Tapered amplifier laser with frequency-shifted feedback

We thank the referees for their helpful comments.\

Here is our response.\

\textbf{referee 1}\

• Reference the abstract, http://adsabs.harvard.edu/abs/2014APS..MAR.C1323L found via Google, in fairness). In the last paragraph of Sec.~1, we changed we demonstrate for the first time an FSF laser that'' towe demonstrate an FSF laser that'' and added ``In mutually independent work, a similar system was developed for broadband laser cooling, as indicated by conference abstract [19].''.\

• Frequency referencing to a rubidium saturated absorption spectrum is worth mentioning in the paper and referencing, e.g. dois:10.1364/OE.22.010494, 10.1063/1.4801996. At the end of Sec.~2.3 we added ``The comb can be stabilized in frequency by locking the seed laser to a desired frequency, e.g. a Rb spectroscopy line. In previous work such stabilization required an additional lock of a comb line to a reference laser, see [13, 22] and references therein.''\

• Comment on the limitations in comparison to other frequency combs. In Sec. 4 we added ``In contrast to other frequency combs [23, 8] our comb has a narrower spectrum. It covers 0.7\,nm, which is a fraction of the 3dB gain bandwidth of our TA (11\,nm). We observe that the spectrum can be broadened by increasing the AOM frequency, although less than proportional to that frequency. Broadening the comb width would be valuable to increase the range of application of our scheme and could be a topic for further research. Still, a comb spanning hundreds of GHz is relevant to many applications and we plan to use the frequency comb of our FSF laser as frequency reference for photoassociation spectroscopy of RbSr molecules [24].''\

• P3 1\,MHz linewidth - on what timescale? rms? a linewidth below 1\,MHz.''$\rightarrow$a linewidth below 1MHz on a timescale of 5us.''\

• P3 344.25\,THz $\rightarrow$ 384.25\,THz done\

• P3 Find and replace groves $\rightarrow$ grooves done\

• P5 Figure 3 has seeded output at ref freq. (c,d) but not described in text until 3.2. We think that a reader should be able to see from the figure caption that Fig. 3b belongs to Sec. 3.2. If we would slit the figure the reader would have a harder time to compare Fig.3a and Fig.3b, which would be a loss. Shifting it to after the start of Sec.3.2 would or shifting it to after the start of Sec. 3.2 would force the reader to flip through more pages when discussing Fig.3a and shift all later figures away from the page on which they are referred to. We chose to leave the figure as is.\

• Figure 3 caption needs capitalisation for each section. done\

• P5 by orders of magnitude'' $\rightarrow$by four orders of magnitude'' done\

• P7 as blue curve $\rightarrow$ as shown in the blue curve. For comparison we also show the spectrum obtained by the grating spectrometer as blue curve in Fig.~4.''->For comparison we also show the spectrum obtained by the grating spectrometer in Fig.~4 (solid blue).''\

• Fig 5 => Please check this is the total integrated power, and not the spectral power density in dB/Hz (if so please correct accordingly). Formerly indeed the spectral power density was given. We now integrated the signals over frequency. The vertical label is changed to Relative peak power'' and in the caption we addedThe relative peak power is obtained by integrating peaks such as the ones shown in Fig. 3a2 and 3b2 over frequency and comparing them to the noise floor.''.\

\textbf{referee 2:}\

• I only have one minor question: Is a qualitative understanding of the "global" properties of the output spectra in Figs.~2 and 3 possible? Am I e.g. correct in assuming that the width of the optical spectrum is mainly determined by (or at least roughly proportional to) the AOM frequency, plus some other smaller contributions? The authors list 370\,GHz as the upper limit for this width in the seeded case, but over what range can this be changed (both in the seeded and unseeded case)?\

We do not have a full understanding of the properties and limitations of our spectrum. We tried to increase the spectrum and found experimentally that increasing the AOM efficiency and frequency helped. But the width of the comb is not proportional to the AOM frequency, it grows less strongly. We speculate that the increased amount of amplified spontaneous emission in a frequency band corresponding to the AOM frequency reduces the number of amplified roundtrips through the cavity. We clarify the relation of spectral width and AOM frequency by the following modifications.\

The full width half maximum of the spectrum is 120 GHz.'' $\rightarrow$The full width half maximum of the spectrum is 120\,GHz independent of the center frequency. The spectrum can be broadened by increasing the AOM frequency, as shown in Sec.\,3.2.'' This points the reader to the already existing text ``Figure 4 shows the spectrum of a frequency comb spanning 370\,GHz. To obtain a frequency comb over this larger spectral width compared to before we find it necessary to increase the AOM frequency and diffraction efficiency. In the following we worked with f$_{\rm AOM}$ = 370\,MHz and optimized the diffraction efficiency to 87\%.''\

Finally we tell the reader that more work is needed to understand the spectrum, especially its width, at the end of Sec.~4, see our response to ``Comment on the limitations in comparison to other frequency combs.'' above.\

\textbf{other changes:}\

Figure labels corrected (To do: list changes)\

The FSF laser exhibits coherent modeless broadband radiation''$\rightarrow$The FSF laser emits coherent modeless broadband radiation''\

``A. Bayerle$^{1,*}$, S. Tzanova$^{2,3}$, P. Vlaar$^1$ B. Pasquiou$^1$ F. Schreck$^{1,y}$"$\rightarrow$"A. Bayerle$^{1,*}$, S. Tzanova$^{2,3}$, P. Vlaar$^1$, B. Pasquiou$^1$, and F. Schreck$^{1,y}$"\

use of a Yb$^{3+}$ doped fiber amplifiers''$\rightarrow$use of Yb$^{3+}$ doped fiber amplifiers"\

P5: "an arm length difference of 26.0(4)\,cm."$\rightarrow$"an arm length difference of 26.00(4)\,cm.''\

added sentence on P6: ``The power of the seed laser at the TA input facet was 80$\mu$W.''\

P5: Each sub-system will be now described in detail.''$\rightarrow$Each sub-system will now be described in detail.''\

P3: used to resolve the mode structure of the FSF laser, is shown in Fig.~1c.''$\rightarrow$used to resolve the mode structure of the FSF laser, is shown in the lower part of Fig.~1c.''\

P6: contains additional sharp lines at integer multiples of f$_{AOM}$''. $\rightarrow$contains additional sharp lines at integer multiples of f$_{/rm AOM}$.''\

P7: is present in the spectrum for all seed intensities presented here.''$\rightarrow$is present in the spectrum for all seed intensities explored here.''\

P5: Fig. 3a and 3b''$\rightarrow$Fig. 3a2 and 3b2''\ P8: Fig. 6'' $\rightarrow$Fig. 5''