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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 11, Iss. 25 — Dec. 15, 2003
  • pp: 3359–3364
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Polarization-independent all-fiber multiwavelength-switchable filter based on a polarization-diversity loop configuration

Yong Wook Lee, Kyung Jun Han, Byoungho Lee, and Jaehoon Jung  »View Author Affiliations


Optics Express, Vol. 11, Issue 25, pp. 3359-3364 (2003)
http://dx.doi.org/10.1364/OE.11.003359


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Abstract

In this paper a polarization-independent all-fiber multiwavelength-switchable filter based on a polarization-diversity loop configuration is newly proposed. The proposed apparatus consists of a polarization beam splitter, high birefringence fibers, and polarization controllers. Our theoretical analysis shows that the apparatus exhibits unique feature which allows it to operate as a polarization-independent multiwavelength periodic filter with a good channel isolation and to make its channel wavelength switchable by varying effective birefringence of the polarization-diversity loop through the proper adjustment of the polarization controllers contained within the loop. Theoretical prediction was experimentally verified.

© 2003 Optical Society of America

1. Introduction

Rapid increase in the capacity of fiber telecommunication systems by orders of magnitude creates strong demands of novel devices such as multiwavelength light sources or all-fiber wavelength-selective filters. Especially, all-fiber filters are attractive components in all-optical network because they have the advantages of low loss, small size, and better performance compared with conventional optical filters based on bulk dispersive components. So far, a great many techniques for implementing all-fiber filters have been proposed, such as the fused fiber filters [1

1. I. J. Wilkinson, “Birefringence control in close-spaced fused-fiber wavelength-division multiplexers: a comparison of three models,” Opt. Lett. 16, 1159–1161 (1991). [CrossRef] [PubMed]

,2

2. M. Eisenmann and E. Weidel, “Single-mode fused biconical couplers for wavelength division multiplexing with channel spacing between 100 and 300 nm,” J. Lightwave Technol. 6, 113–119 (1988). [CrossRef]

], the Fabry-Perot filter [3

3. J. Stone, L. W. Stulz, and A. A. M. Saleh, “Three-mirror fibre Fabry-Perot filters of optimal design,” Electron. Lett. 26, 1073–1074 (1990). [CrossRef]

], the Mach-Zehnder filter [4

4. G. P. Agrawal, Fiber-Optic Communication Systems, 2nd ed. (Wiley, New York, 2002). [CrossRef]

], the fiber grating filter [5

5. D. C. Johnson, F. Bilodeau, B. Malo, K. O. Hill, P. G. J. Wigley, and G. I. Stegeman, “Long-length, long-period rocking filters fabricated from conventional monomode telecommunications optical fiber,” Opt. Lett. 17, 1635–1637 (1992). [CrossRef] [PubMed]

], the acoustooptic filter [6

6. D. A. Smith, J. E. Baran, J. J. Johnson, and K. Cheung, “Integrated-optic acoustically-tunable filters for WDM networks,” IEEE J. Sel. Areas Commun. 8, 1151–1159 (1990). [CrossRef]

], and the birefringent filters based on a Sagnac interferometer [7

7. X. Fang and R. O. Claus, “Polarization-independent all-fiber wavelength-division multiplexer based on a Sagnac interferometer,” Opt. Lett. 20, 2146–2148 (1995). [CrossRef] [PubMed]

,8

8. Y. W. Lee, B. Lee, and J. Jung, “Multiwavelength-switchable SOA-fiber ring laser based on polarization-maintaining fiber loop mirror and polarization beam splitter,” ECOC and IOOC, 568–569, Italy (2003).

]. Generally, the polarization state in conventional fibers is random in communication systems; hence polarization-independent fiber filters are preferred. In this paper a new polarization-independent all-fiber multiwavelength-switchable filter is proposed which is based on a polarization-diversity loop configuration (PDLC). To our best knowledge it is the first time the PDLC is applied for achieving polarization independence in optical filtering, though the polarization-diversity loop by itself is not a new concept [9

9. T. Hasegawa, K. Inoue, and K. Oda, “Polarization independent frequency conversion by fiber four-wave mixing with a polarization-diversity technique,” IEEE Photon. Technol. Lett. 5, 947–949 (1993). [CrossRef]

,10

10. T. Morioka, K. Mori, and M. Saruwatari, “Ultrafast polarization-independent optical demultiplexer using optical carrier frequency shift through crossphase modulation,” Electron. Lett. 28, 1070–1072 (1992). [CrossRef]

]. Compared to the Mach-Zehnder interferometer-based filters, the PDLC-based all-fiber filter is more robust to environmental changes because the two lights rotating the high birefringence fiber (HBF) loop travel along a common light path. The proposed apparatus consists of a polarization beam splitter (PBS), HBF’s, and polarization controllers (PC’s). Our theoretical analysis shows that the apparatus exhibits unique feature which allows it to operate as a polarization-independent multiwavelength periodic filter with a good channel isolation and to make its channel location switchable by varying effective birefringence of the polarization-diversity loop including the HBF through the proper adjustment of the PC’s contained within the loop. Theoretical prediction was successfully demonstrated by experiments.

2. Principle of operation

Figure 1(a) shows the basic structure of the PDLC-based all-fiber filter. The basic components comprising the filter shown in Fig. 1(a) are a PBS and HBF whose fast axis is rotated 45 ° relative to horizontal axis of the PBS. To facilitate discussion, let us assume that the horizontal and vertical axes of the PBS are designated as the x- and y-axes, respectively. First, let us also assume that the light introduced into the port 1 of the filter is x-polarized light (x polarization). Then, as shown in clockwise (CW) path of Fig. 1(b), the input light propagates through the polarizer (x-polarized), the HBF with its fast axis 45 ° oriented with respect to x-axis, and the analyzer (x-polarized) sequentially, rotating in a CW direction. During the passage through the HBF, the x-polarized light is decomposed into two polarization components corresponding to those aligned to the fast and slow axis of the HBF, respectively, and the phase difference Γ (=2πBL/λ) between them is generated due to birefringence of the HBF. Here B is the birefringence, L is the HBF length and λ is the wavelength in vacuum. When these two components come out of the port 2, therefore, they can interfere because we select the same polarization (x polarization) components in the two lights. Similarly, when the input light is y-polarized, the light travels the filter in a counterclockwise (CCW) direction as shown in CCW path of Fig. 1(b) but the same interference spectrum is obtained as the above one. Especially, as an arbitrarily polarized light can always be decomposed into x- and y-polarized components, the transmitted intensity becomes the superposition of intensity spectra of two interference patterns due to x and y input polarizations and thus the transmitted output of the filter becomes independent of the input polarization.

This physical discussion can be supported with the help of the following mathematical formulation. According to Jones matrix formulation [11

11. R. C. Jones, “New calculus for the treatment of optical systems,” J. Opt. Soc. Am. 31, 488–492 (1941). [CrossRef]

] the Jones matrices of the proposed apparatus along the CW and CCW directions are

TCW=[1000]R(π4)[eiΓ200eiΓ2]R(π4)[1000]=[cos(Γ2)000],
TCCW=[0001]R(π4)[eiΓ200eiΓ2]R(π4)[0001]=[000cos(Γ2)]
(1)

where R(θ) is coordinate transformation matrix. If we assume an arbitrary input field Ein as Eq. (2), output field Eout and output intensity Iout of the filter are expressed as follows.

Ein=[abejϕ]
(2)
Eout=TCW·Ein+TCCW·Ein=cos(Γ2)[abejϕ],Iout=12ε0μ0(a2+b2)[1+cos(Γ)]
(3)

where a, b, and ϕ correspond to magnitudes of two field components and phase difference, (which are arbitrary real values), respectively, and ε 0and µ 0 are permittivity and magnetic permeability, respectively. From Eq. (3), it is clear that the output intensity of the light is a wavelength-dependent sinusoidal function and is independent of the input polarization whenever the input intensity is constant.

Fig. 1. (a) Basic structure of the PDLC-based all-fiber filter and (b) schematic of the propagating light path.
Fig. 2. Schematic diagram of the proposed filter for channel wavelength-switching.

In this basic structure, wavelength location of the transmission spectrum can be shifted by varying the phase difference due to the birefringence of the entire polarization-diversity loop, which is composed of the HBF and ordinary fibers spliced to the HBF (except a lead-in and lead-out fiber), in the filter. For example, a change of π rad in the phase difference due to birefringence will move the transmission spectrum from maximum to minimum at a specific wavelength. If we insert a PC within the polarization-diversity loop, therefore, it is expected to be possible to move maxima/minima wavelengths (channel wavelengths) of the transmission spectrum because the effective birefringence of the entire loop can be controlled by adjusting the PC. In order to achieve channel wavelength-switching in the PDLC-based filter, we now consider the case in which PC’s are added to the basic structure of the filter as shown in Fig. 2. Two quarter-wave plates (QWP’s; QWP 1 & QWP 2) and one half-wave plate (HWP) in Fig. 2 are placed for controlling the effective loop birefringence and the orientation of the HBF with respect to the x-axis, respectively. Based on the above Jones matrix formulation, the Jones matrix T (1st term: CW direction, 2nd term: CCW direction) and transmittance TR of the proposed filter are calculated as the following expressions;

T=[1000]TQWP2(θ2)TQWP1(θ1)R(θp)[eiΓ200eiΓ2]R(θp)THWP(θh)[1000]
+[0001]THWP(θh)R(θp)[eiΓ200eiΓ2]R(θp)TQWP1(θ1)TQWP2(θ2)[0001]
(4)
TR=12+12{sin[2(θpθ2θ1)]cos2(θ1θ2)sin[2(θp+θ2θ1)]sin2(θ1θ2)}cos(Γ)
12{sin[2(θ1θ2)]cos(2θ2)}sin(Γ)
(5)

where TQWP 1, TQWP 2, and THWP are Jones matrices of the QWP 1, QWP 2, and HWP, respectively. And θ 1 and θ 2 are fast-axis orientation (azimuthal) angles of the QWP 1 and 2 with respect to x-axis, respectively, and θp is that of the HBF. The fast axis orientation of the HWP (θh) is set to (θp - 45 °)/2 for arbitrary θp to be effectively 45 ° with respect to the x-axis by rotating the polarization plane of the propagating light. In the calculation, ideal PBS and PC’s were assumed and any insertion loss due to optical components that construct the filter was not considered.

Table 1. Four optimal QWP combinations and corresponding intensities

table-icon
View This Table
Fig. 3. Calculated transmission spectrum of the proposed filter for channel wavelength-switching.

3. Experimental results and discussions

To verify the theoretical results, we constructed a PDLC-based all-fiber filter as shown in Fig. 2 and measured the transmission spectrum at four optimal QWP combinations by adjusting each QWP. The proposed filter is composed of a PBS (OZ optics), HBF, one HWP (OZ optics), and two QWP’s (OZ optics). The birefringence and length of the HBF is ~4.8×10-4 and 6.25 m, respectively. The length of the HBF was determined so that wavelength spacing (channel spacing) between transmission maxima becomes 0.8 nm. Figure 4 shows the measured transmission spectrum of the proposed filter at four optimal sets for channel wavelength-switching. As predicted in the theoretical results, the switching displacement between channels, whose spacing at any QWP combination was measured to be ~0.8 nm, could be chosen as one of three displacements (0.2, 0.4, and 0.6 nm) by selecting proper sets of QWP’s. The channel isolation of the implemented filter at all optimal settings was measured to be larger than 20 dB. Insertion loss of the filter was measured to be ~4.2 dB which mainly comes from that of two QWP’s (~1.3 dB) and the PBS (~0.93 dB for one way, totally ~1.86 dB) including fiber fusion splicing loss between the HBF and ordinary fiber. The insertion loss can be diminished by using low-loss PC’s and PBS and by improving the fusion splicing of fiber splices between the HBF and ordinary fiber. Spectral flatness of the transmission in each optimal set was measured to be less than 0.16 dB and spectral flatness variation among four optimal settings was measured to be less than 0.37 dB. Especially, in order to examine the input polarization independence of the proposed filter, the polarization sensitivity of the transmission spectrum was measured by placing an additional PC (Agilent 8169 A) in front of the lead-in fiber (port 1 of the filter). During the measurement, we rotated both one QWP and one HWP, which are contained (with one input polarizer) within the additional PC, in a random way each time, ensuring that the signal polarization had been varied over the entire Poincare sphere. The maximum polarization sensitivity we observed was less than ~0.5 dB, which could be affected by polarization sensitivity of the photodetector and also imperfection of the PBS used in the experiments. This result deviates by less than ~0.5 dB from the theoretical expectation obtained from Eq. (5). In addition, the absolute channel wavelength control of the periodic transmission band can be done by controlling the voltage of the PZT drum wound by the HBF [12

12. Y. Shiquan, L. Zhaohui, D. Xiaoyi, Y. Shuzhong, K. Guiyun, and Z. Qida, “Generation of wavelength-switched optical pulse from a fiber ring laser with an F-P semiconductor modulator and a HiBi fiber loop mirror,” IEEE Photon. Technol. Lett. 14, 774–776 (2002). [CrossRef]

], or by controlling the tension/pressure applied to the HBF [13

13. S. Li, K. S. Chiang, and W. A. Gambling, “Generation of wavelength-tunable single-mode picosecond pulses from a self-seeded gain-switched Fabry-Perot laser diode with a high-birefringence fiber loop mirror,” Appl. Phys. Lett. 76, 3676–3678 (2000). [CrossRef]

].

Fig. 4. Measured transmission spectrum of the proposed filter for channel wavelength-switching.

4. Conclusion

In this paper a new polarization-independent all-fiber multiwavelength-switchable filter based on a PDLC was proposed. The proposed filter is composed of a PBS, HBF, one HWP, and two QWP’s. The spectral characteristics of the proposed filter were theoretically and experimentally studied. Typical values of measured channel isolation and insertion loss of the implemented filter were ~20 dB and ~4.2 dB, respectively. The maximum input polarization sensitivity we observed was less than ~0.5 dB, which could be affected by polarization sensitivity of the photodetector and also imperfection of the PBS used. Particularly, the switching displacement between channels could be chosen as one of three displacements (0.2, 0.4, and 0.6 nm) in the wavelength-switching operation by selecting proper sets of QWP’s. The developed PDLC-based filter can be utilized in many applications in WDM optical network systems as well as in various multiwavelength optical sources.

References and links

1.

I. J. Wilkinson, “Birefringence control in close-spaced fused-fiber wavelength-division multiplexers: a comparison of three models,” Opt. Lett. 16, 1159–1161 (1991). [CrossRef] [PubMed]

2.

M. Eisenmann and E. Weidel, “Single-mode fused biconical couplers for wavelength division multiplexing with channel spacing between 100 and 300 nm,” J. Lightwave Technol. 6, 113–119 (1988). [CrossRef]

3.

J. Stone, L. W. Stulz, and A. A. M. Saleh, “Three-mirror fibre Fabry-Perot filters of optimal design,” Electron. Lett. 26, 1073–1074 (1990). [CrossRef]

4.

G. P. Agrawal, Fiber-Optic Communication Systems, 2nd ed. (Wiley, New York, 2002). [CrossRef]

5.

D. C. Johnson, F. Bilodeau, B. Malo, K. O. Hill, P. G. J. Wigley, and G. I. Stegeman, “Long-length, long-period rocking filters fabricated from conventional monomode telecommunications optical fiber,” Opt. Lett. 17, 1635–1637 (1992). [CrossRef] [PubMed]

6.

D. A. Smith, J. E. Baran, J. J. Johnson, and K. Cheung, “Integrated-optic acoustically-tunable filters for WDM networks,” IEEE J. Sel. Areas Commun. 8, 1151–1159 (1990). [CrossRef]

7.

X. Fang and R. O. Claus, “Polarization-independent all-fiber wavelength-division multiplexer based on a Sagnac interferometer,” Opt. Lett. 20, 2146–2148 (1995). [CrossRef] [PubMed]

8.

Y. W. Lee, B. Lee, and J. Jung, “Multiwavelength-switchable SOA-fiber ring laser based on polarization-maintaining fiber loop mirror and polarization beam splitter,” ECOC and IOOC, 568–569, Italy (2003).

9.

T. Hasegawa, K. Inoue, and K. Oda, “Polarization independent frequency conversion by fiber four-wave mixing with a polarization-diversity technique,” IEEE Photon. Technol. Lett. 5, 947–949 (1993). [CrossRef]

10.

T. Morioka, K. Mori, and M. Saruwatari, “Ultrafast polarization-independent optical demultiplexer using optical carrier frequency shift through crossphase modulation,” Electron. Lett. 28, 1070–1072 (1992). [CrossRef]

11.

R. C. Jones, “New calculus for the treatment of optical systems,” J. Opt. Soc. Am. 31, 488–492 (1941). [CrossRef]

12.

Y. Shiquan, L. Zhaohui, D. Xiaoyi, Y. Shuzhong, K. Guiyun, and Z. Qida, “Generation of wavelength-switched optical pulse from a fiber ring laser with an F-P semiconductor modulator and a HiBi fiber loop mirror,” IEEE Photon. Technol. Lett. 14, 774–776 (2002). [CrossRef]

13.

S. Li, K. S. Chiang, and W. A. Gambling, “Generation of wavelength-tunable single-mode picosecond pulses from a self-seeded gain-switched Fabry-Perot laser diode with a high-birefringence fiber loop mirror,” Appl. Phys. Lett. 76, 3676–3678 (2000). [CrossRef]

OCIS Codes
(060.2330) Fiber optics and optical communications : Fiber optics communications
(260.1440) Physical optics : Birefringence
(260.5430) Physical optics : Polarization
(350.2460) Other areas of optics : Filters, interference

ToC Category:
Research Papers

History
Original Manuscript: November 12, 2003
Revised Manuscript: November 23, 2003
Published: December 15, 2003

Citation
Yong Lee, Kyung Han, Byoungho Lee, and Jaehoon Jung, "Polarization-independent all-fiber multiwavelength-switchable filter based on a polarization-diversity loop configuration," Opt. Express 11, 3359-3364 (2003)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-11-25-3359


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References

  1. I. J. Wilkinson, �??Birefringence control in close-spaced fused-fiber wavelength-division multiplexers: a comparison of three models,�?? Opt. Lett. 16, 1159-1161 (1991). [CrossRef] [PubMed]
  2. M. Eisenmann and E. Weidel, �??Single-mode fused biconical couplers for wavelength division multiplexing with channel spacing between 100 and 300 nm,�?? J. Lightwave Technol. 6, 113-119 (1988). [CrossRef]
  3. J. Stone, L. W. Stulz, and A. A. M. Saleh, �??Three-mirror fibre Fabry-Perot filters of optimal design,�?? Electron. Lett. 26, 1073-1074 (1990). [CrossRef]
  4. G. P. Agrawal, Fiber-Optic Communication Systems, 2nd ed. (Wiley, New York, 2002). [CrossRef]
  5. D. C. Johnson, F. Bilodeau, B. Malo, K. O. Hill, P. G. J. Wigley, and G. I. Stegeman, �??Long-length, longperiod rocking filters fabricated from conventional monomode telecommunications optical fiber,�?? Opt. Lett. 17, 1635-1637 (1992). [CrossRef] [PubMed]
  6. D. A. Smith, J. E. Baran, J. J. Johnson, and K. Cheung, �??Integrated-optic acoustically-tunable filters for WDM networks,�?? IEEE J. Sel. Areas Commun. 8, 1151-1159 (1990). [CrossRef]
  7. X. Fang and R. O. Claus, �??Polarization-independent all-fiber wavelength-division multiplexer based on a Sagnac interferometer,�?? Opt. Lett. 20, 2146-2148 (1995). [CrossRef] [PubMed]
  8. Y. W. Lee, B. Lee, and J. Jung, �??Multiwavelength-switchable SOA-fiber ring laser based on polarizationmaintaining fiber loop mirror and polarization beam splitter,�?? ECOC and IOOC, 568-569, Italy (2003).
  9. T. Hasegawa, K. Inoue, and K. Oda, �??Polarization independent frequency conversion by fiber four-wave mixing with a polarization-diversity technique,�?? IEEE Photon. Technol. Lett. 5, 947-949 (1993). [CrossRef]
  10. T. Morioka, K. Mori, and M. Saruwatari, �??Ultrafast polarization-independent optical demultiplexer using optical carrier frequency shift through crossphase modulation,�?? Electron. Lett. 28, 1070-1072 (1992). [CrossRef]
  11. R. C. Jones, �??New calculus for the treatment of optical systems,�?? J. Opt. Soc. Am. 31, 488-492 (1941). [CrossRef]
  12. Y. Shiquan, L. Zhaohui, D. Xiaoyi, Y. Shuzhong, K. Guiyun, and Z. Qida, �??Generation of wavelengthswitched optical pulse from a fiber ring laser with an F-P semiconductor modulator and a HiBi fiber loop mirror,�?? IEEE Photon. Technol. Lett. 14, 774-776 (2002). [CrossRef]
  13. S. Li, K. S. Chiang, and W. A. Gambling, �??Generation of wavelength-tunable single-mode picosecond pulses from a self-seeded gain-switched Fabry-Perot laser diode with a high-birefringence fiber loop mirror,�?? Appl. Phys. Lett. 76, 3676-3678 (2000). [CrossRef]

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