Developing a Miniaturized Laser Diode Solution for the Medical Market

According to Pantec Biosolutions AG (Liechtenstein, Europe), the global aesthetic market, which includes skin rejuvenation, is expected to grow from a $4.4 billion market in 2010 to a $7.5 billion market in 2015. Meanwhile, the market for transdermal drug delivery is growing rapidly and is expected to be a multi-billion dollar market by 2015. Designed for medical professionals and consumers, these new devices offer a precise delivery method for a variety of medical applications, such as skin rejuvenation, in-vitro fertilization, and vaccinations.

Fig 1 – P.L.E.A.S.E.® Professional, a tabletop medical laser device targeted mainly for the dermatologic and aesthetic markets.

Pantec Biosolutions’ first Oclaro-based product, which is called the P.L.E.A.S.E.^® Professional, is a tabletop system that will be used for aesthetic skin rejuvenation and selected dermatological applications. This device will initially be sold in Europe with plans to bring the product into selected markets worldwide, starting in 2011.

The following article discusses how Oclaro (San Jose, CA) worked with Pantec Biosolutions AG to develop this customized quasi-continuous wave (QCW) sub-assembly laser diode solution.

Market and Application Can Drive End-Product Requirements

An industrial application usually requires higher power and higher brightness, as well as a laser diode lifetime of several ten-thousand hours. Medical devices, on the other hand, tend to have lower requirements on diode lifetime and are more sensitive on parameters such as cost, efficiency, thermal management, and size. In this example, a small handheld battery-powered laser device needed to be developed. Oclaro was able to leverage its full skill set from epitaxial design of semiconductor lasers, wafer processing, optical coating and facet passivation, as well as assembly design. The Pantec Biosolutions’ team was responsible for designing the solid state laser, as well as the overall application know-how, including clinical studies and compliance with laser safety and medical regulations.

This particular project with Pantec Biosolutions focused on its laser-based transdermal delivery platform P.L.E.A.S.E.^® (Precise Laser Epidermal System). The first device using this new platform is the newly-released P.L.E.A.S.E.^® Professional (Fig 1), a tabletop medical laser device targeted mainly for the dermatologic and aesthetic markets. It creates precisely controlled aqueous micropores through the stratum corneum into the epidermis (Fig. 2). An intelligent graphical user interface, together with the CE mark and the laser class 1 grading, make it simple and safe for the medical professional to use. At a later time, this device will be complemented by the P.L.E.A.S.E.^® Private (Fig 4), a battery-powered handheld medical laser device targeted mainly for drug delivery.

Meeting Small Packaging Requirements is Crucial

Fig 2 – Schematic picture of the P.L.E.A.S.E.® Professional handset, illustrating the application of the device.

A small size and the capability to deliver the required pump power at power conversion efficiency values above 60% were pre-requisites for small packaging. Simple thermoelectric cooling enabled the design for the small handpiece of the P.L.E.A.S.E. Professional and for the battery-powered home use device. For the pumping of the Er:YAG solid state laser, a monolithic laser assembly with an overall dimension of just 30×25×17 mm³ and a 970-nm laser diode bar with a 9.5-mmwide emission aperture was designed.

For the epitaxial design of the device, the focus was on optimizing power conversion efficiency and thermal robustness. The resulting structure is characterized by a single InGaAs quantum well, and an AlGaAs wave guide. Oclaro’s E2 facet passivation technology, which protects the front facet of the bar against Catastrophic Optical Damage, enables reliable operation of multimode devices at high ex-facet power densities of 120mW/μm or more [2,3]. The passivated mirror facets are coated with high anti-reflection coatings for optimized device performance.

For qcw bars operating at 500A drive current and about 600W output power, 200-μs pulse width and 5-8% duty cycle was performed with bars of the present 970nm chip configuration. No significant degradation was observed after 370 million pulses. The most critical assembly steps were carried out on fully automated die bonding systems, which ensures a high alignment precision and reproduceable solder interfaces with good thermal properties and a homogeneous stress distribution. The thermal properties were simulated using the finite-element-method. Telecom-grade AuSn (gold tin) hard solder makes the product suitable for the most demanding hard-pulse operation mode. The laser diode sub-assembly is shown in Fig. 3.

A key requirement for the design of a monolithic solid state laser without any active adjustment possibility is to ensure stable operation within a wide range of heat load generated in the laser cavity.

Developing a Miniaturized Diode-Pumped Laser System

Fig. 3 – Photograph of the a) the laser diode and b) the assembled pump unit with vertical beam emission reflecting the actual size of the unit.

The Er:YAG solid state laser is a four-level laser system. Emission at 2.9 μm is of particular interest for applications that target soft and hard tissue because the emission wavelength coincides with the main water absorption line. Most commercially available Er:YAG laser systems are flash lamp pumped and, due to their low efficiency, require large cooling systems. The development of a diode-pumped Er:YAG laser system led to a significant increase in efficiency, allowing miniaturization of the laser, improved beam quality, and reduced maintenance requirements (Fig. 5).

The 1-mm-diameter Er:YAG laser rod is side pumped by a single 10-mm-wide 970nm laser diode bar mounted on a 10×12×5 mm³ conductively cooled platform. The target application requires reliable laser diode operation at 300W, 200μs, a duty cycle of 10%, and a base plate temperature of 20–30 °C. For efficient pumping of the rod, a laser diode without wavelength stabilization was required to be designed for high thermal robustness. The high power of the laser diode and the small laser rod diameter enable side pumping of Er:YAG despite its low emission cross-section. In order to operate the laser efficiently, the losses in the cavity need to be minimized. This has been achieved by using a monolithic laser cavity. As a further advantage, the monolithic cavity requires no alignment, and the required robustness against mechanical shocks is easily obtained without the need for time-consuming adjustments.

Fig 4 – The P.L.E.A.S.E.® Private device pretreats the skin prior to intra-epidermal drug delivery.

The miniaturized diode-pumped Er:YAG solid state laser system has an overall dimension of only 30×25×17 mm³, including the sealed, highly shock- and vibration-resistant package and the thermoelectric coolers (TECs) (Fig. 5). The system generates 2.8 W average power with M2<5, the optical to optical efficiency is higher than 10%.

Present and Future Target Applications

According to Pantec, the target applications for the medical diode-pumped solid state laser system are transdermal delivery of pharmaceutical and cosmeceutical products and Laser Dynamic Therapy® (LDT). Microporation offers advantages in the delivery of topically applied systemic treatments such as large molecule protein drugs to treat precancerous skin lesions that at present cannot be adequately delivered in topical application. In addition, in vitro analyses have shown that P.L.E.A.S.E. microporation can facilitate the rapid transdermal delivery of various topical anticancer drugs at adequate levels for the efficient treatment of pre-cancerous lesions, suggesting accelerated biological response and increased efficiency.

It is also appropriate for combination treatments where transdermal delivery of cosmeceuticals may be appropriate for certain dermatological conditions. This approach is less invasive than conventional alternatives, and offers faster healing and a shorter downtime with the potential for improved clinical outcomes.

Fig. 5 – Schematic drawing of the diode-pumped Er:YAG laser with the 21-mm-wide thermoelectric cooler on top and bottom and the laser crystal in black. Solid state laser light emission is perpendicular to the drawing plane. The laser diode and the pump optics are hermetically sealed inside the package.

Laser Dynamic Therapy is a precisely controlled treatment of pre-cancerous or other skin lesions, using laser-assisted microporation in combination with delivery of specific anticancer drugs. The significantly increased skin permeation enables a much deeper penetration and improved delivery of the active substances.

Pantec Biosolutions expects that the accelerated biological response within the target structure will result in a faster clearance of the treated tumor lesions, compared to known conventional topical treatments. The ultimate goal is to improve patient convenience and compliance by giving dermatologists several options to increase treatment efficiency of pre-cancerous lesions and to reduce treatment time and pain associated with standard photodynamic therapy (PDT) or conventional ointment therapies. The system can either enhance the therapeutic outcome of standard PDT by increased permeation of photosensitizers deeper into the dermis, or by completely replacing the currently used methods with Laser Dynamic Therapy.

Conclusion

In conclusion, the successful development of an application-specific product requires close cooperation on all levels with the tight interaction of R&D teams. The semiconductor as well as laser diode assembly have to be matched to the requirements of the end product, which can vary strongly depending on market and application.

This article was written by Christian Naumer, Senior Product Line Manager for Oclaro, Inc., San Jose, CA. For more information, Click Here

References

M. Krejci, A. Heinrich, J. Müller, T. Bragagna and N. Lichtenstein, "Miniaturized high power Er:YAG solid state laser pumped by a single laser diode bar", Proc. SPIE 7912, 79121D (2011); doi:10.1117/12.873576
D. Jaeggi, C. Naumer, “Diversity is strength for high-power laser diodes”, optics.org/OLE, 1/2009
C. Naumer, N. Lichtenstein, B. Schmidt, P. Bruns, F. Kubacki, P. Harten, O. Lissotschenko,“Brightness Required!”, Laser & Photonik, 3/2007
C.Naumer, D.Jaeggi, N.Lichtenstein, B.Schmidt, “Setting the Pace in Industrial Laser Systems”, Laser & Photonik, 4/2006
N. H. Zech M.D., M. Murtinger M.D., P. Uher M.D., “Pregnancy after ovarian superovulation by transdermal delivery of follicle-stimulating hormone”, Fertility and Sterility, Volume 95, Issue 8, 30 June 2011, Pages 2784-2785
Y.G. Bachhav, S. Summer, A. Heinrich, T. Bragagna, C. Böhler, Y.N. Kalia, „Effect of controlled laser microporation on drug transport kinetics into and across the skin”, Journal of Controlled Release, Volume 146, Issue 1, 17 August 2010, Pages 31-36
J. Yu, Y.G. Bachhav, S. Summer, A. Heinrich, T. Bragagna, C. Böhler, Y.N. Kalia, “Using controlled laser-microporation to increase transdermal delivery of prednisone”, Journal of Controlled Release, Volume 148, Issue 1, 20 November 2010, Pages e71-e73