We demonstrate a novel method for precise control over mode competition in a mode locked oscillator. By manipulating the gain properties in the cavity, we achieve almost complete control over the mode locked oscillation spectrum. The core principal of our method is to tailor the overall gain profile in the cavity, by combining the standard homogeneous gain with a small amount of spectrally shaped inhomogeneous gain, which allows us to selectively boost desired frequencies within the overall mode competition.

One of the basic concepts in the physics of laser oscillators is mode competition, which is the crucial factor in determining the steady state solution of a laser oscillation. Specifically, mode competition renders some possible oscillations unstable in a homogeneously broadened laser, for example – dual color oscillation cannot stably develop, as the colors inherently compete. It is rather simple in a homogeneously broadened laser to obtain mode locked operation with a single tunable spectral lobe, by using spectral filtering to modify the loss spectrum in the cavity. However, loss filtering fails when more structured spectrum (multi lobed) is required, because mode competition between the lobes in the homogeneously broadened gain in many cases selects only one lobe of the allowed set. Thus, although dual-color pulses are appealing for applications such as: Raman spectroscopy, Raman microscopy and direct frequency comb spectroscopy, multi-lobed oscillations  pulses were so far hard to obtain in homogeneously broadened lasers. . Clearly, the spectrum of pulses may be shaped outside the optical cavity, but this shaping is inherently lossy, and for many applications impractical. Several attempts were performed in the past to obtain dual-color oscillations in a single oscillator with minimal success, mainly due to unresolved mode competition problems. Other methods bypass mode-competition by active or passive synchronization of several sources, but are more costly, bulky and require special care for stabilization of timing jitter between the participating pulses.

We present a far simpler method to shape the spectrum of ultrashort pulses using a single mode-locked oscillator, allowing one to develop sources with precise control over the spectral properties of the emitted light pulses. This is achieved by a novel design of the cavity that nulls the competition between colors and enables shaping of the gain spectrum according to the desired oscillation (as opposed to shaping the loss spectrum in standard methods). By placing an additional gain medium in the cavity at a position where the spectrum is spatially dispersed, and by controlling the spatial shape of the pump beam in the gain medium inside the dispersive arm, we are effectively creating an intra-cavity pulse shaper, allowing us to control the emitted spectrum precisely.

Here we present the design and results of a modelocked Ti: Sapphire femtosecond laser based on this novel design of the laser cavity. Mode locking is obtained using the standard Kerr lens technique. The cavity is designed to generate ultrashort optical pulses with a double lobed spectrum, in which the intensity, bandwidth and spectral position of each lobe can be independently tuned within the Ti: Sapphire emission spectrum where dispersion is compensated.