JP2608104B2 - Optical fiber laser device - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an optical fiber laser device capable of providing a laser output having a smooth optical spectrum with very little influence of return light to a laser resonator. is there.

(Prior Art) In recent years, an optical fiber doped with a rare earth element such as Nd (neodymium), Er (erbium), Pr (praseodymium), Yb (ytterbium) (hereinafter referred to as a rare earth element-doped optical fiber) is a laser active material. It has been reported that such a single mode optical fiber laser or optical amplifier has many potential uses in the field of optical sensors and optical communication, and its application is expected.

As an optical fiber laser using a rare earth element-doped optical fiber, a silica-based optical fiber doped with Nd is used as a laser active material, and a semiconductor laser or an Ar laser pumped CW is used.
(Continuous wave) Dye laser as excitation light source, wavelength 1.088μ
Example of confirming CW oscillation, Q-switch oscillation, mode-lock oscillation, etc. with m. Also, using an optical fiber doped with Pr or Er as a laser active material, and using an Ar laser as an excitation light source, CWs with wavelengths of 1.06 μm and 1.54 μm, respectively. An example in which oscillation was confirmed is shown by Earl. J. Meers et al. (J.Mears et al., OFC'86.TUL15
Etc.).

FIG. 2 is a diagram schematically showing a configuration of a conventional device for oscillating the above-described optical fiber laser.

That is, this apparatus comprises a pair of reflecting mirrors 2a and 2b at both ends of a predetermined length rare-earth-doped optical fiber 1 which is a laser active material.
And means for injecting excitation light into the optical fiber 1 are provided. 4 is an optical lens, 4a, 4b, 4c
Denotes a condenser lens for constituting an optical system.

That is, the apparatus has a pair of reflecting mirrors 2a and 2b disposed at both ends of an optical fiber 1 of a predetermined length, which is a laser active substance, and has means for injecting excitation light into the optical fiber 1. In the configuration shown in FIG. 2, the semiconductor laser element 3 provided on the side of the reflecting mirror 2a is provided, and the excitation laser is applied from the semiconductor laser element 3 to the core of the optical fiber 1 through a set of optical lenses 4. It is configured to collect and inject light. As the reflecting mirrors 2a and 2b, a pair of dielectric multilayer films is used, and these form an optical resonator.

By the way, in the conventional device as shown in FIG.
A part of the output light of the optical fiber laser is reflected by the lens 4c, and the light returns to the inside of the laser and the resonator, so that the laser operation becomes unstable.

Such a laser instability phenomenon due to return light is well known as a common problem in all laser devices. Conventionally, YIG (Y ₃ F
_e5 O ₁₂₎ or Bi-substituted garnet _{(B ix (G d L u} ) 3-x F e5 O 12)
Optical isolators utilizing the Faraday effect of crystals such as these have been used. However, when using such an isolator, light is absorbed in the crystal,
There is a problem that light loss of B occurs.

On the other hand, in the case of an optical fiber laser, in a state where there is no return light, for example, in the case of an Nd fiber laser, a broad oscillation characteristic having a spectrum half width of 60 to 70 ° is obtained as shown in FIG. It is expected to be used as a light source to replace Diode).

However, as shown in FIG. 2, when, for example, a condensing rod lens 4c is used at the laser output end, when there is return light from the lens, when there is a composite resonator between the emission end of the optical fiber laser and the lens. The output spectrum has a longitudinal mode characteristic as shown in FIG. 4, for example, and there is a problem that it cannot be applied to a light source for a measurement system using interference.

Now, assuming that the gap between the laser emitting end and the lens 4c is d (μm) in the example of FIG. 2, the longitudinal mode interval Δλ (μm) appearing in FIG. 4 is generally given by the following equation.

Here, λ _{0 is} the oscillation wavelength of the laser, and n ₀ is the refractive index of the medium in the gap d.

In the example of FIG. 2, λ ₀ = 1.09 μm, n ₀ = 1 (air), d =
Since it is 0.6 (mm), Δλ = 9.9 × 10 ⁻⁴ μm = 9.9 ° is obtained. This calculated value was confirmed to be in good agreement with the measured value of 9.8Å.

(Problems to be Solved by the Invention) According to the present invention, the influence of return light to a laser resonator is extremely small, a smooth optical spectrum is obtained, and light that can be applied to a light source for a measurement system using interference. An object of the present invention is to provide a fiber laser device.

(Means for Solving the Problems) As a method for preventing the influence of the reflection as described above,
In the present invention, focusing on the expression (1), the gap d between the laser emitting end and the lens is extremely narrowed, and the longitudinal mode interval Δλ accompanying the complex resonator is sufficiently widened, thereby obtaining a laser output spectrum. Remove vertical mode,
The present invention has devised a technique for efficiently extracting a laser beam to the outside.

That is, the oscillation spectrum Δλ of the optical fiber laser
₀ = approximately 65 ° compared to condition of Δλ that is at least two digits wider Δλ≫Δ
As λ ₀ , Δλ> Δλ ₀ × 10 ² = 6500 (Å) (2) is obtained.

From equations (1) and (2) Is obtained.

Furthermore, if d is reduced to the limit, the condition of d = 0 is given. After all, as a conditional expression to obtain a smooth spectrum as the laser output, was gotten. As a specific example, in the case of an optical fiber laser having an oscillation wavelength at λ ₀ = 1.09 μm, equation (4) becomes 0 ≦ d <5.9 (μm) (5), and the gap must be several μm or less. become.

The other point of the present invention, that is, "the point where laser output is efficiently taken out" can be solved by optically coupling an optical fiber having a mirror-polished end surface and an antireflection coating.

At this time, by using a high-precision optical connector,
The condition of the gap in the equation (4) can be easily satisfied. It is also possible to fill the gap with an optically transparent medium in order to further reduce the reflection at both end faces.

The influence of the reflection from the other end face of the optical fiber connected in this way can be avoided by setting the length 1 of the connected fiber to a length equal to or more than a certain value.

That is, the longitudinal mode interval Δλ due to reflection from the other end of the connected optical fiber is Given by Here, N is the group refractive index of the connected optical fiber. In order to freely use the output light of the laser using the connected optical fiber, a length of at least several tens of cm is required. On the other hand, when l is extremely short, a new composite resonator is formed, and an unnecessary longitudinal mode is generated.

Therefore, by taking l to be somewhat longer, Δλ is made sufficiently narrow, and conditions for obtaining an effective continuous oscillation spectrum are obtained. Since the oscillation line width (half width) of each longitudinal mode shown in FIG. 4 is about 1 to 2 °, a condition of Δλ≪1Å (7) is necessary for these to completely overlap. Therefore, if Δλ is set to a value smaller than each oscillation line width by two digits or more, that is, Δλ <0.01 ＜, Is obtained.

For example, in the case of an optical fiber laser having an oscillation wavelength λ ₀ = 1.09 μm, if N = 1.466, l> 3.98 × 10 ⁵ (μm) = 39.8 cm (9).

As described above, according to the present invention, an optical fiber laser device having a smooth oscillation spectrum is provided by connecting an optical fiber regulated by the expressions (4) and (8) to the output end of the optical fiber laser. it can. Hereinafter, specific examples will be described in detail.

(Embodiment) Embodiment-1 FIG. 1 is a block diagram showing one embodiment of the present invention.
Is an Nd-doped single mode optical fiber, 2a and 2b are dielectric multilayer mirrors formed directly on the end face of the optical fiber, 3a, 3b, 3a ', 3
b 'is an optical connector, 5a and 5b are non-reflective dielectric multilayer films for a wavelength of 1.09 µm, and 6 is a normal single mode optical fiber containing no rare earth.

FIG. 5 is an enlarged schematic view of the ferrule coupling portion between the optical connectors 3b and 3a ', wherein 5a' is a non-reflective dielectric multilayer film, 7a and 7b are zirconia ferrules, and 8a and 8b are single ferrules. A core of a single mode optical fiber, 9 is a gap between optical fiber connection portions, 1 'is a rare earth doped single mode optical fiber strand,
6 'is a normal single mode optical fiber. Also,
The addition amount of Nd was 180 ppm, and the length of the optical fiber 1 was 15.5 m.

In FIG. 5, the end face of each optical fiber was manufactured in the following procedure. First, a small hole having a diameter of 127 μm was made in the center of a 2.5 mmφ ferrule made of zirconia, and an optical fiber was inserted into the hole and fixed with an adhesive, and the end face was mirror-finished.
Then, this end face was further processed by buffing, utilizing the hardness difference between zirconia and the optical fiber, so that the glass end face portion was recessed from the alumina ferrule face as shown in FIG.

FIG. 7 shows the result of measuring the polished end face along the central axis by a surface roughness meter after the polishing is completed. The dent due to buffing was 0.2 μm. A dielectric multilayer mirror was deposited on this, and an FC connector was attached. These two connectors were adjusted so as to maximize the optical coupling efficiency, and were fixed by crimping so that the end faces (zirconia portions) were in contact with each other. The coupling loss at this time was 0.1 dB or less.

The gap d between the end faces of the optical fiber in FIG. 5 was about 0.4 μm from the measurement shown in FIG. With this configuration, a stable optical fiber laser can be formed without damaging the laser resonance mirror.

Next, the processing of the other end face of the connected optical fiber will be described.

The ferrule shown in FIG. 6 has the same structure as that shown in FIG. 5, except that the end face is polished at an angle of θ = 8 ° to prevent reflection from the end face of the non-reflective dielectric multilayer film 5d. It is. After mirror polishing the end face, reflectivity at 1.09μm in λ
0.5% or less of a dielectric multilayer antireflection coating was performed. The length of the single mode optical fiber 6 was set to 2 m.

Through the above steps, an optical fiber laser device as shown in FIG. 1 was constructed.

The characteristics of the laser resonance mirrors 2a and 2b are as shown in Table 1 below.

In order to operate the apparatus shown in FIG. 1, first, a GaAlAs semiconductor laser is driven and incident on the optical fiber 1 by using a condenser lens 4d. Here, sufficient position adjustment is performed so that the incident efficiency is maximized. In this embodiment, the GaAlAs laser output 80
Of the mW, up to 28% of 22.4 mW could be coupled. Wavelength 1.09μ
The oscillation output of m was 11.6 mW.

FIG. 8 shows the obtained optical spectrum.
82Å. The longitudinal mode light as shown in FIG. 4 is not observed, and an extremely smooth optical spectrum is obtained.
The additional effect of the optical fiber 6 shown in the figure could be confirmed.

Example 2 In the second example, an experiment was performed using an experimental system similar to that shown in FIG.

In this embodiment, an epoxy adhesive having a refractive index of 1.47 is filled in the gap 9 of the optical fiber connection portion shown in FIG.
7a and 7b were fixed by crimping. After the adhesive portion was cured by heating at about 80 ° C., a laser oscillation experiment was performed in the same process as in the first embodiment. The length of the optical fiber 6 was 1.2 m. The resulting output is
At 12.1 mW, the spectrum width was 82 mm. In the output spectrum, the same smooth oscillation spectrum characteristics as those in FIG. 8 were obtained. The output characteristics have been improved because the reflection at the gap 9 has been reduced.

Comparative Example 1 In the first example, a similar oscillation experiment was performed with the gap d set to 250 μm. As a result, a longitudinal mode of Δλ = 22 ° was observed, and a required smooth output light spectrum was not obtained.

Embodiment 3 The third embodiment was also implemented with the basic configuration of FIG.

FIG. 9 is an enlarged view of an emission end of an optical fiber laser and an optically coupled portion of an optical fiber connected thereto. 7a ′, 7
b ′ is a ferrule made of zirconia whose end face is mirror-polished with a radius of curvature R = 60 mm, and a dielectric multilayer mirror is vapor-deposited on the mirror face.

The tips of the ferrules 7a 'and 7b' are arranged so as to contact each other to maximize the coupling efficiency.

 The reflectance of the laser mirror was as shown in Table 2 below.

The length of the optical fiber 1 was 12 m, the length of the optical fiber 6 was 2.5 m, and the amount of Nd added to the optical fiber 1 was 220 ppm.
The optical fiber 1 had a panda structure, the difference in refractive index between the core and the clad was 0.52%, and the birefringence was 2.5 × 10 ⁻⁴ .

 Other experimental conditions are the same as in the first embodiment.

Coupling efficiency of GaAlAs laser to optical fiber laser is 41%
And 32.8 mW were coupled. The oscillation output at a wavelength of 1.09 μm was 17 mW, and the threshold value (absorption amount) of the laser was 1.8 mW. No longitudinal mode light due to reflection was observed, and a smooth output light spectrum with a half-width of 77 ° was obtained. In this embodiment, the example of Nd is described, but it is obvious that the present invention can be applied to other rare earth element-doped optical fibers.

(Effects of the Invention) As described above, the optical fiber laser device of the present invention can effectively suppress the disturbance of the output light spectrum due to the return light to the optical fiber laser, so that an extremely smooth output light spectrum can be obtained. Therefore, there is an advantage that a high-output low-coherent light source can be provided to various sensors utilizing interference or an optical sensor such as an optical fiber gyro.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical fiber laser for explaining a first embodiment of the present invention, FIG. 2 is a configuration diagram of a conventional apparatus, and FIG. 3 is a diagram of an optical fiber laser output spectrum when there is no return light. FIG. 4 is a diagram showing an example of an output spectrum of an optical fiber laser when return light is present. FIG. 5 is a diagram showing an optical fiber laser emitting end in the first and second embodiments and connected thereto. FIG. 6 is an enlarged view of the coupling portion of the end face of the optical fiber, FIG. 6 is an enlarged view illustrating the state of the other end face of the connected optical fiber 6 used in the first to third embodiments, FIG. FIG. 8 is a view showing an example of measurement of the polished state of the ferrule end face on the optical fiber laser emitting side used in the first and second embodiments and the ferrule end face of the optical fiber connected to the ferrule end face. FIG. 8 shows the first embodiment. Output spectrum of the obtained optical fiber laser FIG. 9 is an enlarged view of an emission end of an optical fiber laser and an optical coupling portion of an optical fiber connected thereto. 1 ... Rare earth doped optical fiber 1 '... Rare earth doped single mode optical fiber, 2a, 2b, 2b' ... Dielectric multilayer mirror 3a, 3b, 3a ', 3b' ... Optical connector 5a, 5b , 5a ': Non-reflective dielectric multilayer film 6: Single-mode optical fiber 6': Single-mode optical fiber 7a, 7b, 7a ', 7b': Ferrule made of zirconia 8a, 8b: Single Core of one-mode optical fiber 9 Gap between optical fiber end faces

Claims (1)

(57) [Claims] A laser system comprising a laser oscillation medium comprising a single mode optical fiber doped with a rare earth element and a resonance mirror for generating laser resonance, and a light source for exciting the rare earth element of the oscillation medium. An optical fiber laser device comprising: an optical fiber containing no rare earth element is optically connected to an emission end of the optical fiber; and a gap d between the laser emission end and a light receiving end of the optical fiber is The oscillation wavelength of the optical fiber laser is λ ₀ (μm), the refractive index of the transparent material filled in the gap d is n ₀ , And the group index of the optical fiber connected to the optical fiber laser is N, and the length l of the optical fiber is at least An optical fiber laser device configured to satisfy the following conditions.