JP2000354010A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical transmitter which can highly stably perform optical communication by using homodyne transmission method, with a simple and inexpensive constitution. SOLUTION: An optical transmitter first sends a laser light emitted from a laser light source 1 to a polarizer/separator 2. The polarizer/separator 2 separates the laser light into two different polarized waves and sends one of the polarized waves to a modulator 4. The modulator 4 modulates the received polarized waves into modulated waves, by using base-band signals S0 and sends the modulated waves to a synthesizer 5. The polarizer/separator 2 also sends the other polarized waves to the synthesizer 5 as they are, as a reference carrier wave. The combining unit 5 combines the modulated waves with a reference carrier wave and transmits the combined waves to a reception side through an optical fiber 6. On the reception side, a polarizer/separator 7 first separates the combined waves into the modulated waves and reference carrier wave and sends the separated waves to a synchronization detector 8. The detector 8 demodulates the modulated waves to the base-band signals S0 by performing homodyne detection on the modulated waves, based on the reference carrier wave.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission apparatus for modulating laser light with a baseband signal, transmitting the modulated light using an optical fiber, and reproducing and demodulating a baseband signal from the transmitted modulated light. Things.

[0002]

2. Description of the Related Art The mainstream of a transmission system in a conventional optical transmission apparatus is IM-DD (Intensity Modula).
In the IM-DD system, an optical signal intensity-modulated with a baseband signal is transmitted, and the baseband signal is reproduced and demodulated again by direct detection (intensity detection). In this method, information is added to a change in light intensity.

Further, there is a coherent system capable of improving the optical receiving sensitivity by 10 to 25 dB as compared with the IM-DD system. This coherent method is an important technology when constructing an optical network using a wide frequency band of light.

FIG. 3 is a diagram showing the configuration of a conventional coherent optical transmission device. In a coherent optical transmission device, as shown in FIG.
One signal (laser light) is modulated by the modulator 103 by the baseband signal S0 from the input terminal 102 and transmitted by the optical fiber 104. On the receiving side, when the received light (optical received data) is received by the synchronous detector 105 and synchronously detected and demodulated, a heterodyne method and a homodyne method are used.
In the heterodyne method, the received light is
In the homodyne method, the local oscillation light from the local oscillation laser 106 is always mixed with the received light to be mixed with the local oscillation light from the laser and converted to an intermediate frequency and then demodulated to the baseband signal S0. And the baseband signal S
The signal is demodulated to 0.

[0005]

However, in the above-mentioned conventional optical transmission device using the coherent method, in any case,
Laser 10 that is more stable for transmission and reception than M-DD system
In addition to the necessity of 1 and 106, the receiving side is required to perform laser synchronization by the synchronization circuit 107. This made the system complicated and expensive in performing the coherent method.

[0006] The present invention has been proposed in view of the above,
It is an object of the present invention to provide an optical transmission device that can be constructed simply, inexpensively, and stably.

[0007]

In order to achieve the above object, an optical transmission apparatus according to the present invention modulates a laser beam with a baseband signal, transmits the modulated beam using an optical fiber, and transmits the modulated beam. In an optical transmission device that reproduces and demodulates a baseband signal from a modulated light, a laser beam from a laser light source is separated into two different polarizations, and one of the polarizations is modulated using a baseband signal to produce a modulated wave, The other polarization is used as it is as a reference carrier, and after combining the two, the combined wave is sent to the receiving side via an optical fiber, and separated by the receiving side into a modulated wave and a reference carrier again. Is demodulated into a baseband signal by homodyne detection.

[0008]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a diagram showing a configuration of an optical transmission device according to the present invention. An optical transmission device according to the present invention is a device that modulates a laser beam with a baseband signal S0, transmits the modulated beam using an optical fiber 7, and reproduces and demodulates the baseband signal S0 from the transmitted modulated beam. First, the laser light source 1
Is transmitted to the polarization splitter 2. The polarization separator 2 separates the laser light into two different polarizations,
One polarized wave is transmitted to the modulator 4. In this modulator 4,
The baseband signal S0 from the input terminal 3 is applied to the received polarization.
And modulates it into a modulated wave.
To send to. Note that the modulation in this case may be any of phase modulation, frequency modulation, and amplitude modulation. In addition, the polarization separator 2
Is used as the reference carrier with the other polarization as it is,
To send to. The combiner 5 combines the modulated wave and the reference carrier, and transmits the combined wave to the receiving side via the optical fiber 6.

On the receiving side, first, the polarization separator 7 separates the combined wave into a modulated wave and a reference carrier again, and
To send to. The synchronous detector 8 performs homodyne detection of the modulated wave based on the reference carrier, thereby obtaining the baseband signal S0.
Demodulated to

FIG. 2 is a diagram showing a specific configuration example of the optical transmission device of the present invention. In this configuration example, the polarization beam splitters 21 and
71 is adopted. In addition, a polarization beam splitter or the like can be used for the synthesizer 5. Further, the synchronous detector on the receiving side performs homodyne detection to obtain a baseband signal S0. That is, by combining the modulated wave obtained by separation by the polarization beam splitter 71 and a reference carrier which is a carrier component of the modulated wave by an optical frequency synthesizer (mixer) 81, a baseband signal as a beat signal of light is converted. obtain. At this time, the reference carrier input to the optical frequency synthesizer 81 has passed through the quarter-wave plate 83 so as to have the same polarization as the modulated wave. This is received by the light receiving element 82 and converted into an electric signal to obtain a baseband S0 signal as an electric signal.

As described above, in this embodiment, the laser beam is separated into two different polarizations, and one of the polarizations is sent to the receiving side as a reference carrier, and the receiving side uses the reference carrier for homodyne detection. This eliminates the need for a highly stable laser and a circuit for synchronizing the received light, which were required in the past.Thus, optical communication using the homodyne transmission method can be performed in a simple and inexpensive configuration with high stability. Can be done.

Further, since the laser light is separated into two different polarized waves and transmitted simultaneously by the same fiber, the influence of the laser on the transmission side and the influence of noise and the like received during transmission become common, and therefore, the influence on the reception side is increased. These effects can be canceled at the time of synchronous detection, and detection (demodulation) can be performed with high accuracy. In general, there is a difference in the amount of propagation delay of each polarization due to the polarization dispersion of the fiber. However, since this amount of propagation delay does not change in a short time, this device can treat it as a fixed amount and does not cause a communication problem.

[0014]

As described above, according to the optical transmission apparatus of the present invention, a laser beam is separated into two different polarizations, and one of the polarizations is sent to a receiving side as a reference carrier.
On the receiving side, homodyne detection is performed with the reference carrier wave, eliminating the need for a highly stable laser and a circuit for synchronizing the received light, which were required in the past. It can be performed with high stability under an inexpensive configuration.

Further, since the laser light is separated into two different polarized waves and transmitted simultaneously through the same fiber, the influence of the laser on the transmission side and the influence of noise and the like received during transmission become common, and therefore, the influence on the reception side is increased. These effects can be canceled at the time of synchronous detection, and detection can be performed with high accuracy.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an optical transmission device of the present invention.

FIG. 2 is a diagram showing a specific configuration example of an optical transmission device of the present invention.

FIG. 3 is a diagram showing a configuration of a conventional coherent optical transmission device.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 laser light source 2 polarization splitter 3 input terminal 4 modulator 5 combiner 6 optical fiber 7 polarization splitter 8 synchronous detector 9 amplification filter 21 polarization beam splitter 71 polarization beam splitter 81 optical frequency synthesizer 82 light receiving element S0 base Band signal

Claims (1)

[Claims] 1. A method for modulating a laser beam with a baseband signal,
In an optical transmission device that transmits the modulated light using an optical fiber and regenerates and demodulates a baseband signal from the transmitted modulated light, a laser beam from a laser light source is separated into two different polarizations, and one of the polarizations is separated. The wave is modulated using a baseband signal to form a modulated wave, the other polarization is used as it is as a reference carrier, and the two are combined.Then, the combined wave is sent to the receiving side via an optical fiber, and the modulated wave is again formed on the receiving side. By separating into a reference carrier and homodyne detection of the modulated wave based on the reference carrier,
An optical transmission device for demodulating into a baseband signal.