JP2001196698A - Variable wavelength semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem where the upper-limit of variable wavelength characteristics is limited since a conventional variable-wavelength semiconductor laser has a small total loss of a resonator and oscillates with a small threshold gain (first quantum level) and hence has a gain spectrum of only several tens to approximately 100 nm in terms of a wavelength region. SOLUTION: Total loss is increased by increasing only the waveguide loss, a high-order quantum level is stimulated to expand the gain spectrum width of an active layer to 200 nm, thus providing the wide-band variable-wavelength semiconductor laser.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber communication technology used in a trunk line telephone switching network and the like, and more particularly to a wide band required in a wavelength division multiplexing optical communication technology for simultaneously using light of different wavelengths for signal transmission. The present invention relates to a wavelength tunable semiconductor laser, specifically, has a waveguide provided with a sampled grating reflector having a different grating period or a super-periodic structure grating Bragg reflector before and after the quantum well active layer. The present invention relates to a wavelength tunable semiconductor laser provided with a plurality of control electrodes and using laser oscillation.

[0002]

2. Description of the Related Art Although generally applicable to tunable semiconductor lasers, the following description will be made by way of an example of a super-periodic structure diffraction grating Bragg reflector (SS) having different diffraction grating periods.
G-DBR: Super-Structure-Grating Distributed Bra
gg Reflector, hereinafter referred to as “SSG-DBR”. ) The laser will be described. Figure 8 shows the journal Journal of Quantum Electronics (H. Ishii et al.
IEEE Journal of Quantum Electronics, vol. 32, No.
3, 1996, pp. 433-441) is a schematic sectional view showing the configuration of an SSG-DBR laser provided with SSG-DBRs having different diffraction grating periods before and after a quantum well active layer.

In the figure, reference numeral 1 denotes a front super-periodic structure diffraction grating mirror as a front reflector, and 2 denotes a rear super-periodic structure diffraction mirror as a rear reflector. Reference numeral 3 denotes an active layer (also referred to as “quantum well active layer” or “active region”), which is composed of, for example, four quantum wells having a compressive strain of 1.2%. 4 is a phase control area, 6 is a front mirror control current,
7 is an active layer current, 8 is a phase control current, 9 is a rear mirror control current, 56 is an electrode through which a front mirror control current 6 flows, and 5
7 is an electrode through which the active layer current 7 flows, 58 is a phase control current 8
, 59 is an electrode through which the rear mirror control current 9 flows, 10 is an n-InP substrate, 11 is a p-InP cladding layer, 12 is
The p-InGaAsP cap layer 18 is output light.

[0004] Reference numeral 13 denotes an InGaAsP waveguide layer (also referred to as a "waveguide"), which comprises a front super-periodic structure diffraction grating mirror 1, an active layer 3, a phase control region 4, and a rear super-periodic structure diffraction grating mirror 2. I have. Although the active layer 3 is limited to InGaAsP here for convenience of description,
The active layer 3 is not limited to InGaAsP, but may be, for example, AlGaAs. Reference numeral 14 denotes a length corresponding to one period of the front super-periodic structure diffraction grating mirror 1 (referred to as front mirror diffraction grating period), and reference numeral 15 denotes a length corresponding to one period of the rear super-periodic structure diffraction grating mirror 2 (backward). The front mirror grating period 14 and the rear mirror grating period 15 are not the same but different.

[0005] Further, in the front super-periodic structure diffraction grating mirror 1 and the rear super-periodic structure diffraction grating mirror 2, while changing the pitch and phase of the diffraction grating quasi-continuously, the front mirror diffraction grating period 14 and the rear mirror diffraction Lattice period 15
Is repeated (the repetition is not shown). FIG. 8 shows only the cross section of the SSG-DBR laser, but the actual SSG-DBR laser is processed into a ridge shape with a stripe width of 3.5 μm.

[0006] As a wavelength tunable semiconductor laser reflecting mirror having the same reflection characteristics as the SSG-DBR, for example, V. Jayaraman et al. IEEE Journal of Quantum Elec.
tronics, vol. 29, No. 6, 1993, pp1824-1834).
R: Distributed Bragg Reflector, hereinafter referred to as “DBR”. ).

Next, the operation of the SSG-DBR laser shown in FIG. 8 will be described. The spontaneous emission light generated from the active layer 3 by injecting the active layer current 7 from the electrode 57 is multiple-reflected by the front super-periodic structure diffraction grating mirror 1 and the rear super-periodic structure diffraction grating mirror 2 to generate stimulated emission light. It is amplified and laser oscillation occurs. Since the oscillation threshold gain of the wavelength tunable semiconductor laser is equal to the total loss of the resonator, it is the sum of the waveguide loss and the mirror loss.

Here, the term "resonator" is a term focused on a phenomenon in which stimulated emission light is amplified and laser oscillation occurs, and refers to the entire wavelength tunable semiconductor laser. Laser oscillation occurs by injecting an active layer current 7 into the active layer 3 in such an amount as to generate a gain equal to the total loss.

Once the laser oscillation starts, the carrier density in the active layer 3 is fixed at a substantially constant value unless the oscillation threshold gain fluctuates due to a temperature change of the active layer 3 or the like. The output light 18 is mainly emitted from the front super-periodic structure diffraction grating 1 side by setting the reflectivity of the rear super-periodic structure diffraction grating mirror 2 higher than that of the front super-periodic structure diffraction grating 1.

The wavelength tunable semiconductor laser oscillates with a small total gain of the resonator and a low threshold gain, and once oscillates in the sub-band corresponding to the first quantum level, the carrier density in the active layer 3 becomes low. Fixed to When oscillation occurs at the first quantum level in this manner, a higher-order (eg, second quantum level) quantum level of the active layer 3 is not excited. When the active layer 3 oscillates at the first quantum level, the active layer 3 has a gain spectrum from several tens to about 100 nm when viewed in the wavelength region. Variable characteristics are determined.

Incidentally, the total loss of the resonator is the sum of the mirror loss and the waveguide loss, but the mirror loss is increased by lowering the mirror reflectivity or shortening the length of the active layer, thereby increasing the total loss of the resonator. It is not preferable to increase the loss.
This is because, if the mirror loss is increased by lowering the mirror reflectance to increase the total loss, the peak width of the reflection spectrum is widened, which results in an increase in the oscillation line width during laser oscillation and a deterioration in the side mode suppression ratio. It is.

Further, if the total loss is increased by increasing the mirror loss by shortening the length of the active layer 3, the waveguide loss is reduced, and it becomes difficult to obtain a large laser output. Therefore, to increase the total loss, it is advantageous to increase only the waveguide loss without significantly changing the mirror loss.

[0013]

The conventional tunable semiconductor laser has a small total loss of the resonator and oscillates with a low threshold gain (first quantum level). Since the gain spectrum is only about 10 to about 100 nm, a wavelength range from about 100 nm to about 200 nm cannot be obtained, and there is a problem that the upper limit of the wavelength tunable characteristic is limited.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the conventional example, and it is intended to excite a higher-order quantum level and expand the gain spectrum width of the active layer itself. An object of the present invention is to provide a wavelength tunable semiconductor laser having a wide band.

[0015]

The wavelength tunable semiconductor laser according to the present invention comprises a sampled grating reflector having a different diffraction grating period or a super-periodic structure grating Bragg reflector having a different diffraction grating period, having a quantum well active layer. In a wavelength tunable semiconductor laser including a waveguide provided before and after, and a plurality of control electrodes for controlling the wavelength of laser light oscillated in the waveguide, selectively increasing the waveguide loss of the quantum well active layer. Thus, it is possible to make a contribution of a gain from a higher-order quantum level.

A wavelength tunable semiconductor laser according to the present invention comprises:
An impurity diffusion layer reaching at least below the interface between the quantum well active layer and the substrate is formed in a region adjacent to both side surfaces of the quantum well active layer.

The wavelength tunable semiconductor laser according to the present invention comprises:
A first waveguide in which a sampled grating reflector having a different grating period or a super-periodic structure Bragg reflector having a different grating period is provided before and after the quantum well active layer; A wavelength tunable semiconductor laser comprising a plurality of control electrodes for controlling the wavelength of laser light oscillating at a wavelength, wherein the second semiconductor laser is optically coupled to the quantum well active layer and has a high waveguide loss. By providing a waveguide, it is configured to be able to contribute a gain from a higher-order quantum level.

A wavelength tunable semiconductor laser according to the present invention comprises:
The second waveguide has a non-waveguide region provided on an end face located on the side opposite to the side coupled to the first waveguide.

The wavelength tunable semiconductor laser according to the present invention comprises:
The end face of the second waveguide located on the side opposite to the side coupled to the first waveguide is inclined.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1. FIGS.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view for explaining a wavelength tunable semiconductor laser according to a first embodiment of the present invention, and more specifically, a cross section of a wavelength tunable semiconductor laser showing a configuration example using an InGaAsP-based material using an SSG-DBR as a reflector It is a schematic diagram. Here, FIG. 1 is a cross-sectional view of a wavelength tunable semiconductor laser, and FIG.
2 is a sectional view taken along line AA of the wavelength tunable semiconductor laser, and FIG. 3 is a sectional view taken along line BB of the wavelength tunable semiconductor laser shown in FIG.

In FIG. 1, reference numeral 1 denotes a front super-periodic structure diffraction grating mirror as a front reflector, and 2 denotes a rear super-periodic structure diffraction mirror as a rear reflector. Reference numeral 3 denotes an active layer (also referred to as “quantum well active layer” or “active region”), which is composed of, for example, four quantum wells having a compressive strain of 1.2%. Reference numeral 4 denotes a phase control area, 6 denotes a front mirror control current, 7 denotes an active layer current, 8 denotes a phase control current, 9 denotes a rear mirror control current, 56 denotes an electrode through which the front mirror control current 6 flows, and 57 denotes an active layer current 7. A flowing electrode, 58 is an electrode through which the phase control current 8 flows, 59 is an electrode through which the rear mirror control current 9 flows, 10 is an n-InP substrate, 11 is a p-InP cladding layer, 12 is a p-InGaAsP cap layer, and 16 is an impurity. A diffusion region, 17 is an impurity diffusion depth, and 18 is output light.

Reference numeral 13 denotes an InGaAsP waveguide layer (also referred to as a "waveguide"), which comprises a front super-periodic structure diffraction grating mirror 1, an active layer 3, a phase control region 4, and a rear super-periodic structure diffraction grating mirror 2. I have. Although the active layer 3 is limited to InGaAsP here for convenience of description,
The active layer 3 is not limited to InGaAsP, but may be, for example, AlGaAs. Reference numeral 14 denotes a length corresponding to one period of the front super-periodic structure diffraction grating mirror 1 (referred to as front mirror diffraction grating period), and reference numeral 15 denotes a length corresponding to one period of the rear super-periodic structure diffraction grating mirror 2 (backward). The front mirror grating period 14 and the rear mirror grating period 15 are not the same but different.

Further, in the front super-period structure diffraction grating mirror 1 and the rear super-period structure diffraction grating mirror 2, the front mirror diffraction grating period 14 and the rear mirror diffraction are integer-numbered times while changing the pitch and phase of the diffraction grating quasi-continuously. Lattice period 15
Is repeated (the repetition is not shown). In FIG. 2, reference numeral 22 denotes a current confinement layer.

Here, in a region adjacent to both side surfaces of the active layer 3, a region in which impurity diffusion is performed to an impurity diffusion depth 17 reaching at least below the interface between the active layer 3 and the n-InP substrate 10 (impurity diffusion) A diffusion region 16) is provided. Further, since the impurity diffusion region 16 is located in a region adjacent to both side surfaces of the active layer 3, the impurity diffusion region 16 is also formed in the active layer 3.

In FIG. 3, reference numeral 30 denotes the length of the impurity diffusion region 16 when the impurity diffusion region 16 is formed in the active layer 3. 2 and 3, the components denoted by the same reference numerals as those in FIG. 1 are the same or equivalent components.
The tunable semiconductor laser is processed into, for example, a ridge shape having a stripe width of 3.5 microns.

FIG. 4 shows the magazine Electronics Letters.
(H. Tabuchi et al. Electronics Letters, vol. 26, N
o. 11, 1990, pp742-723)
FIG. 4 is a schematic diagram of an emission spectrum from an end face when current is injected into a GaAsP multiple quantum well active layer. In FIG.
Reference numeral 39 denotes an emission spectrum of the quantum well when the injected carrier density is low, 40 denotes an emission spectrum of the quantum well when the injected carrier density is intermediate, and 41 denotes an emission spectrum of the quantum well when the injected carrier density is high.

Next, the SSG-D constructed as described above
The operation of the BR laser will be described. The spontaneous emission light generated from the active layer 3 by injecting the active layer current 7 from the electrode 57 is multiple-reflected by the front super-periodic structure diffraction grating mirror 1 and the rear super-periodic structure diffraction grating mirror 2 to generate stimulated emission light. It is amplified and laser oscillation occurs.

Once the laser oscillation starts, the carrier density in the active layer 3 is fixed to a substantially constant value as long as the oscillation threshold gain does not fluctuate due to a change in the temperature of the active layer 3 or the like. The output light 18 is mainly emitted from the front super-periodic structure diffraction grating 1 side by setting the reflectivity of the rear super-periodic structure diffraction grating mirror 2 higher than that of the front super-periodic structure diffraction grating 1.

In a region adjacent to both side surfaces of the active layer 3, a region where the impurity diffusion is performed to an impurity diffusion depth 17 which reaches at least below the interface between the active layer 3 and the n-InP substrate 10 (the impurity diffusion region 16). ). By controlling the length 30 of the impurity diffusion region 16 extending inside the active layer 3, the degree of the waveguide loss can be adjusted and the waveguide loss can be increased.

By increasing the waveguide loss and increasing the total loss of the resonator and raising the oscillation threshold gain, the oscillation threshold gain can be reached only from the gain from the subband corresponding to the first quantum level. It is not possible to excite a higher-order (for example, second quantum level) quantum level of the active layer. Exciting a higher quantum level by controlling the length 30 of the impurity diffusion region 16 and increasing the waveguide loss to increase the total loss of the resonator and increasing the carrier density in the active layer 3 Therefore, the gain spectrum width of the active layer 3 can be widened.

By the way, in the measurement of the emission spectrum shown in FIG. 4, a non-reflective coating is applied to one end surface of a sample processed into a Fabry-Perot laser shape.
No laser oscillation occurs and the output spectrum roughly corresponds to the gain spectrum. Here, the emission spectrum 39 of the quantum well when the density of injected carriers injected into the active layer 3 is small is that when the injection current is small (20 mA). At this time, the spectrum shape has a light emission peak (maximum value) around a wavelength of 1.57 μm.

The emission spectrum 40 of the quantum well when the injected carrier density is intermediate indicates that the injected current is intermediate (> 50).
mA) is an intermediate value. At this time, in addition to the emission peak near 1.57 μm corresponding to the first quantum level, an emission peak is also observed near 1.42 to 1.44 μm corresponding to the second quantum level.

Further, the emission spectrum 41 of the quantum well when the injection carrier density is large has a larger injection current. At this time, the state density corresponding to the first quantum level is satisfied and the gain tends to be saturated, and the emission peak intensity corresponding to the second quantum level increases. Thus, qualitatively, when the bias current (active layer current 7) is increased, the second quantum level starts to be excited, the gain profile extends to the short wavelength side, and the final gain width reaches 200 nm or more. You can see that there is.

The active layer 3 is configured to increase the waveguide loss at least until a gain from a higher quantum level (for example, a second quantum level) can be contributed. Thus, if the gain spectrum width is viewed in the wavelength region, for example, a wide wavelength region from several tens to about 200 nm can be obtained. The upper limit of the characteristics can be improved.

Even if the total loss of the resonator is somewhat increased by controlling the length 30 of the impurity diffusion region 16, as long as the higher quantum level is not excited, the present invention is the same as the conventional example. It is a matter of course that excitation of a higher-order quantum level is indispensable in the present invention.

By the way, the InGaAsP-based material which has a long wavelength band has been described as an example. However, the present invention is not limited to this. The effect can be obtained. The wavelength in the long wavelength band of the InGaAsP material is, for example, 1.3 μm.
m and 1.55 μm. Examples of the wavelength in the short wavelength band of the AlGaAs-based material include 0.8 μm and 0.9 μm.

Although the case where the SSG-DBR is used as a reflector has been described as an example, the present invention can be applied to a wavelength tunable semiconductor laser using a sampled grating DBR as a reflector. Needless to say. Operation of the above SSG-DBR laser, InGaAs
It is not limited to the first embodiment that the same effect can be obtained by using an AlGaAs-based material without being limited to the P-based material. Although a detailed description is omitted, the same applies to the following embodiments.

Second Embodiment FIG. 5 is a view for explaining a wavelength tunable semiconductor laser according to a second embodiment of the present invention. More specifically, FIG. 5 is a plan view including a top surface of an active layer. It is sectional drawing which cut | disconnected the wavelength variable semiconductor laser. In the figures, the components denoted by the same reference numerals as those in FIGS. 1 to 3 are the same or equivalent.

In the figure, reference numeral 19 denotes a loss-added waveguide that is adjacent to the active layer 3 and corresponds to a second waveguide optically coupled to the active layer 3. (For example, the second quantum level) to increase the waveguide loss until the gain can be contributed. In FIG. 5, the loss adding waveguide 19 has a so-called Y-branch structure as an example. A part of the laser light generated in the active layer 3 is guided to the loss-added waveguide 19 and most of the laser light guided to the loss-added waveguide 19 does not return to the active layer 3 again. Higher losses.

In the wavelength tunable semiconductor laser configured as described above, only the waveguide loss can be selectively increased without changing the mirror loss by the function of the loss adding waveguide 19, so that the higher quantum level ( For example, the second quantum level)
Can be injected in a sufficient amount to excite. As a matter of course, the loss-added waveguide 19 has at least a higher quantum level (for example, a second quantum level) of the active layer.
The configuration is such that the waveguide loss is increased until the gain can be made to contribute.

Further, by adjusting the active layer current 7 injected from the electrode 57, a higher-order quantum level can be excited, so that the oscillation threshold gain can be increased, the oscillation gain width of the active layer 3 can be widened, and the Variable wavelength operation can be realized. The guided light guided to the loss adding waveguide 19 does not usually contribute to laser oscillation.

FIG. 5 shows the case where the loss-added waveguide 19 is adjacent to the side of the active layer 3. However, the loss-added waveguide 19 is provided adjacent to the side, above or below the active layer 3. The same effect can be obtained even if the optical waveguide is optically coupled to the active layer 3 by a directional coupler type configuration in which the loss-adding waveguide 19 and the waveguide layer 13 are brought close to each other.

Third Embodiment FIG. 6 is a view for explaining a wavelength tunable semiconductor laser according to a third embodiment of the present invention, and more specifically, a plan view including the upper surface of an active layer. FIG. 4 is an element top view corresponding to a state where the element is cut and the element is viewed from above. In the figures, the components denoted by the same reference numerals as those in FIGS. 1 to 5 are the same or equivalent.

In the figure, reference numeral 20 denotes a non-waveguide region located on the end face of the loss-added waveguide 19 opposite to the side adjacent to the active layer 3. The non-waveguide region 20 is made of the same semiconductor layer as the loss-added waveguide 19, and its width is widened to such an extent that in-plane light confinement is not performed. It does not contribute to laser oscillation.

At the end face of the loss-added waveguide 19 opposite to the side adjacent to the active layer 3, a part of the guided light traveling through the loss-added waveguide 19 is reflected, which adversely affects the oscillation mode of the laser. there is a possibility. A non-waveguide region 20 that is widened to the extent that in-plane light confinement does not occur is formed on the end face of the loss-added waveguide 19 opposite to the side adjacent to the active layer 3.
Is provided, it is possible to prevent reflection of the guided light traveling through the loss-added waveguide 19, so that the influence of the reflected return light is eliminated, and the guided light guided to the waveguide does not contribute to laser oscillation at all. You can do so.

FIG. 6 shows a case where the non-waveguide region 20 is adjacent to the side of the active layer 3. However, the non-waveguide region 20 is provided with a non-reflective coating (not shown). Can obtain the same effect. The anti-reflection coating portion includes a dielectric thin film, more specifically, for example, Si ₃ N ₄ , SiO ₂ , or Al ₂ O ₃ .

The non-waveguide region 20 or the non-reflection coating eliminates the influence of the reflected return light and excites a higher-order quantum level (for example, the second quantum level).
By widening the oscillation gain width of the above, a wavelength tunable operation in a wide band can be realized.

Fourth Embodiment FIG. 7 is a view for explaining a wavelength tunable semiconductor laser according to a fourth embodiment of the present invention, and more specifically, a plane including the upper surface of an active layer. FIG. 4 is an element top view corresponding to a state where the element is cut and the element is viewed from above. In the figures, the components denoted by the same reference numerals as those in FIGS. 1 to 5 are the same or equivalent.

In the figure, reference numeral 21 denotes an inclined end face located on the end face of the loss adding waveguide 19 opposite to the side adjacent to the active layer 3. Inclined end face 21 of loss-added waveguide 19
The angle θ is preferably 3 to 15 degrees, more preferably about 7 degrees, where θ is the angle between the inclined end face 21 and the end face of the current confinement layer 22.

By providing the inclined end face 21 in the loss adding waveguide 19, the direction of the reflected light from the end face of the loss adding waveguide 19 opposite to the side adjacent to the active layer 3 can be changed to the optical axis of the input light. Thus, since the light is prevented from traveling again through the loss-added waveguide 19, the influence of the reflected return light on the laser oscillation mode can be eliminated, and the guided light guided to the waveguide does not contribute to laser oscillation. Configuration.

The inclined end surface 21 of the loss-added waveguide 19 eliminates the influence of reflected return light, excites a higher-order quantum level (for example, the second quantum level), and reduces the oscillation gain width of the active layer 3. By widening, it is possible to realize a wide-band tunable operation.

[0052]

As described above, according to the present invention, the waveguide loss of the quantum well active layer is selectively increased so that the gain can be contributed from the higher quantum level. By exciting the next quantum level to increase the gain spectrum width itself of the active layer, there is an effect that a wide-band tunable semiconductor laser can be obtained.

Further, according to the present invention, the impurity diffusion layer reaching at least below the interface between the quantum well active layer and the substrate is formed in a region adjacent to both side surfaces of the quantum well active layer. There is an effect that a quantum level can be excited to realize a wavelength tunable operation in a wide band.

Further, according to the present invention, by providing the second waveguide which is optically coupled to the quantum well active layer and has a high waveguide loss, the gain from the higher quantum level can be improved. , It is possible to obtain a wide-band wavelength-tunable semiconductor laser by exciting a higher-order quantum level and broadening the gain spectrum width itself of the active layer.

According to the present invention, the second waveguide is provided with the non-waveguide region on the end face located on the side opposite to the side coupled to the first waveguide. Therefore, there is an effect that a device capable of reliably exciting a higher-order quantum level and realizing a wide-band wavelength tunable operation can be obtained.

Further, according to the present invention, the end face of the second waveguide located on the side opposite to the side coupled to the first waveguide is inclined, so that the influence of the reflected return light is eliminated and the second waveguide is surely eliminated. There is an effect that a device capable of exciting a higher-order quantum level and realizing a wide-band wavelength tunable operation can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a wavelength tunable semiconductor laser according to a first embodiment.

FIG. 2 is a diagram for explaining the wavelength tunable semiconductor laser according to the first embodiment;

FIG. 3 is a diagram for explaining the wavelength tunable semiconductor laser according to the first embodiment;

FIG. 4 is a diagram for explaining the wavelength tunable semiconductor laser according to the first embodiment;

FIG. 5 is a diagram for explaining a wavelength tunable semiconductor laser according to a second embodiment.

FIG. 6 is a diagram illustrating a wavelength tunable semiconductor laser according to a third embodiment;

FIG. 7 is a diagram for explaining a wavelength tunable semiconductor laser according to a fourth embodiment.

FIG. 8 is a diagram for explaining a conventional tunable semiconductor laser.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Front super-period structure diffraction grating mirror 2 Back super-period structure diffraction grating mirror 3 Active layer 4 Phase control area 6 Front mirror control current 7 Active layer current 8 Phase control current 9 Rear mirror control current 10 n-InP substrate 11 p-InP Cladding layer 12 p-InGaAsP cap layer 13 InGaAsP waveguide layer 14 front mirror diffraction grating period 15 rear mirror diffraction grating period 16 impurity diffusion region 17 impurity diffusion depth 18 output light 19 lossy waveguide 20 non-waveguide region 21 inclined end face Reference Signs List 22 Current confinement layer 30 Length 39 Emission spectrum of quantum well when injection carrier density is low 40 Emission spectrum of quantum well when injection carrier density is medium 41 Emission spectrum of quantum well when injection carrier density is high 56-59 electrode

Claims (5)

[Claims] 1. A waveguide in which a sampled grating reflector having a different grating period or a super-periodic structure Bragg reflector having a different grating period is provided before and after a quantum well active layer, and oscillates in the waveguide. In a wavelength tunable semiconductor laser including a plurality of control electrodes for controlling the wavelength of laser light, the contribution of gain from a higher-order quantum level can be increased by selectively increasing the waveguide loss of the quantum well active layer. A wavelength tunable semiconductor laser characterized in that the wavelength tunable semiconductor laser is configured to be able to perform the operation. 2. The impurity diffusion layer according to claim 1, wherein an impurity diffusion layer reaching at least below an interface between the quantum well active layer and the substrate is formed in a region adjacent to both side surfaces of the quantum well active layer. Tunable semiconductor laser. 3. A first waveguide in which a sampled grating reflector having a different grating period or a super-periodic structure Bragg reflector having a different grating period is provided before and after a quantum well active layer, A wavelength tunable semiconductor laser comprising a plurality of control electrodes for controlling the wavelength of laser light oscillated in a first waveguide, wherein the wavelength tunable semiconductor laser is optically coupled to the quantum well active layer. A wavelength tunable semiconductor laser comprising a second waveguide having a high waveguide loss so as to be able to contribute a gain from a higher quantum level. 4. The tunable wavelength according to claim 3, wherein the second waveguide is provided with a non-waveguide region on an end face located on a side opposite to a side coupled to the first waveguide. Semiconductor laser. 5. The wavelength tunable semiconductor laser according to claim 3, wherein an end face of a second waveguide located on a side opposite to a side coupled to the first waveguide is inclined.