Low noise amplification of an optically carried microwave signal: application to atom interferometry
ABSTRACT In this paper, we report a new scheme to amplify a microwave signal carried on a laser light at λ=852nm. The amplification is done via a semiconductor tapered amplifier and this scheme is used to drive stimulated Raman
transitions in an atom interferometer. Sideband generation in the amplifier, due to self-phase and amplitude modulation, is
investigated and characterized. We also demonstrate that the amplifier does not induce any significant phase-noise on the
beating signal. Finally, the degradation of the performances of the interferometer due to the amplification process is shown
to be negligible.
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ABSTRACT: A new principle of lidar-radar is theoretically and experimentally investigated. The proposed architecture is based on the use of an rf modulation of the emitted light beam and a direct detection of the backscattered intensity. Use of a radar-processing chain allows one to obtain range and Doppler measurements with the advantages of lidar spatial resolution. We calculate the maximum range of this device, taking into account different possible improvements. In particular, we show that use of a pulsed two-frequency laser and a spatially multimode optical preamplification of the backscattered light leads to calculated ranges larger than 20 km, including the possibility of both range and Doppler measurements. The building blocks of this lidar-radar are tested experimentally: The radar processing of an rf-modulated backscattered cw laser beam is demonstrated at 532 nm, illustrating the Doppler and identification capabilities of the system. In addition, signal-to-noise ratio improvement by optical pre-amplification is demonstrated at 1.06 microm. Finally, a two-frequency passively Q-switched Nd:YAG laser is developed. This laser then permits two-frequency pulses with tunable pulse duration (from 18 to 240 ns) and beat frequency (from 0 to 2.65 GHz) to be obtained.Applied Optics 10/2002; 41(27):5702-12. · 1.69 Impact Factor
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ABSTRACT: This paper analyzes the use of Raman transitions to select a narrow velocity distribution of atoms. We determine the evolution of the atomic wave function comprised of both the internal state and the external momentum of the atom in the presence of two counterpropagating laser beams. The effects of a single pi pulse, two separated Ramsey pi/2 pulses, and a sequence of four pi/2 pulses are analyzed.Physical Review A 02/1992; 45(1):342-348. · 3.04 Impact Factor
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ABSTRACT: Amplification of ultrashort optical pulses in semiconductor laser amplifiers is shown to result in considerable spectral broadening and distortion as a result of the nonlinear phenomenon of self-phase modulation (SPM). The physical mechanism behind SPM is gain saturation, which leads to intensity-dependent changes in the refractive index in response to variations in the carrier density. The effect of the shape and the initial frequency chirp of input pulses on the shape and the spectrum of amplified pulses is discussed in detail. Particular attention is paid to the case in which the input pulsewidth is comparable to the carrier lifetime so that the saturated gain has time to recover partially before the trailing edge of the pulse arrives. The experimental results, performed by using picosecond input pulses from a 1.52-μm mode-locked semiconductor laser, are in agreement with the theory. When the amplified pulse is passed through a fiber, it is initially compressed because of the frequency chirp imposed on it by the amplifier. This feature can be used to compensate for fiber dispersion in optical communication systemsIEEE Journal of Quantum Electronics 12/1989; · 1.83 Impact Factor
arXiv:1003.0266v1 [physics.atom-ph] 1 Mar 2010
Low noise amplification of an optically carried microwave signal:
application to atom interferometry
T. L´ ev` eque, A. Gauguet∗, W. Chaibi†, and A. Landragin‡
LNE-SYRTE, Observatoire de Paris, CNRS, UPMC,
61 avenue de l’Observatoire, 75014 Paris, FRANCE
(Dated: March 1, 2010)
In this paper, we report a new scheme to amplify a microwave signal carried on a laser light at
λ = 852 nm. The amplification is done via a semiconductor tapered amplifier and this scheme is
used to drive stimulated Raman transitions in an atom interferometer. Sideband generation in the
amplifier, due to self-phase and amplitude modulation, is investigated and characterized. We also
demonstrate that the amplifier does not induce any significant phase-noise on the beating signal.
Finally, the degradation of the performances of the interferometer due to the amplification process
is shown to be negligible.
∗Present address: Department of Physics, Durham University, Rochester Building, South Road, Durham
DH1 3LE, England
†Present address: ARTEMIS - Observatoire de Nice, Boulevard de l’Observatoire, 06305 Nice, France
Optically carried microwave signals are of special interest in a large field of applica-
tions, from optical communication to opto-electronics techniques for detection such as Lidar-
Radar , and to fundamental metrology and spectroscopy. For instance, time and frequency
dissemination at long distances by optical fibres has shown extremely high performances .
Furthermore, this principle is also widely used in atomic physics. As an example, it is at the
basis of the Coherent Population Trapping (CPT) clocks , which benefits from the reduc-
tion of size compared to standard microwave clocks. This method is also used in most of the
atom interferometers to generate Raman lasers for manipulating atomic wave-packets .
In fact, it enables reduction of the propagation noise over the laser paths and systematic
errors. The signal is composed of two optical frequencies separated by a frequency in the
microwave range. This can be achieved by different means: directly from a two frequency
laser , by a single sideband electro-optic modulator , by filtering two sidebands from
an optical comb [6, 7] or from a phase-lock between two independent lasers , as used in
this work. In most of the applications, these sources are not powerful enough and would
benefit from being amplified without adding extra sidebands or extra phase noise onto the
FIG. 1: Laser setup. The frequency-reference laser L1 is locked on a spectroscopy signal. The L2
laser frequency is mixed with the frequency-reference laser and the optical beat note is compared to
a microwave reference and phase-locked through a Digital Phase and Frequency Detector (DPFD).
L1 and L2 are combined on a polarizing beam splitter cube and amplified using the same tapered
amplifier (TA). The output power is injected in a polarizing fibre through an acousto-optical
In this paper, we report a study of the influence of a semi-conductor tapered amplifier on
a two frequency laser system. This setup is dedicated to the generation of a pair of Raman
lasers at λ = 852 nm, with a fixed frequency difference close to 9.192 GHz for further use
in an atom interferometer . The experimental setup is described in section II. Sections
III and IV are dedicated to the characterization and measurement of the optical spectrum
and to the analysis of the spurious sideband generation due to non-linear effects in the gain
medium . Then we measure the extra noise added by the amplifier on the microwave
signal in section V. Finally, the impact on our atom interferometer is quantified in section VI.
II. EXPERIMENTAL SETUP
The laser setup consists of two external-cavity laser diodes using SDL 5422 chip. These
sources are based on an intracavity wavelength selection by an interference-filter  and
benefit from a narrow linewidth (14 kHz) and a wide tunability (44 GHz). The diodes
are regulated around room temperature and supplied by a current of 80 mA to provide an
optical output power of 45 mW. The frequency locks are achieved with a feedback to the
diode current and the length of the external cavity.
The first laser L1 (Fig. 1), is used as an absolute frequency reference. It is locked 300 MHz
red detuned from the atomic transition between the |6S1/2,F = 3? and
|6P3/2,F = 2? states of Caesium (D2 line) using a frequency modulation spectroscopy tech-
The phase difference between L1 and L2 is locked with the method described in  and
summarized in the following. Small amounts of light of L1 and L2 are superimposed on a
fast photoconductor (PhD12, Hamamatsu G4176, bandwidth: 15 GHz). The beat note at
ν12= 9.192 GHz is then mixed with a reference signal, given by a microwave synthesizer .
The output signal is sent to a Digital Phase and Frequency Detector (DPFD, MCH 12140)
which derives an error signal proportional to the phase difference between the two lasers.
After shaping and filtering, this output signal is used to generate the feedback allowing to
phase-lock the laser L2 on L1. In this way, the features of the microwave reference is mapped
on the optical signal with a bandwidth of 3.5 MHz.
In order to provide sufficient optical power, the output signals of L1 and L2 are injected
with the same linear polarization in a GaAs tapered semiconductor waveguide amplifier
(TA, EYP-TPA 0850-01000-3006 CMT03) pumped by a current of I = 2 A and stabilized
to room temperature. A half-wave plate and a polarizing cube allow the power ratio between
the two lasers to be adjusted at the input of the TA. In a normal operation, 11.2 mW of L1
and 16.2 mW of L2 are injected in the TA, which runs in a saturated gain regime. Then
the output beam (with a power of 770 mW) passes through an acousto-optical modulator
(AOM), driven at 80 MHz, from which the first output order is coupled to a polarizing fibre.
This scheme enables the laser light to be pulsed on the atomic system by switching the RF
signal of the AOM.
III. SELF-PHASE AND AMPLITUDE MODULATION
In this part, we study the sideband generation due to simultaneous phase and amplitude
modulation in the gain medium. As two closely detuned optical frequencies are injected
in the TA, it is crucial to determine the spectral impact of potential non-linear effects
in the semiconductor during the amplification process. Indeed, the beat note at ν12 =
9.192 GHz between L1 and L2 induces a strong modulation of the power through the TA
which affects the gain and the optical index in the semiconductor . In this situation, the
resulting sidebands could cause undesirable effects on our experiment: for instance detuned
excitations can shift the energy levels of our atomic system, called light shift , or it could
drive unwanted Raman transitions. For this reason, we conducted simulations which were
compared to experimental measurements.
The total electric field propagating through the TA can be described by,
E (t,z) = A(t,z)e−i(ω0t−kz), (1)
= |A(t,z)|eiψ(t,z)× e−i(ω0t−kz),
where A(t,z) is the wave envelope which varies at the microwave frequency, k is the
wave vector, and ω0is the optical carrier frequency. By referring P (t,z) = |A(t,z)|2to the
envelope power (ψ (t,z) is its phase), we obtain at the TA input the modulation profile,
P (t,z = 0) = P0(1 + m · cos(2πν12t + φ)), (2)
where P0is the nominal modulation power, m is the modulation factor and φ is the phase
difference between L1 and L2 (see Appendix A). In our case, we have P0≃ 27.4 mW and
m ≃ 0.983. The amplifier is then driven between a saturated state and a non-saturated
state at the frequency ν12. In order to calculate the sideband generation expected from the
amplification process, we write, as for any amplification medium, the interaction between
the carriers and the light field equations. This was widely described in reference [16, 17] for
the case of a constant cross section amplifier. Taking into account the amplifier splay (see
appendix A), these equations become,
Esat0(1 + µz),
where τCis the carrier lifetime in the semiconductor, Esat0the saturation energy for the
initial amplifier cross section, µ is the amplifier splay factor, α the linewidth enhancement
factor and vgis the wave group velocity. g refers to the linear gain and the small-signal gain
g0is defined as,
where I0and N0are the current and carrier density required for transparency, a the gain
coefficient and Γ the confinement factor (see Appendix A).
The sideband generation is due to a phase and an amplitude modulation resulting from
a non-linear gain modulation. This gain modulation depends on the field amplitude along
the amplifier and lead to a non-linear distortion of the signal at the output of the TA.
A modification of the optical index (4) induces an optical phase modulation. Indeed, an
increase of the gain modulation exacerbates the phase and the power modulation distortions
at the same time.
The evaluation of this effect requires the set of equations (3-5) to be solved. Since the
relaxation time τc and the excitation characteristic time 1/ν12 are of the same order of
magnitude, the usual adiabatic approximation can’t be used. Therefore, the system has