JPH11337894A - Optical subcarrier frequency modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To deal with a high carrier frequency at a low cost and to widen the modulation band by multiplexing an optical carrier demultiplexed with a wavelength filter with a frequency modulated optical sub-carrier outputted from an optical frequency shifter. SOLUTION: Two waves adjacent at a 60 GHz interval of a laser beam from a mode lock laser diode(MLLD) 1 are cut out by an array waveguide diffraction grating(AWG) 2. An emission beam from a fiber collimator 3a is made incident on an acoustooptical modulator(AOM) 4 as a parallel beam. A modulator 13 applies an output of an oscillator 11 to the AOM 4 when a digital signal is '1', and doesn't output when '0'. The optical sub-carriers transmitted or diffracted by the AOM 4 to be frequency modulated are made incident on a 3 to 1 coupler 6 respectively through the fiber collimators 3b, 3c. The 3 to 1 coupler 6 multiplexes the optical carrier of an optical frequency f0 demultiplexed by the AWG 2 with the optical sub-carrier of the optical frequencies f1 (f2) frequency modulated by the AOM 4 to output it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mobile communication system for transmitting a microwave or millimeter wave high-frequency signal.
The present invention relates to an optical subcarrier frequency modulator that performs frequency modulation with a high frequency signal in an optical domain in an optical subcarrier transmitter used in an AN system and a broadcast network.

[0002]

2. Description of the Related Art In a radio communication system using a microwave or millimeter wave of about several tens of GHz as a carrier, a large number of high-frequency repeaters are required due to a large loss of a high-frequency signal to an antenna base station. Therefore, microwaves or millimeter waves are superimposed on optical carriers as subcarriers,
An optical subcarrier link for optically transmitting an optical fiber has been proposed. However, conventionally, frequency modulation of a high-frequency signal (subcarrier) has been performed in the electric domain.

FIG. 6 shows a configuration example of a conventional optical subcarrier frequency modulator. In the figure, a modulator 21 frequency-modulates a sine wave of several tens of MHz to about 100 MHz output from an oscillator 22 by a data signal output from a signal source 23. The modulated signal and the output of the oscillator 24 having a higher frequency are mixed by the mixer 25 and converted (up-converted) into a predetermined microwave or millimeter wave band signal. Note that the mixer 25 generates a signal having a sum frequency and a signal having a difference frequency, but generally only the signal having the sum frequency is used. The high-frequency signal generated here is input to the light intensity modulator 27. The light intensity modulator 27 modulates the intensity of the laser light output from the laser diode 26 with a high-frequency signal. Thereby, the frequency-modulated high-frequency signal is superimposed on the optical carrier.

[0004]

As shown in FIG. 6, a conventional optical subcarrier frequency modulator performs frequency modulation on a high-frequency signal in an electric domain, and uses an oscillator or a mixer to perform microwave or millimeter-wave band modulation. It was configured to upconvert to a signal. As a result, costs were increased and power consumption was increased. In addition, as a current capability of electronic components, a band of several tens of MHz for analog or a bit rate of several tens of Mbit / s for digital has been a limit.

An object of the present invention is to provide an optical subcarrier frequency modulator which can cope with a high carrier frequency at low cost and has a wide modulation band by performing frequency modulation by a high frequency signal in an optical domain. .

[0006]

An optical subcarrier frequency modulator according to the present invention comprises a multi-wavelength laser light source for generating a plurality of optical spectra, and a wavelength filter for demultiplexing each optical spectrum output from the multi-wavelength laser light source. An optical frequency shifter driven by a high-frequency signal and frequency-modulating an optical subcarrier demultiplexed by a wavelength filter; an optical carrier demultiplexed by a wavelength filter; and frequency-modulated light output from the optical frequency shifter. An optical system for multiplexing the subcarriers. In this configuration, the high-frequency signal may be either an analog signal or a digital signal.

[0007] It is also possible to employ a configuration in which three or more optical spectra are split by the wavelength filter, a plurality of optical frequency shifters are provided, and a plurality of optical subcarriers are frequency-modulated by a plurality of high-frequency signals.

[0008] The optical subcarrier frequency modulator of the present invention comprises:
A multi-wavelength laser light source for generating a plurality of optical spectra, a wavelength filter for demultiplexing each optical spectrum output from the multi-wavelength laser light source, and two optical frequencies by inputting optical subcarriers demultiplexed by the wavelength filter. An optical frequency shifter that outputs components, two optical intensity modulators that optically modulate two optical frequency components with a digital signal, an optical carrier split by a wavelength filter, and two optical intensity modulators And an optical system for multiplexing the frequency-modulated optical subcarrier output from the optical subcarrier.

[0009] Further, an optical spectrum to be demultiplexed by a wavelength filter is three or more, a plurality of sets of optical frequency shifters and two optical intensity modulators are provided, and a plurality of optical subcarriers are respectively frequency-modulated by a plurality of high-frequency signals. It may be.

[0010] The optical subcarrier frequency modulator of the present invention comprises:
A multi-wavelength laser light source for generating a plurality of optical spectra, a wavelength filter for demultiplexing each optical spectrum output from the multi-wavelength laser light source, and two optical frequencies by inputting optical subcarriers demultiplexed by the wavelength filter. An optical frequency shifter that outputs components, a 2 × 1 optical switch that selects two optical frequency components according to a digital signal, an optical carrier that is split by a wavelength filter, and a frequency output from the 2 × 1 optical switch An optical system for multiplexing the modulated optical subcarrier.

[0011] Also, the optical spectrum to be demultiplexed by the wavelength filter is set to three or more, a plurality of sets of optical frequency shifters and 2 × 1 optical switches are provided, and a plurality of optical subcarriers are frequency-modulated by a plurality of digital signals, respectively. Is also good.

[0012]

(First Embodiment) FIG. 1 shows a first embodiment of an optical subcarrier frequency modulator according to the present invention. In this embodiment, the digital frequency modulation (FS
K) is performed.

In FIG. 1, reference numeral 1 denotes a mode-locked laser diode (hereinafter referred to as "MLLD"), 2 denotes an arrayed waveguide grating (hereinafter referred to as "AWG"), 3a, 3b, and 3c.
Is a fiber collimator, 4 is an acousto-optic modulator (hereinafter referred to as “A
OM "), 5 is an optical fiber, 6 is a 3-to-1 coupler,
11 is an oscillator, 12 is a digital signal source, and 13 is a modulator. Oscillator 11, digital signal source 12, modulator 1
Reference numeral 3 denotes an electric circuit system that outputs a high-frequency signal for driving the AOM 4.

Here, MLLD1 used as a multi-wavelength laser light source, AWG used as a wavelength filter
2. AOM4 used as an optical frequency shifter will be described.

The MLLD 1 processes a part of a laser diode electrode as a modulation terminal called a rocker terminal, and generates an optical pulse corresponding to a resonator length corresponding to the bias voltage. The repetition frequency of a general MLLD is approximately 10 to 100 GHz. When viewed on the frequency axis, it can be said that the laser diode emits a plurality of optical spectra at intervals equal to the repetition frequency. If this state is maintained, the repetition frequency changes and becomes unstable due to temperature and current. However, by inputting a high-frequency signal near the repetition frequency specific to the element to the rocker terminal, the frequency can be stabilized. In the figure, a constant current source, a high-frequency oscillator, a temperature control system, and the like for stabilizing the frequency of the MLLD 1 are omitted.

The multi-wavelength laser light source can be realized by, besides the MLLD 1, a master / slave laser composed of two DFB lasers, or a configuration of externally modulating a single-spectrum laser beam with a high-frequency signal.

The AWG 2 is a one-input / multiple-output wavelength filter formed by sequentially connecting an input waveguide, an input slab waveguide, an array waveguide, an output slab waveguide, and an output waveguide. Here, it is used to cut out two predetermined lines from the optical spectrum of the laser light emitted from the MLLD 1. Note that the AWG may be replaced with a resonator type wavelength filter.

The AOM 4 transmits the incident light as it is when no voltage is applied. On the other hand, when a high-frequency voltage is applied, the high-frequency signal propagates as a sound wave in the element to form a diffraction grating. The incident light is diffracted and emitted by the influence of the diffraction grating. At this time, the optical frequency is simultaneously shifted by the frequency of the high-frequency signal. Due to this effect, the present embodiment uses AOM4 as an optical frequency shifter.

The frequency modulation operation of the present embodiment based on the above configuration will be described with reference to FIG. MLLD1
The two light spectra at a predetermined millimeter wave interval are extracted from the light spectrum generated in step (1). These are the two optical frequencies f0 and f1 shown in FIG. This frequency interval f1-f0
Is fm. f0 is an optical carrier used as a frequency reference. When the optical subcarrier of the optical frequency f1 is frequency-modulated, its optical spectrum becomes as shown in FIG.
Assuming that the frequency shift is + fs, the optical spectrum has two original optical frequencies f1 and f2 (= f1 + fs). At this time, as the modulation band expands, the optical spectrum has a certain width from a linear shape. That is, as shown in FIG. 2B, the shape changes from a broken line to a solid line as the band is widened. Finally, the three lights of f0, f1, and f2 are mixed and transmitted to the optical transmission line.

Hereinafter, a specific configuration of each section of the present embodiment will be described. The MLLD 1 has a resonator length equivalent to 60 GHz, and a sine wave voltage of 60 GHz is applied to the rocker terminal. The oscillation wavelength is in the 1.55 μm band. ML
The laser light emitted from the LD 1 is cut by the AWG 2 into two adjacent waves at 60 GHz intervals. One wave (optical subcarrier) of the optical frequency f1 is guided to the fiber collimator 3a, and one wave (optical carrier) of the optical frequency f0 is guided to the optical fiber 5. The light emitted from the fiber collimator 3a enters the AOM 4 as a parallel beam.

On the other hand, the oscillator 11 oscillates at 2 GHz, and the digital signal source 12 outputs a 50 Mbit / s digital signal. The modulator 13 is an on / off modulator that functions as a switch, and applies the output of the oscillator 11 to the AOM 4 when the digital signal is “1”, and does not output the output when the digital signal is “0”. Therefore, when the digital signal is “1”, A
The incident light of OM4 is diffracted and output, and its optical frequency is also f
s = 2 GHz. The digital signal "0"
At this time, since the AOM 4 is in the transmitting state, the incident light does not change its path or optical frequency. The optical subcarriers transmitted or diffracted by the AOM 4 and frequency-modulated in this manner enter the three-to-one coupler 6 via the fiber collimators 3b and 3c, respectively.

The three-to-one coupler 6 multiplexes the optical carrier of the optical frequency f0 demultiplexed by the AWG 2 and the optical subcarrier of the optical frequency f1 or f2 frequency-modulated by the AOM 4, and outputs the multiplexed signal. The multiplexed laser light has the optical spectrum shown in FIG. 2 (b), and if it is detected by a photodiode after transmission, the carrier frequency is 60 GHz and the frequency shift is 2 GHz.
A frequency modulated signal having a z and a bit rate of 50 Mbit / s can be extracted.

In this embodiment, a plurality of sets of the AOM 4, the fiber collimators 3a to 3c, the oscillator 11, the digital signal source 12, and the modulator 13 are provided, and the AWG 2 separates three or more waves and uses a plurality of optical subcarriers. A configuration may be adopted in which the signals are frequency-modulated and multiplexed with the corresponding high-frequency signals.

(Second Embodiment) FIG. 3 shows an optical subcarrier frequency modulator according to a second embodiment of the present invention. Note that the present embodiment is a configuration for performing analog frequency modulation.

In the figure, 1 is MLLD, 2 is AWG,
3a and 3d are fiber collimators, 4 is an AOM, 5 is an optical fiber, 7a and 7b are Fourier transform lenses, 8 is a 2-to-1 coupler, 14 is an analog signal source, and 15 is a voltage controlled variable frequency oscillator. The analog signal source 14 and the voltage-controlled variable frequency oscillator 15 are an electric circuit system that outputs a high-frequency signal for driving the AOM 4.

The functions of MLLD1, AWG2 and AOM4 are the same as in the first embodiment. The analog signal source 14
Here, it is assumed that a video signal having a band of about 5 MHz and an amplitude of 1 Vpp is generated. The voltage controlled variable frequency oscillator 15 is an oscillator having a center frequency of 30 MHz, but has a configuration in which the oscillation frequency changes according to the amplitude when an external signal is applied. When this output is input to AOM4, the optical subcarrier is emitted with the angle of diffraction changed and the optical frequency is also changed to AOM4.
Higher by the frequency change to.

In order to focus the optical subcarrier diffracted or transmitted by the AOM 4 at one point, a Fourier transform lens 7
a, 7b and a fiber collimator 3d. The focal length of each of the Fourier transform lenses 7a and 7b is F (c
m), the distance between the AOM 4 and the Fourier transform lens 7a is F, the distance between the Fourier transform lenses 7a and 7b is 2F,
The distance between the Fourier transform lens 7b and the fiber collimator 3d is F. With this optical system, the optical subcarrier emitted from the AOM 4 can be diffracted or transmitted,
In any case, the light is guided to the fiber collimator 3d.
As described above, since the optical system on the output side of the AOM 4 is configured to be able to stop at one point without depending on the diffraction angle, even the optical subcarrier frequency-modulated by the analog signal can enter the fiber collimator 3d. . The two-to-one coupler 8 combines the optical carrier of the optical frequency f0 demultiplexed by the AWG 2 and the optical subcarrier frequency-modulated by the AOM 4, and outputs the resultant.

In this embodiment, the AOM 4, the fiber collimators 3a and 3d, the Fourier transform lens 7
a, 7b, a plurality of sets of analog signal sources 14, and a voltage controlled variable frequency oscillator 15, and the AWG 2 separates three or more waves,
A configuration may be adopted in which a plurality of optical subcarriers are frequency-modulated and multiplexed with the corresponding high-frequency signals.

(Third Embodiment) FIG. 4 shows a third embodiment of the optical subcarrier frequency modulator according to the present invention. Note that the present embodiment is a configuration for performing digital frequency modulation.

In the figure, 1 is MLLD, 2 is AWG,
3a, 3b, 3c are fiber collimators, 4 is AOM,
5 is an optical fiber, 6 is a 3-to-1 coupler, 9a and 9b are optical intensity modulators, 16 is an oscillator, and 17 is a digital signal source.

The functions of MLLD1, AWG2 and AOM4 are the same as in the first embodiment. The oscillator 16 has a frequency 2
It outputs a sine wave of GHz and is always applied to AOM4. By appropriately controlling the applied voltage, the transmitted light at the optical frequency f1 and the diffracted light at the optical frequency f2 from the AOM 4 have a light intensity ratio of 1.
They can be output simultaneously on a one-to-one basis. The transmitted light of the optical frequency f1 is transmitted to the fiber collimator 3b and the light intensity modulator 9a.
And enters the three-to-one coupler 6. On the other hand, the diffracted light having the optical frequency f2 passes through the fiber collimator 3c and the light intensity modulator 9b and enters the 3-to-1 coupler 6.

The light intensity modulator 9a is a digital signal source 17
The light intensity modulator 9b is connected to the data inversion output of the digital signal source 17. The digital signal source 17 outputs data in the normal phase and the negative phase simultaneously. Thus, when the data signal is "0", the transmitted light of the optical frequency f1 is outputted from the light intensity modulator 9a, and when the data signal is "1", the diffracted light of the optical frequency f2 is outputted from the light intensity modulator 9b. Is output. That is, only one of the two optical subcarriers is output. this is,
Similar to the first embodiment, this is equivalent to a digital signal frequency-modulated in the optical domain. The three-to-one coupler 6 has A
An optical carrier having an optical frequency f0 demultiplexed by WG2 and AO
The optical subcarrier of the optical frequency f1 or f2 frequency-modulated by the M4 and the optical intensity modulators 9a and 9b is multiplexed and output.

The configuration of the present embodiment cannot be used as an analog modulator because the output of the AOM used as an optical frequency shifter must be switched for each optical frequency component. However, since the switching of the AOM itself is not used as in the first embodiment,
The bit rate can be increased to the performance of the light intensity modulator. For example, when a Mach-Zehnder type light intensity modulator made of lithium niobate or an electric absorption type semiconductor optical switch is used as the light intensity modulators 9a and 9b, and the frequency of the oscillator 16 is increased, the performance of several tens Gbit / s can be obtained.

Light intensity modulators 9a and 9b and 3: 1 coupler 6
May be integrated on a single substrate. In addition, it is desirable that the envelope does not change as a frequency-modulated high-frequency signal. For this purpose, the optical frequencies f1, f
Although it is necessary for each of the components 2 to have the same optical path length, the use of an integrated light intensity modulator can reduce the size of the device and easily maintain a constant optical path length.

In this embodiment, the AOM 4, the fiber collimators 3a to 3c, and the light intensity modulators 9a and 9c
b, a plurality of sets of oscillators 16 and digital signal sources 17,
A configuration may be adopted in which three or more waves are split by the AWG2, and a plurality of optical subcarriers are frequency-modulated and multiplexed by corresponding high-frequency signals.

(Fourth Embodiment) FIG. 5 shows a fourth embodiment of the optical subcarrier frequency modulator according to the present invention. Note that the present embodiment is a configuration for performing digital frequency modulation.

In the figure, 1 is MLLD, 2 is AWG,
3a, 3b, 3c are fiber collimators, 4 is AOM,
5 is an optical fiber, 8 is a 2-to-1 coupler, 10 is a 2 × 1 optical switch, 16 is an oscillator, and 18 is a digital signal source.

This embodiment is characterized in that the light intensity modulators 9a and 9b and the digital signal source 1 in the third embodiment are different from each other.
7 is realized by the 2 × 1 optical switch 10 and the digital signal source 18. That is, 2
The x1 optical switch 10 selects the transmitted light of the optical frequency f1 when the output of the digital signal source 18 is "0", and selects "1".
When the diffracted light of the optical frequency f2 is selected,
A digital signal is obtained by frequency-modulating in the optical domain.
The two-to-one coupler 8 has an optical frequency f0 demultiplexed by the AWG2.
AOM4 and 2 × 1 optical switch 10
The optical subcarriers of the optical frequency f1 or f2, which have been frequency-modulated, are multiplexed and output.

[0039]

As described above, the optical subcarrier frequency modulator of the present invention uses optical components such as a multi-wavelength laser light source, a wavelength filter, an optical frequency shifter, an optical intensity modulator, and a 2 × 1 optical switch. As a result, analog and digital frequency modulation can be performed on the optical subcarrier. As a result, a signal with a high bit rate or a wide band can be transmitted at a very high carrier frequency.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of an optical subcarrier frequency modulator according to the present invention.

FIG. 2 shows an optical spectrum of a multi-wavelength laser light source and an optical spectrum of frequency-modulated output light.

FIG. 3 is a block diagram showing a second embodiment of the optical subcarrier frequency modulator of the present invention.

FIG. 4 is a block diagram showing a third preferred embodiment of the optical subcarrier frequency modulator according to the present invention.

FIG. 5 is a block diagram showing a fourth embodiment of the optical subcarrier frequency modulator according to the present invention.

FIG. 6 is a block diagram showing a configuration example of a conventional optical subcarrier frequency modulator.

[Description of Signs] 1 Mode-locked laser diode (MLLD) 2 Arrayed waveguide grating (AWG) 3a, 3b, 3c, 3d Fiber collimator 4 Acousto-optic modulator (AOM) 5 Optical fiber 6 3-to-1 coupler 7 Fourier transform Lens 8 2 to 1 coupler 9 Optical intensity modulator 10 2 × 1 optical switch 11 Oscillator 12 Digital signal source 13 Modulator 14 Analog signal source 15 Voltage controlled variable frequency oscillator 16 Oscillator 17 Digital signal source 18 Digital signal source

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04J 14/00 14/02 (72) Inventor Hiroshi Matsuoka 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (8)

[Claims] 1. An optical subcarrier frequency modulator that frequency-modulates an optical subcarrier and superimposes a high-frequency signal, comprising: a multi-wavelength laser light source that generates a plurality of optical spectra; and each light output from the multi-wavelength laser light source. A wavelength filter for splitting a spectrum, an optical frequency shifter driven by the high-frequency signal and frequency-modulating an optical subcarrier split by the wavelength filter; an optical carrier split by the wavelength filter; An optical system for multiplexing the frequency-modulated optical subcarrier output from the frequency shifter. 2. The optical subcarrier frequency modulator according to claim 1, further comprising a plurality of optical frequency shifters, each of which modulates the frequency of each of the plurality of optical subcarriers with a plurality of high-frequency signals, and multiplexes the signals with an optical system. An optical subcarrier frequency modulator, characterized in that: 3. An optical subcarrier frequency modulator that frequency-modulates an optical subcarrier and superimposes a digital signal, comprising: a multi-wavelength laser light source for generating a plurality of optical spectra; and each light output from the multi-wavelength laser light source. A wavelength filter for splitting a spectrum, an optical frequency shifter for inputting optical subcarriers split by the wavelength filter and outputting two optical frequency components, and complementing the two optical frequency components with the digital signal An optical system for multiplexing two optical intensity modulators for performing optical intensity modulation, an optical carrier split by the wavelength filter, and a frequency-modulated optical subcarrier output from the two optical intensity modulators An optical subcarrier frequency modulator comprising: 4. The optical subcarrier frequency modulator according to claim 3, further comprising a plurality of sets of an optical frequency shifter and two optical intensity modulators, each of the plurality of optical subcarriers being frequency-modulated by a plurality of digital signals, An optical subcarrier frequency modulator having a configuration for multiplexing by an optical system. 5. An optical subcarrier frequency modulator that frequency-modulates an optical subcarrier and superimposes a digital signal, comprising: a multi-wavelength laser light source that generates a plurality of optical spectra; and each light output from the multi-wavelength laser light source. A wavelength filter that splits a spectrum, an optical frequency shifter that inputs optical subcarriers split by the wavelength filter and outputs two optical frequency components, and converts the two optical frequency components according to the digital signal. A 2 × 1 optical switch to be selected; an optical carrier split by the wavelength filter;
An optical system for multiplexing a frequency-modulated optical subcarrier output from one optical switch. 6. The optical subcarrier frequency modulator according to claim 5, comprising a plurality of sets of an optical frequency shifter and a 2 × 1 optical switch.
An optical subcarrier frequency modulator, wherein a plurality of optical subcarriers are frequency-modulated by a plurality of digital signals, respectively, and multiplexed by an optical system. 7. The optical subcarrier frequency modulator according to claim 1, wherein the multi-wavelength laser light source is a mode-locked laser diode. 8. The optical subcarrier frequency modulator according to claim 1, wherein the optical frequency shifter is an acousto-optic modulator.