Maintaining high operational value while minimizing system costs is a constant challenge in the technology industry. This is particularly true for advanced research and manufacturing systems that employ laser light for purposes such as analysis, sensing and fabrication.
The good news is that the cost of lasers has decreased over time to the point where more common lasers can be purchased at a more cost-effective price, regardless of their power output. This simplifies the choices for optics in many laser applications, as the use of a higher power laser can compensate for lower quality optics. However, this is not the case for ultrafast laser applications where success or failure is highly dependent upon high-quality, specialty lasers and optical components.
Lasers chosen for cutting-edge applications must produce coherent light with precise, application-specific characteristics. The parameters that define these characteristics include:
The properties of ultrafast laser light at the target are strongly affected by phenomena such as dispersion within the optical path. This is why ultrafast laser selection requires careful consideration of the optical properties of the reflective and refractive components in the optical path to the target.
In this white paper, we explore the “false economy” provided by lower quality, lower cost optic components and its impact on the ultimate success or failure of research experiments and other ultrafast laser applications.
While the prices for ultrafast lasers have been dramatically reduced in recent years, the costs remain significant, especially for research applications where funding may be limited. If a researcher has invested most of the available funding into the laser system of an analysis, sensing or fabrication system, there may be a strong temptation to opt for less expensive optics to complete the laser light delivery subsystem. This is almost always a detrimental choice for ultrafast laser applications.
Components in the optical path that have poor optical characteristics will degrade many critical quality parameters of the laser light delivered to the target. Moreover, the erosion of the laser light quality in ultrafast laser systems is often subtle and not easily detected without dedicated specialty instrumentation such as spectrum, power or pulse width analyzers.
When inexpensive optics are used in an ultrafast laser system, the laser light pulse most often suffers from dispersive effects. Dispersion of the laser light pulse as it passes through the optical train results in:
Lengthening or distortion of the laser light pulse can have serious negative impacts for research experiments and other applications. For example, peak power is a critical laser parameter in applications such as nonlinear optical investigations. It is directly and proportionately affected by an increase in the duration of the laser light pulse. A laser pulse length that changes from 20 fs to 30 fs duration – easily the case when using non-specialized optics – produces an approximate 30% decrease in the peak power of the pulse at the target.
This, in turn, produces concomitant changes in the results of measurements or manufacturing processes using the laser system. If a corrective dispersive mirror solution cannot be implemented in the optical path, optical pulse compressors (Figure 1) can be employed. However, optical compressors are expensive and are composed of optical components that can be challenging to align. They also produce additional power losses in the optical path (prism compressors normally account for 30-40% loss) and they may fail to compensate for higher order dispersion effects.
Inexpensive optics can create dispersion in the optical path for ultrafast systems that require compensation for best performance. Dispersion can be avoided by using specially designed optical components that may be more expensive in the short term, but create more successful outcomes overall.
Specially designed optics can keep dispersion in check, eliminating the need for costly, difficult-to-align optical compressors. When selecting such specialty optics, it is important to consider certain critical specifications for use in ultrafast laser systems:
The two graphs in Figure 2 clearly demonstrate the distortion that occurs when an ultrashort pulse is reflected by a standard dielectric mirror (left) as compared to the same pulse reflected by a mirror optimized for use with an ultrafast laser (right).
It’s important to note that any of the reflective optics discussed in this section may be used; however, the unique needs of your particular application should be carefully considered in the decision process.
A typical ultrafast laser optics configuration may have, in addition to polarizing and focusing elements, at least 10 reflective mirrors between the laser light source and the target. The high number of mirrors makes it imperative that the specific reflectivity of the mirrors be maximized since any optical losses due to reflection are cumulative and these losses reduce the optical power at the target.
It is important to consider that standard metallic mirrors such as gold or silver work well in the ultrafast regime only so long as the protective over-coat is designed for ultrafast laser system use. A bare metal coating has minimal GDD because the light does not penetrate deeply into the metal before being reflected, thereby minimizing dispersion. However, bare metals are not recommended because of degradation over time.
Most metallic mirrors have dielectric over-coats for environmental protection and reflectivity enhancement; however, improper design of these coatings may significantly distort the GDD at certain wavelengths within the high reflectance range.
Also, metallic mirrors typically have poor reflectivity at specific wavelengths as compared to dielectric coatings. Ultimately, the use of a dielectric or metallic mirror with no GDD specifications risks distortion of the pulse or reduced broadband reflectivity within the optical train.
As a model, consider some different options shown here for a mirror optimized for 800 nm laser light pulses in an ultrafast spectroscopy experiment. The different mirrors in this list are rated from “OK” to “Best” for this application.
Ultimately, the investment made depends upon your particular application and needs.
Legend:
R = absolute reflectivity, typically at normal angle of incidence
Rp = reflectivity of P-polarized light, typically at 45° angle of incidence
Choosing the best optics for an ultrafast laser application can be a challenging proposition. The choices made for the ultrafast laser, particularly for the optical components, will be critical for determining the success or failure of research experiments and other ultrafast laser applications.
It is always prudent to take the time to understand the optical requirements of the application and to study the specifications of the optics selected to meet these specifications prior to purchasing the system components. The least expensive choice may not always be the most cost-effective or technologically suitable one, resulting in higher costs – and more failures – in the long run
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