JPS6235084B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention provides a wavelength multiplexed optical fiber transmission system,
In particular, the present invention relates to a wavelength multiplexing optical fiber transmission system suitable for a case where the light source is a semiconductor laser and the number of multiplexed channels is three or more.

Wavelength multiplexing optical fiber transmission technology, which uses the different wavelengths of light to multiplex multiple lights and transmit them through a single optical fiber, is capable of economical use of transmission line optical fibers and is easy to use in existing electrical circuits. It is possible to transmit large amounts of data that can break through walls, and has high system flexibility; for example, channels can be added by simply changing the optical transmitter/receiver and optical multiplexing/demultiplexing circuit, regardless of the optical fiber used for the transmission route. It also has various advantages such as being able to multiplex transmission of different types of signals such as analog signals and digital signals, and is suitable for various transmission systems ranging from short-distance low-speed transmission systems to long-distance high-speed transmission systems. It has been actively researched and developed recently.

For optical multiplexing circuits (also called optical multiplexing circuits) and optical demultiplexing circuits, there are interference film types that take advantage of the wavelength dependence of the light reflection and transmission characteristics of dielectric multilayer films (interference films), and diffraction gratings. Generally, a diffraction grating type circuit is often used, which takes advantage of the fact that the angle of diffracted light has wavelength dependence.

However, the interference film type has the disadvantage that the wavelength interval cannot be narrowed because the interference film is not sharp.
When the number of channels is large, the interference film type circuit must be multi-staged, resulting in a large insertion loss and a complicated structure, so the practical upper limit for the number of channels is about 4. .

On the other hand, with the diffraction grating type, it is easier to increase the number of channels than with the interference film type, but the core diameter of the optical fiber on the side where the light is emitted is set to be smaller than the core diameter of the optical fiber on the side where the light enters. If it is not made sufficiently large, the diffracted light will not enter the output side optical fiber when the wavelength of the light source changes, resulting in disadvantages such as a slight change in wavelength will greatly increase insertion loss. However, in diffraction grating type optical multiplexing circuits, the core diameter of the transmission line optical fiber, which is the optical fiber on the output side, is often uniquely determined by system design from the viewpoint of frequency band, etc., so the above drawbacks cannot be overcome. If the core diameter of the optical fiber on the input side is made sufficiently small in order to avoid this problem with this optical multiplexing circuit, a drawback arises in that the coupling efficiency between the optical fiber and the light source deteriorates outside the circuit. In particular, when the transmission line optical fiber is a single mode optical fiber, the core diameter is generally 10 μm or less, making it difficult to realize a diffraction grating type optical multiplexing circuit. In addition, in a diffraction grating type optical demultiplexing circuit, the light output from the optical fiber on the output side is generally guided to a photodetector having a receiving diameter larger than the core diameter of the transmission line optical fiber. It is easy to increase the core diameter, and those with more than 10 channels have been developed.

From the above points of view, in wavelength multiplexing optical fiber transmission systems with a channel number of 4 or less, an interference film type is generally used for the optical multiplexing circuit, and an interference film type or a diffraction grating type is used for the optical demultiplexing circuit. There is. In systems with five or more channels, the diffraction type cannot be used at all as an optical multiplexing circuit, and the interference film type also has a large circuit insertion loss, which reduces the system's allowable transmission line loss and shortens the transmission line distance. This is unavoidable, because in this system, the wavelength interval between channels cannot be narrowed, so a wide wavelength range is required, and for this reason, the wavelength range where the transmission line optical fiber has a large loss must also be used. To,
The disadvantage was that the transmission path distance became even shorter.

There is another reason why a diffraction grating type cannot be used in an optical multiplexing circuit. That is, in this case, there was a drawback that the insertion loss of the optical multiplexing circuit increased due to temperature changes in the wavelength of the light source. For this reason, considering a certain operating temperature range, there are disadvantages in that the permissible transmission line loss of the system cannot be lowered and the transmission line distance cannot be increased. This drawback is particularly severe when the transmission line optical fiber is a single mode optical fiber, and a system using a diffraction grating type optical multiplexing circuit is impossible.

Therefore, an object of the present invention is to reduce the number of channels to 3.
In the above case, it is an object of the present invention to provide a wavelength division multiplexing optical fiber transmission system that allows the transmission line distance to be longer than the conventional one.

According to the present invention, at least three semiconductor lasers having different oscillation wavelengths, an optical multiplexing means for wavelength-multiplexing the output lights of these semiconductor lasers, a transmission line optical fiber for propagating the wavelength-multiplexed light, and the propagated light In an optical fiber transmission system including a demultiplexing circuit that demultiplexes the light of each wavelength, and a photodetector that detects the demultiplexed light of each wavelength, the optical multiplexing means
an optical multiplexing means that receives the output light of the semiconductor laser as an input and includes at least one wavelength-dependent optical multiplexing circuit; and an output of the at least one wavelength-dependent optical multiplexing circuit and other wavelength-dependent optical multiplexing circuits; The output of an optical multiplexing circuit or a single wavelength-independent optical multiplexing circuit that combines the output lights of the inputted semiconductor lasers and outputs the wavelength-multiplexed light; A wavelength multiplexing optical fiber transmission system is obtained in which the wavelengths of at least two laser beams assigned to the wavelength-dependent optical multiplexing circuit of the means are different in order of magnitude by at least one wavelength.

Next, a detailed description will be given with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of the most preferred embodiment of the wavelength division multiplexing optical fiber transmission system of the present invention. The first of the first to sixth optical transmitters 1 to 6
The electrical signals input to the ~sixth signal input terminals 11~16 are transmitted to the first~sixth transmitter circuits 21~
26, the signals are amplified, etc., and applied to the first to sixth semiconductor lasers 31 to 36, respectively, and converted into optical signals. Here, the wavelengths λ ₁ , λ ₂ , ...... λ of the output lights of the first to sixth semiconductor lasers 31 to 36 are
₆ is 850nm, 860nm, 870nm, 880nm, 890nm
are set respectively. These first to sixth semiconductor lasers 31 to 36 have a double heterostructure in which the active layer is GaAs and Al _x Ga _1-x As layers are provided on both sides of the active layer. By controlling the minute amount of Al mixed in, light of the desired wavelength in the 800 nm band as described above is output.

First to sixth semiconductor lasers 31 as described above
The output light of 36 is connected to the first to sixth coupling circuits 41 to 46.
The light is condensed and coupled to the first to sixth optical fibers 51 to 56, respectively. These first to sixth optical fibers 51 to 56 are polarization maintaining optical fibers having an elliptical core with a long axis length of 10 μm and a short axis length of 7 μm. It has a characteristic of outputting linearly polarized light with the same polarization plane when it is input.

Among the first to sixth optical fibers, the first,
The third and fifth optical fibers 51, 53, and 55 are connected to a first optical multiplex circuit 201 having interference type wavelength dependence.
The output lights of the first, third, and fifth semiconductor lasers 31, 33, and 35 are wavelength-multiplexed and the linearly polarized light having a plane of polarization parallel to the plane of the paper in FIG. 57, similarly, the second,
The fourth and sixth optical fibers 52, 54, and 56 are connected to a second optical multiplexing circuit 202 which also has interference film type wavelength dependence, and the second, fourth, and sixth semiconductor lasers 32, The linearly polarized lights of the output lights 34 and 36 whose plane of polarization is perpendicular to the paper plane of FIG. here number 7
Both of the optical fibers and the eighth optical fiber have the same characteristics as the first to sixth optical fibers.

The seventh and eighth optical fibers 57 and 58 are connected to a third optical multiplexing circuit 203 that has no wavelength dependence and has a polarization plane optical multiplexing circuit configuration, and are connected to two linearly polarized lights whose polarization planes are orthogonal to each other. are multiplexed and guided to a transmission path optical fiber 59. Transmission line optical fiber 5
9 has a core diameter of 50 μm, a fiber outer diameter of 125 μm,
It is made of focused multimode optical fiber with a numerical aperture (NA) of 0.2.

The light propagated through the transmission line optical fiber 59 is input to the optical branch circuit 204 . The optical branch circuit 204 is a diffraction grating type branch circuit, and a transmission line optical fiber 59 is connected to its input side, and a step type multimode optical fiber with a core diameter of 100 μm and an NA of 0.25 is connected to its output side. Certain 9th to 14th optical fibers 6
1 to 66 are connected, and the transmission line optical fiber 5
9 output light with wavelengths λ ₁ to λ ₆ is split into the first
~6th optical receivers 101 to 106, respectively.

In the first to sixth optical receivers 101 to 106, the output lights from the ninth to fourteenth optical fibers 61 to 66 are connected to the first to sixth optical fibers by the seventh to twelfth coupling circuits 71 to 76.
are input to the photodetectors 81 to 86, respectively, where they are converted into electrical signals, and then amplified by the first to sixth receiving circuits 91 to 96, and output to the first to sixth electrical signal output terminals 111 to 111. 116.
The first to sixth photodetectors 81 to 86 each have a receiving diameter.
A Si avalanche photodiode with a diameter of 300 μm is used.

As can be seen from the above explanation, the optical multiplexing circuit in this wavelength division multiplexing optical fiber transmission system can be summarized as: a wavelength-dependent optical multiplexing circuit;
It consists of a combination of optical multiplexing circuits with no wavelength dependence. In this case, the first optical multiplexing circuit wavelength-multiplexes the light of every other wavelength, and the second optical multiplexing circuit wavelength-multiplexes the light of the remaining wavelengths. After wavelength multiplexing, the output lights of the first and second optical multiplexing circuits are multiplexed by an optical multiplexing circuit that is not wavelength dependent. Therefore, since the wavelength interval of the transmitted light can be made 1/2 of the wavelength interval of the light wavelength multiplexed by each of the first and second optical multiplexing circuits, the wavelength used by each of the first and second optical multiplexing circuits is The first and second wavelength ranges are approximately the same as the range.
The number of channels that can be multiplexed by each optical multiplexing circuit is 2.
It is possible to multiplex transmit twice the number of channels.
On the other hand, to put this in another way, if the number of transmission channels is constant, the wavelength range used can be reduced to about 1/2, so it is possible to use a wavelength range with less loss in the optical fiber for transmission. The road distance can be increased.

Next, I will explain the performance of the system in detail.
an optical multiplex circuit 201, a second optical multiplex circuit 202,
The insertion loss of the third optical multiplexing circuit 203 is approximately 1 dB, and the insertion loss of the optical demultiplexing circuit 204 is approximately 1.5 dB.
Therefore, the total insertion loss of the optical multiplexing/demultiplexing circuit system was about 3.5 dB. Furthermore, the wavelength range required for each of the first and second optical multiplexing circuits 201 and 202 is 40nm (20nm×
Considering the margin of 20nm in 2), the total is 60nm for each.
become. Note that the wavelength range in which the transmission line optical fiber 59 is used is 10 nm in 50 nm (10 nm x 5) because the wavelength interval of each channel to be transmitted is 10 nm.
Considering the margin, it will be 60nm. The loss of the transmission line optical fiber 59 was greatest for a channel having a wavelength of λ ₁ =840 nm, which was approximately 2.8 dB/Km.

On the other hand, if the optical multiplexing circuit system consisting of the first to third optical multiplexing circuits is constructed only from conventional interference film type optical multiplexing circuits, then the wavelength interval of the channels multiplexed by the interference film type optical multiplexing circuits is 20nm, so
The wavelength range required for this interference film type optical multiplex circuit is 120 nm. In addition, the transmission path optical fiber 59
The wavelength range used is also 120 nm, which is the same wavelength range required for interference film type optical multiplex circuits.

Generally, it is difficult to reduce the loss of an interference film over a wide wavelength range due to the effects of ripples, etc. Therefore, if the wavelength range required for an interference film type optical multiplex circuit is wide, the insertion loss increases. For example, 800n when the number of channels is 6, the wavelength interval is 20nm, and the wavelength range is 120nm.
The insertion loss of an m-band interference film type optical multiplex circuit is approximately 4 dB. Furthermore, if the wavelength range in which the transmission line optical fiber 59 is used is wide, a wavelength range with a large loss must also be used, and the loss of the transmission line optical fiber 59 becomes large for a certain channel. For example, the wavelengths λ ₁ , λ ₂ , ...... λ ₆ of the output lights of the first to sixth semiconductor lasers are set to 790 nm, 810 nm, etc.
m, 830nm, 850nm, 870nm, and 890nm, respectively, the loss of the transmission line optical fiber is λ ₁ =
The largest for the 790nm channel is approximately
It becomes 3.5dB/Km.

As described above, in the wavelength division multiplexing optical fiber transmission system of this embodiment, the insertion loss of the optical multiplexing/demultiplexing circuit system can be reduced by about 2 dB compared to the conventional one. In addition, the wavelength range in which transmission line optical fibers are used has been narrowed by 50 nm, so in channels where transmission line optical fibers have the highest loss, the transmission line optical fiber loss has decreased from approximately 3.5 dB/Km to approximately 2.8 dB/Km. decreased. For these reasons, when the bit rate is 100 Mb/s, for example, the transmission line distance, which was conventionally about 10 km, can be lengthened by about 3 km to about 13 km by adopting the configuration of this embodiment.

In addition, in the above embodiment, the transmission path optical fiber 59
We used a focused multimode optical fiber with a core diameter of 50 μm and an NA of 0.2, but the core diameter was approximately 10 μm.
Optical fibers of other parameters may also be used, such as single mode optical fibers. Further, the third optical multiplexing circuit 203
Although a polarization plane optical multiplexing circuit was used as the method, any circuit without wavelength dependence may be used. For example, March 1973
Even if it is something like the double lens type optical multiplex circuit proposed by Shikada et al. in Volume 4 of the Proceedings of the National Conference of the Institute of Electronics and Communication Engineers published on May 5th, 1978, page 4-112, number 855. good. If the third optical multiplexing circuit 203 is not a polarization optical multiplexing circuit as described above, it is not necessary to use polarization maintaining optical fibers for the first to eighth optical fibers 51 to 58.

Moreover, the first to sixth semiconductor lasers 31 to 36 are
Although it is an AlGaAs semiconductor laser that outputs light in the 800 nm band, the material of the semiconductor laser is InGaAsP.
It may be made of other materials such as, and the wavelength band may be another wavelength band such as the 1 μm band.

Furthermore, although the number of transmission channels is six, it is not particularly limited to six, and may be any number of channels as long as it is three or more. In connection with this, the wavelength interval of the wavelengths of the output lights of the first to sixth semiconductor lasers 31 to 36 is
Although it is assumed that the wavelength interval is 10 nm, this wavelength interval may be appropriately determined in consideration of the number of channels, the wavelength change range of the semiconductor laser in the operating temperature range, etc., and is not particularly limited to the above value.

Furthermore, the optical multiplexing circuit has wavelength-dependent
It is assumed that the configuration is composed of a combination of first and second optical multiplexing circuits 201 and 202 and a third optical multiplexing circuit 203 that has no wavelength dependence. Various other combinations with optical multiplex circuits are also possible. For example, when the number of channels is 3, after wavelength-multiplexing the light of channel 1 with a wavelength of λ ₁ and the light of channel 3 with a wavelength of λ ₃ using a wavelength-dependent interference film type optical multiplexing circuit, Even if this wavelength-multiplexed light and the light of channel 2 whose wavelength is λ ₂ (however, λ ₁ < λ ₂ < λ ₃ ) are multiplexed by a polarization plane optical multiplexing circuit, which is an optical multiplexing circuit without wavelength dependence, good.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of the most preferred embodiment of the wavelength division multiplexing optical fiber transmission system according to the present invention. Explanation of symbols: 1 to 6 are optical transmitters, 11 to 16 are electrical signal input terminals, 21 to 26 are transmission circuits, 31
~36 is a semiconductor laser, 41~46 is a coupling circuit,
51 to 58 are optical fibers, 59 are transmission line optical fibers, 61 to 66 are optical fibers, 71 to 76 are coupling circuits, 81 to 86 are photodetectors, 91 to 96 are receiving circuits, and 101 to 106 are optical receivers. , 111-11
Reference numeral 6 represents an electrical signal output terminal, 201 to 203 represent an optical multiplexing circuit, and 204 represents an optical demultiplexing circuit.

Claims (1)

[Claims] 1 At least three semiconductor lasers with different oscillation wavelengths, an optical multiplexing means for wavelength-multiplexing the output lights of these semiconductor lasers, a transmission line optical fiber for propagating the wavelength-multiplexed light, and a means for separating the propagated light into wavelengths. In an optical fiber transmission system including a demultiplexing circuit that generates a wave, and a photodetector that detects the demultiplexed light of each wavelength, the optical multiplexing means inputs the output light of the semiconductor laser and transmits at least one wavelength-dependent light. an optical multiplexing means including a wavelength-dependent optical multiplex circuit, and an output of the at least one wavelength-dependent optical multiplex circuit and an output of another wavelength-dependent optical multiplex circuit or output light of the input semiconductor laser; and a wavelength-independent optical multiplexing circuit that synthesizes the wavelength-multiplexed light and outputs the wavelength-multiplexed light. A wavelength multiplexing optical fiber transmission system characterized in that the wavelengths are separated by at least one wavelength in order of size.