JPH04234710A - Optical fiber - Google Patents

Abstract

PURPOSE:To reduce the reflected component on the end surface of an optical fiber returning to the optical fiber again by providing a glass plate on the end surface of the optical fiber. CONSTITUTION:On the end surface of the optical fiber 10 the glass plate 30 having a surface wider than the end surface and same or approximately same refractive index as that of the optical fiber 10, is provided. Thus, light transmitted inside the optical fiber 10 is not reflected on the end surface, but made indent on the glass plate 30. Since the incident light is diffused inside the glass plate 30, the component returned to the optical fiber 10 in the light reflected on the end surface without being projected from the end surface of the glass plate 30, is a little.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber for transmitting light, and more particularly to an optical fiber with a modified end.

0002

2. Description of the Related Art In recent years, optical fibers have been widely used for purposes such as transmitting light through free paths and transmitting optical signals that are resistant to electromagnetic noise. One way to use this optical fiber is when it is necessary to transmit light in both directions. Light is transmitted bidirectionally using fibers. Examples include the optical fiber laser Doppler vibrometer described below and other bidirectional communication systems using optical fibers.

FIG. 2 is a schematic diagram of an optical fiber laser Doppler vibrometer as an example of a system for bidirectionally transmitting light through a single optical fiber. Here, an optical fiber laser Doppler vibrometer refers to a laser beam that is reflected from a vibrating object to be measured and that is frequency-modulated by the vibration.
A beat corresponding to the vibration frequency and amplitude of the object to be measured is generated by interfering the object light with a reference light of a constant frequency that interferes with the object light to generate a beat, and detecting this beat with a photodetector. Among laser Doppler vibrometers that obtain signals, this refers to one configured to transmit laser light through an optical fiber in order to provide flexibility in the positional relationship between the main body of the vibrometer and the object to be measured.

In FIG. 2, a laser beam 2 emitted from a laser light source 1 is split into two by a beam splitter 3 into transmitted light and reflected light. This beam splitter 3 is different from a polarizing beam splitter (hereinafter abbreviated as "PBS") and splits the laser beam 2 into two regardless of the polarization direction of the laser beam 2. The light 2a transmitted through the beam splitter 3 reaches the PBS 4. This PBS 4 is arranged to transmit the laser beam 2 (transmitted light 2a) which is originally polarized, and the transmitted light 2a transmits through this PBS 4 and is focused by the fiber input lens 5 to preserve the plane of polarization. The light enters the optical fiber 6 from its one end surface 6a and is transmitted to the sensor head section 20. Here, what is polarization maintaining optical fiber?
A single mode optical fiber that transmits only light polarized in two predetermined directions that are orthogonal to each other. The light transmitted to the sensor head section 20 by the polarization-maintaining optical fiber 6 is emitted from the end face 6b of the polarization-maintaining optical fiber 6, passes through the λ/4 plate 21, the objective lens 22, and then passes through the object to be measured 2.
Irradiate 3. Here, it is assumed that the object to be measured 23 is repeatedly vibrating in the direction A-B, which is inclined by an angle θ with respect to the optical path of the irradiated light.

The above-mentioned irradiation light is reflected by the object to be measured 23, and this reflected light (this reflected light is referred to as "object light") passes through the objective lens 22 and the λ/4 plate 21 again to the polarization-maintaining optical fiber. 6 from its end surface 6b. Here, the light that irradiates the object to be measured 23 is the light that is converted into circularly polarized light by passing through the λ/4 plate 21 once, and the object to be measured 2
Since the object light reflected from 3 passes through the λ/4 plate 21 again, this object light passes through the end face 6b of the polarization maintaining optical fiber 6.
It becomes linearly polarized light whose polarization direction differs by 90 degrees from the light emitted from the . The object light incident on the polarization-maintaining optical fiber 6 is transmitted by the polarization-maintaining optical fiber 6, exits from the end face 6a, and is emitted from the fiber input lens 5.
The beam is collimated by , and enters the PBS 4 . As described above, the polarization direction of this object light has been rotated by 90 degrees, so it is reflected by the PBS 4, and the component of this object light that has passed through the beam splitter 7 enters the photodetector 31 in the signal processing section 30. .

On the other hand, the light reflected by the beam splitter 3 is optically detected via the following optical path in order to maintain coherence with the object light by making the optical path length approximately the same as that of the object light side. The light is input to the vessel 31. That is, the light reflected by the beam splitter 3 (hereinafter referred to as "reference light") passes through the PBS 8, is focused by the fiber input lens 9, and is sent to the polarization maintaining optical fiber 10. The light enters from one end surface 10a, is transmitted to the sensor head section 20, is emitted from the other end surface 10b of the polarization preserving optical fiber 10, is converted into circularly polarized light by passing through the λ/4 plate 24, and is converted into circularly polarized light through the reference light lens. 25 and is irradiated onto a reference mirror 26. The reference light reflected by the reference mirror 26 passes through the reference light lens 25 again, passes through the λ/4 plate 24, and enters the polarization maintaining optical fiber 10 from its end face 10b. At this time, the reference light has its polarization direction rotated by 90 degrees with respect to the light emitted from the end face 10b of the polarization-maintaining optical fiber 10, as in the case of the irradiation light (object light) described above. The reference light that enters the polarization-maintaining optical fiber 10 from the end face 10b is transmitted through the polarization-maintaining optical fiber 10,
The light is emitted from the end face 10a of the polarization-maintaining optical fiber 10, collimated by the fiber input lens 9, and P
It enters BS8. As mentioned above, the polarization direction of the reference light incident on this PBS8 has been rotated by 90 degrees, so this PBS8
The frequency is shifted by being reflected by the BS 8 and passing through the acousto-optic light modulator 11 driven by the driver 32 in the signal processing section 30, and the beam splitter 7
The reflected component enters the photodetector 31 together with the aforementioned object light. The object light and reference light that entered the photodetector 31 as described above interfere on this photodetector 31,
This interfered light is detected by the photodetector 31 to obtain a beat signal of frequency fb, and the preamplifier 33
After being amplified, it is input to a signal processing circuit (not shown),
The vibration frequency and the like of the object to be measured 23 are determined.

In the optical fiber laser Doppler vibrometer configured as described above, the wavelength and frequency of the laser beam 2 emitted from the laser light source are λ and fo, respectively, and the vibration velocity of the object to be measured 23 is V (time t), the frequency shift amount fd of the object light reflected from the object to be measured 23 from the laser beam 2 is fd=(2・V/λ)・cosθ... (1
), therefore, the frequency fs of the object light is
fs=fo+fd=fo+(2・V/λ)・c
osθ... (2). Also, the reference light is
As mentioned above, since the acousto-optic light modulator 11 undergoes a frequency shift, if the modulation frequency of the acousto-optic light modulator 11 is fm, the reference light after passing through the acousto-optic light modulator 11 is The frequency fr is fr=fo+fm
...(3).

Since the object light and the reference light interfere on the photodetector 31, a sum frequency and a difference frequency of the above equations (2) and (3) appear, but on the photodetector 31, The frequency of this difference is detected, and the frequency fb of this difference is the above (
From equations 2) and (3), fb=|fr−f
s| = |fm-(2・V/λ)
・cos θ | ... (4). The frequency fb of this difference is detected as a beat signal, and thereby the velocity Vcosθ of the object to be measured 23 in the optical axis direction of the irradiated light is detected.

[0009]

[Problems to be Solved by the Invention] With the above-described configuration, an optical fiber laser Doppler vibrometer is realized, but in the optical fiber Doppler vibrometer, the optical fiber 6 is It plays the role of transmitting the light before being modulated, and transmitting the object light modulated by the object to be measured 23 on the return path. However, for example, when the light incident on the optical fiber 6 exits from the end face 6b, a portion of the light is reflected and this reflected light is transmitted to the photodetector 31 together with the object light. In other words, light that has not been Doppler-shifted by the object to be measured 23 is mixed into the object light that returns to the photodetector 31, and this mixed light interferes with the reference light, causing so-called carriers whose frequency does not change. This poses a problem in that it remains behind, leading to measurement errors.

[0010] In view of the above-mentioned problems, the present invention improves the end face of an optical fiber so that out of the light that has been transmitted within the optical fiber, it is reflected at the end face of the optical fiber and passes through the optical fiber. An object of the present invention is to provide an optical fiber in which components that return as they are are reduced.

[0011]

[Means for Solving the Problems] A first optical fiber of the present invention for achieving the above object has a surface on the end face thereof having a larger area than the end face and having the same refractive index as the optical fiber. Alternatively, it is characterized by being attached with a glass plate having a similar refractive index. Further, in the second optical fiber of the present invention, the end face of the optical fiber is
It is characterized in that it is formed obliquely to a direction perpendicular to the length direction of the optical fiber. Furthermore, the third optical fiber of the present invention has a surface having a larger area than the end surface on the end surface of the second optical fiber, and one surface of the surface that is in contact with the end surface is at the same angle as the end surface. It is characterized by being attached with a glass plate having a refractive index that is the same as or similar to the refractive index of the optical fiber, and whose other surface extends in a direction perpendicular to the length direction of the optical fiber. be. Here, it is preferable to apply an antireflection film to the tip end surface of the glass plate in the first optical fiber or the third optical fiber, or to the end surface of the second optical fiber.

0012

[Operation] The first optical fiber of the present invention has a wide glass plate attached to its end face having a refractive index that is the same as or similar to the refractive index of the optical fiber. The light enters the glass plate with almost no reflection at the end face of the optical fiber. In addition, the light that is emitted from the optical fiber and enters the glass plate is
Since the light spreads within the glass plate, the light is not emitted from the end face of the glass plate and only a small amount of the light reflected from the end face returns into the optical fiber.

Furthermore, since the end face of the second optical fiber of the present invention is formed obliquely, the light reflected from the end face is reflected obliquely with respect to the direction in which the optical fiber extends. Light leaks from the optical fiber and is attenuated while being transmitted through the optical fiber.

Furthermore, the third optical fiber of the present invention has a glass plate with a large area attached to the end face of the obliquely formed optical fiber like the second optical fiber, and the optical fiber side of the glass plate is attached to the end face of the optical fiber. Since the back surface (tip surface) seen from the optical fiber extends in a direction perpendicular to the direction in which the optical fiber extends, it functions as both the first optical fiber and the second optical fiber of the present invention, and this third The reflected light returning within the optical fiber is reduced.

[0015] If an anti-reflection coating is applied to the tip surface of the glass plate of the first optical fiber or the third optical fiber or the end surface of the second optical fiber, the reflected light itself at the tip surface or end surface is reduced. Therefore, the reflected light returning within the optical fiber is further reduced.

[0016]

[Examples] Examples of the present invention will be described below. FIGS. 1(a), (b), and (c) are the first and second embodiments of the present invention, respectively.
FIG. 7 is a schematic cross-sectional view showing an end portion of an optical fiber with a ferrule, which is each of the second and third embodiments of the optical fiber. (a)
, (b), and (c), components that correspond to each other are indicated by the same numbers or the same numbers with a dash, and common explanations are represented by the explanations in the figure (a). That's it.

FIG. 1(a) is a schematic cross-sectional view showing the end of an optical fiber with a ferrule, which is an embodiment of the first optical fiber of the present invention. 11 and a covering 12 covering the core wire 11
It is composed of. The part where the coating 12 near the tip of the optical fiber 10 is removed and the core wire 11 is exposed is covered with a ceramic body 21 constituting the ferrule 20.
The distal end surface 21a of the ceramic body 21 forms the same plane as the distal end surface 11a of the core wire 11.

A glass plate 3 having a refractive index almost the same as that of the core wire 11 is disposed on the tip end surface 11a of the core wire 11.
0 is also pasted with a transparent adhesive having almost the same refractive index as that of this core wire 11. Further, the periphery of the glass plate 30, the periphery of the ceramic body 21, and the portion covered by the coating 12 slightly backward from the tip of the optical fiber 10 are covered with a stainless steel body 22 constituting the ferrule 20.

When light is transmitted from the right side to the left side in the figure through the ferruled optical fiber constructed in this manner, the tip surface 11a of the core wire 11 has a refractive index that is almost the same as that of the core wire 11. Since it is in contact with the adhesive having a refractive index and the glass plate 30, very little light is reflected by this tip surface 11a and returns into the optical fiber on the right side of the figure. Furthermore, the light emitted from the tip surface 11a and entering the glass plate 30 spreads within the glass plate 30, so that only a small amount of the light reflected by the tip surface 30a of the glass plate 30 returns to the core wire 11. Therefore, the amount of light that is reflected by the end face of the ferruled optical fiber and returns inside the optical fiber is significantly reduced compared to when the glass plate 30 is not attached.

[0020] If an anti-reflection film is attached to the front end surface 30a of the glass plate 30, the amount of light reflected at the front end surface 30a will be reduced compared to the case without the anti-reflection film.
The amount of light returning into the core wire 11 is further reduced.

FIG. 1(b) is a schematic cross-sectional view showing an end portion of an optical fiber with a ferrule, which is an embodiment of the second optical fiber of the present invention. Although a glass plate is not attached to the tip of this ferrule-equipped optical fiber, the core wire 11
The distal end surface 11a' is formed obliquely. Therefore, among the light transmitted in the optical fiber from the right to the left in the figure, the light reflected at the tip surface 11a' is reflected by the core wire 1.
1 at an angle, and as it is transmitted through the core wire 11, it leaks out from the core wire 11 and is attenuated. It goes without saying that it would be more effective to apply an antireflection film to the obliquely formed tip surface 11a'. However, since the tip end surface 11a' of the core wire 11 is formed obliquely, this tip surface 11a' is formed obliquely.
The light emitted from a' will travel in a direction different from the direction in which the optical fiber extends, and if this is inconvenient, it will be necessary to correct the optical path using an optical system other than this optical fiber.

FIG. 1(c) is a schematic cross-sectional view showing an end portion of a ferrule-equipped optical fiber, which is an embodiment of the third optical fiber of the present invention. In this optical fiber with a ferrule, the tip surface 11a' of the core wire 11 is formed obliquely like the tip surface of the core wire of the optical fiber with a ferrule shown in FIG. 1(b). One side 30b
A glass plate 30' is attached to the glass plate 30', which is formed at the same angle as the tip surface 11a' and whose other surface (tip surface) 30a' extends in a direction perpendicular to the direction in which the optical fiber 10 extends. There is. This glass plate 30' and the adhesive for pasting this glass plate 30' have almost the same refractive index as the core wire 11, as shown in the embodiment shown in FIG. 1(a). Same as in case.

With this configuration, the reflection at the tip end surface 11a' of the core wire 11 is slight, and even this slight reflected light enters the core wire 11 obliquely, and as described above, as it returns inside the core wire 11, This results in attenuation. Furthermore, the light emitted from the tip surface 11a' of the core wire 11 and incident on the glass plate 30' is
′, so the tip surface 30a of this glass plate 30′
Only a portion of the light reflected by the core wire 11 is incident on the core wire 11, so that the amount of light that returns inside the core wire 11 is significantly reduced. Further, in the embodiment shown in FIG. 1(c), the glass plate 30'
Since the light emitted from the distal end surface 30a' of the optical fiber 10 travels in the same direction as the extending direction of the optical fiber 10, the optical path correction optical system required in the embodiment shown in FIG. 1(b) is unnecessary. . It should be noted that it is more effective to apply an anti-reflection film to the front end surface 30a' of this glass plate 30', as in the case of the above-mentioned embodiment. Here, in the embodiment shown in FIGS. 1(a) and 1(c), the tip end surface 11a of the core wire 11 is
, 11a' with an adhesive, but the glass plates 30, 30' may be fixed with the stainless steel body 22 instead of using an adhesive. Furthermore, the embodiments shown in FIGS. 1(a), (b), and (c) are examples of optical fibers with ferrules in which a ferrule is attached to the tip of the optical fiber 10, but the optical fiber of the present invention It is not necessarily necessary to have a ferrule attached to the tip of the core wire 11.
The positional relationship between the tip surfaces 11a, 11a' of the core wire 11a' and the glass plates 30, 30', or the angle at which the tip surface 11a' of the core wire 11 widens may be as described above. The method and the method of fixing the tip of the optical fiber are not limited to a specific method.

[0024]

Effects of the Invention As explained in detail above, the optical fiber of the present invention has the advantage that glass is attached to the end face of the optical fiber, or the end face of the optical fiber is formed obliquely, or Since the end face of the optical fiber is formed obliquely and a glass plate is attached to the end face,
The light amount ratio of the light that is not emitted from the optical fiber but is reflected and returns to the light that has been transmitted through the optical fiber is reduced, and measurements caused by this return light, for example, in the optical fiber laser Doppler vibrometer mentioned above, are reduced. Negative effects such as errors are reduced. Furthermore, by applying an antireflection film to the tip end surface of the glass plate or the tip end surface of the optical fiber, reflected light can be reduced even more effectively.

[Brief explanation of the drawing]

[Fig. 1] A schematic cross-sectional view showing the end portion of an optical fiber with a ferrule, which is each embodiment of the first, second, and third optical fibers of the present invention.

[Figure 2] Schematic configuration diagram of an optical fiber laser Doppler vibrometer as an example of a system that transmits light in both directions using a single optical fiber.

[Explanation of symbols]

10 Optical fiber 11 Core wire 11a, 11a' Tip surface of core wire 12 Coating 20 Ferrule 21 Ceramic body 22 Stainless steel body 30, 30' Glass plate 30a, 30a' Tip surface of glass plate

Claims (4)

[Claims] 1. An optical fiber characterized by having a glass plate attached to an end face of an optical fiber, the glass plate having a surface having a larger area than the end face and having a refractive index that is the same as or similar to the refractive index of the optical fiber. fiber. 2. An optical fiber characterized in that the end face of the optical fiber is formed obliquely with respect to a direction perpendicular to the length direction of the optical fiber. 3. The optical fiber according to claim 2, wherein the end surface of the optical fiber has a surface having a larger area than the end surface, one surface of the surface in contact with the end surface spreads at the same angle as the end surface, and the other surface. 1. An optical fiber comprising a glass plate having a refractive index that is the same as or similar to that of the optical fiber, and which extends in a direction perpendicular to the longitudinal direction of the optical fiber. 4. An optical fiber characterized in that an anti-reflection film is applied to the distal end surface of the glass plate in the optical fiber according to claim 1 or 3, or to the end surface in the optical fiber according to claim 2.