Research Papers Blue Laser Diode

1. Introduction

Since the first practical demonstration of a laser by Theodore Maiman in 1960 [1], the range of applications has heavily increased. With improvements in production as well as performance, diode lasers also became increasingly attractive. Due to direct electrical pumping, diode lasers are by far the most efficient light sources currently available [2, 3]. Being based on chip technology, they can be manufactured in high numbers and at low cost. Their dimensions of only a few mm3 enable very compact laser systems. All these features increase their application potential, including biomedical applications. Applications range from imaging and diagnostics, e.g., optical coherence tomography [4], fluorescence lifetime imaging [5], diffuse optical imaging [6], THz imaging [7], laser Doppler imaging [8] or Raman spectroscopy, to direct treatment such as photocoagulation [10], photo-dynamic therapy [11] or biomodulation and bioactivation [12].

Compared to lasers limited to specific atomic transitions, diode lasers cover a much wider spectral range. Depending on the used compound semiconductors and their composition, the emission wavelengths of typical III-V compound semiconductors range from blue to near-infrared (400 nm – 2 µm, [13]). Although spectral side modes are sufficiently suppressed at higher currents [14], the application of diode lasers may be limited by their spectral characteristics. In these cases, the emission bandwidth can be narrowed, e.g., by intrinsic [15] or external feedback [16]. The latter also enables single-mode emission tunable over several tens of nanometers [17], in addition to the tunability obtained by adjusting the injection current or the laser temperature.

Despite the number of wavelengths that can be accessed with diode lasers, the output power might not be sufficient. In addition, other wavelengths, especially in the visible range, may not be achievable due to lack of available laser structures. One option also to achieve these wavelengths, or to increase the output power at a certain spectral region, is nonlinear frequency conversion [18], as discussed in this article. Other options are optically pumped semiconductor lasers [19] or solid-state lasers [20], although not covered in the present review. Due to the optical excitation these lasers show reduced optical efficiencies compared to electrically pumped diode lasers [21].

The output power and the beam propagation parameters (M2) of the diode lasers strongly depend on the design of the semiconductor structures. Nearly diffraction-limited beams are obtained with ridge-waveguide (RW) and tapered diode lasers. While the output power of RW lasers is limited to 1–2 W [22], more than 10 W are achieved with tapered lasers [23]. High-power emission is also obtained with broad area (BA) diode lasers [24] or diode laser bars and stacks [25]. However, these devices typically show reduced beam qualities that may be improved by additional feedback [26].

All these devices are edge emitting diode lasers, i.e., the propagation of the generated laser emission is in plane with the substrate surface [27]. In surface emitting diode lasers, also known as vertical cavity surface emitting lasers (VCSEL), the propagation direction is normal to the substrate surface. Their optical cavities are short and the facet reflectivities are high, resulting in low threshold currents [27]. The output power is typically in the milliwatt range but can be increased significantly by optical pumping in so-called vertical external cavity surface emitting lasers (VECSEL) [28]. In comparison to edge-emitting diode lasers described in this article, the challenging factor towards high-power, near diffraction-limited emission from VCSELs is the proper heat removal from the active region [29].

In addition to continuous wave (CW) emission, diode lasers may also be operated in pulsed mode. Pulsed emission is achieved by mode-locking [34], Q-switching [31] or in a more direct manner by gain-switching [32]. These techniques enable the generation of pico- to femtosecond pulses with repetition rates in the GHz range. Compared to other mode-locked or Q-switched lasers, the lower upper-state lifetime of nanoseconds [33] reduces the obtained pulse energies of diode lasers [34]. However, generated pulses with up to 50 W peak power [35] are more than sufficient for applications such as fluorescence measurements, which will be discussed in the present review.

The above-mentioned characteristics, i.e., output power, beam properties, wavelength spectral coverage and tunability, compactness and low cost, make diode laser technology versatile and increasingly applicable in the biomedical field, in particular. In this review, we provide an overview of state-of-the-art edge emitting diode lasers and their use within key biomedical applications. At the end we give an outlook on the future perspective of diode lasers for emerging applications within the biomedical field.

2. Edge-emitting diode lasers

Two key advantages of diode lasers are their capability of covering a wide spectral range and the possibility of realizing different layouts with individual features. In this section we introduce the required material structures and explain how diode lasers are built up. At the end we focus on the performance of diode lasers with respect to the applications discussed in the article.

2.1. Material structures and fabrication of diode lasers

Several compound semiconductors have to be applied in order to cover the above mentioned spectral range between 400 nm and 2 µm. A coarse selection of the laser wavelength λ can be performed by adjusting the composition of the material, later given as molfraction x, y, z. An overview on the available group III-V-compound materials is shown in Fig. 1. In the blue to green spectral range InxAlyGa1–x–yN material is used [36, 37]. Red emitting diodes between 615 nm and 750 nm are based on InxAlyGa1-x-yP. Between 670 nm and 890 nm AlxGa1-xAs1-yPy is used as an active layer. Longer wavelengths up to 1.2 µm can be reached using InxGa1-xAsyP1-y or In1-x-yAlxGayAs, grown on GaAs [38, 39]. Grown on InP substrates, the latter materials can cover the range up to 2.3 µm. Even longer wavelengths can be addressed by using antimonide based layer structures, lead salt lasers or more recently quantum cascade lasers.

The basic design of typical vertical layer structures is given in Fig. 2. The active layer consists of one or more quantum wells or quantum dots. This layer is embedded into a p- and n-side waveguide, which is surrounded by cladding layers. The p-side is completed by a highly doped contact layer. The layer structures are typically grown by metal organic vapour phase epitaxy (MOVPE) [40–44] or molecular beam epitaxy (MBE) [45, 46] on different substrates with a diameter between 2–4 inch.

The wafers are processed into laser devices applying different lithographic, etching and plating technologies. Typically, a first lithography defines the stripe width along which the light is guided through the device. Two mechanisms can contribute to this guiding, index-guiding and gain-guiding. In order to achieve index-guiding, a ridge can be etched into the p-side waveguide layer as shown in Fig. 3a. The deep etching causes a step in the refractive index leading to lateral confinement. Typical RW lasers provide nearly diffraction-limited beam quality at output powers in the lower watt-range. For gain-guiding, typical for BA lasers, a conductive stripe is defined in the contact layer (Fig. 3b). This is done by destroying the conductivity outside the stripe using ion implantation or by etching a low MESA structure. Hereby the carrier injection and laser emission are limited to this area. The stripe width can be in the range from some 10 µm up to 800 µm. Such BA lasers reach significantly higher output powers up to some 10 W, but suffer from poor beam qualities with M2 values typically in the range 10–50. To reach even higher output power several emitters can be combined within one laser bar (Fig. 3c), which reaches CW output powers of several 100 W.

In order to obtain high output power emission with good beam quality, one of the most promising concepts is the tapered laser. Within one chip, the diffraction-limited radiation of a RW section is coupled into a flared section (Fig. 3d), which can be realized index-guided or typically gain-guided. This section acts ideally as a passive amplifier [47–52]. In the flared section the mode-area is slowly broadened while the single-transverse-mode profile is mostly maintained.

The emission linewidth of a diode laser can be stabilized and narrowed by introducing an internal grating into the resonator [53]. In the case of distributed feedback lasers (DFB), the grating spans over the entire resonator length [54–56, 57–59]. Alternatively, it is possible to implement the grating as distributed Bragg reflector (DBR), acting as a wavelength-selective resonator mirror [60–63].

Having defined and fabricated these structures, an appropriate metallization of the p-side of the device is performed, followed by a thinning of the substrate and the rear side metallization.

The processed wafer is cleaved according to the desired resonator length and facet coated. In order to achieve high output power, special care has to be given to the cleaning and passivation of the laser facets [64–66]. This process step is followed by an optical coating of the facets. For laser devices, the rear facet is coated with a high reflectivity Rr ≈ 96%, whereas the front facet is anti-reflection coated with Rf ≤ 30%. Using the devices as a gain medium in external cavity configurations, one side of the device is anti-reflection coated with an extremely low reflectivity Rf < 5 × 10−4. For devices used as amplifiers both sides have this low reflectivity.

In order to operate the devices, they are mounted on special heat sinks, providing an efficient heat removal. The most common approach is to mount the diodes p-side down, reducing the thermal resistance [67]. Long lasers with low thermal resistance can also be mounted p-side up, lowering the mounting induced stress [23]. First the laser is soldered on a submount. Depending on the laser devices different materials can be used. If the devices are mounted without any significant strain between the semiconductor and the mount, a submount material with a comparable thermal expansion coefficient might be selected, such as CuW, BN [68] or BeO [69, 70]. If heat removal is crucial and the devices are tolerant against strain, submount materials such as chemical vapor deposition (CVD) diamond [71] can be used. Alternatively, AlN can be applied; a relatively cheap material and easy to handle. As solder AuSn is often used, which guarantees a reproducible soldering process. Finally the diode laser submount is mounted on a copper block of different geometries. These copper blocks can be cooled passively (i.e. conductively) or actively using micro-channel coolers. The fabricated laser diodes exhibit very long lifetimes up to several 10,000 h. Examples of such tests and the analysis of failures are reported in [72, 73].

2.2. Performance characteristics of diode lasers

All biomedical laser applications require certain parameters to be fulfilled. These can be, for example, wavelength, output power, beam quality, size and cost-efficiency of the laser systems. Diode lasers have proven their superior performance in these aspects. An overview on achieved maximum CW output powers at wavelengths between the blue and near-infrared spectral region is given in Fig. 4.

It is evident from Fig. 4 that diode lasers cover a large spectral range with increasing output power towards the near-infrared. Up to 25 W were obtained with broad area devices between 800–1000 nm [81]. This wavelength range coincides with a local maximum in the absorption spectra of blood. Even though the beam quality is rather poor, these lasers are extensively used in dermatology, because output power and wavelength rather than beam quality are the key parameters, as explained in Section 'Direct application of high-power diode lasers in dermatology'.

In the red spectral range up to 5.6 W were reported for BA lasers [78]. Around 1 W was achieved with tapered devices [91]. The red to near-infrared wavelength region is preferred for diffuse spectroscopy and imaging, discussed in Section 'Diffuse near-infrared spectroscopy and imaging using diode lasers'. Due to their size, efficiency, and power requirements in the order of milliwatts, diode lasers are preferably applied in these applications.

As Fig. 4 shows, obtaining high-power laser emission at shorter wavelengths in the visible range is still challenging. In the green spectral range up to 170 mW were demonstrated using ridge waveguide lasers [87]. With high-power green light being of high importance, for example, in dermatology and direct pumping of ultrashort pulsed lasers, frequency conversion represents a solution to increase the power at these wavelengths, as described in Chapter 5. Up to 12 W with near diffraction-limited beams were reported with tapered lasers at 978 nm [94] and 1060 nm [95]. Both types of devices were based on intrinsic DBR gratings as rear-end mirrors. Due to the high reflectivity of the intrinsic gratings, the rear facets of the lasers require antireflection coating. Therefore, spurious spectral modes are not reflected back into the tapered section and the spectral linewidth is significantly narrowed [23]. Due to their output power and their excellent spatial and spectral characteristics these devices are ideal for frequency conversion into the blue-green spectral range. Combined with the large number of material compositions, frequency conversion of diode lasers also enables access to new wavelengths, currently not covered.

In the blue spectral range up to 1.6 W were shown with direct-emitting BA devices [74]. These wavelengths are preferably applied, for example, in fluorescence measurements typically requiring low power emission. One major challenge is to obtain yellow emission. This is mainly due to missing material structures for edge-emitting diode lasers around 590 nm or 1180 nm.

For any given structure or wavelength, pulsed emission is obtained simply by modulating the diode injection current. This enables generating pulses with adjustable pulsewidths and repetition rates suitable for applications, such as fluorescence based imaging. As explained in Section 'Gain-switched diode lasers generating optical pulses down to the picosecond range', the obtained peak power may be reduced compared to other laser systems, but still sufficient with respect to the sensitivity of biological targets.

It is obvious that based on their performance diode lasers become increasingly applicable in the biomedical field, and the following sections emphasize their advantages within key biomedical applications.

3. Direct application of high-power diode lasers in dermatology

As pointed out earlier, diode lasers provide increased output power in the near-infrared range. In dermatology these wavelengths combined with the absorption by blood are used to treat different diseases, such as vascular malformations and hemangiomas. Due to reduced absorption and scattering coefficients in tissue, corresponding diode lasers allow for longer penetration depths and the treatment of deeper-lying vessels. In addition, diode lasers address the need for compact and efficient systems. Their flexibility in wavelengths and the direct control of laser emission enable optimizing treatment parameters with respect to specific chromophores, the treatment outcome and reduction of side-effects.

3.1. Short introduction of selective photothermolysis

The application of lasers in the biomedical field is strongly related to light-tissue interactions. Such interactions enable both imaging as well as direct treatments. Light-tissue interactions can mainly be described and quantified by four different parameters: the refractive index, the scattering coefficient, the scattering phase function, and the absorption coefficient [96], respectively. While the scattering coefficient defines the probability of photon scattering events, the absorption coefficient provides information about the amount of energy being extracted from an incident light wave. Their wavelength dependence [97] and the ratio between the scattering coefficient and the sum of the scattering and absorption coefficients, called the albedo [98], determine the penetration depth and therefore the optimum wavelengths for different applications.

In the visible range (400–600 nm) the absorption is dominated by oxy- and deoxy-hemoglobin, and melanin (Fig. 5). Above 1300 nm water is the main absorber. Within that window (≈ 600–1300 nm) the absorption coefficients are reduced by 1–4 orders of magnitude.

The tissue response depends on the heat generated by absorption. Anderson and Parrish introduced selective photothermolysis, suggesting that selective tissue absorption within the so-called thermal relaxation time of the tissue leads to selective destruction of the target [100]. The thermal relaxation time is the time in which the targeted tissue dissipates 50% of the generated heat and it scales with the square of the target diameter. It therefore depends on the absorption coefficient and size of the target, as well as on the laser wavelength and pulse duration. The optimum pulse duration should be equal to or slightly less than the thermal relaxation time, in order to avoid damaging the surrounding tissue. For each target there is a critical temperature. Temperatures exceeding that value will lead to coagulation, vaporization, and finally ablation of the tissue, respectively [101].

3.2. Diode lasers for photocoagulation

In dermatology, selective photothermolysis is chosen for applications, such as hair removal [102], skin rejuvenation [103], or photocoagulation [104], respectively. The latter is based on absorption of photon energy by blood and shall be the main application discussed in this section. Considering Fig. 5 the most obvious wavelengths for photocoagulation are in the green-yellow spectral range. Possible choices of lasers are, e.g., frequency doubled solid-state lasers, providing > 100 W of output power in CW or pulsed mode [105, 106]. These lasers tend to be bulky and expensive and thus alternative solutions are required. In addition, the very high absorption of blood in the green-yellow spectral range limits the penetration depth and the size of the vessels treated. Lasers with lower absorption are preferred to enhance volume heating of deeper-lying, larger vessels. In addition, a lower absorption in melanin has the potential to cause less damage to the skin. The main attenuation stems from the light scattering, which is reduced inversely proportional with wavelength. Hence, increasing the wavelength enhances penetration depth.

Accordingly, in Fig. 5 a trade-off solution to this problem is shown. The hemoglobin absorption curves also exhibit a local maximum in the range between 800–1000 nm and, simultaneously, the absorption in melanin is reduced. At these wavelengths, light experiences less scattering [97], increasing the penetration. However, the absorption coefficient of hemoglobin is reduced by more than one order of magnitude but still sufficient to obtain the effect. Available diode lasers are capable of emitting multiple tens of watts [81, 82] and can easily be coupled into multimode fibers for direct delivery. By providing sufficient optical energy in the most efficient and compact manner, whilst the beam propagation parameters not being crucial, these high-power, near-infrared diode lasers have become very attractive light sources for photocoagulation.

3.3. Treatment of vascular malformations and hemangiomas with diode lasers

3.3.1. Endovenous laser treatment of vascular malformations

Vascular malformations are disorders of blood or lymphatic vessels causing reddish or bluish lesions underneath the skin [104]. For example, venous malformations are common disorders where the valves within the veins are unable to prevent the reflux of blood causing swelling, pain and muscle cramps. The surgical treatment of choice is ligation and stripping of the veins leading to complications such as trauma, bleeding and scars, as well as increased hospital costs and long recovery times [107]. The non-surgical procedure is sclerotherapy, which can also cause pigment changes and scarring [108].

An alternative method is endovenous laser treatment (EVLT), a minimally invasive method introduced for the treatment for varicose veins [109]. The heat generated by absorption diffuses through the blood and vessel walls initiating the development of steam bubbles that cause thermal injury and vessel occlusion [110].

The light energy of a high-power, long-pulsed, fiber-coupled laser is delivered directly into the vein through the fiber and guided by ultrasound imaging. The light pulses are initiated while the fiber is slowly withdrawn causing vessel closure. Compared to sclerotherapy, EVLT enables a more precise control of vein wall damage, lowering the recanalization rates of the closed vessels.

Diode lasers are the lasers of choice for EVLT. They provide the necessary power level and wavelengths in fiber-coupled packages enabling compact and cost-efficient laser systems for the treatment. Furthermore, the amount of energy can be precisely controlled directly by the laser current. The first demonstration of a diode laser EVLT was carried out using a 14 W, 810 nm laser [109]. The actual procedure was carried out with 3–4 W delivered in 1–2 second pulses, required due to the blood flow dissipating the heat. The treated veins had mean diameters of 5 mm and lengths of 20 cm. The immediate results indicated an excellent closure rate of 100% comparing favorably to other minimally invasive techniques. These results were confirmed by other groups [107, 110-113]. A study of the short-term efficacy of EVLT showed that 99% out of 90 cases still showed vessel closures after 9 months follow-up [107]. The patients were instructed to walk immediately after the procedure and continue their normal daily activities, indicating the viability of the procedure. The risk factors for nonocclusion are not only related to laser parameters, such as fluence (energy per cm2) or irradiation time, but also to physiological parameters such as the vein diameter and the distance of the thrombus to a larger vessel after the procedure [114]. Therefore, accurate diagnosis is of paramount importance in determining the proper laser and its parameters, in order to optimize the outcome of these treatments and minimize side-effects.

3.3.2. Treatments of vascular malformations applied externally

While EVLT requires the light to be delivered through a fiber via minimally invasive surgery, other procedures, such as the treatment of port-wine stains or telangiectasia [115], rely on the energy being delivered directly through the skin. The success of these treatments relies on the combination of light absorption and penetration depth. For small vessel sizes, green-yellow lasers like solid state lasers or dye lasers are chosen [116]. For larger vessels, deeper penetration is required. As discussed above, deeper penetration is achieved at longer wavelengths, obtained, for example, with near-infrared diode lasers. However, diode lasers are typically not preferred for treatments of these mostly superficial malformations. Nevertheless, a few groups did examine their capability in that field [117–121].

In one example, vascular abnormalities were treated with 150 ms pulses of a 980 nm laser at 300–500 J/cm2 [120]. As mentioned above, longer wavelengths and short pulses increase the potential causing less damage to the skin. In another study laser therapy was combined with radiofrequency. In that case, the absorption of 250 ms laser pulses preheated the blood vessel and created conditions for selective radiofrequency applications [121]. As a consequence, this combination allowed reducing the laser fluence (80–100 J/cm2), lowering the risk of possible damages to the epidermal layer even further. The overall response of the patients in that study was excellent showing 75–100% lesion clearance.

3.3.3. Treatment of hemangioma by diode laser surgery

In comparison to vascular malformations, hemangiomas are vascular tumors developing after birth and regressing after a couple of years [104]. However, in case of symptoms such as bleeding, pain or functional compromise, treatment is strongly recommended. One preferred treatment is endolesional diode laser surgery [122]. As mentioned above, diode lasers provide sufficient output power in fiber-coupled packages, enable compact and cost-efficient laser systems, and the amount of energy at the desired wavelength can be controlled by the laser injection current.

Using a 980 nm diode laser delivering 3–12 watts in continuous or long-pulsed mode, 160 pediatric patients were treated with head and neck hemangioma up to 7 cm in size. The results showed that diode laser treatment improves individual results for lesions up to 5 cm. A similar laser was used performing soft tissue surgery of oral hemangioma [123]. The diode laser was chosen due to its ability to cut with high ablation capacity and reduced bleeding rates, while simultaneously coagulating soft tissue [124, 125]. It was noted that the removed specimens can have a size ≤ 5 mm to still enable a reliable histopathological diagnosis [126]. The diode laser emission led to a sufficient hemostasis and precise incision margins without the need for suturing after surgery [127]. Compared to competing lasers the same group concluded that diode lasers enabled cutting comparable to CO2 lasers and coagulation similar to Nd:YAG lasers [127]. All these results confirm that diode lasers are competitive choices in soft tissue surgery.

Based on their advantages high-power diode lasers are increasingly preferred within applications in dermatology. The range of wavelengths that are accessed with diode lasers open a range of new opportunities, compared to competing systems. Combined, these wavelengths, the resulting penetration depths and the obtained output powers enable addressing individual treatment parameters in a highly efficient manner, while satisfying the need for compact, portable and low-cost laser systems. These advantages combined with the continuous work in diode laser technology will increase the number of direct diode laser applications in the biomedical field even further.

4. Wavelength-swept diode laser systems for optical coherence tomography

Optical coherence tomography (OCT) is an interferometric technique that generates cross-sectional images of scattering material with a typical depth resolution of a few micrometers [128]. Rapidly wavelength-swept laser light sources, or simply swept sources, make ultra-fast OCT image acquisition possible. Semiconductor diodes are ideal gain media for these swept sources, as they permit broadband wavelength tuning at very high speed.

4.1. Optical coherence tomography

Due to the unique ability to image the morphology of biologic tissues non-invasively (Fig. 6, left), OCT has become a well-established tool for biomedical research and clinical diagnostics [129]. It is used on regular basis for early detection of retinal pathologies and for monitoring treatment of those. Another clinical application is examining atherosclerotic plaques and coronary stents in cardiac blood vessels with endoscopic OCT systems. OCT is being used in many other fields of medical and biologic research, but also for technical purposes, such as non-destructive material testing or contact-free metrology [130].

An OCT system probes the sample with a beam of light (typically near-infrared), and obtains a depth-resolved reflectivity profile from the backscattered fraction. One such measurement is called an A-scan in analogy to ultrasound imaging. By scanning the beam laterally over the sample, a two- or three-dimensional image can be assembled from a number of adjacent A-scans. Most state-of-the-art OCT systems acquire A-scans in the frequency domain, i.e. by detecting the spectrum of the backscattered light after interference with a reference beam. They employ either broadband illumination and a spectrometer or a tunable narrowband light source and a fast photodetector [131, 132]. In the latter scheme (Fig. 6, right), the light source performs rapid sweeps over a broad wavelength range [133, 134], hence this method is termed swept-source OCT (SS-OCT). While SS-OCT requires more complex light sources than spectrometer-based OCT, it offers a number of advantages, such as longer imaging depth range [135], lower susceptibility to artifacts caused by sample motion [136], and the possibility of ultra-high speed image acquisition [137, 138].

4.2. Special properties of swept sources

Most swept sources are tunable lasers in highly specialized configurations that meet the requirements for OCT. State-of-the-art swept sources feature sweep repetition rates ranging from 100 kHz up to several MHz. The tuning bandwidth can be well above 100 nm, which is desirable since the OCT depth resolution improves proportionally with the bandwidth [129]. On the other hand, the dynamic linewidth, i.e. the instantaneous width of the narrowband spectrum while it is being tuned, is rather broad compared to classical CW laser lines. Up to several 10 GHz may be acceptable, which results in an OCT imaging depth range of a few millimeters [134, 139, 140]. In recent years, however, considerable efforts went into the development of swept sources with narrower dynamic linewidth in order to increase the imaging depth range [141-144].

The very high tuning speeds of 107–108 nm/s can only be realized using semiconductor laser gain media, which feature a short excited-state lifetime on the order of nanoseconds. Other swept source configurations based upon doped crystals or fibers did not show good performance at high sweep rates [134, 145, 146].

Semiconductor gain media have also a number of other advantages. They are available for many different wavelength ranges and offer broad gain bandwidths as well as unmatched flexibility for tailoring the gain spectrum. Due to direct electrical pumping, light sources can become very efficient and compact. It also permits straight-forward arbitrary shaping of the light source spectrum, which is useful for optimizing the OCT signal acquisition [147-149] and allows to correct for spectrally dependent transmittance of optical media in the probing beam path [150].

4.3. Swept source technology

Today, most swept sources in practical applications are external cavity tunable lasers (ECTLs) using a semiconductor optical amplifier (SOA) – i.e. a diode with single-mode waveguide and anti-reflection coated facets – as gain medium and a tunable band-pass filter in either a free-space or fiber-based setup. Free-space resonators can be very compact [151], especially in conjunction with a tunable filter based upon micro-electro-mechanical systems (MEMS) [152, 153].

Fiber-based setups (Fig. 7), on the other hand, which offer uncomplicated implementation of stable, alignment-free light sources, are preferred by the research community [139, 140, 154]. Furthermore, by using a long fiber resonator (several 100 to 1000 meters) one can synchronize the sweep rate of the tunable filter with the resonator roundtrip frequency [155]. Using this technique, called Fourier domain mode-locking (FDML), one can overcome the tuning speed limitation given by the time required to build up laser light from spontaneous emission. Whereas ECTLs with short resonators have been demonstrated with sweep rates up to 400 kHz [156], more than 5 MHz could be achieved with an FDML laser [137].


Solid-state lighting (SSL) is now the most efficient source of high color quality white light ever created. Nevertheless, the blue InGaN light-emitting diodes (LEDs) that are the light engine of SSL still have significant performance limitations. Foremost among these is the decrease in efficiency at high input current densities widely known as “efficiency droop.” Efficiency droop limits input power densities, contrary to the desire to produce more photons per unit LED chip area and to make SSL more affordable. Pending a solution to efficiency droop, an alternative device could be a blue laser diode (LD). LDs, operated in stimulated emission, can have high efficiencies at much higher input power densities than LEDs can. In this article, LEDs and LDs for future SSL are explored by comparing: their current state-of-the-art input-power-density-dependent power-conversion efficiencies; potential improvements both in their peak power-conversion efficiencies and in the input power densities at which those efficiencies peak; and their economics for practical SSL.

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