JPS6120383A - Driving circuit for distributed feedback laser diode - Google Patents

Abstract

PURPOSE:To make side-mode components difficult to be contained in output light by detecting the variation of side-mode components contained in monitor light with time, and then controlling the bias current according to the result. CONSTITUTION:A monitor light 3 from a distributed feedback laser diode 1 is delivered by means of a photo-branching element 5; an output light 5b is converted into electric signals by means of a photo detector 6; the bias current is controlled in feedback by means of an automatic photo-power control circuit 12, so that the level of a signal-transmission light 4 may be kept constant. Then, a side mode 5' is taken out by making the other output light 5a from the photo- branching element 5 incident by means of a wavelength filter 7, converted into electric signals through a photo detector 8, and then amplified in the amplifier 9; thereafter, the result is inputted to a comparator 14 together with the output of an amplifier 10. An output corresponding to the output ratio of the amplifiers 9 and 10 can be obtained from the comparator 14. Finally, an automatic photo- power control circuit 11 controls the bias current I1 according to the output of the comparator 14.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a distributed feedback laser diode drive circuit, and particularly to control of bias current.

In optical communication using a single mode fiber, when performing phase modulation, two-frequency modulation, etc. that require wavelength stability, a distributed feedback laser diode that oscillates in a substantially single longitudinal mode is used. If you use a normally distributed feedback laser diode whose oscillation center wavelength is at the fiber's zero dispersion (minimum group delay between propagation modes) wavelength (around 1.3 μm), the optical power loss of the fiber will be somewhat large. , the wavelength with the least loss (near 1.5 μm) is used. However, since the wavelength of this lowest loss is in the fiber's finite dispersion region (where group delay between propagation modes appears), if the laser diode does not oscillate in a complete single longitudinal mode, mode splitting noise will occur after fiber transmission. waveform deterioration occurs. In order to prevent the above phenomena, even in the finite dispersion wavelength range of the fiber, it is necessary to suppress the generation of wavelengths (side modes) adjacent to the central oscillation wavelength of the laser beam so that group delay between propagation modes does not occur. There is.

[Prior Art] FIG. 3 is a diagram explaining the prior art, in which 1 is a distributed feedback laser diode, and 2 is an electric waveform output device.

3 is a monitor light, 4 is an output light for signal transmission, 6 is a photodetector, 10 is an amplifier, 12 is an automatic optical power control circuit (A
PC). In the prior art, a signal pulse current from an electrical waveform output device 2 is superimposed on a bias current to create an electrical waveform, and the zero distribution feedback laser diode 1 that drives the distributed feedback laser diode 1 converts the current into a signal. After being amplified by the signal transmission device 10, it is input to the automatic optical power control circuit 12 to control the bias current to the distributed feedback laser diode 1 so that the output light 4 is always constant.

[Problem that the invention seeks to solve]

The configuration described above only keeps the output level constant and does not take into account side mode suppression of the laser beam at all. Furthermore, because the distributed feedback laser diode emits wavelength light in the finite dispersion region of the fiber for the above-mentioned reasons, mode distribution noise causes waveform deterioration on the receiving side, which can cause deterioration in communication quality.

[Means for the invention to solve the problem]

In order to solve the above-mentioned problems, the present invention detects temporal changes in side mode components included in monitor light and controls the bias current according to the results so that side mode components are less likely to be included in output light. do.

[Effect]

In general, distributed feedback laser diodes belong to gain-guided lasers, so when the bias current value is increased, the wavelength spectrum width of the output light is narrowed, the optical power of the wavelength component corresponding to the side mode is reduced, and the optical power of the wavelength component corresponding to the side mode is reduced. It has the property of increasing power.

Therefore, it is conceivable to suppress the side mode by sufficiently increasing the bias current value, but since the extra current is used, power consumption increases and a large amount of heat is generated. On the other hand, the optical power of the side mode may undergo large fluctuations within a short period of time due to temporal changes in the gain distribution inside the laser light of the distributed feedback laser diode due to temperature fluctuations, external feedback light, and the like.

Therefore, the present inventors reduce power consumption by applying a bias current to a level that does not cause quality deterioration when communicating the optical power ratio of the side mode and center mode. In addition, in order to prevent waveform deterioration caused by temporal fluctuations in the optical power of the side mode, for example, the average optical power Pa of the monitor light and the average power Ps of the side mode are respectively detected, and the two are compared using a comparator. The bias current value that minimizes the error rate or noise is fed back. As a result, the side mode power Ps
increases, the bias current increases and the center mode power P
O increases, and the ratio with the side mode can be maintained above a certain value.

〔Example〕

The present invention will be explained below using the embodiment shown in FIG.

Figure Note: The same numbers as in Figure 3 are the same parts, and 5 is the light component.

The monitor light 3 from the distributed feedback laser diode 1 is distributed by the optical branching element 5, and the output light 5b is sent to the photodetector 6.
The bias current is converted into an electric signal by the automatic optical power control circuit 12, and the bias current is feedback-controlled to keep the level of the signal transmission light 40 constant.

Then, the other output light 5a of the optical branching element 5 enters the wavelength filter 7, and the side mode 5a' is extracted.
The photodetector 8 converts the signal into an electrical signal, the amplifier 9 amplifies it, and then inputs it to the comparator 14 together with the output of the amplifier 10 . An output corresponding to the ratio of the outputs of amplifiers 9 and 10 is obtained from the comparator 14. Then, the automatic optical power control circuit 11 outputs a bias current of 1. control.

Specifically, the bias current 11 is controlled using the relationship expressed by equation (1).

ΔI, −6 to (P s / P a ) −
(tl, where K is a positive proportionality constant, Ps is the average optical power of the side mode and all modes, and this ratio is passed through Δ(Ps
/Pa) increases, the bias current is controlled to increase. Also, in equation +11, force/P s /
P a as the following 121. (Even if it is replaced like 31, it can be controlled in the same way.

Δ I, = Nimu (P s / P o ) −42
1Δ L = Kb (Pa/Po) −(3
In Equation 1 (2), the average optical power of the side mode and the center mode are compared, and in order to keep this ratio within a range that does not cause interference during communication, control is performed so that the bias current is increased when the ratio increases. However, for PO, a filter 7' must be inserted between the optical branching element 5 and the photodetector as shown in FIG. 1 in order to make the selection.

In equation (3), in order to keep the ratio of the optical average power of the center mode and all modes within a range that does not cause interference during communication,
Control is performed to increase the bias current when the ratio increases. In this case, for Po, a filter with center wavelength selectivity is used as the wavelength filter 7.

This is a coated light branching element, and the same numbers as in FIG. 1 are the same members. The configuration shown in Figure 2 is the signal transmission light 4. This is a method in which an optical branching element that functions as a filter is provided on the side to select wavelengths. That is, in the figure, the multilayer coated optical branching element I5 separates the optical signal transmission light 4 into branched lights 13a and 13b. Of these, branch %13b is reduced to only side mode components by filtering. In this configuration, in addition to optical branching, it also serves as a filter, so the first
Compared to the case shown in the figure, the circuit can be expected to be smaller in size compared to the circuit.

〔Effect of the invention〕

The present invention corresponds to (1) the ratio of the average power of the side mode to all modes, or (2) the ratio of the average power of the side mode to the center mode, or (3) the ratio of the average power of the center mode to all modes. Since the bias current is changed and the side mode is suppressed, the transmission signal quality can be improved.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining the present invention in detail. 2nd FM is a diagram for explaining a modification of FIG. 1; FIG. 3 is a diagram for explaining the prior art. In the figure, 1 is a distributed feedback laser diode m, 2 is an electric waveform output device, 3 is a monitor light, 4 is an optical output for signal transmission, 5 is an optical branching element, 6.8 is a photodetector, 7.7' is a filter,
9. lO amplifier. 11.12 is APC, 13a and 13b are branched lights. 14 is a comparator, and 15 is a multilayer coated optical branching element. (−”−'−”'1

Claims (1)

[Claims] 1. The distributed feedback laser diode drive circuit that controls the average power value of the laser output light to be constant by monitoring the laser light from the distributed feedback laser diode and feedback controlling the bias current. A distributed feedback laser diode drive circuit characterized in that the distributed feedback laser diode drive circuit detects a temporal change in a side mode component contained in the laser light and controls the bias current according to the detection result. 2. The method according to claim 1, wherein the temporal change in the side mode component is detected by detecting a change in the ratio of the average optical power of the side mode component to the average optical power of all modes. Distributed feedback laser diode drive circuit. 3. The distributed feedback according to claim 1, wherein the temporal change in the side mode component is detected by detecting a change in the ratio of the side mode average optical power to the center wavelength average power. Laser diode drive circuit. 4. The method according to claim 1, wherein the temporal change in the side mode component is detected by detecting a change in the ratio of the average optical power of the center oscillation wavelength to the average optical power of all modes. Distributed feedback laser diode drive circuit.