Actively phased matched frequency doubling optical waveguide

An actively phase matched waveguide structure suitable for use as a frequency doubling device is described. The phase matching of the waveguide is controlled by application of a control voltage. The waveguide includes first and second non-linear optical layers of material having differing indexes of refraction forming a waveguide. Disposed on the surface of the waveguide is a layer of transparent semiconductor material disposed between two transparent electrodes. The index of refraction of the transparent semiconductor material varies with the potential applied to the electrode and thereby varies the phase matching of the waveguide as a whole.

BACKGROUND OF THE INVENTION 
This invention relates to nonlinear optical devices and more particularly 
to devices for doubling the frequency of electromagnetic radiation passing 
therethrough. 
Optical digital data storage devices, such as compact discs, have recently 
come into common usage. Typically, such discs are read and written to by 
means of a light emitted by a semiconductor laser (i.e. a laser diode). 
However, the light generated by semiconductor laser diodes generally falls 
within the lower end of the electromagnetic frequency spectrum (i.e. red 
or infrared). The use of higher frequency light, i.e. at the blue end of 
the spectrum, to read and write to optical storage medium would result in 
greatly increased storage density. Unfortunately, however, there are yet 
no practical blue semiconductor lasers. To date, the only blue lasers are 
large gas lasers which are obviously unsuitable for use in compact and 
inexpensive optical storage read/write devices. 
Accordingly, a device capable of converting the light emitted by readily 
available semiconductor laser diodes to blue light is greatly desired. 
Laser diodes that emit infrared light are inexpensive and widely 
available. The frequency of blue light is twice that of infrared 
radiation. Accordingly, a device capable of doubling the frequency of 
infrared radiation has considerable commercial potential. The present 
invention is directed to providing an inexpensive frequency doubling 
device that may be used in conjunction with an infrared semiconductor 
laser to provide blue light suitable for use in reading and writing 
optical storage media. 
The field of the non-linear optics has provided a number of devices used as 
frequency doublers, generally through the means of second harmonic 
generation (SHG) of a fundamental frequency. Such devices include bulk 
materials and stacks of non-linear crystals. A particularly effective 
doubling device is a non-linear optical waveguide. As a light beam passes 
through the waveguide the non-linear optical effect causes the generation 
of a lightwave of the second harmonic of the input lightwave. Such optical 
waveguides can be quite efficient in providing frequency doubling. 
However, efficient frequency doubling requires accurate phase matching 
between the fundamental and harmonic waves. If the frequency doubling 
device is not properly phase matched interference effects will cause 
attenuation of the second harmonic. In a waveguide the tolerance 
requirements for accurate phase matching between the geometrical and 
physical properties of the waveguide are very difficult to achieve. A 
number of different structures have been proposed to provide phase 
matching. Phase matching has been attempted by both passive and active 
means. Passive phase matching has been accomplished by, for example, the 
addition of a periodic structure to a frequency doubling device. However, 
such devices are incapable of responding to changeable conditions and may 
lose accuracy over time. In contrast, the parameters of an active 
structure can be controlled in response to the measured output of the 
doubled light. Accordingly, it is desirable to be able to control the 
phase matching properties in an active manner. However, active phase 
matching devices have either been impractical or incapable of controlling 
the phase matching to a sufficient degree. 
Proposals for active phase matching of the waveguide have been made in, for 
example, U.S. Pat. No. 4,427,260 (Puech et al) issued Jan. 24, 1984 and in 
the article "Electric Field Tuning of Second-Harmonic Generation in A 
Three-Dimensional LiNbO.sub.3 Optical Waveguide", Applied Physics Letters, 
34(1), Jan. 1, 1979. These proposals achieve phase matching by means of 
electro-optic tuning of the material forming the waveguide. However, these 
approaches are constrained by the fact that the index of refraction of the 
waveguide will undergo only relatively small changes by means of the 
electro-optic effect. Accordingly, such means are able to compensate for 
only relatively small changes in the geometrical or physical properties of 
the waveguide. 
The present application is directed to overcoming the difficulties of the 
prior art. Specifically, the structure and methodology of the present 
invention is capable of achieving phase matching over a relatively wider 
range of geometric and physical changes to the waveguide. 
SUMMARY OF THE INVENTION 
The present invention is directed to providing an inexpensive device and 
method for doubling the frequency of electromagnetic radiation. 
Specifically, the structure utilizes active control of phase matching by 
an applied electric field. The applied electric field is applied across a 
semiconductor material which is disposed along a surface of a frequency 
doubling non-linear optical waveguide. The index of refraction of the 
semiconductor material is altered by the "Franz-Keldysh effect" in the 
presence of an applied electric field. The Franz-Keldysh effect is capable 
of modifying the index of refraction of the semiconductor layer to a 
relatively large degree. Accordingly, the phase matching of the waveguide 
structure as a whole is controllable to a greater degree than that of the 
prior art utilizing the electro-optic effect to control the index of 
refraction of the waveguide itself. 
Specifically, the present invention is directed to a tunable waveguide 
structure which has a first waveguide constructed from first and second 
layers of non-linear optical material. The first and second layers have 
differing index of refraction and thus form a waveguide. Located on one 
surface of the waveguide structure is a semiconductor structure comprising 
first and second transparent electrodes disposed on either side of a 
transparent layer of semiconductor material. A voltage applied to the 
semiconductor layer by means of the electrode controls the index of 
refraction of the semiconductor layer and thus controls the phase matching 
of the waveguide as a whole. 
As can be appreciated, a frequency doubling device constructed in 
accordance with the present invention is advantageous in that it is 
relatively easy to manufacture because the waveguide and semiconductor 
structure is not complex. Furthermore, since the phase matching is active 
the waveguide will frequency double over a wide range of environmental 
conditions. Accordingly, the device will operate efficiently in various 
environments and for relatively long periods of time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 illustrate an embodiment of the inventive actively phased 
matched waveguide 10. The main body of the waveguide comprises a slab of 
non-linear optical material 12 such as by way of example only, potassium 
titanate phosphate (KTiOPO.sub.4) known as "KTP". In a second layer 14, 
Thallium (T1) is incorporated into the KTP by diffusion or other 
techniques. The incorporation of thallium into KTP forms a frequency 
doubling waveguide since the thallium causes an increase in the index of 
refraction of the KTP (see the refractive index profile at FIG. 2), thus 
forming a non-linear optical waveguide capable of second harmonic 
generation. 
Disposed on top of layer 14 is buffer layer 16 of insulating material such 
as silicon dioxide (SiO.sub.2) which serves to separate the waveguide of 
layers 12 and 14 from the electrode and semiconductor layers disposed 
above. Disposed on top of buffer layer 16 is a first electrode layer 18 of 
a transparent conductor. Disposed on a layer 18 is a layer 20 of 
transparent semiconductor material whose index of refraction varies in 
accordance with an applied electrical field. Disposed above layer 20 is a 
second transparent conductive electrode 22, which with electrode 18 is 
used to apply an electrical field to semiconductor layer 20 to vary its 
index of refraction. Suitable material for the transparent electrodes 18, 
22 is indium tin oxide (InSnO). Suitable semiconductor materials which 
have an index of refraction which varies in accordance with an applied 
electric field are II-VI materials such as Zinc Selenide (ZnSe). Glasses 
doped with II-VI particles are a possibility as well. 
The structure of device 10 is such that a change in the index of refraction 
of semiconductor layer 20 effects the propagation of light through the 
waveguide as a whole. Thus, changes to the index of refraction of layer 20 
by means of the field applied to layers 18, 22 controls the phase matching 
of the waveguide. Since the voltage applied between electrodes 18, 22 
controls the index of refraction of layer 20 which in turn controls the 
phase matching of the entire device 10, control over phase matching is 
accomplished by varying the potential applied between electrodes 18 and 
22. Accordingly, electrodes 18, 22 are connected to a variable voltage 
supply 30 the potential of which controls phase matching. Power supply 30 
has a control input which in turn is connected to control circuitry 32 
which has an input connected to a device 34 for detecting the intensity of 
light of the second harmonic of the input light. Depending on the 
intensity of the output light device 34 causes control circuit 32 to 
correct the output voltage of voltage supply 30 thus ensuring phase 
matching. The actual circuitry used in voltage supply 30 control 32 and 
light intensity meter 34 are well known to those skilled in the art and 
need not be described further. 
The materials used in the above-described structure are not to be construed 
as limiting but merely exemplary. The above-described materials namely, a 
KTP and thallium doped KTP waveguide utilizing a zinc selenide 
semiconductor layer for phase matching control is suitable for use in the 
frequency doubling of infrared to blue light, which as described above, is 
important in the field of optical storage devices. However, other 
materials may be used for doubling in other frequency ranges. For example, 
the substrate could be comprised of lithium niobiate (LiNbo.sub.3) and the 
semiconductor layer could be constructed from III-V materials. 
Furthermore, electrodes 18 and 22 and semiconductor 20 could be replaced 
by a P-I-N multi-quantum well structure which would provide enhanced 
changes in the index of refraction. Finally, the thallium doped KTP layer 
could be replaced by a second ZnSe layer (inactive in frequency doubling) 
which eliminates the need for KTP diffusion. However, overall efficiency 
is likely to be reduced. 
Although the present invention has been described in conjunction with 
preferred embodiments, it is to be understood that modifications and 
variations may be resorted to without departing from the spirit and scope 
of the invention, as those skilled in the art will readily understand. 
Such modifications and variations are considered to be within the purview 
and scope of the invention and the appended claims.