Single-element optical injection locking of diode-laser arrays

By optically injecting a single end-element of a semiconductor laser array, both the spatial and spectral emission characteristics of the entire laser array is controlled. With the output of the array locked, the far-field emission angle of the array is continuously scanned over several degrees by varying the injection frequency.

FIELD OF THE INVENTION 
The present invention relates to the field of control of laser beam 
radiation, and more particularly to a method of and apparatus for 
controlling the spatial and spectral emission of a laser beam from a 
semiconductor laser array. 
BACKGROUND OF THE INVENTION 
The substantial potential for high-power continuous wave and pulsed lasing 
in coherently coupled diode-laser arrays has led to considerable interest 
in their development. Attention has focussed on the problem of controlling 
the spatial emission characteristics of these devices with the aim of 
promoting single-lobed far-field emission patterns. The approaches most 
often used entail a `chriping` of the gain profile across the device by 
varying the stripe widths or the spacing between strips. Alternatively, 
such a spatially chirped gain has also been realized by fabrication of a 
diode-laser array structure in which the gain of each stripe could be 
independently controlled via separate current contacts. Although the above 
approaches have demonstrated some degree of success in producing 
single-lobed far-field patterns, they provide no improvement in the 
spactral characteristics of the devices, which in the case of gain-guided 
arrays, oscillate in several longitudinal modes. 
Semiconductor diode-laser arrays provide an intense and efficient source of 
laser radiation. However, these devices have two inherent drawbacks which 
limit their usefulness. They generally emit their radiation in a two-lobed 
far-field pattern which makes beam focusing and/or propagation difficult, 
and the spectral distribution of their emission is spread over many 
angstroms. In addition, it is difficult to cause the output of the laser 
array to scan even a limited field. 
Injection-locking is well known for controlling the spectral distribution 
of single-channel diode lasers. Furthermore, control of both the spectral 
and spatial distribution of diode-laser arrays has been demonstrated using 
a single-frequency diode laser as the master oscillator. In L. Goldberg, 
H. F. Taylor, J. F. Weller, and D. R. Scifres, "Injection-Locking of 
Coupled-Stripe Diode Laser Arrays", Applied Physics Letters, 46, 236 
(1985), injection locking is applied to a diode laser array to control the 
spatial profile of light emitted by the array. Because all the elements of 
the array are injected uniformly, injection is applied to the array at an 
angle of 4 degrees off the array axis in order to produce a phase tilt 
across the array leading to emission in a single beam. 
As the injection array frequency was varied, a frequency interval (locking 
bandwidth) was observed over which the single-lobed emission from the 
array could be maintained. When a single element of the array was injected 
(again at an angle of 4 degrees to the array axis), there was a much 
smaller locking range (less than 1 GHz). There was not, however, any 
reference to or realization of scanning of the emission beam from an 
injection-locked array by varying the injection frequency of the array. 
Previously, it has been observed that by incorporating a semiconductor 
laser array into a cavity containing a grating as a tuning element, the 
emission angle of the two-lobed far-field beam changes as the grating is 
tuned. In this case the far-field beam consists of two-lobes of nearly 
equal intensity centered about the axis of the diode-array with the result 
that there was no net angular steering of the total beam. The prior art 
does not disclose scannable optical injection of radiation for steering 
the emission of a single-lobed far-field diode laser beam. 
Various means of scanning laser beam output using the input frequency as a 
medium to bring about the scan are known. For example, in U.S. Pat. No. 
3,541,471 to Kaminow et al, scanning is limited to a fixed frequency which 
requires the scanning frequency to be equal to the transverse mode 
separation frequency. 
Generally in such systems, separate phase modulators and/or frequency 
shifters are required. These separate external or intracavity phase 
modulators and/or frequency shifters are undesirable since they introduce 
additional losses in the optical power available and require additional 
components and circuitry to monitor and control their performance. 
In U.S. Pat. No. 3,691,483 to Klein, phase control is integrated into the 
semiconductor structure itself. However, the use of a separate electrical 
input signal to control the relative phase shift of each laser in the 
array and a computer to synchronize and control the overall phase tilt of 
the array are required. 
In U.S. Pat. No. 3,626,321 to Smith and in U.S. Pat. No. 3,691,483 to 
Klein, generation of multiple input beams, together with associated 
mirrors or optical distributors as required to generate the multiple input 
beams, is disclosed. 
In general, it is desirable to scale up the diode laser array to a very 
large number of emitting elements (in the form of a one- or 
two-dimensional array). This scaling is very difficult and costly, if not 
impossible, with the prior art. In U.S. Pat. No. 3,626,321 to Smith, for 
example, a plurality of laser beams are generated by means of interference 
effects, and the intensity of any one beam is therefore correspondingly 
reduced. 
In U.S. Pat. No. 3,541,471 to Kaminow et al, the spatial intensity profile 
of the emitted beam varies with the scan angle since the selection of the 
transverse modes which are excited is angle depenent. Such dependence of 
spatial intensity of the emitted laser beam upon the scan angle is 
undesirable. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide an 
apparatus and method for optically steering the output beam from a 
diode-laser array and for scanning the beam over an angle of several 
degrees. 
Another object of the invention is to provide a techniue involving optical 
injection of radiation for steering the emission of a single-lobed 
far-field laser beam. 
Another object is to provide a system for optically injected diode laser 
scanning which is not limited to a fixed frequency. 
Still another object of the invention is to provide an optically injected 
diode laser scanning system wherein the separate phase modulators and/or 
frequency shifters required in the prior art are eliminated. 
An additional object of the invention is to permit scaling of the diode 
laser array to a very large number of emitting elements (in the form of a 
one- or two-dimensional array) which requires only one or at the most a 
very few master oscillators to control the performance of the array. 
Another object of the invention is to provide a laser beam whose spatial 
intensity profile is relatively insensitive to the angle of emission of 
the laser beam. 
Additional objects, advantages, and novel features of the invention will be 
set forth in part in the description that follows and in part will become 
apparent to those skilled in the art upon examination of the following or 
may be learned with the practice of the invention. The objects and 
advantages of the invention may be realized and attained by means of the 
instrumentalities and combinations particularly pointed out in the 
appended claims. 
To achieve the foregoing and other objects, and in accordance with the 
purposes of the present invention as described herein, an improved 
apparatus and method are provided not only to control the spectral and 
spatial distribution of the output of diode-laser arrays, but also to 
steer the output beam from a diode-laser array and to scan it over an 
angle of several degrees. This ability to optically control the beam angle 
of diode-laser arrays opens up the possibility of high-speed direct 
optical or electronic scanning and pointing of high-power diode-laser 
arrays without the need for mechanical or electro-optical scanning 
elements. 
In accordance with the invention, to achieve mode control and beam steering 
in a diode-laser array, the optical radiation emitted by a frequency 
tunable master optical oscillator (for example a narrow band tunable 
continuous wave dye laser which has a bandwidth of less than 2 MHz and 
which is electronically scannable over many tens of gigahertz) is injected 
into a single end-element of a multi-element gain-guided semiconductor 
diode-laser array. By injecting a single end-element, the emission of the 
entire array can be locked. This locking narrows the emission bandwidth of 
the array from several longitudinal modes spread over hundreds of 
gigahertz to a single longitudinal mode with a very narrow emission 
bandwidth (less than 2 MHz for injection with a narrow band dye laser 
master oscillator). 
In accordance with another aspect of the invention, the angle scanning of 
the diode-laser array can be accomplished electronically by maintaining a 
fixed optical injection frequency from the master oscillator and varying 
the electric current supplying the diode-laser array. 
In accordance with the above embodiment of the invention, a single 
end-element of a diode laser array is optically injected along the array 
axis. In this manner, the array is forced to emit in a single beam. 
Furthermore, with this method, the phase tilt across the entire array can 
be controlled and varied simply by changing the frequency of the injected 
radiation.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Reference is made to the block diagram of FIG. 1 wherein system 10 of the 
invention, shown in an overview, is comprised of a controlled variable 
master optical oscillator 12 that applies an output signal through a 
focussing lens 14 to a semiconductor diode laser array 16. A source 17 of 
injection current is applied to the array 16. 
The block diagram of FIG. 2 details a system to record a variety of system 
parameters obtained in accordance with the invention. Master oscillator 12 
in the preferred embodiment is a frequency-stabilized continuous wave dye 
laser (Coherent, Model 599-21, with LD700 dye) with a measured linewidth 
of less than 2 MHz. The dye laser is pumped with the red output from a 
krypton ion laser. Oscillator 12 alternatively may be any other suitable 
type of laser, such as a semiconductor laser diode or color-center. 
Furthermore, the oscillator 12 may be provided by a pulsed as well as 
continuous wave laser. Scanning Fabry-Perot interferometers 20 (150 MHz 
and 1.5-GHz free-spectral-ranges) and a 1-meter spectrometer are used to 
monitor the optical injection bandwidth and wavelength. Isolation between 
the dye laser 12 and the diode-laser array 16 is provided by a 
permanent-magnet Faraday isolator 24. The intensity of the injected beam 
can be continuously varied without affecting either the dye laser 12 or 
the location of the focal spot on the diode array 16 emission surface by 
using a variable attenuator 26 consisting of a polarization rotator and a 
linear polarizer. A 12-mm-focal-length lens 14 is then used to focus this 
beam onto the selected facet of the diode-laser array 16. Alternatively, 
an optical waveguide can be applied for this purpose. 
The injection of radiation from the dye laser 12 into the diode laser array 
16 is monitored using a CCD-camera 28 and associated optical system which 
utilizes a camera focusing lens 30 to collect the backscattered radiation 
(depicted as rays 32) from the surface of the diode-laser array 16. This 
camera 28 is also used to measure the near-field intensity distribution of 
the diode array emission. An off-axis mirror 34 located near the laser 
focusing lens 14 (orthogonal to the diode-array junction) directs a 
portion of the diode-array emission to a silicon photodiode-array 36 (512 
elements with 50-micrometers spacing) which measures the far-field angular 
distribution of the array output. 
The spectral distribution of the diode-array 16 output is measured with the 
1/4 meter monochromator spectrometer 22 and the series of scanning 
Fabry-Perot interferometers 20 (375-GHz, 15-GHz, and 150-MHz free-spectral 
range). 
The diode laser arrays 16 used for carrying out the invention are 
commercial ten-element gain-guided devices (Spectra Diode Labs, Model 
SDL-2410-D1, 6-micrometer stripe width and 10-micrometer center-to-center 
spacing) which operate at the 100-mW continuous wave level with a center 
wavelength near 815 nm. The devices are mounted on a thermoelectrically 
cooled stage (not shown) which permits translation along these orthogonal 
axes and rotation about two of these axes. 
It should be noted that, although discrete components comprising the laser 
array and light directing optics are shown in FIG. 2, the components may 
alternatively be provided in the form of an integrated optical circuit. 
During experimentation using the system of FIG. 2, the dependence of the 
optical injection-locking on the input injection angle was examined. The 
injection angle was varied in the range (+) or (-) 5 degrees to the array 
axis. However, no strong angular dependence was observed, and all the 
measurements disclosed herein were made with an injection angle along the 
array axis. 
FIG. 3 shows the far-field angular distribution of the diode-laser array 16 
operating at the 100-mW level (360-mA current, 1.49xI(threshold)) for 
various injection power levels. In the free-running mode (i.e. no injected 
power), the far-field emission consists of two broad lobes centered near 
(+) or (-) 4 degrees from normal. FIGS. 3B-3D show the narrow single-lobed 
far-field distribution which is produced when a single end-element of the 
diode-laser array 16 is injected end-on (along the array axis) at a 
wavelength corresponding to the higher frequency end of a free-running 
longitudinal mode cluster. It should be noted that the emission angle 
(from normal) of the single-lobed far-field beam is slightly larger than 
that of the free-running emission. Furthermore, the single-lobed emission 
is directed away from the end-element which is being injected. 
As shown in FIG. 3D with an injection power of 12 mW (measured at the diode 
facet), the angular width of the far-field emission is measured to be 0.43 
degrees (full-width at half-maximum). Under these conditions, the output 
power of the array is increased by 10-15% from its free-running 100 mW 
level. The diameter of the focussed injection-beam on the diode-laser 
array facet (1/e.sup.2 intensity points, where the letter "e" is the 
universally accepted designator for the base for natural logarithms and is 
a constant of value e-2,718) was measured to be slightly less than 5 
microns, and the coupling efficiency into the diode array is estimated to 
be about 20%. 
Numerical modeling of the injection-locking process indicates that the 
single-lobed emission is very similar in nature to that produced by 
chirping the gain of the array. In the present case, injection of 
radiation into an end-channel leads to a tilting of the phase fronts 
emanating from the individual elements which in turn produces an off-axis 
single-lobed far-field pattern radiated at an angle slightly larger than 
that which characterizes one of the free-running lobes. 
The high degree of spectral purity which is characteristic of the optical 
injection-locked semiconductor laser array of the invention is seen in the 
scanning Fabry-Perot interferometer graph shown in FIG. 4. Here the less 
than 2 MHz bandwidth of the emitted radiation is characteristic of the 
spectral width of the injected radiation and is more than an order of 
magnitude narrower than that observed previously using a diode-laser 
master oscillator in Goldberg et al discussed above. 
The frequency range over which the diode-array 16 could be locked was 
measured at various injection power levels. As in Goldberg, et. al., the 
degree of locking was determined by measuring the peak intensity of the 
single-lobed output beam. The locking range was defined as the frequency 
range corresponding to the half-power points of the single-lobed far-field 
beam. These results are plotted in FIG. 5. Examination of the results for 
the 3 mW injection power level shows that locking occurs at a number of 
distinct frequency intervals with maxima separated by approximately 10 GHz 
(0.22 A). This frequency spacing is consistent with the spacing of the 
high-order supermodes observed in high-resolution spectra of free-running 
diode-laser arrays. A number of asymmetric locking ranges are observed 
with the single-lobe peak intensity increasing abruptly on the 
low-frequency side of each locking range and decreasing more gradually on 
the high-frequency side. This asymmetric behavior has also been observed 
in injection-locking studies of single-element laser diodes and has been 
attributed to the carrier density dependent index of refraction in the 
active region of diode lasers. 
In FIG. 5, a locking range of approximately 6 GHz (FWHM) is observed for 
the strongest intensity peak at the 3 nW injection level. As the injected 
power level is increased, the peak of this strongest locking range shifts 
to lower frequency with a square-root power dependence, as observed in 
single-stripe diode lasers. 
With 6 mW of injected power, the individual locking ranges begin to 
overlap; and at the 12 mW level a continuous locking range of over 60 GHz 
is observed. 
In addition to examining the effect of injection frequency on locking 
characteristics, an examination of the effect of injection frequency on 
the far-field emission was conducted. At a 12 mW injection level, the 
far-field beam angle (emission angle from normal) is observed to increase 
linearly from 4.7 degrees to 7.0 degrees as the injection frequency is 
scanned over 100 GHz (see FIG. 6). This corresponds to an angular tuning 
rate of 2.3.times.10.sup.-2 degrees/GHz (1.04 degrees/A). At the 
half-intensity points of the locking range, (indicated by vertical arrows 
in FIG. 6), the total scan angle is 1.39 degrees. 
The observed change in the far-field beam angle with injection frequency is 
consistent with our current understanding of the injection-locking 
process. Any change in the resonance conditions for the injected radiation 
produces a corresponding change in the phase-fronts emanating from the 
individual channels in the array. We have verified this by varying the 
diode-array injection current while maintaining a constant wavelength for 
the injected beam. As the injection current was decreased from the normal 
operating current, 360 mA (1.49.times.I (threshold)), to 299 mA 
(1.23.times.I(threshold)), we observed a decrease in the far-field beam 
angle of about one degree. 
Our study of single-element injection locking of diode laser arrays reveals 
behavior not previously observed for single-stripe diode lasers. The 
occurrence of a number of discrete natural frequencies, presumably due to 
the existence of supermodes, provides a multitude of locking ranges which, 
at high injection power levels, can be merged into a continuum covering 
many tens of gigahertz. Furthermore, the angle of the far-field 
single-lobed output beam can be scanned over a range of several degrees by 
varying the injection wavelength thus demonstrating the possibility for 
active optical control of diode-laser arrays. 
Thus, in accordance with the principles of the invention described 
heretofore, at an injection power level of 12mW, mode control of a 
ten-element diode array is obtained by single-channel injection locking 
over a frequency locking bandwidth of 60 GHz at half-power levels while 
maintaining a single-lobe far-field emission beam. This power dependent 
locking bandwidth is a factor of 80 greater than previously demonstrated 
for single-channel injection locking of similar diode-laser arrays. 
By varying the optical injection frequency over the 60 GHz frequency 
locking range, the single-lobed far-field beam emitted by the array can be 
scanned over an angle of 1.39 degrees at a rate of 2.3.times.10.sup.-2 
degrees/GHz (1.04 degrees/Angstrom). Larger angle scans (up to 2.3 
degrees) are also obtainable with a reduction in the single-lobe power 
with increasing angle by the subject method and apparatus for laser beam 
steering by injection-locking. 
The angular scanning observed with single-channel injection-locking is 
largely independent of the optical injection power. The scanning rate 
(degrees/GHz) varies by only 10% as the injected power is varied by a 
factor of four. This relationship between injection power and scanning 
rate seems to indicate that the mechanism for the angular scanning is not 
gain saturation in the semiconductor array, but rather the change in the 
phase-fronts of the optical waves propagating in the array due to the 
presence of the injected wave. 
Angle scanning of the diode-laser array is accomplished electronically by 
maintaining a fixed optical injection frequency from the master oscillator 
and varying the electric current supplying the diode-laser array. In this 
case, a change in the diode-laser current of 61 mA (from 299 mA to 360 mA) 
increases the emission angle of the single-lobed far-field beam by 
approximately one degree. 
In the present case, a single end-element of a diode laser array is 
optically injected along the array axis. In this manner, the array is 
forced to emit in a single beam. Furthermore, with this method, the phase 
tilt across the entire array can be controlled and varied simply by 
changing the frequency of the injected radiation. Under these conditions 
the locking range is 60 GHz, and the array emission beam can be scanned 
linearly with injection frequency over an angle of 2.3 degrees. 
The injection-locking technique of the invention is applicable to 
one-dimensional diode-laser arrays of arbitrary dimensions and also to 
two-dimensional arrays. The technique of the invention has been 
demonstrated with a stripe-geometry gain-guided diode-laser array. The 
techniques of the invention are believed to be applicable to arrays of 
other construction (e.g. broad-area arrays, index-guided arrays, etc.) 
thereby providing a means for directly controlling the spatial and 
spectral emission characteristics of these arrays and also scanning the 
output beam from the arrays. It is expected that the optical and/or 
electronic scanning capability afforded by injection-locking techniques of 
the invention will play an essential role in the development of large 
scale diode arrays for commercial and military applications. 
Numerous additional benefits are obtained by employing the principles of 
the invention. For example, with the invention, beam steering of a 
diode-laser array occurs primarily from the injection of an optical beam 
into the array rather than by electronic means (although electronic 
scanning is also possible and has been disclosed). This use of optical 
scanning allows for scanning at much faster speeds than is possible with 
electronic scanning. In addition, the subject invention is not limited to 
scanning at a fixed frequency. 
With the invention, the laser beam control and scanning apparatus is vastly 
simpler and more compact than the prior art since it utilizes the active 
gain medium of a closely coupled diode laser array to act both as a source 
of high power laser radiation and also as the phase modulator required to 
steer the emission angle of the laser radiation. The simplicity and direct 
integration achieved with the subject invention eliminates the separate 
phase modulators and/or frequency shifters required in the prior art. 
Further, the relative phases of the various emitters required to cause the 
lasing beam to be emitted at a given angle are automatically and 
efficiently achieved by the use of a single input laser beam from a master 
oscillator injected into a single end-element of a multiple-element array. 
In order to scan the emission angle of the diode array output beams, a 
change in the frequency of the master oscillator is all that is required. 
Thus with the invention, the multiple input beams and the associated 
mirrors or optical distributors required to generate them, as required in 
the prior art U.S. Pat. No. 3,626,321 to Smith and U.S. Pat. No. 3,691,483 
to Klein are eliminated. 
In addition, with the invention, the use of a single input beam allows the 
number of lasing elements in the scanned laser array to be changed at will 
without necessitating any change in the control apparatus (master 
oscillator and input beam optics). Thus, scaling of the diode laser array 
to a very large number of emitting elements (in the form of a one- or 
two-dimensional array) requires only one or at the most a very few master 
oscillators to control the performance of the array. This scaling would be 
very difficult and costly, if not impossible, with the prior art. 
Furthermore in accordance with the invention, a single, 
nearly-diffraction-limited laser beam is generated by the injection-locked 
diode laser array. The spatial intensity profile of this beam is 
relatively insensitive to the angle of emission of the beam. These 
characteristics are desirable in order to concentrate the emitted laser 
power into an intense beam which can be used for any number of 
applications. 
Another benefit of the invention is that it can be implemented as a single 
compact unit in the form of an integrated optical circuit. 
Still another benefit resulting from employing the principles of the 
invention is that a conventional two-lobed far-field beam pattern is 
converted to a single narrow beam having a near-diffraction-limited 
angular width in the dimension of the array. These benefits are 
accomplished with no reduction in the total output power of the 
diode-laser array. 
The foregoing description of the invention has been presented for purposes 
of illustration and description. It is not intended to be exhaustive or to 
limit the invention to the precise form disclosed. Obvious modifications 
or variations are possible in light of the above teachings, for example, 
as described in the detailed description of FIG. 2.