Exploring light-emitting diode pumped luminescent concentrators in solid-state laser applications

In the past, there were limited efforts to use light-emitting diodes (LEDs) for pumping solid-state lasers. However, these attempts were overshadowed by the introduction of laser diodes, which offered more favourable pumping conditions. Nevertheless, recent advancements in high-power LEDs, coupled with the utilization of luminescent concentrators (LC), have paved the way for a novel approach to pump solid-state lasers. The combination of LEDs and LC in this LED-LC system presents several advantages, including enhanced ruggedness, stability, and cost-effectiveness compared to other laser pumping methods. This review explores the various techniques employed to pump solid-state lasers using LED-LC as a pump source, along with improvements made to enhance the brightness of LEDs in this context.


Introduction
In 1961, T Maiman achieved the first demonstration of LASER (Light Amplification by Stimulated Emission of Radiation) using a ruby laser that was pumped by a flashlight [1].A laser system comprises three essential components: an active lasing material or gain medium, an external energy source, and an optical resonator.The classification of lasers often depends on the type of gain medium employed.For instance, in solid-state lasers, the gain medium is a solid material, hence the term 'solid-state' laser.The external energy source provides the energy to create population inversion in the lasing medium called 'pumping'.Pumping can be achieved in numerous ways, including electrical discharge, optical method, etc. Amongst these pumping methods, light-emitting diodes (LEDs) have arisen as a potential pumping source for solidstate lasers at a lower cost.The first (LED) pumped laser was demonstrated using a calcium fluoridedoped dysprosium (CaF 2 : Dy) crystal in 1964 [2].This work used luminescence from GaAs x P 1-x LEDs at pumped-helium temperatures of 77 K to pump the crystal.Lasing was achieved for an optical pump power of 0.1 W but only for 0.2 s due to heating issues.It was observed that cryogenic temperatures were needed as in ruby, where the rate of spin-lattice relaxation (due to two-phonon Raman scattering of Kramers-type paramagnetic ions -like chromium (Cr) ions in sapphire lattice) scales with temperature [3].
In 1969, R.B. Allen and S.J. Scalise demonstrated a continuous operation of a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser using GaAs x P 1−x LEDs at liquid-nitrogen temperatures [4].The peak absorption occurred at 810 nm for the transition from 4 I 9/2 to 4 F 5/2 with a laser output of 40 mW at 1.0641 μm.Later, A R Reinberg et al demonstrated the first observation of a laser at 1.029 μm obtained by pumping ytterbium-doped YAG (Yb: YAG) with silicon-doped gallium arsenide (Si: GaAs) LEDs [5], where the LEDs were operated at 80 K.They observed a strong spectral match between the Si: GaAs LED emission and single Yb 3+ absorption band in Yb: YAG, allowing them to demonstrate a peak power of 0.7 W with the pulsed operation.
As laser diodes (LDs) gained prominence, LED pumping gradually lost traction primarily because meeting the cryogenic temperature requirements necessary for LED pumping proved challenging.LDs emerged as a versatile method for laser pumping.However, when LDs are utilized as pump sources, the narrow emission line of the diode laser must align with the strongest absorption line of the lasing medium to achieve optimal efficiency.Any deviation in the emission line, caused by thermal effects or variations in pump current, can lead to instability in the pump source.Conversely, to maximize laser efficiency, the laser system should be designed to effectively concentrate and transfer the pump light into the lasing medium using suitable geometrical configurations.
With the longer lifetime, more robustness, much lower cost, and the re-emergence of LEDs with the ability to be operated at room temperature drew attention back to the long-forgotten LEDs as laser pumping sources.Further, LEDs have moderately broad emission, often overlapping with a group of high absorption lines in a laser material, making it an ideal candidate as a pumping source.
However, the brightness theorem also known as the law of etendue states that brightness (W mm −2 .Sr) cannot be increased by any passive optical system [6].This means no irradiance enhancement is possible for LED-like Lambertian pump sources using conventional optical methods.However, luminescence conversion techniques offer a workaround to this limitation.In these techniques, the light emitted by the source is not reflected or refracted but absorbed by a luminophore (or lumophore) material.The absorbed light is then re-emitted at a different wavelength through a luminescent process.Lumophores can be either fluorophores or phosphors, depending on the luminescent material embedded inside a host matrix.Fluorophores are generally fluorescent chemical compounds such as dyes.Phosphors are generally madeup of mostly transition metals or rare-earth compounds in which an activated material is added to a host material.One example is Ce: Y 3 Al 5 O 12 (Ce: YAG), discovered in 1967 for color television cathode ray tubes [7].Ce: YAG showed bright yellow emission with absorption bands in the ultraviolet and in the blue.This gives the opportunity to pump it with blue LEDs [8].
The luminescence conversion technique could make the LEDs considerably brighter via luminescence concentration, producing a broad spectrum that offers the flexibility to match applications.The paper proposes first a description of the brightness enhancement by luminescent concentrators.Next, it gives an overview of the existing LED-pumped luminescent concentrators.Finally, it presents a review of the lasers pumped by luminescent concentrators.

Luminescent concentrators (LCs)
Luminescent concentrators (LCs) are typically constructed using a thin rectangular strip composed of a higher index material embedded with luminophores.This design allows for efficient absorption of pump light by the luminophores by the large surface area of the LC.The re-emitted light, now at a higher wavelength, is guided through total internal reflection (TIR) towards the edges of the LC. Figure 1 illustrates various Ce-doped LCs illuminated by ultraviolet light, showcasing the effectiveness of the luminescent concentrator.Luminescent concentrators can be categorized based on the type of pump source they utilize.There are solar-pumped LCs, also known as luminescent solar concentrators (LSC), and LED-pumped concentrators (LED-LC).

Luminescent solar concentrators (LSC)
Achieving a balance between higher power conversion efficiencies and reduced fabrication costs is essential in the photovoltaic industry.Most of the research using luminescent concentrators has concentrated mainly on harvesting solar energy to increase the lightharvesting efficiency of photovoltaic cells.A schematic representation of LSC is shown in figure 2. In 1975, C. E. Backus stated that any device that redirects sunlight in a way that increases the solar energy flux on the desired absorber is called a solar concentrator [9].Later in 1977, A. Goetzberger and W. Greubel proposed and theoretically evaluated the concept of luminescent solar concentrators (LSC) in traditional Si solar cells [10].They showed the collection of solar radiation and the concentration of the collected radiation in LSC through fluorophores doped inside a transparent sheet of a chosen material [10].Typically, the solar spectrum is broad; converting this broad spectrum to monochromatic light can increase the efficiency of PV devices.Normally, LSC is a fluorophore-embedded, optically transparent waveguide material.These fluorophores can absorb incident solar light and re-emit at a higher wavelength.The reemitted light is transported to the edge via TIR, which can then be collected in an adjacent solar cell.LSC research peaked in 1980 and was then hindered due to the lack of fluorescent materials available at that time.

LED-pumped luminescent concentrators (LED-LCs)
Despite LED technology improvements since the 1970s, the LED irradiance is still lower compared to LD.In recent years, the utilization of LEDs to pump LCs enables achieving irradiances that could not be achieved with LEDs alone but at the cost of wavelength shift.
Typically, LCs are designed as thin rectangular strips with high aspect ratios between the front face and end face (figure 3).With the assumption that the LC is pumped uniformly through the two surfaces with the largest surface area, the optical processes within the LC are shown in figure 3 for an example of a Ce:YAG LC pumped by blue LEDs.Common loss mechanisms in this configuration include reflection of incident light from the incident surface (5), light loss through the escape cone (or loss cone) defined by the critical angle losses (for light emitted within the Brewster angle), (6), non-radiative decay (7), self-absorption of re-emitted light by luminophores with spectral overlap (8), parasitic absorption by the host material which does not re-emit light (9), directly transmitted light with no photon absorption (10), scattering within the LC (11) and surface loss (12).
The luminophores inside the LC absorb the incoming light and re-emit light at a higher wavelength (Stokes shift).Part of the re-emitted light falls within the loss cone (where the angle is smaller than the critical angle) and is lost at the surface of the LC.The light rays with an angle greater than the critical angle are guided inside the waveguide through TIR.
The critical angle of a material with refractive index n surrounded by air is given by; The output power of an LC can be expressed as  the LC.Optical conversion efficiency can be expressed as [22,23]; where, R is the surface reflection coefficient, abs h is the fraction of light absorbed by the luminophores, PLQY h is the photoluminescence quantum yield of luminophores (number of emitted photons to each absorbed photon), stokes h is the Stokes efficiency, which is the energy loss between the absorption and emission, trap h is the efficiency of light trapping (light with an angle greater than the critical angle), self h is the efficiency of self-absorption by the luminophores, host h is the efficiency of the host material, taking into account the absorption and scattering inside the host, and TIR h is the portion of light emitted by the luminophore totally internally reflected towards the small facet of LC.
To increase the , o o h / loss channels such as heat generated inside the LC, QE, waveguiding (i.e., light falls within the loss cone, parasitic scattering, and absorption), and self-absorption of the converted light should be minimised.
The main difference between LSC and LED-LCs is the size of the concentrator which is much smaller for LED-LC to be compatible with the size of the pump source, in this case, commercially available LEDs.

LED-pumped luminescent concentrators in use
LED-LCs can be sorted by luminescent materials.The most used are Ce doped YAG pumped by blue LEDs.Laser materials have also been proposed as LED-LC exploiting spontaneous emission instead of stimulated emission.

Ce:yag
Ce 3+ doped YAG slabs are the most common luminescent material used within LED-pumped LCs to increase LED irradiance due to their high photoluminescence quantum yield.This luminescence arises from the transition of Ce 3+ ions from the 5d level to the two 4f levels.The well-separated Stokes shift, high photoluminescent quantum yield ∼95% ( PLQY h ), short emission lifetime, and high thermal conductivity make Ce:YAG a promising candidate as an LC.In addition, Ce: YAG demonstrated thermal quenching at high temperature (600 K), potentially simplifying the management of the heat created by the pumping.
In 2007, T. Brukilacchio and his colleague A.R. Conner were awarded a patent on luminescent concentrators based on ceramic luminescent material, a Ce:YAG rod pumped by GaN blue LEDs.The incident light from the LEDs was absorbed by the luminophores inside the luminescent rod and re-emitted at a higher wavelength.This luminescent light is trapped inside the rod by the TIR light-guiding mechanism and extracted via indexed matched output optics for use in various applications [22].
In 2015, J Sathian et al filed a patent on luminescent concentrators with further improvements to the design.Here, they used a Stokes shifting material in the slab or strip design, allowing greater optical output whilst allowing for effective heat removal from the concentrator [23].
In 2016, Barbet et al [24] reported one of the first demonstration of Ce: YAG LED-LC.A 9 mm ´100 mm ´1 mm Ce: YAG was pumped by 350 LEDs, closely coupled to the top and bottom surface of the LC with a fill factor of 19.4%.The LEDs were operated at pulse regime with a 3.75 A current with a pulse duration of 100 μs yielding a maximum power of 650 W. The output peak power of the LC was recorded as 43 W with an irradiance 2.6 higher than one LED.The target application was laser pumping (refer section 4.2).
The same year, De Boer et al [25] presented a Ce: YAG pumped by 56 blue LEDs in continuous wave mode.The YAG host matrix has a high refractive index (n = 1.83) and inevitably, the critical angle for TIR is as low as θ c = 33°.This limits the extraction efficiency to 8% per small exit edge or 16% with a mirror put on the opposite face.To improve the extraction efficiency, a concentrator parabolic compound was glued at the output.The device then provides 18 W for a blue pump power of 84 W. The target application was solid-state lighting (video projection).
In 2017, J Sathian et al reported a luminescent concentrator, a 304 mm long Ce:YAG waveguide in the form of a thin rectangular strip optically pumped from both sides by InGaN blue LEDs.The concentrator in this system is air-cooled, as in figure 4(a) [21].Ce 3+ ions dispersed in the LC absorb the peak LED emission at 460 nm (figure 4(b)).This absorbed light is re-emitted within the concentrator at 548 nm and is TIR-guided to the smallest end facet of the concentrator.An output power of 20 W was obtained in continuous wave.

Yb, Er:glass
Laser material can be revisited to produce spontaneous emission instead of stimulated emission.P Pichon et al in 2018 [26] proposed to make an LC with a co-doped Yb, Er glass generally used for laser operation at 1.55μm.This was the first LED-LC in the short-wave infrared (SWIR).They used low-cost, highly compact LEDs with peak emission at 940 nm to pump a ytterbium (21% doped), erbium (0.5% doped) codoped glass (Yb, Er:glass) LC.This LC is capable of absorbing light in the 900 nm-1000 nm region.Yb, Er:glass is very popular in the telecommunication sector as the Er 3+ emission wavelength at 1550 nm falls in the fiber optic communication window (figure 5(b)) [27].Figure 5(a) depicts the high intense lasing signal at 1550 nm resulting from Er 3+ transition from the first excited state to the ground state ( 4 I 13/2 → 4 I 15/2 ).Er 3+ ions are co-doped with Yb 3+ ions to increase the absorption efficiency (Yb 3+ absorption occurs from 2 F 7/2 → 2 F 5/2 ) and for energy transfer to 4 I 11/2 from Er 3+ due to energy overlap of the transition dipoles of the two elements.
In P. Pichon's LED-pumped LC design, they use LEDs with an emitting surface dimension of 1 mm ´1 mm [26].By using 64 LEDs on a single LC with dimensions 50 mm ´2 mm 0. ´5 mm, (length (L) ´width (w) ´thickness (h)), they were able to achieve a 51% filling factor.The LEDs were driven by 100 mA continuous current resulting in a total emitted power (optical) of the LED around 6.272 W and measured output electrical power of 141 mW at 1550 nm.This yields an optical conversion efficiency of 2.25%.The concentration factor (C) is calculated for LC, assuming that both input and output irradiance are Lambertian emitters, by taking the ratio of the LC output irradiance over the LED irradiance.'C' depends on the LED filling factor (the percentage of how much of the pump surface is covered by the LED emitting surface) and the dimensions of the LC.
In the work published by P Pichon et al, the geometrical factor (G) was calculated as 25 and the concentration factor, C reached 0.29 [26].The concentration value for LED-LC with Yb,Er:glass was lower than the typical values obtained with LED-LC using Ce:YAG crystals.The reason comes from the reabsorption by Er ions, in close relation to the quasi-three level nature of this laser material.Therefore, the concentration was investigated and, in trying to optimise the concentration factor, two LCs were bonded together (increasing G to 50), setting a mirror in the opposite edge of the LC (increasing trap h by reflecting the outgoing light otherwise lost through that facet) and finally, a parabolic compound is used at the output end to improve the .extract h With these additions, they attained a C of 0.65 for an optical conversion efficiency of 2.56%.

CTH:YAG
The most recent development in LED-LC is reported by L Lopez et al using a cascade LC design in the SWIR region using a Chromium, thulium, and holmium doped YAG (CTH:YAG) crystal [29].CTH:YAG was first proposed by B M Antipenko et al in 1985 [30].CTH: YAG is a popular laser medium in SWIR, with a broad emission from 1.7 μm to 2.1 μm, where the energy transfer occurs via the following steps; energy transfer from Cr 3+ ions to Tm 3+ ions, relaxation exchange between 3 F 4 to 4 H 6 and 3 H 6 to 3 H 4 in Tm 3+ ions and transfer energy from Tm 3+ ions to Ho 3+ ions [31].
In the L Lopez et al cascade LC design [29], blue LED coupled Ce:YAG was chosen to pump the CTH: YAG crystal due to good overlap between the emission of Ce:YAG with the absorption band of CTH:YAG.A 3 mm wide CTH:YAG crystal (n = 1.8) was attached to the small end of a 1 mm × 14 mm × 220 mm Ce:YAG crystal using optical adhesive with a refractive index of 1.5.This cascade LC was pumped with 2240 LEDs.A gold-coated mirror was attached to the opposite end to increase the output power of the cascade LC.The LEDs were pumped at 260 μs at a frequency of 10 Hz for a quasi-continuous signal to be delivered at a maximum power of 3.15 W per LED.A peak power of 1.7 kW was delivered to the CTH:YAG crystal resulting in a peak power of 50 mW in the 1.8 μm-2.1 μm spectral region.Regardless of the low efficiency of this novel cascade LC design, it provides a route to new possibilities and demonstrates the potential for enhancing the efficiency further, such as increasing LED pulse duration compatible with the CTH:YAG lifetime (8.5 ms) and introducing a heat draining system to the LC design.

Lasers pumped by luminescent concentrators
As aforementioned, an LC is a narrow transparent matrix slab with a high refractive index containing luminophores dispersed in it.These luminophores promptly absorb light falling onto the LC surface with the largest surface area and re-emit light at a redshifted wavelength.The re-emitted light is TIR-guided to the thin edge of the slab.This allows to increase in the output irradiance, which is a key parameter to reach the threshold in a laser oscillator.

Polymer laser pumped by LC
In 2009, Y Yang et al demonstrated that LCs could be used to pump polymer lasers.A mix of coumarin 6 and coumarin 338 fluorochromes embedded in SU8 photoresist with a refractive index of 1.62 is spin-coated on a coverslip (n = 1.5) [32].The active layer of the LC is illuminated using a 442 nm He-Cd laser beam for characterization and then pumped by an optical parametric oscillator at 450 nm for laser demonstration.
After demonstrating that a high light concentration ratio (ratio between the output irradiance from one edge to the pump irradiance) can be achieved, they explored the possibility of using this LC to pump a polymer laser with a gain medium consisting of poly [2-methoxy-5-(20-ethylhexyloxy)−1,4-phenylenevinylene] (MEH-PPV) that matches with the photoluminescence spectrum of the optimised LC.They observed that the LC could transform the pump beam from a circular spot to a thin stripe, therefore, the effect of pump beam shape on the lasing threshold density was investigated.Figure 6 depicts an LCpumped polymer laser setup.For 2D distributed feedback (DFB) lasers, with a laser wavelength at 620 nm (close to the superluminescence), the pump threshold density was reduced by a factor of 4.8 with the use of LC compared to the direct pumping with a circular spot of density 5.3 kWcm 2 .For the 1D DFB laser, the LC further reduced the pump threshold density by a factor of 4.5.
In 2014, A Barbet et al were the first to report a NIR LED pumped Nd:VYO 4 laser [34].They achieved an output energy of 40 μJ at 1064 nm for an input pump energy of 7.4 mJ corresponding optical efficiency of 0.5%.In 2019, H. Xiao and group demonstrated LEDpumped Nd:YVO 4 laser in quasi-continuous wave (QCW) and passively Q-switched regime (PQS) [35].They used a 4 mm´4 mm´25 mm a-cut 1% Nd doped YVO 4 gain medium side pumped with near IR LEDs (810 nm centre) as depicted in figure 7.With a pump energy of 6.28 mJ, the designed Nd:VYO 4 laser has an output energy of 206 μJ at 1064 nm in the QCW regime corresponding to an optical conversion efficiency of 4.1%.In the PQS regime, a pulse energy of 2.5 μJ was obtained.The laser pulse duration was 897 ns with an output energy stability of 97.54%.Despite these demonstrations, the pump power in direct LED-pumping is limited by the surface available around the laser crystal.This is the direct consequence of the small irradiance of LEDs.To overcome this drawback, LED-LC is proposed as described in the following.
The first LED-LC pumped solid-state laser was demonstrated by A Barbet et al using a Nd:YVO 4 crystal [24].The LC consisted of an optically polished YAG slab doped with Ce 3+ ions.This pumping system was sturdier than LDs and independent of the external environment conditions.The chosen dimensions of the slab were 100 mm ´1 mm ´1 mm (already described in section 3.1).The design allows the LEDs to illuminate the top face of the slab, and the re-emitted light by the embedded luminophores is TIR-guided to the narrow edge of the slab to be outcoupled (figure 8).To extract the light more efficiently, the LC was bonded directly on the laser crystal using an optical adhesive with a refractive index greater than the air.
The absorption band of the chosen lasing medium shows a good overlap with the emission band of Ce: YAG crystal (figure 9) [24].A current with a pulse duration of 100 μs to match the laser medium lifetime to limit the thermal effects of the uncooled crystals.
The estimated peak output power transferred to the Nd:YVO 4 reaches 165 W, with a pump energy of 16.5 mJ, an efficiency o o h / of 25.2%.A maximum output energy of 360 μJ at 1064 nm was recorded with a 3% transmission output coupler recording an optical efficiency of 2.2% related to the LC output and 0.6% related to the total LED pump energy [24].´cm −1 .There is an overlap between the emission band of the Ce and the absorption of the Nd: YAG as seen in figure 10.The small overlap leads to low reabsorption losses for the light re-emitted by Ce 3+ ions.The initially chosen dimensions of the slab were 100 mm ´1 mm ´1 mm, which then evolved to a slab with dimensions 100 mm ´9 mm ´1 mm to increase the pumping surface in order to minimise the propagation losses.The doping concentration of Ce was 0.25 0.05%,  and the thickness of the YAG slab was chosen as 1 mm to optimise the absorption of LEDs for the given doping concentration.
The pumping LEDs have a narrow emission spectrum centred at 450 nm with an irradiance of each LED around 90 W/cm 2 for 1 A driving current.By delivering the current as pulses with 260 μs intervals, the irradiance increased to 350 W cm −2 .They used 1120 of such LEDs to pump the LC slab with a width of 14 mm and 320 LEDs to pump the LC slab with a width of 2.5 mm, achieving output peak powers of 294 W and 64 W, respectively.The concentration factors and optical efficiencies of these two slabs were 6% and 7.2% for the 14 mm slab and 7.0% and 6.1% for the 2.5 mm slab, respectively.With the optical adhesive having a refractive index of 1.5, they have estimated that the optical efficiency should be greater than 15%.The radiance of a single LED was measured at 100 W cm −2 /Sr and was sequentially increased to 668 W cm −2 /Sr after coupling with the LC exhibiting a higher radiance (at 550 nm).
The laser crystal used in this work is Nd:YAG with 1 at.% of Nd 3+ concentration to avoid concentration quenching.This lasing crystal is an attractive choice as it can be grown of high quality at low cost and can operate in a passively Q-switched regime.
As shown in figure 11, the laser crystal (1 mm ´2.5 mm ´14 mm) was pumped in 14 mm ´1 mm facet and 2.5 ´1 mm facet.The designed laser first experimented on free-running mode.LEDs were pulsed at 5 A with 250 μs (comparable with Nd:YAG lifetime) at 10 Hz yielding 204 mJ from LED-LC.The output energy of the laser was 5 mJ with an optical efficiency of 2.5% with a multimode profile.The maximum laser peak power reached 24.4 W, and assuming TEM 11,0, the calculated radiance is 31.2MW m −2 /Sr.
The peak power of the system was further improved to 263 μJ energy in a 33 ns pulse duration with the highest power of 8 kW by passively  Q-switching the system with two Cr:YAG saturable absorbers [36].This allowed a maximum radiance of 706 GW cm −2 Sr −1 to be achieved.

First tunable LED-LC pumped solid-state laser
The Alexandrite laser exploits a semiprecious stone called Alexandrite as the gain medium.The Alexandrite laser (chrysoberyl [BeAl 2 O 4 ] doped with 0.01%-0.4% chromium ions [Cr 3+ ]) is a broadly tunable solid-state vibronic system with broadband emission spectrum centred at 750 nm enabling tuning between 700 and 800 nm [37][38][39].Since the absorption lies within the visible range, it facilitates optical pumping via flash lamps or red laser diodes.Further, alexandrite crystal has high mechanical strength, high thermal conductivity around 23 Wm −1 K −1 and minimal passive losses.J C Walling, R C Morris et al were granted US patent in 1976 for the first working threelevel alexandrite laser put into operation at 680 nm [39].A few years later, it was found that the alexandrite laser could be operated as a four-level laser at higher wavelengths [38].Currently, Alexandrite crystals are produced at a limited number of companies due to high toxicity in beryllium and high cost.What is more significant is the temperature-dependent spectral properties of alexandrite crystals.Elevated temperature reduces fluorescence lifetime, increases emission cross-section, and causes a red shift in the emission band owing to the structure of the Alexandrite energy levels.
Cr 3+ ions replace the atoms in a beryllium crystal, so an asymmetrical distribution of the atoms in the crystals occurs.This asymmetry causes vibrations in the crystal, yielding Alexandrite, to be known as a vibronic laser [40].The broadband lasing in the Alexandrite crystal happens through the vibronically widened 4 T 2 → 4 A 2 transition.The intrinsic lifetime of the upper 4 T 2 laser level (around 6.6 μs) is insufficient for lasing.However, the lower-lying 2 E level is a metastable level (lifetime ∼1.54 ms).The energy difference between the 4 T 2 and 2 E levels is only a few kT at room temperature (which can be determined using Boltzmann statistics), which enables the 4 T 2 energy level to be coupled to the lower-lying metastable 2 E level.This coupling raises the effective radiative lifetime (τ R ) of the 4 T 2 level to ∼ 260 μs.As a result, sufficient population density is  achieved in the 4 T 2 state via pulsed pumping with a broader emission (figure 12(a)) [40].
At higher temperatures, the ratio of ions in the 4 T 2 level of the Alexandrite rises due to increased phonon energy, which enhances the effective emission crosssection of the 4 T 2 level (σ em ). Figure 12(b) shows this process where a strong broadband emission occurs from the 4 T 2 → 4 A 2 transition.P Pichon et al developed the first tunable solidstate laser pumped using an LED pumped Ce doped YAG concentrator design [41].They used blue LEDs with an emission spectrum centred at 450 nm to pump the luminescent concentrator.Ce ions were doped in the YAG slab and then re-emitted inside the slab centred at 550 nm.The emission of Ce ions (∼550 nm) has a good overlap with the absorption of the Alexandrite crystal (see figure 13 below).
To design the LC, a Ce-doped YAG crystal with dimensions 14 mm ´1 mm ´100 mm was utilised.The surface with the highest area was selected to pump the LC with LEDs, and the emitted light was collected via the surface with the smallest area (1 mm ´14 mm), creating a large aspect ratio between the input and output surfaces.
The irradiance of each LED is 90 W cm −2 (for 1 mm 2 emitting area) when driven by 1 A continuous drive current.The highest LED irradiance reached 315 W cm −2 after pulsing the drive current (5 A) in a square shape for 260 μs (to match the Alexandrite lifetime).The heat from LC was effectively removed using a water-cooled heat sink.1120 LEDs were used to pump the LC with a fill factor of 41%.When the LEDs were pulsed at 10 Hz with a duration of 260 μs, the measured output power at the LC surface (in the air) was 2.1 kW cm −2 and 5.3 kW cm −2 inside the optical adhesive with a refractive index 1.5 that used to bond the LC to the Alexandrite crystal.Based on the assumption that both the LED and LC have Lambertian emission, the LC demonstrates an optical efficiency of 7.2% (in the air) and 18% with an indexmatching optical adhesive.Similarly, the concentration factor is 6 in air and 15 in adhesive.The LC provides 167 mJ output power, sufficient to pump an alexandrite crystal.The experimental setup is shown in figure 14.At 748 nm wavelength, the maximum laser energy of 2.9 mJ, with an output beam of TEM 00 (diameter 125 μm), was achieved with the 2.0% transmission output coupler [41].The laser pulse duration is 108 μs with a build-up time of 152 μs achieving a peak power of 26 W. For 167 mJ pump energy from LC, the optical efficiency of the laser was 1.7%, and 0.29%, related to the total LED output energy [41].This successful demonstration of an LED-pumped tunable laser with Cr 3+ doped crystal is a significant achievement in transition metal lasers.
Power scaling was performed by doubling the Ce: YAG concentrator length to 200 cm by bonding two LCs using UV curing adhesive.This configuration can achieve a maximum of 268 mJ to lasing medium, with maximum irradiance reaching 8.5 kW cm −2 .The small signal gain linearly increased and reached 1.28 at the maximum power.
Encouraged by the above results, the group performed small signal gain measurements in amplifier configuration using eight pass multipass amplifier whose injection can be tunable using a CW Ti-sapphire.They achieved a maximum gain of 4 at 750 nm, tunable between 700 nm-830 nm.There is a possibility to improve temperature altered-gain performances by increasing the temperature of the Alexandrite crystal.

LED-LC pumped Ti:sapphire laser
After the discovery of Ti:sapphire (Ti:Al 2 O 3 ) laser pumped using coumarin 504 dye laser in 1982 by Peter Moulton, working in MIT Lincoln Laboratory, the Ti: sapphire laser has been extensively investigated as a lasing material [42].It is well established that Ti: sapphire has a broadband absorption band in the 400-600 nm blue-green wavelength range peaking at 490 nm and associated emission in the 650-1100 nm red-IR range enabling ultrashort pulsing and wavelength tuning [43].In a Ti:sapphire system, sapphire (Al 2 O 3 ) is the host material.Some trivalent titanium ions (Ti 3+ ) replace the Al 3+ ions in the host lattice by adding titanium (III) oxide to the melted sapphire.
With the ultrashort upper state lifetime of Ti 3+ ions in sapphire (3.2 μs at room temperature), a high pump flux is required to reach the lasing medium's threshold.Therefore, energy must be stored in the pump source, and light should be pumped in microsecond pulses closer to Ti 3+ lifetime within the mJ to J energy range.Parasitic absorption can be quantified using the matric called 'Figure of merits' (FOM).For the grown Ti:sapphire laser crystal, this is defined as the ratio between the coefficient of absorption at the pump wavelength of around 490 nm and the coefficient of absorption of laser wavelength associated with the parasitic absorption by Ti 3+ -Ti 4+ pairs at about 800 nm, respectively [43].A high FOM value can be achieved with a lower Ti 2 O 3 doping concentration in Al 2 O 3 (0.03-0.25 wt%).Ti:sapphire lasers have been pumped using flashlights [44,45], argon-ion lasers [46], frequency-doubled diode-pumped solid-state lasers (DPSSL) [47] and semiconductor laser diodes [43].Rapid progress in GaN semiconductor material paves the way for the direct pumping of Ti:sapphire lasers.Even though blue-green LDs are perfect for Ti: sapphire oscillators, they are not compatible with amplifiers with enough storage.In 2009, P.W. Roth and his colleagues demonstrated the first direct LED pumped Ti:sapphire laser in a compact design using a 1 W, 452 nm GaN diode laser, achieving 19 mW of continuous wave output power [48].They identified an additional loss channel called 'photodarkening' loss at around 800 nm related to the blue pump wavelengths below 478 nm [48].
LEDs are more rugged, and comparatively indifferent to the environment, unlike LDs.But LEDs have limited irradiance, typically 100 W cm −2 .Therefore, LEDs are still an order of magnitude below the required laser threshold for Ti:sapphire.To overcome this issue, LEDs can be coupled with a luminescent concentrator.The first LED-LC pumped Ti:sapphire laser was reported by P Pichon et al in 2018 [49].They demonstrate pumping using a cerium-doped lutetium aluminum garnet (Ce:LuAG) slab of dimensions 14 mm ´1 mm ´100 mm to match the Ti:sapphire absorption band.
Highly compact blue LEDs with an emission spectrum peak at 450 nm were chosen to match the absorption spectrum of Ce:LuAG, shown in figure 15.For a continuous current of 1 A and the emitting surface of 1 mm ´1 mm, these LEDs corresponded to an irradiance of 90 W cm −2 .The peak LED irradiance was further improved to 425 W cm −2 by pulsing the current (7 A at 15 μs).The emitted blue light from the LEDs is absorbed by the concentrator and re-emits at a higher wavelength in the green region.This green emission is TIR guided inside the slab and is collected from one end.The geometrical concentration factor reached 200 owing to the higher aspect ratio between the pump and the collecting surfaces (4 mm ´100 mm and 1 mm ´14 mm, respectively).A total of 1120 LEDs were used on two pumping surfaces to pump the LC with a 41% fill factor.The Ti:sapphire lasing crystal was attached to the emitting end of the using UVcured optical adhesive with a refractive index of 1.7 (figure 16).The total irradiance of the LEDs reached 2.6 kW cm −2 in air and 6.6 kW cm −2 in high refractive indexed optical adhesive when pulsed at 10 Hz, 15 μs, and 7 A. This system achieved optical conversion efficiencies of 7.8% (in the air) and 19% (in optical adhesive).The LC radiance was 15 times higher than that of LED.With 1120 LEDs used in the LC, it provided 3.9 mJ in a 15 μs pulse.
Another similar LC was combined with the initial one, yielding a length of 200 mm.When the LC was operated in the same mode of operation as earlier, this resulted in a geometrical factor of 200, optical efficiencies of 5.8% (in the air), 14% (inside optical adhesive), resulting in concentration factors of 9.5 (in the air), 23 (inside optical adhesive) and radiance of 240 kW cm −2 and 3150 kW cm −2 inside the bonded Ti:sapphire crystal.The calculated total energy delivered to the lasing crystal was 20.9 mJ.A 1 mm ´1 mm ´14 mm Ti:sapphire crystal (0.25 at.%Ti-doped) was used as the gain medium and attached to the LC's end facet using UV-cured optical adhesive.The schematic diagram of the setup is shown in figure 16.The system reached a maximum lasing energy of 32 μJ at 790 nm using a 2.0% transmission output coupler with a  tunability between 755 nm-845 nm.The optical efficiency of the laser was 0.15% for an LC pump energy of 20.9 mJ.The LC was pumped by 2240 LEDs accounting for an energy of 143 mJ, equating to an overall optical efficiency of 0.023%.They achieved TEM 00 operation with a 2% transmission output coupler; yielding 18 μJ as the highest laser output.

LED-LC pumped Cr:LiSAF laser
Transition metal doped lasers with Cr 3+ impurities are promising due to the resistance of the trivalent state of Cr 3+ towards oxidation and reduction, making it easier to incorporate into various host materials/ growing environments.Further, the resistance of the third excited state of the Cr 3+ to nonradiative transition results in near-unity PLQY under an ambient environment [50].Cr 3+ :Colquiriites such as Cr:LiSAF [50][51][52], Cr:LiCAF [53,54], and Cr: LiSGaF [55], belong to the family of such transition metal doped lasers.
As shown in figure 17, the energy transitions in the low-field Cr 3+ have ions excited into the metastable state 4 T 2 by pumping in the red and blue absorption bands [54].This resulted in laser transition to the excited vibrational states of 4 A 2 level.Figure 15 also shows the Excited-state absorption (ESA) originating from the upper laser level, reducing the intrinsic laser efficiency of Cr 3+ laser materials.
Cr:LiSAF has favourable properties like broad tunability (from 730 to 1000 nm), enabling generation of 10 fs pulses, and large emission cross section for efficient extraction of stored energy [56].The Cr 3+ lifetime reaches as high as 67 μs at 300 K allowing more energy storage as well as enabling Q-switching/amplifier configurations [54].On the other hand, Cr:LiCAF, even though its absorption and emission spectra are not as broad as Li:SAF, has a higher upper laser level lifetime (170 μs) compared to Li:SAF, therefore, offers better energy storage compared to Li:SAF [54].
The LC used in the experiments consisted of two 1 mm ´14 mm ´100 nm Ce:YAG slabs bonded together using UV curing adhesive.2240 blue light LEDs with emission centred at 450 nm were placed close to the large facets.The LED emitting area was 1 mm 2 and corresponded to a fill factor of 41%.LEDs were driven with a maximum 5 A current using a pulsed regime at 10 Hz with a pulse duration of 250 μs corresponding to a maximum peak irradiance of 315 W cm −2 .The energy emitted per LED was 0.79 mJ leading to a total energy of 1.76 J to the LC.The energy reaching the Cr: LiSAF was 257 mJ leading to a conversion efficiency of 14.6%.The irradiance increased from 315 W cm −2 of LED to 7.3 kW cm −2 at the LC output edge.The lasing media absorbed 43.5% of the pumped light.
The Cr:LiSAF crystal with 5.5 at% was used as the lasing media with dimensions 2.5 mm ´1 mm ´14 mm.All laser facets were cut at Brewster's angle to minimise light escaping through the sides.The crystal axis orientation was chosen to maximise the light absorption based on the polarisation axis.
After this first demonstration, Pichon et al proposed to implement this new laser technology in more complex laser systems.They developed a tunable source in the UV based on a cavity dumped LED-LCpumped Cr:LiSAF [58].Next, they demonstrated a LED-LC-pumped femtosecond regenerative amplifier [59] and a first Master Oscillator Power Amplifier (MOPA) based on two LED-pumped Cr:LiSAF [60].In all cases, the laser systems were able to produce mJ pulses.The repetition rate was fixed at 10Hz in a first step, in order to reduce thermal effect in the Cr:LiSAF crystal with is very temperature sensitive.By appropriate thermal management using a Cr:LiSAF slab, they managed to reach a repetition rate of 100 Hz.This first investigation of the LED-LC-pumped lasers system starts to reveal the potential of this new pumping technology.

Developments in LC designs and alternative methods for light extraction
Luminescent concentrators enable achieving high geometrical concentration values circumventing the brightness theorem.A major loss in the system occurs when light is extracted from high refractive indexed LED-LC to air.The classical approaches to mitigate this issue were using extraction optics such as parabolic compounds [61], surface texturing and photonic crystal nanostructures [62].
T Gallinelli et al investigated the effect of simple wedge output facet on the light extraction theoretically as well as experimentally [63].They performed intensity profile angular measurements on LCs with different wedge angles as depicted in figure 18 below.The results revealed that the power extraction as well as directional emission can be increased and heavily depend on the wedge angle.Further, they observed that the loss cone was reduced with increasing wedge angle.
Previously, in section 3.2, LED pumped LC in SWIR region was discussed.In that section, some of the strategies P Pichon et al utilised to improve the concentration factor of the LED LC design were outlined [64].They investigated the light recycling in 2D by increasing the light trapping efficiency.In a classical 2D-LC, the factor C is governed by the factor G. Furthering this, increased the brightness additional order of magnitude was achieved by confining light in 3D.
When a highly reflective mirror is added to the unused end facet, the output light otherwise lost can be reflected into the LC, recycling the light increasing the h trap in 2D luminescent concentrator.The main difference between 2D LC and 3D LC is, in the 3D LC case, instead of having an open output facet like in 2D LC, it is partially closed by a highly reflective mirror.The aperture shape can be arbitrary chosen keeping in mind on the aperturing ratio r S s; = / where S is the output face surface area (w t ´) and s is the aperture area (figure 19).
In the experimental setup shown in figure 20 above, a Ce:YAG slab of 1 mm × 14 mm × 100 mm with Ce 3+    [59] concentration of 1.2 10 19 ´cm −3 was used [64].Highly reflective (R 99.4% = ) mirrors were attached to small edge facets.The D-shaped closing mirrors attached to the output edge partially confine the light perpendicular to the output face.The width of the LC did not have any effect on the C in classical 2D-LC other than the area of the pumping surface.But, in 3D-LC design, the width (w) in the C as aperturing ratio (S) depends on the w.The LC was pumped using a blue laser diode 20 cm away from the LC pumping surface illuminating it homogeneously.They experimentally demonstrated that, with this new design, the brightness of the LC was enhanced by an additional order of a magnitude.
In 2021, M Nourry-martin et al pumped similar setup as in 3D-LC, using 1120 high power blue LEDs with emission spectrum centred at 450 nm [65].They achieved a 42% LED fill factor in their design.Two LCs were bonded together resulting LC of 1 mm × 14 mm × 200 mm.The Ce:YAG slab has Ce 3+ concentration of 9.9 10 18 ´cm −3 .The estimated average pump intensity on the LC slab was around 1.8 W mm −2 .High reflective mirrors covered the small facets; 2Dshaped closing mirrors were attached to the output facet using a digital calliper enabling precise control in the aperture length.For an aperture output surface of 1 mm 2 , the peak power and brightness of LC were 145 W and 4.6 kW cm −2 /Sr respectively.If we compare this with the LED used experimentally, there is a factor of 35 enhancement in the 3D-LC design compared to that of an LED with same emitting area.Furthermore, the system resulted in a luminous efficacy of 525 lm W −1 ; with luminous flux of 7.6 10 4 ´lm and brightness of 2.4 10 4 ´cd/mm 2 marking it as one of the brightest incoherent light sources to date.The discrepancy observed in expected proportional behaviour in the output power for different pump powers; especially for the higher aperture ratios; was attributed to the excited state absorption (ESA).ESA reduces the population inversion in excited state as well as creates additional loss channels.It was observed that, due to ESA, the loss coefficient is increased three times at maximum pump power.
In this comprehensive review, we have extensively explored scenarios where the laser crystal is directly bonded to the luminescent crystal.Notably, Juna Sathian and colleagues are at the forefront of pioneering alternative techniques for enhancing the extraction of trapped light from Ce:YAG luminescent concentrators (LCs).In their ongoing experiments, they are employing optical adhesives to affix a compound parabolic concentrator (CPC) to the end facet of a Ce:YAG LC, aiming to achieve more efficient light extraction.The selection of the CPC material is informed by Monte Carlo-based ray tracing simulations conducted with LightTools.This innovative approach not only facilitates the convergence of light from the LC but also focuses it onto optical fibers while preserving a robust and portable design.While the use of optical adhesives with CPCs has been demonstrated previously, particularly in the context of digital projection [64], this work uniquely emphasizes the improvement of extraction efficiency from the LC.It is important to note that this work is currently under publication in another journal.Their focus extends to optimizing light focusing and beam shaping to align with optical fibers, a crucial aspect for excitation and illumination applications.

Summary
This review briefly discusses the fundamental principle and properties of luminescent concentrators.A summary of the LED-LC pumped lasers is given below in the table 1.
The ability to utilize simple and cost-effective LEDs in combination with light concentration methods to create high-brightness Lambertian light sources holds significant importance for advancements in laser pumping and other applications requiring high brightness.Traditionally, LEDs alone are not capable of achieving high brightness levels.However, by employing wavelength conversion techniques to overcome the limitations imposed by the law of etendue, phosphors pumped by LEDs have achieved considerable peak powers and output irradiance in specific wavelength ranges, such as yellow and short-wave infrared (SWIR).The review highlights the use of materials selected from scintillators (e.g., Ce: YAG) and laser media (e.g., Yb:Er: glass, CTH:YAG) as luminescent concentrators.Furthermore, the exploration of other materials across the optical spectrum can be pursued.
The review compiled the research works carried out in LED-LC pumped lasers.Since their initial demonstrations, LED-LC pumped lasers have progressed to a new stage with the development of complex laser systems.The highest potential of LED-LC pumped lasers is at a repetition rate of 100 Hz, that cannot be addressed by flashlamp because of lifetime and that is difficult to address with laser diode pumping because of the cost of laser diodes.With a cost of less than one euro per LED, LED can provide a high amount of energy at a moderate total cost.The next step in improving LED-LC pumped laser systems involves enhancing the efficiency of luminescent concentrators, which currently range from 10%-20%.Ongoing efforts focus on improving light extraction and confinement within the concentrators to further enhance the performance of LED-LC systems.

( 1 )( 3 )( 4 )
Blue LED pump light is incident on the surface of the LC with the largest surface area (2) A portion of the transmitted light is absorbed by a luminophore (cyan colour sphere) and immobilised inside the LC Re-emission of absorbed light at a higher wavelength spectrum Part of this light in (3) is TIR-guided to the small end facet of the LC

Figure 3 .
Figure 3. Cross-sectional diagram of a general LC (Ce:YAG) with light interactions inside the concentrator depicted.Adopted and modified with permission from [21] © 2022, Optical Society.

4. 3 .
LED-LC pumped Nd:YAG passively Q-switched laser P Pichon et al were the first to show that LED-LC can pump Nd:YAG laser in free running mode and passively Q-switched mode[36].Similarly, they also utilise a Ce-doped YAG slab.The YAG growing process is well-optimised to have very low internal losses.They measured internal losses of around 1.62 10 2

Figure 11 .
Figure 11.Setup of the experiment.LEDs are placed on the top and the bottom of Ce:YAG concentrators.Reprint with permission from [36] © 2022, Elsevier.

Figure 12 .
Figure 12.(a) Energy band diagram of Alexandrite lasing medium (b) Normalised absorption and emission cross section of Alexandrite at 25 °C.Normalised emission spectrum of Alexandrite at 400 °C is also given.All spectra are given for E//b polarisation.Reprint with permission from [40] © 2022, Optical Society.

Figure 13 .
Figure 13.Narrow emission spectrum of InGaN LEDs (blue) and broad fluorescence of Ce:YAG (green) pumped in pulsed regime at RT. Absorption coefficients (a) of Ce:YAG (red) and Alexandrite (black) for light polarised parallel to the b axis.Reprint with permission from [41] © 2022, Optical Society.

Figure 14 .
Figure 14.Top view of the experimental setup.Printed LED circuits mounted on top and the bottom of the Ce:YAG crystal.Reprint with permission from [41] © 2022, Optical Society.

Figure 20 .
Figure 20.Illustration of 3D-LC.The closing mirrors can adjust the output area of the end facet.Reprint with permission from [64] © 2022, Optical Society.

Table 1 .
Summary of LED-LC pumped solid state lasers.