Michael Dr. Kühnelt

Visible lasers for mobile projection

Visible laser sources are attracting considerable interest to enable ultra-small, embedded laser scanning projection devices. We report recent progress on the development of red, green and blue semiconductor based laser sources. Red and blue colours are achieved by edge-emitting laserdiodes, whereas green uses frequency doubled optically pumped semiconductor lasers. Green lasers turned-out to be on the critical path for the technical and commercial success of laser displays. Because all current approaches are based on frequency doubling, the green source is the major contributor of cost, size and power consumption. Important parameters like size, efficiency, output power, beam quality, and modulation bandwidth are discussed.

Experimental study of the output dynamics of intracavity frequency doubled optically pumped semiconductor disk lasers

The dynamic behavior of the continuous wave output of an intracavity frequency doubled optically pumped semiconductor disk laser is described. These lasers typically exhibit stable emission. The reasons are found to be the tendency to single-mode operation and the high gain coupling of different modes due to an extremely short gain medium and the resonant periodic gain structure. A dependence of the fluctuation amplitude on gain decoupling of different fundamental modes is demonstrated. With the presented laser setup, more than 80% of conversion efficiency with respect to the available fundamental power has been achieved.

52.4: High Speed Green Frequency Converted Semiconductor Laser for Projection Displays

Recent progress on compact green laser for projection applications based on frequency doubled semiconductor structures will be presented. Avoiding external acousto- or electro-optical modulation for scanning laser displays, the green laser prototypes can be directly modulated in the 20–40MHz range.

Recent advances in VECSELs for laser projection applications

Laser projectors integrated in portable devices offer a new platform for media display but put strong demands on the laser sources in terms of efficiency, modulation band width, operating temperature range and device cost. Osram Opto Semiconductors has developed and produces synthetic green lasers for projection applications on which the latest results are reported. Based on vertical external cavity surface emitting laser (VECSEL) technology and second harmonic generation an output power of >75mW has been achieved. The maximum output power is to a large extent limited by the high thermal resistance of the monolithic VECSEL chip used. To overcome the thermal limitations a new thinfilm VECSEL chip design is proposed where the epitaxial layers are transferred to a silicon carrier and processed on wafer level, thus significantly lowering the thermal resistance and improving the maximum output power.

MgO-doped PPLN with cascaded structure for intracavity frequency doubling of optically pumped semiconductor disk lasers

A magnesium-oxide doped periodically poled lithium-niobate with segments of cascaded poling periods as a nonlinear crystal for intracavity frequency doubling of an optically pumped semiconductor disk-laser to the green spectral range is presented. Results show temperature-dependent second harmonic generation and a maximum output power of 62 mW at 525 nm corresponding to a 24% conversion efficiency.

IR lasers optimised for high-temperature operation in frequency-doubled green module for mobile laser projection

The above presented data of a green laser module regarding output power of 100mW based on an 850nm pump laser, 1060nm VECSEL and intra-cavity second-harmonic generation prove an excellent performance of the whole module with the overall WPE of as high as 10% at 40°C. It is the highest value of all cutting edge technologies enabling green lasing achieved up to date with capability of high-speed modulation in MHz range. Reliability and WPE of a pump die at high temperatures are of crucial importance for this kind of device. We demonstrate reliable operation up to 90°C junction temperature of the pumping IR laser diode at current density of 1.4kA/cm2, which is also used as a single emitter delivering 0.5–1W output power after being mounted in TO-can under the same junction temperature and current load.

Compact and efficient green laser modules

This paper presents a miniature green laser source targeted for mobile display applications based on an intra-cavity frequency doubled optically pumped semiconductor disk laser (OPS). The OPS laser consists of an IR pump laser, pump optics, a semiconductor gain chip and an external resonator with intra-cavity frequency doubling. By using micro-optics for pump and cavity optics, a module volume of less than 0.5 cm3 has been scaled down. Typical output power of green lasers required for mobile displays range from 20 mW to about 50 mW.