the modes into the other, with frequency shifting also taking place as described above. Mode filtering can then be used to isolate the desired (frequency shifted) mode.
Fibre acousto-optic devices of this type are generally more suitable than the Bragg cell for use in an all-fibre system. However, the maximum acoustic frequency that can be applied is limited by the fibre dimensions, and is typically restricted to about 10 MHz (megahertz). The light is generally confined to the fibre core, but the acoustic wave must cause vibration of the entire fibre. Thi
REFERENCES:
patent: 5345522 (1994-09-01), Vali et al.
patent: 5379357 (1995-01-01), Sentsui et al.
T.A. Birks, et al.; "Four-port fiber frequency shifter with a null taper coupler" Optics Letters/vol. 19, No.23/Dec. 1, 1994; pp. 1964-1966.
Birks Timothy Adam
Pannell Christopher Noel
Russell Philip St John
Ngo Hung N.
University of Southampton
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