A semiconductor laser device

Abstract: A semiconductor laser device (1) includes a semiconductor laser element having an active layer (2) comprising a quantum well structure or a multi-quantum well structure which allows n levels of quantum states (n ≧ 2) from the first quantum level to the n-th quantum level, in which the front facet reflectivity (5) and the rear facet reflectivity (6) are made asymmetric or low so that an oscillation occurs at the n-th quantum level, a reflecting mirror (8) having a reflectivity which enables that an oscillation of the semiconductor element occurs at a quantum level lower than the n-th quantum level, and a reflecting mirror moving means for varying the oscillation wavelength of the semiconductor laser element (1) by arranging the reflecting mirror (8) in the neighborhood of the front facet or rear facet of the semiconductor laser element. Therefore, a semiconductor laser device which oscillates at desired light outputs at two or more wavelengths by quite a simple construction can be obtained.

Description: The present invention relates to a semiconductor laser device and, more particularly to one comprising a single element capable of oscillating at two or more different wavelengths. Figure 4(a) shows a structure of a prior art two-wavelength oscillation semiconductor laser device recited in, for example "Y. Tokuda et al., Appl. Phys. Lett. 49(24), pp. 1629-1631, (1986)". Figure 4(b) shows the light output vs. current characteristics thereof and figure 4(c) shows an energy band diagram in the vicinity of the active layer thereof. In figure 4, on a p type or n type substrate 701, a p type or n type buffer layer 702, a p type or n type lower cladding layer 703, a p type or n type light confinement layer 704, an undoped quantum well active layer 705, an n type or p type light confinement layer 706, an n type or p type upper cladding layer 707, and an n type or p type contact layer 708 are successively disposed. A SiO 2 film 709 is disposed on a region on the upper cladding layer 707 where the stripe-shaped contact layer 708 is not formed. An electrode 710 is disposed on the rear surface of the substrate 701 and an electrode 711 is disposed on the contact layer 708 and the SiO 2 film 709. Reference numeral 712 designates an impurity diffusion region. A description is given of the operation. When a current is injected to the semiconductor laser device, first of all, spontaneous emission light (i in figure 4(b)) is emitted. Then current is further injected, induced emission arises when the gain owing to the current injection and the total loss in the semiconductor laser are equal to each other (ii in figure 4(b)). Then, an oscillation owing to the transition of the quantum well at the bottom state (n = 1) is obtained as shown in figure 4(c). When current is still further injected, the number of electrons and holes occupying the state of n = 2 of the quantum well increases and an oscillation owing to the transition of n = 2 shown in figure 4(c) is obtained (iii in figure 4(b)). Figure 5 shows a perspective view of a prior art array type semiconductor laser device which emits a plurality of different wavelength laser lights, recited in, for example Japanese Patent Laid-open publication No. 61-242093. In figure 8, on an n type GaAs substrate 110, an n type GaAs buffer layer 111, an n type Al x Ga 1-x As (x = 0.4) layer 112, an Al y Ga 1-y As (y = 0.2) layer 113, an Al z Ga 1-z As (z = 0.1 to 1) layer 114, an Al y Ga 1-y As (y = 0.2) layer 115, a p type Al x Ga 1-x As (x = 0.4) layer 116 and a p + type GaAs layer 117 are successively disposed. Furthermore, a common n side electrode 118 is disposed on the rear surface of the substrate 110 and p side electrodes 119 and 120 are disposed on the p + type GaAs layer 117. This array type semiconductor laser device is provided with a first laser light generating region 121 and a second laser light generating region 122, and a reflection coating 125 is executed to the portion 123a of the first laser light generating region 121 at the device facet 124 while no reflection coating is executed to the portion 123b of the second laser light generating region 122 at the device facet 124. A description is given of the operation of this array type semiconductor laser device. As discussed above, since the reflection coating 125 is executed to the facet 123a of the first laser light generating region 121 while no reflection coating is executed to the facet 123b of the second laser light generating region 122, optical loss in the first laser light generating region 121 is larger than that in the second laser light generating region 122. As a result, an oscillation of wavelength λ 1 at quantum level of n = 1 occurs in the second laser light generating region 122 having less optical loss while an oscillation of wavelength λ 2 at quantum level of n = 2 occurs in the first laser light generating region 121 having high optical loss. In this way, the optical loss in the respective laser light generating regions 121 and 122 are changed by changing the reflectivity at the respective laser facets 123a and 123b, whereby a monolithic device oscillating at a plurality of wavelengths is realized. Figure 6 shows a cross-sectional view of a prior art semiconductor laser device which emits a plurality of different wavelength laser lights, recited in, for example Japanese Patent Laid-open Publication No. 63-312688. In figure 9, on an n + type GaAs substrate 201, an n type Al z Ga 1-z As cladding layer 202, an n type Al z Ga 1-z As (z → y) parabolic type diffraction index distribution layer 203, an Al x Ga 1-x As active layer 204a, an Al y Ga 1-y As barrier layer 205, an Al x Ga 1-x As active layer 204b, a p type Al z Ga 1-z As (z → y) parabolic type diffraction index distribution layer 206 and a p type Al z Ga 1-z As cladding layer 207 are successively disposed. An n side electrode 211 is disposed on the entirety of the rear surface of substrate 201. Furthermore, a p + type GaAs cap layer 208a is disposed on A region on the cladding layer 207 and a p + type GaAs cap layer 208b is disposed on B region on the cladding layer 207. P side electrodes 212a and 212b are disposed on the cap layers 208a and 208b, respectively. This semiconductor laser device comprises a quantum well active layer part A and a light absorption amount control part B, which controls the oscillation by applying an electric field to the control part B using the electrodes 211 and 212b. Figure 7 shows a perspective view of a prior art array type semiconductor device which emits a plurality of different wavelength laser lights, recited in, for example Japanese Patent Laid-open Publication No. 63-32986. In figure 7,, on an n type GaAs substrate 305, an n type AlGaAs cladding layer 306, a quantum well active layer 307 and a p type AlGaAs cladding layer 308 are successively disposed, and p type GaAs contact layers 309a, 309b and 309c are disposed on the cladding layer 308 in parallel with each other. Insulating films 310 are disposed at regions on the cladding layer 308 where the contact layers 309a, 309b and 309c are not disposed. An n side electrode 303 is disposed on the entirety of the rear surface of substrate 5 and p side electrodes 304a, 304b and 304c each having different length are disposed corresponding to the contact layers 309a, 309b and 309c, respectively. Current terminals 301, 302a, 302b and 302c are respectively connected to the n side electrode 303, p side electrodes 304a, 304b and 304c. In addition, dotted line 311 shows a diffusion front of p type impurities. It is well known that when a current is injected to the quantum well active layer and the injection carrier density is increased to generate the band filling, as the energy level of the quantum level is higher, the amplification gain is higher. In a laser array in which each laser element has the same cavity length, that the length of the gain region is changed by changing the length of the electrode is equivalent to that the loss is changed, and as the laser has shorter electrode length, a higher amplification gain is required to oscillate. Accordingly, as the length of the electrode is shorter, the higher energy quantum level is required to obtain a laser oscillation. In this array type semiconductor laser device, the laser having shorter electrode for injecting current oscillates at higher quantum level, whereby the oscillation wavelengths of respective lasers can be varied. Figure 8 shows a cross-sectional view of a semiconductor quantum well laser device which oscillates at a high quantum level with increasing the cavity loss, recited in, for example Japanese Laid-open Patent Publication No. 63-54794. In figure 8 on an n type GaAs substrate 402, an n type AlGaAs cladding layer 403, a GaAs quantum well active layer 404, a p type AlGaAs cladding layer 405 and a p type GaAs contact layer 406 are successively disposed. An n side electrode 401 is disposed on the rear surface of substrate 402 and a p side electrode 407 is disposed on the contact layer 406 except for the absorption region 409. In this semiconductor laser device, the absorption region 409 is provided at a part of the device to increase the loss of whole device, thereby enabling an oscillation at higher energy level of quantum well. When the size of this absorption region 409 is adjusted, it is possible to switch the lights of both wavelengths of n = 1 and n = 2 or to output the lights of both wavelengths of n = 1 and n = 2 at the same time by changing the quantity of injection current. Figure 9 shows a perspective view of a prior art semiconductor laser device in which the current injection electrode is divided into plural and the current level to be injected to the divided electrodes are controlled to enable oscillation at various quantum levels, recited in Japanese Patent Laid-open Publication No. 63-32985. In figure 9 on an n + type GaAs substrate 505, an n type AlGaAs cladding layer 506, a quantum well active layer 507, a p type AlGaAs cladding layer 508 and a p type GaAs contact layer 509 are successively disposed. An n side electrode 503 is disposed on the rear surface of the substrate 505 and p side electrodes 504A and 504B are disposed on the contact layer 509. Current terminals 501, 502A and 502B are connected to these electrodes 503, 504A and 504B, respectively. In addition, dotted line 513 shows a diffusion front of p type impurities. In this semiconductor laser device, the quantity of injection current from the electrode 504B is changed to control whether the oscillation occurs at the quantum level of n = 1 or the quantum level of n = 2 in the quantum well active layer 507. That is, when current is not injected to the electrode 504B, an oscillation does not occur at a gain for the quantum level of n = 1. By increasing the quantity of current injected to the electrode 504A, the gain for the quantum level of n = 2 is increased and a laser oscillation occurs at a wavelength corresponding to the quantum level of n = 2. When current is injected to the electrode 504B in this state, the loss inside the laser element decreases and the gain for the quantum level of n = 1 exceeds the loss, and then a laser oscillation occurs at a wavelength corresponding to the quantum level of n = 1. In this conventional device, the current level injected to the current injection electrode divided in plural is controlled, whereby oscillations at a plurality of wavelengths are realized. Figure 10 shows a prior art semiconductor laser device in which the oscillation wavelength is varied by varying the refractive index of the electro-optic crystal provided at the laser facet, recited in Japanese Laid-open Patent Publication No. 1-208884. In figure 10, a reflecting mirror 602 is provided at one side facet of a laser diode 601 and an electro-optic crystal 603 is provided at the other facet. A control circuit 604 for controlling the refractive index of the electro-optic crystal 603 is connected to the electro-optic crystal 603. In this semiconductor laser device, the oscillation wavelength is determined by the oscillation mode of the laser diode 601 and the oscillation mode of the electro-optic crystal 603. The oscillation mode of the electro-optic crystal 603 can be varied by that a voltage is applied to the electro-optic crystal 603 by the control circuit 604 to change the refractive index of the electro-optical crystal 603, whereby the oscillation wavelength of the laser can be varied. The prior art semiconductor laser devices capable of oscillating at two or more wavelengths are constructed so that the oscillation wavelength is varied by the current injected to the laser element or the wavelength controlling region formed integrally with the laser element or that oscillations occur at different wavelengths in different light emitting regions of the array type laser device. In such laser device, there is provided no element separated from the semiconductor laser element to make the semiconductor laser element oscillate at different wavelength. It is an object of the present invention to provide a multi-wavelength oscillation semiconductor laser device capable of oscillating at two or more wavelengths from one semiconductor laser element and to obtain a desired light output at respective wavelengths by providing an element separated from the semiconductor laser element. Another object of the present invention is to provide a multi-wavelength oscillation semiconductor laser device capable of changing wavelength at high speed. Other object and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. Semiconductor laser devices in accordance with the present invention are defined in appended claims 1 and 5. In the accompanying drawings: Figure 1 is a perspective view showing a structure of a semiconductor laser device in accordance with an embodiment of the present invention; Figure 2 is a perspective view showing an alternative embodiment of the present invention; Figure 3 is a diagram for explaining the embodiments of the present invention; and Figures 4 to 10 are diagrams showing prior art semiconductor laser devices. Embodiments of the present invention will be described in detail with reference to the drawings. Figure 1 shows a semiconductor laser device in accordance with an embodiment of the present invention. In figure 1, reference numeral 1 designates a semiconductor laser element having a quantum well or multi-quantum well active layer which allows two or more quantum states. The quantum well or multi-quantum well active layer 2 is disposed at the center of the laser element 1. A lower side electrode 3 is disposed on the bottom surface of the semiconductor laser element 1 and an upper side electrode 4 is disposed on the upper surface of the semiconductor laser element 1. A film 5 of low reflectivity or non-reflectivity (anti-reflective (AR) film) is coated on the front facet of the semiconductor laser element 1. A film 6 of high reflectivity is coated on the rear facet of the semiconductor laser element 1. A metal wiring 7 for injecting current is disposed on the upper side electrode 4. The semiconductor laser element 1 is mounted on a conductive base (not shown) and electric current flows from the metal wiring 7 to the conductive base through the laser element 1. In figure 1, layers other than the active layer 2 which constitute the laser structure of the laser element 1 are not shown for simplification. A semiconductor plate 9 to both surfaces of which anti-reflecting films or low reflecting films 10a and 10b are attached so as to make the semiconductor plate 9 non-reflecting or low reflecting in a state where current is injected is arranged in the neighbourhood of the low reflecting film 5 of the laser element 1. The semiconductor plate 9 is of p type or n type or undoped, an electrode 11 for injecting carriers is attached thereto, and a carrier injection metal wiring 12 is connected to the electrode 11. The semiconductor laser device of this embodiment utilizes a change in refractive index due to carrier injection to the semiconductor plate 9 in order to electrically vary the reflectivity. The semiconductor laser element 1 has a quantum well or multi-quantum well active layer 2 which allows quantum states of two or more quantum levels similarly as the first embodiment. Here, the reflectivity of the front facet reflecting film 5 is Rf and the reflectivity of the rear facet reflecting film 6 is Rr and these reflectivities satisfy Rf · Rr < Rth 1,2 . As described above, coating films 10a and 10b which make the semiconductor plate 9 non-reflecting or low reflecting in a state where current is injected into the semiconductor plate 9 are provided on the surfaces of the semiconductor plate 9. Accordingly, when carriers are injected into the semiconductor plate 9, light does not return to the semiconductor laser and an oscillation of short wavelength of the state of n = 2 is obtained. On the other hand, when no current is injected into the semiconductor plate 9, the refractive index becomes large as compared with when current is injected. Then, the reflection from the semiconductor plate 9 becomes large, thereby obtaining an oscillation at long wavelength of the state of n = 1. According to geometrical optics, when a substance of refractive index n m having a thickness of λ/4 n m is arranged between a semiconductor or dielectric substance of refractive index n m and air, non-reflecting is obtained against light of wavelength λ. Therefore, the structure shown in figure 3(a) in which materials 41a and 41b of refractive index n f (= n m ) having a thickness of λ/4n f are arranged as low reflecting films at the both sides of the semiconductor or dielectric substance 40 becomes a non-reflecting film against wavelength of λ. However, since it is difficult to obtain a material of reflectivity n f = n m practically, not a perfect non-reflecting film but a low reflectivity film close to non-reflecting is obtained. This low reflecting film does not change the reflectivity largely when the wavelength λ changes from the wavelength of quantum level of n = 1 to the wavelength of quantum level of n = 2, which changing value is several tens nm. Therefore, this structure has no wavelength selectivity. When carriers are injected or an electric field is applied to the semiconductor or dielectric substance, the refractive index of the semiconductor or dielectric substance changes from n m to n m + Δn m . The reflectivity R of the structure shown in figure 3(a) at this state is given by the following formula. The change in refractive index against the quantity of injected carriers (Δn m /ΔN, ΔN: the quantity of injected carriers) is -4.0 X 10 -21 cm 3 in a case of AlGaAs. In the structure of figure 1, when Al x Ga 1-x As (x = 0.1) is used for the semiconductor plate 9, n m = 3.52. When the quantity of injected carriers is 1 X 10 20 cm 3 , Δn m = - 0.4. Furthermore, when Zr 2 O 3 of refractive index 2.2 is used for the low reflecting film, the reflectivity under that the carrier is injected becomes 4.8%, which is about twice as high as the reflectivity 2.5% obtained under that the carrier is not injected. This is an example and it is possible to change the reflectivity largely by selecting materials and the injection carrier quantity. In the above embodiment, the reflecting mirror element comprising the semiconductor plate 9 and the low reflecting films 10a and 10b, separated from the laser element 1 is arranged in the neighborhood of the facet of the laser element 1, and the refractive index of the reflecting mirror element is varied by controlling the quantity of carriers injected to the semiconductor plate 9, thereby to change the reflectivity of the reflecting mirror element. Therefore, the mirror loss of the laser can be controlled and an oscillation of the quantum well laser at different quantum levels is obtained. In addition, since the switching of the wavelength is performed by the carrier injection to the reflecting mirror element which is separated from the laser element, quite a high speed switching characteristics is obtained. In the above-described embodiment, a reflecting mirror element which becomes non-reflecting or low reflecting in a state where carriers are injected thereto is used, a reflecting mirror element which becomes non-reflecting or low reflecting in a state where carriers are not injected can be used with the same effect as described above. Figure 2 shows a semiconductor laser device as an alternative embodiment of the present invention. This embodiment utilizes a change in refractive index owing to an electro-optic effect in order to change the reflectivity. In figure 2 the same reference numerals as those shown in figure 1 designate the same or corresponding parts. The front facet and the rear facet of the semiconductor element 1 are asymmetrically coated by the low reflecting film 5 and the high reflecting film 6, respectively. A semiconductor or dielectric substance 30 having electro-optic effect to both surfaces of which anti-reflecting films or low reflecting films 31a and 31b are attached so as to make the semiconductor or dielectric substance 30 non-reflecting or low reflecting in a state where an electric field is not applied is arranged in the neighborhood of the low reflecting film 5 of the semiconductor element 1. In this embodiment, the refractive index of the semiconductor or dielectric substance 30 is varied not due to carrier injection but due to an electro-optic effect by an application of an electric field differently from the embodiment of figure 1. However, the principle of realizing the multi-wavelength oscillation is totally the same. That is, when an electric field is not applied to the semiconductor or dielectric substance 30, the reflecting mirror element comprising low reflecting films 31a and 31b and the semiconductor or dielectric substance 30 becomes non-reflecting or low reflecting and, therefore, the laser oscillates at the state of n = 2. When an electric field is applied, the refractive index of the semiconductor or dielectric substance 30 changes and the reflectivity increases, thereby obtaining an oscillation at the state of n = 1. Since the switching of the wavelength is performed by an application of an electric field, quite a high speed switching characteristics can be obtained similarly as in the embodiment of figure 1. While in the above-described embodiment a reflecting mirror element which becomes non-reflecting or low reflecting in a state where an electric field is not applied thereto is used, a reflecting mirror element which becomes non-reflecting or low reflecting when an electric field is applied thereto can be used with the same effects as described above. In addition, when the semiconductor laser element is allowed up to i (> 2)th quantum levels, it is possible to obtain oscillations up to i-th levels by changing the amplitude of the electric field applied to the semiconductor or dielectric substance.

Claims: A semiconductor laser device comprising: a semiconductor laser element (1) having an active layer (2) comprising a quantum well structure or a multi-quantum well structure which has n quantum state levels, n ≥ 2, from a first quantum level to an n-th quantum level, in which the front facet reflectivity (Rf) and the rear facet reflectivity (Rr) are asymmetric or low so that oscillation occurs for the n-th quantum level; a reflecting mirror element (9-12), comprising a material (9) the refractive index of which can be changed by injection of carriers to change the reflectivity thereof, located either in front of the front facet (5) or behind the rear facet (6) of said semiconductor laser element (1); wherein said semiconductor laser element (1) can be forced to oscillate for a different quantum level upon the injection of said carriers. A semiconductor laser device in accordance with claim 1 wherein said reflecting mirror element (9-12) comprises: a semiconductor plate (9) the refractive index of which changes due to carrier injection; means (11,12) for injecting carriers into said semiconductor plate (9); and low reflecting film or anti-reflection film coatings (10a, 10b) formed on both surfaces of said semiconductor plate (9). A semiconductor laser device in accordance with claim 2 wherein said low reflecting film or anti-reflection film coatings (10a,10b) are such that said reflecting mirror element (9-12) is either low reflecting or non-reflecting in a state where current is injected thereto. A semiconductor laser device in accordance with claim 2 wherein said low reflecting film or anti-reflection film coatings (10a,10b) are such that said reflecting mirror element (9-12) is either low reflecting or non-reflecting in a state where no current is injected thereto. A semiconductor laser device comprising: a semiconductor laser element (1) having an active layer (2) comprising a quantum well structure or a multi-quantum well structure which has n quantum state levels, n ≥ 2, from a first quantum level to an n-th quantum level, in which the front facet reflectivity (Rf) and the rear facet reflectivity (Rr) are asymmetric or low so that oscillation occurs at the n-th quantum level; a reflecting mirror element (30-33), comprising a material (30) the refractive index of which can be changed by application of electric field to change the reflectivity thereof, located either in front of the front facet (5) or behind the rear facet (6) of said semiconductor laser element; wherein said semiconductor laser element (1) can be forced to oscillate for a different quantum level upon application of said electric field. A semiconductor laser device in accordance with claim 5 wherein said reflecting mirror element (30-33) comprises: a semiconductor plate (30) the refractive index of which changes due to application of an electric field; means (32,33) for applying an electric field to said semiconductor plate (30); and low reflecting film or anti-reflection film coatings (31a,31b) formed on both surfaces of said semiconductor plate (30). A semiconductor laser device in accordance with claim 6 wherein said low reflecting film or anti-reflection film coatings (31a,31b) are such that said reflecting mirror element (30-33) is either low reflecting or non-reflecting in a state where electric field is applied thereto. A semiconductor laser device in accordance with claim 6 wherein said low reflecting film or anti-reflection film coatings (31a,31b) are such that said reflecting mirror element (30-33) is either low reflecting or non-reflecting in a state where no electric field is applied thereto.

Inventors: AOYAGI TOSHITAKA, NAGAI YUTAKA, SHIGIHARA KIMIO, AOYAGI, TOSHITAKA, NAGAI, YUTAKA, SHIGIHARA, KIMIO, AOYAGI, TOSHITAKA, MITSUBISHI DENKI K.K., OPTELEC., NAGAI, YUTAKA, MITSUBISHI DENKI K.K., OPTELEC., SHIGIHARA, KIMIO, MITSUBISHI DENKI K.K., OPTELEC.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1991
Main classification: H01S3
IPC codes: H01S3, H01S5