GB2353397A

GB2353397A - A quasi-phase matching (QPM) nonlinear optical material, and a solid state laser system using a QPM

Info

Publication number: GB2353397A
Application number: GB0017437A
Authority: GB
Inventors: Yen-Chieh Huang
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-07-16
Filing date: 2000-07-17
Publication date: 2001-02-21
Also published as: FR2796500A1; TW443644U; GB0017437D0; DE20011396U1

Abstract

A quasi phase matching (QPM) nonlinear optical material 201 comprises, a QPM single grating region 2011 with a first QPM grating period along a first direction. A QPM multiple grating region 2012 has plurality of inner QPM gratings 20121, 20122, 20123, arranged in parallel along a second direction perpendicular to the first direction. Each of the inner gratings 20121, 20122, 20123, has a corresponding QPM grating period different from the others along the first direction. The two QPM regions are monolithically integrated along the first direction. The first QPM region of the body can be either the single grating region 2011 or the multiple grating region 2012. The non linear optical material, may be a ferroelectric material which is one of LiNbO<SB>3</SB>, LiTaO<SB>3</SB>, KNbO<SB>3</SB>, KTP, RbTiOAsO<SB>4</SB> and RbTiOPO<SB>4</SB>. A solid state laser (Figure 4) including the non linear QPM material (201, Figure 4) is pumped by a laser source (301, Figure 4). In the laser system the non linear frequency conversion may include second harmonic generation (SHG), third harmonic generation (THG), difference frequency generation (DFG), sum frequency generation (SFG), optical parametric generation (OPG), optical parametric amplification (OPA), and optical parametric oscillation (OPO).

Description

2353397 Monolithically Integrated Quasi-phase-matching Nonlinear Optical

Materials for Serial and Parallel Laser Generation The present invention is related to a laser material design, and especially to a monolithically integrated quasi-phase-matching nonlinear optical material design for serial and parallel laser generation. It also relates to the laser sources utilizing such a material design.

Most conventional laser generation relies on quantum energy transitions and therefore produces radiation at fixed wavelengths or frequencies. Many applications require wavelength-tunable laser sources. For example, laser environmental sensing employs tunable laser sources for monitoring trace materials by using the laser spectroscopic technique. Also, the display technology needs tri-color (red, green, blue) sources in certain wavelength ranges. Although laser sources such as dye laser, free-electron laser, and certain nonlinearcrystal lasers are wavelength-tunable, they are not easily accessible due the issues on cost, size, efficiency, and complexity.

In recent years, nonlinear optics has made significant progress in tunable laser generation. The method involves the interaction of two input laser signals (in some cases, the two signals are divided from one signal) propagating co-linearly through a nonlinear optical material to generate a third, desired laser radiation at a different frequency. The second-order nonlinear frequency conversion includes second harmonic generation (SHG), difference frequency generation (DFG), sum frequency generation (SFG), optical parametric generation (OPG), optical parametric amplification (OPA), and optical parametric oscillation (OPO). Among all, the optical parametric generation produces the desired laser radiation by amplifying the vacuum noise and 2 usually requires a higher pump threshold. In the nonlinear frequency conversion process, when the desired laser signal is phase-matched to the other two laser signals, the intensity of the desired laser signal grows inside the nonlinear optical material along the laser propagation direction. If the three laser signals are not phase matched, the intensity of the desired laser signal oscillates in amplitude along the path of propagation. In the nonlinear optical materials, the distance within which the desired laser signal continues to grow is called the coherence length. Therefore in the non-phase-matched situation the maximum output energy of the desired laser signal can only be accumulated over one coherence distance. The phase-matching condition among the three signals is primarily determined by the dispersive characteristics of the material. Since temperature can change the dispersive characteristics of the material, varying the temperature of the nonlinear optical material is one of several ways of tuning the output wavelengths.

Recently quasi -phase-matching (QPM) nonlinear lasers have been invented to improve the efficiency of tunable laser generation. Fejer at al. has described this technique in "Quasi -phase-matc hed Second Harmonic Generation: Tuning and Tolerances," IEEE Journal of Quantum Electronics, vol. 28, 1992, pp. 2631-2654. Additional information can be found, for example, from U.S. Pat. No. 5,036,220, No. 5,800,767, No. 5,714,198, and No. 5,838,702. In this very scheme, the laser frequency of a pump laser is converted into another frequency in a nonlinear optical material with the second-order nonlinear coefficient reversing its sign every coherence length. The periodic sign change of the nonlinear coefficient ensures the phase matching or momentum conservation in the laser frequency conversion process, whiling allowing the choice of the largest nonlinear coefficient and thus 3 the highest conversion efficiency in that particular nonlinear material. The periodic sign change is typically done by periodically changing the spontaneous polarization direction of a ferroelectric nonlinear optical material. The spatial period of the alternating ferroelectric domain, along with the pump laser wavelength and the operating temperature, determines the desired laser wavelength. With a fixed pump laser wavelength and a ferroelectric domain grating period, the output laser wavelength can be tuned over a certain range by varying the temperature. Many ferroelectric nonlinear crystals are good quasi-phase-matching materials, such as LiNb03, LiTa03, KTiOP04 (KTP), RbTiOASO4.

(RTA), RbTiOP04, etc.

As shown in Fig. 1, a first kind of conventional QPM nonlinear optical material 101 only has a single grating period so that only one nonlinear optical effect may take place. See for example: "Quasi-phase- matching 1.064 tm-pumped Optical Parametric Oscillator in Bulk Periodically Poled LiNb03". published by Myers et al. in Optics Letters, Vol. 20 No. 1, January 1, 1995, pp. 52-54. Another kind of conventional nonlinear optical material 101 consists of two half parts with two different single grating periods integrated monolithically so that two nonlinear optical effects may take place successively. Rosenberg has described this kind of scheme in "2.5-W continuous-wave, 629-nm solid- state laser source," Optics Letters, Vol. 23 No. 1, February 1, 1998, pp. 207-209. Additional reference can be found in the U.S. Pat. No. 5,768,302 issued on June 16, 1998. However, monolithically integrated two single gratings are under the same temperature at a time. When varying the temperature, one may phase match the first grating but mismatch the second one. This difficulty prevents one from tuning the laser wavelength over a wide range.

4 It is therefore attempted by the applicant to resolve the above difficulty encountered in the prior art. This invention discloses a novel laser generation scheme, which allows successive nonlinear frequency conversions and a wide range of wavelength tuning.

An object of the present invention is to provide a nionolithically integrated quasi-phase-matching nonlinear optical material for serial and parallel laser frequency conversion.

Another object of the present invention is to provide a nonlinear optical material which can performs successive laser frequency conversion at two or more different temperatures.

The nonlinear optical material of the present invention includes a quasi-phase-matched (QPM) region having a single QPM grating period along a first direction, and a QPM region having plural QPM gratings arranged in parallel along a second direction. Preferably, the first direction and the second direction are perpendicular to each other.

These two QPM regions are monolithically fabricated on a single nonlinear optical material.

A further object of the present invention is to provide a solid-state laser system utilizing the monolithically integrated QPM nonlinear optical material discussed above.

The solid-state laser system of the present invention includes a QPM nonlinear optical material and a pump laser source. The QPM nonlinear optical material consists of a QPM single-grating region monolithically integrated with a QPM multiple-grating region with their grating vectors aligned along the laser propagation direction. The first QPM region, either the single-grating or the multiple-grating region, first converts the pump laser frequency into another, and the second QPM region continues to generate other laser frequencies different from those already in the first QPM region.

According to the present invention, the solid-state laser system further includes a temperature-contro I ling oven for changing the dispersive characteristics of the QPM device and thus extending the wavelength tuning range.

According to the present invention, the solid-state laser system further includes a resonance cavity, which is formed by two resonator mirrors surrounding the QPM material or by two optical coatings on the free ends of the QPM material.

The present invention may be best understood through the following description with reference to the accompanying drawings, in which:

Fig. I schematically shows a conventional QPM nonlinear optical material in a frequency conversion laser resonator; Fig. 2 schematically shows a first preferred embodiment of the nonlinear optical material according to the present invention; Fig. 3 schematically shows other preferred embodiments of the nonlinear optical material according to the present invention; - Fig. 4 schematically shows the first preferred embodiment of the solid-state laser system without any resonator mirrors; Fig. 5 schematically shows the second preferred embodiment of the solid-state laser system with resonator mirrors; Fig. 6 schematically shows the third preferred embodiment of the solid-state laser system with integrated resonator mirrors; and Fig. 7 schematically shows the fourth preferred embodiment of the solid-state laser system with a seeding laser signal.

Figure 2 schematically shows the nonlinear optical material 201 of the present invention. The nonlinear optical material 201 is preferably 6 made by a monolithically periodically poled ferroelectric crystal, such as LiNb03, LiTa03, KTiOP04, RbTiOAsO4 and RbTiOP04. The gratings can be grouped into two parts - one part is a single QPM grating region 2011 and the other is a multiple QPM grating region 2012. The single grating region 2011 has a single grating period along a first direction, the grating-vector direction or the laser propagating direction. The multiple-grating region 2012 has several inner QPM gratings 2012 1, 20122 20123 arranged in parallel along a second direction, the direction perpendicular to the first grating vector or to the laser propagation direction. The grating periods in the inner gratings of the multiple QPM grating region are different.

As Fig. 2 shows, the inner gratings 20121, 20122, 20123 in the multiplegrating region 2012 are arranged in parallel and are arranged perpendicular to the laser propagation direction. Between two neighboring inner gratings, there is usually, although not necessarily, a constant space 2013 separating the two QPM gratings. The laser generated in the first region can be further converted into other laser with different wavelengths in the second region or vice versa, as shown in Fig. 3(A). Fig. 3(B) shows another preferred nonlinear optical material according to the present invention. The material design in Fig. 3(B) can be considered to be two or more PM sections in Fig. 2 or Fig. 3(A) monolithically integrated together. It is to be understood that material design of the present invention can have various modifications and similar arrangements and should not be limited to the disclosed embodiments.

This invention can be best appreciated from the following two laser generation configurations. The first configuration, termed the parallel laser generation for the present invention, employs a pump laser with its 7 laser beam size fully covering the single-grating region and all the gratings in the multiple-grating region. This way, the single-grating region first generates a laser (say, from SHG) or two lasers (say, from OPG) via nonlinear frequency conversion, and the second grating region utilizes some or all the lasers in the single-grating region to further convert the lasers to other frequencies. Consequently all the QPM gratings in the second region provide lasers at different frequencies, yielding a very broad band laser radiation.

The second configuration, termed the serial laser generation for the 10 present invention, is to circumvent the temperature-tuning difficulty associated with two or more cascaded single gratings cited in U.S. Pat. No. 5,768,302. If the monolithic QPM device is formed by two single gratings in series, the phase matching conditions in both gratings might not be satisfied simultaneously when the temperature is tuned away from a certain pre-designed value. This limits the wavelength tuning range. In the second configuration of the present invention, the pump lasei- beal-n size is no more than the width of each QPM grating in the multiplegrating region. At some temperature T1, the pump laser samples the gratings 2011 in the single-grating region and 20121 in the multiple- grating region, and generates a desired laser wavelength at the OUtrUt. At another temperature T2, the pump laser may sample the gratings 2011 and 20122 to ensure the phase matching condition in both reions- for efficient laser generation at another laser wavelength. Likewise, at some temperature T3, a third laser wavelength can be generated with an appropriate combination of gratings from the single-grating region and the multiple-grating region. The alignment can be done by mechanically sliding the monolithic nonlinear optical element side way or deflecting 8 the pump laser beam into the channel having the right grating combination.

Figure 4 illustrates the first preferred embodiment of the solid-state laser system according to the present invention. The pumping laser source 301 provides the input laser signal with the first wavelength to pump the monolithic QPM device 201, and the optical element 601 or an optical system focuses- the pump laser signal and aligns the beam position. The monolithic QPM device of the present invention 201 performs the aforementioned parallel or serial laser frequency conversion and generates laser radiation at the desired wavelengths. The QPM device is preferably enclosed in a temperature-control oven 40 for changing the QPM characteristics of the material. Therefore one may extend the wavelength tuning range by varying the temperature. For the serial laser generation configuration, a precision pusher can be further installed to mechanically select the QPM gratings in the multiple-grating region.

Figure 5 schematically shows a second preferred embodiment of the solid-state laser system according to the present invention. This embodiment is an extension of the single-pass configuration in Fig. 4. A resonator, consisting of two resonator mirrors 501 and 502, circulates one or more of the laser signals to reduce the pump laser threshold and to enhance the overall laser conversion efficiency.

Figure 6 schematically shows a third preferred embodiment of the solid-state laser system according to the present invention. To reduce the size of the laser system, two optical coatings 503, 504 on the free ends of the material 201 replace the two resonator mirrors in Fig. 5. The optical coating may have arbitrary spectral reflectivities and radii of curvature, according to the system requirements.

9 Figure 7 schematically shows a fourth preferred embodiment of the solid-state laser system according to the present invention. In this example, the material 201 serves as an amplifier to provide amplification to the seed laser 303, while performing serial and parallel frequency conversions by using the pump laser 302. The dichroic, mirror 604 combines the seed laser 303 and the pump laser 302 before they enter the material 201. The optical elements or optical systems 602 and 603 focus the pump laser 302 and seed laser 303 respectively. The pump laser 302 and the seed laser 303 may exchange their locations, if necessary.

EXPERIMENT The invention is verified with the configuration in Fig. 2. Since all the examples in Figs. 3-7 have lower pump-threshold energies compared to that in Fig. 2, the experiment in Fig. 2 is representative and may provide a direct confirmation to those in Figs. 3-7. In other words, the configuration in Fig. 2 requires the highest pumping laser intensity, because multiple laser frequency conversions are completed in a single transit without any s-eeding signal or feedback mirror.

In the experiment, the monolithic QPM material is made by electrically poling a z-cut congruent lithium niobate wafer with a 500 Am thickness. The single-grating region is I cm long and the multiple grating region is 4 cm long. The single grating has a QPM period of 20 Am, which is suitable for the third-order second harmonic generation of the 1064-nm pump laser at 40.6'C. The multiple-grating region consists of five QPM gratings with their grating periods equal to I I Am, 11.25 Am, 11.50 tm, 11.75 Am, and 12 Am. When pumped by a 532 nm wavelength laser at the temperature 40.6'C, the 11 Am, 11.25 Am, 11.50 tm, 11.75 tm, and 12 tm gratings are suitable for generating paired photons via optical parametric process, with their wavelengths at [621.2 nm, 3704.7 nm], [615.3 nrn, 3828.8 nm], [609.4 nm, 4186.8 nm], [603.3 nrn, 4501.2 nm], [596.2 rim, 5941.0 nm] respectively.

The pump laser in the experiment is a passively Q-switched Nd:YAG laser, producing 8 P/pulse at a 8.3 kHz repetition rate. The pump wavelength is 1064 mn wavelength and the pump pulse width is about 600 psec. Since the peak power of the pump laser is more than 13 kilowatt, almost 100% of the pump energy is converted into the 5 3 2-nm second harmonic energy in the single-grating region. The 532-nm green laser continues to traverse the multiple-grating region and generate different-wavelength laser radiations from different QPM gratings through the optical parametric generation process. The generated laser pulse width is shortened to about 150 psec due to the nonlinear conversion mechanism. For different wavelengths at the output, the laser conversion efficiencies are different. In particular, for the 12-tm grating in the multiple-grating region, one of the paired photons has a 5329-nm wavelength that is already in the absorption band of lithium niobate. As a result, the pump threshold is higher and the com-ersion efficiency is lower for this particular grating period. However it the output of the I 1-tm grating, the 1064-nm laser to 618-nm laser conversion efficiency is approximately 5% with this single-pass, OPG configuration. For the examples in Figs. 3-7, the conversion efficiency can be significantly higher.

The broadband laser generation at multiple temperature settings is further verified in the following experiment. The monolithic QPM material is again made by a single-crystal lithium niobate wafer with a 500 ptm thickness. The single grating region is 3 cm long and has a I I grating period of 30 gm. The multiple-grating region is 2 cm long, consisting of two inner gratings with 19 trn and 19.25 Vtm periods. When pumped by the 1064-nm wavelength passively Q-switched laser, a 1555 nm laser is generated in the single-grating region at 25% conversion efficiency, if the QPM grating is kept at 60'C. At the same temperature, the 19 gm-period QPM grating in the multiple-grating region fur-ther frequency double the 1555-nm wavelength signal to a 777.5-nm wavelength signal at the output. When the oven temperature is raised to 100.2'C, the single-grating region generates 1570 nm- wavelength laser, which can be frequency-doubled to produce 785 nm laser if the 19.25 gm QPM grating in the multiple-grating region is moved into the pump laser path. With the 8 J/pulse pump laser, 10% overall photon efficiency is achieved for this laser up-conversion process. This experiment proves the usefulness of the present invention in generating tunable laser radiation.

12

Claims

CLAIMS:

1. A quasi-phase-matching (QPM) nonlinear optical material (201) having a material body comprising:

a QPM single-grating region (2011) with a first QPM grating period along a first direction; and a QPM multiple-grating region (2012) having a plurality of inner QPM gratings (20121, 20122, 20123) arranged in parallel along a second direction perpendicular to the first direction, wherein each of the inner gratings (20121, 20122, 20123) has a corresponding QPM grating period different from the others along the first direction; wherein the two QPM regions are monolithically integrated along the first direction, and a first QPM region of the body can be either the single-grating region (2011) or the multip le- grating region (2012).

2. The nonlinear optical material according to claim 1, characterized in that the monolithic material (201) is a ferroelectric material.

3. The nonlinear optical material according to claim 2, characterized in that the ferroelectric material is one of LiNb03, LiTa03, Li103, KNb03, KTP, RbTiOAS04 and RbTiOP04

4. A solid-state laser system comprising:

a monolithic quasi-phase-matching (QPM) nonlinear optical material (201) having a material body including a QPM,single grating region (2011) with a first grating period along a first direction and a QPM multiple-grating region (2012) having a plurality of inner gratings (20121, 20122, 20123) arranged in parallel along a second direction perpendicular to the first direction, wherein each of the inner gratings has a corresponding grating period along the first direction; and 13 a pump laser source (3 0 1) for providing a first laser signal with a first wavelength to the material (201) along the first direction; wherein a first QPM region of the body can be either the single grating region (2011) or the multiple-grating region (2012), and wherein the first QPM region converts the pump laser wavelength to others and the second QPM region further converts those wavelengths generated in the first QPM region to more other wavelengths.

5. The solid-state laser system according to claim 4 further comprising a temperature-control oven (40) for controlling the temperature of the material (20 1) such that more laser wavelengths can be generated.

6. The solid-state laser system according to claim 4 further comprising a resonance cavity surrounding the QPM nonlinear optical material (201) for lowering the pump laser threshold during wavelength conversion.

7. The solid-state la.ser system according to claim 6, characterized in that the resonance cavity is formed by a first mirror (501) and a second mirror (502) surrounding the material, with the two mirrors coated to resonate any of the wavelengths involved in the frequency conversion.

8. The solid-state laser system according to claim 6, characterized in that the resonance cavity is formed by two optical coatings (5.03, 504) on two free ends of the material, with the two coatings designed to resonate any of the wavelengths involved in the frequency conversion.

9. The solid-state laser system according to claim 4, characterized in that the nonlinear frequency conversion includes second harmonic generation (SHG), third harmonic generation (THG), difference 14 frequency generation (DFG), sum frequency generation (SFG), optical parametric generation (OPG), optical parametric amplification (OPA) and optical parametric oscillation (OPO).

1O.The solid-state laser system according to claim 4, characterized in that the pump laser source (301) is a passively Q-switched laser.

1 LA device substantially as hereinbefore described with reference to the accompanying drawings as shown.