JP3444516B2 - Frequency modem - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency modulation / demodulation device, and more particularly to a frequency modulation / demodulation device (hereinafter referred to as an FM modulation / demodulation device) for generating and demodulating a high frequency and wide band frequency modulation signal by an optical signal processing technique. Call).

[0002]

2. Description of the Related Art In recent years, an FM modulation circuit for generating a frequency modulation signal (hereinafter referred to as an FM modulation signal) by utilizing an optical frequency modulation operation of a semiconductor laser and heterodyne detection,
F including an FM demodulation circuit for demodulating the FM modulated signal
An M modulator / demodulator is being researched and developed. Such an FM modulated signal is better than that generated by a normal electrical method.
It has a large frequency shift amount and a wide band. Therefore, this FM modulator / demodulator is regarded as a promising device that collectively converts AM-FDM (amplitude modulation-frequency division multiplexing) multi-channel signals used in CATV and the like into FM modulated signals and reconverts them. In the following, as a conventional FM modulator / demodulator, "Reference; K. Kikushimam" is given.
a et. al. "150-km Non-Repea
ted 60-Channel AM-Video T
transmission Mission Opt
ical Heterodyne AM / FM Con
verter ", ECOC'95, Th.L.3.1.
Brussels, 1995 ”will be taken up for explanation.

FIG. 4 is a block diagram showing the configuration of a conventional FM modulator / demodulator. In FIG. 4, the FM modulation / demodulation device includes an FM modulation circuit 1001 and an FM demodulation circuit 1002. The FM modulation circuit 1001 includes a signal source 101
And an FM modulation laser (hereinafter, FM laser) 102,
The FM optical demodulation circuit 1002 includes a first optical waveguide unit 103, a local light source 104, a second optical waveguide unit 105, and a photodetector unit 106, and an identification unit 107, a delay unit 108, and an AN.
It includes a D-gate 109 and a filter 110. In addition, FIG. 4 also shows signal waveforms (a) to (e) of each part in the FM demodulation circuit 1002.

In the FM modulator / demodulator constructed as described above, in the FM modulator circuit 1001, the signal source 101 is
An electric signal in a predetermined frequency band, which is an original signal to be FM-modulated, is output. The FM laser 102 is composed of, for example, a semiconductor laser, and has a wavelength λ in a steady state with a constant injection current.
When the light of 1 is oscillated and the injection current is amplitude-modulated, the output light intensity is modulated (IM modulation), and the oscillation wavelength (or optical frequency) is also modulated, so that the output signal from the signal source 101 is It is converted into an optical frequency modulation signal centered on wavelength λ1 and output. The first optical waveguide section 103 guides the output optical signal from the FM laser 102. Local light source 104
Outputs a light (unmodulated light) having a wavelength λ0 which differs from the oscillation wavelength λ1 of the FM laser 102 by a predetermined amount Δλ. The second optical waveguide section 105 guides the unmodulated light from the local light source 104. First optical waveguide section 103 and second optical waveguide section 105
Both of the two lights guided by are input to the photodetector unit 106.

The photodetector section 106 is composed of a photodiode or the like having a square-law detection characteristic, and has a light intensity (I
M) The modulation component is converted into a current amplitude modulation component (hereinafter, referred to as IM-DD component), and when two lights having different wavelengths are input, a frequency corresponding to the wavelength difference between the two lights is obtained. It has the property of generating a beat component (this operation is called heterodyne detection). The optical detection unit 106
The output optical signal from the FM laser 102 and the local light source 104
When the output light from the light is input, a beat signal is output at a frequency corresponding to the wavelength difference Δλ between these two lights.
This beat signal coincides with the FM-modulated signal of the electric signal output from the signal source 101.

In the FM demodulation circuit 1002, the identification section 107
Inputs an FM modulation signal (see (a)) output from the FM modulation circuit 1001, identifies this with a constant threshold Vref, and converts it into a pulse signal (pulse conversion). Here, the threshold value Vref is generally selected as the amplitude center or average value of the FM modulation signal. This pulse signal is branched into two, and one of the pulse signals (see (b)) is AND gate 10
9 is input. The other pulse signal is input to the AND gate 109 after being given a predetermined time delay td in the delay unit 108 (see (c)). The AND gate 109 receives the two input pulse signals ((b),
The logical sum signal (see (c)) (see (d)) is generated and output. The filter 110 passes only the component of a predetermined frequency band similar to the output signal from the signal source 101 to the logical sum signal and outputs it (see (e)). The output signal from the filter 110 has its amplitude variation uniquely corresponding to the frequency variation of the FM modulation signal (see (a)), and becomes an FM demodulation signal.

In the FM modulation circuit 1001 described above, by using appropriate ones for the FM laser 102 and the local oscillation light source 104, it is difficult to realize a center frequency (carrier wave) of several GHz or more, which is difficult to realize by an FM modulation circuit of a general electric system. It is possible to generate a high frequency and wide band FM modulated signal having a frequency deviation of several hundred MHz or more. Also,
In the FM demodulation circuit 1002, the identification unit 107 and AND
By configuring the gate 109 using, for example, a high-speed logic element for 10 Gbps transmission, a demodulation circuit compatible with such a wide band FM modulation signal can be realized.

However, in the conventional FM modulator / demodulator as described above, the FM modulation signal generated by the FM modulation circuit 1001 is an optical intensity modulation component generated together with the optical frequency modulation signal in the FM laser 102, so that the amplitude fluctuation and It has a mean value fluctuation. The amplitude fluctuation and the average value fluctuation become a factor that significantly distorts the waveform of the FM demodulation signal output from the FM demodulation circuit 1002. Hereinafter, this will be described in detail with reference to FIGS. 5 and 6.

FIG. 5 shows the FM modulation circuit 1001 shown in FIG.
FIG. 6 is a schematic diagram for explaining the relationship between the spectrum and the waveform of the FM modulation signal generated by the above. Figure 5 (a) shows
The spectrum and the waveform of an ideal FM modulation signal are shown. FIG. 5B shows a spectrum and a waveform of a signal (FM + IM signal described later) that is IM-modulated at the same time as the FM modulation. FIG. 5C shows an FM-IM signal with IM-D.
The spectrum and waveform of the signal in which the D component is superimposed are shown. FIG. 6 is a diagram for explaining the influence of the amplitude variation and average value variation of the FM modulated signal on the FM demodulation operation.

When the optical signal output from the FM laser 102 has only an optical frequency modulation component, the generated F
As shown in FIG. 5A, the waveform of the M-modulated signal becomes an ideal one with neither amplitude variation nor average value variation, and its spectrum is the oscillation wavelength difference (λ1−λ0) between the FM laser 102 and the local light source 104. ) Shows a symmetrical shape with a frequency fc corresponding to () as the center. On the other hand, as described above, the FM laser 10
When the optical signal output from 2 is subjected to the optical frequency modulation and the optical intensity modulation (IM modulation) at the same time, the spectrum shape of the heterodyne detection component becomes asymmetrical as shown in FIG. Accompanying amplitude fluctuation (IM modulation component) (hereinafter, such a signal is referred to as FM + IM signal). Further, in the present FM modulator / demodulator, such FM +
A component (IM-DD component) obtained by square-detecting the light intensity modulation component in the photodetection unit 106 is superimposed on the IM signal.
Therefore, as shown in FIG. 5C, F such that the average value (or low frequency component) varies depending on the IM-DD component.
The M + IM signal is output from the FM modulation circuit 1001. That is, although the FM modulation circuit in the FM modulator / demodulator shown in FIG. 4 generates a very high frequency and wide band FM modulation signal, it is difficult to generate an ideal FM modulation signal, and the amplitude fluctuation and average value fluctuation Will output an FM modulated signal. Therefore, with the configuration of the conventional FM demodulation circuit that discriminates the FM modulation signal with the fixed constant threshold Vref, as is clear from FIG. 6, it is no longer possible to accurately discriminate the FM demodulation signal. Waveform distortion will occur.

[0011]

As described above, a fixed constant signal is applied to a wide band FM modulation signal generated by an FM modulation circuit constructed by utilizing the optical frequency modulation operation of a semiconductor laser and heterodyne detection. In the FM demodulation circuit that performs differential detection after identifying the FM modulation signal with the threshold value,
There was a problem that accurate demodulation was difficult.

Therefore, an object of the present invention is to generate an FM modulation signal by utilizing the optical frequency modulation operation of a semiconductor laser and heterodyne detection, and furthermore, to demodulate the FM modulation signal and obtain a waveform of an FM demodulation signal. F that can reduce distortion
An M modulation / demodulation device.

[0013]

Means for Solving the Problems and Effects of the Invention The first invention is a frequency modulation signal (hereinafter referred to as an FM modulation signal).
An FM modulation / demodulation device comprising an FM modulation circuit for generating and outputting and an FM demodulation circuit for inputting and demodulating the FM modulation signal, wherein the FM modulation circuit is a signal source for outputting an electric signal having a predetermined frequency band. And has the property of oscillating light of wavelength λ1 in a steady state where the input electric signal does not change, and uniquely converting the amplitude change of the input electric signal into a change in light intensity and a change in optical frequency. A frequency modulation laser (hereinafter referred to as an FM laser) that converts an output signal from a signal source into an optical signal and outputs the optical signal, a first optical waveguide unit that guides the output optical signal from the FM laser, and an FM Due to the square wave detection characteristic, the local oscillation light source that outputs the light of wavelength λ0 that differs from the oscillation wavelength λ1 of the laser by the predetermined amount Δλ, the second optical waveguide that guides the output light from the local oscillation light source, Change in intensity to change in current amplitude The first optical waveguide has the property of generating a beat component at a frequency corresponding to the wavelength difference between the two lights when the two lights of different wavelengths are input while being converted and output. The output optical signal guided by the second optical waveguide section and the output optical signal guided by the second optical waveguide section are input, and a beat signal is generated at a frequency corresponding to the wavelength difference Δλ between the output optical signal and the output light. The FM demodulation circuit includes an optical detection unit that generates and outputs an FM modulated signal.
A branching unit that branches the FM modulated signal output from the M modulation circuit into a first signal and a second signal; a first filter that allows only a component of a predetermined frequency band to pass through the second signal; With the instantaneous level of the output signal from the first filter as a threshold, the first signal is discriminated and pulsed to generate the first pulse signal, and the discriminator outputs the signal in two branches. A delay unit that gives a predetermined delay to one of the first pulse signals, the other of the first pulse signals output from the identifying unit, and the first pulse signal that has been delayed by the delay unit By performing a calculation, the timing at which the other first pulse signal changes is detected, and a pulse output unit that outputs a second pulse signal at the detected timing, and a second pulse signal, Of the components of a given frequency band The and a second filter which passes.

An FM frequency modulated signal generated by modulating the injection current of the FM laser and heterodyne-detecting the FM frequency modulated signal is amplitude-varied by the light intensity modulation operation in the FM laser. (IM modulation component) and mean value variation (IM-DD component). Therefore, in the FM demodulation circuit of the first invention, the first filter extracts and outputs the IM-DD component as the component of the predetermined frequency band from the second signal output from the branching unit. The discriminating unit receives the first signal and the output signal from the first filter, discriminates the first signal using the instantaneous level of the output signal from the first filter, which changes with time, as a threshold value, and pulsates the signal. To do. After that, an FM demodulated signal is obtained by performing differential detection. As described above, in the identifying unit 107, the FM modulated signal (first signal) is identified using the threshold value that changes similarly to the amplitude variation and average value variation of the FM modulated signal (first signal). As a result, the FM demodulation circuit can generate an FM demodulation signal with less waveform distortion.

A second invention is the same as the first invention, except that the FM
The demodulation circuit is installed in the propagation path of the first or second signal, and the propagation time t1 until the first signal reaches the identification unit from the branch unit and the second FM signal from the branch unit to the first A delay adjusting section for delaying the input first or second signal so that the propagation time t2 until reaching the identifying section via the filter becomes equal, and a propagation path for the first or second signal. Installed in the discriminating section and the average value fluctuation amount of the first signal input to the discriminating unit and the instantaneous level fluctuation amount of the second signal input to the discriminating unit are equal to each other.
And an amplitude adjusting unit that adjusts the amplitude of the signal.

According to the second aspect of the invention, the delay adjusting section includes the first adjusting section.
Propagation time t from the branch signal to the discrimination section
1 and the mean value variation component of the first signal by adjusting so that the propagation time t2 from the branching portion to the identification portion via the first filter becomes equal to The phase of the IM-DD component extracted from the second signal is matched. Further, the amplitude adjustment unit is configured to change the average value of the first signal input to the identification unit and the second amount input to the identification unit.
It is adjusted so that the instantaneous level fluctuation amount of the IM-DD component extracted from the signal of is equal to. As a result, the identification unit 107 causes the FM modulation signal (first
Signal) can be identified, and the FM demodulation circuit can generate an FM demodulation signal with less waveform distortion.

In a third aspect based on the first or second aspect, the pulse output section comprises an AND gate, and the AND gate delays the other first pulse signal output from the identifying means and the delay section. Is generated and a logical sum signal with one of the first pulse signals is generated, and the logical sum signal is output as a second pulse signal.

In the third aspect of the invention, the pulse output section is constituted by the AND gate. This makes it possible to reduce the circuit scale of the pulse output unit.

[0019]

1 is a block diagram showing the configuration of a frequency modulation / demodulation apparatus (hereinafter referred to as an FM modulation / demodulation apparatus) according to a first embodiment of the present invention. In FIG. 1, F
The M modulation / demodulation device includes an FM modulation circuit 1001 and an FM demodulation circuit 1002. The FM modulation circuit 1001 includes a signal source 101 and an FM modulation laser (hereinafter, FM laser).
102, a first optical waveguide section 103, and a local light source 104
The FM demodulation circuit 1002 includes an identification unit 107, a delay unit 108, an AND gate 109, and a second filter 11 and a second optical waveguide unit 105 and an optical detection unit 106.
0, a branching unit 111, and a first filter 112 are included. Further, FIG. 1 also shows signal waveforms (a) to (f) of each part in the FM demodulation circuit 1002.

Next, the operation of the FM modulator / demodulator shown in FIG. 1 will be described with reference to FIG. FIG. 2 is a diagram for explaining an ideal threshold value Vref for the identification unit 107 shown in FIG. 1 to identify an FM modulated signal. FM modulation circuit 100
The operation of No. 1 is the conventional FM modulation circuit described above (see FIG. 4).
The signal source 101 outputs an electric signal in a predetermined frequency band which is an original signal to be FM-modulated. The FM laser 102 is typically composed of a semiconductor laser or the like,
When the injection current is oscillated in a steady state where the injection current is constant and the injection current is amplitude-modulated by the output signal from the signal source 101, the output light intensity is modulated and the oscillation wavelength (optical frequency) is also modulated. It receives and outputs an optical frequency modulation signal centered on the wavelength λ1. First optical waveguide 103
Guides the optical signal output from the FM laser 102. The local oscillation light source 104 has an oscillation wavelength λ of the FM laser 102.
It outputs light of wavelength λ 0 (unmodulated light) that differs from 1 by a predetermined amount Δλ. The second optical waveguide section 105 guides the unmodulated light from the local light source 104. First optical waveguide 103 and second
Both of the two lights guided by the optical waveguide unit 105 are input to the optical detection unit 106. The optical detection unit 106 is configured by a photodiode or the like, square-square-detects the light intensity modulation component of the output optical signal from the FM laser 102 and outputs it as an IM-DD component, and also performs the FM by heterodyne detection.
A beat signal (FM modulation signal) is output at a frequency corresponding to the wavelength difference Δλ between the output optical signal from the laser 102 and the output light from the local light source 104.

In the FM demodulation circuit 1002, the branch unit 111
Inputs an FM modulated signal (see (a)) and outputs it as a first
Signal and the second signal are output. The first filter 112 inputs the second signal, passes only the component (IM-DD component) in a predetermined frequency band corresponding to the electric signal output from the signal source 101, and outputs the component (see (b)). ). The identification unit 107 determines the instantaneous level of the second signal (IM-DD component) that has passed through the first filter 112 as a threshold value V.
The first signal is identified as ref (see FIG. 2), pulsed, and then branched into two and output. The delay unit 108
A predetermined time delay td is given to one of the signals output from the identifying unit 107 (see (d)). AND gate 109
Inputs the other signal (see (c)) output from the discriminating unit 107 and the output signal from the delay unit 108 (see (d)), and generates an OR signal (see (e)). To do.
The second filter 110 passes only the component of a predetermined frequency band to the logical sum signal (pulse signal) output from the AND gate 109 and outputs it (see (f)). As described above, the identifying unit 107 uses the threshold Vref corresponding to the amplitude variation and the average value variation of the FM modulated signal to accurately identify the FM modulated signal (first signal).
The M demodulation circuit 1002 can generate an FM demodulation signal with little waveform distortion.

FIG. 3 shows an F according to the second embodiment of the present invention.
It is a block diagram showing a configuration of an M modulation and demodulation device. The FM modulator / demodulator shown in FIG. 3 is different from the FM modulator / demodulator shown in FIG. 1 only in that an amplitude adjusting unit 301 and a delay adjusting unit 302 are added to the FM demodulating circuit 1002. Other configurations are similar to those of the FM modulator / demodulator shown in FIG.
The illustration and description of the FM modulation circuit 1001 are omitted, and corresponding parts in the FM demodulation circuit 1002 are designated by the same reference numerals and the description thereof is omitted.

The amplitude adjusting section 301 is inserted in the propagation path of the first signal and passes through the first filter 112 and the average value fluctuation amount of the first signal inputted to the identifying section 107, and the identifying section 1 passes through.
The amplitude of the first signal is adjusted so that the instantaneous level fluctuation amount of the second signal (IM-DD component) input to 07 is equal. The delay adjustment unit 302 is inserted in the propagation path of the second signal, and the first signal is output from the branch unit 111 to the identification unit 10.
7 and the second signal from the branch unit 111 via the first filter 112 to the identification unit 1
The delay amount of the second signal is adjusted so that the propagation time t2 until reaching 07 becomes equal, and the phase matching between the first signal and the second signal in the identifying unit 107 is realized. With the above configuration, the identification unit 107 causes the threshold Vre that more accurately corresponds to the amplitude variation and the average value variation of the FM modulated signal.
With the configuration in which the FM modulated signal (first signal) is accurately identified using f 2, the FM demodulation circuit 1002 can generate an FM demodulated signal with less waveform distortion.

Further, in FIG. 3, the amplitude adjusting unit 301 is the second
The delay adjusting unit 302 is inserted in the propagation path of the first signal in the propagation path of the signal, but is inserted in the propagation path of the first signal and the propagation path of the second signal, respectively, if necessary.

In the first and second embodiments, the AND gate 109 is used to perform the differential detection. However, the other signal output from the discriminating unit 107 and the discriminating unit 107 are output. In addition, by performing a logical operation using one of the signals delayed by the delay unit 108, the timing at which the signal changes is detected, and a signal is output at the detected timing, for example, exclusive It is also possible to use a logical OR circuit or the like.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an FM modulation / demodulation device according to a first embodiment of the present invention.

FIG. 2 is a diagram for explaining an ideal threshold value Vref for the identification unit 107 shown in FIG. 1 to identify an FM modulated signal.

FIG. 3 is a block diagram showing a configuration of an FM modulation / demodulation device (only an FM demodulation circuit is shown) according to a second embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of a conventional FM modulator / demodulator.

5 is a schematic diagram for explaining a relationship between a spectrum and a waveform of an FM modulation signal generated by the FM modulation circuit 1001 shown in FIG.

FIG. 6 is a diagram for explaining the influence of the amplitude variation and average value variation of the FM modulated signal on the FM demodulation operation.

[Explanation of symbols]

101 ... Signal source 102 ... FM modulation laser (FM laser) 103 ... First optical waveguide 104 ... Local light source 105 ... Second optical waveguide 106 ... Optical detection unit 107 ... Identification unit 108 ... Delay unit 109 ... AND gate 110 ... second filter 111 ... Branch 112 ... the first filter 301 ... Amplitude adjustment unit 302 ... Delay adjustment unit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI H04B 10/18 (72) Inventor Koji Kikushima 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Nihon Telegraph and Telephone Corporation ( 56) References Japanese Patent Laid-Open No. 9-326769 (JP, A) Yu Fuse et al., CNR characteristics of optical video transmission system using wideband FM modulation technology, IEICE Transactions BI, Japan, Japan Electronics Informatics IEICE, September 1998, Vol. J81-BI, No. 9, pp. 557-565 Naoya Sakurai et al., Optical image distribution system using optical processing technology, IEICE technical research report, Japan, The Institute of Electronics, Information and Communication Engineers, February 15, 1996, Vol. 511, pp. 19-24 (58) Fields surveyed (Int.Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 JISST file (JOIS)

Claims (3)

(57) [Claims] 1. A frequency modulation circuit for generating and outputting a frequency modulation signal (hereinafter referred to as an FM modulation signal), and the FM.
A frequency modulation / demodulation device comprising a frequency demodulation circuit for inputting and demodulating a modulation signal, wherein the frequency modulation circuit comprises a signal source for outputting an electric signal having a predetermined frequency band, and a wavelength in a steady state where the input electric signal does not change. It has the property of oscillating light of λ1 and uniquely converting the change in the amplitude of the input electric signal into the change in the light intensity and the change in the optical frequency. A frequency modulation laser (hereinafter referred to as FM) which is converted into a signal and output.
A laser), a first optical waveguide section for guiding an output optical signal from the FM laser, and a local light source for outputting light having a wavelength λ0 different from the oscillation wavelength λ1 of the FM laser by a predetermined amount Δλ, A second optical waveguide that guides the output light from the local light source and a square-law detection characteristic convert a change in the light intensity of the input light into a change in the current amplitude and output the same, and also input two lights of different wavelengths. In this case, a beat component is generated at a frequency corresponding to the wavelength difference between the two lights, and the output optical signal guided by the first optical waveguide section and the second optical waveguide are generated. The output light guided by the wave unit is input, and an optical detection unit that generates and outputs an FM modulated signal that is a beat signal at a frequency corresponding to the wavelength difference Δλ between the output optical signal and the output light. The frequency demodulation circuit includes the frequency modulation circuit. A FM filter that outputs the FM modulated signal into a first signal and a second signal; a first filter that passes only the component of the predetermined frequency band with respect to the second signal; A discriminator that discriminates the first signal into pulses to generate a first pulse signal by using the level of the output signal from the first filter as a threshold, and outputs the first pulse signal in two branches; A delay unit that gives a predetermined delay to one of the first pulse signals, and the other of the first pulse signals output from the identifying unit,
By performing a logical operation using the first pulse signal delayed by the delay unit, the other first
Detecting the timing when the pulse signal of changes,
A frequency modulation / demodulation device comprising: a pulse output unit that outputs a second pulse signal at the detected timing; and a second filter that passes only the component of the predetermined frequency band with respect to the second pulse signal. 2. The frequency demodulation circuit is installed in the propagation path of the first or second signal, and the propagation time t1 from the branching section to the identifying section, and A delay is given to the input first or second signal so that the propagation time t2 from the branching portion of the second FM signal to the identification portion via the first filter becomes equal. A delay adjustment unit, an average value variation amount of the first signal that is installed in the propagation path of the first or second signal and that is input to the identification unit, and a level of a second signal that is input to the identification unit The frequency modulation / demodulation device according to claim 1, further comprising an amplitude adjustment unit that adjusts the amplitude of the input first or second signal so that the fluctuation amount becomes equal. 3. The pulse output unit includes an AND gate, and the AND gate outputs the other first pulse signal output from the identifying unit and one of the first pulse signals delayed by the delay unit. Generate a logical sum signal with a pulse signal,
The frequency modulation / demodulation device according to claim 1, wherein the logical sum signal is output as the second pulse signal.