JP3479925B2 - Distributed feedback semiconductor laser and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a distributed feedback suitable as a light source in an optical communication system.
The present invention relates to an ed fedback (DFB) type semiconductor laser and a method for manufacturing the same.

In recent years, in a light source used in a sub-carrier multiplex analog optical communication system, a DF having a good linearity of optical output characteristics (hereinafter referred to as IL characteristics) with respect to a laser current.
A B-type laser is required, and D of the present invention is required.
The FB type laser can meet the demand.

[0003]

2. Description of the Related Art Generally, a light source for a sub-carrier multiplex analog optical communication system has a light wavelength of 1.3 μm.
A m-band DFB laser is used.

The reason for this is that the wavelength dispersion of the optical fiber is the smallest, that is, the wavelength band of the light that has the smallest optical waveform deterioration due to the optical fiber transmission is the 1.3 μm band, and the multimode oscillation. This is because the DFB laser of single mode oscillation has lower noise than that of the Fabry-Perot type laser.

FIG. 8 is a fragmentary perspective view showing a standard DFB type laser.

In the figure, 1 is a substrate, 2 is a diffraction grating, and 3
Indicates an optical guide layer, and 4 indicates an active layer. still,
An AR coat (anti-ref) is formed on the emitting end face of the laser beam.
reflective coating is applied, and an HR coat (high-reflective) is provided on the rear end surface.
coating) is applied.

In the DFB type laser, as shown in the figure, the refractive index period is generated by forming the diffraction grating 2 in the optical waveguide, and as a result, single mode oscillation is realized. .

Further, in the DFB type laser, the normalized coupling constant κL in which the linearity of the IL characteristic, which is important as a light source for an analog optical communication system, is determined by the depth of the diffraction grating 2 or the like.
Strongly depends on (if necessary, "DFB Laser
diode and Module for Ana
log Application "H. Yoneta
niet. al. FJITSU SCI. Tech.
J. , 29, 4, 1993 (reference 2)).

FIG. 9 is a diagram showing the relationship between the current and the optical output applied to a standard DFB type laser and the differential efficiency. The horizontal axis represents the current [mA] applied to the DFB type laser and the left vertical direction. The optical output [mW] is plotted on the axis, and the differential efficiency [mW / mA] is plotted on the right vertical axis.

When the standardized coupling coefficient κ · L is small, that is, when the periodical change in the refractive index is small, a sub-linear IL characteristic is obtained. On the contrary, when κ · L is large,
That is, when the periodical refractive index change is large, super-linear IL characteristics are obtained. Here, sub-liner is
As the horizontal axis increases, the slope of the characteristic line becomes more gradual and becomes more convex, and Super-Linear means that the slope of the characteristic line becomes steeper and lower as the horizontal axis increases. It means to be convex.

From the figure, when .kappa..multidot.L is about 0.7, an IL characteristic having good linearity can be obtained until the optical output becomes 0, and such an IL characteristic is required. Then, the electric signal is faithfully converted into an optical signal without distortion.

The above-mentioned phenomenon is caused by axial hole burning,
This phenomenon is caused by the effect called "," which is a phenomenon in which the light intensity distribution in the resonator direction changes as the light output increases, and has a great influence on the linearity of the IL characteristic.

FIG. 10 is a diagram showing the light intensity distribution in a λ / 4 shift DFB type laser. In the figure, DF
The light intensity distribution in the cavity direction is shown by changing the bias current applied to the B-type laser.

In the figure, (A) is a case where κ · L = 1,
(B) shows the case of κ · L = 2, and (C) shows the case of κ · L = 3.

In the case of (C), that is, when .kappa.L = 3, which is large, the end face of the DFB type laser, that is, as the normalized bias current shown in the figure increases, that is, The light intensity distribution becomes relatively large when the intracavity position in the figure is -L / 2 or + L / 2.

In the case of (B), that is, when κ · L = 2,
The light intensity distribution is smaller than that of (C), and in the case of (A), that is, when κ · L = 1, the light intensity distribution is further smaller than that of (B). -It has a linear IL characteristic.

From the above, the normalized coupling constant κ ·
L is optimized to a value at which the light intensity distribution in the cavity direction becomes the flattest (if necessary, see Reference 3). Specifically, the depth of the diffraction grating 2 or the light guide layer 3 shown in FIG.
The composition is to optimize the refractive index distribution of the optical waveguide.

[0018]

As described above, even if the normalized coupling constant κ · L is optimized, the light intensity distribution in the resonator direction is not always flat depending on the end face phase of the diffraction grating. I won't.

11 and 12 are diagrams showing the light intensity distribution in the resonator direction depending on the phase of the end face of the diffraction grating.

These data are asymmetric end face reflectance type D.
It was obtained by analyzing an FB type laser, and the front end face reflectance is 0 [%], the rear end face reflectance is 80 [%], and all are uniform κ = 27 [cm ⁻¹ ]. .

In FIG. 11, (A) is the case where the phase φ of the diffraction grating at the rear end face is 0, (B) is the case where the phase φ is π / 2, and in FIG. 12, (A) ) Is the case where the phase φ of the diffraction grating on the rear end face is π, and (B) is the case where the phase φ is 3π / 2.

In each figure, the horizontal axis represents the position inside the resonator, and the vertical axis represents the relative light intensity.

From the figure, it can be seen that the flatness of the light intensity distribution in the cavity direction changes greatly depending on the phase of the diffraction grating on the rear end face.

When manufacturing the DFB type laser as described above, reactive ion etching (reactive) using a C ₂ H ₆ gas as an etching gas is performed.
ion etching (RIE) method is applied to form a uniform diffraction grating on the entire surface of the substrate (if necessary, "GaI by C ₂ H ₆ -based reactive ion etching" is used).
Formation of Diffraction Grating and Stripe for nAsP / InP Semiconductor Laser "Matsuda, et al. IEICE Transactions Vo
l. J77-CI No. 5 pp. 250-259
1992 (reference 4)), and, for example, metal organic chemical vapor deposition (metalorganic c
chemical vapor deposition
n: MOCVD method is applied to form a uniform optical waveguide layer over the entire surface of the substrate.

As described above, the diffraction grating having a uniform normalized coupling constant κL has a limit in flattening the light intensity distribution in the cavity direction.

According to the present invention, by a simple means, a diffraction grating whose coupling constant κ increases from the rear end face to the front end face is formed, and the light intensity distribution in the cavity direction is flattened to obtain the IL characteristic. Try to improve the linearity.

[0027]

According to the present invention, the coupling coefficient is increased from the rear end face of the DFB laser (for example, the end face provided with the high light reflection film) toward the front end face (for example, the end face provided with the low light reflection film). To form a diffraction grating in which κ gradually increases,
A semiconductor with a periodic structure that is a diffraction grating is surrounded by a semiconductor with a refractive index different from that of the semiconductor, that is, a so-called "island-like" diffraction grating is adopted. The basic principle is to gradually increase the thickness of the island-shaped diffraction grating from the rear end face toward the front end face (light emission end face).

From the above, the DFB according to the present invention
In the type laser and the manufacturing method thereof, (1) the semiconductor layer has an exposed region along the laser stripe, and the exposed width of the semiconductor layer is wide on one end side of the laser end face and on the other end side. And a semiconductor layer having a structure that is formed by a metal organic chemical vapor deposition method using a selective growth mask having a narrow width and has a thin film thickness on the opposite end face side compared to the output end face side of the laser stripe, A diffraction grating provided in the semiconductor layer and forming a resonator , the diffraction grating
Is based on a semiconductor having a refractive index different from that of the diffraction grating.
Surrounded by islands , or

(2) On the first semiconductor layer (for example, n-InP substrate 11) which is the base, along the laser stripe, and the exposed width of the first semiconductor layer is such that one end side of the laser end face is wide and the other end side is To form a selective growth mask (for example, selective growth mask 12A) that exposes the laser stripe region with a narrow width, and a refractive index different from that of the first semiconductor layer on the exposed first semiconductor layer. And a second semiconductor layer (for example, InGaAsP diffraction grating layer 13) capable of forming a diffraction grating and a third semiconductor layer (for example, n-) having the same refractive index as the first semiconductor layer and covering the second semiconductor layer. A step of selectively growing the InP buffer layer 15) by a metal organic chemical vapor deposition method to obtain a second semiconductor layer and a third semiconductor layer which are thin on one end side and thick on the other end side of the laser end face; Diffraction grating on semiconductor layer A step of forming a selective etching mask (for example, a resist film 16) for forming, and the second semiconductor layer by selectively etching the third semiconductor layer and the second semiconductor layer through the selective etching mask Forming a diffraction grating and exposing the surface of the first semiconductor layer, the side surface of the second semiconductor layer and the side surface of the third semiconductor layer (formation of diffraction grating and formation of mesa), and the exposed first semiconductor. A fourth semiconductor layer (for example, an n-InP spacer layer) having the same refractive index as that of the first semiconductor layer is grown on the layer surface, the second semiconductor layer side surface, and the third semiconductor layer side surface to form the second semiconductor layer. A step of enveloping the diffraction grating consisting of the first semiconductor layer, the third semiconductor layer and the fourth semiconductor layer in an island shape, and a fifth semiconductor layer (for example, Activity consisting of quantum wells 19), a step of growing a sixth semiconductor layer (for example, p-InP clad layer 21) forming a clad layer with a refractive index for optically confining the active layer on the fifth semiconductor layer, On the sixth semiconductor layer, a seventh semiconductor layer (for example, p-I) having a narrower band gap than the sixth semiconductor layer and serving as a contact layer is formed.
a step of growing an nGaAsP contact layer 27), or

(3) In the above (2), the first semiconductor layer, the third semiconductor layer and the fourth semiconductor layer are made of InP, and the second semiconductor layer is made of InGaAsP.
A protective layer made of GaAs or InGaAs (for example, the diffraction grating protective layer 14) is interposed between the second semiconductor layer and the third semiconductor layer.

Usually, in the DFB type laser, the light intensity tends to increase from the center of the resonator toward the rear end face, and decreases toward the front end face. Thus, it is possible to form a diffraction grating in which the coupling constant κ gradually increases from the rear end face to the front end face in the resonator direction. Therefore, compared with the case of a diffraction grating with a uniform coupling constant κ, the diffraction grating The light intensity distribution in the cavity direction can be flattened regardless of the end face phase.

13 and 14 are diagrams showing the light intensity distribution in the cavity direction for explaining the superiority of the DFB type laser of the present invention.

In each of the figures, the solid line is the non-uniform inclination κ = 13.
The light intensity distribution in the cavity direction in the case of [cm ^-1 ] to 40 [cm ^-1 ] is represented, and the broken line represents the light intensity distribution in the cavity direction in the case of uniform κ = 27 [cm ^-1 ]. ing.

In FIG. 13, (A) is the case where the phase φ of the diffraction grating at the rear end face is 0, (B) is the case where the phase φ is π / 2, and in FIG. 14, (A) ) Is the case where the phase φ of the diffraction grating on the rear end face is π, and (B) is the case where the phase φ is 3π / 2.

In each of the figures, the horizontal axis represents the position inside the resonator, and the vertical axis represents the relative light intensity.

Non-uniform inclination κ = 13 [cm ⁻¹ ] to 40 [c
In the case of m ⁻¹ ], compared with the case of uniform κ = 27 [cm ⁻¹ ], regardless of the phase of the diffraction grating on the rear end face,
It can be seen that the light intensity distribution in the cavity direction is flat.

FIG. 15 is a diagram showing the relationship between the normalized coupling coefficient κ · L and the flatness of the light intensity distribution in the cavity direction in an asymmetric end face reflectance type DFB type laser.

In the figure, the horizontal axis represents the normalized coupling coefficient κ.
L is taken, but ∫κ in case of non-uniform inclination κ
(Z) dZ is shown, and the vertical axis shows the flatness F of the light intensity distribution in the cavity direction, where the flatness F is

[0039]

[Equation 1]

∘ indicates uniform κ, Δ indicates non-uniform inclination κ, and κ is 0.5 × κ.
It shows a state where the change is up to 1.5 × κ.

As is clear from the figure, in the case of uniform κ, that is, in the case of ∘, there is large variation in any normalized coupling coefficient κ · L of 0.6, 0.8 and 1.0. Be taken care of. That is, in the case of uniform κ, the manufacturing variation is large even if κ · L is adjusted by changing the thickness of the diffraction grating or the like.

On the other hand, in the case of non-uniformity κ, that is, in the case of Δ, κL is particularly small, κL = 0.6 or 0.8.
In such cases, the distribution is concentrated, there is little variation, and it is understood that manufacturability is excellent.

In both the DFB type laser formed with uniform κ and the DFB type laser formed with non-uniform κ, a non-reflection film having a reflectance of 0% is formed on the light output end face side. Therefore, the end face phase is 0 degree, and the end face on the side opposite to the light output end face side is formed with a highly reflective film having a reflectance of 80 [%], and the end face phase is 2π × ( n / 8)
(However, n is an integer of 1 to 8) degrees.

[0044]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a plan view showing the principal part of a DFB type laser in the process steps for explaining an embodiment according to the present invention, and FIGS. FIG. 3 is a side sectional view showing an essential part of a DFB type laser in a process key point for explaining an embodiment according to the invention, which will be described below with reference to these figures. It should be noted that FIGS. 2 and 3 show a plane cut along a line XX seen in FIG.

See FIGS. 1 and 2A. 1- (1) Chemical vapor deposition
position (CVD) method.
A SiO ₂ film having a thickness of, for example, about 1000 [Å] is formed on the InP substrate 11.

1- (2) By applying a resist process in the lithographic technique and a wet etching method using a hydrofluoric acid-based etchant as an etchant, the S
Masking 12 for selective growth by patterning the iO ₂ film
Form A and 12B.

The lengths of the selective growth masks 12A and 12B in the Y direction are, for example, 400 [μm], and the distance between the adjacent selective growth masks, for example, the selective growth masks 12A in the Y direction is, for example. It is 200 [μm]. Also,
The width of the selective growth masks 12A and 12B in the X direction is, for example, 250 [μm], and the width of the stripe 11A is 50.
[Μm].

As is apparent from FIG. 1, there is a stripe 11A between the selective growth masks 12A and 12B for forming an "island" diffraction grating.
2A and 12B approximately in the center, that is, chip separation line X2-X
The ridge of the "island" should be the highest in the vicinity of 2, and the ridge of the "island" should be the lowest in the vicinity of the chip separation line X1-X1 or X3-X3.

1- (3) InGaAs whose energy band gap wavelength λg is 1.20 [μm] is obtained by applying the MOCVD method.
The P diffraction grating layer 13 is grown. The thickness of the diffraction grating layer 13 is 5 [nm] in the resonator direction (a portion outside the selective growth masks 12A and 12B, that is, the chip separation line X1-X1 or the chip separation line X3-X3. 25 nm to 25 nm (selective growth masks 12A and 1)
2B approximately in the center, that is, near the chip separation line X2-X2)
Gradually changes between.

1- (4) Subsequently, the n-InGaAs diffraction grating protective layer 14 having 5 to 6 atomic layers and the n-type having a thickness of 5 [nm] to 25 [nm].
The InP buffer layer 15 is grown. The buffer layer 1
The thickness of 5 also changes like the diffraction grating layer 13.

1- (5) The selective growth masks 12A and 12B are removed. Here, by forming the buffer layer 15, the state of the “island” diffraction grating can be favorably preserved during the growth of the spacer layer 17 in a later step. That is, the surface of the diffraction grating layer 13 is covered with the buffer layer 15, so that the reflectance of the diffraction grating is determined at that stage.

For example, in the step of forming a diffraction grating in the diffraction grating layer 13, that is, in the etching step, when the surface of the diffraction grating layer 13 is in direct contact with the selective etching mask, the surface cleanliness of the diffraction grating is Is deteriorated, it is difficult to obtain the expected reflectance.

The spacer layer 17 is made of InP, but its growth temperature is usually about 450 [° C.] when the MOCVD method is applied. The spacer layer 17 is formed after the diffraction grating is formed on the diffraction grating layer 13 and grows so as to be embedded therein, and at that time, the shape of the diffraction grating is thermally deformed.

This is the InG forming the diffraction grating layer 13.
This is due to the high vapor pressure of phosphorus (P) contained in aAsP.

Therefore, in the present embodiment, it is possible to grow the InP at a growth temperature of 450 [° C.] or lower, and it is difficult to thermally deform even at 450 [° C.] or higher, that is, compared with the vapor pressure of P. As a thin film semiconductor layer containing an element having a low vapor pressure, for example, GaAs, InGaAs or the like is grown on the diffraction grating layer 13 as the diffraction grating protection layer 14.

Accordingly, even if the InP spacer layer 17 is grown by the MOCVD method after the diffraction grating is formed by etching, the surface of the diffraction grating is covered with a material which is not easily thermally deformed. Therefore, the shape can be well preserved.

Since the diffraction grating protective layer 14 is extremely thin, the effective reflectance of the diffraction grating is
And the buffer layer 15 (if necessary, "MOV
PE grow of wire-shaped
InGaAs on corrugated InP "
T. Fujii et. al. , See).

Refer to FIG. 2B. 2- (1) A resist pattern in a lithography technique including an interference exposure method.
By applying the process, a resist film 16 for forming a diffraction grating is formed.

See FIG. 3A. 3- (1) By applying the RIE method using C ₂ H ₆ gas as an etching gas, the resist film 16 is used as a mask and the depth is at least 50 [nm]. Above, that is, from the surface to the substrate 1
Etching to reach 1 is performed to form a diffraction grating.

Referring to FIG. 3B, 3- (2) The resist film 16 is removed, and then the MOCVD method is applied to form the spacer layer 17 and the separate confinement hetero structure.
A structuring layer (SCH) layer 18, an active layer 19, an SCH layer 20, and a cladding layer 21 are sequentially grown.

The main data regarding each of the semiconductor layers grown here is exemplified as follows. About the spacer layer 17 Material: n-InP Impurity concentration: 5 × 10 ¹⁷ [cm ⁻³ ] to 1 × 10 ¹⁸ [cm ⁻³ ] Thickness: 330 [nm]

About the SCH layer 18 Material: i-InGaAsP λg: 1.05 [μm] Thickness: 30 [nm]

Regarding the active layer 19 ○ Barrier layer Material: i-InGaAsP λg: 1.10 [μm] Thickness: 10 [nm] Number of layers: 8

○ Well layer Material: i-InGaAsP λg: 1.36 [μm] Thickness: 6.0 [nm] Number of layers: 7

About the SCH layer 20 Material: i-InGaAsP λg: 1.05 [μm] Thickness: 10 [nm]

Clad layer 21 Material: p-InP Impurity concentration: 5 × 10 ¹⁷ [cm ⁻³ ] to 3 × 10 ¹⁸ [cm ⁻³ ] Thickness: 500 [nm]

By passing through the steps described above, a semiconductor laminated body structure which can be used for producing the DFB type laser according to the present invention is obtained. Therefore, the semiconductor laminated body structure is processed into the FBH embedded structure and the DFB type laser is manufactured. To complete.

FIGS. 4 to 6 are front sectional views showing a main part of the DFB type laser in the process steps for explaining the embodiment according to the present invention. Hereinafter, with reference to these drawings, FIG. explain. 4 to 6, a cross section in a direction orthogonal to the cross section seen in FIGS. 2 and 3 is taken, and the same symbols as those used in FIGS. 1 to 3 are the same. They represent parts or have the same meaning.

See FIG. 4 4- (1) By applying the CVD method and the lithography technique,
SiO for performing mesa etching on the clad layer 21
A mask film 22 having stripes of ₂ is formed.

4- (2) By applying a wet etching method using a bromine-based etching solution as an etchant, the portion of the semiconductor laminate exposed laterally of the mask film 22 is exposed from the surface to the substrate 1
1 is etched to form a mesa.

See FIG. 5 5- (1) By applying MOCVD while leaving the mask film 22, selective regrowth is performed to embed the mesa.
The burying layer 23, the block layer 24, and the surface layer 25 are formed.

It is needless to say that the mesa is embedded so that the structure has a current blocking effect so that the driving current flows only in the mesa. Here, the current is blocked by using the pn reverse junction. ing.

The main data regarding each semiconductor layer for embedding the mesa is illustrated as follows.

About the buried layer 23 Material: p-InP Impurity concentration: 1 × 10 ¹⁸ [cm ⁻³ ] Thickness: 0.5 [μm]

Regarding the block layer 24: Material: n-InP Impurity concentration: 5 × 10 ¹⁸ [cm ⁻³ ] Thickness: 0.5 [μm]

Regarding the surface layer 25: Material: p-InP Impurity concentration: 1 × 10 ¹⁸ [cm ⁻³ ] Thickness: 0.5 [μm]

Although the semiconductor layers are formed to have the same thickness, the buried layer 23 is drawn thicker in the figure because the growth of the buried layer 23 depends on the substrate 11.
From both the surface and the side of the mesa, due to the doubling of the thickness in its vicinity.

As described above, when the mesa is buried, a pn reverse junction is formed between the block layer 24 and the buried layer 23, so that the current injection is blocked and the current injected from the surface layer 25 side. Will flow into the mesa.

See FIG. 6 6- (1) After removing the mask film 22, the planarization layer 26 and the contact layer 27 are sequentially grown by applying the MOCVD method.

The main data regarding each semiconductor layer grown here is illustrated as follows. The scale of each semiconductor layer is changed in comparison with other portions in the drawing. Regarding the flattening layer 26 Material: p-InP Impurity concentration: 1 × 10 ¹⁸ [cm ⁻³ ] Thickness: 1 [μm]

Contact layer 27 Material: p-InGaAsP Impurity concentration: 3 × 10 ¹⁸ [cm ⁻³ ] Thickness: 0.3 μm

6- (2) Thereafter, by applying a normal technique, a passivation film is formed on the surface, and a p-side electrode and an n-side electrode are formed to fabricate a DFB type laser.

6- (3) Wafer cutting lines Y1-Y1 and Y2-Y2 shown in FIG. 1 are used for the wafer in which the DFB type laser is formed as described above.
Since a rod-shaped body in which a large number of DFB lasers are connected in series is obtained by cutting along ... Chip separation lines X1-X1, X seen in FIG.
A chip is formed by cleaving along 2-X2, X3-X3, ..., And a high light reflection film is formed on the cleavage surface of the thinnest side.
In addition, a low light reflecting film is applied to the cleavage surface on the thickest side to complete the process.

FIG. 7 shows a DFB completed by carrying out the present invention.
It is a principal part cut-away perspective view showing a type laser.

In the figure, 41 is an n-InP substrate, and 42 is
Is n-InGaAsP diffraction grating layer, 43 is n-InGa
As diffraction grating protective layer, 44 is n-InP spacer layer, 4
4A is an n-InP buffer layer, 45 is i-InGaAs
P-SCH layer, 46 is an active layer, 46B is a barrier layer made of i-InGaAsP forming the active layer 46, 46W is a well layer made of i-InGaAsP forming the active layer 46, and 47 is an i-InGaAsP-SCH layer. , 48 is p
-InP clad layer, 49 is p-InP buried layer, 5
0 is an n-InP block layer, 51A is a p-InP surface layer, 51B is a p-InP flattening layer, and 52 is p-InGa.
AsP contact layer, 53 is a passivation film made of SiO ₂ , 54 is a p-side electrode laminated with Ti / Pt / Au, 55 is an n-side electrode made of AuGe, and 56 is a separation groove.

Although not shown, a low light reflecting film is formed on the cleavage surface on the side where the diffraction grating layer 42 is thick, and a high light reflecting film is formed on the cleavage surface on the opposite side.

From FIG. 7, it can be clearly seen that the thickness of the diffraction grating layer 42 gradually increases from the rear end face to the front end face. Therefore, according to this configuration, the gradient nonuniform coupling coefficient κ · Since L is generated and the light reflection due to the diffraction grating layer 42 increases from the rear end face to the front end face, the light intensity distribution that normally decreases from the rear end face to the front end face is corrected, and the inside of the resonator is corrected. The light intensity distribution becomes substantially constant.

The present invention is not limited to the embodiment described above, and many other modifications can be realized.

For example, the SCH layer 1 found in the above embodiment.
8 and 20 are not indispensable constituent elements and may be formed as needed.

Further, although the active layer 19 is composed of multiple quantum wells, it is needless to say that it may be a single quantum well, and is further composed of a thick film having a thickness of, for example, about 1 [μm]. It may be a so-called bulk active layer.

InG is used as the material of the diffraction grating layer 13.
Although aAsP is used, it goes without saying that this may be appropriately changed according to the required oscillation wavelength.

The substrate 11, the buffer layer 15, and the spacer layer 17 function as the diffraction grating if the refractive index is different from that of the diffraction grating. Therefore, the material is not particularly limited to InP, and the substrate 11 is not limited to InP. It can be appropriately changed depending on what kind of material is used, and as for the semiconductor material forming the other portions, a desired material may be selected as necessary.

[0093]

In the DFB type laser and the method of manufacturing the same according to the present invention, the diffraction grating serving as the laser resonator is thicker along the laser stripe and from the output end face side to the opposite end face side of the laser stripe. Is formed so as to decrease.

DF obtained by adopting the above configuration
Since the B-type laser has a diffraction grating in which the coupling constant κ gradually increases from the rear end face to the front end face in the cavity direction, a conventional DFB type laser having a diffraction grating with a uniform coupling coefficient κ is formed. Compared with the laser, the light intensity distribution in the cavity direction is flat, regardless of the end face phase of the diffraction grating, and as a result, the linearity of the IL characteristic becomes good.

Further, in the step of growing the semiconductor layer forming the diffraction grating, a mask for selective growth (for example, 12 A) is used.
And 12B), the thickness of the diffraction grating can be easily changed in the stripe direction.

[Brief description of drawings]

FIG. 1 is an explanatory plan view of relevant parts of a DFB type laser in a process essential part for explaining an embodiment according to the present invention.

FIG. 2 is a side sectional view showing a main part of a DFB type laser in a process main part for explaining an embodiment according to the present invention.

FIG. 3 is a side sectional view showing an essential part of a DFB type laser in a process essential part for explaining an embodiment according to the present invention.

FIG. 4 is a fragmentary front view showing a DFB type laser in a process essential part for explaining an embodiment according to the present invention.

FIG. 5 is a fragmentary front view showing a DFB type laser in a process main part for explaining an embodiment according to the present invention.

FIG. 6 is a fragmentary front view showing a DFB type laser in a process main part for explaining an embodiment according to the present invention.

FIG. 7 is a fragmentary perspective view showing a DFB type laser completed by carrying out the present invention.

FIG. 8 is a fragmentary perspective view showing a standard DFB type laser.

FIG. 9 is a diagram showing a relationship between a current flowing through a standard DFB type laser and an optical output.

FIG. 10 is a diagram showing a light intensity distribution in a λ / 4 shift DFB type laser.

FIG. 11 is a diagram showing a light intensity distribution in a cavity direction depending on an end face phase of a diffraction grating.

FIG. 12 is a diagram showing a light intensity distribution in a resonator direction depending on an end face phase of a diffraction grating.

FIG. 13 is a diagram showing the light intensity distribution in the cavity direction for explaining the superiority of the DFB type laser of the present invention.

FIG. 14 is a diagram showing the light intensity distribution in the cavity direction for explaining the superiority of the DFB laser of the present invention.

FIG. 15 is a diagram showing the relationship between the normalized coupling coefficient κ · L and the flatness of the light intensity distribution in the cavity direction in an asymmetric end face reflectance type DFB laser.

[Explanation of symbols]

41 n-InP substrate 42 n-InGaAsP diffraction grating layer 43 n-InGaAs diffraction grating protective layer 44 n-InP spacer layer 44A n-InP buffer layer 45 i-InGaAsP / SCH layer 46 active layer 46B i constituting the active layer 46 -Barrier layer 46W made of InGaAsP Well layer 47 made of i-InGaAsP constituting the active layer 46 i-InGaAsP / SCH layer 48 p-InP clad layer 49 p-InP buried layer 50 n-InP block layer 51A p-InP surface Layer 51B p-InP flattening layer 52 p-InGaAsP contact layer 53 passivation film 54 made of SiO ₂ p-side electrode 55 laminated with Ti / Pt / Au n-side electrode 56 made of AuGe separation groove

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-280394 (JP, A) JP-A-5-102597 (JP, A) JP-A-6-310806 (JP, A) JP-A-6- 196798 (JP, A) Japanese Patent Laid-Open No. 3-198392 (JP, A) Special Table 4-505687 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 5/00-5 / 50

Claims (3)

(57) [Claims] 1. A selective growth mask having a semiconductor layer exposed region along a laser stripe and having an exposed width of the semiconductor layer wide on one end side of the laser end face and narrow on the other end side. A semiconductor layer formed by the used metal organic chemical vapor deposition method and having a structure in which the film thickness is smaller on the end face side opposite to the output end face side of the laser stripe, and a resonator is provided in the semiconductor layer. and a diffraction grating, the diffraction grating comprises a semiconductor having a refractive index different from that of the diffraction grating
A distributed feedback semiconductor laser, characterized in that it is surrounded by an island and forms an island shape . 2. A laser stripe region is formed on a first semiconductor layer which is a base along a laser stripe, and the exposed width of the first semiconductor layer is such that one end side of the laser end face is wide and the other end side is narrow. Forming an exposed selective growth mask, and a second semiconductor layer and a first semiconductor on the exposed first semiconductor layer that can form a diffraction grating with a refractive index different from that of the first semiconductor layer A third semiconductor layer having the same refractive index as the layer and covering the second semiconductor layer is selectively grown by metal organic chemical vapor deposition to be thin on one end side and thick on the other end side of the laser end face. Layer and the third semiconductor layer, a step of forming a selective etching mask for forming a diffraction grating on the third semiconductor layer, and the third semiconductor layer and the second semiconductor layer through the selective etching mask. Before selectively etching the semiconductor layer of Shaping the second semiconductor layer into a diffraction grating and exposing the first semiconductor layer surface, the second semiconductor layer side surface and the third semiconductor layer side surface, and the exposed first semiconductor layer surface and second A fourth semiconductor layer having the same refractive index as that of the first semiconductor layer is grown on the side surface of the semiconductor layer and the side surface of the third semiconductor layer, and a diffraction grating formed of the second semiconductor layer is formed on the first semiconductor layer and the third semiconductor layer. Island-shaped surrounding with the semiconductor layer and the fourth semiconductor layer, growing a fifth semiconductor layer to be an active layer on the fourth semiconductor layer, and forming an active layer on the fifth semiconductor layer. A step of growing a sixth semiconductor layer forming a clad layer with an optical confinement refractive index; and a seventh semiconductor layer on the sixth semiconductor layer, which has a narrower band gap than the sixth semiconductor layer and becomes a contact layer. And a step of growing a semiconductor layer. Method for producing a distributed feedback semiconductor laser. 3. The first semiconductor layer, the third semiconductor layer, and the fourth semiconductor layer are made of InP, and the second semiconductor layer is I.
3. The method of manufacturing a distributed feedback semiconductor laser according to claim 2, wherein nGaAsP is used as a material, and a protective layer made of GaAs or InGaAs is interposed between the second semiconductor layer and the third semiconductor layer.