JPH11163471A - External cavity type semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide stability in electric current/optical characteristics against changes in environmental temperature, etc., and to provide stability in oscillation wavelength when a fiber grating(FG) of low reflectivity is used, by so forming a refractive index distribution of a fiber grating(FG) as to draw an envelope along the advancing direction of propagating light. SOLUTION: A distribution of refractive index in FG is so formed as to draw an envelope along the advancing direction of propagation light. In a semiconductor laser, an optical lens 31 is provided on the optical path between an SOA10 and an optical fiber 20, which allows the light reciprocating between the SOA10 and the optical fiber 20 to be condensed, so that the optical coupling efficiency is improved for efficient generation of laser light. As a result, electric current/optical characteristics are stable with respect to changes in environmental temperature, etc., and furthermore, oscillation wavelength is stable even when the FG of low reflectivity is used. Thus, the lager can be utilized as a light source with stable oscillation wavelength used for wavelength multiplex system of optical communication and excitation for a fiber amplifier.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an external cavity type semiconductor laser. More specifically, the present invention relates to a semiconductor laser including an optical amplifier (hereinafter, referred to as “SOA”) and a resonator, and particularly a part of the resonator is a fiber grating (hereinafter, referred to as “FG”). The present invention relates to a novel configuration of a semiconductor laser provided with a reflector formed by the method described above.

[0002]

2. Description of the Related Art A semiconductor laser comprises a semiconductor light emitting device having a specific layer structure including an active layer, and a pair of reflectors for reflecting light emitted from the semiconductor light emitting device at regular intervals. At least.

FIG. 7 is a diagram showing one typical configuration of an external cavity semiconductor laser.

As shown in FIG. 7A, this semiconductor laser is a light emitting device and has an SOA 10 having an amplifying function.
And an optical fiber 20 also serving as an emission port.

Here, the SOA 10 has an active layer 11 formed therein and a pair of electrodes 12 and 13 for supplying an injection current to the active layer 11. It is configured to emit light horizontally from the active layer by the driving current supplied through the active layer. Furthermore, on the side of SOA10,
Dielectric multilayer films 16a and 16b are formed respectively. Of these dielectric multilayer films 16a and 16b, the dielectric multilayer film 16 formed on the end face on the side where the optical fiber 20 described later is disposed
b is an anti-reflection film. On the other hand, the dielectric multilayer film 16a formed on the end face opposite to the optical fiber 20 has a high reflectance, and constitutes one of the reflectors in the resonator of the semiconductor laser.

On the other hand, the optical fiber 20 is formed of the dielectric multilayer film.
On the side opposite to 16, the end face is arranged so that the radiation emitted from the SOA 10 is injected. An FG 23 is formed near the end of the core 21 of the optical fiber 20. That is, in the FG23 portion, as shown in FIG. 7B, the refractive index is distributed so as to change periodically, and light of a specific frequency is reflected. Therefore, a resonator is formed by the FG 23 and the dielectric multilayer film 16 described above. The distal end 24 of the optical fiber 20 is formed so that at least the end face of the core 21 is spherical, and has a function of converging propagating light.

[0007]

FIG. 8 is a graph showing the relationship between the mirror loss and the oscillation phase condition in the external cavity semiconductor laser having the above-described configuration.

As shown in FIG. 8A, the semiconductor laser satisfies the resonance condition that the phase of one round trip of the resonator coincides with the original phase, and oscillates at the wavelength where the mirror loss is the smallest. (For the sake of simplicity, the wavelength dependence of the gain is ignored here). As a result, as shown in the figure, a plurality of peaks called side lobes b and c occur on both wings for one main peak a.

In a semiconductor laser having such characteristics,
When the characteristics of the SOA and the resonator change due to some factors such as a change in the environmental temperature, the resonance oscillation conditions change as shown in FIG. 8B, so that the side lobe changes from the longitudinal mode a near the main peak wavelength of the reflector. Hopping of the wavelength occurs in the nearby longitudinal mode b or c. Further, since the oscillation wavelength and the mirror loss greatly change discontinuously, a kink occurs in the current / optical characteristics.

When the reflectivity of the FG is reduced, the difference between the mirror loss near the main peak a and the mirror loss in the side lobes b and c becomes small.
As shown in (b), a plurality of laser oscillations can occur in the range of the width δR under the constant resonance condition. Therefore, FIG.
As shown in (a), there is also a problem that so-called multimode oscillation occurs and the oscillation wavelength range is widened.

Therefore, the present invention solves the above-mentioned problems of the prior art, stabilizes the current / light characteristics even when the environmental temperature changes, and stabilizes the oscillation wavelength even when a low-reflectivity FG is used. It is an object of the present invention to provide an external cavity type semiconductor laser.

[0012]

According to the present invention, there is provided a semiconductor optical amplifier for generating and amplifying light, and a resonator formed by a pair of reflectors for mutually reflecting the optical output of the semiconductor optical amplifier. Wherein at least one of the reflectors forming the resonator is a fiber grating having a periodic refractive index distribution, and the refractive index distribution of the fiber grating is a An external resonator type semiconductor laser is provided which is formed so as to draw an envelope along a direction.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The external cavity type semiconductor laser according to the present invention is characterized mainly by the unique refractive index distribution of FG constituting one reflector of the cavity. That is,
In the semiconductor laser according to the present invention, the distribution of the refractive index in the FG is configured to draw an envelope along the traveling direction of the propagating light. In an external resonator type semiconductor laser in which a resonator is formed using an FG having such a refractive index distribution, side lobes are very small in reflection characteristics, as described later.

That is, in the ordinary FG, since the length of the FG forming portion is finite, the change in the refractive index of the portion where the FG is not formed and the change of the refractive index of the FG forming portion are steep. It causes large side lobes. On the other hand, in the FG in which the refractive index distribution describes an envelope, a sharp change in the refractive index does not occur, so that the side lobe is reduced. It should be noted that the envelope referred to here includes both triangles and Gaussian shapes.

Further, according to a preferred embodiment of the present invention, F
Optical components can be inserted between G and SOA to improve the efficiency of the reflector. Such optical components include:
Although an optical lens supplied as a single component can be used, an end face of an optical fiber used as an external reflector and an output port can be processed into a convex shape and replaced. Further, both may be used in combination.

Hereinafter, the present invention will be described more specifically with reference to the drawings. However, the following disclosure is merely one embodiment of the present invention, and does not limit the technical scope of the present invention.

[0017]

FIG. 1 is a diagram showing a specific configuration example of an external cavity semiconductor laser according to the present invention. Components common to those of the conventional semiconductor laser shown in FIG. 8 are denoted by common reference numerals.

As shown in FIG.
The basic configuration is the same as that of the conventional semiconductor laser shown in FIG. However, the refractive index distribution in the FG 26 formed on the optical fiber 10 is different.

FIG. 2 is a graph showing the refractive index distribution in the FG used in the semiconductor laser according to the present invention.

As shown in FIG. 2A, in this semiconductor laser, the refractive index distribution of the FG draws an envelope (apotize curve) along the traveling direction of the propagating light. In the FG having such a refractive index distribution, as will be described later, since the difference between the main peak of the mirror loss and the silobe is sufficiently large, multi-mode oscillation hardly occurs. Therefore, as shown in FIG. 2B, a single prominent peak appears at the oscillation wavelength.

According to a preferred embodiment of the present invention, the refractive index distribution in the actual FG is formed such that the average refractive index in each section becomes constant as shown in FIG. Is preferred. By forming such a refractive index distribution, Fabry-Perot resonance in the FG can be avoided, so that the difference between the main peak a and the side lobes b and c further increases,
The peaks of the side lobes b and c fall outside the range of the width δR of the resonance condition. Therefore, multi-mode oscillation does not occur.

FIG. 3 is a graph showing a relationship between a mirror loss and a phase condition in a semiconductor laser having an FG having the above-described characteristics.

As shown in FIG. 3A, in the semiconductor laser constructed according to the present invention, the main peak that occurs at a specific wavelength that satisfies the resonance condition and has the smallest mirror loss is different from the side lobe that occurs on both wings. It has a very large head.

Therefore, in this semiconductor laser, the SO
When the characteristics of A and the resonator change, the hopping of the wavelength does not occur even if the resonance oscillation condition changes as shown in FIG. Further, for this reason, discontinuous changes in the oscillation wavelength and the mirror loss that cause kink in the current / optical characteristics do not occur.

FIG. 4 is a diagram showing another embodiment of the external cavity type semiconductor laser according to the present invention. In the figure, components common to those of the other drawings are denoted by common reference numerals, and duplicate description is omitted.

As shown in FIG. 1, in this semiconductor laser, an optical lens 31 is disposed on an optical path between the SOA 10 and the optical fiber 20. By condensing the light reciprocating between the SOA 10 and the optical fiber 20 by the optical lens 31, it is possible to improve the optical coupling efficiency and efficiently generate the laser light. Further, by forming the end face 25 of the optical fiber 20 at an angle, the end face reflection of the end face of the optical fiber can be eliminated.

FIG. 5 is a view showing still another embodiment of the external cavity type semiconductor laser according to the present invention. Also in this figure, the same components as those in the other drawings are denoted by the same reference numerals, and redundant description is omitted.

As shown in FIG. 1, in this semiconductor laser, a plurality of optical lenses 31 and 32 are arranged on an optical path between the SOA 10 and the optical fiber 20. These optical lenses 31, 32
By condensing the light reciprocating between the SOA 10 and the optical fiber 20, the laser can be generated efficiently. Further, similarly to the embodiment shown in FIG. 4, the end face 25 of the optical fiber 20 can be formed obliquely to eliminate end face reflection. Further, by using a plurality of lenses, the tolerance becomes gentle, and the deterioration of the optical coupling efficiency due to the misalignment of the optical fiber can be reduced.

An external cavity semiconductor laser according to the present invention having the structure shown in FIG. 1 and having a fiber grating having a refractive index distribution that draws a Gaussian curve as shown in FIG. 2C was fabricated. . An InGaAsP thin film was grown on an InP substrate to form a light emitting device having an active layer of a quantum well structure composed of quaternary crystals. An SiN thin film was applied as a dielectric multilayer film to the front end face of the light emitting device so that the reflectance on the optical fiber side was 1% or less. As the optical fiber, a single mode fiber with a core diameter of 10 μm is used.
A fiber grating was formed on the core near the incident end over a length of 6 mm. The maximum refractive index difference in the fiber grating was in the range of 10 ^-4 to 0, and the average refractive index of the entire fiber grating was set to 1.46. For comparison, a fiber grating having a monotonic refractive index distribution with a maximum refractive index difference of 10 ⁻⁴ was separately manufactured.

An external cavity type semiconductor laser was assembled using the above fiber grating, and the output of each was measured with a spectrum meter while changing the temperature. FIG. 6 shows the measurement results. As shown in the figure, in the external cavity type semiconductor laser having the conventional configuration, the transmission wavelength changes at a certain temperature. On the other hand, it has been confirmed that the external resonator type semiconductor laser according to the present invention has a stable transmission wavelength even with a large change in temperature.

[0031]

As described above in detail, the external cavity type semiconductor laser according to the present invention has a stable current / optical characteristic even when the ambient temperature changes due to the unique refractive index distribution of the FG. Further, the oscillation wavelength is stable even when FG having a low reflectance is used. Therefore, it can be suitably used as a light source having a stable oscillation wavelength used for exciting a wavelength multiplexing system or a fiber amplifier in optical communication.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration example of an external cavity semiconductor laser according to the present invention.

FIG. 2 is a diagram showing a refractive index distribution of FG and an oscillation wavelength in the external cavity semiconductor laser according to the present invention.

FIG. 3 is a graph illustrating the relationship between the mirror loss and the phase condition of oscillation in the external cavity semiconductor laser according to the present invention, particularly when the phase condition changes.

FIG. 4 is a diagram showing another configuration example of the external cavity semiconductor laser according to the present invention.

FIG. 5 is a diagram showing another configuration example of the external cavity semiconductor laser according to the present invention.

FIG. 6 is a graph showing the results of measuring the characteristics of an actually manufactured external cavity semiconductor laser.

FIG. 7 is a diagram showing a typical configuration of a conventional external cavity semiconductor laser.

FIG. 8 is a graph illustrating a relationship between a mirror loss and a phase condition of oscillation, particularly when the phase condition changes.

FIG. 9 is a graph showing a relationship between a change in FG reflectivity and an oscillation wavelength of a conventional external cavity semiconductor laser.

[Explanation of symbols]

10 ... SOA, 11 ... Active layer, 12, 13 ... Electrode, 14 ... Diffraction grating (distributed feedback type reflector), 15, 17,
18 ・ ・ ・ Diffraction grating (distributed reflection type reflector), 16a, 16b ・
..Dielectric multilayer film, 20 ... optical fiber layer, 21
・ Core, 22 ・ ・ ・ Clad, 23 ・ ・ ・ Fiber grating

Claims (4)

[Claims] 1. A semiconductor laser comprising: a semiconductor optical amplifier for generating and amplifying light; and a resonator formed by a pair of reflectors for reflecting output light of the semiconductor optical amplifier. At least one of the reflectors constituting the device is a fiber grating having a periodic refractive index distribution,
An external resonator type semiconductor laser characterized in that the refractive index distribution of the fiber grating is formed so as to draw an envelope along the traveling direction of propagating light. 2. The external cavity semiconductor laser according to claim 1, wherein a refractive index distribution in said fiber grating is formed such that an average of the refractive index is constant in an arbitrary section of said fiber grating. An external cavity type semiconductor laser characterized in that: 3. An external cavity type semiconductor laser according to claim 1, wherein at least one optical lens is provided on an optical path between said semiconductor optical amplifier and said optical fiber including said fiber grating. An external cavity type semiconductor laser comprising: 4. The external cavity semiconductor laser according to claim 3, wherein said optical lens is an end face of an optical fiber formed into a convex shape.