WO2011140641A1

WO2011140641A1 - Packaging method of laser and nonlinear crystal and its application in diode pumped solid state lasers

Info

Publication number: WO2011140641A1
Application number: PCT/CA2011/000549
Authority: WO
Inventors: Ye Hu
Original assignee: Ye Hu
Priority date: 2010-05-11
Filing date: 2011-05-11
Publication date: 2011-11-17
Also published as: CN102893465A; CN102893465B

Abstract

The present invention is related to methods of packaging optical nonlinear crystal with a periodically domain inversion structure (e.g. periodically poled MgO doped lithium niobate), together with a laser crystal (e.g. Nd doped YVO₄) and to achieve low-cost compact efficient second harmonic generation in an intra-cavity configuration.

Description

1. FIELD OF THE INVENTION

The present invention relates to methods of packaging a laser crystal together with an optical nonlinear crystal based on the quasiphase matching (QPM) technique, which can be used to generate light in a wavelength range from UV to mid-IR.

2. DESCRIPTION OF THE RELATED ART

Many lasers can be built using the diode pumped solid state (DPSS) laser and the frequency doubling technologies, in which a laser crystal and nonlinear crystal are included. Typically, a pumping laser diode 1 (e.g. a semiconductor laser diode lasing at 808 nm). a GRIN lens 2, a laser crystal 3 (e.g. Nd doped YVO4), a nonlinear crystal 4 (i.e. frequency doubling component), and a coupling mirror 5 are required for a DPSS laser system, as shown in Fig.1. The facets of the laser crystal and the nonlinear crystal are properly coated with either high reflection (HR) or anti-reflection (AR) films so that the fundamental light is confined in the laser cavity while the SHG light is coupled out the laser cavity efficiently, as shown in Fig. 1(a). The nonlinear crystal acts as a second harmonic generator (SHG) or frequency doubler. To achieve efficient SHG, the phase matching condition has to be satisfied. The phase matching (PM) is usually achieved by selecting proper incident and polarization angle (i.e. phase matching angle) of the light beam into the nonlinear crystal so that n2<,,,_v» - iVa,_tf ~ 0 or Π2«_Λ- - η_ω.„ = 0, where ιΐ2_ω,₀, ii2<. -, n₍„.₀ and n₍, . are refractive indices at SH and fundamental light for ordinary and extraordinary beam, respectively.

The phase matching can also be achieved by employing the quasi-phase matching (QPM) technique, in which a periodical domain inversion grating is formed along the light propagation direction so as to satisfy the QPM condition. By pumping a laser crystal (i.e. Nd doped YV(¾) with a pump laser diode with a lasing wavelength of 808 nm, fundamental light of a wavelength λ (i.e. 1064 nm) is generated within a laser cavity. If the PM or QP condition is satisfied, a second harmonic light at a wavelength of λ 12 ( i.e. 532 nm) can be generated efficiently. The period of the domain inversion grating Λ is decided by the QPM condition (i.e. 2 (¾<„-! ) - λ/Α. where ιΐ2_ω and ti_m arc refractive indices at SH and fundamental light, respectively ).

To achieve efficient laser operation, surfaces of the laser crystals and nonlinear crystals are properly coated. Usually all the components are mounted on a copper holder 10 in order to efficiently remove the heal generated in the lasing process.

In fact, the above described technique using the bonded nonlinear crystal is wel l known and has been disclosed in a number of literatures, such as

The above mentioned laser system with discrete components, as shown in Fig.l(a), has the following disadvantages. First, there are too many components involved in the laser, which increases cost of the laser. Second, it is difficult to align the components (especially to achieve high parallelism of the input laser crystal and the output facet of the nonlinear crystal), and thus increases the laser assembling cost and reduces laser performance.

In fact, the above described technique using the bonded nonlinear crystal is well known and has been disclosed in a number of literatures, such as U.S. Patent No. 5,365,359, Nov.15, 1994. Mooradian, et al., Microchip laser; PCT/US94/09393, Aug.22, 1994. Hargis, et al., Deep blue microlaser; PCT EP97/05241 , Sept. 27, 1996. Nagele, et al, Frequency-doubling diode-pumped solid-state laser; T. Y. Fan, et al., "Efficient GaAIAs diode-laser-pumped operation of Nd:YLF at 1 .047 m with intracavity doubling to 532.6 nm", Optics Letters, vol. 1 1 , p.204 (1986); T. Ypkoyama, et al., "Compact and Highly Efficient Intracavity SHG Green Light Source with Wide Operation Temperature Range Using Periodically Poled Mg:LiNb03", The Review of Laser Engineering, Supplemental Volume, p.1046 (2008); A. Harada, et al., "Intracavity frequency doubling of a diode-pumped 946-nm Nd:YAG laser with bulk periodically poled MgO:LiNb03'\ Optics Letters, vol.22, p.805 (1997); D.Y. Shen, et al., "Efficient operation of an intracavity-doubled Nd:YV04 KTP laser end pumped by a high-brightness laser diode", Applied Optics, vol.37, p.7785 ( 1998); S. W. Chu, et al., "High-Efficiency Intracavity Continuous- Wave Green-Light Generation by QuasiphaseMatching in a Bulk Periodically Poled MgO:LiNb03 Crystal", Advances in OptoElectronics, vol. 2008, Article ID 151487.

To reduce size, cost, of the DPSS lasers and to increase the performance of the DPSS lasers, laser ciystal and nonlinear crystal employed in DPSS can be bonded together either through glue or optical contact, as shown in Fig.l (b). In this bonded structure, the laser crystal 3 (e.g. Nd doped YV0₄) and nonlinear crystal 4 (e.g. KTP or MgO:PPLN ) is bonded together, as shown in Fig.1 (b). To confine the fundamental light within the laser cavity, reduce coupling loss of pump power and couple SH light efficiently from the cav ity, the laser crystal 3 is coated with a film 6, which has HR at wavelengths of fundamental and SH light (e.g. 1064 nm and 532 nm) but AR at the wavelength of the pumping light (e.g. 808 nm), while nonlinear crystal 4 is coated with a film 9, which has HR at fundamental light (e.g. 1064 nm) and AR at SH light (e.g. 532 nm).

In fact, the above described technique using the bonded nonlinear crystal is well known and has been disclosed in a number of literatures, such as Moravian, et al, U.S. Patent No. 4,953, 166, Microchip laser. February 9. 1989: J. J. Zayhowski et al., "Diode-pumped passively Q-switched picosecond microchip lasers'^". Optics Letters, vol. 19, p. 1427 ( 1994); R. Fluck, et al, "Passively Q-switched 1.34-micron Nd:YV0₄ microchip laser with semiconductor saturable-absorber mirrors," Optics Letters, vol. 22, p. 991 (1997); U.S. Patent No. 5,295.146. March 1 5, 1 994. Gavrilovie, et al.. Solid state gain mediums for optically pumped monolithic laser; U.S. Patent No. 5,574, 740. August 23, 1994. Hargis, et al., Deep blue microlaser; U.S. Patent No. 5.802,086, September 1 , 1998. Hargis, et al., High-efficiency cavity doubling laser; U.S. Patent No. 7, 149,23 1 , December 12, 2006. Afzal, et al., Monolithic, side-pumped, passively Q-switehed solid-state laser; U.S. Patent No. 7,260, 1 33 , August 21 , 2007. Lei, et al.. Diode-pumped laser; U.S. Patent No. 7.535,937, May 1 9, 2009. Luo, et al., Monolithic microchip laser with intra-cavity beam combining and sum frequency or difference frequency mixing; U.S. Patent No. 7,535,938, May 19, 2009; Luo, et al.. Low-noise monolithic microchip lasers capable of producing wavelengths ranging from IR to UV based on efficient and cost-effective frequency conversion; U.S. Patent No. 7.570,676. August 4, 2009. Essaian, et al.. Compact efficient and robust ultraviolet solid-state laser sources based on nonlinear frequency conversion in periodically poled materials; USPC Class: 372 10, IPCS Class: AH01 S31 1FI, Essaian. et al.: R.F. Wu. et al., "High-power diffusion-bonded walk-off-compensated KTP OPO", Proc. SPIE, Vol. 4595, 1 1 5 (2001 ); Y.J. Ma, et al., "Single-longitudinal mode Nd:YV0₄ microchip laser with orthogonal-polarization bidirectional traveling- waves mode", 10 November 2008, Vol. 1 , No. 23, OPTICS EXPRESS 1 8702; C.S. Jung, et al., "A Compact Diode-Pumped Microchip Green Light Source with a Built-in Thermoelectric Element^'', Applied Physics Express 1 (2008) 062005. The bonding can be achieved by using either adhesive epoxy or the direct bonding technique. Since epoxy can be damaged at high optical power, the direct bonding or optical bonding technique has to be used for high power SHG lasers although the process of adhesive epoxy bonding is much easier than that of the direct bonding.

The bonded nonlinear crystal can be traditional nonlinear crystal such as KTP or periodically poled crystal such as PPLN. The laser employing the bonded nonlinear crystal can either based on second harmonic generation ( SHG) or sum frequency generation (SFG) or difference frequency generation ( DFG). Since nonlinear coefficient of KTP is much less than that of PPLN, it is preferred to use PPLN as a nonlinear crystal in the SHG lasers from laser efficiency point of view.

Advantages of the lasers with bonded crystals include simplified structure, low cost, and compact size. However there are some disadvantages for the bonded crystals such as reduced stability due to the heat transfer from the laser crystal to nonlinear crystal, limited output power due to the poor bonding condition and/or weak bonding strength of the surfaces (- 1 mW green for glue bonded crystals, - 100 mW for optical bonded crystals), and increased cavity loss due to the reflection at the bonded surface.

3. SUMMARY OF THE INVENTION

The objective of the present invention is to provide methods to overcome the problems involved in DPSS lasers including a nonlinear crystal and a laser crystal. In these methods, the laser crystal and nonlinear crystal are separated with each other and are bonded on a heat sink through a highly thermal conductive layer to remove the heat generated in the crystals, to prevent heat transfer from the laser crystal to the nonlinear crystal, to reduce packaging cost of the laser, to reduce cavity loss of the laser, and to increase stability of the laser.

According to one aspect of the present invention, as shown, in Fig.2, a laser crystal 1 and a nonlinear crystal 2 are first bonded with a heat sink 8. through a highly thermal conductive layer 9. The gap between the laser crystal 1 and the nonlinear crystal 2 is set properly depending on laser output power. The facets of the laser crystal and the nonlinear crystal are properly coated with either high reflection (H R) or ami-reflection ( AR) films 3-6 so that the fundamental light is confined in the laser cavity while the SHG light is coupled out the laser cavity efficiently. In the case of green DPSS lasers, film 3 has HR. at wavelengths of fundamental and SH light (e.g. 1064 nm and 532 nm) but AR at the wavelength of the pumping light (e.g. 808 nm). film 4 has AR at fundamental light (e.g. 1064 nm) and AR at SH light (e.g. 532 nm), film 5 has AR at fundamental light (e.g. 1064 nm) and AR at SH light (e.g. 532 nm), and film 6 has HR at fundamental light (e.g. 1064 nm) and AR at SH light (e.g. 532 nm). The second harmonic generation occurs only in the nonlinear crystal 2 in which the phase matching condition is satisfied. By pumping a laser crystal (i.e. Nd doped YV0 ) with a pump laser diode at a lasing wavelength of 808 nm, fundamental light of a wavelength λ (i.e. 1064 nm) is generated within a laser cavity. If the nonlinear crystal is selected properly so that the phase matching condition is satisfied, a second harmonic light at a wavelength of λ ,-2 (i.e. 532 nm) can be generated efficiently. 4. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detailed description given herein below, taken in conjunction with the accompanying drawings.

in the drawings:

FIG. l is a schematic drawing of a prior art of a DPSS SHG laser with (a) separated crystals; and (b) bonded crystals.

FIG.2 is a schematic diagram for explaining the concept of one method described in the first preferred embodiment to achieve a DPSS laser according to the present invention. FJG.3 is a schematic diagram for explaining the concept of one method described in the second preferred embodiment to achieve bonding of laser crystal and nonlinear crystal with a heat sink with high parallelism of input laser crystal input facet and output nonlinear crystal facet according to the present invention.

FI.G.4 is a schematic diagram for explaining the concept of one method described in the third preferred embodiment to achieve a DPSS laser according to the present invention.

FIG.5 is a schematic diagram for explaining the concept of one method described in the forth preferred embodiment to achieve a DPSS laser according to the present invention.

5. DAILED DESCRIPTION OF PREFFERED EMBODIMENTS

The present invention solves the foregoing problems by means described below.

In the first preferred embodiment, a bonding structure for DPSS lasers is shown in Fig.2. A laser crystal (e.g. Nd:YV0₄) 1 and a nonlinear crystal (e.g. MgO:PPLN) 2 are first bonded with a heat sink (e.g. A IN substrates) 8. The typical thickness of the laser crystal and nonlinear crystal can be used here (e.g. 0.5 mm), while the thickness of the AlN substrate is properly selected (e.g. 0.5 mm - 10 mm) so that the bonded crystals can be handled easily. The bonding between crystal 1 , 2 and AlN substrate 8 can be done using a long bar to reduce the overall manufacturing cost. The AlN substrate 8 has high thermal conductivity. The bonding between the laser crystal 1 and AlN substrate 8, and between nonlinear crystal 2 and AlN substrate 8 can be done through a highly thermal conductive layer (e.g. Indium film). To enhance bonding strength, all the bonding surfaces are coated with a layer of metal (e.g. gold). The input facet, of the laser crystal and the output surface of the nonlinear crystal are in parallel. The laser crystal and nonlinear crystal are properly coated with either high reflection (HR) or anti-reflection (AR) films 3-6 so that the fundamental light is confined in the laser cavity while the SHG light is coupled out the laser cavity efficiently. There are two types of coatings that can be used. As shown in Fig.2(a). in the first approach, film 3 has HR at wavelengths of fundamental (e.g. 1064 nm) but AR at the wavelength of the pumping light (e.g. 808 nm), film 4 has AR at fundamental light (e.g. 1064 nm) and HR at SH light (e.g. 532 nm), film 5 has AR at fundamental light (e.g. 1064 nm) and AR at SH light (e.g. 532 nm). and film 6 has HR at fundamental light (e.g. 1064 nm) and AR at SH light (e.g. 532 nm). On the other hand, as shown in Fig.2(b). in the second approach, film 3 has HR at wavelengths of fundamental and SH light (e.g. 1064 nm and 532 nm) but AR at the wavelength of the pumping light (e.g. 808 nm), film 4 has AR at fundamental light (e.g. 1064 nm) and AR at SH light (e.g. 532 nm), film 5 has AR at fundamental light (e.g. 1064 nm) and AR at SH light (e.g. 532 nm), and film 6 has HR at fundamental light (e.g. 1064 urn) and AR at SH light (e.g. 532 nm). The gap between the laser crystal and nonlinear crystal is properly selected between 0.01 mm and 10 mm so that heat transfer from the laser crystal to the nonlinear crystal is negligible while the gap is as smal l as possible. As a result, the gap is dependent on power of- the laser. The second harmonic generation occurs only in the nonlinear crystal 2 in which the QPM condition is satisfied. By pumping a laser crystal (i.e. Nd doped YV(¾) with a pump laser diode with a lasing wavelength of 808 nm, fundamental light of a wavelength λ (i .e. 1064 nm) is generated within a laser cavity. If the nonlinear crystal is selected properly so that the phase matching condition is satisfied, a second harmonic light at a wavelength of λ 12 (i.e. 532 nm) can be generated efficiently.

Based on the description above, it is easy to understand that the invented structure has the following advantages. First, the heat generated in the laser crystal and nonlinear crystal can be removed easily due to the high thermal conductivity of A1N heat sink and copper metal mount. Second, since there is an air gap between the laser crystal and nonlinear crystal, heat transfer from the laser crystal to the nonlinear crystal is suppressed and thus stability of the laser is improved. Third, the output facet 4 of the laser crystal and the input facet 5 of the nonlinear crystal are AR coated at fundamental light wavelength. As a result, cavity loss at the fundamental light is reduced and thus high fundamental l ight power can be expected within the cavity. This contributes high SHG efficiency of the laser. Forth, the bonding between the laser crystal, nonlinear crystal and heat sink is conducted in long bars. As a result, after dicing, many bonded chips (which include laser crystal, nonlinear crystal and heat sink) can be obtained with a single bonding process. This results signi ficant cost reduction on packaging.

In the second preferred embodiment of the present invention, laser crystal bar 1 and nonlinear crystal bar 2 is bonded with a heat sink 5 through a highly thermal conductive indium film 6. Spacers 3, 4 are inserted at each end of the laser crystal and nonlinear crystal bars. After bonding, the bonded laser crystal and nonlinear crystal bars are diced into small chips following the dotted lines as shown in Fig.3. The diced chips can then mounted, on a copper metal holder.

Based on the description above, it is easy to understand that the invented structure has the following advantages. First, the input facet of the laser crystal precisely in parallel with the output facet of the nonlinear crystal can be ensured, which is very important to achieve efficient laser and stable lasing. Second, precise control of the gap can be achieved between the laser crystal and nonlinear crystal by properly selecting spacers with proper thickness. Third, this invention makes it possible that multiple bonded laser ciystal and nonlinear crystal with parallel input and output facets in one single bonding process, and thus reduce manufacturing cost significantly.

In the third preferred embodiment of the present invention, a bonding structure for DPSS lasers is shown in Fig.4. The basic structure is similar to that shown in Fig.l, except that another highly thermal conductive heat sink 11 (e.g. A1N substrate) is used to bond the crystals from another side of the laser and nonlinear crystal through an Indium film 12. The thickness of the A1N can be selected between 0. 1 mm and 10 mm.

Based on the description above, it is easy to understand that the heat removing efficiency- can be enhanced but the packaging cost is slightly increased as compared with the structure shown in Fig.2. In the forth preferred embodiment of the present invention, a preferred bonding structure for DPSS lasers is shown in Fig.5. The basic structure is similar to that shown in Fig. I, except that another metal holder 1 1 (e.g. copper) is used to bond the oystals from another side of the laser and nonlinear crystal through an Indium film 12. The thickness of the copper holder can be selected between 0.5 mm and 50 mm.

Based on the description above, it is easy to understand that the heat removing efficiency can be enhanced. The packaging cost is slightly increased as compared with the structure shown in Fig.2, but is cheaper than that shown in Fig.4.

The above embodiments have described the bonded MgO:PPLN nonlinear crystal for green laser with the intra-cavity configuration. Of course, the methods described in the present invention can be applied to other bonded nonlinear crystals such as MgO:PPLT. PPKTP. etc.

The above embodiments have described SHG green laser with the bonded nonlinear crystal and the intra-cavity configuration. Of course, the methods described in the present invention can be applied to other SHG lasers such as SHG blue lasers, etc.

The above embodiments have deseribcd SHG lasers using the bonded nonlinear crystal. Of course, the methods described in the present invention can also be applied to other optical nonlinear processes such as optical parametric oscillation, difference frequency generation, etc.

Claims

CLAIMS What is claimed is :

1 . A method for packaging a laser crystal and an optical nonlinear crystal which are bonded on a highly thermal conductive heat sink and mounted on a metal holder to achieve efficient wavelength conversion in an intra-cavity configuration.

2. The laser crystal and nonlinear crystal in Claim 1 are first bonded on the heat sink with certain gap, in which the input facet of the laser crystal and the output facet of the nonlinear crystal are kept in parallel.

3. The bonded laser crystal and nonlinear crystal in Claim 2 are then bonded to a metal holder together with the heat sink.

4. The bonding surface of the laser crystal, nonlinear crystal, and heat sink in Claim 1 are deposited with a layer of metal before the bonding.

5. The laser crystal and the nonlinear crystal in Claim 1 are cut in a form of a long bar before bonding.

6. A spacer with the same thickness is inserted at each end of the bars in the bonding to ensure the parallelism of the input facet of the laser crystal and the output facet of the nonlinear crystal in Claim 2.

7. The two facets of the laser crystal and the nonlinear crystal in Claims 1 are properly coated so that the fundamental light is confined within a laser cavity, while the second harmonic light can be extracted efficiently from the output facet of the nonlinear crystal.

8. The input facet of the laser crystal in Claim 7 is high reflection (HR) coated at fundamental light wavelength and anti-reflection (AR) coated at the wavelength of the pumping light; the output facet of the laser crystal is AR coated at fundamental light and HR coated at SH light; while the input facet of the nonlinear crystal is AR coated at fundamental light and AR coated at SH light; the output facet of the nonlinear crystal is HR coated at fundamental light and AR coated at SH light.

9. The input facet of the laser crystal in Claim 7 is HR coated at both fundamental and SH light and AR coated at the wavelength of the pumping light; the output facet of the laser crystal is AR coated at both fundamental and SH light; while the input facet of the nonlinear crystal is AR coated at both fundamental and SH light; and the output facet of the nonlinear crystal is HR coated at fundamental light and AR coated at SH light.

10. The bonded laser crystal and nonlinear crystal in Claim 1 are then bonded to the second heat sink.

1 1. The bonded laser crystal and nonlinear crystal in Claim 1 are then bonded to the second metal holder.

12. The bonding surfaces of the laser crystal, nonlinear crystal, and the heat sink in Claim 10 are deposited with a layer of metal before the bonding.

13. The bonding surfaces of the laser crystal and nonlinear crystal in Claim 1 1 are deposited with a layer of metal before the bonding.

14. The bonding of the laser ciystal and nonlinear ciystal with the heat sink in Claim 1 and Claim 10 is achieved through either direct bonding of two metal films or highly thermal conductive film of a soft metal or epoxy.

15. The bonding of the heat sink with the metal holder in Claim 1 and Claim 1 1 is achieved through either direct bonding of two metal films or highly thermal conductive film of a soft metal or epoxy.