A widely tunable wavelength converter based on nonlinear polarization rotation in a

carbon-nanotube-deposited D-shaped fiber

K. K. Chow*1

, S. Yamashita1, and Y. W. Song

2

1Department of Electrical Engineering and Information Systems, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

2Center for Energy Materials Research, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea *Corresponding author: kkchow@sagnac.t.u-tokyo.ac.jp

Abstract: We demonstrate widely tunable wavelength conversion based on cross-phase modulation induced nonlinear polarization rotation in a carbon nanotubes (CNTs) deposited D-shaped fiber. A 5-centimeter-long CNT-deposited D-shaped fiber is used as the nonlinear medium for wavelength conversion of a 10 Gb/s non-return-to-zero signal. Wavelength tunable converted signal over 40 nm is obtained with around 2.5-dB power penalty in the bit-error-rate measurements.

2009 Optical Society of America

OCIS codes: (060.4370) Nonlinear optics, fibers; (070.4340) Nonlinear optical signal processing

References and links

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2. A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu,Y. H. Lee, S. G. Kim, D. T. Colbert, G. Scuseria, D. Tománek, J. E. Fischer, and R. E. Smalley, “Crystalline ropes of metallic carbon nanotubes,” Science 273, 483–487 (1996).

3. Ph. Avouris, M. Freitag, and V. Perebeinos, “Carbon Nanotube Optics and Optoelectronics,” Nat. Phton. 2, 341-350 (2008).

4. S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, “Ultrafast fiber pulsed lasers incorporating carbon nanotubes,” IEEE J Select. Top. Quantum Electron. 10, 137-146 (2004).

5. S. Yamashita, Y. Inoue, S. Maruyama, Y. Murakami, H. Yaguchi, M. Jablonski, and S. Y. Set, “Saturable absorbers incorporating carbon nanotubes directly synthesized onto substrates/fibers and their applications to mode-locked fiber lasers,” Opt. Lett. 29, 1581-1583 (2004).

6. Y. C. Chen, N. R. Raravikar, L. S. Schadler, P. M. Ajayan, Y. P. Zhao, T. M. Lu, G. C. Wang, and X. C. Zhang, “Ultrafast optical switching properties of single-walled carbon nanotube polymer composites at 1.55

µm,” App. Phys. Lett. 81, 975-977 (2002).

7. S. Tatsuura, M. Furuki, Y. Sato, I. Iwasa, M. Tian, and H. Mitsu, “Semiconductor carbon nanotubes as ultrafast switching materials for optical telecommunications,” Adv. Mater. 15, 534–537 (2003).

8. S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, “A noise suppressing satiable absorber at 1550 nm based on carbon nanotube technology,” in Proc. OFC 2003, paper FL2, Atlanta, GA, USA (2003).

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10. Y. W. Song, S. Y. Set, and S. Yamashita, “Novel Kerr shutter using carbon nanotubes deposited onto a 5-cm D-shaped fiber,” in Proc. CLEO 2006, paper CMA4, Long Beach, CA, USA (2006).

11. K. Kashiwagi, S. Yamashita, H. Yaguchi, C. S. Goh, and S. Y. Set, “All optical switching using carbon nanotubes loaded planar waveguide,” in Proc. CLEO 2006, paper CMA5, Long Beach, CA, USA (2006).

12. S. J. B. Yoo, “Wavelength conversion technologies for WDM network applications,” J. Lightwave Technol. 14, 955-966 (1996).

13. G. P. Agrawal, Nonlinear Fiber Optics, 3rd ed. (New York: Academic, 210-216, 2001).

1. Introduction

Owing to its unique optical properties, carbon nanotube (CNT) technology has recently drawn much research attention [1-3]. Applications and studies of CNT including mode-locked lasers [4, 5], ultra-fast optical response [6, 7], and optical noise suppression [8] had been

(C) 2009 OSA 27 April 2009 / Vol. 17, No. 9 / OPTICS EXPRESS 7664#106726 - $15.00 USD Received 22 Jan 2009; revised 22 Apr 2009; accepted 23 Apr 2009; published 24 Apr 2009

investigated. In particular, CNTs can be employed as a fiber-based nonlinear medium due to their ultra-high nonlinear coefficient [4, 9]. Previously, our group had reported optical switching based on a Kerr shutter using a few centimeters of CNT-deposited D-shaped fiber [10] as well as optical loop mirror incorporated with CNT-loaded planar waveguide [11], which are significant for various system applications especially wavelength conversion. In a wavelength-routed optical network, wavelength conversion plays a major role in providing the wavelength flexibility and avoiding wavelength blocking [12]. For fiber-based wavelength converter the nonlinear medium usually favors compact, high nonlinearity, and splicable to standard single-mode fiber (SMF). In this paper, we further investigate the optical switching properties of the CNTs and experimentally demonstrate tunable wavelength conversion using a short piece of CNT-deposited D-shaped fiber for the first time. With the cross-phase modulation (XPM) induced nonlinear polarization rotation effect in the fiber, wavelength conversion is obtained with a wavelength tuning range over 40 nm. Also, the D-shaped fiber is fully spliced to standard SMF without free space coupling so that the converter can have a stable operation. A power penalty of 2.5 dB is measured for 10 Gb/s non-return-to-zero (NRZ) signal in the bit-error-rate (BER) measurements.

2. Design and fabrication of CNT-deposited D-shaped fiber for nonlinear applications

The CNT-deposited D-shaped fiber works with the interaction between CNTs and the evanescent field of propagating light in the fiber. Figure 1(a) shows the schematic illustration of the D-shaped fiber and the corresponding SEM image of the deposited CNTs. In our experiment, the CNTs are made by a bulk production method called high-pressure CO conversion. Since the isolation of individual CNT is critical to obtain the maximum nonlinearity, the diameter and the diameter distribution of the CNTs are well controlled. The CNTs are then dispersed in a solvent without any significant agglomeration, and only the homogeneous part is taken after the centrifugal separation before spraying on the D-shaped fiber. The D-shaped fiber is prepared by polishing a segment of standard SMF held by a V-grooved block. The fiber together with the V-grooved block is polished with 4 steps in order to ensure the non-cracked and smooth surface of the D-shaped area, thus minimize the beam scattering through the polished face. The insertion loss is monitored during polishing therefore the amount of optical power leakage through the polished face can also be monitored. The CNTs are then deposited on the D-shaped surface by spray method, and the fiber sample is finalized by adding a protection layer above the deposited CNTs. Figure 1(b) shows the photo of the finished device with the fiber pigtails. The overall insertion loss of the CNT-deposited D-shaped fiber adopted in our experiment is 12 dB with a CNT-light interaction length of around 5 cm. Note that the V-grooved block is a few cm longer as a buffer for protecting the junctions between the D-shaped region and the SMF pigtails. Since the CNTs are randomly sprayed on the D-shaped area, the fiber is polarization sensitive with around 4-dB power variation to the polarization-dependent resonance of individual CNTs. It is worth noting that since the D-shaped fiber is made by standard SMF, the splicing loss of the device to sub-systems or laser cavities can be nearly neglected.

Fig. 1. (a) Schematic illustration of D-shaped fiber with carbon nanotubes (CNTs) deposited (SEM image: magnification 50K) and (b) photo showing the CNT-deposited D-shaped fiber in a V-groove block with single-mode fiber pigtails.

(b)

Fiber Core

D-shaped Fiber

CNT Layer

Polished Surface

(a)

(C) 2009 OSA 27 April 2009 / Vol. 17, No. 9 / OPTICS EXPRESS 7665#106726 - $15.00 USD Received 22 Jan 2009; revised 22 Apr 2009; accepted 23 Apr 2009; published 24 Apr 2009

3. Experiment and results on wavelength conversion

The experimental setup on wavelength conversion using CNT-deposited D-shaped fiber is shown in Fig. 2. In this session, the modulator is initially turned off and the continuous-wave (cw) output of the external cavity laser (ECL1) serves as the pump for nonlinear effect. The cw pump is then amplified by an erbium-doped fiber amplifier (EDFA) followed by an optical bandpass filter for eliminating ASE from the EDFA. The amplified pump is then combined with a cw probe light from the ECL2 using a 3-dB coupler. The launched optical power of the pump and the probe light into the CNT-deposited D-shaped fiber are estimated to be 21 dBm and -3 dBm, respectively. The combined light then undergoes XPM induced nonlinear polarization rotation in the CNT-deposited D-shaped fiber and the probe light is modulated and filtered out by the polarizer. The switching out of the probe light is originated from the relative states of polarization (SOP) between the probe light and the polarizer. When the SOP of the probe light and the polarizer are orthogonal to each other and the pump light is aligned

to have a SOP 45° with respect to that of the polarizer, with appropriate pump power level the birefringence of the fiber is modulated and the probe light will be switched out (on state) or blocked (off state) by the polarizer corresponding to the input signal. The transmittivity of the probe light in such configuration can be expressed as [13]:

(1)

where T is the transmittivity of the probe light and ∆φ is the phase different between the pump

and the probe light. T becomes 100% when ∆φ = π or an odd multiple of π where the probe

light is blocked completely when ∆φ equals to even multiples of π. In our experiment, the SOP of the pump and the probe light are optimized by the polarization controllers in order to

obtain π phase shift and maximize the switching efficiency. Finally, we use an optical bandpass filter to completely filter out the pump light and wavelength conversion is obtained.

CNT Deposited

D-shaped Fiber

Receiver/

BERT

EDFA

`

3 dB

coupler

ECL2

Filter

ECL1

10 Gb/s

Pattern

Generator

MZ

Modulator Polarizer

Filter

Fig. 2. Experimental setup on wavelength conversion using CNT-deposited D-shaped fiber. ECL: external cavity laser; EDFA: erbium-doped fiber amplifier; BERT: bit-error rate test set.

Fig. 3. Output spectra obtained after the CNT-deposited D-shaped fiber and the polarizer with (a) pump turned off and (b) pump turned on.

1540 1545 1550 1555 1560

Wavelength (nm)

Spectr

al In

tensity (

10 d

B/d

iv)

(a)

Probe

1540 1545 1550 1555 1560

Wavelength (nm)

Spectr

al In

tensity (

10 d

B/d

iv)

(b)

Probe

Pump

∆=

2sin2 φ

T

(C) 2009 OSA 27 April 2009 / Vol. 17, No. 9 / OPTICS EXPRESS 7666#106726 - $15.00 USD Received 22 Jan 2009; revised 22 Apr 2009; accepted 23 Apr 2009; published 24 Apr 2009

Figure 3 shows the spectra obtained after the polarizer. In our experiment the pump and the probe light are set at 1555.0 nm and 1545.0 nm, respectively. Figure 3(a) depicts the spectrum with the pump light turned off. Note that the probe light power is suppressed to minimum by adjusting its SOP orthogonal to that of the polarizer by the polarization controllers. With the input pump turned on and polarization appropriately adjusted, the probe light experiences XPM induced nonlinear polarization rotation and its SOP becomes parallel with that of the polarizer, leading to the power increase of the probe light output as shown in Fig. 3(b). From the spectra it is observed that the maximum extinction ratio obtained between on and off state is 16 dB.

0

0.2

0.4

0.6

0.8

1

0 25 50 75 100 125 150 175 200 225 250

Input pump power (mW)

Tra

nsm

ittivity o

f p

robe (

100%

)

Experimental results

Theoretical fitting

Fig. 4. Plot of probe light transmittivity against input pump power.

The normalized transmittivity of the probe light against input pump power is plotted in Fig. 4. A theoretical fitting of Eq. 1 is also depicted. The results show that with the launched

pump power of around 125 mW π phase shift between the pump and the probe light is obtained and the characteristics is consistent with the theoretical fitting. The pump power required for 100% probe light transmission can be expressed as [13]:

(2)

where Aeff is the effective core area, n2 is the Kerr coefficient, and L is the fiber length. With the launched pump power of around 125 mW and fiber length of 5 cm, by Eq. 2 the estimated

nonlinear coefficient of our sample can be as high as 5.02×105 W

-1km

-1, which is attractive for

numerous nonlinear applications. Note that this estimation is concerning the effective nonlinear coefficient of the fabricated fiber device instead of the nonlinear coefficient of the deposited CNTs only. It is worth noting that four-wave mixing effect is also observed in another experiment using the same piece of CNT-deposited D-shaped fiber, which indicates that phase matching between the pump and the probe light is able to obtain. From Eq. 2, it is suggested that adopting a smaller core fiber such as dispersion-shifted fiber or dispersion compensating fiber for fabricating the D-shaped fiber can in principle decrease the required pump power. However, it is a trade off concerning the mechanical difficulties of precise polishing of the D-shaped fiber.

Ln

AP

eff

22

λ=

(C) 2009 OSA 27 April 2009 / Vol. 17, No. 9 / OPTICS EXPRESS 7667#106726 - $15.00 USD Received 22 Jan 2009; revised 22 Apr 2009; accepted 23 Apr 2009; published 24 Apr 2009

Fig. 5. Optical spectra obtained (a) after the CNT-deposited D-shaped fiber and the polarizer with 10 Gb/s NRZ input signal; and corresponding close up of (b) input signal and (c) converted signal.

Fig. 6. Plot of bit-error rate against received optical power. Inset (upper) and (lower) show the 10 Gb/s eye-diagrams of input and converted signal, respectively.

In order to investigate the system performance of the wavelength converter, the modulator is turned on and the ECL1 output is modulated to be a 2

31-1 bits pseudorandom

NRZ signal at 10 Gb/s as shown in Fig. 2. Similar to the previous case, the 10 Gb/s signal is amplified and combined with the cw light from the ECL2 and launched into the CNT-deposited D-shaped fiber. Figure 5(a) shows the output spectrum obtained after the polarizer where Fig. 5(b) and 5(c) depict the magnified spectra of the input signal and the converted signal, respectively. It is observed that the spectrum of the converted light is spectrally-broadened and modulated to be a 10 Gb/s signal. Note that no intensity modulation of the converted wavelength is observed before the light passing through the polarizer. The performance of the wavelength converter is further investigated by performing BER measurements. Figure 6 plots the output BER against the received optical power with the inset showing the 10 Gb/s eye diagrams of the input signal and the converted signal. In this measurement the wavelengths of S and C are the same with those shown in Fig. 5(a). The Fig. shows the results of a 10-nm down-conversion and the power penalty is measured to be 2.5 dB at 10

-9 BER level. One possible source of the power penalty is believed to be originated

from the defect of the hand-polished D-shaped fiber surface. It is believed that a better performance can be obtained with the fiber polished by precise machining.

1554.6 1554.8 1555 1555.2 1555.4

Wavelength (nm)

Sp

ectr

al In

ten

sity (

10

dB

/div

)

1544.5 1544.7 1544.9 1545.1 1545.3 1545.5

Wavelength (nm)

Sp

ectr

al In

tensity (

10

dB

/div

)

(b)

(c)

1540 1545 1550 1555 1560

Wavelength (nm)

Spectr

al In

tensity (

10 d

B/d

iv)

(a) Input signal (S)

Converted signal (C)

-10

-9

-8

-7

-6

-5

-4

-3

-42 -40 -38 -36 -34 -32 -30 -28 -26 -24 -22 -20

Received Power (dBm)

log (

BE

R)

back-to-back

converted signal

(C) 2009 OSA 27 April 2009 / Vol. 17, No. 9 / OPTICS EXPRESS 7668#106726 - $15.00 USD Received 22 Jan 2009; revised 22 Apr 2009; accepted 23 Apr 2009; published 24 Apr 2009

0

5

10

15

20

25

1520 1530 1540 1550 1560 1570

Wavelength (nm)

Extinction R

atio (

dB

)0

1

2

3

4

5

6

7

8

Pow

er

Penalty (

dB

)

Fig. 7. Plot of extinction ratio and power penalty at 10-9 BER level against different converted wavelength.

Figure 7 further plots the output extinction ratio between on and off state against different converted wavelength with the input signal fixed at 1555.0 nm. Note that the conversion range has covered all the C-band over 40 nm while maintaining over 14 dB extinction ratio. The Fig. also depicts the power penalty at 10

-9 BER level against different

converted wavelength in the 10 Gb/s BER measurements. It is observed that the power penalty is kept at around 2.5 dB for the whole tuning range with less than 1 dB variation. Note that the large increase of the power penalty approaching the shorter wavelength side is originated from the converted wavelength being outside the gain bandwidth of the EDFA for BER measurements. The wavelength tuning range is actually limited by the performance of the ECL adopted in the experiment and it is expected that wider conversion range with similar BER performance can be obtained with suitable tunable laser source.

4. Conclusion

A widely tunable wavelength converter has been experimentally demonstrated using cross-phase modulation induced nonlinear polarization rotation in a carbon nanotubes deposited D-shaped fiber. The converted signal has over 14 dB extinction ratio all over C-band. Also, a power penalty of 2.5 dB with less than 1 dB variation for the whole tuning range is measured in the 10 Gb/s BER measurements. It is expected if the polishing of the D-shaped fiber can be further improved, lower power penalty can be obtained. Owing to the ultra-fast response properties of the CNTs, future work on high repetition rate RZ signal pulse processing is also expected. The results show that such compact fiber device with a length of 5 cm is promising for wavelength conversion applications in all-optical networks and other nonlinear applications.

Acknowledgment

This work was supported by Strategic Information and Communications R&D Promotion Programme (SCOPE) of The Ministry of Internal Affairs and Communications (MIC).

(C) 2009 OSA 27 April 2009 / Vol. 17, No. 9 / OPTICS EXPRESS 7669#106726 - $15.00 USD Received 22 Jan 2009; revised 22 Apr 2009; accepted 23 Apr 2009; published 24 Apr 2009