WO2008153488A1 - Diode -pumped laser with a volume bragg grating as an input coupling mirror. - Google Patents

Diode -pumped laser with a volume bragg grating as an input coupling mirror. Download PDF

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Publication number
WO2008153488A1
WO2008153488A1 PCT/SE2008/050689 SE2008050689W WO2008153488A1 WO 2008153488 A1 WO2008153488 A1 WO 2008153488A1 SE 2008050689 W SE2008050689 W SE 2008050689W WO 2008153488 A1 WO2008153488 A1 WO 2008153488A1
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Prior art keywords
laser
bragg grating
cavity
volume bragg
wavelength
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PCT/SE2008/050689
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French (fr)
Inventor
Fredrik Laurell
Björn Jacobsson
Valdas Pasiskevicius
Jonas HELLSTRÖM
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Fredrik Laurell
Jacobsson Bjoern
Valdas Pasiskevicius
Hellstroem Jonas
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Publication of WO2008153488A1 publication Critical patent/WO2008153488A1/en

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    • 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
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/0941—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode
    • H01S3/09415—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode the pumping beam being parallel to the lasing mode of the pumped medium, e.g. end-pumping
    • 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
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/0915—Processes or apparatus for excitation, e.g. pumping using optical pumping by incoherent light
    • H01S3/092—Processes or apparatus for excitation, e.g. pumping using optical pumping by incoherent light of flash lamp
    • H01S3/093—Processes or apparatus for excitation, e.g. pumping using optical pumping by incoherent light of flash lamp focusing or directing the excitation energy into the active medium
    • 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
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/08059—Constructional details of the reflector, e.g. shape
    • 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
    • H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
    • H01S3/10023—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by functional association of additional optical elements, e.g. filters, gratings, reflectors
    • 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
    • H01S3/105—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length
    • H01S3/1055—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length one of the reflectors being constituted by a diffraction grating
    • 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
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • 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
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/0627—Construction or shape of active medium the resonator being monolithic, e.g. microlaser
    • 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
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
    • H01S3/0811—Construction or shape of optical resonators or components thereof comprising three or more reflectors incorporating a dispersive element, e.g. a prism for wavelength selection
    • H01S3/0812—Construction or shape of optical resonators or components thereof comprising three or more reflectors incorporating a dispersive element, e.g. a prism for wavelength selection using a diffraction grating
    • 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
    • H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
    • H01S5/041—Optical pumping
    • 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 innovation relates to a diode-pumped laser.
  • Diode-pumped solid-state lasers are the best option for obtaining high laser power in combination with a good beam quality.
  • This type of lasers finds many application areas including cutting, welding, electronics, graphical applications and medicine.
  • the most fundamental limitation for the performance in these lasers is the heat generation that is taking place in the laser medium as a consequence of the difference in wavelength between the pump wavelength and laser wavelength.
  • the energy difference between the pump and laser photons is converted to heat that remains in the laser medium. Unless this heat can be efficiently removed, it is detrimentat for the laser performance.
  • One way to reduce this heat generation is to let the pump wavelength and laser wavelength be spectrally close, although this is not so easy to achieve. Usually, two problems occur.
  • these lasers should be longitudinally pumped, meaning that the pump and laser light travels in the same direction through the laser medium.
  • an input coupling mirror is needed that transmits the pump radiation but reflects the laser radiation.
  • dichromatic dielectric multilayer mirrors have been used for this purpose, but this type of mirrors become increasingly more difficult to produce and more expensive, the closer spectrally the pump wavelength and the laser wavelength are. Eventually, when the wavelength separation between pump and laser is very small, these dielectric mirrors cannot be made.
  • a second problem that must be handled is to enforce the laser wavelength to be spectrally close to the pump wavelength, since this does not occur otherwise. Traditionally, this is done by introduction into the laser cavity of some sort of filter, which filter can cause problems with substantial loss and a complicated and unstable setup. Disclosure of the invention
  • One object of the present innovation is to solve the two problems described above of finding an appropriate input coupler and enforce the laser wavelength to be spectrally close to the pump wavelength by usage of a new type of input coupler for the pump light into the laser cavity, namely by usage of a volume Bragg grating.
  • Volume Bragg gratings can be manufactured with such a narrow bandwidth so that the pump wavelength and laser wavelength can be as spectrally close as within 0.1%. Still, the cost of such a grating is not more than for a conventional dielectric mirror.
  • the ratio (r) between the laser wavelength and the pump wavelength is preferably at most 110%, more preferably at most 105% and most preferably at most 102%.
  • the grating acts as a spectral filter and locks the laser wavelength as spectrally close to the pump wavelength as is desired.
  • the heat generation in the laser medium can be eliminated without complicating the design of the laser.
  • the present innovation enables the construction of new types of lasers with significantly better performance than conventional lasers, at the same or at a lower price.
  • the volume Bragg grating can for example be: a volume Bragg grating that is permanently written in a thermo-photo-refractive glass [Oleg M. Efimov, Leonid B. Glebov, Vadim I. Smirnov, United States Patent 6,673,497 B2. 6 Jan.
  • a B 2 m no ⁇ cos ⁇ , (1)
  • A denotes the period of the grating
  • m is the order of the Bragg reflection
  • n 0 denotes the average refractive index of the glass
  • ⁇ is the angle inside the glass between the incident radiation and the normal of the grating's periodical structure
  • the change of refractive index is created in a crystal with a mainly one-dimensional structure, where openings for the ion exchange are formed by a mask on the surface of the crystal
  • crystals are crystals from the KTP family
  • the ion exchange is usually taking place in a salt in liquid phase, e g RbNbO 3
  • this type of gratings have previously been used to create waveguide structures [IEEE Photonics Technology Letters 9, 470 (J997)]
  • the index modulation has a step-like profile in the direction of propagation, which enables also the use of higher order (m> ⁇ ) gratings for frequency locking of the laser
  • AB is the wavelength of the Bragg-matched light at the angle ⁇ Q
  • «O denotes the average refractive index of the glass
  • n ⁇ denotes the amplitude of the refractive index modulation
  • A denotes the period of the grating
  • d is the thickness of the grating along the propagation direction of the light
  • the grating bandwidth is thus fractions of a nanometre This enables a laser with a locked wavelength and thereby also a laser with stable output power
  • this narrow bandwidth is also important since these frequency conversion processes usually have such a narrow acceptance bandwidth that the wavelength shifts that can be found in conventional lasers then lead to amplitude fluctuations.
  • the so called "green noise” where the competition between longitudinal modes in a broad laser spectrum in an intra-cavity conversion scheme leads to chaotic amplitude fluctuations.
  • the bandwidth of the spectrum can be narrowed so that this effect disappears or becomes insignificant. In some circumstance, however, the grating should be combined with an etalon or a Lyot filter.
  • Figure 1 shows a first embodiment of the invention.
  • Figure 2 shows a second embodiment of the invention, wherein the laser comprises a monolithic construction.
  • Figure 3 shows a third embodiment of the invention, wherein the laser comprises two volume Bragg gratings.
  • Figure 4 shows an embodiment of the invention, wherein the laser has a folded cavity.
  • Figure 5 shows an embodiment of the invention, wherein the laser comprises a volume Bragg grating at an angle placed in a linear, unfolded cavity.
  • Figure 6 shows a variation of the embodiment according to Figure 4.
  • Figure 7 shows a variation of the embodiment according to Figure 5.
  • Figure 8 shows a spectrum for the pump wavelength and laser wavelength for a laser according to the present innovation.
  • Figure 9 shows the laser output power as a function of pump power for a laser according to the present innovation.
  • the laser medium is comprised of ions of lanthanide metals, such as Nd, Tm, Ho, Er, Yb or combinations of these, or ions of transition metals, such as Ti, Cr, Co or combinations of these, which ions have been doped in a dielectric crystal, in a glass or in a ceramic material.
  • the laser medium could alternatively be a semiconductor.
  • the shown embodiments can also contain additional components.
  • One such additional component can be a nonlinear crystal, which is placed in the laser cavity for conversion of the generated laser light into another wavelength.
  • An example of this is a crystal for frequency doubling.
  • the nonlinear crystal could also be a crystal for sum frequency generation or difference frequency generation, for which one more laser beam is inserted into the cavity, for example coupled into the cavity by means of a volume Bragg grating or another of the laser cavity mirrors.
  • the crystal could also be a crystal for optical parametric generation of light, such as an optical parametric oscillator, generator or amplifier.
  • one more laser signal beam is inserted into the laser cavity, for example coupled into the cavity through the volume Bragg grating or another of the laser cavity mirrors, similar to the sum frequency generation or difference frequency generation.
  • the laser construction according to the invention which is more stable, has better mode quality and higher power than conventional lasers.
  • the laser configuration according to the invention can be combined with an etalon or a Lyot filter in the cavity to obtain oscillation for just a single longitudinal mode.
  • Another type of functional component that can be arranged in the laser according to the invention is a component for control of whether the laser operates to give a continuous wave output or to give a pulsed output.
  • Examples of pulsed operation of a laser includes Q-switching, mode locking and cavity dumping.
  • Q-switching can either be performed with a passive technique, for example with the help of a saturable absorber, or be performed with an active technique, for example with an acousto-optic or an electro-optic modulator, or a combination of theses techniques.
  • Modelocking can for example be obtained with a passive technique, for example with the help of a saturable absorber or with the help of a Kerr lens, or be performed with an active technique, for example with an acousto-optic or an electro-optic modulator, or a combination of these techniques.
  • Figure 1 shows an embodiment of the invention where a laser crystal Ic is pumped with a pump Ia through an input coupling grating Ib.
  • the input coupling grating is according to the invention a volume Bragg grating that acts as an input coupling mirror of the cavity.
  • a volume Bragg grating can be manufactured with a very narrow reflection band, which then leads to the possibility of the pump wavelength and the laser wavelength to be spectrally close to each other.
  • the generated laser radiation Id oscillates in the cavity between the input coupling grating Ib and the output coupling mirror Ie, at which the output coupling mirror couples out part of the oscillating laser radiation If.
  • one or more functional components Ig can be arranged in the cavity.
  • Figure 2 shows an embodiment where the laser comprises a monolithic component, whereby the components of the laser have been joined by bonding, glue or in some other way.
  • a laser crystal 2c is pumped by a pump 2a through an input coupling grating 2b.
  • the generated laser radiation 2d oscillates in the cavity between the input coupling grating 2b and the output coupling mirror 2e, at which the output coupling mirror couples out part of the oscillating laser radiation 2f.
  • one or more functional components 2g can be arranged in the cavity.
  • Figure 3 shows an embodiment with two volume Bragg gratings, where a laser crystal 3c is pumped by a pump 3a through an input coupling grating 3b.
  • the generated laser radiation 3d oscillates in the cavity between the input coupling grating 3b and the output coupling grating 3e, at which the output coupling grating couples out part of the oscillating laser radiation 3f.
  • These two volume Bragg gratings 3 b, 3 e can have the same or partly different spectral characteristics.
  • one or more functional components 3g can be arranged in the cavity.
  • Figure 4 shows an embodiment with a folded cavity, where the volume Bragg grating 4b is used at an angle and together with two mirrors 4e and 4f forms the cavity.
  • a laser crystal 4c is pumped by a pump 4a through an input coupling grating 4b.
  • the generated laser radiation 4d oscillates in the cavity between a mirror 4f and the output coupling mirror 4e, at which the output coupling mirror couples out part of the oscillating laser radiation 4g.
  • This type of cavity more freedom is available for control of the laser transverse mode by the help of mirrors 4f and 4e.
  • the strength of the spectral filtering of the volume Bragg grating is increased since the laser light passes the said grating twice per roundtrip.
  • one or more functional components 4h can be arranged in the cavity.
  • Figure 5 shows an embodiment where the volume Bragg grating 5b is at an angle, and placed in a linear (unfolded) cavity.
  • this volume Bragg grating 5b is intended for reflecting the pump radiation 5a.
  • a laser crystal 5c is pumped by a pump 5a via the input coupling grating at an angle 5b.
  • the generated laser radiation 5d oscillates in the cavity between a mirror 5f and the output coupling mirror 5e, at which the output coupling mirror couples out part of the laser radiation 5g.
  • one or more functional components 5h can be arranged in the cavity.
  • FIG. 6 shows a variation of the laser according to Figure 4.
  • the version according to Figure 6 contains two gratings at an angle 6b, 6i, for input coupling of the pump.
  • the laser can be pumped from two different directions, whereby higher gain can be obtained.
  • a laser crystal 6c is pumped by a pump 6a through an input coupling grating 6b from one direction, and at the same time the laser crystal 6c is pumped by another pump 6j through an input coupling grating 6i from the opposite direction.
  • the generated laser radiation 6d oscillates in the cavity between a mirror 6f and the output coupling mirror 6e, at which the output coupling mirror couples out part of the laser radiation 6g.
  • one or more functional components 6h can be arranged in the cavity.
  • FIG. 7 shows a variation of the laser according to Figure 5.
  • the version according to Figure 7 contains two gratings at an angle 7b, 7i, for input coupling of the pump. Instead of reflecting the laser radiation, these volume Bragg gratings 7b, 7i are intended for reflecting the pump radiation 7a, 7j. Thereby the laser medium can be pumped from two different directions, whereby higher gain can be obtained.
  • the two input coupling gratings 7b, 7i are used at an angle but placed in a linear (unfolded) cavity.
  • a laser crystal 7c is pumped by a pump 7a via an input coupling grating at an angle 7b from one direction, and at the same time the laser crystal is pumped by another pump 7j via an input coupling 7i from the opposite direction.
  • the generated laser radiation 7d oscillates in the cavity between a mirror 7f and the output coupling mirror 7e, at which the output coupling mirror couples out part of the laser radiation 7g.
  • one or more functional components 7h can be
  • Figure 8 the spectrum for the pump wavelength and laser wavelength is shown in a laser according to the invention.
  • the laser cavity that was used is of the type shown in Figure 1.
  • the pump light around 980 nm is shown and to the right of Figure 8 the laser radiation around 998 nm is shown.
  • the input coupling grating was a volume Bragg grating with a narrow reflection peak at 998 nm.
  • the volume Bragg grating forced the laser to oscillate at 998 nm, while at the same time there was good transmission into the laser cavity and onto the laser crystal of the pump radiation around 980 nm.
  • Figure 9 shows the output power as a function of the incident pump power for this laser.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optics & Photonics (AREA)
  • Lasers (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

Abstract

A laser is disclosed, comprising a laser medium that can be optically pumped and a cavity in which the said laser medium is placed, which laser comprises a volume Bragg grating that is arranged so that optical pumping of the laser medium can be done through the said volume Bragg grating.

Description

Diode-pumped laser with a volume Bragg grating as an input coupling mirror
Technical field
The present innovation relates to a diode-pumped laser.
Technical background
Diode-pumped solid-state lasers are the best option for obtaining high laser power in combination with a good beam quality. This type of lasers finds many application areas including cutting, welding, electronics, graphical applications and medicine. The most fundamental limitation for the performance in these lasers is the heat generation that is taking place in the laser medium as a consequence of the difference in wavelength between the pump wavelength and laser wavelength. The energy difference between the pump and laser photons is converted to heat that remains in the laser medium. Unless this heat can be efficiently removed, it is detrimentat for the laser performance. One way to reduce this heat generation is to let the pump wavelength and laser wavelength be spectrally close, although this is not so easy to achieve. Usually, two problems occur.
First, in order to build an efficient and not too complicated and expensive laser, these lasers should be longitudinally pumped, meaning that the pump and laser light travels in the same direction through the laser medium. To make longitudinal pumping possible, an input coupling mirror is needed that transmits the pump radiation but reflects the laser radiation. Previously, dichromatic dielectric multilayer mirrors have been used for this purpose, but this type of mirrors become increasingly more difficult to produce and more expensive, the closer spectrally the pump wavelength and the laser wavelength are. Eventually, when the wavelength separation between pump and laser is very small, these dielectric mirrors cannot be made.
A second problem that must be handled is to enforce the laser wavelength to be spectrally close to the pump wavelength, since this does not occur otherwise. Traditionally, this is done by introduction into the laser cavity of some sort of filter, which filter can cause problems with substantial loss and a complicated and unstable setup. Disclosure of the invention
One object of the present innovation is to solve the two problems described above of finding an appropriate input coupler and enforce the laser wavelength to be spectrally close to the pump wavelength by usage of a new type of input coupler for the pump light into the laser cavity, namely by usage of a volume Bragg grating. Volume Bragg gratings can be manufactured with such a narrow bandwidth so that the pump wavelength and laser wavelength can be as spectrally close as within 0.1%. Still, the cost of such a grating is not more than for a conventional dielectric mirror. The ratio (r) between the laser wavelength and the pump wavelength is preferably at most 110%, more preferably at most 105% and most preferably at most 102%. At the same time, the grating acts as a spectral filter and locks the laser wavelength as spectrally close to the pump wavelength as is desired. Thereby, the heat generation in the laser medium can be eliminated without complicating the design of the laser. In this way, the present innovation enables the construction of new types of lasers with significantly better performance than conventional lasers, at the same or at a lower price.
Examples of laser wavelengths and pump wavelengths that can be obtained with the present innovation are: pumping of Nd: YAG at 869 nm and lasing at 885 nm (r = 101.8%); pumping of Yb: KYW at 980 nm and lasing at 998 nm (r=101.8%); and pumping of Eπglass at 1480 nm and lasing at 1530 nm (r=103.4%). The volume Bragg grating can for example be: a volume Bragg grating that is permanently written in a thermo-photo-refractive glass [Oleg M. Efimov, Leonid B. Glebov, Vadim I. Smirnov, United States Patent 6,673,497 B2. 6 Jan. 2004; Oleg M. Efimov, Leonid B. Glebov, Larissa N. Glebova, Vadim I. Smirnov, United States Patent 6,586,141 Bl. 1 July 2003] or a periodically ion-exchanged structure in a crystal. In the first case, a periodical structure is written in the form of a change of the refractive index of the glass, a so called Bragg grating, by means of a holographic technique. Then the mentioned Bragg grating will act as a wavelength selective filter that only reflects radiation within a narrow wavelength range. The wavelength of the reflected radiation, /IB, is given by the Bragg condition as
AB = 2 m no Λ cosθ, (1) where A denotes the period of the grating, m is the order of the Bragg reflection, n0 denotes the average refractive index of the glass and θ is the angle inside the glass between the incident radiation and the normal of the grating's periodical structure This type of volume gratings in a thermo-photo-refractive glass have the advantage of being relatively small (of the order of millimetre size), that they do not degrade with time and that they can withstand high optical powers It is to prefer if the order m is low, preferably m=\, so that the efficiency of the reflection is high
In the other case, with periodical ion exchange, the change of refractive index is created in a crystal with a mainly one-dimensional structure, where openings for the ion exchange are formed by a mask on the surface of the crystal Examples of such crystals are crystals from the KTP family The ion exchange is usually taking place in a salt in liquid phase, e g RbNbO3 Examples of this type of gratings have previously been used to create waveguide structures [IEEE Photonics Technology Letters 9, 470 (J997)] For this type of gratings, the index modulation has a step-like profile in the direction of propagation, which enables also the use of higher order (m>\) gratings for frequency locking of the laser
An important effect that is obtained by usage of a volume Bragg grating to lock the laser is that the laser emission spectral range can be restricted to be much more narrow than if a conventional mirror is used The bandwidth of the grating is given by
Figure imgf000004_0001
where AB is the wavelength of the Bragg-matched light at the angle ΘQ, «O denotes the average refractive index of the glass, n\ denotes the amplitude of the refractive index modulation, A denotes the period of the grating and d is the thickness of the grating along the propagation direction of the light
With typical lengths, d, of a few millimetres, the grating bandwidth is thus fractions of a nanometre This enables a laser with a locked wavelength and thereby also a laser with stable output power In the area of optical frequency conversion (frequency doubling, sum frequency generation, difference frequency generation, optical parametric oscillators, generators and amplifiers) this narrow bandwidth is also important since these frequency conversion processes usually have such a narrow acceptance bandwidth that the wavelength shifts that can be found in conventional lasers then lead to amplitude fluctuations. Especially common is the so called "green noise", where the competition between longitudinal modes in a broad laser spectrum in an intra-cavity conversion scheme leads to chaotic amplitude fluctuations. With a volume Bragg grating the bandwidth of the spectrum can be narrowed so that this effect disappears or becomes insignificant. In some circumstance, however, the grating should be combined with an etalon or a Lyot filter.
Brief description of the drawings
Embodiments of the invention and examples of the performance will be described in detail below, with reference to the accompanying drawings, on which: Figure 1 shows a first embodiment of the invention.
Figure 2 shows a second embodiment of the invention, wherein the laser comprises a monolithic construction.
Figure 3 shows a third embodiment of the invention, wherein the laser comprises two volume Bragg gratings.
Figure 4 shows an embodiment of the invention, wherein the laser has a folded cavity. Figure 5 shows an embodiment of the invention, wherein the laser comprises a volume Bragg grating at an angle placed in a linear, unfolded cavity.
Figure 6 shows a variation of the embodiment according to Figure 4. Figure 7 shows a variation of the embodiment according to Figure 5. Figure 8 shows a spectrum for the pump wavelength and laser wavelength for a laser according to the present innovation.
Figure 9 shows the laser output power as a function of pump power for a laser according to the present innovation.
Detailed description of the drawings With reference to the attached drawings, a number of preferred embodiments of the present invention will next be described. For all embodiments it is beneficial if the laser medium is comprised of ions of lanthanide metals, such as Nd, Tm, Ho, Er, Yb or combinations of these, or ions of transition metals, such as Ti, Cr, Co or combinations of these, which ions have been doped in a dielectric crystal, in a glass or in a ceramic material. The laser medium could alternatively be a semiconductor.
The shown embodiments can also contain additional components. One such additional component can be a nonlinear crystal, which is placed in the laser cavity for conversion of the generated laser light into another wavelength. An example of this is a crystal for frequency doubling. The nonlinear crystal could also be a crystal for sum frequency generation or difference frequency generation, for which one more laser beam is inserted into the cavity, for example coupled into the cavity by means of a volume Bragg grating or another of the laser cavity mirrors. The crystal could also be a crystal for optical parametric generation of light, such as an optical parametric oscillator, generator or amplifier. In the latter case, one more laser signal beam is inserted into the laser cavity, for example coupled into the cavity through the volume Bragg grating or another of the laser cavity mirrors, similar to the sum frequency generation or difference frequency generation. In all the cases of frequency conversion in a laser according to the present innovation it is beneficial with the laser construction according to the invention which is more stable, has better mode quality and higher power than conventional lasers. In order to maximize the stabilization of the laser, the laser configuration according to the invention can be combined with an etalon or a Lyot filter in the cavity to obtain oscillation for just a single longitudinal mode.
Another type of functional component that can be arranged in the laser according to the invention is a component for control of whether the laser operates to give a continuous wave output or to give a pulsed output. Examples of pulsed operation of a laser includes Q-switching, mode locking and cavity dumping. Q-switching can either be performed with a passive technique, for example with the help of a saturable absorber, or be performed with an active technique, for example with an acousto-optic or an electro-optic modulator, or a combination of theses techniques. Modelocking can for example be obtained with a passive technique, for example with the help of a saturable absorber or with the help of a Kerr lens, or be performed with an active technique, for example with an acousto-optic or an electro-optic modulator, or a combination of these techniques.
Figure 1 shows an embodiment of the invention where a laser crystal Ic is pumped with a pump Ia through an input coupling grating Ib. The input coupling grating is according to the invention a volume Bragg grating that acts as an input coupling mirror of the cavity. As is described above, a volume Bragg grating can be manufactured with a very narrow reflection band, which then leads to the possibility of the pump wavelength and the laser wavelength to be spectrally close to each other. The generated laser radiation Id oscillates in the cavity between the input coupling grating Ib and the output coupling mirror Ie, at which the output coupling mirror couples out part of the oscillating laser radiation If. Optionally, one or more functional components Ig can be arranged in the cavity.
Figure 2 shows an embodiment where the laser comprises a monolithic component, whereby the components of the laser have been joined by bonding, glue or in some other way. A laser crystal 2c is pumped by a pump 2a through an input coupling grating 2b. The generated laser radiation 2d oscillates in the cavity between the input coupling grating 2b and the output coupling mirror 2e, at which the output coupling mirror couples out part of the oscillating laser radiation 2f. Optionally, one or more functional components 2g can be arranged in the cavity.
Figure 3 shows an embodiment with two volume Bragg gratings, where a laser crystal 3c is pumped by a pump 3a through an input coupling grating 3b. The generated laser radiation 3d oscillates in the cavity between the input coupling grating 3b and the output coupling grating 3e, at which the output coupling grating couples out part of the oscillating laser radiation 3f. These two volume Bragg gratings 3 b, 3 e can have the same or partly different spectral characteristics. Optionally, one or more functional components 3g can be arranged in the cavity.
Figure 4 shows an embodiment with a folded cavity, where the volume Bragg grating 4b is used at an angle and together with two mirrors 4e and 4f forms the cavity. A laser crystal 4c is pumped by a pump 4a through an input coupling grating 4b. The generated laser radiation 4d oscillates in the cavity between a mirror 4f and the output coupling mirror 4e, at which the output coupling mirror couples out part of the oscillating laser radiation 4g. With this type of cavity, more freedom is available for control of the laser transverse mode by the help of mirrors 4f and 4e. At the same time the strength of the spectral filtering of the volume Bragg grating is increased since the laser light passes the said grating twice per roundtrip. Optionally, one or more functional components 4h can be arranged in the cavity.
Figure 5 shows an embodiment where the volume Bragg grating 5b is at an angle, and placed in a linear (unfolded) cavity. Instead of reflecting the laser radiation, this volume Bragg grating 5b is intended for reflecting the pump radiation 5a. A laser crystal 5c is pumped by a pump 5a via the input coupling grating at an angle 5b. The generated laser radiation 5d oscillates in the cavity between a mirror 5f and the output coupling mirror 5e, at which the output coupling mirror couples out part of the laser radiation 5g. With this type of cavity, more freedom is available for control of the laser transverse mode by the help of mirrors 5f and 5e. Optionally, one or more functional components 5h can be arranged in the cavity.
Figure 6 shows a variation of the laser according to Figure 4. The version according to Figure 6 contains two gratings at an angle 6b, 6i, for input coupling of the pump. With this embodiment the laser can be pumped from two different directions, whereby higher gain can be obtained. A laser crystal 6c is pumped by a pump 6a through an input coupling grating 6b from one direction, and at the same time the laser crystal 6c is pumped by another pump 6j through an input coupling grating 6i from the opposite direction. The generated laser radiation 6d oscillates in the cavity between a mirror 6f and the output coupling mirror 6e, at which the output coupling mirror couples out part of the laser radiation 6g. Optionally, one or more functional components 6h can be arranged in the cavity.
Figure 7 shows a variation of the laser according to Figure 5. The version according to Figure 7 contains two gratings at an angle 7b, 7i, for input coupling of the pump. Instead of reflecting the laser radiation, these volume Bragg gratings 7b, 7i are intended for reflecting the pump radiation 7a, 7j. Thereby the laser medium can be pumped from two different directions, whereby higher gain can be obtained. The two input coupling gratings 7b, 7i are used at an angle but placed in a linear (unfolded) cavity. A laser crystal 7c is pumped by a pump 7a via an input coupling grating at an angle 7b from one direction, and at the same time the laser crystal is pumped by another pump 7j via an input coupling 7i from the opposite direction. The generated laser radiation 7d oscillates in the cavity between a mirror 7f and the output coupling mirror 7e, at which the output coupling mirror couples out part of the laser radiation 7g. Optionally, one or more functional components 7h can be arranged in the cavity.
In Figure 8 the spectrum for the pump wavelength and laser wavelength is shown in a laser according to the invention. The laser cavity that was used is of the type shown in Figure 1. To the left of Figure 8 the pump light around 980 nm is shown and to the right of Figure 8 the laser radiation around 998 nm is shown. In this case the input coupling grating was a volume Bragg grating with a narrow reflection peak at 998 nm. The volume Bragg grating forced the laser to oscillate at 998 nm, while at the same time there was good transmission into the laser cavity and onto the laser crystal of the pump radiation around 980 nm. Figure 9 shows the output power as a function of the incident pump power for this laser.

Claims

1. A laser, comprising a laser medium that can be optically pumped and a cavity in which the said laser medium is placed, which laser comprises an input coupling mirror in the form of a volume Bragg grating that is arranged so that optical pumping of the laser medium can be done through the said volume Bragg grating.
2. The laser of claim 1, wherein the said volume Bragg grating is positioned so that it acts as a mirror for the laser radiation generated in the laser.
3. The laser of claim 1 or 2, wherein the said volume Bragg grating is arranged so that the ratio between the laser wavelength and the pump wavelength is at most 110%.
4. The laser of claim 3, wherein the said volume Bragg grating is arranged so that the ratio between the laser wavelength and the pump wavelength is at most 105%.
5. The laser of claim 4, wherein the said volume Bragg grating is arranged so that the ratio between the laser wavelength and the pump wavelength is at most 102%.
6. The laser of any one of the preceding claims, wherein the components of the laser cavity, comprising input coupling grating, laser medium and output coupling mirror, are joined to form a monolithic unit.
7. The laser of claim 6, wherein the monolithic unit is joined with a diffusion bonding joint or a joint of glue.
8. The laser of any one of the preceding claims, wherein also the output coupling mirror of the laser cavity consists of a volume Bragg grating.
9. The laser of any one of the preceding claims, wherein the said volume Bragg grating is arranged to act as a folding mirror for the said laser cavity.
10. The laser of any one of the preceding claims, wherein the cavity also contains an optically nonlinear crystal for frequency conversion of the generated laser radiation
11 The laser of claim 10, wherein the optically nonlinear crystal is designed for frequency doubling, sum frequency generation, difference frequency generation or optical parametric generation using the generated laser radiation
12 The laser of any one of the preceding claims, further comprising a spectral filter arranged in the cavity to obtain single longitudinal mode operation of the generated laser light
13 The laser of any one of the preceding claims, wherein the laser medium comprises ions of a Lanthanide metal which are doped into a host material
14 The laser of any one of claims 1-13, wherein the laser medium comprises ions of a transition metal which are doped into a host material
15 The laser of claim 13 or 14, wherein the host material is a dielectric crystal, glass or ceramic material
16 The laser of claim 13, wherein the Lanthanide metal is Nd, Tm, Ho, Er, Yb or a any one combination of these
17 The laser of claim 14, wherein the transition metal is T, Cr, Co or any one combination of these
18 The laser of any one of claims 1-12, wherein the laser medium is a semiconductor
19 The laser of any one of the preceding claims, further comprising a functional component in the cavity for obtaining pulsing of the generated laser radiation
20 A laser, comprising a laser medium that can be optically pumped and a cavity in which the said laser medium is placed, which laser comprises a volume Bragg grating that is arranged so that optical pumping of the laser medium can be done through the said volume Bragg grating.
PCT/SE2008/050689 2007-06-15 2008-06-10 Diode -pumped laser with a volume bragg grating as an input coupling mirror. WO2008153488A1 (en)

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