JP2014239222A - External resonator light-emitting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress optical power variation by suppressing mode hop, and enhancing wavelength stability, without using a Peltier element.SOLUTION: An external resonator light-emitting device includes a light source for oscillating semiconductor laser light, and a grating element constituting the light source and an external resonator. The light source includes an active layer for oscillating semiconductor laser light. The grating element includes an optical waveguide having an incidence surface on which the semiconductor laser light impinges and a light emission surface for emitting light of a desired wavelength, a Bragg grating formed in the optical waveguide, and a propagation part provided between the incidence surface and Bragg grating. Relationship of expressions (1)-(4) is satisfied.

Description

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

  As a semiconductor laser, a Fabry-Perot (FP) type is generally used which constitutes an optical resonator 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.

  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 outside. 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. In addition, since the temperature change of the refractive index of the semiconductor is different, the temperature stability can be improved by designing 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.

IEICE Transactions C-II Vol.J81, No.7 pp.664-665, July 1998 IEICE technical report LQE, 2005 Vol. 105, No. 52, pp.17-20

JP 49-128689 JP 56-148880 WO2013 / 034813 JP2000-082864 JP 2006-222399 JP2002-134833

ここで、λ0は初期状態でのグレーティング反射波長を表す。
また、グレーティング反射波長の変化δλGは、下式で表される。 Non-Patent Document 1 mentions a mode hop mechanism that impairs the wavelength stability associated with a temperature rise, and an improvement measure thereof. The wavelength variation δλs of the external cavity laser due to the temperature is the change in refractive index Δna of the active layer region of the semiconductor, the length La of the active layer, the refractive index variation Δnf of the FBG region, the length Lf, and the temperature variation δTa. , ΔTf is expressed by the following equation from the standing wave condition.

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

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

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

Expression (E) is established from Expression (C) and Expression (D).

  In order to suppress the mode hop, it is necessary to use the temperature within ΔTall or less, and the temperature is controlled by a Peltier device. In the formula (E), when the refractive index changes of the active layer and the grating layer are the same (Δna / na = Δnf / nf), the denominator becomes zero, the temperature at which the mode hop occurs is infinite, and the mode hop is Indicates that it will disappear. 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 a mode hop, 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. 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.

  An object of the present invention is to suppress mode hopping, increase wavelength stability, and suppress fluctuations in optical power without using a Peltier element.

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 surface on which the semiconductor laser light is incident and an output surface that emits outgoing light of a desired wavelength, a Bragg grating formed in the optical waveguide, and the incident surface and the Bragg grating. Is provided, and the relationship of the following formulas (1) to (4) is satisfied.
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. )

（式（５）において、dλ_G／ｄＴは、ブラッグ波長の温度係数である。
dλ_ＴＭ／ｄＴは、外部共振器レーザの位相条件を満足する波長の温度係数である。） In a preferred embodiment, the relationship of the following formula (5) is further satisfied.

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

In a preferred embodiment, the following formulas (6) to (8) are satisfied.
L _WG ≦ 600 μm (6)
1 μm ≦ L _g ≦ 10 μm (7)
20 μm ≦ L _m ≦ 100 μm (8)

(In Formula (6), _LWG is the length of the grating element.
In Expression (7), L _g is the distance between the exit surface of the light source and the entrance surface of the optical waveguide layer.
In Expression (8), L _m is the length of the propagation unit. )

  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.

FIG. 1 is a schematic diagram of an external resonator type light emitting device. FIG. 2 is a cross-sectional view of the grating element. FIG. 3 is a perspective view schematically showing the grating element. FIG. 4 is a cross-sectional view of another grating element. FIG. 5 is a diagram for explaining a mode hop mode according to a conventional example. FIG. 6 is a diagram for explaining a mode hop mode according to a conventional example. FIG. 7 is a diagram illustrating a mode hop mode according to an example of the present invention. FIG. 8 shows reflection characteristics (gain conditions) and phase conditions in a conventional structure. FIG. 9 shows reflection characteristics (gain conditions) and phase conditions in the structure of the present invention.

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

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

  As shown in FIGS. 1 and 3, the grating element 7 is provided with an optical waveguide 11 having an incident surface 11 a on which the semiconductor laser light A is incident and an emission surface 11 b that emits the emitted light B having a desired wavelength. C is reflected light. A Bragg grating 12 is formed in the optical waveguide 11. A propagation part 13 without a diffraction grating is provided between the incident surface 11 a of the optical waveguide 11 and the Bragg grating 12, and the propagation part 13 faces the active layer 5 with a gap 14 therebetween. Reference numeral 7B denotes an antireflective film provided on the incident surface side of the optical waveguide 11, and reference numeral 7C denotes an antireflective film provided on the output surface side of the optical waveguide 11. In this example, the optical waveguide 11 is a ridge-type optical waveguide and is provided on the substrate 10. The optical waveguide 11 may be formed on the same surface as the Bragg grating 12 or may be formed on an opposing surface.

  The reflectance of the non-reflective layers 7A, 7B, and 7C may be a value smaller than the grating reflectance, and is preferably 0.1% or less.

  As shown in FIG. 2, in this example, the high refractive index 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 high refractive index layer 11. Is formed. For example, a pair of ridge grooves 19 are formed in the high refractive index 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.

  4, the high refractive index 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 high refractive index layer 11. Has been. For example, a pair of ridge grooves 19 are formed on the substrate 10 side of the high refractive index 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.

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

  The ridge-type optical waveguide is obtained by physically processing and molding, for example, by cutting with a peripheral blade or laser ablation.

  The Bragg grating can be formed by physical or chemical etching as follows.

  As a specific example, a metal film such as Ni or Ti is formed on 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 high refractive index layer, at least one selected from the group consisting of magnesium (Mg), zinc (Zn), scandium (Sc) and indium (In) is used to further improve the optical damage resistance of the optical waveguide. Metal 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 high refractive index 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 above-described adhesive layer is unnecessary.

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

  The reflectance of the non-reflective layer 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 end face reflection. Further, although the grating element and the support substrate are bonded and fixed in the example of FIG. 2, they may be directly bonded.

Hereinafter, the meaning of the conditions of Formula (1)-Formula (8) is 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 αa and αb are the loss coefficients of the active layer and the grating layer, respectively, La and Lb are the lengths of the active layer and the grating layer, respectively, and r1 and r2 are the mirror reflectivities (r2 is the grating) Cout is a coupling loss between the grating element and the light source, ζ _t g _th is a gain threshold of the laser medium, φ 1 is a phase change amount by the laser side reflection mirror, φ 2 Is a phase change amount in the grating portion.

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

  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. 5, and FIG. 6, the reflection characteristic of the grating is about Δλg = 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 becomes discrete, and it is designed so that there are 2-3 points of the expression (2-3) within Δλg. For this reason, the thing with a long active layer length of a laser medium is needed, and the thing of 1 mm or more is used.

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

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

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

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

  From the above, in actual operation, the conventional external cavity type 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 formula (2-4) must be 0.03 nm / ° C or less. Specific materials include gallium arsenide (GaAs), lithium niobate (LiNbO ₃ ), and tantalum oxide (Ta ₂ O). ₅ ), zinc oxide (ZnO), and alumina oxide (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. 7 shows an example of this.

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.
△ λG = 0.4nm
Vertical mode interval △ G _TM = 0.2nm
Grating length Lb = 3mm
LD active layer length La = 600μm
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. 5, 6, and 7, 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 is maximized 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 reflectivity peak is set to 0.8 nm or more is to make the reflectivity 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 the full width at half maximum [Delta] [lambda] _G and 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.

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. This is because a material having a large refractive index has a large temperature change of the refractive index, and T _mh in the equation (2-4) can be increased. From this viewpoint, _nb is more preferably 1.9 or more. The upper limit of n _b is not particularly preferably 4 or less since the formed grating pitch becomes too small it is difficult.

In addition, the condition shown in Formula (5) is 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.

  Therefore, by satisfying the equation (5), the temperature at which the mode hop occurs can be increased and the mode hop can be effectively suppressed. The numerical value of the formula (5) is more preferably 0.025 or less.

The length of the grating element _{L WG} also to 600μm or less (formula 6). It is a premise of the present invention to make this as short as Lb. From this _{viewpoint, L WG} is preferably 400μm or less, more preferably 300μm or less. Further, _LWG is preferably 50 μm or more.

The distance L _g between the light exit surface of the light source and the light guide layer entrance surface is 1 μm or more and 10 μm or less (formula (7)). As a result, stable oscillation is possible.
The length _{L m} of the propagation unit, 20 [mu] m or more and 100μm or less (equation 8). As a result, stable oscillation is possible.

(Example)
An apparatus as shown in FIGS. 1 to 3 was produced.
Specifically, Ni 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 Λ180 nm and a length of Lb of 100 μm was formed by reactive ion etching using the Ni pattern as a mask. The groove depth of the grating was 300 nm. In order to form an optical waveguide for y-axis propagation, an excimer laser was used to form a groove with a width of Wm 3 μm and a Tr of 0.5 μm in the grating portion. Further, a buffer layer 17 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 bonded using a black LN substrate as a supporting substrate.

  Next, the black LN substrate side was attached to a polishing surface plate, and the back surface of the LN substrate on which the grating was formed was precisely polished to a thickness (Ts) of 1 μm. Thereafter, the surface plate was removed and the polished surface was sputtered to form a buffer layer 16 made of SiO 2 with a thickness of 0.5 μm.

Then, it cut | disconnected in bar shape with the dicing apparatus, both ends were optically polished, both ends were formed with an AR coat of 0.1% or less, and finally the chip was cut to produce a grating element. The element size was 1 mm wide and L _wg 500 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 3%, and a full width at half maximum Δλ _G of 1.3 nm were obtained.

  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 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 250μm
Mounting specifications:
Lg: 3μm
Lm: 20μm

  When the module was mounted and driven by current control (ACC) without using a Peltier device, 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 60 ° C., and the power output fluctuation was within 1% (FIGS. 5 and 7).

(Comparative example)
Similarly to the example, Ni 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 Λ180 nm and a length of Lb of 1000 μm was formed by reactive ion etching using the Ni pattern as a mask. The groove depth of the grating was 300 nm. Further, in order to form an optical waveguide for y-axis propagation, a groove with a width of Wm 3 μm and a Tr of 0.5 μm was formed on the grating portion with an excimer laser.

  Further, a buffer layer 17 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 bonded using a black LN substrate as a supporting substrate.

Next, the black LN substrate side was attached to a polishing surface plate, and the back surface of the LN substrate on which the grating was formed was precisely polished to a thickness (Ts) of 1 μm. Thereafter, the surface plate was removed and the polished surface was sputtered to form a buffer layer 16 made of SiO 2 with a thickness of 0.5 μm. Then, it cut | disconnected in bar shape with the dicing apparatus, both ends were optically polished, both ends were formed with an AR coat of 0.1% or less, and finally the chip was cut to produce a grating element. The element size was 1 mm wide and L _wg 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, in order to evaluate the characteristics of the external resonator type laser using this grating element, a laser module as shown in another figure was mounted. 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:
Lg: 3μm
Lm: 20μm

  When the module was mounted and driven by current control (ACC) without using a Peltier device, 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%.

The present invention relates to a light source that oscillates a laser beam, and provided with a grating element constituting the light source and the external resonator, an external resonator type light emitting device you single-mode oscillation,
The light source includes an active layer that oscillates the semiconductor laser light;
The grating element includes an optical waveguide having an incident surface on which the semiconductor laser light is incident and an output surface that emits outgoing light of a desired wavelength, a Bragg grating formed in the optical waveguide, and the incident surface and the Bragg grating. Is provided, and the relationship of the following formulas (1) to ( 5 ) is satisfied.
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. )

（式（５）において、dλ_G／ｄＴは、ブラッグ波長の温度係数である。
dλ_ＴＭ／ｄＴは、外部共振器レーザの位相条件を満足する波長の温度係数である。）

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

Claims (7)

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 surface on which the semiconductor laser light is incident and an output surface that emits outgoing light of a desired wavelength, a Bragg grating formed in the optical waveguide, and the incident surface and the Bragg grating. An external resonator type light emitting device comprising a propagation section provided between the two, and satisfying the relations of the following formulas (1) to (4).

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 light source and the grating element are directly optically connected, the length L _b of the Bragg _grating, the length L _a of the active _layer, and the incident surface of the optical waveguide layer and the exit surface of the light source The apparatus according to claim 1, wherein the sum of the distance L _g and the length L _m of the propagation part is 910 μm or less. The apparatus according to claim 1, wherein a relationship of the following formula (5) is satisfied.

(In the formula (5), dλ _G / dT is a temperature coefficient of the Bragg wavelength.
dλ _TM / dT is the temperature coefficient of the wavelength that satisfies the phase condition of the external cavity laser. ) The apparatus according to any one of claims 1 to 3, wherein a relationship of the following formulas (6) to (8) is satisfied.

L _WG ≦ 600 μm (6)
1 μm ≦ L _g ≦ 10 μm (7)
20 μm ≦ L _m ≦ 100 μm (8)

(In Formula (6), _LWG is the length of the grating element.
In Expression (7), L _g is the distance between the exit surface of the light source and the entrance surface of the optical waveguide layer.
In Expression (8), L _m is the length of the propagation unit. ) The reflectance of the Bragg grating is the reflectance of an antireflective film provided on the end face of the active layer on the grating element side, the reflectance of an antireflective film provided on the incident surface side of the optical waveguide, and the The device according to any one of claims 1 to 4, wherein the device has a reflectance higher than that of a non-reflective film provided on the exit surface side of the optical waveguide.   The material according to any one of claims 1 to 5, 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 oxide. The device described in 1.   The device according to claim 1, further comprising a buffer layer provided on the optical waveguide.

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