Using multimode Fabry-Perot laser without
external-injection for wavelength conversion
C.H. Yeh, C.W. Chow, C.H. Wang, F.Y. Shih, Y.F. Wu and
S. Chi
Proposed and experimentally investigated is an all-optical wavelength converter using a multi-quantum-well multi-longitudinal-mode Fabry-Perot laser diode without any external probe beam or external built-in cavity. The input wavelength can be converted at 2.5 Gbit/s by the converter and the power penalty of 3.5 dB is observed at a bit error rate of 1029. The proposed converter apparatus is simple and
cost-effective for wavelength division multiplexing network applications.
Introduction: Wavelength division multiplexing (WDM) is an import-ant technology for broadband fibre-optic transmissions, because the capacity is easy to increase by adding a new wavelength that determines the signal destination in the WDM transmission network. The wavelength-blocking problem in large networks can be overcome by using wavelength converters. The wavelength converter is also required for flexible wavelength management in fixed wavelength systems[1, 2]. Therefore, wavelength converters are the important components allow-ing transparent interoperability, contention resolution and wavelength routing for WDM applications[3]. Several techniques for the wave-length conversions have recently been reported and studied, such as using the nonlinear optical gating inside fibre loop, cross-gain modulation, cross-phase modulation and four-wave mixing techniques based on the semiconductor optical amplifier (SOA) [3, 4]. Furthermore, an all-optical wavelength converter using a low-cost Fabry-Perot laser diode (FP-LD) has also been proposed based on injection locking techniques[5, 6]for 2.5 Gbit/s signal conversion. In accordance with[3 – 5], most of the all-optical wavelength converters require an additional probe wavelength. It would increase the cost and complexity of the converter. In addition, using an external built-in cavity inside the FP-LD also complicates the laser scheme[6].
In this Letter, we propose and demonstrate a simple scheme for a wavelength converter using a multi-longitudinal-mode (MLM) FP-LD. Based on the proposed wavelength converter, the input wavelength can be converted at 2.5 Gbit/s with 3.5 dB power penalty at a BER of 1029.
Experiment and discussion: Fig. 1shows the experimental setup for the wavelength converter. The proposed wavelength converter consists of a multi-quantum-well MLM FP-LD, a polarisation controller (PC), an optical circulator (OC) and a bandpass filter (BF). In this measurement, the optical spectrum and output power are measured by an optical spec-trum analyser (OSA) with a 0.05 nm resolution and a power meter (PM).
wavelength converter FP-LD PC BF OC lconverted linput
Fig. 1 Experimental setup for wavelength converter scheme
In this proposed converter architecture, the threshold current of the MLM FP-LD used is 9 mA and the mode spacing (Dl) is 1.1 nm. The BF is used to filter the corresponding mode of FP-LD that we want for wavelength converting and to suppress the other sidemodes. The 3 dB bandwidth and average insertion loss of the BF used are around 0.4 nm and 5 dB, respectively, and the central wavelength is set at 1537.30 nm. The PC is placed between the FP-LD and OC to adjust the polarisation state of the input signal to maximise the conver-sion efficiency.Fig. 2shows the output spectra of the free-running MLM FP-LD, which is illustrated by the short dash line; and input wavelength (linput) of 1537.32 nm, which is illustrated by the solid line. The input
signal is modulated by non-return-to-zero (NRZ) data at 2.5 Gbit/s, 1.25 Gbit/s and 625 Mbit/s. Its average power is 0.6 dBm. In this experiment, the linput will pass through the proposed converter for
wavelength conversion. The bias current of the FP-LD used is 24 mA at a temperature of 258C and the central wavelength and gain-bandwidth of the FP-LD are 1542.87 nm (power ¼ 27.2 dBm) and around 20 nm, respectively. 1530 –75 –60 –45 –30 –15 15 0 FP-LD (free-runing) 1535 1540 1545 po w e r, dBm l, nm linput 1550 1555 1560
Fig. 2 Output spectra of free-running MLM FP-LD operating at 24 mA at temperature of 258C and input wavelength of 1537.32 nm with 0 dBm output power
In this experiment, we select the central output wavelength of the FD-LD (with maximum power level) to act as the converted wavelength (lconverted). Whenlinputinjects into the FP-LD, the output spectrum of
the FP-LD without passing through the BF is shown inFig. 3by the dashed line.Fig. 3 also shows the two highest peak wavelengths at 1537.32 and 1542.87 nm, after the launching oflinput. Compared with
the third higher peak wavelength inFig. 3, the sidemode suppression ratio (SMSR) of the selectedlconvertedandlinputare 23 and 18 dB.
1530 –75 –60 –45 –30 –15 15 18 dB 23 dB 0 1535 1540 1545 po w e r, dBm l, nm lconverted 1550 1555 1560 FP-LD (after-injection)
Fig. 3 Output spectra of FP-LD after external injection of 1537.32 nm (linput) without (short dashed line) and with (solid line) using BF to filter
central wavelength (lconverted) of 1542.87 nm
By comparing the optical spectra of the FP-LD before (Fig. 2) and after the launching oflinput(Fig. 3), we can see that this conversion
mechanism is different form the wavelength conversion based on pump-probe wavelength injection locking or self-injection locking. We can see that the input signal does not suppress other sidemodes of the FP-LD, but its power has been depleted to the made mode (central wavelength) of the FP-LD; that is, the power of thelconvertedis increased
owing to this mechanism. The output spectrum of the FP-LD after passing through the BF,lconverted, is also illustrated inFig. 3by the
solid line.
In this measurement, the input signallinputis modulated by an
exter-nal Mach-Zehnder (MZ) modulator at 625 Mbit/s, 1.25 Gbit/s and 2.5 Gbit/s, using an NRZ pseudorandom binary sequence (PRBS) with a pattern length of 2312 1. Fig. 4 shows the eye diagrams at
2.5 Gbit/s, 1.25 Gbit/s and 625 Mbit/s data rate of the input signal linputand the converted signallconverted. They are all clear and widely
opened under various modulated rates. To realise the wavelength con-version performance, bit-error-rate measurements are performed. Fig. 5shows that the converted operation leads to a3.5 dB power penalty at a BER of 1029under 2.5 Gbit/s NRZ modulation. It is also
worth mentioning that the wavelength conversion is not based on
pump-probe wavelength injection locking or self-injection locking. Hence, it is not sensitive to the polarisation of the input signal. We observe that polarisation dependence of the input signal is less than 2 dB at a BER of 1029. 625 Mbit/s lcon v e rt ed linput 1.25 Gbit/s 2.5 Gbit/s
Fig. 4 Output eye diagrams at 625 Mbit/s, 1.25 Gbit/s and 2.5 Gbit/s data rate, with input signallinput before and after passing through proposed
wavelength converter –26 10–10 10–9 10–8 10–7 10–6 10–5 10–4 –25 –24 –23 –22 received power, dBm BER (log) –21 –20 –19 –18 –17 –16 lconversion linput
Fig. 5 Bit error rate curve for 2.5 Gbit/s NRZ modulation with pattern length of 2312 1
Conclusions: We propose and experimentally investigate an all-optical wavelength converter using a MLM FP-LD without any external probe wavelength or external built-in cavity operating. The input wavelength linputcan be converted at 2.5 Gbit/s even if the input wavelength is
far away from the central wavelength of the FP-LD. A power penalty of 3.5 dB is observed at a bit error rate (BER) of 1029in the wavelength converted signal. Therefore, the proposed wavelength converter has the benefits of being simple and cost-effective.
#The Institution of Engineering and Technology 2009 23 November 2008
doi: 10.1049/el.2009.3365
C.H. Yeh (Information and Communications Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 31040, Taiwan, Republic of China)
E-mail: [email protected]
C.W. Chow, C.H. Wang, F.Y. Shih and S. Chi (Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan, Republic of China)
Y.F. Wu and S. Chi (Department of Electro-Optical Engineering, Yuan Ze University, Chungli, Taoyuan 32003, Taiwan, Republic of China) References
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