Repetition Rate Control Research Articles

Energy-Preserving Arbitrary Repetition-Rate Control of Periodic Pulse Trains Using Temporal Talbot Effects

The temporal Talbot effect comprises a set of self-imaging phenomena in which a pulse train undergoes various coherence revivals after propagation through a dispersive medium. Besides the intrinsic physical interest of the phenomenon itself, the effect has found practical applications in various scientific areas. In optical signal processing, the temporal Talbot effect has been used to multiply the repetition-rate of periodic pulse sequences by integer factors. Because it operates coherently on the frequencies that comprise a pulse train, the temporal Talbot effect has been shown to mitigate noise such as reduction of pulse-to-pulse timing jitter and amplitude variation, as well as real-time optical averaging. Recently, there has been renewed interest in the temporal Talbot effect due to its use in arbitrary repetition rate multiplication and division, not just integer multiplication. Arbitrary repetition-rate control is based on a suitable combination of temporal phase-modulation and spectral-phase filtering using temporal Talbot conditions. By adjusting the phase-modulation profile and group-velocity dispersion, the multiplication and division factors can be tuned to be any desired values- fractional or integer. One of the main advantages of this methodology compared to traditional approaches of repetition rate control is its energy efficiency. Because temporal and spectral phase filtering are lossless processes, pulse trains using Talbot-based repetition rate control only suffer insertion loss from the system. In this article, we first present an overview of the theory behind temporal Talbot effects and then review recent work on its application for realizing arbitrary control of the repetition-rate of periodic optical pulses. This platform enables the creation of simple, versatile optical pulse sources for diverse applications where customized repetition rates are necessary.

Characterization of pseudosingle bunch kick-and-cancel operational mode

Author(s): Sun, C; Robin, DS; Steier, C; Portmann, G | Abstract: Pseudosingle-bunch kick-and-cancel (PSB-KAC) is a new operational mode at the Advanced Light Source of Lawrence Berkeley National Laboratory that provides full timing and repetition rate control for single x-ray pulse users while being fully transparent to other users of synchrotron radiation light. In this operational mode, a single electron bunch is periodically displaced from a main bunch train by a fast kicker magnet with a user-on-demand repetition rate, creating a single x-ray pulse to be matched to a typical laser excitation pulse rate. This operational mode can significantly improve the signal to noise ratio of single x-ray pulse experiments and drastically reduce dose-induced sample damage rate. It greatly expands the capabilities of synchrotron light sources to carry out dynamics and time-of-flight experiments. In this paper, we carry out extensive characterizations of this PSB-KAC mode both numerically and experimentally. This includes the working principle of this mode, resonance conditions and beam stability, experimental setups, and diagnostic tools and measurements.

Fourier-transform spectroscopy using an Er-doped fiber femtosecond laser by sweeping the pulse repetition rate.

Femtosecond lasers allow for simultaneous detection of multiple absorption lines of a specimen over a broad spectral range of infrared or visible light with a single spectroscopic measurement. Here, we present an 8-THz bandwidth, 0.5-GHz resolution scheme of Fourier-transform spectroscopy using an Er-doped fiber femtosecond laser. A resolving power of 1.6 × 104 about a 1560-nm center wavelength is achieved by sweeping the pulse repetition rate of the light source on a fiber Mach-Zehnder interferometer configured to capture interferograms with a 0.02-fs temporal sampling accuracy through a well-stabilized 60-m unbalance arm length. A dual-servo mechanism is realized by combining a mechanical linear stage with an electro-optic modulator (EOM) within the fiber laser cavity, enabling stable sweeping control of the pulse repetition rate over a 1.0-MHz scan range with 0.4-Hz steps with reference to the Rb clock. Experimental results demonstrate that the P-branch lines of the H13CN reference cell can be observed with a signal-to-noise ratio reaching 350 for the most intense line.

Ultra-low noise all polarization-maintaining Er fiber-based optical frequency combs facilitated with a graphene modulator

High bandwidth carrier phase and repetition rate control are critical for the construction of low phase noise optical frequency combs. Here we demonstrate the use of a graphene modulator for the former and a bulk electro-optic modulator for the latter enabling record low phase noise operation of an Er fiber frequency comb. For applications that do not require carrier phase control, we show that the form factor of a fiber comb can be reduced by adapting a graphene modulator for rapid repetition rate control. Moreover, the whole system demonstration is performed with all-polarization maintaining Er fiber frequency combs, highly suitable for applications in the field.

A carbon nanotube-embedded fiber-optic tunable coupler for flexible repetition rate control of a passively Q -switched fiber laser

We propose a carbon nanotube (CNT)-embedded fiber-optic tunable coupler as a passive Q-switch that can provide flexible control of the output pulse repetition rate in passively Q-switched fiber lasers, and experimentally demonstrate its feasibility. The device is implemented by sandwiching a thin CNT layer between two side-polished fibers. Using the coupler, it was experimentally shown that repetition-rate-tunable Q-switched pulses can readily be produced from an erbium-doped fiber ring cavity by adjusting the cross-coupling ratio through control of the core alignment mismatch angle between the two side-polished fibers. Flexible tuning of the pulse repetition rate is shown to be achievable from 20.96 to 12.98 kHz at a fixed pump power by adjusting the core alignment mismatch angle while the pulse width is varied from 4.98 to 5.44 μs.

Kinetics of generation, relaxation, and accumulation of electronic excitations under two-photon interband picosecond absorption in tungstate and molibdate crystals

Under two-photon 523.5 nm interband picosecond laser excitation, we measured the kinetics of induced absorption in PbWO4, ZnWO4, and PbMoO4 crystals with 532 to 633 nm continuous probe radiation. We obtained real-time information about the dynamics of the generation, relaxation, and accumulations of electronic excitations over a wide time range (from picoseconds to hundreds of seconds) and the 77–300 K temperature range. For the studied crystals, exponential temperature-independent growth of the induced absorption (IA) with 60 ns rise time reflects the dynamics of the generation of electronic excitation. The kinetics of the IA exponential growth with temperature-dependent 3.5–11 μs time constants reflect the dynamics of energy migration between neighboring tungstate (molibdate) ions to traps for the studied crystals. The multiexponential relaxation absorption kinetics strongly depend on temperature, and the relaxation decay time of induced absorption increased from tens to hundreds of milliseconds to seconds under crystal cooling from 300 to 77 K. We found that the increase in the laser pump repetition rate (0–10 Hz) leads to the accumulation of electronic excitations. Control of the repetition rate and the number of excitations allowed us to change the relaxation time of the induced absorption by more than two orders of magnitude. Due to accumulation of excitations at 77 K, the absorption relaxation time can exceed 100 s for PbWO4 and PbMoO4 crystals. In the initially transparent crystals, two-photon interband absorption (2PA) leads to crystals opacity at the 523 and 633 nm wavelengths. (An inverse optical transmission of the crystals exceeds 50–55 at a 50–100 GW/cm2 pump intensity.) Measured at ∼1 mW probe radiation of 532 and 633 nm wavelengths, the induced absorption values are comparable with those obtained under two-photon absorption at ∼5 kW pump power. An optical 2PA shutter for the visible spectral range is proposed with a variable shutting time from hundreds of microseconds to tens of seconds.

High precision laser ranging by time-of-flight measurement of femtosecond pulses

Time-of-flight (TOF) measurement of femtosecond light pulses was investigated for laser ranging of long distances with sub-micrometer precision in the air. The bandwidth limitation of the photo-detection electronics used in timing femtosecond pulses was overcome by adopting a type-II nonlinear second-harmonic crystal that permits the production of a balanced optical cross-correlation signal between two overlapping light pulses. This method offered a sub-femtosecond timing resolution in determining the temporal offset between two pulses through lock-in control of the pulse repetition rate with reference to the atomic clock. The exceptional ranging capability was verified by measuring various distances of 1.5, 60 and 700 m. This method is found well suited for future space missions based on formation-flying satellites as well as large-scale industrial applications for land surveying, aircraft manufacturing and shipbuilding.

Time-of-flight measurement with femtosecond light pulses

The time-of-flight of light pulses has long been used as a direct measure of distance1,2, but state-of-the-art measurement precision using conventional light pulses or microwaves peaks at only several hundreds of micrometres3,4. Here, we improve the time-of-flight precision to the nanometre regime by timing femtosecond pulses through phase-locking control of the pulse repetition rate using the optical cross-correlation technique5,6. Our experiment shows an Allan deviation of 117 nm in measuring a 0.7-km distance in air at a sampling rate of 5 ms once the pulse repetition is phase-locked, which reduces to 7 nm as the averaging time increases to 1 s. This enhanced capability is maintained at long range without periodic ambiguity, and is well suited to lidar applications such as geodetic surveying7, range finders8 and absolute altimeters9. This method could also be applied to future space missions involving formation-flying satellites for synthetic aperture imaging10,11 and remote experiments related to general relativity theory12. Scientists improve the precision of time-of-flight measurements from several hundreds of micrometres to the nanometre regime by timing femtosecond pulses through phase-locking control of the pulse repetition rate using the optical cross-correlation technique. This result looks set to benefit synthetic aperture imaging for future space missions of formation-flying satellites and remote experiments involving the general theory of relativity.

A multi-branch, fiber-based frequency comb with millihertz-level relative linewidths using an intra-cavity electro-optic modulator

We demonstrate that fiber-based frequency combs with multi-branch configurations can transfer both linewidth and frequency stability to another wavelength at the millihertz level. An intra-cavity electro-optic modulator is employed to obtain a broad servo bandwidth for repetition rate control. We investigate the relative linewidths between two combs using a stable continuous-wave laser as a common reference to stabilize the repetition rate frequencies in both combs. The achieved energy concentration to the carrier of the out-of-loop beat between the two combs was 99% and 30% at a bandwidth of 1 kHz and 7.6 mHz, respectively. The frequency instability of the comb was 3.7x10(-16) for a 1 s averaging time, improving to 5-8x10(-19) for 10000 s. We show that the frequency noise in the out-of-loop beat originates mainly from phase noise in branched optical fibers.

Reduction of Nonlinear Absorption in Li2B4O7 by Temperature- and Repetition Rate-Control

We address the critical issue of thermal dephasing of the UV harmonic generation process due to nonlinear absorption in Li2B4O7 crystals. Reduction of nonlinear absorption is achieved by increasing the crystal temperature up to 200 °C and reducing the pulse repetition rate. Our results show the way for significant improvement of the conversion efficiency for the fourth harmonic generation of Nd lasers using Li2B4O7 crystals.

20 GHz arbitrary radio-frequency waveform generator based on incoherent pulse shaping

We demonstrate a new approach of photonically assisted radio-frequency (RF) waveform generation using a spectrally incoherent light source. The system is based on the so-called generalized frequency-to-time mapping operation. In this work, external modulation of the source is done by concatenating two electro-optic Mach-Zehnder modulators properly biased to achieve short pulse gates which allow for broad bandwidth electrical signals. Also, the spectral shaping stage is performed prior to O/E conversion. A detailed theoretical analysis demonstrates that even in this case the frequency-to-time mapping is preserved. The key is that the noise level is greatly reduced because the amplitude filter serves as noise rejecter. Experimental results show up to approximately 20 GHz electrical bandwidth signals using 10 Gb/s standard telecommunication equipment with the nice feature of repetition rate control on the generated electrical waveform.

Measurement of the comb dynamics for feedback control of an etalon-based coupled optoelectronic oscillator

The response of an optical frequency comb from an etalon-based coupled optoelectronic oscillator to changes in drive current, optoelectronic loop phase, modulator bias, and laser cavity length has been measured. It is found that controlling the phase of the optoelectronic loop is best suited for control of the pulse repetition rate, whereas controlling the laser cavity length is best for stabilization of the optical carrier frequency. Moreover, by measuring the instabilities of the carrier frequency at the fixed-point frequency of the optoelectronic phase, changes to the optoelectronic phase can be decoupled from changes to the laser cavity.

A racetrack mode-locked silicon evanescent laser

By utilizing a racetrack resonator topography, an on-chip mode locked silicon evanescent laser (ML-SEL) is realized that is independent of facet polishing. This enables integration with other devices on silicon and precise control of the ML-SEL's repetition rate through lithographic definition of the cavity length. Both passive and hybrid mode-locking have been achieved with transform limited, 7 ps pulses emitted at a repetition rate of 30 GHz. Jitter and locking range are measured under hybrid mode locking with a minimum absolute jitter and maximum locking range of 364 fs, and 50 MHz, respectively.

Repetition rate control in continuous-wave-pumped, passively Q-switched solid-state lasers

We describe a technique for the adjustment and control of repetition rates in continuous-wave-pumped, passively Q-switched solid-state lasers. The method uses a movable intracavity lens to modify the mode matching and hence the gain of the laser beam per pass. The technique was applied to a diode-pumped Nd3+:YVO4 laser passively Q-switched with a Cr4+:YAG saturable absorber at 1064 nm. At a fixed pump power of 5.4 W, we were able to continuously adjust the repetition rate between 13.8 and 25 kHz by translating an intracavity converging lens. We also demonstrate that by adjusting the lens position, the repetition rate can be kept at a desired value as the pump power varies. In particular, as the pump power was increased from 3.95 to 5.9 W, the lens position was varied by 0.91 cm to keep the repetition rate constant at 13.8 kHz. Rate-equation formalism was used to investigate the variation of the repetition rate as a function of lens position, and very good agreement was obtained between experiment and theory.

High-power figure-of-eight fiber laser with passive sub-ring loops for repetition rate control

The operation of an erbium figure-of-eight all-fiber laser with a fundamental repetition rate of 5.68 MHz, peak power of 0.7 kW and pulse width of 420 fs is reported. Four times the fundamental repetition rate is achieved with two passive sub-ring loops at a repetition rate of 22.8 MHz. The sub-ring loop consists of only a single-mode fiber coupler with an input port and output port connected so as to make the time delay an odd multiple half-period of the input pulse period. This method is simple and inexpensive when compared with conventional methods employing sub-ring cavity or special fiber Bragg grating to control repetition rate.

High-resolution spectroscopy with a femtosecond laser frequency comb

The output of a mode-locked femtosecond laser is used for precision single-photon spectroscopy of 133Cs in an atomic beam. By changing the laser's repetition rate, the cesium D1 (6s 2S(1/2)-->6p 2P(1/2)) and D2 (6s 2S(1/2)-->6p 2P(3/2)) transitions are detected and the optical frequencies are measured with accuracy similar to that obtained with a cw laser. Control of the femtosecond laser repetition rate by use of the atomic fluorescence is also implemented, thus realizing a simple cesium optical clock.

Orthogonal control of the frequency comb dynamics of a mode-locked laser diode.

We have performed detailed studies on the dynamics of a frequency comb produced by a mode-locked laser diode (MLLD). Orthogonal control of the pulse repetition rate and the pulse-to-pulse carrier-envelope phase slippage is achieved by appropriate combinations of the respective error signals to actuate the diode injection current and the saturable absorber bias voltage. Phase coherence is established between the MLLD at 1550 nm and a 775-nm mode-locked Ti:sapphire laser working as part of an optical atomic clock.

TiO 2 thin films prepared by PLD for photocatalytic applications

Photocatalytic titanium oxide (TiO 2) thin films are synthesized by the Nd:YAG pulsed laser deposition (PLD) technique. The influence of laser repetition rate and substrate temperature on the crystalline structure is investigated by X-ray diffraction (XRD) measurement. Films deposited at a substrate temperature of 150 °C exhibit the anatase form, regardless of the repetition frequency. In films deposited at 300 °C, however, when the repetition rate is increased the crystal form changes from rutile to anatase. The change in crystal form is probably caused by a drop in the surface temperature of the growing film. The effect is very sensitive to repetition rate, which changes the deposition rate. The results suggest that control of the laser-pulse repetition rate in PLD is a very important factor for obtaining highly efficient photocatalytic TiO 2 thin films.

Short pulse generation and control in Er-doped frequency-shifted-feedback fibre lasers

Novel features of frequency-shifted-feedback (FSF) fibre lasers are demonstrated. Dual-wavelength-switching FSF Q-switching and picosecond pulse fibre laser is reported by exploiting bandwidth of erbium fibre amplifier for pulse trapping. Wavelength switching between the states that have the spectra stably trapped by edges of the gain spectrum was accomplished by modulating the RF power applied to the acousto-optic frequency shifter. Also, a FSF fibre laser is demonstrated that utilizes a Michelson interferometer for pulse repetition rate control. The laser enables obtaining 1.64 ps pulses with a repetition rate up to 150 GHz. Finally we report the generation of 620 fs full-width at half-maximum pulses from a Ti:sapphire pumped Er-doped FSF fibre laser by exploiting nonlinear polarization rotation effects. We believe that this correspond to the shortest pulse ever obtained with a FSF fibre laser.

Control system for ALID-7 linear accelerator used in radiation processing

The data acquisition and control system for the electron linear accelerator ALID-7 of 5.5 MeV and 0.7 kW, built in Romania, used on pilot-scale radiation processing, is discussed. The system provides: personnel and sensitive device protection against dangerous events; programmed interlocking and warning signals during accelerator operation; single electron pulses or electron pulse trains with small variations in pulse dose; control of electron pulse length and repetition rate, electron beam intensity, magnetron frequency, high voltage level on magnetron and electron-gun modulators, sweeping amplitude and frequency, conveyor velocity, irradiation time, and electron pulses number; simultaneous electron beam and microwave treatment. An important feature of the installation is an original control technique for obtaining programmed beam single shots and pulse trains with programmed pulse number, pulse repetition frequency, and pulse duration, from a diode gun linear accelerator, by discrete pulse temporal position modulation of the gun electron pulses and the magnetron microwave pulses. It is particularly useful for automatic control of absorbed dose rate level, irradiation process control as well as in pulse radiolysis studies, single pulse dose measurement or for research experiments where pulse-to-pulse reproducibility is required.