JP3412584B2 - External cavity type semiconductor laser - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an external cavity type semiconductor laser. More specifically, the present invention relates to a novel structure of an external resonator type semiconductor laser including a laser resonator formed by combining a reflection surface formed on an end surface of a light emitting element and a diffraction grating formed on an optical fiber.

[0002]

2. Description of the Related Art A semiconductor laser is basically composed mainly of a light emitting element as a light source and a laser resonator having a pair of reflectors that mutually reflect the light emitted from the light emitting element. There is.

In the semiconductor laser as described above, one of the pair of reflectors constituting the laser resonator is often an end face of the light emitting element itself or a dielectric reflection film formed on the end face. On the other hand, the other reflector of the laser resonator uses a diffraction grating provided near the end of the optical fiber optically coupled to the light emitting element, in addition to the case where it is formed on the end face of the light emitting element. There is a case.

That is, by forming a periodic refractive index distribution in the vicinity of the incident side end of the optical fiber, it can be used as a reflector for reflecting light of a specific diffraction wavelength. Since a diffraction grating formed in such an optical fiber is called a fiber grating (hereinafter referred to as “FG”), a semiconductor laser provided with a laser resonator having such a configuration is called a fiber grating laser (FGL). Has been. Further, since one of the reflectors is outside the light emitting element, it is classified as an external cavity type semiconductor laser.

In the semiconductor laser having the above structure, F
The oscillation wavelength is determined by the diffraction wavelength of G and the longitudinal mode wavelength. Therefore, it is known that an FG can be manufactured with extremely high accuracy by an interference exposure method or the like, and thus there is an advantage that a desired oscillation wavelength can be realized relatively easily.

FIG. 5 is a diagram schematically showing the basic structure of the external cavity type semiconductor laser as described above.

As shown in FIG. 1, the external cavity type semiconductor laser includes a semiconductor optical amplifier 1 as a light source, a series of optical systems 2a and 2b for converging light emitted from the semiconductor optical amplifier 1, and an optical system 2a. It is mainly composed of an optical fiber 3 optically coupled to the semiconductor optical amplifier 1 via 2b.

Here, a high reflection film 1a is deposited on one end surface of the semiconductor optical amplifier 1 and ideally completely reflects the light generated by the semiconductor optical amplifier 1 itself. on the other hand,
An extremely low reflection film 1b is applied to the surface facing the high reflection film 1a, and the light generated by the semiconductor optical amplifier 1 is emitted from this surface.

On the other hand, a diffraction grating 3a is formed near the incident end of the optical fiber 3. The diffraction grating 3a has a periodic refractive index distribution and reflects light of a specific wavelength.
Therefore, the high-reflection film 1a and the diffraction grating 3a formed on the semiconductor optical amplifier 1 described above mutually reflect light to cause laser oscillation. Normally, the end surface of the optical fiber 1 is obliquely polished or a low reflection film is applied so that the reflected light generated at the end surface of the optical fiber 3 does not return to the semiconductor optical amplifier 1.

In the external resonator type semiconductor laser as described above, one of the reflectors (FG3a) constituting the laser resonator is arranged outside the semiconductor optical amplifier 1, so that the physical properties of the laser resonator are not provided. There is no choice but to increase the length. For this reason, it has been known that the relaxation oscillation time taken from the introduction of light into the laser resonator to the generation of coherent light becomes long, and there is a limit in increasing the modulation frequency. Therefore, various ideas have been proposed and implemented, for example, by processing the end face of the optical fiber 3 into a spherical shape and omitting the insertion of the optical system.

In the external cavity type semiconductor laser as described above, the oscillation wavelength is the gain characteristic of the semiconductor optical amplifier,
It is determined by the reflection bandwidth of the diffraction grating, the longitudinal mode of the laser cavity, and the longitudinal mode of the semiconductor optical amplifier. If the reflectance of the non-reflective surface 1b of the semiconductor optical amplifier 1 is completely zero, it is not necessary to consider the longitudinal mode of the semiconductor optical amplifier. However, in reality, it takes an inevitable finite value and cannot be ignored.

[0012]

By the way, when the reflection band width of the diffraction grating is narrowed in order to narrow the oscillation wavelength in the semiconductor laser, the number of longitudinal modes existing in this band gradually decreases. . However, when the current for driving the semiconductor optical amplifier changes or the element temperature changes while the longitudinal mode is about several, the longitudinal mode wavelength changes. Therefore, in the reflection band of the diffraction grating, a phenomenon called "mode hopping" occurs in which one longitudinal mode jumps to another longitudinal mode. When such mode hopping occurs, discontinuity called kink occurs in the IL characteristic (driving current-emission output characteristic) of the semiconductor laser, which is not preferable.

Therefore, conventionally, when the semiconductor laser is actually used, it is necessary to incorporate a temperature control element or a circuit into the semiconductor laser module to manage the temperature, but the scale of the laser device becomes large. In addition, many man-hours and adjustments are required, which is one of the causes for increasing the cost of the semiconductor laser.

Conversely, mode hopping can be suppressed by expanding the reflection bandwidth of the diffraction grating so that a plurality of longitudinal modes are always included in the bandwidth. However, in this case, the oscillation spectrum is also broadened, and therefore, particularly in a system in which wavelength combining is performed by an interferometric multiplexer, the loss in the multiplexer becomes extremely large, so that it cannot be used practically.

Further, by forming a diffraction grating used as one of the reflectors at a position away from the end face of the optical fiber, the resonator length is increased and the longitudinal mode interval is narrowed, so that it exists in the reflection spectrum of the diffraction grating. It is also considered to increase the number of vertical modes. However, in this case, the characteristics of the diffraction grating are easily affected by the outside,
For example, when stress is applied to the optical fiber, an unexpected problem occurs such that the oscillation wavelength shifts.

Therefore, it is an object of the present invention to solve the above-mentioned problems of the prior art and to provide a novel semiconductor laser having an oscillation wavelength of a sufficiently narrow band without expanding the circuit scale.

[0017]

According to the present invention, one of a pair of end faces facing each other has a high reflection surface and the other has a low reflection surface, and a semiconductor optical amplification element and a diffraction grating functioning as a reflector. And an optical fiber optically coupled to the low reflection surface side of the amplification element, and the high reflection surface of the semiconductor amplification element and the diffraction grating are configured to form an optical resonator. An external cavity semiconductor laser,
An external resonator type semiconductor laser is provided, which is provided outside the optical resonator in the optical fiber and includes a return light generation reflector for returning reflected light into the optical resonator.

As the return light generating reflector, a second diffraction grating formed separately from the diffraction grating functioning as a part of the laser resonator can be used. Alternatively, it is also possible to form a region where the refractive index specifically changes in the core of the optical fiber and use the reflection generated in this region as the return light.

Here, the reflectance r of the return light generation reflector at the oscillation wavelength λ of the external cavity type semiconductor laser is
It is preferable that the reflectance of the diffraction grating forming the optical resonator is smaller than R at the oscillation wavelength and larger than 10 ⁻⁴ / (1−R). That is, when the reflectance of the return light generation reflector is higher than the reflectance R of the diffraction grating, the reflection at this reflector becomes dominant, and as a result, one end face of the laser resonator is not the diffraction grating. It effectively becomes this second reflector. On the other hand, when it is smaller than 10 ⁻⁴ / (1−R), the amount of reflected return light becomes insufficient and the effect of the return light cannot be expected.

Further, by tilting the diffraction axis of the second diffraction grating with respect to the optical axis of the optical fiber, the distance between the second diffraction grating and the diffraction grating functioning as one end face of the laser resonator is increased. Can be suppressed. That is,
The normal line of at least one of the reflection surface of the diffraction grating forming the optical resonator and the reflection surface of the return light generation reflector is inclined within a range of 2 ° or less with respect to the optical axis of the optical fiber. Is preferred.

The angle of 2 ° was determined as follows. That is, the core of the silica optical fiber has a refractive index of 1.448 for light having a wavelength of 1.55 μm, and the cladding has a refractive index of 1.444. The angle θ _{t at} which light is totally reflected in the optical waveguide formed of such a material is θ _t = sin
It is known to be 4 ° from ^-1 (1.444 / 1.448).
Therefore, in order to propagate the propagating light in the optical fiber,
The incident angle of the propagating light at the core / clad interface is required to be smaller than the angle θ _t . On the other hand, when the angle of the normal line and an optical fiber of the returning light generating reflector and theta _r, the reflected light will be at an angle of 2 [Theta] _r to the propagation optical axis of the optical fiber. Therefore, in order for the propagating light to be reflected by the reflector and propagate in the optical fiber, the condition that 2θ _r is smaller than the above θ _t is essential. That is,
[2θ _r <θ _t = 4 °], and the condition of θ _r <2 ° needs to be satisfied.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described more specifically with reference to the drawings, but the following description is merely one example of the present invention, and does not limit the technical scope of the present invention. is not.

FIG. 1 is a partially cutaway view showing a typical structure of an external cavity type semiconductor laser which is a subject of the present invention.

As shown in the figure, this semiconductor laser is
A semiconductor optical amplifier 31, a monitor light receiving element 32, a semiconductor optical amplifier temperature adjusting element (Peltier effect element), etc. are housed in a so-called butterfly type package 10. In addition, a single mode optical fiber 20 formed with a diffraction grating
Are coupled to the semiconductor optical amplifier 31 through a series of members 41, 42, 43, and these members 41, 42, 43 are respectively positioned so as to obtain a good optical coupling between them. .

FIG. 2 is a sectional view of the semiconductor laser shown in FIG. 1 taken along a vertical plane including the propagation optical axis.

As shown in the figure, the semiconductor optical amplifier 31 is
It is mounted on a submount 31a made of a good heat conductor material, and the submount 31a is mounted on the base 30. The monitor light-receiving element 32 is mounted on another submount 32a which is also made of a good heat conductor material, and this submount 32a is also on the common base 30. Further, the base 30 is mounted on the Peltier effect element 34.

Further, the front end of the base 30 is raised upward, and a member group for adjusting the optical axis is provided at the rising portion.
41 to 43 are fixed.

That is, the optical fiber 20 on which the diffraction grating is formed.
Is a position adjustment in the Z-axis direction after being inserted into the ferrule 21 (an operation for determining the relative distance to the semiconductor optical amplifier,
Usually, the distance between them is set to 0.1 mm or less), and they are fixed to the member 41 at the bonding points 21a so as to maintain the state. Next, the member 41 is slid on the base rising portion 30b to align the optical axes of the fiber 20 and the semiconductor optical amplifier 31 in the XY directions, and after adjustment, they are fixed at the bonding points 41a. Further, the member 42 is placed on the member 41 and fixed at the adhesion point 42a. Here, the member 43 is first fixed to the ferrule 21 at the bonding point 21a, and then the optical axes of the fiber 20 and the semiconductor optical amplifier 31 are adjusted again. This is a readjustment necessary due to the positional deviation that occurs when welding the bonding point 41a.

In the above optical axis adjustment, the bonding point 21a is used as the fulcrum, the bonding point 21b is used as the power point, and the tip of the optical fiber 20 is used as the point of action. Appears as a small displacement at the tip of the optical fiber 20, so fine adjustment is possible.

Further, each fixing is performed by welding using a YAG laser, for example. Therefore, the members 41 to 43, the ferrule 21,
The material of the base 30 and the like is preferably made of metal (for example, SUS) capable of YAG welding. However, when a resin material is used, it can be fixed with an adhesive.

In the semiconductor laser configured as described above, the diffraction grating and the refractive index discontinuity used as the reflector are formed in the region of the fiber 20 housed in the ferrule 21. That is, the diffraction grating is formed by using a phase grating mask after removing the coating of the fiber, and then inserted into the ferrule.

In some cases, the tip of the optical fiber 20 is curved to give the function of a condenser lens. This is to shorten the length of the external optical resonator formed between the semiconductor optical amplifier rear end and the semiconductor optical amplifier side diffraction grating,
By giving an optical function to the tip of the optical fiber, it becomes possible to omit a part of the optical component. In particular, when operating the external cavity type semiconductor laser at a high speed, such a configuration is preferable in order to shorten the relaxation time depending on the cavity length. On the other hand, in the case where high-speed modulation is not required like the excitation light source of an optical amplifier, such consideration is unnecessary. The present invention is effective in any of its applications.

[Embodiment 1] FIG. 3 is a diagram schematically showing the functional structure of an external cavity type semiconductor laser according to the present invention, which is produced by the physical structure shown in FIGS. 1 and 2. Is.

As shown in the figure, this semiconductor laser differs from the conventional semiconductor laser shown in FIG. 5 in that a second diffraction grating 3b is provided in the optical fiber 3. The remaining structure is common to the conventional semiconductor laser shown in FIG.

For the sake of convenience of explanation, in the following description, the diffraction grating 3a conventionally provided for constructing a laser resonator is referred to as a "main diffraction grating", and the present invention is used to generate return light. The diffraction grating 3b newly provided in 1. will be referred to as "sub-diffraction grating".

The sample 1 having the above-described structure is a semiconductor laser which can be used as a pump light source for an optical fiber amplifier having an emission wavelength of 0.98 μm. Here, as each diffraction grating (main diffraction grating and sub diffraction grating), the diffraction wavelength
A 0.98 μm diffraction grating was formed by irradiating the optical fiber with ultraviolet rays using a phase shift mask. Here, the reflection bandwidth of the main diffraction grating (half-width: FWH
M = Full Width at Half Maxima) was 1 nm, and the maximum reflectance was 5%. On the other hand, the sub-diffraction grating has a half width of 5
nm and maximum reflectance was 1%. The distance between them was 1 mm. Table 1 shows the specifications of Sample 1 collectively. In addition, the measured characteristics of Sample 1 are shown in FIG.

[0037]

[Table 1]

Here, the diffraction wavelength in the 0.98 μm band is ±
5 nm is allowed. Further, the main / sub diffraction wavelengths can be made different within this range. Furthermore, the maximum reflectance of the main diffraction grating is determined in relation to the light intensity to be extracted (the light intensity decreases as the reflectance increases), and the full width at half maximum is the active material used in the optical amplifier (in this case, Er:
Erbium) is determined by the characteristics of the quantum level (diffraction wavelength is the same).

Sample 2 was produced as a semiconductor laser which can be used as a pump light source having an emission wavelength of 1.48 μm. Both of the two diffraction gratings had a diffraction wavelength of 1.48 μm, the main diffraction grating had a half width of 1 nm and a maximum reflectance of 3%, and the sub diffraction grating had a half width of 5 nm and a maximum reflectance of 1%. The distance between them is 0.
It was set to 5 mm. Table 2 shows the specifications of Sample 2 collectively. Further, the measured characteristics of Sample 2 are shown in FIG.

[0040]

[Table 2]

Even in the 1.48 μm band, the diffraction wavelength is allowed to have a width of ± 10 nm or more (because the width of the excitation level of erbium is wide), and the diffraction wavelength should be different for the main and the sub. Is also possible in this range.

Therefore, samples 3 and 4 were also manufactured by changing the specifications as shown in Tables 1 and 2 below. The characteristics of these samples are shown in FIGS. 8 and 9.

[0043]

[Table 3]

[0044]

[Table 4]

As can be seen from FIGS. 6 to 9, 0.01 to 0.1 W / A was obtained as the maximum value of the kink in the region where the injection current of the semiconductor optical amplifier was 600 mA or less in each of the above samples. On the other hand, this value is 0.5 W / A or more in an ordinary semiconductor laser having no return light generation means, and it was confirmed that the configuration according to the present invention contributes to output stability. Here, the kink value is IL
(Applied current-output light intensity) dL when the curve is first differentiated
The maximum value of / dI.

The distance between the main / sub diffraction gratings is not particularly limited to the above value. However, when the sub-diffraction grating is provided outside the ferrule, the sub-diffraction grating is directly affected by the stress applied to the optical fiber, so that the diffraction characteristics, reflectance, etc. may change. On the other hand, if the effective reflection position of the sub-diffraction grating is outside the resonator, the same effect can be obtained by forming the main / slave diffraction gratings in the same region.

[Embodiment 2] In this embodiment, a sample in which any one of the main diffraction gratings is tilted so that the normal line of the iso-refractive index surface does not coincide with the optical axis of the sub-diffraction grating. Was produced. The tilted diffraction grating can be formed simply by arranging the phase shift mask obliquely with respect to the optical axis of the fiber and irradiating with ultraviolet light. The rest of the configuration is common to that of the semiconductor laser shown in FIG.

In Sample 5, the normal line of the iso-refractive index surface of the main diffraction grating is formed with an inclination of 0.5 ° with respect to the optical axis. With such a configuration, multiple reflection between the main and sub diffraction gratings is suppressed. Table 5 shows the specifications of the sample 5 having such a configuration.

[0049]

[Table 5]

The inclination angle at this time is as follows:
In order to satisfy the condition of total reflection at the interface of the clad,
The selection is preferably made so as to satisfy the following formula.

[0051]

[Formula 1] 90 ° -2θ> θ _c = sin ^-1 (n ₂ / n ¹ ) = sin ^-1
(1.444 / 1.448) ≈ 86 °

Here, n ₁ is the refractive index of the core, and n ₂ is the refractive index of the cladding.

The distance between the main and sub diffraction gratings must be 70 μm or more under the same conditions. However, if the diffraction grating is manufactured using a normal phase shift mask, the latter condition can be satisfied without any problem. .

Therefore, in Sample 6, the iso-refractive index surface of the sub-diffraction grating is inclined by 1 ° with respect to the optical axis of the optical fiber. Further, in Sample 7, both the main and sub diffraction gratings were tilted by 1 ° in directions opposite to each other with respect to the optical axis. The specifications of these samples 6 and 7 are shown in Table 6 and Table 7, respectively.

[0055]

[Table 6]

[0056]

[Table 7]

The characteristics of the measured samples 5, 6 and 7 are shown in FIG. 11 and FIG.
12 and 13 respectively. As can be seen from FIGS. 11 to 13,
When the uniform refractive index surface of the diffraction grating was tilted, the maximum value of kink of each semiconductor laser was 0.01 to 0.05 W / A. The improvement in the kink value compared to the sample in which the main / sub diffraction gratings are not tilted is considered to be because the influence of multiple reflection between both diffraction gratings was avoided.

[Embodiment 3] FIG. 4 is a diagram schematically showing the functional structure of another example of the external cavity type semiconductor laser according to the present invention.

As shown in the figure, in the present embodiment, with respect to the semiconductor laser shown in FIG. 3, a refractive index discontinuity region 3c is formed in the optical fiber 3, and this is used for reflection light generation for returning light. It is configured to be used as a container. The rest of the configuration is common to that of the semiconductor laser shown in FIG.

As a semiconductor laser having the above structure,
Samples 8 to 14 were actually manufactured. The refractive index discontinuous portion 3c can be formed by placing an ultraviolet ray mask on the optical fiber and irradiating the specific portion with the ultraviolet ray through the mask. The position and characteristics of the refractive index discontinuous portion 3c can be arbitrarily set by appropriately selecting the size and shape of the opening of the mask used.

Among the samples prepared here, Sample 8 to Sample
Reference numeral 11 shows the case of a normal diffraction grating whose main refractive index surface is perpendicular to the optical axis of the fiber. At this time, the refractive index discontinuity surface can be arranged anywhere outside the external optical resonator formed by the main diffraction grating. Therefore, in Samples 8 to 10, the distance from the main diffraction grating is set to about several mm, but in Sample 11, it is formed so as to overlap the main diffraction grating. The specifications of Samples 8 to 11 are shown in Tables 8 to 11, respectively.

[0062]

[Table 8]

[0063]

[Table 9]

[0064]

[Table 10]

[0065]

[Table 11]

On the other hand, in Samples 12 to 14, at least one of the uniform refractive index surface and the refractive index discontinuous surface of the main diffraction grating is used for the propagation light of the optical fiber, as in the case of the second embodiment. Tilted with respect to the axis. At this time, the inclination angle is required to be within 2 ° as described above. Sample 12 to Sample 14
Tables 12 to 14 show the specifications of each.

[0067]

[Table 12]

[0068]

[Table 13]

[0069]

[Table 14]

The measured characteristics of Samples 8 to 14 are shown in FIGS.
Shown in 19 respectively. As can be seen from the results shown in FIGS. 14 to 19, the kink value of the IL curve of the sample of this example is 0.01 to 0.2 W / in the case of Sample 8 to Sample 10 in which the reflecting surface has no inclination.
A, 0.01 in the case of Sample 12 to Sample 14 with the reflecting surface inclined
All were very good at 0.1 W / A.

In fact, in a system using a semiconductor laser, a discontinuity of the refractive index always exists on the optical path. That is, for example, reflection of propagating light occurs at a refractive index discontinuity formed inevitably at a connector connection point, a fiber-fiber connection point, or the like.
However, the reflection of light at such a discontinuity eventually becomes a mere noise source. The reason is that the reflected return light caused by such a discontinuity has a random polarization direction, and when it is returned to the optical resonator having polarization dependence, its influence varies. Is.
On the other hand, in the external resonator type semiconductor laser according to the present invention, the return light generation reflector is provided in the vicinity of the resonator to make the polarization direction of the return light constant and reduce the kink value in the IL curve. It realizes the stabilization of the optical output.

[0072]

As described in detail above, the external cavity type semiconductor laser according to the present invention suppresses the occurrence of kinks without using additional elements or circuits as in the case of temperature control. A stable IL characteristic is realized. Further, since such an action can be obtained regardless of the FG formation position, the specifications of the semiconductor laser are not limited.

Further, the basic characteristic of FGL that it is easy to realize a desired oscillation frequency is maintained as it is, and it can be advantageously used in a wavelength division multiplexing optical communication system or the like.

[Brief description of drawings]

FIG. 1 is a partially cutaway view showing a physical configuration example of an external cavity type semiconductor laser according to the present invention.

FIG. 2 is a cross-sectional view of the external cavity type semiconductor laser shown in FIG.

FIG. 3 is a diagram schematically showing a functional configuration example of an external cavity type semiconductor laser according to the present invention.

FIG. 4 is a diagram schematically showing a functional configuration example of an external cavity type semiconductor laser according to the present invention.

FIG. 5 is a diagram schematically showing a typical configuration of a conventional external cavity type semiconductor laser.

FIG. 6 is a semiconductor laser sample 1 manufactured according to the present invention.
It is a graph which shows the characteristic of.

FIG. 7 is a semiconductor laser sample 2 manufactured according to the present invention.
It is a graph which shows the characteristic of.

FIG. 8 is a semiconductor laser sample 3 manufactured according to the present invention.
It is a graph which shows the characteristic of.

FIG. 9 is a semiconductor laser sample 4 manufactured according to the present invention.
It is a graph which shows the characteristic of.

FIG. 10 is a graph showing characteristics of a semiconductor laser sample 5 manufactured according to the present invention.

FIG. 11 is a graph showing characteristics of a semiconductor laser sample 6 manufactured according to the present invention.

FIG. 12 is a graph showing characteristics of a semiconductor laser sample 7 manufactured according to the present invention.

FIG. 13 is a graph showing characteristics of a semiconductor laser sample 8 manufactured according to the present invention.

FIG. 14 is a graph showing characteristics of a semiconductor laser sample 9 manufactured according to the present invention.

FIG. 15 is a semiconductor laser sample manufactured according to the present invention.
11 is a graph showing 10 characteristics.

FIG. 16 is a semiconductor laser sample manufactured according to the present invention.
11 is a graph showing characteristics of 11.

FIG. 17 is a semiconductor laser sample manufactured according to the present invention.
13 is a graph showing 12 characteristics.

FIG. 18: Semiconductor laser sample manufactured according to the present invention
13 is a graph showing characteristics of 13.

FIG. 19 is a semiconductor laser sample manufactured according to the present invention.
14 is a graph showing 14 characteristics.

[Explanation of symbols]

1 ... Semiconductor optical amplifier, 2a, 2b ... Optical system, 3 ... Optical fiber, 3a, 3b ... Diffraction grating, 10 ... Package, 20 ... optical fiber, 30 ... base, 31 ... Semiconductor optical amplifier, 31a, 32a ... Submount, 32 ... Monitor light receiving element

Claims (4)

(57) [Claims] 1. A semiconductor optical amplifier having one of a pair of end faces facing each other as a high reflection surface and the other as a low reflection surface, and a diffraction grating functioning as a reflector are formed. An external resonator type semiconductor laser having an optical fiber optically coupled to a low reflection surface side, and the high reflection surface of the semiconductor amplification element and the diffraction grating are configured to form an optical resonator. Te, outside the optical resonator in the optical fiber, comprising a return light generation reflector to return the reflected light to the optical resonator,該戻Ri light
Oscillation waves of this external-cavity semiconductor laser of the generating reflector
The reflectivity r in the length is the diffraction pattern that forms the optical resonator.
Smaller than the reflectance R of the child at the oscillation wavelength, 10 ⁻⁴ /
An external cavity type semiconductor laser, which is larger than (1-R) . 2. The external resonator type semiconductor laser according to claim 1, wherein the return light generation reflector is a second diffraction grating formed in the optical fiber. Type semiconductor laser. 3. The external resonator type semiconductor laser according to claim 1, wherein the return light generation reflector is a region where the refractive index is discontinuously changed in the core of the optical fiber. External cavity type semiconductor laser. 4. The by external cavity type semiconductor laser according to any one of claims 1 to 3, the reflection plane or the return light generation reflector gratings forming the optical resonator The external cavity semiconductor laser is characterized in that at least one of normal lines of the reflecting surfaces makes an angle with the optical axis of the optical fiber of 2 ° or less and greater than 0 °.