Distributed feedback semiconductor laser

[0002] This application claims the benefit of Japanese Patent Application No. 10-30546, filed Jan. 28, 1998, which is hereby incorporated by reference.

[0003] 1. Field of the Invention

[0004] The present invention relates to a semiconductor laser, and more particularly, to a distributed feedback semiconductor laser (hereinafter referred to as a DFB semiconductor laser) having a wavelength varying function.

[0005] 2. Discussion of the Related Art

[0006] The DFB semiconductor laser is known as a device that has various applications in optical communication systems such as in optical cable television (CATV) technology, pumping light sources of an SHG short-wavelength laser for high-density information recording and of a small size solid-state laser, optical measurements, and the like. The conventional DFB semiconductor laser is commonly manufactured by a 2-stage (or more) epitaxial growth. Such DFB lasers are manufactured by forming a diffraction grating on a waveguide layer and then growing other layers epitaxially on the waveguide.

[0007] To avoid the complexity involved with an epitaxial growth of two or more stages, DFB semiconductor lasers have been developed that are manufactured having a 1-stage epitaxial growth. In recent years, the importance of providing such DFB semiconductor lasers with variable oscillation wavelengths has continued to increase.

[0008] Accordingly, DFB semiconductor lasers having variable oscillation wavelengths incorporating a chirped diffraction grating structure have been proposed. The chirped diffraction grating structure is classified into, for example, (a) a structure in which the diffraction grating of a DFB semiconductor laser is given a non-uniform pitch, and (b) a structure in which the diffraction grating of a DFB semiconductor laser is given a constant pitch and a bent waveguide is formed on a waveguide layer. The oscillation wavelength is varied in those manners.

[0009] However, method (a) requires a complex and difficult laser manufacturing process and hence it is not suitable for mass-production because a non-uniform diffraction grating needs to be formed. Method (b) results in a large loss of light because the light traveling direction is not perpendicular to the diffraction grating.

[0010] Accordingly, the present invention is directed to a distributed feedback (“DFB”) semiconductor laser and manufacturing method that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

[0011] An objective of the present invention is to provide a DFB semiconductor laser that is capable of varying its oscillation wavelength, has a simple structure, is easy to manufacture and suitable for mass production, and results in a small loss of light.

[0012] Additional features and advantages of the present invention will be set forth in the description which follows, and will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure and process particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0013] To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a distributed feedback semiconductor laser of the present invention includes a semiconductor substrate having a bottom surface and a top surface; an active layer formed on the top surface of the semiconductor substrate; a ridge stripe formed on the active layer and having a first surface and extending in a first direction; a periodic structure that is periodic in the first direction; a plurality of p-type electrodes formed on the first surface of the ridge stripe; and an n-type electrode formed on the bottom surface of the semiconductor substrate, wherein the first surface of the ridge stripe is parallel with a growth plane of the active layer, the first surface having at least two different widths in a direction perpendicular to the first direction.

[0014] In another aspect, the present invention provides a distributed feedback semiconductor laser that includes a semiconductor substrate; a first cladding layer formed on the semiconductor substrate; a first guide layer formed on the first cladding layer; an active layer formed on the first guide layer; a second guide layer formed on the active layer; a second cladding layer formed on the second guide layer; a contact layer formed on the second cladding layer; a ridge stripe, including the second cladding layer and the contact layer, extending in a light emitting direction; a diffraction grating formed on the second cladding layer; a plurality of p-type electrodes formed on the ridge stripe; and an n-type electrode formed on a bottom surface of the semiconductor substrate, wherein the ridge stripe has at least two different widths in a direction perpendicular to a ridge stripe extending direction.

[0015] In a further aspect of the present invention, a method of manufacturing a distributed feedback semiconductor laser includes the steps of forming a first cladding layer on a semiconductor substrate; forming a first guide layer on the first cladding layer; forming an active layer on the first guide layer; forming a second guide layer on the active layer; forming a second cladding layer on the second guide layer; forming a contact layer on the second cladding layer; providing a ridge stripe, including the second cladding layer and the contact layer, extending in a light emitting direction; forming a diffraction grating on the second cladding layer; forming a plurality of p-type electrodes on the ridge stripe; and forming an n-type electrode on a bottom surface of the semiconductor substrate, wherein the step of providing a ridge stripe includes the step of providing the ridge stripe with at least two different widths in a direction perpendicular to a ridge stripe extending direction.

[0016] It is to be understood that both the foregoing general description and the following detailed description arc exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

[0017] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

[0018] In the drawings:

[0019] FIG. 1 is a sectional view of a distributed feedback (“DFB”) semiconductor laser (before electrode formation) according to an embodiment of the present invention;

[0020] FIG. 2 is a perspective view of the DFB semiconductor laser of FIG. 1;

[0021] FIGS. 3 and 4 illustrate steps of a manufacturing process of a DFB semiconductor laser according to an embodiment of the present invention;

[0022] FIG. 5 illustrates the shape of a ridge stripe of a DFB semiconductor laser according to the present invention;

[0023] FIGS. 6-8 illustrates steps of a manufacturing process of a DFB semiconductor laser according to the present invention;

[0024] FIG. 9 is a graph showing oscillation wavelength vs. relative intensity spectrum of the DFB semiconductor laser of the present invention;

[0025] FIG. 10 is a graph showing a relationship between the ridge stripe width of the DFB semiconductor laser of the present invention and the effective refractive index of the DFB semiconductor laser of the present invention; and

[0026] FIG. 11 is a perspective view of a DFB semiconductor laser (before electrode formation) according to another embodiment of the present invention.

[0027] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

[0028] In the present invention, an InGaAsP/InP-type DFB semiconductor laser is formed on an indium phosphorus (hereinafter referred to as InP) substrate by, for example, an organic metal compound growth method (MOCVD) using an organic metal compound gas as a material, and is characterized by layers that constitute a ridge stripe.

[0029] The device structure, manufacturing method, and various characteristics of a DFB semiconductor laser according to the invention will be hereinafter described with reference to the accompanying drawings.

[0030] (I) Device Structure of DFB Semiconductor Laser

[0031] FIG. 1 is a sectional view of a DFB semiconductor laser according to an embodiment of the present invention. In FIG. 1, reference numerals 1 and 2 denote a substrate of InP and a cladding layer of n-InP, respectively. Reference numerals 3 and 5 denote guide layers of InGaAsP for confinement of light in an InGaAsP active layer 4. Numeral 6 denotes a cladding layer of p-InP that constitutes a ridge stripe. Numeral 7 denotes a contact layer of p-InGaAsP or p-InGaAs that constitutes the ridge stripe. Numeral 8 denotes insulator layers made of SiO₂or the like.

[0032] The above-described layers are grown so as to be laid one on another in the order shown in FIG. 1, whereby a laser substrate having a laser structure is formed.

[0033] Although not shown in FIG. 1, a diffraction grating 9 is formed on the guide layer 5 in a pitch corresponding to an oscillation wavelength. The diffraction grating 9 is formed by electron beam lithography (described later) or the like.

[0034] FIG. 2 is a perspective view of the DFB semiconductor laser of FIG. 1. In this embodiment, respective surfaces, parallel with the growth planes, of the cladding layer 6 and the contact layer 7 that constitute the ridge stripe have an approximately trapezoidal shape whose bases are perpendicular to the ridge stripe extending direction, that is, the laser light emission direction. That is, a ridge stripe width W_fof the end face from which laser light is emitted and a ridge stripe width W_rof the opposite end face are base lengths of the approximately trapezoidal shape. Although not shown in FIG. 2, the insulator layers 8 occupy both sides of the cladding layer 6 and the contact layer 7.

[0035] In the DFB semiconductor laser of FIG. 2, laser light is emitted from the portion of the end face having the ridge stripe width W_fin the vicinity of the active layer 4. The laser light emission end face is made having a non-reflective surface and the end face opposite to it is formed with a high-reflectance film. As shown in FIG. 2, the lines of the diffraction grating 9 are formed at a predetermined constant pitch so as to extend in a direction approximately perpendicular to the laser light emission direction. FIGS. 1 and 2 show a state before electrode formation. A final form of the DFB semiconductor laser is as shown in FIG. 8.

[0036] Manufacturing Method of DFB Semiconductor Laser

[0037] FIGS. 3-4 and 6-8 illustrate steps of a DFB semiconductor laser manufacturing method according to the invention.

[0038] (a) Lamination Step

[0039] First, as shown in FIG. 3, an InP substrate 1 is prepared and its surface is cleaned by chemical etching. Then SCH (separate confinement heterostructure) regions 3-5, and a cladding layer 6 and a contact layer 7 that later constitute a ridge stripe are formed by a prescribed epitaxial growth method (liquid-phase growth, organic metal vapor-phase growth, molecular beam growth, or the like). In the invention, for example, the materials shown in FIGS. 1 and 3 are used.

[0040] (b) Ridge Stripe Forming Step

[0041] Next, as shown in FIG. 4, a prescribed stripe mask of SiO₂, TiO₂, or the like is formed, by photolithography or the like, on the contact layer 7 so that a ridge stripe will extend in the laser light emission direction. The portions of the cladding layer 6 and the contact layer 7 excluding the portions where the stripe mask has been formed are etched out, whereby a ridge stripe 11 having side flat portions 10 and a flat top portion 11A is formed. In the invention, as shown in FIG. 5, the prescribed stripe mask is formed so that the respective surfaces, parallel with the growth planes, of the cladding layer 6 and the contact layer 7 that will constitute a ridge stripe will have an approximately trapezoidal shape whose bases are perpendicular to the laser light emission direction. That is, the stripe mask is formed so that a stripe mask width W_fat the front end face from which laser light is to be emitted and a stripe mask width W_rat the opposite, rear end face will be base lengths of an approximately trapezoidal shape. The stripe mask may be formed so as to assume an exact trapezoid rather than an approximately trapezoidal shape.

[0042] As for the resulting ridge stripe, a relationship W_f<W_rholds.

[0043] The stripe mask is removed after execution of this step.

[0044] (c) Diffraction Grating Forming Step

[0045] In the invention, a diffraction grating 9 is formed by, for example, electron beam lithography. First, a protective film (not shown) of SiO₂or the like is formed so as to cover the side flat portions 10 and the flat top portion 11A. Next, a resist layer (not shown) is formed by coating the protective film on the side flat portions 10 and the flat top portion 11A with an electron beam resist and then performing a heat treatment. Then, as shown in FIG. 6, a diffraction grating pattern having a pitch A in the ridge stripe extending direction (resonator direction) is formed on the resist layer by drawing, with an electron beam, lines in the direction perpendicular to the laser light emission direction. The pitch A corresponds to a desired laser oscillation wavelength.

[0046] The diffraction grating pattern of the resist layer is transferred to the SiO₂protective film by CF₄dry etching. A diffraction grating 9 is then formed in the side flat portions 10 by wet-etching the side flat portions 10 selectively with a hydrochloric acid type etchant. The protective film is thereafter removed.

[0047] Although in FIG. 6 the diffraction grating 9 is also formed in the side faces of the ridge stripe, it may instead be formed only in the side flat portions 10. Furthermore, although in this embodiment both dry etching and wet etching are employed in the diffraction grating forming step, it is possible to instead employ only dry etching. In such a case, the stripe mask that was used in the ridge stripe forming step is left even after the formation of the ridge stripe 11 and removed after completion of the dry etching.

[0048] In general, in the DFB semiconductor laser, a periodic structure that varies at a pitch Λ in the laser light traveling direction, and hence the refractive index also varies periodically and laser oscillation occurs at a wavelength at which the reflectance of light reflected periodically is high (Bragg reflection). Therefore, the oscillation wavelength of the DFB semiconductor laser is determined by the pitch Λ of the periodic structure and the refractive index n. In general, a single longitudinal mode is obtained when a Bragg condition

[0049] Λ=mλ/2n  (1)

[0050] is satisfied, where m is an integer, λ is an oscillation wavelength (in a vacuum), and n is the refractive index of the semiconductor laser medium.

[0051] (d) Electrode Forming Step

[0052] As shown in FIG. 7, an insulator layer 8 of SiO₂or the like is formed so as to entirely cover the ridge stripe 11 and the diffraction grating 9, and is then removed partially until the flat top portion 11A is exposed. A top p-type electrode 12 is formed by depositing an alloy metal such as Au/Zn on the exposed flat top portion 11A of the contact layer 7 and the insulator layers 8.

[0053] Finally, as shown in FIG. 8, the p-type cladding layer 6, the contact layer 7, and the top p-type electrode 12 are divided by etching approximately at the middle position in the laser light emission direction. As a result, an electrode separation groove 13 is formed and the top p-type electrode 12 is divided into p-type electrodes 12A and 12B.

[0054] The top p-type electrode 12 is divided for a wavelength varying control (described later). The number of divisions is not limited to two and is determined properly in accordance with the wavelength varying method. That is, the intended operation is obtained by forming electrodes in portions having different ridge stripe widths.

[0055] Although in this embodiment the p-type cladding layer 6, the contact layer 7, and the top p-type electrode 12 are divided, only the contact layer 7 and the top p-type electrode 12 may be divided. Then, a bottom n-type electrode layer 14 is formed by depositing an alloy metal such as Au/Ge on the bottom surface of the substrate 1. A DFB semiconductor laser is thus completed.

[0056] The principle of wavelength varying of the DFB semiconductor laser manufactured by the above method will be described below in detail.

[0057] FIG. 9 is a graph showing how the oscillation wavelength is shifted in the DFB semiconductor laser of the invention. In FIG. 9, the horizontal axis represents the oscillation wavelength λ of the semiconductor laser and the vertical axis represents the relative intensity.

[0058] Thin lines “a” represent possible oscillation modes within the semiconductor laser gain curve (without the diffraction grating), and a thick line “b” and a broken line “c” represent single-mode oscillation modes that occur when predetermined currents are caused to flow through the top p-type electrodes 12A and 12B (see FIG. 8).

[0059] For example, if the same current that enables laser oscillation is caused to flow through each of the top p-type electrodes 12A and 12B, because of the filtering effect of the diffraction grating 9, the DFB semiconductor laser emits laser light having a prescribed wavelength corresponding to the current value, for example, the wavelength of the thick line b in FIG. 9.

[0060] Under such a situation, the wavelength is shifted to the shorter wavelength side or the longer wavelength side by changing the currents flowing through the top p-type electrodes 12A and 12B. For example, when a shift occurs to the shorter wavelength side, laser light having the wavelength of the broken line c in FIG. 9 is emitted.

[0061] Specifically, the laser oscillation wavelength is shifted to the shorter wavelength side (leftward in FIG. 9) when the following relationship is satisfied:

[0062] (current value of top p-type electrode 12A)<(current value of top p-type electrode 12B)  (2).

[0063] The laser oscillation wavelength is shifted to the longer wavelength side (rightward in FIG. 9) when the following relationship is satisfied:

[0064] (current value of top p-type electrode 12A)<(current value of top p-type electrode 12B)  (3).

[0065] That is, in the DFB semiconductor laser of the present invention, the wavelength varying control can be performed only by varying the currents flowing through the top p-type electrodes 12A and 12B. This wavelength varying control of the DFB semiconductor laser of the invention relates to the width of the ridge stripe.

[0066] FIG. 10 shows a variation in effective refractive index n_edue to a variation in the ridge stripe width W of the DFB semiconductor laser of the invention. In FIG. 10, the horizontal axis represents the ridge stripe width W and the vertical axis represents the effective refractive index n_eof the entire semiconductor laser. As shown in FIG. 10, the effective refractive index n_edecreases as the ridge stripe width W decreases.

[0067] Since the pitch Λ of the diffraction grating of the DFB semiconductor laser is given by the above Equation (1), the oscillation wavelength λ satisfies

[0068] λ=2nΛ/m  (4).

[0069] The refractive index of the DFB semiconductor laser of the invention is determined by geometrical structures such as the shapes of the respective layers of the semiconductor laser, physical structures of the layers of the SCH structure region, the cladding layer, the contact layer, the insulator layer, etc., and other factors. The effective refractive index n_eis defined so as to include the refractive indices relating all of those factors.

[0070] Therefore, the oscillation wavelength of the DFB semiconductor laser of the invention is given by

[0071] λ=2n_eΛ/m  (5).

[0072] It is seen from Equation (5) that the oscillation wavelength of the DFB semiconductor laser varies in proportion to the effective refractive index n_ewhen the pitch Λ of the diffraction grating is constant. That is, the oscillation wavelength becomes shorter as the effective refractive index n_edecreases, and becomes longer as the effective refractive index n_eincreases.

[0073] Since the effective refractive index n_evaries depending on the ridge stripe width W as described above (see FIG. 10), it is understood from Equation (5) that the oscillation wavelength varies in accordance with the ridge stripe width W.

[0074] Since the ridge stripe width W is such that the width W_fof the front end face (laser light emission face) is narrower than the width W_rof the rear end face (see FIG. 5), the wavelength becomes shorter according to Equation (5) if the current values are controlled so as to satisfy Equation (2). Conversely, the wavelength becomes longer if the current values are controlled so as to satisfy Equation (3).

[0075] The fact that the effective refractive index n_evaries with the ridge stripe width W as shown in FIG. 10 is due to the above-described geometrical structure of the semiconductor laser. That is, in a portion where the ridge stripe is narrow, the light intensity distribution leaks more to the insulator layers 8 than in a portion where the ridge stripe is wide, and hence a larger part of the insulator layers 8 having a smaller refractive index than the layers constituting the ridge stripe is involved, to lower the entire effective refractive index.

[0076] As described above, in the DFB semiconductor laser of the invention, the oscillation wavelength is made variable only by forming the ridge stripe so that it assumes an approximately trapezoidal shape whose bases are perpendicular to the laser light emission direction and controlling the currents flowing through the respective electrodes.

[0077] Although in the above embodiment the ridge stripe is formed so as to assume an approximately trapezoidal shape whose bases are perpendicular to the laser light emission direction, the shape of the ridge stripe is not limited to it. The oscillation wavelength can be varied if the ridge stripe has at least two width values.

[0078] Although the above embodiment is directed to the lateral coupling ridge-type DFB semiconductor laser, the invention is not limited to such a case and can be applied to all kinds of DFB semiconductor lasers having a diffraction grating inside.

[0079] Although in the above embodiment no diffraction grating exists under the ridge stripe, the invention can also be applied to a ridge-type DFB semiconductor laser having a diffraction grating under the ridge stripe.

[0080] Although in the above embodiment the diffraction grating has a uniform pitch, the invention can also be applied to a case where the diffraction grating has a non-uniform pitch. In this case, if the non-uniform pitch of the diffraction grating is set so as to become shorter toward the laser light emission side, because of the multiplier effect of the varied ridge stripe width and the non-uniform pitch, a larger wavelength shift is obtained for the same variation in current than in the case of the uniform diffraction grating pitch.

[0081] The above embodiment is directed to a DFB semiconductor laser having a ridge stripe. FIG. 11 shows another embodiment in which the invention is applied to an active layer buried stripe DFB semiconductor laser. That is, an active layer and all the layers above it are shaped like a trapezoid and a diffraction grating is formed on a guide layer.

[0082] In summary, a distributed feedback semiconductor laser has a lamination structure of a semiconductor substrate, an active layer formed on the semiconductor substrate, and layers constituting a ridge stripe and formed on the active layer. A structure that is periodic is formed in a ridge stripe extending direction. A plurality of p-type electrodes are formed on the top surface of the ridge stripe, and an n-type electrode is formed on the bottom surface of the semiconductor substrate. The ridge stripe has a top surface that is parallel with the growth planes of the active layer and the layers constituting the ridge stripe. The top surface of the ridge stripe has at least two different widths in the direction perpendicular to the ridge stripe extending direction.

[0083] As described above, in the DFB semiconductor laser according to the invention, the oscillation wavelength of the semiconductor laser is varied only by forming the ridge stripe so as to assume an approximately trapezoidal shape whose bases are perpendicular to the laser light emission direction and controlling currents flowing through the respective electrodes.

[0084] The configuration of the DFB semiconductor laser of the instant invention results in a laser that is simple in structure, easy to manufacture (i.e., suitable for mass-production), and small in loss of light. The above distributed feedback laser may be configured in such a manner that the width of the first surface decreases toward a laser light emission side of the semiconductor laser. In the above distributed feedback laser, the first surface may have an approximately trapezoidal shape in which a base on the laser light emission side is shorter than a base on a side opposite to the laser light emission side.

[0085] It will be apparent to those skilled in the art that various modifications and variations can be made in the distributed feedback semiconductor laser of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.