Nanosecond optical parametric oscillator with midinfrared intracavity difference-frequency mixing in orientation-patterned GaAs.

We report on efficient midinfrared difference-frequency generation (DFG) in orientation-patterned GaAs by intracavity mixing the signal and idler pulses of a narrowband nanosecond optical parametric oscillator based on periodically poled LiNbO3. The maximum average DFG output power reached 215 mW at 8.15 µm for a repetition rate of 35 kHz. The temperature tuning range spanned over 8026-8710 nm. The maximum overall conversion efficiency from 1 to 8 µm amounted to ∼1.3%.

Shot-noise limited tunable dual-vibrational frequency stimulated Raman scattering microscopy.

We present a shot-noise limited SRS implementation providing a >200 mW per excitation wavelength that is optimized for addressing two molecular vibrations simultaneously. As the key to producing a 3 ps laser of different colors out of a single fs-laser (15 nm FWHM), we use ultra-steep angle-tunable optical filters to extract 2 narrow-band Stokes laser beams (1-2 nm & 1-2 ps), which are separated by 100 cm-1. The center part of the fs-laser is frequency doubled to pump an optical parametric oscillator (OPO). The temporal width of the OPO's output (1 ps) is matched to the Stokes beams and can be tuned from 650-980 nm to address simultaneously two Raman shifts separated by 100 cm-1 that are located between 500 cm-1 and 5000 cm-1. We demonstrate background-free SRS imaging of C-D labeled biological samples (bacteria and Drosophila). Furthermore, high quality virtual stimulated Raman histology imaging of a brain adenocarcinoma is shown for pixel dwell times of 16 µs.

Simultaneous stimulated Raman gain and loss detection (SRGAL).

The fidelity of stimulated Raman scattering (SRS) microscopy images is impaired by artifacts such as thermal lensing, cross-phase modulation and multi-photon absorption. These artifacts affect differently the stimulated Raman loss (SRL) and stimulated Raman gain (SRG) channels making SRL and SRG image comparisons attractive to identify and correct SRS image artifacts. To provide answer to the question: "Can I trust my SRS images?", we designed a novel, but straightforward SRS scheme that enables the dectection of the stimulated Raman gain and loss (SRGAL) simultaneously at the same pixel level. As an advantage over the conventional SRS imaging scheme, SRGAL doubles the SRS signal by acquiring both SRL as well as SRG and allows for the identification of SRS artifacts and their reduction via a balanced summation of the SRL and SRG images.

Background-suppressed SRS fingerprint imaging with a fully integrated system using a single optical parametric oscillator.

Stimulated Raman Scattering (SRS) imaging can be hampered by non-resonant parasitic signals that lead to imaging artifacts and eventually overwhelm the Raman signal of interest. Stimulated Raman gain opposite loss detection (SRGOLD) is a three-beam excitation scheme capable of suppressing this nonlinear background while enhancing the resonant Raman signal. We present here a compact electro-optical system for SRGOLD excitation which conveniently exploits the idler beam generated by an optical parametric oscillator (OPO). We demonstrate its successful application for background suppressed SRS imaging in the fingerprint region. This system constitutes a simple and valuable add-on for standard coherent Raman laser sources since it enables flexible excitation and background suppression in SRS imaging.

Narrow-band periodically poled lithium niobate nonresonant optical parametric oscillator.

We report on a narrowband, nonresonant periodically poled lithium niobate (PPLN) optical parametric oscillator using a volume Bragg grating (VBG) as the spectral narrowing element. Pumping by a Nd:YVO4 laser at 1.06 µm, a maximum output power of 4.75 W is achieved at a repetition rate of 20 kHz for a conversion efficiency of 47.5%. Both signal and idler spectra are narrowed to less than 2 nm, at good beam quality and stability.

Single-step sub-200 fs mid-infrared generation from an optical parametric oscillator synchronously pumped by an erbium fiber laser.

We demonstrate the single-step generation of mid-infrared femtosecond laser pulses in a AgGaSe2 optical parametric oscillator that is synchronously pumped by a 100 MHz repetition rate sub-90 fs erbium fiber laser. The tuning range of the idler beam in principle covers ∼3.5 to 17 µm, only dependent on the choice of cavity and mirror design. As an example, we experimentally demonstrate idler pulse generation from 4.8 to 6.0 µm optimized for selective vibrational resonant molecular spectroscopy. We find an oscillation threshold as low as 150 mW of pump power. At 300 mW pump power and a central wavelength of ∼5.0 µm, we achieve an average infrared power of up to 17.5 mW, with a photon conversion efficiency of ∼18%. A pulse duration of ∼180 fs is determined from a nonlinear cross-correlation with residual pump light. The single-step nonlinear conversion leads to a high power stability with <1% average power drift at <0.5% rms noise over 1 h.

Difference-frequency generation of ultrashort pulses in the mid-IR using Yb-fiber pump systems and AgGaSe(2).

We employ AgGaSe(2) for difference-frequency generation between signal and idler of synchronously-pumped picosecond / femtosecond OPOs at 80 / 53 MHz. Continuous tuning in the picosecond regime is achieved from 5 to 18 µm with average power of 140 mW at 6 µm. In the femtosecond regime the tunability extends from 5 to 17 µm with average power of 69 mW at 6 µm. Maximum single pulse energies of >1 nJ in both cases represent the highest values at such high repetition rates.

Difference-frequency generation of fs and ps mid-IR pulses in LiInSe(2) based on Yb-fiber laser pump sources.

Difference-frequency generation between signal and idler of Yb-fiber laser synchronously pumped femtosecond and picosecond optical parametric oscillators (SPOPOs) operating at 53 and 80 MHz, respectively, is investigated in the wide bandgap chalcogenide crystal LiInSe(2). Single pulse energies in excess of 1 nJ are demonstrated, and the continuous tuning extends from 5 to 12 µm.