IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 11, NO. 3, MAY/JUNE 2005
567
Laser Beam Combining for High-Power,
High-Radiance Sources
T. Y. Fan, Senior Member, IEEE
(Invited Paper)
Abstract—Beam combining of laser arrays with high efficiency
and good beam quality for power and radiance (brightness) scaling
is a long-standing problem in laser technology. Recently, significant
progress has been made using wavelength (spectral) techniques and
coherent (phased array) techniques, which has led to the demonstration of beam combining of a large semiconductor diode laser
array (100 array elements) with near-diffraction-limited output
(M2 ∼ 1.3) at significant power (35 W). This paper provides an
overview of progress in beam combining and highlights some of
the tradeoffs among beam-combining techniques.
Index Terms—Diode lasers, fiber lasers, laser arrays, laser beam
combining, phased arrays, wavelength-division multiplexing.
I. INTRODUCTION
HE development of high-average-power laser systems
with nearly ideal beam characteristics has been an ongoing
effort since the earliest days of laser technology. In the development of solid-state lasers, progress has been slow, largely limited
by thermooptic effects that cause distortion in the laser beam,
which sets a limit on the diffraction-limited average power. This
development has also been expensive because these have been
one- or few-of-a-kind laser systems involving substantial research and development costs. An alternative approach to building high-power lasers is to use arrays of relatively lower power
lasers; however, this approach requires that the beams from the
array elements be combined to have the propagation characteristics of a single beam. Semiconductor and fiber gain elements
have attractive attributes for beam-combined systems because of
their ease in building in array formats, their high efficiency, and
the ability to get near-diffraction-limited beams from individual
elements. With recent advances in beam-combining technology,
laser arrays are becoming a viable alternative for high-power,
good-beam-quality laser systems.
Beam-combining techniques on laser arrays can be characterized in three broad classes, notionally illustrated in Fig. 1.
The first is side-by-side beam combining in which the array
elements may (or may not) operate at the same wavelength, but
nothing is done to try to control the relative spectra or phases of
the elements. Conventional diode-laser arrays (linear bars and
two-dimensional arrays) fall into this class. This class of beam
combining is not addressed in this paper, as the radiance of these
types of sources cannot be any greater than the radiance of a
single array element.
T
Manuscript received February 11, 2005; revised April 15, 2005.
The author is with the Quantum Electronics Group at MIT Lincoln Laboratory,
Cambridge, MA 02139 USA.
Digital Object Identifier 10.1109/JSTQE.2005.850241
Fig. 1. Notional schematics of the broad classes of beam combining. (top)
Side-by-side beam combining. (middle) Coherent beam combining with tiledaperture implementations on the left and with filled-aperture implementations
on the right. (bottom) Wavelength beam combining using serial implementations
on the left and parallel implementations on the right.
The next class is coherent beam combining (CBC) or phased
arrays in which all of the array elements operate with the same
spectrum and the relative phases of the elements are controlled
such that there is constructive interference. Historically, most
of the effort to obtain good beam quality through laser beam
combining has attempted to use this class of techniques. This is
the analog of phased-array transmitters in the radio-frequency
(RF) and microwave portions of the electromagnetic spectrum,
but in the optical domain CBC has proven to be difficult because of the shortness of an optical wavelength. The phases of
the array elements need to be controlled to a small fraction of a
wavelength (2π phase); for the optical portion of the spectrum,
the wavelength is on the order of 1 µm. CBC has been demonstrated for small arrays, but identifying robust simple phasedarray approaches for combining large arrays (tens to hundreds
of elements) with nearly ideal beam quality has been elusive.
The last class is wavelength beam combining (WBC), in
which the array elements operate at different wavelengths and
then a dispersive optical system is used to overlap the beams
from the elements in the near and far fields. WBC has also been
used historically, but not nearly as often as CBC. Wavelengthdivision multiplexing in optical communications falls into this
class, in which multiple wavelengths are put into a singlemode optical fiber. Again, the difficulty has been identifying approaches that robustly combine large laser arrays in a
1077-260X/$20.00 © 2005 IEEE
Authorized licensed use limited to: MIT Libraries. Downloaded on January 26, 2010 at 16:19 from IEEE Xplore. Restrictions apply.
568
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 11, NO. 3, MAY/JUNE 2005
simple manner, but in WBC, the development of external-cavity
grating-stabilized beam-combining implementations has led to
the demonstration of near-ideal combining, M 2 = 1.3, on large
laser arrays, 100 elements [1]. Finally, polarization multiplexing
is a form of beam combining, but it is not of interest for large
arrays, as the improvement is limited to two.
The key metrics for the outputs of these types of systems are
the power and beam quality, which can be used to express the
radiance (or brightness), defined as power per unit area per unit
solid angle. The radiance B is
B=
CP
λ2 (M 2 )2
(1)
where P is the power, λ is the wavelength, M 2 is the beam
quality (assuming a radially symmetric beam), and C is a constant that depends on the definition of beam size and divergence
angles. If a Gaussian beam definition is chosen, then C = 1.
In side-by-side beam combining, the brightness (radiance) is
no better than that of a single element (brightness-theorem limited [2]), and, therefore, this type of beam combining cannot
be used to obtain near-diffraction-limited outputs from laser arrays. In CBC, the radiance ideally scales as ff N , where ff is an
efficiency factor less than or equal to 1 and N is the number of
elements. WBC scales similarly as fg N , where fg again is an
efficiency factor less than or equal to 1.
Another useful metric for these systems is the Strehl ratio
S, which characterizes the on-axis far-field intensity of a beam
propagated from a near-field hard aperture. S is defined as the
ratio of this on-axis intensity to that of an ideal (in phase and amplitude), equal-power top-hat beam filling the same hard aperture, and S cannot be larger than 1. It is particularly useful for
describing nonidealities in CBC systems. Another way to look
at the nonideality of CBC systems is to recognize that, for the
ideal phasing of an array of N elements, the on-axis far-field
intensity will be N times higher than for the same array with no
fixed phase relations (incoherent) among the elements [3]. This
can be simply understood by recognizing that the radiance of
the incoherent array is at best that of a single element and that
an ideally phased system will have a radiance of N times this
amount.
As a note, it has been often stated that the on-axis intensity of
CBC systems scales as N 2 (e.g., see [4]), which appears to be
at odds with the stated brightness scaling. The on-axis intensity
does scale as N 2 , but only if the emitting aperture grows proportional to N . If instead the aperture size is constrained to a fixed
size, then the on-axis intensity scales only as N . Another way
of looking at this is that the beam quality cannot be better than
diffraction limited or the Strehl ratio cannot be larger than 1.
Excellent reviews on CBC systems [2], [5], [6], particularly
on semiconductor laser arrays [3], [7]–[9], are available. Consequently, this paper is not meant to be an exhaustive review. It discusses in the next section the fundamental requirements imposed
on CBC and WBC systems to achieve near-diffraction-limited
output. WBC has fewer and simpler requirements. This discussion is followed by sections on implementations and results of
CBC and WBC systems. The final section makes some comparisons between coherent- and wavelength-combined systems.
TABLE I
FUNDAMENTAL REQUIREMENTS FOR ACHIEVING IDEAL BEAM COMBINING
II. REQUIREMENTS AND CHARACTERISTICS BY
BEAM-COMBINING CLASS
CBC and WBC fundamentally impose different requirements
on the output of the array elements because electric fields are
vectorally summed in CBC, whereas in WBC, powers are added.
The fundamental requirements on array-element output for
attaining ideal combining are summarized in Table I. These
fundamental requirements are independent of the exact implementation. However, in some specific implementations, it might
be desirable for additional requirements to be levied against
array-element characteristics. For example, in WBC, it might
be desirable to have the elements have the same polarization because some optical components might be polarization sensitive;
however this is an implementation-specific desire as opposed
to a fundamental requirement. Clearly, CBC fundamentally imposes more requirements on element output than WBC.
Given that WBC simply adds powers (i.e., is incoherent combining), it is easy to see why the fundamental requirements are
relaxed. One fundamental requirement is that the power spectra
of the elements not overlap with each other. If there is an overlap in power spectra, then to beam-combine effectively, phases
would need to be controlled in the overlapping portion of the
spectrum, which, of course, is coherent combining. In CBC, the
electric fields of the elements must constructively add, which
then imposes requirements that the polarizations must be controlled and the amplitudes of the elements must be controlled
at every instant in time. The output power spectra need to be
the same, and the phases need to have the correct relationships
to constructively add. As element properties deviate from that
listed in Table I, the efficacy of beam combining decreases
(i.e., S decreases).
As illustrated in Fig. 1, CBC implementations can be divided
into two subsets characterized by the output formatting: tiledaperture and filled-aperture implementations. In tiled-aperture
implementations, individual elements have outputs that are
adjacent to each other, and there is interference only in the
far field. This type of implementation can be thought of as a
synthesized plane wave. Clearly, to minimize side lobes and to
obtain the maximum far-field intensity, the fill factor must approach unity (i.e., the spaces between tiles must be minimized).
In filled-aperture implementations, the interference occurs in
the near field. The beam combiner in a filled-aperture system
can be thought of as the inverse of a beamsplitter, and proper
phase, amplitude, and polarization relations among the multiple
beams must be maintained for efficient combining. Alternatively, in a fiber system, the beam combiner can be thought of
as a 1–N splitter being run in reverse. There is a direct equivalence [2] between tiled- and filled-aperture (called aperture
Authorized licensed use limited to: MIT Libraries. Downloaded on January 26, 2010 at 16:19 from IEEE Xplore. Restrictions apply.
FAN: LASER BEAM COMBINING FOR HIGH-POWER, HIGH-RADIANCE SOURCES
filled and superposition, respectively, in [2]) systems, and so the
basic element-control requirements and the effects of nonideal
element properties are essentially the same.
III. COHERENT COMBINING
This section presents the requirements of CBC in more detail,
particularly as the array-element outputs degrade from being
ideal. This discussion is followed by a review of some of the
implementations and results.
S is degraded by errors in relative phase control, relative amplitude control, relative polarization control, relative element
beam pointing, and a less-than-unity fill factor in tiled-aperture
systems (or less than ideal near-field overlap in filled-aperture
systems). The most difficult part of successful coherent combining is the need to control the phase. The other main source of
nonideal performance in tiled-aperture systems is a less-thanunity fill factor, so the primary focus here is on these two error
sources.
First, consider the phase-control requirements for coherent
combining. Nabors [10] and Leger [2] analyzed these requirements in the context of a tiled-aperture system. The simplest
case is what Nabors calls the uncorrelated case. In this case,
the errors in the phase control of the elements can be described
as being relative to a common reference plane. In the limit
of large N , the relative root-mean-square (rms) phase errors
can be viewed as being equivalent to an rms wavefront error
for a continuous phase sheet (the continuum limit). To limit
the degradation of the Strehl ratio caused by phase errors to
0.7, the rms phase errors must be limited to ∼λ/10 or better
(in the continuum limit), the same as the requirement on rms
wavefront error for a continuous phase sheet.
Nabors [10] also defines the correlated case. In this case, the
phase-control mechanism is such that phase control acts between
adjacent array elements, and therefore, the phase errors are defined as occurring between adjacent elements. The degradation
in S is dependent on the number of array elements in the continuum limit. To maintain a fixed S, the allowed rms phase error
scales proportional to N −1/2 in the continuum limit. In other
words, as the array gets larger, less phase error between adjacent
elements is allowed for a given S to be attained. This occurs in
the correlated case because effectively the phase random-walks
across the aperture.
Next, consider the effect of a less-than-unity fill factor on
the on-axis intensity. The effect of near-field beam patterns on
the far field in tiled implementations is well known from array
antennas [11]. There is a very simple relation between S and
the fill factor, assuming an otherwise ideal phased array; S of
a tiled-aperture system is simply given by the fill factor [2].
Here, the fill factor is defined for tiles with top-hat intensity
distributions, with the fill factor calculated by dividing the area
of the tiles by the total area of the transmitting aperture. Another
aspect of tiled-aperture systems is that the fraction of power in
the central lobe and the on-axis intensity are not affected by
exact placement of the tiles within the aperture; the placement
of the tiles only affects the sidelobe structure [2]. For efficient
beam combining (S approaching 1), the near-field placements
569
of the elements need to be controlled to a small fraction of the
near-field tile size to achieve unity fill. In contrast, if the fill
factor is small, then the near-field placement does not need to
be controlled precisely, at the expense of a reduction in S.
Finally, consider the amplitude-control and far-field pointing
requirements. Leger [2] looks at the amplitude-control requirements in the context of a tiled-aperture system. For S to approach
1, the amplitude nonuniformity across the aperture needs to be
minimized. In fact, in the case of uniform phase, S is simply the
square of the mean amplitude across the aperture divided by the
mean of the amplitude squared (intensity).
Errors in relative far-field beam pointing among the elements
can be viewed as equivalent to tilts of the wavefront in the
near field. These tilts can be expressed as an rms wavefront
error, which leads to a decrease in S. To have S approach 1 in
a tiled-aperture implementation, the far fields of the elements
must be pointed in the same direction to within a small fraction
of the far field of a single element. The combination of this
far-field pointing requirement and the need to control the nearfield placement to within a small fraction of an array-element
near-field size leads to the requirement on diffraction-limited
element alignment in Table I.
A. Implementations of CBC
CBC has been applied to arrays of semiconductor, solid-state,
gas, and fiber gain elements. Until recently, much of the efforts
in CBC can be described as being marginally successful at best.
Early demonstrations were not particularly robust against perturbations and had unclear scalability to large arrays and higher
power. Some of the difficulties arose because of a lack of analysis of the expected performance of various implementations.
Much of the work in the mid-1980s through the 1990s was on
CBC diode laser arrays, driven by the potential inherent simplicity of semiconductor systems, but limited success was achieved.
There are excellent reviews on these efforts [2], [3], [7]–[9]. Advances in understanding of the requirements, a more accurate
analysis of specific implementations, and a better appreciation
of the difficulties in scaling to large arrays have led to recent
demonstrations of coherent combining that appear to be scalable
both to array size and power. Many implementations of CBC
have been reported and often fall into one of the following approaches notionally illustrated in Fig. 2: common resonator,
evanescent-wave or leaky-wave coupling, self-organizing,
active feedback, and nonlinear optical.
In common-resonator approaches, the array elements are
placed inside an optical resonator, and feedback from the resonator is used to couple together the elements [12]–[20]. This
implementation can be viewed as being a spatially sampled version of a bulk resonator. Consequently, in analogy to a bulk
resonator, the challenge for the resonator is to force lowest order transverse-mode operation. In a bulk resonator this might
be done by using an intracavity spatial filter. In CBC using
common resonators, mode selection has been done using intracavity spatial filters and the Talbot effect. Although these
common-resonator approaches have been successful at low average power, as the power increases, typically there has been
Authorized licensed use limited to: MIT Libraries. Downloaded on January 26, 2010 at 16:19 from IEEE Xplore. Restrictions apply.
570
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 11, NO. 3, MAY/JUNE 2005
Fig. 2. Schematics of notional approaches to CBC. From the top: common
resonator, evanescent or leaky wave, self-organizing or supermode, active feedback, and nonlinear optical (phase conjugation).
difficulty obtaining low-order transverse mode operation. One
issue is variation in the optical path length, known as piston
error, among the array elements particularly at higher powers,
which can be viewed as being the equivalent of wavefront distortion in a bulk optical element. Piston error makes it difficult
to attain lowest order transverse mode operation, in analogy to
distorted optical media in bulk lasers. This common-resonator
approach has been more successful with CO2 lasers [17]–[20]
than with diode or solid-state lasers because of the much longer
10-µm wavelength of the CO2 laser. This lower piston error
(in number of waves) has enabled an 85-element CO2 laser
array to be phase-locked [20].
Evanescent-wave or leaky-wave coupling approaches [7]–[9],
[21]–[26] have been used extensively, particularly in scaling to
CBC semiconductor laser arrays. In this approach the array
elements are placed sufficiently close together that their field
distributions overlap and thereby couple the elements. In-phase
coupling of the array elements is desired to obtain high onaxis far-field intensity; however, it has been observed that the
coupling often is predominantly π out-of-phase, giving a power
null on-axis. For out-of-phase coupling, there is a null between
the array elements that, compared with in-phase coupling, tends
to lead either to minimum loss, particularly if the space between
elements is lossy, or higher gain because the spatial overlap of
the mode with the array elements is better. The other difficulty
in evanescent-wave or leaky-wave approaches is scaling to large
arrays because they are typical of the correlated case of Nabors
[10] discussed previously.
In the self-organizing, also known as supermode, approach,
the array is composed of elements with very different optical
path lengths, and the optical spectrum self-adjusts to minimize
the loss of the array [27]–[35]. This approach is essentially a
Michelson interferometric resonator [27], [28], generalized to
arrays of more than two elements. There are multiple ways
of understanding this type of resonator. One is to think about
the reflectivity of the resonator as a function of wavelength as
seen from the output coupler. The wavelengths of the reflectivity maxima will change as the array-element path lengths
vary, and if a sufficiently high reflectivity occurs at a wavelength within the gain bandwidth of the array elements, then
the array will oscillate. Another way of viewing this approach
is to consider each of the array elements as a separate optical resonator (from the point of view of axial-mode positions).
The array elements mutually injection-lock each other at an
optical frequency that is within the injection-locking range for
every array element. Demonstrations have been done using this
technique up to ∼10 elements using fiber lasers. However, the
beam-combining efficiency appears to fall off as the number of
elements increases, and prospects for scaling this implementation to large arrays are unclear [32]. In addition, for successful
implementation, there is a need to define key design parameters,
such as the required differences in optical path lengths among
elements.
In active-feedback implementations, path-length differences
among array elements are detected, and then feedback is used to
equalize the optical path lengths modulo 2π [36]–[41]. This
approach can be thought of as being equivalent to using a
deformable mirror to actively correct the wavefront distortion
in a bulk gain element. This type of implementation has been
used mostly in master-oscillator power-amplifier (MOPA) architectures. Some of the key issues include defining the method
of detection of differences in optical path length, understanding
the dynamics of variations in optical path length, and designing a servo system with an actuator with sufficient bandwidth
and dynamic range that can correct for these variations. No
et al. [36], [37] phased a 100-element array of semiconductor
amplifiers in a MOPA architecture, but the power in the central
Authorized licensed use limited to: MIT Libraries. Downloaded on January 26, 2010 at 16:19 from IEEE Xplore. Restrictions apply.
FAN: LASER BEAM COMBINING FOR HIGH-POWER, HIGH-RADIANCE SOURCES
571
Fig. 3. CBC of two fiber amplifiers using heterodyne detection of the error
signal and active feedback. From [41].
lobe was only 1.6 W out of a total of 7.9 W. The correction was
only done statically.
More recent demonstrations have utilized servo loops to correct the path differences in real time. For example, an array of
19 fiber-pigtailed semiconductor lasers was injection-locked to
force the array elements to operate with the same spectrum [39].
The fiber pigtails were brought together to a tiled aperture,
and the fiber pigtail lengths were actively controlled to produce constructive interference in the far field. Intensity under
servo control was measured to be 13 times that for no phase
control (compared with an ideal of 19 times). Based on the
far-field pattern, the fill factor in this demonstration was low.
Recently, arrays of fiber amplifiers have also been phased using
active feedback techniques, as illustrated in Fig. 3 [40], [41]. In
these implementations a master oscillator is input to the array
of fiber amplifiers. A sample of the array output is heterodyned
against a reference to extract an error signal. This error signal
is fed back to a phase actuator to provide phase control. Nearideal phase control and far-field intensity patterns were demonstrated [40], [41], although these tiled-aperture implementations
had significant sidelobes.
Nonlinear optical approaches to beam combining have
included phase conjugation and Raman beam combining
[42]–[56]. Many of the CBC efforts using phase conjugation
relied on stimulated Brillioun scattering in bulk media, which
has a relatively high threshold requiring high-peak power lasers.
More recently, lower thresholds have been obtained by using
guided-wave configurations [56]. Key issues with nonlinear optical beam combining include scaling to large numbers of elements, having a low threshold, and handling the bandwidth and
dynamic range of the required phase corrections.
Finally, it is useful to discuss the phase-control requirements
in the context of fiber gain elements. The path-length variation
(phase noise) of commercial 10-W Yb-doped fiber amplifiers
has been reported, and these results are shown in Fig. 4 [41]. At
fiber amplifier turn on, the path length goes through thousands
of waves, primarily driven by heating of the fiber. In thermal
steady state, the path length in millisecond time scales varies
a few tenths of a wave in a quiet laboratory environment, although this variation can be much larger in acoustically noisy
Fig. 4. Path-length (phase) noise in waves at 1.07-µm wavelength of linearly
polarized 10-W Yb-doped fiber amplifiers. (a) Turn-on transient regime. (b)
Thermal steady-state regime in an acoustically quiet environment. Top trace at
10-W output power and bottom trace at 1-W output power. From [41].
environments. Clearly, these path-length changes are large
enough that they must be compensated for in order to perform CBC successfully. Any CBC implementation must be able
to accommodate these types of fluctuations, both in terms of
their bandwidth and their dynamic range. On the other hand,
these fluctuations are sufficiently small that they cause negligible linewidth broadening for GHz linewidths and, thus, no
compensation for these effects is anticipated to be needed in
WBC systems.
IV. WAVELENGTH COMBINING
Although WBC has been investigated far less than CBC for
power and radiance scaling, it has been used for attaining nearly
ideal combining on large laser arrays. In this section, various
implementations of wavelength combining are discussed along
with implications for element control. This discussion is followed by a review of recent results. The structure of this section is different from the preceding one because the requirements on element control are not really driven by fundamental
Authorized licensed use limited to: MIT Libraries. Downloaded on January 26, 2010 at 16:19 from IEEE Xplore. Restrictions apply.
572
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 11, NO. 3, MAY/JUNE 2005
requirements, in contrast to CBC, but instead are more heavily
driven by implementation specifics.
A. WBC Implementations
WBC implementations can be divided into two subsets, serial
and parallel, characterized by the beam combiner, as shown
notionally in Fig. 1. An early implementation of wavelength
combining that was proposed and demonstrated used dichroic
interference filters to serially combine six diode lasers operating
at different wavelengths [57]. In this implementation, each diode
laser or channel operated at a different wavelength. The output of
an individual diode was transmitted through an interference filter
that passed its wavelength, but reflected all other wavelengths.
Concepts for parallel implementations of WBC for power and
radiance scaling using gratings were developed at a similar time
to series implementations. For example, a pair of gratings, called
a grating rhomb, was used to wavelength-combine diode lasers
operating at different wavelengths [58], [59]. The difficulty with
this implementation was that conventional Fabry–Perot diode
lasers were utilized and their output spectra were not stable,
even with temperature control. The lack of sufficient wavelength
stabilization degraded the output beam quality.
In the early 1990s, wavelength-combining techniques were
also being developed in the area of wavelength-divisionmultiplexing (WDM) transmitters for optical communications
[60], [61]. In these WDM transmitters, also called multichannel
grating cavity lasers, a one-dimensional array of semiconductor
lasers is beam-combined by sharing a laser cavity that contains a grating. This method of WBC is attractive because the
combination of the grating and optical feedback performs the
two functions of controlling the wavelength of each individual array element and simultaneously combining the beams so
that they overlap spatially. However, in WDM transmitters, the
focus has been on getting multiple wavelength channels into
a single-mode fiber, and consequently, power, brightness, and
efficiency have not been particularly important drivers. For example, in many of the embodiments of these transmitters, the
laser array, grating, and laser resonator are integrated onto a
monolithic substrate. The difficulty is that monolithic guidedwave implementations of gratings and laser resonators are lossy,
which limits the efficiency. In addition, the monolithic substrate
implementations have limits on power handling.
At MIT Lincoln Laboratory, we invented a low-loss freespace WBC implementation that simultaneously provides wavelength control and nearly ideal beam combination for large (hundreds of elements) laser arrays [1], [62]–[68]. A schematic of
this implementation with a linear diode laser array is shown
in Fig. 5; however, an array of fiber gain elements could be
substituted for the diode laser array. By using optical feedback,
the spectrum of each element is controlled to be different from
the others and to be right for ideal beam combination. Each
of the laser gain elements is inside a laser resonator, in which
one resonator mirror is on one end of the gain element and at
the output end of the laser resonator is the partially reflective
output coupler. At the interface between the laser gain elements
and free space, there is an antireflection coating or an angled
Fig. 5. Free-space implementation of parallel WBC using optical feedback for
gain element spectrum control.
facet to prevent reflections at this interface. The transform lens,
grating, and output coupler are common optical elements of
the external resonator shared by each of the laser array elements. The transform lens acts to transform the position of
an array element into an angle of incidence on the grating,
provided that the lens is located one focal length from the array. Spatial overlap of the beams from each element is ensured
by placing the grating one focal length away from the transform lens. Codirectional propagation of the individual beams
is forced by the flat output coupler, because the directions of
propagation of the output beams are all normal to this mirror.
Because the incidence angles on the grating for the beams from
each array element differ, the external resonator selects different
wavelengths for each array element as needed to force coaxial
propagation.
Another way to view the operating principle of this externalcavity laser is to consider a single array element. A single array
element can be tuned in this resonator by translating the array
element in the plane of the page and perpendicular to the optical axis of the lens. When this array element is translated, the
propagation direction of the output does not change because the
output coupler forces propagation normal to its surface; neither
does the position of the beam footprint on the grating change
because it is located a focal length away from the transform
lens. Consequently, if we instead put additional array elements
along this path, then each array element will operate at a different wavelength with beams that are coaxial with each other.
Yet another way of viewing the operation of this architecture
is via analogy to a grating spectrometer. In a grating spectrometer, typically broadband radiation is incident on the grating
(propagating in a direction opposite to the combined laser output beam). The grating disperses wavelength into diffraction
angle off the grating, and then a transform lens or mirror converts the propagation angle into position at the focal plane, such
that different wavelengths fall onto different locations. Essentially, the spectrally combined array can be viewed as a grating
spectrometer run in reverse. This implementation works with
one-dimensional arrays. In principle, spectral combining can be
extended to two-dimensional arrays by using crossed gratings,
as is done in spectrometer systems that use CCD imagers as
detectors [69].
We have performed some estimates as to the number of array
elements that could be combined using this architecture, and it
appears that hundreds to thousands of elements can be combined
Authorized licensed use limited to: MIT Libraries. Downloaded on January 26, 2010 at 16:19 from IEEE Xplore. Restrictions apply.
FAN: LASER BEAM COMBINING FOR HIGH-POWER, HIGH-RADIANCE SOURCES
573
Fig. 6. Output spectrum as a function of near-field position along the array for
a WBC combined diode array [1].
under reasonable assumptions. It can be shown that the dimensional extent of the gain element array d is related to the focal
length of the transform lens f , the total wavelength spread of
the optical output ∆λ, and the dispersion of the grating dβ/dλ
by the expression
d ≈ f (dβ/dλ)∆λ.
Fig. 7. Beam-combined output power and beam quality as a function of current
to the array [1].
(2)
The dispersion of the grating relates the change in diffraction
angle to the change in optical wavelength. This dispersion, in
turn, is related to the grating groove spacing a and the diffraction
angle β by
dβ/dλ = 1/(a cos β).
(3)
A typical value for dispersion for a 2000-lines/mm grating
at 1-µm wavelength is around 4 rad/µm. For f = 20 cm and
a total wavelength spread of 25 nm across the array, which is
achievable in fiber and semiconductor gain media near 1-µm
wavelength, then d is around 2 cm, assuming 4 µm/rad grating
dispersion. For array elements spaced on 250-µm centers, such a
design accommodates around 80 gain elements. Tighter element
spacing or larger focal length should enable even larger arrays
to be combined.
This external-cavity implementation has been used to
wavelength-combine diode [1], [63], [64] and fiber laser arrays [62], [65]–[67]. In a recent demonstration [1], an array
of 100 slab-coupled optical waveguide diode lasers was beamcombined with an output beam quality of M2 ∼ 1.3 and 35-W
output power, which is the highest radiance diode array to our
knowledge. Fig. 6 shows the spectrum as a function of position
along the array, and Fig. 7 shows the output power and beam
quality as a function of current to the array. In fiber laser beam
combining, both oscillator [62], [66], [67] and master-oscillator
power-amplifier (MOPA) architectures [65], as shown schematically in Fig. 8, have been demonstrated. MOPA architectures
separate temporal and spectral waveform control from power
generation, as it is often observed that fiber laser oscillators
pulse and have undesirable spectral broadening effects. Using
a MOPA architecture, an array of five 2-W Yb-doped fiber
amplifiers was combined with a beam quality of M2 = 1.14,
showing that essentially ideal WBC can be achieved with fiber
arrays [65]. Using oscillator architectures, an array of four Tmdoped fiber lasers with 11-W output power and an unspecified
beam quality was demonstrated [66], and an array of three Ybdoped fiber lasers with 104 W and M2 = 2.7 was demonstrated
using a fused-silica transmission grating for the dispersive
element [67].
Fig. 8.
Schematic of MOPA architecture for WBC.
B. Element-Control Requirements Imposed by WBC
Implementations
Series and parallel WBC implementations pose challenges
in spectrum control and element alignment for scaling to large
arrays. Here, we review some of these challenges and solutions.
In serial approaches the spectrum-control problem applies
to both array elements and filters. As N increases and the
wavelength spacing between elements decreases, manufacturing efficient filters becomes increasingly difficult. Second,
the series arrangement requires that the angular positioning
of the interference filters has tight tolerances because the
laser at the end of the array accumulates a large number of
bounces. Errors in angular positioning lead to smearing of the
output in the far field and degradation of the on-axis intensity.
Clearly, the near and far fields need to overlap to a small fraction
of a diffraction-limited beam to achieve a combined beam
with near-diffraction-limited output; hence, the requirement in
Table I for diffraction-limited beam alignment. However, this
basic approach of serial combination is used in WDM of transmitters for fiber-optic communication, which was enabled by
developments in distributed feedback lasers, fiber Bragg gratings, and single-mode optical fibers. Efficiency is less important
in the WDM transmitter application than in power and radiance
scaling applications, so losses are more tolerable. The errors
in angular positioning and near-field positioning are eliminated
by the use of single-mode optical components, at the expense
of optical loss if fiber couplers or splices are less than ideal.
In the parallel implementations, the need for diffractionlimited alignment implies that the far-field pointing of the
Authorized licensed use limited to: MIT Libraries. Downloaded on January 26, 2010 at 16:19 from IEEE Xplore. Restrictions apply.
574
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 11, NO. 3, MAY/JUNE 2005
TABLE II
OUTPUT CHARACTERISTICS
elements and the optical system must be arranged such that
the beams have good spatial overlap on the grating. If the transform optic is exactly a focal length from the array, this means
that the far-field pointing of the array elements must be the same
to within a small fraction of the far-field beam divergence of a
single element. Bochove [68] has analyzed such effects. In the
implementations of such systems as those in Fig. 5, there is
a requirement on the element spectrum that is coupled to the
near-field placement. The spectrum must be sufficiently narrow that diffraction by the grating of a finite-spectral-width
beam adds far-field beam divergence that is small relative to
the diffraction-limited beam divergence. The placement of an
array element in the near field must be controlled to be correct,
given the wavelength of the element, or conversely the wavelength of the element must be controlled to be correct, given the
near-field placement of the element. The use of optical feedback
automatically controls array elements to operate at a wavelength
and spectral extent set by the near-field placement in the array
plane. Effectively, this lifts the requirement on near-field placement in this plane, or equivalently, feedback control is being
used to adjust the wavelengths to match the near-field position.
Placement in the orthogonal direction (out of the plane of the
array) is important; smile effects in a linear array will lead to
degradation in the beam quality in the noncombining plane.
V. COMPARISONS BETWEEN CBC AND WBC
Although power and radiance ideally scale proportionally
with N in both CBC and WBC, there are important differences
in their difficulty in achieving ideal beam combining, output
characteristics, and number scalability within fixed optical systems. These differences can drive the choice between coherent
and wavelength techniques. Table II lists the basic output characteristics of CBC and WBC systems.
The most obvious difference in the output characteristics is
the output spectrum. For a CBC system, the spectrum does not
inherently need to vary as the number of elements changes. In
contrast, in WBC systems the spectrum is inherently multiwavelength and does inherently vary with the number of elements.
This multiwavelength output characteristic may rule out WBC
for some applications (e.g., coherent laser radar), but make it
desirable for others. For many applications, the spectral characteristics are not particularly important.
The other important distinguishing characteristic is the
change in near- and far-field beam patterns as N varies, which
leads to a fundamental difference in number scalability between
CBC and WBC systems. In WBC, the number of elements can
vary without changing the near- and far-field beam patterns,
and the on-axis intensity scales with the number of elements
Fig. 9. Effect when one element fails in a two-element WBC array showing
that the on-axis far-field intensity scales with the number of elements.
without any optical-system reconfiguration (assuming the optical system can accommodate the changes in spectrum with
N ). In contrast, in CBC systems, optical-system changes are
required to achieve on-axis intensity scaling with the number
of elements. Clearly, in tiled CBC systems, the near- and farfield beam patterns change as the number of elements change,
except for the ideal case of S = 1. The need to change the optical system, assuming a fixed output aperture, as the number
of elements changes can be easily illustrated for the ideal case
of S = 1. In an optical system with a fixed-output aperture, a
telescope needs to be changed or the tiles need to be reformatted for every change in the number of tiles in order to fit the
tiles within the fixed aperture and maintain S = 1. In filledaperture CBC systems the near and far fields can be invariant
with N . However, for the on-axis intensity to be proportional
to the number of elements in filled-aperture systems, the beam
combiner needs to be changed as the number of elements varies
because the efficiency of a beam combiner designed for combining M beams changes with the number of beams. An M × 1
combiner, whether a free-space combiner or fiber coupler, is
only optimized for M elements, not for any other number of
elements, and it is possible to get unity efficiency of combining
the array elements only for M elements. As N varies from M ,
the efficiency of the combiner decreases.
One of the attractive features of beam-combined arrays is the
possibility of graceful degradation because the failure of a single
element does not cause the output to cease. The considerations
above dictate differences between CBC and WBC in the gracefulness of this degradation. Once a beam-combined source is
implemented in a fixed optical system, the degradation of the
on-axis intensity is more graceful with WBC than with CBC.
This can be seen using the simplest examples of combining of
two elements. Fig. 9 shows the use of a dichroic mirror to perform WBC of two elements. Assume that both elements have
identical beam quality and the alignment is ideal. If one element
fails, then the power transmitted to the far field is reduced to
half the value with no change in far-field pattern, and, consequently, the on-axis intensity in the far field decreases to half
the value. This can easily be generalized to the case of large N
by considering the implementation of Fig. 5. Failure of a single
element does not change the far- or near-field beam patterns.
Consequently, if we define the fraction of working elements as
F (assuming equal-power elements), then the on-axis far-field
intensity scales like F .
Now consider CBC of two elements, as illustrated in Fig. 10
for both tiled- and filled-aperture implementations, and assume
Authorized licensed use limited to: MIT Libraries. Downloaded on January 26, 2010 at 16:19 from IEEE Xplore. Restrictions apply.
FAN: LASER BEAM COMBINING FOR HIGH-POWER, HIGH-RADIANCE SOURCES
575
Fig. 10. Effect when one element fails in two-element CBC arrays for both tiled and filled apertures. The on-axis intensity scales as F 2 in a fixed optical system.
A scaling of F can be recovered only by reconfiguration.
ideal phase control and alignment. In the filled-aperture case,
a 50/50 beamsplitter is used as the combining element. If one
element fails, then only 25% of the power goes into the desired
far-field direction, but the far-field pattern does not change.
Consequently, the on-axis far-field intensity has dropped to 25%
of the original value (i.e., F 2 scaling), not 50%. This appears to
be at odds with the stated radiance scaling with N . It is possible
to recover to F scaling, but only by physical reconfiguration
of the optics, in this case by removing the beamsplitter. With
the beamsplitter removed, the far-field intensity now is back
to 50% of the original, in agreement with the radiance-scaling
law. In the tiled implementation, similarly the on-axis intensity
drops to 25% of the original value when one element fails, or
F 2 scaling. Here, the argument is that the power transmitted
to the far field drops to half the original value, and the beam
divergence increases to double the original value because there
is no longer constructive interference between the two elements.
This combination leads to a drop of the on-axis intensity to 25%
of the original value. By physical reconfiguration of the optics,
in this case by adding a telescope, the far-field intensity can be
made to scale as F .
It is easy to generalize this argument to the case of a larger
number of elements in tiled-aperture systems. One factor of F
comes about simply because there is less power being transmitted. The second factor of F arises from the proportionality of
the Strehl ratio with the fill factor. Only by physical reconfiguration can the on-axis intensity be made to scale as F . In a
tiled-aperture system this reconfiguration might include moving and resizing the tiles. By equivalence, this F 2 law applies
to filled-aperture implementations as well. Here the F scaling
can be recovered by changing the beam combiner.
VI. SUMMARY
Important advances have been made over the past few years
in beam combining of laser arrays for higher power and bright-
ness sources using both CBC and WBC techniques. These advances have led to combining of a 100-element diode array with
nearly ideal beam quality. CBC inherently levies more difficult
element-control requirements than WBC. Scalability with the
number of elements is more graceful with WBC than CBC.
ACKNOWLEDGMENT
The author would like to acknowledge A. Sanchez, S. Augst,
A. Goyal, and B. Chann, all of MIT Lincoln Laboratory, for
their helpful discussions and insights.
REFERENCES
[1] B. Chann, R. K. Huang, L. J. Missaggia, C. T. Harris, Z. L. Liau, A.
K. Goyal, J. P. Donnelly, T. Y. Fan, A. Sanchez-Rubio, and G. W. Turner,
“High-power, near-diffraction-limited diode laser arrays by wavelength
beam combining,” Opt. Lett, to be published.
[2] J. R. Leger, “External methods of phase locking and coherent beam addition of diode lasers,” in Surface Emitting Semiconductor Lasers and
Arrays, G. A. Evans and J. M. Hammer, Eds. Boston, MA: Academic,
1993, pp. 379–433.
[3] S. R. Chinn, “Review of edge-emitting coherent laser arrays,” in Surface
Emitting Semiconductor Lasers and Arrays, G. A. Evans and J. M. Hammer, Eds. Boston, MA: Academic, 1993, pp. 9–70.
[4] G. S. Mecherle, “Laser diode combining for free space optical communication,” Proc. SPIE Opt. Technol. Commun. Satellite Applicat., vol. 616,
pp. 281–291, 1986.
[5] J. R. Leger, M. Holz, G. J. Swanson, and W. Veldkamp, “Coherent beam
addition: An application of binary optics,” Lincoln Lab. J., vol. 1, pp. 225–
245, Fall 1988.
[6] A. F. Glova, “Phase locking of optically coupled lasers,” Quantum Electron., vol. 33, pp. 283–306, Apr. 2003.
[7] D. Botez, “Monolithic phase-locked semiconductor laser arrays,” in Diode
Laser Arrays, D. Botez and D. R. Scrifres, Eds. Cambridge, U.K.: Cambridge Univ. Press, 1994, pp. 1–67.
[8] D. F. Welch and D. G. Mehuys, “High-power coherent, semiconductor
laser, master oscillator power amplifiers and amplifier arrays,” in Diode
Laser Arrays, D. Botez and D. R. Scrifres, Eds. Cambridge, U.K.: Cambridge Univ. Press, 1994, pp. 72–122.
[9] I. S. Goldobin, N. N. Evtikhiev, A. G. Plyavenek, and S. D. Yakubovich,
“Phase-locked integrated arrays of injection lasers,” Sov. J. Quantum Electron., vol. 19, pp. 1261–1284, Oct. 1989.
Authorized licensed use limited to: MIT Libraries. Downloaded on January 26, 2010 at 16:19 from IEEE Xplore. Restrictions apply.
576
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 11, NO. 3, MAY/JUNE 2005
[10] C. D. Nabors, “Effect of phase errors on coherent emitter arrays,” Appl.
Opt., vol. 33, pp. 2284–2289, 1994.
[11] T. C. Cheston, J. Frank, “Array antennas” in Radar Handbook, M. I. Skolnik, Ed., New York: McGraw-Hill, 1970.
[12] E. M. Philipp-Rutz, “Spatially coherent radiation from an array of GaAs
lasers,” Appl. Phys. Lett., vol. 26, pp. 475–477, Apr. 1975.
[13] J. R. Leger, M. L. Scott, and W. B. Veldkamp, “Coherent addition of
AlGaAs lasers using microlenses and diffractive coupling,” Appl. Phys.
Lett., vol. 52, pp. 1771–1773, May 1988.
[14] J. R. Leger, G. J. Swanson, and W. B. Veldkamp, “Coherent laser addition
using binary phase gratings,” Appl. Opt., vol. 26, pp. 4391–4399, Oct.
1987.
[15] C. J. Corcoran and R. H. Rediker, “Operation of five individual diode
lasers as a coherent ensemble by fiber coupling into an external cavity,”
Appl. Phys. Lett., vol. 59, pp. 759–761, Aug. 1991.
[16] Y. Kono, M. Takeoka, K. Uto, A. Uchida, and F. Kannari, “A coherent
all-solid-state laser array using the Talbot effect in a three-mirror cavity,”
IEEE J. Quantum Electron., vol. 36, no. 5, pp. 607–614, May 2000.
[17] A. F. Glova, Yu. A. Dreizin, O. R. Kachurin, F. V. Lebedev, and V.
D. Pis’mennyi, “Phase locking of a two-dimensional array of CO2 waveguide lasers,” Sov. Tech. Phys. Lett. (USA), vol. 11, pp. 102–103, Feb.
1985.
[18] L. A. Newman, R. A. Hart, J. T. Kennedy, A. J. Cantor, A. J. DeMaria,
and W. B. Bridges, “High power coupled (CO2 ) waveguide laser array,”
Appl. Phys. Lett., vol. 48, pp. 1701–1703, Jun. 1986.
[19] O. R. Kachurin, F. V. Lebedev, and A. P. Napartovich, “Properties of an
array of phase-locked (CO2 ) lasers,” Sov. J. Quantum Electron. (USA),
vol. 15, pp. 1128–1131, Sep. 1988.
[20] V. V. Vasil’tsov, V. S. Golbev, Y. V. Zelenov, Y. A. Kurushin, and
D. Yu. Filimonov, “Using diffraction optics for formation single-lobe farfield beam intensity distribution in waveguide CO2 -lasers synchronized
arrays,” Proc. SPIE, vol. 2109, pp. 122–128, 1993.
[21] D. G. Youmans, “Phase locking of adjacent channel leaky waveguide CO2
lasers,” Appl. Phys. Lett., vol. 44, pp. 365–367, Feb. 1984.
[22] D. R. Scifres, R. D. Burnham, and W. Streifer, “Phase-locked semiconductor laser array,” Appl. Phys. Lett., vol. 33, pp. 1015–1017, Dec. 1978.
[23] L. J. Mawst, D. Botez, C. Zmudzinski, M. Jansen, C. Tu, T. J. Roth, and
J. Yun, “Resonant self-aligned-stripe antiguided diode laser array,” Appl.
Phys. Lett., vol. 60, pp. 668–670, Feb. 1992.
[24] M. Oka, H. Masuda, Y. Kaneda, and S. Kubota, “Laser-diode-pumped
phase-locked Nd:YAG laser arrays,” IEEE J. Quantum Electron., vol. 28,
no. 4, pp. 1142–1147, Apr. 1992.
[25] S. Saunders, R. Waarts, D. Nam, D. Welch, D. Scifres, J. C. Ehlert,
W. J. Cassarly, J. M. Finlan, and K. M. Flood, “High power coherent
two-dimensional semiconductor laser array,” Appl. Phys. Lett., vol. 64,
pp. 1478–1480, Mar. 1994.
[26] S. Menard, M. Vampouille, A. Desfarges-Berthelemot, V. Kermene,
B. Colombeau, and C. Froehly, “Highly efficient phase locking of four
diode pumped Nd:YAG laser beams,” Opt. Commun., vol. 160, pp. 344–
353, Feb. 1999.
[27] M. DiDomenico Jr., “Characteristics of a single-frequency Michelsontype He-Ne gas laser,” IEEE J. Quantum Electron., vol. QE-2, no. 8,
pp. 311–322, Aug. 1966.
[28] P. W. Smith, “Mode selection in lasers,” Proc. IEEE, vol. 60, no. 4,
pp. 422–440, Apr. 1972.
[29] A. Shirakawa, T. Saitou, T. Sekiguchi, and K. Ueda, “Coherent addition
of fiber lasers by use of a fiber coupler,” Opt. Express, vol. 10, no. 21,
pp. 1167–1172, Oct. 2002.
[30] D. Sabourdy, V. Kermene, A. Desgarges-Berthelemont, L. Lefort,
A. Barthe´le´my, C. Mahodaux, and D. Pureru, “Power scaling of fibre lasers
with all-fibre interferometric cavity,” Electron. Lett., vol. 38, pp. 692–693,
Jul. 2002.
[31] M. L. Minden, H. Bruesselbach, J. L. Rogers, M. S. Mangir, D. C. Jones, G.
J. Dunning, D. L. Hammon, A. J. Solis, and L. Vaughan, “Self-organized
coherence in fiber laser arrays,” Proc. SPIE, vol. 5335, pp. 89–97, 2004.
[32] A. Shirakawa, K. Matsuo, and K. Ueda, “Power summation and bandwidth narrowing in coherently-coupled fiber laser array,” presented at the
Conference on Lasers and Electro-Optics, Optical Society of America,
Washington, DC, 2004.
[33] L. Liu, Y. Zhou, F. Kong, and Y. C. Chen, “Phase locking in a fiber laser
array with varying path lengths,” Appl. Phys. Lett., vol. 85, pp. 4837–4839,
Nov. 2004.
[34] A. A. Ishaaya, N. Davidson, L. Shimshi, and A. A. Friesem, “Intracavity
coherent addition of Gaussian beam distributions using a planar interferometric coupler,” Appl. Phys. Lett., vol. 85, pp. 2187–2189, Sep. 2004.
[35] A. A. Ishaaya, L. Shimshi, N. Davidson, and A. A. Freisem, “Coherent
addition of spatially incoherent light beams,” Opt. Express, pp. 4929–
4934, Oct. 2004.
[36] K. H. No, R. W. Herrick, C. Leung, R. Reinhart, and J. L. Levy, “One
dimensional scaling of 100 ridge waveguide amplifiers,” IEEE Photon.
Technol. Lett., vol. 6, no. 9, pp. 1062–1066, Sep. 1994.
[37]
, “Two dimensional scaling of ridge waveguide amplifiers,” Proc.
SPIE, vol. 2148, pp. 80–90, 1994.
[38] J. S. Osinski, D. Mehuys, D. F. Welch, R. G. Waarts, J. S. Major, K.
M. Dzurko, and R. J. Lang, “Phased array of high-power, coherent, monolithic flared amplifier master oscillator power amplifiers,” Appl. Phys.
Lett., vol. 66, pp. 556–558, Jan. 1995.
[39] L. Bartelt-Berger, U. Brauch, A. Giesen, and H. Opower, “Power-scalable
system of phase-locked single-mode diode lasers,” Appl. Opt., vol. 38,
pp. 5752–5760, Sep. 1999.
[40] J. Anderegg, S. Brosnan, M. Weber, H. Komine, and M. Wickham, “8-W
coherently phased 4-element fiber array,” Proc. SPIE, vol. 4974, pp. 1–6,
2003.
[41] S. J. Augst, T. Y. Fan, and A. Sanchez, “Coherent beam combining
and phase noise measurements of ytterbium fiber amplifiers,” Opt. Lett.,
vol. 29, pp. 474–476, Mar. 2004.
[42] N. G. Basov, V. F. Efmkov, I. G. Zubarev, A. V. Kotov, A. B. Mironov,
S. I. Mikhailov, and M. J. Smimov, “Influence of certain radiation parameters on wavefront reversal of a pump wave in a Brillouin mirror,” Sov. J. Quantum Electron. (USA), vol. 9, pp. 455–458, Apr.
1979.
[43] N. G. Basov, V. F. Efmkov, I. G. Zubarev, A. V. Kotov, and S. I. Mikhailov,
“Control of the characteristics of reversing mirrors in the amplification
regime,” Sov. J. Quantum Electron. (USA), vol. 11, pp. 1335–1337, Oct.
1981.
[44] D. L. Carroll, R. Johnson, S. J. Pfeifer, and R. H. Moyer, “Experimental
investigations of stimulated Brillouin-scattering beam combination,” J.
Opt. Soc. Amer. B, Opt. Phys., vol. 9, pp. 2214–2224, Dec. 1992.
[45] H. J. Kong, J. Y. Lee, Y. S. Shin, J. O. Byun, H. S. Park, and H. Kim, “Beam
recombination characteristics in array laser amplification using stimulated
Brillouin scattering phase conjugation,” Opt. Rev., vol. 4, pp. 277–283,
Mar.–Apr. 1997.
[46] B. C. Rodgers, T. H. Russell, and W. B. Roh, “Laser beam combining and
cleanup by stimulated Brillouin scattering in a multimode optical fiber,”
Opt. Lett., vol. 16, pp. 1124–1126, Aug. 1999.
[47] A. F. Vasil’ev, A. A. Mak, V. Mit’kin, V. A. Serebryakov, and V. E. Yashin,
“Correction of thermally induced optical aberrations and coherent phasing
of beams during stimulated Brillouin scattering,” Sov. Phys.-Tech. Phys.
(USA), vol. 31, pp. 191–193, Feb. 1986.
[48] D. A. Rockwell and C. R. Giuliano, “Coherent coupling of laser gain
media using phase conjugation,” Opt. Lett., vol. 11, pp. 147–149, Mar.
1986.
[49] K. V. Gratsianov, A. F. Komev, V. V. Lyubimov, A. A. Mak, V. G. Pankov,
and A. I. Stepanov, “Investigation of an amplifier with a composite active
element and a stimulated Brillouin scattering mirror,” Sov. J. Quantum
Electron. (USA), vol. 16, pp. 1544–1546, Nov. 1986.
[50] V. M. Leont’ev, V. G. Novoselov, Y. P. Rudnitskii, and I. V. Chemyshava,
“Solid-state laser with a composite active element and diffraction-limit
divergence,” Sov. J. Quantum Electron. (USA), vol. 17, pp. 220–223, Feb.
1987.
[51] K. V. Gratsianov, A. F. Komev, V. V. Lyubimov, and V. G. Pankov, “Laser
beam phasing with phase conjugation in Brillouin scattering,” Opt. Spectrosc. (USA), vol. 68, pp. 360–361, Mar. 1990.
[52] A. F. Vasli’ev, S. B. Gladin, and V. E. Yashin, “Pulses-periodic Nd:YAlO3
laser with a phase-locked aperture under conditions of phase conjugation
by stimulated Brillouin scattering,” Sov. J. Quantum Electron. (USA),
vol. 21, pp. 494–497, May 1991.
[53] D. S. Sumida, D. C. Jones, and D. A. Rockwell, “An 8.2 J phase-conjugate
solid-state laser coherently combining eight parallel amplifiers,” IEEE J.
Quantum Electron., vol. 30, no. 11, pp. 2617–2627, Nov. 1994.
[54] R. H. Moyer, M. Valley, and M. Cimolino, “Beam combination through
stimulated Brillouin scattering,” J. Opt. Soc. Amer. B, Opt. Phys., vol. 5,
pp. 2473–2489, Dec. 1988.
[55] M. H. Smith, D. W. Trainor, and C. Duzy, “Shallow angle beam combining
using a broad-band XeF laser,” IEEE J. Quantum Electron., vol. 26, no. 5,
pp. 942–949, May 1990.
[56] T. H. Russell, S. M. Willis, M. B. Crookston, and W. B. Roh, “Stimulated
Raman scattering in multimode fibers and its application to beam cleanup
and combining,” J. Nonlin. Opt. Phys., Mater., vol. 11, pp. 303–316, Sep.
2002.
Authorized licensed use limited to: MIT Libraries. Downloaded on January 26, 2010 at 16:19 from IEEE Xplore. Restrictions apply.
FAN: LASER BEAM COMBINING FOR HIGH-POWER, HIGH-RADIANCE SOURCES
[57] K. Nosu, H. Ishio, and K. Hashimoto, “Multireflection optical
multi/demultiplexer using interference filters,” Electron. Lett., vol. 15,
pp. 414–415, Jul. 1979.
[58] P. O. Minott and J. B. Abshire, “Grating rhomb diode laser power combiner,” Proc. SPIE Opt. Technol. Space Commun. Syst., vol. 756, pp. 38–
48, 1987.
[59] J. A. R. Rall, P. L. Spadin, R. K. Zimmerman, and W. Maynard, “Test results of a diffraction grating beam combiner,” Free-Space Laser Commun.
Technol., vol. 1218, pp. 264–275, 1990.
[60] I. H. White, “A multichannel grating cavity laser for wavelength division
multiplexing applications,” J. Lightwave Technol., vol. 9, no. 7, pp. 893–
899, Jul. 1991.
[61] M. C. Farries, A. C. Carter, G. G. Jones, and I. Bennion, “Tunable multiwavelength semiconductor laser with single fibre output,” Electron. Lett.,
vol. 27, pp. 1498–1499, Aug. 1991.
[62] C. C. Cook and T. Y. Fan, “Spectral beam combining of Yb-doped fiber
lasers in an external cavity,” in OSA Trends in Optics and Photonics, Vol.
26, Adv. Solid-State Lasers, M. M. Fejer, H. Injeyan, and U. Keller, Eds.
Washington, DC: Optical Society of America, 1999, pp. 163–166.
[63] V. Daneu, A. Sanchez, T. Y. Fan, H. K. Choi, G. W. Turner, and C. C. Cook,
“Spectral beam combining of a broad-stripe diode laser array in an external
cavity,” Opt. Lett., vol. 25, pp. 405–407, Mar. 2000.
[64] C. Hamilton, S. Tidwell, D. Meekhof, J. Seamans, N. Gitkind, and
D. Lowenthal, “High power laser source with spectrally beam combined
diode laser bars,” Proc. SPIE, vol. 5336, 2004.
[65] S. J. Augst, A. K. Goyal, R. L. Aggarwal, T. Y. Fan, and A. Sanchez,
“Wavelength beam combining of ytterbium fiber lasers,” Opt. Lett., vol. 28,
pp. 331–333, Mar. 2003.
[66] W. A. Clarkson, V. Matera, T. M. J. Kendall, D. C. Hanna, J. Nilsson, and
P. W. Turner, “High-power wavelength-combined cladding-pumped Tmdoped silica fibre lasers,” in OSA Trends in Optics and Photonics (TOPS),
Vol. 56, Conf. on Lasers and Electro-optics (CLEO 2001), Washington,
DC: Optical Society of America, 2001, pp. 363–364.
[67] M. Reich, J. Limpert, A. Liem, T. Clausnitzer, H. Zellmer, E. B. Kley,
and A. Tünnermann, “Spectral beam combining of ytterbium-doped fiber
lasers with a total output power of 100 W,” in Europhys. Conf. Abstracts,
vol. 28, 2004, C Fib-10137.
577
[68] E. J. Bochove, “Theory of spectral beam combining of fiber lasers,” IEEE
J. Quantum Electron., vol. 38, no. 5, pp. 432–445, May 2002.
[69] S. S. Vogt and G. D. Penrod, “HIRES: A high resolution echelle spectrometer for the Keck 10-m telescope,” in Instrumentation for GroundBased Optical Astronomy: Present and Future, L. B. Robinson, Ed. Berlin,
Germany: Springer-Verlag, 1988, pp. 68–103.
T. Y. Fan (S’82–M’87–SM’96) received the S.B. degree from the Massachusetts
Institute of Technology (MIT), Cambridge, in both electrical engineering, and
materials science and engineering and the M.S. and Ph.D. degrees in electrical
engineering from Stanford University, Stanford, CA.
He is the Assistant Leader of the Quantum Electronics Group at MIT Lincoln
Laboratory. He joined MIT Lincoln Laboratory as a Staff Member in 1987. He
has contributed broadly in solid-state laser and nonlinear optics technology. He
is widely recognized for his work in diode-pumped solid-state lasers and in the
development of Yb:YAG lasers and for advances in laser beam combining.
Dr. Fan is a Fellow of the Optical Society of America (OSA). He served as
an Elected Member of the IEEE/LEOS Board of Governors from 1994 to 1996
and was the Topical Editor, Lasers for Optics Letters from 1994 to 1999. He is
currently serving as Division Editor for the Lasers, Photonic, and Environmental
Optics Division of Applied Optics. He has also served on numerous program
committees of conferences, including as Chair of the LEOS Annual Meeting
and the OSA Topical Meeting on Advanced Solid-State Lasers.
Authorized licensed use limited to: MIT Libraries. Downloaded on January 26, 2010 at 16:19 from IEEE Xplore. Restrictions apply.