KR20060092647A - Quantum well laser diode having wide band gain - Google Patents

Abstract

According to the present invention, a quantum including an active layer having a multi-quantum well structure for converting injected current into light, a PN junction structure of a compound semiconductor formed with the active layer interposed therebetween, and an electrode for injecting current In a well laser diode, the quantum well laser diode is disclosed, wherein the multiple quantum wells of the active layer have a structure in which the thickness of the quantum wells is not constant.

DFB LD, active layer, multiple quantum well, diffraction grating, DH

Description

Quantum well laser diode with wide-band gain {QUANTUM WELL LASER DIODE HAVING WIDE BAND GAIN}

The following drawings attached to this specification are illustrative of preferred embodiments of the present invention, and together with the detailed description of the invention to serve to further understand the technical spirit of the present invention, the present invention is a matter described in such drawings It should not be construed as limited to

1 is a block diagram of a distributed feedback laser diode according to the prior art.

Figure 2 is a structure diagram of a multi-quantum well of a distributed feedback laser diode according to the prior art.

Figure 3 is a gain profile of the distributed feedback laser diode according to the prior art.

4 is a cutaway perspective view showing the configuration of a laser diode according to a preferred embodiment of the present invention.

5 is a structural diagram of a multi-quantum well of FIG. 4.

6 is a gain profile of a laser diode according to a preferred embodiment of the present invention.

7 is a graph showing changes in gain peaks and Bragg wavelengths according to temperature changes of a distributed feedback laser diode according to the prior art.

8 is a graph showing changes in gain peaks and Bragg wavelengths according to temperature changes of the distributed feedback laser diode according to the present invention;

<Description of main reference numerals in the drawings>

100 ... InP substrate 101a ... n type electrode

101b ... p-type electrode 102 ... multiple quantum wells

103 ... current blocking layer 104 ... p-InP cladding layer

105 ... InGaAs layer 106 ... Insulation layer

107 ... U-channel 108 ... Diffraction grating

109 ... High Reflective

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode (LD), and more particularly, to a laser diode having a multiple quantum well (MQW) structure that provides a broadband gain.

In particular among semiconductor laser diodes, an uncooled distributed feedback laser diode (Uncooled DFB LD) typically consists of a Planar Buried Heterostructure (PBH) structure having a buried active layer and a flat surface.

Referring to FIG. 1, a typical distributed feedback laser diode includes an InP substrate 10 serving as a base, an active layer 13 for converting injected current into light, and an active layer 13 so that light is well constrained. The photoresist layers 12 and 14 of the InGaAsP material provided at both sides, the diffraction grating 11 provided between the InP substrate and the active layer 13 to form a single mode wavelength, and the optical confinement layer 14 The p-InP cladding layer 15, the InGaAs layer 16, and the InP layer 17 are sequentially provided.

In particular, the active layer 13 is a portion where light is generated. In recent years, in order to improve the performance of a laser diode, a multi-quantum well structure in which a quantum well 13a and a barrier layer 13b are repeatedly formed is generally provided.

The multiple quantum wells provided in the conventional laser diode have a structure in which the quantum wells have the same thickness, as shown in FIG. 2. Since the energy states are similar and the energy states of the holes in the valence band all have similar values, the energy (E _g ) produced by the electrons in the conduction band combined with the holes in the valence band is present in the same wavelength range. (See gain profile in FIG. 3).

Looking at the operation of the laser diode, on the other hand, in the initial application of the forward voltage, the laser diode acts like an LED, which means that the carrier of the active layer in the low voltage region is not enough to be a density inversion (Spontaneous). This is because emission is dominant. As the voltage increases, density inversion occurs in the active layer, and at the threshold voltage point at which stimulated emission predominates, the loss of light in the laser diode and the gain due to the optical amplification are balanced and the threshold When the current flows, a change from LED behavior to laser oscillation occurs. In the injection current above the threshold current, coherent light is emitted from the laser diode by the stimulus emission, and the wavelength spectrum is the resonance condition satisfying the Fabry-Perot mode and the multi-quantum well structure. The gain spectral profile determined by the includes the wavelength of the multiple mode.

The distributed feedback laser diode (DFB LD) has a structure having a diffraction grating near the active layer of the Fabry-Perot laser diode. The reflectance index is changed by the pitch of the diffraction grating so that the gain spectrum is increased. Selectively outputs only a specific Bragg wavelength for the diffraction grating period. That is, only one mode among the multiple Fabry-Perot modes is selectively taken to enable the single mode wavelength spectrum (DFB mode).

In general, the temperature coefficients of the DFB mode and the gain peak (Febri-Perot mode) have values of about 0.1 nm / ° C. and 0.4 nm / ° C., respectively. These differences in temperature coefficients often limit the operating range of the DFB mode with temperature changes. Normally, the gain peak moves three to five times faster than the DFB mode when the temperature changes, so if the gain peak coincides with the DFB mode, the DFB mode will be separated from the gain peak at lower or higher temperatures, and if the DFB mode and gain peak are severe, Because of the inability to combine, the Fabry-Perot mode oscillates. The temperature operating range in DFB mode is a function of the coupling coefficient and increases as the coupling coefficient increases. The large coupling coefficient has the advantage of keeping the threshold current density low and widening the operating temperature range of the DFB mode, but it does not increase the coupling coefficient because it shows nonlinear current-light output characteristics or kink characteristics. Therefore, in order to make an uncooled distributed feedback laser diode capable of DFB oscillation in the temperature range of about -40 to 85 ° C., the distance between the gain peak and the DFB mode oscillation wavelength while maintaining the coupling coefficient at an appropriate value is known. A method of controlling the temperature range of DFB mode oscillation was used by adjusting the detuning appropriately.

However, in the conventional uncooled distributed feedback laser diode, the width of the gain peak at -3 dB (the width of the half point of the gain peak), since the thicknesses of the quantum wells constituting the multiple quantum wells are all uniform. Because of this narrowness, the allowable range of the detuning value becomes small even if the detuning point is obtained for a value satisfying the entire temperature range (-40 to 85 ° C). In addition, when growing multiple quantum wells on a semiconductor substrate, there is a vulnerability that must strictly control the gain peak uniformity of the entire wafer.

SUMMARY OF THE INVENTION The present invention has been made in view of the above, and an object thereof is to provide a laser diode having a multi-quantum well structure having a broadband gain to widen a use temperature range.

In order to achieve the above object, the present invention provides an active layer having a multi-quantum well structure, which converts the injected current into light, a PN junction structure of a compound semiconductor formed with the active layer interposed therebetween, and injection of current. In the quantum well laser diode comprising an electrode for, the multi-quantum well of the active layer, characterized in that the thickness of the quantum well has a constant structure.

The multiple quantum wells may be configured such that thicknesses of quantum wells are different from each other.

Alternatively, the multi quantum well may have a structure having a different thickness for each group of quantum wells having the same thickness.

The present invention may further include an InP substrate serving as a base and a diffraction grating interposed between the substrate and the active layer to make light generated in the active layer into a single mode wavelength.

Preferably, the diffraction grating may be an index coupled type, a gain coupled type, a loss coupled type, or a complex coupled type.

The single mode wavelength produced by the diffraction grating may be included in the range from the visible light region to the infrared region.

As the waveguide structure provided in the present invention, it is preferable to adopt a ridge type or buried heterostructure.

Preferably, strain may be applied to the multiple quantum well or its barrier layer.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, terms or words used in the specification and claims should not be construed as having a conventional or dictionary meaning, and the inventors should properly explain the concept of terms in order to best explain their own invention. Based on the principle that can be defined, it should be interpreted as meaning and concept corresponding to the technical idea of the present invention. Therefore, the embodiments described in the specification and the drawings shown in the drawings are only the most preferred embodiment of the present invention and do not represent all of the technical idea of the present invention, various modifications that can be replaced at the time of the present application It should be understood that there may be equivalents and variations.

4 is a cutaway perspective view showing the configuration of a quantum well laser diode according to a preferred embodiment of the present invention.

4, the quantum well laser diode according to the preferred embodiment of the present invention is provided between the PN junction structure of the compound semiconductor, the active layer 102 having a multi-quantum well structure, and the electrode for the injection of current ( 101a and 101b), and the thickness of the quantum wells constituting the multi quantum well is not constant. As the waveguide structure provided in such a laser diode, a well-known raised or buried heterostructure can be preferably employed.

More specifically, the laser diode according to the preferred embodiment of the present invention is formed by etching the active layer 102 in a mesa shape having a width of about 1 ~ 1.5㎛ on the InP substrate 100, the etched active layer ( 102) The current blocking layer 103 of the p-InP layer and the n-InP layer is grown on both sides, and the p-InP cladding layer 104 is grown on the active layer 102 again. Here, the current blocking layer 103 serves to block the injected current from leaking to regions other than the active layer 102.

Preferably, a diffraction grating 108 may be provided between the InP substrate 100 and the active layer 102 to create a single mode wavelength. Such a diffraction grating 108 is preferably a known exponential coupling type, gain coupling type, loss coupling type or complex coupling type. In addition, the single mode wavelength produced by the diffraction grating is preferably included in the range from the visible light region to the infrared region.

In addition, to reduce the parasitic capacitance, a U-channel 107 formed by etching the p-InP layer 104 and the current blocking layer 103 in a U-shape is provided, and on top of the U-channel 107. After the InGaAs layer 105 and the insulating layer 106 are deposited, the insulating layer 106 inside the U-channel 107 is selectively removed, and the n-type electrode 101a on the bottom surface of the InP substrate 100 is removed. The corresponding p-type electrode 101b is formed in a certain pattern.

After the multilayer structure is formed as described above, an antireflective film (not shown) is coated on the front side of the cutting surface (Facet), which is formed through a wafer cutting process, and a high reflective film 109 is coated on the rear side to improve light output efficiency. Will be higher.

In particular, the active layer 102 converts the current injected through the electrodes 101a and 101b into light, and has a multi-quantum well structure. Here, the quantum wells constituting the multi-quantum well structure have different thicknesses or different thicknesses for each group of quantum wells having the same thickness.

The multi-quantum well has a structure in which a quantum well and a barrier layer are repeatedly formed, and a strain may be applied to each quantum well or the barrier layer to control the characteristics of the laser diode.

As illustrated in FIG. 5, when the quantum wells are formed to have different thicknesses, the energy states of the quantum wells in the conduction band region are distributed in various ways, and the energy states of the holes in the valence band region all have different values. Therefore, since the energy E _g generated by the electrons in the conduction band region combined with the holes in the valence band region is present over a wide wavelength region, a gain profile according to the wavelength is shown in FIG. It has a relatively wide gain range compared to the prior art described.

On the other hand, when manufacturing a non-cooled distributed feedback laser diode with a constant thickness of the quantum wells according to the prior art, as shown in Figure 7, the gain peak (see the center profile) and the DFB mode Bragg wavelength (at room temperature T2) When A) is matched, at low temperature T1 (see left profile) or at high temperature T3 (see right profile), the gain peak travels faster than the Bragg wavelength (T1 is B and T3 is C). The gain value of the Bragg wavelength at T3 is smaller than the DFB threshold gain value (see baseline I) so that DFB oscillation does not occur and the Fabry-Perot wavelength oscillates at the peak point of the low or high temperature gain profile.

However, according to the present invention, when the thickness of the multi-quantum well is formed as shown in FIG. 5, the gain width is relatively wide, and thus, when the uncooled distributed feedback laser diode is manufactured, the gain peak (center) at room temperature T2 is shown in FIG. 8. When the Bragg wavelength (A) is matched with the Bragg wavelength (A), the shift speed of the gain peak (T1 is on the left, T3 is on the right profile) at the low temperature T1 or T3 is the shift of the Bragg wavelength (T1 is B, T3 is C) Although faster than speed, the gain profile is wider, so the Bragg wavelength gain at low temperature T1 or T3 is sufficiently larger than the DFB threshold gain (see Baseline I), resulting in DFB oscillation and no Fabry-Perot wavelength oscillation.

Therefore, if all configurations except the quantum well are the same condition, when using the multi-quantum well used in the present invention, DFB mode oscillation is possible in a wider temperature range than in the prior art.

In the present invention, the process of manufacturing the quantum well itself may be performed using conventional semiconductor processing techniques.

Although the present invention has been described above by means of limited embodiments and drawings, the present invention is not limited thereto and will be described below by the person skilled in the art to which the present invention pertains. Of course, various modifications and variations are possible within the scope of the claims.

Since the distributed feedback laser diode according to the present invention has a wider gain profile than in the related art, DFB oscillation is possible over a wide use temperature range.

In addition, there is an advantage in that it is not necessary to strictly control the gain peak uniformity of the entire wafer during manufacturing because of using a nonuniform multi-quantum well structure.

Claims (8)

In a quantum well laser diode comprising an active layer having a multi-quantum well structure for converting the injected current into light, a PN junction structure of a compound semiconductor formed between the active layer and an electrode for injection of current , The quantum well laser diode of the active layer has a structure in which the thickness of the quantum wells is not constant. The method of claim 1, The multi-quantum well, quantum well laser diode, characterized in that the thickness of each quantum well different. The method of claim 1, The multi-quantum well laser diode, characterized in that having a different thickness for each group of quantum wells having the same thickness. The method of claim 1, A base InP substrate is provided, And a diffraction grating interposed between the substrate and the active layer to make light generated in the active layer into a single mode wavelength. The method of claim 4, wherein The diffraction grating is a quantum well laser diode, characterized in that the exponential coupling type, gain coupling type, lossy coupling type or complex coupling type is employed. The method of claim 4, wherein The single mode wavelength generated by the diffraction grating is included in the range from the visible region to the infrared region quantum well laser diode. The method of claim 1, A quantum well laser diode, wherein a raised or buried heterostructure is employed as the waveguide structure. The method of claim 1, Strain is applied to the multi-quantum well or its barrier layer.

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