WO2013034813A2

WO2013034813A2 - Wavelength conversion unit

Info

Publication number: WO2013034813A2
Application number: PCT/FI2012/050874
Authority: WO
Inventors: Janne Konttinen; Pietari Tuomisto
Original assignee: EpiCrystals Oy
Current assignee: EpiCrystals Oy
Priority date: 2011-09-07
Filing date: 2012-09-07
Publication date: 2013-03-14
Anticipated expiration: 2014-03-07
Also published as: WO2013034813A3

Abstract

A device (400, NLC) for second harmonic generation comprises: - a wavelength-converting portion (MCP) for converting energy of first light (B1) into second light (B2) at a conversion efficiency, the second light (B2) having a shorter wavelength than the first light (B1), the conversion efficiency having a maximum at a temperature-dependent spectral location (λCE), - a diffractive grating (G1) for reflecting a spectrally selected portion of the first light (B1), said portion being determined by a reflection band of the grating (G1), and - a strain-inducing element (SE1, SE2, SE3, SE4), wherein the optical device (400, NLC) is arranged to operate such that: - the temperature of the wavelength-converting portion (MCP) and the temperature of the grating (G1) depend on an operating temperature of the device (400, NLC), - a change (∆T) in the operating temperature causes a first spectral shift (∆λCE) in the spectral position (λ_CE) of the maximum conversion efficiency and a second spectral shift in (∆λ_PB) the spectral position (λ_PB) of the reflection band, and - the strain-inducing element (SE1, SE2, SE3, SE4) is arranged to induce temperature-dependent strain in the wavelength-converting portion (MCP) and/or in the diffractive grating (G1) so as to reduce a difference between the first spectral shift (∆λ_CE) and the second spectral shift (∆λ_PB).

Description

WAVELENGTH CONVERSION UNIT

FIELD

The present invention relates to providing light by sum frequency generation.

BACKGROUND

It is known that the optical energy of infrared light may be converted to visible light by sum frequency generation in a nonlinear crystal.

In order to maximize the conversion efficiency, the wavelength of the infrared light should match with the spectral properties of the nonlinear crystal.

SUMMARY An object of the present invention is to provide a wavelength conversion unit based on nonlinear material and adapted to operate at an extended temperature range. An object of the present invention is to provide a light source comprising the wavelength conversion unit. An object of the present invention is to provide a method for generating light by using the wavelength conversion unit.

According to a first aspect of the invention, there is provided a device according to claim 1 . According to a second aspect of the invention, there is provided a method according to claim 14.

According to third aspect of the invention, there is provided a spectrally selective device according to claim 15. According to fourth aspect of the invention, there is provided a nonlinear crystal according to claim 17.

Second light may be generated from first light by sum frequency generation. The first light may be emitted from a laser light emitting unit. The first light may be e.g. infrared light (wavelength longer than 760 nm in vacuum), and the second light may be e.g. visible light (wavelength in the range of 400 nm to 760 nm in vacuum). In particular, the wavelength of the first light may be substantially equal to 1064 nm and the wavelength of the second (green) light may be substantially equal to 532 nm.

The wavelength of light provided by a laser light emitting unit may be stabilized by spectrally selecting a part of light emitted from the light emitting unit, and coupling the selected part back into the light emitting unit as an optical feedback signal. The feedback signal may be selected by using a diffraction grating.

The wavelength of light provided by the laser light emitting unit may be converted by using a wavelength conversion unit, which comprises nonlinear material. The wavelength conversion unit has a conversion efficiency, which depends on the wavelength of light provided by the laser light emitting unit. Most of the optical power (energy) of the second light may be generated in a main converting portion of the wavelength conversion unit. The spectral position of the reflection band may be selected to match with the spectral position of the maximum conversion efficiency at a predetermined operating temperature. A variation in the operating temperature may cause temperature-induced shift (Δλ₀Ε) of the spectral position of the maximum conversion efficiency. The variation in the operating temperature may also cause temperature-induced shift (Δλ_ΡΒ) of the spectral position of the reflection band of the grating. A variation in the operating temperature may cause a mismatch between the spectral position of the reflection band and the spectral position of the maximum conversion efficiency. This in turn may cause instability and/or reduction of output power. One or more strain-inducing elements (SE1 , SE2, SE3, SE4) may be arranged to induce temperature-dependent strain in the wavelength- converting portion (MCP) and/or in the diffractive grating (G1 ) so as to reduce a difference between a temperature-induced shift (Δλ₀Ε) of the spectral position of the maximum conversion efficiency and a temperature-induced shift (Δλ_ΡΒ) of the spectral position of the reflection band.

Without using at least one strain-inducing element, a temperature-induced shift (Δλ₀Ε) of the spectral position of the maximum conversion efficiency of a periodically poled lithium niobate crystal may be e.g. 0.08 nm/°C. Without using the strain-inducing element, a temperature-induced shift (Δλ_ΡΒ) of the spectral position of the reflection band may be e.g. 0.03 nm/°C in case of a grating implemented on a lithium niobate substrate. Thus, the difference could be e.g. in the order of 0.05 nm/°C. Thanks to the strain-inducing element, the shift Δλ₀Ε may be decreased and/or the shift Δλ_ΡΒ may be increased such that the difference between the temperature-induced shift (Δλ₀Ε) of the spectral position of the maximum conversion efficiency and the temperature-induced shift (Δλ_ΡΒ) of the spectral position of the reflection band may be e.g. substantially smaller than 0.05 nm/°C, preferably smaller than 0.03 nm/°C. Thanks to the strain-inducing element, the shift Δλ₀Ε may be decreased and/or the shift Δλ_ΡΒ may be increased such that the shift Δλ_ΡΒ is e.g. in the range of 50% to 150% of the shift Δλ₀Ε, preferably in the range of 80% to 120% of the shift Δλ_0Ε- The nonlinear material of the nonlinear crystal is typically ferroelectric. The strain-inducing element may have a different coefficient of thermal expansion (CTE) than the ferroelectric material of the nonlinear crystal. The mismatch between the coefficient of thermal expansion of the strain-inducing element and the coefficient of thermal expansion of the crystal may introduce temperature-dependent compressive strain or tensile strain in the material of the crystal. Compressive strain may increase the refractive index of a ferroelectric material and tensile strain may decrease the refractive index of the ferroelectric material. The magnitude of the spectral shift may depend on the refractive index of the nonlinear material. Consequently, the strain- inducing element may have an effect on the magnitude of the spectral shift Δλ₀Ε and/or on the magnitude of the spectral shift Δλ_ΡΒ. Thanks to the strain-dependent refractive index of the material, the magnitude of the temperature-dependent spectral shift Δλ₀Ε and/or Δλ_ΡΒ may be substantially different from a spectral shift in a reference situation where the material of the nonlinear crystal or the material of the grating is not subjected to temperature-dependent strain.

The ferroelectric material may also be piezoelectric. The strain-inducing element may be e.g. a coating layer, which has been implemented on a selected area of a nonlinear crystal. In order to create localized temperature-dependent strain, the same coating layer should not cover both the main converting portion and the grating. A strain-inducing element may rather easily implemented on the top of the main converting portion (MCP) or on top of the diffractive grating (G1 ), e.g. in wafer-level manufacturing. However, a strain-inducing element may also be on the side of the main converting portion or on the side of the grating. A strain-inducing element may also be located beneath the main converting portion or beneath the grating.

The position, dimensions, and material of the strain-inducing element may be selected so as to modify the magnitude of the temperature-dependent shift of the spectral position (Δλ_ΡΒ) of the reflection band and/or so as to modify the magnitude of the temperature-dependent shift (Δλ₀Ε) of the spectral position of the maximum conversion efficiency.

The embodiments and their benefits will become more apparent to a person skilled in the art through the description and examples given herein below, and also through the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS In the following examples, the embodiments will be described in more detail with reference to the appended drawings in which shows a light source comprising a light-emitting unit, a wavelength converting unit and a wavelength-selective unit, wherein the wavelength-selective unit comprises a grating, shows, in a side view, a wavelength converting unit comprising a grating, shows, in a side view, a light source comprising a light-emitting unit, a wavelength converting unit, and a grating, shows spectral dependency of amplification in a gain medium, spectral dependency of conversion efficiency of a nonlinear crystal, and spectral dependency of the reflection band of a grating, shows the effect of temperature on the spectral position of the maximum amplification, the effect of temperature on the spectral position of the maximum conversion efficiency, and the effect of temperature on the spectral position of the reflection band, shows how the spectral position of the maximum amplification may depend on the temperature in case of temperature- dependent strain, shows how the spectral position of the reflection band may depend on the temperature in case of temperature- dependent strain, shows, in a side view, a wavelength converting unit having a grating, wherein a strain-inducing element is arranged to cause strain in the grating, shows, in an end view, the unit of Fig. 6a, shows, in a top view, the unit of Fig. 6a, shows, in a three-dimensional view, the unit of Fig. 6a, shows, in a side view, a wavelength converting unit, wherein a strain-inducing element is arranged to cause strain in the main converting portion, shows, in an end view, the unit of Fig. 7a, shows, in a top view, the unit of Fig. 7a, shows, in a three-dimensional view, the unit of Fig. 7a, shows, in a top view, a wavelength converting unit comprising strain-inducing elements disposed on the sides of the main converting portion, shows, in an end view, the unit of Fig. 8a, shows, in a three-dimensional view, the unit of Fig. 8a, shows, in a top view, a wavelength converting unit comprising strain-inducing elements disposed on the sides of the grating, shows, in an end view, the unit of Fig. 9a, shows, in a three-dimensional view, the unit of Fig. 9a, shows, in a top view, a wavelength converting unit comprising strain-inducing elements arranged to create transverse strain, shows, in a side view, a wavelength converting unit having a strain-inducing element, shows, in a side view, a wavelength-selective unit having a strain-inducing element, shows, in a side view, a wavelength converting unit, which is optically coupled to a wavelength-selective unit, shows, in a three-dimensional view, a light source having a straight configuration, shows, in a side view, a light source having a folded configuration, and shows, in a three-dimensional view, an encapsulated light source.

DETAILED DESCRIPTION

Referring to Fig. 1 , a light source 400 may comprise a light emitting unit LD1 , a wavelength converting unit NLC, and a wavelength-selective unit 80. The light emitting unit LD1 may be arranged to emit first light B1 . The first light B1 may be coupled into the wavelength converting unit NLC.

The light emitting unit LD1 may be a semiconductor laser, e.g. a diode laser.

The wavelength converting unit NLC comprises nonlinear material, which may be arranged to generate second light B2 by sum frequency generation (SFG), in particular by second harmonic generation (SHG). Thus, the optical frequency of the second light B2 may be higher than the optical frequency of the first light B1 . In particular, the optical frequency of the second light B2 may be equal to two times the optical frequency of the first light B1 . In other words, the wavelength λ of the first light may be equal to two times the wavelength λ₂ of the second light (when the lights B1 and B2 hypothetically propagate in vacuum). The optical frequency of the second light B2 may also be equal to three times or four times the optical frequency of the first light B1 . The nonlinear material may be e.g. LiNb0₃ (lithium niobate), MgO:LN (magnesium oxide-doped lithium niobate), lithium tantalite, potassium titanyl phosphate (also known as KTP), or lithium triborate (LBO). The nonlinear material may be ferroelectric. The nonlinear material may be periodically poled so as to provide quasi phase matching (QPM).

The term "nonlinear" refers to the capability of providing harmonic optical frequencies. In this context, the term "nonlinear" does not refer to the geometrical form of the material.

The wavelength converting unit NLC may also be called as a "nonlinear crystal". The spectrally selective unit 80 may be arranged to provide optical feedback to the light emitting unit LD1 so as to stabilize wavelength (and the optical frequency) of the first light B1 . The spectrally selective unit 80 may also be called as the optical feedback unit. In particular, the optical feedback unit 80 may be arranged to reflect a predetermined part R1 of the spectrum of the first light B1 through the nonlinear material of the wavelength converting unit NLC back to the light emitting unit LD1 .

The spectrally selective unit 80 may comprise a grating G1 . The grating G1 may be e.g. a Bragg grating, a grating having a spatially modulated period length, or a resonant grating.

The spectral form of the reflection band of the grating G1 may be tailored by spatially modulating the period length of the grating G1 , instead of using a constant period length.

SX, SY, an SZ denote orthogonal directions (the direction SY is shown e.g. Fig. 7d).

Referring to Fig. 2, a wavelength converting unit NLC may comprise a grating G1 for providing the spectrally selective optical feedback R1 . In other words, the grating G1 may be integrated in the wavelength converting unit NLC. The grating G1 may comprise e.g. a plurality of diffractive features 83, which are periodically arranged so as to form a diffractive grating. In particular, the grating may be a Bragg grating. The diffractive features 83 may be e.g. microscopic ridges or grooves. The diffractive features 83 may be e.g. substantially parallel to the direction SY. A_G denotes the length of the period of the grating G1 . The period length A_G(z) may be spatially constant, or the period length A_G(z) may slightly depend on the position z so as to modify the form of the spectral response provided by the grating G1 . z denotes the position in the direction SZ. The diffractive features 83 may have e.g. a rectangular (i.e. binary), sinusoidal, blazed, or trapezoidal grating profile.

The first light B1 may propagate in the direction SZ through the wavelength converting unit NLC. The wavelength converting unit NLC may comprise a waveguide 92, which in turn may comprise the nonlinear (ferroelectric) material. The waveguide 92 maybe arranged to confine the light B1 so as to increase the efficiency of the sum frequency generation. The waveguide 92 may be implemented on a substrate 96. The diffractive features 83 of the grating G1 may also be implemented in a waveguide 92 e.g. by laser scribing.

The nonlinear material of the wavelength converting unit NLC may be periodically poled so as to increase the efficiency of the sum frequency generation.

The wavelength converting unit NLC may have a main converting portion MCP comprising the nonlinear material. At least 80% of the optical power of the light B2 may be generated in the main converting portion MCP. The main converting portion MCP may be located such that mechanical strain in the main converting portion MCP depends on the temperature in a different manner than the mechanical strain in the grating G1 .

The main converting portion MCP may also be called as the "wavelength converting portion". Also the grating G1 may comprise the nonlinear material. In particular, the diffractive features 83 of the grating G1 may be implemented on the waveguide 92. Thus, a small fraction of the power of the second light may be generated also in the material of the grating G1 . L1 denotes the length of the main converting portion MCP, and L2 denotes the length of the grating G1 in the direction SZ. L2 may be substantially shorter than L1 . For example L2 may be smaller than 20% of L1 . In this case, even if the grating G1 comprises the nonlinear material, at least 80% of the conversion may take place in the main converting portion MCP. The length L1 may be e.g. in the range of 2 mm to 10 mm.

Referring to Fig. 3, a light emitting unit LD1 of a light source 400 may comprise:

- a waveguide 24 having an electrically pumped gain region 20,

- a saturable absorber 40, and

- a reflector 60.

The combination of the saturable absorber 40 and the reflector 60 is also known as a semiconductor saturable absorber mirror (SESAM).

The saturable absorber 40 and the gain region 20 may be arranged to emit first pulsed light B1 , which may be coupled into the wavelength converting unit NLC. The light B1 may be coupled into wavelength converting unit NLC by a light-concentrating structure 120. The light-concentrating structure 120 may collimate or focus light into the nonlinear material of the wavelength converting unit NLC. The light concentrating structure 120 may be e.g. a refractive lens or a diffractive lens.

The light source 400 may be adapted to emit short light pulses B1 and/or B2 at a high repetition rate. The duration of the light pulses may be e.g. in the range of 500 fs to 1 ns. The repetition rate of the pulses may be e.g. in the order of 100 MHz to 100 GHz. Furthermore, the output of the light source 400 may be modulated extremely fast by adjusting the bias voltage of the saturable absorber 40. The waveguide 24, the saturable absorber 40 and the reflector 60 may be implemented on a substrate 12. The substrate 12 (or the substrate 10 in Fig. 12b) may be e.g. gallium arsenide (GaAs), gallium indium arsenide (GalnAs) or Indium phosphide (InP).

Second pulsed light B2 may be generated in the wavelength converting unit NLC by sum frequency generation (SFG). A residual portion of the first light B1 and the generated second light B2 may impinge on the grating G1 . A portion R1 of the first light B1 defined by the reflection band of the grating G1 may be reflected backwards through the wavelength converting unit NLC. The second light B2 is transmitted in the forward direction SZ.

A portion of the reflected light R1 may be guided back to the gain region 20 via the waveguide 92 and via the light concentrating structure 120. It is emphasized that it is not necessary to couple all optical power of the reflected light into the gain region 20, in order to obtain wavelength stabilization.

The grating G1 may be arranged to stabilize the optical frequency of said first light B1 by providing optical feedback R1 to the gain region 20 through the wavelength converting unit NLC. Consequently, this may provide very short pulses and maximum intensity of the first light B1 in the nonlinear material.

The light emitting unit LD1 may comprise light-amplifying medium, i.e. a gain region 20. Seed light propagating in the gain region 20 may induce stimulated emission of first light B1 .

Optical feedback R1 coupled through the main converting portion MCP may facilitate stable operation of the light source 400. Attenuation at high intensity levels in the main converting portion MCP may be high, and attenuation at low intensity levels in the main converting portion MCP may be low, respectively. This effect may stabilize the output power of the light source 400. Optical feedback R1 coupled through the main converting portion MCP may facilitate stable operation in a wide range of operating powers of the light source 400. Intensity of reflected light coupled into the gain region 20 through the main converting portion MCP may be attenuated less at low power levels. This may facilitate stable operation at low power levels. The light source 400 comprising the saturable absorber 40 may be adapted to emit short light pulses at a high repetition rate. The successive light pulses may have a short coherence length and they may be substantially noncoherent with each other. Consequently, the pulsed light may create a lower speckle contrast than light provided by a continuously operating laser. An image formed by coherent light typically creates annoying speckle patterns when viewed visually.

In particular, very short light pulses may be generated by a Q-switched arrangement when the wavelength-selective optical feedback R1 is provided to the gain region 20 through the main converting portion MCP by the grating G1 . The conversion efficiency of the main converting portion MCP depends on the intensity of the light B1 in non-linear manner. Thus, the reflectivity of the combination of the main converting portion MCP and the grating G1 may be substantially reduced at high intensity values, which may allow generation of very short light pulses by cavity dumping. Optical feedback R1 provided by the combination of the main converting portion MCP and the grating G1 is substantially smaller for the high-intensity light pulses than for the low- intensity light. Thanks to the intensity-dependent feedback, the fall time of the generated pulses may be very short. Consequently, very short and intense light pulses of visible light may be generated at a high efficiency.

The speckle contrast may be substantially reduced when the light source provides short light pulses at a high repetition rate. Thanks to the short duration of the pulses, the pulses may have a broad spectrum which further reduces speckle contrast.

Thanks to the pulsed operation, the peak power may be substantially higher than the peak power of a continuously operating laser device, when the devices have the same average power. The peak optical power may be e.g. greater than 10 times the average optical power, or even greater than 100 times the average optical power. Consequently, the efficiency of conversion in the nonlinear medium may be substantially increased.

Thanks to the pulsed operation at a high repetition rate, the light source 400 may consume less electrical power than a continuously operating device providing the same optical power at the same visible wavelength. Consequently, the light source 400 may operate at a lower temperature and the operating reliability may be higher. Consequently, the weight and the size of the required cooling units may be reduced.

Referring to the uppermost curve of Fig. 4, the optical amplification I -I/ISEED in the gain region 20 is wavelength-dependent. \-_\ denotes the intensity of the first light B1 generated in the gain region 20, and I_SEED denotes the intensity of the seed light. The optical gain has a maximum at a wavelength λ_ΑΜΡ-

Referring to the second curve of Fig. 4, the optical conversion efficiency \₂/\-_\ of the wavelength converting unit NLC may also be wavelength-dependent. The conversion efficiency may have a maximum Eff_MAx at a wavelength λ₀Ε- denotes the intensity of first light B1 propagating in the nonlinear medium NLC and l₂ denotes the intensity of second light B2 generated in the nonlinear medium NLC.

Referring to the lowermost curve of Fig. 4, the grating G1 may have a wavelength-dependent reflectance l_R/ . I_R denotes the intensity of light R1 reflected by the grating G1 and \-_\ denotes the intensity of first light B1 impinging on the grating G1 . The reflection band may have a maximum at a wavelength λ_ΡΒ. Fig. 4 shows a reflection band for the reflected light R1 .

The FWHM width ΔλρννΗΜ,ΡΒ of the reflection band may be e.g. in the range of 0.1 nm to 1 nm, or in the range of 1 nm to 10 nm. FWHM is an acronym for full width at half maximum.

In practice, the spectral FWHM width of the first light B1 may be arranged to be slightly smaller than the FWHM width ΔλρννΗΜ,ΡΒ of the reflection band. The FWHM width ΔλρννΗΜ,ΡΒ of the reflection band (passband) may be e.g. in the range of 50% to 150% of the FWHM width ΔλρννΗΜ,οΕ of the conversion efficiency curve (the second curve of Fig. 4) in order to minimize the speckle contrast and/or in order to ensure operation in a wide range of operating temperatures. Advantageously, the FWHM width ΔλρννΗΜ,ΡΒ of the reflection band may be in the range of 50% to 120% of the FWHM width Δλ_ΡννΗΜ,οΕ of the conversion efficiency curve. A broader spectrum of the first light B1 may provide a lower speckle contrast of the second light B2 than a narrower spectrum. Furthermore, a broader reflection band may at least partially overlap the conversion efficiency curve in a wide temperature range when the width ΔλρννΗΜ,ΡΒ of the reflection band is in the same order of magnitude as the width Δλ_Ρ νΗΜ,οΕ of the conversion efficiency curve.

On the other hand, the FWHM width ΔλρννΗΜ,ΡΒ of the reflection band may be smaller than or equal to the FWHM width ΔλρννΗΜ,οΕ of the conversion efficiency curve (the second curve of Fig. 4) in order to stabilize the light source 400 to an operating point which is near the maximum conversion efficiency, i.e. to maximize the optical output power. In particular, the FWHM width of the reflection band may be smaller than or equal to 50% of the FWHM width of the conversion efficiency curve.

The spectral position λ_ΡΒ of the reflection band may be selected or adjusted by selecting the period A_G and/or by selecting the materials of the grating G1 . The temperature of the grating G1 may have an effect on the period A_G, and consequently on the spectral position λ_ΡΒ.

The spectral position λ₀Ε of maximum conversion efficiency may be selected e.g. by selecting the poling period of a periodically poled nonlinear crystal and/or by selecting the material of the nonlinear crystal NLC. The temperature of the nonlinear crystal may have an effect on the spectral position λ₀Ε- The temperature-induced shift of λ₀Ε per unit temperature change may be e.g. in the order of 0.09 nm/°C.

Without stabilization, the temperature-induced shift of the wavelength λ_ΑΜΡ of emitted first light B1 per unit temperature change could be e.g. in the order of 0.3 nm/°C (when the wavelength λ_ΑΜΡ is in the vicinity of 1064 nm) An increase in the temperature of the main converting portion MCP may cause an increase in the wavelength (spectral position) λ₀Ε- An increase in the temperature of the grating G1 may cause an increase in the wavelength (spectral position) λ₀Ε- Thus, the spectral position λ_ΡΒ of the reflection band and the wavelength of the first light B1 may follow (i.e. track) the spectral position λ₀Ε of maximum conversion efficiency of the nonlinear crystal NLC provided that the temperature of the main converting portion MCP is kept close to the temperature of the grating G1

The grating G1 may be positioned such that the difference between the temperature of main converting portion MCP and the temperature of the grating G1 is smaller than or equal to 10°C in steady state operation when the light source 400 is operated at maximum power. Said difference is advantageously smaller than 3°C, and preferably smaller than 1 °C. The grating G1 may be thermally coupled to the main converting portion MCP. The grating G1 may be directly or indirectly connected to the main converting portion MCP such that thermal conductivity between the grating G1 and the main converting portion MCP is high enough. For example, the grating G1 and the main converting portion MCP may be mechanically connected to each other. For example, the grating G1 and the main converting portion MCP may be mechanically connected to the same heat-conducting component. In particular, the wavelength converting unit NLC may be arranged to operate such that the temperature of the grating G1 is substantially equal to the temperature of the main converting portion MCP.

The main converting portion MCP may be thermally coupled to the grating G1 and/or to a further component, e.g. to an electric heater or to a light- emitting unit LD1 . The grating G1 may be thermally coupled to the main converting portion MCP or to said further component. The spectral position λ₀Ε of maximum conversion efficiency may depend on the temperature of the main converting portion MCP. The spectral position λ_ΡΒ of the reflection band may depend on the temperature of the grating G1 . The temperature of the main converting portion MCP and the temperature of the grating G1 may in turn depend on an operating temperature. Said "operating temperature" may be e.g. a temperature of said further component, the temperature of the main converting portion MCP, or the temperature of the grating G1 . The light source 400 may be arranged to operate such that the temperature of the grating G1 depends on the temperature of the main converting portion MCP. For example, the wavelength converting unit NLC and the grating G1 may be positioned in the same housing 412 (See Fig.12c). They may even be mounted on the same heat-conductive body. The light source 400 may be arranged to operate such that the temperature of the grating G1 is substantially equal to the temperature of the nonlinear crystal NLC.

The temperature of the grating G1 may be arranged to follow the temperature of the main converting portion MCP (or vice versa) e.g. by enclosing the grating G1 and the main converting portion MCP in a thermally conductive housing (see e.g. Fig. 12c). The "vice versa" means herein that the temperature of the main converting portion MCP may be arranged to follow the temperature of the grating G1 . The temperature of the grating G1 may be arranged to closely follow the temperature of the main converting portion MCP (and/or vice versa) e.g. by implementing the grating G1 and the main converting portion MCP on the same substrate 96. This may also facilitate implementing accurate optical coupling of the main converting portion MCP to the grating G1 . In particular, when a portion of a waveguide 92 is arranged to operate as main converting portion MCP, the grating G1 may be implemented in another portion of said waveguide 92 by forming a plurality of diffractive features 83 in the waveguide 92, on the waveguide 92 and/or in the vicinity of the waveguide 92.

The grating G1 and the main converting portion MCP may be parts of the same component, and the operating temperature of the main converting portion MCP may be substantially equal to the operating temperature of the grating G1 . The temperature of the grating G1 and the temperature of the main converting portion MCP may be arranged to be dependent on the temperature of the gain region 20 of the light emitting unit LD1 . For example, the grating G1 and the main converting portion MCP may be thermally coupled to a heatsink of the light emitting unit LD1 such that the difference between the temperature of the grating G1 and the temperature of the main converting portion MCP is smaller than 3°C, preferably smaller than 1 °C (when the light emitting unit LD1 is operating). Referring to Fig. 5a, the spectral position λ_ΑΜΡ of maximum amplification, the spectral position λ₀Ε of maximum conversion efficiency, and the spectral position λ_ΑΜΡ of optical gain may depend on the temperature.

The temperature of the grating G1 may be arranged to follow the temperature of the main converting portion MCP (or vice versa). Thus, in steady state operation of the light source 400, the temperature of the grating G1 may be expressed as a function of the temperature of the main converting portion MCP. Consequently, when the operating temperature is changed, the spectral position λ_ΡΒ of the reflection band and the wavelength of the first light B1 may follow the spectral position λ₀Ε of maximum conversion efficiency.

Thus, the intensity of the second light B2 generated by the light source 400 depends less on the operating temperature of the main converting portion MCP and on the operating temperature of the gain medium 20 than without using the grating G1 for wavelength stabilization.

However, the spectral position λ_ΑΜΡ of maximum amplification and the spectral position λ₀Ε of maximum conversion efficiency may depend on the temperature such that a spectral shift of λ_ΑΜΡ is substantially greater than a spectral shift Δλ₀Ε of λ₀Ε (or vice versa). In other words, the slope of the curve λοΕ(Τ) may be different from the slope of the curve λ_ΡΒ (T). As a consequence, the wavelength of the first light B1 emitted by the light emitting unit LD1 may drift so that the intensity of the second light B2 generated in the nonlinear crystal NLC becomes unstable. The problem may get even worse when the output of the light emitting unit LD1 is modulated and/or combined with an output of another light source.

The slope of the curve λ₀Ε(Τ) is equal to Δλ₀Ε(Τ)/ΔΤ, where ΔΤ denotes a change of temperature, and Δλ₀Ε(Τ) denotes a spectral shift of the wavelength Δλ₀Ε caused by the change ΔΤ. The slope of the curve λ_ΡΒ(Τ) is equal to Δλ_ΡΒ(Τ)/ΔΤ, where ΔΤ denotes a change of temperature, and Δλ_ΡΒ(Τ) denotes a spectral shift of the wavelength Δλ_ΡΒ caused by the change ΔΤ.

The spectral position of the first light B1 generated in the gain region 20 may be stabilized by defining the wavelength of the optical feedback (seed light) coupled into the gain region 20 of the light-emitting unit LD1 . A part of the first light B1 may be reflected by the grating G, and the reflected light R1 may be used as the seed light in the gain region 20.

The grating G 1 may be dimensioned such that the spectral position λ₀Ε of maximum conversion efficiency coincides with the spectral position λ_ΡΒ of the reflection band of the grating G 1 when the operating temperature of the grating G 1 and the operating temperature of the main converting portion MCP are equal to a (predetermined) temperature T_c.

When the operating temperature of the grating G 1 is substantially equal to the operating temperature of the main converting portion MCP, and when the difference between the wavelengths λ₀Ε and λ_ΡΒ should be kept smaller than a predetermined value Δλ_ΜΑχ, the operating temperature may be varied between a minimum temperature T_MIN and a maximum temperature T_MAX- RAN denotes the (useful/applicable) temperature range defined by the temperatures T_MIN and T_MAX-

However, the range RAN may be too narrow for some applications. For example, the operating temperature of the main converting portion MCP may depend on the operating power of the light emitting unit LD1 . Keeping the operating temperature of the main converting portion MCP close to the predetermined temperature T_c may require the use of an additional heating element, which in turn may increase the costs of the light source 400. In certain applications, it may be difficult to maintain the operating temperature of the main converting portion MCP close to the operating temperature of the grating G1 .

At least one strain-inducing element may be arranged to reduce the difference between the spectral shifts Δλ₀Ε and Δλ_ΡΒ.

When the grating period A_G and/or the refractive index n_G are changed, the corresponding spectral shift Δλ_ΡΒ of the reflection band may be solved from the diffraction equation, in particular from the Bragg equation. The spectral position λ_ΡΒ of the reflection band of the grating G1 may be given by the diffraction equation

where n_G denotes the refractive index of the (ferroelectric) material of the grating G1 , A_G denotes the grating period, θι denotes the angle of incidence of the first light B1 , 0_R denotes the angle of diffraction of the reflected (diffracted) light R1 , and m is an integer. The angles θι and 0_R define the angle between the direction of propagation of light and the normal of the grating surface. The diffractive features 83 (e.g. linear ridges or linear defects) may be substantially perpendicular to the direction of propagation of the light B1 , and the diffractive features 83 may be positioned in a plane (i.e. grating surface), which is substantially parallel to the direction of propagation of the light B1 . The angles θι and 0_R may be substantially equal to 90°. In this case, the diffraction equation is reduced to the Bragg equation 2ηοΛο=ηΊλ_ΡΒ.

In a hypothetical reference situation, the positions of the diffractive features 83 (and consequently the length of the grating period) of the grating G1 are determined only by thermal expansion of a ferroelectric material of the grating G1 caused by the change ΔΤ of temperature. If temperature- dependent variation of refractive index and stress-dependent variation of refractive index are not taken into consideration, the relative spectral shift ΔλρΒ_,ΒΕρ ρΒ is substantially equal to the temperature-induced relative change of the period length A_G according to equation (1 a):

where AA_G,REF denotes thermal expansion of the period length A_G of the grating G1 in the reference situation. Δλ_ΡΒ,ρ,ΕΡ denotes spectral shift of the reflection band of the grating G1 caused by thermal expansion in the reference situation. A_G denotes the period length before (or after) the change of temperature and λ_ΡΒ denotes the spectral position of the reflection band before (or after) the change of temperature.

When the material is not subjected to stress, the relative change of the period length A_G is proportional to the coefficient of thermal expansion CTE_G of the grating G1 :

^AAG,REF _{= CTE AT (1 B)}

AG ΔΤ denotes a change of temperature.

If the change of refractive index is not taken into consideration, the relative shift ΔλρΒ,ρ,Ερ λρΒ may be proportional to the coefficient of thermal expansion CTE_Gi of the ferroelectric material of the grating G1 :

^¾EF = CTE_G1 - AT (l c)

λ_ρΒ

The refractive index of the material may depend on the temperature even when the material is not subjected to stress. Optical path length in the material is equal to geometrical length multiplied by refractive index. Thus, the temperature coefficient of refractive index may contribute to the variation in an effective grating period experienced by the first light B1 inside the material, in addition to the contribution of the change of grating period caused by thermal expansion. The temperature variation of refractive index may make an additional contribution to the relative shift Δλ_ΡΒ,ρ,Ερ λρΒ according to the following equation: •ΔΤ (i d)

where n_G denotes the refractive index of the ferroelectric material of the grating G1 . (1 /n_G)(dn_G/dT) denotes the relative change of refractive index caused by unit temperature change in homogeneous unstressed (ferroelectric) material of the grating G1 . The parameter dn_G/dT is also known as the "temperature coefficient of refractive index" or "temperature variation of refractive index". For crystalline materials such as lithium niobate, the parameters n_G and dn_G/dT may depend on the orientation of the crystal. Typical values for lithium niobate are n_e= 2.21 , n_o=2.30, dn_e/dT=37-1 0^"6/°C, dn_o/dT=3.3- 10-6/°C. ne refers to the refractive index for extraordinary rays, and no refers to the refractive index of ordinary rays.

Equation (1 d) may now define a hypothetical reference situation, where the (ferroelectric) material of the grating G1 is not subjected to stress.

Mechanical stress has an (additional) effect on the refractive index of the ferroelectric material. Consequently, the equation (1 d) is not valid if the ferroelectric material is subjected to temperature-dependent stress. In case of temperature-dependent stress, the spectral shift Δλ_ΡΒ may be substantially larger (or smaller) than the reference value Δλ_ΡΒιΡιΕρ given by the equation (1 d). The spectral shift Δλ_ΡΒ may be e.g. greater than or equal to 1 .2 times the spectral shift Δλ_ΡΒιΡιΕρ given by the equation (1 d). The spectral shift Δλ_ΡΒ may even be greater than or equal to two times the spectral shift Δλ_ΡΒιΡιΕρ given by the equation (1 d).

Referring to Figs. 5b and 5c, an extended temperature range EXTRAN may be provided by reducing the difference between the slope of the curve λ₀Ε(Τ) and the slope of the curve λ_ΡΒ(Τ). In case of Fig. 5b, the slope of the curve λ_ΡΒ(Τ) is increased with respect to the situation shown in Fig. 5a. In case of Fig. 5b, the slope of the curve _CE{T) is reduced with respect to the situation shown in Fig. 5a. The refractive index of a ferroelectric crystal (material) may be modified by introducing strain. Strain can be induced e.g. by depositing a surface film at an elevated temperature, wherein the thermal expansion coefficient of the surface film may be smaller than the thermal expansion coefficient of the substrate. When cooled to room temperature, this may induce a compressive strain in the film and tensile strain in the substrate, due to the mismatch in thermal expansion. "Strain induced optical waveguides in lithium niobate, lithium tantalate, and barium titanate", O. Eknoyan et al., Appl. Phys. Lett. 60 (4), 27 January 1992, discloses a method of producing channel waveguides in LiNbO, LiTaO, and BaTiO, ferroelectric crystals by depositing thick Si0₂ films at an elevated temperature and patterning them by reactive ion etching. The static strain resulting from the large thermal expansion mismatch between the substrate and film may cause a localized increase in the refractive index via the strain-optic effect. In addition, an electro-optic contribution to the index increase is believed to result from a surface charge distribution which compensates the electric field due to the piezoelectric effect. So, an optical waveguide may be built by modifying the refractive index in a predefined region of a substrate. The main converting portion MCP may comprise ferroelectric nonlinear material, whose refractive index depends on mechanical strain induced in said material. Also the grating G1 may comprise ferroelectric material, whose refractive index depends on mechanical strain induced in said material. The material may be e.g. lithium niobate, lithium tantalite, potassium titanyl phosphate (also known as KTP), or lithium triborate (LBO). Compressive strain may cause an increase in the refractive index, while tensile strain may reduce the refractive index. In particular, the grating G1 and the waveguide 92 may comprise the same ferroelectric material. The slope of the curve λ_ΡΒ(Τ) and/or the slope of the curve λ₀Ε(Τ) may be modified by using one or more strain-inducing elements SE1 , SE2, SE3, SE4 (see Figs. 6a-1 1 b).

The material of the strain-inducing element SE1 , SE4 may be selected to have a lower coefficient of thermal expansion than the (ferroelectric) material of the main converting portion. The material of the strain-inducing element SE1 , SE4 may be selected to have a lower coefficient of thermal expansion than the (ferroelectric) material of the grating G1 .

For example, the coefficient of thermal expansion (CTE) of the ferroelectric material may be e.g. in the range of 5·10^"6/Κ to 20 ·10^"6/Κ, and the coefficient of thermal expansion of the strain-inducing element may be e.g. in the range of 0.0-10^"6/K to 3.0 ·10^"6/Κ. For example, the ferroelectric material may be lithium niobate and the strain-inducing element may comprise Si0₂ (quartz) or Si (silicon).

The coefficient of thermal expansion of Si0₂ is 0.5-10^"6/K. The coefficient of thermal expansion of Si is 2.6-10^"6/K. The coefficient of thermal expansion of lithium niobate is anisotropic. The coefficient of thermal expansion of lithium niobate is 14.4-10^"6/K in the direction of the a-axis. The coefficient of thermal expansion of lithium niobate is 7.5-10^"6/K in the direction of the c-axis.

For example, the thickness hi of the strain-inducing element SE1 , SE2, SE3, SE4 may be e.g. in the range of 0.1 μιτι to 50 μιτι. The suitable thickness of the strain-inducing element may be selected based on the difference between the coefficient of thermal expansion of the strain-inducing element and the coefficient of thermal expansion of the ferroelectric material, based on the elastic modulus of the strain-inducing element and/or based on the elastic modulus of the ferroelectric material. The thickness of the strain-inducing element may be small (e.g. in the range of 0.1 μιτι to 5 μιη) when the difference between the coefficients of thermal expansion is large and/or when the elastic modulus of the strain-inducing element is high. The thickness of the strain-inducing element may be higher (e.g. in the range of 3 μιτι to 50 μιτι) when the difference between the coefficients of thermal expansion is small and/or when the elastic modulus of the strain-inducing element is low.

Referring to Fig. 6a, a strain-inducing element SE1 may be arranged to cause temperature-dependent mechanical strain in the material of the grating G1 . A strain-inducing element SE1 may be implemented e.g. on the diffractive features of the grating G1 . A strain-inducing element SE1 may be deposited on the diffractive features.

The material of the strain-inducing element SE1 , SE4 may be selected to have a lower coefficient of thermal expansion than the (ferroelectric) material of grating G1 .

The strain-inducing element SE1 may be implemented on the grating G1 at a temperature T|_Mp, which is lower than an operating temperature of the grating G1 .

Thus, at least a part of the grating G1 may be under compressive strain CS when the grating G1 is opeating at a temperature higher than the implementation temperature T|_Mp. The strain-inducing element SE1 is under tensile strain TS, respectively.

The strain-inducing element SE1 mechanically restricts the thermal expansion and/or contraction of the grating G1 . In particular, the strain- inducing element SE1 mechanically restricts the thermal expansion and/or contraction of the ferroelectric material of the grating G1 . The strain-inducing element SE1 has a different coefficient of thermal expansion than the (base) material of the grating. The strain-inducing element SE1 may have a different coefficient of thermal expansion than the ferroelectric material of the grating. Thus, a variation in the temperature of the grating G1 and/or in the temperature of the strain-inducing element SE1 may cause temperature- dependent strain in the grating G1 . Thus, the mechanical stress of the grating G1 may depend on the temperature of the grating G1 .

In particular, an increase in the operating temperature of the grating G1 may cause an increase in compressive strain, which in turn may increase the slope of the curve λ_ΡΒ(Τ) as schematically shown in Fig. 5b.

The base material of the grating G1 may mean a transparent material, which is located such that the first light B1 and the reflected light R1 propagate in said transparent material, and which is located such that thermal expansion of said transparent material has an effect on the grating period A_G. In particular, a part of said transparent material may be arranged to operate as the waveguide 92.

Fig. 6b shows an end view of the wavelength converting unit NLC of Fig. 6a. A waveguide 92 may be implemented on the side of a block of nonlinear material e.g. by annealed-proton-exchange (APE) or by diffusion, e.g. by zinc or titanium diffusion. The lateral dimension of the waveguide 92 may be defined e.g. by etched grooves 98. Residual side portion 99 may remain after the material of the grooves 98 has been etched away.

The waveguide 92 may also be formed by mechanical machining. The diffractive features 83 of the grating G1 may be formed e.g. by etching or laser scribing. The features 83, i.e. microscopic ridges/grooves may be formed by curing a lacquer in a mold.

The strain-inducing element SE1 may be implemented on the grating G1 e.g. by physical vapor deposition, by chemical vapor deposition, by epitaxy, and/or by sputtering. When the diffractive features 83 are microscopic ridges, the resulting relief pattern may substantially improve adhesion of the strain-inducing element SE1 to the material layers of the grating G1 .

Fig. 6c shows a top view of the wavelength converting unit NLC of Fig. 6a. Fig. 6d shows a three-dimensional view of the wavelength converting unit NLC of Fig. 6a.

If desired, a larger area of the wavelength converting unit NLC may be first covered with the material of the strain-inducing element SE1 , and the material may be e.g. etched away from areas where the strain-inducing element SE1 is not needed.

If desired, adhesion of the strain-inducing element SE1 may be improved by an auxiliary layer. For example, adhesion of silicon on lithium niobate may be improved by using an interlayer of iron (Fe). The thickness of the interlayer may be e.g. in the range of 100 nm to 5 μιτι, preferably in the range of 1 μιτι - 2 μιη.

If desired, the grooves of the grating G1 may be filled with a suitable filler material before implementing the strain-inducing element SE1 over the grating G1 .

The strain-inducing element SE1 may be implemented at a temperature, which is in the range of 20°C to 60°C, in particular at a temperature, which is substantially equal to the "room temperature" (25°C).

The strain-inducing element SE1 may also be implemented at a temperature, which is higher than the operating temperature. In that case, the grating G1 is under tensile strain at the operating temperature lower than the implementation temperature T|_Mp.

Referring to Fig. 7a, a strain-inducing element SE2 may be arranged to cause strain in the main converting portion MCP. The strain-inducing element SE2 may be positioned such that a strain caused by the element SE2 in the main converting portion MCP is substantially different from the strain caused by the element SE2 in the grating G1 .

In particular, the element SE2 may be positioned such that it does not cause strain in the grating G1 .

In this case, the strain-inducing element SE2 may have higher coefficient of thermal expansion than the ferroelectric material of the main converting portion MCP. For example, the strain-inducing element SE2 may comprise a polymer, which has a higher coefficient of thermal expansion than the ferroelectric material.

If the strain-inducing element SE2 is implemented at a temperature, which is lower than the operating temperature, the main converting portion MCP is subjected to tensile strain at the operating temperature higher than the implementation temperature T|_Mp. The tensile strain may increase with increasing temperature. Thus, the refractive index of the ferroelectric material of the main converting portion MCP may be reduced with increasing temperature, which in turn reduces the slope of the curve λ₀Ε(Τ) as shown in Fig. 5c. The strain-inducing element SE1 may also be implemented at a temperature, which is higher than the operating temperature. In that case, the grating G1 is under compressive strain at an operating temperature lower than the implementation temperature T|_Mp. Fig. 7b shows an end view of the wavelength converting unit NLC of Fig. 7a. Fig. 7c shows a top view of the wavelength converting unit NLC of Fig. 7a. Fig. 7d shows a three-dimensional view of the wavelength converting unit NLC of Fig. 7a. Referring to Figs. 8a-8c, one or more strain-inducing elements SE3 may be implemented on the side or sides of the main converting portion MCP in order to cause temperature-dependent strain in the main converting portion MCP. The (one or more) element(s) SE3 may be positioned such that the magnitude and/or direction of strain caused by the element(s) SE3 in the main converting portion MCP is substantially different from the magnitude and/or direction of strain caused by the element SE2 in the grating G1 . In particular, the (one or more) element(s) SE3 may be positioned such that they do not cause strain in the grating G1 . In this case, the strain-inducing elements SE3 may have higher coefficient of thermal expansion than the ferroelectric material of the main converting portion MCP. For example, the strain-inducing elements SE3 may comprise a polymer or a metal, which has a high coefficient of thermal expansion. For example, the strain-inducing element(s) SE3 may comprise metallic copper, metallic aluminum, metallic silver, metallic indium or metallic tin. In particular, a strain-inducing element SE3 comprising one or more of these metals may be used in combination with a main converting portion MCP, which comprises lithium niobate. If the strain-inducing elements SE3 are implemented at a temperature, which is lower than the operating temperature, the main converting portion MCP is subjected to tensile strain at the operating temperature higher than the implementation temperature T|_Mp. The tensile strain may increase with increasing temperature. Thus, the refractive index of the ferroelectric material of the main converting portion MCP may be reduced with increasing temperature, which in turn reduces the slope of the curve λ₀Ε(Τ) as shown in Fig. 5c.

The strain-inducing elements SE3 may also be implemented at a temperature, which is higher than the operating temperature. In that case, the main converting portion MCP is under compressive strain at an operating temperature lower than the implementation temperature T|_Mp.

Fig. 8b shows an end view of the wavelength converting unit NLC of Fig. 8a. Fig. 8c shows a three-dimensional view of the wavelength converting unit NLC of Fig. 8a.

Referring to Figs. 9a - 9c, one or more strain-inducing elements SE4 may be implemented on the side or sides of the grating G1 in order to cause temperature-dependent strain in the grating G1 . The (one or more) element(s) SE4 may be positioned such that a strain caused by the element(s) SE4 in the main converting portion MCP is substantially different from the strain caused by the element(s) SE4 in the grating G1 . In particular, the elements SE4 may be positioned such that they do not cause strain in the main converting portion MCP.

The material of the strain-inducing elements SE4 may be selected to have a lower coefficient of thermal expansion than the (ferroelectric) material of grating G1 . The strain-inducing elements SE4 may be implemented on the sides of the grating G1 at a temperature T|_Mp, which is lower than an operating temperature of the grating G1 . Thus, at least a part of the grating G1 is under compressive strain CS when the grating G1 is at a temperature higher than the implementation temperature T_!Mp. The strain-inducing elements SE4 are under tensile strain TS, respectively. The strain-inducing elements SE4 may also be implemented at a temperature, which is higher than the operating temperature. In that case, the grating G1 is under tensile strain at the operating temperature lower than the implementation temperature T|_Mp.

A strain-inducing element SE4 may be implemented in a groove 98.

Referring to Fig. 10, the strain caused by the strain-inducing elements S1 , S2, S3, and/or S4 does not need to be parallel to the direction of the light beam B1 (i.e. parallel to the direction SZ).

Thermal expansion of the nonlinear material is typically anisotropic, which may provide a way for creating anisotropic strain in the nonlinear material. The direction of the induced strain may be selected by selecting the orientation of the crystal planes of the material with respect to the orientation of the strain-inducing element. Also the strain-inducing element may comprise material, which has anisotropic thermal expansion.

Referring to Figs 1 1 a and 1 1 b, the grating G1 does not need to be integrated in the wavelength conversion unit NLC. In other words, the wavelength selective unit 80 may be separate from the wavelength conversion unit NLC. The wavelength selective unit 80 and the wavelength conversion unit NLC may be manufactured and delivered separately. The wavelength conversion unit NLC may comprise strain-inducing elements SE2 and/or SE3, which may be positioned e.g. as shown in Figs. 7d and 8c.

The wavelength selective unit 80 may comprise strain-inducing elements SE1 , SE4, which may be positioned as e.g. shown in Figs. 6d and 9c.

The wavelength selective unit 80 does not need to comprise a strain-inducing element if the wavelength conversion unit NLC comprises at least one strain- inducing element. The wavelength conversion unit NLC does not need to comprise a strain-inducing element if the wavelength selective unit 80 comprises at least one strain-inducing element. When used as a part of a light source 400, the wavelength conversion unit NLC may be optically coupled to the wavelength selective unit 80 as shown in Figs. 1 and 1 1 c. The spectrally selective unit 80 may be separate from the wavelength conversion unit NLC. However, optical alignment of the system and/or temperature control may be more complex than when using the integrated units shown in Figs. 6a-10. When using a wavelength selective unit 80, which is separate from the wavelength conversion unit NLC, it is not necessary to couple all optical power of the reflected light R1 into the waveguide 92 of the wavelength conversion unit NLC, in order to obtain wavelength stabilization. However, the grating G1 may be in the vicinity of the waveguide 92 of the wavelength conversion unit NLC to ensure sufficient optical feedback. The distance d1 between the waveguide 92 of the wavelength conversion unit NLC and the wavelength selective unit 80 may be e.g. smaller than or equal to 1000 μιτι.

The distance d1 may also be e.g. smaller than or equal to ten times the thickness d₉₂ of the core of the waveguide 92 in order to ensure sufficient optical feedback. In particular, the distance d1 may be e.g. smaller than or equal to the thickness d₉₂ of the core of the waveguide 92. The waveguide 92 may be rather wide in the direction SY. d₉₂ refers to the smallest dimension of the core of the waveguide 92. The smallest dimension of the core may be e.g. in the range of 3 to 10 μιτι.

The light source 400 may also be arranged such that the distance d1 is e.g. greater than or equal to 10 μιτι at each operating temperature of the light source 400. This ensures that the wavelength conversion unit NLC and the wavelength selective unit 80 do not contact each other due to thermal expansion. The contact might damage the wavelength conversion unit NLC and/or the wavelength selective unit 80.

Alternatively, the wavelength selective unit 80 may be directly attached to the wavelength conversion unit NLC, i.e. to the nonlinear crystal. Fig. 12a shows a light source 400 having a straight (i.e. linear) configuration. The nonlinear material of the wavelength converting unit NLC may be periodically poled in order to increase conversion efficiency. The wavelength converting unit NLC may comprise a plurality of periodically poled zones (domains) 91 a, 91 b. Λ_ΡΡ denotes the period of the poling. The light-emitting unit LD1 and the wavelength converting unit NLC may be attached on a base plate 14. The light source 400 may comprise a light-concentrating structure 120 to collect the first light B1 into the nonlinear crystal NLC, in particular into the waveguide 92.

Fig. 12b shows a light source 400, which has a folded configuration. The light source 400 of Figs 12a has a folded optical cavity.

The light source 400 may have a coupling structure M45 which is arranged to change the direction of the first light B1 emitted from the light-emitting unit LD1 before it impinges on the wavelength converting unit NLC. The coupling structure M45 may be arranged to change the direction of the first light B1 by an angle βι which is e.g. in the range of 70 to 1 10 degrees. The coupling structure M45 may be e.g. an etched facet. The coupling structure M45 may be arranged to change the direction of the first light B1 before passing through the light-concentrating structure 120.

The light emitting unit LD1 may comprise:

- a waveguide 24 having an electrically pumped gain region 20,

- a saturable absorber 40,

- a coupling structure M45,

- a light-concentrating structure 120,

- a substrate 10, and

- a reflector 60.

The saturable absorber 40 and the gain region 20 may be arranged to emit first pulsed light B1 , which may be guided into the main converting portion MCP by the coupling structure 45 and the light-concentrating structure 120. The light-concentrating structure 120 and the coupling structure M45 may also guide the reflected light R1 back to the gain region 20. When compared with the straight configuration, the folded configuration may be more compact and stable. The folded configuration may facilitate alignment of the light-concentrating structure 120 with respect to the light- emitting unit LD1 . In particular, the light-concentrating structure 120 may be etched on the substrate 10 of the light-emitting unit LD1 .

Referring to Fig. 12c, the light emitting unit LD1 and the wavelength converting unit NLC may be positioned in a housing comprising e.g. a shell 412 and a base 410. The light source 400 may optionally comprise a window 419 attached to the housing so as to form a hermetic (gas-tight) enclosure for the light emitting unit LD1 and the wavelength converting unit NLC. The base 410 and/or the shell 412 may be used as a heat sink for the light emitting unit LD1 . The housing, in particular the shell 412, may be arranged to equalize the temperatures of the main converting portion MCP and the grating G1 .

The housing, in particular the shell 412, may be arranged to operate such that the temperatures of the main converting portion MCP and the grating G1 follow the operating temperature of the light emitting unit LD1 . In general, the grating G1 may be positioned such that the difference between the operating temperature of the main converting portion MCP and the operating temperature of the grating G1 is smaller than or equal to 10°C in steady state operation when the light source 400 is operated at maximum power.

The light source 400 may even be arranged such that a difference between the operating temperature of the grating G1 and the operating temperature of the nonlinear crystal NLC is smaller than 3°C when the light emitting unit 400 operates at a maximum power rated for continuous operation. Preferably, said difference is smaller than 1 °C.

Thus, the spectral position λ_ΡΒ of the reflection band and the wavelength of the first light B1 may follow the spectral position λ₀Ε of maximum conversion efficiency of the main converting portion MCP. The spectral shift (Δλ₀Ε) of maximum conversion efficiency of the main converting portion MCP per unit temperature change ΔΤ may be e.g. in the range of 50% to 150% of the spectral shift (Δλ_ΡΒ) of the reflection band of the grating G1 per unit temperature change ΔΤ. Advantageously, the spectral shift (Δλ₀Ε) of maximum conversion efficiency of the main converting portion MCP per unit temperature change ΔΤ may be e.g. in the range of 80% to 120% of the spectral shift (Δλ_ΡΒ) of the reflection band of the grating G1 per unit temperature change ΔΤ. The dimension of spectral shift per unit temperature change may be e.g. nm/°C.

For example, a strain-inducing element (SE2, SE3) may be arranged to decrease the spectral shift Δλ₀Ε (per unit temperature change) so that it is smaller than 0.06 nm/°C, wherein the main converting portion MCP may comprise lithium niobate. The spectral shift Δλ₀Ε per unit temperature change may be smaller than 0.06 nm/°C when the wavelength of the first light B1 is in the range of 1000 nm to 1 100 nm in vacuum. In particular, the spectral shift Δλ₀Ε per unit temperature change may be smaller than 0.06 nm/°C when the wavelength of the first light B1 is substantially equal to 1064 nm in vacuum. The relative spectral shift Δλ₀Ε ( οΕ ·ΔΤ) may be smaller than 56-10^" ⁶/°C

A strain-inducing element (SE1 , SE4) may be arranged to increase the spectral shift Δλ_ΡΒ (per unit temperature change) so that it is greater than 0.05 nm/°C, wherein the grating G1 may comprise lithium niobate. The spectral shift Δλ_ΡΒ per unit temperature change may be greater than 0.05 nm/°C when the wavelength of the first light B1 is in the range of 1000 nm to 1 100 nm in vacuum. In particular, the spectral shift Δλ_ΡΒ per unit temperature change may be greater than 0.05 nm/°C when the wavelength of the first light B1 is substantially equal to 1064 nm in vacuum. The relative spectral shift Δλ_ΡΒ/(λ_ΡΒ ·ΔΤ) may be greater than 47-10^"6/°C

When at least one of the converting portion MCP and the grating G1 is stressed by a strain-inducing element and when both the converting portion MCP and the grating G1 comprise lithium niobate, the difference between Δλ₀Ε and Δλ_ΡΒ (per unit temperature change) may be smaller than 0.03 nm/°C when the wavelength λ of the first light B1 is in the range of 1000 nm to 1 1 00 nm in vacuum. In particular, the difference between Δλ₀Ε and Δλ_ΡΒ (per unit temperature change) may be smaller than 0.03 nm/°C when the wavelength λ of the first light B1 is substantially equal to 1 064 nm in vacuum. The relative difference (Δλ₀Ε-Δλ_ΡΒ)/(λ_ΡΒ ·ΔΤ) may be greater than 28-1 0^"6/°C. (In operation, the wavelength λ_ΡΒ may be approximately equal to the wavelength _CE) -

The grating G1 may be dimensioned such that the spectral position λ₀Ε of maximum conversion efficiency substantially coincides with the spectral position λ_ΡΒ of the reflection band in at least one operating point of the light source 400. The operating point of the light source 400 may be characterized e.g. by the temperature of the main converting portion MCP, by the temperature of the grating G1 , by the temperature of the light-emitting unit LD1 , or by the temperature of an electric heater element. The spectral positions λ₀Ε and λ_ΡΒ may coincide when the temperature of the main converting portion MCP is equal to a predetermined operating temperature of the main converting portion MCP, and the temperature of the grating G1 is equal to a predetermined operating temperature of the grating G1 . The grating G1 may be dimensioned such that the spectral position λ₀Ε of maximum conversion efficiency coincides with the spectral position λ_ΡΒ of the reflection band when the temperature of the grating G1 is equal to the temperature of the main converting portion MCP, and wherein the difference between the temperature T_MAX and the temperature of the main converting portion MCP is smaller than or equal to 1 0°C.

The grating G1 may be dimensioned such that the spectral position λ₀Ε of maximum conversion efficiency coincides with the spectral position λ_ΡΒ of the reflection band when the difference between the temperature of the main converting portion MCP and the temperature of the grating G1 is smaller than or equal to 3°C, and wherein the difference between the temperature T_MAX and the temperature of the main converting portion MCP is smaller than or equal to 1 0°C. The grating G1 may be dimensioned such that the spectral position λ₀Ε of maximum conversion efficiency of the main converting portion MCP coincides with the spectral position λ_ΡΒ of the reflection band of the grating G1 at a predetermined temperature of the main converting portion MCP. Said predetermined temperature may be e.g. equal to the temperature T_c shown in Figs. 5a-5, or it may be slightly different. Said predetermined temperature may be e.g. in the range of T_MAX-10°C to T_MAX+10°C, where T_MAX denotes the temperature of the main converting portion MCP when the light emitting unit 400 operates at the maximum power rated for continuous operation.

In particular, the grating G1 may be dimensioned e.g. such that the spectral position λ₀Ε of maximum conversion efficiency of the main converting portion MCP substantially coincides with the spectral position λ_ΡΒ of the reflection band of the grating G1 when the light emitting unit 400 operates at a maximum power rated for continuous operation. The term "continuous operation" may also include a situation where the second light B2 is emitted as pulsed light. "Maximum power" may refer herein to the maximum average optical power of visible light which can be generated by the light source during a period of 100 hours. The wavelength converting unit NLC advantageously comprises a waveguiding layer 92 for confining light and for increasing the conversion efficiency. Confining the light may facilitate subsequent focusing of the generated light B2 onto a display or projecting the light B2 onto a projection screen. However, the wavelength converting unit NLC may also be implemented without a waveguiding layer 92. In particular, the main converting portion MCP may be implemented without a waveguiding layer 92.

Referring back to Fig. 2, a grating G1 positioned after the main converting portion MCP does not reduce the conversion efficiency.

However, a grating G1 may also be positioned between the gain region 20 and the main converting portion MCP. In particular, the orientation of the wavelength conversion unit NLC may be reversed with respect to the orientation shown in Fig. 2. Positioning of the grating G1 between the light emitting unit LD1 and the main converting portion MCP may provide a strong feedback. However, in that case the grating G1 may reduce the intensity of the first light B1 coupled into the main converting portion MCP, thereby reducing the conversion efficiency.

The light source 400 may be used to implement e.g. a display device, in particular a virtual display device, or an image projector. The display device may be arranged to display graphics and/or text.

An image projector may comprise e.g. three light sources 400. The image projector may comprise e.g. one light source 400 for generating red light R, one light source 400 for generating green light G and one light source 400 for generating blue light. Unstable output may cause problems with color reproduction when a light source 400 is used as a part of the image projector. Thanks to the one or more strain-inducing elements, the output power level of the light source 400 may be stabilized. Thanks to the strain-inducing elements, temperature dependence of the output power level of the light source 400 may be significantly reduced.

Thanks to the one or more strain-inducing elements, substantially large manufacturing tolerances may be allowed. Consequently, the wavelength converting units may be mass-produced cost-effectively. In addition, the method may be fully compatible with wafer-level manufacturing.

The effect of mechanical strain on the refractive index of ferroelectric materials has been discussed e.g. in the following articles: O.Eknoyan, H.F.Taylor, Z.Tang, V.P.Swenson, J.M.Marx, "Strain induced optical waveguides in lithium niobate, lithium tantalite and barium titanate", Appl.Phys.Lett. 60(4), 27 January 1992, P.A.Kirkby, P.R.Selway, L.D:Westbrook, "Photoelastic waveguides and their effect on stripe- geometery GaAs/Ga_{1 -x}AlxAs lasers", J.Appl.Phys. 50(7), July 1979, "L.C.D.Goncalves, A.N.R. da Silva, N.I.Morimoto, A.L.Cortes, J.C.Santos, "Integrated optical structures obtained by the photoelastic effect", Annals of Optics - 25th National Meeting of Condensed Matter Physics of the Brazilian Physical Society (XXV ENFMC), 2002. Bonding of silicon on lithium niobate has been discussed e.g. in the following article: M.M.R:Howlader, T.Suga, Room temperature bonding of silicon and lithium niobate, Applied Physics Letters 89, 031914 (2006). The light emitting units LD1 shown in Figs. 3 and 12b may be arranged to provide a highly polarization-stable output for a periodically poled nonlinear crystal NLC.

The light source 400 may comprise a light emitting unit LD1 described e.g. in WO 2008/087253. The patent publication WO 2008/087253 is herein incorporated by reference.

The light emitting unit LD1 may also be e.g. a VCSEL (vertical-cavity surface- emitting) laser or a DPSS (diode pumped solid state) laser.

The period length Λ_ΡΡ of the periodic poling of the main converting portion MCP may be spatially varied (as a function of the position coordinate z in the direction SZ) in order to modify the spectral form of the conversion efficiency function. For example, the period length Λ_ΡΡ may increase with increasing distance from an end of the nonlinear crystal, in order to provide chirped poling. For example, the period length Λ_ΡΡ may decrease with increasing distance from an end of the nonlinear crystal.

The period length A_G of the grating period of the grating G1 may be spatially varied (as a function of the position coordinate z in the direction SZ) in order to modify the spectral form of the reflectance function. For example, the period length A_G may increase with increasing distance from an end of the grating G1 , in order to provide a chirped grating. For example, the period length A_G may decrease with increasing distance from an end of the grating G1

The grating G1 may be a resonant grating, whose orientation is substantially perpendicular to the direction of propagation of the first light B1 . Theories explaining the operation of resonant gratings G1 have been discussed e.g. in articles "Coupled-mode theory of resonant grating filters", by S.M.Norton, T.Erdogan, and G.M.Morris, in J.Opt.Soc.Am A, Vol. 14, No. 3, March 1997, pp. 629-638, and "Phenomenological theory of filtering by resonant dielectric gratings", by A.-L. Fehrembach, D.Maystre, A.Sentenac, in J.Opt.Soc.Am A, Vol. 19, No.6, June 2002, pp.1 136-1 144. The spectral properties of the resonant grating G1 may be designed and/or optimized by using the rigorous diffraction theory. The grating G1 may be doubly periodic resonant grating. The design of doubly periodic resonant gratings and two-dimensional resonant gratings have been discussed e.g. in articles "Angular tolerant resonant grating filters under oblique incidence", by A.Sentenac and A.-LFehrembach, in J.Opt.Soc.Am A, Vol. 22, No. 3 March 2005, pp. 475-479, and "Increasing the angular tolerance of resonant grating filters with doubly periodic structures, in Optics Letters, Vol. 23, No. 15, August 1 , 1998, pp. 1 149-1 151 . In case of a divergent beam, the direction of propagation of light may refer to the centerline of the beam.

For the person skilled in the art, it will be clear that modifications and variations of the devices according to the present invention are perceivable. The figures are schematic. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.

Claims

1 . An optical device (400, NLC), comprising :

- a wavelength-converting portion (MCP) for converting energy of first light (B1 ) into second light (B2) at a conversion efficiency, the second light (B2) having a shorter wavelength than the first light (B1 ), the conversion efficiency having a maximum at a temperature-dependent spectral location (λοε),

- a diffractive grating (G1 ) for reflecting a spectrally selected portion of the first light (B1 ), said portion being determined by a reflection band of the grating (G1 ), and

- a strain-inducing element (SE1 , SE2, SE3, SE4),

wherein the optical device (400, NLC) is arranged to operate such that:

- the temperature of the wavelength-converting portion (MCP) and the temperature of the grating (G1 ) depend on an operating temperature of the device (400, NLC),

- a change (ΔΤ) in the operating temperature causes a first spectral shift (Δλ₀Ε) in the spectral position ( _CE) of the maximum conversion efficiency and a second spectral shift in (Δλ_ΡΒ) the spectral position (λ_ΡΒ) of the reflection band, and

- the strain-inducing element (SE1 , SE2, SE3, SE4) is arranged to induce temperature-dependent strain in the wavelength-converting portion (MCP) and/or in the diffractive grating (G1 ) so as to reduce a difference between the first spectral shift (Δλ₀Ε) and the second spectral shift (Δλ_ΡΒ).

2. The device of claim 1 wherein the second spectral shift is in the range of 50% to 1 50% of the first spectral shift, preferably in the range of 80% to 120% of the first spectral shift.

3. The device of claim 1 or 2 wherein the difference between the first spectral shift (Δλ₀Ε) divided by the change (ΔΤ) in the operating temperature and the second spectral shift (Δλ_ΡΒ) divided by the change (ΔΤ) in the operating temperature is smaller than 0.03 nm/°C when the wavelength (λ) of the first light (B1 ) is in the range of 1 000 nm to 1 1 00 nm.

4. The device according to any of the claims 1 to 3 wherein the wavelength- converting portion (MCP) comprises a ferroelectric material, a strain-inducing element (SE2, SE3) is arranged to induce strain in the ferroelectric material of the wavelength-converting portion (MCP), and a coefficient of thermal expansion (CTE) of the strain-inducing element (SE2, SE3) is larger than a coefficient of thermal expansion (CTE) of the ferroelectric material of the wavelength-converting portion (MCP).

5. The device according to any of the claims 1 to 4 wherein the grating (G1 ) comprises a ferroelectric material, a strain-inducing element (SE1 , SE4) is arranged to induce strain in the ferroelectric material of the grating (G1 ), and a coefficient of thermal expansion (CTE) of the strain-inducing element (SE1 , SE4) is smaller than a coefficient of thermal expansion (CTE) of the ferroelectric material of the grating (G1 ).

6. The device according to claim 5, wherein the temperature-induced second spectral shift (Δλ_ΡΒ) is greater than or equal to 1 .2 times a reference spectral shift ΔλρΒ,ΒΕΡ given by the following equation:

^AVB,REF = ^CTEGI ' ^ΔΤ ' B

where CTE_Gi denotes the coefficient of thermal expansion of the ferroelectric material, ΔΤ denotes a change of temperature of the ferroelectric material, λ_ΡΒ denotes a spectral position of the reflection band, n_G denotes refractive index of the ferroelectric material, and dn_G/dt denotes the temperature coefficient of refractive index of the ferroelectric material.

7. The device according to any of the claims 1 to 6 wherein the grating (G1 ) is in thermal contact with the wavelength-converting portion (MCP).

8. The device according to any of the claims 1 to 7 wherein the grating (G1 ) and the wavelength-converting portion (MCP) have been implemented on the same substrate (96).

9. The device according to any of the claims 1 to 8 wherein the grating (G1 ) and the wavelength-converting portion (MCP) are enclosed within a thermally conductive enclosure (412).

10. The device according to any of the claims 1 to 8 wherein the wavelength- converting portion (MCP) is a portion of a waveguide (92) comprising ferroelectric material, and the grating (G1 ) has been implemented in or on said waveguide (92).

1 1 . The device according to any of the claims 1 to 10 wherein the wavelength-converting portion (MCP) is a portion of a waveguide (92) and the strain caused by the strain-inducing element (SE1 , SE2, SE3, SE4) is anisotropic and has a maximum in a direction which is substantially perpendicular to the waveguide (92).

12. The device according to any of the claims 1 to 1 1 wherein the wavelength-converting portion (MCP) is a portion of a waveguide (92), and the strain caused by the strain-inducing element (SE1 , SE2, SE3, SE4) is anisotropic and has a maximum in a direction which is substantially parallel with the waveguide (92).

13. The device according to any of the claims 1 to 12 comprising a light emitting unit (LD1 ) arranged to provide the first light (B1 ), wherein the grating

(G1 ) is arranged to reflect a spectrally selected portion of the first light (B1 ) back to the light emitting unit (LD1 ) in order to stabilize the wavelength of the first light (B1 ).

14. A method for generating light (B2), the method comprising:

- emitting first light (B1 ),

- converting energy of first light (B1 ) into second light (B2) by a wavelength- converting portion (MCP) at a conversion efficiency, the second light (B2) having a shorter wavelength than the first light (B1 ), the conversion efficiency having a maximum at a temperature-dependent spectral location (λοε),

- reflecting a spectrally selected portion of the first light (B1 ) by a grating (G1 ), said portion being determined by a reflection band of the grating (G1 ), and

- inducing temperature-dependent strain in the wavelength-converting portion (MCP) or in the grating (G1 ) by a strain-inducing element (SE1 , SE2, SE3,

SE4), wherein

- a change in the operating temperature causes a first spectral shift (Δλ₀Ε) in the spectral position ( _CE) of the maximum conversion efficiency and a second spectral shift (Δλ_ΡΒ) in the spectral position (λ_ΡΒ) of the reflection band, and

- the strain caused by the strain-inducing element (SE1 , SE2, SE3, SE4) is arranged to reduce a difference between the first spectral shift (Δλ₀Ε) and the second spectral shift (Δλ_ΡΒ) .

1 5. A spectrally selective optical device (80, G1 ), comprising:

- a diffractive grating (G1 ) for reflecting a spectrally selected portion of light (B1 ), said portion being determined by a reflection band of the grating (G1 ), and

- a strain-inducing element (SE1 , SE4),

wherein the optical device (80, G1 ) is arranged to operate such that a change (ΔΤ) in the temperature of the grating (G1 ) causes a spectral shift (Δλ_ΡΒ) in the spectral position (λ_ΡΒ) of the reflection band, and the strain-inducing element (SE1 , SE4) is arranged to induce temperature-dependent strain in a ferroelectric material of the grating (G1 ) such that the temperature-induced shift (Δλ_ΡΒ) is greater than or equal to 1 .2 times a reference spectral shift Δλ_ΡΒιΒΕρ given by the following equation :

^A B,REF = ^CTEG1 ^{' ΔΤ ' Λ}ΡΒ + · ^ΔΤ

1 6. The device of claim 1 5 wherein the ferroelectric material comprises lithium niobate and the spectral shift (Δλ_ΡΒ) divided by the corresponding change (ΔΤ) in the temperature is greater than 0.05 nm/°C when the wavelength (λ) of the light (R1 ) reflected by the grating (G1 ) is 532 nm.

1 7. An optical device (400, NLC), comprising :

- a strain-inducing element (SE2, SE3),

wherein the optical device (400, NLC) is arranged to operate such that a change (ΔΤ) in the temperature of the wavelength-converting portion (MCP) causes a spectral shift (Δλ₀Ε) in the spectral position ( _CE) of the maximum conversion efficiency, and

- the strain-inducing element (SE2, SE3) is arranged to induce temperature- dependent strain in the wavelength-converting portion (MCP) such that the spectral shift (Δλ₀Ε) divided by the corresponding change (ΔΤ) in the temperature is smaller than 0.06 nm/°C when the wavelength (λ) of the first light (B1 ) is in the range of 1 000 nm to 1 1 00 nm.

18. The device of claim 1 7, wherein the spectral shift (Δλ₀Ε) divided by the corresponding change (ΔΤ) in the temperature is smaller than 0.06 nm/°C when the wavelength (λ) of the first light (B1 ) is equal to 532 nm.

1 9. The device of claim 1 7 or 1 8, wherein the wavelength-converting portion (MCP) comprises lithium niobate.