Amplitude-squeezed light with intensity fluctuations 29%  below the standard quantum limit (SQL) is produced from a pump-suppressed room-temperature semiconductor laser, corresponding to 41% below the SQL after correction for detection efficiency. Excess noise, which degrades the observed squeezing, appears to be associated with the presence of weak longitudinal side modes

Kitching, J.

Provenzano, D.

Yariv, A.

English

Caltech Authors - Main

2526 OPTICS LETTERS / Vol. 20, No. 24 / December 15, 1995Generation of amplitude-squeezed light from a
room-temperature Fabry–Perot semiconductor laser
J. Kitching,* D. Provenzano, and A. Yariv
Department of Applied Physics, California Institute of Technology, 128-95, Pasadena, California 91125
Received July 3, 1995
Amplitude-squeezed light with intensity f luctuations 29% below the standard quantum limit (SQL) is produced
from a pump-suppressed room-temperature semiconductor laser, corresponding to 41% below the SQL after
correction for detection eff iciency. Excess noise, which degrades the observed squeezing, appears to be
associated with the presence of weak longitudinal side modes.
PACS numbers: 42.50.Dv, 42.50.Lc, 42.55.Px, 42.62.Eh.  1995 Optical Society of AmericaSqueezed states of the electromagnetic field are
beginning to create new possibilities for precision
measurements near the standard quantum limit
(SQL). Amplitude-squeezed states generated from
pump-suppressed semiconductor lasers offer a number
of desirable features such as large squeezing band-
widths, ease of implementation, and the fact that
the squeezing is superimposed upon a large coherent
optical field. Although measurements of amplitude
squeezing performed on lasers cooled to cryogenic
temperatures have resulted in substantial amounts of
squeezing,1 the generation of amplitude-squeezed light
from room-temperature devices has been considerably
less successful. To our knowledge, the largest amount
of squeezing that has been measured from an unmodi-
fied semiconductor laser at room temperature was
only 7% below the SQL.2 Several other experiments
on both room-temperature and cryogenically cooled
devices3 – 7 have reported substantial amounts of excess
noise, which tends to obscure the squeezing.
As there appear to be no fundamental reasons why
room-temperature lasers should exhibit signif icantly
less squeezing than cryogenically cooled devices, there
has been some effort recently to determine why the
generation of amplitude squeezing at room tempera-
ture has been so difficult. This has been motivated in
part by a desire to understand the excess noise sources
that occur in semiconductor lasers. More important,
however, is the lure of being able to produce large
amounts of squeezing from off-the-shelf commercial de-
vices. Several experiments were performed recently
that were designed to increase the squeezing from com-
mercial room-temperature lasers by the use of exter-
nal optical modifications such as injection locking and
optical feedback.6,7 Although these experiments have
been, on the whole, successful in reducing the ampli-
tude noise to as much as 50% below the SQL, the added
complexity is, in principle, unnecessary.
In this Letter we present measurements, using a
balanced detector, of amplitude squeezing 29% below
the SQL from a room-temperature semiconductor
laser. This corresponds to a noise level 41% below the
SQL at the output facet of the laser when correction
is made for optical losses present in the detection
system. These results demonstrate that substantial
squeezing can indeed be obtained from a commercial0146-9592/95/242526-03$6.00/0device operating at room temperature with no external
components or modifications other than those used
to detect the light. The excess noise that degrades
the squeezing is also investigated, and it appears that
the source of most of this noise under a wide range of
operating conditions is associated with the presence of
longitudinal side modes. Our measurements indicate
that, under optimal conditions, the amount of squeez-
ing is limited primarily by the device and detection
eff iciencies, in accordance with the standard single-
mode theory of amplitude noise in semiconductor
lasers.8,9
The experimental setup is shown in Fig. 1. The
semiconductor laser used (SDL-5402-H1) was an
actively temperature-stabilized quantum-well index-
guided structure with an external differential
quantum efficiency of 68% at room temperature.
The output light from the laser was sent through
an optical isolator (providing .60 dB of isolation)
to a delay line balanced receiver3 for measurement
of the amplitude noise. Common mode rejection
of more than 45 dB was obtained at a frequency of
28 MHz. Figure 2 shows the measured photocurrent
noise power spectrum (trace A) when the photocurrents
at 28 MHz were added. The electronic delay present in
one arm of the receiver caused a frequency-dependent
phase shift between the two signals entering the differ-
ential amplifier with the result that the output photo-
Fig. 1. Experimental setup: LD, laser diode; L, lens;
M, mirror; ly2, half-wave plate; P, polarizer; PBS, polar-
izing beam splitter; D’s, detectors (Hamamatsu S3994);
PA’s, preamplif iers (Minicircuits ZFL-500LN); A, differen-
tial amplifier (Tektronix 7A24). 1995 Optical Society of America
December 15, 1995 / Vol. 20, No. 24 / OPTICS LETTERS 2527current noise power from the receiver varied, as a
function of frequency, between the actual laser noise
level and the SQL.3 We checked the level of the SQL
by shining the light from high-power light-emitting
diodes (LED’s) onto the detectors to produce the same
photocurrent as did the laser. The photocurrent noise
power spectrum obtained from the LED’s is shown in
trace B. It can be seen that, at 28 MHz, the laser
photocurrent noise power is 1.4 dB below the SQL
(assumed, here, to be determined by the LED),
indicating that the laser amplitude f luctuations were
squeezed by this amount. Whereas LED’s have also
been shown to exhibit squeezed intensity f luctua-
tions,10 the low efficiency of the devices s,10%, current
to current) and the close agreement of the LED noise
level with the SQL measured with the laser noise
spectrum (see Fig. 2, trace A, at 7 and 20 MHz)
indicated that the LED noise was indeed close to
the SQL. For the measurement in Fig. 2 the laser
injection current was 66 mA, the current-to-current
differential efficiency was 48%, and the laser was
cooled to ,5 –C. The resolution bandwidth for this
measurement was 100 kHz, and the background ampli-
fier noise level (typically .10 dB below the signal level
for the higher detector photocurrents) was subtracted
from all measured signals.
Saturation of the detector response at high frequen-
cies when the detector is illuminated with intense
optical fields can result in errors in the measure-
ment of the laser noise or in the calibration of the
SQL. However, previous experiments6,7 with the
detectors used in this experiment demonstrated their
excellent saturation properties, and no evidence for
signif icant detector saturation was observed in this
experiment within the range of photocurrents and
frequencies at which measurements were made. At
detector currents up to 14 mA the shot-noise level
at 28 MHz, calibrated with both the LED’s and the
laser, was found to be linear in the dc photocurrent
to within roughly 1%. As an additional check on the
level of squeezing, the amplitude noise was measured
as a function of the optical attenuation.7 As expected,
the noise power, normalized to the SQL, was found
to increase in a linear fashion toward unity as the
attenuation was made stronger.
The squeezing could be obtained only within a
narrow range of laser temperatures and injection cur-
rents; at most combinations of these two parameters
the laser operated multimode and the amplitude noise
was far above the SQL. It appeared that particu-
larly good side-mode suppression was required in order
generate squeezed light from the laser. The longitudi-
nal mode spectrum was therefore measured simultane-
ously with the amplitude noise, and a typical spectrum,
obtained when the amplitude noise was 1.4 dB below
the SQL, is shown in Fig. 3. It seems likely that the
excellent side-mode suppression sø30 dB) observed in
this laser was the reason for the large squeezing mea-
sured, whereas other, similar, lasers would not produce
an equivalent amount of amplitude squeezing under
comparable operating conditions.6,7,11
The photocurrent noise at 28 MHz was then mea-
sured as a function of the laser injection current. The
results, normalized to the SQL, are plotted in Fig. 4against the pump rate R ­ iyith 2 1. Also shown
in the figure is the prediction of the single-mode
theory9 in the low-frequency limit with the pump noise
assumed to be completely suppressed. The minimum
noise obtained with this laser was 1.5 dB, or 29% below
the SQL.
If the laser temperature was altered slightly,
both the mode spectrum and the amplitude noise were
found to change in a correlated way. By far the largest
change in the side-mode power occurred for the two
longitudinal side modes closest to the lasing mode
in wavelength. A clear correlation between the
optical power in these two side modes and ex-
cess amplitude noise can be seen in Fig. 5; the
laser noise was found to increase to ten times the
Fig. 2. Photocurrent noise power spectrum from the laser
(trace A) and the LED (trace B) when the differential
amplifier added the detector photocurrents at 28 MHz.
Squeezing is indicated by the laser noise dropping below
the LED noise (SQL) at certain frequencies.
Fig. 3. Longitudinal mode spectrum for the laser when
the amplitude noise was squeezed by 1.4 dB.
Fig. 4. Normalized photocurrent noise power spectrum
dependence on injection current. The solid curve is the
prediction of the single-mode theory of Ref. 9.
2528 OPTICS LETTERS / Vol. 20, No. 24 / December 15, 1995Fig. 5. Dependence of the laser amplitude noise on the
fraction of power in the two side modes closest to the lasing
mode. The solid curve is a third-order polynomial fit to
the measured data.
SQL when 5% of the optical power was present
in the two measured side modes. An extrapola-
tion of a third-order polynomial fit (solid curve) to
zero side-mode power gives excellent agreement with
the prediction of the single-mode theory (indicated
by the dashed–dotted line). This suggests that if
the excess noise associated with the side modes were
eliminated the laser noise would approach the funda-
mental limit determined, primarily, by the device and
detection efficiencies. We note, however, that the
theory of Ref. 9 applies only to a laser for which both
facet ref lectivities are close to unity. This was not
the case for the laser used in this experiment, which
had one facet antiref lection coated. Recent numerical
calculations using traveling-wave models12 have shown
that dramatic changes in the theoretically predicted
amplitude noise power can occur when the good-cavity
assumption is relaxed.
The mechanism by which the side modes generate
the excess noise is still unclear. However, the second-
order term in the fit dominates over the range of
side-mode powers shown, indicating that the excess
noise is proportional to the square of the side-mode
power in this range. Attempts to fit the multimode
theory described in a previous publication7 to the data
were not particularly successful. At side-mode powers
above 1% of the total power the fit in Fig. 5 no longer
agreed well with the measured data.
It is known that an extremely small amount of op-
tical feedback can signif icantly affect the amplitude
noise of the laser.7 We therefore performed several
checks under conditions of maximum squeezing to de-
termine whether spurious optical feedback was present
and, if so, was inf luencing the laser noise. First, the
laser temperature and the injection current were var-
ied by an amount not large enough to cause a signif-
icant change in the side-mode suppression but large
enough to vary the laser frequency by a few gigahertz.
The idea here was that if optical feedback were present
with an intensity large enough to change the amplitude
noise, then changing the laser frequency would change
the phase of the feedback, thereby generating oscilla-
tions in the amplitude noise. The amplitude noise
was found to vary by less than the measurement
error of 0.1 dB, however. In addition, we varied the po-
sitions of both the optical isolator (which was found to
be the offending component in a previous experiment7)and the collimating lens over several wavelengths by
placing their translation stages on piezoelectric trans-
ducers, and again no significant change in the ampli-
tude noise was observed. The above checks seem to
indicate that optical feedback from the tested compo-
nents was not playing a significant role in reducing the
amplitude noise.
In summary, amplitude squeezing 29% below
the SQL has been measured from a commercial
room-temperature Fabry–Perot semiconductor laser.
Excess noise, which degrades the squeezing, appears
to be associated with the presence of longitudinal side
modes. Although the measured amount of squeezing
is still 10–20% smaller than predicted by the standard
single-mode theory, an extrapolation of the data
to zero side-mode power gives agreement with the
theory to within ø3%. Careful stabilization of the
laser temperature and injection current are impor-
tant factors in keeping the side-mode power to below
acceptable levels. However, other than the control of
optical feedback from the detection system, no other
modifications to the laser appear to be necessary to
yield significant amplitude squeezing.
The authors gratefully acknowledge useful com-
ments from Y. Shevy and support from the National
Science Foundation, the Advanced Research Projects
Agency, and the U.S. Office of Naval Research. The
laser used in the experiment was generously donated
by Spectra Diode Laboratories, Inc.
*Present address, National Institute of Standards
and Technology, M.S. 847.10, 325 Broadway, Boulder,
Colorado 80303.
References
1. W. H. Richardson, S. Machida, and Y. Yamamoto, Phys.
Rev. Lett. 66, 2867 (1991).
2. S. Machida, Y. Yamamoto, and Y. Itaya, Phys. Rev.
Lett. 58, 1000 (1987).
3. G. L. Abbas, V. W. S. Chan, and T. K. Yee, Opt. Lett.
8, 419 (1983); S. Machida and Y. Yamamoto, Opt. Lett.
14, 1045 (1989).
4. W. H. Richardson and R. M. Shelby, Phys. Rev. Lett.
64, 400 (1990).
5. M. J. Freeman, H. Wang, D. G. Steel, R. Craig, and
D. R. Scifres, Opt. Lett. 18, 379 (1993).
6. H. Wang, M. J. Freeman, and D. G. Steel, Phys. Rev.
Lett. 71, 3951 (1993); M. J. Freeman, H. Wang, D. G.
Steel, R. Craig, and D. R. Scifres, Opt. Lett. 18, 2141
(1993).
7. J. Kitching, A. Yariv, and Y. Shevy, Phys. Rev. Lett.
74, 3371 (1995).
8. D. E. McCumber, Phys. Rev. 141, 306 (1966); H. Haug
and H. Haken, Z. Phys. 204, 262 (1967).
9. Y. Yamamoto, S. Machida, and O. Nilsson, Phys. Rev.
A 34, 4025 (1986); 35, 5114 (1987).
10. P. R. Tapster, J. G. Rarity, and J. S. Satchell, Euro-
phys. Lett. 4, 293 (1987); H.-A. Bachor, P. Rottengatter,
and C. M. Savage, Appl. Phys. B 55, 258 (1992).
11. M. J. Freeman, D. C. Kilper, D. G. Steel, R. Craig, and
D. R. Scifres, Opt. Lett. 20, 183 (1995).
12. B. Tromberg, H. E. Lassen, and H. Olesen, IEEE J.
Quantum Electron. 30, 939 (1994); D. D. Marcenac and
J. E. Carroll, IEEE J. Quantum Electron. 30, 2064
(1994).


Generation of amplitude-squeezed light from a room-temperature Fabry-Perot semiconductor laser

http://authors.library.caltech.edu/2846/1/KITol95.pdf

Generation of amplitude-squeezed light from a room-temperature Fabry-Perot semiconductor laser

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main