A Review of Tunable Orbital Angular Momentum Modes in Fiber: Principle and Generation

Round 1

Reviewer 1 Report

The authors present an overview on methods for the generation of tunable orbital angular momentum beams. The generation modes are systematically sorted with respect to polarization and wavefront helicity properties by using Poincaré sphere representation.  Three basic types of modes are distinguished.

The paper is relevant and interesting for the community.  The style of writing has still some potential for improvements. The manuscript could be considered for a publication after minor revision.

Comments

(1) In the abstract, the abbreviation “(PS)” should be given behind “Poincaré spheres”.

(2) The title of the paper promises a review. In Table 1, fiber-based methods are listed with corresponding literature. It is a pity that a similar Table is missing for free-space papers.

(3) It has to be mentioned that, for a transfer of ultrashort pulses (which seems to be an important point for future communication systems), free space solutions have essential advantages because of avoiding frequency chirping and pulse lengthening. A technical solution may be tunable MEMS or LCoS-SLMs which were applied to shape femtosecond lasers.

(4) p. 6, headline 3.3: For the reader, it is difficult to get what means “…from l to (l+n) and then to n”…

(5) Because of some weakness in language style, proofreading by a native speaker is highly recommended.

Typos and style (examples):

- Reference [6]: One author is “Ritsch-Marte” instead of “Ritschmarte”. The order of the names is also wrong (F, J, B, R is the right one).

- p. 12, line 372: “the challenges” should be “the challenge”

- p. 12, line 373: it must be “converter” instead of “convertor”

- not all references contain complete required data, see, e.g., ref. [54] where a paper number is missing; or ref. [15] where the volume is missing; or refs. [24] and [41]. Citations have to be carefully checked.

Author Response

we have submitted the response in word form.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript "A Review of tunable orbital angular momentum 3 modes: principle and generation" serves as a review paper for creating OAMs. The work summarizes current progress in OAM generation study and provides good references to each sub-category of the topic. The field is well plowed and contributed by countless researchers with the authors included. However, as a review paper for the field, the comprehensiveness of the paper raises big concern. The introduction is too short, and the main test should properly address the earliest contributors in each category (the references are a bit "fresh"). A stronger conclusion also helps to glue all the introduced topics together, which is currently missing. Overall, I recommend major revision and re-review of the manuscript.

To help the authors deliver a more comprehensive review paper, the following comments are given for consideration.

Introduction, paragraph 1: expand with short explanations regarding how OAMs benefits the listed technical area such as "optical tweezers" and others. Also, for optical communication (fiber and free space), OAMs are pursuit in both directions  in large-capacity and long ranges due to its partial robustness against turbulence. The authors may pay attention to additional use of OAMs in fields of importance.

It may sounds better to restrict the title or topic to fiber based OAM generation, as more emphasized in the context. Nevertheless, as the use of mode patterns are purely mathematical, they apply to non-fiber based OAM signal generations. Yet the authors should be aware of different realization approaches and add flexibility in addressing certain research areas. The approaches include the simple use of a spatial light modulator for tunable OAM beams have been mentioned, but not elaborated with its principles and effectiveness in combination with other works mentioned in section 4.1. Limitations of SLM method can also be mentioned as have been discussed frequently in literature. New work such as "phase and amplitude beam shaping with two deformable mirrors implementing input plane and Fourier plane phase modifications," Appl. Opt. 57 (2018) also reveals solutions of tunable OAMs when discrete phase modulation is not available.

Part 2 can be significantly shorter. As the OAM mode is most popular, it sounds better to introduce it mainly and express the other two as alternative methods of math representation.

*A severe writing problem happened to section 4, which is more of a hand-waving style of introducing different realization approaches. For example, the review paper should at least provide a concise illustration on each of the methods shown in figure 6. It is also worthwhile to make comparisons or list fundamental modules required for each method. In this way, readers have a good view of the complexity and capacity of each established approach. Otherwise, the pictured platforms look disjoint from the paper and cause confusions.

A strong conclusion is needed, not simply "the usefulness" of OAM signals. It sounds more practical to favor one particular method (such as the fiber based generation), or address the cost issue for tunable OAM generation despite its unique properties in application (which is actually the problem now, correct?). In this fashion, the readers could gain more intuition of the subject. 

There are minor issues in writing, yet they can be corrected through revision and the second review period.

Author Response

We have already submitted the response in Word form.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The detailed response letter properly handles all the reviewer's concerns. The current manuscript is a much better presentation of their work. The paper qualifies a comprehensive review article on fiber based OAM mode generations.