Semiconductor Saturable Absorber Mirrors | previous | next | feedback |
You can buy SESAMs from:
- BATOP, offering various types of saturable absorber mirrors for mode locking and noise suppression
- RefleKron, offering customized semiconductor saturable absorbers for mode-locked and Q-switched lasers
Ask RP Photonics for advice on how to use SESAMs for Q switching or mode locking of any kind of laser. Dr. Paschotta has a very deep experience in this field.
(Acronym: SESAM)
Definition: saturable semiconductor absorber devices acting as nonlinear mirrors
A semiconductor saturable absorber mirror (SESAM) (or simply SAM = saturable absorber mirror) is a mirror structure with an incorporated saturable absorber, all made in semiconductor technology. Such devices are mostly used for the generation of ultrashort pulses by passive mode locking of various types of lasers.
Typical Structure of a SESAM
Typically, a SESAM contains a semiconductor Bragg mirror and (near the surface) a single quantum well absorber layer. The materials of the Bragg mirror have a larger bandgap energy, so that essentially no absorption occur in that region. Such SESAMs are sometimes also called SBRs. For obtaining a large modulation depth, as required e.g. for passive Q switching, a thicker absorber layer can be used. Also, a suitable passivation layer on the top surface can increase the device lifetime.
Figure 1: Structure of a typical SESAM for operation around 1064 nm. On a GaAs substrate, a GaAs/AlGaAs Bragg mirror is grown. Within the top layers, there is an InGaAs quantum well absorber layer, which may e.g. be 10 nm thick.
The penetration of the optical field into a SESAM can be calculated with the same matrix method as applied to other types of dielectric mirrors. Of particular importance is the optical intensity in the region where the saturable material is placed. This influences the modulation depth as well as the saturation fluence (see below). However, the design of the structure also influences the bandwidth and the chromatic dispersion.
Figure 2: Refractive index profile and optical intensity distribution within a SESAM with anti-resonant design, as is often used. The intensity distribution has a maximum at the absorber position (indicated by the vertical gray line).
There are also some more exotic types of semiconductor saturable absorbers, which can be based e.g. on quantum dots embedded in glass [5,6] or on carbon nanotubes [7].
Resonant and Nonresonant SESAM Designs
As there is a Fresnel reflection at the semiconductor-air interface, this together with the Bragg reflection leads to a cavity effect (resonance effect). In most cases, this cavity is designed to be antiresonant for the operation wavelength of the device (see also Figure 2). Such devices exhibit a relatively broad wavelength range with a more or less constant degree of saturable absorption and with small chromatic dispersion. Compared with devices with an anti-reflection coating, antiresonant designs have a lower field penetration into the absorber and thus a lower modulation depth as well as a higher saturation fluence and higher damage threshold. (The latter, however, is no advantage, because higher pulse fluences are needed to saturate such a device.)
In relatively rare cases, resonant designs are used. These have a higher modulation depth and lower saturation fluence, and a smaller range of operation wavelengths.
By varying the material composition as well as certain design parameters, the macroscopic parameters of a SESAM (in particular, the operation wavelength, the modulation depth, the saturation fluence, and the recovery time) can be tailored for operation in very different regimes.
Physical Mechanism of Saturable Absorption
Figure 3: Excitation and relaxation of carriers in a semiconductor.
The saturable absorption is related to an interband transition: the energy of absorbed photons is transferred to electrons, which are brought from the valence band to the conduction band. There is first some quite rapid thermalization relaxation within the conduction and valence band within e.g. 50-100 fs, and later on (often on a time scale of tens or hundreds of picoseconds) the carriers recombine, often with the aid of crystal defects.
For low optical intensities, the degree of electronic excitation is small, and the absorption remains unsaturated. At high optical intensities, however, electrons can accumulate in the conduction band, so that initial states for the absorbing transition are depleted while final states are occupied (→ Pauli blocking). Therefore, the absorption is reduced. After saturation with a short pulse, the absorption recovers, first partially due to intraband thermal relaxation, later on completely via recombination.
Figure 4: Reflectivity change of a semiconductor saturable absorber, hit by a short pulse at t; = 0. Part of the reflectivity change disappears rather quickly after the pulse, whereas another part takes many picoseconds to recover. Such curves can be recorded with pump-probe measurements.
Important Properties of SESAMs
The most important characteristics of a SESAM as used e.g. for passive mode locking or Q switching are the following:
- The modulation depth is the maximum nonlinear change of reflectivity. It depends on the thickness of the absorber, on the material, the optical wavelength, and the degree of optical field penetration into the absorber structure.
- The saturation fluence is the fluence of an incident short pulse which is required for causing significant absorption saturation. It depends on the absorber material, the wavelength, and on the field penetration into the absorber structure. Also, there can be a "roll-over" of the saturation curve (i.e., a reduction in reflectivity for high fluence values), which can be caused by two-photon absorption (for sub-picosecond pulses) or by other effects.
- The recovery time is the exponential time constant of absorption recovery after a saturating pulse. It is normally between a few picoseconds and hundreds of picoseconds. Note, however, that the recovery is often not of exponential form (see Figure 4). The recovery time is strongly influenced by the defect density in the absorber, and possibly in nearby structures.
- There are normally some nonsaturable losses, which are unwanted, since they only lead to device heating while not contributing to the pulse shaping. Generally, nonsaturable losses tend to be higher for SESAMs with a larger modulation depth and faster recovery, but there are exceptions.
Additional details concern the lateral homogeneity, the optical damage threshold and device lifetime. The latter is often difficult to assess and strongly depends on the operation conditions. Furthermore, it can be important that a SESAM can tolerate a certain heat load. Thermal issues become important not only at high average power levels, but also for operation with very high pulse repetition rates.
Semiconductor Materials for SESAMs
The by far most common type of SESAM is used in lasers emitting in the 1-μm wavelength region. Here, the saturable absorber is an InGaAs quantum well (or sometimes multiple quantum wells), where the indium content is adjusted to achieve an appropriate value of the bandgap. The mirror structure is based on GaAs and AlAs, grown on a gallium arsenide wafer. The lattice mismatch of InGaAs on GaAs and AlAs causes significant compressive strain in the absorber layer, and particularly for high indium content this can cause the formation of defects. The effect of defects may even be helpful, as it reduces the recovery time and may thus allow for shorter pulses or better pulse stability in a mode-locked laser. The defect concentration is therefore often increased by low temperature growth of the absorber layer. For too low growth temperature and/or a high indium content, however, nonsaturable losses can become too high. The recovery time may also be reduced by bombardment with fast ions after growth (→ ion implantation). Partial annealing of defects at some elevated temperature can help to find a better compromise between nonsaturable losses and recovery time.
For use at shorter wavelengths, e.g. for passive mode locking of titanium-sapphire lasers emitting around 800 nm, GaAs quantum wells can be used. The use of GaAs then has to be avoided in the mirror structure; it is common to use a Bragg mirror made of AlGaAs/AlAs. For very short pulse durations, the reflectivity bandwidth of a Bragg mirror is not sufficient; in such cases, special broadband SESAM designs containing a metallic mirror are sometimes used.
At longer wavelengths such as the bands around 1.3 μm or 1.5 μm, InGaAs quantum wells can still be used, but they then have a very high built-in strain. Therefore, GaInNAs (dilute nitride) absorbers have been developed, which allow for very low nonsaturable losses. It is also possible to use indium phosphide-based absorbers in devices grown on InP wafers. Various types of Bragg mirrors are used in the 1.5-μm region, partially depending on the type of absorber layer.
Dispersive SESAMs
Although most SESAMs exhibit only moderate amounts of chromatic dispersion for reflected light, dispersion of any sign can be engineered into a SESAM via the multilayer structure. Such dispersive SESAMs may then serve the purpose of dispersion compensation in a laser resonator, in addition to the function of a passive mode locker. However, such methods have only relatively rarely been applied, mostly because the need to control dispersion introduces certain design conflicts. For example, the wanted dispersion may only appear in a quite limited optical bandwidth, and wavelength-dependent losses of the device may drive the laser to operate outside that bandwidth. Also, it is quite restricting to work with SESAMs which offer certain fixed combinations of saturable absorption and dispersion.
Applications of SESAMs
SESAMs are widely used for passive mode locking of lasers, particularly for solid-state bulk and fiber lasers. They work with a wide range of laser parameters and usually allow for reliable self-starting mode locking. They can be used even at very high output power levels of tens of watts, provided that the overall laser design allows them to be operated in the appropriate regime. Another application is passive Q switching e.g. of microchip lasers or fiber lasers.
A general condition for the successful use of SESAMs in lasers is the selection of a suitable SESAM design as well as the adjustment of a number of laser parameters, in particular the cavity mode size on the absorber. The use of a SESAM with inappropriate device and operation parameters often leads to problems in the form of various instabilities or SESAM damage.
SESAMs can also be used for certain methods of nonlinear filtering and signal processing, e.g. in the context of optical fiber communications.
Bibliography
| [1] | M. N. Islam et al., "Color center lasers passively mode locked by quantum wells", IEEE J. Quantum Electron. 25, 2454 (1989) |
| [2] | S. Tsuda et al., "Mode-locking ultrafast solid-state lasers with saturable-Bragg reflectors", IEEE J. Sel. Top. Quantum Electron. 2, 454 (1996) |
| [3] | U. Keller et al., "Semiconductor saturable absorber mirrors (SESAMs) for femtosecond to nanosecond pulse generation in solid-state lasers", IEEE J. Sel. Top. Quantum Electron. 2, 435 (1996) |
| [4] | I. D. Jung et al., "Semiconductor saturable absorber mirrors supporting sub-10-fs pulses", Appl. Phys. B 65 (2), 137 (1997) |
| [5] | P. T. Guerreiro and S. Ten, "PbS quantum-dot doped glasses as saturable absorbers for mode locking of a Cr:forsterite laser", Appl. Phys. Lett. 71 (12), 1595 (1997) |
| [6] | A. M. Malyarevich et al., "Glass doped with PbS quantum dots as a saturable absorber for 1-μm neodymium lasers", J. Opt. Soc. Am. B 19 (1), 28 (2002) |
| [7] | S. Y. Set et al., "Ultrafast fiber pulsed lasers incorporating carbon nanotubes", J. Sel. Top. Quantum Electron. 10 (1), 137 (2004) |
| [8] | O. Okhotnikov and M. Pessa, "Dilute nitride saturable absorber mirrors for optical pulse generation", J. Physics: Condensed Matter 16, S3107 (2004) |
| [9] | R. P. Prasankumar et al., "Design and characterization of semiconductor-doped silica film saturable absorbers", J. Opt. Soc. Am. B 21 (4), 851 (2004) |
| [10] | G. Paunescu et al., "In situ characterization of semiconductor saturable absorber mirrors in an operating Yb:KGW mode-locked laser", Opt. Lett. 30 (20), 2799 (2005) |
| [11] | A. Rutz et al., "Parameter tunable GaInNAs saturable absorbers for mode locking of solid-state lasers", J. Crystal Growth 301-302, 570 (2007) |
| [12] | M. Haiml et al., "Optical characterization of semiconductor saturable absorbers" Appl. Phys. B 79, 331 (2004) |
| [13] | S. Schön et al., "Dilute nitride absorbers in passive devices for mode locking of solid-state lasers", J. Crystal Growth 278, 239 (2005) |
| [14] | R. Grange et al., "Antimonide semiconductor saturable absorber for passive mode locking of a 1.5-μm Er:Yb:glass laser at 10 GHz", IEEE Photon Technol. Lett. 18 (7), 805 (2006) |
| [15] | U. Keller, "Semiconductor nonlinearities for solid-state laser modelocking and Q-switching", in Semiconductors and Semimetals, Vol. 59A, edited by A. Kost and E. Garmire, Academic Press, Boston, 1999 |
See also: saturable absorbers, passive mode locking, self-starting mode locking, Q switching, pulse generation
Categories: photonic devices, pulses
This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) could become very valuable for your business!
