GB2165640A - Gas or vapour concentration monitoring - Google Patents
Gas or vapour concentration monitoring Download PDFInfo
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- GB2165640A GB2165640A GB08425918A GB8425918A GB2165640A GB 2165640 A GB2165640 A GB 2165640A GB 08425918 A GB08425918 A GB 08425918A GB 8425918 A GB8425918 A GB 8425918A GB 2165640 A GB2165640 A GB 2165640A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
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Abstract
A gas or vapour monitoring apparatus employing differential absorption spectroscopy without the use of a monochromator drives an injection laser 10 cyclically between three different drive current levels. Two of the levels are employed in a feedback circuit incorporating a reference absorption cell 18, to stabilise the laser operating wavelength in the vicinity of the peak of a spectral absorption line when driven with these two levels. The third level is employed to stabilise the laser operating wavelength at a point removed from the absorption line when driven with this current level, and hence provide a reference point for the absorption measurements made with the aid of a test absorption cell 15. <IMAGE>
Description
SPECIFICATION
Gas or vapour concentration monitoring
This invention relates to a method of monitoring the concentration of a gas or vapour in a test absorption cell by a differential absorption measuring technique, and also relates to apparatus for performing this monitoring technique.
The basic principle of differential absorption measurements of this sort involves directing through the test cell a first beam of light whose wavelength registers with a spectral absorption peak of that gas or vapour, and using a photodetector to compare the resulting attenuation with that provided by directing a second beam of light along the same path through the test cell, this second beam having a wavelength close to that of the first, but in a region of the spectrum at which the gas or vapour is relatively transparent.
Provided that there is a spectral absorption line in the appropriate part of the spectrum, the test cell may be located at a station remote from the source and the detector, to which it is optically coupled by means of optical fibres. Such an apparatus is described by
K. Chan et al in a paper entitled, 'Optical Remote Monitoring of CH4 gas using low-loss optical Fiber link and InGaAsP light-emitting diode in 1.33um region', (Appl. Phys. Lett Vol.
43 No. 7 pp 624-6, 1st October 1983). The authors of that paper employed as the optical source a light emitting diode whose spectral line width was broad compared with the spectral line width of the relevant methane absorption line, and hence it was necessary for the apparatus to include a monochromator.
The present invention is particularly concerned with apparatus in which there is no need for a monochromator. This is achieved by using an injection laser as the optical source.
A paper by P. Pokrowsky and W. Herrmann entitled, 'Sensitive detection of hydrogen chloride by derivative spectroscopy with a diode laser', (Optical Engineering January/February 1984/Vol. 23 No. 1 pp 88-91) describes the use of an injection laser in apparatus for detecting the concentration of a vapour by absorption spectroscopy. However, it may be noted that the method does not involve making measurements at two wavelengths as in conventional differential absorption spectroscopy, its source emits in a spectral region for which there are at present no low-loss optical fibres, and the source has a spectral line structure necessitating the use of a monochromator.
According to the present invention there is provided a monochromatorless differential optical absorption monitoring apparatus for monitoring the concentration of a gas or vapour present in a test absorption cell by reference to the absorption at a wavelength registering with a spectral absorption line of that gas or vapour, wherein the optical source for the apparatus is provided by an injection laser cyclically driven between three different drive current levels, two of which levels are employed in a feedback circuit incorporating a reference absorption cell to stabilise the laser operating wavelength in the vicinity of the peak of a spectral absorption line when driven with these two levels, the third level being employed to stabilise, during its currency, the laser operating wavelength at a point removed from any absorption line.
The invention also resides in a monochromatorless differential optical absorption monitoring apparatus for monitoring the concentration of a gas or vapour present in a test absorption cell by reference to the absorption at a wavelength registering with spectal absorption line of that gas or vapour, wherein the test cell is optically coupled by means of optical fibres with a source and a first detector,
wherein the source is an injection laser provided with a current driver constructed to supply in cyclic repetition a current (i,+Ai) for a first time period of the cycle, a current (i1-Ai) for a second period, and then a current i2 for a third and final period of the cycle, wherein the width of the spectral absorption line is large compared with the laser emission frequency shift occasioned by a drive current change of 2Ai and small compared with the shift occasioned by a change of (i1-i2) wherein the laser is provided with feedback means for stabilising its output frequency which means incorporates a second detector positioned to receive light from the laser via a reference absorption cell containing a quantity of the gas or vapour, which second detector is constructed to be responsive to the magnitude of laser light it receives during the first and second period of the laser drive current cycle such that, in the transition from the first period to the second, the feedback means is adapted to drive the laser emission frequency from an operating point on one side of the spectral absorption peak to an equivalent position on the other side,
and wherein the first detector is constructed to be responsive to the difference between the magnitude of laser light it receives during the first and second period of the cyclic current drive.
The invention further resides in a monochromatorless method of monitoring the concentration of a gas or vapour present in a test absorption cell by differential optical absorption, wherein light from an optical source is directed to the test cell via a first optical fibre to emerge therefrom and be directed by a second optical fibre to a first detector,
wherein an injection laser driven by a cyclic repetition of three current levels (i1+Ai), (i2-Ai) and L2 is used as the source, wherein light from the laser is directed through a reference absorption cell containing a quantity of the gas or vapour and on to a second detector which forms part of a feedback path which controls the emission of the laser such that, as the drive current changes from (i,+Ai) to (i1 -Ai), the frequency is shifted from an operating point on one side of a spectral absorption line of the gas or vapour to an equivalent position on the other, and wherein i2 is sufficiently different in value from i, for the emission of the laser while driven with i2 to be at a frequency removed from any spectral absorption line of the gas or vapour.
There follows a description of apparatus, and its method of use, embodying the invention in a preferred form. The description refers to the accompanying drawings in which
Figure 1 is a schematic diagram of the apparatus,
Figure 2 is a diagram of the laser drive current waveform employed in the apparatus of
Fig. 1, and
Figures 3 and 4 are graphs illustrating how the drive levels of Fig. 2 are employed in relation to a spectral absorption line possessed by the gas or vapour under analysis.
The emission wavelength of an injection laser is affected by temperature. One of the factors concerned is that the peak wavelength of spontaneous emission, Ap, is a function of temperature. Another factor is that the optical path length of the laser cavity is also a function of temperature, and hence the wavelength of any particular longitudinal mode, Am, of an injection laser is also a function of temperature. For an InGaAsP laser about 0.3mm long di.p/dT~0.5nm/ C, while dAm/dT~0.12nm/ C.
A packaged laser diode convected by way of a Peltier heat pump to a heat sink can readily be stabilised to a temperature of +0.05 C by using a negative temperature coefficient thermistor mounted on the laser package as the sensor element of a feedback loop. Hence the output of such a laser operating in single longitudinal mode can be stabilised to about +2.5X10 2nm. Comparing this with a value of about 0.5nm for the spectral line width of one of the absorption lines in the CH4 spectrum at about 1300nm, it is clear that in principle the emission of the laser can relatively readily be held to the peak of such an absorption line by control of its temperature.Then, by temporarily increasing the laser drive current in a stepwise manner so as to produce a temporary rise of about 4"C in the temperature of the laser, the laser emission wavelength can be shifted off the absorption line to a nearby region at which absorption is low. A significantly smaller shift in temperature is sufficient to achieve the same purpose if that shift is such as to engender mode hopping to an adjacent longitudinal mode, but normally it is not necessary to arrange for such mode hopping to occur because the magnitude of temperature change to give an adequate wavelength change in the absence of any mode hopping is well within operational limits.
Referring now to Fig. 1 of the drawings, an injection laser package 10 is mounted on the cold side of a Peltier heat pump 11 which in turn is mounted with its hot side in thermal contact with heat sink 12. Light from the laser is launched into an optical fiber 13 and directed to a remote location where the emergent light is collimated by a lens 14 and directed through a test absorption cell 15 containing the gas or vapour sample to be analysed. Near the launch end of the optical fibre 13 is located a directional coupler 16 for tapping off a proportion of the launched power which is directed via a collimating lens 17 into a reference absorption cell 18 containing a quantity of the gas or vapour to be analysed.
Light emerging from the test cell 15 is launched into a second optical fibre 19 by means of a lens 20 for transmission back from the remote location to a photodetector 21. Similarly light emerging from the reference cell 18 is directed by lens 22 upon photodetector 23. The two photodetectors feed phase sensitive detectors 24 and 25 respectively.
Both these phase sensitive detectors receive timing signals from a current driver 26 which powers the laser 10.
The output of phase sensitive detector 25 is used in a feedback control loop to regulate the operating point of the laser. This may be achieved by using the output to control the rate of heat extraction from the laser provided by the Peltier heat pump (indicated in Fig. 1 by the connection 27), or alternatively (as indicated by the broken line connection 28), it may be achieved by using the output to add (or subtract) a d.c. level to the current drive waveform supplied to the laser by the current driver 26.
The current driver 26 for powering the laser 10 is constructed to supply in cyclic repetition a current (i,+Ai) for a first time period t1, a current (i1-Ai) for a second time period t2, and a current i2 for a third time period t3. (Fig.
2). The laser operating conditions are chosen so that a current drive of i1 would produce an emission close to the peak of an absorption line, with the current drives of (i1+Ai) and (i1-Ai) producing emission wavelengths within the bounds of the absorption line but on either side of its peak, as depicted in Fig. 3.
Because (in Fig. 2) the emission wavelength corresponding to i1 is not exactly centred on the peak of the absorption line, the attenuation A' that occurs in time period t1 when the laser is driven with a current (i1 +Ai) is greater than the attenuation A" that occurs in the time period t2 when the laser is driven with a current (i1-Ai). Photodetector 23 receives light transmitted through the reference absorption cell 18, and so its output reproduces this difference. The control signals from the current driver 26 that regulate the operation of phase sensitive detector 25 make it compare the photodetector output during t with that during t2.The phase sensitive detector output is then used as the control signal of a feedback control loop regulating the operation of the laser to bring it to the balanced operating point indicated in Fig. 4 where the same attenuation
A1 is provided in time periods t1 and t2 Photodetector 21 receives light transmitted through the test absorption cell 15. If there is any of the gas or vapour with this spectral absorption line present in the test cell, then the light received by photodetector 21 will be equally attenuated during time periods t1 and t2 in proportion to the concentration of that gas or vapour. During time period t3 the gas or vapour will provide substantially no attenuation because the emission of the laser has been shifted off the absorption line.Hence the signal output from photodetector during time period t and t2 provides a measure of the concentration of the gas or vapour when compared with the reference point provided by the output during time period t3. Accordingly, the control signals from the current driver that regulate the operation of phse sensitive detector 24 make it compare the photodetector output during t, and t2 with that during t3, and in this way a signal output is provided at 29.
For the detection of methane, one of the lines of the absorption bands in the region of 1.33 microns is suitable because this band is well separated from nearby water vapour absorption bands (at 1.13 and 1.38 microns), and also from nearby carbon dioxide absorption bands (at 1.22 and 1.43 microns) and hence the rotation-vibration bands for the v2+2v3 combination can be relatively readily be isolated. An injection laser suitable for operation in the region of 1.33 microns is one based on lnGaAsP. Within this absorption band the Q band (AJ=O) is the most intense, and is at 1.3312 microns. If this is an inconveniently long wavelength having regard for instance to price and availability, attention may be directed to the R branch (AJ= 1 1) where the absorption is around half that of the Q branch, but where lines down to 1.318 microns can be employed adjacent lines in this branch being separated by about 2nm.
It should be clearly understood that the invention is not limited in application to the detection and measurement of methane, but is generally applicable to the detection and measurement of a variety of gases and vapours, some of which are liable to require the use of lasers constructed from Ill-V semiconductor systems other than that of InGaAsP.
Claims (9)
1. A monochromatorless differential optical absorption monitoring apparatus for monitoring the concentration of a gas or vapour present in a test absorption cell by reference to the absorption at a wavelength registering with a spectral absorption line of that gas or vapour, wherein the optical source for the apparatus is provided by an injection laser cyclically driven between three different drive current levels, two of which levels are employed in a feedback circuit incorporating a reference absorption cell to stabilise the laser operating wavelength in the vicinity of the peak of a spectral absorption line when driven with these two levels, the third level being employed to stabilise, during its currency, the laser operating wavelength at a point removed from any absorption line.
2. A monochromatorless differential optical absorption monitoring apparatus for monitoring the concentration of a gas or vapour present in a test absorption cell by reference to the absorption at a wavelength registering with spectral absorption line of that gas or vapour, wherein the test cell is optically coupled by means of optical fibres with a source and a first detector,
wherein the source is an injection laser provided with a current driver constructed to supply in cyclic repetition a current (i1+Ai) for a first time period of the cycle, a current (i1hi) for a second period, and then a current i2 for a third and final period of the cycle, wherein the width of the spectral absorption line is large compared with the laser emission frequency shift occasioned by a drive current change of 2Ai and small compared with the shift occasioned by a change of (i1-i2) wherein the laser is provided with feedback means for stabiiising its output frequency which means incorporates a second detector positioned to receive light from the laser via a reference absorption cell containing a quantity of the gas or vapour, which second detector is constructed to be responsive to the magnitude of laser light it receives during the first and second period of the laser drive current cycle such that, in the transition from the first period to the second, the feedback means is adapted to drive the laser emission frequency from an operating point on one side of the spectral absorption peak to an equivalent position on the other side,
and wherein the first detector is constructed to be responsive to the difference between the magnitude of laser light it receives during the first and second period of the cyclic current drive.
3. Apparatus as claimed in claim 1 or 2, wherein the feedback control is constructed to regulate the extraction of heat from the laser.
4. Apparatus as claimed in claim 1 or 2, wherein the feedback control is constructed to regulate the d.c. component of drive current supplied to the laser.
5. Differential absorption gas or vapour monitoring apparatus substantially as hereinbefore described with reference to the accompanying drawings.
6. A monochromatorless method of monitoring the concentration of a gas or vapour present in a test absorption cell by differential optical absorption, wherein light from an optical source is directed to the test cell via a first optical fibre to emerge therefrom and be directed by a second optical fibre to a first detector,
wherein an injection laser driven by a cyclic repetition of three current levels (il+Ai), (i1 -Ai) and 2 is used as the source, wherein light from the laser is directed through a reference absorption cell containing a quantity of the gas or vapour and on to a second detector which forms part of a feedback path which controls the emission of the laser such that, as the drive current changes from (i,+Ai) to (i1-Ai), the frequency is shifted from an operating point on one side of a spectral absorption line of the gas or vapour to an equivalent position on the other, and wherein i2 is sufficiently different in value from ii for the emission of the laser while driven with 2 to be at a frequency removed from any spectral absorption line of the gas or vapour.
7. A method as claimed in claim 6, wherein the feedback regulates the extraction of heat from the laser.
8. A method as claimed in claim 6, wherein the feedback regulates the d.c. component of drive current supplied to the laser.
9. A method of monitoring gas or vapour by differential absorption, which method is substantially as hereinbefore described with reference to the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08425918A GB2165640B (en) | 1984-10-13 | 1984-10-13 | Gas or vapour concentration monitoring |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08425918A GB2165640B (en) | 1984-10-13 | 1984-10-13 | Gas or vapour concentration monitoring |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8425918D0 GB8425918D0 (en) | 1984-11-21 |
| GB2165640A true GB2165640A (en) | 1986-04-16 |
| GB2165640B GB2165640B (en) | 1988-05-18 |
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Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08425918A Expired GB2165640B (en) | 1984-10-13 | 1984-10-13 | Gas or vapour concentration monitoring |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2165640B (en) |
Cited By (18)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0263931A2 (en) * | 1986-10-04 | 1988-04-20 | Kernforschungszentrum Karlsruhe Gmbh | Method and device for measuring continuously the concentration of a gas component |
| EP0303293A2 (en) * | 1987-08-14 | 1989-02-15 | AT&T Corp. | Laser temperature modulation and detection method |
| WO1989003028A1 (en) * | 1987-09-22 | 1989-04-06 | The Victoria University Of Manchester | Gas detection method and apparatus |
| WO1990000732A1 (en) * | 1988-07-07 | 1990-01-25 | Altoptronic Ab | Method and apparatus for spectroscopic measurement of the concentration of a gaz |
| EP0364642A1 (en) * | 1988-09-22 | 1990-04-25 | The Victoria University Of Manchester | Gas detection method and apparatus |
| WO1991010895A1 (en) * | 1990-01-10 | 1991-07-25 | Mütek Gmbh | Process and device for determining the concentration of isotopes |
| EP0708322A1 (en) * | 1994-10-14 | 1996-04-24 | Spectral Sciences Inc. | Diode laser system for monitoring chemical species |
| EP0729565A1 (en) * | 1993-04-16 | 1996-09-04 | MCCAUL, Bruce W. | Gas spectroscopy |
| US5572031A (en) * | 1994-11-23 | 1996-11-05 | Sri International | Pressure- and temperature-compensating oxygen sensor |
| DE19525415A1 (en) * | 1995-07-12 | 1997-01-16 | Wissenschaftlich Tech Optikzen | Gas concentration determination method esp. for water vapour in industrial oven - by setting laser light source by temperature or current regulation to selected absorption wavelength with smaller temperature dependence |
| US5742399A (en) * | 1996-04-18 | 1998-04-21 | American Air Liquide, Inc. | Method for stabilizing the wavelength in a laser spectrometer system |
| US5818578A (en) * | 1995-10-10 | 1998-10-06 | American Air Liquide Inc. | Polygonal planar multipass cell, system and apparatus including same, and method of use |
| US5835230A (en) * | 1997-07-10 | 1998-11-10 | American Air Liquide Inc. | Method for calibration of a spectroscopic sensor |
| US5880850A (en) * | 1996-04-18 | 1999-03-09 | American Air Liquide Inc | Method and system for sensitive detection of molecular species in a vacuum by harmonic detection spectroscopy |
| US5949537A (en) * | 1996-04-18 | 1999-09-07 | American Air Liquide Inc. | In-line cell for absorption spectroscopy |
| US5963336A (en) * | 1995-10-10 | 1999-10-05 | American Air Liquide Inc. | Chamber effluent monitoring system and semiconductor processing system comprising absorption spectroscopy measurement system, and methods of use |
| US6084668A (en) * | 1997-07-10 | 2000-07-04 | American Air Liquide Inc. | In-line cell for absorption spectroscopy |
| US6442736B1 (en) | 2000-10-03 | 2002-08-27 | L'air Liquide Societe Anonyme A Directoire Et Conseil De Surveillance Pour L'etude Et L'expolitation Des Procedes Georges Claude | Semiconductor processing system and method for controlling moisture level therein |
-
1984
- 1984-10-13 GB GB08425918A patent/GB2165640B/en not_active Expired
Cited By (27)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0263931A3 (en) * | 1986-10-04 | 1988-12-21 | Kernforschungszentrum Karlsruhe Gmbh | Method and device for measuring continuously the concentration of a gas component |
| US4849637A (en) * | 1986-10-04 | 1989-07-18 | Kernforschungszentrum Karlsruhe Gmbh | Method and apparatus for continuously measuring the concentration of a gas component |
| EP0263931A2 (en) * | 1986-10-04 | 1988-04-20 | Kernforschungszentrum Karlsruhe Gmbh | Method and device for measuring continuously the concentration of a gas component |
| EP0303293A3 (en) * | 1987-08-14 | 1990-08-22 | AT&T Corp. | Laser temperature modulation and detection method |
| EP0303293A2 (en) * | 1987-08-14 | 1989-02-15 | AT&T Corp. | Laser temperature modulation and detection method |
| WO1989003028A1 (en) * | 1987-09-22 | 1989-04-06 | The Victoria University Of Manchester | Gas detection method and apparatus |
| US5173749A (en) * | 1988-07-07 | 1992-12-22 | Altoptronic Ab | Method and apparatus for spectroscopic measurement of the concentration of a gas |
| WO1990000732A1 (en) * | 1988-07-07 | 1990-01-25 | Altoptronic Ab | Method and apparatus for spectroscopic measurement of the concentration of a gaz |
| EP0364642A1 (en) * | 1988-09-22 | 1990-04-25 | The Victoria University Of Manchester | Gas detection method and apparatus |
| WO1991010895A1 (en) * | 1990-01-10 | 1991-07-25 | Mütek Gmbh | Process and device for determining the concentration of isotopes |
| EP0729565A4 (en) * | 1993-04-16 | 1998-01-28 | Bruce W Mccaul | Gas spectroscopy |
| EP0729565A1 (en) * | 1993-04-16 | 1996-09-04 | MCCAUL, Bruce W. | Gas spectroscopy |
| US6150661A (en) * | 1993-04-16 | 2000-11-21 | Bruce W. McCaul | Gas spectroscopy |
| EP0708322A1 (en) * | 1994-10-14 | 1996-04-24 | Spectral Sciences Inc. | Diode laser system for monitoring chemical species |
| US5572031A (en) * | 1994-11-23 | 1996-11-05 | Sri International | Pressure- and temperature-compensating oxygen sensor |
| DE19525415A1 (en) * | 1995-07-12 | 1997-01-16 | Wissenschaftlich Tech Optikzen | Gas concentration determination method esp. for water vapour in industrial oven - by setting laser light source by temperature or current regulation to selected absorption wavelength with smaller temperature dependence |
| US5818578A (en) * | 1995-10-10 | 1998-10-06 | American Air Liquide Inc. | Polygonal planar multipass cell, system and apparatus including same, and method of use |
| US5963336A (en) * | 1995-10-10 | 1999-10-05 | American Air Liquide Inc. | Chamber effluent monitoring system and semiconductor processing system comprising absorption spectroscopy measurement system, and methods of use |
| US6154284A (en) * | 1995-10-10 | 2000-11-28 | American Air Liquide Inc. | Chamber effluent monitoring system and semiconductor processing system comprising absorption spectroscopy measurement system, and methods of use |
| US6493086B1 (en) | 1995-10-10 | 2002-12-10 | American Air Liquide, Inc. | Chamber effluent monitoring system and semiconductor processing system comprising absorption spectroscopy measurement system, and methods of use |
| US5742399A (en) * | 1996-04-18 | 1998-04-21 | American Air Liquide, Inc. | Method for stabilizing the wavelength in a laser spectrometer system |
| US5880850A (en) * | 1996-04-18 | 1999-03-09 | American Air Liquide Inc | Method and system for sensitive detection of molecular species in a vacuum by harmonic detection spectroscopy |
| US5949537A (en) * | 1996-04-18 | 1999-09-07 | American Air Liquide Inc. | In-line cell for absorption spectroscopy |
| US6188475B1 (en) | 1996-04-18 | 2001-02-13 | American Air Liquide, Inc. | In-line cell for absorption spectroscopy |
| US5835230A (en) * | 1997-07-10 | 1998-11-10 | American Air Liquide Inc. | Method for calibration of a spectroscopic sensor |
| US6084668A (en) * | 1997-07-10 | 2000-07-04 | American Air Liquide Inc. | In-line cell for absorption spectroscopy |
| US6442736B1 (en) | 2000-10-03 | 2002-08-27 | L'air Liquide Societe Anonyme A Directoire Et Conseil De Surveillance Pour L'etude Et L'expolitation Des Procedes Georges Claude | Semiconductor processing system and method for controlling moisture level therein |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2165640B (en) | 1988-05-18 |
| GB8425918D0 (en) | 1984-11-21 |
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| PCNP | Patent ceased through non-payment of renewal fee |