Atomic frequency standard laser pulse oscillator

Details Atomic frequency standard laser pulse oscillator Atomic frequency standard laser pulse oscillator

1999-12-14

2001-12-25

Arroyo, Teresa M. (Department: 2881)

Coherent light generators

Particular beam control device

Control of pulse characteristics

C372S026000, C372S038020, C372S038070, C372S018000, C372S009000

Reexamination Certificate

active

06333942

ABSTRACT:

BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to an atomic frequency standard laser pulse oscillator which can generate an ultra-short optical pulse train with a precise repetition frequency which is stabilized at a higher level than that of a time standard obtained by a conventional atomic oscillator.
DESCRIPTION OF THE PRIOR ART
FIG. 3
is a diagram showing the structure of a laser pulse oscillator described in Japanese Patent Application Laid-open No. 8-18139 (1996).
In
FIG. 3
, reference numeral
1
indicates an optical fiber doped with rare-earth ions (hereinafter referred to as “rare-earth ion doped optical fiber”),
2
is a pumping optical source for generating pump light for the rare earth ion doped optical fiber,
3
is an optical coupler for coupling the pump light to the rare-earth ion doped optical fiber
1
,
4
is an optical beam splitter for extracting a laser output,
5
is an optical isolator for limiting the light propagation to a single direction,
6
is an optical modulator, and
7
is an optical filter.
1
,
3
,
4
,
5
,
6
and
7
are coupled in a ring configuration to construct a ring laser cavity. The laser output split by the optical beam splitter
4
is divided again with an optical beam splitter
8
into an optical output terminal and the driver of the optical modulator
6
. The driver of the optical modulator
6
comprises a clock extraction circuit
9
, a phase shifter
10
, and a microwave electrical amplifier
11
.
The operation mechanism of this laser pulse oscillator, which has an optical pulse train with a high repetition rate, is as follows. When the pump light output from the pumping optical source
2
is coupled into the rare-earth ion doped optical fiber
1
through the optical coupler
3
, continuous wave oscillation occurs in the propagation direction of the optical isolator
5
through the bandwidth of the optical filter
7
. This laser output is extracted through the optical beam splitter
4
, and part thereof is input through the optical beam splitter
8
to the clock extraction circuit
9
comprising a photo detector, a narrow-band electrical filter and an electrical amplifier. Thus, a sinusoidal wave clock signal is extracted with this clock extraction circuit
9
, and the phase timing of the clock signal is adjusted by the phase shifter
10
. After that, the clock signal is amplified by the microwave electrical amplifier
11
, and then input into the optical modulator
6
. As a result, the light in the laser cavity is intensity-modulated at a frequency that is synchronized with the clock signal.
In general, where L is the cavity length, n the refractive index of optical fiber, and c the speed of light, amplitude modulation of a fundamental frequency fo=c/(nL) determined by L is applied to the laser cavity, and mode locking at a fundamental repetition rate (a single optical pulse in the cavity) is achieved. When a modulation frequency f is set at an integral multiple of fo such that f=qfo=qc/(nL), where q is the integer, harmonically mode-locked oscillation can be achieved at a frequency of q times the fundamental frequency. That is, equally-spaced q optical pulses are produced in the cavity simultaneously, generating an optical pulse train having a repetition frequency of qfo.
A consideration is made for a case where a clock extraction circuit
9
at 10 GHz is used in FIG.
3
. Since a clock signal in the vicinity of 10 GHz which does not coincide with an integral multiple of the fundamental frequency fo cannot generate a stable optical pulse train, it eventually disappears. In contrast, when the clock signal coincides with an integral multiple of the fundamental frequency fo, the modulation frequency and the repetition frequency of the optical pulse are completely synchronized, and therefore stable pulse oscillation is gradually enhanced. When this process is repeated, only the clock signal with the integral multiple of the fundamental frequency, which was initially noisy, remains. That is, the optical modulator
6
is driven only by a pure sinusoidal wave clock signal with no spurious longitudinal modes, and stable 10 GHz harmonic mode-locking is achieved. This prior art is referred to as harmonic and regenerative mode-locking.
In the prior art harmonically and regeneratively mode-locked laser pulse oscillator, when the cavity length fluctuates because of a temperature change in the laser cavity, the fundamental frequency fo changes with time (above patent publication). Since the modulation frequency f which is applied to the optical modulator is automatically controlled by the change in the cavity length, the repetition frequency of the generated optical pulse train fluctuates following the change in cavity length. For example, where L=200 m and f=10 GHz, when the temperature in the cavity is changed by 0.01° C., the cavity length L changes by 20 &mgr;m and the repetition frequency changes by 1 KHz. However, the jitter and low-drift characteristics of the harmonically and regeneratively mode-locked laser pulse oscillator have been well investigated, and it is known that, although it has low frequency drift, its high frequency jitter is less severe than that of ordinary electrical synthesizers and is as small as 80 fs at a repetition frequency of 10 GHz. By contrast, the jitter of an ordinary electrical synthesizer is about 200-400 fs.
To stabilize the repetition frequency of the harmonically and regeneratively mode-locked laser pulse oscillator, a phase-locked loop method (PLL method) has been proposed in which the repetition frequency of the laser is controlled by synchronizing it with an external clock signal (Japanese Patent Application Laid-open No. 10-74999 (1998)).
FIG. 4
is a diagram showing the configuration of the prior art laser pulse oscillator described in Japanese Patent Application Laid-open No. 10-74999 (1998). The components newly added to the laser pulse oscillator shown in
FIG. 3
are a synthesizer (a standard signal generator)
12
, a phase shifter
13
, a phase comparator
14
, an electrical filter
15
, a negative feedback circuit
16
, an electrical amplifier
17
, and a piezoelectric transducer (PZT)
18
.
In
FIG. 4
, the phase difference between an external signal generated from the synthesizer
12
and the clock signal of a laser pulse oscillator under a free running condition (output of the clock extraction circuit
9
) is converted into a voltage error signal by the phase comparator
14
consisting of a double balanced mixer (DBM). This error signal is negatively fed back through the electrical filter
15
, the negative feedback circuit
16
, and the electrical amplifier
17
to the piezoelectric transducer
18
in the cavity. This operation automatically controls the cavity length so that the repetition frequency of the laser pulse oscillator synchronizes with the external signal, and the repetition frequency is stably locked to the oscillation frequency of the synthesizer
12
.
However, the stability of the repetition frequency of the laser pulse oscillator shown in
FIG. 4
is limited by the stability of the externally supplied clock signal, that is, the stability of the synthesizer. An optical pulse train with an ultra-stable repetition frequency that can be used as a time standard has not heretofore been obtained. However, it is important to note that the high frequency jitter characteristic of the present laser pulse oscillator shown in
FIG. 4
is superior to that of the synthesizer. We use this excellent feature in the present invention.
Apart from the laser pulse oscillator, an atomic oscillator has been already proposed in which the oscillation frequency is locked to a resonance frequency of an atom (such as cesium (Cs) or rubidium (Rb)) (ref. Yoshimura, Koga, Oura, “Frequency and time/fundamentals of atomic clock/mechanism of atomic time”, Society of Electronic Information Communications).
FIG. 5
is a diagram showing the configuration of a conventional Cs atomic oscillator. In
FIG. 5
, reference numeral
21
indi

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