JP2009283531A - Wavelength-variable light source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an external resonator-type wavelength-variable light source capable of securing excellent wavelength selectivity with a small number of parts and taking out light output efficiently over a wide wavelength band range even when using a semiconductor laser having a far field pattern narrow in a horizontal direction. <P>SOLUTION: The external resonator-type wavelength-variable light source selecting the wavelength of light from a semiconductor laser using a diffraction grating is characterized in that an achromatic prism produced by laminating two types of glass materials with Abbe numbers different from each other, is provided between the semiconductor laser and the diffraction grating. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a wavelength tunable light source, and more particularly, to improve the wavelength selectivity of an external resonator type wavelength tunable light source such as a Littrow arrangement or a Littman arrangement that uses a diffraction grating to select light from a semiconductor laser and output light. The present invention relates to a wavelength tunable light source that efficiently outputs light.

  The external resonator type wavelength tunable light source performs wavelength dispersion and wavelength selection of light from a semiconductor laser with a diffraction grating, and feeds back the wavelength-selected light to the semiconductor laser to cause laser oscillation at a desired wavelength. Examples of the external resonator type tunable light source include a Littrow-arranged tunable light source and a Littman-arranged tunable light source.

  The far-field pattern of light emitted from a semiconductor laser used in such an external resonator type negative feedback amplifier is generally a perfect circle, but for example a special semiconductor laser with as little end-face reflection as an elliptical pattern. In some cases, a beam having a beam shape that is short with respect to the groove arrangement direction (wavelength dispersion direction, hereinafter referred to as the horizontal direction) and long with respect to the groove direction is used.

  By the way, the magnitude of the chromatic dispersion by the diffraction grating is mainly determined by the number of grooves of the diffraction grating, and the wavelength resolution (wavelength selectivity) becomes larger as the irradiation width of light in the horizontal direction of the diffraction grating is larger (the number of irradiated grooves is larger). Also gets higher. Therefore, the beam shape of light emitted from the semiconductor laser is shaped from an ellipse to a circle to improve the wavelength selectivity of the diffraction grating.

  FIG. 7 is a configuration example of a conventional wavelength variable light source of an external resonator type (see Patent Document 1). In FIG. 7, in the semiconductor laser 1, one end face 1a is subjected to non-reflection treatment (for example, AR coating), and light is emitted from the AR-coated end face 1a.

  The lens 2 emits the output light of the semiconductor laser 1 as parallel light, and collects the returned light (hereinafter also referred to as return light) on the AR-coated end face 1 a of the semiconductor laser 1.

  The diffraction grating 3 wavelength-disperses the light from the lens 2 and emits the light to the mirror 4, or wavelength-disperses the reflected light from the mirror 4 again and emits the light to the lens 2.

  The mirror 4 selects light of a desired wavelength from the diffracted light wavelength-dispersed by the diffraction grating 3 and reflects it to the diffraction grating 3. Further, the mirror 4 rotates around a predetermined point so as to be a Littman arrangement.

  The beam splitter 5 is provided between the lens 2 and the diffraction grating 3. A part of the return light from the diffraction grating 3 is branched and output as output light, and the other is fed back to the semiconductor laser 1 through the lens 2. .

  The beam expander 6 has an anamorphic prism pair (anamorphic prisms 61 and 62), is provided between the beam splitter 5 and the diffraction grating 3, and shapes the beam shape of light from the semiconductor laser 1 into a circle. Then, the light is emitted to the diffraction grating 3.

The operation of such an apparatus will be described.
The light emitted from one end face 1a of the semiconductor laser 1 is converted into parallel light by the lens 2, passes through the beam splitter 5, and is shaped by the beam expander 6 (the beam shape is expanded in the horizontal direction). The beam-shaped light is wavelength-dispersed by the diffraction grating 3 and emitted to the mirror 4. Further, only light having a desired wavelength is reflected by the diffraction grating 3 by the mirror 4, and the wavelength of the reflected light is selected again by the diffraction grating 3.

  Then, the light whose wavelength has been selected twice is fed back to the semiconductor laser 1 through the beam expander 6, the beam splitter 5, and the lens 2, and an external resonator is formed by the other end face of the semiconductor laser 1 and the mirror 4. The laser oscillates. Further, by rotating the mirror 4, the resonator length and reflection wavelength are also changed, and stable single mode oscillation is performed at a desired wavelength.

  Further, the beam splitter 5 branches the light that is fed back to the semiconductor laser (that is, the return light that has been wavelength-selected twice), and uses a part as output light. As a result, laser light with extremely high unity from which spontaneous emission light generated by the semiconductor laser 1 itself is removed is output as output light.

Japanese Patent Laid-Open No. 5-198881

  Thus, by providing the beam expander 6 between the semiconductor laser 1 and the diffraction grating 3, the irradiation width in the horizontal direction to the diffraction grating 3 is improved, and the wavelength selectivity is increased. Further, by providing the beam splitter 5 between the semiconductor laser 1 and the diffraction grating 3, spontaneous emission light generated by the semiconductor laser 1 itself can be removed, and output light with extremely high unity can be obtained.

  However, the anamorphic prisms 61 and 62 in the beam splitter 5 and the beam expander 6 are refractive media such as glass having a higher refractive index than air. Therefore, the provision of these refractive media 5, 61, 62 in the optical path increases the external resonator length, narrows the mode interval, and degrades the wavelength selectivity.

  Further, by providing the beam splitter 5 for taking out the optical output, there is a problem that the optical power loss in the external resonator increases and the optical power efficiency deteriorates.

  Further, as shown in FIGS. 8A and 8B, by performing beam shaping using the anamorphic prisms 61 and 62 alone, the thickness of the prisms 61 and 62 as the refractive medium is minimized. Therefore, as a result, the increase in the length of the external resonator is minimized, and a favorable wavelength selectivity can be obtained as compared with the case where the anamorphic prism pair 6 and the beam splitter 5 are configured as shown in FIG.

  However, when the output light is coupled to the optical fiber and taken out, the coupling efficiency decreases due to the chromatic dispersion of the refractive index generated by the prisms 61 and 62, which becomes a serious problem particularly in a tunable light source having a wide wavelength band. .

  The present invention solves these problems, and its purpose is to ensure excellent wavelength selectivity with a small number of components and a wide wavelength band even for a semiconductor laser having a narrow horizontal far-field pattern. It is to realize an external resonator type wavelength tunable light source that can efficiently extract light output over a wide range.

The invention described in claim 1
In an external resonator type tunable light source that selects light from a semiconductor laser using a diffraction grating,
An achromatic prism in which two kinds of glass materials having different Abbe numbers are bonded is provided between the semiconductor laser and the diffraction grating.

The invention according to claim 2 is the invention according to claim 1,
The latter stage of the achromatic prism is formed of a high refractive material.

The invention according to claim 3 is the wavelength tunable light source according to claim 1 or 2,
The achromatic prism is a laminate of two glass materials having the same refractive index.

  According to the present invention, by providing the achromatic prism in which two kinds of glass materials having different Abbe numbers are bonded in the external resonator, the beam shaping of the output light of the semiconductor laser is performed and the refraction angle when passing through the prism is set. It can be kept constant regardless of the wavelength, and the diffracted light output can be efficiently coupled to the optical fiber and taken out over a wide wavelength band.

  In addition, since the number of components and the optical path length that can be inserted into the external resonator can be reduced, the increase in the external resonator length can be reduced, and as a result, the mode interval can be prevented from being narrowed, and the wavelength selectivity is a negative factor. Can be minimized.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram showing a first embodiment of the present invention. Components identical with those shown in FIG. 7 are designated by like reference characters and need not be described again. In FIG. 1, an achromatic prism 7 is provided instead of the beam splitter 5 and the beam expander 6 shown in FIG.

  The achromatic prism 7 is obtained by bonding prisms 71 and 72 made of at least two kinds of glass materials having different Abbe numbers (wavelength dispersion of refractive index), and is provided on the optical path of the lens 2 and the diffraction grating 3. As a specific glass material example, for example, KVC89 (wedge angle 24.9 deg) is used as the prism 71, and KPSFN3 (wedge angle 16.2 deg) is used as the prism 72.

  In the achromatic prism 7, the surface on which the output light of the semiconductor laser 1 is incident is defined as an incident surface 7a, and the surface from which the incident light from the incident surface 7a is emitted to the diffraction grating 3 (that is, the return light from the diffraction grating 3 is incident). The outgoing surface 7b is defined as the outgoing surface 7b.

  Here, by setting the incident angle of light incident on the incident surface 7a from the semiconductor laser 1 to the Brewster angle, the reflectance can be made substantially zero, and the antireflection film on the incident surface 7a and the emission surface 7b can be reduced. The coating of the partial reflection film can be made unnecessary. That is, since the light from the semiconductor laser 1 is linearly polarized light, the incident light from the semiconductor laser 1 to the incident surface 7a is totally reflected without being reflected by the incident surface 7a.

  The semiconductor laser 1, the diffraction grating 3, the mirror 4 and the like in the embodiment of FIG. 1 are configured to have a Littman arrangement. Since refractive media such as the achromatic prism 7 and the lens 2 are present in the external resonator, the position serving as a reference for the rotational movement of the mirror 4 is different from the case where these refractive media are not present.

The operation of such an apparatus will be described.
The light emitted from one end face (non-reflecting end) 1 a of the semiconductor laser 1 is collimated by the lens 2 to become parallel light and is incident on the achromatic prism 7. At this time, the collimated light from the lens 2 is incident on the achromatic prism 7 without being reflected by the incident surface 7 a of the achromatic prism 7.

  The collimated light is beam shaped by the achromatic prism 7 so as to expand the beam shape in the horizontal direction, and is emitted from the emission surface 7 b to the diffraction grating 3. The diffraction grating 3 performs the first wavelength dispersion for the light thus shaped and emits the light to the mirror 4. Only the light having a desired wavelength is selected by the mirror 4 and reflected by the diffraction grating 3, and the diffraction grating 3 performs wavelength selection by wavelength-dispersing the reflected light again and emits it to the achromatic prism 7.

  The return light whose wavelength is selected twice by the diffraction grating 3 is incident on the exit surface 7 b of the achromatic prism 7. At this time, part of the return light is reflected by the exit surface 7b due to the difference in refractive index between the refractive index of the achromatic prism 7 and the refractive index of air, but most of the return light is reflected by the exit surface 7b. Without being incident on the achromatic prism 7.

  The return light incident on the achromatic prism 7 is emitted from the incident surface 7a and fed back to the semiconductor laser 1 through the lens 2, and an external resonator is formed by the other end surface of the semiconductor laser 1 and the mirror 4 to form a laser. Oscillates. By rotating the mirror 4, the resonator length and the reflection wavelength are also changed, and stable single mode oscillation is performed at a desired wavelength.

  FIG. 2 is an explanatory diagram of wavelength dispersion when the mirror 4 is rotated according to the wavelength. FIG. 3 is an explanatory diagram of the prism portion of FIG. 2. FIG. 2A is a diagram using an anamorphic prism 61 alone. A conventional example is shown, and (B) shows an example of the present invention using an achromatic prism 7. In the conventional example shown in (A), when the mirror 4 is rotated according to the wavelength, the refraction angle when passing through the prism also changes according to the wavelength, and chromatic dispersion occurs. On the other hand, according to the present invention shown in (B), even if the mirror 4 is rotated according to the wavelength, the refraction angle when passing through the prism can be kept constant regardless of the wavelength, and chromatic dispersion does not occur. .

  According to the present invention, as shown in (B), of the return light from the diffraction grating 3, the light reflected by the exit surface 7b of the achromatic prism 7 is extracted and output as output light. At this time, the achromatic prism is output so that the output light is output substantially in parallel with the zero-order light from the diffraction grating 3 (the optical axis of the zero-order light and the optical axis of the reflected light of the achromatic prism 7 are parallel). 7 emission surfaces 7b are set. Note that there are two types of zero-order light from the diffraction grating 3, one based on light incident on the diffraction grating 3 from the achromatic prism 7 and one reflected on the mirror 4. The zero-order light here is achromatic. This is for light incident on the diffraction grating 3 from the prism 7.

  4A and 4B are explanatory diagrams when the output light of the wavelength tunable light source of FIG. 2 is coupled to an optical fiber. FIG. 4A shows a conventional example in which the output light has chromatic dispersion, and FIG. 4B shows the wavelength of the output light. An example of the present invention without dispersion is shown. Since the output light of the conventional example shown in (A) has wavelength dispersion, even if the output light is condensed by the lens 8, only a part of the wavelength is coupled to the optical fiber 9. However, since the output light of the present invention shown in (B) has no wavelength dispersion, almost all wavelengths can be coupled to the optical fiber 9 by collecting the output light with the lens 8.

  FIG. 5 is an explanatory diagram of the coupling efficiency of FIG. 4 and shows the ratio when the coupling efficiency peak at each wavelength is zero. In the conventional example shown in (A) where the output light has chromatic dispersion, the coupling efficiency peaks at a specific wavelength λ, and the coupling efficiency decreases at the short wavelength side λ−α and the long wavelength side λ + α. On the other hand, in the case of the present invention shown in (B) in which the output light has no chromatic dispersion, the peak of the coupling efficiency is shown at a specific wavelength λ, and the entire region of the short wavelength side λ−α and the long wavelength side λ + α The coupling efficiency is almost flat.

  With such a configuration, the achromatic prism 7 provided between the semiconductor laser 1 and the diffraction grating 3 serves both as beam shaping of collimated light and extraction of output light from the second diffracted light. The number of parts of the refractive medium in the external resonator can be reduced as compared with the conventional device shown in FIG.

  Therefore, as shown in FIG. 6, the increase in the length of the external resonator can be reduced, the mode interval can be suppressed from being narrowed, and the wavelength selectivity can be improved.

  Further, the loss of optical power can be suppressed by reducing the number of parts of the refractive medium, the wavelength selectivity can be improved and the output light can be output efficiently.

  Further, since the number of parts can be reduced as compared with the conventional configuration, the cost can be reduced and the size can be reduced.

  In addition, since there is no parallel plane portion between the components, the influence of interference and stray light due to multiple reflection can be suppressed.

  Further, by setting the incident angle of the light incident on the incident surface 7a of the achromatic prism 7 to the Brewster angle, the antireflection film coating on the incident surface 7a side can be omitted, and the manufacturing cost can be further reduced.

  Further, since the achromatic prism 7 reflects a part of the light fed back to the semiconductor laser (that is, the return light whose wavelength is selected twice) to the output surface 7b, it is generated by the semiconductor laser 1 itself. Laser output light with extremely high unity from which spontaneous emission light has been removed can be obtained.

  The output light can be easily coupled to the optical fiber, and the structure can be simplified and downsized.

  As described above, according to the present invention, it is possible to ensure excellent wavelength selectivity with a small number of components and efficiently output light over a wide wavelength band even in a semiconductor laser having a narrow horizontal far field pattern. An external resonator type wavelength tunable light source that can take out the light can be realized, and is suitable for a light source in the field of optical communication.

It is a block diagram which shows one Example of this invention. It is a beam shaping explanatory drawing by the achromatic prism 7. FIG. 2 is an operation explanatory diagram of FIG. FIG. 2 is an operation explanatory diagram of FIG. FIG. 2 is an operation explanatory diagram of FIG. FIG. 2 is an operation explanatory diagram of FIG. It is a structural example figure of the conventional wavelength variable light source of an external resonator type. It is conventional operation explanatory drawing.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2, 8 Lens 3 Diffraction grating 4 Mirror 7 Achromatic prism 71, 72 Prism 7a Incident surface 7b Output surface 9 Optical fiber

Claims (3)

In an external resonator type tunable light source that selects the wavelength by making the output light of the semiconductor laser incident on the diffraction grating
A wavelength tunable light source, wherein an achromatic prism in which two kinds of glass materials having different Abbe numbers are bonded is provided between the semiconductor laser and the diffraction grating.   The wavelength tunable light source according to claim 1, wherein a stage subsequent to the achromatic prism is formed of a highly refractive material.   3. The wavelength tunable light source according to claim 1, wherein the achromatic prism is obtained by bonding two kinds of glass materials having the same refractive index.

Images