Distributed Bragg reflector laser

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Photomicrographs of a DBR laser DBR laser schematic.jpg
Photomicrographs of a DBR laser

A distributed Bragg reflector laser (DBR) is a type of single frequency laser diode. Other practical types of single frequency laser diodes include DFB lasers and external cavity diode lasers. A fourth type, the cleaved-coupled-cavity laser has not proven to be commercially viable. VCSELs are also single frequency devices. [1] The DBR laser structure is fabricated with surface features that define a monolithic, single mode ridge waveguide that runs the entire length of the device. A resonant cavity is defined by a highly reflective DBR mirror on one end, and a low reflectivity cleaved exit facet on the other end. Within the cavity is a gain ridge portion, where current is injected to produce a single spatial lasing mode. The DBR mirror is designed to reflect only a single longitudinal mode. As a result, the laser operates on a single spatial and longitudinal mode. The laser emits from the exit facet opposite the DBR end. The DBR is continuously tunable over approximately a 2 nm range by changing current or temperature. The temperature coefficient is approximately 0.07 nm/K, and the current coefficient is approximately .003 nm/mA. [2] DBR lasers are stable, low noise optical sources. When operated with a low noise power supply at constant temperature, edge emitting DBR lasers have a linewidth of less than 10 MHz. Power levels typically can run up to several hundred milliwatts.

A distributed Bragg reflector (DBR) is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with varying refractive index, or by periodic variation of some characteristic of a dielectric waveguide, resulting in periodic variation in the effective refractive index in the guide. Each layer boundary causes a partial reflection of an optical wave. For waves whose vacuum wavelength is close to four times the optical thickness of the layers, the many reflections combine with constructive interference, and the layers act as a high-quality reflector. The range of wavelengths that are reflected is called the photonic stopband. Within this range of wavelengths, light is "forbidden" to propagate in the structure.

Laser diode semiconductor laser

A laser diode, (LD), injection laser diode (ILD), or diode laser is a semiconductor device similar to a light-emitting diode in which the laser beam is created at the diode's junction. Laser diodes can directly convert electrical energy into light. Driven by voltage, the doped p-n-transition allows for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation, in the form of an emitted photon is generated. This is spontaneous emission. Stimulated emission can be produced when the process is continued and further generate light with the same phase, coherence and wavelength.

A distributed feedback laser (DFB) is a type of laser diode, quantum cascade laser or optical fiber laser where the active region of the device contains a periodically structured element or diffraction grating. The structure builds a one-dimensional interference grating and the grating provides optical feedback for the laser. This longitudinal diffraction grating has periodic changes in refractive index that cause reflection back into the cavity. The periodic change can be either in the real part of the refractive index, or in the imaginary part. The strongest grating operates in the first order - where the periodicity is one-half wave, and the light is reflected backwards. DFB lasers tend to be much more stable than Fabry-Perot or DBR lasers and are used frequently when clean single mode operation is needed, especially in high speed fiber optic telecommunications. Semiconductor DFB lasers in the lowest loss window of optical fibers at about 1.55um wavelength, amplified by Erbium-doped fiber amplifiers (EDFAs), dominate the long distance communication market, while DFB lasers in the lowest dispersion window at 1.3um are used at shorter distances.

DBR lasers are often confused with DFB lasers. [3] Both exhibit narrow linewidth and stable single frequency operation. However, the location of the feedback element (the grating) causes the DBR and the DFB to have distinct operational characteristics. Because the grating is distributed all along the gain region in the DFB, the grating and gain region experience similar conditions as the device is tuned with current and temperature. The DFB can exhibit a continuous tuning range of 2 nm or more. However, over a sufficiently long current or temperature range, the emitted wavelength will suddenly jump to a longer wavelength (red shift), [4] leaving a gap in the tuning range.

Because the DBR laser has a passive grating region, its tuning characteristic is different from that of the gain region. Increasing current in the gain region causes a red shift in laser output due to heating. The reflectivity curve of the passive grating does not change. As a result, the grating will experience loss of reflectivity at the longer wavelengths, and eventually will induce a discontinuous blue shift in the wavelength to find a higher gain mode. The blue shift ensures that the wavelength characteristic will repeat itself with increasing temperature or current, and no gaps will occur in the tuning. [5]

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Dye laser

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Vertical-cavity surface-emitting laser

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Tunable diode laser absorption spectroscopy (TDLAS) is a technique for measuring the concentration of certain species such as methane, water vapor and many more, in a gaseous mixture using tunable diode lasers and laser absorption spectrometry. The advantage of TDLAS over other techniques for concentration measurement is its ability to achieve very low detection limits. Apart from concentration, it is also possible to determine the temperature, pressure, velocity and mass flux of the gas under observation. TDLAS is by far the most common laser based absorption technique for quantitative assessments of species in gas phase.

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Here, is a list of initialisms and acronyms used in laser physics, applications and technology.

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Output coupler

An output coupler (OC) is the component of an optical resonator that allows the extraction of a portion of the light from the laser's intracavity beam. An output coupler most often consists of a partially reflective mirror, allowing a certain portion of the intracavity beam to transmit through. Other methods include the use of almost-totally reflective mirrors at each end of the cavity, emitting the beam either by focusing it into a small hole drilled in the center of one mirror, or by redirecting through the use of rotating mirrors, prisms, or other optical devices, causing the beam to bypass one of the end mirrors at a given time.

A Random Laser (RL) is a laser in which optical feedback is provided by scattering particles. As in conventional lasers, a gain medium is required for optical amplification. However, opposite to Fabry-Perot cavities and Distributed FeedBack lasers, neither reflective surfaces nor distributed periodic structures are used in RLs, as light is confined in an active region by diffusive elements that either can or cannot be spatially distributed inside the gain medium.

A superluminescent diode is an edge-emitting semiconductor light source based on superluminescence. It combines the high power and brightness of laser diodes with the low coherence of conventional light-emitting diodes. Its emission band is 5–700 nm wide.

Laser linewidth is the spectral linewidth of a laser beam.

In physics, a high contrast grating is a single layer near-wavelength grating physical structure where the grating material has a large contrast in index of refraction with its surroundings. The term near-wavelength refers to the grating period, which has a value between one optical wavelength in the grating material and that in its surrounding materials.

References

  1. Hecht, Jeff (1992). The Laser Guidebook (Second ed.). New York: McGraw-Hill, Inc. pp. 317–321. ISBN   0-07-027738-9.
  2. "Wavelength Tuning in DBR Lasers". www.photodigm.com. Retrieved 2 December 2014.
  3. "Distributed Feedback Lasers". RP Photonics Encyclopedia. Retrieved 27 August 2014.
  4. Klehr, A.; Wenzel, H.; Brox, O.; Erbert, G.; Nguyen, T-P.; Trankle, G. (2009). "High power DFB lasers for D1 and D2 rubidium absorption spectroscopy and atomic clocks". Proc. SPIE. 7230: 72301I-1–8. doi:10.1117/12.805858.
  5. Spencer, John; Young, Preston. "Photodigm Applications Note: Contrasting the Photodigm DBR Laser Diode Architecture with Competing DFB Designs". photodigm.com. Retrieved 27 August 2014.