Optical Frequency Comb Source Research Articles

Experimental demonstration of a multi-core fiber seeded comb optical network (MCF-SCON)

We propose and experimentally demonstrate an optical network architecture that uses wideband optical frequency comb (OFC) sources synchronized with transmitted network broadcast seed signals. All network nodes are connected by multi-core fibers (MCFs) which are used for data transmission and seed distribution. The nodes that connect the data transmission and seed distribution layers contain an OFC that may be operated in master or slave mode to provide carriers both for modulation and local oscillators for coherent detection. Nodes are able to switch, amplify, split, and regenerate the seed lightwave for full network distribution over large areas. Using OFCs with 25 GHz spacing covering the C- and L-bands, we experimentally investigate a range of scenarios compatible with metro or inter-data-center network architectures. These scenarios include single span transmission, bi-directional transmission, and multi-hop seed and data transmission including seed amplification, splitting, and regeneration with laser injection locking within network nodes, and a maximum transmission distance of 130 km. We show that MCF seed and data distribution with spatial-switching enables identical network-wide transceiver combs with the potential for reducing the complexity of digital signal processing as well as significant hardware sharing. Data-rates of approximately 100 Tb/s/core and 600 Tb/s per fiber are measured showing the potential of this architecture for high-capacity networking with OFCs replacing hundreds of transmitter and local oscillator lasers in each node.

Nozaki-Bekki solitons in semiconductor lasers.

Optical frequency-comb sources, which emit perfectly periodic and coherent waveforms of light1, have recently rapidly progressed towards chip-scale integrated solutions. Among them, two classes are particularly significant-semiconductor Fabry-Perót lasers2-6 and passive ring Kerr microresonators7-9. Here we merge the two technologies in a ring semiconductor laser10,11 and demonstrate a paradigm for the formation of free-running solitons, called Nozaki-Bekki solitons. These dissipative waveforms emerge in a family of travelling localized dark pulses, known within the complex Ginzburg-Landau equation12-14. We show that Nozaki-Bekki solitons are structurally stable in a ring laser and form spontaneously with tuning of the laser bias, eliminating the need for an external optical pump. By combining conclusive experimental findings and a complementary elaborate theoretical model, we reveal the salient characteristics of these solitons and provide guidelines for their generation. Beyond the fundamental soliton circulating inside the ring laser, we demonstrate multisoliton states as well, verifying their localized nature and offering an insight into formation of soliton crystals15. Our results consolidate a monolithic electrically driven platform for direct soliton generation and open the door for a research field at the junction of laser multimode dynamics and Kerr parametric processes.

Performance of quantum-dash mode-locked lasers (QD-MLLDs) for high-capacity coherent optical communications.

We investigate the capabilities and limitations of quantum-dash mode-locked lasers (QD-MLLDs) as optical frequency comb sources in coherent optical communication systems. We demonstrate that QD-MLLDs are on par with conventional single-wavelength narrow linewidth laser sources and can support high symbol rates and modulation formats. We manage to transmit 64 quadrature amplitude modulation (QAM) signals up to 80 GBd over 80 km of standard single-mode fiber (SSMF), which highlights the distinctive phase noise performance of the QD-MLLD. Using a 38.5 GHz (6 dB bandwidth) silicon photonic (SiP) modulator, we achieve a maximum symbol rate of 104 GBd with 16QAM signaling and a maximum net rate of 416 Gb/s per carrier in a single polarization setup and after 80 km-SSMF transmission. We also compare QD-MLLD performance with commercial narrow-linewidth integrable tunable laser assemblies (ITLAs) and explore their potential for use as local oscillators (LOs) and signal carriers. The QD-MLLD has 45 comb lines usable for transmission at a frequency spacing of 25 GHz, and an RF linewidth of 35 kHz.

Photonic beamforming using a quantum-dash optical frequency comb source.

We demonstrate photonic beamforming using a quantum-dash (QD) optical frequency comb (OFC) source. Thanks to the 25 GHz free spectral range (FSR) and up to 40 comb lines available from the QD OFC, we can implement phased antenna arrays (PAAs) with directional radiation and scanning. We consider two types of PAAs: a uniform linear array (ULA) and a uniform planar array (UPA). By selecting different comb lines with a programmable optical filter, we can tune the FSR of the OFC source and realize a discrete scanning function. We evaluate the beam squint of the ULAs, and the results show that we can achieve broadband operation. Finally, we show that we can achieve both directional radiation and scanning simultaneously using the UPA.

Multi-frequency 5G NR millimeter-wave signal generation over analog RoF link using optical frequency combs

Optical heterodyne analog radio-over-fiber (A-RoF) links can provide a cost-effective and spectrally efficient solution for the generation and distribution of millimeter-wave/Terahertz (mm-wave/THz) carriers for the next-generation wireless systems. The use of an optical frequency comb (OFC) source in such a system can facilitate carrier generation free from frequency fluctuation. However, decorrelation between tones, due to path length difference, can lead to phase noise on the generated mm-wave carrier — limiting the performance of low subcarrier spacing multi-carrier signals specified in the 5G standard. In this work, we analyze the effect of comb tones decorrelation due to path length difference on the performance of 5G compatible 244 kHz subcarrier spacing 59 GHz OFDM signal over an optical heterodyne A-RoF link using a gain switched laser (GSL) OFC source. Furthermore, we demonstrate the ability of the OFC-based A-RoF optical/mm-wave system to support flexible and interoperable mm-wave functionality through signal generation at 27 GHz, 43 GHz and 59 GHz using different pairs of GSL OFC tones for remote heterodyning. The capability of multi-frequency 5G NR compatible mm-wave signal generation and potential for photonic integration make the demonstrated GSL OFC based optical heterodyne A-RoF system a suitable candidate for deployment in the next generation wireless systems.

Distortion-free amplification of 100 GHz mode-locked optical frequency comb using quantum dot technology.

Semiconductor mode-locked optical frequency comb (ML-OFC) sources with extremely high repetition rates are central to many high-frequency applications, such as dense wavelength-division multiplexing. Dealing with distortion-free amplification of ultra-fast pulse trains from such ML-OFC sources in high-speed data transmission networks requires the deployment of semiconductor optical amplifiers (SOAs) with ultrafast gain recovery dynamics. Quantum dot (QD) technology now lies at the heart of many photonic devices/systems owing to their unique properties at the O-band, including low alpha factor, broad gain spectrum, ultrafast gain dynamics, and pattern-effect free amplification. In this swork, we report on ultrafast and pattern-free amplification of ∼100 GHz pulsed trains from a passively ML-OFC and up to 80 Gbaud/s non-return-to-zero (NRZ) data transmission using an SOA. Most significantly, both key photonic devices presented in this work are fabricated from identical InAs/GaAs QD materials operating at O-band, which paves the way for future advanced photonic chips, where ML-OFCs could be monolithically integrated with SOAs and other photonic components, all originated from the same QD-based epi-wafer.

Femtosecond-precision electronic clock distribution in CMOS chips by injecting frequency comb-extracted photocurrent pulses

A clock distribution network (CDN) is a ubiquitous on-chip element that provides synchronized clock signals to all different circuit blocks in the chip. To maximize the chip performance, today’s CDN demands lower jitter, skew, and heat dissipation. Conventionally, on-chip clock signals have been distributed in the electric voltage domain, resulting in increased jitter, skew, and heat dissipation due to clock drivers. While low-jitter optical pulses have been locally injected in the chip, research on effective distribution of such high-quality clock signals has been relatively sparse. Here, we demonstrate femtosecond-precision distribution of electronic clocks using driver-less CDNs injected by photocurrent pulses extracted from an optical frequency comb source. Femtosecond-level on-chip jitter and skew can be achieved for gigahertz-rate clocking of CMOS chips by combining ultralow comb-jitter, multiple driver-less metal-meshes, and active skew control. This work shows the potential of optical frequency combs for distributing high-quality clock signals inside high-performance integrated circuits, including 3D integrated circuits.

Coherently averaged dual-comb spectroscopy with a low-noise and high-power free-running gigahertz dual-comb laser.

We present a new type of dual optical frequency comb source capable of scaling applications to high measurement speeds while combining high average power, ultra-low noise operation, and a compact setup. Our approach is based on a diode-pumped solid-state laser cavity which includes an intracavity biprism operated at Brewster angle to generate two spatially-separated modes with highly correlated properties. The 15-cm-long cavity uses an Yb:CALGO crystal and a semiconductor saturable absorber mirror as an end mirror to generate more than 3 W average power per comb, below 80 fs pulse duration, a repetition rate of 1.03 GHz, and a continuously tunable repetition rate difference up to 27 kHz. We carefully investigate the coherence properties of the dual-comb by a series of heterodyne measurements, revealing several important features: (1) ultra-low jitter on the uncorrelated part of the timing noise; (2) the radio frequency comb lines of the interferograms are fully resolved in free-running operation; (3) we validate that through a simple measurement of the interferograms we can determine the fluctuations of the phase of all the radio frequency comb lines; (4) this phase information is used in a post-processing routine to perform coherently averaged dual-comb spectroscopy of acetylene (C2H2) over long timescales. Our results represent a powerful and general approach to dual-comb applications by combining low noise and high power operation directly from a highly compact laser oscillator.

Ultralow Noise and Timing Jitter Semiconductor Quantum-Dot Passively Mode-Locked Laser for Terabit/s Optical Networks

Diode optical frequency comb lasers are promising compact solutions to generate high-speed optical pulses for applications in high spectral efficiency wavelength division multiplexing transmission with advanced modulation formats. In this paper, an InAs/InP quantum dot (QDot) C-band single-section passively mode-locked laser (MLL) based broadband optical frequency comb source with a free spectral range of 28.4 GHz is presented. The device exhibits less than 1.5 MHz optical linewidth (phase noise) over 56 channels and 2.1 fs pulse-to-pulse timing jitter with a central wavelength of 1550 nm. Using this comb, we demonstrate an aggregate data transmission capacity of 12.5 Terabit/s over 100 km of standard single mode fiber by employing dual-polarization with 16 QAM modulation format. This investigation shows the viability for semiconductor QDot MLLs to be used as low-cost optical source in Terabit/s or higher optical networks.

A tunable optical frequency comb source using cascaded frequency modulator and Mach–Zehnder modulators

Abstract In this work, we demonstrate a tunable optical frequency comb (OFC) source based on a cascaded frequency modulator (FM) and two Mach–Zehnder modulators (MZMs). The setup includes one FM and two MZMs, and a sinusoidal RF signal source that directly drive all these modulators. A Flat OFC source with a high number of comb lines, and tunable frequency spacing and center wavelength is analytically modelled and simulated. The results reveal that 51 comb lines with a frequency spacing of 25 GHz are generated when only FM is used. Thirteen of these lines have power variations of 1 dB. Next, by cascading FM with two MZMs, 127 comb lines are obtained. In addition, 101 of these lines have power variations of 1 dB. An optical frequency comb, with tunable frequency spacing ranging from 10 to 40 GHz is successfully generated. Moreover, the center wavelength of the generated OFC can be tuned from 1310 to 1610 nm.

Reconfigurable microwave photonic filter based on a quantum dash mode-locked laser.

We demonstrate a reconfigurable microwave photonic (MWP) filter using a quantum dash (QDash) mode-locked laser (MLL) that can generate an optical frequency comb (OFC) with ∼50 comb lines and a free spectral range of 25 GHz. Thanks to the large number of comb lines, the MWP filter responses can be easily programmed by tailoring the OFC spectrum. We implement MWP filter responses with Gaussian, sinc, flat-top, and multiple peaks, as well as demonstrate that tuning of the central frequency. We achieve a minimum 3 dB bandwidth of ∼100 MHz for a sinc-shaped MWP filter, while the maximum out-of-band rejection can be up to ∼30 dB with Gaussian apodization. Our results show that the QDash-MLL is a promising OFC source for developing integrated and reconfigurable MWP filters.

Gain-Switched Optical Frequency Comb Source Using a Hybrid Integrated Self-Injection Locking DFB Laser

A hybrid integrated gain-switched (GS) optical frequency comb (OFC) source with eight pure continuous comb lines within 3-dB of the spectral envelope peak and a narrow linewidth of 615 kHz is proposed and demonstrated by coupling a silicon nitride microring reflector (MRR) to a commercially available distributed feedback (DFB) laser. Due to the self-injection locking effect induced by the MRR, the carrier-to-noise ratio (CNR) and phase correlation between comb lines are greatly enhanced. In addition, benefiting from the unique merit of gain switching, the resultant comb can also achieve a large frequency spacing adjustment range from 5.55 GHz to 8.7 GHz. This work explores a cost-efficient and robust OFC source for numerous applications, such as radio over fiber (RoF) and coherent optical communication.

InAs/InP quantum dot mode-locked laser with an aggregate 12.544 Tbit/s transmission capacity.

Chip-scale optical frequency comb sources are ideal compact solutions to generate high speed optical pulses for applications in wavelength division multiplexing (WDM) and high-speed optical signal processing. Our previous studies have concentrated on the use of quantum dash based lasers, but here we present results from an InAs/InP quantum dot (QDot) C-band passively mode-locked laser (MLL) for frequency comb generation. By using this single-section QDot-MLL we demonstrate an aggregate line rate of 12.544 Tbit/s 16QAM data transmission capacity for both back-to-back (B2B) and over 100-km of standard single mode fiber (SSMF). This finding highlights the viability for InAs/InP QDot lasers to be used as a low-cost optical source for large-scale networks.

Multi-wavelength 128 Gbit s−1 λ −1 PAM4 optical transmission enabled by a 100 GHz quantum dot mode-locked optical frequency comb

Semiconductor mode-locked lasers (MLLs) with extremely high repetition rates are promising optical frequency comb (OFC) sources for their usage as compact, high-efficiency, and low-cost light sources in high-speed dense wavelength-division multiplexing transmissions. The fully exploited conventional C- and L- bands require the research on O-band to fulfil the transmission capacity of the current photonic networks. In this work, we present a passive two-section InAs/InGaAs quantum-dot (QD) MLL-based OFC with a fundamental repetition rate of ∼100 GHz operating at O-band wavelength range. The specially designed device favours the generation of nearly Fourier-transform-limited pulses in the entire test range by only pumping the gain section while with the absorber unbiased. The typical integrated relative intensity noise of the whole spectrum and a single tone are −152 and −137 dB Hz−1 in the range of 100 MHz–10 GHz, respectively. Back-to-back data transmissions for seven selected tones have been realised by employing a 64 Gbaud four-level pulse amplitude modulation format. The demonstrated performance shows the feasibility of the InAs QD MLLs as a simple structure, easy operation, and low power consumption OFC sources for high-speed fibre-optic communications.

Coherent 16-QAM bidirectional fiber-wireless hybrid access network with optimal use of the optical local oscillator signal

This work reports on the optimal use of an optical local oscillator signal from a gain-switched vertical cavity surface emitting laser (VCSEL) optical frequency comb (OFC) in wavelength division multiplexed (WDM) fixed-wireless access networks. The system was tested for its ability to transmit full-duplex error-free data signals to and from a fixed/wireless user. The idea of wavelength reuse was used for the first time to our knowledge, for simultaneous upstream, downstream, and photonic up-conversion to high-frequency RF carrier. Two optical carrier signals were filtered from the generated OFC. One of the two optical carriers was modulated with 16-QAM 28-Gbps data signal. The second optical carrier was amplitude-modulated with an 8.5-Gbps on-off keying (OOK) data signal. These two modulated optical carriers were multiplexed and transmitted over 24 km of standard single mode fiber (SSMF). At the remote site, the second optical carrier carrying the 8.5-Gbps data was selected, demodulated, and recovered. The recovered optical wavelength was used both for photonic up-conversion and for upstream data transmission. Error-free performance was obtained for both downstream, upstream, and wireless data transmission. The wireless carrier obtained after photonic up-conversion using the recovered wavelength was transmitted over a 4-m wireless distance. An EVM of 7% was obtained for the 16-QAM signal. These results are attractive as they demonstrate and motivate the possibility of using gain-switched VCSELs as OFC sources in future fixed-wireless WDM access networks. Wavelength reuse improves the spectral efficiency in the optical and RF domain. The use of VCSELs results in power-efficient access networks.

Engineering the spectral bandwidth of quantum cascade laser frequency combs.

Quantum cascade lasers (QCLs) facilitate compact optical frequency comb sources that operate in the mid-infrared and terahertz spectral regions, where many molecules have their fundamental absorption lines. Enhancing the optical bandwidth of these chip-sized lasers is of paramount importance to address their application in broadband high-precision spectroscopy. In this work, we provide a numerical and experimental investigation of the comb spectral width and show how it can be optimized to obtain its maximum value defined by the laser gain bandwidth. The interplay of nonoptimal values of the resonant Kerr nonlinearity and cavity dispersion can lead to significant narrowing of the comb spectrum and reveals the best approach for dispersion compensation. The implementation of high mirror losses is shown to be favorable and results in proliferation of the comb sidemodes. Ultimately, injection locking of QCLs by modulating the laser bias around the round trip frequency provides a stable external knob to control the frequency-modulated comb state and recover the maximum spectral width of the unlocked laser state.

Frequency-offset-tolerant optical frequency comb-based coherent transmission for intra-datacenter interconnections.

In recent years, in order to increase the capacity and scalability of intra-datacenter (DC) transmission, the optical frequency comb (OFC) source has been considered promising to replace discrete lasers, aiming to reduce the cost of wavelength division multiplexing (WDM) transmission within DC. In this paper, an OFC based coherent architecture is proposed. An OFC, in the receiver side, is split by a splitter with a uniform power ratio and separately used as local oscillators (LOs) to detect the demultiplexed signals. The signal spectrum is copied onto every tone of the LO-OFC, and a large frequency offset (FO) tolerance is achieved. In addition, the required ADC sampling rate is the same as a system without FO. Extensive simulations are conducted. In the simulated coherent WDM transmission system, a 3-tone-OFC is used to provide 3 carriers, and an 11-tone-OFC is split and used to provide LO-OFCs. For a 64GBd polarization multiplexing 16 quadrature amplitude modulation (PM-16QAM) WDM transmission, the tolerances of FO are up to about ±0.3THz and ±0.374THz for the 1st/3rd signal, and the 2nd signal, respectively, below the pre-forward error correction (FEC) bit error rate (BER) level of 1.25×10-2. Moreover, the maximum tolerance of FO linearly increases with the number of effective tones in LO-OFC. Further, extensive experiments with back-to-back connection are conducted to verify the performance. The tolerance of FO is up to >36 GHz for 36GBd PM-16QAM transmission with a 3-tone-LO-OFC below the BER level of 1.25×10-2. The proposed OFC based coherent architecture is a promising solution for intra-DC interconnections with a large FO.

Millimeter-wave enabled PAM-4 data transmission over hybrid FSO-MMPOF link for access networks

In this paper, we propose a full duplex architecture based on a hybrid link composed of free space optics (FSO) and multimode plastic optical fiber (MMPOF) for short-range wireless access networks. The proposed architecture employs mode group division multiplexing (MGDM) and wavelength reuse techniques to transmit data between central unit (CU) and radio access units (RAUs). An optical frequency comb source to generate multiple optical sidebands is realized using a single laser source to provide 60 GHz millimeter-wave (mm-wave) signals at each RAU by simultaneously transmitting PAM-4 signal on linearly polarized (LP) modes of each optical sideband. Data rate of 2times12 Gbps at mm-wave frequency of 60 GHz is achieved for both downlink (DL) and uplink (UL) transmissions. The transmission of 2times12 Gbps PAM-4 signal over hybrid FSO-MMPOF link is investigated at different values of refractive index structure parameter (C_n^2) by employing log-normal (LN) FSO channel model to support three RAUs in a ring topology. The acceptable receiver sensitivities below −5 and −0.6 dBm for DL and UL transmissions are achieved, respectively. The proposed hybrid architecture can be a potential candidate for future communication networks.

Single mode fiber-free space optic based hybrid $$32\times 2~\text{Gbps}$$ WDM-PON architecture employing optical frequency comb source and polarization multiplexing

In this paper, we propose a cost-efficient single mode fiber-free space optics (SMF-FSO) based hybrid $$32 \times 2~\text{Gbps}$$ wavelength division multiplexed passive optical network (WDM-PON) employing optical frequency comb (OFC) source and polarization multiplexing. OFC source is realized at optical line terminal (OLT) by modulating a single continuous wave (CW) laser source with radio frequency (RF) sinusoidal signal and dual-drive Mach–Zehnder modulator (DD-MZM). 16 coherent optical carriers are used to split into 32 x and y-orthogonal polarization states which are used to transmit the on-off keying (OOK) data of 32 channels to remotely located optical network units (ONUs). The performance of the channels is analyzed with bit error rate (BER) plots employing Gamma–Gamma (GG) FSO channel model at different turbulence regimes and lengths of FSO link. Moreover, the effect of varying the polarization mode dispersion (PMD) on BER is also evaluated. The cost of the proposed architecture is analyzed and compared with conventional PON architecture. The cost efficiency of the proposed architecture is demonstrated by the use of a single CW laser source to transmit the data of 32 users between remote node (RN) and ONUs through FSO links having reduced installation and maintenance costs.

Simultaneous transmission of clock and data signals in photonic-assisted WDM passive optical networks

This work reports on the use of a gain-switched vertical cavity surface-emitting laser (VCSEL) optical frequency comb (OFC) to generate multiple optical carriers for applications in future wavelength-division multiplexing (WDM) passive optical network (PON) systems. The VCSEL-based OFC system was tested for its ability to simultaneously transmit error-free data signals up to 30 Gbps, and a 50 MHz clock signal proposed to be used for latency monitoring of the network. The system was demonstrated over a fiber distance beyond 20 km. Four optical carrier signals were filtered from the generated OFC. Three of the four optical carriers were combined and amplitude-modulated with 5 and 10 Gbps on–off keying (OOK) data signals. The fourth optical carrier was amplitude-modulated with a 50 MHz clock signal. These four modulated optical carriers were multiplexed and transmitted over 21 km of standard single-mode fiber. When the three data-carrying optical carriers were modulated with 5 Gbps OOK data and multiplexed with the clock-carrying carrier, a negligible transmission penalty was achieved after fiber transmission. When these three data-carrying optical carriers were modulated with 10 Gbps OOK data and multiplexed with a clock-carrying carrier, a maximum transmission penalty of 4.8 dB was achieved after fiber transmission. The system was investigated at a minim bit error rate (BER) of 10 − 9 . A receiver sensitivity of less than − 11 d B m was achieved at both bit rates. The clock signal suffered little effect when transmitted with the three optical carriers at both 5 and 10 Gbps data rate. A power penalty of 15.37 dB, mainly due to fiber attenuation, was experienced in the clock after fiber transmission. Due to the unavailability of the numerical system to quantify the clock performance in terms of phase noise, the phase-noise results could not be provided. Nevertheless, the provided qualitative results were able to show that the transmitted clock integrity was able to be retained after being multiplexed with transmission on the 21 km fiber channel. For the first time, to the best of our knowledge, we have simultaneously reported on data and clock transmission results using a low-power, VCSEL-based OFC to realize a WDM-PON system. These results are attractive as they demonstrate and motivate the possibility of using gain-switched VCSELs as OFC sources in future WDM-PON networks requiring enhanced channel capacity and stringent network latency monitoring to realize intelligent, simple, and power-efficient PON networks.