Microwave Discharge in Hollow Optical Fibers as a Pump for Gas Fiber Lasers
Round 1
Reviewer 1 Report
The authors used a scheme similar to a slot antenna in the wall of a metal microwave waveguide to excite plasma in a HCF. The electrical field in the slot was analyzed. Several capillaries and HCFs were used in the experiments, and the lengths of the plasma reached 25 cm.
There are some questions:
1. Line145, line 217, the reference is missed.
2. Since the MW discharge excitation in HCF has been proposed in previous work, what is the difference between this work and those work?
3. What is the difference between the surfatron and waveguide the author proposed?
4. How long is the length of the HCF or capillary used in the experiment, and how much time does it take to vacuum the HCF?
5. Line 333, “all the above capillaries and HCFs with an inner diameter of 3 mm, 2 mm, and 1.3 mm, 230 μm, 120 μm, and 110 μm”, is confusing. Which numbers belong to HCFs and which numbers belong to capillaries?
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors experimentally demonstrate plasma generation in hollow-core fibers (HCF) using external microwave (MW) excitation using a slot antenna in a MW waveguide. The purpose of the work is to study this setup as a possible approach for the realisation of gas-discharge fiber lasers (GDFL).
In their introduction the authors give a very good and complete overview over previous results published in this area and put their work very well into context. Although plasma generation in HCF using MW excitation has been studied before, the particular excitation scheme based on a slot antenna presented in this work is quite novel and original, and leads to different excited plasma states in the fiber. Hence, the work is significant as it allows to identify the influence of the excitation scheme on the resulting plasma parameters.
The study is very well supported by theoretical considerations of the required breakdown voltage for initiating a gas-discharge in a hollow-core fiber, and by a detailed analysis of the required geometry of the slot to achieve this voltage.
Experimental results are presented in the form of pictures of the discharge and an emission spectrum of the generated argon plasma. However, no attempt is made to make any additional measurements to further characterise the plasma state (e.g. temperature, electron density, etc.), and this is the main weakness of the work. Only rudimentary estimations of these properties are made based on comparison of the discharge spectrum with previous work.
Overall, the paper presents and interesting, non-invasive approach to generate a gas-discharge in HCF, a field that is scarcely explored but could have a significant impact if GDFL would become a reality. While the context and theory are well presented, the experimental results are very basic and lacking the depth required for a full evaluation of the excitation scheme. Nevertheless, I recommend publication of the work.
Questions / Comments:
1) One purpose of the work is to evaluate the excitation scheme for the possible use in GDFLs. The standing wave excitation pattern generates a longitudinally varying electric field, which probably translates also to a longitudinally varying population inversion. This could trigger significant reabsorption of laser radiation. Is this not a significant drawback of the method? Could the standing wave pattern be avoided in the future?
2) There are several missing references, where empty brackets [] are given in the text.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
