Disk shaped tunable optical filter

Abstract: A disk shaped tunable optical filter is formed by a transparent substrate in a disk shape, and a filter layer formed on a top surface of the substrate, having such a filter characteristic that a transmission central wavelength varies along a circumferential direction as a monotonic function of a viewing angle. This disk shaped tunable optical filter can be advatageously utilized in realizing a wavelength discrimination apparatus, a reference wavelength light generation apparatus, an optical packet generation apparatus, and a tunable optical filter module, which are also disclosed.

Description: The present invention relates to an optical filter and is applicable to wavelength discrimination apparatus for varying transmission central wavelength at high speed in an optical communication system, to a reference wavelength light generation apparatus for generating a single mode continuous wave light of a reference wavelength based on reference wavelength grids in a high density wavelength division multiplexing network system, and to an optical packet generation apparatus for generating a time series of monochromatic pulse lights of respective wavelengths over a wide bandwidth in an optical network system that utilizes lights of different wavelengths for different channels. US-A-3 442 572 describes a disk shaped substrate with a multi layer coating having a thickness which changes linearly along a line concentric with the axis of rotation. US-A-5 046 162 describes a disk shaped filter, the surface of which is partitioned into three areas assigned to red, green and blue filters. Another disk shaped filter is described in WO 96/41218. A wavelength division multiplexing type transmission system has been proposed for transmitting a large capacity of signals using lights of different wavelengths. In such a system, it is said that there is a need for a technique to discriminate and monitor each wavelength. A typical conventional technique for wavelength discrimination has been one that utilizes reflection by a grating mirror, which is capable of discriminating each wavelength at high resolution (the minimum resolution currently available being about 0.1 nm) depending on an incident angle with respect to the grating, so that it is widely used for measurement as an optical spectrum analyzer. Also, as a wavelength discrimination apparatus with even higher resolution, there is an apparatus using the Michelson interferometer which is commercially available. These two conventional apparatuses are suitable for a wavelength discrimination apparatus for measurement purpose, but they have sizes that are too large for their use as a component having a function for discriminating and monitoring wavelengths in a system, and there has been a need for a wavelength discrimination apparatus in a more compact size. As a scheme that can meet this requirement. there has been a proposition of a scheme (rotary tunable optical filter) in which transmission central wavelength can be made tunable by rotating a dielectric multi-layer filter. However, this scheme has drawbacks in that the polarization dependency of the transmission efficiency becomes stronger as the incident angle becomes larger and the operable wavelength range is limited by the incident angles. In order to resolve these drawbacks, there has been a proposition of a linear optical filter in which the central wavelength varies along a straight line. When this linear optical filter is used, the central wavelength can be selected according to a position at which the optical beam passes. In addition, in this linear optical filter, there is no change in the incident angle even when the transmission wavelength is changed so that there is hardly any polarization dependency. Also, in these optical filters, in order to realize the high speed wavelength variation, there is a need to provide a high speed driving mechanism. In this regard, the rotation mechanism basically has a high speed characteristic, but the rotary tunable optical filter requires a rotation of a disk plate so that it is difficult to realize a balance in rotation so that the rotation speed is severely limited. On the other hand, in the case of the linear optical filter, there is a need for a large amplitude high speed shuttle motion mechanism in order to realize a wide wavelength variation capability at the same time. This requirement quantitatively amounts to the acceleration of 4 × 10 4 m/s 2 in the case of realizing the shuttle motion with an amplitude of 10 cm and a frequency of 100 Hz, for example, and the realization of this acceleration by the currently available technology would call for a gigantic actuator. Now, in the wavelength division multiplexing network system, it is important to stabilize a reference wavelength light source with respect to reference wavelength grids calibrated to absolute wavelengths. Conventionally, a Distributed Feed-Back Laser Diode (DFB-LD) capable of realizing a wavelength controllable single mode oscillation has been proposed as the reference wavelength light source, and a method utilizing a monochrometer as shown in Fig. 1 has been proposed as a method for stabilizing the reference wavelength of this DFB-LD. Fig. 1 shows a conventional monochrometer in which a desired spectrum is obtained at a diffraction grating 4 on a rotation stage 8 from a light source in a form of a photo-diode (PD) 1 through a slit 2 and a mirror 3, and a collimated beams are obtained by a mirror 5, a slit 6 and a lens 7 and outputted to an optical fiber 9. Namely, the monochrometer has a wavelength discrimination function of a high absolute precision, where the reference wavelength light can be obtained by setting the monochrometer to the specified wavelength and controlling the laser oscillation wavelength such that the intensity of a laser beam to be discriminated becomes maximum. However, in the apparatus for stabilizing the reference wavelength using the monochrometer as shown in Fig. 1 requires an optical system with a long optical beam path because its wavelength discrimination function utilizes the wavelength dependency of the diffraction angle by the grating, so that the apparatus becomes large. In addition, in order to secure the wavelength stabilization, there is a need to make the optical system mechanically strong, and for this reason it is necessary to provide a frame with a very high rigidity. Also, at a time of controlling the oscillation wavelength, it is customary to take a difference between two wavelengths and provide a feed-back so as to nullify this difference, but it is difficult to take the difference between peaks of the spectrum so that the oscillation wavelength is usually modulated at a low frequency (5 to 10 KHz) in a vicinity of the specified wavelength. However, this modulation will be superposed a a intensity modulation component and this intensity modulation may cause some trouble due to noise at a time of clack extraction or the like in the transmission system. Note that, by rotating the diffraction grating at high speed in the monochrometer described above, it is possible to construct a sample servo type wavelength control apparatus for generating good continuous monochromatic lights without wavelength modulation, but such an apparatus has a problem in that the sample servo may not function properly as there arises a limit in making the sampling period smaller because the diffraction grating is associated with a severe limitation on the number of rotations unlike the case of disk. Furthermore, the diffraction grating monochrometer has been widely used for the purpose of carrying out the wavelength spectrum analysis which is the basic technique of the optical measurement. In this case, the specific wavelength is discriminated by diffracting only a wavelength that satisfies the Bragg condition, and the wavelength to be discriminated is selected by rotating the diffraction grating. Here, a correspondence between the wavelength and the rotation angle is defined uniquely so that the high precision measurement becomes possible by accurately controlling the rotation angle. However, it requires a long optical beam path in order to obtain the high resolution and therefore it is difficult to form a module suitable for incorporation into a system, so that its applicability has been limited. Now, the conventional optical packet generation apparatus generates optical packets using a wavelength switch based on AOTF (Acousto-Optic Tunable Filter). Note that, throughout the present specification, the optical packets refers to time series monochromatic pulse lights with wavelengths differing at a constant time interval. This AOTF operates as a narrow bandwidth wavelength selection switch by utilizing surface elastic waves on an electro-optic crystal (such as LiNbO 3 ) as diffraction grating, and is capable of switching wavelength at high speed. The switching speed is as fast as 10 µs or less because it is the electric switching. For example, it has been demonstrated that the switching from 1560 nm to 1552 nm in 6 µs is possible by optimizing the control system of AOTF (see M. Misono et al., "High-speed wavelength switching and stabilization of an acoutooptic tunable filter for WDM network in broadcasting stations", IEEE Photonics Technol. Lett., Vol. 4, pp. 572-574, 1996; and H. Hermann et al., "Low-Loss Tunable Integrated Acoustooptical Wavelength Filter in LiNbO 3 with Strong Sidelobe Suppression", IEEE Photonics Technology Letters, Vol. 10, No. 1, pp. 120-122, January 1998). The wavelength selection function of AOTF utilizes the wavelength dependency of the diffraction grating. For this reason, there arises a limit to a wavelength at which the diffracted light can be coupled to the optical fiber again, so that the operable wavelength bandwidth is limited. This range is currently about 10 nm. On the other hand, the large scale network system containing a CATV broadcast system or the like requires supply of optical packets by switching wavelengths over a wide bandwidth. The actual required wavelength bandwidth depends on the individual system but over 100 nm is desired in general. Hence the bandwidth of the currently available AOTF has been insufficient for this purpose. According to the present invention there is provided a disk shaped optical filter as defined in claim 1. Preferred embodiments of the invention are defined in the dependent claims. Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic diagram showing a configuration of a conventional monochrometer; Fig. 2 is a diagram showing a basic configuration of a high speed wavelength discrimination apparatus not according to the present invention; Fig. 3 is a block diagram showing a specific configuration of a first high speed wavelength discrimination apparatus not according to the present invention; Fig. 4A is a diagram showing a timing detection mark provided on a disk shaped optical filter in the first high speed wavelength discrimination apparatus of Fig. 3; Fig. 4B is a timing chart showing a procedure of the start timing detection according to the timing detection mark shown in Fig. 4A; Fig. 5 is a block diagram showing a specific configuration of a second high speed wavelength discrimination apparatus not according to the present invention; Fig. 6 is a timing chart showing a procedure of the calibration based on reference wavelength grids used in the second high speed wavelength discrimination apparatus of Fig. 5; Fig. 7 is a block diagram showing a specific configuration of a third high speed wavelength discrimination apparatus according to the first embodiment of the present invention; Fig. 8 is a timing chart showing start timing signals and wobble signals used in the third high speed wavelength discrimination apparatus of Fig. 7. A disk shaped optical filter may comprise: a transparent substrate in a disk shape; and a filter layer formed on a top surface of the substrate, having such a filter characteristic that a transmission central wavelength varies along a circumferential direction as a monotonic function of the rotation angle; characterized in that the filter layer has wobble marks formed thereon which are to be used in track controlling when the disk shaped optical filter is set in rotation. The filter layer may have a filter characteristic in which the transmission central wavelength linearly increases or decreases with respect to the rotation angle. The filter layer may include a plurality of layers with mutually different filter characteristics, which are formed concentrically with respect to the axis of rotation of the disk shaped optical filter or the filter layer may be divided into a plurality of fan-shaped regions with mutually different transmission central wavelengths. The filter layer may have a mark formed thereon, which is to be used in detecting a start timing of a filter region provided by the filter layer when the disk shaped optical filter is set in rotation. The filter layer may have a plurality of marks formed thereon along radial directions at prescribed angular intervals between adjacent marks with respect to the axis of rotation of the disk-shaped optical filter, which are to be used in controlling the number of rotations of the disk shaped optical filter when the disk shaped optical filter is set in rotation. The substrate may be made of quartz, the filter layer may be made of dielectric multi-layer, and the substrate may have a wide bandwidth antireflection coating applied on a bottom surface. Referring now to Fig. 2 to Fig. 8, the first embodiment of the present invention will be described in detail. This first embodiment is directed to a high speed wavelength discrimination apparatus according to the present invention. Fig. 2 shows a basic configuration of the high speed wavelength discrimination apparatus which comprises a disk shaped optical filter 12 having such a filter characteristic that the transmission central wavelength varies along a circumferential direction, which is positioned on an optical beam path 11 of collimated lights, and a variable rotation mechanism 13 for rotating the optical filter 12 at high speed in synchronization with external signals. A disk rotation mechanism is adopted for the variable rotation mechanism 13 in order to eliminate the mechanical limitation on the maximum number of rotations. Note that, in Fig. 2, the disk shaped optical filter 12 is arranged to be slightly inclined from a direction perpendicular to an optical axis of the optical beam path 11 (a vertical direction of the optical axis), but the present invention is not necessarily limited to this arrangement, and it is possible to arrange the disk shaped optical filter 12 along the direction perpendicular to the optical axis (the vertical direction of the optical axis). When the disk shaped optical filter 12 is arranged to be slightly inclined from the vertical direction of the optical axis as shown in Fig. 2, there is an advantage that an incident light will not be directly reflected back to the incident side. In such a case, it is preferable to set this inclination to be about several radians in view of a relationship with the filter characteristic. Note that the high speed rotations of a disk are already demonstrated by the optical disk device and the hard disk device, and about 7000 rpm (117 Hz) is currently realizable in practice. Furthermore, unlike the existing disk devices which are subjected to a limitation due to memory capacity, the optical filter 12 of the present invention has essentially no limitation on a disk diameter so that it is also possible to further pursue the conditions for realizing high speed rotations. The disk shaped optical filter 12 comprises a substrate 14 in a form of a quartz disk, and a filter layer 15 with transmission central wavelengths varying along a circumferential direction which is provided on the substrate 14, in order to realize the wavelength discrimination. The filter layer 15 is formed by the usual dielectric multi-layer. The wavelength discrimination range and bandwidth are determined by the configuration of this filter layer 15. Here, the quartz is used for the substrate 14 on which the filter layer 15 is to be formed, because of its high transparency, its high reliability with respect to high speed rotations based on its high mechanical strength, and its excellent stability against temperature. A disk surface of the substrate 14 facing toward the variable rotation mechanism 13 side that is on opposite side of the filter layer 15 is applied with a wide bandwidth antireflection coating. In Fig. 2, fiber collimators 18 each comprising an optical fiber 18 and a collimate lens 17 are arranged face to face and the disk shaped optical filter 12 is inserted in the optical beam path 11 formed between the fiber collimators 18 such that lights pass through the disk shaped optical filter 12. There is a need to make the beam diameter sufficiently small so as not to degrade the filter characteristic, but the usual collimate beam has a beam diameter of 300 µm which is sufficiently small with respect to a disk of 2.5 inches diameter so that this condition can be satisfied. Also, with this beam diameter it is possible to propagate the optical beam through about 60 mm of space under the condition of the coupling loss being 0.5 dB or less, so that it suffices to realize the configuration shown in Fig. 2. Using the disk shaped optical filter 12 shown in Fig. 2, it is possible to realize a wavelength discrimination apparatus in which the transmission central wavelength varies continuously over a wide bandwidth. In particular, by forming the filter layer 15 such that the transmission central wavelength varies linearly along the circumferential direction, It is possible to realize the wavelength discrimination in synchronization with a timing requested from a system at arbitrary central wavelength variation rate, using the number of rotations control and the conventional electric synchronization technique. For example, when multiple wavelength continuous optical beams arranged at equal wavelength interval are incident on this wavelength discrimination apparatus, the incident lights can be converted into output lights in forms of a series of pulse lights arranged at equal interval on a time axis. This amounts to the conversion between the wavelength axis and the time axis so that it becomes possible to realize the measurement of absolute wavelengths on the time axis provided that absolute wavelengths corresponding to the multiple wavelengths of the incident lights are calibrated. Now three specific configurations of a high speed wavelength discrimination apparatus will be described. Fig. 3 shows a configuration of a first high speed wavelength discrimination apparatus , which comprises a disk shaped optical filter 12 formed by a quartz disk 14 and a filter layer 15 which is placed in an optical beam path 11, a variable rotation mechanism 13 connected to the disk shaped optical filter 12, a rotation controller 19 for controlling the variable rotation mechanism 13 according to synchronization signal input 27, fiber collimators 18 each formed by an optical fiber 16 and a collimate lens 17 between which the optical beam path 11 is formed, WDM (Wavelength Division Multiplexing) couplers 20 for coupling an input timing signal light 26 and an input signal light 25 into an input side fiber collimator 18 and decoupling an output signal light 25 and an output timing signal light 26 from an output side fiber collimator 18, a splitter 21 for splitting the output signal light 25, photo-detectors (PD) 22 for converting the output timing signal light 26 from the WDM coupler 20 and the output signal light 25 from the splitter 21 into electric signals, and a synchroscope 23 receiving electric signals from the photo-diodes 22 including a trigger signal 28 obtained from the output timing signal light 26, and mirrors 31 for forming the optical beam path 11 that passes through the disk shaped optical filter 12 from its top side to its bottom side. In the configuration of Fig. 3, the disk shaped optical filter 12 has the transmission central wavelengths that are varying linearly along the circumferential direction so that the wavelength axis can be directly converted into the time axis under the condition of constant rotations that can be realized by the phase comparison using an output of a rotary encoder or the like that monitors the number of rotations and a master clock or the like. Consequently, the high speed wavelength discrimination becomes possible by providing the master clock and synchronizing the rotation system and the measurement system (photo-diodes 22 and the synchroscope 23) to the master clock. The time axis is immediately converted into the wavelength axis according to a central wavelength shift per unit rotation angle that was measured in advance in a stationary system. Here, the key issue is the detection of a timing of a filter layer starting portion, because the conversion of wavelength axis into time axis becomes impossible unless this timing is detected. In order to make this timing detection possible. a timing detection mark (header) 30 is attached at the starting portion of the filter layer 15 as shown in Fig. 4A, such that the start timing can be detected by detecting a timing at which the collimated beam passes this header 30 as indicated in Fig. 4B. In the configuration of Fig. 3, the timing signal light 26 with a wavelength different from that of the signal light 25 is entered at the input side WDM coupler 20 and extracted at the output side WDM coupler 20 after passing through the filter, such that the timing extraction can be realized without affecting the signal light 25. Note that, in the configuration of Fig. 3, a compact configuration is realized by adopting a collimating system in which the optical axis is changed by the mirrors 31. Fig. 5 shows a configuration of a second high speed wavelength discrimination apparatus, in which the configuration similar to that of Fig. 3 described above is provided as a channel-1 (CH1). In addition, the configuration of Fig. 5 further includes a channel-2 (CH2) formed by a multiple wavelength reference light source 32, a timing detection light source 24. a combiner 33 for combining the multiple wavelength reference lights from the multiple wavelength reference light source 32 and a timing detection light from the timing detection light source 24, fiber collimators 38 each formed by an optical fiber 36 and a collimate lens 37 between which an optical beam path 41 is formed, mirrors 39 for forming the optical beam path 41 that passes through the disk shaped optical filter 12 from its top side to its bottom side, and another photo-detector (PD) 22 for converting the CH2 output lights into electric signals and supplying the obtained electric signals to the synchroscope 23. Here, the collimating system of the channel-2 is provided in parallel to the collimating system of the channel-1. In the configuration of Fig. 5, the disk shaped optical filter 12 has the transmission central wavelengths that are varying nonlinearly along the circumferential direction. Here, however, it is assumed that the transmission central wavelengths varies not randomly but according to a smooth monotonic function. When such a filter characteristic is used, it is impossible to convert the wavelength axis into the time axis by simply detecting the start timing so that it is impossible to realize a good wavelength discrimination characteristic. For this reason, in the configuration of Fig. 5, the channel-2 is provided in order to form reference grids 34 on the time axis as shown in Fig. 6, and the wavelengths are calibrated according to the reference grids 34. More specifically, the multiple wavelength reference lights are entered into the channel-2, and the transmitted light intensity signal is detected by the photo-detector 22. Then, intensity peaks corresponding to the reference lights with wavelengths λ 1 , λ 2 , λ 3 , ··· are detected at times t 1 , t 2 , t 3 , ·····. Then, using these detected peaks as the reference grids 34, the relationship between the wavelength and the times are obtained by interpolation. With this calibration, the transmission central wavelength can be obtained in terms of the time by the following equation (1): λc = λ(t) where t is a relative time within one period of the disk rotation that is measured by setting a time corresponding to the mark on the disk as an origin. When the time required for the mark on the disk to be rotated from the beam passing position of the channel-2 to the beam passing position of the channel-1 is ΔT (in which case the viewing angle between the channel-1 and the channel-2 will be ωΔτ where ω is an angular velocity), the transmission central wavelength of the signal light passing in the channel-1 will be given by the following equation (2). λc = λ(τ-Δτ) Fig. 7 shows a configuration of a third high speed wavelength discrimination apparatus according to a first embodiment of the present invention, which corresponds to the case of realizing the wavelength discrimination by changing the filter characteristic such as bandwidth and the wavelength range. It is possible to realize the same by exchanging disks, but the configuration of Fig. 7 is practically quite useful because it is possible to realize the function of the optical spectrum analyzer for realizing measurement while changing the resolution. In the configuration of Fig. 7, the disk shaped optical filter 12 has mutually different first filter layer 15A and second filter layer 15B that are arranged concentrically on the substrate. As a mechanism for selecting one of these two filter layers, the track and servo mechanism used in the conventional optical disk can be utilized. Namely, the seek and tracking operation is carried out by a track controlling actuator 41 that is controllable in a radial direction, to which the fiber collimators 18 similar to those of the configuration of Fig. 3 are attached. In order to maintain the track, the conventionally known sample and servo technique can be utilized. In this way, it is possible to realize the tracking without leaving a trucking groove on the filter layer, so that it is possible to prevent the degradation of the wavelength discrimination characteristic. Also, in the configuration of Fig. 7, the disk shaped optical filter 12 is provided with four track controlling wobble marks 42. Here, the tracking width of several tens of microns is sufficient and the high speed controlling of the actuator is unnecessary. When a tracking and timing control light 43 is entered at the input side WDM coupler 20, the tracking and timing signal 44 from the output side WDM coupler 20 has an exemplary waveform as shown in Fig. 8. which contains start timing signals 45 due to the timing detection mark 30 and wobble signals 46 due to the track controlling wobble marks 42. As described, according to the first embodiment, it is possible to realize the high speed wavelength discrimination by a very simple configuration. This wavelength discrimination mechanism can be utilized in reducing size and cost of the conventional optical spectrum analyzer considerably, or in realizing discrimination of wavelength multiplexed lights synchronized with a system. It is also to be noted that the disk shaped optical filter according to the present invention can be utlized in such a manner that the desired transmission central wavelength can be specified from outside solely in terms of time, using the clock as a reference for example. It is also to be noted that, in the present invention, a combination of the photo-coupler and the marks provided on the disk shaped optical filter such as along the circumference of the disk shaped optical filter can effectively function as a high precision encoder. It is also to be noted that, besides those already mentioned above, many modifications and variations of the above embodiments may be made without departing from the novel and advantageous features of the present invention. Accordingly, all such modifications and variations are intended to be included within the scope of the appended claims.

Claims: A disk shaped optical filter (12), comprising: a transparent substrate (14) in a disk shape; and a filter layer (15) formed on a top surface of the substrate, having such a filter characteristic that a transmission central wavelength varies along a circumferential direction as a monotonic function of the rotation angle; characterized in that the filter layer (15) has wobble marks (42) formed thereon so as to be used in controlling the track of a light beam to be concentric around the rotation axis of the disk shaped optical filter, when the disk shaped optical filter is set in rotation. The disk shaped optical filter of Claim 1, wherein the filter layer (15) has a filter characteristic in which the transmission central wavelength linearly increases or decreases with respect to the rotation angle. The disk shaped optical filter of Claim 1, wherein the filter layer (15) includes a plurality of layers (15A, 15B) with mutually different filter characteristics, which are formed concentrically in a radially adjacent manner with respect to the axis of rotation of the disk-shaped optical filter. The disk shaped optical filter of Claim 1, wherein the filter layer (15) is divided into a plurality of fan-shaped regions with mutually different transmission central wavelengths (λ 1 , λ 2 , ..... λ N ). The disk shaped optical filter of Claim 1, wherein the filter layer has a mark (30) formed thereon, which is to be used in detecting a start timing of a filter region provided by the filter layer (15) when the disk shaped optical filter is set in rotation. The disk shaped optical filter of Claim 1, wherein the filter layer (15) has a plurality of marks (231), formed thereon along radial directions at prescribed angular intervals between adjacent marks with respect to the axis of rotation of the disk-shaped optical filter, which are to be used in controlling the number of rotations of the disk shaped optical filter when the disk shaped optical filter is set in rotation. The disk shaped optical filter of Claim 1, wherein the substrate (304) is made of quartz, the filter layer (303) is made of dielectric multi-layer, and the substrate (304) has a wide bandwidth antireflection coating (305) applied on a bottom surface.

Inventors: ABE HIROSHI, AIDA KAZUO, KATAGIRI YOSHITADA, KAWAI SHINGO, NAGAOKA SHINJI, OBARA HITOSHI, OHIRA FUMIKAZU, SUZUKI KEN-ICHI, TACHIKAWA YOSHIAKI, ABE, HIROSHI, AIDA, KAZUO, KATAGIRI, YOSHITADA, KAWAI, SHINGO, NAGAOKA, SHINJI, OBARA,HITOSHI, OHIRA, FUMIKAZU, SUZUKI, KEN-ICHI, Abe, Hiroshi, NTT Corporation, Aida, Kazuo, NTT Corporation, Katagiri, Yoshitada, NTT Corporation, Kawai, Shingo, NTT Corporation, Nagaoka, Shinji, NTT Corporation, Obara,Hitoshi, NTT Corporation, Ohira, Fumikazu, NTT Corporation, Suzuki, Ken-ichi, NTT Corporation, Tachikawa Yoshiaki, NTT Corporation
Applicants: NIPPON TELEGRAPH ANDTELEPHONE, NIPPON TELEGRAPH ANDTELEPHONE CORPORATION
Application year: 1998
Main classification: G02B5
IPC codes: G02B6, G02B26, G02B27, G02B7