JP2004220008A - Volume type phase grating, its manufacturing method, optical module and semiconductor laser module using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem wherein, especially in the case that light having a wavelength near the oscillation wavelength of a semiconductor laser 12 enters the semiconductor laser 12 directly, the oscillation wavelength becomes unstable under the influence and the spectrum property and the output of the laser become unstable. <P>SOLUTION: This volume type phase grating has phase gratings which have periodic refractive index variation, respectively, and is characterized in that respective phase gratings are inclined at a prescribed angle β to an incident plane. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a volume phase grating using Bragg reflection that reflects and returns only a specific wavelength of incident light, a manufacturing method thereof, and an optical module and a semiconductor laser module using the same.

  Conventionally, as means for returning light of a specific wavelength in an optical fiber, there is fiber Bragg grating (abbreviated as FBG).

As shown in FIG. 12, a periodic refractive index change is provided in the fiber core 24, and the recording light 30 having a wavelength λuv in the ultraviolet region has a mask interval Λ (mask) on the optical fiber 10. Irradiation is performed through the phase mask 36, and the intensity-modulated region due to the interference of the ± 1st-order diffracted light 25 therefrom is transferred and formed on the optical fiber 10. A change in the refractive index was caused, and the phase grating 20 could be manufactured. At that time, the lattice interval Λ (FBG) of the refractive index change period is
Λ (mask) = 2 × Λ (FBG)
In a relationship.

The characteristics of the FBG 14 manufactured in this manner are determined by the amount of change in the refractive index, the lattice spacing Λ (FBG), and the length thereof. The change amount and length of the refractive index affect the reflectance and the bandwidth, and the lattice spacing Λ (FBG) affects the center wavelength. With respect to the longitudinal direction of the optical fiber 10, the reflection center wavelength λb of the FBG 14 having a constant lattice spacing Λ (FBG) is
λb = 2 × n × Λ (FBG)
n: It is indicated by the effective refractive index of the fiber core.

As shown in FIG. 11, an FBG 14 is formed on the output side fiber end 17 of the semiconductor laser module 16 and is coupled via a coupling lens 7 so that a part of light (several% to about 10%) is transmitted to the semiconductor laser 12. To make it act as an external resonator, narrow the output wavelength spectrum characteristic of the semiconductor laser 12, and stabilize the spectrum characteristic. The oscillation spectrum characteristic of the semiconductor laser 12 substantially matches the reflection center wavelength λb of the FBG 14 which is an external resonator.

  Further, the spectrum and output characteristics with respect to a temperature change can be stabilized (see Patent Document 1).

  Also, instead of the FBG 14 constituting the phase grating reflector in the optical fiber 10, the same SiO 2 or glass-based material is used to change the refractive index, so that the phase grating 20 is constituted by a volume type. There is a phase grating. In particular, in the case of using the diffraction direction of the emergence angle of the ± 1st-order diffracted light 25 so that the direction of the angle of the reflected light coincides with the direction of the angle of the reflected light, it is referred to as volume Bragg grating (VBG).

  Although not shown, recording light having a wavelength λuv having a uniform phase (for example, light having a wavelength of 458 to 528 nm in an ultraviolet band by an argon laser) is split into two by a beam splitter or the like, and each light is once converged by a lens. Then, in order to remove unnecessary diffracted light, the light is passed through a pinhole (diameter: 5 to 25 μm) provided at the focal position, and then converted into parallel light (not shown).

  FIG. 10 (a) shows the manufacturing method, in which a phase grating made of an induced-refractive-index medium (thickness such as SiO2 base oxide glass or the like with an additive such as silver added thereto) having optically polished upper and lower surfaces. The optical path length from the beam splitter to the element surface 22 of the phase grating substrate 3 on the element surface 22 of the substrate 3 is made to exactly match each other, and the surface of the phase grating substrate 3 is irradiated and exposed for about 5 to 30 minutes.

  The exposure time depends on the material of the phase grating substrate 3 to be used.

By adjusting the angle θ0 to the phase grating substrate 3, the wavelength of the recording light 30 for arbitrarily setting the grating interval P1 is λuv, and the incident angle is θ0. From the Snell's law, n0 × sinθ0 = n1 × sinθ2
Assuming that the refractive index n0 of air is 1, the angle θ2 in the phase grating substrate 3 is θ2 = sin ⁻¹ {sin θ0 / n1}
n1: Refractive index of the phase grating substrate 3 n0: Refractive index of air (= 1)
The wavelength λm of the refractive index n1 in the phase grating substrate 3 at the wavelength λuv is such that the light speed Cm in the phase grating substrate 3 is 1 / n1 of the light speed Cuv in the air (the frequency f is constant). Cm = Cuv / n1
From Cm = f × λm and Cuv = f × λuv, λm = λuv / n1.

  When two plane-wave lights having the wavelength A of the amplitude A intersect in the phase grating substrate 3 having the refractive index n1, the interference causes the grating interval P1 of the refractive index variation of the phase grating 20 to be reduced to zero when the combined amplitude of each plane wave becomes zero. Is determined by the following formula,

2A × [cos {(2 × π × Y × sin θ2) / λm}] = 0
A: Amplitude of each plane wave Y: Y-axis position Y (k) = {(2 × k + 1) × λm} / (4 × sin θ2)
(K is an arbitrary integer)
It becomes a straight line group shown by.

Therefore, the lattice spacing P1 of each phase grating is given by the following equation: P1 = Y (k + 1) −Y (k)
P1 = λm / {2 × sin θ2}
Indicated by The line of the fixed period is exposed. When the irradiated phase grating substrate 3 is left in a high-temperature environment of about 500 ° C. for several hours, it appears as a periodic refractive index change region. A refractive index variation Δn of about 0.01 to 0.001 can be obtained.

  In FIG. 10B, a volume type phase grating 37 is vertically cut out from the upper and lower surfaces of the phase grating substrate 3 with a width T. The incident surface 21 obtained by optically polishing the cut surface 26 is subjected to AR coating with a dielectric multilayer film to prevent reflection on the incident surface 21.

  As shown in FIG. 10C, a large number of volume type phase gratings 1 having a height of D mm square and a cut width of T mm can be manufactured by the above process. When the incident light 18 having the wavelengths of λa and λb having the Bragg condition is incident on the volume phase grating 1, the light of the wavelength λb is totally reflected, and the light of the wavelength λa becomes the end face reflection 27.

Although not shown, when the light is inclined at an angle α and the light is incident, the diffraction direction and the reflection angle coincide with the configured phase grating surface 20a, and the Bragg diffraction condition is established. The diffraction efficiency depends on the cutting width T (the number of the phase gratings 20). Further, when light having a wavelength different from the wavelength under the Bragg diffraction condition or light is incident at an angle, the diffraction efficiency decreases and the emission angle of the diffracted light also changes.
JP-A-9-283847

  The conventional configuration using a phase grating having such characteristics has the following disadvantages.

  When the FBG 14 of FIG. 11 is attached to the output side fiber end 17 of the semiconductor laser 12, light having a wavelength other than the semiconductor laser 12, particularly light having a wavelength close to the oscillation wavelength of the semiconductor laser 12, directly enters the semiconductor laser 12. As a result, there is a problem that the oscillation wavelength becomes unstable due to the influence, and the spectrum characteristic and the output become unstable.

  In order to eliminate the influence, an optical isolator for removing such light may be attached to the output side of the semiconductor laser 12. However, when an optical isolator (not shown) is attached, the semiconductor from the FBG 14 which is an external resonator is removed. Since the necessary reflected light 19 to the laser 12 is also blocked, it cannot function as an external resonator.

  Also, in order to remove such unnecessary reflected light 19, it is possible to remove the reflected light 19 by attaching an in-line optical isolator (not shown) to the output side of the FBG 14, but it becomes expensive in cost, and a plurality of Since the module has an assembly configuration, a space for parts is required.

  The FBG 14 area in the output fiber end 17 is usually as long as about 10 mm, and when there is a large temperature change, the lattice spacing P1 itself changes due to linear expansion, so that the wavelength of the reflected light 19 changes, There is a problem that the oscillation wavelength of the laser element 12 changes.

  In addition, when the conventional volume phase grating 37 is used, when the incident light 18 is vertically incident thereon, as shown in FIG. 10C, even if each incident surface 21 is AR-coated. Inevitably, the end face reflection 27 is slightly generated, and even unnecessary light such as the wavelength λa other than the specific Bragg condition is reflected, and the reflected light returns to the semiconductor laser 12 as it is, causing unnecessary oscillation in the laser 12 and output. There is a drawback that the spectrum characteristics become unstable.

  In view of the above, the present invention has a phase grating having a periodic refractive index change, wherein each phase grating is inclined at a predetermined angle β with respect to the incident surface.

  Furthermore, the present invention is characterized in that the entrance surface and the exit surface are parallel.

Further, the angle β of the inclination of the phase grating with respect to the incident surface is β = sin ⁻¹ {(sin α) / n1} (α> β)
α: Incident angle of incident light with respect to the incident surface of the volume phase grating n: Refractive index of the substrate forming the product type phase grating, which is characterized by being indicated.

  Further, at least one coupling lens is coupled to the volume phase grating.

  Further, the above-mentioned volume phase grating is provided on one or both sides of the optical isolator element, and is connected to a coupling lens.

  Further, the optical module is connected to a semiconductor laser, and a part of the wavelength of the emitted light of the semiconductor laser is returned to the semiconductor laser to oscillate.

Further, in the method of manufacturing a volume phase grating, when ultraviolet rays having the same phase are incident on the substrate for the volume phase grating from two directions, and respective incident angles are θ0 and θ1,
β = | (θ2−θ3) / 2 |
θ2 = sin ^-1 {(sin θ0) / n1}
θ3 = sin ^-1 {(sin θ1) / n1}
Is satisfied.

  Further, the volume phase grating is cut at an angle α with respect to the upper surface of the element, the cut surface is used as an incident / exit surface, and after polishing the incident surface, an antireflection coat is formed. Things.

  As described above, the present invention has the following effects.

(1) By making the incident light incident on the incident surface of the volume phase grating at an angle α, unnecessary reflected light due to end face reflection is prevented from returning together with the reflected light from the phase grating.

(2) By making the periodic refractive index distribution surface of the volume type phase grating an angle β with respect to the incident surface, the volume type phase grating is perpendicularly incident on the phase grating having a periodic refractive index change in the medium, Unnecessary reflected light is prevented from being reflected on the reflected light from the phase grating.

(3) By mounting the volume phase grating according to the present invention together with the optical isolator, the function of removing unnecessary reflected light and the function as an external resonator of the semiconductor laser are integrated.

(4) By mounting on a semiconductor laser, a semiconductor laser module with a built-in external resonator and optical isolator is realized, and it is not necessary to separately attach an inline optical isolator as in the conventional case.

(5) By attaching lenses to both ends or one end of the volume phase grating according to the present invention, or attaching to a fiber with a lens, it is realized as a filter module having stable characteristics against temperature fluctuation, and applied to various optical modules. Enabled.

  Hereinafter, embodiments of the present invention will be described.

  FIG. 1 shows a cross section of a volume phase grating 1 according to the present invention, and FIG. 1 (b) shows details of a portion A thereof.

  The volume type phase grating 1 has a phase grating 20 having a periodic refractive index fluctuation, and each phase grating 20 is inclined at an angle β with respect to an incident surface 21. The light 18 having a wavelength λb is configured to be reflected.

  The volume phase grating 1 thus configured is tilted, and the incident light 18 having the wavelength λb is made incident at an angle α.

  At this time, the angles α and β are set so that the incident light 18 is incident on the phase grating 1 at an angle β and is perpendicularly incident on the phase grating 20 at a grating interval P2.

  Also, since the grating interval P2 is configured as a Bragg condition only for light of wavelength λb, it has a high reflectance, and only light of wavelength λb is totally reflected and returns to the original optical path as it is. .

  The remaining wavelength λa proceeds as it is, enters the opposite exit surface 28 at an angle β, and the exit light 29 exits at an angle α.

  Here, it is particularly preferable that the entrance surface 21 and the exit surface 28 are parallel.

  Although the incident light 18 and the outgoing light 29 to the volume phase grating 1 are parallel, an optical axis shift △ 1 occurs, and is expressed by the following equation.

Δ1 = T1 × tanγ / cosα
T1: the width of the volume phase grating 1 and γ is the difference between the angle α and the angle β, and is expressed by the following equation.

γ = α-β
β = sin ^-1 {(sinα) / n1}
When set under the above conditions, the incident light 18 is perpendicularly incident on the phase grating 20, only the light of the wavelength λb is Bragg-reflected, and returns to the original optical path as it is. Further, since the incident light 18 is incident on the incident surface 21 at a predetermined angle α instead of perpendicularly incident, the reflected light 19 due to the Bragg reflection is affected by the end surface reflection 27 on the incident surface 21 of the volume phase grating 1. Instead, only the component λb can be returned.

  When the refractive index n1 of the volume phase grating 1 is in the range of 1.5 to 2.0, the angle β satisfies the condition α> β> γ.

FIG. 2 shows a volume type phase grating 2 obtained by cutting the volume type phase grating 1 shown in FIG. 1 into a parallelogram. The angle of the phase grating surface 20a with respect to the element upper surface 22 having a periodically changing refractive index is γ. In this case, the optical axis shift # 2 between the incident light 18 and the outgoing light 29 is:
Δ2 = T2 × tanγ
T2: indicated by the width of the volume phase grating 2.

  In this case, it can be mounted on the mounting substrate without inclination.

FIG. 7A shows a method of manufacturing the volume phase grating 1 according to the present invention shown in FIGS. A phase grating substrate 3 (a glass plate based on SiO ₂ to which silver, Na, K, or the like is added) is prepared, and the upper surface 22 of the element is polished and processed into a plane through which light is transmitted. It is fixed to a substrate holder (not shown), and the recording light 30 having a wavelength λuv with an angle θ0 from the direction A and an angle θ1 from the direction B toward the element upper surface 22 is irradiated with a plane wave ultraviolet light having the same intensity for a certain period of time. Irradiation (for several tens of seconds to several tens of minutes) to cause optical interference in the substrate for phase grating 3 and record by exposure. The refraction angles θ2 and θ3 in the medium due to the angles θ0 and θ1 of the recording light 30 from both directions AB are expressed by Snell's law by the following equation, where n0 = 1.

θ2 = sin ^-1 {(sin θ0) / n1}
θ3 = sin ^-1 {(sin θ1) / n1}
When the refractive index of the phase grating substrate 3 is n1, the wavelength λm of the recording light 30 in the phase grating substrate 3 is
λm = λuv / n1
Indicated by

At that time, the angle β in FIG.
β = | (θ2−θ3) / 2 | (θ3> θ2)
It becomes.

  Next, the phase grating substrate 3 on which the exposure recording has been performed is placed in an electric furnace and heated to about 500 ° C. As a result, the refractive index of the portion that has not been irradiated with ultraviolet light decreases, and a periodic sinusoidal refractive index change occurs.

It is also known that when the phase grating substrate 3 is a SiO ₂ substrate, the amount of change in the refractive index is increased by preliminarily applying a pressure treatment with hydrogen or the like before irradiation with ultraviolet rays, and such a treatment is also effective. It is.

  The lattice spacing P2 of the resulting refractive index fluctuation is expressed by the following equation.

P2 = λm / [2 × sin {(θ2 + θ3) / 2}]
7 (b) and 7 (c), the volume type phase grating 1 is cut from the phase grating substrate 3 in the vertical direction with a cutting width T1, and the cut surface 26 is used as a light input / output surface as shown in FIG. The volume type phase grating 1 is obtained. Further, if the cutting is performed at the cutting width T2 in the direction of the angle α, the volume phase grating 2 shown in the embodiment of FIG. 2 is obtained. The number of layers of the phase grating surface 20 is determined by the cut widths T1 and T2, and the diffraction efficiency can be set. The thinner the cutting widths T1 and T2, the smaller the number of layers of the phase grating surface 20, the lower the diffraction efficiency, and the lower the reflectance. In the case of an external resonator for a semiconductor laser, the diffraction efficiency may be about 10%, and the cut widths T1 and T2 may be small. Since it is necessary to increase the height nearer, the cutting widths T1 and T2 become larger.

  Usually, the cutting widths T1 and T2 for the external resonator for the semiconductor laser are about 1 mm, and for the add / drop module are about 4 to 5 mm. After the cut surfaces 26 of the volume phase gratings 1 and 2 are optically polished, if further spatially independent use is required, antireflection AR coating with a dielectric multilayer film or the like is applied to prevent end face reflection. Through the above process, the volume phase gratings 1 and 2 of the present invention are manufactured and can be applied to various optical modules.

  FIG. 3 shows a volume type phase grating 2 of the present invention mounted on one side of an optical isolator 15.

  The optical isolator 15 is provided on the emission side of the semiconductor laser 12, transmits the emission light 29 in a wavelength range near the emission light wavelength of the semiconductor laser 12, and blocks the reflected light 19. It is installed to prevent the oscillation state from becoming unstable. Generally, a Faraday rotator 4 made of a garnet material is sandwiched between polarizers 5, and a magnet 6 for applying a saturation magnetic field to the Faraday rotator 4 is provided.

  FIG. 3A shows a configuration diagram of the volume phase grating 2 mounted and fixed on a substrate 8. In this case, each element is integrated with a light-transmitting adhesive or a glass solder material, but each element may be separated and fixed. The magnet 6 is unnecessary when a magnetic garnet material is used for the Faraday rotator 4 to be used.

  FIG. 3B shows a case where the coupling lenses 7 are fixedly coupled to both sides of the optical element shown in FIG. 3A. In the semiconductor laser module 16 shown in FIG. It is set on the semiconductor laser 12 and fixed to the substrate 8 on the Peltier element 23.

With such a configuration, light emitted from the semiconductor laser 12 is converted into parallel light by the coupling lens 7, and first enters the volume phase grating 2. At that time, the Bragg reflection wavelength of the volume phase grating 2 is set within the oscillation wavelength spectrum width of the semiconductor laser 12. Thereby, part of the light from the semiconductor laser 12 (about 10% of the output) is reflected.

  Generally, the reflectance of the volume phase grating 2 is set to about 5 to 10%, and the volume phase grating 2 acts as an external resonator of the semiconductor laser 12. By doing so, the laser oscillation is drawn to a wavelength that is equal to the wavelength characteristic of the Bragg reflected light of the volume phase grating 2, and oscillation with stable spectrum characteristics is possible in that range.

  The optical module 13 mounted with the volume phase grating 2 shown in FIG. 6B is mounted on a Peltier element 23 and controlled in temperature, similarly to the semiconductor laser 12, so that the temperature of the environment is largely changed. Even in this case, the cycle of the refractive index fluctuation of the volume phase grating 2 is not affected by the linear expansion due to the temperature change, and the semiconductor laser 12 can oscillate stably. When unnecessary reflected light 19 from the outside, particularly reflected light 19 in a wavelength range near the oscillation wavelength of the semiconductor laser 12 returns to the semiconductor laser 12 via the output side fiber end 17, the oscillation becomes unstable due to the influence. As a result, the wavelength-spectrum output characteristics become unstable, which is not preferable in use.

  In this case, since the optical isolator 15 in the optical module 13 blocks such reflected light 19, the oscillation of the semiconductor laser 12 is not affected by the reflected light 19 at all.

  In this case, in the conventional semiconductor laser module 16 with FBG shown in FIG. 11, the FBG 14 is formed on the output fiber end 17 mounted on the ferrule 31 and is easily affected by a change in ambient temperature. .

  Further, the reflected light 19 from the optical fiber 10 enters the semiconductor laser 12 via a number of phase grating surfaces 20 formed in the FBG 14, and in particular, the reflected light 19 in a wavelength range near the oscillation wavelength of the semiconductor laser 12 is generated. When it returns, the oscillation becomes unstable, and its wavelength-spectrum output characteristics become unstable without being stabilized. In particular, in the case of an excitation light source for an optical fiber amplifier for DWM transmission, high output, wavelength multiplexing, and polarization multiplexing are required, and it is necessary to stabilize the oscillation wavelength and narrow the spectrum width. The semiconductor laser module using the volume phase grating according to the present invention is effective.

  FIG. 4 shows a configuration in which both ends of the phase grating 2 according to the present invention are coupled by coupling lenses 7, and is used on the emission side of the semiconductor laser 12 in FIG. The light is reflected by diffraction and acts as an external resonator, and is used for stabilizing the oscillation wavelength of the semiconductor laser 12. In this case, unlike the case of FIG. 6A, the optical isolator 15 is separated from the volume phase grating 2 and attached to the output-side fiber end 17, whereby the reflected light 19 from the optical fiber 10 can be removed.

  FIGS. 5A and 5B are configuration examples of an in-line optical module in which the volume phase grating 2 is mounted. When light of a plurality of wavelengths is incident, it is used as a band-pass filter that reflects light of a specific wavelength corresponding to the refractive index fluctuation of the volume phase grating 2. Compared to the FBG type band-pass filter, the element length is as short as 2 to 3 mm, the linear expansion due to temperature change is small, and there is almost no change in the lattice spacing. It has a small mounting structure.

  FIG. 5C shows a case in which the volume phase grating 2 is mounted together with the optical circulator element 11, and there is no need to separately attach the FBG 14 to constitute an add / drop circuit as in the related art. Further, the temperature characteristics can be configured to be stable. The volume phase grating 2 according to the present invention is not limited to these, and can be applied to optical modules of various forms.

  The volume phase type device 1 was actually manufactured according to the process of FIG.

A phase grating substrate 3 (refractive index n1 = 1.525) made of a photo-induced refractive index material based on SiO ₂ and having a diameter of 2 inches and a thickness of 2 mm was prepared by optically polishing the upper and lower surfaces of the phase grating substrate 3. As a coherent light source, a water-cooled argon laser having a wavelength of 488 nm and an output of 3 W was used. An argon laser and an optical system for interference exposure were set on a vibration isolation table, and the laser output light was split into equal intensity by a beam splitter on the way, and each beam was passed through a lens and set at a converging position at a pinhole with a diameter of 10 μm. The unnecessary diffracted light was eliminated by. The collimator lens system converts the light into parallel light having an outer diameter of 35 mm (1 / e ^{2 of the} peak intensity), and irradiates the phase grating substrate 3 for about 10 minutes by two-beam interference (holographic method) in a dark room. Exposure was recorded. (Not shown above)
The angles of the recording lights were set to θ0 = 47 ° and θ1 = 54 °, and the angles in the phase grating substrate 3 were set to θ2 = 28.7 ° and θ3 = 32 °.

  The angle of the phase grating 20 with respect to the incident surface 21 is β = 1.65 °, and the angle α is α = 2.5 °.

  Then, the exposure-recorded phase grating type substrate 3 is removed from the holder, heated in an electric furnace at a temperature of about 450 ° C. to 600 ° C. for 3 to 4 hours, and the refractive index of the exposed portion is reduced. Caused a periodic refractive index change. Thereafter, the wafer was vertically cut at a width T of 1 mm.

  Next, the cut surface 26 was optically polished, and a dielectric multilayer film for AR coating was vapor-deposited on the surface to produce the volume phase grating 1.

  The reflection spectrum characteristic is within a range of 1475 nm ± 1 nm, and the reflectance is about 10%.

  Then, the volume type phase grating 1 shown in FIG. 4 was inclined at an angle α = 2.5 °, and both ends were sandwiched between coupling lenses 7 and fixed on a substrate 8 to obtain an optical module 13.

  As shown in FIG. 6B, the semiconductor laser module 16 was fixedly mounted on the substrate 8 on the Peltier element 23 of the semiconductor laser module 16 having the semiconductor laser 12 having an oscillation wavelength of 1472 nm at 25 ° C., slightly deviating from the reflection spectrum characteristic.

  FIG. 8 shows that the output fiber end 17 to which the volume phase grating 1 is attached has a diffraction efficiency of about 10%, and the half-width with respect to the output power peak at an external temperature of 25 ° C. is about 1.5 nm. The semiconductor laser 12 has a spectrum characteristic.

  The power and output spectrum characteristics are monitored in a state where the semiconductor laser module 16 is subjected to the temperature control by the APC (abbreviation of Auto Power Control) and the Peltier element 23 at a temperature change condition of −20 ° C. to 60 ° C. did.

  FIG. 9 shows the amount of wavelength shift of the peak wavelength of the output power. When the volume type phase grating 1 of the present invention is attached (solid line), there is almost no wavelength shift, and the stability is hardly changed. Oscillation state was obtained.

  When a conventional FBG is used (broken line), the temperature cannot be controlled by the APC and the Peltier element 23 as in the embodiment because of its structure. Therefore, the FBG linearly expands due to the influence of the external temperature, and the reflection wavelength shifts to a long wavelength band. As a result, the oscillation wavelength of the semiconductor laser 12 is also drawn into it and shifted to a long wavelength band.

  It has been shown that the volume phase grating 1 of the present invention functions sufficiently as an external modulator and realizes a laser module having stable characteristics even in a place where there is a large temperature change.

(A) is sectional drawing of the volume type phase grating by this invention. FIG. 2B is a detailed view of a portion A in FIG. FIG. 4 is a cross-sectional view of another embodiment of the volume phase grating according to the present invention. 3A is a cross-sectional view of an embodiment in which the volume phase grating shown in FIG. 2 is mounted on an optical isolator, and FIG. 3B is a light in which both ends of the volume phase grating shown in FIG. It is sectional drawing of a module. 1 is a cross-sectional view of an optical module in which both ends of a volume phase grating according to the present invention are connected by lenses. (A) is an embodiment in which the volume phase grating according to the present invention is attached to a fiber collimator, (b) is an embodiment in which the volume phase grating of the present invention is configured in an in-line optical module, (c) () Is a figure which shows each embodiment at the time of mounting the volume type phase grating of this invention with an optical circulator. (A) is a cross-sectional view of a semiconductor laser module mounted with an optical isolator to which the volume phase grating of the present invention is attached, and (b) is a cross-sectional configuration diagram of a semiconductor laser module mounted with the volume phase grating of the present invention. . (A) is a configuration diagram showing a method of manufacturing a volume phase grating of the present invention by an optical interference method, and (b) is a diagram showing a volume phase grating shown in FIGS. 1 and 2 from a volume phase grating substrate of the present invention. FIG. 2C is a configuration diagram showing conditions for cutting into a grating, and FIG. 2C is a configuration diagram of the volume phase grating of the embodiment shown in FIGS. 1 and 2 of the present invention. 5 is a spectrum characteristic of output light of a semiconductor laser module using the volume phase grating of the present invention. FIG. 9 shows the amount of wavelength shift depending on the external temperature of the semiconductor laser module using the volume phase grating of the present invention and the conventional semiconductor laser module using the FBG. (A) is a configuration diagram showing a conventional method for producing a volume phase grating by an optical interference method, (b) is a configuration diagram showing conditions for cutting a volume phase grating from a conventional substrate for a volume phase grating, (C) is a configuration diagram showing a conventional volume phase grating. It is sectional drawing of the conventional semiconductor laser module with FBG. FIG. 7 is a configuration diagram illustrating an example of a conventional FBG manufacturing method.

Explanation of reference numerals

1: Volume phase grating 2: Volume phase grating 3: Phase grating substrate 4: Faraday rotator 5: Polarizer 6: Magnetic field applying magnet 7: Coupling lens 8: Substrate 10: Optical fiber 11: For optical circulator Element 12: Semiconductor laser 13: Optical module 14: FBG
15: Optical isolator 16: Semiconductor laser module 17: Output side fiber end 18: Incident light 19: Reflected light 20: Phase grating 20a: Phase grating surface 21: Incident surface 22: Element upper surface 23: Peltier element 24: Fiber core 25: Diffracted light 26: cut surface 27: end surface reflection 28: emission surface 29: emission light 30: recording light 31: ferrule 33: fiber collimator 34: in-line optical module 35: optical circulator 36: phase mask 37: volume type phase grating α : Angle β: Angle γ: Angle λa: Wavelength λb: Reflection center wavelength λm: Wavelength A: Amplitude T: Cutting width T1: Cutting width T2: Cutting width P1: Lattice spacing P2: Lattice spacing △ 1: Optical axis shift △ 2 : Optical axis deviation Λ (mask): mask interval Λ (FBG): lattice interval λuv: wavelength n1: refractive index n0: refractive index △ u: refractive index change amount Cm: light velocity Cuv: light speed

Claims (8)

A volume phase grating, comprising: a phase grating having a periodic refractive index change, wherein each phase grating is inclined at a predetermined angle β with respect to an incident surface. 2. The volume phase grating according to claim 1, wherein the entrance surface and the exit surface are parallel. 3. The angle β of inclination of the phase grating with respect to the incident surface of the phase grating according to claim 1 is β = sin ⁻¹ {(sin α) / n1} (α> β).
α: Incident angle of incident light with respect to the incident surface of the volume phase grating n: An optical module characterized by being indicated by the refractive index of the substrate forming the product type phase grating. An optical module, wherein at least one coupling lens is coupled to the volume phase grating according to claim 1. An optical module, wherein the volume phase grating according to claim 1 is installed on one or both sides of an optical isolator element and connected to a coupling lens. 6. A semiconductor laser module, comprising connecting the optical module according to claim 3 to a semiconductor laser, returning a part of the wavelength of the emitted light of the semiconductor laser to the semiconductor laser, and causing the semiconductor laser to oscillate. 3. The method of manufacturing a volume phase grating according to claim 1 or 2, wherein ultraviolet rays having the same phase are incident on the volume phase grating substrate from two directions, and the incident angles are θ0 and θ1, respectively.
β = | (θ2−θ3) / 2 |
θ2 = sin ^-1 {(sin θ0) / n1}
θ3 = sin- ^{1 {} (sin θ1) / n1}
A method for manufacturing a volume phase grating, characterized by satisfying the following. The volume phase grating is cut at an angle α with respect to the upper surface of the element, the cut surface is used as an input / output surface, and after polishing the incident surface, an antireflection coat is formed. 8. The method for producing a volume phase grating according to 7.

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