Thomas H. Wood

The quantum well self-electrooptic effect device: Optoelectronic bistability and oscillation, and self-linearized modulation

We report extended experimental and theoretical results for the quantum well self-electrooptic effect devices. Four modes of operation are demonstrated: 1) optical bistability, 2) electrical bistability, 3) simultaneous optical and electronic self-oscillation, and 4) self-linearized modulation and optical level shifting. All of these can be observed at room-temperature with a CW laser diode as the light source. Bistability can be observed with 18 nW of incident power, or with 30 ns switching time at 1.6 mW with a reciprocal relation between switching power and speed. We also now report bistability with low electrical bias voltages (e.g., 2 V) using a constant current load. Negative resistance self-oscillation is observed with an inductive load; this imposes a self-modulation on the transmitted optical beam. With current bias, self-linearized modulation is obtained, with absorbed optical power linearly proportional to current. This is extended to demonstrate light-by-light modulation and incoherent-to-incoherent conversion using a separate photodiode. The nature of the optoelectronic feedback underlying the operation of the devices is discussed, and the physical mechanisms which give rise to the very low optical switching energy (∼4 fJ/ μm2) are discussed.

Novel hybrid optically bistable switch: The quantum well self‐electro‐optic effect device

We report a new type of optoelectronic device, a self‐electro‐optic effect device (SEED), which uses the same GaAs/GaAlAs multiple quantum well material simultaneously as an optical detector and modulator. Using a series resistor and constant voltage bias supply the SEED shows optical bistabilty (OB) of the recently discovered type which relies on increasing absorption and requires no mirrors. OB is seen at room temperature from ∼850–860 nm, at powers as low as 670 nW or switching times as short as 400 ns (limited only by power restrictions) with ∼1‐nJ optical switching energy in a 600‐μm‐diam device. Total energies per unit area (∼18 fJ/μm2) are substantially lower than any previously reported for OB.

Multiple quantum well (MQW) waveguide modulators

Because the electroabsorption effect in semiconductor multiple quantum well material is approximately 50 times larger than in bulk semiconductors, significant interest has been generated in the use of MQWs in optical modulators. Small high-speed devices have been made which show promise for external modulators in optical transmission systems, as well as for encoding and processing components in optical interconnect and signal processing systems. The fact that these modulators are made from III-V semiconductors had led to interest in integration of these components with other active optoelectronic components. Although most devices have operated with light of a wavelength of 0.85 mu m, recently much progress has been made in applying this technology to devices operating near 1.55 mu m. The author reviews the work of the last few years in this field and indicates some future directions. >

An evaluation of architectures incorporating wavelength division multiplexing for broad-band fiber access

Wavelength division multiplexing (WDM) offers the potential to transport very high bit rates in access systems serving residential and small business customers. We describe the options for WDM access systems, and evaluate them on the basis of an international standard that is emerging for passive optical networks (PONs) that use wavelength-independent power splitting. The greatest impediment to meeting the high-bandwidth potential of WDM for downstream (i.e., from the central office to the home) transmission is the need for a moderately priced multiple-wavelength light source, implementing WDM in the upstream (home to central office) direction presents greater hurdles than in the downstream. The technical aspects of overcoming these hurdles can all be met. The real challenge will be in finding technical solutions that can meet the very stringent cost requirements of access systems. Systems that use WDM in the downstream direction only, while using wavelength-independent power-combining in a PON configuration for upstream, may prove economical before full WDM systems. WDM may also prove valuable if upgrades are needed in the future for power-splitting PONs.

131 ps optical modulation in semiconductor multiple quantum wells (MQW's)

A new optical modulator has been fabricated which uses the recently discovered electroabsorption effect in MQWs. Optical pulses 131 ps long were generated when the device was driven with 122 ps electrical pulses. The input-output characteristics of the device show that it has low insertion loss with reasonable modulation depth and drive voltage.

Electric field screening by photogenerated holes in multiple quantum wells : a new mechanism for absorption saturation

We observe saturation in the electroabsorption of InGaAs/InP multiple quantum wells (MQWs) at high optical intensity. Contrary to the mechanism for zero‐field MQWs, we find that saturation occurs due to the presence of trapped photogenerated holes that screen the MQWs from the applied electric field. By carefully measuring the absorption coefficient of the wells and the emission time for holes, we are able to fit the observed electroabsorption saturation with no adjustable parameters.

Room‐temperature excitons in 1.6‐μm band‐gap GaInAs/AlInAs quantum wells

The first observation of strong and well‐resolved exciton peaks in the room‐temperature absorption spectra of infrared band‐gap multiple quantum well structures (MQW’s) is reported. Assignment of the optical resonances in the absorption spectra of GaInAs/AlInAs MQW’s yields the material parameters of this new heterojunction. The discontinuities of the conduction and valence bands are found to be ΔEc=0.44 eV and ΔEv=0.29 eV, respectively.

Strong polarization‐sensitive electroabsorption in GaAs/AlGaAs quantum well waveguides

We report the first measurements of perpendicular field electroabsorption (quantum confined Stark effect) in GaAs/AlGaAs quantum wells for light propagating parallel to the plane of the layers. This geometry is well suited for integrated optics. The absorption edge shifts to longer wavelengths with increasing field by as much as 40 meV, giving a modulation depth>10 dB. The strong dichroism present in this geometry is retained even at high fields, making polarization‐sensitive electro‐optical devices possible. We also demonstrate in the waveguide geometry optical bistability due to the self‐electro‐optic effect with 20:1 on/off ratio.

Two-wavelength WDM analog CATV transmission with low cross talk

We report a two-wavelength WDM analog CATV transmission system with cross talk <-60 dBc at launched power of +9 dBm/wavelength, an improvement of 13 dB over previous results. We are able to achieve this result principally by reducing the channel spacing from 8.6 nm to 2.2 nm. In addition, contrary to expectations for SRS, we find significant reduction of the cross talk by applying a 2-GHz dithering tone to the laser.

Wavelength‐selective voltage‐tunable photodetector made from multiple quantum wells

We show that a pin‐doped multiple quantum well (MQW) diode can be used as a photodetector whose voltage of maximum photocurrent is wavelength dependent. The voltage of maximum photocurrent can be located accurately and related to the wavelength of the incident light, allowing measurements of the wavelength with a precision of 0.03 A=1.2 GHz. This provides a simple, compact, solid‐state device that can be simultaneously used to measure the intensity and wavelength of an optical beam. Furthermore, the device shows high responsivity, low dark current, and fast response.