Distributed feedback laser

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Distributed feedback laser ( English , German laser with distributed feedback ), in German mostly just called DFB laser , are laser diodes in which the active material is periodically structured. The structures of changing refractive index form a one-dimensional interference grating or an interference filter ( Bragg mirror ). The interference leads to wavelength-selective reflection and forms the optical feedback of the laser . Bragg mirrors (DBR lasers) are closely related to the DFB lasers.

properties

DFB and DBR laser diodes have a lower threshold current limit and a better beam quality than conventional laser diodes that work according to the principle of the Fabry-Pérot laser and whose end faces act like a Fabry-Pérot interferometer . This minimizes the secondary modes that arise with the Fabry-Perot laser, which lead to dispersion effects in fiber optic cable transmission .

While conventional laser diodes oscillate in several longitudinal modes, DFB and DBR lasers only work in one longitudinal mode.

The spectral bandwidth of the DFB and DBR lasers is very small. The deviations from the set wavelength are less than 10 −7 . At a wavelength of 2 µm, corresponding to a frequency of approx. 150  T Hz, they are approx. 0.2 pm (0.0002 nm), corresponding to 15 MHz. Values ​​of 2 MHz can be achieved in the laboratory (for comparison: with conventional laser diodes the spectral bandwidth is approx. 1 to 4 nm).

DFB and DBR laser diodes are an inexpensive alternative to wavelength selection processes outside the laser crystal ( external cavity diode laser , ECDL), but do not achieve their even higher stability (less than 1 MHz). Frequency-stabilized DFB lasers are now also available in the infrared wavelength range .

While the Bragg structure in DFB laser diodes is located in the active zone (the amplification zone), in DBR lasers it is located outside the active zone, but in a waveguide integrated on the chip . Both principles can also be applied to fiber lasers .

Both DFB and DBR lasers can be detuned by changing temperature and current. On the one hand, this requires precise thermostatting for high wavelength stability, but on the other hand, it enables the wavelength to be changed or adjusted within a large range. For temperature control with simultaneous heat dissipation, thermoelectric coolers are used, which - with polarity reversal - can also heat.

Due to their accuracy, DFB and DBR lasers are used in DWDM systems ( dense wavelength division multiplex ) for precise length measurement, in optical spectroscopy ( Raman spectroscopy ), for the detection of trace gases (excitation of atomic and molecular resonances ) and used for testing / measuring glass fibers.

Since the refractive index of the semiconductor material depends on the electron density, the wavelength of DBR lasers can also be changed by a current through the Bragg zone. This type of wavelength control is much faster than influencing the temperature. Since the same current flows through the Bragg zone and the amplification zone in the DFB laser, its modulation causes a modulation of the amplitude and the wavelength or frequency at the same time. This rapid change in wavelength is called laser chirp.

For data transmission over long distances, wavelength modulation causes dispersion . Therefore, for higher transmission speeds> 1 Gbps, the DFB lasers are usually operated with constant current and the optical signal is only modulated by a downstream modulator. Electro-absorption (EA) modulators that are already integrated on the DFB chip can be used for the amplitude modulation. These combinations are then called EADFB lasers.

Web links

Individual evidence

  1. ↑ http://www.mdpi.com/1424-8220/10/4/2492/ (public reading: DFB Lasers Between 760 nm and 16 µm for Sensing Applications.)
  2. ↑ http://www.hanel-photonics.com/laser_diode_market_DFB_DBR.html (overview of the available DBR and DFB lasers)
  3. ↑ Examples of DFB and DBR lasers (Eagleyard company, spin-off of the Ferdinand Braun Institute Berlin)