U.S. patent application number 10/328711 was filed with the patent office on 2004-05-13 for double etalon optical wavelength reference device.
Invention is credited to McCallion, Kevin, McDaniel, Don, Tayebati, Parviz, Watterson, Reich.
Application Number | 20040091002 10/328711 |
Document ID | / |
Family ID | 22524508 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040091002 |
Kind Code |
A1 |
Watterson, Reich ; et
al. |
May 13, 2004 |
Double etalon optical wavelength reference device
Abstract
A compact wavelength monitoring and control assembly for a
narrow band (i.e., laser) source is provided, comprising two narrow
bandpass, wavelength selective transmission filter elements of
Fabry-Perot structure through which two separate collimated beams
from a laser source are directed onto two photodetectors. The
spacing of the multiple transmission maxima for one etalon is
chosen to match that of the desired set of frequencies to be used
for locking purposes. The spacing of the transmission maxima for
the second etalon is used, in combination with a dielectric filter,
to generate a wavelength fiducial to denote an absolute frequency.
The spacing of the second etalon is chosen to be much wider than
the frequency grid etalon. A control circuit processes the
simultaneously acquired signals from the two detectors as the laser
wavelength is varied. The device functions as an optical wavelength
discriminator in which the detectors convert optical energy to
current (or voltage) for a feedback loop for controlling the laser
source. Any one of a large number of discrete, predetermined
wavelengths may be chosen for locking using the same device. The
system is compact and may be packaged within the same temperature
controlled laser assembly for maximum performance and minimum
circuit board space requirements.
Inventors: |
Watterson, Reich;
(Lexington, MA) ; Tayebati, Parviz; (Boston,
MA) ; McDaniel, Don; (North Andover, MA) ;
McCallion, Kevin; (Boston, MA) |
Correspondence
Address: |
LAHIVE & COCKFIELD, LLP.
28 STATE STREET
BOSTON
MA
02109
US
|
Family ID: |
22524508 |
Appl. No.: |
10/328711 |
Filed: |
December 23, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10328711 |
Dec 23, 2002 |
|
|
|
09636807 |
Aug 10, 2000 |
|
|
|
6498800 |
|
|
|
|
60148148 |
Aug 10, 1999 |
|
|
|
Current U.S.
Class: |
372/20 |
Current CPC
Class: |
H01S 5/0078 20130101;
G01J 2009/0257 20130101; H01S 5/0687 20130101; H01S 5/18388
20130101; H01S 5/18366 20130101; G01J 9/0246 20130101 |
Class at
Publication: |
372/020 |
International
Class: |
H01S 003/10 |
Claims
What is claimed is:
1. A wavelength reference apparatus for use in calibrating a
tunable Fabry-Perot filter or a tunable VCSEL to a precise,
absolute frequency on a target frequency grid, the wavelength
reference apparatus comprising: a first etalon, wherein the first
etalon is chosen so as to have its transmission peaks spaced at the
target frequency grid; a first detector for detecting the
transmission peaks established by the first etalon; a dielectric
filter and a second etalon, wherein the dielectric filter is chosen
so as to have its transmission peak centered on a peak in the
target frequency grid and the second etalon is chosen so as to have
its transmission peaks spaced significantly further apart than the
target frequency grid; and a second detector for detecting a
transmission peak established by the dielectric filter in series
with the second etalon; whereby when monotonic light is swept
through the apparatus, the transmission peak established by the
dielectric filter and the second etalon will identify a specific
frequency on the target-frequency grid.
2. A wavelength-locking apparatus for use in tuning a tunable
Fabry-Perot filter or a tunable VCSEL to a precise, absolute
frequency on a target frequency grid, the wavelength locking
apparatus comprising: a first etalon, wherein the first etalon is
chosen so as to have its transmission peaks spaced at the target
frequency grid; a first detector for detecting the transmission
peaks established by the first etalon; a dielectric filter and a
second etalon, wherein the dielectric filter is chosen so as to
have its transmission peak centered on a peak in the target
frequency grid and the second etalon is chosen so as to have its
transmission peaks spaced significantly further apart than the
target frequency grid; a second detector for detecting a
transmission peak established by the dielectric filter in series
with the second etalon; whereby when monotonic light is swept
through the apparatus, the transmission peak established by the
dielectric filter and the second etalon will identify a specific
frequency on the target frequency grid; and a controller for tuning
the wavelength of the device by monitoring the transmission peaks
of the first etalon.
3. A method for tuning a tunable Fabry-Perot filter or a tunable
VCSEL, comprising the steps of: (1) simultaneously sweeping the
wavelength of light output by the device in a monotonic manner
through (1) a first etalon so as to generate an inline comb of
optical transmission peaks, the first etalon being chosen so as to
have its transmission peaks spaced at a desired target frequency
grid, and (2) a dielectric filter and a second etalon, where the
dielectric filter is chosen so as to have its transmission peak
centered on a peak in the target frequency grid and the second
etalon is chosen so as to have its transmission peaks spaced
significantly further apart than the target frequency grid; (2)
identifying the frequency of the transmission peak of the
dielectric filter and the second etalon, and a corresponding one of
the transmission peaks of the first etalon; and (3) monitoring the
output of the first-etalon as the device is tuned so as to tune the
device to a desired frequency.
Description
REFERENCE TO PENDING PRIOR PATENT APPLICATION
[0001] This patent application claims benefit of pending prior U.S.
Provisional Patent Application Serial No. 60/148,148, filed Aug.
10, 1999 by Parviz Tayebati et al. for DOUBLE ETALON OPTICAL
WAVELENGTH REFERENCE DEVICE, which patent application is hereby
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates to photonic devices in general, and
more particularly to tunable filters and tunable lasers.
BACKGROUND OF THE INVENTION
[0003] In pending prior U.S. patent application Ser. No.
09/105,399, filed Jun. 26, 1998 by Parviz Tayebati et al. for
MICROELECTROMECHANICALLY TUNABLE, CONFOCAL, VERTICAL CAVITY SURFACE
EMITTING LASER AND FABRY-PEROT FILTER, and in pending prior U.S.
patent application Ser. No. 09/543,318, filed Apr. 5, 2000 by
Peidong Wang et al. for SINGLE MODE OPERATION OF MICROMECHANICALLY
TUNABLE, HALF-SYMMETRIC, VERTICAL CAVITY SURFACE EMITTING LASERS,
which patent applications are hereby incorporated herein by
reference, there are disclosed tunable Fabry-Perot filters and
tunable vertical cavity surface emitting lasers (VCSEL's).
[0004] More particularly, and looking now at FIG. 1, there is shown
a tunable Fabry-Perot filter 5 formed in accordance with the
aforementioned U.S. patent applications Ser. Nos. 09/105,399 and
09/543,318. Filter 5 generally comprises a substrate 10, a bottom
mirror 20 mounted to the top of substrate 10, a bottom electrode 15
mounted to the top of bottom mirror 20, a thin support 25 atop
bottom electrode 15, a top electrode 30 fixed to the underside of
thin support 25, a reinforcer 35 fixed to the outside perimeter of
thin support 25, and a confocal top mirror 40 set atop thin support
25, with an air cavity 45 being formed between bottom mirror 20 and
top mirror 40.
[0005] As a result of this construction, a Fabry-Perot filter is
effectively created between top mirror 40 and bottom mirror 20.
Furthermore, by applying an appropriate voltage across top
electrode 30 and bottom electrode 15, the position of top mirror 40
can be changed relative to bottom mirror 20, whereby to change the
length of the Fabry-Perot cavity, and hence tune Fabry-Perot filter
5.
[0006] Correspondingly, and looking next at FIG. 2, a tunable
vertical cavity surface emitting laser (VCSEL) 50 can be
constructed by positioning a gain medium (or "active region") 55
between bottom mirror 20 and bottom electrode 15. As a result, when
gain medium 55 is appropriately stimulated, e.g., by optical
pumping, lasing can be established within air cavity 45, between
top mirror 40 and bottom mirror 20. Furthermore, by applying an
appropriate voltage across top electrode 30 and bottom electrode
15, the position of top mirror 40 can be changed relative to bottom
mirror 20, whereby to change the length of the laser's resonant
cavity, and hence tune VCSEL 50.
[0007] Tunable Fabry-Perot filters and tunable VCSEL's of the type
disclosed in the aforementioned U.S. patent applications Ser. Nos.
09/105,399 and 09/543,318 are highly advantageous since they can be
quickly and easily tuned by simply changing the voltage applied
across the top electrode and the bottom electrode.
[0008] However, it has been found that tunable Fabry-Perot filters
and tunable VCSEL's of the type disclosed in U.S. patent
applications Ser. Nos. 09/105,399 and 09/543,318 have performance
characteristics which can vary slightly from unit to unit. In
addition, it has also been found that the performance
characteristics of any given unit can vary slightly in accordance
with its age, temperature, etc. Accordingly, it is generally not
possible to precisely predict in advance the exact voltage which
must be applied to a particular device in order to tune that device
to a specific frequency. This can present an issue in some
applications, particularly telecommunications applications, where
the devices may need to be tuned to precise, absolute
wavelengths.
OBJECTS OF THE INVENTION
[0009] As a result, one object of the present invention is to
provide a novel wavelength reference apparatus for calibrating a
tunable Fabry-Perot filter and/or a tunable VSCEL, whereby the
device may be tuned to a precise, absolute wavelength.
[0010] Another object of the present invention is to provide a
novel wavelength-locking apparatus for tuning a tunable Fabry-Perot
filter and/or a tunable VCSEL to a precise, absolute wavelength,
and for thereafter keeping that device tuned to that
wavelength.
[0011] Still another object of the present invention is to provide
a novel method for calibrating a tunable Fabry-Perot filter and/or
a tunable VSCEL, whereby the device may be tuned to a precise,
absolute wavelength.
[0012] Yet another object of the present invention is to provide a
novel method for wavelength-locking a tunable Fabry-Perot filter
and/or a tunable VCSEL, whereby to tune the device to a precise,
absolute wavelength, and for thereafter keeping that device tuned
to that wavelength.
SUMMARY OF THE INVENTION
[0013] These and other objects are addressed by the present
invention.
[0014] In one form of the invention, there is provided a wavelength
reference apparatus for use in calibrating a tunable Fabry-Perot
filter or a tunable VCSEL to a precise, absolute frequency on a
target frequency grid, the wavelength reference apparatus
comprising: a first etalon, wherein the first, etalon is chosen so
as to have its transmission peaks spaced at the target frequency
grid; a first detector for detecting the transmission peaks
established by the first etalon; a dielectric filter and a second
etalon, wherein the dielectric filter is chosen so as to have its
transmission peak centered on a peak in the target frequency grid
and the second etalon is chosen so as to have its transmission
peaks spaced significantly further apart than the target frequency
grid; and a second detector for detecting a transmission peak
established by the dielectric filter in series with the second
etalon; whereby when monotonic light is swept through the
apparatus, the transmission peak established by the dielectric
filter and the second etalon will identify a specific frequency on
the target frequency grid.
[0015] In another form of the invention, there is provided a
wavelength-locking apparatus for use in tuning a tunable
Fabry-Perot filter or a tunable VCSEL to a precise, absolute
frequency on a target frequency grid, the wavelength locking
apparatus comprising: a first etalon, wherein the first etalon is
chosen so as to have its transmission peaks spaced at the target
frequency grid; a first detector for detecting the transmission
peaks established by the first etalon; a dielectric filter and a
second etalon, wherein the dielectric filter is chosen so as to
have its transmission peak centered on a peak in the target
frequency grid and the second etalon is chosen so as to have its
transmission peaks spaced significantly further apart than the
target frequency grid; a second detector for detecting a
transmission peak established by the dielectric filter in series
with the second etalon; whereby when monotonic light is swept
through the apparatus, the transmission peak established by the
dielectric filter and the second etalon will identify a specific
frequency on the target frequency grid; and a controller for tuning
the wavelength of the device by monitoring the transmission peaks
of the first etalon.
[0016] In still another form of the invention, there is provided a
method for tuning a tunable Fabry-Perot filter or a tunable VCSEL,
comprising the steps of: (1) simultaneously sweeping the wavelength
of light output by the device in a monotonic manner through (i) a
first etalon,so as to generate an inline comb of optical
transmission peaks, the first etalon being chosen so as to have its
transmission peaks spaced at a desired target frequency grid, and
(ii) a dielectric filter and a second etalon, where the dielectric
filter is chosen so as to have its transmission peak centered on a
peak in the target frequency grid and the second etalon is chosen
so as to have its transmission peaks spaced significantly further
apart than the target frequency grid; (2) identifying the frequency
of the transmission peak of the dielectric filter and the second
etalon, and a corresponding one of the transmission peaks of the
first etalon; and (3) monitoring the output of the first etalon as
the device is tuned so as to tune the device to a desired
frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] These and other objects and features of the present
invention will be more fully disclosed or rendered obvious by the
following detailed description of the preferred embodiments of the
invention, which is to be considered together with the accompanying
drawings wherein like numbers refer to like parts and further
wherein:
[0018] FIG. 1 is a schematic side view of a tunable Fabry-Perot
filter;
[0019] FIG. 2 is a schematic side view of a tunable VCSEL;
[0020] FIG. 3 is a schematic diagram of wavelength reference
apparatus and wavelength-locking apparatus for tuning a tunable
Fabry-Perot filter and/or a tunable VCSEL to a desired frequency,
and for thereafter keeping that device tuned to that frequency;
[0021] FIG. 4 is a schematic diagram of wavelength reference
apparatus formed in accordance with the present invention; and
[0022] FIG. 5 shows the optical transmission functions of the two
optical branches of the wavelength reference apparatus shown in
FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] Looking next at FIG. 3, there is shown a system 100 which
comprises a wavelength reference apparatus for calibrating a
tunable Fabry-Perot filter and/or a tunable VCSEL, whereby the
device may be tuned to a precise, absolute wavelength. System 100
also provides a wavelength-locking apparatus to keep the tunable
Fabry-Perot filter and/or tunable VCSEL tuned to a precise,
absolute wavelength.
[0024] More particularly, system 100 generally comprises a tunable
Fabry-Perot filter or tunable VCSEL 105, a wavelength reference
apparatus 110, and a controller 115.
[0025] Tunable Fabry-Perot filter or tunable VCSEL 105 preferably
comprises a tunable Fabry-Perot filter or tunable VCSEL of the type
disclosed in the aforementioned U.S. patent applications Ser. Nos.
09/105,399 and 09/543,318. For convenience of description, tunable
device 105 will hereinafter be described in the context of being a
tunable VCSEL; however, it will be appreciated that the present
invention is equally applicable to the situation where tunable
device 105 comprises a tunable Fabry-Perot filter.
[0026] Wavelength reference device 110 is adapted to provide a
precise reference frequency at multiple wavelengths over the
wavelength region of interest. These precise reference frequencies
are located on precise, absolute wavelengths.
[0027] In accordance with a preferred embodiment of the invention,
and looking now at FIG. 4, wavelength reference device 110 is
constructed so that light from VCSEL 105 is input via an optical
fiber 120 and collimated by, for example, a GRIN lens. A beam
splitting device 125 (for example, a non-polarizing beam splitting
cube) divides the light into two portions.
[0028] One portion of the light passes through a Fabry-Perot etalon
130, which generates a comb of transmission peaks spaced at the
desired spacing, e.g., as shown by the comb 135 of transmission
peaks 140 shown in FIG. 5A, where the transmission peaks have a 50
GHz spacing. The light transmitted by first etalon 130 is focused
by a lens and detected by a suitable detector 145 (an InGaAs
detector, for example).
[0029] The second light path proceeds through a 90-degree prism
150. A dielectric filter 155 is applied to the surface of the
prism; this dielectric filter 155 has a transmission profile 158 as
shown in FIG. 5B. Light which has passed through dielectric filter
155 is then passed through a second etalon 160. This second etalon
160 has a distinct, and larger, mode spacing than the
aforementioned first etalon 130 in the first light path. See, for
example, the comb 165 of transmission peaks 170 shown in FIG. 5B,
where the transmission peaks have a 225 GHz spacing. Finally the
light which has passed through both dielectric filter 155 and
second etalon 160 is focused and detected by a second detector
175.
[0030] By placing dielectric filter 155 and second etalon 160 in
series, only light having a wavelength matching the transmission
profiles of both dielectric filter 155 and second etalon 160 can
pass through to second detector 175. In particular, because of the
construction of dielectric filter 155 and second etalon 160, only
light at a single frequency can pass through the transmission
profile of dielectric filter 155 and the transmission profile of
second etalon 160; and, significantly, this single frequency will
always be precisely and absolutely known from the construction of
dielectric filter 155 and second etalon 160. By way of example, but
not limitation, in the example of FIG. 5B, this single known
frequency will exist at the transmission peak 170A of second etalon
160.
[0031] In order to calibrate the tunable VCSEL 105, light from
VCSEL 105 is monotonically swept across the wavelengths of interest
as the first and second detectors 145, 175 are monitored. When
second detector 175 (i.e., the detector monitoring the output of
dielectric filter 155 and second etalon 160) detects an output
peak, the light from VCSEL 105 will be at the wavelength where the
peaks of dielectric filter 155 and second etalon 160 match, i.e.,
at the single known frequency referred to above. Thus, the
wavelength reference device 110 permits calibration of tunable
VCSEL 105 against the single known frequency defined by the
convergence of the transmission profile 158 of dielectric filter
155 and the transmission profile 165 of second etalon 160.
[0032] At the same time, the output of first detector 130 can be
calibrated against this same known frequency, i.e., the specific
peak 140A for the same reference frequency will also be known.
Furthermore, once the specific peak 140A is known, the output of
first detector 130 can be used to tune tunable VCSEL 105 to any
given frequency 140 on the comb 135 of transmission peaks 140.
[0033] Furthermore, once VCSEL 105 has been tuned to a desired
target frequency, the output of detector 145 can be monitored; if
this output drifts off the desired transmission peak (i.e.,
indicating that VCSEL 105 has drifted off the desired target
frequency), the system can adjust the voltage being applied to
VCSEL 105 so as to bring the VCSEL back to the desired
frequency.
[0034] In essence, first etalon 130 provides narrow maximum
transmission peaks for use by a wavelength locking circuit for
locking to any one peak. The second etalon's free spectral range is
chosen in such a manner as to require only a simple, series
dielectric order selection filter in order to isolate a single
known frequency. This single, known wavelength (frequency) is used
by the controlling circuit to determine the proper peak generated
by the first etalon for locking.
[0035] Controller 115 comprises circuitry for reading the output of
detectors 145, 175 and adjusting the voltage applied to VCSEL 105
so as to tune VCSEL 105 to the desired wavelength, and to
thereafter keep it tuned to that wavelength.
[0036] More particularly, the basic wavelength reference device
consists of two air-spaced Fabry-Perot etalons 130, 160 and an
optical dielectric bandpass filter 155. Light introduced into
either etalon will be transmitted at multiple frequencies
(wavelengths). The transmission frequencies will be integer
multiples of the free spectral range ("FSR") defined as FSR=c/2 nL,
where c is the speed of light, n is the refractive index of air,
and L is the physical length of the etalon. The FSR could be chosen
to be equal to the ITU Wavelength Division Multiplexing grid (200
GHz, 100 GHz, 50 GHz, 25 GHz). The corresponding etalon lengths are
approximately 0.75 mm, 1.50 mm, 3.0 mm, and 6.0 mm. In the vicinity
of each such multiple of the FSR, optical frequencies will be
transmitted over a range of frequencies .about.FSR/finesse, where
finese is determined by the reflectivity of the Fabry-Perot
plates.
[0037] The two etalons 130, 160 will be illuminated in parallel.
One etalon, i.e., etalon 130, will provide a grid of narrow peaks
to be used for locking the tunable laser. The width of the peak is
adjusted by the choice of the value of the finesse. The free
spectral range will typically be chosen to match the desired ITU
grid (50 GHz, for example). A combination of manufacturing
tolerances (of the etalon assembly) and optical alignment (angle
tuning) will ensure that an accurate 50 GHz free spectral range is
obtained.
[0038] The second etalon, i.e., etalon 160, is designed in such a
manner as to denote, in combination with a dielectric filter, a
single known wavelength. The control electronics simultaneously
monitor the transmitted optical intensity (via photodetectors) as a
tunable laser source (or broadband light transmitted by a tunable
filter) monotonically varies the wavelength of the light input to
the wavelength reference device 110.
[0039] A single known wavelength is denoted by choosing the free
spectral range of the second etalon 160 to be as large as practical
(225 GHz, for example) and of such a value as to meet two
requirements: (1) one of the desired ITU frequencies must be an
integer multiple of the larger free spectral range, and (2) the
ratio of the free spectral range to the ITU grid spacing should be
as large as possible and half integer (225 GHz/50 GHz=4.5 in this
example). Such a half integer choice will have the result that an
overlap between the large FSR etalon 160 and the 50 GHz etalon 130
will occur every two periods of the large FSR etalon 160, thus
making the design of the dielectric order selection filter simpler.
Possible choices for overlap frequencies (which span the ITU C
band) are: 190.35, 191.25, 192.15, 193.05, 193.95, 194.85, 195.75,
196.65 THz or, in wavelengths: 1574.95, 1567.54, 1560.20, 1552.93,
1545.72, 1538.58, 1531.51, 1524.50 nm.
[0040] A single mode of second etalon 160 will be isolated via the
dielectric filter 155 placed in series with that etalon. Since the
spacing between modes is much larger than the fundamental grid, the
requirements that the dielectric filter must meet are significantly
relaxed. In this example, the optical filter passband may be as
wide as 3 nm, rather than 0.3 nm.
[0041] If desired, dielectric coatings on a single plate may
perform beam splitting.
[0042] And beam collimation may performed using other lens types
(e.g., plano-convex, asphere, etc.).
[0043] Also, beam bending angles may be other than 90 degrees.
[0044] Furthermore, mirrors rather than prisms may be used for beam
splitting and/or bending.
[0045] Also, the dielectric passband filter 155 may be a bandstop
filter.
[0046] And the transmission width may be somewhat different than
the value shown.
[0047] Or a different overlap frequency between the short period
etalon 130 and the long period etalon 160 may be chosen, e.g.,
every third or fourth period.
[0048] The free spectral range of the short period etalon may be
chosen at 50 GHz, 100 GHz or 200 GHz and still meet ITU
requirements.
[0049] The dielectric bandpass filter may be a separate
element.
[0050] The dielectric bandpass filter may be incorporated as part
of a reflective element.
[0051] The dielectric bandpass filter may be much narrower than
discussed above.
[0052] The two detectors may be incorporated into a single
package.
[0053] More than one reference frequency may be generated (outside
or within the band of interest).
[0054] Furthermore, larger diameter detectors may be used without
focusing lenses.
[0055] Also, the dielectric filter may be applied directly on the
input face of etalon 160, thus comprising a single integral
package.
Advantages of the Invention
[0056] Numerous advantages are obtained through the provision of
the present invention.
[0057] For one thing, the present invention provides a stable,
robust, absolute optical wavelength reference for use in wavelength
referencing and locking.
[0058] And the present invention provides a compact physical
design.
[0059] In addition, temperature stabilization will not be required;
and no optical switching is required.
Modifications
[0060] It is to be understood that the present invention is by no
means limited to the particular constructions and method steps
disclosed above and/or shown in the drawings, but also comprises
any modifications or equivalents within the scope of the
claims.
* * * * *