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SiC bolometer laser power sensor has advanced specs

Astronomy

See page 20

A laser-power meter (LPM) that uses a volume absorbing, single-crys- tal, silicon carbide (SiC) bolometer and detects wavelengths from 190 nm to 200 µm has been developed at Firebird Sensors (Reno, NV). Attributes of SiC that are important for LPM sensing include wide

spectral range, high thermal conductivity (5.1 W/cm·K), and high binding energy (4.67 eV). In addition, the SiC sensor lacks mechanisms that cause drift: there is no diffusion of impurities within the SiC up to temperatures >1200°C; there are no grain boundaries; and the electrode structure is stable above 800°C.

In conjunction with heat-sink integration, these SiC attributes yield characteristics that include a dynamic range of 106 (1 mW to 1 kW), no fan or water-cooling requirement to measure powers >1 kW, exceptionally high damage threshold (undam-

aged by peak pulse powers of 10 TW/cm2), high accuracy (±0.1% at full scale), and short time between measurement cycles (~1 s).

Two configurations

The meter head (MH) includes a sensor and an aluminum heat sink (see figure). A detachable handle located on top of the MH is not shown. The so-called “MEGA” MH has two configurations of operation: sensor-sinked or sensor-isolated. The fastresponse (FR) MH is in sinked configuration only.

The sinked configuration minimizes response time but requires that a pulsed laser emit at a pulse rate of 10 Hz or more to give a steady average power reading. The isolated configuration maximizes low-power sensitivity and can measure the average power emitted from a pulsed laser at frequencies down to 2 Hz.

In the sinked configuration, heat is rapidly transferred from the sensor to the heat sink, lowering the sensor’s natural 100% response time to below 6 s. In the isolated configuration, the sensor heat loss to the heat sink is minimized; this extends the

low frequency and low power range by an order of magnitude, but increases the natural 100% response time to about 30 s.

In each configuration, the MEGA and FR MHs can be used with

or without a concave focusing mirror. The purpose of the mirror is to further extend the low-power range by increasing the amount of laser radiation absorbed by the sensor. The sensor’s absorption is a function of wavelength and, to some degree, of temperature.

At wavelength values below and above 10.6 µm, part of the laser radiation passes through the sensor. The sensor surfaces are roughened to disperse reflected and transmitted beams; however, a heat dump

should be located behind the sensor when high powers are being measured or when a mirror is not used. At or near a 10.6 µm wavelength, SiC exhibits the Restrahlen effect, where nearly 100% of the radiation is absorbed near the surface, then nearly 100% of the absorbed radiation is re-emitted from the surface. In this case, the sensor surface should be misoriented from the laser by 5° to 10°, and a heat dump should be placed in the path of the reflected beam.

The sensor’s dynamic range of 106 is achieved by a combination of sensor configuration, adding the mirror when needed, and maximizing the operating-temperature range by heat-sink- ing. The minimum power measurement capability is obtained by isolating the sensor and using the mirror to maximize sensor absorption.

The maximum continuously measurable power at a given wavelength is dictated by the operating-temperature range, which depends on the sensor and heat-sink sizes—high power can be measured in both sinked and isolated configurations. In the sinked configuration, the maximum measurable power is

the power that raises the heat-sink temperature to 200°C (due to a temperature limit dictated by circuitry inside the MH). To date, we have determined that a 17 mm sensor in a 76.2 × 76.2 × 32.3 mm FR heat sink can measure up to 2 kW

of continuous-wave radiation at a 1 µm wavelength. In the isolated configuration, the maximum measurable power is the power that raises the sensor temperature to 800°C.

Damage threshold (DT) at a given wavelength is a function of both the average power in a laser beam cross-sec- tion and the peak pulse power, where, although the average power may be well

below the DT of a LPM sensor, the peak pulse power may still damage the sensor. In both cases, as the average power in

a laser beam increases, the DT of the sensor decreases.

Experimental results

To date we have measured DT at three wavelength (λ) values (see table). DT was reached in two of the three tests, while in the third test, the SiC sensor was not damaged. In all three tests, the damage threshold exceeded that of any other LPM sensor by many orders of magnitude.

A unique capability of the FR MH is its ~1 s waiting times between measurement

cycles. This is valuable when the MH is built into a laser, and a movable mirror is used to toggle the beam between the laser process optics and the LPM sensor. Very short cycle times, applicable to laser manufacturing, are possible in the FR mode, where, if the sensor is zeroed before each measurement, the sensor response to each succeeding laser beam exposure is accurate, whether or not the

MH has returned to its initial temperature. This can be repeated until the MH temperature reaches 200°C. With the proper heat-sink mass, this temperature may never be reached. — Dr. James Parsons, email: jim.parsons@firebirdsensors.com

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How is it possible to calibrate the ultraviolet (UV) channel of an imaging solar coronagraph as it flies through space?

Just look to the stars. Researchers from the National Institute for Astrophys-

ics (INAF) and the University of Florence (both in Florence, Italy) are developing procedures to use the UV emission of stars to calibrate the METIS coronagraph. It is an external-occulted novel coronagraph that will obtain simultaneous UV and polarized-visible-light images of the solar corona aboard the European Space Agency’s (ESA’s) Solar Orbiter satellite that will be launched in 2017.1

In-flight calibration

For the METIS UV channel, in-flight procedures must calibrate the solar corona UV brightness, monitor intensity changes

thanks to its UV interference filter, will select only the part of these spectra lying within a narrow (±10 nm) wavelength

SOLSTICE, SPICAM, and the International UV Explorer (IUE).

Coronal-radiance values from the

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(Newark), and Samsung Advanced Institute of Technology (Suwon-si, South Korea) have fabricated phosphor-free red, orange, yellow, green, and blue InGaN nanowire LED arrays monolithically integrated on silicon (Si). To create different colors, only the nanowire properties need be changed.

Dot-in-a-wire structure

The InGaN/GaN quantum-dot-in-a- wire heterostructures are created via a three-step growth process using radiofrequency plasma-assisted molecular beam epitaxy (MBE) on a large-area patterned Si substrate. The emission wavelengths depend on the sizes and compositions of the dots. The researchers grew different types of multicolor arrays that included emission wavelengths ranging from about 450 to 700 nm (see figure). Nanowire LED array elements with sizes of 300 × 300 μm, 100 × 100 μm, and 50 × 50 μm were fabricated.

In a result that has potential for lighting LEDs, three-color LED pixels were created, consisting of a blue, a green, and an orange/red subpixel, all of which could be separately biased and thus changed in intensity. In one example, color-correla- tion temperatures (CCTs) between 3826 K (warm white) and 6527 K (a very cool, bluish white) are created by the pixels (see table). Other LED combinations produced CCTs between 1900 K and 6800 K.

The researchers plan to boost the carrier-injection efficiency in future devices, as well as fabricate these LED arrays on transparent substrates (via transferral from Si) to enhance the light-extraction efficiency, as well as on copper substrates for more effective thermal management.

“Using high-resolution lithography techniques, the pixel size of this RGB

Researchers grew

different types of

multicolor arrays that

included ~450–700 nm

emission wavelengths.

resonance mode in the single-layer silicon films causes perfect

absorption (> 99%) at the peak absorption wavelengths of

552, 605, 657, and 700 nm for silicon films

with thickness values of 110, 120, 130, and 140 nm, respectively (see figure).

Thermal annealing the devices blue-shifts the absorption wavelengths

and correspondingly changes their colors. The reflectance spectra are measured at different angles of incidence up to 70°. As the angle of incidence is increased, the reflectance slightly increases, the peak absorption wavelengths become slightly shorter, but the peak reflection wavelengths do not shift.

Strong light absorption in nanome- ter-scale germanium thin films on metal surfaces was reported earlier by a group at Harvard University.2 However, the absorption in the nanoscale germanium films was not complete because the group investigated the enhanced optical absorption of the first order optical cavity resonance mode. The researchers at

the University of Alabama in Huntsville used the second-order optical cavity resonance mode to achieve perfect light absorption in single-layer silicon films at designated optical wavelengths using a technique called “selective mode critical coupling.” For achieving perfect light absorption with critical coupling to the second-order cavity mode, the silicon film thickness is slightly increased, but still remains a small fraction of the peak absorption wavelength. —Gail Overton

REFERENCES

1.S. S. Mirshafieyan and J. Guo, Opt. Express 22, 25, 31545 (2014).

2.M. A. Kats et al., Nat. Mat. 12, 1, 20 (2012).

According to research from SPIE, there are about 2750 companies worldwide supplying photonics components and materials, and they employ 700,000 people. These companies sell $156 billion of light sources, optics, and sensors. Of these companies, more than 2000 realize revenues of less than $10 million, accounting for less than 4% of worldwide sales (see figure).

With photonics components at the core of most modern technologies, this is an exciting time for the industry. So, have the stars aligned for more photonics companies to transcend the $10-million plateau? If so, what can a small business do to become a larger market participant?

To explore the question, Linda Smith

of CERES Technology Advisors, an M&A advisory firm working with photonics companies, talks with Jan Melles, president of Photonics Investments and founder of 12 photonics companies.

Linda Smith: Were you surprised by any of the research presented by SPIE on the make-up of the core optics and photonics components market?

Jan Melles: Not really—it’s more a confirmation of many years observing the photonics components market. As the SPIE data clearly illustrate, little has changed in the way the components market has developed over many years.

LS: How has the competitive landscape evolved in that time? In industries such as semiconductor manufacturing, as technology matured, the industry consolidated. With photonics components, however, the technologies matured, but much of the industry remains highly fragmented. What do you see?

JM: There are two main reasons why the industry has developed this way. First, there is an absence of a high-volume market such as is the case with semiconductors. Second, the component market does not add enough value to enable component companies to grow into more substantial organizations. To make the jump from being a component supplier to an organization that manufac-

tures and supplies products that add more value has been a difficult one.

1600

1400

1200

1000

800

600

400

200

0

0were made, but as long as you learn from them, the potential to grow these companies improves.

Company size

Core photonics suppliers worldwide. Less than 10% of companies employ ~77% of the workforce. (Courtesy SPIE)

Laser Focus World www.laserfocusworld.com

LS: A small business looking to grow has many options– expand the product line for the current customers, extend into new markets, move up the food chain to compete

January 2015 27

MarketInsights

with current customers. What are the upsides and downsides of these options for a photonics components company?

JM: I think that extending the product line and entering new geographic markets are obvious objectives under any circumstance. However, adding more value by getting into the subassembly or systems business may indeed have serious consequences for the relationship any company may face with its current OEM customer base, assuming there is overlap. This is an age-old dilemma that requires intensive analysis weighing the benefits of more revenue by selling more added-value products versus the risk of losing business.

Over the years, I have tended to be more conservative and in favor of not stepping on our OEM customers’ toes. It takes lot of hard work to gain that kind of business and, at the same time, in most of the companies I have been involved with, OEM sales represented about 50% of total sales. So there really must be strong arguments to risk it— which of course illustrates exactly why it is so tough to go upscale with the val- ue-added principle.

LS: Do you think that the optics industry is too insular? If so, how does this affect careers and the development of talent?

JM: The photonics components industry distinguishes itself from other tech industries through the loyalty to this market by the people working in it. Walk the trade shows like Photonics West or LASER World of Photonics and you will find people who have been in this industry for a long time—more so than in other fields.

This may be a consequence of the fact that on a global scale photonics is a niche industry. Yet there is also a constant influx of young technical talent from the universities who are attracted by the perception that photonics is a clean technology serving a wide variety of technical and scientific applications. So it may be that photonics technology by itself does not attract all the young talent, but using photonics is a

pathway to other fields or industries.

LS: Would you say that most small optics businesses struggle to build a global distribution network?

JM: That certainly had been the case earlier on, but these days when most companies have their own websites providing global access, it is not so difficult. Having said that, the Web is no alternative to having a representative organization selling your products in global markets. For those companies already using reps, they should consider setting up their own facilities in key markets if they reach a plateau on sales volume, market share, or both.

LS: Many small optics businesses do not have access to growth capital and are limited to the owners’ personal wealth. Taking on outside investment and/or merging with a larger competitor or partner could alleviate these growing pains. Is this a good option?

JM: This is as much an emotional issue as a rational one. Yes, it would make sense to consider these options, but independence and control over your business is too strong a motive for most small company owners to surrender. Personally, I have been able to overcome those problems by either bank financing or entering into investing partnerships with people I trust to help run and grow the business.

For any of the foreign subsidiaries we established, operations were almost always financed locally or with startup capital coming from the parent company. The managers and key employees of these foreign subsidiaries were given stock in the companies to better align the companies’ interests with their personal interests. Furthermore, when I sold such a company, these managers benefited from the same multiples as I did.

LS: Photonics companies are the key enablers to a wide breadth of vertical markets, from life sciences and manufacturing, to information technology and energy. These vertical markets attract almost 10 times the investment of the photonics companies. I know it

is not unusual that profitability and return on investment are more attractive as one moves up the value chain, but is the magnitude of the imbalance unique to the photonics industry? Do you see this trend amplifying?

JM: It is not so much an issue of what application a photonics company is focusing on; it is the value they add to the process. Our discussion here focuses on the photonics components industry, and it is unfortunate that components will only get you so far, which is probably no different from any other industry. I can count the number of photonics components companies that go through the $10-million sales level on the fingers of one hand. One of the usual ways to go “upscale” in a product line is to intensify business connections with OEM customers by supplying them more added-value products, which is beneficial not only for increasing sales, but also for making it more difficult for competitors to compete effectively.

LS: Often guilty of this myself, I find small business owners have a tough time making time to work “on” the business versus to work “in” the business. How did you keep your eye on the big picture, plan with discipline, and stay out of the lab and the coating chamber?

JM: The scene you present is indeed how things often work out in the real world. It is also one of the prime reasons why too many small companies stay small. Most of the time photonics companies are started by an engineer or scientist trying to realize their own company on the basis of technical knowledge from previous studies or work. Where things go wrong is when the founder continues to spend most of their time in the lab and does not pay enough attention to the business aspects of the company.

The founder’s job needs to include two things. First, the founder needs to give employees a clear view of upcoming goals and expectations to meet them. I always presented this in writing to all my employees on a single sheet of paper. And second, they need to realize that without

creating an effective team that allows them to delegate operational responsibilities, they will become ineffective and an obstacle in growing the business.

LS: I appreciate having learned best practices from my large corporate employers, but having had to work outside my comfort zone and learn real-time from my mistakes at a fami- ly-owned optics business and at startups was invaluable. I learned that as a result of an entrepreneurial culture, individuals behave purposefully and can build more value, more quick-

ly. An entrepreneurial culture can go a long way to compensate for lack of funds, brand, and human resources. How can the entrepreneurial cul-

ture that got these small optics companies where they are today help launch them off the $10-million plateau?

JM: It all depends on the aspirations, ambitions, and management skills of the owner. Some owners are perfectly happy to maintain a status quo for a long time, preferring a more comfortable lifestyle but likely not realizing that no company can survive in the long term by sticking to such a philosophy. For those entrepreneurs that have the ambition to grow as much as they can manage, they must realize that no such objective can be achieved without creating an effective team, delegating responsibilities, and inspiring and rewarding the employees.

Jan Melles is the president of Photonics Investments and was the co-founder and later chairman of Melles-Griot. He is currently on the board of numerous pub-

lic and private companies, and invests in and brokers the mergers and acquisitions of photonics companies. e-mail:

jmelles@photonicsinvestments.com; www. photonicsinvestments.com.

Linda Smith has 25 years of experience in mergers & acquisitions, strategic marketing, product management, and sales in the photonics industry. In 2006, she founded CERES Technology Advisors, an M&A advi-

sory firm, to serve technology businesses that are largely unserved or under-served by existing investment banks and business brokerages. e-mail: lindasmith@cerescom.net; www. cerescom.net.