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Compact, high-power, frequency-converted diode laser systems

Periodic Reporting for period 1 - CoDiS (Compact, high-power, frequency-converted diode laser systems)

Reporting period: 2015-06-01 to 2015-10-31


Visible lasers are of interest to a wide variety of industries, including life science, lighting, medical diagnos-tics, laser pumping, and scientific applications. Existing laser technologies do not offer the combination of intrinsic stability, low noise, high beam quality, high power, and wavelength tuneability of the Norlase Tapered Doubled Diode Laser platform.
Norlase is a spin-out company set up to develop and commercialize a new class of visible lasers, tapered diode doubled lasers (TDDL), based on award-winning technology invented at Technical University of Denmark (DTU). Norlase’s top management – including the CEO and the five Board Members – have longstanding experience and a very strong track-record within the photonics and lasers industry, namely in developing start-ups into large, successful businesses and/or heading some of the world-leading companies in the sector.

The TDDL is a unique and simple platform technology that vastly reduces cost of visible lasers for high-volume applications. Based on patented and award-winning technology, the overall objective of Norlase is to manufacture a new class of compact, stable, low-noise visible lasers in the 1 to 20 W output power range that provides designers of OEM equipment unique opportunities in terms of performance and cost effectiveness. The Norlase mission is to become world-leading supplier of visible continuous wave lasers in this power range. Norlase aims at establishing itself as a laser production company based in Europe – thus creating significant tangible economic impacts, namely in terms of job creation, particularly highly-skilled engineering and highly-specialized technical jobs. Despite being a start-up company, Norlase has already completed prototype sales including delivery to several market leading OEM customers, raised BA and VC capital and finally Norlase has won several national grants (incl. EUDP and the Market Maturation Fund) in fields including laser-based lighting and life science applications.

The novel, compact laser is based on semiconductor lasers (diode lasers) and this technology has potential to yield low-cost, reliable products. In market segments representing a few hundred units per year for Norlase, as those mentioned above, the unit cost may be reduced drastically, i.e. a step-change cost-wise. For our OEM customers this step-change will expand existing markets and additionally open new market opportunities. Hence, the Norlase technology provides a huge business opportunity to establish a leading laser manufacturer within Europe thriving on a technology (semiconductors), where Europe holds a strong and world-leading position.

The key objectives for the overall innovation project are:

• to complete the validation of Norlase’s platform technology and to demonstrate and prepare market roll-out of the initial product portfolio – particularly in wavelength regions where development and initial demonstration have already been reached, i.e. green (532 nm) and blue (488 nm);

• to complete 10,000 hrs life-time and reliability tests of products at 532 nm (2W and 4W) and at 488 nm (2W) in order to have a fully certified product for OEM customers;

• to remove the need for expensive optical isolators by innovative designs of semiconductor laser;

• to plan the expansion of Norlase’s platform technology into wavelength regions that cannot be reached by competing technologies, namely yellow, where there is huge market potential.

Accordingly, the main objectives for the CoDiS feasibility study (Phase 1) was:

• to map out market segments in a detailed business plan, where Norlase technology makes the above-mentioned step-change cost-wise allowing sustainable growth of the company, and to prepare activities for Phase 2 project,

• to complete the design phase for electronics, control software, and user interface,

• to elaborate the strategy for further reducing the number of components that in turn will allow further cost reduction, thus enhancing the business opportunity.

Norlase currently has opportunities to provide several different wavelengths. The obvious strength is that irrespectively of the chosen wavelength, the opto-mechanical platform, electronics and control software remains unchanged. However, some remaining risks need to be addressed, namely reducing complexity by eliminating optical isolation, and aligning the development of the technology and the technical specifications to the best route to market, ensuring sustainable growth of the company. Accordingly, the targeted key achievements are:

• To fully develop the market study and compile this into the detailed business plan:
 as part of the assessment the route to market should be mapped out

• To further investigate the need for optical isolation and possible technical solutions circumventing this component that otherwise might pose a bottleneck in increasing profitability;

• this particular component is bulky and relatively expensive even in high volumes, thus solutions without said component will reinforce the Norlase business case.

• To complete the design phase for electronics, control software, and user interface:
 complete the design of the modular driver electronics for the laser head;
 complete the control software programming that would allow accessing laser heads via internet protocols, which is highly useful e.g. for remote trouble shooting.

Accordingly, the two main technical feasibility study activities planned in the work package was the design of control system and interfaces and the investigation of the options for removal of the diode current used for optical isolation. In parallel to these activities, we have achieved significant progress with the platform in the yellow wavelength regions. These positive results have a major impact on our plans for exploitation of the platform.
Technical feasibility study results

1. Control system and interfaces:
The TDDL requires cost-effective and easy-to-operate driver electronics, that can be controlled via a simple user interface. In this task, design of the modular driver electronics for the laser head and the control software is completed.

Laser driver:
In collaboration with an electronics supplier, a one-box solution to control the laser head was developed. This solution is called “AuroraOne”. A picture of the electronic driver can be seen on figure 1.

The controller is in essence 5 separate electronics controllers combined into one. The 5 separate drivers are as follows. Two current controllers (power supplies) to control the current injected into the tapered diode laser, and three temperature controllers that control the temperature of the laser and the two non-linear crystals. The unit also contains an additional temperature monitor used for recording the case temperature of the laser head. Finally, the controller includes a circuit for monitoring the response from a photodiode inside the laser head. The response from this photodiode can be used to calculate the output power from the laser. This feature can either be used to monitor or stabilize the output power from the laser.

On the front of the AuroraOne a power switch turns the device on/off. A key switch disables/enables the laser output. The AuroraOne is connected to the laser head with a single custom made laser cable connected to the back of the device. Also on the back of the device, an external interlock circuit can be connected to the controller, enabling safety shutdown of the laser if needed. Furthermore, the controller can drive an external fan, if forced air-cooling of the laser head is needed.

A USB port on the back of the AuroraOne is used for controlling the driver with a PC.
Compliance testing of European and US regulations were performed with an external regulatory and compliance laboratory. Test certificates have been issued, showing that the AuroraOne complies with the following standards:

a) CE EN 61010-1:2010 – Safety requirements for electrical equipment for measurement, control and laboratory use.

b) CE EN 61326-1:2013 – Electromagnetic compatibility (EMC) requirements for electrical equipment for measurement, control and laboratory use.

c) FCC CFR 47 Part 15 – Emission regulations on electronics device sold inside the United States.

The current status of the AuroraOne is that the first 3 working units have been developed, built and delivered. These units are currently undergoing tests. Minor bugs are identified and fixed. An additional order of 15 devices is placed with the supplier for delivery in Q1 2016.

Control software
To communicate with the AuroraOne and thereby with the laser head, a LabView-based program is made. This program monitors and controls all relevant parameters, such as temperatures, currents and laser output power. All the parameters of the laser are preset, and the laser is turned either on or off by pressing the “start” and “stop” laser button.
Laser emission can be enabled in two modes of operation, “constant current” and “constant power”. In constant current mode, the currents to the laser diode are fixed. External and internal influences on the laser head, such as room temperature or diode ageing are not compensated for. The output power will therefore be fluctuating slightly. In constant power mode, the photodiode inside the laser head is used to monitor the output power. The current setpoints are then dynamically adjusted to obtain a constant and stabile laser output.

Included in the software is an algorithm to turn on the laser in a controlled manner. The algorithm ensures that the laser operates in a “correct” mode of operation when emission is enabled. This ensures that the laser will run for a longer time and not fail after a short time of operation.

The software program constantly monitors a number of parameters of the laser head. These parameters include currents supplied to the laser diode, temperatures (laser diode, crystals and case), output power, etc. All parameters are logged in a data file on the PC, making it possible to investigate the performance of the laser head over time.

2. Optical isolator alternatives
The TDDL comprises one bulky and costly component, i.e. an optical isolator. Presently it is needed to prevent the tapered diode from failing or becoming unstable.
First, a detailed investigation of the feedback sensitivity of the tapered diode lasers has been performed in order to assess the feedback levels where the laser is affected. A tapered diode laser was collimated and directed to a mirror setup before which a variable damping of the laser beam could be inserted. The feedback level could thus be varied in the range 10-7 to 10-3. Different measurement devices were used to investigate the impact of the different levels of feedback. During the investigations, the spatial beam properties, the spectral properties and the intensity noise was monitored. It turned out that feedback at these low levels had negligible influence on the spatial beam properties. However, it was found that both the spectral properties and the intensity noise were affected by the feedback even at relatively low levels. The spectrum of the laser was monitored using an optical spectrum analyser with a spectral resolution of 0.004 nm. The spectrum of the laser is affected at feedback levels as low as 5•10-5, where an additional peak is observed in the spectrum.

In the same measurement series the intensity noise was monitored with the results shown in Figure 5. Here it can also be observed that the intensity noise increases significantly at a feedback level of 5•10-5 in agreement with the spectral measurements. It can also be noted that the intensity noise drops for high feedback levels, where the laser operates in multiple spectral modes. This is to be expected as the competition between many modes gives less noise than competition between few modes.
A more detailed analysis of the feedback determined the exact feedback levels that were enough to make the laser unstable.

The influence of the quality of optical isolators has also been investigated. Within a finished laser module with all components fixed in place, the optical isolator was changed from one with poor performance to one with good performance. The linewidth of the laser was observed under operation with the two isolators using a Fabry-Perot interferometer. It was clearly observed that the laser linewidth is significantly affected by feedback from the remaining optical components when the poor isolator is used. Through detailed analysis, the exact difference in the level of optical feedback to the diode was determined.

Two routes to eliminate the optical isolator have been explored:
Skewing of the crystal: Feedback to the laser originates from all optical interfaces that the light passes downstream from the laser and from scattering inside the optical materials. Collimated or even focused light perpendicularly incident on a plane face of an optical component will provide the largest amount of feedback to the laser. In contrary to this, light incident on angled or curved optical facets only gives a small amount of feedback.
We have investigated the use of crystals with angled facets to eliminate feedback from the nonlinear crystals in the laser modules. Angles of up to 10° on the crystal facets have been employed. It is observed that the laser is still influenced by feedback even at large facet angles on the crystal. We suspect that this feedback originates from scattering inside the crystal material and it is thus difficult to eliminate. It is also observed that a fine adjustment of the isolator rotation can improve the laser performance. When the isolator is not optimally aligned, the feedback from the crystal will make the laser unstable regardless of the crystal facet angle.
Therefore, skewing of the crystal will not solve the challenges with feedback in the present laser configuration.

Modified diode architecture: There are in general two solutions for making monolithically integrated tapered diode lasers. In one structure, the oscillator extends throughout the entire device; the laser light oscillates between the back facet mirror or an integrated grating and the front facet mirror. In the other structure, called a Master Oscillator Power Amplifier (MOPA) structure, a DFB or DBR laser seeds light into a tapered amplifier. In a MOPA, the oscillator occupies only a small part of the device; the larger part is taken up by the amplifier.
The currently used lasers are of the first type with a DBR grating acting as the rear laser reflector and the front facet of the tapered section is the output coupler in the laser. The DBR grating has a relatively high reflectivity (>50%) while the front facet has a low reflectivity (<1%). The very low reflectivity of the front facet makes this structure quite sensitive to external feedback as even small amounts of power coupled through the front facet will constitute a significant disturbance of the laser. This laser structure is therefore more prone to feedback instability.
A MOPA structure could be a more stable solution towards feedback sensitivity. Here, a DFB or DBR laser is operated under constant conditions providing stable power and spectral properties. This laser is injected into a tapered amplifier that boosts the output power while preserving the spectral properties of the seed laser. The tapered amplifier is coated with very low reflectivity to avoid feedback to the seed laser and to avoid the amplifier lasing on its own. Light that is fed back to the tapered amplifier will be amplified in the backward direction and will be fed back into the seed laser. However, as the seed laser has embedded gratings and will have a significantly higher reflectivity, it is expected that the seed laser can tolerate a higher amount of feedback.
The technology owned by the current Norlase supplier of tapered diodes is a BDR laser. This supplier is currently not able to deliver devices of the MOPA type. However, during this project phase, we have been able to locate a supplier that is producing MOPAs and has material of interest in stock. This material is of a relevant technology, but the wavelength provided and power level of the MOPA devices are such that this cannot be used as part of the product. However, the MOPAs are interesting and relevant for a feasibility test, and we have placed a PO to get test devices.
We are currently investigating the MOPA structure and preliminary results shows an increased robustness against feedback.
Selected solution: The preliminary results indicate that a modified laser structure will give the largest improvement in feedback sensitivity. Therefore this is the selected solution. At the same time, we will also minimize the feedback from optical components inserted after the laser in order to lower the feedback from its current level. Observation of the laser properties during alignment of optical components will be employee to ensure a feedback level tolerable to the laser. A combination of these two initiatives is expected to enable elimination of the optical isolator

Progress in yellow wavelengths
A key feature of the concept underlying the TDDL platform is its scalability to new wavelengths. As described in the introduction, we had planned in the innovation project to explore the expansion into new wavelength regions, especially yellow (561 nm).
The wavelength of the laser is determined by half the wavelength by the tapered diode used in the configuration, and replacement of the 1062 nm diode in the AuroraOne with a 1122 nm diode will lead to the desired output wavelength, which was further tested during the feasibility study. We obtained so promising results with this wavelength and the configuration that it is now from a technological point sufficiently mature to be candidate for exploitation in line with the 532 configuration. A market leading initial customer for the 561 nm laser has been attracted and first prototype delivery is planned by end-2015.

3. Market studies
In addition to the technical feasibility studies, we had planned a set activities in select areas where we saw the need for more specific input to our business plan, such as market segments, lead customers and key opinion leaders for the most relevant segments. During the project, as we achieved the positive results in the yellow wavelength region, we had stronger emphasis towards the new segments that now was immediately available for our platform.

Pump lasers
Although the 532 nm Aurora One has potential on a range of market applications, initially we saw the pump laser market as an interesting entry market. Laser pumping is the activity of transferring energy from an external source to the gain medium of a laser. One of the most widespread methods is using another laser as the external source. Such a laser is referred to as a pump laser. The pump laser represents a significant part of the total cost of the laser system. It is also the main performance inhibitor; the beam quality of the pump dictates the potential quality of the images produced and the longevity of the system is usually decided by failure of the pump. Furthermore, the move towards more user-friendly, easy-to-handle systems is com-plicated by the large footprint and sensitivity of current pump laser technology. This makes the pump laser segment a suitable entry market for Norlase. The company has successfully achieved pilot demonstrators with lead customers on the market. The pilots are performing above expectations.

High-power green lasers that are directly replaceable by the Norlase Aurora One 532 (3W), constitute about 50% of the €280M market for pump lasers. This market is split into two approximately equally-sized segments; 1) DPSSLs and OPSLs, and 2) direct diodes. In 2012, the total sales of pump lasers for DPSSL and fibre lasers accounted for ~4% share of the global laser market (Laser Focus World 2012). The DPSSL and OPSL segment is dominated by green lasers that represent around 50% of the total pump laser market. Aurora One 352 (3W) competes directly with DPSSL and OPSL. The total addressable market (TAM) for Norlase in this segment is estimated at €140M (5 % CAGR 2014-22). Norlase has identified several lead customers in this market.

Pump laser lead customers:
As part of the feasibility study, potential lead customers where identified, while the collaboration with previously established lead customers was further explored.

Medical Applications
Medical applications are particularly interesting for Norlase due to the high interest in exotic wavelengths that can hardly be reached by todays dominating technologies. This market area is fairly mature. The overall market for Therapeutic and Aesthetic lasers market in 2014 was ~$890M, according to data from Laser Focus World and Strategies Unlimited. Visible lasers are used for some specific therapeutic applications, particularly in the field of ophthalmology and in cosmetic, dermatology applications; sales of ophthalmic lasers for photocoagulation comprise a large part of this market area, between $120M and $300M. These laser sales are dominated by DPSSL/OPSL lasers. The wavelengths required depend on the absorption of the biological material. For ophthalmic applications (eye-treatments), wavelengths of green (532 nm) and yellow (~ 577 nm, and sometimes down to 560 nm) are used most. For dermatology (skin treatments), 577 nm is the preferred wavelength, but other wavelengths including 532 nm green are most widespread due to the cost and poor performance of relevant 577 nm lasers. Both opthtalmology and dermatology and markets are shifting from green to yellow as the latter becomes more available. This represents a majo
r oppo
Progress since grant agreement
The finding in our feasibility study confirms our hopes and expectation in the technology, the product and the market. The project has not only confirmed and improved the concept, but also led us to set more ambitious objectives for the innovation project and advanced our road map and market expectations. During the project, market traction has increased, both on the existing market for pump lasers and on the new market for medical lasers.

More ambitious innovation project objectives
When we initiated the feasibility study project, our aim for the exploitation of our technology platform was to exploit the market for green lasers, and the objectives for the innovation project (phase 2) set out was:

• to complete the validation of Norlase’s platform technology and to demonstrate and prepare market roll-out of the initial product portfolio – particularly in wavelength regions where development and initial demonstration have already been reached, i.e. green (532 nm) and blue (488 nm);

• to complete 10,000 hrs life-time and reliability tests of products at 532 nm (2W and 4W) and at 488 nm (2W) in order to have a fully certified product for OEM customers;

• to remove the need for expensive optical isolators by innovative designs of semiconductor laser;

• to plan the expansion of Norlase’s platform technology into wavelength regions that cannot be reached by competing technologies, namely yellow, where there is huge market potential.

Thus, the last of these objective was included to have ambitious goals for scalability of the TDDL platform into scales beyond the target for innovation project. As described above, the progress during the feasibilty study activities on the expansion of the technology into the yellow wavelength area convinced us that we should aim directly for this opportunity in the innovation project. Thus, rather than including this as an option plan for expansion, we will move directly to the exploitation of this wavelength domain as the main aim of the innovation project. However, as we keep setting new goals for the expansion of the platform, we will in the innovation project include new ambitious goals for the scalability.

Unchanged concept
The concept behind the technology remains unchanged as we move into the yellow wavelength domain. As it is clear from the descriptions of the TDDL platform above, it is the same patented platform that are used to achieve the yellow wavelengths as the green and blue.

However, the feasibility study has led to improvement to the platform. One of the goals of the study was to find a solution that could obsolete an optical isolator a bulky, a costly component in the design. As described above, the feasibility study has substantiated that a combination of two solutions is a viable path to eliminate this component, which will improve the margin of the product even more. These solutions will need to be further explored and developed to capture the cost-saving potential.

New markets and increased impact
From the perspective of our platform, the change to new wavelengths is thus a continuous innovation of the TDDL technology. However, as described, moving into yellow domain gives us ability to disrupt the market even more compared the initially foreseen target market of green and blue lasers.

At the beginning of the feasibility study, with the TDDL platform applied to green domain, the market for pump lasers was identified as the best market for introduction. This is still the case for this wavelength. However, with the proven feasibility of extending the platform into the yellow domain, even more interesting segment becomes available, namely medical applications such as ophthalmic (eye-treatments) application and dermatology (skin treatments).

Thus, as we change from from green to yellow lasers, we perform a step-change in our route to market strategy. Accordingly, instead of the foreseen pump laser markets, we will in the innovation project focus on the next step in our route to market strategy, the medical applications, and within this, initially the market for laser for ophthalmology applications.

The Innovation project
The SME instrument Phase 2 continues to be the best vehicle for us to support our route-to-market strategy and we are current preparing an application for this purpose. To provide us with the required operational environment to mature our technology, we have identified and contacted several potential lead customers on the medical market, who can take the part as end-user. We have as part of the feasibility study obtained agreement with the most suitable lead customer for the target market.