cesium cell, is not less than 0.1 μW, while the long-term stability is 10
À10
. In QFS,
oscillations with the frequency 4.6 GHz are extracted by the resonator, in which the
cesium cell is placed.
These oscillations pass on the modulator. Control of laser optical frequency is
performed with the modulator. In the compact rubidium QFS, oscillations are
formed by the almost similar control scheme as in QFS on cesium. We can conclude
that in the compact rubidium QFS the adjustment happens with the help of “input”
laser emission with the wavelength 0.795 μm (with generation frequency of
373 THz) on the “output” radio frequency 6.8 GHz (Fig. 2.19d). Here, with some
reservation, QFS represents the “black box”, and we may speak about conversion of
the laser emission of 373 THz into the radio frequency of 6.8 GHz.
2.9.2 A Synthesizer with the Fabry–Perot High-Q Resonator
In the laser synthesized FSS, the stabilized laser, made on the laser base (optical
operating frequency ν o ¼ 456 THz, the wavelength is 658 nm) with the stabilizing
Fabry–Perot high-Q resonator, and the femtosecond laser operating in the synchronous mode of longitudinal laser mode (or “mode-locking”) are used. Control of the
optical frequency with utilization the Fabry–Perot high-Q optical resonator provides
the spectral line width 0.2 Hz of the master laser optical emission, which is the record
index. PSD of the phase noise of the laser optical emission with the frequency
456 THz is À17 dB/Hz at the offset 1 Hz from the optical carrier [61, 75].
The basis of QFS creation, which circuit is shown in [61, 75], is the principle of
radio-frequency oscillation formation by means of adding on the photodetector area
of two synchronized optical oscillations spaced in the frequency by 1 GHz. Modern
lasers operating in this mode are usually constructed according to the diagram shown
in [61, 75]. This diagram contains series-connected into a loop the amplitude electrooptical modulator (or absorption cell with saturation), the pass-band optical filter, the
directional isolator, and the active nonlinear optical element for emission
amplification.
Types of the optical spectra at output of the femtosecond laser and the RF
spectrum at output of FSS are shown in [61, 75].
The FSS structure represents the ideal optoelectronic divider-converter of the
optical frequency of 456 THz (the wavelength in the air is 0.65 μm) by a number
N ¼ 53,200. At the “divider” output, the RF signal of 10 GHz is formed, which PSD
of the phase noise at offset frequency 1 Hz is À112 dB/Hz. This result is record at
frequency offset 1–300 Hz. This synthesizer surpasses by À50 dB/Hz in phase
Fig. 2.19 (continued) oscillator with frequency of 10 kHz. (a) The equivalent diagram of the
optoelectronic part of the quantum frequency standard with cesium cell. (b) Functional diagram of
QFS of cesium cell. (c) Cesium energy levels. (d) Rubidium energy levels [57]
60
2 Nanostructural Optoelectronic Oscillators with the Fiber-Optical Delay Line
À10
. In QFS,
oscillations with the frequency 4.6 GHz are extracted by the resonator, in which the
cesium cell is placed.
These oscillations pass on the modulator. Control of laser optical frequency is
performed with the modulator. In the compact rubidium QFS, oscillations are
formed by the almost similar control scheme as in QFS on cesium. We can conclude
that in the compact rubidium QFS the adjustment happens with the help of “input”
laser emission with the wavelength 0.795 μm (with generation frequency of
373 THz) on the “output” radio frequency 6.8 GHz (Fig. 2.19d). Here, with some
reservation, QFS represents the “black box”, and we may speak about conversion of
the laser emission of 373 THz into the radio frequency of 6.8 GHz.
2.9.2 A Synthesizer with the Fabry–Perot High-Q Resonator
In the laser synthesized FSS, the stabilized laser, made on the laser base (optical
operating frequency ν o ¼ 456 THz, the wavelength is 658 nm) with the stabilizing
Fabry–Perot high-Q resonator, and the femtosecond laser operating in the synchronous mode of longitudinal laser mode (or “mode-locking”) are used. Control of the
optical frequency with utilization the Fabry–Perot high-Q optical resonator provides
the spectral line width 0.2 Hz of the master laser optical emission, which is the record
index. PSD of the phase noise of the laser optical emission with the frequency
456 THz is À17 dB/Hz at the offset 1 Hz from the optical carrier [61, 75].
The basis of QFS creation, which circuit is shown in [61, 75], is the principle of
radio-frequency oscillation formation by means of adding on the photodetector area
of two synchronized optical oscillations spaced in the frequency by 1 GHz. Modern
lasers operating in this mode are usually constructed according to the diagram shown
in [61, 75]. This diagram contains series-connected into a loop the amplitude electrooptical modulator (or absorption cell with saturation), the pass-band optical filter, the
directional isolator, and the active nonlinear optical element for emission
amplification.
Types of the optical spectra at output of the femtosecond laser and the RF
spectrum at output of FSS are shown in [61, 75].
The FSS structure represents the ideal optoelectronic divider-converter of the
optical frequency of 456 THz (the wavelength in the air is 0.65 μm) by a number
N ¼ 53,200. At the “divider” output, the RF signal of 10 GHz is formed, which PSD
of the phase noise at offset frequency 1 Hz is À112 dB/Hz. This result is record at
frequency offset 1–300 Hz. This synthesizer surpasses by À50 dB/Hz in phase
Fig. 2.19 (continued) oscillator with frequency of 10 kHz. (a) The equivalent diagram of the
optoelectronic part of the quantum frequency standard with cesium cell. (b) Functional diagram of
QFS of cesium cell. (c) Cesium energy levels. (d) Rubidium energy levels [57]
60
2 Nanostructural Optoelectronic Oscillators with the Fiber-Optical Delay Line