JP3450180B2 - Tunable laser - Google Patents

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a technology of a wavelength tunable laser for enabling continuous oscillation wavelength tunable operation and performing stable single mode oscillation. 2. Description of the Related Art In recent years, wavelength multiplexing transmission for transmitting light of a plurality of wavelengths to one fiber requires laser light sources of a plurality of different wavelengths as light sources. Also, these light sources need to be controlled very accurately to ensure good transmission quality. At present, these light sources are manufactured by changing the length of the resonator of the laser element. However, the laser element is manufactured again at the time of maintenance due to poor manufacturing yield and a fixed wavelength. There are problems such as necessity. The above problems can be solved by using an external cavity laser. FIG. 9 shows a conventional example of an external cavity laser. In the above conventional example, the semiconductor laser device 1a
The diffraction grating 1b is arranged outside the above, and the wavelength is selected by rotating the diffraction grating 1b, thereby varying the laser oscillation wavelength. Here, 1c denotes a lens and 1d denotes an optical fiber. [0005] However, in this conventional example, since the wavelength selection resolution depends on the pitch of the diffraction grating 1b and becomes a stepwise variable wavelength operation, it is impossible to perform a stepless variable wavelength operation. Further, when the wavelength variable range is set to be large, there is a drawback that it is difficult to reduce the size and the cost because the size of the diffraction grating 1b becomes large. In order to solve these problems, a tunable resonator type tunable laser using a tunable mirror formed by a micromachine technology has been proposed (Optonews).
(1997) No. 2, p122). The proposed configuration is shown in FIG. In the above proposal, a variable mirror (reflection mirror) 2b formed by a micromachine is arranged behind the laser element 2a in parallel with the element end face to form an external resonator. By moving the variable mirror 2b back and forth in the central axis direction, the length of the resonator changes, and the oscillation wavelength of the laser changes. Here, 2c indicates a lens, and 2d indicates an optical fiber. However, in the above proposal, since the variable mirror 2b and the end face of the laser element 2a need to be arranged in parallel, it is difficult to manufacture the mirror and the external resonator. There is a drawback that loss is large and high output cannot be obtained. SUMMARY OF THE INVENTION It is an object of the present invention to overcome the drawbacks of the prior art and to realize a tunable laser that is compact, easy to manufacture, and capable of obtaining high output in a wide tunable range. According to a first aspect of the present invention, there is provided a wavelength tunable laser for converting light emitted from a laser element into parallel light on an optical path for coupling light emitted from the laser element to an optical fiber. Tunable optical bandpass filter with a laser-side lens, a parallel light transmission center wavelength adjustment mechanism, and a half mirror that reflects part of the incident light from the tunable optical bandpass filter and returns it to the laser element A wavelength tunable laser having an external resonator including: a half mirror and an optical fiber, and a fiber-side lens for image conversion for coupling incident light from the half mirror to the optical fiber, A Fabry-Perot micromachined wavelength tunable filter is used as the tunable optical bandpass filter. A narrow band optical band-pass filter having an adjusting mechanism, an elastic supporting thin film made of an insulator arranged so as to be spaced apart from each other, a reflecting portion formed at the center of the elastic supporting thin film, and the reflecting portion Fabri formed in
A pair of reflective mirrors formed of dielectric multilayer films formed in the Fabry-Perot openings and arranged in parallel with each other, and a pair of the mirrors; A gap holding spacer arranged to hold the interval between the reflection mirrors,
It has the function of generating Coulomb force by applying a voltage to the driving electrode and changing the transmission center wavelength by controlling the distance between a pair of reflecting mirrors based on the applied voltage. A means for measuring the capacitance between the displacement sensor electrodes, which is disposed so as to be inclined by a predetermined angle with respect to the central axis of the incident light, and monitoring the gap length between the reflecting mirrors based on the measured value. Means for controlling a voltage applied to the driving electrode so as to have a target gap length, and the end face of the optical fiber is provided with a predetermined angle with respect to the central axis of the incident light in order to suppress reflection of the incident light. The surface is polished so as to have an inclined surface, and the end surface is coated with an anti-reflection coating by a dielectric multilayer film, and the polarization maintaining surface of the optical fiber and the emission polarization of the laser element are coincident, and guided through the optical fiber. The polarization state of Shako is characterized in that linearly polarized light is maintained. Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is an overall configuration diagram of a wavelength tunable laser according to the present embodiment, FIG. 2 is an exploded perspective view of a Fabry-Bellowe type optical wavelength tunable filter, FIG.
FIG. 4 is a graph showing the relationship between the applied voltage and the filter center wavelength. (Embodiment 1) The wavelength tunable laser according to the present embodiment is configured to enable continuous oscillation wavelength tunable operation and stable single mode oscillation. Specifically, as shown in FIG. 1, a laser element 1, a laser-side lens 10, a wavelength-variable optical bandpass filter 2, a half mirror 3, a fiber-side lens 11, and a fiber 4 are arranged on a central axis 5. is there. The laser element 1 is a multimode oscillation type Fabry-Perot laser, and the laser-side lens 10 converts light emitted from the laser element 1 into parallel light. The optical bandpass filter 2 is a narrow band optical bandpass filter having a transmission center wavelength adjusting mechanism, and is arranged obliquely with respect to the central axis 5 as shown in FIG. The half mirror 3 is an element that reflects part of incident light and transmits most of the light, and is arranged perpendicular to the central axis 5.
The lens 11 couples the parallel light to the fiber 4 and the combined light enters the fiber 4. The light emitted from the laser element 1 is converted into parallel light by the laser-side lens 10, and then the wavelength is selected by the wavelength variable filter 2. A part of the wavelength-selected light is reflected by the half mirror 3 and is again returned to the wavelength tunable filter 2.
Through the laser-side lens 10 and returned to the laser element 1. The returned light is amplified inside the laser element 1 and emitted again. By repeating this, an external resonator is formed between the above optical systems, and the laser element 1
Is emitted as a single mode oscillation at the transmission center wavelength of the tunable optical bandpass filter 2. Most of the light oscillated in the single mode is transmitted through the half mirror 3, image-converted by the fiber-side lens 11, and then coupled to the fiber 4. By changing the transmission center wavelength of the tunable optical bandpass filter 2, the oscillation center wavelength of the laser element 1 changes and the wavelength of the light emitted to the fiber 4 also changes. I do. Further, more specific embodiments will be described. The laser element 1 is made of indium gallium
1550nm manufactured with arsenic / phosphorus compound semiconductor
This is a Fabry-Perot type laser that oscillates in multimode with a focus on. The laser-side lens 10 includes the laser element 1
Is converted into parallel light having a beam diameter of 300 μm, and the fiber side lens 11 is an image conversion lens for coupling the parallel light to the fiber 4. In this embodiment, an aspheric lens is used for these lenses in order to prevent a decrease in coupling efficiency due to a change in wavelength. The optical band pass filter 2 is a narrow band optical band pass filter having a transmission center wavelength adjusting mechanism.
In order to suppress reflected stray light from the band-pass filter, it is arranged at an angle of 3 degrees with respect to the central axis 5. In this embodiment, in order to realize downsizing and wavelength tunability over a wide range,
As the tunable optical bandpass filter 2, a Fabry-Perot micromachined tunable optical filter manufactured by silicon micromachine technology is used. FIGS. 2 and 3 show an example of the configuration of the micromachine light wavelength tunable filter. FIG. 2 is an exploded perspective view of the present variable wavelength filter. This filter is an elastic supporting thin film 20 which is an insulator.
1. Fabry-Perot opening 203 formed in reflecting section 202, driving electrode 204, displacement sensor electrode 205,
It is composed of a gap holding spacer 206. In the Fabry-Perot opening 203, a reflection mirror 207 (see FIG. 3) made of a dielectric multilayer film is formed. FIG. 3 is a sectional view of the present wavelength tunable filter. The reflecting mirrors 207 on both sides are held by a gap holding spacer 206 so as to have an interval of about 7 μm. The reflectance of the reflection mirror 207 is 99%. The center wavelength of the filter when no voltage is applied to the driving electrode 204 is 1600 nm, and the full width at half maximum of the filter is 0.4 μm, which is smaller than the mode interval of the laser element 1.
In addition, the resonance period with the next mode of Fabry-Perot (FS
R) is wider than the oscillation range of the laser element, and is approximately 200
nm. FIG. 7 shows the transmission characteristics of the present wavelength tunable filter. The half mirror 3 reflects a part of the incident light,
It is an element that transmits most of the light, and is arranged perpendicular to the central axis 5. In the present embodiment, the half mirror 3 is formed by a dielectric multilayer film on a parallel plate glass having a thickness of 0.5 mm. The half mirror 3 has a reflectance of 8% and a transmittance of 92%. The fiber 4 is a polarization-maintaining single-mode fiber, and the fiber end face is polished so as to have a slope of 5 degrees with respect to the central axis in order to suppress reflection.
The end face is coated with an anti-reflection coating made of a dielectric multilayer film. The polarization maintaining surface of the fiber 4 and the outgoing polarization of the laser element 1 match, and the polarization state of the light guided through the fiber 4 is maintained as linear polarization. (Explanation of Operation) First, the details of the single mode oscillation operation of the present invention will be described with reference to FIG. The light 101 emitted from the laser element 1 is converted into parallel light by the laser-side lens 10 and then passes only through the oscillation mode near the filter transmission center wavelength when passing through the tunable optical bandpass filter 2. Other oscillation modes are reflected by the filter 2 and become stray light 105. The light 102 transmitted through the tunable optical bandpass filter 2 is partially reflected by the half mirror 3. The reflected light 104 again passes through the wavelength tunable optical bandpass filter 2, undergoes image conversion by the laser-side lens 10, and returns to the laser element 1. Light 1 returned
The laser light 04 is amplified inside the laser element 1 and emitted again from the end face of the laser element 1 as light 101. By repeating this, an external resonator is formed by the optical system up to the laser element 1 and the half mirror 3, so that the light 102 is a single mode oscillation light having a very narrow line width at the filter center wavelength. Become. Most of the single mode oscillation light 103 is transmitted by the half mirror 3, image-converted by the fiber side lens 11, and then coupled to the fiber 4. Next, the tunable operation will be described with reference to FIGS. When a voltage is applied to the driving electrodes (1) and (2) in FIG. 3, a Coulomb force is generated between the two driving electrodes (1) and (2). The direction of the generated Coulomb force is a direction in which the interval between the two electrodes is reduced. Therefore, by applying a voltage, the distance between the two mirrors is reduced. From the following equation (1) (Coulomb's law), the applied voltage and the change in the distance between the two mirrors (displacement) are proportional to the square of the voltage and inversely proportional to the square of the distance between the electrodes. Coulomb force: F = aV ＾ 2 / G ＾ 2 = kδG (1) V: applied voltage G: electrode spacing δG: displacement a, k: constant On the other hand, the distance between two mirrors and the filter transmission center wavelength The amount of fluctuation is expressed by the following equations (2) to (4)
It is linear from the equation of Perot resonance), and the center wavelength shifts to the shorter wavelength side when the distance between the mirrors becomes smaller. m = int (2Lcosθ / λc) (2) FSR = 2Lcosθ / m (m + 1) (3) λp = 2Lcosθ / m (4) λp: Filter transmission center wavelength m: Oscillation mode L: Mirror interval θ: FIG. The inclination angle between the central axis 5 and the central axis of the optical filter (3 degrees in the illustrated example) λc: the central wavelength of the variable wavelength band ((minimum wavelength + maximum wavelength) / 2) This embodiment (FSR = 200 nm: Mirror spacing = about 7 μm) is 1500 nm to 1600 nm
Is aimed at realizing the wavelength variable operation in the above, and the mirror movable amount required at that time is １ ／ wavelength (about 0.4 μm in the case of the present embodiment) from the above equation (2). Since this is sufficiently smaller than the distance between the mirrors, it can be ignored. In the above preconditions, the above (1),
From equation (4), the filter transmission center wavelength fluctuation amount changes in proportion to the square of the applied voltage. FIG. 4 shows the relationship between the applied voltage and the center wavelength of the filter. When the transmission center wavelength of the filter changes due to the application of a voltage to the driving electrode, the resonance wavelength of the external resonator changes, so that the wavelength of the light coupled to the fiber 4 also changes. Operate. In the present embodiment, the transmission center wavelength of the tunable optical bandpass filter 2, that is, the interval between the two reflection mirrors 207 determines the oscillation wavelength.
It is necessary to prevent a change in the interval between the reflection mirrors 207 due to an environmental change or the like. Therefore, in the present embodiment, the driving electrode 20
Four displacement sensor electrodes 205 attached to the side of No. 4
The gap length is monitored by measuring the capacitance between the two, and the oscillation wavelength can be stably maintained by forming the feedback loop. (Embodiment 2) Next, another embodiment of the present invention will be described with reference to FIG. In the present embodiment, instead of a micromachine type Fabry-Perot optical filter as a wavelength variable optical bandpass filter, a narrow band optical bandpass filter 21 formed of a dielectric multilayer film,
It comprises a drive unit 22 for driving the narrow band optical bandpass filter 21 to rotate. The narrow-band optical band-pass filter 21 is formed by forming a narrow-band optical band-pass filter with a dielectric multilayer film on a glass having a thickness of 0.3 mm. 1600 when the incident angle is 5 degrees
nm, full width at half maximum is 0.4 nm. The driving section 22 is composed of a rotation mechanism using a motor and a rotation angle detection mechanism using a rotary encoder.
It has a function to change from 50 degrees to 50 degrees. Further, it is possible to detect the light incident angle with a rotary encoder. The relationship between the light incident angle of the narrow band optical bandpass filter 21 formed of a dielectric multilayer film and the center wavelength of the filter is expressed by the following equation (5). λp = xcos θ λp: Filter center wavelength θ: Light incident angle x: Constant In this embodiment, 1500 nm to 160
The aim is to realize a wavelength tunable operation at 0 nm,
The incident angle variation required at that time was about 38 degrees in the case of the dielectric multilayer film used in the present embodiment. FIG. 6 shows an example of the relationship between the incident angle of light rays on the narrow band optical bandpass filter 21 and the center wavelength. The operation of changing the laser oscillation wavelength by changing the center wavelength of the narrowband optical bandpass filter 21 is described below.
This is the same as when a micro-machine type Fabry-Perot optical filter is used. The first technical effect of the present invention is that despite its wide wavelength tunable range, it is small and can perform continuous wavelength tunable operation. The reason is,
This is because a Fabry-Perot type wavelength tunable filter using a micromachine is used for the wavelength selection element. The second technical effect is a high output. The reason is that the optical system forming the external resonator includes a lens for image conversion, and optical loss at the resonator portion is small. A third technical effect is that manufacturing is easy. The reason is that, since a transmission type optical filter is used for the wavelength selection element, the loss increase due to the position and angle shift of the optical element is small. A fourth technical effect is that a wavelength shift does not occur in response to an environmental change. The reason is that mechanical feedback by position detection can be applied to the transmission center wavelength of the tunable filter.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall configuration diagram of a wavelength tunable laser according to a first embodiment of the present invention. FIG. 2 is an exploded perspective view of a Fabry-Perot type optical wavelength tunable filter of the wavelength tunable laser according to the first embodiment of the present invention. FIG. 3 is a sectional view of a Fabry-Perot type optical wavelength tunable filter of the wavelength tunable laser according to the first embodiment of the present invention. FIG. 4 is a graph showing a relationship between an applied voltage of the wavelength tunable laser according to Embodiment 1 of the present invention and a filter center wavelength. FIG. 5 is an overall configuration diagram of a tunable laser according to a second embodiment of the present invention. FIG. 6 is a graph showing a relationship between a light incident angle and a filter center wavelength of the wavelength tunable laser according to the second embodiment of the present invention. FIG. 7 is a graph showing an example of transmission characteristics of the Fabry-Perot type optical wavelength tunable filter according to the first embodiment of the present invention. FIG. 8 is a configuration diagram showing an example of a conventional tunable laser. FIG. 9 is a configuration diagram showing another example of a conventional tunable laser. [Description of Signs] 1 Laser element 2 Wavelength variable optical bandpass filter 3 Half mirror 4 Optical fiber 5 Central axis 10 Laser side lens 11 Fiber side lens 21 Narrow band optical bandpass filter 22 Driving unit 101 Light emitted from laser element 102 Light transmitted through a bandpass filter 103 Light transmitted through a half mirror 104 Light reflected by a half mirror 105 Light reflected by a bandpass filter 201 Elastic supporting thin film 202 Reflecting portion 203 Fabry-Perot 204 Driving electrode 205 Displacement sensor electrode 206 Gap maintaining spacer 1a Laser element 1b Diffraction grating 1c Lens 1d Optical fiber 2a Laser element 2b Reflection mirror 2c Lens 2d Optical fiber

Claims (1)

(1) A laser-side lens for converting light emitted from the laser element into parallel light on an optical path for coupling light emitted from the laser element to an optical fiber; A wavelength tunable optical bandpass filter having a light transmission center wavelength adjusting mechanism, and an external resonator including a half mirror that reflects a part of incident light from the wavelength tunable optical bandpass filter and returns the reflected laser light to the laser element. A wavelength tunable laser disposed, wherein a fiber-side lens for image conversion for coupling incident light from the half mirror to the optical fiber is disposed between the half mirror and the optical fiber; A Fabry-Perot micromachined optical wavelength variable filter is used as the pass filter, and the wavelength variable optical bandpass filter has a transmission center wavelength adjusting mechanism. A band-pass optical band-pass filter, an elastic supporting thin film made of an insulator arranged at a distance from each other, a reflecting portion formed at the center of the elastic supporting thin film, and a fabric formed at the reflecting portion. A pair of reflection mirrors made of a dielectric multilayer film formed in the Fabry-Perot opening, and arranged in parallel with each other, with a Perot opening, a plurality of driving electrodes, and a displacement sensor electrode; A gap holding spacer arranged to hold an interval between the pair of reflecting mirrors, generating a Coulomb force by applying a voltage to the driving electrode, and based on the applied voltage, the pair of reflecting mirrors. Has a function of changing the transmission center wavelength by controlling the interval between them, and is inclined by a predetermined angle with respect to the center axis of the incident light to suppress reflected stray light. Are arranged to, means for measuring the capacitance between the displacement sensor electrode,
Means for monitoring the gap length between the reflection mirrors based on the measured value, and controlling a voltage applied to the driving electrode so as to have a target gap length, wherein the end face of the optical fiber is In order to suppress the reflection of the emitted light, it is polished so as to form a slope at a predetermined angle with respect to the central axis of the incident light, and the end face is provided with an anti-reflection coating by a dielectric multilayer film, A wavelength tunable laser, wherein a polarization maintaining surface and emission polarization of the laser element coincide with each other, and the polarization state of the incident light guided through the optical fiber is maintained as linear polarization.