Widely frequency tunable, ultrafast laser sources are utilized in applications such as coherent anti-Stokes Raman spectroscopy (CARS), multiphoton microscopy, frequency combs and terahertz generation. Moreover, they are a unique opportunity to reach wavelength ranges which are not accessible by typical laser gain media due to their utilization of real energy levels for amplification.

Unfortunately, these energy levels are determined by the crystal material and cannot be adapted to emit different wavelengths. One solution is to utilize an optical parametric oscillator. Noncollinear phase matching allows for tuning over an ultra-broadband frequency range.

In the past, the phase matching inside the crystal material was realized by tilting or heating the crystal, or by changing the cavity length. Unfortunately, most of these method realize only rather slow tuning speeds in the order of seconds.

This problem can be circumvented by introducing a noncollinear angle between the pump and signal beam which enables an ultra-broadband tuning range by varying the cavity length only. With an additional fast translation stage, one can quickly tune the desired signal wavelength continuously through the whole phase matching range. Our group has demonstrated tuning speeds in the order of 100 ns/ms by using a fast piezo actuator in near-infrared noncollinear optical parametric oscillators (NOPOs).

One amazing feature of the crystal material BBO is the ability to adapt this concept to the visible spectral range by shifting the pump photon energy into the UV. The phase matching parameters are changed as well to fit the newly desired tuning range. As a result, a fast tunable, high power femtosecond laser source in the visible can be realized.

The research group led by Prof. Dr. Uwe Morgner from the Institute of Quantum Optics researched the experimental realization of a widely tunable, high power, femtosecond optical parametric oscillator which covers nearly the entire visible spectral range. The results are published in Photonics Research Volume 9, No. 9, 2021 (Robin Mevert, Yuliya Binhammer, Christian M. Dietrich, et al. Widely tunable, high-power, femtosecond noncollinear optical parametric oscillator in the visible spectral range[J]. Photonics Research, 2021, 9(9): 09001715).

The NOPO can be quickly tuned from 440 – 720 nm which corresponds to a range from red to violet colors as it is shown in Fig.1. Moreover, the output power stays above 100 mW for the most part of the tuning range. Because the net intracavity group delay dispersion is positive, the resulting pulses are positively chirped. As a result, pulse durations are in the order of 300 to 400 fs. However, with additional external pulse compression efforts one could compress the pulses below 120 fs over the whole tuning range.

Fig.1 (a) Measured spectra, (b) output power (red) and pulse durations (blue) of the visible NOPO at various cavity lengths.

Another benefit is the scalable output power due to the lack of heat dissipation inside the nonlinear crystal. Therefore, it is only limited by the two-photon absorption of the UV light in the crystal and by the damage threshold of the mirror coatings. By adapting the cavity design to counteract these limitations, a power scaling into the watt-level should be realizable with this concept.

In this paper a quickly tunable, high power, femtosecond noncollinear optical parametric oscillator with covers nearly the entire visible spectral range is, to the best of our knowledge, demonstrated for the first time. Its broad tuning range is a unique opportunity for ultrafast spectroscopy. The additional high tuning speeds creates new opportunities for two-color spectroscopy and imaging.