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
Today, small-form-factor diode-pumped solid-state (DPSS) and direct-diode (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 end-pumped 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-light-level 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, direct-diode 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 edge-emitting 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.