U.S. patent application number 11/561132 was filed with the patent office on 2007-09-27 for high efficiency, wavelength stabilized laser diode using awg's and architecture for combining same with brightness conservation.
This patent application is currently assigned to JDS Uniphase Corporation. Invention is credited to Bruno Acklin, David J. Dougherty, Kenneth M. Dzurko, Len Marabella, Martin H. Muendel, Matthew Glenn Peters, Victor Rossin, Robert B. Sargent, Kuochou Tai, Yongan Wu.
Application Number | 20070223552 11/561132 |
Document ID | / |
Family ID | 37622337 |
Filed Date | 2007-09-27 |
United States Patent
Application |
20070223552 |
Kind Code |
A1 |
Muendel; Martin H. ; et
al. |
September 27, 2007 |
High Efficiency, Wavelength Stabilized Laser Diode Using AWG's And
Architecture For Combining Same With Brightness Conservation
Abstract
The invention relates to high power semiconductor lasers based
on a laser diode array waveguide grating (DAWG) in which the
wavelength is stabilized using an array waveguide grating (AWG) in
an external cavity configuration. Another aspect of the present
invention relates to techniques for efficiently coupling optical
gain element arrays to an AWG. Another feature provides for the
efficient and brightness-conserving combination of multiple high
power DAWG lasers into a single output.
Inventors: |
Muendel; Martin H.;
(Oakland, CA) ; Dougherty; David J.; (Mountain
View, CA) ; Peters; Matthew Glenn; (Menlo Park,
CA) ; Rossin; Victor; (Mountain View, CA) ;
Sargent; Robert B.; (Santa Rosa, CA) ; Marabella;
Len; (Santa Rosa, CA) ; Tai; Kuochou;
(Fremont, CA) ; Acklin; Bruno; (Mountain View,
CA) ; Wu; Yongan; (San Jose, CA) ; Dzurko;
Kenneth M.; (Monte Sereno, CA) |
Correspondence
Address: |
ALLEN, DYER, DOPPELT, MILBRATH & GILCHRIST P.A.
1401 CITRUS CENTER 255 SOUTH ORANGE AVENUE
P.O. BOX 3791
ORLANDO
FL
32802-3791
US
|
Assignee: |
JDS Uniphase Corporation
Milpitas
CA
95035
|
Family ID: |
37622337 |
Appl. No.: |
11/561132 |
Filed: |
November 17, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60737925 |
Nov 18, 2005 |
|
|
|
Current U.S.
Class: |
372/50.12 |
Current CPC
Class: |
H01S 5/4062 20130101;
H01S 5/02251 20210101; H01S 5/0612 20130101; G02B 6/12004 20130101;
H01S 5/146 20130101; H01S 5/2036 20130101; H01S 5/141 20130101;
G02B 6/2813 20130101; H01S 5/148 20130101; H01S 5/4012 20130101;
H01S 5/0268 20130101; H01S 5/026 20130101; H01S 5/0655 20130101;
H01S 5/4087 20130101 |
Class at
Publication: |
372/050.12 |
International
Class: |
H01S 5/00 20060101
H01S005/00 |
Claims
1. A high power laser source comprising: a first chip including a
multi-mode semiconductor waveguide for providing gain in a
predetermined wavelength band, wherein each semiconductor waveguide
has a first reflector at one end; a second chip including an array
waveguide grating having an input port coupled to the semiconductor
waveguide, and an output port; a second reflector disposed to
receive light from the array waveguide grating output port and to
reflect a portion of the light back into the array waveguide
grating, thereby defining a laser cavity with the first reflector;
wherein: the semiconductor waveguide is dimensioned to support
propagation of multiple transverse modes of light having
wavelengths in the predetermined wavelength band; and the multiple
transverse modes on the first chip are optically coupled to the
input port of the array waveguide grating on the second chip.
2. The high power laser source in claim 1, further comprising
additional semiconductor waveguides, wherein spaces between the
semiconductor waveguides are not uniform in order to obtain a
prescribed temperature gradient across the first chip.
3. The high power laser source in claim 2, wherein the prescribed
temperature gradient is constant.
4. The high power laser source in claim 2, wherein the prescribed
temperature gradient is approximately zero.
5. The high power laser source in claim 1, further comprising
coupling means for coupling the semiconductor waveguide to the
array waveguide grating, wherein the coupling means is selected
from the group consisting of a planar lens, a cylindrical lens, a
two-dimensional lens, a multi-mode interference region, an
integrated fast-axis lens etched into the first chip, and an
integrated fast-axis lens etched into the second chip.
6. The high power laser source in claim 1, wherein the second
reflector is located proximate to the output port of the array
waveguide grating.
7. The high power laser source in claim 1, wherein the second
reflector is located in an output optical fiber which is coupled to
the output port of the array waveguide grating.
8. The high power laser source in claim 1, further comprising
coupling means for coupling the semiconductor waveguide to the
array waveguide grating, wherein the coupling means comprises a
planar grin lens integrated in the input port of the array
waveguide grating.
9. The high power laser source in claim 1, wherein the
semiconductor waveguide has a flare increasing in width toward the
input port of the array waveguide grating.
10. The high power laser source in claim 9, wherein the flares
overlap to form a contiguous region at the input port of the array
waveguide grating.
11. The high power laser source in claim 1, wherein the
semiconductor waveguide is optically coupled to a star-coupler
region of the input port of the array waveguide grating.
12. The high power laser source in claim 1, wherein the output port
of the array waveguide grating comprises a single-mode output
waveguide.
13. The high power laser source in claim 1, wherein the input port
of the array waveguide grating comprises a slab waveguide.
14. The high power laser source in claim 1, wherein the input port
of the array waveguide grating comprises a star-coupler region of
an array waveguide grating.
15. The high power laser source in claim 5, wherein the multi-mode
interference region supports multiple wavelengths.
16. The high power laser source in claim 1, wherein the output port
of the array waveguide grating has a focus outside an edge of the
second chip to reduce optical field intensity within the second
chip and to provide optical coupling to an optical fiber.
17. The high power laser source according to claim 12, further
comprising additional array waveguide gratings, wherein each array
waveguide grating transmits therethrough a different wavelength
band and the single-mode output waveguides are combined in a
wavelength combiner into a single-mode combined output waveguide
which is coupled to a single-mode fiber.
18. The high power laser source according to claim 12, further
comprising additional array waveguide gratings, wherein each
single-mode output waveguide is terminated at an edge of the second
chip for coupling to a multi-mode core of a single output
fiber.
19. The high power laser source according to claim 18, wherein the
single-mode output waveguides form a two-dimensional array at the
edge of the second chip.
20. The high power laser source according to claim 18, wherein each
single-mode output waveguide is terminated at the edge of the
second chip for coupling to a rectangular multi-mode core of a
single output fiber.
21. The high power laser source according to claim 12, wherein the
single-mode output waveguides are terminated at an edge of the
second chip to align with single-mode fibers at a first end of a
single-mode fiber array.
22. The high power laser source according to claim 21, wherein a
second end of the single-mode fiber array is configured so that the
single mode fibers form a two-dimensional array for coupling into a
multi-mode core of a single optical fiber.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of U.S. Provisional
Application No. 60/737,925 filed Nov. 18, 2005, entitled
"WAVELENGTH STABILIZED LASER DIODE" which is incorporated herein by
reference for all purposes.
TECHNICAL FIELD
[0002] The present invention relates to wavelength stabilized
semiconductor laser sources in which array waveguide gratings (AWG)
are used as the optical wavelength stabilizing element in an
external cavity, in particular to broad-area (multimode)
semiconductor laser diode arrays and, more particularly, to a
system for scaling power to a 10 watt to kilowatt level into a
single fiber output by combining outputs of multiple laser sources
using wavelength and polarization multiplexing to conserve
brightness.
BACKGROUND OF THE INVENTION
[0003] Semiconductor high power lasers are interesting as
replacements of today's commercially available CW fiber lasers
(FL's) for many reasons, among which compact size and high
efficiency are among the most attractive. High-power single-mode
fiber-coupled (FC) modules potentially have advantages of
wavelength versatility (most FL's are based on Yb at 1050-1100 nm),
compactness, improved efficiency, and enhanced reliability due to
the elimination of the double-clad fiber.
[0004] High-power multimode modules are of interest in less
demanding applications, where output powers in the 100 W range from
a 25-100 micron core fiber would be adequate. They would have the
added advantage of lifting the constraint of single-mode operation,
making these devices smaller, lower cost, and easier to manufacture
than their single-mode counterpart.
[0005] For applications demanding pulsed output or extremely high
brightness, high-power modules can serve as a very cost-effective
pump for Yb fiber lasers or disk lasers. One particular area of
interest is a pump source for amplifying picosecond or femtosecond
pulses in double-clad fiber, where the fiber length must be kept
extremely short to mitigate pulse distortion, and therefore a very
high brightness pump is required.
[0006] A narrow wavelength high brightness pump technology will
improve the fiber laser efficiency in a number of ways. Firstly,
tight wavelength control will allow pumping of the narrow 976 nm
Ytterbium absorption line which has a better quantum defect and
higher absorption than the broader 915 nm line. Secondly, a high
brightness source will enable pumping into a smaller diameter
fiber. Conventional bars and single emitters, with relatively low
brightness, require fiber with a large inner cladding (typically
400-600 microns), while a high-NA polymer outer clad fiber ends up
having a much higher loss per unit length and very long (typically
>50 m). Thirdly, the combination of these improvements will
translate to a shorter overall fiber length with lower overall
absorption and scattering and lower threshold power requirements.
Fourthly, lasing in the 1040 nm band will be support, which has a
lower quantum defect. For example the impact of pumping at 976 nm
and lasing at 1050 nm, versus 915 nm and 1100 nm, will be to
improve the quantum defect from 83% to 93%. The net effect of such
pump laser improvements will enable a fiber laser
optical-to-optical efficiency in the 80% range.
[0007] Finally spectral multiplexing of either the single-mode or
the multimode modules will enable reaching even higher output
powers without impacting beam quality. For example, spectrally
multiplexing multiple multimode modules could permit the
construction of a 2-5 kW industrial cutting/welding direct diode
system that would be extremely reliable and cost-effective. This
would be an appealing alternative to current FL-based architectures
currently being explored, in which a number of 100-200 W
single-mode fiber lasers are combined spatially into a multimode
output.
[0008] The power achievable from a single laser module is limited
to the range of several watt. To achieve the power levels demanded
by various high-power applications mentioned above, a scaling
scheme must be used that can efficiently combine the power from
multiple laser sources.
[0009] Simple optical combiners do not satisfy the efficiency
requirement for fundamental physics reasons. In contrast,
wavelength and polarization combiners can be made with zero
theoretical optical losses. To implement such combining means, the
laser source modules must fulfill certain properties. In addition
to producing high output power and efficiency, their emission must
be wavelength stabilized to within tight limits and be sufficiently
narrow band, as well as possessing a high optical beam quality.
[0010] To obtain high output power and efficiency, broad area laser
diodes are generally used, however due to their dimensions they
tend to oscillate in multiple transverse modes. The broad area
output facet of these lasers also presents a challenge for coupling
into a single mode optical fiber in applications where maximal
brightness is called for.
[0011] Wavelength stabilization techniques are well known in the
art for single mode (narrow stripe) laser diodes. Distributed
feedback gratings as well as distributed Bragg reflectors are
commonly integrated into the laser structure, but these exhibit a
relatively high wavelength shift with temperature, which can be
problematic in high power environments. Other techniques are based
on using thermally stable materials in an external cavity, such as
Fabry-Perot etalons, free-space diffraction gratings and fiber
Bragg gratings.
[0012] Similar techniques have been developed also for broad area
lasers, however coupling them into an external cavity with the
required efficiency is still a major problem. Another stabilization
method involves reflecting a spectrally narrow portion of the
output emission back into the broad area laser.
[0013] One such method has been disclosed by Bardia Pezeshki et al.
in US Patent Application 20020085594 in the name of Santur, Inc.
The embodiment shown in FIG. 1a has a plurality of DFB laser diodes
as used in telecommunications, coupled into an array waveguide
grating (AWG) for combining at a common output. A disadvantage of
this arrangement is that it requires close tracking of the laser
wavelength with the passbands of the WDM device under changing
temperature conditions.
[0014] Another method has been disclosed by Robert G. Waarts et al.
in U.S. Pat. No. 6,212,310, assigned to SDL, Inc. An embodiment is
shown in FIG. 1b where a plurality of optical fiber lasers, each
with a different wavelength, is coupled through a WDM device such
as a fused taper coupler, dichroic mirrors or grating mirrors into
a single output. The output fiber has a Bragg reflector
incorporated within it for providing some feedback to the fiber
lasers. This has a similar disadvantage to the previous
example.
[0015] Thus, an object of the present invention is to stabilize the
wavelength of broad area multi-mode laser diodes.
[0016] Another object of the present invention is to provide an
efficient coupling of such laser diodes to a single-mode waveguide
or fiber.
[0017] A further object of the present invention is to provide a
device in which the optical outputs of an array of laser emitters
is combined while conserving brightness, and furthermore to combine
outputs of multiple arrays of laser emitters into one single-mode
or multi-mode output to achieve high optical power levels in the
tens to hundreds of watt.
SUMMARY OF THE INVENTION
[0018] Accordingly, the present invention relates to high power
semiconductor lasers based on a laser diode array waveguide grating
(DAWG) in which the wavelength is stabilized using an array
waveguide grating (AWG) in an external cavity configuration.
[0019] Another aspect of the present invention relates to
efficiently coupling optical gain element arrays to an AWG.
[0020] Another feature of the present invention provides for the
efficient and brightness conserving combination of multiple high
power DAWG lasers into a single output.
[0021] The present invention relates to a high power laser
comprising:
[0022] a first chip including a multi-mode semiconductor waveguide
for providing gain in a predetermined wavelength band, wherein each
semiconductor waveguide has a first reflector at one end;
[0023] a second chip including an array waveguide grating having an
input port coupled to the semiconductor waveguide, and an output
port;
[0024] a second reflector disposed to receive light from the array
waveguide grating output port and to reflect a portion of the light
back into the array waveguide grating, thereby defining a laser
cavity with the first reflector;
[0025] wherein the semiconductor waveguide is dimensioned to
support propagation of multiple transverse modes of light having
wavelengths in the predetermined wavelength band; and the multiple
transverse modes on the first chip are optically coupled to the
input port of the array waveguide grating on the second chip.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The invention will be described in greater detail with
reference to the accompanying drawings which represent preferred
embodiments thereof, wherein:
[0027] FIG. 1a: Prior art involving an array of semiconductor laser
coupled to an array waveguide grating (AWG);
[0028] FIG. 1b: Prior art showing several lasers sharing a common
cavity reflector through a wavelength division multiplexer
(WDM);
[0029] FIG. 2a: Example of a single-mode diode-AWG (DAWG) module
design;
[0030] FIG. 2b: Example of a multi-mode DAWG module design;
[0031] FIG. 3a, 3b: Schematic of DAWG wavelength control,
stabilization and multiplexing.
[0032] FIG. 4: Example of non-uniform bar pitch for achieving a
prescribed transverse temperature profile in the gain chip;
[0033] FIG. 5a, 5b, 5c: Cross-sectional view of fast axis coupling
techniques between gain chip and AWG chip;
[0034] FIG. 6: Example plan view of slow axis coupling technique
between gain chip and AWG chip;
[0035] FIG. 7a: Basic single-mode laser diode bar (Sbar) AWG
combination using 2-axis lenses;
[0036] FIG. 7b: Advanced single-mode laser diode bar (Sbar)--AWG
combination showing method for eliminating both the fast-axis and
slow-axis coupling optics;
[0037] FIG. 7c: Basic coupling of flared single-mode laser diode
bar (Sbar) to planar lens on AWG input, showing various dimensions
to be optimized;
[0038] FIG. 7d: Advanced coupling of overlapping flare single-mode
laser diode bar (Sbar) directly to star coupler region on AWG
input, showing various dimensions to be optimized;
[0039] FIG. 7e: Example of multi-mode interference (MMI) Sbar to
AWG combination, showing various dimensions to be optimized;
[0040] FIG. 8: Example of an array of planar lenses for collecting
and focusing output of flared diode emitters. Single mode
waveguides on the output connect to the AWG. Trim heaters can
correct alignment errors between the bar and AWG PLC chip;
[0041] FIG. 9a, 9b: Example of technique to avoid high optical
intensities at the multiplexed side of the AWG: the slab guide (a)
can be cut, and the laterally converging wave allowed to focus in
free space (b). A cylindrical lens re-images in the vertical
direction;
[0042] FIG. 10a: L-I-V curves for state-of-the-art JDSU 980 nm and
865 nm narrow stripe single-mode laser diodes;
[0043] FIG. 10b: PCE curves for state-of-the-art JDSU 980 nm and
865 nm narrow stripe single-mode laser diodes;
[0044] FIG. 11a, 11b, 11c: Example cross-section of spatially
combined waveguide output into a multi-mode fiber: 1-dimensional
array (a), 2-dimensional array (b), and 1-dimensional array into
rectangular core (c);
[0045] FIG. 11d, 11e: Example cross-section of fiber array ribbon
having a different arrangement at each end: at AWG end (d), and
output end (e);
[0046] FIG. 12a: Example of on-chip wavelength multiplexing of DAWG
outputs;
[0047] FIG. 12b: Example of spatially combined single-mode
waveguides into multi-mode fiber;
[0048] FIG. 12c: Example of spatially combined single-mode
waveguides into single-mode fiber array on silicon optical
bench;
[0049] FIG. 13a: Overall concept for kilowatt system using passive
wavelength combination;
[0050] FIG. 13b: Schematic of polarization combined DAWG
modules;
[0051] FIG. 13c: Schematic of example 5 kW system showing a
detailed drawing of the 5 kW system and its components. The example
system uses 32 DAWG modules combined via polarization and
wavelength multiplexing;
[0052] FIG. 14: Plot of combined beam propagation factor (BPF)
versus near field aperture with ideal optics. BPF maximizes at 0.98
for an aperture 1.5 times the beam waist;
[0053] FIG. 15: Plot of BPF as a function of aberrations for an
aperture of 1.5 times beam waist. There is some optimization of
aperture depending on type of aberration;
[0054] FIG. 16a: 2-dimensional linear plot of modeled WAVMUX near
field using an aperture 1.5 times the beam waist;
[0055] FIG. 16b: 2-dimensional linear plot of Far-field of WAVMUX
Beam including aberrations, where BPF=0.60;
[0056] FIG. 17: Plot of spectral characteristics of representative
polarization beam combiner;
[0057] FIG. 18: Example chart showing AWG module outputs as a
function of relative frequency and polarization. After polarization
MUX, the 32 AWG outputs are reduced to 16 beams;
[0058] FIG. 19a: Example zigzag multiplexing scheme (beam expanding
lenses and polarization multiplexers are not shown). Each Level 1
multiplexer employs 3 filters;
[0059] FIG. 19b: Example of level 2 multiplexer showing outlines of
four Level 1 multiplexers. The Level 2 multiplexer employs 3
filters;
[0060] FIG. 20: Spectral plot of level 1 multiplexing scheme.
Filter 1 reflects AWG band 1 (centered at 100 GHz relative
frequency), while transmitting AWG band 2 (centered at 200 GHz
relative frequency). Filter 2 transmits AWG band 3, while
reflecting bands 1 and 2, similarly filter 3 adds AWG band 4 into
the beam;
DETAILED DESCRIPTION
[0061] Silica array waveguide gratings (AWG) form the basis of the
design for spectrally multiplexing lasers at narrow channel
spacing. The diode-AWG (DAWG) concept has inherent advantages over
conventional gratings for spectral combining of laser arrays.
Firstly, the AWG has extremely high spectral resolution, enabling a
large number of laser diode emitters to be packed into a narrow
spectral region. Secondly, due to their planar structure, the AWG
is well matched for efficient coupling to a diode bar, and it
uniquely enables disruptive laser architectures, such as flares,
that can greatly increase the optical power per emitter. Thirdly,
the concept is cost-effective, scalable, highly manufacturable, and
precisely controllable since it is based on silicon wafer
processing technology. Finally, it enables a simple, compact,
robust, thermally stable DAWG module with fiber-coupled or free
space collimated output with minimum amount of micro-optics.
[0062] This invention disclosure presents innovative design
concepts for the laser diode bar, the laser to silica waveguide
coupling, the silica AWG and the silica output coupling to fiber or
free space.
[0063] FIG. 2a shows an example of a single-mode DAWG module
design, while an example of a multi-mode DAWG module design appears
in FIG. 2b. Referring to these figures, the DAWG module 200 in
package 260 comprises a diode bar or semiconductor gain chip 201
optically coupled to an input of an AWG chip 202 through coupling
optics 250, here shown as a separate lens, but may also be
integrated on one or both chips 201, 202. In the case of the
multi-mode DAWG module, input and output may be achieved through a
slab waveguide regions 219, with the output coupled directly into
an output fiber 210. For the single-mode counterpart, the input may
have individual waveguides on the input adjacent to the coupling
optics 250 and a single waveguide 209 for coupling to an output
fiber 210.
[0064] The function of a DAWG is related to spectral combining
concepts that have been demonstrated in recent years using
conventional gratings but with some distinct advantages. As seen in
exemplary design in FIG. 3a, each individual laser diode 303a-c is
an external cavity laser where the feedback comes from a broad
spectrum grating (or simple reflector) 311 placed at the far end of
a silica AWG 302. Required coupling optical elements are not shown
for simplicity.
[0065] The exact lasing wavelength of each emitter is determined by
the AWG 302, which functions as an extremely high-order diffraction
grating. Scaling this approach to a diode bar (also referred to as
"SBar" for short) 301 matched to the AWG 302 with many channels
304a-c (three shown in this example) creates substantial
advantages. Typically, 40 channels are not uncommon for DWDM
systems. The AWG 302 not only controls the wavelength and
wavelength spacing of the individual laser diodes 303a-c, but also
multiplexes them into a single waveguide output 309.
[0066] Still referring to FIGS. 3a and 3b, optical feedback for the
external cavity DAWG laser 300 can be provided in the output
waveguide 309 or in the output fiber 310 coupled to the output
photonic light circuit (PLC) facet 312. One option is to use a weak
Bragg grating 311 to provide a few percent of feedback to sustain
oscillation. The grating 311 can either be in the output waveguide
309 using a shallow etch on the core of the guide or in the output
fiber 310 using UV writing technology. The weak grating should have
sufficient bandwidth to capture the range of the AWG spectrum.
Another option is to utilize the Fresnel reflection, R.about.4%, at
the output PLC facet 312 of the AWG chip 302 or at the input facet
of an output fiber. A final option is to position the fiber facet
313 within a few microns of the output PLC facet 312, thus
generating effectively one surface with a reflectivity that is
determined by the gap between the facets according to the
Fabry-Perot formula. The gap can be set at the time of manufacture
in order to create an effective reflectivity in the between 0% and
16%. With this addressable range, it should be possible to adjust
the feedback to optimize the performance of the laser.
[0067] Alternative techniques for feedback may implement a
single-mode Bragg grating with a narrow reflection band. Such a
grating can be written into a photosensitive core of a double-clad
multi-mode fiber or fabricated by etching into a core of a
multi-mode waveguide on an optical chip, such as a silicon or
silica photonic light circuit (PLC). Although such a reflector is
suitable when the double-clad multi-mode fiber or PLC is coupled
directly to a multi-mode semiconductor waveguide e.g. 303a, it
would be of limited advantage to use it as a reflector 311 on the
output waveguide of the AWG chip 302 illustrated in FIGS. 3a and
3b. The reason is that while the AWG chip 302 can provide efficient
coupling with a multi-mode semiconductor waveguide e.g. 303a, it
also restricts the spectral bandwidth of the respective outputs to
different narrowband wavelength bands, which are not necessarily
compatible with the narrow reflection band of a single-mode Bragg
grating.
[0068] There are multiple possibilities for output coupling from
the AWG chip 302, including a fiber output as well as free space
output. The fiber output concept (single-mode, multi-mode,
polarization maintaining, etc.) can be accomplished by focusing the
AWG waveguide output onto the fiber, butt coupling the fiber to the
waveguide facet (possibly in physical contact, e.g. using a convex
polished fiber tip against the waveguide facet), or even bonding or
fusion splicing the fiber into the silica AWG. Another concept is
directing the output through a collimating lens (as part of the AWG
chip 302 or a discrete optical element) and using the collimated
power. The free space option is especially useful for further beam
combining.
[0069] The SBar design discussed here is based on conventional
laser diode structures as shown in FIGS. 3a and 3b. For example,
rectangular shaped gain regions of laser diodes 303a-c have a
particular length 315 (i.e. 1 mm or higher), particular width 316
(i.e. .about.4 .mu.m for single spatial mode or >10 .mu.m for
multi-mode operation) and front 307 (typical R<30%) and rear 305
(typically R>90%) facet reflectivity.
[0070] Special considerations for laser diode design for AWG
coupling fall in a few distinct areas: [0071] Front facet coating
[0072] Layout of laser diodes in a bar or short bar (SBar) format)
[0073] Optimizing design for maximum diode to AWG coupling [0074]
Optimizing active region design for efficient wavelength locking to
the AWG
[0075] Optimizing the laser diode active region for higher gain
includes using a wider gain bandwidth gain material to accommodate
a wider tuning range of the AWG.
[0076] The front facet coating 307 should be modified to be as low
as possible (i.e. <1%) to minimize feedback from
retro-reflections into the cavity and optical interference between
the front facet 307 and any other discontinuities in the AWG
302.
[0077] The front facet 307 of laser diodes 303a-c or input facet
308 of the AWG 302 (for example in FIG. 3a), or both, can also be
angled to reduce unintended optical feedback, as is well known in
the art.
[0078] The layout of the laser diodes 303a-c in bar 301 is
important to DAWG 300 performance. A typical laser diode bar 301 is
1 cm long with evenly spaced laser diode emitters 303a-c. It is
advantageous to reduce bar length 315 in order to reduce smile, and
especially, the misalignment between the laser diode bar 301 and
the AWG 302. In addition, for the DAWG module the spacing between
the laser diodes 303a-c will be determined by the requirements of
the AWG design. For example, a single-mode DAWG pitch might be
around 30 .mu.m in order for the AWG 302 to be a small as possible.
Another possible modification could be a non-uniform pitch as shown
in FIG. 4 by varying distances d.sub.1 through d.sub.6 between
laser diodes 103a through 103g respectively. The non-uniform pitch
could compensate for non-uniform heating and active region
temperature, or be adjusted to achieve some prescribed temperature
profile.
[0079] High coupling efficiency between the laser diode bar 301 and
the silica AWG 302 is important to enable high power output and
reliable operation of the laser. Minimizing excess optical
components is important for reduced parts cost, in addition to
enabling easier alignment or even self-alignment, which is
important for high manufacturing yield and low production cost.
[0080] Optimizing a design for optical coupling includes adjusting
beam divergence in the fast-axis and slow-axis to match the input
requirements of the AWG. Typical fast-axis divergence can be 20
degrees or more. Special wide waveguide or optical cavity designs
can reduce this to less than 10 degrees. Reduction in laser
efficiency may be compensated by an increase in laser diode to AWG
coupling efficiency.
[0081] The first coupling efficiency issue to be considered,
typically the technologically more demanding one, is the fast-axis,
or perpendicular, optical beam divergence at the front facet 507 of
the laser diode bar 501 and at the rear facet 508 of the silica AWG
502 as illustrated in FIG. 5a. The light typically diverges about
15 to 30 degrees upon exit from front facet 507 into free space.
The simplest approach is to use a lens 529 to couple the light
beam, for which a simple rod lens may be sufficient. More advanced
solutions are etching the lens 528 into the silica waveguide 504 as
shown in FIG. 5b, or making a graded index (GRIN) lens 527 as shown
in FIG. 5c, or a vertical taper by varying the effective indices of
the silica layers.
[0082] Another option to further improve the coupling efficiency is
to bond the laser diode bar 501 directly to the silica AWG 502. A
final option is to implement a mode expander section near the
output facet 507 of the diode bar 501. Such a mode expander could
be realized by, for example, re-growing a suitable structure onto
the bar or by etching away parts of the high-index layers in order
to reduce the effective index and allow waveguide modes to
expand.
[0083] The combination of an integrated fast-axis coupling optic
(any of the above options except for a discrete lens such as the
rod lens) with a simple butt coupling in the slow axis, with a gap
of only a few microns or tens of microns, has the added benefit
that the lateral mode structure in the laser bar is not
significantly disturbed during the coupling across into the silica
waveguide. Linking the modes from the laser bar directly into the
silica waveguide is a novel concept that is enabled by the
above-disclosed coupling schemes and opens the possibility of new
optical structures leveraging waveguide modal effects that have not
been previously exploited. Examples of such structures will be
discussed in subsequent disclosures.
[0084] Another coupling efficiency issue to be considered is the
slow-axis, or parallel, optical beam divergence at the front facet
607 of the laser diode bar 601 and at the rear facet 608 of the
silica AWG 602 as illustrated in FIG. 6. The light typically
diverges about 5 to 8 degrees upon exit from front facet 607 into
free space. An advanced solution is to integrate a planar lens 626
into the waveguide 604 on the silica AWG 602. The so-called planar
lens 626 can be realized by either a classical convex lens shape or
by another AWG operating in zero order.
[0085] FIG. 7a shows a conventional approach using lenses 729a-c to
couple the light beam from the front facet 707 of the laser diode
bar 701 and at the rear facet 708 of the silica AWG 702, but each
laser emitter 703a-c and waveguide 704a-c pair, respectively, will
need a separate lens. However a lens array has cost and optical
alignment implications.
[0086] Yet another approach to efficient coupling is presented in
FIG. 7b. The laser diode bar 701 is directly coupled into a freely
diverging region 720 of AWG 702. A simple option is to position the
laser diode bar 701 close enough to the AWG 702 so that there is an
effective lateral butt coupling and no focusing is needed. It
should be possible to accommodate a gap on the order of 10 micron
between front facet 707 of the laser diode bar 701 and at the rear
facet 708 of the silica AWG 702, but this rules out the use of any
discrete optics (e.g. rod lens) for fast-axis coupling, as was
discussed above.
[0087] When the laser emitter waveguides 703a-c are directly
coupled into a star coupler 720, the waveguides should be slightly
converging as they enter the coupler. It would be desirable to bend
the laser emitter waveguides 703a-c slightly so that they
effectively converge at the front facet 707 where the light beam
crosses the gap as it enters the star coupler 720 through rear
facet 708 of the silica AWG 702.
[0088] Another coupling efficiency issue to be addressed is bending
mismatch between the laser diode bar 701 and silica AWG 702,
so-called smile. One concept to alleviate this problem is to use a
shorter laser diode bar 701, i.e. limiting the length to a few
millimeter, whereby the total and percentage of bending can be
maintained at less than a few micron. For smile of constant
curvature, the total smile scales as the square of the bar width,
so a 5 mm bar will have only one-quarter the smile of a lcm bar.
Another concept is to actively bend the silica using bimetallic
strips to match the laser bar smile during alignment.
[0089] AWGs set the level of spectral filtering, which determines,
ultimately, how many single emitters can fit into a particular
assigned bandwidth. With their long, lithographically precise
optical delay lines, AWGs can achieve GHz range channel spacing
over a small free spectral range (FSR), and pack many emitter
wavelengths into a small bandwidth. However, since efficiency is a
central goal, the optical coupling losses from the laser bar and
through the AWG must be made as small as possible. This becomes the
main factor in design trade-offs for the AWG. Table 1 shows an
estimated loss budget for a hybrid DAWG structure. Because laser
diode to AWG coupling ("LD to PLC") is a fundamental challenge, it
is assigned the largest share of the loss budget. TABLE-US-00001
TABLE 1 DAWG Loss Budget Loss Element dB Loss Trans. Comments LD to
PLC 0.7 85% Planar Lens 0.2 95% AWG design loss 0.2 95% Propagation
Loss 0.3 93% 15 cm Long at 0.02 dB/cm PLC to fiber 0.2 95% Total
Efficiency 70%
[0090] AWG design losses are mainly concerned with the amount of
the freely expanding wave-front in the star coupler that is to be
coupled into the single mode waveguides of the AWG. The choice of
number of the array guides immediately determines the amount of
clipping of the expanding wave, and thus power loss. A sufficient
number of guides is generally used to keep the loss level well
below 0.1 dB, though this does impact overall size of the AWG. JDSU
has developed a number of proprietary methods for grading the
transition between the slab and the single mode array waveguides
which result in losses of about 0.1 dB per star coupler. The amount
of light lost to cladding radiation modes can be reduced by making
the transition adiabatic. These techniques are standard in certain
commercial AWG production lines, and have a direct benefit on
achieving low loss.
[0091] A major challenge for low optical loss is the physical size
of the AWG at the narrow channel spacing required. Narrow band
frequencies imply long path length delays, and all of the AWG
designs considered for the high power applications disclosed here
are much larger than commercial telecom devices. Waveguide
propagation loss, which plays a minor role in telecom devices, is
very important here. A commercial standard 0.8% index delta
waveguides have propagation losses of 0.03 dB/cm. This loss is not
due to absorption, but rather due to scattering off the etched
sidewalls of channel waveguides. Optimizing the waveguide size to
minimize modal overlap with the sidewalls, while retaining enough
confinement to provide require bend radii is one way to reduce such
optical loss.
[0092] The total optical path length through the AWG is the biggest
determining factor in the amount of propagation loss a design will
have. The size of different AWG designs can be compared using a
formula that determines R, the length (radius) of the star coupler
regions. R = n s .times. d i .times. d a .times. .DELTA. .times.
.times. .lamda. n s .times. .lamda. 0 .times. FSR ##EQU1##
[0093] Here, d.sub.i is the pitch of the input and output
waveguides at the star coupler, d.sub.a is the pitch of the array
guides, .DELTA..lamda. is the channel spacing, .lamda..sub.o is the
center wavelength, FSR is the free spectral range and n, is the
planar waveguide effective index. It is important to note that a
larger star coupler, for a given input waveguide numerical
aperture, will require more array waveguides to catch the radiating
wave, so this may have the most impact on the AWG total path
length.
[0094] An important trade-off is the pitch of the inputs to the
AWG. As discussed previously, the laser array can be coupled to a
set of individual single mode waveguides which are then directed to
the AWG on the PLC chip. Since the waveguide mode size is on the
order of 6 micron or less, the pitch and star coupler radius can be
small. Going to higher index delta can further reduce the
pitch.
[0095] One other consideration that may be of relevance is the
choice of the AWG substrate. For certain manufacturers,
conventional AWGs are fabricated on a silicon substrate. For a DAWG
device disclosed in accordance with the present invention, another
potential option would be to use a silica substrate. An all-glass
device thus generated may have some advantages from the standpoint
of fabrication and packaging processes, for example, the
possibility to etch a fast-axis lens into the AWG facet as
mentioned above. It could also be more transparent than a silicon
substrate, depending on the laser wavelength, so it may not absorb
as much of the scattered light from the AWG. On the other hand, it
will have a lower thermal conductivity so the light that does get
absorbed may cause more localized heating.
[0096] According to the invention disclosed herein, laser diode
emitters will each generate a single spatial mode output that can
be coupled into a single spatial mode at an AWG output. Single
spatial mode is important for a variety of applications requiring
high brightness, small spot sizes or large depths-of-field.
Multi-mode DAWG designs will also be presented.
[0097] Conventional narrow stripe single spatial mode laser diodes
can be integrated into the DAWG concept. However to enable even
higher single mode efficiency and power there are some new designs
that can take advantage of the AWG properties.
[0098] Exemplary state-of-the-art narrow-stripe single-mode laser
diode output characteristics are shown in FIG. 10a, while the
corresponding terminal voltage characteristics appear in FIG. 10b.
The commercially available 980 nm JDSU Model 6560 laser diode is
rated for 0.65 W operation and the power conversion efficiency
(PCE) peaks at about 55%. JDSU has also made high power 865 nm
single-mode diodes, an example laser diode being operated at more
than 1 W optical output power. The reduction in wavelength from 980
nm to 865 nm corresponds to a higher terminal voltage due to an
increased energy band-gap in the semiconductor gain region, while
slope efficiency (higher photon energy) is also higher, so the net
shift in PCE going from 980 nm to 865 nm is small.
[0099] Two example designs for a narrow-stripe laser DAWG are shown
in FIGS. 7a and 7b. The latter makes use of integrated coupling
optics at the rear facet 708 of the silica AWG 702, a cross-section
of which is illustrated in FIG. 5b, in particular by reference
numeral 528. Both the fast-axis and slow-axis external coupling
optics are thereby obviated. For either case the laser diode design
will need to be optimized for best power coupling, for example, by
adjusting beam divergence. Also the laser must be designed for best
feedback stability using methods such as adjustment of coating
reflectivity values or by modifying the active region structure and
composition.
[0100] To reach higher single mode powers that exceed the facet
catastrophic optical degradation (COD) power density limit, the
output aperture of the device has to be increased while
diffraction-limited single spatial mode lasing is preserved. One
potential design incorporates waveguides 703a-c that have
corresponding flared sections 719a-c (FIG. 7c). Flares 719a-c have
been demonstrated at different wavelengths by many groups. JDSU has
achieved 2.2 W continuous-wave diffraction limited monolithically
integrated master oscillator power amplifier (MOPA) at 854 nm. JDSU
also reported 5.25 W power from a flared amplifier at 860 nm and
has significant Intellectual Property on various flare designs. The
theory behind a flare design is that the single-mode generated in
the back part of the laser diode experiences amplification in the
flared section 719a-c. The flared design has rarely outperformed
traditional laser diode designs for a variety of reasons, such as
low PCE due to non-uniform mode and current injection as well as
astigmatism, but these can be addressed in a DAWG combined design.
As mentioned previously, in the flared oscillator architecture,
planar lenses 720a-c would be implemented at the AWG input to
refocus the diverging output from the flare 719a-c and accommodate
the laser pitch D as shown in FIG. 7c. Various dimensions to be
optimized are designated by D, W.sub.1, L.sub.1 and L.sub.2 (giving
a total length 715).
[0101] The overlapping flare (OLF) illustrated in FIG. 7d is a
novel architecture, being essentially a variation on the flared
oscillator approach in which an array of tapered oscillators 703a-c
merges directly into a first star coupler 720 in the AWG 702. While
the laser diode waveguides 703a-c are not modified appreciably, the
corresponding flared sections 719a-c have been spread to such a
degree where their effective width W.sub.2eff is larger than the
laser diode waveguide pitch D, the overlap being such that the
pumped region is continuous along the front facet 707 of the laser
diode bar 701. The advantages are low optical complexity and higher
throughput, as well as higher laser efficiency and reduced
susceptibility to facet damage due to a larger, more uniformly
filled active region.
[0102] As indicated in FIG. 7d, there is a wide range of parameters
D, W.sub.1, L.sub.1, L.sub.2 and W.sub.2eff over which the OLF can
be optimized. Although the star coupler imposes certain
restrictions on the laser pitch, the geometry can be optimized to
provide a large active region and output aperture on the diode bar.
One of the known problems of flared lasers is beam filamentation
that makes field distribution non-uniform and limits
diffraction-limited power. JDSU has developed approaches to prevent
filamentation by patterned current pumping. In both flared
approaches, various electrical pumping schemes and multiple
contacts can be used in order to optimize efficiency and beam
quality.
[0103] Another novel architecture, which enables single-mode output
from a conventional broad-area laser through the phenomenon of
multimode interference (MMI) in step-index waveguides. Broad-area
lasers have not only much higher power but also higher inherent
efficiency than traditional single-mode lasers and tapered
oscillators since the lasing volume is more uniformly filled.
Furthermore, this approach leverages JDSU's existing investment and
demonstrated expertise in broad-area laser technology, with
demonstrated 14 W optical power output per device at over 70%
electrical-to-optical conversion efficiency.
[0104] FIG. 7e shows the physical layout of laser 700 with multiple
MMI cavity lasers feeding into the input star coupler of an AWG. A
compound MMI region is formed by the length L.sub.1 of laser diode
waveguides 703a-c on laser diode bar 701 plus a certain length
L.sub.2 of silica waveguides 704a-c on the AWG chip 702 such that
the region is self-imaging in double pass. The self-imaging length
for a symmetrically fed MMI with multiple regions of different
refractive indices is given by the following formula:
.SIGMA..sub.iL.sub.i/n.sub.i=mW.sup.2/.lamda..sub.0 [0105] where
n.sub.i is the effective index of refraction, W the waveguide
width, .lamda..sub.0 the free-space wavelength, and m an integer.
For a double-passed, symmetrically-fed 100 mm wide waveguide and a
3 mm long laser chip, the multimode silica waveguide region will be
an integer multiple of 7.4 mm long in order to provide
self-imaging.
[0106] The operation of the MMI cavity laser may be understood as
follows: the AWG 702 provides monochromatic, single-spatial-mode
feedback into each of laser diode waveguides 703a-c via single-mode
waveguides 706a-c from the star coupler (not shown). Many lateral
modes of the compound MMI region are excited, so that the input
field rapidly spreads to fill the MMI laterally. The field crosses
from the silica waveguides 704a-c on the AWG chip 702 into the
laser diode waveguides 703a-c on laser diode bar 701, undergoes
amplification, is reflected at the HR-coated rear laser facet 705
and returns through the laser diode waveguides 703a-c back into the
silica waveguides 704a-c. Following one round trip through the MMI,
the field reaches its self-imaging length and re-forms into the
single spatial mode at the point where the silica waveguides 704a-c
meet the single-mode waveguides 706a-c, respectively.
[0107] As in the other architectures, the AWG converges lasers of
different wavelengths into a single single-mode output and provides
appropriate optical feedback to each of the laser diode waveguides
703a-c. In this way, while there is a relatively small single-mode
spot in the silica waveguide at the start and end of the round trip
through the MMI, the optical field gets spread out laterally during
the time that it is traversing the front laser facet 707 to rear
facet 708 and propagating inside the laser diode waveguides 703a-c.
This enables the full inherent efficiency and optical-power
capability of the broad-area laser to be leveraged.
[0108] In a further novel architecture, fundamentally different
from the preceding one, the multimode output end of the compound
MMI region terminates directly into the star coupler of the AWG
without any intervening section comprising the single-mode
waveguides 706a-c. As a result, there is the equivalent of many
single-mode beams being coupled into each compound MMI region, each
at a slightly different color and lateral position at the face of
the MMI, as determined by the AWG. The imaging properties of the
MMI must be fixed so that each effective single-mode spot images
back onto itself following a round trip through the laser. This is
a lower-symmetry situation than in the preceding architecture, so
the criterion becomes: .SIGMA..sub.iL.sub.i/n.sub.i=8
mW.sub.2/.lamda..sub.0. where all symbols retain the same meaning
as before.
[0109] In one embodiment, a diode bar contains an array of
multimode emitters which are coupled directly across into the star
coupler of an AWG. Sample dimensions satisfying the criterion would
be a 3 mm long laser diode (used in double pass) and an emitter
width of 16 micron. Since a single-mode spot is typically 4 micron
wide, there would be the equivalent of about four resolvable
wavelengths across the width of the face of the MMI. Radiation of
each wavelength rapidly spreads across the MMI waveguide and
intermingles with the other colors as it propagates the length of
the diode and reflects back at the end facet. Upon re-arrival at
the facet, the individual wavelengths reach their imaging length
and thus resolve themselves back into their localized region with
their specific wavelength. The AWG now merges all of the
wavelengths from this MMI, along with all of the wavelengths from
all of the other MMIs, into one single-mode output beam.
[0110] There are several degrees of alignment tolerance to be
considered: vertical (aligning the planes of the laser bar and
silica AWG or PLC), horizontal (aligning the laser diode waveguides
along the SBar to corresponding AWG waveguide inputs), and
rotational, including deviation from perfect flatness (smile).
[0111] The vertical dimension is the most critical for
mode-matching and alignment, as the laser mode is typically on the
order of a micron, and the optical beam divergence is
correspondingly high. Typically this dimension causes the greatest
loss when coupling from single-mode laser diodes. One of the most
successful technologies for high-efficiency coupling of high-power
pump lasers into single mode fiber is the tapered fiber lens. These
tapers terminate in an 8 .mu.m to 9 .mu.m radius of curvature which
acts as a small microlens to couple the dimensions of the laser
(typically 1.times.3 .mu.m) efficiently into the .about.10 .mu.m
single-mode fiber (SMF). One method for forming microlenses is to
laser machine the ends of the fiber using a CO.sub.2 laser.
[0112] In a similar manner, the facet of a PLC chip can be polished
to a sharp point and then the glass can be reflowed with a CO.sub.2
laser. It is preferable to use a silica instead of silicon for the
substrate in order to avoid coefficient of thermal expansion (CTE)
mismatch in this process. The result is a cylindrical facet
microlens whose radius of curvature can be controlled by the polish
angle and the laser intensity. Both the laser facet and the silica
waveguide facet should be ultra-antireflection coated, and the
spacing between them should be a few microns. JDSU has extensive
expertise with tapered fiber lenses, achieving coupling
efficiencies as high as 85% in 980 nm pump laser manufacturing.
Since only one dimension need be controlled on the PLC, even higher
coupling efficiency can be expected.
[0113] The horizontal alignment problem is much less severe because
the dimensions of the diode structures (flares and MMIs) are on the
order of hundreds of micron. More complete descriptions of these
approaches are given in later sections, but a short discussion of
the horizontal alignment considerations are given here. In the
flared-oscillator diode architecture, novel planar waveguide lenses
can incorporated to refocus the diverging output from the flares
into single-mode waveguides that transmit the power into the input
star coupler of the AWG, as shown in FIG. 8. These waveguide lens
structures 820a-c work on the same principle as an AWG, but without
the wavelength dispersion. An input spot enters a star coupler and
spreads out to illuminate a waveguide array. The array guides
redirect the light to form a converging wave-front in output star
couplers 821a-c which re-image the input into output single-mode
waveguides 810a-c respectively. Changing the radii of curvature in
the output star coupler enables different magnifications to be
realized. Trim heaters can be used to provide a phase tilt which
can move the spot along the output focal arc, so that the device
can be adjusted to compensate for alignment errors occurring during
bonding the laser diode chip 801 and AWG PLC chip 802.
[0114] In the overlapping flare architecture, the beam propagates
directly from the diode structure into the input star coupler of
the AWG. No precise lateral alignment is needed, as the lateral
position of the diode bar with respect to the AWG will have only a
slight impact upon the output wavelength. Displacements of up to
several micron will generally cause much less than 1 GHz wavelength
shift.
[0115] In the MMI architecture, a multimode waveguide on the diode
chip needs to be coupled into an equally sized waveguide on the
silica chip. Given the width of the waveguides (on the order of 100
.mu.m) and the very small gap of only a couple of microns between
the waveguides due to the use of the facet microlens to couple in
the vertical axis, a horizontal positioning accuracy on the order
of one micron will be adequate to give very high lateral coupling
efficiency.
[0116] To address smile and rotation, it is preferable to use as
short a bar as possible (2-4 mm) to minimize the total range of
smile. This short bar length can be implemented due to the very
high PCE of the JDSU diodes with the consequent reduction in
thermal load as compared to conventional bars. If necessary, the
PLC may also be flexed slightly using bimetallic strips to match
the laser bar smile. This is another significant benefit of the
DAWG architecture compared to conventional micro-optic coupled
architectures.
[0117] One concern is that the highest intensities will occur in
the multiplexed output guide of the AWG. This must be a single mode
structure to enforce wavelength combining of the laser sources. The
small core size of single mode waveguides (and fibers) warrants a
discussion of their power handling capabilities.
[0118] The bulk silica optical damage threshold quoted in the
literature is about 20 W/.mu.m.sup.2. Typical waveguide mode sizes
(fitted Gaussian waist, 1/e.sup.2 in intensity) for 0.8% and 1.5%
index delta are 4.9 mm and 3.2 mm respectively, which implies that
the guides will be able to handle optical power on the order of 200
W. Even higher power densities have been demonstrated in silica
fiber using kilowatt CW fiber laser output. The analogy between
silica guides and fiber is accurate since they are both fabricated
using chemical vapor deposition (CVD) from the same ultra-pure gas
sources, and use the same doping materials (boron, phosphorus, and
germanium).
[0119] Depending on the feedback and output coupling scheme chosen,
there may also be a need for anti-reflection coatings on waveguide
or fiber facets. Such coatings typically have lower damage
thresholds than the bulk material and may prove to be a limiting
factor.
[0120] In the event that the damage limits are found to be much
lower than required, there is a straightforward mitigation
technique. FIG. 9a shows a typical planar waveguide 902 at the
output of an AWG star coupler in which an output beam is focussed
to a point 909 on a curved focal plane 985. Since the optical wave
is simply freely propagating in the planar waveguide or slab 902,
the slab 902 can be cut short along a line 999 in FIG. 9b, and the
wave allowed to continue focusing in free space to a
high-power-handling, large-core optical fiber 989. A cylindrical
lens 980 can be used to collect and refocus the light in a vertical
direction. Cutting the slab only a few hundred microns from the
focus will reduce the intensities substantially and allow the use
of short focal length cylindrical microlenses, simplifying
packaging and improving reliability.
[0121] Since the AWG controls the wavelength and does not require
single-mode operation, a version using multimode emitters is also
possible and covered in this DI. The output of the AWG, and the
output fiber coupled to it, would typically match the emitter size,
unless magnification or demagnification were implemented in the
AWG. For applications not requiring the highest possible beam
quality, a multimode approach may provide advantages over
single-mode including higher laser bar efficiency, higher coupling
efficiency, and simpler designs and smaller chips because of the
ability to use fewer emitters, each operating at higher powers than
in the single-mode case.
[0122] The AWG could be used to tailor the output beam quality of
the multimode emitters. For example, it could be designed to
transmit and reflect back radiation within a relatively narrow
lateral numerical aperture (NA), compared to the normal NA of about
0.15 for a multimode emitter. Alternatively, it could even be
designed to cause the emitter to lase preferentially in specific
high-order modes in order to optimize the extraction efficiency,
for example. Either of these options could be realized by tailoring
the transmission profile as a function of NA, basically by giving
the various waveguides in the array of the AWG varying transmission
values. The waveguide array could even be tailored to rearrange the
angular power density distribution.
[0123] Since the output of these devices will be multimode
laterally in any case, it is conceivable also to relax the
single-mode constraint in the vertical dimension on the AWG,
without adversely affecting the usefulness of the device. Having
two vertical modes rather than one in the AWG waveguide, for
example, could potentially greatly simplify the challenge of the
single-mode to single-mode coupling in the vertical axis between
the bar and the AWG, and increase the efficiency. However, this
will cause modal dispersion in the AWG corresponding to probably
several nanometers' shift between the two modes. This modal
dispersion could conceivably be compensated by designing the AWG so
that the spectral shift equals the AWG's free spectral range, or a
multiple of it. In this way, both modes would image into the same
spot at the AWG output, even though they would be operating in
different orders in the AWG.
[0124] In accordance with another aspect of the invention disclosed
here, innovative design concepts are presented for combining
single-mode AWG outputs into multi-mode fibers or waveguides for
high power and brightness. The reasons why separating the power
into multiple bars may be preferred can be ascribed to relaxed
cooling requirements (power distributed in larger area),
potentially higher yield from aligning smaller bars (less smile),
as well as a higher yield from matching bars to a required AWG
wavelength range.
[0125] One such concept is to use integrated DAWGs comprising
multiple bars and AWGs and combine the power using wavelength
multiplexing. This concept is illustrated in FIG. 12a, where module
1200 contains two laser diode bars 1205a,b are coupled into two
AWGs 1240a,b respectively, centered at different wavelengths so
that their spectra do not overlap. The optical power in the AWG
output single-mode waveguides 1209a,b is combined in a wavelength
combiner 1242 on the PLC chip 1202 such as a Mach-Zehnder
interferometer. The output, still in one single-mode waveguide
output is suitable for coupling to fiber 1210.
[0126] There are potential advantages to spatially combining narrow
single-mode waveguides from a DAWG module into multi-mode fibers.
Firstly, single-mode laser diodes are more easily cooled using
passive heatsinks or thermoelectric coolers than broad stripe bars,
which generally require water cooling. Secondly, single-mode laser
diodes can achieve higher power density than multi-mode laser
diodes due to absence of multi-mode filaments and non-uniform facet
power density. Finally, the inherent mode stability and selectivity
of single spatial mode narrow laser stripes may permit better laser
performance than multi-mode stripes which can have filaments and
non-uniform mode profiles.
[0127] Another basic concept is shown in 12b. A series of parallel
single-mode laser diode bars 1205a-c are coupled into a series of
AWGs (three only shown in this example) located on AWG chip 1202.
The outputs of single-mode waveguides 1209a-c are made to converge
to an input into a multi-mode fiber 1211. There is a loss of
brightness but an increase in total power. The single-mode
waveguides 1209a-c can be aligned spatially (higher total power but
uniform power density) or focused together to a spot (higher power
density but lower brightness due to different beam angles).
[0128] Examples of how single-mode beams combine in multi-mode
fiber is shown in FIGS. 11a to 11c.
[0129] FIG. 11a shows the simplest realization of the concept. A
linear array of single mode waveguides 1180 fits spatially in a
large core 1160 of multi-mode fiber 1170. A 2D array 1182, as
illustrated in FIG. 11b, could be used to further fill a core 1160
of fiber 1170 either by stacking AWG outputs, combining AWG outputs
using turning mirrors or by fusing rectangular core fiber. FIG. 11c
shows a concept where a linear array of single mode waveguides 1180
from the AWG output is aligned to a matching rectangular core 1162
of fiber 1170 thereby increasing brightness.
[0130] Another possible concept is detailed in 12c. Emission from
laser diode chips 1205a-d of laser system 1200 is collected by
silica AWGs 1255a-d and output on single mode waveguides 1209a-d
respectively, which are directed into a linear array 1212 of single
mode fibers. The linear array of fibers 1109a-d seen in end view in
FIG. 11d is aligned in a suitable support 1150 such as a silicon
optical bench. The other end of fibers 1109a-d can then be bundled
together to form an end view as seen in FIG. 1e to conserve some of
the brightness compared with a similar dimension high numerical
aperture multi-mode fiber 1111. Once bundled, the high power output
can be coupled into or fused with a multi-mode fiber or the power
can be used directly.
[0131] The final concept is an extension of the last method.
Separate DAWG modules can be coupled individually into single mode
fibers. These separate single-mode fibers can be bundled as in FIG.
11e.
[0132] Now an aspect of the invention disclosed will be considered.
By combining the outputs of narrow wavelength range DAWG modules
with wavelength selective filters, kilowatt levels of optical power
can be multiplexed into narrow wavelength bands with superior beam
quality and minimum loss of total brightness. The wideband, flat
transmission and reflection characteristics of the thin film
filters (TFF) are critical for combining the DAWG outputs.
[0133] The JDSU concept to achieve a kilowatt level system with a
narrow wavelength, diffraction limited beam is based upon passive
wavelength combination of high power, single-mode laser diodes. The
system output consists of co-linear, co-propagating,
wavelength-multiplexed single spatial mode beams that fill the
output aperture. This approach obviates the need for complex active
phase control of sub-beams to create a coherently coupled spatial
array.
[0134] This novel approach is enabled by:
[0135] a) High power, high efficiency single-mode laser diode
arrays;
[0136] b) Silica based planar light-wave circuits (PLCs); and
[0137] c) Wavelength division multiplexing beam combiners using
thin film interference filters.
[0138] There are many parameters that must be optimized to a given
set of system requirements. An example 5 kW 10 nm bandwidth system
is given in Table 1. TABLE-US-00002 TABLE 1 Design targets to meet
an example 5 kW in 10 nm system Example Category Parameter 5 kW
Design System Total System Power (kW) 5.0 Requirements System
Wavelength Range (nm) 10 Beam Propagation Factor (BPF) 0.60 Laser
Center Wavelength (nm) 975 Diode Bar Power per Laser Diode (W) 10
Laser Diode Efficiency (PCE) 65% D-AWG Diodes per Bar and AWG 30
Module DAWG Wavelegth Spacing (GHz) 2.0 Diode to AWG Efficiency 65%
Power per AWG Module (W) 195 DAWG modules per polarization 16 Total
number of DAWG Modules 32 WDM Beam Polarization Mulitplexing 2
Combiner Level 1 Wavelength Channels 4 Level 2 Wavelength Channels
4 AWG Module Range (GHz) 60 Total Wavelength Range (GHz) 3189
Wavelength Fill Factor (Light:Total) 1.0:3.3 Beam Combiner
Efficiency 80%
[0139] The laser diode design is targeted to produce a high power
single-mode device. For this example the diodes reach 10 W.
Previous experience in flares and high-efficiency bars indicate
this is difficult but potentially achievable. A shortfall in diode
power can be compensated by tighter spacing of the AWGs or WDMBC
filters.
[0140] For the example, the DAWG combination is targeted to be a 30
channel device with a 2 GHz channel spacing operating up to 70%
optical efficiency and emitting over 200 W in a single-mode
aperture. The 2 GHz designs have been modeled and are feasible
using existing capabilities. The tightest feasible spacing (around
1.5 GHz) is limited by the available 200 mm wafer size. The 65%
optical efficiency is also challenging but achievable. 200 W in a
single-mode silica waveguide and fiber is below the theoretical
damage threshold limit.
[0141] The beam combiner in the example has three levels, including
one polarization and two wavelength levels. To evaluate beam
quality a Beam Propagation Factor (BPF) is defined as the laser
optical output power in a specified far-field bucket divided by the
total optical output power radiating from the defining near-field
exit aperture of the combined laser beam. The far field bucket is
defined as 1.44 times the diffraction-limited spot area defined as
(.pi./4)(.theta..sub.DLf), where .theta..sub.DL=2.44.lamda./D where
.lamda. is the laser center wavelength, f is the focal length of
the optics used to form the far field spot, and D is the defining
exit aperture of the combined laser beam.
[0142] For this example the BPF was targeted at 0.60 which is a
useful number for many applications. This number is attainable
because the beam combiner inputs are all circular single-mode
polarized beams directly from single-mode fiber or collimated AWG
output and there are no intentionally beam-aberrating elements in
the design. However, the requirements and tolerances on the beam
combiner optical components are fairly stringent.
[0143] The system architecture concept is shown in FIG. 13a. First,
a high power, high efficiency laser diode bar 1301 is coupled into
a PLC-based AWG 1302. The AWG controls the wavelength of each
emitter extremely precisely and combines them into one single
spatial mode output. The PLC-based AWG 1302 also provides an
integration platform that enables novel emitter architectures that
scale to multiple watts per emitter.
[0144] Next, the output of AWG 1302 is coupled out via output fiber
1311 into collimating and beam expanding optics 1370, passing on to
polarization beamsplitter 1360 to be combined with another output
from a similar module at orthogonal polarization. The polarization
combined beam is then deflected by a turning mirror 1340a into a
second level wavelength division multiplexing beam combiner (WDMBC)
1380a. The WDMBC 1380a combines this beam with other similar ones
1330 using thin film interference filters (TFFs). TFFs can combine
these narrow wavelength bands efficiently to build up to the
kilowatt level within a tight wavelength range. The output of WDMBC
1380a is input into a third level WDMBC 1380e with the aid of
turning mirrors. This input is further combined with other beams
1350 and finally the aggregate optical power 1390 is output.
[0145] As mentioned above, the first level of a three level beam
combiner is polarization multiplexing. FIG. 13b depicts a concept
for such a polarization multiplexer 1350. Output beams of two DAWGs
1320 are collimated and expanded in the optics 1370 before being
combined with orthogonal polarizations in the polarization
beamsplitter 1360 to produce a combined output beam 1375.
[0146] FIG. 13c shows a schematic of an exemplary 5 kW system and
its components. The example system uses 32 DAWG modules combined in
pairs via polarization multiplex into sixteen modules 1350 (only
one referenced as example), which are further combined by five
wavelength multiplexing modules 1380a-e. At three ports of each
wavelength multiplexing module 1380a-e is a wavelength selective
TFF, with a total of 15 TFFs required for the system. The maximum
number of reflections for any beam off a polarization- or
wavelength-selective optical element is seven, the minimum is zero,
and the average is 31/2. These design choices will change depending
on the exact power level and bandwidth requirements.
[0147] The proposed WDMBC approach provides the specified beam
propagation factor (BPF) of 0.60 without stringent requirement on
phase matching of individual sub-beams. This passive solution
minimizes complexity and enhances robustness. The effects of
near-field fill factor, tilt, curvature, and phase ripple can be
modeled and trade-offs can be made to establish initial beam
quality budgets. The variation of BPF to relative near-field
aperture is illustrated in FIG. 14 and the ideal BPF maximizes at
0.98 for an aperture of 1.5 times the beam waist, w.sub.0. A
smaller near-field aperture clips the beam and adds diffraction
losses, while larger size reduces the size of the far-field bucket
and the fraction collected.
[0148] The AWG modules deliver single spatial mode power via
single-mode fibers (or in free-space) to the beam combiner. Each
beam experiences a number of bounces through the beam combiner
which adds tilts, curvatures, and phase ripple to the combined
output beam. Aberrations that degrade the beam quality and BPF have
been budgeted using a propagation model as summarized in the plots
of FIG. 15 for an aperture 1.5 times the beam waist respectively,
where the effects of tilt 1501, curvature 1502, and phase ripple
1503 (8-cycle sinusoidal) on the BPF are plotted as functions of
magnitude. It is reasonable to assume that these aberrations are
random and uncorrelated so that the output wave-front errors are
proportional to the square root of the number of bounces. These
design curves enable estimation of a preliminary wave-front error
budget, per beam combiner optical element.
[0149] The near-field beam of the system is a Gaussian filling the
near-field aperture as shown in FIG. 16a, where the aperture size
is related to the 1/e.sup.2 beam waist, w.sub.0. The ideal beam
propagation factor (BPF) maximizes at 0.98 for an aperture of
1.5w.sub.0. By including acceptable non-idealities in the optical
system a 0.60 BPF is possible.
[0150] An example modeled far-field of a combined beam is shown in
FIG. 16b, including aberrations, where BPF=0.60. The asymmetry is
due to random optical aberrations (tilt, curvature and phase
ripple). The aberrations are a result of beam misalignment, optics
imperfections and thermally induced effects.
[0151] In summary, beam combination via wavelength multiplexing
leads to achievable optical tolerances and eliminates the need for
complex, active phase control, making it possible to meet the BPF
requirements of this system.
[0152] As mentioned before, the collimated output of the DAWG
modules is combined using a series of polarization and wavelength
beam combiners. These will now be considered in more detail.
[0153] The beam combining system collimates the DAWG output beams
using a lens system. To keep the power per unit area to a
reasonable level the beam will be expanded to dimensions on the
order of millimeters. The target power density is less than several
hundred W/cm.sup.2 at the collimation stage which is low enough to
permit the use of a transmission lens (or lenses). The lenses
should be adjustable to permit precise beam alignment and
adjustment so that the beams from all bands are identical at the
output.
[0154] The first multiplexing stage is preferably polarization
multiplexing for two reasons: 1) the narrow bandwidth at this stage
eases the design of the polarizing filters, and 2) it facilitates
use of a cube beam combining geometry that does not optically
distort the beam.
[0155] A thin-film based polarization multiplexer operates by
transmitting one polarization through a filter while reflecting the
other polarization from the filter, both into a common beam. The
polarization multiplexer may be a single plate. In this case it may
be used at an angle of incidence of approximately 60 degrees. The
theoretical spectrum of such a polarization combiner is shown in
FIG. 17. Another possible implementation would employ a 45 degree
air-spaced cube beam splitter, which reduces induced beam
aberrations.
[0156] The subsequent multiplexing steps are wavelength
multiplexing. After polarization combining, the outputs from the N
AWGs is then in the form of N/2 beams, each of the beams
characterized by a distinct wavelength band as shown schematically
in FIG. 18 for an exemplary system with N=32 DAWG modules.
[0157] By using multiple levels of multiplexing, power in the
kilowatt range can be achieved. In one possible implementation,
each level employs a zigzag multiplexer of the type shown in FIG.
19a. In wavelength multiplexer 1940, AWG bands 1-4, indicated by
1910a-d respectively, are reflected in turn from TFFs 1930a, 1930b
and finally 1930c, the combined beam exiting at port 1920.
[0158] Another possible implementation using two levels of
wavelength multiplexing will first combine DAWG Modules outputs in
groups of four wavelength bands (Level 1 bands) and then to combine
four such bands into a final output (Level 2 bands).
[0159] The exact geometry will be arrived at through trade-offs of
filter performance, power-handling capability of filters, small
device size, etc.
[0160] In the 5 kW example of a Level 1 multiplexer, the filters
must transmit a 60 GHz AWG band while reflecting one or more of the
other AWG bands. The transition region between bands can be 40 GHz.
A plot illustrating Level 1 multiplexing is given in FIG. 20.
Filter 1 reflects AWG Band 1 (centered at 100 GHz relative
frequency), while transmitting AWG band 2 (centered at 200 GHz
relative frequency). Filter 2 transmits AWG band 3, while
reflecting bands 1 and 2. Similarly Filter 3 adds AWG band 4 into
the beam.
[0161] Filters of this type are commonly fabricated for
telecommunications applications in the 1550-nm wavelength region,
commonly having dimensions of 1-2 mm on a side. For high-power
applications, the filters must be scaled up in size to have a clear
aperture (having uniform coating performance) of 20-25 mm, and in
total power-handling capability from a few Watts to a few
kilowatts. Level one multiplexing filters need to handle about 500
W in transmission, and up to about 1500 W in reflection.
[0162] The outputs of four Level 1 devices 1940a-d can be
multiplexed using another zigzag multiplexer of the type shown in
FIG. 19b to produce a combined beam 1925. A geometrical arrangement
may provide for the Level 1 zigzag paths to be orthogonal to the
zigzag path of the Level 2 multiplexer. If such a scheme is
employed, the space occupied by four Level 1 multiplexers and one
Level 2 multiplexer would be on the order of 540 mm.times.180
mm.times.120 mm. FIG. 19b shows the Level 2 multiplexer with
outlines of the four Level 1 multiplexers.
[0163] The Level 1 bands consist of four channels and three
dead-spaces totaling 360 GHz. The Level 2 filters therefore must
combine the 360 GHz bands. The dead-space between Level 1 bands is
preferably 140 GHz. By using several zigzags instead of just one,
the total number of reflections may be reduced as compared with
using a single large zigzag. This decreases the amount of beam
distortion introduced, and therefore should improve beam quality.
In the disclosed architecture, the worst-case beam undergoes 7
reflections: one in the polarization multiplexer, three in the
Level 1 multiplexer, and three in the Level 2 multiplexer. This
compares with 16 for the worst case beam if a single zigzag
combiner is used: one in the polarization multiplexer, 15 in the
zigzag multiplexer.
[0164] Several families of products are expected to be enabled by
the laser diode coupled AWG (DAWG) technology.
[0165] The most straightforward products derived from the DAWG are
potentially high-power single-mode fiber-coupled (FC) modules.
These products could be direct replacements for today's
commercially available CW fiber lasers (FLs). They would bring the
advantages of wavelength versatility (most FL's are based on Yb at
1050-1100 nm), compactness, improved efficiency, and enhanced
reliability due to the elimination of double-clad fiber.
[0166] A second family of products would be a multimode
implementation of the DAWG in which broad-area emitters are
spectrally multiplexed to provide output powers in the 100 W range
from a 25-100 micron core fiber. Lifting the constraint of
single-mode operation can make such devices smaller, cheaper, and
more manufacturable than the single-mode version. They would still
have much higher brightness than conventional multimode FC bars and
would address many applications now being served by fiber lasers.
For applications demanding pulsed output or extremely high
brightness, they would serve as a very cost-effective pump for Yb
fiber lasers or disk lasers. One particular area of interest would
be as a pump source for amplifying picosecond or femtosecond pulses
in double-clad fiber, where the fiber length must be kept extremely
short to mitigate pulse distortion, for which a very high
brightness pump is advantageous. The multimode DAWG may be a key
technology that enables commercially viable, high-average-power
ultrashort-pulse sources.
[0167] The narrow wavelength high brightness pump technology can
improve fiber laser efficiency in a number of ways. Firstly, the
tight wavelength control makes them suitable for optical pumping of
the narrow 976 nm Ytterbium line which has a better quantum defect
and higher absorption than the broader 915 nm line. Secondly, the
high brightness permits pumping into a smaller diameter fiber.
Conventional bars and single emitters, with relatively low
brightness, require a large inner cladding (typically 400-600
microns in diameter) and a high-NA polymer outer clad fiber, which
ends up being very long (typically >50 m) with a much higher
loss per unit length. Thirdly, the combination of these
improvements translates to a shorter overall fiber length with
lower overall absorption and scattering and lower threshold power
requirements. Fourthly, these changes will support lasing in the
1040 nm band which has a lower quantum defect. For example the
impact of pumping at 976 nm and lasing at 1050 nm, versus 915 nm
and 1100 nm, is to improve the quantum defect from 83% to 93%. The
net effect of these improvements can enable a fiber laser
optical-to-optical efficiency in the 80% range.
[0168] Finally spectral multiplexing of either the single-mode or
the multimode DAWG modules will enable reaching higher output
powers without impacting beam quality. For example, it will be
possible to spectrally MUX a number of multimode DAWGs to create a
2-5 kW industrial cutting/welding direct diode solution that would
be extremely reliable and cost-effective. This could be an
appealing alternative to current FL-based architectures being
explored, in which a number of 100-200 W single-mode fiber lasers
are combined spatially into a multimode output.
[0169] The DAWG module is interesting for breakthrough products in
a variety of markets. Single mode DAWGs can replace fiber lasers
for non-metal marking, drilling and other material processing
applications. Multi mode DAWGs can also serve low brightness
material processing applications. Furthermore multi-mode (or
single-mode) DAWGs can be spatially or spectrally combined to reach
the kilowatt level metal material processing market. Finally DAWGs
will enable more efficient, higher brightness, higher power fiber
lasers.
* * * * *