Raman laser
A Raman laser is an optically pumped laser based on stimulated Raman scattering .
Unlike other lasers, the pump radiation does not generate a population inversion before the laser de-excitation takes place, but the photons emit stimulated energy to the medium (typically as lattice oscillations or phonons ) and are re- emitted almost immediately with lower photon energy .
In contrast to other lasers, the frequency of the emitted laser radiation is therefore not determined by the energy transitions of the laser medium, but by the difference between the Raman scattering energy (e.g. phonon resonance) and the energy of the pump photons. The Stokes shift caused by Raman scattering is constant, so the output wavelength of the Raman laser or Raman amplifier can be determined by the pump wavelength.
The laser medium quartz glass (typical glass fiber ), for example, has a phonon resonance maximum at 13.2 THz and thus the output laser radiation should have a wavelength 53 nm longer than the pump radiation for optimal amplification .
If light shifted by the Raman frequency is generated in a Raman fiber amplifier by the process of stimulated emission , the relationship between the pump power P p and the signal power P s can be described by a differential equation system.
In order to construct a Raman fiber laser, frequency-selective Bragg gratings are written into the fiber based on the respective pump wavelength . For example, the fundamental wavelength or the respective arising from the Raman effect Stokes orders resonant reflect. Between these mirrors, the power of the pump light is coupled to the signal wave. In this way, cascaded Raman lasers can be constructed with the resulting first Stokes order pumping a second Stokes order, and so on. Raman fiber lasers can be pumped forwards or backwards like normal fiber lasers , depending on the point at which the pump light is injected. They offer a very good solution for providing light power in a very large wavelength range in a frequency-selective manner.
The first Raman laser was realized in 1962 by Gisela Eckhardt and EJ Woodbury in nitrobenzene , which was used for pumping in a Q-switched ruby laser .
The first fiber amplifiers based on the Raman effect were realized around 1975. A Raman fiber laser in the kilowatt range presented in, for example, consists of a seed laser and a downstream Raman fiber amplifier, which simultaneously generates its pump radiation (1070 nm) by the glass fiber being erbium-doped and pumped using diode laser radiation (976 nm). The output wavelength was chosen to be 1123 nm, which corresponds exactly to the above-mentioned optimal Stokes difference of 53 nm.
literature
- Rainer Engelbrecht: Nonlinear fiber optics. Springer Verlag 2015, pp. 431–492 (chapter Raman fiber laser ).
- Bahaa EA Saleh, Malvin Carl Teich: Fundamentals of Photonics. 2nd completely revised and expanded edition, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2008, ISBN 978-3-527-40677-7 .
Web links
- SUPPRESSION OF STIMULATED RAMAN SCATTERING IN FIBER LASERS (accessed December 22, 2017)
- Lasers in material processing (accessed December 22, 2017)
Individual evidence
- ↑ https://www.rp-photonics.com/raman_gain.html R. Paschotta: Raman Gain (Lexicon of RP Photonics Consulting GmbH), accessed on May 5, 2020
- ^ EJ Woodbury, WK Ng: Ruby laser operation in the near IR . In: Proceedings of the Institute of Radio Engineers . tape 50 , no. 11 , 1962, pp. 2367 , doi : 10.1109 / JRPROC.1962.287964 .
- ↑ Gisela Eckhardt, RW Hellwarth, FJ McClung, SE Schwarz, D. Weiner, EJ Woodbury: Stimulated Raman Scattering From Organic Liquids . In: Physical Review Letters . tape 9 , no. 11 , December 1, 1962, pp. 455-457 , doi : 10.1103 / PhysRevLett.9.455 .
- ↑ https://www.researchgate.net/publication/298912723_Bidirectional_pumped_high_power_Raman_fiber_laser Qirong Xiao et. al .: Bidirectional pumped high power Raman fiber laser , in Optics Express 24 (6): 6758, March 2016, DOI: 10.1364 / OE.24.006758, accessed on May 5, 2020