JPS63202085A - Optical fiber raman soliton laser - Google Patents

Abstract

PURPOSE:To obtain high output light soliton in a specific range which is the lowest loss band of silica optical fiber by adequately combining the pumping source and the optical fiber. CONSTITUTION:With combination of an optical fiber 25 where P2O5 is doped to the core and a YAG laser 21, shift of wavelength of Stokes light can be set large. Therefore, the Stokes light of 1.5mum band ca be obtained with the direct pumping source of the YAG laser 21. Accordingly, a pumping source of large output can be used and the soliton output, several times larger than that of the color center laser, can be realized. Moreover, since the erbium laser 21 provides the oscillation wavelength of about 1.54mum and an output almost equal to that of YAG laser. With combination of the erbium laser 21 and GeO2 doped quartz core of pure quartz core fiber, the Stokes light can be obtained in the vicinity of 1.65mum. The silica optical fiber shows sufficiently low loss even in the wavelength of 1.65mum and therefore it can sufficiently be put into the practical use.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical fiber Raman soliton laser that has an optical pulse width of several picoseconds or less and that is applicable to high-speed optical communications and that generates optical solitons. It is something.

[Prior Art] The basic structure of this type of optical fiber Raman soliton laser has already been described in the literature (for example, ■ Nakazawa, ``Soliton Laser'', Solid State Physics).

Vol, 21. No, 9. pp, 47-57(1
986), ■M, N.

Islam, L.F., Molcnauer,
” Fiber Ramanamplificatio
n 5oliton 1aser,'
XIV International Conference
Rence on Quantum Electron
ics.

Digest of Technical Pa
pers, TUIII, San Fran-ci
sco, 1986).

First, we will give an overview of optical solitons.

When a light pulse whose time width (hereinafter referred to as pulse width) is about 1 picosecond or less and whose peak power is about OmW or less propagates through an optical fiber, the pulse width generally becomes large.

This is due to the effect of so-called group velocity dispersion, in which the propagation speed of light depends on the wavelength of light. If the light emitted from the light source is pulsed, sidebands will occur even with an ideal light source, so this effect cannot be avoided, and if the intensity of the propagating light increases to, for example, about 100 mW or more at its peak value, The refractive index of the portion (called the core) of the optical fiber through which the light passes is a function of the light intensity (called the optical power effect), and propagation is different from the propagation of the light pulse described above. The optical force effect has the effect of shifting the wavelength of the light in the rising part of the optical pulse to the longer wavelength side and the wavelength of the light in the falling part of the optical pulse to the shorter wavelength side, and is called self-phase modulation of light. The quartz-based optical fiber that is used in
The group velocity dispersion becomes zero in the m-band, and the longer wavelength side is called the anomalous dispersion region, and in this region, the longer the wavelength, the slower the propagation speed. From the above, it can be understood that when a strong optical pulse is propagating in the anomalous dispersion region, the self-phase modulation effect and group velocity dispersion are balanced and the pulse width does not change. Such a light pulse is called an optical soliton, and its time waveform is a second hyperpublic.
) is known to have the form

Next, we will provide an overview of stimulated Raman scattering.

When a light pulse or continuous light having a peak intensity of about IW or more is incident and propagated in an optical fiber, light having a frequency different from that of the light that has entered the optical fiber is generated.

This is because the incident light is modulated by the natural vibration of the grating that constitutes the optical fiber. When the light intensity is high, the modulated and generated light will contribute to the vibration of the grating, leading to an oscillation state. In the case of an optical fiber, the newly generated light has a wavelength longer than the wavelength of the incident light and is called Stokes light. In this state, the optical fiber is equivalent to a medium that has a gain at the Stokes wavelength.

Here, a basic configuration diagram of a conventional optical fiber Raman soliton laser is shown in FIG.

In FIG. 2, 11 is an excitation light source that generates a light pulse train, 12 is a dichroic mirror as a light coupler, and 1
3 is an optical fiber, 14.15 and 16 are lenses, 17
, 18, 19 and 110 are rays.

Here, the propagation path of light is between the optical fiber 13 and the lens 26.
and 27 and the dichroic mirror 12 form an optical closed loop. This dichroic mirror 12 is an optical coupler that inputs light into the optical closed loop or extracts light from the optical closed loop.

The excitation light source generates light pulses at regular time intervals,
The train of optical pulses is made to enter a closed optical loop via a dichroic mirror 12 serving as an optical coupler. Stimulated Raman scattered light pulses are generated in the optical fiber 12 in the optical closed loop by the incident light pulses. Here, the optical path length of the optical closed loop is adjusted so that the stimulated Raman TFK random pulse light propagates through the optical fiber 12 and reaches the dichroic mirror 12, and at the same time, other light pulses enter the dichroic mirror 12. I'll keep it.

In FIG. 2, the excitation light source 11 is a color center laser, which generates pulsed output light with a pulse width of about 10 picoseconds and a peak value of about 100 W. Although the wavelength of this output light is variable, it is, for example, 1.45 μm. The dichroic mirror 12 reflects at a wavelength of 1.45 μm, and reflects at a wavelength of 1.55 μm.
It has a wavelength dependence of 30% reflection at m. The output light beam 17 from the excitation light source 11 is guided by a lens 14 to a dichroic mirror 12, and the reflected light beam 19 here is guided via a lens 15 to an optical fiber 13.

The optical fiber 13 is a positive mode polarization maintaining fiber with a length of 500 m, and has a zero dispersion wavelength around 1.3 μm. When the optical pulse from the excitation light source 11 described above propagates through the optical fiber 13, a self-phase modulation effect and stimulated Raman scattering occur, but since the pulse width is wide, the self-phase modulation effect is small.

The excitation light and Stokes light propagate through the optical fiber 13, and the light 45110 collected by the lens 16 enters the dichroic mirror 12 again. The excitation light is reflected by this mirror 12 and emitted to the outside of the optical system as a light beam 18, but a considerable portion of the Stokes light re-enters the optical fiber 13 from the mirror 12 via the lens 15. By adjusting the optical pulse excitation speed or optical path length of the excitation light source so that other optical pulses from the excitation light source 11 are incident at the same time as this re-injection, the Stokes light is transferred to the optical fiber 1.
3 and is amplified by the excitation light.

In a steady state, a Stokes light pulse train with a pulse width and intensity determined by the intensity of the excitation light source 11, the optical loss of the optical system, and the optical path length of the optical system can be taken out of the optical system through the dichroic mirror 12. When the intensity of the Stokes light in the optical fiber 13 becomes large enough to cause a self-phase modulation effect and reach a level that balances the effect of group velocity dispersion, the Stokes light becomes the aforementioned optical soliton. The gain bandwidth of stimulated Raman scattering is as wide as several 1 or+m, and the propagation speed varies depending on the wavelength, so the wavelength of the optical soliton can be varied by changing the optical path length.

In the conventional optical fiber Raman soliton laser, the optical fiber is an optical fiber with a GeO2-doped quartz core or a pure silica core, so the Stokes light has a wavelength of about 45 nm on the long wavelength side.
0 cm-' shift (the shifted view is expressed as a difference in reciprocal wavelengths, and the unit cm-' is called Kaiser).

[Problems to be solved by the invention] On the other hand, the minimum loss wavelength of silica-based optical fiber is 1.55 μm.
Since it is located nearby, in order to obtain Stokes light at this wavelength, the excitation light source wavelength needs to be about 1.45 μm. A color center laser is the only excitation light source with such an oscillation wavelength, but in order to generate optical fiber Raman solitons using such a color center laser, a special laser that is not available for -S is required. Moreover, such a color center laser has the drawbacks of a small output power and a narrow wavelength tunable range.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an optical fiber Raman soliton laser having a large output in the 1.5 μm and 1.6 μm bands, which are the lowest optical loss bands of silica-based optical fibers.

[Means for Solving the Problems] In order to achieve such an object, the first form of the present invention is such that a light propagation path forms an optical closed loop, and the optical closed loop connects an optical fiber to the optical closed loop. The light source generates optical pulses at regular intervals, and the optical pulse train is input from the optical coupler into the optical closed loop. generates a stimulated Raman scattered light pulse in the optical fiber, and at the same time that the stimulated Raman scattered light pulse propagates in the optical closed loop and reaches the optical coupling part, another light pulse in the optical pulse train is transmitted to the optical coupling part. In an optical fiber Raman soliton laser in which the optical path length of the optical closed loop is adjusted so that the light enters the laser beam, the light source has a wavelength of 1.32 μm or 1.34 μm.
It is a YAG laser that oscillates at
It is characterized by containing 2O5.

In a second aspect of the present invention, the light propagation path forms an optical closed loop, the optical closed loop includes an optical fiber and an optical coupler for inputting light into the optical closed loop or extracting light from the optical closed loop, and the light source A light pulse is generated at a fixed time interval from In an optical fiber Raman soliton laser, the optical path length of the optical closed loop is adjusted so that at the same time that the optical pulse propagates within the optical closed loop and reaches the optical coupling section, another optical pulse of the optical pulse train enters the optical coupling section. , the light source is an erbium laser, and the core of the optical fiber contains at least one of GeO2, SiO, and F.

[Function] In conventional technology, a color center laser using a special crystal is used as an excitation light source, but since the color center laser uses a YAG laser as an excitation light source, the output is a fraction of that of a YAG laser. The drawback is that only output can be obtained. In contrast, according to the present invention, the amount of wavelength shift of Stokes light can be increased by combining an optical fiber whose core is doped with pzoa and a YAG laser.
1.5 μcm using YAG laser as a direct excitation light source
'! Stokes light at iF can be obtained. Therefore, in the present invention, a pumping light source with a large output can be used,
As a result, a soliton output several times that of a color center laser can be obtained.

In addition, since the oscillation wavelength of the Elbum laser is around 1.54 μm and the output is comparable to that of a YAG laser, by combining the Elbum laser with a GeO2-doped quartz core or pure silica core fiber, the
Stokes light is obtained near m. Since a silica-based optical fiber has a sufficiently low loss even at 1.65 μm, the present invention has sufficiently high practical utility value when using an erbium laser in this manner.

[Example] The present invention will be described in detail below with reference to the drawings.

FIG. 1 shows the basic configuration of a first embodiment of the present invention, where 21 is a YAG laser as an excitation light source, and its oscillation wavelength is 1.32 μm or 1.34 μm.

22 is an optical pulse compressor. This optical pulse compressor 22
can be constructed from an optical fiber whose male dispersion is greater than the excitation light source wavelength and an inverse dispersion medium such as a grating pair. Note that if the pulse width of the mode-locked output of the YAG laser is 30 picoseconds or less, such optical pulse pressure l1rA device 22 is unnecessary, but if it is about 100 picoseconds, it is necessary. Reference numeral 23 denotes a dichroic mirror, which has a reflection characteristic of reflecting light at a wavelength of 1.5 μm or less, and reflecting light at a wavelength of i, sμI or more at a rate of 30% or less. 24 is an optical fiber, the core of which contains 10 mol% of P2O in addition to 5i02.
The above is included. 25, 26 and 27 are an optical pulse compressor 22 and a dichroic mirror 23, respectively.
This is a lens disposed between the dichroic mirror 23 and both end faces of the optical fiber 24.

Here, the propagation path of light forms an optical closed loop, and the optical closed loop includes an optical fiber 24 and lenses 26 and 27 and an optical coupler for inputting light into the optical closed loop or extracting light from the optical closed loop. dichroic mirror 23. The excitation light source 21 generates light pulses at regular time intervals, the light pulse train is made to enter the optical closed loop from the optical coupler 23, and the entered light pulses generate Lffii Raman scattered light pulses in the optical soya bar 24. . This stimulated Raman scattered light pulse propagates within the optical closed loop and reaches the optical coupling section 23 . At the same time,
The optical path length of the optical closed loop is adjusted in advance so that the other optical pulse of the optical pulse train is incident on the optical coupling section 23.

When the light from the excitation light source 21 enters the optical fiber 24,
Since the excitation light intensity is sufficiently greater than the threshold of stimulated Raman scattering, Stokes light is generated and propagates through the optical fiber 24. The peak value of the optical pulse can easily be several kW or more.

Since P,05 and 5h02 exist in the core of this optical fiber 24, the wavelength of the Stokes light due to 5in2 is generally shifted by about 450 cm-', and I'20
The Stokes light due to 5 is shifted by about 1320 cl'. Length 600 containing 14.2 mol% r'2o in the core
When output light from a YAG laser with a wavelength of 1.32 μm is input into an optical fiber of 1.32 μm and the intensity of the input light is increased, Stokes light is observed at the output end of the optical fiber. Here, when the incident light intensity is less than 70W, Stokes light due to 5i02 is dominant, but when it is more than 70W, P2O
, the Stokes light due to , becomes dominant (in.

5uzuki, ni, Noguchi, and N, Ue
su3i, “Selective stimulus
ed Ilaman scattering i
n highly P2O,-doped 5i
lica single-mode fiber
rs”, 0pticsLetters, V
ol, If, pp, 656-1+58 (1986)
reference).

In FIG. 1, the Stokes light that has propagated through the optical fiber 24 has a wavelength of approximately 1.60 μm, and part of it is reflected by the dichroic mirror 23, and the rest is transmitted back to the optical fiber 24.
incident on . At this time, a light pulse from the excitation light source 21 enters the optical fiber 24 at the same time, and the Stokes light is further amplified. Through such an operation, the Stokes optical power due to P2O becomes strong enough to cause a phase modulation effect in a steady state, and an optical pulse can be extracted from the dichroic mirror 23 as an optical soliton.

In a second embodiment of the present invention, in FIG. 1, the excitation light source 21 is an erbum laser, and the optical fiber 24 is an optical fiber containing Gem2, 5in2, or F in its core. In this case as well, optical solitons can be obtained by the same action as in the first embodiment.

In this case, the oscillation wavelength of the erbium laser is 1.5
4 μm, and the amount of shift of Stokes light by Gem, 5in2 or F is about 450cl', so
Stokes light with a wavelength of 1.66 μm is obtained. The operation during this time is similar to that described above with respect to the combination of the YAG laser 21 and the optical fiber 24 having a core doped with P2O, and an optical soliton with a wavelength of 1.66 μm can be extracted.

In addition, the dichroic mirror 23 has a wavelength of 1.8μ.
Reflects light with a wavelength of 1.6 μm or more, and 30% of light with a wavelength of 1.6 μm or more
Those that reflect at the following rate are suitable.

In the above description, an example was shown in which the core contained GeO2, 5i02, or F, but the same effect can be obtained even if a plurality of these materials are used.

[Effects of the Invention] As explained above, in the optical fiber Raman soliton laser, according to the present invention, by appropriately combining the excitation light source and the optical fiber, the lowest loss band of 1, High output optical solitons can be obtained in the 5-1.6 μm band.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of the optical fiber Raman soliton laser of the present invention, and FIG. 2 is a block diagram showing a conventional example. 11... Color center laser serving as an excitation light source, 12... Dichroic mirror serving as an optical coupler, 13... Optical fiber with τ dispersion wavelength around 1.3 μm, 14.15.16... Lens , 17.18,19,110...ray, 21...YA
G laser or erbium laser, 22... Optical pulse compressor, 23... Dichroic mirror, 24... Optical fiber, 25.26.27... Lens.

Claims (1)

[Claims] 1) The propagation path of light forms a closed optical loop, and the closed optical loop includes an optical fiber and an optical coupler for inputting light into the closed optical loop or extracting light from the closed optical loop, A light pulse is generated from a light source at regular time intervals, the light pulse train is made to enter the optical closed loop from the optical coupler, the incident light pulse generates a stimulated Raman scattered light pulse in the optical fiber, and the stimulated Raman scattering light pulse is generated in the optical fiber. The optical path length of the optical closed loop is such that at the same time that the Raman scattered light pulse propagates within the optical closed loop and reaches the optical coupling unit, another optical pulse of the optical pulse train is incident on the optical coupling unit. In the optical fiber Raman soliton laser, the light source is a YAG laser that oscillates at a wavelength of 1.32 μm or 1.34 μm, and P_2
An optical fiber Raman soliton laser characterized by containing O_5. 2) The propagation path of light forms an optical closed loop, and the optical closed loop includes an optical fiber and an optical coupler for inputting light into the optical closed loop or extracting light from the optical closed loop, and the optical closed loop is connected to the light source at regular time intervals. A light pulse is generated, the light pulse train is made to enter the optical closed loop from the optical coupler, the incident light pulse generates a stimulated Raman scattered light pulse in the optical fiber, and the stimulated Raman scattered light pulse is an optical fiber Raman in which the optical path length of the optical closed loop is adjusted so that another optical pulse of the optical pulse train is incident on the optical coupling unit at the same time that the optical pulse train propagates within the optical closed loop and reaches the optical coupling unit; An optical fiber Raman soliton laser, wherein the light source is an erbum laser, and the core of the optical fiber contains at least one of GeO_2, SiO_2, and F.