CA2064726C

CA2064726C - Monolithic semiconductor harmonic laser sources

Info

Publication number: CA2064726C
Application number: CA002064726A
Authority: CA
Inventors: Richard J. F. Normandin; Francoise Chatenoud; Robin L. Williams
Original assignee: National Research Council of Canada
Current assignee: Individual
Priority date: 1990-06-29
Filing date: 1991-06-27
Publication date: 1996-09-17
Anticipated expiration: 2011-06-27
Also published as: EP0494274B1; CA2064726A1; JP2693269B2; JPH04505224A; EP0494274A1; WO1992000548A1; DE69114257D1; US5051617A

Abstract

Harmonic generation laser sources are disclosed. The sources include a waveguide having a multilayered structure to enhance the harmonic generation. Various embodiments are described, some including an active lasing means built in the waveguide.

Description

2~64726 MONOLITHIC SEMICONDUCTOR UARMONIC LASER SOURCES
Field of the Int7ention The present invention relates to harmonic laser S sources. More ~;rl~lly~ it is directed to coherent light sources in which coherent contra-L..u~d~ g lights in ~ ~l waveguide generates a coherent light at the sum frequency.
l of U T ~-In Optics Letters, Vol. 4, No. 2, February 1979, pp. 58-59, R.
" (the present inventor) et al, reported the nonlinear mixing of oppositely ~u~Li~lg guided waves. The resultant field was coupled to radiation modes and in a direction ~.l ' to the waveguide surface, in the case of equal frequency r ~ ' ~~ In subsequent articles, its application to picosecond 15 signal processing, the creation of all optical transient digitizers and ~LIu~ Lt;l~
d- -- ' the potential usefulness of this work. (See Appl. Phys. Lett. 36 (4), 15 Feb. 1980, pp. 253-255 by R. Normandin et al; 40 (9), 1982, pp. 759-761 by R. Normandin et al, and "Integrated Optical Circuits and C~ edited by L.D. T~ ' Dekker Inc., New York, U.S.A., Chapter 9, by G.I. Stegeman 20 et al.) The overlap of the two oppositely IJlupa~,ali..g fields will give rise to a nonlinear r~ ;,.,. source at the sum frequency. In bulk media such a process is - " v~ due to the ' , ~ . ..; of energy and w.~.v~Liùn in all directions. This is not the case in a waveguide geometry.
u..ru.i ~, since the waves do not grow with distance (no phase 25 matching) the resultant fields are much weaker than that obtained, in a traditional second harmonic generation device. Therefore, this nonlinear interaction has remained largely a laboratory curiosity. However, the present inventor has succeeded to increase this interaction by factors of 107 and obtain efficient conversion in the visible region. With presently available diode laser sources most 30 of the visible spectrum can be reached. Ultrafast b~- ' samplers and monolithic high resoluaon ,~L.~ are possible in the context of fiber optical ~L
I ~

2 ~06472~
systems a~d u~lucl~ integrated circuitry. This invention is thus relevant to optical data storage, display technology, optical radDr and ranging and ~ Lh division ~ u~JLuCI~Llu~ systems, to name a few.
S

Objeds of ' Inve~i~
It is therefore an object of the present invention to provide a coherent light source.
It is a further object of the preDent invention to provide a ' coherent light source which includes active lasing means.
It is yet another object of the present invention to provide a ' coherent light source which comprises a multilayer structure.
Summarv of the Inveotion lS Briefly stated a ' coherent light source according to the present invention includes an optical waveguide for lJIU~I ~,aLillg a light in one dimension under a guiding condition. The waveguide has a plurality of layers of at least one optically non-linear ' material, the plurality of layers being parallel to the dimension. The coherent light source further includes 20 coherent light injection me^Dns for injecting into the waveguide along the dimension contra-~ coherent lights of ' ' ' ~ I h I.~ L~ID to produce a secondary light at the sum frequency of the contra-~lupa6~illg lights in a direction relative to the dimension and determined by the mutual ! 1~ of the contra-g, lights. The layers have the thickness and periodicity d ' by 25 the sum frequency and the refractive indices of the plurality of layers such that the secondary light produced therein is enhanced as it leaves the waveguide in the direction.
Bri~Df r~crriDtion of - - Drawin~s For a more complete, ' ' ~ of the present invention and for 30 further objects and advantages thereof, references may be made to the following description taken in ~- ; - with the ~ . .r;.~g drawings in which:

3 2064726 Figure I is a schematic cross-sectional view of a waveguide Figure 2 is a graph showing the harmonic cross-section An~
for GaAs, according to theoretical ~
Figure 3 is a graph showing An, under more realistic conditions.
Figure 4 is a schematic cross-sectional view of a coherent light source according to one; ' ' of the invention.
Figure 5 shows, in graphs, . u.. in the harmonic cross-section AD~ according to one ' Figure 6 is a graph showing the electric field 1' ' - in the 10 waveguide.
Figure 7 is a dispersion curve of the ~
Figure 8 shows, in graphs, i,"~ ..t~ in the harmonic cross-section A~, according to a further ~ L
Figure 9a is a typical dispersion curve for a multilayer waveguide.
lS Figure 9b and 9c are cross-section curves and graphs indicating illl~U.. in the harmonic cross-section AD, according to a further ..
Figures lOa, lOb and I la, I Ib are actual results obtained in ~.A~
Figures 12 and 13 are schematic perspective views of the ' laser sources according to still further c ...1,~ ' of the invention.
Figure 14 is a schematic view of a further . ..1~' which employs a si~ geometry.

~r - ~ 20S4726 Figu}es I5 and 16 illustrate yet other embodiments of the invention in which holograms are used for phase control.
Figure 17 is a ~ e~iv~ view of an embodiment in which a hologram is used for functions of discrete optical components.
s D~t~ih~-l V~ l;.... of thP Preferred Emhodinn~nt~
As described above, the aforementioned articles by the present inventor report that contra-propagating lights in a guided waveguide produce a second harmonic light in a direction perpendi-o cular to the waveguide axis. The articles deal in detail with theoreticalexplanations. The generation of harmonic light can be expanded to cases where oppositely propagating lights of different wavelengths are coupled in a waveguide.
Figure 1 is a cross-sectional view of a generalized waveguide which is formed by three layers having refractive indices n1, n2 and n3 as shown. The layers are made of optically non-linear materials and satisfy the guiding onditif n~ for the propagating lights.
In the figure, El and E2 indicate the oppositely propagating lights and the resultant light E3 is radiated in the directions ~l and 03 with respect to 20 the axis of the waveguide. The field (li~, ;l,l~li.-l.~ of El and E2 are also shown.
The wavelength and the directions (pl and ~1)3 of emission of the resultant light is determined by the laws of conservation of energy and of momentum. Therefore, if kl, k2 and k3 are, respectively, wave 25 vectors of the contra-propagating lights and the resultant light, and ~l.
A2 and ~3 are, respectively, wavelengths of the lights under discussion, the following equations must be satisfied:
, ~ , kl +k2=k3 1+1=1 - s ~0~2~
As reported in the articles, if ~ 2, ~l and ~3 are 90. It should be noted here that in a certain case where waveguide forms a laser cavity between two mirrors, E1 and E2 are the same laser light travelling in opposite directions and having the same wavelength.
s While it can be generalized by extrapolation as described above, the following discussion deals mainly with the case where the oppositely propagating lights have the same wavelength, e.g. ~ 2. It is now convenient to describe the second harmonic generation in the norm:lli7P-'i form as 121 = AnlII I2 where l2~, Il and I2 are the intensities of the second harmonic light and contra-propagating lights ~ e~liv~ly, and Anl is the harmonic cross-section. Anl for uniform GaAs optical waveguides was calculated and published in Appl. Phys. Letters 38(10),1981, pp 759-760, P.J. Vella et al.
Other optically nonlinear materials, e.g. AlGaAs, AlAs, InGaAs and InP, can be considered for this purpose under different guiding conditions.
As an example, and nPglP~tin~ materials losses, the cross section Anl for a thin, symmetric GaAs layer is shown in Figure 2. This calculation was for 1.06 llm as the fundamental and was performed several years ago. Such a symmetric layer has no guiding cutoff and a strong interference by interfaces between layers is evident. For a fixed input intensity the guided filn~i imPnt:~l field decreases with increasing guide thickness (horizontal axis) resulting in a sharp reduction in radiated harmonic. This is clearly seen as the envelope of Figure 2 and 2s at about 1.0 ,um, the cross-section is ~10-l3W-I which is very small.
A more accurate model is presented in Figure 3. Using the geometry depicted in Figure 1, for air as the top medium, with fully complex indices of refraction (i.e. with full material losses taken in consideration) the harmonic cross-section is presented. Cutoff for guiding at the flln; imPnt~l (1.06 llm again) is around 0.2 llm where data ~ 6 2064726 are absent. The r effects are still present despite being attenuated by absorption losses. U r ~ the cross-section decreases sharply for thicker, more usable films. With such a high index of refraction it is difficult to couple light efficiendy to very thin films. In a fiber optic context it is nearly impossible S to get usable a nounts of light coupled in.
In order to get usable devices the harmonic cross-section must be improved and its decrease with thickness controlled.
It is possible to eliminate the r effects by simply creating a multilayer of nonlinear/lineartnonlinear/linear etc. materials. With the proper 10 spacing all the ~ would arrive in phase at the top interface. This is shown ' 'ly in Figure 4. In the figure, a multilayer waveguide 10 is formed on a buffer 12 of AlGaAs, which is in turn provided on a GaAs substrate 14. A pair of fiber optics 16 and 18 are also . h ~ lly shown. An arrow 20 depicts the radiated harmonic light. In fact, the harmonic light is also radiated in5 the opposite direction but is absorh~ed in tbe buffer and substrate. It can be ' and redirected by a mirror structure built at the interface.
The layer periodicity is taken as the radiated ~ in the guide, of course. The periodicity ~ is determined by Q = )~3/n2 where ~3 is the of the secondary light and n2 is the refrac~ive index of the layer at A3.
20 In the present instance, the periodicity between the crystalline (to be nonlinear) and isotropic media is of the order of 1200 A
and therefore the thickness of each layer is in the ~' ' ' ~ ~ ~ of 600 AA. The overall cross-section is shown in Figure 5 as the higher curve. Thisis compared with the normal, l~ , - GaAs guide at the bottom. Noting 25 this is a l -~ - scale enormous increases are obtained. Note also the reduced falloff as layer thickness is increased.
There is an r ; fact: such an alternating stack of linear vs.
nonlinear layers would be difficult to build in reality. The periodicity is of the order of 600A between the crystalline (to h~e nonlinear) and isotropic media. Since 30 epitaxial techniques are used to grow such ' films the crystalline space groups are identical between layers. Therefore, each layer would have a similar second harmonic coefficient. It is possible to devise solutions to this problem, 7r 7 2a64726 near the bandgap in particular, but the overall performance does not justify the added complications.
The object is to have all nonlinear field contributions add in phase in the far field. Instead of modulating the nonlinear coefficient s a second approach and one of the preferred embodiments of the invention is to simply modulate the phase of the radiating nonlinear field by creating regions of differing index of refraction. Thus, assuming similar nonlinear coefficients for GaAs and AlGaAs, a cross-section r~lr~ tir~n was performed. The result of alternating GaAs and AlGaAs layers in a waveguide is shown in Figure 6, for a guided wavelength of 1.06 llm. The buffer layer is also made of AlGaAs. In all cases the Al content was set at 40%. On the horizontal axis, air, multilayer wave-guide (ML), AlGaAs buffer and GaAs substrate are indicated. The guided TEo electric field (E field) for a 0.6 ,um film is shown on the vertical axis.
The dispersion curve is shown in Figure 7 as a function of the guide thickness up to 2 llm.
The resultant Pnh:~nt~PmPnt is shown in Figure 8. Note the uniform cross-section vs. thickness. In some regions an Pnh ~ncPmPnt of up to one million is obtained. Clearly the amplification or resonance 20 effect of the multilayer overtakes both the absorption losses of the GaAs at 0.53 llm and the reduction in field intensity as the waveguide thickness is inaeased.
A further increase by a factor of ten is possible by increasing the aluminum content of the alloy to 80%. The complex index of 2s refraction imaginary part is then reduced to 0.002 at 0.53 llm greatly reducing optical absorption losses. Furthermore, the increase in index change between the GaAs and AlGaAs allows for more efficient phase control. The result is shown in Figures 9a and 9b. The dispersion curve of the multilayer waveguide at 1.06 llm is shown in Figure 9a. In Figure 30 9b, the upper curve is the effective cross-section for the layered film. En-h~n~ PmPnts by factors over one million are quite evident for thick 8 206~72~
micron sized layers. As a further example, in Figure 9c, the case of AlogGaolAs with Al07GaO3As is computed and compared to a single GaAs film. ~nhAnrl~mPnt of over ten million fold in cross section can be realized.
s Subsequently, experiments were conducted using the geometry shown in Fig. 4. A normal 0.6 ,um thick GaAs waveguide on a 1.6 llm thick AlGaAs buffer was compared to a multilayer stack of AlGaAs/GaAs layer with a resultant thickness of 0.6 !lm, again on a buffer layer of AlGaAs, 1.6 llm thick. Thus both waveguide geometries o are identical, apart from the multilayer.
The first sample was grown with a 40% aluminum content.
Figures lOa and IOb show respectively the harmonic outputs of the multilayer and the single layer configurations. As can be seen in the figures, an 800-fold ~nh~nf~mf~nt in 0.53 llm light emission was obtained. ~'nmr~rin~ with theory, shown in Fig. 8, for 0.6 ~Lm thickness an ~nh~nr~m~nt of d~ u~ll,al~ly 800 was predicted. The mode beating in the harmonic signal was clearly visible when imaging the harmonic, green surface emission. Because of the ori~nt~tinn of the (100) substrate, a mixture of TE and TM must propagate in order to have any harmonic signal generated. Substrates with a (111) nri~nt:ltinn have been ordered to solve this problem. The mode beating gave rise to a surface array of sources spaced a few hundreds of micrometers apart. As this coherent array radiated, several sharp sidelobes could readily be seen by eye.
The next sample was grown with the geometry except for an aluminum content of 80% in the layers. Signal strengths of the multilayer guide and the simple GaAs layer guide are shown in Figures lla and llb respectively. An increase of approximately 3000 is apparent.
The enhancement is most likely greater since the photomultiplier detector was in saturation with the multilayer and near its signal-to-noise limit for the normal layer. The theoretical model predicted an ~nh~nrPm~n~ of over 4000 in this case. This is shown in Figure 9b for a 9 20S~72~
0.6 ,um waveguide. In another experiment, a I llm thick Alo.gGao.lAs/Alo.7Gao.3As multilayer structure was also built. An absolute cross section measurement is shown in Figure 9c. Agreement with theory is excellent as shown by the actual measurement indicated s in the figure.
In all cases the average power coupled in the waveguide was less than 70 IlW with a repetition rate of 7.7 kHz and pulse widths of 200 ns. The 1.06 llm light source was an unpolarized CVI Nd-YAG laser.
In fact, for the 80% Al sample the green radiation field was still visible with the laser running CW at the same average power level. In a true laser cavity the circulating power is, of course, several orders of m~nitll~lr- more intense. In a channel laser geometry there is further confinr-mrnt, in the plane of the guide, thus increasing the power density by another order of m:~nitllAr The electromagnetic field propagating in a laser cavity concictin~ of a waveguide for light ronfin~nmont (typical of geometries used in srmirnn~lllrtr,r diode lasers) can be thought of as two contra-propagating fields of the identical wavelength. This leads to further embodiments of the present invention.
Referring to Figure 12, a further embodiment is srhPm~tic~lly illustrated, using electrically pumped geometries. A
harmonic laser source of this embodiment includes an active region 40 and radiating region 42 whose composition is the same everywhere except for ~rlrlitir,n~l layers 44 needed to form p-n junctions in the active 2s region for lasing. The laser mirrors 46 and 48 are the front and back of the chip. By providing 100~ mirrors for the filnrl~m~nt~l wavelength the overall lasing threshold can be substantially lower than that of a conventional laser. The radiating region is the thinned down region of exposed multilayer 50. The harmonic light 52 is a planar beam emitted perpendicularly to the multilayer structure of the radiating region.
There is no incompatibility between the lasing layers and the harmonic .

generation regions. Thus it is possible to have a quantum well recombination layer (e.g. as shown by the thin region 54 in the figure) for lasing efficiency without disturbing the harmonic generation.
Similarly it is also possible to have an etch stop layer in the laser s structure without affecting its p~lru~ dllce. This etch stop layer is then used to provide a controlled etch down to the multilayer radiative region. Transparent electrodes can be used also.
By using conservative estimates of the ridge width, length and the intra-cavity lasing power density of an InGaAs for example a radiated field of better than 0.1 mW CW is obtained for green operation.
The full visible spectrum can be reached by using InP (red), InGaAs (yellow, green) or GaAs (blue).
Figure 13 shows yet another pumped geometry. It has the advantage of being side pumped. Therefore the full length is available for harmonic radiation, m~imi7in~ device efficiency. Zinc and silicon are implanted or diffused to create the needed p and n contact region and provide a change in refractive index for the quantum well region, thus providing optical lateral ronfinPml~nt The laser mirrors are on the front and back surfaces of the chip and form a laser cavity 60 between 20 them. The harmonic light 62 is emitted perpendicularly with the full length of the laser cavity.
Figure 14 is a schematic view of a further embodiment which solves the difficulty of growing a waveguide with a stack of alternating linear and nonlinear materials or with multilayers of 25 different refractive indices. The geometry shown in the figure permits the construction of optimized structures with a single material, thus ~limin:ltin~ materials constraints. The f~lnrl~mf~nt~l, waveguided light 70 and 72, either from outside inputs or from the monolithically integrated laser action, propagates in a corrugated single mode channel 30 waveguide 74. For guided harmonic light the effective refractive index of the structure will be given by the dispersion relation for its proper materials constants and guide thickness. Therefore the addition of small ribs 76 in the l~ngi~tlfiin:~l direction will locally modify the effective propagation refractive index for the harmonic light 77, without perturbation of the guided fllnrl~mPn~lc in the plane perpendicular direction. The substrate 78 carries a buffer layer 80. The increased height s of a sub-guide 8~ under the ribs will change the effective guided index for the harmonic light. This is the same effect achieved in the stack geometry by the variation in film composition. Without composition restrictions, in this geometry, extremely large changes in effective index are possible simply by controlling the height of the corrugated structure.
While in this embodiment, small ribs are formed by removing material away, it is also possible to create the same effect of local refractive index modification by doping desired areas e.g. by focused ion beam ~Pf hni~lllP
Its periodicity is subject to the same conditions for the harmonic wavelength as in our previous calculation. This therefore implies a resonance of ~ uxill.dtely 65 nm.
Figures 15 and 16 illustrate further embodiments of the invention which make use of holography. By using a spatially controlled phasing a plane wave can be transformed in any desired far field pattern. It is therefore possible to synthesize the effect of various 20 optical components such as lenses, prisms or gratings by holographic techruques. As can be reasoned from the foregoing description, the added layers control the phase of the harmonic light by the alternating variation of the layers' index of refraction. Similarly, the addition of an overcoat can be used to modify the emitted plane wave. This can be 25 accomplished in two ways, first, by a simple film on top; second, by a gradual change through the layers. Therefore, Figure 15 shows a hologram overlayer 90 on the buffer and substrate to control the phase of the radiated harmonic light 9~. Figure 16, on the other hand, indicates implanted hologram 94. Both effects can easily be obtained by 30 focused ion beam (FIB) processing. After proper computing of the holographic pattern for the desired optical effect a FIB written photoresist is used to pattern and etch the surface. In the second case FIB

12 206~726 implantation will cause damage to the multilayer at the desired locations. Selective etching can be used to remove these areas.
Similarly the damaged regions can be left as is, the induced change index due to the implantation and the structural damage being sufficient to s provide the needed lateral phase control for the holographic reconstruction.
Thus this FIB technique can be used to write on the surface of the harmonic multilayer waveguide a synthesized holographic pattern. This holographic pattern provides the needed diffraction effect to replace discrete optical components that would otherwise be needed to process the harmonic field. An example of a lens/grating combination is shown ~n Figure 17.

Claims

We claim:

1. A semiconductor coherent light source, comprising:
an optical waveguide means for propagating a light in one dimension under a guiding condition, coherent light injection means for injecting into the said waveguide means along said dimension contra-propagating coherent lights of fundamental wavelengths, the said waveguide means having a multilayer means for producing and enhancing a secondary light at the sum frequency of said contra-propagating lights in a direction different from the said dimension and determined by the mutual relationship of said contra-propagating lights, said multilayer means comprising a plurality of layers of at least one optically non-linear semiconductor material said plurality of layers being parallel to the said dimension and having a predetermined thickness and periodicity determined by the refractive indices of said layers and by the sum frequency such that the said secondary light produced in the said multilayer means is enhanced as it leaves said waveguide means in said direction.

2. The semiconductor coherent light source according to claim 1 wherein:
all the said layers have a substantially same thickness and alternate layers have a first and a second predetermined refractive index.

3. The semiconductor coherent light source according to claim 1 wherein:
the said contra-propagating lights are of a same fundamental wavelength, and the said secondary light is the second harmonic and is produced in a direction perpendicular to the said layers.

4. The semiconductor coherent light source according to claim 2 wherein:
the said contra-propagating lights are of a same fundamental wavelength and the said secondary light is the second harmonic and is produced in a direction perpendicular to the said layers.

5. The semiconductor coherent light source according to claim 3 wherein:
the said waveguide further comprises a lasing cavity and a laser active means for producing and injecting into the said waveguide along the said dimension contra-propagating lights of the same fundamental wavelength.

6. The semiconductor coherent light source according to claim 4 wherein:
the said waveguide further comprises a lasing cavity and a laser active means for producing and injecting into the said waveguide along the said dimension contra-propagating lights of the same fundamental wavelength.

7. The semiconductor coherent light source according to claim 3 wherein:
the said coherent light injection means is a fiber optic connected to one end of the said waveguide; and a mirror is provided on the other end of the said waveguide.

8. The semiconductor coherent light source according to claim 4 wherein:
the said coherent light injection means is a fiber optic connected to one end of the said waveguide, and a mirror is provided on the other end of the said waveguide.

9. The semiconductor coherent light source according to claim 3 wherein:
the said coherent light injection means is fiber optics connected to both ends of the said waveguide.

10. The semiconductor coherent light source according to claim 4 wherein:
the said coherent light injection means is fiber optics connected to both ends of the said waveguide.

11. The semiconductor coherent light source according to claim 1 wherein:
a holographic layer is provided on the outermost layer of the said waveguide.

12. The semiconductor coherent light source according to claim 2 further comprising:
a holographic layer provided on the outermost layer of the waveguide.

13. The semiconductor coherent light source according to claim 1 wherein:
each of the said layers has a predetermined thickness and predetermined spatial areas having different refractive indices to form an implanted holographic optics.

14. The semiconductor coherent light source according to claim 2 wherein:
each of the said layers has a predetermined thickness and predetermined spatial areas having different refractive indices to form an implanted holographic optics.

15. The semiconductor coherent light source according to claim 2 wherein:
the said coherent light injection means is fiber optics connected to both ends of the said waveguide.

16. The semiconductor coherent light source according to claim 2 wherein:
the said coherent light injection means is fiber optics connected to both ends of the said waveguide.

17. The semiconductor coherent light source according to claim 2 wherein:
each of the said layers is arranged one beside the other on a substrate and has a predetermined width and refractive index.

18. The semiconductor coherent light source according to claim 2 wherein:
each of the said layers is arranged one beside the other on a substrate and has a predetermined width and refractive index.

19. The semiconductor coherent light source according to claim 4 wherein:
each of the said layers is arranged one beside the other on a substrate and has a predetermined width and refractive index.

20. The semiconductor coherent light source according to claim 6 wherein:
each of the said layers is arranged one beside the other on a substrate and has a predetermined width and refractive index.