JPH09211344A - Polarization independent optical external modulation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device which can impose external modulation on an optical fiber in light propagation with efficiency without depending upon the polarization of light made incident on the optical fiber. SOLUTION: While the relative interval between >=2 optical modulators 1A and 1B, having piezoelectric elements 3A and 3B formed of sinters, along the length of the optical fiber, the angles of intersections of the optical modulators with the optical fiber 7, and the angle θof relative intersection of the optical modulators are held, the optical modulators 1A and 1B are set to different distances to the core of the optical fiber 7 and incorporated at the outer periphery of the optical fiber 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization independent optical external modulator for modulating light propagating through a core of an optical fiber.

[0002]

2. Description of the Related Art As a well-known optical modulator for optical communication, a device such as a semiconductor laser or a light emitting diode that directly modulates a light source by an optical modulator is widely used. This optical modulation means is basically based on a one-to-one correspondence between an optical transmission line (optical fiber) and a signal source (light source).

In such an optical modulation means, when each optical signal is made to enter one optical transmission line from a plurality of signal sources, it causes a loss increase, but between these signal sources and the optical transmission line. It is necessary to interpose an optical coupler. Therefore, in the case of the optical modulation means using the direct modulation method, the number of signal sources that can be coupled to one optical transmission line is limited from the viewpoint of suppressing this type of loss increase.

As an alternative to the above direct modulation method, an optical modulation means by an external modulation method has been proposed.
This optical modulation means is one in which an optical modulator is assembled in the middle of an optical transmission line through which light of constant intensity propagates. The transmitted light on the optical transmission line can be modulated through this optical modulator. In particular, an optical modulator utilizing the acousto-optic effect has a small insertion loss when connected to an optical transmission line,
The number of signal sources that can be coupled to one optical transmission line is increased, and practical convenience is improved.

As an optical modulation means by an external modulation system,
Reference can be made to the technology disclosed in Japanese Patent Laid-Open No. 5-40248. The structure is shown in FIGS. 5 to 7, the external light modulator 1 is composed of the piezoelectric section 3 provided on the substrate 2, and the piezoelectric section 3 is
It is composed of a lower electrode 4, a piezoelectric film 5 and an upper electrode 6. When the external optical modulator 1 is assembled to the single mode type optical fiber 7, it is as follows.

As an example, in the case shown in FIGS. 5 and 6, the substrate surface of the external optical modulator 1 which does not have the piezoelectric portion 3 is applied to the outer peripheral surface of the optical fiber 7, and the specific acoustic impedance ( The product of the density of the sound wave propagation medium and the speed of sound) is passed through the binder 8 which is similar to the cladding of the optical fiber 7,
The external light modulator 1 and the optical fiber 7 are coupled to each other. In the case shown in FIG. 7 as another example, the substrate surface having the piezoelectric portion 3 of the external optical modulator 1 (the upper electrode 6 of the piezoelectric portion 3).
Side) is applied to the outer peripheral surface of the optical fiber 7, and the external optical modulator 1 and the optical fiber 7 are coupled to each other via the binder 8 in the same manner as described above. In addition, the lower electrodes 4 and the upper electrodes 6 in each of the above examples have lead wires 9, 10
Are connected to each other, and the external optical modulator 1 is connected to an electric system (not shown) for applying a high frequency signal via the lead wires 9 and 10.

FIG. 8 shows a case in which the optical modulation means by the external modulation method illustrated in FIGS. 5 to 7 is incorporated in the measurement system thereof. In the measurement system of FIG. 8, a light source 11 composed of a laser diode (LD), a polarization element 12 for setting polarization to an optimum state, an external light modulator 1, an intensity modulation light detecting element 13, an optical / electrical converter ( O / E) 14 are optically connected to each other through an optical fiber, and are connected between the external optical modulator 1 and the driving power supply 16 and between the optical / electrical converter 14 and the spectrum. The analyzer 15 and the oscilloscope 17 are electrically connected to each other.

The light emitted from the light source 11 in the measurement system of FIG. 8 first passes through the polarizing element 12 and then reaches the optical transmission line in which the external optical modulator 1 is assembled. At this time, the external optical modulator 1 to which a high frequency signal of a predetermined frequency is applied via the driving power supply 16 changes the polarization state of the light reaching the optical transmission line according to the applied signal. The light thus modulated passes through the light detecting element 13 and becomes intensity-modulated light, and the optical / electrical converter 14
After being converted into an electric signal by, the signal enters the spectrum analyzer 15. In the spectrum analyzer 15, the modulated output input here is subjected to spectrum analysis and spectrum observation, and the polarization element 12 is adjusted based on the analysis result. Therefore, in the above, the light from the polarization element 12 to the external optical modulator 1 is adjusted. The polarization state of is optimally maintained.

In the above-mentioned light modulating means using the external modulation method, when the light to be modulated is viewed from a plane perpendicular to its propagation direction, the following problems occur. For example, perpendicular to the elastic wave generated through the piezoelectric portion 3,
When horizontal linearly polarized light propagates, the propagating light is only phase-modulated and may not be polarized plane-modulated. Moreover, the polarization state of the light propagating through the optical fiber fluctuates due to the external pressure, temperature change, and the like that spread to the optical fiber in actual use, and it is difficult to always maintain this optimally. By the way, in the case of the measurement system described in FIG. 8, optimal polarization is obtained by adjusting the polarization element 12 in the monitor state of the modulation output. This is a difficulty in constructing a modulation system and entails a high equipment cost, which are practical obstacles.

The present applicant has previously proposed a patent of a polarization independent optical external modulation device that solves this practical problem (Japanese Patent Laid-Open No. 7-72435). That is, JP-A-7-7
The polarization-independent optical external modulator proposed in Japanese Patent No. 2435 discloses an optical modulator in which a piezoelectric portion for ultrasonic wave generation including an electric system for applying a high frequency signal is provided on a substrate, and an optical modulator. An external optical modulator comprising a combination of a single-mode optical fiber that modulates the optical polarization plane of the core when receiving ultrasonic waves from the piezoelectric part of Holding the relative distance between the optical modulators along the depth direction, the crossing angle of these optical modulators with respect to the optical fiber, and the relative crossing angle between the optical modulators,
It is attached to the outer circumference of the optical fiber.

In the above, the means for generating ultrasonic waves having different phases from the piezoelectric portion of each optical modulator toward the core of the optical fiber is as follows. As an example, the piezoelectric parts of these optical modulators include an electric system for receiving high-frequency signals having different phases, and the distances from the respective piezoelectric parts to the core of the optical fiber are set to be equal to each other. As another example, the piezoelectric parts of these optical modulators include an electric system for receiving high-frequency signals having the same phase, and the distances from the respective piezoelectric parts to the core of the optical fiber are set to be different from each other. .

[0012]

In any of the above cases, the piezoelectric portion is formed on the substrate, and the piezoelectric portion is firmly bonded to the substrate. For this reason, the piezoelectric portion alone cannot vibrate, and always vibrates integrally with the substrate. Therefore, when driven at a resonance frequency determined by the thickness of the substrate, strong vibration is obtained, and ultrasonic waves can be efficiently applied to the optical fiber. By the way, in the case of the latter example in which the distance from the piezoelectric section to the core of the optical fiber is different from each other, it is inexpensive because the configuration of the electric system section of the optical external modulator can be simplified, but in order to provide a phase difference of ultrasonic waves. When the thickness of the substrate is changed, the resonance frequency also changes, and there is a problem that the optical modulator does not work sufficiently.

The resonance frequency of the piezoelectric portion is, for example, 30
It is a frequency band with a short wavelength such as 0 MHz, and it is not preferable to interpose a spacer, for example, for making the distance different from each other in the propagation path of the ultrasonic wave, because it is easily attenuated at the mismatched portion of the acoustic impedance. Further, for example, more specifically, the thickness of the spacer for imparting a phase difference of 180 ° / n is several μm, and this also poses a problem that it is difficult to form the spacer. In the case of the former example in which the distances from the piezoelectric section to the core of the optical fiber are made equal to each other to electrically provide a phase difference, there is a problem that an electric system for providing a phase difference requires a component and the cost is inevitably increased. is there.

The present invention solves the above problems and does not depend on the polarization of light incident on an optical fiber, and
It is an object of the present invention to provide an inexpensive polarization-independent optical external modulator capable of efficiently modulating the propagation light inside the optical fiber from the outside.

[0015]

The present invention has the following means to solve the above problems.

A polarization-independent optical external modulator according to claim 1 of the present invention is an optical modulator provided with a piezoelectric element for ultrasonic wave generation including an electric system for applying a high frequency signal,
In an optical external modulator comprising a combination with a single-mode optical fiber in which the optical polarization plane of the core is modulated when receiving ultrasonic waves from the piezoelectric element of the optical modulator, the piezoelectric element of the optical modulator is a sintered body. The two or more optical modulators are configured such that the relative spacing between the optical modulators along the length of the optical fiber, the crossing angle of these optical modulators with respect to the optical fiber, and the mutual optical modulators. While maintaining the relative crossing angle and being assembled on the outer circumference of the optical fiber,
As a means for propagating ultrasonic waves having different phases from the piezoelectric elements of each optical modulator toward the core of the optical fiber, the piezoelectric elements of these optical modulators are electrically connected to receive a high-frequency signal having the same phase. In addition to including the system, the distance from each piezoelectric element to the core of the optical fiber is set to be different from each other.

According to a second aspect of the present invention, the polarization-independent optical external modulator is characterized in that spacers are provided so that the distances from the piezoelectric elements to the core of the optical fiber are different from each other.

In the polarization independent optical external modulator according to claim 3 of the present invention, when the number of optical modulators mounted on the outer circumference of the optical fiber is n (where n ≧ 2), each optical modulator is The device is provided with means for propagating each ultrasonic wave having a phase shift of approximately 180 ° / n from these piezoelectric elements to the core of the optical fiber, and the relative crossing angle of these optical modulators is , About 90 ° / n.

According to the polarization independent optical external modulator of the present invention, for example, two optical modulators are assembled on the outer circumference of the optical fiber, and the relative crossing angles of these two optical modulators are set. At 45 °, the two ultrasonic waves emitted from the piezoelectric elements of both optical modulators toward the optical fiber are irrespective of the polarization state of the light incident on the optical fiber. The polarization of the propagation light is reliably and efficiently modulated. That is, even if the polarized light (linearly polarized light) incident on the optical fiber coincides with the propagation direction of the one ultrasonic wave and the polarization plane modulation by the one ultrasonic wave is not applied, the other ultrasonic wave is linear. 45 for polarized light
Since it works at an angle of °, it is efficiently polarization-modulated.

Moreover, since the two ultrasonic waves in this case are independent of each other with a phase difference of 90 °, they do not act as a combined wave and ensure the polarization plane modulation. Therefore, in the case of the polarization independent optical external modulator according to the present invention, it is possible to reliably and efficiently perform polarization plane modulation of the propagation light of the optical fiber using a plurality of ultrasonic waves. Further, since the piezoelectric element of the optical modulator of the polarization independent optical external modulator according to the present invention is made of a sintered body, for example, barium titanate, the substrate for adjusting the propagation distance of ultrasonic waves is Not necessarily required. Even if a substrate is used, the piezoelectric element vibrates independently, so that the resonance condition does not change depending on the substrate thickness, and it can be used as a spacer, for example. Since the sintered body forming the piezoelectric element is sufficiently thicker than the thin film and the resonance frequency is as low as several kHz to 10 MHz, the attenuation is small even if it is used as a spacer. In addition, 1 for ultrasonic
The spacer thickness for giving a phase difference of 80 ° / n is also
For example, in the case of n = 2 and a driving frequency of 10 MHz, if brass is used as the material, the thickness is 0.1 mm, and there is no difficulty in manufacturing.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to examples. 1 to 4 show a polarization-independent optical external modulator according to the present invention. In FIGS. 1 to 4, the two optical modulators 1A and 1B are piezoelectric elements 3A and 3B, respectively. have. Each piezoelectric element 3A, 3
B is composed of an upper electrode 4, a piezoelectric body 5, and a lower electrode 6. In addition, a spacer 2A is formed on the upper electrode 4 in the piezoelectric element 3A. Further, the lead wires 9 and 10 are connected to the upper electrodes 4 and the lower electrodes 6, respectively, as in FIGS. 5 to 7, and both the optical modulators 1A and 1B are connected.
Are respectively connected to an electric system (not shown) for applying a high frequency signal via these lead wires 9 and 10.

Of the members forming the piezoelectric elements 3A and 3B, the piezoelectric body 5 is made of a piezoelectric material composed of a sintered body such as barium titanate. The upper electrode 4, the lower electrode 6, and the lead wires 9 and 10 are made of a well-known metal conductor such as Al, Cr, Au, or Cu, or
It consists of these alloys. The spacer 2A is made of a silicon sheet. Incidentally, the upper electrode 4 and the lower electrode 6 above and below the piezoelectric body 5 are formed by a sputtering method, a vacuum deposition method, a CVD method, or the like.

In FIG. 1 and FIG. 2, the single mode type optical fiber 7 has, for example, a core and a clad made of silica, and another example of the core made of silica and a clad made of plastic. As an example of
The core and clad are made of plastic. When the cladding of the optical fiber 7 is made of quartz, its outer peripheral surface is covered with a plastic coating layer.

The two optical modulators 1A and 1B are connected to the optical fiber 7
At the time of assembly, the spacer 2A of the piezoelectric element 3A and the upper electrode 4 side of the piezoelectric element 3B are applied to the outer peripheral surface of the optical fiber 7, and the optical modulators 1A, 1B and the optical fiber 7 are coupled by the binder 8. It

The binder 8 in the above is made of a material whose inherent acoustic impedance is similar to that of the cladding of the optical fiber 7. More specifically, silica-based glass or a polymer compound to which metal powder is uniformly added is used as the binder 8. Other substances may be used as the binder 8 as long as they satisfy a predetermined specific acoustic impedance. When the polymer compound containing metal powder is used as the binder 8, the uncured polymer compound for binder is applied to the joint between the optical modulators 1A and 1B and the optical fiber 7, and then the polymer compound for binder is applied. Is cured. This coupling means does not matter whether the optical fiber 7 is made of quartz or plastic.

In both the optical modulators 1A and 1B assembled on the outer circumference of the optical fiber 7 as described above, as is apparent from FIGS. 1 and 2, these piezoelectric elements 3A and 3B are the optical fibers. 7 is facing the outer peripheral surface. In such a mode, the two optical modulators 1A and 1B respectively intersect the optical fiber 7 from different directions, and the relative distance L1 between the optical modulators along the length direction of the optical fiber 7, It holds L2 and the relative crossing angle θ between the two optical modulators 1A and 1B.

Functions of both optical modulators 1A and 1B are shown in FIG.
In the case of FIG. 3, since the optical fiber 7 is intentionally shown large, both optical modulators 1A and 1A will be described.
B and the optical fiber 7 are not at the same magnification. In FIG.
The ultrasonic wave emitted from one optical modulator 1A is transmitted to the spacer 2A.
The ultrasonic waves propagated through the core of the optical fiber 7 through the optical modulator 7B also propagate through the core of the optical fiber 7. In this case, the two optical modulators 1A and 1B are the optical fibers 7
Since the speed of light propagating through the optical fiber 7 is sufficiently faster than the speed of sound propagating through the spacer 2A, the optical fiber 7, etc., it is considered that the optical fiber 7 acts on the same position in the longitudinal direction. As an example, the two optical modulators 1A and 1B have a mutual angle θ = 90 ° / n =
When 45 ° is satisfied, even if the polarization (linear polarization) incident on the core of the optical fiber 7 matches the propagation direction of the ultrasonic wave emitted from one optical modulator 1A, the other optical modulator The ultrasonic wave emitted from 1B is 4
Since it acts from the direction of 5 °, polarization plane modulation can be efficiently applied, and therefore, it does not depend on incident light.

The piezoelectric body 5 forming the piezoelectric elements 3A and 3B is a barium titanate sintered body having a thickness of 0.2 mm.
The upper electrode 4 and the lower electrode 6 are 100 μm thick silver paste, the spacer 2A is a 75 μm thick silicon sheet,
The optical fiber 7 is a nylon coated optical fiber core having a diameter of 0.9 mm, and the piezoelectric elements 3A and 3B are set to have a spatial angle difference of 90 ° / 2 = 45 ° in the optical fiber longitudinal direction. The resonance frequency of the piezoelectric elements 3A and 3B having the piezoelectric body 5 having the above-mentioned thickness is 10 MHz, and the wavelength of the ultrasonic wave passing through the spacer 2A is 0.3 mm. Therefore, the piezoelectric elements 3A and 3B have a thickness of 75 μm. Due to the relationship of the optical fiber 7, a quarter wavelength (= 90) between the ultrasonic wave propagation distances L1 and L2.
°) phase difference occurs.

As shown in FIG. 4, a polarization-independent optical external modulator incorporating the piezoelectric elements 3A and 3B splits a signal supplied from the outside into two optical modulators 1A and 1A, respectively.
When input to B, a phase difference of 90 ° is generated, so that the polarization independent optical external modulator which does not require an electric system section for giving a phase difference of 90 ° and has a simple structure.

In the above embodiment, the spacer is provided only on the piezoelectric element 3A, but the piezoelectric element 3A is not provided.
You may provide in both A and 3B. If you want to install both
A phase difference of 90 ° can be generated by setting the thickness difference between the two spacers to, for example, 75 μm when the spacer is a silicon sheet.

[0031]

In the polarization-independent optical external modulator according to the present invention, two or more optical modulators have a relative spacing between the optical modulators along the length direction of the optical fiber, and the optical fiber. These optical modulators are assembled on the outer circumference of the optical fiber while maintaining the crossing angles of these optical modulators and the relative crossing angles of the optical modulators. When a sound wave is emitted, at least one ultrasonic wave reliably and efficiently performs polarization plane modulation on the propagation light of the optical fiber regardless of the polarization state of the light incident on the optical fiber.

Therefore, the propagation light inside the optical fiber can be efficiently modulated from the outside without depending on the polarization of the light incident on the optical fiber.
Besides, since a polarization means for adjusting the polarization state and its related mechanism are not required, this type of system can be constructed easily and economically. Moreover, since the piezoelectric element of the optical modulator of the polarization independent optical external modulator according to the present invention is made of a sintered body, a substrate for adjusting the propagation distance of ultrasonic waves is not necessarily required. A polarization-independent optical external modulator incorporating a piezoelectric element composed of this sintered body splits a signal supplied from the outside and inputs it to each optical modulator to obtain a phase difference of 180 ° / n. Therefore, the polarization-independent optical external modulator is inexpensive and has a simple structure, which does not require an electric system section for giving a phase difference.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of a polarization independent optical external modulator of the present invention.

FIG. 2 is a side view of the polarization independent optical external modulator of FIG.

FIG. 3 is a schematic diagram for explaining the function of the polarization independent optical external modulator of the present invention.

FIG. 4 is a schematic diagram for explaining an example of use of the polarization independent optical external modulator of the present invention.

FIG. 5 is a front perspective view showing an example of a conventional optical external modulator.

6 is a rear perspective view of the optical external modulator shown in FIG.

FIG. 7 is a perspective view showing another example of the optical external modulator seen in the conventional example.

FIG. 8 is a block diagram showing a measurement system of the optical external modulation means.

[Explanation of symbols]

 1A Optical modulator 1B Optical modulator 2A Spacer 3A Piezoelectric element 3B Piezoelectric element 4 Upper electrode 5 Piezoelectric body 6 Lower electrode 7 Optical fiber 8 Binder 9 Lead wire 10 Lead wire

Claims (3)

[Claims] 1. An optical modulator provided with a piezoelectric element for ultrasonic wave generation including an electric system for applying a high frequency signal,
In an optical external modulator comprising a combination with a single-mode optical fiber in which the optical polarization plane of the core is modulated when receiving ultrasonic waves from the piezoelectric element of the optical modulator, the piezoelectric element of the optical modulator is a sintered body. The two or more optical modulators are configured such that the relative spacing between the optical modulators along the length of the optical fiber, the crossing angle of these optical modulators with respect to the optical fiber, and the mutual optical modulators. While maintaining the relative crossing angle and being assembled on the outer circumference of the optical fiber,
As a means for propagating ultrasonic waves having different phases from the piezoelectric elements of each optical modulator toward the core of the optical fiber, the piezoelectric elements of these optical modulators are electrically connected to receive a high-frequency signal having the same phase. A polarization-independent optical external modulator including a system, wherein the distances from each piezoelectric element to the core of the optical fiber are set differently from each other. 2. The polarization independent optical external modulator according to claim 1, wherein spacers are provided so that the distances from the respective piezoelectric elements to the core of the optical fiber are different from each other. 3. When the number of optical modulators assembled on the outer circumference of an optical fiber is n (where n ≧ 2), each optical modulator has a phase shift of approximately 180 ° / n from these piezoelectric elements. And a means for propagating each ultrasonic wave having the following to the core of the optical fiber, and the relative crossing angle of these optical modulators is set to approximately 90 ° / n. The polarization-independent optical external modulator according to claim 1 or 2.