JP4984537B2 - External resonator type tunable light source - Google Patents

Description

  The present invention relates to an external resonator type wavelength tunable light source used in the field of optical communication or the field of optical measurement technology.

  As a light source used in optical communication or optical measurement technology, a light source capable of wavelength stability with a single mode oscillation with a narrow spectral line width and a variable wavelength is required. An external resonator type tunable light source has been developed as one of light sources capable of oscillation wavelength. FIG. 11 is a plan view showing the configuration of the external resonator type wavelength tunable light source according to the first conventional example. As shown in FIG. 11, the external resonator type wavelength tunable light source 100 according to the first conventional example includes a laser diode 101, a collimating lens 102, a diffraction grating 103, and a plane mirror 104. The external resonator type wavelength tunable light source shown in FIG. 11 is an external resonator type wavelength tunable light source including a Littman type external resonator.

  The laser diode 101 is a Fabry-Perot type semiconductor laser in which, for example, a lower cladding layer, an active layer, and an upper cladding layer are sequentially formed on a semiconductor substrate, and parallel end surfaces obtained by cleaving the semiconductor substrate are used as resonators. . The laser diode 101 has a non-reflective film (AR coat) 105 formed on one end face forming a resonator. The laser diode 101 is disposed with the end surface on which the antireflective film 105 is formed facing the collimating lens 102 side.

  The collimating lens 102 is positioned with respect to the laser diode 101 so that one focal point is disposed at the laser light emission position on the end surface of the laser diode 101 where the non-reflective film 105 is formed. The collimator lens 102 collimates the laser light emitted from the laser diode 101 and condenses the laser light diffracted by the diffraction grating 103 at the laser light emission position of the laser diode 101. The diffraction grating 103 has a planar diffraction surface 103a in which a grating is formed in a direction orthogonal to the paper surface, and the laser light collimated by the collimating lens 102 and the laser light reflected by the plane mirror 104 are obtained. Diffracts at an angle corresponding to the wavelength. The plane mirror 104 reflects the laser light diffracted by the diffraction grating 103. The flat mirror 104 can move in a plane parallel to the paper surface and can rotate in the same plane.

  In the above configuration, the laser light emitted from the laser diode 101 is converted into parallel light by the collimator lens 102, then enters the diffraction surface 103a of the diffraction grating 103, and is diffracted at an angle corresponding to the wavelength. Of the laser light diffracted by the diffraction grating 103, the laser light diffracted in the direction in which the plane mirror 104 is disposed is reflected by the plane mirror 104 and then enters the diffraction surface 103 a of the diffraction grating 103 again. This laser light is again diffracted by the diffraction grating 103 and travels in the reverse direction along the original optical path, and is condensed by the collimating lens 102 and enters the laser diode 101. A part of the laser light incident on the laser diode 101 is reflected by the end face 101 a of the laser diode 101 and travels in the reverse direction inside the laser diode 101, and a part thereof is emitted from the laser diode 101.

  As described above, the external resonator type tunable light source according to the first conventional example shown in FIG. 11 has a resonator formed by the end face 101a of the laser diode 101 and the flat mirror 104 provided outside the laser diode 101. ing. Here, when the plane mirror 104 constituting the resonator is moved in a plane parallel to the paper surface and rotated in the same plane so that the light diffracted by the diffraction grating 103 is vertically incident, the plane mirror 104 enters the plane mirror 104. The wavelength of the laser light emitted from the end face 101a of the laser diode 101 can be varied because the optical path length of the resonator changes as the wavelength of the laser light changes. Specifically, when the plane mirror 104 is moved in the direction indicated by the reference numeral D11 in FIG. 11 and appropriately rotated, the wavelength of the laser light can be continuously changed to the long wavelength side, and conversely When the plane mirror 104 is moved in the direction indicated by the reference numeral D12 and rotated as appropriate, the wavelength of the laser beam can be continuously varied to the short wavelength side.

  FIG. 12 is a plan view showing a configuration of an external resonator type wavelength tunable light source according to a second conventional example. As shown in FIG. 12, the external resonator type wavelength tunable light source 200 according to the second conventional example includes a laser diode 101, a collimating lens 102, and a diffraction grating 106. In FIG. 12, the same components as those included in the external resonator type wavelength variable light source 100 shown in FIG. The external resonator type tunable light source illustrated in FIG. 12 is an external resonator type tunable light source including a Littrow external resonator. The diffraction grating 106 provided in the external resonator type wavelength tunable light source 200 shown in FIG. 12 has a planar diffraction surface 106 a in which a grating is formed in a direction orthogonal to the paper surface, and is made parallel by the collimating lens 102. The laser beam is diffracted at an angle corresponding to the wavelength. The diffraction grating 106 can move in a plane parallel to the paper surface and can rotate in the same plane.

  In the above configuration, the laser light emitted from the laser diode 101 is converted into parallel light by the collimator lens 102, then enters the diffraction grating 103, and is diffracted at an angle corresponding to the wavelength. Of the laser light diffracted by the diffraction grating 103, the laser light traveling in the reverse direction along the original optical path is condensed by the collimator lens 102 and enters the laser diode 101. A part of the laser light incident on the laser diode 101 is reflected by the end face 101 a of the laser diode 101 and travels in the reverse direction inside the laser diode 101, and a part thereof is emitted from the laser diode 101.

  As described above, the external resonator type wavelength tunable light source according to the second conventional example shown in FIG. 12 has a resonator formed by the end face 101a of the laser diode 101 and the diffraction grating 1064 provided outside the laser diode 101. ing. Here, when the diffraction waist 106 forming the resonator is rotated in a plane parallel to the paper surface and appropriately rotated in the same plane, the wavelength of the laser beam traveling in the opposite direction is changed and the optical path length of the resonator is changed. Therefore, the wavelength of the laser light emitted from the end face 101a of the laser diode 101 to the outside can be varied. Specifically, when the diffraction grating 106 is moved in the direction indicated by the symbol D21 in FIG. 12 and rotated appropriately, the wavelength of the laser light can be continuously varied to the long wavelength side, and conversely When the diffraction grating 106 is moved in the direction indicated by the symbol D22 and is appropriately rotated, the wavelength of the laser beam can be continuously varied to the short wavelength side.

For details of the conventional external resonator type wavelength tunable light source, see, for example, the following Patent Documents 1 and 2.
JP-A-11-68248 JP 2000-353854 A

  By the way, in the external resonator type wavelength tunable light source 100 shown in FIG. 11, all of the laser light diffracted by the diffraction grating 103 and reflected by the reflected light 104 travels backward in the original optical path and returns to the laser diode 101. Is ideal. In other words, the coupling efficiency between the return laser beam and the laser diode 101 is ideally 100%. The same applies to the external resonator type wavelength tunable light source 200 shown in FIG. However, the coupling efficiency actually decreases due to several causes. When the coupling efficiency decreases, it is considered that the oscillation of the external resonator type wavelength tunable light source becomes multimode.

  One factor that causes a reduction in coupling efficiency is that the focal length varies with wavelength due to chromatic aberration of the collimating lens 102. Here, in the visible light region, it is easy to compensate the chromatic aberration of the collimating lens 102 by combining a plurality of types of lenses. However, for example, in the 1.55 μm wavelength band used for optical communication, a combination of a plurality of types of lenses is used. It is difficult to compensate for chromatic aberration. Thus, for example, a method of compensating a chromatic aberration by forming a diffraction grating on the lens surface of the collimating lens 102 can be considered, but the compensation of the chromatic aberration by such a method is far from a practical level.

  FIG. 13 is an aberration diagram showing an example of the aberration of the collimating lens 102. (a) shows the aberration when the wavelength is 1.4 μm, and (b) shows the aberration when the wavelength is 1.5 μm. (C) shows the aberration when the wavelength is 1.65 μm. In FIG. 13, the horizontal axis indicates the positions (PX, PY) in two directions that are included in the plane orthogonal to the laser beam path, and the vertical axis indicates the aberration amount of the laser beam passing through each position. taking it. Assuming that the collimator lens 102 is designed so that the aberration is optimized for light having a wavelength of 1.5 μm, the aberration is compared for light having a wavelength of 1.5 μm, as shown in FIG. It is considered to be small. On the other hand, as shown in FIGS. 13A and 13C, it can be seen that the aberration increases when the wavelength deviates from the design wavelength.

  Another factor that causes a reduction in coupling efficiency is the alignment accuracy between the laser diode 101 and the collimating lens 102. Since the focal length of the collimating lens 102 is about several millimeters, it is difficult to accurately place one focal point of the collimating lens 102 at the laser light emission position on the end surface of the laser diode 101 where the non-reflective film 105 is formed. It is.

  Due to the above factors and the like, the collimating lens 102 is a single lens having an aspherical surface, and no actual measures are taken to compensate for chromatic aberration. In recent years, the performance of an external resonator type wavelength tunable light source has been improved, and those having a wavelength tunable range of about 200 nm have been developed. In the future, in order to further expand the wavelength variable range, it is necessary to take a fundamental measure that can compensate for the chromatic aberration of the collimating lens 102.

  Here, as described above, it is difficult to compensate for the chromatic aberration itself of the collimating lens 102. Therefore, a method of compensating the chromatic aberration of the collimating lens 102 by making the collimating lens 102 movable by using an actuator or the like and optimizing the position of the collimating lens 102 with respect to the laser diode 101 for each oscillation wavelength is conceivable. However, since the focal length of the collimating lens 102 is short as described above, it is difficult to position with high accuracy.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an external resonator type wavelength tunable light source capable of improving performance deterioration due to chromatic aberration without adding a new movable part.

In order to solve the above problems, an external resonator type wavelength tunable light source according to the first aspect of the present invention includes a light source (11) having an antireflective film (15) formed on one end face thereof, A lens (12) disposed on the end face side on which the reflective film is formed, a diffraction grating (13, 23) that diffracts light from the light source via the lens at an angle according to the wavelength, and the diffraction grating Reflecting mirrors (14, 24) for reflecting a part of the diffracted light toward the diffraction grating, and the light reflected by the reflecting mirror toward the diffraction grating through the lens In the external resonator type wavelength tunable light source (10, 20) incident on the light source, the lens is positioned with respect to the light source so that light emitted from the light source becomes divergent light having a predetermined divergence angle. the grating arrangement direction of the grating First so that the direction the light diffracted in the same wavelength which passes through the plane parallel to the is parallel light, diffractive surface grating intervals in accordance with the divergence angle of the transformed the divergent light by the lens is modulated along ( 13a, 23a), and the shape in the second direction perpendicular to the first direction is a curved surface shape .
According to this invention, the light from the light source is converted into divergent light having a predetermined divergence angle by the lens and then enters the diffraction surface of the diffraction grating. Since the diffraction surface of the diffraction grating is modulated in accordance with the divergence angle of the divergent light from the lens, the divergent light incident on the diffractive surface is diffracted in the direction corresponding to the modulation. Of the light diffracted by the diffraction grating, the light diffracted to the lens side enters the light source .
In order to solve the above-mentioned problem, an external resonator type wavelength tunable light source according to the second invention of the present invention includes a light source (11) having an antireflective film (15) formed on one end face, and the light source of the light source. A lens (12) disposed on the end face side on which the reflective film is formed, and a diffraction grating (33) for diffracting light from the light source via the lens at an angle corresponding to a wavelength, In the external resonator type tunable light source (30) in which diffracted light having a predetermined wavelength is incident on the light source through the lens, the lens has a divergence in which the light emitted from the light source has a predetermined divergence angle. The diffraction grating is positioned with respect to the light source to be light, and the diffraction grating has a curved shape in the second direction perpendicular to the first direction along the arrangement direction of the grating, and diffracted light of the same wavelength is Reverses the diverging light path while focusing As the process proceeds, the lattice spacing in accordance with the divergence angle of the transformed the divergent light by the lens is characterized in that it comprises a modulated diffractive surface (33a).

  According to the present invention, since the aberration of the lens is corrected by the diffraction grating and the mirror, the performance deterioration due to chromatic aberration can be improved without adding a new movable part.

  Hereinafter, an external resonator type wavelength tunable light source according to an embodiment of the present invention will be described in detail with reference to the drawings.

[First Embodiment]
FIG. 1 is a perspective view showing the configuration of an external resonator type tunable light source according to the first embodiment of the present invention, and FIG. 2 is a plan view showing the configuration of the external resonator type tunable light source. As shown in FIGS. 1 and 2, the external resonator type wavelength tunable light source 10 according to the first embodiment of the present invention includes a laser diode 11, a lens 12, a diffraction grating 13, and a mirror 14. The external resonator type wavelength tunable light source shown in FIGS. 1 and 2 is an external resonator type wavelength tunable light source including a Littman type external resonator.

  The laser diode 11 is a Fabry-Perot type semiconductor laser in which, for example, a lower clad layer, an active layer, and an upper clad layer are sequentially formed on a semiconductor substrate, and parallel end surfaces obtained by cleaving the semiconductor substrate are used as resonators. . The laser diode 11 has an anti-reflection film (AR coat) 15 formed on one end face forming a resonator. The laser diode 11 is disposed with the end surface on which the antireflective film 15 is formed facing the lens 12 side.

  The lens 12 is positioned with respect to the laser diode 11 so that one focal point is disposed on the inner side of the radar diode 11 with respect to the laser light emission position on the end surface of the laser diode 11 on which the antireflective film 15 is formed. . Thereby, the lens 12 turns the laser light emitted from the laser diode 11 into divergent light having a predetermined divergence angle. The lens 12 condenses the laser light diffracted by the diffraction grating 13 and makes it incident on the laser diode 11.

  The diffraction grating 13 has a planar diffraction surface 13a on which a grating extending in the vertical direction (direction orthogonal to the paper surface in FIG. 2) is formed, and is reflected by the diverging light from the lens 12 and the mirror 14. Diffracts laser light. The grating interval of the grating formed on the diffraction surface 13 a of the diffraction grating 13 is modulated according to the divergence angle of the diverging light from the lens 12. Specifically, the grating interval is modulated so that diffracted light having the same wavelength passing through a plane parallel to the direction along the arrangement direction of the grating formed on the diffraction surface 13a becomes parallel light.

  Here, since the light incident on the diffraction grating 13 via the lens 12 is divergent light, the incident angle of the divergent light with respect to the diffraction grating 13 differs depending on the incident position. FIG. 3 is a diagram for explaining a difference in incident angle of diverging light incident on the diffraction grating 13. For example, as shown in FIG. 3, the divergent light L1 incident in the upper part of the drawing with respect to the optical axis AX enters the diffraction grating 13 with an incident angle α1. On the other hand, the divergent light L2 incident in the lower part of the figure than the optical axis AX enters the diffraction grating 13 with an incident angle α2 larger than the incident angle α1.

  If the gratings are formed on the diffraction surface 13a of the diffraction grating 13 at regular intervals, the wavelengths of the light incident on the diffraction grating 13 with the incident angle α1 and the light incident on the diffraction grating 13 with the incident angle α2 are the same. Even so, light incident at different angles of incidence will be diffracted at different angles. In the present embodiment, diffraction is performed so that diffracted light of the same wavelength passing through a surface parallel to a direction along the arrangement direction of the grating formed on the diffraction surface 13a (for example, the paper surface in FIGS. 2 and 3) becomes parallel light. The lattice spacing of the surface 13a is modulated.

  Specifically, the modulation is performed so that the grating interval becomes narrower as the incident angle with respect to the diffraction grating 13 becomes larger. For example, when the incident angle of light incident on the diffraction grating 13 is α, the grating interval is modulated in proportion to the incident angle α, or the grating interval is reduced in proportion to a quadratic function of the incident angle α. Modulated so that. Although details will be described later, the chromatic aberration of the lens 12 is corrected by modulating the grating interval of the diffraction grating 13. When modulation is performed so that the grating interval is narrowed in proportion to the incident angle α (when corrected by a linear expression of the incident angle α), the chromatic aberration of the lens 12 is first-order corrected and proportional to the quadratic function of the incident angle α. Thus, when the modulation is performed so that the grating interval is narrowed (when correction is performed using a quadratic expression of the incident angle α), the chromatic aberration of the lens 12 is secondarily corrected.

  The mirror 14 has a cylindrical reflection surface whose upper and lower direction (the direction orthogonal to the paper surface in FIGS. 2 and 3) is a curved surface, and reflects the laser light diffracted by the diffraction grating 13. As described above, the divergent light that has entered the diffraction grating 13 is diffracted light having the same wavelength that passes through a plane parallel to the direction in which the gratings are formed on the diffraction surface 13a (for example, the paper surface in FIGS. 2 and 3). Is diffracted to become parallel light. On the other hand, in the vertical direction (the direction orthogonal to the paper surface in FIGS. 2 and 3), since such diffraction is not performed, the light diffracted by the diffraction grating 13 becomes light that diverges in the vertical direction. In order to collect the diverging light, the reflection surface of the mirror 14 has a curved surface in the vertical direction. The mirror 14 condenses the incident light so that the light diffracted by the diffraction grating 13 travels in the reverse direction on the original optical path after reflection. Further, the mirror 14 can move in a plane parallel to the paper surface of FIG. 2 or FIG. 3 and can rotate in the same plane.

  In the above configuration, the laser light emitted from the laser diode 11 is converted into divergent light having a predetermined divergence angle by the lens 12 and then incident on the diffraction surface 13a of the diffraction grating 13 and diffracted. At this time, since the grating interval of the grating formed on the diffraction surface 13a is modulated, the divergent light incident on the diffraction surface 13a of the diffraction grating 13 is an array of gratings in which light of the same wavelength is formed on the diffraction surface 13a. The light is diffracted to become parallel light in a plane parallel to the direction along the direction.

  Of the light diffracted by the diffraction grating 13, the laser light diffracted in the direction in which the mirror 14 is disposed is reflected by the mirror 14 and then enters the diffraction surface 13 a of the diffraction grating 13 again. This laser light is again diffracted by the diffraction grating 13, travels in the reverse direction along the original optical path, is condensed by the lens 12, and enters the laser diode 11. A part of the laser light incident on the laser diode 11 is reflected by the end face 11 a of the laser diode 11 and travels in the reverse direction inside the laser diode 11, and a part thereof is emitted from the laser diode 11.

  As described above, in the external resonator type wavelength tunable light source 10 according to the first embodiment of the present invention, a resonator is formed by the end face 11 a of the laser diode 11 and the mirror 14 provided outside the laser diode 11. . Here, when the mirror 14 forming the resonator is moved in a plane parallel to the paper surface of FIGS. 2 and 3 and is rotated in the same plane so that the light diffracted by the diffraction grating 13 is vertically incident, Since the wavelength of the laser light incident on the mirror 14 changes and the optical path length of the resonator changes, the wavelength of the laser light emitted to the outside from the end face 11a of the laser diode 11 can be varied. Specifically, when the mirror 14 is moved in the direction indicated by the symbol D1 in FIG. 1 and appropriately rotated, the wavelength of the laser light can be continuously varied to the long wavelength side, and conversely the symbol When the mirror 14 is moved in the direction indicated by D2 and is appropriately rotated, the wavelength of the laser beam can be continuously varied to the short wavelength side.

  FIG. 4 is an aberration diagram showing an example of the aberration of the optical system including the lens 12, the diffraction grating 13, and the mirror 14. FIG. 4A shows the aberration when the wavelength is 1.4 μm. ) Shows the aberration when the wavelength is 1.5 μm, and (c) shows the aberration when the wavelength is 1.65 μm. The aberration diagram shown in FIG. 4 is an aberration when the chromatic aberration of the lens 12 is first-order corrected by modulating the grating formed on the diffraction surface 13a of the diffraction grating 13 so that the grating interval becomes narrower in proportion to the incident angle α. FIG. In FIG. 4, the positions (PX, PY) in two directions which are included in a plane orthogonal to the laser light path and are orthogonal to each other are taken on the horizontal axis, and the aberration amount of the laser light passing through each position is taken on the vertical axis. taking it.

  However, like the collimating lens 102 shown in FIGS. 11 and 12, the lens 12 is designed so that the aberration is optimized with respect to light having a wavelength of 1.5 μm. Comparing FIG. 4B showing the aberration of the optical system according to the present embodiment at a wavelength of 1.5 μm and FIG. 13B showing the aberration of the conventional collimating lens 102, the image height in the PY direction is high. At the position, although there are both a portion where the aberration of the optical system according to the present embodiment is larger and a portion where the aberration is smaller, the amount of aberration does not change much as a whole.

  On the other hand, when FIG. 4A showing the aberration at the wavelength of 1.4 μm is compared with FIG. 13A, the aberration of the optical system according to the present embodiment is significantly larger than that of the conventional one. You can see that it is getting smaller. Further, when comparing FIG. 4C and FIG. 13C showing the aberration at the wavelength of 1.65 μm, the aberration of the optical system according to the present embodiment is significantly smaller than that of the conventional one. I understand that Thus, it can be seen that the chromatic aberration is greatly improved by first correcting the chromatic aberration of the lens 12.

  FIG. 5 is an aberration diagram showing another example of the aberration of the optical system including the lens 12, the diffraction grating 13, and the mirror 14, and (a) shows the aberration when the wavelength is 1.4 μm. (B) shows the aberration when the wavelength is 1.5 μm, and (c) shows the aberration when the wavelength is 1.65 μm. The aberration diagram shown in FIG. 5 is a second-order correction of the chromatic aberration of the lens 12 by modulating the grating formed on the diffraction surface 13a of the diffraction grating 13 in such a way that the grating spacing is reduced in proportion to the quadratic function of the incident angle α. FIG. In FIG. 5, as in FIG. 4, the positions (PX, PY) in two directions which are included in the plane orthogonal to the laser light path and are orthogonal to each other are taken on the horizontal axis, and the laser light passing through each position is shown. The amount of aberration is plotted on the vertical axis.

  Comparing FIG. 5B showing the aberration of the optical system according to the present embodiment with a wavelength of 1.5 μm and FIG. 13B showing the aberration of the conventional collimating lens 102, the optical system according to the present embodiment is compared. It can be seen that the aberration is slightly larger. However, comparing FIG. 5 (a) and FIG. 13 (a) showing aberrations at a wavelength of 1.4 μm, the aberration of the optical system according to the present embodiment is much smaller than that of the conventional one. I understand that Further, comparing FIG. 5C and FIG. 13C showing the aberration at the wavelength of 1.65 μm, the aberration of the optical system according to the present embodiment is significantly smaller than that of the conventional one. I understand that Further, comparing FIG. 5 showing the aberration in the case where the secondary correction is performed and FIG. 4 showing the aberration in the case where the primary correction is performed, the aberration is obtained when the secondary correction is performed at any wavelength. Can be seen to be smaller. Thus, it can be seen that even when the chromatic aberration of the lens 12 is secondarily corrected, the chromatic aberration is greatly improved.

  FIG. 6 is a diagram illustrating an example of the coupling efficiency of return light with respect to the laser diode 11. In FIG. 6, the horizontal axis represents wavelength and the vertical axis represents coupling efficiency. Referring to FIG. 6, it can be seen that the coupling efficiency in the conventional external resonator type wavelength tunable light source 100 shown in FIG. 11 is greatly reduced as the wavelength is separated from 1.5 μm. On the other hand, in the external resonator type wavelength tunable light source 10 of the present embodiment, the coupling efficiency when the wavelength is 1.5 μm in both cases where the primary correction and the secondary correction are performed. Is slightly lower than that of the conventional example, but the coupling efficiency is not significantly reduced as in the conventional example even when the wavelength is away from 1.5 μm. For this reason, according to the external resonator type wavelength tunable light source 10 according to the present embodiment, it is possible to improve performance degradation due to chromatic aberration without adding a new movable part.

[Second Embodiment]
FIG. 7 is a perspective view showing a configuration of an external resonator type wavelength tunable light source according to the second embodiment of the present invention. In FIG. 7, the same components as those of the external resonator type wavelength variable light source according to the first embodiment of the present invention shown in FIG. As shown in FIG. 7, the external resonator type wavelength tunable light source 20 according to the second embodiment of the present invention includes a laser diode 11, a lens 12, a diffraction grating 23, and a mirror 24. Note that the external resonator type wavelength tunable light source 20 shown in FIG. 7 is also an external resonator type wavelength tunable light source including a Littman type external resonator.

  Since the external resonator type wavelength tunable light source 20 of the present embodiment includes the same laser diode 11 and lens 12 as in the first embodiment, divergent light having a predetermined divergence angle is incident on the diffraction grating 23. . The diffraction grating 13 provided in the external resonator type wavelength tunable light source 10 shown in FIG. 1 has a planar diffraction surface 13a, and the mirror 14 is a cylindrical surface with a curved surface in the vertical direction (direction perpendicular to the paper surface in FIG. 2). It had a reflective surface. On the other hand, in the external resonator type wavelength tunable light source 20 of the present embodiment, the diffraction grating 23 has a cylindrical diffraction surface 23a, and the mirror 24 has a planar reflection surface. Hereinafter, the diffraction grating 23 and the mirror 24 will be described in detail.

  The diffraction grating 23 diffracts the divergent light from the lens 12 and the laser light reflected by the mirror 24. Similar to the grating formed on the diffraction surface 13 a of the diffraction grating 13, the grating interval of the grating formed on the diffraction surface 23 a of the diffraction grating 23 is modulated according to the divergence angle of the diverging light from the lens 12. Specifically, the grating interval is modulated so that the diffracted light having the same wavelength passing through the plane parallel to the direction along the arrangement direction of the grating formed on the diffraction surface 23a becomes parallel light. Specifically, the modulation is performed so that the grating interval becomes narrower as the incident angle with respect to the diffraction grating 23 becomes larger.

  Here, in the external resonator type wavelength tunable light source 10 shown in FIG. 1, since the diffraction surface 13a of the diffraction grating 13 is planar, the diverging light incident on the diffraction grating 13 is in the vertical direction (perpendicular to the paper surface in FIG. 2). Diffracted so as to diverge. On the other hand, in the external resonator type wavelength tunable light source 20 shown in FIG. 7, the diffraction surface 23a of the diffraction grating 23 has a cylindrical shape. Therefore, the light diffracted by the diffraction grating 23 is diffracted as parallel light without diverging in the vertical direction (direction corresponding to the direction orthogonal to the paper surface in FIG. 2).

  The mirror 24 reflects the laser light diffracted by the diffraction grating 23. As described above, since the light diffracted by the diffraction grating 23 is parallel light, the light diffracted in the direction in which the mirror 24 is disposed enters the reflecting surface of the mirror 24 perpendicularly. Similarly to the mirror 14, the mirror 24 can move in a plane corresponding to a plane parallel to the paper surface of FIG. 2 and can rotate in the same plane.

  In the above configuration, the laser light emitted from the laser diode 11 is converted into divergent light having a predetermined divergence angle by the lens 12 and then incident on the diffraction surface 23a of the diffraction grating 23 and diffracted. At this time, since the grating interval of the grating formed on the diffraction surface 23a is modulated, the divergent light incident on the diffraction surface 23a of the diffraction grating 23 is an array of gratings in which light of the same wavelength is formed on the diffraction surface 23a. The light is diffracted to become parallel light in a plane parallel to the direction along the direction. Moreover, since the diffraction surface 23a of the diffraction grating 23 is cylindrical, it is also diffracted so as to be parallel light in the vertical direction (the direction corresponding to the direction orthogonal to the paper surface in FIG. 2).

  Of the light diffracted by the diffraction grating 23, the laser light diffracted in the direction in which the mirror 24 is arranged is reflected by the mirror 24 and then enters the diffraction surface 23 a of the diffraction grating 23 again. This laser light is again diffracted by the diffraction grating 23, travels in the reverse direction along the original optical path, is condensed by the lens 12, and enters the laser diode 11. A part of the laser light incident on the laser diode 11 is reflected by the end face 11 a of the laser diode 11 and travels in the reverse direction inside the laser diode 11, and a part thereof is emitted from the laser diode 11.

  As described above, in the external resonator type wavelength tunable light source 20 according to the first embodiment of the present invention, the resonator is formed by the end face 11 a of the laser diode 11 and the mirror 24 provided outside the laser diode 11. . Here, when the mirror 24 is moved in the direction indicated by the reference symbol D1 in FIG. 7 and rotated as appropriate, the wavelength of the laser light can be continuously varied to the long wavelength side, and conversely the reference symbol D2 When the mirror 24 is moved in the attached direction and rotated as appropriate, the wavelength of the laser light can be continuously varied to the short wavelength side.

In the external resonator type tunable light source 20 of the present embodiment and the external resonator type tunable light source 10 of the first embodiment, one of the diffraction grating and the mirror has a cylindrical shape and the other has a planar shape. It is common in a certain point, and the only difference is whether the diffraction surface 23a of the diffraction grating 23 is cylindrical or the reflection surface of the mirror 14 is cylindrical. That is, the diffraction grating 23a of the diffraction grating 23 provided in the external resonator type wavelength tunable light source 20 also modulates the grating interval of the grating, similarly to the diffraction grating 13 provided in the external resonator type wavelength tunable light source 10 of the first embodiment. ing. For this reason, also in the external resonator type wavelength tunable light source 20 according to the present embodiment, it is possible to improve performance deterioration due to chromatic aberration without adding a new movable part, as in the first embodiment.
[Third Embodiment]
FIG. 8 is a perspective view showing a configuration of an external resonator type tunable light source according to a third embodiment of the present invention, and FIG. 9 is a plan view showing a configuration of the external resonator type tunable light source. 8 and 9, the same reference numerals are given to the same components as those of the external resonator type wavelength variable light source according to the first embodiment of the present invention shown in FIG. As shown in FIGS. 8 and 9, the external resonator type wavelength tunable light source 30 according to the third embodiment of the present invention includes the laser diode 11, the lens 12, and the diffraction grating 33. The external resonator type tunable light source 30 shown in FIGS. 8 and 9 is an external resonator type tunable light source including a Littrow type external resonator.

  Since the external resonator type wavelength tunable light source 30 of this embodiment includes the laser diode 11 and the lens 12 similar to those of the first embodiment, divergent light having a predetermined divergence angle is incident on the diffraction grating 33. . In addition, in order to focus the divergent light incident on the diffraction grating 33, the diffraction grating 33 has a cylindrical diffraction surface 33a having a curved surface in the vertical direction (direction orthogonal to the paper surface in FIG. 9).

  The diffraction grating 33 diffracts the divergent light from the lens 12. Similar to the grating formed on the diffraction surface 13a of the diffraction grating 13 shown in FIG. 1, the grating spacing of the grating formed on the diffraction surface 33a of the diffraction grating 33 is modulated according to the divergence angle of the divergent light from the lens 12. Has been. However, the grating interval is modulated so that the diffracted light having the same wavelength passing through the plane parallel to the direction along the arrangement direction of the grating formed on the diffraction surface 33a is collected. That is, the method of modulating the grating interval is different from that of the diffraction grating 13 shown in FIG. Specifically, the grating spacing is modulated so that the diffracted light diffracted by the diffraction grating 33 travels in the opposite direction along the optical path of the diverging light from the lens 12 and is collected. The vertical curvature of the diffraction grating 33 is set so that the diffracted light diffracted by the diffraction grating 33 travels in the reverse direction along the optical path of the diverging light from the lens 12 and is collected.

  In the above configuration, the laser light emitted from the laser diode 11 is converted into divergent light having a predetermined divergence angle by the lens 12 and then incident on the diffraction surface 33a of the diffraction grating 33 and diffracted. At this time, the grating interval of the grating formed on the diffractive surface 33a is modulated, and the diffractive surface 33a has a vertical curvature, so that the diverging light incident on the diffractive surface 33a of the diffraction grating 33 is Diffracted to travel in the opposite direction along the original optical path. The light diffracted by the diffraction grating 33 travels in the opposite direction along the original optical path, is collected by the lens 12 and enters the laser diode 11. A part of the laser light incident on the laser diode 11 is reflected by the end face 11 a of the laser diode 11 and travels in the reverse direction inside the laser diode 11, and a part thereof is emitted from the laser diode 11.

  As described above, the external resonator type wavelength tunable light source 20 according to the first embodiment of the present invention has a resonator formed by the end face 11 a of the laser diode 11 and the diffraction grating 33 provided outside the laser diode 11. Yes. Here, when the diffraction grating 33 is moved in the direction indicated by the symbol D3 in FIG. 7 and appropriately rotated, the wavelength of the laser light can be continuously varied to the long wavelength side, and conversely the symbol D4. When the diffraction grating 33 is moved in the direction indicated by and rotated as appropriate, the wavelength of the laser light can be continuously varied to the short wavelength side.

  The external resonator type wavelength tunable light source 30 of this embodiment is formed on the diffraction surface 33a of the diffraction grating 33 with the external resonator type wavelength tunable light source 10 and the external resonator type wavelength tunable light source 20 of the first embodiment. Although the method of modulating the grating interval of the grating is different, the chromatic aberration of the lens 12 can be corrected also in this embodiment. For this reason, according to the external resonator type wavelength tunable light source 30 according to the present embodiment, it is possible to improve performance deterioration due to chromatic aberration without adding a new movable part.

[Fourth Embodiment]
FIG. 10 is a perspective view showing a configuration of an external resonator type wavelength tunable light source according to the fourth embodiment of the present invention. In FIG. 10, the same components as those of the external resonator type wavelength variable light source according to the first embodiment of the present invention shown in FIG. As shown in FIG. 10, the external resonator type wavelength tunable light source 40 according to the fourth embodiment of the present invention includes a laser diode 11, a collimating lens 42, a diffraction grating 43, and a mirror 44. The external resonator type wavelength tunable light source 40 shown in FIG. 10 is an external resonator type wavelength tunable light source including a Littman type external resonator.

  The external resonator type wavelength tunable light source 40 of this embodiment includes a collimating lens 42 instead of the lens 12 shown in FIGS. The collimating lens 42 and the lens 12 are the same element as a single optical element. However, in the lens 12 shown in FIGS. 1 and 2, one focal point is disposed on the inner side of the radar diode 11 with respect to the emission position of the laser light on the end surface of the laser diode 11 where the non-reflective film 15 is formed. On the other hand, one focal point of the collimating lens 42 is disposed at the emission position of the laser beam on the end surface of the laser diode 11 where the non-reflective film 15 is formed. That is, the collimating lens 42 and the lens 12 are positioned differently with respect to the laser diode 11. The collimator lens 42 collimates the laser light emitted from the laser diode 11 and condenses the laser light diffracted by the diffraction grating 43 at the laser light emission position of the laser diode 11.

  The diffraction grating 43 diffracts the parallel light from the lens 42 and the laser light reflected by the mirror 44. The grating interval of the grating formed on the diffraction surface 43a of the diffraction grating 43 is also modulated, but diffracted light having the same wavelength passing through a plane parallel to the direction along the arrangement direction of the grating formed on the diffraction surface 43a is emitted. The grating spacing is modulated so that it is focused. Further, the diffraction surface 43a of the diffraction grating 43 has a cylindrical shape in the vertical direction (a direction corresponding to a direction orthogonal to the paper surface in FIG. 2), and is convex toward the collimator lens 42 as shown in FIG. Are arranged as follows. For this reason, the parallel light from the lens 43 incident on the diffraction grating 43 is diffracted so as to diverge in the vertical direction.

  The mirror 44 reflects the laser light diffracted by the diffraction grating 43. As described above, the parallel light incident on the diffraction grating 43 is diffracted so that diffracted light having the same wavelength passing through a plane parallel to the direction along the grating arrangement direction is focused. Further, since the diffraction grating 43 has a cylindrical shape in the vertical direction, parallel light from the lens 43 incident on the diffraction grating 43 is diffracted so as to diverge in the vertical direction. For this reason, the reflection surface of the mirror 44 is a toroidal surface. That is, the reflection surface of the mirror 44 has a different curvature in two orthogonal directions.

  Specifically, in the plane parallel to the direction along the arrangement direction of the grating formed on the diffraction surface 43 a of the diffraction grating 43, the reflection surface of the mirror 44 is used to diverge the diffracted light that is focused. It has a curvature that becomes convex toward On the other hand, in order to focus the diffracted diffracted light in the vertical direction, it has a curvature that is convex on the side opposite to the side where the diffraction grating 43 is disposed. The mirror 44 can move in a plane corresponding to the plane parallel to the paper surface of FIG. 2 and can rotate in the same plane as the mirror 14 shown in FIG.

  In the above configuration, the laser light emitted from the laser diode 11 is converted into parallel light by the lens 42 and then incident on the diffraction surface 43a of the diffraction grating 43 and diffracted. At this time, since the grating interval of the grating formed on the diffraction surface 43a is modulated, the diverging light incident on the diffraction surface 43a of the diffraction grating 43 is an array of gratings in which light of the same wavelength is formed on the diffraction surface 43a. Diffracted to converge in a plane parallel to the direction along the direction. On the other hand, it is diffracted so as to diverge in the vertical direction.

  Of the light diffracted by the diffraction grating 43, the laser light diffracted in the direction in which the mirror 44 is disposed is reflected by the mirror 44 and then enters the diffraction surface 43 a of the diffraction grating 43 again. The laser light is again diffracted by the diffraction grating 43 to become parallel light, travels in the reverse direction along the original optical path, is collected by the collimating lens 42, and enters the laser diode 11. A part of the laser light incident on the laser diode 11 is reflected by the end face 11 a of the laser diode 11 and travels in the reverse direction inside the laser diode 11, and a part thereof is emitted from the laser diode 11.

  As described above, in the external resonator type wavelength tunable light source 40 according to the first embodiment of the present invention, the resonator is formed by the end face 11 a of the laser diode 11 and the mirror 44 provided outside the laser diode 11. . Here, when the mirror 44 forming the resonator is moved in a plane corresponding to a plane parallel to the paper surface of FIG. 2 and is rotated in the same plane so that the light diffracted by the diffraction grating 43 is vertically incident. Since the wavelength of the laser light incident on the mirror 44 changes and the optical path length of the resonator changes, the wavelength of the laser light emitted to the outside from the end face 11a of the laser diode 11 can be varied. Specifically, when the mirror 44 is moved in the direction indicated by the symbol D5 in FIG. 10 and rotated appropriately, the wavelength of the laser light can be continuously varied to the long wavelength side, and conversely the symbol When the mirror 44 is moved in the direction indicated by D6 and is appropriately rotated, the wavelength of the laser beam can be continuously varied to the short wavelength side.

  The external resonator type wavelength tunable light source 40 of this embodiment is formed on the diffraction surface 43a of the diffraction grating 43, which is the same as the external resonator type wavelength tunable light sources 10, 20, and 30 of the first to third embodiments described above. Although the method of modulating the grating spacing of the gratings is different, the chromatic aberration of the collimating lens 42 can also be corrected in this embodiment. For this reason, according to the external resonator type wavelength tunable light source 40 according to the present embodiment, it is possible to improve performance deterioration due to chromatic aberration without adding a new movable part.

  The spectroscopic device according to the embodiment of the present invention has been described above. However, the present invention is not limited to the above-described embodiment, and can be freely changed within the scope of the present invention. For example, in the external resonator type wavelength tunable light source 40 shown in FIG. 10, the case where the diffraction grating 43 is arranged so as to be convex toward the collimating lens 42 has been described as an example. The surface facing the side may be a concave surface. However, in such a case, the curvature of the mirror 44 in the vertical direction needs to be a curvature that is convex toward the side where the diffraction grating 43 is disposed. In the above-described embodiment, the case where the grating interval of the diffraction grating is modulated by a primary expression or a quadratic expression has been described as an example, but any modulation method can be used. For example, it is possible to modulate using not only a linear expression and a polynomial, but also a linear function and a high-order function.

It is a perspective view which shows the structure of the external resonator type | mold wavelength variable light source by 1st Embodiment of this invention. It is a top view which shows the structure of the external resonator type | mold wavelength variable light source by 1st Embodiment of this invention. It is a figure for demonstrating the difference in the incident angle of the diverging light which injects into the diffraction grating 13. FIG. FIG. 6 is an aberration diagram showing an example of aberration of an optical system including a lens 12, a diffraction grating 13, and a mirror 14. FIG. 10 is an aberration diagram showing another example of aberration of the optical system including the lens 12, the diffraction grating 13, and the mirror 14. 3 is a diagram illustrating an example of coupling efficiency of return light with respect to a laser diode 11. FIG. It is a perspective view which shows the structure of the external resonator type | mold wavelength variable light source by 2nd Embodiment of this invention. It is a perspective view which shows the structure of the external resonator type | mold wavelength variable light source by 3rd Embodiment of this invention. It is a top view which shows the structure of the external resonator type | mold wavelength variable light source by 3rd Embodiment of this invention. It is a perspective view which shows the structure of the external resonator type | mold wavelength variable light source by 4th Embodiment of this invention. It is a top view which shows the structure of the external resonator type | mold wavelength variable light source which concerns on a 1st prior art example. It is a top view which shows the structure of the external resonator type | mold wavelength variable light source which concerns on a 2nd prior art example. 6 is an aberration diagram illustrating an example of aberration of the collimator lens 102. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 External resonator type wavelength variable light source 11 Laser diode 12 Lens 13 Diffraction grating 13a Diffraction surface 14 Reflection mirror 15 Non-reflective film 20 External resonator type wavelength variable light source 23 Diffraction grating 23a Diffraction surface 24 Reflection mirror 30 External resonator type wavelength variable Light source 33 Diffraction grating 33a Diffraction surface

Claims (2)

A light source having an antireflective film formed on one end face, a lens disposed on the end face side of the light source on which the antireflective film is formed, and an angle corresponding to the wavelength of light from the light source via the lens And a reflection mirror that reflects a part of the light diffracted by the diffraction grating toward the diffraction grating, and reflects the light reflected toward the diffraction grating by the reflection mirror. In an external resonator type wavelength tunable light source that is incident on the light source through the lens,
The lens is positioned with respect to the light source so that light emitted from the light source becomes divergent light having a predetermined divergence angle,
The diffraction grating corresponds to a divergence angle of the divergent light converted by the lens so that diffracted light having the same wavelength passing through a plane parallel to a first direction along the arrangement direction of the grating becomes parallel light. A diffraction surface whose grating interval is modulated, and the shape in the second direction perpendicular to the first direction is a curved surface shape.
External resonator type tunable light source, characterized in that. A light source having an antireflective film formed on one end face, a lens disposed on the end face side of the light source on which the antireflective film is formed, and an angle corresponding to the wavelength of light from the light source via the lens In an external resonator type wavelength tunable light source, the light having a predetermined wavelength diffracted by the diffraction grating is incident on the light source through the lens.
  The lens is positioned with respect to the light source so that light emitted from the light source becomes divergent light having a predetermined divergence angle,
  The diffraction grating has a curved shape in a second direction perpendicular to the first direction along the arrangement direction of the grating, and diffracted light of the same wavelength is converged so as to travel in the reverse direction of the optical path of the divergent light. And a diffractive surface whose grating interval is modulated in accordance with a divergence angle of the divergent light converted by the lens.
  An external resonator type wavelength tunable light source characterized by the above.

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