JPH03127032A - Functional optical waveguide medium - Google Patents

Abstract

PURPOSE:To improve exciting efficiency by specifying the specific refractive index differences to the pure quartz of the 1st core positioned in the central part of cores and the 2nd core formed on the outer periphery thereof. CONSTITUTION:This medium is made of the double structures consisting of the 1st core 1 positioned in the central part of the cores disposed concentrically with each other and the 2nd core 2 formed on the outer periphery thereof. The specific refractive index difference DELTAn1 to the pure quartz of the 1st core is in a 0<=DELTAn1<=0.5% range and the specific refractive index difference DELTAn2 to the pure quartz of the 2nd core 2 is in a 0.7<=DELTAn2<=3.5% range. A rare earth element or transition metal is added only to the 1st core part 1. The scattering losses are, therefore, decreased and the hardening of the laser active material is enhanced. The laser activity effect is executed only in the low-loss central part of the cores where the light energy density is highest. The mechanical waveguide medium having the high efficiency is obtd. in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a functional optical waveguide medium that obtains high excitation efficiency.

(Conventional technology and problems) In recent years, Nd (neodymium), Er (erbium),
Pr (praseodymium), Yb (itteribium)
An optical fiber doped with rare earth elements such as (hereinafter referred to as rare earth element doped optical fiber) is used as a laser active material,
It has been reported that single mode optical fiber lasers or optical amplifiers have many possibilities of use in the fields of optical sensors and optical communications, and their applications are expected.

As an optical fiber laser amplifier using a rare earth element-doped optical fiber, an example in which optical amplification was confirmed at a wavelength of 1.54 μm using an Er-doped silica-based optical fiber as a laser active material and a semiconductor laser as an excitation light source is as follows. R,
J, Mears et al. (J, Mears et al., Elect
Ron, Lett, 23. PP1028-1029
．． (1987).

In order to effectively utilize optical fibers doped with such laser-active substances, it is necessary to have a waveguide structure that allows excitation light to enter optically efficiently and efficiently utilizes these excitation lights. .

As a method to increase the incidence efficiency of an optical fiber, the NA (numerical aperture: NA= (nco”-nd
2) 1/2, neo: fiber core refractive index, nd:
A method of increasing the refractive index of the cladding is known.

That is, increasing the refractive index of nco and ncl effectively increases the incidence efficiency. For this reason, for example, a method is used in which a high concentration of GeO2 is added to a quartz glass core to increase the refractive index.

However, there was a problem that the scattering loss of the glass increased significantly when the core was doped at a high concentration.
There was a problem that it was difficult to obtain an efficient functional waveguide medium.

Figure 2 shows the scattering loss (wavelength 1 μm) of a single mode optical fiber doped with GeO2 in the core and the doping concentration of GeO2 (
As shown in Figure 0, which is an investigation of the relationship between the relative refractive index difference and the relative refractive index difference, a sharp increase in scattering loss appears when the GeO2 doping concentration is 5 mo1% or more. Further, when Al2O3 is used as an additive, there is a problem in that glass crystals are generally formed when 5 mo1% or more is added.

On the other hand, as a method of efficiently using the light excited (pump light) in a single mode optical fiber, as shown in Fig. 3, a laser active material (Er in the example of Fig. 3) is placed only in the center of the core. The method of adding erbium is B, J, A1n
5lie et al. (1988 European International Conference on Optical Communications, Paper Proceedings pp, 62-65, 198g)
, this is the Er
The aim is to increase the excitation efficiency by exciting the .

However, in the configuration shown in Fig. 3, although the excitation efficiency is increased, there is a problem that the loss increases because the portion with high optical energy and the high refractive index region of the core (that is, the high scattering loss region) are optically overlapped. Ta.

Furthermore, in the above example, rare earth elements are used at high concentrations (several hundreds of ppm).
Since the rare earth elements are added in the above amount, there is a drawback that scattering loss is large due to the effect of adding a high concentration of rare earth elements. That is, FIG. 4 shows the concentration of Er added by the inventors and the
This study investigated the relationship between excessive loss (scattering loss) of optical fibers in The elemental addition/concentration dependence is different from other dopants for adjusting the refractive index (e.g. 0e02, A
When co-adding 12O3, etc.), the effect tends to become even more pronounced.

(Object of the Invention) The present invention has been made in view of the above-mentioned problems, and provides functionality for reducing scattering loss, increasing the efficiency of a laser active material, and achieving low loss and high optical energy density while maintaining a high NA. The purpose is to provide an optical waveguide medium.

(Means for Solving the Problems) In order to solve these problems, the present invention includes adding rare earth elements or transition metals having laser transition to the core portion, and forming them on the outer periphery of the core so that is a quartz-based functional optical waveguide medium having a cladding with a low refractive index, the cores being arranged concentrically with each other, a first core located at the center of the core, and a first core formed on the outer periphery of the core. The relative refractive index difference Δn1 of the first core with respect to pure quartz is in the range of 0≦Δn1≦0.5%, and the relative refractive index difference Δn1 with respect to the pure quartz of the second core is in the range of The relative refractive index difference Δn2 is in the range of 0.7≦Δn2≦3.5%, and the rare earth element or transition metal is added only to the first core portion.

That is, the refractive index distribution of the core is changed by adding a first core layer with a relatively low refractive index doped with a laser active substance and a larger N layer.
A second core layer having a relatively high refractive index for obtaining A. In addition, by suppressing the concentration of the laser active substance added to the first core layer, scattering loss is small, clusters do not occur, and dopants for adjusting the refractive index (Ge02, Al2O3, P2
The concentration of O5, etc.) is also kept low, resulting in a glass layer with low optical loss.

As a result, the optical fiber with this structure has an extremely high numerical aperture (NA) due to the effect of the high refractive index layer of the second core layer, while the center part of the core (first core layer) has a high optical energy density. Since the total amount of dopants is kept low, the scattering loss is small and the loss is low overall, and since the laser active material is added only to this low loss region,
It is characterized by the high excitation efficiency of the laser active substance and the high density of light energy acting on the laser active substance, making it possible to provide a highly efficient functional optical waveguide medium (fiber).

A functional optical waveguide medium having such a configuration is suitable for use as, for example, a high-amplification optical fiber amplifier or an optical fiber laser.

FIG. 1(a) is a conceptual diagram of the structure of the functional optical fiber of the present invention. Here, Δn1, Δn2, and Δn3 are the respective relative refractive indices of the first core, second core, and cladding with respect to pure quartz. Further, no is the refractive index of pure quartz.

Here, Δn is the relative refractive index difference of the optical fiber, and Δn=
Δn2+Δn3 (1).

Further, rl and r2 are the radii of the first core layer and the second core layer, respectively, and r1/r2 is 0.
Approximately 2 to 0.4 is desirable.

FIG. 1(b) shows a slightly modified version of the refractive index distribution in FIG. 1(a), in which the shape of the refractive index distribution is changed from a step-like shape to a slightly smooth distribution. The distribution as shown in Figure 1(b) is
Although they can be produced by thermal diffusion of dopants during the fiber manufacturing process or by utilizing dopant evaporation during the vitrification process, they can also be formed using precise distribution control techniques.

In other words, it should be noted that the distribution shown in FIG. 1(b) basically has the same function as the optical fiber of the present invention.An important point in the present invention is that the distribution of light energy density, which is extremely important as a laser active region, has the same function as the optical fiber of the present invention. A low-loss, high-efficiency first core layer with a low dopant content is provided in the center of the high core, which significantly reduces the loss of the entire fiber and increases the efficiency of the laser active region (laser active substance doped region). At the same time, by providing the second core layer, the effective numerical aperture (NA) of the fiber is increased, and as a result, the energy density of the first core layer is significantly improved, thereby making the functional optical fiber The key point is that the excitation efficiency has been significantly improved.

As mentioned above, the relative refractive index Δn1 of the first core of the functional optical waveguide medium according to the present invention with respect to pure quartz is 0≦Δn1≦0.
．． It is 5%. As is clear from FIG. 2, in order to apply a low-loss glass material to the first core layer with high optical energy density, the range Δn1≦0 where there is almost no increase in scattering loss is required.
This is because 5% is essential.

Further, the relative refractive index Δn2 of the second core with respect to pure quartz is 0
．． It is necessary that 7≦Δn2≦3.5%.0 The present inventors have found that extremely high efficiency amplification is possible in a relatively low concentration Er-doped fiber. In such optical amplifiers, the length of the fiber used is at least several tens of meters, or even several hundred meters, so when transmitting high-speed optical signals (e.g. several Gd/s or more), Er-doped fibers are used. You can no longer ignore your own dispersion.
Wavelength range from 1.53 using Er-doped fiber amplifier
In order to achieve a wavelength of zero dispersion in the 1.57 μm band or an extremely low dispersion value (less than 0.1 ps/person-km), the relative refractive index difference of a single mode optical fiber must be at least 0.
．． 7% or more is required.

As the host material of the first core as described above, for example, pure silica glass (Si02), Al2O3P2O5
-SiO2-based quartz glass or the like can be used.

The rare earth elements and transition metals added to the first core are:
In addition to the above Er, other rare earth elements such as Nd, Tm
, Ho, Yb, etc., and one or more transition metal elements such as Ti, Ni, and Cr.

The concentration N of the substance added to the first core is preferably 1 to 150 ppm. When it exceeds 150 pp'm, not only the scattering loss of the glass increases, but also
There is a risk that the amplification efficiency will deteriorate significantly, while if it is less than lppm, the fiber length required to obtain the maximum amplification is
This is because there is a risk of problems in the equipment configuration, such as the need for a small fiber amplifier.

Further, as the host material for the cladding, for example, fluorine-doped silica glass (F-3i02) can be effectively used.

Hereinafter, a detailed explanation will be given using specific examples.

(Embodiment 1) FIG. 5 is a structural diagram of an optical fiber according to a first embodiment of the present invention. 1 is the first core layer, and the glass material is 5i0
2-Er, the amount of Er added is 90 ppm.

The refractive index of the interspace is almost equal to the refractive index of pure quartz (Si02) (Δn x 70) -2 is the second core layer, the glass material is GeO2-3i02, and the amount of dots of GeO2 is 16. 5mo1%, Δn2=1.6%. 3
is the cladding layer, and the glass material is 5i02F, Δ
n3 was 0.6% (-0.6% relative to pure quartz).

The fiber was manufactured using the following procedure.

First, a porous base material with a diameter of 56 mm was produced by the VAD method, and this was vitrified in an Er volatile atmosphere to obtain an Er-doped quartz glass base material with a diameter of 20 mm for the first core layer. After near-stretching with flame, the surface is etched and GeO2 is formed as a second core layer by VAD method.
A 2-SiO2 porous body was attached externally and vitrification treatment was performed. After flame stretching the obtained glass rod, a 5i02 porous body was further formed on the outer periphery of the rod, and this was vitrified in a fluorine atmosphere to obtain an optical fiber base material.

Here, the process of externally attaching the fluorine-doped clad glass is done in a total of four steps, and the outside is further covered with a quartz glass tube.
An optical fiber base material having outer diameter/core diameter=D/r2=56 was obtained, and r1/r2 was 0.28.

The obtained base material was drawn using an optical fiber drawing device equipped with a high-purity carbon heater to obtain a core diameter of (2r2) 2
．． 23μm, outer diameter 125μm, cutoff wavelength 0.89
A μm single mode optical fiber was obtained. The loss of this optical fiber was 1.2 dB/km at a wavelength of 1.2 μm.

In order to investigate the functionality of the optical fiber, an experimental system as shown in FIG. 6 was installed at the bottom of the tank, and an optical amplification experiment using an optical fiber amplifier was conducted.

In FIG. 6, 5 is the signal light source (signal to be amplified) wavelength λs
= 1.537 μm DFB laser, 6, 6' are wavelengths λp
= 1.485 μm excitation light source, 7.7', 7'.

71 is a laser condensing lens; 8 is a polarization beam splitter for polarizing the two beams of laser 6.6°; 9
10.10" is a dichroic mirror for combining the signal light source 5 and excitation light source 6.6';11.10" is an optical connector;
11″.

11", 11" is a single mode optical fiber cord, 12.
12'' is a polarization-independent optical isolator, 13 is the Er-doped single mode optical fiber prepared above, 14 is a filter,
15 is a spectrum analyzer.

Here, the length of the Er-doped fiber was 250 m.

The experimental system shown in FIG. 6 was operated according to the following procedure. First, the signal light source 5 was turned on, and the light was made to enter the optical fiber 11 via the lenses 7'', 7'' and the dichroic mirror 9. At this time, the output at the connector 10 was 54 dBm. Next, the excitation light sources 6 and 6' were turned on, and the polarization beam splitter 8, the optical microscope 5-9, the lens 7.7',
The output of the excitation light at the connector 10' was 18 mW.The above combined light was input to the Er-doped optical fiber 13 via the connector 10''. When the incident amplification characteristics were measured using the spectrum analyzer 15, a net amplification degree of 43 dB was obtained.

(Comparative Example 1) For comparison, the same relative refractive index difference (2, 2
%) was fabricated, and an optical amplification experiment was conducted using an experimental system similar to that shown in FIG. However, the optical fiber fabricated here has Er uniformly distributed within the core of the optical fiber with a step-like distribution.
The glass composition is Ge02-Er-3i02 and contains Er at the same concentration (90 ppm) as in the above example.

The concentration of 0e02 in the core is the same as in the first example <1
6mo1%, the cladding glass is 5i02F type glass, and the relative refractive index difference with respect to pure quartz is -0.62%
, therefore the relative refractive index difference between the core and cladding is 2.22%
Met. Note that pure quartz was used for the jacket layer.

Further, the core diameter of the same fiber was 2.19 μm, and the cutoff wavelength was 0.9 μm. The loss was 5.6 dB/km at a wavelength of 1.2 μm. In addition, the fiber length is 180 μm, which gives the same absorption as in the example at a wavelength of 1.485 μm.
It was set as m.

The measurement results of the amplification characteristics of the above fiber are as follows:
It remained at 32dB at 8mW.

(Comparative Example 2) The glass composition of the core was 0e02-P2O5-Er-3i0
2. The glass composition of the cladding is 5i02-P2O5, Δn
=2.20%, Er doped throughout the core; fi degree 90
ppm, λc = 0.89 μm A stepped single mode optical fiber with a core diameter of 2.2 μm was fabricated and an optical amplification experiment similar to the above was conducted, and the amplification degree of Pp = 18 mW was 2.
It remained at 8dB. Note that the wavelength of this optical fiber is 1.
The loss at 2 μm was 7.8 dB/km, which was larger than that of Comparative Example 1. This is due to the fact that the Q◇2 doping concentration in the core has increased to 20 mo1% and that this highly doped region covers the entire core.

In addition, in the above examples and comparative examples, the wavelength is 1.2μ.
The reason why the loss is compared using m is that absorption of Er has almost no effect on transmission loss near this wavelength, so it is possible to evaluate the loss of the glass including the effect of the waveguide structure.

(Example 2) In the structural diagram of FIG. 1, the glass composition of the first core layer is Al2O3-P2O5Er-3i02, the glass composition of the second core layer is GeO2-P2O5-3i02, and the glass composition of the cladding layer is F. -3i02, the glass composition of the jacket layer is 5io2 (pure quartz), Δn2=2.5%, Δn
The manufacturing method for manufacturing the optical fiber with 1=1.0% and Δn3=0.6% is almost the same as in Example 1. Here,
Δn=Δn2+Δn3=3.1%. Also, rx
/r2=0.32, λC=0.88μm, core diameter=1.
It was 85 μm.

The loss is at wavelength 1. : 3.1 dB/km at 2 tzm.

The amount of Al2O3 added to the first core glass is 2.6 mo
1%, P2O5 was 2.2mol%, and the concentration of Er was 55ppm. Note that when the amount of Al2O3 added was 4.3 mo1% or more (relative refractive index difference of 0.5% or more), some crystals appeared during vitrification, making it impossible to obtain a stable glass.

The above optical fiber was used to construct an optical fiber amplification system shown in FIG. 7, and an optical amplification experiment was conducted. In FIG. 7, 16 is a fiber-stretched WPM (0.98 μm/1.537 μm
m) Optical fiber coupler, 17°17', 17' is 10
Zirconia fiber ferrule with diagonal polishing
8'' is a polarization beam splitter for the 0.98 μm band.

The total length of the Er fiber was 77 m. To operate this, first the signal light 5 (λ
s=1.537 μm> is incident on the Er fiber 13 via the coupler 16. Here, the output level of the signal light at the terminal 10' was -56 dBm.

Next, the excitation light source 6.6' (λp=0.98 μm) is turned on and similarly coupled to the coupler 16. Here, the coupling rate of light with a wavelength of 1.537 μm from terminal 17 to terminal 10' is 97%, and the coupling rate of light with a wavelength of 0.98 μm from terminal 17 to terminal 17' is 97%.
The light coupling rate of czm is 2% (17'+10' is 98%)
The optical loss of the coupler was 0.98. cz
It was 0.2 dB at m, and 0.25 dB at 1.537 μm. Further, the output amount of the connector 10 of the LD having a wavelength of 0.98 μm, which is the excitation light source, was 11 mW. These combined lights were coupled to the Er-doped optical fiber 13 via the connector 10.10'. The amplified signal is sent to the optical spectrum analyzer 1 via the optical fiber 11'.
5 and an optical power meter 18, and the amplification degree was measured.

As a result, the net amplification degree was 46d at a pumping light intensity of 11mW.
B was obtained.

(Comparative Example 3) As a comparative example of the above example, Δn=3.1%, core diameter 1
．． An 82 μm Er-doped optical fiber was produced. Er is added throughout the core. The glass composition of the core is Ge
O2P2O5-3i02, the composition of the cladding glass is F
-3i02.

The obtained optical fiber had a core diameter of 1.85 μm, an Er concentration of 900 ppm, and λc=0.85 μm. Further, the loss (wavelength: 1.2 μm) was 120 dB/km, which was extremely large compared to Example 2.

In addition, the maximum amplification obtained (excitation wavelength 0.98, um,
The excitation light amount (11 mW) remained at 22 dB.

In this way, the reason why the amplification characteristics are inferior to that of Example 2 is that
In this comparative example, high concentrations of GeO2 and E are present in the central region of the core.
As a result of a series of studies by the inventors, it can be concluded that the scattering loss has increased significantly due to r'+, and that clusters have occurred as a result of the addition concentration of Er being too high, and the amplification efficiency has deteriorated. It has been found that in silica glass with an Er concentration of 150 ppm or less, the influence of clusters hardly appears. On the other hand, if the Er concentration is less than 1 ppm, the fiber length required for amplification will be several kilometers, which is inappropriate for practical use.The appropriate value for the Er doping concentration of functional optical fiber is 1≦Er concentration≦150 ppm. Met.

In the above embodiments, the present invention has been described in detail by taking up an optical fiber amplifier as an application example of a functional optical fiber, but it goes without saying that the present invention can be applied to other applications, such as an optical fiber laser.

In addition, other rare earth elements such as Nd, Tm, Ho,
It should be noted that the present invention is also applicable to Yb, etc., and transition metal elements such as Ti, Ni, and Cr.

Note that if the doping amount of the second core layer is 3.5% or more in terms of relative refractive index difference, cracks will occur in the optical fiber base material due to stress, making it difficult to form a fiber, and this will limit the scope of application of the present invention. It turned out to be an inappropriate area.

Furthermore, the advantage of using F-3i02 glass as the cladding glass is that when obtaining an optical fiber with the same NA, the amount of doping in the second core layer can be reduced, which has the effect of reducing loss in the optical fiber. For example, when GeO2 is used as a dopant in the second core layer,
Decrease the amount of GeO2 added by 6 to 7 mo1% (i.e., the relative refractive index difference of F-3i02 to quartz -0.6 to
-0.7%).

(Effects of the Invention) As explained above, according to the functional optical waveguide medium (fiber) of the present invention, the structure of the core portion of a single mode optical fiber is 1
and a second core surrounding the core, and dopants in the first core layer (refractive index adjusting dopants such as GeO2, P2O5, Al2O3, laser active material dopants Er, Nd, Ti, etc.) The concentration of (
By keeping the relative refractive index difference low, scattering loss is reduced and the effectiveness of the laser active material is increased.Furthermore, by increasing the doping amount of the second core layer, which has a relatively low propagation light energy density, the fiber NA can be increased. At the same time, since the second core layer has a structure in which no laser active substance is added, it is possible not only to obtain extremely high optical energy density while maintaining low loss of the optical fiber despite the high NA, but also to increase the optical energy density. Since the laser activation action can be performed only in the center of the low-loss core where the loss is highest, there is an advantage that a highly efficient functional waveguide medium can be provided.

In particular, if the present invention is applied to an optical fiber amplifier, there is an advantage that an optical amplifier with high performance and high multiplication factor can be provided.

[Brief explanation of the drawing]

Figure 1 is a structural diagram of the functional optical waveguide medium of the present invention, and Figure 2 shows the scattering loss and GeO2 doping amount (
Figure 3 is a diagram of the refractive index distribution of a conventional functional optical fiber and its operating principle. Figure 4 is a diagram of the relationship between Er doping concentration and excess loss of an Er-doped optical fiber. Figure 5 shows an example of the structure of the optical fiber of the present invention, Figure 6 shows the experimental system of 1 and 485 μm pumped optical fiber amplifiers to which the optical waveguide medium of the present invention is applied, and Figure 7 shows the structure of the optical fiber 0 to which the optical waveguide medium of the present invention is applied. ,
This is an experimental system for a 98 μm pumping optical fiber amplifier. 1... First core layer 2... Second core layer 3... Cladding layer 4... Jacket a... Core refractive index distribution b... Light energy density in core q. ...Refractive index of SiO2 5...Signal light source 6.6'...Excitation light source T, 7#, 7"ゝ, 7#"...Condensing lens 8...Polarization beam splitter 9... - Dichroic mirrors 10, 10'... Optical connectors 11, 11', 11'', 111'... Single mode optical fiber cords 12. 13, 14, 15, 16, 17. 18, 12'... Polarization-independent optical isolator...Er-doped single mode optical fiber...Filter...Spectrum analyzer...WPM IT, 17"...Optical fiber ferrule...Optical power meter.

Claims (2)

[Claims] (1) A silica-based functional optical waveguide medium in which a rare earth element or transition metal having a laser transition is added to the core part, and a cladding formed on the outer periphery of the core and having a refractive index lower than that of the core. , the cores are arranged concentrically with each other, and a first core located at the center of the cores;
A double structure is formed with a second core formed on the outer periphery, and a relative refractive index difference Δ of the first core with respect to pure quartz is formed.
n1 is in the range of 0≦Δn1≦0.5%, the relative refractive index difference Δn2 of the second core with respect to pure quartz is in the range of 0.7≦Δn2≦3.5%, and the rare earth element or A functional optical waveguide medium, wherein a transition metal is added only to the first core portion. (2) The functional optical waveguide according to claim 1, wherein the concentration N of the rare earth element or transition element added to the first core portion is in the range of 1≦N≦150 ppm. Medium.