In our paper [1], we presented a new type of cavity using chirped fiber Bragg gratings (CFBG) to radically enlarge a resonance mode while maintaining its build-up time for every wavelength within the cavity. This theory was made by supposing a varying propagation phase $\beta z$within a CFBG, with $z={\delta }_1\Delta \lambda$. The resulting linear phase term depending on the chirp enables us to cancel the wavelength dependence on the resonance term, thus enabling the broad-band mode. This term was not shown in previous literature, for it varies rapidly and therefore hard to see in numerical calculation where phase remains within 2π. Experimental measurements agreed perfectly with the theoretical expectation of this supposition, as well as experimental behavior when changing length of cavity, where the center point of the broad-band-mode was expected to shift of ~1 nm per mm of change.

However, this erratum brings to light an experimental artefact that acts the same way as the theory. It was discovered that a de-synchronisation between our scanning laser sampling rate and the free-spectral range of cavity modes of a Fabry-Perot (FP) could yield to the same experimental measurement as shown in Fig. 5 and Fig. 6 of [1]. Indeed, this happens if the sampling rate is a multiple of the free-spectral range (FSR) of such a cavity and the resulting de-synchronisation give the quadratic shape of Fig. 5 and Fig. 6. The lengths of 0.8 m and 0.5 m that were tested, lengths chosen for practical fabrication reasons, do indeed yield approximate FSR of 3 and 1.5 pm respectively. Since, another length was then tested at 0.9 m, which did not show any conclusive broadband mode cavity (BBMC) effect, which therefore casts doubt on our conclusions concerning our experimental results. Indeed, these seem to be caused by a standard FP cavity formed by the CFBG.

We believe that if the BBMC effect as presented by the theory is truly present, it is hidden within this FP response due to a delocalized reflection within the CFBG where the wave “sees” the whole grating and thus leads to no propagating phase variation with wavelength. Therefore, the question of whether this effect is true remains unanswered and further experiments are required to validate this theory.