JP2001036473A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a reduction in a level of an optical signal due to other causes than a decreased output of a light emitting element. SOLUTION: The optical transmitter is provided with an optical module 3 having a light emitting element 1 and an internal light receiving element 2 that receives part of light emitted from the light emitting element 1, an outer light receiving element 4 that receives a light sent from the optical module 3 at the outside of a device 10, a comparator circuit 5 that compares a light receiving level of the internal light receiving element 2 with a light receiving level of the outer light receiving element 4, and a level control circuit 6 that controls a drive current of the light emitting element 1 to control a light emission level of the light emitting element 1, and also with a modulation degree control circuit 7 that controls a degree of modulation of a modulated signal light emitted from the light emitting element 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission device used in an optical communication system, and is suitable for use as an optical transmission device such as a one-way optical CATV system and a two-way optical CATV system. is there.

[0002]

2. Description of the Related Art For example, in a CATV optical communication system, if the optical level received by an optical receiver is low, the C / N characteristics deteriorate. Therefore, conventionally, as shown in FIG. 4, a part of the output light emitted from the light emitting element C of the module B of the optical transmitter A in the center is received by the light receiving element D of the module B, and the automatic light emission level control circuit (APC : Automatic Powe
r Control) E, and the control circuit E controls the bias voltage applied to the light emitting element C so that the light emitting level from the light emitting element C is adjusted to be constant.

[0003]

The above-mentioned conventional optical transmission apparatus has the following problems. 1. Even if the output light emitted from the light emitting element C of the module B is controlled to a constant level as described above, the optical transmission path may be abnormal or may be affected by the outside during transmission, and When the transmission loss of the optical receiver increases, the optical level received by the optical receiver decreases and the C / N characteristics deteriorate. 2. Light emitting element C of module B
When the light emitted from the light emitting element C is directly modulated by applying an RF signal to the light emitting element C, if the light emitting level is increased by increasing the bias current of the light emitting element C by the APC, the level of the RF signal is always constant. In addition, the RF signal level with respect to the light emission level decreases, and the modulation degree decreases. 3. Conventional AP
In C, the light level after transmission cannot be monitored.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is not only to stabilize the light emission level by detecting a change in the light emission level from the light emitting element, but also to output the light even when the light emission level from the light emitting element is constant. It also detects a decrease in the light level afterwards, so that the light level received by the optical receiver is always constant,
An object of the present invention is to provide an optical transmission device capable of preventing a decrease in C / N.

The first optical transmission device of the present application comprises a light emitting element for generating a modulated signal light modulated by an RF signal and an internal light receiving element for receiving a part of the modulated signal light emitted from the light emitting element. An optical module provided with an external light receiving element for externally receiving the modulated signal light transmitted from the optical module; a comparison circuit for comparing the light receiving level of the internal light receiving element with the light receiving level of the external light receiving element; A level control circuit is provided that controls the drive current of the light emitting element based on the comparison to control the level of the modulated signal light emitted from the light emitting element.

The second optical transmission device of the present application includes a light emitting element for generating a modulated signal light modulated by an RF signal and an internal light receiving element for receiving a part of the modulated signal light emitted from the light emitting element. An optical module provided with an external light receiving element for externally receiving the modulated signal light transmitted from the optical module; a comparison circuit for comparing the light receiving level of the internal light receiving element with the light receiving level of the external light receiving element; A level control circuit that controls the drive current of the light emitting element based on the comparison to control the level of the modulation signal light emitted from the light emitting element;
A modulation degree control circuit for controlling a modulation degree of the modulation signal light emitted from the light emitting element, the control circuit controlling a level of an RF signal input to the optical module based on the comparison to modulate the modulation signal light; It controls the degree.

A third optical transmitter of the present application includes a light emitting element for generating a modulated signal light modulated by an RF signal, and an internal light receiving element for receiving a part of light emitted from the light emitting element. The optical module, an external light receiving element that receives the modulated signal light transmitted from the optical module externally, a comparison circuit that compares the light receiving level of the internal light receiving element with the light receiving level of the external light receiving element, and a comparison circuit. A level control circuit for controlling a light emission level from the light emitting element by controlling a driving current of the light emitting element based on the control signal, and a modulation degree control circuit for controlling a modulation degree of a modulation signal light emitted from the light emitting element, the control circuit comprising: Detects the level of the pilot signal contained in the RF signal in the modulated signal light received by the external light receiving element, and controls the level of the RF signal based on the level change to control the level of the RF signal. It is intended to control the furniture.

[0008] The fourth optical transmission device of the present application comprises the third optical transmission device.
The pilot signal is included in the RF signal input from the outside in the optical transmitting device, or the pilot signal is oscillated from a local oscillator in the optical transmitting device and combined with the RF signal.

[0009]

(Embodiment 1) An example in which the first embodiment of the optical transmission apparatus of the present invention is used as a downlink E / O converter installed at the center of a bidirectional CATV system will be described. A description will be given based on FIG. This CATV system converts an electric signal into an optical signal by an optical transmission device 10 (downstream E / O converter 10) of the present invention installed in a center as shown in FIG. The downstream optical signal is converted to an electrical signal by the downstream O / E converter 16 in the node 14 and transmitted to each subscriber, and transmitted from each subscriber to the node 14 via the upstream transmission line 18. The HFC (Hybrid Fiber Coax) method of converting the obtained electric signal into an optical signal by the upstream E / O converter 20 in the same node 14 and transmitting this as an upstream optical signal to the upstream O / E converter 22 in the center Bidirectional CA
It is a TV system.

The down E / O converter 10 in the center
Is a comparison between an optical module 3 having a light emitting element 1 and an internal light receiving element 2 as shown in FIG. 1, an external light receiving element 4, and a light receiving level of the internal light receiving element 2 and a light receiving level of the external light receiving element 4. A circuit 5 and a level control circuit 6 for controlling the drive current of the light emitting element 1 based on the comparison result of the comparison circuit 5
And a modulation degree control circuit 7 for controlling the level of the RF signal input to the optical module 3.

The light emitting element 1 generates a modulated signal light modulated by an input RF signal. Specifically, the driving current output from the level control circuit 6 is R
By superimposing and inputting the F signal, a modulated signal light that is directly intensity-modulated based on the RF signal is generated, and this is used as a downstream optical signal by the downstream E / O converter 1.
Output from 0. The internal light receiving element 2 constitutes one optical module 3 together with the light emitting element 1, receives a part of the modulated signal light generated from the light emitting element 1, converts the light into an electric signal, and converts the light into an electric signal. Input to the control circuit 6.

The external light receiving element 4 is provided in a node 14 provided on a downstream transmission line 12 through which a downstream optical signal output from the downstream E / O converter 10 is transmitted, and a part of the downstream optical signal. Is converted into an electric signal and input to the status monitor 24 in the node 14. The electric signal input to the status monitor 24 is input to the upstream E / O converter 20 to be converted into an optical signal, superimposed on the upstream optical signal, and input to the upstream O / E converter 22 in the center,
Then, it is converted into an electric signal and input to the comparison circuit 5.

The comparison circuit 5 compares the electric signal output from the internal light receiving element 2 with the electric signal output from the external light receiving element 4 to determine the light receiving level of the internal light receiving element 2 and the light receiving level of the external light receiving element 4. And outputs a signal to the level control circuit 6 and the modulation degree control circuit 7, respectively.

The level control circuit 6 to which the signal from the comparison circuit 5 has been input increases or decreases the drive current (bias current) of the light emitting element 1 so that the light receiving level of the internal light receiving element 2 and the light receiving level of the external light receiving element 4 are compared. The light emission level of the light emitting element 1 is adjusted so as to eliminate the difference.

The modulation degree control circuit 7 adjusts the drive level of the light emitting element 1 by changing the level of the RF signal based on the signal input from the comparison circuit 5. That is, in the case of the direct intensity modulation method, if the driving current is increased while the level of the RF signal is kept constant to increase the light emitting level of the light emitting element 1, the RF signal level relative to the light emitting level is relatively reduced. Therefore, the C / N characteristic and the output level on the receiving side (the output level of the downstream O / E converter 16 in the node 14 in FIG. 1) decrease due to the decrease in the modulation degree of the modulated signal light. Therefore, the drive level of the light emitting element 1 is increased by increasing the level of the RF signal by the modulation degree control circuit 7, so that the modulation degree of the modulation signal light is kept constant.

In this case, the modulation degree is slightly lower than the standard modulation degree of the light emitting element 1 (the driving level of the light emitting element 1 is −3).
(approximately dB). In other words, adjusting the light emission level of the light emitting element 1 by increasing or decreasing the drive current means operating the light emitting element 1 at a level other than the standard light output level of the light emitting element 1, so that the distortion characteristic also changes ( Deterioration). Therefore, when the present inventor conducted an experiment by changing the drive current of the light emitting element 1 of FIG. 1 from a standard value to a maximum value, deterioration of the distortion characteristics was confirmed, and the deterioration was related to the C / N characteristics (balance). It has been found that the improvement can be achieved by lowering the degree of modulation.

The electric signal input from the internal light receiving element 2 to the level control circuit 6 can be used to determine whether the decrease in the light level is based on the abnormality of the light emitting element 1 or the abnormality of the optical transmission line. it can. That is, when there is a difference between the light receiving levels of the internal light receiving element 2 and the external light receiving element 4, if the level of the electric signal input from the internal light receiving element 2 to the level control circuit 6 does not change, the light level will not decrease. It can be determined that it is based on the abnormality of the optical transmission line. Further, the driving current of the light emitting element 1 is always controlled based on the electric signal input from the internal light receiving element 2, and only when the light receiving level of the internal light receiving element 2 differs from the light receiving level of the external light receiving element 4, the comparison circuit 5 The driving current of the light emitting element 1 can be controlled based on a signal input from the light emitting element 1.

(Embodiment 2) An example in which the second embodiment of the optical transmitter of the present invention is used as a downstream E / O converter installed at the center of a one-way CATV system will be described.
A description will be given based on FIG. The basic configuration of the optical transmission device 10 (downstream E / O converter 10) of the present invention shown in FIG.
Are the same as those shown in FIG. The difference is the one-way CATV
According to the system, the external light receiving element 4 provided in the node 14 in FIG. 1 is provided inside the downstream E / O converter 10 so that a part of the modulated signal light output from the optical module 3 is converted to an optical signal. That is, the light is branched by the branching device 26, and the light is received by the external light receiving element 4. Also, in FIG. 1, the electric signal output from the external light receiving element 4 is input to the comparison circuit 5 via the upstream transmission line 18 in FIG.
FIG. 2 shows that the data is directly input to the comparison circuit 5.

(Embodiment 3) An example in which the third embodiment of the optical transmission apparatus of the present invention is used as a downstream E / O converter installed at the center of a bidirectional CATV system will be described.
This will be described with reference to FIG. The basic configuration of the optical transmitter 10 (downstream E / O converter 10) of the present invention shown in FIG.
Are the same as those shown in FIG. The difference is the downstream E / O in FIG.
In addition to the external light receiving element 4 provided in the converter, a second external light receiving element 4 is provided in the node 14 and the downstream light output from the downstream E / O converter 10 by the two external light receiving elements 4. That is, a signal is received. The method of inputting the light receiving level of the second external light receiving element 4 to the comparison circuit 5 is the same as the method described in the first embodiment.
The comparing circuit 5 compares the light receiving levels of the three light receiving elements of the internal light receiving element 2, the external light receiving element 4, and the second external light receiving element 4, and the level control circuit 6 determines the light emitting element 1 based on the comparison result. The drive current is controlled, and the modulation degree control circuit 7 controls the level of the RF signal based on the comparison result.

(Other Embodiments) The modulation degree control circuit 7 detects the level of a pilot signal included in the RF signal in the downstream optical signal (modulated signal light) received by the external light receiving element 4, and detects this level. The level of the RF signal can be controlled based on the change. In this case, RF input from outside
If the signal does not include a pilot signal, a local oscillator 29 is provided in the optical transmitter 10 as shown by a virtual line in FIG. It can be combined with a signal and used instead of a pilot signal.

The optical transmitter according to the present invention described in each of the above embodiments has an alarm for notifying when the difference between the light receiving level of the internal light receiving element 2 and the light receiving level of the external light receiving element 4 exceeds a predetermined value. A function of outputting a signal to a center or other predetermined place can be added. The alarm signal may be transmitted by being superimposed on the up / down signal,
A transmission path dedicated to the alarm signal may be provided in advance, and the signal may be transmitted via this dedicated path, or may be transmitted by any other desired method.

[0022]

According to the first invention of the present application, a light emitting element for generating a modulated signal light modulated by an RF signal and an internal light receiving element for receiving a part of the modulated signal light emitted from the light emitting element are provided. An optical module provided with an external light receiving element for externally receiving the modulated signal light transmitted from the optical module,
A comparison circuit for comparing the light receiving level of the internal light receiving element with the light receiving level of the external light receiving element; and controlling the driving current of the light emitting element based on the comparison by the comparison circuit to control the level of the modulated signal light emitted from the light emitting element. The provision of the level control circuit for controlling has the following effects. 1. Even if the output light level of the light emitting element is normal, the received light level is reduced even if the light receiving level of the optical receiving apparatus is reduced due to a shift of the optical axis between the optical transmitting apparatus and the optical transmission path. The light receiving level can be kept constant by adjusting the light emitting level of the light emitting device. 2. Similarly, when the light receiving level is reduced due to an increase in transmission loss due to an abnormality in the optical transmission line, for example, looseness of the connector or deterioration of the fusion point, the light receiving level of the light emitting element is adjusted to keep the light receiving level constant. Can be held. 3. The decrease in the light reception level due to the above-mentioned causes such as the deviation of the optical axis and the looseness of the connector usually progresses relatively slowly in about two weeks at about -0.5 dB. By performing the repair work, the problem can be repaired without bringing down the system.

The second or third invention of the present application comprises a modulation degree control circuit for controlling the level of an RF signal input to the optical module to control the modulation degree of the modulation signal light emitted from the light emitting element. Therefore, the following effects are obtained in addition to the above effects. 1. By increasing the light emission level of the light emitting element, the degree of modulation does not decrease and the C / N characteristics and the output level on the receiving side do not decrease.

According to a fourth invention of the present application, the pilot signal is included in an RF signal inputted from the outside or is oscillated from a local oscillator in an optical transmission apparatus and R
Since the signal is synthesized with the F signal, it has the following effects. 1. When the pilot signal is included in the RF signal input from the outside, the level of the RF signal can be controlled using the pilot signal as it is. 2. When the pilot signal is oscillated from the local oscillator and is synthesized with the RF signal, even if the input RF signal does not include the pilot signal, the level of the RF signal is reduced in the same manner as when the pilot signal is included. Can be controlled.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a first embodiment of an optical transmission device according to the present invention.

FIG. 2 is an explanatory diagram showing a second embodiment of the optical transmission device of the present invention.

FIG. 3 is an explanatory diagram showing a third embodiment of the optical transmission device of the present invention.

FIG. 4 is an explanatory diagram showing an example of a conventional optical transmission device.

[Explanation of symbols]

 Reference Signs List 1 light emitting element 2 internal light receiving element 3 optical module 4 external light receiving element 5 comparison circuit 6 level control circuit 7 modulation degree control circuit

Claims (4)

[Claims] A light emitting element for generating a modulated signal light modulated by an RF signal, and an internal light receiving element for receiving a part of the modulated signal light emitted from the light emitting element.
, An external light receiving element (4) for externally receiving the modulated signal light transmitted from the optical module (3), a light receiving level of the internal light receiving element (2) and an external light receiving element (4). (5) a comparison circuit for comparing with the light receiving level
And the light emitting element (1) based on the comparison in the comparison circuit (5).
An optical transmission device comprising a level control circuit (6) for controlling the level of modulated signal light emitted from the light emitting element (1) by controlling the driving current of the light emitting element (1). 2. A light emitting element (1) for generating a modulated signal light modulated by an RF signal, and an internal light receiving element (2) for receiving a part of the modulated signal light emitted from the light emitting element (1).
, An external light receiving element (4) for externally receiving the modulated signal light transmitted from the optical module (3), a light receiving level of the internal light receiving element (2) and an external light receiving element (4). (5) a comparison circuit for comparing with the light receiving level
And the light emitting element (1) based on the comparison in the comparison circuit (5).
A level control circuit (6) for controlling the level of the modulation signal light emitted from the light emitting element (1) by controlling the drive current of the light emitting element (1);
A modulation degree control circuit (7) for controlling the modulation degree of the modulation signal light emitted from the light emitting element (1) is provided, and the control circuit (7) is configured to input an RF signal to the optical module (3) based on the comparison. An optical transmission device for controlling a modulation level of a modulated signal light by controlling a signal level. 3. A light-emitting element (1) for generating a modulated signal light modulated by an RF signal, and an internal light-receiving element (2) for receiving a part of light emitted from the light-emitting element (1). An optical module (3) and an external light receiving element (4) for externally receiving the modulated signal light transmitted from the optical module (3)
And a comparison circuit (5) for comparing the light receiving level of the internal light receiving element (2) with the light receiving level of the external light receiving element (4); and driving the light emitting element (1) based on the comparison in the comparison circuit (5). A level control circuit (6) for controlling a light emission level from the light emitting element (1) by controlling a current; and a modulation degree control circuit (7) for controlling a modulation degree of a modulation signal light emitted from the light emitting element (1). The control circuit (7) detects the level of the pilot signal included in the RF signal in the modulated signal light received by the external light receiving element (4), and detects the level of the RF signal based on the level change. An optical transmission device for controlling the modulation degree of the modulated signal light by controlling the modulation. 4. The optical transmitter according to claim 3, wherein the pilot signal is included in an RF signal input from the outside, or is oscillated from a local oscillator in the optical transmitter and combined with the RF signal. An optical transmission device, comprising: