Radiation pulse generation

Details Radiation pulse generation Radiation pulse generation

1989-01-31

1990-09-25

Arnold, Bruce Y.

Optics: measuring and testing

Range or remote distance finding

With photodetection

350 9615, 350320, 356 44, 307425, G02B 2700, G02B 626

Patent

active

049589105

DESCRIPTION:

BRIEF SUMMARY
The invention relates to apparatus and methods for generating pulses of radiation.
In the field of optical communications, there is a continuing need to compress optical pulses from those generated by conventional lasers which typically have durations in the order of 100 ps.
In a paper entitled "Fibre Raman soliton laser pumped by a Nd:YAG laser" Optics Letters, vol 12, no. 3, Mar. 1987 pp 181-183, a completely integrated oscillator loop is disclosed which is pumped with 100 ps pulses and provides output pulses as a result of Raman scattering in and optical fibre with pulse widths in the order of 190 fs. One of the drawbacks of this system is the need for an optical loop of closely controlled length and the additional complications of providing feedback.
In accordance with one aspect of the present invention, a radiation pulse generating assembly comprises a radiation generator for generating pulses of radiation; a waveguide into which pulses of radiation at working intensities from the radiation generator are coupled, the waveguide causing the radiation pulses to undergo Ramon scattering to generate reduced width pulses with wavelengths lying within a Raman spectrum; and filtering means into which the reduced width pulses are coupled for selecting reduced width pulses within a desired wavelength band.
In accordance with a second aspect of the present invention, a method of generating pulses of radiation comprises coupling first pulses of radiation at working intensities into a waveguide, the waveguide causing the first pulses to undergo Raman scattering to generate reduced width pulses with wavelengths lying within a Raman spectrum; and selecting those reduced width pulses which have wavelengths lying within a desired wavelength band.
We have discovered surprisingly that pulses with wavelengths within the Raman spectrum have a high degree of coherence so that the phases of the different frequency components are substantially mode locked. Thus in real time the Raman pulse width is given approximately by the inverse of the spectral width. Furthermore, the pulses are extremely stable which is a consequence of the fact that they are non-linear solitary waves which have been amplified and compressed in the generation process.
In contrast to the known arrangement for pulse width reduction described above, our system is a single pass system without feedback, thus simplifying the overall structure. Furthermore, in our system, unlike the known system, it is possible to select pulses with a desired wavelength. The advantage of a single pass system is that the radiation generator can produce an arbitrary sequence of pulses whereas in the previous case the pulses must be generated at a closely controlled frequency.
The waveguide will comprise a non-linear medium in order to cause the incoming pulses to undergo Raman scattering and conveniently the medium should have a broad Raman gain spectrum. The medium should support non-linear solitary waves at the wavelengths within the Raman spectrum.
Preferably, the continuous wave is unstable with respect to the formation of side bands at wavelengths shorter than the Raman gain peak. This condition leads to a lowering of the threshold for the Raman process to begin, thus enabling lower power sources to be used.
Preferably, the filtering means comprises a first filter into which pulses from the waveguide are coupled, the first filter being adapted to pass at least some (preferably all) of the Raman wavelengths but to reject the wavelength of the original pulses from the radiation generator; and a second filter downstream of the first filter with a bandwidth smaller than the width of the Raman spectrum and a centre wavelength tunable over at least part of the range of the Raman spectrum.
Preferably, the assembly further comprises one or more additional waveguides between the one waveguide and the filtering means, the additional waveguide receiving the reduced width pulses and causing the pulses to undergo Raman scattering whereby the pulse widths are compressed.
The invention i

REFERENCES:
patent: 4560246 (1985-12-01), Cotter
patent: 4804264 (1989-02-01), Kirchhofer et al.
patent: 4812682 (1989-03-01), Holmes
patent: 4815804 (1989-03-01), Desurvire et al.
patent: 4823166 (1989-04-01), Hartog et al.
Conference on Lasers and Electro-Optics, 21-24, May 1985, Baltimore, Md., OSA/IEEE (U.S.), pp. 80-81, L. F. Mollenauer et al.: "Experimental Demonstration of Soliton Propagation in Long Fibers": Loss Compensated by Raman Gain.
IEEE Journal of Quantum Electronics, vol. QE-22, No. 12, Dec. 1986, IEEE (New York, U.S.) F. M. Mitschke et al: "Stabilizing the Soliton Laser", pp. 2242-2250.
IEEE Journal of Quantum Electronics, vol. QE-11, No. 13, Mar. 1975 IEEE (New York, U.S.) R. H. Stolen: "Phase-Matched-Stimulated Four-Photon Mixing in Silicafiber Waveguides", pp. 100-103.
Soviet Physics, JEPT, vol. 62, No. 3, Sep. 1985, (New York, U.S.), E. M. Dianov et al: "Generation of Ultrashort Pulses by Spectral Filtering During Stimulated Raman Scattering in an Optical Fiber"; pp. 448-455.
Optics Letters, Mar. 1987, vol. 12, No. 3, Kafka et al: "Fiber Raman Soliton Laser Pumped by a ND:YAG Laser"; pp. 181-183.
Tuesday Poster/Afternoon; Jun. 1986, Islam et al article: "Fiber Raman Amplification Soliton Laser"; pp. 76-77.

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