Spectralus' Periodically Poled Lithium Niobate (PPLN, PPMgOLN) and Tantalate (PPLT) Technology

Periodically-poled Lithium Niobate (Tantalate) Crystalline Components for Generation of Blue-Green Light from InGaAs/GaAs Diode and DPSS Lasers.

Since the invention of the first laser many years ago, the frequency conversion of laser radiation by nonlinear optical crystals has become an important technique widely used in quantum electronics and laser physics for solving various scientific and engineering problems. The fundamental physics of three-wave light interactions in nonlinear optical crystals is now reasonably well understood. This has enabled the production of various harmonic generators, sum- and difference- frequency generators, and optical parametric oscillators based on the nonlinear optical crystals that are now commercially available. At the same time, scientists continue an active search for improved nonlinear optical materials.

In early 1980’s it was recognized that in ferroelectric materials, optical second harmonic generation (SHG) efficiency can be dramatically enhanced by devising of the periodically poled domain (PPD) based quasi-phase matching (QPM). In recent years congruent and stoichiometric Lithium Niobate (LN), Lithium Tantalate (LT), and Magnesium Oxide doped LN and LT (MgO:LN and MgO:LN), KTP crystals have been used for application to QPM SHG. These materials are ferroelectric, which means that below Curie temperature they exhibit a spontaneous electric polarization and domain structure.

Creation of the periodical domain structure by periodic inversion of the spontaneous polarization is called “periodical poling” and provides a means for producing the 180° phase shift required to implement QPM. Periodically poled ferroelectric based nonlinear optical materials are suitable for use in optical devices to convert near infrared (NIR) radiation from a diode or other lasers to light in the blue-green (visible) or near UV region of the optical spectrum.

Further information on the applications and properties of nonlinear crystals, especially LN and MgO doped LN, are given in “Handbook of Nonlinear Optical Crystals” by V.G Dmitriev, G.G Gurzadyan and D.N. Nikogosyan, Springer-Verlag (1999) ISBN3-540-69354-5 292.

Laser light in the blue-green (visible) wavelength region (i.e., 400-550nm) is used in a wide variety of analytical techniques. Currently the argon ion laser is the only generally available source of coherent blue-green light. Argon ion lasers are relatively bulky, delicate and expensive. There is a great need for a solid state blue-green laser. None is currently available, which emits in this spectral region. However there are solid state (semiconductor diode and DPSS) lasers, which emit in the near infra-red (NIR) region.

Frequency doubling (nonlinear frequency conversion) would enable a NIR laser to provide blue-green light. An extensive discussion of blue-green laser technology and applications, including biomedical engineering, spectroscopy, semiconductor wafer inspection, display science, optical data storage, reprographic, color display and undersea communication is contained in “Blue-Green Lasers” by W. Risk, T. Gosnell and A. Nurmikko, Cambridge University Press (2003) ISBN 0-521-52103-3. The discussion of nonlinear frequency conversion using QPM in nonlinear crystals contained therein is incorporated herein by this reference (see especially pages 77-84 and 101-104 and 108).

Conversion of NIR laser source light into light in the green-blue spectral range can be carried out by using SHG, also known as frequency doubling techniques, a technology generally known in the laser-based optical industry. A reasonable goal for single-pass conversion efficiency is that it should be in ~25% range to avoid excessive laser cost. Conversion efficiency is proportional to input power, the square of the effective nonlinear coefficient of the nonlinear element (crystal) and the length of the nonlinear element.

As crystal length is increased, conversion efficiency increases, but the frequency doubling process becomes more sensitive to changes in temperature, strain and other factors affecting the uniformity of the refractive index of the nonlinear element. As a result, length alone cannot be used to compensate for an element’s low nonlinearity. Therefore, the material with the maximum non-linear coefficient should be used to enhance the efficiency of conversion.

A technique that compensates for the difference in phase velocity between the fundamental wave and its harmonic in a nonlinear crystal caused by natural dispersion is known as birefringent phase matching. In this case, the optical anisotropy of a nonlinear crystal is used to find a unique propagation direction, where fundamental and harmonic waves have the same phase velocity. For most of the commercially available nonlinear optical materials (LN, LT, and KTP) the maximum conversion efficiency is about 1-3%/W•cm . LN and LT are particularly attractive materials because of their status as commodity materials, and they have a high nonlinear coefficient (normally referred to as d33) and also are available in relatively large size crystals.

QPM provides the mechanism for an efficient way to generate second harmonic frequencies. The general concept of using QPM as a mechanism for doubling optical frequencies has been known for about forty years. Essentially it is a technique that compensates for the difference in phase velocity between the fundamental wave and its harmonic in a nonlinear crystal caused by natural dispersion. In QPM, the two waves are allowed to have different phase velocities, and they shift out of phase relative to one another over a distance called the coherence length.

At present the most efficient way to create a QPM structure is to use periodically poled single-crystalline ferroelectric materials. In these materials, creating specific micrometer scale domain configurations with a periodically alternating direction of spontaneous polarization are used for this purpose. Due to the polar character of these materials the sign of the non-linear coefficient (d33) can be changed by switching the direction of spontaneous polarization. If the period (L) of the periodically polled domain structure is equal to double the coherent length, the phase difference due to natural dispersion is compensated for by the change of the sign of the non-linear coefficient (d33 Þ- d33) at the domain boundaries, causing the continuous transference of power from the fundamental beam to the harmonic beam throughout the entire length of the crystal.

Effective nonlinear coefficient value, poling period and absorption edge (the range of limited absorption) are the factors which influence the choice of periodically poled materials for SHG applications.

There is considerable interest in compact lasers operating in the blue-green spectral region (0.4-0.55?m) for various applications in science and technology such as biotechnology, data storage, optical image recording and displays. The preferred laser source would be a blue GaN-based diode laser. Although such devices are now commercially available, their output powers are limited (~50mW) and cost is relatively high. Thus, in the present circumstances, the frequency-doubling of near-infrared (0.8-1?m) commercial, low-cost diode lasers represents a competitive technology.

Displayers for consumer application required multi-watt sources that can be manufactured in high volumes for less than $100/watt. To meet the needs of display applications, a new class of nonlinear optical materials with high optical-to-optical efficiencies would be required, and their development becomes the focus of vendors producing nonlinear-optical components.

Two of the most widely used nonlinear materials employed for frequency conversion of near-infrared diode lasers are Lithium Niobate (LiNbO3 or LN) and Lithium Tantalate (LiTaO3 or LT) partly because of their modest cost, but more importantly because they can be periodically poled to create quasi-phase matching conditions (QMC) that can increase the second harmonic generation (SHG) efficiency up to 80% for the a pulse laser. There is a lot maturity with these materials; however cost-effective manufacturing methods that produce stoichiometric LN and LT crystals do not exist yet. Standard congruent material is grown with strong deviation from the perfect stoichiometric composition.

The lithium content is in range of 48%, whereas the ideal chemical formula for stoichiometric balance would be 50% lithium. The fact that the crystal is missing some lithium means that the crystal is rich in defects. Stoichiometric crystals exhibit low absorption, a low coercive field (allowing shorter poling periods) and lower susceptibility to photorefraction. Due to difficulties with the growth of stoichiometric crystals, high crystalline element processing (element orientation, cutting/dicing and fine optical polishing), and high labor cost, the price of a periodically poled LN or LT (PPLN, PPLT) element in USA is ~$1000.

Technical Description

1. Material selection

Tab. 1 shows that the most effective material for SHG application is LN crystal having biggest value of d33. Due to QPM, it is possible to create viable bulk single-pass blue-green light sources for display applications, using LN, since it provides a way to obtain a normalized room temperature conversion efficiency of ~4% /(watt•cm) for 1064nm?532nm SHG and 5%/(watt•cm) efficiency for 920nm?460nm SHG, more than twice the minimum requirement.

LT has a normalized room temperature conversion efficiency of 0.85-1% /(watt•cm), below the display requirement for bulk single-pass 1064nm?532nm SHG. However, for 920nm?460nm SHG, LT has a normalized conversion efficiency of 2% /(watt•cm) and would be suitable for that application. It is important to point out that for the wavelengths of ?410nm LT has higher transparency compared to LN as well and therefore it is material of choice for 300nm-410 nm wavelength conversion (Fig.1).

Two other potential materials in which QPM has been demonstrated for BG light generation are KN and KTP. A normalized conversion efficiency for these materials are ~2.5% /(watt•cm) for KN and 1.5%/(watt•cm) for KTP, for the 852nm?426nm SHG. To achieve 25% single-pass conversion efficiency, 10 W power of 852nm, a KN crystalline element of 2.5 cm and KTP crystalline element of 3.5 cm length are required. However the maximum crystal length in production is 2cm for KN and 3cm for KTP. Therefore, KN and KTP have not been chosen because both crystals are not available in sufficiently long devices size, are prohibitively expensive and suffer from a number of quality issues.

The Spectralus’s strategy for material development is to advance the technology of bulk and waveguide electrical field periodic poling in LN and LT 5-50 mm long, 0.5-2mm thick devices that could be routinely fabricated with good uniformity over an entire 3”-4” diameter wafer. Lithographic technique is used to produce QPM structure and to assure the periodicity, where even small errors in periodicity can substantially degrade conversion efficiency. The ability to define QPM/domain structure with lithographic precision created an opportunity to fabricate SHG-based conversion devices with performances not achievable using non-lithographic techniques.

Tab. 1. Parameters of perspective ferroelectrics for QPM SHG applications

In LN and LT crystals QPM structure is fabricated by creating of the ferroelectric-domain-inverted grating. Electric field poling is used to create the domain grating. Fabrication of LN and LT QPM devices involves first lithographically patterning the domain grating structure. Then the domain-inversion process is produced by applying voltage to lithographically defined periodic electrode structure when the polar component of electric e field is larger than the coercive field (Tab.1). This technique is referred to as electric field periodical poling or simply periodical poling (PP).

Domain periods (?) between 6 ?m and 8 ?m are required for green light generation, and blue light generation requires domain periods between 2 ?m and 5 ?m. To reach a maximal efficiency of conversion, engineering of uniform domain grating with a duty cycle close to 50% in 0.5-2mm thick LN and LT single-crystalline elements is required.

Fig.1. Wavelength accessible using GaInAs/GaAs lasers diodes and periodically LN and LT crystals