Sum and Difference Frequency Generation

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Crystal materials lacking inversion symmetry can exhibit a so-called <$\chi^{(2)}$> nonlinearity. In such nonlinear crystal materials, sum frequency generation ({SFG=sum frequency generation}) or difference frequency generation ({DFG=difference frequency generation}) can occur, where two pump beams generate another beam with the sum or difference of the optical frequencies of the pump beams.

A sum frequency mixer is sometimes called a FASOR (Frequency Addition Source of Optical Radiation).

A special case is sum frequency generation with an original pump wave and a frequency-doubled part of it, effectively leading to frequency tripling. Such a cascaded process can be much more efficient than direct frequency tripling on the basis of a <$\chi^{(3)}$> nonlinearity.

Sum or difference frequency generation processes require phase matching to be efficient. Usually there is no simultaneous phase matching for both processes, so that only one of them can take place.

Typical Applications

Some typical applications of sum frequency generation are:

• generation of red light (→ red lasers), e.g. by mixing the outputs of a 1064-nm Nd:YAG laser and a 1535-nm fiber laser, resulting in an output at 628 nm
• generation of ultraviolet light, e.g. by mixing the output of a 1064-nm Nd:YAG laser with frequency-doubled light at 532 nm, resulting in 355-nm UV light

Difference frequency mixing with pump waves of similar frequency can lead to a mixing product with a long wavelength. Some examples are:

• generation of light around 3.3 μm by mixing 1570 nm from a fiber laser and 1064 nm
• generation of light around 4.5 μm by mixing 860 nm from a laser diode and 1064 nm

Such mid-infrared wavelengths are required, e.g., for the laser spectroscopy of gases.

Difference frequency generation can also be used for generating terahertz waves. For efficient terahertz wave generation, there are special semiconductor-based photomixers, where the terahertz beat note of two similar optical frequencies generates an oscillation of the carrier density in the semiconductor, which is translated into an oscillating current and then into terahertz radiation. That physical mechanism is substantially different from the common one based on a <$\chi^{(2)}$> nonlinearity.

Insight from a Photon Picture

Sum Frequency Generation

In a sum frequency mixer, both pump waves experience pump depletion when the signal becomes intense. For efficient conversion, the photon fluxes of both input pump waves should be similar. If one input wave has a lower photon flux, and its power is totally depleted somewhere in the crystal, there can be backconversion during subsequent propagation.

Difference Frequency Generation

In a difference frequency mixer, the lower-frequency wave is amplified rather than depleted. This is because photons of the beam with highest photon energy (shortest wavelength) are effectively split into two lower-frequency photons, thus adding optical power to both lower-frequency waves. The term parametric amplification emphasizes the aspect of amplification, and the difference frequency mixing product is then called the idler wave.

carrier–envelope Offset Frequencies

For operation with trains of ultrashort pulses, the carrier–envelope offset frequency (CEO frequency) of the output of a sum or difference frequency mixer is essentially the sum or difference, respectively, of those frequencies for the input. (The result may have to be corrected by subtracting the line spacing, which is identical to the pulse repetition rate, to get back to the interval from zero to the line spacing.)

It is interesting to consider what happens if difference frequency generation is applied to the low- and high-frequency components of a broadband frequency comb, which can be generated e.g. with a femtosecond laser, possibly followed by an optical fiber for supercontinuum generation. The CEO frequency of the output is then the difference between two identical frequencies, i.e., zero. This implies that the carrier–envelope offset phase is temporally constant. (In practice, it may still exhibit some drift, but only with a quite limited range.) This principle is realized in some devices for obtaining a more or less constant CEO phase without employing active stabilization methods.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains 12 suppliers for sum and difference frequency generators. Among them:

Artifex Engineering

Artifex Engineering provides finished sum and difference frequency generator crystals for laser applications. Visit our product page for more information. We look forward to your inquiry.

APE

The APE HarmoniXX DFG is designed to extend APE’s product line of wavelength converters into the mid-IR range. It mixes signal and idler output beams of synchronously pumped OPOs and is available for various pump sources. The DFG output wavelength can be changed easily by tuning the OPO signal. It covers a wide wavelength range from 4 μm to >15 μm.

EKSMA OPTICS

EKSMA Optics offers various nonlinear crystal materials that can be used for sum frequency generation.

Covesion

Covesion’s MgO:PPLN free space and fiber coupled up- and down-conversion solutions offer exceptional conversion efficiency for generating visible and mid-IR wavelengths through sum and difference frequency generation.

We can offer a range of custom free space and fiber coupled solutions suitable for one-off orders to large-volume manufacture with customizable options including:

• free space or fiber coupled solutions
• multiple grating, chirped or fan-out designs
• tailored AR coatings
• custom grating periods and apertures
• 2×1 or 2×0 fiber input/output configurations
• resistive or Peltier temperature control
• integrated or external temperature control
• broad coverage of visible and mid-IR wavelengths
• power monitoring, control and output filtering
• compatibility with both CW and pulsed lasers

HC Photonics

HC Photonics (HCP) offers both commercial off-the-shelf (COTS) and custom PPMgO:LN or PPMgO:LT based sum and difference frequency generators (SFG & DFG) for out wavelengths ranging from 355 nm to 5000 nm. The product family includes bare crystals (bulk and waveguide) and plug-and-play fibered mixers.

A reference spec is >2 W at 632 nm from a fibered mixer converting the output of 1064-nm and 1560-nm fiber lasers.

Key features:

• >200 kinds of commercial off-the-shelf (COTS) crystals with oven/holder for shipping today
• output wavelength range: 355–5000 nm
• fibered mixer is single-pass, high-efficiency and optimized for specified input pumps
• available for fiber or free space as input/output coupling interfaces (such as 2×0, 2×1, 1+0×0, 0+0×0; 0 = free space, 1 = one fiber)

Bibliography

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