Fibre optics - laser delivery for the future

Instrument builders are under constant pressure to create more functional, more reliable, higher-performing instrumentation. In this pursuit, single-mode fiber optic delivery of laser beams is increasingly being seen as a key enabler, particularly for high value specialist microscopy applications.

THE beauty of a fibre optic laser delivery system is in the flexibility it gives the instrument designer to route the laser beam. It allows the laser source to be separated from the instrument front end and removes the need for the traditional bulky opto-mechanics. As a result, it offers smaller, neater instrument design.

When used with a diode laser, fibre optic delivery actually improves the quality of the beam by suppressing the higher order transverse modes and transmitting a single traverse mode of light that is almost Gaussian in profile. This removes the need for additional filters (Figures 1a and 1b).

When used with a gas laser such as He-Ne or Argon ion, fibre optic delivery protects the instrument from the heat and vibration generated by the laser. This is because single-mode fibre is a cold light source so heat and vibration at the input are not transmitted to the output.

All types of laser exhibit transverse jitter, or beam pointing instability caused by thermal and opto-mechanical instabilities in the laser cavity – typically in the region of 30 microradians per ?C. Use of a single-mode fibre delivery reduces this dramatically - the best designs offer a pointing beam stability of better than one microradian per ?C regardless of laser technology.

The core diameter of a single-mode fibre operating within the visible wavelengths range is typically about five microns. Because of this, the fibre system is sometimes perceived as being inefficient, difficult to use and potentially unstable.

In fact, the technology has developed significantly in the last 15 years to overcome these objections. Careful design of the input optics to mode-match the laser beam parameter to the fibre achieves excellent results – typically 70% coupling efficiency or better (Figure 2).

The latest systems are exceptionally easy to use and highly repeatable. For example, the Point Source system takes about two minutes to align and suffers less than 0.5%loss over 100 repeat insertions. This has been achieved through a combination of sub-micron factory optical alignment that guarantees beam position and beam angle of less than 100μm and 200 microradians respectively and detailed kinematic design of the laser-to-laser interface (Figures 3a and 3b).

A further objection to single-mode fibre is that, because it has a circular cross-section, it degrades the polarisation of the laser. This is because when it is bent or coiled, the stress optic co-efficient of the glass induces birefringence. The outcome is two modes of propagation, which recombine with an unpredictable phase relationship at the output, resulting in polarisation drifting and fading.

This problem is avoided in modern systems by the use of polarization-preserving fiber, which is not optically symmetrical and has strong internal birefringence cased by stress-applying sectors. The internal birefringence is significantly higher than normal bend-induced levels, so as long as the laser is correctly aligned to either of the two axes, polarisation is preserved.

iFLEX Q3 compact laser diode system for precision optical instrumentation in the biotechnology and semiconductor sectors

Point Source has worked with many OEM customers to integrate single-mode fibre technology solutions into specialist microscopy instrumentation. For example, its technology is now employed in 50% of all the confocal microscopes worldwide, and the company is working with instrumentation builders involved in total internal reflection fluorescence microscopy (TIRF) and related areas such as fluorescence recovery after photo-bleaching (FRAP) investigations and fluorescence ratio determinations.

Following are two examples of how single-mode fibre laser delivery is achieving significant performance benefits in specialist measurement instrumentation in DNA sequencing and protein crystallography.
The speed of development in DNA sequencing technology has been astonishing and facilitated the first ever large-scale biological project. The Human Genome and related projects spanned many continents in an amazing co-operative effort to generate the DNA sequences of many genomes. The focus is now shifting to re-sequencing in an immense effort to establish a link between genotypic variation and phenotype.
Next generation sequencing technologies will make it even cheaper and quicker to sequence a genome. This will allow for complete genome sequences to be determined from many different individuals of the same species. For humans, this will allow us to better understand aspects of human genetic diversity.
To make sequencing a viable technology, instruments have been developed that can run more than one sample at a time. These new platforms are based on parallel sequencing of millions of DNA fragments. High sensitivity fluorescence detection is achieved using laser excitation and total internal reflection optics
Laser lifetime, even of the latest solid state sources, is still a consideration in the design of instruments to minimise downtime and cost of ownership. This means that the critical optical alignment of the laser beam to the high density arrays is a significant overhead burden during manufacture and field replacement.

By employing fibre-optic beam delivery systems in the design, the need for expensive line of sight optics is avoided.  Remote citing of laser head and power supply de-couples sources of heat and vibration from the instrument, and allows a smaller footprint for the instrument head. Opto-mechanical alignment considerations of the laser beam to application are simplified and no optical re-alignment within the instrument head is required during laser replacement, resulting in minimal downtime.
The low beam pointing error inherent in the single-mode fibre solution also reduces possible fluctuations in the signal to noise ratio from Rayleigh and Raman scattering for non-confocal detection architecture (Figures 4a and 4b).
Optic-optic delivery of lasers is a true enabler in the goal of next generation instruments to sequence the entire human genome in less than a day, and for a cost of less than $1,000. This could open the door to a wide range of new and accessible applications, such as assaying disease risk and monitoring therapy response in patients.

Macromolecular crystallography is a powerful tool in revealing the structures and interactions of protein molecules. It provides valuable information that can be used to develop effective pharmaceutical compounds more rapidly. Recent advances in crystallographic technologies have established protein crystallography as the lead instrument in structure-based drug design. However, until now the actual crystal growth has been the largest bottleneck in this area of proteomics.

Figure 1a and 1b: Beam profile (traverse intensity) – before and after coupling. Figure 2: Throughput efficiency (%) – distribution of 1000 fibres. Figure 3a and 3b: Beam position (µm) and beam angle (mrad) – distribution of 1000 fibres. Figure 4a and 4b: Beam pointing stability (µrad) before and after fibre coupling

In the dynamically controlled protein crystal growth (DCPCG) experiment conducted onboard NASA space shuttle mission STS-95 coupled-coupled lasers diodes were used to study protein crystal growth in space. 

When the detrimental gravitational effects, such as convection and sedimentation are eliminated, protein crystal growing conditions are optimised. Ultimately, this can yield larger and more perfect crystals, achieving crystallisation un-obtainable on Earth.

The laser light scattering subsystem used in the experiment provides a method of detecting an aggregation event earlier than by standard video analysis methods, thus enabling more precise control and optimisation of the crystal growth process. 

Specifically, the onset of aggregation leads to an increase in scatter of a collimated laser diode beam directed through the sample cell. This scatter is detected at 90° by a photodiode shielded by a small pinhole aperture. There is a separate laser light scattering system, each with its own laser, for each of the ten growth cell blocks in the experiment, as well as for the control cell.  Because the system activation is determined by scattered light, if the beam moves relative to the detector pinhole, the change in background scatter signal could be misinterpreted as the onset of aggregation.

Apart from obvious benefits brought by diode-based laser systems such as small size and low power consumption, the key to the success of the system is the low beam pointing error enabled by fibre coupling of the laser diode. The stability of direction and point in space means confident detection of the onset of crystal aggregation. The angular pointing error of the beam is less than 1µrad for every degree C rise in temperature. The experiment was housed in a thermally controlled environment, so the fibre optic cables provides the ability to locate the lasers remotely, and hence remove any possibility of thermal perturbation to the crystallisation experiments.

David Pointer is Managing Director and founder of Point Source Ltd, the world leader in Flexible Laser Technology for precision optical instrumentation, based at Hamble, UK.