Thermally stable air-gap etalon for dense...

Details Thermally stable air-gap etalon for dense... Thermally stable air-gap etalon for dense...

1999-07-20

2001-04-10

Scott, Jr., Leon (Department: 2881)

Coherent light generators

Particular temperature control

C372S019000, C372S108000

Reexamination Certificate

active

06215802

ABSTRACT:

TECHNICAL FIELD
The present invention relates to the use and construction of etalon filters. In particular, etalons used in telecommunications data transmission. More specifically the present invention relates to etalons used in Dense Wavelength Division Multiplexing. In addition the principles of the present invention also contemplate use in telescopic filters.
BACKGROUND OF THE INVENTION
In modern telecommunication systems, it is becoming ever more important to increase the density of data transmitted over any particular transmission line. As such it is advantageous to increase the number of the effective bandwidths useable by each transmission line. The advent of the Internet in the past few years has further accelerated the race for higher data transmission density.
When the transmission line is a fiber-optic cable, one method of expanding the effective data transmission capacity is to transmit a number of closely spaced optical frequencies on each cable. Such optical frequencies are also referred to as “carrier frequencies”. By separately modulating and demodulating each such carrier frequency, the amount of information that may be carried on one fiber-optic cable can be substantially increased. This technique is called “Wavelength-Division Multiplexing” (WDM) and, when the spacing between the wavelengths gets very small, the technique is referred to as “Dense Wavelength-Division Multiplexing” (DWDM).
In a typical telecommunications application, laser diodes are used to provide optical signals which are transmitted through fiber optic cables. Presently, these signals are produced by a series of laser diodes whose output is a series of carrier wavelengths (frequencies) separated by a specified amount. For example, several diodes each producing signal at a different wavelength separated by 100 GHz (gigahertz) produce a composite signal which may be directed down a single optical fiber. These carrier frequencies are modulated and multiplexed to carry a multiplicity of signals on the same optical fiber. At the receiving end of the fiber, the carrier frequencies are demultiplexed and demodulated.
Multiplexing and demultiplexing of the carrier signals may be accomplished by various means, including optical gratings or coated optical interference filters. However, the use of such optical gratings and interference filters present certain problems which have not yet been overcome in the industry. Conventional air-spaced etalon filters have also been used to separate carrier signals as well as provide frequency standards used to monitor the lasers generating the carrier signals. However, current methods of producing precision etalons is a highly specialized “craft” more in the nature of an “art”. Furthermore, such etalons are typically manufactured in small quantities of only a few units at a time. Currently, there is no known method of mass producing etalons having sufficient optical quality for this application. As a result, etalons of this type are not suitable for applications requiring large numbers of etalons (e.g. telecommunications applications).
Related Technologies
As explained above, existing methods of carrier frequency separation suffer from a number of limitations. For example, interference filters used to separate carrier frequencies can not achieve the necessary degree of thermal stability. This difficulty is particularly apparent when such filters are used to achieve DWDM with narrow band or dense channel spacing requirements of 100 GHz (0.8 nm) or smaller. The inventor does not know of any interference filters currently capable of meeting the demanding standards set up by the International Telecommunications Union (ITU) grid.
The information contained below is a draft of the proposed ITU frequency grid. It should be noted that this grid has not been finalized:
&lgr; (in nm.)
f (in THz)
1530.33
195.900
1531.12
195.800
1531.90
195.700
1532.68
195.600
1533.47
195.500
1534.25
195.400
1535.04
195.300
1535.82
195.200
1536.61
195.100
1537.40
195.000
1538.19
194.900
1538.98
194.800
1539.77
194.700
1540.56
194.600
1541.35
194.500
1542.14
194.400
1542.94
194.300
1543.73
194.200
1544.53
194.100
1545.32
194.000
1546.12
193.900
1546.92
193.800
1547.72
193.700
1548.51
193.600
1549.32
193.500
1550.12
193.400
1550.92
193.300
1551.72
193.200
1552.52
193.100
1553.33
193.000
1554.13
192.900
1554.94
192.800
1555.75
192.700
1556.55
192.600
1557.36
192.500
1558.17
192.400
1558.98
192.300
1559.79
192.200
1560.61
192.100
1561.42
192.000
1562.23
191.900
1563.05
191.800
1563.86
191.700
1564.63
191.600
1565.50
191.500
Bragg gratings have also been used to solve the carrier frequency separation problem. Unfortunately, Bragg gratings are extremely sensitive to temperature variation and require expensive temperature control mechanisms to stabilize systems using them. Additionally, Bragg gratings require costly optical circulators or an interferometric Mach-Zehnder setup to pass the selected wavelengths.
Another method currently used to separate carrier frequencies is the arrayed waveguide grating (AWG). AWGs suffer from inferior filter passbands, and a rather severe polarization-dependent signal loss, and poor non-adjacent channel isolation as compared to other existing technologies. As a result, AWGs often require the addition of other technologies to achieve separation which extends beyond 16 channels, increasing the overall cost of such systems.
Another technique sometimes used to separate carrier frequencies is by using a Fabry-Perot etalon. Such etalons are the subject of the present invention. One major drawback to using etalons is that, until now, it has not been possible to mass produce etalons which provide the needed bandwidth separation and provide sufficient temperature stability.
Fabry-Perot Etalons
The concept of Fabry-Perot etalons is well known in the art and is discussed in a number of classic texts. For example, M. Bom and E. Wolf, “Principles of Optics” Pergamon Press (1980) incorporated herein by reference. In general, a Fabry-Perot etalon consists of two parallel surfaces separated by a gap. The two surfaces may have an optical coating applied to their surfaces or may be uncoated. The surfaces can be the opposing faces of two separate plates separated by a gap, the gap being filled with air or a vacuum. Such an etalon is referred to as an “air-spaced etalon”. An etalon may also be constructed using two parallel surfaces on opposite sides of a single solid plate. This is referred to as a “solid etalon”. Both types are used extensively in spectral analysis, laser-line narrowing, mode selection, and as integral components in the construction of ultra-narrow band optical filters, as well as many other instances where spectral selection is desired.
An air-spaced etalon can be made extremely thermally stable, whereas a solid etalon is subject to changes in its optical thickness based on changes in ambient temperature, causing etalon passband wavelengths to vary with changing temperature. In the telecommunications industry, it is desirable to have stable passbands over a widely varying conditions, including temperature. Therefore, the use of air-spaced etalons has been favored.
Historically, air-spaced etalons have been constructed using two different designs.
FIG. 1
shows an etalon
1
having two parallel optically flat surfaces (also called plates or etalon plates)
10
,
11
separated by spacers
14
,
15
, which define a gap
17
equal to the length of the spacers
14
,
15
. As the ambient temperature changes, the spacers
14
,
15
expand and contract leading to an expansion and contraction of the gap
17
, effectively changing the passband wavelength of the etalon
1
.
FIG. 2
illustrates an alternative design known as a “re-entrant” etalon
2
. Such etalons feature a third plate known as a risers
19
. Re-entrant etalons
2
are frequently used when a gap
17
of less than about 0.5 millimeters (mm) is desired. The gap
17
in re-entrant etalons is defined by the difference in length between the spacers
14
and
15
and the thickness of the riser
19
. The optical qua

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