IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 10, OCTOBER 2005
2185
Modulation-Dependent Limits to Intensity-Noise
Suppression in Microwave-Photonic Links
Thomas E. Darcie, Fellow, IEEE, and Amol Moye
Abstract—Cancellation of common-mode intensity noise in
microwave-photonic links is found to be limited by the applied
intensity modulation. A model is developed and verified experimentally for the dependence of the maximum noise suppression
on modulation index, for two-tone and multichannel modulation.
For example, for a root-mean-square modulation index of 0.3,
suppression is limited to 5 dB.
Index Terms—Analog links, balanced detector, intensity-noise
suppression, microwave-photonic links (MWPLs), spur-free
dynamic range.
I. INTRODUCTION
M
ICROWAVE-PHOTONIC links (MWPLs) continue to
find widespread application in transmitting analog radio
frequency (RF) or microwave information over optical fiber.
Advantages such as low loss, light weight, high signal fidelity,
and immunity to electromagnetic interference have compelled
a wide assortment of photonic solutions. While the utility of
MWPLs in cable television networks, remote antennas, radar
links, and many other applications is indisputable, increasing
link performance continues to be a priority [1], [2].
For links operating with high signal-to-noise ratios, where received optical power levels are high (e.g., tens of milliwatts),
relative-intensity noise (RIN) is generally a primary limitation.
RIN can be introduced by the source laser or drive electronics,
by beating of the signal with spontaneous emission in an optical
amplifier, or by multiple reflections in the transmission fiber. An
important technique for mitigating the impairment from RIN
involves the use of a dual-output Mach–Zehnder (MZ) intensity modulator, two transmission fibers, and a balanced detection scheme [e.g., [2]–[4]]. The balanced detector provides the
sum of complementary MZ output signals, while RIN present at
the modulator input (common mode) is cancelled.
Numerous examples of RIN suppression in MWPLs have
been reported, but modulation conditions are not generally specified. High common-mode rejection ratios ( 27 dB) have been
reported [5], where the modulation depth is presumably small.
While common-mode suppression has become a standard figure
of merit for balanced detectors and overall link balance, high
suppression has been attained only for links with small or no
modulation. RIN suppression in links operating under modulated conditions has been limited [6] to 10 dB.
Manuscript received April 25, 2005; revised June 28, 2005. This work was
supported by National Sciences and Engineering Research Council and by the
Canadian Institute for Photonic Innovation.
The authors are with University of Victoria, Victoria, BC V8W 3P6A, Canada
(e-mail: tdarcie@uvic.ca; amoye@uvic.ca).
Digital Object Identifier 10.1109/LPT.2005.856416
Fig. 1. Setup for analysis and experiment using MZ modulator showing
tunable filter (TF), variable optical attenuator (VOA), polarization controller
(PC), and balanced photodetector (D1, D2).
In this letter, we develop a simple model describing the dependence of RIN suppression on modulation index, for standard
two-tone and multichannel modulation conditions. Predictions
show excellent agreement with experimental measurements
conducted on a balanced link operating at 2 GHz (two-tone) and
700 MHz (multichannel). It is shown that the maximum RIN
suppression that can be achieved, even with perfect zero-modulation common-mode suppression, decreases with increasing
modulation index. For example, with a root-mean-square (rms)
modulation index of 0.3 (typical of a loaded cable-television
transmitter), the maximum suppression is limited to 5 dB.
II. ANALYSIS
Analysis begins with well-known expressions [7] for source
RIN and signal-spontaneous beat noise from an optical amplifier, applied to the configuration shown in Fig. 1. The MZ is
biased at quadrature such that for an excess modulator loss
and input optical power
, the output power is
(1)
Modulation voltage
is normalized to
(6.2 V for this particular device), and the denotes the two complementary outputs. The rms noise current generated in each detector by RIN
is
(2)
where is the noise bandwidth, the loss between the modthe detector responsivity. Signalulator and detector, and
spontaneous beat noise contributes
1041-1135/$20.00 © 2005 IEEE
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(3)
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 10, OCTOBER 2005
where
is the power at the input the amplifier with gain and
is the photon energy. RMS noise currents
noise figure , and
.
are proportional to the instantaneous applied modulation
At the output of the balanced detectors, subtraction of the
yields the total mean-square noise
noise currents
current
(4)
is the average power received by one detector and the
term in square brackets can be considered an effective intensity
. is related to the conventional common-mode supnoise
pression factor and accounts for imperfect amplitude and phase
. Typically, would be between 0.1
matching. Ideally,
and 0.01, corresponding to between 20 and 40 dB of RIN suppression. The term involving can be averaged over many RF
cycles such that
Fig. 2. Multichannel noise measurements showing the dependence of the
measured intensity noise on normalized modulation index .
(5)
We consider
modulating carriers of the form
(6)
is large (e.g., 10) and
is the single-channel moduIf
, then
is a
lation index
. It
zero-mean Gaussian process with variance
is
can be shown easily that the expected value of
, such that
(7)
Equation (7) describes the increase in noise with increasing
modulation, for any modulation that can be described by the
, for small ,
variance . For a typical two-tone test
(5) and (7) both yield
(8)
III. EXPERIMENT
As shown in Fig. 1, a tunable laser was coupled to an erbiumdoped fiber amplifier (EDFA) (17-dBm output power) and a
tunable filter to minimize spontaneous-spontaneous beat noise,
and a polarization controller. A maximum power of 16 dBm
was available at the input to the modulator at a wavelength of
1.55 m. The balanced detector (Discovery Semiconductors,
Inc.) consisted of two high-speed InGaAs detectors (17-GHz
bandwidth) with the common node matched to 50 and specified common-mode suppression (no modulation) of 30 dB. To
, noise was measured for a variety of received
determine
optical power levels with no modulation. Results were consistent with a source RIN of 152.8 dB/Hz and an EDFA noise
was meafigure of 5 dB (with 6.0-dBm input power).
sured to be 144 dB/Hz, dominated by signal-spontaneous beat
noise. The minimum value for was determined by comparing
Fig. 3. Two-tone noise measurements showing the dependence of the
measured intensity noise on modulation index m.
measured noise for single and balanced detectors to be approximately 0.1. This depended strongly on the exact bias point of
the MZ. Manual control was adequate to compensate for slow
drift of the bias point and maintain below levels for which the
modulation dependent terms in (7) and (8) were dominant (for
or greater than about 5%).
Modulation was applied to the MZ from two synthesized
sweep generators (2.0 and 2.1 GHz) or a Matrix cable-television video-carrier generator (34 channels spaced by 6 MHz
between 541.25 and 751.25 MHz). Modulation depths for each
channel or for the composite multichannel load were measured
on the optical sampling oscilloscope. From the multichannel
Gaussian-shaped histogram, was determined as 0.707 times
half width to the average received optical
the ratio of the
power. Alternatively, could be calculated from (6) using the
single-channel measurements of . Noise measurements were
then taken for a wide range of modulation depths.
IV. RESULTS AND DISCUSSION
Results from multichannel and two-tone tests are shown in
Figs. 2 and 3, respectively, and compared with predictions. RF
using a load resistance of
noise power is calculated from
12.5 , accounting for 50 matching in the balanced detector
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DARCIE AND MOYE: MODULATION-DEPENDENT LIMITS TO INTENSITY-NOISE SUPPRESSION IN MWPLs
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V. SUMMARY
We have demonstrated and quantified how the degree of
intensity-noise suppression in an MWPL is reduced as the
modulation increases. For small or no modulation, suppression
can be high (e.g., 30 dB) if the link is well balanced. But
for increasing modulation, the suppression is decreased with
quadratic dependence on the modulation index ( or ). A
simple model is presented that agrees well with experimental
measurements over the range of modulation conditions likely to
be encountered in most applications, enabling straightforward
determination of the residual intensity noise that would be
expected under modulated conditions.
ACKNOWLEDGMENT
Fig. 4. Variation of the maximum achievable RIN suppression with
modulation depth and zero-modulation common-mode suppression factor S .
and the spectrum analyzer (effective detector impedance is 25 ,
and half the power is measured by the analyzer). Analyzer and
receiver noise are subtracted from the measured noise power.
The figures show results under balanced and unbalanced (one
is domdetector only) conditions. For low modulation,
inated by imperfect cancellation through . As modulation is
increased, the quadratic dependence on becomes dominant.
Under typical operating conditions, the exact value of becomes irrelevant. Both types of modulation produce good agreement with the simple theory. Small differences between theory
and experiment (particularly for two-tone) can be attributed to
experimental error in determining modulation index and calibration of the detectors and analyzer.
Based on these results, it is seen that balanced detection is effective for reducing intensity noise primarily under conditions
of weak modulation. Fig. 4 summarizes the predicted effective
, as a function of
suppression, defined as
. For close to zero, suppression can be as large as the detectors can be balanced in amplitude and phase over the frequency
range of interest. Increasing reduces the suppression such that
, typical for fully loaded cable television systems,
for
one cannot expect suppression better that 5 dB.
The authors would like to thank friends at AT&T Laboratories for the use of the MZ modulator, and Discovery Semiconductors, Inc. for special consideration regarding the balanced
photodetector.
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