JP3451890B2 - Frequency modulation method - 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 system in which a semiconductor laser is directly modulated by a modulated signal to obtain frequency-modulated output light, and in particular, it is not affected by a residual AM component of the laser. , A novel wideband frequency modulation method having good FM modulation characteristics.

[0002]

2. Description of the Related Art As a method of distributing multi-channel video signals in a CATV system or the like, a method of using optical transmission for a backbone system has been adopted instead of the conventional coaxial system. In such a system, optical transmission is used for the backbone system from the center to the node, and the same coaxial system as before is used for transmission from the node to each subscriber, making full use of the low loss and wide bandwidth of optical transmission. A large-scale, high-quality, high-performance service can be realized, and the introduction cost can be kept low by matching with the conventional system. In an optical transmission system, frequency-multiplexed multi-channel AM video signals are directly converted into optical intensity signals and transmitted. Therefore, optical transmission systems are required to have high quality signal transmission in terms of noise characteristics and distortion characteristics, and light emitting elements, light receiving elements, and the like applicable to these transmissions have been developed so far.

On the other hand, in order to make the most of the effectiveness of optical transmission, studies are being actively conducted on a system in which all the lines from the center to each subscriber are connected by optical fibers. In such a system, in addition to the above-mentioned CATV multi-channel video distribution,
High-speed data communication is also possible, and it is expected as an information infrastructure in the multimedia era that can support various services.

In the case of distributing multi-channel AM video signals in such an optical subscriber system, the optical signal transmitted from the center is multi-branched by the star coupler and distributed to each subscriber, thereby sharing the optical transmitting equipment. It is necessary to reduce the cost of the center. However, as described above, the AM video signal is required to have a high signal quality, so that a large number of branches cannot be taken.

Therefore, a method has been proposed in which multi-channel AM video signals are collectively FM-modulated and transmitted (for example, K. Kikushima et. Al., ECOC'95, Th, L.3.1, 1995).
According to this method, the transmission quality necessary for obtaining the desired video quality is greatly mitigated by the FM modulation effect, so that a large number of branches can be taken. In this method, the key is to realize a wide band FM modulator that collectively FM-modulates multi-channel AM video signals. AM with frequency multiplexing
Since the band of the video signal extends to several hundred MHz, it is difficult to collectively perform FM modulation on this with an electronic circuit as in the conventional case. Therefore, the above-mentioned reference proposes an FM modulation method using optical heterodyne detection. FIG. 5 shows the configuration.

The frequency modulation type optical transmitter of FIG. 5 directly modulates the semiconductor laser 1 with an AM video signal,
Obtain output light that is frequency-modulated. This output light and the output light of the laser 2 for local oscillation are multiplexed by the optical coupler 5 and heterodyne detection is performed by the light receiving element 6, so that the oscillation frequency difference fb between the laser 1 and the laser 2 becomes the center frequency. Obtain an FM modulated signal. The FM modulated signal is amplified by the amplifier 10, the transmission laser 11 is driven by the amplified output, and the FM modulated transmission optical signal is output. In order to stabilize the center frequency fb of the FM modulated signal, the light receiving element 7
The signal subjected to the heterodyne detection is demodulated by the FM demodulator 8 and the laser 2 is negatively feedback controlled by the frequency stabilizing circuit 9. The optical attenuators 3 and 4 provided between the outputs of the laser 1 and the laser 2 and the optical coupler 5 adjust the level of each optical signal to be multiplexed.

F using such optical heterodyne detection
Multi-channel AM with wide band by M modulator
It becomes possible to collectively perform FM modulation of video signals, increase the number of optical branches for multi-channel video distribution in an optical subscriber system, and expect to realize a low-cost system.

[0008]

In the FM modulation system using the optical heterodyne detection as described above, the optical frequency modulation by the direct modulation of the semiconductor laser is used. In this case, the optical output intensity is obtained by the direct modulation. Is also modulated. Therefore, the input / output spectrum of the FM modulator is as shown in FIG. 6, for example. As shown in FIG. 6 (a), assuming a single-frequency carrier of frequency fm as an input signal, the FM modulation output obtained by optical heterodyne detection is two as shown in FIG. 6 (b). The spectrum shape is such that a plurality of sideband components having a frequency interval fm are arranged above and below the laser oscillation frequency difference fb as a center. At the same time, an intensity modulation component due to the direct modulation of the laser appears at the frequency fm. If the ratio of the line spectrum intensity of the frequency fb, which is the FM carrier, to the line spectrum intensity of the frequency fm, which is the residual AM component, is defined as the residual AM suppression ratio, signal distortion will occur during demodulation unless this residual AM suppression ratio is set large. .

In order to increase the residual AM suppression ratio, a method has been proposed in which the intensity ratios of two lasers to be combined are set to be largely asymmetrical (Sakurai et al. "Optical image distribution system using optical processing technology"). Technical report SSE95-141, 1995). That is, while the current of the residual AM component detected by the light receiving element 6 is proportional to the light intensity reaching from the laser 1, the current of the beat component of both lasers 1 and 2 which is the FM component is the laser 1.
And is proportional to the square root of the product of the light intensities reaching from the laser 2. Therefore, by making the light intensity reaching the light receiving element 6 from the laser 1 sufficiently smaller than that of the laser 2 by using the optical attenuators 3 and 4 provided at the outputs of the laser 1 and the laser 2, the residual AM component Can be suppressed.

By the way, in order to obtain a good signal-to-noise ratio in such an FM modulator, the phase noise of the laser must be suppressed sufficiently small. That is, since the phase noise of the laser is converted into the intensity noise component by the heterodyne detection, a laser having a sufficiently small oscillation spectrum line width is required. Although the line width is narrowed variously by devising the structure of the laser, in general, the FM modulation efficiency of the laser decreases as the line width becomes narrower, so that the modulation current of the laser required to obtain the required frequency modulation degree increases. , Residual A
The M component becomes large. Therefore, in order to suppress this, the composition ratio of the two laser beams must be made more asymmetrical as described above, and the signal-to-noise ratio after reception is deteriorated. In addition, the nonlinearity of the response due to the increase of the modulation current is also a problem. Therefore, there is a limit to this measure.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to solve the above problems and provide a novel wideband frequency modulation system which is not affected by the residual AM component of the laser and has good FM modulation characteristics.

[0012]

To achieve the above object, the present invention forms a first modulated signal and a second modulated signal having mutually opposite phase relationships from a modulated signal, A current flowing through the first semiconductor laser is modulated by the first modulated signal, a current flowing through the second semiconductor laser is modulated by the second modulated signal, and the output light of the first semiconductor laser and the The output light of the second semiconductor laser is combined by an optical coupler.

A frequency-modulated signal may be obtained by detecting the combined light combined by the optical coupler with a light receiving element.

Either or both of the first modulated signal and the second modulated signal are input to the semiconductor laser via a level adjuster, and the respective semiconductor lasers are combined by the optical coupler. The level may be adjusted so that the light intensity modulation amplitudes of the output lights from are equal.

A variable optical attenuator is inserted between either or both of the first semiconductor laser and the second semiconductor laser and the optical coupler, and the respective semiconductor lasers are combined by the optical coupler. The output light from the semiconductor laser may be attenuated so that the light intensity modulation amplitudes of the output light from the.

The optical coupler may be configured to have a variable branching ratio, and the branching ratio may be changed so that the light intensity modulation amplitudes of the output lights from the respective semiconductor lasers combined by the optical coupler become equal.

A part of the combined light from the optical coupler is branched by the monitor optical coupler, the branched light is detected by the monitor light receiving element, and the modulated signal component is detected from the monitor detection output. The level adjuster may be controlled according to the detection output.

A part of the combined light from the optical coupler is branched by the monitor optical coupler, the branched light is detected by the monitor light receiving element, and the modulated signal component is detected from the monitor detection output. The variable optical attenuator may be controlled according to the detection output.

A part of the combined light from the optical coupler having the variable branching ratio is branched by the monitor optical coupler, the branched light is detected by the monitor light receiving element, and the modulated signal is output from the monitor detection output. The components may be detected, and the branching ratio of the optical coupler configured to change the branching ratio may be controlled according to the detected output.

A pilot signal is superimposed on the modulated signal, a part of the output light from the optical coupler is branched by an optical coupler for monitoring, the branched output light is detected by a light receiving element for monitoring, and this monitor is used. The pilot signal component may be detected from the detection output, and the level adjuster may be controlled according to the detection output.

A pilot signal is superimposed on the modulated signal, a part of the output light from the optical coupler is branched by an optical coupler for monitoring, the branched output light is detected by a light receiving element for monitoring, and the monitor is used. A pilot signal component may be detected from the detection output, and the variable optical attenuator may be controlled according to this detection output.

A pilot signal is superposed on the modulated signal, a part of the output light from the optical coupler having the variable branching ratio is branched by an optical coupler for monitoring, and this branched output light is received by a light receiving element for monitoring. Alternatively, the pilot signal component may be detected from the monitor detection output, and the branching ratio of the optical coupler configured to be variable in the branching ratio may be controlled according to the detection output.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

As shown in FIG. 1, the frequency modulation type optical transmitter of the present invention receives a modulated signal as a signal input, and outputs a first modulated signal and a first modulated signal having mutually opposite phase relationships from the modulated signal. A hybrid 12 forming two modulated signals,
First and second level adjusters 13 and 14 for adjusting the levels of the respective modulated signals, first and second semiconductor lasers 1 and 2 whose currents are modulated by the respective modulated signals,
By detecting the first and second optical attenuators 3 and 4 that attenuate the output light of each semiconductor laser, the optical coupler 5 that combines the output light of the semiconductor lasers that have passed through these optical attenuators, and the combined light A light receiving element 6 for obtaining a frequency-modulated signal, an amplifier 10 for amplifying the frequency-modulated signal, and a transmission laser 11 driven by the amplified frequency-modulated signal are provided. 7 is a light receiving element, 8 is an FM demodulator, and 9
Is a frequency stabilizing circuit.

The operation of the frequency modulation type transmitter of FIG. 1 will be described. First, the modulated signal is input to the hybrid 12 to obtain in-phase and anti-phase modulated signals. The respective modulated signals are input to the semiconductor lasers 1 and 2 via the level adjusters 13 and 14 and directly modulate the respective lasers 1 and 2. The output lights of the lasers 1 and 2 are input to the optical coupler 5 via the optical attenuators 3 and 4, respectively, and are combined by the optical coupler 5. This combined light is detected by the light receiving element 6 to obtain an FM modulation signal. The FM modulated signal is amplified by the amplifier 10, the transmission laser 11 is driven by the amplified output, and the FM modulated transmission signal is output.

In the configuration of FIG. 1, the intensity of the component of the output light of the laser 1 received by the light receiving element 6 is P ₁ , and the intensity of the component of the output light of the laser 2 received by the light receiving element 6 is P ₂ The intensity modulation degree of the laser 1 is m ₁ , and the intensity modulation degree of the laser 2 is m ₂ . At this time, the output current I of the light receiving element 6 is expressed by the following equation.

I = R [(1 + m ₁ ) P ₁ + (1 + m ₂ ).
P ₂ ] + 2R [(1 + m ₁ ) (1 + m ₂ ) P ₁ P ₂ ]
^1/2 sin (2πf _b t) Here, R is a photoelectric conversion coefficient of the light receiving element.

The first term in the above equation is the detection component of the laser light, and the second term is the beat component. The residual AM modulation component is included in the first term, where m ₁ P ₁ = −m ₂ P ₂
When m ₁ and m ₂ are set to satisfy the above condition, the residual AM modulation component is canceled. That is, laser 2 is laser 1
The phase may be reversed so that the intensity modulation degree is m ₂ = −m ₁ P ₁ / P ₂ . Further, since the beat component has a component corresponding to the difference frequency between the oscillation frequencies of the two lasers, P
_{If 1} and P ₂ are equivalent, the intensity modulation degree of each laser required to obtain a required FM modulation signal is about half that of the conventional one.

As an ideal state, m ₁ =-
Considering the case of m ₂ and P ₁ = P ₂ , the optical modulation degree (amplitude ratio between the first term and the second term) of the FM carrier component (frequency f _b component) is 100%. On the other hand, in the conventional method, as described above, in order to suppress the residual AM modulation component, P
_{Since 1} and P ₂ are greatly different, the degree of optical modulation is small and the signal-to-noise ratio is deteriorated.

Further, in the system of the present invention, F
Since the degree of optical modulation of the M carrier component is large, it is not necessary to amplify and photoelectrically convert the FM-modulated signal obtained as in the prior art, and output the optical signal as an optical signal. It is also possible to send the light as it is as transmission light. That is, as shown in FIG. 2, the light receiving element 6, the amplifier 10, and the transmitting laser 11 are omitted from the configuration of FIG.
A simple configuration that allows the combined light from the optical coupler 5 to be transmitted as it is as transmission light becomes possible.

Another embodiment of the present invention will be described.

The frequency modulation type transmitter shown in FIG. 3 has, in addition to the configuration of FIG. 2, a signal source 15 for superimposing a pilot signal on a modulated signal and a monitor for branching a part of the combined light from the optical coupler 5. An optical coupler 17, a monitor light receiving element 18 for detecting the branched light, and a feedback control circuit 19 for detecting a pilot signal component from the monitor detection output and controlling the level adjusters 13 and 14 according to the detection output. Has been. The basic configuration is the same as that shown in FIGS. 1 and 2, and a duplicate description of the operation is omitted. The pilot signal from the signal source 15 is superimposed on the modulated signal, and the lasers 1 and 2 are driven as in the case of FIG. After combining the two laser output lights by the coupler 5, a part thereof is taken out by the monitor optical coupler 17, received by the monitor light receiving element 18, and the pilot signal component in the received signal is detected by the feedback control circuit 19, Either or both of the level adjusters 13 and 14 are controlled based on the detected intensity.

As described above, a part of the output light of the optical coupler is detected as a monitor, and the component showing the difference in the light intensity modulation amplitudes of the two laser output lights is detected from the monitor output before combining the both output lights. The light intensity modulation amplitude of each laser output light after being fed back to the signal and combined by the optical coupler is adjusted. In the configuration of FIG. 3, the pilot signal is preliminarily superimposed on the modulated signal, so that the pilot signal component is used as the component indicating the difference in the light intensity modulation amplitudes of the two laser output lights. The feedback destination is a level adjuster 13 for adjusting the levels of the in-phase and anti-phase modulated signals,
14 By such feedback control, the pilot signal component can always be made zero, and thus the residual AM component in the output light of the optical coupler can be sufficiently suppressed.

Next, a configuration will be described in which the pilot signal is not superimposed on the modulated signal, but the modulated signal component is detected from the monitor output as the component indicating the difference in the light intensity modulation amplitude, and the same effect is obtained. . The frequency modulation type transmitter shown in FIG. 4 omits the signal source 15 from the configuration of FIG. 3, and the feedback control circuit 19 detects the modulated signal component and adjusts the level. The description of the operation overlapping with the case of FIG. 3 is omitted. A part of the combined light from the coupler 5 is extracted by the monitor optical coupler 17, the AM component in the received signal is synchronously detected by the feedback control circuit 19, and the level adjusters 13 and 14 of the level adjusters 13 and 14 are detected based on the detected output. Control either or both. With such feedback control, it is possible to suppress the residual AM component in a stable manner as in the case of FIG. 3 without requiring a pilot signal.

In the embodiments of FIGS. 3 and 4, the method of controlling the drive input of each laser, that is, the modulated signal by the level adjusters 13 and 14 is used as a method of canceling the residual AM component with high accuracy. The light may be adjusted before combining the two output lights. For example, the optical attenuators 3 and 4 arranged on the output side of the lasers 1 and 2 are variable optical attenuators,
The same effect can be obtained by controlling the amount of attenuation. Alternatively, the same effect can be obtained by configuring the optical coupler 5 with an optical coupler having a variable branching ratio and controlling the branching ratio.

[0036]

The present invention exhibits the following excellent effects.

(1) FM with direct modulation of semiconductor laser
Wideband F due to modulation effect and heterodyne detection method
M modulation is easily obtained.

(2) Residual AM which is a problem in direct modulation
The modulation component can be removed by applying the modulated signals to the respective lasers so that the modulated signals have mutually opposite phases, and an FM modulated signal having a good signal-to-noise ratio and distortion characteristics can be obtained.

(3) Since the intensities of the two laser lights to be combined can be set to be equal to each other, the light intensity modulation degree of the FM carrier can be made close to 100%. It is not necessary to photoelectrically convert and send the light, and the combined light can be sent as it is as transmission light.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a frequency modulation type optical transmitter showing an embodiment of the present invention.

FIG. 2 is a configuration diagram of a frequency modulation type optical transmitter in which the configuration of FIG. 1 is simplified.

FIG. 3 is a configuration diagram of a frequency modulation type optical transmitter showing another embodiment of the present invention.

FIG. 4 is a configuration diagram of a frequency modulation type optical transmitter showing another embodiment of the present invention.

FIG. 5 is a configuration diagram of a conventional frequency modulation type optical transmitter.

FIG. 6 is an input / output spectrum diagram of a conventional frequency modulation type optical transmitter.

[Explanation of symbols]

1 semiconductor laser (first semiconductor laser) 2 Semiconductor laser (second semiconductor laser) 3,4 optical attenuator 5 Optical coupler 6 Light receiving element 12 hybrid 13,14 Level adjuster 15 Pilot signal source 17 Monitor Optical Coupler 18 Photodetector for monitor 19 Feedback control circuit

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ identification code FI H04B 10/152 (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 2/00-2/02 H04B 10/12 -10/158 JISST file (JOIS) INSPEC (DIALOG)

Claims (11)

(57) [Claims] 1. A first modulated signal and a second modulated signal having mutually opposite phase relationships are formed from the modulated signal,
A current flowing through a first semiconductor laser is modulated by the first modulated signal , a current flowing through a second semiconductor laser is modulated by the second modulated signal, and the first semiconductor is modulated. In a frequency modulation method in which output light of a laser and output light of the second semiconductor laser are combined by an optical coupler ,
One of the first modulated signal and the second modulated signal, or
Both through the level adjuster into the laser diode.
Each half after being combined by the optical coupler
Light intensity modulation amplitude of output light from conductor laser becomes equal
A frequency modulation method characterized by adjusting the level in such a way. 2. The modulated signals have mutually opposite phase relationships.
Forming a first modulated signal and a second modulated signal that
Flow to the first semiconductor laser by the first modulated signal
A current that is modulated by the second modulated signal.
The current flowing through the conductor laser is modulated to generate the first semiconductor laser.
The output light of the laser and the output light of the second semiconductor laser.
In the frequency modulation method of combining with a coupler,
Any one of the first semiconductor laser and the second semiconductor laser
Insert a variable optical attenuator between either or both and the optical coupler
And each semiconductor after being synthesized by the optical coupler.
The light intensity modulation amplitude of the output light from the body laser becomes equal.
To attenuate the output light from the semiconductor laser.
Characteristic frequency modulation method. 3. The modulated signals have mutually opposite phase relationships.
Forming a first modulated signal and a second modulated signal that
Flow to the first semiconductor laser by the first modulated signal
A current that is modulated by the second modulated signal.
The current flowing through the conductor laser is modulated to generate the first semiconductor laser.
The output light of the laser and the output light of the second semiconductor laser.
In the frequency modulation method that synthesizes with a coupler,
Configure the coupler branching ratio variable, each of the semiconductor of the output light from the laser frequency modulation method you characterized by changing the division ratio so that the optical intensity modulation amplitude is equal after being synthesized by the optical coupler . 4. A part of the combined light from the optical coupler is monitored.
It is branched by the optical coupler for the monitor and this branched light is
Detected by a light receiving element and modulated from this monitor detection output
The signal component is detected and the level adjustment is performed according to the detected output.
The frequency modulation system according to claim 1 , wherein the frequency modulator is controlled . 5. A part of the combined light from the optical coupler is monitored.
It is branched by the optical coupler for the monitor and this branched light is used for the monitor.
Detected by a light receiving element and modulated from this monitor detection output
The signal component is detected, and the variable optical reduction is performed according to the detected output.
The frequency modulation method according to claim 2, wherein the attenuator is controlled . 6. A part of the combined light from the optical coupler having a variable branching ratio is branched by an optical coupler for monitoring, the branched light is detected by a light receiving element for monitoring, and the output detected from this monitor is detected. 4. The frequency modulation system according to claim 3, wherein the modulation signal component is detected, and the branching ratio of the optical coupler configured to change the branching ratio is controlled according to the detection output. 7. A pilot signal is superimposed on the modulated signal.
Then, a part of the output light from the optical coupler is branched by an optical coupler for monitoring, the branched output light is detected by a light receiving element for monitoring, and a pilot signal component is detected from the monitor detection output. detection and frequency modulation scheme according to claim 1, wherein the controller controls the level adjustment <br/> device in accordance with the detection output. 8. A pilot signal is superimposed on the modulated signal.
Then, a part of the output light from the optical coupler is branched by an optical coupler for monitoring, the branched output light is detected by a light receiving element for monitoring, and a pilot signal component is detected from the monitor detection output. Detects the variable optical attenuation according to the detected output
Frequency modulation method according to claim 2, wherein the controlling the vessel. 9. A pilot signal is superposed on the modulated signal, a part of the output light from the optical coupler configured to have a variable branching ratio is branched by an optical coupler for monitoring, and the branched output light is used for monitoring. 4. The frequency according to claim 3, wherein the photodetector detects the pilot signal component from the monitor detection output, and the branch ratio of the optical coupler configured to change the branch ratio is controlled according to the detected output. Modulation method. 10. A combined light received by the optical coupler is received.
The frequency-modulated signal can be detected by detecting with an optical element.
Frequency modulation method according to claims 1 9 or characterized in that to obtain. 11. The first semiconductor laser and the second semiconductor laser.
Of either or both of the semiconductor lasers and the optical coupler
Insert a variable optical attenuator between them and combine them by the optical coupler.
Intensity of output light from each semiconductor laser
The output from the semiconductor laser is adjusted so that the modulation amplitudes become equal.
Frequency modulation method according to claim 1, wherein the attenuating force light.