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 5 (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 5 (b) shows the light output vs. current characteristics thereof and figure 5 (c) shows an energy band diagram in the vicinity of the active layer thereof. In figure 5 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 SiO₂ 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 SiO₂ 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 5 (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 5(b)). Then, an oscillation owing to the transition of the quantum well at the bottom state (n = 1) is obtained as shown in figure 5(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 5(c) is obtained (iii in figure 5 (b)).

Figure 6 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_xGa_1-xAs (x = 0.4) layer 112, an Al_yGa_1-yAs (y = 0.2) layer 113, an Al_zGa_1-zAs (z = 0.1 to 1) layer 114, an Al_yGa_1-yAs (y = 0.2) layer 115, a p type Al_xGa_1-xAs (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 λ₁ at quantum level of n = 1 occurs in the second laser light generating region 122 having less optical loss while an oscillation of wavelength λ₂ 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 7 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_zGa_1-zAs cladding layer 202, an n type Al_zGa_1-zAs (z → y) parabolic type diffraction index distribution layer 203, an Al_xGa_1-xAs active layer 204a, an Al_yGa_1-yAs barrier layer 205, an Al_xGa_1-xAs active layer 204b, a p type Al_zGa_1-zAs (z → y) parabolic type diffraction index distribution layer 206 and a p type Al_zGa_1-zAs 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 8 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 10, 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 9 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 9 , 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 10 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 10 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 11 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 11 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 2.

• Figure 1 is a perspective view showing a structure of a semiconductor laser device in accordance with a first embodiment of the present invention;
• Figure 2 is a diagram showing a perspective view of a structure of a semiconductor laser device used in an experiment for explaining the operation principle of the present invention and an experimental result thereof;
• Figure 3 is a diagram showing the relation between the injection current and the gain in respective quantum level when the reflectivity of the facet of the laser element (mirror loss at the facet) is varied;
• Figure 4 is a perspective view showing an alternative of the first embodiment of the present invention; and
• Figures 5 to 11 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 a first 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. A 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) or predetermined reflectivity is disposed at the front facet of the semiconductor laser element 1, whose reflectivity is Rf. A film 6 of high reflectivity or predetermined reflectivity is disposed at the rear facet of the semiconductor laser element 1, whose reflectivity is Rr. A reflecting mirror 8 of reflectivity Rm is disposed in the neighbourhood of the front facet of the semiconductor laser element 1. This mirror 8 is arranged in the neighbourhood of the front facet of the semiconductor laser element 1 or removed therefrom by a driving mechanism (not shown). A metal wiring 7 for injecting current is disposed on the upper side electrode 4. This 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 constitutes the laser structure of the laser element 1 are not shown for simplification.

A semiconductor laser device in accordance with a first embodiment of the present invention will be described in detail hereinafter. For simplification, two wavelength oscillations of oscillation at a wavelength corresponding to the bottom level (n = 1) and oscillation at a wavelength corresponding to the next level (n = 2)are considered. The laser oscillation conditions for the n = 1 level and the n = 2 level are represented as in the following formulae (1) and (2).

$${\text{g th₁ = α}}_{\text{i1}}\text{+}\frac{\text{1}}{\text{2L}}{\text{ℓ}}_{\text{n}}\frac{\text{1}}{\text{Rf}}\text{·}\frac{\text{1}}{\text{Rr}}\text{ (1)}$$

$${\text{g th₂ = α}}_{\text{i2}}\text{+}\frac{\text{1}}{\text{2L}}{\text{ℓ}}_{\text{n}}\frac{\text{1}}{\text{Rf}}\text{·}\frac{\text{1}}{\text{Rr}}\text{ (2)}$$

   Here, gth₁ and gth₂ are threshold gains required for respective levels, α_i1 and α_i2 are internal loss of the semiconductor laser element corresponding to the wavelengths of respective levels, L is a cavity length of the semiconductor laser, and Rf and Rr are a front facet reflectivity and a rear facet reflectivity of the semiconductor laser, respectively. Although Rf and Rr vary in accordance with the wavelengths of respective level actually, it is assumed that they are equal because respective wavelength are close to each other.

Figure 2(a) shows a structure of a planar stripe semiconductor laser used in an experiment for explaining the operation of this embodiment and figure 2(b) shows a graph for confirming the change in the oscillation wavelength when the reflectivity of the both facets of the semiconductor laser device of figure 2(a) are varied. In the semiconductor laser device of figure 2(a), the active layer is an AlGaAs quantum well layer of about 150 angstroms thickness. As confirmed in figure 2(b), when the product of the front facet reflectivity and the rear facet reflectivity exceeds some value (about 0.07 in the figure), an oscillation occurs at a long wavelength of n = 1. When the product is below that value, an oscillation occurs at short wavelength of n = 2. This value is a threshold reflectivity Rth_1,2 corresponding to the state of n = 1 and the state of n = 2.

Figure 3 is a diagram for explaining the principle of this wavelength change. Figure 3(a) shows a case where Rf· Rr > Rth_1,2. In this case, when the injection current is increased, the state of n = 1 firstly reaches the threshold gain gth₁ and an oscillation occurs at the state of n = 1. On the other hand, as shown in figure 3(b), in a case where Rf · Rr < Rth_1,2, the threshold gains gth₁ and gth₂ becomes large as a whole and when the injection current is increased, the gain against the state of n = 1 is saturated on the way and does not reach the threshold gain gth₁. On the other hand, the gain against the state of n = 2 is not saturated and reaches the threshold gain gth₂, and then the laser oscillates at the state of n = 2.

The first embodiment of the present invention utilizes this principle. In this embodiment device, the reflectivity Rf of the front facet reflecting film 5 and the reflectivity Rr of the rear facet reflecting film 6 of the semiconductor laser element 1 are established to satisfy Rf · Rr < Rth_1,2, and a mirror 8 of reflectivity Rm is arranged in the neighborhood of the front facet of the semiconductor laser element 1. Here, Rm is established to approximately satisfy Rm · Rr > Rth_1,2. In this case, when the mirror 8 is provided, an oscillation occurs at a long wavelength of the state of n = 1 and when the mirror 8 is not provided, oscillation occurs at short wavelength of n = 2 state. By moving this mirror 8 at high speed, it is possible to perform wavelength switching easily. Here, Rf, Rr and Rm can be arbitrary set as far as they satisfy the above condition.

Figure 4 shows a semiconductor laser device as a second embodiment. This laser device oscillates at three or more different wavelengths.

In the laser element 1 of this laser device, it is assumed that quantum levels of i levels or more are allowed. Reference numerals 8a, 8b, 8c, 8d ... designate mirrors of reflectivity Rm_i-1, Rm_i-2, Rm_i-3 ... which are arranged continuously in a straight line and are slidable. When the threshold reflectivities between the state of n = i and the state of n = i - 1, between the state of n = i - 1 and the state of n = i - 2, between the state of n = i - 2 and the state of n = i - 3 ... are Rth_i-1,i, Rth_i-2,i-1, Rth_i-3,i-2..., respectively, the reflectivity of respective mirrors Rm_i-1, Rm_i-2, Rm_i-3 ... are established to satisfy the following formula.

   When there is no mirror, an oscillation occurs at the highest level of n = i. When mirrors 8a, 8b, 8c ... of reflectivities Rm_i-1, Rm_i-2, Rm_i-3 ... are arranged in the neighborhood of the front facet of the laser element 1, oscillations at the level of n = i - 1, the level of n = i - 2, the level of n = i - 3 ... are obtained, respectively.

Since these mirrors are slidable to be easily moved, oscillations at the level of n = 1 to the level of n = i are easily obtained by moving the mirrors. In the figure, although the mirrors are arranged in a straight line to be slidable, they can be arranged circularly to be rotated.