U.S. patent application number 10/144519 was filed with the patent office on 2002-11-28 for method of monitoring an optical signal from a laser.
Invention is credited to O'Connor, Gary.
Application Number | 20020176458 10/144519 |
Document ID | / |
Family ID | 27386110 |
Filed Date | 2002-11-28 |
United States Patent
Application |
20020176458 |
Kind Code |
A1 |
O'Connor, Gary |
November 28, 2002 |
Method of monitoring an optical signal from a laser
Abstract
A method and apparatus are provided for monitoring an optical
signal from a solid state laser. The method includes the steps of
providing a planar substrate defined by opposing planar surfaces
and a substrate edge that together define a substrate volume that
is transparent to an optical signal from the solid state laser,
disposing the solid state laser on one of the opposing planar
surfaces the planar substrate such that a first portion of the
optical signal travels directly through the planar substrate
substantially orthogonal to the opposing planar surfaces and a
second portion of the optical signal is reflected between the
opposing planar surfaces of the planar substrate and passes out of
the substrate through the substrate edge, and disposing an optical
detector proximate the edge of the planar substrate to detect the
second portion of the optical signal.
Inventors: |
O'Connor, Gary;
(Bolingbrook, IL) |
Correspondence
Address: |
Jon P. Christensen
Welsh & Katz, Ltd.
22nd Floor
120 South Riverside Plaza
Chicago
IL
60606
US
|
Family ID: |
27386110 |
Appl. No.: |
10/144519 |
Filed: |
May 13, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60291678 |
May 17, 2001 |
|
|
|
60290815 |
May 14, 2001 |
|
|
|
Current U.S.
Class: |
372/29.02 ;
372/29.011; 372/43.01 |
Current CPC
Class: |
H01S 3/1305 20130101;
H01S 5/02251 20210101; H01S 5/0234 20210101; H01S 5/0683 20130101;
H01S 5/183 20130101; H01S 5/02325 20210101; H01S 5/423
20130101 |
Class at
Publication: |
372/29.02 ;
372/29.011; 372/43 |
International
Class: |
H01S 003/13; H01S
005/00 |
Claims
What is claimed is:
1. A method of monitoring an optical signal from a solid state
laser, such method comprising the steps of: providing a planar
substrate defined by opposing planar surfaces and a substrate edge
that together define a substrate volume that is transparent to an
optical signal from the solid state laser; disposing the solid
state laser on one of the opposing planar surfaces of the planar
substrate, such that a first portion of the optical signal travels
directly through the planar substrate substantially orthogonal to
the opposing planar surfaces and a second portion of the optical
signal is reflected between the opposing planar surfaces of the
planar substrate and passes out of the substrate through the
substrate edge; disposing an optical detector proximate the edge of
the planar substrate to detect the second portion of the optical
signal.
2. The method of monitoring an optical signal from a solid state
laser as in claim 1 further comprising defining the substrate as an
optically transparent substrate.
3. The method of monitoring an optical signal from a solid state
laser as in claim 2 further comprising defining the solid state
laser as a vertical cavity surface emitting laser.
4. The method of monitoring an optical signal from a solid state
laser as in claim 3 further comprising laser ablating a groove in
the substrate.
5. The method of monitoring an optical signal from a solid state
laser as in claim 3 further comprising mechanically scribing a
groove in the substrate.
6. The method of monitoring an optical signal from a solid state
laser as in claim 4 further comprising breaking the substrate along
the groove into two planar sections.
7. The method of monitoring an optical signal from a solid state
laser as in claim 6 further comprising roughening the substrate
edge such that light striking the roughened edge may pass out of
the substrate.
8. An apparatus for monitoring an optical signal from a solid state
laser, such apparatus comprising: a planar substrate defined by
opposing planar surfaces and a substrate edge that together define
a substrate volume that is transparent to an optical signal from
the solid state laser; the solid state laser disposed on one of the
opposing planar surfaces of the planar substrate, such that a first
portion of the optical signal travels directly through the planar
substrate substantially orthogonal to the opposing planar surfaces
and a second portion of the optical signal is reflected between the
opposing planar surfaces of the planar substrate and passes out of
the substrate through the substrate edge; an optical detector
disposed proximate the edge of the planar substrate to detect the
second portion of the optical signal.
9. The apparatus for monitoring an optical signal from a solid
state laser as in claim 8 wherein the substrate further comprises
an optically transparent substrate.
10. The apparatus for monitoring an optical signal from a solid
state laser as in claim 9 wherein the solid state laser further
comprises a vertical cavity surface emitting laser.
11. The apparatus for monitoring an optical signal from a solid
state laser as in claim 10 further comprising a laser ablated
groove disposed in the substrate.
12. The apparatus for monitoring an optical signal from a solid
state laser as in claim 11 further comprising a mechanically
scribed groove disposed in the substrate.
13. The apparatus for monitoring an optical signal from a solid
state laser as in claim 12 further comprising a break in the
substrate along the groove such that the substrate forms two planar
sections.
14. The apparatus for monitoring an optical signal from a solid
state laser as in claim 13 further comprising the substrate edge
being roughened such that light striking the roughened edge may
pass out of the substrate.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefits of U.S. Provisional
Application No. 60/29,678 filed May 17, 2001.
Field of the Invention
[0002] The field of the invention relates to solid state lasers and
more particularly to an output of a solid state laser.
BACKGROUND OF THE INVENTION
[0003] Solid state lasers are generally known. Such devices are
typically constructed by coupling a light-emitting diode to a
resonant cavity.
[0004] A vertical cavity surface emitting laser (VCSEL) is one type
of solid state laser. For example, 850 nm VCSELs may be built in
the AlGaAs/GaAs material system and fabricated on a GaAs substrate.
Like most semiconductor lasers, the active region of the VCSEL
consists of multiple quantum wells, but, unlike edge-emitting
lasers, the mirrors are formed during epitaxial growth using
distributed Bragg reflectors (DBRs). The GaAs substrate functions
to absorb photonic energies greater than the GaAs bandgap.
[0005] Most VCSEL devices are designed to emit light out of only
one of the distributed Bragg reflector (DBR) facets. As such,
associated transmission structures may be coupled directly to those
facets.
[0006] While VCSEL lasers work well, they are still subject to
failure and degradation due to time and temperature. Because of the
importance of optical communications, a need exists for a means of
monitoring VCSEL devices that is not subject to its own inherent
defects.
[0007] VCSELs need some form of power control to maintain a
constant output. Such power control could be performed
automatically by measuring an output of a light emitting device
during operation and using this measurement to control the power
supplied to the light emitting device. Such control may be easily
achieved when the light emitting device is an edge emitting laser
because edge emitting lasers output light from two ends thereof.
Thus, one output may be used for the desired application, while the
other output may be used for the power control.
[0008] Previous attempts to monitor the power of VCSELS typically
involve splitting off a portion of the output beam to use as a
monitor beam. However, such splitting off obscures part of the beam
which may affect the wavefront and imaging, and hence coupling, of
the light. Further, if the intensity distribution changes, such as
when there is a change in lasing mode, the monitored power may
change in a way which does not represent the overall output power
of the VCSEL within a desired lasing mode.
[0009] Additionally, splitting off a portion of the beam may
require that the output of the VCSEL to be increased in order to
maintain the requisite power level at a laser receiver while
allowing the monitoring function. Previous methods of scattering
the beam to create a monitor beam relied on reflection for
directing the beam and did not provide an optimal signal to the
monitor detector. Further, previous scattering did not insure the
entire beam was being monitored. Beam splitting may also require
complex optical reflecting components that can be costly and
involve precise alignment steps.
[0010] In this invention is disclosed a novel method for monitoring
the output performance of a VCSEL array of optical ports. The
invention uses light from the VCSEL that is divergent from the
optical signal entering the optical waveguide. In other words,
light not entering the waveguide because of natural optical losses
is thus utilized for the purpose of monitoring the VCSEL. By using
light that would otherwise be scattered or absorbed (lost light),
more light is transmitted down an optical fiber, and signal
integrity may be preserved.
SUMMARY OF THE INVENTION
[0011] A method and apparatus are provided for monitoring an
optical signal from a solid state laser. The method includes the
steps of providing a planar substrate defined by opposing planar
surfaces and a substrate edge that together define a substrate
volume that is transparent to an optical signal from the solid
state laser, disposing the solid state laser on one of the opposing
planar surfaces of the planar substrate such that a first portion
of the optical signal travels directly through the planar substrate
substantially orthogonal to the opposing planar surfaces and a
second portion of the optical signal is reflected between the
opposing planar surfaces of the planar substrate and passes out of
the substrate through the substrate edge, and disposing an optical
detector proximate the edge of the planar substrate to detect the
second portion of the optical signal.
BRIEF DESCRIPTION OF FIGURES
[0012] FIG. 1 depicts an optical communication system in accordance
with an illustrated embodiment of the invention;
[0013] FIG. 2 depicts a side view of the laser transmitter system
that may be used with the system of FIG. 1;
[0014] FIG. 3 depicts a detailed view of a portion of the laser
transmitter system in FIG. 2;
[0015] FIG. 4a depicts a side view of the substrate and laser
array;
[0016] FIG. 4b depicts a detailed view of the substrate and laser
array of FIG. 4a;
[0017] FIG. 5a depicts another side view of substrate and laser
array;
[0018] FIG. 5b depicts a detailed view of the substrate and laser
array of FIG. 5a; and
[0019] FIG. 6 depicts another view of the laser transmitter
system.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0020] FIG. 1 depicts a simplified laser communication system 10,
shown generally under an illustrated embodiment of the invention.
Under the illustrated embodiment, an information signal is coded
under an appropriate format within an encoder 12. An output of the
coder 12 may be provided as a control signal to a laser driver 14
that may, in turn, provide a driving signal to a solid state laser
16. The laser 16 may convert the electrical driving signal into an
optical signal that may then be transmitted through a waveguide 24
to a remote location.
[0021] At the remote location, a detector 20 may convert the
optical signal back into the electrical domain. A decoder 22 may
retrieve the information signal for use locally.
[0022] In order to maintain transmission efficiency across the
waveguide 24, a feedback and monitoring circuit 18 may be provided
to monitor the output of the laser 16. As an output of the laser 16
changes, a photonics detector 30 may detect the optical signal and
a monitoring circuit 18 may adjust a gain of the driving circuit 14
as appropriate to maintain a constant transmission signal.
[0023] Shown in FIG. 2 is the laser transmitter system 16, 30 of
FIG. 1. Included in the system may be the laser array 16, an
optically transparent substrate 52 to which the laser 16 is
attached, a printed circuit board 56, and a photonics detector 30
(e.g., a PIN photodiode). The laser array 16 may be mechanically
attached to the substrate with an appropriate adhesive (not shown).
The laser array 16 may be electrically attached to the signal
driver 14 (shown in FIG. 1) by conventional electrical traces 62
and stud bumps 72. The stud bumps 72 may also function to
structurally support the laser array 16 to the substrate 52. The
traces 62 may traverse both portions the substrate 52 as shown,
electrically attaching the laser array 16 to the signal driver
shown in FIG. 1). The array 16 could also be mechanically and
electrically attached by solder ball rather than stud bumps.
[0024] The printed circuit board, or PCB, 56 may be any suitable
material such as FR4, ceramic interconnect, or the like. The PCB 56
may have a plurality of electrical and optical devices for signal
processing, as well as electrical traces and electrical pads (not
shown on the PCB 56 in the figure).
[0025] The optically transparent substrate 52 may be attached to
the PCB 56 by any conventional method (i.e., solder, adhesive,
etc.). The substrate 52 may comprise a glass or glass like
structure having suitable optical and structural properties, (other
materials that have been found to display suitable properties
include plastic and ruby crystal). The substrate 52 shown in FIG.2
contains two planar sections 64, 66 that may be separated by a
ninety degree angle (details of the substrate 52 will be discussed
in further detail below).
[0026] It will be understood that the laser array 16 can be any
suitable photonic device or array of photonic devices. Yet, in a
preferred embodiment of the present invention, the laser array is a
vertical cavity surface emitting laser (VCSEL) array. The laser
array 16 may have a number of optical ports 50 (FIG. 6) for
coupling light to a respective optical device, such as a waveguide
24. The number of optical ports 50 in the laser array 16 is not
limited in any way. The array 16 could have 1 or n optical ports
50. The optical ports 50 may provide transmission paths 60 that
pass directly through the substrate 52 to which the laser array 16
is attached, as shown in the FIG. 2.
[0027] Also shown in FIG. 2 is an optical detector 30 that may be
electrically and mechanically attached to the PCB 56. The detector
30 is positioned to receive a portion of light energy from the
laser array 16 as shown. Within the detector 30, the light energy
may be detected and converted into an analog feedback signal. The
analog signal, in turn, may be coupled to an inverting amplifier 32
(FIG. 1). The monitoring circuit 18 may in turn use the signal from
the inverting amplifier 32 to adjust the gain on the laser driver
14 as appropriate.
[0028] During operation of the laser communications system 10, the
feedback signal may be used to maintain a laser output appropriate
to provide an adequate level of energy impinging upon the detector
20. As the laser 16 ages, the level of the feedback signal may
fall. As the level of the feedback signal falls, the inverting
amplifier 32 may increase a gain of the driver 14 thereby
compensating for loss of laser energy.
[0029] The physical structure of the laser transmitter system 16,
30 will now be discussed in further detail below. As previously
stated, the optically transparent substrate 52 may comprise a glass
or glass like structure displaying adequate optical and structural
properties. Turning to FIG. 4a, the substrate 52 may first be
fabricated in a planar form. The optical array 16, and electrical
contacts 62 and 72, may all be disposed on a first surface 80 of
the substrate 52.
[0030] FIG. 2 illustrates the substrate 52 having a ninety degree
bend to allow optical signals to travel substantially parallel to
the PCB 56. As illustrated in FIGS. 4 and 5, the ninety degree bend
in the substrate 52 may be formed by breaking the substrate along a
groove 68 and rotating a portion of the substrate 52 about the
groove 68. After breaking, the substrate 52 may then become a
two-member assembly, having relatively rigid planar elements 64 and
66. The groove 68, shown in the enlarged side view of FIG. 4b, may
be formed on a second surface 82 of the substrate 52. The groove 68
may be formed at any location on the second surface 82.
[0031] The groove 68 could be formed using a conventional laser
ablation, laser scribing, or mechanical scribing process. The
groove 68 may traverse the width while not extending through the
thickness of the substrate 52, as illustrated in FIG. 4b (i.e.,
about 80% through the thickness). If the groove 68 is formed
completely through the thickness of the substrate, the electrical
traces 62 could be damaged or separated. The preferred method of
forming a groove 68 in the substrate is the conventional laser
ablation technique.
[0032] Upon forming the groove 68 partially through the substrate
52, the substrate 52 could be placed in a mechanical fixture that
breaks the substrate 52 by rotating the planar elements 64, 66
about the groove 68. The broken substrate 52 with first and second
planar elements 64 and 66 may then have an abutting common edge 84,
as shown in FIG. 5b. The first and second planar elements 64, 66
may be rotated to any angle, with respect to each other, about the
common edge 84 (e.g., the planar elements may form a desired angle
of ninety degrees on one side and 270 degrees on the other
side).
[0033] The conductive traces 62 traversing the substrate 52 (i.e.,
connecting the two halves 64, 66 of the substrate 52) may
structurally and electrically interconnect the two planar elements
64, 66. The conductive traces 62 traversing the two planar elements
may also form a hinge 86 extending the width of the substrate 52
(the hinge 86 being located along the common edge 84). The vertical
planar element 66 may be rotated along the hinge 86 to any desired
angle. In a preferred embodiment of the present invention, the
vertical planar element 66 is broken and rotated ninety degrees,
forming a ninety-degree angle with the substrate's horizontal
planar element 64 as part of a single manufacturing step. Rotating
of the substrate to the desired angle in a single step could more
quickly and efficiently complete the assembly process of the
substrate 52 and two subsections 64, 66. That is, the planar
substrate 52 could be broken and rotated to the desired angle by
necessarily rotating the second planar element 66 of the substrate
52 about the hinge 86, thus eliminating the separate specific
manufacturing process of breaking the substrate 52. Rotating the
vertical planar element 66 of the substrate 52 to the desired angle
allows the transmission axis 60 of the optical array 16 to be
aligned parallel to the PCB 56 and the horizontal planar element 64
of the substrate 52, further promoting planarity and
manufacturability.
[0034] The vertical planar element 66 may further be held in place
by a conventional adhesive and/or polyimide (not shown) applied to
the first surface 80 of the substrate 52 near the hinge 86 and
common edge 84. In addition, a retaining structure (not shown)
could be placed near the second surface 82 of the vertical planar
element 66 such that the structure substantially prevents the
element 66 from rotating about the hinge 86 from the desired angle.
If the angle of the vertical planar element 66 deviates from its
nominal position (i.e., the desired angle) with respect to the
horizontal planar element 64, optical signals may not be properly
aligned to the optical waveguide 24.
[0035] As light from the optical array 16 is transmitted through
the vertical planar element 66, the majority of the light then
passes through the substrate 66 and into an appropriate optical
waveguide 24. Yet, a portion of the light 58 is not able to escape
the substrate 66. That is, some light enters the substrate 66 and
is internally reflected. This portion of the light 58 has the
critical angle necessary to be reflected within the substrate 66.
The internally reflected light 58 may traverse through the length
of the vertical planar element 66 and may in turn be used as a
feedback signal to monitoring circuit 18, as further described.
[0036] As shown in FIGS. 3-5, the processes of forming the groove
68 in the substrate 52 and breaking the substrate 52 causes
microscopic irregularities to form in the break region 88 planar
elements 64, 66. The irregular surfaces of the planar elements 64,
66 shown in further detail in FIGS. 3, 4b and 5b, together make up
a break region 88. The irregular surfaces in the break region 88
are similar in structure and function to a roughened or unpolished
end of an optical fiber. When light is transmitted down an optical
fiber and strikes an unpolished end on a transparent substrate, the
light is scattered. A portion of the light is reflected back down
the fiber, and a portion of the light is absorbed at the fiber end.
Yet, some light exits the fiber through the roughened surface. This
light may exit the fiber at a different angle than which it struck
the unpolished surface.
[0037] The irregularities in the substrate 52 are then similar in
structure to a roughened optical fiber. A portion of the light 58
striking a broken edge 74 of the vertical planar element 66 may be
allowed to escape the substrate 66 and impinge on the detector 30.
This light 58 in turn may be used as the feedback monitoring signal
coupled to the monitoring circuit 18.
[0038] Shown in greater detail in FIG. 3 is light 58 exiting the
vertical planar element 66 through the break edge 74. The feedback
photonics detector 30 may be disposed on the PCB 56 such that it
receives light 58 exiting the broken edge 74 of the substrate 66.
The feedback photonics detector 30 can be situated at or near the
broken edge 74 of the vertical planar element 66 to collect light
from the optical array 16.
[0039] The detector 30 can be any conventional/suitable photodiode
or photonics detector. The detector 30 could also be attached to
the optically transparent substrate 52, and is not limited to a
specific location. The preferred method of use attaches the
detector 30 to the PCB 56 near the break region 88 of the substrate
52.
[0040] In a preferred embodiment of the present invention, the
feedback photonics detector 30 is used to detect a net change in
optical power output from the optical array 16. FIG. 6 shows a
front view of the laser communications system 10. In this view,
optical signals being transmitted to the optical waveguide 24
(shown in FIG. 2) come out of the page. The detector 30 is adapted
to receive reflected and scattered light 58 from all the optical
ports 50 of the optical array 16. Since the operating
characteristics of optical devices on the same wafer/optical array
16 tend to behave similarly, the detector 30 receives light 58 from
all the optical ports 50, and the monitoring circuit 18 looks for a
change in power output from the optical array 16. As the power
output of the optical array 16 reduces, the light input 58 to the
detector 30 accordingly reduces.
[0041] If the performance in the optical ports 50 as a whole
decreases because of temperature shifts, age, or other similar
causes, then the use of one photodetector 30 can provide an
appropriate feedback signal that can in turn instruct the laser
driver 14 to increase output power accordingly.
[0042] Alternatively, the output of the individual ports 50 may be
measured individually under a multiplexing format. A multiplexing
circuit 90 within the driver 14 may individually activate the ports
50 on start up and measure an output of each port 50.
Alternatively, the multiplexing circuit 90 may monitor for those
events where only a single port 50 or a small number of ports 50
are active. A simple summing equation may then be used to determine
the specific output of each port 50 and to adjust a driver level
accordingly.
[0043] The use of one photodetector 30 to provide feedback for all
the optical ports 50 has cost and manufacturing advantages. If,
instead, one photodiode were to be used to monitor each optical
port of the optical array, the overall cost of the communications
device would increase. In addition, manufacturing yield would
decrease if additional optical components were added to the system,
and optical diodes tend to have a greater fallout rate than passive
optical components.
[0044] Another advantage of the present invention lies in the
source of light providing the feedback signal. Optical signals that
would normally be lost due to scattering and/or reflection are,
instead used to provide optical feedback to improve and optimize
performance of the optical transmitter. As previously stated, the
majority of light from the laser array 16 may be transmitted to the
optical waveguide 24. Some light is naturally lost because of
differences in index of refraction, reflection, and absorption. It
is the intent of the invention to use light that would otherwise be
considered lost, thus maintaining a relatively high optical power
level transmitted to the optical waveguide. This can preserve
optical signal integrity as well.
[0045] A specific embodiment of a method and apparatus for
monitoring an optical signal from a solid state laser has been
described for the purpose of illustrating the manner in which the
invention is made and used. It should be understood that the
implementation of other variations and modifications of the
invention and its various aspects will be apparent to one skilled
in the art, and that the invention is not limited by the specific
embodiments described. Therefore, it is contemplated to cover the
present invention and any and all modifications, variations, or
equivalents that fall within the true spirit and scope of the basic
underlying principles disclosed and claimed herein.
* * * * *