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Abstract
We present monolithically integrated multi-channel coherent L-band transmitter (Tx) and receiver (Rx) photonic integrated circuits (PICs) on InP substrates. The L-band PICs are able to provide post-forward error correction (FEC), error-free operation for dual-polarization (DP) 16-QAM coherent transmission at 33 Gbaud. These transceivers operate at 200 Gbps per channel and support 1.2 Tbps aggregate capacity per 6 channel PIC. We also demonstrate in this work a C + L band communication system with two C-band superchannels (2 x 6λ) and three L-band superchannels (3 x 6λ) over a 600 km link. The received signals all have Q > 7.7 dB, which is well above the error-free threshold of the FEC used in this work.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The demand for internet bandwidth has been steadily increasing for the past two decades and has accelerated with the recent advent of applications such as video streaming, cloud services, and machine-to-machine communications. To satisfy this increasing demand and to provision for 5G and other new services, data center interconnect providers, fixed internet service providers and long-haul providers need to expand capacities in their communication system backbones in a cost-effective manner. Wavelength-level switching with manageable bandwidth is also necessary to efficiently route data streams for many applications. Coherent dense wavelength division multiplexing (DWDM) communication addresses these needs, with spectral efficiency near the theoretic limit, by accessing two degrees of freedom per optical field polarization [1].
High capacity C-band DWDM coherent systems often consist of more than 100 wavelengths with each wavelength spaced by 50 GHz or less [2]. Multi-channel coherent PICs with up to 14 such tunable channels on a single chip have been demonstrated [3,4] and provide a reduced cost and minimum device footprint solution for coherent DWDM systems. Additionally, monolithic InP PICs reduce the number of optical couplings between discrete components and improve reliability [5]. Over the past decade, we have reported increasing PIC channel density as well as DWDM PIC data rates from 10 Gbps per wavelength in 2004 (On-Off-Keyed, OOK) [6] to coherent PICs and systems with up to 100Gbps per channel [7–11] to 200Gbps per wavelength in production modules [12], research results of 600Gbps per wavelength in modules [13], and 880 Gbps (Rx) and 1Tbps (Tx) per wavelength research results in 2018 (coherent with dual-polarization) [14]. With spectral efficiency approaching the theoretical limit, it is difficult or expensive to significantly increase the fiber capacity for current communication systems limited to the C-band. Expanding into the L-band, however, provides a path to readily double the routable fiber capacity for a given modulation format and data rate while avoiding a significant investment in higher bandwidth DSP, electrical and optical component capabilities. Extension into the L-band has become feasible, since the common components used commercially in C-band optical networks, such as erbium-doped fiber amplifiers (EDFAs) and switches, can be extended to cover the L-band.
An advanced C + L-band transmission system using discrete optical components has been previously reported [15]. A four-channel WDM L-band transmitter comprised of four discretely packaged, directly modulated lasers and a four-channel Si PIC that multiplexed and pulse carved the modulated laser outputs has been reported [16]. Other demonstrations of partial integration include integrated passive Si photonics modulators with Ge receivers for coherent transmission and detection using off-chip lasers [17], and 4 x 56 Gbps electro-absorption modulated laser arrays on InP for PAM4 amplitude modulation [18]. However, to date there are few reports combining the benefits of coherent L-band DWDM with monolithically integrated PICs. Most recently, we reported preliminary L-band transceiver results [19], with monolithically integrated PICs.
In this work, we present monolithically integrated 6-channel L-band coherent transmitter and receiver PICs on InP. The L-band PIC modules show post-FEC, error-free operation for DP 16-QAM coherent transmission at 33 Gbaud achieving a data rate of 200 Gbps per channel or 1.2 Tbps per PIC. A C + L PIC-based link consisting of two C-band superchannels (2 x 6λ) and three L-band superchannels (3 x 6λ) with a 600 km reach is also demonstrated. The received signals show a Q > 7.7 dB, well above the FEC error-free threshold.
2. L-band PIC architecture
The L-band transceiver used in this work consists of a transmitter and a receiver PIC pair. PICs, free-space optics, and Tx and Rx application-specific integrated circuit (ASIC) chips are all packaged inside a hermetically sealed transceiver module like the ones in previously reported C-band demonstrations [3,4,20].
The L-band Tx PIC has 6 channels transmitting 200 Gbps per channel for a total data rate of 1.2 Tbps per PIC. A schematic of the monolithic PIC architecture is shown in Fig. 1. Each channel of the Tx PIC includes a widely tunable laser (WTL) with full L-band coverage. The continuous wave (CW) WTL output is fed into two independent, nested I-Q Mach-Zehnder modulator (MZM) circuits, labeled as TE and TE’. Since we use compressively-strained quantum wells in the active layers that provide gain only in the TE polarization, both TE and TE’ signals also have their electric fields aligned in the plane of the epitaxial layers [21,22]. Outside the Tx PIC chip, the TE path maintains its polarization while the TE’ path is rotated by 90 degrees by the free-space optics of the transceiver module, and hence becomes the perpendicular TM polarization of the dual-polarization transmission format inside a fiber. A variable optical attenuator (VOA) is included after each MZM circuit for balancing the signal levels between the TE and TE’ arms of the same channel, and semiconductor optical amplifiers (SOAs) boost the optical power before TE or TE’ channel wavelengths are power-combined (red and blue boxes in Fig. 1.) RF pads are provided in the MZM circuit to achieve modulation at the 33 Gbaud rate shown in this work. InP-based waveguides route the signals between the above described components in the monolithically integrated PIC on an InP substrate.
Fig. 1 Schematic for Tx and Rx PIC architectures. Each PIC has 6 channels with each channel delivering 200 Gbps, or 1.2 Tbps per PIC.
The monolithic L-band Rx PIC also has 6 channels and mirrors the Tx PIC in architecture. The signal entering the transceiver module is split into TE and TM polarizations, and is rotated only on the TM path with free space optics (FSO) to launch the TE’ signal. Similar to the Tx PIC, TE and TE’ signal designations defined to track the signals which are both polarized TE with respect to the epitaxial layers of the Rx chip. TE and TE’ signals from the FSO are coupled into the Rx PIC, power split and broadcast into the multi-mode interference (MMI)-based 4 × 4 hybrids of all channels. Each channel has a local oscillator (LO) that is a WTL with full L-band coverage, supplying light to both TE and TE’ hybrids. Each hybrid supplies mixed LO and signal light to four high-speed photodetectors (PDs) that provide coherent decoding of the I-Q RF signal components for each polarization. An MMI taps some power from each LO for laser characterization. The high-speed RF PD signals are fed into a transimpedance amplifier and a digital signal processor (DSP) chip to recover the originally transmitted signals.
3. L-band tunable laser performance
The L-band for this work is defined as a 4.8 THz band from 186.250 THz to 191.050 THz (~40 nm wide), which is adjacent the red side of the extended C-band which spans 191.325 THz to 196.125 THz (also 4.8 THz wide).
WTLs are well known for their ability to provide continuous tuning over a wavelength range as wide as entire C-Band using the vernier effect of two slightly dissimilar mirrors [23,24]. The WTL design used in this work is described in detail in [19]. The grating- based front- and back-mirrors of each L-band WTL in this work have ~5 nm reflectivity comb spacings but are slightly different from each other. With independent tuning of the front and back comb-like reflectivity spectra, a vernier effect can be achieved, providing more than 40 nm of continuously tunable lasing wavelengths. Figure 2 shows the lasing wavelength maps for all 6 channels of an L-band Tx PIC, operated in CW mode. The horizontal and vertical axes of a map are the estimated effective index changes of the front- and back-mirror sections, labeled as Mirror A and Mirror B, and are calculated as
where ng is the waveguide group effective index, 𝑑λ is the wavelength shift on a mirror from tuning, and λc is the center wavelength of the tuning range of interest. The WTLs are biased at greater than 3 × the threshold current, while the phase section is left floating without bias when the data of Fig. 2 was recorded. These maps show that by appropriate choice of mirror indices, the widely tunable laser can access the entire L-band. For stable continuously tunable operation, the phase section is used to align the cavity modes with the mirror reflectivity peaks. Figure 3 shows the continuous tuning range for all 6 channels of an L-band Tx PIC. All channels have the full 4.8 THz L-band coverage with actual tuning ranges greater than 5.5 THz, offering substantial design and manufacturing margins comparable to the C-band WTLs previously reported [3].Fig. 2 Widely tunable laser wavelength tuning maps for all 6 channels of an L-band Tx PIC as a function of the estimated effective index change of the two mirror sections.
Fig. 3 Full laser tuning range for all 6 channels of an L-band Tx PIC, with mirror and phase sections biased to provide continuous tuning. The L-band range of 4.8 THz is indicated by the gray shaded area. All channels show full L-band coverage with sufficient margin for manufacturability.
Laser linewidth (LW) is critical to coherent communications since LW represents an important component of the overall spectral phase noise [25]. Measured FM noise spectra, extracted from offline processing of the constellation captured with a high-speed oscilloscope similar to [26], is shown in Figs. 4(a) and 4(b) for an L-Band PIC. The FM noise extracted with this method is the total noise, and it includes noise from sources other than the laser. Source driver contribution to phase noise in the 10-100 MHz frequency range is low and this band can be used to estimate the LW of the laser. Figure 4(a) shows the phase noise power spectral density (PSD) for that frequency band for an L-band Tx PIC that is below 8x104 Hz2/Hz for all 6 channels. LW can then be estimated as < 250 kHz for all tested L-band conditions with WTL wavelengths set to 1588 nm, which is near the L-band center.
Fig. 4 Measured widely tunable laser phase noise power spectral density (PSD) for an L-band Tx PIC (a) for all 6 channels at 1588 nm, and (b) for a single channel at the L-band center (1588 nm) and edge wavelengths (1570 nm and 1610 nm). The inferred laser linewidth is ~250 kHz in all cases.
Figure 5 shows the normalized spectra from a single L-band WTL with 100 GHz spacing spanning the full 4.8 THz L-band, from 186.250 THz to 191.050 THz. The L-band WTL side-mode suppression ratio (SMSR) is clearly greater than 48 dB, comparable to that of the C-band WTLs [3,4,20]. 200 Gb/s per wave systems can achieve a total of 27.6 Tb/s from 138 tightly packed waves in the L-band (4.8 THz bandwidth).
Fig. 5 Transmitter normalized DC power vs. frequency for L-band PICs in steps of 100 GHz. WTL SMSR is greater than 48 dB for all frequencies.
4. L-band PIC manufacturing capability
To evaluate L-band PIC capability, key performance parameters of L-band PICs are compared to those of C-band PICs in Fig. 6. Comparison is made for PICs manufactured under similar conditions, with at least 150 PICs per optical frequency band. The manufacturing process for monolithically integrated L-Band PICs is similar to the description in [27]. Values plotted in the cumulative distributions in Fig. 6 are for the limiting channel per PIC and are normalized with respect to the same value for L-Band and C-Band PICs in each sub-plot except for IQ imbalance, which by itself is a normalized value.
Fig. 6 Performance capability comparison of key parameters for L-band PICs and for C-band PICs, with at least 150 PICs in each group. Distributions shown for (a) Tx PIC laser threshold current normalized to the spec (b) Normalized Tx laser power measured with on-chip detectors (c) Tx PIC MZM Vπ deviation from target. (d) Rx PIC laser threshold normalized to spec (e) Normalized Rx laser power measured with on-chip detectors and (f) RX PIC IQ imbalance in dB.
Figure 6(a) shows that the distribution of Tx WTL threshold currents normalized to spec is very similar for L-Band and C-band PICs. Figure 6(b), plotting Tx WTL output powers, also shows similarity between L-band PICs and C-band PICs, enabling similar launch powers as currently deployed C-Band PICs in the field. The MZM Vπ deviation from target is also nearly identical as shown in Fig. 6(c).
Figure 6(d) shows that the Rx PIC WTL laser thresholds for the two optical bands are similar. The WTL LO output power of C-Band and L-Band PICs is measured with on-chip photodiodes (Fig. 6(e)), indicating similar distributions and therefore similar optical gain performance as current C-Band PICs deployed in the field. Finally, Fig. 6(f) shows similar Rx PIC IQ imbalance capability over this large sample size.
5. L-band transceiver back-to-back performance
L-band Tx and Rx PICs were first mounted and wire bonded to ceramic carriers and integrated with the Tx ASICs, Rx ASICs, WTL drivers, MZM drivers and free-space optics in a hermetic transceiver package, similar to previously reported C-band PICs [3,4,20]. Back-to-back RF constellation diagrams were then measured between a pair of these packaged L-band transceiver modules, as shown in Fig. 7. They were operated by running 33Gbaud DP 16 QAM data on all 6 channels simultaneously, with each channel packed on a 37.5 GHz spacing near the center of the L-band. The exact set of channel center frequencies was 188.5563, 188.5938, 188.6313, 188.6680, 188.7063 and 188.7438 THz. The transmitted signals were digitally synthesized Nyquist-shaped subcarriers, known to improve performance against fiber nonlinearities [28]. The constellation diagrams were constructed with offline processing of direct scope capture of the received data. The 33 Gbaud constellations display well-defined symbol locations for both polarizations of all 6 channels.
Fig. 7 Back-to-back DP 16-QAM × 33 Gbaud constellations for all 6 channels between an L-band Tx PIC module and an L-band Rx PIC module. The channel spacing is 37.5 GHz for wavelengths at the center of the L-band.
6. C + L band system demonstration
We have previously reported on experiments demonstrating 27.6 Tbps fiber capacity spanning over 4.8 THz bandwidth of the C-band using near-Nyquist spectral efficiency superchannels [29]. With the same superchannel architecture, the L-band provides another 27.6 Tbps, doubling the total capacity to 55.2 Tbps over a single fiber.
A simplified example of the C + L optical transmission system, with two C-band and three L-band superchannels, is shown in Fig. 8. Each superchannel consists of six 33 Gbaud, 16-QAM DP wavelengths, each separated by 37.5 GHz, and is coded or decoded by a single 6-channel transceiver. Therefore, two 6-channel C-band transceivers and three 6-channel L-band transceivers are used to provide all the demonstrated wavelengths. The two C-band superchannels are centered at 193.70 and 193.95 THz while the three L-band superchannels are centered at 188.40, 188.65 and 188.90 THz. The guard band between superchannels is set to 12.5 GHz. The two C-band superchannels from two different C-band transmitters (in two transceivers) are multiplexed by a wavelength selective switch (WSS) followed by a C-band EDFA to boost the power to −1 dBm per superchannel. The three L-band superchannels are multiplexed by a passive coupler followed by an L-band EDFA to boost the power to −1 dBm per superchannel as well. The already multiplexed C- and L-band superchannels are then further multiplexed by a band coupler and launched into the fiber link. The fiber link consists of six 100 km Corning TXF fiber spools, with hybrid Raman/EDFA amplifiers following each spool, as shown in Fig. 8. To compensate the approximately 17 dB loss from each 100 km fiber span, a backward propagating Raman amplifier (RA) is included at the end of each fiber spool, followed by C- and L-band EDFAs to maintain signal power through the link. After 600 km of transmission, a band splitter routes C-band light to a WSS, which routes the C-band superchannels to two C-band receivers, and sends the L-band light to three L-band receivers.
Fig. 8 Transmission testbed schematic. Wavelength selective switches (WSS) and passive couplers/splitters are used to mux/demux the C-band and L-band wavelengths respectively at the transmitting/receiving ends.
The transmitted and received link spectra also reported previously [19], are shown in Fig. 9. The C-band noise floor is ~5dB lower than for the L-band spectra due to the use of a WSS for multiplexing C-band channels and a passive coupler for L-band channels. The WSS filters out broadband amplified spontaneous emission (ASE) noise, while passive couplers add noise, but a WSS was not available for the L-band when the data were taken. In spite of this additional noise on L-band channels, the received C- and L-band signals all achieve a Q greater than 7.7 dB, which is well above the error-free threshold for the FEC used in this work. The 600 km transmission distance tested in this work is limited by the available number of 100 km spans, but not by the transceivers. More 100 km spans could be added in future work to test the maximum reach for error-free operation.
Fig. 9 C + L band transmitted (top) and received (bottom) spectra for 2 C-band superchannels and 3 L-band superchannels over a 600 km link.
7. Conclusions
In order to double the fiber capacity of optical links, we have successfully demonstrated 6-channel L-band Tx and Rx PICs monolithically integrated on InP substrates for coherent DWDM transmission. Each PIC is capable of supporting 1.2Tb/s aggregate capacity, with 200Gb/s per channel. All active and passive optical components such as WTLs, MZMs, VOAs, SOAs, PDs, MMI-based power splitters, combiners, hybrids, taps and interconnect waveguides are monolithically integrated on the L-band PICs. The transceiver module package includes the PICs along with ASIC driver/amplifier chips and free-space optics for optical field polarization rotation and dual polarization multiplexing.
The L-band Tx and Rx PICs operate over the full L-band with similar manufacturability and performance as their C-band counterparts, enabling post-FEC error-free operation for DP 16-QAM coherent transmission at 33 Gbaud over a 600 km link.
Our demonstration of L-band PIC-based transceivers doubles the fiber bandwidth capacity on an Infinera line system from 27.6 Tbps (C-band only) to 55.2 Tbps (C + L).
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