Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/14—External cavity lasers
  • H01S5/146—External cavity lasers using a fiber as external cavity
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
  • H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
  • H01S3/109—Frequency multiplication, e.g. harmonic generation
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/005—Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping
  • H01S5/0092—Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping for nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/02—Structural details or components not essential to laser action
  • H01S5/022—Mountings; Housings
  • H01S5/0225—Out-coupling of light
  • H01S5/02251—Out-coupling of light using optical fibres
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/14—External cavity lasers
  • H01S5/141—External cavity lasers using a wavelength selective device, e.g. a grating or etalon

Definitions

• the present invention refers to a device for gene ⁇ rating frequency doubled laser light by quasi-phase matching, comprising a laser diode, the active region of which emits coherent radiation at a fundamental wave ⁇ length; a frequency doubling waveguide, receiving said fundamental wavelength radiation and providing frequency doubled radiation, the wavelength of which being half the fundamental wavelength, said waveguide being formed in a non-linear optical element with a quasi-phase matching grating that satisfies the quasi-phase matching condition for frequency doubling with respect to the fundamental wavelength from the laser; and a stabilizing grating which provides frequency stabilizing feedback to the laser diode.
• Diode lasers may be manufac- tured to have very small dimensions, they provide good spectral properties and are easily controlled electronic ⁇ ally at extremely high rates while being able to be manu ⁇ factured at a reasonable cost.
• applications for which frequency doubled diode laser devices are expected to be of great use includes data storage (such as CD-ROM, CD-RAM, storage of sound and/or pictures on compact discs), graphical applications (such as multi colour printers, film development, digital imaging) , telemetries and instruments for medicine and biotech- nology analysis, such as DNA sequencing with flourophore tagged samples.
• a disadvantage of the small and compact design of the diode laser is that the dimensions thereof provide only a limited power output compared with other lasers.
• current single-mode single stripe lasers typically provide output radiation powers in the range from 1 to 300
• other types of lasers such as Nd:YAG
• Frequency doubling is a second order non-linear pro ⁇ cess, for which the optical power of the frequency doub ⁇ led radiation, P 2 ⁇ , is proportional to the square of the optical power of the fundamental wave P ⁇ .
• the conversion efficiency is defined as the relationship between the frequency doubled and the fundamental power, and is therefore proportional to the input power at the funda ⁇ mental wavelength.
• fundamental wave or “fundamental wavelength” refers to the wave and the wavelength, res- pectively, from the laser and “doubled wave” or “doubled frequency” refers to the light wave and the light fre- quency, respectively, generated by said frequency doubling.
• opti ⁇ cal waveguides are utilized for the frequency conversion.
• a waveguide light can be concentrated to a small spot of high intensity and then propagate through the wave ⁇ guide, wherein the high intensity is maintained over a longer interaction distance then what is possible in a conventional bulk crystal, which provides high conversion efficiency.
• the conversion efficiency is about 100 times more efficient in an opti ⁇ mized waveguide as compared to frequency doubling in a bulk crystal with so called confocal focusing.
• QPM quasi-phase matching
• phase matching is required, which means that the polarizations induced at different spatial positions in the crystal emit frequency doubled light coherently. More concretely, this means that light which is emitted from a first position in the crystal at a first point in time, and then propagates through the crystal and reaches a second position at a second point in time, should be in phase with light generated at said second position at said second point in time.
• the two waves ⁇ and 2 ⁇ must experience the same index of refrac ⁇ tion. This is normally not possible, while dispersion makes the index of refraction for the shorter wavelength higher than the index of refraction for the longer wave ⁇ length.
• phase matching If phase matching is not achieved, light generated at the beginning of the non-linear medium will have a successively increasing phase difference compared to light generated at a later position.
• the phase diff ⁇ erence is 180 degrees, the locally generated light will be completely out of phase compared to the light genera- ted at the beginning of the material. These waves will hence interfere destructively, and the energy of the frequency doubled wave will no longer increase, but in ⁇ stead couple back into the fundamental wave.
• the phase difference becomes 360 degrees, all light will be coupled back into the fundamental wave and the process will then be repeated (see Fig IB) .
• phase matching is by utilizing so called birefringent phase matching.
• a non-linear material is used, often a crystal, which has different indices of refraction in different directions.
• the diff ⁇ erence in refractive index makes it possible to provide the desired phase matching.
• this method only has limited use while, for each material, only a given narrow phase matching frequency range can be used.
• Quasi-phase matching is a preferable method in building up the frequency doubled wave in such a manner so as to eliminate the problem with destructive inter ⁇ ference.
• the distance l c is called a "coherence length".
• Quasi-phase matching provides several advantages. For example, any wavelength within the transparent region of the non-linear material may be generated by proper selection of the periodicity of the modulation of the non-linearity. Furthermore, a single polarisation can be utilized and therefore it is not necessary to rely on a material having a suitable birefringence. Furthermore, for many materials, the largest non-linear coefficient cannot be utilized in birefringent phase matching, but only in quasi-phase matching.
• Fig. 1A shows a waveguide arranged at the surface of a non-linear material, in which a quasi-phase matching grating is formed by ferro ⁇ electric domain inversion.
• Fig. IB shows a comparison between the generation of the frequency doubled wave as function of the propagation distance for quasi-phase matching and conventional phase matching.
• a further aspect of quasi-phase matching is that even if the properties of the non-linear material and the dimensions of the quasi-phase matching grating can be controlled with high accuracy, currently available lasers cannot be constructed with corresponding predetermined wavelengths with the same accuracy. Hence, the frequency of the fundamental wave in many cases is not exactly known when the laser is manufactured or bought. To make sure that the above mentioned phase matching condition will be fulfilled, several waveguides are often manufac ⁇ tured in the non-linear material, wherein the grating periods ⁇ of the quasi-phase matching gratings are arran ⁇ ged to be slightly different in the different waveguides.
• the light from the laser is then coupled to one waveguide at a time, and the frequency doubled signal is registe ⁇ red, whereby the waveguide, i.e. the quasi-phase matching grating, providing the largest generation of frequency doubled light can be determined.
• the fundamental wavelength and the grating period ⁇ accommodates the quasi-phase matching condition.
• a prerequisite for achieving frequency doubling with high conversion efficiency is that the radiation has high intensity in the waveguide. This is the reason why the waveguide with the quasi-phase matching grating in the non-linear material should be of a single mode type. With a single mode a smooth wavefront is achieved and the light may be focused into a small, diffraction limited spot.
• the requirement on the conver ⁇ sion efficiency of the system is very high while the output power from diode lasers is relatively low.
• This, together with the requirement of high intensity in the waveguide for efficient frequency doubling, means that the transfer of light from the active region of the diode laser to the waveguide in the non-linear material must have low power loss as well as good coupling to the waveguide.
• Another way of achieving said coupling is to arrange beam shaping and focusing optics between the diode laser and the frequency doubling waveguide.
• Such an arrangement has the advantage that the laser and the waveguide can be safely mounted separated from each other and that the light from the active region of the laser may be collec ⁇ ted and focused onto the waveguide in the non-linear material.
• the coupling efficiency is reduced due to the provision of intermediate optics. It is possible to achieve very stable frequency doubling if the coupling from the laser to the waveguide is maintained steady.
• a disadvantage of the above mentio ⁇ ned arrangements is that the laser and the waveguide are separated. Consequently, if the temperature changes, then the coupling will change as well, while the construction comprises different materials with different thermal expansion coefficients.
• the waveguides have cross sec ⁇ tions of a few micrometers, and a vertical or transverse displacement of the laser relative to the waveguide which amounts to only parts of a micrometer will shift the lasers coupling to the waveguide and hence affect the intensity in the waveguide, which in turn will affect the amplitude of the frequency doubled wave.
• This problem makes the entire construction sensitive to external influence and has in prior art hindered a development beyond the prototype stage for frequency doubled diode laser systems generating blue light.
• the laser spectrum must be stable and reside within the phase matching bandwidth for quasi-phase matching, which typically is of the order of 0.1 nm for conventional waveguides.
• phase matching bandwidth typically is of the order of 0.1 nm for conventional waveguides.
• diode lasers In stable frequency doubling to blue light, it has been found much more difficult to use diode lasers than other solid-state lasers, such as titanium-sapphire lasers. If the laser wavelength changes, then the ampli- tude of the frequency doubled wave will change even if the fundamental power remains constant. Diode lasers usually changes in wavelength with temperature, and the laser spectrum is sensitive to back-reflected light, which often returns from the waveguide. In prior art, frequency stabilization of the diode laser is provided by a so called Bragg grating.
• This grating provides optical feedback which stabilizes the laser at a wavelength which matches the quasi-phase matching bandwidth of the frequency doubling waveguide.
• Such frequency matched optical feedback makes the diode laser less sensitive to back-reflections.
• the Bragg grating can be provided in two ways. Either in the shape of a grating integrated into the diode laser or in the shape of a grating arranged in the quasi-phase matching waveguide. An example of the later is described in US 5 185 752 which will be discussed below.
• DBR and DFB lasers are examples of diode lasers having integrated Bragg gratings for frequency stabili- zation of the laser spectrum, which are formed in a pas ⁇ sive waveguide arranged in conjunction with the active regions of the diode lasers, see Fig. 2A and 2B.
• the US patent 5 185 752 describes a coupling arran ⁇ gement for frequency doubled diode lasers.
• a diode laser and a frequency doubling waveguide in a non-linear material is used.
• the frequency doubling wave ⁇ guide has a first grating providing frequency doubling by quasi-phase matching and a second so called Bragg grating stabilizing the laser.
• the feedback from the Bragg gra- ting makes the diode laser less sensitive to back-reflec ⁇ ted radiation.
• the optical coupling of the light from the diode laser to the waveguide is achieved by means of beam shaping and beam focusing optics.
• the coupling is obtained by simply arranging the laser in front of the waveguide.
• An object with the present invention is to provide a device for generation of frequency doubled laser light by means of quasi-phase matching, the device not exhibiting the disadvantages of prior art and said coupling of the light from the laser to the frequency doubling waveguide is achieved in a stable, effective and reliable manner.
• Another object with the present invention is to pro ⁇ vide a device for generation of frequency doubled laser light by means of quasi-phase matching, wherein the laser spectrum is advantageously stabilized within the quasi- phase matching bandwidth.
• Another object with the present invention is to pro ⁇ vide a device for generation of frequency doubled laser light by means of quasi-phase matching, wherein the light is transferred from the laser to the frequency doubling waveguide without changing the state of polarisation.
• the present invention provides a device for genera ⁇ ting of frequency doubled laser radiation by means of quasi-phase matching, comprising: a laser diode, the active region of which emits coherent radiation of a fundamental wavelength; a frequency doubling waveguide, receiving the fundamental wavelength radiation and emitting a lightwave having a wavelength which is half the fundamental wavelength, said waveguide being formed in a optical non-linear element having a quasi-phase matching grating satisfying the quasi-phase matching condition for frequency doubling with respect to the fundamental wavelength from the laser, and a stabilizing grating providing frequency stabilizing feedback to the diode laser; said device being characterized by an opti- cal fiber having one end coupled to the active region of the diode laser and the other end connected to the wave ⁇ guide in the non-linear element, wherein light being emitted from the active region of the laser diode is transferred through the optical fiber to the waveguide, whereby efficient and stable light transfer is achieved.
• an optical fiber is used to transfer light from the active region of the laser to the waveguide in the non-linear material.
• the optical fiber is mounted against the laser and the waveguide. Because of the optical fiber being flexible, any disloca ⁇ tions of the waveguide with respect to the laser can be accommodated. Sensitivity to external interference in the form of temperature variations or mechanical vibrations is thereby eliminated. This can be done without any sub- stantial degradation of other properties, such as spec ⁇ tral properties, output power or duration.
• the invention is based on the recognition that matching of the material and the dimensions of the opti ⁇ cal fiber with respect to the non-linear material and the refractive index and the dimensions of the waveguide can provide a correspondingly good conversion efficiency for the coupling of light from the optical fiber to the wave- guide.
• the optical fiber maintains polarisation.
• Preservation of the polarisation state can be achieved in several ways.
• the optical fiber may itself maintain pola- rization, or the optical fiber itself may not maintain polarisation but may instead be mounted in a fixed physical position so that the polarisation state provided by the fiber in transferring the light from the laser to the waveguide is kept constant.
• the optical fiber comprises a stabilizing Bragg grating providing frequency stable operation of the laser diode.
• a great advantage in having the stabilizing gra ⁇ ting in the optical fiber and not in conjunction with the frequency doubling waveguide is that it makes it possible to first stabilize the laser at a desired wavelength and then search and test for the waveguide having the fre ⁇ quency doubling grating which gives the largest conver ⁇ sion efficiency for frequency doubling.
• the stabilizing grating is formed in the waveguide together with the frequency doubling grating there is always a possibility that the two gratings will not give the same frequency, and hence that the quasi-phase matching is not achieved.
• the optical fiber is mounted on the waveguide by a glue joint. It has been found that, by using a suitable glue, having a index of refraction which matches the re ⁇ fractive indices of the optical fiber and the waveguide, an improved coupling from the laser to the waveguide is actually achievable. This is because the glue is acting as a kind of "matching medium" between the two refractive indices.
• the glue joint is so thin (a few micrometers) that no substantial loss take place at the glue layer.
• Fig. 1A schematically shows a waveguide in a non ⁇ linear material having a quasi-phase matching grating
• Fig. IB shows, for purpose of comparison, the gene ⁇ ration of the frequency doubled wave as a function of the length of the material with and without quasi-phase matching
• Fig. 2A schematically shows a cross section through a DFB laser
• Fig. 2B schematically shows a cross section through a DBR laser
• Fig. 3 shows a device for the generation of frequen ⁇ cy doubled laser light generated by means of quasi-phase matching according to an embodiment of the present invention
• Fig. 4A and 4B shows a another embodiment of the invention, wherein the laser is stabilized by a Bragg grating
• Fig. 5 shows yet another embodiment of the inven ⁇ tion, wherein the connection of the fiber to the wave ⁇ guide is protected from stresses by means of a support and supporting glue;
• Fig. 6 shows another embodiment of the invention, wherein the arrangement is kept at a predetermined temperature by means of a temperature control plate;
• Fig. 7 shows another embodiment of the invention, in which the coupling of the fiber to the waveguide is reinforced by a holder device.
• Fig. 1A shows a waveguide being arranged at the surface of a non-linear material, wherein a quasi-phase matching grating has been formed by ferroelectric domain inversion.
• Fig. IB shows, for purpose of comparison, the gene ⁇ ration of the frequency doubled wave as function of the length of the material with and without quasi-phase matching.
• Fig. 2A and 2B schematically show cross sections through a DFB and DBR laser.
• Fig. 3 shows a device for the generation of frequen ⁇ cy doubled laser light by means of quasi-phase matching according to an embodiment of the present invention.
• a frequency doubling laser unit comprises a laser diode 1 having an active region emitting coherent radia ⁇ tion at a fundamental wavelength.
• the diode laser shall be of a transverse single mode type and shall preferably be of a longitudinally single mode type or have a narrow spectrum (multi-mode) being matched to the phase matching bandwidth for frequency doubling (typically 0.1 nm) .
• the laser diode 1 is a DFB or a DBR laser providing frequency stabilization of the laser spectrum.
• Light from the active region 1 of the diode laser is transferred to a waveguide 3 in a non-linear element 4 through an optical fiber 2.
• Both the fiber 2 and the waveguide 3 is of a single mode type.
• the optical fiber 2 is connected to the waveguide 3 by means of a glue joint 5.
• the non-linear element is preferably formed by LiNb0 3 , LiTa0 3 or TiOP0 4 . Furthermore, the non-linear element 4 comprises a domain inverted grating arranged in the waveguide 3 providing quasi-phase matching of light from the laser diode 1, whereby a substantial part of the fundamental wave from the laser diode 1 is converted into a wave having the doubled frequency.
• the non-linear element may comprise several waveguides having quasi-phase matching gratings with somewhat different grating periods, wherein the laser diode is connected to the waveguide providing the highest output power at the frequency doubled wave.
• a fiber pig-tail laser diode is used for manufacturing a device as shown in Fig. 1.
• the fiber is brought to the waveguide and permanently fixed with a drop of glue.
• the glue may be UV curable or heat curable so that curing can be initiated when the fiber has been aligned and the coupling optimized.
• a laser diode having a fiber pig-tail typically consists of a laser diode in a pod and a micro lens or a micro lens system mounted centrosymmetrically in front of the pod. The fiber is then centrosymmetrically mounted in front of the lens for optimum light coupling to the fiber pig-tail. Because of the fact that the complete construe- tion is mounted around an axis, the laser emission direc ⁇ tion axis, the construction becomes relatively insensi ⁇ tive to temperature variations.
• the optical fiber 2 is polarisation preserving, so the light entering the waveguide 3 has the same state of polarisation as the light leaving the laser.
• Fig. 4A and 4B shows two examples of frequency sta ⁇ bilization of the laser spectrum at the phase matching wavelength in cases when the diode laser 1 is not inher- ently stabilized (i.e. the laser not being a DFB or DBR laser) .
• the frequency stabilization and feed ⁇ back is provided by means of a Bragg grating 6 formed with UV light in the optical fiber 2
• Fig. 4B the same is provided by means of a grating 6 formed in the frequency doubling waveguide 3.
• Fig 5 shows an embodiment of the invention, wherein the stabilization of the coupling of the optical fiber 2 to the waveguide 3 is provided by means of a support 7.
• the support is arranged at a portion of the fiber close the coupling of the fiber to the waveguide 3, and the fiber portion of the optical fiber 2 is fixed to the sup ⁇ port 7 with glue 8.
• the glue joint 8 and the support 7 relieve and protect the coupling of the fiber to the waveguide.
• the laser diode 1 and the non-linear optical element 4 is mounted on a temperature controlled plate 9. This arrangement allows temperature control and temperature tuning of the entire unit and thereby provi ⁇ des better stabilisation of the laser spectrum as well as improved performance in general.
• Fig. 7 Yet another way of reinforcing the coupling of the optical fiber 2 to the waveguide 3 is shown in Fig. 7.
• the optical fiber is mounted in a capillary, a ferrule, a V-groove or other holder 10.
• the construction is polished to optical quality.
• the holder 10 is then mounted against the waveguide 3 and the non-linear optical element 4 by means of glue 5 so that the optical fiber 2 is aligned with the waveguide 3.
• the strength of the coupling is improved in this arrange ⁇ ment, while the glue in Fig. 7 is acting on a larger glue area.

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• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Optics & Photonics (AREA)
• Semiconductor Lasers (AREA)
• Optical Integrated Circuits (AREA)
• Optical Couplings Of Light Guides (AREA)

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Cited By (6)

Citations (4)

• 1994
  • 1994-12-07 SE SE9404254A patent/SE504584C2/sv not_active IP Right Cessation
• 1995
  • 1995-12-07 AU AU42766/96A patent/AU4276696A/en not_active Abandoned
  • 1995-12-07 WO PCT/SE1995/001474 patent/WO1996018132A1/en active Application Filing

Patent Citations (4)

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