Optically pumped semiconductor laser

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Optically pumped semiconductor lasers , OPSL for short , also semiconductor disk lasers or VECSEL ( Vertical External Cavity Surface Emitting Laser ), are lasers in which, in contrast to semiconductor lasers, the active semiconductor is not excited (pumped) electrically, but optically by another (shorter-wave) laser. becomes. The optical pumping of the semiconductor crystal is usually carried out with infrared radiation from laser diodes .

construction

The semiconductor structure consists of an active zone that sits on a Bragg mirror ( distributed Bragg reflector ) with very high reflectivity . The active zone usually consists of a sequence of periodically arranged quantum films ( quantum wells ) which are surrounded by pumping light absorber layers. The laser resonator is formed by the Bragg mirror and at least one external mirror.

properties

Usually, optically pumped semiconductor lasers emit in the near or mid- infrared , depending on the type of semiconductor used. The pump laser, usually a laser diode , also emits in the near infrared, but has a slightly shorter wavelength. The pump laser is focused on the semiconductor wafer, as a result of which inversion forms in this area and the optically pumped semiconductor laser starts to oscillate. By enlarging the excited surface, the power of the optically pumped semiconductor laser can be scaled into the watt range without the beam quality suffering.

advantages

Advantages over edge-emitting laser diodes are primarily the much better beam quality ( M² <1.2) and the symmetrical beam profile, but also the surface scalability and the external resonator, which u. a. Q-switching, mode coupling and frequency multiplication allowed within the resonator.

Compared to surface emitter laser diodes (VCSEL), OPSL achieve significantly higher output powers.

Compared to solid-state lasers , OPSL have specifically adjustable laser wavelengths due to the composition of the semiconductor, lower manufacturing costs and are more compact.

disadvantage

Disadvantages compared to laser diodes are the lower efficiency, the higher purchase price and the complexity.

Compared to surface emitter laser diodes (VCSEL), OPSL are more complex in construction, more expensive and larger.

Compared to diode-pumped solid-state lasers (DPSSL), OPSL have a lower efficiency, a possibly shorter service life of the semiconductor and a very much lower maximum output power (a few watts compared to many kilowatts).

Applications

With the help of frequency doubling in the external resonator, the laser radiation in the infrared or in the visible spectral range can be converted into visible or ultraviolet light. Since frequency doubling is a non-linear optical effect, a very high intensity is necessary for high efficiency. Very high intensities are present within the resonator, which is why a non-linear optical crystal is placed within the external resonator of the semiconductor wafer laser. The composition of the semiconductor used determines the laser wavelength and can be specifically adjusted within the available material systems. By selecting suitable optically non-linear crystals, the entire visible color spectrum can in principle be covered. Blue, green, yellow and orange frequency-doubled semiconductor disk lasers have already been demonstrated and are commercially available. Red semiconductor disk lasers and ultraviolet frequency doubled lasers are under development.

By integrating saturable absorbers into the laser resonator, pulse operation can be made possible through passive mode locking . This results in very short pulses with a very high repetition rate.

Another application is absorption spectroscopy ( TDLS ).

Optically pumped semiconductors are still used to pump other laser sources, primarily ultra-short pulse or UV lasers.

literature

  • Svent-Simon Beyertt: Quantum film pumps for power scaling of semiconductor disk lasers. Herbert Utz Verlag, Munich 2011, ISBN 978-3-8316-4051-5
  • Thomas Kübler: Modeling and simulation of the semiconductor wafer laser. Herbert Utz Verlag, Munich 2009, ISBN 978-3-8316-0918-5 .
  • Hermann Kahle, Roman Bek, Matthias Heldmaier, Thomas Schwarzbäck, Michael Jetter, Peter Michler: High optical output power in the UVA range of a frequency-doubled, strain-compensated AlGaInP-VECSEL . In: Applied Physics Express . tape 7 , no. 9 , September 1, 2014, p. 092705 , doi : 10.7567 / APEX.7.092705 .

Web links

Individual evidence

  1. ↑ a b Hermann Kahle, Roman Bek, Matthias Heldmaier, Thomas Schwarzbäck, Michael Jetter, Peter Michler: High optical output power in the UVA range of a frequency-doubled, strain-compensated AlGaInP-VECSEL . In: Applied Physics Express . tape 7 , no. 9 , September 1, 2014, p. 092705 , doi : 10.7567 / APEX.7.092705 .