JP2002280647A - Fiber laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fiber laser that makes the band of reflection characteristics narrow for inhibiting the adjacent vertical mode of the fiber laser, and at the same time can generate a difference signal for monitoring a tuning state to the resonator of the vertical mode of the fiber laser. SOLUTION: There are a reflector 1, a laser medium 2, a polarizer 3, a Faraday rotator 4, and the resonator 7 where a phase plate 6 is arranged between a pair of reflectors 5a and 5b as a polarization element on the same light axis 8. In the resonator 7, a natural polarization axis is formed. The natural polarization axis coincides with the polarization axis direction of the phase plate 6, and mutually has a different resonance frequency. A beam branch element 9 between the resonator 7 and polarizer 3 reflects one portion of reflected light 8a of the resonator 7 for generating branching light. The branching light is polarized and analyzed by a 1/4 wavelength plate 11, a polarization separation element 12, and photo detectors 13a and 13b; and a difference signal generation circuit 14 generates the difference signal 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fiber laser that performs laser oscillation using an optical amplification fiber, and more particularly to a fiber laser in which adjacent longitudinal modes are suppressed. Such a fiber laser is suitable for applications such as measurement and optical communication.

[0002]

2. Description of the Related Art Conventionally, an etalon that is isotropic with respect to polarized light is provided in a ring resonator constituted by using an optical amplification fiber doped with a rare earth element, and a narrow band transmission characteristic of the etalon is used. A configuration for suppressing adjacent longitudinal mode laser oscillation is known.

On the other hand, in a technical field different from a fiber laser, Hansch and Couillaud (Opt.
ommun. 35 (3), 441 (1980)). This method makes the polarization characteristics of the resonator anisotropic by inserting a polarizer into the resonator,
The difference signal is generated by detecting the polarization state of the reflected light.

[0004]

As a characteristic required for a laser light source used in measurement and optical communication, it is important to suppress adjacent longitudinal modes, particularly in a fiber laser. Therefore, conventionally, an etalon (resonator) is used, but a difference signal for tuning the longitudinal mode wavelength of the fiber laser resonator to the transmission wavelength of the etalon is obtained from either the transmitted light or the reflected light of the etalon. Cannot be generated directly.

On the other hand, Hansch-Couillaud
In the method, the polarization characteristic of the resonator is made anisotropic, and a difference signal can be formed from the reflected light of the resonator.However, since the resonator is used as a ring arrangement, the narrow band property of the transmission characteristic is used. Therefore, it is practically difficult to configure a laser resonator.
In addition, it is considered that the arrangement of the resonators in a ring is a device for preventing reflected light from the resonators from returning in the direction of the laser medium (light source). In general, the reflection characteristic of a resonator is a characteristic obtained by inverting the transmission characteristic of a narrow band, that is, a characteristic of a wide band, and has an effect of not only suppressing the adjacent longitudinal mode of the laser but also increasing the instability of the oscillation longitudinal mode. I do. Therefore, although this method can generate a difference signal, it cannot construct a laser that suppresses oscillation in an adjacent longitudinal mode using reflection.

SUMMARY OF THE INVENTION It is an object of the present invention to narrow a reflection characteristic of a resonator and to use the same to suppress an adjacent longitudinal mode of a fiber laser and to monitor a tuning state of the longitudinal mode of the fiber laser to the resonator. To provide a fiber laser capable of generating a difference signal for the same.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a fiber laser for performing laser oscillation by inputting excitation light to an optical amplification fiber including a laser medium to which a rare earth element is added. A polarizer, a Faraday rotator, and a resonator in which a polarizing element is disposed between a pair of reflecting mirrors are disposed on the same optical axis, and pass through the polarizer and the Faraday rotator from the laser medium. Then, the direction of the intrinsic polarization axis of the resonator is arranged at an angle other than 0 ° or 90 ° with respect to the polarization direction of the incident light toward the resonator, and one of the polarization components of the incident light is one of the intrinsic polarization axes. When one resonates, the other polarization component is reflected, travels in the same optical axis as the incident light in the opposite direction, returns to the laser medium, and forms a laser resonator with the reflecting mirror. And the resonance And a beam splitting unit that reflects a part of the reflected light on the optical axis between the polarizer and the split light, and a quarter-wave plate on the optical axis of the split light, a polarization separating unit, A light detector for detecting the light intensity of each of the polarization components separated by the polarization separation means, and a means for generating a difference signal between the electric signals output from the light detector. Fiber laser.

Hereinafter, the principle of the present invention will be described with reference to an arrangement example shown in FIG. In FIG. 1, a reflector 1, a laser medium 2, a polarizer 3, a Faraday rotator 4, and a phase plate 6 (for example, a quarter-wave plate) as a polarizing element between a pair of reflectors 5a and 5b. Are arranged on the same optical axis 8. Since the optical lengths of the fast axis and the slow axis of the phase plate 6 are different from each other, by disposing the phase plate 6 inside the resonator 7, the phase axes coincide with the polarization axis direction of the phase plate 6, and the resonance frequencies are different from each other. A polarization axis is formed. The resonator 7 is a linear resonator whose optical axis coincides with the optical axis 8.

On the optical axis 8 between the resonator 7 and the polarizer 3, a beam splitter 9 for reflecting a part of the reflected light 8a of the resonator 7 and generating split light is arranged. . On the optical axis 10 of the branched light, a quarter-wave plate 11, a polarization separation element 12, and photodetectors 13a and 13b that receive the light separated by the polarization separation element 12 are arranged. Detector 13
a difference signal generating circuit 14 for generating an electrical difference signal 15 based on the outputs of the signals a and 13b.

The beam splitting element 9 has a non-zero reflectivity for both polarization axis components and does not add a new phase difference between the polarization components. It has the property of reflecting. The polarization axis of the 波長 wavelength plate 11 and the polarization axis of the polarization separation element 12 are arranged so as to form an angle of 45 ° with each other. When the split light is incident as linearly polarized light, the difference signal generating circuit 14 generates a zero difference signal 15, and generates a non-zero difference signal 15 reflecting the phase difference if the light is elliptically polarized light. Therefore, the polarization state of the reflected light 8a is converted into an electrical difference signal 15.

In the above arrangement, the light passing through the polarizer 3 from the laser medium 2 side becomes linearly polarized light, the polarization of which is rotated by 45 ° by the Faraday rotator 4, and then enters the reflecting mirror 5a.
Here, by arranging the direction of the fast axis of the phase plate 6 at an angle other than 0 ° or 90 ° with respect to the direction of the incident linearly polarized light, the linearly polarized light has two polarization components in the direction of the intrinsic polarization axis. Is decomposed into

When one of the decomposed polarized light components resonates with the resonator 7, the other polarized light component is not resonated and is reflected by the reflecting mirror 5a to become linearly polarized light in the opposite direction on the same optical axis as the incident light. The polarized light is rotated by the Faraday rotator 4, passes through the polarizer 3, and returns to the laser medium 2. On the other hand, since the split light split by the beam splitter 9 is also linearly polarized light, the difference signal 15 becomes zero.

On the other hand, if none of the decomposed polarization components resonates, both polarization components of the incident light are reflected in an elliptically polarized state close to linearly polarized light, and travel in the same optical axis 8 in the opposite direction. Since the major axis direction of the elliptically polarized light rotates 90 ° after passing through the Faraday rotator 4, it hardly passes through the polarizer 3. On the other hand, since the light split by the beam splitter 9 is also elliptically polarized, a non-zero difference signal 15 is generated.

Although the above has been described qualitatively, detailed characteristics can be calculated using a Jones matrix.

P is a polarizer, R (γ) is the polarization rotation angle γ of the Faraday rotator, and R (β) is the direction β of the intrinsic polarization axis of the resonator.
TBS is the transmittance of the beam splitter, RBS is the reflectivity of the beam splitter, R (α) is the direction α of the polarization axis of the beam splitter,
C (π / 2) is the amount of phase shift of the ４ wavelength plate, R (ψ) is the direction 進 of the fast axis of the 波長 wavelength plate, Pa and Pb are decomposed into respective polarization components of the polarization separation element, Let R (φ) be a matrix that represents the direction φ of the polarization axis of the polarization separation element. The matrix MR representing the reflection characteristics of the resonator is a diagonal matrix, and each component of the matrix is represented as follows.

[0016]

(Equation 1)

Here, R1 and R2 are the reflectivities of the reflecting mirrors 5a and 5b, δ is the one-way phase of the resonator with respect to the fast axis, and Δδ is the relative phase of the slow axis.

Using the above matrix, the matrix Mref of light that has passed through the polarizer in the direction of returning to the laser medium is expressed as follows. Mref = P · R (α) · TBS · R (−α) · R (−γ) · R
(β) ・ MR ・ R (−β) ・ R (−γ) ・ R (α) ・ TBS ・ R (−
α) ・ P

A matrix Msig of light reaching the photodetector
a and Msigb are represented as follows. Msiga = R (φ) ・ Pa ・ R (−φ) ・ R (ψ) ・ C (π /
2) ・ R (−ψ) ・ R (α) ・ RBS ・ R (−α) ・ R (−γ) ・
R (β) ・ MR ・ R (−β) ・ R (−γ) ・ R (α) ・ TBS ・ R
(−α) · P Msigb = R (φ) · Pb · R (−φ) · R (ψ) · C (π /
2) ・ R (−ψ) ・ R (α) ・ RBS ・ R (−α) ・ R (−γ) ・
R (β) ・ MR ・ R (−β) ・ R (−γ) ・ R (α) ・ TBS ・ R
(−α) · P

Here, assuming that the Jones vector of light incident on the polarizer is E0 = [Ex0, Ey0] T (T represents transposition), the reflectance Iref is expressed as follows. Iref = │Mref ・ E0│ ^ 2 / │E0│ ^ 2

The light intensities Ia and Ia received by the photodetector
b is expressed as follows. Ia = │Msiga ・ E0│ ^ 2 Ib = │Msigb ・ E0│ ^ 2

Then, the difference signal Isig is expressed as follows. Isig = (Ib−Ia) × η (η: light → current conversion coefficient)

As an example, α = 0 °, γ = 45 °, β = 2
5%, the reflectance of the reflecting mirrors 5a and 5b is 80%, Δδ
= 90 °, the reflectivity of the beam splitting element is 2%, and E0 = [1,
When 0 [T] is set, the reflection characteristics and the difference signal characteristics when the polarizer 3 side is viewed from the laser medium 2 side are shown in FIGS. 2 and 3, respectively. From the figure, it can be seen that a reflection characteristic in a narrow band is obtained, and that at δ = 0 ° and δ = 90 °, which are the tuning phases of the resonator, the reflectance shows a peak and a difference signal crossing zero is obtained.

Therefore, by disposing a laser medium between the optical system and at least one other reflecting mirror, adjacent longitudinal modes are suppressed, and a deviation from a resonance peak of the resonator is detected as a difference signal. Fiber laser that can be constructed.

Furthermore, in the present invention, it is desirable that the polarization rotation angle of the Faraday rotator is not 45 °. As a calculation example, α = 0 °, γ = 43 °, β = 25 °, and the reflecting mirror 5
a, 5b are both 80%, Δδ = 90 °,
When the reflectivity of the beam splitter is 2% and E0 = [1,0] T, the characteristics of the reflectivity and the difference signal are shown in FIGS. 4 and 5, respectively. Compared to FIG. 2 in the case of γ = 45 °, the reflectance in the case of resonance in the slow axis (δ = ± 90 °) and the reflectance in the case of resonance in the fast axis (δ = 0 °, 180 °) It turns out that it becomes larger than a reflectance.

Therefore, by disposing a laser medium between the above optical system and at least one other reflecting mirror, adjacent longitudinal modes are suppressed, and a deviation from the resonance peak of the resonator is detected as a difference signal. Yes, and the fiber laser oscillates at the higher polarization intrinsic polarization axis. As a result, the polarization axis of oscillation can be controlled.

[0027]

FIG. 6 is a front view showing the configuration of a fiber laser according to an embodiment of the present invention. This fiber laser has a Faraday rotator mirror 31 and
The first wavelength multi-wavelength optical device 51a and the optical amplification fiber 32
, A second multi-wavelength multiwave device 51b, a first Faraday rotator 52, a lens 53, a polarizer 33, a beam splitter 39, a second Faraday rotator 34, and a pair of reflecting mirrors 35a. And a resonator 37 including a phase plate 36 are arranged on one optical axis 38 in this order. First
The pump light sources 55a and 55b are connected to the first and second multi-wavelength optical devices 51a and 51b via optical fibers 54a and 54b, respectively. On the optical axis 40 branched by the beam splitter 39, a quarter-wave plate 41, a Glan laser prism 42, and photodetectors 43a and 43b for receiving the light polarized and separated by the Glan laser prism 42 are arranged. , The outputs of the photodetectors 43a and 43b are
It is connected to the.

The optical amplification fiber 32 is composed of an optical fiber (EDF) doped with Er. The pumping light sources 55a and 55b are semiconductor lasers that oscillate laser light having a wavelength of 980 nm. The laser light is multiplexed and absorbed by the optical amplification fiber 32 via wavelength multiplexing optical multiplexers 51a and 51b. The polarization rotation angle of the first Faraday rotator 52 is 45 °, and the first Faraday rotator 52 is arranged in combination with the Faraday rotator mirror 31 so that the intrinsic polarization in the polarizer 33 becomes linearly polarized light. The optical length from the Faraday rotator mirror 31 to the reflecting mirror 35a is about 3
At 8m, the corresponding FSR (Free Spectral Range) is about 4
MHz.

The transmission polarization axis of the polarizer 33 is disposed at an angle of 0 °, and the Faraday rotator 34 has a polarization rotation angle of 46 ° 10.
And the phase difference between the fast axis and the slow axis of the phase plate 36 is 94
In ゜, the angle of the fast axis is set to β. The reflecting mirror 35a is a plane mirror, the reflecting mirror 35b is a concave mirror, and the reflectance is both 98%.
Light radiated from the optical amplification fiber 32 is condensed by the lens 53 and arranged so as to match the TEM00 mode. The optical length of the resonator 37 is about 43 mm, and the FSR is about 3.
At 6 GHz, the finesse is about 150. The beam splitter 39 is a parallel flat glass plate, the incident angle of which is 45 °, the polarization axis of the P polarization is arranged in the direction of 0 °, and a 波長 wavelength plate 41.
Are arranged at an angle of 45 °, and the transmission polarization axis of the Glan laser prism 42 is arranged at an angle of 0 °. The light split by the Glan laser prism 42 is received by the photodetectors 43a and 43b, respectively, and the output is output as a difference signal voltage 45 via a differential amplifier 44.

In the above arrangement, the excitation light sources 55a, 55
5b with the output of each being 150 mW.
When 2 was excited, laser oscillation occurred at a wavelength of about 1560 nm, and a laser output 46 of about 70 mW was obtained.

FIG. 7 shows a spectrum obtained by observing, with a spectrum analyzer, an electric signal obtained by attenuating the laser output 46 with a neutral density filter and receiving the light with a photodetector.
Here, the DC component of the electric signal generated from one longitudinal mode is estimated to be about −30 dBm. The 4 MHz signal is a beat with the nearest vertical mode, and the beat power is about −
Since it was 65 dBm, a suppression ratio of the adjacent longitudinal mode of about 70 dB was obtained.

FIG. 8 is a graph showing the degree of polarization of the laser output 46 with respect to the angle β of the phase plate 36. The vertical axis in the figure is
A positive value indicates that the light is polarized in the fast axis direction, and a negative value indicates that the light is polarized in the slow axis direction. Since the polarization rotation angle of the Faraday rotator 34 was 46 ゜ 10 ′ and not 45 °, polarization oscillation in the fast axis direction was obtained.

FIG. 9 is a graph showing an example of the difference signal.
FIG. 10 is a graph showing an example of the reflection characteristics. Since the characteristics of the difference signal are difficult to understand when the fiber laser is oscillating, the figure removes the fiber amplifier part and replaces it with a DFB-LD that outputs a single-mode laser beam with a wavelength of 1553 nm. And a piezo element is attached to the reflecting mirror 35b.
Is obtained by sweeping the length (phase) by several μm.
A characteristic crossing zero at the resonator length (phase) at which the reflectance peaks is obtained.

In the above example, the polarizing element disposed in the resonator constituted by a pair of reflecting mirrors is not limited to the phase plate described in the embodiment, but may be another type having another polarization characteristic. An element such as a Brewster plate, a polarizer, a polarization-maintaining optical fiber, a single-mode optical fiber that is anisotropically stressed by being wound in a loop, or a crystal generally having birefringence. May be.

The beam splitting element may be arranged not on the optical path between the polarizer and the Faraday rotator but on the optical path between the Faraday rotator and the resonator.

Further, the reflectivity of the reflector of the resonator is not limited to the exemplified value, but exceeds 0% to 100%.
The following reflectance may be set. Furthermore, the respective reflectances of the pair of reflecting mirrors may be different.

Further, the angles of the polarization axes of the polarizer, the beam splitting means, and the polarizing element are not limited to the exemplified values.
It can be set freely to achieve the target optical characteristics.

[0038]

As described above, according to the present invention, the adjacent longitudinal mode of the fiber laser can be suppressed by narrowing the reflection characteristics of the resonator. Further, it is possible to generate a difference signal for monitoring the tuning state of the fiber laser to the longitudinal mode resonator.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a typical arrangement for explaining the principle of the present invention.

FIG. 2 is a graph showing an example of a reflection characteristic in the configuration of FIG.

FIG. 3 is a graph showing an example of a difference signal characteristic in the configuration of FIG. 1;

FIG. 4 is a graph showing another example of the reflection characteristics in the configuration of FIG. 1;

FIG. 5 is a graph showing another example of the characteristic of the difference signal in the configuration of FIG. 1;

FIG. 6 is a front view showing a configuration of a fiber laser according to an embodiment of the present invention.

FIG. 7 is a graph showing an example of a spectrum of a laser output of a fiber laser.

FIG. 8 is a graph showing an example of a polarization characteristic of a laser output of a fiber laser.

9 is a graph showing an example of a difference signal characteristic in the configuration of FIG.

FIG. 10 is a graph showing an example of reflection characteristics in the configuration of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reflecting mirror 2 Laser medium 3,33 Polarizer 4,34,52 Faraday rotator 5a, 5b, 35a, 35b Reflecting mirror 6,36 Phase plate 7,37 Resonator 9 Beam splitting element 11,41 Quarter-wave plate DESCRIPTION OF SYMBOLS 12 Polarization separation element 13a, 13b, 43a, 43b Photodetector 14 Difference signal generation circuit 31 Faraday rotator mirror 51a, 51b Multi-wavelength wavelength filter 32 Optical amplification fiber 53 Lens 39 Beam splitter 42 Gran laser prism 44 Differential amplifier

Claims (2)

[Claims] 1. A fiber laser for performing laser oscillation by inputting excitation light to an optical amplification fiber including a laser medium to which a rare earth element is added, comprising: a light reflecting means; the laser medium; a polarizer; and a Faraday rotator. And a resonator configured by disposing a polarizing element between a pair of reflecting mirrors are disposed on the same optical axis, and incident light from the laser medium passing through a polarizer and a Faraday rotator toward the resonator. The direction of the intrinsic polarization axis of the resonator is disposed at an angle other than 0 ° or 90 ° with respect to the polarization direction of the resonator. When one of the polarization components of the incident light resonates at one of the intrinsic polarization axes, the other Is reflected,
The same optical axis as the incident light travels in the opposite direction and returns to the laser medium, and forms a laser resonator between the reflector and the reflector. On the optical axis between the resonator and the polarizer, Beam splitting means for reflecting a part of the reflected light to be split light, a 波長 wavelength plate on the optical axis of the split light, a polarization splitting means, and a polarized light component separated by the polarization splitting means. A fiber laser comprising: a light detector for detecting each light intensity; and means for generating a difference signal between electric signals output from the light detector. 2. The polarization rotation angle of the Faraday rotator is 4
2. The fiber laser according to claim 1, wherein the angle is not 5 [deg.].