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Spectrum-sliced wavelength conversion using Four-wave mixing from a semiconductor optical amplifier David Forsyth and Michael Connelly Optical Communications Research Group, Dept. of Electronic and Computer Engineering, University of Limerick, Limerick, Ireland. e-mail:
[email protected]
Abstract: Four-wave mixing is an established and useful method of converting data from one wavelength channel to another. In this paper, we achieve four-wave mixing from an incoherent ASE source with reduction in relative intensity noise. 2005 Optical Society of America
OCIS codes: (190.4380) Nonlinear optics, four-wave mixing; (250.5980) Semiconductor optical amplifiers
1. Introduction The increasing desire for high bandwidth access services is a key factor in encouraging the development of low-cost networks. Spectrum slicing is an established method of dividing incoherent light using optical filters to generate large-scale multiwavelength light in bundles [1]. The technique usually makes use of a single, inexpensive multiwavelength broadband source, such as the amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA), with some filtering effects added. Spectrum-slicing therefore provides an attractive and low-cost alternative, with flexible wavelength selection, to the use of conventional WDM fiber optic communication systems employing multiple semiconductor lasers operating at discrete wavelengths as carriers for the different data channels. Performance-optimized WDM spectrum-sliced systems have the potential for use in future subscriber local area network fiber communication systems, requiring only low-cost equipment for applications at improving data rates. A key drawback of the technique is the large degree of inherent intensity noise, which can be reduced using the non-linear gain compression characteristics of saturated [2] semiconductor optical amplifiers (SOA). In future local access networks utilizing spectrum-slicing, there may be a requirement for all optical wavelength conversion for much the same reasons as networks employing lasers [3]. The four-wave mixing (FWM) wavelength conversion technique involves the generation of a signal replica from two inputs (cw pump and modulated probe). Efficient FWM is achieved when both inputs are co-polarized. FWM using coherent light has been studied extensively, but there are limited achievements on the subject using broadband light, except for work on FWM in dispersion-shifted fiber [4] and work using polarized ASE instead of a coherent laser as the probe source into an SOA [5]. In this work, SOA FWM is obtained using a polarized ASE pump and probe source. The FWM signal is shown to have less relative intensity noise (RIN) compared to the input probe signal because of SOA gain compression. 2. Experiment Two polarized 0.5 nm FWHM channels of spectrum-sliced light were created from a single erbium broadband ASE light source, as shown in Figure 1, amplified and input to a SOA to facilitate FWM. The probe signal was modulated with a 622 Mb/s 215–1 pseudo-random-bit-sequence. Polarization controllers PC1 and PC2 matched the state of polarization between pump and probe respectively to enable efficient FWM to occur in the SOA. An additional SOA was used as a preamplifier to detect the FWM signal. The 1.0 nm filter after the SOA ensured complete pump removal and ASE reduction prior to mixing. Figure 2 shows the output spectra obtained from the SOA, FWM 1 being the wave of interest. The SOA bias was 300 mA for maximum noise suppression [2].
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0.5 nm bpf
0.5 nm bpf PC1
EDFA
PUMP ARM SOA Erbium ASE source
1 nm bpf
EDFA MOD
polarizer
0.5 nm bpf
PROBE ARM
PC2 0.5 nm bpf
15
2 –1 PSRB at 622 Mb/s
BERT
1 nm bpf
attenuation
optical receiver P r/x dBm SOA preamplifier receiver system
Fig. 1. Schematic of set-up (bpf: bandpass filter, PC: polarization controller, BERT: bit-error-rate test set).
20
pump
Power (dBm)
10
probe
0 -10
FWM 1
FWM 2
-20 -30 -40 -50 1542
1547
1552
1557
Wavelength (nm) Fig. 2. SOA output spectra and associated FWM signals.
The pump and probe inputs to the SOA were 1.5 dBm (1550.14 nm) and – 4.1 dBm (1552.51 nm) respectively, and the conversion efficiency of the FWM signal (ratio of FWM to input probe power) was approximately –18 dB. Figure 3 shows RIN measurements taken on both the modulated probe input to the SOA and the FWM output from the SOA. These show a clear reduction in the RIN (∼16 dB) due to the strong pump putting the SOA into saturation regime [2]. Figures 4 (a) and (b) show Q factor and extinction ratio (ER) measurements taken on the FWM signal (after pre-amplification) versus the received power (Pr/x). The receiver sensitivity for Q = 6 (corresponding to BER = 10-9) is equal to – 17 dBm.
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-70 -80 probe input
-90
FWM output
RIN (dB/Hz)
-100 -110 -120 -130 -140 -150 0
500
1000
1500
2000
Frequency (MHz)
Fig. 3. Probe input and FWM signal RIN spectra.
6.5
10
6
9.5
(a)
5 4.5
8.5
4
8
3.5
7.5
3 -27
(b)
9 ER (dB)
Q factor
5.5
-22
-17
P r/x (dBm)
-12
7 -30
-25
-20 P r/x (dBm)
-15
-10
Fig. 4. Measurements taken on the FWM signal: (a) Q factor and (b) extinction ratio at 622 Mb/s.
3. Conclusion We have characterized efficient wavelength conversion of a spectrum-sliced signal using four-wave mixing in an SOA. The output FWM signal has improved RIN compared to the input signal. This technique has good potential for application in future local access spectrum-sliced networks, where wavelength conversion may be required. 4. References [1] T. Yamatoya and F. Koyama, “Noise Suppression of Spectrum-Sliced Light Using Semiconductor Optical Amplifiers,” ELECTRON COMMUN JAPAN (PART 2), 86, 28-35 (2003). [2] M. Zhao, G. Morthier and R. Baets, “Analysis and Optimization of Intensity Noise Reduction in SpectrumSliced WDM Systems Using a Saturated Semiconductor Optical Amplifier,” PHOTONIC TECH L, 14, 390392 (2002). [3] D. F. Geraghty, R. B. Lee, M. Verdiell, M. Ziari, A. Mathur and K.J. Vahala, “Wavelength conversion for WDM communication systems using four-wave mixing in semiconductor optical amplifiers,” IEEE J SEL TOP QUANT, 3, 1146 – 1155 (1997). [4] Y. S. Jang and Y. C. Chung, “Four-Wave Mixing of Incoherent Light in a Dispersion-Shifted Fiber Using a Spectrum-Sliced Fiber Amplifier Light Source,” PHOTONIC TECH L, 10, 218-220 (1998). [5] G. Hunziker, R. Paiella, K. J. Vahala and U. Koren, “Measurement of the Stimulated Carrier Lifetime in Semiconductor Optical Amplifiers by Four-Wave Mixing of Polarized ASE Noise,” PHOTONIC TECH L, 9, 907909 (1997).
