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MIcro-pulsed, diode-based LASer

Periodic Reporting for period 1 - MILAS (MIcro-pulsed, diode-based LASer)

Reporting period: 2017-09-01 to 2018-08-31

MILAS project was outlined to meet the needs of Norlase in regard to the development of the next generation of lasers emitting at 560-600 nm for medical applications. The core of Norlase’s technology also known as tapered diode doubled lasers (TDDL) is based on frequency doubling the emission of high-brightness tapered diodes. Norlase has demonstrated emission in continuous wave (CW) in the green-yellow region, for instance, at the green-lime (561 nm) and at the yellow (577 nm) wavelength. Moreover, TDDL can deliver high power (~3 W) in a nearly-diffraction output beam, making it especially attractive to medical fields as ophthalmology, where high-brightness visible lasers are required.

Current ophthalmology treatments also demand modulation of the laser light intensity to avoid excess damage through overheating of the area to be treated. On this background, the MILAS project was conceived in order to develop modulation capability of TDDL and played a fundamental role by pushing the technology further to the market needs.

After the end of the project, the Innovation Associate was hired by Norlase in a permanent research position.
The work performed in MILAS was divided into 5 work packages. The following was performed and achieved within each work package:

Work Package 1: Project Management & dissemination

Task 1.1 Project management, coordination and reporting
No challenges or delays were found between the coordination of the planned activities and the work carried out by the IA. All milestones have been accomplished within the project.

Task 1.2 Recruitment
The recruitment process started at the end of March 2017 and ended on May 1st, 2017. Interviews were carried out during May/June, and the candidate was selected in July 2017. The innovation associate started as scheduled, on September 1st, 2017.

Task 1.3 Risk management and contingency planning
No contingency planning or risk management was needed in the project.

Task 1.4 Dissemination
MILAS project resulted in two contributions to photonics conferences, SPIE Photonics West 2018 in San Francisco, and CLEO San Jose, 2018. At SPIE Photonic West, Norlase presented for the first time a Watt-level yellow laser system . The Innovation Associate participated in the characterization of this laser system and in preparing the manuscript. At CLEO San José, the demonstration of the first Watt-level pulses by dephasing modulation was presented by our CEO . Additionally, a scientific article entitled “2.7 W diffraction-limited yellow lasers by efficient frequency doubling of high-brightness tapered diode lasers” was submitted by the IA as the first author to the Elsevier journal “Optics and communications”.

Work package 2: Training

Task 2.1. In-house training
Two workshops were held at Norlase headquarters. The first one was focused on good-practices at Norlase labs, an introduction to the lab equipment and laser safety precautions. The second one introduced the IA to regulatory issues for medical products using lasers and provided a general overview of quality management.

Task 2.2 Short-terms stays
Two short terms stays were planned at research institutions relevant for the IA. One was completed within the project period, the second was scheduled after the project period.

Task 2.3 Core training events
The IA attended four seminars organized by the European Commission.
Work package 3: Temporal dynamics and feedback sensitivity

Tasks 3.1 Theoretical considerations on laser dynamics
The IA reviewed the state of the art of the tapered diode lasers.

Tasks 3.2 Diagnostics: measurements and characterization
Several experimental techniques were used by the IA to identify and to gain awareness on the laser dynamics as well as on the feedback effects. This was also helpful in understanding how modulation by de-phasing could be attained. The main outcome of this task is that Norlase incorporated in its assembly procedure a new way of determining whether the diode lasers are unstable after collimating the infrared beam.

Work package 4: New laser modulation scheme based on de-phased micro-pulsing

Tasks 4.1 Millisecond pulsing: mechanical shutter
Microsecond pulsing was demonstrated by using a mechanical shutter (NMlaser, LST200SLP). A customized shutter with an aperture larger than the offered in the NMlaser catalogue was required.

Tasks 4.2 Microsecond pulsing: current modulation de-phasing
De-phasing modulation was tested by driving the currents of two diode lasers of 4 and 6-mm long in different crystal lengths: 10, 40 and 80 mm (two-cascaded crystals).

Task 5.1Design select and purchase components.
The breadboard prototype (see Figure 1) was formed by a Norlase laser system, a fiber coupling system, and a mechanical shutter. The mechanical shutter was placed in the beam path before the light was coupled into the fiber (not shown in Figure 1). The breadboard also included custom-made electronics to drive the shutter.

Task 5.2 Construct breadboard prototype test breadboard prototype.
The tests carried out using the mechanical shutter and the electronics showed a stable operation of the shutter. In this case, the generation of the microsecond pulses is usually limited by the shutter lifetime, in the order of billions of counts. On the contrary, the microsecond pulsing is limited by the diode lifetime whenever the driving conditions and temperature setting remain unchanged.
The project set out three overall objectives to go beyond the state of the art, all of which were achieved:

- Provide theoretical understanding of semiconductor structures relevant to Norlase system:

The IA performed a set of measurements identifying the key parameters relevant to the modulation. This was done with the aim of investigating the appropriate strategy for de-phasing modulation. A good theoretical understanding of the tapered diode lasers and their emission characteristics was gained. It was found, for example, that the hysteresis of the emission of the laser diode while sweeping the currents upwards and downwards makes the wavelength jumps difficult to predict.

- Develop and implement a novel micro-pulsing scheme based on de-phasing:

Two setups were built, one in an optical breadboard offering flexibility and access to the different optical components, and the other one was directly using a Norlase laser system. In the former configuration, modulation was tested and investigated using different crystals lengths. This setup provided a solid understanding about the current limitations of the de-phasing modulation. In the latter, a Norlase laser system with two cascaded crystals was tested under de-phasing. As expected, it was found that the wavelength fluctuations while changing the currents of the diodes made challenging the generation of stable and well-defined pulses.

- Demonstrate the innovate and business potential to the Norlase product portfolio by building a breadboard prototype:

An up and running prototype was built. The laser system emitted 2.7 W (CW) at 577 nm. The mechanical shutter, selected among several to comply with the medical safety regulations, was tested providing a minimum pulse duration of 2 ms, and a pulse amplitude corresponding to the maximum attained power of 2.7 W. After this project, Norlase will be capable of offering microsecond pulses along with their laser systems in its portfolio based on the mechanical shutter solution.