Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript reports the self-developed 1 GHz repetition rate Ti:Sa optical frequency comb (OFC) at Argentina’s National Metrology Institute (INTI), which is the first domestic OFC system in Argentina. The authors built a prism-free bow-tie Ti:Sa femtosecond oscillator, generated octave-spanning supercontinuum (500-1100 nm) via PCF nonlinear broadening, and realized full self-referencing by phase-locking both repetition rate frep​ and carrier-envelope offset f0​ to a Cs atomic clock national frequency standard. Long-term locking stability and Allan deviation of two locking loops are systematically characterized. As a practical demonstration, the authors calibrated the frequency instability of a 1064 nm Nd:YAG laser by beat-note measurement of its second harmonic with the comb. The work fills the blank of optical frequency traceability in Argentina, with complete optical setup, clear stabilization schemes and sufficient measured data, matching the positioning of Metrology focusing on national metrology infrastructure and laser frequency calibration. However, multiple deficiencies exist in novelty comparison, stability optimization analysis, metrology performance quantification and application expansion, requiring substantial revisions before acceptance. Detailed comments are listed below.

 

1. The paper mentions 50% supercontinuum conversion efficiency but does not analyze the influence of coupling objective focusing, chirped mirror reflection times and PCF input power on broadening efficiency. Supplement a brief parametric test discussion to optimize the octave-spanning spectrum flatness.
2. The manuscript only measures RF beat-notes of f_repand f₀, without characterizing the optical phase noise of comb teeth. It is recommended to supplement phase noise spectral density curves of the stabilized comb at offset frequencies from 10 Hz to 1 MHz, to fully evaluate spectral purity for high-precision metrology.
3. The literature review lacks recent works on Latin American national metrology institute OFC construction; add regional comparative references to highlight the regional breakthrough of this system. Rearrange Section 5 Conclusions to clearly separate current achievements, existing limitations and detailed future engineering improvement routes.
4. The author should clearly indicate the accurate signal-to-noise ratio in Figures 3b, 4b, and 7b. I believe that Figures 4b and 7b did not reach 35dB and 30dB, respectively. Moreover, the @ 100kHz marked in the figure is not appropriate, and the resolution should not be labeled, but the center frequency should be labeled.
5. Figure 1b marks multiple optical clock transition wavelengths but lacks corresponding power intensity comparison at each characteristic wavelength; add spectral power values at key metrology wavelengths to prove effective beat-note detection potential.

Author Response

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Reviewer 2 Report

Comments and Suggestions for Authors

In the manuscript by Fernando Yapur et al., the authors systematically study the first optical frequency comb based on a home-built Ti:Sa femtosecond laser in Argentina. The result analysis for laser system is interesting. However, in order to be considered for publication, the following comments need to be addressed in detail.

1. The supercontinuum generation is optimized by eight reflections on a chirped mirror pair. What is the criterion for selecting such specific number of reflections during the experiments?
2. The f0​ lock is reported to be limited to ~1 hour due to thermal drifts causing the feedback signal to saturate. What are the specific free-running fluctuation amplitudes of f0​ observed over typical timescales?
3. The supercontinuum generation relies on coupling the ~900 mW, 30 fs pulses into a PCF. How is the long-term coupling efficiency and alignment stability maintained during the experiments? Is the coupling setup enclosed and actively stabilized, or is it manually re-optimized periodically?
4. For the Ti:Sa fs oscillator, is there any active stabilization of the carrier-envelope offset frequency’s free-running value beyond the slow pump control? Over the 16-hour frequency​ lock, does the laser operating in a single mode family (i.e., no mode-hops), or was any monitoring of the mode index performed?

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript entitled “Development of a high repetition rate Ti:Sa optical frequency comb at INTI for laser frequency traceability and calibration” (corresponding author M. del Pilar Campos Marino) submitted to the Metrology journal describes development of a Ti:Sa optical frequency comb that is frequency-stabilized against a Cs reference and the characterization of its metrological performances.

First part of this manuscript addresses the setup of the home-built Ti:Sa oscillator. Its spectrum, broadened with a photonic crystal fiber, is relatively flat over one octave and covers many recommended frequencies of atomic and molecular clocks. The optical comb is frequency-stabilized against the signal of a commercial Cs microwave clock. A phase-locked loop is implemented for f_rep stabilization. Standard f-2f interferometry technique is implemented for carrier-envelope offset frequency stabilization. The performances of the frequency locks are characterized with Allan deviation plots and spectra of in-loop signals. This setup allows to measure frequency instability of a frequency-doubled free-running laser at 1064 nm.

I recommend to the authors to resubmit a major revised version of the manuscript. Remarks and issues on this version of the manuscript are indicated as follows:

Section 1: The references have to be updated to cover extensive literature that is available for applications of stabilized frequency combs, particularly for many developments realized in the last years (ex.: microcombs, tests of variations of fundamental constants,…).  Add references to relevant optical clocks that may be measured with a stabilized and broadened Ti:Sa frequency comb.

Section 1: Check and fix the order of references in the manuscript.

Section 2: There is no indication about the optical design of the home-made Ti:Sa oscillator. Please provide all relevant parameters of the laser cavity mirrors, Ti:Sa crystal, and sizes of the green and infrared optical beams.

Lines 126—127: Explain precisely how the frequency of signal is divided by a factor of 10 (relate to a specific circuit or technical procedure)

Lines 121—123: The assertion is not clear, please explain precisely: which are the correction requirements (ex.: frequency bandwidth and optical pathlength change covered with the PZT) and which is the link with long term stabilization of f_rep.

Figure 4 legend: Again, explain precisely how the frequency of signal is divided by a factor of 128 (relate to a specific circuit or technical procedure)

Lines 74, 122, 153, 154: Fix please typesetting for f_rep

Line 196: Demonstration of the capability of the stabilized optical frequency comb as transfer oscillator is incomplete. This development is aimed towards measurement of optical frequencies against the Cs clock. Please realize and indicate the result for the measurement of the frequency-doubled 1064 nm laser, particularly the absolute optical frequency and its uncertainty.

Reference 23: Please insert a space between the title of this reference and the journal name.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The author has made great improvement and revised the manuscript upon the reviewer's comments from both experiment data and analysis. I believe the revised manuscript is suitable for publish in Metrology

Reviewer 3 Report

Comments and Suggestions for Authors

The authors took all referee’s comments and included in the revised version of the manuscript suitable corrections. The referee proposes publication of this version of the manuscript in the Metrology journal.