Semiconductor laser device

According to one embodiment of the invention, a semiconductor laser device includes a plurality of first unit stacked bodies and a plurality of second stacked bodies. The plurality of first unit stacked bodies have an emission region including a first quantum well layer and capable of emitting a first infrared light by an intersubband transition, and an electron injection region capable of transporting an electron relaxed to a mini-band level in the emission region to a downstream unit stacked body. The plurality of second unit stacked bodies have an emission region including a second quantum well layer and capable of emitting a second infrared light by an intersubband transition, and an electron injection region capable of transporting an electron relaxed to a mini-band level in the emission region of the second quantum well layer to a downstream unit stacked body. The second quantum well layer has at least one well width different from a well width of the first quantum well layer. The first unit stacked body and the second stacked body are stacked with spatial periodicity.

TECHNICAL FIELD

This invention relates to a semiconductor laser device.

BACKGROUND ART

Laser devices emitting infrared light are applied to a broad range of fields such as environment measurement. A quantum cascade laser made of a semiconductor is small in size and has high convenience, and enables high precision measurement.

The quantum cascade laser includes an active layer alternately stacked with, for example, GaInAs and AlInAs, and including a quantum well layer. The quantum cascade laser has a structure where both side surfaces of the active layer are interposed between, for example, InP cladding layers. In this case, the cascade-connected quantum well layer is capable of emitting infrared laser light with a wavelength of 4 to 20 μm by intersubband transition of a carrier.

Various gases included in air have an absorption spectrum peculiar to the gas due to infrared ray radiation. For this reason, type and concentration of the gas can be known by measuring an infrared ray absorption amount. In this case, a wavelength range of the laser light emitted from the quantum cascade laser is required to be wide.

PRIOR ART DOCUMENT

Patent Document

SUMMARY OF INVENTION

Problem to be Solved by Invention

The embodiments of the invention provide a semiconductor laser device capable of emitting infrared light in a wide wavelength band.

Means for Solving Problem

According to one embodiment of the invention, a semiconductor laser device includes a plurality of first unit stacked bodies and a plurality of second unit stacked bodies. The plurality of first unit stacked bodies have an emission region including a first quantum well layer and capable of emitting a first infrared light by an intersubband transition, and an electron injection region capable of transporting an electron relaxed to a mini-band level in the emission region to a downstream unit stacked body. The plurality of second unit stacked bodies have an emission region including a second quantum well layer and capable of emitting a second infrared light by an intersubband transition, and an electron injection region capable of transporting an electron relaxed to a mini-band level in the emission region of the second quantum well layer to a downstream unit stacked body. The second quantum well layer has at least one well width different from a well width of the first quantum well layer. The first unit stacked body and the second unit stacked body are stacked with spatial periodicity.

EMBODIMENTS OF INVENTION

Embodiments of the invention will be described hereinafter with reference to the accompanying drawings.

FIG. 1Ais a schematic perspective view a partially cut semiconductor laser device according to a first embodiment of the invention,FIG. 1Bis a schematic cross-sectional view along A-A line.

The semiconductor laser device includes at least a substrate10, a stacked body20provided on the substrate10, and a dielectric layer40. InFIG. 1A, a first electrode50, a second electrode52and an insulating film42are further included.

The stacked body20includes a first cladding layer22, a first guide layer23, an active layer24, a second guide layer25, and a second cladding layer28. Each of a refractive index of the first cladding layer22and a refractive index of the second cladding layer28is set to be lower any of refractive indices of the first guide layer23, the active layer24and the second guide layer25, and an infrared laser light60is set to be adequately confined in a stacking direction of the active layer24.

The stacked body20has a striped configuration, and can be called a ridge waveguide RG. If two end surfaces of the ridge waveguide RG are assumed to be mirror surface, stimulated emission light is emitted from a light emission surface as the infrared laser light62. In this case, an optical axis62is defined as a line which connects a center of a cross section of an optical resonator having a mirror surface as a resonant surface. That is, the optical axis62coincides with an extending direction of the ridge waveguide RG.

If a width WA in a direction parallel to a first surface24aand a second surface24bof the active layer24is too wide in a cross section perpendicular to the optical axis62, a high-order mode is generated in a horizontal transverse direction, and a high output becomes difficult. If the width WA of the active layer24is, for example, 5 to 20 μm or the like, the horizontal transverse mode is easily controlled. If a refractive index of the dielectric layer40is lower than a refractive index of any layer constituting the active layer24, the ridge waveguide RG can be constituted along the optical axis62by the dielectric layer40provided so as to interpose side surfaces20a,20bof the stacked body20.

FIG. 2is an energy band diagram describing the operation of the semiconductor laser device according to the first embodiment.

The active layer24has a cascade structure where an emission region and an injection region are alternately stacked. Such a semiconductor laser can be called a quantum cascade laser. A first unit stacked body80includes a first emission region82and a first injection region84. The first injection region84includes an electron injection region88and an extraction barrier layer BE. The first injection region84may further include an adjustment quantum well layer90on a downstream. The first emission region82is capable of emitting a first infrared laser light by an intersubband transition of a first quantum well layer86. A carrier (electron in the figure) is injected from the first injection region84into a second emission region94, after the intersuband transition, the electron is extracted from the second emission region94into a second injection region96. The carrier moves from the upstream to the downstream. That is, the first unit stacked body80is located on the upstream. On the other hand, a second unit stacked body92is located on the downstream. For example, it can be said that the first injection region84transports (inject) the carrier (electron) to the second emission region93of the second unit stacked body92.

The second unit stacked body92includes the second emission region93and a second injection region95. The second injection region95includes an electron injection region96and an extraction barrier layer BE. The second injection region95may further include an adjustment quantum well layer98on the downstream. The second emission region93is capable of emitting a second infrared light including the infrared laser light by an intersubband transition of a second quantum well layer94. The second injection region95is capable of relaxing energy of a carrier (electron in the figure) injected from the second emission region93to a mini-band level Lm2.

In the first quantum well layer86and the second quantum well layer94, when the well width W1is narrowed to, for example, 10 nm or less, energy levels are discrete and a subband (high level Lu) and a subband (low level Ll) or the like are produced. The carrier such as an electron injected from an injection barrier layer BI can be effectively confined in the quantum well layer. For this reason, in the case where the carrier transits from the high level Lu to the low level Ll, a photon (hv) corresponding to energy differences (Lu1−Ll1), (Lu2−Ll2) or the like is emitted (transition of carrier such as electron).

The intersubband transition occurs in one of a conduction band or a valence band. That is, recombination of a hole and an electron due to a pn junction is not necessary and light is emitted by transition of only one of the carriers. In the case of the figure, in the semiconductor stacked body, an electron70is injected into a quantum well layer via the injection barrier layer BI by a voltage applied between the first electrode50and the second electrode52, and the intersubband transition occurs.

The unit stacked body has a plurality of mini-bands (also referred to as subband). It is favorable that energy difference in the mini-band is small and the mini-band is close to a continuous energy band. The electron at the low level Ll1of the first emission region86is relaxed to a mini-band level Lm1, passes through the extraction barrier layer BE, is injected into the first injection region88, and is transferred (injected) into the downstream unit stacked body. The electron at the low level Ll2of the second emission region93is relaxed to the mini-band level Lm2, passes through the extraction barrier layer BE, is injected into the second injection region95, and is transferred (injected) into the downstream unit stacked body.

A well layer determining the intersubband transition of the quantum well layers in the emission region is referred to as a first well layer, and the width is expressed by W1. In the first embodiment, the well layer width W1generating the electron transition accompanying with light emission in the second quantum well layer94is different from the well layer width W1generating the electron transition accompanying with light emission in the first quantum well layer86.

FIG. 3shows a graph of a gain to an emission wavelength when changing the width of the first well layer.

The vertical axis represents a gain (1/cm), and the horizontal axis represents an emission wavelength (μm).

In response to increasing the width W1of the first well layer to 6.3 nm (A), 6.4 nm (B), 6.5 nm (C), 6.6 nm (D), the peak of the emission wavelength becomes long to 6.1 μm, 6.15 μm, 6.2 μm, 6.25 μm. The peak of the emission wavelength can be changed by changing the width W1of the first well layer of the unit stacked body.

In the first embodiment, the first unit stacked body80and the second unit stacked body92are stacked with spatial periodicity. Therefore, a quantum cascade laser including a plurality of unit stacked bodies having different width W1of the first well layer and having a wide emission wavelength band can be achieved.

The first unit stacked body80and the second unit stacked body92can be alternately stacked. Otherwise, three types of unit stacked bodies may be stacked periodically like A-B-C-A-B-C . . . . Furthermore, A-A-B-A-A-B . . . may be accepted. The stacked number can be, for example, 20 to 50 or the like.

In the first embodiment, the substrate10can be based on InP or the like. The first cladding layer22and the second cladding layer28can be based on InP or the like. The first guide layer23and the second guide layer25can be based on InGaAs or the like. The active layer24can be based on InGaAs (In0.53Ga0.47As or the like)/In0.52Al0.48As or the like.

The first cladding layer22and the second cladding layer28can have, for example, an n-type impurity concentration of 6×1018cm−3by Si doping, and the thickness can be, for example, 1 μm. The first guide layer23and the second guide layer25can have, for example, an n-type impurity concentration of 4×1016cm−3by Si doping, and the thickness can be, for example, 3.5 μm. A part of the quantum well layer forming the injection region may be doped with Si.

(Table 1) shows an example of a unit stacked body structure constituting a quantum cascade laser according to a second embodiment.

The different energy difference (Lu−Ll) is different between two unit stacked bodies having different widths W1of the first well layers. Therefore, an electron injection efficiency may be decreased and an optical output may be decreased. In the second embodiment, a transition energy level Lt1lower than the mini-band level Lm is created continuously from the adjustment quantum well layer90of the upstream first unit stacked body80to the second quantum well layer94of the second unit stacked body adjacent to the downstream and having different emission wavelength.

In the case of a comparative example (structure A), the width W1of the first well layer is 6.3 nm, and the comparative example does not include the adjustment quantum well layer. The example I of the second embodiment (structure B+adjustment layer1) includes the adjustment quantum well layer90made of one pair of well layer/barrier layer. The example II (structure C+two pairs of well layer/barrier layer) includes the adjustment quantum well layer made of two pairs of well layer/barrier layer. The example III (structure D+three pairs of well layer/barrier layer) includes the adjustment quantum well layer made of three pairs of well layer/barrier layer.

FIG. 4Ais an energy band diagram of an example I of the second embodiment,FIG. 4Bis an enlarged view of the broken line region,FIG. 4Cshows an energy band diagram of an example II,FIG. 4Dis an enlarged view of the broken line region,FIG. 4Eshows an energy band diagram of an example 3,FIG. 4Fis an enlarged view of the broken line region.

In the example I, the example II, the example III, the transition energy level Lt1lower than the mini-band level Lm1is created continuously from the adjustment quantum well layer90of the first unit stacked body80to the second emission region94. Therefore, even if the unit stacked body having different constitution is set to have the cascade structure, the electron injection efficient can be held to be high.

FIG. 5shows a graph of a gain to an emission wavelength of the semiconductor laser device according to the second embodiment.

The injection regions of the example I, the example II, the example III of the second embodiment shown in (Table 1) have the adjustment quantum well layer90stacked with one, two, three layers of a pair of the well layer (thickness 2.5 nm) and the barrier layer (thickness 3 nm), respectively. In the second embodiment, the electron injection efficiency can be increased, and a gain and the optical output can be increased. Therefore, this makes it easy to widen the emission wavelength band. The constitution of the adjustment quantum well layer90is not limited thereto. Depending on the width W1of the first well layer in the emission region cascade-connected on the downstream of the carrier, the width and the repeated periodicity of the well layer/barrier layer constituting the adjustment quantum well layer90can be determined. The cross section of the active layer24is able to be analyzed by TEM (Transmission Electron Microscopy).

For example, if the unit stacked body of the example I and the unit stacked body of the example III are alternately stacked, both gains are added and further wide gain band can be achieved. Since the transition energy levels Lt1, Lt2are created across the two unit stacked bodies, the electron injection efficiency can be increased.

FIG. 6Ais an energy band diagram according to the comparative example (W1=6.4 μm),FIG. 6Bis an enlarged view of the broken line region,FIG. 6Cis an energy band diagram according to the comparative example (W1=6.5 μm),FIG. 6Dis an enlarged view of the broken line region,FIG. 6Eis an energy band diagram according to the comparative example (W1=6.6 μm),FIG. 6Fis an enlarged view of the broken line region.

The comparative example shows the energy band diagram of the unit stacked body not provided with the adjustment quantum well layer. In any of the cases of W1=6.4, 6.5, 6.6 μm, the mini-band levels Lm1of the unit stacked body180are directly reproduced in the quantum well layer186of the next unit stacked body (same constitution180) to form the high levels Lu2. That is, the transition energy level lower than the mini-band levels Lm1making the electron injection easy does not exist near the interface. Therefore, the electron injection efficiency is likely to decrease at the interface of the unit stacked body and the optical output decreases.

In contrast, in the first and second embodiments, at least two unit stacked bodies having different well layer widths are stacked with periodicity. Therefore, a light emitting element capable of emitting infrared light over the wide wavelength band (including quantum cascade laser) is provided.

FIG. 7Ais a configuration view of an exhalation diagnosis device according to the embodiment,FIG. 7Bis a schematic view of an absorption spectrum of a plurality of gases,FIG. 7Cis a view for describing a first adjustment mechanism and a second adjustment mechanism of a wavelength control unit.

The exhalation diagnosis device includes a quantum cascade laser170or the like, a wavelength control unit, a gas cell (corresponding to “chassis”)280, a detection unit287, and a signal processing unit288. The quantum cascade laser170and the wavelength control unit can be referred to as a light source unit191.

The wavelength control unit includes the first adjustment mechanism shifting the wavelength of the infrared laser light or the like into an absorption spectrum of one kind of gas of a plurality kinds of gases included in exhalation of human being or the like, and a second adjustment mechanism shifting within the absorption spectrum of one kind of gas.

In the exhalation diagnosis device, the first adjustment mechanism includes a diffraction grating171or the like. The diffraction grating171is provided so as to cross an optical axis162of the quantum cascade laser170, and constitutes an external resonator. As shown inFIG. 7(c), in exhalation BR including a plurality of gases, an incident angle of the infrared laser light is changed from β1 to β4 or the like depending on the spectrum of respective gases, and the wavelength of the infrared laser light is changed (coarse adjustment).

The diffraction grating171is rotationally controlled about an axis crossing the optical axis162by a stepping motor199and a controller198controlling it. An anti-reflection coating film AR is favorable to be provided on an end surface of the quantum cascade laser70on a side of the diffraction grating171. Furthermore, if a partial reflection coating film PR is provided on an opposite side to the anti-reflection coating film AR, the external resonator can be constituted with the diffraction grating171.

An absorption spectrum of a molecule is discrete, and in order to improve measurement accuracy, a wavelength is needed to meet an absorption peak accurately. In order to avoid absorption of carbon dioxide and water which are main components in the exhalation, and to measure absorption of a molecule to be measured, the wavelength is needed to be tuned in to the absorption peak accurately. However, the wavelengths of the absorption peak of a molecule and the light source may be influenced by measurement environment to shift. Therefore, fine-adjustment by the second adjustment mechanism is favorable.

As shown inFIG. 7C, the second adjustment mechanism does not rotate the diffraction grating171to be constant. Wavelength adjustment can be realized by changing an operating current ILDor duty of the quantum cascade laser170, changing an operating temperature of the quantum cascade laser170by using a Peltier element290or the like, or changing an external resonator length by a piezoelectric element or the like. The second adjustment mechanism may change the operating temperature of the quantum cascade laser170by one of a chiller, a heater and a refrigerant or combination use. The refrigerant may be one of liquid nitrogen, water, ethanol water, and liquid helium.

As shown inFIG. 7B, for example, gas concentrations of acetone (a peak of absorption amount represented by a vertical axis is near 7.37 μm) and methane (a peak of absorption amount is near 7.7 μm) are set to be measured. Absorption spectra of different gases are greatly separated by, for example, generally 0.3 μm or the like. Therefore, in order to measure a plurality of gases in a short time (for example, one minute or the like), it is favorable to sweep quickly the wavelength of infrared laser light and to increase the shift range.

On the other hand, in the case where wavelength adjustment is performed in the absorption spectrum of one gas in the second adjustment mechanism, a shift range may be narrower than a wavelength range in the first adjustment mechanism. However, adjustment accuracy is required to be improved. That is, it is not easy to realize the first adjustment mechanism which is mainly used for coarse adjustment and the second mechanism which is mainly used for fine adjustment by using the same wavelength control mechanism.

A gas cell280includes an exhalation suction port281, an exhalation exhaust port282, an incident window293of the infrared laser light, and an emission window284of the infrared laser light. The laser light from the quantum cascade laser170has a divergence angle. For this reason, it is preferred to provide an optical system272for collimating between the quantum cascade laser170and the incident window283. It is preferred to provide a light focusing system286between the emission window284and the detector287.

The human exhalation BR includes nitrogen, oxygen, carbon dioxide, water or the like as a main component. Simultaneously, extremely small amount of different molecules of 1000 kinds or more are included. A change of small amount of gas serves as an index of disease. For this reason, when the small amount of gas G1included in the exhalation is measured, early detection and prevention of the disease becomes possible. If the exhalation diagnosis device is used like this, the diagnosis can be made in a shorter time and more easily than performing a blood test.

For example, if acetone can be detected as the small amount of gas G1, diabetes or the like can be detected. In this case, detection sensitivity of the ppm degree using the infrared ray having a wavelength of 7 to 8 μm is necessary. If ammonia can be detected as the small amount gas, hepatitis can be detected. In this case, detection sensitivity of the ppm degree using the infrared ray having a wavelength of 10.3 μm is necessary. If ethanol and acetaldehyde can be detected as the small amount of gas, an amount of drinking can be measured.

If the emission wavelength range of the quantum cascade laser170is narrow, a plurality of quantum cascade lasers170and a plurality of external resonators corresponding to the respective quantum cascade lasers are necessary in order to generate the infrared laser light with the wide wavelength range. For this reason, a device is increased in size. In contrast, the quantum cascade laser according to the embodiment has a wide emission wavelength band. Therefore, the infrared laser light with the wide wavelength range can be emitted in one quantum cascade laser, and the device can be downsized.