In optical communication systems, large-capacity transmission based on high-speed transmission and wavelength multiplexing has been promoted. Particularly, in optical communication systems for trunk line, wavelength multiplexing communication has been widely used. In the wavelength multiplexing communication, a wavelength channel interval is prescribed and when the wavelength channel interval within a band of an optical fiber amplifier is 50 GHz, it is possible to use approximately one hundred channels.
Here, when representing a channel interval by Δf [Hz] and representing a transmission rate by B [bit/s], B/Δf [bit/s/Hz] is called a frequency efficiency. For example, when Δf=50 GHz and a transmission rate per channel is 100 Gbit/s, the frequency efficiency results in 2 bit/s/Hz.
Because the amplification band of the optical fiber amplifier is limited, in order to transmit a larger amount of information within a fixed band, it is necessary to enhance the frequency efficiency. Meanwhile, merely increasing the bit rate of a signal in order to enhance the frequency efficiency leads to a problem of cross talk between the channels. Thus, as next generation optical communication formats, an optical multiple-value modulation format and an optical orthogonal frequency-division multiplexing (OFDM) method have been considered.
The optical multiple-value modulation format increases information amount without expanding a frequency band by associating each of the amplitude and the phase of light with, not two values, that is, “0” and “1”, but multiple values. Further, in the optical OFDM, an OFDM signal, which is generated on an electric signal basis, is optical-modulated, and the optical-modulated OFDM signal is multiplexed under the state where optical subcarriers are each caused to be in a mutually orthogonal state. Thus, the optical OFDM enables reduction of the occurrence of problems due to the cross talk, and enhancement of the frequency efficiency.
Through such processing based on electric signals, digital data to be modulated is subjected to a multiple-value conversion process and a multiplexing process, and is transmitted. The transmitted light signal is demodulated into an electric signal at a receiving end.
In an optical modulation circuit of an optical transmitter, a digital/analog (D/A) converter and an optical modulator are needed in order to convert the digital data which is an electric signal into an optical analog signal waveform.
As an optical modulator used when performing such a complicated optical modulation as described above, there is a Mach-Zehnder interferometer. The Mach-Zehnder interferometer is realized as an optical waveguide using, for example, lithium niobate (LiNbO3) as the material. Hereinafter, a modulator configured by the Mach-Zehnder interferometer using the lithium niobate as the material will be referred to as an “LN modulator”.
FIG. 12 illustrates a block diagram of the LN modulator in relation to the present invention. Light is inputted from an optical input port 2 of an LN modulator 300 to an optical divider 3, and lights divided by the optical divider 3 is outputted to a first optical waveguide 4 and a second optical waveguide 5. An electrode is formed on part of each of the first optical waveguide 4 and the second optical waveguide 5. Further, it is possible to, with respect to each of the optical waveguides, vary the phase of light transmitting through the optical waveguide by applying a voltage to the electrode. Hereinafter, the first optical waveguide 4 provided with the electrode and the second optical waveguide 5 provided with the electrode will be referred to as a first phase modulator 17 and a second phase modulator 27, respectively.
Lights outputted from the first optical waveguide 4 and the second optical waveguide 5 are inputted to the optical coupler 8, and from the optical coupler 8, lights are each outputted to a corresponding one of the first optical output port 30 and the second optical output port 31.
The LN modulator 300 applies voltages to the first phase modulator 17 and the second phase modulator 27, and thereby varies the phases of lights transmitting through the waveguides. Further, it is possible to vary the intensities of lights outputted from the first light output port 30 and the second light output port 31 by utilizing a light interference effect in the optical coupler 8.
Meanwhile, an optical modulation function similar to that of the LN modulator 300 shown in FIG. 12 can be also realized by using optical waveguides composed of a semiconductor material. Currently, for realizing such an optical modulator, an indium phosphide (InP) and arsenic indium phosphide gallium (InGaAsP)-origin material and an gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs)-origin material are proposed. Then, there has been proposed an optical modulator including a Mach-Zehnder interferometer composed of one of these semiconductor materials. With respect to such an optical modulator using the semiconductor material, generally, it is possible to lower an operation voltage for the optical modulator to a fraction of the operation voltage of the LN modulator, and further reduce the size of the optical modulator to less than or equal to one tenth as compared with that of the LN modulator. This is because a variation coefficient of a refractive index relative to the voltage with respect to a semiconductor material is larger than or equal to several ten times that of lithium niobate.
Some of optical modulators constituted by optical waveguides have a function of, for the purpose of setting an optimum operational condition, monitoring lights transmitting through the optical waveguides and feeding back the monitoring result to driving conditions for the optical modulator. In patent literature (PTL) 1 to 3, there is disclosed a configuration in which part of a light signal is monitored by using a photodiode (PD) provided at outside. Through such a configuration, characteristic deterioration during operation of the optical modulator is suppressed.
Further, in PTL4, there is disclosed a configuration of an optical FSK/SSB modulator which is capable of adjusting the intensity of a light signal outputted from a sub-Mach-Zehnder waveguide. Further, in PTL5, there is disclosed a configuration of an optical modulator provided with a Mach-Zehnder waveguide for an intensity adjustment.
Generally, the PD for monitoring light transmitting through an optical waveguide is provided at outside of the optical modulator, or, as described in PTL3, the PD is monolithic-integrated at inside of the optical modulator. Even when the PD is integrated, a signal outputted as the result of operation of a Mach-Zehnder interferometer (the signal being a signal at a portion outer than the optical coupler of the optical modulator) needs to be monitored.
Further, in the LN modulator 300 shown in FIG. 12, the configuration that lights radiated from the edges of the waveguides are monitored in order to detect the intensities of lights having transmitted through the first optical waveguide 4 and the second optical waveguide 5, it is impossible to directly grasp the light intensities inside of the optical waveguides. For this reason, with respect to the LN modulator 300 shown in FIG. 12, there has been also a problem that it is difficult to grasp the situation of light transmitting inside thereof with accuracy.
For example, in an optical modulator disclosed in PTL3, a PD for detecting the light intensity is monolithically-integrated inside the optical modulator. Nevertheless, even in the configuration disclosed in PTL3, the light intensity is monitored at a portion outer than the optical coupler. For this reason, in the configuration disclosed in PTL3, there is a problem that it is difficult to grasp the intensity of light inputted to the optical coupler with accuracy.