Optical Comb Source Research Articles

Quantum dot fourth-harmonic colliding pulse mode-locked laser for high-power optical comb generation

Quantum-dot mode-locked lasers have advantages such as high temperature stability, large optical bandwidth, and low power consumption, which make them ideal optical comb sources, especially for wavelength-division multiplexing (WDM) telecommunications, and optical I/O applications. In this work, we demonstrate an O-band quantum dot colliding pulse mode-locked laser (QD-CPML) to generate optical frequency combs with 200 GHz spacing with maximum channels of 12 within 3 dB optical bandwidth. To achieve the high output power of individual comb lines, four channel conditions are implemented at central wavelength of 1310 nm for WDM transmission experiments. Each channel exhibits more than 10 dBm output power with 200 Gb/s PAM-4 and 270 Gb/s PAM-8 modulation capability via thin-film LiNbO3 Mach–Zehnder interferometer modulator without the requirement of any optical amplifications. This high-order QD-CPML is an ideal comb source for power-efficient optical interconnects and large bandwidth optical data transmission.

Wireless Signal Transmission at the 2 and 3 THz-Band Enabled by Photonics-Based Transmitter and Hot Electron Bolometer Mixer

We have demonstrated terahertz (THz) wireless transmission of quadrature phase-shift keying (QPSK) signals at 2 and 3 THz-bands. Despite higher atmospheric attenuation at frequencies above 1 THz, it might be applicable for short-range wireless communications. The proposed system is composed of a photonics-based transmitter using an optical comb source and a heterodyne receiver system using a hot electron bolometer mixer (HEBM). In the transmitter, THz signals are generated by photomixing of optical carriers extracted from broadband optical combs. The receiver system consists of a quasi-optical HEBM and a phase-locked THz quantum cascade laser (THz-QCL) as a local oscillator (LO). Although the emission power of the photonics-based transmitter was on the order of tens of nW, it could be received with sufficient SNRs due to high sensitivity of the HEBM. Demonstrations of signal transmission were carried out with 2 and 3 THz-band. Optical carriers were modulated with basic LTE FDD R9 uplink signals with full filled QPSK, and converted to THz signals, transmitting to the receiver system. The THz signals were successfully demodulated, in which error vector magnitude (EVM) values of the demodulated signal was about 22% and 30% for 2 and 3 THz-band, respectively.

Nanometer resolution of 2D single-shot low-coherence & comb-based profilometry based on the effect of discrete height of spatial phase modulator surface

A 2D single-shot comb-based interferometer is well known as a powerful tool for inline inspection owing to its high speed, extensive measurement range, and anti-vibration. However, nanometer resolution is still a milestone that has not been achieved in comb-based interferometers owing to the precision limitation caused by the discrete profile of the spatial phase modulator (SPM) device, typically in the range of several micrometers. Therefore, this study confirms 2D single-shot comb-based profilometry at sub-micron and nanometer-scale resolutions. The system first investigates the effect of the discrete limitation of the SPM surface profile and then verifies the submicron detection ability of the optical frequency comb-based setup. It was quantitatively shown that the highest resolution can be obtained by properly setting the step size of the spatial phase modulator and the SPM surface resolution of the imaging optics. The highest profilometry resolution was recorded at 26.4 nm in a short measurement range, while the setup using the optical comb source with millimeters measurement range for industrial application also can achieve a resolution of approximately 259.2 nm. The results demonstrate the system feasibility in overcoming the discrete limitations of the SPM profile. It also demonstrates for the first time that the so-called 2D single-shot comb-based interferometer can achieve nanometer-scale profilometry resolution covering the measurement range from tens of millimeters to sub-micrometer detection.

Tunable, coherent optical comb source via on-chip bidirectional coupling.

A tunable comb source is demonstrated through gain switching on a three-sectioned photonic integrated circuit (PIC). The PIC consists of two mutually coupled lasers connected by a passive waveguide. One of these is a tunable, two-section, single mode laser. The second laser is a simple Fabry-Perot cavity laser which can be phase-locked with the single mode laser via bidirectional coupling. Frequency combs are produced by gain switching the Fabry-Perot laser by applying a high-power radio frequency signal. Combs are generated with line spacings ranging from 3.5 to 8 GHz. The on-chip bidirectional coupling causes the comb to also be generated in the two-section device. Despite the lack of on-chip optical isolation between the lasers, the resulting combs are stable. Numerical simulations using a delay-differential model reproduce the results and reveal the important role played by the short delay times inherent to on-chip integration in this stability.

A novel DWDM-based comb source for future optical network

In recent times, there has been enormous growth in internet-based applications such as online entertainment platforms, video conferencing and voice over internet protocol (VoIP) applications which might be the trend in future. In order to support such bandwidth-hungry applications, the speed of the network should be enhanced. Increasing the infrastructure of the network requires heavy investment. Another alternative to increase the capacity of the network is to use dense wavelength division multiplexing (DWDM) which also results in increased costs as the lasers and other equipment needed for generation of wavelengths are required. In this work, an alternative cost effective technique is used to create a DWDM scenario using optical flat comb source (OFCS) by combining amplitude modulator (AM) and single drive mach zehnder modulator (SD-MZM). The spectral lines generated using this procedure can be used in Passive Optical Network (PON) as input sources. In this paper, 51 flat spectral lines are generated and transported over a distance of 100 km which are also capable of supporting a data rate of greater than 10 Gbps in a single spectral line.

Snapshot multi-frame parallel spectral holographic microscopy based on a reconfigurable optical comb.

We present a snapshot multi-frame parallel holographic microscopy system through a reconfigurable optical comb source, which consists of a digital micromirror device (DMD) based spectrum filter system and a spectroscopic Michelson interferometric system. The proposed system allows arbitrarily tuning comb spacing and comb number, and the capturing of multi-frame images without overlap in one exposure. As a result, high-quality spectral holograms can be obtained with less acquisition time. The performance of the system is detailed in the experiment and 45-wavelengths holographic imaging for perovskite micro-platelets is conducted, which proves the system has the ability to realize high-performance four-dimensional (4D) imaging.

Inter-mode soliton linear-wave scattering in a Kerr microresonator.

Soliton microresonator frequency combs (microcombs) have recently emerged as an attractive new type of optical comb source with a wide range applications proposed and demonstrated. To extend the optical bandwidth of these microresonator sources, several previous studies have proposed and studied the injection of an additional optical probe wave into the resonator. In this case, nonlinear scattering between the injected probe and the original soliton enables the formation of new comb frequencies through a phase-matched cascade of four-wave mixing processes. In this work, we expand the relevant analyses to consider soliton-linear wave interactions when the soliton and the probe fields propagate in different mode families. We obtain an expression for the phase-matched idler locations as a function of the dispersion of the resonator and the phase detuning of the injected probe. We confirm our theoretical predictions in experiments performed in a silica waveguide ring microresonator.

On-chip optical comb sources

On-chip integration of optical comb sources is crucial in enabling their widespread use. Integrated photonic devices that can be mass-manufactured in semiconductor processing facilities offer a solution for the realization of miniaturized, robust, low-cost, and energy-efficient comb sources. Here, we review the state of the art in on-chip comb sources, their applications, and anticipated developments.

Hybrid WDM-MDM transmitter with an integrated Si modulator array and a micro-resonator comb source.

We demonstrate a multi-channel silicon photonic transmitter based on wavelength division multiplexing (WDM) and mode division multiplexing (MDM). The light source is realized by a silicon nitride (Si3N4) Kerr frequency comb and optical modulation is realized by silicon electro-optic modulators. Three wavelengths and two modes are employed to increase the optical transmission capacity. The accumulated data rate reaches 150 Gb/s. The dense integration of WDM and MDM components with a compact optical comb source opens new avenues for the future high-capacity multi-dimensional optical transmission.

Space-division multiplexed transmission in the S-band over 55 km few-mode fibers.

Transmission of highly spectral efficient 24.5 GBaud quadrature phase shift keying and 16- and 64-quadrature amplitude modulated signals in the S-band between 1492 nm and 1518 nm wavelength is demonstrated over 55 km few-mode fibers. The carrier lines for S-band transmission were generated by a single wideband optical comb source with more than 120 nm optical bandwidth. While the three-mode fiber was originally optimized for C- and L-band transmission, we show that differential mode delay and mode-dependent loss show only a minor wavelength dependence within the measured S-band channels. However, the transceiver sub-system, including S-band optical amplifiers as well as a reduced optical signal-to-noise ratio of the comb source, leads to a significant Q-factor penalty for channels towards the edges of the S-band optical amplifiers below 1495 nm and above 1515 nm wavelength.

Highly Spectral Efficient C + L-Band Transmission Over a 38-Core-3-Mode Fiber

High capacity transmission with a data rate over 10.66 Pb/s is demonstrated in a 13 km, 38-core-3-mode few-mode multi-core fiber. This was achieved with a single optical comb source to generate 368 × 24.5 GBaud 64- or 256-quadrature-amplitude-modulated signals in a 25 GHz wavelength channel grid in combination with a few-mode multi-core fiber with high uniformity amongst cores. We investigated essential transmission channel characteristics and found that mode-dependent loss strongly decreases the achievable data rates in some cores, while we did not observe a correlation between varying impulse response spread and data rate. We further did not find an influence of the (radial) position of a core within the cladding on the data rate. Finally, we explored the potential of multi-span transmission over the same fiber with transmission distances up to 65 km, showing a strong performance penalty from prototype components with sub-optimal transmission properties.

Video-rate centimeter-range optical coherence tomography based on dual optical frequency combs by electro-optic modulators.

Imaging speed and range are two important parameters for optical coherence tomography (OCT). A conventional video-rate centimeter-range OCT requires an optical source with hundreds of kHz repetition rate and needs the support of broadband detectors and electronics (>1 GHz). In this paper, a type of video-rate centimeter-range OCT system is proposed and demonstrated based on dual optical frequency combs by leveraging electro-optic modulators. The repetition rate difference between dual combs, i.e. the A-scan rate of dual-comb OCT, can be adjusted within 0~6 MHz. By down-converting the interference signal from optical domain to radio-frequency domain through dual comb beating, the down-converted bandwidth of the interference signal is less than 22.5 MHz which is at least two orders of magnitude lower than that in conventional OCT systems. A LabVIEW program is developed for video-rate operation, and the centimeter imaging depth is proved by using 10 pieces of 1-mm thick glass stacked as the sample. The effective beating bandwidth between two optical comb sources is 7 nm corresponding to ~108 comb lines, and the axial resolution of the dual-comb OCT is 158 µm. Dual optical frequency combs provide a promising solution to relax the detection bandwidth requirement in fast long-range OCT systems.

Broadband RF Channelizer Based on an Integrated Optical Frequency Kerr Comb Source

We report a broadband RF channelizer based on an integrated optical frequency Kerr micro-comb source, with an RF channelizing bandwidth of 90 GHz, a high RF spectral slice resolution of 1.04 GHz, and experimentally verify the RF performance up to 19 GHz. This approach to realizing RF channelizers offers reduced complexity, size, and potential cost for a wide range of applications to microwave signal detection.

Advanced RF and microwave functions based on an integrated optical frequency comb source.

We demonstrate advanced transversal radio frequency (RF) and microwave functions based on a Kerr optical comb source generated by an integrated micro-ring resonator. We achieve extremely high performance for an optical true time delay aimed at tunable phased array antenna applications, as well as reconfigurable microwave photonic filters. Our results agree well with theory. We show that our true time delay would yield a phased array antenna with features that include high angular resolution and a wide range of beam steering angles, while the microwave photonic filters feature high Q factors, wideband tunability, and highly reconfigurable filtering shapes. These results show that our approach is a competitive solution to implementing reconfigurable, high performance and potentially low cost RF and microwave signal processing functions for applications including radar and communication systems.

Fully Adaptable Electro-Optic Dual-Comb Generation

In this letter, an all-electronic method that provides a precise control over the number of spectral lines of an electro-optic frequency comb is presented and experimentally validated. This feature, in conjunction with the straightforward configuration of the optical resolution (repetition frequency) inherent to electro-optic combs, enables the generation of optical comb sources with the unique capability of a complete adaptability to the targeted spectral feature/s that could ultimately maximize the SNR of any electro-optic comb-based spectroscopic measurement.

Optical frequency comb generation with ultra-narrow spectral lines.

We demonstrate an optical comb source that generates 550 ultra-narrow spectral lines with a spectral linewidth of 1.5-3kHz, spanning over the C-band. The source originates from a single-mode Brillouin laser processed with phase modulation, pulse compression, and four-wave mixing. As a result, the narrow linewidth of the Brillouin laser improves the phase noise of every spectral line of the frequency comb.

Wavelength multicasting through four-wave mixing with an optical comb source.

Based on four-wave mixing (FWM) with an optical comb source (OCS), we experimentally demonstrate 26-way or 15-way wavelength multicasting of 10-Gb/s differential phase-shift keying (DPSK) data in a highly-nonlinear fiber (HNLF) or a silicon waveguide, respectively. The OCS provides multiple spectrally equidistant pump waves leading to a multitude of FWM products after mixing with the signal. We achieve error-free operation with power penalties less than 5.7 dB for the HNLF and 4.2 dB for the silicon waveguide, respectively.

Butterfly packaged low‐linewidth optical comb source

A packaged optical comb source is demonstrated. A low-linewidth optical comb source is designed and fabricated. The device is packaged in a seven-pin high-speed butterfly package with an subminiature version A (SMA) connector for RF modulation and fibre pigtail. Fibre coupling efficiency is estimated at 43%. The packaged comb source is shown to have a linewidth of 300 kHz for the comb line set when modulated at 4 GHz, with eight comb lines within 3 dB of each other. The prototype packaged comb source has applications in high bandwidth telecommunications.

Demonstration of 16QAM-OFDM UDWDM Transmission Using a Tunable Optical Flat Comb Source

A new approach for designing broad and flattened spectrum multicarriers optical sources is presented leading to a 32 spectral lines source by using a dual-arm Mach-Zehnder modulator (MZM) and a 41 spectral lines source from two-stage MZM. A modified simulated annealing-based optimization method is applied to derive the necessary settings allowing the optical flat comb source (OFCS) to be ultraflat. The OFCS is mooted as a technology to enhance the overall capacity of an access optical network by increasing the number of wavelength division multiplexing (WDM) channels. Here, we demonstrate an ultradense WDM (UDWDM) transmission for application to passive optical networks with (11 × 12.5 Gbps) quadrature amplitude modulation (QAM) based on a 4 b/s/Hz spectral efficiency orthogonal frequency division multiplex (16QAM-OFDM) transmitter and direct detection. We use an OFCS to generate the 11 subcarriers spaced by 6.25 GHz, made of a two-stage MZM. We study the performance of three filtered channels in terms of error vector magnitude in back-to-back (B-to-B) conditions and after propagation through 25 and 100 km standard single mode fiber.

Integrated Gain Switched Comb Source for 100 Gb/s WDM-SSB-DD-OFDM System

We demonstrate a 100 Gb/s short reach system using a multicarrier transmitter based on a gain switched monolithically integrated laser. An optical comb source with 12.5-GHz free spectral range is achieved by gain-switching an integrated passive feedback laser. The 100 Gb/s wavelength division multiplexed, single sideband, direct detection, orthogonal frequency division multiplexed (WDM-SSB-DD-OFDM) system operates over 25 km standard single mode fiber exhibiting a spectral efficiency of 1.8 b/s/Hz. Receiver sensitivity of -14.2 dBm is achieved after 25 km transmission. Performance optimization with phase and amplitude precompensation is employed to improve the SSB OFDM modulation thereby reducing the interchannel interference and overcoming the power fading induced by the optical filter. We also present a theoretical analysis of the SSB-OFDM modulation.