THz Repetition Rate Research Articles

Surface structuring of fused silica by an ultrashort pulse burst with THz repetition rates

Surface structuring of fused silica by an ultrashort pulse burst with THz repetition rates

Terahertz-Rate Kerr-Microresonator Optical Clockwork

Kerr microresonators generate interesting and useful fundamental states of electromagnetic radiation through nonlinear interactions of continuous-wave (CW) laser light. Using photonic-integration techniques, functional devices with low noise, small size, low-power consumption, scalable fabrication, and heterogeneous combinations of photonics and electronics can be realized. Kerr solitons, which stably circulate in a Kerr microresonator, have emerged as a source of coherent, ultrafast pulse trains and ultra-broadband optical-frequency combs. Using the f-2f technique, Kerr combs support carrier-envelope-offset phase stabilization for optical synthesis and metrology. In this paper, we introduce a Kerr-microresonator optical clockwork based on optical-frequency division (OFD), which is a powerful technique to transfer the fractional-frequency stability of an optical clock to a lower frequency electronic clock signal. The clockwork presented here is based on a silicon-nitride (Si$_3$N$_4$) microresonator that supports an optical-frequency comb composed of soliton pulses at 1 THz repetition rate. By electro-optic phase modulation of the entire Si$_3$N$_4$ comb, we arbitrarily generate additional CW modes between the Si$_3$N$_4$ comb modes; operationally, this reduces the pulse train repetition frequency and can be used to implement OFD to the microwave domain. Our experiments characterize the residual frequency noise of this Kerr-microresonator clockwork to one part in $10^{17}$, which opens the possibility of using Kerr combs with high performance optical clocks. In addition, the photonic integration and 1 THz resolution of the Si$_3$N$_4$ frequency comb makes it appealing for broadband, low-resolution liquid-phase absorption spectroscopy, which we demonstrate with near infrared measurements of water, lipids, and organic solvents.

Mode locked Nd3+ and Gd3+ co-doped calcium fluoride crystal laser at dual gain lines

Based on a novel disordered fluoride crystal—Nd3+ and Gd3+ co-doped CaF2, we demonstrate a dual-wavelength synchronously mode locked laser in a single cavity, for the first time. Two gain lines at 1064.2 nm and 1064.7 nm were synchronously mode locked by gain spectrum splitting method, corresponding to a 0.15 THz repetition rate. The mode locked laser shows a 6.5 ps separated pulse duration.

Self-starting harmonic frequency comb generation in a quantum cascade laser

Optical frequency combs establish a rigid phase-coherent link between microwave and optical domains and are emerging as high-precision tools in an increasing number of applications. Frequency combs with large intermodal spacing are employed in the field of microwave photonics for radiofrequency arbitrary waveform synthesis and for generation of THz tones of high spectral purity in the future wireless communication networks. We demonstrate for the first time self-starting harmonic frequency comb generation with a THz repetition rate in a quantum cascade laser. The large intermodal spacing caused by the suppression of tens of adjacent cavity modes originates from a parametric contribution to the gain due to temporal modulations of the population inversion in the laser. The mode spacing of the harmonic comb is shown to be uniform to within $5\times 10^{-12}$ parts of the central frequency using multiheterodyne self-detection. This new harmonic comb state extends the range of applications of quantum cascade laser frequency combs.

Octave-spanning dissipative Kerr soliton frequency combs in Si_3N_4 microresonators

Octave-spanning, self-referenced frequency combs are applied in diverse fields ranging from precision metrology to astrophysical spectrometer calibration. In the past decade, Kerr frequency comb generators have emerged as alternative scheme offering chip-scale integration, high repetition rate and bandwidths that are only limited by group velocity dispersion. The recent observation of Kerr frequency combs operating in the dissipative Kerr soliton (DKS) regime, along with dispersive wave formation, has provided the means for fully coherent, broadband Kerr frequency comb generation with engineered spectral envelope. Here, by carefully optimizing the photonic Damascene fabrication process, and dispersion engineering of $\mathrm{Si_{3}N_{4}}$ microresonators with $1\,\mathrm{THz}$ free spectral range, we achieve bandwidths exceeding one octave at low powers ($\mathcal{O}(100\,\mathrm{mW})$) for pump lasers residing in the telecom C-band ($1.55\,\mathrm{\mu m}$), as well as for the first time in the O-band ($1.3\,\mathrm{\mu m}$). Equally important, we find that for THz repetition rate comb states, conventional criteria applied to identify DKS comb states fail. Investigating the coherence of generated, octave-spanning Kerr comb states we unambiguously identify DKS states using a response measurement. This allows to demonstrate octave-spanning DKS comb states at both pump laser wavelengths of $1.3\mathrm{\,\mu m}$ and $1.55\,\mathrm{\mu m}$ including the broadest DKS state generated to date, spanning more than $200\,\mathrm{THz}$ of optical bandwidth. Octave spanning DKS frequency combs can form essential building blocks for metrology or spectroscopy, and their operation at $1.3\mathrm{\,\mu m}$ enables applications in life sciences such as Kerr comb based optical coherence tomography or dual comb coherent antistokes Raman scattering.

Misalignment measurement of femtosecond electron bunches with THz repetition rate

Generation of a well-aligned bunch train is of vital importance for preserving the beam quality in a plasma wake-field accelerator. The simultaneous measurements of the relative misalignments in both planes, of the individual femtosecond electron bunches with THz repetition rate in a train, are reported and analyzed with simulations. The new method proposed in this paper demonstrates that the measurements can be done quickly by applying a single quadrupole scan together with a radio-frequency deflector (RFD), while the RFD strength and the RFD phase for each bunch do not need to be known

Spectral-temporal encoding and decoding of the femtosecond pulses sequences with a THz repetition rate

Experimental and numerical modeling techniques demonstrated the possibilities of the spectral-time encoding and decoding for time division multiplexing sequence of femtosecond subpulses with a repetition rate of up to 6.4 THz. The sequence was formed as a result of the interference of two phase-modulated pulses. We report the limits of the application of the developed method of controlling formed sequence at the spectral-temporal coding.

Tunable THz radiation source from dielectric loaded waveguide excited by nonrelativistic electron bunch trains

We propose a novel scheme to generate a tunable narrow-band THz radiation. In this scheme, a train of laser pulses with THz repetition rate is used to drive a photocathode direct current (DC) gun, leading to the emission of a train of electron bunches. The electron bunch train is subsequently accelerated by the gun field and applied to selectively excite one of the modes in the dielectric loaded waveguide (DLW) structure, which is located downstream the DC gun. Thanks to the tunability of the repetition rate of laser pulses and the gun voltage, a tunable narrow-band THz radiation source can be obtained. This proposed source has the advantages of compactness, robustness and relatively high power.

Dual-wavelength mode-locked operation on a novel Nd^3+,Gd^3+:SrF_2 crystal laser

We report on efficient continuous-wave (CW) and dual-wavelength mode-locked operations on a novel Nd3+,Gd3+:SrF2 crystal laser for the first time. Codping Gd3+ in the Nd3+:SrF2 crystal significantly increased the fluorescence intensity and aroused and inhomogeneously broadened the emission spectrum. A CW operation with an output power of 992 mW and corresponding slope efficiency as high as 42.7% was obtained. We also obtained dual-wavelength synchronously mode-locked operation, with pulse duration of 1.9 ps. The mode-locked spectrum separated distinctly at 1052.2 nm and 1059.4 nm. Autocorrelation trace clearly showed a 417 fs beating period, and 1.94 THz repetition rate.

Generation of multiple laser lines by sum-frequency mixing of continuous-wave Raman emissions from a dispersion-compensated optical cavity

Three color continuous-wave (CW) laser emissions with constant frequency separation are generated in the near-infrared (NIR) region using a dispersion-compensated optical cavity filled with hydrogen gas. By focusing these laser emissions into second-harmonic generation (SHG) crystals, multiple second harmonic signals and sum-frequency signals are generated in the near-ultraviolet (NUV) with a constant frequency spacing. Up to five colors of these NUV CW laser emissions can be generated simultaneously by using SHG crystals with different orientations. The interference between the second-harmonic signal of one NIR laser emission and the sum-frequency signal of the other two NIR emissions was observed experimentally, indicating mutual phase coherence among the NIR laser emissions. The phase coherence allows the synthesis of a train of ultrashort pulses with a THz repetition rate in both the NUV and the NIR by using the CW emission lines.

Six-dimensional measurements of trains of high brightness electron bunches

Trains of ultrashort electron pulses with THz repetition rate, so-called comblike beams, are assuming an ever growing interest in plasma-based acceleration. In particle-driven plasma wakefield acceleration (PWFA), a train of driver bunches with separation of the order of plasma wavelength, i.e., 300 μm, resonantly excites a plasma wake, which accelerates a trailing witness bunch, injected at the accelerating phase. Comblike beams have great potentialities in different fields of applications. In particular, radiation sources, such as free-electron lasers and THz radiation, take advantage from the possibility to tailor electron beams modulated both in time and energy, to customize emission bandwidth and temporal properties. In these scenarios, the manipulation of longitudinal phase space to investigate different bunch configurations, in terms of energy and time separation, is founded on the knowledge of the 6D phase space of each bunch in the train. In this paper we present the methods developed at the SPARC_LAB test facility in order to fulfill the requirements. Starting from conventional diagnostics, therefore applying well-known tools using more than one diagnostic at the same time, we have completely characterized not only the full 6D phase space of a comblike electron beam with THz repetition rate, but also each single bunch within the train. To our knowledge, this is the first time such a measurement has been performed. Experimental results for multibunch trains in different configurations, suitable for PWFA applications, will be shown and discussed.

Direct measurement of the parameters of a femtosecond pulse train with a THz repetition rate generated by the interference of two phase-modulated femtosecond pulses.

A femtosecond pulse train with THz repetition rate generated by the interference of two phase-modulated pulses has been recorded experimentally. Pulse repetition rates and their duration have been measured. It has been shown that at the 50-fs time delay between phase-modulated pulses the repetition rate is 3.1 THz with a pulse width of 200 fs, while at the 120-fs time delay the repetition rate is 7.1 THz with a pulse width of 67 fs.

Passive mode-locking performance of mixed Nd:La_011Y_089VO_4 crystal

Passive mode locking of a diode pumped Nd:La(0.11)Y(0.89)VO(4) mixed crystal laser with a semiconductor saturable absorber mirror (SESAM) was experimentally investigated for the first time to our knowledge. Stable CW mode-locking has been achieved on both a-cut and c-cut mixed crystals. In case of the a-cut crystal, when a 2% output coupler (OC) was used, the shortest pulse obtained was 4.5 ps and the highest output power was 0.94 W, while when a 6% OC was used, the shortest pulse obtained was 6.8 ps and the highest output power was 5.16 W. In the latter case the optical conversion efficiency is 38% and the slope efficiency is 40.3%, respectively; with the c-cut crystal the shortest pulse achieved was 5.5 ps. Moreover, simultaneous mode locking at two close wavelengths of 1064.3 nm and 1066.2 nm was observed on the c-cut crystal. The mode locked pulse beating generated temporal interference fringe of 0.5 THz repetition rate.

The THz radiation source at SPARC

The linac driven coherent THz radiation source at the SPARC facility is able to deliver broadband THz pulses with femtosecond shaping. In addition, high peak power, narrow-band THz radiation can be also generated, taking advantage from advanced electron beam manipulation techniques, able to generate a train of short electron bunches with THz repetition rate.

Ultrafast Strain Engineering in Complex Oxide Heterostructures

We report on ultrafast optical experiments in which femtosecond midinfrared radiation is used to excite the lattice of complex oxide heterostructures. By tuning the excitation energy to a vibrational mode of the substrate, a long-lived five-order-of-magnitude increase of the electrical conductivity of NdNiO(3) epitaxial thin films is observed as a structural distortion propagates across the interface. Vibrational excitation, extended here to a wide class of heterostructures and interfaces, may be conducive to new strategies for electronic phase control at THz repetition rates.

THz optical pulses from a coupled-cavity quantum-dot laser

Optical pulses are generated from a coupled-cavity quantum-dot (QD) laser consisting of a short QD-waveguide Fabry–Perot (F–P) cavity and three long external fiber Bragg grating (FBG) cavities. When the laser is biased at low operation current, the feedback from the external cavities dominates and laser pulses have a 1.01 THz repetition rate, determined by the equal frequency difference of the three FBGs. We are thus able to decouple the repetition rate of a mode-locked laser from the cavity length. With much higher bias current, the QD F–P cavity dominates and the repetition rate is switched to 43.8 GHz, defined by the length of the F–P cavity.

Dual-wavelength passively mode-locked Nd:LuYSiO_5 laser with SESAM

A diode-end-pumped dual-wavelength mode-locked laser based on Nd:LuYSiO5 crystal is demonstrated. With a SESAM, simultaneous mode locking at the 1075.8 nm and 1078.1 nm is achieved and the dual-wavelength mode locked pulses have a pulse width of 8.9 ps. Due to frequency beating, ultrahigh repetition rate ultrafast pulses with 997 fs pulse width and 0.59 THz repetition rate are further formed. Under 12.7 W absorbed pump power 1.7 W mode-locked output power was obtained, the slope efficiency of the mode locked laser was 24.3%.

Dual-Wavelength Pulse Train Generation Based on Silicon-on-Insulator Optical Waveguides

In this paper, we present and numerically study a method with pump-probe configuration to generate ∼4.17-THz dual-wavelength ultrahigh-repetition-rate pulse train from the continuous-waves (CW) based on the submicron silicon-on-insulator (SOI) optical waveguides. An original ∼4.17 THz repetition rate pulsed pump, with femtosecond pulse duration, at 2900 nm (mid-wave infrared wavelength-MWIR) and 2-channel CW lights at 1450 nm and 1650 nm (near infrared wavelengths) are simultaneously launched into the silicon waveguide. Results show that, when both of CW lights with high intensity pulsed pump are co-propagating along the SOI waveguide, the CW lights will, respectively, experience normal and anomalous group velocity dispersion (GVD) regimes, leading to developing into two inverted ∼4.17 THz repetition rate pulse train by the combined effects of GVD and nonlinear frequency chirp induced by pulsed pump inside the waveguide. Moreover, the outcome pulse trains have short pulse duration and high extinction ratio (ER) comparing with the original pulsed pump if the systems parameters such as input power and waveguide length are judiciously adjusted.

Dual-wavelength synchronously mode-locked Nd:CNGG laser

We have experimentally demonstrated a dual-wavelength synchronously mode-locked Nd:CNGG laser based on the semiconductor saturable absorber mirror technique. Mode locking was achieved simultaneously on two gain bands of the crystal that have a central wavelength separation of 2.4 nm. The fundamental mode-locked pulse train has a repetition rate of 88 MHz and pulse duration of 5 ps, with an average output power of approximately 90 mW. Autocorrelation measurements show that each of the synchronously mode-locked pulses consists of a train of quasi-periodic beat pulses with a 660 fs pulse width and a 0.63 THz repetition rate.

Generation of 1 THz repetition rate, 97 fs optical pulse train based on comb-like profiled fibre

Experimentally generated is a 1 THz, 97 fs optical pulse train at 1.55 cm wavelength region based on a comb-like profiled fibre, which consists of 15 pairs of highly nonlinear fibres (HNLFs) and standard singlemode fibres. Such high-repetition rate pulse generation in the 100 fs regime was made possible by the HNLF with high nonlinearity and small dispersion slope.