JP2821532B2 - Optical amplifier - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical amplifier suitable for use in the fields of optical communication and optical measurement.

"Problems to be Solved by the Invention" Conventionally, optical amplifiers using a semiconductor LD (semiconductor laser diode) have been mainly used. The semiconductor LD amplifier has a non-reflective coating on both end surfaces of a normal semiconductor LD and is used as a traveling wave amplifier. The problems of the semiconductor LD amplifier are that the amplification degree has polarization dependence and that the amplification degree between bits changes in the signal light of a high bit rate due to the pattern effect.

As an optical amplifier that does not have the above-mentioned disadvantages, a fiber amplifier using an optical fiber doped with a rare earth element and utilizing a laser transition of the added rare earth element has attracted attention. The features of this amplifier are that the polarization characteristics of the amplifier can be freely controlled and that there is no pattern effect.
The polarization characteristics of the amplifier can be controlled by, for example, a polarization independent amplifier if the fiber structure is a perfect circular core fiber, or a polarization independent fiber by using an absolute absolute single polarization maintaining fiber. It may be a dependent amplifier.

"Problems to be solved by the invention" By the way, optical fiber amplifiers require optical pumping,
Conventionally, a dichroic mirror is used for multiplexing / demultiplexing the pump light and the signal light (amplified light). For this reason, it is necessary to change the specification of the dichroic mirror depending on the wavelength of the pump light, and there is a problem that the versatility as an amplifier is lacking. In addition, when the wavelengths of the pump light and the signal light are close to each other, there is a practical problem that it is difficult to manufacture a dichroic mirror.

The present invention has been made in view of the above-described circumstances, and has as its object to realize a configuration of an optical amplifier that does not depend on the wavelengths of pump light and signal light.

"Means for solving the problem" In order to solve the above-mentioned problem, in the invention according to claim 1, 0.8, 1.3, 1.5 μm
A first and second pair of optical fibers or optical waveguides in which a rare earth element or a transition metal element having a laser transition level is added to m, and which has a different refractive index in two orthogonal directions in the core cross section. An active medium, an excitation light source for generating excitation light for exciting the first and second active media,
X, which is two orthogonal polarizations of the signal light and the pump light,
After being decomposed into Y polarization, the X polarization of the signal light and the Y polarization of the pump light are combined to form a first combined wave, and the Y polarization of the signal light and the X polarization of the pump light are combined. A first optical element for combining to form a second combined wave, an optical system for guiding the first and second combined waves to the first and second active media, respectively, the first and second combined waves; X polarization of excitation light emitted from active medium and Y
A second optical element that combines the polarization with the X-polarized light and the Y-polarized light of the emitted signal light, and separates and outputs the combined excitation light and signal light. It is characterized by:

In the invention described in claim 2, the first,
The second optical element is characterized in that a polarization beam splitter composed of an optical fiber, an optical waveguide or an individual optical component is used.

The invention according to claim 3 is characterized in that filters are provided in series on the light emission sides of the first and second active media, and pass only the light to be amplified.

A fourth aspect of the present invention is characterized in that an isolator corresponding to the wavelength of the light to be amplified is provided on the light emitting side of the first and second active media.

[Operation] In the first and second active media, first and second combined waves composed of orthogonal X and Y polarized waves are individually amplified. The second optical element combines the X and Y polarizations of the signal light in the light emitted from the first and second active media, and also combines the X and Y polarizations of the excitation light. Are spatially separated and emitted.

Next, an embodiment of the present invention will be described with reference to the drawings.

First, the basic principle of the present invention will be described. First
FIG. 1 is a block diagram showing the basic configuration of the present invention. In this figure, the pump light and the signal light are separated by a polarization beam splitter 1. As a result, a combined wave of the X polarization component of the pump light and the Y polarization component of the signal light is supplied to the optical path 5. The optical path 6 is supplied with a composite wave of the Y polarization component of the pump light and the X polarization component of the signal light. These two combined light beams are guided to the X-polarization optical fiber amplifier 2 and the Y-polarization optical fiber amplifier 3 and are separately amplified. The pump light emitted from the optical fiber amplifiers 2 and 3 and the amplified signal light are combined by the polarization beam splitter 4. At this time, since the signal light and the pump light in the optical paths 5, 6, 5 ', 6' are orthogonal to each other, the signal light and the pump light multiplexed by the polarization beam splitter 4 are spatially separated. Separated and output. That is, in the polarization beam splitter 4, the X polarization component and the Y polarization component of the pump light are combined, and the X polarization component and the Y polarization component of the signal light are combined. Waves are output separately.

As described above, the most significant feature of the present invention is that the polarization combining technique is used for the multiplexing and demultiplexing of the pump light and the signal light.
Note that an optical waveguide can be used instead of the optical fiber amplifiers 2 and 3.

Next, FIG. 2 is a diagram showing a configuration of an optical amplifier according to a first embodiment of the present invention.

Er-doped PANDA type optical fibers 12, 13
A Fabry-Perot type semiconductor laser (oscillation center wavelength 1.48 μm) as the semiconductor lasers 7 and 7 ′ for excitation, and a DFB type semiconductor laser (oscillation wavelength 1.553 μm) as the semiconductor laser 8 for signal light. ing. The two semiconductor lasers 7, 7 'are arranged so that their polarization planes are orthogonal to each other, and the excitation lights output from these are converted into parallel beams by the lenses 16, 16' and multiplexed by the polarization beam splitter 9. You. The multiplexed pump light is composed of X and Y polarized lights having no phase correlation with each other. And the semiconductor laser 8
Is output by the lens 18 to the single-mode optical fiber 14, the emitted light is again converted into a parallel beam by the lens 15, and is incident on the polarization beam splitter 10 (for excitation light + signal light wave). . This polarization beam splitter 10
Then, the X-polarized component of the pump light and the Y-polarized component of the signal light are multiplexed to the lens 17 side, and the Y-polarized component of the pump light and the X-polarized component of the signal light are multiplexed to the lens 17 'side. Emit. The respective output lights from the polarization beam splitter 10 enter the PANDA-type Er-doped optical fiber 12 for X-polarization amplification and the PANDA-type Er-doped optical fiber 13 for Y-polarization amplification via lenses 17 and 17 ', respectively. .

The PANDA type Er-doped optical fibers 12 and 13 have an Er doping concentration of 10%.
It is about 0 ppm, is a single mode at the wavelengths of the pump light and the signal light, and has no difference in the transmission loss of X and Y polarizations in the same wavelength range. The mode birefringence is 0.5 × 10 ⁻⁴ . PAN
The outgoing light (amplified signal light and leakage pumping light) from the DA-type Er-doped optical fibers 12 and 13 are collimated by lenses 19 and 19 ', respectively, and then polarized beam splitter 11 (signal light / pumping light separation). For). The outgoing light of the polarization beam splitter 11 is such that the XY polarized components of the excitation light and the signal light are multiplexed with each other, and the signal light is emitted to the lens 21 side, and the excitation light is emitted to the lens 20 side. , 23.

Here, FIG. 3 shows the spectra of the amplified light 25 and the ASE light (amplified spontaneous emission light) 24. The spectrum shown is measured by putting the fiber 22 (single-mode optical fiber for extracting signal light) shown in FIG. 2 into an optical spectrum analyzer. From the broad spectrum of the ASE light 24, it can be confirmed that the signal light spectrum with a narrow wavelength width is amplified and emitted.

FIG. 4 is a diagram showing the degree of amplification for signal light having circularly polarized light and linearly polarized light. In FIG. 2, the amplification characteristic when the polarization of the signal light on the exit side of the lens 15 is circular polarization is 26.
, And 27 for the case of linearly polarized light. As is clear from the drawing, according to the present embodiment, the amplification characteristics do not depend on the polarization characteristics of the signal light.

Next, FIG. 5 is a diagram showing the configuration of a second embodiment of the present invention. As the optical amplifying medium, Er-doped PANDA type optical fibers 31 and 32 (fiber characteristics such as Er-doped concentration are the same as those in Example 1) are used, and Fabry-Perot type semiconductor lasers are used as the pumping semiconductor lasers 7 and 7 '. 29 (center wavelength 1.48μ
m). The DFB semiconductor laser 28 (the oscillation wavelength is 1.553 μm and the emitted light is circularly polarized) is used as the semiconductor laser for signal light. Light emitted from both lasers is guided by a single-mode optical fiber, multiplexed by a fiber-type polarization beam splitter 30, and then Er-doped PANDA.
The light enters the optical fibers 31 and 32. The X-polarization component of the pump light and the Y-polarization component of the signal light are supplied to the PANDA type Er-doped optical fiber 32 for Y-polarization amplification, and the Y-polarization component of the pump light and the X-polarization component of the signal light are X-polarized. The light is propagated to the PANDA type Er-doped optical fiber 31 for wave amplification. Reference numeral 33 denotes an isolator provided on the emission side of the amplification fiber and adjusted for signal light, thereby removing return light. After the return light is removed, the leakage pump light and the amplified signal light are multiplexed by the fiber-type polarization beam splitter 34. On the signal light extraction side of the output part of the fiber type polarization beam splitter 34, an ASE is provided by a narrow band dielectric multilayer filter 35 (center wavelength 1.553 μm, pass band ± 3 nm) adjusted for the wavelength of the signal light. Removed.

With this configuration, the pump light intensity is 50 mW and the incident signal light intensity is -30.
Under the condition of dBm, an amplification degree of 15 dB was obtained, and it was confirmed that there was no polarization dependence.

FIG. 6 is a diagram showing the configuration of the third embodiment of the present invention. Note that the main components and arrangement are the same as those in the second embodiment, and a description thereof will not be repeated.

The difference from the second embodiment is that an isolator for removing return light is not used, and a birefringence filter 36 is used instead of the narrow band dielectric multilayer filter.

The birefringence filter is a band-pass filter using polarization characteristics, and as an example, is composed of a combination of a wave plate and a polarizer that gives an arbitrary phase difference between XY polarizations. The illustrated birefringent filter 36 is a three-plate birefringent filter composed of three quartz plates which are arranged so as to have a Brewster angle at the signal light wavelength and are mutually rotated.
In the μm band, it operates as a bandpass filter that can obtain a transmission band of 1 nm for an arbitrary wavelength. In this embodiment,
The center wavelength of the birefringence filter 36 is set to 1.553 μm.

With this configuration, the pump light intensity is 50 mW and the incident signal light intensity is -30.
Under the condition of dBm, an amplification degree of 17 dB was obtained, and it was confirmed that there was no polarization dependence.

[Effects of the Invention] As described above, according to the configuration of the present invention, since the polarization combining method is used for multiplexing the pump light and the signal light, even when both wavelengths are very close to each other. Combining can be performed efficiently and easily. Also, in an active medium (amplifying fiber, etc.), the signal light is linearly polarized, so that an isolator having a normal structure can be used, and polarization such as a birefringence filter as a narrow band filter for removing ASE is used. There is an advantage that a filter using wave characteristics can be used.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the basic configuration of the present invention, and FIG.
FIG. 3 is a block diagram showing the configuration of the first embodiment of the present invention. FIG. 3 is a characteristic diagram showing the spectrum of amplified light and ASE light. FIG. 4 shows the amplification degree for signal light having circularly polarized light and linearly polarized light. FIG. 5 is a block diagram showing the configuration of the second embodiment of the present invention, and FIG. 6 is a block diagram showing the configuration of the third embodiment of the present invention. DESCRIPTION OF SYMBOLS 1 ... Polarization beam splitter, 2 ... X polarization optical fiber amplifier, 3 ... Y polarization optical fiber amplifier, 4 ... Polarization beam splitter, 5,5 '... Optical path, 6,6' …… light path,
7, 7 ': semiconductor laser for excitation; 8, DFB semiconductor laser for signal light; 9, polarization beam splitter for polarization synthesis;
10 ... Polarization beam splitter, 11 ... Polarization beam splitter, 12 ... PANDA type Er-doped optical fiber for X polarization amplification, 1
3 …………………………………………………………………………………………………………………………………………………………………………………………………………………. 30
………………………………………………………………………………………… ……………………………………………………………………………………….
DA-type Er-doped optical fiber, 33 ... Isolator, 34 ... Fibre-type polarization beam splitter, 35 ... Narrow band dielectric multilayer filter, 36 ... Tunable birefringence filter.

────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Etsuji Sugita 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-63-289981 (JP, A) JP-A Sho 63-502067 (JP, A) JP-A-58-64088 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01S 3/00 G02F 1/35 501 JICST file (JOIS)

Claims (4)

(57) [Claims] (1) 0.8, 1.3, 1.
A rare earth element or a transition metal element having a laser transition level at 5 μm is added, and a first and second pair of optical fibers or optical waveguides having different refractive indexes in two orthogonal directions in the core cross section are provided. An active medium; an excitation light source for generating excitation light for exciting the first and second active media; and X, which are two orthogonal polarizations of the signal light and the excitation light.
After being decomposed into Y polarization, the X polarization of the signal light and the Y polarization of the pump light are combined to form a first combined wave, and the Y polarization of the signal light and the X polarization of the pump light are combined. A first optical element that combines the two into a second combined wave; an optical system that guides the first and second combined waves to the first and second active media; The X-polarized light and the Y-polarized light of the excitation light emitted from the active medium are combined, and the X-polarized light and the Y-polarized light of the emitted signal light are combined to separate the combined excitation light and signal light. An optical amplifier, comprising: a second optical element that outputs an output signal. 2. An optical amplifier according to claim 1, wherein a polarization beam splitter composed of an optical fiber, an optical waveguide or individual optical components is used as said first and second optical elements. . 3. The optical amplifier according to claim 1, wherein a filter for passing only light to be amplified is provided in series on the light emitting side of the first and second active media. 4. The optical amplifier according to claim 1, further comprising an isolator corresponding to the wavelength of the light to be amplified on the light emitting side of the first and second active media.