KR20040050249A - A polarization switching vertical-cavity surface emitting laser and an optical transmitter employing the laser - Google Patents

Abstract

PURPOSE: A polarization control vertical cavity surface emitting laser and an optical transmission device using the same are provided to perform optical transmission without distortion when generating a high speed modulated optical signal. CONSTITUTION: The vertical cavity surface emitting laser comprises a semiconductor substrate, a bottom mirror layer stacked on the semiconductor substrate, an active layer to give a gain on the bottom mirror layer, and an insulation layer preventing a current from flowing around the active layer. An electrode contact layer contacts an electrode to apply a voltage to the active layer and the insulation layer. A polarization control layer generates a birefringence by the voltage applied to the electrode contact layer. And a polarization electrode contact layer contacts the electrode to apply a polarization voltage.

Description

A polarization controlled surface emitting laser and an optical transmitting device using the same {A POLARIZATION SWITCHING VERTICAL-CAVITY SURFACE EMITTING LASER AND AN OPTICAL TRANSMITTER EMPLOYING THE LASER}

The present invention relates to a polarization control surface emitting laser and an optical transmitting device using the same, and more particularly, a surface-emitting laser (VCSEL) and a polarizing plate, and a frequency of a signal when generating a high-speed modulation signal. The present invention relates to an optical transmitter capable of modulating a signal without distortion.

BACKGROUND OF THE INVENTION An optical transmission device using a surface emitting laser is used as a light source for optical communication and data communication, and is being used in various fields such as optical coupling and optical sensors. In particular, optical transmission devices using surface emitting lasers have high fiber-coupling efficiency, low unit cost due to wafer-based fabrication process, low mounting cost, and two-dimensional array characteristics. It is expected to quickly become a light source for next generation optical communication and signal processing. Such surface-emitting lasers can be fabricated and tested on a wafer scale, and do not require an additional lens system for coupling with optical fibers as compared to conventional edge-emitting laser diodes (LDs). There is an advantage that the manufacturing cost of the optical transmission device can be significantly lowered. However, currently used optical emission devices using surface emitting lasers generate a modulated signal mainly by a direct modulation method (modulating the driving current / voltage itself and applying it to the laser). According to the direct modulation method, frequency chirping occurs due to a change in the internal refractive index of the surface emitting laser in converting an electrical signal of a high frequency into an optical signal, thereby deteriorating a wavelength characteristic of the output light. This problem causes a problem that it becomes impossible to demodulate the modulated signal at the receiving end as the transmission distance and the transmission speed increase during the optical signal transmission.

This frequency chirping may occur even when the driving voltage / current has a constant value (DC), but more seriously occurs when the driving voltage / current changes (AC). Therefore, in the case of the direct modulation scheme widely used in the related art, there is a problem that such a problem of frequency chirping becomes more significant.

1A is a block diagram showing the structure of a light transmitting element using a conventional surface emitting laser. The optical transmitting element shown in FIG. 1A has a surface emitting laser 1 for generating an optical signal by applying a predetermined electrical signal, a lens 3 for effectively coupling the generated optical signal to an optical fiber, and an optical signal. It consists of an optical fiber 4 for transmission. In addition, a driving current (voltage) I _oper (V _oper ) is applied to drive the surface emitting laser 1. In order to generate a high speed modulated optical signal, as shown in FIG. 1B, the driving current (voltage) I _oper (V _oper ) itself is modulated to drive the surface emitting laser, and is emitted from the surface emitting laser 1. The output signal P _out is used as a modulated optical signal. In this way, when the driving current (voltage) I _oper (V _oper ) of the surface emitting laser 1 is modulated and applied to the surface emitting laser 1, the driving current (voltage) I _oper (V _oper ) is applied to the driving current (voltage) I _oper (V _oper ). As a result, a change in the density of the internal carrier (n) of the surface emitting laser (1) occurs, resulting in a chirping phenomenon in which the oscillation frequency (wavelength) of the emitted optical signal is widened or distorted. (AC chirping) occurs. Due to this chirping phenomenon, the line width of the lasing optical signal is increased, and when the optical signal having a large line width is coupled to a predetermined optical fiber and transmitted through the optical fiber, signal distortion occurs due to dispersion. There is a problem.

FIG. 2 is a diagram illustrating a change in the internal carrier density of the surface-emitting laser according to the change in the driving current (voltage) I _oper (V _oper ) of FIG. 1B and a change in wavelength characteristic of the oscillation optical signal accordingly. As shown in FIG. 2, as the drive current (voltage) I _oper (V _oper ) is changed, the internal carrier density n of the surface emitting laser is changed, thereby changing the internal refractive index of the laser. . This series of changes causes the output signal P _out of the surface emitting laser to change, resulting in a wider line width of the output signal P _out . As will be readily appreciated by those skilled in the art, when the line width of the optical signal oscillated by the laser is widened, dispersion according to the characteristics of the optical fiber material is increased. In other words, when the predetermined information is loaded on the optical signal, chromatic dispersion occurs due to the difference in the transmission speed of light according to the wavelength of the optical fiber. When the transmission distance increases more than a certain distance, the optical signal modulated due to the monochromatic dispersion overlaps with other signals around the center wavelength, which causes the receiver to be unable to demodulate the signal transmitted at the transmitter. In addition, this problem becomes more serious when modulation is performed at high speed (when the modulation frequency is very high), and therefore, when generating a high speed modulation signal of 2.5 Gbps or more, the transmittable distance is very limited due to the problem.

The optical transmission device according to the present invention aims to enable optical transmission without distortion when generating a high speed modulated optical signal.

In addition, the optical transmission device according to the present invention is composed of a surface emitting laser and a polarizing plate, it is an object to transmit a modulated optical signal to a long-distance destination without frequency distortion caused by the change of the internal carrier.

It is also an object of the present invention to propose a novel structure of a polarization control surface emitting laser for transmitting a modulated optical signal without frequency distortion caused by internal carrier changes.

In addition, an optical transmission device using a polarization control surface emission laser according to the present invention is to be able to easily check the operating state and performance of the optical transmission device.

1A to 1B are diagrams showing the structure of an optical transmitter using a conventional surface emitting laser and the light output of the optical transmitter.

FIG. 2 is a diagram illustrating a change in internal carrier density and a change in output beam wavelength according to optical signal modulation in a conventional optical transmitting device using a surface emitting laser.

3 is a cross-sectional view illustrating a layer structure of a surface emitting laser according to an embodiment of the present invention.

4A to 4B are views illustrating the structure and light output of the optical transmitting device using the polarization control surface emission laser according to an embodiment of the present invention.

5 is a view showing a change in the internal carrier density and the output beam wavelength according to the optical signal modulation in the optical transmitting device using a polarization control surface emission laser according to an embodiment of the present invention.

6 is a view showing the structure of an optical transmitting device using a surface emitting laser in another embodiment of the present invention.

Description of the main parts of the drawing

1: Surface-emitting laser 1 ': Polarization control surface-emitting laser

2: polarizer 3: lens

4: optical fiber V _Oper (I _Oper ): driving voltage (current)

V _Polar : Polarization Voltage P _Out : Surface Emission Laser Beam Output

P _Out1 : polarization control surface emission laser beam output

P _Out2 : Polarization control surface emission laser light output after passing through polarizer

n: carrier inside the semiconductor laser

11 semiconductor substrate 12 lower mirror layer

13 insulating layer 14 active layer

15 electrode contact layer 16 polarization control layer

17: polarization electrode contact layer

22: upper mirror layer consisting of an electrode contact layer, a polarization control layer, a polarization electrode layer

18 polarization voltage electrode 19 driving voltage electrode

20: driving voltage electrode 35: photodetector

A surface emitting laser according to a preferred embodiment of the present invention is a semiconductor substrate, a lower mirror layer stacked on the semiconductor substrate, an active layer for gaining on the lower mirror layer, to prevent the current flowing around the active layer An insulating layer for forming an electrode contact layer in contact with an electrode for applying a voltage on the active layer and the insulating layer, a polarization control layer for generating a birefringence by a voltage applied to the electrode contact layer, and applying a polarizing voltage And a polarizing electrode contact layer in contact with the electrode, wherein the electrode contact layer, the polarization control layer, and the polarization electrode contact layer constitute an upper mirror layer as a whole.

In addition, the optical transmission device using a surface-emitting laser according to an embodiment of the present invention is a fiber-optic surface-emitting laser (vertical-cavity surface-emitting laser), a polarizer (polarizer), and the optical signal passing through the polarizing plate optical fiber And a lens for coupling to the lens.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

3 is a configuration cross-sectional view showing the structure of a polarization control surface emitting laser according to an embodiment of the present invention. The polarization control surface emitting laser shown in FIG. 3 adjusts the polarization of the output beam through the polarization voltage V _polar independently of the driving current (voltage I _oper (V _oper )) in the long wavelength region of 1.3 to 1.55 _탆 . Active polarization controlled surface emitting laser.

3, the polarization control surface emitting laser according to the preferred embodiment of the present invention is composed of the following layers. The polarization control surface emission laser according to the present invention shown in FIG. 3 is composed of a semiconductor DBR (Distributed Bragg Reflector) in which n-type InP semiconductor substrates 11 and n-type InAlGaAs / InAlAs (InAlGaAs / InP) are alternately stacked. Lower mirror layer 12, insulating layer 13 composed of air or dielectric to prevent current flow, gain layer and spacer layer composed of multilayer quantum wells of InAlGaAs / InGaAs semiconductors to give gain An active layer 14 consisting of p-type InAlGaAs / InAlAs (InAlGaAs / InP) semiconductor DBR for current injection, an electrode contact layer 15 consisting of a semiconductor DBR, and InAlGaAs / InAlAs for generating birefringence without current flow by voltage. A polarization control layer 16 composed of an InAlGaAs / InP) semiconductor DBR and a polarization electrode layer 17 composed of an n-type InAlGaAs / InAlAs (InAlGaAs / InP) semiconductor DBR are laminated in this order. In addition, the polarization control surface emission laser shown in FIG. 3 has an n-type polarization voltage electrode 18, a p-type driving voltage electrode 19, and an n-type driving voltage electrode 20 for applying current and voltage to the laser device. It includes.

An electrode contact layer 15 composed of a p-type InAlGaAs / InAlAs (InAlGaAs / InP) semiconductor DBR for current injection shown by reference numerals 15 to 17, InAlGaAs / for causing birefringence to occur without current flow by voltage. The polarization control layer 16 composed of InAlAs (InAlGaAs / InP) semiconductor DBR and the polarization electrode layer 17 composed of n-type InAlGaAs / InAlAs (InAlGaAs / InP) semiconductor DBR constitute the upper mirror layer 22 as a whole.

In the polarization control surface emission laser shown in FIG. 3, a constant voltage (current) may be applied between the p-type driving voltage electrode 19 and the n-type driving voltage electrode 20 to emit light of a constant output, and the n-type When a predetermined modulation voltage is applied between the polarization voltage electrode 18 and the p-type driving voltage electrode 19, birefringence is generated in the upper mirror layer 22 according to the all-optical effect. The output light may be modulated using this. At this time, since the linearly polarized light is modulated and emitted in a direction perpendicular to each other according to the modulated polarization voltage, the modulated optical signal can be obtained by passing the modulated and emitted light through a polarizing plate passing only linearly polarized light. do.

4A is a block diagram showing the structure of an optical transmitting device according to an embodiment of the present invention. Referring to FIG. 4A, a light transmitting device according to an exemplary embodiment of the present invention includes a polarization control surface emitting laser 1 ′, a polarizing plate 2 for passing only an optical signal polarized in a predetermined direction, and a polarizing plate 2. A lens 3 for coupling an optical signal passing through the optical fiber to the optical fiber, and an optical fiber 4 for transmitting the optical signal to a predetermined destination. The driving current (voltage) I _oper (V _oper ) is applied to the electrode (see FIG. 3) of the polarization control surface emission laser 31, and the polarization voltage V _polar is applied to the polarization electrode (see FIG. 3). . P _out1 shown in FIG. 4A represents the beam output of the polarization control surface emitting laser 1 ', and P _out2 represents the beam output after passing through the polarizing plate 2, respectively.

FIG. 4B illustrates a change in light outputs P _out1 and P _out2 according to a change in driving current (voltage) I _oper (V _oper ) and polarization voltage V _polar applied to the optical transmitting element illustrated in FIG. 4A. Drawing. Since the driving current (voltage) I _oper (V _oper ) that determines the output of the surface emitting laser 1 'remains constant, the total light output P _out1 of the surface emitting laser 1' remains constant. . As the polarization voltage V _polar is changed, the light output P _out2 passing through the polarizer 2 has a modulation characteristic. At this time, the polarizing plate (2) is a constant polarization voltage (V _polar) is authorized to while passing through the light signal polarized in a predetermined direction, or a constant polarization voltage (V _polar) polarized in a predetermined direction in a state in which which is not applied Modulation characteristics of the optical signal P _out2 passing through the polarizing plate 2 may be controlled by passing the old optical signal. The optical signal P _out2 modulated in this manner is coupled to the optical fiber 4 by the lens 3 and transmitted to a predetermined destination.

FIG. 5 is a view illustrating the change in the internal carrier density n of the polarization-controlled surface-emitting laser and the oscillated light according to the change of the driving current (voltage) I _oper (V _oper ) and the polarization voltage V _polar shown in FIG. 4B. It is a figure which shows the wavelength characteristic of a signal. Since the driving current (voltage) I _oper (V _oper ) is constant, there is no change in the internal carrier density n, thereby resulting in a very sharp line width of the light emitted by the surface emitting laser. Therefore, the problem of the AC chirping phenomenon pointed out as the problem of the prior art can be excluded, so that the line width of the oscillated light is not widened or distorted, and signal distortion is caused by optical fiber material dispersion in proportion to the line width of light. Therefore, it is possible to transmit a high speed modulated optical signal over a long distance.

6 is a block diagram of an optical transmission device using a polarization control surface emission laser according to another embodiment of the present invention. Referring to FIG. 6, an optical transmitting device using a surface emitting laser according to another embodiment of the present invention includes a polarization control surface emitting laser 31, a polarized beam splitter 2, and an optical signal with an optical fiber. A lens 33 for coupling, and an optical fiber 34 for transmitting the optical signal to a predetermined destination. The driving current (voltage) I _oper (V _oper ) is applied to the electrode (see FIG. 3) of the polarization control surface emission laser 31, and the polarization voltage V _polar is applied to the polarization electrode (see FIG. 3). . The light component polarized in a predetermined direction among the light polarized according to the polarization voltage (V _polar ) is passed through the polarization beam splitter 32, is applied to the optical fiber 34 through the lens 33, and the polarized light The vertical component is reflected by the polarization beam splitter 32 and applied to the photodetector 35. The photodetector 35 detects the light reflected by the polarization beam splitter 32, which is out of phase with the light passing through the polarization beam splitter 32. Since the polarized modulated signal can be monitored using the detected light, it is possible to easily perform the normal operation of the optical transmitter and the performance measurement of the optical transmitter (such as power monitoring).

According to the optical transmission device according to the present invention, it is possible to obtain a technical effect that the optical transmission is possible without distortion when generating the modulated optical signal.

In addition, according to the optical transmission device according to the present invention, it is composed of a surface emitting laser and a polarizing plate, it is possible to obtain a technical effect that can transmit a modulated optical signal to a long-distance destination without frequency distortion caused by internal carrier change.

In addition, according to the present invention, it is possible to obtain a technical effect that a new structure of a polarization control surface emitting laser for transmitting a modulated optical signal can be obtained without frequency distortion caused by internal carrier density change.

In addition, according to the optical transmission device using the polarization control surface emission laser according to the present invention, it is possible to obtain a technical effect that it is possible to easily check the operating state and performance of the optical transmission device.

As described above, although the present invention has been described by way of limited embodiments and drawings, the present invention is not limited to the above-described embodiments, which can be variously modified and modified by those skilled in the art to which the present invention pertains. It is obvious that modifications are possible. Accordingly, the spirit of the present invention should be understood only by the claims set forth below, and all equivalent or equivalent modifications thereof will belong to the scope of the present invention.

Claims (8)

For surface emitting lasers, Semiconductor substrates; A lower mirror layer stacked on the semiconductor substrate; An active layer for gaining on the lower mirror layer; An insulating layer for preventing current from flowing around the active layer; An electrode contact layer in contact with the electrode for applying a voltage on the active layer and the insulating layer; A polarization control layer in which birefringence is generated by a voltage applied to the electrode contact layer; And Polarization electrode contact layer in contact with the electrode for applying the polarization voltage Wherein said electrode contact layer, said polarization control layer, and said polarization electrode contact layer constitute an upper mirror layer as a whole. 2. The surface-emitting laser according to claim 1, wherein the polarization of the emitted light is controlled by applying a predetermined polarization voltage between the electrode contact layer and the polarization electrode contact layer. The method of claim 1, The semiconductor substrate is an n-type InP semiconductor substrate, The lower mirror layer is composed of a semiconductor DBR (Distributed Bragg Reflector) in which n-type InAlGaAs / InAlAs (InAlGaAs / InP) are alternately stacked. The active layer is composed of a gain layer and a space layer of a multilayer quantum well of InAlGaAs / InGaAs semiconductor, The electrode contact layer is composed of a p-type InAlGaAs / InAlAs (InAlGaAs / InP) semiconductor DBR, The polarization control layer is composed of InAlGaAs / InAlAs (InAlGaAs / InP) semiconductor DBR, And said polarizing electrode contact layer is composed of an n-type InAlGaAs / InAlAs (InAlGaAs / InP) semiconductor DBR. The method of claim 3, The semiconductor DBR layer constituting the lower mirror layer, the electrode contact layer, and the polarizing electrode contact layer is fabricated by alternately growing a semiconductor thin film layer having different refractive indices in a lattice matched structure to InP, and the thickness of one cycle is the optical length. The surface-emitting laser, characterized in that configured to be half of the laser oscillation wavelength. In the optical transmission element, Vertical-cavity surface-emitting lasers; Polarizers; And Lens for coupling the optical signal passing through the polarizing plate to the optical fiber Optical transmitting element comprising a. The method of claim 5, And the polarizing plate is a linear polarizing plate for passing only linearly polarized beams in a predetermined direction. The method of claim 5, A polarization beam splitter for separating light according to polarization; A photodetector for detecting light reflected by the polarization beam splitter Optical transmitting element further comprises. The method according to claim 5 or 7, The polarization control surface emission laser is an optical transmission element, characterized in that the surface emission laser of claim 1.