Single-Wavelength Dark-Optical-Comb Injected Semiconductor Optical Amplifier
4.1 Introduction
Recently, the implementations of all-optical pulsed data transformation in traveling-wave semiconductor optical amplifiers (SOAs) have been comprehensively investigated in combination with versatile techniques such as direct electrical modulation [4.1], four-wave-mixing [4.2], cross-phase modulation, waveguide coupler [4.3] and loop-mirror [4.4] based interferometry. Direct modulation technique inevitably meet bottleneck of low-bit-rate operation due to insufficient bandwidth of the SOA. Four-wave mixing scheme exhibits all-optical and high-speed capabilities but suffers relatively low signal-to-noise ratio and conversion efficiency. The interferometric configurations bear problems either the complicated device fabrication or the requirement on exact delay-time control. To achieve all-optical and high-contrast operations, a temporally gain-sliced SOA was employed to demonstrate pulsed data-format conversion under backward single-wavelength dark-optical-comb injection technique [4.5]. Such a scheme preserves the incoming and transformed data wavelength and provides precise gain control in time domain, thus leading to an enhanced extinction ratio at bit-rate higher than modulation bandwidth of the SOA electrode. However, an extremely large frequency chirping effect usually
accompanies with the cross-gain depletion based all-optical conversion due to large carrier density changed during the signal processing, which inevitably degrade the bit-error-rate performance and reduces the transmission distance without proper wavelength dispersion compensation during propagation [4.6]. In principle, the dynamic frequency chirp of the pulsed data signal from SOA is affected not only by the intense gain modulation in time domain, but also by the reshaped gain profile of the SOA in spectral domain. It is thus worthy of investigating the dynamic gain and chirp behaviors such a SOA under broadband and large duty-cycle optical injection induced cross-gain depletion situation. The theoretical and experimental studies on spectral and temporal gain-shaping of the SOA facilitate a precise control of the dynamic frequency chirp at the pulsed data-stream.
In this work, we establish a dynamic chirp model for the backward dark-optical-comb injected SOA to compare the effect of single- or multi-wavelength injection on both the duty-cycle and the dynamic frequency chirp of the pulsed data all-optical converted by the SOA. By temporally and spectrally reshaping the gain profile of the SOA with such a backward injected multi-wavelength dark-optical-comb, the converted pulse data reveals a smaller frequency chirp than that obtained under the single-wavelength dark-optical-comb injection case. The temporal and spectral dependencies of the SOA gain under single- and multi-wavelength dark-optical-comb injection are derived to elucidate experimental results.
4.2 Experimental Setup
The all-optical pulsed data-format converter is schematically shown in Fig. 4.1, in which a SOA DC-biased at 350 mA with a maximum gain at 1530 nm and an amplified spontaneous emission linewidth of 38 nm at 3-dB decay is employed. The
dark-optical-comb is generated by passing a continuous-wave laser source through an electrical-comb driven Mach-Zehnder intensity modulator (MZM) to temporally reshape the gain-window of the SOA, while the DC-level of the amplified electrical comb pulse at repetition frequency of 10 GHz is slightly offset from zero to obtain maximum modulation depth of the dark-optical-comb. Either a single-wavelength distributed feedback laser diode (DFBLD) with a 3-dB linewidth of 1 MHz, or a multi-wavelength Fabry-Perot laser diode (FPLD) with 3-dB linewidth of 7.297 nm is employed in our experiments for comparison, their corresponding spectra are shown in the inset of Fig. 4.1. A commercial electrical comb generator activated by a 10-GHz sinusoidal clock signal of 30 dBm generates electrical pulse-train of 30-ps pulsewidth, leading to the generation of an dark-optical-comb (see inset of Fig. 4.1) with a duty-cycle of 70 % output from the MZM. After propagating through an erbium-doped fiber amplifier (EDFA) with 20-dB gain and an optical circulator (OC), the dark-optical-comb is used to backward inject and then periodically deplete the gain of SOA for implementing non-return-to-zero to return-to-zero (NRZ-to-RZ) data format conversion. The incoming optical NRZ data-stream is simulated by encoding a tunable laser (TL) with another MZM, which is driven by a pseudo-random-bit-sequence (PRBS) data-stream generator with a pattern length of 223-1. The injection power at the port 2 of the OC is increased to saturate the gain of SOA for obtaining maximum extinction ratio of the converted pulsed data. The wavelength, input power and extinction ratio (defined as the ratio of the “on level power” to the “off level power”) of the incoming optical NRZ PRBS data are 1529.2, -15 dBm and 12 dB, respectively. The dark-optical-comb is set at longer wavelength to achieve a better extinction ratio of the converted signal will be obtained at the output [4.7]. Afterwards, the converted RZ data is analyzed by a chirp analyzer (Advantest
Q7606B) to obtain its dynamic frequency chirp at different dark-optical-comb injection power.
4.3 Results and Discussion
In experiment, the input NRZ data is converted into a pulsed RZ data with an improved extinction ratio of 15.5 dB in the SOA under an dark-optical-comb injection power of 16.5 dBm. Temporal traces of the dark-optical-comb, the converted pulsed RZ data bit, and the corresponding dynamic frequency chirp obtained under multi- and single-wavelength injection conditions are shown in Fig. 4.2 and 4.3, respectively.
The peak-to-peak frequency chirp related to the input NRZ signal, the multi- and single-wavelength dark-optical-comb are determined as 1.0, 2.1 and 1.9 GHz, respectively. As the injection power of the dark-optical-comb increasing from 2.4 to 16.5 dBm, the FWHM of the converted RZ signal is shortened from 41.2 to 31.6 ps, however, the peak-to-peak frequency chirp of the pulsed RZ data bit is concurrently enlarged from 6.7 to 11.1 GHz by using the multi-wavelength dark-optical-comb injection.
In contrast to the multi-wavelength injection, the injection of single-wavelength dark-optical-comb with same power level only consumes the carriers pumped upon the states with energy larger than injecting photons in the SOA. Therefore, the carriers left at low energy levels only accounts for the amplified spontaneous emission, which eventually contributes to the DC-level of the data-stream and causes a limited extinction ratio of the converted RZ pulse. Under the single-wavelength dark-optical-comb injection, the FWHM of the converted RZ signal is shortened from 37.3 to 30.8 ps as the injection power from 2.4 to 16.5 dBm, whereas the peak-to-peak frequency chirp of the pulsed RZ data bit is increased from 9.1 to 13.2 GHz.
Apparently, the single-wavelength dark-optical-comb injection shortens the converted RZ pulsewidth at a cost of enlarged dynamic frequency chirp as compared to the multi-wavelength case. Theoretically, the gain coefficient of SOA exhibits a Lorentzian lineshape described determined by both the pumped carrier concentration and the spectral distribution [4.8], as described in Eq. (2.1.2). The pulsed RZ data-format conversion occurred in SOA under the intense cross-gain depletion process not only results in patterning effect in time domain, but also induces a large chirp with its level proportional to the depth of gain depletion. Nonetheless, the residual gain as well as the gain depletion depth of SOA can be minimized if we further shrink the gain distribution profile of SOA in spectral domain, since the derivative of gain coefficient to its spectral linewidth is always greater than zero,
( )
2( )
3( )
2Therefore, the dynamic frequency chirp of the converted RZ pulse is further reduced by shrinking the gain distribution of SOA in spectral domain. Temporal and spectral slicing on the gain of SOA can simultaneously be implemented by introducing a multi-wavelength dark-optical-comb injection into the SOA.
In more detail, the effect of the backward dark-optical-comb injection power on the gain of the SOA, the converted RZ pulsewidth, and the peak-to-peak chirp performance of the converted pulsed RZ at different injection conditions are analyzed and shown in Fig. 4.4 and 4.5. Figure 4.4 shows the gain and peak to peak chirp as a function of dark-optical-comb injection power. The peak to peak chirp is increased as increasing injection power, since the phase of the converted signal rapidly increases due to carrier-induced index changes. In particular, the fluctuations on measured frequency chirp with changing injection power is due to the interference occurred
between the injected and partially reflected dark-optical-combs in SOA. Such an intense injection induced interference changes the carrier and gain dynamics in SOA and thus affects the pulse shape and dynamic frequency chirp [4.9]. On the other hand, for general optical time-division-multiplexing (OTDM) application, it is requisite to generate a RZ data bit with shorter duty-cycle or pulsewidth, thus enlarging the channel numbers and communication capacity. In principle, the limitation on converted RZ pulsewidth of the SOA-based RZ pulsed data converter is mainly determined by the effective carrier lifetime of τ= [τs-1 + d(gPin)/dn]-1 [4.10]. That is, the rise-/fall-time as well as the duty-cycle of the converted RZ data bit can essentially be shortened due to the decreasing carrier lifetime in a highly biased SOA with strong optical injection. The converted RZ pulsewidth is also plotted as a function of the dark-optical-comb injection power and shown in Fig. 4.5. The observed rising time of the converted RZ pulse remain almost unchanged, however, the falling time was monotonically reduced by increasing the injection power of dark-optical-comb. The evolution of converted RZ pulsewidth with injection power exhibits similar decreasing trend with its falling time.
To provide a fast conversion speed and shortened response, the increase in both the biased current of the SOA and the injection power of dark-optical-comb are mandatory.
Nonetheless, the shortening of converted RZ pulsewidth inevitably results in a large dynamic frequency chirp under a same gain depletion depth of SOA. The increasing trend of the peak-to-peak dynamic frequency chirp with increasing gain of SOA (see Fig. 4.6) is plot as a function of the dark-optical-comb injection power at different biased currents of the SOA are shown in Fig. 4.7. If we consider the effect of dark-optical-comb power on the peak-to-peak frequency chirp, the first-order derivative of Eq. (2.2.6) gives an increasing trend of the dynamic frequency chirp with
the dark-optical-comb injection power Pin. That is,
where C0 is constant. It explains the lower frequency chirp of the converted RZ pulse induced at lower biased current of SOA. As the injection power increases from 2.4 to 16.5 dBm, the increment of dynamic frequency chirp of the SOA converted RZ pulse data are 2.7, 3.2 and 4.4 GHz at SOA biased currents of 150, 250 and 350 mA, respectively. In comparison, it is observed that the multi-wavelength injection can provide better performance than single-wavelength injection on reducing the dynamic frequency chirp at higher injection powers. Since the gain spectrum of SOA becomes narrower at the multi-wavelength injection case, a more significant reduction on the dynamic frequency chirp with a broadband dark-optical-comb injection can thus be expected.
In the practical fiber-optic communication network, even though the bit-error-rate performance of the converted RZ pulse during transmission is subject to the net effect of duty-cycle and frequency chirp, the receiving power penalty induced by improperly compensated chirp of the RZ pulsed data is a much more concerned factor when propagating through a fiber link. Our experiments conclude that the spectral slicing on the gain profile of SOA via multi-wavelength injection can thus be an efficient approach for reducing frequency chirp without seriously sacrificing the duty-cycle of the converted RZ pulse data. These experimental results correlate well with our theoretical simulation that a multi-wavelength-injection induced gain-depletion of SOA can effectively reduce the dynamic frequency chirp in comparison with that obtained under single-wavelength-injection condition. Optimization of the pulsed RZ data-format conversion in the SOA with a temporally and spectrally sliced
gain-window can thus be concluded. Reshaping the gain profile in such a cross-gain depleted SOA can alternatively be achieved by using other broadband optical source, however, which inevitably introduces large relative intensity noise and degrades the transmission as well as bit-error-rate performance of the converted RZ pulse. The multi-wavelength dark-optical-comb injection with same power level are currently the best solution to benefit from advantages such as the complete carrier consumption above SOA bandgap and the lower dynamic frequency chirp induced during data-format conversion.
4.4 Conclusion
We have investigated the pulsewidth and dynamic frequency chirp characteristics of an all-optical RZ data-format converter by using a temporally and spectrally gain-sliced SOA, in which cross-gain depletion is achieved by a backward single- or multi-wavelength dark-optical-comb injection. In experiment, the effects of the dark-optical-comb injection power on both the pulsewidth and the frequency chirp of the converted RZ pulse are theoretically elucidated. To provide a fast conversion speed and shortened response, the increase in both the biased current of the SOA and the injection power of dark-optical-comb are mandatory. Reduction on duty-cycle of the injected dark-optical-comb although shortens the pulsewidth of the converted pulsed data bit, which also induces a larger frequency chirp since the same gain-depletion level is accomplished within a narrower time window. Nonetheless, the residual gain as well as the gain depletion depth of SOA can be minimized if we further shrink the gain distribution profile of SOA by multi-wavelength dark-optical-comb injection in spectral domain. This operation efficiently reduces the dynamic frequency chirp of the converted RZ pulse. Under the same injection power
of 2.4 dBm, the multi-wavelength-injection converted RZ pulse data bit exhibits a peak-to-peak frequency chirp of 6.7 GHz, which is reduced by almost 40% as compared to the single-wavelength injection case. Such a reduction on the dynamic frequency chirp with a broadband dark-optical-comb injection is theoretically explained due to the significant gain narrowing effect in SOA. A broadband dark-optical-comb injection with an appropriate power level is expected as the best solution to benefit from advantages such as the complete carrier consumption above SOA bandgap and the minimized dynamic frequency chirp induced during RZ pulsed data-format conversion.
4.5 References
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1530 1540 1550 1560 1570 -50
-25 -50 -25
FPLD
Power (dBm)
Wavelength (nm) DFBLD
Fig. 4.1 Experimental setup. Amp.: amplifier.; ATT.: optical attenuator; DSO: digital sampling oscilloscope; EDFA: erbium doped fibre amplifier; OBPF: optical band-pass filter; OC: optical circulator; PC: polarization controller; PPG: PRBS pattern generator; TL: tunable laser. Electrical path: solid line. Optical path: dash line.
0 1
-7 0 7
Fig. 4.2 Temporal traces of (a) multi-wavelength dark-optical-comb and (b) converted pulsed RZ signal, (c) and (d) are corresponding chirps at injected powers of 16.5 dBm (solid) and 2.4 dBm (dashed).
0 1
-7 0 7
Fig. 4.3 Temporal traces of (a) single-wavelength dark-optical-comb and (b) converted pulsed RZ signal, (c) and (d) are corresponding chirps at injected powers of 16.5 dBm (solid) and 2.4 dBm (dashed).
2 4 6 8 10 12 14 16
Peak to Peak Chirp (GHz)
Fig. 4.4. Gain and the peak to peak chirp of the converted pulsed RZ signals by the single- / multi-wavelength dark-optical-comb injection as a function of the dark-optical-comb injection power.
2 4 6 8 10 12 14 16
Peak to Peak Chi rp (GHz)
Fig. 4.5 FWHM and the peak to peak chirp of the converted pulsed RZ signals by the single- / multi-wavelength dark-optical-comb injection as a function of the dark-optical-comb injection power.