Abstract:
A wavelength selecting apparatus includes a wavelength selecting filter and that a monitoring filter are similar acousto-optic tunable filters; a light source that outputs light having a specific wavelength to the monitoring filter; a light receiving unit that detects a wavelength of light that has passed the monitoring filter; and a control unit that outputs, to the wavelength selecting filter, a control signal having a control frequency to selectively pass light of a desirable wavelength. The light source includes a light emitting element that emits light of a wide wavelength band; and an optical filter having at least two transmission bands through which the light passes to the monitoring filter. The control unit, based on at least two wavelengths of the light output by the light source, outputs a control signal having a control frequency corresponding to the desirable wavelength.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a wavelength selecting apparatus and a wavelength selecting method.  
         [0003]     2. Description of the Related Art  
         [0004]     A tunable filter is conventionally placed in a photonic network as an important optical element used to select a wavelength required to split signal light. Among such optical elements used to select a wavelength, an acousto-optic tunable filter (AOTF) has particularly attracted attention in recent years. The AOTF is a type of optical filter that has merits, such as enabling a reduction in size and cost based on integration, in addition to high-speed properties of a wavelength selecting operation and extensiveness of a tunable range. A wavelength selecting apparatus based on this AOTF can be already put to practical use. However, an AOTF subsystem that includes a laser output unit and utilizes this laser as a reference light source to improve accuracy of wavelength selection based on the AOTF has also been proposed.  
         [0005]      FIG. 9  is a view of a conventional AOTF subsystem. The AOTF subsystem includes a function unit that judges whether selection of a wavelength by an AOTF is appropriately performed and accordingly executes control. An AOTF subsystem  10  in the drawing includes an AOTF  11  that selects a wavelength, a reference light source unit  12 , a monitoring AOTF  13 , a light receiver  14 , and a controller  15 . This subsystem also includes a temperature controller  16  that executes control according to the temperature dependence of an element serving as a light source.  
         [0006]     The AOTF subsystem  10  is configured to have both a wavelength selecting function of the AOTF, i.e., only transmitting signal light having a specific wavelength based on a radio frequency (RF) control signal input to the AOTF  11  from the controller  15  and a function of using reference light from the reference light source unit  12  to record a corresponding relationships between the RF control signal and the selected wavelength and deriving linear RF control signal-selected light wavelength characteristics to improve a selection accuracy for a light signal. According to such a configuration, when multiplexed light signals (λa to λh) are input to the AOTF subsystem  10 , a light signal (λd) having a selected wavelength alone can be selected and transmitted.  
         [0007]      FIG. 10  is a view showing a configuration of the AOTF. An operation of selecting a wavelength in the AOTF  11  will now be explained with reference to  FIG. 10 . The AOTF  11  and the monitoring AOTF  13  have the same element structure, however, they input signals to different ports depending on application, and hence they are individually depicted. The AOTF  11  includes a polarizing beam splitter (PBS)  20 , a comb-like electrode  21 , and a surface acoustic wave (SAW) waveguide  22  in a circuit manufactured on a lithium niobate (LiNbO 3 ) substrate.  
         [0008]     When an RF control signal is applied to the comb-like electrode  21 , a SAW is excited. In the linear SAW waveguide  22  through which the SAW is transmitted, a polarized wave having a particular wavelength rotates due to an acousto-optic effect. Here, when the PBS  20  integrated at an intersection of the circuit operates without being dependent on the polarized light, input light (λa to λh) can be divided into selected light (λd) and non-selected lights (λa, λb, λc, λe, λf, λg, and λh) and led to different ports, respectively. A wavelength of the selected light is dependent on a frequency of the RF control signal applied to the comb-like electrode  21 , and multiple selected lights can be obtained when multiple RF control signals are input. Selection of a wavelength by the AOTF  11  can be carried out by using the above-explained principle (refer to “Photonic Network” by Terumi Chikama, [online], July, 1999, Magazine FUJITSU, [retrieved Nov. 19, 2004, Internet &lt;URL: http://magazine.fujitsu.com/index2.html&gt;]).  
         [0009]     As the reference light source unit  12 , a unit configured by a light-emitting diode (LED) having a wide emission spectrum is disclosed in Japanese Patent Application Laid-open No. H3-233425, Japanese Patent Application Laid-open No. H6-120605, and Japanese Patent Application Laid-open No. H10-262031.  
         [0010]     However, as a light source element of the reference light source unit  12  in the conventional AOTF subsystem  10 , a distributed feedback-laser diode (DFB-LD) is used for its characteristic of outputting only one wavelength per element. This DFB-LD is advantageous in that a peak of an output waveform is prominent in the light receiver  14 , and detection of the peak by the controller  15  is easy. On the contrary, the DFB-LD has problems, such as reduced reliability of the light source due to a temperature dependence or deterioration with time and a high cost. There is demand to configure the reference light source unit  12  using an element other than the DFB-LD. Furthermore, the peak of an output waveform cannot be detected when the LED alone is used as the reference light source unit  12  and the LED cannot be applied to the AOTF subsystem as it is.  
       SUMMARY OF THE INVENTION  
       [0011]     It is an object of the present invention to at least solve the above problems in the conventional technologies.  
         [0012]     A wavelength selecting apparatus according to one aspect of the present invention includes a wavelength selecting filter that is a first acousto-optic tunable filter; a monitoring filter that is a second acousto-optic tunable filter like the first acousto-optic tunable filter; a light source that outputs light having a specific wavelength to the monitoring filter; a light receiving unit that detects a wavelength of light that has passed the monitoring filter; and a control unit that outputs, to the wavelength selecting filter, a control signal having a control frequency to selectively pass light of a desirable wavelength. The light source includes a light emitting element that emits light of a wide wavelength band, and an optical filter having at least two transmission bands and through which the emitted light passes to be output to the monitoring filter. The control unit outputs the control signal that has the control frequency that corresponds to the light of the desirable wavelength, based on at least two wavelengths of the light output by the light source.  
         [0013]     According to another aspect of the present invention, a wavelength selecting method of outputting a control signal having a control frequency that selectively passes a light of a desirable wavelength through a wavelength selecting filter of a pair of like acousto-optic tunable filters and of outputting the control signal based on a detected wavelength of a reference light of a specific wavelength when the reference light is passed through a monitoring filter of the pair of like acousto-optic tunable filters, includes outputting light of a wide wavelength band as the reference light to the monitoring filter, through an optical filter having at least two transmission bands, from a light emitting element; detecting at least two wavelengths in the specific wavelength of the reference light transmitted through the monitoring filter; calculating characteristic information of the control signal based on at least two of the detected wavelengths and the frequency of the reference signal respectively corresponding thereto; obtaining the control frequency corresponding to the desirable wavelength based on the characteristic information; and outputting the control frequency to the wavelength selecting filter.  
         [0014]     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  
       [0015]      FIG. 1  is a diagram of a wavelength selecting apparatus according to an embodiment;  
         [0016]      FIG. 2  illustrates an output spectrum from a general reference light source unit;  
         [0017]      FIG. 3  illustrates an output spectrum from a reference light source unit according to the embodiment;  
         [0018]      FIG. 4  is a flowchart of a peak detecting operation;  
         [0019]      FIG. 5  is a graph of RF control signal frequency versus photo diode (PD) values;  
         [0020]      FIG. 6  is a diagram of RF control signal frequency versus transmission wavelength characteristics;  
         [0021]      FIG. 7  is a diagram of transmission characteristics of an optical filter used in the embodiment;  
         [0022]      FIG. 8  is a diagram of PD values for each light signal power at the time of input to the PD;  
         [0023]      FIG. 9  is a view of a conventional AOTF subsystem; and  
         [0024]      FIG. 10  is a view of an AOTF. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     Referring to the accompanying drawings, exemplary embodiments according to the present invention are explained in detail below.  
         [0026]      FIG. 1  is a diagram of a wavelength selecting apparatus  100  according to the present embodiment. The wavelength selecting apparatus  100  includes an AOTF  101  for wavelength selection, a reference light source unit  102 , a monitoring AOTF  103 , a light receiver  104 , a controller  105 , and a temperature controller  106 .  
         [0027]     Although the AOTF  101  and the monitoring AOTF  103  are realized by a single AOTF module configured as a module, the AOTF  101  and the monitoring AOTF  103  are individually depicted since they have independent input and output ports. The AOTF  101  selects light having an arbitrary wavelength from multiplexed signal lights (λa to λh) input from the outside and transmits the selected light as output light (λd) to the outside, and is used as a main function unit in the wavelength selecting apparatus  100 .  
         [0028]     Since the wavelength of light selected by the AOTF  101  is dependent on the frequency of the RF control signal applied by the controller  105 , a relationship between the frequency of the RF control signal and the wavelength of the selected light must be derived to accurately select light having the arbitrary wavelength. The relationship between the frequency of the RF control signal and the wavelength of light selected by the AOTF  101  can be represented as a linear primary expression.  
         [0029]     Therefore, light having a known wavelength is input to the monitoring AOTF  103 , and a frequency value of the RF control signal required to allow this light having the known wavelength to be transmitted as selected light through the monitoring AOTF  103  is obtained. In regard to two or more lights having known wavelengths, a primary expression that is used to calculate a frequency of the RF control signal required to allow light having an arbitrary wavelength to be transmitted through the monitoring AOTF  103  is obtained from a frequency value that allows transmission through the monitoring AOTF  103 .  
         [0030]     The primary expression obtained by using the monitoring AOTF  103  is stored in the controller  105  and used when setting the frequency of the RF control signal that is required for transmission of arbitrary selected-light from among multiplexed signal lights input to the AOTF  101 .  
         [0031]     The reference light source unit  102  includes an LED  107  that outputs light having a broadband wavelength and an optical filter  108  having multiple transmission bands. For example, a general-purpose superluminescent-LED (SLED) can be used. The SLED can obtain an optical output that is at least −40 dBm in a broadband of, e.g., 1520 nanometer (nm) to 1590 nm, and is inexpensive (however, the dBm value is a value when performing observation using an optical spectrum analyzer having a resolution of 0.1 nm and is also applied to the following optical power). An emission wavelength can be a wavelength used in an L band based on a change in design as well as a wavelength in a C band. The reference light source unit  102  can be formed of an excitation light source and an erbium-doped fiber (EDF). Reference light output from the reference light source unit  102  includes peaks whose number corresponds to the number of transmission bands of the optical filter  108 . A Fabry-Perot filter (FP filter) having two transmission bands in the optical filter  108  is used in this embodiment. As a result, the reference light source unit  102  outputs reference light having two peaks (wavelengths λ 1  and λ 2 ).  
         [0032]     The reference light is input to the monitoring AOTF  103 , and the RF control signal is also applied from the controller  105 . The frequency of the RF control signal is variable, and the wavelength of light transmitted through the AOTF  101  and the monitoring AOTF  103 , i.e., selected light varies in response to a change in frequency of the RF control signal as explained above.  
         [0033]     The light transmitted through the monitoring AOTF  103  is received by the light receiver  104 . The light receiver  104  includes a PD as a photo detector. The PD outputs a current value whose amount is in proportion to an intensity of light input thereto. Since the reference light (wavelengths λ 1  and λ 2 ) alone is input to the monitoring AOTF  103 , a current value of the PD shows a peak when the RF control signal having a frequency that allows transmission of lights having λ 1  and λ 2  is applied.  
         [0034]      FIG. 2  is a view of an output spectrum from a general reference light source unit, and  FIG. 3  is a view of an output spectrum from the reference light source unit according to this embodiment. The abscissa represents frequency, and the ordinate represents output level. As shown in  FIG. 2 , reference light output from a reference light source unit  12  (see  FIG. 9 ) using two DFB-LD&#39;s as a light source element shows a sharp output spectrum, and a light receiver  14  (see  FIG. 9 ) can accurately detect a peak. Since the detected peak belongs to the reference light (wavelengths λ 1  and λ 2 ), the RF control signal allowing transmission of a light signal having an arbitrary wavelength can be derived from a relationship between the RF control signal at this time and the wavelengths λ 1  and λ 2 , and using the derived signal as an RF control signal applied to the AOTF  101  enables highly accurate wavelength selection.  
         [0035]     As shown in  FIG. 3 , the reference light output from the reference light source unit  102  according to the present embodiment has an output spectrum cut out based on transmission characteristics of the optical filter  108  (see  FIG. 1 ), a full width at half-maximum (FWHM) is wide. If nothing is done, a peak cannot be accurately found by the light receiver  104 .  
         [0036]     This FWHM means a spreading width in a lateral direction at a height that is half of a peak amplitude of a waveform and is used as a value representing a transmissivity of the optical filter  108 . Detection of a peak is easy when the FWHM is narrow. If reference light having such an output spectrum as depicted in  FIG. 2  is a target, a FWHM is very narrow, and a peak can be readily detected. However, when the reference light source  102  includes the LED  107  and the optical filter  108  as in this embodiment, an FWHM is dependent on the transmissivity of the optical filter  108 , and the FWHM can only be reduced to a value according to the performance of the optical filter  108 . At the present time, there is no optical filter having transmission characteristics allowing transmission of a peak like the DFB-LD. Therefore, according to an embodiment of the present invention, the following method is used to accurately detect a peak corresponding to reference light even when the peak has an FWHM to some extent.  
         [0037]      FIG. 4  is a flowchart of a peak detecting operation according to the embodiment. First, the reference light (wavelengths λ 1  and λ 2 ) is input to the monitoring AOTF  103  from the reference light source unit  102  (step S 401 ).  
         [0038]     Then, the controller  105  stores a relationship between the frequency of the RF control signal applied to the monitoring AOTF  103  and the read value of the PD in the light receiver  104  (step S 402 ). At this step, the controller  105  applies the RF control signal, while changing the frequency, to the monitoring AOTF  103  that is receiving the reference light (wavelengths λ 1  and λ 2 ), and the light transmitted through the monitoring AOTF  103  is received by the light receiver  104 . The light receiver  104  includes the PD, and a read value of this PD is supplied to the controller  105 . The read value of the PD means a value of a current that flows when the PD receives light. Since an intensity of the light is in proportion to a current amount, the current value represents a relative light intensity as it is. Therefore, when the controller  105  stores a relationship between the frequency of the applied RF control signal and the read value of the PD corresponding to the frequency, this means that data required to obtain a frequency allowing transmission of the reference light is accumulated.  
         [0039]     Next, two peaks (P 1  and P 2 ) are extracted from the relationship between the frequency of the RF control signal and the read value of the PD, and the extracted peaks are recorded (step S 403 ).  FIG. 5  is a graph of RF control signal frequency and read values of the PD. The abscissa represents frequency (Hz) of the RF control signal applied from the controller  105 , and the ordinate represents the corresponding read value of the PD in the light receiver  104 . By the operation at step S 402 , the controller  105  has such a relationship as depicted in  FIG. 5  recorded therein. The read value of the PD includes pure light transmitted through the monitoring AOTF  103  as well as noise of the PD element itself or noise generated due to light leaked from other ports. At step S 403 , a noise level Pnoise is first determined based on the relationship between the frequency of the RF control signal and the read value of the PD stored in the controller  105 , and then two peaks (P 1  and P 2 ) are extracted from the read value of the PD and recorded.  
         [0040]     Subsequently, the controller  105  obtains P 1   a  (identification point) and P 2   a  (identification point) serving as identification points from respective waveforms of the peak values P 1  and P 2  by using the following expressions 1 and 2 to detect true peaks corresponding to the reference light (wavelengths λ 1  and λ 2 ) by using Pnoise and the peak values P 1  and P 2  stored at step S 403  (step S 404 ). 
 
 P 1 a  (identification point)=( P 1+Pnoise)/2  (1) 
 
 P 2 a  (identification point)=( P 2+Pnoise)/2  (2) 
 
         [0041]     Next, the P 1   a  (identification point) and the P 2   a  (identification point) obtained at step S 404  are assigned to the relationship between the frequency of the RF control signal and the read value of the PD stored in the controller  105  and frequencies f 11  and f 1   h  of the RF control signal at P 1   a  (identification point) and frequencies f 21  and f 2   h  of the RF control signal at P 2   a  (identification point) are obtained (step S 405 ). These f 11 , f 1   h , f 21 , and f 2   h  are values of the two RF control signals (a short-wavelength f 11  or f 21  side and a long-wavelength f 1   h  or f 2   h  side) placed on the respective waveforms at the identification points P 1   a  and P 2   a  as shown in  FIG. 5 .  
         [0042]     Subsequently, frequencies f 1  and f 2  of the RF control signal allowing transmission of the true peaks corresponding to the reference light (wavelengths λ 1  and λ 2 ) are obtained from f 11 , f 1   h , f 21 , and f 2   h  acquired at step S 405  by using the following expressions 3 and 4 (step S 406 ), thereby terminating the peak detecting operation. Accurate detection by this peak detection method when the optical filter  108  having a FWHM of 0.8 nm or below is used has been experimentally demonstrated. 
 
 f 1=( f 11 +f 1 h )/2  (3) 
 
 f 2=( f 21 +f 2 h )/2  (4) 
 
         [0043]      FIG. 6  is a diagram of RF control signal frequency-transmission wavelength characteristics. The ordinate represents RF control signal frequency (Hz), and the abscissa represents a transmission wavelength (nm). Since the reference light (wavelengths λ 1  and λ 2 ) and the frequencies f 1  and f 2  of the RF control signal allowing the reference light (wavelengths λ 1  and λ 2 )) to be transmitted through the monitoring AOTF  103  are determined, a primary line representing RF control signal frequency-transmission wavelength characteristics can be obtained. This primary line can be acquired by using the following expression 5. A frequency f 3  of the RF control signal that allows a light signal having an arbitrary wavelength λ 3  to be transmitted can be set based on this primary line. The primary line represented by the expression 5 can be stored in a non-depicted storage unit provided in the controller  105 . 
   f 3=( f 1− f 2)/(λ2−λ1)×(λ3−λ1)+ f 1  (5)  
         [0044]     Therefore, in the wavelength selecting apparatus  100  depicted in  FIG. 1 , the controller  105  can apply the RF control signal corresponding to a wavelength kd to the AOTF  101  that receives multiplexed light signals (λa to λh) to output from an Out port a light signal having a selected wavelength (λd in the depicted example) alone among the multiplexed light signals input from an In port. In regard to the frequency of the RF control signal applied in this example, expression 5 derived by using the reference light (wavelengths λ 1  and λ 2 ) can be read from the storage unit and used. When outputting light signals having multiple wavelengths, the multiple RF control signals obtained by using expression 5 may be applied. As explained above, a combination of the two reference light values and the frequency values of the control signal allowing transmission of the reference light is used to obtain a slope and an intercept of the primary expression, which represents the relationship between the wavelength light to be transmitted and the frequencies of the control signal. Then, the arbitrary wavelength light λ 3  is input as a variable in the primary expression, expression 5, and the frequency f 3  of the control signal allowing transmission of this arbitrary wavelength light is calculated.  
         [0045]      FIG. 7  is a diagram of transmission characteristics of an example of the optical filter  108  used in the embodiment. The ordinate represents a transmission loss (dB), and the abscissa represents a wavelength (nm) of light to be transmitted. As shown in the figure, transmission characteristics of the optical filter  108  include two peaks. The optical filter  108  is a multilayer film type FP filter (manufactured by, e.g., Santec corporation; see Japanese Patent Application Laid-open No. 2004-138798 and Japanese Patent Application Laid-open No. 2004-177658), and has free spectral range (FSR) characteristics of 60 nm, a transmission loss of 5 dB or below, a C band suppression ratio of 15 dB or above, and a FWHM of 0.5 nm or below. The FSR is a value indicative of a peak interval of a waveform, and an element representing the transmissivity of the optical filter  108  such as an FWHM.  
         [0046]     Commonly, light having a wavelength different from the wavelength band of the light signal that allows transmission is utilized as the reference light (wavelengths λ 1  and λ 2 ), and a wavelength having an arrangement where the two reference lights sandwich the wavelength band of the light signal is required. In present optical networks, light having a C band (1530 nm to 1565 nm) wavelength is mainly used, and characteristics enabling output of light having wavelengths sandwiching a C band are required as with the optical filter  108 . Therefore, a wavelength band of a laser beam input from the LED  107  must be equal to or above an FSR width of the optical filter  108 . In recent years, in addition to the C band, utilization of an L band (1565 nm to 1625 nm) having a longer wavelength has also advanced. When coping with multiplexed signal lights having such a wide band, the optical filter  108  having a wide FSR width sandwiching both the C band and the L band, and the LED  107  having a wavelength band that is equal to or above the FSR width are used.  
         [0047]      FIG. 8  is a diagram of a read value of the PD with respect to each power of a light signal at the time of input to the PD. Performance of the LED  107  (see  FIG. 1 ) constituting the reference light will now be examined with reference to this  FIG. 8 . The abscissa shown in  FIG. 8  represents a control signal frequency (kHz) from the controller  105 , and the ordinate represents a PD read value (relative intensity of light) in the light receiver  104 . A waveform of a read value of the PD varies with respect to each value of a power (input power) of the reference light at the time of input to the PD. With consideration of the results and experimental data depicted in  FIG. 8 , an input power that is not smaller than −65 dBm is required to obtain a waveform that enables discrimination of a peak. As explained above, the transmission loss of the optical filter  108  (FP filter) according to this embodiment is equal to or below 5 dB, and a transmission loss of the AOTF  103  is 5 dB at worst. Therefore, an output power of the LED  107  that is equal to or above −55 dBm may be provided to assure an input power that is equal to or above −65 dBm at the time of input to the PD.  
         [0048]     As explained above, in the LED  107  constituting the reference light source unit  102  according to the embodiment, an output laser bandwidth should be 30 nm or above and an output power should be −55 dBm or above to cope with at least one band (e.g., the C band or the L band). In the optical filter  108 , an FSR is preferably 30 nm or above to generate reference light sandwiching at least one band, and an FWHM is preferably 0.8 nm or below to detect each peak.  
         [0049]     Demand for such conditions can be satisfied by using a general LED element or a general optical filter. The wavelength selecting apparatus  100  according to the present embodiment can be manufactured at a lower cost than that of a conventional AOTF subsystem. Since the optical filter  108  is a passive element, age-related deterioration does not occur. Facets of the LED  107  rarely deteriorate compared with the DFB-LD, and precise wavelength control is not required. Therefore, the LED  107  has less factors of age-related deterioration, and has a long life. Based on these matters, the entire reference light source unit  102  is highly reliable, and its stable performance can be maintained.  
         [0050]     According to the embodiments describe above, a reference light source unit can be configured using a general-purpose optical component and a low cost wavelength selecting apparatus having high reliability with less aged deterioration can be effected.  
         [0051]     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.