Manuscript ID: 409532 Type: research article Title: A wide-range wavelength-tunable photon-pair source for characterizing single-photon detectors Author: Christian Kurtsiefer; National University of Singapore Dear Dr. Kurtsiefer: We are really sorry for our late answer, we waited unsuccessfully for the answer of a referee who had promised us a report. Your manuscript can be accepted for publication in Optics Express provided that you address the reviewer concerns. The reviews are appended below. It is the policy of Optics Express to allow only one revision cycle. Thus, it is important to include as part of your resubmission a detailed response to all of the reviewer points. You will upload your response letter in the online system. Of course, you may not agree with the reviewers on every point and, in that case, your responses and reasoning should be clearly presented. Please note that a response letter is required before a final manuscript decision can be made. 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To do this, the relevant author must click on the “Update Account” link on their Prism homepage or log-in directly to https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2faccount.osa.org&umid=bd52af05-70c5-4bf5-b3d0-2bfc4ea95904&auth=8d3ccd473d52f326e51c0f75cb32c9541898e5d5-79b6e4f2451e7c2d3463dee2015afa2c8b43fc5c, then click the button “Create or Connect your ORCID iD” in the ORCID section of the Participation tab. If the ORCID window does not appear, then change your browser settings to enable pop-ups. --------Reviewer Comments-------- Reviewer 1: This is a remarkable technique to precisely characterize the temporal response of SPADs or other single photon detectors. In particular it achieves a great wavelength accuracy thanks to the use of multiple reference optical filters. The technique is very well described making it reproduceable in other labs. The manuscript also reports for the first time, as an application of the technique, a high resolution characterization of the temporal response of a SPAD as a function of wavelength that surprisingly shows differences over a small range of few tens on nm. PS. I found a small typo in the first page: "we characterize timing the behavior" is written instead of "we characterize the timing behavior" Reviewer 2: The authors describe a tunable photon pair source based on parametric down-conversion used to characterize temporal response of single-photon detectors by measuring cross-correlation between output signal recorded from single photon detectors. The source is based on a BBO non-linear crystal; the photon pair generated can be tuned over a wide range (from 526 nm to 661 nm on the visible side and from 1050 to 1760 nm on the infrared side) changing the phase matching condition. This source allows to cover the two most interesting regions of interest for optical quantum technologies and to use as a reference detector superconducting nanowire detectors (SNSPD). The experimental results shows good temporal response of InGaAs avalanche photododiode, essentially independent from wavelength (1200 nm to 1600 nm), while for thin Silicon avalanche photodiode for the visible range (540 nm to 640 nm) the temporal response is significantly dependent on wavelength and an exponential tail should dominate temporal response at higher wavelength; this is of relevant interest in low jitter or fast fluorescence measurements where good specifications of the detector at one wavelength are inappropriately extended at another wavelength, causing for example, misattributing longer lifetime. It will be interesting to characterize in future work the temporal behavior of thick Silicon avalanche photodiodes, tipically with slower time response, but usually with greater detection efficiency. The paper is well written and relevant within the optical quantum community. I recommend the publication of this work. --------Pre-Production Review-------- Please confirm that all of your funding sources have been added into your Prism submission in the Fees and Funding step. If your manuscript is accepted for publication, the Funding listing in your published paper will be pulled from this section. 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