JP4346694B2 - Method for controlling laser light output for light scattering measuring device by inducing mode hopping and averaging the results and its illumination system - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a light scattering measurement device that measures scattered light from molecules and fine particles contained in a solution in a sample cell.
[0002]
[Prior art]
Accurate measurement of scattered light from molecules and fine particles in a solution is an experimental technique that is convenient and has a wide range of applications for determining various physical properties such as particle size and particle structure in addition to molecular weight and molecular size. .
[0003]
If the light scattering measurement is used in combination with various chromatographic separation devices and concentration detectors, the distribution difference of the various quantities can also be estimated. For the light scattering measurement, measurement conditions, and the results obtained from the measurement, see p. Je. Details are described in the paper of PJWyatt (Analytica Cbimica Acta Vol 272, pp 1-40 (1993)).
[0004]
The space of the laboratory is limited, and such a space is valuable, but it is preferable that the structure of the measuring apparatus is compact. It is desirable to use a semiconductor diode laser light source that is extremely small rather than a gas laser light source that is bulky enough to require a compact structure. However, semiconductor diode lasers have a number of problems that must be overcome for use in light scattering measurement devices and detectors.
[0005]
A sudden change in output level due to mode hopping is also a problem. Even if the action state of the laser changes slightly, the fundamental vibration mode moves from one mode to another mode, so that the laser operation output may change suddenly. This sudden change can be caused by a small change in temperature, drive current, or reflected light to the laser. When the sample is introduced, the additional light is scattered toward the laser, and the changing refractive index of the solution also changes the phase of the scattered light. The additional light may be reflected into the laser from the area of the sample cell through which the laser beam passes. To prevent this scattered reflected light from interacting with the laser, expensive components must be added. In the case of a very complex sample holding structure as described in U.S. Pat.No. 4,616,927, a number of parallel surfaces are provided so as to be orthogonal to the incident laser beam, so that the reflection component is hardly completely removed. It is impossible. When the temperature and drive current are at the mode shift critical point, the laser output can fluctuate up and down by several percent even if there is a slight random change in the level and phase of the reflected light.
[0006]
An important physical property measured by a light scattering measurement device, or more generally, a light scattering photometer, is the ratio of the incident light output per unit area to the sample to the scattered light output per steradian from the sample.
[0007]
In this case, if the laser light source is completely stable, the incident output need only be measured once for calibration. However, the laser output tends to change according to temperature, reflected light, drive current, and usage time. Therefore, in the conventional technique employed for correcting the fluctuation of the laser output, a method of dividing a part of the beam and monitoring the laser output with an optical detector using the divided part has been adopted. Using the laser monitor signal output from the detector, the laser output is stabilized by feedback to the electronic circuit that adjusts the drive current, or the scattered light signal is normalized in the process of mathematically processing the measurement data be able to. In either case, the scattered light signal is divided by the laser monitor signal.
[0008]
However, all of these methods involve small errors. Since mode hopping of the laser occurs very rapidly, the laser output changes over a short period of time until the drive current can be adjusted so that the monitor feedback signal and control circuit can recover the required output level. Similarly, there is neither simultaneity nor signal averaging symmetry between the scattered light detector signal and the laser monitor detector signal, so if the signal changes so rapidly that the monitor cannot track it will be a large error. May occur.
[0009]
Another source of error is the fact that the laser beam does not necessarily have one spatial and temporal component or beamlet. Thus, the laser monitor may prevent the monitor from accurately tracking a signal proportional to the scattered signal in response to a different beamlet combination than the light scattering detector responds to. The reason that the high frequency laser power change must be avoided in many cases against the sudden change caused by mode hopping is due to the problems described above.
[0010]
Mode hopping noise of diode lasers in optical communication signal applications occurs at very high frequencies and is the subject of many research reports and several patents. This very high frequency noise is particularly troublesome in the field of communications, as it occurs at the same frequency as the required communication signal frequency. In the case of light scattering measurements that are often made at very low frequencies, it is only necessary to accurately track the average scatter detector signal and the average monitor signal. Reduction of high-frequency mode hopping noise by 2.5 GHz drive current modulation. J. Vanderwall and J. Reported by J. Blackburn (Optics Let. Vol. 4, No. 9, pp 295-296, Sept 1979). In addition, noise reduction in video disk systems that are driven and modulated at frequencies of 100 MHz or higher is a Hitachi engineer M.M. Ojima (M.Ojima) and S. Reported by S. Yonezawa (Applied Opotics Vol. 25, No. 9, 1 May 1986). In addition, IBM engineer Kay. K. Stubkjaer and M. Small (M. Small) reported noise reduction by 50 to 200 MHz modulation (Electronics Let. Vol. 19, pp388-399 (1983)). Said M. Ojima (M. Ojima) et al. Used 200 MHz to 1 GHz modulation. E. E. Gage and S. In the paper by S. Beckens (Optical Data Storage, SPIE Vol. 1316, pp 199-204 (1990)), the optimum modulation drive frequency for reducing high frequency noise is related to the delay of reflected light to the laser. To make it clear. In this paper, a modulation drive frequency of 100 to 450 MHz was examined. Furthermore, in these papers, extremely high levels of modulation are used, and in practice, driving current below the limit value is applied during part of the modulation cycle, and it is considered that coherent emission stops during this time. ing.
[0011]
Ryoichi Ito's US Patent No. 3,815,045 (1974) “Method of and Device for Modulating Directly a Semiconductor Laser” assigned to Hitachi is used to shift the semiconductor laser between two spatially different modes. A high frequency modulation of the output beam is described using a modulation drive current and optically selecting only one of the spatial modes. Because light scattering photometers require a stable light beam rather than a modulated light beam, this method cannot be applied to light scattering photometers. An optimum laser for a light scattering measurement device operates in only one spatial mode to form a collimated beam. As described in the Ryoichi Ito patent, the problem of stray light becomes serious when operating in other spatial modes.
[0012]
M transferred to Mitsubishi. M. Kohno and J. In J. Itami's Japanese patent application No. 59-9086 and the corresponding German application DE 41 33 772 A1 (May 21, 1992), the reflected light is sufficiently reduced by 500-600 MHz modulation. A compact disk read detection structure is disclosed that is configured to generate In this case, the optimum frequency is selected according to the distance from the reflection area of the disk to the laser light source. However, this method cannot be applied to a light scattering measurement apparatus for several reasons described below. First, the use of a drive current of 500 MHz is problematic and disadvantageous in terms of cost. Second, since the material of the light scattering sample often travels with or within the laser beam on the fluid flow, there is no constant scattering distance, and therefore a single optimal modulation high frequency Does not exist. Third, reducing extremely high fringe noise is almost meaningless in light scattering measurement devices where signal averages of 0.1 to 10 seconds are often used.
[0013]
In the prior art, noise reduction methods, without exception, focus on the laser power itself, and do not directly use the detected signal filtering, signal averaging or signal to laser monitor ratio calculation.
[0014]
[Problems to be solved by the invention]
The present invention pays attention to the overall performance of the entire light scattering measuring device in which only the ratio of the scattered light and the incident laser output is required. This allows for fluctuations in the laser output if it can be effectively tracked or removed after signal detection. The proposal of the present invention for the problem is to shift the frequency of the mode hopping noise above the pass band of the signal processing filter included in the detection system. Therefore, the laser drive modulation, the matching signal average filter, and the scattered light / incident light ratio calculation are the main points of the present invention.
[0015]
An object of the present invention is to make it possible to use a compact solid-state laser light source for a light scattering measurement apparatus. Similar techniques can be applied to other types of laser light sources with high frequency variations, as will be apparent to those skilled in the art. A light scattering measuring device or photometer usually measures scattered light from one or more angles, but it is usually changing the intensity of the incident light source by means of a beam splitter or the like, so it changes slowly. Can also be used with a light source with unsteady output. Due to mode hopping due to reflected light and / or stray light re-entering the laser and mode hopping due to temperature changes, the output of lasers, especially solid diode lasers, is not abruptly averaged and monitored with standard techniques. Strength variations or gradual changes may appear. For example, a change in mode structure can change not only the total laser output, but also the ratio of the laser monitor output to the sample incident power. Such a problem is particularly serious in the case of an extremely thin solution of small molecules with weak scattered light, and since stray light is likely to change with changes in the laser mode, it is necessary to accurately subtract stray light. The present invention helps solve this problem by modulating the intensity of the laser so that the output signal can be effectively filtered and measured with high accuracy in both laser monitors and scattered light detectors. It is about to try. To assess all mode structures, abrupt modulation of the laser drive current eliminates the gradual changes that appear in the averaged signal. As a result, the remaining slow change can be accurately tracked, and an accurate ratio of scattered light to incident light can be obtained. In the case of a laser having an external optical system, in order to change the mode structure, for example, a modulation form such as movement of a laser resonator mirror or optical phase modulation in the laser resonator can be used.
[0016]
[Means for Solving the Problems]
The present invention mitigates the effect of mode hopping on the measured light scattering ratio. Therefore, the present invention is in contrast to the known art in that it seeks to suppress the optical signal formed by the laser itself. The modulated laser power contains abrupt power fluctuations, but by averaging the modulated laser power electronically, the averaged laser power is small and slow, except for sudden power level fluctuations. Leaving only smooth fluctuations. This slowly varying average laser power level is measured using a split beam monitor. Similarly, the light scattering signal is averaged to form a smooth, slowly varying signal, so that the ratio of the scattered light signal to the laser output signal at each monitor angle required for light scattering measurement is obtained. .
[0017]
【Example】
FIG. 1 shows a configuration of light scattering measurement by a light scattering photometer using a fluid passing through a sample cell 1 surrounded by a plurality of collimated scattered light detectors 2. Both the scattered light detector 2 and the detector used for the illumination light source / laser monitor described later are generally hybrid photodiodes incorporating a linear amplifier. Although photomultiplier tubes have been used for this detection, their quantum efficiency is very low at red wavelengths from almost any type of laser diode. The sample is illuminated by a solid state laser 3, and the laser control power supply 4 is modulated by a waveform generator 5. The solid state laser 3 is usually combined with a collimating lens member so as to form a collimating output called a laser beam 10. In the preferred embodiment of the invention, the waveform generator 5 forms a triangular wave. The sample cell 1 may have a configuration similar to that described in U.S. Pat. No. 4,616,927, but other types of sample cells 1 illuminated by the solid state laser 3, such as rectangular or cylindrical cells, may be photometric. Can be used together with the same laser modulation method.
[0018]
Each of the scattered light detectors 2, 2,... Has an amplifier including an analog filter 6. The output signal from the scattered light detector 2 is digitized by individual digitizer circuits including the digital filter 7 to block high frequency signals that are not compatible with the experimental state to be measured. Analog and digital filters are typically low-pass filters that are selected to pass signals of typical frequency from the light scattering signal to be measured and to block frequency signals that are generated as a result of laser modulation. It is. It goes without saying that the same result can finally be obtained by using an analog multiplexer to switch all signals to a single A / D converter. The digital signal thus processed may be multiplexed by the digital multiplexer 8 for transmission to the computer via the serial port of the computer 9.
[0019]
A portion of the laser beam 10 is directed by a beam splitter 11 to a laser monitor 12 having similar analog and digital components associated therewith.
[0020]
Since the driving current of the solid-state laser 3 is an important parameter for controlling the laser output and preventing the diode from being damaged, the conventional configuration often uses a current adjusting power source. It goes without saying that Norton equivalent power supplies and current limiting resistors can also be used. A convenient way to “sample” the available mode states by modulating the laser current is to add a periodic waveform voltage to the reference voltage of the power supply so that the laser current varies with the periodic waveform. Although a waveform other than the triangular wave may be used, the triangular wave is preferable in that the consumption time at each current level is equal. In the preferred embodiment, the peak current can be limited to a safe value for the laser diode by a single controller, and the degree of modulation can be adjusted by a unique controller. The depth of modulation may be adjusted so that the mode hopping noise is smoothed as much as possible.
[0021]
The minimum laser drive current will be referred to as the limiting current. The laser light output increases rapidly as the drive current rises above the limit level. In light of experience, to smooth mode hopping noise, peak-to-peak modulation of currents above the limit level is up to about 50%, but about 15% is generally sufficient. For example, for a diode that is used at 60 ma and has a limiting current of 40 ma, peak-to-peak modulation of 0.15 × (60−40) = 3 ma is required to achieve smoothing. The maximum laser diode driving current of the solid-state laser 3 is limited to a value lower than the instantaneous peak value that damages the laser irreparably. In practice, the laser can be driven with an average current very close to the instantaneous maximum. At drive currents below the maximum value, the modulation takes some time and the average output is low.
[0022]
The advantage of the preferred embodiment of the present invention is that only 15% of the modulation is required. That is, the average output loss is very small. The optimum modulation frequency varies to some extent depending on the system. For example, commercially available from Wyatt Technology Corporation incorporating a 20 mW InGaAIP laser such as the Toshiba TOLD 9140 operating in single transverse mode at 690 nm. In the case of the miniDAWN® photometer, we have found that a modulation frequency of 150 Hz is convenient and effective.
[0023]
This frequency was high enough to handle effectively by photometer analog and digital filters and low enough to effectively sample longitudinal mode hopping. Ideally, the modulation frequency should be 10 to 1000 times the frequency of the measured scattered light signal. The laser output beam used in a light scattering measurement device is usually linearly polarized. However, such polarization is not important to the present invention. In a preferred embodiment of the present invention, incident light is polarized in a vertical direction with respect to a horizontal plane including the scattered light detector 2 when measuring light scattering.
[0024]
FIGS. 2A and 2B show a triangular wave applied voltage used to modulate the laser output and a laser driving current according to the applied voltage, respectively, in a preferred embodiment of the present invention. The measurement frequency of a typical light scattering signal formed as the sample eluent passes through the optical fluid cell of the scattered light detector 2 is a few Hz or less. Based on this fact, the inventors have found that a modulation frequency of about 150 Hz is appropriate for sufficiently smoothing irregularities in laser output caused by mode hopping. If it is necessary to measure a scattered signal with a relatively high frequency, the laser modulation frequency must be correspondingly increased.
[0025]
As already mentioned, the modulation frequency must be roughly one to three orders of magnitude higher than the detection frequency. A frequency of 50 to 5000 Hz is sufficient for typical light scattering measurements made with liquid chromatographic separation of polymer samples. In light of experience, we have found that the optimum frequency and amplitude for modulating the laser current will vary depending on the particular laser and detection system configuration. As will be apparent from the examples below, finding the optimal modulation frequency and amplitude for various types of laser sources that may affect subsequent scatter measurements due to mode hopping noise is a light scatter measurement professional. It will be relatively easy. In the preferred embodiment of the present invention, the laser control power supply 4 is triangularly modulated, but it will be apparent to those skilled in the art that satisfactory results can be obtained using other waveforms. For example, by scanning with simple sine waves, sawtooth waves, and other periodic or random waveforms, any mode that is believed to be responsible for output fluctuations can be sampled.
[0026]
3A-3D show experimental data collected by triangular wave modulation of the preferred embodiment. Data 13 in FIG. 3A shows the unmodulated laser power obtained by instantaneous heating of the diode laser with a heat gun and then natural cooling. When a 15% modulated triangular wave laser current of 150 Hz is used, an output characteristic 14 as shown in FIG. 3B is obtained. Note that since the scatter signal is normalized by division by the beam monitor signal, smooth, long-term fluctuations in the laser output are not important for typical light scatter measurements. Such normalization involves digitizing each analog scattered light and laser monitor signal by an A / D converter, filtering the digital data by a digital signal processor, storing the digitized and filtered values in computer means, The required ratio is calculated by dividing the digitized scattered light signal by the digitized laser monitor signal to obtain a digitized ratio of each scattered light signal. The smoothed signal ratio can also be obtained by analog dividing means in which the output ratio signal itself is an analog value. As an alternative to this general method, those skilled in the art will be able to conceive of, for example, adjusting the average laser power using a laser monitor signal.
[0027]
When a thin polystyrene sample is injected into a flowing solvent through a photometer system as shown in FIG. 1, the light scattering signal 15 of unmodulated scattering 90 ° shown in FIG. 3C and the corresponding laser output shown in FIG. 3C. 16 is obtained. The noise at the peak 17 in FIG. 3C is due to the mode hopping caused by the scattered light being reflected to the laser. The average light scattering signal in the case of laser modulation is indicated by 18 in FIG. 3D and the laser output is indicated by 19. The peak 20 shown in FIG. 3D does not have peak data irregularity like the peak 17 shown in FIG. 3C. The light scattering signal corresponding to the non-modulated laser current affected the laser output itself by reflecting a part of the light to the laser and / or changing the phase of the reflected light.
[0028]
FIG. 4 is a simplified diagram of a preferred embodiment of a circuit for supplying modulation current to a laser diode such as that used in the light scattering photometer shown in FIG. The potentiometer 21 supplies an adjustable DC voltage to the non-inverting input of the operational amplifier 26. The operational amplifier 26 controls the gate of the field effect transistor 28, which controls the current to the laser diode 29. Negative feedback to the operational amplifier 26 for current control is given by the voltage generated in the series resistor 27. Modulation is accomplished by AC coupling a square wave voltage 22 to the current control input via a coupling capacitor 23 and an adjustable coupling resistor 24. The filtering action of the resistor 24 and the coupling capacitor 23 converts the square wave into a wave very close to the intended triangular wave. Modulation AC coupling allows the control of the average DC laser current by the potentiometer 21 and the modulation amplitude by the resistor 24 to be performed independently.
[0029]
Although preferred means for obtaining an effective result by utilizing a laser light source containing essentially noise in a light scattering measurement apparatus have been described above, it will be apparent to those skilled in the art that the preferred embodiment of the present invention described above is applied. On the other hand, it is possible to make various changes, and any change relating to the modulation type, amplitude and frequency is included in the scope of the present invention.
[Brief description of the drawings]
FIG. 1 is a block diagram of a stabilization method that forms part of a light scattering detection scheme as a preferred embodiment of the present invention.
FIGS. 2A and 2B are graphs showing laser drive current triangular wave modulation applied voltage and the resulting laser drive current. FIG.
FIGS. 3A and 3B are graphs showing the effect of a noise reduction method on laser output mode hopping noise caused by temperature drift; FIGS. C and D are graphs showing the effect of a noise reduction method on laser output mode hopping noise caused by reflected light from a light scattering sample.
FIG. 4 is a circuit diagram for typical laser stabilization for a solid-state InGaAIP laser power source operating at 690 nm with a 20 mW peak power.
[Explanation of symbols]
1 Sample Cell 2 Collimated Scattering Light Detector 3 Solid State Laser 4 Control Power Supply 9 Computer 10 Laser Beam 11 Beam Splitter 18 Average Light Scattering Signal 20 by Laser Modulation 20 Peak

Claims (23)

In a laser light irradiation system for a light scattering measurement device incorporating a laser that emits a laser beam that inherently has noise due to mode hopping instability,
a) laser emitting means for emitting the laser beam inherently affected by the mode hopping instability;
b) a laser driving current modulation means capable of modulating the laser driving current with a frequency of about 10 to 1000 times the frequency for measuring the scattered light signal and a maximum amplitude below the peak current level of the laser;
c) means for splitting a portion of the emitted laser beam and means for outputting a laser monitor signal representing the output of the laser beam by detecting the split portion with a monitor detector;
d) Low-pass filter means for passing a signal having the frequency of the scattered light signal to be measured and blocking the signal having a modulation frequency, the fluctuations and modes of the laser monitor signal due to the modulation of the laser beam. Light scattering characterized by comprising a low-pass filter means for laser monitor signals, which simultaneously removes signal fluctuations due to noise caused by hopping and is substantially equal to the low-pass filter used in the light scattering measuring device. Laser light irradiation system for measuring equipment.   2. The laser light irradiation system according to claim 1, wherein the laser driving current modulation means outputs a minimum laser driving current equal to or higher than a limit driving current necessary for starting the laser. 2. The laser driving current modulating means modulates the laser driving current with an amplitude of about 15% of a difference between the laser driving current and a limit value which is a minimum current value for driving a laser. The laser light irradiation system described.   The low-pass filter means for the laser monitor signal includes an A / D converter for converting the laser monitor signal, a digital storage means for storing the digitally converted signal, and a smoothing processing for the stored digital conversion signal. 2. The laser light irradiation system according to claim 1, further comprising: a digital signal processing means for converting the digital signal processing means.   The laser light irradiation system according to claim 1, wherein the laser is a solid-state diode laser.   2. The laser light irradiation system according to claim 1, wherein the laser driving current modulating means modulates the laser driving current at 50 to 5000 Hz.   The laser light irradiation system according to claim 1, wherein the monitor detector is a photodiode.   2. The laser light irradiation system according to claim 1, wherein the laser driving current is modulated with a triangular wave by the laser driving current modulating means. 2. The laser beam according to claim 1, wherein an amplitude of modulation of the laser drive current modulation means is 50% or less of a difference between the laser drive current and a limit value which is a minimum current value for driving the laser. Irradiation system.   The laser light irradiation system according to claim 1, wherein the laser forms a linearly polarized output beam.   6. The laser light irradiation system according to claim 5, wherein the laser operates at a wavelength of about 690 nm. In a method for controlling a laser for light scattering measurement, which inherently has noise due to mode hopping instability,
a) modulating the laser drive current with a frequency approximately 10 to 1000 times the measurement frequency of the scattered light signal and a maximum amplitude below the peak current level of the laser;
b) Dividing a portion of the emitted laser beam and detecting the divided portion with a monitor detector to obtain a value representing the output of the laser beam;
c) The signal from the monitor detector is smoothed by a low-pass filter that passes the signal having the frequency of the scattered light signal to be measured and blocks the signal having the modulation frequency, so that the signal due to noise caused by mode hopping At the same time, fluctuations are removed,
d) detecting the scattered light of the sample illuminated by the modulated laser beam from one or more angles, and using the respective detected scattered light signals at the respective angles in the low range used in step c) Smoothing with a low-pass filter substantially equal to the filter, smoothing the detected scattered light fluctuation caused by the modulation of the laser beam by each low-pass filter and simultaneously removing signal fluctuation due to noise caused by mode hopping,
e) from the step of forming the output ratio of the scattered light from the sample and the incident light on the sample at each measurement angle by dividing the thus smoothed scattered light signal by the similarly processed laser monitor signal. A light scattering measurement method characterized by comprising: 13. The light scattering measurement method according to claim 12 , wherein the minimum value of the modulated laser driving current is larger than a limit driving current necessary for starting the laser. 13. The light scattering measurement method according to claim 12 , wherein the modulation depth of the laser driving current is about 15% of a difference between the laser driving current and a limit value which is a minimum current value for driving the laser. . 13. The light scattering measurement method according to claim 12, wherein the modulation of the laser driving current is about 150 Hz. The light scattering measurement method according to claim 12 , wherein the laser is a solid-state diode laser. 13. The light scattering measurement method according to claim 12 , wherein the modulation is 50 to 5000 Hz. 13. The light scattering measurement of claim 12, wherein the detected and smoothed laser monitor signal and the smoothed scattered light signal are digitized and the output ratio is calculated by computer means. Method. 13. The light scattering measurement method according to claim 12 , wherein each of the scattered light signals is analog-divided by the laser monitor signal to obtain the output ratio. The light scattering measurement method according to claim 12 , wherein the laser driving current is modulated by a triangular waveform. 13. The light scattering measurement method according to claim 12 , wherein a depth of modulation of the laser driving current is 50% or less of a difference between the laser driving current and a limit value which is a minimum current value for driving the laser. . The light scattering measurement method according to claim 12 , wherein the laser beam is linearly polarized. The light scattering measurement method according to claim 12 , wherein the monitor detector is a photodiode.

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