An array of diode lasers, either a two-dimensional array of surface emitting lasers, or a linear array of stripe lasers, is phase locked by a diode laser through a hologram which focuses the output of the diode laser into a set of distinct, spatially separated beams, each one focused onto the back facet of a separate diode laser of the array. The outputs of the diode lasers thus form an emitted coherent beam out of the front of the array

Lesh, James R.

English

NASA Technical Reports Server

United States Patent 1191 
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[11] Patent Number: 4,677,629 
[45] Date of Patent: Jun. 30, 1987 
 
Emitted 
Coherent 
Beorn 
[54] MEANS FOR PHASE LOCKING THE 
OUTPUTS OF A SURFACE EMITTING 
LASER DIODE ARRAY 
[75] Inventor: James R. Lesh, Pasadena, Calif. 
[73] Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. NO.: 781,812 
[22] Filed: Sep. 30, 1985 
[51] Int. (3.4 .............................................. HOlS 3/098 
[52] U.S. Cl. ...................................... 372/18; 372/103; 
372/43; 350/3.73; 350/3.81 
[58] Field of Search ..................... 372/103, 18, 31, 43, 
372/44, 50; 350/3.73, 3.81 
1561 References Cited 
US. PATENT DOCUMENTS 
3,452,296 6/1969 Lockenvitz et al. ............... 33V94.5 
3,579,142 5/1971 Smiley ................................ 331/945 
3,577,093 5/1971 Simpson ............................. 33V94.5 
3,586,995 6/1971 Hubach .............................. 33V94.5 
3,593,190 7/1971 Reinberg ............................ 33V94.5 
3,602,838 8/1971 Stem .................................. 33V94.5 
3,691,483 9/1972 Klein .................................. 332/7.51 
3,704,377 11/1972 Lehovec ............................ 331/94.5 
3,704,427 11/1972 Heywang ........................... 33V94.5 
3,763,441 10/1973 Roess ................................. 33V94.5 
/ 
3,794,929 2/1974 Carr ..................................... 330/4.3 
3,835,414 9/1974 Aheam ............................... 33W94.5 
3,970,963 7/1976 Chester .............................. 33V94.5 
4,096,448 6/1978 Hayes .................................... 372/18 
4,221,468 9/1980 Macken ............................... 350/292 
4,246,548 1/1981 Rutz ...................................... 372/44 
4,321,551 3/1982 Bleil et al. ............................ 330/4.3 
4,479,221 10/1986 Kitamura .............................. 372/75 
OTHER PUBLICATIONS 
Chow; “Phase Locking Lasers by an Injected Signal”; 
Opt. Lett., vol. 7, No. 9, Sep. 1982; p. 417. 
A. Ibaraki, et al., “GaAlAs/GaAs Surface 2 Emitting 
Injection Laser” Thirteenth International Quantum 
Electronics Conference, Jun. 18, 1984, pp. 49-50. 
Primary Examiner-Leon Scott, Jr. 
Attorney, Agent, or Firm-Paul F. McCaul; John R. 
Manning; Thomas H. Jones 
1571 ABSTRACT 
An array of diode lasers, either a two-dimensional array 
of surface emitting lasers, or a linear array of stripe 
lasers, is phase locked by a diode laser through a holo- 
gram which focuses the output of the diode laser into a 
set of distinct, spatially separated beams, each one fo- 
cused onto the back facet of a separate diode laser of the 
array. The outputs of the diode lasers thus form an 
emitted coherent beam out of the front of the array. 
3 Claims, 5 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19870014527 2020-03-23T15:21:31+00:00Z
US. Patent JUIL 30,1987 Sheet 1 of3 4,677,629 
4 
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U.S. Patent JUIL 30,1987 Sheet 2 of 3 4,677,629 
U.S. Patent JUII 30,1987 Sheet 3 of 3 4,677,629 
4.677.629 
1 
MEANS FOR PHASE LOCKING THE OUTPUTS 
OF A SURFACE EMITTING LASER DIODE ARRAY 
ORIGIN OF INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract, and is sub- 
ject to the provisions of Public Law 96-517 (35 USC 
202) in which the Contractor has elected not to retain 
title. 
BACKGROUND OF THE INVENTION 
This invention relates to phase locking diode laser 
arrays, and more particularly to a technique for phase 
locking the outputs of the diode lasers in the array with 
inherent light from a master diode laser. 
Diode lasers do not emit light with sufficient power 
for most applications, such as communications. Conse- 
quently, the practice has been to provide an array of 
2 
a 5 pm channel), coupling of the master laser power 
into the array channels can be accomplished with rea- 
sonably high geometric efficiency using a cylindrical 
lens. 
One of the major drawbacks of present laser diode 
arrays formed in a body of semiconductor material is 
that they are inherently one dimensional, as noted 
above. The laser channels are defined by laying down 
contact stripes on top of thinly grown layers deposited 
10 on a substrate. The channels are defined by contact 
stripes, or other means, such as index guiding of the 
lasing which occurs parallel to the substrate surface 
with outputs occurring at the edges of the sheet of 
grown layers. It is for that reason that these devices 
One solution to the problem of providing more laser 
diodes in an array is to make a two-dimensional, sur- 
5 
l5 have been restricted to one-dimensional arrays. 
face-emitting (SE) array. Both GaAlAs/GaAs and GaI- 
nAsP/InP systems have been used to fabricate a sur- 
edge ekttingstripe lasers, typically 5 pm wide on gpm *O face-emitting injection laser. See A. IbarA, s. 
centers. These lasers are coupled for coherent Opera- IsEkawa, S. Ohko~chi and K. Iga, “ G d l A s / G d s  
tion, Usually by techniques designed into the array Surface Emitting Injection Laser,” Proceedings, Thk- 
structure. However, these edge emitting diode lasers teenth International Quantum Electronic Conference, 
are, at present, inherently one dimensional, with the  hi^^, CA, J~~~ 18, 1984, paper MLL7, pages 
lasing occurring parallel to the substrate. To increase 25 49-50, s. uchiyama and K. rga, L L T ~ ~  ~ i ~ ~ ~ ~ i ~ ~ ~ 1  
the output power, it would be desirable to stack several A~~~~ of Improved ~~i~ Surface-Emitting 
Of these linear arrays; the problem is to provide a way Of 
them to ‘perate in phase. Much work has been Injection Lasers” and Proceedings, 1985 Conference on Lasers and Electro-optics, paper No. T W 6 ,  page 45. In devoted to the analysis, development and fabrication of 
diode laser arrays. D* R* ’‘ifre‘’ R* 
these devices, layers are first grown on a substrate. 
Burnham and w‘ 30 Then etched laser facets (pothole mirrors) are etched streifer, “Phase-Locked Semiconductor Laser 
Appl’ Phys‘ Lett* vO1’ 33, PP. 1015-1017, 1978; D‘ E’ from the bottom (through the substrate) and laser facets are etched or otherwise provided on the top. In such Ackley, “High Power Multiple Stripe Injection Lasers 
with Channel Guides,,, IEEE J. Quantum Electron., 
Vol. QE-18, pp. 1910-1917, 1982; E. Kapon, C. Lind- 35 surface‘ 
sey, J. Katz, s. Margalit and A. Yariv, ‘6Coupling 
Mechanism of Gain-Guided Integrated Semiconductor 
Laser Arrays,” Appl. Phys. Lett. 44, Feb. 15, 1984. 
A desirable property of such arrays is to have the 
output beams from each of the laser stripes phase coher- 40 prevents any phase 
ent (relative to one another). Sometimes this phase co- the only method by which this type of laser could 
herence can be produced by simply placing the laser be phase locked is through injection locking. However, 
stripes close enough together. E. Kapon, J. Katz, C. the geometric efficiency (i-e.9 aggregate area ofthe back 
Lindsey, s. Margalit and A. y k v ,  q-ontrol of Mutual facets divided by the area of the array) is extremely 
Phase Locking ofMonolithical Integrated Semiconduc- 45 small. This is true to a lesser extent even of a linear 
tor Lasers,” phys. Lett. 43, pp. 421-42 (1983); J. array, So that it is difficult to use injection locking there 
Katz, E. Kapon, C. Lindsey, S. Margalit, U. Shreter and as Well because in both cases the geometry makes COW 
A. Y k v ,  “phase Locked Semiconductor Laser Array pling a master laser into every diode of the array.virtu- 
with Separate Contacts,” Appl. Phys. Lett. 43 pp. ally impossible. SO the Problem is to Phase synchronize 
521-523, (1983). However, such coupling can often lead 50 the outputs ofthe diode lasers in the array such that the 
to undesirable supermode patterns in the array far field. combined output is Phase coherent and Contains 
E. Kapon, J. Katz, and A. Yariv, “Supermode Analysis ble far-field supermodes, i.e., produces a single lobe 
of Phase Locked Arrays of Semiconductor Lasers,” far-field pattern, using a master diode laser. 
SUMMARY OF THE INVENTION Opt. Lett., Vol. 9., pp. 125-127, 1984. 
via injection locking where a master laser is focused In accordance with the present invention, an array of 
into the back facet region ofthe array, thereby injecting diode lasers is provided with a single master diode laser 
a phase coherent stimulus into all the array channels. for phase locking the outputs of the diode lasers 
Some success has been achieved experimentally using 
this technique. L. Goldberg, H. F. Taylor and J. IF. 60 
Weller, “Optical Injection Locking of a GaAlAs Phase 
Coupled Laser Array,” presented at XI11 International 
Conference on Quantum Electronics, Postdeadline 
paper PD-B4, Anaheim, CA, June 12, 1984; J. Daher, 
“Injection Locking of an Incoherent Laser Array,” 65 
Summer Internal Summary Report, Jet Propulsion Lab- 
oratory, August 1984. Since the array stripes (channels) 
are quite close together (usually about 9 pm spacing for 
devices, the lasing output occurs normal to the substrate 
Although such devices hold promise for two-dimen- 
sional structures, the separation between laser front 
facets (potholes) is usually 25 to 100 pm. This separa- 
tion, along with the short laser cavity length, effectively 
between the lasers* 
A more positive method to achieve phase locking is 55 
through a hologram prepared to focus light into a set of 
distinct, spatially separated beams, each focused to di- 
rect light from the master diode onto an etched laser 
facet of each of the diode lasers of the array. The holo- 
gram is made using the same mask as used to photolitho- 
graphically define the back laser facets. The same mas- 
ter diode laser, or one of substantially the same wave- 
length is used to produce the hologram in an otherwise 
conventional way. A beam splitter and a mirror are 
used to produce the necessary reference beam. This 
4,677,629 
3 
technique of making and using a hologram to focus 
beams of light from a master diode laser onto the back 
laser facets in an array has its greatest advantage for 
two-dimensional arrays, but may be used to advantage 
for linear arrays as well. 
The novel features that are considered characteristic 
of this invention are set forth with particularity in the 
appended claims. The invention will best be understood 
from the following description when read in connection 
with the accompanying drawings. 
BRIEF DESCRIPTION O F  THE DRAWINGS 
FIG. 1 is a schematic diagram of one embodiment of 
the invention comprised of a two-dimensional array of 
surface emitting diode lasers and a master diode laser 
for phase locking all diode lasers utilizing separate 
beams formed by a hologram from the beam emitted by 
the laser diode. 
FIG. la is a plan view of FIG. 1 illustrating the ar- 
rangement for optimum illumination of the hologram 
for the purpose at hand. 
FIG. l b  is a cross section taken on a line b-b in FIG. 
1. 
FIG. 2 is a schematic diagram of another embodiment 
of the invention comprised of a linear array of diode 
lasers and a master diode laser for phase locking all 
diode lasers utilizing separate beams formed by a holo- 
gram from the beam emitted by the laser diode. 
FIG. 3 illustrates schematically the manner in which 
a hologram is made from a template used to etch or 
otherwise form laser facets on the back of the array. 
DESCRIPTION OF PREFERRED 
EMBODIMENTS 
Referring now to FIG. 1, a two-dimensional array 10 
of surface emitting (SE) diodes 12, is excited by a sepa- 
rate beam of light formed by a hologram 14 from a beam 
emitted by a single diode laser 16. The hologram is 
prepared to focus the output of the master laser into a 
set of distinct, spatially separated beams, each one fo- 
cused onto a separate etched facet in the back side of a 
laser array. Phase coherent light is thus emitted by all of 
the SE diode lasers because of the phase locking 
achieved by back lighting the SE diode lasers of the 
array with a reference beam at an angle as shown in 
FIG. la. The combined output of the laser array is 
phase coherent. 
The requisite holographic technology is quite well 
known, and recently used for such purposes as commer- 
cial telephone equipment, namely a telephone credit 
card validating system using a hologram. When inserted 
into the telephone, and illuminated by a laser, a pattern 
of spatially separated beams is produced which the 
telephone is configured to recognize before it will vali- 
date the credit card. In the proposed laser array injec- 
tion locking scheme, the hologram can also be config- 
ured to compensate for asymmetry in the output beam 
shape of the master diode, thereby eliminating the need 
for comDlex ad iza t ion  oDtics. such as a Drism uair. 
4 
diode laser of substantially the same wavelength, pro- 
vides coherent light used with a holographic emulsion 
plate 14 (an ordinary photographic plate) to produce an 
image of the front laser facets 20 from the mask 24. 
5 During exposure of the holographic emulsion plate, the 
coherent light is divided into two beams by a beam 
splitter 25. Part of the light is directed to illuminate the 
holographic emulsion plate through the mask 24, and 
part (a reference beam) is channeled by a mirror 26 to 
10 illuminate the holographic emulsion plate with a refer- 
ence beam at an angle with respect to the direct beam. 
The two beams produce an interference pattern on the 
holographic emulsion plate which bears no resemblance 
to the pattern on the mask 24 until developed and again 
15 illuminated by a similar coherent reference beam, as 
shown in FIG. 1. Then there is produced an image of 
the photolithography mask 24 on the back of the array 
10, with a separate beam focused on each facet 20. Since 
the facets are all thus illuminated by light from the same 
20 diode laser 16, the output of the laser array is a coherent 
beam made up from the in-phase outputs of the individ- 
ual diode lasers of the array. 
A similar hologram 14 is formed in the same way for 
the linear array 10’ of FIG. 2, shown as a contact stripe 
25 laser, although it could be made by other known stripe 
laser techniques such as the index guided stripe laser. 
The only difference is that the photolithography mask is 
not normally required to fabricate the array, so such a 
mask has to be produced for this purpose. The holo- 
30 gram 14 focuses a beam into the active region of each 
35 
40 
45 
50 
55 
In p rodwig  the two-dimensional diodk lase; array 60 
10, a mask is used to photolithographically etch laser 
facets (pothole mirrors) 18 on the front side 19, as 
shown in FIG. lb. Another mask is similarly used to 
form laser facets 20 on the back 21. The photolithogra- 
phy mask used for the facets on the back is also used to 65 
produce the hologram 14 in an arrangement shown in 
FIG. 3 wherein that mask is identified by the reference 
numeral 24. Light from the diode laser 16, or another 
stripe laser from the back facet 27, and a coherent beam 
is emitted from the front facet 28. 
Although particular embodiments of the invention 
have been described and illustrated herein, it is recog- 
nized that modifications and variations may readily 
occur to those skilled in the art. Consequently, it is 
intended that the claims be interpreted to cover such 
modifications and variations. 
What is claimed is: 
1. An array of injection phase-locked diode lasers 
utilizing as a master a separate laser source, and means 
for coupling light from said master laser source into 
each diode laser of said array of diode lasers, said array 
being fabricated in a body of semiconductor material, 
each diode laser having a back facet on one side of said 
body of semiconductor material and a front facet on an 
opposite side of said body of semiconductor material, 
said light coupling means comprising a hologram pre- 
pared for producing from the output of said master laser 
source a set of distinct, spatially separated beams, each 
one directed into the back facet of each diode laser, 
thereby to assure emission of phase coherent light from 
said array of diode lasers, said hologram being posi- 
tioned between said master laser source and said one 
side of said body of semiconductor material, said holo- 
gram being prepared by illuminating a holographic 
emulsion plate with an image of said array of diode 
lasers using coherent light of substantially the same 
wavelength as said master laser source, and a coherent 
reference beam of substantially the same wavelength as 
said master laser source. 
2. The combination as defined in claim 1 wherein said 
array of diode lasers is comprised of a rectangular array 
of surface emitting diode lasers. 
3. The combination as defined in claim 1 wherein said 
array of diode lasers is comprised of a liner array of 
stripe lasers. * * * * *  


Means for phase locking the outputs of a surface emitting laser diode array

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