Train Of Ultrashort Optical Pulses 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.

The photodetection of ultrashort optical pulse trains for low noise microwave signal generation.

Electrical signals derived from optical sources have achieved record-low levels of phase noise, and have demonstrated the highest frequency stability yet achieved in the microwave domain. Attaining such ultrastable phase and frequency performance requires high-fidelity optical-to-electrical conversion, typically performed via a high-speed photodiode. This paper reviews characteristics of the direct photodetection of optical pulses for the intent of generating high power, low phase noise microwave signals from optical sources. The two most popular types of photodiode detectors used for low noise microwave generation are discussed in terms of electrical pulse characteristics, achievable microwave power, and photodetector nonlinearities. Noise sources inherent to photodetection, such as shot noise, flicker noise, and photocarrier scattering are reviewed, and their impact on microwave phase fidelity is discussed. General guidelines for attaining the lowest noise possible from photodetection that balances power saturation, optical amplification, and amplitude-to-phase conversion, are also presented.

Design and performance of a repetition rate controllable and wavelength tunable L + U-band actively mode-locked erbium fiber laser

Mode-locked lasers draw great interest in the laser research field due to their diverse applications and easy achievement of ultra-short and high-intensity optical pulse trains. Actively mode-locked (AML) fiber lasers are more flexible compared to passively mode-locked fiber lasers owing to their ability to control repetition rates and pulse intervals electronically. The design and development of wavelength tunable AML fiber lasers operating beyond the C-band have attracted great attention from the research community. In this paper, a repetition rate controllable and wavelength tunable L+U-band AML erbium fiber laser (AML-EFL) based on a single standard 1480 nm pump laser and Mach–Zehnder modulator (MZM) as an intra-cavity intensity modulator is demonstrated using numerical simulation. The AML-EFL is created using a MZM driven by electrical Gaussian pulses and a tunable optical bandpass filter that is used to tune the laser cavity at any wavelength in the 1565–1645 nm range. The repetition rate of the AML-EFL is controlled from 500 kHz to 1 GHz. Trains of pulses with pulse widths of 103 ns and 50 ps and pulse intervals of 2 µs and 1 ns are successfully generated at repetition rates of 500 kHz and 1 GHz, respectively. A pulse energy of 626 nJ is obtained for a 500 kHz repetition rate. Signal-to-noise ratios of 30 and 32 dB are observed for trains of mode-locked pulses with repetition rates of 500 kHz and 1 GHz, respectively. In addition, slope efficiencies of around 66% for an L-band wavelength of 1582.1 nm and 30% for a U-band wavelength of 1633.6 nm are obtained considering the optimized parameters.

Image-Rejected Multi-Band Frequency Down-Conversion Based on Photonic Sampling

An image-rejected multi-band frequency down-conversion scheme is proposed and experimentally demonstrated based on photonic sampling. The multi-band radio-frequency (RF) signals to be processed are copied into two replicas in quadrature, which are then sampled by an ultra-short optical pulse train via a polarization-multiplexed modulator. After polarization demultiplexing and detection using a pair of low-speed photodetectors, the multi-band RF signals are simultaneously down-converted to the intermediate frequency (IF) band. The image components can be suppressed by quadrature coupling the two generated IF signals via an electrical 90° hybrid coupler (HC). In the experiment, multi-band RF signals in the frequency range of 6 GHz to 39 GHz are down-converted to the IF band below 4 GHz using a local oscillator (LO) signal at 8 GHz to generate the ultra-short optical pulse train. Image rejection is achieved in the digital domain using digital signal processing to compensate for the amplitude and phase mismatch between the two IF signals and to implement quadrature coupling. In addition, through using an electrical phase shifter, an electrical attenuator, and an electrical 90° HC to achieve quadrature coupling of the two IF signals, image-rejected multi-band frequency down-conversion is also verified in the analog domain.

Multi-band microwave signals generation based on a photonic sampling with a flexible ultra-short pulse source.

We propose and experimentally demonstrate a photonic sampling-enabled multi-band microwave signals generation scheme. The intermediate-frequency input signal is up-converted to high frequency output signals in multiple frequency bands, after being optically sampled by an ultra-short optical pulse train and subsequently being detected by a photodetector (PD). The ultra-short pulse source, realized by using an electro-optic modulation-based time lens with the chirp compensation, can be flexibly adjusted to enhance the performance of multi-band frequency up-conversion. In the experiment, after optimizing the optical ultra-short pulse source with a repetition frequency of 8 GHz, ten phase-coded up-converted microwave signals with a 13-bit Barker code were successfully generated within a frequency range from 7 GHz to 41 GHz. The power fluctuation among the generated microwave signals in multi-band is measured to be 5.9 dB. The experiment results indicate that, our proposed scheme can realize the multi-band up-conversion with a high fidelity.

Photonic sampling analog-to-digital conversion based on time and wavelength interleaved ultra-short optical pulse train generated by using monolithic integrated LNOI intensity and phase modulator.

High-speed analog-to-digital conversion (ADC) is experimentally demonstrated by employing a time and wavelength interleaved ultra-short optical pulse train to achieve photonic sampling and using wavelength division demultiplexing to realize speed matching between the fast optical front-end and the slow electronic back-end. The sampling optical pulse train is generated from a cavity-less ultra-short optical pulse source involving a packaged device that monolithically integrates an intensity modulator and a phase modulator into a chip based on lithium niobate on insulator (LNOI). In the experiment, the fiber-to-fiber insertion loss of the packaged modulation device is measured to be 6.9 dB. In addition, the half-wave voltages of the Mach-Zehnder modulator and the phase modulator in the LNOI-based modulation device are measured to be 3.6 V and 3.4 V at 5 GHz, respectively. These parameters and the device size are superior to those based on cascaded commercial devices. Through using the packaged modulation device, two ultra-short optical pulse trains centered at 1541.40 nm and 1555.64 nm are generated with time jitters of 19.2 fs and 18.9 fs in the integral offset frequency range of 1 kHz to 10 MHz, respectively, and are perfectly time interleaved into a single pulse train with a repetition rate of 10 GHz and a time jitter of 19.8 fs. Based on the time and wavelength interleaved ultra-short optical pulse train, direct digitization of microwave signals within the frequency range of 1 GHz to 40 GHz is demonstrated by using a two-channel wavelength demultiplexing photonic ADC architecture, where the effective number of bits are 5.85 bits and 3.75 bits for the input signal at 1.1 GHz and 36.3 GHz, respectively.

Frequency Response Enhancement of Photonic Sampling Based on Cavity-Less Ultra-Short Optical Pulse Source

The influence of the residual pulse chirp and the spectrum shape from a cavity-less optical pulse source on the sampling performance is theoretically analyzed. According to the theoretical analysis, an optimization method for enlarging the operation bandwidth of a photonic sampling system based on a cavity-less ultra-short pulse source is proposed and experimentally demonstrated. Through biasing the Mach-Zehnder modulator in the cavity-less optical pulse source below its quadrature point, the output spectrum flatness can be greatly improved, which is beneficial for optimizing the photonic sampling frequency response introduced by the ultrashort optical pulse train. In the experiment, the 3-dB bandwidth of the photonic sampling frequency response is increased from less than 10 GHz to more than 35 GHz by using the proposed method without sacrificing the output signal-to-noise ratio.

Frequency response measurement of high-speed photodiodes based on a photonic sampling of an envelope-modulated microwave subcarrier.

An approach to measuring the frequency response of high-speed photodiodes (PDs) is proposed and experimentally demonstrated based on employing an ultrashort optical pulse train to sample an envelope-modulated microwave subcarrier. Through up-and-down conversion sampling, a varying frequency component and a fixed low frequency component can be obtained, where the varying frequency component probes the ultra-wideband response information of PD. Through measuring the relative amplitude between the two frequency components, the frequency response of the PD at the any frequency within ultra-wideband frequency range can be calculated by taking the response at the fixed low frequency component as a reference. Thereinto, the frequency response of the electro-optic modulator is cancelled out, and the uneven comb intensity introduced by the ultrashort optical pulse train can be corrected by choosing the specific frequency of the microwave subcarrier. In the proof-of-concept experiment, the self-calibrated frequency response measurement of a commercial PD is demonstrated by employing an optical pulse train with a repetition rate of 9.954 GHz and an electro-optic frequency sweeping up to 4.977 GHz. The frequency measurement range is achieved up to 49.77 GHz, and the frequency resolution reaches 300 kHz in the rough measurement and 10 Hz level in the fine measurement. The consistency between the proposed method and conventional methods proves the ultra-wideband and hyperfine frequency response measurement of PDs.

Broadband high-resolution microwave frequency measurement based on photonic undersampling via using three cavity-less optical pulse sources with coprime repetition rates.

A photonic-assisted broadband and high-resolution microwave frequency measurement scheme is proposed and demonstrated based on undersampling via using three cavity-less optical pulse sources with coprime repetition rates. After undersampling by three ultrashort pulse trains with repetition rates in the order of gigahertz, input microwave signal is down-converted to three intermediate-frequency (IF) signals located in the first Nyquist frequency range. Through measuring the frequencies of the IF signals via fast Fourier transform after digitization by the commercially available analog-to-digital convertors, the input microwave signal frequency can be retrieved based on the frequency identification algorithm. In the proof-of-concept experiment, three ultrashort pulse trains with repetition rates of 2.99, 3.07, and 3.10 GHz are generated by a cavity-less optical pulse source, where the pulse widths are 9.5, 9.6, and 9.8 ps, respectively. Through using these three ultrashort optical pulse trains, a frequency measurement range up to 40 GHz is realized, where the frequency measurement error is less than ±5kHz, and the spurious-free dynamic range is 91.25dBcHz2/3.

A Molecularly Modulated Mode-Locked Laser

A mode-locked laser operating at a frequency over 10 THz is reported, which is three orders of magnitude greater than a standard mode-locked laser. The system used molecules with a Raman gain as an amplifier, while coherent molecular motions were used for optical modulation. Molecules in a high-finesse optical cavity modulated a continuous-wave beam to produce a train of ultrashort optical pulses at a repetition rate corresponding to the frequency of molecular motion. Phase-locking was achieved by an appropriate compensation of the total dispersion of the optical cavity. Thus, the oscillating multiple longitudinal modes were all coupled under phase-matching conditions of parametric four-wave mixing.

Cross-correlation frequency-resolved optical gating for characterization of an ultrashort optical pulse train

A technique involving cross-correlation frequency-resolved optical gating is applied in characterizing an optical pulse train consisting of several spectral components. An optical beat was used as a reference pulse for measuring the relative spectral phase among the spectral components in the test pulse generated by four-wave Raman mixing in hydrogen gas. It was confirmed that a change in the relative phase can be measured by monitoring the shift in the interference fringe and that Raman emissions generated by four-wave Raman mixing are phase locked.

Investigations of the Talbot effects for a grating illuminated by the ultrashort optical pulses

Based on the theories of the Fresnel diffraction and the signal processing, the Talbot effects for a grating illuminated by a train of ultrashort optical pulses are investigated numerically. Influences of the input pulse parameters and the grating structure on the Talbot images are discussed detailedly. Results demonstrate that the contrast of the high/low field boundary becomes obscure for the optical pulse with shorter temporal duration and chirp. And a grating with smaller duty cycle (DC) is easier to the formation of constructive interference field. In addition, although the high/low field distribution resembles as the image of grating period at the integer Talbot detecting planes, it repeats k-times at the fractional Talbot detecting plane, which can be exploited for obtaining higher pulse repetition rate multiplication.

Kuznetsov–Ma soliton and Akhmediev breather of higher-order nonlinear Schrödinger equation**Project supported by the Key Project of Scientific and Technological Research in Hebei Province, China (Grant No. ZD2015133).

In terms of Darboux transformation, we have exactly solved the higher-order nonlinear Schrödinger equation that describes the propagation of ultrashort optical pulses in optical fibers. We discuss the modulation instability (MI) process in detail and find that the higher-order term has no effect on the MI condition. Under different conditions, we obtain Kuznetsov–Ma soliton and Akhmediev breather solutions of higher-order nonlinear Schrödinger equation. The former describes the propagation of a bright pulse on a continuous wave background in the presence of higher-order effects and the soliton’s peak position is shifted owing to the presence of a nonvanishing background, while the latter implies the modulation instability process that can be used in practice to produce a train of ultrashort optical soliton pulses.

Attosecond timing in optical-to-electrical conversion

The most frequency-stable sources of electromagnetic radiation are produced optically, and optical frequency combs provide the means for high-fidelity frequency transfer across hundreds of terahertz and into the microwave domain. A critical step in this photonic-based synthesis of microwave signals is the optical-to-electrical conversion process. Here, we show that attosecond (as) timing stability can be preserved across the opto-electronic interface of a photodiode, despite an intrinsic temporal response that is more than six orders of magnitude slower. The excess timing noise in the photodetection of a periodic train of ultrashort optical pulses behaves as flicker noise (1/f) with amplitude of 4 as/√Hz at 1 Hz offset. The corresponding fractional frequency fluctuations are 1.4×10−17 at 1 s and 5.5×10−20 at 1000 s. These results demonstrate that direct photodetection, as part of frequency-comb-based microwave synthesis, can support the timing performance of the best optical frequency standards, and thereby opens the possibility for generating microwave signals with significantly better stability than any existing source.

Rapid-scan acousto-optical delay line with 34 kHz scan rate and 15 as precision

An optical fast scan delay exploiting the near-collinear interaction between a train of ultrashort optical pulses and an acoustic wave propagating in a birefringent crystal is introduced. In combination with a femtosecond Er:fiber laser, the scheme is shown to delay few femtosecond pulses by up to 6 ps with a precision of 15 as. A resolution of 5 fs is obtained for a single sweep at a repetition rate of 34 kHz. This value can be improved to 39 as for multiple scans at a total rate of 0.3 kHz.

Analysis of shot noise in the detection of ultrashort optical pulse trains

We present a frequency domain model of shot noise in the photodetection of ultrashort optical pulse trains using a time-varying analysis. Shot-noise-limited photocurrent power spectral densities, signal-to-noise expressions, and shot-noise spectral correlations are derived that explicitly include the finite response of the photodetector. It is shown that the strength of the spectral correlations in the shot noise depends on the optical pulsewidth, and that these correlations can create orders-of-magnitude imbalance between the shot-noise-limited amplitude and phase noise of photonically generated microwave carriers. It is also shown that only by accounting for spectral correlations can shot noise be equated with the fundamental quantum limit in the detection of optical pulse-to-pulse timing jitter.

Exploiting shot noise correlations in the photodetection of ultrashort optical pulse trains

We present a frequency domain model of shot noise in the photodetection of ultrashort optical pulse trains using a time-varying analysis. Shot noise-limited photocurrent power spectral densities, signal-to-noise expressions, and shot noise spectral correlations are derived that explicitly include the finite response of the photodetector. It is shown that the strength of the spectral correlations in the shot noise depends on the optical pulse width, and that these correlations can create orders-of-magnitude imbalance between the shot noise-limited amplitude and phase noise of photonically generated microwave carriers. It is also shown that only by accounting for spectral correlations can shot noise be equated with the fundamental quantum limit in the detection of optical pulse-to-pulse timing jitter.

Ultrashort optical pulse phase stabilization using cross-polarization modulation method

A simple method for phase stabilization of ultrashort optical pulse train is presented. By using cross-polarization effect in high nonlinear photonic crystal fiber, optical pulses from a mode-locked fiber laser are wavelength converted with stable phase character. A differential phase shift keying modulation system is successfully built to test the performance of this method.

Control of quantum pathways for the generation of continuous-wave Raman sidebands

The generation of a multifrequency continuous-wave laser through stimulated Raman scattering and phase-matched four-wave mixing in a medium-filled optical cavity is demonstrated. Three different quantum pathways for the four-wave mixing, two of them degenerate and one of them nondegenerate, can be excited independently by tuning the intracavity dispersion. The results suggest that phase-matched Raman sidebands were generated on the longer wavelength side as well as on the shorter wavelength side, which can be used for the Fourier synthesis of a train of ultrashort optical pulses.

Train of high-power femtosecond pulses: Probe wave in a gas of prepared atoms

We present a new method for generating a regular train of ultrashort optical pulses in a prepared two-level medium. The train develops from incident monochromatic probe radiation travelling in a medium of atoms, which are in a quantum mechanical superposition of dressed internal states. In the frame of used linear theory for the probe radiation, the energy of individual pulses is an exponentially growing function of atom density and of interaction cross section. Pulse repetition rate is determined by the generalized Rabi frequency and can be around 1 THz and greater. We also show that the terms, extra to the dipole approximation, endow the gas by a new property: non-saturating dependence of refractive index on the dressing monochromatic field intensity. Contribution of these nonsaturating terms can be compatible with the main dipole approximation in the wavelength region of about ten micrometers (the range of CO_2 laser) or larger.