JPH0653904A - Optical transmitter - Google Patents

Abstract

PURPOSE:To obtain an optical transmitter in which the deterioration of a transmission due to the wavelength chirp of a semiconductor laser can be reduced without making a band higher. CONSTITUTION:This device is equipped with an exclusive OR circuit 103 which performs the exclusive OR operation of input data and data obtained by delaying the input data in a prescribed time by a delay element 102 instead of NRZ/RZ converting the input data. The output of the exclusive OR circuit 103 is inputted to a semiconductor laser driving circuit 104, and the optical output of a semiconductor laser 105 is intensity-modulated by a optical interference device 106 in which the optical path difference of two optical paths is set to be equal to the delay time slot of the delay element 102, and transmitted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter used for optical communication.

[0002]

2. Description of the Related Art At present, in general optical communication, optical signals are transmitted by direct intensity modulation of a semiconductor laser which is a light source. However, when the transmission speed becomes as high as several gigabits per second, in the case of direct intensity modulation, the oscillation wavelength chirp at the time of modulation becomes a problem. When an optical pulse having a wavelength chirp is transmitted for a long distance in an optical fiber having chromatic dispersion, waveform distortion occurs and the transmission characteristics deteriorate.

This problem can be avoided by causing the semiconductor laser to emit direct current and converting an electric signal into an optical signal by using an external modulator, but there is a problem that a large power is required for the modulator.

On the other hand, as shown in FIG. 3, there is also a modulation method in which the phase of an oscillating light wave is modulated by changing the injection current to the semiconductor laser and is converted into a change in light intensity by self-homodyne shifted by 1 time slot. Being tried. FIG. 3 is a diagram showing a configuration of an optical transmitter according to this method,
The data input from the data input unit 301 is converted into an RZ format signal by the NRZ / RZ conversion circuit 302 and input to the laser drive circuit 303. The drive signal output from the laser drive circuit 303 drives the semiconductor laser 304,
An optical signal in which the transmission wavelength is modulated is output. This optical signal is converted into an optical signal whose optical intensity is modulated by a Mach-Zehnder interferometer whose two optical path differences are one time slot.

However, in the case of this configuration, it is necessary to perform NRZ / RZ conversion of the electric signal once. This NRZ / RZ conversion requires a band twice as large as that of a normal signal processing circuit, and thus there is a problem that it becomes difficult to realize as the transmission speed increases.

[0006]

As described above, the conventional optical transmitter has a problem that when the transmission speed is said to be ultra-high speed, which is several gigabits per second, the transmission deterioration is caused by the wavelength chirp of the semiconductor laser. It was In addition, there is a problem in that the modulation method for converting into light intensity change by self-homodyne is required to once perform NRZ / RZ conversion of an electric signal as input data.

The present invention has been made in order to solve the above problems. Even when the transmission speed is several gigabits per second, the transmission is performed by the wavelength chirp of the semiconductor laser with a simple circuit configuration without increasing the band. It is an object to provide an optical transmitter without deterioration.

[0008]

In order to achieve the above object, the present invention provides an exclusive OR circuit in which an electric signal is inputted to one input terminal and an output of the exclusive OR circuit is an integral multiple of one time slot. A delay element that is delayed and input to the other input terminal of the exclusive OR circuit, a photoelectric conversion element that converts the output from the exclusive OR circuit into an optical signal, and an optical path corresponding to the delay time in the delay element. It is composed of an optical interferometer having a difference and performing intensity modulation of the optical signal from the photoelectric conversion element.

[0009]

[Operation] The optical transmitter having the above-mentioned configuration transmits the input data to the NRZ /
Instead of RZ conversion, the exclusive OR of the input data and the data obtained by delaying the input data by the delay element for a predetermined time is taken, and the exclusive OR output is used as the input signal of the semiconductor laser drive circuit, and the optical path difference between the two optical paths is taken. Is an optical interferometer set to be equal to the delay time slot of the delay element and intensity-modulates the optical output of the semiconductor laser for transmission.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of an embodiment of an optical transmitter according to the present invention.

In FIG. 1, 103 is an exclusive OR circuit and 102
Is an n-bit delay circuit, and 104 and 105 are a laser drive circuit and a semiconductor laser, respectively. Reference numeral 106 is an optical interferometer, for example, a Mach-Zehnder interferometer.

In the optical transmitter configured as described above, the data input from the data input section 101 is input to one input terminal of the exclusive OR circuit 103. A part of the output of the exclusive OR circuit 103 is input to the delay circuit 102, delayed by n time slots of the transmission speed of the input data, and then input again to the input terminal of the other step of the exclusive OR circuit 103. . Another part of the output of the exclusive OR circuit 103 is input to the laser drive circuit 104, and the semiconductor laser 1
05 is driven to convert an electric signal into an optical signal whose transmission wavelength is modulated. The optical signal output from the semiconductor laser 105 is output as an optical signal whose optical path difference between the two optical paths is optically signal-processed by the Mach-Zehnder type optical interferometer 106 which is the n time slot. In the above configuration, the light intensity when two light waves having angular frequencies ω ₀ and ω ₁ and a phase difference δ interfere with each other is cos ² It is proportional to (((ω ₀ −ω ₁ ) t + δ) / 2). Here, δ = (1 + 2m) π, (however,
m is an integer) Furthermore, (ω ₀ −ω ₁ ) t = 2π (where
If t is the time of one time slot), then ω ₀ =
In each case of ω ₁ and ω ₀ ≠ ω ₁ , the relative phase difference and the interference light intensity are as shown in FIGS. 2 (a) and 2 (b). As can be seen from FIG. 2, if ω ₀ is a logical “0” and ω ₁ is a logical “1”, the RZ-type exclusive logic is obtained. Also, the data in time slot i is D
i, if the output of the exclusive OR circuit 103 is Qi, then Qi = Di << + >> Qi-n. However, << + >> shows an exclusive OR operation. At this time, the output of the semiconductor laser 105 is the exclusive OR circuit 10
Since the output of 3 is converted, it is logically expressed by the same formula. On the other hand, since the optical interferometer 106 performs exclusive logic with the signal n time slots before, if the optical signal is Oi, then Oi = Qi << + >> Qi-n = Di << + Qi-n << + >> Qi- n = Di. This means that the optical signal output from the optical signal output unit 107 is logically equal to the input data. That is, instead of NRZ / RZ conversion of the input data, the exclusive OR of the input data and the data obtained by delaying this input data for a predetermined time with a delay element is taken, and the exclusive OR output is used as the input signal of the semiconductor laser drive circuit. By setting the optical path difference between the two optical paths of the optical interferometer to be equal to the delay time slot of the delay element, it can be seen that it becomes equivalent to the case of NRZ / RZ conversion of input data.

Therefore, in the present invention, since it is not necessary to perform NRZ / RZ conversion of an electric signal, the band is not increased and the transmission deterioration due to the wavelength chirp of the semiconductor laser can be reduced.

[0014]

As described above, according to the optical transmitter of the present invention, it is possible to reduce the transmission deterioration due to the wavelength chirp of the semiconductor laser without increasing the band.

[Brief description of drawings]

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

FIG. 2 is a diagram for explaining the principle of the present invention.

FIG. 3 is a diagram showing a configuration of a conventional optical transmitter.

[Explanation of symbols]

Reference numeral 101 ... Data input section, 102 ... N-bit delay circuit, 103 ... Exclusive OR circuit, 104 ... Laser drive circuit, 105 ... Semiconductor laser, 106 ... Optical interferometer, 1
07 ... Optical signal output section

Claims (1)

[Claims] 1. An exclusive OR circuit in which an electric signal is input to one input terminal, and an output of the exclusive OR circuit is delayed by an integral multiple of one time slot and is input to the other input terminal of the exclusive OR circuit. A delay element to be input, a photoelectric conversion element that converts the output from the exclusive OR circuit into an optical signal, and an optical path difference corresponding to the delay time in the delay element, and the intensity of the optical signal from the photoelectric conversion element. An optical transmitter comprising an optical interferometer for performing modulation.