H.-J. Song, J.S. Lee and J.-I. Song
optical RF
fLO
intensity
out
intensity
lLO
optical LO fLO
CW in
time
SOA
optical IF fIF
signal in
intensity
time
remote base station 20 km SMF
1:1 coupler EOM
EDFA
A W G
lLO ATTN lIF
SOA-MZI SOA SOA
ATTN
lIF = 1541.35 nm TLS
EAM
LNA BERT 155.52 Mbit/s clock PRBS 215-1 data
DPSK modulator (fRF = 2.5 GHz)
data clock
DPSK demodulator (fRF = 2.5 GHz)
IF generator
Fig. 2 Experimental setup
Experiments and results: Fig. 2 shows the experiment setup for frequency upconversion of 155.52 Mbit=s DPSK data at 2.5 GHz in the remote base station (RBS). The central station consists of optical LO and optical IF generators. A DFB LD is used for generation of the optical LO signal. The output of the DFB LD, having a wavelength of 1550.12 nm, is modulated by an electro-optic modulator biased at half-wave voltage to generate a dual-mode light signal. In this experiment, the mode interval of the LO signal is set to 24 GHz because of the limited bandwidth of the electrical mixer at the receiver. The optical IF signal having 155.52 Mbit=s DPSK data at 2.5 GHz is generated by using an electroabsorption modulator (EAM). The EAM was appropriately biased to avoid distortion in the modulated signal. The wavelength of the optical IF is 1541.35 nm. The generated optical IF and LO signals are combined and amplified by an erbium-doped fibre amplifier and then transmitted over the 20 km-long SMF. The transmitted signals are demultiplexed by an arrayed waveguide. The demultiplexed LO and IF signals are coupled to the proper input ports of the SOA-MZI and the upconverted output signal is detected by the PD. Before signal upconversion, the power of the optical IF signal is adjusted to an appropriate level to ensure a linear signal mixing. PLO = -20 dBm PIF = -14 dBm
optical power, 5 dB/div.
Introduction: As the demand for broadband communication increases, higher carrier frequencies in wireless networks are utilised to accommodate the bandwidth requirement. The radio over fibre (RoF) system is a promising technology for broadband wireless access applications because of the many advantages of the optical fibre, including low transmission loss and ultra-wide bandwidth. Photonic frequency upconversion of the radio over fibre signal, which can overcome the bandwidth limitation of electrical frequency up-conversion techniques, is one of the key technologies for RoF systems. Recently, signal upconversion by using the cross-gain modulation (XGM) effect in a semiconductor optical amplifier (SOA) having high conversion efficiency was demonstrated [1]. However, it requires a high optical power to saturate the gain of an SOA. In this Letter, we propose a frequency upconversion utilising the cross-phase modulation (XPM) effect in SOAs that requires a lower optical power than that utilising the XGM effect. Frequency upconversion of the signal at 2.5 GHz with 155.52 Mbit=s differential phase shift keying (DPSK) data to 26.5 GHz is demonstrated after transmission over a 20 km-long singlemode fibre (SMF).
SOA
fLO /2 = 12 GHz PC DFB LD lLO = 1550.12 nm
An all-optical frequency upconversion of radio over fibre signal with an optical heterodyne detection that is based on the cross-phase modulation effect in a semiconductor optical amplifier is proposed. Optical IF signal that contains 155.52 Mbit=s differential phase shift keying signal at 2.5 GHz along with 24 GHz optical LO signal are transmitted over a 20 km-long singlemode fibre and then frequency upconverted to 26.5 GHz at a remote base station by using an SOA Mach-Zehnder interferometer. Error-free frequency upconversion is experimentally demonstrated.
lLO
central station LO generator
PD
All-optical frequency upconversion of radio over fibre signal with optical heterodyne detection
lLO
lIF
lIF 1540
Fig. 1 Conceptual diagram of SOA-MZI
Principle: Fig. 1 shows how the optical IF signal is upconverted to a specific RF frequency using an SOA Mach-Zehnder interferometer (SOA-MZI) based on the XPM effect in the SOA. In this upconverter, the optical IF signal that has data at lower frequency band (fIF) and the optical LO signal in a dual-mode light form having a mode interval of fLO are directed to the signal port and the CW port of the SOA-MZI, respectively. Half of the incoming optical LO signal is phase-modulated by the optical IF signal at the bottom SOA. The two outputs of the SOA interfere with each other, resulting in an intensity modulation of the optical LO signal by the optical IF signal. The output signal is then detected by a photodetector (PD) using an optical heterodyne detection scheme, generating a data signal at fIF, a single-frequency signal at fLO, and an upconverted data signal at fLO fIF. Unlike electrical upconverters, this scheme can generate extremely high upconverted frequency (fLO fIF), since the optical LO frequency (fLO) is not limited by the performance of the SOA [2]. In addition, since the phase modulation in the SOA is much more sensitive to optical power from the signal port than the gain modulation [2], efficient signal upconversion can be achieved even if the input optical power is small.
electrical power, 10 dB/div.
time RBW = 1 MHz ATTN = 10 dB
26.0
1545 l, nm a
1550
2 ¥ 155.52 MHz
calculated curve 26.5 frequency, GHz b
27.0
Fig. 3 Measured optical and electrical spectrum of upconverted signal a Optical spectrum before PD b Electrical spectrum after LNA
Figs. 3a and b show the optical spectrum of the signal before the PD and the electrical spectrum of the upconverted 155.52 Mbit=s DPSK signal at 26.5 GHz after the LNA, respectively. As can be seen in Fig. 3a, the optical power at lIF is much lower than that at lLO because
ELECTRONICS LETTERS 4th March 2004 Vol. 40 No. 5
the SOA-MZI is operated in a contra-directional configuration. Also, as can be seen in Fig. 3b, an electrical spectrum clearly representing the 155.52 Mbit=s DPSK signal is observed at the centre frequency of 26.5 GHz. Fig. 4 shows the measured bit error rate (BER) and eye diagram of the demodulated data. A BER less than 10 9 is achieved at the received optical LO power of 19.5 dBm. The recovered clock and the eye diagram at this error-free point are shown in Fig. 4b 0
Acknowledgments: This work was supported, in part, by ITRCCHOAN, BK21, and ERC-UFON programs.
PIF = -14 dBm 20 km-long SMF
# IEE 2004 Electronics Letters online no: 20040219 doi: 10.1049/el:20040219
-3
BER, log
Conclusion: All-optical frequency upconversion of the RoF signal by using the SOA-MZI with the optical heterodyne detection has been proposed and experimentally demonstrated. The optical IF signal having 155.52 Mbit=s DPSK data is transmitted over the 20 kmlong SMF and all-optically upconverted to 26.5 GHz at the RBS. A BER of less than 10 9 is achieved at the received optical LO power of 19.5 dBm.
3 January 2004
H.-J. Song, J.S. Lee and J.-I. Song (Department of Information and Communications, Kwangju Institute of Science and Technology, 1 Oryong-dong, Puk-gu, Gwangju, 500-712, Korea)
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E-mail:
[email protected] References 1
-9
2 -12 -30
-25 -20 -15 received optical LO power, dBm a
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3
Seo, Y.-K., Choi, C.-S., and Choi, W.-Y.: ‘All-optical signal upconversion for radio-on-fiber applications using cross-gain modulation in semiconductor optical amplifier’, IEEE Photonics Technol. Lett., 2002, 14, (10), pp. 1448–1450 Durhuus, T., Mikkelsen, B., Joergensen, C., Danielsen, S.L., and Stubkjaer, K.E.: ‘All-optical wavelength conversion by semiconductor optical amplifier’, J. Lightwave Technol., 1996, 14, (6), pp. 942–954 Song, H.-J., Lee, J.S., and Song, J.-I.: ‘Linearity performance of a signal up-conversion by using cross-phase-modulation in all-optical SOA-MZI wavelength converter’. Proc. Annual Meeting of the IEEE LEOS, October 2003, Vol. 2, pp. 1007–1008
1
2
Freq (C1) Ch 1
1.0 V
155.3 MHz low signal amplitude W
Ch 2
100 mV W
M 2.5 ns 10.0 GS/s IT 2.5 ps/pt A Ch1 20.0 mV
b
Fig. 4 Measured BER and eye diagram a Measured BER b Eye diagram
ELECTRONICS LETTERS
4th March 2004
Vol. 40 No. 5
