WO2015190385A1

WO2015190385A1 - External resonator-type light-emitting device

Info

Publication number: WO2015190385A1
Application number: PCT/JP2015/066177
Authority: WO
Inventors: 近藤　順悟; 山口　省一郎
Original assignee: 日本碍子株式会社
Priority date: 2014-06-12
Filing date: 2015-06-04
Publication date: 2015-12-17

Abstract

[Problem] To suppress, in an external resonator-type laser derived from a grating element, excitation of a higher-order mode between a Bragg grating and a light-emitting surface when heat stress is applied to the grating element. [Solution] An external resonator-type light-emitting device (1) is provided with a semiconductor diode laser light source (2) and a grating element (9) composed of the light source and an external resonator. The light source (2) is provided with an active layer (5) for causing a semiconductor diode laser beam to oscillate. The grating element (9) is provided with: an optical waveguide (11) having an incident-light surface (11a) on which a semiconductor diode laser beam impinges and a light-emitting surface (11b) from which light of a desired wavelength is emitted, the optical waveguide (11) describing a convex shape in cross-section; a Bragg grating (12) comprising projections and recesses formed in the optical waveguide; and an emission-side transport part (20) provided between the Bragg grating (12) and the light-emitting surface (11b). The Bragg grating causes the laser to oscillate in a reflection wavelength region, and a difference exists between the width W_m of the optical waveguide on the Bragg grating side and the width W_out of the optical waveguide on the light-emitting surface side.

Description

External resonator type light emitting device

The present invention relates to an external resonator type light emitting device using a grating element.

As a semiconductor laser, a Fabry-Perot (FP) type is generally used in which an optical resonator is sandwiched between mirrors formed on both end faces of an active layer. However, since this FP type laser oscillates at a wavelength that satisfies the standing wave condition, the longitudinal mode tends to become multimode, and the oscillation wavelength changes when the current or temperature changes, thereby changing the light intensity. To do.

Therefore, for the purpose of optical communication and gas sensing, a single mode oscillation laser with high wavelength stability is required. For this reason, distributed feedback (DFB) lasers and distributed reflection (DBR) lasers have been developed. In these lasers, a diffraction grating is provided in a semiconductor, and only a specific wavelength is oscillated by utilizing the wavelength dependency thereof.

Examples of DBR lasers and DFB lasers that have a monolithic grating in the semiconductor laser and external cavity lasers that have a fiber grating (FBG) attached outside the laser are examples of the realization of wavelength-stable semiconductor lasers. it can. These are the principles of realizing wavelength stable operation by feeding back part of the laser light to the laser by a wavelength selective mirror using Bragg reflection.

The DBR laser realizes a resonator by forming irregularities on the waveguide surface on the extension of the waveguide of the active layer to form a mirror by Bragg reflection (Patent Document 1 (Japanese Patent Laid-Open No. 49-128689): Patent) Document 2 (Japanese Patent Laid-Open No. 56-148880). Since this laser is provided with diffraction gratings at both ends of the optical waveguide layer, the light emitted from the active layer propagates through the optical waveguide layer, a part of which is reflected by this diffraction grating, returns to the current injection part, and is amplified. Is done. Since only the light of a specific wavelength reflects in the direction determined from the diffraction grating, the wavelength of the laser light is constant.

Also, as this application, an external resonator type semiconductor laser has been developed in which a diffraction grating is a component different from a semiconductor and a resonator is formed externally. This type of laser is a laser with good wavelength stability, temperature stability, and controllability. The external resonator includes a fiber Bragg grating (FBG) (Non-patent Document 1) and a volume hologram grating (VHG) (Non-patent Document 2). Since the diffraction grating is composed of a separate member from the semiconductor laser, it has the feature that the reflectance and resonator length can be individually designed, and it is not affected by the temperature rise due to heat generation due to current injection. Can be better. Further, since the temperature change of the refractive index of the semiconductor is different, the temperature stability can be improved by designing it together with the resonator length.

Patent Document 6 (Japanese Patent Laid-Open No. 2002-134833) discloses an external resonator type laser using a grating formed in a quartz glass waveguide. This is to provide a frequency stabilized laser that can be used in an environment where the room temperature changes greatly (for example, 30 ° C. or more) without a temperature controller. Further, it is described that a temperature-independent laser in which mode hopping is suppressed and the oscillation frequency is not temperature-dependent is provided.

JP 49-128689 JP 56-148880 WO2013 / 034813 JP2000-082864 JP 2006-222399 JP2002-134833 Japanese Patent Application No. 2013-120999 Japanese Patent Application 2014-004240 JP2011-75917A

Non-Patent Document 1 mentions a mode hop mechanism that impairs the wavelength stability associated with a temperature rise, and an improvement measure thereof. The wavelength variation δλs of the external cavity laser due to the temperature is the change in refractive index Δna of the active layer region of the semiconductor, the length La of the active layer, the refractive index variation Δnf of the FBG region, the length Lf, and the temperature variation δTa. , ΔTf is expressed by the following equation from the standing wave condition.

Here, λ0 represents the grating reflection wavelength in the initial state.
Further, the change ΔλG in the grating reflection wavelength is expressed by the following equation.

Since the mode hop occurs when the longitudinal mode interval Δλ of the external resonator becomes equal to the difference between the wavelength variation Δλs and the grating reflection wavelength variation ΔλG, the following equation is established.

The longitudinal mode interval Δλ is approximately expressed by the following equation.

Mathematical formula 5 is established from

mathematical formulas

3 and 4.

In order to suppress the mode hop, it is necessary to use within a temperature of △ Tall or less, and the temperature is controlled by a Peltier device. In Equation 5, when the refractive index changes of the active layer and the grating layer are the same (Δna / na = Δnf / nf), the denominator becomes zero, the temperature at which the mode hop occurs becomes infinite, and the mode hop disappears. Is shown. However, in the monolithic DBR laser, since current is injected into the active layer in order to oscillate the laser, the refractive index change between the active layer and the grating layer cannot be matched, resulting in a mode hop.

Mode hop is a phenomenon in which the oscillation mode (longitudinal mode) in the resonator changes from one mode to another. When the temperature and injection current change, the gain and resonator conditions change, the laser oscillation wavelength changes, and the problem arises that optical power fluctuates, which is called kink. Therefore, in the case of an FP type GaAs semiconductor laser, the wavelength usually changes with a temperature coefficient of 0.3 nm / ° C., but when a mode hop occurs, a larger fluctuation occurs. At the same time, the output fluctuates by 5% or more.

For this reason, in order to suppress mode hops, temperature control is performed using a Peltier element. However, this increases the number of parts, increases the size of the module, and increases the cost.

In Patent Document 6, in order to make the temperature independent, the conventional resonator structure is left as it is, and stress is applied to the optical waveguide layer to compensate for the temperature coefficient due to thermal expansion, thereby realizing temperature independence. is doing. For this reason, a metal plate is attached to the element, and a layer for adjusting the temperature coefficient is added to the waveguide. For this reason, there exists a problem that a resonator structure becomes still larger.

The present inventor has disclosed an external resonator type laser structure using an optical waveguide grating element in Patent Document 7. In this application, optical confinement is strengthened by forming an optical material layer on a support substrate, and the refractive index difference formed by the concave and convex grooves is increased. This makes it possible to obtain a desired Bragg reflectivity at a short distance, to realize an external cavity laser with a short cavity length, and when the full width at half maximum Δλ _G of the reflection characteristic of the grating element satisfies a specific formula In addition, laser oscillation with high wavelength stability and no power fluctuation is possible without temperature control.

As a result of investigation by the present inventor, it has been found that the following problems occur. That is, when the environmental temperature changes and thermal stress is applied to the grating element, a higher-order mode may be excited between the Bragg grating and the exit surface.

This is considered to be due to deformation due to warping because this structure has a different thermal expansion coefficient and elastic modulus in the support substrate, the optical material layer, and the buffer layer.

On the other hand, the present inventor disclosed in Patent Document 8 that the excitation of the higher-order mode is suppressed by changing the width of the optical waveguide on the exit surface with respect to the width of the optical waveguide in the Bragg grating.

However, in the ridge-type waveguides described in the examples of

Patent Documents

7 and 8, as described in Patent Document 9, coupling from the optical waveguide mode to the substrate mode (slab waveguide mode) is easy and radiation is easy. For this reason, when the width of the optical waveguide on the exit surface is narrowed, confinement is weakened, the optical electric field leaks in the direction of the substrate, coupling to the substrate mode becomes remarkable, and light propagation loss increases.

An object of the present invention is to suppress excitation of a higher-order mode between a Bragg grating and an emission surface when thermal stress is applied to the grating element in an external resonator type laser using a grating element. is there.

The present invention is a semiconductor laser light source, and an external resonator type light emitting device comprising a grating element that constitutes the semiconductor laser light source and an external resonator,
The semiconductor laser light source includes an active layer that oscillates semiconductor laser light,
The grating element is an optical waveguide having an incident surface on which the semiconductor laser light is incident and an emission surface that emits outgoing light of a desired wavelength, and an optical waveguide having a convex cross section in the optical waveguide. A Bragg grating composed of the formed irregularities, and an emission-side propagation portion provided between the Bragg grating and the emission surface, and lasing in the wavelength range reflected by the Bragg grating, and the Bragg grating in the Bragg grating The width of the optical waveguide is different from the width of the optical waveguide at the exit surface.

The present inventor examined the reason why a higher-order mode is excited between the Bragg grating and the exit surface when thermal stress is applied to the grating element. As a result, the near-field pattern of the laser is greatly deformed in the vicinity of the emission surface of the device, and this has been found to cause high-order mode excitation and a reduction in the coupling efficiency of the emitted light.

That is, the width of the optical waveguide in the Bragg grating is set to be equal to the near-field pattern of the laser in order to increase the coupling efficiency with the semiconductor laser element. The horizontal size of the near field of the semiconductor laser may be 2 μm to 7 μm, for example. In this case, the width of the optical waveguide of the grating element is set to 2 μm to 7 μm.

However, for example, when an optical waveguide is formed in a thin layer made of an optical material, when the thickness of the thin layer made of the optical material is as thin as 0.5 μm to 3 μm, for example, the multimode is the same as in Patent Document 8. Further, there is a problem that a waveguide is formed, and the size of the near field pattern is different between the horizontal direction and the vertical direction, resulting in flattening.

When the optical waveguide in the Bragg grating is in a multimode, the propagation constant is different between the fundamental mode and the higher order mode, so Bragg reflection occurs at different wavelengths. However, by combining the gain characteristics of the laser and the reflection characteristics of the grating, laser oscillation can be selectively performed in the reflection wavelength band in the fundamental mode or the reflection wavelength band in the higher-order mode. That is, laser oscillation in the fundamental mode is possible by matching the gain curve to the reflection wavelength band of the fundamental mode.

In this case, it was found that when the environmental temperature changes and thermal stress is applied to the grating element, a higher-order mode is excited between the Bragg grating and the exit surface. In the grating part, the optical electric field distribution (transverse mode shape) is disturbed by the unevenness. Usually, even if the emitting part is in multimode, the fundamental mode is excited. However, when shrinkage or bending stress is applied to the waveguide portion due to environmental temperature change, the higher-order mode is excited and becomes multimode. This phenomenon also occurs when there is reflection from the end face. This phenomenon becomes more conspicuous as the ratio of the horizontal field size to the vertical direction (flatness) of the near field pattern increases.

Based on these findings, the inventor suppresses deformation of the near-field pattern on the exit surface by changing the width of the optical waveguide on the exit surface relative to the width of the optical waveguide on the Bragg grating, thereby increasing the higher order. We found that mode excitation can be suppressed. Furthermore, it is possible to reduce the light propagation loss by narrowing the width of the optical waveguide at the exit surface.

1 is a schematic diagram of an external resonator type light emitting device 1. FIG. 1 is a plan view schematically showing an external resonator type light emitting device 1. FIG. It is a schematic diagram of another external resonator type light emitting device 1A. 3 is a perspective view schematically showing a grating element 9. FIG. (A), (b) is a schematic diagram which respectively shows the cross-sectional shape of a grating element. (A), (b), (c) is a schematic diagram which respectively shows the cross-sectional shape of a grating element. (A), (b) is a schematic diagram which respectively shows the cross-sectional shape of a grating element. It is a figure explaining the form of the mode hop by a prior art example. It is a figure explaining the form of the mode hop by a prior art example. 4 illustrates an example discrete phase condition in a preferred embodiment. It is a figure explaining laser oscillation conditions.

An external resonator type light emitting device 1 schematically shown in FIG. 1 includes a light source 2 that oscillates a semiconductor laser beam and a grating element 9. The light source 2 and the grating element 9 are mounted on the common substrate 3.

The light source 2 includes an active layer 5 that oscillates semiconductor laser light. In the present embodiment, the active layer 5 is provided on the substrate 4. A reflective film 6 is provided on the outer end face of the substrate 4, and a non-reflective layer 7 A is formed on the end face of the active layer 5 on the grating element side.

As shown in FIGS. 1 and 4, the grating element 9 is provided with an optical waveguide 11 having an incident surface 11a on which the semiconductor laser light A is incident and an output surface 11b that emits an outgoing light B having a desired wavelength. C is reflected light. A Bragg grating 12 is formed in the optical waveguide 11. Between the incident surface 11 a of the optical waveguide 11 and the Bragg grating 12, an incident-side propagation part 13 without a diffraction grating is provided, and the incident-side propagation part 13 faces the active layer 5 with a gap 14 therebetween. Yes. Reference numeral 7B denotes an antireflective film provided on the incident surface side of the optical waveguide 11, and reference numeral 7C denotes an antireflective film provided on the output surface side of the optical waveguide 11.

In a preferred embodiment, the grating reflectivity is greater than the reflectivity at the exit end face of the active layer, greater than the reflectivity at the entrance end face of the optical waveguide, and greater than the reflectivity at the exit end face of the optical waveguide. large. From this point of view, it is preferable that the reflectivity at the exit end face of the active layer, the reflectivity at the entrance end face of the optical waveguide, and the reflectivity at the exit end face of the optical waveguide are each 0.1% or less.

The reflectance of the

non-reflective layers

7A, 7B, and 7C may be a value smaller than the grating reflectance, and is preferably 0.1% or less. However, as long as the reflectance at the end face is smaller than the grating reflectance, the non-reflective layer may be omitted and a reflective film may be used.

In the present invention, the optical waveguide is composed of a core made of an optical material, and a clad surrounds the core. The cross section of the core (cross section in the direction perpendicular to the light propagation direction) is a convex figure.

The convex figure means that a line segment connecting any two points of the outer contour line of the core cross section is located inside the outer contour line of the core cross section. A convex figure is a general geometric term. Examples of such figures include triangles, quadrangles, hexagons, octagons, and other polygons, circles, ellipses, and the like. As the quadrangle, a quadrangle having an upper side, a lower side, and a pair of side surfaces is particularly preferable, and a trapezoid is particularly preferable.

In a preferred embodiment, as shown in FIG. 5A, an optical waveguide 11 made of a core made of an optical material is formed on a substrate 10 via an adhesive layer 30 and a lower buffer layer 16. A lower buffer layer functioning as a clad exists under the optical waveguide 11. An upper buffer layer 23 that also functions as a cladding is formed on the side surface and the upper surface of the optical waveguide 11 to cover the optical waveguide 11. The cross-sectional shape of the optical waveguide 11 is a trapezoid, and the upper surface 11a is narrower than the lower surface 11b. The upper buffer layer 23 includes a side surface covering portion 23 b that covers the side surface of the optical waveguide 11 and an upper surface covering portion 23 a that covers the upper surface.

Further, in the element 9A shown in FIG. 5B, the optical waveguide 11A made of a core made of an optical material is formed on the substrate 10 via the adhesive layer 30 and the lower buffer layer 16. The cross-sectional shape of the optical waveguide 11A is trapezoidal and the lower surface is narrower than the upper surface. The upper cladding layer 23 includes a side surface covering portion 23 b that covers the side surface of the optical waveguide 11 and an upper surface covering portion 23 a that covers the upper surface.

In the element 9B shown in FIG. 6A, the optical waveguide 11 made of a core made of an optical material is formed on the substrate 10 via the lower buffer layer 16, and is separated from the lower buffer layer. There is no adhesive layer. Further, the upper buffer layer is not provided on the side surface and the upper surface of the optical waveguide 11. For this reason, the side surface and the upper surface of the optical waveguide 11 are exposed to the atmosphere, and the atmosphere functions as a cladding.

In the element 9 </ b> C shown in FIG. 6B, the buffer layer 22 is provided on the substrate 10, and the optical waveguide 11 made of a core made of an optical material is embedded in the buffer layer 22. The buffer layer 22 has an upper surface covering portion 22b that covers the upper surface of the optical waveguide, a side surface covering portion 22c that covers the side surface of the optical waveguide, and a bottom surface coating diagram 22a that covers the bottom surface of the optical waveguide.

In the element 9D shown in FIG. 6C, the buffer layer 22 is provided on the substrate 10, and an optical waveguide 11A made of a core made of an optical material is embedded in the buffer layer 22. The buffer layer 22 has an upper surface covering portion 22b that covers the upper surface of the optical waveguide, a side surface covering portion 22c that covers the side surface of the optical waveguide, and a bottom surface coating diagram 22a that covers the bottom surface of the optical waveguide.

7A, an optical waveguide 11 made of a core made of an optical material is formed on a substrate 10 with a lower buffer layer 16 interposed therebetween. An upper buffer layer 23 that also functions as a cladding is formed on the side surface and the upper surface of the optical waveguide 11 to cover the optical waveguide 11. In this example, an adhesive layer separate from the lower buffer layer 16 is not provided.

In the element 9F shown in FIG. 7B, an optical waveguide 11A made of a core made of an optical material is formed on the substrate 10 via the lower buffer layer 16. An upper buffer layer 23 that also functions as a cladding is formed on the side surface and the upper surface of the optical waveguide 11A to cover the optical waveguide 11A. In this example, an adhesive layer separate from the lower buffer layer 16 is not provided.

In the present invention, the Bragg grating may be formed on the upper surface 11a side of the optical waveguide, or may be formed on the lower surface 11b side of the optical waveguide.

When the air layer is in direct contact with the grating, the difference in refractive index can be increased without the presence of a grating groove, and the reflectance can be increased with a short grating length. Further, the upper buffer layer and the lower buffer layer may be made of the same material or different materials.

The buffer layer can function as a clad. From this viewpoint, the buffer layer preferably has a refractive index smaller than that of the material of the core portion. For this reason, the refractive index difference between the core part and the clad part is preferably 0.2 or more, and more preferably 0.4 or more. Examples of suitable materials for forming the cladding include silicon oxide, alumina, tantalum oxide, titanium oxide, and oxidation.

FIG. 3 shows an apparatus 1A according to another embodiment. Most of the apparatus 1A is the same as the apparatus 1 of FIG. The light source 2 includes an active layer 5 that oscillates laser light. However, the antireflection layer 7A is not provided on the end surface of the active layer 5 on the grating element 9 side, and a reflective film 25 is formed instead. This is a form of a normal semiconductor laser.

The oscillation wavelength of the laser light is determined by the wavelength reflected by the grating. If the reflected light from the grating and the reflected light from the end face of the active layer 5 on the grating element side exceed the laser gain threshold, the oscillation condition is satisfied. Thereby, a laser beam with high wavelength stability can be obtained.

In order to further improve the wavelength stability, the feedback amount from the grating may be increased. From this viewpoint, the reflectance of the grating is preferably larger than the reflectance at the end face of the active layer 5. As a result, the gain obtained by the resonator using the grating becomes larger than the gain obtained by the resonator of the original semiconductor laser, and stable laser oscillation can be performed by the resonator using the grating.

Here, in the present embodiment, as shown particularly in FIGS. 2 and 4, an incident-side propagation portion 13 is provided between the incident surface 11 a and the Bragg grating 12, and the Bragg grating 12 and the exit surface are provided. The output side propagation part 20 is provided between 11b. In this example, the output-side propagation unit 20 includes a connecting part 20a continuous from the end of the Bragg grating 12, an output part 20c continuous to the output surface 11b of the optical waveguide, and a taper provided between the connecting part and the output part. A portion 20b is provided.

In this example, the width W _out of the optical waveguide at the exit surface 11 b is smaller than the width W _m of the optical waveguide at the Bragg grating 12. Also, exit side propagating portion 20, the width W _t of the optical waveguide comprises a small tapered portion 20b toward to the exit surface side from the Bragg grating side. In this example, the width W _m of the optical waveguide in the connecting portion 20a is constant, and the width W _out of the optical waveguide in the emitting portion is also constant. Further, W _t becomes the maximum value W _m at the boundary with the connecting portion 20a, and becomes the minimum value W _out at the boundary with the emitting portion 20c.

The width W _{m of the} optical waveguide means the minimum value of the width of the optical waveguide in the cross section. If the shape of the optical waveguides of the upper surface is narrow trapezoid, the width Wm of the optical waveguide is the width of the upper surface, when the shape of the optical waveguides of the lower surface is narrow trapezoid, the width W _m of the optical waveguide lower surface of the width (See FIGS. 5 to 7). This also applies to W _out , W _t and the like.

The width W _m of the optical waveguide in the Bragg grating is set to be equal to the near-field pattern of the laser in order to increase the coupling efficiency with the semiconductor laser element 2. The horizontal size of the near field of the semiconductor laser may be 2 μm to 7 μm, for example. In this case, the width Wm of the optical waveguide is set from 2 μm to 7 μm.
In the optical waveguide structure of the present Therefore, the horizontal size of the width W _m and the near field of the optical waveguide becomes substantially the same value.

When the optical waveguide in the Bragg grating is in a multimode, the propagation constant is different between the fundamental mode and the higher order mode, so Bragg reflection occurs at different wavelengths. However, by matching the gain curve with the reflection wavelength band of the fundamental mode, laser oscillation in the fundamental mode is possible. However, in this case, it was found that when the environmental temperature changes and thermal stress is applied to the grating element 9, a higher-order mode is excited in the outgoing side propagation part between the Bragg grating 12 and the outgoing face 11b. . This phenomenon becomes more conspicuous as the ratio of the horizontal size of the near field to the vertical direction (flatness) increases.

Here, in the present embodiment, by making the width W _out of the optical waveguide on the exit surface smaller than W _m , it is possible to suppress flattening of the near field pattern on the exit surface, thereby exciting higher-order modes. Can be suppressed.

As the light source, a laser with a highly reliable GaAs-based or InP-based material is suitable. As an application of the structure of the present application, for example, when a green laser that is the second harmonic is oscillated using a nonlinear optical element, a GaAs laser that oscillates near a wavelength of 1064 nm is used. Since GaAs-based and InP-based lasers have high reliability, a light source such as a one-dimensionally arranged laser array can be realized. It may be a super luminescence diode or a semiconductor optical amplifier (SOA). In addition, the material and wavelength of the active layer can be selected as appropriate.

Note that a method for stabilizing power by a combination of a semiconductor laser and a grating element is disclosed below.
(Non-Patent Document 3: Furukawa Electric Times, January 2000, No. 105, p24-29)

The optical waveguide can be obtained by, for example, physically processing and molding the optical material layer by cutting with an outer peripheral blade or laser ablation.

The Bragg grating can be formed by physical or chemical etching as follows.
As a specific example, a metal film such as Ni or Ti is formed on the optical material layer, and windows are periodically formed by photolithography to form an etching mask. Thereafter, periodic grating grooves are formed by a dry etching apparatus such as reactive ion etching. Finally, it can be formed by removing the metal mask.

In the optical waveguide, one or more metal elements selected from the group consisting of magnesium (Mg), zinc (Zn), scandium (Sc), and indium (In) are provided in order to further improve the optical damage resistance of the optical waveguide. In this case, magnesium is particularly preferable. The crystal can contain a rare earth element as a doping component. As the rare earth element, Nd, Er, Tm, Ho, Dy, and Pr are particularly preferable.

The material of the adhesive layer may be an inorganic adhesive, an organic adhesive, or a combination of an inorganic adhesive and an organic adhesive.

Further, the optical material layer may be formed by forming a film on the support substrate by a thin film forming method. Examples of such a thin film forming method include sputtering, vapor deposition, and CVD. In this case, the optical material layer is formed directly on the support substrate, and the above-described adhesive layer does not exist.

The specific material of the support substrate is not particularly limited, and examples thereof include glass such as lithium niobate, lithium tantalate, and quartz glass, quartz, and Si.

The reflectance of the non-reflective layer needs to be less than or equal to the grating reflectivity, and the film material to be formed on the non-reflective layer is laminated with an oxide such as silicon dioxide, tantalum pentoxide, magnesium fluoride, calcium fluoride, etc. Films and metals can also be used.

Further, each end face of the light source element and the grating element may be cut obliquely in order to suppress the end face reflection. The grating element and the support substrate are bonded and fixed in the example of FIG. 5, but may be directly bonded.

In the case of the optical waveguide employed in the present invention, the core constituting the optical waveguide and the cladding surrounding the core are separated from each other in material, and the shape of the near field can be changed by changing the width of the core portion. Can be changed. Further, since there is no coupling to the substrate mode (slab mode), it is possible to suppress a reduction in light propagation loss by narrowing the width of the core portion.

However, since the waveguide of the present application has a large refractive index difference between the core and the clad, it has a strong optical confinement structure, so that the waveguide dimension for making a single mode becomes fine.

From the viewpoint of making the propagation light on the emission side into a single mode, the width W _out of the optical waveguide on the emission surface is preferably 3.5 μm or less, and from the viewpoint of suppressing flattening of the near field pattern, The width W _out of the optical waveguide is more preferably 2 μm or less, and most preferably 1 μm or less.

On the other hand, from the viewpoint of suppressing a reduction in light propagation loss at the exit-side propagation portion, the width W _out of the optical waveguide at the exit surface is preferably 0.1 μm or more, and more preferably 0.5 μm or more.
In view of the coupling between the semiconductor laser, the width W _m of the optical waveguide in the Bragg grating is preferably at least 2 [mu] m, further preferably not less than 2.5 [mu] m. For the same reason, the width W _m of the optical waveguide in the Bragg grating is preferably 7 μm or less, and more preferably 6.5 μm or less.

From the viewpoint of the effect of the present invention, the ratio W _out / W _m between W _out and W _m is preferably 1/50 or more, and more preferably 1/10 or more. Moreover, 2/3 or less is preferable and 1/2 or less is more preferable.

Further, in the above-described embodiment, the emission side propagation part 20 is provided with the tapered part 20b, the constant width connecting part 20a, and the constant width emission part 20c. However, the output side propagation part 20 may be composed of a combination of a tapered part 20b and a constant width connecting part 20a. In this case, the output surface is located at the output side end of the tapered part 20b. Alternatively, the output-side propagation unit 20 may include a tapered part 20b and a fixed-width output part 20c. In this case, the output-side end of the Bragg grating 12 and the incident-side end of the tapered part 20b are continuous. .

In a preferred embodiment, the horizontal width of the near field at the entrance surface of the grating element is larger than the horizontal width of the near field at the exit surface of the grating element. Alternatively, the width of the optical waveguide at the entrance surface of the grating element is larger than the width of the optical waveguide at the exit surface of the grating element. As a result, it is possible to suppress the multi-mode excitation in the exit-side propagation unit described above while maintaining high coupling efficiency with the light source.
From this point of view, (the horizontal width of the near field on the entrance surface / the horizontal width of the near field on the exit surface) is preferably 1.3 or more, and more preferably 1.5 or more. . If this becomes too large, the change in the width of the optical waveguide becomes large and the design becomes difficult. Therefore, (the horizontal width of the near field on the incident surface / the horizontal width of the near field on the output surface) is set to 10. It is preferable to set it as follows, and it is more preferable to set it as 5 or less.
The horizontal width of the near-field of the laser beam is measured as follows.
Measure the light intensity distribution of the laser beam and measure the width where the intensity distribution is 1 / e ² (e is the base of natural logarithm: 2.71828) with respect to the maximum value (usually equivalent to the center of the core). This is generally defined as the horizontal width of the near field. In the case of laser light, the mode field is defined because the size differs between the horizontal direction and the vertical direction of the laser element. When it is concentric like an optical fiber, it is defined as a diameter.
In the measurement of the light intensity distribution, it is possible to obtain the light intensity distribution of the spot of the laser light by measuring the beam profile using a near infrared camera or measuring the light intensity with a knife edge.

Hereinafter, preferred embodiments of the apparatus of the present invention will be further described.
Regarding the grating element, generally, when a fiber grating is used, quartz has a small temperature coefficient of refractive index, so dλ _G / dT is small and | dλ _G / dT−dλ _TM / dT | is large. For this reason, there exists a tendency for temperature range (DELTA) T in which a mode hop occurs to become small.

For this reason, in a preferred embodiment, a material having a refractive index of 1.7 or more is used for the optical waveguide in which the grating is formed. As a result, the temperature coefficient of the refractive index can be increased and dλ _G / dT can be increased. Therefore, | dλ _G / dT−dλ _TM / dT | can be decreased, and the temperature range ΔT in which the mode hop occurs can be increased.

In the preferred embodiment, on the premise of this, the full width at half maximum Δλ _G at the peak of the Bragg reflectivity is set to be large, contrary to the common sense of those skilled in the art. In addition, in order to prevent mode hops from occurring, it is necessary to increase the wavelength interval (longitudinal mode interval) that satisfies the phase condition. Therefore, it is necessary to shorten the cavity length, the length L _b of the Bragg grating has been shortened to 300μm or less.

In addition, by adjusting the depth t _d of the unevenness constituting the Bragg grating within a range of 20 nm or more and 250 nm or less, Δλ _G can be set to 0.8 nm or more and 6 nm or less. Within the range of _G , the number of longitudinal modes can be adjusted to 2-5. That is, the wavelengths satisfying the phase condition are discrete, and when the number of longitudinal modes in Δλ _G is 2 or more and 5 or less, mode hops are repeated in Δλ _G , and It will not come off. For this reason, since a large mode hop does not occur, wavelength stability can be increased and fluctuations in optical power can be suppressed.

Hereinafter, the meaning of the conditions of the present embodiment in the configuration as shown in FIG. 11 will be further described.
However, since the mathematical formula is abstract and difficult to understand, first, a typical form of the prior art and this embodiment will be compared briefly to describe the features of this embodiment. Next, each condition of the present embodiment will be described.

First, the oscillation condition of the semiconductor laser is determined by gain condition × phase condition as shown in the following equation.

The gain condition is given by the following equation from equation (2-1).

Where αa, αg, αwg, αgr are the active layer, the gap between the semiconductor laser and the waveguide, the unprocessed waveguide portion on the input side, and the loss factor of the grating portion, respectively, L _a , L _g , L _m L _gr is the length of the active layer, the gap between the semiconductor laser and the waveguide, the unprocessed waveguide section of the grating on the input side, and the length of the grating section, and r1 and r2 are the mirror reflectivities (r2 is the grating reflectivity) C _out is a coupling loss between the grating element and the light source, ζ _t g _th is a gain threshold of the laser medium, φ 1 is a phase change amount by the laser side reflection mirror, φ 2 Is a phase change amount in the grating portion.

From the equation (2-2), if the gain ζ _t g _th (gain threshold value) of the laser medium exceeds the loss, it indicates that laser oscillation occurs. The gain curve (wavelength dependence) of the laser medium has a full width at half maximum of 50 nm or more and has broad characteristics. Further, since the loss part (right side) has almost no wavelength dependence other than the reflectance of the grating, the gain condition is determined by the grating. For this reason, the gain condition can be considered only by the grating.

On the other hand, the phase condition is expressed by the following equation from the equation (2-1). However, φ1 is zero.

As the external resonator type laser, a product using a quartz glass waveguide or FBG as an external resonator has been commercialized. In the conventional design concept, as shown in FIGS. 8 and 9, the reflection characteristic of the grating is about Δλ _G = 0.2 nm and the reflectance is 10%. For this reason, the length of the grating portion is 1 mm. On the other hand, the phase condition, the wavelength which satisfies become discrete, in △ lambda _G, are designed to be (2-3) equation points 2-3. For this reason, the thing with a long active layer length of a laser medium is needed, and the thing of 1 mm or more is used.

In the case of a glass waveguide or FBG, the temperature dependence of λg is very small, and dλ _{G /} dT = about 0.01 nm / ° C. For this reason, the external cavity laser has a feature of high wavelength stability.

However, the temperature dependence of the wavelength satisfying the phase condition is as large as dλ _s /dT=0.05 nm / ° C., and the difference is 0.04 nm / ° C.

In general, the temperature T _{mh at} which the mode hop occurs can be considered as the following equation from Non-Patent Document 1 (considered as Ta = Tf).
ΔG _TM is a wavelength interval (longitudinal mode interval) that satisfies the phase condition of the external cavity laser.

Thus, in the conventional case, T _mh is about 5 ° C. For this reason, mode hops are likely to occur. Therefore, when a mode hop occurs, the power fluctuates based on the reflection characteristics of the grating and fluctuates by 5% or more.

From the above, in actual operation, the conventional external cavity laser using the glass waveguide or FBG performs temperature control using the Peltier element.

On the other hand, in this embodiment, a grating element having a small denominator of the equation (2-4) is used as a precondition. The denominator of formula (2-4) is preferably 0.03 nm / ° C. or less. Specific optical materials include gallium arsenide (GaAs), lithium niobate (LN), lithium tantalate (LT), oxidation Tantalum (Ta ₂ O ₅ ), zinc oxide (ZnO), alumina oxide (Al ₂ O ₃ ), and titanium oxide (TiO ₂ ) are preferable.

It has been found that if there are 5 or less wavelengths satisfying the phase condition within Δλ _G , operation is possible under stable laser oscillation conditions even if mode hopping occurs.

That is, in this embodiment, for example, when using z-axis polarized light of lithium niobate, the oscillation wavelength changes at 0.1 nm / ° C. based on the temperature characteristics of the grating with respect to the temperature change. Even if it occurs, it is possible to make it difficult for power fluctuations to occur. The present structure, △ lambda grating length in order to increase the _G L _b is a 100μm example, △ G _TM to be increased L _a is a 250μm example.

In addition, it supplements about the difference with patent document 6. FIG.
The present application presupposes that the temperature coefficient of the grating wavelength and the temperature coefficient of the gain curve of the semiconductor are close to each other. For this reason, a material having a refractive index of 1.7 or more is used. Further grating groove depth _{t d} of 20nm or more, and 250nm or more, the reflectance of 3% or more, 60% or less, and 0.8nm over the full width at half maximum △ lambda _G, is set to 250nm or less. As a result, the resonator structure can be made compact and temperature-independence can be realized without any additional components. In patent document 6, each parameter is described as follows, and each is in the category of the prior art.
△ λ _G = 0.4nm
Vertical mode interval △ G _TM = 0.2nm
Grating length L _b = 3mm
LD active layer length L _a = 600 μm
Propagation length = 1.5mm

Each condition will be described more specifically below.
0.8 nm ≦ Δλ _G ≦ 6.0 nm (1)
10 μm ≦ L _b ≦ 300 μm (2)
20 nm ≦ t _d ≦ 250 nm (3)
n _b ≧ 1.7 (4)

In the formula (4), the refractive index _{n b} of the material constituting the optical waveguide is set to 1.7 or more.
Conventionally, a material having a lower refractive index, such as quartz, has been generally used. However, in the concept of the present invention, the refractive index of the material constituting the Bragg grating is increased. This is because a material with a large refractive index has a large temperature change in the refractive index, so that T _mh in equation (2-4) can be increased, and the temperature coefficient dλ _G / dT of the grating as described above. It is because it can enlarge. From this viewpoint, it is preferred that the _{n b} is 1.8 or more, and more preferably 1.9 or more. The upper limit of n _b is not particularly preferably 4 or less since the formed grating pitch becomes too small it is difficult.

The full width at half maximum Δλ _G at the peak of the Bragg reflectivity is set to 0.8 nm or more (Formula 1). λ _G is the Bragg wavelength. That is, as shown in FIG. 8 and FIG. 9, when the reflection wavelength by the Bragg grating is taken on the horizontal axis and the reflectance is taken on the vertical axis, the wavelength at which the reflectance becomes maximum is taken as the Bragg wavelength. In the peak centered on the Bragg wavelength, the difference between the two wavelengths at which the reflectance is half of the peak is defined as the full width at half maximum Δλ _G.

The full width at half maximum Δλ _G at the peak of the Bragg reflectance is set to 0.8 nm or more (formula (1)). This is to make the reflectance peak broad. From this viewpoint, the full width at half maximum Δλ _G is preferably set to 1.2 nm or more, and more preferably set to 1.5 nm or more. The full width at half maximum Δλ _G is 6 nm or less, more preferably 3 nm or less, and preferably 2 nm or less.

The length _{L b} of the Bragg grating to 300μm or less (equation 2). The length L _b of the Bragg grating is a grating length in the direction of the optical axis of the light propagating through the optical waveguide. Be shorter than the Bragg grating length L _b below the conventional 300μm is a premise of the design concept of the present embodiment. That is, it is necessary to increase the wavelength interval (longitudinal mode interval) that satisfies the phase condition in order to make mode hopping difficult. For this purpose, it is necessary to shorten the resonator length, and to shorten the length of the grating element. From this viewpoint, it is more preferable that the Bragg grating length L _b and 200μm or less.

Reducing the length of the grating element reduces the loss and can reduce the laser oscillation threshold. As a result, driving with low current, low heat generation, and low energy is possible.

The length L _b of the grating, in order to obtain a reflectance of 3% or more is preferably at least 5 [mu] m, in order to obtain a reflectance of 5% or more, more preferably more than 10 [mu] m.

In the formula (3), t _d is the depth of the irregularities constituting the Bragg grating. By setting 20 nm ≦ t _d ≦ 250 nm, Δλ _G can be set to 0.8 nm or more and 6 nm or less, and the number of longitudinal modes can be adjusted to 2 or more and 5 or less in Δλ _G. . From such a viewpoint, td is more preferably 30 nm or more, and further preferably 200 nm or less. In order to set the full width at half maximum to 3 nm or less, 150 nm or less is preferable.

In a preferred embodiment, the reflectance of the grating element is preferably set to 3% or more and 40% or less in order to promote laser oscillation. This reflectivity is more preferably 5% or more in order to further stabilize the output power, and more preferably 25% or less in order to increase the output power.

As shown in FIG. 11, the laser oscillation condition is established from a gain condition and a phase condition. Wavelengths that satisfy the phase condition are discrete and are shown, for example, in FIG. That is, in this structure, the oscillation wavelength can be fixed within Δλ _G by bringing the temperature coefficient of the gain curve (0.3 nm / ° C. in the case of GaAs) close to the temperature coefficient dλ _G / dT of the grating. Further △ lambda _G number of longitudinal modes are two or more in, when present 5 or less, the oscillation wavelength repeats mode hopping in the △ lambda _G, large because it can reduce the probability of laser oscillation outside the △ lambda _G There is no mode hop, the wavelength is stable, and the output power can operate stably.

In a preferred embodiment, also to 500μm or less length L _a of the active layer. From this viewpoint, it is more preferable to set the length L _a of the active layer and 300μm or less. The length L _a of the active layer with a view to increasing the output of the laser, it is preferable that the 150μm or more.

In equation (6), dλ _G / dT is the temperature coefficient of the Bragg wavelength.
Dλ _TM / dT is a temperature coefficient of the wavelength that satisfies the phase condition of the external cavity laser.
Here, λ _TM is a wavelength that satisfies the phase condition of the external cavity laser, that is, a wavelength that satisfies the above-described phase condition of (Equation 2.3). This is called “vertical mode” in this specification.

The following supplements the vertical mode.
In the formula (2.3), β = 2πn _eff / λ, where n _eff is the effective refractive index of the portion, and λ satisfying this is λ _TM . φ2 is the phase change of the Bragg grating.

ΔG _TM is a wavelength interval (longitudinal mode interval) that satisfies the phase condition of the external cavity laser. lambda _TM Since the plurality of, means the difference of a plurality of lambda _TM.

Therefore, satisfying the equation (6) makes it possible to increase the temperature at which mode hops occur and to effectively suppress mode hops. The numerical value of the formula (6) is more preferably 0.025 or less.

In a preferred embodiment, the length _{L WG} grating element also to 600μm or less. _LWG is preferably 400 μm or less, and more preferably 300 μm or less. Further, _LWG is preferably 50 μm or more.

In a preferred embodiment, when functioning as an external cavity laser, the distance L _g between the light exit surface of the light source and the light entrance surface of the optical waveguide may be zero. However, considering durability against environmental temperature changes, L _g is set to 1 μm or more and 10 μm or less. As a result, stable oscillation is possible.

The length L _m of the entrance-side propagation unit, it is preferable as short as possible from the viewpoint of shortening the resonator length. However, in order to remove the optical electric field component of the non-propagating light that cannot be coupled from the light source to the optical waveguide, the thickness is set to 20 μm or more and 100 μm or less. However, it is not necessary to provide the incident side propagation part.

In order to ensure temperature stability, it is necessary to shorten the distance L (FIGS. 1 and 3) between the highly reflective end face opposite to the output side of the semiconductor laser and the exit end point of the grating. L is preferably 900 μm or less, more preferably 800 μm or less, and most preferably 600 μm or less.

Example 1
An apparatus as shown in FIGS. 1, 2, 4 and 7A was produced.
An optical material layer was formed by depositing 0.5 μm of a lower buffer layer 16 made of SiO ₂ and 1.2 μm of Ta ₂ O ₅ on the lower buffer layer 16 made of SiO ₂ that functions as a clad by a sputtering apparatus on a support substrate 10 made of quartz. Next, Ti was formed on the optical material layer made of Ta ₂ O _5, and a grating pattern was produced by a photolithography technique (EB drawing apparatus). Thereafter, the Bragg grating 12 having a pitch interval of Λ238.5 nm and a length of L _{b of} 100 μm was formed by fluorine-based reactive ion etching using the Ti pattern as a mask. Groove depth t _d of the grating 12 was set to 40 nm.

Further, in order to form an optical waveguide, reactive ion etching was carried out in the same manner as described above, and a width W _{m of} 3 μm and both sides were etched so that the optical material layer was completely cut by 1.2 μm.
Here, the optical waveguide width _{W m} of the Bragg grating 12 was set to 3 [mu] m. At the same time, as shown in FIGS. 1, 2, and 4, a connecting portion 20a having a constant width, a tapered portion 20b, and an emitting portion 20c having a constant width are provided. The dimensions in each part were as follows.
Core part width in the connecting part 20a: 3 μm
Core part width W _{out at the} emitting part 20c: 0.5 μm
Core part width W _{t at the} taper part 20b: 0.5 to 3 μm

Finally, an SiO ₂ layer serving as an upper cladding layer was formed by 2 μm sputtering so as to cover the waveguide.
Then, it cut | disconnected in bar shape with the dicing apparatus, both ends were optically polished, both ends were formed with a 0.1% AR coat, and finally the chip was cut to produce a grating element. The element size was 1 mm wide and L _wg 500 μm long.

Next, the optical characteristics of the grating element are obtained by using a super luminescence diode (SLD), which is a broadband wavelength light source, to input light into the grating element and analyzing the output light with an optical spectrum analyzer. The reflection characteristics were evaluated. As a result, with respect to the TE mode, a center wavelength of 975 nm, a maximum reflectance of 20%, and a full width at half maximum Δλ _G of 2 nm were obtained.

Next, a laser module was mounted as shown in FIGS. The light source element was a normal GaAs laser, and the exit end face was not coated with AR.
Light source element specifications:
Center wavelength: 977nm
Output: 40mW
Half width: 0.1nm
Laser element length 250μm
Mounting specifications:
L _g : 1 μm
L _m : 20μm

After mounting the module, when a semiconductor laser was driven by current control (ACC) without using a Peltier element, without using a monitoring photodiode, it oscillated at a center wavelength of 975 nm corresponding to the reflected wavelength of the grating, and output Was smaller than that without the grating element, but the laser characteristic was 35 mW.

The shape of the near field pattern on the output side end face of the grating element was 0.5 μm in the horizontal direction and 1 μm in the vertical direction, and was almost a perfect circle. The single mode was maintained even when the temperature was changed from 20 ° C to 70 ° C.

(Comparative example)
In Example 1, over the entire length of the optical waveguide 11, the optical waveguide width is constant and 3 [mu] m, a height T _s was constant at 0.5 [mu] m. Thereafter, a grating element was produced by the same method.

After mounting the module, when a semiconductor laser is driven by current control (ACC) without using a Peltier element, without using a monitor photodiode, it oscillates at a center wavelength of 975 nm corresponding to the reflected wavelength of the grating, and outputs. Was smaller than that without the grating element, but the laser characteristic was 30 mW.

The shape of the near field pattern on the output side end face of the grating element was a flat waveguide having an aspect ratio of 3 with a horizontal direction of 3 μm and a vertical direction of 1 μm. Further, when the temperature was changed from 20 ° C. to 70 ° C., multimode was excited at around 70 ° C.

Claims

An external resonator type light emitting device comprising a semiconductor laser light source and a grating element constituting the semiconductor laser light source and an external resonator,
The semiconductor laser light source includes an active layer that oscillates semiconductor laser light,
The grating element is an optical waveguide having an incident surface on which the semiconductor laser light is incident and an emission surface that emits outgoing light of a desired wavelength, and an optical waveguide having a convex cross section in the optical waveguide. A Bragg grating composed of the formed irregularities, and an emission-side propagation portion provided between the Bragg grating and the emission surface, and lasing in the wavelength range reflected by the Bragg grating, and the Bragg grating in the Bragg grating An external resonator type light emitting device, wherein the width of the optical waveguide is different from the width of the optical waveguide at the exit surface.
The apparatus according to claim 1, wherein the width of the optical waveguide at the exit surface is smaller than the width of the optical waveguide at the Bragg grating.
3. The apparatus according to claim 2, wherein the horizontal width of the near field at the entrance surface of the grating element is larger than the horizontal width of the near field at the exit surface.
4. The output side propagation part includes a taper part in which the width of the optical waveguide decreases from the Bragg grating side toward the output surface side. A device according to claim.
The grating element further includes a support substrate, the optical waveguide is provided on the support substrate, and the thickness of the optical waveguide is 0.5 μm or more and 3.0 μm or less, The device according to any one of claims 1 to 4.
6. The material constituting the optical waveguide is selected from the group consisting of gallium arsenide, lithium niobate, tantalum oxide, zinc oxide, alumina oxide, lithium tantalate, and titanium oxide. Apparatus according to any one of the claims.
The device according to any one of claims 1 to 6, wherein a relationship of the following formulas (1) and (2) is satisfied.

10 μm ≦ L b ≦ 300 μm (1)
20 nm ≦ td ≦ 250 nm (2)

In (Equation (1), L b is the length of the Bragg grating.
In Formula (2), td is the depth of the unevenness constituting the Bragg grating. )
The distance between the end surface opposite to the emission side of the active layer of the semiconductor laser and the emission side end point of the Bragg grating is 900 μm or less. The device described in 1.
The device according to any one of claims 1 to 8, wherein a relationship of the following formulas (3) and (4) is satisfied.

0.8 nm ≦ Δλ G ≦ 6.0 nm (3)
n b ≧ 1.7 (4)

(In formula (3), Δλ G is the full width at half maximum at the peak of the Bragg reflectivity.
In the formula (4), n b is the refractive index of the material constituting the optical waveguide. )
The apparatus according to any one of claims 1 to 9, wherein the relationship of the following formula (5) is satisfied.

L WG ≦ 500 μm (5)

(In Formula (5), LWG is the length of the grating element.)
The apparatus according to claim 9 or 10, wherein the full width at half maximum Δλ G includes wavelengths that can satisfy a phase condition of laser oscillation of 2 to 5 inclusive.
The apparatus according to any one of claims 1 to 11, wherein a relationship of the following formula (6) is satisfied.

(In formula (6), dλ G / dT is a temperature coefficient of the Bragg wavelength.
dλ TM / dT is the temperature coefficient of the wavelength that satisfies the phase condition of the external cavity laser. )