Coherent light generators – Particular resonant cavity – Folded cavity
Reexamination Certificate
2001-03-15
2004-05-18
Lee, Eddie (Department: 2815)
Coherent light generators
Particular resonant cavity
Folded cavity
C372S011000, C372S026000
Reexamination Certificate
active
06738408
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a mode-locked laser apparatus that generates an ultra-high-speed light pulse train with a stable repetition rate such as that required for high-capacity communication systems and the like, and particularly to a mode-locked laser apparatus that uses the chromatic dispersion characteristics of the optical path to generate a feedback signal and adjusts the length of the optical path using this signal.
2. Description of the Prior Art
Pulse generators having a high repetition rate and that are optical sources with a uniform repetition rate that can be synchronized to an external clock signal are important in the field of optical communication. Recently, by adapting mode-locking techniques to fiber lasers, research into generating such high-repetition pulse trains has become active.
A known method of generating pulses by mode locking is the method of installing an intensity modulator or phase modulator in a ring laser oscillator and modulating the intensity or phase of the light passing through the modulator. At this time, the modulation frequency f
m
required to achieve the optimal mode locking can be expressed by the following Equation 1.
f
m
=
N
·
(
c
n
L
)
=
N
·
f
r
(
1
)
Here, c is the speed of light, n is the index of refraction of the optical fiber, L is the length of the optical path of the oscillator, f
r
(=c
L) is the fundamental repetition rate of the laser, and N is a positive integer. When f
m
and f
r
have the aforementioned relationship, a periodic light pulse is generated from the laser and the repetition rate of the pulse becomes the same as the modulation frequency f
m
. While one pulse is present within the oscillator in the case of N=1, N pulses are present at equal intervals in the case of N>1. Typically, f
r
is between several hundred kHz and several dozen MHz. For this reason, in order to generate a pulse train with a repetition rate in the GHz band required for optical communications, mode locking is performed by modulation in the state N>>1. Mode locking in the case of N>1 in this manner is typically called harmonic mode locking.
In order to increase the repetition rate of mode-locked pulses, from the above explanation it can be seen that it is sufficient to increase the modulation frequency. However, the modulation frequency has an upper limit given by the bandwidth of the modulator or the radiofrequency (RF) oscillator that generates the modulation signal. In passing, optical modulators with a frequency bandwidth of 40 GHz have recently become commercially available and there are reports of them being used to generate 40-GHz mode-locked pulses by mode locking.
In addition, higher-order mode locking methods that exceed the bandwidth of the modulator and RF oscillator and the like in order to increase the repetition rate of the optical pulses have been proposed in Reference 1 (K. S. Abedin, N. Onodera and M. Hyodo, “Repetition-rate multiplication in actively mode-locked fiber lasers by higher-order FM mode locking using a high-finesse Fabry-Perot filter,” Applied Physics Letters, Vol. 73, No. 10, pp. 1311-1313, 1998), Reference 2 (K. S. Abedin, N. Onodera and M. Hyodo, “Overcoming the repetition-rate-multiplication imposed by free-spectral-range of the Fabry-Perot filter used in higher-order FM mode-locked lasers,” Electronics Letters, Vol. 34, No. 23, pp. 2264-2265) and Reference 3 (K. S. Abedin et al., “Generation of a 64-GHz, 3.3-ps transform-limited pulse train from a fiber laser employing higher-order frequency-modulated mode locking,” Optics Letters, Vol. 24, No. 22, pp. 1564-1566 (1999).) and the like. By means of any one of the above methods, a pulse train with a repetition rate of an integral multiple of the modulation frequency can be generated from a mode-locked laser, so the realization of a higher-order mode-locked laser pulse generator that exceeds the bandwidth limit due to the modulator as above became possible.
FIG. 1
shows one example of such a higher-order mode-locked laser pulse generator. The apparatus shown in
FIG. 1
consists primarily of an optical fiber amplifier doped with rare-earth elements (hereinafter referred to as a “rare-earth-doped fiber amplifier”)
101
, optical filter
102
, polarization controller
103
, optical splitter
104
, optical modulator
105
, optical isolator
106
, Fabry-Perot filter
107
, electrical (RF) oscillator
108
, amplifier
109
and temperature controller
110
.
The rare-earth-doped fiber amplifier
101
consists mainly of an optical fiber doped with a rare-earth element, a pump source which excites the fiber, an optical coupler and an optical isolator. This amplifier is connected in the form of a loop via the optical filter, polarization controller, optical modulator, optical isolator and Fabry-Perot filter, thereby forming a laser resonator
100
.
The aforementioned higher-order mode-locked laser is characterized in that, in contrast to an ordinary mode-locked laser, a Fabry-Perot filter is installed within the resonator, and the modulation frequencies and characteristics of the Fabry-Perot filter or particularly the free spectral range (FSR) are set such that the excitation modes of the optical spectrum overlap with specific ones of the periodic pass spectra of the filter. More specifically, the conditions for performing higher-order (K-order) mode locking can be expressed by the following Equation 2.
K·f
m
=Q·FSR=f
o
(2)
Here, Q and K are positive integers with a mutually prime relationship with respect to each other and thus have no common prime factors. For example, Q=1, K=4; or Q=2, K=5. In Equation 2, f
m
is the modulation frequency and f
o
is the pulse repetition rate.
Also in order to perform higher-order mode locking, as in Equation 1, the modulation frequency must be selected as an integral multiple of the fundamental repetition rate f
r
of the resonator. Moreover, if the relationship between the FSR of the Fabry-Perot filter and the modulation frequency is as in Equation 2, a mode-locked pulse train is generated at a repetition rate K times the modulation frequency f
m
(f
o
=K·f
m
=Q·FSR).
By using a Fabry-Perot filter with an FSR having a relationship with the modulation frequency such as that in Equation 2, in contrast to the case of ordinary mode locking wherein mode locking is applied by means of first-order modulation sidebands of modulation, in a higher-order mode-locked laser, K-order modulation sidebands are involved in mode locking. As a result, the repetition rate of pulses can be made to be K times the modulation frequency. Reference 3 above reports a technique whereby phase modulation is performed at 16 GHz and a Fabry-Perot filter with an FSR of 64 GHz is used (Q=1, K=4) to generate a pulse train with a repetition rate of 64 GHz. As another example, Reference 2 above reports an example wherein phase modulation is performed at a frequency of 5.79 GHz and a Fabry-Perot filter with an FSR of 3.48 GHz is used to generate a pulse train with a repetition rate of 17.4 GHz. In this case, Q=5 and K=3.
As described above, by performing higher-order mode locking, it is possible to generate pulse trains with a high repetition rate that was not possible with ordinary mode locking. For example, if fourth-order mode locking is performed using a 40-GHz phase modulator which has the broadest bandwidth commercially available and a Fabry-Perot filter with an FSR of 160 GHz, it is expected that a pulse train can be generated with a repetition rate of 160 GHz.
However, with a conventional higher-mode mode-locked laser pulse generator as described above, there are problems in that when pulses are generated over a long period of time, the length of the optical path of the resonator changes due to expansion or changes in the optical characteristics due to changes in the temperature of the constituent members, or when used in a vibrating environment,
Communications Research Laboratory, Independent Administrative I
Lee Eddie
Nguyen Joseph
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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