JPH07250031A - Optical bidirectional transmission system - Google Patents

Abstract

(57) [Abstract] [Purpose] It is an object to improve a multiplexing method of a reflection type bidirectional transmission method using a semiconductor modulator and to provide an efficient transmission method. The center device includes an optical transmission means for transmitting light of a plurality of wavelengths including signal light addressed to the user device, transmits a signal from the center device to the user device at one wavelength, and transmits the signal at the other wavelength to the user device. The continuous wave light used for modulation is supplied. The user device includes an optical modulation unit that performs optical modulation and absorption, and an optical waveguide unit that supplies light from the optical transmission line to the optical modulation unit and guides return light from the optical modulation unit to the optical transmission line. including. As the optical modulation means of the user device and the optical waveguide means for guiding the return light to the optical transmission line, there is provided a semiconductor modulator in which a mirror is provided on an end face different from the end face connected to the optical transmission line.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a system used for two-way communication by an optical signal, and more particularly to an optical transmission system in which a center device has a light source and a user device does not have a light source.

[0002]

2. Description of the Related Art In the conventional optical bidirectional transmission system, a method is basically used in which two fibers are drawn between a center device and a user device. However, this method has a problem that the fiber laying is expensive. In order to solve this problem, a method has been adopted in which a single fiber is used between the center device and the user device and bidirectional transmission / reception is performed using a multiplexing technique such as ping-pong transmission. At this time, a transmitter and a receiver are required on the center device side and the user device side, respectively. In this method, since the transmitter / receiver is also provided on the user device side, there is a problem that it takes time to rush for repair when a failure occurs, the recovery time is long, the cost is high, and the like.

In order to solve this, an optical transmission system has been proposed in which a light source for sending a signal from a user device to a center device is not provided in the user device (for example, Electronics Letter).
s, vol.22 No.10, pp.517-518, 1986). In the case of this method, as shown in FIG. 11, the signal transmitted from the center device 90 passes through the directional coupler 92 in the user device 91 and is received by the receiver 93. When transmitting from the user equipment to the center equipment, continuous wave light which is not modulated is sent from the center equipment to the user equipment, and the directional coupler 92 in the user equipment is sent.
And enters the LiNbO ₃ modulator 94 via the. The incident light is reflected by the mirror 95 on the rear surface of the modulator and sent back to the center device. Light is modulated as it travels back and forth within the modulator. However, with this method, the number of elements is large, and it is difficult to adjust the coupling between the elements. Reference numeral 96 is a directional coupler of the center device. When a semiconductor is used as a modulator, the number of elements may be one modulator, unlike the case of a LiNbO ₃ modulator. In this case, in the case of bidirectional multiplexing, a method of performing time division (ping-pong) and a method of changing the frequency to be modulated depending on the direction are used.

[0004]

However, in the above-described bidirectional multiplexing method, when it is difficult to control the transmission / reception timing by time division, and when the transmission takes time and the amount of information increases. However, in modulation frequency multiplexing, there is a problem in that sensitivity cannot be obtained when a modulator is used as a detector. An object of the present invention is to solve such a problem and improve a bidirectional multiplexing method in a reflection type bidirectional transmission method using a semiconductor modulator, simplify a control method, effectively use time, and perform optical detection. It is to provide an efficient transmission method with improved sensitivity.

[0005]

According to a main aspect of the present invention, in a reflective bidirectional transmission system using a semiconductor modulator, a device for transmitting light of a plurality of wavelengths to a center device as a bidirectional multiplexing method. And wavelength division multiplexing.

In the present invention, the electric signal of the center device is converted into an optical signal of one wavelength by the electro-optical converter, and the optical signal is passed through the optical fiber to the user device and received. When transmitting from the user equipment, continuous wave laser light of another wavelength sent from the electro-optical converter in the center equipment enters into the user equipment and is modulated by the user equipment. It returns to the center device via the fiber, enters the photoelectric converter, and is received.

[0007]

In the present invention, if the semiconductor modulator is used in the user device, (1) transmission and reception are performed by one element, and (2) voltage transmission is performed during transmission and reception, so that the laser consumes a normal power. It is smaller than that of (1), (3) the power supply circuit can be reduced, (4) the temperature dependence of the characteristics is small, and (5) transmission and reception can be performed simultaneously. Therefore, the control method can be simplified, the time can be effectively used, and the light detection sensitivity can be improved.

[0008]

1 is a block diagram showing a first embodiment of the present invention. The center device 10 includes a circulator 12, an electro-optical converter 13 that outputs two wavelengths, and an opto-electric converter 14. The user device 11 includes a transmitter / receiver 15 using a semiconductor modulator.
including. The two are connected by an optical fiber 16. The transmitter / receiver 15 using the semiconductor modulator used in the user device 11 will be described in detail. The structure is shown in FIG.
Basically, it has the structure of a semiconductor modulator, and it is a waveguide type modulation / absorption layer 23, a cladding layer 24, a high reflection film 22, a low reflection film.
It consists of 21.

In FIG. 3, a reverse bias is applied to the semiconductor modulator.
The change in absorption coefficient is shown. Apply reverse bias
As the absorption edge shifts to the longer wavelength side
I understand. Actual operating wavelength λ₁, Λ₂With arrow
You To change the reverse bias between 0V and -4V
From the wavelength λ₁Modulates light by changing the absorption coefficient at
can do. The on / off ratio (extinction ratio) is about 2
It is 0 dB. Wavelength λ ₂Has a large absorption coefficient for
In fact, this semiconductor modulator acts as a photodetector.

The operation of the semiconductor modulator during transmission / reception of this system will be described. When transmitting from the user device, continuous wave laser light of wavelength λ ₁ sent from the center device enters from the end face of the low reflection film 21 and is coupled to the waveguide type modulation / absorption layer 23. At this time, since the end face is covered with the low reflection film, the coupling efficiency to the modulation / absorption layer is improved. The light coupled to the waveguide type modulation / absorption layer is reflected by the high reflection film 22, passes through the waveguide type modulation / absorption layer again, and passes through the low reflection film. When light passes through the absorption layer, an externally modulated voltage is applied to control the absorption coefficient of the waveguide type modulation / absorption layer, thereby applying modulation. The modulated light is transmitted to the center device via the optical fiber.

When receiving a signal from the center device,
Wavelength λ sent from the center device ₂Modulated ray of
The light enters from the end face of the low reflection film 21 and is waveguide-type modulation / absorption.
Combine with the confining layer 23. Light coupled to waveguide modulation / absorption layer
Is reflected by the high-reflection film 22, and again the waveguide type modulation / absorption layer is formed.
pass. When light passes through the waveguide type modulation / absorption layer,
In the absorption layer, the wavelength λ₂Light has a large absorption coefficient,
Light is gradually absorbed and converted into photocurrent, which results in photoelectric conversion.
Done.

In the present invention, the electric signal from the center unit is converted into the optical signal of wavelength λ _{2 by} the electro-optical converter 13, passes through the circulator 12, passes through the optical fiber 16 and enters the user unit 11, and is received. To be done. When transmitting from the user device, the continuous wave laser light of the wavelength λ ₁ sent from the electro-optical converter 13 in the center device 10 enters the user device 11, which is modulated by the user device 11 and modulated by the reflection. The light passes through the optical fiber 16, returns to the center device, passes through the circulator 12, enters the photoelectric converter 14, and is received. In this example, since the electro-optical converter and the opto-electric converter and the optical fiber are coupled by the circulator, the transmitted and received light signals can be separated, and the loss of the transmission line can be reduced.

In this embodiment, wavelength multiplexing is used as a bidirectional transmission system between the center device and the user device. That is, the wavelength of the light for transmitting the signal from the center device to the user device and the wavelength of the light for transmitting the signal from the user device to the center device by utilizing the reflected light are separately communicated. Timing charts of this embodiment are shown in FIGS. 4 (a) and 4 (b). In the figure (a), T indicates transmission and R indicates reception. CW represents continuous wave laser (CW) light. Wavelength λ ₂
Transmission from the center device to the user device using the
Transmission from the user equipment to the center equipment using CW light from the center equipment using ₁ is performed at the same time, and transmission and reception are repeated between the center equipment user equipments. In the diagram (a), the center device is idle during the propagation delay time. On the other hand, in the diagram (b), the propagation delay time is effectively used, and the next transmission from the center device to the user device is performed without waiting for the transmission from the user device. In the case of this embodiment,
Communication is possible by any of the timing charts, and as compared with normal TCM (time compression multiplexing), transmission and reception can be performed simultaneously, so that time can be effectively used.

In this embodiment, the circulator is used for switching the opto-electric converter and the electro-optical converter in the center device, but a directional coupler, a Y-branch structure or the like may be used, and the branching ratio thereof. Is not limited to 1: 1 and may be changed according to the situation. However, when a device other than the circulator is used, the propagation loss may increase in both transmission and reception.

FIG. 5 shows a block diagram of the second embodiment.
In this embodiment, single mode optical fibers 51 and 52 are used as an optical transmission line, and a center device 50 and n user devices 54 are connected to each other by the single mode fibers. The optical transmission line from the center device 50 to each user device 54 and the optical transmission line from each user device 54 to the center device 50 are shared. Between the center device 50 and each user device 54,
It comprises a star coupler 53 for distributing the optical signal. As in the first embodiment, the center device 50 includes a circulator, an electro-optical conversion circuit for transmitting light of two wavelengths as a means for generating signal light addressed to the user device, and an optical device for receiving signals from the user device. It has an electric conversion circuit. Each user device
54 comprises a transceiver using a semiconductor modulator.

The operation of this embodiment will be described. The signal light Pi having a wavelength λ ₂ to the i-th user device generated from the opto-electric conversion circuit of the center device passes through the circulator and the single mode optical fiber 51 and passes through the star coupler.
I am at 53. In the star coupler 53, the optical signal is distributed to the n user devices, and in the i-th transmission path, it is incident on the semiconductor modulator of the i-th user device via the single mode fiber 52.

Next, when a signal is sent from each user equipment side to the center equipment side, the center equipment sends out continuous wave laser light having a wavelength λ ₁ and reaches each user equipment via the star coupler. In each user device, the reflected light is modulated as in the first embodiment, passes through the star coupler and the circulator of the center device again, reaches the photoelectric conversion circuit, and is converted into an electric signal.

A timing chart of this embodiment is shown in FIG. The optical signal of wavelength λ ₂ from the center device to each user device is transmitted to all user devices. When sending a signal from the user equipment to the center equipment, the wavelength λ from the center equipment
The continuous wave laser (CW) light of ₁ is reflected and modulated by the user device and sent to the center device. In FIG.
A signal is sent from the center device to the user device after the center device and each user device transmit and receive once. When each user device is far from the center device and it takes time to propagate, the propagation delay time can be effectively used, and the next transmission from the center device to the user device can be performed without waiting for the transmission from the user device. It is possible.

FIG. 7 shows a block diagram of the third embodiment of the present invention. In this embodiment, single-mode optical fibers 71 and 72 are used as an optical transmission line, and a center device 70 and n user devices 74 are connected to each other by the single-mode fibers. The optical transmission line from the center device 70 to each user device 74 and the optical transmission line from each user device 74 to the center device 70 are shared. Between the center device and each user device,
A LiNbO ₃ optical switch 73 is connected as a transmission line switching device. As in the first embodiment, the center device 70 includes a circulator, an electro-optical conversion circuit that sends out light of two wavelengths as means for generating a signal addressed to the user device, and an opto-electric device for receiving a signal from the user device. It has a conversion circuit. Each user device 74 comprises a transceiver using a semiconductor modulator.

Next, the operation of this embodiment will be described.
The signal light Pi having the wavelength λ ₂ to the i-th user device, which is generated from the photoelectric conversion circuit of the center device 70, enters the optical switch 73 via the circulator and the single-mode optical fiber 71. The i-th transmission path is selected in the LiNbO ₃ optical switch 73 and is converted into an electric signal via the single mode fiber 72 by the transmitter / receiver composed of the semiconductor modulator of the user equipment.

Next, when a signal is sent from each user equipment side to the center equipment side, the center equipment sends out continuous wave laser light of wavelength λ ₁ and reaches each user equipment via an optical switch. In each user device, as in the first embodiment, the reflected light is modulated, and the modulated reflected light passes through the circulator of the center device again via the optical switch, reaches the photoelectric conversion circuit, and is converted into an electric signal. To be done. Transmission and reception can be performed simultaneously because the wavelengths are different.

A timing chart of this embodiment is shown in FIG. In FIG. 8, when the i-th optical switch is turned on, the center device and the i-th user device are connected and transmission / reception is performed using two wavelengths. After that, the switches are o
Then, signals are transmitted and received between the center device and each user device. In this embodiment, a LiNbO ₃ optical switch is used as the optical switch.
Although 73 is used, an optical switch using another material may be used.

Next, a fourth embodiment of the present invention will be described. FIG. 9 shows a block diagram. This is almost the same as the second embodiment, but an optical filter 55 for selecting a wavelength for each user equipment.
It is equipped with Further, the electro-optical converter of the center device 50 uses an electro-optical converter that sends out light of n wavelengths instead of light of two wavelengths. In the present embodiment, unlike the embodiments described so far, the wavelength is not divided between the CW light and the modulated light, but the wavelength is assigned to each user apparatus to perform transmission / reception.

FIG. 10 shows a timing chart of this embodiment. When transmitting from the center device to the user device, signal lights of n wavelengths assigned to each user device are transmitted. Each user device selects and detects light of its assigned wavelength by the optical filter 54. When a signal is sent from each user equipment to the center equipment, continuous wave light is sent out at the timing considering the propagation time in the center equipment, and the selected light is modulated at the time of reflection by the optical filter of each user equipment Sent to the device. In the case of this embodiment, when each user equipment transmits a signal to the center equipment, the center equipment transmits the continuous wave light in consideration of the reflection transmission timing at each user equipment, so that the timing can be controlled by the user equipment. Therefore, the user device can be simplified.

[0025]

As described above, in the optical bidirectional transmission system of the present invention, it is possible to realize the simplification of the control method, the effective use of time, the improvement of the photodetection sensitivity, etc. in the user device which is not provided with the light source. .

[Brief description of drawings]

FIG. 1 is a block diagram of a first embodiment of the present invention.

FIG. 2 is a diagram showing a transceiver using a semiconductor modulator.

FIG. 3 is a diagram illustrating voltage dependence of an absorption coefficient of a dynamic modulator and an operating wavelength.

FIG. 4 is a timing chart in the first embodiment.

FIG. 5 is a block diagram of a second embodiment.

FIG. 6 is a timing chart of the second embodiment.

FIG. 7 is a block diagram of a third embodiment.

FIG. 8 is a timing chart of the third embodiment.

FIG. 9 is a block diagram of a fourth embodiment.

FIG. 10 is a timing chart of the fourth embodiment.

FIG. 11 is a diagram for explaining a conventional technique.

[Description of Reference Signs] 10, 50, 70 Center device 11, 54, 74 User device 12 Circulator 13 Electro-optical converter 14 Photo-electric converter 15 Transceiver 16, 51, 52, 71, 72 Single-mode optical fiber 21 Low Reflective film 22 Highly reflective film 23 Modulation / absorption layer 24 Cladding 53 Star coupler 73 Optical switch 55 Wavelength filter

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location H04J 14/06 H04B 10/152 10/142 10/04 10/06 7739-5K H04B 9/00 L

Claims (6)

[Claims] 1. A center device and a user device, which are connected to each other by a bidirectional optical transmission path, wherein the center device includes an optical transmitting means for transmitting light of a plurality of wavelengths including signal light addressed to the user device. The user device includes an optical modulator that performs optical modulation and absorption, and an optical guide that supplies light from the optical transmission line to the optical modulator and guides return light from the optical modulator to the optical transmission line. An optical two-way transmission system characterized by comprising wave means. 2. A semiconductor modulator provided with a mirror on an end face different from an end face connected to the optical transmission line, as an optical modulation unit of the user device and an optical waveguide unit for guiding the return light to the optical transmission line. The bidirectional transmission system according to claim 1, characterized in that 3. The center device has means for transmitting light of two wavelengths, the first wavelength is used for transmitting a signal from the center device to the user device, and the second wavelength is used by the user device. 3. The optical bidirectional transmission system according to claim 1, wherein the optical bidirectional transmission system is used for supplying light used for modulation. 4. The optical signal is distributed to a plurality of user equipments by using a star coupler.
The optical bidirectional transmission method according to any one of 1. 5. The optical bidirectional transmission system according to claim 1, wherein an optical signal is distributed to a plurality of user devices by using an optical switch. 6. The optical bidirectional transmission system according to claim 4, wherein each user equipment is provided with an optical filter.