Tunable Source Produces Microsecond Pulses in the Mid-IR

Breck Hitz

A collaboration among scientists at Nederlands Centrum voor Laser Research bv and the University of Twente, both in Enschede, the Netherlands, and at Laser Zentrum Hannover eV in Germany has resulted in what they believe is the first optical parametric oscillator (OPO) pumped with microsecond pulses from a wavelength-tunable, Q-switched solid-state laser. The laser will have important applications in precision spectroscopy and other applications.

OPOs pumped by solid-state lasers in the 1- to 2-µm region are efficient sources in the 2- to 5-µm spectral region for applications ranging from molecular chemistry and spectroscopy to military countermeasures. But short-pulse (~1 ns or less) OPOs have a large Fourier-limited bandwidth that makes them unsuitable for ultraprecise applications such as the spectroscopic detection of molecular trace gases. To satisfy the narrow-bandwidth requirements of these applications, several research groups around the world have been investigating OPOs pumped by long-pulse, Q-switched lasers. The German/Dutch collaboration improves on previous work by pumping the OPO with a wavelength-tunable laser, significantly enhancing the tunability of the OPO.

The bow-tie, ring-resonator OPO is pumped by a fiber laser Q-switched with an acousto-optic modulator (Figure 1). Thirty meters of double-clad, Yb-doped fiber, pumped with up to 13 W from a 976-nm diode laser, provide the fiber laser's gain medium. An acousto-optic Q-switch operates on the first-order diffraction to ensure adequate modulation depth for stable Q-switching. An acousto-optic tunable filter is the intracavity wavelength-selecting element, and intracavity polarizing optics allow the output coupling to be varied. The ~1.1-µm, 25-kHz pulses emerging from the fiber laser have an average power of approximately 3.6 W and a duration of 2.1 µs. They can be tuned over 19 nm without significant change in pulse duration by adjusting the frequency of the signal applied to the acousto-optic tunable filter.

Figure 1. The long pulses from the Q-switched fiber laser generate narrow-bandwidth, tunable mid-IR idler pulses in the optical parametric oscillator.

The OPO is resonant only at the signal wavelength of ~1.5 µm and is based on a periodically poled lithium-niobate crystal. The 40-mm crystal has four domain gratings with periods of 29.25, 29.5, 29.75 and 30.0 µm. When pumped at 3.6 W from the fiber laser, the OPO produces up to 560 mW of output at the idler wavelength of 3.36 µm. The OPO wavelength can be tuned by adjusting the niobate crystal's temperature while changing the fiber laser's frequency. By varying the crystal temperature from 115 to 215 °C, the researchers caused the tuning ranges of the different gratings to overlap so that the idler could be tuned from 3145 to 3689 nm and the signal from 1518 to 1634 nm.

They also were able to tune the OPO output from 3257 to 3452 nm by adjusting the drive frequency applied to the fiber laser's acousto-optic tunable filter. They tuned the fiber laser over 19 nm, but held the niobate temperature constant (181 °C) and kept the quasi-phase matching in one grating (29.75 µm) while the OPO's idler was tuned over the entire 195-nm range. The corresponding signal wavelength varied only by 2.6 nm (Figure 2).

Figure 2. The solid lines are the theoretical tuning curves for the optical parametric oscillator, based on published Sellmeier equations, and the squares are experimental data. By tuning the pump laser while maintaining constant quasi-phase-matching conditions in the LiNbO₃crystal, the researchers observed only 2.6 nm of variation in the signal wavelength, while the idler tuned across 195 nm.

The measured spectral bandwidth of the OPO's signal was ~50 GHz, but by placing an etalon inside the OPO, the scientists reduced the signal bandwidth to less than their instrumental resolution of 4 GHz. The corresponding bandwidth of the output idler was that of the fiber laser, approximately 130 to 260 GHz, and the output power at the idler wavelength was modestly reduced by the etalon to 430 mW. Because this bandwidth is orders of magnitude greater than the Fourier limit for the 2-³s pulses, the researchers believe that further bandwidth reduction should be possible by reducing the bandwidth of the fiber laser.