DE102010021197B4 - Method for high-precision transmission of time or frequency standards - Google Patents

Abstract

Method for the high-precision transmission of time or frequency standards from a transmitter through the atmosphere to a receiver, comprising the steps of: generating a laser carrier constant carrier frequency, generating a laser carrier frequency shifted in frequency by the time or frequency standard, and transmitting the transmitter Carrier signal and the frequency-shifted carrier signal, wherein the time or frequency normal at the receiver side of the frequency shift between the frequency-shifted carrier signal and the carrier signal is obtained without a receiver-side separate local oscillator for signal demodulation application.

Description

The invention relates to a method for high-precision transmission of time or frequency standards from a transmitter to a receiver.

The state of the art is the transmission of the frequency standard via radio bearers to the partners to be synchronized (typical satellites). On short terrestrial routes, laser transmission is being tested, but these are not yet operational. The procedure is typically as follows: on a very clean oscillating laser wave, the frequency normal is modulated, e.g. by coherent phase or amplitude modulation. This signal is emitted via a transmitting telescope towards the receiver. This collects the light signal again with a receiving telescope and demodulates the frequency signal. This can e.g. done by a classical heterodyne receiver, this superimposes a so-called Local Oscillator (LO) with the incoming wave (EL) and tries to adapt its oscillation frequency and phase exactly to the EL. For this, the LO in phase and frequency must be regulated by appropriate mechanisms (so-called OPLL: Optical Phase-Lock Loop). If the phase lock succeeds, the control signal of the OPLL is a variable for measuring the modulation signal and thus the frequency standard to be transmitted. This method corresponds to the radio FM known, but it is much more expensive in the optical (with carrier frequencies around 300THz). There are other similar methods, simpler methods do not require heterodyne reception and measure e.g. only the amplitude of the incoming laser signal, but this is much less accurate, since the amplitude of the laser is greatly disturbed by the atmospheric refractive index turbulence.

Problems in the transmission of the frequency standard by laser exist in the phase noise of the laser source and the atmospheric refractive index turbulence. Both effects cause the optical carrier signal is not perfect but rushes in its phase. A frequency standard modulated thereon is likewise disturbed by this. The atmospheric refractive index turbulence leads to a stochastic path length change in the μm range, with which the phase of the optical signal can fluctuate by several wavelengths and the frequency transmission is correspondingly worsened.

In addition, the coherent transmission at both ends requires highly stable lasers (for EL and LO) and elaborate methods for stabilizing the laser phase. For this, e.g. So-called frequency combs used which allow a high-precision implementation between electrical and optical frequency, but these are expensive and still relatively bulky.

In order to be able to correct the phase errors due to the refractive index turbulence, in some approaches this refractive index phase-deviation error is measured in a separate structure; through a two-way experiment on a retroreflector, however, this requires a considerable additional effort.

EP 0 016 608 A1 describes an apparatus and method for signal transmission over an optical fiber. A first monochromatic beam is used as the carrier beam, while a second monochromatic beam having a predetermined frequency different from that of the carrier beam is used as the information beam.

The method according to the invention is defined by the features of patent claim 1.

The task solution consists in a coherent transmission of the frequency signal, in which the local oscillator is also transmitted with the actual signal. The receiver therefore does not require a separate LO (always with a phase error and which must be supplied to the receive signal via the OPLL), but the demodulation is done with the supplied EL wave. Since the two signals to be mixed transmit through the same interfering medium, they are also disturbed exactly the same (the atmospheric dispersion at a frequency offset of only about 100 MHz is negligible). Likewise, both originate from the same source and therefore also have the same phase errors of the laser source EL. All this means that the demodulated signal at the difference frequency no longer contains phase errors, since the subtraction drops out all frequency and phase errors.

The transmission or modulating the frequency normal signal can be done by an acousto-optical modulator. This optical element is electrically driven with a sine wave and causes a frequency offset of the continuous laser signal to exactly the electrical oscillation. Thus, one obtains a laser signal which is shifted by exactly the frequency to be transmitted (typ. 100MHz), this is sent to the frequency transmission parallel to a previously split-off part of the original EL laser signal towards the receiver.

In the following an embodiment of the invention will be explained in more detail with reference to FIG.

At the transmitter, the carrier signal cos (ω _EL t) and the carrier signal cos ((ω _EL + ω ₀ ) · t) shifted by the frequency normal ω _{0 are} generated: $$\text{cos}\left({\omega }_{eL}t\right)+\text{cos}\left(\left({\omega }_{eL}+{\omega }_0\right)\cdot t\right)$$

ω_EL :

Frequenz Sendelaser (ca. 200 bis 300 THz)

ω₀:

Normfrequenz (10MHz oder 100MHz)

Δφ_A:

atmosphärischer Phasenfehler

DC:

Signalanteile bei Gleichstrom

HF:

Signalanteile die bei doppelter optischer Trägerfrequenz entstehen können elektrisch nicht detektiert werden.

At the receiver, the entire signal is squared by the photodetector and converted into electrical current: $$\begin{array}{l}{\left[\text{cos}\left({\omega }_{eL}t+\mathrm{\Delta }{\phi }_A\right)+\text{cos}\left(\left({\omega }_{eL}+{\omega }_0\right)\cdot t+\mathrm{\Delta }{\phi }_A\right)\right]}^2=\hfill \\ =2\text{cos}\left({\omega }_{eL}t+\mathrm{\Delta }{\phi }_A\right)\text{cos}\left({\omega }_{eL}t+{\omega }_{0}t+\mathrm{\Delta }{\phi }_A\right)+{\text{cos}}^2\left({\omega }_{eL}t+\mathrm{\Delta }{\phi }_A\right)+{\text{cos}}^2\left({\omega }_{eL}t+{\omega }_{0}t+\mathrm{\Delta }{\phi }_A\right)=\hfill \\ =2\text{cos}\left({\omega }_{eL}t+\mathrm{\Delta }{\phi }_A\right)\text{cos}\left({\omega }_{eL}t+{\omega }_{0}t+\mathrm{\Delta }{\phi }_A\right)+DC+\underset{\begin{array}{c}TermemitdOppelterTräGerFrequenz\\ werdenGedämpf\end{array}}{\underset{\}}{HF\left(2{\omega }_{eL}t...\right)}}=\hfill \\ =2\frac{1}{2}\left[\text{cos}\left({\omega }_{eL}t+\mathrm{\Delta }{\phi }_A+{\omega }_{eL}t+{\omega }_{0}t+\mathrm{\Delta }{\phi }_A\right)+\text{cos}\left({\omega }_{eL}t+\mathrm{\Delta }{\phi }_A-\left[{\omega }_{eL}t+{\omega }_{0}t+\mathrm{\Delta }{\phi }_A\right]\right)\right]+DC=\hfill \\ =\underset{WirdGedämpf}{\underset{\}}{\text{cos}\left(2{\omega }_{eL}t+{\omega }_{0}t+2\mathrm{\Delta }{\phi }_A\right)}}+\text{cos}\left(-{\omega }_{0}t\right)+DC=\text{cos}\left({\omega }_{0}t\right)+DC\hfill \end{array}$$

ω _EL :

Frequency transmission laser (about 200 to 300 THz)

ω ₀ :

Standard frequency (10MHz or 100MHz)

Δφ _A :

atmospheric phase error

DC:

Signal components at DC

HF:

Signal components that occur at twice the optical carrier frequency can not be detected electrically.

Thus, according to the invention, a frequency transmission through the atmosphere can take place without the optical path length fluctuations due to the atmospheric refractive index turbulence having an effect on the transmitted standard frequency. The frequency transmission is possible with a spectrally relatively impure transmission laser, because the phase noise of the transmission laser is subtracted away. There is no need for a reception laser (local oscillator), which means that no optical phase-lock loop (OPLL) is required at the receiver. A separate measurement and correction of the atmospheric phase error is not necessary.

Claims (5)

Method for the high-precision transmission of time or frequency standards from a transmitter through the atmosphere to a receiver, comprising the steps of: Generating a laser light carrier signal with a constant carrier frequency, - Generating a frequency-shifted by the time or frequency normal laser light carrier signal and - Transmission of the carrier signal and the frequency-shifted carrier signal, wherein the time or frequency normal at the receiver side from the frequency shift between the frequency-shifted carrier signal and the carrier signal is obtained without a receiver-side separate local oscillator for signal demodulation application. Method according to Claim 1 , characterized in that on the transmitter side, the power from a carrier signal source is divided into two carrier signals, wherein from one of the two carrier signals, the frequency-shifted carrier signal is generated and wherein the carrier signal and the frequency-shifted carrier signal are transmitted in combined form. Method according to Claim 1 or 2 , characterized in that the carrier signal and the frequency-shifted carrier signal are superimposed by multiplication, in particular in a quadrature receiver, wherein on the receiver side, the time or frequency normal from the beat of the superimposed signal is determined. Method according to one of the preceding claims, characterized in that the frequency shift of the carrier signal takes place on the transmitter side by an acousto-optic modulator. Method according to one of the preceding claims, characterized in that the frequency normal is a highly accurate, preferably generated by an atomic clock radio frequency signal.

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