Repetition Rate Optical Pulse Research Articles

Pulsed microwave photonic vector network analyzer based on direct sampling.

Pulsed VNAs enabling large dynamic measurement at a low pulse duty cycle are necessary for the characterization of active devices due to the overheating damage risk under continuous wave stimulation. In this paper, we proposed a pulsed microwave photonic vector network analyzer (p-MPVNA) based on direct sampling. With the broad system bandwidth of the p-MPVNA and undersampling technique, pulsed signals are received thru asynchronous wideband detection, and pulsed S-parameters are calculated thru vector superposition. In asynchronous wideband detection, the continuous spectrum of the pulsed signal is discretized into multiple frequency components. A low repetition rate optical pulse train undersamples the pulsed signal, and the discretized frequency components are aliased. The frequency components in the main lobe including most energy of the pulse signal are vector superimposed to calculate the pulsed S-parameter. The proposed p-MPVNA has a dynamic range decreases rate of 10log(duty cycle) when pulse duty cycle is below 10% , which is much slower than that of 20log(duty cycle) for classical narrowband detection. An experimental p-MPVNA is established for validation. A 6 to 18 GHz microwave amplifier is measured with continuous and pulsed power supply and the measured gain curves are consistent with the results from a commercial VNA. The system dynamic range decrease with pulse duty cycle is verified by the pulsed S-parameter measurement of a 10 GHz low pass filter under continuous and pulsed stimulation.

All-optical repetition rate stabilization methods for ultrafast Yb-based SESAM mode-locked fiber oscillator

We demonstrate optical to radio frequency (RF) locking technique using all-fiber phase shifter. Phase shift at 1030 nm was achieved by pumping Er-doped fiber incorporated in the resonator. Pump induced refractive index change was fast enough to stabilize optical pulse repetition rate of ultrafast Yb-doped fiber oscillator and achieve sub-200 fs timing jitter. We also investigate the source of phase noise in soliton fiber oscillators and provide guidelines how to further reduce timing jitter.

Phase Noise of Optical Pulse Trains Generated by Talbot Effect in Frequency Shifting Loops

High repetition rate optical pulse trains are essential for a wide range of applications, including photonic-assisted sampling. Among the different solutions proposed so far, Talbot lasers, based on the generation of an optical frequency comb in a frequency shifting loop, are a simple yet efficient source of short transform-limited pulses. By controlling the comb spectral phase, the repetition rate is reconfigurable. It can be set to a (possibly large) multiple of the comb spacing and reach the GHz range, while only requiring a low bandwidth synthesizer (typ. 100 MHz). The noise characteristics of this pulse trains is a key factor for its application. Therefore, we investigate the phase noise properties of Talbot lasers. In particular, we prove theoretically and demonstrate experimentally that the phase noise at high offset frequency from the carrier, does not depend on the multiplication factor $q$ . We also demonstrate that the phase noise at low offset frequencies, which increases as $20 \log q$ , can be strongly reduced by a simple locking technique. Finally, we show that in addition to their reconfigurability, Talbot lasers can provide timing jitters at 8.2 GHz as low as 350 fs (integrated from $10^{5}$ Hz to the Nyquist frequency), making them suitable candidates for time metrology applications.

Programmable optical pulse repetition rate multiplication via spectral phase manipulation.

Flexible phase patterns for optical pulse repetition rate multiplication (PRRM) are proposed and experimentally demonstrated via spectral phase-only manipulation. We introduce formulas of the phase condition for power lossless PPRM with arbitrary multiplication factors and undistorted temporal pulse profiles. For some multiplication factors the solution extends PRRM phase patterns from reported phase conditions to more flexible phase patterns, inspiring potentials of further devices available for PRRM. This flexibility also benefits PRRM when we use the reported devices. As a proof of concept, we numerically and experimentally demonstrate PRRM with multiplication factors up to eight by programming the spectral phase using an optical wave-shaper (OWS), involving different phase patterns. In practice, manipulation of the spectral phase induces spectral amplitude variations due to the intrinsic property limitation of the OWS. We quantitatively characterize this limitation and select a suitable phase pattern from our new solution to achieve a uniform temporal pulse train but with no spectral amplitude trimming.

Compact low-noise passively mode-locked Er-doped femtosecond all-fiber laser with 2.68 GHz fundamental repetition rate.

A passively mode-locked Er-doped fiber laser with a fundamental repetition rate of 2.68GHz is reported. The oscillator operating at a central wavelength of 1558.35nm has a compact, robust structure and low-noise performance. The timing jitter integrated from 30MHz down to 300Hz is 82.5fs, and the timing jitter performance is analyzed based on the theory model. The amplification and compression of the high repetition rate optical pulses are also investigated. After a three-stage amplifier, the average power is boosted to 430mW. Meanwhile, based on the nonlinear self-phase modulation effect, the spectral bandwidth is broadened from 7.56 to 19.2nm, and the corresponding pulse width is compressed to 244fs.

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.

Ultrashort optical pulse repetition rate multiplication based on two-core-fiber Mach–Zehnder interferometer

In this paper, we propose a much compact ultrashort optical pulse repetition rate multiplication approach based on two-core-fiber Mach–Zehnder interferometer. Since the temporal period is approximately inverse-proportional to its spectral spacing, the repetition rate of output pulses can be doubled (Multiplication factor N = 2) by selecting the proper modes and suppressing the undesired spectral lines. Influences of the input frequency deviation and the input/output coupling coefficients (Cin, Cout) on the quality of multiplied pulses are investigated. Results demonstrate that the multiplication scheme works well for the input frequency deviation of 3% and Cin (Cout) ranging from 0.4 to 0.6.

Phosphorus-free mode-locked semiconductor laser with emission wavelength 1550 nm

We have fabricated passive mode-locked laser diodes based on strained InGaAlAs/InGaAs/InP heterostructures with crystal lattice mismatch parameter of +1.0 % between quantum well and barrier. The laser with temperature stabilization at 18 °C was demonstrated 10.027 GHz optical pulse repetition rate with 6 ps pulse duration time. Timing jitter of optical pulses in mode-locked regime was 0.145 ps.

Integrated Photonic Chip Enabled Simultaneous Multichannel Wideband Radio Frequency Spectrum Analyzer

Radio frequency (RF) spectrum analysis is an essential technique for monitoring signal quality in ultrahigh-speed optical transmission, broad bandwidth photonic RF arbitrary waveform generation, and for characterization of high repetition rate optical pulse sources. Whereas the intrinsic bandwidth of electronics limits its use for RF spectrum measurement of high-speed signals, a photonic approach based on ultrafast Kerr nonlinear effects can readily measure the RF spectrum of optical signals having a bandwidth beyond 1 THz. The majority of fiber-based or integrated photonic RF spectrum analyzers reported to date can monitor only one channel at a time. We demonstrate how to harness mode-selective excitation of nonlinear optical effects to perform simultaneous multichannel RF spectrum analysis using a single integrated silicon photonic device. This approach, which can be scaled to a higher number of channels, opens up a new degree of freedom for realizing multichannel signal characterization using a single integrated photonic device.

Nonuniform optical sampling signal reconstruction algorithm

optical pulse generated by mode-locked lasers has lower time jitter than electronics, which is the main advantage of optical sampling. However, by using multiplexer to further increase the repetition rate of optical pulse will cause nonuniformity in amplitude and time delay. Here we discuss the effect brought by this kind of static deviation, and propose the reconstruction algorithm in Nth-order nonuniform optical sampling. Experiment results show that it can effectively build the original signal.

Role of the optical pulse repetition rate in the efficiency of terahertz emitters

Excitation of n-GaAs and p-InAs terahertz emitters by the series of optical pulses is studied by ensemble Monte Carlo simulations. It is found that the spatial separation of photoexcited electrons and holes dramatically reduces the recombination intensity in n-GaAs emitter, the operation of which is based on the surface field effect. The spatial separation of carriers does not affect the recombination intensity in p-InAs emitter, the operation of which is based on the photo-Dember effect. Therefore, the recovery time of equilibrium state after optical pulse in n-GaAs emitter significantly exceeds the corresponding recovery time in p-InAs emitter. This fact leads to a substantial reduction of photocurrent amplitude in n-GaAs emitter excited by the optical pulse series at high repetition rate.

Theoretical model and spectrum characteristics of superimposed photonic crystal fiber grating

The superimposed gratings have attracted considerable interest because they can extend the potential applications of gratings. Superimposed gratings are fabricated by inscribing multiple gratings at the same section of the fiber, and they can demonstrate various features simultaneously. A number of optical devices based on superimposed gratings have been reported, such as multi-wavelength filters, beam shapers, ultrahigh repetition rate optical pulse generators, etc. Photonic crystal fiber (PCF) can bring new optical characteristics by changing the sizes, spacings and arrangements of the air holes in the fiber. In this paper, we present the spectra of the superimposed gratings inscribed in a photonic crystal fiber. A numerical mode is proposed based on the V-I transmission matrices. The traditional cosinoidal variation of refractive index is replaced with a square-type refractive index variation, and the scattering occurs at a localized discrete location. According to the simulations, the reflection spectra and time delays of a superimposed Bragg grating and superimposed chirped Bragg grating are analyzed. A superimposed Bragg grating and a superimposed chirped Bragg grating are fabricated in the single mode photosensitive PCFs under the irradiation of a 193 nm ultraviolet laser. The superimposed Bragg grating is composed of four subgratings with resonance wavelengths at set spacings. And under a phase mask displacement of 1.03 mm, the superimposed chirped Bragg grating has a periodic resonance with a period of 0.82 nm. The results show that the spectrum of superimposed Bragg grating can be flexibly customized by the parameters of each subgrating. Superimposed chirped Bragg gratings have good linear group delays and flat periodic resonance amplitudes, and the resonance period can be adjusted by displacing the phase mask. The grating spectra obtained from experiments are in good agreement with the theoretical analyses. The research results in this paper provide an important basis for designing, fabricating, and applying the superimposed PCF gratings.

Optical pulse repetition rate multiplication and shaping using double-Mach–Zehnder-based ring resonator

In this paper, we propose a double-Mach–Zehnder (MZ)-based ring resonator for optical pulse repetition rate multiplication and shaping. In one hand, by adjusting the ring resonator circumference, optical pulse trains with different repetition factors 2× or 3× are obtained, and all the optical pulses are uniform amplitude and identical shaping. On the other hand, by optimizing phase parameters of the two MZ interferometers to change impulse response amplitude or phase of the ring resonator, the output pulse trains with step envelope or triangular envelope can be achieved when the input pulse width is sufficient narrow. In addition, if the input pulse width is broad enough to interfere with each other, every individual pulse in the optical pulse trains can be shaped into triangular waveform flexibly.

Gigahertz repetition rate, sub-femtosecond timing jitter optical pulse train directly generated from a mode-locked Yb:KYW laser

We show that a 1.13 GHz repetition rate optical pulse train with 0.70 fs high-frequency timing jitter (integration bandwidth of 17.5 kHz-10 MHz, where the measurement instrument-limited noise floor contributes 0.41 fs in 10 MHz bandwidth) can be directly generated from a free-running, single-mode diode-pumped Yb:KYW laser mode-locked by single-wall carbon nanotube-coated mirrors. To our knowledge, this is the lowest-timing-jitter optical pulse train with gigahertz repetition rate ever measured. If this pulse train is used for direct sampling of 565 MHz signals (Nyquist frequency of the pulse train), the jitter level demonstrated would correspond to the projected effective-number-of-bit of 17.8, which is much higher than the thermal noise limit of 50 Ω load resistance (~14 bits).

Suppercontinuum Generation in InP Nano Inner Cladding Fibers

The InP nano inner cladding fiber is fabricated by the method of modified chemical vapor deposition. The thickness of the InP nano thin film is about 16 nm. A 20 dB supercontinuum over 100 nm in the telecommunication band is generated by infecting 1550 nm, 120 fs, 50 MHz repetition rate optical pulse into this kind of fiber. Moreover, the results indicate that the width of the supercontinuum spectra can be influenced by the fiber length and pump power. The longer fiber at higher pump power can produce broader supercontinuum spectrum.

Half-Brewster-prism pair for broadband mode-locked optical pulse compression

A new design, based on a prism pair, is proposed for broadband optical pulse compression. Our results show that it is possible to achieve shorter pulse duration, to have better stabilities of both the CEP and the pulse duration suffering from the fluctuation of the laser beam size and less variation of the CEP due to the fluctuation of the incident angles of the light pulses to the prism pair by using the novel prism pair instead of the conventional Brewster-prism pair for intracavity dispersion compensation. This design is quite suited for intracavity compression of broadband and low repetition rate optical pulses.

Mechanism of Light Detection in Current-Biased Superconducting ${\rm MgB}_{2}$ Microbridges

We report on time-resolved, transient photoimpedance measurements performed on current-biased microbridges excited by 100-fs wide, 800-nm wavelength, and 76-MHz repetition-rate optical pulses. Our bridges had dimensions of 20 mum by 40 mum and were patterned by conventional photolithography and ion milling from 150-nm-thick, epitaxial MgB2 films grown by a hybrid physical-chemical vapor-deposition technique. For high-speed detection experiments, the microbridges were embedded into coplanar-strip transmission lines and connected to a 20-GHz-bandwidth sampling oscilloscope via a semirigid coaxial cable. The test structures were placed inside an optical cryostat and studies were performed in the temperature range between 15 K and 40 K. At temperatures, e.g., 20 K, far below the bridge critical temperature and under low optical excitation, large and fast (130 ps in duration) photoresponse transients were observed, which we associate with the kinetic-inductive (Cooper-pair breaking) mechanism. At temperatures approaching the MgB2 critical temperature and/or under intense light illumination, an additional slow component of the photoresponse with a nanosecond decay time was measured. We interpret this second photoimpedance component as a resistive response, resulting from the hot-electron heating effect. The observed picosecond kinetic photoresponse makes our MgB2 bridges attractive candidates for fast and efficient optical detectors and photon counters.

Flat Supercontinuum Generation at 1550 nm in aDispersion-Flattened Microstructure Fibre Using Picosecond Pulse

The generation of a flat supercontinuum of over 80 nm in the 1550 nmregion by injecting 1.6 ps 10 GHz repetition rate optical pulses intoan 80-m-long dispersion-flattened microstructure fibre is demonstrated.The fibre has small normal dispersion with a variation smaller than1.5 (ps.nm−1.km−1) between 1500 and 1650 nm. Thegenerated supercontinuum ranging from 1513 to 1591 nm has the flatnessof ±1.5 dB and it is not so flat in the range of severalnanometres around the pump wavelength 1552 nm. Numerical simulation isalso used to study the effect of optical loss, fibre parameters andpumping conditions on supercontinuum generation in thedispersion-flattened microstructure fibre, and can be used for furtheroptimization to generate flat broad spectra.

A method of tunable high repetition-rate picosecond optical pulse generation using injection locking of laser diodes

A method of tunable, high repetition-rate optical pulse generation by using optical injection locking of semiconductor lasers has been described in this paper. This method utilizes the superposition of five coherent optical carriers obtained from the spectrum of optical phase modulation, which results in a sharp optical pulse. Theoretical calculations indicate that a pulse width of 1 ps and a repetition rate of 200 GHz can be achieved for a modulator drive frequency of 40 GHz.

Repetition rate and center wavelength-tunable optical pulse generation using an optical comb generator and a high-resolution arrayed-waveguide grating

Optical pulse generation using an optical comb generator and a high-resolution arrayed-waveguide grating with 30 integrated phase and amplitude modulators is successfully demonstrated. The repetition rate and the center wavelength of the Gaussian pulse train can be controlled from 9.7 to 10.3 GHz and from 1527 to 1565 nm, respectively.