Number Of Comb Lines Research Articles

A Microwave Photonic Channelized Receiver Based on Polarization-Division Multiplexing of Optical Signals

Aimed at the problems of optical frequency combs, such as their large number of comb lines, their high flatness, and their lack of ease in generating, as well as the fact that the channelization efficiency of the scheme based on optical frequency combs is low, we proposed a microwave photonic channelization receiver based on signal polarization multiplexing. Using two-line local optical frequency combs with different frequencies to demodulate the RF signal in the orthogonal polarization state, 16 sub-channels with a bandwidth of 1 GHz can be received simultaneously. The experimental results show that the image rejection ratio can reach 28 dB, and the third-order spurious-free dynamic range of the system can reach 96.8 dB·Hz2∕3. This scheme has the advantages of a large number of sub-channels and a high channelization efficiency; it has great application potential in broadband wireless communication, radar, and electronic warfare systems.

Scalable parallel ultrafast optical random bit generation based on a single chaotic microcomb

Random bit generators are critical for information security, cryptography, stochastic modeling, and simulations. Speed and scalability are key challenges faced by current physical random bit generation. Herein, we propose a massively parallel scheme for ultrafast random bit generation towards rates of order 100 terabit per second based on a single micro-ring resonator. A modulation-instability-driven chaotic comb in a micro-ring resonator enables the simultaneous generation of hundreds of independent and unbiased random bit streams. A proof-of-concept experiment demonstrates that using our method, random bit streams beyond 2 terabit per second can be successfully generated with only 7 comb lines. This bit rate can be easily enhanced by further increasing the number of comb lines used. Our approach provides a chip-scale solution to random bit generation for secure communication and high-performance computation, and offers superhigh speed and large scalability.

Flexible broadband optical frequency comb generation assisted by injection locking of semiconductor laser

A broadband optical frequency comb (OFC) generator is proposed and experimentally demonstrated. Based on the injection locking process in a semiconductor laser diode, multiple coherent carriers with large intervals can be generated, which are further combined to give a much broader OFC through the electro-optical modulation (EOM) technique. The corresponding theoretical analysis and simulation have been given in detail. In the experimental demonstration, an OFC with 9 comb lines can be generated on a single carrier by the cascaded MZMs with a multi-harmonic RF drive signal. After tripling the number of carriers, the number of the comb lines becomes 27. By properly adjusting the carrier’s interval and drive signal frequency, the generated OFC shows great flexibility on spacing and the number of comb lines, where a 27-line OFC with FSR of 5 GHz and 6.5 GHz have been demonstrated respectively. In addition, by increasing the number of RF sources, the OFC with up to 43 comb lines can be generated in this system. Compared with the traditional OFC generation implemented by the EOM technique on a single optical carrier, three times optical spectrum extension or the number of comb lines increasing can be achieved. The Multi-carrier broaden spectrum method retains all the advantages of EOM-based OFC generation and overcomes its disadvantages, such as limited spectrum width and the number of comb lines. It provides a new idea for broadband OFC generation.

Dual Electro-Optic Comb Generator by Heterodyning Two Combs for Extending the Optical Bandwidth

A novel dual-electro-optic (DEO) comb generator is proposed by heterodyning the optical frequency combs (OFCs) to achieve a high bandwidth of ~2.74 THz with an efficient and cost-effective structure. This can attract the research community’s interest, particularly for the inherent support in optical communications due to its simplicity and high throughput. The OFC is generated by cascading a continuous wave (CW) laser and Mach Zehnder modulator (MZM) coupled to a second laser source that produces a broad spectrum of frequencies by the frequency heterodyning after passing it through a second MZM. This is the highest number of comb lines produced so far in cascaded configurations of the electro-optic modulators. The frequency difference between the two lasers is carefully selected, equal to 1.38 THz, which combinedly produces around 137 well-flattened OFC lines with 20 GHz frequency spacing and more than 40 dBm tone to noise ratio and an amplitude difference of 0 to 6 dB. The bandwidth of the generated flattened comb source is approximately 2.74 THz, which covers over half of the optical C-band.

Performance investigation for the WDM-passive optical network using multicarrier source

Abstract In this article, the comb wavelength division multiplexing passive optical network (Comb WDM-PON) system has been simulated. Simulation models were created in VPI photonics. The proposed system employs 140 Gbps for seven comb lines. Analytical modelling and simulations are performed for a flat comb source with seven comb lines, variable channel spacing, and a center wavelength. As a result, the performance of the comb WDM-PON system that utilizes a single differential Mach–Zehnder (DiffMZ) modulator comb source is also investigated using different comb line numbers. The total capacity of the system increases as the number of comb lines increases. However, the system performance is degraded. Data rates of 80 Gbit/s can be achieved at the threshold bit error rate of BER (1 × 10−9), over a transmission distance of 200 km using 4 comb lines with a multi-diagonal (MD) code for 4 OLTs.

Analysis of parameter combinations for optimal soliton microcomb generation efficiency in a simple single-cavity scheme

Dissipative Kerr solitons generated in high-Q optical microresonators provide unique opportunities for different up-to-date applications. Increasing the generation efficiency of of such signals is a problem of paramount importance. We provided comprehensive analysis and found optimal conditions providing maximal pump-to-comb conversion efficiency (up to 100\%) for the cases of a free-running and self-injection-locked pump lasers. The dependence of the optimal coupling rate on the pump power was revealed, in addition to the trade-off relations balancing the efficiency and the comb line number. The methods to increase the total comb power were also discussed.

Multi-band triangular frequency modulation signal generation based on gain-switched laser

A multi-band triangular frequency modulation signal generation method is proposed and verified by simulation and experiment. This is a simple photonics method based on distributed feedback gain switching states of semiconductor lasers without the need for complex external modulators. The carrier frequency and bandwidth of the generated triangular frequency modulation signal are N times that of the fundamental signal (N is the number of comb lines). By tuning the bandwidth and duration of the fundamental signal, the product of the number of frequency bands and the time bandwidth (TBWP) of the generated signal can be easily adjusted. The autocorrelation results of the generated waveform are also given. The triangular frequency modulation signals of six bands from S-band to K-band are realized, and have different chirp rates. The maximum TBWP reaches 24000, and the corresponding range resolution reaches 6.3 cm.

Simple multi-band linearly frequency-modulated signal generation with a multiplying bandwidth based on a gain-switching laser.

Multi-band linearly frequency-modulated (LFM) signal generation with a multiplying bandwidth is proposed and experimentally demonstrated. It is a simple photonics method based on the gain-switching state in a distributed feedback semiconductor laser without a complex external modulator and high-speed electrical amplifiers. With N comb lines, the carrier frequency and bandwidth of generated LFM signals are N times those of the reference signal. (N is the number of comb lines.) The number of bands and time-bandwidth products (TBWPs) of the generated signals could be easily adjusted by tuning the reference signal from an arbitrary waveform generator. Three-band LFM signals with carrier frequencies ranging from the X-band to K-band are given as an example, and the TBWP up to 20000. The results of auto-correlations of the generated waveforms are also given.

Broadband Microwave Photonic Channelizer with 18 Channels Based on Acousto-Optic Frequency Shifter

A microwave photonic channelizer can achieve instantaneous reception of ultra-wideband signals and effectively avoid electronic bottleneck; therefore, it can be perfectly applied to a wideband radar system and electronic warfare. In channelization schemes based on an optical frequency comb (OFC), the number of comb lines usually depends on that of the sub-channels. In order to improve the utilization rate of the comb lines of OFC, we propose a scheme to shift the frequency of OFC by using an acousto-optic frequency shifter (AOFS), which can obtain three times the number of sub-channels of the comb lines of an OFC. In order to simplify the experiment, only a three-line OFC is used in the experiment. A three-line local oscillator (LO) OFC is frequency-shifted up and down by two AOFSs, and nine optical LO signals with different frequencies are obtained, thereby realizing the simultaneous reception of eighteen sub-channels. The proposed scheme enjoys a large number of sub-channels and minimal channel crosstalk. Experimental results demonstrate that a 9-GHz bandwidth RF signal covering 10–19 GHz is divided into 18 sub-channels with a sub-bandwidth of 500 MHz. The image rejection ratio of the sub-channels is about 23 dB, and the spurious-free dynamic range (SFDR) of the receiver can reach 98 dB·Hz2/3.

Frequency Stability Requirements in Quasi-Integer-Ratio Time-Expanded Phase-Sensitive OTDR

Time-expanded phase sensitive (TE-ϕ)OTDR is a recently reported technique for distributed fiber sensing that relies on the use of an electro-optic dual frequency comb (DFC) scheme. A distinctive feature of this approach is its ability to provide high spatial resolution (on the centimeter scale) with detection bandwidths orders of magnitude lower than those of conventional ϕOTDR systems. The stringent trade-off between resolution, range and sensing bandwidth that exists in TE-ϕOTDR has demonstrated to be substantially relaxed by implementing two frequency combs with quasi-integer-ratio repetition rates. However, employing very dissimilar line separations (with a ratio between them >100) is challenging due to the need of keeping the coherence over long sequences of interferograms, which eventually limits the attainable range. In this paper, we formulate the requirements for the frequency stability of the reference clock used in a quasi-integer-ratio DFC scheme. This analysis allows us to stablish the limits on the number of comb lines (i.e., on the number of available independent sensing points) for a particular reference clock. By using a rubidium atomic clock (with a relative frequency stability of ∼10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−13</sup> ), we demonstrate up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> sensing points along 2 km of fiber with tens of Hz sensing bandwidth.

Nonlinear dynamics of a semiconductor microcavity laser subject to frequency comb injection.

The nonlinear dynamical behaviors of a semiconductor microcavity laser with frequency comb injection have been experimentally and numerically investigated. The microcavity laser is harmonically locked to a unit fraction of the comb spacing due to the undamped relaxation oscillation at certain conditions, creating additional comb lines with reduced frequency spacing. The stability maps indicating various locking states are obtained based on rate equations, which demonstrates that the locking regions are closely related to the relaxation oscillation. Moreover, the microcavity laser with comb injection leads to spectral broadening of the original comb and the number of comb lines raises from 3 to 13. Owing to the large modulation bandwidth of the microcavity laser, the comb lines and the frequency spacing can be tailored over a wide range by varying the injection parameters.

Optical-Frequency-Comb Generation Based on Single-Tone Modulation and Four-Wave Mixing Effect in One Single Semiconductor Optical Amplifier

We propose a novel optical-frequency-comb (OFC) generation scheme based on single-tone modulation and the four-wave mixing (FWM) effect in one single semiconductor optical amplifier (SOA) modulated by a radio frequency (RF) current. A comprehensive broad-band dynamic model, which considers single-tone modulation and the FWM effect, is presented. The simulated results show that, although only one single continuous-wave light is input into the SOA, an OFC with a large number of frequency components can be achieved as a result of single-tone modulation and the FWM effect in the SOA. The number of comb lines and the spectral bandwidth of the OFC increase by raising the amplitude of the RF modulation current. Increasing the input light power can increase the average optical power of the OFC. The frequency interval is tunable within a certain range by tuning the frequency of the RF modulation current injected into the SOA.

Proposed Scheme for Ultra-Flat Optical Frequency Comb Generation Based on Dual-Drive Mach–Zehnder Modulators and Bidirectional Recirculating Frequency Shifting in Single Loop

Recirculating frequency shifting has attracted much attention for its advantages in the generation of the flexible and high-quality optical frequency comb. A new scheme of ultra-flat optical frequency comb generation system based on single-loop bidirectional recirculating frequency shift is proposed and studied in this paper. The generation system employs two pairs of dual-drive Mach–Zehnder modulators and several polarization devices. Compared with the method of single-loop unidirectional recirculation frequency shift, under the same cycles, the number of comb lines generated by the proposed method is doubled, and the generated optical frequency combs have less noise accumulation and better flatness. The theoretical model is established, and the proposed scheme is verified by software simulation. A 111-line optical frequency comb with the spacing of 12.5 GHz, the flatness of 0.76 dB, and the optical signal-to-noise ratio of 27.39 dB was obtained by adopting the proposed scheme.

Tunable microwave frequency comb generation using a Si3N4-MDR based actively mode-locked OEO.

Microwave frequency combs (MFCs) have important applications in communication and sensing owing to their characteristics of large number of comb lines, wide frequency range, and high precision of comb spacing. In many applications, MFCs are required to emit signals with tunable center frequency and variable comb spacing to accommodate different operating frequency bands and accuracies. Here, we demonstrate a tunable MFC by injecting a low-frequency electrical signal into a tunable optoelectronic oscillator (OEO). Tuning of MFC's center frequency and comb spacing are realized, allowing a frequency tuning range from 1 to 22 GHz and 50 comb lines within a 5 MHz bandwidth obtained in the MFC generator. In addition, the introduction of the silicon nitride micro-disk resonator (Si3N4-MDR) in the system paves the way for the integration of MFC generator.

Generation of Flat Optical Frequency Comb Using Cascaded PMs With Combined Harmonics

Flat optical frequency comb (OFC) can be generated by using single phase modulator (PM), where combined harmonics are applied to drive the PM. However, the number of comb lines and flatness of OFC are limited under this scheme. In this work, we use cascaded PMs with combined harmonics to generate flat OFC. The optimized phases and modulation indices in terms of amplitudes for each harmonic are obtained by solving optimization problem to minimize the fluctuation of OFC with a certain number of comb lines. Both simulation and experiment are carried out to evaluate the feasibility of the proposed scheme. Regarding the case of combined fundamental tone, third harmonic, and fifth harmonic, a 15-line OFC is generated with the flatness of 0.29 dB and 0.65 dB observed in simulation and experiment, respectively. Other feasible solutions to flat OFC with combined harmonics are also investigated.

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.

Investigation of low-power comb generation in silicon microresonators from dual pumps

Here, the influence of pump powers for silicon microresonator-based Kerr comb generation (KCG) was investigated theoretically and experimentally in two types of dual-pumped KCG. One employs dual pumps at different free spectral range (FSR) spacings of the resonator for broadband comb spectrum and the other to produce comb lines at one FSR spacing for low pump powers. The KCG is modeled by including nonlinear absorption and mode interaction into the Lugiato–Lefever equation. Both the theoretical and the experimental results indicate that the KCG with pump powers inside the ring of tens of milliwatts would be limited by high nonlinear absorption even with a 25 V reverse bias to deplete free carriers. The most efficient comb spectrum occurs with recirculating pump power in the ring cavity of 13 mW (0.3 mW pump power in the bus waveguide) at 25 V reverse bias. Generally, for silicon KCG at the telecom wavelengths, the use of low pump powers and reverse bias is essential. At 25 V reverse bias, increasing the stored pump power in the ring to 160 mW would suppress the KCG and produce a similar number of comb lines as 1 mW recirculating pump power. This work advances our understandings of silicon KCG in the telecom band.

Near-infrared frequency comb generation from a silicon microresonator

Optical frequency combs (OFCs) have been widely explored in the silica and silicon nitride platforms, from visible light to mid-infrared wavelength. Although silicon has a larger nonlinear coefficient, it suffers from larger two-photon absorption and consequent free-carrier absorption. Therefore, microresonator-based OFCs have not been experimentally demonstrated in the silicon platform with a single pump in the near-infrared region. Here, we experimentally demonstrated the Kerr comb generation (KCG) in a high-quality-factor silicon microresonator using sub-milliwatt pump power without any reverse bias for removing the free carriers. We experimentally investigated the effects of pump powers and quality factors (Qs) on the comb output power and number of comb lines. Under low pump power, either larger pump power or larger Q can yield higher comb output power and more comb lines. However, at higher pump powers, the comb output power is limited by nonlinear absorptions. The KCG was well established by using pump wavelengths in the range from 1340 nm to 1650 nm. The further combination of this microresonator with a p-i-n diode is possible to obtain considerable comb output powers. This work shows the possibility of microresonator-based OFCs in silicon chips at telecom wavelengths.

Flat frequency comb generation employing cascaded single-drive Mach–Zehnder modulators with a simple analogue driving signal

A scheme, using two-single drive Mach–Zehnder modulators (SD-MZMs) to generate flat optical frequency comb (OFC) by an analogue driving signal, is experimentally demonstrated. The modulator is used to add equidistant sidebands to an injected carrier continuous-wave laser centred at 1550 nm. The flatness of the comb lines is achieved by tuning the DC bias voltages supplied to both modulators. Five comb lines are tested for different frequency spacing of 8.5, 10, 11.5, and 12 GHz and show good frequency tunability. Also, the effect of increasing the radio frequency (RF) modulation signal power on the number of comb lines is investigated. A total of 5 comb lines are obtained with a repetition rate of 10 GHz and 0.77 dB flatness. The proposed system is simple and cost-effective due to the utilization of a simple Mach–Zehnder modulator structure driven by one RF signal generator.