JP2003198021A - Fiber laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a single-frequency fiber laser free from mode hopping with no sideband in its laser frequency. <P>SOLUTION: This fiber laser comprises a first wavelength multiplexing coupler 61a, an optical amplifier fiber 41, a second wavelength multiplexing coupler 61b, a first optical isolator 43a, a band-pass filter 59, a polarization controller 58, a fiber stretcher 57, a Glan laser prism 44, a lens 60, a beam sampler 49, an optical resonator 47 including a Brewster plate 46 interposed between a pair of reflecting mirrors 45a, 45b, and a second optical isolator 43b, all which are arranged on an optical axis 48 to configure the ring resonator fiber laser. <P>COPYRIGHT: (C)2003,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION [0001] TECHNICAL FIELD The present invention relates to an optical amplification fiber.
Fiber laser light source that performs laser oscillation using
Suppresses the oscillation of the longitudinal mode adjacent to the
The present invention relates to a fiber laser in which ping is eliminated. These files
Fiber lasers are suitable for applications such as measurement and optical communication.
You. [0002] 2. Description of the Related Art Conventionally, a light amplification fiber to which a rare earth element has been added is used.
Of a ring-type laser resonator using
Laser emission in adjacent longitudinal mode
Etalon placed inside laser cavity to suppress vibration
And a double ring resonator with a sub-ring
To provide a resonator-type filter element
Proposed. Generally, the longitudinal mode of a fiber laser
Because the frequency spacing is narrow, the
To suppress the oscillation, the characteristics of the filter
And must be. However, this is
The laser frequency fluctuates due to the optical length fluctuation of the resonator.
Tuning to the resonant frequency of the filter element
Easily deviates, and the laser oscillation changes to another longitudinal mode.
Transition, so-called mode hopping may occur
Are known. Eliminate this mode hopping problem
Therefore, the frequency of the fiber laser is
A method of tuning control to the wave number has been adopted. As an example,
By modulating the optical length of the resonator type filter element,
By modulating the intensity of the laser output and performing phase detection
Generates error signal and feedbacks optical length of laser cavity
Have been proposed (Zhang J. et al.,
J.Lightwave Technol.14(1), 104 (1996)). [0003] SUMMARY OF THE INVENTION Especially used in measurement
As a property of fiber lasers, laser at a single frequency
Laser oscillation is required. But, for example,
In the prior art, the optical length of the resonator type filter element was modulated.
100 kHz around the laser frequency
Sidebands over the bandwidth of the order,
Intensity modulating the laser output to generate an error signal
Single-frequency laser light source
Was inadequate for use. An object of the present invention is to eliminate mode hopping
Single-frequency fiber laser with sideband at laser frequency
It is to provide a fiber laser without waves. [0005] According to the present invention, a rare earth element is
Using an optical amplifying fiber and an optical isolator
Configure the resonator and input the pump light to the optical amplification fiber.
In addition to causing laser oscillation, the laser
Propagation direction of laser light is regulated in one direction by the optical isolator
Fiber laser with a first polarizer,
The output side and the input side of the laser light of the optical amplification fiber,
Placed at different ports of the polarizer, the output side
Another port where laser light is output through the polarizer
A second polarizer and a pair of reflecting mirrors are arranged in this order on the optical axis.
An optical resonator configured to be placed in
Polarization direction of incident light incident on the optical resonator with the direction of the polarization axis
At an angle other than 0 ° or 90 ° with respect to the
One of the polarization components of the incident light is at one of the intrinsic polarization axes.
When resonating, the reflected light of the optical resonator
Proceeds in the same optical axis as the light in the opposite direction, and passes through the first polarizer.
Output to a polarization port orthogonal to the polarization of the incident light.
The ring type feedback to the input side of the optical amplification fiber
Forming a laser resonator, the optical resonator and the first polarization
A part of the reflected light is reflected on the optical axis between the
Beam splitting means, and 1/4 wavelength on the optical axis of the split light.
Plate, polarized light separating means, separated by the polarized light separating means
A photodetector that detects the light intensity of each of the polarized components,
Generating an error signal from the electrical signal output by the photodetector;
Circuit for generating and outputting a control signal from the error signal
A control circuit, the laser resonator being responsive to the control signal;
An optical length adjusting means for adjusting the optical length;
Means for tuning the frequency of the optical resonator to the resonance frequency of the optical resonator.
It is a fiber laser characterized in that it is constituted. Hereinafter, the principle of the present invention will be described with reference to an arrangement example shown in FIG.
It will be described with reference to FIG. In FIG. 1, an optical amplification fiber 1
, Optical isolators 3 a and 3 b, polarizer 4,
Brew as a polarizing element 6 between the pair of reflecting mirrors 5a and 5b
An optical resonator 7 on which a star plate is disposed is disposed on an optical axis 8;
A ring-type laser resonator is configured. Output light 18
Is output from one port of the polarizer 4. Excitation light source
The pump light radiated from 2a and 2b is a wavelength multiplexing optical multiplexer.
21a and 21b to the optical amplification fiber 1.
Waves are absorbed. The optical resonator 7 is a linear resonator,
The axis coincides with the optical axis 8. Bruise inside the optical resonator 7
By disposing the star plate 6, the direction of the incident surface and the
The characteristic polarization axis of the optical resonator 7 is formed in a direction orthogonal to this.
You. The optical axis 8 between the optical resonator 7 and the polarizer 4
Above, a part of the reflected light 8b of the optical resonator 7 is reflected and branched.
A beam splitting element 9 which serves as light is arranged. Also branch
On the optical axis 10 of the light, a quarter-wave plate 11 and a polarization separation element
Element 12, photodetectors 13a and 13b, differential amplifier
And an error signal generation system 15.
You. The beam splitter 9 has both orthogonal polarization components.
Has a non-zero reflectance for
Has a characteristic of not adding a new phase difference to the data.
Polarization axis of quarter-wave plate 11 and polarization axis of polarization separation element 12
Are arranged at an angle of 45 ° to each other. Further, a control signal is generated from the error signal and output.
Control circuit 16 and a laser resonator according to a control signal.
And an optical length adjusting means 17 for adjusting the optical length. In the above arrangement, the optical amplification fiber 1
The light output from the optics passes through an optical isolator 3a and passes through a polarizer.
4 enters from one port and becomes linearly polarized light 8a
The light exits from another port and enters the reflecting mirror 5a. Here
The direction of the entrance plane of the Lewster plate 6 is
Make an angle other than 0 ° or 90 ° with the direction
By arranging the light, the light 8a is transmitted along the intrinsic polarization axis of the optical resonator 7.
It is decomposed into two polarization components whose directions are orthogonal to each other. One of the decomposed polarization components is applied to the optical resonator 7.
When resonating, the reflected light 8b of the optical resonator 7 is different from the incident light 8a.
It becomes linearly polarized light having orthogonal polarization components and
A polarization port that travels in the opposite direction and is orthogonal to the light 8a of the polarizer 4
And output as light 8c. Light 8c is an optical isolator
The light is input to the optical amplification fiber 1 via 3b. In contrast
And if none of the decomposed polarization components resonate,
The reflected light 8b is in an elliptically polarized state close to the polarization state of the incident light 8a.
Therefore, the intensity of the light 8c is smaller than that at the time of resonance.
You. Therefore, the light composed of the polarizer 4 and the optical resonator 7
The academic system is designed to output the light 8c in response to the input of the light 8a.
Works as a filter optical system. Optical resonator including polarizing element and error signal generation
For generating an error signal from a combination of configurations of system 15
Has been proposed by Hansch et al. (Opt. Commun.35
(3), 441 (1980)), and this method generates an error signal.
No sideband is required for the laser frequency for formation. A filter comprising a polarizer 4 and an optical resonator 7
Optical system, beam splitter 9 and error signal generating optical system 14
The characteristics of the optical system consisting of
It can be calculated using Trix. P₁And P_TwoAre the light 8a and the light of the polarizer 4, respectively.
8c (P₁And P_TwoAre orthogonal), T_BSTo
Transmission of beam splitter 9, R_BSOf the beam splitter 9
R (α) is the direction of the incident surface of the beam splitter 9, and C (π
/ 2) is the amount of phase shift of the 波長 wavelength plate 11, and R (ψ) is １ ／ wave
Direction of fast axis of long plate 11, P_aAnd P_bEach polarized light separation
Decomposition of the polarization components into the photodetectors 13a, 13b of the element, R
(φ) is the direction of the polarization axis of the polarization separation element, and R (β) is the optical resonator
7 are matrices representing the directions of the unique polarization axes. Ma
Also, a matrix M representing the reflection characteristics of the optical resonator 7_RIs a diagonal matrix
And each matrix element is represented as follows: 11 elements: M_R11= [-√R₁+ (1-α₁)
√R_Twoexp (−i2δ)] / [1- (1-α)₁) √ (R₁R_Two) e
xp (−i2δ)] 22 elements: M_R22= [-√R₁+ (1-α_Two) √R_Twoexp
(-I2δ)] / [1- (1-α_Two) √ (R₁R_Two) exp (-i2
δ)]. Where R₁And R_TwoAre the reflections of the reflecting mirrors 5a and 5b, respectively.
Rate, α₁And α_TwoAre the direction of the plane of incidence of the Brewster plate and
Loss of the optical resonator 7 for polarized light in a direction orthogonal to it,
δ is a one-way optical path phase. Using the above matrix, the rows of the filter optical system
Column M is represented as follows. M = P_Two・ R (α) ・ T_BS・ R (−α) ・ R
(β) · M_R・ R (−β) ・ R (α) ・ T _BS・ R (−α) ・ P₁
  . Also, a matrix of light reaching the photodetectors 13a and 13b
M_a, M_bAre represented as follows. M_a＝ R (φ) ・ P_a・ R (−φ) ・ R (ψ) ・ C
(π / 2) ・ R (−ψ) ・ R (α) ・ R_BS・ R (−α) ・ R (β)
・ M_R・ R (−β) ・ R (α) ・ T_BS・ R (−α) ・ P₁  , M_b＝ R (φ) ・ P_b・ R (−φ) ・ R (ψ) ・ C (π / 2) ・ R
(−ψ) ・ R (α) ・ R_BS・ R (−α) ・ R (β) ・ M_R・ R (-
β) ・ R (α) ・ T_BS・ R (−α) ・ P₁    . Light 8a is input light to the filter optical system, and light 8c is transmitted light.
Then, the transmittance I of the filter optical system is as follows.
Is represented by I = │ME₀│^Two/ │E₀│^Two The light intensity received by the photodetectors 13a and 13b
I_a, I_bAre each I_a = │M_a・ E₀│^Two I_b = │M_b・ E₀│^Two And the error signal I_sigIs I_sig = (I_b-I_a) × η × A Is represented by Where E₀ = [E_x0, E_y0]^T (T is transposed
Represents Jones vector of the light incident on the polarizer 4.
, Η is the light intensity-current conversion coefficient of the photodetector, and A is the differential
This is the degree of amplification of the width unit 14. As an example, α = 0 °, β = 25 °, R₁=
60%, R_Two= 100%, α₁= 0%, α_Two= 30%
4% in both polarization axis directions of the beam splitter, ψ = 45
Η, η = 1, A = 1, and the electric field of light 8a is E₀ = [1,
0]^TAnd the light 8a is reflected by the optical resonator 7 and
8c, the characteristics of the optical system and the characteristics of the error signal
These are shown in FIGS. 2 and 3, respectively. With the above configuration, the laser
Is generated in the vertical mode closest to the peak of the filter characteristic.
Will shake. In addition, the characteristics of the filter optical system
At the optical path phase δ = 0 °,
Error due to deviation of the filter characteristic of the frequency from the peak
The difference signal is obtained and the peak of the filter characteristic of the laser frequency is obtained.
Control optical length of laser cavity to reduce deviation from
can do. As described above, according to the present invention, the mode
A single-frequency fiber laser that eliminates
A fiber laser having no sideband at the frequency can be provided. [0022] FIG. 4 shows an embodiment of the present invention.
It is a front view showing the composition of a certain fiber laser. This file
The fiber laser is provided with a first wavelength multi-wavelength optical device 61a and an optical amplifier.
The width fiber 41, the second wavelength-multiplexed wave device 61b,
1 optical isolator 43a and bandpass filter 59
, Polarization controller 58, and fiber stretcher
-57, the Gran laser prism 44, and the lens 60
, A beam sampler 49, and a pair of reflecting mirrors 45a and 45
b, an optical resonator 47 including a Brewster plate 46;
Two optical isolators 43b are arranged on the optical axis 48,
This constitutes a ring-type resonator fiber laser. First
And the second wavelength multi-wave devices 61a and 61b are provided with an excitation light source.
42a and 42b are respectively connected. Bee
A quarter wave is placed on the optical axis 50 branched by the
Long plate 51, Glan laser prism 52, photodetector 5
3a and 53b are arranged, and each output is a differential amplifier 5
4 is connected. Further, the differential amplifier 54 is controlled
The circuit 56 and the fiber stretcher 57 are connected. The optical amplifying fiber 41 is a light-amplifying fiber
Fiber (EDF). Further, the excitation light sources 42a, 4
2b is a semiconductor laser which outputs a laser beam having a wavelength of 980 nm.
And the laser light is supplied to the wavelength division multiplexing optical multiplexers 61a and 61a.
The light is multiplexed and absorbed by the optical amplification fiber 41 via 1b.
The optical length of the laser resonator is about 50 m per round, and the corresponding FS
R (longitudinal mode frequency interval) is about 6 MHz. The transmission polarization axis of the Glan laser prism 44 is
Arranged in the direction of an angle of 0 ° and incident on the Brewster plate 46
The polarization axis of the plane is arranged in the direction of 45 °. The reflecting mirror 45a
98% reflectivity with concave mirror, 45b with mirror
When the rate is 99.8% or more, the light 48a is collected by the lens 60.
TEM₀₀It is arranged to match the mode. Optical resonance
The optical length of the detector 47 is about 50 mm, and the FSR is about 3 GHz.
z (about 0.024 nm) and finesse is about 250.
The beam sampler 49 is a parallel flat glass plate, and the incident angle is
45 °, the polarization axis of the incident surface is arranged in the direction of 0 °, and 1 /
The fast axis of the four-wavelength plate 51 is at an angle of 45 °,
The transmission polarization axis of the mechanism 52 is arranged at an angle of 0 °. The band-pass filter 59 is a single-layer multilayer film.
In the cavity type, the 3 dB bandwidth is 1 nm. Polarization
The plane controller 58 is directed to the Glan laser prism 44
Adjust the polarization characteristics of the reflected light to the light 48a and the laser output 62.
Used to adjust the power distribution of the vehicle. The fiber stretcher 57 is a laminated type P
An optical fiber is provided on the side of a ZT element.
The optical length of the optical fiber according to the control voltage applied to the
Change. In the above arrangement, the excitation light sources 42a, 4a
The output of each of the optical amplification fibers 4 was set to 150 mW.
Excitation 1 from the peak of the filter characteristic of the laser frequency
Control the optical length of the laser cavity to reduce the deviation
At a wavelength of 1560 nm, the
A user output 46 was obtained. FIG. 5 shows the spectrum of the laser output 62.
Measured with an optical spectrum analyzer with a resolution of 0.01 nm
This is an example. As a comparison, the control loop was opened.
FIG. 6 shows the spectrum of the laser output 62 in the state.
is there. With the control loop open, the F
Laser with multiple wavelengths at about 0.024 nm intervals as SR
Is oscillating. This allows the control loop
Fig. 5 shows a state in which the fiber laser operates at a single frequency.
It can be seen that oscillation is occurring. FIG. 7 shows that the control loop is open and closed.
Observe the error signal in the state with a spectrum analyzer.
It is an example of the measured spectrum. A laser cavity, or
Acoustic vibration applied to the optical resonator 47 and electric power applied to the control circuit
Apart from peaks due to disturbances such as temperament noise,
It can be seen that there are no peaks created in the process. Accordingly
It can be seen that there is no sideband around the laser frequency.
You. In the above example, a pair of reflecting mirrors is used.
The polarization element disposed in the optical resonator is described in the embodiment.
Not limited to Brewster plates
Elements having properties, for example, birefringent crystals, polarized light
A single-mode optical fiber having birefringence
Is also good. The reflectance of the reflecting mirror of the optical resonator is illustrated as an example.
Value is not limited to
For realization, it may be set between 0% and 100%. Further, a polarizer, a beam splitting means, and a polarizing element
The angle of the polarization axis of is not limited to the values illustrated,
It can be set to achieve the required optical characteristics. In this embodiment, the output light 62 is
Output from one port
Crab fiber coupling and take out from fiber
Can also be issued. The optical length adjusting means 17 is a ring.
It may be provided at any location in the cavity. [0034] As described in detail above, according to the present invention,
Oscillates at a single frequency without mode hopping,
Providing fiber lasers without sidebands at the frequency
Can be.

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 reflection characteristics in the configuration of FIG. FIG. 3 is a graph showing an example of a characteristic of an error signal in the configuration of FIG. FIG. 4 is a front view showing a configuration of a fiber laser according to an embodiment of the present invention. 5 is a graph showing an example of a spectrum of a laser output when the control loop is closed in the configuration shown in FIG. 4; FIG. 6 is a graph showing a laser output spectrum when the control loop is opened in the configuration shown in FIG. FIG. 7 is a graph showing an example of a spectrum. FIG. 7 is a graph showing an example of a spectrum of an error signal. DESCRIPTION OF SYMBOLS 1, 41 Optical amplification fiber 4 Polarizer 14, 54 Differential amplifier 5a, 5b, 45a, 45b Reflector 6, 46 Brewster plate 7, 47 Optical resonator 9, 49 Beam splitting element 11, 51 Quarter-wave plate 12 Polarization separation elements 13a, 13b, 53a, 53b Photodetectors 14, 54 Differential amplifiers 16, 46 Control circuit 17 Optical length adjusting means 44 Gran laser prism 49 Beam sampler 52 Gran laser prism 57 Fiber stretcher

Claims (1)

Claims: 1. A laser resonator is constituted by using an optical amplification fiber containing a rare earth element and an optical isolator, and pumping light is input to the optical amplification fiber to cause laser oscillation. In a fiber laser in which the propagation direction of the laser light in the optical amplification fiber is regulated in one direction by the optical isolator, a first polarizer is provided, and the output side and the input side of the laser light of the optical amplification fiber are connected to the polarizer. Arranged on different ports, the output side laser light is on the optical axis of another port output through the polarizer, a second polarizer and a pair of reflectors are arranged in this order. The optical resonator thus configured is disposed, and the direction of the intrinsic polarization axis of the optical resonator is set to 0 ° or 90 ° with respect to the polarization direction of incident light incident on the optical resonator.
When the one of the polarization components of the incident light resonates at one of the intrinsic polarization axes, the reflected light of the optical resonator travels in the opposite direction along the same optical axis as the incident light. In the first polarizer, a ring-type laser resonator is output to a polarization port orthogonal to the polarization of the incident light and returns to the input side of the optical amplification fiber, and the optical resonator and the A beam splitting unit that reflects a part of the reflected light on the optical axis between the first polarizers to be a split light, a quarter-wave plate on the optical axis of the split light, and a polarization separation unit, A photodetector for detecting the light intensity of each of the polarization components separated by the polarization separation means, a circuit for generating an error signal from an electric signal output by the photodetector, and a control signal for generating a control signal from the error signal. A control circuit for outputting, and the Fiber laser, characterized in that an optical length adjusting means for adjusting the optical length of the resonator, and the frequency of the laser oscillation constitutes the tuning means to the resonant frequency of the optical resonator.