JP2001194299A - Cavity ring-down spectral system and method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably lock laser and a ring-down cavity for CRDS with high reliability. SOLUTION: A cavity ring-down spectral system has a sample holding cavity, a light source containing laser incident on the cavity and simultaneously generating locking beam and sampling beam one of which is subjected to S-polarization in the cavity and the other one of which is subjected to P-polarization, a locking device for locking the cavity and the light source using the locking beam reflected by the cavity and a sampling device for detecting the sampling beam emitted from the cavity and determining a ring-down ratio showing the absorption of the sampling beam by a sample.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to cavity ring-down spectroscopy (CRDS), and more particularly to laser and ring-down cavities for CRDS.
cavity locking system and method.

[0002]

2. Description of the Related Art Conventional spectroscopy has a sensitivity of about 100.
The limit was from 1/00 to about 1 / 100,000.
In the conventional CRDS system, the sample (absorbing material)
Are placed in a high precision, stable linear optical resonator. The intensity of the light pulse introduced into the resonator decreases with time. For an empty cavity, the light intensity within the cavity follows a logarithmic decay characterized by the reflectivity of the mirrors, the distance between the mirrors, and the ring-down rate that depends only on the speed of light within the cavity. If the sample is placed in the resonator, the ringdown will be accelerated. Under appropriate conditions, the light decay is still logarithmic.
The absorption spectrum for the sample is obtained by plotting the reciprocal of the ring-down rate against the wavelength of the incident light.

[0003] Conventional pulsed CRDS systems have faced many challenges. In systems using pulsed lasers, the rate of data acquisition was limited by the repetition rate of the pulsed laser source. In addition, the relatively small spectral overlap between the cavity mode and the laser spectral width, and the insignificant light that is integrated into the cavity is insufficient to allow the light introduced into and extracted from the cavity The strength was small.

[0004]

SUMMARY OF THE INVENTION It is a primary object of the present invention to provide a system and method for stably and reliably locking a laser and a ring-down cavity for CRDS. Another object is to provide a CR that allows continuous locking of the laser and ringdown cavity over multiple ringdown measurements.
To provide a DS system. Yet another object is to provide a CRDS system that allows for fast repeated measurements. Another object is to provide a CRDS system in which the frequencies of the locking light and the sampling light are different so that the locking light does not affect the measured value obtained using the sampling light.
Yet another object is to provide a CRDS system that reduces the change in ring-down constant from shot to shot. Another object is to reduce the baseline noise of the CRD.
To provide an S system. Yet another object is to provide a laser and ring-down cavity that can be locked without the need for an external reference cavity.
To provide a DS system. Another object is to provide a CRDS system that can reduce optical feedback from a ring-down cavity to a laser. Yet another purpose is CRDS
It is an object to provide a system which allows a relatively simple coupling of the locking light leaving the cavity to a locking device.

[0005]

SUMMARY OF THE INVENTION The present invention provides a CRDS system that locks light using unique sampling. The sampling light is used to make ringdown measurements, while the locking light is used to lock the frequency of the ringdown resonant cavity and CW laser. The sampling light is turned on and off to make ring-down measurements, while the locking light is maintained continuously. With continuous locking, faster repetitive measurements can be achieved, resulting in lower noise levels. The locking light and the sampling light have different frequencies to effectively separate within the cavity and to ensure that the locking light does not affect ring-down measurements made with the sampling light.

[0006] In a preferred embodiment, the ring-down cavity is a ring-shaped cavity. In a given mode, the ring cavity has separate resonance frequencies and mirror losses for S-polarized light and P-polarized light. The loss in the cavity is smaller than for S-polarized light.
It is preferable that the sampling light be S-polarized with respect to the cavity while the locking light be P-polarized while the sampling light is high with respect to the P-polarized light. The use of a ring-shaped cavity provides the desired separation between the resonance frequencies of a given mode and tunes both the sampling light and the locking light to the same cavity without coupling the sampling light and the locking light. Can be done. Further, the use of the ring-shaped cavity has an effect that optical feedback to the laser can be reduced.

[0007] At least one of the locking light and the sampling light is frequency shifted to ensure decoupling within the cavity. Acousto-optic modulator (A
OM) is preferably arranged in the path of the sampling light. This AOM is used to make the frequencies of the sampling light and the locking light different. AOM
Is also used as a switch for turning on and off the sampling light in order to perform ring-down measurement.

[0008] In another aspect, the ring-down cavity may be straight or folded. At that time, the sampling light and the locking light are in the same transverse mode (for example, TE
M00). Therefore, the frequency shift between the sampling light and the locking light is equal to the free spectral range of the cavity.

[0009]

FIG. 1 is a schematic diagram illustrating a preferred system of the present invention. FIG. 2 is a schematic diagram illustrating the optical components of the system of FIG. FIG. 3 shows a locking member of the system of FIG. FIG. 4 is a view showing a locking member according to another embodiment of the present invention. FIG. 5 shows another embodiment of the present invention that uses a straight ring-down cavity. FIG. 6 is a diagram showing the cavity frequency response to P-polarized light and S-polarized light for the system according to one embodiment of the present invention. FIG. 7 is a diagram illustrating a measured value of a spectral density that fluctuates due to a frequency difference in relation to a frequency difference in a system having the characteristics of FIG. 6. FIG. 8 shows a spectrum of atmospheric water vapor recorded using a system having the features of FIG.

The following description shows an embodiment of the present invention, but the present invention is not necessarily limited to this embodiment. In the following description, unless otherwise specified, the polarization direction is assumed to be relative to the ring-down resonant cavity of the system. Also, unless otherwise specified, the term "ring down" will be interpreted to include both light attenuation and build-up in the resonant cavity.

FIG. 1 shows a preferred system 20 of the present invention.
FIG. Optical connections are indicated by dashed lines and electrical connections are indicated by solid lines. The system 20 includes a tunable monochromatic light source 22 and a ring-down resonant cavity 24 optically connected to the light source 22.
And a sampling detection device 26, a monitoring (diagnosis) device 28 and a locking device 30 optically connected to the cavity 24. The locking device 3
0 is electrically connected to the light source 22 and the cavity 24, and the monitoring device 28 is electrically connected to the sampling detection device 26.

The cavity 24 holds a target sample on an optical path in the cavity. The light source 22 generates a continuous wave input sampling light 32a and a locking light 32b incident on the cavity 24. The input sampling light 32a has a wavelength within the target absorption region of the sample. The input locking light 32b is the sampling light 3
2a has a predetermined wavelength. The sampling light 32a and the locking light 32b have different frequencies in the cavity 24. The sampling light 32a is S-polarized, and the locking light 32a
b is preferably P-polarized.

The output lights 34a to 34a coming out of the cavity 24
4c enters the detection device 26, the monitoring device 28, and the locking device 30, respectively. These output lights 34a-
Reference numeral 34c includes an output sampling light 34a incident on the sampling detection device 26, an output monitoring light 34b incident on the monitoring device 28, and an output locking light 34c incident on the locking device 30. The locking device 30 locks the frequency of the light source 22 and the cavity 24, and ensures that the line width of the sampling light in the cavity overlaps the line width of the target mode of the cavity 24. The sampling detector 26 determines the ring-down rate, absorption spectrum, and nuclide amount of the target sample. The monitoring device 28 monitors the light intensity and the transverse mode structure in the cavity. The light intensity in the cavity is used to characterize the locking of light source 22 and cavity 24. In one embodiment, a measurement of the peak intensity of the light in the cavity is used to adjust the frequency separation between the locking light and the sampling light.

FIG. 2 shows the arrangement of the optics of the system 20. The locking device 30 is shown in FIG. For clarity, various common optical elements, such as lenses and mirrors used for focusing and redirecting light, are well known to those skilled in the art and will not be described in detail. do not do.

Referring to FIG. 2, the light source 22 includes the fundamental light 4.
A continuous wave (CW) laser 40 for producing
An optical element for generating the input sampling light 32a and the locking light 32b is provided. Laser 40
Preferably has a line width on the order of 1 MHz and an output of at least 100 μW. The laser 40 is a tunable external cavity diode laser (ECDL)
It is preferred that Other suitable light sources include solid state and dye lasers, and continuous wave optical parametric oscillators (OPOs). Laser 40, including its controller, is mounted in a Faraday cage (not shown) to shield laser 40 from stray electromagnetic interference that would cause instability in the temperature controller of laser 40.

An optical isolator 44 is optically coupled to the laser 40 and is disposed in the path of the fundamental light 42. The isolator 44 reduces the optical feedback into the laser 40 caused by reflection and adjusts the polarization of the fundamental light 42. Isolator 44 is preferably a Faraday isolator that includes a half-wave plate tuned to produce P-polarized light. A beam expander 46 and a beam splitter 48 are arranged continuously in front of the isolator 44 so as to receive light emitted from the isolator 44. The basic light 42 is transmitted to the beam splitter 4
8, the sampling light 32a and the locking light 32
Before being split into b and b, they are passed through a beam expander 46.

An acousto-optic modulator (AOM) 54 provides a sampling light 32 a between the laser 40 and the cavity 24.
Is located on the route of. The AOM 54 is driven by an RF signal generated by a voltage-controlled oscillator (not shown). The AOM 54 is arranged between the optical elements 56a and 56b for focal light and mode matching. The optical element 56a includes a mode matching lens for selecting TEM00 light passing through the cavity 24, and a focal lens for collimating the sampling light 32a to the AOM 54. The optical element 56b includes a lens that re-collimates the sampling light 32a after passing through the AOM 54. The half-wave plate 57 receives the sampling light 32a.
Sampling light 3 to change the polarization state of
2a. In the illustrated device, the AOM 54 is optimized for modulating the P-polarized light, so the half-wave plate 57 is located behind the AOM 54.

The AOM 54 functions as a switch for modulating the intensity of the sampling light 32 reaching the cavity 24 (for example, switching on and off) so that the sampling light 32a can ring down in the cavity 24. AOM 54 may be used, among other things, to generate a pulse or continuous wave step input. The AOM 54 transmits the S-polarized light 32 a to the cavity 2.
Cavity 24 to be tuned to the desired mode of
Also functions as a frequency shifter for shifting the frequency of the sampling light 32a incident on. AOM54
Shifts the frequency of the sampling light 32a by an amount equal to the frequency of the RF signal driving the AOM 54. A
Preferably, the primary beam from OM 43 is directed to cavity 24. AOM passed through cavity 24
A window 50 is located in the optical path between the AOM 54 and the cavity 24 to select the primary beam from 54 while blocking the zero order beam.

A phase modulator (PM) 58 is disposed on the path of the locking light 32b between the laser 40 and the cavity 24. PM 58 is preferably an electro-optical resonant phase modulator. In the particular device shown, PM 58 is optimized to modulate S-polarized light,
It is arranged between the half-wave plates 60a and 60b. The half-wave plates 60a, 60b allow the PM 58 to maximize the electro-optic effect within the PM 58 for a particular crystal orientation.
Adjust the polarization of the light inside 58. The PM 58 introduces a locking sideband into the locking light 32b. The sidebands have a predetermined frequency ωf greater than the line width of the laser 40 and the desired mode of the cavity 24, and the locking light 3
2b away from the center frequency ωL. The sideband of ωL−ωf is shifted in phase by π (180 °) with respect to the sideband of ωL + ωf. A mode matching lens 62 is disposed on the path of the locking light 32b in order to select a TEM00 mode to be passed through the cavity 24.

A polarizing beam splitter 64 is disposed in the path of the sampling light 32a and the locking light 32b, facing the cavity 24. The beam splitter 64 includes a sampling light 32a and a locking light 32b.
And directed in the direction of the cavity 24. Beam splitter 64, isolator 44 and half-wave plate 5
7, 60a and 60b are S-polarized sampling lights 3
It functions as a polarization adjusting element for selecting the 2a and the P-polarized locking light 32b to pass through the cavity 24.

The cavity 24 includes an input mirror 66a,
It has an intermediate mirror 66b and an output mirror 66c. These mirrors 66a-66c define a close, triangular optical path 68 in the cavity. A gas sample 72 is disposed in the optical path 68 and fills the cavity 24. Sample 72 is an optically immobile material (ie, not an enhancement medium). Input mirror 66a
The output mirror 66c is a flat mirror that reflects light appropriately. The intermediate mirror 66b is a mirror having a high reflectance. The intermediate mirror 66b is mounted on the piezoelectric stage 74. The stage 74 is used as a cavity path length adjusting element. The stage 74 includes a mirror 66
a, 66c to adjust the position of mirror 66b relative to the path length of cavity 24 and cavity 2b.
4. Adjust the resonance frequency of any mode.

The coefficient of thermal expansion and stress tensor of the cavity 24 is preferably optimized to minimize cavity deformation (due to pulling, bending, bending, etc.) and mechanical resonance. Therefore, cavity 2
Preferably, the four optical elements are monolithically integrated. Mirrors 66a, 66c and stage 74
Are fixedly secured to a common block (not shown) of a thermally stabilized material, preferably a low thermal expansion glass such as, for example, mica ceramic. By stably and monolithically attaching the mirrors 66a and 66c and the stage 74, a change in the shape of the cavity 24 due to mechanical vibration and / or temperature vibration can be reduced. Such a shape change causes a drift in the mode frequency of the cavity 24.

The input lights 32a and 32b are input to an input mirror 66.
A enters the cavity 24 via a and generates a light wave traveling in the cavity along the optical path 68. Input light 3
2a and 32b are not perpendicular to mirror 66a, so that the light reflected by mirror 66a is not collinear with the light incident on mirror 66a. The accuracy of the cavity 24 is sufficiently high so that light waves traveling in the cavity can propagate through the cavity 24 multiple times. The locking light (P-polarized) and the sampling light (S-polarized) in the cavity
4 causes different losses and has different resonance frequencies for a given longitudinal mode of the cavity 24. Detection light 34a
And the monitoring light 34b exits the cavity 24 via the output mirror 66c.

A polarizing beam splitter 76 is optically coupled to output mirror 66c. Beam splitter 76
Is disposed on the optical path between the mirror 66c and the sampling detector 82. The beam splitter 76 converts the S-polarized sampling light 34 a into the sampling detector 8.
Go to 2. The detector 82 is described in Harb et al., Ph.
As described in ys. Rev. A. 54: 4370 (1996),
Preferably, the photodiode is optimized to have a high gain. A polarizing separator 78 and a focusing lens 80 are disposed on the optical path between the beam splitter 76 and the detector 82. Suitable polarizing separators include, in particular, Brewster windows, Gran-Taylor, Gran-Thomson or Wollaston prisms, and Thomson beam splitters. Polarizing separator 78
Is selected to transmit the S-polarized sampling light 34a to the sampling detector 82.

The ring-down device 90 is electrically connected to the sampling detector 82. The ring-down device 90 determines the ring-down rate and the corresponding absorption spectrum for the sample 72 by analyzing the ring-down waveform detected by the sampling detector 82. Ring-down device 90 also determines the amount of the target nuclide in sample 72. The intensity of the output sampling light 34a incident on the detector 82 indicates the intensity of the sampling light in the cavity 24, and thus the strength of the interaction of the sample 72 with the in-cavity sampling light. The ring down device 90 is a conventional one.
The ring down 90 may be implemented, for example, using computer software in an experimental setting, or using dedicated hardware in a process control device in a manufacturing setting.

The monitoring detectors 86 and 88 are optically connected to the mirror 66c. The beam splitter 76 directs the P-polarized inter-direction 34b to the monitoring detectors 86 and 88 via the beam splitter 92 and the focusing lens 94. Detector 86 is preferably a photodiode as described in Harb et al., Supra. Detector 86 is used to monitor the power of the light in the cavity. Detector 86 may be connected to AOM 54 via a monitoring servo device (not shown) to adjust the drive frequency of AOM 54 to compensate for changes in the frequency difference between sampling resonance and rocking resonance. . Such changes are caused by sample variations.

The detector 88 includes a charge-coupled device array (CCD)
Camera). Detector 88 is ideally used to monitor the transverse mode structure of light in the cavity to ensure that only TEM00 light propagates in cavity 24. The monitoring processing device 96
It is electrically connected to the detectors 86 and 88 and the ring-down device 90. The monitoring processing device 96 transmits the recorded monitoring data to the ring-down device 90 to store the corresponding ring-down data.

The locking detector 100 is connected to the input mirror 6
6a, which is optically connected to the input mirror 66a and is arranged to catch the output locking light 34c emitted from the input mirror 66a. Locking detector 100 is preferably a photodiode similar to that used for surveillance detector 86, optimized for high bandwidth to minimize locking sidebands with minimal distortion and noise. . The polarizing separator 102 and the focusing lens 104 are
a between the locking detector 100 and the locking detector 100
c is arranged on the optical path. The light emitted from the input mirror 66a has a reflection component reflected by the input mirror 66a and a leak component transmitted from the cavity 24 via the input mirror 66a. The polarization separator 102 selects the P-polarized locking light 34c to be transmitted to the detector 100.

The detector 100 is a part of the locking device 30. FIG. 3 shows a schematic view of the locking device 30. The design of the locking device 30 is based on a pound-dever locking system. Detailed information on pound-drever locking can be found in Drever et al., Ap, which is incorporated herein by reference.
pl. Phys. B 31: 97-105 (1983).

Referring to FIG. 3, a high-pass filter (H
(PF) 108 is electrically connected to the output of the detector 100. The pass threshold of the HPF 108 is smaller than the frequency separation ωf between the center frequency and the locking sideband of the signal detected by the detector 100. A bandpass filter (BPF) 110 is connected to the output of the HPF 108. The pass band of the BPF 110 is centered on ωf. The output of the BPF 110 is connected to the input of the mixer 112. Another input of the mixer 112 is connected to a first output of a voltage controlled oscillator (VCO) 106 that can generate a drive signal at a frequency ωf. A second output of VCO 106 is connected to PM 58 to drive PM 58. The output of the mixer 112 is connected to a low-pass filter (LPF) 114. The pass threshold of LPF 114 is smaller than 2ωf. The output of the LPF 114 is connected to a servo device 116, and the servo device 116 is connected to the stage 74 via an amplifier 118.

The VCO 106 generates a drive signal of ωf to drive the PM 58. PM58 has a frequency ωL
At ± ωf, a locking sideband is inserted into the locking light 32b. Here, ωL is the frequency of the central band of the locking light 32b. The output locking light 34c is constituted by the reflection of the locking light 32b and the locking light leaking from the cavity 24. The width of the sideband of the locking light depends on the tuning of each sideband of the cavity 24, and consequently on the difference between ωL and the corresponding resonance frequency of the intended predetermined mode of the cavity 24.
For example, if ωL is higher than the corresponding resonance frequency of cavity 24, the sideband reflected at ωL−ωf has a lower intensity than the sideband reflected at ωL + ωf.

The locking light 34c detected by the detector 100 includes a frequency component of ωL and a frequency component of ωf. Generally, ωL is about 1014 Hz and ωf is about 107 Hz. Detector 100 is not fast enough to resolve the ωL component of locking light 34c in time. As a result, the electric signal generated by the detector 100 includes a component centered on the zero frequency corresponding to the ωL frequency component of the locking light 34c. The detector 100 can time-resolve the ωf component of the locking light 34c, so that the generated electrical signal includes a component centered at ωf. H
The PF 108 and the BPF 110 select the ωf frequency component to be passed through the mixer 112.

Mixer 112 combines the signals received from VCO 106 and detector 100 to generate an error signal centered at zero frequency. The mixer 112
Effectively calculate the difference between the sideband signals to generate the error signal. The sign of the error signal indicates whether ωL is higher or lower than the resonance frequency of the corresponding cavity 24.
The LPF 114 eliminates harmonics (for example, 2ωf) from the signal generated by the mixer 112 and selects a zero-frequency error signal to be passed to the servo device 116. Servo unit 116 applies an error signal to stage 74 to adjust the path length, and thereby the desired resonant frequency of cavity 24, to ensure that the desired mode of cavity 24 is tuned to ωL. Convert to

The locking light 32b is continuously turned on during operation of the system 20 to ensure continuous locking of the cavity 24 and the light source 22. Sampling light 32a is selectively switched off after sufficient accumulation of light in cavity 24 during continuous wave operation to enable measurement of the ring-down of sampling light 32a in cavity 24. Performing a plurality of ring-down measurements while maintaining the locking light 32b in the on state allows the system 20 to have a relatively high repetition rate (e.g.,
kHz) can be tolerated.

<Another System> FIG. 4 is a schematic diagram showing another system 20 'according to the present invention. The locking device 30 'adjusts the frequency of the light emitted from the laser 40' so as to tune the central band of the locking light 32b to the cavity 24. The output of mixer 112 is LP
It is connected to the frequency adjuster of laser 40 'via F114, servo device 116' and amplifier 118 '. If ωL is higher than the desired resonant frequency of cavity 24, servo device 116 ′ adjusts laser 40 ′ to reduce its illumination frequency to adjust ωL to the desired mode of cavity 24. . Similarly, ω
L is increased if lower than the desired value.

FIG. 4 is a schematic diagram illustrating another system 20 "of the present invention. The system 20" includes a linear ring-down cavity 24 ". A polarizing beam splitter 65 and a quarter wavelength. A plate 67 is continuously arranged between the PBS 64 and the cavity 24 ″ on the optical path of the sampling light and the locking light. PBS6
5 directs the incoming locking light and the sampling light to the cavity 24 "while directing the reflected locking light to the detector 100. The frequencies of the sampling light and the locking light are the same in the transverse mode (eg, T
EM00), but tuned to a different (eg, adjacent) longitudinal mode of the cavity 24 ". The frequency difference between the sampling light and the locking light is
Equal to one free spectral range (FSR) of 4 ". For a typical FSR value of several hundred MHz, a conventional AOM is suitable for introducing the desired frequency difference between the sampling light and the locking light. ing.

The CRDS baseline noise is mostly caused by simultaneous excitation and beating of multiple modes in a ring-down cavity (RDC), or by successive excitations of different modes. Different transverse / longitudinal modes have different resonant frequencies and ring down at different rates in the empty cavity. Ensuring that a single mode is excited in the ring-down cavity can significantly improve the signal-to-noise ratio of the system.

Since the frequency variation in the laser source is similar for the sampling light and the locking light, if the locking light mode is locked to the cavity, the corresponding sampling light mode is also locked to the cavity. The AOM used to switch the sampling light on and off gives the sampling light an appropriate frequency shift so that it can be combined in the same longitudinal mode as the locking light.

There are many advantages to using a ring-shaped ring-down cavity in the system of the present invention. The ring-shaped cavity can provide the desired separation between the resonant frequencies for the sampling light and locking light of different polarizations, simplifying the optics used to couple the locking light in and out of the cavity, In addition, optical feedback to the laser can be reduced.

It is important to note that the non-normal incidence of linearly polarized light on a dielectric interface generally results in different responses to P-polarized light (PPL) and S-polarized light (SPL). Is well known from physical interface physics. Therefore,
An optical ring resonator constructed with a geometry using a non-normal incident reflector (eg, an isosceles triangle) is actually a two superimposed non-degenerate Fabry-Perot cavity, ie, P polarization cavity (PP
C) and an S-polarized cavity (SPC).

Non-normal incidence dielectric mirrors are generally
It has a lower reflectivity for PL than for SPL. As a result, the ring resonator will consist of reduced precision PPC and high precision SPC. The structure of the transverse mode of the ring resonators PPC and PPS is as follows.
They are the same because they are determined only by geometric conditions. The non-uniform phase shift accumulated during the non-perpendicular reflection of the PPL and SPL causes a constant frequency difference between the frequency response of the PPC and the SPC. PPC and SP
The free spectral range of C differs depending on the derivative of the mirror phase shift, and is estimated to be several tens of kHz.

Since orthogonal polarization is easily separated using polarization optics, the simultaneous use of PPL and SPL provides a direct solution to the separation of the cavity locking problem from actual ringdown measurements. Low precision PPC for locking improves the feedback locking signal and relaxes the requirements on servo bandwidth, feedback gain, and differential sensitivity to the frequency difference between the laser and the cavity. The absolute wavelength accuracy at each point is
More than the laser linewidth or cavity mode sweep range,
It is determined by PC accuracy and locking servo quality. The improved frequency stability can greatly increase the resolution achievable with such a system.

The use of a ring cavity can also considerably simplify the optics used to couple the locking light to the locking component. In systems using straight or folded cavities, the optical signal emerging from the cavity input is collinear with the light rays incident on the cavity input. As a result, a half-wave plate,
Optical components such as analyzers and beam splitters are needed to selectively couple the locking light reflected by the cavity input with the locking device.
Further, the ring cavity does not reflect light directly to the light source, thereby greatly reducing the problem of optical feedback.

The following examples illustrate particular implementations of the present invention and should not be construed as limiting the invention.

[0045]

EXAMPLE The system of the present invention was used to generate a spectrum of atmospheric water vapor. The laser is a commercially available E
CDL (New Focus: 6226-H032), 833.2 nm
Tunable up to 862.5 nm. The laser output is
Over the entire tuning range, it varied in the range of about 9-13 nW for a drive current of 60 mA. A 25-35 dB Faraday isolator (New Focus: 5568) was used to reduce residual back reflection to the laser. RDC (CVI: T
The input mirror and output mirror of LM2-800-45S-1037)
It has a reflectivity of 99.93% for SPL at 833 nm and 99.3 for PPL at 833 nm.
% Reflectance. Intermediate RDC mirror (REO: run C
628,7.75 nm, 840-880 nm, 1 ゜ wedge)
It had a reflectivity of 99.95% at 33 nm.

One-fourth of the fundamental light generated by the laser was used for locking and three-fourths were used for sampling. AOM (Brimro
(se: Gpm-400-100-960), and focused at a spot size of 60 μm, and re-parallelized using a lens with a focal length of 5 cm. The AOM was driven by a VCO using a frequency in the range of 300-535 MHz. AOM is 320
A frequency shift of MHz was applied to the input sampling light.
The lens used for TEM00 mode matching between the sampling light and the locking light had a focal length of 127 cm. The phase modulator was a 58.5 MHz electro-optic resonant phase modulator (New Focus: 4001). Cavity T
The laser light coupled to the EM00 mode was ９０90% for PPL and ８５85% for SPL.

The VCO driving the phase modulator is 58.5.
MHz. P passed through the locking detector
A Gran Taylor prism was used to select the PL. The passband of the locking bandpass filter is 9
Centered around 0 MHz. The servo device is a passive notch filter centered at 52 kHz (48 dB max attenuation, 2k
(Bandwidth 3 Hz). A notch filter was used to suppress the vibration of the piezoelectric stage at a resonance frequency of 52 kHz. The use of a notch filter has improved system stability and reduced sensitivity to disturbances such as acoustic noise. The gain of the servo device and associated amplifier is 60 dB, 3 dB at 455 Hz
Had the following bandwidth.

The locking and monitoring photodiode was optimized for a 90 MHz bandwidth. The shot noise limited sideband signal is equal to
It was easily achievable for an optical output of 00 μW. The sampling photodiode was optimized for gain at a bandwidth of 25 MHz. The output sampling light output was about μW, and the detected ring-down attenuation constant was about several hundred ns or more. The monitoring photodiode receives 90% of the monitoring light,
Meanwhile, the surveillance CCD camera received the remaining 10%.

A Glan-Thompson prism was used to select the SPL to pass through the sampling detector. After detection by the sampling detector, the sampling signal is converted to a low-noise, high-speed amplifier (Stanford Research System
s: SR445) and digitized using a 10-bit 1 GHz oscilloscope (Tektronix: 11402). The attenuation waveform is based on Naus et al.'S ILS-XII 12th Interdiscip
linary Laser Science Conference.ed.AP Society
(OSA, Rochester, New York, 1996), p. 122, was used to approximate the initial estimate provided by the linear least squares approximation algorithm for the signal using the Levenberg-Marquardt algorithm. All scans were performed in air at atmospheric pressure and a water pressure of 12 Torr.

FIG. 6 shows the scan results of the cavity frequency response for PPL and SPL. The zero frequency is arbitrary. The resonance frequencies for SPL and PPL were observed to be separated at 282 MHz. The resonance frequency separation changed by several tens of kHz when the cavity mode overlapped the water absorption line. Therefore, the driving frequency of the AOM was adjusted using intensity information from the monitoring device to maintain proper frequency separation between the locking light and the sampling light. For small samples (eg, for tracking nuclide detection), frequency separation due to sample variance is generally small or negligible.

The discrimination gradient of the error signal generated by the mixer, which converts the error signal voltage to a frequency difference, was used to determine the variation in the frequency difference between the laser frequency and the cavity resonance frequency. FIG. 7 depicts measured values of the spectral density of the frequency difference variation (noise with respect to the frequency difference between the laser and the cavity). The total jitter between the laser and the cavity resonance was 1.1 MHz, slightly smaller than the free-running laser linewidth. The locking device reduced noise at low frequencies (<20 kHz). Faster servos may be used to reduce higher frequency components of the noise.

FIG. 8 shows the spectrum of atmospheric water vapor and HITRAN recorded using the system described above.
The corresponding spectra obtained from the 96 database are shown. The resolution of the recorded spectrum was 0.002 nm, except around the absorption peak where the resolution increased to 0.001 nm. Ten averaged decay waveforms (ADW) were recorded at each wavelength. Each ADW
It was determined by averaging 256 shots. The RMS baseline noise was 5 × 10 −9 cm −1 and the nominal sensitivity was 5 ppm. The recorded spectrum was converted to HITRAN 96 in absolute frequency, line intensity and line width.
And compared.

It will be apparent to those skilled in the art that the embodiments described above can be modified in many ways without departing from the scope of the invention. For example, the system of the present invention may be used to cover a wide range of wavelengths and may be used to characterize the type of nuclide of interest. Various locking methods and devices may be used. Therefore, the scope of the present invention should be determined by the appended claims and their equivalents.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a preferred system of the present invention.

FIG. 2 is a schematic diagram showing the optical components of the system of FIG.

FIG. 3 shows a locking member of the system of FIG. 1;

FIG. 4 is a view showing a locking member according to another embodiment of the present invention.

FIG. 5 illustrates another embodiment of the present invention using a straight ring-down cavity.

FIG. 6 is a diagram illustrating a cavity frequency response to P-polarized light and S-polarized light in the system according to the embodiment of the present invention.

7 is a diagram showing a measured value of a spectral density that fluctuates due to a frequency difference in relation to a frequency difference in a system having the characteristics of FIG. 6;

8 shows a spectrum of atmospheric water vapor recorded with a system having the features of FIG. 6;

[Explanation of symbols]

20, 20 ', 20 "cavity ring-down spectroscopic system 22 light source 24, 24" cavity 26 sampling detector 30, 30' locking device 32a, 34a sampling light 32b, 34c locking light 40, 40 'laser 44 isolator (polarization adjusting optics) Device) 48, 64, 65, 76, 92 Beam splitter (beam splitting optical device) 54 AOM (acoustic optical modulator) 57, 60a, 60b Half-wave plate (polarization adjusting optical device) 58 PM (phase modulator) 64 Beam splitter (polarization adjusting optical device) 72 Sample 74 Stage (cavity optical path length adjusting device) 82 Sampling detector 90 Ring down device 100 Locking detector 106 VCO (Oscillator) 114 Mixer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Barbara A. Pardus United States 94086 Sunnyvale, Lakeside Drive 1249 2060 (72) Inventor Charles Sea Herb United States of America 9025 Palo Alto, El Camino Real 39434 (72) Inventor Richard N. Zear United States 94305, California 94305 Santa Ines Street 724 F-term (reference) 2G020 AA03 AA04 CB23 CB42 CB43 CC23 CC47 CC55 CC65 CD04 CD13 CD24 CD32 CD34 CD52 2G059 AA02 AA03 BB01 CC09 EE01 FE01 EE01 EE01 EE01 EE01 EE01 EE01 EE01 EE01 EE01 H HH02 JJ12 JJ19 JJ20 JJ22 JJ30 KK04 LL01 MM03 MM09 NN02 PP05

Claims (10)

[Claims] 1. a) a ring-down ring resonant cavity for holding a sample; and b) locking light and sampling light optically coupled to the cavity and incident on the cavity, one of which is located within the cavity. S-polarized,
A light source including a laser that simultaneously generates locking light and sampling light, the other of which is p-polarized; c) optically coupled to the cavity and electrically coupled to at least one of the light source and the cavity; A locking device for receiving the locking light reflected by the cavity and using the locking light to lock the cavity and the light source; and d) detecting sampling light that is optically coupled to the cavity and exits the cavity. And a sampling device for determining a ring-down rate indicating the absorption of the sampling light by the sample. 2. The light source further comprises: a light source for selecting P-polarized light for the locking light and S-polarized light for the sampling light on an optical path between the laser and the cavity. The system of claim 1, comprising polarization adjusting optics. 3. The method according to claim 1, wherein the light source further comprises: an optical path between the laser and the cavity for tuning at least one of the locking light and the sampling light to a desired mode of the cavity. The system according to claim 1, further comprising a frequency shifting optical device for frequency shifting the locking light or the sampling light. 4. The light source is further disposed on an optical path of the sampling light between the laser and the cavity, and frequency-shifts the sampling light to tune the sampling light with respect to the cavity. ,
2. The system of claim 1, further comprising an acousto-optic modulator that switches the sampling light so that the sampling light can ring down in the cavity. 5. The apparatus according to claim 5, wherein the cavity is electrically connected to the locking device, and further comprises a cavity optical path length adjusting device for adjusting an optical path length of the cavity to lock the cavity to the light source. The system of claim 1, wherein the system comprises: 6. The locking device comprising: a) a locking detector optically coupled to the cavity and detecting locking light reflected by the cavity; and b) electrically coupled to the locking detector. A processing device that generates an error signal indicating a difference between a center frequency of the locking light and a resonance frequency of the cavity, wherein the light source further includes an optical path of the locking light between the laser and the cavity. Wherein the locking device is further electrically connected to the phase modulator, and wherein a first sideband having a frequency higher than the center frequency and a second sideband having a frequency lower than the center frequency are provided. An oscillator for driving the phase modulator to insert the sideband of the phase shifter into the locking light, and electrically connected to the locking detector and the oscillator. And a mixer for calculating a difference between electric signals corresponding to the first sideband and the second sideband to generate the error signal. System. 7. A ring-down spectroscopic ring resonant cavity having an input and an output and holding a sample; b) a continuous wave tunable laser optically coupled to the input and generating a fundamental light; c) arranged on an optical path between the laser and the input, splitting the fundamental light into locking light that is incident on the input and is P-polarized in the cavity and sampling light that is S-polarized; A beam splitting optics; d) optically coupled to the input and electrically coupled to at least one of the cavity and the laser, and providing electrical feedback from reflection of the locking light from the input. A locking device for generating an error signal and using the error signal to lock the laser and the cavity; and e) the beam splitting optics and the input device. A switch disposed on the optical path of the sampling light between the power and a switch that switches the sampling light so that the laser and the cavity are locked while ringing down the sampling light multiple times in the cavity; and f) at the output. A sampling detector that is optically coupled to detect the sampling light emitted from the cavity; g) a ring-down device that is electrically coupled to the detector and determines a ring-down rate for the sampling light; h) a light source optically coupled to the cavity for generating locking light and sampling light of different frequencies to be incident on the cavity; i) optically coupled to the cavity; Electrically connected to at least one of the cavities, A locking device that locks the cavity and the light source using locking light reflected by the tee, wherein the sampling detector exits the cavity while the cavity and the light source are locked. A cavity ring-down spectroscopy system comprising detecting a plurality of ring-downs of the sampling light. 8. The light source comprising: a) a continuous wave tunable laser optically coupled to the cavity and producing fundamental light; b) disposed on an optical path between the laser and the cavity; A beam splitting optical device for splitting fundamental light into the locking light and the sampling light; and c) being arranged on at least one of the optical paths of the sampling light and the locking light, and shifting one of the sampling light and the locking light in frequency. The system of claim 7, further comprising a frequency shifting optic device for causing a shift. 9. A ring-down resonant cavity for holding a sample; and b) a light source optically coupled to the cavity for generating locking light and sampling light at different frequencies to be incident on the cavity. C) locking means optically coupled to the cavity and locking the cavity and the light source using the locking light reflected by the cavity; and optically coupled to the cavity; Detecting means for detecting a number of ring-downs of the sampling light emitted from the cavity while the light source is locked; e) being electrically connected to the detecting means, and detecting a ring-down rate for the ring-down. A ring-down means for determining Ngudaun spectroscopy system. 10. A method for locking a laser and a cavity for ring-down spectroscopy, comprising: a) using the laser to generate mutually different frequency sampling light and locking light for incidence on the cavity. B) locking the cavity and the laser using the locking light; and c) generating the absorption spectrum of a sample disposed in the cavity by using the locking light. Obtaining a plurality of measurements of the ring down of the sampling light when is in a locked state.