DE102013106137A1 - Method for measuring the pulse energy of an optical radiation and measuring device for carrying out the method - Google Patents

Abstract

A method and a measuring device for measuring the pulse energy of an optical radiation are described, with the aid of which a measuring error is reduced, which is related to the uncertainty of an exact pulse generation time between two discrete readings of an analog-digital converter. The benefit is achieved by using a standardization pulse converter, which generates electrical pulse signals in a uniform (standardized) form, which is independent of the radiation pulse shape and of the respective sequence of digital signal processing.

Description

The invention relates to a method for measuring the pulse energy of an optical radiation according to the preamble of claim 1 and to a measuring device for carrying out the method.

The invention can be used for pulse energy measurement of the optical and predominant laser radiation within a large spectral range as well as for devices which measure the energy of the pulsed light radiation. The invention can be used in production facilities as well as in medical devices to determine the energy characteristics of pulsed lasers.

Currently, most radiant energy meters with a wide spectral range of leading manufacturers, such. "Coherent", "Spectrum Detector", "Newport", etc., from highly developed countries based on pyroelectric radiation detectors. This is because they well combine the inherent selectivity inherent in the thermal radiation receivers with a high operating speed. Their speed of operation is comparable in some cases with that of photoreceptors. Its linearity range is even superior to the photoreceiver. Moreover, the pyroelectric receivers have an integral mode as one of their modes. In integral mode, an electrical signal is proportional to the total energy of a radiation pulse. This makes it possible to significantly simplify the circuits of the measuring devices and to increase the measuring accuracy. These facts predestinate the interest for the development of devices based on the pyroelectric radiation receivers and for a method of measuring impulse radiation energy with their help.

A known method for pulse energy measurement by means of a pyroelectric receiver is in Book by VF Kossorotov, LS Kremenchugsky, VB Samoylov and LV Shchedrina "Pyroelectric Effect and Its Practical Applications", Kiev, Naukova Dumka, 1989 (p. 149) described. According to this method, a receiver operated in integral mode is irradiated with a pulse. This pulse is much shorter than the electrical time constant of the receiver (τ _Imp. << τ _el. ). The signal amplitude of the receiver is measured at the pulse end. The measured signal amplitude is multiplied by a calibration factor of the meter. This gives the pulse energy.

The said method can be carried out with the aid of a device which in the Article by O. Touayar et al. "Experimental evaluation of a pyroelectric detector linearity used for pulsed laser energy absolute measurement"("Sensors and Actuators", A 120 (2005), 482-489) is described. The device consists of two units. The first unit is a pyroelectric sensor with a preamplifier. The output pulse signal amplitude of the preamplifier is proportional to the laser pulse energy. The second unit is used to measure this amplitude. It consists of a comparison circuit, two monostable flip-flops and an integrator. The signal amplitude is compared in the comparison circuit with a threshold value. The threshold corresponds to the electrical and the acoustic noise. If the threshold is exceeded, the comparison circuit activates the first monostable flip-flop. The first monostable flip-flop generates a pulse with a shift corresponding to the signal increase time (signal growth time) T _m , and activates the second monostable flip-flop. The second monostable flip-flop activates an integrating circuit. The integrating circuit generates a voltage that is proportional to the pyroelectric signal amplitude.

The disadvantage of this method and the device for its implementation is an insufficient measurement accuracy of the pulse energy. It is due to uncontrollable errors that occur in the formation of temporary shifts in the system of monostable flip-flops. The measurement accuracy of the single-signal amplitude is also influenced by the fact that there is no digital statistical signal and noise processing. This considerably increases the measurement uncertainty, especially in the case of a stepless and slow decrease in the amplitude of the optical pulse or in the presence of piezoelectric oscillations which arise in the receiver space under the effect of a short radiation pulse.

The closest prior art is a method and apparatus for laser pulse energy measurement (Patent "Method and apparatus for measuring laser pulse energy"). US 5,980,101 [PC ⁶ G01 17/00, inventor: JR Unternahrer, FR Staver, 9.11.1999). Here a first laser pulse is sent to an energy generator. The energy generator generates a first electrical signal, which corresponds to the first laser pulse, with the aid of which the time constant of the energy generator is determined. The second laser pulse causes the pyroelectric receiver to generate a second signal. The second signal is used to precisely determine the energy of the second laser pulse in consideration of the time constant of the encoder.

The method is performed by means of a device. The device consists of energy sources, an analog-to-digital converter and a computer. The energy sources receive the laser radiation and generate a signal which corresponds to the laser radiation energy. Then another filtering is done. The computer first calculates the time constant of the encoder based on the converted signal of the first laser pulse. Thereafter, the energy of the second laser pulse is calculated on the basis of the converted signal of the second laser pulse and on the basis of the transmitter time constant. The existing computerized data processing system makes it possible to improve the measurement accuracy in the laser energy measurement by the statistical signal properties, the noise and the acoustic and electrical disturbances are evaluated.

The deficiency of this method and the device for its execution is an insufficient measurement accuracy of the pulse energy. It is due to the fact that for the different pulse shapes and the different pulse duration a matching amplifier with a wide passband for the receiver is necessary. This leads to a noise voltage increase and accordingly to a reduction of the signal-to-noise ratio.

The further deficiency of this method and apparatus is that the analog-to-digital signal conversion associated with the detection of signal strength at discrete (discrete) times, provides no opportunity to determine both the pulse generation timing and the pulse maximum timing. This causes a reduced measurement accuracy and a lack of measurement data security.

It is an object of the invention to increase the measurement accuracy in the measurement of the laser pulse radiation energy by means of a perfection of the method for laser radiation energy measurement and the device for this purpose, by the above-mentioned deficiencies are eliminated.

The stated object is solved by the features of claim 1.

wobei T der Zeitabschnitt zwischen zwei benachbarten Abfragen des AD-Umsetzers ist, τ1 die Zeitkonstante des Eingangskreises des Anpassungsverstärkers, U1 und U2 jeweils die Amplituden der ersten und der zweiten Abfrage ungleich Null sind.The inventive method comprises the following method steps: irradiation of a pyroelectric detector with an optical radiation pulse, filtering and amplification of an electrical signal, transmission of an electrical signal to an analog-to-digital converter (AD converter), transmission of a digital signal image a calculator and a subsequent mathematical processing. The mathematical processing in this case includes a determination of the zero level in the digital signal image, this part always precedes a pulse start, a detection of a signal drop section after a signal extreme value, a signal extrapolation and a calculation of the pulse energy. In this case, the electrical signals corresponding to the pulses of different shapes and different duration, previously converted into electrical pulses of the same shape and duration. The recognized section is approximated by a straight line. Then t _{Anf is} calculated, where t _{Anf is} the start time of the radiation _pulse . It is calculated according to the following equation: t _Anf = t ₀ + Δ, where t _{0 is} the time of origin of the first value read not equal to zero and Δ is the time span between the beginning of the pulse and the reading time U 1:

where T is the period of time between two adjacent samples of the AD converter, τ1 is the time constant of the input of the matching amplifier, U1 and U2 are the amplitudes of the first and second samples, respectively non-zero.

Thereafter, the pulse energy is calculated by taking the value on the approximate line at the pulse start time t _Anf. is multiplied by the calibration factor of the energy meter.

The stated object is also achieved by using a measuring device according to the invention. The meter consists of a pyroelectric detector and a matching amplifier. The output of the matching amplifier is connected to the input of the analog-to-digital converter. The output of the analog-to-digital converter is connected to a computer. The output of the detector is connected to the input of a normalizing pulse transformer. The output of the normalizing pulse transformer is connected to the input of the matching amplifier. The normalization pulse converter is designed so that its amplitude-frequency characteristic in a frequency range between (τ _int. ) ^-1 and the upper Limit of the working frequency range, where τ _{int is} the time constant of the integration circuit of the matching amplifier. The latter ensures that electrical pulses are produced at the output of the matching amplifier whose spectrum is in this frequency range.

To explain the invention in more detail, a signal conversion during the laser pulse energy measurement using the method and measuring device according to the invention is considered. Show it:

• 1 die Form des Stromimpulses, welcher von dem pyroelektrischen Detektor erzeugt wird,
• 2 die Signalform am Ausgang des zusätzlichen Filters mit einer Zeitkonstante von 3 Mikrosekunden und
• 3 die Signalform am Ausgang des Anpassungsverstärkers mit einer Integrierzeitkonstante von 100 Mikrosekunden ist,

1 the response of the pyroelectric converter unit to a 10 nanosecond radiation pulse, wherein

• 1 the shape of the current pulse generated by the pyroelectric detector,
• 2 the waveform at the output of the additional filter with a time constant of 3 microseconds and
• 3 is the waveform at the output of the matching amplifier with an integral time constant of 100 microseconds,

2 the graphical representation of the measured data quantity with the instantaneous pulse values and the description of the methods for a digital data processing as well

unkorrigiert und

unter Berücksichtigung der Korrektur nach der Gleichung (3)). 3 the pulse energy measurement errors depending on the time interval t between the pulse start and the first next read value of the AD converter for the times 0 ≦ t <T (

uncorrected and

considering the correction according to the equation (3)).

In 1 is a response of the pyroelectric transducer unit to a 10 nanosecond radiation pulse (line 1 ). An additional filter with a time constant of 3 μsec. generates electrical pulse signals having a uniform shape (line 2 ). This standardized uniform shape does not depend on the radiation pulse shape over the entire duration range (<10 μsec). The line 3 shows the waveform at the output of the matching amplifier with an integration time constant of 100 microseconds. The amplitude of this signal is proportional to the total radiation energy.

The pulse calculation operation is performed on the basis of a measured data amount with current pulse values. This amount of data is collected for each detected pulse during real time measurement. The graphical representation of the current momentum measurement data set as well as the description of the methods for the digital processing of this data set in order to determine a numerical energy value for this pulse are the 2 refer to.

• • Abschnitt I: in Bezug auf das Nullniveau des Signals,
• • Abschnitt II der Impulsvorderflanke: Es handelt sich dabei um einen Abschnitt vom ersten abgelesenen Wert ungleich Null bis zum Maximalwert (Punkt b),
• • Abschnitt III der Impulsrückflanke.

Each data set consists of 32 instantaneous voltage readings Y (0), Y (1) ... Y (31), which represent a digital image of the respective pulse. The dataset consists of three distinctive sections:

• • Section I: with respect to the zero level of the signal,
• • section II of the leading edge of the pulse: this is a section from the first non-zero reading up to the maximum value (point b),
• • Section III of the pulse trailing edge.

The relative zero level of the signal is indicated by a horizontal line ( 1 ) approximates. The level of this line is obtained by sorting the subset values Y (0) ... Y (12). The relative zero level is the median of the sorting of the subset values Y (0) ... Y (12) (point a of the line ( 1 )). The 2 shows a data amount of current measurement data after the performed sorting operation of the subset Y (0)... Y (12). Thus, the relative zero level of the signal is Y (6).

The leading edge of the pulse is approximated to be the calculated starting _{instant of the leading edge of} the pulse t _Anf. (Point c) to find. In order to reduce the error in the determination of the calculated starting time of the pulse leading edge, a new signal processing method is inventively proposed and executed in the software for the energy meter. This makes it possible t _Anf. with a much smaller error than the time T between the adjacent start times of the AD converter.

wobei es sich um einen kurzen Impuls (τ_Imp. < τ₁) mit einer Amplitude U₀ mit einer Genauigkeit bis auf seine Konstante handelt.It is now considered a general case. Here, the electrical pulse with the pulse duration τ _Imp. From the pyroelectric detector via an input circuit with a time constant τ ₁ to the input of the matching amplifier with a Integrierzeitkonstante τ _{2 is} sent. The ratio τ ₁ << τ _{2 is} maintained. The time function (time dependency) of the output signal U / (t) of the matching amplifier when pulsing to its input looks as follows:

where it is a short pulse (τ _Imp. <τ ₁ ) with an amplitude U ₀ with an accuracy except for its constant.

From equation (1) shows that the shape of the output signal of the amplifier U (t) during the irradiation of the detector with the short pulse from the pulse duration τ _Imp. Does not depend and that they τ completely by the constant combination ₁ and τ ₂ is described.

wobei

Δ

die Zeitspanne zwischen dem Impulsanfang und dem Ablesezeitpunkt U1,

T

der Zeitabschnitt zwischen zwei benachbarten Abfragen des AD-Umsetzers ist.

Taking into account the ratio τ ₁ << τ ₂ for the first two non-zero measured values (U1 and U2, respectively) of the analog-to-digital conversion of the function U (t), a period of time 0 ≤ t <T can be written generally as follows :

in which

Δ

the time interval between the start of the pulse and the reading time U1,

T

is the period of time between two adjacent queries of the AD converter.

The a priori uncertainty of the pulse start time and, accordingly, the magnitude Δ is an essential component of the radiant energy measurement error.

In the solution of system (2) with respect to the size Δ, the result is:

unkorrigiert und

unter Berücksichtigung der Korrektur nach der Gleichung (3)) dargestellt. Die Berechnungen wurden bei folgenden Kenndaten durchgeführt: T = 3,4 μSek., τ₁ = 2,5 μSek. und τ₂ = 100 μSek.In 3 are measurement errors for the pulse energy measurement depending on the time interval t between the pulse start and the first next read value of the AD converter for the times 0 ≦ t <T (

uncorrected and

considering the correction according to equation (3)). The calculations were carried out with the following characteristics: T = 3.4 μsec, τ ₁ = 2.5 μsec. and τ ₂ = 100 μsec.

• • Die Vergrößerung des Zeitabschnittes zwischen dem Impulsanfang und dem Ablesezeitpunkt U1 verursacht bei beiden Verfahren eine Vergrößerung des Fehlers der Impulsenergieermittlung.
• • Der Maximalfehler der Energieermittlung ohne Rücksicht auf die Korrektur Δ beträgt 2,6%.
• • Der Maximalfehler der Energieermittlung mit Rücksicht auf die Korrektur Δ beträgt 0,25%.

From the characteristics, the following can be concluded:

• The increase in the time interval between the start of the pulse and the reading time U1 causes an increase in the error of the pulse energy determination in both methods.
• • The maximum error of the energy determination without regard to the correction Δ is 2.6%.
• • The maximum error of the energy determination with regard to the correction Δ is 0.25%.

The consideration of the correction quantity Δ reduces this component of the measurement error by up to 10 times. Thus, the calculation of the correction quantity Δ according to the equation (3) makes it possible to reduce the error associated with the uncertainty of the pulse start time and to increase the measurement accuracy of the radiation pulse energy.

The pulse trailing edge (period III, 2 ) is approximated by a straight line according to the least squares method. For this, a subset Y (20)... Y (27) is rejected from the measured data quantity of the instantaneous pulse values. This subset is obviously due to the pulse trailing edge. The approximation line (straight line ( 2 )) is passed over the points Y (20) ... Y (27) according to the least squares method. On this approximation line, the coefficient values A and B of this line are looked up.

A, B

die Koeffizienten der Näherungsgerade und

X(c)

die berechnete Impulsanfangszeit sind.

Then the argument value X (c) is inserted in the straight-line equation with the coefficients A and B and the calculated pulse amplitude (point d on the straight line ( 2 )) in volts. U = A * X (c) + B (4) in which

A, B

the coefficients of the approximation line and

X (c)

are the calculated pulse start time.

K

die Volt-Joule-Empfindlichkeit des pyroelektrischen Empfängers über die Kalibrierwellenlänge und

K1

der Koeffizient der relativen Empfindlichkeit über die Strahlungswellenlänge ist.

The pulse energy is calculated as follows: E = U · K · K1 (5) in which

K

the volt-joule sensitivity of the pyroelectric receiver over the calibration wavelength and

K1

is the coefficient of relative sensitivity over the radiation wavelength.

Thus, the application of the above-described method of measuring the pulse energy of the optical radiation and the device therefor allows to reduce the measurement error. This error is related to the uncertainty of the exact pulse generation time between two discrete readings of the analog-to-digital converter. A performance effect is achieved by using a normalizing pulse transformer which generates electrical pulse signals having a uniform (standardized) shape. These pulse signals are independent of the radiation pulse shape and of the respective sequence of digital signal processing.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 5980101 [0007]

Cited non-patent literature

• Book by VF Kossorotov, LS Kremenchugsky, VB Samoylov and LV Schschedrina "Pyroelectric Effect and Its Practical Applications", Kiev, Naukova Dumka, 1989 (p 149) [0004]
• Article by O. Touayar et al. "Experimental evaluation of a pyroelectric detector linearity used for pulsed laser energy absolute measurement"("Sensors and Actuators", A 120 (2005), 482-489) [0005]

Claims (2)

Method for measuring the pulse energy of an optical radiation comprising the following method steps: irradiation of a pyroelectric detector with a pulse, filtering and amplification of an electrical signal, which is then sent to an analog-to-digital converter, a subsequent transmission of a digital signal image to a computer with a subsequent mathematical processing, wherein the mathematical processing consists of the following steps: a determination of the zero level in the digital signal image, this part always precedes a pulse start, a detection of a signal drop section for a signal extremum, a signal extrapolation and a calculation of the pulse energy, characterized in that the electrical signals, which correspond to the pulses of different shapes and different duration, are previously converted into electrical pulses of the same shape and duration that the recognized section by a straight line is approximated that the starting time of the pulse is calculated as follows: t _Anf = t ₀ + Δ, where t _{0 is} the date of origin of the first value read not equal to zero, and

where T is the time interval between two adjacent samples of the AD converter, τ1 is the time constant of the input of the matching amplifier, U1 and U2 are the amplitudes of the first and second samples other than zero, and thereafter the pulse energy is calculated by the value an approximation line to the pulse start time t _Anf. multiplied by a calibration factor of the energy meter. Measuring device for measuring the pulse radiation energy of an optical radiation for performing the method according to claim 1 with a pyroelectric detector and a matching amplifier whose output is connected to the input of an analog-to-digital converter, wherein the output of the analog-to-digital converter is connected to a computer, characterized in that the output of the detector is connected to the input of a normalizing pulse converter whose output is connected to the input of the matching amplifier, and in that the normalizing pulse converter is arranged so that the upper limit of the range of its output pulses is the upper limit of the working frequency range of the Does not exceed the matching amplifier.

Images