With current endeavours in the UK and around the world to foster the development of new Quantum-based technologies (QT) to sense and detect previously unreachable interactions, such as underground structures and gas leakages, the generation of light sources with highly stabilised output wavelengths have become critical. Diode-pumped solid-state (DPSS) and external cavity diode (ECD) lasers are two of the ideal candidates for narrow linewidth single frequency generation.
This paper will discuss some of their spectral performance differences.
Current laser technologies are compared upon SWaP-C (size, weight, power & cost) figures.
Laser operation is supported for multiple lasing modes with a given cavity geometry and lasing medium. The excitation from the pump source is therefore distributed amongst the supported modes. The first area of interest for narrow-linewidth lasers is therefore mode selection to ensure single longitudinal mode (SLM) operation.
In this case, the power in the lasing mode is proportional to the number of photons associated with the spectral position of that mode. While, the total power is given as the sum of the power stored in the lasing mode and the additional spontaneous terms.
The theoretical minimum linewidth or the intrinsic linewidth is given by the Schawlow-Townes limit:
In this equation Δf is the instantaneous linewidth, whilst α is the detuning term first introduced by Lax. The above formula gives an accurate estimate for the linewidth of each system, however Henry showed that for a semiconductor-based system the linewidth can be at least 50 times broader than previously expected. This was explained by the increased spontaneous emission factor relevant to semiconductor lasers.
In this paper we assessed the spectral performance differences of DPSS and ECD lasers. We showed that due to lower side-mode suppression encountered in ECDLs, less power is coupled in the specified linewidth of the system. We reconstructed some of the most recently published narrow-linewidth ECDL spectra to compare that against one of the best Nd:YAG DPSS lasers in the market. We used the term spectral density contrast (s.d.c) to illustrate the percentage of power emitted within the specified linewidth of the system. In the following analysis we converted the spectral data from a logarithmic into a decimal representation.
Fig 1. Illustrating the spectral density contrast estimates for the analysed systems, each subfigure was reconstructed from published data, which was then transformed from a logarithmic to a linear representation. Following captions define (the source, specified linewidth and s.d.c. figure) of each system. (a) Coherent's Mephisto DPSS laser, 1kHz, 80.28%, (b) Kasai et al., 14kHz, 66.34%, (c) Schkolnik, et al., 88kHz, 58.54%, (d) Laurain, et al., 124kHz, 27.73%, (e) Bennets et al., 5.2kHz, 24.93%, (f) Zhu et al., 70kHz, 23.24%.
One application for narrow-linewidth lasers is super-high-resolution spectroscopy. Cold atom gas cells are frequently used as absolute-frequency references in these experiments. Absorption broadening is purely defined by the Doppler effect, which results in a Gaussian-shaped absorption line. In the following graph we illustrate the spectral overlap of an arbitrary laser emission curve with the absorption of a gas cell.
This shows that the application-specific useful power of the system depends on the line shape of the laser.
In this respect we can expect a DPSS laser with a lower specified output power to perform comparatively to an ECDL with a higher output.
We showed that due to the dissimilarities in the line shape broadening between ECD and DPSS lasers, there exists a difference in the spectral density contrast. This consequently indicates that given the same outpower specification, the two platforms might provide different levels of useful power.
For the examples illustrated in this paper, we found that while an s.d.c. of above 80% can be achieved in the DPSS case, for ECDLs this figure can be as low as 25%, meaning that only a quarter of the emitted power is contained within the specified linewidth of the system. This in turn can have implications on the corresponding SWaP-C figures and the overall feasibility of the two laser platforms.
For a full list of references, please refer to the PDF version of this article.
Spectral density contrast in DPSS and ECD lasers for quantum and other narrow-linewidth applications - B. Szutor, F. Karpushko | UniKLasers Ltd., Strathclyde University, EPSRC CDT for Applied Photonics.