Variable optical attenuator

First and second optical fibers are opposed to each other, between which first and second lenses constituting a lens system having an optical axis coincident with those of the optical fibers are arranged with a gap therebetween in the direction of the optical axis. The actuators, ect are used to move the first and second lenses with electrostatic forces, in opposite directions along the optical axes of the optical fibers by the same amount at the same time. Thereby, the spot size of the light incident on the optical fiber on the reception side is changed while maintaining the light propagating between the first optical fiber and the second optical fiber point-symmetric in mode field shape. This changes the coupling efficiency between the first optical fiber and the second optical fiber, allowing an adjustment in light power.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a variable optical attenuator of light for use in optical communications.

2. Prior Art

Presently, with the wavelength multiplexing transmission systems in optical communications, there is known such a system as shown inFIG. 1. This system has a plurality of optical amplifiers31arranged on a transmission line30of wavelength multiplexed light at relaying positions so that the optical amplifiers31amplify the wavelength multiplexed transmission light. Such transmission accompanied with the amplification of wavelength multiplexed transmission light by a plurality of optical amplifiers31allows long-distance wavelength multiplexing transmission.

In the wavelength multiplexing transmission system shown inFIG. 1, a function of amplifying multi-wavelength light collectively is requires of each optical amplifier31. For the wavelength multiplexing transmission system to improve in transmission quality, it is also required that the plurality of optical amplifiers31each perform optical amplification without a great difference in power among the individual wavelengths of the transmission light amplified. To reduce this differences in power among the wavelengths of the transmission light, it has been suggested that a variable optical attenuator having the function of uniformizing the multi-wavelength light to a desired power collectively be arranged, for example, in each of the optical amplifiers31.

Among the requirements for this type of variable optical attenuator are: {circle around (1)} a constant optical attenuation at each wavelength of the multi-wavelength light, or equivalently, no variation or smaller variations in optical attenuation with respect to changes in wavelength; {circle around (2)} an attainable optical attenuation of −30 dB or higher; {circle around (3)} resistance to high optical input power; and {circle around (4)} a compact size.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a compact variable optical attenuator capable of attaining an optical attenuation of, e.g., −30 dB or higher, reducing the wavelength dependency of the optical attenuation, suppressing the occurrence of a polarization dependency loss, and withstanding high optical input power as well.

To achieve the foregoing object, the present invention provides a variable optical attenuator comprising: a first optical part; a second optical part opposed to the first optical part with a predetermined gap therebetween; and optical coupling efficiency adjusting means for adjusting a coupling efficiency between the first optical part and the second optical part while maintaining light propagating between the first optical part and the second optical part point-symmetric in mode field shape, wherein the optical coupling efficiency adjusting means adjusts the coupling efficiency between the first optical part and the second optical part for an adjustment in light power.

DETAILED DESCRIPTION

FIG. 2shows an example of a conventional variable optical attenuator. In this variable optical attenuator, an optical absorption member15having a glass substrate11and an optical absorption film12is arranged on the optical path of light propagating between optical parts, or optical fibers3and4. The glass substrate11is placed on an XY plane generally orthogonal to the Z-axis with the direction of the optical axes of the optical fibers3and4as the Z-axis. The optical absorption film12is deposited on the top side of the glass substrate11. The optical absorption film12has a thickness distribution over the XY plane, and is formed, for example, to gradually increase in the thickness in the direction of the Z-axis as getting closer to the right side of the diagram in the X direction. The top side of the optical absorption film12and the bottom side of the glass substrate11are given anti-reflecting coatings13and14, respectively.

In this conventional variable optical attenuator shown inFIG. 2, when the optical absorption member15is moved along the X-axis as shown by the arrow A in the diagram, the optical absorption film12varies in the thickness on the optical path between the optical fibers3and4. This variation in the thickness of the optical absorption film12causes a change in optical attenuation. Thereby the optical attenuation is controlled.

FIG. 3Ashows another example of a conventional variable optical attenuator. In this variable optical attenuator, Faraday rotators16are arranged on the optical path. Birefringent wedge plates17and permanent magnets18are arranged to sandwich these Faraday rotators16therebetween in the direction of the optical path. In addition, electromagnets19are arranged to sandwich one of the Faraday rotators16therebetween in a direction orthogonal to the optical path. The reference numeral20in the diagram represents a wave plate.

In this conventional variable optical attenuator shown inFIG. 3A, the direction of magnetization of the Faraday rotator16is changed by the current applied to the electromagnets19, so as to control the optical attenuation by means of the Faraday effect. Here, the relationship between the current applied to the electromagnets19and the optical attenuation is shown in the graph ofFIG. 3Bwith the wavelength of light as a parameter.

FIG. 4Ashows still another example of a conventional variable optical attenuator. This variable optical attenuator is provided with a linear shutter plate21to be placed on the optical path of light emitted from an optical fiber3, and a moving mechanism22of this shutter plate21.

In this conventional variable optical attenuator shown inFIG. 4A, the moving mechanism22moves the shutter plate21in the X directions in the diagram to interrupt the optical path of the light emitted from the optical fiber3. The optical attenuation is controlled according to the amount of light interrupted by this shutter plate21. Here, the relationship between the wavelength of the light propagating through the optical fiber3and the optical attenuation is shown in the graph ofFIG. 4B.

Incidentally, the two types of variable optical attenuators shown inFIGS. 2 and 3Ahave already been put to practical use. The variable optical attenuator shown inFIG. 4Ahas been disclosed in IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 1, January/February 1999, pp. 18–25.

In the variable optical attenuator shown inFIG. 2, however, the provision of an optical attenuation of −30 dB or higher by using the optical absorption film12, given the present technologies, requires that the optical absorption film12be increased in thickness. This involves increasing the width Wx of the optical absorption film12to the order of 1 cm. As a result, there has been a problem of difficult miniaturization of the apparatus because such moving means as a motor for moving the optical absorption film12and the rest becomes indispensable. Furthermore, in the variable optical attenuator provided with the optical absorption film12, the optical absorption film12generates heat when the incident light is high in power. Therefore, there has been another problem that incident light having a power above a certain extent might destroy the optical absorption film12.

In the variable optical attenuator shown inFIG. 3A, the optical attenuation has a great dependence on wavelengths as shown by the characteristic lines a, b, and c inFIG. 3B. Accordingly, there has been a problem that even if a desired optical attenuation is obtained at some wavelengths, the desired optical attenuation cannot be obtained at different wavelengths. Here, the characteristic line a shows the optical attenuations at a wavelength of 1535 nm, the characteristic line b the optical attenuations at a wavelength of 1549 nm, and the characteristic line c the optical attenuations at a wavelength of 1565 nm. In addition, since not-shown polarizers and analyzers are required aside from the electromagnets19, the Faraday rotators16, and the permanent magnets18, there have been problems of complicated configuration and difficult miniaturization of the apparatus.

Furthermore, the variable optical attenuator shown inFIG. 4Ahas also had the problem that the optical attenuation has a great dependence on wavelengths as shown by the characteristic lines a and b inFIG. 4B.

The characteristic line a shows the wavelength dependence of the optical attenuation at varying wavelengths with the shutter plate21fixed to a position where an attenuation of 12.2 dB is obtained at a wavelength of 1500 nm. Here, a comparison between the maximum and minimum optical attenuations in the range of wavelengths of 1500–1600 nm finds a difference of approximately 0.8 dB in optical attenuation.

The characteristic line b shows the wavelength dependence of the optical attenuation at varying wavelengths with the shutter plate21fixed to a position where an attenuation of 13.5 dB is obtained at a wavelength of 1500 nm. Here, a comparison between the maximum and minimum optical attenuations in the range of wavelengths of 1500–1600 nm finds a difference of approximately 1 dB in optical attenuation.

As described above, in the variable optical attenuator shown inFIG. 4A, the wavelength dependence of the optical attenuation becomes more significant as the optical attenuation increases. For obtaining an optical attenuation of −30 dB, the wavelength dependence of the optical attenuation becomes still greater than on the characteristic line b. Thus, practical application has been difficult unless this problem about the wavelength dependence of the optical attenuation is solved.

Additionally, in this configuration, the mode field of the light emitted from the optical fiber3is interrupted by the shutter plate21. Therefore, a point-asymmetric diffraction pattern spreading because of diffraction is formed on the end of an optical fiber at the reception side. This diffraction pattern spreads out beyond the size of the optical-fiber end, and the light to be received near the rim of the optical-fiber end varies in reflectance depending on the direction of polarization. This has caused another problem of the occurrence of a polarization dependency loss.

The problem about the wavelength dependence of the optical attenuation is ascribable to the wavelength dependence of the mode field diameter such that the mode field diameter of the light emitted from the optical fiber3increases as the light propagating through the optical fiber3shifts toward longer wavelengths, and the mode field diameter decreases as the light shifts reversely toward shorter wavelengths.

It was found that the polarization dependency loss occurring in such a variable optical attenuator as shown inFIG. 4Atended to increase with an increase in the amount of light interrupted by the shutter plate21. The reason for this seems that as the amount of light interrupted by the shutter21increases, the polarization dependency loss becomes relatively greater because the mode field shape of the light resulting from the diffraction spreads out with further asymmetry and the light power decreases as well.

No polarization dependency loss will occur, however, when the above-described diffraction pattern spreads out in a point-symmetric fashion about the optical axis. For example, as shown inFIG. 5, light TE-polarized at top and bottom ends is TM-polarized at right and left ends, or reversely, light TM-polarized at the tom and bottom ends is TE-polarized at the right and left ends. Eventually, polarization-based differences in reflectance will cancel out each other vertically and horizontally even near the rim of a diffraction pattern if symmetrical portions lie alike vertically and horizontally. Consequently, there will arise no polarization dependency loss.

In view of this, the present inventor provides a configuration for allowing a change in the optical coupling efficiency between first and second optical parts with the mode field shape of light maintained point-symmetric. Hereinafter, a variable optical attenuator according to an embodiment of the present invention will be described with reference to the drawings.

Initially, description will be given of the configuration of an example of the variable optical attenuator according to the present embodiment.

As shown inFIGS. 6A and 6B, the variable optical attenuator according to the present embodiment has a silicon substrate1as a semiconductor substrate. A recess35is formed in the center of this silicon substrate1. V grooves28and29are formed on the silicon substrate1at both sides longitudinally across this recess35. A first optical fiber3, or a first optical part, is inserted and fixed to the V groove28. A second optical fiber4, or a second optical part to be opposed to the first optical fiber3with a predetermined gap therebetween, is inserted and fixed to the V groove29. The connecting end of the first optical fiber3and the connecting end of the second optical fiber4are opposed to each other with a gap of, e.g., about 750 μm.

Two comb-like actuators7aand7bare arranged on the bottom of the recess35in between the first optical fiber3and the second optical fiber4. Hinges5and6are formed on these two actuators7aand7b, respectively, by using semiconductor microfabrication technologies. A first lens8and a second lens9are formed on these hinges5and6, respectively, also by using semiconductor microfabrication technologies. Thus, the first and second lenses8and9are erected upright on the silicon substrate1via the hinges5and6, etc.

These first and second lenses8and9are arranged with a predetermined gap therebetween in the direction of the optical axes of the first and second optical fibers3and4(the Z direction) to constitute a lens system having an optical axis coincident with those of the first and second optical fibers3and4. That is, the optical axes of the first optical fiber3, the first lens8, the second lens9, and the second optical fiber4are put in agreement with one another. Both the first and second lenses8and9have a focal length of about 40 μm, for example.

The comb-like actuators7aand7bare formed on the recess35of the silicon substrate1by micromachining technologies including a sacrificial layer deposition process and an etching process which are known publicly. In addition, springs23for balancing the forces of the actuators7aand7band voltage applying means (not shown) for applying a desired voltage to the actuators7aand7bare provided. Thereby, lens moving means is constituted of the actuators7aand7b, springs23, and voltage applying means for moving the first and second lenses8and9arranged on the actuators7aand7bvia the hinges5and6. The lens moving means is a kind of micro electro mechanical system (MEMS).

Using the lens moving means, the first and second lens8and9are moved along the optical axes of the optical fibers3and4, so that the coupling efficiency between the first optical fiber3and the second optical fiber4is changed for an adjustment in light power. Specifically, a desired voltage is applied to the actuators7aand7bso that the actuators move back and forth in the Z direction due to electrostatic forces, moving the first lens8and the second lens9along the Z direction. Here, the first lens8and the second lens9are moved in opposite directions by the same distance at the same time.

Incidentally, at initial positions, both the distance between the connecting end of the first optical fiber3and the first lens8and the distance between the connecting end of the second optical fiber4and the second lens9, or the fiber-end-to-lens distances, are 330 μm, for example. The voltage applied to the actuators7aand7bcan be varied up to 100 V. Given that this voltage applied is 100 V, the fiber-end-to-lens distances mentioned above decrease to, e.g., 270 μm each.FIGS. 6A and 6Bshow situations with different fiber-end-to-lens distances.

The variable optical attenuator according to the present embodiment is configured as described above. When a voltage is applied to the actuators7aand7bto operate the lens moving means, both the distance between the connecting end of the first optical fiber3and the first lens8and the distance between the connecting end of the second optical fiber4and the second lens9, i.e., the fiber-end-to-lens distances vary by the same amount. This changes the spot size of the light incident on the optical fiber at the reception side while maintaining the light propagating between the first optical fiber3and the second optical fiber4point-symmetric in mode field shape, with a change in the coupling efficiency between the first optical fiber3and the second optical fiber4.

Consequently, as shown inFIG. 7, the optical attenuation changes with varying fiber-end-to-lens distances. In thisFIG. 7, the characteristic lines a and b show characteristics at wavelengths of 1530 nm and 1580 nm, respectively. As is evident from these characteristic lines a and b ofFIG. 7, an optical attenuation of −30 dB and higher is attained. The changes in optical attenuation due to varying fiber-end-to-lens distances are almost identical at a wavelength of 1530 nm and at a wavelength of 1580 nm. Given an optical attenuation of −30 dB, the difference in optical attenuation between wavelengths of 1530 nm and 1580 nm is as small a value as approximately 0.36 dB or less. That is, the optical attenuation has little wavelength dependence.

For further examination on the wavelength dependence of the optical attenuation, the optical attenuation was measured for changes with varying wavelengths at attenuations around 10, 20, and 30 dB. In result, as shown inFIG. 8, little change was found in the optical attenuation for wavelength variations ranging from 1530 nm to 1580 nm at any of the attenuations around 10 dB (characteristic line a), 20 dB (characteristic line b), and 30 dB (characteristic line c). That is, it was confirmed that the optical attenuation has little wavelength dependence.

In addition, the variable optical attenuator according to the present invention was examined for polarization dependency losses at wavelengths. Specifically, in view of the fact that the variable optical attenuator shown inFIGS. 6A and 6Bhad an insertion loss of 0.3 dB, polarization dependency losses were measured for across the range of wavelengths of 1530 nm and 1580 nm, with optical attenuations falling between the foregoing value of −0.3 dB and a value of −31.2 dB which was obtained when 100 V was applied to the actuators7aand7b. Here, the coupling efficiency between the first optical fiber3and the second optical fiber4could be changed without the diffraction effect due to the shutter plate21since the optical mode field was not interrupted by the shutter plate21as in the conventional variable optical attenuator shown inFIG. 4Aabove. In result, as shown inFIG. 9, the polarization dependency losses in this wavelength range were suppressed to approximately 0.2 dB or less at any of the attenuations around 10 dB (characteristic line a), 20 dB (characteristic line b), and 30 dB (characteristic line c).

As has been described, according to the variable optical attenuator of the present embodiment, the first lens8and the second lens9constituting the lens system having an optical axis coincident with those of the first and second optical fibers3and4are moved by the lens moving means including the actuators7aand7b, in opposite directions along the optical axis by the same amount at the same time. This makes it possible to change the spot size of the light incident on the optical fiber on the reception side while maintaining the light propagating between the first optical fiber3and the second optical fiber4point-symmetric in mode field shape. As a result, it becomes possible to change the coupling efficiency between the first optical fiber3and the second optical fiber4for an adjustment in light power.

In this way, the optical attenuation can be freely controlled to attain an optical attenuation of −30 dB or higher. It is also possible to reduce the wavelength dependence of the optical attenuation significantly as compared to the cases of the conventional variable optical attenuators shown inFIGS. 3A and 4Aabove.

Unlike the conventional variable optical attenuator shown inFIG. 4Aabove in which the optical mode field is interrupted by the shutter plate21, there is no chance of being affected by the diffraction resulting from the shutter plate21, nor any diffraction other than that of spreading out as a Gaussian beam. Therefore, the mode field shape point-symmetric about the optical axis can be maintained in the lens system, with a drastic suppression of the occurrence of polarization dependency losses.

Since the optical absorption film12is not used as in the conventional variable optical attenuator shown inFIG. 2above, it is possible to withstand high optical input power.

The first and second lenses8and9formed by using semiconductor microfabrication technologies are erected upright on the silicon substrate1via the hinges5and6which are also formed by using semiconductor microfabrication technologies. Therefore, it is possible to support the lens system consisting of the first and second lenses8and9with precision and achieve the miniaturization of the apparatus as well.

The lens system consisting of the first and second lenses8and9is moved with electrostatic forces by using the lens moving means including the actuators7aand7b. Therefore, the fiber-end-to-lens distances can be controlled with accuracy, to attain, by extension, optical attenuations of favorable control characteristics. In addition, the use of MEMS for the lens moving means eliminates the need for a motor or other moving means as in the conventional variable optical attenuator shown inFIG. 2above. This can contribute to further miniaturization of the apparatus.

Moreover, the first and second optical fibers3and4are fixed to the V grooves28and29formed in the silicon substrate1, respectively. Thus, the optical axes of these first and second optical fibers3and4can be aligned to each other with facility and with precision.

Consequently, the variable optical attenuator according to the present embodiment can be installed, for example, in an optical amplifier for wavelength division multiplexing transmission with the function of uniformizing multi-wavelength light to a desired power collectively, satisfying all the requirements as a variable optical attenuator for smoothing amplification characteristics.

Incidentally, the present invention is not limited to the embodiment described above, and may be practiced in various forms.

The foregoing embodiment has been configured so that the first and second lenses8and9are moved with electrostatic forces by using the lens moving means including the actuators7aand7b. Nevertheless, microelectromagnets may be formed by micromachining technologies including a magnetic film deposition process and an etching process, so as to constitute an electromagnetic drive system in which the first and second lenses8and9are moved with electromagnetic forces from the microelectromagnets. Even in this case, it is possible as in the foregoing embodiment to control the fiber-end-to-lens distances accurately and, by extension, control the optical attenuation with precision, as well as to contribute to further miniaturization of the variable optical attenuator.

Moreover, the foregoing embodiment has used the lens system consisting of the first and second lenses8and9. Nevertheless, the number of lenses to constitute the lens system is not limited to two, and shall be set as appropriate. For example, a single lens is applicable. Three or more lenses may be used. The size and shape of the lens(es) are not limited in particular, and shall be set as appropriate. Besides, the moving distance of the lens(es) such as the first and second lenses8and9is not limited in particular, and shall be set as appropriate. For example, when the lens system consists of a single lens, the variable optical attenuator can be advantageously simplified in configuration.

Furthermore, the foregoing embodiment has used the first and second optical fibers3and4as the first and second optical parts. Nevertheless, either one of the first and second optical parts may be an optical part other than an optical fiber. Both may be optical parts other than optical fibers.

In the foregoing embodiment, the actuators7aand7bmove the first and second lenses8and9along the respective axes of the first and second optical fibers3and4. Alternatively, the variable optical attenuator of the present invention may include, instead of the actuators7aand7b, actuators40aand40b, shown inFIGS. 10A and 10B, which move the first and second optical fibers3and4along the respective optical axes of the first and second lenses8and9.

In this case, the first and second optical fibers3and4are movably received in respective V grooves28and29cut in the silicon substrate1, and the actuators40aand40bmove the corresponding optical fibers independently or in a coordinated manner along the respective optical axes of the first and second lenses8and9. Where the movements of the first and second optical fibers3and4are coordinated, the optical fibers3and4are simultaneously moved in opposite directions by the same amount.

Like the aforementioned actuators7aand7b, the actuators40aand40bare formed on the silicon substrate1by using semiconductor microfabrication technologies.

Instead of using the actuators40aand40b, linear motors (not shown) may be used to move the first and second optical fibers3and4, respectively.

Also in the case where the first and second optical fibers3and4, instead of the first and second lenses8and9, are moved as in the variable optical attenuator ofFIGS. 10A and 10B, functions and advantages similar to those of the attenuator ofFIGS. 6A and 6Bcan be achieved.