Real-time chemical sensing using stimulated Raman scattering in nanofibers

Abstract

A system includes a laser (1) operative to emit a light beam, a beam splitter (2) arranged to split the light beam into a first beam and a second beam, the first beam being directed to a nonlinear converter (8) that generates a signal beam having a Stokes-shifted wavelength, a recombiner (9) arranged to recombine the signal beam with the second beam to form a recombined beam which is directed to a tapered optical fiber (5) located within a material to be monitored, and a detector (7) arranged to detect light emitted by the tapered optical fiber (5) and which uses stimulated Raman spectroscopy to detect a chemical in the material.

Claims

1. A system comprising: a laser (1) operative to emit a light beam; a beam splitter (2) arranged to split said light beam into a first beam and a second beam, said first beam being directed to a nonlinear converter (8) that generates a signal beam having a wavelength shifted by a Stokes shift; a recombiner (9) arranged to recombine said signal beam with said second beam to form a recombined beam which is directed to a tapered optical fiber (5) located within a material to be monitored; and a detector (7) arranged to detect light emitted by said tapered optical fiber (5) and which uses stimulated Raman scattering spectroscopy to detect a chemical in said material.

2. The system according to claim 1, wherein a frequency of said signal beam equals a frequency of said laser (1) minus the Stokes shift, and said second beam has a frequency equal to said laser frequency.

3. The system according to claim 1, further comprising an optical modulator (4) arranged to modulate said signal beam with a modulation before recombining with said second beam, said modulation being transferred to said second beam.

4. The system according to claim 1, further comprising a filter (10) that filters out said signal beam prior to said detector (7).

5. The system according to claim 1, further comprising a demodulation system (6) arranged to detect amplitude of energy transfer of a signal generated by said detector (7).

6. The system according to claim 1, wherein said nonlinear converter (8) is based on stimulated Raman scattering wherein a nonlinear medium of said nonlinear converter (8) is identical to said chemical in said material, said nonlinear medium being of sufficient concentration to generate stimulated Raman emission.

7. The system according to claim 2, further comprising a phase modulator (11) arranged to adjust the said Stokes shift.

8. The system according to claim 2, further comprising heat transfer apparatus (12) arranged to heat or cool said nonlinear converter (8) and adjust said Stokes shift.

9. The system according to claim 6, further comprising another medium different than said nonlinear medium that causes the same Stokes shift as said chemical in said signal beam.

10. The system according to claim 1, wherein said tapered optical fiber (5) comprises a grating for blocking any undesirable wavelength.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:

(2) FIG. 1 is a simplified illustration of a tapered fiber used in an embodiment of the invention, wherein light is evanescent in the tapered region and is in direct contact with the environment.

(3) FIG. 2 is a simplified illustration of a system for real-time chemical sensing using stimulated Raman scattering in nanofibers, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

(4) Reference is now made to FIG. 1, which illustrates a tapered fiber used in an embodiment of the invention.

(5) In regions a and c laser light is confined in the single mode fiber. The fiber's diameter is then adiabatically reduced so that it reaches subwavelength dimension in the region b. Then the wave d is partially confined within the fiber and partially outside the fiber, propagating within the surrounding medium and interacting with it.

(6) Thus, light is coupled to a standard single mode fiber that is then tapered down to a few hundreds of nanometers. In the thin tapered region, light is partly outside the fiber itself, and propagates in fact within the medium surrounding the fiber. It therefore can interact with this medium though linear or nonlinear interaction.

(7) A critical aspect of the prior art is the requirement of two lasers: one for the pump and one for the signal. The present invention provides a solution for eliminating this constraint, as is now described with reference to FIG. 2.

(8) In one embodiment, it is assumed that one is interested in monitoring a given compound that might be found only in traces within the surrounding medium of the fiber. The Raman shift of this compound is well-known, so that two laser wavelengths (pump wavelength and signal wavelength) can be perfectly defined in order to generate the SRS signal.

(9) Light exits from a laser 1 and impinges on a beam splitter 2. One of the beams (the “signal” beam) is directed towards a cell filled with the compound to be detected, through a tapered fiber 3. The signal beam then goes through an optical modulator 4 and is recombined with the initial beam in a recombiner 9, in which the combined beam goes through a cell which contains a tapered fiber 5 in the environment to be tested. One of the beams is filtered out and light is detected by a detector 7. The electrical signal is sent to a demodulation system 6 that detects the amplitude of the energy transfer.

(10) In summary, as opposed to the prior art, only one laser is used and the second wavelength beam is derived through nonlinear interaction of this laser. In order to do so, light from the laser 1 is split into two beams. The strong beam is sent to a wavelength convertor 8 that generates the signal beam whose frequency equals the laser frequency less the Stokes shift. The other beam is defined as the pump beam (whose frequency is the laser frequency). The signal beam is then recombined with the pump beam and both are coupled within the tapered fiber (of FIG. 1) that is located within the medium to be monitored (as seen in FIG. 2).

(11) A spectral filter 10 can be used to remove the signal beam (it can be integrated within the fiber or external to it) and only the pump is detected on the detector 7.

(12) A modulator 4 may be positioned before both beams are recombined in order to modulate the signal beam. This modulation is then transferred to the pump beam and the modulation amplitude is detected using a lock-in demodulation scheme.

(13) In a preferred embodiment, the nonlinear converter 8 is based on stimulated Raman scattering where the nonlinear medium is the same compound as the one that is to be monitored. The difference is that in this cell, the nonlinear medium is at a high concentration level so that stimulated Raman emission can be generated from spontaneous Raman emission.

(14) Since the concentration levels might be very different in the two cells, the Raman shifts might be slightly different, and no energy transfer would be observed. In order to compensate for this possible difference several compensation schemes are possible:

(15) 1) A phase modulator 11 can be introduced before or after the amplitude modulator, or even better, integrated within the amplitude modulator, in order to generate a compensating frequency shift.

(16) 2) The cell in nonlinear converter 8 can be heated or cooled by heat transfer apparatus 12 so that the temperature dependence compensates for the concentration dependence.

(17) 3) A different compound that has exactly the required Stokes shift as the one in the monitored cell is put inside the nonlinear conversion cell.

(18) The compound might be diluted within another medium which will act as the Raman medium and generate the stimulated conversion if its Raman cross-section is stronger than the compound itself. In order to reduce this adverse effect, a grating can be inscribed in the fiber so that the undesirable wavelength is blocked or ejected from the fiber, so as to block the building up of the stimulated emission. Such a grating can be a long period grating or a slanted grating.

(19) The SRS process is cumulative, meaning that the longer the interaction length the higher the conversion rate. Tapered fibers are limited in length because of their fragility (about a few centimeters maximum length). The SRS effect can be reinforced by splicing together several tapered fibers so that the cumulated length can be much longer than a few centimeters, and the stimulated Raman conversion will be much stronger.