Device for detecting optical pulses

Abstract

The present invention relates to a device (10) for detecting optical pulses, the detection device (10) comprising: - a single detector (12) capable of delivering a current signal (ID) upon receiving an optical pulse, - a single measurement chain (14) comprising: - a main branch (20) connected to the single detector (12) so as to receive the current signal (ID), - a first measurement stage (22) capable of measuring the temporal amplitude of the optical pulse as a function of the current signal (ID), and - a second measurement stage (24) capable of measuring the energy of the optical pulse as a function of the current signal (ID).

Description

[0001] TITLE: Optical Pulse Detection Device

[0002] The present invention relates to a device for detecting optical pulses. The present invention also relates to a use of such a detection device.

[0003] For certain applications, it is useful to simultaneously measure the temporal amplitude and energy of an optical pulse. For example, in a rangefinder, it is useful to simultaneously measure the amplitude of the initial pulse and the pulse energy in the laser cavity.

[0004] For this purpose, the state of the art commonly uses two separate detectors for each measurement. The solutions are therefore not very compact.

[0005] Alternatives include using a high-speed sampling device and then digitally calculating the pulse energy. However, the sampling must be very fast to guarantee measurement accuracy, which is not always possible and can lead to measurement inaccuracies.

[0006] There is therefore a need for a device that can measure the temporal amplitude and energy of an optical pulse, which is more compact and more precise.

[0007] To this end, the invention relates to a detection device comprising:

[0008] a single detector capable of delivering a current signal upon receiving an optical pulse on the single detector,

[0009] a single measurement chain comprising:

[0010] o a main branch connected to the single detector so as to receive the current signal,

[0011] o a first measurement stage suitable for measuring the temporal amplitude of the optical pulse as a function of the current signal, the first measurement stage comprising a measuring resistor and a first measuring unit, the measuring resistor being connected in series with the main branch so as to be traversed by the current signal, the first measuring unit being suitable for measuring the voltage across the resistor, o a second measurement stage suitable for measuring the energy of the optical pulse as a function of the current signal, the second measurement stage comprising an integrating circuit and a second measuring unit, the integrating circuit comprising an integrating capacitor and a discharge resistor, the integrating capacitor being connected in series with the main branch so as to be traversed by the current signal, the discharge resistor being connected in parallel across the terminals of the integrating capacitor,The second unit of measurement is specifically designed to measure the voltage across the integrating capacitor; the peak value of the voltage across the integrating capacitor at the end of the optical pulse is a reflection of the energy of the optical pulse.

[0012] According to other advantageous aspects of the invention, the device comprises one or more of the following features, taken individually or in all technically possible combinations:

[0013] - the first unit of measurement has an input capacitance and an input impedance chosen so that the first unit of measurement derives at most a fraction of the signal in current less than 5 percent;

[0014] - the single measurement chain also includes a servo stage designed to maintain a zero potential at the junction point between the measurement resistance of the first measurement stage and the integration capacitor of the second measurement stage, so as to compensate for the leakage current of the single detector;

[0015] The control stage includes:

[0016] - an inverting integrator circuit connected at the input to the integrating capacitor, and

[0017] - a voltage follower circuit whose input is the output of the inverting integrator circuit and whose output is connected to the junction point;

[0018] - the input of the voltage follower circuit of the control stage derives at most a fraction of the current signal less than 0.1 percent;

[0019] - the measuring resistance has a value between 1 and 100 Ohms;

[0020] - the first unit of measurement includes:

[0021] - a first voltage follower circuit connected to one of the terminals of the measuring resistor,

[0022] - a second voltage follower circuit connected to the other terminal of the measuring resistor, and

[0023] - a differential amplifier circuit whose inputs are connected to the output of the first voltage follower circuit and the second voltage follower circuit, and whose output is a voltage proportional to the current through the measuring resistance;

[0024] - the value of the discharge resistance of the integrating circuit is chosen so that the discharge of the integrating capacitor has a duration greater than or equal to 20 microseconds; - the single detector has a capacitance, the ratio between the capacitance of the integrating capacitor and the capacitance of the single detector being greater than or equal to 1; and

[0025] - the second unit of measurement includes an amplifier whose input absorbs or supplies a current of less than 100 nanoamperes.

[0026] The invention also relates to the use of a detection device to measure the temporal amplitude and energy of optical pulses emitted by a laser device.

[0027] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0028] [Fig. 1] Figure 1 is a schematic view of an example of a detection device according to a first embodiment,

[0029] [Fig. 2] Figure 2 is a schematic view of an example of a detection device according to a second embodiment,

[0030] [Fig. 3] Figure 3 is a schematic view of an example of a first unit of measurement, and

[0031] [Fig. 4] Figure 4 is a representation of an example of the voltage outputs as a function of time of a first stage measuring the temporal amplitude of the optical pulse, and of a second stage measuring the energy of the optical pulse.

[0032] A device for detecting optical pulses is illustrated in Figures 1 and 2. The optical pulses originate, for example, from a laser device. The laser device is, for example, a laser or a laser rangefinder.

[0033] Such a detection device 10 is particularly well-suited for measuring the temporal amplitude and energy of pulses emitted by a laser device. For example, in the case of a laser, the detection device 10 enables the measurement of the energy of the emitted optical pulses, for instance, for predictive maintenance, to monitor the evolution of a laser's energy over time, or to control the laser's energy to a setpoint. For example, in the case of a laser rangefinder, the detection device 10 allows simultaneous determination of the temporal profile and energy of the emitted optical pulses.

[0034] The detection device 10 comprises a single detector 12 and a single measuring chain 14.

[0035] The single detector 12 is designed to deliver a current signal ID following the reception of an optical pulse on the single detector 12. The single measurement chain 14 comprises a main branch 20, a first measurement stage 22 and a second measurement stage 24. Optionally, as in Figure 2, the single measurement chain 14 further comprises a servo stage 26.

[0036] The main branch 20 is connected to the output of the single detector 12 so as to receive the current signal ID. The main branch 20 is the only branch of the detection device 10 connected to the single detector 12.

[0037] The first measurement stage 22 is designed to measure the temporal amplitude of the optical pulse as a function of the current signal ID.

[0038] The first measuring stage 22 includes a measuring resistance Rf and a first measuring unit 30.

[0039] The measuring resistor Rf is connected in series with the main branch 20 so that it is traversed by the current signal ID.

[0040] Preferably, the measuring resistance Rf has a value between 1 and 100 Ohms. The measuring resistance Rf is, therefore, of low value.

[0041] The first unit of measurement 30 is used to measure the voltage across the measuring resistor Rf, this voltage being proportional to the current flowing through the measuring resistor Rf. Figure 4 illustrates an example of the evolution of this voltage V1 over time, allowing the time-domain shape of the optical pulse to be obtained.

[0042] Preferably, the first unit of measurement 30 has an input capacitance and input impedance chosen such that the first unit of measurement 30 derives at most a fraction of the signal into current ID less than 5 percent.

[0043] Typically, the input capacitance is chosen to be low (<5pF) and the input draws or supplies a current of less than 100 nA. A low input capacitance also helps to avoid excessively reducing the bandwidth.

[0044] In one example implementation, the first measurement unit 30 comprises one or more operational amplifiers. Preferably, the gain-bandwidth product of the operational amplifier(s) is chosen so that the pulses are minimally attenuated. The gain-bandwidth product is then sufficiently high. For example, depending on the rise time of the pulse to be measured, the gain-bandwidth product is typically on the order of 500 MHz or more.

[0045] In an example implementation illustrated by Figure 3, the first unit of measurement 30 comprises:

[0046] A first voltage follower circuit 32 is connected to one terminal of the measuring resistor Rf. As illustrated in Figure 3, the first voltage follower circuit 32 includes an operational amplifier 33 in a follower configuration. A second voltage follower circuit 34 is connected to the other terminal of the measuring resistor Rf. As illustrated in Figure 3, the second voltage follower circuit 34 includes an operational amplifier 35 in a follower configuration.

[0047] A differential amplifier circuit 36 ​​whose inputs are connected to the outputs of the first voltage follower circuit 32 and the second voltage follower circuit 34, and whose output is a voltage proportional to the current flowing through the measuring resistor Rf. As illustrated in Figure 3, the differential amplifier circuit 36 ​​includes an operational amplifier 37 in differential configuration (connections to resistors R1, R2, R3, R4).

[0048] The first measuring unit 30 in Figure 3 allows the differential measurement of the voltage across Rf (and therefore of the current through Rf), altering the current ID as little as possible after it has passed through Rf, so that this current can be reused by the following stage to measure the pulse energy with a minimum of error.

[0049] Alternatively, the first unit of measurement 30 includes only the differential amplifier circuit 36.

[0050] The second measurement stage 24 is designed to measure the energy of the optical pulse as a function of the current signal ID.

[0051] The second measuring stage 24 includes an integration circuit 38 and a second measuring unit 39.

[0052] The integration circuit 38 is suitable for integrating the signal into current ID.

[0053] The integration circuit 38 includes an integration capacitor Cint and a discharge resistor Ri.

[0054] The integration capacitor Cint is connected in series with the main branch 20 so that it is traversed by the current signal ID.

[0055] We give below an example of the choice of the value of the integration capacitor Cint. This choice is the result of a compromise.

[0056] First, the signal I e The current delivered by the detector undergoes a capacitive current divider, between its own capacitance C dét and the Cint integration capability, so that the resulting current ID is such that:

[0057]

[0058] Let a = Error on the current I e due to the capacitive voltage divider, we have:

[0059] In Cj n “- Cj n "- 1

[0060] — = — — — = 1 — a, we deduce — = — 1

[0061]

[0062] Cint TC d etc d And

[0063] C ■ 1

[0064] For example, for an error < 2% on the initial current

[0065] e > - 1 = 49

[0066] C dét 0.02 Furthermore, the peak amplitude measured across the terminals of the integration capacitor C int depends on the shape of the pulse. Thus, for a pulse in the shape of a square wave with a peak value of I DMax et of width r, I' expression of AV Max East:

[0067] ... ^DMax

[0068] AV Max = — - T

[0069]

[0070] L int

[0071] For example, to measure the peak AV amplitude Max resulting from the measurement on an impulse in the form of a square wave with a peak value (I DMa x)mm = 10p 4 and width r = 100ns, if we estimate that to ensure a quality measurement (AV Max ) mül = 10 mV min , SO:

[0072] (Cint)Max = (lDMax)m ' nT = H22_î * 100 * 1 Q -9 = QOpF k mUMax (AV M ax)min 10*10-3 r If the detector has a capacity of C dét = 20 pF for example, so as seen previously, the error due to the capacitive divider is:

[0073] C int / (C int + C dét ) = 1 − α hence α = C dét / (C int + C dét ) = 20 / (100 + 20) = 17%

[0074] However, it is possible knowing C int etc dét to estimate this error and compensate for it elsewhere, or to calibrate the energy measurement.

[0075] C's choice int It therefore stems from a compromise, because the more C int The larger the error, the smaller the error on the initial current, but also the smaller the peak amplitude AV. Max measured.

[0076] Preferably, the ratio between the capacitance of the integrating capacitor Cint and the capacitance C détThe value of the single detector 12 is greater than or equal to 1, advantageously greater than or equal to 49 for an error on the initial current < 2%. This allows for maintaining good energy measurement accuracy.

[0077] The discharge resistor Ri is connected in parallel across the terminals of the integrating capacitor Cint. The discharge resistor Ri allows for controlled discharge of the integrating capacitor Cint. It is chosen, for example, based on the signal's repetition frequency so that the analog-to-digital converter can easily sample the pulse peak before it has time to decay.

[0078] Preferably, the value of the discharge resistance Ri of the integration circuit 38 is chosen so that the discharge of the integration capacitor Cint has a duration greater than or equal to 20 microseconds, advantageously greater than or equal to a few hundred µseconds (at least 100 microseconds).

[0079] The second measuring unit 39 is designed to measure the voltage across the integrating capacitor Cint. The peak value of the voltage across the integrating capacitor Cint at the end of the optical pulse (when the current signal ID has finished evolving) reflects the energy of the optical pulse. Figure 4 illustrates an example of this peak voltage V2 over time. Preferably, the second measuring unit 39 includes an amplifier whose input draws or supplies a current of less than 100 nA. This low current prevents degradation of the measurement.

[0080] The control stage 26 (optional) is designed to maintain a zero potential at the junction point P between the measuring resistance Rf of the first measuring stage 22 and the integration capacitor Cint of the second measuring stage 24, so as to compensate for the leakage current (darkness current) of the single detector 12. The control stage 26 thus makes it possible to maintain the dynamics of energy measurement, independently of variations in the leakage current of the single detector 12 (the leakage current varying in particular as a function of temperature).

[0081] The control stage 26 is connected at its input to the integrating capacitor Cint, but not to the measuring resistor Rf (therefore not connected to junction point P). The control stage 26 is connected at its output to junction point P.

[0082] In the example illustrated by Figure 2, the control stage 26 includes an inverting integrator circuit 40 and a voltage follower circuit 42.

[0083] The inverting integrator circuit 40 includes an operational amplifier in inverting integrator configuration (and therefore connected with a Roffst resistor and a Coffst capacitor, and one of whose outputs (positive terminal) is connected to ground).

[0084] The inverting integrator circuit 40 is connected at the input to the integration capacitor Cint. On the other hand, the inverting integrator circuit 40 is not connected to the measuring resistor Rf (and therefore not connected to the junction point P).

[0085] The voltage follower circuit 42 includes an operational amplifier in follower configuration.

[0086] The voltage follower circuit 42 is connected at the input to the output of the inverting integrator circuit 40 and is connected at the output to the junction point P.

[0087] Preferably, the input of the voltage follower circuit 42 of the control stage 26 derives at most a fraction of the current signal ID less than 0.1 percent.

[0088] If the first measuring unit 30 is configured as in the implementation example shown in Figure 3, the input of the voltage follower circuit 42 of the control stage 26 can be connected to the first measuring unit 30 at the output of the operational amplifier 35, which is in follower configuration. This avoids any current bypass from the input of the control stage to the measuring capacitor C. int .

[0089] Preferably, the output impedance of the inverting integrator circuit 40 is chosen so that it remains low in the upper part of the measured pulse spectrum, on the order of a few ohms, so that the output potential of the control stage 26 varies little during the onset of the pulse. It is possible to place an RC network at the output of the control stage 26, which passively presents a low impedance at the high frequencies of the pulse; for example, a 50-ohm resistor in series followed by a 100 nF capacitor connected to the ground of the detection device 10.

[0090] In the absence of a control stage 26, the integration capacitor Cint is connected to a fixed potential. The fixed potential is, for example, the electrical ground of the detection device 10.

[0091] An example of a detection process implemented by the detection device 10 will now be described.

[0092] The detection process includes the reception of an optical pulse by the single detector 12, and the generation of a current signal ID as a function of the received optical pulse.

[0093] The detection method includes the measurement, by the first measuring stage 22, of the temporal amplitude of the optical pulse as a function of the current signal ID.

[0094] The detection method also includes the measurement, by the second measurement stage 24, of the energy of the optical pulse as a function of the current signal ID.

[0095] The present detection device and method thus makes it possible to improve the accuracy in energy measurement. Indeed, due to the choice of the input capacitance and input impedance of the first measuring unit 30, all the current delivered by the single detector 12 first passes through the low-value measuring resistance Rf, then enters without loss or almost without loss into the integration capacitor Cint.

[0096] Furthermore, the use of a single detector limits optical adjustments to a single detector in the laser cavity and allows for a gain in compactness.

[0097] The discharge resistance Ri of the integrator circuit allows for a slow discharge of the integrator capacitor. The detection device 10 then behaves as a sample-and-hold circuit, enabling energy measurement with a slow ADC (Analog-to-Digital Converter).

[0098] Finally, in the different embodiments, carrying out the transformations of quantities (detector current to voltage for measurement of the time amplitude, and detector current to Energy) with passive components at the head minimizes noise and allows gain in dynamics compared to a circuit of voltage measurement, then integration from the voltage.

[0099] It is emphasized that figures 1 and 2 are given as an example, the first and second measurement stages being able to be reversed, i.e. that the second measurement stage 24 has its input connected to the output of the detector and the first measurement stage 22 has its input connected to the output of the second measurement stage 24.

[0100] A person skilled in the art will understand that the embodiments and variants described above can be combined with each other provided they are technically compatible.

Claims

DEMANDS 1. Optical pulse detection device (10), the detection device (10) comprising: a single detector (12) capable of delivering a current signal (I D ) following the reception of an optical pulse on the single detector (12), a single measuring chain (14) comprising: • a main branch (20) connected to the single detector (12) so as to receive the current signal (I D ), • a first measurement stage (22) designed to measure the temporal amplitude of the optical pulse as a function of the current signal (I D ), the first measuring stage (22) comprising a measuring resistor (Rf) and a first measuring unit (30), the measuring resistor (Rf) being connected in series with the main branch (20) so as to be traversed by the current signal (I D), the first unit of measurement (30) being suitable for measuring the voltage across the resistor, • a second measurement stage (24) designed to measure the energy of the optical pulse as a function of the current signal (I D ), the second measuring stage (24) comprising an integrating circuit (38) and a second measuring unit (39), the integrating circuit (38) comprising an integrating capacitor (Cint) and a discharge resistor (Ri), the integrating capacitor (Cint) being connected in series with the main branch (20) so as to carry the current signal (I D), the discharge resistance (Ri) being connected in parallel to the terminals of the integration capacitor (Cint), the second unit of measurement (39) being suitable for measuring the voltage across the terminals of the integration capacitor (Cint), the peak value of the voltage across the terminals of the integration capacitor (Cint) at the end of the optical pulse being the image of the energy of the optical pulse.

2. Device according to claim 1, wherein the first measuring unit (30) has an input capacitance and input impedance chosen such that the first measuring unit (30) derives at most a fraction of the signal into current (I D ) less than 5 percent.

3. Detection device (10) according to claim 1 or 2, wherein the single measuring chain (14) further comprises a servo stage (26) suitable for maintaining a zero potential at the junction point (P) between the measuring resistance (Rf) of the first measuring stage (22) and the integrating capacitor (Cint) of the second measuring stage (24), so as to compensate for the leakage current of the single detector (12).

4. Detection device (10) according to claim 3, wherein the control stage (26) comprises: an inverting integrator circuit (40) connected at its input to the integrating capacitor (Cint), and a voltage follower circuit (42) whose input is the output of the inverting integrator circuit (40) and whose output is connected to the junction point (P).

5. Detection device (10) according to claim 4, wherein the input of the voltage follower circuit (42) of the control stage (26) derives at most a fraction of the current signal (I D ) less than 0.1 percent.

6. Detection device (10) according to any one of claims 1 to 5, wherein the measuring resistance (Rf) has a value between 1 and 100 Ohms.

7. Detection device (10) according to any one of claims 1 to 6, wherein the first unit of measurement (30) comprises: a first voltage follower circuit (32) connected to one of the terminals of the measuring resistor (Rf), a second voltage follower circuit (34) connected to the other terminal of the measuring resistor (Rf), and a differential amplifier circuit (36) whose inputs are connected to the output of the first voltage follower circuit (32) and the second voltage follower circuit (34), and whose output is a voltage proportional to the current through the measuring resistance (Rf).

8. Detection device (10) according to any one of claims 1 to 7, wherein the value of the discharge resistance (Ri) of the integrating circuit (38) is chosen so that the discharge of the integrating capacitor (Cint) has a duration greater than or equal to 20 microseconds.

9. Detection device (10) according to any one of claims 1 to 8, wherein the single detector (12) has a capacitance, the ratio between the capacitance of the integrating capacitor (Cint) and the capacitance of the single detector (12) being greater than or equal to 1.

10. Detection device (10) according to any one of claims 1 to 9, wherein the second unit of measurement (39) comprises an amplifier whose input absorbs or supplies a current of less than 100 nanoamperes.

11. Use of a detection device (10) according to any one of claims 1 to 10 for measuring the time amplitude and energy of optical pulses emitted by a laser device.

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