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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 6 — Mar. 15, 2010
  • pp: 6056–6063
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Wavelength switchable flat-top all-fiber comb filter based on a double-loop Mach-Zehnder interferometer

Ai-Ping Luo, Zhi-Chao Luo, Wen-Cheng Xu, and Hu Cui  »View Author Affiliations


Optics Express, Vol. 18, Issue 6, pp. 6056-6063 (2010)
http://dx.doi.org/10.1364/OE.18.006056


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Abstract

A wavelength switchable all-fiber comb filter with flat-top spectral response based on a double-loop Mach-Zehnder (M-Z) interferometer is proposed and demonstrated. The proposed flat-top filter consists of a rotatable polarizer and a double-loop M-Z interferometer composed of two fiber couplers with a polarization controller (PC) in the first loop. In the theoretical analysis, when the second coupler of the M-Z interferometer is a non-3dB one, with proper settings of the polarization state of the input light and the PC, the wavelength switchable comb filter with flat-top passband can be obtained. Theoretical prediction was verified by experimental demonstration. The measured 1 dB bandwidth was 0.51 nm with a channel spacing of 0.98 nm, indicating that the flat-top passband of 1 dB bandwidth extends to about 50% of the comb spacing.

© 2010 OSA

1. Introduction

Optical comb filters have attracted much attention as wavelength selective elements in dense wavelength-division-multiplexed (DWDM) optical fiber communication systems. As the increasing capacity of the communication system, wavelength switchable comb filters are regarded as important optical components for dynamic channel adding/dropping in optical communications. Several methods were used to realize the wavelength switchable comb filters, such as using polarization-diversity loop configuration [1

1. Y. W. Lee, K. J. Han, B. Lee, and J. Jung, “Polarization-independent all-fiber multiwavelength-switchable filter based on a polarization-diversity loop configuration,” Opt. Express 11(25), 3359–3364 (2003). [CrossRef] [PubMed]

], exploiting a semiconductor optical amplifier in Sagnac loop interferometer [2

2. K. L. Lee, M. P. Fok, S. M. Wan, and C. Shu, “Optically controlled Sagnac loop comb filter,” Opt. Express 12(25), 6335–6340 (2004). [CrossRef] [PubMed]

], incorporating a PZT in Hi-Bi Sagnac loop [3

3. S. Yang, Z. Li, X. Dong, S. Yuan, G. Kai, and Q. Zhao, “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(6), 774–776 (2002). [CrossRef]

], employing a Mach-Zehnder (M-Z) filter [4

4. A. P. Luo, Z. C. Luo, and W. C. Xu, “Tunable and switchable multiwavelength erbium-doped fiber ring laser based on a modified dual-pass Mach-Zehnder interferometer,” Opt. Lett. 34(14), 2135–2137 (2009). [CrossRef] [PubMed]

]. One limitation of the wavelength switchable comb filters reported above is that their transmission bands are not flat which have a sinusoid shape. A filter with flat-top passband bandwidth is preferred for signal fidelity and tolerance of signal wavelength drift which can relax the requirements on wavelength control in a DWDM system. Therefore, it is desirable to design a filter with flat-top spectral response. Up to date, many techniques have been proposed to implement flat-top filter operation. For example, by using silicon nitride-based double-ring resonator [5

5. J. F. Song, Q. Fang, S. H. Tao, M. B. Yu, G. Q. Lo, and D. L. Kwong, “Silicon Nitride-based compact double-ring resonator comb filter with flat-top response,” IEEE Photon. Technol. Lett. 20(24), 2156–2158 (2008). [CrossRef]

], Gires-Tournois etalons [6

6. C. H. Hsieh, R. Wang, Z. Wen, I. McMichael, P. Yeh, C. W. Lee, and W. H. Cheng, “Flat-top interleavers using two Gires-Tournois etalons as phase dispersive mirrors in a Michelson interferometer,” IEEE Photon. Technol. Lett. 15(2), 242–244 (2003). [CrossRef]

,7

7. L. Wei and J. W. Y. Lit, “Design optimization of flattop interleaver and its dispersion compensation,” Opt. Express 15(10), 6439–6457 (2007). [CrossRef] [PubMed]

], Sagnac loop with birefringent crystals [8

8. C. W. Lee, R. Wang, P. Yeh, and W. H. Cheng, “Sagnac interferometer based flat-top birefringent interleaver,” Opt. Express 14(11), 4636–4643 (2006). [CrossRef] [PubMed]

], planar lightwave circuit (PLC) [9

9. Q. Wu, P. L. Chu, H. P. Chan, and B. P. Pal, “Polymer-based compact comb filter with flat-top response,” IEEE Photon. Technol. Lett. 17(12), 2619–2621 (2005). [CrossRef]

], cascaded high birefringence fiber [10

10. Y. Lai, W. Zhang, and J. A. R. Williams, “All-fibre multichannel flattop filter based on coherent fibre delay line structure,” Electron. Lett. 38(10), 473–475 (2002). [CrossRef]

], and the fiber Bragg grating [11

11. S. Derevyanko, “Design of a flat-top fiber Bragg filter via quasi-random modulation of the refractive index,” Opt. Lett. 33(20), 2404–2406 (2008). [CrossRef] [PubMed]

], the filters with flat-top spectral passband were successfully demonstrated. Generally, the all-fiber comb filters have the advantages of low insertion loss, low cost, and good compatibility with the fiber communication systems. Furthermore, in some applications, the flat-top comb filter with dynamic wavelength switching operation should also be investigated. However, there are few reports on the all-fiber wavelength switchable flat-top comb filters. Lee et al reported an all-fiber wavelength switchable comb filter with flat-top passband based on a birefringence combination Sagnac loop [12

12. Y. W. Lee, H. T. Kim, J. Jung, and B. Lee, “Wavelength-switchable flat-top fiber comb filter based on a Solc type birefringence combination,” Opt. Express 13(3), 1039–1048 (2005). [CrossRef] [PubMed]

]. By changing the polarization state of the light in the loop, channel wavelength switching operation with flat-top passband was realized. Recently, we have demonstrated a tunable and switchable multiwavelength fiber laser based on a modified M-Z interferometer [4

4. A. P. Luo, Z. C. Luo, and W. C. Xu, “Tunable and switchable multiwavelength erbium-doped fiber ring laser based on a modified dual-pass Mach-Zehnder interferometer,” Opt. Lett. 34(14), 2135–2137 (2009). [CrossRef] [PubMed]

]. However, the comb spectra of the filter in Ref [4

4. A. P. Luo, Z. C. Luo, and W. C. Xu, “Tunable and switchable multiwavelength erbium-doped fiber ring laser based on a modified dual-pass Mach-Zehnder interferometer,” Opt. Lett. 34(14), 2135–2137 (2009). [CrossRef] [PubMed]

]. have a sinusoid shape and no flat-top spectral response was observed. In this paper, we propose and demonstrate a wavelength switchable flat-top all-fiber comb filter based on a double-loop M-Z interferometer. Instead of using the combination of a polarization-dependent isolator and a polarization controller to change the polarization states of the input light in Ref [4

4. A. P. Luo, Z. C. Luo, and W. C. Xu, “Tunable and switchable multiwavelength erbium-doped fiber ring laser based on a modified dual-pass Mach-Zehnder interferometer,” Opt. Lett. 34(14), 2135–2137 (2009). [CrossRef] [PubMed]

], we employed a rotatable polarizer to accurately and simply measure the rotation angle of the input light which also leads to lower insertion loss. The proposed filter consists of a rotatable polarizer and a double-loop M-Z interferometer composed of two fiber couplers with a PC in the first loop. Different coupling ratio combinations have been investigated. When the second coupler is a non-3dB one, by properly adjusting the orientation of the PC and the polarization state of the input light, the wavelength switchable comb filter with flat-top spectral response was successfully obtained. Theoretical prediction was verified by experimental results.

2. Experimental setup and theoretical analysis

Figure 1
Fig. 1 Schematic of the proposed flat-top comb filter.
shows the schematic of the proposed wavelength switchable flat-top comb filter, which consists of a rotatable polarizer and a double-loop M-Z interferometer composed of two fiber couplers with a PC in the first loop. The PC is used to generate the flat-top comb spectral response as well as control the flatness of the comb spectrum. The rotatable polarizer is employed to make the input light linearly polarized and adjust the polarization state. An amplified spontaneous emission (ASE) light source is used to measure the transmission characteristics of the proposed filter.

1) c1=0.5,c20.5. In this case, Eqs. (3) and (4) are simplified to:

T1=2c2(1c2)[(cos2θsin2θcosδ)cos2φ+sinθsin(2α+θ)sinδsin2φ]
(5)
T2=2(12c2)c2(1c2)sin2θcos(2α+θ)sinδsinφ
(6)

2) c10.5,c20.5. In this case, the proposed filter also serves as a flat-top comb filter. However, when the flat-top operation was obtained initially by rotating the PC in a proper position, the wavelength interleaving operation with flat-top spectral response could not be achieved by only adjusting the orientation of the rotatable polarizer. Figure 4
Fig. 4 Calculated wavelength tunable operation with flat-top passband using two non-3dB fiber couplers.
illustrates the output wavelength tunable comb flat-top spectra with the parameters of c1=0.7,c2=0.2, θ=0.663π, α=0.61π (black curve), α=1.11π (red dotted curve). As can be seen in Fig. 4, the transmission passband of the proposed filter can be tuned but it does not locate exactly at the interleaving position. Nevertheless, once the flat-top operation was obtained (θ=0.663π and α=0.61π), the wavelength switching (interleaving) operation was still able to be realized by adjusting the PC with an angle of θ+π while other parameters were fixed, as shown in Fig. 4 with blue curve. It is worthy to note that once the interleaving operation with blue curve was achieved, one can rotate the polarizer with an angle of α+π/2 to further tune the wavelength flat top spectrum, as shown in Fig. 4 with dotted green curve. Therefore, it indicates that the proposed filter can be discretely tuned to the wavelength positions in which cover one period of the channel spacing.

3. Experimental results and discussion

To verify the theoretical prediction, we constructed the proposed flat-top comb filter as shown in Fig. 1 and measured the transmission spectra at different orientations of the polarizer and the PC. First, we concentrated on the case of c1=0.5,c20.5. The coupling ratios of two fiber couplers used in the experiment are 50:50 and 20:80. L is about 60 cm, and the path difference between two arms ΔL=1.63mm indicates that the comb spacing of 0.98 nm can be obtained. When the PC and the polarizer were appropriately adjusted, the wavelength switching operation with flat-top spectral response was obtained, as shown in Fig. 5
Fig. 5 Experimentally measured wavelength switchable flat-top operation by using two couplers with coupling ratios 50:50 and 20:80.
. The insertion loss was measured to be about 3.5 dB in the experiment. As predicted in the theoretical calculation, once the flat-top comb spectrum was obtained, the switching operation of the proposed flat-top filter could be implemented by rotating the polarizer or the PC (θ) with an additional angle of π/2 or π, respectively. The experimentally measured 1 dB bandwidth of 0.51 nm and 3 dB bandwidth of 0.66 nm with a free spectral range of 0.98 nm were obtained.

Then we replaced the first 3 dB coupler by a non-3 dB one, for this experiment, a 30:70 fiber coupler was used. Here, ΔL=1.52mm. The results were shown in Fig. 6
Fig. 6 Experimentally measured wavelength tunable flat-top operation of the proposed filter with a 30:70 and a 20:80 coupler.
. With a proper adjustment of the PC, we could easily obtain the flat-top comb spectrum (black curve). Note that the peak-to-notch contrast ratio in the experimental observation was ~15 dB, which was not as high as the theoretical calculation. It was mainly due to the limited precision of manually controlled PC and the insertion loss between the fiber components. When we further rotated the polarizer with an additional angle of π/2, the wavelength tunable operation (dotted red curve) but not interleaving operation with a ~0.23 nm wavelength shift of about 22% comb spacing was obtained. Nevertheless, as discussed in the theoretical analysis, the wavelength switching (interleaving) operation (black and blue curve) still could be achieved by rotating the PC when the other parameters were fixed in the experiment. As the interleaving operation (blue curve) was achieved, with the further adjustment of the polarizer, we can tune the flat-top spectrum with a wavelength of 22% comb spacing again, as shown in Fig. 6 with dotted green curve. The experimental results are well consistent with the theoretical predictions.

4. Conclusion

In conclusion, we have theoretically and experimentally demonstrated a wavelength switchable flat-top comb filter based on a double-loop M-Z interferometer which consists of two fiber couplers with a PC in the first loop. With proper settings of the polarization state of the input light and the PC, the wavelength switching comb filter with flat-top spectral response can be obtained by using a non-3dB coupler as the second coupler of the M-Z interferometer. In the experiment, the measured 1 dB bandwidth of 0.51 nm with a free spectral range of 0.98 nm was obtained, indicating that the flat-top passband of 1 dB bandwidth extends to about 50% of the channel spacing. Moreover, the proposed filter provides the advantages of simple implement and all fiber design.

References and links

1.

Y. W. Lee, K. J. Han, B. Lee, and J. Jung, “Polarization-independent all-fiber multiwavelength-switchable filter based on a polarization-diversity loop configuration,” Opt. Express 11(25), 3359–3364 (2003). [CrossRef] [PubMed]

2.

K. L. Lee, M. P. Fok, S. M. Wan, and C. Shu, “Optically controlled Sagnac loop comb filter,” Opt. Express 12(25), 6335–6340 (2004). [CrossRef] [PubMed]

3.

S. Yang, Z. Li, X. Dong, S. Yuan, G. Kai, and Q. Zhao, “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(6), 774–776 (2002). [CrossRef]

4.

A. P. Luo, Z. C. Luo, and W. C. Xu, “Tunable and switchable multiwavelength erbium-doped fiber ring laser based on a modified dual-pass Mach-Zehnder interferometer,” Opt. Lett. 34(14), 2135–2137 (2009). [CrossRef] [PubMed]

5.

J. F. Song, Q. Fang, S. H. Tao, M. B. Yu, G. Q. Lo, and D. L. Kwong, “Silicon Nitride-based compact double-ring resonator comb filter with flat-top response,” IEEE Photon. Technol. Lett. 20(24), 2156–2158 (2008). [CrossRef]

6.

C. H. Hsieh, R. Wang, Z. Wen, I. McMichael, P. Yeh, C. W. Lee, and W. H. Cheng, “Flat-top interleavers using two Gires-Tournois etalons as phase dispersive mirrors in a Michelson interferometer,” IEEE Photon. Technol. Lett. 15(2), 242–244 (2003). [CrossRef]

7.

L. Wei and J. W. Y. Lit, “Design optimization of flattop interleaver and its dispersion compensation,” Opt. Express 15(10), 6439–6457 (2007). [CrossRef] [PubMed]

8.

C. W. Lee, R. Wang, P. Yeh, and W. H. Cheng, “Sagnac interferometer based flat-top birefringent interleaver,” Opt. Express 14(11), 4636–4643 (2006). [CrossRef] [PubMed]

9.

Q. Wu, P. L. Chu, H. P. Chan, and B. P. Pal, “Polymer-based compact comb filter with flat-top response,” IEEE Photon. Technol. Lett. 17(12), 2619–2621 (2005). [CrossRef]

10.

Y. Lai, W. Zhang, and J. A. R. Williams, “All-fibre multichannel flattop filter based on coherent fibre delay line structure,” Electron. Lett. 38(10), 473–475 (2002). [CrossRef]

11.

S. Derevyanko, “Design of a flat-top fiber Bragg filter via quasi-random modulation of the refractive index,” Opt. Lett. 33(20), 2404–2406 (2008). [CrossRef] [PubMed]

12.

Y. W. Lee, H. T. Kim, J. Jung, and B. Lee, “Wavelength-switchable flat-top fiber comb filter based on a Solc type birefringence combination,” Opt. Express 13(3), 1039–1048 (2005). [CrossRef] [PubMed]

OCIS Codes
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.2340) Fiber optics and optical communications : Fiber optics components
(350.2460) Other areas of optics : Filters, interference

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: December 1, 2009
Revised Manuscript: February 10, 2010
Manuscript Accepted: March 4, 2010
Published: March 11, 2010

Citation
Ai-Ping Luo, Zhi-Chao Luo, Wen-Cheng Xu, and Hu Cui, "Wavelength switchable flat-top all-fiber comb filter based on a double-loop Mach-Zehnder interferometer," Opt. Express 18, 6056-6063 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-6-6056


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References

  1. Y. W. Lee, K. J. Han, B. Lee, and J. Jung, “Polarization-independent all-fiber multiwavelength-switchable filter based on a polarization-diversity loop configuration,” Opt. Express 11(25), 3359–3364 (2003). [CrossRef] [PubMed]
  2. K. L. Lee, M. P. Fok, S. M. Wan, and C. Shu, “Optically controlled Sagnac loop comb filter,” Opt. Express 12(25), 6335–6340 (2004). [CrossRef] [PubMed]
  3. S. Yang, Z. Li, X. Dong, S. Yuan, G. Kai, and Q. Zhao, “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(6), 774–776 (2002). [CrossRef]
  4. A. P. Luo, Z. C. Luo, and W. C. Xu, “Tunable and switchable multiwavelength erbium-doped fiber ring laser based on a modified dual-pass Mach-Zehnder interferometer,” Opt. Lett. 34(14), 2135–2137 (2009). [CrossRef] [PubMed]
  5. J. F. Song, Q. Fang, S. H. Tao, M. B. Yu, G. Q. Lo, and D. L. Kwong, “Silicon Nitride-based compact double-ring resonator comb filter with flat-top response,” IEEE Photon. Technol. Lett. 20(24), 2156–2158 (2008). [CrossRef]
  6. C. H. Hsieh, R. Wang, Z. Wen, I. McMichael, P. Yeh, C. W. Lee, and W. H. Cheng, “Flat-top interleavers using two Gires-Tournois etalons as phase dispersive mirrors in a Michelson interferometer,” IEEE Photon. Technol. Lett. 15(2), 242–244 (2003). [CrossRef]
  7. L. Wei and J. W. Y. Lit, “Design optimization of flattop interleaver and its dispersion compensation,” Opt. Express 15(10), 6439–6457 (2007). [CrossRef] [PubMed]
  8. C. W. Lee, R. Wang, P. Yeh, and W. H. Cheng, “Sagnac interferometer based flat-top birefringent interleaver,” Opt. Express 14(11), 4636–4643 (2006). [CrossRef] [PubMed]
  9. Q. Wu, P. L. Chu, H. P. Chan, and B. P. Pal, “Polymer-based compact comb filter with flat-top response,” IEEE Photon. Technol. Lett. 17(12), 2619–2621 (2005). [CrossRef]
  10. Y. Lai, W. Zhang, and J. A. R. Williams, “All-fibre multichannel flattop filter based on coherent fibre delay line structure,” Electron. Lett. 38(10), 473–475 (2002). [CrossRef]
  11. S. Derevyanko, “Design of a flat-top fiber Bragg filter via quasi-random modulation of the refractive index,” Opt. Lett. 33(20), 2404–2406 (2008). [CrossRef] [PubMed]
  12. Y. W. Lee, H. T. Kim, J. Jung, and B. Lee, “Wavelength-switchable flat-top fiber comb filter based on a Solc type birefringence combination,” Opt. Express 13(3), 1039–1048 (2005). [CrossRef] [PubMed]

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