Repetition Rate Of Pulse Train Research Articles

Design of an Optimized Terahertz Time-Domain Spectroscopy System Pumped by a 30 W Yb:KGW Source at a 100 kHz Repetition Rate with 245 fs Pulse Duration

Ytterbium laser sources are state-of-the-art systems that are increasingly replacing Ti:Sapphire lasers in most applications requiring high repetition rate pulse trains. However, extending these laser sources to THz Time-Domain Spectroscopy (THz-TDS) poses several challenges not encountered in conventional, lower-power systems. These challenges include pump rejection, thermal lensing in nonlinear media, and pulse durations exceeding 100 fs, which consequently limit the detection bandwidth in TDS applications. In this article, we describe our design of a THz-TDS beamline that seeks to address these issues. We report on the effectiveness of temperature controlling the Gallium Phosphide (GaP) used to generate the THz radiation and its impact on increasing the generation efficiency and aiding pump rejection while avoiding thermal distortions of the residual pump laser beam. We detail our approach to pump rejection, which can be implemented with off-the-shelf products and minimal customization. Finally, we describe our solution based on a commercial optical parametric amplifier to obtain a temporally compressed probe pulse of 55 fs duration. Our study will prove useful to the increasing number of laboratories seeking to move from the high-energy, low-power THz time-domain spectroscopy systems based on Ti:Sapphire lasers, to medium-energy, high-power systems driven by Yb-doped lasers.

Porous nano-grained cuprous selenide (Cu2Se) for mid-infrared photonics

Recently, mid-infrared photonics has attracted tremendous attention for various applications. To address the dilemma of the limited variety of materials for mid-infrared photonics, the mid-infrared optical modulation characteristics of the porous nano-grained cuprous selenide (Cu2Se) were investigated for the first time. The large effective nonlinear absorption coefficient, large modulation depth, and low saturation absorption intensity at 2.3 μm indicate that the porous nano-grained Cu2Se were promising saturable absorbers for the generation of Q-switched pulsed laser in the mid-infrared regime. As an application, the Tm:GdVO4 passively Q-switched laser operating on the 3H4 → 3H5 Tm3+ transition was achieved for the first time by using the prepared porous nano-grained Cu2Se as the saturable absorber. The minimum pulse duration and the maximum repetition rate of the stable Q-switched pulse train were 440 ns and 76.9 kHz, respectively. Our results demonstrated that the porous nano-grained Cu2Se was a promising candidate for the mid-infrared photonic applications.

10 Hz Stable Mode-locked Er-Fiber Laser

Abstract: In this paper, a stable output at the reduction repetition rate from 80 MHz mode-locked Er-doped fiber laser (EDFL) was demonstrated by adjusting the operating point for external LiNbO3 Mach-Zehnder‎ electro-optic intensity modulator to suppress harmonic mode-locked pulses. More than 70% of the laser beam was launched into the external pigtail Mach-Zehnder intensity modulator using single mode fiber with collimating lens and fiber (ferrule connector) FC adapter. The optimum values of Mach-Zehnder modulator bias voltages V0 and Vπ (half wave voltage) which resulted in maximum and minimum laser output power to leave the modulator were found at 6.5 and 0.29 V, respectively. The best modulation was achieved for the RF modulating signal with an opening time of about 8 ns. For driving voltage equal to Vπ, a clean mode-locked pulse train was generated with a pulse shape and energy similar to the input pulse, albeit with a low repetition rate. The minimum repetition rate reached as low as 10 Hz, resulting in pulse energy and peak power of 0.32 nJ and 1.5 kW, respectively. Second-harmonic generation of the low repetition rate pulse train was also produced by using a PPLN crystal. This opens up the potential for using this laser in single photon detection experiments and as a seed laser for many applications. Keywords: Er-fiber laser, Passively mode-locked fiber laser, Nonlinear polarization rotation, Mach-Zehnder modulator, Fast electro-optic modulator. PACS: Fiber lasers, 42.55.Wd, Mode locking, 42.60.Fc.

Real-Time Eye Diagram Monitoring for Optical Signals Based on Optical Sampling

A real-time eye diagram monitoring method for optical signals is proposed and experimentally demonstrated based on optical sampling. In the system, the optical signals under test are directly sampled by an optical sampling pulse train with a narrow pulse width and a high repetition frequency. The sampling pulse train is achieved in a Mach-Zehnder modulator (MZM), gated on-off by an electrical pulse. The sampled optical signals are then broadened and detected by a photodetector (PD). A low-speed electrical analog-to-digital converter (ADC) will then quantify the detected electrical signals. Combining with an algorithm based on the infinitesimal calculus, the quantified data is then used to achieve the eye diagram, according to which more time-domain parameters, such as period, time jitter, Q value, and bit error rate (BER) for the optical signals under test, are obtained. Thanks to the high repetition rate of the optical sampling pulse train, the eye diagram and the time-domain parameters of the optical signals are observed in real time. Experimental results show that a real time of about 350-μs eye diagram monitoring for a 2.5-Gb/s optical signal with a dynamic range from −10 to −22 dBm is achieved. In addition, time jitters are measured to range from 4.3 to 49.8 ps. Q values are estimated to range from 20.4 to 4.3, corresponding to BERs ranging from 2.3 × 10−92 to 8.5 × 10−6. The results are also verified by a commercial real-time oscilloscope.

Designing a single-mode anomalous dispersion silicon core fiber for temporal multiplet formation.

A highly nonlinear single-mode anomalous dispersion silicon core fiber (SCF) is suitably designed and optimized to generate a high repetition rate pulse train in the temporal domain from a single input pulse at a sufficiently shorter optimum length in comparison to silica-based standard fibers used for the same purpose. The large amount of Kerr-induced nonlinearity of a SCF is effectively utilized here such that input Gaussian pulses or pulse trains transform into a highly repetitive temporal multiplet. The effects of free-carrier generation-induced change in absorption and dispersion are included while studying the nonlinear pulse propagation through the SCF. To declare the generated pulse as a superior-graded triplet, a Q parameter, as a function of relative pulse parameters of the individual pulses of a triplet, is defined for the first time, to the best of our knowledge. Different pulse parameters are thoroughly optimized as well as the effect of external gain is examined from the perspective of requirement of shorter fiber length and development of quality triplets. Finally, the work is further extended for the formation of quadruplet pulses by the same type of SCF. It is to be mentioned here that such a methodical study for the generation of a temporal multiplet using a semiconductor core fiber has not been reported earlier.

Locking of microwave oscillators on the interharmonics of mode-locked laser signals.

In this paper, the theory of phase-locking of a microwave oscillator on the interharmonics, i.e. non-integer harmonics, of the repetition rate of the optical pulse train of a mode-locked laser (MLL) is developed. A balanced optical microwave phase detector (BOMPD) is implemented using a balanced Mach-Zehnder modulator and is employed to discriminate the phase difference between the envelope of the optical pulses and the microwave oscillator. It is shown mathematically that the inherent nonlinear properties of BOMPD with respect to the microwave excitation amplitude can be used for interharmonic locking. The characteristic functions of the phase detector for interharmonic locking are derived analytically and are compared with the measurement results. An opto-electronic phase-locked loop (OEPLL) is demonstrated whose output frequency locks on interharmonics of the MLL repetition rate when an appropriate modulator bias and sufficient RF amplitude are applied. Thus, for the first time theory and experiment of reliable locking on interharmonics of the repetition rate of a MLL are presented.

Influence of Magnetic Field on Amplification without Inversion Induced by a Femtosecond Pulse Train

Taking advantage of the short pulse duration and the high repetition rate of the fs pulse train, amplification without inversion (AWI) can be achieved in two- or three-level atomic systems. A modulation method using a static magnetic field to control such AWI gain in an atomic system is proposed. The schematic model of the 52S1/2−52P1/2 transition of 87Rb is adopted, and the results show that the system gain can be significantly enhanced and modulated by the magnetic field, while the population inversion can be almost completely suppressed. Moreover, the pulse number, pulse repetition period, and pulse area can also be utilized to modulate the AWI gain.

Indium Tin Oxide Nanowire Arrays as a Saturable Absorber for Mid-Infrared Er:Ca0.8Sr0.2F2 Laser.

We demonstrated a passively Q-switched Er:Ca0.8Sr0.2F2 laser with indium tin oxide nanowire arrays as an optical modulator in the mid-infrared region. In the Q-switched regime, the maximum output power of 58 mW with a slope efficiency of 18.3% was acquired. Meanwhile, the minimum pulse duration and highest repetition rate of the stable pulse trains were 490 ns and 17.09 kHz, corresponding to single pulse energy of 3.4 μJ and peak power of 6.93 W, respectively. To the best of our knowledge it was the first time that indium tin oxide nanowire arrays were employed as a saturable absorber to make pulse lasers carried out at 2.8 μm. The experimental data show that indium tin oxide nanowire arrays can be employed as a competitive candidate for saturable absorber in the field of mid-infrared solid-state lasers.

Nonlinear pulse reshaping in a typically designed silicon-on-insulator waveguide and its application to generate a high repetition rate pulse train

A silicon-on-insulator (SOI) planar waveguide is designed with a small group velocity dispersion (β 2) ∼2.212 (ps2 m−1) and a quite high nonlinear coefficient (γ) of ∼360.57 (W−1 m−1). The designed waveguide is capable of reshaping a super-Gaussian input optical pulse into a parabolic pulse (PP), without any use of external gain. The same waveguide with a relatively longer length can also generate a triangular pulse (TP) by using a positive chirp at the input. In both cases, PP and TP are created at a much shorter optimum length (L opt) of a few millimeters when compared to previously reported works on normal dispersion optical fibers. The interaction of a pulse pair inside such an SOI waveguide is investigated, for the first time as per our knowledge to generate a high-frequency (∼4.8 THz) pulse train, while lower repetition rate (∼180 GHz) pulses are used at the input. This study as a whole enables potential device applications in the domains of tunable high frequency (THz) pulse generators, optical signal processing, and many more.

One-to-Sixteen Wavelength Multicast of Optical Single-Sideband OFDM Signals Using a Single Pulsed Pump

We demonstrate an effective wavelength multicast scheme to generate 16 new channels with low penalty and high spectral efficiency using a single optical pump. The scheme capitalizes on cross-phase modulation of an input signal by a rate-multiplied pump pulse train to generate multiple wavelength channels. The pulse rate is multiplied in an energy-efficient way via the temporal Talbot effect. In the experiment, we have successfully achieved more than 20 multicast copies of a 40-Gb/s QPSK OFDM single-sideband signal. The copied channels are spaced by 50 GHz which is determined by five times multiplication on the repetition rate of a 10-GHz input pump pulse train. The BER performances of the central 16 channels are evaluated, showing less than 1.3-dB OSNR difference among the channels. Compared to the original single-sideband signal, the central channel experiences only a 0.1-dB OSNR penalty in our approach. The result verifies the effectiveness of our scheme in realizing spectrally efficient and high-channel-count wavelength multicast by using a single pump generated with relatively low-speed electronic resources.

Real-time discrete Fourier transformer with complex-valued outputs based on the inverse temporal Talbot effect.

Discrete Fourier transform (DFT) plays an important role in digital signal processing. In this paper, we present a novel optical real-time discrete Fourier transformer with complex-valued outputs, which is enabled by the inverse temporal Talbot effect. In the system, an input pulse train is first quadratically phase-modulated as in an inverse temporal Talbot system and then split into two channels. In the first channel, the pulse train is further amplitude-modulated pulse-by-pulse by a discrete data sequence to be transformed. In the second channel, a reference signal modulates the pulse train, which is for removing the residual quadratic phase profile in the output pulse train. The pulse trains in the two channels propagate through a shared dispersion medium with a proper dispersion value determined by the inverse temporal Talbot effect. A 90-degree optical hybrid and two balanced photodetectors are employed to retrieve the real and imaginary parts of the DFT results. In this scheme, the pulse repetition rate of the output pulse train is equal to the input one. In addition, we present a full theoretical framework to explain exactly the DFT relationship. We also demonstrate that the input data sequence can be complex-valued with the help of an I/Q modulator.

Passively Q-switched Yb-doped fiber laser based on Ag nanoplates saturable absorber

We experimentally investigated Ag nanoplates as saturable absorber for Q-switched pulse generation in an Yb-doped fiber laser. The pulse train repetition rate increases with the increase of the pump power. At the maximum pump power of 600 mW, the maximum repetition rate and average output power are 184.8 kHz and 10.77 mW, respectively, corresponding to single pulse energy of 58.3 nJ. To the best of our knowledge, it is the first demonstration of the passively Q-switched fiber laser utilizing the material of Ag nanoparticles at the wavelength of 1-μm. Our investigations demonstrate the flexibility of our solution-processed Ag nanoplates-based saturable absorber, making it a promising candidate for a variety of stable and low-cost ultrafast lasers.

Subharmonic Entrainment of Kerr Breather Solitons.

We predict subharmonic entrainment of breather-soliton oscillations to a periodic perturbation at the round-trip time T_{R} in Kerr-nonlinear optical resonators; an integer ratio T_{b}/T_{R}=N≫1 results for breathing period T_{b}. Rigid entrainment is observed with intermediate finesse (F∼30-40) for N up to 20, and we propose a way to realize higher entrainment ratios at higher finesse. This nontrivial synchronization across the widely separated timescales of the photon lifetime and round-trip time points to a new direction for research in this field and may find application, for example, in the measurement of a pulse train repetition rate that is electronically inaccessible.

Self Q-switched mode-locking in compact red Pr3+-doped ZBLAN fiber laser

The self Q-switching mode-locking effect in compact red fiber laser has been investigated. The Q-switched mode-locking state can keep operating with a pump power ranging from 120.5 mW to 564.8 mW. The maximum average output power of this self Q-switched mode-locking laser is 22.5 mW, and the central wavelength of the output optical spectrum is 635.9 nm with a 3 dB bandwidth of 2.4 nm, as well as the fundamental repetition rate of mode-locking pulse trains being 25.8 MHz. This work can provide an attractive method for Q-switched mode-locking pulse generation in visible fiber lasers.

Conductive graphene as passive saturable absorber with high instantaneous peak power and pulse energy in Q-switched regime

Abstract The Q-switched pulse regime is demonstrated by integrating conductive graphene as passive saturable absorber producing relatively high instantaneous peak power and pulse energy. The fabricated conductive graphene is investigated using Raman spectroscopy. The single wavelength Q-switching operates at 1558.28 nm at maximum input pump power of 151.47 mW. As the pump power is increased from threshold power of 51.6 mW to 151.47 mW, the pulse train repetition rate increases proportionally from 47.94 kHz to 67.8 kHz while the pulse width is reduced from 9.58 μs to 6.02 μs. The generated stable pulse produced maximum peak power and pulse energy of 32 mW and 206 nJ, respectively. The first beat node of the measured signal-to-noise ratio is about 62 dB indicating high pulse stability.

Nonlinear Pulse Compression to Sub-40 fs at $4.5~\mu$ J Pulse Energy by Multi-Pass-Cell Spectral Broadening

We report on the pulse compression of an 18.5 MHz repetition rate pulse train from 230 fs to sub-40 fs by nonlinear spectral broadening in a multi-pass cell and subsequent chirp removal. The compressed pulse energy is $4.5~\mu \text{J}$ , which corresponds to 84 W of average power, with a compression efficiency of 88%. This recently introduced compression scheme is suitable for a large pulse energy range and for high average power. In this paper, we show that it can achieve three times shorter pulses than previously demonstrated.

Repetition-Rate-Switchable and Self-Mode-Locked Pulses Generation From a Gain-Switched Thulium-Doped Fiber Laser and Their Amplification Properties

We report on repetition-rate-switchable and self-mode-locked pulses generation from a gain-switched thulium-doped fiber laser. Stable pulse trains were gain-switched by a home-made, pulsed Erbium/Ytterbium codoped fiber amplifier. The pulse repetition rate (PRR) of stable output pulse trains can be switched as 1/4, 1/3, and 1/2 times of that of the pump pulse train. Besides, the gain-switched mode-locked pulses were also achieved in the same cavity. By following three-stage thulium-doped fiber amplifiers, the average output power of the stable gain-switched pulses was scaled to 115 W with the PRR of 100 kHz, corresponding to a pulse energy of about 1.15 mJ. The amplification properties of the gain-switched mode-locked pulses were also characterized and discussed.

The Challenge in Realizing Truly Agile Fiber Lasers

AbstractThe master oscillator power amplifier (MOPA) fiber laser has been around for several years but the bulk of applications use Q‐switched fiber lasers. Now, with the processing capabilities of the Q‐switched fiber laser fully explored, the MOPA fiber laser is getting revived attention. Laser‐diode seeded MOPAs hold promise of arbitrarily modulated high‐power laser output. Pulse‐pumping the amplifiers to handle ON/OFF gating and low repetition rate pulse trains without overshoot or undershoot is of special interest.

Mode-locked laser with pulse interleavers in a monolithic photonic integrated circuit for millimeter wave and terahertz carrier generation.

This Letter presents a photonics-based millimeter wave and terahertz frequency synthesizer using a monolithic InP photonic integrated circuit composed of a mode-locked laser (MLL) and two pulse interleaver stages to multiply the repetition rate frequency. The MLL is a multiple colliding pulse MLL producing an 80 GHz repetition rate pulse train. Through two consecutive monolithic pulse interleaver structures, each doubling the repetition rate, we demonstrate the achievement of 160 and 320 GHz. The fabrication was done on a multi-project wafer run of a generic InP photonic technology platform.

High-speed pulse train amplification in semiconductor optical amplifiers with optimized bias current.

In this paper, we have experimentally investigated the optimized bias current of semiconductor optical amplifiers (SOAs) to achieve high-speed input pulse train amplification with high gain and low distortion. Variations of the amplified output pulse duration with the amplifier bias currents have been analyzed and, compared to the input pulse duration, the amplified output pulse duration is broadened. As the SOA bias current decreases from the high level (larger than the saturated bias current) to the low level, the broadened pulse duration of the amplified output pulse initially decreases slowly and then rapidly. Based on the analysis, an optimized bias current of SOA for high-speed pulse train amplification is introduced. The relation between the SOA optimized bias current and the parameters of the input pulse train (pulse duration, power, and repetition rate) are experimentally studied. It is found that the larger the input pulse duration, the lower the input pulse power or a higher repetition rate can lead to a larger SOA optimized bias current, which corresponds to a larger optimized SOA gain. The effects of assist light injection and different amplifier temperatures on the SOA optimized bias current are studied and it is found that assist light injection can effectively increase the SOA optimized bias current while SOA has a lower optimized bias current at the temperature 20°C than that at other temperatures.