PTB >> Mini CW Lasers Enable Next-Generation Bioinstrumentation

Figure 1: The Excelsior Line of DPSS and DD lasers is designed for bioinstrumentation based on specific biological experimentation requirements, including excitation wavelength, size, stability, and closed-loop output control.

During the past few years, low-cost, continuous-wave (CW) lasers have helped advance a wide range of life and health science applications such as cell sorting, DNA sequencing, confocal microscopy, micro array readers, hematology, and flow cytometry. The bioinstrumentation market continues to evolve, and as it matures, it continues to follow the same trends inherent to the semiconductor and telecommunications markets. Like their counterparts in those other markets, manufacturers of benchtop instruments are looking for robust, cost-effective solutions. They want smaller footprints so that they can decrease the size of their solutions. At the same time, they want to consolidate their supply chain by focusing on proven suppliers that can provide a complete spectrum of wavelengths.

In order to better serve this market, companies like Newport’s Spectra-Physics Lasers Division have concentrated on three goals: reliable performance across the spectrum, smaller footprints, and reducing costs. This article will look at the ways in which laser companies are designing low-cost CW lasers for bioinstrumentation.

DPSS vs. DD Laser Designs

Figure 2: Newport has developed a 3-in-1 power supply for OEM systems that use multiple lasers in a single enclosure.

Today, small-form-factor diodepumped solid-state (DPSS) and directdiode (DD) lasers are available in wavelengths that answer many of the most common bioinstrumentation applications, from spectroscopy to DNA sequencing and confocal microscopy. Many companies, like Spectra-Physics, use a combination of both technologies in order to offer a complete family of wavelengths.

The Spectra-Physics Excelsior products are intra-cavity doubled Nd:YVO4 (532 nm) or Nd:YAG (473 and 561 nm)-based lasers. The laser cavity is endpumped by an 808-nm laser diode. The pump light is absorbed by the gain material, which emits light at the fundamental wavelength of 1.122 microns, 1.064 microns, or 946 nm, respectively. The fundamental light gets frequency-converted by a non-linear crystal, which is located inside the laser cavity. Intra-cavity doubling provides unique advantages such as high harmonic conversion efficiency, very stable output power, and extremely low high-frequency noise — all important parameters for low-lightlevel bioinstrumentation applications. In addition, Newport’s Excelsior lasers include an active light loop to allow the system to monitor laser operation and maintain a constant output. Bioinstrumentation lasers can come with or without collimation optics, depending on the collection method and system application, but in every case, the laser head is temperature-stabilized by a thermoelectric cooler (TEC), which stabilizes the laser resonator structure. The temperature-stabilized laser head ensures reliable product performance over a wide temperature range.

In contrast to the DPSS lasers, directdiode lasers offer greater wavelength flexibility for targeting a wider range of fluorescent tags, for example. Newport’s Spectra-Physics line of direct-diode Excelsior lasers are semiconductor edgeemitting lasers that have been optimized around several key wavelengths, including 375, 405, 440, 635, and 785 nm. The lasers are typically packaged into Can-Type housings and emit a single transverse mode. Due to the short laser cavities, direct-diode products are not available as single-frequency options or narrow linewidth. The raw beam parameters are highly divergent and differ significantly when comparing the horizontal vs. the vertical axis. Additional optics are necessary to provide either an elliptical or round output beam. The spectral linewidth of the direct diodes is fairly broad (~0.5 nm) compared to DPSS lasers (<0.01 pm), which affects the brightness of the light source. However, in most bioinstrumentation applications, the brightness and spectral linewidth are not critical parameters because the laser light is used to excite fluorescent dye markers for downstream measurements.

488 nm - The Critical Wavelength
The most critical wavelength to join the coherent bioinstrumentation spectrum for DNA analysis and flow cytometry is 488 nm. In order to provide OEMs the maximum flexibility, many lasers have emerged that share a consistent footprint. One example is the Spectra-Physics Cyan OEM laser. The laser produces a high-quality optical beam at a wavelength of 488 nm with ultra-lowintensity noise and superior wavelength stability. At a fraction of the size of an Argon ion laser, the Spectra-Physics Cyan OEM laser consumes far less power and features dramatically improved mean-time-to-failure. It is available with output powers from 10 to 50 mW in a package that is only 125 × 70 × 34 mm (L× W × H) with typical power consumption of 6 W or less. While this package supports existing instrument designs, its footprint is significantly larger than the Excelsior line.

To answer the call for a smaller cyan laser that can be combined with other lasers in a single instrument, Spectra-Physics introduced the Excelsior 488 laser, which is 40% smaller than other commercially available lasers in this class. This enables OEM designers to shrink next-generation desktop instruments where size is a driving factor. Like the Cyan laser, the Excelsior 488-nm configuration delivers narrow-linewidth, singlefrequency operation for applications that rely on sub-MHz linewidth, plus solid power and wavelength stability.

In order to generate the 488-nm wavelength, the laser takes a slightly different approach. This Doubled External Cavity Semiconductor Laser (DECSL) uses a telecom-grade single-mode semiconductor gain chip at the fundamental wavelength of 976 nm. One facet of the gain chip and one external mirror form the laser cavity (i.e., external cavity). The fundamental wavelength of 976 nm is frequency-doubled by focusing the light into a non-linear crystal. A small portion of the SHG output is directed onto a photo detector, which is part of an active servo loop that maintains constant output power while operating the laser. Before exiting the hermetically sealed laser head, the output light passes through a telescope for final adjustment of the output beam parameters.

3-in-1 Solutions
While there is ongoing research to reduce the footprint of the laser itself, many companies are also looking at other ways of reducing the footprint of benchtop instruments. One area of focus has been at the power supply. For example, Spectra-Physics has developed a new power supply with the capability of driving up to three Excelsior DPSS or DD lasers in any combination or configuration. Obviously, reducing the space requirements for the power supply is another significant step in shrinking the footprint of next-generation benchtop instruments, but the three-inone power supply enables other benefits including greater product customization capabilities, and improved thermal management.

Manufacturers of bioinstrumentation continue to follow their colleagues in the telecom and semiconductor markets. In order to meet the demand for cutting-edge solutions in life and health sciences, they will need complete solution providers that can help them lower capital equipment costs and reduce the footprint of their benchtop instruments, while simultaneously delivering unique performance such as beam characteristics, line widths, and stable operation.

Laser providers already are focused on factors such as expanding wavelength offerings, turnkey fiber-coupled solutions, footprint size, narrow linewidth, low RMS noise, and low power consumption to drive next-generation bioinstruments. These advancements in DPSS, direct-diode, and doubled diode extracavity lasers will continue to support a wide range of applications that include flow cytometry, cell sorting, DNA sequencing, confocal microscopy, micro array readers, retinal imaging, and hematology.

This article was written by Jürgen Niederhofer, Product Manager, of Newport’s Spectra-Physics Lasers Division in Stahnsdorf, Germany. For more information, contact Mr. Niederhofer at juergen.niederhofer@spectra-physics.com or visit http://info.hotims.com/10968-200.