M. Uebernickel

High-Power DBR-Tapered Laser at 980 nm for Single-Path Second Harmonic Generation

We present experimental results about edge-emitting distributed-Bragg-reflector-tapered diode lasers emitting at 980 nm. The investigated lasers show an output power of up to 12 W with a conversion efficiency of about 45%. The lasers also exhibit a small vertical divergence <15deg full-width at half maximum (FWHM), a nearly diffraction-limited beam quality, and a narrow spectral linewidth with FWHM smaller than 12 pm. These properties allow efficient single-pass second harmonic generation with power levels of more than 1 W at 488 nm.

Compact second-harmonic generation laser module with 1 W optical output power at 490 nm

We demonstrate continues-wave 1 W at 490 nm on a 2.5 cm(3) micro-optical bench using single-path second-harmonic generation with a periodically poled MgO:LiNbO(3) bulk crystal. The pump laser is a distributed Bragg reflector tapered diode laser having a single-frequency spectrum and a pump power of 9.5 W. Based on that 1 W blue light could be achieved resulting in an optical conversion efficiency of 11%. Furthermore, the module has an output power stability of better than 2% and the blue laser beam shows an nearly diffraction limited beam quality of M(2)(sigma) = 1.2 in vertical and M(2)(sigma) = 2 in lateral direction.

Thermal optimization of second harmonic generation at high pump powers.

We measure the temperature distribution of a 3 cm long periodically poled LiNbO₃ crystal in a single-pass second harmonic generation (SHG) setup at 488 nm. By means of three resistance heaters and directly mounted Pt100 sensors the crystal is subdivided in three sections. 9.4 W infrared pump light and 1.3 W of SHG light cause a de-homogenized temperature distribution of 0.2 K between the middle and back section. A sectional offset heating is used to homogenize the temperature in those two sections and thus increasing the conversion efficiency. A 15% higher SHG output power matching the prediction of our theoretical model is achieved.

Pulse-shape improvement during amplification and second-harmonic generation of picosecond pulses at 531 nm

We study second-harmonic generation of picosecond pulses in bulk periodically poled lithium niobate using an all-semiconductor master oscillator-power amplifier with gain-switched seed as our fundamental source. Both during amplification and during the subsequent second-harmonic generation, the signal pulse shape improves, and the resulting pulses at 531 nm are nearly independent of the seed pump conditions. Over a wide range of repetition rates and seed settings, we obtain green pulses with a duration of less than 31 ps FWHM and a peak power of more than 5.1 W. We further investigate the influence of the fundamentals spectral dynamics on pulsed second-harmonic generation efficiency and obtain excellent agreement between our measurements and previously published theoretical treatments.

High-brightness distributed-Bragg-reflector tapered diode lasers: pushing your application to the next level

We demonstrate monolithic distributed-Bragg-reflector tapered diode lasers having an output power up to 12 W, a small spectral width of below ▵λ<10 pm and a beam quality close to the diffraction limit. This results in a brightness close to 1 GWcm-2sr-1. Due to these excellent electro-optical characteristics we achieved visible laser light up to P=1.8 W in a single-path second harmonic generation experiment. This allowed us to develop compact Watt-class (P=1.1 W) visible laser modules having an excellent beam quality (M²<3) with a narrow spectrum (▵λ<30 pm). The entire device is integrated on a micro-optical bench with a volume below 20 cm³. In another application we demonstrate for the first time a femtosecond gigahertz SESAM-modelocked Yb:KGW laser. Such a laser system benefits from the small spectral emission and the focusability of the developed diode laser. A record peak power of 3.9 kW was achived. At the repetition rate of 1 GHz, 281 fs pulses with an average output power of 1.1 W were generated. This Yb:KGW laser has a high potential for stable frequency comb generation.

High-power (1.1W) green (532nm) laser source based on single-pass second harmonic generation on a compact micro-optical bench

We demonstrate a compact high-power green (532nm) laser module based on single-pass second harmonic generation. The pump source is a distributed Bragg reflector tapered diode laser. The frequency conversion is achieved with a 2.5 cm long periodically poled MgO:LiNbO3 bulk crystal. The entire module is integrated on a compact micro-optical bench with a footprint of 2.5 cm3. Up to 1.1 W output green light power is achieved at a pump power of 7.6 W with an optical conversion efficiency of about 15% and a corresponding module wall-plug efficiency of more than 4%. The green laser beam has a relatively good beam quality (measured at output power level of ~0.9 W) with M²σ=1.8 in the vertical direction and M²=4.9 in the lateral direction, respectively. The long-term output power stability is ±10% (tested at output power level of ~0.6 W).

Rayleigh length dependent SHG conversion at 488nm using a monolithic DBR tapered diode laser

We present a study of the single pass SHG conversion as a function of the Rayleigh length (RL) and beam diameter (BD) using a monolithic distributed Bragg reflector (DBR) tapered laser. The DBR tapered laser has a 6th order surface grating and a ridge waveguide. Single longitudinal mode emission at 978nm with a side-mode suppression ratio of more than 40dB and at an output power of 2.7W at 15°C have been obtained in continuous wave operation. The beam was collimated using an aspheric and a cylindrical lens and focused using a variety of lenses with various focal lengths. The resulting caustics were acquired using a camera and used for SHG in a 5cm periodically poled LiNbO3 (PPLN) crystal. This allowed an investigation of the dependency of the SHG conversion efficiency on the RLs and BDs. We obtained 330mW of output power at 488nm using the optimal focus length. The experiments showed that an optimum conversion requires longer focal lengths then forecasted by Boyd-Kleinmans theory, which is explained due to the partial coherence. We developed an extension of that theory to account for that partial coherence, which bases in principle on a mismatch related general Agrawals nonlinear integration kernel. We use this theory to explain the dependence of the SHG efficiency from the beam propagation factor M2.

Compact Watt-class visible light sources using direct frequency-doubled edge-emitting diode lasers

Compact laser light sources in the visible spectral range emitting several Watts are required for display technology, sensor systems and material processing. Second harmonic generation (SHG) using highly brilliant edge emitting infrared lasers is a promising way to fill the spectral gap of directly emitting semiconductor lasers. Newly developed distributed Bragg reflector (DBR) tapered lasers allow a very efficient SHG due to their extraordinary brightness. On an optical bench more than 1 W power at 488 nm was obtained by directly doubling the laser light with a 5 cm long PPLN crystal. Using hybrid integration on a micro-optical benches we now achieved 0.5 W power at 488 nm with a 2.2 cm long PPLN crystal.

Thermal optimization of the second harmonic generation with tapered diode lasers

Recently, hybrid integrated compact laser sources with high optical output powers in the visible range around 488 nm were demonstrated using tapered diode lasers. This was done by single-pass second harmonic generation (SHG) using a periodically poled LiNbO3 crystal of 30 mm length. The conversion efficiency depends on the light source but is also a function of the temperature distribution along the length of the crystal. The maximum conversion efficiency of a given beam is theoretically achieved by a homogenous temperature distribution. Experiments have shown that for high power SHG different absorption mechanisms are causing a temperature gradient in the crystal. This gradient leads to an inhomogeneous poling period, which diminishes the effective crystal length and leads to a smaller conversion efficiency. In this paper we present a method for the optimization of the temperature management during the SHG. This is done by a multizone heater package that can be integrated into compact laser sources. This package can be used to create arbitrary temperature distributions and is especially able to compensate an arising temperature gradient.

Wigner distribution function of DBR tapered diode lasers

In this paper, we utilize the concept of the Wigner distribution function (WDF) on distributed-Bragg-reflector taper lasers (DBR-TPL). The WDF allows the derivation of the phase and the intensity distribution just as well as the spatial coherence properties of the laser beam. For a given single-mode fiber the coupling efficiency for a given beam and optical system can be obtained by means of a simple overlap integral. Simultaneously, this approach delivers the corresponding beam forming requirements to meet the optimum coupling condition. We found a good agreement between the measured coupling efficiencies of the DBR-TPL into a single-mode fiber under varying coupling conditions and the corresponding efficiencies derived from the measured WDF by simulating the same coupling conditions.