Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/005—Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping
• • 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
• • 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
• • 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
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/02—Structural details or components not essential to laser action
  • H01S5/022—Mountings; Housings
  • H01S5/023—Mount members, e.g. sub-mount members
  • H01S5/02325—Mechanically integrated components on mount members or optical micro-benches
  • H01S5/02326—Arrangements for relative positioning of laser diodes and optical components, e.g. grooves in the mount to fix optical fibres or lenses
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/35—Non-linear optics
  • G02F1/37—Non-linear optics for second-harmonic generation
  • G02F1/377—Non-linear optics for second-harmonic generation in an optical waveguide structure
  • G02F1/3775—Non-linear optics for second-harmonic generation in an optical waveguide structure with a periodic structure, e.g. domain inversion, for quasi-phase-matching [QPM]
• • 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
  • H01S5/0014—Measuring characteristics or properties thereof
  • H01S5/0035—Simulations of laser characteristics
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/005—Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping
  • H01S5/0092—Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping for nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
  • H01S5/042—Electrical excitation ; Circuits therefor
  • H01S5/0427—Electrical excitation ; Circuits therefor for applying modulation to the laser
• • 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
  • H01S5/1082—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 with a special facet structure, e.g. structured, non planar, oblique
  • H01S5/1085—Oblique facets

Definitions

• the present invention relates generally to folded laser systems and more particularly folded laser systems with nonlinear optical wavelength conversion, such as frequency doubled green lasers.
• Green laser light can be achieved by non-linear frequency doubling of infrared light.
• a light beam 2 from an infrared diode laser (3) is directed into a non-linear optical crystal 4, such as periodically-poled lithium niobate (PPLN) where it is converted into green light 5.
• PPLN periodically-poled lithium niobate
• One aspect of the invention is a folded laser system having an optical axis, the laser system comprising: (I) a coherent light source; (II) a reflector; (EI) a lens component situated between the light source and the reflector; and (FV) a non-linear optical crystal, wherein the light source and the non-linear optical crystal are separated by a distance d>50 ⁇ m.
• the lens component is positioned to provide a collimated beam when intercepting light from the light source, such that the collimated beam is at an angle ⁇ ' to the optical axis and is structured to provide an image of the coherent light source on the non-linear optical crystal.
• the reflector is situated to intercept the collimated beam and to reflect the collimated beam to the nonlinear optical crystal through the lens.
• the coherent light source and the non-linear optical crystal are separated by an air gap.
• the laser system is a green laser
• the light source is an infra red (ER.) diode laser
• the receiver is a non linear optical crystal, for example SHG (second harmonic generator) for converting IR light to the green light.
• SHG second harmonic generator
• Some advantages provided by the exemplary green laser embodiments of laser system of the present invention are relatively loose alignment tolerances for the optical components; low sensitivity to both heat produced by the diode laser; and maximized green conversion efficiency due to improved coupling between the diode laser and the nonlinear optical crystal.
• Other advantages provided by the exemplary embodiments of the present invention are minimized temperature gradients across the nonlinear optical crystal, and minimized impact of optical feedback from undesirable reflections and/or backscatter off the nonlinear optical crystal that reach the diode laser.
• Figure IA illustrates a prior art laser system
• Figure IB schematically illustrates schematically a folded laser system according to one embodiment of the present invention
• Figure 2 is a folded cavity green laser system according to one embodiment of the present invention.
• Figure 3 is a thermal model that illustrates heat conduction between the diode laser and the nonlinear crystal of Fig. 2;
• Figure 4 shows change in the optical coupling efficiency between a diode waveguide and a crystal waveguide as a function of waveguide to waveguide spacing d;
• Figure 5 A illustrates a cross-sectional side view of an exemplary non-linear crystal
• Figure 5B illustrates a cross-sectional end view of the exemplary non-linear crystal of Fig. 5 A
• Figure 6 is a is a cross-sectional view of a folded cavity green laser system according to yet another embodiment of the present invention.
• Figure 7 illustrates coupling efficiency vs. waveguide spacing d of two different laser system configurations
• Figures 8A and 8B illustrate a nonlinear optical crystal mounted over a diode laser in yet another embodiment of the present invention
• Figure 9 is a plot of coupling efficiency (CE) achievable by commercially available lens components which can be utilized in some embodiments of the present invention.
• Figures 1OA and 1OB illustrate schematically two exemplary embodiments folded cavity green laser system
• Figure 11 is a cross-sectional view of a lens component, a crystal waveguide, and a tilted diode laser waveguide according to one embodiment of the present invention
• Figure 12 is a plot of an optical path length vs. back working distance (BWD) for two exemplary lens components
• Figure 13 is a plot of coupling efficiency vs. BWD
• Figure 14 is a is a cross-sectional view of a lens component according to yet another embodiment of the present invention.
• Figure 15 illustrates coupling performance vs. waveguide spacing d of the lens component according to one embodiment of the present invention and two commercially available lens components;
• Figure 16 is a is a cross-sectional view of a lens component according to another embodiment of the present invention.
• Figure 17 is a is a cross-sectional view of a lens component according to yet another embodiment of the present invention.
• Figure 18 illustrates the evolution of aberrations (wave front error) as a function of the tilt of an exemplary lens component
• Figure 19 illustrates the evolution of aberrations (wave front error) as a function of the tilt of an exemplary lens component
• Figure 20 is a plot of coupling efficiency as a function of the tilt of two exemplary lens components.
• the folded laser system 10 in this exemplary embodiment is a frequency doubled green laser that has a folded cavity configuration.
• light is emitted from a coherent light source 20 in the form of the divergent light beam 22, and is captured and collimated by a single lens component 30.
• the lens component 30 preferably operates in a telecentric condition. That is, the lens 30 is constructed and is situated such that the exit pupil of the optical system is located at infinity.
• coherent light source 20 be small ( ⁇ lcm 3 ), of relatively high power (>10mW) and is modulated at high rates (about lOMHz or higher).
• coherent light source 20 is an infrared (IR) semiconductor laser (IR diode laser 20')-
• the diode laser 20' includes a diode waveguide 20 'A.
• the IR light emanates as the divergent light beam 22 from the output facet of the diode waveguide 20 'A.
• the output facet of the diode waveguide may be formed perpendicular to the waveguide's axis, or it may be cleaved at an angle to the waveguide's axis (not shown).
• the divergent light beam 22 is characterized by the emission half angle ⁇ at 1/e 2 , for example 20° in one direction and 7° in the other (perpendicular) direction.
• the emission half angle ⁇ is measured relative to the average emission angle (beam centroid) provided by the coherent light source.
• the collimated (IR) beam 40 propagates towards reflector 50 at an angle ⁇ ' and is then reflected from the reflector 50 back toward lens component 30.
• the reflector 50 may be, for example, a planar mirror.
• the reflected beam propagates through the lens component 30 towards the image plane 60, where it is focused on the input facet of crystal waveguide 70 'A (the waveguide portion) of a non-linear optical crystal 70'.
• the lens component 30 provides an image of the output facet of the diode waveguide 20 'A on the input facet of the crystal waveguide 70 'A of the non-linear optical crystal 70'.
• the non-linear optical crystal 70 ' may be, for example, a second harmonic generator (SHG) such as a periodically poled lithium niobate (PPLN) crystal.
• SHG second harmonic generator
• PPLN periodically poled lithium niobate
• Other non-linear optical crystals may also be utilized, hi this embodiment the non-linear optical crystal 70' receives the IR light provided to it by the lens component 30 and converts it to a green light 5.
• the lens component 30 has a short focal length (preferably less than 5 mm, and more preferably less than 3 mm, and even more preferably less than 2 mm), and low astigmatism, in order to achieve excellent optical coupling between the coherent light source 20 and the crystal waveguide 70 'A of the non-linear optical crystal 70', while minimizing both (i) defocusing caused by changes of temperature, and (ii) the overall size of the laser system 10.
• the reflector 50 may be a conventional (fixed) planar mirror, or may be a mirror with actuation of its tip/tilt angle, such as a micro-electrical mechanical system (MEMS) mirror.
• MEMS micro-electrical mechanical system
• the coupling of light between the diode waveguide 20 'A and the crystal waveguide 70 'A may be adjusted in two primary ways. First, the position of the lens component 30 may be moved in x, y, or z (focus) axis. Second, the mirror 50 may be tilted. Because the mirror is located in the collimated space for the infrared beam, angular adjustments will cause position (x, y) movements of the reflected and focused beam at the input facet of the crystal.
• the non linear optical crystal e.g., PPLN crystal converts a substantial fraction of the infrared light into green light, which is emitted from the output facet of the crystal waveguide 70'A (Fig IB).
• adjustments in either of the position of the lens component 30, or of the angle of the reflector 50 can be utilized to move the focused spot at the input facet of the crystal waveguide 70' A of the non-linear optical crystal 70'.
• both the light source 20 and the receiver are decentered with respect to the optical axis OA (optical axis of the lens component 30) and are situated symmetrically or approximately symmetrically (departure from symmetry is within ⁇ 100 ⁇ m, preferably within ⁇ 50 ⁇ m) with respect to the optical axis.
• the output facet of the diode waveguide 20 'A of the infrared diode 20' and the input facet of the crystal waveguide 70 'A of the nonlinear optical crystal 70' are separated by a small air gap and by a small distance d compared to the focal length f of the lens 30 (i.e., d «f).
• the focal length f of the lens 30 is 1 to 5 mm (lmm ⁇ f ⁇ 5 mm), for example, lmm, 1.3 mm, 1.5 mm, 1.7 mm, 2 mm or 2.5 mm.
• the focal length f of the lens 30 is 1 to 5 mm (lmm ⁇ f ⁇ 5 mm).
• the separation d between the light source 20 and the non-linear optical crystal 70' is 30 ⁇ m ⁇ d ⁇ 1500 ⁇ m, more preferably 50 ⁇ m ⁇ d ⁇ 750 ⁇ m, more preferably 100 ⁇ m ⁇ d ⁇ 600 ⁇ m, even more preferably 150 ⁇ m ⁇ d ⁇ 500 ⁇ m and most preferably 300 ⁇ m ⁇ d ⁇ 500 ⁇ m.
• the distance d may be 75 ⁇ m, 100 ⁇ m, 125 ⁇ m, 150 ⁇ m, 200 ⁇ m, 250 ⁇ m, 300 ⁇ m, 400 ⁇ m, or 450 ⁇ m.
• the folded laser system configuration described herein has the advantage of reducing the overall length of the laser cavity (and hence reduces package size of the laser), because the optical path is folded upon itself.
• the folded laser configuration also advantageously minimizes the effect of non-symmetric optical aberrations produced by the lens component 30, because the same lens component 30 is used twice - once to collimate the beam and once to refocus the light on the input facet of the crystal waveguide 70 'A of the nonlinear optical crystal70'.
• the laser system 10 may be completely passive (i.e., it may include no moving components). (Such design is illustrated schematically in Figure IB).
• the laser system 10 may easily utilize an adjustable reflector such as a MEMS mirror to actively align the focused beam on the PPLN input facet in two lateral directions.
• the folded laser system configuration utilizes a light source (diode laser 20) and a receiver (non linear crystal 70) that are decentered with respect to the optical axis of the lens component 30, off axis optical aberrations are present and can be difficult to control.
• the optical aberrations must be kept small in order to achieve high coupling from the diode laser 20' into the non-linear optical crystal 70'.
• One advantage of the green laser embodiment 10 of the present invention is that off axis aberrations are kept small, even if the lens component 30 is misaligned.
• close proximity of the diode laser 20' to the non-linear optical crystal 70' may also cause heat to be transferred from the diode laser 20' to the non-linear optical crystal 70'.
• Thermal gradients in the non-linear optical crystal degrade conversion efficiency from the infrared light into the green light.
• One advantage of the green laser embodiments of the present invention is that thermal transfer from the diode laser to the crystal is minimized because the diode waveguide 20 'A of the diode laser 20' and the crystal waveguide 70 'A of the non-linear optical crystal 70'are separated by an air gap AG.
• at least some exemplary embodiments of the laser system 10 do not require actuators to control the focus of the optical beam by moving either the reflector 50 or the lens component 30.
• the laser systems 10 do not defocus (or have a minimal defocus) and do not significantly change lateral positioning of optical components as a function of temperature (otherwise optical coupling between the input facet of the crystal waveguide 70 'A and the diode waveguide 20 'A will be compromised, and the optical output power may be lost).
• the laser system 10 of can also advantageously control or minimize the impact of optical feedback. For example, in the green laser embodiments described herein reflections from the front facet of the crystal waveguide 70 'A of the non-linear optical crystal 70' do not induce undesirable mode hopping behavior from the infrared laser diode 20'.
• Figure 2 illustrates schematically mounted optical components that are assembled into one embodiment of a green laser system 10.
• the non-linear optical crystal 70' (PPLN crystal) is placed above the diode laser 20', with a small air gap AG separating the ends of the two waveguides 70 'A, and 20 'A.
• the existence and size of this air gap AG is important for a several reasons, described below.
• one or more wire bonds 23 attach to various sections of the diode laser 20', in order to provide current and voltage control signals to the diode laser. These wire bonds form loops 23' that have a certain minimum bend radius and thus extend by finite height above the diode laser 20'.
• a minimum wire bond loop height may be, for example, 100 ⁇ m -150 ⁇ m, defining the minimum possible vertical separation between the diode waveguide 20 'A of the infrared diode laser 20' and the input facet of the crystal waveguide 70 'A of the nonlinear optical crystal 70'.
• the air gap AG thermally insulates the non-linear optical crystal 70' from the diode laser 20', which acts as a heat source when operating. Air acts as a good thermal insulator, in particular compared to metals or many other solid materials, preventing heat from the diode laser 20' from reaching the non-linear optical crystal 70'. It is preferable to keep heat from reaching the non-linear optical crystal 70' because the heat can create thermal gradients in the non-linear optical crystal 70', thus negatively affecting the nonlinear conversion efficiency of the crystal waveguide 70 'A.
• a thermal gradient in the non-linear optical crystal 70' can be detrimental, because the temperature affects the refractive index of the crystal waveguide 70 'A within the non-linear optical crystal 70'.
• the wavelength dependence of the green output is a sin(x)/x function (with the exact shape depending on the uniformity of the crystal waveguide 70 'A), and thermal gradients distort this function. (Please note that the symbol x represents the deviations in the IR wavelength ⁇ from the optimal wavelength.)
• Figure 3 is a thermal model that illustrates how heat from the diode laser is conducted in a laser system configuration similar to that shown in Figure 2. More specifically, Figure 3 illustrates a fine element thermal model of the diode laser 20' and the cantilevered non-linear optical crystal 70' separated by the air gap AG. Although the diode laser 20' acts as a source of heat, the air gap thermally insulates it from the shown in Figure 3. The diode laser is supported by a metal package base. As shown in this figure almost all of the heat generated by the diode is conducted into the metal package base.
• this model illustrates that heat is conducted away efficiently by any metal contact, and does not pass from the diode laser 20' to the non-linear optical crystal 70', because of the relatively high thermal impedance of the air gap.
• Experimental data also demonstrated that, due to the presence of the air gap AG, the conversion efficiency of the non-linear optical crystal 70' was not degraded by thermal effects.
• the distance d between the two waveguides 70 'A, 20 'A should be kept as small as is practically possible, because a large distance requires that either the output facet of diode waveguide 20 'A, or the input facet of the crystal waveguide 70 'A of the non-linear optical crystal 70', or both, be substantially decentered (Y axis) with respect to the optical axis (Z axis). Generally the optical axis of the lens component 30 is situated midway between the two waveguides 70 'A, 20 'A.
• This provides both optical coupling of light between the two waveguides 70 'A, 20 'A, and also allows an active mirror 50 (if an active mirror is utilized) to be at the center of its actuation range, so that mirror tilt can be used to compensate for small motions of the waveguides. (These motions may be produced, for example, by temperature and humidity changes.)
• These aberrations include astigmatism, coma, and spherical aberration.
• Figure 4 is an illustrative example of how the coupling efficiency between the two waveguides 20 'A, 70 'A is reduced as the distance d (vertical distance, along the Y axis) between them is increased.
• distance d vertical distance, along the Y axis
• optical aberrations distort the beam, and coupled power between waveguides 20 'A, 70 'A becomes smaller.
• a lens component with a longer focal length will produce an image with less optical aberrations, when the image and the object are displaced by the same distance d', one way of minimizing these aberrations is to use a lens component with a long focal length.
• preferred waveguide separation distance d is larger than 50 ⁇ m, but smaller than 1500 ⁇ m and preferably smaller than 700 ⁇ m.
• distances d of 150 ⁇ m to 450 ⁇ m work well when the focal length of the length component 30 is about 1.5 mm.
• a lens component 30 with a slightly larger or smaller focal length f will work well when distance d is somewhat larger 450 ⁇ m or somewhat smaller than 150 ⁇ m).
• the minimum distance d is determined first by the ability to fit wire bond loops 23' between the diode laser 20' and non-linear optical crystal 70'.
• the crystal waveguide 70 'A may not be located at the outermost edge of the non-linear optical crystal 70', because typical non-linear optical crystal 70' has a "cap" layer 70 'B that can be a few microns to hundreds of microns thick.
• An exemplary cap layer 70 'B and the top layer 70 'C with the crystal waveguide 70 'A therebetween are illustrated schematically in Figures 5 A and 5B. Therefore, the minimum possible separation between the two waveguides is set by the thickness of the cap layer 70 'B (if present) plus the minimum distance needed to accommodate the wire bonds 23.
• the maximum waveguide distance d is determined primarily by the optical aberrations of the lens component 30, because the optical coupling between the two waveguides 20 'A and 70 'A will decrease as distance d increases.
• the non-linear optical crystal 70' need not be located above the diode laser 20'. Instead, the non-linear optical crystal 70' can be located to the side of the diode laser 20'.
• Such a "side- by- side” configuration is shown schematically in Figure 6. This configuration has the advantage of allowing a large amount of vertical space for the laser wire bonds 23. However, it generally requires a wider separation between the two waveguides, because the diode laser's structure has some inherent width (about 300 ⁇ m), and in addition, the crystal waveguide 70 'A may not be located at the edge of the non-linear optical crystal 70'.
• FIG. 7 illustrates coupling efficiency vs. waveguide spacing d of two different laser system configurations.
• the non-linear optical crystal 70' is placed over the laser 20 '(along the Y axis) as shown in Figure 2 (see curve CC) and in another configuration (S- S) the non-linear optical crystal 70' is placed adjacent to the laser 20' (along the X axis) as shown in the "side-by-side” configuration of Figure 6.
• the line with circles corresponds to the "side-by- side" configuration
• the line with rectangles corresponds to the cantilevered configuration.
• the side-by-side configuration of Figure 6 results in higher coupling efficiency at large separation distances d than the cantilevered configuration of Figure 2. Therefore, side-by-side mounting will allow a larger spacing d between the two waveguides, while achieving the same coupling efficiency.
• the separation d between the light source 20 and the non-linear optical crystal 70' in a "side by side" configuration is 30 ⁇ m ⁇ d ⁇ 1500 ⁇ m, more preferably 50 ⁇ m ⁇ d ⁇ 750 ⁇ m, even more preferably 50 ⁇ m ⁇ d ⁇ 500 ⁇ m and most preferably 350 ⁇ m ⁇ d ⁇ 500 ⁇ m.
• the air gap may be characterized by a distance d of 50 ⁇ m, 75 ⁇ m, 100 ⁇ m, 125 ⁇ m, 150 ⁇ m, 200 ⁇ m, 250 ⁇ m, 300 ⁇ m, 400 ⁇ m, or 450 ⁇ m.
• the coupling efficiency difference D of these two waveguides is 0.8% at 350 ⁇ m and at 450 ⁇ m of 2.4%.
• non linear crystal 70' may be advantageous to mount from its top surface, i.e. the surface that is furthest away from the waveguide and that corresponds to the top layer 70 'C of the non-linear optical crystal 70'.
• This is depicted in Figures 8 A (side view) and 8B (input end view).
• the advantage of this top mounting technique is that nonlinear optical crystals 70' of various cap thicknesses (the distance between the crystal waveguide and the bottom surface of the non-linear crystal) may be used interchangeably in the same laser system. Such interchangeability is advantageous because it allows the use of non-linear optical crystal 70' from different sources (vendors), which may have different manufacturing techniques and hence different cap thicknesses.
• the laser system 10 shown in Figures IB, 2, 6, 8A and 8B is designed such that the optical path length OPL between the light source 20 (the output facet of diode waveguide 20 'A) and receiver 70 (input facet of crystal waveguide 70 'A of the SHG crystal) has the same optical path length as the diode laser's cavity.
• the laser system 10 of Figures IB, 2, 6, 8A and 8B is designed to work in a coupled cavity condition, such that the cavity formed between the output facet of the diode waveguide 20 'A of the diode laser 20' and the input facet of the crystal waveguide 70 'A of the non-linear optical crystal 70' has the same optical path length as that of the diode laser's internal cavity.
• the optical path length through the diode waveguide 20'A of the diode laser 20' is 9.5 mm
• the optical path length through the laser system 10 should be 9.5 mm.
• the light source 20 is a diode laser
• the optical path length (OPL) from the light source 20 to the lens component 30, through the lens component 30, and to the reflector 50 is 1 A of the OPL through diode waveguide 20 'A.
• the lens component 30 preferably is used to both collimate the IR light provided by the diode laser 20', and to refocus the light into the crystal waveguide of non-linear optical crystal 70'.
• the lens component 30 is situated to provide magnification M of approximately 1:1.
• the lens component is situated to image the output facet of the diode waveguide 20 'A on the input facet of the crystal waveguide 70 'A at a magnification M and 0.9 ⁇ I M I ⁇ 1.1. More preferably 0.95 ⁇
• the lens component 30 has numerical aperture NA between about 0.35 and about 0.6, and a focal length f of 1 mm to 3 mm, a front worldng distance FWD of 0.3 mm to 3 mm and a back working distance BWD of 0.5 mm to 3 mm.
• the FWD is a distance along the optical axis from the light source 20 to the front surface Sl of the lens component 30 (i.e., the lens surface facing the light source).
• the BWD is the distance from the rear surface S2 of the lens component 30 to the reflector 50.
• the reflector 50 is located in the back focal plane of the lens component 30 which allows optimal optical coupling to be achieved when the direction of the average emission angle (beam centroid) of the light source 20 is parallel to the average beam angle on the receiver 70 (i.e., it is parallel to the centroid of the converging light cone intercepted by the input facet of the non-linear optical crystal 70').
• the lens component 30 is structured to provide a collimated beam such that the collimated beam is at an angle ⁇ ' (with respect to the normal to the reflector surface) such that: 0.05 RAD ⁇ ' is ⁇ 0.2RAD.
• the exemplary lens component 30 is structured to provide on the receiver an image of the light source, the image characterized by (i) astigmatism of more than 0.05 waves RMS, and less than 0.1 waves RMS 3 when the lens component's optical axis is not misaligned with respect to the laser system's axis (midline between the (facets) of two waveguides), or with respect to the average emission angle (beam centroid) of the light source; and (ii) astigmatism of less than 0.05 for tilt angles of 2 to 6 degrees, when the lens component is tilted by of 2 to 6 degrees with respect to the average emission angle of the light source.
• the RMS wave front error on the receiver 70 is ⁇ O.l ⁇ , where ⁇ is the central wavelength provided by the light source 20.
• the astigmatism may be created by: (i) wedge in the lens component, or (ii) decentration of one of the surfaces of the lens component relative to another, or (iii) by one of the surfaces being tilted relative to another.
• the lens component 30 of the embodiments described herein is preferably optimized to allow a relatively wide air gap AG between diode waveguide 20 'A and the crystal waveguide 70 'A, with minimal coupling penalty.
• the lens component 30 retains a high coupling efficiency even though the output facet of the diode waveguide 20 'A and the input facet of the crystal waveguide 70 'A are separated by a relatively large distance d. Because the optical path is folded, and only a single lens component 30 is utilized, the object (output facet of the diode waveguide 20 'A) and image (located at the input facet of the crystal waveguide 70 'A) are situated off the lens component's optical axis.
• the lens component 30 is preferably designed to have low astigmatism (e.g., between O.Ol ⁇ and O.l ⁇ , where is ⁇ the wavelength provided by the diode laser 20') in order to provide minimal distortion of the beam spot at the image plane for the off axis positioned waveguides 20 'A, 70 'A.
• a comparison of the coupling efficiency (CE) vs. LD-SHG vertical distance, achievable with various commercially available lens components is shown in Figure 9.
• a first exemplary lens component (lens #1) has lower astigmatism than the second exemplary component (lens #2), resulting in a greater tolerance to waveguide separation.
• Coupling curves for the first lens component were also calculated for two different lens-to-mirror distances (BWD) of 2 mm and 2.3mm).
• a short focal length lens component 30 is preferable in order to minimize laser system's length.
• the distance between the mirror 50 and the two waveguides 70 'A and 20 'A is approximately two focal lengths.
• a short focal length lens component 30 will have less defocus as a function of temperature than a longer focal length lens component.
• the change in the lens focal length f caused by temperature induced changes in refractive index of the lens component 30 is approximated by df dn f dT dT n ⁇ where /is the focal length, n is the nominal refractive index of the lens material, and dn/dT is the change in refractive index with temperature.
• the focal length f be less than 5 mm, more preferably 1 mm ⁇ f ⁇ 3 mm, and most preferably 1 mm ⁇ f ⁇ 2 mm. Lastly, it is preferable to select the lens material that has low dn/dT values.
• the exact choice of BWD is influenced by several other considerations.
• First consideration is the launch angle of the laser beam (average emission angle of the beam 22, or the angle of beam centroid) from the diode laser 20'. If the diode waveguide 20 'A has a non-flat cleave, then the emitted light can easily be launched a few degrees upward or downward away from the z-axis.
• the optimal BWD will be slightly different than one focal length, which enables the reflected beam enter the input facet of the diode waveguide 20'A at an optimum angle (e.g., normal to the input facet of crystal waveguide 70 'A).
• This is illustrated, for Example in Figure 1OA.
• this departure from the symmetry in the optical system results in tightening of the alignment tolerances for the placement of the two waveguides 70 'A, 20 'A, and in tightening of the positioning tolerances of the lens component 30.
• the laser system would retain symmetry and would be telecentric ( Figure 10B), resulting in looser tolerances for the positioning of the of the two waveguides 70 'A, 20 'A.
• This design simultaneously increases the amount of coupled light by ensuring the proper angle of incidence onto the input facet of crystal waveguide 70 'A, and widens the alignment tolerances of the optical system by making it telecentric.
• the second consideration in selecting the BWD is setting the optical path length of the cavity formed between the output facet of the diode laser and the input facet of the crystal waveguide 70 'A equal to that of the diode waveguide itself.
• the efficiency of the nonlinear conversion process is a sensitive function of IR laser wavelength (with the band width ⁇ being on the order of 0.2 nm). This makes the green output power of the laser system sensitive to small wavelength changes in the IR light provided by the diode laser 20'.
• the input facet of the crystal waveguide 70 'A is anti-reflection coated and angle cleaved to minimize reflections and hence feedback into the diode laser 20'. Even so, there can still be enough reflection and back scatter to influence the mode selection of the diode laser 20'. If this feedback changes as a function of time, through thermal changes of the cavities formed by any of the optical components of laser system 10, or other environmental change, then the diode laser 20'can experience mode hoping, and output power ( green light out put power) of the laser system will fluctuate.
• ⁇ is the laser wavelength (e.g. the IR wavelength of the diode laser) and L is the optical cavity length (e.g., of the diode waveguide), and n is the index of refraction inside the cavity (e.g., inside the diode laser cavity formed by the diode laser 20').
• the mode spacing of a 3 mm long InGaAs infrared diode laser 20' is approximately 0.06 nm. This means that in this example the required OPL between input facet of the crystal waveguide 70 'A and out put facet of diode waveguide 20' can be achieved by using a lens component 30 with focal length f of about 1.5 mm.
• Figure 12 illustrates the optical path length between the diode waveguide 20 'A and the waveguide crystal 70 'A as a function of BWD (lens to mirror spacing or distance), for two different exemplary lens components.
• the desired optical path length is 9.36mm, which matches the cavity mode spacing of the diode laser 20'.
• Figure 14 illustrates the lens component 30 shown in Figure 11.
• the lens component 30 of Figures 2 and 3 is optimized to provide RMS (root mean square) wave front error (WFE) of less than 0. l ⁇ for a ⁇ 200 ⁇ m field at the 1060 nm wavelength, over a numerical aperture NA of 0.4, and to have a combination of the focal length and thickness such that the optical path length between the light source and the receiver is 9.36 mm.
• WFE root mean square wave front error
• the radii of curvature (n, r 2 ), thickness Th (vertex to vertex), and aspheric coefficients of the lens component 30 are selected to advantageously:
• the lens component 30 has a front surface Sl and a rear surface S2.
• the front surface Sl is convex and aspheric with a radius of curvature x ⁇ .
• the rear surface is convex and aspheric with a radius of curvature r 2 such that ri > r 2
• c is the radius of curvature
• r is the radial distance from the lens component's center
• k conic coefficient
• Figure 15 illustrates the performance of the lens component 30 suitable for use in laser systems 10, and also the performance of two exemplary commercial aspherical lenses (1 and 2) typically used for coupling applications. As described above, the output facet of the waveguide of the infrared diode 20' and the front facet of the waveguide of the nonlinear optical crystal 70' are separated by a small distance d. Figure 15 illustrates that the lens component 30 has a higher coupling efficiency than two commercially available aspherical coupling lenses with similar focal lengths.
• the lens component 30 maintains about 90% of the maximum coupling efficiency or higher when output facet of the waveguide of the infrared diode 20' and the front facet of the waveguide of the nonlinear optical crystal 70' are separated by the distance d of up to 450 ⁇ m (0.45 mm), while the other two lenses maintained 90% of the maximum coupling efficiency for d values of only 350 ⁇ m and 215 ⁇ m respectively.
• lens component 30 maintains coupling efficiency of about 80% of the maximum coupling efficiency or higher when output facet of the waveguide of the infrared diode 20' and the front facet of the waveguide of the nonlinear optical crystal 70' are separated by the distance d of about 560 ⁇ m, while the other two lenses maintained 80% of the maximum coupling efficiency for d values of only about 360 ⁇ m and 270 ⁇ m respectively.
• Figure 16 illustrates another exemplary lens component 30 suitable for use in the laser systems 10.
• the lens component 30 of Figure 16 has the following characteristics:
• Figure 17 illustrates the lens component 30 suitable for use in the laser systems 10.
• the lens component 30 of Figures 2 and 3 has the following characteristics: (I) it allows the laser system to be in a coupled cavity condition (OPL between the diode laser and the non linear laser system equals that of the diode laser within +/-0.05 mm;
• a conventional way to optimize lens systems consists in putting all the optical elements in their nominal position and let the optical design software find a local minimum for a given optimization function. Also, in order to make the positioning tolerances of the optical components as large as possible, the usual optimization method consists of minimizing the aberrations in the intermediate spaces (i.e. between the optical components). That is, during typical optimization the lens designer tries to verify that, after each optical surface that provides optical power, the wave front is as close as possible to a perfect (spherical or plane ) wave front. This is usually done by including some constraints over the Seidel coefficients (aberrations) in the intermediate spaces (i.e., in spaces between different surfaces and between optical elements) into the optimization function.
• Figure 18 illustrates lens aberrations vs. lens tilt, and more specifically, the evolution of the total wave front error (WFE) as a function of the tilt of one exemplary lens component 30).
• WFE total wave front error
• the distance d between the diode laser and the PPLN crystal is kept constant (0.35mm) and the focus is adjusted for each value of the lens tilt.
• both the amplitudes of coma (C) and astigmatism (A) (which are the predominant aberrations) increase.
• Figure 20 illustrates the coupling efficiency calculated for both designs (lens design #1 and #2) versus the tilt of the lens and the tilt of the mirror respectively. As can be seen, tolerances are dramatically improved without any significant impact on the coupling when the components are at their nominal position.

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