A 5-kW thin disk laser with a beam parameter product (BPP) of ≤ 2.5 mm×mrad (50-μm processing fiber) has been realized. Target applications of this device include high speed laser cutting and remote (wobble) welding. Furthermore, we present an 8-kW thin disk laser system with a BPP of 4 mm×mrad (100-μm processing fiber) based on one disk. We also present results on a 18-kW thin disk laser based on two disks (125-μm processing fiber). A new line of thin disk lasers with output powers of 1-6 kW is introduced: up to four fiber outputs allow for a wide variety of time and energy sharing schemes.
The disk laser is one of the most important laser concepts for today’s industrial laser market. Offering high brilliance at low cost, high optical efficiency and great application flexibility the disk laser paved the way for many industrial laser applications. Over the past years power and brightness increased and the disk laser turned out to be a very versatile laser source, not only for welding but also for cutting. Both, the quality and speed of cutting are superior to CO<sub>2</sub>-based lasers for a vast majority of metals, and, most important, in a broad thickness range. In addition, due to the insensitivity against back reflections the disk laser is well suited for cutting highly reflective metal such as brass or copper. These advantages facilitate versatile cutting machines and explain the high and growing demand for disk lasers for applications besides welding applications that can be observed today. From a today’s perspective the disk principle has not reached any fundamental limits regarding output power per disk or beam quality, and offers numerous advantages over other high power resonator concepts, especially over fiber lasers or direct diode lasers. This paper will give insight in the latest progress in kilowatt class cw disk laser technology at TRUMPF and will discuss recent power scaling results as well.
In the last decade diode pumped solid state lasers have become an important tool for many industrial materials processing applications. They combine ease of operation with efficiency, robustness and low cost. This paper will give insight in latest progress in disk laser technology ranging from kW-class CW-Lasers over frequency converted lasers to ultra-short pulsed lasers.<p> </p> The disk laser enables high beam quality at high average power and at high peak power at the same time. The power from a single disk was scaled from 1 kW around the year 2000 up to more than 10 kW nowadays. Recently was demonstrated more than 4 kW of average power from a single disk close to fundamental mode beam quality (M²=1.38). Coupling of multiple disks in a common resonator results in even higher power. As an example we show 20 kW extracted from two disks of a common resonator. <p> </p>The disk also reduces optical nonlinearities making it ideally suited for short and ultrashort pulsed lasers. In a joint project between TRUMPF and IFSW Stuttgart more than 1.3 kW of average power at ps pulse duration and exceptionally good beam quality was recently demonstrated. <p> </p>The extremely low saturated gain makes the disk laser ideal for internal frequency conversion. We show >1 kW average power and >6 kW peak power in multi ms pulsed regime from an internally frequency doubled disk laser emitting at 515 nm (green). Also external frequency conversion can be done efficiently with ns pulses. >500 W of average UV power was demonstrated.
We present our latest experimental results in wavelength stabilization of high power laser diode systems by using Volume Holographic (Bragg) Gratings. Such systems are used as optical pumps to increase the efficiency and brightness of Thin Disk Lasers. To achieve a wide locking range from threshold until maximum operation current (for example from 30A to 250A), careful control of laser system alignment is necessary to ensure effective feedback and locking, without using strong gratings which could reduce laser efficiency. For this purpose, we use wavefront correction optics to compensate for laser bar smile and Fast Axis Collimation pointing errors. We reduce the pointing errors from ~ 1 mrad to an average under 0.1 mrad across the bar and across the entire stack. Time resolved spectra are used to investigate the dynamic locking behavior with the goal of achieving a locking speed comparable to the rise time of the current (100 μs). Experimental results for multi-kW laser systems are presented, both in CW and soft pulsed operation modes.
We report on our latest results of near fundamental mode operation of Yb-doped thin-disk lasers. 4 kW of continuous wave output power at M²<1.4 has been achieved by using one disk only. To the best of our knowledge this is the highest cw output power ever extracted from a single disk resonator design aiming for fundamental mode beam quality. Furthermore, a promising optical-to-optical efficiency of up to 56% at peak power has been achieved by pumping at 969 nm. Besides zero phonon line pumping, standard resonator components of our TruDisk thin-disk laser product series have been used such as the laser disk, and the pump optics which allows for 44 passes of the pump light through the laser crystal. It should be noticed that neither aberration correction methods nor a vacuum resonator design have been necessary to achieve this result.
This paper highlights the latest advances of disk laser technology at TRUMPF. The disk laser combines unique properties, especially high output brilliance (at the lowest pump brilliance requirements of any high power platform), power scalability and insensitivity to back reflections. In the latest generation of CW disk lasers, 6kW are extracted from one disk in an industrial product at beam qualities suitable for cutting and welding. Laboratory results with up to 4 kW laser power at nearly diffraction limited beam quality (M<sup>2</sup>=1.38) and 8 kW with a beam quality of 3 mm mrad from a single disk and even higher output power levels with lower beam quality will be presented. Finally, results of a frequency doubled CW disk laser will be shown.
This paper highlights the latest advances of disk laser technology at Trumpf. The disk laser combines unique properties,
especially high output brilliance (at the lowest pump brilliance requirements of any high power platform), power
scalability and broad applicability from cw to ps systems. In the new generation of cw disk lasers, 6kW are extracted
from one disk in an industrial product at beam qualities suitable for welding. Moreover, scaling laser power to 10 kW per
disk and resonators with higher brilliance are discussed. These advances are enabled by a combination of power scaling
and increase of optical-to-optical efficiency. In addition, applications of the disk laser principle to pulsed operation, from
ns to ps duration, at infrared and green wavelengths are discussed. Finally, an outlook on the capabilities of disk lasers
towards highest cw power and ultra-high peak powers of petawatts and beyond is given.
The quasi two-dimensional geometry of the disk laser results in conceptional advantages over other geometries.
Fundamentally, the thin disk laser allows true power scaling by increasing the pump spot diameter on the disk
while keeping the power density constant. This scaling procedure keeps optical peak intensity, temperature,
stress profile, and optical path differences in the disk nearly unchanged. The required pump beam brightness -
a main cost driver of DPSSL systems - also remains constant.
We present these fundamental concepts and present results in the wide range of multi kW-class CW-sources,
high power Q-switched sources and ultrashort pulsed sources.
This paper highlights unique advantages of the disk laser technology for converting the moderate brilliance of laser
diodes into excellent solid state laser beam quality with high efficiency. In contrast to traditional diode pumped
solid state lasers, particularly all relevant fluencies remain constant when the power is scaled by increasing the
active area of the disk. The state of the art TRUMPF disk laser family is presented, including latest results.
Theoretical and practical limits are discussed and an outlook over new disk laser generations including high
power cw, q-switched, and amplified systems is given.
In this work, we investigate the absorption distribution in InGaN-on-sapphire based light-emitting diodes (LEDs). We observed by photothermal deflection spectroscopy (PDS) and transmission measurements that most of the absorption takes place in a thin layer close to the sapphire substrate. The lateral intensity distribution in the surrounding of LED emitters is determined by the photocurrent measurement method. Based on the observations by PDS and transmission, a model for the lateral light propagation in the LED-wafer containing also a thin, strong absorbing layer is presented. It is shown that interference of the mode profiles with the absorbing layer leads to different modal absorption which explains the non-exponential intensity distribution. We are able to estimate the optical thickness of the absorbing layer to be 75 nm. Furthermore, this layer can be identified as one of the major loss mechanism in InGaN-LEDs grown on sapphire substrate due to the large absorption coefficient which is effective at the emission wavelength.
The absorption of lateral guided modes in light emitting diodes is determined by the photocurrent measurement method. A theory for waveguide dispersion is presented and extended by ray-tracing simulations. Absorption coefficients of InGaN-on-sapphire and AlGaInP-based structures is evaluated by comparison with simulation curves. For nitride-based samples with emission wavelengths of 415 nm and 441 nm an absorption of 7 cm<sup>-1</sup> is obtained. It is found that scattering is present in the buffer layer and influences the lateral intensity distribution. The investigated AlGaInP-based sample exhibits an absorption of α = 30 cm<sup>-1</sup> at 650 nm emission wavelength.
We present results on efficient InGaAlP light-emitting diodes using lateral outcoupling taper. This concept is based on light generation in the very central area of a circularly symmetric structure and, after light propagation between two mirrors, outcoupling in a tapered mesa region. We have demonstrated the suitability of this concept on As-based Light-Emitting Diodes emitting at 980 nm. Since the idea is not limited to a certain material system, we fabricated InGaAlP-based LEDs emitting in the red wavelength regime. By adjusting the process flow to the new material system we were able to achieve external quantum efficiencies in the range of 13% for unencapsulated devices. Additionally we present a new concept combining the idea of outcoupling tapers with a waferscale soldering technique. First samples show external quantum efficiencies in the range of 11%.