JPH0969819A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To miniaturize an optical transmitter by reducing the number of parts provided to an optical fiber. SOLUTION: This optical transmitter 1a is constituted of a signal source 4, a semiconductor laser module 9, an optical fiber 2, an amplifier 6, a comparator 7, an adder 10, a signal line 3, a delay line 5 and an optical modulator module 8a. DC light 14 from the semiconductor laser module 9 is modulated by the optical modulator module 8a corresponding to a high frequency signal from the signal source 4 and outputted as signal light 11. The optical signal current outputted from the optical modulator module 8a is subjected to comparison together with the original signal generated from the signal source 4, and a distortion component caused by the non-linearity of the optical modulator is detected as a difference. By inverting this distortion component, superimposing the component onto the original signal and impressing it to the optical modulator module 8a, the non-linearity of the optical modulator module 8a is canceled and the transmission of optical signal, so that the distortion-corrective signal can be transmitted. Thus, in the optical transmitter for CATV necessitating low distortion characteristics, the number of parts provided to the optical fiber can be reduced and the transmitter can be made low in cost and small in size.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter, and more particularly to an optical transmitter capable of reducing the number of parts with an optical fiber to reduce the cost and the size.

[0002]

2. Description of the Related Art Recently, an existing CAT using a coaxial cable has been installed.
In place of the V system, a system using an optical fiber in order to extend the transmission distance and increase the number of channels is rapidly spreading. In such a CATV system, an analog modulation (VSB-AM) method is mainly adopted, and an optical transmitter is required to have low distortion in order to suppress intermodulation distortion.

In order to realize the above requirements, a light emitting element used in an optical transmitter for analog transmission such as CATV is an element which faithfully outputs modulated light corresponding to an input modulated signal, that is, an input electric signal. A light emitting element having a good linearity of an output optical signal is applied.

As a light emitting element, a low distortion DFB laser diode module is generally used. On the other hand, since these low distortion semiconductor lasers are generally expensive, they have a distortion characteristic. A device design is widely used in which a relatively inexpensive semiconductor laser is used as a light source instead of being slightly inferior, and electric signal processing is applied to compensate for insufficient element linearity and correct distortion.

Here, a conventional optical transmitter using distortion correction will be described with reference to FIG. 3 is a diagram showing an example of a conventional optical transmitter of this type.

A conventional optical transmitter 1c using distortion correction is
As shown in FIG. 3, a signal source 4, a semiconductor laser module 9, an optical directional coupler 13, an optical fiber 2, a monitor light receiving module 12, an amplifier 6, a comparison operational amplifier 7, an adder 1
0, a signal line 3, and a delay line 5. The arrow shown in the signal line 3 in FIG. 3 indicates the transmission direction of the electric signal.

In the conventional optical transmitter 1c, the semiconductor laser module 9 is modulated by the high frequency signal generated from the signal source 4 and emits the signal light 11. Then, the signal light 1 output from the semiconductor laser module 9
1 is branched by the optical directional coupler 13, part of which is guided to the monitor light receiving module 12, and the rest is transmitted through the optical fiber 2.

The optical signal current generated from the monitor light receiving module 12 due to the input of a part of the signal light 11 is amplified by the amplifier 6, and then is amplified by the amplifier 6 and then the original signal generated from the signal source 4 and the comparison operational amplifier. 7, the distortion component of the semiconductor laser module 9 is detected as a difference as a result of the comparison operation. This difference, that is, the distortion component is inverted, fed back and superimposed on the original signal, and then the semiconductor laser module
When applied to the laser 9, the non-linearity of the semiconductor laser module 9 is canceled out, and as a result, distortion is corrected,
It becomes possible to transmit an optical signal with low distortion. The delay line 5 is
This is to match the phases of two signals that are compared and calculated.

[0009]

In the case of the above-mentioned conventional optical transmitter 1c, the "semiconductor laser module 9, the optical directional coupler 13, the monitor light receiving module 12", and the light constituting the apparatus are used. The number of parts with fibers increases to three, which has a drawback that it is difficult to reduce the size and cost of the device.

The present invention has been made in view of the above-mentioned drawbacks, and its technical problem (objective) is that CAT
An optical transmission device for which a low distortion characteristic is required, such as V type, is to provide an optical transmission device in which the number of parts with optical fibers is reduced, and the cost and size are reduced.

[0011]

An optical transmitter according to the present invention uses an electroabsorption-type semiconductor optical modulator particularly for light modulation,
The optical signal current output from the semiconductor optical modulator is fed back to the original signal to correct the distortion characteristic of the semiconductor optical modulator, and thus the optical transmitter of the above-mentioned object is provided. It is provided.

That is, according to the present invention, "a semiconductor optical modulator having a light absorption rate with respect to input light that changes according to an applied modulation signal, and the semiconductor optical modulator according to the amount of light input to the semiconductor optical modulator is provided. An optical transmitter, wherein an output of a semiconductor optical modulator is controlled by feeding back a difference obtained as a result of comparing an optical signal current output from a modulator and the modulated signal to the modulated signal. Is the gist.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In the optical transmitter according to the present invention, a semiconductor optical modulator whose optical absorption coefficient with respect to input light changes according to an applied modulation signal is referred to as "electroabsorption type semiconductor optical modulator". Module with output optical fiber
Externally modulated optical modulator module "is preferred, and the use of this optical modulator module is a preferred embodiment of the present invention.

As the semiconductor optical modulator, an "optical source integrated type optical modulator module in which an electroabsorption type optical modulator and a semiconductor laser serving as a light source are integrated on one semiconductor substrate. The use of “le” is a more preferred embodiment of the present invention.

[0015]

EXAMPLES The present invention will now be specifically described with reference to examples of the optical transmitter according to the present invention, but the present invention is not limited to the following examples.

(Embodiment 1) FIG. 1 shows an embodiment of the present invention.
It is a figure which shows the optical transmitter which is (Example 1). As shown in FIG. 1, the optical transmitter 1a according to the first embodiment includes a signal source 4,
Semiconductor laser module 9, optical fiber 2, amplifier 6,
Comparative operational amplifier 7, adder 10, signal line 3, delay line 5,
It is composed of an optical modulator module 8a.

In the first embodiment, the semiconductor laser module 9 steadily emits the DC light 14 without being modulated, and the DC light 14 is sent to the optical modulator module 8a by the optical fiber 2. Is led. The optical modulator module 8a is an external modulation type module in which an input / output optical fiber is attached to an electroabsorption type semiconductor optical modulator to form a module, which absorbs the input light according to an applied electric signal. When the rate changes, the input light is intensity-modulated and output.

In the first embodiment, the DC light 14 is modulated according to the high frequency signal generated from the signal source 4 and output as the signal light 11. Further, the electro-absorption type semiconductor optical modulator converts the absorbed light as a photocurrent and outputs the photocurrent, and thus functions as an optical receiver.

In the first embodiment, the mechanism other than that described above is almost the same as that of the conventional optical transmitter (see FIG. 3 above).

That is, the optical signal current output from the optical modulator module 8a is amplified by the amplifier 6 via the signal line 3 and then input to the comparison operational amplifier 7 together with the original signal generated from the signal source 4. As a result of the comparison calculation, a distortion component caused by the nonlinearity of the optical modulator is detected as a difference. When this difference is inverted, fed back and superimposed on the original signal, and applied to the optical modulator module 8a, the nonlinearity of the optical modulator module 8a is canceled out, and the optical signal whose distortion is corrected is transmitted. Is possible.

In such a low-distortion optical transmitter, a semiconductor optical modulator (optical modulator module 8a) is used, and the optical signal current output from the semiconductor optical modulator is used for distortion correction, thereby The number of fiber-attached parts can be reduced from the conventional three to two, and there is an advantage that the device can be downsized.

(Embodiment 2) FIG. 2 shows another embodiment of the present invention.
It is a figure which shows the optical transmitter which is (Example 2). In the second embodiment, instead of the semiconductor laser module 9 and the optical modulator module 8a of the first embodiment, an optical modulator module 8b is applied, and the functions and configurations of the respective parts are the same as described above. This is the same as in the first embodiment.

Optical modulator module 8 in the second embodiment
Reference numeral b is a light source integrated type in which an electro-absorption optical modulator and a semiconductor laser serving as a light source are integrated on one semiconductor substrate, and the present invention is used as a module in the present invention. By applying it, there is an advantage that the number of parts with an optical fiber forming the optical transmission device can be further reduced and the size can be further reduced as compared with the first embodiment.

[0024]

As described in detail above, the present invention uses an electroabsorption type semiconductor optical modulator for the modulation of light, and the optical signal current output from this semiconductor optical modulator is used as the original signal. By correcting the distortion characteristic of the semiconductor optical modulator by performing feedback, the number of parts with an optical fiber can be reduced in an optical transmitter such as a CATV that requires a low distortion characteristic, and the cost and size can be reduced. The effect of being able to measure is produced.

[Brief description of drawings]

FIG. 1 is a diagram showing an optical transmitter which is an embodiment (Embodiment 1) of the present invention.

FIG. 2 is a diagram showing an optical transmitter according to another embodiment (second embodiment) of the present invention.

FIG. 3 is a diagram showing an example of a conventional optical transmission device using distortion correction.

[Explanation of symbols]

 1a, 1b, 1c Optical transmitter 2 Optical fiber 3 Signal line 4 Signal source 5 Delay line 6 Amplifier 7 Comparison operational amplifier 8a, 8b Optical modulator module 9 Semiconductor laser module 10 Adder 11 Signal light 12 Monitor Light receiving module 13 Optical directional coupler 14 DC light

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H01S 3/103

Claims (3)

[Claims] 1. A semiconductor optical modulator having a light absorptance for input light that changes according to an applied modulation signal, and the semiconductor optical modulator outputs the semiconductor optical modulator according to the amount of input light to the semiconductor optical modulator. An optical transmitter, wherein the output of the semiconductor optical modulator is controlled by feeding back the difference obtained as a result of comparing the optical signal current and the modulated signal to the modulated signal. 2. An external-modulation-type optical modulator module in which an input / output optical fiber is attached to an electro-absorption semiconductor optical modulator to form a module as the semiconductor optical modulator. Item 2. The optical transmitter according to item 1. 3. The light modulator module of the light source integrated type as the semiconductor light modulator, wherein an electro-absorption light modulator and a semiconductor laser serving as a light source are integrated on a single semiconductor substrate. The optical transmission device according to claim 1, wherein the optical transmission device is configured to use a module.