SAM – saturable absorber mirror

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The reflection bandwidth of the SAM has to be larger than the pulse bandwidth. In case ofa SAM with an underlying Bragg-mirror the reflection bandwidth is determined by the ratioof the refractive indices n H /n L of the layers in the thin film stack. More about Braggmirrors...The relative spectral width w = ∆λ/λ of the high reflectance zone of a conventionalsemiconductor AlAs/GaAs thin film stack is about 0.1. Therefore the width of the highreflection zone of an AlAs/GaAs Bragg-mirror with a center wavelength of 1000 nm is about100 nm. From the tables above this results in a minimum pulse duration of about 20 fs. Forshorter pulses other mirror types, for instance dielectric or metallic mirrors has to be used.7.3 Absorption bandwidthAn ideal SAM has a constant saturable absorption for all wavelength of the pulse spectrum.In case of a 5 fs pulse the width of this wavelength interval is some hundreds ofnanometers.SeongKyeong Photonics 12 info@skphotonics.com
Refractive index > GaAs | AlAs | Al x Ga 1-x As | In x Ga 1-x As> Devices > Bragg mirror | SAM | RSAM | SANOS | SOC | PCASOC - Saturable Output Coupler> Contents● Aim of SOC● SOC parameters● Transmittance● Saturable absorption● Relaxation time● Reflectance bandwidth● Absorbance> Aim of SOCUsing a saturable output coupler (SOC), a selfstarting,passively mode-locked diode pumpedsolid-state laser with a very simple layout canbe arranged. A SOC is a combination of thewell known saturable absorber mirror (SAM)with an output coupler.In case of using a SOC instead of a SAM forpassive mode-locking the optical pump powercan be provided through the end mirror of thelaser cavity.Mode locking produces stable and coherentpulsed lasers by forcing the faces of themodes to maintain constant values relative toone another. These modes then combinecoherently. Fundamental mode-locking resultsin a train of optical pulses with a period of 2L/c, where L is the cavity length and c thespeed of the light in free space.Mode locking occurs when laser losses are modulated at a frequency equal to the reverse of the pulse periodc/2L. The SOC is a passive mode locking device without the use of an external drive signal, which spontaneouslylocks the modes with fast material response time.> SOC parametersA SOC consists of a Bragg-mirror on a semiconductor wafer like GaAs, covered by an absorber layer and a moreor less sophisticated top film system, determining the saturable loss. The back side of the SOC wafer isantireflection coated.The most important parameters of a SOC are:●●●●●transmittancesaturable absorptionrelaxation timesaturation fluencereflectance bandwidth.> TransmittanceSeongKyeong Photonics 13 info@skphotonics.com
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SAM – saturable absorber mirror

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