1. Field of the Invention
The present invention is directed towards the field of spatial light modulators (SLM). In particular the present invention is directed towards a directly modulated-SLM.
2. Description of the Related Technology
Gallium arsenide (GaAs) direct bandgap semiconductor material led to the first successful room temperature laser and remains one of the most important types of lasers even today. Its success is largely because it shares nearly the same lattice constant as Ga1-xAlxAs, which serves as a barrier layer for a wide range of x when fabricated into buried heterostructures. Because of both optical and carrier confinement, and because GaAs can be readily p-doped and n-doped, this has made GaAs lasers the most common of all semiconductor lasers. The laser output is centered at 850 nanometer wavelength in the visible red spectral region due to the band gap energy of 4.2 electron volts.
Now turning to Vertical Cavity Surface Emitting Lasers (VCSELs), the fundamental difference between conventional edge-emitting semiconductor laser diodes and VCSELs lies in their geometry. As the name VCSEL implies, it is a device that emits power perpendicularly from its surface. More importantly, VCSEL wafers are fabricated using layer-by-layer deposition methods, followed by chemically-assisted ion beam etching to form planar arrays of pillar-shaped microlasers. The geometrical arrangement of their end reflectors consists of many alternating high/low refractive index layers effectively making up a pair of Fabry-Perot resonator mirrors. These mirrors can have reflectances >99%, deposited directly on both sides of a multiple QW active region. VCSEL arrays are usually grown using Metal-Organic Chemical Vapour Deposition (MOCVD) techniques by sequentially depositing all of their layers and then etching away all layers down to the substrate, leaving a two-dimensional array of microlasers with diameters generally ranging from 5μ to 10μ. These microlasers generally have only a few active quantum well layers (QWs) and therefore have low gain in their light propagation direction, which requires them to have mirror reflectances of >99%. However, since they have a small mirror separation, usually about 8μ, their single frequency operation is guaranteed. Two engineering problems that must be faced are attachment of metallic electrodes within a dense 2D VCSEL array and removal of heat from the array when the VCSEL microlaser array is operated at a high duty cycle. Usually one electrode is attached to the non-emitting end of each microlaser, but the output laser beam must emit through the opposite face where a second electrode is attached and limits separation distance between each microlaser. Typically, VCSELs have threshold injection current densities of Jth=5 to 7 kA/cm2, but due to their small size this translates to actual threshold current values of approximately 1 milliampere per microlaser with a typical power output ≦0.5 milliwatt at 850 nm for a GaAs-based device. One important feature of VCSELs is the shape of the output laser beam, which can be controlled to make it highly circular and symmetric about its axis. This obviates the need for external astigmatic type beam correction that is generally necessary in the case of edge-emitting diode lasers. While large 2D arrays may be etched onto a single substrate, the problem of effectively cooling such large arrays remains.
Lasers are typically thought of as devices that emit optical power due to stimulation of radiation as a result of optical gain produced by some type of pumping mechanism. Such devices may be considered as oscillators that generate external optical power in a highly directional beam within a narrow spectral bandwidth. However, all oscillators are amplifiers with feedback. Lasers are optical amplifiers with feedback provided by two or more mirrors. Those lasers having an open Fabry-Perot type resonator oscillate near a well-defined center frequency νo with adjacent frequencies determined by the mirror spacing L, where such side frequencies are separated by: Δν=c/2L.
If it is desired that the device discussed above should not oscillate at all, a device may be built similar to a laser that suppress oscillation by eliminating any feedback. Such a device can remain as strictly an amplifier without feedback. Semiconductor optical amplifiers (SOAs) have all the features of a laser diode type device but it must be ensured that they do not oscillate by equipping them with antireflective end face coatings and not exceeding pump input levels where they may tend to self-oscillate. The unsaturated gain coefficient in a SOA active region is given by:γo(ν)=(λ2/8πτr)ρ(ν)[fc(E2)−fv(E1)]
where: τr=radiative recombination time; ρ(ν)=joint density of states;
[fc(E2)−fv(E1)]=degree of population inversion due to the difference in occupancy factors for electrons in energy level E2 of conduction band versus electrons in energy level E1 of valence band.
When an SOA is pumped by injected current, it behaves as a four-level device, which means the gain coefficient γo(ν) depends upon injected carrier concentration, but in a totally nonlinear way. This makes analysis difficult, but can be treated by considering operation at high gain, where the peak gain γp(ν) varies nearly linearly with injected carrier density. Then it is approximated:γp(ν)≈α(ν)[Δn/ΔnT−1]
where: α(ν)=absorption coefficient under zero current injection; Δn=injected carrier density; ΔnT=injected carrier density at transparency condition where gain just balances loss. Finally, an expression for overall SOA unsaturated gain for an SOA length L given by:Go(ν)=exp[(Γγo(ν)−α(ν))L]
Here Γ is a confinement factor describing the ratio of power flowing in the active device region versus total power flowing through the entire device. Now consider the nonlinear behavior of an SOA device which is chiefly controlled by the injected carrier density Δn. Specifically changes in Δn can induce changes in phase associated with light passing through an SOA device. Conversely, the passage of an optical signal through an SOA can alter the gain by inducing changes in Δn.
The unsaturated gain coefficient denoted above by γo(ν) becomes saturated when power flows through an SOA. Gain media in which homogeneous broadening occurs is considered, and for which gain saturates in the following manner:γ(ν)=γo(ν)/[1+2[(Φν(+)+Φν(−))/Φνsat] Sin2kz]
where: Φνsat is the saturated photon flux in the z-direction along the device, which is related to the optical intensity by: Iν=hνΦν; and where k=2π/λ. The above expression allows for spatial hole burning in the gain medium, which may become important when SOA VCSEL type devices are considered.
The devices discussed above may be useful in variety of systems, however to date they have not been used to their fullest potential.
Current devices use indirectly modulated SLMs. The laser beam is reflected off of a pixilated surface. For example this may be a mirrored surface such as a Digital Micromirror Device. A laser beam is attenuated as it is illuminated through a pixilated surface, for example a pneumatic liquid crystal. This also requires the conversion of a laser beam with a Gaussian transverse profile into a flat-top profile for even illumination of the image.
Therefore, there is need to provide a device and system that requires no flat-top generator, has a broader range of pixel intensity control, such as gray scale. Has pixel control of optical signal power. Has the ability to change polarization on individual pixels within a page on a holographic storage medium. There is a need for a solid state device with no moving parts that can enable optical signal input for holographic storage from a bus or network.