EP0136306A4

EP0136306A4 - Optical fibres.

Info

Publication number: EP0136306A4
Application number: EP19840900837
Authority: EP
Inventors: Allan Whitenack Snyder; Frank Friedrich Ruhl; David Neil Payne
Original assignee: Australian National University
Current assignee: Australian National University
Priority date: 1983-02-25
Filing date: 1984-02-24
Publication date: 1985-07-30
Also published as: EP0136306A1; JPS60501183A; WO1984003362A1

Abstract

Single mode, single polarisation fibres are produced by the introduction of anisotropy into an optical fibre of otherwise arbitrary geometry and arbitrary but guiding refractive index profiles. The anisotropy, which may be induced by the application of transverse stress to the optical fibre, is sufficient to establish conditions under which one mode of linearly polarised light "leaks" from the fibre. In the case of step-profile optical fibres, having a core (10) and a cladding (11), the anisotropy may be such that one mode of propagation of light is completely eliminated from the fibre. Alternatively, in such fibres, the anisotropy may be such that the leakage of the unwanted mode from the fibre is gradual, so that the fibre effectively attenuates that mode, thereby eliminating the propagation of that mode when the fibre has a sufficient length.

Description

TITLE: "OPTICAL FIBRES"

Technical Field

This invention concerns optical fibres. More particularly, it concerns optical fibres comprising an inner glass core having a first refractive index and a glass cladding surrounding the core, the cladding having a second refractive index, with the core and the cladding having dimensions which permit the transmission through the fibre of a single mode of propagation of electromagnetic radiation in the form of light. (Note: in this specification, the term "glass" is used in a sense to encompass a wide range of materials that are usually termed "glass" in this field, including silica and doped silicas, and "light" includes radiation in the ultraviolet and near infrared frequency bands.) Background art

Optical fibres per se are well known. They are currently used to transmit pulses of light to convey information in communications systems, and they are also used in a wide variety of instruments. They may be constructed in one of several ways. One construction has the optical fibre as a very small bore hollow glass tube filled with a very pure liquid. In another construction, the optical fibre is constructed entirely of glass with the refractive index varying progressively from the centre of the fibre to its outer surface. The most common type of optical fibre currently in use, however, consists of a core of glass having a constant, high refractive index surrounded by a sheath or cladding of glass having a constant, lower refractive index. This is called a "step profile fibre". For convenience, in this specification we will illustrate the invention for step profile fibres but it applies to other types of fibres as well.

Single mode fibres currently produced propagate electromagnetic radiation in the fundamental mode only. However, there are really two fundamental modes, one for each of the two orthogonally polarised states of light. As a consequence of nonuniformities and imperfections in the fibre, there is coupling between these two modes. Hence light launched into the fibre with one state of polarisation is randomly scattered into the other state of polarisation. This is an additive effect (that is, it accumulates with fibre length) and eventually the state of polarisation of the light in the fibre is indeterminate. Moreover the state of polarisation of the output from the fibre fluctuates with changes in environmental conditions, such as temperature and pressure.

There have been attempts to produce single-mode fibres which maintain the input state of polarisation. Such fibres have been called "polarisation maintaining fibres". The approach adopted up until now to make a polarisation maintaining fibre is to produce what is called a "high birefringence fibre". In a high birefringence fibre, asymmetry between the two orthogonally polarised fundamental modes is deliberately introduced. This may be done, for instance, by choosing an asymmetrical fibre geometry or by applying asymmetrical stress to the fibre. The likelihood of coupling between the orthogonally polarised fundamental modes decreases with increasing birefringence. Thus high birefringence fibres do reduce the polarisation coupling effect but they do not eliminate it.

Different approaches to the production of fibres having a high birefringence due to the stress-optic effect have yielded (a) the elliptical core/cladding type of fibre, which is described by I P Kaminow et al in their paper entitled "Strain birefringence in single polarisation germano-silicate optical fibres", which was published in Electronics Letters, Volume 15, pages 677 to 679, 1979, (b) the stress elliptical-jacket type of fibre, which is featured in the paper by T Katsuyama et al in their paper entitled "Low-loss singlepolarisation fibres", which was published in Electronics Letters, Volume 17, pages 473 to 474, 1981, and (c) the "side-pit" type of fibre, produced by T Hosaka et al and described in their paper entitled "Low-loss single-polarisation fibres with asymmetrical strain birefringence", which was published in Electronics Letters, Volume 17, pages 530 to 531, 1981. More recently, however, R D Birch, D N Payne and M P Varnham have noted some drawbacks with the techniques that have been used to produce those three types of optical fibre, and have developed a technique which produces maximum asymmetric stress to a fibre using stress-producing sectors in the fibre which are positioned so that the stress geometry in the crosssection of the fibre resembles the shape of a bow tie. Birch et al have reported this fibre structure, and the mechanism by which it may be produced, in their paper entitled "Fabrication of polarisation-maintaining fibres using gas-phase etching", which appeared in Electronics Letters, Volume 18, No. 25 (25 November 1982) at pages 1036 to 1038. Their fibre production technique uses a complex procedure which involves assymetrical doping of a silica tube which is to be the preform of the fibre, followed by selective hightemperature etching with a fluorine-liberated gas, then collapsing the tube to form the solid optical fibre preform.

Disclosure of the present invention

The main objective of the present invention is to produce a single-mode optical fibre that will propagate only one state of polarised light. This objective is achieved by constructing the fibre so that the unwanted linearly polarised mode of propagation is "leaked" from the fibre. If the leakage of the unwanted mode is very rapid, the fibre effectively will not support this mode at all. If the leakage is less rapid, the fibre becomes an attenuator of the unwanted mode of propagation, and in practice (for example, when a relatively long length of fibre is used in, for instance, communication systems) the unwanted mode is eliminated from the fibre. The present inventors have established that such fibres can be realised by fibres which have a glass core surrounded by a glass cladding, with anisotropy introduced into the fibres. The anisotropy may be induced, for example, by the application of transverse stress to the fibre preform. If the anisotropy of the fibre is appropriately chosen, one of the two orthogonally polarised fundamental modes will effectively not propagate. The present invention may be realised by either of two alternative approaches. Let x and y denote the two orthogonal directions of polarisation of light in the fibre. For convenience, we assume, for the purposes of this specification, that the fibre is to propagate the fundamental x-polarised mode and eliminate the fundamental y-polarised mode.

The first approach to produce the present invention requires the anisotropy induced in the fibre to be such that the refractive index profile for xpolarised light is guiding and the refractive index for y-polarised light is effectively constant throughout the fibre cross-section. (That is, in the case of a step-profile fibre, the refractive indices for ypolarised light in the core and cladding are substantially the same.) In this case the fibre cannot support the y-polarised fundamental mode at all.

The second approach which realises the present invention requires the induced anisotropy in the fibre to be such that the refractive index profile for xpolarised light is guiding and the propagation constant for the fundamental y-polarised mode, β_y, satisfies the relationship β_y <k.n_x2. Here the wavenumber k is 2π/λ and A is the wavelength of light transmitted in the fibre.

Thus, according to the present invention, an optical fibre comprises a single-mode optical fibre with anisotropy induced therein, characterised in that the anisotropy is such that one mode of linearly polarised light effectively leaks from the fibre.

When applied to a step-profile fibre, the present invention can be defined as an optical fibre comprising: a) a core of a first glass having a first refractive index, n₁, when in an unstressed state; and b) a cladding of a second glass having a second refractive index, n₂, when in an unstressed state; characterised in that said core and cladding are stressed so that the first refractive index, n₁, becomes n_x1 for x-polarised light and n_y1 for ypolarised light, and the second refractive index, n₂, becomes n_x2 for x-polarised light and n_y2 for ypolarised light; and one of the following sets of relationships is satisfied:

⁽¹⁾ n_x1≥n_x2; and n_y1 = n_y2, or

[n_y1 - n_y2] « [n_x1 - n_x2] ;

^{(2) n}xl £ⁿx2' and the fibre is sufficiently birefringent that, for a circularly symmetric step profile fibre with Δ_x=Δ_y, n_y2 ≤ n_x2 {1 -Δ_y [1.14 - V_y ^-1]²}, where Δ_y = 1 - [n_y2/n_y1],

V_y = [2π/λ].p.n_y1.[2 Δ_y] ½ , and Δ_x and V_x are defined correspondingly. Here W is the wavelength of light being transmitted and p is, the diameter of the fibre core.

Usually the fabrication technique for such fibres will involve the application of transverse stress to the preform of the fibre. This stress is preserved in the fibre as it is drawn from the preform. With a fibre constructed in accordance with the present invention as defined above, the y-polarised light either is not propagated at all in the fibre or otherwise leaks from the stressed fibre, thus effectively eliminating the y-polarised fundamental mode in a sufficiently long length of fibre.

The features of the present invention will become more apparent from the following description of fibres of the present invention. In the following description, reference will be made to the accompanying drawings.

Brief description of the drawings Figure 1 is a schematic representation of the end of a step profile type of optical fibre;

Figure 2 is a graph showing the variation of the refractive index, n, with the radial distance from the centre of the fibre of Figure 1, after a transverse stress has been applied to the fibre. The refractive index now depends on the state of polarisation of the light;

Figure 3 is a graph showing the variation of the refractive index, n, for a step-profile fibre constructed in accordance with the first approach noted above ; and

Figure 4 is a graph showing the variation of the refractive index, n, for a step-profile fibre constructed in accordance with the second approach noted above.

Detailed description of the illustrated embodiments

For convenience the present invention will be illustrated with reference to its realisation in a step-profile optical fibre of circular symmetry. However, the present inventors emphasise that this invention does not require a particular fibre geometry or refractive index profile. The invention allows for great flexibility in this area and the inventive concept may be embodied in a fibre constructed as simply as a circularly symmetric step-profile fibre (which has been chosen, for simplicity, to illustrate the invention in this specification) or as complex as a "bow-tie fibre". The important feature of the invention is that sufficient anisotropy is introduced into the optical fibre to cause one mode of polarised light to leak from the fibre. There is no limitation on the particular way by which the anisotropy is introduced into the optical fibre. However, for the purposes of this specification, it will be assumed that the anisotropy is stress-induced anisotropy, as stressinduction is currently the most widely used method of creating anisotropic optical fibres. As shown in Figure 1, a circularly symmetric step-profile optical fibre comprises a glass core 10 of circular cross-section surrounded by a cladding 11 of a different kind of glass. For a circularly symmetric step-profile optical fibre the cladding has an annular cross-section. However, as noted previously, the actual fibre geometry is not essential to the present invention. Always the refractive index of the core is higher than the refractive index of the cladding material, and the boundary between the core 10 and cladding 11 is continuous. Typically, the diameter of the core of the fibre, p , ranges from 3 to 20 micrometres.

When a transverse stress is applied to an optical fibre of the type shown in Figure 1 (using one of the stress application techniques that are used by workers in this field), the fibre becomes birefringent and the refractive indices for each form of polarised light become different in both the core 10 and the cladding 11. This is shown in Figure 2, where the solid curve shows the refractive index as a function of radial position for x-polarised light and the dashed curve shows the radial variation in refractive index for ypolarised light. It should be noted, once again, that Figure 2 refers to a circularly symmetric step-profile fibre. The graph would be more complex for, for example, a bow-tie optical fibre.

In the situation shown in Figure 3, the stress applied to the optical fibre has resulted in the refractive index for y-polarised light in the core being the same as the refractive index for y-polarised light in the cladding. In this case the fibre cannot support any y-polarised bound modes, and the ypolarised light will rapidly leak from the fibre, even when a short length of fibre is used to transmit light. Very rapid leakage of the y-polarised light also occurs when n_y2 n _y1 and

[n_x1 - n_x2] » [n_x1 - n_y2]. Figure 4 illustrates the refractive index profile of a circularly symmetric anisotropic step-profile fibre. As a consequence of the anisotropy, the refractive index profile depends on the state of polarisation of the light. The modes of the fibre are not completely linearly polarised. For instance the "x-polarised mode" has also a small y-polarised component and thus "sees" something of the refractive index profile for the y-polarised light and similarly the "y-polarised mode" "sees" something of the refractive index profile for x-polarised light. Two of the present inventors have shown, in their paper entitled "New single-mode single-polarisation optical fibre" , which appeared in Electronics Letters , Volume 19 , pages 185 to 186 , 3rd March 1983 , thatleakage from the y-polarised fundamental mode will occur whenever β_y < ^{k n}x2 .

This situation applies to Figure 4, provided the parameters depicted in the figure obey the relationships noted above in the recitation of the conditions for the second approach to realise the present invention in step-profile fibres. The theory of this "leaky mode design" has been expounded by two of the present inventors in their paper entitled "Singlemode, single-polarisation fibres made of birefringent material", that has been published after the priority date of this application in the Journal of the Optical Society of America, Volume 73, pages 1165 to 1174, September 1983. If the requisite conditions apply , then the y-polarised mode will be attenuated in the fibre. In short lengths of fibre, a small attenuation is of no consequence, but when the transmission of light through the fibre is over long distances (which is normally the case in transmission systems and other applications of the present invention), even a small attenuation will result in the extinction of the transmission of y-polarised light well before the observation of the optical signal at the reception end of the fibre. Consequently, this type of fibre effectively eliminates the transmission, over reasonable distances, of y-polarised light. There is no interference with the transmission of x-polarised light because always n_x2 ≥ n_y2 and hence also β_x>k n_y2. The application of perturbation theory to the mathematical representations of the modal transmission of electromagnetic radiation at optical frequencies through birefringent optical fibres shows that this explanation of the operation of the present invention is correct.

From the information given above, it follows that the minimum birefringence that is necessary for the ypolarised fundamental mode to be leaky is when β_y = k n_x2. For a circularly symmetric step-profile fibre with

Δ_x =Δ_y - Δ , so that V_x = V_y = V this requires a minimum relative birefringence of where δ_yx = 1 - (n_y1/n_x1) . To test the present invention, fibres were constructed and transversely stressed using the "bowtie" stressing technique described in the aforementioned paper by R D Birch, D N Payne and M P Varnham. The fibres were constructed to have a core of germano-silicate glass surrounded by a cladding of fluoro-phosphorous silicate glass. The cladding was integrally bound to an outer region of the same glass as the cladding, but containing zones of borosilicate glass. During the fabrication process of the fibre, these zones produce a transverse stress in the fibre. Due to the stress-optic effect this causes the fibre to become anisotropic. The fibres used for the testing were each 500m long. Using light of wavelength 830 nm, the guided mode

(x-polarised light) experienced a loss of 5 dB/km, while the leaky mode (y-polarised light) was attenuated at the rate of 55 dB/km. Thus the test fibres exhibited both low loss and an extinction ratio of 50 dB/km.

To summarise, the present invention provides an optical fibre in which the y-polarised mode of light transmission is effectively eliminated from the fibre, thus permitting true single-mode, single-polarisation light transmission by the fibre. Industrial Application

Optical fibres which transmit only one mode of linearly polarised light are used in a wide variety of optical fibre applications. They are especially useful in coherent detection systems and in optical fibre interferometers.

Claims

1. An optical fibre comprising a single-mode optical fibre with anisotropy induced therein, characterised in that the anisotropy is such that one mode of linearly polarised light effectively leaks from the fibre.

2. An optical fibre as defined in claim 1, in which the single mode optical fibre comprises a stepprofile optical fibre having a) a core (10) of a first glass having a first refractive index, n₁, when in an unstressed state; and b) a cladding (11) of a second glass having a second refractive index, n₂, when in an unstressed state; characterised in that said core (10) and cladding (11) are stressed so that the first refractive index, n₁, becomes n_x1 for x-polarised light and n_y1 for y-polarised light, and the second refractive index, n₂, becomes n_x2 for x-polarised light and n_y2 for y-polarised light; and one of the following sets of relationships is satisfied: n_x1≥ n_x2; and either n_y1 = n_y2, or

[n_y1 - n_y2] « [n_x1 - n_x2].

3. An optical fibre as defined in claim 1, in which the single mode optical fibre comprises a stepprofile optical fibre having a) a core (10) of a first glass having a first refractive index, n₁, when in an unstressed state; and b) a cladding (11) of a second glass having a second refractive index, n₂, when in an unstressed state; characterised in that said core (10) and cladding

(11) are stressed so that the first refractive index, n₁, becomes n_x1 for x-polarised light and n_y1 for y-polarised light, and the second refractive index, n₂, becomes n_x2 for x-polarised light and n_y2 for y-polarised light; and the following relationships are satisfied: n_x1 ≥ n_x2 ; and the fibre is sufficiently birefringent that the propagation constant β_y for the fundamental y-polarised mode satisfies the relationship β_y ≤ k n_x2.

4. An optical fibre as defined in claim 3 , in which the step-profile optical fibre is a circularly symmetric step-profile fibre with Δ_x = Δ_y, so that n_y2 ≤n_x2 {1 -Δy [1.14 - V_y ^-1]²}' where

Δ _y = 1 - [n_y2/n_y1 ] ,

V_y = [ 2 π/λ].p. n_y1 [ 2 Δ_y] ½. and Δ_x is defined analogously, λ is the wavelength of light transmitted and p is the core diameter. An optical fibre as defined in claim 2, claim 3 or claim 4, in which the transverse stressing of the fibre is achieved by a technique which involves the production of zones of borosilicate glass in a cladding of fluoro-phosphorous silicate glass which surrounds a core of germano-silicate glass.