JPH08125264A - Semiconductor laser - Google Patents

Abstract

PURPOSE: To simultaneously oscillate a semiconductor laser in a prescribed plurality of wavelengths, to miniaturize the laser and to lower the production cost of the laser by a method wherein a periodical phase shift is provided in the diffraction grating of the semiconductor laser and a plurality of reflection peaks are provided in the reflection spectrum of the diffraction grating. CONSTITUTION: A diffraction grating 9 is held formed on the upper surface of an n-type InP clad layer 2 by an electron beam exposure method and etching, for example, prior to a crystal growth of an n-type InGaAsP guide layer 3 and a period A corresponding to the primary diffraction grating is about 240nm. When a periodical phase shift is provided in the diffraction grating, the threshold gain of a semiconductor laser, which is oscillated in a plurality of wavelengths in every period M, is reduced and it becomes possible to produce simultaneously a laser beam of a plurality of wavelengths. Moreover, as this period M is decided by the period of the phase shift, a plurality of wavelengths, which are simultaneously oscillated, can be selected by selecting properly the period of the diffraction grating and the period of the phase shift. Accordingly, a multitude of the gain peak wavelengths of an active layer in correspondence with the wavelengths of the laser, whose threshold gain is reduced, are reduced and the number of wavelengths, which can be simultaneously oscillated, is increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser used as a light source for wavelength-multiplexed optical communication or frequency-multiplexed optical communication, and more particularly to a semiconductor laser which oscillates at a plurality of predetermined wavelengths simultaneously.

[0002]

2. Description of the Related Art In wavelength (or frequency) multiplex optical communication,
A plurality of light sources having different wavelengths are required, and each light source needs to stably oscillate at a predetermined wavelength. As a semiconductor laser having a stable oscillation wavelength, a distributed feedback semiconductor laser or a distributed reflection semiconductor laser has been conventionally known.

[0003]

However, these conventional semiconductor lasers have the following problems. The oscillation wavelength of these semiconductor lasers is determined by the pitch of the diffraction grating, and the oscillation is single mode oscillation. Therefore, at present, a light source for wavelength division multiplexing optical communication is made by using a plurality of semiconductor lasers having different diffraction grating pitches. Therefore, as a matter of course, semiconductor lasers and electric circuits corresponding to the number of wavelengths to be multiplexed are required, and there is a problem that miniaturization and cost reduction cannot be expected.

SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor laser which has been made in view of the above points and which oscillates at a plurality of predetermined wavelengths at the same time and is small in size and low in cost.

[0005]

In order to achieve such an object, the present invention provides a semiconductor laser having an optical resonator using a diffraction grating, wherein the reflection spectrum of the diffraction grating has a plurality of reflection peaks. It is characterized in that it is configured to occur.

[0006]

Function: The diffraction grating of the semiconductor laser is provided with a periodic phase shift to provide a plurality of reflection peaks in the reflection spectrum of the diffraction grating. With such a structure, simultaneous oscillation at a plurality of wavelengths is possible.

[0007]

The present invention will be described in detail below with reference to the drawings.
In this embodiment, an InP-based distributed feedback semiconductor laser using a diffraction grating will be described as an example. FIG. 1 is a structural sectional view showing an embodiment of a semiconductor laser according to the present invention. In the figure, 1 is an n ⁺ type InP substrate, 2 is an n type InP cladding layer, 3 is an n type InGaAsP guide layer, 4 is an undoped InGaAsP active layer, 5 is a p type InP cladding layer, 6
Is a p ⁺ type InGaAsP contact layer, 7 is an n type electrode,
Reference numeral 8 is a p-type electrode, and 9 is a diffraction grating.

On the upper surface of the n ⁺ type InP substrate 1, n type InP is formed.
The clad layer 2, the n-type InGaAsP guide layer 3, the undoped InGaAsP active layer 4, the p-type InP clad layer 5, and the p ⁺ -type InGaAsP contact layer 6 are sequentially stacked, and the n-type electrode 7 is formed on the lower surface of the n ⁺ -type InP substrate 1. On the other hand, on the upper surface of the p ⁺ type InGaAsP contact layer 6, p type electrodes 8 are attached, respectively. The diffraction grating 9 is n
It is formed on the upper surface of the type InP clad layer 2 by making unevenness in the cavity direction (Z direction).

Although the basic operation principle of the semiconductor laser having such a structure is the same as that of the conventional one, only the diffraction grating 9 in this embodiment is different from the conventional distributed feedback semiconductor laser. The diffraction grating 9 will be described in detail below.

The diffraction grating 9 is formed on the upper surface of the n-type InP cladding layer 2 by, for example, an electron beam exposure method and etching prior to the crystal growth of the n-type InGaAsP guide layer 3, and has a period corresponding to a first-order diffraction grating. Λ is about 240
nm.

FIG. 2 is a diagram for explaining the phase of the diffraction grating. FIG. 2A is a diagram shown for comparison, and shows the case of a diffraction grating having no phase shift. FIG. 2B shows the case of a diffraction grating provided with a phase shift 11 of 180 degrees in the center. FIG. 2C shows the case of the diffraction grating of the present invention. The diffraction grating shown in FIG. 2B is a structure that has been conventionally used to improve the single mode property of a distributed feedback semiconductor laser, and is a quarter wavelength (λ / 4) shift diffraction grating or λ. It is called a / 4 shift distributed feedback semiconductor laser.

In the diffraction grating of the present invention shown in FIG. 2C, in addition to the central phase shift 11 shown in FIG.
A plurality of 180 degree phase shifts 12 are provided with a cycle L.
The central phase shift 11 is not essential,
The phase shift may be other than 180 degrees and may not be present. Further, the phase shift amount of the plurality of phase shifts 12 is preferably 180 degrees, but it does not necessarily have to be 180 degrees.

The operation of the semiconductor laser having such a structure is
Next, description will be made with reference to the schematic diagram of the oscillation threshold gain with respect to the wavelength shown in FIG. The solid line in FIG. 3 represents the oscillation threshold gain of the distributed feedback semiconductor laser using the diffraction grating 9 of the present invention shown in FIG. 2C, and the dotted line in FIG. 3 represents the diffraction grating shown in FIG. This is the oscillation threshold gain of the used λ / 4 shift distributed feedback semiconductor laser.

In the conventional λ / 4 shift distributed feedback semiconductor laser, the oscillation threshold gain decreases at one wavelength as shown by the dotted line in FIG. 3, and single mode oscillation occurs at this wavelength. On the other hand, when the diffraction grating is provided with a periodic phase shift, the oscillation threshold gain is reduced at a plurality of wavelengths for each period M as indicated by the solid line in FIG. 3, and it is possible to simultaneously oscillate at a plurality of wavelengths. become. Furthermore, since the period M is determined by the period L of the phase shift 12, it is possible to select a plurality of wavelengths that oscillate simultaneously by appropriately selecting the period Λ of the diffraction grating 9 and the period L of the phase shift 12.

The present invention is not limited to the above embodiment, but can be modified appropriately. For example, as shown in FIG. 4, the active layer 4 of FIG. 1 is replaced by the InGaAsP quantum well layers 41a, 41b, 41c and the InGaAsP barrier layer 42.
A multiple quantum well sandwiched between a, 42b, 42c and 42d may be used. Although the number of quantum wells is three in FIG. 4 for simplification of description, the number of quantum wells is not limited at all.

Since the gain peak wavelength of the quantum well having such a structure depends on the well width and the mixed crystal ratio of the quantum well, the gain peak wavelength of the quantum well can be changed by changing them. The gain characteristic of the multiple quantum well shown in FIG. 4 is shown in FIG. The well width W ₁ , W ₂ , W ₃ of each quantum well layer is set to W
By setting ₁ > W ₂ > W ₃ , it is possible to determine a plurality of gain peak wavelengths of the active layer corresponding to the wavelengths with reduced threshold gains shown by the solid line in FIG. 3, and increase the number of wavelengths that can be simultaneously oscillated. be able to.

Further, as shown in FIG. 6, each quantum well layer 41
a, 41b and 41c are p-type InGaAsP barrier layers 42
e, 42f and n-type InGaAsP barrier layers 42g, 42
and p-type electrodes 43a and 43b on each barrier layer.
Alternatively, the n-type electrodes 44a and 44b may be formed, and the current flowing through each electrode may be changed to independently control the gain of each quantum well.

Although the distributed feedback type semiconductor laser has been described as an example in the embodiment, the present invention can be applied to the distributed Bragg reflector semiconductor laser and the same effect can be obtained.

[0019]

As described above, according to the present invention, a semiconductor laser that simultaneously oscillates at a plurality of predetermined wavelengths can be realized, and the number of semiconductor lasers and electric circuits required for a wavelength division multiplexing optical communication light source can be reduced. It can be reduced and the structure can be simplified, and the cost and size of the light source for wavelength division multiplexing optical communication can be realized.

[Brief description of drawings]

FIG. 1 is a structural sectional view showing an embodiment of a semiconductor laser according to the present invention.

FIG. 2 is a diagram for explaining a phase of a diffraction grating.

FIG. 3 is a schematic diagram of oscillation threshold gain with respect to wavelength.

FIG. 4 is a structural cross-sectional view showing an embodiment of an active layer in the present invention.

FIG. 5 is a diagram showing a gain characteristic of a multiple quantum well.

FIG. 6 is a structural cross-sectional view showing another embodiment of the active layer in the present invention.

[Explanation of symbols]

1 n ⁺ type InP substrate 2 n type InP clad layer 3 n type InGaAsP guide layer 4 undoped InGaAsP active layer 5 p type InP clad layer 6 p ⁺ type InGaAsP contact layer 7, 44a, 44b n type electrode 8, 43a, 43b p Type electrode 9 Diffraction grating 11,12 Phase shift 41a, 41b, 41c InGaAsP quantum well layer 42a, 42b, 42c, 42d InGaAsP barrier layer

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Takaaki Hirata 2-932 Nakamachi, Musashino City, Tokyo Yokogawa Electric Co., Ltd. (72) Yoshihiko Tachikawa 2-932 Nakamachi, Musashino City, Tokyo Within Kawa Denki Co., Ltd.

Claims (4)

[Claims] 1. A semiconductor laser having an optical resonator using a diffraction grating, wherein a plurality of reflection peaks are generated in a reflection spectrum of the diffraction grating so as to simultaneously oscillate at a plurality of wavelengths. A semiconductor laser characterized in that 2. The diffraction grating is provided with a periodic phase shift so that a plurality of reflection peaks are generated in a reflection spectrum of the diffraction grating, so that the diffraction grating can simultaneously oscillate at a plurality of wavelengths. The semiconductor laser according to claim 1. 3. The active layer has a multiple quantum well structure, and the gain characteristics of the active layer are controlled by changing the well width and / or the mixed crystal ratio of the multiple quantum well. The semiconductor laser according to claim 1. 4. The active layer has a multi-quantum well structure, a plurality of electrodes are provided in the multi-quantum well, and a current flowing through each electrode is changed to control a gain characteristic of the active layer. The semiconductor laser according to claim 3, which is characterized in that.