JPH0653581A - Pumping module for erbium-doped fiber photoamplifier - Google Patents

Abstract

PURPOSE:To obtain a module wherein it is small-sized and its amplification efficiency is high by a method wherein beams of light from two lasers can be coupled with good efficiency without using a polarization-plane preservation fiber. CONSTITUTION:In a pumping module for an erbium-doped fiber photoamplifier, a TE-mode laser 21, a pumping light source in which a TE-mode laser 22 is arranged and a beam splitter 24 on which beams of light from the TE-mode laser 21 and the TE-mode laser 22 are incident and in which the beams of light are composed without using a fiber are installed, and pumping light from the beam splitter 24 is made incident on a dielectric multilayer-film filter 25.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifier used in an optical fiber communication system, and more particularly to a method for amplifying an optical fiber doped with an erbium element by injecting a laser beam from a high power semiconductor laser. Erbium Doped Fiber Amplifier (hereinafter abbreviated as EDFA: Erbium Doped Fiber Amp)
ifer) pumping module.

[0002]

2. Description of the Related Art Conventionally, as a technique in such a field,
For example, "Journal of Lightwave
Technology, vol. 8, No. 9 (19
90), P.P. 1350 ~ 1356 Recent Pr
ogress in High-Power GaIn
AsP Lasers ”.

In particular, Section V "Pumpin" in the above paper
g Module For an EDFA ”, and FIG. 12 of this document shows a configuration diagram of an experimental apparatus for measuring amplification characteristics. According to it, an erbium-doped fiber amplifier (EDFA) directly amplifies laser light in the wavelength band of 1.55 μm, which is usually used as signal light, as it is, and as a light source for amplification, a wavelength of 1. Optical output laser light in the 47 to 1.49 μm band (referred to as pumping light) is used. An optical fiber (erbium-doped fiber: ED) to which these lights are added
When incident on F), the laser light of 1.55 μm which is the signal light is amplified. In the following description, all laser light is laser light from a semiconductor laser, and the semiconductor laser is simply referred to as a laser.

FIG. 3 is a block diagram of a system for measuring the gain of such a conventional erbium-doped fiber amplifier. As shown in this figure, the part 1 surrounded by the double-lined frame
0 is a pumping module. Oscillation wavelength 1.55
Signal light from a μm distributed feedback semiconductor laser (DFB laser) 1 passes through a single mode fiber 3 via an attenuator (ATT) 2 and a dielectric multilayer filter (DM).
F) Enter 4. Here, the attenuator 2 is inserted for an experiment in order to change the output of the input light.

Further, the pumping light is a pump (Pum).
p) 1 and pump (Pump) 2 semiconductor laser 6 to polarization maintaining fiber (PMF) 7, beam splitter (PBS) 8, dielectric multilayer filter (DMF) 4 to erbium-doped fiber (EDF) 12 It is incident. The signal light is amplified by the pumping light inside the EDF 12. Usually, pumping light of 50 mW or more is required to obtain optical amplification efficiency of 30 dB or more,
The higher the output, the better the amplification factor.

In this figure, 5 and 14 are collimators, 11 and 13 are isolators, 15 is a dielectric multilayer film filter, and 17 is a spectrum analyzer.
Here, the amplification characteristic when 75 mW pumping light is used is described, and an amplification factor of 30 dBm or more is obtained for 0 dBm (1 mW) signal light.

Two lasers are used to use the pumping light of the optical output. The reason for this is described below, and the method will be described. The output of a high-power laser with a wavelength used for pumping light is about 200 mW at the maximum, but in practice it must be used for tens of thousands of hours.
Must be used at 75% or less of maximum output. Moreover, only about 50% of the light from the semiconductor laser 6 enters the PMF 7. Furthermore, considering the loss in PBS8, DMF4, etc., the output that can directly contribute to the amplification is 50 mW or less. For this reason, it has been devised to use more than one element.

Here, the laser beams of the two semiconductor lasers 6 and 6 are combined with a minimum loss, and the EDF is combined.
12 is used. The propagation mode of the laser light is TE (Transverse Electror).
ic) mode and TM (Transverse Magn)
normal semiconductor laser is TE
Mode oscillation.

Here, the semiconductor lasers 6 and 6 of the pump 1 and the pump 2 have the same wavelength band (1.47 to 1.49 μm).
Is a device that oscillates in the TE mode, and the PMFs 7 and 7 convert the oscillation mode of one laser to the TE mode while the other is converted to the TM mode, and combine it with the PBS 8 to generate E.
It is made incident on the DF12. This allows TE
The loss due to coupling is smaller than that when two mode lasers are used, and the light can be incident on the EDF 12.

[0010]

However, in the method described above, the polarization-maintaining fiber (PMF) 7, 7 is used to couple the light from the two lasers. The length is 1 to 2 m, and there is a problem when a module from the pumping light source to the EDF 12 (EDFA pumping module) is downsized.

In the present invention, since the polarization maintaining fiber described above must be used, in order to eliminate the problem that it cannot be miniaturized, light from two lasers is efficiently coupled without using the polarization maintaining fiber. It is an object of the present invention to provide a pumping module for an erbium-doped fiber optical amplifier that is compact and has high amplification efficiency.

[0012]

In order to achieve the above object, the present invention provides a pumping module for an erbium-doped fiber optical amplifier, which comprises a first semiconductor laser oscillating in TE mode and an oscillation in TM mode. A pumping light source for arranging the second semiconductor laser, and a beam splitter for irradiating and combining the light from the first and second semiconductor lasers without using a fiber are provided.

[0013]

According to the present invention, as described above, in the pumping module of the EDFA, the polarization-maintaining fiber that changes the polarization mode of the laser light is removed, and the two laser lights are directly input to the coupled erbium-doped fiber. It is something that is done.

In this method, a semiconductor laser in which the oscillation mode of one laser is the TE mode and the other one oscillates in the TM mode is used. A normal semiconductor laser is
The TE mode laser oscillates, and this TE mode laser is a device used conventionally, for example, a VIPS laser (V-
grooved inner stripease
rs on p-type substates). For the TM mode laser, a strained quantum well structure is used in the active layer portion of the semiconductor laser.

Therefore, unlike the prior art, it is possible to efficiently combine two laser beams and enter the dielectric multilayer filter without using a polarization-maintaining fiber. As a result, the pumping module of the erbium-doped fiber optical amplifier can be made very small.

[0016]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a block diagram of a pumping module of an erbium-doped fiber optical amplifier according to an embodiment of the present invention. As shown in this figure, a semiconductor laser that oscillates in a TE mode (hereinafter referred to as TE mode laser) 21 and a semiconductor laser that oscillates in a TM mode (hereinafter referred to as TM mode laser) 22 are used. The laser light is directly incident on the beam splitter (PBS) 24 by using 23 or the like.
Although the case where the condenser lens 23 is used is shown here,
The method is not limited to this as long as the method does not use the polarization maintaining fiber, and the light may be incident on the PBS 24 by another method. The pumping light from the beam splitter 24 enters the dielectric multilayer filter 25. Further, 26 is an input port for signal light, and 27, 3
0 is a single mode fiber, 28 and 29 are collimators for aligning the positions of signal light and pumping light, 3
Reference numeral 1 is an output port for signal light.

These semiconductor lasers 21 and 22 are elements having an oscillation wavelength of about 1.47 .mu.m to 1.49 .mu.m. InP single crystal is used as a material crystal for a substrate, and InP is provided on the substrate.
A mixed crystal thin film such as GaInAsP is epitaxially grown. As the TE mode laser, a conventional VIPS laser or the like is used. Since TM mode oscillation cannot be obtained by a laser having a conventional structure, a semiconductor laser having a structure described below is used.

The TM mode laser used in the present invention has a strained multiple quantum well structure (Strained Multi) in the active layer.
apple Quantum Well: Strain
ed-MQW) and strain is 2 parallel to the crystal growth direction.
Design to have axial tensile stress. The amount of strain is such that crystal defects do not enter due to lattice mismatch.

FIG. 2 is a schematic diagram of a crystal for explaining the lattice distortion of the TM mode laser, and FIG. 2 (a) shows the lattice constant a.
The case where a crystal having a lattice constant b is grown on the crystal is shown, and ε is a strain amount, which is a difference in lattice constant. Usually, when the thickness of the growing crystal is less than a certain thickness (critical film thickness), as shown in FIG. 2B, the lattice constants become the same in a plane parallel to the growth direction, The length in the growth direction changes. Here, in the case of growing a material having a small lattice constant, the growth direction is contracted to b ', and tensile stress is applied in the direction indicated by the arrow.

FIG. 2C is a sectional view of a crystal structure for explaining the strained MQW structure. The barrier layer 42 having a large band gap and the well layer 43 having a smaller band gap are formed on the InP substrate 4
A double heterostructure sandwiched between 1 and the cladding layer 44 is shown. The thickness of each of the barrier layer 42 and the well layer 43 is about 10 nm. The composition and thickness are appropriately controlled to obtain the desired oscillation wavelength. If wells having different lattice constants are used, the difference in lattice constant causes strain and stress is generated. This stress changes the band structure of energy. In particular, the band structure of the valence band is significantly changed, and the degeneracy of light holes and heavy holes can be resolved.

In a normal laser, a laser having a quantum well structure without strain, and a quantum well laser having compressive strain, the transition from heavy holes to the conduction band is dominant, and the laser oscillates in the TE mode. However, when tensile stress is applied, the transition between light holes and conduction band electrons becomes dominant, and the TM mode oscillates. In the present invention, when this tensile strained quantum well structure is used, InGaAsP is used as the material of the quantum well layer. Conventionally, InGaAs is used for the InP strained quantum well structure, and in this case, the amount of strain changes depending on the In composition. However, since the amount of strain and the band gap energy are related to each other, that is, the strain and band gap energy are uniquely determined for a certain composition of In. Therefore, the degree of freedom is reduced when controlling the required oscillation wavelength.

When InGaAsP is used, strain and bandgap energy can be controlled independently, so that it can be freely controlled within a composition range suitable for high output. The present invention is not limited to the above-mentioned embodiments, and various modifications can be made based on the spirit of the present invention, and these modifications are not excluded from the scope of the present invention.

[0023]

As described above in detail, according to the present invention, the TE mode laser and the TM mode laser are used as the high output light source of the pumping module for the erbium-doped fiber amplifier. As described above, it is possible to efficiently combine two laser beams and enter the dielectric multilayer filter without using the polarization-maintaining fiber.

As a result, the pumping module of the erbium-doped fiber optical amplifier can be made very small.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a pumping module of an erbium-doped fiber optical amplifier showing an embodiment of the present invention.

FIG. 2 is a schematic diagram of a crystal for explaining lattice distortion of a TM mode laser showing an example of the present invention.

FIG. 3 is a block diagram of a system for measuring the gain of a conventional erbium-doped fiber amplifier.

[Explanation of symbols]

 21 TE mode laser 22 TM mode laser 23 Condensing lens 24 Beam splitter 25 Dielectric multilayer film filter 26 Signal light input port 27,30 Single mode fiber 28,29 Collimator 31 Signal light output port 41 InP substrate 42 Barrier layer 43 well layer 44 clad layer

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI technical display location H01S 3/10 Z 8934-4M 3/18 (72) Inventor Makoto Jouma 1 Toranomon, Minato-ku, Tokyo 7-12 Oki Electric Industry Co., Ltd.

Claims (3)

[Claims] 1. A pumping light source in which a first semiconductor laser oscillating in a TE mode and a second semiconductor laser oscillating in a TM mode are arranged, and (b) the first semiconductor laser and the second semiconductor laser. A pumping module for an erbium-doped fiber optical amplifier, comprising: a beam splitter for entering and combining light from a semiconductor laser. 2. The pumping module for an erbium-doped fiber optical amplifier according to claim 1, wherein the second semiconductor laser has a structure including a GaInAsP strained quantum well having tensile strain in an active layer. 3. A pump for an erbium-doped fiber optical amplifier according to claim 1, wherein the light from the first semiconductor laser and the light from the second semiconductor laser are incident on the beam splitter through a condenser lens. module.