45° AOI 725 - 1000nm Ultrafast Chirped Mirrors
1030nm Highly-Dispersive Broadband Ultrafast Mirrors
GDD (fs²)
R (%), P-Polarization, 5° -50
GDD (fs2), P-Polarization, 5°
950 970 990 1010 1030 1050 1070 1090
-100
-150
-200
-250
-300
-350
-400
-450
-500
(nm)
Figure 14.4: Highly-dispersive mirrors provide both high reflectivities
(red) and negative GDD (blue) with a high magnitude and far less wavelength
dependent oscillations than chirped mirrors
650 - 1350nm, Complementary Chirped Mirror Pair
Coating Reflectivity/GDD Performance
R (%)
99.9
99.8
99.7
99.6
99.5
99.4
99.3
99.2
99.1
99
3000 100
GDD Rp
GDD Rp 3° (L)
GDD Rp 3° (S)
Rp 3° (L)
Rp 3° (S)
Rp
550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400
Wavelength (nm)
Reflectance (%)
99.5
99
98.5
98
97.5
97
96.5
96
95.5
95
www.edmundoptics.co.uk/LO 43
Chirped Mirrors
It is also important to understand chirped mirrors to fully understand
highly-dispersive mirrors. Unlike GTI mirrors, which provide negative
GDD using a resonance effect, chirped mirrors introduce a controlled
negative GDD by a wavelength-dependent penetration depth into the
mirror’s coating. Typical dielectric mirrors are designed to reflect one
particular wavelength, while every coating layer in a chirped mirror
is designed to reflect a different wavelength. The thickness of coating
layers increases from the external mirror surface to the bottom layer,
causing long wavelengths to penetrate deeper into the coating and experience
longer path lengths than shorter wavelengths, counteracting
positive dispersion (Figure 14.2).
Unfortunately, the sharp transitions between different layer thicknesses
in this simple dielectric structure causes oscillations in group delay dispersion
(GDD) as a function of wavelength (Figure 14.3).
Section 14.2:
Highly-Dispersive Mirrors
Ultrafast highly-dispersive mirrors combine wavelength-dependent
penetration, similar to that of chirped mirrors, with a multi-resonance
effect in the coating known as multi-GTI.1 This combination counters
both the bandwidth limitations of traditional GTI mirrors and the GDD
oscillations of chirped mirrors. Highly-dispersive mirrors also provide
high throughput, zero third-order dispersion, and negative GDD with a
large magnitude, making them ideal pulse compressing optics for ultrafast
systems (Figure 14.4).
The several advantages of ultrafast highly-dispersive mirrors over traditional
chirped and GTI mirrors have made them essential elements of
ultrafast laser setups. They are highly advantageous for applications in
high energy ultrafast pulse generation that require a negative GDD with
a large magnitude. These mirrors allow for pulse compression without
increasing the number of bounces off dispersive mirrors, which limits
the alignment sensitivity and round-trip losses.
Section 14.3:
Chirped Mirror Pairs
As shown earlier, highly-dispersive mirrors avoid the third- and higherorder
dispersion experienced by traditional compression methods such
as gratings and prisms. However, they experience GDD oscillations over
bandwidths greater than 100nm, as shown in Figure 14.3. Chirped mirror
pairs may be a better solution for applications where these applications
would be detrimental. The mirrors in a chirped mirror pair are chosen to
have out-of-phase GDD oscillations, leading to an overall oscillation-free
GDD over a large bandwidth (Figure 14.5). This allows for compression
of pulse durations down to <3 fs.
References:
1. Pervak, V., et al. "High-Dispersive Mirrors for Femtosecond Lasers."
Optics Express, vol. 16, no. 14, 2008, pp. 10220–10233., doi:10.1364/
oe.16.010220.
-50
-70
-90
-110
-130
700 750 800 850 900 950
GDD (fs²)
λ (nm)
P-Polarization, 45°
Figure 14.3: Oscillations in the GDD of an ultrafast chirped mirror due
to the discrete switching between different layer thicknesses
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
-200
-400
-600
-800
-1000
500
GDD (fs2)
Figure 14.5: Two chirped mirrors with out-of-phase GDD oscillations
are paired to result in a nearly oscillation-free total GDD over a very wide
range of wavelengths. "L" and "S" are used to distinguish the mirrors
/LO