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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 15 — Jul. 19, 2010
  • pp: 15332–15337

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Phase-transparent flexible waveband conversion of 43 Gb/s RZ-DQPSK signals using multiple-QPM-LN waveguides

Hongbin Song, Osamu Tadanaga, Takeshi Umeki, Isao Tomita, Masaki Asobe, Shuto Yamamoto, Kunihiko Mori, and Kazushige Yonenaga  »View Author Affiliations


Optics Express, Vol. 18, Issue 15, pp. 15332-15337 (2010)
http://dx.doi.org/10.1364/OE.18.015332


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Abstract

We report phase-transparent waveband conversion with polarization insensitivity based on second harmonic (SH) wave pumped difference frequency generation (DFG) using multiple-quasi-phase-matched LiNbO3 (QPM-LN) waveguides. Flexible waveband conversion is demonstrated over the entire C-band using a tunable DFB-LD array (TLA) as a pump source for a multiple-QPM-LN waveguide. The penalty free waveband conversion of 43 Gb/s return-to-zero differential quadrature phase-shift-keying (RZ-DQPSK) waveband signals is successfully achieved.

© 2010 OSA

1. Introduction

The waveband switching node, which groups several wavelengths with the same destinations into a waveband and deals with them as one optical path, was proposed for constructing a cost-effective, large-throughput optical cross connector (OXC) to meet the rapidly increasing data traffic demand in networks [1

K. Sato and H. Hasegawa, “Prospects and challenges of multi-layer optical networks,” IEICE Trans. Commun. E90-B(8), 1890–1902 (2007). [CrossRef]

]. An all-optical waveband converter is needed for the waveband-switching node to avoid the wavelength contention and reduce the power consumption of photonic nodes. In addition, advanced phase modulation formats such as differential binary phase-shift-keying (DPSK) and differential quadrature phase-shift-keying (DQPSK) have been applied to large-capacity optical communication systems [2

P. J. Winzer and R. Essiambre, “Advanced modulation formats for high-capacity optical transport networks,” J. Lightwave Technol. 24(12), 4711–4728 (2006). [CrossRef]

]. Phase-transparency is required for the waveband conversion of such advanced modulation formats. Difference frequency generation (DFG) in a quasi-phase-matched LiNbO3 (QPM-LN) waveguide is an attractive wavelength conversion method, owing to the large bandwidth and modulation format transparency [3

M. H. Chou, I. Brener, M. M. Fejer, E. E. Chaban, and S. B. Christman, “1.5-μm-band wavelength conversion based on cascaded second-order nonlinearity in LiNbO3 waveguide,” IEEE Photon. Technol. Lett. 11(6), 653–655 (1999). [CrossRef]

]. Moreover, variable waveband conversion is also possible by using a wavelength tunable light source as a pump source for a multiple-QPM-LN waveguide [4

M. Asobe, O. Tadanaga, H. Miyazawa, Y. Nishida, and H. Suzuki, “Multiple-quasi-phase-matched device using continuous phase modulation of χ(2) grating and its application to variable wavelength conversion,” IEEE J. Quantum Electron. 41(12), 1540–1547 (2005). [CrossRef]

,5

X. Wu, W. R. Peng, V. R. Arbab, J. Wang, and A. E. Willner, “Tunable optical wavelength conversion of OFDM signal using a periodically-poled lithium niobate waveguide,” Opt. Express 17(11), 9177–9182 (2009). [CrossRef] [PubMed]

]. Among several tunable light sources, we have investigated the performance of a tunable laser diode array (TLA) as a pump source for variable waveband conversion. A TLA is composed of a DFB-LD array, a multi-mode interference (MMI) coupler and a semiconductor optical amplifier (SOA) [6

H. Ishii, H. Oohashi, K. Kasaya, K. Tsuzuki, and Y. Tohmori, “High-power (40 mW) L-band tunable DFB laser array module using current tuning,” in Conference of Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2005), paper OTuE1.

]. Thus, a TLA can generate multiple-wavelength output with discrete spacing and allows high-speed wavelength selection. These features make it an ideal pump source for a multiple-QPM-LN waveguide. We have demonstrated dynamic variable waveband conversion from the C-band to the L-band using multiple-QPM-LN waveguides with four QPM wavelength peaks and a TLA [7

H. B. Song, T. Tadanaga, T. Umeki, I. Tomita, H. Ishii, H. Oohashi, and M. Asobe, “High-quality grouped-wavelength conversion and dynamic switching using multiple-QPM LiNbO3 and TLA,” IEEE Photon. Technol. Lett. 21(13), 854–856 (2009). [CrossRef]

].

In this work, we expand the waveband conversion range by fabricating a new multiple-QPM-LN waveguide with eight QPM peaks. We construct an integrated waveband converter by using a TLA and the new multiple QPM-LN waveguides and demonstrate flexible and polarization insensitive waveband conversion over the entire C-band. We also confirm the phase-transparency of the converter by using 43 Gb/s RZ-DQPSK signals.

2. Multiple-QPM-LN waveguide module with 8 QPM peaks

The multiple QPM-LN waveguides with eight QPM peaks were designed using an asymmetric phase modulation on a periodical domain grating in LiNbO3 [4

M. Asobe, O. Tadanaga, H. Miyazawa, Y. Nishida, and H. Suzuki, “Multiple-quasi-phase-matched device using continuous phase modulation of χ(2) grating and its application to variable wavelength conversion,” IEEE J. Quantum Electron. 41(12), 1540–1547 (2005). [CrossRef]

]. The phase modulation curve and theoretical phase-matching curve are shown in Fig. 1(a) and (b) , respectively. It can be seen that the phase modulation function curve contains a smooth part and a steep phase jump. The period of the phase modulation function was 6 mm and the total length of the waveguide was 48 mm. We designed the pump frequency spacing to be 300 GHz to contiguously convert 12-channel waveband signals with a 50 GHz spacing covering the entire C-band.

Fig. 1 (a) Phase-modulation function, (b) Theoretical phase-matching curve.

We fabricated two multiple-QPM-LN waveguides using the direct bonding method [8

Y. Nishida, H. Miyazawa, M. Asobe, O. Tadanaga, and H. Suzuki, “0-dB wavelength conversion using direct-bonded QPM-Zn:LiNbOs ridge waveguide,” IEEE Photon. Technol. Lett. 17(5), 1049–1051 (2005). [CrossRef]

]. The utilization of Zn-doped LiNbO3 as a core layer made the waveguides highly resistant to the photorefractive damage. The waveguide was assembled in a module with four fiber-optic ports [9

M. Asobe, T. Umeki, O. Tadanaga, K. Yoshino, E. Yamazaki, and A. Takada, “Low crosstalk and variable wavelength conversion using multiple QPM LiNbO3 waveguide module,” Electron. Lett. 45(10), 519–520 (2009). [CrossRef]

]. The 4-port configuration facilitates the bi-directional input and output of signals and second harmonic (SH) waves around 780 nm. A Peltier device is installed in the module to control the waveguide temperature. Thus the phase matching curve can be easily tuned via the temperature controller. The insertion loss of the waveguide is 3 dB at 1550 nm. Figure 2 shows the phase-matching curves of two multiple-QPM-LN waveguide modules at 54 and 56 °C, respectively. The phase matching curves of the modules deviate slightly from the theoretical value in the conversion efficiency balance of the eight peaks and the ripple between the phase-matching peaks. It is reported that the ripple results in the cross talk due to the sum frequency generation (SFG) between the signal and idler wavebands and subsequent DFG between the SF light and the signal [10

I. Tomita, T. Umeki, O. Tadanaga, H. B. Song, and M. Asobe, “Apodized multiple quasi-phase-matched LiNbO3 device for low-cross-talk waveband conversion,” Opt. Lett. 35(6), 805–807 (2010). [CrossRef] [PubMed]

]. These ripples are mainly caused by steep increase in nonlinearity at the edge of waveguide and partially increased by the non-uniformity of the waveguide. All these ripples can be reduced by using the apodized QPM gratings and improving the uniformity of the waveguide [10

I. Tomita, T. Umeki, O. Tadanaga, H. B. Song, and M. Asobe, “Apodized multiple quasi-phase-matched LiNbO3 device for low-cross-talk waveband conversion,” Opt. Lett. 35(6), 805–807 (2010). [CrossRef] [PubMed]

]. The maximum SHG conversion efficiencies including the coupling loss at the signal and pump ports of two modules are 32 and 28.8%/W, respectively.

Fig. 2 Phase matching curve of two modules.

3. Adjacent waveband conversion covering entire C-band

We constructed a polarization-insensitive integrated waveband converter using the multiple QPM-LN modules and a TLA. Figure 3 shows the configuration of the integrated waveband converter. As a cascaded SHG/DFG using one multiple-QPM-LN module is easily to generate crosstalk due to the SFG between the pump and signal wavebands and the subsequent DFG between an SF light and the signal. To obtain a waveband conversion with low crosstalk, we employ SH-pumped DFG using two multiple QPM-LN modules instead of cascaded SHG/DFG using one multiple-QPM-LN module [7

H. B. Song, T. Tadanaga, T. Umeki, I. Tomita, H. Ishii, H. Oohashi, and M. Asobe, “High-quality grouped-wavelength conversion and dynamic switching using multiple-QPM LiNbO3 and TLA,” IEEE Photon. Technol. Lett. 21(13), 854–856 (2009). [CrossRef]

,9

M. Asobe, T. Umeki, O. Tadanaga, K. Yoshino, E. Yamazaki, and A. Takada, “Low crosstalk and variable wavelength conversion using multiple QPM LiNbO3 waveguide module,” Electron. Lett. 45(10), 519–520 (2009). [CrossRef]

]. The two multiple-QPM-LN waveguide modules are tuned to the same phase-matched wavelength peaks by using the temperature controller. The output from the TLA is amplified using an external erbium doped fiber amplifier (EDFA) and injected into multiple-QPM LN 1 to generated an SH wave around 780 nm. The generated SH power of 19.4 dBm is injected into multiple-QPM-LN 2 in both directions via a coupler. A polarization beam splitter (PBS)-based coupler is used to minimize the wavelength dependence of the splitting ratio of the SH wave. The SH waves were delivered from multiple-QPM LN 1 to 2 using polarization maintaining fibers for 780 nm. The waveband signals pass through a circulator and are routed into a PBS. The vertical polarization component is directly input into multiple-QPM-LN 2 in the forward direction. The horizontal polarization component is changed to the vertical direction via the connection of polarization maintaining fibers and injected into multiple-QPM-LN 2 in the backward direction. The idlers corresponding to the two polarization components are combined and routed to the output port.

Fig. 3 Schematic diagram of polarization-independent waveband converter. TLA: Tunable Laser-diode Array; QPM-LN: Quasi-Phase-Matched Lithium Niobate; PBS: Polarization Beam Splitter; EDFA: Erbium doped Fiber Amplifier.

In this experiment, we converted a 12-channel waveband signal with a 50 GHz spacing filtered from a multi-wavelength light source [11

T. Yamamoto, T. Komukai, K. Suzuki, and A. Takada, “Spectrally flattened phase-locked multi-carrier light generator with phase modulators and chirped fiber Bragg grating,” Electron. Lett. 43(19), 1040–1041 (2007). [CrossRef]

]. Figure 4 shows the spectra of the unconverted waveband signals and converted waveband signals. It can be seen that the adjacent waveband conversion was obtained from 1530 nm to 1575 nm covering the entire C-band. A conversion efficiency of > –17 dB was obtained. The waveband conversion spectra for the waveband signals in TM mode and TE mode are shown in Fig. 5 (a) and (b) , respectively. The results confirmed that the polarization sensitivity of the waveband conversion was < 0.9 dB.

Fig. 4 Spectra of waveband signals and idlers.
Fig. 5 Waveband conversion spectra with different polarization states.

The multi-wavelength output with a discrete spacing and fast wavelength selection provided by the TLA provides an ideal pump source for variable waveband conversion based on DFG using a multiple-QPM-LN waveguide. We achieved a dynamic and flexible waveband conversion in a polarization-insensitive configuration by switching the output wavelength of the TLA. We successfully demonstrated the flexible waveband conversion of the 12-channel waveband signals with a 50 GHz spacing.

4. Phase-transparent waveband conversion

DFG based wavelength conversion can preserve the phase information thus it can offer transparent waveband conversion. To confirm this property, we attempted to convert 43 Gb/s RZ-DQPSK based waveband signals. The experimental setup is shown in Fig. 6 . A 5-channel waveband signal with 100 GHz spacing was modulated with a 43 Gb/s RZ-DQPSK modulator, and input into the polarization-insensitive waveband converter. The bit error rate (BER) was investigated using a DQPSK receiver. We measured the BERs of an idler at 1554.2 nm with single-channel and 5-channel signals inputs.

Fig. 6 Configuration of BER experiments using 43 Gb/s RZ-DQPSK signals.

The BER measurement results for back-to-back signals and the corresponding idlers are shown in Fig. 7 . No appreciable power penalty was observed for single-channel or 5-channel signal inputs, which confirmed that no substantial noise is generated during the waveband conversion. Figure 8 shows the eye diagrams of the signal and the corresponding idler. It also indicates that there is no significant degradation in the signal quality caused by the polarization insensitive waveband converter.

Fig. 7 BER measurement result.
Fig. 8 Eye diagrams. (a) Eye diagram of signal at 1535.8 nm (b) Eye diagram of idler at 1554.2 nm.

5. Conclusion

Phase-transparent waveband conversion over the entire C-band was achieved using a multiple-QPM-LN waveguide module with eight QPM wavelength peaks. Flexible waveband conversion was successfully demonstrated by switching the output wavelength of the TLA. The conversion efficiency of > –17 dB and the polarization insensitivity (< 0.9 dB) of the waveband conversion were obtained. The result of BER measurement experiment with 43 Gb/s RZ-DQPSK signals indicated that there was no significant signal degradation induced by the flexible waveband converter, which confirmed the phase-transparency of the polarization insensitive waveband converter.

Acknowledgement

We thank Dr. Hiroyuki Ishii and Dr. Hiromi Oohashi of NTT Photonics Laboratories for providing the TLA. This work was supported by the National Institute of Information and Communication Technology (NICT) of Japan.

References and links

1.

K. Sato and H. Hasegawa, “Prospects and challenges of multi-layer optical networks,” IEICE Trans. Commun. E90-B(8), 1890–1902 (2007). [CrossRef]

2.

P. J. Winzer and R. Essiambre, “Advanced modulation formats for high-capacity optical transport networks,” J. Lightwave Technol. 24(12), 4711–4728 (2006). [CrossRef]

3.

M. H. Chou, I. Brener, M. M. Fejer, E. E. Chaban, and S. B. Christman, “1.5-μm-band wavelength conversion based on cascaded second-order nonlinearity in LiNbO3 waveguide,” IEEE Photon. Technol. Lett. 11(6), 653–655 (1999). [CrossRef]

4.

M. Asobe, O. Tadanaga, H. Miyazawa, Y. Nishida, and H. Suzuki, “Multiple-quasi-phase-matched device using continuous phase modulation of χ(2) grating and its application to variable wavelength conversion,” IEEE J. Quantum Electron. 41(12), 1540–1547 (2005). [CrossRef]

5.

X. Wu, W. R. Peng, V. R. Arbab, J. Wang, and A. E. Willner, “Tunable optical wavelength conversion of OFDM signal using a periodically-poled lithium niobate waveguide,” Opt. Express 17(11), 9177–9182 (2009). [CrossRef] [PubMed]

6.

H. Ishii, H. Oohashi, K. Kasaya, K. Tsuzuki, and Y. Tohmori, “High-power (40 mW) L-band tunable DFB laser array module using current tuning,” in Conference of Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2005), paper OTuE1.

7.

H. B. Song, T. Tadanaga, T. Umeki, I. Tomita, H. Ishii, H. Oohashi, and M. Asobe, “High-quality grouped-wavelength conversion and dynamic switching using multiple-QPM LiNbO3 and TLA,” IEEE Photon. Technol. Lett. 21(13), 854–856 (2009). [CrossRef]

8.

Y. Nishida, H. Miyazawa, M. Asobe, O. Tadanaga, and H. Suzuki, “0-dB wavelength conversion using direct-bonded QPM-Zn:LiNbOs ridge waveguide,” IEEE Photon. Technol. Lett. 17(5), 1049–1051 (2005). [CrossRef]

9.

M. Asobe, T. Umeki, O. Tadanaga, K. Yoshino, E. Yamazaki, and A. Takada, “Low crosstalk and variable wavelength conversion using multiple QPM LiNbO3 waveguide module,” Electron. Lett. 45(10), 519–520 (2009). [CrossRef]

10.

I. Tomita, T. Umeki, O. Tadanaga, H. B. Song, and M. Asobe, “Apodized multiple quasi-phase-matched LiNbO3 device for low-cross-talk waveband conversion,” Opt. Lett. 35(6), 805–807 (2010). [CrossRef] [PubMed]

11.

T. Yamamoto, T. Komukai, K. Suzuki, and A. Takada, “Spectrally flattened phase-locked multi-carrier light generator with phase modulators and chirped fiber Bragg grating,” Electron. Lett. 43(19), 1040–1041 (2007). [CrossRef]

OCIS Codes
(060.2330) Fiber optics and optical communications : Fiber optics communications
(130.3730) Integrated optics : Lithium niobate
(190.4390) Nonlinear optics : Nonlinear optics, integrated optics
(130.7405) Integrated optics : Wavelength conversion devices

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: April 22, 2010
Revised Manuscript: May 24, 2010
Manuscript Accepted: May 28, 2010
Published: July 2, 2010

Citation
Hongbin Song, Osamu Tadanaga, Takeshi Umeki, Isao Tomita, Masaki Asobe, Shuto Yamamoto, Kunihiko Mori, and Kazushige Yonenaga, "Phase-transparent flexible waveband conversion of 43 Gb/s RZ-DQPSK signals using multiple-QPM-LN waveguides," Opt. Express 18, 15332-15337 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-15-15332


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References

  1. K. Sato and H. Hasegawa, “Prospects and challenges of multi-layer optical networks,” IEICE Trans. Commun. E90-B(8), 1890–1902 (2007). [CrossRef]
  2. P. J. Winzer and R. Essiambre, “Advanced modulation formats for high-capacity optical transport networks,” J. Lightwave Technol. 24(12), 4711–4728 (2006). [CrossRef]
  3. M. H. Chou, I. Brener, M. M. Fejer, E. E. Chaban, and S. B. Christman, “1.5-μm-band wavelength conversion based on cascaded second-order nonlinearity in LiNbO3 waveguide,” IEEE Photon. Technol. Lett. 11(6), 653–655 (1999). [CrossRef]
  4. M. Asobe, O. Tadanaga, H. Miyazawa, Y. Nishida, and H. Suzuki, “Multiple-quasi-phase-matched device using continuous phase modulation of χ(2) grating and its application to variable wavelength conversion,” IEEE J. Quantum Electron. 41(12), 1540–1547 (2005). [CrossRef]
  5. X. Wu, W. R. Peng, V. R. Arbab, J. Wang, and A. E. Willner, “Tunable optical wavelength conversion of OFDM signal using a periodically-poled lithium niobate waveguide,” Opt. Express 17(11), 9177–9182 (2009). [CrossRef] [PubMed]
  6. H. Ishii, H. Oohashi, K. Kasaya, K. Tsuzuki, and Y. Tohmori, “High-power (40 mW) L-band tunable DFB laser array module using current tuning,” in Conference of Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2005), paper OTuE1.
  7. H. B. Song, T. Tadanaga, T. Umeki, I. Tomita, H. Ishii, H. Oohashi, and M. Asobe, “High-quality grouped-wavelength conversion and dynamic switching using multiple-QPM LiNbO3 and TLA,” IEEE Photon. Technol. Lett. 21(13), 854–856 (2009). [CrossRef]
  8. Y. Nishida, H. Miyazawa, M. Asobe, O. Tadanaga, and H. Suzuki, “0-dB wavelength conversion using direct-bonded QPM-Zn:LiNbOs ridge waveguide,” IEEE Photon. Technol. Lett. 17(5), 1049–1051 (2005). [CrossRef]
  9. M. Asobe, T. Umeki, O. Tadanaga, K. Yoshino, E. Yamazaki, and A. Takada, “Low crosstalk and variable wavelength conversion using multiple QPM LiNbO3 waveguide module,” Electron. Lett. 45(10), 519–520 (2009). [CrossRef]
  10. I. Tomita, T. Umeki, O. Tadanaga, H. B. Song, and M. Asobe, “Apodized multiple quasi-phase-matched LiNbO3 device for low-cross-talk waveband conversion,” Opt. Lett. 35(6), 805–807 (2010). [CrossRef] [PubMed]
  11. T. Yamamoto, T. Komukai, K. Suzuki, and A. Takada, “Spectrally flattened phase-locked multi-carrier light generator with phase modulators and chirped fiber Bragg grating,” Electron. Lett. 43(19), 1040–1041 (2007). [CrossRef]

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