Temperature-Stable Lithium Niobate Q-switch

Dr. Dieter Jundt, VP Research and Development

G&H offers temperature-stable electro-optic Q-switches for operation over a wide range of temperatures. Our devices solve the problem of pyro-electric charge buildup commonly leading to premature lasing. The charges are dissipated in a thin layer close to the surface where they are generated, making the use of radioactive ionization sources unnecessary.

Temperature Stable Electro-Optic Q-Switch
Temperature Stable Electro-Optic Q-Switch

Lithium Niobate Q-switches have long been used as a cost-effective solution in Nd:YAG laser systems. Portable and on-flight targeting lasers have strict performance criteria including operation at temperatures as low as -40°C. The associated pyro-electric charge buildup can lead to birefringence non-uniformity which causes problems such as low contrast ratio and poor hold-off from premature lasing.

The traditional approach to overcome this problem has been to provide an external ionization source that neutralizes the pyro charges as soon as they are generated. The most common approach uses radioactive americium, a material with complex handling and disposal requirements. There has been growing interest in an alternative approach that induces a weak material conductivity in Lithium Niobate (LN) and thus provide a convenient way to dissipate pyro charges. This approach was pioneered for non-optical applications at our Palo Alto facility over ten years ago (US patent 6,319,430), and has been refined for Q-switch applications over the last few years.

A close collaboration among our sites has resulted in our offering of a variety of temperature-stable (TS) Q-switches. Two TS variants are shown in the figure below together with a traditional LN Q-switch (at left). The image shows diffuse white light transmitted through the 21mm length of the Q-switches. The crystal dimension is 7.4×7.4mm.

White light transmission through various Q-switch models
White light transmission through various Q-switch models

The device in the middle illustrates that the chemical reduction process not only causes conductivity, but also some undesirable optical absorption, a result of the nature of the charge carriers created. Fortunately, the absorption at the typical laser operating wavelength 1064nm is only one fifth of the visible absorption.

The trade-off between absorption and conductivity makes it necessary to work closely with our customers and has led to more than one variety of TS Q-Switches. The middle crystal is produced by thermally annealing a previously finished Q-Switch (shown at left). The degree of reduction can be adjusted and will change both the conductivity (higher is better) as well as the optical insertion loss of the Q-switch (lower is better). We have worked with customers in the US and Europe to optimize the degree of reduction of the Q-switch to match it to the laser where it is used. A high gain laser cavity needs high conductivity to avoid pre-lasing but at the same time can tolerate a higher insertion loss. A lower gain system (e.g. diode pumped) needs lower insertion loss but can tolerate more depolarization without pre-lasing. The device shown has an insertion loss of 4% at 1064nm and is optimal for a very high gain laser system. While such devices are successfully used in ongoing production, the non-uniform profile has potential drawbacks. For example, the reduction layer close to the Y-faces provides a conduction path between the electrodes and this may lead to charge migration and instability in the half-wave voltage.

Work performed over the last year has led to the next generation devices as pictured on the right. The processes are proprietary to G&H and produce a Q-switch that has uniform absorption across the aperture and achieves a higher figure of merit (the ratio of conductivity to absorption) than prior processes. The Q-switch pictured has reduction-related insertion loss of only 0.6%, but higher target conductivities can also be produced as easily. G&H sites in Cleveland, Palo Alto and Ilminster collaborated on optimized process and product conditions.

Laser damage testing has shown that all our TS Q-switches meet typical damage threshold specification of 300MW/cm2. TS versions of both our standard sizes (9x9x25mm and 7.4×7.4x21mm) are available. For further information on G&H TS Q-Switches please contact Product Managers, Arron Campi in the US and in the rest of the world Johnnie Ironside Smith.