Low-noise Brillouin laser on a chip at 1064 nm
Jiang Li, Hansuek Lee, and Kerry J. Vahala*
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA
*Corresponding author: vahala@caltech.edu
Received October 25, 2013; accepted November 15, 2013;
posted December 3, 2013 (Doc. ID 200164); published January 8, 2014
We demonstrate narrow-linewidth-stimulated Brillouin lasers at 1064 nm from ultra-high-
Q
silica wedge disk
resonators on silicon. Fundamental Schawlow
–
Townes frequency noise of the laser is on the order of
0
.
1
Hz
2
∕
Hz
. The technical noise spectrum of the on-chip Brillouin laser is close to the thermodynamic noise limit
of the resonator (thermorefractive noise) and is comparable to that of ultra-narrow-linewidth Nd:YAG lasers. The
relative intensity noise of the Brillouin laser also is reduced by using an intensity-stabilized pump laser. Finally,
low-noise microwave synthesis up to 32 GHz is demonstrated by heterodyne of first and third Brillouin Stokes lines
from a single resonator. © 2013 Optical Society of America
OCIS codes:
(190.5890) Scattering, stimulated; (290.5900) Scattering, stimulated Brillouin; (190.4390) Nonlinear
optics, integrated optics.
http://dx.doi.org/10.1364/OL.39.000287
Low phase (frequency) noise lasers are critical in a vari-
ety of scientific, commercial, and military applications,
including spectroscopy [
1
], optical clocks [
2
], coherent
fiber-optic communication [
3
], microwave photonics
[
4
], and remote sensing [
5
]. Among various laser technol-
ogies to achieve low phase noise, stimulated Brillouin
scattering involves the nonlinear interaction among the
pump, Stokes, and acoustic fields. Due to stronger damp-
ing of the acoustic field relative to the optical fields, the
frequency noise of the Brillouin laser is greatly sup-
pressed relative to the pump laser frequency noise [
6
,
7
].
Brillouin fiber ring lasers with narrow linewidth on the
order of 10 Hz to 1 kHz have been demonstrated [
8
,
9
].
Also Brillouin lasers have been demonstrated from
ultra-high-
Q
bulk microcavities [
7
,
10
] and chip-based
Chalcogenide waveguides [
11
]. Moreover, precise match-
ing of the free-spectral range (FSR) to the Brillouin shift
(necessary for efficient laser operation) has been re-
cently demonstrated using a silica ultra-high-
Q
resonator
on silicon [
12
]. For operation in the 1550 nm band, these
devices also have record-low fundamental frequency
noise (Schawlow
–
Townes noise) for a chip-based laser
[
12
,
13
] and furthermore have been used to demonstrate
microwave synthesis up to 22 GHz using cascaded oscil-
lation in a single resonator [
14
].
In this Letter, we study the operation of these devices
at 1064 nm by pumping using a narrow-linewidth
Ytterbium-doped fiber laser. In addition to characteriza-
tion of the laser frequency noise, we investigate the laser
relative intensity noise (RIN) of these devices for the first
time. Overall, the fundamental and technical frequency
noise of the laser is greatly suppressed relative to the
pump laser and is comparable with that of low-noise
Nd:YAG lasers. Finally, because the Brillouin frequency
shift
ν
B
is inversely proportional to the pump wavelength,
i.e.,
ν
B
2
nV
A
∕
λ
p
, where
n
is the refractive index of
silica,
V
A
is the acoustic velocity in silica, and
λ
p
is the
pump wavelength, low noise microwave generation up to
32 GHz (as compared to 22 GHz using a 1550 nm pump
[
14
]) is demonstrated using cascaded Brillouin laser lines
at 1064 nm.
The experimental schematic is given in Fig.
1
. A tuna-
ble continuous fiber laser is amplified by a Ytterbium-
doped fiber amplifier (YDFA). In order to reduce the
intensity noise induced by the YDFA, a RIN reduction
setup comprised of an acousto-optical modulator
(AOM), photodetector, and servo control is used. The
intensity-stabilized pump laser is coupled to the disk res-
onator by a taper fiber [
15
,
16
]. The disk size (
D
≈
4
mm)
is designed such that its FSR matches the Brillouin shift
frequency at 1064 nm (
ν
B
∼
15
.
9
GHz). As part of this
design process, a frequency-modulation method was
used for FSR measurement [
17
]. Larger disks (
D
≈
8
and 16 mm) with FSR matching half and one-fourth of
ν
B
also were fabricated. A Pound
–
Drever
–
Hall (PDH)
lock scheme
—
consisting of a phase modulator (PM), sig-
nal generator, mixer, and servo feedback
—
is used to
lock the pump laser to the cavity resonance [
18
]. The
SBS laser signal propagates opposite to the direction
of the pump wave and is characterized using a photo-
detector and an optical spectrum analyzer. For laser
frequency noise measurements, a Mach
–
Zehnder inter-
ferometer (MZI) also is used as an optical frequency dis-
criminator (not shown in the figure). Finally, measured
threshold pump power was in the range of 1 mW for
loaded cavity linewidths ranging from 2 to 5 MHz.
Fig. 1. Experimental schematic for characterization of
1064 nm SBS laser. Shown in the setup are PM, phase modula-
tor; YDFA, Ytterbium-doped fiber amplifer; VOA, variable
optical attenuator; AOM, acousto optic modulator; PD, photo-
detector; SG, signal generator; OSA, optical spectrum analyzer.
January 15, 2014 / Vol. 39, No. 2 / OPTICS LETTERS 287
0146-9592/14/020287-04$15.00/0
© 2014 Optical Society of America
The frequency noise spectral density of the 1064 nm
SBS laser was measured by a MZI with a FSR of
7.6 MHz. The laser was held at the quadrature point of
the MZI in order to convert frequency noise to amplitude
noise [
12
,
19
]. The inset of Fig.
2(a)
shows the frequency
fluctuation spectra measured for an 8 mm diameter de-
vice at a series of power levels. Above a few 100 kHz, the
noise spectrum is approximately white, suggesting that
this portion of the spectrum is associated with fundamen-
tal Schawlow
–
Townes noise. The two-sided, fundamen-
tal Schawlow
–
Townes frequency noise of the SBS
laser can be written as [
13
]
S
ST
ν
f
ℏ
ω
3
8
π
2
PQ
T
Q
E
n
T
N
T
1
;
(1)
where
Q
T;E
are the total and external
Q
factors,
P
is the
output power of the Brillouin laser, and
n
T
N
T
is the
number of thermal quanta in the mechanical (optical)
field.
N
T
is negligible for optical frequencies, while
n
T
≈
386
for a phonon frequency of 15.9 GHz at room
temperature. The white noise level is plotted versus laser
power in the main panel of Fig.
2(a)
. A theoretical curve
based on the Schawlow
–
Townes formula agrees very
well with the data.
The technical frequency noise of the 1064 nm SBS laser
also was characterized and is shown in Fig.
2(b)
. The red
curve is the frequency noise of the pump laser, while the
green and blue curves give the frequency noise of SBS
lasers based on 4 and 16 mm disks, respectively. The
pump laser is a narrow linewidth Ytterbium-doped fiber
laser, with an effective linewidth of 1 kHz. Its frequency-
noise spectrum features a broad shoulder from 1 to
100 kHz. Significantly, the SBS laser frequency noise is
suppressed relative to the pump laser for frequencies
above 1 kHz. The technical noise between 1 and 50 kHz
is close to the measured thermorefractive noise of the
disk resonators [
20
]. Indeed, the measured decrease of
the noise spectrum for the 16 versus the 4 mm disk is
consistent with an expected decrease of thermorefrac-
tive noise with an increase of mode volume [
20
,
21
].
For comparison, the black dashed curve shows the cal-
culated frequency-noise spectrum based on adapting a
model of thermorefractive noise in a spherical resonator
[
21
] to a 4 mm disk in a manner similar to that described
in [
22
]. This thermorefractive noise component is added
to a white Schawlow
–
Townes noise in the plot, i.e.,
S
ν
f
S
Thermo
ν
f
S
ST
ν
f
. For even lower frequencies
between 100 Hz and 1 kHz, the SBS laser frequency noise
is believed to be limited by environmental mechanical
and acoustic noise. For comparison, the red square mark-
ers give the frequency noise of a narrow-linewidth com-
mercial Nd:YAG laser (Mephisto Laser, Coherent Inc.,
online data sheet [
23
]).
In addition to frequency noise, we also characterized
the RIN of the on-chip SBS laser. Figure
3
shows the SBS
laser RIN spectrum measured with the RIN reduction for
the pump laser on and off. The servo control bandwidth
for the RIN reduction was around 100 kHz. When the RIN
reduction is off, the SBS laser RIN tracks the RIN shape
of the pump laser
YDFA. The low-frequency behavior
in this spectrum (1
–
100 kHz), as well as the noise spike at
65 kHz, is associated with the YDFA, while the noise
bump between 100 kHz and 10 MHz is the fiber-laser re-
laxation oscillation resonance. The SBS laser RIN
reaches a shot-noise-limited value of
−
141
dBc
∕
Hz for
frequencies greater than 20 MHz, when the incident op-
tical power to the photodetector is 41
μ
W. When the RIN
reduction is on, the pump laser intensity noise is stabi-
lized below the locking bandwidth at 100 kHz, leading
to the reduction of the SBS RIN within 100 kHz by a fac-
tor up to 20 dB. The low-frequency RIN of the SBS laser
with the RIN reduction on is on the order of
−
125
dBc
∕
Hz. Further reduction of the SBS laser RIN
is currently under investigation.
Finally, low-noise microwave generation up to 32 GHz
based on cascaded SBS lines in the 1064 nm band was
demonstrated. When the intracavity power of the first
Stokes line reaches the pump threshold for the
Fig. 2. (a) Inset: Frequency noise spectra for the SBS laser at
1064 nm measured at a series of laser output powers. The
spectra are Schawlow
–
Townes noise limited for offset frequen-
cies greater than a few 100 kHz. The device uses an 8 mm
diameter cavity. Main panel: Schawlow
–
Townes noise level
from the inset plotted versus output power and compared with
the theoretically predicted Schawlow
–
Townes noise level.
(b) Frequency noise spectra measured at offset frequencies less
than 4 MHz for the pump laser (fiber laser, red curve), SBS laser
from a 4 mm disk (green curve) and SBS laser from a 16 mm
disk (blue curve). The Schawlow
–
Townes limit at high offset
frequency is lower for the 16 mm device, as this device featured
a higher optical
Q
factor. The red square markers are the fre-
quency noise data for a narrow linewidth Nd:YAG laser (see
text for reference). The black dashed line shows the calculated
thermorefractive noise for a 4 mm microresonator plus a white
Schawlow
–
Townes noise.
288 OPTICS LETTERS / Vol. 39, No. 2 / January 15, 2014
second-order Stokes, the second Stokes line can start
oscillation. Recently, we have demonstrated low-noise
microwave synthesis up to 22 GHz (K band) based on
heterodyne of the first and third Stokes lines in the
1550 nm band (where the Brillouin shift is around
10.8 GHz) [
14
]. The open- and closed-loop configuration
for microwave generation based on the 1064 nm SBS la-
sers, shown in Fig.
4(a)
, is similar to that in the 1550 nm
band [
14
]. However, a larger Brillouin shift at 1064 nm,
i.e.,
ν
B
∼
1
∕
λ
p
, leads to microwave generation with higher
frequencies in the Ka band up to 32 GHz. In order to char-
acterize the phase noise of the generated microwave sig-
nal, a phase-noise analyzer (PNA, Rohde Schwarz model
number FSUP26) with a bandwidth up to 26.5 GHz is
used. The beat note of 32 GHz measured directly from
the fast photodetector with a bandwidth of 45 GHz is first
divided to 4 GHz by an eightfold frequency divider and
then measured by the PNA.
The center frequency of the microwave signal can be
fine-tuned by varying the pump laser frequency. Thus it
can be regarded as an optical voltage controlled oscilla-
tor (OVCO). The Fig.
4(b)
inset shows the optical spec-
trum for the first and third Stokes lines measured in the
back direction. Figure
4(b)
shows the open-loop (free
running OVCO) and closed-loop (phase lock-loop OVCO
referenced to a low-frequency oscillator at 497 MHz)
single-sideband phase noise of the 4 GHz signal. A phase
noise level as low as
−
121
dBc
∕
Hz at 100 kHz offset for
the open loop case is achieved for the 4 GHz carrier.
In conclusion, we have demonstrated a narrow-
linewidth SBS laser at 1064 nm from an on-chip ultra-
high-Q silica wedge disk resonator. The fundamental
Schawlow
–
Townes frequency noise of the SBS laser is
on the order of
0
.
1
Hz
2
∕
Hz. The technical frequency
noise of the SBS laesr is close to the thermodynamic
limit (thermorefractive noise) of the resonator at room
temperature, with performance comparable to an ultra-
narrow-linewidth Nd:YAG laser. The RIN of the SBS laser
is suppressed by intensity stabilization of the pump laser.
Finally, low-noise microwave synthesis up to 32 GHz
using cascaded SBS lines is demonstrated.
We are grateful for financial support from the DARPA
ORCHID and QUASAR programs, the Institute for Quan-
tum Information and Matter (IQIM), the NSF Physics
Frontiers Center with support of the Gordon and Betty
Moore Foundation, and the Kavli NanoScience Institute.
References
1. R. J. Rafac, B. C. Young, J. A. Beall, W. M. Itano, D. J.
Wineland, and J. C. Bergquist, Phys. Rev. Lett.
85
, 2462
(2000).
2. S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E.
Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates,
K. R. Vogel, and D. J. Wineland, Science
293
, 825
(2001).
3. E. Ip, A. Lau, D. Barros, and J. Kahn, Opt. Express
16
, 753
(2008).
4. T. M. Fortier, M. S. Kirchner, F. Quinlan, J. Taylor, J. C.
Bergquist, T. Rosenband, N. Lemke, A. Ludlow, Y. Jiang,
C. W. Oates, and S. A. Diddams, Nat. Photonics
5
, 425
(2011).
5. C. Karlsson, F. Olsson, D. Letalick, and M. Harris, Appl. Opt.
39
, 3716 (2000).
Fig. 4. (a) Schematic for microwave generation using the first
and third SBS Stokes lines at 1064 nm. (b) Main panel: Single-
sideband phase noise for the generated microwave at 4 GHz
under open loop (free running) and closed loop (phase-lock
loop with reference to a low-frequency reference oscillator
at 497 MHz) cases. Inset: Optical spectrum of the first and third
Stokes lines in the backward propagating direction.
Fig. 3. RIN spectrum for the 1064 nm SBS laser with the pump
laser RIN reduction
“
on
”
(blue curve) and
“
off
”
(red curve). Also
shown is the detector noise (black curve).
January 15, 2014 / Vol. 39, No. 2 / OPTICS LETTERS 289
6. A. Debut, S. Randoux, and J. Zemmouri, Phys. Rev. A
62
,
023803 (2000).
7. I. Grudinin, A. Matsko, and L. Maleki, Phys. Rev. Lett.
102
,
043902 (2009).
8. S. P. Smith, F. Zarinetchi, and S. Ezekiel, Opt. Lett.
16
, 393
(1991).
9. J. Geng, S. Staines, Z. Wang, J. Zong, M. Blake, and S. Jiang,
IEEE Photon. Technol. Lett.
18
, 1813 (2006).
10. M.TomesandT.Carmon,Phys.Rev.Lett.
102
,113601(2009).
11. R. Pant, C. G. Poulton, D. Choi, H. Mcfarlane, S. Hile, E. Li,
L. Thevenaz, B. Luther-Davies, S. J. Madden, and B. J.
Eggleton, Opt. Express
19
, 8285 (2011).
12. H. Lee, T. Chen, J. Li, K. Yang, S. Jeon, O. Painter, and K. J.
Vahala, Nat. Photonics
6
, 369 (2012).
13. J. Li, H. Lee, T. Chen, and K. J. Vahala, Opt. Express
20
,
20170 (2012).
14. J. Li, H. Lee, and K. J. Vahala, Nat. Commun.
4
, 2097 (2013).
15. M. Cai, O. Painter, and K. J. Vahala, Phys. Rev. Lett.
85
,74
(2000).
16. S. M. Spillane, T. J. Kippenberg, O. J. Painter, and K. J.
Vahala, Phys. Rev. Lett.
91
, 043902 (2003).
17. J. Li, H. Lee, K. Y. Yang, and K. J. Vahala, Opt. Express
20
,
26337 (2012).
18. R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M.
Ford, A. J. Munley, and H. Ward, Appl. Phys. B
31
,97
(1983).
19. K. J. Williams, A. Dandridge, A. D. Kersey, J. F. Weller,
A. M. Yurek, and A. B. Tveten, Electron. Lett.
25
, 774
(1989).
20. H. Lee, M. G. Suh, T. Chen, J. Li, S. A. Diddams, and K. J.
Vahala, Nat. Commun.
4
, 2468 (2013).
21. M. L. Gorodetsky and I. Grudinin, J. Opt. Soc. Am. B
21
, 697
(2004).
22. A. Schliesser, G. Anetsberger, R. Riviere, O. Arcizet, and
T. J. Kippenberg, New J. Phys.
10
, 095015 (2008).
23. For example,
“
Mephisto/Mephisto S Ultra-Narrow Line-
width CW DPSS Laser,
”
https://www.coherent.com/
downloads/Mephisto_CoherentDatasheet_Jan2013.pdf
.
290 OPTICS LETTERS / Vol. 39, No. 2 / January 15, 2014