Laser wavelength stabilization

Details Laser wavelength stabilization Laser wavelength stabilization

2001-06-14

2003-06-17

Paladini, Albert W. (Department: 2827)

Coherent light generators

Particular beam control device

Optical output stabilization

C372S029011, C372S029023, C372S032000, C372S034000, C372S038010, C372S038020

Reexamination Certificate

active

06580734

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to lasers, and specifically to stabilization of lasers operating in a single mode.
BACKGROUND OF THE INVENTION
FIG. 1
is a schematic diagram showing operation of a lasing system
18
, as is known in the art. System
18
comprises two mirrors
20
and
22
separated by a distance L. In order for system
18
to laser i.e., to resonate, at a wavelength &lgr;, a medium
24
between mirrors
20
and
22
must provide gain, and an effective optical path length L
eff
between the mirrors must be an integral number of half-wavelengths. Quantitatively,
L
eff
=nL
  (1a)
so that
m·&lgr;/
2=
nL
  (1b)
or
f=m·c
/(2
nL
)  (1c)
wherein m is a positive integer, n is a refractive index of medium
24
, f is the frequency corresponding to the wavelength &lgr;, and c is the speed of light.
From equation (1c), a separation &Dgr;f of lasing frequencies is given by
&Dgr;
f=c
/(2
nL)
  (2)
Each such lasing frequency corresponds to a longitudinal cavity mode. Since f=c/&lgr;, &Dgr;f≅&lgr;−c·&Dgr;&lgr;/&lgr;
2
so that equation (2) can be rewritten to give a separation &Dgr;&lgr; of lasing wavelengths:
&Dgr;&lgr;≅&lgr;
2
/(2
nL
)  (3)
FIG. 2
is a graph of intensity I vs. wavelength &lgr; illustrating cavity modes for system
18
, as is known in the art. A curve
30
represents an overall gain of medium
74
in system
18
. Peaks
32
A and
32
B, with separation &Dgr;&lgr;, show the cavity modes present in system
18
, each node corresponding to a different value of m. As is evident from
FIG. 2
, there are many possible cavity modes for system
1
X.
Optical communications within fiber optic links require that the laser carrier have as small a frequency spread as possible, particularly when multiple wavelengths are to be multiplexed on a single fiber. Thus, for efficient communication only one cavity mode should be used, and optimally the frequency spread within the mode should be minimized. Typically, methods for stabilizing the frequency of the laser include utilizing distributed feedback (DFB) lasers and/or distributed Bragg reflectors (DBR). DFB lasers have a frequency-selection grating built into the laser chip, the grating being physically congruent with the gain medium. The grating in a DBR laser is external to the gain medium. The gratings in DFB and DBR lasers are part of the semiconductor material, which is unstable, DFB and DBR lasers were therefore typically externally stabilized utilizing an external wavelength reference in order to achieve good stability.
FIG. 3
shows the effect of adding a tuning element such as a fiber grating to system
18
, as is known in the art. A curve
34
shows the resonance curve of the fiber grating, which has a bandwidth &Dgr;&lgr;
G
of the same order as &Dgr;&lgr;, the separation between the longitudinal cavity modes. If the grating is optically coupled to system
18
, then mode
32
A is present, and other modes such as mode
32
B, are suppressed.
FIG. 4
is a schematic diagram showing a gain medium
38
coupled to a fiber grating
50
, as is known in the art. Gain medium
38
is formed from a semiconducting gain element
44
having a laser gain region
42
. Light from region
42
exits from a facet
56
of region
42
to a medium
46
, and traverses medium
46
so that a lens
48
collects the light into a fiber optic
52
. Fiber grating
50
is mounted in fiber optic
52
, which grating reflects light corresponding to curve
34
of
FIG. 3
back to region
42
. The mirrors of the laser cavity comprise a rear mirror which in this example is a back facet
57
of the semiconductor gain element, and an output coupling mirror which in this example is the fiber grating. The rear and output coupling mirrors could also be reversed. In the reversed configuration the rear mirror would be the fiber grating and the output coupling mirror would be back facet
57
of the semiconductor gain element. In the reversed configuration the detector would preferably be positioned behind the fiber grating. It is desirable to eliminate parasitic reflections due to surfaces and interfaces internal to the cavity. To eliminate parasitic reflection from the facet of the interfaces internal to the cavity. To eliminate parasitic reflection from the facet of the semiconductor closest to the fiber grating, in this case facet
56
, that facet is usually anti reflection coated. It is also useful to anti reflection coat a tip
49
of the fiber closest to the semiconductor gain element to again reduce parasitic reflections. Preferably, grating
50
is written directly at the end of the fiber optic facing the laser. Alternatively, a length L
f
of a fiber
63
is interposed between lens
48
and fiber optic grating
50
. Thus region
42
, medium
46
, fiber optic
63
and grating
50
form a resonant system
60
corresponding to region
24
of FIG.
1
. This architecture is generally known in the art as an external cavity laser or more specifically as a fiber grating laser (FGL). System
60
has an effective optical path length L
eff
given by:
L
eff
=n
1
−L
1
+n
0
·L
0
+n
f
·L
f
+n
g
·L
gef
  (4)
wherein n
1
is a refractive index of region
42
;
L
1
is a length of region
42
;
n
0
is a refractive index of medium
46
;
L
0
is a length of medium
46
;
n
f
is a refractive index of fiber
63
;
L
f
is the length of fiber
63
.
n
g
is a refractive index of grating
50
; and
L
gef
is an effective length of grating
50
.
Replacing the optical path length nL of equation (1b) by that given by equation (4) leads to the following equation giving cavity modes for the system of FIG.
3
:
m &lgr;/
2=(
n
1
·L
1
+n
0
·L
0
+n
f
L
f
+n
g
·L
gef
)  (5)
In constructing system
60
, it is necessary to adjust and maintain the positions of curve
32
A and
34
to have their peaks at the same wavelength. Changes in temperature and/or changes in injection current into region
42
and/or mechanical changes affect one or more parameters of the optical path length given by equation (4). Such changes can thus cause mode hopping, which refers to the phenomena whereby mode
32
A shifts underneath resonance curve
34
of the fiber grating. When that shift is large enough, an adjacent mode will at some point experience a larger gain and start to lase. These mode hops occur underneath the resonance curve of the fiber grating (curve
34
in
FIG. 3
) resulting in wavelength shifts and intensity noise when the mode hops. For example, referring to
FIG. 3
, mode
32
A.
U.S. Pat. No. 4,786,132 to Gordon, whose disclosure is incorporated herein by reference, describes a semiconductor laser diode coupled to a single mode optical fiber. The fiber comprises a built-in Bragg reflector grating which reflects of the order of 50% of the light from the laser back to the laser. The reflected light provides feedback to the laser so that the laser produces a single frequency output.
U.S. Pat. No. 5,077,816 to Glomb et al., whose disclosure is incorporated herein by reference, describes a narrowband laser source, a portion of the light from which is supplied to a resonant grating region in a fiberoptic, external to the laser. The current through the laser is dithered, causing the frequency of the laser to dither. The correspond in dithered light intensity transmitted by the grating is used in order to adjust the current through the laser so as to maintain the frequency of the laser at the resonant frequency of the grating.
U.S. Pat. No. 5,706,301 to Lagerstrom, whose disclosure is incorporated herein by reference, shows a laser control system which uses a fiber optic grating as a resonant control element. A difference in light intensity between laser light passing through the grating, and light which does not pass through the grating is measured, and the difference is used in order to vary the temperature of a laser generating the light, so as to maintain the frequency of the laser at the resonant freq

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