GB2235986A - Fibre type wavelength conversion element - Google Patents

Abstract

In a fibre type wavelength conversion element, which has fibre core made of non-linear optical crystal and in which wavelength of incident light is converted by non-linear optical effect of the second order on said core, a cladding material is used, which has the refractive index to satisfy the condition of: 0.95 < n clad<2 omega >/ncore < omega >< 1.005 wherein the refractive index of the core to incident light in ncore omega , and the refractive index of the clad to the secondary higher harmonic light is nclad2 omega . <IMAGE>

Description

FIBER TYPE WAVELENGTH CONVERSION ELEMENT The present invention relates to a fiber type wavelength conversion element, and in particular to a fiber type wavelength conversion element using Cherenkov radiation type phase matching.

A wavelength conversion element is already known, in which an optical waveguide is provided using non-linear medium, a light wave is guided into extremely small regions, and the second higher harmonics of light are efficiently generated. This wavelength conversion element is roughly divided into the following two types according to the method to satisfy the phase matching: One is a type, in which the phase velocity of non-linear polarization wave excited from incident light is equalized with that of the second higher harmonics and phase matching is performed between the fundamental wave, i.e. waveguide mode of incident light, and waveguide mode of the second higher harmonics. The other is a type, in which so-called Cherenkov radiation type phase matching is performed between waveguide mode of fundamental wave and radiation mode of the second higher harmonics.

An object of the present invention is to provide a fiber type wavelength conversion element with high efficiency, using Cherenkov radiation type phase matching.

The fiber type wavelength conversion element according to the present invention comprises a core of fiber made of non-linear optical crystal and it is a fiber type wavelength conversion element to convert the wavelength of incident light by the secondary non-linear optical effect in the core. When it is supposed that the refractive index of the core to incident light is ncore and the refractive index of the clad to the secondary higher harmonic light is unclad20 . It is made of the clad material having the refractive index to satisfy the following condition: 0.95 < Nclad2#/Ncore# < 1.005 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically shows the concept of the +(cladding) generation of SH wave in fiber type wavelength conversion element.Fig. 2 is a drawing to show the emission of SH wave from the fiber type wavelength conversion element.

In the following, description is given in detail on the embodiment of the invention in connection with the drawings.

First, description is given on the generation of the secondary higher harmonics (SH wave) in a fiber type wavelength conversion element using Cherenkov radiation type phase matching.

In Fig. 1, when fundamental wave mode is propagated in the core with effective refractive index Ni , nonlinear polarization wave to generate SH wave is also propagated with the same phase velocity C/NX (C: Light velocity). It is supposed that SH wave is generated in the direction where the non-linear polarization wave takes an angle of e with the direction of waveguide at the point A in the figure, and that, after unit time, SH wave is again generated in the direction with an angle of e at point B.If SH wave generated at the point A reaches the point C after unit time propagating in the clad and if o is the angle, at which AC intersects BC perpendicularly, the wave surface of SH wave with nonlinear polarization wave in generated between A and B.As the result, coherent SH wave is generated.

If refractive index of the clad to SB wavelength is nclad2w , the condition for phase matching is: N # = n c l a d2# cos # ... ... ( 1 ).

from the figure. Namely, N # < n c l a d 2# ... ... ( 2 ) SH wave is automatically generated in the direction of o where phase matching is performed. In general, if it is supposed that the refractive indices of the clad and the core to fundamental wave are nclado and ncore respectively and that overlayer is the air, the condition for the propagation of fundamental wave is propagated as mode in the core is: n c l a d# < N# < n c o r e# ... ( 3 ) In the wavelength dispersion of the refractive index of the clad, ncladi < nclad2# .Therefore, if the condition n c l a d# < n c o r e# < n c l a d2# ... ( 4 ) is satisfied, the equation (2) is satisfied to all fundamental modes with the core having any diameter. Even when n c l a d# < n c l a d2# < n c o r e# there exists fundamental wave, which satisfies the equation (2) with film thickness within a certain range.

In this way, in the fiber type wavelength conversion element, which is crystallized on fiber core using nonlinear optical material, when the light from laser light source to emit coherent light such as semiconductor laser or YAG laser is guided to fiber in LP01 mode, and SH wave is generated by non-linear polarization excited by such light, SH wave is propagated as the clad mode to repeat total reflection within the boundary between the clad and the air and it is emitted in conical shape in the direction, making an angle of o with end surface of the fiber.

In the meantime, it is necessary to perform analysis by introducing approximate equation in order to identify the aspect of the generation of SH wave by Cherenkov type radiation phase matching. Accordingly, the approximate equation to describe the generation of SH wave and its solution are given, and discussion is made on the output of SH wave.

If it is supposed that electric field of incident light wave is tE:, dielectric constant under vacuum condition is s o, and magnetic permeability under vacuum condition is 0, SH wave #2# 0 can be approximated as 2w 0 can be calculated from the following equation.

This is the solution of the following inhomogeneous Helmholtz equation.

#2 E 2#o + o (2#)2 #o E2#o = f where f = -4# 2 0 # 0 d(2) ## ## , and d(2) is a constant to express non-linear effect of the second order. As the result, JE2(i) 0 can be interpreted as the solution to describe the physical system when it is supposed that dielectric constant in waveguide or core is equal to that of substrate or clad and is at constant level for the entire space, and that non-linear polarization excited from the primary light is light source.

Utilizing the fact that # is sufficiently far compared with S ', the following equation is obtained.

3-dimensional vector # is expressed by 2dimensional vectors # and z. Namely, it is supposed that r = (, z). If the electric field where light is guided toward the direction of z axis is expressed by: Ew (r) =ew (p) e - @ss@ non-linear polarization within the range of -L/2 # z # L/2 where non-linear medium is present is f = - 4 #2 0 #0 d(2) e# e# (p) e -@2ss@ Although electric field of SH wave can be expressd by the following equation, the range of integration relating to z is -L/2 ' z # L/2, and it is the entire space if # is concerned.

Here, it is supposed that wavelength of fundamental wave under vaccum condition is 1 , and that k is as follows: k = K / 2 = 2 # n c l a d2# / # # = (|#|sin # cos # #, |#|sin # sin #, |#|cos #) When integration is performed on z,

Thus, it is evident that the electric field (E2ct) is strongly radiated toward the direction, which is determined by cost o = B / k . . This expresses well the features of Cherenkov radiation and the approximate equation and its solution describe this well.

Next, the output of SH wave is discussed.

When it is supposed that the power of SH wave is #2# , then,

To obtain the power, 1E2a) . #2# * is calculated.

When # is expressd by polar coordinates as: # = (|r| sin # cos #, |r| sin # sin #, |r| cos #) and it is supposed that # (p, z) Ir (pt, z') When integrated on z,

X # exp [ik|p-p' |sin # cos (# - #o)] {d(2) e# e# (p)} .

{d(2) e# e# (p')} d p d p ' To simplify the equation, the following substitution is performed.

d e f f 2 g (p) # g (p') = {d(2) e# e# (p) } .

{d (2) e# e# (p')} From these and from the equaton (a) below,

# exp [ik | p - p' |sin # cos (# - #o )] g (p) # g (p') d p d p ' d # sin # d # Integrating on @ , the addition theorem of Bessel function is used. At the same time, # and ' are expressed by polar ordinates as: p = (r cos #, r sin #) @ p ' = (r cos #' , r sin #' ) Then,

X g (r, #) # g (@',#') @@' drdr' d#d#' 5@ # d# ......(a) s m is Nemuann's factor, and it is 1 when m = 0 and it is 2 otherwise.

In case of axially symmetric system where nonlinear crystal is used as fiber core and wavelength conversion is performed, 9 (r, # ) does not depend on # and only the term m = 0 remains in the equation (a).

Therefore,

where # represents angular frequency of incident light, c is light velocity under vacuum condition, ss is propagation constant of guided light, L is the length of wavelength conversion element contributing to wavelength conversion, Jo is class 1 Bessel function of 0 order.

Also, a is core radius of the fiber, and deff is nonlinear constant effectively contributing to wavelength conversion. 9(t)can be easily calculated as LP01 mode is guided: K, (W) 2 a2 Ur g(r) = - #2 o2 C2 - # J o ( J 1 (U) 2 U2 a where U= (#2 o #1 - ss2) 1/2 a W= (ss2 - #2 o # 2) 1/2 a 0 is magnetic permeability under vaccum condition, J1 is a class 1 Bessel function of the first order, and K1 is class 1 modified Bessel function. # .1 and 2 are dielectric constants to incident light (# ) of core and clad respectively:: #1 = #o (n c o r e# ) 2 @ #2 = #o (n c l a d# ) 2 C is a constant, which can be obtained from total energy of light guided by the fiber. The power P# of the primary light is divided into the power Pcore# of the light in the core.

P# = P c o r e# + P c l a d# # # o P c o r e# = - (#2 o #1 + ss2 ) C2 .

2P K1 (W) 2 a4 -(Jo (U)2 + J1 (U)2 ) J1 (U) 2 U2 # # o P c l a d# = - (#2 o #2 + ss2 ) C2 2P a (K1 (W) 2 -Ko (W) 2 w2 C can be obtained from the above equations using the power p# of the guided light. In case non-linear material of the core is determined in the fiber type wavelength conversion element, the core radius a of the fiber and refractive index of clad material are changed as parameters, and fiber type wavelength conversion element with high efficiency can be obtained by estimating the output of wavelength conversion element from the above equation through numerical calculation.

In this case, the material having the refractive index satisying the condition of 0.98 < n c l a d @@ / n c o r e @ @@@@@ is selected as clad material.

For example, in case DAN (4-(N, N-dimethylamino)-3acetoamidenitrobenzene) (ncore# = 1.738) is used as nonlinear material of the core, and LAF03 glass (nclado = 1.7176, nclad = 1.7398) is used as the clad material, n c l a d# / n c o r e# = 0.988 n c l a d2# / n c o r e# = 1.001 and the above conditions could be satisfied. The conversion efficiency [%] in this case is 0.23 x 10-2 in case core radius [t m] is 0.5, 0.24 x 10-1 in case it is 0.7, 0.16 in case it is 0.9, 0.17 in case it is 1.1, 0.15 in case it is 1.3, and 0.13 in case it is 1.5.

In case DAN is used as non-linear material for the core, and SF1 glass (nclad# = 1.6925, nclad2# = 1.7254) is used as the clad material, n c l a d# / n c o r e# = 0.973 n c l a d2# / n c o r e# = 0.993 and the above conditions could be satisfied. The conversion efficiency [%] in this case is 0.16 in case core radius [ m] is 0.5, 0.34 in case it is 0.7, and 0.83 in case it is 0.9.

Also, in case MNA(2-methyl-4-nitroaniline) (ncore# = 1.785) is used as non-linear material of the core, and SF14 glass (nclad# = 1.7331, nclad2# = 1.7713) is used as the clad material, n c l a d# / n c o r e# = 0.971 n c l a d2# / n c o r e# = 0.9923 and the above conditions could be satisfied. The conversion efficiency (%) in this case is 7.67 in case core radius [y m) is 0.5, 18.9 in case it is 0.7, and 30.4 in case it is 0.9, showing high efficiency.In any case, these are the results when the power of incident light P# = 40[mW) and the wavelength of incident light A = 1064 [nm].

Further, the material having the refractive index satisfying the condition of 0.95 < nclad 20 / ncoreX < 1.005 is selected as the clad material.

For example, in the case where DMNP (3,5-dimethyl-l (4-nitrophenyl) pyrazole) (ncore# = 1.786) is used as non-linear material of the core, and in the case where the light from semiconductor laser of 870 nm is used as fundamental wave and wavelength is converted, the conversion efficiency, the refractive index ratio r of B [%] (= nclad# / ncoreo ), and the dependency on core radius of the glass of Table 1 below were examined. This was the case where the power of the guided primary light is 40 nW and fiber length is 1 mm, and the value of # x 100 was entered in Table 1.

Table 1

Glass Ratio Core radius [ m] material (r) 0.30 0.35 0.40 0.45 0.50 0.60 0.70 0.80 1.00 1.20 1.40 1.60 of clad.

1 LaSF01 1.012 0.00 0.00 0.00 0.03 0.10 0.16 0.00 0.32 0.81 0.21 0.04 2 SF11 1.022 0.01 0.66 0.40 0.91 1.11 0.40 0.00 0.45 0.85 - - 3 SF14 1.007 0.82 2.69 4.31 4.59 3.61 0.82 0.00 0.20 - - - 4 SF4 1.003 1.46 3.77 5.38 5.49 4.26 0.93 0.04 - - - - 5 LaF05 1.000 0.24 1.20 2.57 3.47 3.37 0.89 0.00 0.20 - - - Conversion 6 SF13 0.994 3.51 6.28 7.71 7.53 5.75 1.11 - - - - - efficiency 7 SF3 0.993 3.54 6.30 7.76 7.65 5.58 1.11 - - - - - 8 SF10 0.986 5.24 7.99 9.47 8.89 7.50 11.2 - - - - - 9 LaF03 0.981 2.34 4.54 6.40 8.14 9.23 8.82 - - - - - 10 SF1 0.979 6.43 9.08 10.9 12.2 10.0 0.77 - - - - - 11 LaF08 0.975 4.51 6.92 9.70 12.4 16.0 - - - - - 12 LaF02 0.974 4.36 6.72 8.86 12.4 17.9 - - - - - - 13 LaF07 0.974 5.10 7.35 9.69 15.8 ,- - - - - - - 14 SF15 0.968 8.03 10.5 13.8 23.5 10.0 - - - - - - 15 LaF01 0.962 5.50 6.93 4.61 - - - - - - - - 16 SF8 0.961 8.39 10.6 14.6 - - - - - - - - 17 SF5 0.951 8.32 8.11 - - - - - - - - - - [x10-2%] 18 SF2 0.936 6.12 - - - - - - - - - Although not ginven in Table 1, when optical glass SF15 (nc1ad2 = 1.729) is used as clad under the same conditions as above, the conversion efficiency was 0.351 % with the core radius of 0.475 it m.

Also, in case optical glass LAF07 (nclad2 = 1.734) is used as clad, the conversion efficiency was 0.117% in case core radius was 0.425 it m, and it was 0.298% in case core radius was 0.475 it m.

Further, in case DAN was used as core material and the light from YAG laser with wavelength of 1064 nm was converted, the glass material as given in Table 2 below was examined. This was the case where the power of the guided primary light was 40 mW and fiber length was 1 mm.

The value of # X 100 when conversion efficiency was n [%] was entered in Table 2.

Table 2

Glass Ratio Core radius [ m] material (r) 0.30 0.35 0.40 0.45 0.50 0.60 0.70 0.80 1.00 1.20 1.40 1.60 of clad 19 SF14 1.019 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.04 0.12 20 SF4 1.015 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.11 0.33 0.65 1.16 1.27 21 SF3 1.006 0.00 0.00 .002 0.03 0.16 1.21 3.60 6.87 10.2 12.8 12.9 8.2 22 SF13 1.004 0.00 0.00 .002 0.02 0.14 1.13 3.42 6.60 9.88 12.3 12.4 7.83 23 LaF05 1.001. 0.00 0.00 0.00 .004 0.04 0.53 2.07 4.55 7.33 9.68 10.7 7.55 24 SF13 0.999 0.00 .009 0.11 0.52 1.40 4.31 7.85 11.5 15.4 19.8 - - Conversion 25 LaF08 0.993 0.00 0.01 0.15 0.63 1.53 4.07 6.71 9.14 11.4 13.9 - - efficiency 26 SF1 0.992 0.00 0.09 0.53 1.44 2.68 5.28 7.53 9.50 11.4 13.4 - 27 SF15 0.982 0.10 0.60 0.51 2.48 3.31 4.29 4.42 2.96 - - - 28 LaF01 0.981 0.03 0.30 0.95 1.78 2.54 3.36 2.87 - - - - 29 SF8 0.976 0.24 0.92 1.79 2.53 3.00 3.05 - - - 30 SF5 0.966 0.52 1.27 1.87 2.13 1.89 - - - - - 31 SF2 0.951 0.80 1.29 1.09 - - - - - - 32 SF7 0.947 0.91 1.16 - - - - - - 33 F5 0.925 0.54 0.52 - - - - - - - [x10-2%] 34 F8 0.921 0.55 - - - - - - As it is evident from Table 1 and Table 2, it is preferable that the refractive index rario Ir is within the range of 0.95 < nclad2# / ncore# < 1.005. Namely, in case the glass material having the value of nclad2# / nCorez exceeding 1.005 is selected, the conversion efficiency of higher harmonics is decreased. In case the value of nclad2# / nCorei is 0.95 or less, the range of the core radius is narrowed down where the guided primary light available as wavelength conversion element is in single mode, and the core radius is more strictly controlled in the practical producition. Or, the coupling efficiency of primary light to wavelength conversion element is decreased, and conversion efficiency is decreased as the result. Or, the coupling to wavelength conversion element of primary light becomes extremely sensitive to temperature change and external vibration, and the device lacks the reliability.

As described above, the fiber type wavelength conversion element according to this invention comprises the material having the refractive index satisfying the condition of: 0.95 z 95 < n Cl id2CO7 n Care" < 1.005 as clad material, and this provides high efficiency of the wavelength conversion.

Claims (11)

1. A fiber type wavelength conversion element, comprising fiber core of non-linear optical crystal, wavelength of incident light being converted by nonlinear optical effect of the second order in said core, characterized in that: a clad material having refractive index satisfies the condition of: 0.95 < n c l a d2# / n c o r e# < 1.005 when it is supposed that the refractive index of the core to incident light is nCore , and that the refractive index of the clad to the secondary higher harmonic light is nclad2# .

2. A fiber type wavelength conversion element according to Claim 1, wherein said non-linear optical crystal is (3.5-dimethyl-l-(4-nitrophenyl) pyrazole).

3. A fiber type wavelength conversion element according to Claim 2, wherein said clad material is optical glass having nclad2 of 1.729.

4. A fiber type wavelength conversion element according to Claim 2, wherein said clad material is optical glass having nclad2# of 1.749.

5. A fiber type wavelength conversion element according to Claim 2, wherein said clad material is optical glass having nclad2o of 1.734.

6. A fiber type wavelength conversion element according to Claim 1, wherein said non-linear optiacal crystal is DAN (4-(N,N-dimethylamino)-3-acetoamidenitrobenzen).

7. A fiber type wavelength conversion element according to Claim 6, wherein said clad material is optical glass having nclad2o of 1.7398.

8. A fiber type wavelength conversion element according to Claim 6, wherein said clad material is optical glass having nclad2o of 1.7254.

9. A fiber type wavelength conversion element according to Claim 1, wherein said non-linear optical crystal is MNA (2-methyl-4-nitroaniline).

10. A fiber type wavelength conversion element according to Claim 9, wherein said clad material is optical glass having nclad2 of 1.7713.

11. A fiber type wavelength conversion element as claimed in claim 1 substantially as herein described.