Incoherent, optically coupled laser arrays with increased spectral width

An optically uncoupled laser array is modified at its current confinement geometry to introduce nonuniformity in effective optical cavity width and/or length among the different lasers comprising the laser array. An array laser comprising a plurality of spaced lasing elements with an optical cavity for generating and propagating radiation under lasing conditions with each of the laser elements being optically uncoupled from one another is enhanced by an extended spectral emission linewidth and reduction in temporal coherence. This extended spectral emission linewidth and reduction in temporal coherence is accomplished by changing the gravity gain or loss for at least a majority of the laser elements relative to each other whereby a different longitudinal mode(s) of operation and frequency of operation exist for each such laser element. The enhancement may be accomplished, for example, by providing nonuniformity in the current confinement width or nonuniformity in the effective current pumped region or a change in optical cavity absorption loss for at least a majority of laser elements. Such nonuniformity or change may be randomly varying or monotonically increasing or decreasing across the laser elements of the array.

BACKGROUND OF THE INVENTION 
This invention relates to optically uncoupled or nonphase locked 
semiconductor laser arrays and more particularly to optically uncoupled 
laser arrays modified to have an overall increased spectral emission 
bandwidth. An array of closely spaced but optically uncoupled diode lasers 
has been proposed and developed for electro-optic line modulators and line 
printers. An example of such a laser array is disclosed in U.S. patent 
application Ser. No. 808,197 filed Dec. 12, 1985 entitled, "INCOHERENT, 
OPTICALLY UNCOUPLED LASER ARRAYS FOR ELECTRO-OPTIC LINE MODULATORS AND 
LINE PRINTERS". In that disclosure, an incoherent diode laser array source 
is employed as a single solid state light source capable of providing a 
sheetlike, uniform and high intensity of light. The uncoupled array 
comprises a series of monolithic index guided diode lasers on a single 
substrate with their index waveguide cavities formed by layer impurity 
induced disordering. The essential ingredient is that the individual 
lasers or emitters of such an laser array operate randomly phased with 
respect to each other, i.e., they are not sufficiently evanescently 
coupled to be in phase with each other. If each laser in the array 
oscillates independently of the other lasers, its optical phase and/or 
frequency will randomly drift and, as a result, permit optical 
interference to exist only within the beam of each individual laser rather 
than among or between the beams of adjacently positioned diode lasers of 
the array. 
Although such laser arrays exhibit the desired single lobe, far field 
radiation patterns desired for electro-optic line modulators, their 
optical spectra are narrower than the emission spectra of an LED. The 
bandwidth or linewidth of these laser arrays are narrower because all the 
lasers in the array are fabricated to be nominally identical and each 
laser oscillates in predominately one longitudinal mode relative to their 
cavities of substantially equal length. Any spectral width in the total 
emission arises only due to small but unintentional differences existing 
in the geometrical and optical parameters between the spatial emitters. 
Also, it is possible that the temporal coherence of these laser arrays 
will be too large to eliminate optical interference effects which arise 
from light scattering due to scratches, dust and other foreign micro 
particles present in the optical system of the modulator or printer. 
It is an object of this invention to provide means in the laser array 
structure to force the individual emitters in the array to lase at 
different longitudinal modes to thereby increase the overall spectral 
emission bandwidth of the array laser. 
It is another object of this invention to reduce temporal coherence in 
optically uncoupled index guided array lasers. 
SUMMARY OF THE INVENTION 
According to this invention, an optically uncoupled laser array is modified 
at its current confinement geometry to introduce nonuniformity in 
effective optical cavity width and/or length among the different lasers 
comprising the laser array. An array laser comprising a plurality of 
spaced lasing elements with an optical cavity for generating and 
propagating radiation under lasing conditions with each of the laser 
elements being optically uncoupled from one another is enhanced by an 
extended spectral emission linewidth and reduction in temporal coherence. 
This extended spectral emission linewidth and reduction in temporal 
coherence is accomplished by changing the gravity gain or loss for at 
least a majority of the laser elements relative to each other whereby a 
different longitudinal mode(s) of operation and frequency of operation 
exist for each such laser element. The enhancement may be accomplished, 
for example, by providing nonuniformity in the current confinement width 
or nonuniformity in the effective current pumped region or a change in 
optical cavity absorption loss for at least a majority of laser elements. 
Such nonuniformity or change may be randomly varying or monotonically 
increasing or decreasing across the laser elements of the array. 
Other objects and attainments together with a fuller understanding of the 
invention will become apparent and appreciated by referring to the 
following description and claims taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Disclosed are two different generic embodiments for reducing temporal 
coherence and cause lasing in different longitudinal modes in an optically 
uncoupled laser array. The first embodiment is shown in FIG. 1 wherein the 
laser emitters of the array are fabricated with unequal current 
confinement or stripe widths or progressively different optical cavity 
widths. The second embodiment is shown in FIGS. 2-7 wherein nonuniform 
pumping is achieved through nonuniform or unequal current confinement 
geometry. U.S. Pat. No. 4,594,719 discloses different center to center 
spacing among laser elements for a particular function unrelated to this 
disclosure and is provided with uniform width current confinement strips. 
In the invention herein, the current confinement widths of the spaced 
laser elements are not identical and the array of laser elements are not 
phase locked. 
In the explanation of these two embodiments, it should be realized that 
every laser element of an array laser need not have a nonuniformity in 
effective optical cavity width and/or length among the different laser 
elements, as it will be realized by those skilled in the art that only a 
substantial number, e.g. a majority, of laser elements in the array laser 
need have a different nonuniformity to be significantly effective in 
extending the spectral linewidth or bandwidth of emission of the laser. 
Reference is now made to FIG. 1 wherein there is disclosed the first 
embodiment of this invention. Optically uncoupled array laser 10 comprises 
a plurality of closely spaced laser elements or emitters 11 formed by 
laser impurity induced disordering (IID) or other techniques as are well 
known in the art. Only four emitters 11 are shown for the sake of 
expediency and simplicity. Obviously, additional emitters may be included. 
Laser 10 comprises a n-GaAs 12 upon which is epitaxially deposited the 
following layers: cladding layer 14 of n-Ga.sub.1-x Al.sub.x As where x 
may, for example, be equal to 0.4 and the layer may be about 1.5 .mu.m 
thick; active region 16 comprising an active layer or a single quantum 
well structure or a multiple quantum well structure, for example, four 
GaAs quantum well layers each 10 nm thick separated by barrier Ga.sub.0.65 
Al.sub.0.35 As layers each 7 nm thick; cladding layer 18 of p-Ga.sub.1-x 
Al.sub.x As where x may, for example, be equal to 0.4 and the layer may be 
about 0.8 .mu.m thick; and cap layer 20 of p+GaAs being about 0.1 .mu.m 
thick. The active region may be p doped, n doped or undoped. 
The processing of laser array 10 begins with the deposition of a film of 
Si.sub.3 N.sub.4 of about 100 nm thick. This film is photolithographically 
patterned to provide windows or openings for forming regions 13 via Si 
diffusion. Next, an approximately 50 nm thick film of silicon is deposited 
on the array, followed by another film of Si.sub.3 N.sub.4 of 
approximately 100 nm thickness. Diffusion is performed at 850.degree. C. 
for about 7.5 hours to disorder the active region in areas adjacent to 
what will eventually become the lasing filaments as represented by 
emitters 11. 
The current confinement spacing for emitters 11 is unequal. The spacing, S, 
between the first two diffusion regions 13 is smaller than the spacing 
between the next two regions, i.e., S+.DELTA., and so on, so that the 
linear progression of increase in current confinement or stripe width is 
S, S+.DELTA., S+2.DELTA., S+3.DELTA., S+n.DELTA.. . . As an example, S may 
equal 1 .mu.m and .DELTA. may be in the range of 0.5 .mu.m, or example. 
However, this range is not at all intended to be exclusive. Further, 
.DELTA. need not be of the same value in between regions 13 but rather may 
be a different value between each pair of diffusion regions 13. The 
forgoing also applies with respect to all subsequent embodiments to be 
discussed. 
The n-type Si diffusion regions 13 preferably extend through active region 
16. It is preferred that the diffusion extend through the active layer and 
any layers that form a part of or function as part of the optical cavity 
of each laser element. In this regard, the diffusion may be best to extend 
partly into lower cladding layer 14 depending, for example, upon the 
percentage of aluminum in the inner cladding layers. If additional inner 
confinement layers are provided with the active region, it is preferred 
that the diffusion extend through such confinement layers since they are 
part of the laser element optical cavity. The result to be realized is 
that the diffusion must extend sufficiently through the optical cavity of 
each laser element to prevent stable phase locking between adjacent 
emitters due to overlap of the evanescent optical wave extending between 
adjacent laser cavities. The diffusion regions 13 provide both optical 
confinement of this wave as well as carrier confinement to the individual 
laser cavities represented by the emitters 11 so that the individual laser 
elements may be closely spaced, starting with S approximately equal to 1 
.mu.m, without being optically coupled to one another. 
After the diffusion, the Si layer and both Si.sub.3 N.sub.4 layers are 
removed by etching in a CF.sub.4 plasma. The entire surface of array 10 
may then be Zn-diffused to reconvert the n-type Si-diffused GaAs cap layer 
20 and part of cladding layer 18 to p-type material (not shown) to avoid 
any functional problems with the parasitic p-n junction that is in 
parallel with the active region lasing junction. 
Laser array 10 is metallized with Cr--Au or Ti--Pt--Au contact 22 on cap 
layer 20 and alloyed with Au--Ge, followed by Cr--Au or Ti--Pt--Au as 
contact 24 on the bottom surface of substrate 12. 
The voltage bias to each laser emitter 11 will be approximately the same 
when applied across contacts 22 and 24. However, since each emitter 11 has 
a different pumping width, lasing in each laser cavity will occur at a 
different carrier density and total current. Due to the employment of 
layers exhibiting quantum size effects in active region 16, enhanced 
bandfilling will occur so that different increments of bandfilling will 
occur for different widths of current confinement to laser emitters 11 
resulting in different emission wavelengths and different spectral 
emission bandwidths for each of the arrays. Thus, the difference in 
current confinement width enhance the differences between the wavelengths 
at which the spatial emitters will emit radiation. 
Reference is now made to FIG. 2 wherein there is disclosed the second 
embodiment of this invention. Optically uncoupled array laser 30 comprises 
a plurality of closed spaced laser elements or emitters 11 formed by layer 
impurity induced disordering (IID) in the manner as previously explained 
in connection array laser 10 or any other technique. However, all of laser 
emitters 32 are uniformly spaced at a distance, S. The contact 
configuration on the top of laser 30 is nonuniform to bring about 
nonuniform current distribution among the several laser elements or 
emitters 32. Metal contact regions 36 and 38 are regions that are current 
pumped relative to each laser element while isolation region 34 is 
insulated from any current penetration. Isolation region 34 may be formed 
by ion implantation or by proton implantment, e.g., proton implant at an 
energy of 70 keV with a dose of 3.times.10.sup.15. Isolation may also be 
accomplished with an insulating layer on the top epitaxial layer of the 
device. To be noted is that central isolation region 34A has a laterally 
varying width relative to adjacent contact regions 36 and 38 so that total 
cavity length for each laser element current pumped beneath contact 
regions 36 and 38 is not identical. Current pumping is confined to contact 
regions 36 and 38 wherein, for example, I.sub.1 =I.sub.2. Current flow 
through only regions 36 and 38 produces nonuniformity in carrier density 
and gain in each laser cavity and, consequently, will characteristically 
provide a different operating current threshold for each laser emitter 32. 
Thus, by varying the degree of nonuniformity across the lateral dimension 
of array laser 30, a lateral nonuniformity in operating current threshold 
of the individual laser elements will occur. These different operating 
current thresholds will cause the individual laser elements to operate at 
different emission wavelengths thereby broadening the total spectral 
linewidth or bandwidth of array laser 30. 
Another aspect of the embodiment of FIG. 2 is that contact regions 36 and 
38 of array laser 30 may be pumped at different currents, I.sub.1 and 
I.sub.2, e.g., I.sub.1 &gt;I.sub.2. This has the effect of also changing, in 
a more enhanced manner, the current density among emitters 32 to broaden 
further the emission wavelengths of the individual emitters. 
Many other contact region geometries are possible for broadening the 
spectral linewidth of an array laser. FIGS. 3-7 disclose other such 
embodiments. 
In FIG. 3, array laser 40 is generally the same as array laser 30 in FIG. 2 
except that the inner edges of metal contact regions 46 and 48 defining 
inner isolation region 44A of isolation region 44 have a stair stepped 
configuration 49. This configuration provides a more abrupt nonuniformity 
in the affected cavity lengths of the different individual laser elements 
42 to be pumped. 
The technique of nonuniform current pumping illustrated in FIGS. 2 and 3 
can also be utilized to fabricate separate but monolithic laser arrays 
which emit with different spectral emission bandwidths, which separate 
bandwidths are centered about different dominate wavelengths of operation 
for the individual arrays. Such a dual wavelength, monolithic laser array 
50 is illustrated in FIG. 4. Monolithic laser array 50 comprises two 
sections of array lasers 51 and 53 comprising independent groupings of 
laser emitters 52 separated by central isolation region 58 forming a part 
of isolation region 54. Each laser section 51 and 53 has metal contact 
region 55 and 57 but the spatial variance between these contacts in 
isolation region 54B of isolation region 54 in laser 51 is closer together 
compared to isolation region 54A for laser 53, so that the maximum loss 
due to the extent of loss of active cavity pumping for laser emitter 52A 
will be greater than that for laser emitter 52B. By controlling the 
average increase in current threshold of each laser array 51 and 53 by the 
spatial variation of separation between the respective pairs of contact 
regions 55 and 57, the center or dominate emission wavelength of each 
array 51 and 53 can selected. This particular dual wavelength embodiment 
has particular utility in increasing the resolution of electro-optic line 
modulators and printers. 
As in the case of previous embodiments, the individual contact regions 55 
and 57 of each laser array 51 and 53 may be each pumped at the same 
current or at different pumping currents, e.g., I.sub.1 and I.sub.2, or 
I.sub.1, I.sub.2, I.sub.3 and I.sub.4. 
FIG. 5 illustrates an array laser 60 with a broad beam width with a 
dominate central emission wavelength. Laser 60 includes a plurality of 
emitters 62 with an isolation region 64 having an inner isolation region 
64A of varying width, between current pumping contact regions 66 and 68. 
Inner isolation region 64A has an increasing spatial variation to the 
center of laser 60 and then a decreasing spatial variation to the opposite 
end of laser 60. 
The techniques of the first and second embodiments described above may be 
combined to effect spectral emission bandwidth of an array laser. FIGS. 6 
and 7 illustrate embodiments of such combinations to achieve this result. 
In FIG. 6, array laser 70 comprises a series of spaced emitters 72 having 
different spatial optical cavity widths which linearly increasing from one 
end 75 of the array, S to S+n.DELTA., laterally across the array to the 
opposite end 77 of the array, in combination with isolation region 74 
having an inner isolation region 74A of increasing spatial variation from 
the same one end 75 of the array laterally across the array to the 
opposite end 77 of the array. 
In FIG. 7, array laser 80 comprises a series of spaced emitters 82 having 
different spatial separation which linearly increase from one end 85 of 
the array, S to S+n.DELTA., laterally across the array to the opposite end 
87 of the array, in combination with isolation region 84 having an inner 
isolation region 84A of increasing spatial variation from the opposite end 
87 of the array laterally across the array to the first mentioned end 85 
of the array. The change in wavelength emission across the array would be 
a monotonical increase in wavelength from left to right across the array. 
In the combination of FIG. 7, the spectral width would be further enhanced 
whereas in FIG. 6, the spectral width would be compensated so that, in end 
result, the effect of broadened spectral width accomplished in the FIG. 7 
embodiment would be of a greater magnitude compared to the FIG. 6 
embodiment. 
Another way of introducing nonuniformity in longitudinal mode operation of 
an optically uncoupled array laser is by introducing a nonuniform loss in 
absorption rather current density or gain among the individual laser 
cavities of the array. This can be accomplished by introducing absorption 
loss along a portion of the optical cavities of some or all of the laser 
elements. One way of accomplishing this loss introduction is by the 
diffusion of an impurity along a portion of the optical cavity of each 
laser element to lower the bandgap therealong. Another manner of 
accomplishing this is to narrow the waveguide width along such optical 
cavity portions. A further way would be to proton implant into the active 
layer or region along such optical cavity portions. In all these cases, 
the length of these portions would be nonuniform or unequal among the 
laser emitters. 
A further way would be nonuniformly coating one of the array laser facets 
along its lateral extent with a reflecting mirror coating of continuously, 
laterally varying level of reflectivity so that the level of optical 
feedback to the respective individual emitters across the array would 
differ from one another. Also, the temporal coherence of an uncoupled 
array laser may be decreased by applying an antireflective (AR) coating to 
one or both mirror facts of the laser or by fabricating the longitudinal 
axis of the array laser optical cavities to be at a small angle relative 
to the plane of the cleaved facets. 
Further, as previously indicated, the nonuniformity or changes across the 
array in current confinement widths, effective pumped regions, optical 
cavity absorption loss or effective optical cavity width may be randomly 
varying laterally across the laser elements of the array or may 
monotonically increase or decrease laterally across the laser elements of 
the array. 
While the invention has been described in conjunction with specific 
embodiments, it is evident to those skilled in the art that many 
alternatives, modifications and variations will be apparent in light of 
the foregoing description. Accordingly, the invention is intended to 
embrace all such alternatives, modifications and variations as fall within 
the spirit and scope of the appended claims.