Patent Application
20100021182
1. Field of the Invention
The present invention relates to an optical transmitter that transmits a phase-modulated optical signal.
Priority is claimed on Japanese Patent Application No. 2008-190991, filed Jul. 24, 2008, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
In recent years, research and development have been made actively to achieve an optical transmission system of a large capacity, which performs long-distance optical transmission. It has been desired to practice the optical transmission systems that transmit and receive phase-modulated optical signals, wherein the optical transmission systems have high maximum transmission rates. The optical transmission systems can use various types of phase modulation such as DPSK (Differential Phase Shift Keying) and DQPSK (Differential Quadrature Phase Shift Keying) in order to rise the maximum transmission rate from 10 Gbps (bit per second) up to 40 Gbps (bit per second).
Japanese Unexamined Patent Applications, First Publications, Nos. 2004-318052 and 2007-82094 each disclose an optical modulator that includes a Mach-Zehnder waveguide for performing optical modulation of a light emitted from a light source. The Mach-Zehnder waveguide is formed on a substrate that is made of a substance exhibiting electro-optical effect, such as lithium niobate (LiNbO3).
Concretely, the optical modulator disclosed in the First Publications is a nest type modulator includes a Mach-Zehnder waveguide performing as a main waveguide and two Mach-Zehnder waveguides performing as first and second sub-waveguides. The Mach-Zehnder waveguide as the main waveguide receives a light from a light source and branches the light into branched lights, so that each of the branched lights passes through in a respective one of mutually different light paths and then the branched lights are combined together. The two Mach-Zehnder waveguides performing as the first and second sub-waveguides are formed in each of the optical paths of the main waveguide. Electrodes to which modulated signals are applied are built on the first and second sub-waveguides of the optical modulator. Optical signals are modulated according to the modulated signals applied to each of the electrodes and the modulated optical signals are output from the optical modulator.
The optical modulator has output characteristics that may vary depending on the temperature of usage environment such as a temperature drift and that may vary over time such as a time-dependant drift. For these reasons, each of the first and second sub-waveguides of the optical modulator has additional electrodes that perform as biased electrodes to which a DC bias is applied for controlling the drifts. The main waveguide also has additional electrodes that perform as biased electrodes. The optical modulator has bias control circuits corresponding to the biased electrodes. Each of the bias control circuits controls the voltage of a DC bias applied to a respective one of the biased electrodes.
Each bias control circuit applies a corresponding one of the bias electrodes with a DC bias that is superimposed with a dither signal that has each different frequency. Each bias control circuit receives optical signals output from the optical modulator. Each bias control circuit synchronously detects each received optical signal separately. The synchronous detection is made using a dither signal superimposed on the DC bias. The synchronously detected signal is used to control the voltage of each DC bias applied to a corresponding one of the biased electrode, so as to suppress the temperature drift and the time-dependant drift.
As described above, each biased electrode is disposed on a corresponding one of the bias electrodes that are disposed on the main waveguide and the first and second sub-waveguides. Dither signals with different frequencies are superimposed on the DC bias to be applied to biased electrodes, wherein the dither signals are used to make the synchronous detection of the received optical signal. Thus, the optical transmitter of the prior art allows the bias control circuits to perform the DC-bias voltage controls simultaneously. But the optical transmitter needs a pair of a dither signal source and a synchronous detection circuit for each of the bias control circuits. That is, the number of the bias control circuits will be equal to the number of pairs of dither signal sources and the synchronous detection circuits. This configuration leads to the increased circuit size as the number of the bias control circuits is increased.
In addition, since the optical modulator is integrated in the optical transmitter, the size of the optical modulator or the area for mounting the optical modulator is restricted to some extent. Here, when the optical modulator itself is made miniaturized adjusting to the mounted area, the biased electrodes disposed on the main waveguide and the first and second sub-waveguides may be limited in its dimension that is defined in a direction along the optical paths of the optical transmitter. When the dimension of the biased electrode becomes short, a DC-bias with a higher voltage (for example, several times higher) is necessary to be applied so as to suppress the drift mentioned above. When the optical modulator is miniaturized, application of a higher voltage may cause an electrical discharge because not only the dimension of the biased electrodes becomes short but also a narrow spatial interval between the bias electrode and other electrodes. The electrical discharge may decrease the reliability of the optical transmitter.
In view of the above circumstances, this invention provides an optical transmitter transmitting a phase-modulated optical signal, wherein the optical transmitter has a smaller circuit size and higher reliability.
In one embodiment, an optical transmitter sets up a phase difference between a first optical signal and a second optical signal acquired by dividing a received input light. The optical transmitter may include, but is not limited to, a modulation unit modulating the first and second optical signals respectively based on first and second data signals input from the outside. The optical transmitter outputs a phase-modulated optical signal acquired by multiplexing the first and second optical signals passing through the modulation unit. The optical transmitter includes an optical receiving unit receiving a phase-modulated optical signal and outputting the received optical signal. The optical transmitter includes a control unit superimposing dither signals which have a predetermined frequency on at least one of the first and second optical signals passing through the modulation unit.
The optical transmitter includes a first modulation unit. The first modulation unit sets up a phase difference between two first branched optical signals acquired by further branching the first optical signal mentioned above and modulates respectively the two first branched optical signals based on two data signals of the first data signals mentioned above. The optical transmitter includes a second modulation unit. The second modulation unit sets up a phase difference between two second branched optical signals acquired by further branching the second optical signal mentioned above and modulates respectively the second branched optical signals based on two data signals of the second data signals mentioned above.
The optical transmitter includes a first control unit that superimposes a dither signal having a predetermined frequency on the first branched optical signal passing through the first modulation unit. The optical transmitter performs control of a phase difference between first branched optical signals that is set up in the first modulation unit. The control of the phase difference is based on a result of detection on the dither signal included in the received signal output from the optical receiving unit. The optical transmitter includes a second control unit that superimposes a dither signal having a predetermined frequency on the second branched optical signal passing through the second modulation unit. The optical transmitter performs control of a phase difference between second branched optical signals that is set up in the second modulation unit. The control of the phase difference is based on a result of detection on the dither signal included in the received signal output from the optical receiving unit. The optical transmitter includes a third control unit. The third control unit performs control of a phase difference between the first and second branched optical signals set up by the modulation unit. The control of the phase difference is based on a result of detection on the dither signal included in the received signal output from the optical receiving unit.
The two signals of the first data signals and the two signals of the second data signals are signals that reverse logic mutually or unrelated individual signals.
The dither signal superimposed on the first optical signal and the dither signal that is superimposed on the second optical signal are different in frequency.
Or the dither signal superimposed on the first optical signal and the dither signal superimposed on the second optical signal are identical in frequency and the dither signals are superimposed on the first and second optical signals with time division.
In one embodiment, the optical transmitter superimposes a dither signal having a predetermined frequency on at least one of first and second optical signal passing through a modulating unit. And the optical transmitter controls a phase difference between first and second optical signals based on a result of detecting a dither signal superimposed on a received signal transmitted from the optical receiving unit. There is an effect that circuit size of the optical transmitter can be made small because a composition superimposing a dither signal to perform control of a phase difference between the first and second optical signals can be omitted though it is necessary in traditional optical transmitters. Moreover, the invention needs not superimpose a dither signal to a bias signal controlling a phase lag between the first and the second optical signals. So, voltage of the control signal can be suppressed by equal to the amount of the amplitude of the dither signal. As a result, there is an effect that an electrical discharge can be prevented and high reliability can be obtained.
Referring now to the attached drawings which form a part of this original disclosure:
Selected embodiments of the present invention will now be described with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
The light source 10 may be realized by, but is not limited to, a laser diode (LD). The light source 10 emits a continuous wave light (CW light). The modulation unit 20 may include a Mach-Zehnder modulator that performs as a main modulator M1. The main modulator M1 has a pair of arms 21a and 21b. The modulation unit 20 receives the continuous wave light from the light source 10. The modulation unit 20 branches the continuous wave light to an optical signal L11 (a first optical signal) passing through the arm 21a and an optical signal L12 (a second optical signal) passing through the arm 21b. The modulation unit 20 receives, from the outside, the first data signals D11 and D12 and the second data signals D21 and D22. The modulation unit 20 modulates the optical signal L11 and L12 based on the first data signals D11 and D12 and the second data signals D21 and D22, respectively. The modulation unit 20 controls the phases of the optical signals L11 and L12 under the control of the bias control unit 50. The modulation unit 20 sets up a phase difference between the optical signals L11 and L12. The modulation unit 20 optically couples the modulated optical signals L11 and L12 together to generate a phase-modulated optical signal.
The main modulator M1 has the arms 21a and 21b. The optical signal L11 passes through the arm 21a. The optical signal L12 passes through the arm 21b. The arm 21a includes another Mach-Zehnder modulator that performs as a sub modulator M2 and a bias electrode 24a. The sub modulator M2 has a pair of arms. The arm 21b includes still another Mach-Zehnder modulator that performs as a sub modulator M3. The sub modulator M3 has another pair of arms.
The sub modulator M2 receives the optical signal L11. The sub modulator M2 branches the optical signal L11 into a pair of first branched optical signals. The sub modulator M2 modulates the paired first branched signals based on the data signals D11 and D12, respectively. The sub modulator M2 controls the phase of one of the paired first branched signals under the control of the bias control unit 50. The sub modulator M2 sets up a phase difference between the pair of first branched optical signals under the control of the bias control unit 50. The sub modulator M2 optically couples the first branched signals together to generate a first output optical signal.
The sub modulator M3 receives the optical signal L12. The sub modulator M3 branches the optical signal L12 into a pair of second branched optical signals. The sub modulator M3 modulates the paired second branched signals based on the data signals D21 and D22, respectively. The sub modulator M3 controls the phase of one of the paired second branched signals under the control of the bias control unit 50. The sub modulator M3 sets up a phase difference between the pair of second branched optical signals under the control of the bias control unit 50. The sub modulator M3 optically couples the second branched signals together to generate a second output optical signal.
The sub modulator M2 has modulating electrodes 22a and 22b and a bias electrode 23a. The sub modulator M2 has the pair of arms, one of which has the modulating electrode 22a and the bias electrode 23a, and another arm having the modulating electrode 22b. The modulating electrode 22a receives an application of the data signal D11. The bias electrode 23a receives an application of a bias signal B1 from the bias control unit 50. The modulating electrode 22b receives an application of the data signal D12. The sub modulator M2 modulates the paired first branched signals based on the data signals D11 and D12 that are applied to the modulating electrodes 22a and 22b, respectively. The sub modulator M2 controls the phase of one of the paired first branched signals based on the bias signal B1 that is applied to the bias electrode 23a. The sub modulator M2 sets up a phase difference between the paired first branched optical signals based on the bias signal B1 that is applied to the bias electrode 23a. The sub modulator M2 optically couples the paired first modulated optical signals together to generate a first modulated optical signal.
The sub modulator M3 has modulating electrodes 22c and 22d and a bias electrode 23b. The sub modulator M3 has the pair of arms, one of which has the modulating electrode 22c and the bias electrode 23b, and another arm having the modulating electrode 22d. The modulating electrode 22c receives an application of the data signal D21. The bias electrode 23b receives an application of a bias signal B2 from the bias control unit 50. The modulating electrode 22d receives an application of the data signal D22. The sub modulator M3 modulates the paired second branched signals based on the data signals D21 and D22 that are applied to the modulating electrodes 22c and 22d, respectively. The sub modulator M3 controls the phase of one of the paired second branched signals based on the bias signal B2 that is applied to the bias electrode 23b. The sub modulator M3 sets up a phase difference between the pair of second branched optical signals based on the bias signal B1 that is applied to the bias electrode 23a. The sub modulator M3 optically couples the paired second modulated optical signals together to generate a second modulated optical signal.
The bias electrode 24a on the arm 21a performs as a phase shifter. The bias electrode 24a receives the first modulated optical signal from the sub modulator M2. The bias electrode 24a receives an application of a bias signal B3 from the bias control unit 50. The bias electrode 24a performs as a phase shifter that shifts the phase of the first optical signal. Thus, the bias electrode 24a as the phase shifter sets up the phase difference between the optical signals L11 and L12.
The arms 21a and 21b are connected at the output of the modulation unit 20. The optical signals L11 and L12 having passed through the arms 21a and 21b respectively are coupled to generate a phase-modulated optical signal as an output optical signal.
The optical receiving unit receives the phase-modulated optical signal from the modulation unit 20. The optical receiving unit generates an electric signal based on the phase-modulated optical signal L1. The optical receiving unit includes the optical splitter such as the optical directional coupler 30 and the photodetector 40. The optical splitter such as the optical directional coupler 30 is disposed on the follower stage of the modulation unit 20. The optical directional coupler 30 receives the phase-modulated optical signal L1 output from the modulation unit 20. The optical directional coupler 30 branches the phase-modulated optical signal at a predetermined branching ratio (for example, a ratio of several hundreds to one). For example, the optical directional coupler 30 reflects the phase-modulated optical signal L1 in part and transmits the same in part. The optical directional coupler 30 may reflect a small portion of the phase-modulated optical signal L1 and may transmit the most of the phase-modulated optical signal L1. The transmitted portion of the phase-modulated optical signal L1 is output from the optical transmitter 1. The reflected portion of the phase-modulated optical signal L1 is transmitted to the photodetector 40. The photodetector 40 receives the reflected portion of the phase-modulated optical signal L1 from the optical directional coupler 30. As the optical directional coupler 30, it is possible for example to use an optical branching device of spatial type such as a beam splitter, an optical branching device of optical fiber type or an optical branching device of planar waveguide type. In addition, a rate of a branching ratio of the optical directional coupler 30 is determined based on a sensitivity of the photodetector 40.
The photodetector 40 receives a phase-modulated optical signal reflected by the optical directional coupler 30. The photodetector 40 generates an electrical signal R1 that indicates the intensity of the phase-modulated optical signal L1. The photodetector 40 may be regarded as a photo-electric converter that converts an optical signal into an electrical signal. A typical example of the photodetector 40 may be, but is not limited to, a photodiode (PD) 40. The photodetector 40 can be realized by any types of device that generate an electrical signal R1 indicating the intensity of the phase-modulated optical signal L1.
The bias control unit 50 receives the phase-modulated optical signal L1 from the light receiving unit, for example, the photodetector 40. The bias control unit 50 generates the bias signals B1, B2, and B3, based on the electrical signal R1 and first and second dither signals Z1 and Z2. The bias control unit 50 superimposes the first and second dither signals Z1 and Z2 onto the bias signals B1, B2, respectively. The bias control unit 50 generates the bias signals B1, on which the dither signal Z1 is superimposed. The bias control unit 50 also generates the bias signals B2, on which the dither signal Z2 is superimposed. The bias control unit 50 further generates the bias signals B3, on which no dither signal is superimposed. The bias signals B3 is free of any dither signal. The bias control unit 50 applies the bias signals B1, B2, and B3 to the optical modulation unit 20 so as to allow the optical modulation unit 20 to control the phase difference between the first and second optical signals L11 and L12 based on the bias signals B1, B2, and B3. The bias control unit 50 may apply the optical modulation unit 20 with the bias signals B1 and B2 at the same timings, and with the bias signal B3 at the different timing from the bias signals B1 and B2. For example, the bias control unit 50 applies the bias signals B1, B2, and B3 to the bias electrodes 23a, 23b and 24a, respectively.
The bias control unit 50 may include dither signal generation units 51a and 51b, synchronous detection circuits 52a and 52b, and bias control circuits 53a, 53b and 53c. The bias control unit 50 applies the bias signals B1 and B2, on which dither signals Z1 and Z2 are superimposed, to bias electrodes 23a and 23b respectively. The bias control unit 50 makes a synchronous detection of the dither signal included in the electrical signal R1 from the photodetector 40. The voltage of the bias signals B1, B2 and B3 are controlled based on the detected signal.
The dither signal generation units 51a and 51b generate dither signals Z1 and Z2 having predetermined frequencies respectively. The frequency of dithering signal Z1 generated in the dither signal generation unit 51a is f1 and the frequency of dithering signal Z2 generated in the dither signal generation unit 51b is f2. The frequency of each of the dither signals Z1 and Z2 can be set to be an arbitrary frequency. In general, the frequency f1 of the dithering signal Z1 may be different from the frequency f2 of the dithering signal Z2.
The synchronous detection circuit 52a receives, from the photodetector 40, the electrical signal R1 that indicates the intensity of the phase-modulated optical signal L1. The synchronous detection circuit 52a further receives the dither signal Z1 having the frequency f1 from the dither signal generation unit 51a. The synchronous detection circuit 52a performs a synchronous detection of the electrical signal R1 output from the photodetector 40 using the dither signal Z1 generated in the dither signal generation unit 51a.
The synchronous detection circuit 52a receives, from the photodetector 40, the electrical signal R1 that indicates the intensity of the phase-modulated optical signal L1. The synchronous detection circuit 52a further receives the dither signal Z2 having the frequency f2 from the dither signal generation unit 51b. The synchronous detection circuit 52b performs a synchronous detection of the electrical signal R1 output from a photodetector 40 using the dither signal Z2 generated in the dither signal generation unit 51b.
The synchronous detection circuit 52a generates a detected signal indicating the voltage of the dither signal Z1 included in the electrical signal R1. The synchronous detection circuit 52b generates a detected signal showing the voltage of the dither signal Z2 included in the electrical signal R1.
The bias control circuit 53a receives the detected signal from the synchronous detection circuit 52a. The bias control circuit 53a also receives the dither signal Z1 from the dither signal generation unit 51a. The bias control circuit 53a superimposes the dither signal Z1 on the bias signal B1. Namely, the bias control circuit 53a generates the bias signal B1 that is superimposed with the dither signal Z1 and applies the bias signal B1 to the bias electrode 23a. The bias control circuit 53a performs control of the voltage of the bias signal B1 based on the detected signal output from the synchronous detection circuit 52a.
The bias control circuit 53b receives the detected signal from the synchronous detection circuit 52b. The bias control circuit 53b also receives the dither signal Z2 from the dither signal generation unit 51b. The bias control circuit 53a superimposes the dither signal Z2 on the bias signal B2. Namely, the bias control circuit 53b generates the bias signal B2 that is superimposed with the dither signal Z2 and applies the bias signal B2 to the bias electrode 23b. The bias control circuit 53b performs control of the voltage of the bias signal B2 based on the detected signal output from the synchronous detection circuit 52b. The bias control circuits 53a and 53b may perform controls of the voltages of the bias signals B1 and B2 at the same timing because the dither signals Z1 and Z2 having the different frequencies are superimposed on the bias signals B1 and B2, respectively.
The bias control circuit 53c receives the detected signal from the synchronous detection circuit 52b. The bias control circuit 53c does not receive either the dither signals Z1 and Z2 from the dither signal generation units 51a and 51b or any other dither signals. The bias control circuit 53c does not superimpose any dither signal on the bias signal B3. The bias control circuit 53c generates the bias signal B3 on which no dither signal is superimposed, so that the bias signal B3 is free of any dither signal. The bias control circuit 53c performs control of the voltage of the bias signal B3 based on the detected signal output from the synchronous detection circuit 52b. The bias control circuit 53c applies the bias signal B3 to the bias electrode 24a disposed on the main modulator M1.
As described above, the dither signal generation unit 51a and the synchronous detection circuit 52a work in cooperation for the bias control circuit 53a. The dither signal generation unit 51b and the synchronous detection circuit 52b work in cooperation for the bias control circuit 53b. The synchronous detection circuit 52b further works alone for the bias control circuit 53c. Namely, the synchronous detection circuit 52b works for both the bias control circuits 53b and 53c.
Any particular dither signal generator that operates for the bias control circuit 53c is not needed. Any particular synchronous detection circuit that operates only for the bias control circuit 53c is not needed. Any dither signal is not superimposed on the bias signal B3. The bias control circuit 53c does not need any dither signal generation unit. The bias control circuit 53c does not need any particular synchronous detection circuit that operates only for the bias control circuit 53c. The bias control circuits 52b and 53c share the synchronous detection circuit 52b commonly.
Namely, the bias control unit 50 includes the first set of the dither signal generation unit 51a and the synchronous detection circuit 52a, and the second set of the dither signal generation unit 51b and the synchronous detection circuit 52b. The bias control unit 50 includes the three bias control circuits 53a, 53b and 53c which generate the bias signals B1, B2, and B3, respectively. The number of the dither signal generation units 51a and 51b is smaller than the number of the bias control circuits 53a, 53b and 53c. The number of the synchronous detection circuits 52a and 52b is smaller than the number of the bias control circuits 53a, 53b and 53c. The bias control unit 50 includes the reduced number of sets of the dither signal generation units 51a and 51b and the synchronous detection circuits 52a and 52b. This configuration reduces the scale of the circuit for a bias control.
In the bias control unit 50, the bias control circuit 53a performs control of voltage of the bias signal B1, and the bias control circuit 53b performs control of voltage of the bias signal B2 at the same time. After the controls of the bias signals B1 and B2, the bias control circuit 53c performs control of voltage of the bias signal B3. Here, signals that the bias control circuits 53a and 53b superimpose on the bias signals B1 and B2 respectively have mutually different frequencies (f1, f2). In the bias control unit 50, the control of voltage of the bias signal B1 by the bias control circuit 53a and the control of voltage of the bias signal B2 by the bias control circuit 53b can be performed at the same time.
The bias control circuit 53c performs control of the bias voltage B3 so that the voltage of the detected signal output from the synchronous detection circuit 52b becomes zero.
The operations of the optical transmitter 1 of the first embodiment are to be explained. The main modulator M1 of the modulation unit 20 receives a continuation light from the light source 10, and the main modulator M1 branches the continuation light to the optical signal L11 passing through the arm 21a and the optical signal L12 passing through the arm 21b by the intensity ratio of 1 to 1 for example. The branched optical signal L11 passing through the arm 21a enters the sub modulator M2 disposed on the arm 21a to be branched furthermore. The branched optical signal L12 passing through the arm 21b enters the sub modulator M3 disposed on the arm 21b to be branched furthermore.
The branched optical signal L11 is further branched into first and second branched lights by the sub modulator M2. The branched optical signal L12 is further branched into third and fourth branched lights by the sub modulator M3. The first branched light is modulated based on the data signal D11 applied to the modulating electrode 22a. The second branched light is modulated based on the data signal D12 applied to the modulating electrode 22b. Moreover, the phase of the first branched light as modulated based on the data signal D11 is then shifted by the bias signal B1 applied to the bias electrode 23a. The phase of the second branched light is set up so that the second branched light has a predetermined phase difference from the first branched light. Then, the first and second branched lights are optically coupled together. Since the bias signal B1 is superimposed on the dither signal Z1, the frequency of the optical signal L11 having passed through the sub modulator M2 includes a frequency component at the frequency f1 of the dither signal Z1.
The third branched light is modulated based on the data signal D21 applied to the modulating electrode 22c. The fourth branched light is modulated based on the data signal D22 applied to the modulating electrode 22d. Moreover, the phase of the third branched light modulated based on the data signal D21 is shifted by the bias signal B2 applied to the bias electrode 23b. The phase of the fourth branched light is set up so that the fourth branched light has a predetermined phase difference from the third branched light. Then, the third and fourth branched lights are optically coupled together. Since the bias signal B2 is superimposed on the dither signal Z2, the frequency of the optical signal L12 having passed through the sub modulator M3 includes a frequency component at the frequency f2 of the dither signal Z2.
A phase of the optical signal L11 passing through the sub modulator M2 is changed by π/2 by the bias signals B3 applied to the bias electrode 24a. As a result, there is a phase difference of π/2 between the optical signal L11 having passed through the sub modulator M2 and the optical signal L12 having passed the sub modulator M3. Then, the optical signal L11 having passed through the arm 21a and the optical signal L12 having passed through the arm 21b are optically coupled together to generate a phase-modulated optical signal. The phase-modulated optical signal is output from the modulation unit 20. The phase-modulated optical signal passes through the optical directional coupler 30, and is transmitted outside as a phase-modulated optical signal L1.
Here, the phase-modulated optical signal which has entered into the optical directional coupler 30 is then received by a photodetector 40, and an electrical signal R1 is output from the photodetector 40. The electrical signal R1 enters into a bias control unit 50. Each of synchronous detection circuits 52a and 52b makes a synchronous detection of the electrical signal R1. As a result, from the synchronous detection circuit 52a, the detected signal showing the voltage of the dither signal Z1 included in the electrical signal R1 is output. And from the synchronous detection circuit 52b, the detected signal showing the voltage of the dither signal Z2 included in the electrical signal R1 is output.
The detected signals of the synchronous detection circuits 52a and 52b are input into bias control circuits 53a and 53b, respectively. Controls of the bias signals B1 and B2 are performed in parallel by the bias control circuits 53a and 53b respectively. Since the detected signals Z1 and Z2 have different frequencies, controls by the synchronous detection circuits 53a and 53b can be performed in parallel. After the control of the bias signals B1 and B2 was completed, control of the bias voltage B3 is performed by the bias control circuit 53c so that the detected signal output from the synchronous detection circuit 52b becomes zero, as is described above with reference to
As described above, the bias control unit 50 includes the dither signal generation units 51a and 51b and the synchronous detection circuits 52a and 52b. The dither signal generation units 51a and 51b generate the dither signals Z1 and Z2 superimposed on the bias signals B1 and B2, respectively. The synchronous detection circuits 52a and 52b perform synchronous detections of the dither signals Z1 and Z2 included in the electrical signal R1 that has received from the photodetector 40. The bias control circuit 53c performs the control of voltage of the bias signal B3 based on the detected signal that has been supplied from the synchronous detection circuit 52b.
Therefore, neither of a dither signal generation unit and a synchronous detection circuit for the bias control circuit 53c is needed, and circuit scale of the bias control unit 50 can be made smaller. Moreover, it is not necessary to superimpose a dither signal on the bias signal B3, and voltage of the bias signal B3 can be made smaller by amplitude of the dither signal, so electric discharge can be prevented and high reliability can be acquired. However, the bias control circuit 53c uses the detected signal from the synchronous detection circuit 52b. So frequencies of the detected signal used in the bias control circuit 53b and the bias control circuit 53c are equal. As a result, control in the bias control circuit 53b and control in the bias control circuit 53c cannot be performed in parallel. It is necessary to perform control in the bias control circuit 53b and control in the bias control circuit 53c at different times.
In addition, in the first embodiment, the case where the bias control circuit 53c performs control of the voltage of the bias signal B3 based on the detected signal output from the synchronous detection circuit 52b is explained as shown in
In the first embodiment, the case where the frequency of the dither signals Z1 and Z2 superimposed respectively on the bias signals B1 and B2 are different is explained. But as long as control by the bias control circuit 53a and control by the bias control circuits 53b are performed at different times, the frequency of the dither signals Z1 and Z2 can be the same. Detected signals of a same frequency can be used in different bias control circuits as long as controls in the bias control circuits are performed at different times. The order of performing the controls in the bias control circuits is arbitrary. When frequency of the dither signals Z1 and Z2 are the same, one dither signal generation unit and one synchronous detection circuit can be shared in the bias control circuits 53a, 53b and 53c, so a circuit scale can be made smaller.
Furthermore, in the embodiment explained above, the case where the data signals D11 and D12 and the data signals D21 and D22 are differential signals reversed in logic mutually and respectively is explained. However, the individual signal which is mutually unrelated can be used as these data signals D11, D12, D21 and D22. In such case, the optical transmitter 1 can transmit the phase-modulated optical signal L1 modulated by 16 values.
The sub modulator M11 receives one of branched lights branched by the sub modulator M2, and branches the branched light into two branched optical signals. The sub modulator M11 sets up a phase difference between the two branched optical signals. The two branched optical signals are modulated respectively based on data signals D11 and D12, then the two modulated optical signals are optically coupled together to be output from the sub modulator M11. The sub modulator M12 receives the other of the branched lights branched by the sub modulator M2, and branches the branched light into two branched optical signals. The sub modulator M12 sets up a phase difference between the two branched optical signals. The two branched optical signals are modulated respectively based on data signals D21 and D22, then the modulated optical signals are optically coupled together to be output from the sub modulator M12.
The sub modulator M13 receives one of branched lights branched by the sub modulator M3, and branches the branched light into two branched optical signals. The sub modulator M13 sets up a phase difference between the two branched optical signals. The two branched optical signals are modulated respectively based on data signals D31 and D32, then the two modulated optical signals are optically coupled together to be output from the sub modulator M13. The sub modulator M14 receives the other of the branched lights branched by the sub modulator M3, and branches the branched light into two branched optical signals. The sub modulator M14 sets up a phase difference between the two branched optical signals. The two branched optical signals are modulated respectively based on data signals D41 and D42, then the modulated optical signals are optically coupled together to be output from the sub modulator M14.
On one arm of the sub modulator M11, a modulating electrode 25a applied with the data signal D11 and a bias electrode 26a applied with a bias signal B11 from the bias control unit 60 are disposed. On the other arm of the sub modulator M11, a modulating electrode 25b applied with the data signal D12 is disposed. Two branched lights are modulated respectively by the data signals D11 and D12 applied respectively to the modulating electrodes 25a and 25b. A phase difference between the two branched lights is set up by the bias signal B11 applied to the bias electrode 26a.
On one arm of the sub modulator M12, a modulating electrode 25c applied with the data signal D21 and a bias electrode 26b applied with a bias signal B12 from the bias control unit 60 are disposed. On the other arm of the sub modulator M12, a modulating electrode 25d applied with the data signal D22 is disposed. Two branched lights are modulated respectively by the data signals D21 and D22 applied respectively to the modulating electrodes 25c and 25d. A phase difference between the two branched lights is set up by the bias signal B12 applied to the bias electrode 26b.
On one arm of the sub modulator M13, a modulating electrode 25e applied with the data signal D31 and a bias electrode 26c applied with a bias signal B13 from the bias control unit 60 are disposed. On the other arm of the sub modulator M13, a modulating electrode 25f applied with the data signal D32 is disposed. Two branched lights are modulated respectively by the data signals D31 and D32 applied respectively to the modulating electrodes 25e and 25f. A phase difference between the two branched lights is set up by the bias signal B13 applied to the bias electrode 26c.
On one of arms of the sub modulator M14, a modulating electrode 25g applied with the data signal D41 and a bias electrode 26d applied with a bias signal B14 from the bias control unit 60 are disposed. On the other of arms of the sub modulator M14, a modulating electrode 25h applied with the data signal D42 is disposed. Two branched lights are modulated respectively by the data signals D41 and D42 applied respectively to the modulating electrodes 25g and 25h. A phase difference between the two branched lights is set up by the bias signal B14 applied to the bias electrode 26d.
In addition, a bias electrode 23a in the sub modulator M2 is applied with a bias signal B21 from the bias control unit 60 and a bias electrode 23b in the sub modulator M3 is applied with a bias signal B22 from the bias control unit 60. And a bias electrode 24a in a main modulator M1 is applied with a bias signal B31 from the bias control unit 60.
The dither signal generation units 61a, - - - , 61d generate dither signals Z1, - - - , Z4. Frequencies of the dither signals Z1, - - - , Z4 differ mutually. The synchronous detection circuits 62a, - - - , 62d make a synchronous detection of the electrical signal R1 output from the photodetector 40 using the dither signals Z1, - - - , Z4 generated by the dither signal generation units 61a, - - - , 61d. As a result, the synchronous detection circuits 62a, - - - , 62d respectively output detected signals which show voltage of the dither signals Z1, - - - , Z4 included in the electrical signal R1.
The bias control circuits 63a, - - - , 63d respectively output the bias signals B11, - - - , B14 which are generated respectively in the dither signal generation units 61a, - - - , 61d and the dither signals Z1, - - - , Z4 are superimposed on respectively. The bias control circuits 63a, - - - , 63d respectively perform control of voltage of the bias signals B11, - - - , B14 based on the detected signals output from the synchronous detection circuits 62a, - - - , 62d respectively. The bias control circuits 63e and 63f respectively perform control of voltage of the bias signals B21 and B22 applied to the bias electrodes 23a and 23b in the sub modulators M2 and M3, based on the detected signals output from the synchronous detection circuits 62b and 62d. The bias control circuit 63b and the bias control circuit 63e use the detected signals from the synchronous detection circuit 62b, and the frequencies of the detected signals used in the bias control circuits 63b and the bias control circuit 63e are equal. Thus, it is necessary to perform controls in the bias control circuit 63b and the bias control circuit 63e at different times. The bias control circuit 63d and the bias control circuit 63f use the detected signals from the synchronous detection circuit 62d, and the frequencies of the detected signals used in the bias control circuit 63d and the bias control circuit 63f are equal. Thus, it is necessary to perform controls in the bias control circuit 63d and the bias control circuit 63f at different times.
The bias control circuit 63g performs control of voltage of the bias signal B31 applied to the electrode 24a in the main modulator M1, based on a detected signal output from the synchronous detection circuit 62d. The bias control circuit 63d, the bias control circuit 63f and the bias control circuit 63g use the detected signals from the synchronous detection circuit 62d, and the frequencies of the detected signals used in the bias control circuit 63d, the bias control circuit 63f and the bias control circuit 63g are equal. Thus, it is necessary to perform controls in the bias control circuit 63d, the bias control circuit 63f and the bias control circuit 63g at different times. As shown in
Fundamental operation of the optical transmitter 2 of the second embodiment is same as that of the optical transmitter 1 of the first embodiment. Concerning about control of bias signals, first, control of the bias signals B11, - - - , B14 is performed respectively in parallel in each of the bias control circuits 63a, - - - , 63d. Frequencies of the detected signals used in the bias control circuits 63a, - - - , 63d are different, so that the controls in the bias control circuits 63a, - - - , 63d can be performed in parallel. Then control of the bias signals B21 and B22 is performed in the bias control circuits 63e and 63f, and finally control of the bias signal B31 is performed in the bias control circuit 63g. However the orders of performing the controls are not limited as long as the controls in the bias control circuits 63a, - - - , 63d and the controls in the bias control circuits 63e and 63f and the control in the bias control circuit 63g are performed at different times.
As described above, in the second embodiment, the bias control unit 60 includes the dither signal generation units 61a, - - - , 61d that generate respectively the dither signals Z1, - - - , Z4 superimposed on the bias signals B11, - - - , B14 respectively. And the bias control unit 60 includes the synchronous detection circuits 62a, - - - , 62d that respectively make a synchronous detection of the dither signals Z1, - - - , Z4 included in the electrical signal R1 output from the photodetector 40. The bias control circuits 63a, - - - , 63d respectively perform controls of voltage of the bias signals B11, - - - , B14 based on the detected signal output from the synchronous detection circuit 62a, - - - , 62d respectively. Frequencies of the detected signals used in the bias control circuits 63a, - - - , 63d are different, so that the controls in the bias control circuits 63a, - - - , 63d can be performed in parallel. The bias control circuits 63e and 63f perform controls of voltage of the bias signals B21 and B22 based on the detected signal output from the synchronous detection circuit 62b. The bias control circuit 63b and the bias control circuit 63e use detected signals from the synchronous detection circuit 62b, and the frequencies of the detected signals used in the bias control circuit 63b and the bias control circuit 63e are equal. Thus, it is necessary to perform controls in the bias control circuit 63b and the bias control circuit 63e at different times. The bias control circuit 63d and the bias control circuit 63f use the detected signals from the synchronous detection circuit 62d, and the frequencies of the detected signals used in the bias control circuit 63d and the bias control circuit 63f are equal. Thus, it is necessary to perform the controls in the bias control circuit 63d and the bias control circuit 63f at different times. And the bias control circuit 63g performs control of the bias signal B31 based on the detected signal output from the synchronous detection circuit 62d. The bias control circuit 63d, the bias control circuit 63f and the bias control circuit 63g use the detected signals from the synchronous detection circuit 62d, and the frequencies of the detected signals used in the bias control circuit 63d, the bias control circuit 63f and the bias control circuit 63g are equal, so it is necessary to perform controls in the bias control circuit 63d, the bias control circuit 63f and the bias control circuit 63g at different times.
Therefore, neither of a dither signal generation unit nor a synchronous detection circuit for the bias control circuits 63e, 63f and 63g is needed, and the circuit scale of the bias control unit 60 can be made smaller. Moreover, it is not necessary to superimpose the dither signals on the bias signals B21, B22 and B31, and the voltage of the bias signals can be made smaller by amplitude of the dither signals, so that any electric discharge can be prevented and high reliability of the optical transmitter can be acquired.
In addition, in the second embodiment described above, the control of the bias signal B21 by the bias control circuit 63e can be performed by the detected signal output from the synchronous detection circuit 62a, and the control of the bias signal B22 by the bias control circuit 63f can be performed by the detected signal output from the synchronous detection circuit 62c. The detected signals used in the bias control circuits 63e and 63f can be selected from any of the detected signals output from the synchronous detection circuits 62a, - - - , 62d as long as the frequencies of the detected signals used in the bias control circuits 63e and 63f are different. And control of the bias signal B31 by the bias control circuit 63g can be performed by any of the detected signals output from the synchronous detection circuits 62a, - - - , 62d. The detected signals used in the bias control circuit 63g can be selected from any of the detected signals output from the synchronous detection circuits 62a, - - - , 62d, as long as the control in the bias control circuit 63g is performed at different time from the controls in the bias control circuits 63a, - - - , 63d.
Moreover, also in the second embodiment, the frequency of the dither signals Z1, - - - , Z4 can be the same as in the first embodiment. When frequency of the dither signals Z1, - - - , Z4 are the same, it is necessary to perform the controls by the bias control circuits 63a, - - - , 63d at different times and to perform controls by the bias control circuits 63e and 63f at different times. The frequencies of the detected signals used in the bias control circuits 63a, - - - , 63g, so it is necessary to perform controls in the bias control circuits 63a, - - - , 63g at different times. The order of performing controls in the bias control circuits 63a, - - - , 63g is arbitrary. When the frequency of the dither signals Z1, - - - , Z4 are the same, one dither signal generation unit and one synchronous detection circuit can be shared in the bias control circuits 63a, - - - , 63d, so that the circuit scale of the control unit can be made smaller.
Furthermore, the data signals D11 and D12, D21 and D22, D31 and D32, and D41 and D42 input from outside may be differential signals which reversed logic mutually and respectively, or may be individual signals which are mutually unrelated. When the data signals D11 and D12, D21 and D22, D31 and D32, and D41 and D42 are differential signals, the optical transmitter 2 can transmit the phase-modulated optical signal L1 modulated by 16 values. When the data signals are individual signals mutually unrelated, the optical transmitter 2 can transmit the phase-modulated optical signal L1 modulated by 256 values.
The optical attenuator 65 makes a compensation for an insertion loss of the sub modulator M13 in the sub modulator M3. The optical attenuator can use a colored glass, a metal thin film, etc. The optical attenuator 65 makes it possible to adjust amount of attenuation of two branched lights branched by the sub modulator M3 to become equal. In addition, an adjustment of a phase change of a branched light caused by the optical attenuator 65 can be performed by the bias signal B22 applied to the bias electrode 23b disposed on the sub modulator M3.
The modulation unit 20 includes a Mach-Zehnder modulator M (a modulator M) having a pair of arms 71a and 71b. The modulator M receives a continuous light from the light source 10, branches the continuous light to an optical signal L11 (a first optical signal) passing through the arm 71a and an optical signal L12 (a second optical signal) passing through the arm 71b. The modulator M sets up a phase difference between the optical signals L11 and L12, modulates the optical signals L11 and L12 respectively based on a data signal D1 (a first data signal) and a data signal D2 (a second data signal) input from outside, and couples the modulated optical signals to be output.
On the arm 71a of the modulator M, an electrode 72a applied with the data signal D1 and a phase shifter 73 applied with a bias signal B3 output from the bias control unit 50 are disposed. On the arm 71b, an electrode 72b applied with a data signal D2 and a bias signal B2 output from the bias control unit 50 are disposed. Two optical signals are modulated respectively by the data signals D1 and D2 applied respectively to the electrodes 72a and 72b. An adjustment of a driving point is performed by the bias signals B1 and B2 applied respectively to electrodes 72a and 72b, and dither signals Z1 and Z2 are respectively superimposed on the optical signals L11 and L12.
The phase shifter 73 gives π/2 of phase difference between the optical signal L11 passing through the arm 71a and the optical signal L12 passing through the arm 71b. A pair of electrodes is disposed on the phase shifter 73 so that a waveguide which the optical signal L11 passes thorough is disposed between the electrodes, for example. And the phase shifter 73 gives π/2 of phase difference between the optical signals L11 and L12 by applying the bias signal B3 to the pair of electrodes. In addition, a fine tuning of a phase difference is made possible by performing a fine tuning of a bias signal applied to a pair of electrodes disposed on the phase shifter 73.
The mixer 74a performs a multiplication of the data signal D1 and the bias signal B1 output from the bias control unit 50. The mixer 74b performs a multiplication of the data signal D2 and the bias signal B2 output from the bias control unit 50. Signals multiplexed in the mixers 74a and 74b are applied respectively to the electrodes 72a and 72b disposed respectively on the arms 71a and 71b of the modulator M.
A fundamental operation of the optical transmitter 3 in the above-mentioned composition is the same as that of the optical transmitter 1 in the first embodiment. Concerning about control of bias signal, first, controls of the bias signals B1 and B2 are performed respectively in the bias control circuits 53a and 53b, and after the controls of the bias signals B1 and B2 are finished, control of the bias signal B3 is performed in the bias control circuit 53c. Order of performing controls in the bias control circuits 53a and 53b and control in the bias control circuit 53c is arbitrary as long as controls in the bias control circuits 53a and 53b and control in the bias control circuit 53c are performed at different times.
As explained above, in the third embodiment, the bias control unit 50 includes the dither signal generation units 51a and 51b and the synchronous detection circuits 52a and 52b like the first embodiment. The dither signal generation units 51a and 51b respectively generate the dither signals Z1 and Z2 superimposed on the bias signals B1 and B2 respectively. The synchronous detection circuits 52a and 52b make a synchronous detection of the dither signals Z1 and Z2 included in the electrical signal R1 output from the photodetector 40. The bias control circuit 53c performs control of voltage of the bias signal B3 based on a detected signal output from the synchronous detection circuit 52b.
Therefore, neither of a dither signal generation unit nor a synchronous detection circuit for the bias control circuit 53c is needed, and circuit scale of the bias control unit 50 can be made smaller. Moreover, it is not necessary to superimpose a dither signal on the bias signal B3, and voltage of the bias signal B3 can be made smaller by amplitude of the dither signal, so electric discharge can be prevented and high reliability can be acquired.
Moreover, also in the third embodiment, control of the bias signal B3 by the bias control circuit 53c can be performed by a detected signal output from the synchronous detection circuit 52a like the first embodiment. When controls by the bias control circuits 53a and 53b are performed at different times, frequency of the dither signals Z1 and Z2 can be the same. When frequency of the dither signals Z1 and Z2 are the same, one dither signal generation unit and one synchronous detection circuit can be shared in the bias control circuits 53a, 53b and 53c, so circuit scale can be made smaller. Furthermore, the data signals D1 and D2 input from outside may be differential signals which reversed logic mutually and respectively, or may be individual signals which are mutually unrelated. And the phase shifter 73 may be disposed on the arm 71b and may be disposed on a previous step of the electrodes 72a and 72b.
The bias control unit 80 has a composition that omits the dither signal generation unit 51a, the synchronous detection circuit 52a, and the bias control circuit 53a from the bias control unit 50 in the third embodiment shown in
While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-190991 | Jul 2008 | JP | national |
