Flat Optical Frequency Comb Generation Research Articles

O-U Band Flat Optical Frequency Comb Generation Based on Dispersion-controlled Fluorotellurite Glass Fiber

O-U Band Flat Optical Frequency Comb Generation Based on Dispersion-controlled Fluorotellurite Glass Fiber

Flat optical frequency comb generation by using one DPMZM and superposed harmonics

In this work, we propose a novel approach to the generation of flat optical frequency comb (OFC) by using dual-parallel Mach–Zehnder modulator (DPMZM) and superposed harmonics, where the superposed harmonics drive only one child Mach–Zehnder modulator of DPMZM. By adjusting precisely the modulation indices and phases of harmonics as well as the biases for DPMZM, flat OFCs are generated. In the experiment, a 9-line OFC with 0.26-dB flatness is achieved when the superposed fundamental tone and third harmonic drive the DPMZM; a 13-line OFC with 0.58-dB flatness is achieved when the fundamental tone, second harmonic, and third harmonic drive the DPMZM simultaneously.

Analysis of the performance of a coherent SAC-OCDMA–OFDM–DWDM system using a flat optical frequency comb generator for multiservice networks

This research proposes the architecture of a SAC-OCDMA–OFDM–DWDM system using a flat optical frequency comb generator and coherent communication. Its aim is to create a new OCDMA communication system that is specifically intended for multiservice networks. The suggested architectural design is numerically simulated and analyzed using OptiSystem and MATLAB. To quantify the performance of the proposed system, SNR, EVM, and BER were used as metrics. Furthermore, a comparison between the two codes, EDW and RD, has been presented. Thus, it has been revealed that the RD code system has better performance in terms of robustness and number of users than the EDW code. Besides, by analyzing the BER for several symbol rates and comparing it to the pre-FEC threshold, the proposed system demonstrates its effectiveness against linear and nonlinear effects. The results also show that optical noise affects signal quality; the mathematical analysis used allowed us to determine the impact of noise on the number of potential active users.

Flat Optical Frequency Comb Generation Based on Monolithic Integrated LNOI Intensity and Phase Modulator

A flat optical frequency comb (OFC) is generated by using a monolithic integrated electro-optic intensity and phase modulator fabricated on lithium niobite on insulator (LNOI) platform. The LNOI-based modulation chip consists of a push–pull Mach–Zehnder modulator (MZM) and a U-shaped phase modulator (PM) connected by a curved optical waveguide. Microwave and optical packaging are implemented for the modulation chip, where the input and output pigtails of the packaged modulation device are polarization-maintaining fibers, with a core diameter of 6.5 μm. The packaged LNOI-based modulation device is featured by a fiber-to-fiber insertion loss as low as 6.97 dB. The half-wave voltages of the MZM and the PM are measured to be 3.6 V and 3.4 V at 5 GHz, respectively. By using the modulation device, an OFC with a tooth spacing of 5 GHz is generated, and the 13 comb teeth in the generated OFC are with a power flatness of 2.4 dB. The measured results of this device indicate that the tooth spacing of the generated OFC can be extended to tens of gigahertz by using a microwave source with a higher output frequency. In addition, the number of the comb teeth can be enhanced beyond 20 by increasing the power of the radio-frequency signal applied to the PM or by further reducing the half-wave voltage of the PM.

Flat Optical Frequency Comb Generator Based on Integrated Lithium Niobate Modulators

Chip-scale electro-optic (EO) frequency combs are expected to play an essential role in future high-capacity optical communications systems and next-generation mobile communications. The application requires integrated EO frequency comb generators featuring good spectral flatness, high modulation bandwidth, low driving voltage and low insertion loss simultaneously. Here, we demonstrate a flat-top optical flat comb (OFC) generator based on high-performance lithium niobate on insulator modulators. The OFC generator shows a low on-chip loss (2.1 dB), a low driving voltage over a broad frequency range, and a large 3-dB EO bandwidth. Moreover, with consuming power of less than 2W, the presented device produces 13 lines with a power variation of less than 1.2 dB and a line spacing of 31 GHz which can further be extended to 67 GHz.

Design of normal dispersion high nonlinear silica fiber and generation of flat optical frequency comb

<sec>The scheme of generating optical frequency comb mainly includes mode-locked laser, electro-optic modulation comb, nonlinear Kerr micro-resonator comb, and nonlinear supercontinuum comb. For the nonlinear supercontinuum comb scheme, the silica-based high nonlinear fiber with near-zero flattened normal dispersion is required. However the fiber dispersion varies along the fiber due to the fabrication inaccuracy. Furthermore, nonlinear supercontinuum comb generation based on the nonlinear fiber has not been systematically studied. In this paper, an optimal design of four-clad flat normal dispersion high nonlinear silica fiber with a triangular core refractive index distribution for the flat optical frequency comb generation is carried out. The effects of the fiber cladding width and refractive index on the fiber dispersion characteristics and cut-off wavelength are studied through using the finite element method mode solver. The optimally designed fiber can obtain relatively flat near-zero normal dispersion in a wavelength range of 1400–1700 nm, the dispersion range is –3–0 <inline-formula><tex-math id="M5">\begin{document}$ \rm{p}\rm{s}/(\rm{k}\rm{m}\cdot \rm{n}\rm{m}) $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M5.png"/></alternatives></inline-formula>, and the dispersion slope is close to 0 at nearly 1550 nm. The effective mode field area of the nonlinear silica fiber is about 11<inline-formula><tex-math id="M6">\begin{document}$ {\text{μm}}^{2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M6.png"/></alternatives></inline-formula>, and the nonlinear coefficient can reach 12.8<inline-formula><tex-math id="M7">\begin{document}$ {\rm{W}}^{-1}{\cdot \rm{k}\rm{m}}^{-1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M7.png"/></alternatives></inline-formula>.</sec><sec>Based on the electro-optic modulation pulse pumping the flat normal dispersion high nonlinear silica fiber, the flat optical frequency comb generation is systematically simulated with the generalized nonlinear Schrödinger equation. The time-frequency evolutions of a hyperbolic secant pulse, a Gaussian pulse and a super Gaussian pulse are simulated by using the X-Frog technology. The time-frequency spectrograms connect the time domain and the frequency domain of the pulse, clearly showing the change of pulse chirp during the propagation. The effects of various parameters on the optical frequency comb are studied, such as the fiber length, second-order dispersion, third-order dispersion, pulse peak power, pulse half width, pulse initial chirp, and pulse shape. An optical frequency comb with 3-dB flatness and about 40-nm bandwidth can be achieved based on hyperbolic secant pulse or Gaussian pulse pumping. Compared with the hyperbolic secant pulse and Gaussian pulse, the super Gaussian pulse can produce a flatter optical frequency comb. An optical frequency comb with 2-dB flatness and about 92-nm bandwidth can be achieved based on the super Gaussian pulse pumping. Therefore, based on the proposed high nonlinear fiber with normal dispersion , it is possible to realize an optical frequency comb with a repetition rate above 10 GHz, power flatness within 3 dB, and spectral bandwidth of about 40–90 nm. The simulation results are beneficial to promoting the localization of normal dispersion high nonlinear silica fiber and its application in flat optical frequency comb.</sec>

Flat optical frequency comb generation employing cascaded dual-drive Mach-Zehnder modulators

The generation of flat optical frequency comb lines (OFCL) of 10 GHz frequency spacing is proposed and demonstrated theoretically employing Dual-Drive Mach Zehnder modulators (DD-MZMs). A theoretical analysis is carried out for ultra flat spectra due to single stage and cascading of DD-MZMs, which is confirmed by simulative results. Employing single DD-MZM, 19 OFCL with flatness 0 dB and 39 OFCL with flatness ≤ 1 dB have been generated at 11.16 Vπ−11.66 Vπ modulation voltages. 37 comb lines are obtained with 0 dB flatness and 49 OFCL with flatness ≤ 0.5 dB using two cascaded dual drive MZMs at 5.61 Vπ–6.11 Vπ–5.61 Vπ, 6.11 Vπ modulation voltages. Combining four DD-MZMs, 34 & 54 OFCL with flatness 0 dB and ≤ 0.5 dB is generated at modulation voltages 2.78 Vπ and 3.33 Vπ. Large numbers of OFCL with very high flatness are achieved by applying low RF voltage.

Flat optical frequency comb generation based on a dual-parallel Mach-Zehnder modulator and a single recirculation frequency shift loop.

To generate a flat optical frequency comb (OFC), a new scheme based on a dual-parallel Mach-Zehnder modulator and a single recirculation frequency shift loop is proposed and analyzed. Compared with the traditional single loop recirculation frequency shift method, the quantity of comb lines is doubled, and the comb flatness is better when the number of cycles is the same. The theoretical analysis model is established, and the simulation results show that a 111-line OFC with frequency spacing of 10 GHz, flatness of 1.32 dB, and optical signal to noise ratio of 27.4 dB can be obtained by adopting the proposed scheme.

Highly flat optical frequency comb generation based on pulse carving and sinusoidal phase modulation

We investigate the relationship between an optical pulse shape and a time lens implemented by means of sinusoidal phase modulation. Based on this investigation, two schemes are proposed to obtain an optical frequency comb (OFC) with exceptionally high flatness and a large number of spectral lines by carving an optical pulse shape to result in a quasilinear chirp via a simple sinusoidal phase modulation technique. The first scheme utilizes an intensity modulator with a single-drive port or with dual-drive ports to carve a narrow pulse. The experimental results show very good spectral profiles with 38 OFC lines at 1.2-dB flatness and 53 lines at 1.5-dB flatness when the intensity modulator is combined with two and three phase modulators for sinusoidal phase modulation, respectively. The second scheme is implemented by replacing the intensity modulator by a dual-parallel Mach–Zehnder modulator (DP-MZM). In this case, we obtain 35 OFC lines at nearly perfect flatness of less than 1 dB and 53 lines at 1.5-dB flatness after combining the DP-MZM with two and three phase modulators, respectively.

Flat optical frequency comb generation based on polarization modulator with RF frequency multiplication circuit and dual-parallel Mach-Zehnder modulator

A new approach to generate optical frequency comb (OFC) with good flatness, large side mode suppression ratio(SMSR) and tunable frequency spacing is proposed. The generator is composed of a polarization modulator (PolM) and a dual-parallel Mach-Zehnder modulator (DPMZM), and the PolM is modulated by a radio frequency(RF) frequency multiplication circuit. The theoretical analysis model of the proposed scheme is established. By adjusting the amplitude and frequency of the RF and the direct current(DC) bias voltage of modulators, 14-, 21- and 35-line OFCs with flatness value lower than 0.5 dB and SMSR value higher than 21 dB can be obtained easily. The stimulation results are in good agreement with the theoretical model.

Very Flat Optical Frequency Comb Generation based on Polarization Modulator and Recirculation Frequency Shifter

AbstractWe propose and experimentally demonstrate a novel scheme to generate a flat and stable optical frequency comb by using recirculation frequency shifter (RFS) loop and polarization-modulator based single-side-band modulator (PSSBM). An ultra-flat 5-carriers generated by a polarization modulator is set as the seed light source of the recirculating loop, and the recirculation times is greatly reduced compared with the regular scheme based on RFS. Through theoretical analysis and experiment simulations, it is shown that the optical frequency comb of high quality with 50 spectral lines and the flatness fluctuation of less than 1 dB can be achieved.

Flat optical frequency comb generation using polarization modulator and frequency shifter with double recirculating frequency shifting loops

A scheme to generate ultra-flat and stable optical frequency comb (OFC) based on multi-wavelength optical seed source and polarization-modulator-based complementary frequency shifter (PCFS) has been proposed and demonstrated. The OFC generation scheme includes two stages: 5-carrier light source is generated by utilizing a polarization modulator at the first stage, and the light source is then used as the seed light in the PCFS at the second stage. Compared with the previous flat OFC generator based on a recirculation frequency shifter, the proposed scheme can greatly reduce the recirculation times, and such a property makes the accumulated noise of the system enormously decrease. In order to achieve the frequency comb with the best flatness, the optical amplifier gain, filter stop-band attenuation and modulator drive voltage are optimized. Experiment simulations show that high-quality OFC with hundreds of frequency lines and power fluctuation within 2 dB can be achieved without direct-current bias control and wavelength-selective-switch.

Operation Strategy of InP Mach–Zehnder Modulators for Flat Optical Frequency Comb Generation

We investigate an InP-based semiconductor Mach-Zehnder modulator (MZM) for the purpose of generating a flat optical frequency comb (OFC). A nine-line OFC having flatness of <;0.8 dB is experimentally obtained by using an InP MZM under an optimized operation condition. To determine the optimized drive voltages, we use a simple model that takes into account nonlinear changes in the refractive index (phase modulation) and optical absorption loss (amplitude modulation) induced in the InP MZM. We confirm that it can reproduce the measured spectra of OFCs generated by the InP MZM. Optimum drive voltages of the InP MZM for flat OFC generation are found by considering the amplitude differences of the linear and nonlinear phase modulations for each arm. It is found that nonlinear phase modulation with a moderate amplitude (0.1-0.2π) flattens the OFC. The OFC becomes less flat at larger amplitudes of the nonlinear phase modulation; however, this deterioration can be suppressed using the amplitude modulation induced by the dependence of the optical absorption loss on the applied voltage. A numerical analysis using the proposed model reveals feasible operation conditions for the InP MZM.

Experimental demonstration of a flat optical frequency comb generation based on cascaded directly modulated distributed feedback laser and polarization modulator

A scheme to generate a flat optical frequency comb (OFC) is proposed and experimentally demonstrated based on a directly modulated distributed feedback (DFB) laser cascaded with a polarization modulator (PolM). In the proposed scheme, the DFB laser is optically injection-locked by a tunable laser source and directly modulated by a radio frequency (RF) signal, which is amplified by a microwave power amplifier. The optical signal is then sent to PolM via a polarization controller (PC) and modulated by the amplified and phase-shifted RF signal from the same source. The optical signal is finally received and measured by an optical spectrum analyzer (OSA) after transmitting through another PC and a polarizer. Here, the OFC with their power variation within 3 dB is desired, and four OFCs with 6, 6, 5, and 4 comb lines are generated using the RF signals with different frequencies, which have a flatness of, respectively, 2.4, 2.5, 0.7, and 0.6 dB. Here, the number of comb lines is decreased, which is due to the RF signal power decrease while its frequency is raised.

A flexible and ultra-flat optical frequency comb generator using a parallel Mach–Zehnder modulator with a single DC bias

A flat optical frequency comb generator utilizing a single dual parallel Mach–Zehnder modulator (DPMZM) is detailed investigated both in theory and experiment. In the scheme, the two sub-modulators of DPMZM are biased at the maximum and minimum transmission points respectively. By adjusting the RF signals power, two optical frequency combs with even harmonics or odd harmonics can be generated. So at the end of DPMZM, 5 and 7-line flat OFCs with a side-comb suppression ratio (SCSR) higher than 18dB are theoretically and experimentally generated. The optical frequency comb generator scheme is relatively simple and valuable due to only one bias voltage may be controlled.

Continuous wave sub‐THz photonic generation with VCSEL‐based optical frequency comb

A simple and energy-efficient photonic system to generate continuously tunable, low phase noise, sub-THz waves based on COTS components is presented. The optical scheme is based on the use of a commercial vertical cavity surface emitting laser under gain switching modulation that provides a very flat optical frequency comb generator (OFCG) with 23 modes in a 20 dB bandwidth. The laser only needs 15 dBm continuous wave radiofrequency input power and 9 mA of bias current to provide this OFCG. Two optical injection locking stages filter and amplify the two desired modes that are detected in a photodiode to produce the desired sub-THz signal at the frequency difference of these two selected modes. As an example, demonstrated is the generation of a very stable 88.2 GHz tone with lower linewidth than 10 Hz using a reference of 4.2 GHz to generate the OFCG.

Flat optical frequency comb generation and its application for optical waveform generation

We demonstrate the generation of a flat optical frequency comb (OFC) using an electro absorption modulator (EAM) and two cascaded phase modulators (PM). Compared with previous flat OFC generation schemes using a Mach–Zehnder modulator (MZM) for amplitude gating of the phase modulated light, an EAM creates much shorter time window for amplitude gating, and more flattened OFC spectrum is obtained using EAM as an amplitude gate. In our experiment, a very flat OFC having 11 lines within 3-dB bandwidth (3.8nm), and 40GHz comb line spacing is generated. Besides the excellent spectral flatness, the strong time-domain linear-chirp of the generated OFC can be simply compensated by a dispersion compensation fiber (DCF) link, which is preferred for optical waveform generation based on spectral manipulation. In this paper, we also demonstrate the OFC application for generation of several optical waveforms by manipulating the amplitude and phase of the comb lines. High repetition-rate optical cosine pulse, optical pulse burst, optical square and triangle pulses are successfully generated.

Generation of Flat Optical-Frequency Comb Using Cascaded Intensity and Phase Modulators

A scheme using one intensity modulator and two phase modulators driven directly by sinusoidal waveform to generate an optical-frequency comb (OFC) is experimentally demonstrated. It is relatively simple, where Bragg grating or specially tailored waveforms are not used. By setting the ratio of direct-current bias to half-wave voltage and phase shifts between sinusoidal waveform applied on the intensity and two phase modulators at appropriate values, 29 comb lines with spectral power variation less than 1.5 dB at 10 GHz are obtained. And the scheme has adjustability; a flat-top OFC with frequency spacing of 9.5-10 GHz is achieved.

Generation of flat optical frequency comb by fiber loop modulation

We propose a novel flat optical frequency comb generation system that employs fiber loop modulation and experimentally demonstrate its operation. Generally, when an external laser beam is injected into a fiber loop system including an optical modulator and an amplifying medium, multiple sidebands are generated exponentially. On the other hand, the fiber loop starts mode-locked oscillation at a center frequency to maximize the round-trip gain in the fiber loop from the seeds of the injected laser sidebands. By locking the sidebands from fiber loop modulation with the mode-locked oscillation in the fiber loop, the synchronous spectrum between the mode-locked oscillation and the sidebands can be shaped into a flat spectrum. Thus, a flat optical frequency comb with a flatness of about ±1 dB is obtained in the spectral bandwidth of about 1 THz.