Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2001-08-21
2003-02-04
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S029020, C372S026000, C372S018000
Reexamination Certificate
active
06516014
ABSTRACT:
BACKGROUND
1. Field of the Invention
This application is related to stabilizing the frequency of a tunable laser and, more particularly, to using spectral hole burning materials to stabilize the frequency of a tunable laser.
2. Description of the Related Art
Lasers emit electromagnetic radiation characterized by the optical range of the spectrum where wavelengths are expressed in nanometers (nm) corresponding to 10
−9
m, and frequencies are expressed in megaHertz (MHz) corresponding to 10
6
Hz, or gigaHertz (GHz) corresponding to 10
9
Hz, or teraHertz (THz) corresponding to 10
12
Hz, where one Hz is one cycle per second.
Previous laser stabilization techniques have relied on frequency references based on Fabry-Perot interferometer resonances or the center frequencies of transitions in atomic or molecular vapors.
In the prior art, for example, the Pound-Drever-Hall laser frequency locking technique (R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward,
Laser Phase and Frequency Stabilization Using an Optical Resonator
, Appl. Phys. B31, 97 (1983), M. Zhu and J. L. Hall,
Stabilization of optical phase/frequency of a laser system: application to a commercial dye laser with an external stabilizer
, J. Opt. Soc. Am. B 10, 802 (1993)) utilizes frequency modulated (FM) spectroscopy techniques (G. C. Bjorklund, M. D. Levenson, W. Lenth, C. Ortiz,
Frequency Modulation
(
FM
)
Spectroscopy: Theory of Lineshapes and Signal
-
to
-
Noise Analysis
, Appl. Phys. B 32, 145 (1983)) to actively lock the laser frequency to a reflection mode of a Fabry-Perot interferometer.
Use of an error signal to stabilize a laser frequency of a tunable laser according to the Pound-Drever-Hall technique is shown in FIG.
1
. In
FIG. 1A
a laser
110
emits a beam along path
161
impinging on a phase modulator
112
The radio frequency (rf) signal generator
142
supplies an electrical signal transmitted by wire to the modulator
112
and to mixer
140
. The modulator
112
phase modulates and passes a first beam portion along path
162
to a beam splitter
114
. Beam splitter
114
passes a second beam portion along a beam path
163
to a cavity
116
between mirrors
115
and
117
. The cavity
116
is tuned to a reference frequency by choice of spacing between mirrors
115
and
117
and the cavity emits a reference beam having an energy peak at the reference frequency along path
163
to splitter
114
. At beam splitter
114
both the modulated beam, which reflects immediately off mirror
115
, and the reference beam are diverted along path
165
to a detector
120
which outputs a detector electrical signal. The detector reference signal contains rf frequency components generated by the interference between the modulated beam and the cavity reference beam. This detector electrical signal is filtered at filter
130
to remove unwanted harmonic frequencies and to generate an error signal that is output to the mixer
140
. At mixer
140
the rf signal from the rf signal generator
142
is combined with the error signal at rf frequencies to demodulate the error signal to lower frequencies where it is used as a control signal that is output to the laser servo electronics
150
. The servo electronics
150
tune the laser
110
in response to the control signal received.
FIG. 1B
shows the output
125
of cavity transmission detector B
122
and the error signal
145
output by error signal detector A
120
combined with the mixer
140
which is sent to the servo electronics
150
as the modulated laser frequency beam
162
is tuned across the cavity reference frequency. The amplitude of the error signal
145
at a given frequency in the vicinity of the lock point
147
is related to the size and direction of the deviation of frequency from the reference frequency at the lock point
147
. Thus the greater the deviation of the laser frequency from the reference frequency, the greater is the error signal and the greater is a component of the input to the servo electronics
150
to tune the laser back toward the reference frequency at the lock point
147
. The stability of the laser frequency is limited by the characteristics of the peak in the cavity transmission signal used as the reference frequency detected by the detector
120
.
To evaluate locking stability, two lasers are locked to adjacent modes of the same cavity. The beat signal between the two laser frequencies is then monitored for fluctuations. A relative locking performance of 1 part in 10
5
of the interferometer linewidth has been recently demonstrated (G. Ruoso, R. Storz, S. Seel, S. Schiller, J. Mlynek,
Nd: YAG laser frequency stabilization to a supercavity at the
0.1
Hz level
, Opt. Comm. 133, 259 (1997)) with a high finesse interferometer cavity having a resonance linewidth of less than 10 kHz, leading to relative stability between two lasers at the sub-Hz level for nominally a minute. The short-term drift of the frequency of either of these lasers arises from thermal length changes, mechanical creep, vibration, or other variation in the length of the reference cavity and hence resonance frequency. A recent notable attempt to approach absolute stability used Fabry-Perot cavities made from sapphire and kept at cryogenic temperatures to achieve a 3 kHz drift over 6 months (R. Storz, C. Braxmaier, K. Jäck, O. Pradl, S. Schiller,
Ultrahigh long
-
term dimensional stability of a sapphire cryogenic optical resonator
, Opt. Lett. 23, 1031 (1998)).
Atomic transitions are commonly used to lock a laser to a specific, absolute frequency. A typical arrangement would substitute a fixed atomic resonance for the cavity resonance and allow the modulated beam to transmit through a gas-phase sample before collecting it on a photodetector. Today's precision clocks and oscillators are based on well-studied microwave transition frequencies of rubidium, cesium, hydrogen and mercury atoms in oscillators and masers. Optical frequency standards in the communications bands of 1.3 and 1.5 micron wavelength are also of interest. Among the principal advantages and difficulties with existing microwave and optical standards are that the transition frequencies of an atom are unique to its type, i.e. predetermined by nature, with consequent limitations on the choice of frequency, and that FM locking techniques typically select the center of those transitions. If the frequencies are not exactly those of interest, they must be transferred through an elaborate chain of precisely controlled optical and radio frequency (RF) synthesis, such as frequency doubling and mixing or parametric oscillation sums and differences. Lastly, strong optical dipole transitions typically have spectral linewidths in the 10's of MHz (megaHertz, i.e., 10
6
Hz)—not especially narrow for use as a precise frequency discriminator for laser locking, though examples of higher multi-pole moment transitions with narrower linewidths do exist.
Another frequency locking technique involves stabilization to a Lamb dip in a gas vapor cell. For the Lamb dip, the gaseous motion of the atoms produces a Doppler shifted distribution of frequencies creating an inhomogeneously broadened absorption-line. A strong laser, intense enough to saturate a particular velocity subset of these atoms can temporarily create a spectral hole anywhere in the absorption profile until it is turned off and the hole disappears (absorption reappears). A less intense probe laser can be locked to the hole created in the presence of the strong laser. The frequency stability of the Lamb dip spectral hole is determined by that of the strong laser. To remove this external stability dependence, the more typical arrangement is to use a single laser divided into two counter-propagating beams, strong and weak. For a lock to occur, the two beams must interact with the same velocity subset of atoms that must be the non-Doppler shifted subset. The lock is then constrained to the center of the inhomogeneous absorption.
Condensed phase spectral hole burning materials are known. Absorption
Carlsten John L.
Cone Rufus L.
Sellin Peter B.
Strickland Nicholas M.
Jr. Leon Scott
The Research and Development Institute Inc.
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