Feeds:
Posts
Comments

Posts Tagged ‘photonics’

Tunable light sources

Larry H. Bernstein, MD, FCAP, Curator

LPBI

 

Putting Tunable Light Sources to the Test

Common goals of spectroscopy applications such as studying the chemical or biological properties of a material often dictate the requirements of the measurement system’s lamp, power supply and monochromator.

JEFF ENG AND JOHN PARK, PH.D., NEWPORT CORP.   http://www.photonics.com/Article.aspx?AID=58302

Many common spectroscopic measurements require the coordinated operation of a detection instrument and light source, as well as data acquisition and processing. Integration of individual components can be challenging and various applications may have different requirements. Conventional lamp-based tunable light sources are a popular choice for applications requiring a measurement system with this degree of capability.

Many types of tunable light sources are available, with differences in individual component performance translating to the performance of the system as a whole. Tunable light sources are finding themselves to be an especially ideal system for one application in particular: quantum efficiency and spectral responsivity characterization of photonic sensors, such as solar cells.

Xenon and mercury xenon lamps, two examples of DC arc lamps.

http://www.photonics.com/images/Web/Articles/2016/2/10/Light_Lamps.jpg

Xenon and mercury xenon lamps, two examples of DC arc lamps.

The tunable light source’s (TLS) versatility as both a broadband and high-resolution monochromatic light source makes the unit suitable for a variety of applications, such as the study of wavelength-dependent chemical or biological properties or wavelength-induced physical changes of materials. These light sources can also be used in color analysis and reflectivity measurements of materials for quality purposes.

Among their unique attributes, the TLS can produce monochromatic light from the UV to near-infrared (NIR). Lamp-based TLSs feature two major components: a light source and a monochromator. Common lamps used in TLSs are the DC arc lamp and quartz tungsten halogen (QTH) lamp. While both of these lamps have a broad emission spectrum, arc and QTH lamps differ in the characteristic wavelength emissions or relatively smooth shape of their spectral output curves, respectively. A stable power supply for the lamp is a critical component since most applications require high light output power stability1.

Smooth spectral output vs. monochromator throughput

DC arc lamps are excellent sources of continuous wave, broadband light. They consist of two electrodes (an anode and a cathode) separated by a gas such as neon, argon, mercury or xenon. Light is generated by ionizing the gas between the electrodes. The bright broadband emission from this short arc between the anode and cathode makes these lamps high-intensity point sources, capable of being collimated with the proper lens configuration.

DC arc lamps also offer the advantages of long lifetime, superior monochromator throughput (particularly in the UV range) and a smaller divergence angle. They are particularly well-suited for fiber coupling applications2. (See Figure 1.)

A xenon arc lamp housed in an Oriel Research lamp housing.

Figure 1. A xenon arc lamp housed in an Oriel Research lamp housing. Photo courtesy of Newport Corp.

Xenon (Xe) arc lamps, in particular, have a relatively smooth emission curve in the UV to visible spectrums, with characteristic wavelengths emitted from 380 to 750 nm. However, strong xenon peaks are emitted between 750 to 1000 nm.

Their sunlike emission spectrum and about 5800 K color temperature make them a popular choice for solar simulation applications. (See Figure 2.)

Arc lamps can have the following specialty characteristics:

Ozone-free: Wavelength emissions below about 260 nm create toxic ozone. Ideally, an arc lamp is operated outdoors or in a room with adequate ventilation to protect the user from the ozone created.

UV-enhanced: For applications requiring additional UV light intensity, UV-enhanced lamps should be used. These lamps provide the same visible to NIR performance of an arc lamp while providing high-intensity UV output due to changes in the material of the lamp’s glass envelope.

High-stability: High-stability arc lamps are made of a higher quality cathode than that typically used for arc lamp construction. As a result, no arc wander occurs, allowing the lamp to maintain consistent output intensity throughout its lifetime.

The spectral output of 3000-W Xe and 250-W QTH lamps used in Oriel’s Tunable Light Sources.

Figure 2. The spectral output of 3000-W Xe and 250-W QTH lamps used in Oriel’s Tunable Light Sources.Photo courtesy of Newport Corp.

QTH lamps produce light by heating a filament wire with an electric current. The hot filament wire is surrounded by a vacuum or inert gas to prevent oxidation. QTH lamps are not very efficient at converting electricity to light, but they offer very accurate color reproduction due to their continuous blackbody spectrum. These lamps are a popular alternative to arc lamps due to their higher output intensity stability and lack of intense UV light emission, spectral emission lines in their output curve and toxic ozone production. These advantages over traditional DC arc lamps make QTH lamps preferable for radiometric and photometric applications as well as excitation sources of visible to NIR light. QTH lamps are also easier to handle and install, and produce a smooth output spectrum. Selecting the most appropriate lamp type is a matter of deciding which performance criteria are most important.

Constant current vs. constant power

The power supply is a vital component for operating a DC arc or QTH lamp with minimum light ripple. The lamps are operated in either constant current or constant power mode and are used in applications such as radiometric measurements, where a stable light output is required for accurate measurement. Providing stable electrical power to the lamp is important since fluctuations in the wavelength and output intensity of the light source impact the accuracy of measurement.

There is very little difference in the short-term output stability when operating an arc lamp or QTH lamp in constant current or constant power mode. However, the differences appear as the lamp ages. For arc lamps, even with a stable power supply, deposits on the inside of the lamp envelope are visible as the electrodes degrade, which causes an unstable arc position, changing the electrical characteristics of the arc lamp. The distance between the cathode and anode of the arc lamp increases, raising the lamp’s operating voltage. For QTH lamps, deposits on the inside of the lamp envelope are visible as the lamp filament degrades, changing the electrical and spectral characteristics of the lamp.

In power mode, the lamp is operated at a constant power setting. As the voltage cannot be changed, the current is raised or lowered to maintain the power at the same level. As the lamp ages, the radiant output decreases. However, lamp lifetime is prolonged.

In current mode, the lamp is operated at a constant current setting. As the voltage cannot be changed, the power is raised or lowered to maintain the current at the same level. As the lamp ages, the input power required for operation is increased. This results in greater output power, which, to some extent, may help compensate for a darkening lamp envelope. However, the lamp’s lifetime is greatly reduced due to the increase in power.

Although power supplies are highly regulated, there are factors beyond the control of the power supply that may affect light output. Some of these factors include lamp aging, ambient temperature fluctuations and filament erosion. For applications in which high stability light output intensity is especially critical, optical feedback control of power supply is suggested in order to compensate for such factors3. (See Figure 3.)

Oriel’s OPS Series Power Supplies offer the option of operating a lamp in constant power, constant current or intensity operation modes.

Figure 3. Oriel’s OPS Series Power Supplies offer the option of operating a lamp in constant power, constant current or intensity operation modes. Photo courtesy of Newport Corp.

Diffraction gratings narrow the wavelength band

Monochromators use diffraction gratings to spatially isolate and select a narrow band of wavelengths from a wider wavelength emitting light source. They are a valuable piece of equipment because they can be used to create quasi-monochromatic light and also take high precision spectral measurements. A high precision stepper motor is typically used to select the desired wavelength and switch between diffraction gratings quickly, without sacrificing instrument performance.

Determining which slit width to use is based on the trade-off between light throughput and the resolution required for measurement. A larger slit width allows for more light throughput. However, more light throughput results in poorer resolution. When choosing a slit width at which to operate the monochromator, both the input and output ports must be set to the same slit width. (See Figure 4.) Focused light enters the monochromator through the entrance slit, and is redirected by the collimating mirror toward the grating. The grating directs the light toward the focusing mirror, which then redirects the chosen wavelength toward the exit slit. At the exit slits, quasi-monochromatic light is emitted4.

A fixed width slit being installed into an Oriel Cornerstone 130 monochromator.

Figure 4. A fixed width slit being installed into an Oriel Cornerstone 130 monochromator.Photo courtesy of Newport Corp.

Measuring quantum efficiencies

Measuring quantum efficiency (QE) over a range of different wavelengths to measure a device’s QE at each photon energy level is an ideal task for a tunable light source. The QE of a photoelectric material for photons with energy below the band gap is zero. The QE value of a light-sensing device such as a solar cell indicates the amount of current that the cell will produce when irradiated by photons of a particular wavelength. The principle of QE measurement is to count the proportion of carriers extracted from the material’s valence band to the number of photons impinging on the surface. To do this, it is necessary to shine a calibrated, tunable light on the cell, while simultaneously measuring the output current. The key to accurate measurement of the QE/internal photon to current efficiency is to accurately know how much scanning light is incident on the device under test and how much current is generated. Thus, measurement of light output with a NIST (National Institute of Standards and Technology) traceable calibrated detector is necessary prior to testing since illumination of an absolute optical power is required.

External quantum efficiency (EQE) is the ratio of the number of photons incident on a solar cell to the number of generated charge carriers. Internal quantum efficiency (IQE) also considers the internal efficiency — that is, the losses associated with the photons absorbed by nonactive layers of the cell. By comparison, EQE is much more straightforward to measure, and gives a direct parameter of how much output current will be contributed to the output circuit per incident photon at a given wavelength. IQE is a more in-depth parameter, taking into account the photoelectric efficiency of all composite layers of a material. In an IQE measurement, these losses from nonactive layers of the material are measured in order to calculate a net quantum efficiency — a much truer efficiency measurement.

Understanding the conversion efficiency as a function of the wavelength of light impingent on the cell makes QE measurement critical for materials research and solar cell design. With this data, the solar cell composition and topography can be modified to optimize conversion over the broadest possible range of wavelengths.

As a formula, it is given by IQE = EQE/(1 − R), where R is the reflectivity, direct and diffuse, of the solar cell. The IQE is an indication of the capacity of the active layers of the solar cell to make good use of the absorbed photons. It is always higher than the EQE, but should never exceed 100 percent, with the exception of multiple-exciton generation. Figure 5 illustrates how the tunable light source is used to illuminate the solar cell to perform an IQE measurement. The software controls all components of the measurement system, including the monochromator and data acquisition5.

A sample QE measurement system using the components of a tunable light source.

Figure 5. A sample QE measurement system using the components of a tunable light source. Photo courtesy of Newport Corp.

To measure quantum efficiency in 10-nm wavelength steps, the slit size of the monochromator is typically hundreds of micron in width. The slit width is reduced approximately half if 5-nm wavelength increments are desired. However, output power of the monochromator is reduced by more than 50 percent if the slit width is halved. Lowering optical power impacts QE measurement since a solar cell responds to this diminished optical power with low output current. This can result in a poor signal-to-noise ratio, making a QE measurement error more likely. The detection of low current requires very sensitive equipment with the ability to measure current down to the pico-ampere level. To make for an easier signal measurement, optical power is typically increased. A DC arc source is the better choice for QE measurements made in 5-nm increments or lower due to the lamp’s arc size resulting in better monochromator throughput. However, a QTH lamp is the better choice if greater than 0.1 percent light stability is required, with the trade-off of not being able to measure in as precise wavelength increments as if an arc lamp was used.

Balance between optical power and resolution is an important consideration as it impacts the quality of the QE measurement. The selection of lamp type and monochromator specifications are important considerations for TLS design. To be considered a suitable component for the majority of spectroscopic applications, high-output power and stability, long lifetime of the lamp, and broadband spectral emission with high resolution capability are required for the TLS.

Meet the authors

John Park, new product development manager at Newport Corp., has designed and developed numerous spectroscopy instruments for the photonics industry for over 10 years. He holds two granted patents and is a graduate from University of California, Irvine, with a Ph.D. in electrical engineering; email: john.park@newport.com. Jeff Eng is a product specialist for Oriel Spectroscopy Products at Newport Corp. His work experience includes application support, business-to-business sales and marketing activity of photonic light sources and detectors. He is a graduate of Rutgers University; email: jeff.eng@newport.com.

References

1. Newport Corp., Oriel Instruments TLS datasheet. Tunable Xe arch lamp sources.http://assets.newport.com/webDocuments-EN/images/39191.pdf.

2. Newport Corp., Oriel Instruments handbook: The Book of Photon Tools, light source section.

3. Newport Corp., Oriel Instruments OPS datasheet. OPS-A series arc lamp power supplies.http://assets.newport.com/webDocuments-EN/images/OPS-A%20Series%20Power%20Supply%20Datasheet.pdf.

4. J. M. Lerner and A. Thevenon (1988). The Optics of Spectroscopy. Edison, N.J.: Optical Systems/Instruments SA Inc.

5. K. Emery (2005). Handbook of Photovoltaic Science and Engineering, eds. A. Luque and S. Hegedus. Chapter 16: Measurement and characterization of solar cells and modules. Hoboken, N.J.: John Wiley & Sons Ltd.

Read Full Post »

Deep brain stimulation for traumatic brain injury

Larry H. Bernstein, MD, FCAP, Curator

LPBI

 

Pulsed laser light turns whole-brain activity on and off

Study may guide deep brain stimulation therapies used for traumatic brain injury and other neurological disorders
December 18, 2015      http://www.kurzweilai.net/pulsed-laser-light-turns-whole-brain-activity-on-and-off

Optogenetic laser light stimulation of the thalamus (credit: Jia Liu et al./eLife)

By flashing high-frequency (40 to 100 pulses per second) optogenetic lasers at the brain’s thalamus, scientists were able to wake up sleeping rats and cause widespread brain activity. In contrast, flashing the laser at 10 pulses per second suppressed the activity of the brain’s sensory cortex and caused rats to enter a seizure-like state of unconsciousness.

“We hope to use this knowledge to develop better treatments for brain injuries and other neurological disorders,” said Jin Hyung Lee, Ph.D., assistant professor of neurology, neurosurgery, and bioengineering at Stanford University, and a senior author of the study, published in the open-access journal eLIFE.

Located deep inside the brain, the thalamus regulates arousal, acting as a relay station to the cortex for neural signals from the body. Damage to neurons in the central part of the thalamus may lead to problems with sleep, attention, and memory.*

Combining light stimulation and fMRI measurements

The observations used a combination of optogenetics and whole-brain functional MRI (fMRI) — known as “ofMRI” — to detect overall effects on the brain, along with EEG and single-unit cell recordings.The researchers noted in the paper that “using targeted, temporally precise optogenetic stimulation in the current study allowed us to selectively excite a single group of neuronal elements and identify their specific role in creating distinct modes of network function.” That could not be achieved with conventional electrode stimulation, the researchers say.

They explain that this method may allow for direct-brain stimulation (DBS) therapeutic methods to be optimized in the clinic “for a wide range of neurological disorders that currently lack such treatment.”

“This study takes a big step towards understanding the brain circuitry that controls sleep and arousal,” Yejun (Janet) He, Ph.D., program director at NIH’s National Institute of Neurological Disorders and Stroke (NINDS), which partially funded the study.

* Further experiments suggested the different effects may be due to a unique firing pattern by inhibitory neurons in a neighboring brain region, the zona incerta, during low frequency stimulation. Cells in this brain region have been shown to send inhibitory signals to cells in the sensory cortex. Electrical recordings showed that during low frequency stimulation of the central thalamus, zona incerta neurons fired in a spindle pattern that often occurs during sleep. In contrast, sleep spindles did not occur during high frequency stimulation. Moreover, when the scientists blocked the firing of the zona incerta neurons during low frequency stimulation of the central thalamus, the average activity of sensory cortex cells increased.


Abstract of Frequency-selective control of cortical and subcortical networks by central thalamus

Central thalamus plays a critical role in forebrain arousal and organized behavior. However, network-level mechanisms that link its activity to brain state remain enigmatic. Here, we combined optogenetics, fMRI, electrophysiology, and video-EEG monitoring to characterize the central thalamus-driven global brain networks responsible for switching brain state. 40 and 100 Hz stimulations of central thalamus caused widespread activation of forebrain, including frontal cortex, sensorimotor cortex, and striatum, and transitioned the brain to a state of arousal in asleep rats. In contrast, 10 Hz stimulation evoked significantly less activation of forebrain, inhibition of sensory cortex, and behavioral arrest. To investigate possible mechanisms underlying the frequency-dependent cortical inhibition, we performed recordings in zona incerta, where 10, but not 40, Hz stimulation evoked spindle-like oscillations. Importantly, suppressing incertal activity during 10 Hz central thalamus stimulation reduced the evoked cortical inhibition. These findings identify key brain-wide dynamics underlying central thalamus arousal regulation.

Read Full Post »

Laser Technology

Larry H. Bernstein, MD, FCAP, Curator

LPBI

 

Laser Focus World   www.laserfocusworld.com

Ultrafast lasers simplify fabrication of 3D hydrogel tissue scaffolds

Multimode holographic waveguides tackle in vivo biological imaging

Mid-infrared Lasers CMOS silicon-on-sapphire process produces broad mid-IR supercontinuum

Looking Back/Looking Forward: Positioning equipment—the challenge of building a solid foundation for optics
Stability and precision have been crucial for optics since the 19th century.
Jeff Hecht

Monolithic DFB QCL array aims at handheld IR spectral analysis
Many QCLs combined on a single chip demonstrate fully electronic wavelength tuning for stand-off IR spectroscopy of explosives and other materials.
Mark F. Witinski, Romain Blanchard, Christian Pfluegl, Laurent Diehl, Biao Li, Benjamin Pancy, Daryoosh Vakhshoori, and Federico Capasso

Quantum dots and silicon photonics combine in broadband tunable laser
A new wavelength-tunable laser diode combines quantum-dot technology and silicon photonics with large optical gains around the 1310 nm telecom window.
Tomohiro Kita and Naokatsu Yamamoto

Computer modeling boosts laser device development
A full quantitative understanding of laser devices is boosted by computer modeling, which is not only essential for efficient development processes, but also for identifying the causes of unexpected behavior.
Rüdiger Paschotta

 

 

Monolithic DFB QCL array aims at handheld IR spectral analysis
MARK F. WITINSKI, ROMAIN BLANCHARD, CHRISTIAN PFLUEGL, LAURENT DIEHL, BIAO LI, BENJAMIN PANCY, DARYOOSH VAKHSHOORI, and FEDERICO CAPASSO

Advances in infrared (IR) laser sources, optics, and detectors promise major new advances in areas of chemical analysis such as trace-gas monitoring, IR microscopy, industrial safety, and security.

One key type of photonic device that has yet to reach its full potential is a truly portable noncontact (standoff), chemically versatile analyzer for fast Fourier-transform infrared (FTIR)quality spectral examination of nearly any condensed-phase material. The unique challenges of standoff IR spectroscopy actually extend beyond advances in IR hardware, requiring the proper combination of several areas of expertise: cutting-edge optical design and laser fabrication, integrated laser electronics, thermally efficient hermetic packaging, statistical signal processing methods, and deep chemical knowledge.

At the core of the approach we have taken at Pendar Technologies is the monolithic distributed feedback (DFB) quantum-cascade laser (QCL) array. Invented in Federico Capasso’s group at Harvard University (Cambridge, MA) and licensed exclusively to Pendar, the continuously wavelength-tunable QCL array source is a highly stable broadband source that can be used for illumination in reflectance spectroscopy. Each element of the array is individually addressable and emits at a different wavelength by design.

The advantages of these QCL arrays over external-cavity (EC) QCLs stem from (1) the monolithic structure of QCL arrays and (2) their fully electronic wavelength tuning— that is, no moving gratings, allowing for much-higher-speed acquisition through improved amplitude and wavelength stability. When integrated into a system, the result is robust, stable, and field-deployable.

One of the key advances that has enabled this technology to be fielded is the high-yield fabrication of each laser ridge in the QCL array from a single wafer such that every channel simultaneously meets the specified wavelength, power, and single-mode suppression ratio. Each of these parameters is critical to both efficient beam combining and to obtaining high-quality molecular spectroscopy once integrated.

With these hurdles largely overcome, the payoff in terms of spectrometer performance lies largely in a demonstrated shot-to-shot amplitude stability in pulsed mode of <0.1%—a factor of 50 more stable than is typical for EC QCLs, even when used in the lab. Most importantly, the DFB QCL noise is random, and averages toward an Allan variance limit quickly such that detector-noise-limited, high-quality spectra can be obtained for trace levels (for example, 1–50 µg/cm2) of typical powders in just 100 ms.

More DFB array advantages While the stability advantage of DFBs vs. EC configurations has been well established, there are a few less-obvious aspects to DFB arrays that make them more suitable to real-world spectroscopy tools and, in particular, portable spectroscopy tools. For one, the laser array as a whole can maintain a 100% duty cycle while each laser in the array requires operation only over a 100/n (%) duty cycle, where n is the number of lasers in the array. Put another way, a laser array consisting of only pulsed QCLs can operate as a truly continuous-wave (CW) system, allowing for high-measurement duty cycle while possibly reducing the cost of fabrication.

In a related way, generating light for an array that has a 100% aggregate duty cycle (by using, for instance, 32 lasers at 3% duty cycle), the thermal heat-sinking requirements of the source are dramatically reduced. Indeed, our packaged prototypes do not even require active cooling to keep the system cool enough to run. A thermoelectric cooler is built into the package only to stabilize the temperature, which therefore stabilizes the 32 wavelengths (see Fig. 1).
FIGURE 1. A 200 cm-1 prototype QCL array with 32 QCLs is shown prior to beam combining and packaging (a), and experimental spectra from 32 adjacent QCLs are seen (b). (Courtesy of Pendar Technologies)
Finally, the arbitrary programmability of the QCL array opens up many new possibilities for experimental optimization. Certain lasers can be skipped, multiple lasers can fire at once, repetition rates and pulse durations can be set for each element, and so on. These advantages are only truly realized when the QCL array is instrumented into a full system.

Looking holistically at how best to integrate this new capability into a full system, it is critical to draft the link equations that govern the use of electrons to produce photons, the collection of photons scattered back, and finally the conversion from raw spectral information to chemical identification. In the case of mid-IR material identification, it becomes clear that three aspects are particularly consequential: (1) How broad a wavelength range is needed for the tool to be of maximum specificity without producing redundant or useless chemical information (that is, how many laser channels should be used, how should they be spaced with respect to one another, and over what total wavelength regime should they be spaced); (2) the mechanical and electro-optical design of the instrument; and (3) how to get the highest performance regressions against reference spectra while maintaining the high-speed identification that the QCL array actually enables.

With regard to the wavelength regions of interest (see Fig. 2), most of the spectral richness of an IR spectrum is centered in two bands, generally referred to as the functional group region (about 3.3–5.5 µm) and the fingerprint region (about 7–11 µm). The first is typically dominated by the stretch modes of certain common bond groups, while the latter includes bending modes of some functional groups as well as lower frequency modes that are characteristic of the macromolecule “backbone”—for instance, the torsional modes of a toluene ring found in many highly energetic materials. With support from the Department of Homeland Security (DHS)’s Widely Tunable Infrared Source (WTIRS) program and from the Army Research Lab, Pendar is developing a compact array module that fully covers 7–11 µm (900–1430 cm-1).

FIGURE 2. An assemblage of IR spectra of many common explosives shows that each has at least one unique absorption feature in the wavelength ranges selected. The blue shaded box indicates strong water interference in the troposphere. The figure intentionally spans beyond 1800 cm-1 so as to illustrate that no new information is gained for this chemical class by shifting the longwave-IR (LWIR) source further to the blue until the midwave-IR (MWIR) is reached.

 

System architecture drivers To maximize signal-to-noise (SNR) while minimizing the required acquisition time, the system architecture is driven by the following first-order considerations: 1. Increasing the laser power enabled by relaxed thermal constraints as the heat load is distributed over several modules (arrays) and laser waveguides. 2. Maximization of the measurement duty cycle enabled by the fast purely electronic control of the array, allowing close to zero-delay switching between lasers— that is, a laser is on at any time. This is also enabled by the distributed heat load among the laser units. 3. Improved source stability, wavelength accuracy, pulse-to-pulse amplitude, and frequency repeatability—all of which are needed to ensure that the source noise is not the limiting form of noise (compared to detector or speckle noise). Other researchers have studied the source-noise problem of commercial EC QCLs as well and concluded that the order-of-magnitude advantage in minimum detectable absorbance (MDA) offered by a DFB QCL carries through the full experiment.

Finally, once the spectra are digitized, the system must use complex chemometrics algorithms to ensure confident identification of threats in the presence of chemical clutter, deliberate interferents, and unknown backgrounds, without the intervention of an expert user. Our approach to real-time chemometrics is centered on the fact that for chemically cluttered situations, spectral libraries alone—no matter how large—cannot constitute the sole basis for chemometric analysis. Microphysics modeling and experimentation are also required, particularly in regard to crystal size distribution, clutter interactions, and chemical photolysis/reactions.

The key advance lies in the incorporation of chemical and physical understanding of the targets and their co-indicators. We are currently developing a four-tiered approach to the spectroscopic algorithms challenge:

1. Physics-based models. Reliable chemical detection from standoff measurements will involve transformation of the chemical signatures in the reference spectral library to reflect the physical and environmental conditions of the experiment. A physics-based model will thus be included in the detection algorithm to help us model the variability in a reference spectrum as a function of effects such as vapor pressure, deliquescence, photochemical lifetime, reactive lifetime, decomposition products, and so on to facilitate better comparison with the measured spectrum.

2. Situational effects. Effects of different substrates and their properties on the chemical signatures and the angular dependence of spectra that are not clearly linked to equations of physics and chemistry will be experimentally evaluated and included in the detection algorithm. In particular, experimentally measuring such variability will help us algorithmically model the variability of chemical signatures from some “gold standard” reference signature, which—in
addition to the physical model—will enable better detection strategies.

3. Feature-based classification. Extraction of relevant feature vectors from the reference library spectra and the knowledge of the chemistry to form a hierarchical decision tree that will help us provide different levels of classification based on the customer requirements. For instance, if a customer is only interested in finding out whether a given chemical is an explosive, then we might save on computational cost by avoiding searching through the leaves of the decision tree to find out the exact chemical.

4. Real-time atmospheric measurements. Once validated, the model will be suitable for field implementation by the inclusion of an integrated sensor suite that simultaneously records atmospheric pressure, temperature, relative humidity, solar flux, wind magnitude, and water-vapor mixing ratio. With these design drivers considered, Pendar recently completed the build of a handheld demonstration system.

Figure 3 shows the experimentally obtained spectra for two nonhazardous chemical targets as a function of stand-off distance. The yellow line in each panel shows the library FTIR (“true”) spectrum for each. Agreements of r2 > 0.9 were typical. With the prototype system as an extrapolation point, continued, focused advances in the technology are now underway to open myriad frontiers in molecular spectroscopy.

 

FIGURE 3. Standoff spectra of of acetaminophen and ibuprofen for three target distances. The black line shows the FTIR of the same using a diffuse reflectance accessory. The only data processing shown is the normalization of the curve areas to a common value.

 

ACKNOWLEDGEMENT Pendar Technologies was formed in August 2015 through a merger between Pendar Medical (Cambridge, MA), a portable spectroscopy company founded by Daryoosh Vakhshoori (who was previously at Ahura Scientific and CoreTek), and QCL sensing startup Eos Photonics (Cambridge, MA), a Harvard spinoff founded by professor Federico Capasso and his postdocs.

 

Quantum dots and silicon photonics combine in broadband tunable laser
TOMOHIRO KITA and NAOKATSU YAMAMOTO

A new wavelength-tunable laser diode combines quantum-dot (QD) technology and silicon photonics with large optical gains around the 1310 nm telecom window and is amenable to integration of other passive and active components towards a truly integrated photonic platform.

A new heterogeneous wavelength-tunable laser diode, configured using quantum dot (QD) and silicon photonics technology, leverages large optical gains in the 1000–1300 nm wavelength region using a scalable platform for highly integrated photonics devices. A cooperative research effort between Tohoku University (Sendai, Japan) and the National Institution of Information and Communication Technology (NICT; Tokyo, Japan) has resulted in the demonstration of broadband tuning of 44 nm around a 1230 nm center wavelength with an ultrasmall device footprint, with many more configurations with various performance metrics possible.

Recently developed high-capacity optical transmission systems use wavelength-division multiplexing (WDM) systems with dense frequency channels. Because the frequency channels in the conventional band (C-band) at 1530–1565 nm are overcrowded, the frequency utilization efficiency of such WDM systems becomes saturated. However, extensive and unexploited frequency resources are buried in the near-infrared (NIR) wavelength regions such as the thousand (T) and original (O) bands between 1000 and 1260 nm and 1260 and 1350 nm, respectively. Quantum dot-based optical gain media have various attractive characteristics, including ultrabroad optical gain bandwidths, high-temperature device stability, and small line width enhancement factors, as well as silicon photonic wire waveguides based on silicon-on-insulator (SOI) structures that are easily amenable to constructing highly integrated photonics devices.1-4

Quantum dot-based optical gain media have various attractive characteristics, including ultrabroad optical gain bandwidths, high-temperature device stability, and small linewidth enhancement factors, as well as silicon photonic wire waveguides based on silicon-on-insulator (SOI) structures that are easily amenable to constructing highly integrated photonics devices.1-4

The photonic devices used for shortrange data transmission are required to have a small footprint and low power consumption. Therefore, compact, low-power wavelength-tunable laser diodes are key devices for use in higher-capacity data transmission systems that have been designed to use these undeveloped frequency bands, and our heterogeneous tunable wavelength laser diode consisting of a QD optical gain medium and a silicon photonics external cavity is a promising candidate.5

Quantum dot optical amplifier Ultrabroadband optical gain media spanning the T- and O-band are effectively fabricated by using QD growth techniques on large-diameter gallium-arsenide (GaAs) substrates. Our sandwiched sub-nano-separator (SSNS) growth technique is a simple and efficient method for obtaining high-quality QDs (see Fig. 1).

 

FIGURE 1. A cross-section (a) shows a quantum dot (QD) device grown using the SSNS technique, resulting in a high-density, highquality QD structure (b) that is used to create a typical SOA (c) using QD optical gain.

 

In the SSNS method, three monolayers (each around 0.85 nm thick) of GaAs thin film are grown in an indium GaAs (InGaAs) quantum well (QW) under the QDs. We had previously observed many large, coalescent dots that could induce crystal defects in QD devices using a conventional growth technique without SSNS. Now, we can obtain high-density (8.2 × 1010 cm-2), high-quality QD structures since the SSNS technique successfully suppresses the formation of coalescent dots.

For single-mode transmission, a ridgetype semiconductor waveguide was fabricated for single-mode transmission. The cross-section of the semiconductor optical amplifier (SOA) has an anti-reflection (AR) coating facet to connect a silicon photonics chip with low reflection and a cleaved facet used as a reflecting mirror in the laser cavity.

To fabricate the SOA, the SSNS growth technique was combined with molecular beam epitaxy. Quantum dots comprised of indium arsenide (InAs) with 20–30 nm diameters were grown within an InGaAs QW. Seven of these QD layers are stacked to achieve broadband optical gain. Subsequently, this QD-SOA is used as an optical gain medium for the heterogeneous laser, which can be complemented by other communication technology devices such as a high-speed modulator, a two-mode laser, and a photoreceiver.6, 7

Silicon photonics ring resonator filter With the QD-SOA fabricated, a wavelength filter is fabricated next using silicon photonics techniques. It includes a spot-size converter that has a silicon oxide (SiOx) core and a tapered Si waveguide that connects the QD-SOA to the Si photonic wire waveguide while minimizing optical reflections and coupling losses (see Fig. 2).

 

FIGURE 2. A microscope image (a) shows a silicon-photonicsbased wavelength-tunable filter. In a transmittance analysis (b), the red and blue dotted lines indicate the transmittance of a small ring resonator with free spectral range FSR1 and a large ring resonator with FSR2, respectively, and the solid line indicates the product of each transmittance. The tuning wavelength range is determined from the FSR difference of the two rings. A smaller difference in the FSR provides a wider wavelength tuning range, even when the transmittance difference between the main and side peaks is small.

 

The wavelength-tunable filter consists of two ring resonators of different size. The Vernier effect of these two ring resonators allows only light of a specific wavelength to reflect to the QD-SOA. Furthermore, Tantalum micro-heaters formed above the resonators provide a means whereby the laser wavelength can be tuned through application of the thermooptic effect.

Essentially, the wavelength tuning operation of the double ring resonator wavelength filter is achieved through Vernier effects wherein a ring resonator acts as a wavelength filter with constant wavelength interval called the free spectral range (FSR), which is inversely proportional to the circumference of the ring. The tuning wavelength range is determined from the FSR difference of the two rings with FSR1 and FSR2.

A smaller difference in the FSR provides a wider wavelength tuning range, even when the transmittance difference between the main and side peaks is small. On the other hand, a sufficiently large transmittance difference is required to achieve stable single-mode lasing and is obtained using large FSR ring resonators.

Silicon photonics allows us to fabricate an ultrasmall ring resonator with large FSR because of the strong light confinement in the waveguide. The ring resonator consists of four circle quadrants and four straight lines and the radius of the circle was chosen to be 10 µm to avoid bending losses. The FSRs of the ring resonators and the coupling efficiency between the bus-waveguide and the ring resonator are optimized to obtain wide wavelength tuning range and sufficient transmittance difference.

The FSRs and the coupling efficiencies of the double ring resonators are designed to obtain a 50 nm wavelength tuning range and 1 dB transmittance difference. We have since fabricated various wavelength-tunable laser diodes, including a broadband tunable laser diode, a narrow spectral-linewidth tunable laser diode, and a high-power integrated tunable laser diode by using a silicon photonics wavelength filter and a commercially available C-band SOA.8, 9

The tunable laser diode Using stepper motor controllers, the QD-SOA—kept at approximately 25°C using a thermoelectric cooler—and the silicon photonics wavelength filter are butt-jointed (see Fig. 3). The lasing wavelength is controlled by the temperature of a micro-heater placed on the ring resonators. With physical footprints of 600 µm × 1 mm and 1 × 2 mm for the wavelength filter and the QD-SOA, respectively, the total device size of the tunable laser diode is just 1 × 3 mm.

 

FIGURE 3. A schematic shows how the heterogeneous wavelengthtunable laser diode is constructed.

 

Measured using a lensed fiber, the laser output from the cleaved facet of the QD-SOA shows single-mode lasing characteristics with a laser oscillation threshold current of 230 mA. Maximum fiber-coupled output power is 0.4 mW when the QD-SOA injection current is 500 mA. As the ring resonator temperature is increased by a heater with 2.1 mW/nm power consumption, the superimposed lasing spectra show a 44 nm wavelength tuning range with more than a 37 dB side-mode-suppression ratio between the ring resonator’s modes. The 44 nm wavelength tuning range of our heterogeneous QD/Si photonics wavelength-tunable laser is, to our knowledge, the broadest achieved to date. The 44 nm tuning range around 1230 nm corresponds to 8.8 THz in the frequency domain, which is far larger than the 4.4 THz frequency that is available within the C-band.

Our heterogeneous laser is suitable for use as a light source on a silicon photonics platform that includes other optical components such as high-speed modulators and germanium (Ge)-based detectors. In addition to application as a single-chip broadband optical transceiver for telecommunications, the laser could also be applied to biomedical imaging applications such as optical coherence tomography (OCT), considering the low absorption of NIR light at 1310 nm in the presence of water.

ACKNOWLEDGEMENTS This research was partially supported by the Strategic Information and Communications R&D Promotion Program (SCOPE), of Japan’s Ministry of Internal Affairs and Communications and a Grant-in-Aid for Scientific Research of the Japan Society for the Promotion of Science.

REFERENCES

1. Y. Arakawam and H. Sakaki, Appl. Phys. Lett., 40, 11, 939–941 (1982).

2. D. L. Huffaker et al., Appl. Phys. Lett., 73, 18, 2564–2566 (1998).

3. R. A. Soref, Proc. IEEE, 81, 12, 1687–1706 (1993).

4. B. Jalai and S. Fathpour, J. Lightwave Technol., 24, 12, 4600–4615 (2006).

5. T. Kita et al., Appl. Phys. Express, 8, 6, 062701 (2015).

6. N. Yamamoto et al., Jpn. J. Appl. Phys., 51, 2S, 02BG08 (2012).

7. N. Yamamoto et al., Proc. OFC, Los Angeles, CA, paper W2A.24 (Mar. 2015).

8. T. Kita et al., Appl. Phys. Lett., 106, 11, 111104 (2015).

9. N. Kobayashi et al., J. Lightwave Technol., 33, 6, 1241–1246 (2015).

 

Computer modeling boosts laser device development
RÜDIGER PASCHOTTA

A full quantitative understanding of laser devices is boosted by computer modeling, which is not only essential for efficient development processes, but also for identifying the causes of unexpected behavior.

Computer modeling can give valuable insight into the function of laser devices. It can even reveal internal details that could not be observed in experiments, and thus allows one to develop a comprehensive understanding from which laser development can enormously profit. For example, the performance potentials of certain technologies can be fully exploited and time-consuming and expensive iterations in the development process can be avoided. Some typical examples clarify the benefits of computer modeling for improved laser device development.

Example 1: Q-switched lasers

FIGURE 1. Evolution of the transverse beam profile (shown with a color scale) and the optical power (black circles, in arbitrary units) in an actively Q-switched laser is simulated with RP Fiber Power software using numerical beam propagation. The color scale is normalized for each round trip according to the timedependent optical power so that the variation of the beam diameter can be seen.

Example 2: Mode-locked lasers

Example 3: Ultrashortpulse fiber amplifiers

FIGURE 2. The evolution of pulse energy and forward ASE powers in a four-stage fiber amplifier system with various types of ASE suppression between the stages, calculated with a comprehensive computer model

FIGURE 3. Form-based software can be used to model laser devices such as a fiber amplifier. It is essential that such forms be made or modified by the user or by technical support, so that they can be tailored to specific applications.

…. more

Documentation and support For any modeling task, documentation of methods and results is essential. The documentation must not only explain details of the user interface, but must inform the user what kind of physical model was used, what simplifying assumptions were made, and what limitations need to be considered. Unfortunately, software documentation is often neglected. In case of doubt, competent technical support should be available—not only for helping with the handling of the software, but also offering detailed technical and scientific advice. For example, a beginner may find it difficult to decide which kind of model should be implemented for a certain purpose and which possibly disturbing effects need to be considered. Such support should come from a competent expert in the field rather than just a programmer.
Rüdiger Paschotta is founder and executive of RP Photonics Consulting, Bad Dürrheim, Germany; e-mail: paschotta@rp-photonics.com; www.rp-photonics.com

 

 

 

Read Full Post »