JP2001168437A - Tunable single frequency laser - Google Patents

Abstract

(57) [Summary] [Problem] A wavelength can be changed in a wide wavelength range,
A tunable single-frequency laser having a single-frequency narrow linewidth. A linear resonator including both ends of reflectors (105, 106) and including a laser gain medium (101) and an optical path (109), and the frequencies of light passing through the laser gain medium (101) in both directions are different from each other. The frequency-increasing converter 103 and the frequency-decreasing converter 102 located on both sides of the laser gain medium 101 inside the resonator so that only one longitudinal mode can oscillate.
And a wavelength-variable first optical filter means (1) capable of changing its own center wavelength to change the oscillation wavelength of the resonator.
02, 103) and a filter 104 having a relatively narrow line width for suppressing mode hopping so that one longitudinal mode is stably maintained in the resonator.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser, and more particularly, to a laser that can change the wavelength in a wide wavelength range and has a single frequency and a narrow line width.

[0002]

2. Description of the Related Art Lasers having a narrow line width of a single frequency and capable of changing the oscillation wavelength in a wide wavelength range have various applications as light sources. In particular,
In the recent optical communication field, a wavelength division optical communication system (Wavelength Division Multiplexed Co
Along with research and development of mmunication systems, there is increasing interest in lasers capable of changing the oscillation wavelength in this wavelength range.

[0003] Among them, a great deal of research on tunable semiconductor lasers has been made, but it is still difficult to commercialize them. On the other hand, although there is relatively little research on tunable optical fiber lasers, optical fiber lasers have advantages such as narrow oscillation line width, small light intensity noise, and high output compared to semiconductor lasers. Also known as

In an optical fiber laser, generally, a phenomenon is observed in which the oscillation frequency is unstable due to hopping in which several longitudinal modes oscillate simultaneously or the oscillation longitudinal mode shifts to another longitudinal mode position.

In the linear resonator, one longitudinal mode inside the resonator forms a standing wave, but a node portion of the standing wave remains in a state where the light intensity is close to 0 and the laser gain is large. On the other hand, the anti-node part of the standing wave has high light intensity and low laser gain. As a result of the spatially different gain distribution, a spatial hole burning phenomenon occurs.

Therefore, in the longitudinal mode that forms the standing wave, the laser gain is consumed only at the end of the standing wave, so that light of a different frequency can share the laser gain in space. As a result, a number of longitudinal modes can oscillate at the same time, and a gain competition between the plurality of modes occurs.
The oscillation frequency becomes unstable due to n). In order to have good characteristics as a light source, the oscillation longitudinal mode of the laser must be limited to one, and its oscillation frequency must be stabilized.

Accordingly, various methods have been proposed for obtaining a single longitudinal mode oscillation frequency with a fiber optic laser. First, a circular resonator is used to transmit light in only one direction. Second, a linear resonator is provided with two phase modulators or two phase modulators.
A method of eliminating standing waves by applying frequency modulators is included.

On the other hand, a method of changing the oscillation wavelength of a laser in a wide wavelength range is as follows.
There is a method of extending the length of an optical fiber using a T (piezo electric transducer) or the like, and a second method of using a Fabry-Perot optical filter having a narrow line width with a circular resonator.

[0009]

However, according to the above-mentioned prior art, it is difficult not only to realize single longitudinal mode oscillation and variable oscillation wavelength at the same time, but also to narrow the oscillation wavelength variable region. There is a point.

Accordingly, it is an object of the present invention to provide a wavelength tunable single frequency laser which can realize single longitudinal mode oscillation and variable oscillation wavelength with a relatively simple device configuration.

Another object of the present invention is to provide a wavelength tunable single-frequency laser capable of changing the oscillation wavelength in the entire region where the laser can oscillate.

[0012]

A tunable single-frequency laser according to the present invention for achieving the above object has a linear resonator including a laser gain medium and an optical path therein, both ends of which are made of a reflector. A laser, wherein the frequencies of light passing through the laser gain medium in both directions are made different from each other so that only one longitudinal mode can oscillate, so that the frequencies located on both sides of the laser gain medium inside the resonator. Increasing conversion means and frequency decreasing conversion means, wavelength-variable first optical filter means capable of changing its own center wavelength to change the oscillation wavelength of the resonator, and one longitudinal mode in the resonator is stabilized. A second optical filter means having a relatively narrow line width for suppressing mode hopping so as to be maintained.

In the present invention, at least one of the frequency increase conversion means and the frequency decrease conversion means can be used also as the wavelength variable first optical filter means.

The laser gain medium includes a rare earth-doped optical fiber, a dye cell, a semiconductor diode amplifier, and Nd-YAG.
Rod, Nd: desirably at least one selected from the group consisting of a glass rod and an Nd rod. When the laser gain medium is a rare earth-doped optical fiber, the rare earth element is: Erbium
More preferably, it is at least one selected from the group consisting of m), niobium, and indium.

Preferably, the optical path is at least one selected from the group consisting of an optical fiber, vacuum, air, an inorganic material waveguide, and an organic material waveguide.

The frequency increasing converter or the frequency decreasing converter is at least one selected from the group consisting of an acousto-optic frequency converter, an electro-optic frequency converter, and an integrated optical frequency converter. It is desirable.

The wavelength variable first optical filter means is at least one selected from the group consisting of a Fabry-Perot optical filter, an acousto-optic optical filter, an electro-optic optical filter, and an integrated optical optical filter. It is desirable.

Further, the second optical filter means having a relatively narrow line width comprises an absorption light filter using a saturated absorber, a Fabry-Perot light filter, an acousto-optic light filter, an electro-optic light filter, and an integrated optical light filter. It is desirable that at least one selected from the group selected.

The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the present invention with reference to the accompanying drawings.

[0020]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
A preferred embodiment of the present invention will be described. FIG. 1 is a schematic configuration diagram of a wavelength tunable single frequency optical fiber laser according to an embodiment of the present invention.

Referring to FIG. 1, it can be seen that the laser resonator is a linear resonator having reflectors 105 and 106 at both ends. As the reflecting mirrors 105 and 106, optical fibers having ends coated with a reflective coating or the like are used. Erbium-doped optical fiber is used as the laser gain medium 101 and is pumped by a laser diode 107 having a wavelength of 980 nm.

The frequency decreasing converter 102 and the frequency increasing converter 103 are all made of an acousto-optic tunable optical filter, and serve not only as frequency converters for converting the frequency of passing light by −F and + F, respectively, but also for resonance. It also functions as a wavelength tunable optical filter for changing the oscillation wavelength of the device. A general single mode optical fiber was used for the optical path 109 forming the laser resonator. In FIG. 1, the optical fiber coupler 1
08 is for observing the output of the laser coming out of the cavity.

In the laser resonator thus configured, the light passing through the frequency reduction converter 102 passes through the intermediate region 110 after its frequency has been reduced by about −F.
The frequency is increased by + F again by the frequency increasing converter 103. This light is reflected by the reflector 106, passes through the frequency increasing converter 103 again, and its frequency is converted by about + F. After passing through the intermediate region 110 again, the frequency is again reduced by about -F by the frequency decreasing converter 102. Experience the process of being converted.

That is, the lights traveling in the opposite directions in the intermediate region 110 have a frequency difference of about 2F, so that a standing wave cannot be generated in the laser gain medium 101.

As a result, the laser gain can be fully used by only the single longitudinal mode oscillation, and the spatial hole burning phenomenon does not occur. Although a single longitudinal mode can be obtained in this way, such a laser longitudinal mode can easily be shifted to another longitudinal mode position by a small perturbation outside the laser. To stabilize this longitudinal mode, a narrow linewidth optical filter 104 is made at one end of the resonator.

As the narrow linewidth optical filter 104, an unpumped erbium-doped optical fiber is used, which operates with a saturated absorber in the laser. The absorption light filter formed by the standing wave in the laser mode by the saturated absorber operates with an optical filter having a narrow line width so as to stabilize the laser oscillation mode.

In the embodiment of the present invention, the frequency decreasing and increasing converters 102 and 103 simultaneously operate as a wavelength tunable optical filter for converting the frequency, and change the center wavelength of the filter to change the oscillation wavelength of the laser. Can be changed.

FIGS. 2 and 3 show the frequency reduction converter 20 used in the wavelength tunable single frequency fiber laser shown in FIG.
FIG. 2 is a schematic diagram illustrating a zero and a frequency increasing converter 250, respectively. These are dual-mode fiber optic acousto-optic frequency converters and simultaneously operate as tunable optical filters.

These operations will be described with reference to FIGS. RF (Radio Frequency) frequency applicator 208
When an RF frequency signal is applied to the acoustic horn 205, a periodic bend is formed in the dual mode optical fiber 210. Such bending provides coupling between the two modes in the dual mode, LP01 mode and LP11 mode, and at this time, frequency conversion of about F occurs at the same time. Further, the mode splitting directional couplers 220 and 270 are provided with the dual mode optical fibers 210 and 290.
Mode coupling between the LP11 mode of the single mode optical fiber 230 and the LP01 mode of the single mode optical fiber 230,280.

Therefore, after the LP01 mode on the input side is changed to the LP11 mode while passing through the dual mode optical fiber, it is converted again to the LP01 mode on the output side. At this time, the frequency of the output light is reduced by about F in the frequency decreasing converter 200 of FIG. 2 and is increased by about F in the frequency increasing converter 250 of FIG. 3. However, such mode conversion is determined. The wavelength at which mode conversion occurs can be changed by changing the frequency F generated at a different wavelength.

FIGS. 4 and 5 are graphs showing characteristics when the frequency converters used in the wavelength-tunable single-frequency optical fiber laser of FIG. 1 operate with an acousto-optic tunable optical filter. FIG. 4 is a graph showing a transmission spectrum of an optical filter according to an applied RF frequency, and FIG.
4 is a graph illustrating a relationship between an RF frequency and a center wavelength of a transmission spectrum of an optical filter.

In FIG. 4, 2.97 MHz, 3.00 MH
z, 3.03 MHz, 3.06 MHz, 3.09 MHz
And when an RF frequency signal of 3.12 MHz is applied to the optical filter, the transmission characteristics of the optical filter are 1, 2, 3,
4, 5 and 6, respectively. The line width of the optical filter was about 6.5 nm.
Referring to FIG. 5, it can be seen that the center wavelength of the optical filter linearly changes according to the value of the applied RF frequency F.

Referring again to FIG. 1, the operation of the wavelength-tunable single-frequency optical fiber laser according to the embodiment of the present invention will be described.

The frequency decreasing converter 102 and the frequency increasing converter 10 which also operate with the fiber optic acousto-optic tunable optical filter.
When an RF signal of frequency F is applied to 3 and the laser gain medium 101, which is an erbium-doped optical fiber, is optically pumped using the laser diode 107, the laser oscillates. At this time, it is possible to observe the output characteristics of the laser coming out of the resonator through the optical fiber coupler 108.

FIG. 6 is a graph showing the output spectrum of the wavelength tunable single frequency optical fiber laser of FIG. This spectrum is obtained by scanning Fabry-Perot having an FSR (Free Spectral Range) of about 6 GHz.
-Perot) Measured using an optical filter.

Referring to FIG. 6, it can be seen that only one longitudinal mode is oscillated by the laser, and the line width of the laser mode is 1 MHz or less of the resolution of the scanning Fabry-Perot optical filter. Can be confirmed.

The wavelength-tunable single-frequency optical fiber laser shown in FIG.
By changing the frequency F of the RF signal applied to 50, the center wavelength of the acousto-optic tunable optical filter can be changed, and the oscillation wavelength of the laser can also be changed.

7 and 8 are graphs showing the characteristics of the wavelength-tunable single-frequency optical fiber laser shown in FIG. 1. More specifically, FIG. 7 is a graph showing the oscillation spectrum characteristics of the laser due to the change in the RF frequency. , FIG. 8 is a graph showing the position of the oscillation wavelength according to the RF frequency.

Referring to FIG. 7, it can be seen that oscillation is possible in the entire wavelength region of the gain of erbium, and the variable oscillation region reaches about 40 nm. Referring to FIG.
It can be seen that the position of the oscillation wavelength due to the RF frequency has a substantially linear relationship.

Therefore, it was confirmed that the laser always oscillates in only one longitudinal mode irrespective of the oscillation wavelength. In this way, a single-frequency tunable laser is realized by a linear resonator. I was able to.

The wavelength variable single frequency laser as described above is realized by an optical fiber laser, but other types of lasers other than the optical fiber laser can be similarly applied.

FIG. 9 is a schematic structural view of a tunable single frequency laser according to another embodiment of the present invention.

Referring to FIG. 9, external energy 607 pumps the laser gain medium 601. The resonator is formed linearly by reflecting mirrors 605 and 606. When the laser starts to oscillate, a laser output 610 that comes out of the resonator through one of the mirrors can be obtained. The laser gain medium 601 is positioned between the frequency increase converter 602 and the frequency decrease converter 603 so that a standing wave is not formed in the laser gain medium 601 and one longitudinal mode is oscillated.

In this embodiment, another wavelength tunable optical filter 609 is provided and its center wavelength is changed, so that the laser oscillation wavelength can be changed. On the other hand, the optical filter 604 having a narrow line width selects a longitudinal mode and prevents mode hopping to another longitudinal mode to obtain a stable laser spectrum, similarly to the embodiment of FIG. Play a role.

Although the present invention has been described in detail with reference to the embodiment, the present invention is not limited to the embodiment.
Those skilled in the art to which the invention pertains will be able to modify or change the invention without departing from the spirit and spirit of the invention.

[0046]

As described above in detail, according to the present invention, it is possible to change the oscillation wavelength in a wide wavelength region with one longitudinal mode in the form of a linear resonator. A tunable single-frequency laser can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic structural view of a wavelength tunable single frequency optical fiber laser according to an embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating a frequency reduction converter used in the tunable single-frequency optical fiber laser of FIG. 1;

FIG. 3 is a schematic diagram illustrating a frequency increasing converter used in the wavelength-tunable single-frequency optical fiber laser of FIG. 1;

4 is a graph showing characteristics (transmission spectrum of an optical filter depending on an applied RF frequency) when frequency converters used in the wavelength-tunable single-frequency optical fiber laser of FIG. 1 operate with an acousto-optic tunable optical filter. It is.

FIG. 5 shows characteristics (relationship between RF frequency and center wavelength of transmission spectrum of optical filter) when frequency converters used in the wavelength-tunable single-frequency optical fiber laser of FIG. 1 operate with an acousto-optic tunable optical filter. FIG.

FIG. 6 is a graph showing an output spectrum of the wavelength-tunable single-frequency optical fiber laser of FIG. 1;

FIG. 7 is a graph showing the characteristics of the wavelength-tunable single-frequency optical fiber laser of FIG. 1 (laser oscillation spectrum characteristics due to RF frequency change).

FIG. 8 is a graph showing characteristics (positions of oscillation wavelengths depending on RF frequencies) of the wavelength-tunable single-frequency optical fiber laser of FIG.

FIG. 9 is a schematic structural view of a tunable single frequency laser according to another embodiment of the present invention.

[Explanation of symbols]

 101, 601 laser gain medium 102, 200, 603 frequency reduction converter 103, 250, 602 frequency increase converter 104, 604 optical filter 105, 106, 605, 606 reflecting mirror 107 laser diode 108 light Fiber coupler 109 optical path 110 intermediate region 205 acoustic horn 208 RF frequency applicator 210, 290 dual mode optical fiber 220, 270 mode splitting directional coupler 230, 280 single mode optical fiber 607 ... Energy 609 ... Tunable optical filter 610 ... Laser output

Claims (9)

[Claims] 1. A laser having both ends formed of a reflector and having a linear resonator including a laser gain medium and an optical path therein, wherein the frequencies of light passing through the laser gain medium in both directions are different from each other. Frequency-increasing conversion means and frequency-decreasing conversion means located on both sides of the laser gain medium inside the resonator so that only one longitudinal mode can oscillate, and its own for changing the oscillation wavelength of the resonator. A wavelength-variable first optical filter means whose center wavelength can be changed; and a second, relatively narrow line width second mode for suppressing mode hopping so as to stably maintain one longitudinal mode in the resonator.
A wavelength tunable single-frequency laser, comprising: an optical filter means. 2. The wavelength tunable unit according to claim 1, wherein at least one of said frequency up conversion means and frequency down conversion means is also used as said wavelength tunable first optical filter means. One frequency laser. 3. The laser gain medium is at least one selected from the group consisting of a rare earth-doped optical fiber, a dye cell, a semiconductor diode amplifier, an Nd-YAG rod, a Nd: glass rod, and an Nd rod. The tunable single frequency laser according to claim 1, wherein: 4. When the laser gain medium is a rare-earth-doped optical fiber, the rare-earth element is any one selected from the group consisting of erbium, niobium, and itbium. A tunable single frequency laser according to claim 3. 5. The optical path according to claim 1, wherein the optical path is at least one selected from the group consisting of an optical fiber, vacuum, air, an inorganic material waveguide, and an organic material waveguide. Item 4. A tunable single-frequency laser according to Item 1. 6. The frequency-increasing conversion means is at least one selected from the group consisting of an acousto-optic frequency converter, an electro-optic frequency converter, and an integrated optical frequency converter. The tunable single-frequency laser according to claim 1, wherein 7. The frequency reduction conversion means is at least one selected from the group consisting of an acousto-optic frequency converter, an electro-optic frequency converter, and an integrated optical frequency converter. The tunable single-frequency laser according to claim 1, wherein 8. The wavelength tunable first optical filter means includes: a Fabry-Perot optical filter, an acousto-optic optical filter,
The tunable single-frequency laser according to claim 1, wherein the tunable single-frequency laser is at least one selected from the group consisting of an electron optical optical filter and an integrated optical optical filter. 9. The second optical filter means having a relatively narrow line width includes an absorption light filter using a saturated absorber, a Fabry-Perot light filter, an acousto-optic light filter, an electro-optic light filter, and an integrated optical light filter. The tunable single-frequency laser according to claim 1, wherein the laser is at least one selected from the group consisting of: