104
4 In the Field of Quantum Technologies
Intracavity Frequency Conversion with VECSELs
Generally, VECSELs are attractive for intracavity frequency conversion schemes
with nonlinear crystals due to their high intracavity optical powers [69–73]. Based on
dual-wavelength operation [74], one can for instance achieve multi-mode THz generation [69, 75] (due to the multi-mode character of the underlying dual-wavelength
operation [75, 76]) or with considerable efforts also single-mode THz generation in a special two-single-frequency VECSEL configuration [70] (e.g. appealing for application as strong-enough THz local oscillator). Such THz sources can
even employ aperiodically-poled nonlinear crystals for widely tunable THz output
around 1 THz [77].
These schemes can further benefit from power-scaling approaches [78]
employing two serially-connected [79] or parallely-connected chips [80]. In fact,
a two-chip TECSEL
7 was recently reported by Guoyu et al. and characterised in
comparison to a single-chip counterpart [81]. While the long-term output stability
of TECSELs over hours of operation time is good [81], the noise from multi-mode
operation and mechanical vibrations on various time scales [82], mode competition
[76] and antiphase intensity fluctuations due to typically-shared spatial and spectral
gain regions [83] have to be taken into account for further improvements and control
of the stability.
Mode-Locked VECSELs
In addition to the aforementioned operation modes, various schemes for modelocking can be realised with VECSELs [84], which can be used for applications such
as multi-photon microscopy [85] or the generation of frequency combs
8 that can
be used for spectroscopy [87]. Ultrashort pulsing down to the 100-fs level has been
reported in recent years [88, 89]. VECSELs have recently also demonstrated their
potential use as ultrafast pump source for quantum emitters with high single-photon
flux [34] (see Fig. 4.2) which are highly desired for high-repetition rate quantum
cryptography schemes.
The Self-Mode-Locking Phenomenon
One recent breakthrough in the field of ultrashort-pulse generation from VECSELs
has been heavily discussed and investigated in recent years, namely the self-starting
SESAM
9 -free pulse formation, “magic” mode-locking or “self-mode-locking” phenomenon in these devices [90–98]. Although not an undisputed phenomenon among
the mode-locked SDL community due to the early claims with incomplete characterisation in support of a mode-locking claim, this remarkable self-mode-locking effect
has been successfully obtained for different VECSEL platforms, which are based on
quantum-well [95] or quantum-dot [94] gain media. Their pulsed light reaches from
the IR over the near-IR (most examples were demonstrated around 1 µm emission
7 Acronym for terahertz external-cavity surface-emitting laser.
8 Frequency combs were first demonstrated by W. Hänsch and co-workers and offer a unique optical
metrology tool for ultra-precise measurements of frequencies with the help of mode-locked lasers.
For background information on this subject, see [86].
9 Acronym for semiconductor saturable absorber mirror.
4 In the Field of Quantum Technologies
Intracavity Frequency Conversion with VECSELs
Generally, VECSELs are attractive for intracavity frequency conversion schemes
with nonlinear crystals due to their high intracavity optical powers [69–73]. Based on
dual-wavelength operation [74], one can for instance achieve multi-mode THz generation [69, 75] (due to the multi-mode character of the underlying dual-wavelength
operation [75, 76]) or with considerable efforts also single-mode THz generation in a special two-single-frequency VECSEL configuration [70] (e.g. appealing for application as strong-enough THz local oscillator). Such THz sources can
even employ aperiodically-poled nonlinear crystals for widely tunable THz output
around 1 THz [77].
These schemes can further benefit from power-scaling approaches [78]
employing two serially-connected [79] or parallely-connected chips [80]. In fact,
a two-chip TECSEL
7 was recently reported by Guoyu et al. and characterised in
comparison to a single-chip counterpart [81]. While the long-term output stability
of TECSELs over hours of operation time is good [81], the noise from multi-mode
operation and mechanical vibrations on various time scales [82], mode competition
[76] and antiphase intensity fluctuations due to typically-shared spatial and spectral
gain regions [83] have to be taken into account for further improvements and control
of the stability.
Mode-Locked VECSELs
In addition to the aforementioned operation modes, various schemes for modelocking can be realised with VECSELs [84], which can be used for applications such
as multi-photon microscopy [85] or the generation of frequency combs
8 that can
be used for spectroscopy [87]. Ultrashort pulsing down to the 100-fs level has been
reported in recent years [88, 89]. VECSELs have recently also demonstrated their
potential use as ultrafast pump source for quantum emitters with high single-photon
flux [34] (see Fig. 4.2) which are highly desired for high-repetition rate quantum
cryptography schemes.
The Self-Mode-Locking Phenomenon
One recent breakthrough in the field of ultrashort-pulse generation from VECSELs
has been heavily discussed and investigated in recent years, namely the self-starting
SESAM
9 -free pulse formation, “magic” mode-locking or “self-mode-locking” phenomenon in these devices [90–98]. Although not an undisputed phenomenon among
the mode-locked SDL community due to the early claims with incomplete characterisation in support of a mode-locking claim, this remarkable self-mode-locking effect
has been successfully obtained for different VECSEL platforms, which are based on
quantum-well [95] or quantum-dot [94] gain media. Their pulsed light reaches from
the IR over the near-IR (most examples were demonstrated around 1 µm emission
7 Acronym for terahertz external-cavity surface-emitting laser.
8 Frequency combs were first demonstrated by W. Hänsch and co-workers and offer a unique optical
metrology tool for ultra-precise measurements of frequencies with the help of mode-locked lasers.
For background information on this subject, see [86].
9 Acronym for semiconductor saturable absorber mirror.