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Short Pulses in Engineered Nonlinear Media
KTH, School of Engineering Sciences (SCI), Physics.
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Short optical pulses and engineered nonlinear media is a powerful combination. Mode locked pulses exhibit high peak powers and short pulse duration and the engineered ferro-electric KTiOPO4 facilitates several different nonlinear processes. In this work we investigate the use of structured, second-order materials for generation, characterization and frequency conversion of short optical pulses.

By cascading second harmonic generation and difference frequency generation the optical Kerr effect was emulated and two different Nd-based laser cavities were mode locked by the cascaded Kerr lensing effect. In one of the cavities 2.8 ps short pulses were generated and a strong pulse shortening took place through the interplay of the cavity design and the group velocity mismatch in the nonlinear crystal. The other laser had a hybrid mode locking scheme with active electro-optic modulation and passive cascaded Kerr lensing incorporated in a single partially poled KTP crystal. The long pulses from the active modulation were shortened when the passive mode locking started and 6.9 ps short pulses were generated.

High-efficiency frequency conversion is not a trivial task in periodically poled materials for short pulses due to the large group velocity mismatch. Optimization of parameters such as the focussing condition and the crystal temperature allowed us to demonstrate 64% conversion efficiency by frequency doubling the fs pulses from a Yb:KYW laser in a single pass configuration. Quasi phase matching also offers new possibilities for nonlinear interactions. We demonstrated that it is possible to simultaneously utilize several phase matched second harmonic interactions, resulting in a dual-polarization second harmonic beam.

Short pulse duration of the fundamental wave is a key parameter in the novel method that we demonstrated for characterization of the nonlinearity of periodically poled crystals. The method utilizes the group velocity mismatch between the two polarizations in a type II second harmonic generation configuration.

The domain walls of PPKTP exhibit second order nonlinearities that are forbidden in the bulk material. This we used in a single shot frequency resolved optical gating arrangement. The spectral resolution came from Čerenkov phase matching, a non-collinear phase matching scheme that exhibits a substantial angular dispersion. The second harmonic light was imaged upon a CCD camera and with the spectral distribution on one axis and the temporal autocorrelation on the other. From this image we retrieved the full temporal profile of the fundamental pulse, as well as the phase. The spectral dispersion provided by the Čerenkov phase matching was large enough to characterize optical pulses as long as ~200 fs in a compact setup. The Čerenkov frequency resolved optical gating method samples a thin stripe of the beam, i.e. the area close to the domain wall. This provides the means for high spatial resolution measurements of the spectral-temporal characteristics of ultrafast optical fields.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , x, 57 p.
Series
Trita-FYS, ISSN 0280-316X ; 2006:74
Keyword [en]
nonlinear optics, KTiOPO4, frequency conversion, mode-locked lasers, ultra-fast lasers, visible lasers, short pulses, ultrafast nonlinear optics, diagnostic applications of nonlinear optics, nonlinear optical materials
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-4234ISBN: 91-7178-540-X (print)OAI: oai:DiVA.org:kth-4234DiVA: diva2:11390
Public defence
2006-12-18, FD5, AlbaNova, Roslagstullsbacken 21, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20100831Available from: 2006-12-15 Created: 2006-12-15 Last updated: 2015-03-30Bibliographically approved
List of papers
1. Generation of 2.8 ps pulses by mode-locking a Nd:GdVO4 laser with defocusing cascaded Kerr lensing in periodically poled KTP
Open this publication in new window or tab >>Generation of 2.8 ps pulses by mode-locking a Nd:GdVO4 laser with defocusing cascaded Kerr lensing in periodically poled KTP
2005 (English)In: Optics express, ISSN 1094-4087, Vol. 13, no 14, 5270-5278 p.Article in journal (Refereed) Published
Abstract [en]

A Nd:GdVO4 laser mode locked by self-defocussing cascaded Kerr lens in PPKTP is presented. A strong pulse shortening mechanism is produced by the interplay of group velocity mismatch and the cavity design. The cavity had a repetition rate of 200 MHz and the mode-locked output power was 350 mW. Pulses as short as 2.8 ps were obtained with a bandwidth of 0.6 nm.

Keyword
Bandwidth, Gadolinium compounds, Lenses, Neodymium lasers, Nonlinear optics, Optical Kerr effect, Ultrafast phenomena
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-6625 (URN)10.1364/OPEX.13.005270 (DOI)000230519100011 ()2-s2.0-22744447507 (Scopus ID)
Note
QC 20100830Available from: 2006-12-15 Created: 2006-12-15 Last updated: 2015-03-30Bibliographically approved
2. Active and passive hybrid mode-locking of a Nd:YVO4 laser with a single partially poled KTP crystal
Open this publication in new window or tab >>Active and passive hybrid mode-locking of a Nd:YVO4 laser with a single partially poled KTP crystal
2006 (English)In: Optics express, ISSN 1094-4087, Vol. 14, no 15, 6675-6680 p.Article in journal (Refereed) Published
Abstract [en]

We report on a Nd:YVO4 laser mode-locked with a hybrid active and passive modulator consisting of a single partially poled KTP crystal. The periodically poled part provides negative cascaded Kerr-lensing, which together with intracavity soft and hard apertures gives passive modulation. Active phase modulation comes from the electro-optic effect by applying a voltage over the unpoled part of the crystal. The active modulation provides pulse lengths of about 95 ps, which initiate pulse shortening and self-sustained passive mode-locking by the cascaded Kerr effect. The repetition rate of the laser was 94 MHz and the output power was 350 mW, with a bandwidth of 0.235 nm and pulse lengths down to 6.9 ps.

Keyword
2.8 ps pulses, 2nd-order nonlinearities, yvo4 laser, locked nd, mirror
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-6626 (URN)000239342300011 ()2-s2.0-33746379038 (Scopus ID)
Note
QC 20100830Available from: 2006-12-15 Created: 2006-12-15 Last updated: 2015-03-30Bibliographically approved
3. Efficient Doubling of Femtosecond Pulses in Aperiodically and Periodically Poled KTP Crystals
Open this publication in new window or tab >>Efficient Doubling of Femtosecond Pulses in Aperiodically and Periodically Poled KTP Crystals
Show others...
2007 (English)In: Optics express, ISSN 1094-4087, Vol. 15, no 3, 1155-1160 p.Article in journal (Refereed) Published
Abstract [en]

Efficient doubling of femtosecond pulses in periodically and aperiodically poled KTP crystals is demonstrated by employing the quasi-phasematched frequency conversion technique. Conversion efficiencies as high as 60% were achieved using either aperiodically or periodically poled KTP nonlinear crystals in an extracavity, single-pass configuration using a diode-pumped femtosecond Yb:KYW laser as the pump source. The temporal characteristics of the frequency-doubled pulses as a function of focusing conditions in a "thick" nonlinear crystal regime have been investigated experimentally and pulses as short as 177 fs have been generated at around 520 nm under strong focusing conditions using a KTP crystal with aperiodic poling.

Keyword
2nd harmonic-generation, flux-grown ktiopo4, 2nd-harmonic generation, blue; index, field
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-6627 (URN)10.1364/OE.15.001155 (DOI)000244682200044 ()2-s2.0-33846875671 (Scopus ID)
Note

QC 20100830

Available from: 2006-12-15 Created: 2006-12-15 Last updated: 2015-03-30Bibliographically approved
4. Simultaneous second-harmonic generation with two orthogonal polarization states in periodically poled KTP
Open this publication in new window or tab >>Simultaneous second-harmonic generation with two orthogonal polarization states in periodically poled KTP
2002 (English)In: Optics letters, ISSN 0146-9592, Vol. 27, 1682-1630 p.Article in journal (Refereed) Published
Abstract [en]

Simultaneous generation of second-harmonic light with orthogonal polarizations in the blue spectral region is demonstrated by use of type II and type I quasi-phase matching for the nonlinear coefficients d(33), d(24), and d(32) in periodically poled KTP. The second-harmonic power ratio in both polarization states can be tuned by the fundamental polarization and (or) the periodically poled KTP temperature. Femtosecond pulse frequency-doubling efficiencies of as much as 39.5% W-1 and 8.1% W-1 are demonstrated in 7-mm periodically poled KTP for type I and type Il processes, respectively. The efficiency limitations caused by various effects of the group-velocity mismatch are discussed.

Keyword
potassium titanyl phosphate, 2nd-harmonic generation, ktiopo4
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-6628 (URN)000177945300014 ()
Note
QC 20100830Available from: 2006-12-15 Created: 2006-12-15 Last updated: 2015-03-30Bibliographically approved
5. Three-dimensional characterization of the effective second-order nonlinearity in periodically poled crystals
Open this publication in new window or tab >>Three-dimensional characterization of the effective second-order nonlinearity in periodically poled crystals
2003 (English)In: Optic letters, ISSN 0146-9592, Vol. 28, no 17, 1555-1557 p.Article in journal (Refereed) Published
Abstract [en]

A novel technique for characterization of the second-order nonlinearity in nonlinear crystals is presented. It utilizes group-velocity walk-off between femtosecond pulses in type II SHG to achieve three-dimensional resolution of the nonlinearity. The longitudinal and transversal spatial resolution can be set independently. The technique is especially useful for characterizing quasi-phase-matched nonlinear crystals, and it is demonstrated in potassium titanyl phosphate.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-8756 (URN)10.1364/OL.28.001555 (DOI)000184836900019 ()
Note
QC 20100830Available from: 2005-11-09 Created: 2005-11-09 Last updated: 2015-03-30Bibliographically approved
6. Ultrashort single-shot pulse characterization with high spatial resolution using localized nonlinearities in ferroelectric domain walls
Open this publication in new window or tab >>Ultrashort single-shot pulse characterization with high spatial resolution using localized nonlinearities in ferroelectric domain walls
2007 (English)In: Optics letters, ISSN 0146-9592, Vol. 32, no 11, 1545-1547 p.Article in journal (Refereed) Published
Abstract [en]

Second-order nonlinearities localized in the regions adjacent to the ferroelectric domain wall in KTiOPO4 are used to realize a single-shot noncollinear frequency-resolved optical gating arrangement for ultrashort pulse characterization with high spatial resolution. Cerenkov phase matching is utilized to achieve spectral dispersion and makes the same nonlinear crystal applicable over wide spectral ranges from the visible to the mid-infrared.

Keyword
Cerenkov counters, Diffraction gratings, Domain walls, Ferroelectric devices
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-6630 (URN)10.1364/OL.32.001545 (DOI)000247756600067 ()2-s2.0-39749093179 (Scopus ID)
Note
QC 20100831Available from: 2006-12-15 Created: 2006-12-15 Last updated: 2010-12-06Bibliographically approved

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