Ti:sapphire broadband laser

(nur in englisch verfügbar)

Powerful light source for the coherence radar

That´s a part of the work to the PhD of Kay Mittler.

One aim of the research project is the investigation of concepts for the setup of a spectrally broadband and powerful light source with good beam quality. Possible applications are in the optical measuring technique for procedures of coherence radar as well as optical coherence tomography.

Schematic setup of the Ti:sapphire laser and pump source

With the schematic setup represented above the Ti:sapphire laser with a frequency-doubled, passively q-switched Nd:YAG laser is longitudinal pumped. The pump source emits pulse trains (Bursts) from 1 ms to 1.5 ms duration and 16 to 41 q-switch-pulses in the burst with a repetition rate of 25 Hz in a TEM₀₀ - mode. The conversion efficiency from 1064 nm (IR) to 532 nm (green) is around 35%. The frequency-doubled and IR-free light is focused with the lens L in the Ti:sapphire crystal on beam radii between 80 and 230 µm. The light of the Ti:sapphire laser is selected by an edge filter and concerning the pulse and burst energy, which examines transversal structure and the spectra of the light over the entire burst as well as the spectra of the single pulses.

Characteristics of the pump source

The pump source of the Ti:sapphire laser supplies 16 q-switch pulses with a pulse duration of approx.. 15 ns, a pulse distance of approx.. 65 µs and a repetition rate of 50 Hz in the TEM₀₀.

Characteristics of the Ti:sapphire laser

Histograms of the output energy of the Ti:Saphir of laser
for different reflectivities of the output coupling mirror

The Ti:sapphire laser supplies a max. output energy of 7.3 mJ ± 3% in a burst with a slope efficiency of 36% with an OC-mirror reflectivity of 80%.

Spectra of the light of the entire pulse train

The spectra of the first pulse in the burst as a function of the
pump energy

The analysis of the spectra of the Ti:sapphire laser as a function of the pumping energy resulted in that the spectrum of the first pulse is almost independent in the burst. However the spectrum of the entire bursts shows a strong dependence concerning the central wavelength and the spectral width.

Spectra of the Nth pulse in the burst at a pump(-burst) energy of
23 mJ

Spectrum and FFT for the light of the entire pulse train

With high pumping energies still another further effect arises. Apart from the substantial widening of the "burst spectrum" one observes a jump of the central wavelength of the later pulses to smaller wavelengths with the simultaneous emerging of higher transversal structures. This effect shows strong dependence on the pumping radius (see below), the pump pulse energy and the temporal distance of the pulses.

Coherence radar measurements

Gray-scale plot of height of a black rubber seal, measured with the
coherence radar of the university of Erlangen-Nuernberg

With the sources of light used so far no dark and strongly curved surfaces and no larger measuring fields could be measured. In first measurements of the coherency radar with source of light presented above could be extended the lateral measuring field on 75 mm x 75 mm with simultaneous increase of the measuring speed on 13.2 to 60 µm/s as well as the longitudinal measuring depth on 11 mm. Only about the half of the available energy was used.

Gray-scale plot of height of a car body sheet,
measured with the coherence radar of the university of Erlangen-Nuernberg

Gray-scale plot of height of a car body sheet,
measured with the coherence radar of the university of Erlangen-Nuernberg

Coherence radar measurements of large and dark objects

Gray scale plot of height and cuts of a black mobile telephon

Measuring speed: 13.2 µm/s
Measuring depth: 5 mm

Field of view: 75 x 75 mm²
Lateral resolution: 147 µm
Power of light on the object: 60 mW

Gray-scale plot of height of a black mobile phone, measured with the
coherence radar of the university of Erlangen-Nuernberg

Coherence radar measurements of large and dark objects

Gray-scale plot of height of a turbine blade

Measuring speed: 26 µm/s
Measuring depth: 8 mm

Field of view: 69 x 69 mm²
Lateral resolution: 135 µm
Power of light on the object: 60 mW

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