Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1998-11-05
2001-03-06
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S283000, C359S324000, C359S484010, C385S006000
Reexamination Certificate
active
06198567
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a Faraday rotation attenuator and, more particularly, to the use of multiple domain garnet, disposed between single mode transmission paths, to form an attenuator of variable strength by varying the external magnetic field applied to the ferromagnetic garnet.
Optical attenuators are useful in a number of different optical system applications. For example, attenuators are used in optical amplifier systems to “balance” the gain across the different operating wavelengths. An optical attenuator may also be inserted in the signal path of an amplifier system beyond the pre-amplifier location to stabilize the saturation level of the power amplifier Wavelength division multiplexed systems may also use optical attenuators in the receiver portion of the system to compensate for variations in received signal power
Most conventional prior art attenuators include a motorized arrangement, using a stepper motor to rotate one or more objects into and out of the optical signal path. Although such arrangements are adjustable and can provide the desired degree of attenuation, they are relatively slow and have reliability concerns related to the need to physically move the objects with respect to the signal path. A non-mechanical optical attenuator is described in the article entitled “Non-Mechanical Variable Attenuator Module using Faraday Effect”, by N. Fukushima et al. appearing in
OSA Trends in Optics and Photonics
, 1996, Vol. 5, at pp. 249-52. In this arrangement, a variable Faraday rotator is disposed between a pair of polarizers. Two magnets are disposed to surround the magneto-optic crystal material in the rotator, a permanent magnet and an electromagnet. The permanent magnet thus defines a certain domain within the material and the polarizers control the polarization state of the input and output signals. The application of a current to the electromagnet is then used to control the degree of attenuation and has been found to provide attenuation in the range of 1.6 to 25 dB.
While the Fukishima et al. arrangement may be considered an advance over the mechanical attenuators, the arrangement is rather bulky, requiring the use of a pair of magnets, as well as the polarizers to control the state of the signals passing through the attenuator.
Thus, a need remains in the art for providing adjustable optical attenuator without the need to physically move system components to achieve the attenuation and which is less bulky than the prior art non-mechanical alternatives.
SUMMARY OF THE INVENTION
The need remaining in the prior art is addressed by the present invention, which relates to a Faraday rotation attenuator and, more particularly, to the use of multiple domain garnet, disposed between single mode transmission paths, to form an attenuator of variable strength by varying the external magnetic field applied to the ferromagnetic garnet.
The adjustable attenuator of the present invention functions to provide transmission of an optical signal from a first (input) single mode optical fiber to a second (output) single mode optical fiber, while providing any “variable” amount of optical attenuation therebetween, that is, provide fiber-to-fiber coupling loss in the range from 1 dB (little attenuation) to greater than 50 dB (significant attenuation). By varying the strength of the magnetic field surrounding the garnet film, the attenuation may be adjusted between these two extremes.
In accordance with one embodiment of the present invention, a variable attenuator comprises an input signal mode fiber, a first focusing/collimating element (such as a GRIN lens, aspheric lens, or the like) for collimating the single mode signal exiting the input fiber, a 90° Faraday rotator comprising a section of garnet material, a second collimating/focusing element disposed beyond the Faraday rotator, and a single mode output fiber for collecting the focused signal exiting the second collimating/focusing element. The garnet is “ferromagnetic”, meaning that in the absence of an applied magnetic field, the garnet exhibits alternating magnetic domains at a microscopic level. That is, the two magnetic domains are balanced such that equal portions of the optical signal passing through will be rotated through each polarization. If the garnet film is of a sufficient thickness to provide a 90° rotation, the two signals associated with the two types of magnetic domain will exit 180° out of the direction of polarization such that their amplitudes will essentially cancel at the output fiber, yielding a large degree of attenuation. The application of a magnetic field results in spreading one magnetic domain type, while causing the other domain type to shrink. Therefore, the portion of the optical signal associated with the latter domain will be diminished and little or no cancellation of the signal associated with the stronger domain will occur, allowing this signal to pass relatively unaffected into the output fiber (that is, little attenuation will occur).
In an alternative embodiment, a 45° Faraday rotator may be used, with a reflector disposed in the signal path beyond the 45° Faraday rotator. In this case, the optical signal will pass through the rotator twice (hence, each beam component will experience a fall 90° rotation) as in the embodiment described above. Again, when no magnetic field is present, the two magnetic domains are equally present, one rotating the plane of linear polarization +45°, and the other rotating the plane of linear polarization −45°. The application of a magnetic field will alter the balance of these two rotations, where a field of sufficient strength will completely suppress one of the domain types. In a reflective arrangement, the return signal may be coupled to a separate output single mode fiber disposed next to the input single mode fiber. Alternatively, the return signal may be coupled back into the input single mode fiber.
Other and further embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
REFERENCES:
patent: 4989938 (1991-02-01), Tamulevich
patent: 5402509 (1995-03-01), Fukushima
patent: 5471340 (1995-11-01), Cheng et al.
patent: 5477376 (1995-12-01), Iwatsuka et al.
patent: 5619367 (1997-04-01), Iwatsuka et al.
patent: 5844710 (1998-12-01), Fukushima
patent: 5867300 (1999-02-01), Onaka et al.
patent: 5889609 (1999-03-01), Fukushima
patent: 5915063 (1999-06-01), Colbourne et al.
N. Fukushima “OSA Trends in Optics and Photonics” vol. 5, pp. 249-252, 1996.
JDS Fitel “VCB Series—Voltage Controlled Attenuators” 1998.
Dicon “Product Summary '96-'97—Switches”.
Ben Loha
Koba Wendy W.
Lucent Technologies - Inc.
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