JPS6242593A - Semiconductor light emission device - Google Patents

Abstract

PURPOSE:To facilitate variation of a TE mode and a TM mode and improve both of light extinguishing ratio and S-N ratio by a method wherein a light emitted from the P-N junction of a light emission device is introduced into a laser resonator to which a modulation current is applied and the modulated light is introduced into an optical fiber through a photodetection unit and lenses. CONSTITUTION:A light emitted from the P-N junction provided in an unshown semiconductor light emission device is introduced into a laser chip 1, to which a bias current IB and a modulation current IS are applied, to be subjected to optical amplification. Then the light emitted from the laser chip 1 is introduced into an optical fiber 6 through a lens 2, a photodetection unit 3 composed of a prism beam splitter (PBS) with an optical monitor 4 and a lens 5 in this order and emitted from the optical fiber 6 to the outside. With this constitution, the intensity of the light from the P-N junction can be freely controlled by the variation of the modulation current IS and high speed long distance transmission can be realized while required TE and TM modes being obtained.

Description

[Detailed Description of the Invention] [Summary] A semiconductor laser in which the plane of polarization of laser light within a laser resonator changes from TE mode to TM mode or from TM mode to TE mode by changing the light injected into the pn junction. and an analyzer that transmits only laser light having polarization of either the TE mode or the TM mode, and a means for extracting the light emitted from the semiconductor laser to the outside through the analyzer. By changing the current, the mode is changed between TE mode and TM mode, and the intensity of the light taken out to the outside is changed to obtain a high-speed light pulse with a large extinction ratio.

[Industrial application field]

The present invention relates to a semiconductor light emitting device, and particularly relates to a TE mode, T
The present invention relates to a light emitting device that uses a laser whose polarization changes between M and M to obtain a light pulse with a high extinction ratio at high speed.

[Conventional technology]

FIG. 8(a) shows the threshold value 1 of a typical semiconductor laser. t
The current-light output characteristics with h are shown. As shown in Figure (b), a laser with such characteristics has I b /1t
A DC bias current Ib of h = 0.8 to 1.0 is applied,
A modulation current Is is superimposed on this to modulate the optical output.

However, in this method, as is clear from the relationship between the amplitude of the modulation current and the extinction ratio of light in FIG. 8(c), in order to lengthen the transmission distance and increase the extinction ratio, the modulation current must be increased. The change in light output must be large. However, it is difficult to fabricate the high power modulators required for this purpose. Furthermore, high power modulation increases stress on the laser, reducing reliability. Furthermore, in order to increase the modulation power, it is necessary to use a high-power transistor, but since the high-power transistor has a large junction capacitance, the response speed becomes poor and it is not possible to increase the speed.

Furthermore, the larger the amplitude of the laser itself, the less likely it is to be modulated. This is because the cutoff frequency Fc of the laser decreases as shown in FIG. 8(d).

Furthermore, when the amplitude modulation width of the laser is increased, carrier density within the laser fluctuates, the monochrome color of the laser decreases, and the spectral width expands. Since the light waves traveling in a fiber differ depending on the wavelength (fiber chromatic dispersion), when the spectral width becomes wider, the waveform becomes distorted due to long-distance transmission.
The product of transmission distance and modulation frequency is limited to a low value.

Therefore, if you increase the amplitude in order to increase the transmission distance, the distance will be limited due to the broadening of the spectrum, and if you decrease the amplitude to decrease the spectrum, the optical output will decrease and the transmission distance will be limited due to fiber loss. A drawback occurs (see the relationship between the spectral width and the modulation amplitude, indicated by ■ in FIG. 7(a)).

On the other hand, by making the bias current rb shown in FIG. 8(b) higher than the L4 value 1th, the frequency characteristics of the laser itself can be improved as shown in FIG. 7(b). Furthermore, it is possible to reduce the spread of the spectrum as shown in Figure 7(a), but the I
When b/Ith is greater than 1, the laser oscillates even at the 0 level of the signal, so the ratio of the light intensity between 0 and 1 (extinction ratio) becomes small, and this method has the disadvantage that it is difficult to increase the speed. be.

[Problem that the invention seeks to solve]

Conventionally, as described above, in order to increase the transmission distance and increase the extinction ratio, it is necessary to increase the modulation current and increase the change in optical output. However, it is difficult to manufacture the high power modulator required for this purpose, and high power modulation increases stress on the laser, reducing reliability. Furthermore, the laser itself has various drawbacks such as the frequency characteristics deteriorate as the amplitude increases, making it difficult to apply modulation, broadening the spectrum width, and shortening the transmission distance.

[Means for solving problems]

In the present invention, by changing the light injected into the pn junction, the polarization plane of the laser light within the laser resonator changes from the TE mode to the T mode.
We focused on the fact that in a semiconductor laser that changes from M mode or 7M mode to TE mode, the transition between TE and TM polarization occurs with a very small current, and utilizes this fact. It has an analyzer that allows only laser light with one of the polarizations to pass through, and a means for extracting the light emitted from the semiconductor laser to the outside through the analyzer. By changing the mode between TM modes and changing the intensity of the light extracted to the outside, a light pulse with a high extinction ratio is obtained at high speed.

[Effect]

DFB lasers have poor polarization selectivity, so the third
As shown in Figures (a) and (b), which show the correlation between the oscillation wavelength of TM and TE and the laser gain, when the magnitude of the gain is made to be the same for both TE and TM, the oscillation mode changes from TE mode to TM. It has been reported that it varies between momos (Autumn 1982, Proceedings of the Japan Society of Applied Physics Academic Conference, p. 9)
92,5 p-P-12, Masafumi Seki et al., DFB-DC
-TE of PBHLD, 7M mode oscillation, 1985 National Conference of the Optical and Radio Division of the Institute of Electronics and Communication Engineers, 260 Haruhiko Tabuchi et al.

(See jump in oscillation axis mode of DFB laser) In particular, if TM is set to be slightly larger than TE near the oscillation threshold as shown in Figure 3 (C), the
Oscillates in M mode.

Next, when the current is increased, the gain shifts to the longer wavelength side, so the oscillation mode shifts to the TE mode at a certain output. T.E.
This is because the mode is normally located at a wavelength 20 to 30 times longer than the 7M mode, as shown in FIGS. 3(a) and 3(c).

If such a laser is used and the bias current Ib is set just before shifting from the 7M mode to the TE mode, modulation between the TE mode and the 7M mode is possible with a small modulation current.

By extracting this modulated light to the outside through an analyzer that selects only TE waves and TM waves, ■The transition of polarization between TE mode and 7M mode occurs with a very small amount of current. TE mode with low modulation current, 7
By using an analyzer, TE mode, and 7M mode modulation, the r
A high extinction ratio can be obtained even when b/rth is sufficiently larger than 1, and by improving the frequency characteristics of the laser by increasing the bias current Ib, high-speed optical modulation is possible.

〔Example〕

In this embodiment, from 7M mode shown in FIG.
It uses a laser that shifts into mode with increasing bias. FIG. 1 shows the optical system of this embodiment, where 1 is the laser chip and an analyzer 3 (PB
S: Leads to Prism Beam Supurinota). The light exiting the PBS 3 passes through a lens 5 and enters an optical fiber 6. Note that 4 is a light receiving element for light monitoring.

In FIG. 2, when biased to point P and modulated by signal current (modulation current) Is, only the change Ps of the TE light is incident on the fiber 6. In this embodiment, Is is
A sufficiently high extinction ratio can be obtained at about 5 mA (peak-to-peak).

The laser used in the present invention, in which the transition between the TE mode and the TM mode occurs, generally has corrugations and is found in lasers that eliminate end face reflection, such as DFB lasers. Its structure is shown in FIG.
It has a striped mesa structure in which an sP contact layer 22 is formed, and this is a p-1nP12. n-1nP13.
It is embedded with n-InGaAs P14. The depth D of the corrugation formed along the guide layer 19 is 5
0-150ns, pitch is 400nm. Further, the laser length LL is 400 μm. The end face is coated with A12Q3 coating 18 to eliminate reflection. Note that 15 is a 5i02 film, and 16.17 is a laser bridge.

Although an example using a laser that transitions from TM mode to TE mode has been shown above, lasers that transition from TE mode to 7M mode are also seen. For example, in the characteristics of the semiconductor laser shown in FIG. 5, a transition from TE mode to 7M mode is observed.

Figure 6 shows the spectrum of the laser, and I/Ith
= 0.8 shows a spectrum with TE and TM separated below the oscillation threshold, I/Ith = 1.8 shows oscillation only in TE mode, and I/Ith = 2.0 shows a spectrum with TE mode separated. It can be seen that the 7M mode appears on the wavelength side.

In the present invention, such a laser is used, and contrary to the optical embodiment, it is passed through an analyzer that only passes polarized waves in the 7M mode, and a change in the injected current causes the TE mode to TM mode to be transmitted to the outside. The intensity of the extracted light can be modulated.

〔Effect of the invention〕

As described above, according to the present invention, the following effects can be obtained.

■Even if the modulation current Is is small, a high extinction ratio can be obtained, and S
/N ratio is improved and transmission distance can be extended.

■Since the amplitude of the modulation current is small, high-speed modulation can be performed relatively easily, and the spectrum width can be expanded quickly, allowing high-speed, long-distance transmission.

(2) Since the bias current can be set at a position sufficiently larger than rth, the frequency characteristics of the laser are improved, and by high-speed modulation, it becomes possible to increase the communication capacity.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is an explanatory diagram of the operation of an embodiment of the present invention, and FIGS. 3(a), (b), and (C) are characteristic diagrams of a laser used in the present invention, respectively. Figure 4 is a sectional view of the main part of the element used in the example, Figures 5 and 6 are characteristic diagrams of the laser transitioning to TE-TM, Figures 7 (a), (b), (c) and 8. (a) to (d) are characteristic diagrams of conventional semiconductor light emitting devices. Main code 1... Laser chip 2.5... Lens 3... PBS 4... Optical monitor 6... Optical fiber

Claims (1)

[Claims] A semiconductor pn junction that generates light by recombining electrons and holes when current is injected, and a laser resonator that guides and amplifies the light generated from the pn junction. , the p
A semiconductor laser in which the plane of polarization of laser light within a laser resonator changes from TE mode to TM mode or from TM mode to TE mode when the light injected into the n-junction is changed, and a laser emitted from the semiconductor laser Out of the light, TE
It has an analyzer that transmits only laser light having polarization of either mode or TM mode, and means for extracting the light emitted from the semiconductor laser to the outside through the analyzer, and changes in the injected current. A semiconductor light emitting device characterized in that the mode is changed between TE mode and TM mode, and the intensity of light extracted to the outside is changed.