JPS6397026A - Method and device for optical communication - Google Patents

Abstract

PURPOSE:To increase an allowable transmission loss and to expand a non-relay transmission distance by transmitting a signal light modulated in terms of binary frequency with an information signal and optically and directly amplifying only one of two kinds of the modulated frequency components to receive in a light reception part. CONSTITUTION:The inrush current of a distributed feedback type semiconductor laser 1 is little modulated with 2Gb/s pulse modulation current from a signal generator 2 and modulated in terms of binary frequency corresponding to the codes 1 and 0 of the information signal. The frequency-modulated signal light passes through a light isolator 31 and condensed by a lens 41 to be combined to a light fiber 5 in single mode. And after propagating the fiber 5, the light is projected into a Fabry-Perot semiconductor laser amplifier 6 through the lens 42 and the light isolator 32. In the amplifier 6 only components of a code 1 is amplified to be outputted and the outputted light is detected by a light detecting unit 7 so that the information signal can be fetched.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical communication method and apparatus, and particularly to an optical communication method in which signal light is directly amplified and then received, and an apparatus for carrying out the same.

[Conventional technology and its problems]

In recent years, with the remarkable improvement in the characteristics of semiconductor lasers and optical fibers, high-speed, long-distance optical transmission with transmission speeds of 2 Gb/s or more and transmission distances of 1100 km or more has become possible. In addition, with the aim of further extending non-repeater transmission distances, active research and development is being carried out on high-power single-axis mode semiconductor lasers, zero-dispersion fibers, and the like.

Now, in such optical communications, a conventionally common method is the so-called direct detection method, in which signal light is intensity-modulated and transmitted, and the increase in intensity is detected.

In this method, in order to extend the non-relay transmission distance, that is, increase the allowable transmission line loss, it is conceivable to increase the power level of the signal light to be transmitted. However, in order to increase the transmission power level, the high-output semiconductor laser must be modulated with a large amplitude, which requires a high-power drive circuit.

Furthermore, when a semiconductor laser is modulated with a large amplitude, large frequency chirping occurs in its output light. As a result, there is a problem in that waveform distortion occurs due to dispersion due to transmission through an optical fiber, which deteriorates reception characteristics.

On the other hand, the permissible transmission path loss can be increased by lowering the reception level of receivable signal light rather than increasing the transmission power level.

As a method for this purpose, an optical heterogain homodyne detection method has been proposed and demonstrated. In this method, an information signal is placed on nine amplitude frequencies or phases of signal light and is transmitted.

Then, local oscillation light having the same or slightly different frequency as the signal light is combined with the signal light in the receiving section, and a beat signal corresponding to the difference frequency between the two is obtained by a photodetector.
An information signal is extracted from this beat signal. Of the future heterogain homodyne detection methods, the FSX heterodyne detection method, which adds an information signal to the frequency of the signal light, can utilize direct frequency modulation of a semiconductor laser, compared to other heterogain methods that require external modulators, etc. It has the advantage of being able to increase the transmission power level.

However, in this optical heterogain homodyne detection method, a polarization control element is required to make the polarization state of the signal light and the local oscillation light the same, and the receiving circuit becomes extremely complicated and expensive. There are drawbacks. Furthermore, there is a problem in that the requirements for the spectral width and frequency stability of the signal light source are very strict.

As another method for lowering the received light level, a method has been considered in which the intensity-modulated and transmitted signal light is directly amplified by an optical amplifier and then received. However, even in this method, it was difficult to increase the transmission power level for the same reason as in the optical direct detection method described above.

An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art, to provide an optical communication method that has a simple configuration, and allows for an increase in allowable transmission line loss and an extension of non-repeater transmission distance compared to the prior art. and to provide a device for carrying out the same.

[Means for solving problems]

In the optical communication method of the present invention, an optical transmitter transmits signal light that is binary frequency modulated by an information signal, and an optical receiver transmits only one of the two types of modulated frequency components. It is characterized by direct amplification and reception.

The optical communication device of the present invention includes an optical transmitter including a signal light source and a means for binary frequency modulating the signal light source with an information signal, and a signal light source transmitted from the optical transmitter.
The present invention is characterized in that it is constituted by an optical receiver including a laser amplifier that directly amplifies only one of the modulated frequency components and a photodetector that receives the output light of this laser amplifier.

Another optical communication device of the present invention includes an optical transmitter including a signal light source and means for binary frequency modulating the signal light source with an information signal, and two modulation frequencies of the signal light from the optical transmitter. A pumping light source for optically directly amplifying only one of the components in a transmission medium, means for separating the amplified signal light from the pumping light and extracting it, and receiving the amplified signal light separated from the pumping light. It is characterized by being configured by an optical receiver including a photodetector and a photodetector.

[Effect]

In the present invention, the optical transmitter transmits binary frequency modulated signal light, and the optical receiver directly amplifies one of the two frequency components of the signal light, converting the information signal into the amplitude or intensity of the signal light. I'm taking it out.

-S In semiconductor lasers, if the bias current is set above the oscillation threshold and the current is minutely modulated, the oscillation frequency can be varied by the temperature fluctuation effect and the carrier density fluctuation effect, and direct frequency modulation can be performed. can. Therefore, by directly frequency modulating the bias current of the semiconductor laser as high as possible without deteriorating it, the transmission power can be significantly increased compared to the conventional method using intensity modulation, despite the small signal current. It can be made higher.

On the other hand, since the optical receiver directly amplifies only one frequency component of the binary frequency modulated signal light, the signal light incident on the photodetector is intensity modulated light. the result,
The conventional optical direct detection method receiving circuit can be used as is, and a simpler system can be constructed compared to the optical heterogain homodyne detection method.

Here, regarding the spectral width and frequency stability of the signal light source, it is sufficient that one of the two types of frequency components of the signal light is included within the optical amplification gain band.

Further, in the present invention, the power level of the received signal light is increased by direct optical amplification. As a result, the influence of noise in the receiving electric circuit can be relatively reduced, and the received light level can be lowered to near the shot noise limit.

In the present invention, there is a possibility that the extinction ratio of the component that is not directly amplified among the two frequency components of the signal light may deteriorate. However, in general, a gain of 20 dB or more can be easily obtained in optical direct amplification, so an extinction ratio of 20 dB or more can be achieved. As a result, deterioration in reception characteristics due to extinction ratio deterioration can be extremely minimized.

〔Example〕

Next, an optical communication method of the present invention and an apparatus for implementing the method will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram of a first embodiment according to the present invention, FIG. 2 is a frequency characteristic diagram for explaining the principle of the first embodiment, and FIGS. FIG. 3 is a diagram showing signal light waveforms at an optical transmitter and an optical receiver in the embodiment.

In FIG. 1, the signal light source 1 of the optical transmitter has a wavelength of 1.55.
um InGaAsP/InP distributed feedback semiconductor laser, optical fiber 5 has a core diameter of 110At and a wavelength of 1.55μ.
Transmission loss at s is 0.2dB/kn+, length is 250k
The laser amplifier 6 of the optical receiving section is an InGaAsP/InP Fabry-Perot semiconductor laser amplifier, and the photodetector 7 is an InGaAs avalanche photodiode (APD). A signal generator 2 that generates a signal for binary frequency modulation using an information signal is connected to the signal light source 1, and an optical isolator 31 and a lens 41 are provided on the output side of the signal light source 1. A lens 42 and an optical isolator 32 are provided between the output terminal of the optical fiber 5 and the laser amplifier 6.

In this embodiment, the injected current of the distributed feedback semiconductor laser 1 is slightly modulated by a 2 Gb/s pulse modulation current from the signal generator 2, so that the information signal code " Binary frequency modulation corresponding to "1" and "0" is performed. Here, the bias current is 210mA
, the amplitude of the pulse modulated current was 10a+A, and the frequency deviation amount at this time was 13 GHz. After passing through an optical isolator 31, this frequency-modulated signal light is focused by a lens 41 and is connected to a single mode optical fiber 5.
is combined with After propagating through the optical fiber, the light passes through the lens 42 and the optical isolator 32 and enters the Fabry-Perot semiconductor laser amplifier 6.

FIG. 2 is a diagram showing the relationship between the spectrum of the binary frequency modulated signal light and the gain of the semiconductor laser amplifier 6 in this embodiment. Here, the component of the information signal with the code "1" has a frequency flB, and the component of the information signal with the code "0" has the frequency f
Each corresponds to the component of l. Furthermore, the gain bandwidth corresponding to one axial mode of the Fabry-Perot semiconductor laser amplifier 6 is approximately 6 GHz when the amplification degree is 35 dB.
Met.

As is clear from this figure, in this example, f of the signal light is
, that is, only the component with code "1" was amplified, and the waveform shown in FIG. 3(b) was obtained as the output light of the semiconductor laser amplifier 6. Then, this amplified signal light is
Information signals are extracted by receiving light at D.

Next, the performance of this embodiment will be explained.

In this example, the power of the signal light emitted from the distributed feedback semiconductor laser was 15 dBm (36 mW), of which 10 dBm could be coupled to the single mode optical fiber 5. As mentioned above, this signal light has a bias current of 210
Since the current is high at mA and the modulation current is small at 10mA,
The frequency distortion was extremely small, and the spectral widths of the frequency components of f and f were approximately 4 GHz. As a result, there was almost no waveform distortion due to fiber dispersion even after 250 km of single mode optical fiber transmission.

On the other hand, in the optical receiver, InG
By increasing the power level of the signal light incident on the aAs-APD, we were able to reduce the noise in the receiving electrical circuit, and as mentioned above, we were able to minimize waveform distortion due to dispersion, resulting in a code error rate of 10. The light receiving level to obtain -9 was set to -42 dBm.

Therefore, in this example, the allowable transmission line loss is 52
dB value, transmission speed 2Gb/s, transmission distance 2
Even in the case of 50 km, a margin of 2 dB could be achieved.

Next, FIG. 4 is a block diagram of a second embodiment according to the present invention, and FIG. 5 is a diagram showing the frequency relationship between pumping light and signal light in the second embodiment.

In this embodiment, the stimulated Brillouin effect, which is one of the nonlinear effects in the optical fiber, is used to amplify the signal light while propagating through the optical fiber.

To amplify signal light using the stimulated Brillouin effect, pump light whose frequency is higher than the frequency of the signal light by the Brillouin shift amount must be introduced into the optical fiber so that it propagates in the opposite direction of the signal light. Here, the amount of Brillouin shift is about 11 GH2 when the wavelength of the excitation light is 1.55 μm.

The effective gain bandwidth Δν of this Brillouin amplification is approximately given by the following equation.

Δshi=ΔshiP+ΔνB (1) However, Δνde is the spectral line width of the excitation light, Δν.

is the natural Brillouin linewidth, which is approximately 100 MHz.

In this second embodiment, the excitation light source 9 of the optical transmitter is I n Ga A S with a wavelength of 1.55 μII+ band.
P/I nP distributed feedback semiconductor laser, the fiber coupler 8 of the optical receiver is a single mode light with a branching ratio of 1:1, which is made by heating and stretching two short single mode optical fibers. A fiber coupler is used. This is then fusion spliced to the optical fiber 5. This connection loss was 0.2 dB or less. A modulation power source 10 for frequency modulating the excitation light is connected to the excitation light source 9, and an optical isolator 33 is provided between this modulation power source and the fiber coupler 8. Furthermore, a lens 43 and a photodetector 7 are provided on the output side of the fiber coupler 8.

The configuration of the transmitter is the same as that of the first embodiment.

Note that the elements other than the excitation light source 9 and the fiber coupler 8 are the same as in the first embodiment.

In this embodiment as well, the distributed feedback semiconductor laser 1 serving as the signal light source is binary frequency modulated by a pulse modulated current from the signal generator 2.

However, in this case, the transmission speed was set to 100 Mb/s and the frequency deviation was set to 5 GHz. This 100 Mb/s binary frequency modulated signal light passes through an optical isolator 31, is focused by a lens 41, and is coupled to a single mode optical fiber 5.

In this embodiment, a distributed feedback semiconductor laser 9, which is an excitation light source, is minutely frequency modulated by a repetitive I MHz sine wave current superimposed on a DC current from a modulated power source 10, so that its spectral width can be changed. There is. Here, in order to realize an effective Brillouin gain band that can amplify the aforementioned 100 Mb/s signal light, the spectral width was set to about 400 MHz.

At this time, the effective Brillouin gain bandwidth was 500 MHz, as predicted from equation (1). Here, in Friuen amplification, the pumping light and the signal light propagate in the opposite directions in the optical fiber 5, so even if the pumping light is modulated at about I MHz, there will be no temporal variation in the amplified signal light. .

In this example, the fiber input of the excitation light was approximately 10 m&I, and its frequency was set as shown in FIG. Note that FIG. 5 shows the relationship between the frequencies of the binary frequency modulated signal light and the pumping light. Here, f in the figure is the frequency of the excitation light. As a result, the frequency f1 component corresponding to the code "1" component of the information signal is transmitted for about 43 d during propagation through the optical fiber.
B could be amplified. Then, by receiving this amplified signal light with an InGaAs-APD 7, which is a photodetector, an information signal could be extracted.

In this second embodiment, the power of the signal light emitted from the distributed feedback semiconductor laser l was 15 dBm, of which 10 dBm could be coupled to the single mode optical fiber 5. On the other hand, in the optical receiver, the received light level is set to 161 d to obtain a bit error rate of 10-9 by this Brillouin amplification.
I was able to make Bm. Therefore, a large value of 71 dB was achieved as the allowable transmission line loss in the second embodiment.

Although the optical communication method according to the present invention and the apparatus for implementing the same have been described above using embodiments, the present invention is not limited to these embodiments, and several modifications can be made.

For example, signal light source 1. As the excitation light source 9, InGaA
Although an sP/InP semiconductor laser is used, semiconductor lasers made of other materials, semiconductor lasers with external mirrors, solid lasers, gas lasers, and other types of lasers may be used. Furthermore, it is clear that the modulation speed and frequency shift amount of binary frequency modulation are not limited to the above embodiments and may be arbitrary.

Furthermore, as the photodetector 7, APD made of other materials,
Photodiodes, photomultiplier tubes, etc. can be used, and as the transmission medium, other types of glass optical fibers and other types of spatial propagation can also be used. Further, the nonlinear effect of the transmission medium may be any effect as long as it can be used for optical amplification, including stimulated Raman scattering and guided photon mixing.

〔Effect of the invention〕

As described above, in the optical communication method according to the present invention and the apparatus for carrying out the same, a binary frequency modulated signal light is transmitted, and in the optical receiving section, one of the two types of modulated frequency components is transmitted. Only the components are optically amplified directly, and the information signal is extracted as the amplitude or intensity of the signal light. As a result, there are advantages in that an optical communication method and a device for carrying out the method are obtained, which have a simple configuration and can increase allowable transmission path loss and extend non-relay transmission distance compared to the conventional method.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a first embodiment of the present invention, FIG. 2 is a frequency characteristic diagram for explaining the principle of the first embodiment of the present invention, and FIG. FIG. 4 is a block diagram of the second embodiment of the present invention, and FIG. FIG. 3 is a diagram showing the frequency relationship between signal light and excitation light in an example. 1... Signal light source 2... Signal generator 31, 32.33... Optical isolator 41, 42.43... Lens 5... Optical fiber 6... ... Laser amplifier 7 ... Photodetector 8 ... Fiber coupler 9 ... Pumping light source 10 ... Modulation power supply

Claims (3)

[Claims] (1) The optical transmitter transmits signal light that has been binary frequency modulated by the information signal, and the optical receiver directly amplifies and receives only one of the two types of modulated frequency components. An optical communication method characterized by: (2) an optical transmitter including a signal light source and a means for binary frequency modulating the signal light source with an information signal; An optical communication device comprising an optical receiver including a laser amplifier that directly amplifies only one component and a photodetector that receives the output light of the laser amplifier. (3) an optical transmitter including a signal light source and a means for binary frequency modulating the signal light source with an information signal, and one of two types of modulation frequency components of the signal light from the optical transmitter; a pumping light source for direct optical amplification of the signal in a transmission medium, means for separating and extracting the amplified signal light from the pumping light, and a photodetector for receiving the pumping light and the separated amplified signal light. An optical communication device comprising an optical receiver.