JPH11163472A - External resonator type semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser wherein wavelength precision of outgoing light is enhanced, while no other characteristic is degraded. SOLUTION: This semiconductor laser comprises a semiconductor optical amplifier 10 and a resonator, formed of a pair of reflectors which mutually reflects the optical output of the semiconductor optical amplifier 10. The resonator comprises a first reflector 14, which having wavelength selectivity is built in near an active layer 11 of the semiconductor optical amplifier 10 and a second reflector 23, allocate outside the semiconductor optical amplifier 10, on its outgoing optical path, with the second reflector 23 formed of optical material.

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 a light emitting unit and a resonator, and more particularly to a novel configuration of a semiconductor laser including an external resonator.

[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.

[0004] As shown in FIG.
An optical amplifier 10 as a light emitting element is combined with an optical fiber 20 also serving as an emission port.

Here, the optical amplifier 10 includes an active layer 11 formed therein and a pair of electrodes 12 and 13 for supplying a current to be injected into the active layer 11. It is configured to emit light horizontally from the active layer by the drive current supplied through the switch 13. Further, dielectric multilayer films 16a and 16b are formed on side surfaces of the SOA 10, respectively. Among these dielectric multilayer films 16a and 16b, the dielectric multilayer film 16b formed on the end face on the side where the optical fiber 20 described later is arranged is an antireflection 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 16a, the end face is arranged so that the radiation emitted from the optical amplifier 10 is injected. Further, a fiber grating 23 is formed near the end of the core 21 of the optical fiber 20. Furthermore, the tip of the optical fiber 20
24 is formed so that at least the end face of the core 21 is spherical.

FIG. 8 is a diagram showing another configuration example of the semiconductor laser. Note that the same components as those in FIG. 7 are denoted by the same reference numerals, and description thereof is omitted.

In this semiconductor laser, a resonator is built in a semiconductor element of the optical amplifier 10. That is, as shown in FIG. 1, a pair of distributed reflection type reflectors 17 and 18 are formed in a layer substantially adjacent to the active layer 11 in the optical amplifier 10. Therefore, laser light is emitted from the side surface of the optical amplifier 10.
Although the reflectors 17 and 18 are formed above the active layer 11 in FIG. 8, the reflectors 17 and 18 function similarly even if they are formed below the active layer 11.

[0009]

In particular, when a semiconductor laser is used for wavelength multiplex transmission, it is often required to increase the wavelength accuracy of the optical output.

In the external cavity type semiconductor laser shown in FIG. 7, the dielectric multilayer film 16a as one of the reflectors has almost no wavelength selectivity. Therefore, it is necessary to narrow the spectrum width of the fiber grating 23 formed on the optical fiber 20. However, in the fiber grating, between the spectral width (δλ) and the length L, the following equation 1 is used.
There is a relationship as shown in

[0011]

[Formula 1] δλ = λ _B ² ｛π ² + (κL) ² ｝ ^0.5 / ( ² nL) Here, λ _B represents a diffraction wavelength, κ represents a coupling constant, and n represents a refractive index.

As can be seen from the above equation, the length L must be increased in order to reduce the spectral width δλ in the fiber grating. Specifically, 0.1
To obtain an accuracy of about 0.3 nm, the fiber grating needs a length of several cm. However, as the length L increases, the effective resonator length also increases, and the time required for light to reciprocate in the resonator increases. For this reason, there is another problem that characteristics are deteriorated particularly in a high frequency band.

On the other hand, in the semiconductor laser shown in FIG. 8, the cavity length is relatively short. However, each reflector forming the resonator is formed as a part of the semiconductor element. As for the material of the semiconductor element, the temperature change of the refractive index n is about one digit larger than that of an optical material such as an optical fiber. On the other hand, the first-order diffraction wavelength λ _B of the distributed reflection type reflector has a relationship as shown in the following Expression 2 with respect to the period Λ of the diffraction grating.

[0014]

[Equation 2] λ _B = 2nΛ

For this reason, when the refractive index changes due to a change in the external temperature, the wavelength of the light output changes.

As described above, in the conventional semiconductor laser, it is difficult to maintain the wavelength accuracy of the emitted light without deteriorating other characteristics. SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art and to provide a novel external cavity semiconductor laser having high and stable optical output wavelength accuracy.

[0017]

According to the present invention, there is provided a semiconductor laser comprising a semiconductor optical amplifier and a resonator formed by a pair of reflectors for mutually reflecting the optical output of the semiconductor optical amplifier. A first reflector having wavelength selectivity built in the vicinity of the active layer of the semiconductor optical amplifier; and a second reflector disposed on the outgoing optical path outside the semiconductor optical amplifier. And the second reflector is formed of an optical material. An external cavity semiconductor laser is provided.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An external cavity type semiconductor laser according to the present invention has a wavelength selectivity built into a semiconductor optical amplifier (hereinafter referred to as "SOA") as one of the reflectors constituting the resonator. The main feature is that a reflector having a stable temperature characteristic is mounted on the outside of the SOA as the other reflector.

That is, in a conventional external cavity semiconductor laser using a fiber grating, one of the reflectors constituting the resonator is used as an end surface of the SOA. Since this reflector has almost no wavelength selectivity, in order to increase the accuracy of the optical output wavelength, the length of the fiber grating, that is, the length of the external resonator is increased, and as a result, the frequency characteristics are deteriorated. Was. On the other hand, in the external cavity semiconductor laser according to the present invention, the wavelength accuracy is improved because the reflector having the wavelength selectivity formed inside the SOA is used as one of the reflectors. Therefore, it is not necessary to increase the length of the resonator.

In a conventional external cavity type semiconductor laser having another configuration, both of the reflectors constituting the resonator are formed of SO.
A. For this reason, the reflector is made of a semiconductor having inferior temperature characteristics as an optical material, which causes deterioration of the temperature characteristics of the semiconductor laser. On the other hand, in the external cavity semiconductor laser according to the present invention, one of the reflectors forming the resonator is a reflector formed of an optical material having excellent temperature characteristics of refractive index. Therefore, the temperature characteristics of the entire resonator do not deteriorate.

Note that the diffraction wavelength of the reflector formed inside the SOA, the diffraction wavelength of the reflector mounted outside the SOA, and S
The difference between the wavelength of the OA gain peak and the wavelength is 20 nm.
The following is preferred. The reason is that, in the semiconductor laser according to the present invention, since each reflector has wavelength selectivity, if the selected wavelength of the reflector is different from the gain peak of the SOA, the reflection loss increases significantly. This is because the efficiency of light output is deteriorated.

In the external cavity type semiconductor laser according to the present invention, the reflector having wavelength selectivity built into the SOA may be a distributed feedback type (hereinafter, referred to as "DFB"). And a distributed reflection type (hereinafter, “DB
R ").

On the other hand, as a reflector mounted outside the SOA, a fiber grating using a silica fiber having a small change in refractive index can be preferably used.
However, it is not limited to this.

Further, according to a preferred embodiment of the present invention, the efficiency of the reflector can be improved by inserting an optical component between the SOA incorporating one of the reflectors and the external reflector. As such an optical component, an optical lens supplied as a single component can be used, but the end face of an optical fiber used as an external reflector and an output port can be processed into a convex shape and replaced. .

Hereinafter, the present invention will be described in more detail with reference to the drawings. However, the following disclosure is merely an example of the present invention, and does not limit the technical scope of the present invention.

[0026]

FIG. 1 is a diagram showing a specific configuration example of an external cavity semiconductor laser according to the present invention. In the figure, components common to those of the conventional semiconductor laser shown in FIG. 7 are denoted by common reference numerals, and detailed description is omitted.

As shown in FIG. 1, the external cavity type semiconductor laser mainly includes an SOA 10 and an optical fiber 20. Here, the SOA 10 has a distributed feedback reflector 14 formed in a layer adjacent to the active layer 11. On the other hand, the optical fiber 20 has a fiber grating 23 near the tip. The distributed feedback reflector 14 and the fiber grating 23 constitute a resonator as a pair of reflectors.

FIG. 2 is a diagram showing another configuration example of the external cavity type semiconductor laser according to the present invention. In FIG.
Components common to those in other drawings are denoted by common reference numerals, and detailed description is omitted. As shown in FIG. 1, in this semiconductor laser, a resonator is formed using a distributed reflection type reflector 15 formed inside the SOA 10 as one reflector.

FIG. 3 is a view showing still another configuration example of the external cavity semiconductor laser according to the present invention. Also in this figure, components common to those in the other figures are denoted by common reference numerals, and detailed description is omitted.

The semiconductor laser shown in FIG. 3 has basically the same configuration as the semiconductor laser shown in FIG. However, the optical lens 30 disposed between the SOA 10 and the optical fiber 20 and the end face shape of the optical fiber 20 have unique characteristics.
That is, by condensing the light reciprocating between the SOA 10 and the optical fiber 20 by the optical lens 30, the coupling efficiency between the SOA and the optical fiber can be improved and the laser can be generated efficiently. Further, by using such an optical lens, the end face 25 of the optical fiber 20 can be formed obliquely. In the end face of the optical fiber thus formed, so-called end face reflection can be eliminated.

FIG. 4 is a graph showing the relationship between the wavelength characteristic of each member of the external cavity semiconductor laser shown in FIG. 1 and the loss due to the reflector.

The reflector formed on the SOA and the reflector formed on the optical fiber as a fiber grating have wavelength selectivity. Therefore, as shown in the figure, at wavelengths deviating from each selected wavelength, the return loss increases significantly. Therefore, the difference between the peak of the gain of the SOA, the diffraction wavelength of the reflector formed in the SOA, and the diffraction wavelength of the reflector mounted outside the SOA is preferably 20 nm or less.

FIG. 5 is a diagram showing still another configuration example of the external cavity semiconductor laser according to the present invention. Also in this figure, components common to those in the other figures are denoted by common reference numerals, and detailed description is omitted.

This semiconductor laser has a unique configuration in that a fiber grating constituting one reflector of the resonator is separated from an optical fiber functioning as an output port from the semiconductor laser. .

That is, the optical fiber 20 arranged on the left side of the SOA 10 in the figure has a fiber grating 23 formed in a part of the core 21 and functions as one reflector of the resonator. On the other hand, the optical fiber 20a arranged on the right side of the SOA in the figure is an optical fiber as a simple optical waveguide. Another reflector in this semiconductor laser is a distributed feedback reflector 14 built in the SOA 10. This reflector can be similarly configured using a distributed reflection type reflector.

In the semiconductor laser thus configured, S
An optical isolator can be easily inserted between the OA and the optical fiber.

An external cavity semiconductor laser according to the present invention having the structure shown in FIG. 1 was actually manufactured. 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 was used, and a 6 mm long core
To form a fiber grating. Also,
For comparison, a semiconductor laser having the conventional structure shown in FIG. 7 was separately manufactured using the same material and used for comparison.

The output of each external cavity semiconductor laser was measured with a spectrum meter while changing the temperature. FIG. 6 shows the measurement results. As shown in the figure, it was confirmed that the transmission wavelength of the external cavity semiconductor laser according to the present invention was stable against a change in temperature.

[0039]

As described above in detail, in the external cavity type semiconductor laser according to the present invention, since all the reflectors constituting the cavity have wavelength selectivity, high frequency characteristics are deteriorated. The wavelength accuracy of the light output can be increased without any problem. Further, since one of the reflectors is made of an optical material having excellent temperature characteristics, the temperature characteristics are stable as a whole.

[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 another embodiment of the external cavity semiconductor laser according to the present invention.

FIG. 3 is a diagram showing still another embodiment of the external cavity semiconductor laser according to the present invention.

FIG. 4 is a graph showing a relationship between wavelength characteristics of each member of the external cavity semiconductor laser shown in FIG. 1 and loss due to a reflector;

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

FIG. 6 is a graph showing measurement results of temperature characteristics of the external cavity semiconductor laser according to the present invention.

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

FIG. 8 is a diagram showing another configuration of a conventional semiconductor laser.

[Explanation of symbols]

10 ・ ・ ・ Optical amplifier, 11 ・ ・ ・ Active layer, 12, 13 ・ ・ ・
Electrode, 14 ... Diffraction grating (distributed feedback type reflector), 1
5, 17, 18 ... Diffraction grating (distributed reflection type reflector), 16
a, 16b: dielectric multilayer film, 20, 20a: optical fiber, 21: core, 22: clad, 23 ...
・ Fiber grating, 30 ・ ・ ・ Optical lens

Claims (7)

[Claims] 1. A semiconductor laser comprising: a semiconductor optical amplifier; and a resonator formed by a pair of reflectors for reflecting output light of the semiconductor optical amplifier, wherein the resonator is a semiconductor optical amplifier. A first reflector having wavelength selectivity built in the vicinity of the active layer, and a second reflector disposed on the outgoing optical path outside the semiconductor optical amplifier, and An external cavity semiconductor laser, wherein the second reflector is formed of an optical material. 2. The external cavity type semiconductor laser according to claim 1, wherein said first reflector is a distributed feedback type reflector arranged on a side of an emission optical path of said semiconductor optical amplifier. An external cavity type semiconductor laser, comprising: 3. An external cavity type semiconductor laser according to claim 1, wherein said first reflector is disposed on an emission optical path of said semiconductor optical amplifier or on an extension thereof. An external cavity type semiconductor laser characterized by the following. 4. The external cavity semiconductor laser according to claim 1, wherein said second reflector has a tip disposed on an output optical path of said optical amplifier. An external cavity type semiconductor laser, which is a fiber grating formed on an optical fiber. 5. The external cavity type semiconductor laser according to claim 1, wherein at least an optical path between the first reflector and the second reflector is provided on the optical path between the first reflector and the second reflector. An external cavity type semiconductor laser comprising one optical lens. 6. The external cavity semiconductor laser according to claim 5, wherein said optical lens is an end face of an optical fiber formed in a convex shape. 7. The external cavity type semiconductor laser according to claim 1, wherein a diffraction wavelength of said first reflector, a diffraction wavelength of said second reflector, and The difference from the peak wavelength of the gain of the semiconductor optical amplifier is
An external cavity type semiconductor laser characterized in that each of them is 20 nm or less.