G. Mak

High-power, 62-fs infrared optical parametric oscillator synchronously pumped by a 76-MHz Ti:sapphire laser

We demonstrate a producing KTiOPO4-based optical parametric oscillator with external-pulse compression, 62-fs pulses at 76 MHz and 175-mW average power (37-kW peak power). The oscillator is pumped synchronously with 800-mW, 110-fs pulses from a Kerr lens mode-locked Ti:Al2O3 laser operating at λ = 765 nm. The oscillator cavity length is actively stabilized, and the total amplitude noise is <1% (rms). With a single mirror set, the signal-beam wavelength is tunable between 1.20 and 1.34 μm. The idler beam tunes from 2.1 to 1.78 μm with an expected (uncompressed) pulse width of <400 fs and ~100-mW average power. Tuning throughout the bandwidth from 0.9 to 4.5 μm should be possible with additional mirror sets.

Externally pumped high repetition rate femtosecond infrared optical parametric oscillator

The first externally pumped high repetition rate optical parametric oscillator (OPO) which produces tunable femtosecond pulses in the near‐infrared is demonstrated. The singly resonant oscillator with a KTiOPO4 (KTP) nonlinear crystal is synchronously pumped by 150‐fs pulses at 645 nm from a mode‐locked dye laser. With a pump power of 280 mW, stable 220‐fs pulses at a 76‐MHz repetition rate, tunable from 1.20 to 1.34 μm with a single set of mirrors, are achieved. The internal power conversion efficiency is 13% and the average infrared power is as large as 30 mW, for the signal beam only. Femtosecond pulses tunable over the wavelength range from 0.9–4.5 μm (with KTP) using multiple mirror sets should be possible. This range includes the technologically important regime from 1.0–1.6 μm.

Subpicosecond, resonant refractive index changes in germanium near 1.5 μm

The refractive index changes have been time resolved in crystalline Ge near the direct‐band‐gap (0.8 eV, 1.55 μm) following femtosecond pulse excitation at room temperature. Measurements were made for 1.485<λ<1.55 μm using degenerate pump‐probe transmission and reflection spectroscopy with 120 fs pulses from an optical parametric oscillator. Resonant excitation leads to large refractive index changes (≳10−3 for 1017 cm−3 carrier density) dominated by band filling and carrier screening effects. Unlike the much slower recovery observed in direct gap semiconductors, the refractive index change disappears with a time constant of 230 fs due to intervalley scattering of electrons to the lower energy L valley in this indirect‐gap semiconductor.

Femtosecond spectroscopy at the direct bandedge of germanium

We time-resolve the transient, photoinduced transmission and reflectivity changes near the direct bandedge of crystalline germanium (at 295 K) with 120-fs, tunable, infrared pulses from a 76-MHz optical parametric oscillator. The data show a rich collection of phenomena including bandgap renormalization, carrier-carrier scattering, carrier cooling and (Gamma) yields L intervalley scattering. An apparent carrier density dependence in the measured intervalley scattering time is attributed to plasma screening of the Coulomb enhanced continuum absorption.

Mid- to far-infrared femtosecond pulses

The emergence of new, efficient, nonlinear crystals and the development of passively mode- locked, solid state lasers, such as the Kerr-lens mode-locked Ti:sapphire laser, has rekindled interest in optical parametric oscillators (OPOs) while opening up new regions of the spectrum to high repetition rate, high average power femtosecond pulses. With synchronous pumping techniques it is now possible to generate pulses as short as 40 fs for (nominally) 1 < (lambda) < 4 micrometers , at repetition rates of 108 Hz, and with average powers measured in 100s of mW. Tuning can be achieved in critically or non-critically phase-matched KTP or LBO via crystal angle, temperature or pump wavelength tuning each of which has its merits in terms of tuning range, ease of use, noise properties, etc.

Tunable ultrashort-pulse generation and amplification using optical parametric techniques

We demonstrate femtosecond pulses from 400 - 1600 nm, both at 76 MHz at high average power and at 1 kHz repetition rate at high pulse energy, using optical parametric techniques. The signal wavelength of the KTiOPO4 (KTP)-based optical parametric oscillator is tunable in the 1.20 - 1.34 micrometers and 1.45 - 1.70 micrometers regions with two mirror sets. This oscillator is synchronously pumped and has a repetition rate of 76 MHz and a signal output power as high as 200 mW. In addition we have used amplified Ti:sapphire femtosecond pulses at 1 kHz repetition rate to generate a white light continuum which extends at least from 400 nm to 1.6 micrometers . Frequency mixing techniques are used to up-convert the continuum to the ultraviolet (UV) region or to amplify the continuum in the infrared (IR) region with LiB3O5(LBO) crystals.