Phase Matching

When the crystal temperature is varied around the optimum point, the phase mismatch and thus the conversion efficiency also vary, as shown in Figure 4. The temperature range in which a high conversion efficiency is obtained is inversely proportional to the crystal length. It also depends on the temperature dependence of the refractive indices involved. Similar relations apply to other nonlinear frequency conversion processes.

Similar curves are obtained e.g. for critical phase matching when the angular orientation of the crystal is varied.

The phase-matching curve actually does not need to be as symmetric as shown in Figure 4. For example, it becomes asymmetric if the crystal temperature is lower at the crystal end faces, as compared with the middle of the crystal. It is possible to quantify the temperature homogeneity in a crystal oven on the basis of the measured phase-matching curve. Based on such data, it can be estimated whether the conversion efficiency could be increased e.g. by using a longer crystal.

The usual technique for achieving phase matching in nonlinear crystals is birefringent phase matching, where one exploits birefringence to cancel the phase mismatch. This technique comes in many variations:

• Type I phase matching means that, e.g., in sum frequency generation the two fundamental beams have the same polarization, perpendicular to that of the sum frequency wave. Conversely, in type II phase matching, the two fundamental beams have different polarization directions; this can be appropriate when the birefringence is relatively strong (overcompensating the dispersion in a type I scheme) and/or the phase velocity mismatch is small. The distinction between type I and type II similarly applies to frequency doubling, and to processes such as degenerate or nondegenerate parametric amplification. The different polarization arrangements can have various practical implications, for example for the combination of several nonlinear conversion stages, or for intracavity frequency doubling.
• Critical phase matching means that an angular adjustment of the crystal (or the beam) is used to find a phase-matching configuration, whereas in noncritical phase matching all polarization directions are along the crystal axes, and the angular position is then not a sensitive parameter.
• The wave vector of all involved beams may have the same direction (collinear phase matching) or different directions (noncollinear phase matching), where, however, the vector sum of the generating beams equals the wave vector of the product beam. A special case is achromatic phase matching where at least one of the interacting beams is angularly dispersed so that each frequency component of the signal is properly phase-matched.

A special technique of significant importance is quasi-phase matching, where real phase matching does not occur, but high conversion efficiencies are nevertheless obtained in a crystal where the sign (or strength) of the nonlinearity varies periodically. Such a periodic variation of nonlinearity can be achieved e.g. by periodic poling. Normally, one then has equal polarization states of all involved waves (as the birefringence is not needed), and this configuration is sometimes called type-0 phase matching.

When phase matching is achieved, the group velocities of the interacting waves are in general still not matched; there is a certain group velocity mismatch, which limits the interaction length for pulses and (for a given interaction length) the spectral range (called phase-matching bandwidth) in which phase matching is achieved. Also, there is only a finite range of beam angles where phase matching works – particularly for critical phase matching. This range of angles is usually called the angular phase-matching bandwidth. See the article on phase-matching bandwidth for more details.

For any given crystal and nonlinear interaction (characterized by the participating wavelengths), there may be multiple phase-matching configurations, the comparison of which involves a number of important properties. Which configuration is most suitable, can depend strongly on the application.

An example is sum frequency generation in an LBO crystal, where two inputs are 1064 and 1535 nm generate a red output at 628 nm. For this interaction, there exist three critical and two noncritical phase-matching schemes, considering only collinear phase matching. Also, the cases with critical phase matching are restricted to a crystal temperature of 20 °C. These schemes are listed in Table 1.

Table 1: Various phase-matching schemes for sum frequency generation in LBO.

Scheme	nonlinearity	walk-off	GVM
XZ I(ee-o), θ = 88.0°	0.85 pm/V	ρ1 = −1.73 mrad ρ2 = −1.82 mrad	GVM13 = −0.30 fs/mm GVM23 = +6.0 fs/mm
XZ II(eo-e), θ = 19.4°	0.50 pm/V	ρ1 = −15.1 mrad ρ2 = −16.3 mrad	GVM13 = +50.4 fs/mm GVM23 = −30.3 fs/mm
YZ II(eo-o), θ = 41.1°	0.51 pm/V	ρ1 = −9.27 mrad	GVM13 = −74.6 fs/mm GVM23 = +51.9 fs/mm
X / ZZY at 13.6 °C	0.85 pm/V	–	GVM13 = −0.29 fs/mm GVM23 = +6.0 fs/mm
Z / XYX at 250 °C	0.67 pm/V	–	GVM13 = +50.4 fs/mm GVM23 = −29.5 fs/mm

As an example of the nomenclature, the critical scheme XZ I(ee-o), θ = 88.0° means that the beams propagate in the XZ plane with an angle of 88.0° to the Z axis. Both inputs have extraordinary (e) polarization (type I), whereas the product wave has ordinary (o) polarization. As an example of a noncritical scheme of type II, take Z / XYX, where the beams propagate in the Z direction and are polarized in the X (1535 nm), Y (1064 nm), and X (628 nm) directions.

It is important to realize that the choice of phase-matching configuration also influences the effective strength of the nonlinearity, because it determines the directions of the electric fields involved with respect to the crystal axes. There are configurations which would offer, e.g., a large gain bandwidth but are hardly usable since the effective nonlinearity would be very weak.

In principle, the noncritical scheme X / ZZY should offer the best performance, having the highest nonlinearity and no spatial walk-off. However, the critical scheme XZ I(ee-o) is actually very similar: starting from the noncritical scheme X / ZZY, one just rotates the crystal by 2° in order to get phase matching at 20 °C instead of 13.6 °C. The walk-off is very weak, as the beam direction is close to the X axis direction.

In the case that a type II scheme is required (e.g. when the input beams are already collinear but have orthogonal polarization directions), the scheme YZ II(eo-o) might be considered, even though the lower group velocity mismatch of scheme XZ II(eo-e) could be slightly more advantageous for ultrashort pulses, because the dispersive pulse broadening is reduced.

The group velocity mismatch (GVM) e.g. between some pump and signal waves can be important for the conversion of ultrashort pulses, as is related to the phase-matching bandwidth. Note that the GVM values vary considerably between different schemes, demonstrating that the choice of the material alone does not determine how important this issue is. For each particular phase-matching scheme, the optimum crystal length (particularly for ultrashort pulses) and beam waist need to be found.

Obviously, it is very useful to have software which can systematically identify all possible phase-matching configurations for a given nonlinear interaction, based on, e.g., Sellmeier equations for the refractive indices of all involved waves. On this basis, the most suitable scheme can be selected, taking into account the specific requirements for the conversion device to be designed.

Note that phase matching is in some cases further complicated by the Kerr effect: the resulting intensity-dependent refractive indices also make the phase-matching relations intensity-dependent, and this effect is sometimes significant, e.g. for four-wave mixing processes in fibers.

Phase matching is relevant not only for χ⁽²⁾ nonlinearities of nonlinear crystal materials, but also for other nonlinear processes. For example, χ⁽³⁾-based nonlinear interactions in fibers (e.g. parametric amplification or four-wave mixing) can be phase matched under certain conditions. Such issues are also of great importance in the context of supercontinuum generation. Some nonlinear processes are automatically phase-matched; an example is self-phase modulation, where all photons involved have the same wavelength.

Your question or comment:

Spam check:

  (Please enter the sum of thirteen and three in the form of digits!)

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.

[1]	P. D. Maker et al., “Effects of dispersion and focusing on the production of optical harmonics”, Phys. Rev. Lett. 8 (1), 21 (1962), doi:10.1103/PhysRevLett.8.21
[2]	M. V. Hobden, “Phase-matched second harmonic generation in biaxial crystals”, J. Appl. Phys. 38 (11), 4365 (1967), doi:10.1063/1.1709130
[3]	R. Eckhardt and J. Reintjes, “Phase matching limitations of high efficiency second harmonic generation”, IEEE J. Quantum Electron. 20 (10), 1178 (1984), doi:10.1109/JQE.1984.1072294
[4]	A. V. Smith et al., “Increased acceptance bandwidths in optical frequency conversion by use of multiple walk-off-compensating nonlinear crystals”, J. Opt. Soc. Am. B 15 (1), 122 (1998), doi:10.1364/JOSAB.15.000122; see also references therein
[5]	A. M. Schober et al., “Broadband quasi-phase-matched second-harmonic generation of ultrashort optical pulses with spectral angular dispersion”, J. Opt. Soc. Am. B 22 (8), 1699 (2005), doi:10.1364/JOSAB.22.001699
[6]	O. Isaienko and E. Borguet, “Generation of ultra-broadband pulses in the near-IR by non-collinear optical parametric amplification in potassium titanyl phosphate”, Opt. Express 16 (6), 3949 (2008), doi:10.1364/OE.16.003949

(Suggest additional literature!)

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. How about a tailored training course from this distinguished expert at your location? Contact RP Photonics to find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, training) and software could become very valuable for your business!