We report on an intense mid-infrared light source that provides femtosecond pulses on a microjoule energy level, broadly tunable in the 3–20-µm wavelength range with pulse durations as short as 50 fs at 5 µm. The pulses are generated by phase-matched difference-frequency mixing in GaSe of near-infrared signal and idler pulses of a parametric device based on a 1-kHz Ti:sapphire amplifier system. Pulse durations are characterized with different techniques including autocorrelation measurements in AgGaS2, two-photon absorption in InSb, and cross-correlation measurements with near-infrared pulses in a thin GaSe crystal. A subsequent zero-dispersion stretcher of high transmission allows for optimum pulse compression, a more detailed amplitude and phase characterization and, ultimately, amplitude shaping of the mid-infrared pulses.

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Generation, shaping, and characterization of intense femtosecond pulses tunable from 3 to 20 µm

We experimentally characterize the two-photon response of a GaAsP photodiode by use of a femtosecond Ti:sapphire laser tuned below the diode bandgap. The photodiode is shown to be highly suitable for real-time second-order autocorrelation measurements of pulses as short as 6fs in duration and with energies as small as a few picojoules.

Autocorrelation measurement of 6-fs pulses based on the two-photon-induced photocurrent in a GaAsP photodiode

By exploiting nonlinear optical effects, a technology of unprecedented flexibility for the production of tunable coherent light has been developed. Referred to as optical parametric generation, it provides sources with spectral coverage extending all the way from the ultraviolet to the mid-infrared, and with temporal coverage extending over all time domains from the femtosecond pulse to the continuous wave. Such sources generate coherent light of outstanding optical quality and are now finding wide-ranging applications.

https://science.sciencemag.org/content/sci/286/5444/1513.full.pdf

Parametric Generation of Tunable Light from Continuous-Wave to Femtosecond Pulses

We demonstrate supercontinuum generation in a photonic crystal fiber with all-normal group velocity dispersion. Pumping a short section of this fiber with compressed pulses from a compact amplified fiber laser generates a 200 nm bandwidth continuum with typical self-phase-modulation characteristics. We demonstrate that the supercontinuum is compressible to a duration of 26 fs. It therefore has a high degree of coherence between all the frequency components, and is a single pulse in the time domain. A smooth, flat spectrum spanning 800 nm is achieved using a longer piece of fiber.

/pdf/coherent-supercontinuum-generation-in-photonic-crystal-fiber-2lwwnxolio.pdf

Coherent supercontinuum generation in photonic crystal fiber with all-normal group velocity dispersion.

In less than fifteen years since the development of the first optical frequency comb (OFC), the device has revolutionized numerous research fields. In spectroscopy, the unique properties of the OFC spectrum enable simultaneous acquisition of broadband spectra while also providing high spectral resolution. Due to the regular structure of its spectrum, an OFC can be efficiently coupled to an optical enhancement cavity, resulting in vastly increased effective interaction length with the sample and absorption sensitivities as low as 1.3×10−11 cm−1 Hz−1/2 per spectral element. This technique, called cavity-enhanced direct frequency comb spectroscopy (CE-DFCS), provides ultra-sensitive absorption measurements simultaneously over a wide spectral range and with acquisition times shorter than a second.

Cavity-enhanced direct frequency comb spectroscopy

We present results showing that, when it is used as a photodetector, a light-emitting diode (LED) has a power-dependent response that can be used for sensitive detection and characterization of picosecond and femtosecond laser pulses. A characterization of a typical LED is presented at 800 nm, and we demonstrate how this effect can be used to construct an extremely compact novel autocorrelator based on a Wollaston prism.

/pdf/light-emitting-diodes-as-measurement-devices-for-femtosecond-2dkimdsae3.pdf

Light-emitting diodes as measurement devices for femtosecond laser pulses

We describe a femtosecond optical parametric oscillator based on periodically poled lithium niobate and pumped by a self-mode-locked Ti:sapphire laser. Signal and idler outputs almost continuously tunable from 975 nm to 4.55 μm were generated by a combination of grating tuning and cavity-length tuning, and an explanation of the tuning properties is given in terms of the gain bandwidth. A threshold of 45 mW was measured and, in the absence of optimized output coupling, signal powers of 90 mW and idler powers of 70 mW were obtained, with 140 mW of green light at 540 nm generated by phase-matched frequency doubling of the signal. Dispersion compensation produced near-transform-limited signal pulses of duration 140 fs. Observations regarding temperature tuning and pump depletion are also presented.

Femtosecond optical parametric oscillator based on periodically poled lithium niobate

We describe the design and operation of a critically phase-matched femtosecond optical parametric oscillator based on KTA. By employing a small pump-signal noncollinear angle, tuning of the idler to beyond 4 /spl mu/m is achieved using a Ti:sapphire pump laser. A Gaussian-beam model is described which can be used to identify the optimal noncollinear phase-matching geometry. Idler and signal pulses are characterized fully both temporally and spectrally and interferometric autocorrelation data at 3.5 /spl mu/m showing idler pulses of only eight optical cycles duration are presented.

Characterization and modeling of a noncollinearly phase-matched femtosecond optical parametric oscillator based on KTA and operating to beyond 4 /spl mu/m

We describe configurations of a novel synchronously pumped femtosecond optical parametric oscillator based on the crystal RbTiOAsO4 and operating with a signal-pulse-repetition frequency as high as 344  MHz. Average signal powers as high as 600  mW and pulse durations of 78  fs are demonstrated at a wavelength of 1.25  µm, and a characterization of the signal output using frequency-resolved optical gating implies asymmetric near-sech2t intensity-profile pulses with significant amounts of spectral cubic phase.

C. McGowan

Papers

Light-emitting diodes as measurement devices for femtosecond laser pulses

Femtosecond optical parametric oscillator based on periodically poled lithium niobate

Characterization and modeling of a noncollinearly phase-matched femtosecond optical parametric oscillator based on KTA and operating to beyond 4 /spl mu/m

Compact, efficient 344-MHz repetition-rate femtosecond optical parametric oscillator.

Femtosecond pulses tunable beyond 4 μm from a KTA-based optical parametric oscillator