Doubled diode offers compact blue source

15 Feb 2006

A new simple design of solid-state blue laser could be an attractive solution for many clinical and industrial applications. Lisa Tsufura and Eric Takeuchi explain.

When it comes to laser technology and creating sources that operate at new wavelengths, sometimes the best approaches are the simplest ones. While numerous companies have developed solid-state lasers that emit in the blue, most are complex designs that are not ideal in terms of manufacturability and performance.

Increased efficiency

An elegant solution for many applications that require a low to moderate power blue source at 488 nm is the doubled diode laser (DDL). This simple design takes light from an infrared diode and passes it through a nonlinear optical crystal to convert it to the blue region. Attractions of the device include improved thermal management and output stability combined with a small footprint and ease of mounting.

Improved reliability

In comparison with other solid-state 488 nm laser technologies, the DDL is unique. Developed specifically with low-cost, volume manufacturability in mind, the module uses far fewer parts, surfaces and coatings than competing solid-state designs. In fact, the DDL relies on just two main components. The first is a Telcordia-qualified 976 nm telecom diode. This well proven device exhibits lifetimes in the order of hundreds of thousands of hours and around one million units have been deployed to date in the fibre-optics industry.

Key advantage

The second component is a second harmonic generator (SHG). It has a history of more than 15 years and was originally developed for use in optical storage applications. It is also qualified for use at significantly higher powers (1 MW/cm²) than it is exposed to in the DDL technology.

Combining these two devices yields an elegant and high-performance source of blue light. In essence, the 976 nm diode output is frequency-doubled to create a 488 nm beam with a power of up to 40 mW and a wavelength variation in the order of just ± 0.25 nm.

The low number of optical surfaces in the design serves to minimize losses and increase system efficiency and reliability. As a result, the diode pump power can be reduced for a given output power and the laser is able to run at lower drive current levels - optimum for high reliability. The output power capability of the DDL is illustrated in 'Output statistics'.

Superior thermal efficiency

One of the key advantages of the DDL technology is its ability to deliver high output power while keeping the system's total power consumption and heat generation to a minimum. The benefit to the OEM system designer is a significant reduction in the size of heat sink required to manage the laser's thermal load.

There is no longer a need for forced convection cooling (which adds vibration and bulk to the system) or large passive heat sinks required by less-efficient systems. This makes the DDL footprint truly compact and mechanical, and thermal design requirements simple. As shown in 'Thermal management', the DDL typically dissipates less than 10 W, even when operated at extreme baseplate temperatures (e.g. 55 °C) compared with the 30-50 W generated by other blue solid-state devices.

Long-term pointing and stability

Since the DDL has few parts, there is little opportunity for movement-induced misalignment within the laser block. This means that the laser is relatively robust to temperature changes, and able to maintain excellent optical output performance and pointing stability over long periods of time. This is important because any beam movement during operation can result in power variations at the end of the optical beam train or at the sample, or cause clipping at modulators and other apertures.

The simple design and efficiency of the DDL technology minimizes the beam-pointing temperature dependence to values in the order of 5 rad/°C. This makes the DDL ideal for use in instruments in clinical or production environments and over large temperature ranges.

Beam pointing is of particular importance in systems relying on the fibre delivery of constant power or in scanning and imaging applications where focused spot or sample location is critically important. In other more complex, free-space optical trains, pointing stability is particularly critical due to the amount of time and energy required to perform alignment during installation. Beam pointing can also ultimately impact instrument accuracy and repeatability and should therefore be a consideration when consistency of measurements and repeatability are important.

Noise

A laser's optical noise is a critical consideration for many scanning and measurement applications because it can impact accuracy, especially when the signal levels being detected are low. The nature of the noise, its magnitude and the frequencies at which it occurs are of interest to system designers who are concerned with the interaction between the laser and the system's associated optical detectors, scan-heads, CCDs or other recording devices.

The noise characteristic of the DDL technology differs from that of the traditional solid-state laser. In place of discrete noise peaks, the optical noise from the DDL appears as a broadband, "white" noise, which does not produce artefacts like banding on images. In many applications, white noise can be averaged out of the measurement or image altogether.

Circular and Gaussian beam quality

The DDL output is designed to achieve ion laser-like beam diameter and divergence characteristics and a circular Gaussian beam profile with an M² of less than 1.1 and minimal astigmatism is achievable. Due to the low number of optical components and reduced opportunities for thermal and stress-related issues, the DDL can achieve polarization extinction ratios exceeding 250:1, which can be advantageous in cellular imaging or separation techniques that utilize scatter or contrast enhancement.

Wavelength accuracy

The DDL approach provides wavelength accuracy that is superior to that of many other optically pumped solid-state lasers. The device's emission wavelength is specified with a spectral width of 20 pm and a wavelength tolerance of ±0.50 nm, although the typical performance is ±0.25 nm. The tight tolerance virtually eliminates the need to calibrate each instrument, shortens assembly time and can improve resolution in wavelength-sensitive measurements.

Applications

Most of the interest in the blue-laser arena continues to be driven by the demands of the biotech and analytical instrumentation markets, fuelled by drug discovery, genomics, clinical applications, process control, and molecular and cell research needs. Many of the applications were originally launched using argon-ion laser technology at the 488 nm excitation wavelength. Today, with solid-state technology, 488 nm is still one of the most popular wavelengths used to excite fluorophores (FITC, Alexa Fluor 488, PE [phycoerythrin], Cy5) used in microscopy, cytometry, microarray scanning and gene-sequencing applications.

Simple yet sophisticated

There are several options when it comes to blue solid-state laser technology. On the surface, many may look alike and few provide the right balance of performance, cost and manufacturability to meet the OEM's needs. The DDL provides the OEM with a simple, truly compact option that is affordable and easily manufactured, along with sophisticated performance characteristics to meet volume desktop device demands.

With its insusceptibility to movement with temperature, the DDL promotes system stability and measurement repeatability. Its high efficiency also ensures minimal heat dissipation, giving the OEM much more design flexibility and smaller footprints to work with.

By exploiting proven diode and SHG technologies, the DDL supports the long-term reliability expected of demanding instrument applications. When you can have optimum wavelength, excellent optical performance and the best balance of performance, cost and manufacturability, the choice is simple.