Temperature stabilization of optical waveguides

Details Temperature stabilization of optical waveguides Temperature stabilization of optical waveguides

2000-05-15

2002-09-10

Scott, Jr., Leon (Department: 2828)

Coherent light generators

Optical fiber laser

C372S064000, C372S096000, C372S034000, C372S102000, C385S047000, C385S013000

Reexamination Certificate

active

06449293

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a method of temperature stabilizing an optical waveguide having a positive thermal optical path length expansion, in particular an optical fiber distributed feed back laser or a distributed Bragg reflector optical fiber laser.
The invention further relates to packaging of fiber lasers to reduce environmental influences, specifically such influences that act to reduce their performance. More specifically yet it relates to packaging techniques for fiber lasers that act to reduce frequency jitter and thus serve to create ultra narrow linewidth fiber lasers.
1. The Technical Field
It is well known in the field of optics that the performance of optical components depends on temperature via induced change in the optical path length. This dependence is due to a change of the refractive index (thermo-optic effect) and strain with temperature. Typically the thermo-optic effect yields the dominant contribution, and for most optical materials the thermo-optic coefficient is positive, i.e. the refractive index increases with increasing temperature. In silica this increase is of the order of +11·10
−6
/° C. For components based on UV-written Bragg gratings in fibers or planar waveguides this results in a temperature drift of the center wavelength of approximately 0.01 nm/° C. Although this figure is approximately 10 times better than what can be obtained in semiconductor based optical components it is still too high for a range of important applications. A notable example is found in optical communication systems based on dense wavelength division multiplexing where the channel spacing may be e.g. 100 GHz/0.8 nm and system administration requires a wavelength drift no higher than 0.001 nm/° C., i.e. 10 times lower than the intrinsic value for UV-written Bragg gratings in fibers or planar waveguides. It is thus necessary to stabilize the wavelength.
Various methods of stabilizing the wavelength have been suggested in the art. In one method the temperature of the device is stabilized actively, e.g. by measuring the device temperature and controlling it through a suitable feedback. The disadvantage of this method is that energy is consumed which will dissipate to the rest of the system.
In other methods the thermo-optic coefficient is manipulated to balance the thermal expansion, or vice versa.
Generally, the temperature dependency of the center wavelength &lgr; of a Bragg grating in an optical fiber on temperature T is given by the following equation (1):
1
λ
·
ⅆ
λ
ⅆ
T
=
1
n
·
∂
n
∂
T
+
α
+
1
n
·
∂
n
∂
ϵ
·
∂
ϵ
∂
T
+
1
Λ
·
∂
Λ
∂
ϵ
·
∂
ϵ
∂
T
(
1
)
where n, &agr; and &egr; are the values for the refractive index, the thermal expansion and the strain. &Lgr; is the Bragg grating period. The 1
st
term including the thermo-optic coefficient
∂
n
∂
T
represents the change in refractive index with temperature, the 2
nd
term represents the thermal expansion coefficient of the optical fiber, the 3
rd
term including the elasto-optic coefficient
∂
n
∂
ϵ
represents the change in refractive index with strain, and the last term represents the change in the Bragg grating period with strain.
From this equation the following methods for temperature stabilization can be suggested:
The thermo-optic coefficient is changed to cancel out the contributions from thermal expansion and strain. In most fiber optical materials these two effects act together to increase the center wavelength with temperature. However, by tailoring the optical material to provide a negative thermo-optic coefficient, the positive contribution from the remaining terms is balanced to provide a stable center wavelength. The disadvantage of this method is that it is not easy to produce an optical material that provides a negative thermo-optic coefficient while maintaining other properties of the material.
Alternatively, the optical fiber can be mounted on a substrate under tension in such a way that its effective thermal expansion becomes negative to compensate the normally positive contribution from the thermo-optic and photo-elastic coefficients. When the optical fiber is mounted under tension the equation (1) reduces to equation (2):
1
λ
·
ⅆ
λ
ⅆ
T
=
1
n
·
∂
n
∂
T
+
α
s
-
1
n
·
∂
n
∂
ϵ
·
α
f
(
2
)
where &agr;
s
and &agr;
t
are the thermal expansion coefficients of the substrate and the optical fiber, respectively. The thermal expansion coefficient of the substrate can be made negative by two methods.
In a method, the substrate can be composed of two materials of different length and having different positive thermal expansion coefficients. The shortest piece of material is made from the material with the highest positive thermal expansion coefficient, and the longest piece is made from the material with the lowest positive thermal expansion coefficient. By fixing one end of the short piece to one end of the long piece, the other ends of the two pieces will approach each other as the temperature is increased. This presumes that the lengths and material parameters are balanced correctly. When an optical fiber is mounted under tension between these ends its effective thermal expansion becomes negative. A disadvantage of this method is that careful adjustments of the lengths and thermal expansions of the two pieces are required in order to ensure that the negative effective thermal expansion compensates the positive thermo-optic coefficient.
In another method, the substrate consists of a single material with an intrinsic negative thermal expansion coefficient. An optical fiber is mounted under tension on the substrate. By selecting and/or designing a substrate material with a suitable value of the negative thermal expansion coefficient, the effective negative thermal expansion compensates the positive contribution from the thermo-optic and photo-elastic coefficients of the optical fiber. This method has the advantage that once the correct material composition has been provided no further adjustments are required in order to achieve a stable center wavelength. Thus, this method has the advantage of simplicity in the mounting process; the exact length of the fiber is not important. Furthermore, depending on the substrate material the mount can be made considerably more robust.
Other than a change in center wavelength, temperature variatons also influence the spectral linewidth of optical waveguide lasers, e.g. optical fiber lasers. The spectral linewidth of lasers, including single frequency rare earth doped fiber lasers, is ultimately determined by optical spontaneous emission noise, corresponding to the Shawlow-Townes limit. For rare earth doped fiber lasers this lies in the Hz to sub-Hz region. In practical implementations, however, environmental effects will affect the cavity stability and lead to linewidths well above the Shawlow-Townes limit. Thus, although long term drift in temperature can be compensated by specialised packaging techniques such as those described above, small and rapid temperature fluctuations cause jitter in the center frequency. The frequency shift due to the thermo-optic effect is approximately 10-5° C.
−1
·&ngr;·&Dgr;T Hz, where &ngr; is the optical frequency and AT is the temperature change. As an example, if the frequency stability is required to be better than 1 MHz at 1550 nm, then the temperature fluctuations must be lower than 10
−3
° C. This way temperature fluctuations in the environment result in an increase in the effective linewidth. Another important contribution to jitter and linewidth increase comes from acoustic vibrations which affect the cavity via the elasto-optic effect. To stabilise the laser frequency and reduce it

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