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Advanced Photonics
Vol. 4, Issue 2, 024002 (2022)

Femtosecond laser-inscribed optical waveguides in dielectric crystals: a concise review and recent advances

Lingqi Li¹, Weijin Kong¹, and Feng Chen^2、*

Author Affiliations

¹Qingdao University, College of Physics Science, Center for Marine Observation and Communications, Qingdao, China

²Shandong University, School of Physics, State Key Laboratory of Crystal Materials, Jinan, China

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DOI: 10.1117/1.AP.4.2.024002 Cite this Article Set citation alerts

Lingqi Li, Weijin Kong, Feng Chen. Femtosecond laser-inscribed optical waveguides in dielectric crystals: a concise review and recent advances[J]. Advanced Photonics, 2022, 4(2): 024002 Copy Citation Text

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Basic geometries of laser writing of waveguides: (a) single-line, (b) double-line, (c) depressed-cladding, and (d) optical-lattice-like configurations. The dark (gray) regions represent the laser-induced tracks.

Fig. 1. Basic geometries of laser writing of waveguides: (a) single-line, (b) double-line, (c) depressed-cladding, and (d) optical-lattice-like configurations. The dark (gray) regions represent the laser-induced tracks.

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$Optical microscope images of laser-induced tracks with (a) different pulse energies and (b) scanning velocity in LiTaO3 crystal. (c) Different refractive index change profiles from positive to negative, depending on the propagation wavelength in ZnSe crystal. Image (c) is reproduced with permission from Ref. 103, Creative Commons Attribution License (CC-BY).$

Fig. 2. Optical microscope images of laser-induced tracks with (a) different pulse energies and (b) scanning velocity in

{LiTaO}_{3}

crystal. (c) Different refractive index change profiles from positive to negative, depending on the propagation wavelength in ZnSe crystal. Image (c) is reproduced with permission from Ref. 103, Creative Commons Attribution License (CC-BY).

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Fig. 3. (a) Microscope images of tracks with different pulse energies and scanning rates in

Nd : {YVO}_{4}

. (b) Influence of different laser polarizations on the tracks and modal profiles in Nd:YAP, perpendicular to the scans (No. 1) and parallel to the scans (No. 2), respectively. Images reprinted from Ref. 75, © 2016 The Optical Society (OSA). (c) Optical images of tracks as the laser writing along different crystalline orientations in

Nd : {GdVO}_{4}

. (d) Laser-induced multifoci in

{LiTaO}_{3}

, corresponding horizontal waveguides, and modal profiles. Reprinted with permission from Ref. 109, © 2019 IEEE.

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Fig. 4. Schematic diagram of direct laser-written cladding waveguides (a) with an ellipsoidal focal spot and (b) with a slit-shaped beam focus. Images (a), (b), and (g) are reprinted with permission from Ref. 40, © 2017 OSA. (c) The phase mask for writing horizontal lines. Microscope image of (d) horizontal tracks, (e) waveguide, and (f) near-field profiles. Images (c)–(f) are reproduced with permission from Ref. 42, CC-BY. (g) Schematic plot of single-scan cladding waveguides utilizing a longitudinal ring-shaped focal field. (h) Calculated 3D isosurface, (i) phase mask, and (j) simulated focal intensity profile. (k) Microscope image and (l), (m) corresponding modal profiles. Images (h)–(m) are reprinted with permission from Ref. 118, © 2019 Chinese Laser Press (CLP).

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Fig. 5. (a) Schematic design of a waveguide-integrated LiQPM grating; (b) SH microscope image of LiQPM grating and waveguide; and (c) helical grating structure. Microscope image of the helical structure: (d) the front face and (e) top view. Images reprinted with permission from Ref. 43, © 2020 OSA.

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Fig. 6. (a) Microscope images of hollow optical-lattice-like structures at different etching times in YAG crystal. (b) Before polished and (c) after polished. Near-field modal profiles at

4 μ m

along (d) TM and (e) TE polarization, respectively. Images reproduced with permission from Ref. 129, © 2020 CLP.

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Fig. 7. (a) Microscope images and modal profiles of tailored multiline waveguides in a

{LiNbO}_{3}

crystal, reproduced with permission from Ref. 91, © 2018 Elsevier. (b) Ring-shaped waveguide based on type-I modification in a BGO crystal, reprinted with permission from Ref. 136, © 2017 OSA. (c) Polarization engineering for dual-line waveguides in a

{LiNbO}_{3}

crystal, reproduced with permission from Ref. 167, © 2020 Elsevier. (d) The “ear-like” waveguide in Nd:YAG crystal, reprinted with permission from Ref. 168, © 2021 OSA. (e) Double-cladding waveguide in

Nd : {YVO}_{4}

crystal, reprinted with permission from Ref. 169, © 2019 OSA.

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Fig. 8. (a) Fabrication and 3D schematic diagram of

Y

-splitters based on rectangular cladding geometry in Ti:sapphire crystal, (b) microscope image of 1-deg branching angle, and (c) intensity distributions at 1064 nm. Images (a)–(c) are reproduced with permission from Ref. 181, © 2018 Elsevier. Microscope images of

Y

-branch with circular cladding structure (d) in top view and (e) in cross section, as well as modal profiles of two arms. Images (d) and (e) are reproduced with permission from Ref. 179, © 2017 Elsevier. (f) 3D beam-splitting structures in a

{LiNbO}_{3}

crystal, reprinted with permission from Ref. 182, © 2018 Optica.

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Fig. 9. (a) Schematic illustration of

1 \times 4

beam splitting and ring-shaped transformation based on photonic-lattice-like structures. Image (a) is reproduced from Ref. 102. (b) Measured evolution of ring-shaped transformation in a Nd:YAG crystal. The scale bar is

50 μ m

. (c) Prototype design and microscope images of

1 \times 3

beam splitters in a

{LiNbO}_{3}

crystal, (d) measured and (e) simulated modal profile. Images (c)–(e) are reprinted with permission from Ref. 145, © 2016 IEEE.

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Fig. 10. (a) Microscope images in top view, (b) end-face of polarization beam splitters, and (c) modal profiles along

n_{0}

,

n_{e}

, and circular polarizations, respectively. Images (a)–(c) are reprinted with permission from Ref. 184, © 2020 IEEE. (d) Schematic plot and microscope images of 3D polarizer. (c) Modal profiles at different polarizations. Images (d) and (e) are reprinted with permission from Ref. 167, © 2020 IEEE.

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Fig. 11. (a) Microscopic pictures of a tapered cladding waveguide in a Nd:YAG crystal and (b) modal profiles at the input radii of

24 μ m

and output of

6 μ m

, respectively. (c) Modal profiles of a straight and tapered waveguide at the same output radii from an incident LED light, reproduced with permission from Ref. 185, CC-BY. (d) Prototype of depressed-cladding 3D waveguide arrays. (e) Optical micrographs at the output face for different separations between the central and adjacent waveguides. Images (d) and (e) are reprinted with permission from Ref. 186, © 2017 IEEE.

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Fig. 12. (a), (b) Geometry and cross-section design in the interaction region of the

3 \times 3

directional coupler in a

{Tm}^{3 +} : YAG

crystal. (c), (d) Top and output view microscope images and (e) output intensity distribution. Images (a)–(e) are reproduced with permission from Ref. 187, CC-BY. (f) Schematic plot of

2 \times 2

directional coupler integrated with 3D microelectrodes in a

{LiNbO}_{3}

crystal. (g) Output intensity profiles with different voltages. Images (f) and (g) are reprinted with permission from Ref. 118, © 2019 CLP.

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Fig. 13. Complex waveguide laser modal profiles at

1 μ m

: (a), (b)

Y

-branches, (c)

1 \times 4

-branch, (d) ring-shaped transformation, and (e) optical-lattice-like. Images (a) and (d) are reproduced with permission from Ref. 100. Images (b) and (c) are reprinted with permission from Ref. 101, © 2016 IEEE. Image (e) is reprinted with permission from Ref. 75, © 2016 OSA. (f) Schematic illustration of the fabrication process of an

S

-curved waveguide, (g) laser spectra of dual-wavelengths at 1064 and 1079 nm, and (h) RF spectrum of modelocking at 31.69 GHz. Images (f)–(h) are reprinted with permission from Ref. 159, © 2020 IEEE.

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Fig. 14. (a) Waveguide-integrated 3D LiQPM scheme, one period, two periods, and four periods in a

{LiNbO}_{3}

crystal. (b) Simultaneous SHG of four wavelengths and the fundamental and second harmonic modal profiles of a single period. Images (a) and (b) are reprinted with permission from Ref. 43, © 2020 OSA. (c) Experimental setup of the ultraviolet SHG process using LiQPM structure in a quartz crystal and (d) the SHG response signal of 177.3 nm. Images (c) and (d) are reproduced with permission from Ref. 124, CC-BY. (e) Schematic diagram of femtosecond laser-written cladding waveguide in a fan-out PPSLT crystal and (f) temperature tuning curves of seven waveguides with different poling periods. Images (e) and (f) are reprinted with permission from Ref. 148, © 2019 OSA.

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Fig. 15. (a), (b) Microscope image and near-field intensity profile of a type-II waveguide in a

\Pr^{3 +} : Y_{2} {SiO}_{5}

crystal. (c) Energy-level structure of

{{}^{3}H}_{4}

ground and

{{}^{1}D}_{2}

excited manifolds. (d) Light-storage experiments using the AFC protocol. Images (a)–(d) are reproduced with permission from Ref. 229, © 2016 American Physical Society (APS). (e), (f) End-face and top-view microscope images of type I and type II waveguides and modal profiles in

\Pr^{3 +} : Y_{2} {SiO}_{5}

crystal, respectively. (g) Time-resolved histogram for signal photons, internal storage efficiency

η_{AFC}

at different storage times, and cross-correlation values between idler photons and stored signal photons. Images (e)–(g) are reprinted with permission from Ref. 52, © 2018 Optica.

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Fig. 16. (a) Experimental setup of coherent optical memory based on an on-chip waveguide. (b) Guided mode intensity distribution of laser-written ridge waveguides in an

{Eu}^{3 +} : Y_{2} {SiO}_{5}

crystal. (c), (d) Top and front view microscope images. Images are reproduced with permission from Ref. 230, © 2020 APS.

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Fig. 17. Schematic illustration of the PLACE fabrication process. (a) Cr thin-film deposition, (b) Cr patterning, (c) CMP, (d) chemical wet etching, and (e) coating

{Ta}_{2} O_{5}

film. Images (a)–(e) are reproduced with permission from Ref. 236, © 2020 Chinese Physical Society (CPL). (f) Camera photo of the 11-cm-long LNOI waveguide, (g) microscope image, and (h) enlarged image. Images (f)–(h) are reproduced with permission from Ref. 232, CC-BY. (i) SEM image of LNOI microdisk. Image (i) is reproduced with permission from Ref. 233, CC-BY.

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Waveguide configuration	Advantages	Disadvantages
Type I	1. Direct writing for 3D micromachining	1. Distorted lattices with degraded bulk features
2. Single-mode guiding structures	2. Bad thermal stabilities
3. Longer wavelength guidance using multiscan technique	3. Guidance only along one polarization
4. Realizable in limited crystals
Double line	1. Well-preserved bulk features	1. No guidance at long wavelength (e.g., mid-IR)
2. Single- or low-order mode structures	2. Guidance only along one polarization in some crystals (e.g., cubic YAG)
3. Being easily achieved in crystals
4. Excellent thermal stabilities	3. Being difficult for 3D waveguides
5. Wide applicability in crystals
Depressed cladding	1. Well-preserved bulk features	1. Relatively longer production time
2. Guidance till long wavelength	2. Being difficult for 3D waveguides
3. Designed geometry and adjustable diameters
4. Very good thermal stabilities
5. High coupling efficiency with fibers
6. Potential guidance along any transverse direction
7. Wide applicability in crystals
Optical-lattice-like cladding	1. Being similar to double line and depressed cladding	1. Special design for different functions and materials
2. 3D device by special designs

Table 1. Advantages and disadvantages of different configurations in transparent material.

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Crystal system	Material	Waveguide configuration	Guiding properties	Ref.
Polarization dependence	Minimum propagation loss (dB/cm)
Cubic crystals	Nd:YAG	Type I (single line)	TE and TM	5@632.8 nm	87
Dual line	TM	0.21@632.8 nm	79
Double cladding	TE and TM	1.3@632.8 nm	132
Optical-lattice like	TE and TM	0.7@1064 nm	100
Cladding + dual line	TE and TM	—	133
Nd:GGG	Dual line	TM	2.0@632.8 nm	134
Depressed cladding	TE and TM	1.7@632.8 nm	135
BGO	Type I (multiscan)	TE and TM	$3.22 @ 4 μ m$	89
Type I (ring shaped)	TE and TM	1.56@1550 nm	136
Dual line	TE and TM	0.5/632.8 nm	137
Depressed cladding	TE and TM	2.1@632.8 nm	137
Tetragonal crystals	$Nd : {YVO}_{4}$	Dual line	TE and TM	0.8@632.8 nm	138
Depressed cladding	TE and TM	1.1@632.9 nm	139
Optical-lattice like	—	—	140
$Nd : {GdVO}_{4}$	Dual line	TM	0.5@1064 nm	141
Depressed cladding	TE and TM	0.7@632.8 nm	142
Hexagonal crystals	6H-SiC	Dual line	TM	0.78@1064 nm	143
Rectangular cladding	TE and TM	1.62@1064 nm	143
Trigonal crystals	${LiNbO}_{3}$	Type I (single line)	TM	2.22@1064 nm	144
Type I (multiline)	TM	1.98@632.8 nm	91
Dual line (vertical)	TM	0.6@1064 nm	63
Dual line (horizontal)	TE	3.25@1550 nm	41
Depressed cladding	TE and TM	1.25@1550 nm	118
Optical-lattice like	TE	1.27@1550 nm	145
Ridge configuration	TM	$3.28 @ 4 μ m$	146
${LiTaO}_{3}$	Type I (single line)	TM	2.67@632.8 nm	147
Dual line (horizontal)	TE	1.7@632.8 nm	109
Depressed cladding	TE and TM	1.56@1550 nm	148
Rectangular cladding	TE and TM	0.12@1550 nm	149
β-BBO	Depressed cladding	TM	0.19@800 nm	150
Sapphire	Type I	TE and TM	2.3@633 nm	151
Dual line	TM	0.65@798.5 nm	152
Depressed cladding	TE and TM	0.37@2850 nm	80
Optical-lattice like	TE and TM	2.9@1064 nm	153
Orthorhombic crystals	KTP	Type I (multiline)	TM	1.0@980 nm	154
Dual line	TE and TM	0.8@633 nm	155
Depressed cladding	TE and TM	1.7@632.8 nm	156
Optical-lattice like	TE and TM	1.2@632.8 nm	157
Nd:YAP	Depressed cladding	TE and TM	0.15@1064 nm	158
Optical-lattice like	TE and TM	1.11@1064 nm	159
Monoclinic crystals	${BiB}_{3} O_{6}$	Depressed cladding	TE and TM	0.6@1064 nm	160
Nd:YCOB	Type I	TM	1,1@1550 nm	85
Depressed cladding	TM and TE	—	161
Nd:GdCOB	Double cladding	TM and TE	0.65@633 nm	162
Nd:KGW	Dual line	TM and TE	2.0@632.8 nm	163
Depressed cladding	TM and TE	1.8@1064 nm	164

Table 2. Summary of latest published works about waveguide configuration and properties of typical crystals in different crystal systems.

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Wavelength band	Gain media	Working wavelength (nm)	Cavity configuration	Operation regime	Laser performance	Ref.
Lasing threshold (mW)	Max. output power (mW)	Slope efficiency
Visible	$N d : Y C O B$	531	Cladding	CW	5	0.1	—	161
$N d : Y A B$	532	Dual line	CW	—	0.032	—	188
$\Pr : {SrAl}_{12} O_{19}$	634.5	Dual line	CW	190	28.1	8%	189
$\Pr, Mg : {SrAl}_{12} O_{19}$	525.3	Dual line	CW	1088	36		190
644	516	1065	37%
724.9	885	504	25%
$\Pr : {LiYF}_{4}$	604	Rhombic cladding	CW	360	25	5.6%	191
720	243	12	2.0%
Ti : sapphire	700 to 870	Dual line	CW	84	143	23.5%	192
798.5	Dual line	CWML (21.25 GHz)	1160.1	87.48	—	152
Near-infrared	$Yb, Na : {CaF}_{2}$	1013.9 and 1027.9	Cladding	CW and $Q$ -switched	152.2	26.6	10%	193
Yb : YAG	1030	$S$ -curved dual line	CW	141	1 W	79%	194
$Y$ -branch dual line	CW	271	2.29 W	52%	97
Dual line	$Q$ -switched	102	5.6 W	74%	195
Dual line	QML (2 GHz)	1800	322	11.3%	196
Double cladding	CW	401.7	45.8	38%	172
Yb : KLuW	1040	Surface cladding	$Q$ -switched	491	680	61%	197
Nd : YAG	1064	Annular ring shaped	CW	191	84	20%	132
Ear-like cladding	CW and $Q$ -switched	10	327	34.4%	168
Cladding	$Q$ -switched	287	102.3	11.9%	198
QML (8.8 GHz)	74	127	26%	199
1061.58 and 1064.18	Cladding	CWML (9.8 GHz)	—	530	—	200
1064	$Y$ -branch cladding	CW	231	172	22.4%	180
1 × 2 splitters	CW	90	333	34%	101
1 × 4 splitters	90	217	22%
Ring shaped	CW and $Q$ -switched	148	224	22%	100
$Nd : {YVO}_{4}$	1064	Cladding	CW	10.3 W	3.4 W	36%	139
$Q$ -switched	57.4	275	37%	201
QML (6.5 GHz)	65	424	56%	202
CWML (6.5 GHz)	19.3	259	30.6%	203
Double cladding	$Q$ -switched	59	397	46%	169
Optical-lattice like	$Q$ -switched	—	85	20%	140
Nd : YAP	1064 and 1079	Cladding	CW	243	199.8	33.4%	158
$S$ -curved cladding	QML (7.9 GHz)	196	77	14.1	159
$S$ -curved optical-lattice like	228	57	10.69
1072 and 1079	Optical-lattice like	CW	384.5	101.3	30.9	75
$Nd : {GdVO}_{4}$	1063.6	Dual line	CW	52	256	70%	141
1064.5	Cladding	CW and $Q$ -switched	178	570	68%	142
Nd : GGG	1061	Dual line	CW	29	11	25%	134
1063	Cladding	CW	270	209	44.4%	135
$Nd : {LuVO}_{4}$	1066.4	Dual line	CW	98	30	14%	204
Nd : KGW	1065	Dual line	CW	141	33	52.3%	163
1067	Cladding	CW	120	198.5	39.4%	164
MIR	$Tm : KLu {({WO}_{4})}_{2}$	1847.4	Surface cladding	CW	52	171.1	37.8%	205
1846.8	$Q$ -switched	500	150	34.6%
1849.6	Cladding	CW	45	247	48.7%	206
1844.8	$Q$ -switched	—	24.9	9.3%
1847	Optical-lattice like	CW	21	46	9.9%	207
1841 to 1848	$Y$ -branch cladding	CW	280	460	40.6%	208
Tm : YAG	1943.5	Cladding	QML (7.8 GHz)	665	6.5	2%	209
$Ho : KGd {({WO}_{4})}_{2}$	2055	Cladding	CW	180	212	67.3%	210
$Tm : {MgWO}_{4}$	2080	Surface cladding	CW	120	132	38.9%	211
Ho : YAG	2091	Cladding	QML (5.9 GHz)	—	170	6.8%	212
2096	CW	100	1775	16%	213
Cr : ZnS	2333	Cladding	CW	450	101	20%	214
Cr : ZnSe	2522	Cladding	CW	—	5200	41%	215
Fe : ZnSe	4070	Cladding	CW	—	995	58%

Table 3. Summary of reported results for waveguide lasers emitting at different wavelengths based on various laser-cavity designs.

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Crystal	Waveguide configuration	Laser regime	$λ_{ω}$ (nm)	$λ_{2 ω}$ (nm)	SHG configuration	$P_{out}$ ( $W^{- 1} {cm}^{- 2}$ )	Norm. efficiency ( $W^{- 1} {cm}^{- 2}$ )	Ref.
BBO	Cladding	CW	800	400	BPM	1.05 mW	0.98%	219
PPKTP	Type I (multiscan)	CW	800	400	QPM	$51 μ W$	0.02%	154
PPSLT	Cladding	CW	800	396 to 401 (tunable)	QPM	0.37 mW	0.39%	221
PPKTP	Dual line	CW	943.18	471.59	QPM	76 mW	4.6%	222
${LiNbO}_{3}$	Cladding	Pulsed	1030	515	BPM	—	—	223
Type I (multiline)	Pulsed	1064	532	BPM	12.45 W (peak)	0.27%	218
Dual line	Pulsed	1064	532	BPM	4.95 W (peak)	0.14%
Cladding	Pulsed	1064	532	BPM	40.40 mW (peak)	0.87%
Cladding	Pulsed	1064	532	LiQPM	25.1 W (peak)	0.0637%	127
Cladding	Pulsed	1065.3, 1064, 1061.6, and 1060.5	532.65, 532, 530.8, and 530.25	LiQPM	1.33 W (peak)	0.64% ( $P_{2 ω} / P_{ω}$ )	43
PPMgSLT	Cladding	CW	1064	532	QPM	$17.3 μ W$	0.74%	224
Cladding	CW	1050	525	QPM	8.5 W	0.16%	220
Cladding	CW	1064	532	QPM	14.87 mW	3.55%	148
Cladding	Pulsed	1064	532	QPM	153 W (peak)	54.3% ( $P_{2 ω} / P_{ω}$ )
${KTiOPO}_{4}$	Optical-lattice like (1 × 4 splitters)	CW	1064	532	BPM	0.65 mW	1.5%	157
Optical-lattice like (straight)	CW	1064	532	BPM	0.67 mW	0.87%
Hybrid optical-lattice	CW	1064	532	BPM	0.8 mW	1.1%	121

Table 4. Summary of latest results for frequency converters in femtosecond laser-written waveguides.

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Lingqi Li, Weijin Kong, Feng Chen. Femtosecond laser-inscribed optical waveguides in dielectric crystals: a concise review and recent advances[J]. Advanced Photonics, 2022, 4(2): 024002

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