Laser Development

Research

Laser Development

For most of our research projects, specific laser systems which are not commercially available are required. Therefore, a significant portion of our work is dedicated to develop and improve laser systems for a broad range of applications. These include both pulsed and cw laser systems, covering a wide range of target wavelengths from the infrared to the ultraviolet spectral range. To achieve the desired wavelengths, many systems utilize self-constructed ECDLs and use frequency conversion employing diffrent kind of nonlinear crystals.

Laser systems and wavelengths

LWI	254nm (FHG of ECDL 1015nm, 2 cavities LBO BBO) 405nm 436nm (ECDL 872nm SHG in cavity PPKTP) 546nm (ECDL 1092nm SHG cavity LiNbO3)
LIDAR	1087nm DFB pulse SHG single pass to 543nm in KTP
SPARC	515nm and 257nm pulse and cw LBO and BBO
MOT	254nm (ECDL 1015nm, FHG LBO BBO)
Qhub	1550nm

CW Nd-doped fiber amplifiers

We demonstrated amplifying the output of a MISER to several Watt power without increasing its line width.

Picture: AG LQO

Output power as a function of pump power. The output power was limited by the available pump power.

Picture: AG LQO

Spectra of the output of the fiber amplifier. (a) broadband spectrum (b) Spectrum of scanning Fabry-Perot interferometer (the inset shows the spectrum of the MISER) (c) Heterodyne beat note from output with second MISER

CW Yb-doped fiber amplifiers

Absorption and Emission spectra of Ytterbium.

We use Yb-doped fiber amplifier to generate high output powers at 1014.8nm and 1029nm for different applications. Recently we have demonstrated up to 30W of output power using a Yb-doped fiber amplifier, as can be seen in the graph.

Output power using Yb-doped fiber amplifier

Recently we have demonstrated up to 30W of output power using an Yb-doped fiber amplifier, as can be seen in the graph.

Slope of a Yb-doped fiber amplifier at 1029nm.

Pulsed Fiber Amplifier

A four stage Yb-doped fiber amplifier with Fourier-transform limited pulses in the ps-regime has been successfully implemented. Using photonic crystal fibers for the two last stages, output powers up to 57W can be achieved regardless of the pulse duration, as can be seen in the graph.

A three stage Yb-doped fiber amplifier exhibiting Fourier-transform limited pulses in the ns-regime has been demonstrated. The power was limited by the onset of stimulated Brillouin scattering. A careful optimization of fiber parameters was necessary in order to achieve the energy levels in question.

Slope of the fourth Yb-doped fiber amplifier stage at 1030nm for two different puls duration.

Picture: AG LQO

Peak power and energy per pulse as function of the pulse duration

Picture: AG LQO

Output energy of the third stage at the fundamental and after SHG

References

Kai Schorstein and Thomas Walther
A high spectral brightness Fourier transform limited nanosecond Yb-doped fiber amplifier, Applied Physics B 97 (2009)591Applied Physics B 97 (2009)591
R. Nicolaescu, Th. Walther, E. S. Fry and M. Muendel, Ultra-narrow linewidth, efficient amplification of low power seed sources by a fiber amplifier, Appl. Opt. 38 (1999) 1784 (opens in new tab)

Let's assume we have a laser beam at a given wavelength impinging on a non-linear crystal. It is possible to observe a second laser beam coming out of the crystal with twice the frequency of the original laser beam, which then is the second harmonic. This effect comes from the nonlinear polarization of the crystal induced by the strong electric field of the laser beam. Also other nonlinear effects can occur such as sum frequency generation (SFG), difference frequency generation (DFG) and optical rectification (OR). The exact process depends on the type of non-linear crystal the geometry at which the crystal is cut polarization of the input beams. But two other conditions must be fulfilled: the conservation of energy and the conservation of momentum, which is called phase matching. The phases of the second harmonic wave must be in phase throughout the crystal such that the different parts interfere constructively at the end of the crystal and a macroscopic wave at the second harmonic emerges form the crystal. This phase matching condition can be fulfilled by the different refractive indices of a birefringent crystal.

CW Type-II Second Harmonic Generation without Temperature Control

Setup of the build-up cavity. By placing a lambda/2 plate inside the cavity, the cavity is only closed after the second round trip.

References

Y. Emery, A. Fleischhauer, Th. Walther and E.S. Fry, Angle Tuned, type-II, external cavity frequency doubling without temperature stabilization, Appl. Opt. 38 (1999) 972

CW Type-I Second Harmonic Generation

We routinely frequency double and quadruple (via two consecutive SHG) the output radiation of our home built laser systems such as fiber amplified ECDL sources.

To achieve high conversion efficiencies and high output powers, we design enhancement cavities around the nonlinear crystals. Since many of our wavelengths of interest are deep in the ultraviolet spectral range, Beta Barium Borate (BBO) is the nonlinear medium of choice. Since BBO crystals are prone to degradation, the cavities are elliptically focusing enhancement cavities, reducing the effect of degradation while maintaining a high conversion efficiency. We recently demonstrated output powers of up to 18W at 515nm by employing LBO as the nonlinear medium. We were also able to demonstrate an output power of up to 2.45W at 257nm without degradation of the BBO crystal.

References

Jens Gumm, Denise Schwarz, Thomas Walther

High Power UV Lasers Employing Elliptically Focusing Enhancement Cavities

Review of Scientific Instruments, vol. 96, issue 3 (2025)

Daniel Preißler, Daniel Kiefer, Thorsten Führer, Thomas Walther

Evolutionary algorithm‐assisted design of a UV SHG cavity with elliptical focusing to avoid crystal degradation

Appl. Phys. B 25 (2019) 220 (opens in new tab)

Picture: Jens Gumm

Slope of the green power against the incident infrared light.

Picture: Jens Gumm

Slope of the UV power against the incident green light.

Pulsed SHG/THG/FHG

By a customized, very compact design of our frequency conversion unit we achieved high conversion efficiencies for second harmonic, third harmonic and fourth harmonic generation. Dual-wave plates (acting as lambda waveplates for the fundamental and lambda/2-plates for the harmonic) are placed in between non-linear crystals in order to use type-I processes in each step.

We recently showed SHG processes with conversion efficiencies up to 60% reaching an output power of up to 34W at 514nm with picosecond pulses. With that we were able to demonstrate output powers of up to 5.3W at 257nm by employing two BBO crystals.

References

T. Beck, Master Thesis, Darmstadt, 2009
C. Tian, Th. Walther, R. Nicolaescu, X.J. Pan, Y. Liao and E.S. Fry, Synchronous, dual-wavelength, injection-seeded amplification of 5 ns pulses in a flashlamp-pumped Ti:sapphire laser, Opt. Lett. 24 (1999) 1496-1498
D. Depenheuer, J. Kohl-Landgraf, H. Gläßer, Th. Walther, A pulsed laser system with large spectral coverage by nonlinear frequency conversion, appeared Applied Physics B (2009)

Picture: Benedikt Langfeld

Slope of the green power against the incident infrared light for different pulse durations.

Picture: Benedikt Langfeld

Solpe of the UV power against the incident green light with different BBO setups.

Research