JPH03211781A - Outside resonator type variable wavelength semiconductor laser - Google Patents

Abstract

PURPOSE:To make it possible to increase a variable range of wavelength to a wider band by comprising a light source of monochrome coherent light which covers a wide range and an outside resonator and laying out in series an active layer whose composition comprises two types and more, which are different from each other, used as a semiconductor laser element having a wider light emitting range, which is the light source, along a wave guide passage or laminating the elements in the direction of their thickness. CONSTITUTION:Photons, generated in a first active layer 3 and a second active layer 4, are wave-guided along the first active layer 3 and the second active layer 4. One part of photons are reflected on a cleavage plane 5 and return to the first active layer 3 and the second active layer 4 while the other part of photons penetrate a reflection prevention film 6. The photons, which are penetrated the film, are collectled by a concave lens 7 and selectively reflected by a diffraction grating 8 and returns to the first active layer 3 and the second active layer 4 again, and get oscillated, making multiple reflection with the cleavage plane 5 and the diffraction grating 8. The oscillated light is outputted from the cleavage from the cleavage plane 5 and the diffraction grating 8 is controlled by a diffraction grating scanning means 9 so that only a required wavelength maybe selectively reflected thereon.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an external cavity type wavelength tunable semiconductor laser used as a light source for coherent optical communications, interference type optical fiber sensors, optical application measuring instruments, etc.

[Conventional technology]

A structural diagram of a conventional external cavity type wavelength tunable semiconductor laser is shown in FIG. Light generated in an active layer formed on a substrate passes through an antireflection film, is emitted from a semiconductor laser element, is focused by a condensing lens, and is selectively reflected by a diffraction grating. The light reflected by the diffraction grating is condensed by a condenser lens, passes through an antireflection film, and enters a semiconductor laser element. The incident light is reflected by the cleavage plane and emitted from the semiconductor laser element again. During this time, the incident light is amplified by stimulated emission in the active layer and undergoes multiple reflections between the diffraction grating and the cleavage plane, resulting in diffraction. Laser oscillation occurs at wavelengths that are selectively reflected by the grating. To select the wavelength for laser oscillation, use the fact that the reflection angle on the diffraction grating differs depending on the wavelength of the incident light, and rotate the diffraction grating so that the light of the required wavelength is directed in exactly the opposite direction to the incident light. This is achieved by changing the reflection angle so that the beam is sent out. However, the wavelength range in which oscillation is possible is limited by the gain spectrum of the active layer of the semiconductor laser device, and is approximately 70 nm for a 1.55 μm band semiconductor laser device. Furthermore, it is desirable that the antireflection film formed on one end face of the active layer has a low reflectance, but the wavelength range in which a low reflectance of 0.1% or less can be obtained for light in the 1.3 μm band is In order to create an anti-reflection film with a wavelength of about 40 nm and such a low reflectance,
Since the error in film thickness must be kept within ±60 Å,
The reflectance and wavelength range often varied. Therefore, it has been difficult to widen the variable wavelength range that can be selected by rotating the diffraction grating.

[Problem to be solved by the invention]

That is, with only an active layer having a single composition, the wavelength range that can be oscillated is limited mainly by the gain spectrum that depends on the composition and shape of the active layer. In addition, in the case of a structure in which only an anti-reflection film is applied to one end face of the active layer, low reflectance must be achieved with only the anti-reflection film, but the permissible range of anti-reflection film thickness that achieves low reflection is Because it was narrow, variations in reflectance were likely to occur. Furthermore, the wavelength range in which low reflectance characteristics can be obtained is narrow, and wavelength variations tend to occur, making it impossible to fully utilize the characteristics of the active layer.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an external cavity type wavelength tunable semiconductor laser that can obtain broadband and wavelength tunable coherent light.

[Means to solve the problem]

In order to solve the above problems, in the external cavity type wavelength tunable semiconductor laser of the present invention, 1) a semiconductor laser element with a wide emission frequency range is used, and 2) only a predetermined wavelength is selected from the light emitted from the semiconductor laser element. A structure was adopted in which the aircraft was transported back to Japan.

Note that the wavelength selection means is controlled to select only a predetermined wavelength.

In addition, as a semiconductor laser device with a wide emission frequency range, (a) two or more active layers with different compositions are connected in series along a waveguide, (b) or stacked in the thickness direction (C ) or a combination of (a) and (b).

In particular, the external cavity type wavelength tunable semiconductor laser according to claim (2) is a semiconductor laser element comprising: 1) an active layer extending from the emission end face and a window end face structure following the active layer; 2) a window end face structure. 3) A structure is adopted in which only a predetermined wavelength of the light emitted after passing through the window end face structure and the antireflection film is selected and returned.

Note that the wavelength selection means is controlled to select only a predetermined wavelength as in claim (1).

[Function] According to the external cavity type wavelength tunable semiconductor laser configured in this way, it is possible to obtain a light source of monochromatic coherent light that can cover a wide range. In particular, oscillation can be obtained in a wider wavelength range than when the active layer has only a single composition. By using a semiconductor laser element, the above effect can be further enhanced.

Moreover, according to the window end face structure of claim (2), the reflectance can be stably reduced in a wide wavelength range. For example, in the window area, the relative reflectance can be reduced to 0.3% over a wide wavelength range, so the performance requirements for the reflectance of the anti-reflection coating can be reduced.
The allowable range for the thickness of the anti-reflection film is expanded, and the accuracy of the reflectance of the anti-reflection film can be relaxed during manufacturing. Specifically, the reflectance of 0.005% or less is 1100 in combination with the anti-reflection film.
It can be obtained in a wavelength range of about n, and the permissible range of the antireflection film thickness at this time is about ±200. In other words, the broadband gain spectrum of the active layer can be effectively utilized.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 is a structural diagram showing an embodiment of an external cavity type wavelength tunable semiconductor laser of the present invention. In the first embodiment, a diffraction grating, a diffraction grating rotation means for rotating the diffraction grating, and a convex lens were used as the wavelength selection means, the control means, and the condensing means, respectively. By injecting a current between the electrode 1 and the electrode 2, photons generated in the first active layer 3 and the second active layer 4 are guided along the first active layer 3 and the second active layer 4. However, one photon is reflected by the cleavage plane 5 and returns to the first active layer 3 and the second active layer 4, and the other photon is reflected by the antireflection film 6.
The transmitted photons are focused by the convex lens 7,
The light is selectively reflected by the diffraction grating 8, returns to the first active layer 3 and the second active layer 4, and undergoes multiple reflections by the cleavage plane 5 and the diffraction grating 8, causing oscillation. The oscillated light is emitted from the cleavage plane 5. Note that the diffraction grating 8 is controlled by a diffraction grating scanning means 9 so that only desired wavelengths can be selectively reflected.

FIG. 2 is a diagram showing the manufacturing process of the semiconductor laser element of the external cavity type wavelength tunable semiconductor laser of the present invention. For the first growth, liquid phase epitaxy (LPE) method and vapor phase epitaxy (VP
E, MO-CVD) method, or molecular beam epitaxy (M
A p-type InP bumper layer 11 and a first non-doped InGaAs layer are formed on the p-type InP substrate 10 by B E ) method or the like.
P active layer (λ: 1.5 μm composition) 3, anti-melt back layer (λ: 1.48 μm) 12, n-type 1nP layer on top of it
The cladding layer 13 is grown thin. Next, the second active layer 4
In order to remove the part where the Si
An Nx film is formed, and unnecessary n-type 1nP cladding layer 13, anti-meltback layer 12 and first non-doped InGaAsP active layer 3 are removed by etching. As the second growth, a second non-doped InGaAsP active layer (λ: 1.55 μm composition) 4, an anti-meltback layer 12, and an n-type InP cladding N13 are grown thinly. Next, remove Yask 17 and grow n for the third time.
A 1nP type cladding layer 13 is grown. Next, after etching leaving only the stripe part of the part grown by mesa etching, n-type 1nP% p-type In
The layers are grown in the order of P to form a buried hetero (BH) structure. Electrodes 1 and 2 are placed on both sides of the substrate after growth.
is vapor-deposited and scribed into chip shapes to form an antireflection film 6. The semiconductor laser element 15 manufactured through such a process is incorporated into an external resonator to constitute an external resonator type wavelength tunable semiconductor laser.

(Second Embodiment) FIG. 3 is a structural diagram showing a second embodiment of the external cavity type wavelength tunable semiconductor laser of the present invention. In a second embodiment,
In the first embodiment, the first active layer 3, the second active layer 4
The electrodes 1 formed on the upper surface of the semiconductor laser element are formed separately on the upper surfaces of the first active layer 3 and the second active layer 4 to form divided electrodes 1a and 1b. Thereby, the current injected into each active layer can be independently controlled, and variations in gain depending on wavelength can be reduced.

(Third Embodiment) FIG. 4 is a structural diagram showing a third embodiment of the external cavity type wavelength tunable semiconductor laser of the present invention. In a third embodiment,
The first active layer 3 and the second active layer 4 of the first embodiment are stacked in the thickness direction, and the process leading to oscillation is the same as that of the tenth embodiment.

(Fourth Embodiment) FIG. 5 is a structural diagram showing a fourth embodiment of the external cavity type wavelength tunable semiconductor laser of the present invention. In a fourth embodiment,
In the semiconductor laser device 15 of the first embodiment, a window end face structure 16 is provided on the end face of the active layer between the second active layer 4 and the antireflection film 6. Conventionally, it has been difficult to achieve a low reflectance using only the anti-reflection film 6, but with this configuration, the reflectance of the semiconductor laser element end face can be controlled by the window end face structure 16 and the anti-reflection film 6, so that the reflectance can be reduced to 0. 003
% can be obtained over a wide wavelength range, and therefore the broadband characteristics of the active layer can be effectively utilized. (Fifth Embodiment) FIG. 6 is a structural diagram showing a fifth embodiment of the external cavity type wavelength tunable semiconductor laser of the present invention. In a fifth embodiment,
In the semiconductor laser device 15 of the third embodiment, a window end face structure 16 is provided on the end face of the active layer between the first active layer 2, the second active layer 4, and the antireflection film 6. .

(Sixth Embodiment) FIG. 7 is a structural diagram showing a sixth embodiment of the external cavity type wavelength tunable semiconductor laser of the present invention. In a sixth embodiment,
In the fourth embodiment, three waveguides 17 are provided adjacent to the first active layer 3 and the second active layer 4. Due to this structure, a portion of the photons generated in the first active layer 3 and the second active layer 4 have a forbidden band width of the first active layer 3 and the second active layer 4.
The waveguide 17 has a composition wider than the active layer 4 and narrower than the InP substrate, and is guided. In this case, the first active layer 3
It is possible to reduce the effect of photons generated in the second active layer 4 being absorbed by the second active layer 4 having a composition with a narrower forbidden band width.

(Seventh Embodiment) FIG. 8(a) is a structural diagram showing a seventh embodiment of the external cavity type wavelength tunable semiconductor laser of the present invention, FIG. 8(b) is a detailed diagram of the quantum well structure, FIG. 8(c) is a characteristic diagram showing the relationship between wavelength and optical output. In the seventh embodiment, the active layer in the third embodiment has a quantum well structure consisting of several well layers having different forbidden band widths. This structure allows
Since the injected carriers are confined in each well layer, gain can be generated in a wavelength range corresponding to the quantum level of each well layer. Therefore, the larger the number of well layers, the wider the wavelength band that can be oscillated.

In each of the embodiments, a diffraction grating was used as the wavelength selection means; however, the transmission center wavelength is controlled to a predetermined value by using not only a diffraction grating but also a continuously variable wavelength filter.

(2) Using an optical piezoelectric crystal, voltage is applied to the electrodes of the optical piezoelectric crystal, and control is performed to selectively deflect only a predetermined wavelength.

It goes without saying that this can be achieved with etc.

[Effects of the Invention] As explained above, the external cavity type wavelength tunable semiconductor laser of the present invention is composed of a light source of monochromatic coherent light that covers a wide range and an external cavity, and the emission frequency range of the light source is As a wide semiconductor laser element, two or more types of active layers having different compositions are arranged in series along a waveguide or stacked in the thickness direction. Therefore, since the wavelength range in which photons incident on the active layer can be amplified is widened, the wavelength variable range can be made wider.

Further, in the external cavity type wavelength tunable semiconductor laser according to claim (2), the semiconductor laser element has a structure including an active layer extending from the emission end face and a window end face structure continuing from the active layer. That is, the reflectance of the semiconductor laser element end face is set to 0.
In addition, it is possible to realize an end face that can stably obtain low reflection over a wide band. Therefore,
The broadband characteristics of the active layer can be effectively utilized.

By using such a semiconductor laser device with a window end facet structure, it is possible to stably produce an external cavity type wavelength tunable semiconductor laser having a narrow spectral linewidth, low noise, and a wavelength tunable over a wide band.

[Brief explanation of drawings]

Fig. 1 is a structural diagram showing the first embodiment of the present invention, Fig. 2 is a diagram showing the manufacturing process of the invention, Fig. 3 is a structural diagram showing the second embodiment of the invention, Fig. 4 is a structural diagram showing a third embodiment of the present invention, FIG. 5 is a structural diagram showing a fourth embodiment of the present invention, FIG. 6 is a structural diagram showing a fifth embodiment of the present invention, and FIG. FIG. 8 is a structural diagram showing a sixth embodiment of the present invention, FIG. 8 is a structural diagram showing a seventh embodiment of the present invention, and FIG. 9 is a structural diagram showing a conventional external cavity type wavelength tunable semiconductor laser. . 1.2... Electrode, 1a, ■b... Divided electrode, 3.
4... Active layer, 5... Cleavage plane, 6... Antireflection film, 7... Condensing means (convex lens), 8... Wavelength variable means (diffraction grating), 9... Control Means (diffraction grating scanning means), 10... Substrate, 11... Buffer layer, 12... Anti-meltback layer, 13... Clad layer, 14... Mask, 15...
Semiconductor laser element, 16... Window end surface structure, 17...
waveguide.

Claims (2)

[Claims] (1) A semiconductor laser device having two or more types of active layers, a condensing means for condensing light emitted from the semiconductor laser device, and a condensing means for condensing light emitted from the semiconductor laser device; an external resonator type wavelength, characterized by comprising a wavelength selection means for selecting only a predetermined wavelength and transmitting the wavelength in a direction opposite to that of the incident light; and a control means for controlling the wavelength selection means so as to select only a predetermined wavelength. Tunable semiconductor laser. (2) Claim characterized in that the semiconductor laser element includes an active layer extending from the emission end face, a window end face structure following the active layer, and an antireflection film provided on the outside of the window end face structure. 1. The external cavity type wavelength tunable semiconductor laser according to 1.