Diode laser
A diode laser is a laser whose light is generated with laser diodes , i.e. with semiconductor materials. Diode lasers can generate light outputs of over 60 kW.
Construction methods and use
Single emitter
In its simplest form, a diode laser consists of just one laser diode , possibly with collimation and focusing optics. Single-emitter laser diodes are used, for example, in laser pointers , for optical data transmission or in CD and DVD scanners or burners. Individual emitters are manufactured with outputs of up to a few watts, are available mounted on a heat sink and often already contain a beam collimation with a spherical or aspherical lens or even a fiber coupling ( pigtailed ).
Single-emitter diode lasers often have special designs that are not used in the multi-emitter diode lasers described below. The most important special designs of single emitter lasers (see also laser diode ) are:
- DFB laser : laser diode with external Bragg mirror
- Diode laser with external resonator ( English external cavity diode laser , ECDL)
- Surface emitter (VCSEL) see also laser diode
Ingots
Since individual laser diodes can be made only to outputs of several watts and emit widely divergent laser radiation, often several laser diodes are combined electrically and optically: One uses bars (Engl. Bar ), side by side contained on a strip-chip multiple single emitter. These are operated electrically in parallel and mounted on a heat sink.
Such diode lasers, also known as submounts , have the following typical parameters:
- Operating current: 30 A to 100 A in continuous operation, approx. 150 A pulsed
- Threshold current: 3.5 A to 15 A.
- Optical output power: up to 90 W in continuous operation at typical wavelengths of 808 to 980 nm
- Voltage: approx. 2 V
The 10–20 individual emitters of such a bar each emit a laser beam that has a beam angle of approx. 40 ° ( fast axis ) in one direction and approx. 12 ° ( slow axis ) in the other direction . The fast axis has the highest possible beam quality (diffraction limited), while the slow axis has a relatively poor beam quality. In order to combine these individual beams, after the collimation of the fast axis (by means of a micro-optical cylinder lens) they are geometrically rotated by means of further micro- optics and arranged next to one another, and then the so-called slow-axis collimation is carried out.
A “laser beam” generated in this way actually consists of several individual laser beams and has a significantly poorer beam quality than other lasers of the same power.
Manufacturers offer both submounts and hermetically sealed lasers, some with fixed fiber optic cables or connections for fiber optic connectors (for example an F-SMA socket ).
Applications of such lasers: metal and plastic welding, selective hardening, soft and hard soldering, build-up welding, pumping of solid-state lasers , especially fiber lasers .
stack
Several of such bars can (engl. Stacks stacks ) are combined, wherein the bars are electrically connected in series and the individual beams are again combined optically. Such stacks can generate optical powers of 0.5–1 kW. Due to the high packing and power flux density, the submounts used have to be cooled with water using so-called microchannel heat sinks. The optical power of such stacks is used, for example, to pump solid-state lasers .
Multi-kilowatt diode laser
The linear polarization and different wavelengths of several stacks are used to further increase performance: Using dichroic and polarization-dependent mirrors, the radiation from several stacks (for example four stacks with two different wavelengths, each mounted orthogonally to one another) can theoretically be mirrored into one another without loss of quality and performance. Laser beam sources with several kilowatts of optical power are obtained with a comparatively very high degree of efficiency (20–30%).
Such high performances are used for material processing (metal welding, hardening, remelting, powder application).
Diode laser with external resonator
The line width of the radiation is a laser diode, without further measures in the range of a few MHz, and the position of its maximum is also dependent on temperature and current. It can be greatly reduced and stabilized with wavelength-selective feedback: if only one frequency is fed back with priority, the wavelength can be determined in this way, since the longitudinal mode competition, which is determined by the short crystal and its properties, is eliminated. To such a frequency selective elements added constructions are always external cavity diode laser ( E Xternal C avity D iode L aser).
Housed laser diodes are known which have already integrated a fiber Bragg grating and thus achieve a temperature-independent narrow-band emission. For example, they are used as seed lasers (see under fiber lasers ) for fiber lasers.
Due to the small line width and also good detunability, the following ECDL are preferred for use in spectroscopy :
Littrow construction
The emerging beam is collimated and hits a diffraction grating with a high number of lines. In the Littrow arrangement, the first order of diffraction is reflected back into the diode, while the zero order of diffraction is coupled out as a useful beam. The rear facet, together with the grating, now forms the resonator. By rotating the grid, e.g. B. with the help of a piezo actuator , the wavelength of the laser can be detuned.
Littmann body
In the Littman arrangement, the first order of diffraction is not reflected back into the diode, but hits a mirror which reflects the light back into the diode via the grating. The laser is detuned by rotating the mirror. The advantage of this somewhat more complex arrangement is that swiveling of the coupled-out beam is avoided if the laser is detuned.
Advantages and disadvantages
Advantages of diode lasers:
- Very compact design
- Simple pumping using electrical power
- Comparatively high electrical / optical efficiency of 25 to over 50%
- Long maintenance intervals compared to lamp-pumped lasers
- Coupling and transport of the radiation in fiber optic cables possible
- Very long service life with sometimes more than 30,000 hours
- Low performance degradation; much less than 1% / 1000 h when operated with nominal current
Disadvantages of diode lasers:
- In comparison to other lasers, the beam quality is poorer (especially at high powers), therefore hardly suitable for cutting and only suitable to a limited extent for deep laser welding of metals
- Strong beam divergence if this is not corrected by suitable optics
- Costs: Laser diodes, their assembly on a heat sink and the adjustment of the micro-optics are still a high cost factor of a diode laser system