1. Introduction

Extending the capacity of optical networks with minimal increase of costs and energy consumption has become a high-priority task in order to support the current growth of Internet traffic. As transmission bottlenecks approach data centers and metro/access networks, use of wavelength division multiplexing (WDM) and advanced modulation formats in these systems appears to be more and more attractive [1]. Yet the need of tens of wavelength-stabilized distributed feedback (DFB) lasers to produce optical carriers for WDM constitutes a severe limitation for a widespread deployment of this technology. A promising solution to this problem consists in replacing a multitude of independent oscillators with a single optical frequency comb (OFC) generator, providing several equally spaced wavelength channels [2]. Using OFCs, multiple terabit per second data transmissions have been achieved with the help of advanced modulation formats, such as quadrature phase-shift keying (QPSK) and 16-state quadrature amplitude modulation (16-QAM), and bandwidth-efficient frequency multiplexing techniques such as optical orthogonal frequency division multiplexing (OFDM) [3] and Nyquist WDM [4–7]. The technologies employed in these experiments, however, may prove to be too complex to be practically implemented in cost sensitive systems, such as data centers or networks for high-performance computing (HPC). As a consequence, the most appealing approaches for these applications currently involve the use of arrays of directly modulated DFB lasers [8,9] or vertical-cavity surface-emitting lasers (VCSEL) [10]. In these communication systems, the optical links within and between data centers are anticipated to be no longer than 2 and 80 km [11], respectively, and to encode data only on a single polarization state of light. Moreover, indications are that transmitter lasers used in data center interconnects will likely adopt the IEEE 802.3 GBase standard that tolerates −128 dB/Hz of relative intensity noise (RIN) [12]. Therefore, ad hoc transmission architectures have to be devised in order to comply with the above requirements. Intensity modulated and directly detected (IM/DD) OFDM in combination with OFCs from semiconductor passively mode-locked lasers (PMLLs) is a promising technology for high data rate transmissions over relatively short links (tens of kilometers down to hundreds of meters). IM/DD OFDM, in fact, allows for a simple receiver architecture and high spectral efficiency, owing to the overlapping of electrical subcarriers [13]. Moreover, it provides excellent tolerance to chromatic dispersion through the use of a suitable cyclic prefix [13,14] and the possibility of employing compatible single side-band (SSB) modulation [15,16] to significantly reduce the signal bandwidth and, therefore, dispersion induced power fading. Semiconductor PMLLs, on the other hand, are ideal candidates for hybrid integration on the emerging silicon photonics platform for data center interconnects [17], owing to their compactness and the reduced number of optical interfaces compared to DFB laser arrays. In such framework, PMLLs based on III-V semiconductors can be connected to Si chips using, for example, photonic wire bonding technology [18], while their longitudinal modes can be individually demultiplexed and modulated, respectively, by means of Si-based ring resonator filters and modulators [19].

Recently, using IM/DD SSB-OFDM in combination with the terahertz-wide OFCs from single-section InAs/InP quantum dash based PMLLs, we have demonstrated Tb/s data transmission in a scenario compatible with intra-data center communications [20]. However, the large number of comb lines generated by these devices at a given output power level results in a relatively low power per optical carrier, leading to an increased sensitivity of the system to intensity noise and a reduced transmission distance. These limitations can be overcome by using quantum well (QW) based PMLLs, providing a large output power over a relatively narrow spectral bandwidth.

In this paper, we report for the first time on the successful use of a single-section QW PMLL as a comb source for WDM data transmission employing IM/DD SSB-OFDM data signals. Sixteen comb lines spaced by 37.6 GHz are modulated using 25 Gb/s compatible SSB-OFDM signals and transmitted over 50 km of standard single-mode fiber (SSMF) with bit error ratio (BER) performance below the 7% forward error correction (FEC) limit (BER = 4.4 × 10⁻³ [21]). As the modulation scheme adopted is particularly vulnerable to intensity noise due to its inherently high carrier-to-signal ratio [22], for our investigations we use a single QW laser, especially designed to reduce coupling of spontaneous emission with lasing modes [23]. In this work, specific device characteristics such as the mode beating spectrum and the RIN of the laser are studied in detail prior to performing transmission experiments. Then, the system performance is evaluated on the basis of these preliminary characterizations and compared to those achievable with a single-channel external cavity laser (ECL). A net data rate of ~400 Gb/s using single-polarization IM/DD SSB-OFDM is demonstrated with a compact, inexpensive MLL source in a scenario compatible with inter- and intra-data center communications.

2. Device fabrication and basic characterization

The device employed for our study is a 1-mm-long, single-section ridge waveguide laser with as-cleaved facets. The component is fabricated from an epitaxial structure grown by gas source molecular beam epitaxy (GSMBE) on an S-doped (001) InP substrate and consisting of a single InGaAsP strained quantum well embedded in 214-nm-thick InGaAsP barriers. This design provides a low optical confinement factor of about 1%, resulting in a reduced impact of spontaneous emission on the amplitude and phase noise of laser longitudinal modes [23]. A modal gain of ~13 cm⁻¹ and internal loss of ~7 cm⁻¹ is extracted from measurements on broad area lasers processed from the grown structure. The investigated laser has a ridge width of 2 μm, ensuring single transverse mode operation, and is mounted on a copper base for thermal management purposes. The operating temperature is kept constant at 20 °C using a Peltier cooler and a temperature controller with a stability of ± 1 mK.

Light-current characteristic as well as the mappings of optical and radio-frequency (RF) spectra measured for the device are shown in Fig. 1. The laser, emitting around 1.56 μm, features a threshold current of 16 mA and output power at saturation of 20 mW. For injection currents above 50 mA, a relatively flat comb spectrum with FWHM on the order of 4 to 6 nm is observed. Strong phase correlation between the comb lines is demonstrated by the narrow RF line shape at the MLL repetition frequency f_r = 37.6 GHz. RF linewidth FWHM on the order of 100 kHz and below, exhibited by the device for a current range as large as 150 mA, indicates good mode-locking performance [24]. The laser repetition frequency is observed to mainly increase with current over the whole operating range, revealing discrete bias points at which sharp transition of the emission spectrum toward longer wavelengths is accompanied by larger values of the beat-note frequency. This hopping behavior, already observed for single-section multiple QW MLLs [25], is typically associated with the nonlinear gain of the laser [26], while the increase of repetition frequency is attributed to the normal dispersion of the semiconductor material, resulting in lower group refractive indices for longer wavelengths [25]. At these bias points at which sharp transition of the emission spectrum occur, stability of both RF frequency and comb lines frequencies is compromised. However, far from these current values, a reduced fluctuation of these quantities is observed. In this work, the device is operated at a bias current of 155 mA, which provides an average power in the fiber of 10 dBm. The optical and RF spectra at this bias point are shown in Figs. 2(a) and 2(b), respectively. Optical linewidth for the different longitudinal modes varies from 5 to 7 MHz, as measured with delayed self-heterodyne technique [27], while an RF linewidth of 59 kHz is deduced from a Lorentzian peak fit.

 

Fig. 1 (a) Light-current characteristic, (b) Optical spectrum and (c) RF spectrum mapping as a function of current for the 1-mm-long, single-section QW laser.

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Fig. 2 (a) Optical spectrum and (b) RF spectrum of the single-section QW laser at 155 mA.

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For transmission experiments, the output of the MLL is amplified to 19 dBm using a low noise-figure (NF = 3.8 dB) Erbium-doped fiber amplifier (EDFA) and an optical bandpass filter (OBPF) is used to select 16 comb lines as WDM optical carriers. Figure 3(a) shows the filtered lines, numbered with increasing wavelength, together with the corresponding optical-carrier-to-noise ratio (OCNR), measured prior to amplification with an optical spectrum analyzer (resolution bandwidth 20 pm). For all comb lines the OCNR is found to be greater than 49 dB, owing to the relatively high output power distributed over a narrow bandwidth. To further characterize the intensity noise of the different wavelength channels, we measure the RIN spectra over the band 0-10 GHz for the full spectrum and for a given set of individual comb lines (Ch. 1, 4, 8, 12, 16). Results shown in Fig. 3(b) indicate the presence of significant mode partition noise, typical of passively mode-locked semiconductor lasers [28], as the RIN of individual modes is on average −131.2 dB/Hz while that of the full comb is −145.6 dB/Hz. The comb lines exhibit analogous intensity noise spectra, with a pronounced RIN component at frequencies below 2 GHz. These features may have a detrimental effect on system performances at low frequencies, even for OCNR as large as 50 dB [29].

 

Fig. 3 (a) Filtered comb after amplification: optical carrier-to-noise ratio before EDFA is indi- cated with red circles, modes are numbered with increasing wavelength. (b) Relative intensity noise spectra (0-10 GHz) for the entire comb and for a number of comb lines (Ch. 1, 4, 8, 12, 16).

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3. Experimental setup for IM/DD SSB-OFDM transmission

System experiments are carried out using the setup shown in Fig. 4. The amplified and filtered comb lines reported in Fig. 3(a) are modulated with a single dual-drive Mach-Zehnder modulator (DD-MZM). In practical installations, each wavelength sub-channel would be separated by a demultiplexer and individually modulated. In addition, the EDFA used to compensate for the large insertion loss of the OBPF and the modulator (~14 dB overall) would be replaced by an integrated semiconductor optical amplifier (SOA), following the approach described in [30]. The DD-MZM is biased at the quadrature point and electrically driven with amplified SSB-OFDM signal waveforms, derived from an arbitrary waveform generator (AWG, Tektronix AWG70002A) operating at 25 GSa/s and constructed as reported in [31]. The OFDM signal is comprised of 76 sub-carriers, each with a bandwidth of 97.66 MHz. Each sub-carrier is encoded with 16-QAM modulation. The total bandwidth of the OFDM signal is ~7.5 GHz with a raw data rate of ~29.6 Gb/s. An additional frequency offset of 800 MHz from the carrier frequency is applied, so the highest frequency component of the modulated signal is 8.3 GHz. This allows avoiding the strongest components of harmful signal-signal beating, characteristic of IM/DD OFDM systems [15]. The peak-to-average power ratio (PAPR) of the signal is about 11.5 dB. The OFDM signal includes 10 training sequences and a cyclic prefix of 6.25% of the IFFT size (which has 256 inputs). In addition an overhead of 7% is assumed for FEC. After overhead subtraction, the net data rate of each OFDM sub-channel is ~25 Gb/s, which over the 16 sub-channels results in a total super-channel data rate of ~400 Gb/s in a single polarization and a spectral efficiency of 0.66 bit/s/Hz.

 

Fig. 4 Schematic of the experimental setup for IM/DD SSB-OFDM transmission.

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In order to decorrelate adjacent channels, odd and even sub-channels are split with tunable cascaded disinterleavers based on asymmetric Mach-Zehnder interferometers (AMZI). The even channels are then passed through a 5 m fiber patchcord, prior to being recombined with the odd channels. The overall signal is subsequently amplified with an EDFA operated in constant power mode and transmitted over 50 km of SSMF. At the receiver side, the individual sub-channels are selected at a pre-amplified filtered stage consisting of a tunable OBPF, an EDFA and a narrow bandwidth, tunable OBPF. Each filtered channel is detected with a 10 GHz receiver composed of a PIN photodiode and an integrated trans-impedance amplifier (TIA), and the signal is recorded using a real-time oscilloscope (RTO) with a sampling rate of 50 GSa/s. Digital processing of the received signal, and BER calculations are then performed offline using Matlab.

4. System performance

The bit-error ratio (BER) performance of the 16 sub-channels determined by error counting over 128 OFDM symbols is shown in Fig. 5(a). For all the sub-channels, the BER after the 50 km SSMF transmission is still below the 7% FEC limit of 4.4 × 10⁻³ [21] (shown as the red line). The performance of the system degrades with increase of RIN and reduction of OCNR [29], and BER results of each sub-channel are in good agreement with the corresponding OCNR values reported in Fig. 3(a). While a larger number of OFDM symbols could be used to improve the statistical significance of BER evaluation, the 23600 bits simulated in total per sub-channel are enough for this purpose as the BER level is above 10⁻³. The insets of Fig. 5(a) show the 16-QAM constellation diagrams for sub-channel number 10, exhibiting a BER of 3 × 10⁻³, and sub-channel number 13, with a BER of 4.2 × 10⁻³, respectively. Also, for the sake of comparison, the transmission performance of a single-channel external cavity laser (ECL, Emcore ITLA module) through the test bed was measured, and is indicated here with a green line at a BER of 1.3 × 10⁻³. The higher BER exhibited by the QW MLL compared to the ECL is attributed to larger RIN of the comb source, due to mode partition noise on the filtered comb lines, and lower OCNR.

 

Fig. 5 (a) Bit error ratio (BER) performances for the QW sub-channels compared to that of a single-channel external cavity laser (ECL). In the insets: 16-QAM constellation diagrams of all the 76 OFDM sub-carriers for sub-channels number 10 and 13. (b) Error-vector magnitude (EVM) of the 16-QAM constellations of the 76 ODFM sub-carriers for QW sub-channels number 10 and 13 and ECL. RIN spectrum of Ch. 10 is also included for comparison.

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The effect of RIN on system performance is made clear when the error-vector magnitude (EVM) of the 16-QAM constellations of the 76 ODFM sub-carriers is investigated more closely. Figure 5(b) compares the EVM performance of sub-channels number 10 (red circles) and 13 (green triangles) and the single-channel ECL (blue squares). The best achievable performance for the test bed, as indicated by the ECL, shows that the EVM degrades linearly with increasing frequency (increasing sub-carrier number). This degradation is not a consequence of digital-to-analog converter (DAC) performance deterioration with frequency, as we could verify by evaluating the system performance for shorter lengths of SSMF [32]. Rather, it is due to the non-linear phase shift among OFDM sub-carriers introduced by the data amplifiers used after the AWG to drive the DD-MZM, significantly impairing the ability to correctly reconstruct the OFDM signal at higher frequencies. While this deterministic effect could be compensated at the transmitter side to attain optimal system performances and allow a fine analysis of the impact of the high-frequency RIN of the PMLL, such a limitation does not prevent to conclude on the role of mode partition noise on the measurement. Indeed, even though at frequencies above 2 GHz, the EVM performance of Ch. 10 and Ch. 13 is similar to that of the ECL and limited mostly by the response of the electrical amplifiers, below 2 GHz the EVM performance decreases with decreasing frequency (or decreasing OFDM sub-carrier number) for both Ch. 10 and Ch. 13. Figure 5(b) also shows the RIN spectrum of Ch. 10, and it is clear that as the intensity noise increases at the lower frequencies, the EVM increases accordingly. By comparison, the average RIN of the ECL is −140 dB/Hz, with a flat power spectrum. This indicates that low-frequency intensity fluctuations are the main cause of performance degradation when using semiconductor passively MLLs as comb sources for IM/DD SSB-OFDM transmission. Yet a low confinement factor epitaxial structure and laser mode-locking in single-section configuration are valuable solutions to achieve better RIN than standard two-section MLLs [29]. These, in fact, allow maximizing the optical power per comb line and reducing the impact of spontaneous emission noise on the lasing modes.

5. Conclusion

In the present work, we have investigated the use of a 37.6 GHz repetition rate, single-section mode-locked laser based on a single InGaAsP/InP quantum well, as comb source for multi- carrier OFDM transmission. The laser, suitably designed to have a low optical confinement factor, presents relatively low intensity and phase noise, as well as sufficient stability for WDM communications. By modulating 16 comb lines with ~25 Gb/s compatible SSB-OFDM signals, data transmission over 50 km of SSMF at a total net data rate of ~400 Gb/s in a single polarization has been achieved. BER performances compatible with the 7% FEC limit are found to be limited mainly by the mode partition noise of the device, being more pronounced at low frequencies. Yet the average RIN values below −130 dB/Hz measured for the longitudinal modes of the QW MLL under test are consistent with the requirements within the IEEE 802.3 transmission standards.

Using the approach described here, the overall capacity and spectral efficiency can be further improved by using Fabry-Pérot lasers with lower free-spectral ranges, provided that the output power of the devices scales with the number of comb lines. Free-spectral range can also be defined lithographically by closing the laser cavity with distributed Bragg reflectors, as demonstrated in [30]. Single quantum well lasers, being mature from a manufacturing point of view and providing reduced noise and large output power over a relatively narrow bandwidth, are good candidates as optical multicarrier sources for optical interconnects. Their compact size and the capability of generating tens of comb lines at rather low injection currents make these devices very attractive for inter- and intra-data center communications, as well as for hybrid integration on the emerging silicon photonics platform.