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Diode Pumped Laser SystemsDiode Pumped Laser Systems

Contents: The DPLs 2 Characteristics of DPLss 3 Typical applicationss 4 Laser components s 5 DPL designss 6 Principless 7 Special versionss 8 Tools for laser designs 9 Commercial systemss 10 Future developmentss 11 Referencess 12

Background:Q-switched lasers 13

Mode locked lasers 14

Tunable lasers 15

Femtosecond lasers 16

DPL-componentss 17

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What’s so special about DPLs?

• DPL is a new generation of lasersIt is based on two major advancements:

• low cost, reliable semiconductor diodes• improved quality of optical nonlinear components

A whole new family of lasers has emerged:• single-chip lasers, e.g. IR or green• medium size all-purpose lasers, IR and green• high power IR lasers for e.g. medical applications• very high power IR lasers for machining• DPLs integrated into other systems;

– Q-switched lasers

– mode locked lasers

– tunable lasers

– femtosecond lasers

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Characteristics of DPLs

• DPLs are smaller, cheaper, more robust and use less power than the older generations of lasersIts characteristic properties are:

• low quantum defect• good spectral match between pump and laser• only few parasitic interactions between laser light and excited state

population• low thermal distortion gives good quality laser light• high efficiency• low cost• long life• mechanically robust• small size• excellent laser beam properties

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Some typical applications

• DPLs are used in hundreds of applications. Some of them are:

• pointing and aligning• laboratory work• medical diagnostics and therapy• dentistry• surgery• light machining (cutting and drilling)• heavy machining (cutting and welding)• spectroscopy• printing industry• environmental sensing• 3-D prototyping• inside industrial products (e.g. CDs, printers)

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Components of DPLs

• The essential components of a DPL are:• the pump laser• the collimating and focusing optics• the laser crystal• the temperature control and heat removal system• the laser resonator• internal nonlinear components• the mechanical structure and mounts

• See more here!

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Different designs

• DPLs have been constructed in a great number of ways:

• axially pumped

• side pumped

• zig-zag pumped

• radially pumped

• diffusely pumped

• thin-disc version

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Principle of DPL

• Three versions of DPLs are shown here: A linear (standing wave) laser , a “linear” ring laser and a prism ring laser.

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Diode pumped prism ring laser with composite crystal

Output couplerAR for pumpHR for circulating field

/4-p late /4-p laterh pol.

lh pol.

Linear DPL“Linear” ring DPL

Prism ring DPL

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Different special versions

• Here we show a few of the most famous designs:

• the single-chip laser

• the NPRO

• the bow-tie laser

• the thin-disk laser

• a very high power laser

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Design tools for lasers

• In our lab we have developed a range of tools for the design and diagnostics of DPLs:

• Mathematica model for laser performance

• ABCD Gauss beam analysis

• Jones matrix polarization analysis

• ray tracing of pump light and absorption

• thermal profiles of laser crystal

• beam propagation analysis of resonator losses

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Commercial DPL systems

• Some of the better known commercial DPL-systems are:The Coherent “Verdi”-serie 5 - 10 W CW 532 nm

The Lightwave NPRO 100-200 mW 532 nm

Spectra-Physics Millennia 2 - 10 W 532 nm

Spectra Diode Labs tunable SDL

CASIX 10 - 200 mW 532 nm

• Some remarkable, but less well known commercial DPL-systems are:The Torsana-laser (Torsana Laser Technologies A/S)

ASAH Medico A/S

GIGA A/S

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Future prospects for the diode-pumped laser

• R. S. Petersen said: It is difficult to forecast - especially the future, but these facts may be used as a basis for anyone’s guess:

• semiconductor laser prices will continue to go down • sdl’s will continue to improve performance and life• optical components will continue to improve• new nonlinear materials with improved performance will appear • new design ideas will be invented

however, competing systems may take over:• fibre lasers• Upconversion/down conversion lasers • polymer lasers• improved performance semiconductor lasers including nonlinear

optics

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References

• Laser monographs:• O. Svelto, “Principles of Lasers”, Plenum Press 1998.

• Proceedings• Advanced Solid-State Lasers 1999, TOPS Vol. XIX

• Catalogues• Best found on the internet

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Q-switched laser

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Q-switching by electrooptic effect.When the voltage is switched on, the electrooptic modulator turns the polari-zation and the light may pass through the Glan-Thomson prism. The laser emits the stored energy in a pulse. The pulse typically lasts a few nanoseconds.

Switching the Q stands for quickly reducing the losses in the laser cavity. The laser will emit the energy stored in the excited state population as a giant pulse.

Q-switching methods:– electrooptic (Pockel’s cell)

– acoustooptic (Bragg cell)

– mechanical (e.g. rotating mirror)

– piezoelectric (coupled cavity)

– passive Q-switching (saturable absorber)

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Mode locked laser

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Mode locking by means of fast internal shutter. Placed at one end, the pulse repetition rate is c/2L. At other locations different pulse rates result, see the figure. The pulse typically lasts a few picoseconds. In the ring laser the position of the shutter is irrelevant.

Mode locking a laser means using an internal switch to modulate the light synchronously with the round trip time.

Example:

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Tunable laser

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The tilted etalon (a plane parallel glass plate) transmits only widely spaced frequency bands, see lower figure. Thus the laser frequency with the highest gain is selected. When the etalon is tilted, the transmission peaks continually change frequency selecting new laser frequencies. However, the laser frequency hops between resonance frequencies. To obtain continuous tuning, the laser resonator must also be adjusted, e.g. by a piezo-mount for one of the mirrors.

A gain medium with a wide gain profile can be used for a tuned laser. A frequency selective element is inserted into the cavity.

Example: Tilted etalon.

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Femtosecond laser

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A very wide gain medium can be made to lase in very short pulses. The technique applies sophisticated pulse stretching and pulse compression.An example from ASSL 1999:

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DPL components

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A diode-pumped solid-state laser contain typically the following components.

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1. Semiconductor pump laser2. Focusing optics for the pump laser3. Laser resonator mirror4. Laser crystal in cooled mount5. Nonlinear crystal in temperature stable oven.6. Laser output mirror

The pump laser is cooled to avoid damage and to tune pump frequency. The laser crystal is cooled to improve efficiency and to avoid damage. Cooling may be by running water through copper mounting plates and/or by thermo-electric cooling (Peltier elements).The nonlinear crystal is phase matched by temperature control. The temperature should be stable and uniform to within 0.01 C.

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DPL technology

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DPL technology

back Lasers an Introduction Diode pumped lasers Nonlinear optics Waveconversion systems

10380 Biomedical Optics

DPL technology

back Lasers an Introduction Diode pumped lasers Nonlinear optics Waveconversion systems

10380 Biomedical Optics

DPL technology

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back Lasers an Introduction Diode pumped lasers Nonlinear optics Waveconversion systems

10380 Biomedical Optics

DPL technology

10445 Biomedical Optics

back Lasers an Introduction Diode pumped lasers Nonlinear optics Waveconversion systems

10380 Biomedical Optics

DPL technology

back Lasers an Introduction Diode pumped lasers Nonlinear optics Waveconversion systems