Radiation resistant fibers with depressed claddings for fiber optic gyro sensing coil

Kurbatov AM1 Kurbatov RA

Department of Center for terrestrial space infrastructure objects exploiting

Kuznetsov Research Institute for Applied Mechanics 55 Aviamotornaya St Moscow 111123 Russia

ABSTRACT

A two kinds of single mode optical fiber are described three-layer with depressed fluorine clad (W-fiber) Panda with Nitrogen core and isotropic fiber with pure (undoped) quartz core and two fluorine claddings (narrow and wide) Both of these fibers could be used in fiber optic gyro sensing coil operating in space radiation environment

Keywords single-mode optical fibers radiation resistance

1 INTRODUCTION

Radiation induced losses in fiber optic gyro (FOG) sensing coil fiber are one of the general problem for FOG

Solutions are known applying fibers with pure (undoped) silica core12 or with Nitrogen core2 Earlier we described a Ge-doped core fiber3 based on widely known refractive index (RI) W-profile4 having a

specifically narrow and deep depressive fluorine cladding and two kinds of fiber for FOG sensing coils basing on this profile polarization maintaining (РМ) fiber with low materialbending losses and polarizing (PZ) fiber One of the basic reasons of these fibers improved characteristics is a fundamental mode tight packing in the physically uniform core when this mode is under much more weak influence of non-uniformities an the core and fluorine clad boundary and at the boundaries of stress-applying rods Beside this a material losses in these rods is eliminated which in convenient fibers may be significant5 Finally according to calculations these fibers should be effective modal filters

Further it is known that Nitrogen-core fiber is radiation-resistant2 Thus changing the Ge-doped core in earlier described3 W-fiber to a Nitrogen core should make this fiber also to be radiation-resistant In present work a first sample of such Nitrogen Panda fiber is described

Also basing on earlier described3 W-profile a construction is created with pure silica core and two fluorine claddings (narrow and wide) which may give one more kind of radiation-resistant fiber for FOG sensing coil First sample of such kind of fiber is also described in the present work

Note that all related to this work calculations were carried out using mathematical models which are schematically described by us earlier3 For additional modal filtering estimation we also used one more model for asimutally symmetric W-fibers with coating having high RI (154) Here we also starting from scalar wave equation Coating is put to have infinite diameter and the field within it is a cylindrical outgoing wave In this case a propagation constant of any mode is a complex number with imaginary part characterizing the propagation losses of this mode

2 NITROGEN-CORE W-PROFILE PANDA-FIBER

On Fig 1 (a) and (b) are shown three RI profiles in W-fiber preform having a Nitrogen core (beginning middle

and the end of preform) and cross section photograph of Panda fiber drawn from it Preform was manufactured by ldquoFiberusrdquo company6 specialists (Moscow) according to profile calculated by the present work authors

1 E-mail akurbatov54mailru

Fig 1(а) W-profiles of Nitrogen core fiber preform

Fig 1(b) Nitrogen core Panda W-fiber cross section photograph

From Fig 1(a) it is clear that RI changes in preform cross section are large enough so they could be reached only by SPCVD-methid2 A dip in the profile center reduces a single-mode region comparing with earlier described3 Ge-core fibers because it reduces fundamental mode effective RI stronger that that of the second-order mode Thus while having the same fundamental mode bending losses a second-mode cutoff wavelength in fiber with dip will be higher than that in the the fiber without dip in the RI profile However according to calculations modal filtering at the operating wavelength (155 μm) should be effective enough Dip also increases a fundamental mode field diameter (MFD) due to its field distortion but still this MFD is lower than the core diameter which means a tight mode packing in it The drawn fiber

measured standard characteristics are listed in Table Further it is possible to noticeably improve waveguide parameters of

described fibers type Beside this in described fiber a Nitrogen content in the core is considerably lower than in high-aperture Nitrogen fibers from original paper2 from which it is also possible to conclude that this is an advantage at least in the ldquoslowrdquo space radiation environment

3 PURE-SILICA CORE ISOTROPIC FIBER WITH DOUBLE FLUORINE CLAD

Another kind of radiation-resistant fibers are the fibers with pure silica core and wide fluorine cladding12 Here

it is possible to combine the low bending losses and effective modal filtering In this case the core should be single mode and the fluorine cladding should be seven and more times wider than the core However in this case fundamental mode is always not tightly packed in the core (see above) Also according to the calculation here it is at least very difficult to reach a noticeable dichroism

On the Fig 2(a) a three RI profiles are presented in the beginning middle and in the end of preform for new fiber which also has a pure silica core but two fluorine claddings (narrow and wide) On Fig 2(b) a cross section photograph of drawn from it first isotropic fiber sample is presented having the diameter 200 μm (profile 1 on Fig 1(a)) This preform is also manufactured by ldquoFiberusrdquo company specialists (Moscow) according to profile calculated by the present work authors

Table Parameter Value

Fiber diameter (μm) 110 Insertion losses (dBkm) 18 h-parameter (1m) 210-5

Linear birefringence 310-4

Cutoff (μm) 142

Fig 2(a) RI profiles in preform for pure silica core fiber at two ends (1 and 3) and in the middle (2)

Fig 2(b) Isotropic fiber first sample cross section (with diameter 200 μm) drawn from the preform part having profile 1 on

Fig 2(a) Drawn fiber diameter (200 μm) could be reduced to 80 μm by removing the part of external quartz cladding without noticeable influence on fiber optical parameters This fiber has the core diameter 9 μm material losses 075 dBkm (as it is shown by spectral losses measurements in this fiber a water concentration (losses peak at 139 μm) is large and bend losses become noticeable only at loop diameter less than 10 mm Further MFD calculated value is 76 μm which is against the 9-μm core means fundamental mode tight packing in it Generally basing on such kind of RI profile a fibers are possible with MFD in the region 6-15 μm which are bend resistant and effective high order modes filters

Described earlier3 W-fiber is bend resistant because its fundamental mode mathematical cutoff7 is approximately equal to 22 μm At the same time fundamental mode sharp spectral losses growth (physical cutof) in a straight 1-m fiber according to calculations should begin near 21 μm and in the straight 1000-m fiber near 18 μm Thus fiber bend resistance is guaranteed by the distance of these values from operating wavelength (155 μm) The same could be said about Nitrogen core W-fiber described above

In the fiber on Fig 2(b) role of laquofundamentalraquo mode is played by one of the modes of overall fiber cross section It has practically the same field form as the fundamental mode of W-fiber3 but in this case it is always under the mathematical cutoff (as the rest of the modes) This is what differs this fiber from W-fiber3 and from Nitrogen core W-fiber However as for ldquofundamentalrdquo mode physical cutoff its threshold in the fiber from Fig 2(b) according to calculations is approximately equal to 22 μm (1-m fiber) and 18 μm (1000-m fiber) ie there is no difference from W-fibers3 from this point of view Due to this fact fiber from Fig 2(b) is also bend resistant in spite of the fact that its core RI due to fluorine penetration is even slightly lower than hat of the quartz external cladding

Generally according to calculations for fixed core and first fluorine cladding it is possible to apply a lot of second fluorine cladding kinds some of which are presented at Fig 3 giving the same ldquofundamentalrdquo mode bending loss

1 2 3

Fig 3 RI profiles giving the same bending loss

Here a W-profile3 is shown (profile 1) We carried out a comparative calculations for profiles 1-4 from the point of view of dichroism and high order modes filtering For the first case profile 1 is most preferable and profile 4 is least preferable In the second case of mode filtering situation is different moving from 1 to profile 4 mode filtering starting from large enough efficiency for profile 1 then grows to even larger values then reaches maximum (profile 2) and after that begin to monotonically decrease coming to be very inefficient at the profile 4 (there is no contradiction with above statement about possible effective mode filtering in the case of single wide fluorine cladding because above we made an accent that core itself should be single mode and all the profiles of Fig 3 have few-mode core)

Returning to Fig 2(a) one may see that all its three profiles are close enough to profile 2 on Fig 3 Further it is planned to manufacture Panda fibers basing on these profiles

4 CONCLUSION

In conclusion we described two kinds of fiber based on well-known refractive index (RI) W-profile 1)

Polarization maintaining (PM) Panda fiber with Nitrogen core and fluorine depressed clad 2) Isotropic fiber with pure silica core and two fluorine claddings (narrow and wide) The latter should give the good base for polarizing (PZ) pure-silica core fiber unlike the conventional pure-silica core fiber with single wide fluorine depressed cladding Both of these fibers are bend resistant and according to literature data should be radiation resistant so they may be used in fiber-optic gyro sensing coil under space radiation environment

5 AKNOWLEGMENTS

Authors are grateful to Golant K M for preform manufacturing for fibers described in the present work

REFERENCES [1] Alam M Abramczyk J Farroni J Manyam U Guertin D ldquoPassive and active optical fibers for space and terrestrial applicationsrdquo Proc SPIE 6308 630808-1-630808-14 (2006) [2] Tomashuk AL Golant KM ldquoRadiation-resistant and radiation-sensitive silica optical fibersrdquo Proc SPIE 4083 188-201 (2000) [3] Kurbatov AM Kurbatov RA ldquoNew optical W-fiber Panda for fiber optic gyroscope sensitive coilrdquo Technical Physics Letters 36(9) 789-791 (2010) [4] Kawakami S Nishida S ldquoCharacteristics of a doubly clad optical fiber with a low-index inner claddingrdquo IEEE Journal of Quantum Electronics 10(12) 879-887 (1974) [5] Tajima K Sasaki Y J ldquoTransmission loss of a 125-μm diameter PANDA fiber with circular stress-applying partsrdquo Journal of Lightwave Technology 7(4) 674 (1989) [6] httpwwwfiberusruaboutphp [7] M Monerie ldquoPropagation in doubly clad single-mode fibersrdquo IEEE Journal of Quantum Electronics 18(4) 535-542 (1982)

1

4

23

n(r)

Fig 1(а) W-profiles of Nitrogen core fiber preform

Fig 1(b) Nitrogen core Panda W-fiber cross section photograph

From Fig 1(a) it is clear that RI changes in preform cross section are large enough so they could be reached only by SPCVD-methid2 A dip in the profile center reduces a single-mode region comparing with earlier described3 Ge-core fibers because it reduces fundamental mode effective RI stronger that that of the second-order mode Thus while having the same fundamental mode bending losses a second-mode cutoff wavelength in fiber with dip will be higher than that in the the fiber without dip in the RI profile However according to calculations modal filtering at the operating wavelength (155 μm) should be effective enough Dip also increases a fundamental mode field diameter (MFD) due to its field distortion but still this MFD is lower than the core diameter which means a tight mode packing in it The drawn fiber

measured standard characteristics are listed in Table Further it is possible to noticeably improve waveguide parameters of

described fibers type Beside this in described fiber a Nitrogen content in the core is considerably lower than in high-aperture Nitrogen fibers from original paper2 from which it is also possible to conclude that this is an advantage at least in the ldquoslowrdquo space radiation environment

3 PURE-SILICA CORE ISOTROPIC FIBER WITH DOUBLE FLUORINE CLAD

Another kind of radiation-resistant fibers are the fibers with pure silica core and wide fluorine cladding12 Here

it is possible to combine the low bending losses and effective modal filtering In this case the core should be single mode and the fluorine cladding should be seven and more times wider than the core However in this case fundamental mode is always not tightly packed in the core (see above) Also according to the calculation here it is at least very difficult to reach a noticeable dichroism

On the Fig 2(a) a three RI profiles are presented in the beginning middle and in the end of preform for new fiber which also has a pure silica core but two fluorine claddings (narrow and wide) On Fig 2(b) a cross section photograph of drawn from it first isotropic fiber sample is presented having the diameter 200 μm (profile 1 on Fig 1(a)) This preform is also manufactured by ldquoFiberusrdquo company specialists (Moscow) according to profile calculated by the present work authors

Table Parameter Value

Fiber diameter (μm) 110 Insertion losses (dBkm) 18 h-parameter (1m) 210-5

Linear birefringence 310-4

Cutoff (μm) 142

Fig 2(a) RI profiles in preform for pure silica core fiber at two ends (1 and 3) and in the middle (2)

Fig 2(b) Isotropic fiber first sample cross section (with diameter 200 μm) drawn from the preform part having profile 1 on

Fig 2(a) Drawn fiber diameter (200 μm) could be reduced to 80 μm by removing the part of external quartz cladding without noticeable influence on fiber optical parameters This fiber has the core diameter 9 μm material losses 075 dBkm (as it is shown by spectral losses measurements in this fiber a water concentration (losses peak at 139 μm) is large and bend losses become noticeable only at loop diameter less than 10 mm Further MFD calculated value is 76 μm which is against the 9-μm core means fundamental mode tight packing in it Generally basing on such kind of RI profile a fibers are possible with MFD in the region 6-15 μm which are bend resistant and effective high order modes filters

Described earlier3 W-fiber is bend resistant because its fundamental mode mathematical cutoff7 is approximately equal to 22 μm At the same time fundamental mode sharp spectral losses growth (physical cutof) in a straight 1-m fiber according to calculations should begin near 21 μm and in the straight 1000-m fiber near 18 μm Thus fiber bend resistance is guaranteed by the distance of these values from operating wavelength (155 μm) The same could be said about Nitrogen core W-fiber described above

In the fiber on Fig 2(b) role of laquofundamentalraquo mode is played by one of the modes of overall fiber cross section It has practically the same field form as the fundamental mode of W-fiber3 but in this case it is always under the mathematical cutoff (as the rest of the modes) This is what differs this fiber from W-fiber3 and from Nitrogen core W-fiber However as for ldquofundamentalrdquo mode physical cutoff its threshold in the fiber from Fig 2(b) according to calculations is approximately equal to 22 μm (1-m fiber) and 18 μm (1000-m fiber) ie there is no difference from W-fibers3 from this point of view Due to this fact fiber from Fig 2(b) is also bend resistant in spite of the fact that its core RI due to fluorine penetration is even slightly lower than hat of the quartz external cladding

Generally according to calculations for fixed core and first fluorine cladding it is possible to apply a lot of second fluorine cladding kinds some of which are presented at Fig 3 giving the same ldquofundamentalrdquo mode bending loss

1 2 3

Fig 3 RI profiles giving the same bending loss

Here a W-profile3 is shown (profile 1) We carried out a comparative calculations for profiles 1-4 from the point of view of dichroism and high order modes filtering For the first case profile 1 is most preferable and profile 4 is least preferable In the second case of mode filtering situation is different moving from 1 to profile 4 mode filtering starting from large enough efficiency for profile 1 then grows to even larger values then reaches maximum (profile 2) and after that begin to monotonically decrease coming to be very inefficient at the profile 4 (there is no contradiction with above statement about possible effective mode filtering in the case of single wide fluorine cladding because above we made an accent that core itself should be single mode and all the profiles of Fig 3 have few-mode core)

Returning to Fig 2(a) one may see that all its three profiles are close enough to profile 2 on Fig 3 Further it is planned to manufacture Panda fibers basing on these profiles

4 CONCLUSION

In conclusion we described two kinds of fiber based on well-known refractive index (RI) W-profile 1)

Polarization maintaining (PM) Panda fiber with Nitrogen core and fluorine depressed clad 2) Isotropic fiber with pure silica core and two fluorine claddings (narrow and wide) The latter should give the good base for polarizing (PZ) pure-silica core fiber unlike the conventional pure-silica core fiber with single wide fluorine depressed cladding Both of these fibers are bend resistant and according to literature data should be radiation resistant so they may be used in fiber-optic gyro sensing coil under space radiation environment

5 AKNOWLEGMENTS

Authors are grateful to Golant K M for preform manufacturing for fibers described in the present work

REFERENCES [1] Alam M Abramczyk J Farroni J Manyam U Guertin D ldquoPassive and active optical fibers for space and terrestrial applicationsrdquo Proc SPIE 6308 630808-1-630808-14 (2006) [2] Tomashuk AL Golant KM ldquoRadiation-resistant and radiation-sensitive silica optical fibersrdquo Proc SPIE 4083 188-201 (2000) [3] Kurbatov AM Kurbatov RA ldquoNew optical W-fiber Panda for fiber optic gyroscope sensitive coilrdquo Technical Physics Letters 36(9) 789-791 (2010) [4] Kawakami S Nishida S ldquoCharacteristics of a doubly clad optical fiber with a low-index inner claddingrdquo IEEE Journal of Quantum Electronics 10(12) 879-887 (1974) [5] Tajima K Sasaki Y J ldquoTransmission loss of a 125-μm diameter PANDA fiber with circular stress-applying partsrdquo Journal of Lightwave Technology 7(4) 674 (1989) [6] httpwwwfiberusruaboutphp [7] M Monerie ldquoPropagation in doubly clad single-mode fibersrdquo IEEE Journal of Quantum Electronics 18(4) 535-542 (1982)

1

4

23

n(r)

Fig 2(a) RI profiles in preform for pure silica core fiber at two ends (1 and 3) and in the middle (2)

Fig 2(b) Isotropic fiber first sample cross section (with diameter 200 μm) drawn from the preform part having profile 1 on

Fig 2(a) Drawn fiber diameter (200 μm) could be reduced to 80 μm by removing the part of external quartz cladding without noticeable influence on fiber optical parameters This fiber has the core diameter 9 μm material losses 075 dBkm (as it is shown by spectral losses measurements in this fiber a water concentration (losses peak at 139 μm) is large and bend losses become noticeable only at loop diameter less than 10 mm Further MFD calculated value is 76 μm which is against the 9-μm core means fundamental mode tight packing in it Generally basing on such kind of RI profile a fibers are possible with MFD in the region 6-15 μm which are bend resistant and effective high order modes filters

Described earlier3 W-fiber is bend resistant because its fundamental mode mathematical cutoff7 is approximately equal to 22 μm At the same time fundamental mode sharp spectral losses growth (physical cutof) in a straight 1-m fiber according to calculations should begin near 21 μm and in the straight 1000-m fiber near 18 μm Thus fiber bend resistance is guaranteed by the distance of these values from operating wavelength (155 μm) The same could be said about Nitrogen core W-fiber described above

In the fiber on Fig 2(b) role of laquofundamentalraquo mode is played by one of the modes of overall fiber cross section It has practically the same field form as the fundamental mode of W-fiber3 but in this case it is always under the mathematical cutoff (as the rest of the modes) This is what differs this fiber from W-fiber3 and from Nitrogen core W-fiber However as for ldquofundamentalrdquo mode physical cutoff its threshold in the fiber from Fig 2(b) according to calculations is approximately equal to 22 μm (1-m fiber) and 18 μm (1000-m fiber) ie there is no difference from W-fibers3 from this point of view Due to this fact fiber from Fig 2(b) is also bend resistant in spite of the fact that its core RI due to fluorine penetration is even slightly lower than hat of the quartz external cladding

Generally according to calculations for fixed core and first fluorine cladding it is possible to apply a lot of second fluorine cladding kinds some of which are presented at Fig 3 giving the same ldquofundamentalrdquo mode bending loss

1 2 3

Fig 3 RI profiles giving the same bending loss

Here a W-profile3 is shown (profile 1) We carried out a comparative calculations for profiles 1-4 from the point of view of dichroism and high order modes filtering For the first case profile 1 is most preferable and profile 4 is least preferable In the second case of mode filtering situation is different moving from 1 to profile 4 mode filtering starting from large enough efficiency for profile 1 then grows to even larger values then reaches maximum (profile 2) and after that begin to monotonically decrease coming to be very inefficient at the profile 4 (there is no contradiction with above statement about possible effective mode filtering in the case of single wide fluorine cladding because above we made an accent that core itself should be single mode and all the profiles of Fig 3 have few-mode core)

Returning to Fig 2(a) one may see that all its three profiles are close enough to profile 2 on Fig 3 Further it is planned to manufacture Panda fibers basing on these profiles

4 CONCLUSION

In conclusion we described two kinds of fiber based on well-known refractive index (RI) W-profile 1)

Polarization maintaining (PM) Panda fiber with Nitrogen core and fluorine depressed clad 2) Isotropic fiber with pure silica core and two fluorine claddings (narrow and wide) The latter should give the good base for polarizing (PZ) pure-silica core fiber unlike the conventional pure-silica core fiber with single wide fluorine depressed cladding Both of these fibers are bend resistant and according to literature data should be radiation resistant so they may be used in fiber-optic gyro sensing coil under space radiation environment

5 AKNOWLEGMENTS

Authors are grateful to Golant K M for preform manufacturing for fibers described in the present work

REFERENCES [1] Alam M Abramczyk J Farroni J Manyam U Guertin D ldquoPassive and active optical fibers for space and terrestrial applicationsrdquo Proc SPIE 6308 630808-1-630808-14 (2006) [2] Tomashuk AL Golant KM ldquoRadiation-resistant and radiation-sensitive silica optical fibersrdquo Proc SPIE 4083 188-201 (2000) [3] Kurbatov AM Kurbatov RA ldquoNew optical W-fiber Panda for fiber optic gyroscope sensitive coilrdquo Technical Physics Letters 36(9) 789-791 (2010) [4] Kawakami S Nishida S ldquoCharacteristics of a doubly clad optical fiber with a low-index inner claddingrdquo IEEE Journal of Quantum Electronics 10(12) 879-887 (1974) [5] Tajima K Sasaki Y J ldquoTransmission loss of a 125-μm diameter PANDA fiber with circular stress-applying partsrdquo Journal of Lightwave Technology 7(4) 674 (1989) [6] httpwwwfiberusruaboutphp [7] M Monerie ldquoPropagation in doubly clad single-mode fibersrdquo IEEE Journal of Quantum Electronics 18(4) 535-542 (1982)

1

4

23

n(r)

Fig 3 RI profiles giving the same bending loss

Here a W-profile3 is shown (profile 1) We carried out a comparative calculations for profiles 1-4 from the point of view of dichroism and high order modes filtering For the first case profile 1 is most preferable and profile 4 is least preferable In the second case of mode filtering situation is different moving from 1 to profile 4 mode filtering starting from large enough efficiency for profile 1 then grows to even larger values then reaches maximum (profile 2) and after that begin to monotonically decrease coming to be very inefficient at the profile 4 (there is no contradiction with above statement about possible effective mode filtering in the case of single wide fluorine cladding because above we made an accent that core itself should be single mode and all the profiles of Fig 3 have few-mode core)

Returning to Fig 2(a) one may see that all its three profiles are close enough to profile 2 on Fig 3 Further it is planned to manufacture Panda fibers basing on these profiles

4 CONCLUSION

In conclusion we described two kinds of fiber based on well-known refractive index (RI) W-profile 1)

Polarization maintaining (PM) Panda fiber with Nitrogen core and fluorine depressed clad 2) Isotropic fiber with pure silica core and two fluorine claddings (narrow and wide) The latter should give the good base for polarizing (PZ) pure-silica core fiber unlike the conventional pure-silica core fiber with single wide fluorine depressed cladding Both of these fibers are bend resistant and according to literature data should be radiation resistant so they may be used in fiber-optic gyro sensing coil under space radiation environment

5 AKNOWLEGMENTS

Authors are grateful to Golant K M for preform manufacturing for fibers described in the present work

REFERENCES [1] Alam M Abramczyk J Farroni J Manyam U Guertin D ldquoPassive and active optical fibers for space and terrestrial applicationsrdquo Proc SPIE 6308 630808-1-630808-14 (2006) [2] Tomashuk AL Golant KM ldquoRadiation-resistant and radiation-sensitive silica optical fibersrdquo Proc SPIE 4083 188-201 (2000) [3] Kurbatov AM Kurbatov RA ldquoNew optical W-fiber Panda for fiber optic gyroscope sensitive coilrdquo Technical Physics Letters 36(9) 789-791 (2010) [4] Kawakami S Nishida S ldquoCharacteristics of a doubly clad optical fiber with a low-index inner claddingrdquo IEEE Journal of Quantum Electronics 10(12) 879-887 (1974) [5] Tajima K Sasaki Y J ldquoTransmission loss of a 125-μm diameter PANDA fiber with circular stress-applying partsrdquo Journal of Lightwave Technology 7(4) 674 (1989) [6] httpwwwfiberusruaboutphp [7] M Monerie ldquoPropagation in doubly clad single-mode fibersrdquo IEEE Journal of Quantum Electronics 18(4) 535-542 (1982)

1

4

23

n(r)