Patent Application: US-81012897-A

Abstract:
an optical fiber has a cladding glass layer surrounding a glass core , in which a region of the optical fiber partially overlapping the cladding and / or the core is formed of photosensitive glass .

Description:
fig1 a is a schematic cross section of an optical fibre 10 ; fig1 b is a schematic representation of the fibre &# 39 ; s refractive index profile ; and fig1 c is a schematic representation of the fibre &# 39 ; s photosensitivity profile . the fibre 10 comprises a light - guiding core 20 and a glass cladding 30 . as shown in fig1 b , the core has a raised refractive index with respect to that of the cladding . in the cross section of fig1 a , the core is represented by the area enclosed by the innermost circle , and the cladding by the area enclosed between the innermost circle and the outermost circle . typically , the diameter of the core in these embodiments is ˜ ( about ) 5 μm ; the outer diameter of the cladding ˜ 125 μm ; and the refractive index difference between the core and cladding ˜ 0 . 013 . fig1 c schematically illustrates the variation in photosensitivity across the diameter of the fibre 10 . an annular ( or open - cylindrically - shaped ) region 40 about 3 μm thick and surrounding the core 20 is made photosensitive by doping with ge and b , whereas the core 20 and most of the cladding 30 are substantially not photosensitive ( or at least much less photosensitive than the region 40 ). in the examples of fig1 a to 1h , the photosensitive region is annular . however , circularly asymmetrical photosensitive regions are also possible . the dopants used to render the region 40 photosensitive are matched so that the refractive index of the region 40 is similar ( or as identical as reasonably possible ) to that of the remainder of the cladding . so , in the absence of any photo - induced refractive index variation in the region 40 , the fibre would behave optically as though the region 40 were formed of the same glass as the remainder of the cladding . however , a small discrepancy can be tolerated . using this fibre configuration , a grating can be written into the region 40 by a standard technique ( for example , by transverse exposure to ultraviolet interference fringes or by exposure through a phase mask ). the grating formed in the region 40 of the fibre then provides normal grating properties , such as narrow - band reflection . a benefit of this arrangement is that the glass composition used in the core can be selected for most efficient operation as , for example , a laser , and does not have to be compromised by photosensitive dopants to allow a grating to be written into the core . the fibre 10 supports only one spatial mode at the laser wavelength . the thickness of the b / ge / si ( si : silicon ) ring is typically larger than the core radius . an example of the fibre 10 has a na ( numerical aperture ) of approximately 0 . 2 and supports a single spatial mode above 1250 nm . fig1 d to 1f schematically illustrate an optical fibre 10 &# 39 ; according to a second embodiment of the invention . the fibre 10 &# 39 ; has a core 20 &# 39 ; and a cladding 30 &# 39 ; of the same size as those of the fibre 10 of fig1 a . again , fig1 e illustrates that the core 20 &# 39 ; has a raised refractive index with respect to that of the cladding 30 &# 39 ;. referring to fig1 f , an annular region 40 &# 39 ; is made photosensitive by appropriate doping ( e . g . with b or ge ). the region 40 &# 39 ; is within the core 20 &# 39 ;, at the radial periphery of the core 20 &# 39 ;. the dopants are selected and balanced so that the refractive index of the region 40 &# 39 ; is substantially the same as that of the core 20 &# 39 ;, so that in the absence of an impressed refractive index modulation , the fibre behaves optically as though the core glass composition was uniform across the core . fig1 g to 1i show corresponding features of a fibre 10 &# 34 ; according to a third embodiment of the invention , having a core 20 &# 34 ; and a cladding 30 &# 34 ;. in the fibre 10 &# 34 ;, an annular photosensitive region 40 &# 34 ; overlaps part of the core and part of the cladding . again , the doping is such that the refractive index of the region 40 &# 34 ; is matched ( for an inner part of the region 40 &# 34 ;) to that of the core 20 &# 34 ;, and ( for an outer part of the region 40 &# 34 ;) to that of the cladding 30 &# 34 ;. in these embodiments , two or more photosensitizing dopants are used in the annular region . the dopants are selected so that they affect the refractive index of the doped glass in opposite senses ( directions ). in this way , the glass of the region can be made photosensitive but without necessarily changing its refractive index with respect to adjacent non - doped glass . so , the intention is that the refractive index of glass of the annular region which lies within the core should be substantially equal to the refractive index of the remainder of the core ; and that the refractive index of glass of the annular region which lies within the cladding should be substantially equal to the refractive index of the remainder of the cladding . 1 ) photosensitizing dopants which depress the refractive index of silica glass : b 2 o 3 : refractive index change is - 4 . 65 × 10 - 4 per mol % of b 2 o 3 f : refractive index change is - 4 . 5 × 10 - 3 per mol % of f 2 ) photosensitizing dopants which raise the refractive index of silica glass : geo 2 : refractive index change is + 1 . 32 × 10 - 3 per mol % of geo 2 sno 2 : refractive index change is + 2 . 06 × 10 - 2 per mol % of sno 2 so , an example of a suitable formulation of dopants to achieve substantially zero change in refractive index ( i . e . so the annular region &# 39 ; s index matches that of the respective adjacent glass ) would be 20 mol % b 2 o 3 and 7 mol % geo 2 . fig2 a illustrates grating formation in a fibre 10 by transverse exposure of the fibre to a fringe pattern from a 248 nm krf ( krypton fluoride ) excimer laser . the grating length is 15 mm ( millimeters ), writing time is approximately 16 minutes at a pulse energy of approximately 0 . 3 j / cm 2 and a repetition rate of 40 hz ( hertz ). fig2 a shows the normalised total reflection of the grating during its formation against time . the normalised total reflection reaches a peak of about 90 % once the writing laser has been turned off . as shown in fig2 b , the resulting grating has a peak transmission loss larger than 35 db ( decibels ). fig3 is a graph schematically illustrating the measured refractive index across a diameter of the fibre 10 before exposure , and the refractive index changes when a grating is impressed on the photosensitive region of the fibre . the curve illustrating the refractive index before exposure shows the structure of the core , where the refractive index is ( in this example ) raised with respect to that of the cladding . the radius of the core is seen to be about 3 . 5 μm . the curves illustrating the refractive index change are measured across the diameter of the fibre 10 along two orthogonal axes . one of these axes (&# 34 ; 0 °&# 34 ;) is substantially parallel to the light of the grating writing beam , and the other (&# 34 ; 90 °&# 34 ;) is substantially perpendicular to the writing beam . it can be seen that the greatest refractive index change occurs at the photosensitive annular region , and that between the two curves the greater change occurs along the writing beam . fig4 a and 4b schematically illustrate a dbr laser formed using the fibre 10 , and fig4 c schematically illustrates the performance of such a dbr laser . referring to fig4 a and 4b , the output port of a wdm ( wavelength division multiplexing ) coupler 100 is spliced to the dbr laser 110 which consists of two gratings 120 , 130 spaced 20 mm apart . the grating 120 nearer to the splice has a reflectivity of 90 % and a length of 5 mm . this is referred to as an output coupler grating . the second grating 130 has a reflectivity larger than 99 % and a length of 15 mm , and is referred to in fig4 b as the high reflector grating . the gratings are written by a 248 nm krf excimer laser with an interferometer arrangement as described above . pump light at 980 nm is coupled to the laser through one input port 112 of the wdm coupler 110 and the laser output is coupled out through the other port 114 of the wdm coupler 110 . the lasing wavelength of the laser is at 1535 nm . referring to fig4 c , the lasing threshold is approximately 4 mw and the slope efficiency is approximately 25 % after taking into consideration of the 1 db loss of the isolator and approximately 0 . 5 db of splice loss . the laser operates in a single frequency and single polarisation mode for output powers up to 8 mw , beyond which the second polarisation mode starts lasing as well . in another prototype embodiment of a dbr laser with lower output coupler coupling ( higher reflectivity of the first grating ), the threshold is found to be sub - mw , attesting to the low intrinsic loss of these laser cavity . fig5 a schematically illustrates a dfb laser formed in the fibre 10 . the configuration of the laser is similar to that of the dbr laser , apart from the fact that a single fibre grating 200 having a phase discontinuity at a point along its length is used for the laser cavity . the grating length is 50 mm , and its reflectivity centered at 1550 . 2 nm . it is written by transverse exposure to fringes from a frequency doubled ar ( argon ) ion laser with a phasemask arrangement . despite the higher threshold ( 14 mw ) for this laser ( shown in fig5 b ) the efficiency with respect to launched pump power is also approximately 25 %, after taking into consideration again the 1 db isolator insertion loss and approximately 0 . 5 db splice loss . the dfb laser operates in a single frequency and single polarisation mode up to 6 mw of output power . the single polarisation mode operation of the lasers is in contrast to the dual polarisation mode operation observed in previous lasers fabricated with the photosensitive region in the core . single polarised lasers are desired for most applications , i . e . as source for use in optical fibre transmission systems . the efficiency of the lasers are primarily limited by the efficiency of the fibre itself , which was measured to be approximately 25 % in a simple cavity configuration where the cleaved end face was used as the reflectors . although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings , it is to be understood that the invention is not limited to those precise embodiments , and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims . 1 . g a ball and w w morey : &# 34 ; 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