JP4381182B2 - Induced Raman spectroscopy method and apparatus - Google Patents

Description

  The present invention relates to a method and apparatus for measuring molecular vibrations of a terahertz band of a biomolecule or a polymer by stimulated Raman spectroscopy using a laser.

  Biomolecules such as DNA and protein, and polymers have a molecular resonance spectrum in the terahertz band. However, when these molecules are in an aqueous solution, since the absorption of terahertz waves by water is large, it is difficult to obtain an absorption spectrum of terahertz waves directly. Water absorption increases with increasing terahertz frequency and is problematic at higher frequencies.

  As a method of observing terahertz vibration in an aqueous solution or organic solution, natural Raman scattering spectroscopy is difficult to measure in the terahertz band due to the presence of florescence.

  In contrast, coherent anti-Stokes Raman (CARS) spectroscopy has been shown to be effective for observing THz band vibrations. However, CARS requires pump light, Stokes light, and anti-Stokes light phase matching. Therefore, the angle between the three beams must be finely matched, and adjustment is required for each sample. The control of the system had to be complicated and subtle.

  In contrast, the stimulated Raman spectroscopic measurement method uses only pump light and signal light and does not require phase matching, so the optical system is simple. However, since the Raman amplification coefficient due to terahertz band vibration of various molecules is small, the pump light intensity has to be extremely increased. Two wavelength-tunable mode-locked Ti-sapphire lasers are used. One is a pump laser and the other is a signal laser. It was difficult.

  The present invention provides a method and apparatus for highly sensitively measuring terahertz band vibrations of biomolecular crystals and molecules in an aqueous solution by stimulated Raman spectroscopy with an extremely simple optical system.

  There are two problems with the stimulated Raman amplification spectroscopy that uses two conventional mode-locked Ti-sapphire lasers. One is that since the mode-lock pulse width τ is a narrow pulse of picosecond or less, the frequency resolution ½πτ becomes 100 GHz or more and the sharp spectrum of the molecule cannot be measured. The second problem is that since there is a fluctuation in the intensity of the incident pulse of signal light, an amplification intensity ΔI smaller than that fluctuation cannot be observed. Usually, observation becomes difficult when ΔI / I is 0.1 (10%) or less.

  In the present invention, a tunable pulse laser having a pulse width within a range of 0.05 ns (50 ps) to 100 ns, such as an optical parametric oscillator (OPO), is used as pump light, while a signal laser is replaced with a pulse laser. A continuous wave (cw) laser is used.

  As the cw laser, a DFB (distributed feedback) laser diode or a DBR (distributed Bragg reflector) laser diode is most preferable because there is no intensity noise due to mode instability. When the output is low, a light source with a laser diode amplifier can be used. A cwYAG laser excited by a laser diode is also suitable because it has low intensity noise and high output.

Since the signal light is not a pulse, the amplification ΔI is observed only during the time when the pump light is present. Since the stimulated Raman effect is an amplification process, ΔI is proportional to the signal intensity I. If the intensity of the cw signal light is large to some extent and low noise, it is possible to detect even a very small amplification degree of ΔI / I of 10 ⁻⁴ or less, so that the sensitivity is 1000 times that of the prior art. Since the resolution can be the pulse width of the pump light, a sharp resolution of 100 MHz or less can be obtained.

  According to the present invention, it is possible to obtain a stimulated Raman amplification spectroscopy method and apparatus capable of observing terahertz band molecular vibration with high resolution and high sensitivity using a very simple mechanism. This facilitates terahertz measurement, especially for molecules in aqueous solution, and is widely used for terahertz spectroscopy of biomolecules, food / pharmaceutical / toxic substance inspection, diagnosis and treatment by terahertz wave irradiation of cancer tissue, etc. can do.

Best Mode for Carrying Out the Invention, Example 1

  In FIG. 1, the pump light source 1 is an OPO, is excited by a triple wave of the YAG laser 2, is tunable, has a pulse width of about 5 ns, and a line width of 3 GHz. The signal light source 3 is obtained by amplifying the output of a DFB or DBR laser diode with a laser diode amplifier. It is a high output single frequency with a cw output of 100 mW or more, and the intensity noise is extremely low.

  There is an aqueous solution 5 of biomolecules in the quartz sample cell 4. FIG. 1 shows the case of backscattering, where signal light and pump light enter the sample cell in a direction close to 180 degrees. Although not shown in the figure, in the case of observing by forward scattering, the angle formed by the signal light and the pump light may be arranged close to 0 degrees.

  By rotating the half-wave plate 6, the polarization direction of the signal light can be made orthogonal or parallel to the pump light. Information on the form of vibration can be obtained from the Raman scattering selection rule by comparing the spectral intensities in the orthogonal direction and the parallel direction. A measured Raman amplification spectrum can be obtained by measuring ΔI or ΔI / I while sweeping the wavelength of OPO.

  The pulse output is integrated using the boxcar integrator 7 to increase the S / N. The grating spectroscope 8 is used as a filter for preventing pump light from becoming stray light and entering the detector and causing noise, and guides only the signal light wavelength to the pulse light detector 9. Reference numeral 10 denotes an isolator for preventing the signal light laser from becoming unstable due to return light and increasing noise.

  In order to obtain a spectrum, the pump light frequency may be swept so that the difference frequency between the frequency (wavelength) of the pump light and the signal light frequency is in the range of 30 THz to 0.1 THz.

  To see the fine structure of the spectrum, a method can be added in which the frequency of the pump light is fixed near the resonance line and the temperature of the DFB or DBR laser diode is changed by a Peltier controller. Since the oscillation frequency of the DFB (DBR) laser diode decreases at a slope of 20 GHz per 1 ° C., a fine structure within an interval of only 20 GHz can be detected by gradually changing the temperature by 1 ° C.

FIG. 2 shows an example of measurement of an induced Raman spectrum in a benzene liquid. The wavelength of the signal light is 976 nm, and an output of 50 mW is incident on the quartz sample cell. The pump light is a pulse of about 4 mJ and a repetition of 10 Hz, and 29.8 THz by sweeping the wavelength variable range of the pump light (OPO) from 189 nm to 889 nm in 0.01 nm steps (3.7 GHz steps). The vibrational spectrum of the benzene molecule at (992 cm ⁻¹ ) is obtained. It can be seen that because of this high resolution, the spectral width is only 54 GHz (1.8 cm ⁻¹ ).

  FIG. 3 shows an induced Raman spectrum of an aqueous solution of trehalose, which is one of the disaccharides, and a terahertz band spectrum peculiar to trehalose is observed between 6 THz and 30 THz. Thus, measurement with an aqueous solution is possible.

  The pump light is OPO as in the first embodiment, but the signal light source is a cwYAG laser (wavelength 1064 nm) excited by a laser diode. Since this laser is excited by a cw laser diode and has low noise characteristics with little fluctuation in output, it meets this purpose. A high output of 300 mW can be obtained.

  FIG. 4 is a measurement example for carbon tetrachloride. The wavelength of OPO is measured from 1042 nm to 1006 nm in 0.5 nm steps, and a spectrum between 6 THz and 16 THz is obtained.

  A pulsed YAG laser excited by a laser diode is used as a pump light source. The pulse output is about 1 mJ, the repetition period is 1 kHz, and the pulse width is 50 ns to 100 ns. Since the pump light wavelength is fixed at 1064 nm, an external resonator type high-power laser diode having a grating is used in order to make the wavelength of the signal light source variable. The laser output is taken out from the diode, and the oscillation wavelength is selected by the grating. Although an output of 100 mW can be obtained, it is desirable to have a double isolator in order to obtain stable operation.

A mode-locked Ti-sapphire laser is used as the pump light. A normal mode-locked Ti-sapphire laser has a pulse width of picoseconds or less, which is 100 GHz or more when converted to a frequency bandwidth, and the spread of the stimulated Raman amplification spectrum is too large, which is not desirable. In particular, the pulse width can be designed to be longer than 50 ps (50 picoseconds) by increasing the length, so that a resolution higher than 2 GHz can be obtained by using such a mode-locked Ti-sapphire laser as a pump light source. it can.
As the signal light, a cwDBR (DFB) laser diode is optimal as in the first embodiment.

  1 is a diagram illustrating a configuration of Example 1. FIG.   It is a figure which shows the example of the induced Raman spectroscopy measurement in a benzene liquid.   It is a figure which shows the example of the induced Raman spectroscopy measurement in a trehalose aqueous solution.   6 is a diagram illustrating an example of stimulated Raman spectroscopy measurement in a carbon tetrachloride liquid having the configuration of Example 2. FIG.

Explanation of symbols

1 ... OPO
DESCRIPTION OF SYMBOLS 2 ... YAG laser 3 ... Signal light source 4 ... Quartz sample cell 5 ... Aqueous solution 6 ... Half-wave plate 7 ... Boxcar integrator 8 ... Grating spectrometer 9 ... Pulse light detector 10 ... Isolator

Claims (4)

A pump laser emitting pump light having a pulse width between 50 ps and 100 ns ;
A signal laser which emits a signal light of a continuous-wave low-noise,
A detector for detecting signal light that has been stimulated Raman-amplified by molecules in the sample to be measured on which the pump light and the signal light are incident.
With the door, at least one frequency between the frequency of the pump light the signal laser beam, by the difference between the frequency of said signal light of the pump light is swept between the 30THz from 0.1 THz, the signal A stimulated Raman spectroscopic apparatus , wherein a Raman amplification spectrum is obtained by detecting a pulse intensity of light with the detector . 2. The stimulated Raman spectroscopic apparatus according to claim 1, wherein the signal laser is a DFB laser diode, a DBR laser diode, an external resonator type tunable laser diode, or a cw laser diode pumped YAG laser. The stimulated Raman spectroscopic apparatus according to claim 1, wherein the pump laser is an optical parametric oscillator (OPO), a Ti-sapphire laser, or a YAG laser.   At least of the pump light and the signal light, the difference between the frequency of the pump light having a pulse width between 50 ps and 100 ns and the frequency of the low noise and continuous wave signal light is between 0.1 THz and 30 THz. One frequency is swept, the pump light and the signal light are incident on the sample to be measured,
  A stimulated Raman spectroscopic method, wherein the signal light detects a pulse intensity obtained by stimulated Raman amplification by molecules in the sample to be measured to obtain a Raman amplified spectrum.

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