Optical Comb Research Articles

Remote Residual Instability Evaluation of Comb-Based Precise Frequency Transmission for Optical Clock

Optical clocks can be used as the absolute frequency reference due to their high accuracy and stability. In a precise optical clock transmission system, the instability loss of the link is usually evaluated by beating the remote optical signal with the reference clock, which is not suitable for real frequency distribution applications. Therefore, it is necessary to assess the performance directly at the remote site for the optical frequency transfer, because the two sites of the link are usually not co-located. In this paper, we proposed a comb-based remote residual instability evaluation scheme. Two coherent optical combs with different wavelengths were extracted from a frequency stable comb and transmitted after multiplexing. The residual instability was evaluated directly at the remote site by measuring the phase fluctuation difference between the two combs. We achieved 8.61 × 10−19 at 40,000 s over a 10 km fiber link, reaching the instability of optical clocks. These results revealed that our scheme can evaluate optical clock frequency transmission directly at the remote site, which made the method truly practical.

Kerr microresonator dual-comb source with adjustable line-spacing.

Optical microresonators offer a highly-attractive new platform for the generation of optical frequency combs. Recently, several groups have been able to demonstrate the generation of dual-frequency combs in a single microresonator driven by two optical pumps. This opens the possibility for microresonator-based dual-comb systems suitable for measurement applications such as spectroscopy, ranging and imaging. Key to the performance of these systems are the parameters of the radio-frequency comb spectrum that arises from the interference of the two optical combs. In this work, we present a simple mechanism to enable the discrete fine-tuning of these parameters by driving the two optical combs with optical pumps with different azimuthal mode numbers. The mechanism consists of tuning the difference in azimuthal mode number between the two pumps by selection of the pumps' frequencies. We are able to implement this technique when the two counter-propagating pumps are set to drive resonances of the same spatial mode family, as well as different mode families. In each case, we experimentally observe ∼1 MHz of discrete tunability in the line-spacing of the radio-frequency comb as the frequency offset between the two pumps is scanned between 0 to 80 free-spectral-ranges.

PANDA Fiber Sagnac Interferometer for Temperature Sensor Application

Fiber Sagnac loop mirror has great potential for use in various application such as multi-wavelength laser source, strain and temperature sensors. This paper demonstrates an optical fiber temperature sensor based on two lengths of PANDA polarization maintaining fibers (PMFs) in a fiber Sagnac interferometer. The temperature sensing is based on the change of spectral spacing of the generated optical comb with the temperature difference between two PMFs. The performance of the system was characterized numerically and experimentally by measuring the spectral spacing when raising the temperature of one of the PMFs. A high temperature sensitivity up to 0.36 nm/°C is obtained, with detuning range of 43 nm. This makes it a good candidate for various in-field sensing application.

Bright and dark Talbot pulse trains on a chip

Temporal Talbot effect, the intriguing phenomenon of the self-imaging of optical pulse trains, is extensively investigated using macroscopic components. However, the ability to manipulate pulse trains, either bright or dark, through the Talbot effect on integrated photonic chips to replace bulky instruments has rarely been reported. Here, we design and experimentally demonstrate a proof-of-principle integrated silicon nitride device capable of imprinting the Talbot phase relation onto in-phase optical combs and generating the two-fold self-images at the output. We show that the GHz-repetition-rate bright and dark pulse trains can be doubled without affecting their spectra as a key feature of the temporal Talbot effect. The designed chip can be electrically tuned to switch between pass-through and repetition-rate-multiplication outputs and is compatible with other related frequencies. The results of this work lay the foundations for the large-scale system-on-chip photonic integration of Talbot-based pulse multipliers, enabling the on-chip flexible up-scaling of pulse trains’ repetition rate without altering their amplitude spectra.

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.

Harmonic Mode‐Locked Er‐Doped Fiber Laser Based on a Microfiber‐Based PbS Nanoparticle Saturable Absorber

AbstractLead sulfide (PbS) is a nanomaterial with excellent optical and chemical properties, such as a narrow bandgap (0.37 eV), high thermal damage threshold, and high stability. Obviously, it is appropriate as a saturable absorber (SA) device for ultrafast photonics. However, PbS nanoparticles (NPs) as the SA of ultrashort harmonic mode‐locked pulse still haven't been demonstrated at present. In this paper, the PbS NPs are made into an SA‐device‐based microfiber by optical deposition method and connected in an integrated Erbium‐doped fiber laser. And both characteristics and nonlinear optical properties of PbS NPs have been systemically investigated. A fundamental frequency mode‐locked pulsed laser is proposed, whose central wavelength is 1560 nm, and the pulse width is 1 ps. In addition, high repetition rate operations are achieved, with a maximum repetition rate of 833 MHz. This is the first time that PbS NPs are used to generate 96th‐order harmonic mode‐locking, and the corresponding pulse duration is 987 fs. It is demonstrated that PbS NPs are a kind of SA photonic material with excellent performance. It can improve the communication capacity by applying fiber communication, and it has potential application value even in material processing and optical comb.

Control and stabilization of Kerr cavity solitons and breathers driven by chirped optical pulses

We investigate the impact of chirped driving fields on the dynamics and generation of Kerr cavity breathers and solitons. Synchronous phase and amplitude modulation of the pumping field can be exploited in order to control soliton dynamics. Here we show that using a phase-modulated super-Gaussian pump permits to stabilize the oscillations of breathing solitons. Moreover, our scheme permits to obtain new dynamical attractors, with a prescribed temporal intra-cavity pattern. Straightforward applications are the deterministic generation of optical frequency soliton combs, optical tweezers, and more generally, all-optical manipulation of light pulses.

1550‐nm Band Soliton Microcombs in Ytterbium‐Doped Lithium‐Niobate Microrings

AbstractMicro‐optical frequency combs are miniaturized coherent light sources exhibiting a tremendous influence on precision spectroscopy, optical clocks, and high‐speed optical communications. The rare‐earth‐doped lithium niobate (LN) is a promising platform to integrate the lasers and comb sources on the same chip of single material. However, microcombs generated in rare‐earth‐doped LN thin film (LNTF) have not yet been fully explored. To explore the protocols of generating optical combs in ytterbium‐doped LNTF, the comb‐like laser is studied in the 1060 nm band, and the dissipative Kerr soliton is experimentally demonstrated in the telecom band in a Z‐cut wafer. The over‐coupled fundamental transverse electric mode with anomalous dispersion is excited at an on‐chip pump power of 75.7 mW to generate the robust bright soliton. The broadband spectrum of the Kerr soliton ranges from 1480 to 1660 nm with 110 comb lines, whose repetition rate is 197.8 GHz. The mode‐locking of the soliton can be achieved as a result of the photorefractive effect of ytterbium‐doped LNTF instead of complex feedback schemes. Numerical simulations are highly accordant with the experimental results. This work expands the microcomb‐generating platform and increases the potential for integrated on‐chip applications including precision ranging and frequency division.

Multi-Tone Optical Source Generation for Applications in Next-Generation Passive Optical Networks using Photonic Structures

This study presents the design and simulation of an integrated multi-carrier optical source with a 227 GHz bandwidth for passive optical network (PON) applications. The optical comb generation attained using a photonic structure known as a micro-ring resonator fabricated in silicon nitride (Si3N4) facilitates cost reduction when produced on a large scale. Additionally, the generated optical comb accomplishes non-uniform tones in terms of the optical signal-to-noise ratio (OSNR), which allows for the dynamic assignment of carriers to retainable customers as a function of the data rate and transmission distance requirements. The design and simulation demonstrate the generation of frequency combs with optical carriers in a range of 5-40 tones, an OSNR range of 20-80 dB, and a free spectral range (FSR) of 50-3 610 GHz. To achieve these features, a geometric design of the device is proposed, and its response to variations of input laser parameters is described. In summary, the device uses two optical micro-resonators with radii of 100 and 450 µm and controls the power and the tuning of laser parameters. The proposed method allows generating a deterministic and reliable path to the frequency combs. Finally, the characteristics of the obtained combs are tested to determine their potential use in PON transmissions.

Flexible channel selection method based on optical combs for broadband signals receiving.

A flexible channel selection method based on optical combs is proposed for reconfigurable optical channels in this paper. Optical-frequency combs with a large frequency interval are used to modulate broadband radio frequency (RF) signals, and an on-chip reconfigurable optical filter [Proc. of SPIE, 11763, 1176370 (2021).10.1117/12.2587403] is used to perform periodic carrier separation of wideband and narrowband signals and channel selection. In addition, flexible channel selection is achieved by presetting the parameters of a fast-response programmable wavelength-selective optical switch and filter device. Channel selection only relies on the combs through the Vernier effect of the combs and the passbands for different periods and does not require the use an additional switch matrix. Finally, flexible switching between and selection of specific channels for 13 GHz and 19 GHz broadband RF signals are experimentally verified.

Investigation of the 6s6p3P2-6s7s3S1 transition of ytterbium atoms with modulation-transfer spectroscopy

We perform the precise measurement of the 6s6p3P2-6s7s3S1 transition frequencies of the neutral ytterbium (Yb) atoms by using the modulation transfer spectroscopy. High-resolution dispersive signals of all isotopes except low-abundance 168Yb (F = 2-F = 1) and 171Yb (F = 5/2-F = 3/2) are obtained. The frequencies of locked laser are measured using an optical comb, and the obtained results are traced to the absolute transition frequency of the 171Yb 1S0-3P0 clock. We measure the isotope shifts relative to 174Yb and evaluate the systematic shifts. The absolute frequency of the 6s6p3P2-6s7s3S1 transition in 174Yb is determined to be 389260605.8 ± 3.2 MHz, which has a huge deviation from the existing data in the studies. Based on the obtained transition frequencies, we calculate the hyperfine constants of the 3P2 and 3S1 states. This research will provide valuable data for the ytterbium-related experiments and improve the reported transition frequencies with higher precision.

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.

Controllable plasmonic vortex sequence with on-chip discrete-slit-based metalens

Like free-space vortex beams, surface plasmon polaritons can carry orbital angular momentum to form plasmonic vortices (PVs). Recently, research interest in PV fundamentals and applications has increased. However, generating and manipulating the topological charges of PVs over wide ranges using on-chip devices remains challenging. Here, we propose an on-chip plasmonic metalens structure to generate tunable PV sequence with controllable topological charges at discrete wavelengths. When compared with conventional spiral-slit structures, the designed metalens has additional structural parameters that bring more degrees of freedom to control the range and interval of the topological charge distribution of the PV sequence. Analytical and simulation methods are used to verify the metalens’ functionality. It is proved that the topological charges of the generated PV sequence are symmetrically distributed about the fundamental mode (l = 0), which cannot be realized by a traditional Archimedean helix. In addition, the normalized powers of the PV sequence are all above 0.8, showing that the designed metalens structure has potential for use as an on-chip optical vortex comb device. This work has potential applications in on-chip optical information processing, integrated optical communications, and optical tweezers.

광섬유 빗살 필터의 사분파장 지연기 연속 조합을 이용한 협대역 다파장 스펙트럼의 연속 주파수 조정에 관한 연구

We propose an optical fiber comb filter based on a polarization-diversity loop structure, which can continuously adjust the narrowband multiwavelength spectrum. The proposed comb filter consists of a polarization beam splitter(PBS), a successive combination of quarter-wave retarders(QWRs), a set of a half-wave retarder and a QWR, and two polarization-maintaining fiber(PMF) segments of equal length. For the convenient adjustment of the narrowband multiwavelength spectrum, the successive combination of QWRs was located between the PBS and the first PMF segment. The transmittance function of the proposed filter was theoretically derived using the Jones calculus. Based on the filter transmittance, we theoretically predicted the orientation angle sets of four wave retarders, which allow the extra phase difference φ of the transmittance function to vary from 0˚ to 315˚ with increments of 45˚. Then, the proposed filter successfully demonstrated the predicted results by obtaining ∼0.0915 ㎚ average wavelength shift and a high linearity of 0.99878.

Rydberg-Atom-Based Electrometry Using a Self-Heterodyne Frequency-Comb Readout and Preparation Scheme

Atom-based radio-frequency (rf) electromagnetic field sensing using atomic Rydberg states is a promising technique that has recently attracted significant interest. Its unique advantages, such as extraordinary bandwidth, self-calibration, and all-dielectric sensors, are a tangible improvement over antenna-based methods in applications such as test and measurement and development of broad-bandwidth receivers. Here, we demonstrate how an optical-frequency comb can be used to acquire data in the Autler-Townes regime of Rydberg-atom-based electrometry in a massively parallel fashion, eliminating the need for laser scanning. Two-photon electromagnetically induced transparency readout and preparation of cesium is used for the demonstration. A flat quasicontinuous optical comb is generated with the probe laser at 852 nm using an electro-optic modulator and arbitrary waveform generator. A single-frequency coupling laser at 509 nm is tuned to the Rydberg launch state. An enhanced transmission signal is obtained using self-heterodyne spectroscopy, where the comb signal is beat against a local oscillator derived from the single-frequency probe laser on a fast photodiode. The transmission of each probe-laser comb tooth is observed. We resolve electromagnetically induced transparency peaks with line widths below 5 MHz, with and without laser locking. Radio-frequency electromagnetic fields as low as $66\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}{\mathrm{Vcm}}^{\ensuremath{-}1}$ are detected, with sensitivities of $2.3\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}{\mathrm{Vcm}}^{\ensuremath{-}1}{\mathrm{Hz}}^{\ensuremath{-}1/2}$. The method offers a significant advantage for reading out electromagnetically induced transparency and Autler-Townes splitting, as neither laser needs to be scanned and slow frequency drifts can be tolerated in some applications. The method enables the detection of the amplitude of a pulsed rf electromagnetic field when the incoming pulse Autler-Townes splits the electromagnetically induced transparency peak.

1.57 Tb/s PDM-layered 80QAM-OFDM in super-channel transmission based on self-homodyne comb

In this paper, polarization division multiplexing-layered 80-ary quadrature amplitude modulation-orthogonal frequency division multiplexing (PDM-layered 80QAM-OFDM) is proposed and demonstrated in super-channel system based on self-homodyne comb. The composition of the super-channel is derived from the optical comb generated by a single laser-driven dual polarized Mach-Zehnder modulator (DP-MZM) modulated by odd sideband suppression. Since the optical comb generated by the transmitter is used as a local oscillator (LO) signal for self-homodyne coherent detection. This scheme effectively reduces the system cost and alleviates the performance constraints caused by phase noise. Moreover, the bit error rate (BER) and generalized mutual information (GMI) of layered 24QAM/48QAM/80QAM-OFDM are evaluated to obtain the highest transmission rate and the laser linewidth tolerance of the system. The results show that in Additive White Gaussian Noise (AWGN) channel, layered QAM signal realized by low-complexity constellation layering technique can significantly improve the BER performance at a small entropy cost compared with ordinary QAM signals. Meanwhile, the layered 80QAM can achieve a signal-to-noise ratio (SNR) gain of 2.7 dB. In addition, after 40 km standard single mode fiber (SSMF) transmission, the total data rate of the system can reach 1.57 Tbit/s and the corresponding spectral efficiency (SE) is 12 bit/s/Hz when the BER of the super-channel system meets soft decision-forward error correction (SD-FEC) threshold.

Novel absolute length measurement method for etalon sensor using optical comb pulsed interference and harmonic etalon reflections

A Fabry–Perot interferometer, also called an etalon, is used in various applications such as resonant structures for lasers, wavelength filters, length and frequency standards, and sensors. When the etalon is used as sensor, both the high resolution and simplicity of the measurement mechanism are required. We propose a method using an optical comb and an etalon harmonically. Using this method, it is possible to achieve measurements with resolution of less than a few nanometers and improved sensitivity. This method can be implemented with a simple system, and traceability can be easily achieved from the characteristics of the optical comb without the need for a high-precision wavelength meter. In this study, in addition to proposing a method, we implemented measurement system including a repetition frequency scanning optical comb and verified it experimentally. The repeatability of the measurement was approximately 3.0 nm (σ = 2) when 175 mm etalon was used. The measured absolute value of the etalon length was compared to the coordinate measuring machine (CMM) measurement results and difference between the two values was <1 μm that indicated the absolute values agreed well with the maximum permissible length error range of the CMM. The etalon displacement was measured, and the result was compared to that of the conventional He–Ne interferometer. The difference between the two was less than 1 nm, indicating the validity of the measurement and its applicability as sensor. These results demonstrated that the proposed measurement method is feasible and functional for etalon sensor.

Wideband Frequency and Angle-of-Arrival Measurement System Based on Optical Subsampling

We proposed a system that simultaneously supports measuring the frequency and the angle of arrival (AOA) of the radio frequency (RF) signal. In the proposed system, the RF signals from different antennas are loaded on different arms of a dual-polarization Mach-Zehnder modulator (Dpol-MZM) and are subsampled by dual optical combs, respectively. The frequency of the receiving RF signal can be obtained from two intermediate frequencies (IF), and AOA can be obtained from the phase difference between the upper- and lower-channel IF signals. In the experiment, 0.5–40 GHz frequency measurement range with an error of ±0.06 MHz and ±180 phase shift measurement range with an error of 1.8 can be achieved based on a low-frequency electronic analog-to-digital converter (ADC). The processing of both broadband signal and multitone signal and the system stability are also verified.