JPH02140028A - Multiplex wavelength optical communication system - Google Patents

Abstract

PURPOSE:To compensate the initial value deviation of a light source wavelength and its temperature dependency by tuning a wavelength selected by a Fabry- Perot variable interference filter with a wavelength of an optical signal to be received and allowing a photodetector to receive a light. CONSTITUTION:A signal from callers 101-103 is converted into an optical signal by light emitting diodes 11-13. A photo star coupler 35 collects lights of each wavelength into one and split by an optical branching device. The optical signal is sent from a transmission station 1 to a reception station 2 by optical fibers 40-42. The optical signal sent to the reception station 2 is subject to wavelength selection by a Fabry-Perot type variable interference filter 50 and the result is made incident in a photodetector 70. A filter control circuit 92 scans the transmission spectrum of the filter 50 so as to be approached to the light wavelength, repeats the scanning to be tuned with the light wavelength. Even when the light wavelength is changed due to temperature change of the light emitting diode or the like, since the filter 50 traces the wavelength, the reception state is unchanged.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a wavelength division multiplexing optical system in which wavelength division multiplexing transmission is performed between one transmitting station and one or more receiving stations via an optical fiber. Regarding communication systems.

<Prior art> Optical fiber communications that transmit optical signals through optical fibers include systems that receive all of the transmitted information, and systems that select and receive the necessary information from the transmitted information. There is something I did. An example of the former is a telephone, and an example of the latter is a system in which multiple terminals such as broadcasting, telephones, and facsimiles receive optically distributed information from the same optical fiber. be.

There is wavelength division multiplexing optical communication, which employs wavelength division multiplexing transmission for the latter type of optical fiber communication. This wavelength division multiplexing optical communication is capable of bidirectional transmission using a single optical fiber, can be made into a large capacity boat, can transmit signals of different types and speeds independently, and has a number of signal channels. It has the advantage of being easy to expand, and by using a variable wavelength selection element, it is possible to realize an extremely economical and expandable communication system.

Conventionally, there is a wavelength division multiplexing optical communication system as shown in FIG.
Publication No. 9). In this communication system, a caller 1 is sent to a transmitting station l.
01, 102, 103 are collected, and each signal is transmitted to the light emitting elements 11, 12 .
13 each will be powered by humans. These light emitting elements 11, 12.
The light emitted from 13 is mixed by an optical star coupler 35 and distributed to optical fibers 40, 41, and 42.

The light transmitted through the optical fiber 40 and reaching the receiving station 2 is made into a parallel beam by the lens 85, and then passed through the interference filter 55.
incident on . This interference filter 55 transmits only light of a specific wavelength, but the transmitted wavelength can be varied by changing the incident angle of the light. Here, the inclination of the interference filter 55 with respect to the light beam is changed by the drive motor 56. When the receiver selects a signal using the switch 91, the inclination of the interference filter 55 is controlled, and only the light of the wavelength that transmits the signal is selectively transmitted and reaches the light receiving element 70, where the optical signal is transmitted. is converted into an electrical signal,
It is sent to receiver 90.

This optical communication system has the following advantages by using a variable wavelength selection element. One is the simplification of the receiving side system. In conventional wavelength-multiplexed optical communication systems, all wavelength-multiplexed optical signals are converted into electrical signals, and then electrical switches are used to select the signals to be received.According to this system, Since the selection is made at the optical signal stage, the number of parts such as the branching element and light receiving element of the branching filter can be significantly reduced. One more
One advantage is that the multiplicity can be easily expanded.

For example, even if a transmitting station starts transmitting using a new wavelength,
The receiving side can receive the data as is without making any changes to the system.

<Problems to be Solved by the Invention> By the way, the above-mentioned conventional wavelength multiplexing optical communication system uses an interference filter 55 that needs to change the inclination with respect to the light beam as a variable wavelength selection element, and in order to change the inclination, It is driven by an external motor. For this reason, it is necessary to combine it with a precise mechanical mechanism.
There are problems in that the device is large-scale, susceptible to vibration, and expensive. Further, even if a prism or a diffraction grating is used as a variable wavelength selection element other than an interference filter, it is necessary to change the inclination of the element, and this does not solve the problem.

In addition, in wavelength-division multiplexing optical communications, increasing the degree of multiplexing is important for improving system efficiency, but because the emission wavelengths of semiconductor lasers, which are currently mainly used as light sources, vary widely, When combined with a wavelength optical multiplexer/demultiplexer, it is difficult to increase the wavelength multiplexing degree.

As a method to solve this drawback, Japanese Patent Application Laid-Open No. 551587
As disclosed in Japanese Patent Application Laid-Open No. 5i7-26720, the demultiplexing characteristics of the diffraction grating type demultiplexer can be shifted as a whole, or as disclosed in Japanese Patent Application Laid-Open No. 62-90042. It is conceivable to use a diffraction grating type demultiplexer and a photodiode array to create a programmable demultiplexer, as shown in the figure.

The former method makes it possible to compensate by utilizing the property that the temperature dependence of the emission wavelength is almost constant regardless of the wavelength used, but there is a problem that the initial deviation of the emission wavelength cannot be compensated for.

In addition, in the latter method, light from a light source with a certain wavelength is focused on multiple photodiodes, and by changing the combination of photodiodes to accommodate variations in wavelength, Although it is possible to obtain good reception conditions and to compensate for the initial deviation of the light source wavelength and temperature dependence at the same time, the analog switch circuit connected after the photodiode becomes large-scale.
The problem is that this method is too complex to be applied to a communication system that selectively receives a small number of wavelength signals among a plurality of transmitted wavelength signals.

As a method to solve these problems, the conventional wavelength division multiplexing optical communication system described above uses a variable wavelength selection element and tunes the selected wavelength of the variable wavelength selection element to the wavelength of the light to be received. However, as mentioned above, the interference filter used as a variable wavelength selection element in this system requires a large-scale driving device, so the response speed is slow and it is not suitable for high-speed operation such as the above-mentioned tuning operation. be.

Therefore, an object of the present invention is to use a small, vibration-resistant variable wavelength selection element to tune the selected wavelength to the wavelength of the light to be received, thereby compensating for the initial deviation of the light source wavelength and its temperature dependence. The purpose of the present invention is to provide a wavelength division multiplexing optical communication system.

Means for Solving the Problems In order to achieve the above object, the present invention includes a plurality of light sources with different emission wavelengths, and a light introduction means for introducing the light rays emitted by the plurality of light sources into an optical fiber. one transmitting station,
one or more wavelength tunable filters that transmit light of a wavelength selected from among the light emitted from the optical fiber; a control means that controls the selected wavelength of the wavelength tunable filter; and a control means that transmits the wavelength tunable filter. In the wavelength multiplexing optical communication system, the wavelength multiplexing optical communication system transmits an optical signal via the optical fiber between one or more receiving stations having a light receiving element that receives the filtered light, the wavelength tunable filter It is characterized by being a Fapley-Perot type variable interference filter. Further, in the present invention, the control means includes:
The wavelength tunable filter is controlled to increase the amount of transmitted light at each of the shorter wavelength side and longer wavelength side than the selected wavelength, and the output level of the light receiving element at the short wavelength side and the long wavelength side is compared. If the output level on the long wavelength side is higher than the output level on the long wavelength side, the selected wavelength is changed to the shorter wavelength side, while if the output level on the longer wavelength side is higher than the output level on the short wavelength side, the selected wavelength is changed to a longer wavelength side. By changing the wavelength side, the selected wavelength can be tuned to the wavelength of the optical signal to be received.

<Operation> In the above configuration, the control means controls the selected wavelength of the Fapley-Perot variable interference filter, and the light receiving element receives the light transmitted through the Fapley-Perot variable interference filter. Alternatively, the control means controls the variable wavelength filter to increase the amount of transmitted light on each of the shorter wavelength side and longer wavelength side than the selected wavelength, and Compare the output levels and
If the output level on the short wavelength side is higher than the output level on the long wavelength side, the selected wavelength is changed to the shorter wavelength side, while if the output level on the long wavelength side is higher than the output level on the short wavelength side, the selected wavelength is changed. The selected wavelength is tuned to the wavelength of the optical signal to be received by changing it to the longer wavelength side, and the light receiving element receives the light that has passed through the Fabry-Perot variable interference filter. Therefore, the initial deviation of the light source wavelength and its temperature dependence can be compensated for.

<Example> Hereinafter, the present invention will be explained in detail with reference to illustrated embodiments.

First Embodiment FIG. 1 is a system configuration diagram of this embodiment. This wavelength division multiplexing optical communication system uses a Fapley-Perot variable interference filter 50 instead of the interference filter 55 in the conventional example shown in FIG. It is controlled so that the wavelength of the signal is transmitted.

The rest of the structure is the same as that of the conventional example, and the same parts are given the same numbers.

In this embodiment, semiconductor lasers with center wavelengths of 780, 830, and 890 nm are used as the light emitting elements 11, 12, and 13, respectively, and the Fabry-Perot variable interference filter 50 also operates in this wavelength range.

In addition, there are wavelength bands centered around 1.3 μm, or 1
．． Wavelength multiplexing optical communication may be performed in a wavelength band centered on 55 μm, and since this corresponds to the low loss region of optical fibers, it is considered to be particularly advantageous in long-distance optical communication systems.

The optical star coupler 35 may have a configuration in which light of each wavelength is combined into one optical fiber using a normal optical multiplexer, and further the optical fiber is divided using an optical splitter.

For the optical fibers 40, 41, 42, an optical branch may be inserted between the transmitting station 1 and the receiving station 2, and another receiving station may be connected from the optical branch via a new optical fiber. It may also be used commonly for signal transmission from the subscriber terminal side to the transmitting station side.

The transmitting station 1 may be a relay station that does not directly collect signals from senders 101, 102, and 103, but relays signals collected by irregular transmitting stations.

A cross-sectional view of the Fapley-Perot variable interference filter 50 used in this example is shown in FIG. 2(a).

This variable interference filter 50 is manufactured as follows.

First, on a transparent substrate 51O3520 made of glass,
Semi-transparent reflective films 511 and 521 are formed. These reflective films may be metal films such as Ag, Aρ, Au, etc., but here a nine-layer alternating multilayer film of dielectrics T i Ot and 5iOy is used. The spectral reflectance of this reflective film is shown in Figure 3 (
Shown in a). That is, this reflective film has a wavelength of 830n+
It is designed to have a maximum reflectance at ++, and has a reflectance of 90% or more in the wavelength range of 780 to 890 nm. Next, electrostatic drive electrodes 515 and 525 and spacers 530 are formed on the reflective films 511 and 521 by a thin film forming method, and the substrate 520 is attached to the reflective films 511 and 521.
The substrates are brought into contact with the spacer 530 so that they face each other, and both substrates are bonded by electrostatic bonding.

There is a cavity between the opposing reflective films 511 and 521, and the cavity interval d changes due to deformation and displacement in response to the force F applied to the glass substrate 520.

The cavity spacing when F=O is d. Then, the proportionality constant is α
As, d-do-αF...(There is the following relationship.The above force F is an electrostatic force generated between the electrodes 515 and 525 when a voltage is applied between the electrodes 515 and 525 from the voltage source 570.

In general, a Fabry-Perot interferometer has an internal optical path length nd
(n is the refractive index of the reflective interlayer medium), a plurality of specific wavelengths λ1. λ2. λ3...Has a characteristic of transmitting nearby light and hardly transmitting other light.

λm=...(2). However, m is the resonance order (+n=1.2, 3, etc.).Equation (2) ignores the phase shift effect of light that occurs when reflected on the reflective film.This (2) By the formula, the transmission wavelength λ
It can be seen that m can be changed by changing the cavity spacing d or the refractive index n of the medium. In the variable interference filter of this embodiment, the central transmission wavelength λm is controlled by controlling the cavity spacing d. Although air is used here as the medium, fluids (vacuum,
There is no particular restriction as long as it is a gas or liquid). Furthermore, this embodiment was designed to use the resonance wavelength of m=4 as the central transmission wavelength. At this time, the wavelength range used is 780~8
Assuming 90 nm, the cavity spacing d is approximately 1560 to 178
It is sufficient if it can be adjusted within the range of 0r+m.

The variable interference filter characteristics described above are shown in FIG. 3(b).

The horizontal axis is the wavelength, and the vertical axis is the spectral transmittance. In the figure, curve A,
B and C each have a center transmission wavelength of 890°830.7
This is a spectral transmittance curve when adjusted to 80 nm. Each of these curves has a sub-transmission band A゛.

[3'' (and B''), C'', etc. exist, but these are outside the wavelength range used (shaded area), so there is no problem.

From this figure, it can be seen that by using this filter, good transmission characteristics can be obtained for the emission wavelength that matches the central transmission wavelength, and good blocking characteristics can be obtained for other emission wavelengths. . However, if we attempt to sufficiently increase the wavelength multiplicity, it is expected that light with slightly non-tuned wavelengths will be received due to the overlap of the bases of the spectral transmittance curves (portions where the cutoff characteristics are not perfect). In order to eliminate this effect, a pulse modulation method (pulsed FM) is used as a signal modulation method for optical communication, which is less susceptible to minute fluctuations in the light level.

It is desirable to use PCM, etc.).

Note that as the Fabry-Perot type variable interference filter, those shown in FIG. 2(b) or FIG. 2(c) can also be used.

FIG. 2(b) is a cross-sectional view of a variable interference filter in which the interference cavity spacing is controlled by an external force. This variable interference filter does not have electrodes 515, 525 as compared to the variable interference filter shown in FIG.
20, and the other configuration is the same as the variable interference filter shown in FIG. 2(a).

Examples of the external force generating tank include an electromagnetic force generating mechanism.

Further, the portion corresponding to the external force transmitting portion 575 itself may be made of a bimorph that generates an external force.

FIG. 2(c) shows a Fabry-Perot variable filter having a structure in which opposing glass plates 510 and 520 on which semi-transparent reflective films 511 and 521 are formed are supported by an external holder 535. The cavity spacing is controlled by a piezoelectric element 585 incorporated in the holder, and electrodes 586 at both ends of the piezoelectric element 585.
This is done by utilizing the expansion and contraction according to the voltage applied between 587 and 587.

The operating method of the Fabry-Perot variable interference filter and the emission wavelength tracking tuning method will be described below.

FIGS. 4(a) to 4(c) are graphs showing the relationship between the laser light spectrum of the emission wavelength λQ and the transmission spectrum of the variable interference filter, where the horizontal axis is the wavelength, and the vertical axis is the light intensity with respect to the emission spectrum. Spectral transmittance is shown for the transmission spectrum. Furthermore, FIG. 5 is a graph showing temporal changes in the center transmission wavelength λL of the wavelength tunable filter, where the horizontal axis is time;
The vertical axis is the center transmission wavelength. (a) shown on the horizontal axis,
The time domains of (b) and (c) are shown in Fig. 4 (
Corresponds to a), (b), and (c).

Initially, the filter control circuit 92 (FIG. 1) operates as shown in FIG.
), the transmission spectrum (center wavelength λt) of the variable interference filter is scanned so as to approach the emission wavelength λg (
(See Figure 5). When the wavelength λt approaches λC to a certain extent, the amount of light transmitted through the variable interference filter starts to increase, and when it reaches a certain level, the control circuit 92 starts the emission wavelength tracking tuning operation.

By this operation, the center transmission wavelength λ of the variable interference filter
t is a frequency f(
For example, it is modulated at 5l-1z to 5kHz).

In this example, λt+ΔλL is alternately generated using a rectangular wave of frequency f.

It is assumed that the waveform is λL - Δλt. However, λt is the time average of the central transmission wavelength λt. The transmission spectra at this time are shown in FIG. 4(b), each having one curve S.
Indicated by

The control circuit 92 compares the average received light IN+ and IS when the transmission spectrum is curved (center wavelength = λL + Δλt) and when the transmission spectrum is curved S (center wavelength = λ - Δλt). If IS:lL, the average center wavelength λt is shifted slightly to the shorter wavelength side, and then again shifted to the longer or shorter wavelength by ±Δλ. By repeating this operation, the average center wavelength, 1
1 is tuned to the point where l5=IL, that is, the emission wavelength λQ (FIG. 4(c) solid line and FIG. 5).

The emission wavelength λQ may change due to changes in the temperature of the light emitting element, etc.
Even if the average central transmission wavelength λt also tracks λQ', the receiving condition does not change (dotted line in FIG. 4(c) and FIG. 5). In the manner described above, stabilization of the reception state is achieved.

What must be noted in this operation is that the level of the optical signal needs to be approximately constant on average. In general, for digital signals, there is no problem if a coding method in which "0" and "0" are not consecutive is used, but it may not be applicable to a direct intensity modulation signal of an analog signal.

In this example, FIGS. 2(a), (b),
The Fabry-Perot type variable interference filter shown in (c) was used, but it has a responsivity that allows the modulation method shown in this example to be applied, has only one selected wavelength peak within the used wavelength band, and Other types of wavelength tunable filters may be used as long as they have a high transmittance for the centrally selected wavelength and can sufficiently block wavelengths outside the selected wavelength band. For example, a Fabry-Perot interferometer that uses a medium that has an electro-optic effect and changes the internal optical path length by changing its refractive index, or an electric light that changes the birefringence depending on the applied voltage between two orthogonal polarizers. It is also possible to use a method such as that in which an optical material is inserted. (Reference: “Electro-objective spectral filters: review”,
W.J. Gunning, Optical Engineering, p837. Vo12ONo, 6 (19
81) (“E 1 electro-optically
tuned spectral filters: a
review, W.J., Gunning.

0ptical Engineering, p83
7, Vol. 20° No. 6 (1981)) Second Embodiment FIG. 6 is a diagram showing the configuration of the receiving side in the wavelength division multiplexing optical communication system of this embodiment. Here, the transmitting side is omitted because it is the same as the configuration shown in the first embodiment (FIG. 1).

In FIG. 6, mixed light from light sources 11, 12, and 13 having different wavelengths is emitted from an optical fiber 40, and after being made parallel by a lens 85, a Fabry-Perot interference filter 50
-1. The reflected light is changed in direction by a reflecting mirror 52 and enters a filter 50-2. The light reflected by this filter 50-2 is transmitted to a reflecting mirror 52.
The direction of the light is changed by , and the light enters the filter 50-3. In this way, the light emitted from the optical fiber 40 passes through the three Fabry-Perot variable interference filters 50-150-2.
50-3 sequentially.

These Fabry-Perot variable interference filters 50-1
．． 50-2.50-3 are light sources 11°12.13 respectively
It is tuned to the wavelength of , and only transmits light of that tuned wavelength. Each transmitted light is converted into an electric signal by the light receiving elements 71, 72, and 73, respectively.

Each of the filters 50-1.50-2.50-3 is independently controlled by the reception stabilization method described in the first embodiment. Taking the filter 50-2 as an example, a signal at the tuning wavelength of this filter is converted into an electrical signal by the light receiving element 72, and is input to the filter control circuit 92-2. This control circuit controls the tuning wavelength of the filter 50-2 based on the received light signal using the reception stabilization method shown in the first embodiment. As described above, since reception can be stabilized for each wavelength, reception is stabilized against deviations and arbitrary fluctuations in the emission wavelength of the light source.

<Effects of the Invention> As is clear from the above, the wavelength division multiplexing optical communication system of the present invention allows the receiving station to select only the necessary optical signals from the optical signals of multiple wavelengths sent from the transmitting station via the optical fiber. Since a Fabry-Perot type variable interference filter is used to selectively receive the information, the device is compact and resistant to vibration. Because it has a structure in which two are bonded together, it is easy to mass produce and can be lowered in price.

In addition, the wavelength division multiplexing optical communication system of the present invention is capable of high-speed operation because the above-mentioned Farpley-Perot variable interference filter has virtually no moving parts, and the selected wavelength of the above-mentioned Farpley-Perot variable interference filter is The control means controls the variable wavelength filter to increase the amount of transmitted light at each of the shorter wavelength side and longer wavelength side than the selected wavelength, and Compare the output levels, and if the output level on the short wavelength side is higher than the output level on the long wavelength side, change the selected wavelength to the shorter wavelength side, and if the output level on the long wavelength side is higher than the output level on the short wavelength side. If it is large, the selected wavelength is changed to the longer wavelength side and the selected wavelength is tuned to the wavelength of the optical signal to be received, so extremely stable reception is achieved regardless of the initial deviation of the emission wavelength or temperature dependence. This makes it possible to make the emission wavelengths of the light sources sufficiently adjacent and increase the wavelength multiplicity.

[Brief explanation of the drawing]

FIG. 1 is a system configuration diagram of the first embodiment of the present invention, FIG. 2 is a sectional view showing the structure of the Fabry-Perot variable interference filter in the above embodiment, and FIG. 3 is a Fabry-Perot variable interference filter in the above embodiment. Figures 4 and 5 are diagrams showing the characteristics of the interference filter. Figures 4 and 5 are explanatory diagrams of the operation of the Fapley-Perot variable interference filter in the above embodiment. Figure 6 is the system configuration on the receiving side in the second embodiment of the present invention. 7 are system configuration diagrams of conventional examples. 1... transmitting station, 2... receiving station, ! 1.12.13...Light emitting element, 35...Optical star coupler, 40.41°42...Optical fiber, 50...Fapley-Perot type variable interference filter, 70
... Light receiving element, 90 ... Receiver, 91 ... Signal selection switch 92 ... Filter control circuit. Patent applicant: Sharp Corporation Agent
Patent attorney: Haka 1 (cited above) Figure 4 (a) Mitsuyuki Imamitsu (%) Spectral reflection (%) Figure 5 Time Figure 6

Claims (2)

[Claims] (1) Select from one transmitting station having a plurality of light sources with different emission wavelengths and a light introducing means for introducing the light beams emitted by the plurality of light sources into the optical fiber, and the light emitted from the optical fiber. 1, comprising: one or more wavelength tunable filters that transmit light of wavelengths that have been transmitted; a control means that controls the selected wavelength of the wavelength tunable filters; and a light receiving element that receives the light that has been transmitted by the wavelength tunable filters. A wavelength multiplexing optical communication system configured to transmit optical signals between one or more receiving stations via the optical fiber, wherein the wavelength tunable filter is a Fabry-Perot type tunable interference filter. Wavelength multiplexing optical communication system. (2) In the wavelength division multiplexing optical communication system according to claim 1, the control means controls the wavelength tunable filter to increase the amount of transmitted light at each of a shorter wavelength side and a longer wavelength side than the selected wavelength. Compare the output level of the light receiving element on the short wavelength side and long wavelength side, and if the output level on the short wavelength side is higher than the output level on the long wavelength side, change the selected wavelength to the short wavelength side. On the other hand, if the output level on the long wavelength side is higher than the output level on the short wavelength side, the selected wavelength is changed to the longer wavelength side, and the selected wavelength is tuned to the wavelength of the optical signal to be received. A wavelength division multiplexing optical communication system characterized by: