JP7453519B2 - Oscillation adjustment method - Google Patents

Description

The present invention relates to a method for adjusting oscillation, and particularly to a method for adjusting oscillation of an external cavity semiconductor laser using a volume Bragg grating.

An external cavity semiconductor laser is known that operates in a single longitudinal mode by constructing an external cavity using a volumetric Bragg diffraction grating (for example, see Patent Document 1). In an external cavity semiconductor laser, laser oscillation with a narrow linewidth is possible only at a specific wavelength by selectively optically feeding back only a part of the spectrum using a wavelength selection grating. In particular, compared to conventional methods (for example, methods that use the wavelength selectivity of the diffraction grating), external cavity semiconductor lasers using a volumetric Bragg diffraction grating can oscillate even with angular shifts because they use Bragg diffraction. It has properties such as its wavelength is difficult to change.

A typical diffraction grating diffracts an incident light beam into different angles depending on the wavelength. Since diffraction is visible over a wide range of incident angles, oscillation adjustment can be made using the diffracted beam as a guide. On the other hand, a volume Bragg diffraction grating is based on Bragg diffraction and therefore has a high degree of wavelength selectivity. A volumetric Bragg diffraction grating diffracts only a wavelength component satisfying a Bragg condition (Bragg wavelength) at a predetermined incident angle, and transmits other light. When an external resonator is constructed using a volumetric Bragg grating, only a small portion of the light is diffracted because the spectral width of a single semiconductor laser is wider than the wavelength width of the volumetric Bragg grating. Therefore, it is difficult to see the diffracted light that serves as a guide, and it is difficult to adjust the laser oscillation.

Japanese Patent Application Publication No. 2015-88505

An object of the present invention is to facilitate the oscillation adjustment of an external cavity semiconductor laser using a volumetric Bragg grating.

In one aspect of the present invention, the oscillation adjustment method includes:
branching light from a semiconductor laser into a first optical path and a second optical path, forming an external resonator in the first optical path and causing single longitudinal mode oscillation;
A volume Bragg diffraction grating is arranged in the second optical path, and an angle of the volume Bragg diffraction grating with respect to the optical axis of the second optical path is adjusted so that the diffracted light by the volume Bragg diffraction grating returns to the semiconductor laser. ,
After adjusting the angle of the volume Bragg diffraction grating, the first optical path is blocked and the position of the volume Bragg diffraction grating is adjusted so that the volume Bragg diffraction grating oscillates.

The above method facilitates oscillation adjustment of an external cavity semiconductor laser using a volumetric Bragg grating.

FIG. 3 is a schematic diagram of the first step of the oscillation adjustment method according to the embodiment. FIG. 6 is a schematic diagram of the second step of the oscillation adjustment method according to the embodiment. FIG. 6 is a schematic diagram of the third step of the oscillation adjustment method according to the embodiment. FIG. 7 is a schematic diagram of the fourth step of the oscillation adjustment method according to the embodiment. FIG. 3 is a schematic diagram showing single longitudinal mode oscillation by a reflector, laser output light, and a design wavelength of a volume Bragg diffraction grating. FIG. 2 is a schematic diagram illustrating the characteristics of a volume Bragg diffraction grating. FIG. 2 is a schematic diagram illustrating the characteristics of a volume Bragg diffraction grating. It is a flow chart of an oscillation adjustment method of an embodiment.

1A to 1D are schematic diagrams showing the procedure of the oscillation adjustment method according to the embodiment. As mentioned above, a Volume Bragg Grating (hereinafter abbreviated as "VBG" as appropriate) differs from a normal diffraction grating in that it diffracts light only when light of a specific wavelength is incident at a specific angle. . Therefore, when an external resonator is configured with VBG and a semiconductor laser is caused to oscillate in a single longitudinal mode, it is difficult to see the light that serves as a guide for adjustment even with a visible light laser, making it difficult to adjust the oscillation. Therefore, in the embodiment, the oscillation adjustment is facilitated by concentrating the spectrum in the vicinity of the Bragg wavelength in advance.

In the embodiment, the oscillation adjustment of an external cavity laser diode (hereinafter referred to as "ECLD") using VBG is facilitated by adjusting according to the procedures shown in FIGS. 1A to 1D.

(1st step)
In FIG. 1A, output light from a semiconductor laser 1 (hereinafter referred to as "LD1") is divided into a first optical path P1 and a second optical path by a polarizing beam splitter 4 (hereinafter referred to as "PBS4"), a partial reflection mirror, etc. The light is branched into an optical path P2, and an external resonator is formed in the first optical path P1 to cause single longitudinal mode oscillation. One end surface of the LD 1 is an antireflection (AR) surface or a partially reflective (PR) surface 1a, and the other end surface is a high reflection surface 1b. The reflectance of the AR surface is preferably less than 5%, more preferably 1% or less. The reflectance of the PR surface is preferably 5% to 50%, more preferably 10% to 30%. The reflectance of the highly reflective surface is preferably 95% or more, more preferably 98% or more.

A wavelength selective reflector 3 is arranged on the first optical path P1, and a resonator is formed by the reflector 3 and the high reflection surface 1b of the LD1. As the reflector 3, a normal diffraction grating may be used, or a configuration in which either an interference filter or a distributed Bragg reflector is combined with a mirror may be adopted. For convenience, the external resonator formed by the reflector 3 is referred to as a "first resonator (hereinafter referred to as "ER1")." This is to distinguish it from an external resonator that will be constructed between VBG and LD1 in a later step.

A half-wave plate 2 may be inserted between the LD1 and the PBS4. The half-wave plate 2 gives a phase difference of 180 degrees to two orthogonal linearly polarized components of the incident light. As a result, when linearly polarized light is incident, the plane of polarization can be rotated to any angle. By rotating the polarization direction of the output light from the LD 1, the angle of incidence and the angle of reflection on the PBS 4 can be adjusted. Furthermore, a collimating lens may be inserted between the LD 1 and the half-wave plate 2.

The branching ratio of the PBS 4 is, for example, 1:1, but is not limited to this, and an appropriate branching ratio can be designed. The polarized light component reflected by the PBS 4 enters the reflector 3, and the return light from the reflector 3 is reflected by the PBS 4, passes through the half-wave plate 2, and enters the active layer of the LD 1. The repetition of light between the reflector 3 and LD1 causes LD1 to oscillate in a single longitudinal mode at the resonant wavelength of ER1.

A wavelength meter 5 is placed on the second optical path P2, and measures the transmitted light of the PBS 4 to monitor the oscillation wavelength of a single longitudinal mode using ER1. The wavelength selection range of the reflector 3 with respect to the optical axis of the first optical path P1 is adjusted so that the wavelength measured by the wavelength meter 5 becomes the target wavelength. The first step is a procedure for causing LD1 to oscillate in a single longitudinal mode. In other words, this is a procedure for narrowing down the broad wavelength spectrum of the LD 1 before oscillation to a specific wavelength range that includes the Bragg wavelength. Even when using an LD1 that outputs visible light, it is difficult to match the oscillation wavelength of the LD1 to the narrow VBG wavelength from the beginning. By performing the first step, the spectrum of the output light of the LD 1 can be narrowed down to an oscillation wavelength near the Bragg wavelength.

The angle of the reflector 3 is such that the resonant wavelength of ER1, that is, the center wavelength of single longitudinal mode oscillation, is slightly on the shorter wavelength side than the design wavelength of the VBG (Bragg wavelength where the direction of incidence is equal to the direction of diffraction). It may be adjusted to shift to . In a later step, when incorporating the VBG into the second optical path P2, the amount of light diffracted by the VBG is smaller than the amount of incident light. When the light diffracted by the VBG oscillates in a single longitudinal mode and follows the same optical path as the light incident on the VBG toward LD1, the diffracted light from the VBG is buried in the incident laser light, and the diffracted light is transferred to the active layer of LD1. Adjustment to return to normal becomes difficult. By setting the resonant wavelength monitored by the wavelength meter 5 to be slightly shorter than the design wavelength of the VBG, the diffracted light of the VBG can be returned to the LD 1 while being slightly deviated from the optical path of the incident light. The details will be described later with reference to FIG. 1B.

FIG. 2 is a diagram illustrating narrowing down of the oscillation wavelength by ER1. When LD1 is a general gain waveguide type laser diode, the spectrum of output light from LD1 before single longitudinal mode oscillation includes many peaks. If we take the average distribution of multiple longitudinal modes, we get a spectrum that spreads over a wide wavelength range, as shown in FIG.

By arranging the reflector 3 at an appropriate angle in the first optical path P1, the LD1 can be caused to oscillate in a single longitudinal mode at the resonant wavelength of the ER1. The angle of the reflector 3 is adjusted so that the resonance peak wavelength of ER1 is on the slightly shorter wavelength side than the design wavelength λ ₀ of the VBG.

If the diffraction wavelength width of VBG is Δλ, the center wavelength of resonance of ER1 λresnt is
λresnt=λ ₀ −bΔλ (0.5<b<3.0)
The angle of the reflector 3 may be adjusted so that. Here, b is a coefficient. The value of b is, for example, greater than 0.5 and less than 3.0, more preferably 1.0 or more and 2.5 or less. When the value of b is 0.5 or less, it is difficult to distinguish between the VBG diffracted light and the incident light. When the value of b exceeds 3.0, the direction of the diffracted light from the VBG deviates too much from the second optical path P2, making it difficult to return the diffracted light to LD1. By appropriately adjusting the angle of the reflector 3 in the first optical path P1, the single longitudinal mode oscillation wavelength of the LD1 by the ER1 can be set to a slightly shorter wavelength side than the wavelength of the diffracted light of the VBG. .

(Second step)
After causing LD1 to oscillate in a single longitudinal mode, as shown in FIG. 1B, VBG7 is placed in second optical path P2, and VBG7 is adjusted relative to the optical axis of second optical path P2 so that the diffracted light Ldif by VBG7 returns to LD1. Adjust the angle.

The VBG 7 is a diffraction grating in which thousands of layers with refractive index anisotropy are formed in glass, and the thickness of the layers (interfaces) in the stacking direction is so thick that it can be called three-dimensional. In the embodiment, VBG7 of 5 mm x 5 mm x 2.5 mm is used as an example. Diffraction occurs under conditions where the light reflected at the interface inside the VBG 7 strengthens each other.

3 and 4 are schematic diagrams illustrating the characteristics of the VBG7. FIG. 3 is a top view of the VBG7. In the VBG 7, many refractive index anisotropic layers 71 are formed at a constant period Λ. Due to the periodic refractive index distribution inside the VBG 7, only light having a limited combination of incident angle θ and wavelength λ _B is diffracted, and other light is transmitted. Generally, the refractive index anisotropic layer 71 is not arranged parallel to the incident plane of the VBG 7, but is formed at a predetermined angle.

In FIG. 4, to simplify the explanation, consider the case where a large number of refractive index anisotropic layers are provided parallel to the incident plane of the VBG 7. The conditions for Bragg diffraction to occur in VBG7 are:
2n ₀ Λcosθ=λ _B
This is when the requirements are met. Here, n ₀ is the refractive index of the glass body. Here, the description will be made assuming that the incident angle (the angle formed by the normal line of the incident surface and the incident light) used in the field of general optics is θ. This condition is called the Bragg condition.

When θ=0, that is, when the incident light is perpendicular to the diffraction surface, λ _B becomes maximum (cos θ=1). When the wavelength λ of the incident light satisfies λ ₀ =2n ₀ Λ, θ=0, and the light is diffracted in the direction of incidence. Here, this λ ₀ is defined as the design wavelength of the VBG.

Actually, the wavelength λ _{in of} the incident light to VBG7 is

を満たすとき、入射光は効率よく回折される。同様に、

When the equation is satisfied, the incident light is efficiently diffracted. Similarly,

の範囲の入射角θinで入射した光が効率よく回折される。

Light incident at an incident angle θin in this range is efficiently diffracted.

In the embodiment, as an example, VBG7 with a design wavelength λ ₀ of 496.23±0.12 nm and a diffraction wavelength width Δλ of about 0.05 nm is used. In the second step, the angle of the diffraction surface of the VBG 7 is adjusted at a position farther from the LD 1 than the final arrangement position of the VBG 7. The reason why the angle of VBG7 is adjusted at a position farther from LD1 is that the farther VBG7 is from LD1, the easier it becomes to distinguish between the light incident on VBG7 on the second optical path P2 and the diffracted light Ldif of VBG7. In the embodiment, as an example, the VBG 7 is placed about 30 cm away from the LD 1 and placed on the second optical path P2.

The light that passes through the PBS4 from the LD1 and enters the VBG7 is light oscillated in a single longitudinal mode by the ER1. As described above, the center wavelength of the light oscillated in a single longitudinal mode is set to be slightly shorter than the design wavelength of the VBG 7. Therefore, almost all of the light contained in the incident laser beam is diffracted in a direction slightly deviating from the optical axis of the incident laser beam, which satisfies the Bragg condition. By increasing the distance from LD1 to VBG7, it becomes easier to recognize the deflection of the diffracted light Ldif. With the diffracted light Ldif visible, the angle of the VBG 7 is adjusted to align the direction of the diffracted light Ldif with the second optical path P2, thereby making the diffracted light Ldif enter the active layer of the LD1 with high precision.

By adjusting the angle of VBG7, the incident light to VBG7 and the diffracted light Ldif from VBG7 are at the same height position (position in the Y direction). Of the light incident on the VBG 7, only the light of the VBG wavelength that is incident at a predetermined angle is diffracted, deviates slightly from the optical axis of the incident light, and propagates in the XZ plane toward the LD1.

The angle of VBG7 may be adjusted so that the oscillation power and monitor current value of LD1 are maximized. When monitoring the oscillation power, a power meter may be placed on the transmission side of the VBG 7 to measure the power. When measuring the current of the output of the LD 1, the angle may be adjusted so that the current value of the photodiode (PD) 8 installed in the LD package is maximized.

The current value monitored by the PD8 represents the intensity of the light that is the sum of the light with the center wavelength λresnt obtained by the ER1 and the diffracted light with the center wavelength _λ0 obtained by the VBG7. Since the angle of the reflector 3 is adjusted when generating single longitudinal mode oscillation in the first step, the angle of VBG7 that maximizes the current value of PD8 is the one that returns the diffracted light Ldif to the active layer of LD1. This is the correct angle.

(3rd step)
In FIG. 1C, the first optical path P1 in which the reflector 3 is placed is blocked, and the VBG 7 is gradually brought closer to the LD1 along the second optical path P2 from the position where the angle was adjusted in the second step. Adjust so that it oscillates in a single longitudinal mode. At this point, the first optical path P1 is interrupted while the reflector 3 remains placed on the first optical path P1. This is because when the diffracted light Ldif from the VBG 7 is lost, the first optical path P1 is immediately optically connected to cause the LD1 to oscillate in a single longitudinal mode.

When blocking the first optical path P1, rotate the half-wave plate 2 in the X-Z plane and adjust so that almost all of the output light of the LD 1 is transmitted to the second optical path P2 where the VBG 7 is arranged. You may.

By adjusting the angle in the second step, the diffracted light Ldif from the VBG7 is adjusted to enter the active layer of the LD1. In the third step, by bringing the VBG 7 close to the LD 1 to an appropriate position, the LD 1 can be caused to oscillate in a single longitudinal mode using only the VBG 7, and the size of the final laser device can be made compact.

For example, a power meter 12 is connected to the transmission side of VBG 7, and the position of VBG 7 is adjusted so that a single peak occurs at maximum power. As an example, the VBG 7 may be moved from a position approximately 30 cm away from the LD 1 to a distance of approximately 5 mm. During the movement of the VBG 7 along the second optical path P2, or once the final position of the VBG 7 is determined, the angle of the VBG 7 with respect to the placement plane of the VBG 7 is adjusted such that the intensity of a single peak is maximized. You may also adjust it again by rotating it in a parallel plane.

(4th step)
In FIG. 1D, after single longitudinal mode oscillation of LD1 with VBG7 alone, unnecessary optical elements except VBG7 and LD1 are removed. In the configuration of the embodiment, the half-wave plate 2, reflector 3, and PBS 4 are removed. LD1 and VBG7 may be arranged within the package of the laser device with adjusted positional and angular relationships. As a result, a high-intensity, narrow-band external cavity type semiconductor laser device can be obtained.

FIG. 5 is a flowchart of the oscillation adjustment method according to the embodiment. The output light of the LD is branched into a first optical path (P1) and a second optical path (P2) (S1). In the embodiment described above, the PBS4 was used to split the reflected light of s-polarized light and the transmitted light of p-polarized light, but a non-polarizing beam splitter with a 1:1 splitting ratio splits the emitted light of the LD into the first one. It may be divided into an optical path (P1) and a second optical path (P2).

Next, an external resonator is formed in the first optical path (P1) to cause the LD to oscillate in a single longitudinal mode (S2). In the embodiment, an ordinary diffraction grating was used as a reflector disposed in the first optical path (P1), and an external resonator was formed between it and the high reflection surface of the LD, but a combination of an interference filter and a mirror , a combination of a distributed Bragg reflector and a mirror, etc. may also be used.

As mentioned above, when oscillating LD1 in a single longitudinal mode with a reflector, by adjusting the angle of the reflector with respect to the first optical path (P1), the oscillation wavelength of the single longitudinal mode can be made to be shorter than the VBG wavelength. It may be set to a slightly shorter wavelength side.

Next, a VBG is placed in the second optical path (P2), and the angle of the diffraction surface with respect to the second optical path (P2) is adjusted (S3). It is desirable that the angle adjustment be performed at a position where the VBG is sufficiently far away from the LD. By making the design wavelength of the VBG slightly longer than the wavelength of the incident light oscillated in a single longitudinal mode, it becomes easier to distinguish the diffracted light of the VBG from the incident light.

Next, the first optical path (P1) is blocked and the position and/or angle of the VBG is adjusted so that the VBG alone oscillates (S4). This position adjustment causes the ECLD to oscillate with a single, maximum peak.

Finally, unnecessary optical elements such as reflectors and beam splitters are removed, and high-intensity, narrow-band laser light is output from the laser device formed by the VBG and LD (S5). The oscillation adjustment method of the embodiment facilitates oscillation adjustment of a semiconductor laser using VBG.

A laser device whose oscillation is adjusted in this manner can output light with a very narrow spectral width, and can be applied to various fields such as environmental measurement, spectroscopic analysis, and medical care.

1 Semiconductor laser 2 Half-wave plate 3 Reflector 4 Polarizing beam splitter 5 Wavemeter 7 Volume Bragg grating 12 Power meter

Claims (6)

branching light from a semiconductor laser into a first optical path and a second optical path, forming an external resonator with a reflector disposed in the first optical path, and causing single longitudinal mode oscillation;
measuring the split transmitted light with a wavelength meter placed in the second optical path to measure the oscillation wavelength of a single longitudinal mode;
Adjusting the angle of the reflector so that the oscillation wavelength measured by the wavelength meter is near the Bragg wavelength and is on the shorter wavelength side than the design wavelength of the volumetric Bragg grating used;
The volume Bragg diffraction grating is arranged in the second optical path, and the angle of the volume Bragg diffraction grating with respect to the optical axis of the second optical path is adjusted so that the diffracted light by the volume Bragg diffraction grating returns to the semiconductor laser. death,
After adjusting the angle of the volume Bragg grating, the first optical path is blocked and the volume Bragg grating is moved along the second optical path so that the volume Bragg grating alone oscillates in a single longitudinal mode. adjusting the position by moving it closer to the semiconductor laser ;
An oscillation adjustment method characterized by: The position adjustment of the volumetric Bragg diffraction grating includes, simultaneously with the movement or after the movement, adjusting the volumetric Bragg diffraction grating to be rotated within a plane parallel to a plane in which the volumetric Bragg diffraction grating is arranged. The oscillation adjustment method according to claim 1 . 3. The oscillation adjustment method according to claim 1 , wherein the reflector is removed after the position adjustment of the volumetric Bragg grating is completed. The oscillation adjustment method according to claim 1 , wherein the reflector is a diffraction grating. The oscillation adjustment method according to claim 1, wherein the reflector comprises either an interference filter or a distributed Bragg reflector, and a mirror. splitting the light from the semiconductor laser into the first optical path and the second optical path by a polarizing beam splitter;
The oscillation adjustment method according to any one of claims 1 to 5 , characterized in that the polarization beam splitter is removed after the position adjustment of the volumetric Bragg grating is completed.

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