Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2002-07-17
2003-11-25
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S026000, C372S029020
Reexamination Certificate
active
06654394
ABSTRACT:
BACKGROUND
1. Field of the Invention
This application relates to a device whereby the frequency and phase of a laser is actively stabilized by locking to a transient spectral hole in a solid state or other condensed phase reference material.
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 utilizes frequency modulated (FM) spectroscopy techniques 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
FIGS. 1A and 1B
. 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
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
, usually enclosed in a vacuum chamber and extremely well isolated from vibrations. The cavity
116
is tuned to a reference frequency by choice of spacing between mirrors
115
and
117
and the cavity stores and re-emits a reference beam having an energy peak at the reference frequency along path
163
to splitter
114
. At the 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.
The required frequency modulation, achieved in the above realization by use of the modulator
112
, also may be achieved by other means, such as current modulation of the laser diode driver current under control of the radio frequency (rf) signal generator
142
, when a laser diode is used as the optical source
110
. For other lasers, other equivalent means of modulation also may be understood.
FIG. 1B
shows the output
125
of cavity transmission detector
122
and the error signal
145
output (showing one example, from a class of possible frequency modulation quadrature signals) by error signal detector
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
and the quality of the entire feedback system of
FIGS. 1A and 1B
.
When quoting the degree of stability of a laser, one simultaneously quotes the degree of stabilization and the time period of observation.; the two are correlated in ways that depend on each laser system. 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 can be accomplished 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, when substantial effort is made to vibrationally and thermally isolate the cavity from its surrounding environment. 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. Attempts have been made to approach absolute stability using Fabry-Perot cavities made from sapphire and kept at cryogenic temperatures to achieve a 3 kHz drift over 6 months.
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, carbon dioxide lasers, cold calcium-stabilized diode lasers, lasers stabilized to optical transitions of iodine, cesium, rubidium, mercury, and ytterbium among others. Optical frequency standards in the communications bands of 1.3 and 1.5 micron wavelength are also of interest. Among the principal advantages of existing microwave and optical standards are that the transition frequencies of an atom are unique to its type, i.e. predetermined by nature and that FM locking techniques typically select the center of those transitions with high precision. A difficulty is that when the frequencies are not exactly those of interest, they must be transferred through a potentially elaborate chain of precisely controlled optical and radio frequency (rf) synthesis, such as frequency doubling and mixing or parametric oscillation sums and differences. That complication may be simplified by use of new optical frequency combs based on femtosecond mode-locked lasers and nonlinear effects in optical fibers. 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 but weaker transitions do exist; one such transition is the
2
S
1/2
−
2
D
5/2
electric-quadrupole transition of a single trapped
199
Hg
+
ion (wavelength of 282 nm and natural linewidth of 2 Hz). The present optimum realization of the
199
Hg
+
ion standard requires operation of a Paul ion trap at liquid helium temperatures.
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 abso
Carlsten John L.
Cone Rufus L.
Sellin Peter B.
Strickland Nicholas M.
Jr. Leon Scott
McDermott & Will & Emery
The Research and Development Institute Inc.
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