Crystal technology aids development of OPOs

07 Dec 2005

Optical parametric oscillators offer a convenient source of coherent light that can be tuned across the ultraviolet, visible and infrared. Günter Warmbier describes the principle of operation and key characteristics of these useful devices.

The first optical parametric oscillator (OPO) was demonstrated by Giordmaine and Miller at Bell Laboratories, US, in 1965, only a few years after the invention of the laser. Over the next two decades the OPO was subject to detailed theoretical and experimental investigations. However, development was hampered by the lack of nonlinear optical materials with appropriate optical and mechanical properties.

What is an OPO?

This situation did not change until the late 1980s when new nonlinear materials such as beta bariumborate (BBO) and lithiumborate (LBO) became available. With their unique properties (high nonlinearity, wide transparency range and high damage threshold), BBO and LBO are still the materials of choice for many applications. OPOs made from either BBO or LBO crystals provide an all-solid-state light source which combines both high efficiency and high output power with a wide tuning range in the ultraviolet, visible and near infrared.

Fig. 1

Besides the need for suitable nonlinear crystals, stable operation of an OPO also requires a high-quality pump beam which has a single transverse mode (TEM₀₀) and is almost diffraction limited. As a result, advances in laser technology have also played an important role in the development of reliable OPOs for scientific applications or integration into technical systems such as LIDAR.

Compact solution

Today, most commercially available OPOs are pulsed systems pumped by Q-switched Nd:YAG lasers operating with nanosecond pulses at a repetition rate of 1-100 Hz. The OPO output has the same repetition rate as the pump laser pulses but a slightly reduced pulse width. OPOs that offer continuous-wave (CW) operation or emit ultrashort (picosecond or femtosecond) pulses are also available.

Characteristics

Tuning range
The tuning range of an OPO depends primarily on the wavelength of the pump laser beam and the type of nonlinear crystal in the cavity. Pumped by the third harmonic (355 nm) of an Nd:YAG laser, the output of a BBO-OPO is widely tunable in the visible and near infrared. For example, by changing the phase-matching angle θ by only 10 ° (23.1-33.1 °) the tuning range of such an OPO can span the visible from 400 to 710 nm (signal wave) and the near infrared from 710 nm to 3 mm (idler wave).

Inside the box

To generate radiation at longer wavelengths, Nd:YAG-pumped OPOs require crystals made from materials such as potassium niobate (KNbO₃) or lithium niobate (LiNbO₃), which have better transparency in the infrared. With these materials the generation of near infrared radiation at wavelengths of up to 5 μm is possible.

OPO's output

Threshold
The threshold of an OPO (the pump power at which operation commences) depends on several factors - the length of the crystal, the reflectivity of the mirrors and the wavelength of the signal and idler wave. For a BBO-OPO pumped by 10 ns long 355 nm laser pulses, the pump energy density at threshold is as low as 0.1-0.2 J cm^-2 which corresponds to pump pulse energies of 5-7 mJ. The power densities are in the range of 20-40 MW cm^-2 and thus well below the BBO damage threshold of several GW cm^-2.

Efficiency and output power
Besides the large tuning range, high efficiency and high output power are the other attractions of OPO devices. With pump pulse energies of 55 mJ (easily generated by commercial small size Nd:YAG systems) the BBO-OPO output exceeds 24 mJ. This efficiency of 43% corresponds to a crystal-internal efficiency of more than 50%.

At higher input powers, the output and efficiency increases significantly. For example, for input energies of 300 mJ, OPO output can exceed 150 mJ which corresponds to a total efficiency of 50% and a slope efficiency as high as 60%.

Emission bandwidth
The bandwidth (spectral width of the emitted beams) of an OPO's output depends on the wavelength, the parameters of the resonator and both the linewidth and power of the pump.

For example, consider a 355 nm pumped OPO operating at three times its pulse energy threshold. For such a device, the line-width of a BBO-OPO signal wave increases with wavelength from about 0.2 nm at 410 nm to almost 4 nm at 650 nm.

Many applications require narrowband operation and fortunately, the linewidth of an OPO can be reduced significantly by simple means. One way to achieve this is to use type II "midband" phase-matching and reflect the unconverted pump radiation back into the OPO-crystal. In contrast to broadband type I phase-matching (where the generated OPO beams have the same polarization), in type II phase-matching the polarization of the generated OPO beams are orthogonal. This type II midband operation can enable a linewidth of less than 5 cm^-1 throughout the tuning range, which is sufficient for numerous applications such as differential absorption LIDAR or cavity ring-down spectroscopy.

Another convenient method to achieve narrow linewidth operation, which avoids the insertion of lossy wavelength selective elements into the OPO cavity, is injection seeding. The injection of low-power narrowband radiation into the OPO cavity has several benefits.

As well as helping control the OPO's bandwidth and wavelength of operation, the technique reduces the oscillator build-up time and OPO threshold by almost a factor of two. Successful seeding has been demonstrated with low-power pulsed or continuous-wave sources and tunable diode lasers. Pulse energies of a few nanojoules or CW power levels of less than 1 mW are usually sufficient. The bandwidths achieved through seeding are typically less than 100 MHz and can even approach the Fourier limit which is about 10 MHz for 10 ns long pulses.

Recent developments
Over the last decade the optical quality of OPO crystals (like KTP, KTA, LiNbO₃ or KNbO₃) has improved significantly and new promising crystals (like bismuth borate) have become available. Nevertheless, BBO and LBO are still the most reliable materials for powerful OPO systems.

An exciting recent development is the demonstration of microstructured nonlinear crystals which exhibit very strong optical nonlinearity and can be engineered to operate at almost any wavelength.

These crystals are ferroelectrics (like lithium niobate or lithium tantalate) that have been exposed to a strong electric field pattern to create periodically inverted domains.

The periodic structure provides phase-matching over longer crystal lengths to enable highly efficient nonlinear conversion processes like harmonic generation or optical parametric amplification. The main advantage of these crystals is their high optical nonlinearity and the fact that a proper choice of the periodicity of the ferroelectric domains provides phase-matching at any wavelength within the transparency range of the crystal material.

These microstructured crystals allow the construction of highly efficient, ultrafast (ns, ps or fs) OPOs that are synchronously pumped by modelocked Ti:sapphire lasers, for example. The wavelength of the generated ultrashort pulse trains is controlled by tuning the wavelength of the modelocked pump laser.

With an appropriate microstured crystal (like stoichiometric MgO:LiNbO₃) such an OPO generates short pulses in the whole range of 890 nm to 5.4 μm by tuning the Ti:sapphire laser within its operating range of 700-900 nm.

Since the second harmonic of such OPOs covers the gap (500-700 nm) between the fundamental and the second harmonic of the Ti:sapphire laser, they are an ideal supplement to modelocked Ti:sapphire lasers.

Microstructured crystals even allowed the operation of single-frequency CW OPOs pumped by diode laser systems with high spectral and spatial power density. These OPOs have the potential for very compact sources of single-frequency radiation with wavelengths tunable throughout the near infrared.

In summary, OPOs have matured to become reliable sources for widely tunable coherent light. As all-solid-state systems, they are highly reliable, and easy and cost-effective to operate. Due to their unique optical properties they are now used routinely in a large number of scientific and technical applications.