Generation of two-color femtosecond pulses by self-synchronizing Ti:sapphire and Cr:forsterite lasers

Ultra-low timing jitter, Ti:Al2O3 synchronization for stimulated Raman scattering and pump-probe microscopy

SIGNIFICANCE

Stimulated Raman scattering (SRS) and pump-probe microscopy are implementations of multiphoton microscopy that acquire high-resolution, label-free images of live samples encoded with molecular contrast. Most commercial multiphoton microscopes cannot access these techniques since they require sample illumination by two temporally synchronized ultrafast pulse trains. We present a compact and robust way of synchronizing an additional Ti:sapphire laser with a conventional single-beam multiphoton microscope to realize an instrument that can acquire images with enhanced molecular specificity.

AIM

A passive optical synchronization scheme for a pair of commercially available, unmodified modelocked Ti:sapphire lasers was developed. The suitability of this synchronization scheme for advanced biomedical microscopy was investigated.

APPROACH

A pair of modelocked Ti:sapphire lasers were aligned in master-slave configuration. Five percent of the master laser output was used to seed the modelocking in the slave laser cavity. The timing jitter of the master and slave pulse trains was characterized using an optical autocorrelator. The synchronized output of both lasers was coupled into a laser scanning microscope and used to acquire spectral focusing SRS and pump-probe microscopy images from biological and nonbiological samples.

RESULTS

A timing jitter between the modelocked pulse trains of 0.74 fs was recorded. Spectral focusing SRS allowed spectral discrimination of polystyrene and polymethyl methacrylate beads. Pump-probe microscopy was used to record excited state lifetime curves from hemoglobin in intact red blood cells.

CONCLUSION

Our work demonstrates a simple and robust method of upgrading single-beam multiphoton microscopes with an additional ultrafast laser. The resulting dual-beam instrument can be used to acquire label-free images of sample structure and composition with high biochemical specificity.

High-contrast, fast chemical imaging by coherent Raman scattering using a self-synchronized two-colour fibre laser

Coherent Raman scattering (CRS) microscopy is widely recognized as a powerful tool for tackling biomedical problems based on its chemically specific label-free contrast, high spatial and spectral resolution, and high sensitivity. However, the clinical translation of CRS imaging technologies has long been hindered by traditional solid-state lasers with environmentally sensitive operations and large footprints. Ultrafast fibre lasers can potentially overcome these shortcomings but have not yet been fully exploited for CRS imaging, as previous implementations have suffered from high intensity noise, a narrow tuning range and low power, resulting in low image qualities and slow imaging speeds. Here, we present a novel high-power self-synchronized two-colour pulsed fibre laser that achieves excellent performance in terms of intensity stability (improved by 50 dB), timing jitter (24.3 fs), average power fluctuation (<0.5%), modulation depth (>20 dB) and pulse width variation (<1.8%) over an extended wavenumber range (2700-3550 cm-1). The versatility of the laser source enables, for the first time, high-contrast, fast CRS imaging without complicated noise reduction via balanced detection schemes. These capabilities are demonstrated in this work by imaging a wide range of species such as living human cells and mouse arterial tissues and performing multimodal nonlinear imaging of mouse tail, kidney and brain tissue sections by utilizing second-harmonic generation and two-photon excited fluorescence, which provides multiple optical contrast mechanisms simultaneously and maximizes the gathered information content for biological visualization and medical diagnosis. This work also establishes a general scenario for remodelling existing lasers into synchronized two-colour lasers and thus promotes a wider popularization and application of CRS imaging technologies.

Wavelength-spacing controllable, dual-wavelength synchronously mode locked Er:fiber laser oscillator based on dual-branch nonlinear polarization rotation technique

A wavelength-spacing controllable, dual-wavelength synchronously mode locked Er:fiber laser oscillator based on dual-branch nonlinear polarization rotation (NPR) technique was presented. The center wavelengths were at 1542 nm and 1561 nm, which had pulse durations of 1.38 ps and 1.70 ps, respectively. Experimentally, the synchronous mode locking was achieved by precisely adjusting the cavity length of one branch. A tolerance in the cavity length mismatch of 0.46 mm for synchronous mode locking was demonstrated. The frequency difference of the two pulse trains was measured to be less than 1 mHz. Additionally, this synchronously mode locked dual-wavelength laser had a wavelength tunable range of about 5.6 nm, and a controllable wavelength spacing from 10.5 nm to 28.2 nm, corresponding to a tunable frequency difference from 1.32 THz to 3.26 THz. To the best of our knowledge, this is the first demonstration of synchronously mode locked dual-wavelength output directly from a Er:fiber laser oscillator, using dual-branch NPR technique.

Dissipative soliton and synchronously dual-wavelength mode-locking Yb:YSO lasers

We experimentally demonstrate the dissipative soliton mode-locking operation of a Yb:YSO laser by using an all-normal dispersion cavity. Strongly chirped pulses are obtained with pulse duration of 9.3 ps at a repetition rate of 113.4 MHz. The central wavelength is 1082 nm with 3.1 nm FWHM bandwidth. A dual-wavelength synchronously mode-locking operation at central wavelengths of 1059.2 nm and 1082.2 nm is also reported. Stable mode-locked pulses are achieved with pulse duration of 10 ps and total average output power of 164 mW. Periodic ultrashort beat pulses with pulse duration of 169 fs at an ultrahigh repetition rate of 1.4 THz can be distinctly observed from the measured autocorrelation trace. To our knowledge, this is the first demonstration of dual-wavelength synchronously mode-locking operation from a Yb:YSO laser.

Generation of single-cycle mid-infrared pulses via coherent synthesis

A new approach for the generation of single-cycle mid-infrared pulses without complicated control systems is proposed, which is based on direct coherent synthesis of two idlers generated by difference frequency generation (DFG) processes. It is found that the waveform of synthesized pulses is mainly determined by the spectra superposition, the carrier-envelope phase (CEP) difference, the relative timing and the chirp ratio between the idlers. The influences of these parameters on the synthesized waveform are also numerically calculated and analyzed via second-order autocorrelation, which offers general guidelines for the waveform optimization. The single-cycle synthesized mid-infrared pulses, which are centered at 4233 nm with the spectrum spanning from 3000 nm to 7000 nm, are achieved by carefully optimizing these parameters. The single-cycle mid-infrared laser source presents the possibility of investigating and controlling the strong field light-matter interaction.

Dual-wavelength passively mode-locked Nd:LuYSiO5 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%.

Attosecond relative timing jitter and 13 fs tunable pulses from a two-branch Er:fiber laser

We present what is believed to be the first direct measurement of the relative timing jitter between the two parallel pulse trains of a two-branch femtosecond erbium-doped fiber laser, operated without active stabilization. The system provides independently tunable pulses in the near infrared with durations down to 13 fs. Using an interferometric optical cross-correlator, the phase-noise spectral density is measured with high sensitivity in a range from 1 Hz up to the Nyquist frequency of 24.5 MHz. We find an integrated jitter of 11 attoseconds directly after the amplifier stages and 43 as after propagation through free-space optics and nonlinear fibers for frequency conversion.

Ultralow-jitter passive timing stabilization of a mode-locked Er-doped fiber laser by injection of an optical pulse train

The pulse timing of a mode-locked Er-doped fiber laser was stabilized to a reference pulse train from a Cr:forsterite mode-locked laser by all-optical passive synchronization scheme. The reference pulses were injected into a ring cavity of the fiber laser by using a 1.3-1.5 mum wavelength-division multiplexer. The spectral shift induced by cross-phase modulation between copropagating two-color pulses realizes self-synchronization due to intracavity group-delay dispersion. The rms integration of timing jitter between the fiber laser pulse and the reference pulse was 3.7 fs in a Fourier frequency range from 1 Hz to 100 kHz.

Independently tunable 1.3 W femtosecond Ti:sapphire lasers passively synchronized with attosecond timing jitter and ultrahigh robustness

A stable passively synchronized femtosecond laser has been realized by coupling two 1.3 W mode-locked Ti:sapphire lasers with a Kerr medium. An ultralong tolerance of 10 microm for the cavity length mismatch and a timing jitter of less than 0.4 fs were obtained. The relative carrier-envelope phase slip was directly observed by measuring the heterodyne output between the two lasers.

100-attosecond timing jitter between two-color mode-locked lasers by active-passive hybrid synchronization

We have demonstrated a reduction of the timing jitter between passively synchronized Ti:sapphire and Cr:forsterite mode-locked lasers into a 100-attosecond (as) regime by suppressing slow fluctuations with the use of active slow-bandwidth extracavity feedback. This active-passive hybrid synchronization scheme permits the achievement of timing jitters of 98 +/- 18 as at a bandwidth of 100 kHz and of 126 +/- 20 as at a bandwidth of 1 MHz for as long as 100 s.

Passively synchronized erbium (1550-nm) and ytterbium (1040-nm) mode-locked fiber lasers sharing a cavity

Erbium and ytterbium fiber lasers were firmly synchronized by nonlinear interaction in active media placed in the same cavity. A two-color femtosecond-picosecond pulse train at largely separate wavelengths of 1.55 and 1.04 microm was generated. Optimizing the laser cavity to enhance the cross-phase modulation in the gain materials has yielded a large mismatch of 20 microm between the two laser cavities.

All-optical phase locking of two femtosecond Ti:sapphire lasers: a passive coupling mechanism beyond the slowly varying amplitude approximation

Two independently tunable femtosecond Ti:sapphire lasers are passively synchronized with a stable relative carrier-envelope offset phase. By heterodyning the spectral overlap of the two frequency combs, we observe multiple regimes for the cavity length difference in which the relative round-trip phase slip is effectively locked to zero. The strong correlation of the femtosecond pulse trains is maintained over minutes without any external stabilization, and relative cavity length variations of 50 nm are compensated. The phase synchronization relies on phase-dependent cross-phase modulation, taking full advantage of the nonresonant optical nonlinearity of the shared gain medium, which is much faster than the optical cycle.

Self-stabilization of carrier-envelope offset phase by use of difference-frequency generation

Self-stabilized carrier-envelope offset phase is achieved by use of difference-frequency (DF) generation. The spectrum from a Ti:sapphire oscillator is broadened in a photonic crystal fiber, and a DF (900 nm) between the blue component (490 nm) and the infrared component (1080 nm) is generated. The beat signal between the fundamental and the DF signal is clearly observed. The wavelength of the DF signal can be tuned down to 780 nm, and hence the signal can be used for injection seeding of a Ti:sapphire oscillator.

Broadband phase-coherent optical frequency synthesis with actively linked Ti:sapphire and Cr:forsterite femtosecond lasers

We link the output spectra of a Ti:sapphire and a Cr:forsterite femtosecond laser phase coherently to form a continuous frequency comb with a wavelength coverage of 0.57-1.45 microm at power levels of 1 nW to 40 microW per frequency mode. To achieve this, the laser repetition rates and the carrier-envelope offset frequencies are phase locked to each other. The coherence time between the individual components of the two combs is 40 micros. The timing jitter between the lasers is 20 fs. The combined frequency comb is self-referenced for access to its overall offset frequency. We report the first demonstration to our knowledge of an extremely broadband and continuous, high-powered and phase-coherent frequency comb from two femtosecond lasers with different gain media.

Optical phase locking among femtosecond subharmonic pulses

We stabilized the relative carrier-envelope phase slip among the pump pulse and its subharmonic signal and idler pulses in a femtosecond optical parametric oscillator, resulting in long-term phasecoherence among the pulses. The stabilized beat signal corresponding to the relative carrier-envelope phase slip among subharmonic pulses had an accumulated phase error of 0.24 rad in the 1-mHz-1-MHz region. The fluctuation of the beat frequency measured by a 1-s-averaged counter was less than 1 mHz in a 1480-s measurement.

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.

Control of relative carrier-envelope phase slip in femtosecond Ti:sapphire and Cr:forsterite lasers

We were able to control relative carrier-envelope phase slip among mode-locked Ti:sapphire and Cr:forsterite lasers by employing electronic feedback. The pulse timings of these lasers were passively synchronized with our crossing-beam technique. Since the optical-frequency ratio of Ti:sapphire and Cr:forsterite is approximately 3:2, we can observe the phase relation by superimposing the third harmonic of Cr:forsterite and the second harmonic of Ti:sapphire lasers in time and in space. The spectrum width of the locked beat note was less than 3 kHz, which corresponds to the controlled fluctuation of a cavity-length difference of less than 10 pm.

Chromium-doped forsterite: dispersion measurement with white-light interferometry

Using a Michelson white-light interferometer, we measure the group-delay dispersion and third-order dispersion coefficients, d2(phi)/d(omega)2 and d3(phi)/d(omega)3, of chromium-doped forsterite (Cr:Mg2SiO4) over wavelengths of 1050-1600 nm for light polarized along both the c and b crystal axes. In this interval, the second-order dispersion for the c axis ranges from 35 fs2/mm to -14 fs2/mm, and the third-order dispersion ranges from 36 fs3/mm to 142 fs3/mm. For the b axis the second-order dispersion ranges from 35 fs2/mm to -15 fs2/mm and the third-order from 73 fs3/mm to 185 fs3/mm. Our data are relevant for the development of optimized dispersion compensation tools for Cr:Mg2SiO4 femtosecond lasers. These measurements help to clarify previously published results and show some significant discrepancies that existed, especially in the third-order dispersion. Our results should furthermore be useful to build up an analytic expression for the index of refraction of chromium forsterite.

Relative carrier-envelope phase dynamics between passively synchronized Ti:sapphire and Cr:forsterite lasers

We observed and measured the relative carrier-envelope phase difference per round trip between synchronized femtosecond Ti:sapphire and Cr:forsterite mode-locked lasers. The relative carrier-envelope phase slip was directly recorded by heterodyning of the Cr:forsterite laser with the supercontinuum from the Ti:sapphire laser. We also obtained another phase relation by superimposing the third harmonic of the Cr:forsterite laser with the second harmonic of the Ti:sapphire laser. In the latter case we obtained a stable beat signal with a signal-to-noise ratio larger than 30 dB and found a dependence of the beat frequency on the cavity length.

Passive synchronization of two independent laser oscillators with a Fabry-Perot modulator

By optical modulation of the reflectivity of an intracavity nonlinear Fabry-Perot semiconductor mirror, the pulse train from a passively mode-locked picosecond Nd:YVO(4) laser oscillator is synchronized to an independent femtosecond-mode-locked Ti:sapphire laser. We obtain stable synchronized pulse trains at central wavelengths of 1064 and 850 nm, and the Ti:sapphire laser is still independently tunable over a large wavelength range. The tolerable cavity-length difference between the two laser oscillators exceeds 20mu;m .