JPH0252536A - Digital optical transmitter - Google Patents

Abstract

PURPOSE:To reduce the coherence length of a laser diode by using a sent digital signal and driving the laser diode with a current being the result of switching a high frequency signal having a sufficiently high frequency. CONSTITUTION:A digital signal to be sent is given to a digital signal input terminal 2 of a transmitter 1 and a high frequency signal having a sufficiently higher frequency than the signal band of the digital signal to be sent is given to a high frequency signal input terminal 3, the transmitter 1 uses the digital signal to be sent to switch the high frequency signal thereby generating a laser diode(LD) drive current. When the LD 4 receives modulation with a sufficiently high frequency, the multi-longitudinal mode oscillation is attained and the width of each longitudinal mode line is increased and the luminescent spectrum is processed as the multi-mode, then the coherence length of the signal light is selected to be several m or below. Thus, the length of the single mode optical fiber 5 is selected to be nearly 10m, the production of modal noise at discontinuous points of optical connectors 8, 9 or the like on a transmission line is suppressed sufficiently lower.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a digital optical transmission device using a laser diode as a light source and a single mode optical fiber having a cutoff wavelength longer than the oscillation wavelength of the light source as a transmission line. It is related to.

[Prior art] Fig. 5 shows, for example, '5 sources and system.
ms: 8DDNM Transmission
on 1300 NM SM Fiber”
(M.

Romeiser and M, 5tern, F.O.
/LAN '87 & MFOC-WEST,
1987. 388-391) is a diagram showing a conventional digital optical transmission device shown in (pp. 388-391);
24) is an optical transmitter, (5), (6) are single mode optical fibers, (8), (9), (26), (28) are optical connectors, (27) is a multimode optical fiber, (11) )
is an optical receiver, (25) is a mode filter, and FIG. 6 is a diagram showing various characteristics of a light source for optical communication.

Next, the operation will be explained. The optical transmitter (24) switches a laser diode in response to a manually inputted digital signal and outputs a digital binary optical signal. In this embodiment, a laser diode (hereinafter abbreviated as LD) with an oscillation wavelength of 0.85 μm is considered. The optical output from the optical transmitter (24) is transmitted through a single mode optical fiber (5
), (6) and the optical connectors (8), (9) to the mode filter (25). Single mode optical fibers (5) and (6) are commonly used optical fibers capable of single mode transmission at 1.3 μm, and 0.85 μm.
LP is the basic mode for μm LD light. , resulting in r'++ two-mode transmission. Therefore, in terms of transmission characteristics, two terms, moat dispersion and modal noise, become problems.

Mode dispersion is LP. This is a phenomenon in which the pulse spreads due to the difference in group velocity between , and LP, and is 1.2 to 1.7 ns/km.
has the value of In a system of 100 Mb/s or more, the number will be affected by +n, but by installing a moat filter (25) on the surface of the optical receiver (II), the optical signal propagated by the LP and mote can be removed. to suppress pulse broadening. The optical receiver (11) has optical connectors (26), (2
8) and a multimode optical fiber (27) to receive an optical signal with suppressed pulse spread. The influence of mode dispersion is sufficiently suppressed by the mode filter (25),
The deterioration in reception sensitivity during +50Mb/S 8m single mode optical fiber transmission is less than 1 dB.

Modal noise, which is the second cause of deterioration of transmission characteristics, is noise caused by fluctuations in the amount of light coupled due to fluctuations in speckle patterns that occur at discontinuous points on the transmission path (such as optical connectors) that are shorter than the coherent length of the light source. It is. As shown in FIG. 6, the coherent length of an LD generally depends on the full width at half maximum of the emission spectrum, and is on the order of several meters to one meter.

In order to suppress the effects of modal noise, it is necessary to make the distance between the optical transmitter and a connection point such as a connector or splice on the transmission path longer than the coherent length. Therefore, for a flexible system configuration, it is necessary to select an LD with a large full width at half maximum and a small coherence length.

[Problems to be Solved by the Invention] As mentioned above, in conventional digital optical transmission devices, the LD is switched by the transmitted digital signal, so at low bit rates, the full width at half maximum of the CD emission spectrum is only the characteristic of the LD. determined by Therefore, in order to reduce modal noise, it is necessary to select an LD with a large full width at half maximum and a small coherent length.

This invention was made to solve the above-mentioned problems, and it is an object of the present invention to shorten the coherent length by increasing the full width at half maximum of the emission spectrum of an LD, and to obtain a digital optical transmission device free from the influence of modal noise. With the goal.

[Means for Solving the Problems] A digital optical transmission device according to the present invention has a bit rate f. an optical transmitter that drives a laser diode with a modulated current obtained by manually generating a digital signal of f and a pulse train having a frequency f that is sufficiently higher than that of the digital signal, and a cutoff wavelength that is longer than the oscillation wavelength of the laser diode. It is composed of a single-mode optical fiber with a single-mode optical fiber and an optical receiver.

The LD is driven by a modulated current obtained by switching a pulse train of a frequency f1 that is sufficiently higher than the digital signal to be transmitted.

[Function] The digital optical transmission device of the present invention increases the full width at half maximum of the emission spectrum of the LD by driving the LD with a modulated current obtained by switching a pulse train with a frequency f sufficiently higher than that of the digital signal to be transmitted. Shorten the coherent length and eliminate the immersion of mortal noise.

[Actual hh Example] FIG. 1 is a diagram showing an embodiment of a digital optical transmission device according to the present invention, and in the figure, (1) indicates a bit rate f. (2) is a digital signal input terminal; (2) is a digital signal input terminal;
3) is the high frequency input terminal, (4) is the LD, (5), (6
), (7) is a single mode optical fiber having a cutoff wavelength longer than the oscillation wavelength of the laser diode, (
8), (9) are optical connectors, (10) are light receiving elements, (1
1) is an optical receiver.

FIG. 2 shows a specific example of the configuration of the optical transmitter (1), which includes a first transistor (17) whose base receives the digital signal; A second transistor (18) constituting a differential pair with the first transistor, and the first and second transistors (18)
7), a transistor (19) as a first constant current source whose collector is connected to the emitter of (18), and a signal with a frequency f1 sufficiently higher than the above digital signal is input to the base, and the emitter of the first transistor (17);
The collector is connected to the above laser diode (
4) is connected to the third transistor (23), and a fourth transistor (23) forming a differential pair with the third transistor (23) is connected to the third transistor (23).
23). Note that (20) is the resistance, (2
1) is a drive current setting terminal, and (2a) and (3a) are bias setting terminals.

Furthermore, FIG. 3 is a waveform diagram of each part of the digital optical transmission device shown in FIG. 1, and (a) shows the digital signal (12).
, (b) is a high frequency signal (13), and (c) is an LD drive current waveform (14). FIG. 4 is a waveform chart of the LD emission spectrum, in which (a) shows the emission spectrum without high frequency input, and (b) shows the emission spectrum with high frequency input.

Next, the operation will be explained using the above-mentioned Fig. 3.4. A digital signal (12) to be transmitted is input to the digital signal input terminal (2) of the transmitter (1), and a frequency f1 which is sufficiently higher than the signal band of the digital signal (12) is input to the high frequency signal input terminal (3). A high frequency signal (13) having .

The transmitter (1) is a digital signal (12) that transmits a high frequency signal (
13) to generate an LD drive current (14). In general, when an LD (4) is modulated at a sufficiently high frequency of 500 MHz or higher, it has the characteristics of multi-longitudinal mode oscillation and an increase in the line width Δλ of each longitudinal mode. Fourth
The figure shows the LD when the high frequency signal (13) is input and when the high frequency signal (13) is not input in the optical transmitter (1) shown in Figure 1.
(4) shows an example of the emission spectrum of LD
As (4), an LD for compact discs with an oscillation wavelength of 0.78 μm is used. The drive current of L[) (14) when the high frequency signal (13) is not input is similar to the digital signal (12). In the emission spectrum (15) when there is no high frequency input, the main longitudinal mode oscillates at a level 10 dB higher than other longitudinal modes, and has a linewidth of almost a single longitudinal mode. In the emission spectrum (lllb) when high-frequency human power is applied, the levels of the main longitudinal mode and other longitudinal modes are close to each other, and the full width at half maximum is about IJnm. The frequency f of the high frequency signal (13) in this case
, is 800MHz. By converting the emission spectrum of the LD (4) into other modes, the coherent length of the signal light of the LD (4) can be made several meters or less. Therefore, by making the length of the single mode optical fiber (5) about 10 m, the amount of modal noise generated at discontinuous points such as optical connectors (8), (9) on the transmission path can be suppressed to a sufficiently low level.

The signal light of LD (4) is transmitted through a single mode optical fiber (
5), (6), and (7), and then photoelectrically converted by the light receiving element (10). It is assumed that the light receiving element (lO) has a sufficiently wide band and has sufficient sensitivity even at the frequency f of the high frequency signal (13). At this time, the output current of the light receiving element (10) is L
The shape is almost similar to the D drive current (14). Optical receiver (1
1) is a normal optical receiver that has only the band necessary to receive the digital signal (12), and has a light receiving element (10).
) is integrated and the output current of the original digital signal (12)
is played.

FIG. 2 shows a specific configuration example of the optical transmitter (1), in which the first transistor (17), the second transistor (18), the first constant current source transistor (19), and the resistor (20) are A current selector switch is configured to generate a current corresponding to the digital signal (12) at the collector of the first transistor (17). The third transistor (22) and the fourth transistor (23) constitute a differential pair, and perform a switching operation at a frequency fl in response to a high frequency signal (13) input to the high frequency signal input terminal (3). As a result, the signal current generated in the collector of the first transistor (17) is a pulse train of frequency f1 as a digital signal (12).
The current waveform is switched by , and the LD drive current (14) is obtained in the third transistor (22).

Note that in the above embodiment, the first constant current transistor (19) and the resistor (20) are used as the constant current source of the differential pair of the first transistor (17) and the second transistor (18). However, a configuration using only the resistor (20) may also be used, and the same effects as in the above embodiment can be achieved.

[Effects of the Invention] As described above, according to the present invention, by driving the LD with a current obtained by switching a high-frequency signal having a frequency sufficiently higher than the digital signal to be transmitted, the full width at half maximum of the emission spectrum of the LD can be increased. You can ask a lot of questions,
This has the advantage that digital optical transmission without the influence of modal noise can be performed without selecting LDs.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a digital optical transmission device according to an embodiment of the present invention, FIG. 2 is a block diagram showing an embodiment of a transmitter in the digital optical transmission device shown in FIG. 1, and FIG.
Figures (a) to (c) are waveform diagrams of various parts of the digital optical transmission device shown in Figure 1, Figures 4 (a) and (b) are LD emission spectrum waveform diagrams, and Figure 5 is the conventional digital optical transmission. FIG. 6 is an explanatory diagram showing various characteristics of a light source for optical communication. In the figure, (1) is a transmitter, (2) is a digital signal input terminal, (3) is a high frequency signal input terminal, (4) is a laser diode (LD), (5) to (7) are single mode fibers, (11) is a receiver. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] an optical transmitter that receives a digital signal with a bit rate f_0 as an input and drives a laser diode with a modulation current obtained by switching a pulse train with a frequency f_1 sufficiently higher than the digital signal using the digital signal; and a cutoff that is longer than the oscillation wavelength of the laser diode. 1. A digital optical transmission device comprising a single mode optical fiber having a wavelength and an optical receiver.