Ti:sapphire Lasers: Past, Present and Future

Institute of Physics ASCRPrague, Czech Republic

November 6, 2013

Peter MoultonQ-Peak, Inc.

Outline

• Quick review of transition-metal spectroscopy• The ruby laser and its consequences• Divalent transition-metal lasers• Ti:sapphire background and invention• Impact of Ti:sapphire lasers to date• Energy scaling in future systems• Alternate approaches to high-intensity lasers

3d ions in solids

Number ofd electrons

Ion(s)

1 Ti3+

2 Ti2+, V3+

3 Cr3+, V2+

4 Cr2+, Mn3+

5 Fe3+, Mn2+

6 Fe2+, Co3+

7 Co2+, Ni3+

8 Ni2+

9 Cu2+

H

Li

Na

K Ca Sc

Rb Sr

Be

Mg

Ti V

Y

Cr Mn Fe Cu Zn Ga Ge

Zr Nb Mo

As

B C N

Al Si P

Tc Ru Cd In Sn SbRh Pd Ag

NiCo

Transition metals

H

Li

Na

K Ca Sc

Rb Sr

Be

Mg

Ti V

Y

Cr Mn Fe Cu Zn Ga Ge

Zr Nb Mo

As

B C N

Al Si P

Tc Ru Cd In Sn SbRh Pd Ag

NiCo

Transition metals

d-electron orbitals – 5-fold degenerate in free space

Energy levels of ions with 3 d-shell electrons

Outline

• Quick review of transition-metal spectroscopy• The ruby laser and its consequences• Divalent transition-metal lasers• Ti:sapphire background and invention• Impact of Ti:sapphire lasers to date• Energy scaling in future systems• Alternate approaches to high-intensity lasers

Ruby quantum efficiency was thought by some to be low (Maiman disagreed)

First publication on laser

Stimulated Optical Radiation in RubyT. H. MAIMAN

Hughes Research Laboratories, A Division of Hughes Aircraft Co., Malibu, California.

Schawlow and Townes1 have proposed a technique for the generation of very monochromatic radiation in the infra-red optical region of the spectrum using an alkali vapour as the active medium. Javan2 and Sanders3 have discussed proposals involving electron-excited gaseous systems. In this laboratory an optical pumping technique has been successfully applied to a fluorescent solid resulting in the attainment of negative temperatures and stimulated optical emission at a wave-length of 6943 Å. ; the active material used was ruby (chromium in corundum).

1. Schawlow, A. L. , and Townes, C. H. , Phys. Rev., 112, 1940 (1958).2. Javan, A. , Phys. Rev. Letters, 3, 87 (1959).3. Sanders, J. H. , Phys. Rev. Letters, 3, 86 (1959).4. Maiman, T. H. , Phys. Rev. Letters, 4, 564 (1960).

Nature 187, 493 - 494 (06 August 1960)

Pictures of first ruby laser at Hughes

Sapphire (corundum, Al2O3) enabled ruby laser

Legacy of early ruby laser development

• First laser• First Q-switched laser• First laser-driven nonlinear optics (harmonics, Raman, etc.)• First use of cryogenic cooling to improve thermo-optical and

spectral characteristics• First demonstration of laser pumping of a solid-state laser

– Argon-ion-pumped ruby laser– Ruby-laser-pumped Sm:CaF2 laser (first 5d-4f laser?)

Outline

• Quick review of transition-metal spectroscopy• The ruby laser and its consequences• Divalent transition-metal lasers• Ti:sapphire background and invention• Impact of Ti:sapphire lasers to date• Energy scaling in future systems• Alternate approaches to high-intensity lasers

Isoelectronic traps in Te-doped CdS- try for a tunable laser, but Auger-process won

Rediscovery of first broadly tunable lasers,handicapped by cryogenic operation

Energy levels of divalent transition metals

Divalent Co in MgF2 :properties at 77 K

pump

Co:MgF2 boule and assorted TM-doped crystals grown at MIT Lincoln Laboratory

Cryogenic laser designs at MIT/LL

Cryogenic operation of Co:MgF2 laser

First room-temperature operation from Co:MgF2

Outline

• Quick review of transition-metal spectroscopy• The ruby laser and its consequences• Divalent transition-metal lasers• Ti:sapphire background and invention• Impact of Ti:sapphire lasers to date• Energy scaling in future systems• Alternate approaches to high-intensity lasers

Bill Krupke suggested a possible material for a lamp-pumped fusion-driver laser – but no gain

Excited-state absorption (ESA) a pervasive problem

Ce3+

Example of complexity in ESA calculations

Color-center laser levels inspired search for systems without ESA

Energy levels of single d electron in crystal

Number ofd electrons

Ion(s)

1 Ti3+

2 Ti2+, V3+

3 Cr3+, V2+

4 Cr2+, Mn3+

5 Fe3+, Mn2+

6 Fe2+, Co3+

7 Co2+, Ni3+

8 Ni2+

9 Cu2+

Early work on Ti in sapphire (1962)

MIT efforts studied defect diffusion using Ti

J. Am Ceramic Soc. 52, 331 (1969)

Ti:sapphire absorption/emission (1982)

400 500 600 700 800 900 1,0000

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

WAVELENGTH (nm)

ABSO

RPT

ION

CO

EFFI

CIE

NT

(arb

. uni

ts)

FLU

OR

ESC

ENC

E IN

TESI

TY (a

rb. u

nits

)

Fluorescencelifetime3.2 usec

First Ti:sapphire laser operation

Ti:sapphire - early photos in 1982-3

MIT couldn’t afford (!) to patent Ti:sapphire

Parasitic absorption was a party spoiler

400 600 800 1,000 1,2000

1E-20

2E-20

3E-20

4E-20

5E-20

6E-20

7E-20

WAVELENGTH (nm)

CR

OSS

SEC

TIO

N (c

m^2

)

ABS.

CO

EFFI

CIE

NT

(arb

. uni

ts)

PI

SIGMA

Work at LL examined Ti3+-Ti4+ as culprit

Crystal growth: Czochralski (Kokta) and HEM

Outline

• Quick review of transition-metal spectroscopy• The ruby laser and its consequences• Divalent transition-metal lasers• Ti:sapphire background and invention• Impact of Ti:sapphire lasers to date• Energy scaling in future systems• Alternate approaches to high-intensity lasers

My own group’s work on Ti:sapphire

Tuning curve of Titan-CW laser pumped by argon-ion laser

700 750 800 850 900 950 1,000 1,050 1,100 1,1500

0.5

1

1.5

2

Wavelength (nm)

7 W pump power

Rare-earth levels and Ti:sapphire tuning

Key tool in development of Er:fiber amplifiers

The good (bad) old days of ultrashort pulses

Original Ippen-Shank modelocked dye laser ca ~1980

Pump: 20 W Argon laser: 60 kW lineand 5 GPM chilled water

Ti:sapphire gain bandwidth support 5 fs pulses

600 700 800 900 10000

0.2

0.4

0.6

0.8

1

WAVELENGTH (nm)

INTE

NSI

TY (a

rb. u

nits

)

GAINPI

SIGMA

98 THz (4.4 fs)

Kerr-lens modelocking (KLM) providesa fast switch to enable fs-pulse modelocking

Ti:sapphire ultrafast lasersreplaced dye lasers in the 90’s

Counting optical cycles

Significance of femtosecond lasers

"for his studies of the transition states of

chemical reactions using femtosecond spectroscopy"

The Nobel Prize in Chemistry 1999

Ahmed H. Zewail Egypt and USA

California Institute of Technology (Caltech)

Pasadena, CA, USA

b. 1946

Time Domain ↔ Frequency Domain

2πδ= ∆φ frep

I(f)

f

δ

0

frepI(f)

f

δ

0

frep

•Frequency modes of the fs pulse are offset from fn=0=0 by δ

Frequency Domain

TimeDomain

2∆φ

t

E(t)

• How can we control the absolute frequencies (and hence the group-phase velocities)? Self-referencing

460 480 500 520 540

Fundamental-Second Harmonic

Beats

Repetition Rate

RF P

ower

(10

dB/d

iv)Frequency (MHz)

D. J. Jones et al, Science 288 p 635 28 April 2000

J. Reichert et al., Opt. Comm. 172 pp 59–68 15 Dec 1999

H. Telle et al., Appl. Phys. B 69, 327–332 8 Sept 1999

Locking via Self-ReferencingTechnique

Beat frequency at overlap = δ

StockholmDecember 10, 2005

Hansch and Hall win Nobel Prize for Optical Combs

Chirped pulse amplification (CPA)

Courtesy: Wikipedia1985 (G.Mourou & D.Strikland)

Setup of typical CPA Ti:Sapphire system

femtosecond KLM mode-locked

Ti:S laser50 fs, 10 nJ, 800 nm

100 MHz

Pulse compressor

Ti:S regenerativeamplifier

1 mJ, 800 nm

Pulse stretcher100 fs 100 ps

SHG Nd:doped Q-switched pump laser

5 mJ, 532 nm

SHG CW Nd:doped pump laser

5 W at 532 nm

output 800 nm, 50-100 fs,

?? mJ

Ti:Samplifier800 nm

SHGNd:doped

Q-switchedpump laser

Table-top CPA system (compressor missing)

The NRL TFL laser is a table-top Ti:sapphire CPA laser system that produces laser pulses at a wavelength of 810 nm with 50 mJ of laser energy in a 50-fs pulse. It is operated at a repetition rate of 10 Hz.

Attosecond pulses, high-harmonic generation

Outline

• Quick review of transition-metal spectroscopy• The ruby laser and its consequences• Divalent transition-metal lasers• Ti:sapphire background and invention• Impact of Ti:sapphire lasers to date• Energy scaling in future systems• Alternate approaches to high-intensity lasers

CPA pushes to a Zettawatt (courtesy Mourou)

10 PW System under construction in Saclay, France

Apollon 10P: Driver

Apollon 10P: Output stages

Five multiple-pass Ti:sapphire stages800 J of green energy needed for last three

Uses Nd:glass slab amplifiers

Apollon 10P: Ti:sapphire crystals

First step in systems for Romania

Present system: 100-mm diameter Ti:sapphire crystal (from Crystal Systems)

Long-term goal: Two, 10-PW systems, 1 Hz rate

New technique (?) for Ti:sapphire growth

Outline

• Quick review of transition-metal spectroscopy• The ruby laser and its consequences• Divalent transition-metal lasers• Ti:sapphire background and invention• Impact of Ti:sapphire lasers to date• Energy scaling in future systems• Alternate approaches to high-intensity lasers

Wide variety of broadly tunable Yb-doped crystals

Material Δt (fs) λ (nm) σem (x10-20 cm2) τ fluo (ms) K (W/m/K)

Yb:CALGO 47 1050 0.75 0.42 6.3-6.9

Yb:phosphate

glass

58 1080 0.05 1.3 0.85

Yb:YVO4 61 (KLM) 1050 0.14 0.3 5.2-5.11

Yb:BOYS 69 1062 0.3 1.1 1.8

Yb:SYS 70 1070 0.44 0.44 1.6-2.8

Yb:KYW 71 (KLM) 1025 3 0.6 3.3

Yb:GdCOB 89 1045 0.35 2.6 2.1

Yb:KGW 100 1037 2.8 0.35 3.3

Yb:CaF2 150 1047 0.25 2.4 9.4

Major advantage: Direct diode pumping Major disadvantages: limited BW, low gain

Yb:CaF2 has operated cw at multi-W levelsbecause of good thermal, mechanical properties

A. Lucca et al., “High-power tunable diode-pumped Yb:CaF2 laser,” Opt. Lett. 29, 1879 (2004).

Yb-doped CaGdAlO4 crystal (Yb:CALGO)

Under the right phase-matching conditions parametric amplifiers have a large gain bandwidth

Low-energy optical parametric chirped pulse amplifier (OPCPA)

10 PW OPCPA system (planned, but delayed)

Ti:sapphire laser - highlights

• Broadly tunable (650-1100 nm) output used widely for scientific and applied linear and nonlinear spectroscopy of gases and condensed media, atmospheric research

• Mode-locked output <10 fs has probed ultrafast dynamics of media (Zewail awarded Nobel Prize in Chemistry for work on molecules)

• Mode-locked systems also can generate new optical frequency standards and allow measurement accuracies of a part in 1018

• Amplified mode-locked lasers (with CPA) have reached Petawatt (1015 W) of output (25 J in 25 fs) to study laser-matter interactions at extremely high intensities, generate x-rays

• Commercial laser sales are on the order of 6000 systems, about $500 million (and will exceed $1B)

• Future: 10 PW systems under construction

• Competition: fiber lasers, Yb:doped crystals, OPCPAs