My 10 best ideas

Mainly during my 10 years spent in Academia I have been able to contribute to several activities in the field of optical fibres, working often in collaboration with several others researchers from many countries. However, in the course of my activity, in some cases the results that have been generated have been a direct consequence of my personal ideas.
I have summarized here what I think can be considered my 10 best personal ideas, according to their novelty and their implications. These are my most relevant personal contributions to the field of optical fibers:

1.            A novel hybrid fiber type
2.            An "old" glass for novel hollow core fibers
3.            A novel design for low loss hollow core optical fibers
4.            A new  method for reducing bending loss in hollow antiresonant fibers
5.            A new method for improving gas filling and detection in hollow core optical fibers
6.            A novel design for hollow antiresonant fibers
7.            A novel design for highly birefringent fibers with large mode area.
8.            A double air clad fabrication approach for Photonic Crystal Fibers.
9.            A novel design for ultra-broad flat dispersion in optical fibers
10.         A “novel” application for microstuctured optical fiber

1.  A novel hybrid fiber type

It was already in early June 2014 that I considered the possibility of coating the core boundary of a hollow core optical fibre. The basic concept came out from a discussion with William Wadsworth in Bath who needed a fibre guiding light far in the mid-infrared for making an electrically pumped fibre laser. Certainly if the fibre was made in semiconductor rather than in glass that would be perfect. But in reality only (or mostly) glass fibres can be drawn. Can you then somehow think about a novel structure capable of preserving at least some of the properties of the additional material? This was the main question at that time. The answer came quite early by some numerical simulations I made which allowed me to find that indeed not only such a fibre could be made, but especially that it could have been an hollow antiresonant fibre just like any other single material hollow fibre. I studied thoroughly this aspect and I finally formulated the equivalence between any composite material hollow fibre and a single material hollow antiresonant fibre with a well-defined wall thickness. I wrote a research project with my colleague Pier Sazio for exploring the feasibility of this fibre in late 2015 and in the spring of 2016 the result was there: an hollow core optical fibre with a core boundary made in silicon. And it was guiding light just as a normal antiresonant fibre !!! Would it then this magic fibre be able to control its own antiresonant guidance properties with light or electrical signals ? 

W. Belardi, P. J. Sazio, F. De Lucia,  F. Poletti, "Composite Material Hollow Antiresonant Fibers,", FiO, Rochester 17-21 Oct. 2016.
W. Belardi, F. De Lucia,  F. Poletti, P.J . Sazio"Composite Material Hollow Antiresonant Fibers,", Optics Letters 42, p.2535-2538 (2017).

2.  An “old” glass for novel hollow core fibers

In March 2015 I was working on one of the compound glass fibre towers at the Optoelectronics Research Centre, for adapting it to the fabrication of hollow core fibres. I was planning to investigate new type of glasses for making hollow fibres, maybe materials with better properties. I was also doing some simulations which were showing the secondary relevance of the material attenuation of the glass. In the evening I re-watched an old movie: “Back to the future”. In the movie a scientist goes to the future and finds out that he doesn’t need plutonium anymore for operating its powerful “time machine”, but just garbage ! One wonderful idea came suddenly to my mind ! If I take, I thought, a glass bottle from the bin and draw it into a fibre could I hope to obtain the same properties than by using ultrapure silica glass? In the following months I had to adapt the fabrication technique to borosilicate glass and, thanks also to my colleagues, largely improving our drawing tower facility. But in October 2015 the final result was there: borosilicate hollow fibres for the visible range had the same level of attenuation of those based in silica !!  Yes just borosilicate, a glass with the same level of attenuation of the glass that was existing (in France) in the middle age !! If they knew that, they could have done optical fibers at that time !!!

W. Belardi, N. White, J. Lousteau, X. Feng and F. Poletti, "Hollow core anitresonant fibers in borosilicate glass," WSOF 2015, Hong Kong 4-6 Nov. 2015. 

3.  A novel design for low loss hollow core optical fibers

Double Antiresonant Fiber (September 2013)
In August 2013, while I was lying on the beaches of Italy, I had an idea for reducing the losses of hollow core optical fibers. Until then the antiresonance mechanism was exploited merely as a way for enlarging the transmission spectra of hollow core fibers or for reducing the overlap between the optical mode and glass material. The performances of hollow antiresonant fibers were limited by confinement loss. Both “1 ring” or “multiple ring”(“Kagomé”) structures were behaving overall in a comparable manner. However something this time could become different. The inverted (“negative”) curvature structure, introduced by Pryamikov et al. , would allow the addition of extra antiresonant elements in the cladding space, which would be in contact with the rest of the existing structure only at a far point from the fibre core. In previous studies it was shown that the presence of these contacts points and their proximity to the core is what determines the loss of the fundamental-like mode.  So I had generated a radically new structure that takes advantage of the use of multiple antiresonant elements which are in contact to each other at far points from the fibre core. Now confinement losses could have been reduced to negligible levels.
In September 2013 I simulated this structure and fabricated the first prototypes. I published these results at the OFC conference and on the Optics Letters Journal in 2014. I still didn’t have time and money to demonstrate experimentally better performances than other hollow core optical fibers.

W. Belardi and J. C. Knight, "Negative curvature fibers with reduced leakage loss," OFC 2014, San Francisco 9-13 Mars 2014. (see also Supplementary material)
W. Belardi and J. C. Knight, "Hollow antiresonant fibers with reduced attenuation",  Opt. Letters 39, 1853-1856 (2014). 

4.  A new method for reducing bending loss in hollow antiresonant fibers

Reduction of bending loss in Free Boundary Fibers
(November 2013)
Since November 2012 I had fabricated hollow antiresonant fibers with a free core boundary but no real advantage of this new structure was resulting from my investigations. I had already demonstrated, by using numerical simulations, that their performances in the mid-infrared would have  been  dominated by material attenuation, and that in the near-infrared and visible spectral range the gap between adjacent cladding tubes needed to be reduced to a minimum level, ideally 0, for reducing confinement losses. So why to keep studying this structure? I had almost quit this research topic when in November 2013 I put together these thoughts and my last results on the study of the impact of the core boundary curvature on the performance of antiresonant fibers. What I understood was that the gap among the cladding tubes of the structure would have played a key role in determining the coupling between the fundamental-like mode and the fibre cladding modes. One of the implications of this relation, but surely not the only one (!) was that bending loss of hollow antiresonant fibers with very large core sizes could have been reduced to negligible levels. In late November I simulated this case and fabricated an hollow antiresonant fiber with low bending loss. These results were published on the Optics Express Journal in April 2014.

W. Belardi and J. C. Knight "Effect of core boundary curvature on the confinement losses of hollow antiresonant fibers" Opt. Express 2013 Vol. 21(19) pp. 21912-21917
W. Belardi and J. C. Knight "Hollow antiresonant fibers with low bending loss" Opt. Express 2014, Vol. 22 (8) pp. 10091-10096 

5.  A new method for improving gas filling and detection in hollow core optical fibers

Side cuttting of Free Boundary Fiber
(January 2014)
In January 2014 I have thought about a new way for filing a gas within an hollow core optical fibre. The method itself is actually “old” (lateral gas filling in hollow core fibers was adopted already several years ago) but the fibre that I thought to use is “new”. It is a “Free Boundary Fibre”, a fibre in which the cladding holes are separated to each other. Then what is the advantage of cutting this fibre on the side, letting a gas accessing the fibre core, as compared to other hollow core optical fibers ? Well, now the cutting operation will not introduce any additional optical loss since the cutting will not concern the fibre core boundary but only the external glass jacket. As a consequence you can think to cut the fibre over quite long lateral sections providing fast and large volume gas filling  and , especially, with almost NO LOSS !!! 

W. Belardi“Design and properties of hollow antiresonant fibers for the visible and near infrared spectral range”, IEEE J. Lightwave Technology 2015, Vol. 33(21) pp. 4497-4503

6.  A novel design for hollow antiresonant fibers

Free Boundary Fiber
(November 2012)
In September 2012, I was looking for a way to reduce optical loss of hollow antiresonant fibers in the near infrared wavelength spectral range. Previous experiments had shown that the main problems in this regime were arising from some light coupling at the contact points between the cladding tubes. Then why not fabricating a novel fibre structure where the cladding tubes where separated from each other? After a few prototypes I made my final fibre for the near infrared regime in November 2012.
The fibre was only weakly guiding over a couple of meters and it was clear that was not a such evident task showing some advantages of this novel structure over his more “conventional” counterpart. I had to wait 1 more year to understand and demonstrate the key advantage of this novel structure.
Interestingly, in parallel to this work, Kolyadin et al. at the Russian Accademy of Science were performing an independent work on the same structure. They published their results in April 2013 on a similar structure performing, in the mid-infrared, in a similar way to the more “conventional” antiresonant structure with touching cladding tubes.

A. N. Kolyadin, A. F. Kosolapov, A. D. Pryamikov, A. S. Biriukov, V. G. Plotnichenko, and E. M. Dianov, "Light transmission in negative curvature hollow core fiber in extremely high material loss region," Opt. Express 21, 9514-9519 (2013) 
W. Belardi and J. C. Knight "Hollow antiresonant fibers with low bending loss" Opt. Express 2014, Vol. 22 (8) pp. 10091-10096 

7.  A novel design for highly birefringent fibers with large mode area
Elliptical Hollow Photonic Crystal Fiber
(April 2005)
In September 2004 I was addressing new possibilities for Photonic Crystal Fibers with high birefringence. I had to realize that all types of form-induced birefringent fibres were conceived to work with small mode area fibres. Instead for large mode area fibers the current technique adopted in order to increase fibre birefringence concerned the use of  Stress Applying Parts to the fibre cladding. I had then the idea of making a large mode area photonic crystal fibre whose birefringence was induced by the presence of an elliptical hole in its fiber core. I simulated this case in late 2004.
I and Géraud Bouwmans made this fibre in April 2005, and this idea was then demonstrated.
The results were published on the Journal of Quantum Electronics in December 2005 and at the OFC conference in 2006.

W. Belardi, G. Bouwmans, L. Provino and M. Douay “Form-induced birefringence in elliptical hollow photonic crystal fiber with large mode area” IEEE J. Quantum Electr. 41,1558-1564 (2005)
W. Belardi, G. Bouwmans, L. Provino, V. Pureur, and M. Douay, " A Large Mode Area Elliptical Hollow Photonic Crystal Fiber," in OFC Conference, Technical Digest (2006), paper OFC3. 

8.  A double air clad fabrication approach for Photonic Crystal Fibers

Double Air Clad Photonic Crystal Fiber
(October 2000)
In the early years of Photonic Crystal Fibre (PCFs) technology I was starting my doctoral studies at the University of Southampton (in 2000). Two different guidance mechanisms were identified for PCFs. The first photonic bandgap fibre had been made only a couple of years before and was based on a perfectly periodic structure. The other guidance mechanism, that I started to study, was the modified total internal reflection (MTIR) effect. PCFs using the MTIR effect didn’t need a perfect periodicity and it was being shown that only the most inner hole rings had an effect on the fibre performances, fabrication and use. The rest of the fibre structure (the most peripheral zones) was completely neglected.
I didn’t neglect it! And I introduced the idea that a modification of the peripheral zone would have had beneficial effect on the fibre fabrication, on the fibre temperature stability and that some further investigations may have identified some advantages in terms of fibre performances.
Indeed several months later my colleagues Jayanta Sahu & others, from the ORC, picked up this double clad fabrication approach that I introduced in order to demonstrate the first dual air-clad large mode area laser.

W. Belardi, “Holey optical fibers for high nonlinearity devices”, PhD Thesis (University of Southampton), 2003.
W. Belardi, K. Furusawa, T. M. Monro, D. J. Richardson, P. W. Turner, “Holey optical fibres“ International publication number : WO 0216980 (2002).
J. K. Sahu, C. C. Renaud, K. Furusawa, R. Selvas, J. A. Alvarez-Chavez, D. J. Richardson, and J. Nilsson. “Jacketed air-clad cladding pumped ytterbium-doped fibre laser with wide tuning range”. Electron. Lett., 37:1116–1117, 2001.

9.  A novel design for ultra-broad flat dispersion in optical fibers

Circular Hollow Photonic Crystal Fiber
(September 2005)
In June 2005, after having finished my work on the Elliptical Hollow Photonic Crystal Fibre (EH-PCF), I thought that the inclusion of air defects within the inner structure of a photonic crystal fibre may have been used also for controlling its dispersion properties. I then started an important numerical study on this novel PCF structure with an inner circular hole. I got my results in October 2005, an almost zero (slightly negative) dispersion between 1.2 and 1.6mm wavelength. At the end of October I was ready to submit these results for publication. Too late !! Kunimasa Saitoh & others, on the other side of the world had basically the same idea, completely independently, and had already published their work, only 2 weeks before, on the Optics Express Journal…That is research!

K. Saitoh, N. Florous, and M. Koshiba, "Ultra-flattened chromatic dispersion controllability using a defected-core photonic crystal fiber with low confinement losses," Opt. Express 13, 8365-8371 (2005) 

10.  A “novel” application for microstuctured optical fiber

Distributed Anti-Stokes Raman Thermometry with Photonic Crystal Fiber
(July 2010)
In several occasions new ideas come from revisiting old concepts with a new mind. This is probably the case of Distributed Anti-Stokes Raman Thermometry (DART) with a high numerical aperture PCF. I had this idea in January 2010 while I was addressing some business opportunity of high temperature sensors. The idea came from the fact that PCFs had two advantages over conventional fibre technology: the absence of dopant (which would diffuse within the fibre at temperatures of 1000degrees or more) and the possibility of having a high numerical aperture (which increases the capture coefficient of the Raman backscattered signal). I assigned this project to one of my students and so this idea could be demonstrated.

Jean-Yves Schnitzler, "Etude et réalisation d’un capteur de haute température à fibre photonique ", Projet de fin d’étude (Supervisors : Walter Belardi (Osyris R&D) and Cyril Paranthoen (INSA Rennes))