Balanced Cross-correlation Research Articles

Arrival time fluctuation of the SwissFEL photocathode laser: characterization by a single color balanced cross correlator.

The arrival time jitter and drift of the photocathode drive laser has an important impact on the performance of a Free-Electron-Laser (FEL). It adversely affects the beam energy jitter, bunch length jitter and bunch arrival time jitter, which becomes especially important for pump-probe experiments with femtosecond time resolution. To measure both parameters background free and stabilize the drift of the Yb:CaF2 based laser we use a well designed balanced optical cross correlator. In this paper we present our results using this device and focus particularly on the performance of the amplifier. We achieve a laser drift of less than 200 fs during 60 h, a 4.5 fs rms jitter of the amplifier relative to its seeding oscillator and 11 fs rms for the whole laser relative to a reference clock integrated from 2 mHz to 100 Hz.

Highly stabilized delay optical path with a long fiber toward absolute distance measurement

A stabilized delay optical path (DOP) toward absolute distance measurement is proposed based on the principle of optical balanced cross-correlation (OBCC). An OBCC signal is a subtraction from two-second harmonics generated by a periodically polarized crystal when the fundamental frequency pulses pass through periodically polarized crystal twice from the front and back. The steep slope of signal near the zero point is 67 mV nm−1 and is used to lock the fiber stretcher in the DOP by a servo controller. Thereby the purpose of stabilizing the DOP has been completed. The DOP is composed of a long fiber with two different applied lengths of ∼466 and ∼2000 m. The pulse numbers difference between delay and reference fibers are 227 and 978. The long-term repeatability of measurement deviations is 1.6 μm and 2.9 μm over 10 min, respectively. Therefore, this highly stabilized DOP can be considered as a stable device of the optical path.

Absolute distance measurement to coaxial multi-targets based on optical balanced cross-correlation using a single femtosecond laser.

We describe a high-precision ranging method based on an optical balanced cross-correlation system with a scanning repetition rate using a single femtosecond laser. By scanning the repetition rate of a laser, measuring pulses can be overlapped with reference pulses. It is an effective method to make reference pulses overlap with coaxial multiple target pulses without additional mechanical devices. The overlapped pulses are launched to the optical balanced cross-correlation system, which improves the time resolution measurement to the attosecond level. Two nominal distances are measured, and an additional commercial laser interferometer is used as a comparison to evaluate the accuracy of our measurement system. Moreover, the thickness of three stacked glasses is measured by our measurement system to verify that this system can measure coaxial multiple targets more quickly than conventional optical balanced cross-correlation systems using a single optical frequency comb.

Parallel determination of absolute distances to multiple targets by time-of-flight measurement using femtosecond light pulses.

Distances to multiple targets are measured simultaneously using a single femtosecond pulse laser split through a diffractive optical element. Pulse arrival from each target is detected by means of balanced cross-correlation of second harmonics generated using a PPKTP crystal. Time-of-flight of each returning pulse is counted by dual-comb interferometry with 0.01 ps timing resolution at a 2 kHz update rate. This multi-target ranging capability is demonstrated by performing multi-degree of freedom (m-DOF) sensing of a rigid-body motion simulating a satellite operating in orbit. This method is applicable to diverse terrestrial and space applications requiring concurrent multiple distance measurements with high precision.

Reduction of timing jitter and intensity noise in normal-dispersion passively mode-locked fiber lasers by narrow band-pass filtering.

Fiber lasers mode-locked with normal cavity dispersion have recently attracted great attention due to large output pulse energy and femtosecond pulse duration. Here we accurately characterized the timing jitter of normal-dispersion fiber lasers using a balanced cross-correlation method. The timing jitter characterization experiments show that the timing jitter of normal-dispersion mode-locked fiber lasers can be significantly reduced by using narrow band-pass filtering (e.g., 7-nm bandwidth filtering in this work). We further identify that the timing jitter of the fiber laser is confined in a limited range, which is almost independent of cavity dispersion map due to the amplifier-similariton formation by insertion of the narrow bandpass filter. The lowest observed timing jitter reaches 0.57 fs (rms) integrated from 10 kHz to 10 MHz Fourier frequency. The rms relative intensity noise (RIN) is also reduced from 0.37% to 0.02% (integrated from 1 kHz to 5 MHz Fourier frequency) by the insertion of narrow band-pass filter.

One-femtosecond, long-term stable remote laser synchronization over a 3.5-km fiber link.

Long-term stable timing distribution over a 3.5-km polarization maintaining (PM) fiber link using balanced optical cross-correlators (BOC) for optical-to-optical synchronization is demonstrated. Remote laser synchronization over 40 hours showed a residual timing jitter and drift of 2.5 fs for the whole locking period and only 1.1 fs integrated from 100 μHz to 1 MHz. This result corresponds to the lowest jitter and drift achieved to date for a multi-km fiber link and remote timing synchronization operating continuously over multiple days.

Long-term stable, sub-femtosecond timing distribution via a 12-km polarization-maintaining fiber link: approaching 10^−21 link stability

Long-term stable, sub-femtosecond timing distribution over a 1.2-km polarization-maintaining (PM) fiber-optic link using balanced optical cross-correlators for link stabilization is demonstrated. Novel dispersion-compensating PM fiber was developed to construct a dispersion-slope-compensated PM link, which eliminated slow timing drifts and jumps previously induced by polarization mode dispersion in standard single-mode fiber. Numerical simulations of nonlinear pulse propagation in the fiber link confirmed potential sub-100-as timing stability for pulse energies below 70 pJ. Link operation for 16 days showed ~0.6 fs RMS timing drift and during a 3-day interval only ~0.13 fs drift, which corresponds to a stability level of 10(-21).

Coherent Synthesis of ultra-broadband Optical Parametric Amplifiers

We report on coherent synthesis of outputs from two ultra-broadband optical parametric amplifiers; their timing is locked to sub-30-as by a balanced cross-correlator. Synthesised pulses have octave-spanning (500-1000 nm) spectra and nearly single-cycle 3.8-fs duration.

Long range absolute distance measurement based on femtosecond laser balanced optical cross-correlation

High-precision distance measurement in a long range is critical for many advanced applications, such as satellite formation flying, free space optical communication and large scale machining. A 52 m absolute distance measurement in free space based on time of flight of femtosecond laser is demonstrated. The timing offset between target-reflected and the reference pulses is precisely characterized by balanced optical cross-correlation method. The balanced cross correlation signal is used for the feedback control of the cavity length and tightly locks the distance under test to multiple of pulse separation. As a result, the time of flight of the target-reflected pulse is determined by the repetition rate of the femtosecond laser, which effectively avoids the loss of timing resolution caused by direct access of pulse time-of-flight from photo-detection. In the experiment, a Yb -doped high repetition rate mode-locked fiber laser working at 1.04 μm is used as a femtosecond laser source, and a measurement precision of 12 nm is achieved in an average time of one second.

Attosecond-resolution timing jitter characterization of free-running mode-locked lasers

Timing jitter characterization of optical pulse trains from free-running mode-locked lasers with attosecond resolution is demonstrated using balanced optical cross correlation in the timing detector and the timing delay configurations. In the timing detector configuration, the balanced cross correlation between two mode-locked lasers synchronized by a low-bandwidth phase-locked loop is used to measure the timing jitter spectral density outside the locking bandwidth. In addition, the timing delay configuration using a 325 m long timing-stabilized fiber link enables the characterization of timing jitter faster than the delay time. The limitation set by shot noise in this configuration is 2.2 x 10(-8) fs(2)/Hz corresponding to 470 as in 10 MHz bandwidth.

Long-term femtosecond timing link stabilization using a single-crystal balanced cross correlator

We demonstrate a self-aligned balanced cross correlator based on a single type-II phase-matched periodically poled KTiOPO4 crystal. The birefringence of the crystal generates a walk-off between the two orthogonally polarized pulses. This enables the balancing of the cross correlator with input pulses at the same center wavelength. As a first application of this single-crystal balanced cross correlator, we stabilized a 310 m long optical fiber link for timing distribution with long-term stable 10 fs precision.

Jitter reduction of two synchronized picosecond mode-locked lasers using balanced cross-correlator with two-photon detectors

The authors have developed a highly synchronized picosecond mode-locked laser system. A balanced cross-correlator using two-photon detectors was employed to observe femtosecond order timing jitter between two picosecond lasers (1.26fs with 150Hz bandwidth and 7.14fs with 1kHz bandwidth), and a signal from the correlator was used as a feedback control signal to reduce the timing jitter. The timing jitter between the two lasers was reduced to 8fs through a low-pass filter with 150Hz bandwidth.

Attosecond active synchronization of passively mode-locked lasers by balanced cross correlation.

A balanced cross correlator, the optical equivalent of a balanced microwave phase detector, is demonstrated. Its use in synchronizing an octave-spanning Ti:sapphire laser and a 30-fs Cr:forsterite laser yields 300-attosecond timing jitter measured from 10 mHz to 2.3 MHz. The spectral overlap between the two lasers is strong enough to permit direct detection of the difference in carrier-envelope offset frequency between the two lasers.