DE102012102521A1 - Method for ultrafast reading of photosensors - Google Patents

Abstract

Shown are a method and apparatus for ultrafast reading of a plurality N of photodetectors spaced apart and illuminated simultaneously by an optical pulse such that each of the N photodetectors generates an electrical output pulse in response to the optical pulse at least approximately simultaneously. The N output pulses are delayed by a respective delay time by means of a delay means. The N delayed output pulses are merged to produce a pulse train signal, and the pulse train signal is read by a readout circuit.

Description

FIELD OF THE INVENTION

The present invention is in the field of photosensitivity. More particularly, it relates to a method and apparatus for ultrafast reading of at least two spaced-apart photodetectors and a spectrometer utilizing such an apparatus and method.

RELATED ART

In a variety of applications both in industry and in basic research, temporal changes or the movements of objects are detected by means of optical detectors. Applications include production monitoring, quality control, chemical analysis, safety engineering, biosensing and medical diagnostics.

To monitor the products moving on conveyor belts in production, for example, they are guided past a permanently installed detector line of a line scan camera. The detector line, which consists of a large number of photodetectors, is read out at short intervals at a specific sampling rate. When recording very fast moving objects so-called TDI-CCD cameras (TDI = Time Delay Integration) are used, which consist of a plurality of detector lines and have an increased sensitivity. Here, the signals of different lines are summed up at different times. Thus, the sensitivity of the recording is increased.

Another application, in which a plurality of photodetectors is read out, forms the spectroscopic examination of substances. In a spectrometer, the radiation to be analyzed is spatially separated so that different spectral radiation components run on different optical paths after separation. For this purpose, for example, a diffractive optical element can be used. The spatial intensity distribution thus provided can then be detected by optical detectors which are spaced from each other. For signal analysis, the signal generated by the detectors is further processed, for which purpose the detectors are read out.

In many cases, said spectrometric applications are used to observe time-varying processes. A prerequisite for this is the possibility of a time-resolved performance of the optical detection measurements on the spaced-apart detectors. If the processes to be investigated run on very short time scales, it is also essential that the signals are generated quickly and sensitively on the one hand, and that they are further processed correspondingly quickly after generation, on the other hand.

One possibility for time-resolved analysis is the use of pulsed radiation whose spectral information - after interaction with the sample to be examined - is determined. Time-sequential optical pulses characterize the sample at different times and can be captured by a photodetector as "snapshots". Furthermore, in a spectrometer, after a spatial separation of the spectral components of the optical pulse at specific photodetector positions, individual spectral sections can be received in a time-resolved manner.

When a photodetector receives optical radiation, electric charges are generated therein. For reading the photodetector in a sample and hold circuit, the charges are latched in response to a sample signal for the period of the sample signal. The signal strength of the analogue output signal corresponds to the charge temporarily stored at the respective time. Thus, the signal strength of the optical radiation is approximated by a stepped, analogue output signal, the step width corresponding to the period of the scanning signal. This output signal can then be converted by an analog-to-digital converter into a digital signal and further processed for analysis. Such a reading is in the patent US 3,486,822 described for a spectrometer.

Another possibility for recording and reading of spatial intensity distributions offer spatially extended CCD sensors. In these, the optical detector and the readout circuit form a common, integrated component. To read such detectors, clock signals in the MHz range can be used. The sampling rate essentially results from the product of the inverse of the number of pixels and the clock signal frequency. Dead-time clock cycles in which dark pixels are output are not taken into account. The sampling rates are for very fast line detectors usually in the range of some 10 kHz, ie they correspond to a time resolution of about 100 microseconds. The use of actively switched components for signal processing has a relatively high noise level result. These components have a complex circuit with its own timing, which limits the sampling rate. In order to prevent smearing of the data by the readout technique, have CCD Recording systems a mechanical shutter or are modulated illuminated. The modulation can be generated with an electro-optical modulator, with an acousto-optical modulator or with a rotating perforated disc. In this case, it is necessary to synchronize the sampling rate with the modulator. In practice, the mechanical shutter or the modulating components limit the technically achievable sampling rate.

Alternatively, spatial intensity distributions can also be measured by mechanically adjustable detection mechanisms. For this purpose, for example, a photodiode used for detection can be moved by means of a displacement table, as described in the patent application US 4,124,297. Another possibility describes the patent application US 4,732,476, in which the scanning of a spectral intensity distribution by rotating an optical grating takes place. Here, the recording duration of a spectrum is in the range of milliseconds to minutes. For a temporal analysis of spectra in the microsecond range, these recording methods are therefore unsuitable.

Another method uses a spatial arrangement of photodiodes, for example a photodiode array, and uses for reading each photodiode each have their own preamplifier and a separate analog-to-digital converter. The digital signals thus generated contain the optical information of the detected spectrum at the location of the photodiode at the time of detection. Very high sampling rates can be achieved with this method. However, this method is very expensive, especially if additionally lock-in amplifiers are used to process the photodiode signals.

The previously known methods for time-resolved reading of a detector device for recording a spatial intensity distribution have the disadvantage that the time resolution is either limited to a range of about 100 microseconds or can only be achieved at considerable expense.

SUMMARY OF THE INVENTION

The present invention has for its object to provide a method that allows a large number of spatially spaced photodetectors, which are illuminated by optical pulses with repetition rates up to the MHz range, inexpensive and low noise read. Another object is to provide a device that uses this method.

This object is achieved by a method according to claim 1 and by a device according to claim 9. Advantageous developments are specified in the independent claims.

Modern ultrashort pulse lasers emit optical pulses with typical pulse durations in the nano-, pico- or femtosecond range with repetition rates in the MHz range. When a plurality N of spaced-apart photodetectors are illuminated with optical pulses, each photodetector generates an output electrical pulse approximately simultaneously in response to each of these optical pulses. The repetition rate of the electrical output pulses generated by a photodetector corresponds to the repetition rate of the optical pulses with which the photodetector is illuminated. Thus, upon detection of a pulsed spatial intensity distribution, a plurality N of pulsed electrical signals are generated. So far, it is not possible to read these signals as quickly as they are provided in a cost effective manner.

• – Verzögern der N elektrischen Ausgangspulse mithilfe eines Verzögerungsmittels um eine jeweilige Verzögerungszeit T_V,i, i = 1...N, wobei mindestens N – 1 der Verzögerungszeiten T_V,i von Null verschieden sind,
• – Zusammenführen der N verzögerten Ausgangspulse, um ein aus den N verzögerten Ausgangspulsen überlagertes Pulszugsignal zu erzeugen, wobei die Verzögerungszeiten T_V,i so gewählt sind, dass die Beiträge der einzelnen Ausgangspulse im Pulszugsignal zeitlich im Wesentlichen nicht überlappen, und
• – Auslesen des Pulszugsignals mithilfe einer Ausleseschaltung, die geeignet ist, einem jeden Fotodetektor einen zeitlichen Abschnitt des Signals zuzuordnen, der der Verzögerungszeit T_V,i des zugehörigen Verzögerungsmittels entspricht.

The present invention solves this problem by a method of reading out a plurality N of photodetectors, where N is an integer ≥ 2 and the photodetectors are spaced apart and illuminated simultaneously by an optical pulse so that each of the N photodetectors respond in response to the photodetectors optical pulse generates at least approximately simultaneously an electrical output pulse. The method comprises the following steps:

• Delaying the N electrical output pulses by means of a delay means by a respective delay time T _{V, i} , i = 1... N, wherein at least N-1 of the delay times T _{V, i} are different from zero,
• Combining the N delayed output pulses to generate a pulse train signal superimposed on the N delayed output pulses, wherein the delay times T _{V, i} are chosen so that the contributions of the individual output pulses in the pulse train signal do not substantially overlap in time;
• - Reading the Pulse train signal by means of a read-out circuit, which is adapted to assign each photodetector a temporal portion of the signal corresponding to the delay time T _{V, i of} the associated delay means.

The fact that the photodetectors are spaced from each other and thus located at different spatial positions, the information contained in the optical pulse can be detected location-dependent by the plurality N of photodetectors. Each photodetector thus detects optical information, for example the intensity of a specific radiation component, at a specific spatial position. The optical pulse can, for example, radiation from the visible region, the near infrared (NIR) region, the infrared (IR) region, the ultraviolet (UV) region and / or other radiation regions. Preferably, the present invention is used in the visible and NIR range and with photon energies of preferably ≦ 100 eV, preferably ≦ 10 eV, and more preferably ≦ 3 eV.

The information contained in the optical pulse is represented by the temporal and spatial spectral intensity distribution. With spaced-apart photodetectors, the spatial spectral information can be detected either without or with prior spectral separation. If the spectral sensitivity ranges of the photodetectors are selected differently from each other and narrow with respect to the spectrum of the optical pulse, only a corresponding spectral section of the spectrum of the optical pulse is detected at the location of each photodetector. In this case, a preliminary spectral separation is not necessary. On the other hand, if the spectral components of the optical pulse are spatially separated before detection, they can be detected with identical spaced-apart photodetectors which are sensitive in the entire spectrum of the optical pulse. The spectral separation can be made by means of an optical grating or a refractive optical element.

The N photodetectors generate a plurality of N output electrical pulses in response to the incident optical pulse. Each of these electrical output pulses contains the information of the optical pulse at the location of the photodetector. Since the optical pulse impinges on the plurality N of photodetectors approximately simultaneously, the plurality of output electrical pulses are also generated approximately simultaneously in response to this optical pulse. Delays in the propagation of an optical pulse propagating at the speed of light or else different response times of the photodetectors can lead to different times of impingement of the optical pulse on the photodetectors or at different production instants of the electrical output pulses. For the present invention, however, these differences can be neglected, otherwise this influence can also be compensated by a suitable choice of the delay means. In the present specification, the timing relationship between generation timings is referred to as "approximately simultaneous".

The N output pulses which are generated at least approximately simultaneously are delayed in a first method step by respective delay times T _{V, i} , i = 1... N, so that N delayed output pulses are generated. The N delay times T _{V, i} are different from each other in pairs, so that the delayed output pulses are offset from each other in pairs in time. It is also possible that one of the delay times is zero, ie all output pulses except one are delayed in time.

In a second method step, these N delayed output pulses are combined to form a pulse train in a pulse train signal. Thus, N individual signals are combined into a single signal, the pulse train signal. Each electrical output pulse is represented in the pulse train signal in the form of a contribution from a particular photodetector. Because of the different delay times T _{V, i} , successive contributions of a pulse train in the pulse train signal have a time interval from each other which corresponds to the difference between the associated delay times T _{V, i} . Furthermore, the contributions in the pulse train signal occupy a specific time interval, which is determined by the corresponding delay time T _{V, i} and a specific offset value.

After the delayed output electrical pulses have been merged into the pulse train signal, the pulse train signal is read out using a readout circuit. The reading takes place in such a way that a specific time segment of the pulse train signal, which belongs to a specific delay time T _{V, i} , is assigned to each photodetector. If several optical pulses are read out, the abovementioned offset value can be determined by means of a trigger signal so that each contribution can be assigned to the corresponding detector and the corresponding optical pulse. The trigger signal can be generated, for example, from the output signal of an additional photodetector or from a signal which has been diverted to one of the N instantaneous or delayed electrical output pulses. Alternatively, the trigger signal can also be generated by the control of the pulsed laser.

So that the individual contributions in the pulse train signal can be identified by the read-out circuit and thus assigned to the respective photodetector, it is necessary that the contributions in the pulse train signal do not substantially overlap in time. This is achieved by a suitable choice of the delay times T _{V, i} . The term "substantially not overlapping in time" in this context means that successive contributions in the pulse train signal are so far apart in terms of time that the read-out circuit still distinguishes them from each other and thus can associate the information contained with the respective photodetector. The time interval, which is necessary so that successive amounts of the same pulse train does not substantially overlap in time, can thereby vary depending on the pulse width of the posts. The pulse width usually corresponds to the half width, ie the full width of the contribution at half the amount of the contribution. For example, the time interval for contributions with a small pulse width may be smaller than for contributions with a comparatively large pulse width. The critical time interval, ie the distance at which successive contributions of the same pulse train do not substantially overlap in time, may depend on the further processing of the pulse train signal. Regardless of the pulse width can be ensured by suitable choice of the delay times T _{V, i} , that the time interval is equal to or greater than the critical time interval.

The method according to the invention thus converts information from a spatial intensity distribution into a single pulse train signal. The spatial position of a photodetector is translated into the position of a temporal section in the pulse train signal. By assigning a specific time segment to each photodetector during readout, the spectral information contained in a contribution is assigned to the corresponding spatial position. In this way, with the method of the present invention, the information of the optical pulse is read out without complicated and cost-intensive electronic components and without averaging over several sampling cycles. The read speed corresponds to the speed at which the information is provided. When using an ultrashort pulse laser with repetition rates in the MHz range, the frequency at which the pulse trains are generated in the pulse train signal is also in the MHz range. The frequency of the contributions in the pulse train signal is greater by the factor N.

Delaying and merging can take place, for example, temporally and spatially separated from one another. For this purpose, the N output pulses are delayed by means of the delay means first individually and in pairs separately from each other to the N delayed output pulses. Subsequently, these N delayed output pulses can then be merged.

However, the first process step of the deceleration and the second process step of the merging do not necessarily take place successively but can also be combined or combined with one another. For this purpose, for example, a certain number of delay sections are connected in series and different photodetectors are each connected to different points of this series connection. Thus, different electrical output pulses each enter the series circuit at different points and each undergo a different number of delay sections.

For generating a pulse train signal by combined delaying and combining, for example, the N-th electric output pulse is delayed by one time (T _{V, N} -T _{V, N-1} ) and then with the (N-1) th instantaneous electric output pulse to one Signal merged with two contributions. This two-contribution signal is delayed by time (T _{V, N-1} - T _{V, N-2} ) and merged with the (N-2) th instantaneous electrical output pulse to form a three-contribution signal. These steps are repeated until finally a signal with N-1 amounts is delayed by the time (T _{V, 2} -T _{V, 1} ) and then merged with the first instantaneous electrical output pulse to the pulse train signal.

Delaying is preferably done using passive delaying means. "Passive" means that for the deceleration no active switching, such as is done using a clock signal, is necessary. A passive delay can be effected, for example, by the propagation delay in a cable. To delay this, no actively switched components are necessary, which require its own clock frequency and thereby limit the time resolution. The use of passive delay means allows a readout in quasi-real time, i. H. the signal reading is done at virtually the same speed as the signal providing and is only delayed by skews. On the other hand, a limitation of the time resolution due to active delay, such as due to buffering or due to a clock signal frequency, does not exist.

In a preferred embodiment, the N delay times T _{V, i have} a value which corresponds to a multiple of a predetermined time interval Δt. Preferably, these result from T _{V, i} = (i-1) .DELTA.t for i = 1 to N. Thereby, the time interval between two consecutive contributions of the same pulse train is constant and corresponds to the value of the predetermined time interval .DELTA.t.

Furthermore, it is advantageous to detect not only one but a plurality of optical pulses which successively strike the plurality N of photodetectors at a repetition rate f _rep . If a pulsed laser source is used to scan the sample to be examined and the photodetectors are illuminated with this plurality of optical pulses interacting with the sample, a pulse train signal is generated containing a pulse train of N contributions for each optical pulse. The contributions of the same pulse train take a temporal range in the pulse train signal T _PT , the size of which depends on the choice of the delay times T _{V, i} . To ensure that the contributions of successive pulse trains do not substantially overlap in time, the delay times are chosen such that the temporal range T _{PT is} less than 1 / f _rep . With the abovementioned delay times of T _{V, i} = (i-1) .DELTA.t, the relationship N .DELTA.t <1 / f _rep is thus required.

A typical repetition rate with which the method of the present invention can be used is in the range of f _rep = 10 MHz. This corresponds to a pulse interval 1 / f _rep of 100 ns. The read-out rate with which the plurality N of photodetectors are read out in the present invention is ≥ 1 kHz, preferably ≥ 10 kHz, and more preferably ≥ 100 kHz. So that neither the contributions of the same pulse train nor the contributions of successive pulse trains essentially overlap in time, on the one hand T _PT must be less than 1 / f _rep , on the other hand the difference (T _{V, i + 1} - T _{V, i} ) for i = 1. ..N - 1 can not be smaller than the critical distance that depends on the pulse width of the contributions. In order to be able to read as many photodetectors as possible, the critical distance should be as small as possible and thus the pulse width of the contributions should be as narrow as possible. The pulse width of the contributions is in the range of the reaction time T _{R of} the photodetector and is limited by this. The response time T _{R of} a photodetector is usually defined by the time it takes for a signal to increase from 10% to 90% of the maximum value. The reaction time T _{R of} a photodetector is approximately determined by the cut-off frequency f _c by T _R ≈ 0.35 / f _c , wherein the cut-off frequency f _{c is} that frequency at which the signal value has dropped to around 50% of the maximum value. In the context of the invention, therefore, fast photodetectors are preferably used which have reaction times T _R which are preferably ≦ 50 ns, preferably ≦ 5 ns, particularly preferably ≦ 0.1 ns. Accordingly, the photodetectors used have cut-off frequencies f _c which are preferably ≥ 0.007 GHz, preferably ≥ 0.07 GHz, particularly preferably ≥ 3.5 GHz. The delay times are chosen such that the predetermined time interval Δt can preferably be chosen to be less than 100 ns, preferably less than 10 ns and particularly preferably less than 100 ps.

Preferably, the pulse train signal is amplified by means of a preamplifier, before the assignment of the temporal sections to the respective photodetector takes place. In a further advantageous development, the pulse train signal is digitized with the aid of an analog-to-digital converter before the assignment of the time segments to the respective photodetector takes place. Note that only a single preamplifier and a single analog-to-digital converter is needed to read the N photodetector signals for further processing. This shows a particular advantage of generating the analog pulse train signal from the plurality of delayed individual signals. If, on the other hand, one digitized the output signal of each photodetector directly, it would be necessary to use N preamplifiers and N analog-to-digital converters, which would considerably increase the design effort and the production costs. Furthermore, identical preamplifiers have different noise and not exactly the same gain. This would increase the noise when using N preamplifiers. In addition, when N analog-to-digital converters are used, N individual signals must also be synchronized with each other. For this purpose, a precise consideration of the maturity is necessary, which further increases the effort. Therefore, there are significant advantages associated with the use of only one analog-to-digital converter and only one preamplifier as discussed above.

Preferably, the method according to the invention is used in a spectrometer for determining the spectrum of an optical pulse. In the spectrometer, the spectral components of the optical pulse are spatially separated. This separation is made, for example, as previously mentioned, by means of a refractive optical element or by means of an optical grating. The spectrometer comprises a plurality of N photodetectors which are spaced apart and illuminated with different spectral components of the optical pulse. The readout of the photodetectors is carried out as described above. The contributions in the pulse train signal contain the spectral information of the optical pulse at the location of the photodetector. This information can be represented, for example, by the amplitude of the respective contribution, the width of the respective contribution, the integrated amplitude of the respective contribution or by another quantity of the respective contribution of the pulse train signal which is indicative of the intensity of the light received at the associated photodetector. By determining this quantity, the detected radiation intensity at the location of the photodetector is determined.

In addition to the method described above, the invention also includes a device for reading out the plurality N of photodetectors using this method.

• – Verzögerungsmittel zum Verzögern von mindestens N – 1 der N elektrischen Ausgangspulse um eine Verzögerungszeit T_V,i, i = 1...N,
• – Mittel zum Zusammenführen der N verzögerten Ausgangspulse, um ein aus den N verzögerten Ausgangspulsen überlagertes Pulszugsignal zu erzeugen, wobei die Verzögerungsmittel dazu eingerichtet sind, die elektrischen Ausgangspulse derart zu verzögern, dass die Beiträge der einzelnen Ausgangspulse im Pulszugsignal zeitlich im Wesentlichen nicht überlappen, und
• – Mittel zum Auslesen des Pulszugsignals, die dazu eingerichtet sind, einem jeden Fotodetektor einen zeitlichen Abschnitt des Signals zuzuordnen, der der Verzögerungszeit T_V,i des zugehörigen Verzögerungsmittels entspricht.

The device according to the invention comprises the following elements:

• Delay means for delaying at least N-1 of the N electrical output pulses by a delay time T _{V, i} , i = 1 ... N,
• Means for combining the N delayed output pulses to generate a pulse train signal superimposed on the N delayed output pulses, the delay means thereto are arranged to delay the electrical output pulses such that the contributions of the individual output pulses in the pulse train signal substantially do not overlap in time, and
• - means for reading the pulse train signal, which are adapted to each photodetector to assign a temporal portion of the signal corresponding to the delay time T _{V, i of} the associated delay means.

The delay means can be realized, for example, by cables of different lengths, so that the electrical output pulses generated by the photodetectors travel different distances through the cables. Due to different lengths of maturity, the electrical output pulses are delayed by different delay times T _{V, i} and thus offset in time. Another possibility is to use commercially available passive delay elements that have a rise time of no more than 1 ns and delay times in the range of about 100 ps to nanoseconds. If necessary, several of these elements can be connected in series to achieve higher delay times. Another embodiment provides a propagation delay directly in the semiconductor material of the photodetector. Due to a relatively low propagation speed of the electrical output pulses in semiconductors, therefore, only a relatively small distance is required.

The means for combining the delayed output pulses may be, for example, commercially available combination elements. These include, for example, N input cables which are connected together at their outputs and terminate in a single output cable.

In an advantageous development, the means for reading comprise an analog-to-digital converter. With this an analog pulse train signal can be converted into a digital signal and then further processed with a suitable data processing system.

Preferably, the means for reading comprises a preamplifier. With this, the contributions of the pulse train signal can be amplified, whereby the further processing is simplified.

In a further development, the device comprises a plurality N of photodetectors for detecting the optical pulse and for generating the electrical output pulses. For use with visible light, for example, silicon photodiodes or photodiode cells can be used.

An advantageous development of the invention relates to a spectrometer for determining the spectrum of an optical pulse by means of a plurality N of photodetectors, wherein N is an integer ≥ 2 and the photodetectors are spaced from each other and arranged so that they each light of an associated spectral range of the optical pulse receive. The spectrometer comprises the aforementioned delay means, means for merging and means for reading. Furthermore, the spectrometer comprises means for determining a size of the pulse train signal which is indicative of the intensity of the light received at the associated photodetector. The determined variable can be assigned to the spatial position of the corresponding photodetector and thus the spectrum of the optical pulse can be determined.

BRIEF DESCRIPTION OF THE FIGURES

Further advantages and features of the invention will become apparent from the following description in which the invention with reference to preferred embodiments with reference to the accompanying drawings is explained in more detail. Show:

1 a schematic representation of a preferred embodiment of the readout device according to the invention,

2 a schematic representation of a pulse train signal,

3 a schematic representation of an alternative embodiment of the readout device according to the invention,

4 a schematic representation of the structure for measuring a pulse train signal,

5 a measured pulse train signal using two photodetectors.

DESCRIPTION OF THE PREFERRED EMBODIMENT

1 shows a schematic representation of a preferred embodiment of the readout device according to the invention 8th , with a pulse train signal 10 as it is in 2 is shown, can be generated. Exactly shows 1 on the left side an optical pulse 12 and right next to it a plurality N of photodetectors 14 , The photodetectors 14 are with delaying agents 18 connected. These are followed by means 22 for merging delayed output pulses 20 to which an analog-to-digital converter 24 connected to a data processing system 26 follows. In every photodetector 14 is in response to the optical pulse 12 an electrical output pulse 16 generated in 1 is shown schematically. In the direction of passage behind the delay means 18 are also delayed output pulses 20 and behind the means 22 for merging a pulse train signal 10 shown schematically.

The following is the function of the readout device 8th described. On the left in 1 is shown as the optical pulse 12 to a plurality N of photodetectors 14 incident. The optical pulse 12 is represented by a plurality of arrows. An arrow corresponds to a spatial section of the optical pulse 12 pointing to a particular photodetector of the plurality N of photodetectors 14 incident. In response to the optical pulse 12 Each one of the plurality generates N of photodetectors 14 almost simultaneously an electrical output pulse 16 , By the delay means 18 become the electrical output pulses 16 by a delay time T _{v, i} = (i-1) .DELTA.t, for i = 1 ... N, delayed, where .DELTA.t is a predetermined time interval, so that for each electrical output pulse 16 a delayed output pulse 20 is produced. In the language of the present disclosure, the output pulse would be 16 of the first photodetector 14 a "delayed" output pulse delayed by a delay time zero to standardize the description. Thus, the delayed output pulse 20 of the ith photodetector 14 to the delayed output pulses 20 of the (i + 1) -th and (i-1) -th photodetectors 14 temporally offset by the predetermined time interval .DELTA.t. By the means 22 to merge the delayed output pulses 20 to a pulse train signal 10 merged. The analog pulse train signal 10 is by means of an analog-digital converter 24 digitized and the digitized signal to a data processing system 26 forwarded.

2 shows the time course of the signal strength of a pulse train signal 10 as it is with the in 1 illustrated readout device 8th is produced. The mapped pulse train signal 10 contains three pulse trains 48 , each containing a number N of posts 28 contain. The totality of the contributions 28 the same pulse train 48 takes in the pulse train signal 10 a temporal range 30 the duration T _PT . The posts 28 each have a pulse width 44 that the full width of a post 28 at half the height of the contribution. The temporal distance of successive contributions 28 the same pulse train 48 is Δt and is for the distance between the first contribution 28 and the second post 28 the right pulse train 48 in 2 drawn by way of example. This distance results from the delay times of T _{v, i} = (i-1) · Δt.

In 2 It can be seen that the time interval Δt is sufficiently greater than the pulse width 44 is such that the pulses of the same pulse train 48 not in time, or at least substantially not overlapping. Here, "essentially not overlapping in time" means that successive contributions 28 in the pulse train signal 10 are so far apart in time from each other that these in the further processing of the pulse train signal 10 can still be separated from each other, so that in each post 28 contained information to the corresponding photodetector 14 can be assigned. The special case that the pulses do not overlap in time is explicitly included. The temporal range 30 has a duration of T _PT = (N-1) · Δt. Furthermore, in 2 the pulse train distance 32 between the contributions 28 the same photodetector of successive optical pulses 12 located. The pulse train interval 32 corresponds to the distance of successive optical pulses 12 , At the pulse train signal 10 out 2 is the pulse train distance 32 greater than T _PT + Δt = N · Δt, so that the contributions 28 different pulse trains 48 essentially do not overlap in time.

3 shows a schematic representation of an alternative embodiment. As well as in 1 meets an optical pulse 12 to a plurality N of photodetectors 14 on. According to the numbering of the N photodetectors used below 14 and the associated signals are the components of the optical pulse 12 in 3 from bottom to top with the numbers "1" to "N" occupied. The N photodetectors 14 are over N connections - represented by lines - with means 34 connected for merging and delaying. The means 34 for merging and delaying, N-1 include individual delay sections 35 of which only four are drawn. The N-1 delay sections 35 are connected in series, so that the output (at the bottom of each) of a delay section 35 with the input (top) of the subsequent delay section 35 is connected, and from bottom to top - in the opposite direction of the signals through the series connection - numbered from "2" to "N". For i = 2 to N, the i-th of the N photodetectors 14 with the input of the ith delay section 35 connected. The first of the N photodetectors 14 is with the output of the second delay section 35 and with an analog-to-digital converter 24 connected. Furthermore, a data processing system 26 which is the analog-to-digital converter 24 is downstream.

In contrast to the read-out device 1 in which the delay means 18 and the funds 22 for merging consist of separate components, the function of these two components in the alternative readout device is made 3 in a component, namely the means 34 to merge and delay, united. The Nth of the N photodetectors 14 in the middle 34 to merge and delay incoming Nth electrical output pulse 16 (not shown) goes through the Nth delay section 35 and is delayed by the time .DELTA.t. Subsequently, the Nth output pulse delayed by Δt becomes the (N-1) th instantaneous output electric pulse 16 (not shown) are combined into a two-contribution signal which is the (N-1) -th delay section 35 passes through and in turn is delayed by the time .DELTA.t. These steps are repeated until finally a signal with N-1 amounts repeats the second delay section 35 goes through it, is delayed by the time .DELTA.t, and then with the first instantaneous electrical output pulse 16 to the pulse train signal 10 is merged. As each contribution 28 when passing through a delay section 35 is delayed by the time .DELTA.t, results in the pulse train signal 10 for the i-th contribution 28 - as in 1 A delay time of T _{v, i} = (i-1) Δt.

4 shows a schematic representation of an experimental design, which is intended to demonstrate the operability of the present invention. At the top is a laser system 36 which provides pulsed radiation with a wavelength of 1.55 μm, with a repetition rate of 4 MHz and with a pulse duration of 100 femtoseconds. Under the laser system 36 is a frequency doubling element 38 for doubling the frequency of the emitted laser radiation imaged. Among them is a plurality of photodetectors 14 for detecting an optical pulse and for generating electrical output signals. To the majority of photodetectors 14 is a readout circuit 40 connected, in turn, with an oscilloscope 42 connected is. In 4 not shown is a sample to be examined with the optical pulse 12 can interact. As indicated by the illustrated construction 4 only the functionality of the present invention is to be shown, is the pulsed spatial laser radiation directly, ie without prior reflection of a sample to be examined or prior transmission through a sample to be examined on the photodetectors 14 directed.

The of the laser system 36 emitted radiation is first through the frequency doubling element 38 , which consists for example of a beta-barium borate crystal, passed around the central wavelength of the optical pulses 12 to halve to a wavelength of about 775 nm. This is necessary to be within the sensitivity range of the silicon photodetectors used in the illustrated embodiment 14 to get. The single silicon photodetectors 14 are now using optical pulses 12 at a wavelength of about 775 nm at a repetition rate of 4 MHz and in response generate output electrical pulses 16 to the readout circuit 40 be passed on. The readout circuit 40 includes the delay means 18 and the funds 22 to merge 1 or the funds 34 to merge and delay 3 (in 4 not shown) to get out of the electrical output pulses 16 a pulse train signal 10 (both in 4 not shown) connected to the oscilloscope 42 passed on and graphed by this. It should be noted that in this structure, in which primarily the functionality is to be demonstrated, no sample is provided in the beam path.

5 shows one with the oscilloscope 42 from 4 illustrated pulse train signal 10 , The pictured temporal section shows seven pulse trains 48 with two contributions each 28 , The pulse train interval 32 between the contributions 28 same photo detector 14 of successive optical pulses 12 is about 250 ns. The temporal range 30 the posts 28 the same pulse train 48 have a duration of about T _PT = 75 ns. The pulse width 44 the contributions 28 is about 15 ns. Furthermore, in 5 reflections 46 to recognize their signal strength about 20% of the signal strength of the posts 28 is. These reflections can arise, for example, at connection points of the cables.

The experimental setup 4 and the pulse train signal 10 out 5 prove the operability of the present invention. At the present pulse train interval 32 of about 250 ns and at the present pulse width 44 the contributions 28 of about 15 ns can be the number of illuminated photodetectors 14 that in the experimental setup 4 is only 2, already without any other modifications to be increased substantially without the contributions 28 in the pulse train signal 10 essentially overlap in time. Depending on the data processing system could in the described structure, the number of photodetectors 14 already increased to 10 or more. Due to a narrower pulse width 44 and / or a larger pulse train spacing 32 The number of detectors can even be increased considerably. This can be done, for example, by the use of photodetectors 14 can be achieved with shorter reaction times and / or by using a slightly lower laser repetition rate. Here, the pulse train signal 10 if necessary, be optimized with regard to the required temporal and spatial resolution. The lower the requirement for the temporal resolution, the lower the laser frequency can be selected. This has a longer pulse train interval 32 entails, so that a pulse train 48 more time essentially non-overlapping contributions 28 may include. Thus, the number of photodetectors 14 and thus increase the spatial resolution. Conversely, the temporal resolution can also be increased by reducing the spatial resolution.

Although preferred embodiments have been shown and described in detail in the drawings and foregoing description, this should be considered as illustrative and not restrictive of the invention. It should be understood that only the preferred embodiments are shown and described and changes and modifications that are presently and in the future within the scope of the invention should be protected. The features shown may be of importance in any combination.

LIST OF REFERENCE NUMBERS

8th

readout device

10

pulse train

12

Optical pulse

14

Plurality N of photodetectors

16

Electric output pulse

18

retardants

20

Delayed output pulse

22

Means to merge

24

Analog to digital converter

26

Data processing system

28

contribution

30

Time range

32

Pulse headway

34

Means for merging and delaying

35

delay section

36

laser system

38

Frequency doubling element

40

readout circuit

42

oscilloscope

44

pulse width

46

reflections

48

pulse train

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 3486822 [0007]

Claims (20)

Method for reading a plurality N of photodetectors ( 14 ), where N is an integer ≥ 2 and the photodetectors ( 14 ) are spaced apart from each other and at the same time by an optical pulse ( 12 ), so that each of the N photodetectors ( 14 ) in response to the optical pulse ( 12 ) at least approximately simultaneously an electrical output pulse ( 16 ), the method comprising the steps of: delaying the N output electrical pulses ( 16 ) by means of a delay means ( 18 . 34 ) by a respective delay time T _{V, i} i = 1 ... N, wherein at least N - 1 of the delay times T _{V, i} are different from zero, combining the N delayed output pulses ( 20 ) to one of the N delayed output pulses ( 20 ) superimposed pulse train signal ( 10 ), wherein the delay times T _{V, i} are selected such that the contributions ( 28 ) of the individual output pulses ( 16 ) in the pulse train signal ( 10 ) substantially do not overlap in time, and reading the pulse train signal ( 10 ) using a readout circuit ( 40 ), which is suitable for each photodetector ( 14 ) a temporal portion of the pulse train signal ( 10 ), the delay time T _{V, i of} the associated delay means ( 18 ) corresponds. The method of claim 1, wherein the step of delaying the N output electrical pulses ( 16 ) by means of a delay means ( 18 ) before the step of merging the N delayed output pulses ( 20 ) he follows. The method of claim 1, wherein the step of delaying the N output electrical pulses ( 16 ) with the step of merging the N delayed output pulses ( 20 ) using a means ( 34 ) is combined for merging and delaying, the agent ( 34 ) for merging and delaying a plurality of successively connected delay sections ( 35 ) and different electrical output pulses ( 16 ) each have a different number of delay sections ( 35 ) run through. Method according to Claim 1 or 3, in which the step of delaying by means of an agent ( 34 ) for merging and delaying is combined with the step of merging such that for i = 1 ... N-1 an i-th signal is delayed by time (T _{V, N + 1-i} - T _{V, Ni} ) and subsequently with the (N-i) th of the N output pulses ( 16 ) is merged into an (i + 1) -th signal, the first signal being represented by the Nth of the N electrical output pulses ( 16 ) and the pulse train signal ( 10 ) is formed by the Nth signal delayed by the time T _{V, 1} . Method according to one of the preceding claims, in which the delaying by means of a passive delay means ( 18 . 34 ), in particular without the use of a clock signal occurs. Method according to one of the preceding claims, in which the delay times T _{V, i are given} by T _{V, i} = (i-1) · Δt, where Δt is a predetermined time interval. Method according to one of the preceding claims, in which a multiplicity of optical pulses ( 12 ) successively at a repetition rate f _rep to the plurality N of photodetectors ( 14 ), wherein the delay times T _{V, i} are chosen such that the contributions ( 28 ) of the same optical pulse ( 12 ) in said pulse train signal ( 10 ) a temporal range ( 30 ) of duration T _PT that is less than 1 / f _rep . A method according to claim 6 or 7, wherein N · Δt <1 / f _rep , where Δt is a predetermined time interval and f _{rep is} the repetition rate of the plurality N of photodetectors ( 14 ) impinging optical pulses ( 12 ). Method according to one of claims 6 to 8, wherein Δt ≤ 100 ns, preferably Δt ≤ 10 ns, particularly preferably Δt ≤ 100 ps. Method according to one of the preceding claims, in which the pulse train signal ( 10 ) using an analog-to-digital converter ( 24 ) is digitized. Method according to one of the preceding claims, in which the pulse train signal ( 10 ) is amplified by means of a preamplifier. Method according to one of the preceding claims, in which the readout of the plurality N of photodetectors is carried out with a read-out rate that is ≥ 1 kHz, preferably ≥ 10 kHz, and is particularly preferably ≥ 100 kHz. Method for determining the spectrum of an optical pulse ( 12 ) using a spectrometric device, the N spaced apart photodetectors ( 14 ), where N is an integer ≥ 2 and the photodetectors ( 14 ) are arranged so that they each light of an associated spectral range of the optical pulse ( 12 ), comprising the steps of: reading the plurality N of photodetectors ( 14 ) according to one of claims 1 to 12, determining a size of the pulse train signal ( 10 ) in said photodetectors ( 14 ) associated with the photodetectors ( 14 ) to determine corresponding spectral components, wherein the size of the pulse train signal ( 10 ) in the mentioned time periods indicative of the intensity of the associated photodetector ( 14 ) received light is. Contraption ( 8th ) for reading out a plurality N of photodetectors ( 14 ), where N is an integer ≥ 2 and the photodetectors ( 14 ) are spaced apart from each other and at the same time by an optical pulse ( 12 ), so that each of the N photodetectors ( 14 ) in response to the optical pulse ( 12 ) at least approximately simultaneously an electrical output pulse ( 16 ), the device comprising the following elements: delay means ( 18 . 34 ) for delaying at least N - 1 of the N output electrical pulses ( 16 ) by a delay time T _{V, i} , i = 1 ... N, means ( 22 . 34 ) for merging the N delayed output pulses ( 20 ) to one of the N delayed output pulses ( 20 ) superimposed pulse train signal ( 10 ), the delay means ( 18 . 34 ) are adapted to the electrical output pulses ( 16 ) in such a way that the contributions ( 28 ) of the individual output pulses ( 16 ) in the pulse train signal ( 10 ) do not substantially overlap in time, and means for reading out the pulse train signal ( 10 ), which are adapted to each photodetector ( 14 ) a temporal portion of the pulse train signal ( 10 ), the delay time T _{V, i of} the associated delay means ( 18 . 34 ) corresponds. Contraption ( 8th ) according to claim 14, wherein the means for reading out an analog-to-digital converter ( 24 ). Apparatus according to claim 14 or 15, wherein the means for reading comprises a preamplifier. Contraption ( 8th ) according to one of claims 14 to 16, in which the plurality N of photodetectors ( 14 ) is formed by silicon photodiodes. Contraption ( 8th ) according to one of claims 14 to 17, in which the delay means ( 18 . 34 ) Cables and different delay times are generated by different cable lengths. Device according to one of Claims 14 to 18, in which the photodetectors ( 14 ) Have reaction times T _R which are ≦ 50 ns, preferably ≦ 5 ns, particularly preferably ≦ 0.1 ns, and / or limit frequencies f _c which are preferably ≥ 0.007 GHz, preferably ≥ 0.07 GHz, particularly preferably ≥ 3 , 5 GHz are. Spectrometer for determining the spectrum of an optical pulse ( 12 ) using a plurality N of photodetectors ( 14 ), where N is an integer ≥ 2 and the photodetectors ( 14 ) are spaced apart from one another and arranged so that they each have light of an associated spectral range of the optical pulse ( 12 ), the spectrometer comprising the following elements: a device ( 8th ) for reading the plurality N of photodetectors ( 14 ) according to one of claims 14 to 19, means for determining a size of the pulse train signal ( 10 ) in said photodetectors ( 14 ) associated with the photodetectors ( 14 ) to determine corresponding spectral components, wherein the size of the pulse train signal ( 10 ) in the said temporal sections characterizing the intensity of the photodetector ( 14 ) received light is.

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