JPH06326415A - Semiconductor laser modulating method - Google Patents

Abstract

PURPOSE:To provide the modulating method for a semiconductor laser having the wide frequency region of flat modulation frequency dependency of frequency modulation efficiency. CONSTITUTION:The title modulating method is the method for modulation of a distributed feedback semiconductor laser on which the electrode on one side is divided into four or more parts. The first electrode 4 is connected to the independently formed first terminal 1, all the even number elect rodes 5 and 7 are connected to the second terminal 2, and all odd number electrodes 6 and 8, excluding the first electrode, are connected to the third terminal 3. The semiconductor laser is oscillated in single mode by injecting a constant current to the first to third terminals respectively, and frequency is modulated by injecting a modulation current to the second terminal 2 and the third terminal 3 respectively in such a manner that the phase of the modulation current becomes an inverse phase.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser modulation method having excellent frequency modulation characteristics.

[0002]

2. Description of the Related Art In frequency modulation of a semiconductor laser, the flatter the modulation frequency dependence of the modulation efficiency, the higher the usefulness. In the conventional modulation method of a multi-electrode distributed feedback semiconductor laser, a constant current is applied to each of the divided electrodes to cause single mode oscillation, and then the modulation current is injected into only one electrode.

[0003]

FIG. 4 shows an example of frequency dependence of the modulation efficiency by this method.

As shown in the figure, in the conventional method, the modulation efficiency largely depends on the frequency, and the modulation efficiency becomes large on the low frequency side of about 10 kHz. In general, the wavelength change caused by injecting a current into one electrode of a semiconductor laser causes a long wavelength shift due to the increase in the refractive index of the semiconductor due to the effect of heat due to the increase in the current injection amount, and the laser resonance due to the increase in the current injection amount. There are both mechanisms of short wavelength shift due to change of carrier distribution in the chamber and change of oscillation state. The effect of heat becomes remarkable in the low frequency region, and the effect of carriers has little dependence on the modulation frequency. Therefore, the frequency modulation efficiency is reduced in the region of 10 kHz to 100 kHz where these two effects compete with each other, and the flatness of the modulation frequency dependence of the modulation efficiency is limited.

An object of the present invention is to provide a method of modulating a semiconductor laser having a wide frequency range in which the frequency dependence of frequency modulation efficiency is flat.

[0006]

A method for modulating a semiconductor laser according to the present invention is a method for modulating a distributed feedback semiconductor laser in which an electrode on one side is divided into four or more electrodes. The electrode number of the
, Nth, the first electrode is connected to the independent first terminal, and all the even-numbered electrodes except the Nth electrode are the second
All the odd numbered electrodes except the first electrode and the Nth electrode are connected to the third terminal, and the Nth electrode is connected to the second terminal or the third terminal depending on whether N is an even number or an odd number. In the case where it is connected or is connected to the fourth terminal, the constant currents I ₁ , I ₂ and I ₃ are respectively injected into the first to _third terminals, and the Nth electrode is connected to the fourth terminal. by injecting a constant current I ₄ thereto, the semiconductor laser a single mode is oscillated, and the respective modulation current IM ₂ and IM ₃ to the second terminal and the third terminal, the modulation current IM ₂ and I
This is to perform frequency modulation by injecting so that the phase of M ₃ becomes the opposite phase.

[0007]

In the laser modulation method of the present invention, the modulation current is injected into one electrode group, and the current having the opposite phase is injected into different electrode groups at the same time, so that the long wavelength shift due to heat generation is canceled. be able to. Therefore, there is no significant increase in modulation efficiency in the low frequency range,
A flat modulation characteristic can be obtained in a wide frequency range.

Further, in the present invention, since the modulation current is not injected into the electrode closest to the emitting end face, there is almost no fluctuation in the output intensity due to the modulation current. That is, almost pure frequency modulation can be performed. The fourth terminal in the above configuration exhibits usefulness when the output light from both end faces is used.

[0009]

The present invention will be described in more detail by the following examples.

<Embodiment 1> A first embodiment of the present invention is shown in FIG.
Shown in. FIG. 1 shows a multi-electrode distributed feedback laser and its connection diagram, where 1 is a first terminal, 2 is a second terminal, 3 is a third terminal, 4 is a first electrode, 5 is a second electrode, 6 is a 3rd electrode, 7 is a 4th electrode, 8 is a 5th electrode, 9 is InGaAsP
A cap layer made of InP, an upper cladding made of InP, a diffraction grating 11, a guide layer made of InGaAsP, an active layer made of InGaAsP, a buffer layer made of InGaAsP, and a lower cladding made of InP. 16 is the lower electrode, 17A and 17
B is an end face non-reflective coating, and 18 is an electrode separation groove. That is, in the semiconductor laser of this embodiment, the buffer layer 14, the active layer 13, the guide layer 1 are formed on the lower clad 15.
2, a diffraction grating 11 and an upper clad 10 are sequentially provided,
Further, a lower electrode 16 and an upper electrode are provided on the lower side and the upper side, respectively. In the present embodiment, the upper electrode is divided into five parts, that is, the first electrode 4, the second electrode 5, the third electrode 6, the fourth electrode 7 and the fifth electrode 8 by the four electrode separation grooves 18, The first to fifth electrodes 4 to 8 are divided into almost the same length. Also, the terminals are the first terminal 1, the second
There are three terminals, the second terminal 3 and the third terminal 3, which can operate independently. The first electrode 4 is connected to the first terminal 1, the second and fourth electrodes 5 and 7 are connected to the second terminal 2, and the third and fifth electrodes 6 and 8 are connected to the third terminal 3, respectively. is there. Further, both ends of the semiconductor layer are provided with end face antireflection coatings 17A and 17B to form a resonator, and the total resonator length is 600 μm. The end face provided with the end face non-reflective coating 17A serves as the emission end.

In the multi-electrode distributed feedback laser having such a structure, the first terminal 1 has a current of 40 mA and the second terminal 2 has a current of 80 m.
A current of 80 mA was injected into each of A and the third terminal 3 to cause single mode oscillation. The conditions for single-mode oscillation are not particularly limited, and each of the first to third
Different currents or constant currents may be passed through the terminals 1 to 3. The conditions for single-mode oscillation differ depending on each laser solid, but it is sufficient to select appropriate conditions while oscillating the laser and perform single-mode oscillation.

Next, modulation was performed by injecting modulation currents having an amplitude of 20 mA and opposite phases to the second terminal 2 and the third terminal 3. The frequency dependence of the modulation efficiency in this case is shown in FIG. Figure 2 shows modulation efficiency from 10kHz to several 100M
It shows that a flat frequency characteristic can be obtained up to Hz. This is because an antiphase modulation current was applied to the second and third terminals 2 and 3, so that the wavelength shift due to heat generation was canceled and only the wavelength shift due to the effect of carriers was taken out. This result shows that the semiconductor laser modulation method of the present invention is effective in obtaining good frequency characteristics. The amplitude of the modulation current applied to the second and third terminals 2 and 3 is basically 1:
1, but 2: 3 or 3: 2 depending on the laser solid
It suffices to obtain a good frequency characteristic by appropriately selecting from such conditions.

<Embodiment 2> A second embodiment of the present invention is shown in FIG.
Shown in. In FIG. 3, 19 is a first terminal, 20 is a second terminal, 21 is a third terminal, 22 is a fourth terminal, 23 is a first electrode, 24 is a second electrode, 25 is a third electrode, and 26 is a fourth electrode. , 27 is a fifth electrode, 28 is a sixth electrode, 29 is a seventh electrode, 30 is an electrode separating groove, 31 is a cap layer, 32 is an upper cladding, 33 is a diffraction grating, 34 is a guide layer, 35 is an active layer, 36 is a buffer layer, 37 is a lower cladding, 38A
Reference numerals 38B and 38B are end-face antireflection coatings, 39 is a lower electrode, and the composition of each semiconductor layer is the same as in Example 1.
In this embodiment, the upper electrode is formed by the six electrode separation grooves 30 into the first electrode 23, the second electrode 24, the third electrode 25, and the fourth electrode 2.
6, the fifth electrode 27, the sixth electrode 28, and the seventh electrode 29, and each of the first to seventh electrodes 23 to 29 is divided into substantially the same length. The terminal is the first terminal 19,
The four terminals are the second terminal 20, the third terminal 21, and the fourth terminal 22, the first electrode 23 is the first terminal 19, and the second, fourth, and sixth electrodes 24, 26, and 28 are the second terminals 20. To
The third and fifth electrodes 25 and 27 are connected to the third terminal 21,
And the seventh electrode 29 is connected to the fourth terminal 22, respectively. It is to be noted that the fact that the resonator is configured by applying the end-face antireflection coatings 38A and 38B on both end faces of the semiconductor layer is the same as in the above-mentioned embodiment, and the total resonator length is 600 μm. Although the emission end is the end face to which the end face non-reflection coating 38A is applied, in the present embodiment, the frequency-modulated optical output without intensity fluctuation is taken out from the opposite end face as well, as will be described later. be able to.

In such a multi-electrode distributed feedback laser,
30 mA for the first terminal 19, 90 mA for the second terminal 20, 60 mA for the third terminal 21, and 30 mA for the fourth terminal 22.
The respective currents were injected to cause single mode oscillation. Note that, as described above, the conditions for single mode oscillation are not particularly limited.

Next, modulation currents of opposite phases were applied to the second and third terminals 20 and 21. The amplitude is the second terminal 20
Is 30 mA and the third terminal 21 is 20 mA. The ratio of the amplitudes of the modulation currents needs to be appropriately selected depending on the laser solid as described above.

The frequency dependence of the modulation efficiency in this case is 10
It was flat from kHz to several 100 MHz. this is,
This is because the wavelength shift due to heat generation was canceled out because only the wavelength shift due to the effect of carriers was taken out because the modulation current having the opposite phase was applied to the second and third terminals 20 and 21. This result shows that the semiconductor laser modulation method of the present invention is effective in obtaining good frequency characteristics. Further, in the present embodiment, since the modulation current is not applied to the electrodes near both end faces, it is possible to take out the optical output in which only the frequency is modulated, with almost no intensity fluctuation, from both end faces. If it is necessary to obtain such an effect, the seventh electrode 29 may be connected to the third terminal 21 so that the modulation current is also injected to the seventh electrode.

The method of the present invention is not limited to the embodiment described above. The composition and configuration of the semiconductor layer of the semiconductor laser are not particularly limited, and for example, InGaAs may be used as the active layer.
A multiple quantum well structure composed of P and InGaAsP or InGaAs may be adopted.

[0018]

As described above, according to the semiconductor laser modulation method of the present invention, a modulation current is injected into one electrode group, and a current having an opposite phase is injected into different electrode groups at the same time. Since it is possible to cancel the wavelength changes due to each other, it is possible to obtain a flat frequency characteristic.

[Brief description of drawings]

FIG. 1 is a schematic view showing the structure and connection of a semiconductor laser according to a first embodiment of the present invention.

FIG. 2 is a graph showing frequency dependence of modulation efficiency by the laser modulation method of the present invention.

FIG. 3 is a schematic diagram showing the structure and connection of a semiconductor laser according to a second embodiment of the present invention.

FIG. 4 is a graph showing frequency dependence of modulation efficiency by a modulation method of a laser having a conventional structure.

[Explanation of symbols]

 1 1st terminal 2 2nd terminal 3 3rd terminal 4 1st electrode 5 2nd electrode 6 3rd electrode 7 4th electrode 8 5th electrode 9 Cap layer 10 Upper clad 11 Diffraction grating 12 Guide layer 13 Active layer 14 Buffer layer 15 Lower Clad 16 Lower Electrode 17 Endless Reflective Coating 18 Electrode Separation Groove 19 First Terminal 20 Second Terminal 21 Third Terminal 22 Fourth Terminal 23 First Electrode 24 Second Electrode 25 Third Electrode 26 Fourth Electrode 27 Fifth Electrode 28 Sixth electrode 29 Seventh electrode 30 Electrode separation groove 31 Cap layer 32 Upper clad 33 Diffraction grating 34 Guide layer 35 Active layer 36 Buffer layer 37 Lower clad 38 End face anti-reflection coating 39 Lower electrode

Claims (1)

[Claims] 1. A method of modulating a distributed feedback semiconductor laser, wherein one electrode on one side is divided into four or more, wherein the electrode numbers of the divided electrodes of the semiconductor laser are first, second and , Third, Nth, the first electrode is connected to an independent first terminal, all the even-numbered electrodes except the Nth electrode are connected to the second terminal, and the first electrode and the Nth electrode are excluded Connect all odd numbered electrodes to the third terminal, and for the Nth electrode,
Depending on whether it is an even number or an odd number, it is connected to the second terminal or the third terminal, or is connected to the fourth terminal, and the constant currents I ₁ , I ₂ and I ₃ are injected into the _first to third terminals, respectively, Further, when the Nth electrode is connected to the fourth terminal, a constant current I ₄ is injected into the fourth terminal to oscillate the semiconductor laser in single mode, and the modulation currents IM ₂ and IM are supplied to the second terminal and the third terminal, respectively. _A method of modulating a semiconductor laser, wherein ₃ is injected so that the phases of the modulation currents IM ₂ and IM ₃ are opposite phases to perform frequency modulation.