WO2010127060A1

WO2010127060A1 - Folded lasers system

Info

Publication number: WO2010127060A1
Application number: PCT/US2010/032882
Authority: WO
Inventors: Etienne Almoric; Jacques Gollier; Lawrence C. Hughes Jr.; Garrett A Piech
Original assignee: Corning Incorporated
Priority date: 2009-04-30
Filing date: 2010-04-29
Publication date: 2010-11-04
Also published as: KR20120024682A; JP2012525610A; TW201110489A; CN102422494A; CN102422494B

Abstract

A folded laser system having an optical axis, the laser system comprising: (I) a coherent light source; (II) a reflector; (III) a lens component situated between the light source and the reflector; and (IV) a non-linear optical crystal, wherein the light source and the non-linear optical crystal are separated by a distance d>50µm. 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 Θ' to the optical axis, the reflector is situated to intercept the collimated beam and to reflect the collimated beam to the non-linear optical crystal through the lens; and the lens component is structured to provide an image on the non-linear optical crystal.

Description

FOLDED LASERS SYSTEM BACKGROUND

[0001] 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.

Technical Background

[0002] Generation of green laser light can be achieved by non-linear frequency doubling of infrared light. Typically, as illustrated in Figure IA, 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.

[0003] The practical challenges in the fabrication of this type of a laser arise from a number of issues. First, because of the small optical waveguides used to confine the light in both the diode laser and the nonlinear optical crystal, alignment tolerances for the components (lens, the nonlinear crystal and the diode laser) are on the order of a few tenths of microns. This presents a challenge both for initial assembly of the laser and for mamtaining alignment over the laser's lifetime. Second, the output power from the nonlinear optical crystal is sensitive to fluctuations of temperature and to shifts in the infrared optical wavelength provided by the laser. Temperature gradients across the nonlinear optical crystal may cause a reduction in output power of the green laser (i.e., in the output power exiting the nonlinear optical crystal).

SUMMARY OF THE INVENTION

[0004] 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>50μm. 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 Θ' 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. [0005] Preferably, the coherent light source and the non-linear optical crystal are separated by an air gap.

[0006] According to some embodiments the laser system is a green laser, the light source is an infra red (ER.) diode laser, and the receiver is a non linear optical crystal, for example SHG (second harmonic generator) for converting IR light to the green light. [0007] 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.

[0008] Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. [0009] It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure IA illustrates a prior art laser system;

[0011] Figure IB schematically illustrates schematically a folded laser system according to one embodiment of the present invention;

[0012] Figure 2 is a folded cavity green laser system according to one embodiment of the present invention;

[0013] Figure 3 is a thermal model that illustrates heat conduction between the diode laser and the nonlinear crystal of Fig. 2; [0014] 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;

[0015] Figure 5 A illustrates a cross-sectional side view of an exemplary non-linear crystal;

[0016] Figure 5B illustrates a cross-sectional end view of the exemplary non-linear crystal of Fig. 5 A;

[0017] 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;

[0018] Figure 7 illustrates coupling efficiency vs. waveguide spacing d of two different laser system configurations;

[0019] Figures 8A and 8B illustrate a nonlinear optical crystal mounted over a diode laser in yet another embodiment of the present invention;

[0020] 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;

[0021] Figures 1OA and 1OB illustrate schematically two exemplary embodiments folded cavity green laser system;

[0022] 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;

[0023] Figure 12 is a plot of an optical path length vs. back working distance (BWD) for two exemplary lens components;

[0024] Figure 13 is a plot of coupling efficiency vs. BWD;

[0025] Figure 14 is a is a cross-sectional view of a lens component according to yet another embodiment of the present invention;

[0026] 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;

[0027] Figure 16 is a is a cross-sectional view of a lens component according to another embodiment of the present invention;

[0028] Figure 17 is a is a cross-sectional view of a lens component according to yet another embodiment of the present invention;

[0029] Figure 18 illustrates the evolution of aberrations (wave front error) as a function of the tilt of an exemplary lens component; [0030] Figure 19 illustrates the evolution of aberrations (wave front error) as a function of the tilt of an exemplary lens component; and

[0031] Figure 20 is a plot of coupling efficiency as a function of the tilt of two exemplary lens components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Reference will now be made in detail to the present preferred embodiment(s) of the invention, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. One exemplary embodiment of the laser system of the present invention is shown in Figures IB and 2, and is designated generally throughout by the reference numeral 10.

[0033] The folded laser system 10 in this exemplary embodiment is a frequency doubled green laser that has a folded cavity configuration. In the laser system 10 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. It is preferable that coherent light source 20 be small (<lcm³), of relatively high power (>10mW) and is modulated at high rates (about lOMHz or higher). In this embodiment 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 Θ at 1/e², for example 20° in one direction and 7° in the other (perpendicular) direction. The emission half angle Θ 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 Θ' and is then reflected from the reflector 50 back toward lens component 30. Preferably, according to some embodiments, 0.05 Rad≤ Θ'<0.2 Rad, and more preferably 0.09 Rad≤ Θ'<0.17 Rad. 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'. That is, 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'. [0034] The non-linear optical crystal 70 'may be, for example, a second harmonic generator (SHG) such as a periodically poled lithium niobate (PPLN) crystal. 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. [0035] Preferably, 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.

[0036] 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. 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). Thus, 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'.

[0037] In tliis example, both the light source 20 and the receiver (non-linear optical crystal 70') 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 μm, preferably within ±50 μm) with respect to the optical axis. More specifically, in order to minimize aberrations of the light beam at the input facet of the crystal waveguide 70 'A of the nonlinear optical crystal 70', 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). Preferably, 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. Preferably, the focal length f of the lens 30 is 1 to 5 mm (lmm≤ f < 5 mm). Preferably the separation d between the light source 20 and the non-linear optical crystal 70' is 30 μm < d < 1500 μm, more preferably 50 μm < d < 750 μm, more preferably 100 μm < d < 600 μm, even more preferably 150 μm < d < 500 μm and most preferably 300 μm < d < 500 μm. For example, the distance d may be 75 μm, 100 μm, 125 μm, 150 μm, 200 μm, 250 μm, 300 μm, 400 μm, or 450 μm. Thus, in this embodiment, the light source 20 (diode laser 20') and the receiver 70 (non-linear optical crystal optical crystal 70') are decentered along the Y axis, with respect to the optical axis, by a distance d'~d/2, for example by a distance d'= d/2±100μm. Preferably the decenter distance d' equals d/2 or is within 50μm of d/2 (i.e., d'= d/2 ± 50μm).

[0038] The folded laser system configuration described herein (for example, see Figures IB, 2, 6, 8 A and 8B) 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'. Given stable and accurate attachment techniques, the laser system 10 may be completely passive (i.e., it may include no moving components). (Such design is illustrated schematically in Figure IB). Alternatively, as described above, 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.

[0039] Practical realization of folded configurations creates a number of challenges. First, because 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. Second, 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. Third, 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. These embodiments 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). Finally, 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'. [0040] 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.

[0041] First, 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 μm -150 μm, 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'.

[0042] Second, 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. More specifically, 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'. In general, 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 λ from the optimal wavelength.)

[0043] 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. That is, while the exact thermal conditions will depend on the materials and specific design, 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.

[0044] Third, 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.) The further either one of the two waveguides is located off the optical axis of the lens, the more optical aberrations will be introduced into the focused spot at the input facet of the crystal waveguide 70 'A of the non-linear optical crystal 70'. 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. As distance d increases, optical aberrations distort the beam, and coupled power between waveguides 20 'A, 70 'A becomes smaller. Because 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. However, we seek to keep the laser package size to a minimum, which dictates that we should use a lens component with the shortest focal length possible. For example the lens component 30 may have a focal length f of about 1.3-1.7 mm (e.g., f=1.5 mm). That is, it is preferable that the lens component 30 has the short focal length, and provides minimum amount of aberrations at the input facet of the crystal waveguide 70 'A, and that the laser system 10 has a high coupling efficiency determined the optimal center-to- center separation d between the two waveguides 20 'A and 70 'A. (Please note that in Figure 4 the peak coupling at d= 110 μm rather at zero (no separation) is due to the angled emission of the diode laser 20.)

[0045] We have determined that preferred waveguide separation distance d is larger than 50 μm, but smaller than 1500 μm and preferably smaller than 700 μm. For example, distances d of 150 μm to 450 μm 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 μm or somewhat smaller than 150 μm). 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'. In addition, 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. For example, if 150 μm height is needed for wire bond loops 23', and if the non-linear optical crystal 70 ' has a 200 μm thick cap layer 70 'B, then the minimum possible waveguide separation distance d (center to center) is 350 μm (200 μm + 150 μm=350 μm). 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.

[0046] Alternatively, 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 μm), and in addition, the crystal waveguide 70 'A may not be located at the edge of the non-linear optical crystal 70'. When utilizing Figure 6 configuration, in order to prevent thermal crosstalk, separation should be made between the diode laser and the non-linear crystal to provide for an air gap AG between them. This "side-by- side" configuration is very similar to that shown in Figure 2, except that in this embodiment the locations of diode laser 20' and the non-linear optical crystal 70' are rotated by 90 degrees so the separation is a horizontal one (X axis), rather than vertical. The small air gap AG is used to ensure thermal isolation between the diode laser 20' and the non-linear optical crystal 70'. The exemplary green laser system 10 shown in Figure

6 also has the advantage of making the system work along the low numerical aperture or horizontal direction of the diode laser (i.e., the beam 22 has less divergence along the x-axis than along the y axis), such that the coma aberration will degrade the optical coupling more slowly than along the vertical axis. This is illustrated in Figure 7. More specifically, Figure

7 illustrates coupling efficiency vs. waveguide spacing d of two different laser system configurations. In one configuration 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. Because the numerical aperture of the diode laser beam 22 is smaller (less divergent) in horizontal direction, 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. Preferably the separation d between the light source 20 and the non-linear optical crystal 70' in a "side by side" configuration is 30 μm < d < 1500 μm, more preferably 50 μm < d < 750 μm, even more preferably 50 μm < d < 500 μm and most preferably 350 μm < d < 500 μm. For example, the air gap may be characterized by a distance d of 50 μm, 75μm, 100 μm, 125 μm, 150 μm, 200 μm, 250 μm, 300 μm, 400 μm, or 450 μm. The coupling efficiency difference D of these two waveguides is 0.8% at 350 μm and at 450 μm of 2.4%.

[0047] Furthermore, it may be advantageous to mount the non linear crystal 70' 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. As long as the distance d between the top of non-linear optical crystal 70' and the diode laser 20' is unchanged, the separation between diode waveguide 20 'A of the diode laser 20' and the crystal waveguide 70 'A of the non-linear optical crystal 70' will remain constant. This type of top mounting technique may also be applied to the side-by-side mounting configuration shown in Figure 6, with the crystal mounting surface being the one furthest away from the diode laser. [0048] 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. (OPL=Di x Nj , where Di is a distance between surfaces of different components, and Nj is the index of refraction of between these surfaces.) That is, 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. Thus, for example, if the optical path length through the diode waveguide 20'A of the diode laser 20' is 9.5 mm, than the optical path length through the laser system 10 (from the light source to the receiver) should be 9.5 mm. Thus, preferably, if the light source 20 is a diode laser, than 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 ¹A of the OPL through diode waveguide 20 'A. The advantage of this configuration is to minimize laser wavelength instability created by the parasitic reflections from the input facet of crystal waveguide 70 'A of the nonlinear crystal 70'.

[0049] 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. Preferably, 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 < | M | < 1.05. Preferably, 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. Preferably, 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').

[0050] Preferably, if the light source 20 provides a diverging beam with a maximum half angle Θ, and the reflector 50 is located in the back focal plane of the lens component 30, such that the direction of the average emission angle of the light source is parallel to the average beam angle on the receiver. Preferably, when a decentered light source is located in a focal plane of the lens component, and up to 750 μm off axis, the lens component 30 is structured to provide a collimated beam such that the collimated beam is at an angle Θ' (with respect to the normal to the reflector surface) such that: 0.05 RAD <Θ' is < 0.2RAD. [0051] 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 RMS₃ 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. Thus, advantageously, even if the lens component 30 is misaligned (e.g., slightly tilted or decentered) during assembly of the laser system 10, the RMS wave front error on the receiver 70 is ≤O.lλ, where λ is the central wavelength provided by the light source 20. [0052] It is noted that 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. [0053] 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. That is, 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. As discussed above, the lens component 30 is preferably designed to have low astigmatism (e.g., between O.Olλ and O.lλ, where is λ 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).

[0054] hi addition, 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). Furthermore, a short focal length lens component 30 will have less defocus as a function of temperature than a longer focal length lens component. To a first approximation, 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. Thus, shorter focal length lens components will provide less movement in the focal position (smaller df/dT). Therefore, it is preferable that 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.

[0055] While the approximate spacing between the lens component 30 and mirror 50 (i.e., the back working distance BWD) is about one focal length, 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. This means that 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. However, 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. If the launch angle of the laser beam 22 was instead parallel to the optical axis, then 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. Hence it is advantageous to physically mount the diode laser 20' such that the emitted IR beam 22 is parallel to the lens optical axis of the lens component 30. This can be done by mounting the diode laser 20' at an angle θ as is shown in Figure 11. In this embodiment, the mounting angle θ = 3.3° and the cap layer is 200 μm thick. 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. [0056] 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 mode spacing of the cavity formed by back reflections from the crystal itself. The efficiency of the nonlinear conversion process is a sensitive function of IR laser wavelength (with the band width Δλ 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'. Because diode lasers are extremely sensitive to even minute amounts of feedback, 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. One way of minimizing the impact of these changes is to ensure that the external cavity (formed by the input facet of the crystal waveguide 70 'A and output facet of diode waveguide 20 'A) is approximately the same free spectral range as that of the diode laser itself. The mode spacing, or the spectral range of the an optical cavity is determined by the following equation:

^{FSR "} 2nL where, λ 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'). For example, 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.

[0057] 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 13 illustrates the coupling efficiency as a function of BWD for the same two exemplary lens components. As is shown in Figure 13, only small adjustments (hundreds of microns or less) of the BWD away from the optimal coupling distance of are needed to create the optimal optical path length. For example, Figure 13 illustrates spacings that provides OPL=9.36 mm.

Examples

[0058] The invention will be further clarified by the following examples.

EXAMPLE 1

[0059] Figure 14 illustrates the lens component 30 shown in Figure 11. hi this exemplary embodiment 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. lλ for a ±200 μm 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.

[0060] The radii of curvature (n, r₂), thickness Th (vertex to vertex), and aspheric coefficients of the lens component 30 are selected to advantageously:

1. minimize coma and astigmatism, (the 2 worst aberrations for system performance);

2. obtain a large field of view: low field aberration combined to a large aperture (e.g., N A=O.4), such that the laser system has good coupling efficiency for separation of 400 μm (d=350 μm, ±25 μm) between the waveguide portion of diode laser 20' and the waveguide portion of non-linear optical crystal 70'; and

3. provide proper combination of focal length and thickness to allow the laser system 10 to work in coupled cavity condition, such that the cavity formed between the output facet of the diode laser 20' and the input facet of the non-linear optical crystal 70' (e.g., second harmonic generating (SHG) crystal) has the same optical path length as that of the diode laser cavity.

[0061] As described above, the lens component 30 has a front surface Sl and a rear surface S2. Preferably, the front surface Sl is convex and aspheric with a radius of curvature x_\. Preferably, the rear surface is convex and aspheric with a radius of curvature r₂ such that ri > r₂

[0062] The lens component 30 of Figure 14 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 optical crystal equals that of the diode laser within +/-0.05 mm): OPL= (0.9 mm + 1.744 mm xl.5 + 1.18 mm) x 2 = 9.39 mm; and (II) has the following parameters: (i) FWD = 0.90 mm; (ii) thickness T (vertex to vertex) of 1.74 mm; (iii) focal length: f = 1.76 mm; (iv) glass index of refraction N at 1060 nm is 1.5; (v) effective diameter of the front surface Sl is 1.4 mm; (v) effective diameter of the rear surface S2 is 2 mm; (vi) NA = 0.61; (vii) outer diameter of the lens component is 2.5 mm to 3 mm. [0063] The surface sag of the surfaces si and s2 is given by the following equation:

C* ^ γ z = + αlx r² + α2x r⁴ + α3x r⁶ + α4x r⁸ + ...

where c is the radius of curvature, r is the radial distance from the lens component's center and k = conic coefficient.

[0064] The surface parameters of the lens component 30 of Figure 14 are given in Table 1 below.

Table 1

[0065] 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. For example, 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 μm (0.45 mm), while the other two lenses maintained 90% of the maximum coupling efficiency for d values of only 350 μm and 215 μm respectively. Similarly, 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 μm, while the other two lenses maintained 80% of the maximum coupling efficiency for d values of only about 360 μm and 270 μm respectively.

EXAMPLE 2

[0066] 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:

(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;

(II) and has the following parameters: (i) FWD = 0.568 mm; (ii) thickness Th (vertex to vertex) of 1.82 mm; (iii) focal length: f = 1.4 mm; (iv) glass index of refraction N at 1060 nm is 1.784; (v) NA = 0.4.

[0067] The surface parameters of the lens component 30 of Figure 16 are given in Table 2 below.

Table 2

EXAMPLE 3

[0068] 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;

(II) and has the following parameters: (i) FWD = 1.01 mm; (ii) thickness Th (vertex to vertex) of 1.578 mm; (iii) focal length: f = 1.789 mm; (iv) glass index of refraction N at 1060 nm is 1.5; (v) NA = 0.4.

[0069] The surface parameters of the lens component 30 of Figure 17 are given in Table 3 below.

Table 3

Aspherical Parameters

Sag

Sag = C h ^Λ2 / (l+( ( l-(l+K)^χC^A2^χh^A2))^Λ0.5) +A4h^Λ4+A6h^Λ6+ ^{■ • ■} +A16h^Λ16 h; radius

Lens Component optimization

[0070] 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. [0071] By applying this method to the folded configuration, we obtained excellent results with diode laser to PPLN crystal distances d of up to 0.5 mm. Unfortunately, all the designs that we obtained using this type of optimization had very tight manufacturing and assembly positioning tolerances, the most critical being probably the tilt of the lens component, which was limited to about 1 degree or less.

[0072] The impact of the slight tilt on either the lens component 30 or the mirror 50 is mostly to introduce some coma and astigmatism, both of which contribute to a coupling degradation (less efficient coupling between the diode laser and the non linear optical crystal). [0073] 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). In this calculation, 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. As the tilt increases, both the amplitudes of coma (C) and astigmatism (A) (which are the predominant aberrations) increase. The consequence is that the wave front error is very small when there is not lens tilted, but it rapidly degrades as the tilt increases. [0074] In order to relax the tolerances, we tried another optimization method. The lens component that resulted from that optimization is very similar to the previous one in term of shape and aspherization. However, tolerance analysis indicates that tolerance on lens tilt, mirror angle, are relaxed by factor of 5. In order to understand where that relaxation is coming from, we calculated again the variation of the aberrations as a function of the tilt of the lens. Figure 19 also illustrates lens aberrations vs. lens tilt. As can be seen on Figure 19, the astigmatism curve (A) is not at a rninimum when the tilt is at zero and actually decreases as the tilt of the lens increases. This basically means that, when all the components are at their nominal position without tilt, the design presents some residual astigmatism that compensates the one that appears when the tilt of the lens increases.

[0075] The consequence is that the total aberrations remain relatively flat over a wide range of angular tilt of the lens. In other words, the nominal design can accommodate a much larger range of positioning tolerances. [0076] 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.

[0077] This analysis shows that positioning tolerances can be dramatically improved by introducing some residual astigmatism into the design. That astigmatism compensates the one that is generated by the components misalignments which makes the system much more forgiving to positioning tolerances.

[0078] It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. A folded laser system having an optical axis, said laser system comprising:

(I) a coherent light source;

(II) a reflector; and

(EH) a lens component situated between the light source and the reflector; (TV) a non-linear optical crystal, wherein the light source and the non-linear optical crystal are separated by a distance d>50μm and an air gap; wherein: (a) 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 Θ' to the optical axis, (b) the reflector is situated to intercept the collimated beam and to reflect the collimated beam to the non-linear optical crystal through the lens; and; and (c) the lens component is structured to provide an image of the coherent light source on the non-linear optical crystal.

2. A folded laser system having an optical axis, said laser system comprising:

(I) a coherent light source;

(II) a reflector; and

(HI) a lens component situated between the light source and the reflector;

(IV) a non-linear optical crystal, wherein the light source and the non-linear optical crystal are situated substantially symmetrically with respect to the optical axis, and are separated by a distance d>50μm and an air gap; wherein: (a) 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 Θ' to the optical axis, (b) the reflector is situated to intercept the collimated beam and to reflect the collimated beam to the non-linear optical crystal through the lens; and; and (c) the lens component is structured to provide an image of the coherent light source on the non-linear optical crystal.

3. The laser system according to claim 2, wherein the coherent light source is a diode laser, and the non-linear optical crystal and the diode laser are tilted with respect to one another.

4. The laser system according to claim 1, wherein the non-linear crystal is cantilevered over the diode.

5. The laser system according to claim 2, wherein the coherent light source is a diode laser and the non linear crystal is held using the face of the crystal that is furthest away from the diode laser.

6. The laser system according to claim 2, wherein d <1500 μm.

7. The laser system according to claim 6, wherein d < 500 μm.

8. The laser system according to claim 7, wherein 150 μm < d < 500 μm.

9. The laser system according to claim 2, wherein said laser system is characterised by astigmatism of more than 0.05 waves RMS, and less than 0.1 waves RMS.

10. The laser system according to claim 2, wherein the reflector is located in the focal plane of the lens component, such that the direction of the average emission angle of the light source is parallel to the average beam angle on the receiver.

11. The laser system of claim 1 wherein said lens component is situated to image at a magnification M and 0.9 < | M | < 1.1.

12. The laser system of claim 2, wherein said light source is a diode laser, and the OPD from the receiver to the diode laser essentially equals OPD inside the diode laser.

13. The laser system of claim 12, wherein optical path distance OPD is: less than 10 mm.

14. The laser system according to claim 2, said lens component is a bi-aspheric singlet with a numerical aperture NA of 0.35 to 0.6; and focal length f, whereinl mm < f < 3 mm.

15. The laser system according to claim 1, wherein the front working distance FWD is 0.3 mm to 3 mm.

16. The laser system according to claim 2, wherein the back working distance BWD is 1.5 mm to 3 mm.

17. The laser system of claim 2, wherein the light from said light source traverses the lens component twice before it reaches said receiver.

18. The laser system of claim 2, wherein

(a) the lens component is positioned to provide a collimated beam when intercepting light from the light source; and

(b) the reflector is situated to intercept the collimated beam and to reflect the collimated beam to the non-linear optical crystal through the lens; and such that the collimated beam is at an angle Θ' to the optical axis wherein angle Θ' is: 0.05 RAD <Θ is < 0.2RAD, when the light source is located in a focal plane of said lens, and is decentered from the optical axis of the lens component by a distance d' <750 μm.

19. The laser system according to claim 18, wherein said angle Θ' is: 0.09 RAD <Θ' is < 0.17 RAD.

20. The laser system according to claim 2, said lens component having

(a) a front surface and a rear surface, the front surface is convex and aspheric with a radius of curvature n , the rear surface is convex and aspheric with a radius of curvature r₂

J ;

(b) NA of 0.35 to 0.5; and

(c) focal length of 1 mm to 3 mm.

21. The laser system according to claim 2, wherein the reflector is located in the image focal plane of the lens component, such that the direction of the average emission angle of the light source is parallel to the average beam angle on the receiver.

22. The laser system according to claim 1, wherein the diode laser is mounted at angle that compensates for the cleave angle of the diode, such that centroid of the emitted beam from the laser diode is parallel to that of the lens component's optical axis.