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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 8 — Apr. 17, 2006
  • pp: 3484–3490
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Complete polarization controller based on magneto-optic crystals and fixed quarter wave plates

Yang Zhang, Changxi Yang, Shiguang Li, He Yan, Jingchan Yin, Claire Gu, and Guofan Jin  »View Author Affiliations


Optics Express, Vol. 14, Issue 8, pp. 3484-3490 (2006)
http://dx.doi.org/10.1364/OE.14.003484


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Abstract

We propose and demonstrate a polarization controller, which is a concatenation of three Faraday rotators based on magneto-optic crystals separated by two fixed quarter wave plates. Comparing with former schemes, this polarization controller is fast, accurate and stable because it is completely driven by electric signals and has no mechanically moving parts. It is simple-structured and low-cost. Moreover, it is programmable to convert an arbitrary state of polarization (SOP) from the input to any designated SOP at the output. It can also be used as a polarization scrambler. It has potential applications in the research of the polarization dependence of optical communication systems or the compensation of polarization mode dispersion (PMD), etc.

© 2006 Optical Society of America

1. Introduction

Polarization control is a crucial technique in achieving optimum performance in coherent fiber optic communication systems, which require the state of polarization (SOP) of the received and local oscillator signals to be matched [1

1. N. G. Walker and G. R. Walker, “Polarization control for coherent communications,” J. Lightwave Technol. 8, 438–458 (1990). [CrossRef]

]. The polarization dependence of optical components in dense wavelength division multiplexed (DWDM) systems becomes a serious issue with the increasing transmission capacity [2

2. A. E. Willner, S. M. R. M. Nezam, L. Yan, Z. Pan, and M. C. Hauer, “Monitoring and control of polarization-related impairments in optical fiber systems,” J. Lightwave Technol. 22, 106–125 (2004). [CrossRef]

]. Polarization controllers are used along with stable light sources and optical power meters to evaluate the polarization dependence. Moreover, as the bit rate climbs from 10 to 40 Gb/s per wavelength division multiplexed channel and beyond, polarization mode dispersion (PMD) is becoming a major system impairment in high speed and long distance optical fiber transmission systems. Polarization controlling is one of the most important techniques in PMD compensation devices [3

3. R. Noé, D. Sandel, and V. Mirvoda, “PMD in high-bit-rate transmission and means for its mitigation,” J. Sel. Top. Quantum 10, 341–355 (2004). [CrossRef]

]. As a result, novel polarization controllers that are fast, accurate, programmable and immune to environmental vibrations are attracting much research interest.

Various polarization-controlling techniques have been proposed. Conventional polarization controllers like the rotating wave-plate type and squeezed fiber type have mechanically moving parts that limit their speed and accuracy [4

4. W. H. J. Aarts and G. D. Khoe, “New endless polarization control method using three fiber squeezers,” J. Lightwave Technol. 7, 033–1043 (1989). [CrossRef]

]. To solve these problems, J. Prat et al. proposed an all-fiber polarization controller that consists of two fiber Faraday rotators with a fiber quarter wave plate (QWP) between them [5

5. J. Prat, J. Comellas, and G. Junyent, “Experimental demonstration of an all-fiber endless polarization controller based on Faraday rotation,” Photon. Technol. Lett. 7, 1430–1432 (1995). [CrossRef]

]. However, this device requires a long fiber length that leads to undesired effects. Moreover, its output SOPs can cover the surface of the entire Poincare sphere only when the input SOP is linear. In other words, it is not a complete polarization controller. Other polarization controllers reported recently are based on liquid crystal rotatable waveplates [6

6. T. Chiba, Y. Ohtera, and S. Kawakami, “Polarization stabilizer using liquid crystal rotatable waveplates,” J. Lightwave Technol. 17, 885–890 (1999). [CrossRef]

], PLZT rotatable-variable waveplates [7

7. K. Hirabayashi and C. Amano, “Feed-forward continuous and complete polarization control with a PLZT rotatable-variable waveplate and inline polarimeter,” J. Lightwave Technol. 21, 1920–1932 (2003). [CrossRef]

], PLZT electro-optic wave plates [8

8. H. Shimizu and K. Kaede, “Endless polarisation controller using electro-optic waveplates,” Electron. Lett. 24, 412–413, (1988). [CrossRef]

], magnetic field rotating type Faraday rotators [9

9. T. Saitoh and S. Kinugawa, “Magnetic field rotating-type Faraday polarization controller,” Photon. Technol. Lett. , 15, 1404–1406 (2003). [CrossRef]

], etc. Ikeda et al. reported an endless tracking polarization controller using variable Faraday rotators (VFRs), which had superior optical characteristics and high reliability [10

10. K. Ikeda, T. Takagi, T. Hatano, H. Kazami, Y. Mimura, and H. Matsuura, “Endless tracking polarization controller,” Furukawa Review 23, 32 (2003).

]. Goldring et al. fabricated a tunable optical polarization controller (TIOPC), which consisted of two magneto-optic (MO) rotators and a QWP. The QWP was placed between the two MO rotators [11

11. D. Goldring, Z. Zalevsky, G. Shabtay, D. Abraham, and D. Mendlovic, “Magneto-optic-based devices for polarization control,” J. Opt. A: Pure Appl. Opt. 6, 98 (2004); [CrossRef]

]. Recently, Yao et al. reported an all-solid-state polarization generator, in which an additional linear polarizer was inserted in Goldring’s configuration [12

12. X. Yao, L. Yan, and Y. Shi, “Highly repeatable all-solid-state polarization-state generator,” Opt. Lett. 30, 1324 (2005). [CrossRef] [PubMed]

]. Yao’s polarization generator generated only four linear polarization states and two circular polarizations, i.e., right-handed circular and left-handed circular polarization states. The intrinsic shortcoming is that a linear polarizer was used, which leads to an additional 3 dB insertion loss for arbitrary input polarization states.

In this paper, we propose and demonstrate a polarization controller, which is a concatenation of three Faraday rotators based on magneto-optic crystals separated by two fixed quarter wave plates. Comparing with former schemes, this polarization controller is fast, accurate and stable because it is completely driven by electric signals and has no mechanically moving parts. It is simple-structured and low-cost. Moreover, it is programmable to convert an arbitrary state of polarization (SOP) from the input to any designated SOP at the output. It can also be used as a polarization scrambler. It has potential applications in the research of the polarization dependence of optical communication systems or the compensation of polarization mode dispersion.

2. Operation principle

We propose a complete polarization controller (PC) based on three magneto-optic crystals separated by two fixed QWPs. The configuration of the PC is show in Fig. 1. In this configuration, the x axis of the Jones space is chosen as the horizontal axis in the experimental setup. The slow axes of the QWPs are fixed and made parallel to the x-axis of the Jones space, i.e., horizontal in the laboratory. Three Faraday rotators using magneto-optic crystals are concatenated via two fixed QWPs. The rotation angles of the Faraday rotators are controlled by a single chip or a computer within the ranges of ±45°, ±90±, and ±45° in Jones space (i.e.±90°, ±180°, and ±90° in Stokes space), respectively. This novel polarization controller has working wavelength at 1550nm, response time of 150μs, and total power consumption of less than 1.2 watt and insert loss of about 3dB. It is simple-structured and operates accurately and stably because it is completely driven by electric signals and has no mechanically moving parts. Moreover, it is programmable to convert an arbitrary SOP from the input to any designated SOP at the output. The polarization controller can be applied in the research of the polarization dependence of optical communication systems or the compensation of polarization mode dispersion, etc.

Fig. 1. Configuration of the polarization controller. FR: Faraday Rotator; QWP: quarter wave plate.

Throughout this work, the input light is assumed to be completely polarized and the normalized Stokes vector [S1, S2, and S3] is used to describe the instantaneous Stokes parameters (dimensionless) of the light. As a result, the absolute value of Si (i=1, 2, 3) is less than one, and the input and output SOPs correspond to points on the surface of the Poincare sphere. The principle of operation of the polarization controller is shown in Fig. 2. The first Faraday rotator rotates an arbitrary input SOP (A) around the S3 axis of the Poincare sphere (i.e. along the latitude line) by an appropriate angle to point B where S2B = 0 (S2B is the second coordinate of the SOP of point B), so that the following QWP will bring it to the equator. The following QWP rotates the output SOP of the first Faraday rotator (B) around the S1 axis by 90° to make the output linearly polarized (C on the equator). The second Faraday rotator rotates the linearly polarized light (C) around the S3 axis (i.e. along the equator) by another appropriate angle to point D such that the following QWP will bring it to the same latitude as the final SOP. The following QWP rotates the output SOP of the second Faraday rotator around the S1 axis by 90° to point E where the elliptically polarized light (including any circularly polarized light) has the final designated ellipticity. Finally, the last Faraday rotator rotates the SOP around the S3 axis (i.e. along the latitude line) by a third appropriate angle to the final designated output SOP (F).

The effect of a Faraday Rotator can be expressed as a Jones matrix J or a rotation matrix S in Stokes space as follows,

J=[cosφsinφsinφcosφ]
S=[cos2φsin2φ0sin2φcos2φ0001]

When the input SOP is [a, b, c] and the output SOP is designated to be [x, y, z], the computer controlled rotation angles of the Faraday rotators can be calculated as

φ1=arctan(ba)2
(1)
φ2=2πfix(Δθπ)fix(Δθπ)Δθ2
(2)
φ3=arctan(yx)2
(3)

where φi (i = 1, 2, 3) is the rotation angle of the ith Faraday rotator. The fix function returns the integer portion of a number. For example, fix (2.98) =2; fix (-3.4) =-3.

Δθ=(1sgn(a)2)π+arctancsgn(a)a2+b2(1sgn(x)2)π+arctanzsgn(x)x2+y2
(4)

with sgn(x) = -1 if x < 0; sgn(x) = 0 if x = 0; sgn(x) = 1 if x > 0.

Fig. 2. An example of the polarization controlling scheme shown on the Poincare sphere. A: input SOP, B: output SOP after the first Faraday rotator, φ1=-26.5651°, C: output SOP after the first QWP, D: output SOP after the second Faraday rotator, φ2=87.1304°, E: output SOP after the second QWP, F: final output SOP, φ3=45.0000°. (a) Three-dimensional view. (b) Projection on the S1=0 plane. (c) Projection on the S2=0 plane. (d) Projection on the S3=0 plane.

In the calculation above, φ 1 and φ 3 are deduced to make S2B=S2E=0, where S2X is the second coordinate of the states of polarization of point X [see Fig. 2(d)]. This ensures point C and point D are on the equator of the Poincare sphere, as the rotations from B to C and from D to E are 90° around S1 axis by the effect of QWPs. Δθ is the algebraic angle difference between point C and point D (the range of Δθ is within -360° ≤ Δθ ≥ +360°), which is then modified to φ 2 according to the position relationship between point C and point D on the Poincare sphere [see Fig. 2(d)], -90°φ 2 ≤ +90°) to decide the magnitude and direction of the rotation from point C to point D.

The slow axes of the QWPs can also be aligned at an angle of 45° or other angles with respect to the x axis of the Jones space. In these cases, the controlling algorithm should be modified accordingly. The example shown in Fig. 2 has the following parameters: the input SOP (A) is [0.6000, 0.3000, -0.7416] and the designated output SOP (F) is [-0.5000, -0.5000, 0.7071]. The first Faraday rotator rotates the input SOP by -26.5651° to [0.6708, 0, -0.7416] (B). The following QWP rotates (B) by 90° to [0.6708, 0.7416, 0] (C). The second Faraday rotator rotates the linearly polarized light by 87.1304° to [-0.7071, 0.7071, 0] (D). The following QWP rotates (D) by 90° to [-0.7071, 0, 0.7071] (E). Finally, the last Faraday rotator rotates the SOP by 45.0000° to (F).

In practice, if the input SOP is fluctuating, a feed-forward control algorithm can be introduced to optimize the rotation angles and stabilize the output SOP by monitoring the realtime input SOP and calculate the controlling voltage accordingly. The responding time will be mainly determined by the speed of SOP monitoring since the calculation and respond of the polarization controller is relatively negligible.

Fig. 3. Output SOPs of 5000 independent samples when the Faraday rotators are randomly controlled. (Input SOP : [0.3000, 0.8660, 0.4000])

This novel polarization controller can also be used as a polarization scrambler since it can generate an output light with any SOP. When the input SOP is fixed and the three rotation angles of the Faraday rotators are controlled by a computer to vary randomly in their respective controlling range, random output SOPs will be generated to cover the surface of the entire Poincare sphere. Figure 3 demonstrates the output SOPs of 5000 independent samples that randomly cover the surface of the entire Poincare sphere, while the input SOP is fixed to be [0.3000, 0.8660, 0.4000].

Moreover, magnetic garnet can be introduced to other applications. A compact optical roll angle sensor with large measurement range and high sensitivity has also been reported, which is based on magnetic garnet single crystal [13

13. S. Li, C. Yang, E. Zhang, and G. Jin, “Compact optical roll-angle sensor with large measurement range and high sensitivity,” Opt. Lett. 30, 242–244 (2005). [CrossRef] [PubMed]

].

Fig. 4. Experimental results shown on Poincare sphere. (a) Output SOPs when all the rotation angles are randomized; input SOP is [0.2673, -0.6118, 0.7424]. (b) Output SOPs when only the 1st Faraday rotator is controlled while the other two are fixed. (c) Output SOPs when only the 2nd Faraday rotator is controlled while the other two are fixed. (d) Output SOPs when only the 3rd Faraday rotator is controlled while the other two are fixed.

We have fabricated the Faraday rotators in the polarization controller by Bi-substituted rare-earth iron garnet [14

14. S. Li, C. Yang, E. Zhang, and G. Jin, Dynamic performance of magneto-optical Bi-substitutued rare-earth iron garnet, Chin. Opt. Lett. 3, 38–41 (2005)

, 15

15. W. Zhao, “Magneto-optic properties and sensing performance of garnet YbBi:YIG,” Sens. and Actuators, A. 89, 250–254 (2001). [CrossRef]

] since it has large magneto-optic effect and small insertion loss. The dimension of the device is 43.6×22.1×12(mm). The total optical path of the device is around 40 mm. The beam diameter of the fiber collimators is 0.5 mm. Ideally, the beam coming out of the fiber needs to be collimated before entering the polarization controller. Practically, less than ideal collimation can be tolerated. In our experiments, the polarization states can be very well controlled by using the present collimators with a beam divergence of 0.25 degrees. The parameters of the Faraday rotators are shown in table.1. Figure 4(a) shows the output SOPs when all the rotation angles are randomized in their respective controlling ranges and the input SOP is [0.2673, -0.6118, 0.7424]. The surface of the entire Poincare sphere is covered, which demonstrates that any designated output SOP can be generated. In this case, the device can be used as a polarization scrambler. Figures 4(b)~4(d) show the traces of the output SOPs when only the 1st, 2nd or 3rd Faraday rotator is controlled to rotate the SOP in its controlling range and all the other parts are fixed. The traces are semicircle, circle, and semicircle around S3, respectively. From these figures it is demonstrated that the controlling ranges of the Faraday rotators are ±45°, ±90°, and ±45°, respectively. This polarization controller has working wavelength at 1550nm, response time of 150μs, and total power consumption of less than 1.2 watt. However, it’s total insertion loss of about 3dB and fluctuating between 1~5dB, which is caused by the characteristic of the magneto-optic material and the polarization dependent loss (PDL) of 0.2~1.4dB. However, this insertion loss and PDL is only experienced when the magnetic field is off or changing, in other words, when the polarization controller is in the off state or the magnetic field is being switched. Large insertion loss fluctuations are caused by the random magnetic domains in the Faraday rotators during the switching. The random magnetic domains in the Faraday rotators scatter the incident light beam, leading to the loss. The insertion loss fluctuation can be largely suppressed by introducing a rotating magnetic field on the Faraday rotators [9

9. T. Saitoh and S. Kinugawa, “Magnetic field rotating-type Faraday polarization controller,” Photon. Technol. Lett. , 15, 1404–1406 (2003). [CrossRef]

].

Table. 1. Parameters of the fabricated Faraday rotator

table-icon
View This Table

3. Conclusion

In conclusion, a novel polarization controller, which is a simple-structured concatenation of three Faraday rotators based on magneto-optic crystals via two fixed quarter wave plates, is proposed and demonstrated. It has working wavelength at 1550nm, response time of 150 μs, total power consumption of less than 1.2 watt and insert loss of about 3dB. Compared with former schemes, this polarization controller is fast, accurate and stable because it is completely driven by electric signals and has no mechanically moving parts. It can convert an arbitrary SOP from the input to any designated SOP at the output, controlled by programmable single-chip or computer. It can also be used as a polarization scrambler. It has potential applications in the research of the polarization dependence of optical communication systems or the compensation of polarization mode dispersion, etc. This polarization controller can also be potentially fabricated in fiber or waveguide structures.

Acknowledgments

This work is supported, in part, by the Trans-Century Training Programme Foundation for the Talents by the Ministry of Education of China. Claire Gu would like to acknowledge the National Science Foundation (ECS-0401206) for partial support.

References and links

1.

N. G. Walker and G. R. Walker, “Polarization control for coherent communications,” J. Lightwave Technol. 8, 438–458 (1990). [CrossRef]

2.

A. E. Willner, S. M. R. M. Nezam, L. Yan, Z. Pan, and M. C. Hauer, “Monitoring and control of polarization-related impairments in optical fiber systems,” J. Lightwave Technol. 22, 106–125 (2004). [CrossRef]

3.

R. Noé, D. Sandel, and V. Mirvoda, “PMD in high-bit-rate transmission and means for its mitigation,” J. Sel. Top. Quantum 10, 341–355 (2004). [CrossRef]

4.

W. H. J. Aarts and G. D. Khoe, “New endless polarization control method using three fiber squeezers,” J. Lightwave Technol. 7, 033–1043 (1989). [CrossRef]

5.

J. Prat, J. Comellas, and G. Junyent, “Experimental demonstration of an all-fiber endless polarization controller based on Faraday rotation,” Photon. Technol. Lett. 7, 1430–1432 (1995). [CrossRef]

6.

T. Chiba, Y. Ohtera, and S. Kawakami, “Polarization stabilizer using liquid crystal rotatable waveplates,” J. Lightwave Technol. 17, 885–890 (1999). [CrossRef]

7.

K. Hirabayashi and C. Amano, “Feed-forward continuous and complete polarization control with a PLZT rotatable-variable waveplate and inline polarimeter,” J. Lightwave Technol. 21, 1920–1932 (2003). [CrossRef]

8.

H. Shimizu and K. Kaede, “Endless polarisation controller using electro-optic waveplates,” Electron. Lett. 24, 412–413, (1988). [CrossRef]

9.

T. Saitoh and S. Kinugawa, “Magnetic field rotating-type Faraday polarization controller,” Photon. Technol. Lett. , 15, 1404–1406 (2003). [CrossRef]

10.

K. Ikeda, T. Takagi, T. Hatano, H. Kazami, Y. Mimura, and H. Matsuura, “Endless tracking polarization controller,” Furukawa Review 23, 32 (2003).

11.

D. Goldring, Z. Zalevsky, G. Shabtay, D. Abraham, and D. Mendlovic, “Magneto-optic-based devices for polarization control,” J. Opt. A: Pure Appl. Opt. 6, 98 (2004); [CrossRef]

12.

X. Yao, L. Yan, and Y. Shi, “Highly repeatable all-solid-state polarization-state generator,” Opt. Lett. 30, 1324 (2005). [CrossRef] [PubMed]

13.

S. Li, C. Yang, E. Zhang, and G. Jin, “Compact optical roll-angle sensor with large measurement range and high sensitivity,” Opt. Lett. 30, 242–244 (2005). [CrossRef] [PubMed]

14.

S. Li, C. Yang, E. Zhang, and G. Jin, Dynamic performance of magneto-optical Bi-substitutued rare-earth iron garnet, Chin. Opt. Lett. 3, 38–41 (2005)

15.

W. Zhao, “Magneto-optic properties and sensing performance of garnet YbBi:YIG,” Sens. and Actuators, A. 89, 250–254 (2001). [CrossRef]

OCIS Codes
(060.4510) Fiber optics and optical communications : Optical communications
(260.5430) Physical optics : Polarization

ToC Category:
Optical Devices

History
Original Manuscript: February 14, 2006
Revised Manuscript: March 27, 2006
Manuscript Accepted: April 3, 2006
Published: April 17, 2006

Citation
Yang Zhang, Changxi Yang, Shiguang Li, He Yan, Jingchan Yin, Claire Gu, and Guofan Jin, "Complete polarization controller based on magneto-optic crystals and fixed quarter wave plates," Opt. Express 14, 3484-3490 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-8-3484


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References

  1. N. G. Walker and G. R. Walker, "Polarization control for coherent communications," J. Lightwave Technol. 8, 438-458 (1990). [CrossRef]
  2. A. E. Willner, S. M. R. M. Nezam, L. Yan, Z. Pan, and M. C. Hauer, "Monitoring and control of polarization-related impairments in optical fiber systems," J. Lightwave Technol. 22, 106-125 (2004). [CrossRef]
  3. R. Noé, D. Sandel, and V. Mirvoda, "PMD in high-bit-rate transmission and means for its mitigation," J. Sel. Top. Quantum 10, 341-355 (2004). [CrossRef]
  4. W. H. J. Aarts and G. D. Khoe, "New endless polarization control method using three fiber squeezers," J. Lightwave Technol. 7, 033-1043 (1989). [CrossRef]
  5. J. Prat, J. Comellas, and G. Junyent, "Experimental demonstration of an all-fiber endless polarization controller based on Faraday rotation," Photon. Technol. Lett. 7, 1430-1432 (1995). [CrossRef]
  6. T. Chiba, Y. Ohtera, and S. Kawakami, "Polarization stabilizer using liquid crystal rotatable waveplates," J. Lightwave Technol. 17, 885-890 (1999). [CrossRef]
  7. K. Hirabayashi and C. Amano, "Feed-forward continuous and complete polarization control with a PLZT rotatable-variable waveplate and inline polarimeter," J. Lightwave Technol. 21, 1920-1932 (2003). [CrossRef]
  8. H. Shimizu and K. Kaede, "Endless polarisation controller using electro-optic waveplates," Electron. Lett. 24, 412-413, (1988). [CrossRef]
  9. T. Saitoh and S. Kinugawa, "Magnetic field rotating-type Faraday polarization controller," Photon. Technol. Lett.,  15, 1404-1406 (2003). [CrossRef]
  10. K. Ikeda, T. Takagi, T. Hatano, H. Kazami, Y. Mimura, and H. Matsuura, "Endless tracking polarization controller," Furukawa Review 23,32 (2003).
  11. D. Goldring, Z. Zalevsky, G. Shabtay, D. Abraham, and D. Mendlovic, "Magneto-optic-based devices for polarization control," J. Opt. A: Pure Appl. Opt. 6,98 (2004); [CrossRef]
  12. X. Yao, L. Yan, and Y. Shi, "Highly repeatable all-solid-state polarization-state generator," Opt. Lett. 30,1324 (2005). [CrossRef] [PubMed]
  13. S. Li, C. Yang, E. Zhang, and G. Jin, "Compact optical roll-angle sensor with large measurement range and high sensitivity," Opt. Lett. 30, 242-244 (2005). [CrossRef] [PubMed]
  14. S. Li, C. Yang, E. Zhang, and G. Jin, Dynamic performance of magneto-optical Bi-substitutued rare-earth iron garnet, Chin. Opt. Lett. 3, 38-41 (2005)
  15. W. Zhao, "Magneto-optic properties and sensing performance of garnet YbBi:YIG," Sens. and Actuators, A. 89, 250-254 (2001). [CrossRef]

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