JP3582561B2 - Broadband FM modulator - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a wideband FM modulator, and more particularly to a wideband FM modulator that generates an FM signal with reduced distortion and noise.
[0002]
[Prior art]
FIG. 4A shows a conventional optical video transmission system, in which an AM video signal generating circuit 60 for generating a multi-channel AM video signal 61 and a wide band for converting the multi-channel AM video signal 61 into an FM signal 62 collectively. An FM modulator 100, a laser diode 20 for emitting an optical signal corresponding to the FM signal 62, an optical distributor 30 for distributing an optical signal from the laser diode 20 to a plurality of optical fibers 40, and an optical signal from the optical fiber 40 Is entered by the subscriber 50. The subscriber 50 is based on a photodiode 51 that converts an optical signal into an electric signal, an FM demodulator 52 that demodulates an electric signal from the photodiode 51 to generate a multi-channel AM video signal 63, and a multi-channel AM video signal 63. And a receiver 53 for displaying an image.
[0003]
In the above configuration, the multi-channel AM video signal 61 is collectively converted into an FM signal 62 by the FM modulator 100, and directly modulates the laser diode 20 to emit an optical signal. The optical signal is distributed to a large number in the optical distributor 30 and sent to the subscriber 50 through the optical fiber 40. In the subscriber 50, the optical signal is converted into an electric signal by the photodiode 51, then demodulated into a multi-channel AM video signal 63 by the FM demodulator 52, and distributed to the receiver 53.
[0004]
4B shows a multi-channel AM video signal 61 output from the AM video signal generation circuit 60, FIG. 4C shows an FM signal 62 modulated by the FM modulator 100, and FIG. Indicates a multi-channel AM video signal 63 demodulated by the FM demodulator 52.
[0005]
FIG. 5 shows the broadband FM modulator 100 of the optical signal image transmission system of FIG. 4A, which includes a DFB-LD (distributed feedback laser diode) 170 and a local oscillation laser diode (LD) of an external resonance type. It has an optical light source 120, an optical multiplexer 130, and a photodiode 140. The DFB-LD 170, the local oscillation light source 120, the optical multiplexer 130, and the photodiode 140 are connected by an optical fiber 150. A current obtained by superimposing a modulation signal current on a bias current is applied to the DFB-LD 170. At this time, the amplitude of the modulation signal is made sufficiently smaller than the entire forward current. When the current fluctuates, the oscillation wavelength of the DFB-LD 170 slightly fluctuates according to the current fluctuation due to the chirp of the DFB-LD 170, and this becomes an optical wavelength modulation signal. The local oscillation light source 120 emits light at a wavelength near the oscillation wavelength of the DFB-LD 170, and is multiplexed with the optical wavelength modulation signal by the optical multiplexer 130. The reason why the external resonance type semiconductor laser is used as the local oscillation light source 120 is that local oscillation light having high spectral purity is required. When the two lights are combined, a beat is generated. If the beat is detected by the photodiode 140, a broadband FM electrical signal having a carrier frequency equal to the difference between the optical frequencies of the two lights can be obtained. Since signal processing is performed in a very high frequency region, such as light, a wideband FM modulator, which is difficult with an electric circuit, can be realized with a simple configuration.
[0006]
Generally, the FM signal is more resistant to noise than the AM signal, so that the optical input level to the optical receiver is small, and even if the signal-to-noise ratio of the received signal is low, the deterioration of the image is small. Therefore, if transmission is performed using the FM signal, the number of optical branches can be increased, and the transmission system cost per subscriber can be reduced. However, since a home receiver usually has only an AM demodulator and not an FM demodulator, if an FM video signal is directly delivered to a home, it is necessary to attach an FM demodulator to each receiver. As shown in FIG. 4, the broadcasting station collectively converts the multi-channel AM video signal 61 into an FM signal 62, transmits only the optical transmission portion by FM, and demodulates the FM demodulator 52 of the subscriber 50 collectively. Then, the signal input to the receiver of the subscriber 50 becomes the same multi-channel AM video signal 63 as in the conventional terrestrial broadcasting. Therefore, the subscriber 50 can select and view a channel in the same manner as viewing and listening to conventional CATV and terrestrial broadcasting.
[0007]
Since it is necessary for the FM modulator 100 to secure the AM video signal 61 of several tens of channels, the modulation signal has a frequency of several hundred MHz. The noise resistance due to FM transmission improves as the occupied bandwidth of the FM signal increases, and in order to obtain a practical advantage in this system, it is necessary to secure an FM signal occupied bandwidth of several GHz. Since both the modulation frequency band and the FM signal band are very wide, a commonly used electric FM modulation circuit cannot be used. In addition, since the multi-channel AM video signal 61 is handled, it is necessary to suppress distortion over this wide bandwidth.
[0008]
[Problems to be solved by the invention]
However, according to the conventional wideband FM modulator shown in FIG. 5, since the DFB-LD 170 is used as a light source for modulation, there is an advantage that the wavelength of output light is easily modulated by chirp. Since the LD 170 has a larger phase noise of the output light and a wider wavelength line width than a light source such as an external resonance type, the noise of the generated FM signal increases. Therefore, the number of optical branches cannot be increased. Further, since the linearity of the current-wavelength conversion of the DFB-LD 170 is low, signal distortion is likely to occur during modulation. Since the modulation of the wavelength is performed by changing the current of the DFB-LD 170, the amplitude of the output light as well as the wavelength slightly changes at the same time. For this reason, there is also a problem that distortion due to the amplitude fluctuation is likely to occur.
[0009]
As described above, in the method of performing optical wavelength modulation using the chirp of the DFB-LD 170, a wideband FM modulator can be easily realized with a simple configuration, but it is difficult to realize low noise and low distortion. There is a problem. For this reason, not only the number of optical branches cannot be increased so much, but also a circuit for distortion compensation or the like is required practically, which may lead to an increase in cost.
[0010]
Therefore, an object of the present invention is to provide a broadband FM modulator that realizes low noise and low distortion and that realizes low cost of an applied optical video transmission system.
[0011]
[Means for Solving the Problems]
The present invention, in order to solve the above problems,
An optical amplifier driven by a predetermined bias current, and a first laser light source having a wavelength-selective reflector optically coupled to the optical amplifier and driven by an input signal voltage;
A second laser light source that emits a second laser light that is multiplexed with the first laser light emitted from the first laser light source;
An optical multiplexer for multiplexing the first and second laser lights, and a photoelectric conversion unit for converting an optical signal multiplexed by the optical multiplexer into an FM electric signal;
Said wavelength-selective reflector, the electrode is applied to the input signal voltage, have a waveguide means formed by material of which the refractive index is changed by an electric field based on the input signal voltage, to the first laser light source The input signal is also input to the phase inverting circuit, and the output of the phase inverting circuit is connected to the input of the second laser light source. The second laser light source has the same optical amplifying element as the first laser light source, and the second laser light source. A broadband FM modulator comprising the same wavelength-selective reflector as one laser light source .
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the wideband FM modulator according to the present invention will be described in detail.
[0013]
FIG. 1 shows an embodiment of a wavelength modulator applied to a broadband FM modulator according to the present invention. The wavelength modulator 110 includes a wavelength-selective reflector 111 made of lithium niobate and a semiconductor optical amplifier 115. The semiconductor optical amplifier 115 has a reflection surface 117 at one end and a transmission surface 118 at the other end. an active layer 116 that is biased current _{I 1.} A wavelength-selective reflector 111 is arranged on the transmission surface 118 side, and a laser resonator that oscillates only at a specific wavelength as a whole is configured. The reflection wavelength of the wavelength selective reflector 111 is determined by the grating in the optical waveguide 112 formed therein. The laser light is focused by the focusing lens 114 and output to the optical fiber 119.
[0014]
The refractive index of lithium niobate changes depending on the electric field. When the electrode 113 formed on the wavelength-selective reflector 111 applies a signal voltage V ₂ corresponding to the multi-channel video signal, an electric field is applied to the light waveguide 112 consisting of lithium niobate, the refractive index of the optical waveguide 112 is changed , The reflection wavelength changes. Therefore, the wavelength of the output light of the external resonance type laser light source can be changed according to the video signal applied to the electrode 113, and operates as a wavelength modulator.
[0015]
FIG. 2 shows an embodiment of the broadband FM modulator of the present invention using the wavelength modulator 110 shown in FIG. This broadband FM modulator includes a wavelength modulator 110, a local oscillation light source 120, a multiplexer 130, and a photodiode 140. Since the local oscillation light needs to have a narrow line width, an external resonance type laser light source is used as the local oscillation light source 120. However, the mechanism for changing the oscillation wavelength as shown in FIG. 1 may not be provided. When the two laser lights are multiplexed by the optical multiplexer 130, a beat equal to the difference between the oscillation frequencies of the respective laser lights is generated. When the beat is detected by the photodiode 140 via the optical fiber 150, an FM-modulated electric signal is obtained. Is obtained. Here, since the carrier frequency of the FM signal 141 becomes equal to the frequency difference between the two laser lights, the FM signal 141 having an arbitrary carrier frequency can be obtained by changing the wavelength of the local oscillation light.
[0016]
Since the wavelength modulator 110 is an external resonance type laser light source, the phase noise can be easily reduced as compared with a conventionally used DFB-LD. Further, since the change in the refractive index of the lithium niobate with respect to the electric field has high linearity, the distortion can be suppressed to a small value. Further, since the intensity of the laser light does not fluctuate due to the modulation signal, the distortion caused by the change in the intensity of the laser light, which has conventionally been a problem, does not occur. Accordingly, a significantly lower noise and lower distortion FM signal can be obtained than in the past, and thus the cost reduction of the optical video transmission system can be efficiently realized.
[0017]
FIG. 3 shows another embodiment of the broadband FM modulator of the present invention, in which the same wavelength modulator 110A as the wavelength modulator 110 is used as the local oscillation light source, and the wavelength modulation generated by the phase inversion circuit 160 is used for this. In this configuration, a signal having a phase opposite to that of the signal input to the device 110 is added. In such a configuration, one wavelength modulator is added signal amplitude same field case, the frequency transition of the FM signal obtained becomes about twice can enlarge the dynamic range.
[0018]
In the system shown in FIG. 4, as the frequency transition of the FM signal, that is, the wider the bandwidth, the stronger the noise, the efficiency of the optical video transmission system can be improved. Also, by adjusting the frequency shift amounts of the two wavelength modulation laser light sources to be the same, the secondary distortion can be canceled. When the frequency transition amount of the FM signal is the same, the level of the modulation signal input to each modulation laser can be reduced, which also has the effect of suppressing distortion.
[0019]
【The invention's effect】
As described above, according to the wideband FM modulator of the present invention, low noise and low distortion can be realized, and the cost of the applied optical video transmission system can be reduced.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing an embodiment of a wavelength modulator serving as a resonance type laser light source applied to a broadband FM modulator of the present invention.
FIG. 2 is a block diagram showing an embodiment of a broadband FM modulator according to the present invention configured using the wavelength modulator of FIG. 1;
FIG. 3 is a block diagram showing another embodiment of the broadband FM modulator according to the present invention configured using the wavelength modulator of FIG. 1;
FIG. 4A is an explanatory view showing a conventional optical video transmission system.
(B) Explanatory diagram showing a multi-channel AM video signal before modulation in a conventional optical video transmission system.
(C) Explanatory drawing which shows the FM signal modulated in the conventional optical video transmission system.
(D) Explanatory drawing which shows the demodulated multi-channel AM video signal in the conventional optical video transmission system.
FIG. 5 is an explanatory diagram showing a wideband FM modulator of a conventional optical video transmission system.
[Explanation of symbols]
20, laser diode 30, optical distributor 40, optical fiber 50, subscriber 51, photodiode 52, FM demodulator 53, receiver 61, multi-channel AM video signal 62, FM signal 63, multi-channel AM video signal 100, FM modulators 110 and 110A, wavelength modulator 111, wavelength selective reflector 112, optical waveguide 113, electrode 114, lens 115, semiconductor optical amplifier 116, active layer 117, light reflecting surface 118, light transmitting surface 119, optical fiber 120, local oscillation light source 130, optical multiplexer 140, photodiode 150, optical fiber 160, phase inversion circuit 170, modulation light source

Claims (2)

A first laser light source having an optical amplifying element driven by a predetermined bias current, optically coupled to the optical amplifying element, and having a wavelength-selective reflector driven by an input signal voltage; and A second laser light source that emits a second laser light that is combined with the emitted first laser light;
An optical multiplexer for multiplexing the first and second laser lights;
Photoelectric conversion means for converting an optical signal multiplexed by the optical multiplexer into an FM electric signal;
Said wavelength-selective reflector, possess the electrodes applied to the input signal voltage, the optical waveguide means formed by material of which the refractive index is changed by an electric field based on the input signal voltage, the first laser light source And the output of the phase inversion circuit is connected to the input of the second laser light source, and the second laser light source has the same light as the first laser light source. A broadband FM modulator comprising an amplifying element and the same wavelength selective reflector as the first laser light source . The light amplification element is a semiconductor light amplification element having a light reflection surface at one end and a light transmission surface at the other end,
The wavelength-selective reflector optically couples with the semiconductor optical amplifying element via the light transmitting surface, and has optical waveguide means formed of lithium niobate whose refractive index changes by the electric field. 2. The wideband FM modulator according to claim 1 , wherein a grating is formed on the means .

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