WO2016093187A1

WO2016093187A1 - External resonator type light-emitting device

Info

Publication number: WO2016093187A1
Application number: PCT/JP2015/084242
Authority: WO
Inventors: 近藤　順悟; 山口　省一郎; 隆史吉野; 武内　幸久
Original assignee: 日本碍子株式会社
Priority date: 2014-12-10
Filing date: 2015-12-07
Publication date: 2016-06-16

Abstract

Provided is an external resonator type light-emitting device provided with a light source 1 that causes semiconductor laser light to oscillate, and a grating element 9 constituting, together with the light source 1, an external resonator, the device producing a single longitudinal mode oscillation. The light source is provided with an active layer 7 which causes the semiconductor laser light to oscillate. The grating element 9 is provided with: an optical waveguide 18 including an incident portion 49 on which the semiconductor laser light is incident, and an emitting portion 31 that emits a desired wavelength of emitted light; a Bragg grating 12 formed in the optical waveguide 18; a propagation portion 18a disposed between the incident portion 49 and the Bragg grating 12; and an optical path modification unit that bends the optical path of the semiconductor laser light, in the grating element. The relationships according to expressions (1) to (4) are satisfied.

Description

External resonator type light emitting device

The present invention relates to an external resonator type light emitting device.

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.

For this reason, a single mode oscillation laser with high wavelength stability is required for purposes such as optical communication and gas sensing. 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.

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 one wavelength of light 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. Moreover, 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.

Patent Document 7 (Japanese Patent Laid-Open No. 2010-171252) discloses an optical waveguide having SiO ₂ , SiO _1-x N _x (x is 0.55 to 0.65), or Si and SiN as a core layer, and the optical waveguide Discloses an external cavity laser in which a grating is formed. This is an external cavity laser that keeps the oscillation wavelength constant without precise temperature control. For this purpose, it is a precondition that the temperature change rate of the reflection wavelength of the diffraction grating (temperature coefficient of the Bragg reflection wavelength) is reduced. In addition, it is described that the power stability can be realized by setting the laser oscillation to the longitudinal mode multimode.

Patent Document 8 (Patent No. 3667209) discloses a laser as an external resonator using a grating formed on an optical waveguide made of quartz, InP, GaAs, LiNbO ₃ , LiTaO ₃ , or polyimide resin. This is because the reflectivity at the light exit surface of the semiconductor laser as the light source is the effective reflectivity Re (substantially 0.1 to 38.4%), and the laser oscillation is set to the longitudinal mode multimode. It is described that power stability can be realized.

JP 49-128689 JP 56-148880 WO2013 / 034813 JP2000-082864 JP 2006-222399 JP2002-134833 JP2010-171252 Patent No. 3667209 Patent No.3404985 JP 2014-222331 JP2000-171650 JP2010-151973

Non-Patent Document 1 mentions a mode hop mechanism that impairs the wavelength stability associated with a temperature rise, and an improvement measure thereof. Wavelength variation [delta] [lambda] _s of the external cavity laser according to the temperature, the refractive index change of the semiconductor of the active layer region △ n _a, the length of the active layer L _a, the refractive index change of the FBG region △ n _f, the length L _f, Each temperature change ΔT _a and ΔT _f is expressed by the following equation from the standing wave condition.

Here, λ ₀ represents the grating reflection wavelength in the initial state.
Further, the change Δλ _G 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 change amount Δλ _s and the grating reflection wavelength change amount Δλ _G , the following equation is established.

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

From Equation 3 and Equation 4, Equation 5 is established.

In order to suppress the mode hop, it is necessary to use the temperature within ΔT _all or less, and the temperature is controlled by a Peltier element. In Formula 5, when the refractive index changes of the active layer and the grating layer are the same (Δn _a / n _a = Δn _f / n _f ), the denominator becomes zero, the temperature at which the mode hop occurs becomes infinite, and the mode It indicates that there are no hops. However, in the monolithic DBR laser, since current is injected into the active layer in order to oscillate the laser, the change in refractive index between the active layer and the grating layer cannot be matched, resulting in a mode hop.

Mode hopping is a phenomenon in which the oscillation mode (longitudinal mode) in the resonator shifts 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.

Patent Document 9 discloses a structure in which a waveguide and a light receiving element are integrated on a semiconductor substrate for miniaturization and simplification as an example of an optical integrated circuit as an application of a semiconductor laser. A curved optical waveguide, a tapered waveguide, and a light receiving element are mounted on the surface of the semiconductor substrate, and a semiconductor laser or an optical fiber is connected to the side surface portion, whereby a compact optical transceiver can be realized.

In Patent Document 10, a plurality of linear waveguides that are periodically poled on a lithium niobate substrate are serially connected by a curved waveguide to realize a compact and high-output wavelength conversion element.

Patent Document 11 discloses an optical integrated circuit in which an optical waveguide formed of SiO ₂ and a mirror are planarly formed on a silicon substrate.

In

Non-Patent Documents

3 and 4 and Patent Document 12, in order to reduce the bending loss of a bent optical waveguide, a straight waveguide and a curved waveguide with a uniform bending radius are shifted from each other in the central axis of each waveguide. A connection (offset connection) method is disclosed. Moreover, in the description of these documents, offset connection is performed in an optical waveguide formed in a semiconductor such as quartz glass, lithium niobate, or InP. However, the radius of curvature of the curved portion of the optical waveguide is 1 mm or more.

Non-Patent Document 4 uses an offset technique, but there is a description that the radius of curvature of the curved portion can be 250 μm.
However, recently, it has been desired to further reduce the size of the optical waveguide element, and for this reason, it has become necessary to make the curvature radius of the curved portion of the optical waveguide 100 μm or less. However, it is difficult to realize such a bending radius in the channel type optical waveguide of the optical waveguide substrate.

Further, Non-Patent Document 3 discloses a structure using a core having high refractive index of silicon photonics. In this case, the SiO ₂ having a refractive index of 1.45 was formed as a cladding on silicon, the Si of refractive index 3.8 to form a core thereon, thereby forming a SiO ₂ having a refractive index of 1.45 as overclad An embedded waveguide is used. The refractive index difference Δn between the core and the clad is 2 or more, and the cross section of the core is 0.3 μm × 0.3 μm. This is such a dimension in order to set the transverse mode to the single mode because the difference in refractive index between the core and the clad is large.

Silicon photonics has a problem that an ultrafine semiconductor patterning technology is required for device manufacturing, and that a coupling loss increases when a semiconductor laser or an optical fiber is connected. Furthermore, it is a problem that it cannot be used at a wavelength of 1 μm or less due to absorption of silicon.

The problem of the present invention is that, without using a Peltier element, mode hopping is suppressed, wavelength stability is increased, optical power fluctuation is suppressed, and a semiconductor laser or a passive element is mounted in a planar manner by bending an optical path. It is to be able to be downsized and simplified.

Also, an object of the present invention is to provide an optical waveguide substrate having a structure in which the curvature radius of the curved portion can be 100 μm or less in the optical waveguide substrate including the curved portion of the channel type optical waveguide.

The present invention is an external resonator type light emitting device including a light source that oscillates a semiconductor laser beam, and a grating element that constitutes the light source and an external resonator,
The light source includes an active layer that oscillates the semiconductor laser light;
The grating element includes an optical waveguide having an incident portion where the semiconductor laser light is incident and an output portion that emits outgoing light of a desired wavelength, a Bragg grating formed in the optical waveguide, and the incident portion and the Bragg grating. An optical path changing unit that bends the optical path of the semiconductor laser light in the grating element, and satisfies the relations of the following formulas (1) to (4): And

Δλ _G ≧ 0.8 nm (1)
L _b ≦ 500 μm (2)
L _a ≦ 500 μm (3)
n _b ≧ 1.8 (4)

(In formula (1), Δλ _G is the full width at half maximum at the peak of the Bragg reflectivity.
In Expression (2), L _b is the length of the Bragg grating.
In the formula (3), _{L a} is the length of the active layer.
In the formula (4), n _b is the refractive index of the material constituting the Bragg grating. )

The present invention also includes a channel-type optical waveguide having an elongated core for propagating light and a clad in contact with the core, and a curved portion where the core is curved when viewed from the main surface of the optical waveguide substrate. An optical waveguide substrate comprising:
The cross section of the core has a convex shape, the refractive index of the core is 1.7 or more and 3.5 or less, and the difference Δn between the refractive index of the core and the refractive index of the cladding is 0. 3 or more, the width of the core is 1.5 μm or less, the thickness of the core is 0.5 μm or more and 2.0 μm or less, and the curvature radius of the curved portion is 100 μm or less. To do.

According to the present invention, mode hops can be suppressed, wavelength stability can be increased, and optical power fluctuations can be suppressed without using a Peltier element.
In general, 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, the temperature range ΔT in which the mode hop occurs becomes small, and the mode hop is likely to occur.

For this reason, in the present invention, a material having a refractive index of 1.8 or more of the waveguide substrate on which the grating is formed is used. 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. Furthermore, this can increase the difference in refractive index between the optical waveguide (core portion) and the peripheral portion (cladding portion), so that excessive loss due to bending when the optical path changing portion is formed can be suppressed.
And without using Peltier elements, mode hops are suppressed, wavelength stability is increased, optical power fluctuations are suppressed, and semiconductor lasers and passive elements are mounted in a plane by bending the optical path, downsizing, It can be simplified.

The distance L _g between the exit surface and entrance surface of the optical waveguide of the light source, the coupling efficiency of the light source and the optical waveguide may be less than 1μm in terms of maximizing. However, from the viewpoint of operating in a wide temperature range, it is necessary to prevent mechanical interference due to thermal expansion, and in a preferred embodiment, equation (6) is satisfied.
1 μm ≦ L _g ≦ 10 μm (6)
(In Formula (6), L _g is the distance between the exit surface of the light source and the entrance surface of the optical waveguide.)

In a preferred embodiment, the exit portion of the emitted light is the end face of the optical waveguide.
In a preferred embodiment, the incident portion and the emission portion are not provided on the side surfaces facing each other of the grating element, and are, for example, on the same surface or on two adjacent side surfaces.
In a preferred embodiment, the Bragg grating is downstream of the optical path changer.
In a further preferred embodiment, in order to reduce the influence of the optical axis shift of the light source, the propagation part provided between the incident part and the Bragg grating has a tapered structure that changes the width of the optical waveguide. It may be.
In a preferred embodiment, the external resonator type light emitting device of the present invention oscillates in a single mode in the longitudinal mode.

In a preferred embodiment, the formula (5) may be further satisfied.

In equation (5), dλ _G / dT is the temperature coefficient of the Bragg wavelength, and dλ _TM / dT is the temperature coefficient of the wavelength that satisfies the phase condition of the external cavity laser.

FIG. 1 is a plan view schematically showing an external resonator type light emitting device. FIG. 2 is a side view schematically showing the apparatus of FIG. 3 is a cross-sectional view showing a grating element 9. FIG. It is a cross-sectional view showing the grating element 9A. It is a cross-sectional view showing the grating element 9B. It is a figure which shows a part of Bragg grating. It is a top view which shows typically another external resonator type light-emitting device. It is a top view which shows typically another external resonator type light-emitting device. It is a top view which shows typically another external resonator type light-emitting device. It is a top view which shows typically another external resonator type light-emitting device. It is a side view which shows the apparatus of FIG. 10 typically. It is a top view which shows typically another external resonator type light-emitting device. It is a top view which shows typically another external resonator type light-emitting device. It is a top view which shows typically another external resonator type light-emitting device. It is a top view which shows typically another external resonator type light-emitting device. It is a top view which shows typically another external resonator type light-emitting device. It is a top view which shows typically another external resonator type light-emitting device. It is a top view which shows typically another external resonator type light-emitting device. 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. It is a figure explaining the form of the mode hop by the example of this invention. The reflection characteristics (gain conditions) and phase conditions in the conventional structure are shown. 3 shows reflection characteristics (gain conditions) and phase conditions in the structure of the present invention. (A), (b), (c) is a schematic diagram which shows the cross section of the

grating elements

21A, 21B, and 21C using the elongated striped

optical waveguides

30 and 30A, respectively. (A), (b) is a schematic diagram which shows the cross section of the

grating elements

21D and 21E using the elongate stripe-shaped

optical waveguides

30 and 30A, respectively. It is a schematic diagram of the optical waveguide of another form. (A), (b) shows the mirror which is an optical path changing part of an optical waveguide, respectively. It is a top view which shows an example of a curved optical waveguide. It is a top view which shows an example of a straight optical waveguide. It is a top view which shows typically the external resonator type light-emitting device concerning other embodiment. It is a top view which shows typically the external resonator type light-emitting device concerning other embodiment. It is a schematic diagram which shows the cross section of the optical waveguide board | substrate 75 using the elongate stripe-shaped optical waveguide 30. FIG. An example of the pattern of the optical waveguide containing a curved part is shown. W _b: is a graph showing the waveguide propagation efficiency in the case of a 100μm from the radius of curvature 10μm in the case of 0.5 [mu] m. It is a graph which shows the propagation efficiency calculated by varying the offset amount of Offset1 about W _b : 0.5 μm. W _b: is a graph showing the propagation efficiency calculated by varying the offset amount of Offset2 about 0.5 [mu] m. W _b: is a graph showing the propagation efficiency calculated by varying the offset amount of Offset3 about 0.5 [mu] m. The cross-sectional shape of the high mesa type optical waveguide adopted in Reference Example 5 is shown. The calculation result of the waveguide propagation efficiency when the refractive index of the core is 1.5 and the refractive index of the clad on the side surface is 1 is shown. The result of calculating the waveguide propagation efficiency when the refractive index of the core is 1.7 and the refractive index of the cladding is 1.4 is shown.

An external resonator type light emitting device 1 schematically shown in FIGS. 1 and 2 includes a light source 1 that oscillates a semiconductor laser beam and a grating element 9 (or 9A, 9B). The light source 1 and the grating element 9 are preferably mounted on a common substrate (not shown).

The light source 1 includes an active layer 7 that oscillates semiconductor laser light. Here, the light source 1 can be a light source capable of laser oscillation independently. This means that the light source 1 oscillates itself even without a grating element.

The light source 1 preferably has a single mode oscillation in the longitudinal mode when laser oscillation is performed independently. However, in the case of an external resonator type laser using a grating element, the reflection characteristic can be given wavelength dependency, so that the longitudinal mode can be set to the multimode independently by controlling the shape of the wavelength characteristic. Even if it oscillates, it can oscillate in a single mode as an external resonator type laser.
In this case, a highly reflective film 3A is provided on the outer end face of the light source 1, and a film 4 having a reflectance smaller than that of the grating is formed on the end face on the grating element side.

The light source 1 may be a super luminescence diode or a semiconductor optical amplifier (SOA) that does not oscillate alone. In this case, the highly reflective film 3A is provided on the outer end face of the light source substrate, and the antireflective film 4 is formed on the end face on the grating element side.

FIG. 1 shows a case where a semiconductor laser and an optical element are mounted on the same surface. In the grating element 9 (9A, 9B), an optical waveguide 18 through which light from the semiconductor laser light source 1 propagates and a Bragg grating 12 are formed. An external resonator is configured between the semiconductor laser and the Bragg grating, and the laser oscillates at a wavelength that satisfies the Bragg diffraction conditions of the grating.

The grating element 9 has four

side surfaces

9a, 9b, 9c, and 9d, and the light source 1 and the optical waveguide element 21 are attached to the side surface 9a so as to face each other. 3B is a reflective film or a non-reflective film. The optical waveguide 18 includes an incident side propagation part 18a, a Bragg grating 12, and emission

side propagation parts

18b, 18c, 18d on the downstream side thereof. A Bragg grating is not formed in the propagation part. A mirror 8A is installed at the end of the propagation part 18b, and a propagation part 18c intersecting the propagation part 18b is provided. Further, a mirror 8B is installed between the

propagation portions

18c and 18d. The light incident from the incident surface 49 of the optical waveguide propagates through the incident-side propagation part 18a, passes through the Bragg grating, undergoes wavelength selection, propagates through the propagation part 18b, is reflected by the

mirrors

8A and 8B, and is emitted from the emission surface 31. To the outside of the device. In this example, the entrance surface and the exit surface of the optical waveguide are on the same side surface 9a.

The light emitted from the emission surface 31 can be incident on the optical waveguide 22 of the separate optical waveguide element 21.

Reference numerals

5 and 6 denote non-reflective films.

Such an optical waveguide element or optical element is not particularly limited, but may be an optical waveguide device such as a wavelength conversion element, an optical modulator, an optical filter, an optical isolator, or an optical fiber, or a photodiode. Also good.

In a preferred embodiment, the reflectivity of the Bragg grating is larger than the reflectivity of the emission end of the light source, the reflectivity of the entrance surface of the grating element, and the reflectivity of the exit surface of the grating element. From this point of view, it is preferable that the reflectance at the light emitting end of the light source, the reflectance at the entrance surface of the grating element, and the reflectance at the exit surface of the grating element are 0.1% or less. The reflectance of the non-reflective layer 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.

The light source 1 includes an active layer 7 that oscillates laser light. However, a non-reflective layer is not provided on the end surface of the active layer 7 on the grating element 9 side, and a reflective film can be formed instead. In this case, 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 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 7.

The sectional structure of the grating element and the sectional structure of the ridge type optical waveguide are not particularly limited, but an example is shown below.
In the example shown in FIG. 3, the optical material layer 11 is provided on the support substrate 10. The optical material layer 11 may be formed on the same surface as the Bragg grating 12 or may be formed on an opposite surface.

In the example shown in FIG. 3, the optical material layer 11 is formed on the substrate 10 via the adhesive layer 15 and the lower buffer layer 16, and the upper buffer layer 17 is formed on the optical material layer 11. For example, a pair of ridge grooves 19 are formed in the optical material layer 11, and a ridge-type optical waveguide 18 is formed between the ridge grooves. In this case, the Bragg grating may be formed on the flat surface 11a or may be formed on the 11b surface. From the viewpoint of reducing the shape variation of the Bragg grating and the ridge groove, it is preferable that the Bragg grating and the ridge groove 19 are provided on the opposite side of the substrate by forming the Bragg grating on the surface 11a.

In the element 9A shown in FIG. 4, the optical material layer 11 is formed on the substrate 10 via the adhesive layer 15 and the lower buffer layer 16, and the upper buffer layer 17 is formed on the optical material layer 11. Yes. For example, a pair of ridge grooves 19 are formed on the substrate 10 side of the optical material layer 11, and a ridge-type optical waveguide 18 is formed between the ridge grooves 19. In this case, the Bragg grating may be formed on the flat surface 11a side, or may be formed on the 11b surface having the ridge groove. From the viewpoint of reducing the shape variation of the Bragg grating and the ridge groove, it is preferable to provide the Bragg grating and the ridge groove 19 on the opposite side of the substrate by forming the Bragg grating on the flat surface 11a surface side. Further, the upper buffer layer 17 may be omitted, and in this case, the air layer can directly contact the grating. As a result, 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.

5, the optical material layer 11 is formed on the substrate 10 via the lower buffer layer 16, and the upper buffer layer 17 is formed on the optical material layer 11. For example, a pair of ridge grooves 19 are formed in the optical material layer 11, and a ridge-type optical waveguide 18 is formed between the ridge grooves 19. In this example, an adhesive layer is not provided, and a buffer layer and an optical material layer are sequentially formed on the substrate 10 by a vapor phase method.

FIG. 6 is a perspective view showing an example of the form of the Bragg grating. td is the grating depth and Λ is the period.

FIG. 7 shows a case where the semiconductor laser and the optical element are mounted on the same surface. In the grating element 29A, an optical waveguide 28 through which light from the semiconductor laser light source 1 propagates and a Bragg grating 12 are formed. An external resonator is configured between the semiconductor laser and the Bragg grating, and the laser oscillates at a wavelength that satisfies the Bragg diffraction conditions of the grating.

The grating element 29A has four

side surfaces

29a, 29b, 29c, and 29d, and the light source 1 and the optical waveguide element 21 are attached to the side surface 29a so as to face each other. A reflective film and a non-reflective film provided on the end face of each element are not shown. The optical waveguide 28 includes incident-

side propagation portions

28a, 28b, 28c, a Bragg grating 12, and an emission-side propagation portion 28d. A Bragg grating is not formed in each propagation part. A mirror 8A is installed between the

propagation units

28a and 28b, and a miper 8B is installed between the

propagation units

28b and 28c. In this example, the entrance surface and the exit surface of the optical waveguide are on the same side surface 9a.

In this example, the Bragg grating 12 is downstream of the mirror that is the optical path changing unit. In such an optical path changing unit, the fundamental mode of the transverse mode propagates efficiently through the optical waveguide. Therefore, the fundamental mode can be selectively oscillated by arranging the grating in the subsequent stage.

In the example of FIG. 8, the grating element 29B has four

side surfaces

29a, 29b, 29c, and 29d, and the light source 1 and the optical waveguide element 21 are attached to the side surface 29a so as to face each other. A reflective film and a non-reflective film provided on the end face of each element are not shown. The optical waveguide 32 includes an incident-side propagation part 32a, a Bragg grating 12, and emission-

side propagation parts

32b, 32c, and 32d. In this example, the

propagation portions

32a, 32b, and 32d extend straight, and the propagation portion 32c is curved to form an optical path changing portion. As a result, the optical waveguide 32 is bent by 180 °, and the incident surface 49 and the emission surface 31 of the optical waveguide 32 are provided on the same side surface 29a.

In the example of FIG. 9, the optical waveguide 33 includes incident-

side propagation portions

33a, 33b, 33c, a Bragg grating 12, and an emission-side propagation portion 33d. In this example, the

propagation portions

33a, 33c, and 33d extend straight, and the propagation portion 33b is curved to form an optical path changing portion. As a result, the optical waveguide 33 is bent by 180 °, and the incident surface 49 and the emission surface 31 of the optical waveguide 33 are provided on the same side surface 29a. A reflective film and a non-reflective film provided on the end face of each element are not shown.

In this example, the Bragg grating 12 is downstream of the curved portion 33b which is an optical path changing portion. Such an optical path changing unit can suppress the propagation of the multimode in the transverse mode, and therefore, it is possible to selectively oscillate the fundamental mode by arranging the grating in the subsequent stage.

10 and 11 show the case where the semiconductor laser and the optical element are mounted on the same surface. In the grating element 39, an optical waveguide 40 through which light from the semiconductor laser light source 1 propagates and a Bragg grating 12 are formed. A recess 42 is formed in the grating element 39 by etching or polishing, and the light source 1 is mounted in the recess 42. An external resonator is formed between the light source 1 and the Bragg grating 12, and laser oscillation is performed at a wavelength satisfying the Bragg diffraction condition of the grating.

The grating element 39 has four

side surfaces

39a, 39b, 39c and 39d, and is attached so that the optical waveguide element 21 faces the side surface 39a. A reflective film and a non-reflective film provided on the end face of each element are not shown. The optical waveguide 40 includes an incident-side propagation part 40a, a Bragg grating 12, and emission-

side propagation parts

40b, 40c, and 40d. A mirror 8A is installed between the

propagation units

40b and 40c, and a mirror 8B is installed between the

propagation units

40c and 40d.

In this example, the entrance surface 49 of the optical waveguide is on the side surface 39e facing the recess 42, and the exit surface is on the side surface 39a.

In the example of FIG. 12, the light source 1 is mounted in the recess 42 of the grating element 39A. The optical waveguide 43 includes incident-

side propagation portions

43a, 43b, 43c, a Bragg grating 12, and an emission-side propagation portion 43d. A mirror 8A is installed between the

propagation parts

43a and 43b, and a mirror 8B is also installed between the

propagation parts

43b and 43c. A reflective film and a non-reflective film provided on the end face of each element are not shown.

In the example of FIG. 13, the light source 1 is mounted in the recess 42 of the grating element 39B. The optical waveguide 44 includes an incident side propagation part 44a, a Bragg grating 12, and emission

side propagation parts

44b and 44c. In this example, the

propagation portions

44a and 44c extend straight, and the propagation portion 44b is curved to constitute an optical path changing portion. As a result, the optical waveguide 44 is bent by 180 °, and the emission surface 31 of the optical waveguide is provided on the side surface 39a. A reflective film and a non-reflective film provided on the end face of each element are not shown.

In the example of FIG. 14, the optical waveguide 45 of the element 39C includes incident-

side propagation portions

45a, 45b, 45c, a Bragg grating 12, and an emission-side propagation portion 45d. In this example, the

propagation portions

45a, 45c, and 45d extend straight, and the propagation portion 45b is curved to constitute an optical path changing portion. As a result, the optical waveguide 45 is bent by 180 °, and the output surface 31 of the optical waveguide is provided on the side surface 39a. A reflective film and a non-reflective film provided on the end face of each element are not shown.

In the example of FIG. 15, the grating element 29A has four

side surfaces

29a, 29b, 29c, and 29d, and is attached so that the light source 1 faces the side surface 29a. The optical waveguide 28A includes incident-

side propagation portions

28a, 28b, 28c, a Bragg grating 12, and emission-

side propagation portions

28d, 28e. A mirror 8A is installed between the

propagation units

28a and 28b, a mirror 8B is installed between the

propagation units

28b and 28c, and a mirror 8C is installed between the

propagation units

28d and 28e. In this example, the incident surface 49 of the optical waveguide is on the side surface 29a, and the exit surface 31 is on the side surface 29d adjacent to the side surface 29a. A reflective film and a non-reflective film provided on the end face of each element are not shown.

In the example of FIG. 16, the optical waveguide 33A of the element 29C includes incident

side propagation portions

33a, 33b, 33c, a Bragg grating 12, and emission

side propagation portions

33d, 33e. In this example, the

propagation portions

33a, 33c, and 33e extend straight, and the

propagation portions

33b and 33d are curved to form an optical path changing portion. As a result, the optical waveguide 33A is bent twice. The incident surface 49 of the optical waveguide 33A is on the side surface 29a, and the exit surface 31 is on the adjacent side surface 29d.

In the example of FIG. 17, a plurality of

curved portions

50b and 50d are formed on the upstream side of the Bragg grating 12 of the element 29E. In other words, the optical waveguide 50 includes straight

propagation Bragg gratings

50a, 50c, 50e, and 50f and two

curved portions

50b and 50d. By forming a plurality of optical path changing portions before the Bragg grating in this way, the transverse mode can suppress the propagation of higher-order light, so that the laser oscillation of the fundamental mode can be promoted and the laser oscillation of the higher-order mode can be suppressed. . In this case, the side surface 29a on which the light source 1 is provided and the side surface 29b on which the optical waveguide element 21 is provided can be opposed to each other. A reflective film and a non-reflective film provided on the end face of each element are not shown.
Moreover, in the example of FIG. 17, the structure which uses a mirror instead of a curved waveguide may be sufficient.

In each example described above, a mirror or a curved portion was installed in the optical waveguide to bend the optical path. However, the mirror and the curved portion can be used in combination.

Also, an optical path changing unit and an external emitting unit that emits the emitted light after changing the optical path to the outside of the device can be provided on the downstream side of the emitting unit of the emitted light. FIG. 18 relates to this embodiment.

FIG. 18 shows a case where the semiconductor laser and the optical element are mounted on the same surface. A separate grating portion 52 is mounted on the grating element 53. In the grating portion 52, an optical waveguide 51 through which light from the light source 1 propagates and a Bragg grating 12 are formed. The optical waveguide 51 includes an incident side propagation part 51a and an emission side propagation part 51b. An external resonator is formed between the light source 1 and the Bragg grating 12, and laser oscillation is performed at a wavelength satisfying the Bragg diffraction condition of the grating. A reflective film and a non-reflective film provided on the end face of each element are not shown.

The grating element 53 has four side surfaces, and the light source 1 and the optical waveguide element 21 are attached to one side surface. The light emitted from the optical waveguide exit surface 31 of the grating element 52 propagates through the optical path 54A provided in the space, changes the optical path by the mirror 8A, propagates through the optical path 54B, and further changes the optical path by the mirror 8B. Propagate and enter the optical waveguide element 21 from the external emitting portion 55. 53 a is the surface of the element 53.

In the present invention, a laser with a highly reliable GaAs-based or InP-based material is suitable as the light source. 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. Since the temperature change of the Bragg wavelength increases as the wavelength becomes longer, the laser oscillation wavelength is particularly preferably 990 nm or less in order to improve the wavelength stability. On the other hand, since the refractive index change Δna of the semiconductor becomes too large when the wavelength is shortened, the laser oscillation wavelength is particularly preferably 780 nm or more in order to improve the wavelength stability. In addition, the material and wavelength of the active layer can be selected as appropriate.

A ridge-type optical waveguide is obtained by, for example, physical processing and molding by cutting with an outer peripheral blade or laser ablation processing.

The buffer layer can function as a cladding layer of the optical waveguide. From this viewpoint, the refractive index of the buffer layer is preferably lower than the refractive index of the optical material layer, and the refractive index difference is preferably 0.2 or more, and more preferably 0.4 or more.
In addition, when a curved portion (bent portion) is provided in the waveguide, the refractive index difference between the buffer layer and the optical material layer is preferably further increased in order to reduce the bending loss, and 0.5 or more is most preferable. preferable.

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 a high refractive index substrate, 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 material layer, one or more metals selected from the group consisting of magnesium (Mg), zinc (Zn), scandium (Sc), and indium (In) are used to further improve the optical damage resistance of the optical waveguide. Elements may be included, in which case magnesium is particularly preferred. 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 11 may be formed by forming a film on a supporting 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 11 is directly formed on the support substrate, and the above-described adhesive layer does not exist.
In this case, the optical material layer 11 may be further formed after forming the lower buffer layer on the support substrate. Further, an upper buffer layer may be formed on the upper surface of the optical material layer 11.
The thickness of the optical material layer is more preferably 0.5 to 3.0 μm.

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, Si, alumina, aluminum nitride, and sapphire.

The reflectance of the non-reflective layer must be less than or equal to the grating reflectivity. As the film material to be formed on the non-reflective layer, a film laminated with an oxide such as silicon dioxide or tantalum pentoxide, or metal is also used. Is possible.

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. In addition, the grating element and the support substrate may be bonded together by adhesion or direct bonding.

Hereinafter, the meaning of the conditions of the formulas (1) to (6) will be further described.
However, since the mathematical expressions are abstract and difficult to understand, first, typical features of the prior art and embodiments of the present invention will be compared briefly to describe the features of the present invention. Next, each condition of the present invention 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 α _a and α _b are the loss factors of the active layer and the grating layer, respectively, L _a and L _b are the lengths of the active layer and the grating layer, respectively, and r ₁ and r ₂ are mirrors The reflectance (r ₂ is the reflectance of the grating), C _out is the coupling loss between the grating element and the light source, ζ _t g _th is the gain threshold of the laser medium, and φ 1 is the laser side reflecting mirror , And φ2 is the amount of phase change at 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, in the comparison table, 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.

When the light source 2 is oscillating, the above equations (2-1), (2-2), and (2-3) become complex equations to become a complex resonator, and the laser oscillation It can be considered as a guide.

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 Table 1, FIG. 22, and FIG. 23, 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, with respect to the phase condition, the satisfied wavelength is discrete and is designed so that there are two to three points in (2-3) within Δλ _g . 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 λ _g 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 = dλ _TM /dT=0.05 nm / ° C., and the difference is 0.04 nm / ° C.

Also, when using Si0 ₂ and SiO (1-x) Nx as the core layer, the temperature change rate of the refractive index △ n _f is as small as 1 × 10 ^-5 / ℃, the temperature dependency of the wavelength 1.3 .mu.m lambda _g Is very small, dλ _G /dT=0.01 nm / ° C. On the other hand, regarding the temperature coefficient of the wavelength (oscillation wavelength) that satisfies the phase condition of the external resonator, when using an InGaAsP laser, the equivalent refractive index of the light source is 3.6, the temperature change of the refractive index is 3 × 10 ^-4 / ° C, and the length If La = 400 μm, the equivalent refractive index of the diffraction grating is 1.54, 1 × 10 ⁻⁵ / ° C., and the length is 155 μm, then dλ _G / dT = dλ _TM /dT=0.09 nm / ° C. Therefore, the difference is 0.08 nm / ° C.

The spectral waveform of the laser light thus laser-oscillated has a line width of 0.2 nm or less. In order to oscillate in a wide temperature range, in order to further widen the temperature range in which mode hopping is not performed, the laser oscillation wavelength by an external resonator at room temperature of 25 ° C. should be shorter than the center wavelength of the grating reflectivity. preferable. In this case, as the temperature rises, the laser oscillation wavelength shifts to the longer wavelength side and laser oscillation occurs on the longer wavelength side than the center wavelength of the grating reflectivity.

Further, in order to oscillate in a wide temperature range, in order to further widen the temperature range in which mode hopping is not performed, the laser oscillation wavelength by the external resonator at room temperature of 25 ° C. is longer than the oscillation wavelength of the light source 1 at the same temperature. It is preferable to oscillate at. In this case, as the temperature rises, the laser oscillation wavelength by the external resonator oscillates on the shorter wavelength side than the oscillation wavelength of the light source 1.

The difference between the laser oscillation wavelength by the external resonator at room temperature and the oscillation wavelength of the light source 1 is preferably 0.5 nm or more, and may be 2 nm or more from the viewpoint of widening the temperature tolerance of laser oscillation. However, if the wavelength difference is increased too much, the temperature variation of the power increases, so from this viewpoint, it is preferably 10 nm or less, and more preferably 6 nm or less.

Generally, the temperature T _{mh at} which the mode hop occurs can be considered as in the following equation from Non-Patent Document 1 (considered as T _a = T _f ).
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, the present invention uses a grating element having a small denominator of the equation (2-4) as a precondition. The denominator of equation (2-4) must be 0.03 nm / ° C or less. Specific materials for forming the grating include gallium arsenide (GaAs), lithium niobate (LiNbO ₃ ), and oxidation. Tantalum (Ta ₂ O ₅ ), zinc oxide (ZnO), and alumina (Al ₂ O ₃ ) are preferable. For example, when using lithium niobate (LiNbO ₃ ), Δλ _G is designed to be about 1.3 nm, and the length of the active layer is 250 μm so that there are two wavelengths within Δλ _G that satisfy the phase condition. ΔG _TM is, for example, 1.2 nm, T _mh is 60 ° C., and the operating temperature range can be widened. FIG. 21 shows an example of this.
Since the Bragg grating is formed in an optical waveguide, the material of the optical waveguide preferably has the refractive index described above, and the materials exemplified as the grating material are preferable.

If there are five or less wavelengths satisfying the phase condition within Δλ _G , mode hops are unlikely to occur, and stable laser oscillation conditions and oscillation can be performed with the longitudinal mode being the single mode. The spectral width of the output laser oscillated under such conditions is 0.1 nm or less.

That is, in the structure of the present invention, the oscillation wavelength changes at 0.05 nm / ° C. based on the temperature characteristics of the grating with respect to the temperature change, but mode hopping can be made difficult to occur. The present structure, the grating length Lb is set to 100μm in order to increase the △ lambda _G, is La in order to increase the △ G _TM is set to 250 [mu] m.

In addition, it supplements also about the difference with patent document 6. FIG.
The present application realizes temperature independence by bringing the temperature coefficient of the grating wavelength and the temperature coefficient of the longitudinal mode close to each other. For this reason, the resonator structure can be made compact and an additional one is unnecessary. 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

Hereinafter, each condition of the present invention will be further described.
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). λ _G is the Bragg wavelength. That is, as shown in FIGS. 19, 20, and 21, 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 the Bragg wavelength. In peak centered at the Bragg wavelength, the difference between the two wavelengths at which the reflectance becomes half the peak full width at half maximum [Delta] [lambda] _G.

The reason why the full width at half maximum Δλ _G at the Bragg reflectance peak is set to 0.8 nm or more is to make the reflectance peak broad as shown in FIG. From this viewpoint, it is preferable to be at least 1.2nm full width at half maximum [Delta] [lambda] _G, it is further preferable to 1.5nm or more. Further, it is preferable that less 5nm a full width at half maximum [Delta] [lambda] _G, more preferably to 3nm or less, it is preferable to 2nm or less.

The length _{L b} of the Bragg grating to 500μm 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 500μm is a premise of the design concept of the present invention. From this viewpoint, it is more preferable to the Bragg grating length L _b and 300μm or less. Further, _{L b} is more preferably set to 200μm or less.

The length of the active layer _{L a} also a 500μm or less (equation 3). It is also a prerequisite for the design concept of the present invention made shorter than the conventional 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 is preferably set at 150μm or more.

Refractive index _{n b} of the material of the Bragg grating is 1.8 or more (Equation 4). 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. The reason for this is that 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 can be increased. It is. From this viewpoint, _nb is more preferably 1.9 or more. The upper limit of n _b is not particularly, although the grating pitch is 4 or less from the formation becomes too small it is difficult, it is preferably more than 3.6 or less. From the same viewpoint, the equivalent refractive index of the optical waveguide is preferably 3.3 or less.

In addition, the condition shown in the equation (5) is also important.
In equation (5), 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.
Since (2.3) is φ2 + 2βLa = 2pπ and β = 2π / λ, λ satisfying this is λ _TM . φ2 is the phase change of the Bragg grating and is calculated by the following equation.

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. Previously used △ lambda equals △ G _TM, λ _s is equal to lambda _TM.

Therefore, satisfying the equation (5) makes it possible to increase the temperature at which the mode hop occurs and to effectively suppress the mode hop. The numerical value of the formula (5) is more preferably 0.025 or less.

In order to suppress the mode hopping and further widen the operating temperature range where the power fluctuation is small, a plurality of gratings having different grating diffraction pitches may be arranged in series.

In a preferred embodiment, the light source and the grating element are directly optically connected, and a resonator structure is formed between the outer end surface opposite to the emission surface of the active layer and the Bragg grating, and the active layer The length between the outer end face of the light source and the exit end point of the Bragg grating is 900 μm or less. Since light is gradually reflected at the grating portion, it is not possible to observe a clear reflection point like a reflection mirror. Although the effective reflection point can be defined mathematically, it exists on the laser side from the end point on the emission side of the Bragg grating. For this reason, in the present application, the length of the resonator is defined at the end point on the emission side. According to the present invention, even with a very short resonator length, light of a target wavelength can be oscillated with high efficiency. From this viewpoint, the length between the outer end face of the active layer and the exit end point of the Bragg grating is more preferably 800 μm or less, and particularly preferably 700 μm or less. From the viewpoint of increasing the laser output, this length is preferably 300 μm or more.

In each of the above examples, the optical waveguide is a ridge type optical waveguide including a ridge portion and at least a pair of ridge grooves forming the ridge portion. In this case, the optical material is left under the ridge groove, and extending portions made of the optical material are also formed outside the ridge groove.

However, in the ridge-type optical waveguide, the strip-shaped elongated core can be formed by removing the optical material under the ridge groove. In this case, the ridge-type optical waveguide is composed of an elongated core made of an optical material, and the cross section of the core forms a convex figure. A buffer layer (cladding layer) and an air layer exist around the core, and the buffer layer and the air layer function as a clad.

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. 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.

24 and 25 relate to this embodiment.
In the grating element 21 A of FIG. 24A, the buffer layer 16 is formed on the support substrate 10, and the optical waveguide 30 is formed on the buffer layer 16. The optical waveguide 30 is composed of a core of an optical material having a refractive index of 1.8 or more as described above. The cross section of the optical waveguide (cross section in the direction perpendicular to the light propagation direction) is trapezoidal, and the optical waveguide is elongated. In this example, the upper side surface of the optical waveguide 30 is narrower than the lower side surface. In the optical waveguide 30, the incident side propagation part, the Bragg grating, and the emission side propagation part as described above are formed.

In the grating element 21 B of FIG. 24B, the buffer layer 22 is formed on the support substrate 10, and the optical waveguide 30 is embedded in the buffer layer 22. The cross section of the optical waveguide (cross section in the direction perpendicular to the light propagation direction) is trapezoidal, and the optical waveguide is elongated. In this example, the upper side surface of the optical waveguide 30 is narrower than the lower side surface. The buffer layer 22 includes an upper buffer 22 b on the optical waveguide 30, a lower buffer 22 a, and a side buffer 22 c that covers the side surface of the optical waveguide 30.

In the grating element 21 C of FIG. 24C, the buffer layer 22 is formed on the support substrate 10, and the optical waveguide 30 A is embedded in the buffer layer 22. The optical waveguide 30A includes a core made of an optical material having a refractive index of 1.8 or more as described above. The cross section of the optical waveguide (cross section in the direction perpendicular to the light propagation direction) is trapezoidal, and the optical waveguide is elongated. In this example, the lower side surface of the optical waveguide 30A is narrower than the upper side surface.

In the grating element 21 D of FIG. 25A, the buffer layer 16 is formed on the support substrate 10, and the optical waveguide 30 is formed on the buffer layer 16. The optical waveguide 20 is included and embedded by another buffer layer 23. The buffer layer 23 includes an upper buffer 23a and a side buffer 23b. In this example, the upper side surface of the optical waveguide 30 is narrower than the lower side surface.

In the grating element 21 E of FIG. 25B, the buffer layer 16 is formed on the support substrate 10, and the optical waveguide 30 A is formed on the buffer layer 16. The optical waveguide 30 A is included and embedded by another buffer layer 23. The buffer layer 23 includes an upper buffer 23a and a side buffer 23b. In this example, the lower side surface of the optical waveguide 30A is narrower than the upper side surface.
The width W _m of the optical waveguide, the width of the narrowest part of the width in the cross section of the optical waveguide.

The shape of the optical waveguide may be a high mesa structure as shown in FIG. In this structure 60, a lower buffer layer 63 is formed on a support substrate 61, an optical material layer 64 is formed thereon, and an upper buffer layer 65 is formed thereon. At this time, the width of the upper buffer layer is narrower than the width of the lower buffer layer, thereby forming a ridge shape.

In any case, the ridge-type waveguide has strong light confinement, and the transverse mode is easily converted into a multimode. In particular, even in the case where the thickness of the optical material layer is 0.5 μm or more and 3 μm or less, a multimode is obtained when the ridge width is larger than 1 μm.

When the optical path is bent by combining an optical waveguide and a mirror, the propagation loss is higher in the higher order mode when the fundamental mode and the higher order mode are compared. It is unclear whether this is due to the loss caused by the mirror part, but the loss due to the longer propagation distance is greater in the higher order mode. Therefore, when an external resonator is configured, the oscillation threshold is higher in the higher order mode, and laser oscillation is less likely.

The mirror material is preferably a metal material with small loss due to reflection. As a specific material, noble metals such as gold, silver and platinum are preferable, but metals such as copper, aluminum, molybdenum, tungsten, tantalum, nickel and chromium may be used. Further, it may be a dielectric multilayer film.

Further, the shape of the reflecting surface of the mirror may be a flat surface or a concave surface. As shown in FIG. 27A, the shape of the waveguide may be the same as or different from the width of the ridge before and after reflection. Furthermore, it may be a tapered waveguide as shown in FIG.

That is, as shown in FIG. 27A, when the optical path is changed by the mirror 8 between the

propagation portions

67a and 67b of the optical waveguide, the width of the propagation portion 67a and the width of 67b should be approximately the same. it can. Alternatively, as shown in FIG. 27B, when the optical path is changed by the mirror 8 between the

propagation portions

68a and 68f of the optical waveguide, the

wide portions

68c and 68d are formed in the contact portion with the mirror 8,

Tapered portions

68b and 68e whose width gradually changes can be provided between the wide portion and the

propagation portions

68a and 68f, respectively.

On the other hand, when the optical path is bent using a bent waveguide, it is preferable to make the bending radius as small as possible. In general, in the region near the minimum curvature radius of the waveguide, since the higher-order mode is cut off, only the fundamental mode can propagate. This is a phenomenon resulting from weak confinement because the effective refractive index of the higher order mode is smaller than that of the fundamental mode.

In the case of the structure of the present application, the bending radius is preferably 200 μm or less, and the radius is preferably 100 μm or less, and most preferably 70 μm or less in order to greatly reduce the propagation loss in the higher-order mode.

In order to reduce the bending loss, a mode conversion loss occurs in which the mode shape is deformed in the outer circumferential direction. In order to improve this, a structure may be adopted in which the bent waveguide connection portion is offset (offset) in the width direction of the ridge waveguide.

In the grating element of the present invention, the width of the optical waveguide at the light incident portion can be made larger than the width of the optical waveguide at the curved portion of the optical waveguide. The propagation loss of light can be reduced by reducing the width of the optical waveguide in the curved portion. At the same time, the coupling loss of incident light can be reduced by relatively increasing the width of the optical waveguide at the light incident portion.

Further, in the grating element of the present invention, the width of the optical waveguide in the light emitting portion can be made larger than the width of the optical waveguide in the curved portion of the optical waveguide. The propagation loss of light can be reduced by reducing the width of the optical waveguide in the curved portion. At the same time, by relatively increasing the width of the optical waveguide at the light emitting portion, the coupling efficiency to the optical component on the emitting side can be improved.

For example, in the example of FIG. 30, the grating element 29D has four

side surfaces

29a, 29b, 29c, and 29d, and the light source 1 and the optical waveguide element 21 are attached to the side surface 29a so as to face each other. A reflective film and a non-reflective film provided on the end face of each element are not shown.

The optical waveguide 75 includes an incident side propagation part 75a, a Bragg grating 12, a taper part 75b, a curved part 75c, a taper part 75d, and an emission side propagation part 75e. In this example, the propagation parts 75a and 75e extend straight. The width W _b of the curved portion 75c is smaller than the optical waveguide width W _in of the incident side propagation portion 75a and the width W _m of the emission side propagation portion 75e. In this example, a tapered portion 75b in which the optical waveguide width gradually decreases is provided between the incident side propagation portion and the curved portion, and the optical waveguide width gradually increases between the curved portion and the emission side propagation portion. A taper portion 75d that is large is provided.

In the grating element 29E of FIG. 31, the optical waveguide 76 includes an incident side propagation part 76a, a taper part 76b, a curved part 76c, a linear propagation part 76d, a taper part 76e, and an emission side propagation part 76f. Optical waveguide width _{W in} the entrance-side propagating portion 76a, than the width _{W m} at the exit side propagating portion 76f, the width _{W b} of the curved portion 76c is smaller.

In this example, the Bragg grating 12 is downstream of the curved portion 76c that is the optical path changing portion. Such an optical path changing unit can suppress the propagation of the multimode in the transverse mode, and therefore, it is possible to selectively oscillate the fundamental mode by arranging the grating in the subsequent stage.

From the viewpoint described above, the width of the optical waveguide at the light entrance and exit is preferably 1.6 μm or more, and more preferably 2.0 μm or more. The width of the optical waveguide in the curved portion is preferably 1.6 μm or less, and more preferably 1.5 μm or less, from the viewpoint of reducing loss. On the other hand, if the width of the optical waveguide becomes too small, the propagation loss increases. Therefore, the width of the optical waveguide in the curved portion is preferably 0.3 μm or more.

Further, the difference between the optical waveguide width at the incident portion and the outgoing portion and the optical waveguide width at the curved portion is preferably 1.0 μm or more from the above viewpoint. On the other hand, if the difference between the optical waveguide width at the incident part and the outgoing part and the optical waveguide width at the curved part becomes too large, the loss at this connection part becomes large, so this difference is preferably 4.0 μm or less.

An optical waveguide substrate according to another invention includes a channel-type optical waveguide having an elongated core for propagating light and a clad in contact with the core, and when viewed from the main surface of the optical waveguide substrate, The core includes a curved portion that is curved. The cross section of the core has a convex shape, the refractive index of the core is 1.7 or more and 3.5 or less, and the difference Δn between the refractive index of the core and the refractive index of the clad is 0. 3 or more, the core width is 1.5 μm or less, the core thickness is 0.5 μm or more and 2.0 μm or less, and the curvature radius of the curved portion is 100 μm or less.
This optical waveguide substrate can be suitably used for an isolator, a polarizing element, an optical switch, an optical modulator, a wavelength conversion element, an optical amplifier, and an optical filter in addition to a grating element.

In such a curved portion, although the radius of curvature is 100 μm or less, which is remarkably small compared to the prior art, it is epoch-making in that the propagation loss in the curved portion can be kept low.
The radius of curvature can be made smaller as the effective refractive index of the optical waveguide is larger even if the refractive index difference between the core and the clad is the same. From this, the radius of curvature can be reduced as the refractive index of the core increases. From this viewpoint, the refractive index of the core material is preferably 1.7 or more, and more preferably 1.9 or more. However, if the refractive index of the core is too large, the cross-sectional dimension for obtaining a single mode will be small and the light spot diameter will be small. This causes a problem that the coupling loss with the semiconductor laser or the optical fiber increases. From this viewpoint, the refractive index of the core is preferably 3.5 or less, and more preferably 3.0 or less.

The difference Δn between the refractive index of the core and the refractive index of the cladding is preferably 0.3 or more, and more preferably 0.5 or more, from the viewpoint of reducing the propagation loss in the curved portion.
In addition, the width of the core in the curved portion is 1.5 μm or less from the viewpoint of reducing the propagation loss in the curved portion by setting the transverse direction as a single mode, but is preferably 1.3 μm or less, and more preferably 1.0 μm or less. In addition, if the core width in the curved portion is too small, it becomes unstable near the cutoff and tends to increase the loss. Therefore, it is preferably 0.3 μm or more, and more preferably 0.5 μm or more. preferable.

Further, the thickness of the core is set to 2.0 μm or less from the viewpoint of reducing the propagation loss in the curved portion, but more preferably 1.5 μm or less. Further, from the viewpoint of reducing propagation loss, the thickness of the core is set to 0.5 μm or more, more preferably 0.7 μm or more.

The radius of curvature of the curved portion is 100 μm or less, but can be 50 μm or less.
In addition, as long as the effect of reducing propagation loss as compared with the conventional optical waveguide can be obtained, the lower limit of the radius of curvature of the curved portion is not particularly limited, but from the viewpoint of generally improving the propagation loss, The curvature radius of the curved portion may be 10 μm or more, or 15 μm or more.

In a preferred embodiment, the optical waveguide substrate includes a support substrate that supports the core and the clad.
In a preferred embodiment, the material of the core is selected from the group consisting of gallium arsenide, lithium niobate single crystal, tantalum oxide, zinc oxide, and alumina.
In a preferred embodiment, the cladding material is selected from SiO2, polyimide, SiO2 glass, and MgF, but is not limited thereto.

This optical waveguide substrate uses an elongate core as an optical waveguide portion as described with reference to FIGS.
That is, in the ridge type optical waveguide, the striped elongated core can be formed by removing the optical material under the ridge groove. In this case, the ridge-type optical waveguide is composed of an elongated core made of an optical material, and the cross section of the core forms a convex figure. A buffer layer (cladding layer) and an air layer exist around the core, and the buffer layer and the air layer function as a clad.

As an example of this, in addition to what is illustrated in FIGS. 24 and 25, a form as shown in FIG. 32 can also be illustrated.
That is, in the optical waveguide substrate 75 of FIG. 32A, the buffer layer 16 is formed on the support substrate 10, and the optical waveguide (core) 30 is formed on the buffer layer 16. The optical waveguide 30 is preferably composed of a core made of an optical material having a refractive index of 1.8 or more as described above. The cross section of the optical waveguide (cross section in the direction perpendicular to the light propagation direction) is trapezoidal, and the optical waveguide is elongated. In this example, the upper side surface of the optical waveguide 30 is narrower than the lower side surface. Further, grooves 77 are formed on both sides of the optical waveguide 30, and extending portions 76 are formed outside the grooves 77, respectively. An upper clad 17 is formed so as to cover the optical waveguide 30 and the extending portion 76, respectively.
In this example, the optical waveguide width _{W m} is at 1.5μm or less, thickness Ts is 0.5 ~ 2.0 .mu.m.

The optical waveguide substrate of the present invention can be suitably applied to a grating element. For example, the curved portion of the optical waveguide shown in FIGS. 8, 9, 13, 14, 16, 17, 28, 30, and 31. Is applicable. For example, the present invention can be applied to an optical waveguide 81 described later as shown in FIG. However, in each of these examples, only the planar pattern of the optical waveguide is illustrated.

The light source mounting portion etching method as shown in FIGS. 10 to 14 can be performed by the following method.
First, a metal mask pattern for forming a metal such as Ti, Ni, etc. on the entire surface of the grating element (actually in a wafer state), applying a resist, and etching the outer peripheral area of the semiconductor laser with a mask aligner The semiconductor laser mounting portion can be formed by etching the support substrate by dry etching with a fluorine-based gas. The incident-side end face of the optical waveguide can make an angle of 89 ° or more with respect to the optical axis, and can also be a mirror surface. Thereafter, the input end face can be coated without reflection.

(Example 1)
Devices as shown in FIGS. 1, 2, 4, and 6 were produced.
Specifically, Ti was deposited on a z-plate MgO-doped lithium niobate crystal substrate, and a grating pattern was produced in the y-axis direction by photolithography. Then, by reactive ion etching of the fluorine-based and the Ti pattern as a mask, to form the grating grooves 12 of the pitch lambda 222 nm, the length L _b 100 [mu] m. The groove depth t _d (FIG. 6) of the grating was 40 nm.

Subsequently, an optical waveguide 18 was formed by reactive ion etching in the same process as described above, and a ridge groove 19 having a width W _{m of} 3 μm and a _{Tr of} 0.5 μm was formed. Further, a buffer layer 16 made of SiO ₂ was formed on the groove forming surface by a sputtering apparatus to a thickness of 0.5 μm, and a black LN substrate was used as the support substrate 10 to adhere the grating forming surface.

Next, the support substrate 10 was attached to a polishing surface plate, and the back surface of the layer on which the grating was formed was precisely polished, so that the thickness (T _s ) of the optical material layer 11 was 1.0 μm. Thereafter, the obtained assembly is removed from the surface plate, and reactive ion etching using a Ti mask is performed in the same manner as described above to form a mirror for bending by 90 °, and 1 μm-thick lithium niobate is formed in the mirror forming portion. Etched. Thereafter, gold was deposited by vapor deposition to form

mirrors

8A and 8B. Next, 0.5 μm of a buffer layer 17 made of SiO _{2 was formed by} sputtering.

After that, the assembly is cut into a bar shape with a dicing machine, the end surface on which the semiconductor laser and the wavelength conversion element are mounted is optically polished, an AR coat of 0.1% or less is formed on this end surface, and finally the chip is cut, and the grating An element was produced. The element size was 1 mm × 1 mm square.

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

Next, a laser module was mounted as shown in FIG. 1 in order to evaluate the characteristics of an external resonator type laser using this grating element. A GaAs laser that oscillates alone was used as the light source element.
Light source element specifications:
Center wavelength: 977nm
Output: 50mW
Half width: 0.1 nm
Laser element length 300μm
Mounting specifications:
(Length of incident side propagation part) Lm: 20 μm
(Distance between light source exit surface and light guide entrance surface) Lg: 1 μm

When the module was mounted and driven by current control (ACC) without using a Peltier element, the laser characteristics were a center wavelength of 975 nm and an output of 30 mW. In order to evaluate the operating temperature range, a module was installed in a thermostatic chamber, and the temperature dependence of the laser oscillation wavelength, the temperature at which the mode hop occurred, and the output fluctuation were measured. As a result, the temperature coefficient of the oscillation wavelength was 0.05 nm / ° C., the mode hop temperature was 60 ° C., and the power output fluctuation was within 1%.

(Reference Example 1)
Similarly to Example 1, Ti was deposited on a z-plate MgO-doped lithium niobate crystal substrate, and a grating pattern was produced in the y-axis direction by photolithography. Thereafter, a grating groove having a pitch interval of Λ 180 nm and a length of L _{b of} 1000 μm was formed by fluorine-based reactive ion etching using the Ti pattern as a mask. The groove depth of the grating was 300 nm. In addition, in order to form an optical waveguide for y-axis propagation, a groove with a width W _{m of} 3 μm and a Tr of 0.5 μm was formed on the grating portion with an excimer laser.
Further, a buffer layer 16 made of SiO ₂ was formed on the groove forming surface by a sputtering apparatus to a thickness of 0.5 μm, and the grating forming surface was adhered using a black LN substrate as a supporting substrate.

Next, the support substrate was attached to a polishing surface plate, and the back surface of the MgO-doped lithium niobate crystal substrate on which the grating was formed was precisely polished to a thickness (T _s ) of 1 μm. Thereafter, the assembly was removed from the surface plate, and the buffer layer 17 made of SiO _{2 was formed} to a thickness of 0.5 μm by sputtering on the polished surface. Then, it cut | disconnected in bar shape with the dicing apparatus, both ends were optically polished, both ends were formed with AR coating of 0.1% or less, and finally the chip was cut to produce a grating element. The element size was 1 mm wide and 1500 μm long.

Optical characteristics of the grating element are reflected from its transmission characteristics by using a super luminescence diode (SLD), which is a broadband wavelength light source, and inputting light into the grating element and analyzing the output light with an optical spectrum analyzer. Characteristics were evaluated. As a result, with respect to polarized light in the x-axis direction (ordinary light), a center wavelength of 800 nm, a maximum reflectance of 10%, and a full width at half maximum Δλ _G of 0.2 nm were obtained.

Next, a laser module was mounted in order to evaluate the characteristics of an external resonator type laser using this grating element. A light source element having a GaAs laser structure, a highly reflective film on one end face, and an AR coat having a reflectance of 0.1% on the other end face was prepared.
Light source element specifications:
Center wavelength: 800nm
Laser element length: 1000μm
Mounting specifications:
(Length of incident side propagation part) Lm: 20 μm
(Distance between light source exit surface and light guide entrance surface) Lg: 1 μm

After mounting the module, it was driven by current control (ACC) without using a Peltier device, and it was found that the laser characteristics were a center wavelength of 800 nm and an output of 50 mW. In order to evaluate the operating temperature range, a module was installed in a thermostatic chamber, and the temperature dependence of the laser oscillation wavelength, the temperature at which the mode hop occurred, and the output fluctuation were measured. As a result, the temperature coefficient of the oscillation wavelength was 0.05 nm / ° C., the mode hop temperature was 6 ° C., and the power output fluctuation was 10%.

(Example 2)
An apparatus as shown in FIGS. 9, 6 and 24A was produced.
Specifically, a SiO ₂ layer 16 that is a lower clad layer is formed on a support substrate 10 made of quartz by a sputtering apparatus to a thickness of 0.5 μm, and a Ta ₂ O ₅ film of 1.2 μm is formed on the SiO ₂ layer 16. A material layer was formed.

Next, Ti was formed on the optical material layer, and a grating pattern was produced by an EB drawing apparatus. Then, by reactive ion etching of the fluorine-based and the Ti pattern as a mask, to form a Bragg grating of pitch Ramuda239nm, the length L _b 100 [mu] m. Groove depth t _d of the grating was set to 40 nm.

The planar shape of the optical waveguide is shown in FIG. The optical waveguide 70 was formed by reactive ion etching in the same manner as described above. 70a is a connecting portion between the light source, 70b are tapered portion, the width of the propagation unit 70c is constant in _{W b,} 70d are curved portion. Further, the

propagation unit

70e, 70f, Bragg grating 12 is straight, the width is _{W m.}

Width _{W in} the coupling portion 70a of the light source is 3μm, and the _{W b} is such that the width _W m 2 [mu] m for 2 [mu] m, and the width of the grating portion 12, to form a ridge groove. The curved portion 70d is offset by 0.1 μm outward from the

propagation portions

70c and 70e. The curvature radius R of the curved portion 70d was 100 μm. The thickness T _s of the optical waveguide 70 is 1.2 μm.

Finally, a buffer layer made of SiO _{2 serving as the} upper clad was formed by 2 μm sputtering so as to cover the optical waveguide 70.
Thereafter, the assembly is cut into a bar shape by a dicing apparatus, the end face on which the semiconductor laser and the wavelength conversion element are mounted is optically polished, this end face is optically polished, and a 0.1% AR coating is formed on both end faces. Finally, the chip was cut to produce a grating element. The element size was 1 mm × 1 mm square. The distance between the semiconductor laser and the output end of the grating was 800 μm.

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

Next, a laser module was mounted as shown in FIG. 9 in order to evaluate the characteristics of an external resonator type laser using this grating element. A GaAs laser that oscillates alone was used as the light source element.
Light source element specifications:
Center wavelength: 977nm
Output: 50mW
Half width: 0.1nm
Laser element length 300μm
Mounting specifications:
Lg: 1μm

After mounting the module, when driven by current control (ACC) without using a Peltier element, it oscillates at a center wavelength of 975 nm corresponding to the reflection wavelength of the grating, and the output is smaller than that without the grating element, but 30 mW It was a laser characteristic. In addition, in order to evaluate the operating temperature range, a module was installed in a thermostat and the temperature dependence of the laser oscillation wavelength and output fluctuation were measured. As a result, the temperature coefficient of the oscillation wavelength was 0.03 nm / ° C., the temperature range where the output fluctuation due to the mode hop was large was 45 ° C., and the power output fluctuation within this temperature range was within 1% even when the mode hop occurred.
In this case, in the grating, the transverse mode becomes multimode, and the reflection wavelength of the primary mode exists at 971 nm with respect to the reflection wavelength of 975 nm of the fundamental mode due to the difference in effective refractive index, but the temperature range of 45 ° C. Did not hop from the basic mode to the primary mode.

(Reference Example 2)
An apparatus as shown in FIGS. 6, 24A and 29 was produced.
Specifically, a SiO 2 layer 16 that is a lower clad layer is formed on a support substrate 10 made of quartz by a sputtering apparatus to a thickness of 0.5 μm, and a Ta ₂ O ₅ film of 1.2 μm is formed thereon to form an optical material layer. Formed.
Next, Ti was formed on the optical material layer, and a grating pattern was produced by an EB drawing apparatus. Thereafter, a Bragg grating having a pitch interval of Λ239 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 was set to 40 nm.

Furthermore, in order to form the optical waveguide 71 shown in FIGS. 29 and 24A, reactive ion etching was performed by the same method as described above. Width _{W in} the 3μm next coupling portion 71a of the semiconductor laser, propagation unit 71c, so that the width _{W m} of the Bragg grating 12 and the exit side propagating portion 71d is 2 [mu] m, was etched to incise the complete optical material layer . A tapered portion 71d is provided between the coupling portion 71a and the propagation portion 71c. The thickness T _s of the optical waveguide is 1.2 μm.

Finally, a buffer layer made of SiO _{2 serving as the} upper clad was formed by 2 μm sputtering so as to cover the optical waveguide.

After that, it is cut into a bar shape by a dicing machine, the end face on which the semiconductor laser and the wavelength conversion element are mounted is optically polished, this end face is optically polished, and both end faces are formed with 0.1% AR coating. A grating element was manufactured by cutting the chip. The element size was 1 mm × 1 mm square. The distance between the semiconductor laser and the output end of the grating was 800 μm.

Next, a laser module was mounted in order to evaluate the characteristics of an external resonator type laser using this grating element. A GaAs laser that oscillates alone was used as the light source element.
Light source element specifications:
Center wavelength: 977nm
Output: 50mW
Half width: 0.1nm
Laser element length 300μm
Mounting specifications:
Lg: 1μm

After mounting the module, when driven by current control (ACC) without using a Peltier element, it oscillates at a center wavelength of 975 nm corresponding to the reflection wavelength of the grating, and the output is smaller than that without the grating element, but a 30 mW laser It was a characteristic. In addition, in order to evaluate the operating temperature range, a module was installed in a thermostat and the temperature dependence of the laser oscillation wavelength and output fluctuation were measured. As a result, the temperature coefficient of the oscillation wavelength was 0.03 nm / ° C., the temperature range where the output fluctuation due to the mode hop was large was 35 ° C., and the power output fluctuation within this temperature range was within 1% even when the mode hop occurred.

In this case, in the grating, the transverse mode becomes multimode, and the reflection wavelength of the primary mode exists at 971 nm with respect to the reflection wavelength of 975 nm of the fundamental mode due to the difference in effective refractive index, and the above temperature range of 35 ° C. It was found that power fluctuations occurred when the mode was exceeded from the basic mode to the primary mode.

(Example 3)
A grating element having the form shown in FIGS. 28, 30, and 32 was produced.
Specifically, a SiO ₂ layer to be a buffer layer 16 serving as a lower cladding is formed on a support substrate 10 made of quartz by a sputtering apparatus, and a Ta ₂ O ₅ film is formed thereon to a thickness of 1.2 μm. Thus, an optical material layer was formed.

Next, Ti was formed on the optical material layer, and a grating pattern was produced by an EB drawing apparatus. Thereafter, a Bragg grating 12 having a pitch interval of Λ239 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.

Next, the optical waveguide 70 shown in FIG. 28 was formed by reactive ion etching in the same manner as described above. Width _{W in} the coupling portion between the light source (incident portion) 70a is 3μm, and the

propagation unit

70c, 70e, the optical waveguide width _{W b} of the curved portion 70d becomes the width 3μm for 0.5μm, and the width of the grating section 12 Thus, a ridge groove was formed.
Here, in the bending portion 70d, the

propagation portions

70c and 70e are offset by +0.15 μm at Offset 1 (80A), 0 μm by Offset 2 (80B), and −0.15 μm by Offset 3 (80C). The curvature radius R of the curved portion 70d was 10 μm. The thickness T _s of the optical waveguide 70 is 1.2 μm.

Finally, the buffer layer 17 made of SiO _{2 serving as the} upper clad was formed by 1 μm sputtering so as to cover the optical waveguide 70.
Thereafter, the assembly is cut into a bar shape by a dicing apparatus, the end face on which the semiconductor laser and the wavelength conversion element are mounted is optically polished, this end face is optically polished, and a 0.1% AR coating is formed on both end faces. Finally, the chip was cut to produce a grating element. The element size was 1 mm × 1 mm square. The distance between the semiconductor laser and the output end of the grating was 800 μm.

Next, a laser module was mounted as shown in FIG. 30 in order to evaluate the characteristics of the external resonator type laser using this grating element. A GaAs laser that oscillates alone was used as the light source element.
Light source element specifications:
Center wavelength: 977nm
Output: 50mW
Half width: 0.1nm
Laser element length 300μm
Mounting specifications:
Lg: 1μm

After mounting the module, it was driven by current control (ACC) without using a Peltier device, and oscillated at a central wavelength of 975 nm corresponding to the reflection wavelength of the grating. In order to evaluate the propagation efficiency due to the bending, a comparison was made between the case where there was a bend and the case where there was a bend.

(Reference Example 3)
In the same manner as in Example 3, the elements shown in FIGS. 28, 30, and 32 were produced.
However, the offset amounts in the

offsets

80A, 80B, and 80C are all zero. The curvature radius R of the curved portion 70d was 10 μm. The thickness T _s of the optical waveguide 70 is 1.2 μm.

Finally, a buffer layer made of SiO _{2 serving as the} upper clad was formed by 1 μm sputtering so as to cover the optical waveguide 70.
Thereafter, the assembly is cut into a bar shape by a dicing apparatus, the end face on which the semiconductor laser and the wavelength conversion element are mounted is optically polished, this end face is optically polished, and a 0.1% AR coating is formed on both end faces. Finally, the chip was cut to produce a grating element. The element size was 1 mm × 1 mm square. The distance between the semiconductor laser and the output end of the grating was 800 μm.

After mounting the module, it was driven by current control (ACC) without using a Peltier device, and oscillated at a central wavelength of 975 nm corresponding to the reflection wavelength of the grating. In order to evaluate the propagation efficiency due to the bending, the amount of light was reduced by 50% when there was a bending as compared with the case where there was a bending.

(Simulation experiment)
A simulation was conducted to optimize the design of the curved part. Three-dimensional analysis by BPM (beam propagation method) using Simulated Bend method and two-dimensional analysis by FDTD (time domain difference method) were performed.

The cross-sectional structure of the optical waveguide substrate was the structure shown in FIG. 32, and the planar shape was the shape shown in FIG. However, in FIG. 33, the optical waveguide 81 includes an incident-side propagation part 81a, a taper part 81b, a linear propagation part 81c,

curved parts

81d and 81e, and a linear propagation part 81f having a relatively large width. The curve is 90 degrees as a whole. There is an offset 80A (Offset 1) between the propagation portion 81c and the curved portion 81d, an offset 80D (Offset 2) is between the

curved portions

81d and 81e, and an offset between the curved portion 81e and the linear propagation portion 81f. There is 80C (Offset3). The offset direction indicated by the arrow in the drawing is positive.

In the three-dimensional analysis by BPM using the simulated bend method, in the waveguide structure shown in FIG. 32, the core is Ta ₂ O ₅ (refractive index 2.14), the thickness is 1.2 μm, and the cladding layer is SiO ₂ (1. 45), and the ridge angle was 70 °. In FIG. 33, the calculation was performed using the ridge line width W _b and each offset amount as parameters.

Example 4
FIG. 34 shows the waveguide propagation efficiency at a wavelength of 800 nm when the bending radius is 10 μm to 100 μm when W _b is 0.5 μm. In this calculation, the refractive index of the core was fixed at 2.15, and the refractive index Nc of the cladding was changed from 1.45 to 2.05 at 0.1 intervals. The offset amount was set to 0 μm (no offset) for Offset1 to Offset3. As a result, when the refractive index Nc of the cladding is 1.85 (Nc185) or less, a bending propagation efficiency of 80% or more can be obtained with a radius of 15 μm or more.

(Example 5)
35, 36, and 37 show propagation efficiencies calculated by varying the offset amounts of Offset1, Offset2, and Offset3 with respect to W _b : 0.5 μm. As a result, it is understood that the bending propagation efficiency is improved by setting the offset to the positive side for Offset1, the offset to near zero for Offset2, and the negative side for Offset3.

(Example 6)
Next, the waveguide propagation efficiency was calculated at a bending radius of 10 μm to 100 μm when the offset amount was set to Offset 1: 0.15 μm, Offset 2: 0 μm, and Offset 3: −0.15 μm for W _b : 0.5 μm. The results are shown in Table 2. As a result, it is possible to propagate with an efficiency of 80% or more even at a radius of 10 μm, and to propagate with an efficiency of 80% or more in a region up to 50 μm.

The offset is effective in suppressing a reduction in propagation loss due to the optical electric field shifting in the outer peripheral direction due to bending. For this reason, the efficiency is improved in the vicinity of the minimum bend radius where the efficiency is reduced without offset, and the bend radius can be further reduced.
However, since the coupling loss is increased by shifting the optical axis, if the offset amount is increased too much, an adverse effect is obtained.

(Example 7)
In Example 6, the optical waveguide width _{W b} of the curved portion 70d was 1.0 .mu.m. Other than that, the experiment was conducted in the same manner as in Example 3. As a result, the results shown in Table 3 were obtained for the waveguide propagation efficiency at a bending radius of 10 μm to 100 μm.

(Example 8)
In Example 6, the optical waveguide width _{W b} of the curved portion 70d was 1.5 [mu] m. The others were tested in the same manner as in Example 3. As a result, the results shown in Table 4 were obtained for the waveguide propagation efficiency at a bending radius of 10 μm to 100 μm.

(Reference Example 4)
In Example 6, the optical waveguide width _{W b} of the curved portion 70d was 2.0 .mu.m. Other than that, the experiment was conducted in the same manner as in Example 3. As a result, the results shown in Table 5 were obtained for the waveguide propagation efficiency at a bending radius of 10 μm to 100 μm.

(Reference Example 5)
A simulation was performed using the high mesa structure 60A of FIG. In this structure 60 A, the lower buffer layer 70 is formed on the support substrate 61. The lower buffer layer 70 includes a base layer 70 a that covers the surface of the support substrate 61, and a protrusion 70 b. An optical material layer 64 is formed on the protrusion 70b, and an upper buffer layer 65 is formed thereon. The width of the upper buffer layer 65 is narrower than the width of the protrusion 70b of the lower buffer layer.

The refractive index of the core part 64 is 1.5, and the side cladding layer is air (refractive index 1). The refractive index difference between the two is 0.5. The refractive index of the material between the support substrate and the core and the material immediately above the core was 1.4. FIG. 39 shows the result of calculating the waveguide propagation efficiency at a wavelength of 800 nm when the bending radius is 10 μm to 100 μm when W _{b is} 0.5 μm in this waveguide. The offset amount was set to 0 μm (no offset) for Offset1 to Offset3.
As a result, it was found that the bending propagation efficiency of 80% or more cannot be obtained unless the radius is 40 μm or more. This propagation efficiency is significantly inferior to the propagation efficiency of the example of the present invention shown in FIG.

Example 9
In Example 4, the refractive index of the core was 1.7, and the refractive index Nc of the cladding was 1.4. The refractive index difference between the two is 0.3. FIG. 40 shows the result of calculating the waveguide propagation efficiency at a wavelength of 800 nm when the bending radius is 10 μm to 100 μm when Wb is 0.5 μm in this waveguide. The offset amount was set to 0 μm (no offset) for Offset1 to Offset3. As a result, it was found that a bending propagation efficiency of 80% or more can be obtained with a radius of 15 μm or more.

Claims

A light source that oscillates a semiconductor laser light, and an external resonator type light emitting device including a grating element that constitutes the light source and an external resonator,
The light source includes an active layer that oscillates the semiconductor laser light;
The grating element includes an optical waveguide having an incident portion where the semiconductor laser light is incident and an output portion that emits outgoing light of a desired wavelength, a Bragg grating formed in the optical waveguide, and the incident portion and the Bragg grating. An optical path changing unit that bends the optical path of the semiconductor laser light in the grating element, and satisfies the relations of the following formulas (1) to (4): An external resonator type light emitting device.

Δλ G ≧ 0.8 nm (1)
L b ≦ 500 μm (2)
L a ≦ 500 μm (3)
n b ≧ 1.8 (4)

(In formula (1), Δλ G is the full width at half maximum at the peak of the Bragg reflectivity.
In Expression (2), L b is the length of the Bragg grating.
In the formula (3), L a is the length of the active layer.
In the formula (4), n b is the refractive index of the material constituting the Bragg grating. )
2. The apparatus according to claim 1, wherein the emitting portion is an end face of the optical waveguide.
3. The apparatus according to claim 1, wherein the incident portion and the emission portion are not provided on opposite side surfaces of the grating element.
The device according to any one of claims 1 to 3, wherein the Bragg grating is provided between the optical path changing unit and the emitting unit.
3. The apparatus according to claim 1, further comprising: an external emission unit that emits the optical path changing unit and the emitted light after the optical path change to the outside of the apparatus on the downstream side of the emission unit.
The light source and the grating element are directly optically connected, and the external resonator is formed between the outer end surface opposite to the emission surface of the active layer and the Bragg grating, and the active layer The apparatus according to any one of claims 1 to 5, wherein a length between the outer end face and an exit end point of the Bragg grating is 900 袖 m or less.
The apparatus according to any one of claims 1 to 6, wherein the optical waveguide includes a ridge portion and at least a pair of ridge grooves forming the ridge portion.
The invention according to any one of claims 1 to 6, wherein the optical waveguide is composed of an elongated core, the cross section of the core has a convex shape, and a clad in contact with the core is provided. The device described.
The material according to any one of claims 1 to 8, wherein the material of the Bragg grating is selected from the group consisting of gallium arsenide, lithium niobate single crystal, tantalum oxide, zinc oxide, and alumina. The device described.
The apparatus according to any one of claims 1 to 9, wherein the relationship of the following formula (5) is satisfied.

(In Equation (5), dλ G / dT is the temperature coefficient of the Bragg wavelength, and dλ TM / dT is the temperature coefficient of the wavelength that satisfies the phase condition of the external resonator laser.)
The device according to any one of claims 1 to 10, further comprising a buffer layer provided on the optical waveguide.
The optical waveguide includes a curved portion that is curved when the grating element is viewed from the main surface, the refractive index of the core is 1.7 or more and 3.5 or less, and the refractive index of the core and the core The difference Δn from the refractive index of the cladding is 0.3 or more, the width of the core is 1.5 μm or less, the thickness of the core is 0.8 μm or more and 2.0 μm or less, and the curved portion The device according to any one of claims 8 to 11, characterized in that the radius of curvature of is not more than 100 袖 m.
The material according to any one of claims 8 to 12, wherein the material of the core is selected from the group consisting of gallium arsenide, lithium niobate single crystal, tantalum oxide, zinc oxide, and alumina. apparatus.
An optical waveguide substrate comprising a channel-type optical waveguide having an elongated core for propagating light and a clad in contact with the core,
The core includes a curved portion that is bent when viewed from the main surface of the optical waveguide substrate, the core has a convex cross section, and the refractive index of the core is 1.7 or more and 3.5 or less. The difference Δn between the refractive index of the core and the refractive index of the cladding is 0.3 or more, the width of the core is 1.5 μm or less, the thickness of the core is 0.8 μm or more, An optical waveguide substrate having a curvature radius of 2.0 μm or less and a radius of curvature of the curved portion of 100 μm or less.
The optical waveguide substrate according to claim 14, further comprising a support substrate that supports the core and the clad.
16. The optical waveguide substrate according to claim 14, wherein a material of the core is selected from the group consisting of gallium arsenide, lithium niobate single crystal, tantalum oxide, zinc oxide, and alumina.