Mode-locked Lasers >> |
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A mode-locked laser is a laser to which the technique of active or passive m ode locking is applied, so that a periodic train of ul trashort pulses is emitted. See the article on m ode locking for more details on mode-locking techniques; the present article focuses more on the lasers themselves. The article on ultrafast lasers also gives some idea about current developments in ultrashort pulse generation.
As ultrashort pulses have a certain bandwidth , mode-locked lasers for short pulses (particularly in the sub-picosecond region) require a gain medium with a large gain bandwidth . Other desirable features are a not too high nonlinearity and chrom ati c
dispersion , and (particularly for passive mode locking) high enough laser cross sections in order to avoid Q-switching instabilities.
Types of Mode-locked Lasers
The following types of lasers are attractive for mode locking:
∙ In the 1970s, dye lasers were routinely used, which were pumped with argon ion lasers . Laser dyes have a broad gain bandwidth , allowing for very short pulses. However , dye lasers have been largely replaced with solid-state lasers once these were able to deliver similar or better performance.
∙ Solid-statebulk lasers , based on ion-doped crystals or glasses , are today the dom inant type of mode-locked lasers. They allow for very short pulses, very high pulse energies and/or average output powers, high or loss pulse repetition rates, and high pulse quality. Some record achievem
ents are listed below.
Figure 1: Resonator setup of a typical femtosecond mode-locked solid-state bulk laser with low or medium output power. The gain medium can be made of a crystal or of glass . A prism pair is used for dispersion compensation, and passive mode locking is achieved with a SESAM .
∙ Fiber lasers can also be m ode-lock ed for generating very short pulses with potentially cheap setups. See the arti cle on m ode-locked fiber lasers for more details. High output powers are typically not achieved directly, but by using fiber amplifiers . The achieved pulse durations of ultrafast fiber lasers are often limited by nonlinearities or by higher-order dispersion , rather than by the gain bandwidth.
∙ Sem iconductor lasers can be built as m ode-locked diode lasers , m ostly for applications in optical fiber comm unications . More recently, optically pum ped passively m ode-lock ed VECSELs have been dem onstrated which can rival other solid-state lasers, particularly if a combination of relati vely high output power , a m ulti-gigahertz pulse repetition rate, and possibly a short pulse duration (a few picoseconds or less) is required.
Design Issues
The design of a mode-locked laser is generally a non-trivial task, and particularly so if extrem e param eter regions for the pulse param eters are targeted. There is a com plicated interplay of many effects, including dispersion and several nonlinear effects, and changing one design parameter often influences several others. (For example, in a soliton mode-locked laser a change in mode size in the laser crystal or of the cavity length changes the balance of nonlinearity and dispersion, and thus also the pulse duration.) As a result, it can be difficult to achieve simultaneously very short pulses, stable operation, and a high power efficiency . For given parameters of the gain medium , there can be certain restrictions on the achievable pulse parameters. A relatively trivial one is that a gain medium with a small gain bandwidth is not suitable for generating very short pulses. Certainly more surprising is e.g. the finding that mode-locked solid-state lasers have difficulties in combining a high pulse repetition rate with a high average output power, and that the additional requirement of generating sub-picosecond pulses m akes this trade-off even m uch m ore dem anding. Such const raints arise from a com bination of effects and issues such as Q-switched mode locking and other kinds of instabilities, details of pulse shaping, and limitations of saturable absorbers, and are also influenced by seem ingly totally unrelated issues such as the beam quality of the available pump source.
For such reasons, a very system ati c process of laser developm ent , based on a solid quantitative understanding of all the relevant physical details and on deep experience with typical limitations, is essential for efficient product development. A key point is to work out a detailed laser design and to check quantitatively a number of issues before engaging in experimental investigations. Without such preparations, there is a risk of getting into a combination of problems which can not simply be solved step by step. Some Special Achievements
Som e special achievements with passively mode-lock ed solid-state lasers are:
∙ The very shortest pulses wi th durations below 10 fs (few-cycle pulses ) are usually achieved with Kerr lens mode locking of a Ti:sapphire laser [6, 5].
∙ High average output powers of up to ≈ 80 W in sub-picosecond pulses [14] and pulse energies above 10 μJ have been obtained from passively m ode-locked thin-disk lasers.
∙ Very high pulse repetition rates have been obtained with passively mode-locked miniature bulk lasers [10, 15, 20] and also with harm onically mode-locked fiber lasers . Even higher values of > 1 THz are possible with small laser diodes [4]. ∙ Various kinds of lasers (normally with high pulse repetition rates) have reached quantum-lim ited timing jitter performance, thus outperforming many high-quality electronic oscillators.
Figure 2: Miniature Er:Yb:glass laser setup for a pulse repetition rate of 50 GHz [15]. The cavity length is only 3 mm (from the output coupler to the SESAM). A modified setup allowed for even 100 GHz [20]. Higher Pulse Energies with Cavity Dumping
As explained in detail in the article on cavi ty dumping , a mode-locked laser can generate higher pulse energies of e.g. several microjoules at lower pulse repetition rates (e.g. 100 kHz or 1 MHz) by incorporation of a cavity dumper in the laser resonator. The basic principle is to form a high-energy pulse within the resonator while having low resonator losses, and then to couple out of the energy with the cavity dumper .
Typical Applications of Mode-locked Lasers
The following list gives some impression of the manifold appli cations of mode-locked lasers:
∙ The high pulse intensities are used for applications in material processing, such as microm achining, surface treatment, drilling holes, and three-dimensional laser prototyping.
∙ In the m edical dom ain, mode-lock ed lasers m ay again be used for a kind of m aterial processing, e.g. as a laser scalpel or in ophthalmology. However, there are also photochem ical effects used e.g. for certain skin treatm ents, and im portant applications in imaging.
∙ Various m ethods of im aging, m icroscopy and spectroscopy, as applied in different domains, greatly profit from short pulses for various reasons.
∙ Short pulses allow for time-resolved measurem ents, e.g. electro-optic sampling m easurem ents on integrated electronic circuits, or pum p –probe measurem ents on sem iconductor devices such as SESAMs .
∙ In the field of m etrology , m ode-locked lasers can be used for distance m easurem ents, but also in frequency metrology (time keeping) and other fields. ∙ A number of processes for nonlinear frequency conversion are greatly facilitated by the high peak powers of mode-lock ed lasers, and are important e.g. for laser projection displays.
∙ Other fields with a large potential are microwave, millim eter-wave and terahertz opti cs, and picosecond optoelectroni cs .
Mode-locked Lasers >> |
Feedback
A mode-locked laser is a laser to which the technique of active or passive m ode locking is applied, so that a periodic train of ul trashort pulses is emitted. See the article on m ode locking for more details on mode-locking techniques; the present article focuses more on the lasers themselves. The article on ultrafast lasers also gives some idea about current developments in ultrashort pulse generation.
As ultrashort pulses have a certain bandwidth , mode-locked lasers for short pulses (particularly in the sub-picosecond region) require a gain medium with a large gain bandwidth . Other desirable features are a not too high nonlinearity and chrom ati c
dispersion , and (particularly for passive mode locking) high enough laser cross sections in order to avoid Q-switching instabilities.
Types of Mode-locked Lasers
The following types of lasers are attractive for mode locking:
∙ In the 1970s, dye lasers were routinely used, which were pumped with argon ion lasers . Laser dyes have a broad gain bandwidth , allowing for very short pulses. However , dye lasers have been largely replaced with solid-state lasers once these were able to deliver similar or better performance.
∙ Solid-statebulk lasers , based on ion-doped crystals or glasses , are today the dom inant type of mode-locked lasers. They allow for very short pulses, very high pulse energies and/or average output powers, high or loss pulse repetition rates, and high pulse quality. Some record achievem
ents are listed below.
Figure 1: Resonator setup of a typical femtosecond mode-locked solid-state bulk laser with low or medium output power. The gain medium can be made of a crystal or of glass . A prism pair is used for dispersion compensation, and passive mode locking is achieved with a SESAM .
∙ Fiber lasers can also be m ode-lock ed for generating very short pulses with potentially cheap setups. See the arti cle on m ode-locked fiber lasers for more details. High output powers are typically not achieved directly, but by using fiber amplifiers . The achieved pulse durations of ultrafast fiber lasers are often limited by nonlinearities or by higher-order dispersion , rather than by the gain bandwidth.
∙ Sem iconductor lasers can be built as m ode-locked diode lasers , m ostly for applications in optical fiber comm unications . More recently, optically pum ped passively m ode-lock ed VECSELs have been dem onstrated which can rival other solid-state lasers, particularly if a combination of relati vely high output power , a m ulti-gigahertz pulse repetition rate, and possibly a short pulse duration (a few picoseconds or less) is required.
Design Issues
The design of a mode-locked laser is generally a non-trivial task, and particularly so if extrem e param eter regions for the pulse param eters are targeted. There is a com plicated interplay of many effects, including dispersion and several nonlinear effects, and changing one design parameter often influences several others. (For example, in a soliton mode-locked laser a change in mode size in the laser crystal or of the cavity length changes the balance of nonlinearity and dispersion, and thus also the pulse duration.) As a result, it can be difficult to achieve simultaneously very short pulses, stable operation, and a high power efficiency . For given parameters of the gain medium , there can be certain restrictions on the achievable pulse parameters. A relatively trivial one is that a gain medium with a small gain bandwidth is not suitable for generating very short pulses. Certainly more surprising is e.g. the finding that mode-locked solid-state lasers have difficulties in combining a high pulse repetition rate with a high average output power, and that the additional requirement of generating sub-picosecond pulses m akes this trade-off even m uch m ore dem anding. Such const raints arise from a com bination of effects and issues such as Q-switched mode locking and other kinds of instabilities, details of pulse shaping, and limitations of saturable absorbers, and are also influenced by seem ingly totally unrelated issues such as the beam quality of the available pump source.
For such reasons, a very system ati c process of laser developm ent , based on a solid quantitative understanding of all the relevant physical details and on deep experience with typical limitations, is essential for efficient product development. A key point is to work out a detailed laser design and to check quantitatively a number of issues before engaging in experimental investigations. Without such preparations, there is a risk of getting into a combination of problems which can not simply be solved step by step. Some Special Achievements
Som e special achievements with passively mode-lock ed solid-state lasers are:
∙ The very shortest pulses wi th durations below 10 fs (few-cycle pulses ) are usually achieved with Kerr lens mode locking of a Ti:sapphire laser [6, 5].
∙ High average output powers of up to ≈ 80 W in sub-picosecond pulses [14] and pulse energies above 10 μJ have been obtained from passively m ode-locked thin-disk lasers.
∙ Very high pulse repetition rates have been obtained with passively mode-locked miniature bulk lasers [10, 15, 20] and also with harm onically mode-locked fiber lasers . Even higher values of > 1 THz are possible with small laser diodes [4]. ∙ Various kinds of lasers (normally with high pulse repetition rates) have reached quantum-lim ited timing jitter performance, thus outperforming many high-quality electronic oscillators.
Figure 2: Miniature Er:Yb:glass laser setup for a pulse repetition rate of 50 GHz [15]. The cavity length is only 3 mm (from the output coupler to the SESAM). A modified setup allowed for even 100 GHz [20]. Higher Pulse Energies with Cavity Dumping
As explained in detail in the article on cavi ty dumping , a mode-locked laser can generate higher pulse energies of e.g. several microjoules at lower pulse repetition rates (e.g. 100 kHz or 1 MHz) by incorporation of a cavity dumper in the laser resonator. The basic principle is to form a high-energy pulse within the resonator while having low resonator losses, and then to couple out of the energy with the cavity dumper .
Typical Applications of Mode-locked Lasers
The following list gives some impression of the manifold appli cations of mode-locked lasers:
∙ The high pulse intensities are used for applications in material processing, such as microm achining, surface treatment, drilling holes, and three-dimensional laser prototyping.
∙ In the m edical dom ain, mode-lock ed lasers m ay again be used for a kind of m aterial processing, e.g. as a laser scalpel or in ophthalmology. However, there are also photochem ical effects used e.g. for certain skin treatm ents, and im portant applications in imaging.
∙ Various m ethods of im aging, m icroscopy and spectroscopy, as applied in different domains, greatly profit from short pulses for various reasons.
∙ Short pulses allow for time-resolved measurem ents, e.g. electro-optic sampling m easurem ents on integrated electronic circuits, or pum p –probe measurem ents on sem iconductor devices such as SESAMs .
∙ In the field of m etrology , m ode-locked lasers can be used for distance m easurem ents, but also in frequency metrology (time keeping) and other fields. ∙ A number of processes for nonlinear frequency conversion are greatly facilitated by the high peak powers of mode-lock ed lasers, and are important e.g. for laser projection displays.
∙ Other fields with a large potential are microwave, millim eter-wave and terahertz opti cs, and picosecond optoelectroni cs .