4. Etch and remove photoresist:
5.2 PUMPING METHODS
In terms of its usefulness as a laser medium the narrow linewidths signify a large stimulated emission cross section and a low pumping threshold. However, since the absorption bands are narrow, then the pump bands must be well matched for efficient transfer of pump energy into them. In optical pumping the light from the pump source is absorbed by the active material and the atoms are then pumped into the upper pump level. A laser beam may be used to pump another laser (laser pumping). The directional properties of a laser beam make it very convenient for pumping another laser, and special pumping chambers are not then required. The absorption spectrum of Nd-doped materials does not vary significantly from one host to another, since the absorption is a result of electron transitions between the inner shells of the atom. Thus the absorption spectrum of Nd:MgO:LiNbOg would not be expected to be much different from that of Nd:YAG. Figure 5.4 is an absorption spectrum of a proton exchange channel guide, of length 1 cm, in Nd:MgO:LiNbOg, with the electric field parallel to the c-axis. The spectrum was obtained with a white-light source spectrometer. Firstly, as a reference a transmission spectrum was obtained with
coupling fibres butted directly to each other. Then the sample was placed between the coupling fibres and the output plotted. The data from these two measurements was used to obtain the attenuation of the incident input signal. After subtracting the non wavelength dependent losses (coupling and scattering loss) the absorption spectrum for the waveguide was obtained.
6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Spectral Bandpass 2nm S
I
o 500 550 600 650 700 750 800 850 900 Wavelength (nm)Figure 5.4. Absorption spectrum of a waveguide in x-cut Nd:MgO:LiNbOg.
The spectrum exhibits the characteristic absorption peaks which occur in Nd-doped crystal systems In order that the absorption peaks are well matched it is necessary for the pump source to have an output in this range. A dye laser operating at 600 nm or a semiconductor-diode laser operating near 815 nm were used as the pump sources in the experiments. We will consider each of these in turn:
5.2.1. Dye laser
Dye lasers rely on optical excitation of an organic dye dissolved in a liquid solvent. Organic dyes have strong absorption bands in the ultraviolet or visible, and when excited by light of the appropriate wavelength, display a broad-band fluorescence spectrum. This broad-band fluorescence spectrum allows dyes to absorb and emit light over a broad range of wavelengths. A dye such as Rhodamine 6G, having a peak fluorescence at around 590 nm, is pumped by the 515 nm line of the argon-ion laser. The dye can absorb virtually all of the visible output and convert over 20% of the input energy into coherent output at the peak of its emission band. The intense
absorption and subsequent heating of a small volume of dye as well as a build-up of unwanted population states requires a continuous and rapid change of the pumped volume. The laminar-flow dye laser avoids this problem by passing the dye, via a nozzle, through the pump beam and into a reservoir which recirculates the dye. To stop the smooth dye stream from breaking up upon leaving the nozzle, ethylene glycol is used as a solvent. The complete configuration is shown in figure 5.5
Fold mirror
\
Biréfringent filter Output coupler Dye jet High reflector PUMP BEAM Pump mirrorFigure 5.5. Schematic of a laminar-flow dye laser system [6]
5.2.1.1 Tuning the output
Tuning of the output wavelength was achieved by using a biréfringent filter. A biréfringent filter consists of several quartz waveplates of different thicknesses, figure 5.6. The plates are oriented at Brewster's angle such that vertically polarised light in the cavity experiences no loss by reflection at the plate surfaces. Consider the action of a single plate on the light in the cavity. The crystal axes of the quaitz are oriented such that the plate acts as a full waveplate for vertically polarised light if the wavelength X satisfies the relation
d(nsiow-nfast) = mlo
where m is an integer, and d is the distance travelled by light in the plate.
(5.2-1)
For other wavelengths, passage through the plate results in elliptical polarisation. After reflection at the end mirror, this elliptically polarised light experiences loss by reflection at the next encounter with the quartz surface. This loss prevents lasing at wavelengths significantly different from those that satisfy the full-wave condition. A
linewidth of -0.3 nm is possible with a single waveplate. The addition of more waveplates reduces the linewidth still further. Tuning the laser was accomplished by rotating the plates about the normal to the plate surfaces. Because the plates are inclined to the optic axis, rotation effectively changes the slow axis refractive index from n^j^^ to n’^j^^, changing the preferred wavelength to
^ slow ^ fast ) / ^ (5.2-2)
Quartz plate Prefened
polaiisation Optic axis N -Normal F-Fast axis S-Slow axis
Figure 5.6. Tuning control for the dye laser
5.2.2. Diode laser
An holosteric laser system has the advantages of efficiency, ruggedness, and compactness. In addition, the longer pump wavelength of a diode laser -815 nm minimises photorefractive damage in Nd:MgO:LiNbOg.
5.2.2.1 Laser action
A semiconductor laser requires that there be a region of the p-n junction where both electrons and holes are present. Optical radiation can occur in this naiTow region. When an electron in the conduction band recombines with a hole in the valence band, a quantum of radiation, with energy equal to the difference in energy between the two states, is released. A population inversion is created when there are more electrons in the conduction band than holes in the valence band, and then the recombination may be stimulated. The wavelength of the transition is determined by the band gap size. An electron raised from the top of the valence band to the bottom of the conduction band has potential energy eVg, where e is the electronic charge, and Vg is the electrical potential required to promote the electron to the conduction band. This energy must equal the photon energy emitted upon recombination.
We therefore have
E = hv = eV„ (5.2-3)
In order that the semiconductor junction lases we merely increase the current until a population inversion is reached and add mirrors to provide feedback. The two end faces are made parallel, by cleaving along crystal planes, and are not normally provided with reflecting coatings since the Fresnel reflection at the semiconductor-air interface is sufficiently large (n=3.6 for GaAs, and therefore R -35%). In modern continuous-wave laser diodes the simple structure suggested above ie. the GaAs homojunction laser is not used, since the threshold current density is too high -40,000 A cm'", due to poor optical and carrier confinement. Most common laser diodes have a double heterostructure ie. two materials aie used to form the junction. Figure 5.7 shows a buried heterostructure diode laser, of the type used in the experiment
300 pm
_ p-GaAs - p-AlGaAs
*p-AlGaAs (Active layer) n-AlGaAs
emission region
n-GaAs (substrate)
Figure 5.7. Buried double heterostructure (Sony TAPwS laser)
The double heterostructure reduces the threshold current density to -2000 A cm'- permitting room-temperature continuous-wave operation.
5.2.2.2 Stripe confinement
Laser action can be limited to a narrow stripe either by restricting cunent flow through the laser or by creating an optical waveguide. Controlling current flow produces a population inversion only in the stripe and also creates a subtle waveguiding effect called gain guiding. Optical waveguides are fomted by creating patterns in the junction plane, the difference in refractive index confines the light to a naiTow stripe, which is called index guiding. Figure 5.8 is an end view of the front facet of the two types of
diode laser. Observe the waveguide groove, in the index guided laser, running adjacent to the actual active layer.
p^GaAlA GaAs p GaAlAs' n G aA lA s Electrode Active junction waveguide groove
gain-guided laser diode index-guided laser diode
Figure 5.8. Stripe geometry for two types of diode laser .
5.2.2.3 Electrical characteristics
The output power of a diode laser is a function of the current flowing across the active junction. The figure below shows a plot of the optical power output as a function of
the input cunent for the Sony TAPS (TAPered Stripe) laser
E