JPH09162468A - Laser oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To output high-power single longitudinal mode laser applicable to optical communication by including a second optical transmission line containing a laser amplifying medium that is stimulated by residual stimulating light and is thus capable of optically amplifying laser light. SOLUTION: An optical resonator 3 consumes only part of stimulating light emitted from a stimulating light source 4 because of its short Er-doped fiber 1. When the rest of the stimulating light is input as residual stimulating light together with laser to a light amplifier 12 in the next position, one 13 of two wavelength-division multiplexing directional couplers separates the laser and the residual light. The laser beam is directed into a Er-doped fiber 5 and the residual stimulating light is directed into an optical fiber 5. The other wavelength-division multiplexing directional coupler 14 launches the residual stimulating light, launched into the optical fiber 15, into the Er-doped fiber 5 inversely with the laser. As a result, in the light amplifier 12 the laser input is propagated to the Er-doped fiber 5 excited by the residual stimulating light, and is thereby optically amplified. This makes it possible to output tens of mW of laser light in the single mode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser oscillator capable of oscillating a laser beam having an output which can be used as a light source of an optical communication system.

[0002]

2. Description of the Related Art In recent years, many wavelength division multiplexing communication systems have been studied in the optical transmission technology of 10 Gbit / sec or more. Optical fiber lasers have attracted attention as a light source for wavelength multiplexing communication for this purpose, and in particular, as a light source for the 1.55 μm band, an optical fiber laser using an erbium (Er) -doped fiber as a laser medium has been actively studied. In wavelength division multiplexing, several to several tens of signal lights with different wavelengths (currently many are 4 or 8) are used at wavelength intervals of about 2 nm, so each signal light has a narrow wavelength width. Therefore, there is a need for a light source capable of oscillating light with a narrow wavelength width. At present, semiconductor DFB lasers (distributed feedback lasers) are mainly used in wavelength division multiplexing experiments. On the other hand, since the optical fiber laser has a high gain of the amplification medium, a large finesse can be obtained inside the resonator, and the oscillation spectrum line width can be narrowed to several KHz, so that it can be used for wavelength division multiplexing communication. Is expected. Furthermore, since oscillation with an optical fiber laser using an Er-doped fiber has a continuous gain characteristic at 1.53 to 1.56 μm, attempts have been made to oscillate a plurality of wavelengths in one resonator. Therefore, it has a great potential as a light source for wavelength division multiplexing communication.

Next, a conventional technique of an optical fiber laser will be described. In a general optical fiber laser, a semitransparent reflector is provided at both ends of an Er-doped fiber as a laser medium to form an optical resonator, and Er in the Er-doped fiber of the optical resonator is excited by pumping light. Then, the population inversion for laser oscillation is caused, and the laser light is oscillated and amplified in this optical resonator. At present, there are two types of optical fiber lasers such as a ring type shown in FIG. 8 and a Fabry-Perot type shown in FIG. 9 depending on the difference in optical resonator configuration. Both types are being studied.

The ring-type optical fiber laser shown in FIG. 8 oscillates by oscillating pumping light output from a pumping light source A into a ring-type optical resonator D composed of an optical fiber B and an erbium-doped fiber C. The laser beam oscillated by this is taken out and output by the optical coupler E. A bandpass filter F for removing unnecessary light in the resonator D and an optical isolator G for removing light in the opposite direction are inserted in the optical resonator D.

In the Fabry-Perot type optical fiber laser of FIG. 9, the pumping light output from the pumping light source A is incident on a Fabry-Perot type optical resonator I composed of an erbium-doped fiber C and a fiber grating H. Laser oscillation is caused to occur, and the laser light oscillated in the resonator I is extracted by the optical coupler E and output. An optical isolator G and a polarization controller J are also attached to this fiber laser.

In the optical fiber laser, a large number of longitudinal modes corresponding to the period of the cavity length oscillate in the gain band of the laser medium. The frequency difference between the adjacent resonance frequencies is called a free spectral range, and is expressed by the equation (1) in the Fabry-Perot type optical fiber laser of FIG. FSR = c / 2nL (1) c: speed of light n: refractive index of fiber core L: resonator length

For example, in an optical resonator made of an Er-doped fiber used as a laser medium and added with Al or the like, 15
The gain spectrum in the 50 nm band spreads gently, and the gain band becomes 30 nm or more. Since a large number of longitudinal modes can exist within this band at the intervals obtained by the above equation (1), in order to perform single mode oscillation in this optical resonator, a large number of longitudinal modes existing in the optical resonator are required. You must select any one oscillation mode from the longitudinal modes,
For that purpose, a mode selection element must be inserted in the resonator. The Fabry-Perot etalon (JLZyskind, JWSulhof
f, J. Stone, DJ Digiovanni, LWStulz, HMPresby,
A.Piccirilli and PEPramayon, Electron.Lett., 2
7, No. 21, 1950 (1991)) and Mach-Zehnde interferometer (EWD
ianov, TRMartirosian, OGOkhotnikov, and VMPa
ramonov, OFC / IOOC '93 Technical Digest WG6 p103)
In addition, there is an example of using a fiber grating as a mode selection element in a Fabry-Perot resonator (for example, SVChernikov, and JR Taylor, Op.
t. Lett. 18, No. 23, 2023 (1993)). The fiber grating is a Bragg reflection diffraction grating formed on an optical fiber, and is a narrow band reflection filter capable of selectively reflecting only a specific wavelength.

[0008]

The Fabry-Perot type optical fiber laser has the following problems when used for wavelength division multiplexing. In wavelength division multiplexing, it is necessary to narrow the oscillation line width, stabilize the frequency, and increase the output (to obtain an output of about ten and several mW necessary for optical communication). In the Fabry-Perot resonator, one mode can be selected by shortening the resonator length and widening the longitudinal mode spacing. Especially, when a fiber grating is used as a mode selection element, the wavelength width is about 0.1 nm (half It is possible to output a laser having a price range). As can be seen from the equation (1), in order to select one longitudinal mode, a mode selection element having a width equal to the free spectrum interval is required. On the contrary, when the limit wavelength width of the mode selection element is 0.1 nm, the cavity length of the laser capable of oscillating a single mode is about 1.6 cm in the 1.55 μm band. However, when an Er-doped fiber is used as a laser medium, a fiber length of several cm cannot obtain an output of tens of mW required as a light source for optical communication. In order to avoid this and increase the output, a high concentration of Er ions is added, the energy absorption efficiency is improved by co-adding Yb, and a strong excitation light source is used (wavelength 980 nm
Although there is a countermeasure such as Ti: sapphire laser), it becomes a problem in providing an inexpensive and practical laser oscillator.

It is an object of the present invention to provide a laser oscillator for outputting a high-power (tens of mW or so) single longitudinal mode laser which can be used for optical communication in a Fabry-Perot type optical fiber laser. is there.

[0010]

In the laser oscillator according to the first aspect of the present invention, as shown in FIGS. 1 to 3, a grating 2 is formed at both ends in the transmission direction of a first optical transmission line 1 containing a laser medium. Optical resonator 3 formed as described above, pumping light source 4 for inputting pumping light for pumping the laser medium from one end of this optical resonator 3, and laser light and residual pumping light output from the other end of optical resonator 3 Is input and is excited by the residual excitation light, and the second optical transmission line 5 including a laser amplification medium capable of optically amplifying the laser light is provided.

As shown in FIG. 4, a laser oscillator according to a second aspect of the present invention is an optical resonator in which a grating 2 is formed at both ends and inside of a first optical transmission line 1 including a laser medium in the transmission direction. 3 and a pumping light source 4 for inputting pumping light for pumping the laser medium from one end of the optical resonator 3,
A second optical transmission line 5 including a laser amplification medium into which the laser light and the residual pumping light output from the other end of the optical resonator 3 are input and which is excited by the residual pumping light and is capable of optically amplifying the laser light is provided. It will be.

In the laser oscillator according to claim 3 of the present invention, the first optical transmission line 1 is a rare earth-doped fiber doped with a laser medium such as rare earth metal ions.

In the laser oscillator according to claim 4 of the present invention, the first optical transmission line 1 is a bulk type waveguide to which a laser medium such as rare earth metal ions is added.

In the laser oscillator according to claim 5 of the present invention, the second optical transmission line 5 is a rare earth-doped fiber doped with a laser amplification medium such as rare earth metal ions.

In the laser oscillator according to claim 6 of the present invention, the second optical transmission line 5 is a bulk type waveguide to which a laser amplification medium such as rare earth metal ions is added.

[0016]

First Embodiment FIG. 1 shows a first embodiment of a laser oscillator according to the present invention. In FIG. 1, 4 is an excitation light source,
Numerals 10 and 11 are optical isolators, 1 is an Er-doped fiber (first optical transmission line) doped with erbium ions (hereinafter referred to as Er ions) as a laser medium (laser active medium), 2a and 2b are Er-doped fibers 1 Reference numeral 12 denotes a fiber grating (Bragg reflection diffraction grating) formed at both ends of the optical amplifier, and 12 denotes an optical amplifier including an Er-doped fiber (second optical transmission line) 5 doped with Er ions as a laser amplification medium.

Fiber gratings 2a and 2b are provided at both ends.
The Er-doped fiber 1 in which is formed constitutes the Fabry-Perot type optical resonator 3 itself. The optical resonator 3 has a length (length of the Er-doped fiber 1) that allows oscillation of a single longitudinal mode laser,
For example, when a single longitudinal mode with a wavelength width of 0.1 nm is generated in the 1.55 μm band, the length of the optical resonator 3 is about 1.6 c.
m. In the optical resonator 3, the pumping light of the pumping light source 4 is incident on the end of the fiber grating 2a via the optical isolator 10, and when the pumping light is absorbed by the Er-doped fiber 1, the Er-doped fiber 1 emits the light. Of the emitted light, only the wavelength components reflected by the fiber gratings 2a, 2b at both ends are lased, and only some of the wavelength components not reflected by the ends of the fiber grating 2b are emitted from the optical resonator 3. Is output. The optical resonator 3 consumes only a part of the pumping light incident from the pumping light source 4 because the Er-doped fiber 1 constituting the optical resonator 3 is short, and most of the rest is output as residual pumping light together with the laser. Then, it is input to the next optical amplifier 12. The fiber grating has a smaller insertion loss than a semi-transparent reflecting mirror, and can efficiently generate a laser with a low threshold and a high output.

The optical amplifier 12 has the structure shown in FIG. 2 or FIG. The optical amplifier 12 shown in FIG. 2 includes wavelength division multiplexing directional couplers (WDM couplers) 13 and 14 and Er-doped fiber (second optical transmission line) 5, and one wavelength division multiplexing directional coupler. Reference numeral 13 separates the laser and residual pumping light input from the optical resonator 3 so that the laser enters the Er-doped fiber 5 and the residual pumping light enters the optical fiber 15, respectively, and the other wavelength division multiplexing directional coupler. 14 reverses the residual pumping light incident on the optical fiber 15 to Er in the opposite direction to the laser.
It is configured to enter the doped fiber 5. In this optical amplifier 12, the laser input from the optical resonator 3 is propagated to the Er-doped fiber 5 that is excited by the residual pump light input from the optical resonator 3 to be optically amplified.

The optical amplifier 12 of FIG. 3 comprises an Er-doped fiber 5 and a wavelength selection filter 16. This optical amplifier 1
In 2, the laser input from the optical resonator 3 is optically amplified by the Er-doped fiber 5 excited by the residual excitation light,
Only the optically amplified laser is output through the wavelength selection filter 16.

FIG. 5 shows an oscillation frequency spectrum of a laser oscillated by the laser oscillator, and a single longitudinal mode laser having an output level required for optical communication is output.

[0021]

Embodiment 2 FIG. 4 shows a second embodiment of the laser oscillator according to the present invention, in which the Fabry-Perot optical resonator 3 shown in FIG. It can be used as a light source.

The optical resonator 3 of FIG. 4 has a plurality of fiber gratings 2 (2c, 2d, 2e, 2f) formed in the Er-doped fiber (first optical transmission line) 1 similar to that of the first embodiment. is there. These fiber gratings 2
c, 2d, 2e, and 2f have different wavelength selectivity, and the grating 2c is set to be able to reflect all wavelength components reflected by 2d, 2e, and 2f. Therefore, in this optical resonator 3, the gratings 2d, 2e, 2
The laser of the wavelength selected by f is output. The lasers of a plurality of wavelengths oscillated by the optical resonator 3 are optically amplified by the optical amplifier 12 having the configuration shown in FIG. 2 or 3 and output.

In each of the embodiments described above, the optical fibers (Er-doped fiber) are used as the first and second optical transmission lines 1 and 5, but a bulk type optical waveguide may be used, for example. In particular, since the first optical transmission line 1 is made relatively short, it is possible to easily make a bulk type. In addition, the first optical transmission line 1 uses E as a laser medium.
Although the r ion is added, it is possible to add a rare earth metal other than Er or a laser medium other than Er. Alternatively, the laser medium can be directly formed into a fiber type or a bulk type and used. In addition, the second optical transmission line 5 is E
Although the r ion was added to have the light amplifying effect, it is also possible to add another substance to obtain the light amplifying effect. In any case, an appropriate substance can be used according to the oscillation wavelength of the laser and other conditions.

[0024]

[Experimental Results] The output level of laser light from the laser oscillators shown in FIGS. The pumping light output from the pumping light source 4 is absorbed by the laser medium (laser active medium) in the optical resonator 3. The amount of pumping light absorbed in the optical resonator 3 varies depending on conditions such as the concentration of rare earth metal added in the laser medium (rare earth metal ion-doped fiber) used in the resonator 3 and the fiber length. In the experimental example in, the Er-doped fiber has a wavelength of 1.48 μm.
When the excitation light of No. 2 was entered, the absorption coefficient of the excitation light in the Er-doped fiber was several dB / m. In the present invention, the length of the optical resonator 3 (Er-doped fiber) is several cm to several m at the longest. By the way, a semiconductor laser having a maximum output of 70 mW (18.45 dBm) or more is used in an optical amplifier. When this laser is used as the pumping light source 4 in FIGS. 1 and 4, residual pumping light emitted from the optical resonator 3 is used. The light is around 30 mW (14.77 dBm). At this time, the laser light intensity emitted from the optical resonator 3 is 0.01
It is about mW (−20 dBm) and does not reach the light intensity (tens of mW) required for optical communication (FIG. 6). FIG. 9 shows the relationship between the amplification factor and the pumping light intensity of the optical amplifier 12. When 30 mW of excitation light and 0.01 mW of laser light are incident on the laser light, the laser light is amplified by about 32 dB and the output is 12 dBm (15.8 mW). Therefore, the laser intensity required for optical communication can be obtained. .

[0025]

The laser oscillator of the present invention has the following effects. 1. Laser light with a narrow wavelength width can be output. It can output laser light in a single longitudinal mode. 3. It can output tens of mW of laser light that can be used for optical communication. Light loss due to the mode selection element (grating) is small, and a low threshold and high output laser can be efficiently generated. 5. Since the residual pumping light not consumed by the optical resonator is reused to increase the output power of the laser light, the energy conversion efficiency is high.

Particularly, in the laser oscillator according to the second aspect, it is possible to simultaneously oscillate lasers of multiple wavelengths by using a plurality of gratings, which is useful as a light source for wavelength division multiplexing communication.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a first example of an embodiment of a laser oscillator of the present invention.

FIG. 2 is a schematic diagram showing a first example of an optical amplifier in the laser oscillator of FIG.

3 is a schematic diagram showing a second example of an optical amplifier in the laser oscillator of FIG.

FIG. 4 is a schematic diagram showing a second example of the embodiment of the laser oscillator of the present invention.

5 is an explanatory diagram showing an example of an oscillation frequency spectrum by the laser oscillator of FIG.

FIG. 6 is an explanatory diagram illustrating a relationship between output light intensity and excitation light intensity in the optical resonator of the laser oscillator of FIGS.

FIG. 7 is an explanatory diagram illustrating the relationship between the amplification factor of laser light and the pumping light intensity in the optical amplifier of the laser oscillator of FIGS.

FIG. 8 is a schematic diagram showing a conventional example of an optical fiber laser having a ring resonator.

FIG. 9 is a schematic diagram showing a conventional example of an optical fiber laser having a Fabry-Perot resonator.

[Explanation of symbols]

 1 First Optical Transmission Line 2 Grating 3 Optical Resonator 4 Excitation Light Source 5 Second Optical Transmission Line

Claims (6)

[Claims] 1. An optical resonator (3) comprising a grating (2) at both ends in the transmission direction of a first optical transmission line (1) containing a laser medium, and an optical resonator (3) from one end of the optical resonator (3). Excitation light source for inputting excitation light for exciting the laser medium (4)
And the laser light and the residual pumping light output from the other end of the optical resonator (3) are input, and the second optical transmission includes a laser amplification medium that is excited by the residual pumping light and is capable of optically amplifying the laser light. Laser oscillator, characterized in that it comprises a path (5). 2. An optical resonator (3) comprising a grating (2) at both ends and inside of a first optical transmission line (1) including a laser medium in the transmission direction, and an optical resonator (3) of the optical resonator (3). A pumping light source (4) for inputting pumping light for pumping the laser medium from one end, and laser light and residual pumping light output from the other end of the optical resonator (3) are input and pumped by the residual pumping light. And a second optical transmission line (5) including a laser amplification medium capable of optically amplifying laser light. 3. The laser oscillator according to claim 1, wherein the first optical transmission line (1) is a rare-earth-doped fiber doped with a laser medium such as rare-earth metal ions. 4. The laser oscillator according to claim 1, wherein the first optical transmission line (1) is a bulk type waveguide to which a laser medium such as rare earth metal ions is added. . 5. The laser oscillator according to claim 1 or 2, wherein the second optical transmission line (5) is a rare earth-doped fiber doped with a laser amplification medium such as rare earth metal ions. 6. The laser oscillator according to claim 1, wherein the second optical transmission line (5) is a bulk type waveguide to which a laser amplification medium such as rare earth metal ions is added. .