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IV. Laser Diode (LD) or Semiconductor Laser

IV. Laser Diode (LD) or Semiconductor Laser. Operation Mechanism Characteristics of LD LD Design (1) : control of electronic properties LD Design (2) : control of optical properties Advanced LD Structures Applications of LD. Introduction to the Semiconductor Laser.

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IV. Laser Diode (LD) or Semiconductor Laser

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  1. IV. Laser Diode (LD) or Semiconductor Laser • Operation Mechanism • Characteristics of LD • LD Design (1): control of electronic properties • LD Design (2): control of optical properties • Advanced LD Structures • Applications of LD

  2. Introduction to the Semiconductor Laser • LASER — Light Amplification by Stimulated Emission of Radiation • The Laser is a source of highly directional, monochromatic, coherent light. • The Laser operates under a “stimulated emission” process. • The semiconductor laser differs from other lasers (solid, gas, and liquid lasers): • small size (typical on the order of 0.1 × 0.1 × 0.3 mm3) • high efficiency • the laser output is easily modulated at high frequency by controlling the junction current • low or medium power (as compared with ruby or CO2 laser, but is comparable to the He-Ne laser) • particularly suitable for fiber optic communication • Important applications of the semiconductor lasers: • optical-fiber communication, video recording, optical reading, high-speed laser printing.high-resolution gas spectroscopy, atmospheric pollution monitoring.

  3. From LED to LD: Improvement by an Optical Cavity

  4. Laser Diode Stimulated radiation narrow linewidth coherent higher output power a threshold device strong temperature dependence higher coupling efficiency to a fiber LED Spontaneous radiation broad spectral incoherent lower output power no threshold current weak temperature dependence lower coupling efficiency Comparison between an LD and LED

  5. Stimulated Emission • Stimulation emission • The two basic requirements for a stimulated emission process to occur: (1) providing an optical resonant cavity to build up a large enough photon field • a very large photon field energy density (12) will enhance the stimulated emission over spontaneous emission (2) obtaining population inversion condition • under the population inversion condition (n2 > n1) the stimulated emission is to dominate over absorption of photons from the radiation field

  6. Optical Resonant Cavity • Optical resonant cavity • parallel reflecting mirrors to reflect the photons back and forth, allowing the photon energy density to build up. • The Fabry-Perot faces (cavity) • The reflecting ends of the laser cavity • The gain in photons per pass between the Fabry-Perot faces must larger than the losses (such as the transmission at the ends, scattering from impurities absorption, and others) • In the semiconductor laser, optical resonant cavity is made by cleaving. • Cleave the oriented sample (GaAs) along a crystal plane (110), letting the crystal structure itself provide the parallel faces.

  7. Resonant modes of a laser cavity • Longitudinal modes • determine the output-light wavelength • Lateral modes • leading to subpeaks on the sides of the fundamental modes, and resulting in “kinks” in the output-current curve. • suppressed by the “stripe-geometry” structure • Transverse modes • generating “hot spots” • suppressed by “thin active layer “ design • Suppressing lateral and transverse mode is necessary to improve the performance of lasers. • Single-mode laser: the laser operates in the fundamental transverse and lateral modes but with several longitudinal modes. • Single-frequency laser: the laser operates in only one longitudinal mode.

  8. Longitudinal modes of a laser cavity • For stimulated emission, the length L of the cavity must satisfy the condition (for resonant): m [ 0/ 2n] = L or m 0 = 2 n L • m is an integral number and is the refraction index in the semiconductor corresponding to the wavelength 0 (n is generally a function of 0) • The separation 0 between the allowed modes in the longitudinal direction is • Since dn/d0 is very small,  002 / 2Ln (for m = 1) • For typical GaAs laser of 0 = 0.94 nm, n = 3.6 and L = 300 m,  0 = 4 Å.

  9. Population Inversion (1) • Forward biasing a p-n junction formed between degenerate semiconductors under high-injection condition.  Population inversion appears about the transition region • The condition necessary for population inversion is (EFC - EFV) > Egwhere EFC, andEFV are the quasi-Fermi levels • In the figure shows then energy diagrams of a degenerate p-n junction (a) at thermal equilibrium (b) under forward bias (c) under high-injection condition

  10. Population Inversion (2) (a) incoherent (spontaneous) emission  EFC - EFV > h> Eg (b) laser modes at threshold  There modes correspond to successive numbers of integral half-wavelengths fitted within the cavity (c) dominant laser mode above threshold  h= Eg

  11. Carrier and Optical Confinement • Carrier and Optical Confinement can be obtained by using the heterostructure design in the LD • Carrier Confinement • reduce the threshold current density • laser can operate continuously at room temperature • Optical Confinement • confinement factor  : the ratio of the light intensity within the active layer to the sum of light intensity both and outside the active layer  = 1 - exp ( - C n d ) n :the difference in the reflective index d :the thickness of the active layer • the larger the n and d are, the higher the  will be • Optical confinement provides effective wave-guide for optical communication

  12. Homojunction and Heterojunction Laser • Homojunction Laser • pulse mode output • large threshold current density • operated at low temperature • broad spectral width of output light • Improvement Heterojunction Laser • Heterojunction Laser (1) Single-Heterojunction Laser (SH Laser) (2) Double-Heterojunction Laser (DH Laser) (3) Stripe-geometry DH Laser (4) Single quantum well (SQW) Laser (5) Multiple quantum well (MQW) Laser (6) Strained layer superlattice (SLS) structure

  13. Double-Heterojunction (DH) Laser

  14. Threshold Current Density • Gain (g) • the incremental optical energy flux per unit length • Threshold Gain • the gain satifies the condition that a light wave makes a complete traveral of the cavity without attenuation •  is the confinement factor,  is the loss per unit length, L is the length of the cavity, R is the reflectance of the ends of the cavity • Threshold Current Density(Jth) • the minimum current density required for lasing to occur • To reduce Jth, we can increase , , L, R and reduce d, 

  15. Characteristics of the DH laser • Threshold current density vs. active layer thickness • The threshold current density decreases with decreasing d, reaches a minimum, and then increases. The increase of Jth at very narrow active thickness is caused by poor optical confinement. • Output power vs. diode current • The light-current characteristics is quite linear above threshold. • Temperature dependence • The threshold current increases exponentially with temperature  Jth ~ exp [ T/T0]

  16. Emission Spectra of the typical DH laser • Emission spectra of a perfect laser • above the threshold, the laser may approach near-perfect monochromatic emission with a spectra width in the order of 1 to 10 Å. • High-resolution emission spectra(of a typical stripe-geometry DH laser) • Sub-peaks, which are evenly spaced with a separation of  = 7.5 Å, appear in the spectra. belong to the longitudinal modes. • Because of these longitudinal modes, the stripe geometry laser is not a spectrally pure light source for optical communication.

  17. Design considerations for laser diode performance • Low threshold current • low threshold can be generated by electronic devices which can be modulated at high speed to provide a high speed modulation in the output (1) reducing the active layer thickness (d) ↣Quantum-Well (~ 50 - 100 Å), Strain Quantum-Well (2) N-doped active region (3) Stripe geometry • Lateral confinement • to avoid the “kink” effect, which produces noise in the optical transmitter • reduce the lateral dimension of the Fabry-Perot cavity • (1) Stripe geometry (Gain-guided cavity) (2) Buried heterostructures • Selective Optical Cavity • to reduce the laser linewidth • (1) Distributed Feedback (DFB) structures (2) Buried heterostructures

  18. Stripe Geometry Laser • Using the “gain-guided cavity” to carry out the lateral confinement • Advantages of a stripe geometry structure • Removing side peaks from the main modes by suppression of the lateral mode. • Reducing the threshold current • less stringent demands on fabrication (because of the smaller active volume and the greater protection offered by isolating the active region from an open surface along two sides) • Fundamental mode operation is valid for all stripe widths below 10 - 15 µm. • Different types of stripe-geometry structure: • oxide stripe • implantation • selective diffusion • Mesa stripe • buried heterostructures • ridge structures

  19. Single Frequency Laser • Single frequency lasers is desirable in the optical fiber communication system to increase the bandwidth of an optical signal. • This is because light pulses of different frequencies travel through optical fiber at different speeds thus causing pulse spread. • Dispersion mechanisms for a step-index fiber:(1) intermodal dispersion(2) waveguide dispersion(3) material dispersion • Dispersion effects can be minimized by using long wavelength sources of narrow spectral width (a single frequency laser) in conjunction with single mode fibers. • Methods to achieve the single frequency lasers: (1) Frequency Selective Feedback • External Grating, Distributed-Feedback (DFB), Distributed Bragg Reflector (DBR) (2) Coupled Cavity • Cleaved Coupled Cavity (C3) laser

  20. Distributed Feedback (DFB) Laser • In periodic structures, special effects occur when the wavelength of the wave approaches the wavelength of the periodic structure. In semiconductor crystals, this leads to bandgaps and Bragg reflections. • The wavelength selective periodic grating with a corrugated structure, made by E-beam lithography and RIE, is incorporated into to the laser. • The period of the grating is d = 2qB /2n where B is the Bragg wavelength give by where 0 is the oscillating wavelength • DFB lasers have been made with sawed end facets or with antireflection coating to suppress the Fabry-Perot modes. • The DFB laser’ main advantage is its very small temperature dependence.

  21. Distributed Bragg Reflector (DBR) Laser • In the DBR laser, the period reflecting mirror stack is placed outside the active lasing region. • The advantages of the DBR lasers: • high coupling efficiency between the active lasing region and the passive waveduide structures. • the wavelength of the output light is tunable. • The reflective index of the stack is alterable by current injection. • The wavelengths that get the highest feedback must satisfy B = 2 q (nr1 d1 + nr2 d2)where is a positive integer • The values of nr1 d1 and nr2 d2 can be altered electronically, therefore can have a certain degree of wavelength tunability

  22. Cleaved-Coupled-Cavity (C3) Laser • The C3 laser consists of two standard Fabry-Perot cavity laser diodes which are self-aligned and very closely coupled to form a two-cavity resonator. • Because the laser light has to travel through an additional cavity (modulator), the only radiation that is reinforced is at a wavelength that resonates both in the laser’s cavity and also in the modulator. • The two cavities can have their currents controlled independently and this is the main advantage of the C3 laser.

  23. Quantum Well Laser • If the thickness of the active region Ly is made small enough (Ly ~ the “de Broglie wavelength” = h/p < 500Å, depending on the materials for GaAs, Ly ~ 20 nm), the carriers are confined in a finite potential well in which the energy band splitting into a “staircase” of discrete levels (the quantization effect) • E-h recombination can only occur with “n = 0 transition” in the quantum well. • In a quantum well (QW), a large number electrons all of the same energy can recombine with a similar block of holes. • Hence, a QW laser should gives a much narrower output wavelength, unlike the other lasers with the bulk effect, where recombining carriers are distributed in energy over a parabolically varying density of states

  24. Multiple Quantum Well (MQW) Laser • Several single quantum wells are coupled into a “multiple quantum well (MQW)” structure. • The significantly reduced temperature sensitivity of MQW lasers has been related to the staircase density of states distribution and the distributed electron and photon distributions of the active region. • This optical confinement helps to contain the otherwise large losses from a narrow active region, leading to low threshold currents. • An MQW is the active region of a laser that can emit a single frequency at several different wavelengths, known as a multiple array grating integrated cavity (MAGIC) laser.

  25. Graded Index Separate Confinement Heterostructure (GRINSCH) Laser • GRaded INdex Separate Confinement Heterostructure (GRINSCH) Laser • A narrower carrier confinement region (d) of high recombination is separated from a wider optical waveguide region • Optical confinement can be optimized without affecting the carrier confinement • GRINSCH-SQW and GRINSCH-MQW • The threshold current for a GRINSCH is much lower than that of a DH laser • For a standard DH laser, both mirror and absorption losses increase rapid for thin active region, leading to very high threshold current.

  26. GRINSCH Laser

  27. Vertical Cavity Surface Emitting Laser (VCSEL) • The structure of an VCSEL is very much like a standard heterojunction LED. • Advantages of the VCSEL: • the possibility of single frequency operation due to the short cavity • the removal of the fragile cleavage process that creates the end mirrors in a standard laser. • The success of the VCSEL depends on incorporating high reflectivity mirrors in the structures • The incorporations of DBR and MQW structures highly improve the performance of the VCSEL. • Various DBRs in the VCSEL: • crystalline BRD • amorphous DBR stacks • MgF/ZnSe DBR

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