JP2005121922A - Light modulation equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide light modulation equipment which easily suppresses light carrier wave power of an output signal of a light modulator. <P>SOLUTION: The light modulation equipment is equipped with the light modulator (an light SSB modulator 3) generating a light modulation signal by modulating a light carrier wave with a transmission signal and outputs a light signal in which the light carrier wave is suppressed by halving-branching the light carrier wave generated in a light source, generating the light modulation signal by inputting one branched light carrier wave to the light modulator and modulating it with the transmission signal and multiplexing the other branched light carrier wave and output light from the light modulator in such a way that they are mutually canceled. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical modulation device for transmitting a transmission signal through an optical fiber network.

  Frequency conversion of broadcast waves transmitted by optical fiber to millimeter wave band, such as existing apartment houses that cannot receive broadcast waves directly and it is difficult to pull in cables for joint reception, or when a river crosses an optical fiber installation route Research on wireless transmission is underway. Among such transmission systems, after converting transmission signals such as broadcast waves and high-frequency signals in the millimeter wave band to optical signals, they are multiplexed and optically transmitted, and the “transmission signal” and the frequency in the millimeter-wave band are received at the receiving end. There has been proposed a method of outputting both or one of the transmission signals converted (self-heterodyne detection) (see Non-Patent Document 1).

  In general, the light modulation method is the "direct modulation method", which modulates the light intensity by modulating the current injected into the laser diode with an electrical signal, and the laser diode is operated at a constant output and modulated by an external modulation element. There is an “external modulation method” to be performed. Although the direct modulation method is a simple method, the refractive index of the semiconductor largely fluctuates (chirping) due to current injection, resulting in dynamic wavelength broadening, which may cause distortion of the optical waveform at the receiving end. On the other hand, the external modulation method can suppress the dynamic wavelength spread sufficiently low.

When high-frequency signals are transmitted by optical intensity modulation with a normal external modulator, arrival time differences occur in the optical upper wave, optical carrier wave, and lower optical wave due to the effects of chromatic dispersion. It is known that there is a problem that a signal cannot be received. As one method for solving this problem, a single sideband modulation (SSB) system that does not transmit one optical sideband that causes interference has been proposed. The optical SSB modulator can be composed of a Mach-Zehnder (MZ) optical modulator using a lithium niobate (LiNbO ₃ ) optical modulator (see Non-Patent Document 2).

  Further, FIG. 2 of Patent Document 1 discloses a configuration method of an optical SSB modulator using a Mach-Zehnder (MZ) optical modulator.

Kenji Suzuki, 3 others "Millimeter-wave optical fiber transmission experiment of digital terrestrial broadcasting", The Institute of Image Information and Television Engineers Technical Report, Vol.27, No.45, PP.1-6, BCT2003-13 (Jul.2003) G. H. Smith, D. Novak, and Z. Ahmed: "Novel technique for generation of optical SSB with carrier using a single MZM to overcome fiber chromatic dispersion", MWP96, PDP-2, 1996

JP 2001-264714 A

  As a result of examining the prior art, the present inventor has found the following problems.

  Lithium niobate external modulators (LN external modulators) used for optical SSB modulators are known to have nonlinearity in electrical / optical conversion characteristics. Since the instantaneous value of the amplitude of an OFDM (Orthogonal Frequency Division Multiplexing) signal used in terrestrial digital broadcasting is much larger than the average value, the influence of third-order intermodulation distortion caused by non-linearity is significant during multi-channel transmission. Third-order intermodulation distortion (IM3) occurring in the transmission signal band equivalently degrades the CN ratio (carrier power to noise power ratio) of the transmission signal. Increasing the degree of optical modulation increases the CN ratio of the transmission signal, while the non-linearity of the optical modulator decreases the CI ratio (carrier power to third-order intermodulation distortion power ratio), effectively increasing the CN ratio. Reduce. In other words, the degree of light modulation is limited in order to avoid the influence of nonlinearity of the light modulator.

  In general, in order to increase the CN ratio at the receiving end after long-distance transmission, the output of the optical modulator is amplified by an optical amplifier and then input to the optical fiber transmission line or in the middle of the optical fiber transmission line. A method such as amplification is used.

  The output signal of the optical SSB modulator is divided into an optical carrier signal and an optically modulated transmission signal (single sideband), but the optical carrier power is larger than the power of the transmission signal. There is a problem that the modulated transmission signal is not sufficiently amplified.

  As described above, the optical SSB system has a feature that it is hardly affected by chromatic dispersion, but the optical modulation degree cannot be deepened as it is. In this case, even if an optical amplifier is used for long-distance transmission, amplification of the transmission signal is limited due to the large power of the optical carrier wave. In order to increase the optical modulation degree of the transmission signal, it is necessary to amplify only the power of the optically modulated transmission signal or suppress only the power of the optical carrier wave.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an optical modulation device capable of easily suppressing the optical carrier power of the output signal of the optical SSB modulator. is there.

  The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

  In order to solve the above-described problems, the present invention provides an optical modulation device including an optical modulator that generates an optical modulation signal by modulating an optical carrier wave with a transmission signal, and uses two optical carrier waves generated by a light source. A branching means; one of the branched optical carriers is input to the optical modulator and modulated with a transmission signal to generate an optical modulation signal; the other of the branched optical carriers and the output light of the optical modulator; And a means for outputting an optical signal in which the optical carrier wave is suppressed by multiplexing so as to cancel each other.

The optical modulator of the present invention is an optical SSB modulator.
In the present invention, the degree of suppression of the optical carrier wave is determined by adjusting the DC bias current of the optical SSB modulator and adjusting the phase of the output light of the optical SSB modulator.

  In the present invention, the optical carrier wave is suppressed in a range where the optical carrier power of the optical signal obtained by multiplexing does not become smaller than the transmission signal power.

  Thus, in the present invention, the output of the light source is divided into two, one is input as an optical carrier signal to the optical SSB modulator, and the other is combined with the output signal of the optical SSB modulator.

  The optical carrier power can be suppressed by adjusting and synthesizing the phases so that the optical carrier wave passing through the optical SSB modulator and the optical carrier wave not passing through the optical SSB modulator cancel each other. At this time, the phase of the optical carrier wave after the multiplexing also changes, but the detection at the receiving end is not affected.

  The adjustment of the phase difference of the optical carrier can be easily realized by adjusting the DC bias current of the optical SSB modulator.

  When an optical DSB (Double Side Band) modulator is used instead of the optical SSB modulator, the phase variation of the optical carrier wave after the multiplexing affects the detection at the receiving end. For this reason, the amplitude of the optical carrier on the cancellation side must be adjusted so that the phase does not change even after multiplexing, and the phase must be adjusted so that the phase is accurately reversed, which complicates the configuration of the apparatus. A new problem such as difficulty in setting occurs.

  By using the present invention, the optical carrier power of the optical modulator output signal can be easily suppressed with a simple configuration without using a steep optical bandpass filter.

  Since the optical carrier power is suppressed and the optically modulated transmission signal can be amplified by an optical amplifier, the optical modulation device is particularly effective when a multiwave signal is transmitted over a long distance.

Hereinafter, embodiments of the invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram for explaining a schematic configuration of a light modulation device according to an embodiment of the present invention.

  As shown in FIG. 1, the light modulation device of the present embodiment includes a light source 1 that outputs unmodulated laser light having an oscillation frequency fc as an optical carrier a, and an electric signal source that outputs an electric signal b having a center frequency fs. 2, an optical SSB modulator 3 to which one carrier wave f obtained by branching the optical carrier a output from the light source 1 by an optical distributor and an electric signal b, and an optical carrier a are branched into two. The other optical carrier c obtained in this way and an optical synthesizer for combining the optical modulation signal (optical signal j) output from the optical SSB modulator 3 are provided.

  The optical SSB modulator 3 is configured with a Mach-Zehnder (MZ) optical modulator, and the input optical carrier f is branched into two optical waveguides and combined after propagating through the optical waveguides. ing. An electrode is disposed in each optical waveguide, an electric signal d having the same phase as the electric signal b is applied to one electrode, and an electric signal g obtained by shifting the electric signal b by π / 2 phase is applied to the other electrode. It becomes the composition which is done. Also, a DC bias current 4 is connected to one electrode, and a DC bias signal source 5 is connected to the other electrode, so that the optical synthesis phase of the optical signals h and i propagating through each optical waveguide is π / 2. A DC bias current for setting and a DC bias current for adjusting the phase of the optical signal j with respect to the branched optical carrier c are applied.

Hereinafter, the operation of the light modulation device of this embodiment will be described with reference to FIG.
The light source 1 outputs an oscillation frequency fc and an unmodulated optical carrier wave a. The optical carrier a is branched into an optical carrier f and an optical carrier c, and the optical carrier f is input to the optical SSB modulator 3. On the other hand, an electrical signal b is output from the electrical signal source 2 and input to the optical SSB modulator 3.

  In the optical SSB modulator 3, the optical synthesis phase of h and i of the MZ optical modulator is set to π / 2, and the electrical signals d and g having a phase difference of π / 2 are used to generate the optical SSB modulator 3. Phase modulation is performed in the two optical waveguides.

  An optical signal k obtained by combining the optical signal j output from the optical SSB modulator and the optical carrier c becomes the output of the optical modulator.

  A schematic diagram of the amplitude frequency characteristics for each of the optical carrier a, electrical signal b, optical carrier c, optical signal j, and optical signal k is shown in the balloon.

  The light source 1 and the electric signal source 2 may be configured to be supplied from the outside of the apparatus, not as the components of the apparatus.

  In this way, among the optical carrier a output from the light source 1, the optical carrier f input to the optical SSB modulator 3 after branching is branched into two optical waveguides, as in the conventional optical SSB modulator. Is done.

  The optical carrier branched into one optical waveguide is phase-modulated with an electrical signal d in phase with the electrical signal b output from the electrical signal source 2 (optical signal h), and the optical carrier branched into the other optical waveguide is The electric signal b output from the electric signal source 2 is phase-modulated with an electric signal g obtained by shifting the phase by π / 2 (optical signal i). At this time, a DC bias current from the DC bias power sources 4 and 5 is also applied to the electrodes to which the electric signal d and the electric signal g are applied, and the optical synthesis phase of the optical signal i and the optical signal h becomes π / 2.

  Therefore, the optical signal j obtained by combining the phase-modulated optical signals h and i is transmitted at the center frequency fc-fs which is an optical carrier of frequency fc and one of its sidebands, as shown in the balloon. It becomes an optical signal consisting of the signal.

  In addition, the optical signals h and i are supplied from the DC bias signal sources 4 and 5 to the optical carrier wave of the optical signal j of the optical SSB modulator 3 together with the DC bias current for setting the phase difference at the time of multiplexing to π / 2. A DC bias current for adjusting the phase of the optical signal j is also applied so that the component and the branched optical carrier c cancel each other.

  As a result, by combining the optical signal j and the optical carrier c, only the optical carrier component of the frequency fc in the optical signal j is suppressed, and as shown in the balloon, the center frequency fc− of the same power as the optical signal j is suppressed. An optical signal k composed of the fs transmission signal and the optical carrier component suppressed from the optical signal j is obtained. In the optical modulation device according to the present embodiment, for example, in order for the optical carrier power of the output optical signal k to be preferable power for detection on the receiving side, the frequency fc of the optical signal k obtained by multiplexing is set. It is desirable that the optical carrier power does not become smaller than the transmission signal (single sideband) power of the center frequency fc-fs. For this purpose, the direct current bias current of the optical SSB modulator is adjusted to change the optical carrier phase of the optical signal j so as to cancel the optical carrier c. The DC bias current is adjusted by changing the DC voltage applied to the electrode of the Mach-Zehnder optical modulator.

  As described above, in the optical modulation device according to the present embodiment, the optical carrier a input to the optical SSB modulator 3 is branched, and the optical carrier power of the optical signal j is suppressed using the branched optical carrier c. Since the optical signal k can be obtained, the optical carrier power of the optical SSB modulator output signal can be easily suppressed with a simple configuration.

  Since the optical carrier power of the optical signal k is suppressed, the optically modulated transmission signal can be sufficiently amplified by an optical amplifier, which is particularly effective when a multiwave signal is transmitted over a long distance.

  In the optical modulation device of the present embodiment, the phase for suppressing the optical carrier power fc is controlled on the optical SSB modulator 3 side, but the present invention is not limited to this, and after branching Needless to say, the phase of the optical carrier c may be controlled.

  In the present embodiment, the optical carrier wave in the optical SSB modulator is suppressed, but it is naturally not limited to SSB modulation, and can be applied to a modulation method in which a carrier wave remains after modulation.

  The invention made by the present inventor has been specifically described based on the embodiment of the invention, but the invention is not limited to the embodiment of the invention and does not depart from the gist of the invention. Of course, various changes can be made.

It is a figure for demonstrating schematic structure of the optical modulation apparatus which is one embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Light source 2 ... Electric signal source 3 ... Optical SSB modulator 4, 5 ... DC bias signal source

Claims (4)

An optical modulation device comprising an optical modulator that generates an optical modulation signal by modulating an optical carrier wave with a transmission signal,
Means for bifurcating the optical carrier wave generated by the light source;
One of the branched optical carriers is input to the optical modulator and modulated with a transmission signal to generate an optical modulation signal so that the other of the branched optical carriers and the output light of the optical modulator cancel each other. Means for outputting an optical signal in which the optical carrier wave is suppressed by being combined with
A light modulation device.   The optical modulator according to claim 1, wherein the optical modulator is an optical SSB modulator.   The optical modulation device according to claim 2, wherein the degree of suppression of the optical carrier wave is determined by adjusting a DC bias current of the optical SSB modulator and adjusting a phase of output light of the optical SSB modulator. The optical modulation device according to any one of claims 1 to 3, wherein the optical carrier wave is suppressed in a range in which the optical carrier power of the optical signal obtained by the multiplexing does not become smaller than the transmission signal power.

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