Download 10-GHz Bandwidth RF Spectral Analyzer With MHz Resolution

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 11, NOVEMBER 2005
2385
10-GHz Bandwidth RF Spectral Analyzer
With MHz Resolution Based on Spectral Hole
Burning in Tm : YAG
G. Gorju, V. Crozatier, I. Lorgeré, J.-L. Le Gouët, and F. Bretenaker
Abstract—We demonstrate the first 10-GHz instantaneous
bandwidth radio-frequency spectrum analyzer based on spectral
hole burning in Tm : YAG. It exhibits 10 000 frequency channels
and a resolution better than 1 MHz. Thanks to the fast and linear
chirping capabilities of the laser used, it has a potential 100%
probability of interception and a response time in the millisecond
range. Its linear dynamic range of 16 dB is presently essentially
limited by the modest amount of optical power available and can
be further improved.
Index Terms—Radio-frequency (RF) spectrum analysis, spectral
hole burning (SHB).
I. INTRODUCTION
T
HE proliferation of spectrally dense signals spanning
gigahertz of bandwidth in battle field environment or
(sub)millimeter astronomy shows the growing need in developing processing techniques able to analyze such signals.
For instance, radio-frequency (RF) analyzers must have the
capability to Fourier analyze multigigahertz (multi-GHz) signals (10 GHz for instance), with submegahertz (sub-MHz)
resolution, very short response times (millisecond timescale),
and a 100% probability of interception. Because they present
broad bandwidth capabilities more naturally than electronics,
optical solutions are under active development for RF signals
processing. In this context, one of the most mature optical solutions is the acoustooptic spectrometer [1] which uses acoustic
waves to deflect an optical beam at an angle proportional to
the RF signal frequency. However, acoustic wave generation
and attenuation currently limit the bandwidth of these devices
to 2 GHz. Alternatively, the emerging spectral hole burning
(SHB) technology has the potential to cope with this increasing
bandwidth demand. Indeed, the absorption bands of rare earth
doped crystals are as broad as 200 GHz, together with a spectral
resolution in the range of 100 kHz at low temperature (about
4 K) [2]. These potentialities of SHB for spectrum analysis
have been first demonstrated by Lavielle et al. [3]. In this setup,
the different RF spectral components are also spatially separated. Indeed, one optically engraves a set of angle multiplexed
monochromatic gratings that diffract the RF signal carried by
another beam in different directions. A 3.3-GHz bandwidth
Manuscript received April 12, 2005; revised July 18, 2005. This work was
supported by an ONR/NICOP program.
The authors are with the Laboratoire Aimé Cotton, Centre National de la
Recherche Scientifique, F-91405 Orsay Cedex, France (e-mail: [email protected]).
Digital Object Identifier 10.1109/LPT.2005.857593
has been demonstrated with a capacity of 100 channels. This
spectrometer has the advantages to exhibit a large dynamic
range (35 dB), to have a Fourier transform limited response
time, and a 100% probability of interception. However, it suffers from three main drawbacks: 1) its spatial demultiplexing
principle makes the channel capacity difficult to increase; 2) it
is unable to cope with two-dimensional RF imaging spectrometer applications; 3) its optical scheme is rather complicated to
implement. In addition, the recently demonstrated photon echo
chirp transform Fourier processor [4] is still limited to relatively
low bandwidths and does not seem to be able to easily reach a
100% probability of interception. A more direct and completely
different approach has recently been proposed [5]. It consists
of exposing the SHB crystal (as a photosensitive material) to
the optically carried RF signal under investigation and read
out the resultant spectral photograph with a monochromatic
laser scanned over the altered absorption profile. Thus, by just
measuring the SHB crystal transmission previously illuminated
by an optically carried RF signal, we can achieve a spectral analyzer with buffer memory. Even if this “photographic scheme”
does not directly lead to a dark background measurement,
its simplicity and potentially very large number of spectral
channels make it attractive. The aim of this letter is to report on
: YAG
the first demonstration of a 10-GHz bandwidth Tm
photographic spectral analyzer with 10 000 channels.
II. EXPERIMENTAL PROCESS AND SETUP
The spectrum analysis experiment based on this photographic
scheme is presented in Fig. 1. The key element is the SHB
material which is a 2.5-mm-long 0.5-at.%-doped Tm : YAG
H absorpcrystal. At low temperature (4.5 K), the H
tion transition of Tm ions at 793 nm (peak absorption 85%)
exhibits the required characteristics: a 25-GHz inhomogeneous
linewidth and a homogeneous linewidth of about 150 kHz. We
use this crystal as a spectral photographic plate in which the RF
spectrum is engraved. Since the lifetime of this spectral photograph is about 10 ms [6], it must be read by scanning the frequency of a laser source at 793 nm through the whole bandwidth in less than a few milliseconds. This is done using a frequency agile external cavity diode laser (ECDL). Its frequency
is chirped by applying a voltage ramp on an intracavity electrooptic crystal (EOC) [7]. This provides us with the needed fast
and large chirps (10 GHz in a few milliseonds) whose linearity
can be well controlled [8]. After spatial filtering in a monomode
fiber, the laser beam is focused on acoustooptic modulator AO1
1041-1135/$20.00 © 2005 IEEE
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 11, NOVEMBER 2005
Fig. 1. Experimental setup. AO1 and AO2 operate at 80 and 75 MHz,
respectively, leading to a heterodyne signal at 5 MHz on PD. The crystal is
cooled at 4.5 K. The whole experiment is synchronized by a pulse generator
labeled “Synchro.”
which serves as an optical switch. Its first-order beam is imm), as shown in
aged onto the crystal (beam waist
Fig. 1. The transmitted intensity is detected on a PIN photodiode
(PD) and amplified by a 10-dB gain amplifier. The zeroth-order
beam at the output of AO1 is focused on another modulator
AO2 whose first-order beam is mixed with the beam transmitted
by the crystal. As illustrated later, this heterodyne detection at
5 MHz gives access to both the absorption and dispersion components of the atomic response that are, respectively, in phase
and in quadrature with the probe field. In a real spectral analyzer, the spectrum would be engraved in the crystal using another fixed frequency laser modulated by the RF signal. For the
present demonstration, we simply mimic the engraving of the
optically carried RF spectrum using the same ECDL as in the
reading step. By applying different voltages on the EOC of the
ECDL, we can create different tones equivalent to an optical carried RF signal. Their amplitudes and durations are controlled by
the arbitrary waveform generator.
III. EXPERIMENTAL RESULTS
An example of a 10-GHz bandwidth spectral analysis is reproduced in Fig. 2. The engraved spectrum consists in a series
of 16 spikes each lasting 150 s (pulse energy 450 nJ) with the
laser tuned to 16 different frequencies. The reading is performed
1.6 ms later, with a 10-GHz bandwidth scanned in 2 ms. During
the reading phase, the optical power incident on the crystal is reduced down to 750 W, avoiding erasing the engraved spectrum.
The resulting signal is demodulated and its amplitude is normalized to the unsaturated transmission of the crystal. Among the
16 engraved peaks, 15 are equally spaced by 620 MHz all over
the 10-GHz bandwidth [see Fig. 2(a)]. The slow increase of their
amplitude versus time in Fig. 2(a) is essentially due to the lifetimes of the populations of the excited and the metastable states
of Tm [6]. The 16th engraved peak is located 5 MHz apart
from one of the 15 equally separated peaks. This doublet is perfectly resolved by our analyzer, as can be seen in Fig. 2(b). The
linewidth of each peak corresponds to 2 MHz in this experiment.
Of course, this resolution depends on the chirp rate , as can be
Fig. 2. (a) Experimental evolution of the demodulated signal versus time
during the readout chirp of 10 GHz in 2 ms. The crystal has been engraved with
15 equally spaced tones spanning the 10-GHz bandwidth. (b) The two doublet
components of the doublet are separated by 5 MHz.
Fig. 3. (a) Experimental (open circles) and theoretical (thick line) evolutions
of the linewidth (full-width at half-maximum) of a single frequency readout
. The inset shows the
signal versus chirp rate . The thin line is just
two quadratures of the demodulated signal for two engraved lines separated
by 1.2 MHz. (b) Experimental (circles and squares) and theoretical (full
line) evolutions of the detected amplitue of a peak versus the engraving
optical energy. The engraving times are 400 (open circles) or 600 s (filled
squares). The inset shows the distribution of the signal value in the background
absorption region (thin line) together with a Gaussian fit (gray line) leading to
a standard deviation of 50 V.
seen from the experimental results reproduced in Fig. 3(a). The
open circles in Fig. 3(a) represent the measured evolution of the
width of a single peak engraved in 200 s (pulse energy 600 nJ)
versus the reading chirp rate. Indeed, it is well known [9] that
of any spectral feature of interest
as soon as the width
is not much larger than
, the readout gets distorted and, in
particular, broadened. If we consider a Lorentzian lineshape of
(in hertz) probed by a light beam of varying dewidth
(exact resonance occurs at
), the resulting
tuning
time evolution
of the transmitted field amplitude normalized to the undistorded one is given by [10]
(1)
stands for the complementary error function. This
where
equation leads to the thick line of Fig. 3(a) obtained with
kHz, which is in very good agreement with the
is larger than the absolute
measurements. This value of
GORJU et al.: 10-GHz BANDWIDTH RF SPECTRAL ANALYZER WITH MHz RESOLUTION BASED ON SHB
limit given by twice the homogeneous linewidth of the ions at
kHz
kHz) because of the contribution of
4.5 K (
becomes
the laser frequency jitter. This limit linewidth
relevant when it is larger than
whose value is reproduced
as a thin line in Fig. 3(a). This shows that a sub-MHz resolution
can be reached if the 10-GHz bandwidth is probed in 10 ms,
leading to a number of frequency channels equal to 10 000.
The inset in Fig. 3(a) is an illustration of this resolution. The
upper trace corresponds to the probing of two engraved tones
MHz/ms.
separated by 1.2 MHz with a chirp value
The two peaks are well resolved. The other quadrature of the
demodulated signal, corresponding to the dispersive part of the
atomic response, is reproduced as the lower trace of this inset.
An other important property of a spectrum analyzer is its dynamic range. It can be extracted from the experimental results
reproduced in Fig. 3(b). This figure represents the evolution
of the amplitude of a single peak readout versus its engraving
optical energy. The experimental points have been obtained for
two values of the engraving pulse duration (400 and 600 s)
and by varying the engraving power. The engraved peak is
read after a time delay equal to 1.3 ms. These measurements
exhibit a typical saturation behavior, as confirmed by their
good agreement with the theoretical curve represented as a full
line in Fig. 3(b) which leads to a saturation energy of 75 nJ.
We estimate the linear part of this response to correspond to a
signal voltage between 0 and 2 mV. To compare this signal with
the typical signal noise, we record 10 333 successive samples
separated by 8 ns while the laser is chirped in a region where no
peak has been engraved. The inset in Fig. 3(b) then reproduces
the distribution of the signal value together with a Gaussian
fit. The average value of the signal (5.15 mV) corresponds to
the transmitted signal when the absorption is not saturated.
The standard deviation of this Gaussian noise is found to be
equal to 50 V. The main components of this noise are 1) the
thermal noise of the detector and amplifier, due to the low value
of the detected optical power (a 750- W power is incident on
the crystal during the reading stage) and 2) the low-frequency
components of the laser intensity noise. This leads to a value of
16 dB for the linear dynamic range in terms of optical field.
IV. CONCLUSION AND DISCUSSION
We have reported the first demonstration of a 10-GHz bandwidth Tm : YAG spectrum analyzer with 10 000 channel capacity and a resolution in the MHz range. This spectrum analyzer has a potential 100% probability of interception and a fast
response time. Future developments include, on the one hand, a
servo-control of the reading laser chirp linearity along the lines
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drawn in [8]. This will allow us to control the frequency to the
same level of precision as the analyzer resolution, i.e., below
1 MHz. On the other hand, to improve the signal-to-noise ratio
of the analyzer, we are now designing a noncollinear experiment
in which the RF signals are engraved as a spatial diffraction
grating using two beams. The reading chirped beam is then diffracted on this grating. The first order of diffraction leads to the
useful signal, which is then obtained on a dark background. This
should lead to a much lower noise level and, together with the
heterodyne detection demonstrated here which permits to eliminate the dispersive part of the atomic response [see the inset
of Fig. 3(a)], to a dramatic increase of the linear dynamic range
of the spectrum analyzer. Finally, the practical demonstration of
a probability of interception of 100% just requires the use of a
second frequency-fixed laser and a fast modulator.
ACKNOWLEDGMENT
The authors gratefully acknowledge discussions with
F. Schlottau, K. Wagner, D. Dolfi, and S. Tonda-Goldstein.
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