of 7
ARTICLE
Received 20 Mar 2013
|
Accepted 4 Jun 2013
|
Published 28 Jun 2013
Microwave synthesizer using an on-chip
Brillouin oscillator
Jiang Li
1
, Hansuek Lee
1
& Kerry J. Vahala
1
Low-phase-noise microwave oscillators are important to a wide range of subjects, including
communications, radar and metrology. Photonic-based microwave-wave sources now provide
record, close-to-carrier phase-noise performance, and compact sources using microcavities
are available commercially. Photonics-based solutions address a challenging scaling problem
in electronics, increasing attenuation with frequency. A second scaling challenge, however, is
to maintain low phase noise in reduced form factor and even integrated systems. On this
second front, there has been remarkable progress in the area of microcavity devices with
large storage time (high optical quality factor). Here we report generation of highly coherent
microwaves using a chip-based device that derives stability from high optical quality factor.
The device has a record low electronic white-phase-noise floor for a microcavity-based
oscillator and is used as the optical, voltage-controlled oscillator in the first demonstration of
a photonic-based, microwave frequency synthesizer. The synthesizer performance is com-
parable to mid-range commercial devices.
DOI: 10.1038/ncomms3097
OPEN
1
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA. Correspondence and requests for m
aterials
should be addressed to K.J.V. (email: vahala@caltech.edu).
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T
he generation of low-phase-noise radio, microwave and
optical signals relies upon resonant devices that feature large
storage times or equivalently high-Q factors
1,2
. Maintaining
large storage time, in compact or integrated circuits, becomes
increasingly challenging with decreasing size and increasing
frequency
3,4
. These two scaling laws of performance create
challenges for compact or integrated microwave and millimeter-
wave sources, wherein performance (in terms of phase noise at a
given offset frequency) steadily degrades with increasing oscillation
frequency
4
. As a result, state-of-the-art microwave oscillators are
larger, discrete devices based on dielectric resonators
5
.The
introduction of optical fibre for long-distance communication
had a positive impact on the frequency scaling side of this problem.
First, the distribution of microwaves over optical fibre has led to
remarkable progress on transfer of low-phase-noise microwaves
onto optical carrier waves
3,6
. Second, fibre delay lines provided a
way to create long storage time with no frequency-dependent
attenuation in hybrid optoelectronic oscillators
7
. These high-
performance optoelectronic oscillators represented a paradigm
shift in the generation of microwaves and were followed by optical
resonator-based versions that employed an optical microcavity to
achieve the requisite oscillator storage time
8,9
. Generally, the ability
to achieve long storage times in compact, high-Q resonators has
experienced a major leap forward during this same period
10–15
.
On another front, both fibre and optical resonators have also
been applied to create microwaves by heterodyne of coherent
laser lines
16,17
. In this case, co-oscillation occurs within the same
cavity so that a high level of common-mode noise suppression
leads to good performance microwave generation
16,18
. Sideband
injection locking in a master-slave laser configuration has also
been applied to enable broadband multiplication of a supplied
electronic microwave signal
19
. A remarkable, recent advance has
been the introduction of optical dividers in the form of frequency
combs to create record low close-to-carrier phase-noise
microwave signal sources
20
. Frequency combs have previously
been applied to stabilize optical carrier frequencies for
distribution of microwaves over optical fibre
21–23
, however, in
this new approach an optical signal serves as the root for the
ultra-high-stability microwave signal itself. Although the
technique is complex, emerging technologies like frequency
microcombs
24,25
and compact reference cavities
26–29
could one
day lead to chip-based photonic microwave sources that exceed
the performance of the best electronic sources. In a related
development, microcavity-based microcombs and parametric
oscillators are providing a new way to synthesize microwaves
directly
30–35
. Although the performance is well off from the
optical divider approach, the method is less complex.
In this work, we report the application of cascaded Brillouin
oscillation to microwave frequency synthesis at a level comparable
to mid-range commercial, all-electrical synthesizers. Although
fibre-based Brillouin lasers have been studied for microwave
generation
36–40
, the present device is the first chip-based Brillouin
microwave source. Moreover, the device has a record low-white-
phase-noise floor (

160 dBc Hz

1
) for any microcavity-based
microwave source (even including non-Brillouin-based methods).
It can also generate coherent microwave power in excess of 1 mW
without any optical or radio frequency (RF) amplification, a
significant simplification as it eliminates the need for microwave
amplification stages after the photodetector. Moreover, being chip-
based devices, the current sources offer integration opportunities
for control and additional functions.
Results
Microwave generation using chip-based Brillouin lasers
.An
ultra-high-Q planar silica-on-silicon disk resonator
15
is used for
cascaded Brillouin oscillation and microwave generation.
Microcavity-based Brillouin lasers have only recently been
demonstrated
41–43
. The present devices are silicon-chip based
and they feature very high coherence and high quantum
efficiency
15,44
. Both high-power and low-phase-noise microwave
generation from the disk resonator are achieved by cascaded
stimulated Brillouin lasing and subsequent photomixing of pairs
of Brillouin laser as illustrated in Fig. 1a. A narrow-linewidth fibre
laser (effective linewidth 1 kHz) is used as the pump laser and is
coupled to the disk resonator (see photomicrograph in Fig. 1b)
through a taper fibre technique
45
. The size of the disk resonator
(
B
6 mm) is carefully designed such that the free spectral range of
the cavity matches the Brillouin gain shift
15,44
. Once the coupled
pump power reaches the stimulated Brillouin threshold, the first-
order Stokes wave is excited in the backward direction and is
routed through a fibre circulator to an optical spectral analyser,
a monitor photodetector (bandwidth 125 MHz) and a fast
photodetector (bandwidth 50 GHz). Further increase of the
pump power leads to cascaded Brillouin lasing, for example,
9th-order Stokes has been demonstrated in our previous work
44
.
These cascaded lines feature Schawlow–Townes noise
o
0.1 Hz
2
Hz

1
(refs 15,44) and low technical frequency noise.
As a result, they are suitable for high-stability microwave
synthesis by photomixing. Fine control of this frequency is
possible by adjustment of the pump frequency using acousto-
optic modulation. The detected Brillouin laser can therefore
function as the voltage-controlled oscillator (VCO) in a
microwave synthesizer as illustrated in Fig. 1c.
Φ
detector
Loop
filter
Low-
frequency
reference oscillation
RF out
μ
Disk
PD
Vtune
AOM
Laser
μ
Disk
c
b
a
3rd
2nd
Pump
1st
1/
N
Figure 1 | Explanation of microwave synthesizer using chip-based
Brillouin laser.
(
a
) A Brillouin microwave oscillator showing pump (blue)
and second Stokes (orange) waves, and the first (green) and third
(red) Stokes waves incident on the detector for microwave generation.
(
b
) Photograph of an
B
6 mm ultra-high-Q planar silica disk resonator on a
silicon chip. Scale bar, 1 mm. (
c
) Block diagram for a closed-loop Brillouin
microwave oscillator. The open-loop Brillouin microwave oscillator (shown
in the upper dashed box) functions as a voltage-controlled photonic
oscillator. All hardware in the upper dashed box replaces an electrical
VCO in a conventional electrical synthesizer. Components in the upper
box include a pump laser, acousto-optic modulator (AOM), circulator,
high-speed detector and ultra-high-Q disk resonator. PD, photodetector.
ARTICLE
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The Brillouin shift in the silica disk cavity is around 10.8 GHz
(ref. 44), so that microwave generation from around 10.8 to
86 GHz is feasible in the present device by photomixing pairs of the
cascaded Brillouin lines. We have focused on the generation of
K-band (
B
21.5–21.8 GHz) microwave signals by photomixing the
first and third Stokes lines. Operationally, these lines feature high
coherence with very low Schawlow–Townes noise and technical
noise; they also provide high-power levels and propagate in the
same direction. It is important to note that besides the common-
mode noise suppression that occurs upon photomixing because the
Brillouin laser lines share a common laser cavity, combinations of
laser lines that share the same optical path (that is, odd–odd
and even–even Stokes pairs) also enable cancellation of any
optical-path length jitter effects.
Phase noise of optical VCO
. To measure the phase noise of the
K-band microwave signal, a phase-noise analyser (Rohde-
Schwartz FSUP Signal Source Analyzer) is used. Figure 2 shows
the single-sideband phase noise (
L
(
f
)) of a 21.7-GHz signal gen-
erated using the first and third Stokes laser lines for a series of
output optical power levels of the 3rd Stokes (

16.5,

10.2,

8.2,

4.5,

1.4 and 1.2 dBm from upper to lower curves).
Significantly, the phase noise of this signal has 1/
f
2
dependence
over five decades (200 Hz to 20 MHz), which corresponds to
white frequency noise. Moreover, this 1/
f
2
noise has an inverse
dependence on the third Stokes laser line power as shown in the
inset to Fig. 2 (phase noise is measured at 100 kHz frequency
offset, and power is varied by changing the pump power). The
inset of Fig. 2 also shows a calculated phase noise based on the
Schawlow–Townes frequency noise of the third Stokes line power.
For a Brillouin laser, the two-sided Schawlow–Townes frequency
noise
44
can be rewritten as:
S
ST
n
ð
f
Þ¼
kT
8
p
2
tt
ex
P
c
2
nV
a
ð
1
Þ
where
P
is output power of the Brillouin laser,
t
(
t
ex
) is the total
(external) cavity photon life time,
c
is the speed of light,
n
is the
refractive index of silica and
V
a
is the sound velocity in silica. This
equation gives the limit of the Schawlow–Townes noise formula
44
when the number of thermal quanta of the Brillouin phonons is
far greater than unity, which is the case for measurements taken
at room temperature (thermal quanta
¼
569). It is important to
note that the Schawlow–Townes noise is independent of both the
pump frequency and the Brillouin frequency. As a result, at an
equivalent power level, the phase-noise levels measured in Fig. 2
would be expected if the pump frequency was higher resulting
in a higher-frequency Brillouin shift. For the calculation of
the Shawlow–Townes noise, measured cavity
t
of
t
¼
28 ns and
t
ex
¼
34 ns are used (that is, no free parameters are used), and
L
(
f
)
¼
10log
ð
S
I
f
ð
f
Þ
=
2
Þ
. Here
S
I
f
ð
f
Þ
is the one-sided power spectral
density of phase noise and is related to the two-sided power
spectral density of frequency noise
S
II
n
ð
f
Þ
as:
S
I
f
ð
f
Þ¼
2
S
II
n
ð
f
Þ

f
2

.
Because the output optical power of the first Stokes line varies
over 2.5–5.0 dBm from the upper to lower measurement curves
(that is, power levels much greater than for the third Stokes
wave), the Schawlow–Townes noise of the first Stokes line is not
expected to be a significant component of noise in the photomix-
generated carrier wave. Indeed, the good agreement between the
measurement values and calculation in the inset of Fig. 2 shows
that the phase noise of the Brillouin microwave signal is mainly
contributed by the Schawlow–Townes noise of the third Stokes
line. As a general comment relevant to the discussion of phase
noise in the introduction, the results in Fig. 2 confirm that the
very high-Q factor of the Brillouin laser (high storage time)
leads to the low-microwave phase noise upon photomixing of the
Stokes laser lines. Ultimately, the cascade process sets the
limit of phase noise in these devices (see Supplementary
Note 1). Finally, at low-offset frequencies in Fig. 2, the phase
noise exhibits a non-Schawlow–Townes component. Although
Brillouin lasers suppress technical phase noise from the
pump
44,41
, it is possible that this noise is contributed by the
pump. It is also possible that this noise is environmental related.
The closed-loop control described in the next section suppresses
this noise.
Phase-locked operation of oscillator
. The Brillouin microwave
oscillator can be operated in two distinct ways: open loop (no
feedback control) and closed loop (phase-lock-loop control). In
open-loop operation, the first and third Stokes lines from the
resonator are directly photomixed on a fast photodetector with no
other control. This is the method used for the spectra in Fig. 2.
Alternatively, the centre frequency of the free-running Brillouin
microwave oscillator can be finely varied in the range of 100 s kHz
to a few MHz, by adjusting the pump laser frequency either
through a piezo control of the pump laser cavity or with an
external acousto-optic modulator. Thus, the Brillouin oscillator
constitutes a VCO, and the device can be configured into a phase-
lock-loop control design (that is, closed-loop operation). Phase-
lock-loop control of optically generated microwave tones has been
previously accomplished by reference to a high-performance
microwave reference
46–48
. In these earlier works, the objective was
to use the the high-stability microwave electrical reference to
stabilize the optical generated tone. In the present case, the original
microwave tone from the open-loop stimulated Brillouin scattering
(SBS) oscillator is already of sufficient stability at shorter times
(higher offset frequencies), so that the phase-lock-loop (PLL) is
used only to obtain long-term stability. Moreover, the loop
control does not use another microwave oscillator to achieve
stability, but rather a low-frequency quartz oscillator via electrical
frequency division. Indeed, the divider used here not only allows
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
−160
−140
−120
−100
−80
−60
−40
−20
0
20
Offset frequency (Hz)
L
(
f
) (dBc Hz
–1
)
−20
−15
−10
−5
0
5
−105
−100
−95
−90
−85
−80
3rd Stokes power (dBm)
L
(
f
) at 100 kHz (dBc Hz
–1
)
Measured
Calculated
Figure 2 | Power dependence of microwave phase noise.
The microwave
signal is produced by photomixing the first and third Stokes lines on a high-
speed detector. The output optical power of the third Stokes line is
increasing from top to bottom curves (

16.5,

10.2,

8.2,

4.5,

1.4
and 1.2 dBm, respectively). The phase noise shows 1/
f
2
dependences for
five decades from 200 Hz to 20 MHz. The dashed lines are 1/
f
2
fits to the
measured phase-noise spectra. Inset: plot showing inverse power
dependence of the phase noise of the Brillouin microwave signal
(100 kHz offset frequency) on the optical power of the third Stokes line.
Circles, measured values; dashed line, calculated values based upon
the Schawlow–Townes formula.
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stabilization using a low-frequency reference oscillator but also
converts the SBS microwave signal into nearly any microwave
signal frequency
o
21.8 GHz. As a result, this device constitutes a
microwave synthesizer with performance comparable to good
commercial electrical synthesizers. The essential elements of the
SBS microwave synthesizer are shown in the experimental
schematic of Fig. 3a. The synthesizer is analysed by a phase-
noise analyser and an Allan deviation tester (ADEV), and a mutual
time base is shared between the frequency reference, phase-noise
analyser and the ADEV.
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
−160
−140
−120
−100
−80
−60
−40
−20
0
20
Offset frequency (Hz)
L
(
f
) (dBc Hz
–1
)
Open loop 21.7 GHz
Closed loop 21.7 GHz
Reference scaled to 21.7 GHz
−100
−50
0
50
100
−100
−90
−80
−70
−60
−50
−40
−30
−20
−10
0
kHz + 21.7 GHz
RF power (dBm)
10
−3
10
−2
10
−1
10
0
10
1
10
2
10
−16
10
−14
10
−12
10
−10
10
−8
10
−6
ADEV
Open loop
Closed loop
Counter limit
1,549.4
1,549.6
1,549.8
1,550
1,550.2
−80
−70
−60
−50
−40
−30
−20
−10
0
Wavelength (nm)
Optical spectrum (dBm)
μ
wave output
AOM
−100
−50
0
50
100
−80
−70
−60
−50
−40
−30
−20
−10
0
10
20
Hz + 21.7 GHz
RF power (dBm)
RBW 1 Hz
RBW 100 Hz
b
a
Pump
Pump
2nd
PD
1
SBS disk
3rd
1st
Ref In
10 MHz clk
Low frequency
reference oscillator
Servo
Ref In
Divider
Amp
Splitter
f
/
N
PD
2
OSA
λ
PNA
ADEV
f
/
N
Pump
laser
ef
d
c
1st
3rd

(s)
Figure 3 | Performance of the synthesizer for both open and closed-loop operation.
(
a
) Experimental schematic for the closed-loop Brillouin
microwave oscillator. AOM, acouto-optic modulator;
f
/
N
, frequency divider; PD, photodetector; OSA, optical spectrum analyzer. A mutual time base is
shared between the phase-noise analyser (PNA), low-frequency reference oscillator and the ADEV. (
b
) Open-loop (red curve) and closed-loop (blue curve)
single-sideband phase noise of the Brillouin oscillator at 21.7 GHz. The green dashed line is the phase noise of the reference oscillator scaled up to 2
1.7 GHz.
(
c
) Optical spectrum of the 1st-Stokes and 3rd-Stokes laser emission lines. The pump wave is also visible, but is weak on account of propagation opposite
to
the direction of the 1st and 3rd Stokes lines. Also visible is the 5th Stokes line in the spectrum as well as the first anti-Stokes wave. (
d
) RF spectrum of an
open-loop Brillouin oscillator at 21.7 GHz. The span is 200 kHz and the resolution bandwidth (RBW) is 100 Hz. (
e
) RF spectrum of a closed-loop Brillouin
oscillator at 21.7 GHz. The span is reduced to 200 Hz and the RBW is 1 Hz. (
f
) Allan deviation (ADEV) of the Brillouin microwave oscillator. Circle markers
represent the open-loop operation, square markers represent the residual Allan deviation for the close-loop operation and the diamond markers give
the
counter limit.
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The optical spectrum of the generated Stokes lines in the back-
propagating direction is shown in Fig. 3c and includes the first and
third Stokes lines that are used to generate the microwave tone of
the present work. The incident optical power levels of the first and
third Stokes lines on the high-speed photodiode are 3.7 and
1.7 mW, respectively (the coupled optical pump power to the
microresonator was 31 mW). For comparison purposes, the open-
and closed-loop phase-noise spectra are presented in Fig. 3b
together with the scaled-up phase noise of the low-frequency
reference oscillator. Phase-noise levels (open loop) of

90 dBc Hz

1
at 10 kHz offset and

110 dBc Hz

1
at
100 kHz are obtained for the 21.7 GHz carrier. The measured
white-phase-noise floor of the oscillator (

156 dBc Hz

1
4
100MHz) is a record for a microcavity-based microwave source
by several orders of magnitude
9,30,35
. The calculated shot noise floor
is

158 dBc Hz

1
for the 21.7 GHz carrier, with 2.2 mA average
photocurrent and a generated RF power of

15.5 dBm, and is in
good agreement with the measured result. Using another device,
both a higher RF power of 2.9 dBm (1.9 mW, no optical or RF
amplification) and yet lower white-phase-noise floor of

160 dBc/
Hz were obtained. These results are within 19 dB of the very lowest
white-phase-noise floors recently demonstrated using high-
performance frequency combs and high-power photodiodes
49
.
Further details on this measurement as well as the power
calculations themselves are provided in the Supplementary Note 2.
For closed-loop operation, an integer frequency divider (1/32) is
used to divide the free-running microwave carrier to 678 MHz for
phase comparision with a low-frequency reference oscillator at
678MHz.ThebluecurveinFig.3bshowsthephasenoiseofthe
closed-loop Brillouin oscillator. The close-to-carrier phase noise is
now greatly reduced. The phase noise of the frequency reference is
also shown in Fig. 3b as the green dashed line. A factor of 20

log32
¼
30 dB has been added to this line to account for phase-noise
scaling with frequency, and thereby enable comparison with the
phase noise of the closed-loop oscillator. As expected, for low-offset
frequencies, the closed-loop oscillator tracks the scaled-up reference
noise. The corresponding RF spectra for the open-loop (span
200 kHz and resolution bandwidth 100 Hz) and closed-loop (span
200 Hz and resolution bandwidth 1 Hz) oscillators are given in
Fig. 3d,e, respectively. To characterize the long-term frequency
stability of the Brillouin oscillator, ADEV measurements were
carried out for open-loop and closed-loop operation using gate
times from 1 to 400 s. The measurement results (normalized to the
Brillouin oscillator frequency) are shown in Fig. 3f. For these
measurements, a sub-harmonic (1/256) at 84.6 MHz of the Brillouin
oscillator is measured, as the maximum input frequency of the
ADEV is 400MHz. It can be seen that the long-term drift of the
open-loop VCO is greatly suppressed. More than 80dB
reduction of the ADEV (with respect to the 10-MHz mutual
time base) is obtained at 400s gate time. Also, a residual Allan
Deviation of 5

10

13
is achieved at 1 s. Overall, the closed-loop
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
−180
−160
−140
−120
−100
−80
−60
−40
−20
Offset frequency (Hz)
L
(
f
) (dBc Hz
–1
)
Closed-loop 21.7 GHz
Div by 32 = 678 MHz
Div by 256 = 85 MHz
Div by 512 = 42 MHz
Div by 1024 = 21 MHz
Div by 2048 = 11 MHz
−1
0
1
2
3
−0.2
−0.1
0
0.1
0.2
2.7 GHz
−2
−1
0
1
2
−0.2
−0.1
0
0.1
0.2
Voltage (a.u.)
1.35 GHz
−4
−2
0
2
4
−0.5
0
0.5
t
(ns)
678 MHz
b
a
Figure 4 | RF frequency synthesis by the K-band low-noise Brillouin microwave oscillator.
(
a
) Phase noise of a series of RF frequencies generated by
frequency division (Div) in the synthesizer. (
b
) Typical sine-wave, time-domain traces of the synthesizer output at 2.7 GHz, 1.35 GHz and 678 MHz (from
top to bottom). All frequencies lower than the base oscillator frequency of 21.7 GHz are possible, and the traces selected here were limited by the
bandwidth of oscilloscope (4 GHz).
10
−3
10
−2
10
−1
10
0
10
1
10
2
10
−16
10
−15
10
−14
10
−13
10
−12
10
−11
10
−10
10
−9

(s)
Out-of-loop
In-loop
Reference
ADEV
Figure 5 | Allan deviation measurements.
The Brillouin microwave
oscillator (21.7 GHz) is phase locked to an ultra-low-noise OCXO at
400 MHz as reference. The blue square markers are the residual Allan
deviation (ADEV) of the Brillouin microwave oscillator with regard to the
reference OCXO. The green diamond markers are the out-of-loop Allan
deviation, measured against a second low-noise OCXO at 10 MHz. The red
circle markers are the Allan deviation of the reference OCXO at 400 MHz
measured against the second OCXO at 10 MHz.
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