Raman fiber laser

Details Raman fiber laser Raman fiber laser

2001-02-02

2003-09-23

Lee, Eddie (Department: 2815)

Coherent light generators

Raman laser

C372S006000, C372S102000, C372S099000

Reexamination Certificate

active

06625180

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of laser engineering and fiber optics and is industrially applicable in fiber communication systems for pumping optical amplifiers used in wide-band fiber-optical communication systems. Furthermore, the proposed device may be used as a source of radiation in fields where the spectral-selective action of radiation on a substance in the near infrared range is required, in particular in medicine, and also for diagnosis of the environment, in chemistry.
BACKGROUND OF THE INVENTION
It is well known that the effect of stimulated Raman scattering (the scattering of light on intramolecular oscillations) may be used to amplify optical radiation (see Y. R. Shen, The Principles of Nonlinear Optics, A Wiley-Interscience Publication John Wiley & Sons, New York-Chichester-Brisbane-Toronto-Singapore, © John Wiley & Sons, Inc., 1984, chapter 10, pp. 141-186.
In the specification of the proposed device the concept of Raman amplification factor in a voluminous sample of the material and in a single-mode fiber with a core and cladding of a predetermined makeup is given substantial consideration.
Here and below the Raman amplification factor G
R
is understood to mean the Raman amplification factor in a voluminous material, as it is defined, for example, in Y. R. Shen, The Principles of Nonlinear Optics, A Wiley-Interscience Publication John Wiley & Sons, New York-Chichester-Brisbane-Toronto-Singapore, © John Wiley & Sons, Inc., 1984, chapter 10, pp. 143-146.
The Raman amplification factor in a fiber g
o
is used to describe the amplification properties of fiber in particular. This amplification factor is related to the value G
R
for the material of the core of the fiber by the relationship g
o
=G
R
/A
eff
, where A
eff
is the effective area of the core of the fiber (see, for example, Govind P. Agrawal, Nonlinear Fiber Optics, [Quantum electronics—principles and applications], Academic Press, Inc., Harcourt Brace Jovanovich, Publishers, Boston-San Diego-New York-Berkley-London-Sydney-Tokyo-Toronto, 1989, Chapter 8, pp. 218-228). The dimension of the factor g
o
is 1/(m W) or dB/(km W). In particular, for a number of communication fibers, the value of g
o
was measured experimentally (V. L. da Silva, J. R. Simpson. Comparison of Raman Efficiencies in Optical Fibers. Conference on Optical Fiber Communications, 1994, OFC'94 Technical Digest, WK13, pp. 136-137, 1994) and was ~5 10
−4
1/(mW) or, which is the same, ~2.2 dB/(km W).
Constructions of fiber lasers are known, the action of which is based on the effect of stimulated Raman scattering in a fiber, and which are actually converters of the frequency (or wavelength) of optical radiation. In particular, a Raman fiber laser based on a germanosilicate fiber is presented in the paper (S. G. Grubb, T. Strasser, W. Y. Cheung, W. A. Reed, V. Mizhari, T. Erdogan, P. J. Lemaire, A. M. Vengsarkar, D. J. DiGiovanni, D. W. Peckham, B. H. Rockhey. High-Power 1.48 &mgr;m Cascaded Raman Laser in Germanosilicate Fibers, Optical Ampl. and Their Appl., Davos, USA, 15-17 June 1995, pp. 197-199).
The five-stage Raman laser presented in this paper is designed to convert optical radiation with a wavelength of &lgr;
o
=1.117 &mgr;m (and frequency &ugr;
o
=8950 cm
−1
) into radiation having a wavelength of 1.48 &mgr;m (6760 cm
−1
). GeO
2
is the main impurity dope in the core of the fiber. The pumping source is an ytterbium laser with a generation wavelength of 1.117 &mgr;m. The Raman fiber laser comprises five pairs of distributed Bragg fiber gratings as reflectors for the wavelengths &lgr;
1
=1.175 &mgr;m (&ugr;
1
=8511 cm
−1
), &lgr;
2
=1.24 &mgr;m (&ugr;
2
=8065 cm
−1
), &lgr;
3
=1.31 &mgr;m (&ugr;
3
=7634 cm
−1
), &lgr;
4
=1.40 &mgr;m (&ugr;
4
=7143 cm
−1
), &lgr;
5
=1.48 &mgr;m (&ugr;
5
=6760 cm
−1
), forming respectively 5 resonators enclosed one in another, which comprise a germanosilicate fiber, for the 1st, 2nd, 3rd, 4th and 5th Stokes components of the Raman scattering in a germanosilicate fiber. When pumping radiation is launched into the aforesaid germanosilicate fiber, amplification of the optical radiation in the frequency range shifted toward the long-wave side relative to the pumping radiation by a value of about 450 cm
−1
occurs due to the effect of stimulated Raman scattering. The magnitude of the shift and the bandwidth of the amplification are determined by the characteristics of the oscillations of molecules of the fiber material which is used in the Raman laser as the active medium, in this particular case by the properties of germanosilicate glass. When the value of the amplification of the optical radiation at a frequency of &ugr;
1
reaches some threshold value, laser generation occurs at that frequency. The 1st stage of the Raman laser works in this way. After initiation of generation at the frequency &ugr;
1
, the radiation with the frequency &ugr;
1
will already serve as a pump for the Raman laser, the resonator of which is tuned to the frequency &ugr;
2
, and so on to the fifth conversion stage. Thus, each stage of the Raman laser under consideration shifts the generation frequency by a value of ~450 cm
−1
.
A drawback of this laser is the relatively low efficiency of radiation conversion in the 5th Stokes component, which is due to losses in the optical fiber at wavelengths of the Stokes components because of the large number of stages of conversion as a result of the small value of the Raman shift of the radiation frequency in standard fibers, in particular because of the presence of optical resonators in the shortwave range of the spectrum (in this particular case a 1.175 &mgr;m resonator), where, as is known, the optical losses in a fiber have a large value as compared with the range of about 1.55 &mgr;m.
U.S. Pat. No. 5,323,404, IPC H 01 S 3/30, published Jul. 21, 1994, comprises a disclosure of a similar device. It is noted here that a Raman laser of this type is capable of generating practically any predetermined wavelength due to the large relative width of the Raman amplification band in a germanosilicate fiber. But the device has the same drawback as in the preceding example.
The device most similar to the claimed device is the known Raman fiber laser disclosed in U.S. Pat. No. 5,838,700, IPC H 01 S 3/30, published Nov. 17, 1998, which comprises as the active medium a length of a fiber that comprises an oxide array on the base of SiO
2
, the makeup of which includes phosphorus oxide, a laser with a generation wavelength of 1.0-1.1 &mgr;m as a pumping source, and two refractive index Bragg gratings as distributed reflectors for a wavelength range from 1.20 to 1.28 &mgr;m, forming a resonator for the first Stokes component and two more Bragg gratings as distributed reflectors for a wavelength range from 1.46 to 1.50 &mgr;m, forming a resonator for the second Stokes component. In accordance with the instant invention, at a pumping radiation wavelength of 1.0-1.1 &mgr;m, radiation with wavelengths of 1.2-1.28 &mgr;m and 1.46-1.50 &mgr;m may be obtained in Raman lasers on a phosphosilicate fiber as a result of only one- or two-stage conversion, respectively. As a result, a higher efficiency of conversion may be achieved than in a Raman laser based on a germanosilicate fiber (which is demonstrated, for example, in the paper of E. M. Dianov, I. A. Bufetov, M. M. Bubnov, A. V. Shubin, S. A. Vasiliev, O. I. Medvedkov, S. L. Semjonov, M. V. Grekov, V. M. Paramonov, A. N. Gur'yanov, V. F. Khopin, D. Varelas, A. Iocco, D. Constantini, H. G. Limberger, R. -P. Salathe, “CW Highly Efficient 1.24 &mgr;m Raman Laser Based on Low-loss Phosphosilicate Fiber,” OFC'99, Technical Digest Series, Postdeadline Papers, PD-25, 1999). The spectrum of a spontaneous Raman scattering in a phosphosilicate fiber (see, for example, E. M. Dianov, M. V. Grekov, I. A. Bufetov, S. A. Vasiliev, O. I. Medvedkov, V. G. Plotnich

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