Number Of Comb Lines Research Articles

Optical frequency comb and picosecond pulse generation based on a directly modulated microcavity laser.

Optical frequency comb (OFC) and picosecond pulse generation are demonstrated experimentally based on a directly modulated AlGaInAs/InP square microcavity laser. With the merit of a high electro-optics modulation response of the microcavity laser, power-efficient OFCs with good flatness are produced. Ten 8-GHz-spaced optical tones with power fluctuation less than 3 dB are obtained based on the laser modulated by a sinusoidal signal. Moreover, the comb line number is enhanced to 20 by eliminating the nonlinear dynamics through optical injection locking. Owing to the high coherence of the OFC originating from the directly modulated microcavity laser, a 6.8 ps transform-limited pulse is obtained through dispersion compensation. The optical pulse is further compressed to 1.3 ps through the self-phase modulation effect in high nonlinear fiber.

Recent progress in integrated electro-optic frequency comb generation

Optical frequency combs have emerged as an important tool enabling diverse applications from test-and-measurement, including spectroscopy, metrology, precision distance measurement, sensing, as well as optical and microwave waveform synthesis, signal processing, and communications. Several techniques exist to generate optical frequency combs, such as mode-locked lasers, Kerr micro-resonators, and electro-optic modulation. Important characteristics of optical frequency combs include the number of comb lines, their spacing, spectral shape and/or flatness, and intensity noise. While mode-locked lasers and Kerr micro-resonators can be used to obtain a large number of comb lines compared to electro-optic modulation, the latter provides increased flexibility in tuning the comb spacing. For some applications in optical communications and microwave photonics, a high degree of integration may be more desirable over a very large number of comb lines. In this paper, we review recent progress on integrated electro-optic frequency comb generators, including those based on indium phosphide, lithium niobate, and silicon photonics.

Microwave Photonic Filter by Using Recirculating Frequency Shifter to Generate Optical Frequency Comb

In this paper, a continuously tunable microwave photonic filter (MPF) with optical frequency comb (OFC) generated by recirculating frequency shifter (RFS) based on an inphase/quadrature (I/Q) modulator is proposed and experimentally demonstrated. A chirped fiber Bragg grating (CFBG) is inserted into the RFS loop to act as an optical bandpass filter. The central frequencies of two passbands of the MPF in 20 GHz range have been tuned linearly from 7.67 GHz to 9.04 GHz, and 15.34 GHz to 18.03 GHz, respectively by changing the frequency of the driving microwave signal onto the I/Q modulator from 20 GHz to 17 GHz. When the frequency of the driving microwave signal is decreased, OFC with smaller tone spacing is generated, and then the MPF's passbands move to higher frequency. So that, the proposed MPF's passbands with high frequencies can be achieved by applying driving microwave signal with low frequency to generate the OFC. By varying the bandwidth of the CFBG, the number of comb lines can be changed, which results in different bandwidth of the MPF's passband. Furthermore, in our experiment by carefully adjusting bias voltage of the Mach-Zenhder modulator, when the frequencies of driving microwave signal onto the I/Q modulator is 20 GHz and 18 GHz, one passband in the 20 GHz range can be suppressed at the frequency of 15.34 GHz, and 8.52 GHz, respectively.

Optical frequency comb based on a distributed feedback semiconductor laser with direct injection current multi-frequency modulation

Direct modulation of a semiconductor laser is a simple and low-cost method to generate compact optical frequency combs (OFCs). Herein, the use of direct injection current multi-frequency modulation (MFM) on a distributed feedback (DFB) semiconductor laser is proposed to generate OFCs. The number of modulation signals, frequency components, and modulation indices of the MFM signal are calculated according to the desired number of comb lines of the OFCs. The feasibility of this method was demonstrated by a series of experiments. OFCs composed of 9, 15, and 27 comb lines were generated with flatness values of 0.58 dB, 1.24 dB, and 2.53 dB, respectively, using a 1.58-μm DFB laser. The proposed method is easy to implement, it offers high flexibility in the selection of the repetition frequency. The proposed method is inexpensive and can improve the OFC quality with no laser-power loss.

Analysis of Non-Idealities in the Generation of Reconfigurable Sinc-Shaped Optical Nyquist Pulses

Optical sinc-shaped Nyquist pulses are widely used in microwave photonics, optical signal processing, and optical telecommunications due to their numerous advantages, like rectangular shape in the frequency domain, the orthogonality and the consequential possibility to use these pulses to transmit data with the maximum possible symbol rate. Ideal sinc pulses with the rectangular spectrum are just a mathematical construct. However, high-quality sinc pulse sequences offer the same advantages and can be generated by a phase-locked rectangular frequency comb with mode-locked lasers, intensity modulators, and integrated devices. Nevertheless, any non-idealities in the pulse and comb generation might lead to a degradation of the system performance, especially for metrology. Here, we investigate and analyze the effect of three major non-idealities, namely, the roll-off factor, the side band suppression ratio (SSR), and the ripple of sinc-shaped reconfigurable optical Nyquist pulse sequences based on 3, 5, and 9-line optical phase-locked frequency combs. We compare these results with the existing literature for the three-line comb followed by the experimental verification of the simulation results. We illustrate that by increasing the number of comb lines, the pulse sequences have superior performance and contribute to lesser root-mean-square (r.m.s.) error. We also discuss the trade-off between the r.m.s. error and the optical power loss for increasing the SSR.

Noise and distortion analysis of dual frequency comb photonic RF channelizers.

Dual frequency combs are emerging as highly effective channelizers for radio frequency (RF) signal processing, showing versatile capabilities in various applications including Fourier signal mapping, analog-to-digital conversion and sub-sampling of sparse wideband signals. Although previous research has considered the impact of comb power and harmonic distortions in individual systems, a rigorous and comprehensive performance analysis is lacking, particularly regarding the impact of phase noise. This is especially important considering that phase noise power increases quadratically with comb line number. In this paper, we develop a theoretical model of a dual frequency comb channelizer and evaluate the signal to noise ratio limits and design challenges when deploying such systems in a high bandwidth signal processing context. We show that the performance of these dual comb based signal processors is limited by the relative phase noise between the two optical frequency combs, which to our knowledge has not been considered in previous literature. Our simulations verify the theoretical model and examine the stochastic noise contributions and harmonic distortion, followed by a broader discussion of the performance limits of dual frequency comb channelizers, which demonstrate the importance of minimizing the relative phase noise between the two frequency combs to achieve high signal-to-noise ratio signal processing.

Sub-100 fs all-fiber broadband electro-optic optical frequency comb at 1.5 µm.

A sub-100 fs all-fiber broadband optical frequency comb seeded from a 12.5 GHz electro-optic modulated pulse is presented. Combining pulse reshaping, nonlinear mixing with dispersion compensation processes, a frequency comb with the main pulse width of 86 fs was achieved. The frequency comb has a 6 dB spectral bandwidth spanning over 150 nm which corresponding to more than 1500 comb tones. The measured average power of the broadband comb is over 550 mW, and the calculated average power of each comb line is roughly -4 dBm. To illustrate the whole spectral broadening process, a numerical investigation was also brought out, showing a very good match with the experiments. With a delayed self-heterodyne interferometer, the evolutions of the seed comb linewidths and the broadened comb linewidths were measured revealing the same parabolic trend. Specifically, the linewidths of the 20 seed comb lines are less than 10 kHz, while the linewidths of the 400 broadened comb lines are less than 1 MHz. The results also indicate that the nonlinear mixing led to an accumulation of the phase noise with respect to the comb line number, indicating that a low phase noise RF source or phase locking technique is essential to produce ultra-low phase noise broadband electro-optic combs.

RF and Microwave Fractional Differentiator Based on Photonics

We report a photonic radio frequency (RF) fractional differentiator based on an integrated Kerr micro-comb source. The micro-comb source has a free spectral range (FSR) of 49 GHz, generating a large number of comb lines that serve as a high-performance multi-wavelength source for the differentiator. By programming and shaping the comb lines according to calculated tap weights, arbitrary fractional orders ranging from 0.15 to 0.90 are achieved over a broad RF operation bandwidth of 15.49 GHz. We experimentally characterize the frequency-domain RF amplitude and phase responses as well as the temporal responses with a Gaussian pulse input. The experimental results show good agreement with theory, confirming the effectiveness of our approach towards high-performance fractional differentiators featuring broad processing bandwidth, high reconfigurability, and potentially greatly reduced size and cost.

Characterization of a multifunctional active demultiplexer for optical frequency combs

We report on a multifunctional active demultiplexer for optical frequency combs. The proposed technique, based on injection locking, combines the functionality of a tunable demultiplexer, gain equalizer and an optical amplifier, all in one device. Validation of the concept is experimentally demonstrated through simultaneous demultiplexing and amplification of combs with various free spectral ranges (2.5 to 15 GHz), achieving an adjacent channel suppression ratio > 35 dB and an output power of 7.5 dBm. The functionality of the demultiplexer is extended to that of a gain equalizer, by filtering comb tones with powers 10 dB below the spectral peak, whilst maintaining a suppression ratio > 30 dB and a constant output power. This feature increases the number of comb lines suited for data modulation (i.e. usable lines), by a factor of ~1.7. Finally, we test the stability of such a demultiplexer, by measuring the frequency drift, beat tone power variations and phase correlation between demultiplexed tones.

Optical injection dynamics of frequency combs

We analyze the nonlinear dynamics of a semiconductor laser with optical injection from a frequency comb. When varying both the injection parameters (detuning and injection ratio) and the comb properties (number of comb lines and comb spacing), we identify and select several dynamics, including (1) injection locking with selective amplification of the comb line that shows the smallest detuning from the injected laser, (2) unlocked time-periodic dynamics that correspond to a new frequency comb with a broadened optical spectrum, thereby sharpening the pulse generated at the injected laser output, and (3) unlocked chaotic dynamics.

Manipulation microwave photonic filter response of single microring by using optical frequency combs

We proposed a reconfigurable microwave photonic filter combining the multiple resonances of single microring resonator and optical frequency combs. By manipulating the number of comb lines, the frequency spacing of the comb lines and the relative power between the comb lines, the MPF can be reconfigured from a notch filter with ultra-high rejection ratio to a flat bandstop filter. The mechanism of the reconfigurable microwave photonic filter was clarified theoretically. And in experiments, a notch filter with an ultra-high RF rejection ratio exceed 60 dB and a flat stopband filter with a 10 dB rejection ratio and a stopband ripple less than 0.53 dB were easily reconfigured by manipulating the optical frequency comb lines.

Microwave and RF Photonic Fractional Hilbert Transformer Based on a 50 GHz Kerr Micro-Comb

We report a photonic microwave and RF fractional Hilbert transformer based on an integrated Kerr micro-comb source. The micro-comb source has a free spectral range (FSR) of 50GHz, generating a large number of comb lines that serve as a high-performance multi-wavelength source for the transformer. By programming and shaping the comb lines according to calculated tap weights, we achieve both arbitrary fractional orders and a broad operation bandwidth. We experimentally characterize the RF amplitude and phase response for different fractional orders and perform system demonstrations of real-time fractional Hilbert transforms. We achieve a phase ripple of < 0.15 rad within the 3-dB pass-band, with bandwidths ranging from 5 to 9 octaves, depending on the order. The experimental results show good agreement with theory, confirming the effectiveness of our approach as a new way to implement high-performance fractional Hilbert transformers with broad processing bandwidth, high reconfigurability, and greatly reduced size and complexity.

Optical frequency comb-based multichannel parallel continuous-variable quantum key distribution.

Continuous-variable quantum key distribution (CVQKD) provides an approach for secure communication in optical fiber communication systems. However, its practical implementation has been hindered by low secret key bit rates that are usually limited to several bits/s to hundreds of kbits/s at distances of more than 25 kilometers. In this paper, we use a pair of optical frequency combs (OFCs) for both multiple parallel transmission and coherent reception, which assign multiple sub-channels involving multiple independent secret keys in a single fiber to increase the key bit rate. The first and last sub-channels are selected for propagating phase references to compensate the phase offset between two free-running combs. We analyze possible excess noise caused by dispersive walk-off in the transmission, imperfect phase compensation in the reception and photon leakage from the phase references. Compared to the previous single-channel CVQKD method, simulation results show more than a factor of 20 increase in the secret key rate at a transmission distance of 35 km and the number of comb lines of 35.

Optical 1.56 Tbps coherent 4-QAM transmission across 60 km SSMF employing OFC scheme

We propose a cost proficient approach to generate Optical Frequency Comb (OFC) lines with Intensity Modulators (IM) in cascaded configuration. The proposed OFC setup utilizes a Directly Modulated Laser (DML) source instead of a conventional approach of using continuous wave laser. A single phase modulator and two Mach Zehnder modulators are used in cascaded connectivity and configured with best parametric values to generate the maximum amount of flat comb lines. The RF source is determined by a sine wave with a signal frequency of 25 GHz where the driving force of the source is 26.9 dBm. The DML is directed by a pseudorandom bit sequence generator and a non-return-to-zero pulse generator having a frequency of 193.1 THz. 78 Comb lines have been achieved successfully having high tone to noise ratio, which is an enormous number of comb lines given the scenario. The proposed scheme is then applied at the OLT side of a coherent 4-Quadrature Amplitude Modulation (QAM) transmission system where 20 Gbps data is transmitted across each carrier frequency for downlink transmission. Overall, 1.56 Tbps data has been successfully transmitted across 25 km, and 60 km SSMF having minimum power penalties, good optical signal to noise ratio with neat and clear constellation diagrams ensuring the transmission.

Exploring the limits of semiconductor-laser-based optical frequency combs.

We report a study on the performance limits of stabilized optical frequency combs from semiconductor mode-locked diode lasers. Operating characteristics such as the number of comb lines, comb tooth linewidth, the physical parameters that affect the independent control of pulse repetition rate and offset frequency, and the potential for self-stabilization, are explored.

Microwave Photonic Channelizer Based on Polarization Multiplexing and Photonic Dual Output Image Reject Mixer

Microwave photonic channelizer based on coherent frequency combs (OFCs) enables processing of ultra-wideband RF signals using low-frequency components. The channel number usually equals to the comb line number of the OFCs and the bandwidth for the microwave input is determined by the frequency spacing of the OFCs. However, the generation of coherent OFCs with large comb lines and large frequency spacing is extremely challenging, limiting greatly the potential of the OFC-based channelizer. In this paper, a microwave photonic channelizer based on polarization multiplexing and photonic dual output image reject mixing is proposed and demonstrated. A broadband signal can be divided into 8 channels using only one optical carrier and a pair of dual-polarization local oscillators (DP-LOs). Each DP-LO has only 2 spectral lines with orthogonal polarization states. In addition, by introducing a pair of OFCs with $N$ comb lines, $8N$ channels can be output. The requirement of the comb line number is efficiently reduced, and the in-band interference is greatly suppressed by introducing the polarization multiplexing and the photonic dual-output image-reject mixing. A proof-of-concept experiment is carried out. An RF signal with 9.6-GHz bandwidth centered at 14 GHz is successfully divided into 8 channels with 1.2-GHz bandwidth, and the in-band interference is effectively suppressed for each channel. In addition, RF signals with 2.4-GHz bandwidth centered at 24.8 and 30.8 GHz are also successfully channelized, respectively.

Generating orthogonally polarized dual frequency combs with slow megahertz repetition rates by a low-nanowatt-level pump

Dual frequency comb generation has been the focus of many recent research efforts owing to its viability in a myriad of applications, ranging from precision measurement to high-speed optical communications. Here, with the state of the art of a single quantum dot (QD) coupled to a pillar microcavity [Nat. Nanotechnol. 12, 663 (2017)], we put forward an alternative setup for the generation of orthogonally polarized dual frequency combs and their corresponding control by using coherent two-tone cavity driving, i.e., a control laser and a probe laser. The present QD-pillar microcavity system operates in the weak-coupling regime, and this makes the experimental realization of the scheme easy. The underlying nonlinear process of the system, thanks to the QD-pillar interaction, for example, degenerate and nondegenerate four-wave mixing, results in efficient frequency comb formation. The dual combs consist of equidistant spectral lines whose spectral spacing coincides with the frequency difference between the control and probe lasers. So, programming this frequency difference between them enables us to tune both the comb-line spacing (i.e., repetition rate) and comb-line number in both vertical ($V$) and horizontal ($H$) polarizations. As compared with the previous approaches for dual microcomb generation, the present scheme has attractive features such as a tunable, slow repetition rate (as slow as a few megahertz), low input power (as low as a few nanowatts), and integrated compact nature in experimentally realistic operating regimes. The achievable low repetition rates are desirable to resolve narrower spectral lines and to enable use of low-power signal processing electronics.

Four-wave mixing cascades seeded by a multiwavelength Brillouin–erbium fiber laser

Four-wave mixing (FWM) cascades which are seeded by a multiwavelength Brillouin–erbium fiber laser (MBEFL) are experimentally demonstrated in this work. The MBEFL, which is based on virtual mirror feedback, is successful in providing the seed for the FWM cascades. The stimulated Brillouin scattering and parametric gain for the development of the MBEFL and FWM cascades, respectively, are provided by a 12 km dispersion-shifted fiber (DSF). By launching the MBEFL into the DSF, as many as 80 comb lines can be generated when the Brillouin pump (BP) wavelength is tuned at 1560 nm. Analysis of FWM cascades as a function of BP wavelength shows that the number of comb lines is greater at the anomalous-dispersion region of the DSF, suggesting that the phase mismatch is minimized in that regime.

Broadband optical frequency comb generator based on driving N-cascaded modulators by Gaussian-shaped waveform

A new approach for realizing a wideband optical frequency comb (OFC) generator based on driving cascaded modulators by a Gaussian-shaped waveform, is proposed and numerically demonstrated. The setup includes N-cascaded MZMs, a single Gaussian-shaped waveform generator, and N-1 electrical time delayer. The first MZM is driven directly by a Gaussian-shaped waveform, while delayed replicas of the Gaussian-shaped waveform drive the other MZMs. An analytical model that describes the proposed OFC generator is provided to study the effect of number and chirp factor of cascaded MZM as well as pulse width on output spectrum. Optical frequency combs at frequency spacing of 1 GHz are generated by applying Gaussian-shaped waveform at pulse widths ranging from 200 to 400 ps. Our results reveal that, the number of comb lines is inversely proportional to the pulse width and directly proportional to both number and chirp factor of cascaded MZMs. At pulse width of 200 ps and chirp factor of 4, 67 frequency lines can be measured at output spectrum of two-cascaded MZMs setup. Whereas, increasing the number of cascaded stages to 3, 4, and 5, the optical spectra counts 89, 109 and 123 frequency lines; respectively. When the delay time is optimized, 61 comb lines can be achieved with power fluctuations of less than 1 dB for five-cascaded MZMs setup.

Ultraflat and broadband optical frequency comb generator based on cascaded two dual-electrode Mach–Zehnder modulators

A novel scheme for the generation of ultraflat and broadband optical frequency comb (OFC) is proposed based on cascaded two dual-electrode Mach–Zehnder modulators (DE-MZM). The first DE-MZM can generate a four-comb-line OFC, then the OFC is injected into the second DE-MZM as a carrier, which can increase the number of comb lines. Our modified scheme finally can generate a broadband OFC with high flatness by simply modifying the electrical power and the bias voltage of the DE-MZM. Theoretical analysis and simulation results reveal that a 16-comb-line OFC with a frequency spacing that two times the frequency of the RF signal can be obtained. The power fluctuation of the OFC lines is 0.48 dB and the unwanted-mode suppression ratio (UMSR) can reach 16.5 dB. Additionally, whether the bias drift of the DE-MZMs has little influence on the power fluctuation is also analyzed. These results demonstrate the robustness of our scheme and verify its good accuracy and high stability with perfect flatness.