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Abstract
I report on a fiber chromatic dispersion compensator comprising an array of 100 fixed delay lines sandwiched by a pair of arrayed-waveguide grating-type wavelength filters. The requisite delay for the dispersion compensation is allocated to each demultiplexed wavelength component after the first wavelength filter, and the wavelength components are again multiplexed at the second filter. The flexible capability of delay pattern arrangement enables us to realize various fixed dispersion compensation characteristics. I show transmittance and relative delay time characteristics of a multichannel dispersion compensator for short-distance wavelength division multiplexing communication, and its application to dispersion compensation for a pulse amplitude modulation four signal. I also demonstrate the realization of a dispersion slope compensator for a dispersion-shifted fiber with this configuration.
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1. Introduction
Optical communication, which implements main signal processing with electrical-domain digital signal processing (DSP), has been vigorously developed and is extensively utilized [1]. While on the other hand, optical-domain signal processing is also important in view of future high-speed and low-power-consumption all-optical networks [2]. Simple and easy-to-operate optical domain signal processing such as compensation for chromatic dispersion of an optical fiber is preferred when transmitting high-speed intensity-modulated optical signals including an on-off keying (OOK) signal and a multilevel pulse amplitude modulation (PAM) signal, for which phase information cannot be extracted in the direct detection receiver. A dispersion-compensation fiber (DCF) has superior characteristics including low loss and wide bandwidth [3]. However, it is unsatisfactory on several counts since its size is large, and it causes signal delay. The research and development on integrated-optic chromatic dispersion compensators are being pursued because of its merits including compactness and low latency compared to the DCF, and the possibility for the load reduction of the DSP [4–6].
In this paper, I report on a silica waveguide-type chromatic dispersion compensator in which an array of 100 fixed delay lines and two arrayed-waveguide grating (AWG)-based wavelength filters [7] are monolithically integrated. The basic configuration of this type of compensator was already reported and its usefulness was numerically clarified [8]. I achieved a multichannel compensator and a dispersion slope compensator with the use of this configuration. The AWGs function as multi/demultiplexers of 100 wavelength components, and the delay lines are used to assign specific delay to each wavelength component. The compensator consists of widely used and highly reliable material and components, and is completely passive, that is, it does not employ any phase adjustment parts including thermo-optic phase shifters. Other type of AWG-based dispersion compensator, which was equipped with phase filters [9] or phase shifters [10], was reported. The former compensator required attachment process of the phase filters to the AWG, which were composed of different material from the AWG waveguide [9]. The latter compensator had a benefit of characteristics tunability, but it also needed another process to install active phase shifters on the waveguides [10]. Although my compensator is not tunable, its flexible capability of delay pattern arrangement enables us to realize various fixed dispersion compensation characteristics.
I first explain the configuration and operating principle of a multichannel compensator for short-distance wavelength division multiplexing (WDM) communication and show its transmittance and delay characteristics. The compensator was designed to compensate for anomalous fiber dispersion at ten channels in the 1.55 µm band. I then demonstrate dispersion compensation for optical 40 Gsymbol/s (80 Gbit/s) PAM4 signals, which were distorted through a single-mode fiber (SMF), with the use of this multichannel dispersion compensator. I also present the realization of a dispersion slope compensator for a dispersion-shifted fiber (DSF, zero-dispersion wavelength: 1.55 µm band) with the combination of the AWGs and fixed delay lines.
2. Configuration and operating principle of chromatic dispersion compensators
2.1 Multichannel compensator for short-distance WDM communication
Figure 1(a) shows the schematic configuration of our integrated-optic multichannel chromatic dispersion compensator for WDM communication. I monolithically integrated two AWGs 1 and 2, and 100 fixed delay lines into one chip. I fabricated the compensator with silica waveguide technology (core: SiO2-GeO2, cladding: SiO2) [4,7,9–14]. The relative index difference Δ and core size of the waveguide were 2.3% and 3.0 µm x 3.0 µm, respectively. The total compensator size was 50 mm x 50 mm. I designed the channel spacing, 3 dB-down channel bandwidth, and free spectral range of each AWG to be 24.9 GHz, 24.2 GHz, and 4778 GHz, respectively. Output ports of the AWG1 were jointed to input ports of the AWG2 through the delay lines. Each wavelength component demultiplexed at the AWG1 is assigned with delay, which is required for achieving the dispersion compensation characteristics, and the component is again multiplexed at the AWG2. As the relative index difference of the silica waveguide was 2.3%, the bending radius of the waveguide was 1.5 mm and the total device size became large as already explained. In this large size device, the in-plane fluctuation of the core size and relative index proceeded from the fabrication process limit. Therefore, I arranged the two AWGs close to each other so that they had the same wavelength characteristics [15].
Fig. 1. Schematic configuration of (a) integrated-optic multichannel chromatic dispersion compensator for short-distance WDM communication and (b) delay line unit pattern in compensator.
Figures 1(b) indicates the configuration of a delay line unit pattern for substantializing the dispersion compensator. In Fig. 1(a), I regarded successive ten delay lines as one unit and set the delay in each unit so that it decreases versus a wavelength. In each unit, the delay difference between the adjacent waveguides and the maximum delay difference were designed to be 10.0 ps and 90.0 ps, respectively. As the refractive index of the silica waveguide core is about 1.45, the delay difference 10.0 ps corresponds to the length difference about 2 mm [11,12]. I allocated ten units as shown in Fig. 1(a). Thus, the compensator in Fig. 1(a) has normal chromatic dispersion (about −50 ps/nm) with a cycle of about 250 GHz and is applicable to the signal distortion compensation in the WDM communication. The compensator can equalize the dispersion of about a 2.9 km long SMF with the typical dispersion value of 17 ps/nm/km at 1.55 µm. In consideration of crosstalk from neighboring and other AWG channels, I set out each AWG so that its channel bandwidth was nearly equal to the channel spacing and the flat transmittance could possibly be obtained through the lightwave interference. In addition, the optical length difference between two adjacent delay lines was designed to be the integral multiple of the average value of demultiplexed two center wavelengths in the delay lines. I designed the operational wavelength range to be 1540.15 to 1559.85 nm. In summary, I assigned ten delay lines to one signal channel and intentionally designed so that the demultiplexed wavelength components at the AWG1 overlapped each other to obtain the smoother transmittance and delay characteristics. I utilized the ten delay line units with the same delay pattern to compensate for the dispersion of ten WDM channels. I did not take into account the phase change caused by the AWG passband shape when I designed the dispersion compensator and a dispersion slope compensator in the next section.
2.2 Dispersion slope compensator for DSF
Figure 2 shows the configuration of a dispersion slope compensator for the DSF, which also consisted of multi/demultiplexing AWGs and an array of 100 waveguides for the delay assignment. The compensator was mainly intended to recover the distortion of an ultra-high speed time division multiplexing (TDM) signal with the symbol rate of more than 100 Gsymbol/s and the bandwidth of more than one nm, which was transmitted through the DSF [16,17]. The chip size, Δ, and AWGs parameters were the same as the compensator in Fig. 1. In Fig. 2, the delay line lengths were designed to compensate for the group delay of a 45 km long DSF that had the typical dispersion slope 0.07 ps/nm2/km [18] and the zero-dispersion wavelength 1552.60 nm. The operational wavelength range was set to 1540.15 nm to 1559.85 nm. I approximated the fiber group delay by a three-term Sellmeier function [19] and assigned requisite delay calculated by the function to each delay line in the compensator.
3. Experimental results
3.1 Characteristics example of AWG in compensator
Figure 3 shows an example of used AWGs transmittance (AWG1 in Fig. 1(a)). The characteristics were measured with an amplified spontaneous emission (ASE) light generated from an erbium-doped fiber amplifier (EDFA). The transmittance of 102 input ports was evaluated by inputting the ASE light into an output monitor port. The fiber-to-fiber loss and loss deviation between center wavelengths of the ports were 5.3 dB and 0.7 dB, respectively. I confirmed that the AWG channel bandwidth surely gave close agreement with the AWG channel spacing.
3.2 Wavelength characteristics of dispersion compensators
Figures 4(a) and 4(b) indicate measured transmittance and relative delay time characteristics versus wavelengths, respectively, relating to the WDM multichannel dispersion compensator shown in Fig. 1(a). I evaluated the transmittance and delay characteristics with the use of the EDFA-based ASE light and a modulation phase shift method (modulation frequency: 3 GHz) [13], respectively. In Fig. 4(b), as designed, the delay characteristics repeat themselves about every 250 GHz and the number of usable channels was ten. Table 1 summarizes measured losses, dispersion, and bandwidth relating to all the ten channels of the compensator. The bandwidth of channels (CHs) 2 to 9 was defined as the negative slope range of the delay time versus the wavelength. I determined short and long wavelength bandwidth edges at CHs 1 and 10, respectively, by the transmittance edges. In Fig. 4(a), the obtained fiber-to-fiber loss was 11.1 dB, and the loss variation within the channel relating to the obtained ten channels was 3.0 dB on average. I mainly attribute the loss variation to the non-ideal interference between the lightwaves of the adjacent delay lines at the AWG2 input. The non-ideal interference stemmed from the length difference deviation between the adjacent delay lines from the ideal value. The intersections between the densely integrated delay lines also caused the loss variation [14]. In Fig. 4(b), the average dispersion value, average delay ripples within the channel, and average bandwidth relating to the ten channels were −47.5 ps/nm, ± 12.7 ps, and 209 GHz, respectively. The compensator can equalize the dispersion of about 2.8 km long SMF on average. The delay characteristics changed in a discontinuous manner (about 333 ps) between CHs 2 and 3 since the used method reset the delay measurement every 2π phase shift [13].
Fig. 4. Measured (a) transmittance and (b) relative delay time characteristics of WDM multichannel dispersion compensator in Fig. 1(a).
Table 1. Summary of Measured Characteristics Relating to All Channels of WDM Multichannel Dispersion Compensator
Figures 5(a) and 5(b) show measured transmittance and relative delay time of the dispersion slope compensator for the DSF shown in Fig. 2, respectively. The fiber-to-fiber loss and dispersion slope were 9.5 dB and −3.03 ps/nm2, respectively. The zero-dispersion wavelength and operational wavelength range were 1552.61 nm and 1540.07 nm to 1559.87 nm, respectively. The operational frequency range was defined by two edges of the transmittance. The loss deviation within the operational frequency range was 3.5 dB. The compensator can equalize the dispersion slope of about a 43.3 km long DSF with the dispersion slope of 0.07 ps/nm2/km. The delay ripple within the operational frequency range was ±22.6 ps.
Fig. 5. Measured (a) transmittance and (b) relative delay time characteristics of dispersion slope compensator in Fig. 2.
3.3 Signal response of multichannel compensator for WDM communication
Figure 6 shows an experimental set-up for compensating for a distorted 1.55 µm PAM4 optical signal passing through the SMF with the use of the WDM multichannel dispersion compensator in Fig. 1(a). The lightwave intensity from a tunable laser diode (LD) was modulated with a signal from a pulse pattern generator (PPG), whose bit rate and pseudo-random bit sequence (PRBS) were 40 Gbit/s and 27−1, respectively. I input the generated optical OOK signal into an integrated-optic PAM signal emulator consisting of a tunable asymmetric Mach-Zehnder interferometer to produce an optical 40 Gsymbol/s (80 Gbit/s) pseudo-PAM4 signal [20]. As I could not substantialize sufficiently long length difference between two arms of the interferometer to produce a PAM4 signal with high randomness, I adopted the short PRBS length [20]. The produced PAM4 signal entered an SMF (length: 3 to 7 km every 1 km, dispersion at 1.55 µm: 17.2 ps/nm/km) and the compensator. I evaluated an optical signal after an EDFA by utilizing an optical sampling oscilloscope with the bandwidth of 65 GHz.
Fig. 6. Experimental set-up for compensating for distorted 1.55 µm PAM4 optical signal passing through SMF with use of WDM multichannel dispersion compensator in Fig. 1(a).
Figure 7(a) shows bit error rates (BERs) of the signals after the compensator when the LD wavelength in Fig. 6 was tuned in 1548.95 nm (center wavelength of CH5). The intensity of the received optical signals was −3.4 dBm. The BERs tended to show floors at the received optical intensity of more than −3.4 dBm. The BERs ranged 1.0 × 10−9 to 6.5 × 10−3, and minimum BER of the back-to-back signal was 7.1 × 10−10 when the received optical signal level was −3.4 dBm. I estimated the BERs by utilizing off-line processing of acquired numerical data with the sampling oscilloscope. I assumed that each sampled level value had a Gaussian probability density function and optimized each threshold value to minimize the BER [20–22]. Figure 7(b) shows measured eye diagrams of the compensated signals when the BERs in Fig. 7(a) were achieved. I obtained the BERs below the KP4 forward error correction (FEC) threshold 2.2 × 10−4 [23] when the SMF length was less than or equal to 5 km. Figure 8(a) shows BERs of the back-to-back signal and the signal after the 5 km long SMF and the compensator when CH5 of the compensator was utilized. The SMF length, which was completely equalized with the compensator, was about 2.8 km. I made a judgement decision of the dispersion compensation based on whether the obtained BER was below the KP4 FEC threshold. Therefore, as the BER was below the FEC threshold when the SMF length was less than 6 km as shown in Fig. 7(a), I set the SMF length at 5 km. Figure 8(b) indicates measured eye diagrams of the back-to-back signal, the signal after the SMF, and the compensated signal. The eyes of the back-to-back and compensated signals were measured when the BERs shown in Fig. 8(a) were minimum. The BER of the compensated signal marked the minimum value of 1.1 × 10−6. The BER of the severely distorted signal after the SMF could not be evaluated. The eye of the compensated signal was open, and the distorted signal after the SMF was recovered to some extent with the compensator although it did not completely equalize the SMF dispersion and had some characteristics imperfectness including transmittance (loss) variation and delay time ripples within the channel.
Fig. 7. (a) BERs and (b) eye diagrams versus SMF length relating to compensated optical 40 Gsymbol/s PAM4 signals with CH5 of WDM multichannel dispersion compensator.
I next investigated compensation effect of all the compensator channels shown in Fig. 4(b). Figure 9 summarizes estimated BERs of 40 Gsymbol/s PAM4 signals, which passed through the 5 km long SMF and were compensated for with the ten channels of the compensator. The received optical intensity for the measurement was −3.4 dBm. The BERs ranged 5.7 × 10−8 to 6.0 × 10−6, which were below the KP4 FEC threshold. The BERs tended to increase with increasing wavelength due to the impact of the SMF dispersion slope. I did not consider the compensation for the fiber dispersion slope when I designed the compensator. The BERs when using CHs 2, 6 and 8 were smaller than those when using their peripheral channels because dispersion compensation values in these channels were relatively larger as shown in Table 1.
Fig. 8. (a) BERs and (b) eye diagrams of back-to-back and compensated optical 40 Gsymbol/s PAM4 signals.
Fig. 9. BERs of compensated optical 40 Gsymbol/s PAM4 signals with ten channels of WDM multichannel dispersion compensator.
4. Conclusion
I demonstrated a completely passive integrated-optic chromatic dispersion compensator, which consisted of 100 fixed delay lines and two AWG-type wavelength filters. I realized two kinds of dispersion compensators (a multichannel dispersion compensator for short-distance WDM communication and a dispersion slope compensator for a DSF) using the configuration. The multichannel compensator showed dispersion compensation characteristics (average dispersion: −47.5 ps/nm) at ten channels in the 1.55 µm band, whose average bandwidth and interval were 209 GHz and 250 GHz, respectively. I confirmed, through BERs and eye diagrams measurement, that the compensator could equalize distorted 40 Gsymbol/s (80 Gbit/s) PAM4 signals, which passed through a 5 km long SMF (dispersion: 86 ps/nm at 1.55 µm), with all the ten channels. All the estimated BERs after the compensation were below the KP4 FECthreshold (2.2 × 10−4). The obtained dispersion slope and bandwidth of the dispersion slope compensator were −3.03 ps/nm2 and 19.8 nm, respectively. The slope compensator can equalize the dispersion slope of about a 43.3 km long DSF with the dispersion slope of 0.07 ps/nm2/km.
Funding
Japan Society for the Promotion of Science (22H00219, 23H04811); Telecommunications Advancement Foundation.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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