KR100861719B1 - Transverse Magnetic Mode Laser Diode - Google Patents

Abstract

The present invention relates to a TM mode laser diode that stably provides a TM mode oscillation, the TM mode laser diode is an n-type doped layer formed on a substrate, an active layer formed on the n-type doped layer, and on the active layer An optical waveguide including a p-type doping layer formed on the optical waveguide; A photonic crystal mirror formed of at least one end of both ends of the optical waveguide in which the n-type doped layer, the active layer, and the p-type doped layer are stacked to reflect a transverse magnetic mode; An n-type electrode formed under the substrate; And a p-type electrode formed on the p-type doping layer, wherein the photonic crystal is a semiconductor film or a plurality of semiconductor bars periodically disposed in an air layer including a plurality of air holes formed periodically, and the TM of the photonic crystal The mode is determined by the area occupied by the air holes or the semiconductor rods of the total area of the photonic crystal and the frequency passing through the photonic crystal. Accordingly, a photonic crystal that reflects the TM mode and transmits the TE mode can be used as a mirror so that the TE mode can have a high threshold gain due to large mirror loss, thereby stably oscillating the TM mode.

Laser Diode, TM Mode, Photonic Crystal, Photonic Crystal Mirror

Description

TM mode laser diode {Transverse Magnetic Mode Laser Diode}

FIG. 1A is a diagram of one embodiment of a two-dimensional photonic crystal structure to be employed in the laser diode of the present invention, and FIG. 1B is a diagram of another embodiment of a two-dimensional photonic crystal structure to be employed in the laser diode of the present invention.

FIG. 2A is a side cross-sectional view illustrating a schematic structure of a general laser diode, and FIG. 2B is a view for illustrating a polarization direction of a general laser diode.

3 is an optical bandgap diagram according to the width of the entire air hole included in the photonic crystal structure of FIG.

4 is a schematic side cross-sectional view of a laser diode according to an embodiment of the present invention in which a photonic crystal mirror of a photonic crystal structure having an optical bandgap for a TM mode is employed on one side.

FIG. 5 is a schematic side cross-sectional view of a laser diode according to another embodiment of the present invention in which a photonic crystal mirror of a photonic crystal structure having an optical bandgap for a TM mode on both sides is employed.

** Explanation of symbols on the main parts of the drawing **

11: air layer 12: semiconductor rod

13: semiconductor medium 14: air hole

20, 40, 50: semiconductor laser diode

21, 41: n-type electrode 22, 42: substrate

23, 43: n-type doped layer 23a: n-waveguide layer

23b; n-clad layers 24 and 44: active layer

25, 45 p-type doped layer 25a: p-waveguide layer

25b: p-clad layer 26: p-type electrode

a: electric field vibration direction in TM mode

b: Electric field vibration direction in TE mode

c: TM mode optical bandgap d: TE mode optical bandgap

47, 51, 53: photonic crystal mirrors 48, 52, 54: antireflective coating layer

The present invention relates to a TM mode laser diode, and more particularly, to a TM mode laser diode that stably outputs light in a transverse magnetic (TM) mode using a photonic crystal.

Due to the development of information and communication environment, optical devices are used in various fields. Especially, laser diodes are relatively small and the threshold current for laser oscillation is smaller than that of general laser devices. It is widely used as an optical device for high speed data transmission and high speed data writing and reading in the field. In addition, a laser diode is used as a means for exciting the plasmon when generating a long period surface plasmon in the metal waveguide to transmit the signal.

At this time, the surface plasmon is generated only in the TM mode of the polarization of the light output of the laser diode, the active layer of the laser diode is used as a quantum well having a tensile stress to produce the TM mode. When the quantum well is subjected to tensile stress, the energy transition by the heavy-hole is larger than that in the light-hole, so that the probability of transition is reduced, and thus the light-hole The transition predominantly causes the laser diode to operate in TM mode.

However, as described above, even if the energy of the light-hole is lowered by using tensile stress, the energy is changed by the quantum effect in the quantum well structure. In this case, the energy increase of the light-hole having a low effective mass is increased and the energy of the heavy-hole is increased. Since the increase is smaller than that, it may occur that the direction of energy change of the effect of the tensile stress and the quantum effect is opposite to each other. Accordingly, the TM mode operation is possible when the effect of the tensile stress is large, but the TM mode operation may not be performed when the quantum effect is larger. In particular, in the case of a long wavelength laser diode, the quantum effect It is not easy to perform the TM mode operation because can be larger.

Accordingly, the present invention is an invention devised to solve the above-mentioned problems, and an object of the present invention is to form a mirror used for at least one end of an optical waveguide in a laser diode including an optical waveguide, and selectively reflect only a TM mode. It is to provide a laser diode that performs a stable TM mode operation.

According to an aspect of the present invention for achieving the above object, the present invention is an n-type doping layer formed on a substrate, an active layer formed on the n-type doped layer, and p-type doping formed on the active layer An optical waveguide comprising a layer; A photonic crystal mirror formed of at least one end of both ends of the optical waveguide in which the n-type doped layer, the active layer, and the p-type doped layer are stacked to reflect a transverse magnetic mode; An n-type electrode formed under the substrate; And a p-type electrode formed on the p-type doping layer, wherein the photonic crystal has a form in which a plurality of semiconductor bars are periodically disposed in a semiconductor film or air layer including a plurality of air holes formed periodically and the TM of the photonic crystal The mode is determined by the area occupied by the air holes or the semiconductor rods of the total area of the photonic crystal and the frequency passing through the photonic crystal.

Preferably, it further comprises an antireflective coating layer formed on the exposed surface of the photonic crystal mirror. The n-type doped layer includes an n-clad layer and an n-waveguide layer, and the p-type doped layer includes a p-clad layer and a p-waveguide layer.

delete

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings so that those skilled in the art may easily implement the technical idea of the present invention.

In general, a photonic crystal refers to an artificial crystal in which materials having different dielectric constants are periodically arranged to form a photon band gap (PBG) in the energy spectrum of electromagnetic waves. When light is reflected in a photonic crystal For most wavelengths, light passes through the photonic crystal without scattering, but at certain wavelengths (or frequencies) bands there is a reflecting region where the light cannot pass, which is called the photon band gap (or optical band). Gap).

When light having a wavelength (or frequency) that falls within the photon band gap is incident into the photonic crystal, the light does not propagate into the photonic crystal and is reflected. This photon bandgap can be used to confine light into a cavity or into a waveguide. Photonic crystals are formed by periodically arranging dielectric materials, and the size or position of the photon band gap varies depending on the refractive index and the periodic structure. By using the optical crystal characteristic as described above, it can be applied to optical devices such as branch filters, optical waveguides, optical delay devices, and lasers.

FIG. 1A is a diagram of one embodiment of a two-dimensional photonic crystal structure to be employed in the laser diode of the present invention, and FIG. 1B is a diagram of another embodiment of a two-dimensional photonic crystal structure to be employed in the laser diode of the present invention. The photonic crystal structure shown in FIG. 1A is a structure in which semiconductor rods 12 are periodically and repeatedly arranged in a medium 11 made of air, and the photonic crystal structure shown in FIG. 1B is a bar made of air in a medium 13 made of semiconductor. (14, hereinafter, air holes) are arranged periodically. Since the two photonic crystal structures have two-dimensional refractive index periods, a photon band gap phenomenon occurs in which a reflection region in which light in a specific range of wavelength bands does not travel in a specific direction is generated.

FIG. 2A is a side cross-sectional view illustrating a schematic structure of a general laser diode, and FIG. 2B is a view for illustrating a polarization direction of a general laser diode. Referring to FIG. 2A, a semiconductor laser diode 20 is formed on a substrate 22, a substrate 22, and an n-doped layer 23 including an n-clad layer 23b and an n-waveguide layer 23a. ), an active layer 24 formed on the n-doped layer 23, a p-doped layer 25 formed on the active layer 24 and including a p-waveguide layer 25a and a p-clad layer 25b. ), An n-type electrode 21 formed under the substrate 22, and a p-type electrode 26 formed on the p-doped layer 25.

Referring to FIG. 2B, the above-described laser diode 20 has an optical waveguide, that is, an electric field passing through the n-waveguide layer 23a, the active layer 24, and the p-waveguide layer 25a is parallel to the optical waveguide. In the direction, the transverse electric wave (TE) mode (b) is defined. In the direction perpendicular to the active layer 24, the transverse electromagnetic wave (TM) mode (a) is defined. If the direction of the electric field (or magnetic field) is different, it affects not only the shape of the mode formed in the optical waveguide region but also the change of the refractive index according to the path of light propagation. That is, the reflection or refraction pattern of light differs depending on the TE mode (b) or the TM mode (a).

3 is an optical bandgap diagram according to an area ratio of air in the photonic crystal structure of FIG. 1B. Referring to FIG. 3, the horizontal axis represents an air hole factor (%), and the vertical axis represents a normalized frequency (u = a / λ). The air hole factor shown on the horizontal axis represents the ratio of air holes to the total area of the photonic crystal, and the vertical axis represents the specific frequency range provided accordingly. Referring to the graph shown, it can be seen that the frequency of light that does not pass through the photonic crystal varies depending on the area of the air hole. The optical bandgap is shown only for the TM mode (or the TE mode) depending on the conditions.

According to the graph, the air hole factor ranges from 0.2 to 0.6 (ie, 20 to 60% of the total photonic crystal area), and when the frequency is 0.5 or more, the TM mode optical bandgap c is shown. When a user uses a photonic crystal having the above conditions at a desired wavelength as a mirror, a mode-selective mirror having a high reflectance with respect to the TM mode of the wavelength and transmitting the TE mode is provided. Of course, when the air hole factor range is 0.2 to 0.6 (i.e., 20 to 60% of the total photonic crystal area) and the frequency is 0.4 or less, the TE mode optical bandgap (d) is represented. Using a photonic crystal with a condition as a mirror becomes a mode-selective mirror that has a high reflectance for the TE mode of that wavelength and transmits the TM mode. According to FIG. 3, since the normalized frequency (u = a / λ) is in the range of 0.45 to 0.6 and the TM bandgap is in the air hole factor of 0.2 to 0.7, assuming that the wavelength of use is 0.8 to 1.5 m, The period (a) of the air hole grating is in the range of 0.36 to 0.9 mu m.

4 is a schematic side cross-sectional view of a laser diode according to an embodiment of the present invention in which a photonic crystal mirror of a photonic crystal structure having an optical bandgap for a TM mode is employed on one side.

Referring to FIG. 4, the laser diode 40 includes an optical waveguide region including an substrate 42, an n-doped layer 43, an active layer 44, and a p-doped layer 45, and an n-type electrode 41. ) And p-type electrode 46. In addition, the present laser diode 40 includes a photonic crystal mirror 47 formed on one side of an optical waveguide region constituting the laser diode 40 and an antireflective coating layer 48 formed on an exposed surface of the photonic crystal mirror 47. It includes more. The photonic crystal mirror 47 is composed of a photonic crystal having a TM mode optical band gap to stably reflect the TM mode, the anti-reflective coating layer 48 is the light transmitted through the photonic crystal 47 made of photonic crystal is reflected in the cross section It is a structure to prevent the incident again inside the optical waveguide region. The antireflective coating layer may be made of TiO ₂ and SiO ₂ , and hardly reflects, and has a reflectance of 0.0001 to 1% or less. Although not shown in the present exemplary embodiment, a mirror made of photonic crystal or a reflective surface not made of photonic crystal may be further formed on the other side of the optical waveguide region.

Therefore, as shown in FIG. 3, if the photonic crystal (mirror) is designed such that only the TM mode exists in the optical bandgap at a specific wavelength, the TE mode having a weak feedback is relatively harder to laser than the TM mode. The conditions for the laser to oscillate are as follows.

…(식)

… (expression)

Α _i is the internal loss, L is the length of the optical waveguide (resonator), and R ₁ and R ₂ represent the reflectances of the mirrors provided in the laser diode, respectively. In order for the laser to oscillate, the gain of the mode must overcome the total loss indicated by the right side of the equation. In the embodiment of FIG. 4, a mirror in which R ₁ (or R ₂ ) differs greatly depending on the mode is formed using a photonic crystal and applied to a Fabry-Perot laser diode. As described above, if R ₁ (or R ₂ ) is large in the TM mode and small in the TE mode, the second term on the right side becomes large in the TM mode, so that oscillation can be performed at a lower gain.

FIG. 5 is a schematic side cross-sectional view of a laser diode according to another embodiment of the present invention in which a photonic crystal mirror of a photonic crystal structure having an optical bandgap for a TM mode on both sides is employed.

Referring to FIG. 5, the laser diode 50 according to the present embodiment is formed on the photonic crystal mirrors 51 and 53 made of photonic crystals reflecting the TM mode in both end regions and the exposed surfaces of the mirrors 51 and 53, respectively. Antireflective coating layers 52 and 54. As shown in FIG. 3, if the photonic crystal used in this embodiment is also a photonic crystal designed to have a TM band optical bandgap at a specific wavelength, the TE mode with weak feedback exits the optical waveguide region of the laser diode without feedback. TM mode reciprocates the optical waveguide with high reflectance, resulting in a more stable TM mode laser diode.

Although the above has been described with reference to the accompanying drawings, the technical idea of the present invention is illustrative of the best embodiment of the present invention and is not intended to limit the present invention. In addition, any person having ordinary skill in the art may make various modifications without departing from the scope of the technical idea of the present invention.

According to the above-described configuration, the present invention can stably oscillate the TM mode by making one or both mirrors of an optical waveguide (resonator) in a laser diode into a photonic crystal having an optical bandgap only for the TM mode.

Claims (5)

An optical waveguide including an n-type doped layer formed on a substrate, an active layer formed on the n-type doped layer, and a p-type doped layer formed on the active layer; A photonic crystal mirror formed of at least one end of both ends of the optical waveguide in which the n-type doped layer, the active layer, and the p-type doped layer are stacked to reflect a transverse magnetic mode; An n-type electrode formed under the substrate; And A p-type electrode formed on the p-type doping layer Including, The photonic crystal has a form in which a plurality of semiconductor bars are periodically disposed in a semiconductor film or air layer including a plurality of periodically formed air holes, and the TM mode of the photonic crystal is occupied by the air holes or the semiconductor bars in the entire area of the photonic crystal. TM mode laser diode determined by area and frequency passing through the photonic crystal. The method of claim 1, TM mode laser diode further comprising an anti-reflective coating layer formed on the exposed surface of the photonic crystal mirror. delete delete The method of claim 1, The n-type doped layer includes an n-clad layer and an n-waveguide layer, and the p-type doped layer includes a p-clad layer and a p-waveguide layer.

Images