Polarization-insensitive and widely tunable ... - OSA Publishing

Polarization-insensitive and widely tunable wavelength conversion for polarization shift keying signal based on four wave mixing in highly non-linear fiber Md. Nur-Al-Safa Bhuiyan, Motoharu Matsuura*, Hung Nguyen Tan, and Naoto Kishi Department of Information and Communication Engineering, The Center for Frontier Science and Engineering*, The University of Electro-Communications, Tokyo, 182-8585, JAPAN [email protected]

Abstract: A fiber-based polarization-insensitive and widely tunable all-optical wavelength conversion for polarization shift keying (PolSK) signal is experimentally demonstrated by means of four-wave-mixing (FWM) in highly non-linear fiber. Polarization sensitivity of our scheme is compared with the conventional one and the conversion performance of the PolSK signal is investigated. In our scheme, preservation of state of polarization (SOP) of input PolSK signal is possible on both of the converted spectral components in FWM spectrum. In addition, over 23 nm of wavelength tunable operation with high conversion performance is achieved and detailed bit-error-rate (BER) characteristics are measured for both the up- and down-converted signal. © 2009 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (190.4380) Nonlinear optics, four-wave mixing; (250.4745) Optical processing devices.

References and links 1. S. Benedetto, R. Gaudino, and P. Poggiolini, “Polarization recovery in optical polarization shift-keying systems,” IEEE Trans. Commun. 45, 1269-1279 (1997). 2. S. Benedetto, R. Gaudino, and P. Poggiolini, “Direct detection of optical digital transmission based on polarization shift keying modulation,” IEEE J. Sel. Areas Commun. 13, 531-542 (1995). 3. S. Benedetto and P. Poggiolini, “Theory of polarization shift keying modulation,” IEEE Trans. Commun. 40, 708-721 (1992). 4. A. Carena, V. Curri, R. Gaudino, N. Greco, P. Poggiolini, and S. Benedetto, “Polarization modulation in ultralong haul transmission systems: A promising alternative to intensity modulation,” in Proc. ECOC 1998, paper noWdA24, Madrid, Spain (1998), pp. 429-430. 5. E. Hu, Y. Hsuen, K. Wong, M. Marhic, L. Kazovsky, K. Shimizu, and N. Kikuchi, “4-Level direct-detection polarization shift-keying (DD-PolSK) system with phase modulators,” in Proc. OFC 2003, paper no. FD2, Atlanta, USA (2003). 6. N. Chi, S. Yu, L. Xu, and P. Jappesen, “Generation of Polarization shift keying signal and its application in optical labeling,” in Proc. ECOC 2005, paper no. Mo 4.4.4, Glasgow, Scotland (2005). 7. P. Martelli, P. Boffi, M. Ferrario, L. Marazzi, P. Parolari, R. Siano, V.Pusino, P. Minzioni, I. Cristiani, C. Langrock, M. M. Fejer, M. Martinelli, V. Degiorgio, “All-Optical Wavelength Conversion of a 100- Gb/s PolarizationMultiplexed Signal,” Opt. Express 17, 17758-17763 (2009). http://www.opticsinfobase.org/oe/ abstract.cfm?URI=oe-17-20-17758.

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Received 11 Sep 2009; revised 24 Oct 2009; accepted 12 Nov 2009; published 22 Jan 2010

1 February 2010 / Vol. 18, No. 3 / OPTICS EXPRESS 2467

8. J. Yu, M. Huang, and G. Chang, “ Wavelength conversion based on copolarized pumps generated by optical carrier suppression,” IEEE Photon. Technol. Lett. 21, 392-394 (2009). 9. M.-F. Huang, J. Yu, and G.-K. Chang, “Polarization insensitive wavelength conversion for 4 × 112 Gbit/s polarization multiplexing RZ-QPSK signals,” Opt. Express 16, 21161-21169 (2008). http://www. opticsinfobase.org/oe/abstract.cfm?URI=oe-16-26-21161. 10. E. Ciaramella, A. D’Errico, R. Proietti, and G. Contestabile, “WDM-POLSK transmission systems by using semiconductor optical amplifiers,” J. Lightwave Technol. 24, 4039-4046 (2006). 11. N. Chi, L. Xu, S. Yu, and P. Jeppesen, “Generation and transmission performance of 40 Gbit/s polarization shift keying signal,” Electron. Lett. 41, 547- 549 (2005). 12. S. Benedetto, and P. T. Poggiolini, “Multilevel polarization shift keying: optimum receiver structure and performance evaluation,” IEEE Trans. Commun. 42, 1174-1186 (1994). 13. R. J. Blaikie, D. P. Taylor, and P. T. Gough, “Multilevel differential polarization shift keying,” IEEE Trans. Commun. 45, 95-102 (1997). 14. L. Han, H. Wen, H. Zhang, and Y. Guo, “All-optical wavelength conversion for polarization shift keying signal based on four-wave mixing in a semiconductor optical amplifier,” Opt. Eng. Lett. 46, pp. 090501 (2007). 15. Z. Wang, N. Deng, C. Lin and C. Chan “Polarization-insensitive widely tunable wavelength conversion based on four-wave mixing using dispersion-flattened high-nonlinearity photonic crystal fiber with residual birefringence” in Proc. ECOC 2006, We3.p.18, Cannes, France (2006). 16. Md. Bhuiyan, M. Matsuura, H. N. Tan, and N. Kishi, “Polarization-insensitive wavelength conversion for polarization shift keying signal based on four wave mixing in highly non-linear fiber,” in Proc. OECC 2009, paper no. FE2, Hong Kong (2009). 17. J. M. Tang and K. A. Shore, “A simple scheme for polarization insensitive four-wave mixing in semiconductor optical amplifiers,” IEEE Photon. Technol. Lett. 11, 1123-1125 (1999). 18. G. P. Agrawal, “Population pulsation and nondegenerate four-wave mixing in semiconductor lasers and amplifiers,” J. Opt. Soc. Amer. B, 5, 147-159 (1988). 19. K. Inoue, “Polarization effect on four-wave mixing efficiency in a single-mode fiber,” IEEE J. Quantum Electron. 28, 883-894 (1992). 20. J. P. R. Lacey, M. A. Summerfield, and S. J. Madden, “Tunability of polarization-insensitive wavelength converters based on four-wave mixing in semiconductor optical amplifiers,” J. Lightwave Technol. 16, 2419-2427 (1998). 21. K. K. Y. Wong, M. E. Marhic, K. Uesaka, and L. G. Kazovsky, “Polarization-independent one-pump fiber-optical parametric amplifier,” IEEE Photon. Technol. Lett. 14, 1506-1508 (2002). 22. 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). 23. M. W. Maeda, W. B. Sessa, W. I. Way, A. Yi-Yan, L. Curtis, R. Spicer, and R. I. Laming, “The effect of four-wave mixing in fibers on optical frequency-division multiplexed systems,” J. Lightwave Technol. 8, 1402-1408 (1990).

1.

Introduction

Polarization shift keying (PolSK) is an attractive modulation format that utilizes the state of polarization (SOP) of optical signal as the information bearing parameter. Binary PolSK signal, represented by two antipodal points (orthogonal to each other) on the poincare sphere, corresponding to electrical signal “0” and “1”, are used for the signal constellation [1, 2]. Polarization modulation has been theoretically and experimentally demonstrated in number of researches [16] and has been found to have significant advantages. Recently significant researches have been published related with polarization multiplexed signal [7-9]. There are also some works have been done on the transmission of the PolSK signal [10, 11]. The main advantage of PolSK is its constant envelope, thus showing reduced sensitivity to self-phase-modulation (SPM) and crossphase-modulation (XPM). Because of its constant power characteristics, PolSK signal is more resistant to nonlinear fiber effects and eliminate the patterning effect in semiconductor optical amplifier (SOA). PolSK also shows reduced sensitivity to laser phase noise [3, 11]. Furthermore, this type of modulation format also supports multilevel coding and enhances spectral efficiency, which is claimed to be one of the key issues in future high-capacity transoceanic links [12, 13]. Therefore, PolSK modulation format is very useful in the future photonic network. Wavelength conversion, a key technique, will be very much required in such kind of network. As the information is encoded in the SOP of the PolSK signal, the converted signal should preserve this input SOP. However, by using conventional wavelength conversion schemes like #117064 - $15.00 USD

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cross-gain-modulation (XGM) or XPM, it is very difficult to preserve the polarization information of the input signal. In a previous research, simple four-wave-mixing (FWM) process involving input binary PolSK signal and pump was considered for the wavelength conversion of PolSK signal as the signal SOP was preserved in this effect [14]. However, there are some drawbacks of the system. Firstly, the converted PolSK signal mainly suffers from signal to noise ratio degradation caused by low FWM conversion efficiency and amplified spontaneous emission (ASE) noise of SOA. Secondly, the change of SOP of the total constellation of the input PolSK signal may degrade the converted signal quality. Moreover, in their work, it was not possible to preserve the SOP of input PolSK signal on both of the converted spectral components in FWM process. In the previous study, preservation of input signal SOP was possible only with one of the converted FWM spectral components, namely, then non-linear converted spectral component with respect to the input signal. Hence it is interesting to employ the linear converted spectral component in the FWM spectrum in order to preserve the SOP of the input signal. Thus, a practical technique will be required to realize the polarization-insensitive wavelength conversion of PolSK signal and preserving the SOP of input signal on both of the converted spectral components in FWM spectrum. On top of that, it is worth investigating the wavelength tunability of the converter, to observe the conversion capability range of the system [15]. In this paper, we have demonstrated, for the first time, a fiber-based polarization-insensitive and widely tunable wavelength conversion for PolSK signal [16]. Preservation of input signal information on both of the converted spectral components is possible in the proposed scheme. Polarization sensitivity of the diversity scheme is compared with the conventional one. Polarization sensitivity is significantly improved in the diversity scheme. Conversion performances of both of the converted PolSK signal are investigated in diversity scheme. In our proposed diversity scheme the wavelength tunable range of 23 nm is obtained with high conversion efficiency. Bit-error-rate (BER) characteristics of the PolSK signal for both up- and down-conversion are also investigated. 2.

Preservation of SOP and polarization diversity of the wavelength converter

There are two features of the proposed wavelength converter. Firstly, it can preserve the SOP of the input PolSK signal on both of the converted spectral components in FWM process. Secondly, it is polarization insensitive to input PolSK signal. As the information is encoded in Input PolSK signal (

ωs )

FWM in non-linear media

CW Pump ( ω p )

Data signal

Pump (Fp )

(Fs )

Conv.1 (F1 )

2ωp − ωs

Output with Converted PolSK signals

Conv.2 (F2 )

ωp

ωs

2ω s− ωp

Fig. 1. Schematic diagram of the wavelength conversion for the PolSK signal using simple FWM. Inset: output FWM Spectra for PolSK signal. Conv.1: 1st converted signal and Conv.2: 2nd converted signal. #117064 - $15.00 USD

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the SOP, it is important that the change of SOP of the converted signal should follow that of the input PolSK signal. It is well known that the polarization state of the input optical signals involving in a FWM process determines the SOP of the converted signal. Figure 1 shows the simple FWM between PolSK signal and pump. From this Figure following equations are satisfied [17]. F1 = (F p · F∗s )F p r(ω p − ωs )e j(2ω p −ωs )t

(1)

F2 = (Fs · F∗p )Fs r(ω p − ωs )e j(2ωs −ω p )t

(2)

Where F p , Fs , F1 and F2 are vectors representing the fields of the pump, the input signal and the two converted spectral components, Conv.1 and Conv.2, respectively. r(ω p − ωs ) is the relative conversion efficiency coefficient. It can be written from these equations that the nonlinear coupling between the wave F p and Fs produces population pulsation at beat frequency (ω p − ωs ) which in turn produces temporal gain and index gratings. As a result of scattering the wave F p , by the gratings, the converted wave F1 is thus created from Eq. (1). Thus, the polarization state of the converted wave F1 is parallel to that of the input wave F p . Similarly, from Eq. (2) the polarization state of F2 is parallel to that of the input wave Fs [17-20]. From the above discussion, it can be said that, the converted signal F2 holds the SOP of the input PolSK signal. On the other hand, the converted signal F1 follows the SOP of the pump signal, which has a fixed SOP. Thus, the converted signal F1 is unable to preserve the SOP of the input PolSK signal. In the previous research without polarization diversity [14], the spectral component, Conv.2 in Fig. 1 was given the major importance, as it could follow the SOP of the input signal. However, if the SOP of the total constellation of the input PolSK signal changes, the quality of the converted signal is degraded. In this conventional scheme, following equations are satisfied for FWM between input PolSK signal and pump [20]. P(Conv.1) = [γ Pp Leff ]2 · Ps cos θ r(ω p − ωs ) e−α L · η

(3)

P(Conv.2) = [γ Ps cos θ Leff ]2 · Pp r(ω p − ωs ) e−α L · η

(4)

Where, P(Conv.1) and P(Conv.2) are the converted wavelength at the frequency of 2ω p − ωs and 2ωs − ω p , respectively. Pp , Ps , α , L, Leff , r(ω p − ωs ), η , γ and θ are Pump power, signal power, attenuation coefficient, fiber length, effective fiber length, relative conversion efficiency coefficient, FWM efficiency, nonliear coefficient and angle between input signal and pump, respectively. From eq. (4) it can be seen that, the power of the converted signal is proportional to the square of the cosine of the angle between pump and signal. Hence, this non-linear converted spectral component degrades more rapidly to the change of SOP of the total constellation of the input PolSK signal than the linear converted spectral component shown in eq. (3). In these situations, polarization diversity operation as well as preservation of SOP of the input PolSK signal on the linear converted spectral component (Conv.1 in Fig.1) in the FWM spectrum is useful [20-22]. Our proposed wavelength converter consists of a polarization beam splitter (PBS) and Sagnac loop with highly non-linear fiber (HNLF) as shown in Fig. 2. As shown in the inset of Fig. 2, the total constellation of the input PolSK signal is composed of the two orthogonal SOP components , S1 and S2 . The change of SOP of the total constellation of the input PolSK signal means that the change of SOP for both the orthogonal components of the PolSK signal at the same time. In each SOP we have an usual non return to zero (NRZ) intensity modulated signal (the two sequence being inverted). When one SOP component is present, the orthogonal SOP component is absent. The PBS splits both pump and signal on both the directions of the loop. Each orthogonal polarization component of the PolSK signal is splitted by the PBS and #117064 - $15.00 USD

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(a)

Data signal

Pump

10 Gb/s NRZ Data Stream Conv.1 PPG 1558 nm ECL

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Polarization Rotator ECL CW Pump 1552 nm

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EDFA : Er-Doped Fiber Amplifier PC : Polarization Controller PBS : Polarization Beam Splitter BPF : Band Pass Filter ECL : External-Cavity Laser PPG : Pulse Pattern Generator POL : Polarizer HNLF : Highly Non-Linear Fiber BER : Bit Error Rate PD : Photo Detector

90

O

X

0.6 nm 0.6 nm

Oscilloscope

BPF

S2

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PC3

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θ

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Fig. 2. Experimental setup for the polarization-insensitive wavelength conversion of PolSK signal. Inset: (a) output optical spectrum from port D of the PBS, (b) pump and PolSK signal at input port A of the PBS. S1 : One SOP component and S2 : orthogonal SOP component of the input PolSK signal.

undergoes separate FWM process in the HNLF. As the SOP of pump and signal remain parallel to each other in a particular direction inside the loop, the SOP of both of the converted signals is assumed to follow the same SOP. Thus both of the converted spectral components can preserve the input signal information. Considering the SOP component, S1 of PolSK signal, it is splitted by PBS along with the pump. With the variation of SOP for the SOP component, S1 of input PolSK signal, the power of the input signal varies in the two directions inside the loop. If same pump powers are injected in both the directions of the loop, from Fig. 2, following equations are satisfied at the converted frequency of 2ω p − ωs (Conv.1) [20, 23]. Pp ) Leff ]2 · Ps sin2 θ r(ω p − ωs ) e−α L · η (5) 2 Pp Pfwm(ccw) = [γ ( ) Leff ]2 · Ps cos2 θ r(ω p − ωs ) e−α L · η (6) 2 Where Pfwm(cw) and Pfwm(ccw) are the power of the converted signal in the clock wise and counter clock wise direction inside the loop, respectively. Powers of the converted signals in both the directions inside the loop are combined or multiplexed. We can get the total converted power at frequency, 2ω p − ωs as follows. Pfwm(cw) = [γ (

Pp ) Leff ]2 · Ps r(ω p − ωs ) e−α L · η (7) 2 Hence, the converted power is independent of change of SOP for the SOP component, S1 of the input PolSK signal. For the orthogonal SOP component, S2 similar argument is applicable. The total output converted signal coming out of PBS is composed of the converted signals for the two orthogonal SOP components, S1 and S2 of the input PolSK signal. Therefore, the power of the converted signal at frequency, 2ω p − ωs (Conv.1) is independent of the change of SOP of the total constellation of the input PolSK signal. P(Conv.1) = [γ (

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For the converted signal, Conv.2 at frequency 2ωs − ω p following equation are satisfied in the clock wise and counter clock wise direction inside the loop, respectively. Pfwm(cw) = [γ Ps sin2 θ Leff ]2 · (

Pp ) r(ω p − ωs ) e−α L · η 2

(8)

Pp ) r(ω p − ωs ) e−α L · η (9) 2 From eq. (8) and eq. (9), we can get the total converted power at frequency, 2ωs − ω p as follows. Pfwm(ccw) = [γ Ps cos2 θ Leff ]2 · (

Pp ) r(ω p − ωs ) e−α L · η [sin4 θ + cos4 θ ] (10) 2 It can be seen form the above eq. (10) that the converted signal, Conv.2 is polarization sensitive with the change of the SOP of input PolSK signal. Hence, the converted signal, Conv.1 at frequency, 2ω p − ωs is considered to be the better choice for the output converted signal. P(Conv.2) = [γ Ps Leff ]2 · (

3.

Experimental setup

The experimental setup for the diversity scheme is shown in Fig. 2. A LiNbO3 phase modulator (Photline Technologies, Ins., PS-LN polarization rotators) as polarization rotator is used for encoding a continuous wave light at the wavelength of 1558 nm with 10 Gb/s (Pseudorandom bit sequence of 231 − 1) non return to zero (NRZ) data stream. Another laser source provides pump light at the wavelength of 1552 nm. The input PolSK signal and pump power are 4 dBm and 6 dBm, respectively. The combined signal is launched into an erdium doped fiber amplifier (EDFA) and the output power is set to produce a total injected power of 22 dBm. A segment of HNLF of 500 m-long is placed in the loop for FWM process. The characteristics of the HNLF is shown in Table 1. The amplified signal enters the PBS through an isolator. Using the polarization controller (PC1), the pump light is polarized at 45◦ with respect to the PBS axis to split the pump power equally in both directions. The extinction ratio of the PBS is 25 dB. The combined signal of input and pump enters the PBS and travels both of the directions inside the loop before entering in the HNLF. Therefore the signal undergoes separate FWM effect in the HNLF. Using PC2, the SOP of the output signal is controlled to come out from port D of the PBS. At the receiving end, the converted PolSK signal at 1546 nm, is selected using two cascaded BPFs each having 3-dB bandwidth of 0.6 nm. The selected signal is amplified to estimate the quality of the signal. PC3 is used to make a linear-polarized signal at the input of the ratatable polarizer (POL). The converted PolSK signal is demodulated to conventional OOK (On-Off Keying) signal using the POL. The extinction ratio of the ratatable POL is 35 dB. By carefully rotating the POL, both of the orthogonal SOPs of the converted signal are detected. The converted signal at the wavelength of 1564 nm can also be selected by tuning the pass band of the BPFs. Table 1. Characteristics of highly non-linear fiber.

Parameter Length Attenuation Dispersion at 1552 nm Dispersion slope at 1552 nm Nonlinear coefficient (γ ) Aeff

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Value 500 0.47 −0.08 0.032 12.6 11

Unit m dB/km ps/nm/km ps/nm2 /km W−1 · km−1 µ m2



4.

Results and discussion

4.1. Polarization diversity characteristics of the wavelength converter and its performance To investigate polarization sensitivity of our scheme and the conventional one, half wave plate is used to rotate the SOP of the total constellation of the input PolSK signal linearly. In conventional scheme of the previous research [14], the combined signal of input and pump light was sent directly through an SOA. In our experiment, the combined signal is sent directly through the HNLF in the conventional case. In the conventional scheme, it was impossible to measure the polarization sensitivity of the converted signal at 1546 nm (Conv.1 in Fig. 2), as it could not preserve the SOP of the input signal. In the diversity scheme, on the other hand, both of the converted signals at 1546 nm (Conv.1) and 1564 nm (Conv.2) can preserve the SOP of the input PolSK signal which enables to measure the polarization sensitivity at both of these wavelengths. -5

-5

(b) Converted signal power (dBm)

Converted signal power (dBm)

(a) -10 One SOP component

-15 Orthogonal SOP component

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-25

Conventional at 1564 nm (Conv.2) (One SOP component) 90 180 270

0

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360

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-15 Orthogonal SOP component

-20

-25

0

Conventional at 1564 nm (Conv.2) (Orthogonal SOP component) 90 180 270

360

Input signal polarization angle (Degree)

Fig. 3. Comparison of power variation between conventional, at the converted wavelength of 1564 nm (Conv.2) and diversity scheme at (a) the converted wavelength of 1546 nm (Conv.1) and (b) 1564 nm (Conv.2) for both of the orthogonal SOP of PolSK signal.

Figure 3(a) and (b) shows the comparison of power variation between the conventional and diversity schemes for both of the orthogonal SOP components of the PolSK signal. While measuring the power variation of the converted signal with input SOP, The polarizer axis is changed with the change of SOP of the input signal using half wave plate, to observe the maximum eye opening of the converted signal. The axes for both of the cases are measured relative to the same reference. The eye pattern and the power variation are observed simultaneously using a 3-dB coupler. For conventional scheme, the power variation is found to be 13.48 dB and 13.27 dB for both the orthogonal SOP components of the PolSK signal, respectively at the converted wavelength of 1564 nm (Conv.2). In diversity scheme, power variations are 1.92 dB and 2.17 dB at converted wavelength of 1546 nm and 2.95 dB and 3.13 dB at the converted wavelength of 1564 nm for both the orthogonal SOP of the PolSK signal, respectively. Hence, the polarization sensitivity has significantly been improved in the diversity scheme. We observed in the diversity scheme that the power variation of Conv.2 is larger than that of Conv.1. It can be realized from the theoretical explanation in eq. (7) and eq. (10). Thus, the converted spectral component, Conv.2 has larger polarization dependence on the change of SOP of the total constellation of the input PolSK signal than the other converted spectral component, Conv.1. For these reasons, in the diversity scheme, the converted signal at the wavelength of 1546 nm (Conv.1) is considered to be the best choice for the output converted signal. #117064 - $15.00 USD

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3

5 One SOP component (20 ps/div.)

6 7 8 9 10 11 -25

Orthogonal SOP component (20 ps/div.)

-24 -2 3 -22 -21 -20 -19 -18 Received Power (dBm)

Back to Back (One SOP component) Converted (one SOP component) Back to Back (Orthogonal SOP component) Converted (Orthogonal SOP component)

4 -log(BER)

4 -log(BER)

3

Back to Back (One SOP component) Converted (one SOP component) Back to Back (Orthogonal SOP component) Converted (Orthogonal SOP component)

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-18

Fig. 4. BER characteristics at (a) the converted wavelength of 1546 nm and (b) the converted wavelength of 1564 nm for both of the orthogonal SOP of PolSK signal in the diversity scheme. Insets: demodulated eye diagrams for the converted signal.

In the proposed diversity scheme, the FWM spectrum at the output of PBS has been observed. The optical signal to noise ratio (OSNR) for the converted wavelength of 1546 nm (Conv.1) and 1564 nm (Conv.2) is found to be 31.04 dB and 19.63 dB, respectively. To evaluate the performance of the converted wavelengths in detail, the bit error rate (BER) characteristics are investigated. The obtained results for the BER are shown in Fig. 4(a) and (b) at the converted wavelength of 1546 nm and 1564 nm, respectively. Clear eye openings are observed and the power penalties at the BER of 10−9 are found to be 1.27 dB and 1.40 dB at the converter wavelength of 1546 nm and 1.95 dB and 2.06 dB at the converted wavelength of 1564 nm, respectively for both of the orthogonal SOPs of the PolSK signal. Power penalties observed for both of the converted wavelength is primarily the result of OSNR degradation. However, it can be observed from the BER characteristics of the converted signal that, there exists some error floor at the BER of 10−9. The power penalty can be affected by the non-linear polarization rotation occurring at the higher level of received power. It can be seen from the result of Fig. 3 that even for diversity scheme, the converted output signal has small power variations with the change of input SOP. With this small power variation, small variation in power penalty is expected. It should be noted that, the BER characteristics and power penalties for both of the converted wavelengths are considered for best case of input SOP. The difference in power penalty between the best and the worst case of input SOP at the converted wavelength of 1546 nm for one orthogonal SOP component is investigated. The difference in power penalty is found to be 0.43 dB as it can be realized that the power variation at that converted wavelength (1546 nm) is 1.92 dB. For the other orthogonal SOP component of the PolSK signal at that converted wavelength similar variations in power penalties can be obtained as the power variation is 2.17 dB . Hence, the signal quality is not affected by the small power penalty variation, with the change of SOP of the input signal. If highly non-linear polarization maintaining fiber (HNLPMF) is used as a non-linear medium in our experiment, the SOP of the input signal and pump could be further ensured remain the same throughout the medium and polarization sensitivity and conversion performance will also be improved. 4.2. Wavelength tunable operation For wavelength tunable operation, we fix the signal wavelength to 1552 nm and tune the pump wavelength (from 1542 nm to 1562 nm) to investigate the conversion performance at different

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Power [dBm]

Power [dBm]

20 10 (a) Pump (1558 nm) Input (1552 nm) 0 Converted -10 (1564 nm) -20 -30 -40 -50 -60 -70 1540 1545 1550 1555 1560 1565 1570

20 10 (b) Pump (1548 nm) 0 Input (1552 nm) -10 Converted (1544 nm) -20 -30 -40 -50 -60 -70 1535 1540 1545 1550 1555 1560 1565 1570

Wavelength [nm]

Wavelength [nm]

Fig. 5. Output FWM spectra showing (a) 12 nm down-conversion and (b) 8 nm upconversion.

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Conversion efficiency (dB)

operating wavelengths. The FWM-spectra for both 12 nm down-conversion (1564 nm) and 8 nm up-conversion (1544 nm) is shown in Fig. 5. The relationship between the conversion efficiency and the converted wavelength is shown in Fig. 6. A 3-dB tuning range of 23 nm is achieved with the peak conversion efficiency of −20.85 dB. Fig. 6 also shows the receiver sensitivities at BER=10−9 as the function of converted signal wavelength for one SOP component of the PolSK signal. Between the converted wavelengths from 1542 nm to 1564 nm, the variation of receiver sensitivity is less than 1 dB. Beyond this range the receiver sensitivity is largely degraded as the conversion efficiency becomes smaller. The wavelength tuning range could be made wider by employing a HNLF with flatter dispersion slope.

Converted signal wavelength (nm) Fig. 6. Output conversion efficiency and receiver sensitivity at BER=10−9 vs. converted wavelength.

The BER of both up- and down-conversion for the fixed input signal wavelength of 1552 nm are measured. Fig. 7 (a) shows the BER characteristics for 12 nm down-converted (1564 nm) and 8 nm up-converted (1544 nm) signals for one SOP component of PolSK signal. The power penalty at the error free level of BER=10−9 is found to be 1.11dB and 1.41 dB, respectively. The #117064 - $15.00 USD

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3

3 Back to Back (1552 nm) 12 nm down-converted (1564 nm) 8 nm up-converted (1544 nm)

-log (BER)

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1544 nm (20 ps/div)

-25 -24 -23 -22 -21 -20 -19 -18 -17 -16 Received power (dBm)

Fig. 7. BER characteristics of (a) one SOP component and (b) orthogonal SOP component of the PolSK signal for both 12 nm down-converted (1564 nm) and 8 nm up-converted (1544 nm) signal. Insets: demodulated eye diagrams for the converted signals.

BER results for the orthogonal SOP component of the PolSK signal at those wavelengths are shown is Fig. 7(b) and the power penalties are found to be, 0.93 dB and 1.62 dB, respectively. The power penalties measured for all these wavelengths are ranging from 0.93 dB to 1.62 dB, which shows that with in the whole tuning range similar results for BER characteristics can be obtained for other converted wavelengths. 5.

Conclusion

We have demonstrated a fiber-based polarization-insensitive and widely tunable wavelength converter for polarization shift keying signal. High polarization insensitivity and conversion performances have been achieved in our proposed scheme. In our system, it is possible to preserve the SOP of the input PolSK signal on both of the converted spectral components in FWM spectrum, which is very important regarding polarization sensitivity issues. The system is capable of operating at higher bit-rate because of the fast response of the fiber and is also applicable to the higher level of PolSK signal. A wavelength tuning range up to 23 nm of the converted signal over 1542-1565 nm has been achieved. Conversion performances also have been measured for both the up- and down-converted signal. The results show that, the proposed wavelength converter with polarization diversity and wide tuning range is suitable for PolSK transmission system in future photonic network.

#117064 - $15.00 USD

(C) 2010 OSA

