SE468453B - DEVICE IN THE FORM OF COHERENT LIGHT CELL BASED ON FREQUENCY CONVERSION OF LIGHT FROM LASAR LASER - Google Patents

Description

and s.k. channel waveguide, can a focused laser beam kept limited to a small cross-sectional area (and thus high intensity) over large lengths without differentiating.

This means that in a waveguide in a good optical Non-linear material can also have good efficiency low power lasers. It for a practical application the limiting factor is mainly that the so-called phase matching the condition (“phase-matching condition”), the choir) must be met.

Lithium niobate (LiNb03) is a material that has both relatively high non-linearity and in which one can operate high-quality long waveguides (several cm long). 10 The so-called. the phase matching condition, which must be met, in order for efficient wavelength conversion to be l5, can be used for frequency mixing (sum or difference frequency generation) is written: k »= k; i k; (10) 2 N '2 N' 2 N or equivalent: n 264 ”=“ Measure ”Mä-z (lb) I 1 1 with. _ i - ä: i i; (2) Where Ä1 and Ia are the two pump wavelengths (X, SÄ,), while Oh, is the generated wavelength, to which the plus signs correspond against sum frequency generation and the minus signs against difference frequency generation. AQIU D fl ßz and A fl m, are the so-called effective refractive indices in the waveguide at respective 25 wavelength. In the special case of frequency doubling, the the sign and Ä, = 2, and hQm, = PL fi ,. Phase matching means that the generated light shall propagate with the same phase as the driving non-linear polarization.

The phase velocities are determined by the effective refractive indices, 30 and for the phase matching condition to be met must the refractive indices at the respective wavelengths meet the condition according to eq (lb). In the case of frequency doubling 10 15 20 25 30 35 468 453 this corresponds to the effective refractive index being the same for both the fundamental and the frequent veins doubled the wavelength. The same equations are valid 'by parametric oscillation, provided plus sign and that k, corresponds to the pump wavelength, where Ä., and Ä., they correspond generated the wavelengths. In case of degenerate parametric oscillation only a new wavelength is generated, ie Ä1 = Ä ,.

To achieve phase matching in waveguides utilizes in the first place the birefringence of the material, whereby they interacting waves propagate with different polarization.

When sum frequency generation in lithium niobate (LiNbO,) has then the light at the two pump wavelengths a polarization against the ordinary refractive index while generating the light gets extraordinary polarization. Of dispersion- the curves of the ordinary and the extraordinary refraction the sindex in LiNbO3, schematically represented in Fig. 1, is shown that it is only possible with frequency doubling and sum frequency generation to wavelengths about 540 nm. Certain tunability can, however, be obtained through different choices of the two the pump wavelengths at sum frequency generation and through variation of the working temperature. In the first case the nonlinearity of the dispersion curve is used while others are based on different temperature dependencies of the ordinary ones resp extraordinary refractive index. By doping sub- the strat material with eg MgO or proton exchange and design of the waveguides (which affects the waveguide dispersion) can one also achieve a certain wavelength shift. It is known that waveguides in LiNbO3 are suitable for both frequency doubling, sum frequency generation, difference frequency generation, parametric oscillation as parametric strengthening. However, it has almost exclusively gas lasers and solid state lasers have been used for such experiments.

It has e.g. shown that with light from a Nd: YAG- laser (with wavelength 1,064 pm) can have high efficiency for frequency doubling for relatively low pump effects in -fx C).

CG 10 15 20 25 30 -F> (fl LN lithium niobate waveguides in both doped and undoped substrate substrates erial. However, due to phase matching problems, frequency doubling has in this way in lithium niobate waveguides not yet applied to semiconductor lasers. É Today, reliable semiconductor lasers are available mainly in three wavelength ranges: about 0.8 pm (based on GaAs), about 1.3 μm (based on InP) and about 1.55 pm (based on InP). none of these wavelengths can be phase matched for frequency doubling in conventional tional lithium niobate waveguides.

However, it is known that you can double the frequency of light from diode lasers with the wavelength Ä = Oß pm in s.k. proton- replaced LiNbO, waveguide, with the frequency doubled light generated in the form of so-called Cerenkov radiation. Then generated the light as a mode of radiation down into the substrate. On that this way, they have succeeded in building a small compact light source with the wavelength in the visible range. A disadvantage of this method is that the non-linear process is not equal effective as when the connection is made between two wavy modes and that in practice it is required that the waveguide has one extremely small cross-sectional area. This leads to very high light intensities in the waveguide and tendency to light induction changes which have a male effect on the frequency the stability of the doubling, especially with continuous pumping ning. Furthermore, the light comes out in a cone under the waveguide, so that the light becomes difficult to collimate and focus.

Another known technique for frequency conversion in wave is to use the so-called quasi-phase matching in periodic domain-used pathways in, for example, LiNbO, or LiTaO3 (lithium talat). Quasi-phase matching is a more generally useful one phase matching method by the possibility of achieving the phase matching chning here is not limited depending on the birefringence size in the material. However, it is technically more difficult 10 15 20 25 30 468 453 to realize the required waveguide structure. Quasi- phase-matching waveguides can be used to double e.g. 0.85 pm and 1.3 pmzs wavelength, to blue respectively red. In contrast, quasi-phase-matched frequency doubling not be used to generate green light due to lack on semiconductor lasers at the then required wavelength.

It is further known that current non-linear optical substrate materials such as LiNbO 2, LiTaO 2, KTP (KTiOPO 2), KNbO 2, (potassium niobate) etc. also have high electro-optical materials. coefficients, why the electro-optical effect with advantage can be used for control and modulation of the light. There are known methods to with different types of integrated optical components i.a. partly fixed or controllably switching light from one channel waveguide to one on the other hand by modulating the intensity, phase or 'of the light phase velocity in a channel waveguide. For regulation and modu- .lation, metal electrodes are generally used as arrangements. on the crystal surface on top of or next to the channel waveguide, and to which electrical drive voltages are applied. The current type of regulation and modulation, which utilizes the electro-optical effect, can be done with low electric driving voltage and high modulation frequency, and requires none mechanically moving parts. It is also known that many non- integrated optical components, controllable and adjustable ones as well as passive, are wavelength selective. ' In order to realize a small semiconductor laser pumped frequency vein doubles based on conventional waveguides in LiNbO3 a relatively strong diode laser at ~ 1.08 pmzs is needed wave-length. It's just this wavelength (approximately) for which the frequency doubling can be phase matched in LiNbO¿ waveguides at room temperature and today no such lasers exist.

Corresponding limitation to a narrow wavelength range for the fundamental wavelengths which in practice can vensdubblas also applies to waveguides in other nonlinear materials such as KNbO3, KTP, LiTaO3, BBQ (B-BaB¿L), LBO 10 15 20 25 30 (LiB¿g), etc. This illustrates the main problem at frequency conversion which is that in such a general way as possible solve the phase matching problem in order to utilize available laser frequencies and nonlinear material.

A side condition is that diode lasers exist today, as are easily accessible, single mode, have a long service life and relatively high output power, mainly in three limited wavelength ranges: 780-850 nm (based on GaAs) and about 1300 and 1550 nm (in the latter two cases based on InP).

To be able to integrate light sources of different wavelengths the same substrate, would further be of great value to many applications such as displays, optical scanning, optical registration, image generation, etc.

For a number of applications, it is also a strong one desire to be able to easily control the intensity of the light source and / or selecting its wavelength from a number of options or control the color balance of the light source, by varying them the mutual intensity shares for a number of constituents wavelength components. The object of the present invention is to provide compact light sources, mainly in wavelength ranges not covered by available semiconductor lasers, by to achieve efficient frequency conversion in waveguides of semiconductor lasers (or other compact, low-power lasers) ), thereby solving the phase matching problem so that as many wavelengths as possible can be -erated from available laser wavelengths and available non-linear optical materials. the purpose is further to enable the generation of more wavelengths from same device (same substrate) and simple with electric control signals be able to modify the characteristics of the light source: select an outgoing wavelength among a number available, control 10 15 20 25 30 the mutual color balance between a number in the light source constituent wavelength components, rapidly varying light sources lance intensity etc.

The invention solves the present problems thereon manner set out in the characterizing parts of the five attached claims. In this case, it is assumed that a fre- frequency convergence is arranged to take place in an optical non- linear material such as LiNbo3, LiTao3, KTP, KTioAso4, KNbO3, BBO, LBO, NYAB or equivalent.

The invention is characterized in that two lasers (semiconductor lasers or other low power lasers) are used for pumping the non-linear frequency mixture (sum, alternatively differential frequency generation) in a waveguide structure in a substrate of any of said materials, and that the lasers used have such wavelengths as to the matching condition is met either by exploitation of the birefringence of the material in combination with the waveguide the dispersion or by using so-called quasi-phase matching technique.

The invention is further characterized in that the same device can give light at several different wavelengths, partly through that in the frequency mixing there is, in addition to the new generated frequency, including the two original pump frequencies frequencies present, partly because the two pump frequencies also individually, preferably in separate channel waveguides, can be frequency doubled or downconverted frequency by parametric oscillation (or speci- otherwise degenerate parametric oscillation).

The invention is furthermore characterized by the possibility that combine the channel waveguides where the frequency conversion takes place with integrated optical components, based on known $> Q \ CD 10 15 20 25 30 35 01 C-J techniques, which may be wavelength selective and preferred electro-optically controllable, in order to fine-tune the phase feed and switch the light on, off and between 5 the channel waveguides, in such a way that with the tric control signals can vary the characteristics of the light source (alternatively it is possible to design / construct the device for certain predetermined properties) in one or more of mainly the following respects: select an outgoing wavelength from one number available (consisting of both pump wavelengths and the new wavelengths generated in the device), vary light source intensity, separate and control radiation of different available wavelengths to different outgoing channel waveguides and vary the intensity of each wavelength, direct radiation with different wavelengths to same or adjacent outputs, control it with each other the intensity or color balance between a number in light source constituent wavelength components. A special case consists of the integrated optical components for fixed, predetermined functions. _ For phase matching based on the use of birefringent gives material and waveguide dispersion together with the temperature dependence of the refractive indices at the current wavelengths the wavelengths that can be phase matched by formulas (1) and (2).

In the case of quasi-phase matching, the relationship still applies between the wavelengths according to equ (2). Equation X1) needs however, not be fulfilled without the "phase mismatch" that is compensated in a known manner by appropriate selection of the periodicity of the quasi-phase matching waveguide.

The invention involves a combination of different components, which each in most cases in itself corresponds to known technology. However, this invention is also based on a in-depth analysis, both theoretical and experimental, for to define combinations that are of practical interest, and under what conditions they are realistic. 10 15 20 25 30 35 468 453 We have e.g. through calculations found that one can achieve sum frequency generation between GaAs lasers and InGaAsP lasers at temperatures around and just above room temperature in waveguides made by titanium sion in LiNbO, (in undoped or MgO-doped form). With two diode lasers (at common wavelengths: approx 0.85 μm and about 1.3 μm or 1.55 μm, respectively) and a LiNbO waveguide, you can thus build a small compact laser light source with wavelengths in the visible (green) the wavelength range, which is of great practical interest. IN a laboratory experiment we have also demonstrated this principle by mixing radiation from two diode lasers with the wavelengths 0.85 pm resp. 1.31 pm in a channel waveguide produced in lithium niobate by titanium diffusion and thereby generated green light at the wavelength 0.508 μm. Here double birefringence of the material is used, in combination with waveguide dispersion, to meet the phase matching conditions the choir. By utilizing in this way the sum frequency generation, the invention thus makes it possible to partly meet the phase matching condition at a temperature close room temperature, with available materials and semiconductor laser wavelengths in cases where the phase matching condition for frequency doubling of the individual available loads the wavelengths cannot be met, partly it becomes possible to generate green light from semiconductor lasers which is not possible by frequency doubling due to lack on the laser diode at a suitable wavelength.

Another application example is that according to the invention the sum frequency generation can also be combined with frequency doubling of the individual light sources, so that if for example, two infrared laser diodes are used as pump light sources there is thus the possibility of generating on the same substrate three different wavelengths in the visible range. If two half- conductor lasers with wavelengths around 0.85 and 1.3 μm respectively used, can thus three different wavelengths in it Ü \ OO 10 15 20 25 30 $> U1 04 -10- visible area is generated: partly blue and red through frequency doubling of the wavelengths 0.85 and 1.3 μm, respectively green by sum frequency generation. It is obvious many applications for such a compact Blue-Green-Red light source.

The invention is described in more detail below with reference to the attached Figures 2-15.

Figure 2 shows the invention in one of its simplest embodiments (embodiment 1). A channel guide 10 is made in the surface layer of a substrate 1 of an optical nonlinear material. Light from two laser diodes 20 and 21, together via the lenses 30 and 31, respectively, and a wavelength selection active beam splitter 32 and is connected via a lens 33 into the channel waveguide 10 in the substrate 1. What is generated in the waveguide the light of the new wavelength passes out through the end surface of the channel waveguide, possibly together with non converted light of the two pump wavelengths. The two later the wavelengths can be removed if desired with the aid of a optical component 34, in the form of a filter or a polarizing sator, at the end surface of the channel waveguide.

The light from the two pump light sources can be on several are combined in the channel guide where the frequency conversion the ringing takes place. The most obvious way is based on one dichroic beam splitter 32 which transmits one wavelength and reflects the other, as already described according to Fig. 2. Examples of other methods for merging the light is illustrated in Figs. 3-5. All figures 2-5 thus illustrates different variants of embodiment 1 of the invention.

Figures 3 and 4 show variants, which are based on couplers integrated in the waveguide substrate. In Fig. 3 are merged the light from the two channels 13 and 14 purely geometric via a coupler 40 (ï-coupler) into the common main the waveguide 10. 10 15 20 25 30 35 468 453 _11- In Fig. 4, a wavelength selective coupler of known type with the design parameters selected so that the light off both the input channel waveguides 13 and 14 end up in it common straight main waveguide 10, where the non-linear the frequency mixing takes place. The wavelength selective coupler is illustrated in the figure in the form of a directional coupler (whose interaction length is then selected to get the switching of light in the desired way) but also other types of integration Prior art optical couplers can be used (eg Mach-Zehnder interferometers, symmetrical and asymmetric X-switches, TIR switches, BOA switches, three-waveguide couplers, combination of directional couplers and Mach-Zehnder interferometer, Y-switches, s.k. digital switches etc.). To adjust the degree of handover in this coupler can be used by the electro-optical the effect.

In both cases, the two lasers can be mounted immediately outside the crystal and use microlenses to connect in the light through the end surfaces of the channel waveguides. Alternatively can direct connection with the laser diodes near the end surfaces of the channels and without intermediate optics used. The light can too connected via short optical fibers (so-called "fiber pig- tails ").

Alternative to filter or polarizer (34 in Fig. 2) to remove the pump wavelengths is an integrated optical polarizer / polarization absorber or integrated optical wavelength selective component.

A third variant illustrated in Fig. 5 is based on a fiber optic coupler 51 is used to bring together the light from the two lasers 20 and 21 with the light from each of the lasers is connected to each of the two components fibers 52 and 53, and that the common outgoing fiber 54 is mounted ("pigtailas") against the LiNbO3 substrate or the light from this fiber 54 is otherwise coupled into the channel waveguide. The state of polarization must be Jä- Gx r ~ IN 10 15 20 25 30 $> 01 LN _12- well controlled, which may require the use of polarization preservative fiber. In addition, the fiber should be single mode for both pump wavelengths. The fiber ends in which the laser the light is switched on can be provided with so-called "tapers" to optimize the connection efficiency. $ if mentioned above, the integrated coupler 41 according to Figure 4 is fine-tuned electro-optically, to achieve the desired handover. This technology can also be used coat for modulation of light. An embodiment of the invention (embodiment 2) which utilizes electro-optically controlled couplers are illustrated in Figures 6-9. According to prior art, the individual couplers are provided with metal electrodes, and electrical voltages applied to them affects the switching of the two wavelengths. One design of such electrodes is schematically illustrated for the coupler 42 in Figure 6 and the couplers 42 and 43 in Figure 7. The design of the waveguide structure in Figure 7 gives i in relation to the embodiment in Figure 6 better possibilities to independently vary the degree of connection from each light source via the input waveguides 13 and 14, respectively into the main waveguide 10. The switches / switches are illus- treated in the form of so-called directional couplers provided with each two metal electrodes 70 and 71 (72 and 73). In order to - facilitate the regulation of the function, the electrodes can be differently designed, for example divided into several along the channel waveguides. Furthermore, there are as mentioned above a large number of alternative types of switches / switches.

The electro-optical control is of value to reduce the requirements for the manufacturing tolerances of the coupler. You can then obtain the desired transmission by means of a correction voltage despite a certain manufacturing owner. the apple can also be used to modulate the generated the light. For example, the generated light can in this way pulsed in time even though the laser diodes are lit continuously. 10 15. 20 25 30 468 453 -13- In a variant of this second embodiment illustrated in Figs. 8 and 9, the interconnection of the light from the the waveguides (13 and 15 in Figures 8 and 13 and 14 respectively in figure 9) to the main waveguide 10, as well as the disconnection of light from the main waveguide 10 to adjacent output channel waveguides 80 and 81, via electro-optical controlled integrated optical couplers / modulators 42, 43, 44, 45 (which are wavelength selective). The invention is is characterized by the light then coming out of the main channel wave output 85 can be selected, using an electric control signal, to consist of one or more of the three available wavelengths (wavelength of mixing frequency and the two pump wavelengths), and further by that of the light amplitude can be electrically modulated. This embodiment also means that you can separate the three available the wavelengths and obtain these each separately in the three different output channel waveguides 80, 85 and 81, respectively.

In cases where the frequency generation can not be phase matched with with the help of birefringence one can instead (according to a third embodiment illustrated in Figures 10 and 11) of the invention use a so-called quasi-phase matching guide 90 there the periodicity of this waveguide is selected in a manner known to it desired frequency generation. See Figs. 10 and 11, as in otherwise are analogous to Figures 8 and 9.

A fourth embodiment involves combining it described frequency mixture with frequency doubling alternative native down-conversion in frequency genome (degenerate) parametric oscillation of the light from each of them both semiconductor lasers are preferably in separate channel waveguides on the same substrate, whereby primarily but not necessarily quasi-phase matching technology out there; çjas. see Figs. 12 and 13. The light from the two semiconductor lasers (inco-8 wired via the input waveguides 13 and 14, respectively) can then partly is connected to the channel guide 90 where sum or 468 453 10 15 20 25 30 35 _14- difference frequency generation takes place, and light from each their semiconductor laser, or part of its power, is connected into a separate waveguide channel, 91 and 92, respectively, for frequency doubling or parametric oscillation. If two semiconductor lasers with wavelengths around 0.85 and 1.3 ßm are used, can thus three different wavelengths in the visible the area is generated: blue and red by frequency doubling of the wavelengths 0.85 and 1.3 μm respectively, partly green by sum- frequency generation. Furthermore, it is schematically illustrated in the figure 13 and 14 how the output channel waveguides 86, 85 and 87 can be brought together to form a small light source in which the intensity of the three colors and the balance between them can be regulated electro-optically and with high speed. In Fig. 13, the output channels have only to not happens without brought together relatively close to each other, and so on necessarily some link between them each of them emits light of the respective wavelength and that the remote field for everyone can be influenced by design of each channel. In Fig. 14, the light of the three wavelengths instead using integrated optical couplers 46, 47, 48 and 49 are brought together in one and the same outgoing channel 85, so as to obtain a single true point light source (common to all wavelengths) as opposed to the variant in Fig. 13.

Integrated optical components in combination with further channel waveguides can further be analogous to the figure 8 is used to separate out unconverted portions of the effects at the pump wavelengths.

For all embodiments of the invention, comes the exact phase matching with temperature tuning of the prior art nonlinear crystal (eg with Peltier elements) optionally supplemented with optical fine tuning using metal electrodes along the length of the waveguide. For the lower waveguide channel 92 in Figure 15 shows an embodiment of such electrodes 74 and 10 15 20 25 30 _NYAB (Neodymium sodium borate) and CMT 468 453 _15- 75 illustrated. These electrodes can be advantageously divided up in several sections along the guides. This type of electrodes can also be used to indirectly modulate the generated light. _ In the manufacture of laser diodes, you get a certain scattering in the wavelength of the lasers. To get it for phase matching desired wavelength can be heated or cooled the laser diode with known methods.

To the nonlinear optical materials that are useful in this context include: LiNbO3, LiNbO3 doped with e.g. MgO (MgO: LiNbO 3) or with Nd and MgO (Nd: MgO: LiNbO 3), LiTaO LiIO3, KDP, KTP (KTiOPO,), with KTP isomorphic crystals' such as Kfrioaso ”further; I-: Nboy Bao (ß-Bas, o,), Lso (Lisgos), (cadmium quick- silvertelluride). Also organic crystals, such as m-NA, mA, MAP, PoM, MNMA, coANP, NPP, DAN, DANS, PNP), polymeric materials and semiconductor materials, such as GaAs and InP, as well as are current.

To increase the efficiency of the frequency conversion you can use waveguide resonators. One can be off standing wave type (Fabry-Perot) or ring resonator type, and is in the case of (degenerate) parametric oscillation necessary for the function. A Fabry-Perot resonator can realized in the form of mirror coatings on the waveguides end surfaces or in the form of lattice reflectors on the waveguides. IN Figure 15 shows the upper channel waveguide 91 formed as one Fabry-Perot resonator with mirror coatings 60 and 61 on the end surfaces. The lower channel waveguide 92 is designed as one resonator utilizing grating reflectors 62 and 63 etched i vàgledarytan.

For some applications it may be appropriate to connect a short optical fiber to the output of the main waveguide to be able to conveniently bring the light to the desired place. Ü \ 10 15 20 J> CT! _16- In the examples above of embodiments of the invention the main emphasis in the description is on conversion to ' * shorter wavelengths: mainly conversion to visible light based on semiconductor laser wavelengths in the near infrared range. As mentioned can for other applications conversion in an analogous manner and using the same type of components occur at longer wavelengths, for example from those ordinary semiconductor laser wavelengths to longer wavelengths in the infrared range. For example, you can start from the wavelengths 0.8 μm and 1.3 μm through the difference frequency generation generates light at about 2.1 pm wavelength.

In the detailed descriptions above, further to semiconductor lasers as pump light sources. Inventive can of course also be used in combination with other coherent pump light sources. One such option is s.k. semiconductor laser pumped solid state lasers, which even these are relatively small and compact. Another alternative natives are semiconductor laser pumped fiber lasers.

In some parts of the description, the term light is assumed also include invisible to the eye infrared and ultraviolet light radiation.

Claims (4)

468 453 _ ']' 7_ ÉÉÉÉÉÉÉÉÉÉ Device in the form of a coherent light source based on frequency conversion of the light from lasers, preferably easily accessible diode lasers, into an optical non-linear material such as LiNbo3, doped L1Nbo3, L1Tao3, Li: o3, KDP, KTP, KTiOAs04, KNbBO3, KNbO3, , NYAB, CMT or similar material, characterized in that two lasers (20, 21) are used to generate a new laser frequency by frequency mixing (sum or difference frequency generation) in a waveguide structure (10; 90) in a substrate (1 ) of any of said materials, and in that the two lasers (20, 21) give radiation at wavelengths such that the so-called the phase matching condition of the optically non-linear frequency mixture is fulfilled in the said waveguide (10; 90). Device according to claim 1, characterized in that said waveguides (10; 90) are arranged in such a way and the two lasers are selected so that the wavelengths of their radiation allow the said phase matching condition to be fulfilled by utilizing the crystal material. birefringence in combination with the waveguide dispersion. Device according to claim 1, characterized in that the channel waveguide where the frequency mixing takes place is designed in the form of a quasi-phase matching waveguide (90) where the periodicity of the waveguide is selected in a known manner to meet the above-mentioned phase matching condition for the frequency mixture. Device according to one of Claims 1 to 3, characterized in that the frequency mixture in the waveguide (10; 90) is combined with frequency conversion of radiation from the two pump light sources separately, preferably in separate