JP2003287725A - Light intensity modulator and optical frequency shifter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light intensity modulator and an optical frequency shifter of which the restriction of frequency shift is eliminated and the drift of a modulation operating point is suppressed. <P>SOLUTION: The light intensity modulator modulates the light intensity of an optical signal made incident as a carrier wave (a) from a light source 1 on the basis of the modulating signal b of the predetermined frequency inputted from a modulation control section MC, and outputs the light intensity- modulated wave c. The modulation control section MC is provided with a beat detecting part BS which detects the beat components of the carrier wave and the light intensity-modulated wave, and a bias control section BC which adjusts bias voltage to output a modulated wave so that the beat components is minimized. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical intensity modulator and an optical frequency shifter for intensity-modulating an optical signal of a predetermined optical frequency, which is incident from a light source, to shift the frequency of the optical frequency.

[0002]

2. Description of the Related Art An optical frequency shifter intensity-modulates a source light of a single optical frequency emitted from a light source, and generates a sideband component of a light intensity-modulated wave obtained by the intensity modulation. The frequency is changed by extracting. In this case, the amount of frequency displacement is defined by the frequency of the modulation signal (electrical signal) used for intensity modulation. In such an optical frequency shifter, a Mach-Zehnder type optical modulator is usually used as an optical intensity modulator. The Mach-Zehnder type optical modulator is excellent in high-speed response, but at the modulation operating point. Has a drawback that it easily drifts due to the influence of ambient temperature.

Japanese Patent Laid-Open No. 2000-12015 discloses that
As a technique for overcoming the drawbacks of the Mach-Zehnder interferometer type optical modulator, a technique for feedback controlling the bias voltage of the modulation signal according to the frequency fluctuation of the sideband component is disclosed. However, this technique has a problem that a high frequency modulation signal cannot be used due to the band limitation of a photodetector that converts a light intensity modulated wave into an electric signal, that is, a frequency displacement amount is limited. .

The present invention has been made in view of the above-mentioned problems, and an object thereof is to suppress the drift of the modulation operating point while eliminating the limitation of the frequency displacement amount.

[0005]

In order to achieve the above object, in the present invention, as a first means relating to an optical intensity modulator, an optical signal incident as a carrier from a light source is inputted from a modulation control section. A light intensity modulator that outputs a light intensity modulated wave by performing light intensity modulation based on a modulation signal of a predetermined frequency, wherein the modulation control section detects beat components of the carrier wave and the light intensity modulated wave. And a bias controller that adjusts the bias voltage so as to minimize the beat component and outputs a modulated wave.

As a second means relating to the optical intensity modulator, in the first means, the beat detection section has a carrier optical demultiplexer for demultiplexing a part of the carrier and a light intensity modulated wave. An optical demultiplexer for a modulated wave that demultiplexes a part, and an optical combiner that combines a carrier wave input from the optical demultiplexer for a carrier wave and an optical intensity modulated wave input from an optical demultiplexer for a modulated wave An optical receiver, a light receiver for converting the combined light output from the optical multiplexer into a combined electric signal, and a low-pass filter for selectively outputting only low-frequency components from the combined electric signal output from the light receiver. Adopt a configuration consisting of.

As a third means relating to the light intensity modulator,
In the second means, instead of the photodetector and the low-pass filter, a configuration is used in which a frequency selective photodetector having a photosensitivity for only low frequency components is used.

As a fourth means relating to the light intensity modulator,
In any one of the above first to third means, a configuration of a Mach-Zehnder type optical intensity modulator is adopted.

On the other hand, in the present invention, as a first means relating to the optical frequency shifter, the sideband wave component is extracted from the light intensity modulated wave output from the light intensity modulator described in any one of the first to fourth aspects. A configuration is provided in which a frequency selection unit that outputs as an optical frequency shift signal is provided.

Further, as a second means relating to the optical frequency shifter, in the first means, the frequency selecting means is an optical bandpass filter for selectively transmitting the lower sideband or the upper sideband. To do.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a light intensity modulator and an optical frequency shifter according to the present invention will be described below with reference to the drawings.

FIG. 1 is a functional block diagram of an optical intensity modulator and an optical frequency shifter in this embodiment. In this figure, reference numeral 1 is a light source, 2, 4 and 5 are optical branching devices, 3 is an external modulator, 6 is an optical multiplexer, 7 is a BPF (band pass filter), 8 is a light receiver, and 9 is an LPF (low pass). filter),
Reference numeral 10 is an A / D (A / D converter), 11 is a control unit, 12 is a D / A (D / A converter), 13 is a bias circuit, and 14 is a signal source.

Among these constituent elements, the optical branchers 4 and 5 surrounded by the two-dot chain line, the optical multiplexer 6, the photodetector 8, the LPF 9, A
/ D10, control unit 11, D / A12, bias circuit 13
The signal source 14 constitutes a modulation controller MC. In addition, among the constituent elements of the modulation control unit MC, the optical splitters 4 and 5, the optical multiplexer 6, the light receiver 8 and the LP, which are surrounded by dotted lines, are included.
F9 constitutes the beat detection unit BS, and the remaining A / D1
0, the control unit 11, the D / A 12, the bias circuit 13, and the signal source 14 form a bias control unit BC.

The light source 1 is a light source light (carrier wave a) having a frequency ν.
Is generated and output to the optical branching device 2. The optical splitter 2 splits and outputs the carrier wave a to the external modulator 3 and the optical splitter 4. The external modulator 3 is a Mach-Zehnder type optical modulator, intensity-modulates a carrier wave a with a modulation signal b (electrical signal) input from the bias circuit 13, and outputs it to the optical branching device 5 as a light intensity modulated wave c. The optical branching device 4 branches the carrier wave a, and outputs one to the outside as main line output light and outputs the other to the optical multiplexer 6. The optical branching device 5 branches the optical intensity modulated wave c and outputs one to the BPF 7 and the other to the optical multiplexer 6.

The optical multiplexer 6 has a carrier wave a and a light intensity modulated wave c.
And are combined and output to the light receiver 8 as combined light d. BP
The F7 separates only the upper sideband component or the lower sideband component from the light intensity modulated wave c and outputs it as the main line output light to the outside. The light receiver 8 converts the combined light d into an electric signal (light reception signal) and outputs it to the LPF 9. The LPF 9 extracts only the DC vicinity component (beat signal) of the received light signal and outputs it to the A / D 10. The A / D 10 converts the beat signal into a digital signal (beat data) and outputs it to the control unit 11.

The control unit 11 outputs bias data corresponding to the beat data to the D / A 12. Here, the bias data is data set by the control unit 11 so that the beat data sequentially decreases in time series. D
/ A12 converts such bias data into an analog signal (bias voltage) and outputs it to the bias circuit 13. The bias circuit 13 superimposes a bias voltage, which is a DC component, on the modulation signal (only the AC component) input from the signal source 14 and outputs it to the external modulator 3. The signal source 14 oscillates a modulation signal of a single frequency f (only the AC component) and outputs it to the bias circuit 13.

Next, the operation of the optical intensity modulator and the optical frequency shifter thus configured will be described in detail with reference to FIGS.

First, FIG. 2 is a diagram showing operating characteristics of the external modulator 3. As shown in this figure, the external modulator 3
In the operating characteristics of, the optical output changes sinusoidally with respect to the bias voltage. For such operating characteristics, the control unit 11
Performs the feedback control of the bias voltage in sequence so that the beat signal has a minimum value, that is, the fundamental wave component a0 (the component of the same frequency as the carrier wave a) included in the light intensity modulated wave c is minimized. That is, the control unit 11 operates so as to maintain the point A at which the optical output has the minimum value as the modulation operating point. The operating point of the light intensity modulator is generally set to, for example, a point B whose operating characteristics are close to a straight line, but in the external modulator 3 of the present embodiment, the point A at which the optical output has the minimum value is the modulation operation. Set to a point.

On the other hand, FIG. 3 shows that the optical output of the external modulator 3 whose operating point is set to the point A, that is, the light intensity modulated wave c.
It is a figure which shows the spectrum of. The light intensity modulated wave c is obtained by intensity-modulating a carrier a having a frequency ν with a modulation signal having a frequency f with the modulation operating point as a point A.
The fundamental wave component c0 having the same frequency as 0, the upper sideband wave component c1 of the frequency (ν + f) shifted upward by the frequency f from the fundamental wave component c0, and the frequency shifted downward by the frequency f (ν
-F) Since the lower sideband component c2 and point A are set in the non-linear region in the operating characteristics of the external modulator 3, the upper and lower sideband components of higher order (not shown because the level is small)
Composed of.

Incidentally, FIG. 3A shows the spectrum component a 0 of the frequency ν corresponding to the carrier wave a. Further, FIG. 3B shows the spectrum of the light intensity modulated wave c,
That is, the fundamental wave component c0, the upper sideband component c1 and the lower sideband component c2 are shown. Further, FIG. 3C shows the beat components db, d1 and d2 of the combined light d. Combined light d
Is the sum of carrier wave a and light intensity modulated wave c,
The beat component db of the fundamental wave component a0 of the carrier a having the same frequency ν and the fundamental wave component c0 of the light intensity modulated wave c, and the fundamental wave component a0 of the carrier a and the upper sideband component c1 of the light intensity modulated wave c.
And beat components d1 and d2 with the lower sideband component c2 are generated.

FIG. 4 is a diagram showing the level relationship between the fundamental wave component c0 and the upper and lower sideband wave components c1 and c2 in the optical output of the external modulator 3. As shown in FIG. 4, when the bias voltage is changed, the levels change opposite to each other. That is, when the fundamental wave component c0 has the maximum value, the upper and lower sideband wave components c1 and c2 have the minimum value, and the fundamental wave component c
When 0 has the minimum value, the upper and lower sideband components c1 and c2 have the maximum values. Moreover, the fundamental wave component c0 and the upper and lower sideband wave components c
For 1 and c2, the rate of change before and after taking the minimum value is steep, while the rate of change before and after taking the maximum value is extremely gentle. The modulation operating point A of the external modulator 3 described above corresponds to the point A at which the fundamental wave component c0 has the minimum value as shown in FIG.

Here, each of the beat components db, d1, d2
Of these, only the beat component db, which is the DC vicinity component, is LP
The signal passes through F9 and is input to the A / D converter 10. The beat component db is generated by adding the fundamental wave component a0 of the carrier wave a and the fundamental wave component c0 of the light intensity modulated wave c by the optical multiplexer 6, so that its level is the same as the fundamental wave component a0. It depends on the level of the wave component c0. Since the level of the fundamental wave component a0 is the component of the carrier wave a, it is constant, whereas the fundamental wave component c0 is the component of the light intensity modulated wave c, and therefore depends on the bias voltage set by the control unit 11. Since the control unit 11 feedback-controls the bias voltage so that the beat component db, that is, the fundamental wave component c0 that changes depending on the bias voltage, takes the minimum value, as a result, the modulation operating point of the light intensity modulation wave c is obtained. Is maintained at point A.

According to this embodiment, the light intensity modulated wave c is modulated based on the beat signal obtained by receiving the beat component db of the combined light d output from the optical multiplexer 6 with the photodetector 8. Since the operating point is feedback-controlled, that is, the light intensity modulated wave is not directly input to the light receiver to be converted into an electric signal as in the prior art, but the beat component db of the combined light d output from the optical multiplexer 6 is used. Since the light receiver 8 receives the light,
It is possible to suppress the drift of the modulation operating point.

Moreover, since the modulation operating point A is set to a point where the rate of change of the fundamental wave component c0 of the light intensity modulated wave c as shown in FIG. 4 changes sharply, the stability of the modulation operating point is extremely high. high. Further, the upper and lower sideband components c of the light intensity modulated wave c
Since 1 and c2 are maximum values at this modulation operating point A,
It is possible to obtain high-level upper and lower sideband components c1 and c2, that is, sideband components c1 and c2 having a good S / N ratio.

Although the Mach-Zehnder type optical modulator is used as the external modulator 3 in the above-described embodiment, the present invention is not limited to this and can be applied to other types of optical modulators. It is possible.

[0026]

As described above, according to the present invention,
A light intensity modulator that outputs a light intensity modulated wave by performing light intensity modulation on the basis of a modulation signal having a predetermined frequency, which is input from a light source as a carrier wave, from a modulation control unit,
Since the modulation control unit includes a beat detection unit that detects the beat component of the carrier wave and the light intensity modulated wave, and a bias control unit that adjusts the bias voltage so that the beat component is minimized and outputs the modulated wave. It is possible to suppress the drift of the modulation operating point while eliminating the limitation of the frequency displacement amount due to the light receiver in the beat detection unit.

[Brief description of drawings]

FIG. 1 is a block diagram showing a functional configuration of an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing an operation of one embodiment of the present invention.

FIG. 3 is a characteristic diagram showing spectra of various signals according to an embodiment of the present invention.

FIG. 4 is a characteristic diagram showing a level relationship between a carrier wave and a side band wave in the embodiment of the present invention.

[Explanation of symbols]

MC ... Modulation control unit BS ... Beat detection unit BC ... Bias control unit 1 ... Light source 2 ... Optical splitter 3 ... External modulator (light intensity modulator) 4 ... Carrier optical demultiplexer 5 ...... Modulation wave optical demultiplexer 6 ...... Optical multiplexer 7 ...... BPF (optical band pass filter; frequency selection means) 8 ...... Receiver 9 ...... LPF (low pass filter) 10 ...... A / D 11 ... ... Control unit 12 ... D / A 13 ... Bias circuit 14 ... Signal source

Continued front page    (72) Inventor Yoshiyuki Sakairi             529-3 Kamata, Ota-ku, Tokyo Andoden             Ki Co., Ltd. F-term (reference) 2H079 AA02 AA12 BA01 BA03 CA05                       CA24 EA05 FA01 FA03 FA04                       HA23 KA18 KA19                 2K002 AA02 AB12 BA06 DA08 EB12                       EB15 GA03 HA02

Claims (6)

[Claims] 1. An optical intensity modulation is performed by modulating an optical signal incident from a light source (1) as a carrier wave (a) based on a modulation signal (b) having a predetermined frequency input from a modulation control unit (MC). A light intensity modulator that outputs a wave (c), wherein the modulation control unit (MC) includes a beat detection unit (BS) that detects a beat component between the carrier wave and the light intensity modulated wave, and the beat component A light intensity modulator, comprising: a bias control unit (BC) that adjusts a bias voltage so as to minimize it and outputs a modulated wave. 2. The beat detecting section (BS) comprises a carrier wave optical demultiplexer (4) for demultiplexing a part of the carrier wave and a modulated wave optical demultiplexer for demultiplexing a part of the light intensity modulated wave. (5)
An optical multiplexer (6) for combining the carrier wave input from the carrier wave optical demultiplexer (4) and the light intensity modulated wave input from the modulated wave optical demultiplexer (5), A light receiver (8) for converting the combined light output from the optical combiner (6) into a combined electric signal, and selectively outputting only low frequency components from the combined electric signal output from the light receiver (8) 2. A light intensity modulator according to claim 1, characterized in that it comprises a low-pass filter (9). 3. The light intensity modulator according to claim 2, wherein a frequency selective photodetector having a photosensitivity only to a low frequency component is used in place of the photodetector (8) and the low pass filter (9). . 4. The light intensity modulator according to claim 1, wherein the light intensity modulator is a Mach-Zehnder type light intensity modulator. 5. A frequency selection means (7) for extracting a sideband component from a light intensity modulated wave output from the light intensity modulator according to any one of claims 1 to 4 and outputting it as an optical frequency shift signal. Optical frequency shifter characterized by. 6. The optical frequency shifter according to claim 5, wherein the frequency selecting means (7) is an optical bandpass filter for selectively transmitting the lower sideband or the upper sideband.