DE102012024692B3 - Phase noise measuring instrument - Google Patents

Abstract

The invention relates to a phase noise measuring device for measuring a phase noise of electromagnetic radiation, in particular of light, with a radiation input (12) for coupling a primary beam (14) of coherent electromagnetic radiation, the primary beam (14) having a primary beam phase noise to be measured, a secondary beam generating device (16) for generating a secondary beam (18) from the primary beam (14), the secondary beam (18) having a secondary beam phase noise that is uniquely dependent on the primary beam phase noise, a secondary beam stabilization device (27 ) having a passive frequency reference embodiment (26) representing a frequency reference and a control device for controlling a secondary beam frequency to the frequency reference, for generating a stabilized secondary beam (18 ') and a phase difference measuring device (36) is set up for measuring e Iner phase difference between primary beam (14) and stabilized secondary beam (18 '), so that the primary beam phase noise can be determined from the phase difference.

Description

The invention relates to a phase noise measuring device and an associated phase noise measuring arrangement.

The spectral power density of the phase-frequency noise is an important parameter for the characterization of oscillators, especially lasers.

For measuring the phase noise, it is known from "Characterization of Clocks and Oscillators" by D. B. Sullivan et al, January 1990, Nat. Institute of Standards and Technology Note 1337 known to use a reference laser, which has a particularly low phase noise. By superposing the primary beam of laser light whose phase noise is to be determined with the reference laser beam of the reference laser, beat signal is generated. The beat signal is detected by means of a photodiode. The signal thus obtained contains the desired phase noise information and can be further investigated, for example by a Fourier transformation.

This method allows measurements of the spectral power density of the phase noise for Fourier frequencies between 1 millihertz and several 100 kilohertz. The upper limit of the Fourier frequencies is given by the control bandwidth of the reference laser. A disadvantage of this method is that reference lasers with low phase noise are very complex and therefore expensive.

The self-heterodyne method is known from the article "Novel method for high resolution measurement of laser output spectrum", Electron Lett., Vol. 16, pages 630-631, (1980) by T. Okoshi et al. In the self-heterodyne method, the primary beam whose phase noise is to be measured is compared to an uncorrelated and frequency-shifted copy of itself. This is possible by means of a fiber-based interferometer with very different arm lengths. The light directed to one of the arms is shifted by an acousto-optic modulator by, for example, 10 MHz and superimposed with the unshifted light conducted into the other arm. The superposition yields a beat signal, from which the phase noise spectrum can be determined.

A disadvantage of this method is that the arm length difference in the interferometer expediently has to be greater than the coherence length of the light of the primary beam to be examined. If the phase noise of modern fiber lasers is to be determined, the arm length difference must be more than 100 km. Such arm length differences in turn lead to a phase noise that can not be separated from the actually measured phase noise of the primary beam. Another disadvantage is that the measurement accuracy in the self-heterodyne method is limited by the intrinsic noise of the interferometer. These are fiber lasers with linewidths in development whose phase noise is so small that it can no longer be measured with the self-heterodyne method. Another disadvantage of known systems lies in the limitation of the lowest limit of the Fourier frequencies. This is given by the reciprocal of the transit time difference. At 100 km, the runtime is about 500 μs. A phase noise measurement is therefore only possible from a lowest frequency of 2 kHz.

From the DE 60 2004 008 111 T2 An optical measuring system is known in which liquid crystal over silicon spatial light modulators (LCOS SLM) are used to locally introduce phase changes in a light field. By Fourier transformation, the local phase deviation can be calculated from the resulting spatial phase distribution. Highest accuracies are difficult to achieve with this measuring system.

In the DE 102 44 413 B4 a device for measuring a phase shift characteristic is described, in which the input device to be tested with a variable wavelength input radiation is applied, wherein the input radiation is modulated by means of a light modulator. The radiation thus obtained is passed to the test specimen and compared the radiation emitted by this radiation with the irradiated radiation. The disadvantage of this is that a light source with very low phase noise is necessary.

The invention has for its object to improve the measurement of the phase noise.

The invention solves the problem by a phase noise measuring device for measuring a phase noise of electromagnetic radiation, in particular of light, having (a) a radiation input for coupling a primary beam of coherent electromagnetic radiation, the primary beam having a primary phase noise to be measured, (b) a secondary beam generating device for generating a Secondary beam from the primary beam, the secondary beam having a secondary beam phasing noise that depends in a unique, in particular one-to-one manner on the primary beam phase noise, (c) a secondary beam stabilization device comprising a passive reference embodiment that embodies a frequency reference and a Regulating device for controlling a secondary beam frequency to the frequency reference, for generating a stabilized secondary beam and a phase measuring device which is adapted to measure a phase difference between the primary beam and the stabilized secondary beam, so that the primary beam phase noise can be determined from the phase difference.

According to a second aspect, the invention solves the problem by an associated method according to the independent method claim.

An advantage of the invention is that the phase noise can be determined with very high accuracy even for Fourier frequencies well below 1 kHz.

Another advantage is the small footprint of the phase noise meter according to the invention. Thus, the phase noise meter can be installed in a standard 19-inch stretch feeder. An important advantage is the large measuring range. The invention makes it possible to perform a phase noise measurement within a wavelength range of about 100 nm, without the need of a frequency comb.

In the context of the present description, the radiation input means, in particular, a location and / or a device into which a laser beam or another beam of electromagnetic radiation is coupled during operation. For example, the radiation input is an end of a glass fiber line, through which the laser beam to be measured can be coupled.

Under the secondary beam generating device is understood in the simplest case, a beam splitter. But it is also possible that the secondary beam generating device comprises a laser, the laser beam is phase-locked to the primary beam.

The secondary beam stabilization device is understood in particular to mean a device which changes the secondary beam frequency of the secondary beam such that the phase noise is reduced by coupling to the frequency reference embodiment.

A frequency reference embodiment is understood as meaning, in particular, an embodiment of a frequency which serves as a reference. An embodiment is, in particular, an object that is such that the frequency can be reproduced reproducibly.

The phase difference measuring device comprises, for example, a mixing device, in particular a beam splitter, on which the primary beam and the stabilized secondary beam are brought into interference. The result is a beat signal from which the phase noise can be determined. Since the stabilized secondary beam has a particularly low phase noise, the phase noise thus determined is, to a good approximation, equal to the primary beam phase noise.

Since noise of the frequency can be described as phase noise, the phase noise meter is also a frequency noise meter. Since the phase noise and a frequency noise are interconvertible, in the context of the present description, a frequency noise is always also meant when a phase noise is mentioned. In particular, the determination of the phase noise is understood as meaning the determination of characteristic quantities of the phase noise, in particular the determination of the spectral power density.

According to a preferred embodiment, the passive frequency reference embodiment comprises a passive resonator. It is favorable if a finesse of the passive resonator is greater than 10,000. The resonator (including mirror) is preferably made of a material with a coefficient of linear expansion of at most 10 ^-8 / Kelvin. It is favorable if the passive resonator is temperature-stabilized. In particular, the resonator may comprise a heat stabilizing device. Preferably, the heat stabilizing device is formed so that the resonator experiences a temperature fluctuation of at most 1 microkelvin per second. For example, the heat stabilizing device comprises at least one heat shield. It is also favorable if the passive resonator is arranged in a vacuum.

Preferably, the control device comprises a Pound Drever Hall controller. By means of such a control device, the secondary beam frequency can be coupled with high accuracy to the natural frequency of the resonator.

Alternatively or in addition to a passive resonator, the passive frequency reference embodiment includes a fiber interferometer (Jiang, M .; Kéfélian, F. Leomonde, P., Clairon, A. & Santarelli, G .: "An agile laser with ultra-low frequency noise and high sweep linerarity "Optics Express, 2010, 18, 3284-3297). It is particularly favorable if the fiber interferometer has an arm length difference of at least 2 km. By the arm length difference, the difference of the optical path lengths between a light beam passing through the first arm, on the one hand, and a light beam passing through the second arm, on the other hand, is understood. In other words, it does not have to be real arm lengths, so it is possible that a ray of light passes through an arm several times.

The invention also solves the problem by a phase noise meter having (a) a radiation input for coupling a primary beam of coherent electromagnetic radiation, the primary beam having a primary beam phase noise to be measured, (b) a primary beam stabilization device having a passive frequency reference embodiment comprising a frequency reference, and a control device for controlling a secondary beam frequency to the frequency reference by means of a control signal, for generating a stabilized primary beam and (c) an electrical evaluation unit arranged to determine the primary beam phase noise from the control signal.

According to the invention is also a phase noise measuring arrangement with a laser for generating the primary beam, and a phase noise measuring device according to the invention, in whose radiation input the laser emits the primary beam.

According to a preferred embodiment, the secondary beam stabilizing device of the phase noise measuring device comprises a low-pass filter for generating a low-frequency component of a control signal of the control device, the laser having a laser-frequency varying device connected to the low-pass filter for reducing drift of the laser. In this way, the phase noise can also be determined on drifting lasers.

In the following invention will be explained in more detail with reference to the accompanying drawings. It shows

1 a phase noise measuring device according to the invention according to an embodiment and

2 a phase noise measuring device according to the invention according to a second embodiment.

3 shows a phase noise measuring device according to the invention according to a third embodiment.

1 shows a phase noise measuring device according to the invention 10 with a radiation input 12 for coupling a primary beam 14 whose phase noise is to be measured. The primary beam 14 has a primary beam frequency f ₁₄ (t), which is called f ₁₄ (t) = f _14.0 + Δf ₁₄ (t) 1 can be described with

Here, φ ₁₄ (t) is the phase noise.

The phase noise meter 10 also includes a secondary beam generating device 16 , in the present case in the form of a beam splitter, which has a secondary beam 18 generated. The primary beam 14 will be over a mirror 20 on a beam splitter 22 guided.

The secondary beam 18 passes through an acousto-optic modulator 24 and then enters a frequency reference embodiment 26 , which is formed in the present case by a highly stable optical resonator. The reference frequency embodiment 26 has a natural frequency f ₂₆ , which is only a distance of two mirrors 28.1 . 28.2 the frequency reference embodiment 26 depends and therefore has a special low noise. The passive reference frequency embodiment 26 has a finesse of in the present case 100000.

The acoustooptic module star 24 and the frequency reference embodiment 26 are part of a secondary beam stabilization device 27 , which also has a schematically drawn Pound-Drever-Hall control device 30 includes. The operation of the Pound Drever Hall control device 30 is described in the literature, for example in Drever, RWP et al., "Laser Phase and frequency stabilization using an optical resonator", Appl. Phys. B 31, 97-105 (1983) and is therefore not explained here.

The secondary beam stabilization device 27 is designed so that a stabilized secondary beam 18 ' is generated for its secondary beam frequency f ₁₈ (t) = f 14.0 + Δf ₁₈ (t) 3 applies, where for frequencies f ₁₄ <f _{border, 30} , | Δf ₁₈ | <| Δf ₁₄ | 4 where f is _{limit, 30 is} the bandwidth of the control device.

The stabilized secondary beam 18 ' is from the frequency reference embodiment 26 is reflected and therefore as a secondary beam 18 ' _r denotes. This secondary beam 18 ' _r is via a second beam splitter 32 also on the first beam splitter 22 directed where he is with the primary beam 14 interferes. The resulting beat signal is from a photodiode 34 measured.

The photodiode 34 is part of a phase difference measuring device 36 , which also has a frequency counter 38 and an FFT calculating device for calculating a Fourier transform of the power spectral density of the phase noise. The counter determines the number of zero crossings in the oscillation of the electric beat signal in a fixed time window T (gate time). From this, the frequency f of the beat signal can be calculated by means of f = n / 2T. This assumes that the counter measures dead time. In addition, counters which are preferably used can also determine a fraction of entire oscillations, so that n is not necessarily an integer. The FFT calculating device 40 generates an electrical output signal 42 that encodes this spectral power density of the phase noise.

The phase noise meter 10 forms together with a laser 44 a phase noise measurement arrangement 46 ,

With the arrangement described above, one obtains the spectral power density of the phase noise for Fourier frequencies up to about 1 kilohertz. In order to be able to measure the phase noise spectrum even at higher Fourier frequencies, that of the photodiode 34 recorded vibration signal can be recorded directly with an RF spectrum analyzer. The signal thus obtained can then be converted into the spectral power density. The Fourier frequency components measurable with the construction described above are by the bandwidth of the control device 30 limited in the present case at about f _{limit, 30} = 100 kilohertz.

The advantage of the illustrated FFT method lies in the possibility of being able to measure the phase noise even at very low frequencies. The lowest frequency is then given by the duration of the recorded frequency track. A combination of frequency counter and FFT on the one hand and a spectrum analyzer on the other hand then allows the measurement of the phase noise spectrum in a frequency range from 1 millihertz to 100 kilohertz.

In order to measure the spectral power density of the phase noise also for Fourier frequencies above the control bandwidth, the passive frequency reference embodiment 26 be used as an additional low pass. In addition the in 1 dashed lines transmitted transmitted stabilized secondary beam 18 ' _t used. This transmitted stabilized secondary beam 18 ' _t is on a third beam splitter 48 with the primary beam 14 brought to interference. The measuring range for the Fourier frequencies of the spectral power density of the phase noise thus expands to several megahertz.

The phase noise meter 10 can thus measure the noise spectrum of the phase noise in a range from 1 millihertz to several 100 kilohertz.

If the laser 44 drifts, the phase noise can be measured well when the secondary beam stabilization device 27 a low pass filter 49 includes and a low pass filtered control signal 50 , which is indicated by a dashed line, to the laser 44 emits. The laser 44 In this case, it includes a laser frequency varying device 52 , For example, a piezoelectric actuator, the mirror spacing of Resanatorspiegeln in the laser 44 can change. The laser frequency changing device 52 adjusts the mirrors of the laser 44 then so that a long-term drift is compensated.

2 shows an alternative embodiment of a phase noise measuring device according to the invention 10 in which the frequency reference embodiment 26 a fiber interferometer 54 includes. The fiber interferometer 54 includes a first arm 56 which is formed by a glass fiber, and a second arm 58 which is also formed by a glass fiber. The optical path length of the first arm 56 is significantly longer than the second arm, for example, at least 2 km longer. The through the two arms 56 . 58 running beams of light are on a beam splitter 60 brought to interference and the interference signal from a photodiode 62 added. That of photodiode 62 generated electrical signal is from the control device 30 evaluated the the acousto-optic modulator 24 so controls that the phase noise in the secondary beam 18 is largely suppressed. As with the in 1 shown construction is the stabilized secondary beam 18 ' with the primary beam 14 on the beam splitter 22 brought to interference and the resulting interference signal is transmitted through the photodiode 34 recorded.

With the in 1 As shown, the system can achieve an Allan standard deviation of 10 ^-13 / τ for integration times between τ = 1 millisecond and τ = 0.1 seconds. For an integration time between τ = 0.1 second and τ = 10 seconds, Allan standard deviation of less than 10 ^{-14 is} reached.

The system achieves inherent noise, which does not reach the minimum achievable phase noise of reference laser systems, but is at least two orders of magnitude away from the phase noise of available fiber lasers.

3 shows an alternative embodiment of a phase noise measuring device according to the invention 10 with a radiation input 12 for coupling a primary beam 14 coherent electromagnetic radiation, the primary beam 14 having a primary beam phase noise to be measured, a primary beam stabilization device 35 , which is a passive frequency reference embodiment 26 representing a frequency reference, and a control device 30 for controlling a primary beam frequency f ₁₄ to the frequency reference f ₂₆ by means of a control signal, for generating a stabilized primary beam 14 ' and an electrical evaluation unit 64 adapted to determine the primary beam phase noise from the control signal.

LIST OF REFERENCE NUMBERS

10

Phase noise measuring instrument

12

radiation input

14

primary beam

16

Secondary beam generating device

18

secondary beam

18 '

stabilized secondary beam

18 ' _r

reflected stabilized secondary beam

_18't

transmitted stabilized secondary beam

20

mirror

22

beamsplitter

24

acousto-optic modulator

26

Frequency reference incarnation

27

Secondary beam stabilizing device

28

mirror

30

Pound-Drever-Hall-control device

32

second beam splitter

34

photodiode

35

Primary beam-stabilizing device

36

Phase difference measuring device

38

frequency counter

40

FFT computation device

42

output

44

laser

46

Phase Noise Measurement arrangement

48

third beam splitter

49

Low Pass Filter

50

control signal

52

Laser frequency-changing device

54

fiber interferometer

56

first arm

58

second arm

60

beamsplitter

62

photodiode

64

evaluation

f ₁₄

Primary beam frequency

f ₂₆

Natural frequency of the resonator

Claims (9)

Phase noise measuring device for measuring a phase noise of electromagnetic radiation, in particular of light, having (a) a radiation input ( 12 ) for coupling a primary beam ( 14 ) coherent electromagnetic radiation, the primary beam ( 14 ) has a primary beam phase noise to be measured, (b) a secondary beam generation device ( 16 ) for generating a secondary beam ( 18 ) from the primary beam ( 14 ), the secondary beam ( 18 ) has a secondary beam phase noise that is uniquely dependent on the primary beam phase noise, (c) a secondary beam stabilization device ( 27 ), which - a passive frequency reference embodiment ( 26 ), which represents a frequency reference, and - comprises a control device for regulating a secondary beam frequency to the frequency reference, for generating a stabilized secondary beam ( 18 ' ) and (d) a phase difference measuring device ( 36 ) arranged to measure a phase difference between primary beam ( 14 ) and stabilized secondary beam ( 18 ' ), so that the primary beam phase noise can be determined from the phase difference. Phase noise measuring device according to claim 1, characterized in that the passive frequency reference embodiment ( 26 ) comprises a passive resonator. Phase noise measuring device according to claim 2, characterized in that the regulating device is a Pound-Drever-Hall control device ( 30 ). Phase noise measuring device according to one of the preceding claims, characterized in that the passive frequency reference embodiment ( 26 ) a fiber interferometer ( 54 ). Phase noise measuring device with (a) a radiation input ( 12 ) for coupling a primary beam ( 14 ) coherent electromagnetic radiation, the primary beam ( 14 ) has a primary beam phase noise to be measured, (b) a primary beam stabilization device ( 35 ), which - a passive frequency reference embodiment ( 26 ), which represents a frequency reference, and - a control device ( 30 ) for controlling a primary beam frequency (f ₁₄ ) to the frequency reference (f ₂₆ ) by means of a control signal, for generating a stabilized primary beam ( 14 ' ) and (c) an electrical evaluation unit ( 64 ) configured to determine the primary beam phase noise from the control signal. Phase noise measuring arrangement with (i) a laser ( 44 ) for generating a primary beam ( 14 ) and (ii) a phase noise meter ( 10 ) according to one of the preceding claims, in whose radiation input ( 12 ) the laser the primary beam ( 14 ). Phase noise measuring arrangement according to claim 6, characterized in that - the secondary beam stabilization device ( 27 ) of the phase noise measuring device ( 10 ) comprises a low-pass filter for generating a low-frequency portion of a control signal of the control device, and - the laser ( 44 ) a laser frequency changing device ( 52 ) associated with the low-pass filter for reducing drift of the laser ( 44 ) connected is. Phase noise measuring arrangement according to one of claims 6 or 7, characterized in that the laser ( 44 ) is an infrared laser and / or a fiber laser. Method for determining phase noise of electromagnetic radiation, in particular of light, comprising the steps of: (a) generating a secondary beam ( 18 ) from a primary beam ( 14 ), whose phase noise is to be measured, so that the secondary beam ( 18 ) has a secondary beam phase noise that is unambiguously dependent on the primary beam phase noise, (b) stabilizing the secondary beam ( 18 ) by means of a passive frequency reference embodiment ( 26 ), which represents a frequency reference, by controlling a secondary beam frequency of the secondary beam ( 18 ) on the frequency reference, and (c) measuring a time-dependent phase difference between primary beam ( 14 ) and stabilized secondary beam ( 18 ' ) and (d) determining the primary beam phase noise from the phase difference.

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