Femtosecond operation and random quasi-phase-matched self-doubling of ceramic Cr:ZnSe laser

Abstract

A 100-fs Cr:ZnSe laser, based on a ceramic active medium, generates 100 mW at 215 MHz repetition rate around 2450 nm. Co-propagating second-harmonic pulse at 1225 nm is simultaneously generated due to random quasi-phase-matching process. 2010 Optical Society of America OCIS codes: 140.3070 Infrared and far-infrared lasers, 140.7090 Ultrafast lasersIn the last few years we evidence a growing interest to Cr 2+ -doped lasers of the II-VI family [1] operating in the "molecular fingerprint" range between 2 and 3.5 μm. The Cr:ZnSe laser has the broadest bandwidth ∆λ/λ among all known room-temperature solid-state active media [1]. The first femtosecond operation (80 fs) using SESAM has been reported in 2006 [2] and KLM mode-locking has been reported in June 2009 [3]. Ceramic Cr:ZnSe laser has been realized in the active mode-locked regime [4]. A special peculiarity of the ZnSe type crystals as active media is the presence of the second-order nonlinearity [5], which makes simultaneous generation of femtosecond pulses at the fundamental and second harmonic (SH) wavelengths possible. The intracavity SH generation from a Cr:ZnSSe laser in continuous-wave regime was observed in [6].In this work, we present a ceramic femtosecond Cr2:ZnSe laser, generating ultrashort pulses at the fundamental (2.45 μm) and the second harmonic wavelength (1.22 μm) simultaneously. This is the first laser of this type, opening up interesting application opportunities. Figure 1: Experimental set-up, mode-locked output spectrum at 2450 nm and a corresponding round-trip group-delay dispersion of the cavity. The "noise" features at the spectrum are due to the intracavity water vapour absorption lines [7,8].The experimental setup is shown in Fig. 1. For dispersion compensation, a single 3-mm thick sapphire plate was used, that was sufficient to compensate the 2-mm thick ceramic sample (Cr concentration 5·10 18 cm -3 ) from Photonics Innovations, Inc. The samples were edge-mounted without active cooling and absorbed ~65% of pump radiation. Reliable self-starting of mode-locking operation (i.e. restarting after interrupting the pump or intracavity beams) was observed at 1.8-2.1 W of pump power, although the laser could be modelocked in the 0.9 -2.2 W pump power range, delivering 45 to 120 mW of average output power at 215 MHz repetition rate. All measurements were performed at ambient atmosphere with relative humidity 40-50%. The autocorrelation (AC) was measured by a collinear autocorrelator using two-photon absorption in an InGaAs photodiode.

Femtosecond operation and self-doubling of Cr:ZnS laser

“…An interesting from the nonlinear-optical standpoint observation was made, when the single crystalline Cr:ZnS is replaced by a ceramic sample. Similarly to ceramic Cr:ZnSe [15] the autocorrelation trace then reveals that besides the main pulse at 2380 nm, the laser also emits radiation at second harmonic around 1190 nm (Fig. 3).…”

Mid-Infrared Femtosecond Lasers

“…molecular spectroscopy [10,11], frequency conversion by synchronously pumped optical parametric oscillator at 4.5-5.5 µm [12], and mid-IR supercontinuum generation in nonlinear fibers [13]. Some nonlinear-optical effects that we have observed are specific to this kind of lasers: intracavity second-harmonic conversion based on a random quasi-phase matching in the non-centrosymmetric host crystal [14], intracavity highresolution molecular spectroscopy with signal enhancement by 1-2 orders of magnitude [15,16].…”

47-fs Kerr-lens mode-locked Cr:ZnSe laser with high spectral purity

“…Table 1 summaries the best pulse train characteristics at different repetition frequencies. The combined effects of high-intracavity pulse intensity and random quasi-phase matching in the polycrystalline Cr:ZnSe active crystal results in a strong second harmonic generation (SHG) and further three-wave mixing processes, such as third and fourth harmonic generation and parametric sum and difference frequency generation [11,14,15]. The strongest SHG signal with an average power of 22 mW was measured at the output of the spherical chirped mirror, as indicated in Fig.…”