JP2702572B2 - Optical fiber amplifier - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical fiber amplifier having high efficiency and a high amplification factor.

(Prior Art) In recent years, Nd (neodymium), E _r (erbium), P _r (praseodymium), Yb (ytterbium) optical fiber (hereinafter, referred to as a rare earth element doped optical fiber.) Doped with a rare earth element such as the It has been reported that a single mode optical fiber laser or an optical amplifier as a laser active material has a possibility of being widely used in the field of optical sensors and optical communication, and its application is expected.

An example of an optical fiber laser amplifier using this rare earth element-doped optical fiber, in which _Er- doped quartz-based optical fiber is used as a laser active material, and a semiconductor laser is used as an excitation light source and light amplification is confirmed at a wavelength of 1.54 μm. Is Earl
J. Meers et al. (RJMears et al, Electron. Let
t., 23, pp 1028-1029, 1987).

FIG. 5 shows an example of an optical fiber laser amplifier as described above, wherein 1 is a distributed feedback (DFB) laser having a wavelength of 1.552 μm, 2 is a drive signal, 3, 3 'is a condenser lens, ,
Reference numeral 4 'denotes an excitation light source having a wavelength of 1.485 μm, and reference numeral 5 denotes a dichroic mirror (for wavelengths of 1.552 μm and 1.485 μm). 6 is
An _Er- doped optical fiber, 9 is a polarization beam splitter (PBS) for polarization-synthesizing the output lights of the light sources 4, 4 ', and 10 is an 8 ° obliquely polished optical fiber connector. Reference numeral 11 denotes a polarization-independent optical isolator, and reference numeral 12 denotes a narrow band-pass filter (Δλ = 1 nm) having a wavelength of 1.552 μm. 13 is a spectrum analyzer, and 14 is an optical power meter. 16 ″ is an optical fiber that couples the polarization-independent optical isolator 11 and the narrow-band transmission filter 12, and 16 is the filter 12 and the spectrum analyzer.
13 is an optical fiber that couples Optical power meter 14 can receive the light output through the optical fiber 16 ^V from narrow band pass filter 12.

Here, the excitation light sources 4 and 4 'are turned on, and each excitation light passes through the lens 3, the polarization beam splitter (PBS) 9 and the dichroic mirror 5, and further passes through the condenser lens 3' and the optical fiber connector 10. The light enters the _Er- doped optical fiber 6. The drive signal 2 is applied to a DFB laser 1 as a signal source to be amplified, and the output light is passed through a condenser lens 3, a dichroic mirror 5, a condenser lens 3 'and an optical fiber connector 10 to an _Er- doped optical fiber 6. Incident on.

Here, the optical signal having a wavelength of 1.552 μm of the DFB laser is traveling wave amplified in the _Er- doped optical fiber 6. This is observed by an optical spectrum analyzer 13 via a polarization independent optical isolator 11 and a narrow band filter 12, and the output is measured by an optical parameter 14. In this way, an amplification degree of usually about 10 to 30 dB is obtained.

However, in the conventional optical fiber amplifier having the structure shown in FIG. 5, filters such as the polarization independent optical isolator 11 and the narrow band transmission filter 12 are incorporated as independent components, respectively. An optical fiber 16 ″ has to be interposed for these couplings.

As a result, the total insertion loss of the polarization-independent optical isolator 11 and the narrow-band transmission filter 12 not only increases from 6 dB to 10 dB, but also increases the number of components constituting the optical amplifier. , A major obstacle to economic realization.

In addition, since a fixed filter is usually used for the narrow band transmission filter 12, it cannot cope with a slight difference or change in the wavelength of the signal light to be amplified, which is not only inferior in versatility but also maximizes the performance of the optical fiber amplifier. There was a drawback that it could not be demonstrated.

FIG. 6 is a diagram showing an optical isolator, a narrow band filter, a pumping light rejection filter, and a connection optical fiber used in a conventional optical fiber amplifier.

In FIG. 6, 17, 17 'are zirconia ferrules whose end faces are polished to 8 mm, 18, 18', 18 ", 18 are condenser lenses, 19 is an isolator crystal, and 20, 20 'are zirconia ferrules. , 21 is a cut filter in the wavelength band of 1.40 μm to 1.50 μm, and 22 is a narrow band having a center wavelength of 1.552 μm (Δλ = 1
nm) filter, 23 is a single mode optical fiber (without _Er addition), 32, 32 'are rutile crystals, and 33 is a YIG crystal.

Here, the ferrule 17 is polished diagonally
This is for suppressing end face reflection. Filter 21
A filter for removing light output without being absorbed by the _Er- doped optical fiber 6 out of the excitation light. Reference numeral 22 denotes amplified spontaneous emission (Amplified Spontaneous Emission).
This is a filter for removing.

Now, if the problems in the configuration shown in FIG. 6 are arranged and listed, the number of parts is as large as 14 and the parts are large.

Therefore, many reflection points and optical coupling systems occur, resulting in a large insertion loss.

Multiple internal reflections cause oscillation of the amplifier and generate noise.

And so on.

(Problems to be Solved by the Invention) An object of the present invention is to provide a small-sized and high-performance optical amplifier while reducing the internal loss of the optical fiber amplifier.

(Means for Solving the Problems) The optical fiber amplifier of the present invention has been made in view of the above-mentioned drawbacks, and includes ferrules 17, 17 ', 20, 20', lenses 18, 1
8 ', 18 "18
Except for 16 ″, the functions of the filters 21 and 22 are further added to the optical path 18-1.
It is configured to be housed in the 8 'to solve the above-mentioned problem. Further, the filter 22 is newly provided with an adjusting function.

As a result, the loss of the optical system shown in FIG. 6 is greatly reduced, the reflection is suppressed, and the function of finely adjusting the operating wavelength can be maintained, thereby achieving high-performance amplification characteristics. .

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Embodiment 1 FIG. 1 is a conceptual diagram of a first embodiment of the present invention,
25 is a rejection filter film in the wavelength band of 1.4 μm to 1.5 μm (wavelength 1.
A plano-convex lens with 26 formed directly on the surface, 25 dB at 48 μm, 27
Is a narrow-band transmission filter made of a dielectric multilayer film with a thickness of 80 μm ((center wavelength: 1.56 μm), bandwidth: 0.8 nm), and 28 is a minute filter for slightly rotating this filter from a direction perpendicular to the optical axis 30. A rotating device, 29 is a clamp for fixing this rotation, 32, 32 'are rutile crystals, and 33 is a YIG crystal. Here, the insertion loss of this component was 2.5 dB at a wavelength of 1.535 μm.

The composite component of the present invention shown in FIG. 1 is mounted in a section PP ′ instead of the polarization independent optical isolator 11, the optical fiber 16 ″ and the narrow band transmission filter 12 of the optical amplifier shown in FIG. An optical amplification experiment was performed.

Here, the wavelength of the DFB laser 1 is 1.51 μm to 1.59 μm.
A continuously variable light source that is variable in the range is used.

In the experiment, the wavelength of the light source 1 was fixed at 1.535 μm, and the lens
The light was incident on the optical fiber 6 via the 3, 3 'and dichroic mirrors 5. Here, the incident light power to the optical fiber 6 was -48 dBm.

Then, the excitation light source (wavelength: 1.48 μm) and 4, 4 ′ were turned on, and coupled to the _Er- doped optical fiber 6 via the lenses 3, 3 ′, the polarization beam splitter 9, and the dichroic mirror 5. The combined optical power at this time was 30 mW. The _Er- doped optical fiber 6 is a single mode optical fiber in which _Er is added to the core portion at 75 ppm, and its length is 220 m. This output is observed by a spectrum analyzer via an optical isolator 24 with a built-in filter shown in FIG. 1, and the micro-rotating device 28 is driven while the optical output intensity is measured by a power meter so that the output is maximized. The angle of the filter (the angle with respect to the optical axis 30) was tilted and finely adjusted so that
Here, since the narrow-band transmission filter 27 is thin, displacement of the optical axis due to rotation does not matter. As a result, the obtained maximum net gain (amplification degree) was 43.5 dB.
It was also found that the level of spontaneous emission light (ASE) was reduced to -60 dBm or less.

Comparative Example 1 In the optical amplification system shown in FIG. 5, the optical isolator and the optical filter shown in FIG. 6 were inserted in the section PP ′, and an optical amplification experiment was performed under the same conditions as in the first embodiment. At this time, the insertion loss of the polarization independent optical isolator 11 at a wavelength of 1.535 μm is 1.6 dB, and the insertion loss of the narrow band transmission filter 12 is 4.5 dB.
And the total insertion loss is polarization independent optical isolator 11
And 6.4 dB including the coupling loss of the narrow band transmission filter 12.

The gain obtained was only 37 dB at a wavelength of 1.535 μm. The difference between the first embodiment and the first comparative example is due to the difference in the center wavelength of the narrow band transmission filter with respect to the operating wavelength (here, 1.535 μm) together with the difference in the insertion loss. is there. In the present invention, as described above, the structure in which the fine adjustment mechanism is provided at the center wavelength of the narrow band transmission filter is provided, so that the fine adjustment can be made to the operating wavelength. It has a configuration that can be used to the fullest.

Embodiment 2 FIG. 2 is a block diagram of an optical isolator with a filter used in a second embodiment of the present invention. Reference numeral 31 denotes a narrow band filter made of a dielectric multilayer film having a center wavelength of 1.552 μm;
32 'is a rutile crystal, 33 is a YIG crystal, and 34 is a filter composed of a dielectric multilayer film, which is a filter for blocking 0.88 μm wavelength band.

Here, a plano-convex lens formed with a narrow band transmission filter 31
No. 25 was selected from a number of filters manufactured by vapor deposition processing, which matched the required light source wavelength of 1.552 μm. The insertion loss of the obtained optical isolator 24 ′ with a built-in filter was 1.8 dB at a wavelength of 1.552 μm. The rejection ratio in the excitation wavelength band of 0.93 μm to 1.03 μm was 60 dB or more.
This is due to the fact that the filter 34 made of the dielectric multilayer film has effectively acted and the YIG crystal for the isolator has extremely large absorption in this wavelength band.

The components of the optical isolator (24 'shown in FIG. 2) with a built-in filter thus mounted were used and mounted on the optical fiber amplifier shown in FIG.

FIG. 3 is a conceptual diagram of a second embodiment of the present invention,
1 is a DFB laser having a wavelength of 1.552 μm, 2 is a drive signal, and 4,4 ′
Is an excitation light source (a GaAs strained quantum well structure laser having a wavelength of 0.98 μm), and 9 is a polarization behim splitter (operating wavelength of 0.98 μm).
μm) and 15 are WDs operating at wavelengths of 0.98 μm and 1.55 μm
M optical fiber coupler.

To operate this, the DFB laser 1
Is turned on to couple to the terminal fiber 16 of the optical fiber coupler 15, and then the excitation light sources 4, 4 'are turned on, and the output is coupled to the terminal fiber 16 ″ of the optical fiber coupler 15.
The output of the optical fiber 16 ^V at this time is a signal light (wavelength λ _S
Of the amplified light) was -48 dBm, and the pump light (? _P ) was 32 mW.

This is coupled to an _Er- doped single-mode optical fiber 6 ( _Er : 70 ppm, length 40 m), and the amplified signal is passed through a spectrum analyzer 13 ′ via an optical isolator 24 ′ with a built-in filter shown in FIG. And the output level was measured with the power meter 14. As a result, 29 dB was obtained as the amplification degree of the net.

Comparative Example 2 In the optical system shown in FIG. 3, a polarization independent optical isolator 11 and a narrow band transmission filter 12, which are individual components, were used instead of the optical isolator 24 'with a built-in filter.
An optical amplification experiment was performed under the same conditions as those of the example. Here, the narrow band filter 12 is manufactured under the same conditions as the narrow band filter used in the second embodiment, and the transmission wavelength is 1.552 μm.
The insertion loss was 3.5 dB, and the insertion loss of the optical isolator was 2.2 dB as in the first embodiment. The overall insertion loss was 6 dB, including the loss of 0.3 dB at the joint between the polarization independent optical isolator 11 and the narrow band transmission filter 12.

The net gain (amplification) obtained under the above conditions is 22.9
stayed in dB. Further, in this comparative example, the storage space for the components was increased by a factor of 2.5 in the case of Example 2.

Embodiment 3 FIG. 4 is a configuration diagram of an isolator with a filter used in a third embodiment of the present invention, and has a structure detachable in a direction as shown in FIG. Reference numeral 27 denotes a narrow band transmission filter, and reference numeral 37 denotes a fixed clamp of the filter. The narrow band transmission filter 27 and the fixed clamp 37 have an integral structure, and a plurality of filters are prepared so as to match a required input signal wavelength. Further, slight adjustment of the center wavelength is performed by adjusting the angle of the narrow-band transmission filter 27 with respect to the optical axis.

Using these components, the same effects as in Examples 1 and 2 were confirmed.

(Effect of the Invention) As explained above, the optical fiber amplifier of the present invention
A configuration in which a polarization-independent optical isolator, a narrow-band filter, and a pumping light rejection filter, which are essential for the operation of a stable and low-noise optical amplifier, are densely mounted, for example, by directly forming the optical components constituting the optical isolator. And has a mechanism for adjusting the center wavelength of the filter over the details of the center wavelength of the signal light.Therefore, the number of components is smaller and smaller than that of the conventional device, the insertion loss is small, In addition, there is an advantage that the amplification degree is high.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a first embodiment of the present invention, FIG. 2 is a configuration diagram of an optical isolator with a filter used in a second embodiment of the present invention, and FIG. 3 is a second embodiment of the present invention. FIG. 4 is a block diagram of an optical isolator with a filter used in a third embodiment of the present invention, FIG. 5 is a block diagram of a conventional optical fiber amplifier, and FIG. 6 is a block diagram of a conventional optical fiber amplifier. FIG. 3 is a diagram illustrating an optical isolator, a narrow band filter, an excitation light rejection filter, and a connection optical fiber that are used. 1 DFB laser 2 Driving signal 3, 3 'Condensing lens 4, 4' Excitation light source 5 Dichroic mirror 6 _Er- doped optical fiber 9 Polarized beam splitter 10 Optical fiber connector 11… Polarization independent optical isolator 12… Narrow band transmission filter 13… Spectrum analyzer 14,14 ′, 14 ″… Optical power meter 15… Optical fiber coupler 16,16 ′, 16 ″, 16, 16 ^1V, 16 ^V ... optical fiber 17, 17 '... ferrules 18, 18', 18 ", 18 ... lens 19 ... isolator crystal 20, 20 '... ferrule 21 ...... wavelength 1.40μm Cut filter in wavelength 1.50 μm band 22 Narrow band filter with center wavelength 1.552 μm 23,23 'Single mode optical fiber ( _Er- free) 24,24'24 "Optical isolator with built-in filter 25 … Plano-convex lens 26… blocking filter film 27… narrow-band transmission filter 28… micro-rotating device 29 … Clamp 30… Optical axis 31… Narrow bandpass filter composed of dielectric multilayer film 32, 32 ′… Rutile crystal 33… YIG crystal 34… Filter composed of dielectric multilayer film 35… Divider 36 …… Fused joint 37 …… Fixed clamp

Claims (3)

(57) [Claims] An optical amplification medium comprising a single mode optical fiber doped with a laser active substance such as a rare earth element or a transition metal element; an excitation light source for generating excitation light for exciting the laser active substance; An optical system that optically couples the pump light and the amplified signal light and guides the single-mode optical fiber, and polarization-independent light coupled to an output side or an input / output side of the single mode optical fiber. An optical fiber amplifier comprising an isolator and optical filters optically coupled to a stage subsequent to the optical isolator on the output side, wherein the optical isolator and the optical filters are connected to an input / output end of the optical isolator. An optical fiber amplifier which is packaged and mounted together in a set of optical imaging systems respectively arranged. 2. A lens surface used in the optical imaging system,
2. The optical fiber amplifier according to claim 1, wherein a dielectric multilayer film having a part or all of the filter characteristics possessed by the filters is directly formed. 3. The optical fiber amplifier according to claim 1, further comprising a mechanism for changing a center wavelength of a narrow band filter for selectively passing an amplified signal light among said filters. .