CC BY
5ALT'22
LD-O-15
LASER DIAGNOSTICS AND SPECTROSCOPY
Interference method for the selection of homogeneous lines in the spectrum of a superluminescent source
S. Morshnev12, N. Starostin1,2, Y. Przhiyalkovskiy12, A. Sazonov1,2
1 - Kotelnikov Institute of Radio Engineering and Electronics of RAS, Fryazino, Russia
2 - SPC Profotech, Skolkovo innovation center area, Bolshoi bulvar 42, Moscow, Russia
E-mail: morshnev@profotech.com
It is known [1,2] that an optical detector with an adjustable spectrum resolution 5X can play the role of a tunable spectral filter with a width 5X. An increase in the filter bandwidth 5X leads to a reduction in the coherence time of the received wave packet. In particular, it is so for the Yokogawa AQ6370C spectrum analyzer, which has a set of discrete spectral resolutions from 5X = 0.02 nm to 5X= 2 nm. By introducing a time delay between two orthogonally polarized wave packets and interfering them, we actually control the degree of overlap of wave packets using visibility of the interference pattern (IP). At the same time, the IP is observed on the spectral analyzer using a wavelength sweep. If the coherence time t is determined by transmission spectrum of the filter, then an increase in the bandwidth 5X of the filter leads to a reduction in the coherence time t, a decrease in the overlap of wave packets and, consequently, to a decrease in the visibility V, which is the same across the entire spectrum of the source. In this situation, the homogeneous structure of the source spectrum does not manifest itself, everything is determined by the spectral width of the spectral filter of the detector.
The width of the spectrum of the homogeneously broadened lines of the superluminescent Er-Yb light source can also limit the coherence time of the detected wave packets if the filter bandwidth becomes wider than the spectrum of the uniformly broadened line. In this case, the coherence time of the wave packet is determined by the homogeneous line width in this region of the spectrum. It is clear that in different regions of the spectrum, different homogeneously broadened lines may have different widths, which will entail different visibility values V(X) for different wavelengths within the source spectrum. In a situation when the source spectrum determines the visibility function V(X), and the width of the detector filter 5X is only greater than a certain value 5XO, it can be argued that 5XO is a uniform width of the source line.
X (nm)
Fig.1. The dependence of visibility on the wavelength. a) circles - 5X = 0.05 nm; triangles - 5X = 0.1 nm; squares - 5X = 0.2 nm; rhombuses - 5X = 0.5 nm.
From the analysis of Fig.1, it can be seen that up to the filter width 5X = 0.5 nm, there is not spectral dependence of the IP visibility, but its value decreases due to a reduction in the overlap of interfering wave packets. Starting from 5X > 0.5 nm, there is a spectral dependence of visibility within the boundaries of the inhomogeneous spectrum of the erbium source.We estimate the coherence time of the wave packet to be t = 1.6 * 10-11 s and the length of wave packet to be 4.8 mm >> X in vacuum. The work was carried out within the framework of the state task of the Kotelnikov Institute Radio Engineering and Electronics of RAS.
[1] Mandel L., Wolf E., "Optical coherence and quantum optics",M., Fizmatlit, (2000).
[2] Mandel L., Wolf E., Proc. Phys. Soc., 80, 894, (1962)
