Abstract:
A radio-frequency-signal generator generates an RF signal. An optical monitor monitors a first intensity of a reference signal and a second intensity of a drop signal. A reference-frequency determining unit determines, based on the first intensity, a first frequency of an RF signal that causes the AOTF to output the reference signal. A temperature detecting unit detects a temperature of the AOTF. A frequency calculating unit calculates a second frequency of an RF signal that causes the AOTF to output a drop signal of a desired wavelength. A control unit controls the RF-signal generator to generate the RF signal of the second frequency calculated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-156304, filed on May 27, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a technology for controlling an acousto-optic tunable filter (AOTF) having a temperature characteristic. More particularly, the present invention relates to a technology for causing the AOTF to selectively output a signal of a predetermined wavelength by correcting the frequency deviation, which is due to the temperature characteristic, of a radio-frequency (RF) signal to be input to the AOTF.  
         [0004]     2. Description of the Related Art  
         [0005]     With a purpose of building a multimedia network, an optical communication device that enables long distance transmission of large-amount data has been demanded. To achieve increased data-transmission capacity, research and development on a wavelength-division multiplexing (WDM) has been carried out because the WDM has advantage in which a broadband property or a large capacity property of an optical fiber can be efficiently utilized.  
         [0006]     In the optical communication network, a function of transmitting, dropping and adding an optical signal at each point on the network as necessary, an optical routing function for selecting an optical transmission path, and a cross-connect function are necessary. For this reason, an optical add/drop multiplexer (OADM) that transmits, drops, and adds an optical signal has been developed. The OADM includes a fixed-wavelength type and a selectable-wavelength type. The fixed-wavelength type can add/drop only an optical signal having a fixed wavelength. The selectable-wavelength type can add/drop an optical signal having an arbitrary wavelength.  
         [0007]     Conventionally, an acousto-optic tunable filter (AOTF) is used to realize an OADM of the selectable-wavelength type. The AOTF acts as to extract only a light having a selected wavelength. Therefore, unlike a fiber grating in which a selected wavelength is fixed, it is possible to arbitrarily select a wavelength. Since the AOTF functions as a tunable wavelength-selecting filter, the AOTF can be applied to a tunable wavelength-selecting filter for a tributary station that adds/drops an optical signal between terminal stations. With such reasons, the OADM using the AOTF is being developed (see, for example, Japanese Patent Application Laid-Open No. H11-218790).  
         [0008]     In the AOTF, a radio frequency signal (hereinafter, “RF signal”) having a frequency band of 160 megahertzs (MHz) to 180 MHz applied to the AOTF functions as a control signal, and the AOTF outputs an optical signal according to the frequency. However, since the AOTF has temperature-dependent properties, even if an identical RF signal is applied to the AOTF, a wavelength of an optical signal to be output varies depending on temperature. Therefore, an AOTF subsystem to obtain an RF signal to output a desirable wavelength based a reference light having a predetermined wavelength output from a reference light source has been proposed.  
         [0009]     However, as described above, with the AOTF, even if an RF signal has an identical frequency, the wavelength of the optical signal to be output varies if ambient temperature changes. Thus, a wavelength that can be obtained also changes, specifically, the wavelength obtained shifts 0.8 nanometer (nm) as the ambient temperature changes each 1° C. This amount of wavelength shift reaches an amount of interval between the selected wavelength and adjacent wavelengths.  
         [0010]     In a method of selecting a wavelength in the AOTF subsystem described above, preparing a reference light having the shortest wavelength and a reference light having the longest wavelength, and by tracking the reference wavelengths, a desirable frequency of the RF signal is calculated based on number of wavelengths and difference between RF frequencies of the reference lights. However, in this method, it is necessary to prepare two reference light sources. As a result, cost increases.  
         [0011]     Moreover, there is another problem if wavelength selection is to be performed only with a single reference light source, that is, temperature-dependent frequency-pulling effect.  FIG. 7  is a schematic for illustrating the temperature-dependent frequency-pulling effect of an AOTF of a dropping type.  
         [0012]     A chart  701  illustrates a wavelength arrangement for output signals λ 1  to λn and reference lights λref 1  and λref 2  when a WDM signal is input to an input port and the reference lights are input to ports for a reference light of the AOTF. As shown in the chart  701 , the WDM transmission signal is formed with optical signals having a frequency interval (grid) of 100 gigahertz (GHz). For example, in a wavelength light having a C band (1530 nm to 1565 nm), 32 optical signals are multiplexed. The reference light λref 1  has a wavelength keeping the wavelength interval of 100 GHz from the optical signal λ 1  having the shortest wavelength. Similarly, the reference light λref 2  has a wavelength keeping the wavelength interval of 100 GHz from the optical signal λn having the longest wavelength.  
         [0013]     A chart  702  illustrates a wavelength arrangement for the optical signals λ 1  to λn and the reference lights λref 1  and λref 2  when the optical signal λ 2  is output from one of output ports of the AOTF when the temperature of the AOTF is 25° C. A solid line indicates the optical light being output and broken lines indicate optical lights not being output and the reference lights.  
         [0014]     A chart  703  illustrates a frequency arrangement for RF signals F 1  to Fn and Fref 1  and Fref 2  to output the optical signals and the reference lights input to the AOTF. A solid line shown in the chart  703  indicates the RF signal F 2  that is applied to the AOTF when the optical signal λ 2  shown in the chart  702  is to be output. The RF signals F 1  to Fn are RF signals to output the optical signals λ 1  to λn when temperature of the AOTF is 25° C. Since the optical signals are arranged at regular intervals, the RF signals are also arranged at regular intervals of Δf 1 . Each of the optical signals shown in the chart  702  with the broken lines are output by applying the RF signal corresponding to each of the optical signals.  
         [0015]     However, when the temperature of the AOTF changes, relationship between a frequency of the RF signal to be applied and a wavelength of the optical signal to be output also changes. A chart  704  illustrates frequencies of RF signals F 1 ′ to Fn 1 ′ and Fref 1 ′ and Fref 2 ′ when the output signal λ 2  is output from one of the output ports when the temperature of the AOTF is 45° C. An RF signal F 2 ′ shown with a solid line is the RF signal to output the optical signal λ 2 . The RF signals F 1 ′ to Fn′ and Fref 1 ′ and Fref 2 ′ shown with broken lines are RF signals to output each of the optical signals and the reference lights. The RF signals F 1 ′ to Fn′ and the Fref 1 ′ and Fref 2 ′ are arranged at regular intervals of Δf 2  to output the optical signals arranged at the regular interval of 100 GHz. The interval Δf 2  is smaller than the interval Δf 1 , which is an interval at which the RF signals are arranged when the temperature of the AOTF is 25° C.  
         [0016]     Thus, when the temperature increases from 25° C. to 45° C., a frequency interval of the RF signals to output an optical signal having an identical frequency changes from Δf 1  to Δf 2  (Δf 2 &lt;Δf 1 ). Such a phenomenon is the temperature-dependent frequency-pulling effect. There is a problem caused by the temperature-dependent frequency-pulling effect when the wavelength selection is to be achieved only with a single reference-wavelength light source.  
       SUMMARY OF THE INVENTION  
       [0017]     It is an object of the present invention to solve at least the above problems in the conventional technology.  
         [0018]     An apparatus according to an aspect of the present invention controls an acousto-optic tunable filter. The acousto-optic tunable filter includes a plurality of input ports for a reference signal and a wavelength-division-multiplexed signal, and a plurality of output ports for the reference signal and a drop signal dropped from the wavelength-division-multiplexed signal. The apparatus includes: a radio-frequency-signal generator that generates a radio-frequency signal; an optical monitor that monitors a first intensity of the reference signal and a second intensity of the drop signal output from the output ports; and a field-programmable gate array. The field-programmable gate array includes a reference-frequency determining unit that determines, based on the first intensity, a first frequency of the radio-frequency signal that causes the acousto-optic tunable filter to output the reference signal; a temperature detecting unit that detects a temperature of the acousto-optic tunable filter; a frequency calculating unit that calculates a second frequency of the radio-frequency signal that causes the acousto-optic tunable filter to output a drop signal of a desired wavelength based on the second intensity, the first frequency, the temperature detected, and a temperature-dependent output characteristic of the acousto-optic tunable filter measured in advance; and a control unit that controls the radio-frequency-signal generator to generate the radio-frequency signal of the second frequency calculated.  
         [0019]     A method according to another aspect of the present invention is a method of selecting an acousto-optic tunable filter. The acousto-optic tunable filter includes a plurality of input ports for a reference signal and a wavelength-division-multiplexed signal, and a plurality of output ports for the reference signal and a drop signal dropped from the wavelength-division-multiplexed signal. The method includes: generating a radio-frequency signal; monitoring a first intensity of the reference signal and a second intensity of the drop signal output from the output ports; determining, based on the first intensity, a first frequency of the radio-frequency signal that causes the acousto-optic tunable filter to output the reference signal; detecting a temperature of the acousto-optic tunable filter; calculating a second frequency of the radio-frequency signal that causes the acousto-optic tunable filter to output a drop signal of a desired wavelength based on the second intensity, the first frequency, the temperature detected, and a temperature-dependent output characteristic of the acousto-optic tunable filter measured in advance; and generating the radio-frequency signal of the second frequency calculated.  
         [0020]     The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a schematic of an acousto-optic tunable filter (AOTF) control device according to an embodiment of the present invention;  
         [0022]      FIG. 2  is a schematic of an AOTF;  
         [0023]      FIG. 3  is a block diagram of a radio-frequency (RF)-signal generator;  
         [0024]      FIG. 4  is a block diagram of an optical monitor;  
         [0025]      FIG. 5A  is a table of a frequency interval AF between RF signals when temperature (TAOTF) of the AOTF varies;  
         [0026]      FIG. 5B  is a plot of the temperature and the frequency interval;  
         [0027]      FIG. 6  is a flowchart of a wavelength selection performed by the AOTF control device; and  
         [0028]      FIG. 7  is a schematic for illustrating temperature-dependent frequency-pulling effect. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     Exemplary embodiments of the present invention will be explained in detail with reference to the accompanying drawings.  
         [0030]      FIG. 1  is a schematic of an AOTF control device  100  according to an embodiment of the present invention. As shown in  FIG. 1 , the AOTF control device  100  includes an optical filter unit  107  and a demultiplexer  108 . The optical filter unit  107  includes an AOTF  101  of an integrated dropping type, an RF-signal generator  102 , an optical monitor  103 , a field-programmable gate array (FPGA)  104 , a digital signal processor (DSP)  105 , and an optical tap  106 .  
         [0031]     The AOTF control device  100  can control the AOTF  101  to output only an optical signal having a predetermined wavelength from among plural optical signals (k 1  to kn) input through a WDM-signal input port In to any one of ports  1  to  4 . A reference light λref is for controlling the RF signal to be applied to the AOTF  101 . The reference light λref is input through a reference-signal input port In to apply an RF signal to be a reference to the AOTF  101 , and then, the reference light λref is output through a port  5 . The ports  1  to  5  are an optical-signal output port Out.  
         [0032]     The RF-signal generator  102  generates the RF signal to be applied to the AOTF  101 . The optical monitor  103  monitors an optical signal output to each output port.  
         [0033]     The FPGA  104  is a programmable large-scale-integration (LSI). The FPGA  104  calculates a frequency of the RF signal to output an optical signal having a predetermined wavelength, and inputs a signal for instructing generation of the RF signal to the RF-signal generator  102 . A value that indicates temperature of the AOTF  101  that has been input to the FPGA  104  is used as a variable in the calculation. The DSP  105  performs a switching control of the RF signal applied from the RF-signal generator  102  based on values of the optical signal and the reference light that are obtained by monitoring. The optical tap  106  splits each of the optical signals output to the ports  1  to  4  and the reference light output to the port  5  to output to the optical monitor  103 .  
         [0034]      FIG. 2  is a schematic of an AOTF. The AOTF is a ferroelectric crystal, and includes a substrate  1 - 7  of lithium niobate (LiNbO3), which is piezoelectric, and two optical waveguides  1 - 1  and  1 - 2  formed by titanium diffusion, as shown in  FIG. 2 . The optical waveguides  1 - 1  and  1 - 2  cross each other at two points. At the two points, polarization beam splitters (PBS)  1 - 3  and  1 - 4  of a waveguide type are provided. Over the optical waveguides  1 - 1  and  1 - 2 , a saw guide  1 - 6 , which is a metallic film, is formed as shown in  FIG. 2 . A surface acoustic wave propagates through the saw guide  1 - 6 . The surface acoustic wave is generated when the RF signal is applied to an interdigital transducer (IDT)  1 - 5 , which is an electrode having teeth engaged with each other.  
         [0035]     As shown in  FIG. 2 , when lights having wavelengths λ 1  to λ 3  are input to the port  1  of the AOTF, the light having both polarized modes of a transverse electric (TE) mode and a transverse magnetic (TM) mode is split into a light in the TE mode and a light in the TM mode to propagate along the optical waveguides  1 - 1  and  1 - 2  respectively. When the surface acoustic wave generated by applying an RF signal f 1  having a predetermined frequency propagates through the saw guide  1 - 6 , refractive indexes of the optical waveguides  1 - 1  and  1 - 2  periodically change due to an acousto-optic (AO) effect at portions at which each of the optical waveguides  1 - 1  and  1 - 2  crosses with the saw guide  1 - 6 .  
         [0036]     Thus, among the lights input, only in a light having a predetermined wavelength that interacts with such a periodic change of the refractive index, the polarized mode rotates to be switched between the TE mode and the TM mode. The TE mode is a waveguide mode that does not have an electric field in a direction of propagation, and the TM mode is a waveguide mode that does not have a magnetic field in a direction of an electric field. An amount of rotation of the polarized mode is proportional to a length of the interaction between a light in the TE mode or in the TM mode and the change of the refractive index, and to a power of the RF signal. The length of the interaction is controlled by an interval between absorbers  1 - 8  and  1 - 9  that are formed over the optical waveguides  1 - 1  and  1 - 2  to absorb the surface acoustic wave.  
         [0037]     In other words, with optimized length of the interaction and optimized power of the RF signal, the light in the TM mode is converted into the light in the TE mode in the optical waveguide  1 - 1 , and the light in the TE mode is converted into the light in the TM mode in the optical waveguide  1 - 2 . The PBS  1 - 4  changes directions of propagation of the light in the TE mode and the light in the TM mode obtained by conversion, and only the light having the wavelength that has interacted with the change is selected as a split light. A light having a wavelength that has not interacted with the change pass through and is output through an output port  2  as an output light. In an example shown in  FIG. 2 , optical signals having wavelengths λ 1  and λ 2  are acted on by RF signals f 1  and f 2 , therefore, are selected as the split lights to be output through an output port  3 .  
         [0038]     Thus, with the AOTF, it is possible to select only a light having a wavelength corresponding to a frequency of the RF signal to be split. The light to be selected can be changed to a light having a different wavelength by changing the frequency of the RF signal. In addition, since the output light output from the output port  2  is an optical signal from which the light having the wavelength corresponding to the frequency of the RF signal is removed, the AOTF is regarded as having a rejection function. The AOTF  101  of the integrated dropping type is formed by integrating five pieces of the AOTFs explained above.  
         [0039]      FIG. 3  is a block diagram of the RF-signal generator  102 . The RF-signal generator  102  includes a direct digital synthesizer (DDS)  301 , a band-pass filter (BPF)  302 , and a radio frequency amplifier (RF-AMP)  303 .  
         [0040]     The DDS  301  outputs a sine wave signal or a cosine wave signal according to information for setting a frequency, a phase, or an amplitude that is input from the FPGA  104 . The BPF  302  allows only a signal having a frequency within a predetermined frequency band to pass through. The RF-AMP  303  amplifies the signal input from the BPF  302  to output the RF signal to the AOTF  101 . A frequency of the RF signal output from the RF-signal generator  102  is controlled by the BPF  302  to be from 160 MHz to 180 MHz.  
         [0041]      FIG. 4  is a block diagram of the optical monitor  103 . The optical monitor  103  includes a photo diode (PD)  401 , a log amplifier  402  for current-voltage conversion, a non-reverse amplifier  403 , a low-pass filter (LPF)  404 , and an analog-to-digital converter (ADC)  405 .  
         [0042]     The PD  401  generates an electric current to convert an optical signal split by the optical tap  106  into an electric signal. The electric current to be generated by the PD  401  corresponds to a light input to the optical monitor  103 . The log amplifier  402  converts the electric current input from the PD  401  into a voltage according to a characteristic of a log. The non-reverse amplifier  403  amplifies the voltage applied from the log amplifier  402 . The LPF  404  allows only a low-frequency component to pass though. Therefore, a high-frequency component of the voltage is removed. The ADC  405  converts an analog signal indicating the voltage output from the LPF  404  into a digital signal to output to the DSP  105 .  
         [0043]     According to the present invention, with the AOTF control device  100  structured as described above, it is possible to perform wavelength selection on the WDM transmission signal to output an optical signal having a desirable wavelength.  
         [0044]      FIG. 5A  is a table of a frequency interval ΔF between RF signals when temperature T AOTF  of an AOTF of an integrated dropping type varies. The frequency interval ΔF indicates an interval between frequencies of RF signals to output optical signals arranged at regular intervals of 100 GHz.  FIG. 5B  is a plot of the temperature and the frequency interval. In  FIG. 5B , a vertical axis represents the frequency interval ΔF (Hz) and a horizontal axis represents the temperature (° C.) of the AOTF  101  shown in  FIG. 1 , and the frequency interval ΔF corresponding to the temperature shown in  FIG. 5A  is plotted.  
         [0045]     As described for the conventional technology, the AOTF has the temperature-dependent frequency-pulling effect. Therefore, as the temperature of the AOTF  101  increases, the frequency interval ΔF decreases. As shown in the plot shown in  FIG. 5B , a change in the frequency interval AF due to the temperature-dependent frequency-pulling effect is linear. Accordingly, in the embodiments of the present invention, a frequency of the RF signal to output an optical signal having a desirable wavelength is calculated using the plot shown in  FIG. 5B  to achieve the wavelength selection.  
         [0046]      FIG. 6  is a flowchart of the wavelength selection by the AOTF control device  100  according to the embodiments of the present invention. The WDM transmission signal (λ 1  to λn) input through the WDM-signal input port In is split into four signals to be input to the ports  1  to  4 . To the port  5 , the reference light having an arbitrary reference wavelength λref 1  is input.  
         [0047]     Under such a condition, fixed values and variables for performing the wavelength selection are prepared in a memory of the DSP  105  (step S 601 ). Specifically, the fixed values are as follows:  
         [0048]     a: a value of the frequency interval ΔF when the temperature of the AOTF  101  is 0° C.;  
         [0049]     b: a coefficient of a slope in a characteristic line of the frequency interval ΔF when the temperature of the AOTF  101  changes for 1° C.; and  
         [0050]     λref: a frequency of the reference light (hereinafter, “reference wavelength”).  
         [0000]     The variables are as follows:  
         [0051]     T AOTF : a current temperature of the AOTF  101  (measured value);  
         [0052]     Fref: a current frequency of the RF signal corresponding to the reference frequency (measured value); and  
         [0053]     λx: a wavelength to be selected (input value).  
         [0054]     Then, while decreasing a frequency of the RF signal to be input to the port  5  from 180 MHz by 1 kHz, the optical monitor  103  monitors the signal and reads values. Thus, a frequency Fref of the RF signal causing the AOTF to output the reference signal having the reference wavelength λref is detected (step S 602 ). To detect the reference wavelength λref, a maximum value in the values read at monitoring while decreasing the frequency of the RF signal is detected, and a frequency of the RF signal at the time when the maximum value is read is determined as the frequency Fref to obtain the reference wavelength λref.  
         [0055]     The frequency Fref detected at step S 602  is stored in the memory of the DSP  105  (step S 603 ). To maintain an optimal frequency of the RF signal corresponding to variation due to a change in ambient temperature or fluctuations in the reference wavelength, an optimal power of the RF-signal at which the value read by the optical monitor  103  becomes the maximum value is obtained. The optimal power can be obtained by performing a frequency tracking and a power tracking of the RF signal (step S 604 ). Thus, the frequency of the RF signal to obtain the current reference wavelength is always updated to a latest value by the frequency tracking process.  
         [0056]     Moreover, when the frequency Fref is updated, the temperature T AOTF  of the AOTF  101  is monitored and stored in the memory of the DSP  105  (step S 605 ). In other words, the temperature T AOTF  is also always updated to a latest value.  
         [0057]     Then, the frequency interval ΔF of the RF signals to output the optical signals arranged at the intervals of 100 GHz at a current state is calculated from Eq. 1 below (step S 606 ). 
 
Δ F=a+b×T   AOTF   (1) 
 
         [0058]     Then, it is determined whether the wavelength selection is requested (step S 607 ). When the wavelength selection is not requested (“NO” at step S 607 ), the process is suspended to be stand-by. When the wavelength selection is requested (“YES” at step  607 ), a frequency Fx of the RF signal that corresponds to a wavelength to be selected is calculated (step S 608 ). The frequency Fx is calculated from Eq. 2 based on parameters of the fixed values and the variables that have been prepared at step S 601 , and the frequency Fref and the temperature T AOTF  that are updated at steps S 602  to S 605 . 
 
 Fx=Fref+ΔFx (λ ref−λx )/0.8  (2) 
 
         [0059]     The frequency Fx calculated and the optimal power level of the RF-signal obtained through the power tracking process are output to the AOTF  101  to selectively output the wavelength requested (step S 609 ).  
         [0060]     Finally, it is determined whether to end the wavelength selection (step S 610 ). When the wavelength selection is to be ended (“YES” at step S 610 ), the process is finished. When the wavelength selection is to be repeated (“YES” at step S 610 ), the process returns to step S 601  to repeat the same processes. Thus, the wavelength selection can be performed again while updating information corresponding to a change in the temperature.  
         [0061]     As described above, according to the AOTF control device and the method of selecting a wavelength, it is possible to achieve accurate wavelength selection for outputting a desirable optical signal with a single reference light source by using predetermined fixed values and variables.  
         [0062]     Since the wavelength selection can be performed with only one reference light source, it is also possible to reduce a size of a device and cost for manufacturing the device. Moreover, since the frequency of the RF signal for selecting a desirable wavelength is acquired by calculation, an arbitrary wavelength can be selected easily and freely without being limited to a wavelength of a fixed channel in the WDM.  
         [0063]     The method of selecting a wavelength explained in the embodiments of the present invention is implemented by executing a computer program, which is prepared in advance, by a computer such as a personal computer and a workstation. The computer program is recorded on a computer-readable recording medium, such as a compact-disk read-only memory the (CD-ROM), a magneto-optical disk (MO), and a digital versatile disk (DVD), and is executed by the computer reading out from the recording medium. The computer program may be a transmission medium that is distributed through a network such as the Internet.  
         [0064]     According to the present invention, it is possible to achieve highly accurate wavelength selection capable of coping with a temperature change with a single reference-wavelength light.  
         [0065]     Moreover, according to the present invention, it is possible to downsize a device and reduce manufacturing cost of the device.  
         [0066]     Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.