lithium niobateLiNbO3
PPLN Λ = 6.82 µm · 1064 → 532 nm · 2 px/µm
lithium niobate / 07 History / 7.5

7.5 Light in a channel

ΔnDEPTH1973–1991

L1 Narrative

1973-1991 · 11 events

To make a modulator small and fast, light must be confined to a channel a few micrometers across, right under the electrodes. In 1973 Bell Labs published “Thin-film LiNbO3 electro-optic light modulator” [Kaminow et al. 1973 rec]. The “thin film” was not a separate film. It was the crystal's own surface, heated until lithium oxide escaped; the lithium-poor skin has a higher extraordinary index and traps light [Carruthers et al. 1974 rec]. In 1974 titanium stripes diffused into the crystal gave the Ti:LN waveguide [Schmidt & Kaminow 1974 rec], the type used in the commercial modulators of 7.7; in 1982 an acid bath that swaps lithium for hydrogen gave another [Jackel et al. 1982 rec]. In 1985 Crystal Technology announced the first commercially available guided-wave modulator [Becker 1988 rec]. In 1989 a French team reported a neodymium-doped LN channel-waveguide laser [Lallier et al. 1989 rec]; in 1991 Paderborn reported an erbium-doped one at 1.532 µm [Brinkmann et al. 1991 rec].

Diffusion profile — For proton exchange, sliders set time and temperature and the step-index profile and guided mode update; for Li2O out-diffusion and Ti in-diffusion no diffusion constants are held, so only their reported depths are drawn. Thin film then and now: What “thin film” meant.
Index increase vs depth, 633 nm
00.050.10.1505101520depth (µm)Δne
249 °C
 4.0 h
Axis
Exchanged depth d (d2 = r t) derived2.34 µm
Mode effective index (ne = 2.2022)2.3188
Rate data rangeinside
Proton exchange: Δne = 0.12 step, 1.37 µm2/h at 249 °C, 0.37 µm2/h at 217 °C Jackel et al. 1982; whether the rate is d2/t or D is not confirmed from the full text. Anchors: Carruthers et al. 1974, Schmidt & Kaminow 1974; no diffusion constants held for Li2O or Ti. ne: Zelmon et al. 1997.
00.050.10.1505101520depth (µm)Δne
249 °C
 4.0 h
Exchanged depth
2.34 µm
Mode effective index
2.3188

Mode intensity at 633 nm, normalized (red curve). Proton-exchange rates Jackel et al. 1982; no diffusion constants are held for Li₂O or Ti, so those show their sourced depths only (Carruthers et al. 1974, Schmidt & Kaminow 1974). Whether the reported rate is d2/t or D is not confirmed from the full text; depth here uses d2 = r t. Substrate ne = 2.2022 at 633 nm Zelmon et al. 1997.

What "thin film" meant: 1973 vs 2018

1973 vs 2018 cross-sections

L1

In 1973 Bell Labs titled a paper “Thin-film LiNbO3 electro-optic light modulator” [Kaminow et al. 1973 rec]. The film was the crystal's own skin, made by baking out lithium oxide so the surface index rose [Carruthers et al. 1974 rec]. In 2018 “thin-film LN” meant a separate slice of crystal well under a micrometer thick (680 nm in the 2004 Zurich films [Rabiei & Gunter 2004 rec]), peeled from a wafer by ion implantation and bonded onto silicon dioxide (1998-2004 methods [Levy et al. 1998 rec; Rabiei & Gunter 2004 rec]). The same words describe guides about 18 times different in thickness and drive voltages about an order of magnitude apart (not normalized for length).

1973 vs 2018 — Two cross-sections side by side at one scale: 1973, a graded index skin in bulk LN with the mode spread over micrometers; 2018, a sub-micrometer rib on SiO2 on Si with a tightly confined mode and electrodes micrometers away. A scale slider zooms from 50 µm to 1 µm field of view without moving either drawing's origin. Readouts: guide thickness, voltage for π phase, bandwidth.
Cross-sections, same scale and originfield of view   50 µm
10 µm197310 µm2018
  50 µm
1973-19742004-2018
Guide12 µm0.68 µm
V(π)~24 V*1.4 V
BW3.2 GHzN/A**
Guide: effective thickness of the single-mode out-diffused guide Carruthers et al. 1974 rec; film Rabiei & Gunter 2004 rec. V(π), BW: Kaminow et al. 1973 / Wang et al. 2018b (20 mm). * π / 0.13 rad/V derived, device length not in the abstract. ** bandwidth not reported in the text read. One device per value; not normalized for length. Profile, rib, oxide, electrodes and mode outlines: schematic.
1973 the crystal's own skin
10 µm1973
2018 an ion-sliced film on SiO₂
10 µm2018
  50 µm
Guide thickness12 µm0.68 µmabout 18× derived
Voltage for π phase~24 V*1.4 V* derived from 0.13 rad/V, length not given
Bandwidth3.2 GHzN/A**** not reported in the text read

Sources: Carruthers et al. 1974, Rabiei & Gunter 2004, Kaminow et al. 1973, Wang et al. 2018b. Index profile, rib, oxide, electrode gap and mode outlines (IR, false color) are schematic; the 12 µm and 0.68 µm are to scale. One device per value; not normalized for length.

L2

1973-19742004-2022
GuideLi2O out-diffused skin in bulk LN; ne raisedIon-sliced single-crystal film on SiO2
Thicknesssingle-mode guide effective thickness 12 µm [Carruthers et al. 1974 rec]680 nm film [Rabiei & Gunter 2004 rec]
Ratioabout 18x (derived, 12 µm / 0.68 µm)
Voltage0.13 rad/V, so about 24 V for π (derived; length not given) [Kaminow et al. 1973 rec]Vπ 1.4 V at 20 mm [Wang et al. 2018b rec]
Bandwidth3.2 GHz, 50 ohm load [Kaminow et al. 1973 rec]> 106 GHz 3 dB electrical (bonded hybrid) [Weigel et al. 2018 rec]; 110 GHz [Xu et al. 2022 rec]
LossN/A (not reported in the abstract)< 0.5 dB on-chip [Wang et al. 2018b rec]

Note: the 12 µm figure is from the 1974 kinetics paper by the same group, not the 1973 device; the 24 V figure has no length normalization. Both comparisons are order-of-magnitude.

L3

LNOI dates from 1998 (ion slicing) and 2004 (bonded sub-micrometer films), not from 2018 [Levy et al. 1998 rec; Rabiei & Gunter 2004 rec]. The 1973 guide is the same Li out-diffusion that chapter 04 treats as a fabrication hazard [Carruthers et al. 1974 rec].

L2 Numbers and mechanisms

Light in a channel

Out-diffusion (1973-1974). Kaminow et al.: “An out-diffused planar optical waveguide in LiNbO3 has been used to produce an efficient broad-band electro-optic phase modulator”; modulation index per volt 0.13 V−1, base bandwidth 3.2 GHz with a 50 ohm load, 0.2 mW/MHz for 1 rad [Kaminow et al. 1973 rec]. Derived: the voltage for π phase is π / 0.13 V−1, about 24 V, for a device whose length the abstract does not give. Carruthers, Kaminow and Stulz: out-diffusion of Li2O raises ne enough for single- and multimode guides; diffusion activation energy in LiNbO3 68 ± 1.2 kcal/mol; a single-mode guide with effective thickness 12 µm [Carruthers et al. 1974 rec].

Ti in-diffusion (1974). Ti, V and Ni films diffused into LiNbO3 give low-loss TE and TM guides; Ti gives single-mode guides confining light within about 1 µm of the surface, with diffusion parameters controlling the waveguide independently [Schmidt & Kaminow 1974 rec].

Epitaxial films (1974). Tien, Riva-Sanseverino and Ballman deflected a beam up to 4 degrees in an epitaxial LiNbO3 film waveguide [Tien et al. 1974 rec].

Proton exchange (1982). Benzoic acid melt at 200-250 C; step index increase 0.12 for extraordinary polarization only; diffusion 1.37 µm2/h at 249 C and 0.37 µm2/h at 217 C on x-cut; losses < 0.5 dB/cm at 633 nm; damages y-cut surfaces [Jackel et al. 1982 rec].

Products and lasers. Becker (1988): active guided-wave devices reported since about 1975, mostly Ti:LiNbO3; in 1985 Crystal Technology announced “the first commercially available product - a high-speed, efficient, intensity modulator” [Becker 1988 rec]. Alferness reviewed waveguide EO modulators in 1982 [Alferness 1982 rec]. Paderborn implanted erbium into LN waveguides (1990) [Brinkmann et al. 1990 rec] and reported what the authors called “the first integrated optical laser in LiNbO3” (a Nd:MgO:LiNbO3 channel-waveguide laser had been published in 1989-10 [Lallier et al. 1989 rec]; D13): Er3+ in-diffused, Ti channel guide, colour-centre pump at 1.477 µm, CW at 1.532 µm, threshold about 8 mW absorbed [Brinkmann et al. 1991 rec].

L3 Sources and status

16 sources cited on this page
ClaimSourceStatus
1973 “thin film” is an out-diffused guide in bulk LN; 0.13 V−1; 3.2 GHzKaminow et al. 1973 abstract (Crossref)rec
Li2O out-diffusion raises ne; 12 µm single-mode guideCarruthers et al. 1974 abstractrec
Ti guides within ~1 µm of surfaceSchmidt & Kaminow 1974 abstractrec
Proton exchange Δ ne = 0.12Jackel et al. 1982 abstractrec
First commercial guided-wave modulator 1985Becker 1988 abstract (single source)rec
Nd:MgO:LN channel-waveguide laser 1989; Er:Ti:LN waveguide laser 1991 (“first” is the 1991 authors' claim)Lallier et al. 1989 (Crossref), Brinkmann et al. 1991 abstractV; D13

References

  1. Kaminow et al., Applied Physics Letters 22 540-542 (1973). Thin-film LiNbO3 electro-optic light modulator. Issued 1973-05-15. doi:10.1063/1.1654500
  2. Carruthers et al., Applied Optics 13 2333 (1974). Diffusion Kinetics and Optical Waveguiding Properties of Outdiffused Layers in Lithium Niobate and Lithium Tantalate. Issued 1974-10-01. doi:10.1364/ao.13.002333
  3. Schmidt & Kaminow, Applied Physics Letters 25 458-460 (1974). Metal-diffused optical waveguides in LiNbO3. Issued 1974-10-15. doi:10.1063/1.1655547
  4. Jackel et al., Applied Physics Letters 41 607-608 (1982). Proton exchange for high-index waveguides in LiNbO3. Issued 1982-10-01. doi:10.1063/1.93615
  5. R.A. Becker, MRS Bulletin 13 21-23 (1988). Optical-Guided-Wave Modulators. Issued 1988-08. doi:10.1557/s0883769400064630
  6. E. Lallier et al., Electronics Letters 25 1491-1492 (1989). Laser oscillation of single-mode channel waveguide in Nd:MgO:LiNbO3. Issued 1989-10-26. doi:10.1049/el:19891000
  7. R. Brinkmann et al., Electronics Letters 27 415-417 (1991). Continuous-wave erbium-diffused LiNbO 3 waveguide laser. Issued 1991-02-28. doi:10.1049/el:19910263
  8. P. K. Tien et al., Applied Physics Letters 25 563-565 (1974). Light beam scanning and deflection in epitaxial LiNbO3 electro-optic waveguides. Issued 1974-11-15. doi:10.1063/1.1655311
  9. Alferness, IEEE Transactions on Microwave Theory and Techniques 30 1121-1137 (1982). Waveguide Electrooptic Modulators. Issued 1982-08. doi:10.1109/tmtt.1982.1131213
  10. R. Brinkmann et al., Integrated Photonics Research PD1 (1990). Annealed Erbium-implanted single-mode LiNbO3 waveguides. Issued 1990. doi:10.1364/ipr.1990.pd1
  11. Rabiei & Gunter, Applied Physics Letters 85 4603-4605 (2004). Optical and electro-optical properties of submicrometer lithium niobate slab waveguides prepared by crystal ion slicing and wafer bonding. Issued 2004-11-15. doi:10.1063/1.1819527
  12. Levy et al., Applied Physics Letters 73 2293-2295 (1998). Fabrication of single-crystal lithium niobate films by crystal ion slicing. Issued 1998-10-19. doi:10.1063/1.121801
  13. Wang et al., Nature 562 101-104 (2018). Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages. Issued 2018-09-24. doi:10.1038/s41586-018-0551-y
  14. Peter O. Weigel et al., Optics Express 26 23728 (2018). Bonded thin film lithium niobate modulator on a silicon photonics platform exceeding 100 GHz 3-dB electrical modulation bandwidth. Issued 2018-09-03. doi:10.1364/oe.26.023728
  15. Mengyue Xu et al., Optica 9 61 (2022). Dual-polarization thin-film lithium niobate in-phase quadrature modulators for terabit-per-second transmission. Issued 2022-01-20. doi:10.1364/optica.449691
  16. Zelmon et al., Journal of the Optical Society of America B 14 3319 (1997). Infrared corrected Sellmeier coefficients for congruently grown lithium niobate and 5 mol% magnesium oxide-doped lithium niobate. Issued 1997-12-01. doi:10.1364/JOSAB.14.003319

Events of 7.5

11 events; status per event
DateEventWhoRefSt
1973-05-15'Thin-film LiNbO3 electro-optic light modulator': an out-diffused planar guide in bulk LN, 3.2 GHzI. P. Kaminow, J. R. Carruthers +2DOIrec
1974-10-01Diffusion kinetics of Li2O out-diffused waveguides in LiNbO3 and LiTaO3J. R. Carruthers, I. P. Kaminow +1DOIrec
1974-10-15Metal (Ti) in-diffused optical waveguides in LiNbO3R. V. Schmidt, I. P. KaminowDOIrec
1974-11-15Beam scanning in epitaxial LiNbO3 film waveguidesP. K. Tien, S. Riva-Sanseverino +1DOIrec
1982-08Waveguide electro-optic modulators (review)R. C. AlfernessDOIrec
1982-10-01Proton exchange in benzoic acid for high-index LiNbO3 waveguidesJ. L. Jackel, C. E. Rice +1DOIrec
1985First commercially available guided-wave modulator (high-speed intensity modulator)DOIrec
1989-03-13Photorefraction in Ti:LiNbO3 waveguides studied at Sumitomo Metal MiningT. Fujiwara, S. Sato +1DOIrec
1989-10-26Laser oscillation in a single-mode Nd:MgO:LiNbO3 channel waveguideE. Lallier, J. P. Pocholle +7DOIrec
1990Annealed erbium-implanted single-mode LiNbO3 waveguides (postdeadline)R. Brinkmann, Ch. Buchal +3DOIrec
1991-02-28Er-doped Ti:LiNbO3 waveguide laser at 1.532 µm ('first integrated optical laser in LiNbO3' per the authors)R. Brinkmann, W. Sohler +1DOIrec