Experimental Study on Km-Range Long-Distance Measurement Using Silicon Photomultiplier Sensor with Low Peak Power Laser Pulse
Round 1
Reviewer 1 Report
The paper is about the joint use of a SiPM sensor and a low power laser source emitting at 905 nm for the measurement of distances in the order of kilometres.
The subject is interesting in itself but the paper is lacking in some aspects that can be significantly improved to provide a clearer picture of the experiment carried out and the results achieved.
The writing in English needs to be reviewed as a whole as it is unclear. There are numerous grammatical errors.
As far as the scientific part is concerned, the following critical aspects are highlighted:
No consideration is given on the factors that led to the use of either a particular detector or laser source.
Lack of details concerning one of the fundamental elements of the experiment presented, i.e. the SiPM. The paper does not provide any details of its electrical or optical characteristics (e.g. active area, PDE, responsivity?).
The paper also lacks considerations regarding the optical coupling of the various constituent elements and some essential features to evaluate the system. For example, the IFOV of the tested instrument is not indicated (substantially dependent on the active area of the detector and the focal length of the collection lens) also in relation to the divergence of the collimated laser (in general when reference is made to the divergence it must be indicated if reference is made to the half or full divergence and if this is evaluated at FWHM). Some considerations should also be made about the size of the various targets in relation to IFOV and the size of the laser spot.
Some more specific issues:
"Introduction" section
"Another researches [2, 3] ...are conducted...". Clarify whether this is one or more researches and to which reference is made in the last sentence
"Semiconductor lasers also have some inherent shortcomings..." Is it advisable to indicate what these solutions might be, e.g. anamorphic prisms?
Experimental design" section
“MICRORB-SMTPA-10020”. Clarify if reference is made to the card or the card with detector. Enter a table or describe the main features of the detector, especially the wavelength of use.
"Beam divergences parallel and perpendicular to the axis of propagation. The divergences are parallel and perpendicular to the junction, not to the axis of propagation. How are the divergences specified: full? Half? FWHM?
"Measured energy of the laser at diode laser": How was it measured?
"The Tx and Rx unit have both the same high precision aspheric lens with a focal length of 97 mm." Please specify the manufacturer and model of the lenses used. Specify if optimized for the wavelength of interest. Justify the use of the same lenses for Rx and Tx also in relation to the laser divergence and the active area of the detector. These considerations are essential to evaluate the optical coupling of the system and any critical issues that can be resolved. Clarify whether the f-number is low enough to best match the laser for the axis with the greatest divergence. If not, quantify the losses, at least approximately. Also indicate how the optical alignment of the various elements has been optimised.
"Collimated laser beam is in line-rectangular shape and its long edge divergence is approximately 3 mrad. Specify the divergence for both directions and how it has been evaluated.
"In front of the receiver, to effectively filter out background light, band-pass (FL905-10, Thorlabs) and long-pass filters (FEL850-10, Thorlabs) were mounted. Clarify the need for the long-pass filter and justify the placement of the filters in a part of the optical system where the beam is very convergent.
"Signal Extraction algorithms" section
"In experimental studies...". Specify
"are accompanied by the noises of all kind...". Specify the noises referred to.
"For this, the basic desired signal has been defined first (...) value wiener filter coefficients are found". This part needs to be better clarified. What is the M parameter?
"First target has the size of 2 by 2 meters of black painted wooden plane and measured to be 1840 meters Away". Specify the IFOV corresponding to this distance and the size of the laser spot. Both of them appear to be larger than the size of the target. Clarify what the effects of this may be. For example, a reduced sensitivity? Effects of other objects in the background? The same considerations should be made for the target at 2435 meters. Describe how the targets were pointed.
"2 thousand and 4 thousand. Write in figures
"the signal with almost fixed error of approximately 10 meters". Check the value provided that does not seem compatible with the reported distances. In general it would have been appropriate to calibrate this offset of the instrument by calibrating it with targets placed at shorter distances for which you have better SNR.
Figure 8, 9 and 10. Target images are completely useless because the targets themselves are not distinguishable.
Author Response
We would like to thank to reviewer for his/her valuable comments and critics.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This is an interesting piece of work to explore the use of SiPMs in laser range finding. This is potentially of interest to a number of different fields and thus is worthy of publication. The work itself seems to have been done well. However the write up and conclusions should be improved before publication in the following ways:
- Figures 1, 2, 4, and 6 should be replaced with higher resolution versions. As it is, the reader cannot see the detail in the images necessary to draw the conclusions the authors want to be drawn.
- The illustration on there right in Fig 2 should be expanded to include the target.
- More detail, and more clarity, needs to be included in the description of the data analysis. Presumably, the measurements are pulse heights and times, but which quantity is used, and when, and how?
- For example, the detailed diagram in Fig 5 is hard to understand because there is no explanation of what X and Y are.
- Figure 7 should include goodness of fit information (the data seem to be non-Gaussian)
- There is no description of what is actually plotted in Figure 7, or how it was normalised.
- There should be a brief description of the the Wiener filter, why it is useful for this purpose, and how it is applied.
- The conclusions should include an estimate of the accuracy of the range finding, and some discussion of that. Is it good compared to other methods? If not, how can it be improved?
Author Response
We would like to thank to reviewer for their valuable comments and critics.
