Claims
- 1. Apparatus for use in an instrument that measures the absorption of a sample by passing radiation through the sample, the sample having more than one absorption line including an absorption line at wavelength .lambda..sub.o, said apparatus serving to positively identify the absorption line being measured as the absorption line at wavelength .lambda..sub.o, said apparatus comprising:
- source means including a diode laser responsive to an applied electric current to produce radiation and emitting a part of that radiation in a first direction toward the sample, and emitting another part of that radiation in a second direction;
- a first detector positioned to receive radiation that was emitted in the first direction after it has passed through the sample, and generating a first electrical signal related to the intensity of the radiation received by said first detector;
- a filter positioned to intercept the radiation emitted in said direction and having a narrow rejection band centered at wavelength .lambda..sub.o and substantially transmitting radiation of wavelengths outside the narrow rejection bank;
- a second detector positioned to receive radiation that has passed through said filter, and generating a second electrical signal related to the intensity of the radiation received by said second detector;
- first means connected to said diode laser for scanning the wavelength of the radiation emitted through a wavelength interval in a continuous manner; and,
- second means, connected to said first means and to said second detector, and responsive to the second electrical signal generated by said second detector to determine at what instant in the scanning the wavelength of the radiation emitted by said diode laser equals .lambda..sub.o.
- 2. The apparatus of claim 1 wherein said first means further comprise means for systematically varying the current applied to said diode laser, thereby scanning the wavelength of the emitted radiation in a systematic manner.
- 3. The apparatus of claim 1 wherein said first means further comprise means for systematically cooling and heating said diode laser, thereby scanning the wavelength of the emitted radiation in a systematic manner.
- 4. Apparatus for use in an instrument that measures the absorption of a sample by passing radiation through the sample, the sample having more than one absorption line including an aboorption line at wavelength .lambda..sub.o, said apparatus serving to positively identify the absorption line being measured as the absorption line at wavelength .lambda..sub.o, said apparatus comprising:
- source means including a diode laser responsive to an applied electric current to produce radiation and emitting a part of that radiation in a first direction toward the sample, and emitting another part of that radiation in a second direction;
- a first detector positioned to receive radiation that was emitted in the first direction after it has passed through the sample, and generating a first electrical signal related to the intensity of the radiation received by said first detector;
- a filter positioned to intercept the radiation emitted in said second direction and having a narrow pass band centered at wavelength .lambda..sub.o and substantially opaque to radiation of wavelengths outside the narrow pass band;
- a second detector positioned to receive radiation that has passed through said filter, and generating a second electrical signal related to the intensity of the radiation received by said second detector;
- first means connected to said diode laser for scanning the wavelength of the radiation emitted through a wavelength interval in a continuous manner; and
- second means, connected to said first means and to said second detector, and responsive to the second electrical signal generated by said second detector to determine at what instant in the scanning the wavelength of the radiation emitted by said diode laser equals .lambda..sub.o.
- 5. The apparatus of claim 4 wherein said first means further comprise means for systematically varying the current applied to said diode laser, thereby scanning the wavelength of the emitted radiation in a systematic manner.
- 6. The apparatus of claim 4 wherein said first means further comprise means for systematically cooling and heating said diode laser, thereby scanning the wavelength of the emitted radiation in a systematic manner.
- 7. Apparatus for controlling the wavelength of the radiation emitted by a diode laser that is mounted on a heat sink, to equal a particular wavelength .lambda..sub.c where the wavelength is a known function of the temperature of the junction of the diode laser, and T.sub.c is the temperature corresponding to .lambda..sub.c said apparatus comprising:
- a coarse control system for comparing the temperature of the heat sink with T.sub.c and for applying heat to or removing heat from the heat sink as required to cause the temperature of the heat sink to approach T.sub.c ; and,
- a fine control system for altering the diode laser current in a systematic way to maximize the transmission of a sample of the radiation through a narrow band pass filter having its pass band centered at the wavelength .lambda..sub.c.
- 8. Apparatus for controlling the wavelength of the radiation emitted by a diode laser that is mounted on a heat sink, to equal a particular wavelength .lambda..sub.c where the wavelength is a known function of the temperature of the junction of the diode laser, and T.sub.c is the temperature corresponding to .lambda..sub.c said apparatus comprising:
- a coarse control system for comparing the temperature of the heat sink with T.sub.c and for applying heat to or removing heat from the heat sink as required to cause the temperature of the heat sink to approach T.sub.c ; and,
- a fine control system for altering the diode laser current in a systematic way to minimize the transmission of a sample of the radiation through a narrow band rejection filter having its rejection band centered at the wavelength .lambda..sub.c.
BACKGROUND OF THE INVENTION
The present application is a continuation-inpart of U.S. patent application Ser. No. 837,605 filed on Mar. 7, 1986 now abandoned for OXYGEN MEASUREMENT USING VISIBLE RADIATION.
The present invention is in the field of gas analysis and more specifically relates to apparatus for measuring the concentration of gaseous oxygen present in a volume by measuring the absorption of visible radiation passing through the gaseous sample.
The present application is concerned with an entirely new way of measuring the concentration of gaseous oxygen, and is particularly suitable for use in compact instruments such as might be used in medical applications. As will be described below, previously known ways of measuring oxygen concentration have suffered from poor accuracy, slow response time, and interference by other gases. With the exceptions of mass spectrometry and gas chromotography, the various methods of measuring gaseous oxygen can be classed into three main groups: paramagnetic, thermoconductive, and electrochemical. These techniques will now be briefly described.
The paramagnetic technique makes use of the paramagnetism of oxygen. The permeability of oxygen at a pressure of 1 atmosphere and at 20 degrees centigrade is 1.00000179. In the so-called Pauling method, the gas is introduced into a cell in which a small dumbbell is suspended on a taut platinum ribbon. The cell is held in a nonuniform magnetic field. The torque on the dumbbell is proportional to the volume magnetic susceptibility of the gas around the dumbbell. This torque is counteracted by the electromagnetic effect of a current which is made to flow through a single turn of platinum wire wound on the dumbbell. The current required to do this is proportional to the original torque and is therefore a measure of the susceptibility of the sample gas. This restoring current is maintained at the correct value automatically by means of a twin photocell which detects the position of a beam of light reflected from a mirror on the suspended dumbbell. The electrical outputs are derived from the restoring current.
There are several drawbacks to this Pauling method. First, its response is slow (typically 10 seconds for 90 percent of full scale). Second, it is nonspecific in the sense that significant interferences are caused by other paramagnetic gases, namely NO and NO.sub.2. Third, since the position of the dumbbell at rest determines the readout and any gas flow blows the dumbbell away from the correct position, this method is not suitable for the measurement of flowing oxygen gas.
The thermoconductivity method is based on the rate at which different gases remove heat from a hot wire. Oxygen conducts heat at a different rate than nitrogen. The rate at which a temperature-sensitive thermistor is cooled in the sample chamber therefore deoends on the oxygen concentration in the chamber. The rate of cooling of this thermistor is compared with that of a similar thermistor in a reference chamber by means of a Wheatstone bridge. The difference is displayed as a meter reading of the oxygen concentration. Silica gel is utilized to equalize the content of water vapor in both the sample and reference chambers so that the readings are not affected by the water vapor.
The thermoconductivity method suffers from a rather slow response (typically 10 seconds from 0 to 90 percent of full scale reading) and cannot be used for monitoring flowing oxygen due to the fact that the rate of cooling depends on the flow rate. Like the Pauling paramagnetic technique, the thermoconductivity method permits only intermittent analysis due to the need for manually introducing the gas into the sampling chamber.
All of the commercially-available continuous oxygen monitors operate on the electrochemical principle. There are two basic types of these instruments: the polarographic and the galvanic. Both of these have porous metal sensing electrodes (anode and cathode) with a gas-tight conducting electrolyte between them. The gas-tight electrolyte prevents mixing of gases between the anode chamber and the cathode chamber of the cell, but allows electrochemical oxygen transfer between anode and cathode. Transoort of electrochemical oxygen (either in the form of cations or oxide ions) between the cathode and anode chambers (one of which is at a fixed oxygen partial pressure for reference) generates an electrical signal which is directly proportional to the partial pressure of oxygen in the sample chamber. Since the diffusion of electrochemical oxygen through the electrolyte depends on temperature, a thermistor is used to regulate the current so that the only variable measured is oxygen concentration in the sample chamber. The difference between the polarographic and the galvanic operation is that the former requires a polarizing voltage from an external power supply for the oxygen transport, whether as the latter acts as a fuel cell and derives its polarizing voltage internally. Oxygen monitors operating on the electrochemical principle are usually slow, although response times of 100 milliseconds have been obtained with the use of very high temperature electrolyte for speeding up the transport of electrochemical oxygen. The adaptation of electrochemical techniques to flowing oxygen measurement is difficult because of the inevitable masking of the sample electrode by the condensations of water vapor such as might be present in a medical application.
In addition to the three main methods discussed above, the possibility of measuring the gaseous oxygen concentration through ultraviolet absorption has been explored by the present inventor in U.S. Pat. No. 4,096,388, and by Kronick, et al. in U.S. Pat. No. 4,192,996. The major problem with the ultraviolet absorption technique is interference by other gases which also absorb ultraviolet radiation in the same portion of the spectrum.
Thus, methods of measuring the concentration of oxygen in a gaseous sample have suffered from a number of deficiencies which have limited the practical usefulness of the techniques, particularly in medical applications.
There is no known strong absorption band for O.sub.2 in the visible and near infrared. However, the existence of three very weak absorption bands of O.sub.2 located at 760 nm, 1.07 .mu.m and 1.27 .mu.m respectively has been known since the early 1960's. The 760 nm band, also called the "A" system, lies at the very edge of the red end of the visible spectrum and arises from the electronic-rotational X.sup.3 .SIGMA..sub.g.sup.- .fwdarw.b'.SIGMA..sub.g.sup.+ transition of the oxygen molecule. This is a spin-flip transition involving the spin change of a .pi..sub.g.sup.- electron. The weakness of this system indicates that it is a magnetic dipole transition. The O--O band of the "A" system of oxygen spans approximately from 759 nm to 773 nm and comprises 72 sharp lines making up four distinct branches designated as P.sub.P, P.sub.Q, R.sub.R and R.sub.Q respectively (see FIG. 1). The equivalent widths of the strongest of these sharp lines ranges between 0.1 and 0.15 nm.
The infrared atmospheric oxygen bands at 1.07 .mu.m and 1.27 .mu.m, represent a magnetic dipole '.DELTA..sub.g .fwdarw..sup.3 .SIGMA..sub.g.sup.- transition and comprises eight distinct branches (P, R, Q.sub.P, S.sub.R, Q.sub.R, P.sub.Q, R.sub.Q and Q.sub.Q) The absorption strengths of these bands are even less than those observed for the "A" system.
The implementation of an oxygen monitor using the aforementioned visible and infrared atmospheric oxygen bands in an absorption technique has heretofore been considered unfeasible because of the extraordinary weakness of these bands and the lack of adequate source, detector, sample chamber and methodology.
The present invention consists of apparatus that permits the monitoring of O.sub.2 concentration using a novel optical absorption technique operating in the 760 nm O--O band of the oxygen "A" system.
The present invention is made feasible by the relatively recent advent of semiconductor laser diode light sources such as the AlGaAs system with emission wavelengths spanning the O--O band of the oxygen "A" system, the availability of the silicon photodiode detector which has optimum response in the 760 nm region, the use of a special sample chamber design that permits long path lengths to be obtained in a relatively compact space and the application of a novel spectral scanning technique.
Due to the extreme weakness of the oxygen "A" system at 760 nm (the intensity modulation for a 2.5 nm band pass filter and a path length of three meters has been deduced from experimental measurements as being on the order of 1.67.times.10.sup.-5 per atmosphere per cm) the use of a relatively broadband source (>10 nm) such as an incandescent lamp or an LED with a narrow bandpass filter to cover all the sharp lines of the O--O band of the oxygen "A" system does not yield sufficient intensity modulation to render feasible an absorption technique for the detection of this gas.
Instead a much narrower spectral source such as a single mode or a multi-mode laser whose emission line widths match closely to those of the oxygen sharp lines and cover only one or at most several of the strongest absorption lines of the O--O band is necessary in order to provide the minimum needed modulation.
The use of a reflecting integrating sphere as a novel sample chamber for providing a long and adjustable path length ensures the fact that sufficient modulation is available if required.
The use of novel optical and thermal feedbacks working in conjunction with the laser diode provides a stable spectral output, which is needed for the absorption scheme.
Finally, a novel laser current drive scheme is used to achieve a thermally-driven spectral scanning of the laser output in and out of the oxygen absorption lines to provide the "reference" and "sample" conditions for the measurement of oxygen in the sample chamber.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which a preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.
US Referenced Citations (1)
| Number |
Name |
Date |
Kind |
|
4192996 |
Kronick et al. |
Mar 1980 |
|
Non-Patent Literature Citations (4)
| Entry |
| Norton et al, ". . . Wavelength Tuning . . . of Lasers", Applied Phys. Let., vol. 18, #4, (Feb.-71), p. 158. |
| Clark et al, ". . . Wavelength Tuning . . . of GaAlAs . . . ", IEEE Jour. of Quant. Elect., vol. QE-18, #2 (1982), p. 199. |
| Vaucher et al, ". . . Tunable Pulses from . . . Laser", IEEE Jour. of Quant. Elect., vol. Q-E, #2 (1982), p. 187. |
| Anzin et al, "Freq. Tuning . . . by Pressures & Temp.", Sov. J. Quant. Elect., vol. 7, #6 (1977), p. 793. |
Continuation in Parts (1)
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Number |
Date |
Country |
| Parent |
837605 |
Mar 1986 |
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Patent Grant
4730112
