KR20030045252A - Long wavelength vertical cavity surface emitting laser diode - Google Patents

Abstract

PURPOSE: A long wavelength surface emitting semiconductor laser diode is provided to manufacture a vertical cavity surface emitting laser(VCSEL) capable of oscillating a single longitudinal and transverse modes by using a material to have a grating matching with an InP substrate difficult to manufacture distributed Bragg reflector(DBR) of a high reflection due to the small difference of refraction index. CONSTITUTION: A long wavelength surface emitting semiconductor laser diode includes an InP substrate(5), a cavity(3) provided with an active layer and a clad layer formed on the InP substrate(5), an upper semiconductor distributed Bragg reflector(DBR)(4) and a lower semiconductor DBR(1) formed the upper and the lower portions of the cavity(3), a lower insulator DBR(1a) formed in contact with the lower semiconductor DBR(1) and an upper insulator DBR(4a) formed on the top portion of the substrate(5).

Description

Long wavelength surface-emitting semiconductor laser diode {LONG WAVELENGTH VERTICAL CAVITY SURFACE EMITTING LASER DIODE}

The present invention relates to a surface emitting semiconductor laser diode, and more particularly, to a long wavelength surface emitting semiconductor laser diode for implementing a long wavelength semiconductor laser using a vertical resonator type surface emitting laser.

Edge emitting laser diodes have a resonant structure parallel to the stacking surface of the device and oscillate the laser beam in a direction parallel to the stacking surface, whereas a vertical resonator type surface emitting laser diode is a stacking surface of the device. The laser oscillates in the vertical direction of the stacked surface of the device while having a resonance structure perpendicular to the device. The VCSEL-Vertical Cavity Surface Emitting Laser has a lower driving current and a smaller beam divergence than the edge emitting laser diode, so that optical communication, optical information recording and holographic memory ( Holographic memory is widely used in the trend.

In addition, the VCSEL not only oscillates single longitudinal mode, but also is easy to manufacture and testable on a wafer, which is superior to edge emitting laser diodes in production, capable of high-speed modulation and oscillating circular beams. Therefore, the coupling with the optical fiber is easy, so it is advantageous to be used as an optical communication device.

The vertical resonator type surface emitting laser (VCSEL) not only performs single-mode mode oscillation of very narrow spectrum, but also has a small radiation angle of the beam, which leads to high connection efficiency and easy integration with other devices due to its structure. The feature makes the VCSEL suitable for optical communication light sources. In addition, the VCSEL can be manufactured much more easily than the distributed feed-back (DFB) edge-emitting laser diode fabricated for narrow-spectrum oscillation in the manufacturing surface, thereby greatly reducing the cost of the device. In addition, it is very easy to apply to WDM (Wavelength Division Multiplex), which is expected to form a rapid market in the future due to its surface-emitting characteristics.

However, for the VCSEL to operate, a very high reflecting Bragg reflector (DBR) mirror is required. DBR required for VCSEL is produced by alternately stacking two materials with different refractive indices at a thickness of 1/4. To obtain high reflectance, the difference in refractive index between the two materials must be large. In addition, in order to inject current into the active layer through the DBR layer may be made of a semiconductor material. The most representative material is made of GaAs / AlAs because GaAs and AlAs are not only large in refractive index difference but also lattice match is possible. For this reason, VCSELs have been developed around short-wavelength VCSELs based on GaAs.

On the other hand, in order to make a device used in the optical communication for the medium-to-long distance transmission, it is necessary to oscillate a long wavelength of 1300nm or 1550nm, which is mainly made of InP as a substrate. When InP is used as a substrate, the most commonly used semiconductor DBR is made of InP / InGaAsP.

However, because the difference in refractive index between the two materials is small, very large stacks (40-50 pairs or more) are required to obtain high reflectivity, and the limit of the highest reflectance that can be obtained is higher than that obtained using GaAs / AlAs. small. In addition, InGaAsP, which is a four-way mixed crystal system, has a low thermal conductivity, and when stacked thickly, heat generated in the active layer is not easily released, thereby deteriorating thermal characteristics of the device. Due to these material limitations, the long-wavelength VCSEL that is currently commercially available has not been developed.

Because of these problems, the long-wavelength VCSEL has been tried to wafer fusion to GaAs / AlAs DBR after separating one or both sides of the cavity consisting of InGaAsP active layer from InP substrate. However, this technology also has technical difficulties in separating very thin cavities, wafer fusion is not suitable for mass production, and electrical ohmic characteristics are not obtained at the bonded interface. .

Another attempt has been made to use a new material instead of InP / InGaAsP DBR, that is, InGaAlAs or Sb compound. However, they also have difficulties in realizing a long wavelength VCSEL that can be put to practical use due to their inherent problems.

Meanwhile, since it is known that 1300nm oscillation is possible using a new compound semiconductor called GaInNAs, development of 1300nm VCSEL using GaAs substrate and GaInNAs active layer is accelerated and commercialized. GaInNAs compounds can generate long wavelengths of 1300 nm or more by lattice matching with GaAs by increasing the N content.

However, the increase of N content is very difficult technically, and the absorption by p-GaAs Free Carrier is rapidly increased at the wavelength above 1300 nm. In fact, using GaAs as a substrate and fabricating 1500nm laser using GaAs / AlAs DBR impossible.

In order to solve the above problems, an object of the present invention is to implement a 1.3 ~ 1.55 ㎛ long-wavelength semiconductor laser used for communication by using a VCSEL-Vertical Cavity Surface Emitting Laser. have.

Another object of the present invention is to provide a long wavelength surface-emitting semiconductor laser diode configured to oscillate with a small number of pairs and thereby to easily cool the heat generated during the oscillation.

It is still another object of the present invention to provide a long wavelength surface emitting semiconductor laser diode which can be used for the use of WDM by facilitating the variation of the oscillation wavelength.

Accordingly, the present invention provides an InP substrate;

A cavity including an active layer and a cladding layer formed on the InP substrate;

Upper and lower semiconductor DBRs formed on and under the cavity;

A lower insulator DBR formed in contact with the lower semiconductor DBR;

An upper insulator DBR formed on the substrate,

And a lens fabricated on the substrate in contact with the upper insulator DBR.

1 is a schematic diagram showing the configuration of a long wavelength surface-emitting semiconductor laser diode according to an embodiment of the present invention.

Figure 2 is a schematic diagram showing the configuration of a tunable laser diode according to an embodiment of the present invention.

<Description of Major Symbols in Drawing>

1: bottom semiconductor DBR1a: insulator bottom DBR

2: current limiting layer 3: cavity including active layer and cladding layer

4: upper semiconductor DBR4a: insulator upper DBR

5: substrate (InP) 6: second type electrode

7: type 1 electrode 8: semiconductor insulating layer

9a: first electrode for controlling wavelength

9b: second electrode for wavelength control

The light oscillated in the present invention may be adjusted to come out up or down by adjusting the relative reflectance value of the DBR.

As shown in Fig. 1, the present invention provides a semiconductor DBR (1) designed so that the reflectance is in the range between 10 and 90%, respectively, on the upper and lower portions of the cavity 3 including an active layer oscillating a wavelength of 1300 to 1600 nm. (4) is provided. In this case, the active layer typically uses InGaAsP, but a semiconductor having a bandgap corresponding to GaInNAs or a long wavelength may be used. Of course, the semiconductor DBR may also use InP / InGaAsP, but the lattice match with InP is epitaxy. Materials such as Sb compound, InAlAs, InAlGaAs and the like capable of epitaxial growth may be used. At this time, as described above, the fabrication of the DBR having a reflectance of 99% or more necessary for VCSEL oscillation while forming lattice matching with the InP is not easy, and therefore, additional DBRs made of Dielectric out of the substrate 5 toward the lower and upper sides of the lower DBR. (1a) and (4a) are provided.

In this embodiment, since the oscillated light comes out as an example, the lower DBR, that is, the reflectances of (1) and (1a) are designed to be higher than the reflectance of the upper DBR, that is, (4) and (4a). For example, the former, that is, the sum of the reflectances of (1) and (1a) is 99.9%, and the latter, that is, the sum of the reflectances of (4) and (4a) is 99.6%.

In the above configuration, the DBR composed of the insulator supplements the reflectance of the semiconductor DBR, and as a result, makes it possible to easily secure a very high high reflectance.

At this time, as shown in Figure 1, (1) and (1a) is in physical contact with each other there is no problem, but (4) and (4a) are physically spaced apart by the substrate (5) The thickness of can range from several tens of micrometers or more than 100 micrometers.

In this structure, the substrate (5) is surrounded by two DBRs, namely (4) and (4a), which acts as another cavity, because its thickness is considerably larger than the oscillation wavelength. Longitudinal mode will exist.

On the other hand, the original cavity (3) should have a thickness of the oscillation wavelength or an integer multiple of the oscillation wavelength, so that one or several longitudinal modes exist as in the general VCSEL.

As a result, in the present invention, a coupled cavity structure in which two cavities are coupled to each other is coupled. In such a coupled cavity, only a matching longitudinal mode of the longitudinal modes of the two cavities is generated. Therefore, even if there are multiple species modes by (5), only one or three or fewer modes are allowed in (3), so the number of oscillating species modes is limited and the gain distribution of the active layer is obtained. It is easy to design so that there is only one species mode within. Thus, it is one of the main features of the present invention to configure a coupled cavity to oscillate a narrow spectrum single species mode.

In this case, the relative reflectances of (1) and (1a) may be determined relatively freely, taking into account other matters, that is, the thermal problem of the device, the current spreading problem, etc., but the relative reflectance between (4) and (4a) has a coupled cavity effect. Consider it and decide.

In the present invention has been described the principle of the single species mode oscillation. Referring now to the principle of lateral mode oscillation, the upper insulator DBR of the present invention is provided on a substrate processed to have a constant radius of curvature as shown in FIG.

If the upper insulator mirror is provided on a flat substrate such as a normal VCSEL, the resonance itself is difficult and it is impossible to obtain a single transverse mode. However, in the present invention, the substrate is processed to serve as a lens so that the reflected light returns to a narrow oscillation region, thereby realizing a single transverse mode. The radius of curvature of the lens can be designed to fit the wave front of the laser beam and the fabrication of the lens can be easily obtained through dry or wet etching.

Another feature of the present invention will be described with reference to FIG. At present, there is an attempt to widen bandwidth by minimizing additional costs by using a plurality of different light sources in one optical line, which is WDM. To enable WDM, not only lasers with regular intervals are required but also tunable lasers that can cover the WDM wavelength band with several backup lasers. In the present invention, such a tunable laser can be easily implemented, which is illustrated in FIG. 2. FIG. 2 is to place the second type electrode on the lower side in the structure shown in FIG. In addition, two electrodes 9a and 9b are provided on the substrate side to allow a current to flow through the substrate. The insulator layer 8 prevents current flowing to the substrate from flowing toward the laser operating layer.

The electrodes 9a and 9b are supplied with a current that is separate from the current required for laser operation. The current is then changed in the refractive index of the substrate, resulting in a change in the cavity mode caused by the substrate. This eventually changes the oscillating coupled cavity mode, which allows fine tuning of the wavelength. In the present invention, the current limiting structure can be used as it is, the conventional technique used in the general VCSEL structure, that is, a method such as selective side oxidation or ion implantation.

As described above, the VCSEL capable of single transverse and longitudinal mode oscillation can be manufactured by using an InP substrate having a small refractive index difference, which makes it difficult to produce a high reflectance DBR, and a material that is lattice matched thereto. In addition, the present invention uses a semiconductor DBR with a limited reflectance and an insulator mirror that can obtain a very high reflectance even with a small number of pairs, so that the oscillation can be performed with a small number of pairs of semiconductor DBRs. It can cool, and the characteristic of an element improves. In addition, the present invention can be easily used for the use of WDM because the oscillation wavelength is variable.

Claims (7)

An InP substrate; A cavity including an active layer and a cladding layer formed on the InP substrate; Upper and lower semiconductor DBRs formed on and under the cavity; A lower insulator DBR formed in contact with the lower semiconductor DBR; And a top insulator DBR formed on top of the substrate. The method of claim 1, The reflectance of the upper semiconductor DBR and the upper insulator DBR is different from the reflectance of the lower semiconductor DBR and the lower insulator DBR. The method of claim 2, The reflectance of the lower semiconductor DBR and the lower insulator DBR is designed to be higher than the reflectivity of the upper semiconductor DBR and the upper insulator DBR. The method of claim 1, A long wavelength surface emitting semiconductor laser diode characterized by depositing an upper insulator mirror on a lens-etched substrate to obtain a single lateral mode oscillation. The method of claim 1, A long wavelength surface-emitting semiconductor laser diode characterized in that it is possible to apply a current for wavelength adjustment on a substrate to obtain a variable oscillation wavelength. The method of claim 1, A semiconductor laser consisting of only a semiconductor DBR that is lattice matched with InP without using a hybrid mirror of semiconductors and insulators in the underlying DBR. The method of claim 1, Semiconductor laser that consists of GaAs / AlGaAs (AlAs) DBR by wafer fusion without using hybrid mirror composed of semiconductor and insulator in the lower DBR