JP7740968B2 - Optical Transmission System - Google Patents

Description

The present invention relates to an optical transmission system.

According to the technology disclosed in Patent Document 1, when optical transmission is performed using wavelength division multiplexing, which transmits data using light of multiple wavelengths, optical pulses are sent out for each wavelength channel on the transmitting side via an optical filter, and optical signals are received and detected on the receiving side via an optical filter.

Japanese Patent Application Laid-Open No. 2005-45415

However, with the technology disclosed in Patent Document 1, in addition to the optical filter used on the transmitting side, an optical filter is also required on the receiving side to extract light of a specific wavelength. Furthermore, a photodetector is required for each wavelength channel. Therefore, with the technology disclosed in Patent Document 1, there is a risk that the costs of providing optical filters and photodetectors will increase.

The present invention was made in consideration of the problems inherent in conventional technology. The object of the present invention is to provide an optical transmission system that can reduce the cost of providing an optical filter and a photodetector for each signal when multiple signals are superimposed and optically transmitted.

An optical transmission system according to an aspect of the present invention comprises a first light emitter, a second light emitter, a transmitter, a light receiver, and an extractor. The first light emitter outputs a first optical signal on which a first signal in a first frequency band is superimposed. The second light emitter outputs a second optical signal on which a second signal in a second frequency band different from the first frequency band is superimposed. The transmitter inputs the carrier light obtained by multiplexing the first optical signal and the second optical signal into an optical fiber. The light receiver receives the carrier light after passing through the optical fiber and converts it into an output signal. The extractor extracts the first signal and the second signal based on the output signal.

The present invention provides an optical transmission system that can reduce the cost of providing an optical filter and light receiving unit for each signal when multiple signals are superimposed and optically transmitted.

1 is a block diagram showing a configuration of an optical transmission system according to an embodiment of the present invention; 3 is a diagram for explaining an example of the relationship between frequency and amplitude in an optical signal of the optical transmission system according to the present embodiment. FIG. 3A and 3B are diagrams for explaining a first example of the relationship between frequency and amplitude in a carrier light of the optical transmission system according to the present embodiment. 10A and 10B are diagrams for explaining a second example of the relationship between frequency and amplitude in the carrier light of the optical transmission system according to the present embodiment. 10A and 10B are diagrams for explaining a third example of the relationship between frequency and amplitude in the carrier light of the optical transmission system according to the present embodiment.

The optical transmission system according to this embodiment will be described in detail below using the drawings. Note that the dimensional proportions in the drawings have been exaggerated for the sake of explanation and may differ from the actual proportions. In addition, in the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[Configuration example of optical transmission system]
1 is a block diagram showing the configuration of an optical transmission system according to this embodiment. The optical transmission system includes a light emitting unit 140 (first light emitting unit, second light emitting unit), an optical coupler 150 (transmitting unit), an optical fiber 300, a light receiving unit 210, and an extracting unit 220.

Here, the light-emitting unit 140 includes a light-emitting element that emits an optical signal. Power is supplied to the light-emitting element from the DC power supply 110, which is a direct current power source, via the bias-tee 130. In other words, the bias-tee 130 is used to supply power from the DC power supply 110 to active elements such as the light-emitting element in a high-frequency circuit.

For example, the light-emitting element provided in the light-emitting unit 140 is configured as a VCSEL (Vertical Cavity Surface Emitting Laser). Note that the configuration of the light-emitting element is not limited to this embodiment. For example, the light-emitting element may be configured as a distributed feedback (DFB) laser or an LED (Light Emitting Diode).

A signal in a predetermined frequency band is input to the light-emitting unit 140 from the input unit 120, and the light-emitting unit 140 outputs an optical signal on which the signal in the predetermined frequency band is superimposed. For example, as shown in FIG. 1, an electrical signal on which the signal in the predetermined frequency band is superimposed may be supplied to the light-emitting element provided in the light-emitting unit 140 via a bias tee 130. The light-emitting element itself may then output an optical signal on which the signal in the predetermined frequency band is superimposed.

Alternatively, an optical signal output from a light-emitting element may be modulated to generate an optical signal on which a signal of a predetermined frequency band is superimposed, and the light-emitting unit 140 may output the optical signal on which the signal of the predetermined frequency band is superimposed. Modulation may be performed using an external modulator such as an electroabsorption modulator (EAM) or a Mach-Zehnder.

In FIG. 1, input units 120a to 120d are shown as input unit 120, bias tees 130a to 130d are shown as bias tees 130, and light-emitting units 140a to 140d are shown as light-emitting unit 140. While FIG. 1 shows four signal lines extending from input unit 120 to light-emitting unit 140, this embodiment is not limited to this. For example, there may be two or more signal lines extending from input unit 120 to light-emitting unit 140.

In the following, a signal input from one of the multiple input units 120 will be referred to as a first signal, and a signal input from another input unit will be referred to as a second signal. Furthermore, the frequency band of the first signal will be referred to as the first frequency band, and the frequency band of the second signal will be referred to as the second frequency band. Furthermore, an optical signal on which the first signal is superimposed will be referred to as a first optical signal, and an optical signal on which the second signal is superimposed will be referred to as a second optical signal. Furthermore, the light-emitting unit 140 that outputs the first optical signal will be referred to as a first light-emitting unit, and the light-emitting unit 140 that outputs the second optical signal will be referred to as a second light-emitting unit. Note that the multiple signals input from the multiple input units 120 are signals in different frequency bands.

The frequency bands of the signals passing through each signal line are different. For example, assume that input unit 120a outputs a signal with a center frequency of 2.5 GHz and a bandwidth Δ of 5 GHz. Therefore, as shown in Figure 2, emitter 140a outputs an optical signal in which signals with a center frequency of 2.5 GHz and a bandwidth Δ of 5 GHz are superimposed.

Let us assume that light-emitting unit 140b outputs an optical signal on which a signal with a center frequency of 7.5 GHz and a bandwidth Δ of 5 GHz is superimposed. Let us also assume that light-emitting unit 140c outputs an optical signal on which a signal with a center frequency of 12.5 GHz and a bandwidth Δ of 5 GHz is superimposed. Let us also assume that light-emitting unit 140d outputs an optical signal on which a signal with a center frequency of 17.5 GHz and a bandwidth Δ of 5 GHz is superimposed.

In addition, the signals passing through each signal line may use different modulation methods. For example, signal modulation methods include phase shift keying (PSK), frequency shift keying (FSK), amplitude shift keying (ASK), and quadrature amplitude modulation (QAM).

The modulation levels of the signals passing through each signal line may also be different. For example, if PSK is used to modulate the signals, any of BPSK (binary phase shift keying), QPSK (quadrature phase shift keying), 8PSK (8 phase shift keying), OQPSK (offset QPSK), π/4 shift QPSK, PLL-QPSK, and π/2 shift BPSK may be used. PSK with even greater modulation levels may also be used.

Furthermore, when QAM is used to modulate a signal, the modulation level is not limited; for example, 16QAM, 64QAM, or 256QAM may be used. QAM with an even higher modulation level may also be used.

In addition, the modulation level for modulating the signal passing through each signal line may be set based on the signal-to-noise ratio of the optical signal when passing through the optical fiber 300. For example, if the signal-to-noise ratio of the first optical signal when passing through the optical fiber 300 is smaller than the signal-to-noise ratio of the second optical signal when passing through the optical fiber 300, the modulation level for the first signal may be set to be smaller than the modulation level for the second signal.

For example, consider an optical signal in which signals in four frequency bands with center frequencies of 2.5 GHz, 7.5 GHz, 12.5 GHz, and 17.5 GHz are superimposed. In this case, the signal in the 2.5 GHz frequency band, which has the lowest center frequency, will be affected by all the other frequency bands and will suffer from large second-order intermodulation distortion. Furthermore, the signal in the 17.5 GHz frequency band, which has the highest center frequency, will be affected by the system's frequency response characteristics.

In other words, the signal-to-noise ratio of signals in the two frequency bands with center frequencies of 2.5 GHz and 17.5 GHz is smaller than the signal-to-noise ratio of signals in the two frequency bands with center frequencies of 7.5 GHz and 12.5 GHz. Therefore, 16QAM signals with a smaller modulation level may be used for signals in the two frequency bands with center frequencies of 2.5 GHz and 17.5 GHz. Furthermore, 64QAM signals with a larger modulation level may be used for signals in the two frequency bands with center frequencies of 7.5 GHz and 12.5 GHz.

In addition, the wavelengths of the light emitted from the light-emitting elements provided in each light-emitting unit 140 may be different. Considering the effects of environmental changes, it is better for the difference in wavelength between the multiple light-emitting elements to be large, in order to reduce noise caused by optical beat signals generated by optical interference when multiple wavelengths of light are combined.

However, the range in which the light receiving unit 210 (described below) has light receiving sensitivity is fixed, and if the optical fiber 300 is long, it will be affected by chromatic dispersion of the optical fiber 300. It is necessary to select the wavelength of the light from the light emitting element provided in each light emitting unit 140 within a range in which these effects are small. For the sake of explanation, let f be the maximum frequency of the first frequency band or the second frequency band, λ1 be the wavelength of the light of the first optical signal, λ2 be the wavelength of the light of the second optical signal, and c be the speed of light. In this case, the relationship "(f λ1 λ2)/c > |λ1 - λ2|" may be satisfied.

For example, when transmitting a signal with a maximum frequency of 20 GHz using a VCSEL with a wavelength in the 850 nm band, the above relationship indicates that it is sufficient to use multiple VCSELs with wavelength differences of 0.1 nm or more.

In addition, the intensity from the light-emitting element provided in each light-emitting unit 140 may be set based on the gain of light passing through the optical fiber 300. For example, if the gain of the first optical signal passing through the optical fiber 300 is smaller than the gain of the second optical signal passing through the optical fiber 300, the intensity of the first optical signal may be set to be greater than the intensity of the second optical signal.

The optical coupler 150 combines multiple optical signals output from the light emitters 140 to generate carrier light, and inputs the generated carrier light into the optical fiber 300. For example, the optical coupler 150 combines multiple optical signals output from the light emitters 140a to 140d, and inputs the resulting carrier light into the optical fiber 300.

Instead of the optical coupler 150, an arrayed-waveguide grating (AWG) filter, which can combine multiple optical signals to generate carrier light, may also be used.

Figure 3 is a diagram illustrating a first example of the relationship between frequency and amplitude in the carrier light of the optical transmission system according to this embodiment. Figure 3 shows a state in which signals with a bandwidth Δ of 5 GHz and center frequencies of four frequency bands, namely 2.5 GHz, 7.5 GHz, 12.5 GHz, and 17.5 GHz, are superimposed on the carrier light.

Figure 4 is a diagram illustrating a second example of the relationship between frequency and amplitude in the carrier light of the optical transmission system according to this embodiment. While Figure 3 shows a state in which the intensities of the signals superimposed on the carrier light are approximately the same, as shown in Figure 4, the intensities of the signals superimposed on the carrier light may be different. In particular, different intensities may be set for each signal superimposed on the carrier light based on the gain of light passing through the optical fiber 300.

In Figure 4, the signal strength in the frequency band with a center frequency of 17.5 GHz is set to be greater than the signal strength in the three frequency bands with center frequencies of 2.5 GHz, 7.5 GHz, and 12.5 GHz. By setting the strength in this way, even if the signal gain in the frequency band with a center frequency of 17.5 GHz is small, noise and other issues caused by the small gain can be reduced.

Figure 5 is a diagram illustrating a third example of the relationship between frequency and amplitude in the carrier light of the optical transmission system according to this embodiment. While Figures 3 and 4 show that the bands of multiple signals superimposed on the carrier light are continuous, as shown in Figure 5, the bands of multiple signals superimposed on the carrier light do not have to be continuous.

The optical fiber 300 transmits the input carrier light and guides it to the light receiving unit 210, which will be described later. For example, if the light output from the light emitting unit 140 has a single wavelength, a single-mode optical fiber may be used as the optical fiber 300. Also, if the light output from the light emitting unit 140 does not have a single wavelength, a multi-mode optical fiber may be used as the optical fiber 300.

The length of the optical fiber 300 may be 100 m or less. The shorter the length of the optical fiber 300, the less the effect of chromatic dispersion on the carrier light passing through the optical fiber 300, and the more noise can be suppressed.

The light receiving unit 210 receives the carrier light after it has passed through the optical fiber and converts it into an output signal. For example, the light receiving unit 210 is a phototransistor, photodiode, avalanche photodiode, etc.

The extraction unit 220 extracts signals in a predetermined frequency band based on the output signal from the light receiving unit 210. For example, as shown in Figure 3, if signals in four frequency bands with center frequencies of 2.5 GHz, 7.5 GHz, 12.5 GHz, and 17.5 GHz are superimposed on the carrier light, the extraction unit 220 extracts signals for each frequency band.

The extraction unit 220 may perform a discrete Fourier transform of the output signal to extract signals in each frequency band, or it may extract signals using a filter that transmits electrical signals in each specified frequency band.

[Effects of the embodiment]
As described above in detail, the optical transmission system according to this embodiment includes a first light emitter, a second light emitter, a transmitter, a light receiver, and an extractor. The first light emitter outputs a first optical signal on which a first signal in a first frequency band is superimposed. The second light emitter outputs a second optical signal on which a second signal in a second frequency band different from the first frequency band is superimposed. The transmitter inputs a carrier light obtained by multiplexing the first optical signal and the second optical signal into an optical fiber. The light receiver receives the carrier light after passing through the optical fiber and converts it into an output signal. The extractor extracts the first signal and the second signal based on the output signal.

This reduces the cost of providing an optical filter and light-receiving unit for each signal when multiple signals are superimposed and optically transmitted. In particular, wideband signals can be transmitted easily and at low cost using a single light-receiving unit, which is fewer than the number of signals.

Furthermore, in the optical transmission system according to this embodiment, the optical frequency of the first optical signal and the optical frequency of the second optical signal may be different. This reduces the influence of noise due to beat signals generated by optical interference between the respective light-emitting elements. As a result, high-quality signal transmission is possible.

Furthermore, in the optical transmission system according to this embodiment, the maximum frequency of the first or second frequency band is f, the wavelength of the light of the first optical signal is λ1, the wavelength of the light of the second optical signal is λ2, and the speed of light is c. The frequency fb of a beat signal generated by two different wavelengths of light is the difference between the frequencies of the two lights, fb = |f1 - f2|. Since overlapping of this beat signal frequency with the signal can affect signal quality, this effect can be eliminated by making fb higher than the maximum frequency f of the signal used. Since the frequencies of the respective lights are expressed as f1 = cλ1 and f2 = cλ2, the relationship (f λ1 λ2)/c < |λ1 - λ2| may be satisfied. This allows the beat signal generated by the interference of light between the light-emitting elements to have a higher frequency than the frequency band of the signal being used. This reduces the effect of noise due to the beat signal. As a result, high-quality signal transmission is possible. It is desirable that λ1 and λ2 be within the range of the light-receiving sensitivity of the light-receiving unit and within a range that is not affected by chromatic dispersion due to the optical fiber.

Furthermore, in the optical transmission system according to this embodiment, the modulation level of the first signal may be different from the modulation level of the second signal. This allows the modulation level to be set taking into account the frequency response characteristics of the system in optical transmission using signals in multiple frequency bands, making it possible to transmit large volumes of information.

Furthermore, in the optical transmission system according to this embodiment, consider a case where the signal-to-noise ratio of the first optical signal passing through the optical fiber is smaller than the signal-to-noise ratio of the second optical signal passing through the optical fiber. In this case, the modulation multi-level of the first signal may be set to be smaller than the modulation multi-level of the second signal. For example, on the low-frequency side, the influence of second-order intermodulation distortion is large, resulting in a higher noise level than in other frequency bands and a lower signal-to-noise ratio. Furthermore, on the high-frequency side, the signal-to-noise ratio may be lowered due to the influence of the system's frequency response characteristics. Because the modulation multi-level is set based on the signal-to-noise ratio, high-quality signal transmission is possible. In particular, it is possible to set up frequency bands with high signal-to-noise ratios to perform more data communication.

Furthermore, in the optical transmission system according to this embodiment, if the gain of the first optical signal when passing through the optical fiber is smaller than the gain of the second optical signal when passing through the optical fiber, the intensity of the first optical signal may be set to be greater than the intensity of the second optical signal. This makes it possible to reduce noise and other issues caused by the small gain after passing through the optical fiber.

Furthermore, in the optical transmission system according to this embodiment, the optical fiber may be a multimode optical fiber. This allows for more accurate optical transmission and higher-quality signal transmission even when the wavelength of the light output from the light-emitting unit is not single.

Furthermore, in the optical transmission system according to this embodiment, the length of the optical fiber may be 100 m or less. This allows noise to be suppressed. The shorter the optical fiber, the less the effect of chromatic dispersion on the carrier light passing through it, thereby suppressing noise.

The present embodiment has been described above, but it is not limited to this, and various modifications are possible within the scope of the gist of the present embodiment.

140 Light emitting unit (first light emitting unit, second light emitting unit)
150 Optical coupler (transmitter)
210 Light receiving unit 220 Extraction unit 300 Optical fiber

Claims (7)

a first light emitting unit that outputs a first optical signal on which a first signal in a first frequency band is superimposed;
a second light emitting unit that outputs a second optical signal on which a second signal in a second frequency band different from the first frequency band is superimposed;
a transmitter that inputs a carrier light obtained by multiplexing the first optical signal and the second optical signal into an optical fiber;
a light receiving unit that receives the carrier light after passing through the optical fiber and converts it into an output signal;
an extractor that extracts the first signal and the second signal based on the output signal;
Equipped with
The modulation multi-level number of the first signal is different from the modulation multi-level number of the second signal.
Optical transmission system. a first light emitting unit that outputs a first optical signal on which a first signal in a first frequency band is superimposed;
a second light emitting unit that outputs a second optical signal on which a second signal in a second frequency band different from the first frequency band is superimposed;
a transmitter that inputs a carrier light obtained by multiplexing the first optical signal and the second optical signal into an optical fiber;
a light receiving unit that receives the carrier light after passing through the optical fiber and converts it into an output signal;
an extractor that extracts the first signal and the second signal based on the output signal;
Equipped with
When the signal-to-noise ratio of the first optical signal when passing through the optical fiber is smaller than the signal-to-noise ratio of the second optical signal when passing through the optical fiber, the modulation multilevel number of the first signal is set to be smaller than the modulation multilevel number of the second signal.
Optical transmission system. 3. The optical transmission system according to claim 1, wherein the optical frequency of the first optical signal is different from the optical frequency of the second optical signal. The maximum frequency of the first frequency band or the second frequency band is f,
The wavelength of the light of the first optical signal is λ1,
The wavelength of the light of the second optical signal is λ2,
Let the speed of light be c,
4. The optical transmission system according to claim 1 , wherein the relationship (f·λ1·λ2)/c<|λ1−λ2| is satisfied. 5. The optical transmission system according to claim 1, wherein when a gain of the first optical signal passing through the optical fiber is smaller than a gain of the second optical signal passing through the optical fiber, the intensity of the first optical signal is set to be greater than the intensity of the second optical signal. 6. The optical transmission system according to claim 1, wherein the optical fiber is a multimode optical fiber. 7. The optical transmission system according to claim 1, wherein the length of the optical fiber is 100 m or less.