JPH047618B2 - - Google Patents

Description

Detailed Description of the Invention [Technical Field to which the Invention Pertains] The present invention is directed to a transmitting device in which a plurality of electrical pulse signals are input, multiplexed optical pulse signals are transmitted to an optical fiber, and a receiving device receives a plurality of electrical pulse signals. This invention relates to an optical multiplex transmission system that converts electrical pulse signals into electrical pulse signals. In particular, a semiconductor laser is used as a transmitter, and the output light wavelength changes depending on the level of the electrical signal injected by the semiconductor laser.Using this characteristic and the dispersion characteristics of the optical fiber, The present invention relates to a communication system that obtains so-called chirp pulses at the receiving output end of an optical fiber.

[Description of prior art]

A plurality of semiconductor lasers are provided corresponding to a plurality of input electric pulse signals, and each semiconductor laser is driven by slightly different phases of each input electric pulse signal,
When the output lights of each semiconductor laser are combined into one by a multiplexer, it is possible to obtain an optical pulse signal in which information of the plurality of input electric pulse signals is arranged in time series. This optical pulse signal is transmitted through a single optical fiber, and the receiving device can restore output electrical pulse signals corresponding to the plurality of input electrical pulse signals from the optical pulse signals arranged in time series. Even if the width of the optical pulse signal is narrowed in such a device, there is always a tail at the rise and fall of the optical pulse, and there is a limit to the communication speed when it becomes impossible to distinguish between adjacent pulses.

On the other hand, when a semiconductor laser is driven with an electric pulse signal, many longitudinal mode oscillations occur. To suppress this, the semiconductor laser is distributed feedback type (DFB).
Alternatively, a distributed reflection type (DRB) technology has been developed. Although this technique suppresses many longitudinal mode oscillations, such semiconductor lasers have a characteristic that the output light wavelength changes depending on the magnitude of the injected current.
It is known that when such output light is transmitted through an optical fiber having dispersion characteristics, the signal waveform changes because the transmission speed differs depending on the wavelength. This phenomenon is described in detail in the following literature.

(1) K.Kishino etal: IEEE, JQE Vol 18 No.3
pp. 343-351 March 1982 (2) K. Iwashita etal: Chirp Pulse
Transmission Through Single Mode-
Fiber, Electronics Letters Vol 18 No.20
Pages 873-874 September 30, 1982 [Object of the Invention] The present invention makes active use of this phenomenon to narrow the pulse width of the received optical pulse and improve the multiplicity or The purpose is to improve communication speed.

[Features of the invention]

The first aspect of the present invention is to sample input electrical pulse signals using timing signals having different phases, and each semiconductor laser has an output wavelength that changes depending on the level of the injected electrical signal, and its characteristics are An optical fiber with a negative chromatic dispersion coefficient is used when the wavelength of the output light changes to the shorter wavelength side when the level of the electrical signal injected by the semiconductor laser increases. An optical fiber having a positive chromatic dispersion coefficient is used when the wavelength of the output light changes to the longer wavelength side when the level of the electrical signal injected by the semiconductor laser increases. do.

The second invention of the present invention is to sample an input electric pulse signal with a timing signal having the same phase, and each semiconductor laser has an output wavelength that changes depending on the level of the injected electric signal. The optical fiber has a negative chromatic dispersion coefficient when the wavelength of the output light changes to the shorter wavelength side when the level of the electrical signal injected by the semiconductor laser increases. An optical fiber that uses an optical fiber and has a positive chromatic dispersion coefficient when the wavelength of the output light changes toward longer wavelengths when the level of the electrical signal injected by the semiconductor laser increases. It is characterized by using.

[Explanation based on examples]

FIG. 1 is a block diagram of an embodiment of the present invention. In order to make the explanation easier to understand, an example will be described in which three input electric pulse signals are multiplexed and transmitted.

Three input electrical pulse signals are input from input terminal _1-1 to
_1-3 is supplied. This input electrical pulse signal is
Each of the signals is sampled by sampling circuits _2-1 to _2-3 . The sampling circuits _2-1 to _2-3 are sequentially supplied with timing signals having slightly different phases from the timing signal generation circuit 3. The outputs of the sampling circuits _2-1 to _2-3 are applied as modulation inputs to semiconductor lasers _4-1 to _4-3 , respectively. Each semiconductor laser _4-1 to _4-3 sends out an optical pulse signal in response to this modulation input.

This optical pulse signal is multiplexed by a multiplexer 5, inputted into an optical fiber 7, and transmitted. The optical pulse signal appearing at the receiving end output of the optical fiber 7 is converted into an electric pulse signal by a photoelectric converter 8, separated by a separation circuit 10, and sent to three output terminals 11-1 _{to 11-1} .
Sent on _11-3 .

FIG. 2 is a diagram showing an example of the configuration of the multiplexer 5. As shown in FIG. This multiplexer 5 is a device for simply combining and inputting three incident lights into an optical fiber 7 using two half mirrors 13 and 14.

FIG. 3 is a diagram showing an example of the configuration of the separation circuit 10. Separate the frame synchronization signal from the input signal,
A common control circuit generates timing signals. The input signal is divided into three signals by the switching circuit 16 according to this timing signal. Switching circuit 1
Each output signal of 6 is sent to a speed conversion circuit _17-1 .
The speed is converted by ~ _17-3 and the terminal 11-1 ~ ₁
Sent on _1-3 .

Here, the semiconductor lasers _4-1 to _4-3 are semiconductor lasers that generate so-called chirp pulses in which the generation of unnecessary longitudinal modes is suppressed and the output light wavelength changes in accordance with changes in the injected current. here,
An AlGaAs-based DFB (distributed feedback) laser was used. This semiconductor laser has a characteristic that when the injection current increases, the output light wavelength changes slightly to the shorter wavelength side. Therefore, at the rising edge of the output optical pulse, the output optical wavelength gradually becomes shorter and reaches the peak value, and at the falling edge, it gradually becomes longer from the peak value.

The optical fiber 7 used had a negative wavelength dispersion coefficient. As mentioned above, when an optical pulse whose output light wavelength changes at the rise and fall is passed through such an optical fiber, the propagation speed of the shorter wavelength portion is greater than the propagation speed of the longer wavelength portion within the fiber. The optical pulse that appears at the receiving output end of the device has a property that the peak portion is shifted toward the rising edge, resulting in a pulse with a sharp waveform.

The apparatus configured in this way will be explained with reference to the time chart shown in FIG. 4A to 4I are waveform diagrams of points a to i shown in FIG. 1. In this waveform diagram, in order to make the principle easier to understand, the pulse signal is drawn in the form of a rectangular wave with no roundness.

The output lights of the semiconductor lasers _4-1 to _4-3 are generated with their phases slightly shifted and partially overlapped, as shown in FIG. 4a, b, and c. Therefore, when multiplexed by multiplexer 5, the output light is the fourth
The waveform becomes a multivalued pulse as shown in Figure d. When this optical pulse signal propagates through the optical fiber 7, the peak portion of the output light from each of the semiconductor lasers _4-1 to _4-3 moves forward to form a sharp waveform as described above, and at the receiving output end, the waveform shown in FIG. The result is a separate and distinguishable waveform as shown in .

By converting this into an electric signal using the photoelectric converter 9, the signal shown in FIG. i is restored.

The input electrical pulse signals may each have a frame configuration. That is, one frame consists of a frame identification code of a small number of bits, a control code for that frame following that, and a large number of information bits following that, and the receiving device identifies this frame and synchronizes it. , it is better to correctly reproduce the received code.

As mentioned above, when chirped pulses are used, even if the pulses are brought closer together beyond the limit of conventional equipment, which would have made them indistinguishable to the transmitting device, the pulses gradually become sharper and separated as they propagate within the optical fiber, allowing the light to be separated. At the output end of the fiber, its separation is sufficiently possible. Therefore, signals can be transmitted at a higher transmission rate than in the conventional method, or the degree of multiplexing can be increased.

FIG. 5 is a block diagram of an embodiment of the second invention of the present invention. In this method, three sampling circuits _2-1 ~
_2-3 is characterized in that it is sampled using clock signals of the same phase. Furthermore, the three semiconductor lasers _4-1 to _4-3 are characterized in that they each have slightly different wavelengths of output light. This semiconductor laser also has the characteristics that unnecessary longitudinal modes are suppressed and the output light wavelength changes according to the magnitude of the injected current. In this example,
Semiconductor laser _4-1 has the shortest output wavelength, and semiconductor laser _4-3 has the longest output wavelength.

The structure of the multiplexer 5 and the structure of the separation circuit 10 are similar to those of the embodiment of the first invention described above.

The operation of the apparatus configured as described above will be explained using the time chart shown in FIG. Figure 6 a~
i is a waveform diagram of points a to i shown in FIG. Since the sampling circuits _2-1 to _2-3 perform sampling in synchronization with the same timing signal, each semiconductor laser 4-
The output lights ₁ to _4-3 are generated at the same time as shown in FIG. 6a, b, and c. Therefore, the combined signal has a high level as shown in FIG. 6d.

However, as mentioned above, the semiconductor laser _4-1 is arranged so that the output light wavelength becomes longer in the order of signals a, b, and c.
Since the characteristics of ~ _4-3 are selected, when this optical pulse signal propagates through the optical fiber 7 whose chromatic dispersion coefficient is negative, the propagation speed becomes faster as the wavelength becomes shorter. The pulse signals gradually separate. Moreover, as mentioned above, each of the semiconductor lasers _4-1 to _4-3 has a property that the output light wavelength becomes slightly shorter as the injected power increases, so when the laser beams are propagated through the optical fiber 7 having a negative wavelength dispersion coefficient,
The peak portion of the optical pulse signal approaches the rising portion, and the rising waveform becomes sharp. Therefore, at the reception output end of the optical fiber 7, it becomes possible to separate and identify the three optical pulse signals, as shown in FIG. 6e.

This optical pulse signal is converted by the photoelectric converter 9 into an electric signal f, which is further converted into three electric pulse signals g to i by the separation circuit 10.

In this way, when optical pulses of different wavelengths are propagated through an optical fiber with chromatic dispersion, even if the transmitting device transmits simultaneously, it becomes possible for the receiving device to temporally distinguish between the two. That is, it becomes possible to multiplex signals that could not be identified with conventional devices, and the efficiency of using the transmission path can be increased.

The wavelength difference Δλ between adjacent optical pulse signals is Δλ≧1/n・f ₀ m・L where n is the number of semiconductor lasers, f ₀ is the bitrate, m is the chromatic dispersion coefficient, and L is the optical fiber length. be.

In each of the above examples, the filter used for the path of the optical pulse signal is omitted, but if a filter such as a multilayer film filter or an interference filter is used to remove unnecessary waves or select the optical pulse signal of each wavelength, signal noise The ratio can be further improved.

Next, test results of the method according to the present invention will be shown.

FIG. 7 and FIG. 8 are diagrams showing the measurement results. Here, a distributed feedback semiconductor laser with an output wavelength of 1.54 μm was used. In this semiconductor laser, the wavelength changes by 0.7 nm from the rise of the optical pulse to the peak value depending on the injected current. The optical fiber used was a silica-based single mode optical fiber with a core diameter of 10 μm and a refractive index difference of 0.25% in this wavelength range. The wavelength dispersion coefficient is -15 ps/Km·nm.

FIG. 7 shows the measurement results of wavelength changes when driving a semiconductor laser. FIG. 7B is a waveform diagram showing the total optical pulse signal on the time axis, and is a diagram showing the waveform taken with an oscilloscope rotated by 90 degrees.
This time axis is 500 ps/div. FIG. 7A is a spectrum distribution diagram in which wavelength components are detected by a spectrum analyzer at six time points indicated by arrows along this time axis. Figure 7 A-6
Each of the waveforms indicates the intensity of the wavelength component for the wavelength shown on the horizontal axis. Furthermore, FIG. 7C is a spectrum distribution diagram in which wavelength components from the rise to the fall of the total optical pulse signal are observed. 7th
From FIG. A, it can be seen that the wavelength of the optical pulse moves toward shorter wavelengths as time passes, that is, as the injected current changes.

FIG. 8 shows pulse width versus transmitted fiber length. 1.7nS pulse width at transmission input end is 104Km
It can be seen that the width narrowed down to 0.35 nS after passing through the optical fiber. Therefore, by combining four optical signals modulated at 400 Mb/s,
It is possible to extract an output waveform of 1.6 Gb/s.

In the above example, the semiconductor laser 12 has the characteristic that the wavelength of its output light changes to the shorter wavelength side when the injection current increases, but conversely, when the injection current increases, the wavelength of the output light changes to the longer wavelength side. If the optical fiber has a positive wavelength dispersion coefficient as the transmission line, the propagation time of the long wavelength component will be shortened. The components move toward the front, resulting in a sharp pulse signal waveform at the output end of the optical fiber.

[Explanation of effects]

As explained above, since multiplexing is performed using chirp pulses, the circuit does not require high-speed operation,
In addition, by using an optical pulse with a wide pulse width on the transmitting side, the pulse peak value of the received waveform at the input of the discriminator becomes equivalently high, and the S/N (signal-to-noise ratio) can be improved. Furthermore, since the pulse width is narrowed on the receiving side, there is an advantage that multiplexed high-speed pulses can be transmitted by inserting multiple sequences of signals on the transmitting side.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the first invention of the present invention. FIG. 2 is a diagram showing an example of the configuration of a multiplexer. FIG. 3 is a diagram showing a configuration example of a separation circuit. FIG. 4 is an operational time chart of the first embodiment of the present invention. FIG. 5 is a block diagram of an embodiment of the second invention of the present invention. FIG. 6 is an operational time chart of an embodiment of the second invention. FIG. 7 is a diagram showing output light characteristics when driving a semiconductor laser. FIG. 8 is a diagram showing measurement results of pulse width with respect to optical fiber transmission distance.

Claims (1)

[Claims] 1. A transmitting device that transmits an output optical pulse signal in which information of a plurality of input electric pulse signals is multiplexed, an optical fiber that transmits this optical pulse signal, and an optical fiber that is transmitted to this optical fiber. a receiving device that receives a pulse signal as input and obtains an output electrical pulse signal corresponding to the plurality of input electrical pulse signals; the transmitting device includes a means for sampling the plurality of input electrical pulse signals; and an output of the means. In an optical multiplex transmission system comprising a plurality of semiconductor lasers, each of which is intensity-modulated by an electric pulse signal obtained from the sample, The sampling means is configured to sample the plurality of input electrical pulse signals using timing signals having equal frequencies and different phases, and the plurality of semiconductor lasers have output optical wavelengths that vary depending on the level of the injected electrical signals. The optical fiber used is one that changes and has almost the same characteristics, and the optical fiber has a characteristic that the wavelength of the output light changes to the shorter wavelength side when the level of the electrical signal injected by the semiconductor laser increases. In some cases, an optical fiber with a negative chromatic dispersion coefficient is used, and in other cases, an optical fiber with a positive chromatic dispersion coefficient is used, in which the wavelength of the output light changes toward longer wavelengths when the level of the electrical signal injected by the semiconductor laser increases. An optical multiplex transmission system characterized in that optical fibers having coefficients are used. 2. A transmitting device that transmits an output optical pulse signal in which information of a plurality of input electric pulse signals is multiplexed, an optical fiber that transmits this optical pulse signal, and an optical pulse signal transmitted to this optical fiber as an input and the above-mentioned. a receiving device that obtains output electrical pulse signals corresponding to a plurality of input electrical pulse signals; the transmitting device includes means for sampling the plurality of input electrical pulse signals; and an electrical pulse signal obtained as an output of the means. In an optical multiplex transmission system comprising a plurality of semiconductor lasers, each of which is intensity-modulated by The plurality of semiconductor lasers are configured to sample the input electric pulse signal using timing signals having the same frequency and the same phase, and each of the plurality of semiconductor lasers has an output optical wavelength that changes depending on the level of the injected electric signal. When the optical fiber has a characteristic that the wavelength of the output light changes to the shorter wavelength side when the level of the electrical signal injected by the semiconductor laser increases, An optical fiber with a negative wavelength dispersion coefficient is used, and when the wavelength of the output light changes to the longer wavelength side when the level of the electrical signal injected by the semiconductor laser increases, a positive wavelength dispersion coefficient is used. An optical multiplexing transmission system characterized in that an optical fiber having the following characteristics is used.