New Optical Fiber Preserves Coherence

Patterns of transverse distribution of optical radiation intensity in the output beam. (Source: MIPT)

Scientists from the Moscow Institute of Physics and Tech­nology (MIPT) and the Kotel­nikov Institute of Radio Engi­neering and Elec­tronics (IRE) of the Russian Academy of Sciences (RAS), in colla­boration with their colleagues from Finland, have developed a new type of optical fiber that has an extremely large core diameter and preserves the coherent properties of light. The results of the study are promising for con­structing high-power pulsed fiber lasers and amplifiers, as well as polari­zation-sensitive sensors.

When it comes to optical fiber appli­cations, preserving the pro­perties of light is crucial. There are two principal para­meters that often need to be preserved: the distri­bution of light intensity in cross section and the polari­zation of light. In their study, the researchers managed to fulfill both conditions. “Optical fiber research is one of the most rapidly developing fields of optics. Over the last decade, numerous technological solutions have been proposed and imple­mented. For instance, researchers and engineers at IRE RAS can now produce optical fiber of almost any diameter with arbi­trary trans­verse structure,” says Vasily Ustimchik, a senior research scientist at IRE RAS and the Russian Quantum Center.

“In the course of this study, a specific structure was formed in the optical fiber. It varies along two ortho­gonal axes, and its diameters change propor­tionally along the fiber. Individually, such solutions are already widely used, so it is critical to continue to work in this direction.”, Ustimchik said. At first glance, an optical fiber seems to be a rather simple system, but in practice, we are confronted with a number of major issues limiting its appli­cations, the first being signal atte­nuation in fiber-optic lines. The solution to this problem has long been found, paving the way for fiber-optic communi­cations.

However, communi­cations are not the only area where optical fibers can be applied. Today, one of the most common types of lasers are based on fiber-optic tech­nology. A fiber laser, just like any other, incor­porates an optical resonator, which causes light to travel back and forth repeatedly. The geo­metrical parameters of the fiber resonator allow for only a limited set of transverse patterns of light intensity distri­bution in the output beam. Naturally, one would want to control the mode structure of the light, and in fact, when it comes to practice, researchers and engineers are mostly seeking to excite nothing but one pure funda­mental mode that does not change with time.

In order to maintain single-mode operation, the fiber must consist of a core and a cladding, made of materials with different refractive indexes. Ordinarily, the thickness of the fiber core, through which radia­tion propa­gates, normally has to be less than 10 micrometers. An increase in the optical power of the light propa­gating in the fiber results in a greater amount of energy being absorbed. This translates into a change in the properties of the fiber. Specifically, it causes uncon­trolled variation of the re­fractive index of the fiber material. This gives rise to para­sitic non­linear effects, resulting in addi­tional spectral lines of emission etc., which limits the strength of the optical signals that are trans­mitted. An existing solution to the problem lies in the variation of the core and outer diameters along the length of the fiber.

If the expansion of the fiber occurs adia­batically, it is possible to reduce the amount of energy transferred to other modes to less than 1 percent, even with a core diameter of up to 100 micrometers which is excep­tionally large for single-mode fibers. Moreover, the fact that the core diameter is large and varies along the fiber increases the threshold for nonlinear effects occurrence. To achieve the second goal, to preserve the polari­zation state of the light the researchers made the cladding of the fiber anisotropic: The width and the height of the inner cladding are different (the cladding is elliptical), which means the propa­gation speed of light with different field oscil­lation directions is not the same.

In a structure like this, the process of trans­ferring energy from one polarized mode to another is almost entirely disrupted. The researchers have shown that the geometric length of the path traveled by light through the fiber at which the oscil­lations of the two different polari­zations are in antiphase depends on the fiber core diameter: It decreases as the diameter is increased. This length, known as the polari­zation beat length, corresponds to one complete rotation of the linear polari­zation state in the fiber. In other words, if you launch linearly polarized light into a fiber, it will be linearly polarized again after traveling precisely this distance. The ability to measure this parameter is in itself evidence of the fact that the polari­zation state in the fiber is preserved.

In order to inves­tigate the properties related to light polari­zation in the fiber, the method of optical frequency-domain reflec­tometry was used. It involves launching an optical signal into the fiber and detecting the back­scattered signal. The reflected signal contains a lot of infor­mation. This method is normally used to determine the location of defects and impurities in optical fibers, but it can also determine both the coherence length and the spatial distri­bution of polari­zation beat length. Coherence reflec­tometry techniques are widely used to monitor the state of optical fibers. However, the method used in this study is notable for enabling data collec­tion at a high resolution of up to 20 micro­meters along the fiber length.

Sergey Nikitov, the leader of the research group, commented: “The fiber samples we obtained have demonstrated great results, indicating good prospects for further develop­ment of such tech­nological solutions. They will find use not only in laser systems but also in optical fiber sensors, where the change of polari­zation charac­teristics is known in advance, since they are determined by external environ­mental factors, such as tempera­ture, pressure, biological and other impuri­ties. Besides, they have a number of advan­tages over semi­conductor sensors. For example, they need no elec­trical power and are capable of carrying out distri­buted sensing, and that is not a complete list.” (Source: MIPT)

Reference: V. E. Ustimchik et al.: Anisotropic tapered polarization-maintaining large mode area optical fibers, Optics Exp. 25, 10693 (2017); DOI: 10.1364/OE.25.010693

Link: Moscow Institute of Physics and Technology (MIPT), Moscow, Russia

 

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