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We describe a 1.28 Tbit/s single-channel transmission in the 1.1 μm band that uses a high-Δ single-mode step-index fiber (SIF) and ytterbium-doped fiber amplifiers (YDFAs). A 650 fs signal pulse was generated from a 10 GHz mode-locked Yb fiber laser (Yb MLFL) that was soliton-compressed by using an anomalous dispersion photonic crystal fiber (PCF). The dispersion of the SIF was compensated over a bandwidth as broad as 2.1 THz with two cascaded chirped fiber Bragg gratings (FBGs). In addition, an enhanced pre-chirping technique was newly adopted for the precise elimination of the large waveform distortion caused by the dispersion slope. As a result, 320 Gbit/s-161 km and 1.28 Tbit/s-58 km transmissions were successfully achieved.
© 2013 Optical Society of America
1. Introduction
Motivated by the ever-growing demand for ultrahigh capacity optical transport networks, it has become increasingly important to exploit new transmission bands other than the conventional 1.55 and 1.3 μm regions to overcome the capacity limitation [1]. Among several candidates, such as the 2 μm window [2], the 1.1 μm band has received considerable attention recently as an ultrawide transmission window. In particular, ytterbium (Yb)-doped fiber amplifiers (YDFAs) [3] can provide ultrabroad-band gain over 7.8 THz, which makes the 1.1 μm band particularly attractive. In addition, a single-mode fiber transmission line can also be realized with a high-Δ step-index fiber (SIF), or with specialty fibers such as photonic crystal fiber (PCF) or hole-assisted fiber (HAF). With these specialty fibers, it is possible to reduce the group-delay dispersion (GVD) greatly by increasing the waveguide dispersion with appropriate design of the air-hole geometry in the cross section, so that the large material dispersion of the silica at 1.1 μm can be compensated [4].
By taking advantage of these properties, several groups have already demonstrated 10~40 Gbit/s single-channel and WDM transmissions at 1.1 μm [5–9]. We previously reported the first 160 Gbit/s-300 km single-channel transmission by employing the OTDM of picosecond optical pulses [10]. This involved the use of several key components including a 10 GHz harmonically mode-locked Yb fiber laser (Yb MLFL) as a picosecond optical pulse source [11], and a nonlinear optical loop mirror (NOLM) as an all-optical demultiplexer [12]. A major impairment as regards ultrashort pulse transmission is the GVD and dispersion slope. In particular, the dispersion slope is difficult to reduce or compensate for even with a combination of different types of fibers since it is dominated by the material dispersion of the silica. We employed a chirped fiber Bragg grating (FBG) and a pre-chirping technique [13, 14] to compensate for the large dispersion and dispersion slope of high-Δ single-mode SIF.
In this paper, we present the first demonstration of the single-channel terabit/s transmission of a subpicosecond optical pulse at 1.1 μm. The subpicosecond optical pulse was generated from a 10 GHz Yb MLFL and was compressed with the soliton effect. As a wideband GVD compensator, we used two cascaded chirped FBGs to increase the dispersion while maintaining the bandwidth. Furthermore, we greatly improved the performance of the dispersion slope compensation with a novel pre-chirping technique, which uses two LN phase modulators in a round-trip configuration and a suitable combination of 10 and 20 GHz phase modulations. As a result, we successfully achieved a 1.28 Tbit/s/ch polarization-multiplexed OTDM transmission at 640 Gbaud over 58 km.
2. Key technologies for single-channel Tbit/s transmission at 1.1 μm
The 10 GHz Yb MLFL that we developed emits a 1.1 ps transform-limited sech pulse at 1.1 μm. To generate the subpicosecond optical pulses required for terabit/s single-channel transmission, we employed pulse compression with the soliton effect. Spectral broadening by self-phase modulation in a normal dispersion and dispersion-flattened fiber is generally used for pulse compression. However, it is difficult to realize such fibers in the 1.1 μm band. We therefore adopted the soliton effect by using an anomalous dispersion PCF [15]. Figure 1(a) shows the configuration we used for soliton compression. A 1.1 ps optical pulse from the 10 GHz MLFL was amplified and launched into a highly nonlinear PCF. The loss, dispersion and nonlinear coefficient of the PCF were 3 dB, + 12 ps/nm/km, and 16 W−1km−1, respectively. The peak power required for a fundamental soliton Psoliton and the soliton period Zsp are given by
where c is the velocity of light, γ is a nonlinear coefficient, D is the dispersion of the PCF and τFWHM is the full width at half maximum (FWHM) of the input pulse. In the present PCF, Psoliton and Zsp were calculated to be 1.2 W and 84 m, respectively. Figure 1(b) shows the output pulse width against the launched power into an 80 m-long PCF. When the launched power was 13.4 dBm (Ppeak = 2.0 W), which corresponds to the soliton order N = (Ppeak/Psoliton)1/2 = 1.29, a pedestal-free 650 fs pulse was successfully obtained as shown in Fig. 1(c). This 650 fs pulse was used as a signal pulse for a 1.28 Tbit/s transmission. Figure 1(d) shows the minimum pulse width of 400 fs obtained at a launched power of 2.8 W. This 400 fs pulse was used as a control pulse for all-optical demultiplexing with an NOLM.
Fig. 1 (a) Configuration of the soliton compression, (b) output pulse width versus launched power (PCF length = 80 m), (c) autocorrelation waveform of a 650 fs compressed pulse (Ppeak = 2.0 W), and (d) autocorrelation waveform of a 400 fs compressed pulse (Ppeak = 2.8 W).
Since the bandwidth of a 650 fs pulse may be as large as 1.8 nm (472 GHz), it is very important to employ a broadband GVD compensator. We previously used a chirped FBG with a dispersion of 720 ps/nm and a bandwidth of 341 GHz in a 160 Gbit/s/ch-300 km transmission [10]. To broaden the bandwidth, we employed two chirped FBGs as shown in Fig. 2(a). We set the FBG bandwidth at 8 nm (2.1 THz) to sufficiently cover the bandwidth of a 650 fs pulse, which was realized by cascading two FBGs with a four-port circulator. As a result, a dispersion of + 392~ + 444 ps/nm was realized as shown in Fig. 2(b), which is capable of compensating for the dispersion of a 10.9~12.3 km SIF.
As a dispersion slope compensator, we employed a pre-chirping technique that was originally developed for 1.5 μm transmission use [13, 14], and we modified its configuration to compensate for a large dispersion slope at 1.1 μm. Figure 3 shows the configuration of the pre-chirping unit consisting of a GVD medium (SIF) and an LN phase modulator, for which we newly adopt a round-trip configuration to increase the chirp. A 10 Gbit/s pulse linearly dispersed with a GVD medium was sinusoidally phase modulated to compensate for the phase shift caused by the dispersion slope during transmission. The sinusoidal phase modulation applied to a linearly dispersed pulse is given by ϕsin(ω) = ϕssin{A(ω−ω0)} in the frequency domain. Here, A = R0Ts/Fbw, R0 = 10 GHz is the modulation frequency, Ts is the linearly dispersed pulse width in the pre-chirping unit, Fbw is the bandwidth of the signal pulse, and ϕs is the amplitude of the phase modulation. From the Taylor series expansion, ϕsin(ω) contains the (ω−ω0)3 term, which is given by (ϕsA3/6)(ω−ω0)3. This allows us to cancel the phase shift caused by the dispersion slope (β3L/6)(ω−ω0)3 (β3: third-order dispersion, L: fiber length) by setting ϕsA3 = β3L [13].
Fig. 3 Setup of dispersion slope compensator with an LN phase modulator in a round-trip configuration.
Since A3 decreases rapidly for narrower pulse widths due to the 1/Fbw dependence of A, it is necessary to increase ϕs to realize dispersion slope compensation for a short pulse. Therefore, we used LN phase modulators in a round-trip configuration as shown in Fig. 3, which enabled a two-fold increase in the chirp. The loss of the dispersion slope compensator was 7 dB.
We evaluated the effect of dispersion slope compensation in a short optical pulse propagation at 1.1 μm. Figure 4 shows the experimental setup. A pre-chirped 10 GHz optical pulse was amplified and launched into a recirculating loop composed of an 11.5 km single-mode high-Δ SIF. The loss and the dispersion of the SIF were 1.5 dB/km and −36 ps/nm/km, respectively, which were compensated for with a YDFA and a wideband GVD compensator as shown in Fig. 2. The dispersion slope of the SIF was 0.175 ps/nm2/km. Figures 5(a) and 5(b) show the cross correlation waveform of a 1.1 ps pulse before and after a 58 km propagation (β3L = 5.19 ps3) without pre-chirp. The waveform was greatly distorted due to the dispersion slope as seen in Fig. 5(b). Figure 5(c) shows the waveform after propagation with pre-chirp. Here, we set ϕs = 9.3π and Ts = 23 ps corresponding to a 290 m SIF as a GVD medium in the pre-chirping unit. The ripples were removed and the pulse width was reduced to 1.3 ps.
Fig. 5 Cross correlation waveform of a 1.1 ps pulse after 58 km propagation at 1.1 μm. (a) Before transmission, (b) without dispersion slope compensation, (c) with dispersion slope compensation.
Next we propagated a 650 fs pulse through a 58 km SIF. Figure 6 shows the result of dispersion slope compensation. As shown in Fig. 6(c), residual pulse width broadening remains after dispersion slope compensation, which indicates that the dispersion slope was not fully compensated for.
Fig. 6 Cross correlation waveform of a 650 fs pulse after 58 km propagation at 1.1 μm. (a) Before transmission, (b) without dispersion slope compensation, (c) with dispersion slope compensation.
To enhance the dispersion slope compensation capability for a subpicosecond optical pulse, which is required for 1.28 Tbit/s/ch transmission, we adopt a novel configuration as a pre-chirping unit as shown in Fig. 7. The new pre-chirping unit consists of two cascaded LN phase modulators with a round-trip configuration, which are driven by 10 and 20 GHz sinusoidal signals. In this case, the sinusoidal phase modulation is given by ϕsin(ω) = ϕs1sin{A(ω−ω0)}−ϕs2sin{2A(ω−ω0)}, and can be expanded by the Taylor series as follows:
The phase shift caused by the dispersion slope can be calculated by setting (ϕs1−8ϕs2)A3 = β3L so that the (ω−ω0)3 term becomes zero. In conventional slope compensation with only 10 GHz modulation, an unnecessary fifth-order component (ϕs1/120)A5(ω−ω0)5 is inevitably added, which induces residual waveform distortion as an equivalent fifth-order dispersion. This is not negligible for a subpicosecond optical pulse. In the present scheme, by virtue of the additional 20 GHz modulation, the fifth-order component can be cancelled by setting ϕs1−32ϕs2 = 0.We constructed the proposed dispersion slope compensator based on the configuration shown in Fig. 7. We used two LN phase modulators with Vπ = 2.7 V in a round-trip configuration, which enabled a four-fold increase in the chirp. A 10 GHz phase modulation was applied to three of the four RF ports of the modulator to increase the magnitude of ϕs1, and a 20 GHz modulation was applied to the remaining RF port. The loss of the dispersion slope compensator was 15 dB. We first carried out a numerical simulation of dispersion slope compensation with the proposed scheme. Figure 8 shows a numerical result for the pulse width after dispersion slope compensation with only 10 GHz modulation and 10 and 20 GHz hybrid modulation. Here, the input pulse width was 650 fs and the dispersion slope β3L was 5.19 ps3 (corresponding to the SIF length of 58 km). With only 10 GHz modulation, the pulse width decreases as the amplitude ϕs1 increases, but the effect of compensation saturates at ϕs1>20π, and the pulse width cannot be reduced to less than 1 ps. This is attributed to the fifth-order component induced in the chirp as shown in Eq. (2), which becomes larger as ϕs1 increases. By contrast, with the hybrid modulation, the pulse width was reduced to 840 fs at ϕs1 = 20π and ϕs2 = 0.625π. Figures 9(a) and 9(b) show the experimental results obtained at ϕs1 = 20π with a 10 GHz modulation and a hybrid modulation, respectively. By employing the improved pre-chirping technique, the residual pulse width recovered to 860 fs as shown in Fig. 9(b).
Fig. 9 Cross correlation waveform of a 650 fs pulse after 58 km propagation over a 1.1 μm SIF with dispersion slope compensation. (a) Only 10 GHz and (b) 10 + 20 GHz modulation.
3. 320 Gbit/s~1.28 Tbit/s single-channel OTDM transmission with wideband GVD and dispersion slope compensators
We demonstrated a 320 Gbit/s~1.28 Tbit/s single-channel transmission using the wideband GVD and slope compensator described in section 2. Figure 10 shows the experimental setup for 1.1 μm, 320 Gbit/s~1.28 Tbit/s single-channel OTDM transmission. A 10 GHz Yb MLFL was used as an optical pulse source whose wavelength and pulse width were 1070 nm and 1.1 ps, respectively. In a 1.28 Tbit/s (640 Gbaud) transmission, the pulse width was externally compressed to 650 fs as shown in Fig. 1. The signal pulse was OOK-modulated at 10 Gbit/s with a 27−1 PRBS, and pre-chirping was applied for slope compensation. The 320 Gbit/s transmission employed the configuration shown in Fig. 3, while the enhanced pre-chirping with hybrid phase modulation was adopted for 640 Gbit/s and 1.28 Tbit/s transmissions. After pre-compensation for the dispersion slope, the 10 Gbit/s signal was multiplexed to 320 or 640 Gbit/s using a fiber delay line. Figures 11(a) and 12(a) show the waveforms of the generated 320 and 640 Gbit/s OTDM signals (measured without pre-chirping). In the 1.28 Tbit/s transmission, the 640 Gbit/s signal was polarization multiplexed with a polarization beam combiner (PBC). The OTDM signal was launched into a recirculating loop, whose configuration is the same as that shown in Fig. 4. The launched power was set at 9, 12, and 15 dBm for 320, 640 Gbit/s and 1.28 Tbit/s transmission, respectively. After the transmission, the OTDM signal was launched into the NOLM switch for demultiplexing to 10 Gbit/s (via PBS for polarization demultiplexing in the 1.28 Tbit/s transmission). As a control pulse source for the NOLM, we used 600 or 400 fs pulses compressed with the soliton effect from another 10 GHz Yb MLFL, whose center wavelength was 1065 nm. The repetition rate was synchronized with a 10 GHz clock signal extracted from the transmitted OTDM data. The NOLM was composed of a 6 m-long highly nonlinear fiber (HNLF) with a dispersion of − 41.6 ps/nm/km, a dispersion slope of 0.192 ps/nm2/km and a nonlinear coefficient of 33 W−1km−1.The switching gate width was 900 fs ~2.2 ps depending on the control pulse width, and the extinction ratio was 14 dB. The insertion loss of the NOLM was 12 dB. Figure 11(b) shows the 10 Gbit/s signal demultiplexed from 320 Gbit/s data with a 2.2 ps switching gate under a back-to-back condition, and Fig. 12(b) is the result demultiplexed from 640 Gbit/s with a 900 fs switching gate. Finally, after removing the control pulse with a 1 nm optical filter, the demultiplexed signal was received by an InGaAs PD and the bit error rate (BER) was measured.
Fig. 11 (a) 320 Gbit/s cross correlation waveform and (b) demultiplexed 10 Gbit/s signal after the NOLM under a back-to-back condition.
Fig. 12 (a) 640 Gbit/s cross correlation waveform and (b) demultiplexed 10 Gbit/s signal after the NOLM under a back-to-back condition.
Figure 13(a) shows the optical spectra of 320 Gbit/s signals after 104 and 161 km transmissions, which were measured without bandpass filters in the transmission line to evaluate the OSNRs. The OSNRs were 19.5 and 14 dB after 104 and 161 km transmissions, respectively. The ASE level is not flat due to the gain characteristics of the YDFAs. Figure 13(b) shows the BER characteristics at 104 and 161 km. Here, the pulse widths after dispersion slope compensation were 1.5 and 1.7 ps at 104 and 161 km, respectively. A BER of 10−9 was successfully achieved for a 104 km transmission with a power penalty of 7 dB. After 161 km, the BER had degraded to 1x10−4, but it was still below the standard FEC limit of 2x10−3. This BER performance is one order of magnitude larger than that estimated from the OSNR as shown by the dashed curve. This indicates that the BER value was determined by both the OSNR degradation, and the residual pulse width broadening after transmission since the dispersion slope compensation was insufficient.
Fig. 13 (a) Optical spectra and (b) BER characteristics for 320 Gbit/s signal after 104 and 161 km transmissions.
Figures 14(a) and 14(b) show the optical spectra and BER performance for a 58 km transmission, respectively. By employing a wideband dispersion slope compensator, we achieved a BER of 6.3x10−6 for single polarization and 2x10−4 with polarization multiplexing. In the polarization-multiplexed transmission, the spectrum suffers from a crosstalk as large as 14 dB due to the polarization-mode dispersion (PMD) of the SIF, which degraded the BER value together with the residual pulse width broadening. However, the BER of 2x10−4 was below the standard FEC limit (2x10−3), indicating an error free transmission at a net bitrate of 1.20 Tbit/s by taking the 7% overhead into account.
4. Conclusion
We reported a 1.1 μm, single-channel terabit/s transmission, which is the first demonstration of a subpicosecond pulse transmission at this wavelength. With enhanced GVD and slope compensation over a large bandwidth, we achieved a 320 Gbit/s/ch OTDM transmission over a 160 km high-Δ single-mode SIF. Furthermore, we successfully demonstrated a 1.28 Tbit/s/ch-58 km transmission with a novel wideband dispersion slope compensator. The present results indicate the possibility of ultrahigh capacity optical transmission in the 1.1 μm band.
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