light from a tapered laser diode, while avoiding significant thermal effects. Besides constituting the highest SH power yet achieved using a laser diode, this demonstrates that the concept successfully combines the high efficiency of the first stage with the good power handling properties of the subsequent stages. The concept is generally applicable and can be expanded with more stages to obtain even higher efficiency, and extends also to other combinations of nonlinear media suitable for other wavelengths.

Abstract-In this paper, we investigate the noise performance and transfer in a titanium sapphire (Ti:S) laser system. This system consists of a DBR tapered diode laser, which is frequency doubled in two cascaded nonlinear crystals and used to pump the Ti:S laser oscillator. This investigation includes electrical noise characterizations of the utilized power supplies, the optical noise of the fundamental light, the second harmonic light, and finally the optical noise of the femtosecond pulses emitted by the Ti:S laser. Noise features originating from the electric power supply are evident throughout the whole transfer chain. It is demonstrated that improving the electrical noise provides an easy method for reducing the relative intensity noise (RIN) in all stages. The frequency doubled light is shown to have a higher RIN than the fundamental light. In particular, the cascaded system is seen to exhibit higher RIN than a setup with only a single nonlinear crystal. The Ti:S is shown to have a cut-off frequency around 500 kHz, which means that noise structures of the pump laser above this frequency are strongly suppressed. Finally, the majority of the Ti:S noise seems to originate from the laser itself, which partly can be explained by the effect of relaxation oscillation frequency.

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Passive S-bend waveguides have been thoroughly studied in the past, however active S-bend structures such as those in Y-branch distributed Bragg reflector (DBR) ridge waveguide (RW) lasers have been paid little attention. In this work, different S-bend based Y-branch DBR lasers emitting at 976 nm are developed, and their electro-optical, spectral and spatial characteristics are characterised. In total, two types of active S-bend (sinusoidal, cosinusoidal) as well as singlebend cosinusoidal Y-branch DBR-RW laser structures are investigated and their performances are compared to a reference straight DBR-RW laser. It is shown that among the investigated devices, the Cos S-bend based Y-branch laser provides the highest single-mode output power. However, when considering the spatial characteristics, the experimental results suggest that the Sin S-bend based Y-branch laser offers the best beam quality, and should be used in applications where good spatial characteristics are important. Finally, the experimental beam parameters are compared to passive waveguide simulations.