As photonics markets drive the development of ever higher powers for a range of laser systems, traditional laser materials are increasingly challenged in dealing with the high optical power densities and large amounts of waste heat. Novel solutions are needed.

Diamond has long been known to have exceptional properties across a range of areas, from low absorption at a number of key wavelengths, the highest thermal conductivity of any bulk material, to excellent mechanical properties, however from historic sources it was difficult to source high quality materials with reliably large dimensions. High pressure, high temperature (HPHT) synthetic diamond has been available for more than fifty years, however its properties and available sizes has limited its uptake in applications outside of mechanical applications.

The purity of the process has enabled a range of applications over the years due to its high quality and availability in large sizes...

An alternate method of making polycrystalline and single-crystal diamond that addresses some of the limitations is via chemical vapor deposition (CVD), being increasingly commercially viable over the last 15 years. In this process gases containing carbon species in a bulk hydrogen carrier are heated by microwaves to temperatures greater than 2000 K. With careful control of the system parameters and feedstock quality, this method has made possible diamond with exceptionally high purity, with, for example, background nitrogen-defect levels in single-crystal diamond measured to be less than 5 parts per billion and diameters of polycrystalline material up to 140 mm. The purity of the process has enabled a range of applications over the years due to its high quality and availability in large sizes, including high power optical windows, windows for wideband infrared spectroscopy, heat spreaders for the semiconductor industry, conducting boron doped electrodes, and gyrotron windows for the development of fusion power.

Diamond has a wide bandgap of 5.45 eV, meaning the short-wavelength cut-off for diamond is around 230 nm, while the material is largely transparent well into the microwave region due to the symmetry of the bonding. Therefore the optical applications cover a wide range of wavelengths, enabling different laser systems to exploit diamond’s high thermal conductivity in a range of ways.

High power CO2 laser output couplers and beamline optics

The most mature optical application of high purity CVD diamond is as output couplers of CO2 lasers, with these parts having been sold for over 15 years. More recently, as power densities in systems have become higher, parts of the 10.6 μm beamline such as beamsplitters and lenses could be made from diamond. The application relies on four key properties:

1. The low absorption coefficient at 10.6 μm. The exceptionally low defect levels possible in CVD diamond keep material near the low intrinsic absorption of diamond at this wavelength.

2. High fracture strength means a typical diamond window designed to safely hold a given pressure can be more than four times thinner than a similar window made from ZnS; keeping the total absorbance in the optic low.

3. The highest thermal conductivity of any bulk material (>2000 W/m.K) means all absorbed power in the window is rapidly moved out to the cooling system, minimising temperature gradients across the window. Note that the heat comes not only from the bulk window material, but usually is more significant from coatings and surfaces.

4. The low coefficient of thermal expansion (CTE) ensures that the windows do not significantly alter their shape during use, minimizing thermal lensing and stress build up in windows during operation.

Of course, it should be noted that in some uses the polycrystalline nature of the optical element would limit its use; however, diamond’s cubic structure allows the impact of multiple crystalline orientations in the film on performance to be minimal. However at shorter wavelengths there are some applications where the birefringence and scatter typical in polycrystalline diamond would limit efficacy, and single crystal solutions are required.

Inside the cavity

Since the invention of the thin disc laser in the 1990s it has become a favored tool for increasing power density from a given gain material. From high power cutting lasers from YAG-based systems, to higher powers at harder to reach wavelengths from semiconductor gain materials in a vertical external cavity surface emitting laser (VECSEL) setup, diamond allows excellent beam qualities at high power due to the axial heat flux through a short dimension into a heat sink.

Due to high thermal conductivity polycrystalline diamond has long been used as a heat spreader on the outside of the cavity in these systems; however as power densities increase even diamond cooling on the rear of the gain material and mirror stack is not sufficient and beam quality falls. Historically the birefringence and scatter of polycrystalline diamond has limited its use in the cavity of short wavelength lasers (typically operating around 1 µm).

For further power scaling a low absorption, low birefringence, higher area diamond product is required with extremely high thermal conductivity that can be placed on the thin disk, inside the cavity. Due to their smaller spot sizes, this has already been realised in VECSEL lasers and has demonstrated to deliver up to two orders of magnitude of increase in power before breakdown. As areas available are increasing, other disc lasers could soon see similar scaling of power density.