CA1256310A - Bandwidth enhancement of multimode optical transmission lines - Google Patents
Bandwidth enhancement of multimode optical transmission linesInfo
- Publication number
- CA1256310A CA1256310A CA000467549A CA467549A CA1256310A CA 1256310 A CA1256310 A CA 1256310A CA 000467549 A CA000467549 A CA 000467549A CA 467549 A CA467549 A CA 467549A CA 1256310 A CA1256310 A CA 1256310A
- Authority
- CA
- Canada
- Prior art keywords
- fiber
- refractive index
- core
- region
- short length
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Abstract
BANDWIDTH ENHANCEMENT OF MULTIMODE OPTICAL TRANSMISSION LINES
Abstract of the Disclosure To improve pulse dispersion of a multimode fiber transmission line, it is spliced to a short length of fiber adapted to strongly attenuate mode groups propagating near the core center.
- i -
Abstract of the Disclosure To improve pulse dispersion of a multimode fiber transmission line, it is spliced to a short length of fiber adapted to strongly attenuate mode groups propagating near the core center.
- i -
Description
~L~2~6 3 ~3 This invention relates to a method for improving the bandwidth of multimode fiber lengths and to multimode transmission lines having improved bandwidths.
One of the signal distortion mechanisms existing in fibers used in light wave communications is inter-modal dispersion.
Such dispersion arises because light passing along a fiber is distributed into a number of modes which vary in propagation velocity.
At a particular wavelength, each mode group has its own velocity and since a coupled light source generally splits into a number of mode groups, a light pulse transmitted along the fiber broadens towards the output end.
To some extent the propagation velocity of individual modes or mode groups can be equalized by appropriately tailoring the refractive index profile of the fiber for a particular wavelength or wavelength range. An acceptable refractive index profile has the highest refractive index at the fiber core and a variation of the refractive index within the core which is substantially parabolic.
Many thousands of kilometers of multimode fiber have been installed in which the refractive index profile is nominally a modified-parabolic distribution. A method of making fiber having this index gradient is the modified chemical vapour deposition (MCVD~ method patented by McChesney et al, U.S. patent 4,217,027. In this method, germania doped silica is deposited on the inside of a pure silica tube as a soot. Subsequently the soot is fused at high temperature and the tube is collapsed and drawn into a t`iber. The germania doped silica forms the core of the fiber and the pure silica its claddingO By suitably altering the content of germania during the deposition phase . ~
~l~56~
the refractive index gradient of the fiber core can be made parabolic.
As previously mentioned, much of the multimode fiber currently in operating fiber optic transmission lines is made by this method.
One inevitable anomaly within the refractive index of S fiber made using the MCVD process is a central refractive index dip which detracts from the desired modified-parabolic index gradient. The refractive index dip results from dopant loss during the consolidation and collapse stages of the manufacturing process.
Investigations of standard fiber made by the McChesney et al method show that mode groups responsible for increased inter-modal dispersion are spatially localized. On scanning the output end of a fiber into which a pulse has been coupled a pre-pulse may be seen which is localized generally on the fiber axis, the pre-pulse slightly preceding the main output pulse. Since there is little mode conversion occuring at wavelengths of interest to light wave communications, very little light in a mode travelling essentially along the fiber axis is converted into the modes travelling in the outer part of the core and vice versa. Typically when a laser, modulated at a particular frequency, is spliced to an 8 kilometer length of a standard graded index multimode fiber having a -3dB
bandwidth of 600 MHz-km, there is a difference in arrival time of the light of almost S nanoseconds between the center and the outside of the fiber core. This results in a minimum in the received 100 MHz AC
signal at 6 microns from the core center. This is due to the near cancellation of the two mode groups which are of approximately equal power at this position and are 180 out of phase.
According to one aspect of the invention, there is ~563~
provided a method of increasing bandwidth of a multimode optical fiber transmission line by coupling it to a short length of fiber adapted to strongly attenuate certain ~ode groups without attenuating other mode groups of different order.
Preferably the length oF fiber functions to attenuate low order modes propagating substantially along the core axis.
Attenuation can be produced by a fiber having a highly attenuating central region to absorb these centrally travelling modes.
Alternatively, the attenuation can be achieved using a fiber with a large diameter central dip in the refractive index profile to inhibit the propagation of these modes. Typically such a large diameter central dip is of the order of 12 microns in diameter. The fiber can be attached to the multimode transmission line either at the beginning of a line or can be inserted into the line. Several such lengths of fiber can be coupled along the length of a transmission line.
According to another aspect of the invention, there is provided a multimode fiber transmission line having coupled thereto a short length of fiber adapted to strongly attenuate certain mode groups without attenuating other mode groups of different order.
Particularly for use in improving the bandwidth of a multimode fiber there is provided a compensating fiber having a core and a cladding the core having a generally parabolic refractive index profile, the profile dipping at the center of the fiber, the central low refractive index region having a diameter of at least 12 microns and having a refractive index substantially lower than the index of an immediately adjacent part of the core.
~L~2~ 3 ~L~
Preferably for silica based transmiss;on lines, the refractive index at the center of the core ;s about 1.458 at 589 mm being essentially that of silica and the refractive index immediately adjacent to the low index central region is in the range 1.465 to 1.480.
The numerical apertures of the transmission line and the compensating fiber should be nominally equal to avoid splice losses.
An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:-Figure 1 is a schematic view of a multimode optical transmission line in which modal dispersion is reduced by a method according to the invention;
Figure 2 shows the relative phase of light received atthe output end of a multimode optical transmission line;
Figure 3 shows the refractive index profile of fiber used to increase bandwidth of a multimode optical transmission line;
Figure 4 shows the bandwidth of a multimode optical transmission line to which compensating fiber is coupled, and, in contrast, the same transmission line to which no compensating fiber is coupled;
Figure 5 shows in graphical form the near field output of a multimode transmission line, firstly in which no compensating fiber is coupled to the transmission line (Figure 5a) and then with a compensating fiber coupled to the transmission line (Figure 5b); and Figure 6 shows an alternative compensating element for use in selectively treating difFerent mode groups in light propagating along a transmission line.
~S~3~
Referring in detail to Fiyure 1, ~ 1 meter section of a compensating fiber 10 is spliced into a transmission link between a pigtail fibar 12 from a semiconductor laser 14 and a multimode transmission fiber 16.
Omitting the compensating fiber 10~ the Relative Phase at the end of an 8 kilometer length of the transmission line 16 is as shown in Figure 2. Within a central axial region of the transmission line corresponding generally to a central dip in the refractive index profile, the light arrives at the end point of the transmission line about 5 nanoseconds before light in the outer part of the core. As shown in Figure 2 for the AC level, at a modulating Erequency of 100 megahertz, a minimum is produced in the modulated signal at a region of about 6 microns from the fiber central axis. At this point the two dominant mode groups are exactly 180 out of phase and cancellation occurs. At this point, as shown by the DC Level of the modulating signal, the signal levels of the two mode groups are approximately equal. It is clear from Relative Phase plot that if the signal content is removed from the central core region to a diameter of 12 microns, then light is transmitted only in those mode groups which are more-or-less in phase.
Referring to Figure 3 a compensating fiber for removing the centrally propagating modes has a cladding diameter of 125 microns with a cladding refractive index of 1.458, and a core having an index variation corresponding essentially to the equation n(r) = n(0) sech Ar where n(r) is the refractive index at a distance r from the ~iber core, n(0) is the refractive index a the core, and A is a constant.
However this profile is modified by a central dip at which the refractive index ~2563~1 has an average value of 1.45~ over a diameter of 12 microns. The fiber can be made using a particular form of the modified chemical vapour deposition method in which several anti-diffusion layers of fluorine doped silica are initially deposited onto the inside sur~ace of a silica tube. Subsequently several layers of germania doped silica are deposited on the inside of the tube, with germania content increasing gradually to form a desired index profile, the germania doped silica being followed by several more layers of pure silica or fluorine doped silica. The dopant content and layer thickness are chosen to achieve the profile shown in Figure 3. A tube so prepared is heated to consolidate the deposited soot into fused glass and the tube is then collapsed and drawn into a fiber.
A length of compensating fiber of 1 meter has been found sufficient to exclude the centrally travelling modes from the transmission line.
The compensating element is placed at the input end of the fiber rather than at the output end in order to establish the preferred modal distribution at the beginning of the link.
As an alternative to the low index central region of the fiber, it can merely be made lossy by including in the deposited silica several parts per million of a dopant such as iron which is strongly absorbent in the spectral region of interest.
Referring to Figure 4, there is shown an example of the improvement which can be obtained by using the compensating fiber of the invention. As shown in the graph of Figure 4, a 25 kilometer standard MCVD produced multimode link has a light output (L), frequency (f) plot as shown at (a). However with a 1 meter length of the ~L~S~
compensating Fiber spliced between the laser pigtail fiber and the transmission fiber the bandwidth is improved as shown in the plot (b).
As shown in Figure 5 which indicates the actual near field output (a) without, and (b) with, compensating fiber, again for a 25 kilometer optical transmission line, the received light is concentrated more Gentrally in (a) than in (b) which confirms that a redistribution of the modal power has been effected by the compensating fiber.
The basis of the invention is to exclude the mode groups which travel through the fiber at a speed different from that of other mode groups.
In an alternative embodiment shown in Figure 6, a compensating element 22 is inserted at spaced points along the multimode optical transmission line 24, the compensating element having a plurality of separate filaments 26, 28 arranged in a bundle, the bundle being placed with its end against the core 30 of the optical transmission line. As shown in Figure 6, in a bundle of silica filaments of outer diameter 17 microns and core diameter of 13 to 15 microns, the filament or filaments 28 at the center of the bundle are longer than the filaments 26 in the outer region oF the bundle so causing the faster travelling modes to travel further than the other modes. In this case the light mode groups at the transmission line axis, instead of being suppressed, are retarded to render them coincident with the other mode groups and so restore the light pulse at the fiber output end.
The tendency for mode groups to become spatially separated arises because of the criticality of the index profile near 3L~S 6 3 ~L~3 the central axis of the fiber. Any deviation of the index from the optimum profile in the central region has a far greater influence on modal dispersion than does a corresponding deviation in refractive index in an outer region of the core since a larger Fraction of the light is carried in the former region. As rnentioned previously, MCVD
fiber is characterized by a specific index dip. However any flattening of the index profile at the central region can have a corresponding effect on the modal dispersion.
One of the signal distortion mechanisms existing in fibers used in light wave communications is inter-modal dispersion.
Such dispersion arises because light passing along a fiber is distributed into a number of modes which vary in propagation velocity.
At a particular wavelength, each mode group has its own velocity and since a coupled light source generally splits into a number of mode groups, a light pulse transmitted along the fiber broadens towards the output end.
To some extent the propagation velocity of individual modes or mode groups can be equalized by appropriately tailoring the refractive index profile of the fiber for a particular wavelength or wavelength range. An acceptable refractive index profile has the highest refractive index at the fiber core and a variation of the refractive index within the core which is substantially parabolic.
Many thousands of kilometers of multimode fiber have been installed in which the refractive index profile is nominally a modified-parabolic distribution. A method of making fiber having this index gradient is the modified chemical vapour deposition (MCVD~ method patented by McChesney et al, U.S. patent 4,217,027. In this method, germania doped silica is deposited on the inside of a pure silica tube as a soot. Subsequently the soot is fused at high temperature and the tube is collapsed and drawn into a t`iber. The germania doped silica forms the core of the fiber and the pure silica its claddingO By suitably altering the content of germania during the deposition phase . ~
~l~56~
the refractive index gradient of the fiber core can be made parabolic.
As previously mentioned, much of the multimode fiber currently in operating fiber optic transmission lines is made by this method.
One inevitable anomaly within the refractive index of S fiber made using the MCVD process is a central refractive index dip which detracts from the desired modified-parabolic index gradient. The refractive index dip results from dopant loss during the consolidation and collapse stages of the manufacturing process.
Investigations of standard fiber made by the McChesney et al method show that mode groups responsible for increased inter-modal dispersion are spatially localized. On scanning the output end of a fiber into which a pulse has been coupled a pre-pulse may be seen which is localized generally on the fiber axis, the pre-pulse slightly preceding the main output pulse. Since there is little mode conversion occuring at wavelengths of interest to light wave communications, very little light in a mode travelling essentially along the fiber axis is converted into the modes travelling in the outer part of the core and vice versa. Typically when a laser, modulated at a particular frequency, is spliced to an 8 kilometer length of a standard graded index multimode fiber having a -3dB
bandwidth of 600 MHz-km, there is a difference in arrival time of the light of almost S nanoseconds between the center and the outside of the fiber core. This results in a minimum in the received 100 MHz AC
signal at 6 microns from the core center. This is due to the near cancellation of the two mode groups which are of approximately equal power at this position and are 180 out of phase.
According to one aspect of the invention, there is ~563~
provided a method of increasing bandwidth of a multimode optical fiber transmission line by coupling it to a short length of fiber adapted to strongly attenuate certain ~ode groups without attenuating other mode groups of different order.
Preferably the length oF fiber functions to attenuate low order modes propagating substantially along the core axis.
Attenuation can be produced by a fiber having a highly attenuating central region to absorb these centrally travelling modes.
Alternatively, the attenuation can be achieved using a fiber with a large diameter central dip in the refractive index profile to inhibit the propagation of these modes. Typically such a large diameter central dip is of the order of 12 microns in diameter. The fiber can be attached to the multimode transmission line either at the beginning of a line or can be inserted into the line. Several such lengths of fiber can be coupled along the length of a transmission line.
According to another aspect of the invention, there is provided a multimode fiber transmission line having coupled thereto a short length of fiber adapted to strongly attenuate certain mode groups without attenuating other mode groups of different order.
Particularly for use in improving the bandwidth of a multimode fiber there is provided a compensating fiber having a core and a cladding the core having a generally parabolic refractive index profile, the profile dipping at the center of the fiber, the central low refractive index region having a diameter of at least 12 microns and having a refractive index substantially lower than the index of an immediately adjacent part of the core.
~L~2~ 3 ~L~
Preferably for silica based transmiss;on lines, the refractive index at the center of the core ;s about 1.458 at 589 mm being essentially that of silica and the refractive index immediately adjacent to the low index central region is in the range 1.465 to 1.480.
The numerical apertures of the transmission line and the compensating fiber should be nominally equal to avoid splice losses.
An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:-Figure 1 is a schematic view of a multimode optical transmission line in which modal dispersion is reduced by a method according to the invention;
Figure 2 shows the relative phase of light received atthe output end of a multimode optical transmission line;
Figure 3 shows the refractive index profile of fiber used to increase bandwidth of a multimode optical transmission line;
Figure 4 shows the bandwidth of a multimode optical transmission line to which compensating fiber is coupled, and, in contrast, the same transmission line to which no compensating fiber is coupled;
Figure 5 shows in graphical form the near field output of a multimode transmission line, firstly in which no compensating fiber is coupled to the transmission line (Figure 5a) and then with a compensating fiber coupled to the transmission line (Figure 5b); and Figure 6 shows an alternative compensating element for use in selectively treating difFerent mode groups in light propagating along a transmission line.
~S~3~
Referring in detail to Fiyure 1, ~ 1 meter section of a compensating fiber 10 is spliced into a transmission link between a pigtail fibar 12 from a semiconductor laser 14 and a multimode transmission fiber 16.
Omitting the compensating fiber 10~ the Relative Phase at the end of an 8 kilometer length of the transmission line 16 is as shown in Figure 2. Within a central axial region of the transmission line corresponding generally to a central dip in the refractive index profile, the light arrives at the end point of the transmission line about 5 nanoseconds before light in the outer part of the core. As shown in Figure 2 for the AC level, at a modulating Erequency of 100 megahertz, a minimum is produced in the modulated signal at a region of about 6 microns from the fiber central axis. At this point the two dominant mode groups are exactly 180 out of phase and cancellation occurs. At this point, as shown by the DC Level of the modulating signal, the signal levels of the two mode groups are approximately equal. It is clear from Relative Phase plot that if the signal content is removed from the central core region to a diameter of 12 microns, then light is transmitted only in those mode groups which are more-or-less in phase.
Referring to Figure 3 a compensating fiber for removing the centrally propagating modes has a cladding diameter of 125 microns with a cladding refractive index of 1.458, and a core having an index variation corresponding essentially to the equation n(r) = n(0) sech Ar where n(r) is the refractive index at a distance r from the ~iber core, n(0) is the refractive index a the core, and A is a constant.
However this profile is modified by a central dip at which the refractive index ~2563~1 has an average value of 1.45~ over a diameter of 12 microns. The fiber can be made using a particular form of the modified chemical vapour deposition method in which several anti-diffusion layers of fluorine doped silica are initially deposited onto the inside sur~ace of a silica tube. Subsequently several layers of germania doped silica are deposited on the inside of the tube, with germania content increasing gradually to form a desired index profile, the germania doped silica being followed by several more layers of pure silica or fluorine doped silica. The dopant content and layer thickness are chosen to achieve the profile shown in Figure 3. A tube so prepared is heated to consolidate the deposited soot into fused glass and the tube is then collapsed and drawn into a fiber.
A length of compensating fiber of 1 meter has been found sufficient to exclude the centrally travelling modes from the transmission line.
The compensating element is placed at the input end of the fiber rather than at the output end in order to establish the preferred modal distribution at the beginning of the link.
As an alternative to the low index central region of the fiber, it can merely be made lossy by including in the deposited silica several parts per million of a dopant such as iron which is strongly absorbent in the spectral region of interest.
Referring to Figure 4, there is shown an example of the improvement which can be obtained by using the compensating fiber of the invention. As shown in the graph of Figure 4, a 25 kilometer standard MCVD produced multimode link has a light output (L), frequency (f) plot as shown at (a). However with a 1 meter length of the ~L~S~
compensating Fiber spliced between the laser pigtail fiber and the transmission fiber the bandwidth is improved as shown in the plot (b).
As shown in Figure 5 which indicates the actual near field output (a) without, and (b) with, compensating fiber, again for a 25 kilometer optical transmission line, the received light is concentrated more Gentrally in (a) than in (b) which confirms that a redistribution of the modal power has been effected by the compensating fiber.
The basis of the invention is to exclude the mode groups which travel through the fiber at a speed different from that of other mode groups.
In an alternative embodiment shown in Figure 6, a compensating element 22 is inserted at spaced points along the multimode optical transmission line 24, the compensating element having a plurality of separate filaments 26, 28 arranged in a bundle, the bundle being placed with its end against the core 30 of the optical transmission line. As shown in Figure 6, in a bundle of silica filaments of outer diameter 17 microns and core diameter of 13 to 15 microns, the filament or filaments 28 at the center of the bundle are longer than the filaments 26 in the outer region oF the bundle so causing the faster travelling modes to travel further than the other modes. In this case the light mode groups at the transmission line axis, instead of being suppressed, are retarded to render them coincident with the other mode groups and so restore the light pulse at the fiber output end.
The tendency for mode groups to become spatially separated arises because of the criticality of the index profile near 3L~S 6 3 ~L~3 the central axis of the fiber. Any deviation of the index from the optimum profile in the central region has a far greater influence on modal dispersion than does a corresponding deviation in refractive index in an outer region of the core since a larger Fraction of the light is carried in the former region. As rnentioned previously, MCVD
fiber is characterized by a specific index dip. However any flattening of the index profile at the central region can have a corresponding effect on the modal dispersion.
Claims (10)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An apparatus, comprising: a multimode optical fiber transmission line having a light input end, and a short length of fiber having the characteristic of strongly attenuating at least one light mode group passed through said fiber without attenuating other mode groups of different order, said short length of fiber having a low refractive index central core region for attenuating an axially propagating mode group and having a light output end coupled to said light input end of said multimode optical fiber transmission line.
2. An apparatus as claimed in claim 1 in which the low refractive index central core region of the short length of fiber occupies a diameter of about 12 microns.
3. An apparatus as claimed in claim 1 in which the short length of fiber has a refractive index profile which is substantially parabolic increasing from a minimum at an outer part of the core to a maximum at a region about 5 microns from the center of the core, the parabolic refractive index profile being modified by a low index central region.
4. An apparatus as claimed in claim 3 in which the low index central region is silica doped with an index depressing material and the part of the core immediately around said central low index region is silica doped with an index raising material.
5. An apparatus as claimed in claim 1 in which the central region of the short length of fiber is strongly absorbing.
6. An apparatus as claimed in claim 5 in which the absorbing region comprises silica containing a light absorbing dopant.
7. An apparatus as claimed in claim 3 wherein said multimode optical transmission line has a substantially parabolic refractive index which dips at the center.
8. A method, comprising: providing a multimode optical fiber transmission line; and coupling to said line a short length of fiber having the characteristic of strongly attenuating at least one light mode group passed through said fiber without attenuating other mode groups of different order and having a low refractive index central core region for attenuating an axially propagating mode group.
9. A method as claimed in claim 8, including the steps of forming said short length of fiber with a core and a cladding, forming the core with a generally parabolic refractive index profile which dips at the center of the short length of fiber and has a central low refractive index region with a diameter of at least 12 microns and a refractive index substantially lower than the index of an immediately adjacent part of the core.
10. A method as claimed in claim 9, including the steps of forming said multimode optical transmission line using an MCVD process to have a substantially parabolic refractive index which dips at the center.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000467549A CA1256310A (en) | 1984-11-09 | 1984-11-09 | Bandwidth enhancement of multimode optical transmission lines |
| JP24910285A JPS61120105A (en) | 1984-11-09 | 1985-11-08 | Fiber connection of optical fiber transmission of optical fiber transmission line and transmission line and fiber |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000467549A CA1256310A (en) | 1984-11-09 | 1984-11-09 | Bandwidth enhancement of multimode optical transmission lines |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1256310A true CA1256310A (en) | 1989-06-27 |
Family
ID=4129121
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000467549A Expired CA1256310A (en) | 1984-11-09 | 1984-11-09 | Bandwidth enhancement of multimode optical transmission lines |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPS61120105A (en) |
| CA (1) | CA1256310A (en) |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS51123650A (en) * | 1975-04-21 | 1976-10-28 | Nec Corp | An optical cable |
| JPS5280133A (en) * | 1975-12-26 | 1977-07-05 | Nec Corp | Mode filter |
| JPS5337438A (en) * | 1976-09-18 | 1978-04-06 | Nec Corp | Optical mode filter |
-
1984
- 1984-11-09 CA CA000467549A patent/CA1256310A/en not_active Expired
-
1985
- 1985-11-08 JP JP24910285A patent/JPS61120105A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPS61120105A (en) | 1986-06-07 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |