Boris Rosenstein

0.7 MW output power from a two-arm coherently combined Q-switched photonic crystal fiber laser.

We demonstrate a high peak power, Q-switched pulsed, intracavity coherently combined fiber laser system. The system is based on two Yb-doped, rod-type, photonic crystal fibers which are passively phase-locked and combined into the single output beam in a power oscillator configuration. Experimental evidence indicate that this oscillator system provides record high peak power of ∼ 0.7 MW with pulse duration of ∼ 10 ns at 1 kHz repetition rate. The measured beam quality shows near-diffraction-limited operation of the coherently combined system.

Overcoming peak-power limitations of fiber lasers

Fiber laser sources have become one of the most popular and rapidly developed laser technologies over the past decade. The main reason for this is that fiber lasers present several key advantages over bulk solid-state lasers, including good heatdissipation handling, high gain, ease of alignment, and singlemode wave guiding.1 However, for applications that require high peak power with good beam quality, these lasers cannot yet offer a good alternative due to material damage and deleterious nonlinear effects. These limiting factors arise from the very high powers within the relatively small core of the fiber. To overcome these limitations and increase peak power, it is possible to either increase the effective single-mode area of the core, or to coherently combine several fiber laser sources. To increase the effective area of a core, we can use large mode area step-index fibers,2 photonic-crystal fibers (PCFs),3 or chirally coupled core fibers.4 However, even with these large core fibers, the output peak power is eventually limited. The second approach consists of combining several ‘independent’ lasers (or amplifiers) into a single coherent output, while preserving the beam quality. To do this, the relative phases of the combined lasers should be locked, either by active or passive locking. Active techniques work by dynamically and continuously adjusting the phase of each independent laser source by means of electronic feedback loops.5 Passive techniques, on the other hand, consist in having the independent laser sources self-phaselocked by means of a passive coupling mechanism.6–9 Passive techniques tend to be limited in terms of the maximum number of combined laser channels,10 but they are simpler and more robust in comparison with the active techniques, which require smart and fast electronic loops and algorithms. It is also possible to increase the peak power of fiber lasers by simultaneously exploiting both approaches by coherently Figure 1. Schematic configuration of the Q-switched intracavity coherently combined fiber laser. PCF: Photonic-crystal fibers. AOQ: Acousto-optic Q-switch. HR: Highly reflective. BS: Beam splitter. OC: Output coupler.

Characterization of Coherently Combined High Peak Power Photonic Crystal Fiber Lasers

We experimentally demonstrate efficient Q-switched operation of two intracavity coherently combined, high average power, rod-type photonic crystal fiber lasers. Furthermore, temporal and frequency content imposing of one channel on the other is investigated.

Cladding Amplification of Very Large Area Modes in a Novel Double-Clad Fiber

We propose and investigate theoretically a new concept for single mode amplification in active double-clad fibers. We show that the guided fundamental mode field diameter can reach 80 μm with good modal discrimination.

High Power Off-axis Pumping of a Flexible Photonic Crystal Fiber Laser

We investigate a new, free-space, off-axis pumping method for high power PCF lasers and amplifiers. Experimental results show high coupling efficiency and superior performance of the laser, along with other advantages such as diode feedback-protection.