Patent Application
20130071082
In some applications, polarization variations can induce output beam instabilities. It is thus desired to stabilize the polarization state of the output beam, which has been done using polarization-maintaining optical fibers. Among known types of polarization-maintaining fibers, PANDA optical fibers are widely used and have at least one, and more typically two stress members extending parallel to the core and inducing polarization maintaining asymmetry. PANDA optical fibers can be manufactured as active or passive polarization-maintaining fibers since their manufacturing process allows the separate manufacturing of the stress applying parts, or stress members, and the fiber optic preform with a core left undoped or doped with rare-earth ions for laser or amplifier applications for instance.
Although satisfactory to a certain degree, former polarization-maintaining optical fibers met limitations in terms of the amount of power they could handle.
For high-power applications, it is convenient to use multi-clad large-mode-area optical fibers. These fibers have a large rare-earth-doped core. Light delivered by the laser pump diode is injected in the cladding of the optical fiber, which is called the pump-guide, and then coupled into the rare-earth-doped core (i.e. having a rare earth dopant doping the core). The larger surface of large rare-earth-doped core helps managing damage in active optical fibers which can occur given the high injected power of the focused beam. A lower core numerical aperture and coiling of the fiber can then be used to still maintain the possibility to operate the fiber in a single-mode (or more likely quasi-single-mode) core regime.
In order to provide such a low core numerical aperture, a multiple-clad configuration may be used. A first cladding, called the pedestal, is inserted between the rare-earth-doped core and a silica cladding which is called the pump-guide. The main advantage of this design is the possibility to have a highly doped core which can be operated in single mode operation. The core and the pedestal are made of doped silica glasses. To produce polarization maintaining optical fiber having high birefringence capacity, one avenue was to apply a PANDA configuration which such optical fibers.
However, since birefringence levels (which affect the efficiency of the polarization maintenance characteristics) are related to the distance between the stress member and the core, and the diameter of the stress members, and given requirements of pedestal diameter, using the PANDA configuration on such fibers resulted in the stress member(s) to extend partially in the cladding portion and partially in the pedestal to reach satisfactory birefringence levels.
PANDA optical fibers are typically fabricated by the rod-in-tube process, which involves drilling channels in the optical fiber preform, in which the stress members are later inserted. The assembled preform is then drawn into PANDA optical fiber in which the proportions (relative diameters of claddings and position of stress members) of the preform are essentially maintained. During the drilling process, the channel was drilled in a section of silica glass of the preform which had two distinct portions. In the pedestal portion, the silica is typically doped for increasing the refractive index or refraction relative to the cladding portion, which is typically left undoped. Drilling in this heterogeneous section of glass led to occurrences of irreversible damage which made the preform unworkable for drawing effectively into optical fiber.
One possible reason for such damage is that the refractive index-increasing dopants used in the pedestal typically have the side effect of increasing the thermal expansion coefficient of the pedestal material. Since the surrounding cladding is typically left without such dopants, thereby yielding a refractive index step, the surrounding cladding did not suffer from the increase in thermal expansion coefficient and thus had a different thermal expansion coefficient than the pedestal portion. Heat being typically generated during the drilling operation, it is likely that the damage to the preform during drilling was a consequence of uneven thermal expansion of the pedestal and the cladding when subjected to the drilling heat.
A further dopant was added to the pedestal to counter the side effect the refractive index-increasing dopant had on the thermal expansion coefficient, and thereby bringing the thermal expansion coefficient of the pedestal as close as feasible to the thermal expansion coefficient of the cladding while maintaining a satisfactory refractive index step between the cladding and the pedestal. This further dopant is referred to herein as a thermal expansion coefficient-reducing dopant. The tests have shown that this successfully limited the occurrence of damage when drilling a channel in the heterogeneous pedestal/cladding portion.
Henceforth, in accordance with one aspect, there is provided a preform of an optical fiber having a core, a pedestal having both a refractive index-increasing dopant and a thermal expansion coefficient reducing dopant, a cladding around the pedestal, and at least one channel drilled partially in the cladding and partially in the pedestal and housing a stress-member. The thermal expansion coefficient of the pedestal can thus be close to the thermal expansion coefficient of the cladding.
In accordance with another aspect, there is provided a multi-clad polarization-maintaining optical fiber comprising: a core; a pedestal surrounding the core and having a pedestal refractive index, the pedestal being made of silica glass doped both with a refractive index-increasing-dopant having a thermal-expansion-coefficient-increasing side-effect and with a thermal-expansion-coefficient-reducing dopant; a cladding surrounding the pedestal and having a cladding refractive index significantly lower than the pedestal refractive index; at least one a stress member extending parallel to the core into both the pedestal and the cladding and adapted to produce birefringence in the core for polarization-maintaining; an outer cladding having a refractive index significantly lower than the cladding refractive index; and a jacket surrounding the outer cladding.
In accordance with another aspect, there is provided a multi-clad polarization-maintaining optical waveguide comprising: a core with a first refractive index and made with a glass which is doped; a pedestal cladding disposed around said core and having a second refractive index lower than said first refractive index, said pedestal cladding being made of said glass doped at least with a first dopant increasing a refractive index of said glass and increasing a coefficient of thermal expansion of said glass, and a second dopant decreasing said coefficient of thermal expansion of said glass to at least partly cancel said increasing a coefficient of thermal expansion of said glass; a pump-guide cladding disposed around said pedestal cladding and made at least of said glass and having a third refractive index lower than said second refractive index; an outer cladding disposed around said pump-guide and having a fourth refractive index lower than said third refractive index; and at least one stress member extending along said optical waveguide at least partly in said pedestal cladding to produce birefringence in said polarization-maintaining optical fiber.
In accordance with another aspect, there is provided a polarization-maintaining optical waveguide comprising: a core with a first refractive index made with a glass which is doped; a first cladding layer disposed around said core and having a second refractive index lower than said first refractive index, said first cladding layer being made of said glass doped at least with a first dopant increasing a refractive index of said glass and increasing a coefficient of thermal expansion of said glass, and a second dopant decreasing said coefficient of thermal expansion of said glass to at least partly cancel said increasing a coefficient of thermal expansion of said glass; and at least one stress member extending along said optical waveguide at least partly in said first cladding layer to produce birefringence in said polarization-maintaining optical fiber.
In accordance with another aspect, there is provided a method of manufacturing a preform for manufacturing a polarization-maintaining optical fiber from a multi-clad preform having a core region, a pedestal region surrounding the core and having a pedestal refractive index, the pedestal region being of silica glass doped both with a refractive index-increasing-dopant having a thermal-expansion-coefficient-increasing side-effect and with a thermal-expansion-coefficient-reducing dopant at least partially countering thermal-expansion-coefficient-increasing side-effect; and a cladding region surrounding the pedestal and having a cladding refractive index significantly lower than the pedestal refractive index, said method comprising: drilling at least one circular-cross-section channel in a heterogeneous section of the multi-clad preform including both a portion of the pedestal region and a portion of the cladding region.
In accordance with another aspect, there is provided a multi-component silica glass for use in the manufacturing of optical waveguides, the multi-component silica glass comprising: a glass matrix; a first dopant increasing a refractive index of said glass and increasing a coefficient of thermal expansion of said glass matrix; and a second dopant decreasing said coefficient of thermal expansion of said glass to at least partly cancel said increasing a coefficient of thermal expansion of said glass matrix.
Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.
In the figures,
It will be noted that throughout the appended drawings, like features are identified by like reference numerals.
Now referring to the drawings,
In this example, the optical fiber 100 is an active fiber adapted for continuous high-power power applications. It has a large rare-earth-doped core 1 doped with at least one rare-earth dopant generating laser or amplifier effect; a first cladding, referred to as the pedestal 2, surrounding the large rare-earth-doped core 1; and a second cladding, referred to as the pump-guide 3, surrounding the pedestal. An outer cladding 5 made of glass or polymer with a lower refractive index is also used here. In operation, optical signal power can be propagated in the core 1 while optical pump power is propagated in the pump-guide 3. A PANDA polarization-maintaining configuration is obtained using stress applying parts, referred to as stress members 4 which have a sufficient diameter, and are disposed sufficiently close to the core 1 to produce satisfactory birefringence in the polarization-maintaining optical fiber 100. It will be noted here that in alternate embodiments, the polarization-maintaining optical fiber have only one stress members, or more than two stress members. Further, in some polarization maintaining optical fibers the stress-members have a cross-sectional shape other than circular. Henceforth the typical two circular stress member configuration is shown here only for purpose of providing an example embodiment.
As can be seen from
The use of a pedestal in rare-earth-doped optical fibers offers many advantages. First, it is known that co-doping with various dopants like aluminum, phosphor, cerium or a mixture thereof can prevent photodarkening in ytterbium-doped optical fibers. Such dopants increase the refractive index of the core. By using a pedestal, a lower numerical aperture of the core can be reached which allows single-mode or quasi-single mode regime.
Second, in the case of erbium-ytterbium co-doped optical fibers, Yb3+ ions act as sensitizer ions and Er3+ ions as acceptors. Phosphorus oxide P2O5 is then added in the core to increase the energy transfer from Yb3+ ions to Er3+ ions. The pedestal around the core is used to compensate for the refractive index increase due to P2O5 doping by reducing the numerical aperture.
Third, such a triple-clad design can make a highly doped single-mode core regime of operation possible.
It is noted that other embodiments can be passive optical fibers which are not doped with rare-earth ions. PANDA triple-clad passive optical fibers may be used as relay fibers for example.
Based on stress analysis of PANDA optical fibers, birefringence B is typically given by the following equation:
where k is a characteristic constant of the optical fiber, Δα is the difference in the thermal expansion coefficients of the stress members 4 and the silica cladding 3, h is the diameter of the stress members 4, e is the distance between the center of each stress member 4 and the center of the core 1 and Φ is the diameter of the silica cladding 3.
Equation (1) shows that birefringence increases when the distance e decreases. Accordingly, in order to obtain high birefringence, the stress members 4 should be inserted as close as possible to the core 1.
In the case of PANDA fibers, stress members 4 are inserted in the optical fiber 100 at the time of manufacturing the preform which is later used to draw the optical fiber 100. In order to insert the stress members 4, channels are drilled in the preform and stress rods are then inserted in the channels. As mentioned above, the pedestal 2 should have a diameter of about three to five times larger than the diameter of the core 1. Accordingly, for the stress member 4 to be large enough and close enough to the core 1 to obtain high birefringence, the preform is drilled partly in the pedestal 2, and partly in the cladding 3. However, the refractive index of the pedestal 2 should be selected to obtain a high numerical aperture between the pedestal 2 and the pump-guide silica cladding 3. The targeted refractive index is typically reached in the pedestal 2 by adding germanium oxide GeO2, or another refractive index-increasing dopant, in the glass composition, which increases the refractive index but also has the side-effect of increasing the thermal expansion coefficient of the silica glass. As shown in
It is therefore proposed herein to modify the composition of the pedestal 2 to decrease the thermal expansion coefficient of the pedestal 2 in order to reduce this stress and allow safe drilling of the preform as close as possible to the core 1. This is done by co-doping the silica glass of the pedestal 2 with titanium oxide (TiO2), or another thermal-expansion-coefficient-reducing dopant, which may also have an effect on the refractive index but decreases the thermal expansion coefficient of the silica glass, thereby at least partly canceling the thermal expansion coefficient increase due to other co-dopants like GeO2. Henceforth, a ratio of refractive index-increasing dopant(s) and thermal-expansion-coefficient-reducing dopant(s) can be selected to achieve satisfactory refractive index while yielding a thermal expansion coefficient equal to, or at least operatively close to the surrounding cladding. The expression operatively close to, in this context, means close enough to maintain damages caused by thermal expansion during drilling under an acceptable threshold given the intended application.
Back to
As shown in
Equation (1) shows that birefringence is proportional to the thermal expansion coefficient difference Δα between stress members 4 and the pump-guide silica cladding 3.
Accordingly, one factor in obtaining satisfactorily high birefringence is using a high thermal expansion coefficient in the stress members 4. Stress members 4 are typically made of silica glass doped with phosphorus oxide (P2O5) and boron oxide (B2O3) or with boron oxide only. Boron oxide increases the thermal expansion coefficient of silica glass and it also significantly reduces its refractive index. In fact, birefringence increases linearly with the increase of the boron doping level. Also, as mentioned above, in order to obtain high birefringence, the stress members 4 should be disposed as close as possible to the core 1 and be sufficiently large. Accordingly, the stress members 4 typically overlap the pedestal 2. More specifically, the distance e′ between the centre of the optical fiber 100 and the nearest edge of the stress members 4 is somewhere between the radius r1 of the core 1 and the radius r2 of the pedestal 2.
An example of a process that can be used to manufacture a polarization-maintaining optical fiber based on the configuration described above is now discussed. The process is now described for the particular example of the triple-clad polarization-maintaining optical fiber 100 as shown in
1) Manufacturing a Precursor Perform
The pedestal region 602 is fabricated inside the low-OH fused silica tube. Successive layers of the glass composition SiO2—GeO2—TiO2 are deposited inside the low-OH fused silica tube by heating with a H2/O2 burner a mixture of SiCl4, GeCl4 and TiCl4 precursors. The number of layers and the flow of the precursors are selected to satisfy the desired ratio between the pedestal diameter (Φ2) and core diameter (Φ1), such as Φ2/Φ1>3.
It is noted that according to Paul et al. in “Chemistry of titanium incorporation in silica glass of optical preform for making of Ti doped optical fibre by the MCVD process”, Optical Materials, 30, pp 1538-1548 (2008) which studies the kinetic of TiO2 incorporation in silica glass using the MCVD process with TiCl4 as the gaseous precursor, the deposition temperature has a less than significant effect on the titanium concentration. However, it is preferable to deposit TiO2 at low temperature to avoid the parasite reduction of Ti(IV) in Ti(III) since this reaction induces a brown coloration of the glass which increases the attenuation. Paul et al. also recommend that TiO2 concentration should not be larger than 2 mol % because too high concentration gives rise to brown coloration.
Next, the core region 601 of the preform is fabricated. In the case of an active optical fiber, porous silica soot layers with or without various refractive index raising co-dopants like germanium oxide and/or phosphorus oxide are deposited inside the tube. Rare-earth elements are then doped inside the porous layers by the solution doping process, as known in the art. Rare-earth elements may include thulium (Tm), erbium (Er), ytterbium (Yb), etc., and a mixture thereof. The solution is made with rare-earth salts precursors. Al2O3 or P2O5 are known to increase the solubility of the rare-earth elements inside the silica glass network. Aluminum salt and/or orthophosphoric acid (H3PO4) are therefore also added to the solution. After drying, the rare-earth doped silica soot layers are sintered at high temperature. The tube is then collapsed by increasing the temperature of the burner.
In the case of a passive optical fiber, successive glass layers of silica doped with raising refractive index dopants like GeO2 and/or P2O5 are deposited inside the tube 603 by heating with a H2/O2 burner a mixture of precursors like SiCl4, GeCl4 and/or POCl3. The tube is then collapsed by increasing the temperature of the burner.
The precursor preform is then etched with hydrofluoric acid or overclad with a low-OH fused silica tube to get the right ratio diameter between the pump-guide cladding region 603 and the core region 601.
2) Drilling Channels in the Precursor Perform
A section of the precursor preform is then cut for the drilling step. Using an ultra-sonic milling machine, two channels are drilled. The key drilling parameters are the diameter h of the holes and the position of the holes. In fact, equation (1) shows that stress member diameter h and the distance e between the center of the stress members and the center of the optical fiber have a great influence on the birefringence. A large stress member diameter h and a short distance e are required to obtain high birefringence. The addition of titanium oxide in the pedestal allows drilling the precursor preform quite close to the core in order to obtain a low distance e between the center of the stress members and the center to the optical fiber such that the distance e′ between the centre of the optical fiber and the nearest edge of the stress member satisfies the condition: r1<e′<r2.
3) Polishing the Inner Surfaces of the Stress Member Channels
The inner surface of the stress member channels is then polished with a low intensity H2/O2 flame to avoid deformation of the holes.
4) Manufacturing the Stress Members
The stress members are fabricated separately, such as with the MCVD process. Equation (1) shows that birefringence is proportional to the coefficient of thermal expansion difference between stress members and pump-guide cladding noted Δα. Accordingly, stress members are typically made of silica glass which is doped to increase its thermal expansion coefficient in order to obtain high birefringence. The stress members are typically made using silica glass doped with P2O5 and B2O3 or with B2O3 only. The stress members are then drawn into smaller diameter rods to fit in the drilled stress member channels.
5) Inserting the Stress Members in the Channels
The stress members are then inserted into the channels in the preform. That assembled preform is then drawn into PANDA optical fiber.
6) Drawing the Preform into an Optical Fiber
In the case of triple-clad polarization-maintaining optical fiber designed for high power applications, an outer cladding made of a low refractive index polymer such as fluoroacrylate polymer coating, Teflon™ coating, or silicone coating is typically added around the optical fiber in the drawing process. The low refractive index cladding may also consist of a specialty silica glass doped with fluorine, for example, and which is deposited around the preform before drilling. An air-clad cladding may also be used to provide a low refractive index cladding by assembling one or more rings of silica capillaries around the preform and overcladding it with a silica tube. In any case, a protective polymer coating is finally added. The final diameter of the triple-clad polarization-maintaining optical fiber is selected as a function of the application for which it is designed.
A preform was fabricated by the MCVD process. First, the multi-component pedestal GeO2—TiO2 doped silica glass was deposed on a low-OH fused silica tube. GeO2 and TiO2 concentrations were targeted to get a difference of thermal expansion coefficient between the pedestal and the silica cladding operatively close to zero. The GeO2 doped silica core was then deposed. φ2 and φ1 are respectively the pedestal and the core diameters. Core and pedestal layers were optimised such as φ2/φ1 larger than 3. The preform was then sintered.
After the preform fabrication, two channels were drilled in a 200 mm long section of the preform by using an ultra-sonic milling machine. The PANDA geometry was designed for a 125 microns diameter optical fiber such as:
No cracking of the preform was observed during the drilling. After drilling, both channels were fire-polished and B2O3 doped silica stress members were inserted into the channels.
This assembled preform was then drawn into a 125 microns optical fiber. A UV cured low refractive index fluoroacrylate polymer is deposed as the outer cladding and the optical fiber is protected by a UV cured standard acrylate polymer.
The obtained triple-clad PANDA optical fiber had the following specifications:
The measured GeO2 and TiO2 concentrations and their contributions to the silica glass thermal expansion coefficient were used to calculate the difference in thermal expansion coefficient between the pedestal and the silica cladding. We get 0.1×10−7/° C. As targeted, this value is close to zero and resulted in drilling of the preform being done without noticeable damage. It is further noted that the core and pedestal diameters satisfy the condition φ2/φ1>3, and the distance e′ satisfies the condition r1<e′<r2.
Although the test was made on a passive fiber, we soundly predict that this would also work in the case of some active fibers. Given the theory that the disappearance of irreversible damage is caused by the use of the thermal expansion coefficient-reducing dopant in the pedestal which brings the thermal expansion coefficient operatively close to the thermal expansion coefficient of the surrounding cladding, it is believed that adding a lasing-effect or other dopant to the core should not have any significant effect when drilling the channels, since the drilling is effected in the cladding and the pedestal.
It is noted that titanium oxide is used in the above described embodiments because of a combination of its optical and mechanical properties in silica glass. One will understand that other dopant(s) having counteracting effects on the thermal expansion coefficient, such as niobium or tantalum (Nb2O5, Ta2O5) for instance, may be used instead.
Depending on the specific application, the values of birefringence reached can vary between 2×10−4 and 5×10−4, for instance.
As can be seen therefore, the examples described above and illustrated are intended to be exemplary only. The scope is indicated by the appended claims.
This application claims priority of U.S. provisional application No. 61/334,218, filed May 13, 2010 by applicants, the contents of which are hereby incorporated by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA2011/050291 | 5/11/2011 | WO | 00 | 11/12/2012 |
| Number | Date | Country | |
|---|---|---|---|
| 61334218 | May 2010 | US |
