Device and method for spectral analysis

Abstract

The invention relates to a device for the spectral analysis of an electromagnetic measurement signal using an optoelectronic mixer, wherein the optoelectronic mixer is designed to generate the electrical superimposition signal by superimposing the electromagnetic measurement signal and a reference signal with at least one known frequency (fo). The device comprises the following features: a signal input for receiving an electrical superimposition signal from the optoelectronic mixer, a low-pass filter, a rectifier, and a read-out unit. The low-pass filter is designed to generate a filtered superimposition signal from the electrical superimposition signal by filtering out frequency portions above an upper cutoff frequency (fG). The rectifier is designed to generate a rectified superimposition signal from the filtered superimposition signal. The read-out unit is designed to determine a match of the known frequency (fo) of the reference signal with the electromagnetic measurement signal based on the rectified overlay signal.

Claims

1. A spectrum analyzer for an electromagnetic measurement signal, the spectrum analyzer comprising: at least one laser source, which is designed to produce a first continuous wave signal with a first frequency and a second continuous wave signal with a second frequency and to combine them into a reference signal, wherein the reference signal is a difference signal of the first continuous wave signal and the second continuous wave signal and the at least one laser source is tunable so as to change the first frequency and/or the second frequency in a predetermined domain, wherein the predetermined domain comprises frequencies from 0 to 10 THz; an optoelectronic mixer, which is designed to produce an electrical superposition signal through a superposition of the electromagnetic measurement signal and the reference signal, wherein the optoelectronic mixer is configured as a photoconductor comprising a finger electrode structure with a distance of less than 20 m between the contact fingers; and a device for a spectral analysis comprising: a signal input for receiving of the electrical superposition signal from the optoelectronic mixer; a low-pass filter, which is designed to produce a filtered superposition signal from the electrical superposition signal by filtering out frequency components above an upper cutoff frequency; a rectifier, which is designed to create a rectified superposition signal from the filtered superposition signal; and a read-out unit, which is designed to determine a correspondence of the known frequency of the reference signal with the electromagnetic measurement signal based on the rectified superposition signal.

2. The spectrum analyzer according to claim 1, which additionally comprises an amplifier, so as to amplify the electrical superposition signal or the filtered superposition signal.

3. The spectrum analyzer according to claim 2, wherein the low-pass filter is part of the amplifier, so that the amplifier has a reduced amplification for signals with frequencies above the cutoff frequency.

4. The spectrum analyzer according to claim 1, wherein the cutoff frequency of the low-pass filter is controllable.

5. The spectrum analyzer according to claim 1, which additionally comprises a lock-in amplifier and/or a modulator, so as to modulate the electrical superposition signal with a signal of known frequency and phase, and wherein the read-out unit is designed to read out the modulated electrical superposition signal based on the lock-in method.

6. The spectrum analyzer according to claim 1, which additionally comprises a high-pass filter, which is designed to filter out frequency components of the electrical superposition signal lying below a lower cutoff frequency, wherein the high-pass filter is located along a signal path before the rectifier and wherein the rectifier particularly comprises a precision rectifier or an operational amplifier circuit.

7. The spectrum analyzer according to claim 1, wherein the upper cutoff frequency of the low-pass filter depends on the line width of the reference signal.

8. The spectrum analyzer according to claim 1, wherein the first laser signal and/or the second laser signal have a wavelength in the range between 500 nm and 1700 nm.

9. The spectrum analyzer according to claim 1, wherein the optoelectronic mixer additionally comprises an antenna for receiving the electromagnetic measurement signal and/or a lens for focusing the electromagnetic measurement signal.

10. The spectrum analyzer according to claim 1, wherein the laser source for producing the first laser signal and the second laser signal is a multimode laser.

11. The spectrum analyzer according to claim 1, wherein the at least one laser source includes a first laser source for generating a first laser signal and a second laser source for generating a second laser signal, wherein the reference signal is a superposition of the first laser signal and the second laser signal.

12. The spectrum analyzer according to claim 1, wherein the optoelectronic mixer is configured as a photoconductor and the device for the spectral analysis is further configured to determine a spectral power of the electromagnetic measurement signal.

13. A method for analyzing an electromagnetic measurement signal by using an optoelectronic mixer, wherein the optoelectronic mixer is configured as a photoconductor comprising a finger electrode structure with a distance of less than 20 m between the contact fingers, wherein the optoelectronic mixer is designed to produce an electrical superposition signal by means of superposition of the electromagnetic measurement signal and a reference signal with at least one known frequency, wherein the reference signal is generated by at least one laser source by combining a first continuous wave signal with a first frequency and a second continuous wave signal with a second frequency, wherein the at least one laser source is tunable to modify the first frequency or the second frequency so that a frequency difference between the first frequency and the second frequency lies in a range extending up to 10 THz, comprising the following steps: receiving of an electrical superposition signal from the optoelectronic mixer; generating of a filtered superposition signal out of the electrical superposition signal by means of low-pass filtering so as to filter out frequency components above a cutoff frequency; generating of a rectified superposition signal by rectifying the filtered superposition signal; and determining of a correspondence of the known reference frequency of the reference signal with the electromagnetic measurement signal.

14. The method according to claim 13, wherein the reference signal comprises a beat of two laser signals produced by one or more laser sources and the method furthermore comprises: tuning of at least one of the laser sources until a frequency of a part of the superposition signal is below the cutoff frequency, and determining a frequency of the measurement signal.

Description

SHORT DESCRIPTION OF THE FIGURES

(1) The example embodiments of the subject technology are better understood by way of the following detailed description and the attached drawings of the different example embodiments, which should however not be understood as limiting the disclosure to these specific embodiments, but rather merely serve the purpose of explaining and understanding the subject technology.

(2) FIG. 1 shows a device for the spectral analysis of the electromagnetic measurement signal in accordance with an example embodiment of the subject technology.

(3) FIG. 2 illustrates the measurement principle for determining a spectrum of the electromagnetic measurement signal.

(4) FIG. 3 shows further details of the photoconductor.

(5) FIG. 4 shows a flow chart of a method for the spectral analysis of the electromagnetic measurement signal in accordance with the subject technology.

DETAILED DESCRIPTION

(6) Example embodiments of the subject technology serve the purpose of analyzing an electromagnetic measurement signal with an unknown spectrum, which can contain one or more unknown frequencies.

(7) FIG. 1 shows such a device 100, which is suitable for the spectral analysis of the electromagnetic measurement signal 20 by using of a photoconductor 60 (as an example of an optoelectronic mixer). The photoconductor 60 is designed to produce an electrical superposition signal 24 through a superposition of the electromagnetic measurement signal 20 and an optical reference signal 40, with the reference signal having at least one known frequency.

(8) The device 100 has the following characteristics: a signal input 105, a low-pass filter 110, a rectifier 120 and a read-out unit 130. The signal input 105 is designed to receive the electrical superposition signal 24 from the photoconductor 60. The low-pass filter 110 is designed to produce a filtered superposition signal 114 from the electrical superposition signal 24 by filtering out frequency components above an upper cutoff frequency. The rectifier 120 is designed to create a rectified superposition signal 124 from that filtered superposition signal 114. The read-out unit 130 is designed to determine a correspondence of the known frequency of the reference signal 40 with the electromagnetic measurement signal 20 based on the rectified superposition signal 124.

(9) The device 100 can additionally be grounded via a ground connection 80, so that relevant voltages can be measured with respect to ground.

(10) The signal input 105 receives the electrical superposition signal 24 from the photoconductor 60, for example via relevant electrical lines. The photoconductor 60 is, for example, designed to receive the electromagnetic measurement signal 20 on one side and the reference signal 40 on the other side. The reference signal 40 is, for example, produced by a source (reference radiation source) 30, but it can be created by superimposing several optical signals from different sources. The source 30, which is not necessarily part of the spectral analysis device 100, can for instance comprise one or more laser sources or a multi-mode laser, which outputs a superposition of one or more laser signals as a reference signal 40.

(11) A photoconductor is also known as a photoresistor and changes its electrical resistance depending on the incident electromagnetic radiation. It often consists of a semiconductor material in which incident electromagnetic radiation creates electron-hole pairs and thereby changes the conductivity. A superposition of the individual signals occurs when different radiation sources irradiate the photoconductor. The reference signal 40 is for example superimposed in the photoconductor 60 with the measurement signal 20 and an electrical superposition signal 24 is output to the device 100.

(12) It is advantageous if the photoconductor 60 reacts sufficiently quickly to changes of the irradiating spectrum so as to thus also capture high frequency signals. The photoconductor 60 can therefore be characterized by the fact that it has a charge carrier lifetime of less than 500 ps or less than 100 ps or less than 50 ps or less than 1 ps. The photoconductor 60 can additionally comprise an antenna 70, which is designed to receive the electromagnetic measurement signal 20 as efficiently as possible. The photoconductor 60 can optionally also comprise a lens 90, which is designed to bundle the electromagnetic measurement signal and direct it toward a sensitive region of the photoconductor.

(13) FIG. 2 portrays the measurement principle for determining whether a certain frequency or a certain frequency range is present in the electromagnetic measurement signal 20 or not. A spectral signal power P for a part S of the superposition signal 24 with a center frequency f and a line width L2 is represented, where the center frequency f=fsfo is the difference between the frequency of the electromagnetic measurement signal 24 and the reference signal frequency fo. Since the reference signal and its frequency fo are known, an analysis of the electromagnetic measurement signal 20 is performed via an analysis of the part S. Only a part of S is shown for the sake of simplicity, but the superposition signal 24 generally has additional frequency components. As stated before, several frequencies are additionally superimposed in the superposition signal 24: a reference signal frequency fo and the frequency fs of the measurement signal 20 that is to be studied, with the device of FIG. 1 being particularly sensitive to the frequency differences: fsfo. The spectral frequency sum share has a very high frequency that does not need to be captured by the system.

(14) The unknown frequency fs of the measurement signal 20 can be determined by the device 100 as follows.

(15) The low-pass filter 110 first filters a frequency component lying below a cutoff frequency fG from the superposition signal 24. For example, if the part of S has a frequency f>fG, the low-pass filter 110 will filter out this part of S. The filtered signal 114 then always comprises the always present noise.

(16) To determine the frequency fs (=f+fo) of the measurement signal 20, the reference radiation source 30 can be detuned so that fo and thus the frequency f of the superposition signal 24 changes accordingly. This movement of the part of S in the frequency range is represented by the double arrow 220. Because of the detuning of the reference radiation source 30, the part S of the superposition signal 24 thus wanders in the frequency range either to higher or to lower frequenciesdepending on whether the reference signal frequency fo comes close to the frequency fs of the measurement signal or gets further away from it.

(17) If, as a consequence of this tuning, the frequency f of the part S is located within the frequency band of the width L.sub.1 (f<fG) provided by the low-pass filter 110, the corresponding measurement signal S is output as a filtered signal 114 and the read-out unit 130 can produce a corresponding detection signal. This detection signal indicates that the electromagnetic measurement signal 20 has a frequency fs, which is (approximately) equal to the known reference signal frequency fo, in fact up to an accuracy defined by the cutoff frequency fG or the bandwidth of the low-pass filter 110. In order to achieve an accuracy that is as high as possible, it is, for example, possible to adjust the line width of the reference signal and the bandwidth L.sub.1 of the low-pass filter 110. If the width L.sub.1 of the low-pass filter is selected to be small (for example of the order of magnitude of the line width of the reference signal source), it can thus be determined by simply detecting a discrete signal that the electromagnetic measurement signal 24 has a frequency component (or a spectral line) with a center frequency fo.

(18) FIG. 3 shows additional details of an example photoconductor 60 and an optional antenna 70. The antenna 70 has a first section 71 and a second section 72, which respectively widen trapezoidally toward a first connector 73 and a second connector 74. The first section 71 has a first finger contact structure with several fingers at the shorter trapezoid side. The second section 72 likewise has a second finger contact structure 78 with several fingers at the shorter side of the trapezoid. The finger contacts 77, 78 extend into each other with a minimum distance d (see FIG. 3, bottom) and are formed along with the first antenna section 71 and the second antenna section 72, at least in part, on a semiconductor substrate, which constitutes the photoconductor 60. The antenna shapes are not trapezoidal in other example embodiments. It is self-understood that the invention is not to be limited to the tapered shape shown.

(19) The bottom of FIG. 3 shows a cross-sectional view along the cross-sectional line A-A, which passes through the semiconductor material of the photoconductor 60 and on the surface of which the first finger structure 77 and the second finger structure 78 extend. The maximum distance d between the fingers of the first finger structure 77 and the fingers of the second finger structure 78 can, for example, be selected so that the signal detection is as efficient as possible. This distance d can, for example, be <20 m (or d<100 m or d<50 m or d<10 m).

(20) The reference signal 40 in the illustrated example embodiment irradiates the semiconductor substrate of the photoconductor 60 from the front, while the electromagnetic measurement signal 20 which is to be measured is, for instance, directed toward the back side (i.e. to the side opposite the electrode finger structures 77, 78). However, the direction of irradiation and/or the angle of irradiation can be selected to be different in other example embodiments.

(21) The example embodiments shown have a relatively simple structure, which can be implemented economically based on commercially available components. This constitutes a significant advantage of the subject technology compared with known systems. Tuning of the reference signal source 30 furthermore allows for the frequency response (sweep oscillator) to be measured. If, for example, the electromagnetic measurement signal 20 has multiple lines in its spectrum (or has a continuous spectrum), the lines are successively detected as signals by tuning the reference signal source 30 (if the line gets to the pass band of the low-pass filter 110). A frequency line in the electromagnetic measurement signal 20 can be assigned to each detuning level of the reference signal source 30 (i.e. of a known frequency).

(22) As stated previously, the reference signal 40 can be an overlay (beat) of signals from two example laser sources, which can respectively be in a wavelength range between 500 and 1700 nm (or between 400 nm and 3000 nm), so that a difference frequency between the two laser beams in the terahertz range, which can be used for the present measurement, arises from the superposition. Other example embodiments are, for example, based on a modified continuous wave terahertz system with two continuous wave lasers (for example two so-called DFB lasers; DFB=Distributed Feedback Laser), i.e. continuous wave laser technology is used for generating the reference signal 40. The frequency analysis for this system can be accomplished as follows.

(23) It is, for example, possible to superimpose laser signals from two laser sources, whose frequencies
f.sub.1, f.sub.2=f.sub.1+f.sub.s+f
are detuned by the measurement signal f.sub.s to be detected in the terahertz range, except for a small deviation f. The deviation f can be set deliberately by the detuning of the laser source(s). A beat in the visual signal results from the overlay of the two laser signals at a frequency difference in accordance with:
P.sub.L=0.5*P.sub.0*(1+cos .sub.Lot),
where .sub.Lo=2(f.sub.2f.sub.1). The mixed laser signal subsequently irradiates the photoconductor 60 and thereby modulates its conductivity in accordance with:
P.sub.L=0.5*P.sub.0*(1+cos .sub.Lot).

(24) The photoconductor 60 additionally receives the measurement signal 20, which is to be studied and has a frequency fs, by way of its antenna 70, which couples to the semiconductor substrate. For improved coupling, the photoconductor 60 can, in accordance with other example embodiments, have a lens 90 (e.g. a silicon lens) or similar visual components, which for example serve the purpose of bundling the electromagnetic measurement signal 20. The received measurement signal 20 is thus converted into a potential, whose voltage is proportional to the received electromagnetic field strength, with the following applying:
U.sub.THzE.sub.THz*cos(.sub.st+),
where .sub.s=2f.sub.s and is any phase.

(25) Since both signals are input to the photoconductor 60, a current according to the following formula results:
IU.sub.THz*0.5*P.sub.0*E.sub.THz*(1+cos .sub.Lot)*cos(.sub.st+).

(26) Apart from the terahertz component, this current also has a component at the frequency difference between the terahertz beat of the optical signal and the frequency of the measurement signal to be characterized
f=|f.sub.Lof.sub.s|
where f.sub.Lo=f.sub.1f.sub.2 (corresponding to the previously defined fo), which oscillates with
I.sub.IFE.sub.THz*cos(2ft+).

(27) If both frequency components lie very close to one another, the resulting frequency component f is a very low frequency and can generally be read out with an RF spectrum analyzer. For example, if the frequency fs of the signal is unknown, it cannot be ensured that the frequency range f lies in the measurement range of the RF spectrum analyzer. The frequency difference of the lasers must consequently be detuned in order to analyze another frequency range and to again record an RF spectrum. This would have to be iterated for the entire THz range and the resulting spectra would have to be assembled. But a spectrum analyzer requires a certain amount of time to record an RF spectrum, so that the required measurement time can become very long.

(28) This procedure is complex and error-prone because of the many steps. It is much simpler if, as in the subject technology, only one signal were to be analyzed and if it only were necessary to pass through the frequency difference of the lasers. The relative phase between the beat on the laser signal and the THz signal is unfortunately not constant since both signals (the optical laser signal and the measurement signal) are incoherent with respect to one another. The phase thus fluctuates randomly, so that no simple DC component that can be read easily arises.

(29) In accordance with example embodiments, the DC component can be created by the device 100 as follows (also see the description relating to FIG. 2).

(30) An optional high-pass filter with a relatively low-pass frequency (for example a few kHz) is first installed behind the photoconductor 60 in order to filter out the low frequency noise. Low-pass filtering with the low-pass filter 110 with a cutoff frequency fG, e.g. within the range of the line width of the lasers (for example, a few MHz), or optionally even more follows thereafter. The measurement bandwidth is thus determined. In doing so it is not important whether the filtering actually takes place via the filter 110 or via the maximum frequency of a possibly following low noise amplifier. The low noise amplifier can for example be expedient for performing a pre-amplification of the frequently very small signals. A better signal to noise ratio is thus achieved. Photodiodes (of different types, e.g. uni-travelling carrier photodiodes) can be used as mixers with the optional high-pass filter.

(31) The thusly band-filtered signal 114 is subsequently rectified by means of a rectifier 120 (e.g. a fast diode or by means of an operational amplifier circuit, also see FIG. 1) so that a positively defined signal 124 results. If the measurement signal 20 is then close to the frequency difference of the lasers and thus within the band filter, a measurable DC signal results from the rectification, in fact regardless of the arbitrary phase between the measurement signal 20 and the laser frequency difference fo. If the frequency fs of the measurement signal 20 is too far away from the beat frequency fo of the laser, the low-pass filter filters out the mixed signal (part S in FIG. 2) and only the noise in the selected pass band is measured.

(32) The frequency difference of the lasers can be tuned thereafter and the DC component can thus be accounted for. It is for example possible to use the so-called lock-in method for this, wherefore the signal would be additionally modulated in this case. It is an advantage of this procedure that only one measuring point per frequency difference of the lasers is to be measured (the measuring point corresponds to the detection of the DC signal at a given reference f.sub.Lo).

(33) This procedure, which agrees with procedure shown in FIG. 2, offers the following advantages: It is clearly more simple than having to measure and evaluate an RF spectrum in each case. A further advantage results from the fact that only a photomixer 60 is needed instead of a photomixer for THz generation and a detector for mixing. A further advantage of the example embodiment consists of the fact that the resulting system is essentially compatible with conventional continuous wave photomixing systems and only a small effort is needed for modifying the spectrum analyzer. Although only a comparatively low frequency resolution, which lies, for example, in the range of the line width of the laser (typically a few MHz) is possibly attainable, example embodiments offer fast and simple tunability. This makes it possible to realize a rapidly operating spectral analyzer. It is in addition possible to analyze extremely wide-band THz signals. The noise threshold depends on the material quality of the photoconductor and should lie in the range fW-pW/Hz for frequencies below 1 THz. The noise increases at higher frequencies. However further measurements should nevertheless be possible if the measurement signal is strong enough. Since continuous wave lasers are very affordable compared with frequency combs, only a THz photo mixer is needed and no RF spectrum analyzers with wide measurement bandwidths are needed, the system is comparatively economical.

(34) FIG. 4 shows a flow chart for a procedure for the spectral analysis of an electromagnetic measurement signal using a photoconductor 60, with the photoconductor being designed to produce the electrical superposition signal 24 by superimposing the electromagnetic measurement signal 20 and a reference signal 40 with at least one known frequency fo. This procedure comprises the following steps: receipt S105 of an electrical superposition signal 24 from the photoconductor 60, production S110 of a filtered superposition signal 114 from the electrical superposition signal 24 by low-pass filtering, so as to filter out frequency fractions above an upper cutoff frequency fG, production S120 of a rectified superposition signal 124 by rectifying a filtered superposition signal 114, and determination S130 of an accordance of the known frequency fo of the reference signal 40 with the electromagnetic measurement signal 20.

(35) All previously described functions of the devices can be implemented in the process as further optional process steps.

(36) The process can be implemented, at least in part, in the form of software on a data processing facility, which makes it possible to control the device 100 and/or the reference signal 30. Additional example embodiments therefore also comprise a storage medium with a computer program stored thereon, which is configured to cause a device to perform the aforesaid process when it runs on a processor (processing unit). The storage medium can be a machine-readable medium, which contains mechanisms for storing and transferring data in a form that is readable by a machine (e.g. a computer). The software-implemented procedure can, for instance, be implemented on a control module with a processor on which the computer program runs.

(37) Important aspects of this invention can be summarized as follows:

(38) Example embodiments relate to a system comprising two continuous wave or quasi-continuous wave laser signals and additionally an ultrafast photoconductor (for example with a charge carrier lifetime of less than 100 ps). The system furthermore comprises a subsequent low-pass filter 110 and a subsequent rectifier 120, as well as read-out electronics 130 for the rectified signal.

(39) In another example embodiment, at least one of the lasers is tunable.

(40) In another example embodiment, the frequency difference of the lasers can be tuned between direct current up to a maximum of 10 THz.

(41) In another example embodiment, an amplifier is inserted after the photoconductor 60 or after the low-pass filter.

(42) In other example embodiments, the low-pass filter 110 is implemented by way of the cutoff frequency of an amplifier.

(43) In other example embodiments, the low-pass filter 110 is tunable.

(44) In other example embodiments, the laser frequency is in the range between 500 and 1700 nm.

(45) In other example embodiments, the measurement signal 20 is additionally modulated and the read-out electronics 130 are based on lock-in technology.

(46) In other example embodiments, the photoconductor 60 has finger contact electrode structures 77, 78 with a distance d of, e.g., less than 20 m.

(47) In other example embodiments, the photoconductor 60 is coupled to an antenna 70.

(48) In other example embodiments, the photoconductor 60 is affixed to a lens 90.

(49) In other example embodiments, a high-pass filter for filtering low-frequency components is additionally implemented after the photomixer 60 or after the low-pass filter 110 or after the amplifier.

(50) In other example embodiments, the laser signals are produced by a multi-mode laser.

(51) In other example embodiments, a continuous wave frequency comb is used as a laser source 30.

(52) The characteristics of the invention disclosed in the description, the claims and the figures can be essential, both individually and in arbitrary combination, for the implementation of the invention.

REFERENCE SYMBOL LIST

(53) 20 Electromagnetic measurement signal

(54) 24 Electrical superposition signal

(55) 30 Source (reference radiation source)

(56) 40 Reference signal

(57) 60 Photoconductor

(58) 70, 71, 72 Antenna

(59) 73, 74 Antenna connectors

(60) 77, 78 Finger contacts

(61) 80 Ground connection

(62) 90 Lens

(63) 100 Device for spectral analysis

(64) 105 Signal input

(65) 110 Low-pass filter

(66) 114 Filtered superposition signal

(67) 120 Rectifier

(68) 124 Rectified superposition signal

(69) 130 Read-out unit

(70) 220 Frequency shift of a spectral component

(71) fo Reference signal frequency

(72) fs Measurement signal frequency

(73) f1, f2 Laser frequencies

(74) fG Cutoff frequency