Coherent light generators – Optical fiber laser
Reexamination Certificate
1998-11-25
2001-08-14
Font, Frank G. (Department: 2877)
Coherent light generators
Optical fiber laser
C372S006000, C372S018000, C372S005000, C372S011000, C372S026000, C372S031000, C372S102000, C372S022000
Reexamination Certificate
active
06275512
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the amplification of single mode light pulses in multi-mode fiber amplifiers, and more particularly to the use of multi-mode amplifying fibers to increase peak pulse power in a mode-locked laser pulse source used for generating ultra-short optical pulses.
BACKGROUND OF THE INVENTION
Background Relating to Optical Amplifiers
Single-mode rare-earth-doped optical fiber amplifiers have been widely used for over a decade to provide diffraction-limited optical amplification of optical pulses. Because single mode fiber amplifiers generate very low noise levels, do not induce modal dispersion, and are compatible with single mode fiber optic transmission lines, they have been used almost exclusively in telecommunication applications.
The amplification of high peak-power pulses in a diffraction-limited optical beam in single-mode optical fiber amplifiers is generally limited by the small fiber core size that needs to be employed to ensure single-mode operation of the fiber. In general the onset of nonlinearities such as self-phase modulation lead to severe pulse distortions once the integral of the power level present inside the fiber with the propagation length exceeds a certain limiting value. For a constant peak power P inside the fiber, the tolerable amount of self-phase modulation &PHgr;
nl
is given by
Φ
n1
=
2
π
n
2
PL
λ
A
≤
5
,
where A is the area of the fundamental mode in the fiber, &lgr; is the operation wavelength, L is the fiber length and n
2
=3.2×10
−20
m
2
/W is the nonlinear refractive index in silica optical fibers.
As an alternative to single-mode amplifiers, amplification in multi-mode optical fibers has been considered. However, in general, amplification experiments in multi-mode optical fibers have led to non-diffraction-limited outputs as well as unacceptable pulse broadening due to modal dispersion, since the launch conditions into the multi-mode optical fiber and mode-coupling in the multi-mode fiber have not been controlled.
Amplified spontaneous emission in a multi-mode fiber has been reduced by selectively exciting active ions close to the center of the fiber core or by confining the active ions to the center of the fiber core. U.S. Pat. No. 5,187,759, hereby incorporated herein by reference. Since the overlap of the low-order modes in a multi-mode optical fiber is highest with the active ions close to the center of the fiber core, any amplified spontaneous emission will then also be predominantly generated in low-order modes of the multi-mode fiber. As a result, the total amount of amplified spontaneous emission can be reduced in the multi-mode fiber, since no amplified spontaneous emission is generated in high-order modes.
As an alternative for obtaining high-power pulses, chirped pulse amplification with chirped fiber Bragg gratings has been employed. One of the limitations of this technique is the relative complexity of the set-up.
More recently, the amplification of pulses to peak powers higher than 10 KW has been achieved in multi-mode fiber amplifiers. See U.S. Pat. No. 5,818,630, entitled Single-Mode Amplifiers and Compressors Based on Multi-Mode Fibers, assigned to the assignee of the present invention, and hereby incorporated herein by reference. As described therein, the peak power limit inherent in single-mode optical fiber amplifiers is avoided by employing the increased area occupied by the fundamental mode within multi-mode fibers. This increased area permits an increase in the energy storage potential of the optical fiber amplifier, allowing higher pulse energies before the onset of undesirable nonlinearities and gain saturation. To accomplish this, that application describes the advantages of concentration of the gain medium in the center of the multi-mode fiber so that the fundamental mode is preferentially amplified. This gain-confinement is utilized to stabilize the fundamental mode in a fiber with a large cross section by gain guiding.
Additionally, that reference describes the writing of chirped fiber Bragg gratings onto multi-mode fibers with reduced mode-coupling to increase the power limits for linear pulse compression of high-power optical pulses. In that system, double-clad multi-mode fiber amplifiers are pumped with relatively large-area high-power semiconductor lasers. Further, the fundamental mode in the multi-mode fibers is excited by employing efficient mode-filters. By further using multi-mode fibers with low mode-coupling, the propagation of the fundamental mode in multi-mode amplifiers over lengths of several meters can be ensured, allowing the amplification of high-power optical pulses in doped multi-mode fiber amplifiers with core diameters of several tens of microns, while still providing a diffraction limited output beam. That system additionally employed cladding pumping by broad area diode array lasers to conveniently excite multi-mode fiber amplifiers.
Background Relating to Modelocked Lasers
Both actively modelocked lasers and passively modelocked lasers are well known in the laser art. For example, compact modelocked lasers have been formed as ultrashort pulse sources using single-mode rare-earth-doped fibers. One particularly useful fiber pulse source is based on Kerr-type passive modelocking. Such pulse sources have been assembled using widely available standard fiber components to provide pulses at the bandwidth limit of rare-earth fiber lasers with GigaHertz repetition rates.
Semiconductor saturable absorbers have recently found applications in the field of passively modelocked, ultrashort pulse lasers. These devices are attractive since they are compact, inexpensive, and can be tailored to a wide range of laser wavelengths and pulsewidths. Quantum well and bulk semiconductor saturable absorbers have also been used to modelock color center lasers.
A saturable absorber has an intensity-dependent loss l. The single pass loss of a signal of intensity I through a saturable absorber of thickness d may be expressed as
l=
1−exp(−&agr;
d
)
in which &agr; is the intensity dependent absorption coefficient given by:
&agr;(
I
)=&agr;
0
/(1+
I/I
SAT
)
Here &agr;
0
is the small signal absorption coefficient, which depends upon the material in question. I
SAT
is the saturation intensity, which is inversely proportional to the lifetime (&tgr;
A
) of the absorbing species within the saturable absorber. Thus, saturable absorbers exhibit less loss at higher intensity.
Because the loss of a saturable absorber is intensity dependent, the pulse width of the laser pulses is shortened as they pass through the saturable absorber. How rapidly the pulse width of the laser pulses is shortened is proportional to |dq
0
/dI|, in which q
0
is the nonlinear loss:
q
0
=l
(
I
)−
l
(
I=
0)
l(I=0) is a constant (=1−exp(−
0
d)) and is known as the insertion loss. As defined herein, the nonlinear loss q
0
of a saturable absorber decreases (becomes more negative) with increasing intensity I. |dq
0
/dI| stays essentially constant until I approaches I
SAT
becoming essentially zero in the bleaching regime, i.e., when I>>I
SAT
.
For a saturable absorber to function satisfactorily as a modelocking element, it should have a lifetime (i.e., the lifetime of the upper state of the absorbing species), insertion loss l(I=0), and nonlinear loss q
0
appropriate to the laser. Ideally, the insertion loss should be low to enhance the laser's efficiency, whereas the lifetime and the nonlinear loss q
0
should permit self-starting and stable cw modelocking. The saturable absorber's characteristics, as well as laser cavity parameters such as output coupling fraction, residual loss, and lifetime of the gain medium, all play a role in the evolution of a laser from startup to modelocking.
As with single-mode fiber amplifiers, the peak-power of pulses from mode-locked single-mode lasers has
Flores Ruiz Delma R.
Font Frank G.
Imra America Inc.
Knobbe Martens Olson & Bear LLP
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