JP7164504B2 - Optical modulator and optical transmitter - Google Patents

Description

The present invention relates to the generation of optical single sideband (SSB) signals.

2. Description of the Related Art Optical communication systems are used in which electrical signals carrying information are amplitude-modulated and transmitted through continuous light. In this case, the modulated light transmitted by the optical transmitter becomes an optical double sideband (DSB) signal having optical signal components corresponding to information on both sides of the optical carrier in the frequency domain. The optical receiving device photoelectrically converts the optical DSB signal to generate an electrical signal obtained by synthesizing a first beat signal of the optical carrier wave and the upper sideband component and a second beat signal of the optical carrier wave and the lower sideband component. Get the signal.

Here, due to the influence of chromatic dispersion, the modulated light transmitted through the optical fiber undergoes different phase rotations depending on the wavelength (frequency). Therefore, in the case of the optical DSB signal, the amount of phase rotation differs between the upper sideband component and the lower sideband component. Here, if the phases of the first beat signal and the second beat signal are reversed at the installation position of the optical receiving device, the optical receiving device cannot acquire the electric signal even if photoelectric conversion is performed.

The above problem is solved by having the optical transmitter perform optical SSB modulation and transmit an optical SSB signal. Non-Patent Document 1 discloses a configuration for generating an optical SSB signal by removing one sideband with an optical filter. Non-Patent Document 2 discloses a configuration for performing optical SSB modulation by using a two-electrode Mach-Zehnder modulator (DD-MZM).

J. Park, et. al. , "Elimination of the fiber chromatic dispersion penalty on 1550 nm millimeter-wave optical transmission", Electron. Lett. , vol. 33, pp. 512-513, 1997 W. Peng, et. al. , "Spectrally Efficient Direct-Detected OFDM Transmission Incorporating a Tunable Frequency Gap and an Iterative Detection Techniques", in Journal of Lightwave Technology. 27, no. 24, pp. 5723-5735, December 15, 2009

However, the configuration of Non-Patent Document 1 requires a filter with a steep filtering characteristic, and it is necessary to shift the filtering characteristic in accordance with fluctuations in the emission wavelength of the light source. In addition, in the configuration of Non-Patent Document 2, Hilbert transform must be performed by digital signal processing, which increases the cost of the device.

The present invention provides a technique for generating an optical SSB signal with a simple configuration.

According to one aspect of the present invention, an optical transmission device includes a first modulation that modulates a first continuous light with a first electrical signal that includes an electrical signal to be transmitted and a first sinusoidal signal and outputs the first modulated light. modulating the second continuous light by means of a second electrical signal including means, the electrical signal, and a second sine wave signal having a phase difference of π/2 from the first sine wave signal to produce a second modulated light; a second modulation means for outputting; a first conversion means for outputting a third electrical signal by processing including photoelectric conversion of the first modulated light; and a fourth electrical signal by processing including photoelectric conversion of the second modulated light. a second conversion means for outputting, phase-modulating the third continuous light with the third electrical signal to generate a third modulated light, and phase-modulating the third continuous light with the fourth electrical signal to perform the fourth modulation and a third modulating means for generating light and combining and outputting the third modulated light and the fourth modulated light, wherein the third modulating means generates the third electrical signal and the fourth modulated light. It is characterized in that the phase difference between the third modulated light and the fourth modulated light is set to π/2 when no electrical signal is input.

According to the present invention, an optical SSB signal can be generated with a simple configuration.

1 is a configuration diagram of an optical transmission device according to an embodiment; FIG. FIG. 4 is a diagram showing frequency components at each position in the optical transmission device; FIG. 4 is a diagram showing frequency components at each position in the optical transmission device; 1 is a configuration diagram of an optical transmission device according to an embodiment; FIG. 1 is a configuration diagram of an optical transmission device according to an embodiment; FIG. FIG. 4 is a diagram showing frequency components at each position in the optical transmission device;

Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. It should be noted that the following embodiments do not limit the invention according to the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more of the features described in the embodiments may be combined arbitrarily. Also, the same or similar configurations are denoted by the same reference numerals, and redundant explanations are omitted.

<First Embodiment>
FIG. 1 is a configuration diagram of an optical transmitter according to this embodiment. The electrical signal corresponds to information to be transmitted. For example, the electrical signal is an intermediate frequency band signal. The electrical signal is branched into two, the first electrical signal on one side is input to the adder 11 and the second electrical signal on the other side is input to the adder 12 . Oscillator 61 generates a first sine wave signal and outputs it to adder 11 and phase shifter 70 . The phase shifter 70 shifts the phase of the first sine wave signal generated by the oscillator 61 by π/2, and inputs the phase-shifted second sine wave signal to the adder 12 . Note that the difference between the frequency of the first sine wave signal generated by the oscillator 61 and the center frequency of the electrical signal is ft. Note that the difference ft is greater than half the bandwidth of the first electrical signal. The adder 11 inputs a third electrical signal including the first electrical signal and the first sine wave signal to the modulator 21 . Meanwhile, the adder 12 inputs a fourth electrical signal including the second electrical signal and the second sine wave signal to the modulator 22 .

FIGS. 2A and 2B show frequency components of the third electrical signal and the fourth electrical signal, respectively. In this example, the frequency of the first sine wave signal generated by the oscillator 61 is higher than the frequency band of the electric signal. The arrows in FIG. 2A correspond to the first sine wave signal, the squares correspond to the first electrical signal, the arrows in FIG. 2B correspond to the second sine wave signal, and the squares to the second electrical signal. corresponds to Above each sine wave signal is shown a phase difference based on the first sine wave signal. Since the negative frequency component is the complex conjugate of the positive frequency component, the phase of the negative frequency component of the second sinusoidal signal is -π/2. For the positive or negative frequency components of the electrical signal indicated by squares, uniform phases and amplitudes cannot be defined, but one phase and amplitude are indicated by complex numbers for the purpose of explanation. In the following, this illustrative phase and amplitude of the positive frequency component of the electrical signal will be referred to as "a+bj". Since the phase of the negative frequency component is the complex conjugate of the positive frequency component, the phase and amplitude of the negative frequency component are a−bj.

The modulator 21 amplitude-modulates the continuous light (not shown) with a third electrical signal to output a first modulated light, and the modulator 22 amplitude-modulates the continuous light (not shown) with a fourth electrical signal to produce a second modulated light. to output A photoelectric conversion unit (O/E) 31 photoelectrically converts, that is, directly detects, the first modulated light and outputs a fifth electrical signal. An O/E 32 photoelectrically converts, that is, directly detects, the second modulated light. and output the sixth electrical signal.

FIGS. 2(C) and 2(D) show the frequency components of the fifth electrical signal and the sixth electrical signal, respectively. With direct detection, the center frequency of the bandwidth of the fifth and sixth electrical signals is |ft|. In this example, since the frequency of the first sine wave signal is higher than the frequency band of the electrical signal, the negative frequency components of the fifth electrical signal and the sixth electrical signal are the negative frequency components of the third electrical signal and the fourth electrical signal. The positive frequency components of the fifth electrical signal and the sixth electrical signal result from the product of the negative frequency components of the third electrical signal and the fourth electrical signal. Since the phase of the first sinusoidal signal is 0, no phase rotation occurs in the first electrical signal, so as shown in FIG. 2(C), the positive frequency component of the fifth electrical signal is A) is the same as the negative frequency component of the third electrical signal, and the negative frequency component of the fifth electrical signal is the same as the positive frequency component of the third electrical signal. On the other hand, since the phase of the positive frequency component of the second sine wave signal is π/2, the phase of the negative frequency component of the sixth electrical signal is the positive frequency of the fourth electrical signal shown in FIG. The component is rotated by π/2, ie, -b+aj as shown in FIG. 2(D). Also, since the phase of the negative frequency component of the second sine wave signal is -π/2, the phase of the positive frequency component of the sixth electrical signal is the negative phase of the fourth electrical signal shown in FIG. The frequency component is rotated by -π/2, that is, -b-aj as shown in FIG. 2(D).

Filter 41 removes unnecessary signals generated by direct detection in O/E 31 and allows only the signal components shown in FIG. 2(C) to pass. Filter 42 removes unnecessary signals generated by direct detection in O/E 32 and allows only the signal components shown in FIG. 2(D) to pass.

The fifth electrical signal filtered by filter 41 is applied to the first electrode of DD-MZM 90 and the sixth electrical signal filtered by filter 42 is applied to the second electrode of DD-MZM 90 . Also, the light source 80 generates continuous light and inputs it to the DD-MZM 90 .

The DD-MZM 90 splits the input continuous light into a first continuous light and a second continuous light, changes the phase of the first continuous light based on the voltage applied to the first electrode, and converts it into third modulated light. A fourth modulated light is obtained by changing the phase of the two continuous lights based on the voltage applied to the second electrode. Then, the DD-MZM 90 outputs modulated light obtained by combining the third modulated light and the fourth modulated light. The direction of phase rotation when the voltage applied to the first electrode is positive is the same as the direction of phase rotation when the voltage applied to the second electrode is positive.

Also, although not shown, a bias voltage is applied to the first electrode and the second electrode, respectively, and the fifth electric signal and the sixth electric signal are not applied to the first electrode and the second electrode. Also (even if the voltages of the fifth electric signal and the sixth electric signal are 0), the DD-MZM 90 gives the first continuous light and the second continuous light a phase change corresponding to the bias voltage. When the fifth electrical signal and the sixth electrical signal are not applied to the first electrode and the second electrode, the bias voltage applied to the first electrode and the second electrode is the phase of the third modulated light and the fourth modulated light. are set to differ by π/2. For example, the bias voltage applied to the first electrode is 0, that is, the phase change due to the bias voltage is 0, and the second electrode is applied with a bias voltage Vb that delays (rotates) the phase by π/2. Applying the bias voltage Vb to the second electrode and applying the sixth electric signal shown in FIG. ) to the second electrode. FIG. 2(E) is obtained by delaying the phase of the signal of FIG. 2(D) by π/2 (multiplied by -j). As shown in FIGS. 2(C) and 2(E), the positive frequency components of the fifth electrical signal and the sixth electrical signal have the same amplitude and are phase-inverted, so they cancel each other out. On the other hand, since the negative frequency components of the fifth electrical signal and the sixth electrical signal have the same amplitude and the same phase, they are not cancelled. The modulated light is thus an optical SSB signal having only the lower sideband corresponding to the negative frequency components of the fifth electrical signal and the sixth electrical signal. When a bias voltage that advances the phase by π/2 is applied to the second electrode, an optical SSB signal having only the upper waveband is obtained.

The same applies to the case where the frequency of the first sine wave signal generated by the oscillator 61 is lower than the frequency band of the electrical signal, which will be described with reference to FIG. FIGS. 3A and 3B show frequency components of the third electrical signal and the fourth electrical signal, respectively. 3(C) and 3(D) respectively show the frequency components of the fifth electrical signal and the sixth electrical signal. In this example, since the frequency of the first sinusoidal signal is lower than the frequency band of the electrical signal, the positive frequency components of the fifth electrical signal and the sixth electrical signal are the positive frequency components of the third electrical signal and the fourth electrical signal. Resulting from the product of the frequency components, the negative frequency components of the fifth electrical signal and the sixth electrical signal result from the product of the negative frequency components of the third electrical signal and the fourth electrical signal. Since the phase of the first sine wave signal is 0, the phase rotation of the first electrical signal does not occur. Therefore, as shown in FIG. 3C, the positive and negative frequency components of the fifth electrical signal are It is the same as the positive and negative frequency components of the third electrical signal shown in A). Since the phase of the positive frequency component of the second sine wave signal is π/2, the phase of the positive frequency component of the sixth electrical signal corresponds to the positive frequency component of the fourth electrical signal shown in FIG. It is rotated by π/2, that is, -b+aj as shown in FIG. 3(D). Also, since the phase of the negative frequency component of the second sine wave signal is −π/2, the phase of the negative frequency component of the sixth electrical signal is the negative phase of the fourth electrical signal shown in FIG. The frequency component is rotated by -π/2, that is, -b-aj as shown in Fig. 3(D).

For example, the bias voltage applied to the first electrode of the DD-MZM 90 is 0, that is, the phase change due to the bias voltage is 0, and the second electrode is given a bias voltage Vb that delays (rotates) the phase by π/2. shall be Applying the bias voltage Vb to the second electrode and applying the sixth electrical signal shown in FIG. ) to the second electrode. FIG. 3(E) is obtained by delaying the phase of the signal of FIG. 3(D) by π/2 (multiplied by -j). As shown in FIGS. 3(C) and 3(E), the negative frequency components of the fifth electrical signal and the sixth electrical signal have the same amplitude and are phase-inverted, so they cancel each other out. On the other hand, the positive frequency components of the fifth electrical signal and the sixth electrical signal have the same amplitude and the same phase, so they are not cancelled. Therefore, the modulated light becomes an optical SSB signal having only the upper sideband corresponding to the positive frequency components of the fifth electrical signal and the sixth electrical signal. When a bias voltage that advances the phase by π/2 is applied to the second electrode, an optical SSB signal having only the lower sideband is obtained.

As described above, an optical SSB signal can be generated with a simple configuration.

<Second embodiment>
Next, the second embodiment will be described, focusing on differences from the first embodiment. FIG. 4 is a configuration diagram of an optical transmission device according to this embodiment. The optical transmitter of the present embodiment is obtained by replacing the optical modulator 21 and O/E 31 in the first embodiment with a square detector 101 and replacing the optical modulator 22 and O/E 32 with a square detector 102 . Square-law detectors 101 and 102 respectively perform square-law detection of the third electrical signal and the fourth electrical signal of the first embodiment, and output fifth electrical signal and sixth electrical signal. Other configurations are the same as those of the first embodiment.

<Third embodiment>
Next, the third embodiment will be described, focusing on differences from the first and second embodiments. FIG. 5 is a configuration diagram of an optical transmission device according to this embodiment. Note that the configuration upstream of the filters 41 and 42 of the optical transmitter according to this embodiment is the same as in the first and second embodiments, and therefore will be omitted.

The oscillator 62 generates a third sine wave signal of frequency fc and outputs it to the multipliers 51 and 52 . The multiplier 51 multiplies the fifth electrical signal and the third sine wave signal to output a seventh electrical signal, and the multiplier 52 multiplies the sixth electrical signal and the third sine wave signal to output an eighth electrical signal. do.

FIGS. 6(A) and 6(B) respectively show the seventh electrical signal and the sixth electrical signal generated by frequency-shifting the fifth electrical signal and the sixth electrical signal shown in FIGS. 2(C) and 2(D). 8 shows the frequency components of the electrical signal. The seventh electrical signal has the negative and positive frequency components of the fifth electrical signal on both sides of frequency fc. That is, the seventh electrical signal has a frequency component whose frequency is the sum of the frequencies of the fifth electrical signal and the third sine wave signal (hereinafter referred to as the frequency sum component), and the frequency difference between the fifth electrical signal and the third sine wave signal. has a frequency component (hereinafter referred to as a frequency difference component) whose frequency is . Similarly, the eighth electrical signal has the negative and positive frequency components of the sixth electrical signal on either side of frequency fc. That is, the eighth electrical signal has a frequency sum component of the sixth electrical signal and the third sine wave signal and a frequency difference component of the sixth electrical signal and the third sine wave signal.

The BPF 111 extracts one of the frequency sum component and the frequency difference component included in the seventh electrical signal and removes the other. The BPF 112 extracts one of the frequency sum component and the frequency difference component included in the eighth electrical signal and removes the other. Note that the BPF 111 and the BPF 112 extract frequency components on the same side of the frequency fc. In this example, it is assumed that the frequency sum component is taken out. 6(C) and 6(D) show the ninth electrical signal output by the BPF 111 and the tenth electrical signal output by the BPF 112, respectively.

Since the frequency components of the ninth electrical signal and the tenth electrical signal are similar to the fifth electrical signal and the sixth electrical signal shown in FIGS. is applied, a ninth electrical signal is applied to the first electrode of the DD-MZM 90, and a tenth electrical signal is applied to the second electrode of the DD-MZM 90, thereby generating an optical SSB signal. It should be noted that the same is true even if the frequency difference component is taken out.

By performing frequency conversion on the fifth electrical signal and the sixth electrical signal in this way, it becomes possible to control the frequency of the sideband of the optical SSB signal.

[Other embodiments]
Although the configurations of FIGS. 1, 4 and 5 are used as optical transmitters in each of the above embodiments, the configurations of FIGS. 1, 4 and 5 are also optical SSB modulators.

The invention is not limited to the above embodiments, and various modifications and changes are possible within the scope of the invention.

21, 22: modulators 31, 32: photoelectric converters 90: two-electrode Mach-Zehnder modulators

Claims (8)

a first modulating means for modulating the first continuous light with a first electric signal including the electric signal to be transmitted and the first sine wave signal to output the first modulated light;
a second electrical signal including the electrical signal and a second sine wave signal having a phase difference of π/2 from the first sine wave signal to modulate the second continuous light to output the second modulated light; 2 modulating means;
a first conversion means for outputting a third electrical signal by processing including photoelectric conversion of the first modulated light;
second conversion means for outputting a fourth electrical signal by processing including photoelectric conversion of the second modulated light;
phase-modulate the third continuous light with the third electrical signal to generate a third modulated light; phase-modulate the third continuous light with the fourth electrical signal to generate the fourth modulated light; a third modulating means for combining and outputting the modulated light and the fourth modulated light;
and
The third modulating means is configured to set the phase difference between the third modulated light and the fourth modulated light to π/2 when the third electric signal and the fourth electric signal are not input. An optical transmitter characterized by: The first conversion means is
a first photoelectric conversion means for outputting a fifth electrical signal by photoelectric conversion of the first modulated light;
first frequency conversion means for frequency-converting the fifth electrical signal with a second sine wave signal to output a seventh electrical signal;
filtering the seventh electrical signal to obtain a frequency component whose frequency is the sum of the frequencies of the fifth electrical signal and the second sine wave signal and a frequency whose frequency is the difference between the frequencies of the fifth electrical signal and the second sine wave signal; a first filter means for extracting one of the frequency components and outputting it as the third electrical signal;
with
The second conversion means is
a second photoelectric conversion means for outputting a sixth electrical signal by photoelectric conversion of the second modulated light;
second frequency conversion means for frequency-converting the sixth electrical signal with the second sine wave signal to output an eighth electrical signal;
filtering the eighth electrical signal to obtain a frequency component whose frequency is the sum of the frequencies of the sixth electrical signal and the second sine wave signal and a frequency whose frequency is the difference between the frequencies of the sixth electrical signal and the second sine wave signal; second filter means for extracting a frequency component on the same side as the side extracted by the first filter means and outputting it as the fourth electrical signal;
2. The optical transmitter according to claim 1, comprising: a first detection means for outputting a third electrical signal by processing including square-law detection of a first electrical signal including the electrical signal to be transmitted and the first sine wave signal;
A second detection that outputs a fourth electrical signal by processing including square-law detection of the second electrical signal including the electrical signal and a second sine wave signal having a phase difference of π/2 from the first sine wave signal. means and
phase-modulating the continuous light with the third electrical signal to generate a third modulated light; phase-modulating the continuous light with the fourth electrical signal to generate a fourth modulated light; a third modulating means for combining and outputting the fourth modulated light;
and
The third modulating means is configured to set the phase difference between the third modulated light and the fourth modulated light to π/2 when the third electric signal and the fourth electric signal are not input. An optical transmitter characterized by: The first detection means is
first square-law detection means for square-law-detecting the first electric signal and outputting a fifth electric signal;
first frequency conversion means for frequency-converting the fifth electrical signal with a second sine wave signal to output a seventh electrical signal;
filtering the seventh electrical signal to obtain a frequency component whose frequency is the sum of the frequencies of the fifth electrical signal and the second sine wave signal and a frequency whose frequency is the difference between the frequencies of the fifth electrical signal and the second sine wave signal; a first filter means for extracting one of the frequency components and outputting it as the third electrical signal;
with
The second detection means is
a second square-law detection means for square-law-detecting the second electric signal and outputting a sixth electric signal;
second frequency conversion means for frequency-converting the sixth electrical signal with the second sine wave signal to output an eighth electrical signal;
filtering the eighth electrical signal to obtain a frequency component whose frequency is the sum of the frequencies of the sixth electrical signal and the second sine wave signal and a frequency whose frequency is the difference between the frequencies of the sixth electrical signal and the second sine wave signal; second filter means for extracting a frequency component on the same side as the side extracted by the first filter means and outputting it as the fourth electrical signal;
4. The optical transmitter according to claim 3, comprising: 5. The optical transmitter according to claim 1, wherein said third modulating means is a two-electrode Mach-Zehnder modulator. When the third electrical signal and the fourth electrical signal are not input due to the bias voltage applied to each electrode of the two-electrode Mach-Zehnder modulator, the phase difference between the third modulated light and the fourth modulated light is π/ 6. The optical transmitter according to claim 5, wherein the number of the optical transmitter is 2. a first modulating means for modulating the first continuous light with a first electric signal including the electric signal to be transmitted and the first sine wave signal to output the first modulated light;
a second electrical signal including the electrical signal and a second sine wave signal having a phase difference of π/2 from the first sine wave signal to modulate the second continuous light to output the second modulated light; 2 modulating means;
a first conversion means for outputting a third electrical signal by processing including photoelectric conversion of the first modulated light;
second conversion means for outputting a fourth electrical signal by processing including photoelectric conversion of the second modulated light;
phase-modulate the third continuous light with the third electrical signal to generate a third modulated light; phase-modulate the third continuous light with the fourth electrical signal to generate the fourth modulated light; a third modulating means for combining and outputting the modulated light and the fourth modulated light;
and
The third modulating means is configured to set the phase difference between the third modulated light and the fourth modulated light to π/2 when the third electric signal and the fourth electric signal are not input. An optical modulator characterized by: a first detection means for outputting a third electrical signal by processing including square-law detection of a first electrical signal including the electrical signal to be transmitted and the first sine wave signal;
A second detection that outputs a fourth electrical signal by processing including square-law detection of the second electrical signal including the electrical signal and a second sine wave signal having a phase difference of π/2 from the first sine wave signal. means and
phase-modulating the continuous light with the third electrical signal to generate a third modulated light; phase-modulating the continuous light with the fourth electrical signal to generate a fourth modulated light; a third modulating means for combining and outputting the fourth modulated light;
and
The third modulating means is configured to set the phase difference between the third modulated light and the fourth modulated light to π/2 when the third electric signal and the fourth electric signal are not input. An optical modulator characterized by:

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