Continuously and smoothly variable optical attenuator

Abstract: A first optical attenuation filter (2) is arranged in a predetermined optical path and attenuates light to be attenuated with a first variable attenuation amount. A second optical attenuation filter (3) is arranged in the predetermined optical path and attenuates, with a second variable attenuation amount, the light which has been transmitted through the first optical attenuation filter. The first and second optical attenuation filters (2, 3) can be driven in the forward and reverse directions and each has an attenuation amount characteristic which changes monotonously in both the forward and reverse directions. An attenuation amount setting section (1) sets a desired attenuation amount to the light to be attenuated. First and second drivers (4A, 5A, 9A, 4B, 5B, 9B) are coupled to drive the first and second optical attenuation filters (2, 3) in the forward and reverse directions, respectively. A controller (13A, 13B) controls driving directions and driving amounts of the first and second optical attenuation filters (2, 3) through the first and second drivers (4A, 5A, 9A, 4B, 5B, 9B) in accordance with the desired attenuation amount so that an algebraic sum of the first and second attenuation amounts finally becomes equal to the desired attenuation amount and a variation in attenuation amount is decreased as a whole, thereby providing a smooth attenuation characteristic.

Description: The present invention generally relates to an optical attenuator apparatus and, more particularly, to an optical attenuator, used in optical communication, for attenuating light incident from a light source by a desired amount and outputting attenuated light so as to prevent optical level variation during attenuation amoung switching. The article in Applied Optics, Vol. 19, No. 14, July, 1980, pages 2408 to 2412 discloses a computer controllable optical attenuator. The apparatus comprises first and second optical attenuator filters in the form of wedged plates arranged in the optical path. The wedged plates are driven by step motors to predetermined angles to effect a predetermined attenuation. Control means are provided for driving pairs of wedges by means of stepper motors geared through anti-backlash worm gears. Fig. 19 shows a general schematic arrangement of another optical attenuator 40 used in optical communication. Schematically, the optical attenuator 40 has incident- and exit-side connectors 42 and 43 provided to two sides of an attenuator body 41, two sets of lenses 45 and 46 for collimating light supplied from a light source 44 to the incident-side connector 42 and guiding parallel light to the exit-side connector 43, and two attenuation filters 49 and 50, arranged on the optical path between the lenses 45 and 46, for attenuating light by a desired amount and outputting the attenuated light to the exit-side connector 43. As the attenuation filters 49 and 50, those shown in Figs. 20 and 21 are conventionally generally used. More particularly, attenuation filters 49 and 50 shown in Fig. 20 have attenuation plates 49a and 50a. In the attenuation plate 49a, a metal film whose transmittance changes continuously in the circumferential direction is formed on a disc-shaped glass substrate in order to obtain continuous attenuation amounts (e.g., 0 to 10 dB). In the attenuation plate 50a, filters 50aa having predetermined different attenuation amounts are provided at several portions of a disc-shaped metal substrate in order to obtain a plurality of fixed attenuation amounts (e.g., 10, 20,..., 50 dB). The attenuation plates 49a and 50a are arranged to partially overlap each other on an optical path along which light is transmitted. Attenuation filters 49 and 50 shown in Fig. 21 have attenuation plates 49b and 50b. In the attenuation plate 49b, a metal film whose transmittance changes continuously along the circumferential direction is formed on a disc-shaped glass substrate to obtain continuous attenuation amounts (e.g., 0 to 10 dB). In the attenuation plate 50b, a plurality of filters 50bb having predetermined fixed attenuation amounts are provided on a disc-shaped glass substrate at predetermined angular intervals to obtain a plurality of fixed attenuation amounts (e.g., 10, 20,..., 50 dB). The attenuation plates 49b and 50b are arranged to partially overlap each other on an optical path along which light is transmitted. In Fig. 19, to attenuate light, supplied from the light source 44 and incident through the incident-side connector 42, by the attenuation filters 49 and 50 described above and output the attenuated light from the exit-side connector 43, the respective attenuation plates 49a, 49b, 50a, and 50b are rotated by drive sections 47 and 48, e.g., motors, to positions to obtain a desired attenuation amount. However, in the optical attenuator 40 using the conventional attenuation filters 49 and 50 shown in Fig. 21, when the attenuation plates 49a and 50a are rotated to vary the attenuation amount, the optical path is blocked by the metal substrate of the attenuation plate 50a, and the attenuation amounts at the blocking portions become infinite, as shown by the attenuation characteristic of Fig. 22. This blocking occurs every time the attenuation plate is rotated to switch the attenuation amount. In the optical attenuator 40 using the conventional attenuation filters 49 and 50 shown in Fig. 21, since the glass substrates are used in both the attenuation plates 49b and 50b, light blocking described above does not occur. However, a level variation occurs at transfer portions of the respective fixed attenuation amounts of the attenuation plate 50b, as shown by the attenuation characteristic shown in Fig. 23. Level variation differs depending on a difference in attenuation amount from the adjacent attenuation plate 49b and occurs inevitably at transfer portions. In the optical attenuator 40 of this type, although not shown, the main shafts of the respective attenuation filters 49 and 50 are coupled to the main shafts of drive sections 47 and 48 each comprising a motor and a angular position detecting potentiometer which outputs a rotational angle of the corresponding attenuation filter as a voltage signal obtained from a change in resistance of a variable resistor. When the drive sections 48 and 48 are operated to rotate the corresponding attenuation filters 49 and 50 by desired amounts and stop, the respective attenuation filters 49 and 50 are stopped by a rotational friction force of the drive sections 48 and 49 within a range of play of the drive sections 48 and 49, and a backlash always occurs. The backlash is caused mainly by the hysteresis of the potentiometer. Fig. 24 shows a relationship between a preset angle (shift amount) and an attenuation amount obtained when the attenuation filter of the optical attenuator having a backlash caused by hysteresis of a potentiometer is continuously rotated in the forward direction, and a relationship between a preset angle and an attenuation amount obtained when the attenuation filter is continuously rotated in the reverse direction. More specifically, an error of about, e.g., 0.05 to 0.2 dB occurs between the attenuation amounts obtained when the attenuation filter is rotated in the forward and reverse directions even when the preset angles are the same, as shown in Fig. 24. In other words, the influence of the backlash appears to exhibit the same attenuation amount even if the shift amounts of the potentiometer are different (D b and D a ). When the two attenuation filters 49 and 50 are controlled to obtain a desired attenuation amount, no problem occurs if the respective attenuation filters 49 and 50 are constantly rotated only in the forward or reverse direction. However, to obtain a desired attenuation amount within a short period of time by eliminating an unnecessary rotation, the attenuation filters 49 and 50 must be rotated in directions closer to target attenuation amounts from current positions, and the forward and reverse rotational directions of the attenuation filters 49 and 50 are controlled in accordance with the target attenuation amounts. Accordingly, even if the target attenuation amounts are the same, the actual attenuation amounts are different depending on the rotational directions of the attenuation filters, and desired attenuation amounts cannot be constantly obtained by the same value. It is, therefore, an object of the present invention to provide a new and improved optical attenuator apparatus in which an instantaneous change in attenuation amount is decreased by removing an influence of backlash so that a level variation does not occur when the attenuation amount is to be changed, thereby obtaining a substantially smooth attenuation characteristic. According to the present invention, an optical attenuator apparatus is provided as defined in claim 1. Embodiments of the apparatus are given in claims 2 to 7. An advantage of the arrangement is that the attenuation amounts of the first and second optical attenuation filters are shared such that their sum becomes a preselected attenuation amount. The attenuation amounts of the first and second optical attenuation filters are set in opposite directions. In this case, the attenuation amount of the first optical attenuation filter is set in a decreasing direction, and that of the second optical attenuation filter is set in an increasing direction. When the attenuation amounts of the first and second optical attenuation filter means are to be changed, they are changed from initial attenuation amounts before change to corresponding target attenuation amounts after change. That is, in the process (final access process) of changing the attenuation amount of the first optical attenuation filter means to an amount immediately before the corresponding constant target attenuation amount, the amount is changed from an amount larger than the target attenuation amount to reach the target attenuation amount. Similarly, in the final access process of the second optical attenuation filter means, the attenuation amount is changed from an amount smaller than the corresponding target attenuation amount to reach the target attenuation amount. The transient change amounts of the two optical attenuation filters from their initial amounts until their final access processes may be set such that their algebraic sum changes monotonically (including invariant flat portions in the change processes). To change an attenuation amount to a desired attenuation amount, the first attenuation filter unit is driven in one direction (e.g., in a direction to increase the attenuation amount) to pass a preset position and thereafter in an opposite direction to return it to the preset position. Along with this operation, the second attenuation filter unit is driven in the same direction as that of the first attenuation filter unit to immediately before a preset position and thereafter in the same direction to the preset position. The desired attenuation amount can be obtained by the sum of the attenuation amounts of the two attenuation filter units. A variation in attenuation amount is gradual in the process of changing the attenuation amount, thereby obtaining a substantially smooth attenuation characteristic. To completely remove the influence of the backlash and the variation in attenuation amount to constantly obtain a smooth attenuation characteristic, it is preferable that movements of the first and second attenuation filter units are controlled so as not exceed the preselected attenuation amount when the attenuation amount is to be increased and so as not to be decreased below the preselected attenuation amount when the attenuation amount is to be decreased, thereby controlling to synchronize the final movements in the opposite directions. This invention can be more fully understood from the following detailed description of embodiments when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing a schematic arrangement of an optical attenuator according to the first embodiment of the present invention; Fig. 2 shows a rotational arrangement of continuous attenuation filters used in the optical attenuator of Fig. 1; Figs. 3A, 3B, and 3C are graphs showing the operations of the continuous attenuation filters when the attenuation amount is increased by the filters shown in Fig. 2; Figs. 4A, 4B, and 4C are graphs showing the operations of the continuous attenuation filters when the attenuation amount is decreased by the filters shown in Fig. 2; Fig. 5 shows a rotational arrangement of continuous attenuation filters used in an optical attenuator according to the second embodiment of the present invention; Figs. 6A, 6B, and 6C are graphs showing the operations of the continuous attenuation filters when the attenuation amount is increased by the filters shown in Fig. 5; Figs. 7A, 7B, and 7C are graphs showing the operations of the continuous attenuation filters when the attenuation amount is decreased by the filters shown in Fig. 5; Figs. 8A, 8B, and 8C are graphs showing the operations of continuous attenuation filters of an optical attenuator according to the third embodiment of the present invention when the attenuation amount is to be increased by removing an amount exceeding a desired attenuation amount; Figs. 9A, 9B, and 9C are graphs showing the operations of continuous attenuation filters of the optical attenuator according to the third embodiment of the present invention when the attenuation amount is to be decreased by removing an amount exceeding a desired attenuation amount; Fig. 10 shows a rotational arrangement of continuous attenuation filters of an optical attenuator according to the fourth embodiment of the present invention; Figs. 11A, 11B, and 11C are graphs showing the operations of the continuous attenuation filters when the attenuation amount is changed by the filters shown in Fig. 10; Fig. 12 is a block diagram of an optical attenuator according to the fifth embodiment of the present invention; Fig. 13 shows an arrangement of continuous attenuation filters of an optical attenuator according to the sixth embodiment of the present invention; Fig. 14 shows an arrangement of continuous attenuation filters of an optical attenuator according to the seventh embodiment of the present invention; Fig. 15 is a view for explaining the principle of the optical attenuator according to the present invention and shows an arrangement of continuous attenuation filters used in the seventh embodiment of the present invention; Figs. 16A, 16B, and 16C are graphs for explaining the principle of the optical attenuator according to the present invention and show the operations of the optical attenuator when the attenuation amount is to be increased by using the continuous attenuation filters of Fig. 15; Fig. 17 shows a panel layout when the optical attenuator according to the present invention is fabricated as a programmable optical attenuator; Figs. 18A, 18B, and 18C are graphs showing an output characteristic at a preset attenuation amount obtained by the optical attenuator according to the present invention and the conventional optical attenuator; Fig. 19 shows a general schematic arrangement of an optical attenuator used in optical communication; Fig. 20 shows an arrangement of a conventional attenuation filter; Fig. 21 shows an arrangement of another conventional attenuation filter; Fig. 22 is a graph showing an attenuation characteristic of the attenuation filter shown in Fig. 20; Fig. 23 is a graph showing an attenuation characteristic of the attenuation filter shown in Fig. 21; and Fig. 24 is a graph showing a relationship between a preset angle and an attenuation amount obtained when an attenuation filter of an optical attenuator having a backlash in a drive system of the attenuation filter is continuously rotated in the forward direction, and a relationship between a preset angle and an attenuation amount obtained when the attenuation filter is continuously rotated in the reverse direction. Reference will now be made in detail to the presently preferred embodiments of the invention as illustrated in the accompanying drawings, in which like reference characters designate like or corresponding parts throughout the several drawings. Fig. 1 is a block diagram showing a schematic arrangement of an optical attenuator according to the first embodiment of the present invention, Fig. 2 shows a rotational arrangement of continuous attenuation filters used in the optical attenuator of Fig. 1, Figs. 3A, 3B, and 3C are graphs showing the operations of the continuous attenuation filters when the attenuation amount is increased by the filters shown in Fig. 2, and Figs. 4A, 4B, and 4C are graphs showing the operations of the continuous attenuation filters when the attenuation amount is decreased by the filters shown in Fig. 2. An optical attenuator 10 according to the first embodiment has, in an attenuator body 10a thereof, a setting section 1, first and second continuous attenuation filters 2 and 3, potentiometers 5A and 5B, a memory section 6, operation control sections 7A and 7B, converting sections 8A and 8B, and drive sections (drive units) 9A and 9B. The setting section 1 sets an attenuation amount to a desired value. The potentiometers 5A and 5B detect rotational angles of motors 4A and 4B to which the continuous attenuation filters 2 and 3, respectively, are mounted. The memory section 6 stores attenuation amount data of the continuous attenuation filters 2 and 3 at two reference wavelengths λ1 and λ2. The operation control sections 7A and 7B such as CPUs read out the corresponding attenuation amount data from the memory section 6 on the basis of a preset value set by the setting section 1, output control signals for designating rotational angles in both forward and reverse directions of the continuous attenuation filters 2 and 3, and calculate wait time required for driving the motors 4A and 4B. The converting sections 8A and 8B such as D/A converters convert the control signals (digital data) from the operation control sections 7A and 7B to analog signals. The drive sections 9A and 9B drive the motors 4A and 4B in predetermined directions on the basis of the signals from the converting sections 8A and 8B. The continuous attenuation filter 2 (3), the motors 4A and 4B, and the potentiometers 5A and 5B constitute first and second continuous attenuation filter units 11 and 12. The memory section 6, the operation control sections 7A and 7B, and the converting sections 8A and 8B constitute control units 13A and 13B. Incident- and exit-side connectors 16 and 17 are arranged on two sides of the attenuator body 10a in order to attenuate light incident from a light source 14 by a predetermined amount and to output the attenuated light to, e.g., an optical power meter 15. Two sets of lenses 18 and 19 for collimating light supplied from the light source 14 to the incident-side conductor 16 and guiding the parallel beam to the exit-side connector 17 are arranged on an optical path connecting the connectors 16 and 17. The two continuous attenuation filters 2 and 3 are arranged in an optical path between the lenses 18 and 19. In each of the continuous attenuation filters 2 and 3, a metal film whose transmittance changes continuously in a circumferential direction is formed on a glass disc so that an attenuation amount changes monotonously in the rotational direction. That is, in the first embodiment, a metal film is formed in each of the continuous attenuation filters 2 and 3 such that the attenuation amount is increased in a counterclockwise direction of the circumference. The continuous attenuation filters 2 and 3 are arranged to partially overlap each other in an optical path along which light is transmitted. The main shafts of the continuous attenuation filters 2 and 3 are coupled to the main shafts of the motors 4A and 4B. The continuous attenuation filters 2 and 3 are controlled independently of each other by the motors 4A and 4B on the basis of the control signals supplied from the operation control sections 7A and 7B. The operation of the optical attenuator having the above arrangement will be described by way of a case in which the attenuation amount is changed from 20 to 22 dB. Assume that the attenuation amount data corresponding to an attenuation amount of 20 dB at a preset wavelength X of the respective continuous attenuation filters 2 and 3 are A0 and B0. When the attenuation amount is set at 22 dB by the setting section 1, the operation control sections 7A and 7B read data A1, A2, B1, and B2 corresponding to 22 dB of the continuous attenuation filters 2 and 3 at the two reference wavelengths λ1 and λ2 from the memory section 6. The data A1, A2, B1, and B2 are written in the memory section 6 in advance in units of continuous attenuation filters 2 and 3. The read data A1, A2, B1, and B2 are subjected to correction calculation by using the two reference wavelengths λ1 and λ2 with respect to the preset wavelength X by the operation control sections 7A and 7B in accordance with the following equation, thereby obtaining attenuation amounts of the continuous attenuation filters 2 and 3: A = A1 - {A1-A2} x-λ1 λ1-λ1 , B = B1 - {B1-B2} x-λ1 λ2-λ1 Subsequently, the operation control sections 7A and 7B calculate data A' and B' of corrected attenuation amounts obtained by adding an overrun amount V to the calculated attenuation amounts, and a wait time S = |D(A0-A') + E| or S = |D(B0-B') + E|, which is necessary for driving the motors 4A and 4B, where D and E are constants. The data A1 and B1 of the attenuation amounts from which the overrun amounts are removed are written in the converting sections 8A and 8B by the operation control sections 7A and 7B. The converting sections 8A and 8B activate the drive sections 9A and 9B based on the stored data A1 and B1. The drive sections 9A and 9B drive the motors 4A and 4B to rotate the continuous attenuation filters 2 and 3 to reach predetermined positions where the wait time S becomes 0. The operations of the continuous attenuation filters subjected to rotational control will be described. To increase the attenuation amount from 20 to 22 dB by the continuous attenuation filters 2 and 3, between time ta and time tc of Fig. 3A, the first continuous attenuation filter 2 (the filter on the left side in Fig. 2) is controlled to overrun in a direction (counterclockwise direction) to increase the attenuation amount from a desired rotational angle (corresponding to an attenuation amount of 11 dB) by a predetermined rotational angle until the attenuation amount is 11.2 dB. Then, between time tc and time td, the continuous attenuation filter 2 is controlled to return in a direction to decrease the attenuation amount by a desired rotational angle, thereby obtaining an attenuation amount of 11 dB. Along with this operation, between time ta and time tb of Fig. 3B, the second continuous attenuation filter 3 (the filter on the right side of Fig. 2) is rotated in a direction (counterclockwise direction) to increase the attenuation amount to reach immediately before a desired rotational angle (corresponding to an attenuation amount of 11 dB), thereby obtaining an attenuation amount of 10.8 dB. Thereafter, between time tb and time tc, the second continuous attenuation filter 3 is stopped to keep the attenuation amount of 10.8 dB. Then, between time tc and time td of Fig. 3B, the second filter 3 is rotated, in synchronism with the return operation of the first continuous attenuation filter 2, in a direction to decrease the attenuation amount by the same returning angle as the filter 2 to reach a desired rotational angle, thereby obtaining an attenuation amount of 11 dB. In this manner, the attenuation amount is increased to 22 dB by the synthesis of the both continuous attenuation filters 2 and 3, as shown in Fig. 3C. As a result, light incident from the light source 14 through the incident-side connector 16 is attenuated to a desired attenuation amount by the continuous attenuation filters 2 and 3 and emerges to the exit-side connector 17. At this time, by the rotating operations of the continuous attenuation filters 2 and 3 in opposite directions in a period A of Figs. 3A to 3C, in a process of changing the attenuation amount, the attenuation amount does not vary instantaneously to remove the influence of the backlash, thereby obtaining a constantly smooth attenuation characteristic. Since the continuous attenuation filters 2 and 3 are constituted by rotary discs, a space required for the movement of the filters and 3 can be minimized. To decrease the attenuation amount from 22 to 20 dB by the continuous attenuation filters 2 and 3 as shown in Fig. 4C, between time ta and time tc of Fig. 4B, the second continuous attenuation filter 3 (the filter on the right side in Fig. 2) is controlled to overrun in a direction (clockwise direction) to decrease the attenuation amount from a desired rotational angle (corresponding to an attenuation amount of 10 dB) by a predetermined rotational angle until the attenuation amount is 9.8 dB. Then, between time tc and time td in Fig. 4B, the continuous attenuation filter 2 is controlled to return in a direction to increase the attenuation amount by a desired rotational angle, thereby obtaining an attenuation amount of 10 dB. Along with this operation, between time ta and time tb of Fig. 4A, the first continuous attenuation filter 2 (the filter on the left side of Fig. 2) is rotated in a direction (clockwise direction) to decrease the attenuation amount to reach immediately before a desired rotational angle (corresponding to an attenuation amount of 10 dB), thereby obtaining an attenuation amount of 10.2 dB. Thereafter, between time tb and time tc of Fig. 4A, the first continuous attenuation filter 2 is stopped to keep the attenuation amount of 10.2 dB. Then, between time tc and time td of Fig. 4A, the filter 2 is rotated, in synchronism with the return operation of the second continuous attenuation filter 3, in a direction to increase the attenuation amount by the same returning angle as the filter 3 to reach a desired rotational angle, thereby obtaining an attenuation amount of 10 dB. In the operation described above, the continuous attenuation filters 2 and 3 are controlled at the same speed. Therefore, one of the filters which does not overrun (corresponding to, e.g., the filter 3 when the attenuation amount is to be increased) is stopped at a position of a predetermined attenuation amount while the other filter overruns (corresponding to the time tc). However, the wait time generated by the stop may be eliminated, as indicated by an alternate long and short dashed line in Figs. 3B and 3C, and the filter which does not overrun may be controlled to rotate. Fig. 5 shows a rotational arrangement of continuous attenuation filters used in an optical attenuator according to the second embodiment of the present invention, Figs. 6A, 6B, and 6C are graphs showing the operations of the continuous attenuation filters when the attenuation amount is increased by the filters shown in Fig. 5, and Figs. 7A, 7B, and 7C are graphs showing the operations of the continuous attenuation filters when the attenuation amount is decreased by the filters shown in Fig. 5. In the optical attenuator of the second embodiment, although the operation of continuous attenuation filters 2 and 3 is different from that of the first embodiment, the arrangement of the filters 2 and 3 and other components is the same as that in the first embodiment. More specifically, to increase the attenuation amount from 20 to 22 dB by the continuous attenuation filters 2 and 3, between time ta and time tc of Fig. 6A, a first continuous attenuation filter 2 (the filter on the left side in Fig. 5) is controlled to overrun in a direction (counterclockwise direction) to increase the attenuation amount from a desired rotational angle (corresponding to an attenuation amount of 11 dB) by a predetermined rotational angle until the attenuation amount is 11.2 dB. Then, between time tc and time td of Fig. 6A, the continuous attenuation filter 2 is controlled to return in a direction to decrease the attenuation amount by a desired rotational angle, thereby obtaining an attenuation amount of 11 dB. Along with this operation, between time ta and time tb of Fig. 6B, the second continuous attenuation filter 3 (the filter on the right side of Fig. 5) is rotated in a direction (counterclockwise direction) to increase the attenuation amount to a desired rotational angle (corresponding to an attenuation amount of 11 dB), thereby obtaining an attenuation amount of 11 dB. Thereafter, between time tb and time tc of Fig. 6B, the second continuous attenuation filter 3 is returned in a direction (clockwise direction) to decrease the attenuation amount by a predetermined rotational angle, thus obtaining an attenuation amount of 10.8 dB. Then, between time tc and time td of Fig. 6B, the second filter 3 is rotated, in synchronism with the return operation of the first continuous attenuation filter 2, in a direction to increase the attenuation amount by the same returning angle as the filter 2 to reach a desired rotational angle, thereby obtaining an attenuation amount of 11 dB. As a result, the attenuation amount is increased to 22 dB by the synthesis of the both continuous attenuation filters 2 and 3, as shown in Fig. 6C. At this time, the influence of the backlash is removed by the rotating operations of the continuous attenuation filters 2 and 3 in opposite directions in a period A of Figs. 6A to 6C. To decrease the attenuation amount from 22 to 20 dB by the continuous attenuation filters 2 and 3 as shown in Fig. 7C, between time ta and time tc of Fig. 7B, the second continuous attenuation filter 3 (the filter on the right side in Fig. 5) is controlled to overrun in a direction (clockwise direction) to decrease the attenuation amount from a desired rotational angle (corresponding to an attenuation amount of 10 dB) by a predetermined rotational angle until the attenuation amount is 9.8 dB. Then, between time tc and time td in Fig. 7B, the continuous attenuation filter 2 is controlled to return in a direction to increase the attenuation amount by a desired rotational angle, thereby obtaining an attenuation amount of 10 dB. Along with this operation, between time ta and time tb of Fig. 7A, the first continuous attenuation filter 2 (the filter on the left side of Fig. 5) is rotated in a direction (clockwise direction) to decrease the attenuation amount to reach a desired rotational angle (corresponding to an attenuation amount of 10 dB), thereby obtaining an attenuation amount of 10 dB. Thereafter, between time tb and time tc of Fig. 7A, the first continuous attenuation filter 2 is returned in a direction (clockwise direction) to increase the attenuation amount by a predetermined rotational angle, thereby obtaining an attenuation amount of 10.2 dB. Then, between time tc and time td of Fig. 7A, the filter 2 is rotated, in synchronism with the return operation of the second continuous attenuation filter 3, in a direction to increase the attenuation amount by the same returning angle as the filter 3 to reach a desired rotational angle, thereby obtaining an attenuation amount of 10 dB. In the operation described above, during the process of obtaining a desired attenuation amount, the attenuation amount partially slightly exceeds the desired attenuation amount. However, the excessive amount is very small and does not directly influence the attenuation characteristic. Figs. 8A, 8B, and 8C and Figs. 9A, 9B, and 9C are graphs for showing the operations of continuous attenuation filters of an optical attenuator according to the third embodiment of the present invention when the attenuation amount is to be increased and decreased, respectively, by removing an amount exceeding a desired attenuation amount. More specifically, in the third embodiment, to increase the attenuation amount, before time tb in Fig. 8A, when a first continuous attenuation filter 2 which overruns has an attenuation amount exceeding an attenuation amount of 11 dB, a second continuous attenuation filter 3 is rotated to reach an angle where the attenuation amount is 11 dB. Similarly, to decrease the attenuation amount, before time tb in Fig. 9A, when the second continuous attenuation filter 3 has an attenuation amount lower than an attenuation amount of 10 dB, the first continuous attenuation filter 2 is rotated to reach an angle where the attenuation amount is 10 dB. In this manner, in a process of changing the attenuation amount to a desired value, the attenuation amount can be changed without exceeding a desired value while the influence of backlash is removed by the rotating operations of the continuous attenuation filters 2 and 3 in opposite directions in a period A of Figs. 8A to 8C and Figs. 9A to 9C. Fig. 10 shows a rotational arrangement of continuous attenuation filters of an optical attenuator according to the fourth embodiment of the present invention, and Figs. 11A, 11B, and 11C are graphs showing the operations of the continuous attenuation filters when the attenuation amount is changed by the filters shown in Fig. 10. In the optical attenuator of the fourth embodiment, although the operation of continuous attenuation filters 2 and 3 is different from that of the first embodiment, the arrangement of the filters 2 and 3 and other components is the same as that in the first embodiment. In the fourth embodiment, to change the attenuation amount, during a period A (between time tc and time td) of Figs. 11A to 11C, the first continuous attenuation filter 2 is controlled to overrun in a direction (counterclockwise direction) to increase the attenuation amount from a rotational angle corresponding to a desired attenuation amount by a predetermined rotational angle, and is then returned in a direction to decrease the attenuation amount. The filter 2 is rotated by a desired rotational angle in this manner. Along with this operation, the second continuous attenuation filter 3 is controlled to rotate in a direction (clockwise direction) to decrease the attenuation amount from a rotational angle corresponding to a desired attenuation amount by a predetermined rotational angle, that is, to rotate in an opposite direction to that of the first continuous attenuation filter 2 and thus to overrun, and is returned in a direction to increase the attenuation amount. The filter 3 is rotated by a desired rotational angle in this manner. More specifically, to decrease the attenuation amount, in the period A, the motor 4 drives to rotate the continuous attenuation filters 2 and 3 such that the attenuation amounts change in opposite directions, thus rotating the filters 2 and 3 to positions to obtain a desired attenuation amount. This embodiment is practiced when a difference between an actual attenuation amount and a desired set attenuation amount falls within a range of an overrun operation. Therefore, in the fourth embodiment, similar to the first and second embodiments, in the process of changing the attenuation amount by rotating the continuous attenuation filters 2 and 3 in opposite directions in the period A, the attenuation amount does not change instantaneously and the influence of the backlash is thus eliminated, thereby constantly obtaining a smooth attenuation characteristic. The fourth embodiment exemplifies a case in which the first and second continuous attenuation filters 2 and 3 are controlled to increase and decrease, respectively, the attenuation amount. The same effect can be obtained even if the rotational directions of the first and second continuous attenuation filters 2 and 3 are reversed. Fig. 12 is a block diagram of an optical attenuator according to the fifth embodiment of the present invention. An optical attenuator 20 according to the fifth embodiment is different from the optical attenuator 10 of the first embodiment described above in the arrangements of control units 29A and 29B having a memory section 26, operation control sections 27A and 27B, and converting sections 28A and 28B, respectively. More specifically, the memory section 26 stores rotational angle data (indicating a preset shift amount) corresponding to attenuation amounts in units of rotational directions (forward and reverse rotations), as shown in Fig. 24. This data is measured in advance in units of the continuous attenuation filters 2 and 3 to be used by actually operating the filters 2 and 3. The operation control sections 27A and 27B have discriminating sections 27a1 and 27a2 and control sections 27b1 and 27b2. The discriminating sections 27a1 and 27a2 discriminate increases/decreases in attenuation amounts of the continuous attenuation filters 2 and 3, respectively, by comparing a preset value of the setting section 1 and a current attenuation amount. The control sections 27b1 and 27b2 read corresponding rotational angle data from the memory section 26 based on the discrimination results and output control signals to the converting means 27A and 27B, respectively. The continuous attenuation filters 2 and 3 are rotated by driving motors 4A and 4B in accordance with signals supplied from the converting sections 28A and 28B based on the control signals. For example, assume that the setting section 1 sets to increase the attenuation amount of the first continuous attenuation filter 2 by 1 dB and to decrease the attenuation amount of the second continuous attenuation filter 3 by 1 dB. Then, the operation control sections 27A and 27B read rotational angle data Da corresponding to 1 dB of an increase, indicated by a line A in Fig. 24, in attenuation amount of the first continuous attenuation filter 2 which increases the attenuation amount, and rotational angle data Db corresponding to 1 dB of a decrease, indicated by a line B in Fig. 24, in attenuation amount of the second continuous attenuation filter 3 which decreases the attenuation amount. The continuous attenuation filters 2 and 3 are rotated by the motors 4A and 4B in accordance with the readout rotational angle data Da and Db, respectively, to reach angular positions corresponding to the rotational angle data. In the process of rotational control for the continuous attenuation filters 2 and 3 described above, the preset wavelengths are subjected to correction calculation by using two reference wavelengths in the same manner as in the first embodiment. In the fifth embodiment described above, the memory section 26 stores the rotational angle data (refer to the lines A and B of Fig. 24) corresponding to two different attenuation amounts for cases in which the attenuation amount is increased/decreased by rotating the motors 4A and 4B in the forward/reverse directions. Fig. 13 shows an arrangement of continuous attenuation filters of an optical attenuator according to the sixth embodiment of the present invention. The optical attenuator according to the sixth embodiment has three continuous attenuation filters 30, 31, and 32. The two continuous attenuation filters 30 and 31 are controlled as a pair in order to widen a variable range of the attenuation amount. The pair of two continuous attenuation filters 30 and 31 and the remaining continuous attenuation filter 32 are controlled so that their ratios of rotational angles coincide with each other. More specifically, in the period A of Figs. 3A to 3C, when the pair of two filters 30 and 31 and one filter 32 are controlled to rotate in directions of opposite attenuation amounts, if the pair of two filters 30 and 31 are rotated in units of 1° in a direction to increase the attenuation amount, the remaining filter 32 is rotated in units of 2° in a direction to decrease the attenuation amount. Of the pair of two continuous attenuation filters 30 and 31, one filter 30 (31) may not be operated but stopped. Fig. 14 shows an arrangement of continuous attenuation filters of an optical attenuator according to the seventh embodiment of the present invention. In the embodiments described above, the respective disc-shaped continuous attenuation filters are controlled to rotate. In contrast to them, in the optical attenuator according to the seventh embodiment of the present invention, two elongated plate-like continuous attenuation filters 33 and 34 are linearly moved to be parallel to each other by racks 35 and 36, pinions 37 and 38, motors 39A and 39B, and a drive control section 40 so as to partially overlap each other. A metal film is formed on the surface of each of the continuous attenuation filters 33 and 34 whose attenuation amount monotonically changes along the moving direction. In the seventh embodiment, although the continuous attenuation filters 33 and 34 need a space in the longitudinal direction, their widthwise space can be greatly decreased when compared to the rotating disc-shaped continuous attenuation filters. Therefore, the types of filters can be efficiently selected in accordance with the internal space of the apparatus body. The drive control section 40 controls the shift amounts of the respective filters 33 and 34 as in the embodiments described above. Fig. 15 and Figs. 16A, 16B, and 16C are a view and graphs for explaining the principle of the optical attenuator according to the present invention, in which Fig. 15 shows an arrangement of continuous attenuation filters used in the first to sixth embodiments of the present invention, and Figs. 16A, 16B, and 16C are graphs for showing the operations of the optical attenuator when the attenuation amount is to be increased by using the continuous attenuation filters of Fig. 15. As is apparent from Figs. 16A to 16C, when the continuous attenuation filters are merely caused to overrun in a direction to increase the attenuation amount and are then returned to obtain preset attenuation amounts, the attenuation amount changes instantaneously in the process of changing the attenuation amount to a desired level. Hence, in the optical attenuator according to any of the embodiments described above, this instantaneous change in attenuation amount is decreased to obtain a substantially smooth attenuation characteristic. As has been described in the first to seventh embodiments, according to the optical attenuators of the present invention, in the process of changing the attenuation amount to a desired level, the influence of the backlash can be removed, and an instantaneous change in attenuation amount is decreased, thereby obtaining a substantially smooth attenuation characteristic. Also, according to the optical attenuator of the present invention, movements of the first and second attenuation filter units are controlled so as not to exceed a desired attenuation amount in the process if the attenuation amount is to be increased, and so as not to decrease the attenuation amount below a desired attenuation amount in the process if the attenuation amount is to be decreased. Therefore, an instantaneous change in attenuation amount can be removed in the process of changing the attenuation amount to the desired level. According to the optical attenuator of the present invention, the final movements of the respective attenuation filter units in opposite directions are controlled to synchronize. Therefore, when the attenuation amount is to be decreased, the space required for the movements of the respective filters can be minimized. Thus, in the process of changing the attenuation amount to a desired level, the influence of the backlash can be completely removed and an instantaneous change in attenuation amount is removed, thereby constantly obtaining a smooth attenuation characteristic. According to the optical attenuator of the present invention, the widthwise space of the attenuation plate constituting the attenuation filter unit can be minimized, and either the plate-like attenuation filter unit or the disc-shaped attenuation filter unit can be efficiently selected in accordance with the internal space of the apparatus body. According to the optical attenuator of the present invention, since both or either one of the first and second attenuation filter units comprises two or more filter members, the variable range of the attenuation amount to be changed can be increased. According to the optical attenuator of the present invention, an attenuation filter unit for removing the influence of the backlash need not be driven, and an instantaneous change in attenuation amount does not occur in the process of changing the attenuation amount to a desired level, and a smooth attenuation characteristic can be obtained. Therefore, the attenuation amount can be quickly changed to the desired level. Fig. 17 shows a panel layout when the optical attenuator according to the present invention is fabricated as a programmable optical attenuator. Referring to Fig. 17, 201: attenuation amount/a wavelength/offset setting dial 202: attenuation amount/wavelength/offset digit setting keys When the key 202 is depressed, a lamp corresponding to it under a display is turned on. 203: attenuation amount setting key (ATTENUATION) 204: attenuation amount relative display key (RELATIVE) The relative and attenuation amount setting keys are depressed simultaneously to display a reference preset value of the relative value. 205: offset setting key (OFFSET) Capable of changing an offset value within a range of ±99.99 dB. Capable of directly reading an output power from the optical attenuator by inputting a light source output value and a loss value of the optical attenuator. 206: wavelength setting key (WAVELENGTH) Capable of switching the wavelength between 1310 and 1551 nm upon key depression. Capable of setting in units of 1 mm by a rotary encoder. 207: step width setting key (STEP) Capable of setting a multiple of 1, 2, and 5 when an attenuation amount, a wavelength, or an offset value is to be changed. 208: internal shutter open/close key (SHUTTER) 209: attenuation amount/wavelength/offset value display 210: optical input/output sections (INPUT, OUTPUT) 211: optical monitor output section (MONITOR OUTPUT) 212: GP-OB remote cancel key Figs. 18A, 18B, and 18C are graphs showing an output characteristic at a preset attenuation amount obtained by the optical attenuator according to the present invention and the conventional optical attenuator. More specifically, the output characteristic at a preset attenuation amount shown in Fig. 18A of the optical attenuator according to the present invention indicates that, according to the present invention, an optical level can be smoothly set and output without accompanying a short break, i.e., with no short break when compared to those obtained by the conventional optical attenuator. The same effect can be obtained by the optical attenuators according to the first to seventh embodiments described above. Additional embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the present invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope of the present invention being indicated by the following claims.

Claims: An optical attenuator apparatus comprising: incident means (16, 18) for guiding light to be attenuated to a predetermined optical path; first optical attenuation filter means (2), arranged in the optical path, for attenuating the light to be attenuated by a first variable attenuation amount, said first optical attenuation filter means (2) being capable of being driven in forward and reverse directions and having an attenuation characteristic which changes monotonically in both forward and reverse directions; second optical attenuation filter means (3), arranged in the optical path, for attenuating by a second variable attenuation amount, the light which has been transmitted through said first optical attenuation filter means (2), said second optical attenuation filter means (3) being capable of being driven in the forward and reverse directions and having an attenuation characteristic which changes monotonically in both forward and reverse directions; optical output means (17, 19) arranged in the optical path for causing light after transmission through said first and second optical attenuation filter means (2, 3) to emerge having an algebraic sum of the first and second variable attenuation amounts; attenuation amount setting means (1) for setting a preselected attenuation amount of the incident light to be attenuated; first drive means (4A, 5A, 9A) coupled to drive said first optical attenuation filter means (2) in the forward and reverse directions; second drive means (4B, 5B, 9B) coupled to drive said second optical attenuation filter means (3) in the forward and reverse directions; and control means (13A, 13B) for controlling driving directions and driving amounts of said first and second optical attenuation filter means (2, 3) through said first and second drive means (4A, 5A, 9A, 4B, 5B, 9B) in accordance with the preselected attenuation amount set by said attenuation amount setting means (1) so that the algebraic sum of the first and second attenuation amounts finally becomes equal to the preselected attenuation amount; characterized in that said control means (13A, 13B) includes means responsive to said preselected attenuation amount for controlling said first and second drive means (4A, 5A, 9A, 4B, 5B, 9B) by driving said first optical attenuation filter means (2) firstly in one direction and thereafter secondly in an opposite direction by said first drive means (4A, 5A, 9A) and by driving said second optical attenuation filter means (3) firstly in one specific direction and thereafter secondly in the same or opposite direction by said second drive means (4B, 5B, 9B), thereby providing the preselected attenuation amount; and said control means (13A, 13B) are constructed such that said second driving of the first and second drive means compensates for overrun and backlash of the first and second drive means. A apparatus according to claim 1, wherein said control means (13A, 13B) drive said second attenuation filter means (3) in said one specific direction, then in a direction opposite to that of said first attenuation filter means (2), and thereafter again in said one specific direction by said second drive means (4B, 5B, 9B), thereby providing the preselected attenuation amount. An apparatus according to claim 1 or 2, wherein said control means (13A, 13B) includes means for controlling said first and second attenuation filter means (2, 3) such that the attenuation amount will not exceed the preselected amount when the attenuation amount is to be increased, and such that the attenuation amount will not be decreased below the preselected amount when the attenuation amount is to be decreased. An apparatus according to any one of the claims 1 to 3, wherein each of said first and second optical attenuation filter means (2, 3) comprises a rotary disc formed by a metal film on a transparent substrate such that the transmittance thereof changes continuously, and the control means (13A, 13B) are adapted to control the final driving operations of the filter means (2, 3) in opposite directions in synchronization. An apparatus according to any one of claims 1 to 3, wherein each of said first and second optical attenuation filter means (2, 3) comprises a linearly movable plate formed by a metal film on a transparent substrate such that the transmittance thereof changes continuously. An apparatus according to any one of claims 1 to 3, wherein at least one of said first and second optical attenuation filter means (2, 3) includes a plurality of optical attenuation filter members. An apparatus according to any one of claims 1 to 3, wherein said control means (13A, 13B) includes a memory section (26) for storing in advance data indicating a relationship between an attenuation amount and a shift amount of said first and second optical attenuation filter means (2, 3) and attenuation amount increasing and decreasing directions in units of said first and second optical attenuation filter means, a discriminating section (27al, 27a2) for discriminating whether a preselected attenuation amount is in the increasing or decreasing direction from a current attenuation amount, and a control section (27bl, 27b2) for controlling movements of said first and second optical attenuation filter means (2, 3) based on a moving direction of said first and second optical attenuation filter means (2, 3) discriminated by said discriminating section, the attenuation amount stored in said memory section (26), and shift amount data of said first and second optical attenuation filter means (2, 3).

Inventors: ITO FUMINOBU, KOMAZAWA HIROSHI, SASAKI YOSHIHARU, ITO, FUMINOBU, KOMAZAWA, HIROSHI, SASAKI, YOSHIHARU
Applicants: ANRITSU CORP, ANRITSU CORPORATION
Application year: 1992
Main classification: G02B26
IPC codes: G02B26