JPH0266985A - Semiconductor laser and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain a compact, stable semiconductor laser excellent in single mode property and narrow in light emitting spectrum-ray width by forming an active region and an outer resonator on the same substrate. CONSTITUTION:A cap layer 12 is etched, and an active region is formed. A clad layer 11 is further etched, and ridges 13 and 14 are formed. A first waveguide and a second waveguide are formed in the region. The second waveguide is formed so as to pass a region where a diffraction grating 10 is provided. The region where the diffraction grating is provided becomes a Bragg reflector 15. This element is provided with the following parts: an active region which is formed with a semiconductor, i.e., a region where the cap 12 is provided; and an outer resonator which reflects the output light from the active region and feeds back the light into the active region. The outer resonator includes the first waveguide in which the output light from the active region is inputted and the second waveguide in which the light beams are combined between the first waveguide and the second waveguide. The Bragg reflector 15 which reflects the light having specified wavelength that is included in the light from the first waveguide is included in the second waveguide.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application]
This invention relates to a semiconductor laser using a reflector as an external resonator.

INDUSTRIAL APPLICABILITY The present invention is suitable for use as a light source for optical communication and optical measurement.

〔overview〕

The present invention provides a semiconductor laser that uses an ROR as an external resonator, and by monolithically integrating the active region and the ROR, a compact and stable semiconductor laser with excellent single mode properties and a narrow emission spectrum linewidth is achieved. The purpose is to provide

[Conventional technology]

One way to cause a semiconductor laser to oscillate in a single mode is to give the reflectance of a resonator a frequency characteristic. In such a laser, by sharpening the frequency characteristics of the reflector, that is, by increasing the Q value, the single mode property can be improved and the spectral linewidth can be narrowed.

A Bragg reflection laser is known as a device in which a reflector with frequency characteristics is monolithically integrated with a semiconductor laser. Also, as a reflector with a high Q value, ROR
has been proposed, and semiconductor lasers using this as an external cavity have also been reported.

Regarding ROR, see Rudolf F. Kazarinoff, Charles H. Henry and F. Anders Olsson, “Narrowband Resonant Optical
"Reflectors and Resonant Optical Transformers for Laser Stabilization and Wavelength Division Multibranching", 1EEE Journal of Claontum Electronics Vol. 0123 No. 9, 19
September 1987 (Rudolf F., Kazarinov,
Charles H, Henry, and N, An
ders 01sson, 'Narrow-Ban
dRe5onant Opt+-cal Refle
ctors and Re5onant Transf
ormers for Laser 5tabiliza
tion and Wavelength Divis
ionMultiplexing”, ■εEB J
Our own of Quantum Blec
-tronics, Vol, QB-23, No. 9.
September 1987).

Regarding semiconductor lasers using ROR as an external cavity, F.I. Olson, C.H. Henry, R.F. Kazarinoff, H.J. Lee, B.H. Johnson, and K.J. Orlowski. , "Narrow Line Width 1.5μ Semiconductor Laser with a Resonant Optical Reflector", Applied Physics Letters, Vol. 51, No. 15, October 12, 1987 (N,
A, 01sson, C,)I.

Henry, R.F., Kazarinov, H.J.
, Lee, B.H., Johnsonand K.J.
Orlowsky, “Narrow linewid”
th 1.5 Btnsemiconductor 1
aser with a resonant opti
calreflector”, Appl, Phys,
Lett, 5H15), 12 Oct.

1987).

[Problem that the invention seeks to solve]

Generally, by monolithically integrating a reflector with a semiconductor laser, the coupling loss between the semiconductor laser and the reflector can be reduced, and single mode property can be improved.
The emission spectrum linewidth can be narrowed. Further, fluctuations in the distance between the semiconductor laser and the external resonator due to mechanical vibration can be eliminated, and stability can be improved. However, a semiconductor laser having a monolithically integrated ROR structure has not yet been reported.

The reason for this is that there are problems of coupling efficiency between the active region of the semiconductor laser and the waveguide of the ROR, and problems of waveguide loss of the ROR, and it is difficult to satisfy both of these problems.

An object of the present invention is to overcome this problem and provide a semiconductor laser in which an ROR is monolithically integrated.

[Means for solving problems]

The semiconductor laser of the present invention is characterized in that the active region and the ROR are formed on the same substrate.

The ROR includes a first waveguide into which light enters, a second waveguide into which light is coupled mutually between the first waveguide, and a second waveguide in which light is coupled from the first waveguide to the second waveguide. It is equipped with a Bragg reflector that reflects light of a specific wavelength included in the light.

To manufacture such a semiconductor laser, a layered structure including a quantum well structure is formed, and the quantum well structure in a portion other than the active region is disordered to serve as a waveguide layer. That is, a quantum well structure is formed on a semiconductor substrate, an active region is formed in a part of this quantum well structure, and the quantum well structure in a part other than the active region is disordered, and the disordered region is formed by this process. a first waveguide into which the output light from the active region enters; a second waveguide into which light is coupled mutually between this first waveguide; and a second waveguide in which light is coupled to the second waveguide from the first waveguide. A Bragg reflector is formed to reflect light of a specific wavelength included in the reflected light.

Alternatively, the waveguide layer may be selectively grown. That is, a layered structure including an active layer is formed on a semiconductor substrate, this active layer is etched to form an active region, and a first conductor is formed in which output light from the active region is incident on the etched region. A second waveguide in which light is mutually coupled between the waveguide and the first waveguide, and a Bragg reflection that reflects light of a specific wavelength included in the light coupled from the first waveguide to the second waveguide. It may also be formed into a container.

To form a Bragg reflector, before forming the first waveguide and the second waveguide, a diffraction grating is formed in a part of the area where the second waveguide is to be formed.

When a second waveguide is formed after this, the region where the diffraction grating is formed becomes a Bragg reflector.

In the ROR, it is desirable to provide a groove between the Bragg reflector and the first waveguide to prevent coupling of light.

The first waveguide and the second waveguide of the ROR can also be integrated in different layers.

A current or voltage can also be supplied to the Bragg reflector of the ROR to control its equipolar refractive index.

This controls the wavelength reflected by the Bragg reflector,
The oscillation wavelength of a semiconductor laser can be controlled.

[For production]

By monolithically integrating the active region and the ROR, the semiconductor laser can be made smaller and the optical coupling efficiency between the active region and the external resonator can be increased. Furthermore, the influence of mechanical vibrations and the like can be removed. Therefore, a compact and stable semiconductor laser with good single mode properties and narrow emission spectrum linewidth can be obtained.

〔Example〕

FIG. 1 shows perspective views of each manufacturing process of a semiconductor laser according to a first embodiment of the present invention. In this drawing, necessary parts are exaggerated in order to clarify each part of the element and the layer structure. .

FIG. 1(a) shows the process of forming a quantum well structure on a semiconductor substrate. That is, a buffer layer 2, a cladding layer 3, a graded index layer 4, and a quantum well layer 5 are formed on a substrate 1.
, a diagonal index layer 6 and a waveguide layer 7 are grown in this order.

FIG. 1(b) shows a step of forming an active region in a part of this quantum well structure and disordering the quantum well structure in a portion other than this active region. That is, a mask 8 is provided in a region on the waveguide layer 7 where an active region is to be formed, and ion implantation is performed. Thereafter, a heat treatment is performed to disorder the quantum well layer 5 in a portion other than the active region.

FIGS. 1(C) to (e) show a first waveguide through which output light from the active region enters the disordered region, and a second waveguide through which light is mutually coupled between this first waveguide. The process of forming two waveguides and a Bragg reflector 15 that reflects light of a specific wavelength included in light coupled from the first waveguide to the second waveguide is shown.

In the step shown in FIG. 1(C), two diffraction gratings 10 are carved into the waveguide layer 7. Region 9 indicates a non-disordered region.

In the step shown in FIG. 1(d), a cladding layer 11 and a cap layer 12 are grown on the waveguide layer 7.

In the step shown in FIG. 1(e), the cap layer 12 is etched leaving a portion above the region 9 to form an active region.
Furthermore, the cladding layer 11 is etched to form a ridge. This ridge 13, 14 effectively changes the refractive index and forms a first waveguide and a second waveguide in that region. The second waveguide is formed to pass through a region where the diffraction grating 10 is provided, and the region where the diffraction grating is provided becomes the Bragg reflector 15.

FIG. 1(f) shows a state in which electrodes are provided on the element of FIG. 1(e).

An electrode 16 is provided on the cap layer 12 of the active region, and an electrode 17 is provided on the Bragg reflector 15. Further, electrodes are provided on the back side of the substrate 1 corresponding to these electrodes 16 and 17.

By supplying current to the quantum well layer 5 from the electrode 16, the quantum well layer 5 emits light. Further, by supplying current or voltage from the electrode 17 to the Bragg reflector 15, the equipolarized refractive index of that region can be controlled, and the reflected wavelength can be controlled. Thereby, the emission wavelength in the quantum well layer 5 can be controlled.

FIG. 2 shows a plan view of the semiconductor laser shown in FIG. 1(e).

This device includes an active region formed of a semiconductor, that is, a region provided with a cap layer 12, and an external resonator that reflects output light from the active region and returns it to the active region. This external resonator is an ROR, and has a first waveguide (the part where the ridge 13 is provided) into which the output light from the active region enters, and a second waveguide in which the light is mutually coupled between this first waveguide. This second waveguide includes a Bragg reflector 15 that reflects light of a specific wavelength included in the light coupled from the first waveguide. The active region and R and OR are monolithically integrated. Further, the active region includes a quantum well structure, and the first waveguide and the second waveguide include a layer in which an extended portion of the quantum well structure is disordered as a waveguide layer.

Here, the operation of ROR will be explained. The length of the Bragg reflector 15 is LD, and the coupling coefficient is LD. , the electric field reflectivity of the Bragg reflectors 15 is RD, the distance between the Bragg reflectors 15 is defined as RD, and the propagation constant between the Bragg reflectors 15 is defined as β. Further, it is assumed that the coupling between the first waveguide and the second waveguide occurs only in the central length portion, and the coupling coefficient is set as .

Further, in the coupling portion, the propagation constant of the first waveguide and the propagation constant of the second waveguide are assumed to be equal.

At this time, the electric field reflectance r of ROR is R, sin' (to cL) exp (-2jβL)・-・
・(1) becomes. However, (2) μ=[nio'+(jΔβ+α/2)2]l/2.

Here, α is the waveguide loss, and Δβ is the deviation from the Bragg wavelength.

Here, the oscillation wavelength is set to 0.85 μm, and the mixed crystal ratio of each layer (
It is assumed that the ratio of aluminum Af to gallium Ga) and the film thickness are the values shown in Table 1.

Table 1 It is also assumed that the diffraction grating of the Bragg reflector 15 is first-order with respect to the wavelength, and its shape is triangular. It is also assumed that the height of the diffraction grating is t = 5Q nm. At this time, the coupling coefficient of the Bragg reflector 15. =96cnr'.

Further, the difference in equipolarized refractive index between the portion where the diffraction grating is formed and the portion where it is not formed is Δn□=0.002.

Assuming that the interval L between the Bragg reflectors 15 is 106 μm, Δ
n□·L=λ/4 -- (4) holds true, and the ROR resonates at the Bragg wavelength of the Bragg reflector 15.

FIG. 3 shows the distance W between the first waveguide and the second waveguide and the coupling coefficient between the waveguides. An example of the relationship is shown below. Here, the equipolarized refractive index difference in the lateral direction caused by the ridges 13 and 14 was set to 0.01. As shown in this figure, the interval W is set to 1.
If it is 5μm, it will be the coupling coefficient. becomes 33c++r'.

FIG. 4 shows the IRI of the Bragg reflector 15 with respect to oLo (coupling coefficient x length) of the Bragg reflector 15.

An example of the value of is shown below. Here, Δβ=0 and α=0.

In order to use the ROR as an external resonator, it is necessary that the absolute value IRII+ of the electric field reflectance be close to "1". Referring to FIG. 4, when LD=3, l Rn
l=0.995. To obtain this value, =95 cm-', so L=311 μm.

The above conditions are summarized in Table 2.

Table 2, Figure 5 shows an example of the values of IRDI'' and 1r12 for the deviation Δβ of the propagation constant from the Bragg wavelength. These values are the values under the conditions shown in Table 2.
The waveguide loss α was set to 0.

As shown in FIG. 5, the full width at half maximum of "lrl" is 0.
It is approximately O2 nm, and a very sharp frequency characteristic can be obtained.

Figure 6 shows the fifth waveguide loss when α = lc + rr'.
This is a diagram equivalent to the figure. In this case, the value of Ir 2 is small. Therefore, the waveguide loss is l c
It is necessary to keep it to about m-'.

FIG. 7 shows perspective views of each manufacturing process of a semiconductor laser according to a second embodiment of the present invention. Similar to FIG. 1, this figure also exaggerates necessary parts in order to clarify each part of the element and the layer structure.

FIG. 7(a) shows a step of forming a layered structure including the quantum well layer 5 on a semiconductor substrate. That is, on the substrate 1, a buffer layer 2, a cladding layer 3, a graded index layer 4,
quantum well layer 5, dilated index layer 6, waveguide layer 7,
A cladding layer 11 and a cap layer 12 are grown in order.

FIG. 7Q)) shows the step of etching the quantum well layer 5 to form an active region. In this step, a mask is provided in the region where the active region is to be formed, and the cap layer 12, Crab layer 11, waveguide layer 7,
The graded index layer 6, active layer 5 and graded index layer 4 are removed by etching.

FIGS. 7(C) to 7(e) show a first waveguide through which output light from the active region enters the etched region, and a second waveguide through which light is mutually coupled between this first waveguide. 3 shows a process of selectively growing a waveguide and forming a Bragg reflector that reflects light of a specific wavelength included in light coupled from the first waveguide to the second waveguide.

In the step shown in FIG. 7(C), a cladding layer 18, a waveguide layer 19, and a cladding layer 20 are selectively grown in the etched region.

In the step shown in FIG. 7(d), two diffraction gratings 10 are carved on the surface of the cladding layer 18.

In the step shown in FIG. 7(e), the cladding layer 18 is etched to form ridges 13,14. At this time, the diffraction grating 10 is left on the surface of the ridge 14, and this is used as a Bragg reflector. The ridges 13, 14 effectively change the refractive index and form a first waveguide and a second waveguide in that region.

FIG. 8 shows a plan view of a semiconductor laser according to a third embodiment of the present invention.

In this embodiment, a groove 80 is provided between the Bragg reflector 15 and the first waveguide (the area defined by the ridge 13) to prevent light coupling.

Further, in this embodiment, the bridges 13 and 14 are formed in a straight line. Therefore, the first waveguide and the second waveguide are also straight, and it is possible to reduce loss caused by bending of the waveguide.

FIG. 9 shows a sectional view of a semiconductor laser according to a fourth embodiment of the present invention.

In this embodiment, the first waveguide and the second waveguide are integrated in different layers. That is, a cladding layer 92, a quantum well layer 93, a waveguide layer 94, and a cladding layer 95 are grown on a substrate 91. Here, the quantum well layer 93 and the waveguide layer 94 are shown as the same layer.

Further, the buffer layer and the delayed index layer are omitted. A cladding layer 95 is provided in the region of the waveguide layer 94.
A waveguide layer 96 is grown on the waveguide layer 96, and a Bragg reflector 15 is formed in a part of the waveguide layer 96. A cladding layer 97 is further grown on the waveguide layer 96.

FIG. 10 shows a sectional view of a semiconductor radar according to a fifth embodiment of the present invention.

In this embodiment, the central portion of the waveguide layer in the fourth embodiment is curved toward the waveguide layer 96 side, and the cladding layer 95 in that region is made thinner. This allows the coupling between the two waveguides to be weak in the region of the Bragg reflector 15 and strong in the center.

〔Effect of the invention〕

As explained above, in the semiconductor laser of the present invention, an ROR having a high Q value of reflection characteristics is monolithically integrated. Therefore, it is possible to obtain a compact and stable semiconductor laser with excellent single mode properties and a narrow emission spectrum width.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of each manufacturing process of a semiconductor laser according to a first embodiment of the present invention. FIG. 2 is a plan view of the first embodiment. FIG. 3 is a diagram showing the relationship between @W between the waveguides and the coupling coefficient G between the waveguides. FIG. 4 is a diagram showing the value of 1Rnl of the Bragg reflector with respect to the coupling coefficient of the Bragg reflector [l×length Ln. FIG. 5 is a diagram showing the values of lR,12 and lr2 with respect to the deviation Δβ of the propagation constant from the Bragg wavelength. FIG. 6 is a diagram equivalent to FIG. 5 when the waveguide loss α=1 cm−'. FIG. 7 is a perspective view of each manufacturing process of a semiconductor laser according to a second embodiment of the present invention. FIG. 8 is a plan view of a semiconductor laser according to a third embodiment of the present invention. FIG. 9 is a sectional view of a semiconductor laser according to a fourth embodiment of the present invention. FIG. 10 is a sectional view of a semiconductor laser according to a fifth embodiment of the present invention. L91...Substrate, 2...Buffer layer, 3, II, 1
8.20.92.95.97...Clad layer, 4.6
... Graded index layer, 5.93 ... Quantum well layer, 7.19.94.96 ... Waveguide layer, 8 ... Mask, 10 ... Diffraction grating, 12 ... cap layer, 1
3.14...ridge, 15...Bragg reflector, 1
6.17... Electrode. Patent applicant Optical Measurement Technology Development Co., Ltd. Agent Patent attorney Nao Ide Takayoshizu W (4m) D 1 mouthful of heat Δf'3 (c Shioki

Claims (1)

[Claims] 1. An active region formed of a semiconductor, and an external resonator that reflects output light from the active region and returns it to the active region, the external resonator comprising: a first waveguide into which the output light is incident, and a second waveguide into which light is coupled mutually between the first waveguide, and the second waveguide is coupled from the first waveguide to the second waveguide. What is claimed is: 1. A semiconductor laser including a Bragg reflector that reflects light of a specific wavelength included in the light, wherein the active region and the external resonator are formed on the same substrate. 2. The semiconductor laser according to claim 1, wherein the active region includes a quantum well structure, and the first waveguide and the second waveguide include a layer in which an extension of the quantum well structure is disordered as a waveguide layer. 3. The semiconductor laser according to claim 1, wherein the first waveguide and the second waveguide include selectively grown waveguide layers. 4. The semiconductor laser according to claim 1, wherein the external cavity includes a groove for preventing light coupling between the Bragg reflector and the first waveguide. 5. The semiconductor laser according to claim 1, wherein the first waveguide and the second waveguide are in different layers of a laminated structure. 6. The semiconductor laser according to claim 1, further comprising means for supplying current or voltage to the Bragg reflector to control its equivalent refractive index. 7. A step of forming a quantum well structure on a semiconductor substrate, a step of forming an active region in a part of this quantum well structure, and a step of disordering the quantum well structure in a part other than this active region; a first waveguide through which the output light from the active region enters the disordered region; a second waveguide through which light is mutually coupled to the first waveguide; and forming a Bragg reflector that reflects light of a specific wavelength included in the light coupled from the first waveguide. 8. A step of forming a layered structure including an active layer on a semiconductor substrate, a step of etching the active layer to form an active region, and a step of transmitting light output from the active region to the region etched by this step. A first waveguide into which light is incident, a second waveguide where light is coupled mutually between this first waveguide, and a specific wavelength included in the light coupled from the first waveguide to this second waveguide. and forming a Bragg reflector that reflects light.