Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
1999-12-17
2002-04-30
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
C359S260000
Reexamination Certificate
active
06379984
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to optical fiber communication systems, and more particularly, to a high-precision etalon for use in monitoring the frequency of an optical laser and method of construction thereof.
BACKGROUND OF THE INVENTION
Optical communication systems utilize optical fibers as information carrying channels. Their low attenuation properties make optical fibers a high-quality transmission medium and allow them able to achieve high data rates. Information is transmitted onto and received from an optical network via optical links which implement a transmitter, receiver, or both. An optical transmitter is implemented with a laser, which is a very high frequency optical oscillator constructed from an amplifier and an appropriate amount of positive feedback.
An etalon is the passive resonant structure often used as an optical filter to establish the wavelength of a transmitted laser beam at a predefined frequency. As known by those skilled in the art, an etalon is an optical resonator comprising two plane-parallel, flat mirrors placed a fixed distance apart. Because its intercavity spacing is fixed, the peak transmission frequencies defined by a particular etalon are unchangeable. The etalon is characterized by a series of equally spaced resonant frequencies that cause it to operate as a comb filter when used as a transmitter. Hence, the etalon is an ideal device for ensuring the proper separation of channel frequencies on a multiple-channel high-resolution optical fiber.
FIG. 1
is a schematic diagram of a conventional etalon optical filter (hereinafter “etalon”)
2
. Etalon
2
includes two plane-parallel partially transmissive feedback mirrors
4
a
and
4
b
positioned to form a cavity
6
therebetween. Mirrors
4
a
and
4
b
are formed as a pair of flat transparent substrates
12
a
and
12
b
to each of which a reflective coating
14
a
and
14
b
has been applied. It is common practice to form the substrates
12
a
,
12
b
as slightly wedge-shaped and to coat the outer faces of substrates
12
a
and
12
b
with an anti-reflective layer to prevent the substrates themselves from acting as optical resonators
Cavity
6
is typically air-filled, or may comprise a crystalline or glassy solid, insulating material, or semiconductor material When used as an optical filter, a laser beam is transmitted through one of the mirrors
4
a
,
4
b
, which reflects between the mirrors
4
a
and
4
b
in the etalon cavity
6
, resulting in constructive interference and a repetitive series of high transmission spikes. The peak transmission of light passing through the etalon
2
occurs at resonant frequencies f
m
.
The transmission characteristics of the etalon
2
are illustrated in FIG.
2
. The frequencies f
m
of maximum transmission satisfy the equation:
f
m
=
m
c
0
2
nl
cos
θ
,
where m is an integer, c
0
is the velocity of light in a vacuum, n is the refractive index of the cavity
6
, l is the spacing between the mirrors
4
a
and
4
b
, and &thgr; is the angle of refraction of the incident light, and &lgr; is the wavelength of the light between the reflectors
4
a
and
4
b
. Adjacent frequencies at which the etalon shows maximum transmission are separated by a frequency.
Δ
f
=
c
0
2
n
cos
θ
,
where &Dgr;f is called the free spectral range (FSR) of the etalon. The frequencies f
m
of maximum transmission are equally spaced. A device that has this characteristic is called a comb filter.
The free spectral range FSR is constant for a given wavelength, which is dependent upon the spacing l of the gap of cavity
6
. Techniques such as wedge tilting exist, in which the angle of the incident light is adjusted, for shifting the maximum transmission frequencies f
m
a small degree (e.g., less than 15°). Accordingly, in applications which require a precise FSR, the spacing l of the gap of the cavity
6
of the etalon
2
must be within very tight tolerance limits.
FIG. 3
illustrates a typical application of an optical filter etalon. In particular,
FIG. 3
is a block diagram of a portion of a fiber optic network that employs an etalon
2
to monitor the wavelengths of a multiple-channel optical fiber
18
. In this application, the goal is to lock the frequency of the transmitted laser beam to a frequency that overlaps one of the resonant frequency f
m
of the etalon
2
.
In operation, a laser
15
generates a laser beam
25
at a frequency f
x
. Laser beam
15
is transmitted onto optical fiber
18
a
, which passes through a coupler
5
. Coupler
5
couples most of the signal (e.g., 90%) onto optical fiber
18
b
, which couples to optical cable
18
for transmission to other destinations Coupler
5
couples the remaining portion of the laser beam signal
25
onto optic fiber
18
c
, which is received by wave blocker
16
. Wave blocker
16
comprises a lens
17
, etalon
2
, and a detector
19
. Lens
17
focuses the laser beam
25
received from optic fiber
18
c
for transmission through etalon
2
. Etalon
2
filters the received beam
25
according to its resonant frequencies f
m
. Detector
19
detects whether or not the frequency f
x
of the laser beam
25
overlaps one of the etalon's resonant frequencies f
m
. The detector output is used by a laser frequency controller
9
to adjust the output frequency f
x
of the laser beam
25
.
The etalon filters are manufactured such that the resonant frequencies f
m
overlap the standard channels set up and regulated by the Federal Communication Commission (FCC) or International Telecommunications Unions (ITU) for fiber optic transmission. The standard channel definition provides for the communication of data across optical fibers between local, national, and even international data exchanges.
Etalons must be manufactured with a cavity gap
6
spacing l of a precise length to allow the maximum transmission frequencies f
m
(i.e., the transmission peaks in
FIG. 2
) to overlap the defined standardized channel. Although the exact values of the maximum transmission frequencies f
m
can be changed slightly via etalon wedge tilting techniques, if the spacing &Dgr;f of the transmission peaks is not exact, it is impossible to line up all the peaks to overlap with the defined channels even with tilting.
Recently, a strong commercial interest has centered around the use of temperature stable etalons for telecommunications applications in the 1.55 um wavelength range. To achieve the high level of required temperature stability, these etalons are typically manufactured using mirrors separated with a fixed air gap whose length is determined by a precision polished spacer made from a low thermal expansion material such as ULE or Zerodur. For the intended applications, the etalons must also have very tight control over the effective spacing l of the mirrors. This requires that the polishing process in which the spacers are fabricated be controlled precisely in order to hold the finished length of the spacers to within typically a few fractions of a micron or less of the desired value l. In a volume manufacturing process this requirement poses serious difficulties in terms of both metrology and process control, causing the manufacturing process to be very expensive. Specifically, because of the difficulty in controlling the polishing process to the degree of accuracy required in these applications (i.e., to within a fraction of a micron), the current wafer polishing technique cannot meet the demands for manufacture of high-precision etalons. Accordingly, a need exists for an alternative high-volume method for manufacturing high-precision etalons.
SUMMARY OF THE INVENTION
The present invention is a novel high-precision manufacturing process that greatly facilitates high-volume economic manufacture of air spaced etalons in which the mirror spacing must be controlled within a fraction of a micron. The invention utilizes coating deposition processes, which are fundamentally easier t
Lalezari Ramin
Sandberg Jon C.
Costa Jessica
Research Electro-Optics, Inc.
The Law Offices of Jessica Costa, P.C.
Thompson Craig
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