Realisierung und elektronische Stabilisierung von Diodenlaser gepumpten einfach-resonanten kontinuierlich-emittierenden optisch parametrischen Oszillatoren aus periodisch gepoltem Lithiumniobat (English shop)

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Singly-resonant continuous-wave optical parametric oscillators (cw-SRO) are excellent coherent radiation sources for applications requiring a narrow spectral width and a wide wavelength tuning range. These include, for example, applications in radiometry, trace gas analysis in molecular spectroscopy, or high-precision frequency measurements. Particularly for trace gas analysis, wide continuous tuning ranges in the GHz regime are necessary on the one hand, in order to cover the largest possible portion of the frequency range under investigation. On the other hand, the smallest possible linewidth is important in order to be able to determine precisely the exact positions of the detected absorption lines. The aim of the present work was therefore to realise a widely tunable cw-SRO and to stabilise its wavelength to the maximum of a molecular line. The wavelength tuning is to be carried out purely electronically and to cover a larger range than previously known systems.

A diode laser with a DFB structure at an emission wavelength of 923 nm was used as the pump system for the cw-SRO. The power of the emitted DFB radiation was 50 mW and was amplified to 3 W by means of a tapered amplifier. The oscillator–amplifier combination is referred to as a MOPA system. The pump radiation of the MOPA system was single-frequency with a spectral width of less than 5 MHz and nearly diffraction-limited. By changing the excitation current of the DFB laser diode between 100 mA and 200 mA, the wavelength of the pump radiation could be tuned continuously and without mode hops by more than 125 GHz.

The cw-SRO is based on a lithium niobate crystal periodically poled for quasi-phase matching (QPM). QPM has the advantage of allowing the use of the largest nonlinear coefficient of the material. The conversion efficiency is increased and the threshold pump power is reduced. This furthermore leads to a significant increase in the output power of the cw-SRO. The wavelength of the converted radiation depends only on the poling period in the QPM crystal. It is therefore possible to shift the emitted wavelengths into new ranges solely by varying the poling period. In this way, spectral ranges can be covered that have so far not been accessible with lasers.

The cw-SRO was designed as a ring resonator and, at a threshold pump power of 1.85 W, achieved a maximum output power of the idler wave of more than 420 mW at 2.1 µm. The generated signal and idler radiation was single-mode with a spectral width of less than 10 MHz in each case. The wavelengths of the signal and idler waves were first tuned coarsely by changing the crystal temperature and by varying the poling period in the crystal. In this way, the complete wavelength range limited by the resonator mirrors, between 1.55 µm and 1.7 µm (signal wave) and 2.02 µm and 2.28 µm (idler wave), could be covered. Fine wavelength tuning was accomplished by changing the diode current of the DFB laser. By varying the diode current between 100 mA and 200 mA, the idler wave could be tuned by 88.4 GHz. This is the largest mode-hop-free tuning range achieved to date for an SRO pumped directly by a diode laser.

In order to demonstrate the applicability of the cw-SRO to molecular spectroscopy, absorption measurements on nitrous oxide were carried out. The position and linewidth of a single molecular line could be measured exactly. The pressure broadening coefficient of 5.9 MHz/mbar and the absorption coefficient of 4.3 • 10−2 cm−1 agree very well with values known from the literature. The absorption line could be unambiguously assigned to a rotational line from the R branch of the (0,0,0,0) – (2,0,0,1) transition at 2.103 µm (4754 cm−1). In a second experiment, five adjacent absorption lines of nitrous oxide were recorded simultaneously over an idler wavelength tuning range of 48 GHz. The line spacings agree very well with values known from the literature. At the same time, the resolution is higher by a factor of 30 compared with an FTIR measurement.

Lock-in technique was used to stabilise the idler wavelength to the maximum of an absorption line. An error signal was generated from the absorption signal, by means of which the electronics can control the diode current of the DFB diode such that the frequency of the idler wave always corresponds to the maximum of the absorption profile. In this way, a frequency stability of the idler wave of ± 30 MHz could be achieved. Thus a compact system was developed which is purely electronically tunable and which, by virtue of its properties such as spectral width, tunability and frequency stability, offers great potential for spectroscopic investigations.

In conclusion, it can be summarised that the system presented in this work represents an alternative to previous SRO systems or diode lasers for spectroscopy. Compared with systems pumped by a solid-state laser, the enormous compactness of the system presented in this work should be emphasised. The efficiency is comparable for both variants. The wide tuning range, the narrow spectral width and the good frequency stability make its use possible in high-precision frequency measurement, laser cooling and trace gas analysis. The selection of the wavelength is accomplished purely electronically and is therefore insensitive to external disturbing influences such as vibrations. By simply varying the poling period of the QPM crystal, it is possible to freely choose the wavelength range between 1 µm and 5 µm.