André Müller

Frequency-doubled DBR-tapered diode laser for direct pumping of Ti:sapphire lasers generating sub-20 fs pulses

For the first time a single-pass frequency doubled DBR-tapered diode laser suitable for pumping Ti:sapphire lasers generating ultrashort pulses is demonstrated. The maximum output powers achieved when pumping the Ti:sapphire laser are 110 mW (CW) and 82 mW (mode-locked) respectively at 1.2 W of pump power. This corresponds to a reduction in optical conversion efficiencies to 75% of the values achieved with a commercial diode pumped solid-state laser. However, the superior electro-optical efficiency of the diode laser improves the overall efficiency of the Ti:sapphire laser by a factor > 2. The optical spectrum emitted by the Ti:sapphire laser when pumped with our diode laser shows a spectral width of 112 nm (FWHM). Based on autocorrelation measurements, pulse widths of less than 20 fs can therefore be expected.

Efficient concept for generation of diffraction-limited green light by sum-frequency generation of spectrally combined tapered diode lasers.

In order to increase the power of visible diode laser systems in an efficient manner, we propose spectral beam combining with subsequent sum-frequency generation. We show that this approach, in comparison with second harmonic generation of single emitters, can enhance the available power significantly. By combining two distributed Bragg reflector tapered diode lasers we achieve a 2.5-3.2 fold increase in power and a maximum of 3.9 W of diffraction-limited green light. At this power level, green diode laser systems have a high application potential, e.g., within the biomedical field. Our concept can be expanded combining multiple diode lasers to increase the power even further.

16 W output power by high-efficient spectral beam combining of DBR-tapered diode lasers

Up to 16 W output power has been obtained using spectral beam combining of two 1063 nm DBR-tapered diode lasers. Using a reflecting volume Bragg grating, a combining efficiency as high as 93.7% is achieved, resulting in a single beam with high spatial coherence. The result represents the highest output power achieved by spectral beam combining of two single element tapered diode lasers. Since spectral beam combining does not affect beam propagation parameters, M2-values of 1.8 (fast axis) and 3.3 (slow axis) match the M2-values of the laser with lowest spatial coherence. The principle of spectral beam combining used in our experiments can be expanded to combine more than two tapered diode lasers and hence it is expected that the output power may be increased even further in the future.

400 mW external cavity diode laser with narrowband emission at 445 nm

A high-power external cavity diode laser (ECDL) system with narrowband emission is presented. The system is based on a commercially available high-power GaN laser diode. For the ECDL, a maximum optical output power of 400 mW in continuous-wave operation with narrowband emission is achieved. Longitudinal mode selection is realized by using a surface diffraction grating in Littrow configuration. A spectral width of 20 pm at 445 nm with a side-mode suppression ratio larger than 40 dB is achieved. This concept enables diode laser systems suitable for subsequent nonlinear frequency conversion into the UV spectral range.

Simultaneous dual wavelength eye-tracked ultrahigh resolution retinal and choroidal optical coherence tomography.

We demonstrate an optical coherence tomography device that simultaneously combines different novel ultrabroad bandwidth light sources centered in the 800 and 1060 nm regions, operating at 66 kHz depth scan rate, and a confocal laser scanning ophthalmoscope-based eye tracker to permit motion-artifact-free, ultrahigh resolution and high contrast retinal and choroidal imaging. The two wavelengths of the device provide the complementary information needed for diagnosis of subtle retinal changes, while also increasing visibility of deeper-lying layers to image pathologies that include opaque media in the anterior eye segment or eyes with increased choroidal thickness.

Wavelength-Stabilized Compact Diode Laser System on a Microoptical Bench With 1.5-W Optical Output Power at 671 nm

A wavelength-stabilized compact diode laser system emitting at 671 nm mounted on a microoptical bench with the dimensions of 13 mmtimes4 mm is presented. A reflecting Bragg grating was aligned on the rear side of a broad-area gain medium for wavelength stabilization at 671 nm. A maximum output power of 1.5 W was obtained together with a spectral width of 40 pm (full-width at half-maximum). At 1.0 W, a center wavelength stability below 20 pm over 5 h was determined. With these features, the devices are well-suited for spectroscopic applications.

Power Scaling of Nonlinear Frequency Converted Tapered Diode Lasers for Biophotonics

Diode lasers have proven to be versatile light sources for a wide range of applications. Nonlinear frequency conversion of high brightness diode lasers has recently resulted in visible light power levels in the watts range enabling an increasing number of applications within biophotonics. This review provides an overview of the developments within nonlinear frequency converted high power laser diodes in the visible spectral range. Single-pass nonlinear frequency doubling is presented as a nonsophisticated method to achieve watt-level output powers and possible routes to higher power and efficiency are included. Application examples within pumping of mode-locked Ti:sapphire lasers and implementation of such lasers in optical coherence tomography are presented showing the application potential of these lasers.

Wavelength stabilized 785 nm DBR-ridge waveguide lasers with an output power of up to 215 mW

Wavelength stabilized distributed Bragg reflector (DBR) diode lasers at an emission wavelength of 785 nm will be presented. The devices have a 14 nm thick GaAsP single quantum well as active layer, which is embedded in Al0.65Ga0.35As waveguide layers and Al0.7Ga0.3As cladding layers. The DBR structures are realized as deeply etched tenth order gratings using I-line wafer stepper lithography. The devices have a stripe width of 2.2 µm and a cavity length of 3 mm including a DBR grating with a length of 500 µm. The devices are mounted p-side up on C-mounts. At a temperature of 25 °C a continuous wave output power of 215 mW and a conversion efficiency of 28% are measured. Up to 140 mW single mode operation with a small tuning range below 190 pm is observed. At 50 mW an aging test was performed showing reliable operation over 1000 h.

Single-pass UV generation at 222.5 nm based on high-power GaN external cavity diode laser.

We demonstrate a compact system for single-pass frequency doubling of high-power GaN diode laser radiation. The deep UV laser light at 222.5 nm is generated in a β-BaB2O4 (BBO) crystal. A high-power GaN external cavity diode laser (ECDL) system in Littrow configuration with narrowband emission at 445 nm is used as pump source. At a pump power of 680 mW, a maximum UV power of 16 μW in continuous-wave operation at 222.5 nm is achieved. This concept enables a compact diode laser-based system emitting in the deep ultraviolet spectral range.

Dual-wavelength diode laser with electrically adjustable wavelength distance at 785 nm.

A spectrally adjustable monolithic dual-wavelength diode laser at 785 nm as an excitation light source for shifted excitation Raman difference spectroscopy (SERDS) is presented. The spectral distance between the two excitation wavelengths can be electrically adjusted between 0 and 2.0 nm using implemented heater elements above the distributed Bragg reflector (DBR) gratings. Output powers up to 180 mW at a temperature of 25°C were measured. The spectral width is smaller than 13 pm, limited by the spectrum analyzer. The device is well-suited for Raman spectroscopy, and the flexible spectral distance allows a target-specific adjustment of the excitation light source for shifted excitation Raman difference spectroscopy (SERDS).