JP2608325B2 - Semiconductor laser - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device useful for wavelength-division multiplexed optical information transmission / processing / recording, etc. And a semiconductor laser that emits a laser beam.

[Prior Art] In recent years, technological advances in the fields of optical communication and optical information processing have been remarkable, and wavelength multiplexing transmission and frequency modulation transmission methods for expanding the transmission band in the communication field, wavelengths in the optical recording field, etc. As a function required for a semiconductor laser, such as improvement in recording density by multiplexing, a high-performance wavelength variable function has become important.

Therefore, in order to make the wavelength tunable, recently, a wavelength tunable semiconductor laser using a ground level and a higher level in the quantum well layer (Japanese Patent Laid-Open No. 63-32985) and a plurality of quantum compounds having different compositions or widths are used. A wavelength tunable semiconductor laser using each level of a well layer (Japanese Patent Laid-Open No. 63-312688) has been proposed.

 The principle of changing the wavelength will be described below.

First, in the case of an active layer having a quantum well structure, as the injection current increases, the gain curve in the active layer shows
Figure, the maximum value of the gain as shown in FIG. 3 in the latter case the low-energy formic guy flop (hereinafter, referred to as E _g) may be moving from (long wavelength) side to the high E _g (short wavelength) side Intellectual Has been.

In a semiconductor laser having such an active layer having a quantum well structure, long-wavelength light oscillates when current is uniformly injected in the resonance direction. That is, while low total losses in the resonator, is the oscillating light corresponding to the low E _g.

Also, in this semiconductor laser, when current is injected non-uniformly, short-wavelength light oscillates. At this time, since the gain decreases in the loss region, a large amount of current injection is performed to increase the gain in the gain region. In particular, since the gain on the short wavelength side of the loss region is very small, the injection current into the gain region is made very large so that the total gain in the resonator becomes the maximum in the short wavelength light.

Here, the uniform injection in the resonance direction means that in the resonance direction of light in the active waveguide where light and electrons in the active layer are concentrated,
This refers to a state where injection is performed at the same current density. On the other hand, even if the current density is uniform over the length in the resonance direction in a certain region, the state in which the current density is different in each region in all the resonators is non-uniform injection. Further, in order to make the injection current into each region different, a separation portion is required between the regions. No electric current is injected into this separated portion even during uniform injection. However, if the separated portion has a length of 10 μm or less, it can be regarded as a substantially uniform implantation state in consideration of the spread of carriers inside the active layer. As described above, the conventional variable wavelength system oscillates long-wavelength light during uniform injection and oscillates short-wavelength light during nonuniform injection.

[Problems to be solved by the invention]

However, in the above-mentioned conventional example, there is a problem that the difference between the current value during uniform injection and the current value during non-uniform injection is very large. For example, the oscillation threshold current at the time of non-uniform injection is more than several tens of times at the time of non-uniform injection. Therefore, the temperature of the element rises and the life of the element is shortened.

Further, in the above conventional example, the length of the absorption region is very important. If the absorption region at the time of non-uniform injection is long, the loss amount of short-wavelength light becomes large, and the gain of the active region is greatly increased. In some cases, short wavelength light did not oscillate. This is because the gain on the short wavelength side in the loss region decreases significantly as the injection current decreases, but the gain on the short wavelength side in the gain region tends to saturate as the injection current increases. This is because a state in which the loss of short wavelength light cannot be compensated may occur.

[Means for Solving the Problems]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a tunable semiconductor laser capable of easily controlling an oscillation wavelength without greatly changing an injection current density.

The above-mentioned object of the present invention has a resonator formed of semiconductors including an active layer having a quantum well structure and a plurality of electrodes arranged in parallel in the resonance direction of the resonator, and is injected from these electrodes. In a semiconductor laser that oscillates light having different quantization energies from each other by independently controlling the current densities of different currents, it is possible to cope with large quantization energies when injection is performed with the same current density from the plurality of electrodes. This is achieved by setting the facet loss of the resonator such that light that oscillates and oscillates light corresponding to a small quantization energy when injection is performed at a different current density depending on the electrode.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

First, in order to theoretically show the present invention, as a simple example, in an active layer composed of a single quantum well layer (SQW), a semiconductor laser in which an injection current density can be changed into two regions (I, II) in a resonance direction. Consider the oscillation conditions of

Since the gain g in each region is a function of the injection carrier density n and the wavelength λ, it can be written as g (n _I , λ _i ). Here, assuming that the injection current I, the layer thickness d of the active layer, the width w of the active region, and the cavity length L, I = ndwL. The length of each region is L _I and L _II , and the total resonator length is L L _I + L _II . At this time, the oscillation condition at a certain wavelength λi is In the state, G _i = 0. Where Γ is SQW
Is the optical confinement coefficient in α, α is the internal loss in the resonator, R _f and R _b
Represents the end face reflectivity of the front and rear faces.

First, a case where the current is injected uniformly in the resonance direction will be described. As the wavelength lambda ₂ of light is oscillated, equation (1) is _{n I = n II = n 0} , Becomes This is the formula for a normal semiconductor laser, where the gain is the internal loss α and the end face loss. Oscillates in a state equal to the sum of In order for the wavelength λ ₂ oscillated during this uniform injection to be a short wavelength corresponding to high E _g , when the wavelength corresponding to low E _g is λ ₁ , the gain at λ ₂ in FIG. , Λ ₁ . That is, g (n ₀ , λ ₁ ) <g (n ₀ , λ ₂ ). If n ₋ <n ₀ , the oscillation threshold gain has not yet been reached here. Needs to be

To meet these conditions, Should be fine. In other words, it is sufficient that the total loss of the resonator is greater than when the respective gains at λ ₁ and λ ₂ are equal. This is the reason that if the internal loss or the facet loss is large, the short wavelength λ ₂ corresponding to the high E _g can be oscillated. In general, an increase in internal loss causes a thermal rise in the resonator and instability of the mode of light. Therefore, in the present invention, the object is achieved by increasing the end face loss.

Basically, a method of reducing the resonator length L or a method of reducing the end face reflectances R _f and R _b can be used.

FIG. 9 schematically shows changes in the oscillation wavelength with respect to the cavity length L of a semiconductor laser having an active layer having a quantum well structure. As L decreases, light oscillating with light corresponding to E _g1 shifts to shorter wavelengths due to the band-filing effect as the threshold gain increases, and
In the resonator length L _1, the gain corresponding to the E _g1 and E _g2 are equal, or oscillate either or simultaneously two wavelengths, 2
One wavelength switching occurs. When L further decreases, light corresponding to E _g2 oscillates and shifts to the shorter wavelength side.

As described above, the oscillation wavelength changes with the resonator length, and L ₁
The light corresponding to different E _g is oscillated at the boundary.

The present invention effectively utilizes this fact.
The cavity length is set smaller than L ₁ so that the light (short-wavelength light) corresponding to E _g2 oscillates during uniform current injection. Injecting this semiconductor laser into a plurality of regions at different current densities,
In other words, if the current is injected non-uniformly in the resonance direction, the gain for E _g2 is small in the loss region where the current density is small, so that the total gain for E _g1 is more than the total gain for E _g2 in the resonator. Threshold gain is reached sooner,
Light (long-wavelength light) corresponding to E _g1 will oscillate.

It is difficult to clarify the range in which the cavity length is short. Because of crystal quality, end face reflectivity, use (measurement)
This is because the threshold gain strongly depends on temperature and the like. Therefore, the range of the cavity length of the present invention is appropriately expressed as a range in which light having a wavelength corresponding to high E _g oscillates during uniform injection. According to the present invention, since the change in the injection current at the time of wavelength switching is small, there is no adverse effect on the device.

Next, the principle of the present invention when the end face reflectances R _f and R _b are reduced will be described. FIG. 10 schematically shows a change in oscillation wavelength with respect to the end face reflectance of a semiconductor laser having an active layer having a quantum well structure. Also in this case, the behavior similar to L is exhibited, and the light (long wavelength light) corresponding to E _g1 shifts to the short wavelength side as R _f R _b decreases. Then, at the reflectances R _f1 and R _b1 , the gains corresponding to E _g1 and E _g2 become equal, and oscillation occurs at one of the two wavelengths or at the same time, or switching of the two wavelengths occurs. When the reflectance further decreases, light corresponding to _Eg2 oscillates and shifts to shorter wavelengths.

As described above, the oscillation wavelength changes according to the end face reflectance, and R
_f1, and the R _b1 bordering oscillates light corresponding to different E _g. This is effectively utilized by another method of the present invention.In a semiconductor laser having a certain cavity length, end face reflection so that light corresponding to E _g2 oscillates when current is uniformly injected. Set the rate. This method can be easily realized by depositing a dielectric material on the end surface and coating it with a sputter or the like. Usually, the reflectance of the end face is often smaller than the reflectance of the cleaved face of about 32%. If the current is injected non-uniformly in the resonance direction of this semiconductor laser, the gain for E _g2 is small in the loss region where the current density is small, so the total gain for E _{g1 is} better than that for E _g2 in the cavity. However, the threshold gain is quickly reached due to an increase in current injection into the gain region, and long-wavelength light corresponding to E _g1 oscillates.

In this case, it is possible to reduce the change in the injection current when switching the oscillation wavelength.

In the above, the resonator length and the end face reflectivity have been described independently. However, in practice, the present invention can be implemented by optimizing and changing both.

In the above description, E _g1 and E _g2 are meant two E _g of E _g for quantized energy within the active layer of a quantum well structure. Specifically, in the case of a single quantum well structure or a multiple quantum well structure, the quantum levels in the well n = 1, n
= 2, n = 3... In the case of a structure composed of a plurality of quantum well layers having different compositions or widths, some energy existing in the active layer is used.
_{E g11, E g12, ...,} E g21, E g22, ..., which means the two levels of the E _g31 .... Here, E _ij is the j-th energy level of the i-th well layer.

FIG. 1 is a schematic structural view of a wavelength tunable semiconductor laser using the present invention. The left side is a sectional view perpendicular to the resonance direction, and the right side is a sectional view parallel to the resonance direction. Hereinafter, the configuration of this laser will be described.

The film _{^{configuration, n + -GaAa n-Al 0.5}} Ga of 0.5μm of n ⁺ -GaAs buffer layer 2,1.5μm on the substrate 1 _0.5 As Kuratsudo layer 3, p-Al _0.5 Ga _0.5 active layer 4,1.5μm An As cladding layer 5 and a 0.5 μm p ⁻ -GaAs cap layer 6 are laminated by a molecular beam epitaxy method, and an Au / Cr electrode 7 is vapor-deposited on the p-side and an Au-Ge / Au electrode 8 is vapor-deposited on the n-side. It has been alloyed to make ohmic contact.

The active layer consists of a barrier layer B of Al _0.3 Ga _0.7 As 150Å separating them quantum well layer W ₂ and the quantum well layer W ₁ and Al _0.08 Ga _0.92 As 160Å of GaAs 80 Å as shown in Figure 2.
Furthermore, a GRIN-SCH layer having a thickness of 400 mm is formed outside the quantum well layer.
The structure is such that G ₁ and G ₂ gradually change from Al _0.3 Ga _0.7 As to Al _0.5 Ga _0.5 As to confine light and electrons. The p-side light / electron confinement layer and barrier layer are doped p-type.

The lateral structure perpendicular to the resonance direction of the semiconductor laser of this embodiment uses a ridge waveguide structure as shown in FIG. 1, and the ridge has a depth of 1.8 μm from the upper surface. The width is 3 μm. Both sides of the ridge are
A Si ₃ N ₄ film 9 is formed by plasma CVD, and etching processing is performed so that the upper portion serves as a current injection window.
Has been deposited. The p-side electrode 7 is separated into two parts in the resonance direction. The total length of the resonator is 260 μm, the front part is 172 μm, the rear part is 78 μm, and the separation part is 10 μm.

First, the resonator length, which is the first feature of the present invention, will be described. FIG. 4 shows experimental values of the relationship between the resonator length and the oscillation wavelength of the semiconductor laser of this embodiment. As the cavity length becomes shorter, the oscillation wavelength becomes shorter, and L
At 300 μm and L 150 μm, the wavelength shifted to a discontinuous short wavelength. As described above, the shortening of the cavity increases the end face loss inside the laser and enables the shortening of the wavelength by increasing the threshold gain.

In the semiconductor laser of the present embodiment, when the cavity length is set to 260 μm, when current is uniformly injected in the cavity direction, the current value is slightly above the threshold and the wavelength is 815.
Oscillated at 7 nm. This wavelength corresponds to the ground level of the W ₂ of AlGaAs well layers. FIG. 3 shows the wavelength dependence of the gain obtained by adding the gains of the two well layers at the time of the uniform injection in consideration of the optical confinement coefficient. The figure shows the injection carrier density, which is n ₀ during uniform injection. At this time, the wavelength dependence of the gain has maximum values at λ ₁ and λ ₂ ,
_Since the peak of the gain for ₂ is the largest, the short wavelength light λ ₂ is oscillating.

Next, the behavior of oscillation when current is nonuniformly injected into this semiconductor laser will be described.

In the loss region, the implanted carrier density is n ₋ and less than n ₀ . Regarding the wavelength dependence of the gain at this time, as shown in FIG. 3, the gain for the wavelength λ ₂ is remarkably reduced as compared with the case of uniform injection, and is considerably smaller than the gain for λ ₁ .

On the other hand, in the gain region, in order to obtain a gain that compensates for the decrease in the gain in the loss region, a larger injection carrier density n ₊ is required than in uniform injection. The wavelength dependence of the gain at this time is such that the gain on the shorter wavelength side becomes larger than n ₀ , as shown by n ₊ in FIG. However, the total intracavity gain at λ ₂ is smaller than that at λ ₁ , and eventually λ
The gain of ₁ reaches the oscillation threshold gain first, and the long wavelength light oscillates.

 The embodiment will be theoretically analyzed below.

As described above, the gain g is therefore a function of the injected carrier density n and energy formic guy flop E _g, can be expressed when the wavelength corresponding to the E _g and lambda g (n, lambda) and. Regarding the semiconductor laser of the present invention which is capable of injecting current by being divided into two regions in the resonance direction, the lengths of the gain region and the loss region in the resonance direction are set.
When L _I and L _{II are set} and the length of the separating portion is set to a length that can be sufficiently ignored, the total resonator length L is approximately L _I + L _II .

Next, gain is considered for each well layer. The well layer W _1, each gain in the well layer _{W 2 g 1 (n 1,} λ), g 2 (n 2, λ) expressed by the optical confinement factor in each well layer, the light with respect to E _g For λ, Γ ₁ (λ) and Γ ₂ (λ), respectively.

From this, the following expression is satisfied inside the resonator for the light of λ _i .

Here, α _i represents the loss inside the resonator of the light of wavelength λ _i , and R _f and R _b represent the front and rear facet reflectivities. Also, since the wavelength dependence of R _f and R _b is small, it is ignored here. G = 0 is the oscillation condition. In the equation (2), when a uniform current is injected, the first term on the right side and the second term in {} are equal, and the carrier change of the gain of the two quantum well layers may be considered.

At the time of uniform injection, if the injection carrier is n0,
The injection carriers into the well layers W ₁ and W ₂ are n ₁ and n ₂ , respectively. λ ₁ is given by the following equation.

However, since there is no gain of λ ₁ due to the well layer W ₂ , g
₂ (n ₂ , λ ₂ ) = 0. Similarly, for λ ₂ , the following equation is obtained.

The wavelength dependence of the optical confinement coefficient depends on the wavelength dispersion of the refractive index, but for simplicity, Γ ₁ (λ ₁ ) Γ ₁ (λ ₂ ),
Γ ₂ (λ ₁ ) Γ ₂ (λ ₂ ). Also, the internal loss is α ₁ α ₂ . At this time, G ₁ is _{g 1 (n 1, λ 1} ) is determined by, G ₂ is _{g 1 (n 1, λ 2} ) and _{g 2 (n 2, λ 2} ) so determined, if n ₀ is increased G ₂ > G ₁ . Here, when the end face loss is large, that is, when L is small, or when R _f and R _b are small, G ₁ <0 = G ₂ occurs. At this time, λ ₂ oscillates.

Next, consider the oscillation wavelength λ _i when carriers are injected nonuniformly in the resonance direction. Gain area and loss area
When injected with carrier densities of n ₊ and n ₋ , currents with densities of n ₁ and n _{2 in the} gain region and n ₁ ′ and n ₂ ′ in the absorption region are injected into the quantum well layers W ₁ and W ₂ . Again, if the changes in Γ ₁ , Γ ₂ , and α with wavelength are small, Becomes

When λ _i = λ ₂ , g ₁ ′ (n ₁ ′, λ ₂ ) <0, g ₂ ′
(N ₂ ′, λ ₂ ) 0, g ₁ (n ₁ , λ ₂ ) and g ₂ (n ₂ , λ ₂ )
If ₂₎ can be very large and the G _i <0.

On the other hand, when λ _i = λ ₁ , g _i ′ (n ₁ ′, λ ₁ )> 0,
Since g ₂ ′ (n ₂ ′, λ ₁ ) = 0, g ₁ (n ₁ , λ ₁ ) and g ₂ (n
₂ , λ ₁ ) can easily be made large enough to satisfy G ₁ = 0.

Thus, the well layer W ₁ to form the small E _g is a loss region during nonuniform injection to show a large loss to the short wavelength light lambda _2, it is not occur easier for oscillation of the long wavelength light.

Above, two wavelengths, λ ₁ and λ ₂ , have been extremely described. However, in actuality, in FIG. 3, a change in the slope of the gain curve between λ ₁ and λ ₂ with respect to wavelength when the amount of injected carrier is changed is shown. As can be seen from the above, in the wavelength between λ ₁ and λ ₂ , oscillation can be performed at a wavelength at which the total gain in the resonator becomes maximum. The wavelength in the threshold state obtained by the experiment is shown in FIG. During uniform injection, front region (L _f = 17
2 μm) I _f = 50 mA, rear region (L _b = 78 μm) I _b = 23 m
When the current of A is injected, the threshold is reached and the shortest wavelength is 81
Oscillated at 5.7 nm.

Non-uniform current injection, I _f =
When a current of 30 mA and I _b = 130 mA was injected with the rear region as the gain region, oscillation occurred at λ = 834.7 nm in the threshold state. On the contrary, in the case of injecting a current of I _f = 245 mA with the front region as the gain region and I _b = 3.5 mA with the rear region as the loss region,
It oscillated at a wavelength of 838.9 nm. When the non-uniform injection was varied, as shown in FIG. 5, the shortest wavelength light oscillated at the time of the uniform injection, and the wavelength became longer as the non-uniformity increased, resulting in two well layers W ₁ and W _2. varied continuously between corresponding wavelengths of E _g. This is because, as described above, due to the difference in gain curve with respect to wavelength in the loss region and the gain region, the wavelength with a large total gain in the resonator selectively oscillates.

FIG. 6 shows the injection current _If to the front region and the injection current to the rear region when the semiconductor laser of this embodiment is in the oscillation threshold state.
It shows the relationship with I _b .

From FIG. 6, when one of the regions is set as a loss region, the other region becomes a gain region which compensates for it, and
Considering in conjunction with FIG. 5, it can be seen that the wavelength can be changed regardless of which region is the loss region. Thus, the quantum well layer with high E _g during uniform injection is
By setting the resonator length to be shorter than a certain length so that the short wavelength light corresponding to E _g oscillates, the oscillation wavelength does not change so much even when nonuniform injection is performed. The wavelength can be made longer, and a good wavelength tunable semiconductor laser can be realized.

In the above experiment, as the nonuniformity of the current injection in the resonance direction increases, the wavelength is continuously lengthened while showing the wavelength shift of the Fabry-Perot mode.
The wavelength could be changed to 38.9 nm.

Furthermore, it is possible to selectively oscillate only a specific wavelength on the gain curve by setting the quantum well layer and barrier layer in the active layer and changing the structure such as the length of the resonator and the length of the gain region and the loss region. It is possible to change the wavelength discontinuously. Also, according to certain conditions, it is also possible to switch only two corresponding wavelengths to two E _g. For example,
As a method of using the semiconductor according to the present embodiment, it is possible to freely oscillate a temporally different wavelength from one element by setting an injection current into two regions in advance and switching between them. .

In the embodiment described above, only the resonator length L has been described. However, similarly to L in the above-described equations (1) and (2), the threshold gain can be reduced by reducing the end face reflectivities R _f and R _b. Is effective as a selective oscillating means for short wavelength light at the time of uniform injection. In this case, it is easy to coat R _f , R
_b can be reduced. Further, it is desirable that R _b > R _f for higher output and higher oscillation efficiency, but the present invention is also effective when R _b ≦ R _f .

FIG. 7 illustrates another embodiment of the present invention in which the active layer is a single quantum well layer (SQW) having a SCH layer. FIG. 7 is a diagram schematically showing the band structure in the SQW layer, in which the light of wavelength λ ₃ is changed to n = by the transition of n = 1.
The light of wavelength λ ₄ oscillates due to the transition of 2. The laser structure may be the same as in FIG. 1 or another form. FIG.
4 shows the injection carrier density dependence of the gain curve with respect to wavelength. As is well known, when the injection carrier density is small, the gain in light corresponding to the ground level is large, but when the carrier density is further increased, a peak of the gain starts to occur in the higher level, and finally, The gain of the ground level is saturated and the gain of the higher level increases.

In FIG. 3, since there are two types of SQWs as shown in FIG. 8, the gain is determined by adding them, but the change of the gain curve with respect to the change of the injection carrier density shows almost the same behavior. There is.

As described above, the wavelength tunable according to the present invention is also possible in the semiconductor laser having the active layer of the SQW structure.

That is, in the present embodiment, by reducing the resonator length L ₁ or the end face reflectivities R _f and R _b , the oscillation threshold gain in the resonator at the time of uniform injection is increased, and the wavelength for the higher-order level is increased. The resonator is configured to oscillate λ ₄ . The gain curve in this case is λ as shown by n ₀ in FIG.
_Since the gain is the largest at the wavelength of ₄ , it oscillates here.

If performed unevenly current injection to the resonator direction, a loss region, since a state carrier density is small, in FIG. 8 n _-
As shown in FIG. 5, the gain is maximum at λ ₃ , decreases at λ ₄ , and sometimes becomes negative. On the other hand, in the gain region, it is necessary to have a gain to compensate for the loss of light in the loss region, so the injection carrier density increases to n ₊ , resulting in λ
The gain on the ₄ side increases, and the gain on the λ ₃ side saturates. Thus, when the carrier is injected unevenly, λ ₄
Loss in the loss region in becomes dominant, to less likely the oscillation of lambda _4, it will reach a threshold gain earlier total of lambda ₃ of gain resonator, wavelength lambda ₃ Light oscillates.

As described above, even when an active layer is used as the SQW layer, a change in the oscillation wavelength can be made with a small current change by uniformly and non-uniformly injecting the current in the resonance direction.

Of course, not only in the case of the single quantum well structure shown here, but also in the case of a plurality of equivalent quantum well structures, exactly the same explanation is made. The optical / electron confinement layer is effective for lowering the threshold value in the case of a quantum well structure, but the present invention can be applied even when there is no such layer.

Next, another embodiment of the setting of the resonator length L, which is the greatest feature of the present invention, will be described.

FIG. 11 shows the current injection regions (III) and (IV) in the resonance direction.
And there is a waveguide loss region (V) between them.

The length of each region is L _III , L _IV , L _V , and the total resonator length L is L = L _III + L _IV + L _V. If L _V is about a few μm, considering the internal spread of the carrier, L _V
Can be neglected, so it can be explained by the example shown above, but if it has a region larger than about 10 μm,
(V) acts on the light as an absorption region, resulting in an increase in the total loss in the resonator and an increase in the oscillation threshold gain. This example is regarded as an element in which the loss region is provided in the resonator in advance. That is, in this case, the entire cavity length is longer than the cavity length that oscillates a short wavelength corresponding to high E _g at the time of uniform injection.
When carriers are injected into I) and (IV) at the same current density, short-wavelength light is emitted. When carriers are injected into (III) and (IV) at different current densities, long wavelength light oscillates.

Such an absorption region can exist anywhere in the resonator direction, but if it is near the end face, it has a window structure and is suitable for high output.

FIG. 12 shows an embodiment in which three electrodes are separately formed in the resonance direction. However, the electrodes on the front end face side and the rear end face side are short-circuited, and injection can be performed with the same carrier density. Therefore, it can be regarded as two regions, that is, the (VI) and (VIII) regions and the (VII) region.

In addition, the loss region of (VI) and (VIII) during non-uniform injection,
If (VII) is used in the gain region, the wavelength can be changed without increasing the light density on the end face so much, so that high output can be achieved.

In addition to these, various electrode configurations for injecting current uniformly or non-uniformly in the resonance direction are conceivable, but basically, as the degree of non-uniformity increases, the oscillation wavelength becomes longer.

As another embodiment of the present invention, FIG. 13 shows an energy band diagram of a conduction band when the active layer is constituted by three different quantum well layers.

The energy gear corresponding to each well is E _g1 ,
E _g2 , E _g3 , and E _g1 <E _g2 <E _g3 .

By applying the present invention and reducing the resonator length L or the end face reflectances R _f and R _b , the end face loss in the resonator can be reduced. If is increased, a wavelength corresponding to E _g3 is oscillated during uniform injection. If the current is non-uniformly injected into this laser, E _g3
From the wavelength corresponding to E _g2 to the wavelength corresponding to E _g2
The wavelength can be increased to a wavelength corresponding to _g1 and the wavelength tunable width can be increased as compared with the case of two well layers. FIG. 14 is an energy band diagram of a conduction band when the active layer is formed of two different quantum well layers and each of the higher levels exists.

N = 1 level energy gap of one quantum well layer
E _g11 , n = 2 level energy _gap E ₁₂ , the n = 1 level energy _gap E _g21 , n = 2 of the other quantum well layer
_Assuming that the energy _gap of the _level is E _g22 , in this example, E _g11 <E _g21 <E _g12 <E _g22 .

Even in such a case, the present invention is effective and the end face loss Is set appropriately, and the current injection is made uniform and non-uniform in the resonance direction, so that adjacent energy gaps, for example,
A wavelength tunable operation between wavelengths corresponding to E _{g21 to} E _g12 becomes possible.

If the end face loss is increased so that _Eg22 oscillates during uniform injection, E
_{g22 → E g12 → E g21 →} E 11 and the oscillation wavelength in a wavelength corresponding to the respective is changed.

In the above-mentioned embodiment, the cross-sectional structure perpendicular to the laser resonance direction is the Ridge waveguide type structure, but it is also applicable to the gain waveguide type structure such as the electrode stripe type structure and the buried type structure. The present invention is also effective in a refractive index waveguide type structure. Further, even if the substrate is n-type, p-type or semi-insulating, a laser structure may be formed, and a laser structure such as a lateral injection type may be used.

Further, although an AlGaAs-based semiconductor laser device has been described as the above-described embodiment, the present invention can be applied to a laser of any material such as an InGaAaP-based laser and an AlGaInP-based laser.

Further, the laser device of the present invention can be used as a high-efficiency optical amplifier operating in a wide wavelength range.

That is, a current slightly smaller than the threshold current at which laser oscillation occurs is injected into each electrode of the laser element of the present invention so that the gain becomes positive over a wide wavelength range. Light having a predetermined wavelength is coupled to the inside of the resonator of the device from the outside.

As a method of injecting and coupling light into the resonator, there are a method of injecting light through one end face of the element, a method of injecting light laterally perpendicular to the resonance direction or from above the element.

When light of a predetermined wavelength is thus incident, light having the same wavelength can be emitted from the end face.

Further, the laser device of the present invention can be used as a highly efficient optical wavelength converter having a wavelength tunable function in a wide wavelength range. That is, a current slightly smaller than a threshold current value at which light of a desired wavelength oscillates is injected into each electrode of the laser element of the present invention. When light inside the resonator of the element is incidentally coupled to this, light having a desired wavelength controlled by an injection current into the element can be emitted from the end surface.

In the present invention, the variable range of the desired wavelength to be emitted is wide, and it is only necessary to control and bias the degree of non-uniformity of the injection current in the resonance direction, so that it is easy to change the wavelength.

When the semiconductor laser of the present invention is used as an optical amplifier or an optical wavelength converter, it is preferable that the end face of the laser element be supplemented with a non-reflective coating.

〔The invention's effect〕

As described above, according to the semiconductor laser of the present invention, it is possible to change the wavelength in a large width with a small injection current difference. Therefore, according to the present invention, a semiconductor laser having a small amount of heat generation and a long life can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an embodiment of the semiconductor laser of the present invention, FIG. 2 is an energy band diagram of an active layer having two different quantum well layers, and FIG. 3 is a wavelength in the active layer of FIG. FIG. 4 shows the relationship between the cavity length and the oscillation wavelength, FIG. 5 shows the relationship between the front injection current and the oscillation wavelength, and FIG. 6 shows the front injection in the oscillation state. FIG. 7 is a diagram showing the relationship between the current and the rear injection current, FIG. 7 is an energy band diagram of an active layer having a single quantum well layer,
FIG. 8 is a diagram showing the gain curve with respect to wavelength in the active layer of FIG. 7, FIG. 9 is a diagram showing the relationship between the resonator length and the oscillation wavelength, and FIG. 10 is a diagram showing the end face reflectance of the resonator and the oscillation wavelength. FIG. 11, FIG. 11 and FIG. 12 are schematic cross-sectional views showing other embodiments of the present invention, and FIGS. 13 and 14 are energy band diagrams showing still another embodiment of the present invention. Is. 1 ...... n ⁺ -GaAs substrate 2 ...... n ⁺ -GaAs buffer layer _{3 ...... n-Al 0.5 Ga 0.5} As Kuratsudo layer 4 ...... active layer _{5 ...... p-Al 0.5 Ga 0.5} As Kuratsudo layer 6 ...... p ⁺ -GaAs cap layer 7 ... p-side electrode 8 ... n-side electrode 9 ... insulating layer

Claims (4)

(57) [Claims] 1. A resonator comprising a stacked semiconductor including an active layer of a quantum well structure and a plurality of electrodes arranged in parallel in the resonance direction of the resonator, and a current injected from these electrodes. Independently controlling the density of each of the semiconductor lasers, the corresponding quantization energies oscillate light different from each other, the light corresponding to the large quantization energy when the injection is performed at the same current density from the plurality of electrodes. Wherein the end face loss of the resonator is set such that light is oscillated and light corresponding to small quantization energy is oscillated when injection is performed at a different current density according to the electrode. 2. The semiconductor laser according to claim 1, wherein the active layer comprises at least one quantum well layer having a plurality of oscillating quantum levels. 3. The method according to claim 1, wherein said active layer comprises a plurality of quantum well layers having quantum level structures different from each other.
9. The semiconductor laser according to item 1. 4. The semiconductor laser according to claim 1, wherein a reflectance of said end face is reduced by coating a dielectric film on at least one end face of said resonator.