WAVEGUIDE OPTICAL FIBER FOR AMPLIFICATION WITHIN S-BAND OPTICAL RANGE

WAVEGUIDE OPTICAL FIBER FOR AMPLIFICATION WITHIN S-BAND OPTICAL

RANGE

BACKGROUND OF THE INVENTION

1. Field of the Invention The invention is directed to a single mode optical waveguide fiber for use in telecommunication systems and more particularly, a waveguide optical fiber allowing amplification within the S-band optical range.

2. Technical Background The continuous growth of bandwidth requirements in optical based communication systems has resulted in a large demand for systems able to operate within several optical wavelength ranges including the S-band optical range, the C-band optical range and the L- band optical range. The S-band is defined as the wavelengths between 1465 nm and 1525 nm, which lies below the C-band wavelength range which extends between 1525 nm and 1560 nm, which in turn lies just below the L-band wavelength range which extends between 1560 nm and 1600 nm. In order to create a viable operating bandwidth, a large bandwidth must be obtained within each of the operating wavelength ranges. Currently, most telecommunications systems utilize the C-band and L-band ranges, while pumping within the S-band optical frequency range has been limited by a "dead" band lying between 1491 nm and 1497 nm within which signal amplification has been limited. More specifically, the use of distributed Raman amplification within the L-band in erbium doped fibers creates a "dead" band between 1491 nm and 1497 nm for the Raman pump, thereby limiting the effectiveness of the L-band.

Parametric amplification is a method previously utilized within the C-band wavelength range to amplify a large bandwidth. Parametric amplification is a non-linear process based on 4-wave mixing in high non-linearity dispersion shifted fibers. In short, parametric amplification is a means of amplifying optical waves whereby an intense coherent pump wave is made to interact with a non-linear optical crystal to produce amplification on two other optical wavelengths. Four-wave-mixing based on parametric amplification is efficient when phase-matching between the pump signal and the amplified signals is approached. By phase-matching the amplified signals and pump signal, the distance over which the coherent transfer of energy between the waves is increased, thereby increasing the possible gain. Phase-matching is most easily achieved when working around the zero point of dispersion in a dispersion shifted fiber.

SUMMARY OF THE INVENTION This invention meets the need for a single mode optical waveguide fiber that allows amplification of an optical signal in the S-band optical range. More specifically, the invention meets the need for a single mode optical waveguide fiber that provides a zero dispersion operating wavelength within the range of between about 1491 nm and about 1497 nm. In a first embodiment, an optical waveguide fiber, includes a core region having a relative refractive index percent and an inner and an outer radius, and an inner clad layer surrounding and in contact with the core region, the inner clad layer having a relative refractive index percent and an outer radius. The optical waveguide fiber also includes an outer clad layer surrounding and in contact with the inner clad layer, the outer clad layer having a relative refractive index. The relative refractive index percent and the radii of the core region, the inner clad layer and the outer clad layer are chosen from the following ranges: the relative index of the core region within the range of from about 1.332% to about 1.628%; the relative index of the inner clad layer within the range of from about 1.301% to about 1.591%; the relative index of the outer clad layer within the range of from about 1.305% to about 1.595%; the outer radius of the core region within the range of from about 1.71 μm to about 2.09 μm; and, the outer radius of the inner clad layer within the range of from about 4.41 μm to about 5.39 μm.

In a second embodiment, an optical waveguide fiber includes a core region having a relative refractive index percent and an outer radius, and an inner clad layer surrounding and in contact with the core region, the inner clad layer having a relative refractive index percent and an outer radius. The optical waveguide fiber also includes an outer clad layer surrounding and in contact with the inner clad layer, the outer clad layer having a relative refractive index percent. The relative refractive index percent and the radii of the core region, the inner clad layer and the outer clad layer are chosen from the following ranges: the relative index of the core region within the range of from about 1.338% to about 1.636%; the relative index of the inner clad layer within the range of from about 1.307% to about 1.597%; the relative index of the outer clad layer within the range of from about 1.311% to about 1.603%; the outer radius of the core region within the range of from about 2.61 μm to about 3.19 μm; and, the outer radius of the inner clad layer within the range of from about 5.31 μm to about 6.49 μm.

The present invention also includes methods for constructing optical waveguide fibers and an optical communication system employing the fibers in accordance with the embodiments described above. Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Exemplary embodiments of the segmented core refractive index profile of the present invention is shown in each of the figures.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram of a waveguide fiber refractive step index profile of an optical waveguide having three segments;

Fig. 2 is a diagram of a waveguide fiber refractive graded index profile of an optical waveguide having three segments; Fig. 3 is a graph of total dispersion versus wavelength of the optical waveguides of

Fig. 1 and Fig. 2;

Fig. 4 is a graph of mode field diameter versus wavelength for the optical waveguides of Fig. 1 and Fig. 2; and,

Fig. 5 is a schematic view of a fiber optic communication system employing an optical fiber of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.

It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview of the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of the specification. The drawings illustrate further features and embodiments of the invention which, together with their description, serve to explain the principles and operation of the invention. Definitions The following terminology and definitions are commonly used in the art.

- The radii of the segments of the core are defined in terms of the index of refraction, or the material of which the segment is made. A particular segment has a first and a last refractive index point. A central segment has an inner radius of zero because the first point of the segment is on the center line. The outer radius of the central segment is the radius drawn from the waveguide center line to the last point of the refractive index of the central segment. For a segment having a first point away from the center line, the radius of the waveguide center line to the location of its first refractive index point is the inner radius of that segment. Likewise, the radius from the waveguide center line to the location of the last refractive index point of the segment is the outer radius of that segment.

The segmented radii maybe conveniently defined in a number of ways. In this application, radii are defined in accordance with the figures, described in detail below. The definitions of segment radius and refractive index, used to describe refractive index profile, in no way limit the invention. Definitions are given herein because in carrying out model calculations, the definitions must be used consistently. The model calculations set forth in the tables below are made using the geometrical definitions labeled in the figures and described in the detailed description.

- The relative index of a segment, Δ%, as used herein, is defined by the equation,

2 2 2

Δ% = 100 X (n I, - n c ) '/2n c ,' wherein n l. is the maximum refractive index of the index profile segment denoted as i, and n , the referenced refractive index, is taken to be the minimum index of the clad layer. Every point in the segment has an associated relative index. The maximum relative index is used to conveniently characterize a segment whose general shape is known.

- The term refractive index profile or index profile is the relation between Δ% or refractive index and radius for a selected segment of the core. The term α profile, a k/a graded index profile, refers to refractive index profile that may be expressed by the equation, n(r) = n (1-Δ) (r/a) , where r is core radius, Δ is defined above, a is the last point in the profile segment, the value of r at the first point of the α-profile is chosen to accord with the location of the first point of the profile segment, and α is an exponent which defines the profile shape. Other index profiles include a step index, a trapezoidal index and a rounded step index, in which the rounding is usually due to dopant diffusion in regions of rapid refractive index change. - Total dispersion is defines as the algebraic sum of waveguide dispersion and material dispersion. Total dispersion is also referred to as chromatic dispersion in the art. The units of total dispersion are ps/nm-km.

- A refractive index profile in general has an associated effective refractive index profile that is different in shape. An effect refractive index profile may be substituted, for its associated refractive index profile without altering the waveguide performance.

- The propagation equations for the injected optical signal and the pump signal, as well as the idler wave created in the four-wave-mixing process, are defined by the equations:

where Ε , Ε , and Ε are the signal and idler fields, and α , α , and α are the passive losses for the three fields. It is assumed that the passive losses are the same for the pump, signal and idler waves, thereby resulting in α = α = α = α. - The phase mismatch, Δ^ between the pump, signal and idler waves is defined as,

where λ_Q is the zero-point of dispersion, dD/dλ is the slope of the dispersion at the wavelength of the zero-point of dispersion and λ and λ are the wavelengths of the pump and the signal, respectively.

- The non-linear coefficient of the fiber gamma is defined by the equation: γ= 2πn_NL/(λA_eff), where n_NL is the non-linear refractive index of the core, λ is the wavelength used, and A_eff is the mode field effective area.

The segmented core optical waveguide described and disclosed herein has a generally segmented core. Each of the segments is described by a refractive index profile, relative refractive index percent, Δ.%, and an outside radius, r.. The i and r on Δ refers to a particular segment. The segments are numbered 1 through n beginning with the inner most segment which includes the waveguide long axis center line. A clad layer having a refractive index of n c surrounds the core. The radius, relative refractive index %, and refractive index profile of each segment of the core are selected to provide a zero dispersion wavelength within the range 1491 nm to 1497 nm.

A general representation of the core refractive index profile is illustrated in Fig. 1, which shows relative refractive index percent charted versus waveguide radius. Although Fig. 1 shows only 3 discreet segments, it is understood that the functional requirements may be met by forming an optical fiber having more than 3 segments. However, embodiments having fewer segments are usually easier to manufacture and are therefore preferred.

The step index profile structure characteristic of the novel waveguide fiber 10 is shown by a core segment 12, a depressed cladding 14, and an outer cladding 16. The refractive index of the core, depressed cladding and cladding are each positive. The refractive index profile associated with each segment may be adjusted to reach a fiber design which provides the required waveguide fiber properties.

Fig. 2 illustrates a variation of the novel waveguide fiber design. In this design, the index profile structure has a graded index and includes a core segment 22, a depressed cladding 24, and an outer cladding 26. Again, properties associated with the fiber segments can be altered as described below.

EXAMPLE 1 The diagram of Fig. 1 is an embodiment of the novel waveguide fiber which provides a zero-dispersion wavelength of between 1491 nm and 1497 nm and which has three segments 12, 14 and 16. The central core or first segment 12 has a relative index, Δ., of within the range of from about 1.332% to about 1.628%, and an outer radius 30, r., of within the range of from about 1.71 μm to about 2.09 μm. The central core 12 has a preferred relative index, A., of about 1.48% and a preferred outer radius of about 1.9 μm. The first surrounding annual segment or second segment 14 has a relative index, Δ₂, of within the range of from about 1.301% to about 1.591%, and a preferred refractive index of about 1.446%. The outer radius 30, r_{] ;} of the central segment 12 is also the inner radius of the first annular segment 14. The radius 30, r^ therefore, is the intersection of the central segment 12 and the first annular segment 14. This convention will be used consistently in all the examples and corresponding figures.

The outer radius 32, r₂, of the first annular segment 14, is within the range of from about 4.41 μm to about 5.39 μm, and is preferably of from about 4.9 μm. The second surrounding annular segment 16 has a refractive index of within the range of from about

1.305% to about 1.595%, and is preferably about 1.450%. The outer radius 32 r₂, of the first annular segment 14 is also the inner radius of the second annular segment 16. The radius 32 r₂, therefore, is the intersection of the first surrounding annular segment 14 and the second surrounding annular segment 16. The outer radius 34 of the second annular segment 16 is approximately 125 μm, however, this radius can vary greatly.

EXAMPLE 2 Another embodiment of a waveguide fiber providing a zero dispersion wavelength of between 1491 nm and 1497 nm has three wavelengths, including a central segment 22, a second segment or first annular segment 24, and a third segment or second annular segment 26, and is shown in Fig. 2.

The central segment 22 has a refractive index of within the range of from about 1.338% to about 1.636%, and is preferably about 1.487%. The first annular segment 24 has a refractive index value of within the range of from about 1.307% to about 1.597%, and is preferably about 1.452%. The second annular segment 26 has a refractive index value of within the range of from about 1.311% to about 1.603%, and is preferably about 1.457%. The radii for the second embodiment are calculated using the conventions set forth in Fig. 1. The radius 40, r_{] ;} of the central segment 22, is within the range of from about

2.61 μm to about 3.19 μm, and is preferably about 2.9 μm. The outer radius 42 r₂, of the first annular segment 24 is within the range of from about 5.31 μm to about 6.49 μm, and is preferably about 5.9 μm. The outer radius 44 of the second annular segment or outer cladding is approximately 125 μm, however, this radius can vary greatly.

Fig. 3 graphs total dispersion versus wavelength for the fibers of examples 1 and 2 above and shows the dispersion slope of each fiber. It should be noted that the step index fiber 10 has a lower dispersion slope value within the 1491 nm to 1497 nm wavelength range as compared to the graded index fiber 20. The lower dispersion slope value provides a more efficient amplification of the associated source signal by reducing the gain ripple effect.

Fig. 4 graphs mode field diameter versus wavelength for the fibers of examples 1 and 2 above. Fig. 4 shows that the mode field diameter for the graded index fiber 20 is greater than that for the step index fiber 10 at a given wavelength. The advantage of the increase mode field diameter of the graded index fiber 20 is a smaller internal stress region between the core and the cladding as compared to the step index fiber 10.

The refractive indices and the cross-sectional profile of the fibers made according to the present invention can be accomplished using manufacturing techniques known to those skilled in the art. As shown in Fig. 5, and in accordance with the present invention, the optical waveguide fiber 10 (or 20) is constructed and configured in accordance with the present invention and used in an optical communication system 50. System 50 includes a transmitter 52 and a receiver 54, wherein transmitter 52 transmits an optical signal via optical fiber 10 (or 20), which in turn is received by 54 after being amplified within optical fiber 10 (or 20).

It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.