A low phase noise frequency comb generated from a continuous-wave seed is experimentally demonstrated across continuous C- and L-bands. Parametrically generated carriers with optical signal-to-noise ratio in excess of 45dB were used to generate 16-ary quadrature amplitude modulated signals. We characterize 20 GBaud channels’ performance that was varied by only 1.7 dB across the combined C/L band.
© 2014 Optical Society of America
The importance of frequency combs has been recognized in both scientific and engineering applications . Apart from metrology [2–5], optical frequency combs provide unique capability in sensing [6, 7], astrophysics , as well as in arbitrary waveform generation [9, 10]. Although previously suggested for wavelength division multiplexed (WDM) transmission, successful demonstrations of comb-based transceivers have been scarce, and past reports particularly lacked the performance necessary for demonstration of high-cardinality modulation formats. Recognizing that WDM systems are at the core of high capacity information transfer , it is necessary to develop frequency combs capable of supporting the modulation formats that are currently commonly realized with high performance single-frequency lasers.
In general, methods of comb generation rely on feedback mechanisms and/or nonlinear signal manipulation. The latter can be divided into techniques that rely on nonlinear modulation techniques (e.g. employing phase modulation) and nonlinear frequency mixing. The former frequency generation class encompasses combs derived from mode-locked lasers [12–14] as well as micro-cavities [15–17]. A device intended for telecom application requires frequency adjustment, a property not shared by cavity-based approaches. Even in case when fixed frequency plan can be accepted, resonator-seeded combs impose strict optical signal to noise ratios (OSNRs) that is well below standard telecommunication lasers (40dB). On the other hand, modulation-based solutions [18–23] are characterized by frequency agility and are free from cavity imposed restrictions. This class of generator, however, is restricted in spectral span by E/O modulation bandwidth, and signal-to-noise ratio due to modulation loss. As a consequence of performance deficiency, there has been limited success in adopting frequency combs in high-capacity coherent communications. Prior demonstrations had either limited spectral coverage , or were restricted to simple modulation formats due to unmanageable carrier noise .
In contrast, parametric combs combine the three key properties: the generation mechanism is cavity-free, unrestricted by E/O conversion, and includes distributed gain mechanism allowing for an efficient, yet band-flexible frequency generation. Indeed, parametric combs have been demonstrably capable of spanning nearly octave bandwidths in pulsed operation , and/or covering the entire 1525-1620nm telecom band in continuous-mode operation, while maintaining high OSNRs [25–30]. However, preliminary report on the performance of higher-order coherent channels carried by parametric comb revealed significant noise aggregation towards the outer edges of the comb spectral span, thus resulting in sensitivity penalty up to 9 dB . This report describes the characteristics of a new frequency comb that significantly reduced noise aggregation. The key to noise suppression in this work was precision dispersion management in the parametric mixer that reduces excessive nonlinear noise coupling. Consequently, we demonstrate 20 GBaud 16QAM generation and detection using 100-GHz pitched parametric frequency comb covering the entire C- and L-bands. The quality of this coherent, multi-tone signal generator was quantified by measuring bit-error-ratios to find maximal variation of 1.7 dB in performance quality over the entire operational band.
2. Advanced design parametric comb phase noise distribution
Taking into account of practical considerations imposed on a coherent lightwave link, a respective candidate source must satisfy a number of highly demanding requirements. Firstly, the source must provide narrow and sufficiently stable carrier linewidth, defined by the complexity of the modulation format as well as the transmitted symbol rate . Secondly, the signal integrity generation at the transmit side mandates sources with high output power as well as high OSNR, simplifying the transceiver implementation and ultimately enabling longer reach . To date, only few comb implementations have jointly fulfilled all of the above requirements.
In particular, the phase stability of each constituent frequency tone plays a critical role in evaluating the comb source compatibility with higher order (coherent) modulation formats. Furthermore, in terms of maintaining the phase stability across the complete emission spectrum, frequency combs represent a particularly challenging construct. Indeed, frequency combs generated via a nonlinear action, regardless of the original seeding mechanism, are inherently characterized by phase noise increase with the comb tone order. The latter property is a consequence of the seed emission phase uncertainty, preventing a penalty-free frequency generation. In this respect, traveling parametric mixers offer the most accessible platform for phase noise inhibition since they provide access to frequency tones with hundreds of lines [27, 28] while preventing resonant (higher-order) mixing processes from occurring. Recent development of shock-wave mixer (SWM) design has relied on such generation mechanism [25–28], allowing scaled, high-OSNR frequency generation. However, early demonstrations of SWM-based combs were not aimed at increased phase stability and suffered from a considerable deterioration of the phase integrity with the ascending line order (i.e from the comb center and towards the spectral edges). In contrast to mixers aided by resonant mechanism, the SWM mixer does not need to operate in highly anomalous regime. With distributed dispersion engineering, the parametric mixer can suppress excess noise even in the absence of spectral confinement simply by alternating regions with anomalous and normal dispersion. A three-stage parametric SWM comb generator, engineered to produce carriers with OSNR in excess of 45dB was constructed and used in the measurements, as shown in Fig. 1.
The comb generator was seeded by a low phase noise external cavity laser (LM), which was frequency-split into distinct waves and regenerated by an injection-locked distributed feedback (DFB) laser diode pair (LS1 and LS2). The pumps created by LS1 and LS2, after amplification and filtering, were jointly coupled into the parametric mixer comprising the following stages. The first stage was made of a highly-nonlinear fiber (HNLF1, γ = 22 W−1km−1) which was tensioned to suppress stimulated Brillouin scattering. A subsequent standard single-mode fiber (SMF1) provided adequate anomalous dispersion to compress the output field of HNLF1, thereby increasing its peak power. The peak power boost availing shock-wave mediated spectral expansion was further enhanced in the second nonlinear fiber-dispersive fiber tandem (HNLF2 – SMF2). The mixer is concluded by an additional mixing stage (HNLF3) in which the final 100nm-wide comb was created. A noteworthy distinction of this mixer from previous designs rests on the longitudinally-engineered dispersion in the last nonlinear stage – its dispersion along the fiber span was controlled to avoid entering anomalous dispersion regime, in which unmanaged noise accumulation would occur . The comb output is shown in Fig. 2. The average power for tones within 1530 – 1600 nm was 3 dBm. While lower than discrete laser diodes providing 10-dBm output power, the comb output power budget can be improved by reducing the splicing loss in the mixer which amounted to 3 dB in current iteration in conjunction with elevated pump power. The performance of a new comb source with higher power per tone will be presented in future report.
The phase stability of the comb’s constituent lines was characterized by filtering a desired line from the cascade and extracting its phase characteristics by homodyne detection with an aid of an external cavity laser (ECL) serving as a local oscillator (Lorentzian FWHM linewidth ≈30 kHz). The characterization setup is illustrated in Fig. 3. Using a real-time oscilloscope with 12.5 GHz bandwidth, the phase evolution of the comb line was recorded in 40-μs-long homodyne traces. The phase error variance of each line was subsequently estimated for 20 GBaud symbol rate by down-sampling the homodyne signal to 20 Gsamples/s, followed by carrier phase recovery using standard Viterbi-and-Viterbi algorithm with a phase-averaging filter window length of 101 to remove slow LO frequency drift . The characterization results are shown in Fig. 4, covering both the C-and L-band regions. Compared to the previous noise unmanaged parametric comb realizations  (phase error variance = 4.9 × 10−3 rad2 at 1530 nm), the phase variance of the new design shown in Fig. 1 is decreased by a factor of five over the entire band of operation.
3. Experimental results: comb-based quadrature amplitude modulation performance characterization
The wide-band comb qualification for coherent transmission was validated for a 20 GBaud 16QAM modulation format. The modulation format was chosen so as to specifically investigate the phase integrity of the generated multi-wavelength source. The corresponding experimental setup is shown in Fig. 5.
A comb line under test was extracted by a cascade of filters and was used as a carrier for QAM modulation. An arbitrary waveform generator was used for 20 GBaud multi-level signal generation. The 16 QAM modulation was imprinted onto the carrier with a 28GHz nested Mach-Zehnder modulator, whereas the generated signal was amplified to 10 dBm ensuring performance characterization unimpeded by the receiver thermal noise. The thus generated signal was noise loaded and filtered by a 0.6nm optical band pass filter, whereas an amplified 100kHz external cavity laser (ECL) was used as a local oscillator in a standard (single polarization) intra-dyne [34,35] detection arrangement. The intra-dyne signal was captured on a real-time oscilloscope, with digital signal processing having been performed on a personal computer (PC). The PC-controlled setup perfectly emulates the bit-error-ratio testing (BERT) system with off-line processing. The digital signal processing (DSP) procedures encompassed standard LO frequency offset removal, the maximum likelihood carrier phase recovery , as well as symbol detection. The BERs measurements were performed for a 215 de-Brujin sequence (with appropriately delayed regular and inverted waveforms generating the 16QAM signal stream in two quadratures of the electric field)and for a range of OSNRs. In the characterization, one out of every five lines across the comb emission spectrum had been subject to testing. The smooth ascending phase noise characteristic (see Fig. 4) fully justifies the adopted reduced set characterization approach. The BER measurement results are shown in Fig. 6.A total deviation of 1.7 dB in the OSNR penalty was recorded in C and L bands, respectively. We note that different sets of amplifiers and filters were used in the characterizations in the two bands, owing to their respective underlying band-specific limitations. A set of 40 GHz discrete balanced detectors and hybrids were used in the two bands, of which we note that a nominal 1 dB performance difference (in favor of the C-band receivers) was rigorously established prior to the experiment, the consequence of which is recognized by a 1dB performance offset observed between the results in C- and L-bands, shown in Fig. 6.
Figure 7 shows a comparative performance throughout the comb emission spectrum as the OSNR (with the noise power level measured in a 0.1nm bandwidth) required to reach the BER level of 10−3. In both transmission bands, we observe only a minor performance deterioration towards the edges of the spectrum (i.e. from the center of the comb towards the outliers in either direction), as expected from the direct phase noise characterization results (see Fig. 4). As implied before, the measurable loss of performance within outer comb emission boundaries is a consequence of the spectral tone linewidth increase originating in non-ideal seed-seed generation and intra-mixer noise accumulation. While measurable, this penalty was negligible when measured with 16QAM modulation. Specifically, the measured sensitivity at BER = 10−3 varied less than 1.7 dB throughout the entire C/L band and represents the smallest phase stability excursion measured to date for optical frequency combs spanning the telecom band. While the increased phase variation toward the edge of the C/L band contributed to the penalty variation, we have also observed the variance in receiver components performance, whereas the two contributions were not separable at the time of the measurement.
We report the characterization of 20 GBaud 16-QAM signal generation based on high-OSNR, low-noise frequency comb source encompassing contiguous C- and L- telecom bands. A noise-managed shock-wave parametric mixer was constructed to achieve low phase noise in the 100-GHz spaced comb spanning 8.75 THz. The carrier fidelity was characterized by direct phase noise characterization, as well as the BER measurements of a 16QAM signal. While the phase error variance increased towards the spectral edges, the penalty associated with this effect was limited to only a 1.7dB penalty variation across the entire C/L band. As a consequence, the results provide a strong argument for the use of CW-diode derived optical frequency combs as a scalable carrier solution in high-capacity lightwave coherent links.
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