JPH03184388A - Tm laser - Google Patents

Abstract

PURPOSE:To provide a TM laser which is liable in manufacture, less temperature dependency, and high in a response speed, by forming a lasing region and a TE/TM mode conversion region in a monolithic manner and controlling the polarization state of emitted light by the control of a magnetic field applied to the TE/TM mode conversion region. CONSTITUTION:A laser diode is constructed which has a multiple quantum well layer 5 as an active region and a waveguide, and right and left end surfaces 14, 15 of which serve as moirrors of a Fabry-Perrot resonator. A current is injected between electrodes 11, 12 for lasing in the lasing region MQW layer 5. Guided light goes and returns between the end surfaces 14, 15 and interact with a superlattice layer 9 subjected to a magnetic field 16 for mode conversion thereof from a TE to a TM mode by Faraday rotation. Herein, the wavelength is so adjusted that the absorption coefficient of the TE light is greatly larger than that of the TM light.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to optical functional devices that can be used in areas such as optical communication and optical memory, and particularly to optical functional devices that can emit TM light.
[Prior art related to M laser] Conventionally, one method of changing the polarization state of laser light is to utilize the electro-optic effect of a substance such as an electro-optic crystal whose optical properties change depending on an external electric field. However, this method has the drawback that a very large electric field is required to obtain a certain degree of change in the polarization state. Therefore, devices using such electro-optic effects become very large and impractical, and are completely unsuitable for optical integration. In addition to this, their response is too slow to be useful in some applications.

Another method of changing the polarization of TE waves to TM waves is to use the acousto-optic effect. Here, acoustic waves are excited in the crystal of the material to induce lattice vibrations, and by selecting appropriate values of the frequency and amplitude of the induced acoustic waves, the polarization of the input optical beam can be changed. Changes in state can be controlled. However, the problem with methods that utilize this effect is that the response is too slow.

Furthermore, there is a great demand for devices that can easily switch between TE light and TM light, so recently, TE/T devices that perform this switching by changing the diode current have been developed.
Converter laser diodes for M-mode conversion have also been proposed (see US Pat. No. 4,612,645).

[Problem to be Solved by the Invention] The principle of this device is based on the fact that the TM mode has a larger gain in the diode current. For this reason, in this device, internal strain exists between the active layer and the substrate. However, the need to create this strain makes it difficult to produce high-quality crystals compared to crystals that have no or only small strains.

Also, since TM gain mainly depends on the magnitude of distortion,
By varying this strain, mode operation is controlled, and this is done by varying the injection current and varying the temperature. That is, if the current is increased, more heat is likely to be generated, resulting in greater distortion. However, the problem with controlling the TM mode gain with temperature is that the device becomes highly temperature dependent. Switching from a pure TE mode of operation to a pure TM mode of operation corresponds to a temperature change of 1 DEG C., thus making the conversion highly temperature dependent.

Therefore, when a device is driven continuously with pulse trains of low and high level injection currents corresponding to TE mode and TM mode, respectively, the heat generated by the current is transferred to the TM mode and TE mode.
must be removed by some cooling means to ensure that the temperature difference between them is maintained.

TM spikes may also occur during TE mode when the device is switched with a finite rise time current.

Furthermore, since mode switching is controlled by the magnitude of the injection current, the full injection current range is not available for pure TM or TE mode operation, and therefore the injection current in TM mode is lower than that in TE mode. This means that the output in TM mode will always be greater than the output in TE mode. That is, it is impossible to generate TE light and TM light that correspond to the same magnitude of injection current and have the same output intensity.

This means that the mode forms differ between the TE mode and the TM mode.

Also, since this device is based on thermal effects, the response will always be limited due to thermal conduction. That is, the thermal process is slow and this device cannot be used as a high speed modulation system.

Therefore, an object of the present invention is to provide a TM laser that is easy to manufacture, has low temperature dependence, and has a fast response.

[Means for Solving the Problems] In the present invention that achieves the above object, a laser oscillation region and a TE/TM mode conversion region are monolithically formed inside a resonator, and the polarization state of emitted light is changed. , T
It is controlled by controlling the magnetic field applied to the E/TM mode conversion region.

More specifically, the conversion region includes a diluted magnetic semiconductor layer and is formed on the same substrate as the active region of the laser oscillation region, or at least one of the oscillation region and the conversion region is located at the TM tip at the wavelength of the laser light. It may have a multi-quantum well structure waveguide whose absorption rate is smaller than that of TE light.

Further, the diluted magnetic semiconductor layer may be formed on the same plane as the active region of the oscillation region, or may be formed on a different plane.

Furthermore, it is also possible to adopt a configuration in which the diluted magnetic semiconductor layer also serves as a multi-quantum well structure waveguide having the above absorption rate.

[Function] In the present invention having the above configuration, mode conversion is mainly performed by a magnetic field without using internal strain, so it is possible to create a device with a layered structure with little strain, and it is not necessary to change the current injected into the laser oscillation region for mode conversion. Nor. Therefore, it is not necessary to use an injection current pulse having a rise time for mode conversion, and the outputs do not differ between the two by injecting different currents in the TM%TE mode.

Since the laser light emitted in the laser oscillation region passes through the mode conversion region many times while traveling back and forth within the resonator, TE/TM mode conversion is performed each time (for example, due to the Faraday effect), and the final It is injected after an appropriate degree of mode conversion is performed. At this time, if the absorption rate of the waveguide (with respect to the laser wavelength) is larger for the TE wave than for the TM wave, TE/TM mode conversion will be performed more efficiently during the round trip.

[Embodiment] FIG. 1 is a side view of a first embodiment of the present invention. In the figure, a P0-doped GaAs substrate is placed on a P-doped GaAs substrate l.
s layer 2, P-doped AlGaAs lower cladding layer 3, undoped AlGaAS lower cladding layer 4, Ga As
/A'hGaAs multiple quantum well waveguide layer5. A.I.Ga.
AS waveguide upper waveguide upper cladding layer 6AlGa A
s Upper Clarado layer 7% N9 doped GaAS cap layer 8
grow. After the growth of this laser diode, the right half is removed by selective etching up to the N11 layer 7, and CdTe/C is deposited on the upper cladding layer 7.
A dMnTe superlattice layer 9 is formed.

On the back surface of the substrate 1, an ohmic contact electrode, Cr/AuJ@11, is deposited as an anode, and on the cap layer 8, an AuGe/Au layer 12 as a cathode is deposited. Layer 13, which is the cathode of the laser diode, becomes a common ground electrode when this device becomes an optical IC chip.The device thus formed constitutes a laser diode with multiple quantum well layer 5 as an active region and a waveguide. However, the left and right end faces 14 and 15 in FIG. 1 serve as mirrors of the Fabry-Bello resonator. The reflective surfaces 14,15 may be spaced or etched, but when the device is used in an integrated manner, one or both surfaces may be etched.

When used alone, it is better to separate it.

In this embodiment, the laser area on the left side of the notch part 20 is 3
00gm, the mode conversion range on the right side of the notch part 20 is 100gm.
~200μm, and the overall device length is ~400μm.
It is 500 μm.

External magnetic field 1 used to control the magnitude of the Faraday rotation angle, which will be described later, in the mode conversion region on the right side of Figure 1.
6 is parallel to the multiple quantum well waveguide layer (MQW layer) 5.

The operation of the first embodiment having the above configuration is as follows.

By injecting a current between the electrodes 11 and 12, laser light is oscillated in the MQW layer 5 in the laser region, and this laser light propagates through the MQW layer 5, but a portion of it is as shown in Figure 1. It also seeps into the superlattice layer 9.

Therefore, while the guided light travels back and forth between the end faces 14 and 15, it interacts with the superlattice layer 9 which is under the action of the magnetic field 16, and the TE wave of the guided light becomes a TM wave due to Faraday rotation (due to the Faraday effect). and the mode is converted. In this way, each time the light travels back and forth within the Fabry-Perot resonator, it passes through the superlattice layer 9 and the TE wave is converted into a TM wave.

Since the magnitude of the Faraday rotation is controlled by controlling the external magnetic field 16, controlling the magnetic field 16 controls switching between the TE wave and the TM wave of light emitted from the device.

By the way, A I G aA s/ G aA sMQ
A typical absorption spectrum of the W layer 5 is as shown in FIG. 2, and the laser oscillation wavelength is as follows.

As shown in , it is near the absorption edge, and the TM mode and TE mode have different absorption coefficients at this wavelength. That is, T.E.
mode has a slightly larger absorption coefficient than TM mode. For TE light, the peak 22 due to excitons of light holes and the peak 23 due to excitons of heavy holes remain almost unchanged, but for TM light, the exciton peak of heavy holes collapses and the peak 23 due to excitons of heavy holes collapses and The exciton peak of is enhanced.

Therefore, if the wavelength of the laser oscillation light is changed from 'LL to a wavelength region corresponding to 1.45 eV, which corresponds to the exciton peak 22 of the light hole in the TE light, the absorption coefficient of the TE light will be lower than that in the TM region. significantly larger and the device's T E/
TM mode conversion efficiency will be greatly improved.

In the first embodiment, the multiple quantum well structure of the MQW layer 5 and the superlattice layer 9 is
If the design is made so that the absorption spectrum is as shown above,
In this way, mode conversion efficiency is greatly improved.

FIG. 3 shows that by changing the current flowing through the electromagnet 25,
A device for magnetically controlling the proportion of the TE or TM mode of the emitted light 26, ie, controlling the magnitude of the external magnetic field 16 of FIG. 1, is shown. The device 27, such as the first embodiment described above, confined within the cylindrical electromagnet 25 is further enclosed in an aluminum cylinder 28. The strength of the magnetic field by the electromagnet 25 is controlled by varying the current to the coil 29, so that the emitted light 26 will be polarization modulated by the electrical signal 31 passed through the cable 30.

FIG. 4 shows a second embodiment. In the second embodiment, P9
On a doped GaAs substrate 41, a P'' doped GaAs buffer @ 42, a P doped AlGaAs lower cladding 143, an undoped AI G5As waveguide lower cladding layer 44, an undoped A1GaAs/GaAS multiple quantum well waveguide layer 45, an undoped A I G a A S waveguide top cladding layer 46 and undoped CdTe/Cd
In the first step, the MnTe superlattice layer 47 is grown in sequence.
The left part of the device is selectively etched down to the lower cladding layer 43, and a P-doped AI G layer is deposited on the buffer layer 42.
aAs lower cladding layer 48. Undoped AI Ga
As waveguide lower clarad layer 49, undoped AlGaA
s/GaAs multiple quantum well waveguide layer 50. Top of undoped AlGaAs waveguide! 51. N-doped AlGa
The As upper cladding layer 52 and the N-doped GaAs cap layer 53 are regrown.

In the above configuration, the TE of the MQW layer 45 in the mode conversion region
The optical absorption peak (see reference numeral 22 in FIG. 2) is designed to match the oscillation wavelength of the MQW layer 50 in the laser region.

Further, in order to promote coupling between the laser region and the mode conversion loading, a SiO2 region 54 is formed in both loadings. This is formed by removing the same amount of the laser region and the mode conversion region by ion beam etching or the like, and filling this region with SiOz.

This SiO,F! ! The width of L54 has an optical length equal to the laser half wavelength, thus making the interface an anti-reflection surface to promote coupling between the laser region and the mode conversion region.

In addition, 55 is an A u G film formed on the cap layer 53.
e/A u ohmic electrode, 56 is the substrate 4
1 is a Cu/AU ohmic electrolytic electrode deposited on the back surface of 1, and 57 is an external magnetic field.

In the second embodiment, the laser beam oscillated from the MQW layer 50 in the laser region propagates through the S i Ow region 54 into the central layer of five of the four MQW layers in the mode conversion region with good coupling. Mode conversion from TE light to TM light (due to interaction with superlattice layer 47) occurs at T
More E light is absorbed (due to the large TE light absorption coefficient for the oscillation wavelength of the MQW layer 45), and polarization modulation is performed according to the strength of the external magnetic field 57. In this way, T
Conversion between E waves and TM waves is performed, and laser light with an appropriate proportion of TM waves is emitted.

FIG. 5 shows a third embodiment. In the third embodiment, AlGaAs/GaAsMQW of the second embodiment! 45 is Cd T
It has a structure that doubles as an e/CdMnTe superlattice layer.

In FIG. 5, a P'' doped GaAs substrate 61 is
0-doped GaAs buffer layer 62, P-doped AlGa
As lower cladding layer 63, undoped Cd M n T
eT7 section Clara layer 64, Cd M n T e / C
d T e multiple quantum well waveguide layer (mode conversion layer) 65,
Then, a CdMnTe waveguide upper cladding layer 66 is grown.

Thereafter, the left part of FIG. 5 is etched, and an undoped AlGaAs lower cladding layer 67 is formed on the lower cladding layer 63, and an undoped AlGaAs/GaAs layer is formed on the lower cladding layer 63.
Multiple quantum well layer 68, undoped AIGaAs
Upper Clarad layer 69, N0 doped GaAs cap layer 7
0 is allowed to regrow.

With the above configuration, MQWF in the mode conversion region for the oscillation wavelength in the laser region! The absorption coefficient of TE light at t65 is set to be significantly larger than that of TE light. This can be done by setting the absorption spectrum as shown in FIG. 2, but it is also done by appropriately setting the thickness of the well and barrier and the Mn ratio in the barrier.

In FIG. 5, 71 is an A u G e / A u electrode, 72 is a wire, and 73 is a Cu / A u electrode.
It is a u electrode, and 74 is a SiO□ region.

In the third embodiment, most of the laser light from the MQW layer 68 in the laser region interacts with the MQW layer 6 in the mode conversion region, which is a diluted magnetic semiconductor, so that mode conversion is performed more effectively. .

In other respects, the operation is substantially the same as that of the second embodiment.

Incidentally, in the above embodiments, the configuration of the active layer in the laser region and the configuration of the leading wave layer and mode conversion layer in the mode conversion region are merely examples, and other configurations may be used.
The MQW layer may be a bulk layer or the like.

[Effects of the Invention] As explained above, according to the present invention, since the control is based on a magnetic field, the ratio of the TM mode of the emitted laser light can be continuously controlled, and the ratio between the TE mode and the TM mode can be controlled continuously. It is also possible to perform high-speed modulation. Moreover, since it is sufficient to stack lattice-matched layers with similar lattice constants,
Devices with high-quality crystal structures can be easily manufactured.

In addition, since the injection current can be kept constant by controlling the magnetic field and modulating it, there is no need for cooling means, and the modulation can be performed using TM,
This can be done without any difference in output in TE mode.

Furthermore, TM mode operated lasers can also be monolithically integrated on optical chips.

[Brief explanation of drawings]

Fig. 1 is a schematic side view of the configuration of the first embodiment of the present invention, Fig. 2 is a graph showing a typical absorption spectrum of a multiple quantum well structure, and Fig. 3 is a schematic cross-sectional view of the configuration of a device for controlling an external magnetic field. , FIG. 4 is a schematic side view of a second embodiment of the present invention, and FIG. 5 is a schematic perspective view of a third embodiment of the present invention.

Claims (6)

[Claims] 1. A laser oscillation region and a TE/TM mode conversion region are monolithically formed inside the resonator, and the polarization state of emitted light is controlled by controlling the magnetic field applied to the TE/TM mode conversion region. TM laser featuring. 2. 2. The TM laser according to claim 1, wherein said TE/TM mode conversion region includes a diluted magnetic semiconductor layer and is formed on the same substrate as an active region of said laser oscillation region. 3. 3. The TM laser according to claim 1, wherein the laser oscillation region has a multi-quantum well structure in which the absorption rate of TM light is lower than the absorption rate of TE light at the wavelength of the laser light. 4. 4. The TM laser according to claim 1, wherein the TE/TM mode conversion region has a multi-quantum well structure having a lower absorption rate of TM light than that of TE light at the wavelength of the laser light. 5. 3. The TM laser according to claim 2, wherein said diluted magnetic semiconductor layer is formed on the same plane as said active region. 6. 6. The TM laser according to claim 5, wherein the diluted magnetic semiconductor layer also serves as a multi-quantum well structure in the TE/TM mode conversion region in which the absorption rate of TM light is lower than the absorption rate of TE light.