Optical waveguides – Polarization without modulation
Reexamination Certificate
1999-10-19
2001-07-24
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Polarization without modulation
C359S490020
Reexamination Certificate
active
06266456
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a polarization scrambler, more specifically to an optical fiber polarization scrambler which effectively decreases degree of polarization of the output light using at least two optical fiber birefringence modulators.
The present invention also relates to an operating parameter input method of an optical fiber polarization scrambler, more specifically to a method for inputting a predetermined values of parameters such as a modulation frequencies and modulation amplitudes established by experiments to effectively operate the optical fiber polarization scrambler.
BACKGROUND ART
Polarization scramblers are devices that converts highly polarized light into light with a scrambled polarization by modulating the state of polarization(“SOP”). Degree of polarization(“DOP”) represents the ratio of polarized light to total light. For example, the DOP of perfectly polarized light is 100%, whereas the DOP of completely depolarized light is 0%. The function of the polarization scrambler is to decrease the DOP in time average. An ideal polarization scrambler forces input light of 100% DOP to output light of 0% DOP.
When light goes through an optical component having a polarization-dependent loss, output power depends on the input SOP of the light. In this case, constant time-averaged output power can be obtained regardless of the input SOP by inserting a polarization scrambler in front of the optical component. Enhanced signal-to-noise ratio can also be obtained with the polarization scrambler in a fiber-optic sensor, optical measurement system and long-haul optical transmission system.
The output SOP is evolved by a birefringence medium, especially the amount of the birefringence and the angle of the birefringence axis when polarized input light is propagating along the optical fiber. For the light which has been transmitted along a long optical fiber over a few meters, however, the output SOP is apt to change since the birefringence of the fiber is easily affected by small environmental perturbation such as temperature and pressure. This leads to output power fluctuation of the light which went through an optical device having certain polarization-dependent loss. However, the fluctuation of the output light could be prevented if it is averaged in time by decreasing the DOP of the output light through polarization modulation.
If the polarization direction of the input light coincides with the direction of the birefringence axis of the birefringence modulator, the output SOP will not vary even with any change of the birefringence. Therefore, in order to induce polarization modulation of the output light regardless of the input SOP using birefringence amplitude modulation, at least two polarization modulators whose birefringence axes form an angle of 45° must be used.
In the prior art, a polarization scrambler is implemented by inducing birefringence modulation in an integrated optical circuit, such as a lithium niobate (LiNbO
3
) optical waveguide. However, this kind of the polarization scrambler has the disadvantages of low efficiency, difficulty in connecting two or more birefringence modulators, and high insertion loss.
U.S. Pat. No. 4,923,290 discloses a basic concept of the polarization scrambler using an optical fiber. However, since the method of directly exerting pressure onto the optical fiber is used to implement the scrambler, the device for inducing the birefringence has a low efficiency with scrambling frequency lower than few hundred Hz, which is too low. Also the birefringence axis is subject to easy change and the output SOP deteriorates. This renders the realization of the polarization scrambler with good performance difficult.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide an optical fiber polarization scrambler which can efficiently carry out polarization modulation and make the time-averaged DOP zero regardless of the input SOP and an operating parameter input method therefor.
Another object of the present invention is to provide an optical fiber polarization scrambler with very low insertion loss and stable performance regardless of the input SOP using enhanced optical fiber birefringence modulators whose modulation frequencies are in orders of hundred kHz to MHz and an operating parameter input method therefor.
A further object of the present invention is to provide an optical fiber polarization scrambler comprising a strand of optical fiber whose structure can counterbalance the effect of the circular birefringence caused by twisting the optical fiber between the neighboring birefringence modulators, while providing the most efficient operating parameter input method therefor.
In order to achieve the above-mentioned objects, the optical fiber polarization scrambler according to the present invention comprises optical fiber birefringence modulators and alternating voltage sources. The optical fiber birefringence modulators include at least two hollow cylindrical piezo devices and a continuous optical fiber wound around outer walls of the piezo devices. The alternating voltage sources respectively apply alternating voltage to each optical fiber birefringence modulator to induce birefringence modulation. The continuous optical fiber is wound around the piezo devices without any twisting in axial direction of the optical fiber to prevent inducing of circular birefringence thereat. But the section of the optical fiber linking the neighboring birefringence modulators is twisted so that the input light passed the front birefringence modulator with its polarization plane being parallel to a principal axis of the front birefringence modulator can be launched into the back birefringence modulator with its polarization plane forming an angle of 45±90n degrees with respect to a principal axis of the back birefringence modulator, where n is an integer.
According to the present invention, the hollow cylindrical piezo devices can have different wall thickness to make the resonance frequency of the thickness mode of each piezo device of the optical fiber birefringence modulators different.
Also, it is preferable that the means for compensating effect of circular birefringence caused by twisting applies further twist to the section of the optical fiber linking neighboring birefringence modulators. Additional twist is of the angle corresponding to 8% of (45±90n) degrees and the total angle between the principal axes of the neighboring birefringence modulators becomes 1.08×(45±90n) degrees which also means the total twist of the linking optical fiber, where n is an integer.
Preferably, the optical fiber is a single-mode optical fiber whose intrinsic birefringence is 5×10
−6
or less.
Also, preferably, the linear birefringence induced by tension winding and bending of the optical fiber on the outer wall of the piezo device is maintained 30 times or more than the intrinsic birefringence of the optical fiber, and more preferably the wound optical fiber is annealed to eliminate spurious birefringence.
In addition, the cross-section of the optical fiber is in a shape of D, and the optical fiber can be wound with the flat plane of the optical fiber abutting the outer walls of the piezo devices to prevent twist of the optical fiber in the axial direction.
On the other hand, the optical fiber polarization scrambler according to the present invention can further comprise means for measuring temperature of the birefringence modulator; and means for determining modulation amplitude corresponding to the temperature sensed by the means for measuring the temperature.
In another structure of optical fiber polarization scrambler according to the present invention, the optical fiber polarization scrambler comprises: optical fiber birefringence modulators including at least two hollow cylindrical piezo devices and an optical fiber wound around outer walls of the hollow cylindrical piezo devices; and alternating voltage sources applying alternating voltage to each optical fiber birefringenc
Kim Byoung Yoon
Koh Yeon Wan
Lee Bong Wan
Yeo Young Bae
Connelly-Cushwa Michelle R.
Donam Systems Inc.
Marger & Johnson & McCollom, P.C.
Palmer Phan T. H.
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