External resonator-type light emitting device

Abstract

An external resonator type light emitting system includes a light source oscillating a semiconductor laser light by itself and a grating device providing an external resonator with the light source. The system performs oscillation in single mode. The light source includes an active layer oscillating the semiconductor laser light. The grating device includes an optical waveguide having an incident face to which the semiconductor laser is incident and an emitting face of emitting an emitting light of a desired wavelength, a Bragg grating formed in the optical waveguide, and a propagating portion provided between the incident face and the Bragg grating. Formulas (1) to (5) are satisfied.

Claims

1. An external resonator type light emitting system which does not comprise a Peltier device, the system comprising a light source oscillating a semiconductor laser light having a wavelength between 780 nm or higher and 990 nm or lower by itself and a grating device providing an external and composite resonator with said light source, said light emitting system emitting an external resonator laser light in single mode; said light emitting system operating in composite resonator mode oscillating both of said external resonator laser light and sad semiconductor laser light; wherein said light source comprises an active layer oscillating said semiconductor laser light and a reflection layer formed on an end face of said active layer on the side of said grating device; wherein said light source oscillating said semiconductor laser light of longitudinal mode and single mode by itself; wherein said grating device comprises an optical waveguide comprising an incident face to which said semiconductor laser light is incident and an emitting face configured to emit light having a desired wavelength, a Bragg grating formed in said optical waveguide, and a propagating portion provided between said incident face and said Bragg grating; wherein said Bragg grating comprises a material selected from the group consisting of gallium arsenide, lithium niobate single crystal, tantalum oxide, zinc oxide, and aluminum oxide; wherein said light source and said grating device are optically connected to each other directly; wherein said external resonator is constituted between said Bragg grating and an outer side end face of said active layer on the opposite side to an emitting face of said active layer; and wherein a length between said outer side end face of said active layer and an emitting side end point of said Bragg grating is 700 m or smaller, wherein said optical waveguide comprises a core; wherein a cross section of said core is of a convex shape; wherein said optical waveguide contacts a clad; wherein a refractive index of said clad is lower than a refractive index of said optical waveguide by 0.2 or larger; wherein five or less wavelengths satisfying phase matching conditions are present within .sub.G; wherein said optical waveguide has a thickness of 0.5 to 3.0 m; and wherein a full width at half maximum of a peak of a Bragg reflectance .sub.G0.8 nm; wherein a length of said Bragg grating L.sub.b500 m; wherein a length of said active layer L.sub.s500 m; wherein a refractive index of a material forming said Bragg grating n.sub.b1.8; wherein $\begin{matrix} .Math. \frac{d_{G}}{dT} - \frac{d_{TM}}{dT} .Math. 0.03 nm / C ., \end{matrix}$ wherein .sub.G/dT represents a temperature coefficient of a Bragg wavelength, and .sub.TM/dT represents a temperature coefficient of a wavelength satisfying a phase condition of an external resonator laser; and wherein a length of said grating device L.sub.WG600 m.

2. The system of claim 1, wherein the following formulas are satisfied; 1 ma distance L.sub.g between an emitting face of said light source and said incident face of said light source10 m; and 20 ma length L.sub.m of said propagating portion100 m.

3. The system of claim 1, wherein a reflectance of said Bragg grating is higher than each of reflectances at an emitting face of said light source, at said incident face of said grating device and at said emitting face of said grating device.

4. The system of claim 2, wherein a reflectance of said Bragg grating is higher than each of reflectances at an emitting face of said light source, at said incident face of said grating device and at said emitting face of said grating device.

5. The system of claim 1, further comprising a buffer layer provided on said optical waveguide.

6. The system of claim 2, further comprising a buffer layer provided on said optical waveguide.

7. The system of claim 3, further comprising a buffer layer provided on said optical waveguide.

8. The system of claim 4, further comprising a buffer layer provided on said optical waveguide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a view schematically showing an external resonator type light emitting system.

(2) FIG. 2 is a cross sectional view of a grating device.

(3) FIG. 3 is a perspective view schematically showing the grating device.

(4) FIG. 4 is a cross sectional view showing another grating device.

(5) FIG. 5 is a diagram for illustrating pattern of mode hopping according to a prior art.

(6) FIG. 6 is a diagram for illustrating pattern of mode hopping according to a prior art.

(7) FIG. 7 is a diagram for illustrating pattern of mode hopping according to the inventive example.

(8) FIG. 8 shows spectrum of light amount of a light source and spectrum of a system obtained by adding a grating device to the light source, according to example 2.

(9) FIG. 9 shows reflection characteristics (gain condition) and phase condition in a prior structure.

(10) FIG. 10 shows reflection characteristics (gain condition) and phase condition in a structure of the invention.

(11) FIGS. 11(a), 11(b) and 11(c) are diagrams schematically showing cross sections of grating devices 21A, 21B and 21C, respectively, utilizing elongate strip-shaped optical waveguides 30, 30A.

(12) FIGS. 12(a) and 12(b) are diagrams schematically showing grating devices 21D and 21E, respectively, utilizing elongate and stripe-shaped optical waveguides 30 and 30A.

EMBODIMENTS OF THE INVENTION

(13) An external resonator type light emitting system 1, schematically shown in FIG. 1, includes a light source 1 oscillating a semiconductor laser light, and a grating device 9. The light source 2 and grating device 9 are mounted on a common mount 3.

(14) The light source 2 includes an active layer 5 oscillating the semiconductor laser light. According to the present embodiment, the active layer 5 is provided on a substrate 4.

(15) Here, as the light source 2, it is used a light source capable of oscillating laser by itself. It means that the light source 2 oscillates laser by itself without the need of a grating device.

(16) It is preferred that the light source 2 oscillates laser by itself in longitude mode and single mode. However, in the case of an external resonator type laser utilizing a grating device, the refractive property can be made dependent on wavelength. By controlling the pattern of the dependence on the wavelength, it is possible that the external resonator type laser can oscillate laser in single mode, even when the light source 2 oscillates by itself laser in longitudinal mode and in multi mode.

(17) A high reflection film 6 is provided on an outer end face of the substrate 4, and a reflection layer 20 is formed on an end face of the active layer 5 on the side of the grating device.

(18) As shown in FIGS. 1 and 3, in the grating device 9, it is provided an optical material layer 11 including an incident face 11 a to which a semiconductor laser light A is incident, and an emitting face 11 b of emitting emission light B of a desired wavelength. C represents reflected light. A Bragg grating 12 is formed in the optical material layer 11. A propagating portion 13 without a diffraction grating is provided between the incident face 11 a of the optical material layer 11 and the Bragg grating 12, and the propagating portion 13 opposes to the active layer 5 through a spacing 14. 7B represents an antireflection film provided on the side of the incident face of the optical material layer 11, and 7C represents an antireflection film provided on the side of the emitting face of the optical material layer 11. According to the present example, the optical material layer 11 is of a ridge type optical waveguide, and provided on a substrate 10. The optical material layer 11 may be formed on the same face or on the opposing face as the Bragg grating 12.

(19) According to a preferred embodiment, the reflectance value of the Bragg grating is larger than those at the light emitting end of the light source, at the light incident face of the grating device and at the light emitting face of the grating device. On the viewpoint, the reflectance values, at the light incident face of the grating device and at the light emitting face of the grating device may preferably be 0.1 percent or lower. It is thus preferred that antireflection layers 7B and 7C are formed on the incident and emitting faces of the grating device. The reflectance of the antireflection layer is lower than the reflectance of the grating and may preferably be 0.1 percent or lower. However, in the case that the reflectance at the end face is lower than the reflectance of the grating, the antireflection film is not necessary and a reflective film may be used.

(20) As shown in FIG. 2, according to the present example, an optical material layer 11 is formed on the substrate 10 through an adhesive layer 15 and a lower buffer layer 16, and an upper buffer layer 17 is formed on the optical material layer 11. For example, a pair of ridge grooves 19 is 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 a flat face 11a or face 11b. On the viewpoint of reducing deviation of shape of the Bragg grating or ridge groove, the Bragg grating is preferably formed on the face 11a so that the Bragg grating and ridge grooves 19 are provided on the opposite sides of the substrate.

(21) Further, according to a device 9A shown in FIG. 4, the optical material layer 11 is formed on the substrate 10 through the adhesive layer 15 and lower buffer layer 16, and the upper buffer layer 17 is formed on the optical material layer 11. For example a pair of the ridge grooves 19 is formed on the side of the substrate 10 in the optical material layer 11, and the ridge type optical waveguide 18 is formed between the ridge grooves 19. In this case, the Bragg grating may be formed on the side of the flat face 11a or on the face 11b with the ridge grooves formed. On the viewpoint of reducing the deviation of shape of the Bragg grating or ridge groove, the Bragg grating may preferably be formed on the side of the flat face 11a so that the Bragg grating and ridge grooves 19 are provided on the opposite sides of the substrate. Further, the upper buffer layer 17 may be omitted, and in this case, air layer can be directly contacted with the grating. It is thus possible to increase a difference of the refractive indices in the cases that the grating groove is present and absent, so that it is possible to make the reflectance larger with the grating having a smaller length.

(22) In this case, the oscillation wavelength of the laser beam is decided on the wavelength reflected by the grating. In the case that the light reflected by the grating and the light reflected at the end face of the active layer 5 on the side of the grating device exceed the threshold value of the laser gain, it is satisfied the oscillation condition. It is thus possible to obtain laser beam having high stability of wavelength.

(23) For further improving the stability of wavelength, it is effective that loop gain from the grating is made larger. On the viewpoint, it is preferred that the reflectance of the grating is larger than the reflectance at the end face of the active layer 5.

(24) As the light source, it is preferred a laser of a GaAs series material or InP series material having high reliability. As an application of the inventive structure, for example, in the case that a non-linear optical device is utilized to oscillate green-light laser as a second harmonic wave, it is to be used laser of GaAs series oscillating at a wavelength around 1064 nm. As the reliability of the GaAs series or InP series laser is excellent, it is possible to realize a light source such as a laser array or the like composed of lasers arranged one-dimensionally. As the wavelength becomes longer, the temperature dependence of the Bragg wavelength becomes larger. It is thus particularly preferred that the oscillation wavelength of the laser is 990 nm or lower for improving the stability of the wavelength. On the other hand, as the wavelength becomes shorter, the change na of the refractive index of the semiconductor becomes too larger. It is thus particularly preferred that the oscillation wavelength of the laser is 780 nm or higher for improving the stability of the wavelength. Further, materials and wavelength of the active layer may be appropriately selected.

(25) The ridge type optical waveguide may be obtained by physically processing and shaping with, for example, grinding by means of a peripheral cutting edge or laser ablation.

(26) The buffer layer may function as the clad layer for the optical waveguide. On the viewpoint, the refractive index of the buffer layer may preferably be lower than that of the optical material layer, and the difference of the refractive indices may preferably be 0.2 or larger and more preferably be 0.4 or larger.

(27) The Bragg grating may be formed by physical or chemical etching as follows.

(28) Specifically, a metal film such as Ni or Ti is formed on the substrate of a high refractive index, and windows are periodically formed by photolithography to form a etching mask. Thereafter, a dry etching system, such as a reactive ion etching system, is utilized to form the grating grooves periodically. At last, the metal mask is removed to form it.

(29) In the optical material layer, for further improving the resistance against optical damage of the optical waveguide, it may be contained one or more metals selected from the group consisting of magnesium (Mg), zinc (Zn), scandium (Sc) and indium (In), and in this case, magnesium is most preferred. Further, in the crystal, it may be contained a rare earth element as a dopant. The rare earth element may preferably be Nd, Er, Tm, Ho, Dy or Pr.

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

(31) Further, the optical material layer 11 may be formed by a film-forming method on a supporting body. Such film-forming method includes sputtering, vapor deposition or CVD. In this case, the optical material layer 11 is directly provided on the supporting body and the above described adhesive layer is not present.

(32) Further, the thickness of the optical material layer may more preferably be 0.5 to 3.0 m.

(33) Specific material of the supporting body is not particularly limited, and includes lithium niobate, lithium tantalate, a glass such as quartz glass, quartz, Si or the like.

(34) The reflectance of the antireflective film is necessarily lower than the reflectance of the grating. The material of the film forming the antireflective film includes a film formed by lamination of oxides such as silicon dioxide, tantalum pentoxide or the like and a metal.

(35) Further, the end faces of the light source device and grating device may be cut in an inclined direction for reducing the reflection at the end faces, respectively. Further, although the grating device and supporting body may be joined by fixing by adhesion according to the example of FIG. 2, they may be directly joined with each other.

(36) The meaning of the conditions defined in the formulas (1) to (8) will be further described below.

(37) As mathematical formulas are abstract and difficult to understand, however, typical embodiments of a prior art and the present invention will be directly compared with each other first to describe the characteristics of the present invention. The conditions of the present invention will be then described.

(38) First, condition for oscillating a semiconductor laser is decided on the gain condition and phase condition, as the following formula.
(C.sub.out.sup.2).sup.4|r.sub.1r.sub.2|exp{(.sub.tg.sub.th.sub.a)L.sub.a.sub.bL.sub.b}exp{j(.sub.1.sub.22L.sub.a)}=1 (2-1)

(39) The gain condition is expressed as the following formula from the formula (2-1).

(40) $\begin{matrix} _{t} g_{th} =_{a} L_{a} +_{b} L_{b} + \frac{1}{L_{a}} \ln (\frac{1}{.Math. r_{1} .Math. .Math. r_{2} .Math. C_{out}^{2}}) & Formula (2 - 2) \end{matrix}$

(41) Besides, a and b are loss coefficients of the active layer and grating layer, respectively, La and Lb are lengths of the active layer and grating layer, respectively, r1 and r2 are reflectances of a mirror (r2 represents a reflectance of the grating), Cout represents a connection loss of the grating device and light source, .sub.tg.sub.th represents a gain threshold value of a laser medium, 1 represents an amount of change of phase due to a reflection mirror on the side of the laser, and 2 represents an amount of change of phase in the grating portion.

(42) The formula (2-2) indicates that laser oscillation occurs in the case that the gain .sub.tg.sub.th (gain threshold value) of the laser medium exceeds the loss. The gain curve (dependency on wavelength) of the laser medium provides a full width at half maximum of 50 nm or larger and exhibits broad characteristics. Further, the loss part (right column) shows hardly any dependency on wavelength other than the reflectance of the grating, so that the gain condition is decided on the grating. As a result, as shown in the comparison table, the gain condition can be evaluated only by the grating.

(43) On the other hand, the phase condition is as shown in the following formula from the formula (2-1). However, 1 becomes zero.
.sub.2+2L.sub.a=2p (p represents an integer) Formula (2-3)

(44) In the case that the light source 2 oscillates laser, composite resonator mode is provided and the formulas (2-1), (2-2) and (2-3) described above are modified to complex formulas, respectively, However, they may be considered as standards of the laser oscillation.

(45) As to the external resonator type laser, it has been commercialized those utilizing an external resonator including quartz glass optical waveguide or FBG. According to prior design concept, as shown in table 1 and FIGS. 5 and 6, the reflection characteristic of the grating was g of about 0.2 nm and a reflectance of 10 percent. The length of the grating portion was thereby made 1 mm. On the other hand, as to the phase condition, the wavelength satisfying it takes discrete values and it is designed so that the formula (2-3) is satisfied at two or three points within g. It becomes thereby necessary the active layer of the laser medium whose length is large, and it has been used the active layer having a length of 1 mm or larger.

(46) TABLE-US-00001 TABLE 1 Prior structure Present invention Reflection FIG. 9 FIG. 10 Characteristics (gain condition) and Phase condition Material FBG, LN (used for glass waveguide ordinary light), GaAs, Ta2O5 ZnO, Al2O3 Length of grating Comparative Example: 100 m Lb Example: 1 mm Length of LD Comparative Example: 300 m Active layer Example; 2.1 mm Mode hopping Comparative Example: 60 C. Temperature Example; 5 C. (operational temperature range) Change of wavelength 0.01 nm/ C. 0.05 nm/ C. Deviation of power Deviation of power 3% or smaller by mode hopping; 5% or larger Notes Temperature control Peltier device is not with Peltier device is needed needed

(47) In the case of a glass waveguide or FBG, the dependency of g on temperature is very small, and d.sub.G/dT becomes about 0.01 nm/ C. As a result, the external resonator type laser has the characteristics of stability on wavelength.

(48) Contrary to this, the dependency of the wavelength satisfying the phase condition on temperature is large and d.sub.G/dT is 0.05 nm/ C., and the difference reaches 0.04 nm/ C.

(49) Further, in the case that the core layer is composed of SiO.sub.2 or SiO.sub.(1-x)N.sub.x, the change rate nf of the refractive index on temperature is as low as 110.sup.5/ C., and the change rate of g on temperature is very low at a wavelength of 1.3 m and d.sub.G/dT becomes 0.01 nm/ C. On the other hand, as to the temperature coefficient of the wavelength (oscillation wavelength) at which the phase matching condition of the external resonator is satisfied, in the case that an InGaAsP type laser is used, and provided that an equivalent refractive index of the light source is 3.6, the change rate of the refractive index on temperature is 310.sup.4/ C., the length La is 400 m, the equivalent refractive index of the grating is 1.54, the change rate is 110.sup.5/ C., and the length is 155 m, d.sub.G/dT becomes 0.09 nm/ C. The difference becomes, therefore, 0.08 nm/ C.

(50) The waveform of the spectrum of the laser beam thus oscillated has a line width of 0.2 nm or smaller. On the viewpoint of enabling laser oscillation in a wide temperature range and making a temperature range free from the mode hopping broader, it is preferred that the laser oscillation wavelength of the external resonator at room temperature (25 C.) is shorter than the central wavelength of the grating reflectance. In this case, as the temperature rises, the laser oscillation wavelength is shifted to a longer wavelength side, so that laser is oscillated at a wavelength longer than the central wavelength of the grating reflectance.

(51) Further, on the viewpoint of enabling the laser oscillation in a wide temperature range and of enlarging the temperature range free from the mode hopping, it is preferred that the laser oscillation wavelength of the external resonator at room temperature (25 C.) is longer than the oscillation wavelength of the light source 2 at the same temperature. In this case, as the temperature rises, the laser oscillation wavelength of the external resonator becomes shorter than the oscillation wavelength of the light source 2.

(52) The difference between the laser oscillation wavelength of the external resonator and the oscillation wavelength of the light source 2 at room temperature may preferably be 0.5 nm or larger and further may be 2 nm or larger, on the viewpoint of enlarging the temperature range permitting the laser oscillation. However, if the difference of the wavelengths becomes too larger, the temperature dependence of the power becomes large. On the viewpoint, the difference may preferably be 10 nm or smaller and more preferably be 6 nm or smaller.

(53) Generally, the temperature Tmh at which the mode hopping takes place can be considered as the following formula based on the non-patent document 1 (It is provided that Ta=Tf).

(54) G.sub.TM is a spacing (longitudinal mode spacing) of the wavelengths satisfying the phase condition of the external resonator type laser.

(55) $\begin{matrix} T_{mh} = \frac{G_{TM}}{.Math. \frac{d_{G}}{dT} - \frac{d_{TM}}{dT} .Math.} & Formula (2 - 4) \end{matrix}$

(56) As a result, T.sub.mh becomes about 5 C. according to a prior art, so that it is susceptible to the mode hopping. In the case that the mode hopping occurs, the power is deviated based on the reflection characteristics of the grating by 5 percent or more.

(57) As described above, in actual operation, a Pertier device has been used to perform temperature control in the prior external resonator type laser utilizing the glass waveguide or FBG.

(58) Contrary to this, the present invention utilizes the grating device in which the denominator of the formula (2-4) becomes small as a precondition. It is needed that the denominator of the formula (2-4) is made 0.03 nm/ C. or lower, and specific material may preferably be gallium arsenide (GaAs), lithium niobate (ordinary light), tantalum oxide (Ta.sub.2O.sub.5), zinc oxide (ZnO), or aluminum oxide (Al.sub.2O.sub.3). For example, in the case that lithium niobate (ordinary light) is used, that .sub.G is designed at about 1.3 nm and that the length of the active layer is made 250 m for making two wavelengths satisfying the phase condition are present within .sub.G, G.sub.TM becomes 1.2 nm and Tmh becomes 60 C. for example, so that it is possible to enlarge the operational temperature range. FIG. 7 shows this example.

(59) In the case that the wavelengths satisfying the phase condition are present at five or less points within .sub.G, the mode hopping can be prevented, so that the operation can be made at stable laser oscillating condition, and the oscillation takes place with longitudinal mode as single mode. The spectrum width of the output of the laser oscillation under the condition becomes 0.1 nm or lower.

(60) That is, according to the inventive structure, although the oscillating wavelength is changed at 0.05 nm/ C. based on the temperature characteristics of the grating with respect to the temperature change, it is possible to make the mode hopping difficult to take place. According to the inventive structure, the length Lb of the grating is made 100 m for enlarging .sub.G, and La is made 250 m for enlarging G.sub.TM.

(61) Besides, the difference over the patent document 6 is supplemented.

(62) The present invention is to realize the non-dependency on temperature by making the temperature coefficient of the wavelength of the grating closer to the temperature coefficient of the longitudinal mode, so that it is possible to make the resonator structure compact without the necessity of an additional part. According to the patent document 6, each parameter is described as follows, which is within a range of a prior art.

(63) .sub.G=0.4 nm

(64) Spacing of longitudinal mode G.sub.TM=0.2 nm

(65) Length of grating Lb=3 mm

(66) Length of LD active layer=600 m

(67) Length of propagating portion=1.5 mm

(68) Each condition of the present invention will be described further below.

(69) The full width at half maximum .sub.G in a peak of Bragg reflectance is made 0.8 nm or higher (formula 1). .sub.G represents Bragg wavelength. That is, as shown in FIGS. 5, 6 and 7, in the case that the horizontal axis is assigned to the reflection wavelength by the Bragg grating and the vertical axis is assigned to the reflectance, the wavelength at which the reflectance takes the maximum is assigned to the Bragg wavelength. Further, in the peak whose center is positioned at the Bragg wavelength, a difference of two wavelengths at which the reflectance takes a half of the peak maximum is assigned to a full width at half maximum .sub.G.

(70) The full width at half maximum .sub.G at the peak of the Bragg reflectance is made 0.8 nm or larger so that the peak of the reflectance is made broad as shown in FIG. 7. On the viewpoint, the full width at half maximum .sub.G may preferably be made 1.2 nm or larger and more preferably be made 1.5 nm or larger. Further, full width at half maximum .sub.G may preferably be made 5 nm or smaller, 3 nm or smaller and 2 nm or smaller.

(71) The length Lb of the Bragg grating is made 500 m or smaller (formula 2). The length Lb of the Bragg grating is a length of the grating in the direction of an optical axis of light propagating in the optical waveguide. It is a precondition of the inventive design concept to shorten the length Lb of the Bragg grating to 500 m or smaller, which is shorter than that in a prior art. On the viewpoint, the length Lb of the Bragg grating may preferably be made 300 m or smaller. Further, Lb may more preferably be made 200 m or smaller.

(72) The length La of the active layer is also made 500 m or smaller (formula 3). It is also a precondition of the inventive design concept to shorten the length La of the active layer than that in a prior art. On the viewpoint, the length La of the active layer may preferably be made 300 m or smaller. Further, the length La of the active layer may preferably be made 150 m or larger.

(73) The refractive index n.sub.b of a material forming the Bragg grating is made 1.8 or higher (formula 4). According to a prior art, it has been generally used a material having a lower refractive index such as quartz. According to the concept of the present invention, the refractive index of the material forming the Bragg grating is made higher. The reason is that the material having a larger refractive index provides a larger dependency of the refractive index on temperature, and that Tmh of the formula (2-4) can be made larger, and that the temperature coefficient d.sub.G/dT of the grating can be made larger. On the viewpoint, n.sub.b may more preferably be 1.9 or higher. Further, although the upper limit of n.sub.b is not particularly defined, it may preferably be 4 or lower because the formation of the grating may be difficult in the case that the grating pitch is too small. nb may more preferably be 3.6 or lower. Further, on the same viewpoint, the equivalent refractive index of the waveguide may preferably be 3.3 or lower.

(74) In addition to this, the condition defined in the formula (5) is important.

(75) In the formula (5), d.sub.G/dT represents a temperature coefficient of the Bragg wavelength.

(76) Further, d.sub.TM/dT represents a temperature coefficient of wavelength satisfying the phase condition of the external resonator laser.

(77) Here, .sub.TM represents a wavelength satisfying the phase condition of the external resonator laser, that is, the wavelength satisfying the phase condition of the formula (2-3) as described above. This is called longitudinal mode in the specification.

(78) The longitudinal mode will be supplemented below.

(79) Since 2+2La=2p and =2/ according to the formula (2-3), satisfying them is assigned to .sub.TM. 2 represents a change of phase of the Bragg grating, and is calculated according to the following formula.

(80) $r_{2} = \frac{- j \tanh (L_{b})}{+ (/ 2 + j) \tanh (L_{b})} .Math. r_{2} .Math. \exp (- j)$

(81) G.sub.TM represents a spacing (longitudinal mode spacing) of the wavelengths satisfying the phase condition of the external resonator laser. Since a plurality of .sub.TM are present, it means a difference of a plurality of .sub.TM. previously used is equal to G.sub.TM and s is equal to .sub.TM.

(82) Therefore, by satisfying the formula (5), it is possible to make the temperature of mode hopping higher to prevent the mode hopping in a practical view. The numerical value of the formula (5) may more preferably be made 0.025 or lower.

(83) The length L.sub.WG of the grating device is made 600 m or smaller (formula 6). It is also a precondition of the present invention to shorten it as Lb. On the viewpoint, L.sub.WG may preferably be 400 m or smaller and more preferably be 300 m or smaller. Further, L.sub.WG may preferably be 50 m or larger.

(84) The distance Lg between the emitting face of the light source and incident face of the optical waveguide is made 1 m or larger and 10 m or smaller (formula 7). The stable oscillation can thereby be realized.

(85) The length Lm of the propagation portion is made 20 m or larger and 100 m or smaller (formula 8). The stable oscillation can thereby be realized.

(86) According to a preferred embodiment, the light source and grating device are directly and optical coupled to each other, the Bragg grating and the outer end face opposite to the emitting face of the active layer constitute a resonator structure, and a length between the outer side end face of the active layer and the end point on emitting side of the Bragg grating is 900 m or smaller. As light is gradually reflected in the grating portion, it is not possible to observe the reflection point clearly as a reflective mirror. Although the effective reflection point can be mathematically defined, it is present on the side of the laser with respect to the end point of the Bragg grating on the emitting side. Considering this, according to the present invention, the length of the resonator is defined at the end point on the emitting side. According to the present invention, even when the length of the resonator is very short, it is possible to oscillate light of a target wavelength at a high efficiency. On the viewpoint, the length between the outer side end face of the active layer and the end point of the Bragg grating on the emitting side may preferably be 800 m or smaller and more preferably be 700 m or smaller. Further, on the viewpoint of improving the output of the laser, the length of the resonator may preferably be 300 m or larger.

(87) According to each of the examples described above, the optical waveguide is a ridge type optical waveguide including a ridge portion and at least a pair of ridge grooves defining the ridge portion. In this case, an optical material is left under the ridge grooves, and elongated portions of the optical material are also left on the outside of the ridge grooves, respectively.

(88) However, in the ridge type optical waveguide, it is possible to form an elongate and stripe-shaped core, by removing the optical material under the ridge grooves. In this case, the ridge type optical waveguide is composed of an elongate core of an optical material, and the cross section of the core is defined by a convex shape. A buffer layer (clad layer) or air layer is present around the core and functions as a clad.

(89) The convex shape means that line segments each connecting optional two points on an outer profile line of the cross section of the core are present inside of the outer profile line of the cross section of the core. Such figure includes polygons such as triangle, quadrilateral, hexagon. heptagon or the like, circle, ellipse or the like. Quadrilateral may preferably be that having an upper side, a lower side and a pair of side lines, and more preferably be trapezoid.

(90) FIGS. 11 and 12 relate to this embodiment.

(91) According to a grating device 21A of FIG. 11(a), a buffer layer 16 is formed on a supporting substrate 10, and an optical waveguide 30 is formed on the buffer layer 16. The optical waveguide 30 is composed of a core composed of the optical material having a refractive index of 1.8 or higher. The shape of the cross section (cross section in the direction perpendicular to the propagating direction of light) of the optical waveguide is trapezoid, and the optical waveguide is extended and elongated. According to the present example, the upper side face of the optical waveguide 30 is narrower than the lower side face. The incident side propagating portion, Bragg grating and emitting side propagating portion as described above are formed in the optical waveguide 30.

(92) According to a grating device 21B of FIG. 11(b), a buffer layer 22 is formed on the supporting substrate 10, and the optical waveguide 30 is embedded inside of the buffer layer 22. The shape of the cross section (cross section in the direction perpendicular to the propagating direction of light) of the optical waveguide is trapezoid, and the optical waveguide is extended and elongated. According to the present example, the upper side face is narrower than the lower side face of the optical waveguide 30. The buffer layer 22 includes an upper side buffer 22b and a lower side buffer 22b of the optical waveguide 30 and side face buffers 22c covering side faces of the optical waveguide 30.

(93) According to a grating device 21C of FIG. 11(c), a buffer layer 22 is formed on the supporting substrate 10, and the optical waveguide 30A is embedded inside of the buffer layer 22. The optical waveguide 30A is composed of the core composed of the optical material having a refractive index of 1.8 or higher. The shape of the cross section (cross section in the direction perpendicular to the propagating direction of light) of the optical waveguide is trapezoid, and the optical waveguide is extended and elongated. According to the present example, the lower side face of the optical waveguide 30A is narrower than the upper side face.

(94) According to a grating device 21D of FIG. 12(a), a buffer layer 16 is formed on the supporting substrate 10, and the optical waveguide 30 is formed on the buffer layer 16. Then, the optical waveguide 30 is surrounded by and embedded in another buffer layer 23. The buffer layer 23 includes an upper side buffer 23a and side face buffers 23b. According to the present example, the upper side face of the optical waveguide 30 is narrower than the lower side face.

(95) According to a grating device 21E of FIG. 12(b), a buffer layer 16 is formed on the supporting substrate 10, and the optical waveguide 30A is formed on the buffer layer 16. Then, the optical waveguide 30A is surrounded by and embedded in another buffer layer 23. The buffer layer 23 includes an upper side buffer 23a and side face buffers 23b. According to the present example, the lower side face of the optical waveguide 30A is narrower than the upper side face.

(96) Then, the width Wm of the optical waveguide is defined as a width of the narrowest part in the cross section of the optical waveguide.

EXAMPLES

Inventive Example 1

(97) It was fabricated the system shown in FIGS. 1 to 3.

(98) Specifically, Ti film was formed on a substrate composed of z-plate of lithium niobate crystal doped with MgO, and photolithography technique was utilized to produce grating pattern in the direction of y-axis. Thereafter, fluorine-based reactive ion etching was performed using the Ti pattern as a mask to form grating grooves at a pitch spacing of 180 nm and a length Lb of 100 m. The depth of the grating groove was 300 nm. Further, for forming the optical waveguide for propagation in y-axis, the grooves each having a width Wm of 3 m and Tr of 0.5 m were processed in the grating portion by means of excimer laser. Further, the buffer layer 17 made of SiO.sub.2 and of 0.59 m was formed by a sputtering system on the face with the grooves formed thereon, and a black LN substrate was used as the supporting body to adhere the face with the grating formed thereon.

(99) Then, the black LN substrate was adhered onto a polishing surface plate and the back face of the LN substrate with the grating was subjected to precision polishing to a thickness (Ts) of 1 m. Thereafter, it was removed off from the surface plate and the buffer layer 16 composed of SiO.sub.2 and of 0.5 m was formed on the polished face by sputtering.

(100) Thereafter, the assembly was cut into bars by means of a dicing equipment and both end faces of the bar was subjected to optical polishing. AR coatings of 0.1% or lower were formed on the both end faces, respectively, and the bar was cut into chips to produce the grating devices. The size of the device was a width of 1 mm and a length L.sub.WG of 500 m.

(101) As to the optical characteristics of the grating device, a super luminescence diode (SLD), a light source for wide band wavelength, was used to input light into the grating device and its output light was analyzed by an optical spectrum analyzer to evaluate the reflection characteristics based on the transmitting characteristics. As a result, it was obtained the characteristics that the central wavelength was 800 nm, maximum reflectance was 3 percent and full width at half maximum .sub.G was 1.3 nm with respect to polarized light (ordinary light) in the direction of x axis.

(102) Then, for evaluating the characteristics of the external resonator laser utilizing the grating device, the laser module was mounted as shown in FIG. 1. As the light source device, it was prepared one having a GaAs series laser structure, in which a high refractive index film was provided on its one end face and an AR coating of a reflectance of 0.1% was provided on the other end face.

(103) Specification of the Light Source Device:

(104) Central wavelength; 800 nm

(105) Length of laser device; 250 m

(106) Specification of Mounting

(107) Lg: 3 m

(108) Lm; 20 m

(109) After mounting the module, the device was driven under current control (ACC) without utilizing a Peltier device to obtain the laser characteristics of a central wavelength of 800 nm and an output power of 50 mW. Further, the module was set in a thermostatic bath for evaluating the operational temperature range to measure the temperature dependency of the laser oscillating wavelength, the temperature with the mode hopping occurred and the deviation of output power. As a result, the temperature coefficient of the oscillating wavelength was 0.05 nm/ C., the temperature of the mode hopping was 60 C., and the deviation of the output power was within 1 percent (FIGS. 5 and 7).

Comparative Example 1

(110) As the Inventive Example 1, Ti film was formed on a substrate composed of z-plate of lithium niobate crystal doped with MgO, and photolithography technique was utilized to produce grating pattern in the direction of y-axis. Thereafter, fluorine-based reactive ion etching was performed using the Ti pattern as a mask to form grating grooves at a pitch spacing of 180 nm and a length Lb of 1000 m. The depth of the grating grooves was 300 nm. Further, for forming the optical waveguide for propagation in y-axis, the grooves each having a width Wm of 3 m and Tr of 0.5 m were processed in the grating portion by means of excimer laser.

(111) Further, the buffer layer 17 made of SiO.sub.2 and of 0.5 m was formed by a sputtering system on the side with the grooves formed thereon, and a black LN substrate was used as the supporting body to adhere the face with the grating formed thereon.

(112) Then, the black LN substrate was adhered onto a polishing surface plate and the back face of the LN substrate with the grating was subjected to precision polishing to a thickness (Ts) of 1 m. Thereafter, it was removed off from the surface plate and the buffer layer 16 composed of SiO.sub.2 and of 0.5 m was formed on the polished face by sputtering. Thereafter, the assembly was cut into bars by means of a dicing equipment and both end faces of the bar was subjected to optical polishing. AR coatings of 0.1% or lower were formed on the both end faces, respectively, and the bar was cut into chips to produce the grating devices. The size of the device was a width of 1 mm and a length L.sub.WG of 1500 m.

(113) As to the optical characteristics of the grating device, a super luminescence diode (SLD), a light source for wide band wavelength, was used to input light into the grating device and its output light was analyzed by an optical spectrum analyzer to evaluate the reflection characteristics based on the transmitting characteristics. As a result, it was obtained the characteristics that the central wavelength was 800 nm, maximum reflectance was 10 percent and full-width at half maximum .sub.G was 0.2 nm with respect to polarized light (ordinary light) in the direction of x axis.

(114) Then, for evaluating the characteristics of the external resonator laser utilizing the grating device, the laser module was mounted as shown in a separate figure. As the light source device, it was prepared one having a GaAs series laser structure, in which a high refractive index film was provided on its one end face and an AR coating of a reflectance of 0.1% was provided on the other end face.

(115) Specification of the Light Source Device:

(116) Central wavelength; 800 nm

(117) Length of laser device; 1000 m

(118) Specification of Mounting

(119) Lg: 3 m

(120) Lm; 20 m

(121) After mounting the module, the device was driven under current control (ACC) without utilizing a Pertier device to obtain the laser characteristics of a central wavelength of 800 nm and an output power of 50 mW. Further, the module was set in a thermostatic bath for evaluating the operational temperature range to measure the temperature dependency of the laser oscillating wavelength, the temperature with the mode hopping occurred and the deviation of output power. As a result, the temperature coefficient of the oscillating wavelength was 0.05 nm/ C., the temperature of the mode hopping was 6 C., and the deviation of the output power was 10 percent.

Inventive Example 2

(122) It was fabricated the system shown in FIGS. 1 and 4.

(123) Specifically, Ti film was formed on a substrate composed of z-plate of lithium niobate crystal doped with MgO, and photolithography technique was utilized to produce grating pattern in the direction of y-axis. Thereafter, fluorine-based reactive ion etching was performed using the Ti pattern as a mask to form grating grooves at a pitch spacing of 214 nm and a length Lb of 100 m. The depth of the grating groove was 40 nm. Further, for forming the optical waveguide for propagation in y-axis, the grooves each having a width Wm of 3 m and Tr of 0.5 m were processed in the grating portion by means of excimer laser. Further, the buffer layer 17 made of SiO.sub.2 and of 0.5 m was formed by a sputtering system on the face with the grooves formed thereon, and a black LN substrate was used as the supporting body to adhere the face with the grating formed thereon. The black LN means lithium niobate whose oxygen content is depleted so that generation of electric charges due to pyroelectricity can be prevented. It is thus possible to prevent cracks of the substrate due to surge resistance responsive to temperature change.

(124) Then, the supporting body was adhered onto a polishing surface plate and the back face of the supporting body with the grating was subjected to precision polishing to a thickness (Ts) of 1 m. Thereafter, it was removed off from the surface plate and the buffer layer 16 composed of SiO.sub.2 and of 0.5 m was formed on the polished face by sputtering.

(125) Thereafter, the assembly was cut into bars by means of a dicing equipment and both end faces of the bar were subjected to optical polishing. AR coatings of 0.1% or lower were formed on the both end faces, respectively, and the bar was cut into chips to produce the grating devices. The size of the device was a width of 1 mm and a length L.sub.WG of 500 m.

(126) As to the optical characteristics of the grating device, a super luminescence diode (SLD), a light source for wide band wavelength, was used to input light into the grating device and its output light was analyzed by an optical spectrum analyzer to evaluate the reflection characteristics based on the transmitting characteristics. As a result, it was obtained the characteristics that the central wavelength was 945 nm, maximum reflectance was 20 percent and full width at half maximum .sub.G was 2 nm with respect to TE mode.

(127) Then, for evaluating the characteristics of the external resonator laser utilizing the grating device, the laser module was mounted as shown in FIG. 1. As the light source device, it was prepared one having a GaAs series laser structure, in which a high refractive index film was provided on its one end face and an AR coating of a reflectance of 0.1% was provided on the other end face.

(128) Specification of the Light Source Device:

(129) Central wavelength; 950 nm

(130) Output power: 20 mW

(131) Half value width: 50 nm

(132) Length of laser device; 250 m

(133) Specification of Mounting

(134) Lg: 1 m

(135) Lm; 20 m

(136) After mounting the module, the device was driven under current control (ACC) without utilizing a Peltier device to obtain the laser characteristics of a central wavelength of 945 nm and an output power of 50 mW. FIG. 8 shows the characteristic spectrum of the laser. Further, the module was set in a thermostatic bath for evaluating the operational temperature range to measure the temperature dependency of the laser oscillating wavelength and the deviation of output power. As a result, the temperature coefficient of the oscillating wavelength was 0.05 nm/ C., the temperature range with the large deviation of output power in the temperature range due to the mode hopping was 80 C., and the deviation of the output power was within 1 percent even in the case that the mode hopping took place.

Inventive Example 3

(137) It was formed the grating grooves at a pitch spacing of 222 nm and a length Lb of 100 m, according to the same procedure as the example 2. The depth of the grating groove was 40 nm. As to the optical characteristics of the grating device, a super luminescence diode (SLD), a light source for wide band wavelength, was used to input light into the grating device and its output light was analyzed by an optical spectrum analyzer to evaluate the reflection characteristics based on the transmitting characteristics. As a result, it was obtained the characteristics that the central wavelength was 975 nm, maximum reflectance was 20 percent and full width at half maximum .sub.G was 2 nm with respect to TE mode.

(138) Then, the laser module was mounted as shown in FIG. 1. As the light source device, it was applied a conventional GaAs series laser whose emitting face was not covered with an AR coating.

(139) Specification of the Light Source Device:

(140) Central wavelength; 977 nm

(141) Output power: 50 mW

(142) Half value width: 0.1 nm

(143) Length of laser device; 250 m

(144) Specification of Mounting

(145) Lg: 1 m

(146) Lm; 20 m

(147) After mounting the module, the device was driven under current control (ACC) without utilizing a Peltier device to obtain the laser characteristics that it was oscillated at a central wavelength of 975 nm responsive to the reflection wavelength of the grating and the output power was 40 mW, although the output power was smaller than that obtained in the case that the grating device is not provided. Further, the module was set in a thermostatic bath for evaluating the operational temperature range to measure the temperature dependency of the laser oscillating wavelength and the deviation of output power. As a result, the temperature coefficient of the oscillating wavelength was 0.05 nm/ C., the temperature with the large deviation of output power in the temperature range due to the mode hopping was 80 C., and the deviation of the output power was within 1 percent even in the case that the mode hopping took place.

Comparative Example 2

(148) In the case that the grating device is not provided in the inventive example 3, the temperature coefficient of the laser oscillating wavelength was as large as 0.3 nm/ C. and the mode hopping temperature was about 10 C. The deviation of the power was large and the deviation of output power was larger than 10 percent, at 10 C. or higher.

Inventive Example 4

(149) Ta.sub.2O.sub.5 film was formed on a quartz substrate for 1.2 m by a sputtering system to provide the optical material layer. Then, Ti film was formed on Ta.sub.2O.sub.5 and photolithography technique was utilized to produce grating pattern in the direction of y-axis. Thereafter, fluorine-based reactive ion etching was performed using the Ti pattern as a mask to form grating grooves at a pitch spacing of 232 nm and a length Lb of 100 m. The depth of the grating groove was 40 nm. Further, it was produced the optical waveguide having the shape shown in FIGS. 2 and 3 of the specification, by the reactive ion etching according to the same procedure described above. As to the optical characteristics of the grating device, a super luminescence diode (SLD), a light source for wide band wavelength, was used to input light into the grating device and its output light was analyzed by an optical spectrum analyzer to evaluate the reflection characteristics based on the transmitting characteristics. As a result, it was obtained the characteristics that the central wavelength was 945 nm, maximum reflectance was 20 percent and full width at half maximum .sub.G was 2 nm with respect to TE mode.

(150) Then, the laser module was mounted as shown in FIG. 1. As the light source device, it was applied a conventional GaAs series laser with a 0.1% AR coating formed on the emitting face.

(151) Specification of the Light Source Device:

(152) Central wavelength; 950 nm

(153) Output power: 20 mW

(154) Half value width: 50 nm

(155) Length of laser device; 250 m

(156) Specification of Mounting

(157) Lg: 1 m

(158) Lm; 20 m

(159) After mounting the module, the device was driven under current control (ACC) without utilizing a Peltier device to obtain the laser characteristics that it was oscillated at a central wavelength of 945 nm responsive to the reflection wavelength of the grating and that the output power was 40 mW, although the output power was smaller than that obtained in the case that the grating device is not provided. Further, the module was set in a thermostatic bath for evaluating the operational temperature range to measure the temperature dependency of the laser oscillating wavelength and the deviation of output power. As a result, the temperature coefficient of the oscillating wavelength was 0.03 nm/ C., the temperature range with the large deviation of output power due to the mode hopping was 50 C., and the deviation of the output power in the temperature range was within 1 percent even in the case that the mode hopping took place.

Inventive Example 5

(160) It was produced the system shown in FIGS. 1 and 3, according to the same procedure as the Example 1. However, the cross sectional shape of the grating device 21D was made as shown in FIG. 12(a).

(161) Specifically, an SiO.sub.2 layer 16 forming the lower clad layer was formed on a supporting substrate 10 made of quartz in a thickness of 0.5 m by a sputtering system, and Ta.sub.2O.sub.5 was formed thereon for 1.2 m to provide the optical material layer.

(162) Then, Ti film was formed on Ta.sub.2O.sub.5 and grating pattern was produced by an ED drawing system. Thereafter, fluorine-based reactive ion etching was performed using the Ti pattern as a mask to form grating grooves at a pitch spacing of 238.5 nm and a length Lb of 100 m. The depth of the grating groove was 40 nm.

(163) Further, for forming the optical waveguide 30, refractive ion etching was performed according to the same procedure as described above to completely cut the optical layer to leave the optical waveguide of a width Wm of 3 m and the both sides. The thickness Ts of the optical waveguide 30 was made 1.2 m.

(164) Finally, the buffer layer 23 made of SiO.sub.2 was formed as the upper clad in 2 m by sputtering to cover the optical waveguide 30.

(165) Thereafter, the assembly was cut into bars by means of a dicing equipment and both end faces of the bar was subjected to optical polishing. AR coatings of 0.1% or lower were formed on the both end faces, respectively, and the bar was cut into chips to produce the grating devices. The size of the device was a width of 1 mm and a length L.sub.WG of 500 m.

(166) As to the optical characteristics of the grating device, a super luminescence diode (SLD), a light source for wide band wavelength, was used to input light of TE mode into the grating device and its output light was analyzed by an optical spectrum analyzer to evaluate the reflection characteristics based on the transmitting characteristics. It was obtained the characteristics that the central wavelength was 975 nm, reflectance was 18 percent and full width at half maximum .sub.G was 2 nm as to the thus measured grating device.

(167) Then, the laser module was mounted as shown in FIG. 1. As the light source device, it was applied a conventional GaAs series laser without an AR coating on the emitting face.

(168) Specification of the Light Source Device:

(169) Central wavelength; 977 nm

(170) Output power: 50 mW

(171) Half value width: 0.1 nm

(172) Length of laser device; 250 m

(173) Specification of Mounting

(174) Lg: 1 m

(175) Lm; 20 m

(176) After mounting the module, the device was driven under current control (ACC) without utilizing a Peltier device to obtain the laser characteristics that it was oscillated at a central wavelength of 975 nm responsive to the reflection wavelength of the grating and that the output power was 40 mW, although the output power was smaller than that obtained in the case that the grating device is not provided. Further, the module was set in a thermostatic bath for evaluating the operational temperature range to measure the temperature dependency of the laser oscillating wavelength and the deviation of output power. As a result, the temperature coefficient of the oscillating wavelength was 0.03 nm/ C., the temperature range with the large deviation of output power due to the mode hopping was 40 C., and the deviation of the output power in the temperature range was within 1 percent even in the case that the mode hopping took place.

External resonator-type light emitting device

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H01S5/02326

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G02B6/124

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H01S5/141

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G02B6/124

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Abstract

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