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We experimentally demonstrate all-fiber optical Manchester code generation at 10 Gbit/s using nonlinear polarization rotation in a single 1km highly-nonlinear fiber with the nonlinearity of 20.4 W-1km-1. 33-dB extinction ratio is achieved in a -4-dBm CW dummy channel by co-injecting orthogonally aligned two 8-dBm pumps into the Kerr medium. Our encoder functions with 10 Gbit/s NRZ data stream and 10 GHz optical clock as the pumps. We present the resultant waveform as well as the optical spectrum of the Manchester-coded output.
© 2006 Optical Society of America
1. Introduction
In the high-speed transparent optical network, employing a suitable modulation format benefits the robust signal transmission against various optical signal degradation effects such as dispersions in the channel as well as nonlinearity-induced inter-channel crosstalks in wavelength-division-multiplexed (WDM) systems.
Manchester code, the simplest form of pulse-position-modulation (PPM), is highly attractive for high-bit-rate optical transmission systems due to its simple clock extraction and high-level intensity fluctuation tolerance [1,2], as well as for wireless communication systems due to better performance against the extensive additive noise and better timing extraction [3]. Since it prevents the resonance of consecutive adjacent pulses in a data stream, the Manchester code also can suppress a cross-correlation between the pulses; thereby restrains unwanted pulses on the zero levels [4]. Combined with phase-shift-keying (PSK), it is shown that Manchester code provides enhanced tolerance to beat interference noise in a passive optical network [2]. Furthermore, the technique to generate the Manchester code can be applied for optical code division multiple access (O-CDMA) systems in secured optical communication networks with reduced multiple access interference [5–8].
Conventionally, Manchester code can be obtained in an electrical domain using costly XOR logic gates that need inefficient OE/EO conversion. Moreover, the electrical components so far may limit the operation speed to 40 Gbit/s [9]. Recently, the Manchester code is demonstrated using a push-pull type intensity modulator with the electrical data and the clock controlling the phase matching condition of an optical channel [10]. However, unfortunately, this scheme still stays in the electrical domain in which the incoming optical data stream should be converted to the electrical signals to feed into the modulator. Thus, for the fiber-based reconfigurable high-speed optical network, an all-fiber solution for the Manchester encoding is highly desirable with the added advantages of (i) no need for OE/EO conversion, (ii) ultrafast nonlinear response time (~2–4 fs) of Kerr effect in the fiber [11], and (iii) excellent fiber compatibility.
We experimentally demonstrate the all-fiber Manchester code generation using nonlinear polarization rotation in a single 1-km highly-nonlinear fiber (HNLF) with the nonlinearity of 20.4 W-1km-1. The scheme is based on the XOR functionality of the Kerr shutter [12] in which a pump induces birefringence in the HNLF so that the state of polarization (SOP) of dummy channel rotates resulting in “1” or “0” through the polarizer located after the HNLF. We obtain 33-dB extinction ratio of output in a -4-dBm continuous wave (CW) dummy channel using orthogonally aligned two pumps (10 Gbit/s optical signal and 10 GHz optical clock) with the optical power of 8-dBm each. Here, the second pump co-injected into the HNLF acts as a “compensator” for the birefringence induced by the first pump. We present the modulated waveform along with the optical spectrum obtained via our all-fiber scheme. As illustrated with sin2 characteristics of the Kerr shutter, we expect to improve the result controlling the parameters including the length of HNLF, the nonlinear coefficient of the fiber, and the pump power level.
2. All-fiber optical manchester code generation
A beam with significant power induces Kerr effect in the fiber. In order to illustrate the effect, we can consider a pump-probe scheme. A birefringence induced by the pump causes a phase difference between TE and TM modes of the probe channel at different wavelength that co-propagates with the pump through the same fiber. As a result, the SOP of the probe rotates due to the presence of the pump. In order to manage the SOP rotation (i.e., birefringence in the fiber), we add an additional pump that is orthogonally aligned with the existing pump. Since the additional pump provides the phase change with the same amount and the opposite sign of the birefringence, it can compensate the phase difference induced by the existing pump completely. Thus, the rotated SOP of the probe can be brought back to the original state using the additional pump
Our Kerr shutter functions based on the 2-pump and probe configuration. Figure 1 illustrates the concept of the all-fiber optical Manchester code generation using the Kerr shutter. The pump-1 is orthogonally aligned with pump-2, and the CW dummy channel is located between two pumps maintaining the angles of - 45° and + 45° with respect to the pump-1 and the pump-2, respectively (see Fig. 1(a)). All three channels have different wavelengths considering both chromatic dispersion and interchannel crosstalk. The dummy channel (i.e., the probe channel) has the shortest wavelength because the nonlinear effect is inversely proportional to the wavelength [11]. A polarizer located at the end of the HNLF filters out optical signals only with a designed polarization state. When the pumps are off, the initial “on” state of the output is set in case the polarizer is in parallel with the SOP of CW dummy channel as can be seen in the Fig. On the other hand, the initial “off” state can be set with the polarizer perpendicularly aligned with the SOP of CW dummy channel when both pumps are off. Although we can choose either initial “on” or “off” state, there should be opportunity cost of either extinction ratio or average output power as long as the Kerr medium has limited nonlinearity. Figure 1(b) illustrates the formation of the optical Manchester code using the non-return-to-zero (NRZ) data and the optical clock. Manchester code is generated on the CW dummy channel when NRZ data and/or optical clock rotate the SOP of the channel with respect to the polarizer located at the end of the transmitter providing “on” and “off” states in a half-bit-time scale. The resulting Manchester code has the half-bit on the right hand side in case the signal is “1” in the NRZ data stream, and has the half-bit on the left hand side when the signal is “0” as shown in the figure.
Fig. 1. (a) Operation concept of the all-fiber Manchester code generation, and (b) waveforms of two input pumps (NRZ and Clock) and encoded output (Manchester code). Tb is bit-time.
Figure 2 depicts the experimental setup. λ1 (1548.8 nm) is the dummy CW, λ2 (1552.1 nm, pump-1) is the incoming 10 Gbit/s NRZ signal, and λ3 (1554.0 nm, pump-2) is the 10 GHz optical clock. A polarization beam combiner (PBC) guarantees the perpendicular SOP alignment of the two pumps. The power of dummy channel into the HNLF is -4 dBm. The high pump power is given by Er-doped fiber amplifiers (EDFAs). Bandpass filters are employed after the amplifier to narrow down the amplified pumps with 0.5 nm and 0.3 nm bandwidths. The SOPs of pumps are adjusted to the input ports of PBC by polarization controllers (PCs). The SOP of dummy channel is aligned to 45° with respect to both pumps, which can maximize Kerr effect in the fiber [11,13]. We assume that the polarization mode dispersion (PMD) value is so low that the polarization coupling effect between two polarization components of the dummy channel is negligible. The HNLF has the nonlinear coefficient of 20.4 W-1km-1 and the length of 1-km. After the HNLF, a polarizer and a tunable bandpass filter are used as the SOP filter and the wavelength filter, respectively. The bandpass filter is tuned for λ1, the Manchester encoded output.
3. Results and discussion
The combination of the optical data and the optical clock provides the all-fiber optical Manchester code generation by virtue of the nonlinear SOP rotation in the 1-km HNLF. We measure the SOP rotation effect in the HNLF as shown in Fig. 3. The initial SOP of the dummy channel is aligned such that the output is fully suppressed by the polarizer at the end of the transmitter in order to set the initial “off” state of the final output (see Fig. 3(a)). When the pump-1 rotates the SOP of the dummy channel, there is a significant output through the polarizer resulting in the output power of about -38 dBm (see Fig. 3(b)). Figure 3(c) explains that the pump-2 compensates the birefringence induced by the pump-1, therefore suppresses the output back to the initial power level by rotating the SOP of the dummy channel back to the initial state.
Fig. 3. Demonstration of SOP rotation effect in the dummy channel using two 8-dBm pumps; (a) without pump, (b) with pump-1 only, and (c) with both pump-1 and pump-2.
Unfortunately, the on/off function with the high extinction ratio occurs in the low power region (~-60~-30 dBm) by setting the initial “off” state. In order to achieve the same high extinction ratio in a high power region with the initial “on” state, as illustrated in the conceptual figure (see Fig. 4), we need more polarization rotation, thus more birefringence. In other words, for the condition of ERoff = ERon, higher value of polarization rotation is required in the “initial on” state (PRon) than that in the “initial off” state (PRoff) as described in the figure This can be explained with following Eq. (1) and (2) for the dummy output channel through the polarizer.
Where, TP is probe transmittivity, Δϕ is phase difference between the polarization states of the dummy channel, L is the length of the HNLF, ΔnL is the refractive index change caused by the fiber structure, n2B is Kerr coefficient, and |Ep|2 is the pump intensity. Since we use a fiber with symmetric core shape, ΔnL is negligible here meaning that the nonlinear part is dominant. Knowing from the equations, we can improve the extinction ratio using a novel Kerr medium with higher nonlinear coefficient and/or high pump power.
Fig. 4. Conceptual explanation of “initial on” and “initial off” schemes. Where, PR and ER represent the polarization rotation and the extinction ratio, respectively.
Figures 5(a) and 5(b) show the achieved extinction ratio of the dummy channel with respect to the power of the optical pump co-injected into the HLNF. Figure 5(a) describes that the output power change as a function of the pump-1 power. As the pump power increases the output power, thereby extinction ratio also increases showing an up-curved distribution of data points that corresponds to the sin2 characteristics of the output. The maximum extinction ratio of 33 dB is obtained with the pump power of 8 dBm. Figure 5(b) illustrates that the pump-2 suppresses back the dummy output that is generated by the pump-1 with the fixed power of 8-dBm. The output is suppressed almost to the same power level of initial state by adjusting the power and the polarization state of the pump-2. The measured data points follow the same trend shown in Fig. 5(a) illustrating the effective compensation of the birefringence.
Figures 6(a)–(c) show the waveforms of two injected pumps (the optical NRZ data and the optical clock), and the generated optical Manchester code at 10 Gbit/s. For the synchronization of the pumps, a phase shifter is added to the pump-2. In case the optical clock is switched with the series of “1s” in return-to-zero (RZ) format, we expect better Manchester coded output waveform owing to the shorter rising and fall times. Moreover, with a short pulsed optical source, more complex PPM wave form could be obtained using the same technique. As a result, the encoded data stream shows successful demonstration of our all-fiber Manchester code generation technique. The spectrum of the Manchester code output amplified by an EDFA is shown in Fig. 6(d). Since the Manchester code is formed with the half-bit-time-based structure, the 10 G as well as the 20 G components can be found in the spectrum.
We note that the operation of our method is affected by fluctuation of surroundings since the technique is realized based on the relatively long length of optical fibers. Especially, polarization sensitivity is one of the obstacles that should be overcome in the method. It can be improved dramatically by introducing novel fibers that have higher nonlinear coefficients, thereby shortening the length of the fibers.
Fig. 5. Extinction ratio of the output with respect to (a) pump-1, and (b) pump-2 when the pump-1 power is fixed to 8 dBm.
Fig. 6. Waveforms of (a) NRZ input, (b) clock input, and (c) Manchester encoded output. (d) Spectrum of the encoded output.
4. Conclusion
We demonstrate the formation of all-fiber optical Manchester code using nonlinear polarization rotation in a HNLF. Our all-fiber configuration ensures the highly efficient encoding without O/E and E/O conversion. We expect that our scheme also can be employed in other applications (especially O-CDMA systems) for non-blocking transmission in the future all-optical reconfigurable fiber network.
References and links
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