Semiconductor modulators offer high gain

Two new optically addressed spatial light modulators (SLMs) have been commercially introduced by CoreTek Inc. (Burlington, MA). One is intended for real-time holographic applications, while the other, designed for incoherent-to-coherent-light conversion, will compete directly with the liquid-crystal light valve made by Hughes (Danbury, CT).

Both CoreTek modulators are based on a novel structure that enables the detection of write beams to be kept separate from readout beams—this allows better modulation depth, gain, and sensitivity than is available in most similar devices. In addition, the company has developed a method of pulsing the electric field across the quantum-well stack that allows for frame rates of more than 10 kHz. These devices are the first multiple-quantum-well-based SLMs on the market.

Multiple-quantum-well (MQW) structures consist of a stack of thin semiconductor layers with alternating bandgaps. The lower bandgap layers act as potential wells, trapping electrons that are freed by incoming photons to leave a distribution of electrons and holes. The optical properties of a MQW structure are determined by a voltage applied across it, but this field is manipulated by the photocarriers. As a result, the altered field changes the absorption and refractive index of the device and modulates any incoming probe beam.

In a conventional MQW device, the intensity of the probe (read) beam must be kept very low, or it would displace electrons and erase an existing pattern—this means the device can provide little optical gain. The CoreTek SLMs, however, are based on a structure that isolates the write beam and its effects from the read beam, thereby removing this constraint.¹ A thick detector region absorbs all incoming write light, but is transparent at the readout wavelength (see figure). The aluminum gallium arsenide (AlGaAs) then produces a photocurrent that locally changes the voltage across the modulator and alters the MQW absorption/index.

The separation of read and write functions does not, in itself, eliminate the problem caused by high read-beam in tensity. This is achieved by growing a modulator at low temperature with higher quantum-well barriers built in. The resulting structure has lower conductivity—so that charges cannot move easily—and a high trap density, which means the charges cannot move as far before being retrapped. The result is ultrafast carrier trapping; this minimizes the effect of the read beam on the modulator.

Eliminating problems

Another problem that had to be overcome was speed. The MQW could be made to switch quickly, but would take longer to revert to its original state. The long (90 ms for one device) recovery time was attributed to charges remaining trapped even after the write illumination was removed. To eliminate this problem, the basic device architecture was retained, but its addressing was changed. Previously, the MQWs had been used with pulsed optical inputs. Instead, CoreTek developers changed to continuous illumination and a pulsed electric field. They found that, when the voltage was switched to zero, the incoming light would erase the field produced by the trapped charges. This reduced the recovery time to 1 µs and consequently made high-frame-rate operation possible.

Both commercially available modulators have relatively low operating voltages and operate around 850 nm. The photorefractive SLM has been optimized to provide high spatial resolution for holographic recording, with a minimum feature size of about 4 µm. It also has a high—for a MQW—output diffraction efficiency of up to 2%. The optically ad dressed SLM for incoherent-coherent conversion operates at more than 10 kHz and has a 60% modulation depth.

According to Ergun Canoglu, a senior researcher at CoreTek, the devices cost about $1500-$3000, depending on specification. They will be used mainly as preprocessors—they can perform image thresholding and subtraction—and as input devices and filters in optical correlators.