Interference filter having a glass substrate

Details Interference filter having a glass substrate Interference filter having a glass substrate

2002-01-07

2004-11-30

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S078000, C428S426000, C428S064200, C428S065100

Reexamination Certificate

active

06825142

ABSTRACT:

Substrates that demand high expansion with good chemical durability are often manufactured from optical glasses. Optical glasses may be employed in various applications, such as substrates for thin-film interference filters used in fiber optic systems. Generally, these interference filters are fabricated from multiple layers of conducting and insulating materials or films that together result in a filter that passes only a narrow bandwidth of incident radiation. Such filters are described, for example, in
Optical Filter Design and Analysis—A Signal Processing Approach
by Christie K. Madsen and Jian H. Zhao published by John Wiley & Sons, 1999.
In one particular application, there is a strong demand for a glass substrate capable of being incorporated into an interference filter for wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) applications. Thin-film interference filters for WDM and DWDM applications have high requirements in terms of the narrow bandwidth of light transmittal (
Introduction to DWDM Technology
by Stamatios V. Kartalopoulos, published by IEEE Press, 2000). Such bandwidths are expressed as a width in passed frequency, typically 200 GHz, 100 GHz, 50 GHz, or less, with smaller values indicating a narrower bandwidth of transmission. For example, a 100 GHz filter within the 1.5 &mgr;m telecommunications band corresponds to a wavelength spread of 0.8 nm; and, a 50 GHz filter within the same 1.5 &mgr;m telecommunications band corresponds to a wavelength spread of 0.4 nm. These filters preferably have bandwidths of less than 200 GHz pass frequency in the 1.5 &mgr;m wavelength region. An optical designer can fabricate useful telecommunications modules using such filters. For example, an optical demultiplexer can be constructed using a multitude of such thin-film interference filters, each one of which separates out a particular wavelength of interest.
Most desirably, the substrate is characterized by high transmission in the near IR where DWDM systems operate, i.e., wavelengths at or near 1.5 &mgr;m, a refractive index value at 587.6 nm, nd, of between 1.50 and 1.70, and a high transformation temperature, T
g
, exceeding 350° C., most preferably exceeding 400° C. High transmission at 1.51 &mgr;m is characterized by a value of digital transmittance, including Fresnel reflection loss, exceeding 88%, more preferably equivalent to or exceeding 90% at 1.5 &mgr;m through a 1.0 mm thickness. Preferably, these filters have minimal wavelength drift with change in temperature. Glass substrates with high thermal expansion, CTE, and high values of Young's modulus, E, allow for decreased amounts of thermally-induced drift (d&lgr;/dT) in the transmission wavelengths of interest, e.g., 1450-1620 nm, 1480-1620 nm, and 1450-1550 nm. A particularly desirable range of thermal expansion values is from 90 to 140×10
−7
/° C., particularly 110 to 140×10
−7
/° C., over a temperature range of −30° C. to +70° C. coupled with a Young's modulus >80 GPa. More preferably, the thermal expansion should lie in the range of 100 to 130×10
−7
/° C. over the same temperature range in combination with a Young's modulus value >85 GPa.
Such narrow bandwidths are highly demanding and difficult to achieve and push the limits of available coating technology. Consequently, the substrate properties are becoming more demanding, and the advanced coating industry desires to have new substrate glasses available that offer enhanced or optimized properties for applications at less than 200 GHz bandwidth range.
Thus, a desired embodiment of the invention is a glass making available an interference filter for a fiber optic system including a substrate and a film coating the substrate. Typically, the substrate is coated with a series of layers of differing materials having properties, e.g., indices of refraction, producing interference effects achieving the desired wavelength transmission spectrum. Fiber optic systems comprise one or more light sources, fiber optic transmission components, filters and end use components, e.g., detection, amplifiers, etc. Glasses of the invention and their properties are described in the following tables:
TABLE 1
Composition (mol %) and Properties
Oxide/Property
Preferred
Most Preferred
SiO
2
35-75
40-70
GeO
2
 0-10
0-5
B
2
O
3
0-8
0-5
Al
2
O
3
0-8
0-5
Li
2
O
>0-25
>0-25
Na
2
O
 0-60
 0-35
K
2
O
0-6
0-5
MgO
 0-35
 0-25
&Sgr; BaO, SrO, CaO, ZnO, PbO
 0-10
0-5
TiO
2
0-5
0-3
La
2
O
3
 0-30
 0-12
RE
2
O
3
 0-12
 0-10
Y
2
O
3
>0-30
>0-25
As
2
O
3
  0-0.5
  0-0.3
F
0-5
0-3
Sum R
2
O
3
, R = Al, B, La and RE
 0-40
 0-40
n
d
>1.5
 1.50-1.70,
especially
1.50-1.65
T (%) at 1550 nm for 1 mm
>88
>90
CTE
−30/+70
[× 10
−7
/C.]
≧90, especially
>100, especially
≧110
>110
Tg [C.]
≧350 C.
≧400 C.
E [GPa]
>80
>85
TABLE 2
Composition (mol %) and Properties
SiO
2
40-60
GeO
2
 0-40
B
2
O
3
 0-10
Al
2
O
3
0-4
Li
2
O
> 0-26
Na
2
O
> 0-26
K
2
O
 0-15
MgO
 0-15
&Sgr; BaO, SrO, CaO, ZnO, PbO
 0-10
TiO
2
0-9
ZrO2
0-2
La
2
O
3
0-4
RE
2
O
3
0-4
Y
2
O
3
>0-5 
Sc
2
O
3
0-4
Nb
2
O
5
0-2
F
0-5
&Sgr; R
2
O
3
, R = Al, B, La and RE
 0-25
As
2
O
3
  0-0.5
Oxide/Property
Preferred
n
d
>1.5
T (%) at 1550 nm for 1 mm
>88
CTE
−30/+70
[× 10
−7
/C.]
≧90
Tg [C.]
≧350
E [Gpa]
> 80
TABLE 3
Composition (mol %) and Properties
SiO
2
45.0-58.0%
B
2
O
3
0.0-5.0%
Al
2
O
3
0.0-3.0%
Li
2
O
 6.5-16.5%
Na
2
O
 7.0-24.0%
K
2
O
 0.0-12.0%
MgO
0.0-8.0%
CaO
0.0-8.0%
SrO
0.0-8.0%
BaO
0.0-8.0%
TiO
2
 0.0-12.0%
ZrO
2
0.5-5.5%
Ta
2
O
5
0.0-1.0%
Gd
2
O
3
+ La
2
O
3
+ Y
2
O
3
2.70-3.30%
As
2
O
3
 0.0-0.15%
Oxide/Property
Preferred
Most Preferred
n
d
>1.5
1.50-1.70
T (%) at 1550 nm for 1 mm
>88
>90
CTE
−30/+70
[× 10
−7
/C.]
≧90
>100
Tg [C.]
400-485
420-480
E [Gpa]
>80
>85
TABLE 4
Composition (mol %) and Properties
SiO
2
45.0-58.0%
B
2
O
3
0.0-5.0%
Li
2
O
 6.5-16.5%
Na
2
O
 7.0-24.0%
K
2
O
 0.0-12.0%
MgO
0.0-8.0%
CaO
0.0-8.0%
SrO
0.0-8.0%
BaO
0.0-8.0%
TiO
2
 0.0-12.0%
ZrO
2
0.5-5.5%
Ta
2
O
5
0.0-1.0%
Gd
2
O
3
+ La
2
O
3
2.70-3.30%
As
2
O
3
 0.0-0.15%
Oxide/Property
Preferred
Most Preferred
n
d
>1.5
1.50-1.70
T (%) at 1550 nm for 1 mm
>88
>90
CTE
−30/+70
[× 10
−7
/C.]
≧90
>100
Tg [C.]
400-485
420-480
E [Gpa]
>80
>85
RE=rare earth ions, excluding La, that do not impart unacceptable absorption at the wavelength of interest (e.g., 1450-1550 nm, especially 1480-1620 nm), i.e., do not degrade T overall beyond the numbers given above. As a more preferred example of acceptable absorption, RE allows for an internal transmission of >0.99 for a 1 mm thickness sample, thereby allowing for an insertion loss of <0.9 dB. Nd and Ho are non-limiting examples of rare earth ions that may be used in the current application.
Without being bound by theory, it is believed that the individual components of the glasses affect certain properties. It is believed that in glasses of the present invention SiO
2
and GeO
2
, both are network formers and Y
2
O
3
and La
2
O
3
are intermediates that do participate as network formers. Na
2
O is a network modifier that typically affects index, expansion, and transformation temperature. Li
2
O is a network modifier that affects index expansion, transformation temperature, Young's modulus, and thermal conductivity. MgO is a network modifier that affects index, expansion, transformation temperature, Young's modulus, and thermal conductivity. Sc
2
O
3
and other rare earth oxides in the prescribed amounts can be directly substituted for Y
2
O
3
and La
2
O
3
. The addition of T

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