Ultrashort Laser Pulse Phenomena, 2nd Edition

Solid-State Lasers 367

Table 6.4

Room temperature physical properties of Cr:Forsterite and Cr:Cunyite lasers.

Property Cr:Mg

2

SiO

4

Cr:Ca

2

GeO

4

Units Ref.

Nonlinear index 2 10

−16

1.5 10

−16

cm

2

/W [72,105]

Dispersion (k" at 1280 nm) 185 fs

2

/cm [106]

Peak absorption at 670 nm

Peak gain (1240 nm) 14.4 80 10

−20

cm

2

[72]

Fluorescence lifetime [101]

τ

F

2.7 15 µs [102,107]

Tuning range from 1167 1350 nm [72]

to 1345 1500 nm

Thermal conductivity 0.03 W/cm/K

Ca

2

GeO

4

(cunyite) [101,102]. The properties of these two laser materials are

compared in Table 6.4. Forsterite-based lasers have become important because

they operate in the 1.3 µm range (1167 to 1345 nm) and can be pumped with

Nd:YAG lasers. Attempts have also been made at diode pumping [103]. By care-

ful intracavity dispersion compensation with a pair of SF58 prisms complemented

by double-chirped mirrors, a pulse duration of 14 fs was obtained [104]. This

laser, pumped by a Nd:YAG laser, had a threshold of 800 mW for cw operation

and 4 W for mode-locked operation. 100 mW output power could be achieved

with a pump power of 6 W.

The forsterite laser produces pulses short enough to create an octave spanning

spectral broadening in fibers as discussed in Section 13.4.1.

4

A prismless compact

ring cavity was designed with combination of chirped mirrors (GDD of −55 fs

2

from 1200 to 1415 nm) and Gires–Tournois interferometer mirrors (GDD of

−280 fs

2

from 1200 to 1325 nm) as sketched in Figure 6.12. This laser, pumped

by a 10 W fiber laser, combined short pulse output (28 fs) with a high repetition

rate of 420 MHz [98].

6.7.5. YAG Lasers

The crystal Y

3

Al

5

O

12

or YAG is transparent from 300 nm to beyond 4 µm,

optically isotropic, with a cubic lattice structure characteristic of garnets. It is one

of the preferred laser hosts because of its good optical quality and high thermal

conductivity. Some of the physical–optical properties are listed in Table 6.5. The

two most important lasers using YAG as a host are Nd:YAG and Yb:YAG.

4

Germanium doped silica fiber with a small effective area of 14 µm

2

nonlinear coefficient of

8.5 W

−1

km

−1

, zero dispersion near 1550 nm.

368 Ultrashort Sources II: Examples

Chirped

ROC  5 cm

Chirped

ROC  5 cm

GTI

OC 1.5%

HR

Chirped

Cr:F

10 mm

f  3.8 cm

10 W

Fiber laser

1075 nm

Figure 6.12 Compact ring cavity of a Cr:forsterite laser used in conjunction with HNLF fibers to

generate an octave spanning continuum in the near IR. (Adapted from Thomann et al. [98].) The

mirrors of 5 cm radius of curvature as well as the first folding mirror (HR) have chirped multilayer

coatings. The second folding mirror is a Gires–Tournois Interferometer (GTI), the third one a standard

high reflector, and the output coupler has a transmission of 1.5%.

Table 6.5

Room temperature physical properties of YAG. The

second-order dispersion is calculated from the derivative of

the Sellmeier equation: n

2

= 1 + 2. 2779λ

2



/(λ

2



− 0. 01142)

with λ



in µm. The data are compiled

from [70,72,104,108–110]

Property YAG Units

Index of refraction 1.064 µm 1.8169

Index of refraction 1.030

µm 1.8173

Dispersion (k



)at1.064µm 733 fs

2

/cm

Dispersion (k



)at1.030µm 760 fs

2

/cm

Nonlinear index 12.4 10

−16

cm

2

/W

Thermal expansion

Ref. [100] 8.2 10

−6

K

−1

Ref. [110] 7.7 10

−6

K

−1

Ref. [111] 7.8 10

−6

K

−1

Thermal conductivity 0.129 W cm

−1

K

−1

dn/dT 8.9 10

−6

K

−1