Patent Grant
8242767
The present application claims the benefit of and priority from German Patent Application Serial No. DE 10 2008 045 359.9, filed Aug. 22, 2008, the entire disclosure of which is hereby incorporated by reference herein.
1. Field of the Invention
The present invention relates to a method and an apparatus for detecting changes in an interval of time between an optical or electrical signal and an optical or electrical reference signal. In addition, the invention relates to a use of the method for synchronizing an optical or electrical signal with an optical or electrical reference signal.
2. Discussion of the Prior Art
It is important to synchronize optical or electrical signals in a highly precise manner in numerous time-critical fields of application, for example telecommunications, data transmission, surveying technology, navigation systems or in large research systems. In particular applications, it may be necessary to synchronize an optical or electrical signal with an optical or electrical reference signal in the range of femtoseconds, that is to say 10−15 s. For such precise synchronization, it is necessary to detect changes in the interval of time between two signals in a highly precise manner in order to then be able to stabilize the interval of time between two signals.
Since, in one femtosecond, light covers a path length of only approximately 0.3 μm, it immediately becomes clear that even minimal changes in length, for example as a result of thermal expansion of optical components, may result in changes in the interval of time between an optical signal and an optical reference signal. This concerns, in particular, the transmission of light signals in a long glass fibre optical waveguide. In order to be able to correct any changes in the length of the transmission path, the change in the interval of time between an optical signal and an optical reference signal must be detected to the femtosecond.
In particular, in order to operate tree electron lasers in the UV or X-ray range, for example the free electron laser in Hamburg (FLASH) and the European free electron laser (XFEL), it is necessary to synchronize various components in the accelerator to the femtosecond. In the case of the XFEL, the components to be synchronized are at a distance of up to 3.5 km from one another, with the result that coaxial distribution systems reach their limits.
A reference pulse laser is typically used to transmit a common optical reference signal to all components to be synchronized. The reference pulse laser itself is usually synchronized with an electrical original reference signal which is predefined by a microwave oscillator, for example. The components to be synchronized with the reference pulse laser beam use either optical or electrical signals which have to be synchronized with the optical reference signal from the reference pulse laser. Such a component in an accelerator could be, for example, an arrival time monitor which is used to determine the arrival time of electron pulses. For this purpose, the arrival time monitor requires an optical or electrical signal which is synchronized, for example, with the signals from other arrival time monitors at other locations in the accelerator. Therefore, all arrival time monitors use the common optical reference signal from the reference pulse laser. However, the problem in this case is that each branch of the reference signal to a component is exposed to different external conditions, for example temperature influences, and the path lengths of the reference signal to the individual components are therefore subjected to fluctuations which are not correlated with one another and interfere with the synchronization of the signals.
It is known from the prior art to use a non-linear crystal to correlate two optical pulse signals which overlap and to use a steep edge of the correlation for highly precise synchronization. However, the disadvantage of the known methods is that the correlation is dependent on the polarization of the signals. In addition, the method is highly dependent on the pulse lengths which, for the rest, have to overlap in terms of time.
Accordingly, the object of the present invention is to provide a method and an apparatus, which overcome the disadvantages of the prior art and provide an improved use for synchronizing optical or electrical signals to the femtosecond.
According to a first aspect of the present invention, this object is achieved by a method for detecting changes in an interval of time between an optical or electrical signal and an optical or electrical reference signal using a photodetector. The method comprises the following steps:
The method can be used in four different modes which are shown in the following table:
In the case of an electrical signal or an electrical reference signal, that is to say in all method modes apart from the optical-optical mode, it is first of all necessary to modulate an optical signal or an optical reference signal on the basis of the electrical signal or electrical reference signal. In this case, the amplitude of the optical signal or optical reference signal is preferably modulated on the basis of the electrical signal or electrical reference signal. It is noted at this point that the interval of time refers to the period of time between an original optical or electrical signal and the original optical or electrical reference signal. In the case of an electrical signal or an electrical reference signal, it may therefore be the case that a change in this interval of time is not expressed in the form of a change in the interval of time between the modulated optical signal and the optical reference signal but rather only in the form of amplitude modulation, for example. If the signal and the reference signal, the interval of time between which is to be detected, are optical, that is to say are in the optical-optical method mode, the modulation steps are not needed.
The optical signal, and the optical reference signal are received using the same photodetector. This avoids differences between different photodetectors and minimizes systematic errors when detecting the interval of time. It is noted at this point that the optical signal and the optical reference signal may have a common source and/or may be branches of the same optical signal.
The inventive method has the advantage over known methods that, inter alia, it is independent of the polarization of the optical signal or of the optical reference signal and is also independent of the respective pulse widths over a wide range. In addition, the pulses of the signal and the pulses of the reference signal need not overlap in terms of time. The proposed method provides a multiplicity of possible temporal offsets between an optical signal and an optical reference signal which are suitable for detecting the temporal change. Only insignificant additional path lengths therefore have to be inserted in order to ensure a suitable operating point.
The optical signal and/or the optical reference signal is/are preferably generated by one or more mode-coupled short-pulse lasers. The optical signal and/or the optical reference signal is/are preferably periodic pulse signals with a pulse width which is relatively small in comparison with the period duration, for example a pulse width of a fraction of a picosecond. In contrast, with a short-pulse laser which is usually operated at a pulse frequency of 50 to 250 MHz, the period duration is 4 to 20 nanoseconds, which corresponds to a path length of the light of 1.2 to 6 meters. Therefore, it is a great advantage of the invention that such long path lengths need not be inserted in order to ensure that the pulses overlap with a width corresponding to a path length of the light of less than 0.3 millimeters.
It is advantageous if the interval of time is set to a value in the range from 0.4 to 0.6, preferably 0.45 to 0.55, of the period duration of the optical signal. It has been found that this makes it possible to achieve a maximum degree of sensitivity to changes if the harmonic is selected appropriately. The selected harmonic is preferably a high-order harmonic, that is to say of the order 5 or higher, for example. This is because it has likewise been shown that the sensitivity to changes is particularly high with the higher-order harmonics, in particular of the order 5 or higher, and a multiplicity of intervals of time can be used as expedient operating points. The largest possible order which can be selected is limited by the bandwidth of the photodetector and the filter width of the filter unit since this restricts the number of orders whose amplitude can still be expediently measured or filtered.
The frequency spectrum may result from the time signal, for example with the aid of Fourier analysis or transformation, the time signal being able to be represented as the sum of harmonics, for example:
where A(t) is the joint signal comprising the optical signal and the optical reference signal in the form of an amplitude A as a function of the time t, n is the order of the harmonic, An is the amplitude of the n-th order harmonic, f0 is a fundamental frequency and φn is the phase shift of the n-th order harmonic. The discrete frequency spectrum than contains the amplitudes An of the respective frequency components as a function of the frequency nf0 which corresponds to the frequency of the n-th order harmonic. If the optical signal and the optical reference signal have the same period duration T0 or the same pulse frequency f01/T0, the same amplitude A, and an interval of time ΔT, then: A0=cAt, Ak=0, A2k=0, if ΔT=T0/(2k) for k=1, 2, . . . , N and φ0=0. It is noted at this point that the invention is not restricted to harmonics in the representation of equation (1) but rather may have any desired representation.
There are different possibilities for detecting changes in the interval of time from changes in the amplitude of the selected harmonic. One simple possibility is for a change in the amplitude of the selected harmonic to be used as a direct measure of the change in the interval of time. Since the frequency spectrum depends on the interval of time, the envelope changes on the basis of the interval of time. It is now advantageous if a harmonic at a frequency at which the magnitude of the gradient of the envelope of the frequency spectrum is at a maximum is selected for filtering. The amplitude of the selected harmonic is then most sensitive to changes in the interval of time. As an alternative to the suitable selection of the harmonic, the interval of time may also be set in such a manner that a harmonic desired for selection has this property.
The disadvantage of this possibility is the dependence of the optical signal or optical reference signal on amplitude fluctuations. A change in the amplitude of the selected harmonic is only suitable as a direct measure of the change in the interval of time when the amplitude of the optical signal or optical reference signal is very constant. Otherwise, amplitude fluctuations in the optical signal or optical reference signal would be incorrectly interpreted as a change in the interval of time.
Therefore, it may be advantageous if a second selected harmonic is additionally filtered from the frequency spectrum of the electrical response signal which has been output and a change in the difference between the amplitude of the selected harmonic and the amplitude of the second selected harmonic is used as a measure of the change in the interval of time. This is because the difference between the amplitude of the selected harmonic and the amplitude of the second selected harmonic is independent of amplitude fluctuations to the greatest possible extent since the latter have the same effect on the two selected harmonics. The second selected harmonic is preferably of an order which is one smaller or greater than the order of the selected harmonic. This is because it has been found that the difference in amplitude of harmonics of adjacent orders, in particular with an interval of time close to half the period duration, is particularly sensitive to changes in the interval of time. It has likewise been found that possible errors caused by the photodetector and/or the downstream electronics and/or the filter unit are particularly small for adjacent harmonics.
For metrological reasons, it may also be advantageous to select a harmonic or to set the interval of time in such a manner that the magnitude of the envelope of the frequency spectrum is at a minimum at the frequency of the selected harmonic. If the amplitudes of the optical signal and optical reference signal are the same and with a suitable interval of time, the selected harmonic may be erased, with the result that the amplitude can be measured at the zero point, which is advantageous for particular applications. However, the disadvantage is that the signal can be synchronized with the reference signal only using further aids since the change in amplitude at the zero point does not contain any information relating to the direction of a change in the interval of time.
In order to determine the direction of a change in the interval of time, it may be advantageous if the method comprises the following further steps:
The reference harmonic and the selected filtered harmonic are preferably multiplied during mixing. If both oscillations are passed into the mixer in phase, the mixer can be used as an “amplitude detector”. The product of the reference harmonic and the selected filtered harmonic is an output signal which oscillates around a particular amplitude at twice the frequency. The signed change in amplitude of the output signal can be extracted, for example, using a low-pass filter which removes the oscillating component of the output signal. In this case, the change in amplitude of the output signal has a sign which depends on the direction of the change in the interval of time, with the result that the direction of the change in the interval of time can be determined from the output signal and can be regulated in a corresponding manner.
It is also advantageous if a delay device is used to delay the optical signal and/or the optical reference signal by a selected period of time. Such a delay device may be, for example, an extension of the path length of the optical signal and/or the optical reference signal.
A second aspect of the invention provides a use of the above-described method for synchronizing an optical or electrical signal with an optical or electrical reference signal, the interval of time being regulated on the basis of the change in the interval of time detected by the method. The interval of time is preferably regulated by means of feedback. It may be particularly advantageous to regulate the difference between the amplitudes of two selected harmonics of adjacent orders to zero.
A third aspect of the invention provides an apparatus for detecting changes in an interval of time between an optical or electrical signal and an optical or electrical reference signal, said apparatus comprising a photodetector, a filter unit and a measuring device,
The photodetector preferably has a wide bandwidth, with the result that the frequency spectrum of the electrical response signal which has been output comprises at least 5 harmonics. The temporal detection resolution is limited by the measurement accuracy of the measuring device, with the result that it is advantageous if the measuring device has a measurement accuracy of at least δA=/A=10−3, preferably of at least δA/A=10−4, for the amplitude of the selected harmonic.
It may also be advantageous if the apparatus comprises second filter unit which is connected to the output of the photodetector and is designed to filter a second selected harmonic from the frequency spectrum of the electrical response signal which has been output, the measuring device being connected to the second filter unit and being designed to detect changes in the interval of time from changes in the difference between the amplitude of the selected harmonic and the amplitude of the second selected harmonic. This apparatus makes it possible to carry out the above-described method in such a manner that the detection of changes in the interval of time is independent of amplitude fluctuations in the optical signal or optical reference signal.
At least one filter unit is preferably integrated in the measuring device, that is to say the connection between at least one filter unit and the measuring device is ensured inside the measuring device. It may also be advantageous if the apparatus comprises a delay device which is designed to delay the optical signal and/or the optical reference signal by a selected period of time. The interval of time can thus be set as desired. Such a delay device may be, for example, an extension of the path length of the optical signal and/or the optical reference signal.
In one advantageous embodiment, the apparatus comprises a second photodetector, a further filter unit and a mixer,
The mixer and the further filter unit may be integrated in the measuring device. Furthermore, the measuring device may be connected to a control unit via feedback, the control unit being designed to regulate the interval of time. The control unit may control, for example, the repetition rate of the reference laser. This is expedient, for example, when the reference laser itself is intended to be synchronized with an electrical reference signal from a microwave oscillator, that is to say the apparatus is intended to carry out the method in the optical-electrical mode. On the other hand, the control unit may also readjust an electrical signal which is intended to be synchronized with the optical reference signal from the reference laser, the apparatus thus being intended to carry out the method in the electrical-optical mode.
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description of the preferred embodiments. This summary is not intended to identity key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
Various other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures.
Preferred embodiments of the invention are described in more detail below with reference to the accompanying
The present invention is susceptible of embodiment in many different forms. While the drawings illustrate, and the specification describes, certain preferred embodiments of the invention, it is to be understood that such disclosure is by way of example only. There is no intent to limit the principles of the present invention to the particular disclosed embodiments.
If the photodetector 5 now receives the optical signal 1 and the optical reference signal 3, it outputs an electrical response signal 15 at an output 13 of the photodetector 5. The electrical response signal 15 has a frequency spectrum which depends on the interval of time ΔT. The filter unit 7 is now used to filter a selected harmonic from the frequency spectrum of the electrical response signal 15 which has been output and the amplitude thereof is measured using the measuring device 9. Changes in the interval of time ΔT can then be detected from changes in the measured amplitude of the selected harmonic. For example, this is possible as a direct measure from changes in the measured amplitude if the amplitude At of the optical signal 1 and of the optical reference signal 3 is constant over time.
By way of example,
It becomes clear from
A(f,ΔT)=0.5·A0(1+cos(ΔT/T0·2πf/f0)). (2)
This means that the envelope 17 has a period length of f0·T0/ΔT. With an interval of time of, for example, ΔT−T0/2, precisely every second harmonic is erased, namely those of an uneven order.
It becomes immediately clear from
As shown in
It also becomes clear that the sensitivity is particularly high for values of the interval of time in the vicinity of ΔT=T0/2, that is to say in the range from 0.4 to 0.6, or preferably 0.45 to 0.55, since the envelope 17 has a short period length and thus steep gradients in this range.
It becomes immediately clear from equation (3) that the sensitivity is greater for higher-order harmonics than for lower-order harmonics. A preferred operating point (illustrated as a black dot in
The second photodetector 33 is designed to receive a branched optical signal 1 and to output a second electrical response signal 39 at an output 41 of the second photodetector 33. The second electrical response signal 39 also has a frequency spectrum in this case. The further filter unit 35 is connected to the output 41 of the second photodetector and is designed to filter a selected reference harmonic from the frequency spectrum of the second electrical response signal 39 which has been output. In this case, the reference harmonic is of the same order as the selected harmonic from the frequency spectrum output by the first photodetector 5 with the electrical response signal 15. The mixer 37 has a first input 43, a second input 45 and an output 47, the first input 43 being connected to the first filter unit 7 and the second input 45 being connected to the further filter unit 35. The mixer 37 is designed to mix the reference harmonic and the selected filtered harmonic and to output the output signal at the output 47 of the mixer 37, a change in the interval of time ΔT being able to be detected from the signed change in amplitude of the output signal. The mixer 37 and the further filter unit 35 may also be integrated in a measuring device 9.
In a similar manner to the third embodiment, it is then possible to regulate to a zero value or minimum value of the amplitude of the selected harmonic which is erased at a desired value of the interval of time of ΔT=0. In this case, the reference harmonic received by the second photodetector 33 and filtered using the further filter unit 35 is of the same order as the selected harmonic from the frequency spectrum output by the first photodetector 5 with the electrical response signal 15. Since, in embodiments of the method in the optical-electrical or electrical-optical mode in which the optical reference signal 3 or the optical signal 1 is amplitude-modulated, a change in the interval, of time ΔT is not expressed by a change in the path difference between the pulses of the optical signal 1 and those of the optical reference signal 3, the sensitivity to changes in the path difference should be minimized in this case. A change in the path difference may be caused, for example, by a change in the length of the path of the optical signal 1 or of the optical reference signal 3. For these embodiments, it may therefore be advantageous if a low-order harmonic is selected in order to minimize, for example, the influence of changes in the length of the path of the optical signal 1 or of the optical reference signal 3. In order to also detect a change in the interval of time ΔT here from a change in a signed change in amplitude of an output signal from a mixer 37, provision is also made here of a mixer 37 having a first input 43, a second input 45 and an output 47, the first input 43 being connected to the first filter unit 7 and the second input 45 being connected to the further filter unit 35. The mixer 37 is designed to mix the reference harmonic and the selected filtered harmonic and to output an output signal at the output 47 of the mixer 37, a change in the interval of time ΔT being able to be detected from the signed change in amplitude of the output signal. The mixer 37 and the further filter unit 35 are integrated in a measuring device 9 here.
In the case of the synchronization of the repetition rate of the short-pulse laser 23 with the electrical reference signal from the microwave oscillator 51, as shown in
Apart from the feedback,
The preferred forms of the invention described above are to be used as illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention.
The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and access the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention set forth in the following claims.
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| 10 2008 045 359 | Aug 2008 | DE | national |
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