F3-6: Multiple RF-carrier Generation Using the

MWP 2005

F3-6

Multiple RF-carrier Generation Using the Wavelength-dependent Stokes Shift and Selective Amplification of Stimulated Brillouin Scattering Chul Soo Park1, Choong Keun Oh1, Chung Ghiu Lee2, and Chang-Soo Park1 1

Department of Information and Communications, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea 2

Photonics System Lab., Korea Photonics Technology Institute (KOPTI), Gwangju 500-460, Republic of Korea

Abstract — We present an optical method that can simultaneously generate multiple RF-carriers. The generated RF-carriers meet the requirements of channel separation in fixed wireless services. The proposed method utilizes the wavelength-dependent Stokes shift and selective amplification of stimulated Brillouin scattering in an optical fiber. Four RF-carriers at 10.851, 10.831, 10.811, and 10.791 GHz, which have the narrow linewidth and the 20-MHz separation, are generated using four optical sources spaced 2.78 nm apart. Index Terms — RF-carrier generation, stimulated Brillouin scattering, selective amplification, wavelengthdependent Stokes shift.

microwave signal sources in the system will become a burden. Therefore, the optical generation of multiple RFcarriers that meets the requirements of channel separation without using several expensive signal sources is required for the flexibility of the future BWA network. In this paper, we propose an optical method that can simultaneously generate multiple RF-carriers, which meets the requirements of channel separation, through the wavelength-dependent Stokes shift and selective amplification of stimulate Brillouin scattering (SBS) in an optical fiber.

I. INTRODUCTION

II. PRINCIPLE OF MULTIPLE RF-CARRIER GENERATION USING STIMULATED BRILLOUIN SCATTERING

In fixed broadband wireless access (BWA) network, multiple radio-frequency (RF) carriers are assigned to each channel by adopting the frequency-arrangement plan [1]. For BWA network such as local multipoint distribution service (LMDS), the different carrier frequencies that follow the channel separation are assigned to each sector in a cell. Also, for the fixed service that uses 10 GHz band, multiple RF-carriers are simultaneously used according to the number of channels adopted in the system [1], [2]. Though, the division of the frequency band into several channels is one of the methods that is devised to fully utilize the given frequency spectrum, it still suffers the bandwidth limitation in each channel due to the fixed channel separation in the electrical system. To overcome the bandwidth limitations imposed on the electrical system, several optical methods that generate RF-carriers and distributing it through optical fiber have been proposed [3]−[5]. Apart from their broad bandwidth and low losses, previous results avoid the dispersions occurred when the microwave signal passes through the optical fiber by generating only two optical waves that have identical phase variations. However, for the system application that adopts several channels, there still requires multiple RF signal sources used for each channel [6]. Considering the high number of channels in the system, constructing multiple

ISBN 89-950043-3-9 93560

2005 KICS

SBS is a nonlinear process that degrades the performance of the optical transmission system due to the low threshold power [7]. However, SBS have beneficial applications such as frequency-selective amplifications originated from the narrow bandwidth of the Brillouin gain spectrum. Moreover, several applications such as carrier generation [8] and gyroscope [9] using the Brillouin fiber laser have been reported. The proposed method which can simultaneously generate multiple RFcarriers is based on the wavelength-dependent Stokes shift and selective amplification of SBS. The refractive index gratings caused by the acoustic waves, generated from the pump waves, induce SBS. And, the backward Stokes wave is generated at the lower frequencies relative to the pump wave as follows [10]; ν stokes ,λ p =

2nυ A λp

(1)

where υ A and n are the velocity of the acoustic wave and the refractive index of the core in an optical fiber, respectively. And, λp is the pump wavelength. From (1), Stokes frequency shift (νstokes,λp) is inversely related to the pump wavelength (λp), i.e. the shorter Stokes frequency shift for the longer pump wavelength. So, if the wavelength-dependent Stokes shift has a linear changes correspond to the pump wavelength changes, the

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generation of the Brillouin gain spectrum and modulated optical spectrum, as depicted in Fig. 1(a) and (b), the path of the optical sources is divided into two ways. One path is directed into the electro-optic modulator (EOM). The other is directed into the erbium-doped fiber amplifier (EDFA) used to provide sufficient power for the generation of the Brillouin gain spectrum. The average optical power injected into the SMF for the Brillouin gain generation is set to 10.0 dBm. The EOM is driven by the amplified mixed-signal of 10.831 GHz and 20 MHz. A single-mode fiber (SMF) that has a 20-km span is used as a nonlinear medium. Optical isolator and circulator are used for the selective amplification. Finally, the selectively amplified signals with their own pump waves are detected by photodiode (Discovery DSC10ER) and RF spectrum analyzer (Anritsu MS2668C). The DC block capacitor is used before the RF spectrum analyzer.

deviations of the Stokes frequency shift originated from the wavelength spacing between pump waves can be simply extracted. Fig. 1 shows the conceptual process of multiple RFcarrier generation method that is based on the wavelength-dependent Stokes shift and selective amplification of SBS. As shown in Fig 1(a), the Brillouin gain spectra correspond to each pump wave have different quantities of the Stokes shift, i.e. νstokes,λp-2, … , νstokes,λp+1. If the wavelength-dependent Stokes shift follows the relation of ‘s’-Hz/nm, the frequency difference between the generated Stokes shifts will be ‘w×s’-Hz for the ‘w’-nm spaced pump waves. When the modulated optical spectrum is co-propagating with the Stokes waves (Brillouin gain spectrum) as depicted in Fig. 1(b), optical sidebands that are overlapped within the Brillouin gain spectra (dashed line) will be selectively amplified. But, the amplitude of optical sidebands that are not located inside of the Brillouin gain spectra will be relatively reduced. Finally, the selectively amplified sidebands with their own pump waves are detected as depicted in Fig. 1(c). When those are beaten by the photodiode, multiple RFcarriers will be generated while the generated RF-carriers have different frequencies if the frequency separation of the sidebands is exactly matched to the deviations of the Stokes frequency shift considering the linear wavelengthdependent Stokes shift.

EDFA

OC

PD

RF-SA DC-block Optical sources

Iso PC coupler

EOM

Iso

SMF

RF-Amp Mixer

20MHz

10.831GHz

νstokes,λ p-2

νstokes,λ p-1

νstokes,λ p

νstokes,λ p+1

Fig. 2. Experimental setup (EDFA: Erbium-doped fiber amplifier; EOM: Electro-optic modulator; Iso: Optical Isolator; OC: Optical circulator; PC: Polarization controller; PD: Photodiode; RF-SA: RF spectrum analyzer; RF-Amp: RFamplifier; SMF: Single mode fiber).

(a)

νp-2

νp-1

νp

νp+1 Opt. freq.

(b)

w-nm

Opt. freq.

IV. RESULTS AND DISCUSSION

(c)

Fig. 3 shows the wavelength-dependent Stokes shift measured for three different optical fibers. The Stokes shifts at the pump wavelength of 1553.17 nm (νp) were 10.831, 10.511, and 9.451 GHz for an SMF (20 km), DSF (17 km), and DCF (10 km), respectively. The causes of such difference among optical fibers are related with the acoustic mode characteristics of fiber geometries [11]. Also, the Stokes shift has the slope of −7.2 MHz/nm for the pump wavelength range of 1530 to 1560 nm, which is same as previously reported [12]. The measured gain of the Brillouin amplifier was 11 dB for an average pump power of 10.0 dBm. Considering the linear slope of the wavelengthdependent Stokes shift, −7.2 MHz/nm, the center frequencies of the Brillouin gain spectrum have the 20MHz spacings for the pump waves spaced amount of 2.78 nm. In our experiment, the generated Brillouin gain spectrum for the pump wave at 1550.39 nm (νp+1) was shifted to the 20-MHz higher frequency than that of the

Opt. freq.

Fig. 1. The principle of multiple RF-carrier generation (a) the wavelength-dependent Stokes shift and its Brillouin gain spectrum (dashed line), (b) the modulated optical spectrum with Brillouin gain spectrum, and (c) the selectively amplified sidebands with pump waves.

III. EXPERIMENTAL SETUP Fig. 2 shows the experimental setup used for the generation of multiple RF-carriers. Four optical sources at 1550.39 (Newfocus 6328), 1553.17 (Alcatel DFB laser diode), 1555.95 (Ando AQ4321D), and 1558.73 nm (Newfocus 6328), separated 2.78 nm apart, are used for the generation of 20-MHz separated RF-carriers. Optical isolators are used in front of each optical source to prevent interference caused by reflections. For the

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Here, the modulated optical spectra using mixed electrical signals of 10.831 GHz and 20 MHz, as depicted in Fig. 1(b), were generated using EOM with a harmonic mixer. Due to the backward propagating characteristic of SBS, the Brillouin gain spectra were copropagated with the modulated optical signals. Thereby, it gives a gain to specific sidebands that is located within the Brillouin gain spectrum. Consequently, as shown in Fig. 4(a), there were pump waves with selectively amplified optical sidebands that were amplified by each Brillouin gain. And, each Brillouin gain spectrum corresponding to the pump wavelengths of 2.78-nm separations has the frequency deviations of 20 MHz relatively to each neighboring spectrum. Since the modulated optical sidebands have 20-MHz spacing, each Brillouin gain spectrum select different sidebands corresponding to the pump wavelength. So, after beating through the photodiode, four RF-carriers were simultaneously generated which had the 20-MHz separations as shown in Fig. 4(b). As can be expected, the proposed method also can avoid the dispersion since there are four pairs of the pump wave and selectively amplified sideband. Also, considering the optical separations, the bandwidth expansions can be achieved if optical spectrum is detected by the photodiode after filtering out for specific pump waves (specific channel frequency) using optical filters.

pump wave at 1553.17 nm (νp). On the other hand, the Brillouin gain spectra for pump waves at 1555.95 (νp-1) and 1558.73 nm (νp-2) had lower Stokes frequency shift compared with that of the pump wave at 1553.17 nm (νp) to the amount of 20 and 40 MHz, respectively. Moreover, the measured 3-dB bandwidth of the Brillouin gain spectrum was less than 15 MHz, which was well matched to the previous report [10], [11]. 11.2

Stokes shift (GHz)

11.0

SMF 20km

10.8

DSF 17km

10.6 10.4 10.2

-7.2 MHz/nm

10.0 9.8

DCF 10km

9.6 9.4 9.2

1530

1535

1540

1545

1550

1555

1560

Pump wavelength (nm) Fig. 3. The wavelength-dependent Stokes shift for three different optical fibers (SMF: single-mode fiber; DSF: dispersion shifted fiber; DCF: dispersion compensation fiber). -5 -10

Amplitude (dBm)

-15 -20 -25 -30 -35 -40 -45 -50 -55 -60 -65

(a)

(b)

(c)

(d)

1550 1551 1552 1553 1554 1555 1556 1557 1558 1559

Wavelength (nm)

(a)

Fig. 5. Measured RF-carriers at (a) 10.791 GHz, (b) 10.811 GHz, (c) 10.831 GHz, and (d) 10.851 GHz with the 500 kHz span and 100 Hz resolution bandwidth in the RF-spectrum analyzer.

Fig. 5 shows RF-carrier spectra with the condition of the 500-kHz span and the 100-Hz resolution bandwidth (RBW) in a RF spectrum analyzer. Each RF-carrier generated through the wavelength-dependent Stokes shift and selective amplification of SBS has different noise characteristics added near the carriers depending on the characteristic of sources used, though, generated multiple

(b)

Fig. 4. Measured spectrum of (a) the selectively amplified optical sidebands with pump waves, and (b) the generated four RF-carriers after beating.

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Technol. Lett., vol. 16, no. 1, pp. 254-256, January 2004. [7] A. R. Chraplyvy, “Limitations on Lightwave communications imposed by optical-fiber nonlinearities,” J. Lightwave Technol., vol. 8, no. 10, pp. 1548-1557, October 1990. [8] D. Culverhouse, K. Kalli, D. A. Jackson, “Stimulated Brillouin scattering ring resonator laser for SBS gain studies and microwave generation,” Electron. Lett., vol. 27, no. 22, pp. 2033-2035, October 1991. [9] F. Zarinetchi, S. P. Smith, and S. Ezekiel, “Stimulated Brillouin fiber-optic laser gyroscope,” Opt. Lett., vol. 16, no. 4, pp. 229-231, February 1991. [10] G. P. Agrawal, Nonlinear fiber optics, New York: Academic, 1995. [11] A. Yeniay, J. M. Delavaux, J. Toulouse, “Spontaneous and stimulated Brillouin scattering gain spectra in optical fibers,” J. Lightwave Technol., vol. 20, no. 8, pp. 1548-1557, August 2002. [12] G. Y. Lyu, S. S. Lee, D. H. Lee, C. S. Park, M. H. Kang, and K. Cho, “Simultaneous measurement of multichannel laser linewidths and spacing by use of stimulated Brillouin scattering in optical fiber,” Opt. Lett., vol. 23, no. 11, pp. 873-875, June 1998. [13] M. Nikles, L. Thevenaz, and P. A. Robert, “Brillouin gain spectrum characterization in single-mode optical fibers,” J. Lightwave Technol., vol. 15, no. 10, pp. 1842-1851, October 1997.

RF-carriers have the narrow linewidth considering the 500 kHz span. The maximum number of multiple RF-carriers that can be simultaneously generated is limited by the operating wavelength-bandwidth of the EOM and EDFA used to generate Brillouin gain spectra, and the 3-dB bandwidth of the Brillouin gain spectrum. If the operating wavelength of 30 nm with the Brillouin gain bandwidth of 15 MHz is considered, the proposed method can simultaneously generate totally 10 RF-carriers spaced amount of 20 MHz at the 10 GHz band for an SMF. In our experiment, however, we demonstrated the generation of four simultaneous RF-carriers that have 20MHz separations using the wavelength-dependent Stokes shift and selective amplification of SBS with four optical sources. Finally, for the amplitude stability of the generated RF-carrier, there requires temperature compensations since the Brillouin gain spectrum has the temperature dependent drifts of 1.36 MHz/°C [13].

V. CONCLUSIONS We presented an optical method that can simultaneously generate multiple RF-carriers using the wavelength-dependent Stokes shift and selective amplification of SBS in an optical fiber. Four RF-carriers of 20 MHz spacing were generated using four optical sources spaced 2.78 nm apart. The frequency spacing of the generated RF-carriers can be controlled by properly choosing the wavelengths. And, it has RF gains from the selective amplification REFERENCES [1] ITU Rec. F.746-7, “Radio-frequency arrangements for fixed service systems,” Geneva, Switzerland, 2003. [2] B. Fong, N. Ansari, A. C. M. Fong, G. Y. Hong, and B. R. Predrag, “On the scalability of fixed broadband wireless access network deployment,” IEEE Commun. Mag., vol. 42, no. 9, pp. s12-s18, September. 2004. [3] G. H. Smith, D. Novak, and Z. Ahmed, “Technique for optical SSB generation to overcome dispersion penalties in fibre-radio systems,” Electron. Lett., vol. 33, no. 1, pp. 72-74, January 1997. [4] J. J. O’Reilly, P. M. Lane, R. Heidemann, and R. Hofstetter, “Optical generation of very narrow linewidth millimeter wave signals,” Electron. Lett., vol. 28, no. 25, pp. 2309-2311, December 1992. [5] L. A. Johansson and A. J. Seeds, “Millimeter-wave modulated optical signal generation with high spectral purity and wide-locking bandwidth using a fiber-integrated optical injection phase-lock loop,” IEEE Photon. Technol. Lett., vol. 12, no. 6, pp. 690692, June 2000. [6] C. M. A. Nirmalathas, C. Lim, M. Attygalle, D. Novak, B. Ashton, L. Poladian, W. S. T. Rowe, T. Wang, and J. A. Besley, “FBG-Based optical interface to support a multisector antenna in a spectrally efficient fiber radio system,” IEEE Photon.

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