Patent Application
20180076597
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2016-179140, filed on Sep. 14, 2016; the entire contents of which are incorporated herein by reference.
Embodiments described herein relate generally a quantum cascade laser device.
A quantum cascade laser device emits infrared in a wider wavelength range by using the intersubband transition of carriers such as electrons, etc.
The infrared wavelength is determined by the energy level of quantum well layers having a cascade connection. For example, the controllability and reproducibility of the thicknesses of the well layers, the mole ratios of the constituent elements, etc., between wafers can hardly be called sufficient in the case where the thickness of each of the well layers is thin and is 10 nm or less. Therefore, the infrared wavelength may shift from the prescribed range.
In general, according to one embodiment, a quantum cascade laser device includes a substrate, a semiconductor stacked body and a first electrode. The semiconductor stacked body includes an active layer and a first clad layer. The active layer is provided on the substrate and is configured to emit infrared laser light by an intersubband optical transition. The first clad layer is provided on the active layer. A ridge waveguide is provided in the semiconductor stacked body. A distributed feedback region is provided along a first straight line at an upper surface of the first clad layer. The ridge waveguide extends along the first straight line. The first electrode is provided at an upper surface of the distributed feedback region. A diffraction grating is arranged along the first straight line in the distributed feedback region. The distributed feedback region includes at least one increasing region where a length of the diffraction grating along a direction orthogonal to the first straight line increases from one end portion of the distributed feedback region toward another end portion of the distributed feedback region.
Embodiments of the invention will now be described with reference to the drawings.
As illustrated in
The semiconductor stacked body 20 is configured to emit infrared laser light 90 by an intersubband optical transition, includes an active layer 12 provided on the substrate 10, and includes a first clad layer 14 provided on the active layer 12.
The ridge waveguide RG is provided in the semiconductor stacked body 20.
A distributed feedback region 40 is provided at the upper surface of the first clad layer 14 along the first straight line (parallel to line A-A) 50 in which the ridge waveguide RG extends. The first electrode 30 is provided at the upper surface of the distributed feedback region 40. A dielectric layer 60 also may be provided at the two side surfaces of the ridge waveguide RG and the upper surface of the substrate 10. In the case where the substrate 10 is conductive, a second electrode 32 may be provided at the back surface of the substrate 10.
As illustrated in
In other words, in the first embodiment, W1<W2 for the length of the diffraction grating 14a. By setting 1.5W1≦W2≦3W1, etc., it is easy to control the temperature distribution while maintaining the horizontal direction beam spread angle in the prescribed range.
By providing a highly reflective film 43 at the end surface of the one end portion 40a and by providing a low reflectance film 44 at the another end portion 40b, the infrared laser light 90 is emitted in the right direction of
For example, the trend of the temperature can be reversed by increasing the heat dissipation by providing a buried layer of InP, etc., in the two side-surface directions (parallel to line B-B) of the ridge waveguide RG.
The vertical axis is the gain (1/cm); and the horizontal axis is the wavelength (μm). The configuration of the quantum cascade laser device having such a gain dependence is described below.
At the position P1 where the temperature is lowest, the wavelength where the gain has a peak is taken to be about 5.85 μm. At the position P3 where the temperature is highest, the wavelength where the gain has a peak is taken to be about 6.15 μm. At the position P2 of the intermediate temperature, the wavelength where the gain has a peak is taken to be about 6.0 μm. For example, the gain of the active layer at the peak wavelength is taken to be about 100 cm−1. The bandwidth where the gain at each position is 50 cm-1 or more is narrow and is about 0.16 μm.
However, when considering the active layer to be made of three regions having the positions P1, P2, and P3 as centers, the active layer may be considered to include the three gain bandwidths in which the peak wavelengths are about 5.85, 6.0 and 6.15 μm. Actually, there is no boundary between the regions; therefore, the gain dependence on the wavelength has the curve envelope shape illustrated by the broken line. For example, the bandwidth where the gain is 50 cm-1 or more widens from 5.78 μm to 6.25 μm. However, in the case where the temperature distribution inside the active layer is increased, the peak value of the gain may decrease; and the bandwidth may change.
The temperature dependence of the peak wavelength of the quantum cascade laser device that can oscillate with a single mode due to the distributed feedback region is, for example, 0.5 to 1.0 nm/° C. If the temperature dependence is 1 nm/° C., the peak wavelength at the position P3 can be set to about 200 nm longer than the peak wavelength at the position P1 by setting the temperature at the position P3 to be about 200° C. higher than the temperature at the position P1.
In other words, according to the first embodiment, the gain can effectively have a wide bandwidth by changing the length of the diffraction grating inside the chip and by providing a temperature difference along the first straight line (the direction of the optical axis) 50.
In the comparative example, a length W3 of a diffraction grating 114a is taken to be uniform. Because the generated heat is substantially uniform along the first straight line 50, the change of the temperature at the position X along the first straight line 50 is nearly flat. Therefore, the gain bandwidth is narrow compared to that of the first embodiment.
According to the simulation illustrated by the ♦ symbols, the peak wavelength lengthens as the change ratio of the well layer thickness changes from the negative side to the positive side. The peak wavelength average value of a first wafer in which the target value of the well layer thickness is set to minus 7% with respect to the reference thickness of the InGaAs well layer is about 3.7 μm (the ∘ symbol). The peak wavelength average value of a second wafer in which the target value of the well layer thickness is set to plus 3% with respect to the reference thickness of the InGaAs well layer is about 3.9 μm (the ▴ symbol). The wavelength difference is large and is about 0.2 μm.
For actual crystal growth processes that use MBE (Molecular Beam Epitaxy) and/or MOCVD (Metal Organic Chemical Vapor Deposition), the peak wavelength difference between wafers is about this level. In the case where a single-mode operation is performed using the diffraction grating, it is not easy to set the gain bandwidth of the active layer to be wider than 0.2 μm.
The desired intersubband optical transition does not occur if the set single-mode wavelength is outside the gain bandwidth of the active layer. In other words, the manufacturing yield of the laser element having the desired peak wavelength decreases.
An example of the quantum well structure of the quantum cascade laser device according to the first embodiment (Table 1) will now be described.
15
The unit quantum well layer includes an injection barrier layer, a light-emitting region, an extraction barrier layer, and an electron injection region. The light-emitting region includes four well layers made of In0.53Ga0.47As and barrier layers made of In0.52Al0.48As that are stacked alternately. The electron injection region that is provided with an extraction barrier layer interposed includes eight well layers made of In0.53Ga0.47As and barrier layers made of In0.52Al0.48As that are stacked alternately. For example, several ten to several hundred layers of this unit quantum well layer are connected by a cascade connection.
The vertical axis is the gain (1/cm); and the horizontal axis is the wavelength (μm). The length of the diffraction grating is the same. The gain has a peak value of about 100 cm−1 when the wavelength is 6.0 μm. The bandwidth where the gain is about ½, i.e., 50 cm−1, is about 0.15 μm.
The gain is 0 or less when the wavelength is about 5.8 μm or less and the wavelength is 6.15 μm or more. In other words, in the case where the length of the diffraction grating is set to be uniform, the gain bandwidth is 0.15 μm and is narrow. For example, the peak wavelength of the gain can be changed by changing the thickness W1 of the well layer included in the light-emitting region.
Conversely, in the first embodiment, the gain bandwidth inside the active layer is widened by increasing the temperature distribution. In other words, as illustrated in
As illustrated in
As illustrated in
As illustrated in
As illustrated in
According to the embodiments of the invention, a quantum cascade laser device is provided in which the controllability and the reproducibility of the light emission wavelength are increased. These quantum cascade laser devices can be used widely in various gas analyzers, detectors of designated substances, etc.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modification as would fall within the scope and spirit of the inventions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-179140 | Sep 2016 | JP | national |
