Transition-metal-ion Doped Tunable Crystalline Fiber Lasers (Invited)

Yi-Hsun LI; Chun-Yi KUO; Sheng-Lung HUANG

doi:10.3788/gzxb20204911.1149010

Abstract

Broadly tunable lasers are useful for basic spectroscopy studies， as well as a wide range of applications from optical communications to biomedical imaging. Transition-metal-ions doped solid-state gain media are eminently suitable for generating broadband emissions. Ti³⁺：sapphire and Cr⁴⁺：YAG crystals are 2 successful examples that are now widely used. Glass-clad Ti³⁺：sapphire and Cr⁴⁺：YAG crystal fibers have shown superior performance for broadly tunable lasers in the near infrared wavelength ranges. The tunable Ti³⁺：sapphire crystal fiber lasers are efficient， and have demostrated the lowest threshold over a 180 nm tuning range. The Cr⁴⁺：YAG crystal fiber laser shows a tuning range of 170 nm， limited by the excited state absorption. To celebrate the 60^th anniversary since laser invention， a brief historical review and the latest developments of the Ti³⁺：sapphire and Cr⁴⁺：YAG crystal fiber lasers are discussed in the manuscript. The tunable Ti³⁺：sapphire crystal fiber laser's wavelength sweeping speed is envisioned， and the optical properties are compared with that of the Cr⁴⁺：YAG crystal fiber. With well-developed crystalline cores and clads for broadly tunable lasers， it is expected that novel applications， such as ultra-broadband optical fiber communications and cellular-resolution optical coherence tomography， could be evolved to meet the high data rate and high image resolution needs in future.

Keywords

Biomedical imaging Broadband optical communication Cr4+：YAG Crystal fiber Laser Ti3+：sapphire

0 Introduction

Ti 3+ ：sapphire crystal was first demonstrated as a widely tunable laser gain medium at the Lincoln Laboratory ［ 21 ］ . The Ti 3+ ：sapphire crystals used in the initial experiments exhibited significant scattering and an unidentified absorption at the laser wavelength. These losses affected the efficiency of the laser， and only pulsed operation was possible. As high-quality crystals became available， a series of tunable lasers based upon Ti 3+ ：sapphire appeared， and a 235 nm tuning bandwidth was demonstrated with one mirror set ［ 22 ］ . For waveguide Ti 3+ ：sapphire lasers， continuous-wave channel waveguide lasers were demonstrated and the emission wavelength was tuned over a 170 nm range by using a birefringent filter in an external cavity ［ 23 ］ .

Among all the Cr 4+ doped gain media， Cr 4+ ：YAG has been shown high concentration of tetrahedrally coordinated Cr 4+ ions and high emission cross section in fiber communication bands. Cr 4+ ：YAG tunable laser has been developed for thirty years， and the first demonstrated was operated in gain-switched mode with a tuning range from 1.35 to 1.45 μm ［ 24 ］ . Then， there were tunable lasers been demonstrated in continuous-wave mode ［ 25 ］ . These systems used high power solid-state laser as pumping source， which was massive and high cost. Although laser diode could reduce the cost and volume， the threshold was still in the Watt level ［ 26 ］ . Another issue was the thermal loading in crystal with bulk structure. The lifetime would suffer from the thermal loading and the thermal lensing effect in the crystal ［ 27 ］ . To overcome the thermal problem and poor pump/signal beam overlapping in bulk crystal， a 120 μm-diameter Cr 4+ ：YAG crystal fiber was developed with a tuning range of 180 nm at a threshold pump power above 2 W ［ 28 ］ .

By far， active glass-clad crystal fibers have shown the state-of-the-art performance among transition-metal-ions doped solid-state waveguide lasers ［ 29 - 31 ］ . In this paper， the latest development of glass-clad Ti 3+ ：sapphire and Cr 4+ ：YAG crystalline fibers based tunable lasers are presented with a focus on the tuning range and wavelength sweeping capabilities.

1 Crystal fiber growth and cladding

Various oxide crystals have been drawn into the fiber forms. For Ti：sapphire and Cr：YAG crystals， the melting points are 2 050 oC and 1 970 oC， respectively. The high melting points make LHPG advantageous to avoid crucible contamination. As shown in Fig. 1（a）， an expanded CO2 laser beam entered the growth chamber and was converted into a donut shape by the reflaxicon. It was then focused by the paraboloidal mirror onto the CF. The inner cone was supported by a ZnSe plate， which is transparent to CO2 laser beam. The 360o axial symmetry prevented cold spots in the growth molten zone. The source and the seed crystal rods were fixed on the lower and upper motorized stages. During stable growth， the dependence of the molten-zone length and shape on the heating CO2 laser were analyzed under both the maximum and the minimum allowed powers［32］. To avoid the high convection rate inside the floating molten zone due to mass-transfer and thermocapillary convections， the CO2 laser stability and the mechanical resonant of the motorized stage should be well controlled to produce high quality grown fibers. Due to the long CO2 laser wavelength， it is difficult to reduce the fiber diameter down to 10 μm by direct focused heating. The co-drawing approach has shown core diameter in the order of 10 μm. In the co-drawing method， the crystalline core was prepared by the LHPG technique， while the claddings were made from various glass capillaries， such as borosilicate， aluminosilicate， flint glass， or even high temperature fused silica. Using YAG Double-Clad Crystal Fiber （DCCF） growth as an example， a schematic of the co-drawing LHPG technique is depicted as shown in Fig.1（b）. With two diameter reduction steps by the LHPG technique， 68 μm-diameter YAG single CFs were initially prepared from a 0.5 mol.% doped Cr：YAG source rod in <111> crystal orientation with a cross section of 500 μm×500 μm. When the Cr：YAG CFs were grown at a lower speed， nonepitaxial Cr3-δO4 crystallites with spinel-type structure were formed on the ｛1 1 2｝ side surfaces. The deposition of Cr3-δO4， rather than segregation of Cr and Ca codopants， can be rationalized by a high crystal field stabilization of octahedrally coordinated Cr3+ attained in rather refractory and close packed oxide［33］. The YAG CF was then inserted into a fused silica capillary with 76- and 320-μm inner and outer diameters for the co-drawing process by the same LHPG system to form the double-clad structure. The 1970 °C melting temperature of the YAG is comparable to the 1600 °C soften temperature of the fused silica. The heating of the co-drawing LHPG method caused a strong interdiffusion between the YAG core and the fused-silica capillary， resulting in an inner cladding layer made of the mixtures. In particular， the DCF core diameter can be well-controlled by just altering the CO2 laser power and the relative growth speed.

2 Tunable laser using glass-clad Ti³⁺：sapphire crystal fiber as the gain medium

As shown in Fig.2 ， an external cavity laser configuration was employed to find out the optical characteristics of the glass-clad Ti 3+ ：sapphire CF. The input and output ends of a 24 mm glass-clad Ti 3+ ：sapphire CF were coated with dielectric films. At the input end， it was anti-reflection coated at 520 nm and high-reflection coated at wavelengths of 715~860 nm. At the output end， it was coated with a transmittance of greater than 99% from 715 to 860 nm. A 1 W， 520 nm Laser Diode （LD） was used as the pump light source. A 60\timesaspheric lens （ f =2.8 mm， NA=0.65， New Focus） was used to couple the pump LD into an 18 μm crystal fiber core， followed by a achromatic lens with f =7.5 cm to collimate the light. Flat output couplers with various transmittance levels were attempted. A filter （NG610， Thorlabs） was used to remove the residual pump power. The backward lasing spectrum was measured using an optical spectrum analyzer （U4000， Ocean Optics） through a dichroic mirror （LPD01-633RS-25， Semrock） to eliminate the pump wavelength.

To model the optical characteristics of the glass-clad Ti 3+ ：sapphire CF， a distributed scheme on the pump， signal， and meta-stable state population is employed as shown below.

\pm \frac{d P_{p}^{\pm} (z)}{d z} = - [Γ_{p} N_{g} (z) σ_{a} + α_{p l}^{p}] P_{p}^{\pm} (z)

\pm \frac{d P_{s}^{\pm} (λ_{i}, z)}{d z} = [Γ_{s} N_{2} (z) σ_{e} (λ_{i}) - α_{p l}^{s}] P_{s}^{\pm} (λ_{i}, z) + N_{2} (z) A_{c o r e} S_{s p} (λ_{i}) ∆ λ_{i}

\frac{d N_{2} (z, t)}{d t} = N_{0} (z, t) W_{03} - N_{2} (z, t) (W_{21} + \frac{1}{τ_{f}})

where ， ， ， and ， respectively denote the pump and signal powers， and electron densities of the ground state and meta-stable state. represents each of the signal wavelength within the gain bandwidth. denotes the counter propagation directions. ， ， ， and represent the absorption and emission cross sections， and propagation losses at signal and pump wavelengths， respectively. and are the confinement factors. is the crystal core area. The spontaneous emission spectral density （）， the transition probability of the ground-state absorption rate （）， and the stimulated emission rate （） can be expressed as follows

S_{s p} (λ_{i}) = \frac{4 π n_{c o r e} (n_{c o r e} - n_{c l a d}) h c^{2}}{λ^{5}} σ_{e} (λ_{i})

W_{03} (z) = \frac{I_{p} σ_{a} λ_{p}}{h c}

W_{21} (z) = \sum \frac{I_{s} (λ_{i}) σ_{e} (λ_{i}) λ_{i}}{h c}

where and are the refractive indice of the core and cladding， respectively. is Planck's constant. λ， ， and represent the wavelength， pump intensity， and signal intensity. According to the lifetime measurement experiment， the average （lifetime） was 3 μs. The pump and signal attenuation coefficients were measured at 0.946 cm-1， 0.017 cm-1， respectively.

Fig.3 shows the comparison between measurement and simulation at various output couplers. The fitted parameters are listed in Table 1 ， and will be compared with those obtained from the Cr 4+ ：YAG crystal fiber.

Parameters	Symbol	Ti³⁺:sapphire	Cr⁴⁺:YAG
Cross sectional mage	-
Core diameter	-	Short-axial: 12 μm Long-axial: 16 μm	16 μm
Refractive index	$n$	Core: 1.77 Cladding: 1.497@446 nm	Core: 1.809 Inner-clad: 1.64 Outer-clad: 1.45@1 450 nm
Active ion concentration	$N$	1.65 $\times$ 10¹⁹ cm^-3	3.41 $\times$ 10¹⁷ cm^-3
Absorption cross section	$σ_{a}$	5.7 $\times$ 10^-20cm²@ 532 nm	22 $\times$ 10^-19cm²@ 1 064 nm
Emission cross section	$σ_{e}$	2.4 $\times$ 10^-19cm²@ 790 nm	2.67 $\times$ 10^-19cm²@ 1 431 nm
Pump excited state absorption	$σ_{e s a}^{p}$	-	5.1 $\times$ 10^-19cm²@ 1064 nm
Signal excited state absorption	$σ_{e s a}^{e}$	-	1.28 $\times$ 10^-19cm²@ 1 431 nm
Fluorescence lifetime @300 K	$τ_{f}$	3.15 μs	4.2 μs
Propagation loss	$α_{p l}$	0.045 dB/cm	0.027 dB/cm

Table 1. Summary of the physical and optical properties of the glass-clad Ti³⁺:sapphire and Cr⁴⁺:YAG crystal fibers

View all Tables

To further study the wavelength tuning properties， a Littrow blazed grating （10RG1800-500-1， Newport） was employed as the wavelength tuning element and the output coupler， as shown in Fig.4. Diffraction efficiency of the grating and the angular dispersion were 88% and 13.81 nm/°， respectively. Output powers were measured after filtering out the residual pump power by a long-wavelength-pass filter. The tuning bandwidth was 183 nm covering 693 nm to 876 nm， as depicted in Fig. 5. Continuous smooth wavelength tuning was achieved all over the wavelength range and no discontinuity nor multiple lines were observed. The tuning range was limited by the reflectance of the CF input coating.

To find out the tuning speed capability of the Ti：sapphire CF laser， the grating based tuning mechanism is modeled by tunable filter in a time-dependent distributed laser setting. The rotating grating is equivalent to a tunable filter with a full width at half maximum bandwidth of 2.78 nm. The governing equations of the pump and signal in Eqs. （1） and （2） are revised as Eqs. （7） and （8）.

\pm [\frac{\partial P_{p}^{\pm} (z, t)}{\partial z} \pm \frac{n}{c} \frac{\partial P_{p}^{\pm} (z, t)}{\partial t}] = - [Γ_{p} N_{g} (z, t) σ_{a} + α_{p l}^{p}] P_{p}^{\pm} (z, t)

\pm [\frac{\partial P_{s}^{\pm} (λ_{i}, z, t)}{\partial z} \pm \frac{n}{c} \frac{\partial P_{s}^{\pm} (λ_{i}, z, t)}{\partial t}] = [Γ_{s} N_{2} (z, t) σ_{e} (λ_{i}) - α_{p l}^{s}] P_{s}^{\pm} (λ_{i}, z, t) + N_{2} (z, t) A_{c o r e} S_{s p} (λ_{i}) ∆ λ_{i}

Depending on the rotating speed of the grating， the laser shows pulsed output with time-dependent lasing wavelength. At a pump power of 1.8 W， the simulation result with a sweep rate of 20 kHz is shown in Fig. 6 . The typical pulse time intervals are 0.5~0.6 μs. This represents about 2 nm spacing of the comb shaped spectra at a tuning range of 180 nm.

3 Tunable laser using glass-clad Cr⁴⁺：YAG crystal fiber as the gain medium

Compared with Ti 3+ ：sapphire CF， the Cr 4+ ：YAG CF is double-cladded. Cr 4+ ：YAG Double-Clad Crystal Fiber （DCCF） with low propagation loss （~0.02 dB/cm） was cladded with fused silica by the sapphire-tube-assist co-drawing LHPG method ［ 19 ］ . At a core-diameter of 16 μm， the stress-induced emission cross section reduction is minimized ［ 34 - 35 ］ . To develop the Cr 4+ ：YAG DCCF tunable laser， an external-cavity laser was built-up， as shown in Fig. 7 . The 4.7 cm-long Cr 4+ ：YAG DCCF was pumped by an polarized 1 064 nm laser diode （LD-1064-BF-600， Innolume）. The DCCF was mounted on a metal holder and glued using a silver adhesive for passive heat dissipation. The input end was coated with high-reflection （ R >99%， from 1 347 to 1 554 nm） dielectric thin film for broadband signal light. The signal output was collected by an aspheric lens and reflected by a planer output coupler （ T =4.4%）. The laser output power was measured at both sides of the cavity.

Fig.8 shows the measured laser output power. The threshold pump power is only 51 mW. The slope efficiencies are 10.7% and 1.0% from the forward and backward ends， respectively. To model the laser， Eqs. （1） and （2） need to be modified with the addition of Cr 4+ ：YAG's Excited State Absorption （ESA）. The distributed simulation model is described in Section 2 with the addition of the ESA terms in Eqs. （9） and （10）. The experimental result agrees well with the simulation.

\pm \frac{d P_{p}^{\pm} (z)}{d z} = - [Γ_{p} (N_{g} (z) σ_{a} + N_{2} (z) σ_{e s a}^{p}) + α_{p l}^{p}] P_{p}^{\pm} (z)

\pm \frac{d P_{s}^{\pm} (λ_{i}, z)}{d z} = [Γ_{s} N_{2} (z) [σ_{e} (λ_{i}) - σ_{e s a}^{s} (λ_{i})] - α_{p l}^{s}] P_{s}^{\pm} (λ_{i}, z) + N_{2} (z) A_{c o r e} S_{s p} (λ_{i}) ∆ λ_{i}

where and are the pump and signal ESA cross sections， respectively.

Table 1 summarizes the simulation parameters of the Ti 3+ ：sapphire SCCF and Cr 4+ ：YAG DCCF lasers. Though the Cr 4+ concentration （3.41\times10 17 cm -3 ） is about 2 orders of magnitude less than that of the Ti 3+ ， the Cr 4+'s high absorption cross section avoids the need for a long Cr 4+ ：YAG DCCF. The high pump and signal ESAs do not significantly affect the Cr 4+ ：YAG DCCF laser efficiency； however， it does limits the tuning bandwidth of Cr 4+ ：YAG laser， and it will be discussed below.

The setup of the Cr4+：YAG crystal fiber tunable laser is shown in Fig. 9. A reflective holographic Littrow-configurated grating （53004BK02-246H， Richardson Grating） operated at the 1st-order diffraction was used as the tuning element and output coupler. The grating with a pitch density of 1 050 pairs/mm has high diffraction efficiency （~97%） in 1.3 to 1.6 μm.

Fig. 10 shows the output characteristics of the Cr 4+ ：YAG DCCF tunable laser. The threshold was less than 100 mW， which was more than one order lower than non-CF based Cr 4+ ：YAG lasers in literature. The linewidth was 0.029 nm. The spectrum of the individual wavelength shown in Fig.11 was recorded by the optical spectrum analyzer every few nanometers. In Fig.11 ， the tuning range was 170 nm， from 1 353 to 1 523 nm. In all tuning range， the side-mode suppression ratio was more than 45 dB. It should be noted that the Cr 4+ ：YAG DCCF tunable laser exhibited narrow linewidth and low amplified spontaneous emission noise compared to the Bismuth-doped fiber tunable laser operated in similar wavelength range ［ 36 ］ .

The tuning ranges at different pump powers were measured as shown in Fig.12 . The maximum tuning range of 170 nm （1 353 to 1 523 nm） was occurred at 232 mW pumping power. Then， the tuning range decreased to 150 nm and became stable at 150 nm when increasing the pump power. It was due to the signal ESA at longer wavelength with high intensity in the Cr 4+ ：YAG DCCF. To reveal the limitation， the gain of the Cr 4+ ：YAG DCCF is numerically simulated. The tuning range becomes larger with the increasing gain. After 300 mW pump power， the tuning range became stable and the gain gradually saturated. The ESA limitation near the long-wavelength end could be mitigated by longer DCCF or higher concentration.

4 Conclusion

Forth years had been past since the first active CF was grown. Due to the superior optical， mechanical， and thermal properties， CF based light sources have shown advantages in areas where high power and high brightness are needed. It is advantageous to draw transition-metal-ions doped crystals into fiber so that the heat dissipation， pump/signal interaction can be improved for efficient laser operation. Both Ti 3+ ：sapphire SCCF and Cr 4+ ：YAG DCCF have shown tuning range of greater than 170 nm at relatively low pump powers. Though the gains of transition-metal-ions doped solid-state laser crystals are in general much less than the semiconductor lasers， fast wavelength sweep is still quite feasible at tens of kHz speed for various applications.

For future ultra-broadband fiber communication system， a broadly tunable laser is essential to offer wavelength-on-demand， dynamic wavelength ports， and simplified inventory managements. In the emerging biomedical imaging field， optical coherence tomography has now become a standard of care impacting the treatment of millions of people every year. Due to the breakthroughs in broadband light sources for reaching the deep and/or interior tissues and organs， cellular-resolution optical coherence tomography could enable optical virtual biopsy in future.

At present， it is challenging to develop a single-mode glass-clad CF mainly due to the large indices of refraction of YAG and sapphire crystals. Using solution based dip coating techniques on CF， followed by high temperature sintering process， could be a viable way to fabricate a fully crystalline clad for single-mode transmission. Such a fully crystalline core and clad fiber could also escalate the optical damage thresholds of the silica fiber based lasers， and could further enable novel applications.

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