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Comparison of dual-wavelength Y-branch DBR single chip diode lasers and diode laser arrays at 785 nm
At T = 25°C and 20 mW pump power, diffraction limited laser emission with 0.5 W optical output power and beam propagation parameters of 1.3 (M24σ) are obtained. At both emission wavelengths of 784.6 nm and 785.2 nm, spectral bandwidths below 0.02 nm at full width at half maximum and side mode suppression ratios of 30 dB are measured. A negligible wavelength shift of < 0.02 nm/A between threshold and maximum power corresponds to a temperature rise during operation of only 0.3 K. This indicates a low thermal influence from the PA to the MO and allows a free choice of excitation power for applications. Compared to previously reported free-running Y-branch diode lasers, the MOPA does not show lateral spatial tilts between the two far field intensity distributions at both wavelengths.
This compact MOPA allows addressing applications such as shifted excitation Raman difference spectroscopy under in-situ conditions and confocal Raman microscopy without the need of a spectral recalibration during the measurements. In addition, simultaneous dual-wavelength operation also enables terahertz frequency generation.
In this paper, diode lasers with customized designs according to the spectral requirements of the applications will be presented. As basis for all devices, Y-branch diode lasers with an integrated grating for wavelength stabilization are realized. The emission of the two branches is combined in an implemented Y-shaped coupler. The bent waveguides are sine shaped S-bends. The spectral tuning is performed via implemented heater elements next to the Distributed Bragg Reflector (DBR) gratings or via the injection current when using a Distributed Feedback (DFB) grating. Powervoltage current characteristics, spectral and tuning properties will be shown.
The devices emitting at 671 nm and 785 nm are used for SERDS, whereas devices at 965 nm were tested as seed sources for pulsed master oscillator power amplifiers (MOPA) suitable for the detection of water vapor. Devices at 785 nm are also suitable for the generation of THz radiation using difference frequency generation. A widely tunable Y-branch diode laser near 972 nm is used for the sum frequency generation in an up-conversion system.
For red emitting devices, around 637 nm an output power up to about 1 W is achieved, limited by the properties of the semiconductor material. Devices emitting in the longer wavelength range around 1000 nm reach output powers up to 15 W. For all the above manufactured diode lasers about 80% of the emitted laser light is within the diffraction limited central lobe. Here, e.g. 10 W diffraction limited power was measured.
The devices are successfully implemented into experiments for non-linear frequency conversion, e.g. SHG and the pumping of an OPO, as an excitation source for absorption spectroscopy, as pump source for fs-lasers, and their emission is efficiently coupled into single mode fibers.
The pump source, a DFB laser emitting at 976 nm, and a periodically poled lithium niobate (PPLN) ridge waveguide crystal is used for the second harmonic generation (SHG). Both components are mounted on a μ-Peltier-element for temperature control. Here, a common wavelength tuning of the pump wavelength and the acceptance bandwidth of the SHG crystal via temperature is achieved.
With the results the light source is suitable for portable Raman and SERDS experiments with a flexible spectral distance between both excitation wavelengths for SERDS with respect to the sample under investigation.
The monolithic devices reach output powers up to 215 mW with emission widths of about 20 pm. At 200 mW the conversion efficiency is 20%, i.e. the electrical power consumption is only 1 W. The spectral distance between the two laser cavities is about 0.6 nm, i.e. 10 cm-1 as targeted. The side mode suppression ratio is better than 50 dB. Amplifying these devices using a ridge waveguide amplifier an output power of about 750 mW could be achieved maintaining the spectral properties of the master oscillator.
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