WO2006088293A1

WO2006088293A1 - Quantum well laser diode having wide band gain

Info

Publication number: WO2006088293A1
Application number: PCT/KR2006/000424
Authority: WO
Inventors: Bon-Jo Koo; Yong-Kwan Kim; Kwang-Soo Huh; Han-Wook Song
Original assignee: Ls Cable Ltd.
Priority date: 2005-02-18
Filing date: 2006-02-06
Publication date: 2006-08-24
Also published as: JP2008530814A; KR20060092647A; KR100693632B1

Abstract

A quantum well laser diode includes an active layer with a MQW (Multiple Quantum Well) structure for converting an injected current into a light, compound semiconductor PN junction structures formed on both sides of the active layer, and electrodes for injecting a current. The MQW of the active layer is configured so that quantum wells therein have irregular thicknesses.

Description

QUANTUM WELL LASER DIODE HAVING WIDE BAND GAIN

Technical Field

[1] The present invention relates to an LD (Laser Diode), and more particularly to an

LD having MQW (Multiple Quantum Well) that gives a wide band gain. Background Art

[2] Among semiconductor laser diodes, an uncooled DFB LD (Distributed Feedback

Laser Diode) is particularly configured in PBH (Planar Buried Heterostructure) that generally has a buried active layer and a flat surface.

[3] Referring to FIG. 1, a general DFB LD includes an InP substrate 10 acting as a base, an active layer 13 for converting an applied current into light, light restricting layers 12, 14 made of InGaAsP material and provided to both sides of the active layer 13 so that light is well restricted, a diffraction grating 11 provided between the InP substrate 10 and the active layer 13 to make a single-mode wavelength, and a p-InP clad layer 15, an InGaAs layer 16 and an InP layer 17 subsequently provided above the light restricting layer 14.

[4] In particular, the active layer 13 allows generation of light, and it generally adopts a

MQW (Multiple Quantum Well) structure in which quantum wells 13a and barrier layers 13b are repeatedly formed in order to improve performance of LD.

[5] In MQWs provided in conventional LDs, the quantum wells are always configured to have the same thickness. If the thickness of quantum wells is identical as shown in FIG. 2, electrons restricted in a conduction band region of all quantum wells show similar energy states, and energy states of holes in a valence band show similar values.

Thus, energies E made by coupling the electrons in the conduction band with the holes g in the valence band will all exist in similar wavelength ranges (see a gain profile of FIG. 3).

[6] Meanwhile, seeing operations of LD, LD is operated like LED (Light Emitting

Diode) in an initial stage for applying a forward voltage because, in a low voltage region, carriers in the active layer are too insufficient to carry out population inversion, and thus spontaneous emission is superior in that region. However, as a voltage increases, population inversion occurs in the active layer, and at a threshold voltage point where stimulated emission becomes superior, loss of light in the LD becomes equivalent with gains obtained by light amplification. In addition, when a threshold current flows, LD changes from LED operation into laser oscillation. For an applied current over the threshold current, coherent light is emitted from LD by means of stimulated emission. At this time, the wavelength spectrum includes multiple mode wavelengths due to the resonance condition satisfying the Fabry-Perot mode and the gain spectrum profile determined by the MQW structure.

[7] The DFB LD is configured to have a diffraction grating at a place near the active layer of the Fabry-Perot LD. In the DFB LD, a reflective index is changed according to a pitch of the diffraction grating, so the DFB LD selectively outputs only a specific Bragg wavelength suitable for the diffraction grating cycle. That is to say, among several Fabry-Perot modes, only one mode is taken to enable a single-mode wavelength spectrum (namely, a DFB mode).

[8] Generally, the DFB mode and the gain peak (or, the Fabry-Perot mode) have temperature coefficients of 0.1 nm/°C and 0.4 nm/°C respectively. Due to this difference of temperature coefficients, an operation range of the DFB mode is sometimes restricted by temperature change. In general cases, the gain peak is changed 3 to 5 times faster than the DFB mode when temperature changes. Thus, if the DFB mode is coincided with the gain peak, the DFB mode is separated from the gain peak at a lower or higher temperature, and in a worse case, the Fabry-Perot mode is oscillated since the DFB mode is not coupled with the gain peak. The operation temperature range of the DFB mode is a function of a coupling coefficient, and it increases as the coupling coefficient increases. A great coupling coefficient advantageously keeps a threshold current population in a lower level and broadens an operation temperature range of the DFB mode, but it shows a non-linear current-light output characteristic or a Kink characteristic, so the coupling coefficient should be not so great. Thus, in order to make an uncooled DFB LD allowing DFB oscillation in a temperature range of -40 to 80°C, an interval between the gain peak and the DFB mode oscillation wavelength, namely a detuning, has been suitably adjusted with keeping the coupling coefficient in a suitable value so as to control a temperature range of the DFB mode oscillation.

[9] However, the conventional uncooled DFB LD has a narrow gain peak width at -3dB

(or, a width at a point corresponding to a half of the gain peak) since all quantum wells composing the MQW have the same thickness. Thus, though a detuning point is taken for a value satisfying the entire temperature range (-40 to 80°C), an allowable range of the detuning value is narrow. In addition, there is a drawback that uniformity of the gain peak for the entire wafer should be strictly controlled when MQW is grown on a semiconductor substrate.

Disclosure of Invention Technical Problem

[10] The present invention is designed in consideration of the above problems, and therefore it is an object of the invention to provide an LD (Laser Diode) with MQW (Multiple Quantum Well), which has a wide band gain so as to broaden a usable temperature range. Technical Solution

[11] In order to accomplish the above object, the present invention provides a quantum well laser diode, which includes an active layer with a MQW (Multiple Quantum Well) structure for converting an injected current into a light, compound semiconductor PN junction structures formed on both sides of the active layer, and electrodes for injecting a current, wherein the MQW of the active layer is configured so that quantum wells therein have irregular thicknesses. [12] The MQW may have quantum wells whose thicknesses are different from each other. [13] As an alternative, the MQW may have quantum well groups in each of which quantum wells have the same thickness, and each group has a thickness different from other groups. [14] The quantum well laser diode of the present invention may further include an InP substrate acting as a base, and a diffraction grating interposed between the substrate and the active layer to make a light generated in the active layer into a single-mode wavelength. [15] Preferably, the diffraction grating is one selected from the group consisting of an index coupled diffraction grating, a gain coupled diffraction grating, a loss coupled diffraction grating, and a complex coupled diffraction grating. [16] The single-mode wavelength made by the diffraction grating may be included in the range from a visible ray region to an infrared ray region. [17] The quantum well laser diode preferably has a ridged or buried heterostructure as a light waveguide structure. [18] Preferably, strain is applied to the MQW or a barrier layer thereof.

Brief Description of the Drawings [19] These and other features, aspects, and advantages of preferred embodiments of the present invention will be more fully described in the following detailed description, taken accompanying drawings. In the drawings:

[20] FlG. 1 shows a conventional DFB LD (Distributed Feedback Laser Diode);

[21] FIG. 2 shows a MQW (Multiple Quantum Well) structure of the conventional DFB

LD;

[22] FIG. 3 shows a gain profile of the conventional DFB LD;

[23] FIG. 4 is a partially-sectioned perspective view showing an LD (Laser Diode) according to a preferred embodiment of the present invention; [24] FIG. 5 shows a MQW structure of the LD shown in HG. 4;

[25] FIG. 6 shows a gain profile of the LD according to the preferred embodiment of the present invention;

[26] FlG. 7 is a graph showing the change of gain peak and Bragg wavelength according to a temperature change of the conventional DFB LD; and

[27] FIG. 8 is a graph showing the change of gain peak and Bragg wavelength according to a temperature change of the DFB LD according to the present invention. Best Mode for Carrying Out the Invention

[28] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.

[29] FIG. 4 is a partially-sectioned perspective view showing a quantum well laser diode according a preferred embodiment of the present invention.

[30] Referring to FIG. 4, the quantum well laser diode according this embodiment includes an active layer 102 provided between PN junction structures of a compound semiconductor and having a MQW (Multiple Quantum Well) structure, and electrodes 101a, 101b for injecting current. The quantum well laser diode according this embodiment is also configured so that quantum wells composing the MQW have irregular thicknesses. A waveguide structure provided in this LD may preferably adopt a well-known ridged or buried heterostructure.

[31] More specifically, the LD according to the preferred embodiment of the present invention is configured so that an active layer 102 is formed by etching on an InP subs trate 100 in a mesa shape with a width of about 1 to 1.5 D, current cut-off layers 103 having p-InP layer and n-InP layer are grown on both sides of the etched active layer 102, and a p-InP clad layer 104 is then grown above the active layer 102. Here, the current cut-off layers 103 play a role of preventing an injected current from being leaked out of the active layer 102.

[32] Preferably, a diffraction grating 108 may be provided between the InP substrate 100 and the active layer 102 so as to make a single-mode wavelength. This diffraction grating 108 preferably employs an index coupled type, a gain coupled type, a loss coupled type, or a complex coupled type. In addition, the single-mode wavelength made by the diffraction grating 108 is preferably included in the range from a visible ray region to an infrared ray region.

[33] In the LD according to the preferred embodiment of the present invention, a U- channel 107 formed by etching the p-InP layer 104 and the current cut-off layer 103 into a substantial U shape is prepared so as to reduce a parasitic electrostatic capacitance.

[34] In addition, an InGaAs layer 105 and an insulation layer 106 are deposited on the

U-channel 107, and then the insulation layer 106 inside the U-channel 107 is selectively removed. In the region where the insulation layer 106 is removed, a p-type electrode 101b corresponding to a n-type electrode 101a on a lower surface of the InP substrate is formed with a predetermined pattern.

[35] The above structure having multi layers is cut into a size suitable for an LD in the wafer cutting process. Then, a front facet of the cut structure is coated with a non- reflective film (not shown), and a rear facet is coated with a high-reflective film 109 so as to enhance light output efficiency further.

[36] In particular, the active layer 102 plays a role of converting a current injected through the electrodes 101a, 101b into a light, and it has a MQW structure. Here, the MQW structure is configured with quantum wells whose thicknesses are different from each other. As an alternative, the MQW structure may include several quantum well groups in each of which quantum wells have the same thickness, and each group has a thickness different from other groups.

[37] The MQW has a structure in which quantum wells and barrier layers are repeatedly formed, and strain may be applied to each quantum well or each barrier so as to control characteristics of the LD.

[38] As shown in FlG. 5, if the quantum wells have different thicknesses from each other, the quantum wells in a conduction band have various distributed energy states, and similarly the holes in the valence band have different energy values from each other. Thus, energies E made by coupling the electrons in the conduction band with g the holes in the valence band exist over a wide wavelength range, so the gain profile according to the wavelength has a relatively wider gain width as shown in FIG. 6, in comparison to the conventional one shown in FIG. 3.

[39] Meanwhile, in case an uncooled DFB LD is made using quantum wells having the same thickness according to the prior art, the gain peak and the Bragg wavelength are changed as shown in FIG. 7. That is to say, when the gain peak (see a center profile) and the DFB mode Bragg wavelength A are coincided at a normal temperature T2, a moving speed of the gain peak is faster than a moving speed of the Bragg wavelength (see B at Tl, C at T3) at a low temperature Tl (see a left profile) or a high temperature T3 (see a right profile). Thus, a gain of the Bragg wavelength is smaller than a DFB threshold gain (see a criterion line I) at a low temperature Tl or a high temperature T3, so DFB oscillation is not generated but the Fabry-Perot wavelength at a peak point of the gain profile at a low or high temperature is oscillated.

[40] Meanwhile, if the MQWs have thickness as shown in FIG. 5 according to the present invention, a gain width is relatively broad. Thus, in case of making an uncooled DFB LD, the gain peak and the Bragg wavelength are changed as shown in FIG. 8. That is to say, when the gain peak (see a center profile) and the Bragg wavelength A are coincided at a normal temperature T2, a moving speed of the gain peak is faster than a moving speed of the Bragg wavelength (see B at Tl, C at T3) at a low temperature Tl (see a left profile) or a high temperature T3 (see a right profile). However, since the gain profile has a great width, the gain of the Bragg wavelength at a low temperature Tl or a high temperature T3 is sufficiently greater than the DFB threshold gain (see a criterion line I), so DFB oscillation is generated but the Fabry- Perot wavelength at a peak point of the gain profile is not oscillated.

[41] Thus, if all configurations have the same condition except quantum wells, the LD having MQW employed in the present invention allows DFB mode oscillation in a wider temperature range in comparison to the conventional one.

[42] In the present invention, a quantum well making process may employ a conventional semiconductor process.

[43] The present invention has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. Industrial Applicability

[44] The DFB LD according to the present invention has a gain profile with a broader width than the conventional one, thereby allowing DFB oscillation in a wider temperature range.

[45] In addition, since an irregular MQW structure is employed, there is no need to strictly control uniformity of gain peaks of the entire wafer during the manufacturing procedure.

Claims

[1] A quantum well laser diode, which includes an active layer with a MQW

(Multiple Quantum Well) structure for converting an injected current into a light, compound semiconductor PN junction structures formed on both sides of the active layer, and electrodes for injecting a current, wherein the MQW of the active layer is configured so that quantum wells therein have irregular thicknesses.

[2] The quantum well laser diode according to claim 1, wherein the MQW has quantum wells whose thicknesses are different from each other.

[3] The quantum well laser diode according to claim 1, wherein the MQW has quantum well groups in each of which quantum wells have the same thickness, and each group has a thickness different from other groups.

[4] The quantum well laser diode according to claim 1, further comprising: an InP substrate acting as a base; and a diffraction grating interposed between the substrate and the active layer to make a light generated in the active layer into a single-mode wavelength.

[5] The quantum well laser diode according to claim 4, wherein the diffraction grating is one selected from the group consisting of an index coupled diffraction grating, a gain coupled diffraction grating, a loss coupled diffraction grating, and a complex coupled diffraction grating.

[6] The quantum well laser diode according to claim 4, wherein the single-mode wavelength made by the diffraction grating is included in the range from a visible ray region to an infrared ray region.

[7] The quantum well laser diode according to claim 1, wherein the quantum well laser diode has a ridged or buried heterostructure as a light waveguide structure.

[8] The quantum well laser diode according to claim 1, wherein strain is applied to the MQW or a barrier layer thereof.