JP2018085416A - Wavelength-variable mirror and wavelength-variable laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To materialize a wavelength-variable mirror which enables high-speed and high-finesse selective extraction of light with a desired wavelength from incident light of a wide wavelength band and to realize a high-speed wavelength-variable laser.SOLUTION: A wavelength-variable mirror comprises: an air etalon having a first high-reflection mirror and a second high-reflection mirror which are opposed to each other through an air gap located therebetween in parallel and arranged to allow a predetermined wavelength of incident light to pass therethrough; a third high-reflection mirror operable to reflect the incident light having passed through the air etalon; and a mechanism operable to mechanically drive the first high-reflection mirror or the second high-reflection mirror in an optical axial direction of the incident light. In the wavelength-variable mirror, a reflection surface of the third high-reflection mirror is perpendicular to the optical axis of the incident light; and a reflection surface of the first high-reflection mirror, and a reflection surface of the second high-reflection mirror are arranged to be inclined to a plane perpendicular to the optical axis of the incident light.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a tunable mirror having a tunable light reflection characteristic using a ferroelectric material having an electrostrictive effect or an inverse piezoelectric effect, and a tunable laser using the same.

  In recent years, it has been desired to realize a narrow-band high-speed tunable optical filter in various fields. For example, in the communication field, optical communication is now applied to access networks, such as the introduction of optical fibers to homes in addition to the increase in capacity, speed and functionality of optical communication systems. It has become. In the future, higher functionality of the network will be required, and it is expected that effective use of wavelengths will advance.

  In high-density wavelength division multiplexing communication, communication capacity is increased by multiplexing a plurality of different lights at very short wavelength intervals and transmitting them through a single waveguide. On the transmission side, for example, a plurality of lights having different wavelengths at very short intervals of 0.8 nm are multiplexed over a range of about several tens of nanometers, whereby a plurality of signals are transmitted with high density. Correspondingly, on the receiving side, a necessary signal is extracted by selectively receiving only light having a predetermined wavelength from the plurality of lights.

  In order to obtain only a specific signal from an optical signal transmitted at such a high density with a high degree of accuracy, the transmission band is sufficiently secured and the cross spectrum with the adjacent channel is sufficiently small. An optical filter must be used. Such an optical filter is not limited to extracting only light having a desired wavelength from a network in which a large amount of light having different wavelengths is transmitted, and transmitting light having a specific wavelength when multiplexing a plurality of lights. It is indispensable even when monitoring the situation. Furthermore, if a specific wavelength can be filtered and switched at high speed, a large-capacity and high-speed network can be realized.

  In the non-communication field, for example, a plurality of visualization techniques using light have been developed. Optical coherence tomography (OCT) for non-destructively observing a cross section of a living body or an electronic device using low coherence interference is used. A high-quality image can be obtained with a resolution having a resolution of about several tens of μm at the cell level within a range of about several mm in the depth direction from the surface.

  As methods for obtaining an OCT image, TD-OCT (Time domain optical coherence tomography) and FD-OCT (Spectral domain optical coherence tomography) are known. The TD-OCT method is generally a method in which an SLD (Super Luminescent Diode) is used as a light source to irradiate the surface to be observed to extract information in the surface and further scan in the depth direction to obtain three-dimensional information. It is.

  On the other hand, as the FD-OCT method, a three-dimensional image can be obtained by obtaining information in the depth direction using a high-speed wavelength swept light source and scanning in the remaining two axial directions.

  In recent years, high-speed wavelength swept light source technology of several hundred kHz or more has rapidly developed, and the FD-OCT method is dominant from the viewpoint of image acquisition speed. As a configuration of a high-speed wavelength swept light source used, a configuration that filters the wavelength of a broad light source with a wide bandwidth at high speed is typical, but there is also a demand for a wavelength-tunable optical filter that performs sharp filtering.

  As such a wavelength tunable optical filter, an optical element called an etalon that utilizes multiple interference that occurs when two translucent mirrors face each other in parallel has been known. It is known that the transmittance characteristic of etalon has transmittance peaks at a plurality of wavelengths. The space (cavity, gap) between the mirrors constituting the etalon may be a cavity (air etalon) or may be filled with a transparent medium having a predetermined refractive index.

  For example, as a device that variably selects and transmits light of a desired wavelength from incident light in a wide wavelength band, liquid crystal is filled in a cavity between mirrors forming a Fabry-Perot etalon, and voltage is applied to the liquid crystal. Thus, there has been proposed a wavelength tunable filter (hereinafter referred to as a liquid crystal etalon) that can change the optical optical path length of the inter-mirror cavity by changing the refractive index (see, for example, Patent Document 1 below). .

  By using this liquid crystal etalon as an element for wavelength selection and placing it in the laser resonator together with the gain medium, it is possible to select and emit laser light of only the desired wavelength channel according to the magnitude of the applied voltage Tunable lasers have been reported.

In addition, a deflector is manufactured using a KTN (KTa _1-x Nb _x O ₃ ) crystal having an electro-optic effect, and a reflection type filter is formed by changing an incident angle to the diffraction grating. A wavelength tunable laser can also be realized by disposing it inside. A broadband wavelength tunable laser capable of selecting and emitting laser light of only a desired wavelength channel according to the magnitude of the voltage applied to the KTN crystal has been realized, and high-speed operation at 200 kHz has been reported (for example, , See Patent Document 2 below).

JP 2006-19514 A JP 2012-74597 A

  However, in the liquid crystal etalon as in Patent Document 1, since there is optical loss in the cavity between the mirrors constituting the liquid crystal etalon, the Finesse (index indicating the wavelength resolution of the etalon, the transmission wavelength period FSR of the optical filter). (Free Spectral Range) divided by the half-value width of the transmission wavelength peak) was 100 or less. Furthermore, the response speed of the liquid crystal was determined from the material characteristics, and the wavelength change speed was also limited to milliseconds to sub-milliseconds.

  Further, although the KTN deflection method as in Patent Document 2 satisfies the requirements for high speed, a high finesse type has not been reported. Accordingly, no coherence length characteristic of a wavelength tunable laser realized by using these conventional devices has been reported to extend over several centimeters.

  The present invention has been made to solve such problems, and includes a tunable mirror that can selectively extract light of a desired wavelength from incident light in a wide wavelength band at high speed with high finesse, and The present invention provides a high-speed wavelength tunable laser used.

  In order to achieve such an object, the present invention is characterized by having the following configuration.

(Structure 1 of the invention)
An air etalon having a first high reflection mirror and a second high reflection mirror facing in parallel across an air gap and transmitting incident light of a predetermined wavelength;
A third highly reflective mirror that reflects incident light transmitted through the air etalon;
A wavelength tunable mirror comprising: a mechanism for mechanically driving the first high reflection mirror or the second high reflection mirror in an optical axis direction of incident light,
The reflective surface of the third highly reflective mirror is perpendicular to the optical axis of the incident light, and the reflective surface of the first highly reflective mirror and the reflective surface of the second highly reflective mirror are the optical axes of the incident light. A wavelength tunable mirror, wherein the mirror is arranged to be inclined with respect to a plane perpendicular to.

(Configuration 2)
At least one of the first to third high-reflection mirrors has a spectral reflectance characteristic of a desired pass bandwidth that is equal to or less than the wavelength width of the FSR of the air etalon with respect to the incident light. The wavelength tunable mirror according to Configuration 1 of the invention.

(Structure 3 of the invention)
The driving mechanism is
A ferroelectric material having an electrostrictive effect or an inverse piezoelectric effect, and a mechanical strain generated in an optical axis direction of the incident light by applying a voltage to the ferroelectric material, thereby causing the air gap of the air etalon to The wavelength tunable mirror according to Configuration 1 or 2, wherein the interval is changed.

(Configuration 4)
The first high-reflection mirror is provided on a parallel plane substrate that is tilted and supported with respect to a plane perpendicular to the optical axis of the incident light,
The second high reflection mirror is provided on the light incident side surface of the wedge-shaped substrate,
The wavelength tunable mirror according to Configuration 3 of the invention, wherein the third high reflection mirror is provided on a surface of the wedge-shaped substrate facing the surface on the light incident side.

(Structure 5 of the invention)
The ferroelectric material is a KTN crystal;
5. The wavelength tunable mirror according to any one of Structures 3 to 4, further comprising temperature control means for controlling the temperature of the KTN crystal so as to maintain a cubic crystal.

(Structure 6 of the invention)
The first high reflection mirror and the second high reflection mirror have spectral reflectance characteristics that reflect light in the entire wavelength band of the incident light,
Any one of configurations 2 to 5 according to the invention, wherein the third high reflection mirror has a spectral reflectance characteristic of a desired pass bandwidth that is equal to or less than a wavelength width of the FSR of the air etalon with respect to the incident light. The wavelength tunable mirror according to item 1.

(Configuration 7)
A resonator having two reflective surfaces opposed in parallel, and a gain medium provided on the optical path between the two reflective surfaces for performing optical amplification;
The resonator includes the wavelength tunable mirror according to any one of configurations 1 to 6, and at least one of the two reflecting surfaces of the resonator includes the wavelength tunable mirror. A wavelength tunable laser comprising: a third reflective surface of a third highly reflective mirror, wherein the wavelength of emitted light is selectively controlled according to an applied voltage.

(Configuration 8)
Of the two reflection surfaces of the resonator, the reflection surface on the light output side is composed of a light output mirror having a spectral reflectance characteristic that transmits in a predetermined wavelength bandwidth. The wavelength tunable laser described in 1.

(Configuration 9)
9. The wavelength tunable laser according to Configuration 7 or 8, wherein the gain medium is composed of a semiconductor optical amplifier having a high optical gain in a predetermined wavelength band.

  The wavelength tunable mirror and wavelength tunable laser according to the present invention can realize a high-speed wide-band wavelength tunable mirror with high finesse and a high-speed wavelength swept light source having a wide band and a high coherence length.

It is sectional drawing of the example of 1 structure of the wavelength variable mirror which concerns on Embodiment 1 of this invention. It is sectional drawing of the example of 1 structure of the wavelength variable laser which concerns on Embodiment 2 of this invention. It is a figure explaining the optical characteristic of the wavelength variable mirror which concerns on Embodiment 1 of this invention. It is a figure explaining the optical characteristic of the wavelength variable laser which concerns on Embodiment 2 of this invention. It is sectional drawing of another Example of the wavelength tunable mirror which concerns on Embodiment 1 of this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the embodiment, a ferroelectric material having an electrostrictive effect or a reverse piezoelectric effect for mechanically driving one of two opposing mirrors constituting an air etalon is provided. Examples of ferroelectric materials include potassium tantalate niobate (KTN: KTa _1-x NbxO ₃ (0 <x <1)) crystal, or K _1-y Li _y Ta _1-x Nb _x O doped with lithium. ₃ is characterized by using a Fabry-Perot etalon type filter made of a crystal material having a composition of 0 <x <1, 0 <y <1 (hereinafter collectively referred to as KTN crystal).

  The KTN crystal changes the crystal system from tetragonal to cubic with increasing temperature, and has a large electrostrictive effect in the cubic. In particular, in a temperature range close to the phase transition temperature to tetragonal crystal, a phenomenon in which the relative permittivity diverges occurs, and the electrostrictive effect proportional to the square of the permittivity is very large and advantageous. Of course, the material is not limited to KTN as long as it exhibits a large electrostrictive effect.

(Embodiment 1: Wavelength variable mirror)
FIG. 1 shows a tunable mirror 10 according to Embodiment 1 of the present invention.

  In FIG. 1, the light incident from the optical window 15 of the casing 16 of the wavelength tunable mirror 10 passes through the space between the fixing members 13 and 14 that support the parallel plane substrate 5 to be inclined and enters the parallel plane substrate 5. To do. The fixing members 13 and 14 may be configured as an integral fixing member as a transparent member, and the optical path of incident light may be secured inside the fixing member.

  Incident light is wavelength-selectively transmitted by an air etalon including the high reflection mirror 1, the air gap 9, and the high reflection mirror 2, and is provided on a surface facing the light incident side surface of the wedge-shaped substrate 8. To 3. The high reflection mirror 3 forms a reflection surface perpendicular to the optical axis of the incident light, and the incident light reflected here follows a path opposite to that of the incident light as reflected light and exits from the left optical window 15. .

  One mirror of the air etalon is formed as the first high reflection mirror 1 on the transparent parallel flat substrate 5 supported by the fixing members 13 and 14. The parallel plane substrate 5 is supported by being slightly inclined by an angle θ with respect to a plane perpendicular to the optical axis of incident light.

  The other mirror that forms a pair of air etalons faces the first high-reflection mirror 1 across the air gap 9 and faces the second high-reflection mirror 2 on the light incident side surface of the wedge-shaped substrate 8. It is formed as.

Therefore, the reflection surface of the first high reflection mirror and the reflection surface of the second high reflection mirror are disposed so as to be inclined at an angle θ with respect to a surface perpendicular to the optical axis of the incident light.
An antireflection film 4 is formed on the surface of the parallel flat substrate 5 on the optical window 15 side, and a third highly reflective mirror 3 is perpendicular to the optical axis of incident light on the surface of the wedge-shaped substrate 8 on the KTN crystal 6 side. Is formed. Therefore, the apex angle of the wedge-shaped substrate 8 is θ.

  Electrodes 11 and 12 for applying a driving voltage are formed on both side surfaces of the KTN crystal 6 serving as a mirror driving mechanism. The third highly reflective mirror 3 formed on the wedge-shaped substrate 8 is joined to a surface perpendicular to the electrode surface of the KTN crystal 6. Since no light is incident on the KTN crystal 6, the bonding method may be a direct bonding method or bonding using an adhesive.

  By applying a driving voltage to the KTN crystal 6 from the power source (not shown) via the electrodes 11 and 12, the mechanical strain generated by the electrostrictive effect or the inverse piezoelectric effect of the KTN crystal 6 which is a ferroelectric material is used. Then, the KTN crystal 6 is stretched and deformed in the optical axis direction of the incident light. Then, the high reflection mirror 2 and the high reflection mirror 3 on the wedge-shaped substrate 8 bonded to the KTN crystal 6 are mechanically driven in the optical axis direction of incident light. Since the high reflection mirror 2 is driven back and forth in the optical axis direction while maintaining the parallelism of the air gap 9 with respect to the high reflection mirror 1, the size of the air gap 9 can be varied and the transmission wavelength of the air etalon can be variably controlled. .

  Here, although not specifically described, in order to maximize the electrostrictive effect, the temperature of the KTN crystal 6 is controlled by the temperature control means so as to maintain a cubic crystal, and a desired dielectric constant is maintained. As a temperature control method, as shown in FIG. 1, a temperature control means such as a Peltier element 7 may be bonded directly to the KTN crystal 6 and thermally connected thereto, and fixed to the housing 16. The entire housing 16 may be sealed and the temperature of the entire space in the housing 16 may be adjusted.

(Another example of Embodiment 1)
In order to adjust the size (interval) of the air gap 9, the high reflection mirror 1 side may be driven. As in another example of the embodiment 1 shown in FIG. 5, the KTN crystal 6 is moved to the optical window 15 side and installed on the wall surface of the casing 16, and the parallel flat substrate 5 and the high reflection mirror 1 are arranged in the optical axis direction. It may be mechanically driven. The KTN crystal 6 may be trapezoidal in cross-sectional shape, and the parallel plane substrate 5 may be fixed to maintain the tilt angle. However, the cross-sectional shape of the KTN crystal 6 is rectangular as shown in FIG. A wedge-shaped substrate 8a having a θ shape may be provided and held in the opposite direction.

  In this case, since the high reflection mirror 2, the wedge-shaped substrate 8, and the high reflection mirror 3 are directly fixed to the wall surface of the housing 16, the reflection surfaces of these mirrors can be mechanically stably held with respect to the optical axis. . Since the KTN crystal 6 is transparent, an optical path from the optical window 15 to the parallel plane substrate 5 may be secured in the KTN crystal 6. The temperature control means (not shown in FIG. 5) for adjusting the temperature of the KTN crystal 6 may be connected to the side surface of the KTN crystal 6 by means of thermal coupling in consideration of electrical insulation and mechanical deformation of the crystal. Good.

(Relationships and numerical examples of air etalon design parameters)
The size L of the air gap 9 of the air etalon is determined according to the required wavelength variable width. For example, when a wavelength variable width of 100 nm is required, the FSR (Free Spectral Range) also needs to be 100 nm or more. In the FSR, the wavelength is λ, the air gap length is L, and θ is an angle formed by the reflection surfaces of the first and second high reflection mirrors and the reflection surface of the third high reflection mirror (the wedge-shaped substrate 8 (optical According to the following (Formula 1) as the apex angle θ) of the glass).

  For example, when FSR = 100 nm and cos θ≈1, the air gap length L is calculated to be 8.45 μm at the wavelength λ = 1.3 μm, and L = 5.5 μm at the wavelength λ = 1.06 μm.

  In addition, the finesse of air etalon (Finess: an index representing wavelength resolution, FSR / half-value width of transmission wavelength peak) is given by the following (Equation 2). High reflectivity R close to 1 is required for the high reflection mirrors 1 and 2 constituting the air etalon.

The transmission spectrum T (λ) of the air etalon shown in FIG. 1 is expressed by (Equation 3).

The wavelength λ _m that gives the maximum peak of the transmittance T (λ) of the air etalon according to the above (Equation 3) is given by the following (Equation 4) where the diffraction order is m, so the amount of gap change necessary for the wavelength change Δλ _m ΔL is given by the following (formula 5).

For example, if Δλ _m = 108 nm, λ _m = 1300 nm, L = 7.8 μm, θ = 3 °, the required gap change amount ΔL = 0.6 μm.

Further, if Δλ _m = 100 nm, λ _m = 1060 nm, L = 5 μm, θ = 3 °, the required gap change amount ΔL = 0.47 μm.

  When L = 7.8 μm, FSR = 108 nm, and the gap length L changes. When L = 8.4 μm, FSR = 100 nm. Even if the gap length changes, the FSR always exceeds 100 nm over the entire variable range. It can be secured.

  When L = 5 μm, FSR = 112 nm, and the gap length L changes. When L = 5.47 μm, FSR = 102 nm. Even if the gap length changes, the FSR is always 100 nm or more in the entire variable range. Can be secured.

The variables represented by the above formulas 1 to 5 are summarized as follows.
λ: wavelength of incident light R: reflectance L (of the first and second high reflection mirrors) L: air gap length ΔL: air gap change amount θ: reflection surface of the first and second high reflection mirrors and the third height Angle formed by the reflecting surface of the reflecting mirror (vertical angle of the wedge-shaped substrate 8 (optical glass) in FIG. 1)
m: Diffraction order λm: Wavelength giving the maximum peak of air etalon transmittance Δλm: Wavelength change (wavelength change with air gap change)
T (λ): Air etalon transmission spectrum

(Electrostriction coefficient and displacement of KTN crystal)
By the way, when the electrostriction coefficient of the cubic KTN crystal is Q, the polarization is P, the electric field is E, and the dielectric constant is ε, the strain vector x is expressed by the following (formula 6).

here,

As values of the electrostriction coefficient of the KTN crystal of the above (formula 6), Q ₁₁ = 7.3 × 10 ⁻² m ⁴ C ⁻² and Q ₁₂ = −4.7 × 10 ⁻² are known.

When an electric field is applied in the principal axis direction of the KTN crystal, Q ₁₁ gives a crystal displacement in the electric field direction, and Q ₁₂ gives a displacement in a direction perpendicular to the electric field. From the relationship of the signs, the displacement in the electric field direction is the direction in which the crystal extends, and the displacement in the direction perpendicular to the electric field is in the direction in which the crystal shrinks.

For example, when a thickness of 1 mm between the positive and negative electrodes of the KTN crystal and a relative dielectric constant εr = 17500 and a voltage of 400 V is applied between the positive and negative electrodes, a uniform electric field of 400 V / mm is generated in the thickness direction within the crystal. At this time, the amount of displacement of the electric field perpendicular to the direction of KTN crystal (optical axis direction) determined to be 0.72 [mu] m using a Q _12, can provide a sufficient amount of displacement ΔL of the wavelength variable 100 nm.

(Effect of tilting the air etalon)
In the first embodiment, the effect of tilting the air etalon with respect to the optical axis will be described.

  In FIG. 1, when parallel light is incident on the wavelength tunable mirror 10 so as to be perpendicularly incident on the reflection surface of the third high reflection mirror 3, the incident light is reflected by the third high reflection mirror 3. The light beam emitted from the wavelength tunable mirror 10 through the same optical path is reflected by a solid arrow, the reflection surface of the first high reflection mirror 1 of the air etalon, the reflection surface of the second high reflection mirror 2, and the like. Light rays emitted from the variable mirror are indicated by broken lines.

  Since the reflection surface of the first high reflection mirror 1 and the reflection surface of the second high reflection mirror 2 and the reflection surface of the third high reflection mirror 3 form an inclination angle θ, the first high reflection of the air etalon is performed. The traveling direction of the light reflected by the reflecting surface of the mirror 1 and the reflecting surface of the second highly reflecting mirror 2 is different from that of the light reflected by the third highly reflecting mirror 3.

  Similarly, the multiple reflection light by the first high reflection mirror 1 and the third high reflection mirror 3 and the multiple reflection light by the second high reflection mirror 2 and the third high reflection mirror 3 are the same as the third reflection light. The traveling direction is different from the light reflected by the high reflection mirror 3. Therefore, it is possible to realize the wavelength tunable mirror 10 that can separate and extract only the light that is perpendicularly incident on the third high-reflecting mirror 3 and reflected on the same optical path as the incident light, and can perform wavelength selection with high accuracy.

  The first high-reflection mirror 1 and the second high-reflection mirror 2 are configured as multilayer reflection films in which high-refractive index dielectric films and low-refractive index dielectric films are alternately stacked, and according to finesse in a wide wavelength band. Thus, the reflectance is determined. The first high reflection mirror and the second high reflection mirror have characteristics that reflect light in the entire wavelength band of incident light.

  Similarly, the third high reflection mirror 3 is configured as a multilayer reflection film in which high refractive index dielectric films and low refractive index dielectric films are alternately laminated. The high reflection mirror 3 can be designed as a reflection type bandpass filter different from the air etalon, having a desired high reflectance (spectral reflectance characteristic) with a pass band width equal to or smaller than the wavelength width of the FSR.

  Alternatively, a spectral reflectance characteristic corresponding to such a bandpass filter characteristic may be provided in either the first or second high-reflecting mirror that is similarly configured with a multilayer reflective film.

  That is, at least one of the first to third high reflection mirrors can have a spectral reflectance characteristic of a desired pass bandwidth that is equal to or smaller than the wavelength width of the FSR with respect to incident light.

(Embodiment 2: wavelength tunable laser)
Hereinafter, a tunable laser 20 using the tunable mirror 10 according to the first embodiment of the present invention will be described as a second embodiment of the present invention. FIG. 2 is a schematic diagram showing a configuration of the wavelength tunable laser 20 according to the second embodiment of the present invention.

  The tunable laser 20 according to the embodiment of the present invention is provided on the optical path between the tunable mirror 10 and the laser output optical mirror 21 constituting the two reflecting surfaces of the laser resonator, and the two reflecting surfaces. And an optical gain medium 23 for optical amplification.

  The gain medium 23 is provided between the wavelength tunable mirror 10 and the optical output mirror 21, and is configured by, for example, a semiconductor optical amplifier. The light output mirror 21 has spectral characteristics (spectral reflectance characteristics) that partially transmit and partially reflect light in the wavelength variable band.

  Here, the first high reflection mirror 1 of the wavelength tunable mirror 10 faces the gain medium 23 side, and the third high reflection mirror 3 and the light output mirror 21 constitute a laser resonator. Are arranged in parallel. An AC power source (not shown) generates an AC voltage and applies the generated AC voltage to the ferroelectric crystal (KTN crystal 6) via the electrodes 11 and 12.

  When a semiconductor optical amplifying element is used as the gain medium 23, a condensing lens or the like may be disposed in the laser resonator in order to ensure the coupling efficiency between the semiconductor optical amplifying element and the light beam above a predetermined value. An antireflection film 22 is provided on the surface of the gain medium 23 on the wavelength tunable mirror 10 side.

  By using the wavelength tunable mirror 10 as a mirror mirror for the resonator of the wavelength tunable laser 20, it is possible to obtain a gain exceeding the oscillation threshold only in a desired wavelength band. As a result, the wavelength tunable laser 20 emits laser light having only a wavelength in a desired wavelength band. Here, by applying an AC voltage to the ferroelectric crystal 6 constituting the wavelength tunable mirror 10 and adjusting the applied voltage, the wavelength tunable laser 20 that changes the wavelength of the laser emission light can be obtained.

  As described above, the wavelength tunable laser 20 according to the second embodiment of the present invention can realize a wavelength tunable laser having a high speed wavelength tunable and a high coherence length as compared with the conventional wavelength tunable laser.

  Specific examples of the wavelength tunable mirror 10 according to the first embodiment of the present invention and the wavelength tunable laser 20 according to the second embodiment of the present invention will be further described below.

(Example 1: Variable wavelength mirror)
Example 1 of the wavelength tunable mirror 10 according to Embodiment 1 of the present invention shown in FIG. 1 will be described. As described above, one of the mirrors constituting the air etalon was formed as the first highly reflective mirror 1 made of a dielectric multilayer film on the parallel flat substrate 5 made of transparent quartz. The other second high reflection mirror 2 was formed as a high reflection mirror 2 made of a similar dielectric multilayer film on a wedge-shaped substrate 8 made of transparent quartz. Here, the apex angle θ of the wedge-shaped substrate 8 was 3 °.

  An antireflection film 4 was formed on the other surface of the parallel plane substrate 5, and a bandpass filter high reflection mirror (third high reflection mirror 3) was formed on the wedge-shaped substrate 8. The reflectivity R of the first and second reflecting mirrors was set to 0.997 in order to realize the finesse 1000.

  As shown in FIG. 1, driving electrodes 11 and 12 (Pt / Au) were formed on both sides of the KTN crystal 6 by vapor deposition or sputtering. A third high reflection mirror 3 having a bandpass filter characteristic formed on the bottom surface of the wedge-shaped substrate 8 was joined to a surface perpendicular to the electrode of the electrode-attached KTN crystal 6. Here, bonding was performed using an adhesive. The temperature of the KTN crystal 6 is controlled so as to maintain a cubic crystal so as to maintain a desired dielectric constant. As a temperature control method, the Peltier element 7 was directly bonded to the KTN crystal 6 and the crystal temperature was controlled at about 30 ° C. The relative dielectric constant at this time was 17500.

  The specific crystal size of the KTN crystal 6 was 3 mm × 4 mm × 1 mmt, and the electrodes 11 and 12 were provided on the entire surface of 3 mm × 4 mm in order to apply a voltage in the direction of 1 mmt.

  In order to set the air gap 9 between the first high reflection mirror 1 and the second high reflection mirror 2 to 7.8 μm, the first mirror 1 is formed while monitoring the parallelism and the gap length between the mirrors. The fixing members 13 and 14 for supporting the parallel plane substrate 5 made of quartz and the KTN crystal 6 for supporting the wedge-shaped substrate 8 made of quartz on which the second mirror 2 is formed are respectively provided on the opposing inner surfaces of the housing 16. Fixed to the wall.

  The fixing members 13 and 14 that support the parallel flat substrate 5 fix the optical window 15 of the housing 16 at a position where the optical window 15 is not blocked. Alternatively, a transparent member may be configured as an integral fixing member, and an optical path of incident light may be secured inside the fixing member. Alternatively, a quartz wedge-shaped substrate 8a having the same shape as the wedge-shaped substrate 8 may be attached in the reverse direction, as in the other embodiment of FIG.

  Further, as shown in FIG. 1, a temperature adjusting Peltier element 7 may be interposed between the KTN crystal 6 and the housing 16 as a temperature control means. The Peltier element 7 may be disposed on the side surface of the KTN crystal 6.

(Optical characteristics of tunable mirror)
FIG. 3 (a) is a diagram showing the optical characteristics of wavelength versus transmittance of a normal air etalon without bandpass filter characteristics. It has a plurality of transmittance peaks at wavelength positions separated by the FSR interval.

  On the other hand, FIG. 3 (b) shows a wavelength tunable etalon (wavelength tunable mirror 10) provided with a third highly reflective mirror 3 having bandpass filter characteristics according to the first embodiment of the present invention manufactured by the method described above. FIG. Since there is a bandpass filter characteristic (spectral reflectance characteristic) of a desired pass bandwidth equal to or smaller than the wavelength width of the FSR shown in the dotted line by the third high reflection mirror, the peaks other than the central peak are suppressed, and the wavelength is suppressed. Even if it is changed, only the transmittance peak is shown. By changing the voltage applied to the KTN crystal from 0 V to 400 V, this transmittance peak can vary from 1250 nm to 1350 nm with a finesse of about 1000.

(Example 2: Characteristics of wavelength tunable laser)
FIG. 4 illustrates characteristics of the wavelength tunable laser 20 (FIG. 2) according to the second embodiment of the present invention.

  In the second embodiment, the optical output mirror 21 for the laser resonator is a spectral characteristic (bandpass filter characteristic, spectral reflection) in which the reflectance is 10% and the remaining 90% is transmitted in the wavelength region from 1250 nm to 1350 nm. The one having a rate characteristic) is used.

  Further, as the gain medium 23, as shown in FIG. 4A, a semiconductor optical amplifying element having a high optical gain in a wavelength band from 1230 nm to 1370 nm is used.

  When a current is injected from an electrode (not shown) into the semiconductor optical amplifying element of the gain medium 23 that performs optical amplification to increase the injection current, the optical gain of the gain medium 23 increases, and the optical loss in the laser resonator and the gain medium A laser oscillation threshold determined by the optical gain is reached, laser oscillation occurs, and laser oscillation light is emitted from the light output mirror 21 side.

  Therefore, if the wavelength range of the light output mirror 21 is set from 1250 nm to 1350 nm as shown by the dotted line in FIG. 4B, laser oscillation can be performed only in this range.

  FIG. 4C is a diagram showing the wavelength change of the laser output light when the voltage V applied to the KTN crystal 6 constituting the wavelength tunable mirror 10 is changed. When a voltage is applied, the gap length increases, so that a sweep from a short wavelength to a long wavelength occurs as the voltage is applied. In the present invention, a wavelength sweep can be performed by applying a sawtooth wave having a frequency of 200 kHz to the KTN crystal 6.

  Since the gap length increased in proportion to the square of the applied electric field, wavelength sweeping proportional to the square of the voltage could be confirmed. It was confirmed that a wavelength variable characteristic exceeding 100 nm can be realized by applying a voltage of 400 V. Furthermore, it was confirmed that the coherence length was several cm or more.

  The wavelength tunable mirror and wavelength tunable laser of the present invention can realize a high-finesse, high-speed wide-band tunable mirror, a high-speed wide-band tunable laser and a wavelength swept light source with a high coherence length, laser processing, microscope, printer, display It can be used for optical devices such as optical communication, sensing, and medical measurement.

DESCRIPTION OF SYMBOLS 1 1st high reflection mirror 2 2nd high reflection mirror 3 3rd high reflection mirror 4 Antireflection film 5 Parallel plane substrate 6 KTN crystal 7 Peltier device 8, 8a Wedge-shaped substrate 9 Air gap 10 Wavelength variable mirrors 11, 12 Electrodes 13 and 14 Fixing member 15 Optical window 16 Housing 20 Wavelength variable laser 21 Optical output mirror 22 Antireflection film 23 Gain medium

Claims (9)

An air etalon having a first high reflection mirror and a second high reflection mirror facing in parallel across an air gap and transmitting incident light of a predetermined wavelength;
A third highly reflective mirror that reflects incident light transmitted through the air etalon;
A wavelength tunable mirror comprising: a mechanism for mechanically driving the first high reflection mirror or the second high reflection mirror in an optical axis direction of incident light,
The reflective surface of the third highly reflective mirror is perpendicular to the optical axis of the incident light, and the reflective surface of the first highly reflective mirror and the reflective surface of the second highly reflective mirror are the optical axes of the incident light. A wavelength tunable mirror, wherein the mirror is arranged to be inclined with respect to a plane perpendicular to. At least one of the first to third high-reflection mirrors has a spectral reflectance characteristic of a desired pass bandwidth that is equal to or less than the wavelength width of the FSR of the air etalon with respect to the incident light. The tunable mirror according to claim 1. The driving mechanism is
A ferroelectric material having an electrostrictive effect or an inverse piezoelectric effect, and a mechanical strain generated in an optical axis direction of the incident light by applying a voltage to the ferroelectric material, thereby causing the air gap of the air etalon to The wavelength tunable mirror according to claim 1, wherein the interval is changed. The first high-reflection mirror is provided on a parallel plane substrate that is tilted and supported with respect to a plane perpendicular to the optical axis of the incident light,
The second high reflection mirror is provided on the light incident side surface of the wedge-shaped substrate,
4. The wavelength tunable mirror according to claim 3, wherein the third high reflection mirror is provided on a surface of the wedge-shaped substrate facing the light incident side surface. 5. The ferroelectric material is a KTN crystal;
5. The wavelength tunable mirror according to claim 3, further comprising a temperature control unit configured to control the temperature of the KTN crystal so as to maintain a cubic crystal. The first high reflection mirror and the second high reflection mirror have spectral reflectance characteristics that reflect light in the entire wavelength band of the incident light,
6. The third high-reflection mirror according to claim 2, wherein the third high-reflection mirror has a spectral reflectance characteristic of a desired pass bandwidth that is equal to or less than a wavelength width of the FSR of the air etalon with respect to the incident light. The wavelength tunable mirror described in the item. A resonator having two reflective surfaces opposed in parallel, and a gain medium provided on the optical path between the two reflective surfaces for performing optical amplification;
The resonator includes the tunable mirror according to any one of claims 1 to 6, wherein at least one of the two reflecting surfaces of the resonator includes the tunable mirror. A wavelength tunable laser comprising a reflecting surface of a third highly reflecting mirror having a wavelength, and selectively controlling the wavelength of emitted light in accordance with an applied voltage. 8. The light output side reflection surface of the two reflection surfaces of the resonator is composed of a light output mirror having a spectral reflectance characteristic that transmits in a predetermined wavelength bandwidth. The tunable laser described. 9. The wavelength tunable laser according to claim 7, wherein the gain medium comprises a semiconductor optical amplifying element having a high optical gain in a predetermined wavelength band.