the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Frequency control and monitoring of the ALOMAR RMR lidar's pulsed high-power Nd:YAG lasers
Abstract. Doppler wind measurements in the middle atmosphere by ground-based lidar are challenging and benefit from precise spectral characterization of the laser source. We present a system for frequency control and monitoring of pulsed commercial high-power Nd:YAG lasers, which is entirely software controlled, automated, and works in real-time. It basically consists of an embedded controller handling the cavity control of the injection-seeded power laser and an embedded controller based spectrometer performing the spectral analysis of each individual power laser pulse using a Fabry-Perot etalon. The power laser cavity length is optimized by pulse build-up time minimization, yielding a stable long-term single-mode operation. The spectrometer is able to analyze continuous-wave as well as pulsed lasers with repetition rates of 100 Hz and resolves frequency changes of less than 300 kHz.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-1995', Anonymous Referee #1, 04 Aug 2025
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AC1: 'Reply on RC1', Jens Fiedler, 23 Sep 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1995/egusphere-2025-1995-AC1-supplement.pdf
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AC1: 'Reply on RC1', Jens Fiedler, 23 Sep 2025
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RC2: 'Comment on egusphere-2025-1995', Anonymous Referee #2, 14 Aug 2025
Summary
In the paper „Frequency control and monitoring of the ALOMAR RMR lidar’s pulsed high-power Nd:YAG lasers“ the authors give a technical overview on the build-up-time resonator stabilization and real time frequency monitoring of their lidar‘s Nd:YAG laser. The frequency stabilization of the injection seeder by iodine spectroscopy is described in another paper (line 54), therefore the title might be considered misleading, because the part of frequency control is described in another paper. The main scope of the paper is performance monitoring of the injection seeder build up time resonator stabilization. Of course, injection seeding can be seen as “frequency control”, but the resonator control mostly controls, which adjacent longitudinal modes to the injection seeder start to oscillate and how fast they start (build up time). Additionally, the Piezo shifts the whole longitudinal mode spectrum. So dependent on how good the resonator length is stabilized to the injection seeder, the bandwidth of the pulsed laser is reduced to nearly single longitudinal mode operation and low frequency offset to the injection seeder.
For the application the bandwidth and the relative frequency control is crucial, but bandwidth is only discussed in figure 7. For a full analysis of the presented system tuning the piezo of the laser over a full spectral range (½ µm) would have been interesting.
Furthermore, clarification about the setup is needed, since not all components mentioned in the publication are depicted in the setup.
Therefore, I advise major revisions.
Comments on the individual sections:
Section 2: System setup
- In figure 1 components are missing (e.g. diffuser/lens/fiber couplers). The components should be described in this section.
Section 3: Pulsed Nd:YAG laser frequency control
- In line 76 the authors write: “When the seed laser radiation is within the bandwidth of a longitudinal mode of the power laser resonator, it is resonantly amplified in the Nd:YAG rod. “
Is the true? When the Q-switch is operated only one round trip of seed laser radiation will be present in the resonator which is specially broadened due to the Q-switch So the adjacent longitudinal modes will always receive a higher start energy than other longitudinal modes (in classical laser theory they start for 0.5 photons :) ). I’ll add some measurements in the appendix. As the resonator mode is better matched to the seed laser wavelength and mode, other modes get suppressed more until the gain of the laser is used up (works with homogeneous broadened gain media).
- In Figure 2 the authors depict the build-up-time and the piezo control voltage after switching the laser on. The histogram of the laser build-up-time is shown but nowhere discussed. An evaluation is difficult because the build-up time (minimum) is not only dependent on the resonator length control but also on the gain in the laser itself. And the gain might be dependent on the temperature of the pump diodes. For this a synchronous plot of the output energy could explain the 2 ns increase in minimal build up time at 8:20.
Section 4: Laser pulse spectrometer
- The authors should motivate their design considerations and add a literature study on the topic.
E.g. a fast real time wavemeter for injection seeded lasers using a fabry-perot as pulsed wavemeter was presented in 1993 by Hahn et. al. „Fabry–Perot wavemeter for shot-by-shot analysis of pulsed lasers“ DOI:10.1364/AO.32.001095. or ”A simple real-time wavemeter for pulsed lasers”, Ja-Yong Koo and I Akamatsu DOI 10.1088/0957-0233/2/1/009. - Why did they choose a camera instead of a line camera? The camera produces more data, which might be redundant and is therefore more demanding on the signal processing.
- Why an Etalon and not a Fizeau interferometer was chosen for the Laser pulse spectrometer?
E.g. in „An absolute frequency reference unit for space borne spectroscopy “, by H. Schäfer et. al. DOI: 10.1117/12.2536012 a fiber coupled wavemeter using a collimator and a Fizeau wedge is presented to compare the wavelength of the injection seeder with the injection seeded pulse from an OPO – they even omitted means of chopping out the cw signal of the injection seeder due to different integration times.
Further literature to be considered:
- Fizeau wavemeter for pulsed laser wavelength measurement, Mark B Morris, Thomas J. McIlrath, and James J. Snyder, https://doi.org/10.1364/AO.23.003862
- Low-cost wavemeter with a solid Fizeau interferometer and fiber-optic input, Benedikt Faust and Lennart Klynning, https://doi.org/10.1364/AO.30.005254
- A simple real-time wavemeter for pulsed lasers, Ja-Yong Koo and I Akamatsu, https://doi.org/10.1088/0957-0233/2/1/009
Further minor questions the authors should consider:
- Why a FSR of 1 GHz was chosen for the Etalon (line 107)?
- What was the reflectivity of the mirrors of the Etalon?
- How is the seed laser light coupled out of the fiber (line 111) – is there a collimator used?
- What is the beam size of the laser and the seed laser on the diffuser?
- Where is the ground glass diffuser (line 113) depicted in Figure 1?
- Where is the 500mm lens depicted in Figure 1?
- Is the chopper necessary? (E.g. Schäfer et. al. did not need this)
- In line 113 the authors describe the lens is imaging the interference patterns. Actually, it transforms the angular interference pattern into a spacial interference pattern. An angle of 1mrad is transformed into a displacement of 0.5mm. So, 0.8 mrad would be 0.4mm ~57-66 pixels for a 6-7 µm camera pixel pitch.
- A line camera would not require such “overkill” hardware like a FPGA running RT-Linux and resource hungry LabVIEW… of course a small FPGA could evaluate a line camera in real time with a defined latency. The authors should focus on describing the technical necessities or concepts and then the details of their implementation (as implementations might changes – but keeping in mind the review criteria:
Scientific significance:
Does the manuscript represent a substantial contribution to scientific progress within the scope of Geoscientific Instrumentation, Methods and Data Systems (substantial new concepts, ideas, methods, or data)?.
E.g. in line 126 the authors write: “Then, for each peak of this function the position and amplitude of the maximum as well as the full width at half maximum (FWHM) are determined. “. The accuracy of the frequency estimation is dependent on the FWHM and the SNR of the measurement – the authors should give a Cramer-Rao limit for their estimator and compare their observations with this.
4.1 Calibration
The method is well described.
4.2 Sensitivity
- The authors should state which slope is used for stabilization of the seed-laser. I suppose the increasing slope in the figure 4 (UT 12:16) is used. Here the type of evaluation (fitting/center of gravity measurement etc...) is important, since a sub pixel evaluation is performed and plotted in figure 5. The chosen criteria in lines 165 and following do not express anything about thermal drift within the Etalon. There is no place where the authors discuss the sensitivity of the laser pulse spectrometer on pressure and temperature. Because it is used for relative measurements, this does not matter too much.
But for the sensitivity the bandwidth of the laser might be important, the bandwidth of the cw seed laser is small compared to the bandwidth of the 10-12ns pulses q-switched laser. The bandwidth of the q-switched laser with 10-12ns is 50-100 Mhz (dependent on the time bandwidth product of the pulse). With a Finesse of 20 and an FSR of 1 GHz the Airy linewidth of the interference pattern is approximately 50 MHz, therefore the expected peak FWHM of the lase pulse including the instrument function of the Etalon would be 2-3 times higher than the peak FWHM of the cw-laser. This would decrease the ‘sensitivity’.
5 FCaM Performance
- In line 180 the authors note:”The individual measurements of the power laser frequency stability reproduce the sinusoidal variation of the cavity length nearly perfectly.” and in line 209 the authors write: “The imprinted cavity length modulation for the BuT minimization method results in approx. ±10 MHz frequency modulation around the mean frequency of the power laser, which potentially can be reduced.”
The authors should discuss why they do not consider using the measured frequency offset with the spectrometer for cavity control. E.g. in DOI: 10.1117/12.2536012 this is the proposed way for a space bourne system. It is a much cleaner signal which is not subject to build-up-time jitter (due to residual inversion etc..).
- In lines 189 ff. the authors begin to speculate about a single event – whether the numerical evaluation worked properly cannot be determined without the raw data of the event.
The sentence: “Destructive interference of adjacent longitudinal modes (mode beating) could explain the observed reduced pulse intensity but should result into a spectral broadening instead of narrowing. In the end, it is unclear which process led to the observed behavior.” could/should be verified by opening the control loop of the laser and scanning the Piezo over a full spectral range (532 nm). With a frequency deviation of 40MHz~1/4 FSR detuning I would expect mode beating between two adjacent modes – furthermore I would expect a lower energy/intensity in the main peak. How stable is the FWHM fit when the intensity is reduced and a second peak appears 160 Mhz away from the main peak? - The peak should be still separated but close to each other due to the finesse and spectral width of the pulse.
Therefore, I would advise the characterization of the measurement setup for all possible detunings of the laser cavity.
Appendix: Unpublished measurements of a laser with the legendary Lightwave Electronics 101 injection seed laser (measured 2005)
Influence of the laser resonator length on the impulse form and spectrum (Image in the supplement.pdf)
The left figure shows the temporal intensity of q-switched pulses of an injection seeded laser with a resonator length of approximately 80 cm. The pulses were measured using a photodetector with ~2GHz bandwidth. The cavity length is controlled using a Piezo translator where the cavity end mirror is mounted on. Additionally, an unseeded laser pulse is generated by blocking the seed laser.
In the middle the Fourier transform power spectra of the measured pulses is displayed. The laser pulse with the lowest build up time shows the least mode beating. As the cavity is more detuned higher order mode beating appears.
In the right figure the measured laser build-up-time (the rate is approximately 0.05 V/ns) by the injection seeder electronics for different cavity detuning is shown. The measured pulses are indicated by horizontal lines.
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AC2: 'Reply on RC2', Jens Fiedler, 23 Sep 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1995/egusphere-2025-1995-AC2-supplement.pdf
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AC3: 'Reply on RC2', Jens Fiedler, 23 Sep 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1995/egusphere-2025-1995-AC3-supplement.pdf
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Reviewer Recommendation: Accept with minor revisions
Summary:
The submitted manuscript describes a system used to stabilize the output frequency of a high powered Nd:YAG laser. The laser design and the system are described in detail as well as the testing done on the system to ensure functionality and long term stability.
In my opinion, the submitted manuscript is very well written from a technical perspective. I also believe the manuscript is within the scope of AMT. However, given how the manuscript is written, I personally would have submitted it to a more optics/laser focused journal. The sourcing is lean but I think sufficient as well. I have no major issues with the technical detail from manuscript as written and would suggest it be published subject to some very minor revisions that I think would clarify a few confusing details.
One issue worth mentioning in a general comment is that while I think the manuscript is within the scope of AMT, the reader needs to bring motivation and knowledge of the atmosphere with them. As I read this manuscript, the laser is clearly targeted at a scientific application (Doppler wind measurements) but the description seems to have almost surgically removed the application. For example, while NLCs and winds are mentioned in the intro in a general way, it seems odd to me to have only mentioned wind accuracy in the conclusion. From an engineering perspective, the content is great. From an atmospheric science perspective, several things remain unclear to me: does this locking method improve your measurements noticeably or are other sources of error (for example shot noise) dominant? What was the accuracy you achieve before this system? Does this system enable scientific exploration that is otherwise inaccessible? These seem to be the most relevant questions to me as an AMT reader.
Major Comments:
Minor Comments: