Abstract
We present bi-frequency continuous wave oscillation in a semiconductor disk laser through direct writing of loss-inducing patterns onto an intra-cavity high reflector mirror. The laser is a Vertical External Cavity Surface Emitting Laser which is optically pumped by up to 1.1 W of 808 nm light from a fibre coupled multi-mode diode laser, and oscillates on two Hermite-Gaussian spatial modes simultaneously, achieving wavelength separations between 0.2 nm and 5 nm around 995 nm. We use a Digital Micromirror Device (DMD) enabled laser ablation system to define spatially specific loss regions on a laser mirror by machining away the Bragg layers from the mirror surface. The ablated pattern is comprised of two orthogonal lines with the centermost region undamaged, and is positioned in the laser cavity so as to interact with the lasing mode, thereby promoting the simultaneous oscillation of the fundamental and a higher order spatial mode. We demonstrate bi-frequency oscillation over a range of mask gap sizes and pump powers.
Highlights
Coherent continuous wave laser sources that simultaneously oscillate on two discrete optical frequencies find applications in spectroscopy [1], radar systems, distance measurements, microwave and terahertz frequency generation [2,3,4] and sensing applications [5], and there exists a strong foundation of research into bi-frequency sources [6, 7]
The technique is remarkably fast: a simple set of cross masks may be fabricated in as little as 30 to 60 minutes. Because both modes share the same external cavity, and because there exists a small spatial region of the gain area that is common to both spatial modes due to their partial overlap [22], noise is common to both oscillating modes and coherence may be enforced between them
The Vertical External Cavity Surface Emitting Laser (VECSEL) gain structure that we used was fabricated by Metallo-Organic Chemical Vapour Deposition (MOCVD) and is composed of a bottom Distributed Bragg Reflector (DBR), consisting of 31 pairs of AlAs/GaAs, followed by a 13λ/2 thick microcavity gain region comprised of 6 strain-compensated InGaAs/GaAsP quantum wells, a top DBR consisting of 4 pairs of Al0.18Ga0.82As/AlAs and a GaAs capping layer
Summary
Coherent continuous wave laser sources that simultaneously oscillate on two discrete optical frequencies find applications in spectroscopy [1], radar systems, distance measurements, microwave and terahertz frequency generation [2,3,4] and sensing applications [5], and there exists a strong foundation of research into bi-frequency sources [6, 7]. The Vertical External Cavity Surface Emitting Laser (VECSEL) is an optically pumped semiconductor quantum well (QW) laser technology which can offer narrow emission linewidths on account of its class A dynamics and low Schawlow-Townes limit [8,9,10,11]. The combination of these factors can lead to low phase noise and good long term stability of lasing operation. Bi-frequency operation has been demonstrated through the inclusion of a birefringent filter for VECSELs [17, 18] and for a mode-locked integrated external cavity surface emitting laser (MIXSEL) [19,20,21] which creates two different effective cavity lengths for the two available orthogonal linear polarisation states
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