Generation Of Optical Pulse Trains Research Articles

Integrated thin-film lithium niobate electro-optic frequency comb for picosecond optical pulse train generation

In recent years, high-performance thin-film lithium niobate (TFLN) electro-optic (EO) modulators boost the fast development of highly integrated, low loss, and large comb spacing EO frequency combs. Furthermore, ultra-short optical pulse trains (USOPTs) can be generated by the temporal domain compression of the optical frequency comb, which play an essential role in photonic sampling analog-to-digital conversion. Here, we demonstrate a flat and broadband EO frequency comb based on a packaged TFLN chip including a monolithic integrated intensity modulator, a phase modulator, and edge couplers. The 25 comb lines with a power fluctuation less than 3 dB are presented successfully. Moreover, we obtain a 10 GHz repetition rate USOPT, the pulse width of which is compressed to 2.67 ps. Our device may find its applications in the fields of ultrafast measurement, wavelength-division-multiplexing optical communication, or high-precision photonic sampling.

Plug-and-play QKD architecture with a self-optical pulse train generator.

The commercialization of quantum key distribution (QKD), which enables secure communication even in the era of quantum computers, has acquired significant interest. In particular, plug-and-play (PnP) QKD has garnered considerable attention owing to its advantage in system stabilization. However, a PnP QKD system has limitations on miniaturization owing to a bulky storage line (SL) of tens of kilometers. And, the secure key rate is relatively low because Bob transmits the signal pulses only at the dedicated time slots to circumvent backscattering noise. This study proposes a new method that can eliminate the SL by realizing an optical pulse train generator based on an optical cavity structure. Our method allows Alice to generate optical pulse trains herself by duplicating Bob's seed pulse and excludes the need for Bob's strong signal pulses that trigger backscattering noise as much as the conventional PnP QKD. Accordingly, our method can naturally overcome the miniaturization limitation and the slow secure key rate, as the storage line is no longer necessary. We conducted a proof-of-concept experiment using our method and achieved a key generation rate of 1.6×10-3 count/pulse and quantum bit error rate ≤ 5%.

Extending thermal stability of short-living soliton states in silicon nitride microring resonators

Dissipative Kerr solitons in microresonators enable on-chip chip generation of low-noise optical pulse trains with high repetition rates, finding applications in optical communication, distance measurement, spectroscopy and radiofrequency generation. However, the most common photonic integrated platforms often show very short living, hence difficult to achieve, soliton states. Here, we exploit an auxiliary resonance to access soliton regime in Si3N4 microresonators by simple wavelength scanning. We increase the likelihood of single soliton formation by more than two times using backward tuning of the laser, and we show that the increased thermal stability allows soliton formation by thermal tuning of the whole sample, keeping the laser at a fixed frequency.

Picosecond pulse generation from continuous-wave light in an integrated nonlinear Bragg grating

Abstract The generation of optical pulse trains from continuous-wave light has attracted growing attention in recent years because it provides a simple way to obtain high repetition rate ultrashort pulses. While pulse generation has been extensively demonstrated in optical fibers, pulse train generation from weak, continuous wave light in photonic chips has posed significant challenges because of the short interaction length and therefore difficulty in acquiring sufficient new frequency content, and/or absence of the appropriate dispersion environment. In this manuscript, we report the pulse train generation of a low continuous-wave signal to 18 ps, by leveraging cross-phase modulation induced by co-propagating pump pulses with a peak power of 3.7 W in an ultra-silicon-rich nitride grating. The pulse train generation dynamics are documented both experimentally and theoretically to arise from cross-phase modulation-induced generation of new spectral content, and dispersive re-phasing. This is a new approach in which picosecond pulse generation may be achieved from low power, continuous-wave light.

On-chip optical pulse train generation through the optomechanical oscillation.

This paper proposes a novel on-chip optical pulse train generator (OPTG) based on optomechanical oscillation (OMO). The OPTG consists of an optical cavity and mechanical resonator, in which OMO periodically modulates the optical cavity field and consequently generates optical pulse trains. The dimensionless method are introduced to simulate the OMO-based OPTG with reduced analysis complexity. We investigate the optomechanical coupling and the dynamic back-action processes, by which we found a dead zone that forbids the OMO, and derived the optimal laser detuning and the minimum threshold power. We analysed the OMO-based OPTG in terms of the pulse shape distortion, extinction ratio (ER) and duty-cycle (DC). Increasing input power, mechanical and optical Q-factors will increase ER, reduce DC and produce sharper and shorter optical pulses. We also discuss the design guidance of OMO-based OPTG and explore its application in distributed fibre optical sensor (DFOS).

Passively mode-locked semiconductor quantum dot on silicon laser with 400 Hz RF line width.

Mode-locked InAs/InGaAs quantum dot lasers emitting optical frequency combs centered at 1310 nm are promising sources for high-speed and high-capacity communication applications. We report on the stable optical pulse train generation by a monolithic passively mode-locked edge-emitting two-section quantum dot laser based on a five-stack InAs/InGaAs dots-in-a-well structure directly grown on an on-axis (001) silicon substrate by solid-source molecular beam epitaxy. Optical pulses as short as 1.7 ps at a pulse repetition rate or inter-mode beat frequency of 9.4 GHz are obtained. A minimum pulse-to-pulse timing jitter of 9 fs, corresponding to a repetition rate line width of 400 Hz, is demonstrated. The generated optical frequency combs yield exceptional low amplitude jitter performance and comb widths exceed 5.5 nm at a -3 dB criteria, containing more than 100 comb carriers.

Low-Loss Optical Nyquist Pulse Train Generation Using Nonauxiliary Wavelength Selective Switch in Communication Band

We report low-loss optical Nyquist pulse train generation using a non-auxiliary wavelength selective switch (WSS). The typical approach for optical Nyquist pulse train generation involves two procedures. The first is conversion from a laser output Gaussian pulse to a Nyquist pulse via spectral filtering with a WSS. The second is multiplexing to generate a Nyquist pulse train with an optical circuit. To generate a high optical signal-to-noise ratio (OSNR) Nyquist pulse train, the first procedure was improved by developing a high-power laser and improving the filtering process by using nonlinear effects in a highly nonlinear fiber. The second procedure also has the potential to further improve the OSNR, because an optical circuit typically causes an optical loss of 9 dB in the case of eight-multiplexing. In this study, we demonstrate optical loss reduction for multiplexing using a nonauxiliary WSS approach without an auxiliary optical circuit. The experimental results show that the optical loss for the Nyquist eight-pulse train is successfully reduced to 2.8 dB.

Highly tunable repetition-rate multiplication of mode-locked lasers using all-fibre harmonic injection locking

Higher repetition-rate optical pulse trains have been desired for various applications such as high-bit-rate optical communication, photonic analogue-to-digital conversion, and multi-photon imaging. Generation of multi GHz and higher repetition-rate optical pulse trains directly from mode-locked oscillators is often challenging. As an alternative, harmonic injection locking can be applied for extra-cavity repetition-rate multiplication (RRM). Here we have investigated the operation conditions and achievable performances of all-fibre, highly tunable harmonic injection locking-based pulse RRM. We show that, with slight tuning of slave laser length, highly tunable RRM is possible from a multiplication factor of 2 to >100. The resulting maximum SMSR is 41 dB when multiplied by a factor of two. We further characterize the noise properties of the multiplied signal in terms of phase noise and relative intensity noise. The resulting absolute rms timing jitter of the multiplied signal is in the range of 20 fs to 60 fs (10 kHz–1MHz) for different multiplication factors. With its high tunability, simple and robust all-fibre implementation, and low excess noise, the demonstrated RRM system may find diverse applications in microwave photonics, optical communications, photonic analogue-to-digital conversion, and clock distribution networks.

Multifunctional Wavelength-Selective Switch for Nyquist Pulse Generation and Multiplexing

We demonstrate optical Nyquist pulse train generation using a wavelength-selective switch (WSS) without an optical multiplexing circuit. The representative approach generating optical Nyquist pulse trains is based on spectral filtering with a WSS and optical time-domain multiplexing (OTDM) with an optical circuit. The use of the optical circuit causes difficulties when extending optical Nyquist pulse train technology to the near-infrared band because this band has no well-established OTDM modules. To resolve this problem, we assign the role of multiplexing to the WSS, in addition to spectral filtering. Furthermore, the WSS-only approach is expected to reduce optical losses and increase pulse-train parameter flexibility. Using only the WSS, we successfully generate optical Nyquist pulse trains in the near-infrared band and realize flexible control of pulse number, width, and interval. In the case of a 10-pulse train, optical loss due to multiplexing is improved by 8.0 dB compared with the conventional method.

Combined Optical and Electrical Spectrum Shaping for High-Baud-Rate Nyquist-WDM Transceivers

We discuss the benefits and limitations of optical time-division multiplexing (OTDM) techniques based on the optical generation of a periodic train of sinc pulses for wavelength-division multiplexing (WDM) transmission at high baud rates. It is shown how the modulated OTDM spectrum bandwidth is related to the optical comb parameters and the pulse shaping of the modulating waveforms in the electrical domain. Such dependence may result in broadening of the modulated spectra, which can degrade the performance of Nyquist-WDM systems due to interchannel crosstalk penalties. However, it is shown and experimentally demonstrated that the same technique of optical pulse train generation can be allied with digital pulse shaping to improve the confinement of the modulated spectrum toward the Nyquist limit independently of the number of OTDM tributaries used. To investigate the benefits of the proposed approach, we demonstrate the first WDM Nyquist-OTDM signal generation based on the periodic train of sinc pulses and electrical spectrum shaping. Straight line transmission of five 112.5-Gbd Nyquist-OTDM dual-polarization quadrature phase-shift keying (QPSK) channels is demonstrated over a dispersion uncompensated link up to 640 km, with full-field coherent detection at the receiver. It is shown that such a design strategy effectively improves the spectral confinement of the modulated OTDM signal, providing a minimum intercarrier crosstalk penalty of 1.5 dB in baud-rate-spaced Nyquist-WDM systems.

Photonic High-Power Continuous Wave THz-Wave Generation by Using Flip-Chip Packaged Uni-Traveling Carrier Photodiodes and a Femtosecond Optical Pulse Generator

The design, analysis, and demonstration of flip-chip bonding packaged uni-traveling-carrier photodiodes (UTC-PDs) with THz (dc to 315 GHz) 3-dB bandwidth and high-power performance are reported. The high-frequency roll-off (up to 0.4 THz) of the flip-chip bonding structure and device heating under high power operation are both minimized through properly downscaling the area of the bonding pad and minimizing the solder distance to the active area of the miniaturized UTC-PD. In order to suppress the serious space-charge screening effect in miniaturized UTC-PDs under high-current density (~180 kA/cm 2 ) operation, an n-type charge layer is inserted into the collector. The detailed dynamic measurement results of these packaged PD modules indicate that non-equilibrium electron transport plays an important role in determining the maximum speed and THz output power. In addition, a femtosecond (fs) optical pulse train generator with a ~300 fs pulse-width output and repetition rate up to ~0.3 THz is also developed to further boost the photo-generated THz-power. Compared with using an optical signal with a sinusoidal envelope for PD excitation, the short-pulse approach can offer a 3-dB enhancement in output power under the same output photocurrent and operating frequency (around 0.3 THz). By utilizing such an fs light source and our PD module, a continuous wave output power as high as 1 mW at an operating frequency of ~0.3 THz is successfully demonstrated.

All-fibre photonic signal generator for attosecond timing and ultralow-noise microwave.

High-impact frequency comb applications that are critically dependent on precise pulse timing (i.e., repetition rate) have recently emerged and include the synchronization of X-ray free-electron lasers, photonic analogue-to-digital conversion and photonic radar systems. These applications have used attosecond-level timing jitter of free-running mode-locked lasers on a fast time scale within ~100 μs. Maintaining attosecond-level absolute jitter over a significantly longer time scale can dramatically improve many high-precision comb applications. To date, ultrahigh quality-factor (Q) optical resonators have been used to achieve the highest-level repetition-rate stabilization of mode-locked lasers. However, ultrahigh-Q optical-resonator-based methods are often fragile, alignment sensitive and complex, which limits their widespread use. Here we demonstrate a fibre-delay line-based repetition-rate stabilization method that enables the all-fibre photonic generation of optical pulse trains with 980-as (20-fs) absolute r.m.s. timing jitter accumulated over 0.01 s (1 s). This simple approach is based on standard off-the-shelf fibre components and can therefore be readily used in various comb applications that require ultra-stable microwave frequency and attosecond optical timing.

Gate-controlled generation of optical pulse trains using individual carbon nanotubes.

In single-walled carbon nanotubes, electron–hole pairs form tightly bound excitons because of limited screening. These excitons display a variety of interactions and processes that could be exploited for applications in nanoscale photonics and optoelectronics. Here we report on optical pulse-train generation from individual air-suspended carbon nanotubes under an application of square-wave gate voltages. Electrostatically induced carrier accumulation quenches photoluminescence, while a voltage sign reversal purges those carriers, resetting the nanotubes to become luminescent temporarily. Frequency-domain measurements reveal photoluminescence recovery with characteristic frequencies that increase with excitation laser power, showing that photoexcited carriers provide a self-limiting mechanism for pulsed emission. Time-resolved measurements directly confirm the presence of an optical pulse train synchronized to the gate voltage signal, and flexible control over pulse timing and duration is also demonstrated. These results identify an unconventional route for optical pulse generation and electrical-to-optical signal conversion, opening up new prospects for controlling light at the nanoscale.

Periodic and aperiodic pulse generation using optically injected DFB laser

Generation of optical pulse trains using semiconductor lasers has attracted significant attention due to their applications in optical communications and signal processing. A pulse generator based on the nonlinear dynamics of an optically injected distributed feedback laser is presented. The proposed system does not require electrical excitation and can be configured to operate in a periodic or aperiodic pulsation regime by controlling the injection conditions. Experiments show periodic pulsation with a repetition rate of 1.034 GHz and aperiodic pulsation with an average repetition rate of 5.618 GHz.

Simulation analysis of an improved optical triangular-shaped pulse train generator based on quadrupling RF modulation incorporating fiber dispersion-induced power fading

We report an improved approach to generate optical triangular-shaped pulse train using quadrupling RF modulation and fiber dispersion-induced power fading. In the proposal, quadrupling RF modulation (via a dual-parallel Mach–Zehnder modulator) is employed to generate four primary optical sidebands (±2nd and ±6th) in spectrum. Then a piece of single mode fiber is connected as the dispersive media. Because of the power degradation induced by fiber dispersion, the undesired 8th order harmonic in optical intensity can be removed. It is found that when the modulation index is adjusted to a proper value (m=4.438), optical intensity with its expression corresponding to the Fourier expansion of idea triangular-shaped waveform can be found. Since the quadrupling RF modulation technique is employed, repetition rate of the target pulse train is four times of the driving frequency, which makes pulse train generation with higher repetition rate or smaller pulse duration possible.

High-quality optical pulse train generator based on solitons on finite background

We report a simple method to exploit the typical properties of solitons on a finite background in order to generate high-repetition-rate and high-quality optical pulse trains. We take advantage of the nonlinear evolution of a modulated continuous wave toward localized structures upon a nonzero background wave in anomalous dispersive fiber. After a stage of nonlinear compression, a delay-line interferometer enables the annihilation of the finite background and simultaneously allows the repetition-rate doubling of the pulse train.

Generation of WDM adaptive-rate pulse bursts by cascading narrow/wideband tunable optical dispersion compensators

We demonstrate passive generation of optical pulse trains with each pulse having distinct center carrier and spectra using tunable group delay (GD) staircase transfer functions. The GD steps result from opposite and equal magnitude GD slopes from narrowband and wideband tunable optical dispersion compensators. We use this technique to split the spectrum of a femtosecond pulse to a pulse burst with precise control of pulse time separation.

Time- and wavelength-interleaved optical pulse train generation based on dispersion spreading and sectional compression

A method to generate time- and wavelength-interleaved optical pulse trains based on dispersion spreading and sectional compression is proposed and demonstrated. A 4×2 GHz time- and wavelength-interleaved pulse train is generated from an input 2 GHz mode-locked pulse train. The advantages of the proposed scheme are its simplicity and robustness, since no microwave component or multiwavelength laser source is required. In addition, we demonstrate supercontinuum generation of an ultraflat 18 nm bandwidth spectrum with less than 0.5 dB fluctuation over the 3.2 nm central bandwidth.

Experimental demonstration of microring-based optical pulse train generator

Recently, we proposed the concept of a microring-based optical pulse train generator, which uses a series of microrings coupled to a waveguide and time-interleaves a trigger pulse propagating on this waveguide to generate an optical pulse train. When each ring is electrically modulated, this electronic-photonic integrated circuit (EPIC) can be used for optical arbitrary waveform generation (OAWG) and ultrafast electro-optic (EO) modulation. This paper presents the proof-of-concept experimental demonstration of this technique with two four-stage chip prototypes fabricated on silicon-on-insulator (SOI) substrate. Device fabrication and testing of the chip prototypes are presented. The measurement results show that from a 20-ps-wide input trigger pulse, two circuits generate four sequential pulses at the output with a constant stage delay of 55 ps and 30 ps respectively, which translate into pulse repetition rates of 18 GHz and 33 GHz.

High-speed optical pulse train generation based on line-by-line spec-tral intensity coding

We have built a simple line-by-line spectral intensity coder with 4 GHz resolution at 1.55 µm using a grating, a collimating lens, and an intensity mask. By applying a 10 GHz mode-locked spectral comb to the coder, we have generated relatively stable 20 - 100 GHz pulse trains with 10 GHz steps. The characteristics of generated pulse trains are examined quantitatively by measuring the corre- sponding optical spectra, time-domain waveforms, autocorrelation traces, and ra- dio-frequency spectra.