We report on the generation of a stable continuous-wave low-frequency
squeezed vacuum field with a squeezing level of $3.8\pm0.1$ dB at 1064 nm, the
wavelength at which laser interferometers for gravitational wave (GW) detection
operate, using periodically poled KTiOPO$_4$ (PPKTP) in a sub-threshold optical
parametric oscillator. PPKTP has the advantages of higher nonlinearity, smaller
intra-crystal and pump-induced seed absorption, user-specified parametric
down-conversion temperature, wider temperature tuning range, and lower
susceptibility to thermal lensing over alternative nonlinear materials such as
MgO doped or periodically poled LiNbO$_3$, and is, therefore, an excellent
material for generation of squeezed vacuum fields for application to laser
interferometers for GW detection

Goda, Keisuke

Mavalvala, Nergis

Mikhailov, Eugeniy E.

Miyakawa, Osamu

Saraf, Shailendhar

Vass, Stephen

Weinstein, Alan

English

arXiv

Keisuke Goda

Eugeniy E. Mikhailov

Osamu Miyakawa

Shailendhar Saraf

Stephen Vass

Alan Weinstein

Nergis Mavalvala

Crossref

Generation of a stable low-frequency squeezed vacuum field with periodically poled KTiOPO_4 at 1064 nm

We report the generation of a stable continuous-wave low-frequency squeezed vacuum field with a squeezing level of 7.4±0.1 dB at 1064 nm, the wavelength at which laser-interferometric gravitational wave (GW) detectors operate, using periodically poled KTiOPO4 (PPKTP) in a subthreshold optical parametric oscillator. The squeezing was observed in a broad band of frequencies above 700 Hz where the sensitivity of the currently operational GW detectors is limited by shot noise. PPKTP has the advantages of higher nonlinearity, smaller pump-induced seed absorption, and wider temperature tuning range than alternative nonlinear materials such as MgO-doped or periodically poled LiNbO3, and is, therefore, an excellent material for generation of squeezed vacuum fields for application to laser interferometers for GW detection

Weinstein, Alan J.

Caltech Authors - Main

92 OPTICS LETTERS / Vol. 33, No. 2 / January 15, 2008Generation of a stable low-frequency squeezed
vacuum field with periodically poled KTiOPO4
at 1064 nm
Keisuke Goda,1 Eugeniy E. Mikhailov,2 Osamu Miyakawa,3 Shailendhar Saraf,4 Stephen Vass,3
Alan Weinstein,3 and Nergis Mavalvala1,*
1LIGO Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2The College of William & Mary, Williamsburg, Virginia, 23187, USA
3LIGO Laboratory, California Institute of Technology, Pasadena, California 91125, USA
4Rochester Institute of Technology, Rochester, New York 14623, USA
*Corresponding author: nergis@ligo.mit.edu
Received July 31, 2007; accepted November 21, 2007;
posted December 5, 2007 (Doc. ID 85919); published January 3, 2008
We report the generation of a stable continuous-wave low-frequency squeezed vacuum field with a squeezing
level of 7.4±0.1 dB at 1064 nm, the wavelength at which laser-interferometric gravitational wave (GW) de-
tectors operate, using periodically poled KTiOPO4 (PPKTP) in a subthreshold optical parametric oscillator.
The squeezing was observed in a broad band of frequencies above 700 Hz where the sensitivity of the cur-
rently operational GW detectors is limited by shot noise. PPKTP has the advantages of higher nonlinearity,
smaller pump-induced seed absorption, and wider temperature tuning range than alternative nonlinear ma-
terials such as MgO-doped or periodically poled LiNbO3, and is, therefore, an excellent material for genera-
tion of squeezed vacuum fields for application to laser interferometers for GW detection. © 2008 Optical So-
ciety of America
OCIS codes: 270.6570, 190.4970.Injection of squeezed vacuum states into the output
port of laser interferometers for gravitational wave
(GW) detection is a promising technique for enhanc-
ing the sensitivity of future detectors, such as Ad-
vanced LIGO [1], which are expected to be opera-
tional in the next few years. Large squeeze factors in
the GW band 10 Hz to 10 kHz, and stable, long-
term operation are among the key requirements that
a squeeze source must satisfy to be used in long-
baseline GW detectors with a large duty factor [2].
Squeezed vacuum fields are typically generated by
using nonlinear crystals in subthreshold optical
parametric oscillators (OPOs). Consequently, the
choice of nonlinear material is critical, since it deter-
mines several important parameters, such as nonlin-
earity, phase-matching type, absorption loss, pump-
induced-seed absorption loss, laser damage
threshold, photorefractive damage threshold, and
susceptibility to thermal lensing. Moreover, since all
GW detectors currently use high-power Nd:YAG la-
sers sources at 1064 nm, generating squeezed
vacuum states at 1064 nm is also essential.
Currently, squeezed state sources for GW detectors
at 1064 nm use OPOs comprising magnesium oxide
(MgO)-doped lithium niobate LiNbO3 crystals [3,4].
LiNbO3 is a widely used nonlinear material with
good long-term stability. MgO doping increases its la-
ser damage threshold and reduces the effect of green-
induced infrared absorption [5], but also increases
impurity and inhomogeneity in the crystal, thereby
increasing intracrystal absorption and scattering
losses, which place a limit on the attainable level of
squeezing. In this Letter, we report the generation of
a squeezed vacuum field using an alternative mate-
0146-9592/08/020092-3/$15.00 ©rial, periodically poled KTiOPO4 (PPKTP) at
1064 nm.
KTiOPO4 (KTP) offers several advantages over
LiNbO3. It has a higher laser damage threshold,
higher resistance to photorefractive damage, and
lower susceptibility to thermal lensing. These proper-
ties are especially critical for the long-term stability
of OPOs pumped by cw visible sources [6]. KTP has
an effective nonlinear coefficient, deff, comparable
with that of LiNbO3 at 1064 nm and has been shown
to generate relatively high levels of quantum correla-
tions [7,8]. Furthermore, recent progress in the elec-
tric field poling of flux-grown KTP has made periodi-
cally poled KTP (PPKTP) an even more promising
candidate. With its high nonlinearity of deff
10.8 pm/V, PPKTP is a competitive alternative to
periodically poled LiNbO3 (PPLN) and LiTaO3
(PPLT). Poling-induced losses of PPKTP are much
lower than those of PPLN. For these reasons, higher
squeezing levels are attainable with PPKTP at real-
istically available pump powers.
Since birefringent phase matching is not used in
periodically poled crystals, temperature tuning of
their refractive indices in the ordinary and extraordi-
nary axes is not required. The only requirement on
the temperature control is to maintain the grating
period against thermal expansion. The typical
FWHM of the effective parametric downconversion
temperature for PPKTP at 1064 nm is 5°C, whereas
it ranges from a few tens to several hundred milli-
kelvins for LiNbO3 (depending on its length). This
property is particularly vital for the long-term stabil-
ity requirement of GW observatories, since the sys-
tem is robust against temperature variations, unlike
2008 Optical Society of America
January 15, 2008 / Vol. 33, No. 2 / OPTICS LETTERS 93the birefringent phase-matching case. The low phase-
matching temperature (typically around room tem-
perature) and large effective temperature range of
PPKTP also significantly reduce difficulties in fabri-
cation of ovens for temperature control.
Observation of squeezing in the GW band has been
reported by McKenzie et al. [3] and Vahlbruch et al.
[4]. Both groups used MgO:LiNbO3, achieving
squeezing levels of about 3–6 dB. Here we demon-
strate the generation of a squeezed vacuum field at
audio frequencies using PPKTP at 1064 nm. This is,
to our knowledge, the first time that squeezing at
1064 nm using PPKTP is reported. This squeeze
source has all the advantages of stability and ease of
operation that PPKTP offers, at a wavelength suit-
able for GW detectors.
PPKTP has previously been used in a cw OPO to
generate squeezed states, but not at 1064 nm. Aoki et
al. reported the generation of squeezed light in side-
band modes of cw light at 946 nm using a PPKTP
crystal in an OPO and observed the squeezing level of
5.6±0.1 dB and the antisqueezing level of
12.7±0.1 dB [9]. More recently, Takeno et al. ob-
served 9.01±0.14 dB of cw squeezing at 860 nm us-
ing a PPKTP crystal in a bow-tie cavity [10]. Hirano
et al. also observed the generation of pulsed squeezed
light from a single-pass degenerate optical paramet-
ric amplifier with a PPKTP crystal pumped by a cw
second-harmonic field and reported the squeezing
level of 3.2 dB and the antisqueezing level of 6.0 dB
at 1064 nm [11]. These are among the highest levels
of pulsed and cw squeezing ever obtained.
The schematic of the experiment is shown in Fig. 1.
The apparatus is composed mainly of (i) the second-
harmonic generator (SHG), (ii) the OPO, (iii) the ho-
modyne detector, (iv) the frequency and intensity
stabilizer, and (v) the phase-locked subcarrier
(LASER2). The main light source is a Nd:YAG master
oscillator power amplifier (MOPA) laser (LASER1).
The laser is frequency and intensity stabilized by us-
ing the LIGO prestabilized laser system [12].
The SHG is a cavity composed of a 6.5 mm long 5%
MgO:LiNbO3 crystal (Photon LaserOptik Inc.) with
an antireflection-coated flat surface, a high-reflective
curved surface, and an output coupling mirror with a
radius of curvature of 50 mm. The radius of curva-
ture and reflectivity of the curved surface of the crys-
tal are, respectively, 8 mm and 99.95% at both 1064
and 532 nm. The output coupler, with reflectivities of
95.0% at 1064 nm and 4.0% at 532 nm, is mounted
on a piezoelectric transducer (PZT) as an actuator
that controls the cavity length, using the Pound–
Drever–Hall technique. The SHG cavity is enclosed
in an oven to maintain the crystal at 114.05°C for op-
timal SHG conversion efficiency. It is pumped by
1.1 W of the 1064 nm from the MOPA laser and gen-
erates 320 mW at 532 nm, which is used as a pump
field in the OPO cavity.
The OPO is a cavity composed of a 10 mm long
PPKTP crystal (Raicol Inc.) with antireflection-
coated flat surfaces and two coupling mirrors, each
with a radius of curvature of 10 mm. The reflectivi-ties are 99.95% at both 1064 and 532 nm for the in-
put coupler and 92.0% at 1064 nm and 4.0% at
532 nm for the output coupler. The OPO cavity
length is 2.3 cm. The crystal is maintained at 33.5°C
to optimize the 1064/532 parametric downconver-
sion. When pumped at frequency 20 and operated
below threshold, the OPO correlates the upper and
lower quantum sidebands of a vacuum field that en-
ters the OPO around the center frequency 0 [13].
The correlation of the quantum sidebands appears as
a squeezed vacuum field. A second laser beam
(LASER2) that is shifted by 642 MHz with respect to
0, and is orthogonally polarized to the vacuum field
that seeds the OPO, is used to lock the OPO cavity in
a TEM00 mode.
The generated squeezed vacuum field is triggered
by the coherent local oscillator (LO) field and mea-
sured with a homodyne efficiency of 99% by the ho-
modyne detector, composed of a 50/50 beam splitter
and a balanced photodetector with two photodiodes
(JDS Uniphase ETX500T) with matched quantum ef-
ficiencies of 93%. The difference between the two op-
tical responses is amplified and measured by a spec-
trum analyzer (HP8591E). The squeeze angle is
locked by the noise-locking technique [14].
We measured the fixed-frequency, zero-span spec-
tra of the shot noise and squeezed/antisqueezed shot
noise, shown in Fig. 2, when scanning the squeeze
angle. The periodic oscillation of the squeeze angle
can be seen in the figure. We also measured the spec-
tra of the shot noise and squeezed shot noise when
the squeeze angle was noise locked. The result is
Fig. 1. (Color online) Schematic of the experiment.
LASER1, Nd:YAG MOPA laser; LASER2, Lightwave 126
laser; FI, Faraday isolator; MC, mode-cleaning fiber; PM,
electro-optic phase modulator; DBS, dichroic beam splitter;
PBS, polarizing beam splitter; BS, 50/50 beam splitter;
BD, bean dump; PO, pickoff mirror;  /4, quarter-wave
plate; PD1–PD8, photodiodes; OC1–OC5, LOs (13.3 MHz,
16.7 MHz, 49.5 MHz, 624 MHz, and 13.5 kHz). Solid lines,
coherent fields; dotted line, squeezed vacuum field. The
low-frequency spectrum analyzer (Stanford Research Sys-
tems SR785), which is not shown in the figure, is used to
measure the broadband spectrum of the balanced photode-
tector output.shown in Fig. 3. Broadband squeezing of 7.4±0.1 dB
94 OPTICS LETTERS / Vol. 33, No. 2 / January 15, 2008at frequencies above 3 kHz is observed. The noise in-
crease at low frequencies is due to scattering of the
LO light from the homodyne detector. To observe
squeezing at lower frequencies requires careful
shielding of the LO- or ambient-light-induced scat-
tered photons from coupling to the OPO cavity. This
was demonstrated very well in [3,4], and is not part
of the initial goals of the present experimental dem-
onstration.
In conclusion, we have demonstrated the genera-
tion of a stable cw low-frequency squeezed vacuum
field with a squeezing level of 7.4±0.1 dB at
Fig. 2. (Color online) Fixed-frequency spectra of shot noise
and squeezed/antisqueezed noise power when the squeeze
angle was scanned as a function of time. The measure-
ments were done at 900 kHz with zero frequency span. The
resolution bandwidth is 100 kHz, and the video bandwidth
is 3 kHz. The squeeze angle was scanned by the PZT
(PZT3) with a ramp function at 10 Hz. The electronic noise
was 17.4 dB below the shot-noise level.
Fig. 3. (Color online) Broadband spectra of shot noise,
squeezed shot noise, and electronic noise. The spectra were
averaged 2000 times. The squeeze angle was noise locked
without any coherent light. The spikes at 13.5 kHz and
15.7 kHz are due to the modulation of the PZT (PZT3) for
the noise-locking technique.1064 nm, suitable for laser interferometers for GW
detection, using PPKTP in a subthreshold OPO. The
attained level of squeezing was limited by the escape
efficiency (or pump power) and detection efficiency.
Since PPKTP is not susceptible to photorefractive
damage and thermal lensing to some extent, if a
higher pump power were available, the escape effi-
ciency could be increased by using lower output cou-
pler reflectivity. Similarly, improvement of 85% over-
all detection efficiency achieved would also lead to a
still higher observed squeezing level. The large
squeezing levels and excellent stability of PPKTP
make it an excellent material for use in squeezing-
enhanced GW detectors. Although gray tracking is a
known problem for KTP, we have not observed it in
this demonstration, likely because we use moderate
pump power and no coherent seed light.
We thank our colleagues at the LIGO Laboratory
and our collaborators at the Australian National Uni-
versity. We are especially grateful to C. Wipf for de-
sign and fabrication of the balanced photodetector
and to K. McKenzie for valuable comments. We also
thank A. Furusawa at University of Tokyo for encour-
aging use of PPKTP. We gratefully acknowledge sup-
port from National Science Foundation grants PHY-
0107417 and PHY-0457264.
References
1. http://www.ligo.caltech.edu/docs/M/M060056-07/
M060056-07.pdf.
2. B. Barish and R. Weiss, Phys. Today 52(10), 44 (1999).
3. K. McKenzie, N. Grosse, W. P. Bowen, S. Whitcomb, M.
B. Gray, D. E. McClelland, and P. K. Lam, Phys. Rev.
Lett. 93, 161105 (2004).
4. H. Vahlbruch, S. Chelkowski, B. Hage, A. Franzen, K.
Danzmann, and R. Schnabel, Phys. Rev. Lett. 97,
011101 (2006).
5. Y. Furukawa, K. Kitamura, A. Alexandrovski, R. K.
Route, M. M. Fejer, and G. Foulon, Appl. Phys. Lett.
78, 1970 (2001).
6. T. Y. Fan, C. E. Huang, B. Q. Hu, R. C. Eckardt, Y. X.
Fan, R. L. Byer, and R. S. Feigelson, Appl. Opt. 26,
2394 (1987).
7. J. Laurat, T. Coudreau, N. Treps, A. Maitre, and C.
Fabre, Phys. Rev. Lett. 91, 213601 (2003).
8. S. Feng and O. Pfister, Opt. Lett. 29, 2800 (2004).
9. T. Aoki, G. Takahashi, and A. Furusawa, Opt. Express
14, 6930 (2006).
10. Y. Takeno, M. Yukawa, H. Yonezawa, and A. Furusawa,
Opt. Express 15, 4321 (2007).
11. T. Hirano, K. Kotani, T. Ishibashi, S. Okude, and T.
Kuwamono, Opt. Lett. 30, 1722 (2005).
12. J. Rollins, D. Ottaway, M. Zucker, and R. Weiss, Opt.
Lett. 29, 1876 (2002).
13. C. M. Caves and B. L. Schumaker, Phys. Rev. A 31,
3068 (1985).
14. K. McKenzie, E. E. Mikhailov, K. Goda, P. K. Lam, N.
Grosse, M. B. Gray, N. Mavalvala, and D. E.
McClelland, J. Opt. B 7, S421 (2005).


Generation of a stable low-frequency squeezed vacuum field with periodically poled KTiOPO4 at 1064 nm

http://authors.library.caltech.edu/9726/1/GODol08.pdf

Generation of a stable low-frequency squeezed vacuum field with
  periodically-poled KTiOPO$_4$ at 1064 nm

Generation of a stable low-frequency squeezed vacuum field with periodically-poled KTiOPO $_4$ at 1064 nm

Abstract

Similar works

Full text

Available Versions

Crossref

Caltech Authors - Main

Caltech Authors - Main

Generation of a stable low-frequency squeezed vacuum field with periodically-poled KTiOPO4_44​ at 1064 nm

Abstract

Similar works

Full text

Available Versions

Crossref

Caltech Authors - Main

Caltech Authors - Main

Generation of a stable low-frequency squeezed vacuum field with periodically-poled KTiOPO $_4$ at 1064 nm