Optical filter device operating in a wavelength-synchronised mode

Abstract: An optical processing device for converting a wavelength of an optical signal comprises an optical processing unit (11) supplied with an input optical beam carrying thereon one or more optical signals with respective wavelengths that are different from each other, the optical processing unit being further supplied with a control optical beam having a stabilized reference wavelength (λ i ) and selecting the optical signal that has a first wavelength and outputting the same with a second wavelength that is specified by the reference wavelength; and a control unit (12) for controlling the optical processing unit by specifying the first wavelength of the optical beam to be selected.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to optical processing systems and more particularly to a wavelength-synchronized optical processing device that produces an optical signal with a wavelength that is synchronized to a stabilized wavelength of a reference optical beam and various optical information processing systems that use such an optical processing device. With the extensive deployment of optical telecommunication network, studies are made on the optical transmission and reception in the optical exchange systems and optical subscriber systems. Among others, there is a proposal to transmit a plurality of optical signals having different wavelengths on a common optical fiber or optical waveguide in the wavelength multiplex mode. In such a wavelength multiplex transmission of optical signals, it is necessary to superpose the optical signals with sufficient wavelength interval such that a reception side can demultiplex the received optical signal into individual optical signal components with reliability. In such wavelength multiplex systems, it will be easily understood that the change or fluctuation of the oscillation wavelength occurring in the optical signals cause a disastrous effect in the operation of the reception side systems. FIG.1 shows the block diagram of a conventional optical processing system that uses a wavelength converter 1a. Referring to FIG.1, the wavelength converter 1a is supplied with an input optical signal having a wavelength of λ 0 and produces an output optical signal with a wavelength of λ 1 . There, the wavelength converter converts the wavelength of the input optical signal to a second wavelength, and the optical signal having the wavelength λ 1 is divided out from the converted optical signal in response to a reference optical beam given externally with the wavelength λ 1 . More specifically, the output optical signal of the wavelength converter 1a is supplied to an optical divider 2a that divides the incident optical beam into a first output beam corresponding to the output optical signal and a second output beam, and the second output beam is supplied to a wavelength comparator 3a. The wavelength comparator 3a is further supplied with the reference optical beam and produces an electric output indicative of the difference between the wavelength of the output optical signal of the converter 1a and the wavelength λ 1 of the reference optical beam. The output electric signal of the comparator 3a is supplied to a controller 4a that in turn produces a control signal for controlling the operation of the wavelength converter 1a. According to this system, one can convert the wavelength λ 0 of the incident optical signal to the wavelength λ 1 by controlling the converter 1a such that the output of the wavelength comparator 3a becomes zero. The wavelength converter 1a may be formed by using a DFB laser diode that can change the oscillation wavelength by controlling the bias current or temperature. FIG.2 shows another conventional wavelength conversion system wherein an input optical signal having a wavelength λ 0 is supplied to a photoelectric converter 1b that produces an electric output in response to the incident optical signal. The output electric signal is supplied to a clock extraction circuit 2b wherein a clock signal is extracted from the information that is modulated on the input optical signal. The clock extraction circuit further discriminates the logic level of the binary information signal modulated on the input optical beam with a timing given by the clocks, and drives an optical modulator 3b. The optical modulator 3b is thereby supplied with a reference optical beam with the wavelength λl and modulates the same in accordance with the electric output of the clock extraction circuit 2b. As a result, an optical output signal is obtained with the wavelength ; λ1. In any of these conventional optical processing systems, there has been a problem in that the normal operation of the system is not achieved when there is a fluctuation in the wavelength of the input optical beam. It should be noted that, in the telecommunication systems, there is no guarantee that the transmission side uses the stabilized optical source with respect to the oscillation wavelength. Further, such a fluctuation of the wavelength of the optical signal may be caused as a result of the dispersion of optical pulses that occur in the optical fibers. Thus, the optical transmission system is not only required to eliminate the fluctuation of the wavelength of the optical source at the transmission side but is also required to have an ability to adapt to the fluctuation of wavelength and to eliminate the same at the reception side or at any intermediate locations between the transmission side and the reception side. SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to eliminate the foregoing problems by providing an optical filter device for selectively passing an input optical signal component included in a plurality of input optical. signal components having respective, mutually different wavelengths, in response to a control optical signal component selected from one or more control optical signal components having respective, mutually different wavelengths, characterized by a first filtering means comprising one or more filtering elements forming first group filtering elements, said first group filtering elements having respective, mutually different optical passbands (λ a - λ d ) corresponding to said wavelengths of said control optical signal components, each of said first filtering elements being supplied with said plurality of control optical signal components for selectively passing one of said control optical signal components therethrough; a second filtering means comprising a plurality of filtering elements forming second group filtering elements, said second group filtering elements having respective, mutually different optical passbands (λ 1 ' - λ 4 '), each of said second filtering elements being supplied with said plurality of input optical signal components for selectively passing one of said input optical signal components therethrough; a transmission control means comprising a plurality of transmission control elements provided in correspondence to said second group filtering elements, each of said transmission control elements being supplied with an input optical signal component from a corresponding second group filtering element and a control optical signal component from a corresponding first group filtering element, for selectively passing therethrough said input optical signal component in response to said control optical signal component; and an optical synthesis means supplied with output optical signal components from said transmission control means for synthesizing the same to produce a single output optical beam. According to one aspect of the invention, the optical passbands (λ 1 ' - λ 4 ') of said second filtering elements are tuned to said wavelengths of said input optical signal components. According to another aspect of the invention, each of said optical transmission control elements comprises a photodetector supplied with an input optical signal component from a corresponding second group filtering element for producing an electrical signal in response thereto, a drive circuit supplied with said electrical signal from said photodetector for producing a drive signal of a laser amplifier, and a laser amplifier supplied with an input optical signal component from a corresponding second group filtering element, said laser amplifier being further supplied with said drive signal from said drive circuit for amplifying said input optical signal component in response to said drive signal. In a preferred embodiment, each of said second filtering elements comprises first optical switching means supplied with an input optical signal and a reference optical signal for selectively outputting one of said input optical signal and said reference optical signal as an output optical signal; tunable optical filter device having an optical passband, said tunable optical filter device being supplied with said output optical signal of said first optical switching means for filtering the same according to said optical passband; second optical switching means supplied with an output optical signal from said tunable optical filter device for selectively outputting the same from one of first and second optical output ports ; and control means supplied with an output optical signal from said second output port for controlling said optical passband of said tunable optical filter device such that said optical passband of said tunable optical filter device is tuned to the wavelength of said reference optical signal. Advantageously, the second optical switching means selects said second optical output port when said first optical switching means selects said reference optical signal, said second optical switching means selects said first optical output port when said first optical switching means selects said input optical signal. In a preferred embodiment, said tunable optical device comprises a distributed feedback laser diode biased below the threshold level of laser oscillation. Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a block diagram showing the construction of a conventional wavelength conversion circuit used in an optical transmission system; FIG.2 is a block diagram showing another conventional wavelength conversion circuit; FIG.3 is a block diagram showing the construction of the wavelength-synchronized optical processing device according to a first embodiment of the present invention; FIG.4 is a block diagram showing the construction of a variable filter used in the optical processing device of FIG.3; FIG.5 is a block diagram showing another example of the variable filter of FIG.4; FIG.6 is a block diagram showing still another construction of the variable filter of FIG.4; FIG.7 is a diagram showing the structure of a semiconductor optical device used in the variable filter of FIG.6; and FIG.8 is a diagram showing the operational characteristics of the device of FIG.7. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG.3 shows the general construction of the wavelength-synchronized optical processing device according to a first embodiment of the present invention. The optical processing device of FIG.3 is an optical demultiplexer that selectively recovers an optical signal component from a wavelength-multiplex optical signal modulated on an input optical beam, in response to a reference optical beam that specifies the wavelength of the optical signal to be recovered. Referring to FIG.3, the optical processing device includes an optical processing unit 11 and a control unit 12, wherein the optical processing unit 11 is generally formed from a variable-wavelength band pass filter 13, a photodetector 14 and an optical modulation unit 15. The band pass filter 13 of the optical processing unit 11 is supplied with an incident optical beam and controlled by the control unit 12 such that only the optical signal that has a particular wavelength specified by the control unit 12 is passed through the filter 13. The output optical beam of the filter 13 is supplied to the photodetector 14 wherein the photodetector 14 converts the optical beam to an electric signal. The output electric signal of the photodetector 14 is supplied to the optical modulation unit 15 and the optical modulation unit 15 modulates the reference optical beam in response to the electric output of the photodetector 14. As will be described later in detail, the filter 13 may be formed of a DFB laser amplifier and changes the wavelength of the optical beam passing therethrough in response to a bias current that is controlled below the threshold of laser oscillation. The optical modulation unit 15 may be a Franz-Keldysh effect device or a laser diode of which gain can be changed in response to the output of the photodetector 14. In operation, an input optical beam that carries thereon optical signals having respective wavelengths λ l ' - λ n ' in the wavelength-multiplex mode is supplied to the filter 13 that in turn is controlled by the control unit 12 to pass the optical signal having the wavelength λ i ' selectively. Meanwhile, the wavelength of the reference beam is well stabilized by using a stabilized optical source such as a laser diode of which operational temperature and bias current are compensated for with respect to any variation. Thereby, one obtains the output optical signal from the optical modulation unit 15 with the wavelength λ i in correspondence to foregoing original optical signal of the wavelength λ i '. The circuit 11 can be formed as an integrated circuit including the control unit 12. FIG.4 shows the construction of the filter 13 used in the system of FIG.3. Referring to FIG.4, the filter 13 includes a number of filter elements 13 1 - 13 4 respectively passing the optical beams of the wavelengths λ a - λ d , as well as a number of filter elements 13 5 - 13 8 respectively passing the optical beams of the wavelengths λ 1 ' - λ 4 ', wherein the wavelength-multiplexed input optical beam is supplied to each of the filter elements 13 5 - 13 8 parallel with each other. Further, the reference optical beam is supplied parallel to each of the filter elements 13 1 - 13 4 . The output optical beams of the filter elements 13 5 - 13 8 are supplied to corresponding variable transmittance controllers 13 9 - 13 12 each having a transmittance that is controlled in response to the output of the corresponding filter element. Thus, when the reference optical beam is given with the wavelength of λ a , the transmittance of the controller 13 9 is selectively increased and the optical signal having the wavelength λ 1 ' and passed through the filter element 13 5 is selectively outputted as an output optical beam of the filter 13. In the present embodiment, one may employ any of the multi-layered dielectric films, optical waveguides, or the spatial filters that use the diffraction grating, for the filter element 13 5 - 13 9 . Preferably, the wavelengths λ a , λ b , λ c and λ d may be set coincident to the wavelengths λ 1 ', λ 2 ', λ 3 ' and λ 4 ' such that λ a = λ 1 ', λ b = λ 2 ', λ c = λ 3 ' and λ d = λ 4 '. This, of course, is not the essential requirement for the operation of the device. FIG.5 shows an example of the variable transmittance controllers 13 9 - 13 12 , wherein the input optical signal from the filter 13 5 - 13 9 is supplied to a laser amplifier 13 23 for amplification by the stimulated emission caused therein in response to the passage of the wavefront of the input optical beam. The operation of the laser amplifier 13 23 is controlled by an electronic control circuit 13 22 that in turn is controlled in response to an electric signal produced by a photodetector 13 21 to which the reference optical beam is supplied. More specifically, the electronic circuit 13 22 produces a bias signal to the laser amplifier 13 23 when there is an incident reference optical beam to the photodetector 13 21 . In response to the bias signal, the laser amplifier 13 23 amplifies the incident input optical signal, while when there is no bias signal, the laser amplifier 13 22 absorbs the input optical signal. For the variable transmittance controllers, one may use, in addition to the foregoing laser amplifier, a Franz-Keldysh effect device or MQW optical modulator that uses the quantum confinement Stark effect. FIG.6 shows another embodiment of the variable-wavelength filter 13 wherein there is provided an optical switch 13 31 that has a first input port A for receiving the reference optical beam and a second input port B for receiving the input optical beam, wherein one of the optical beams at the input port A and the input port B is supplied selectively to a filter device 13 32 of which wavelength is controlled by a controller 13 34 . The optical beam that has passed through the filter device 13 32 is supplied to a second optical switch 13 33 that has a first output port C connected to an output waveguide (not shown) and a second output port D connected to the controller 13 34 . In operation, the first optical switch 13 31 is set at first such that the input port B is selected. Further, the second optical switch 13 33 is set such that the output port D is selected. In this state, the reference optical beam supplied to the input port B is transferred to the controller 13 34 after passing through the filter device 13 32 and the switch 13 33 . Thereby, the controller 13 34 sets the state of the filter device 13 32 such that the transmittance of the optical beam becomes maximum. After such a training of the filter device 13 32 is completed, the state of the optical switches 13 31 and 13 33 switched such that the input optical beam at the port A is outputted at the port C after passing through the filter device 13 32 . Thereby, only the optical signal that has the wavelength coincident to the wavelength of the filter device 13 32 alone appears at the output port C. In other words, the system 13 acts as the variable-wavelength filter that passes the optical signal having a wavelength coincident to the wavelength of the reference optical beam selectively. FIG.7 shows the construction of the filter device 13 32 . Referring to FIG.7, the filter device 13 32 is formed of a DFB laser diode that has an active layer 13 41 and a diffraction grating 13 42 . In operation, the device is biased below the threshold of laser oscillation. Thereby, the incident optical beam causes a stimulated emission in the active layer 13 41 upon passage therethrough. FIG.8 shows the band-pass characteristics of the laser amplifier of FIG.7 for different bias voltages. As can be seen in FIG.8, the peak of the transmittance shifts in response to the bias voltage. By using this effect, one can set the pass-band or pass-wavelength in the filter device by means of the controller 13 34 . According to the construction of FIG.6, one can maintain the stable operational characteristics for the filter 13 as a whole even when there is a change in the operational characteristics of the laser amplifier itself. Obviously, this is due to the feedback control of the laser amplifier 13 32 as described above. For the optical switches 13 31 and 13 33 , one may use the directional coupler and saturable optical absorption switch that changes the transmittance in response to the injection of carriers. Further, the present invention is not limited to the embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the invention.

Claims: An optical filter device for selectively passing an input optical signal component included in a plurality of input optical signal components having respective, mutually different wavelengths, in response to a control optical signal component selected from one or more control optical signal components having respective, mutually different wavelengths, characterized by : first filtering means comprising one or more filtering elements (13 1 - 13 4 ) forming first group filtering elements, said first group filtering elements having respective, mutually different optical passbands (λ a - λ d ) corresponding to said wavelengths of said control optical signal components, each of said first filtering elements being supplied with said plurality of control optical signal components for selectively passing one of said control optical signal components therethrough; second filtering means comprising a plurality of filtering elements (13 5 - 13 8 ) forming second group filtering elements, said second group filtering elements having respective, mutually different optical passbands (λ 1 ' - λ 4 '), each of said second filtering elements being supplied with said plurality of input optical signal components for selectively passing one of said input optical signal components therethrough; transmission control means (13 9 - 13 12 ) comprising a plurality of transmission control elements provided in correspondence to said second group filtering elements, each of said transmission control elements being supplied with an input optical signal component from a corresponding second group filtering element and a control optical signal component from a corresponding first group filtering element, for selectively passing therethrough said input optical signal component in response to said control optical signal component; and optical synthesis means supplied with output optical signal components from said transmission control means for synthesizing the same to produce a single output optical beam. An optical filter device as claimed in claim 1, characterized in that said optical passbands (λ 1 ' - λ 4 ') of said second filtering elements (13 5 - 13 8 ) are tuned to said wavelengths of said input optical signal components. An optical filter device as claimed in claim 1, characterized in that each of said optical transmission control elements (13 9 - 13 12 ) comprises a photodetector (13 21 ) supplied with an input optical signal component from a corresponding second group filtering element for producing an electrical signal in response thereto, a drive circuit (13 22 ) supplied with said electrical signal from said photodetector for producing a drive signal of a laser amplifier, and a laser amplifier (13 23 ) supplied with an input optical signal component from a corresponding second group filtering element (13 5 - 13 8 ), said laser amplifier being further supplied with said drive signal from said drive circuit for amplifying said input optical signal component in response to said drive signal. An optical filter device as claimed in claim 1, characterized in that each of said second filtering elements (13 5 - 13 8 ) comprises : first optical switching means (13 31 ) supplied with an input optical signal and a reference optical signal for selectively outputting one of said input optical signal and said reference optical signal as an output optical signal; tunable optical filter device (13 32 ) having an optical passband, said tunable optical filter device being supplied with said output optical signal of said first optical switching means for filtering the same according to said optical passband; second optical switching means (13 33 ) supplied with an output optical signal from said tunable optical filter device for selectively outputting the same from one of first and second optical output ports (C, D); and control means (13 34 ) supplied with an output optical signal from said second output port for controlling said optical passband of said tunable optical filter device such that said optical passband of said tunable optical filter device is tuned to the wavelength of said reference optical signal. An optical filter device as claimed in claim 4, characterized in that said second optical switching means (13 33 ) is adapted to select said second optical output port (D) when said first optical switching means (13 31 ) selects said reference optical signal, and said second optical switching means is adapted to select said first optical output port (C) when said first optical switching means selects said input optical signal. An optical filter device as claimed in claim 4, characterized in that said tunable optical device comprises a distributed feedback DFB laser diode (13 32 ) biased below the threshold level of laser oscillation.

Inventors: FUJIMOTO NOBUHIRO, KUROYANAGI SATOSHI, NOBUHARA HIROYUKI, ROKUGAWA HIROYUKI, TANAKA KAZUHIRO, WAKAO KIYOHIDE, FUJIMOTO, NOBUHIRO, KUROYANAGI, SATOSHI, NOBUHARA, HIROYUKI, ROKUGAWA, HIROYUKI, TANAKA, KAZUHIRO, WAKAO, KIYOHIDE, Fujimoto, Nobuhiro, c/o Fujitsu Limited, KUROYANAGI, SATOSHI, C/O FUJITSU LIMITED, Nobuhara, Hiroyuki, c/o Fujitsu Limited, Rokugawa, Hiroyuki, c/o Fujitsu Limited, Tanaka, Kazuhiro, c/o Fujitsu Limited, Wakao, Kiyohide, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: H04Q11
IPC codes: H04Q11, H04B10