This invention relates to optical systems, and more particularly to polarization dependent loss in optical systems.
During the past few years, the transmission capacity of fiber optic networks has increased tremendously. As with much of technology, however, more is better, and the limitations of today's technology are impeding the ability of fiber optic networks to fulfill bandwidth demands.
Polarization dependent loss (PDL) is one of the limiting factors in high-capacity wavelength-division-multiplexing (WDM) systems. Many optical network components and erbium-doped fiber amplifiers (EDFA) have non-negligible PDL.
PDL leads to a number of problems in optical networks, such as fluctuation in the optical power and signal to noise ratio (SNR) and enhanced degradation of systems due to the interaction with polarization-mode dispersion (PMD).
Current efforts to control or compensate for PDL are based on the principal that the incidence angle of a crystal or glass is polarization-dependent. Therefore, by tuning the input angle or the position of the crystal or glass, the induced PDL value changes. Here, the induced PDL value is the difference between the maximum and minimum insertion loss for all possible input states of polarization (SOPs). However, such efforts are unable to provide limited variation in the PDL of a system.
Therefore, it is desirable to provide a variable PDL device that may be used to control and/or compensate for PDL in an optical system.
According to an embodiment of the invention, a variable PDL device includes a beam splitter, a first polarization controller, and a beam combiner, where the beam splitter splits incoming light into a first beam with a first polarization state and a second beam with a second polarization state. The input beam has an input intensity, while the component of the input beam with the first polarization state has a first input intensity and the component of the input beam with the second polarization state has a second input intensity.
After the input beam is split, the first beam has its polarization rotated by a first polarization controller, which may be a fiber squeezer or other controller such as a Faraday rotator, liquid crystal, rotatable waveplate, a combination of a variable retardation plate (made of electro-optic or electro-ceramic material, for example) and a quarter wave plate, or other controller. In some embodiments, a redirection element such as a prism is used to redirect the first beam to the polarization controller after the beams are split.
The first and second beams are combined in a beam combiner. In some embodiments, a second redirection element such as a prism is used to redirect the first beam from the polarization controller to the beam combiner. A portion of the beams is transmitted out of the device on an output port. The portion of the output beam that has the first polarization state has a first output intensity, while the portion of the output beam that has the second polarization state has a second output intensity. The output beam has an output intensity.
The device may include an input port, by which the input beam is provided to the device. The polarization of the second beam may be rotated by a second polarization controller, which may also be a fiber squeezer or other controller such as a Faraday rotator.
The polarization controller may be variable. For example, it may be a fiber squeezer whose rotation angle is varied by changing the voltage on a piezoelectric mechanism. It may be a Faraday rotator whose rotation angle is varied by changing the current through an electromagnet.
The ratio of the first output intensity to the output intensity may be different than the ratio of the first input intensity to the input intensity. The ratio of the second output intensity to the output intensity may be different than the ratio of the second input intensity to the input intensity.
In some embodiments, a beam splitter/combiner may be used rather than a separate beam splitter and beam combiner. A first mirror may be used to reflect the first beam back through a first polarization controller to a redirection element and then to the beam splitter/combiner. A second mirror may be used to reflect the second beam back through a second polarization controller and then to the beam splitter/combiner. The mirrors may be simple reflective mirrors, 90 degree Faraday rotator mirrors, or other mirrors.
A method for controlling polarization dependent loss may include splitting an input beam into a first beam and a second beam, rotating the polarization of the first beam and the second beam, and directing a portion of the first beam and a portion of the second beam to an output port.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
a) is a side view of a variable PDL device, according to an embodiment of the invention;
b) illustrates a relationship between an initial state of polarization and the x and y axes;
c) is a side view of a variable PDL device, according to an embodiment of the invention;
Like reference symbols in the various drawings indicate like elements.
According to an embodiment of the invention, a variable PDL device 100 is shown in
In some embodiments, the first polarization state and the second polarization state are orthogonal. For example, for light traveling in the z-direction, the light with the first polarization state may be x-polarized, while light with the second polarization state may be y-polarized. Alternately, the first polarization state and second polarization state may lie along non-orthogonal directions, as long as their vector cross product is in the direction of travel of the light beam.
A polarization beam splitter (PBS) 110 splits the incoming light into a first beam 101 and a second beam 102, where the component of the input beam having a first polarization is split off as first beam 101, while the component of the input beam having a second polarization is split off as second beam 102. For the example shown in
After first beam 101 and second beam 102 are split, first beam 101 has intensity I1(split)=Iin*f1(θ), while second beam 102 has I2(split)=Iin*f2(θ), where θ is the angle between the input SOP and an optical axis of PBS 110. For example, when the optical axis of PBS 110 is in the x-direction,
First beam 101 passes through a first polarization controller 140, where its polarization is rotated by an angle φ. According to an embodiment, the polarization controller is a fiber squeezer, such as the polarization controller described in the co-owned U.S. Pat. No. 6,389,184, entitled “Fiber squeezer polarization controller with low activation loss,” X. Steve Yao, filed on Sep. 30, 2000, Ser. No. 09/676,557, which is hereby incorporated by reference in its entirety. In other embodiments, other polarization controllers such as Faraday rotators, liquid crystals, rotatable waveplates, combinations of a variable retardation plate (made of electro-optic or electro-ceramic material, for example) and a quarter wave plate, or other controllers may be used. First beam 101 is reflected by prism 120. First beam 101 is then incident on a polarization beam combiner (PBC) 125.
In the embodiment shown in
Similarly, second beam 102 passes through a second polarization controller 145, which may be a fiber squeezer polarization controller or other polarization controller, where its polarization is rotated by an angle β. The angles φ and β may be the same or different. Second beam 102 is then incident on PBC 125.
A portion equal to some, none, or all of second beam 102 may be transmitted out of device 100 through output port 130. For example, if second polarization controller 145 rotates the polarization of second beam 102 by 0° or 180° (i.e. it is still in the second polarization state), substantially all of second beam 102 will be transmitted through PBC 125 and out of device 100 through output port 130. If second polarization controller 145 rotates the polarization of second beam 102 by ±90° (i.e. it is in the first polarization state), substantially none of second beam 102 will be transmitted out of device 100 through output port 130. For other angles, a portion of second beam 102 will be transmitted out of device 100 through output port 130.
As shown in
The intensity at output port 130 is equal to Iout=I1(split)*f1(φ)+I2(split)*f2(β). Substituting the values of I1(split) and I2(split) above, then Iout=Iin[f1(θ) f1( )+f2(θ) f2(β)]. For different initial polarization states, (that is, for different values of θ), the loss is different. Therefore, the system exhibits polarization dependent loss that is controllable by controlling the angles φ and β. In some embodiments, the initial polarization θ may be controlled as well. For example, a polarization controller may be provided prior to PBS 110 in device 100 (see below).
In terms of the intensities of the components with the first polarization state and second polarization state, I1(output)=Iinf1(θ)f1(φ), while I2(output)=Iinf2(θ)f2(β). Thus the fraction of the output light in the first polarization state is equal to I1(output)/Iout, while the fraction of the output light in the second polarization state is equal to I2 (output)/Iout.
In some embodiments, a single polarization controller is used rather than multiple polarization controllers (e.g., a polarization controller for each beam as shown in
In
In some embodiments, first beam 101 and second beam 102 are propagated through free space between optical elements such as polarization controller 140, etc. In other embodiments, first beam 101 and second beam 102 are propagated on a physical medium such as an optical fiber along part or all of their optical path through device 100 (or device 200 of
After being split by PBSC 220, first beam 201 has intensity I1(split)=Iin*f1(θ), while second beam 202 has intensity I2(split)=Iin*f2(θ), where θ is the angle between the input SOP and the optical axis of PBSC 220. According to an embodiment, first beam 102 has a first polarization state and is reflected by PBSC 220, while second beam 202 has a second polarization state and is transmitted through PBSC 220, as shown in
First beam 201 is reflected by a first prism 230, then passes through a first polarization controller 240, which may be a fiber squeezer polarization controller or other controller. Controller 240 rotates the polarization of first beam 201. First beam 201 is then reflected by a first mirror 250. The position of first mirror 250 with respect to a second mirror 260 should be such that the differential group delay (DGD) between first beam 201 and second beam 202 is minimized or eliminated. The DGD may be minimized or eliminated either by mechanical design or by using a simple optical component such as a prism, as is known in the art. First beam 201 passes back through controller 240. Controller 240 again rotates the polarization of first beam 201.
First beam 201 is reflected by first prism 230 and is then incident on PBSC 215. The portion of first beam 201 with a first polarization may then be reflected by PBSC 215, while the portion with a second polarization is transmitted through PBSC 215 to output port 225. The portion that is reflected by PBSC 215 is incident on isolator 210. Since the light is traveling in the “wrong” direction, it is absorbed by isolator 210 rather than transmitted out of device 200; e.g. transmitted out of device 200 through an input fiber.
Second beam 202 is transmitted through PBSC 215, then through a second polarization controller 260, which may be a fiber squeezer polarization controller or other controller. Controller 260 rotates the polarization state of second beam 202. Second beam 202 is then reflected by a second mirror 270. Second beam 202 is transmitted through controller 260 again in the reverse direction, where its polarization is again rotated. First mirror 250 and second mirror 270 may be simple reflective mirrors, 90 degree Faraday rotator mirrors, or other mirrors.
Second beam 202 is then incident on PBSC 215. The portion of second beam 202 with a first polarization is reflected by PBSC 215, and travels out of device 200 through output port 225. The portion of second beam 202 with a second polarization is transmitted through PBSC 215, and is then incident on isolator 210. Since this light is traveling in the “wrong” direction, it is absorbed by isolator 210 rather than transmitted out of device 200; e.g. transmitted out of device 200 through an input fiber.
The intensities of first beam 201 and second beam 202 at output port 225 and the total output intensity Iout can be expressed in terms of the initial intensity Iin. If first controller 240 rotates first beam 201 by an angle φ each time first beam 201 passes through controller 240, and second controller 260 rotates second beam 202 by an angle β each time second beam 202 passes through controller 260, and if the initial polarization state had an angle θ with respect to the x-axis, then the intensity at output port 225 is equal to Iout Iin[f1(θ) f1(2φ)+f2(θ) f2(2β)].
As with the embodiment shown in
In terms of the intensities of the components with the first polarization state and second polarization state, I1(output)=Iinf2(θ)f2(β), while I2(output)=Iinf1(θ)f1(φ).
In an embodiment, one of the polarization controllers of
Light is incident on device 300 through an isolator 310. Isolator 310 allows light to pass in the forward direction but generally not in the reverse or “wrong” direction. Light may then be transmitted through a variable polarization controller 320. Controller 320 may be used to rotate the initial polarization state of light received by device 300. Controller 320 may be controlled by applying a voltage such as voltage V1 of
The light is then incident on PBSC 330, which splits the light into a first beam 301 of a first polarization and a second beam 302 of a second polarization. First beam 301 is reflected by a prism 340. First beam 301 is then transmitted through a variable polarization controller 350, where its polarization state may be rotated by an angle φ. Note that the angle φ may be dynamically changed by altering a voltage such as V2, shown in
A portion of first beam 301 having a first polarization may be reflected by PBSC 330, while the remaining portion of first beam 301 having a second polarization may be transmitted through PBSC 330. The portion of first beam 301 that is reflected by PBSC 330 is transmitted through controller 320, where its polarization is rotated, and then into isolator 310. Since the beam is traveling through isolator 310 in the “wrong” direction, substantially none of the light passes out of device 300 through the input.
The portion of first beam 301 that is transmitted through PBSC 330 is then output through output port 375, where it may be further transmitted. For example, the light may be transmitted on an output fiber 385.
After PBSC 330 splits first beam 301 and second beam 302, second beam 302 is transmitted through a variable polarization controller 380, where its polarization state may be rotated by an angle β. Note that the angle β may be dynamically changed by altering a voltage such as V3, shown in
A portion of second beam 302 having a first polarization may be reflected by PBSC 330, while the remaining portion of second beam 302 having a second polarization may be transmitted through PBSC-330. The portion of second beam 302 that is transmitted through PBSC 330 is then transmitted through controller 320, where its polarization is rotated, and then into isolator 310. Since the beam is traveling through isolator 310 in the “wrong” direction, substantially none of the light passes out of device 300 through the input.
The portion of second beam 302 that is reflected by PBSC 330 is then output through output port 375, where it may be further transmitted. For example, the light may be transmitted on an output fiber 385.
The intensity at output port 375 is equal to Iout=Iin[f1(θ) f1(2φ)+f2(θ) f2(2β)]. However, the angles θ, φ, and β may be controlled by altering the appropriate voltage. Therefore, the polarization dependent loss may be tailored to compensate for an uncontrolled polarization dependent loss in another part of the system. In terms of the intensities of the components with the first polarization state and second polarization state, I1(output)=Iinf2(θ)f2(2β), while I2(output)=Iinf1(θ)f1(2φ).
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, different types of polarization controllers, beam splitters, and beam combiners may be used. Accordingly, other embodiments are within the scope of the following claims.
This application claims the benefit of provisional application Ser. No. 60/309,227, filed on Jul. 31, 2001, which is hereby incorporated by reference in its entirety.
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