Passive III-V electro-optical components

The FBH develops passive electro-optical components based on GaAs optoelectronic semiconductor technology. The final aim here is to replicate complex electro-optical setups required, for example, for cold atom quantum sensors in a monolithic fashion, i.e., on the chip. This requires the development of chip-scale building blocks of complex setups, e.g., of optical power splitters or combiners, and of phase- and amplitude-modulators. Design and simulation of these components are carried out cooperatively by the Joint Lab Laser Metrology and the Optoelectronics Department at FBH. For electro-optical characterization and integration into electro-optical modules the Joint Lab Laser Metrology applies its specialized measur­ement capabilities and hybrid integration technologies.

MMI
Fig. 1: REM picture of a 1:2 MMI GaAs coupler for single-mode operation at 780 nm. Design and fabrication is by FBH

Functional principle – MMI couplers

Multi-mode interference (MMI) couplers are based on the self-imaging concept. The input signal from a single-mode ridge waveguide is launched into a multi-mode ridge waveguide (supports ≥ 2  lateral modes) allowing higher order modes to be excited. Each multi-mode waveguide has a key parameter that is called the beat length. At this length, all the phase differences between the higher order modes and the fundamental mode are multiples of 2π. As a result, a projection of the feed signal (self-image) can be created at the beat length. Multiple images can be generated at fractional lengths of the beat length. The exact lateral position and the number of these self-images are defined by the geometry of the cross section of the multi-mode waveguide, the modal refractive indices, and by the lateral position of the feed signal.

Based on the self-imaging concept, devices with multiple input/output ports (1xN, M:N) can be designed. As an example, Fig. 1 shows the two output ports of a GaAs-based 1x2 MMI coupler. In comparison to conventional devices such as directional couplers and Y-couplers, MMI couplers are tolerant to fabrication imperfections which make them a very attractive substitute for building PICs.

REM Ridge
Fig. 2: REM picture of the cross section of a GaAs phase modulator for operation at 780 nm. Design and fabrication is by FBH. The center shows the ridge which allows for single mode wave guiding.

Functional principle – phase modulators

GaAs-based phase modulators offer a compact and robust solution to substitute crystal-based modulators. The vertical multi-layer of the modulator consists of a GaAs/AlGaAs double heterostructure. The cross section of such a structure is shown in Fig. 2. The electro-optic effects resulting in a phase modulation are the linear electro-optic (LEO) effect, the quadratic electro-optic (QEO) effect caused by the Franz–Keldysh effect, and carrier-density-related effects. The typical design wavelength of these devices is at 780 nm for rubidium spectroscopy applications. Devices can be optimized for operation wavelengths ranging from 780 nm to 1110 nm. 

Applications

Building-blocks for chip-scale replication of complex electro-optical setups, e.g., for cold atom-based quantum sensors

  • laser phase and frequency stabilization
  • laser phase modulation, e.g., for atomic spectroscopy and coherent communication 

Typical wavelengths

  • 780 nm
  • 1064 nm
  • 1070 nm  

Technology and packaging

  • GaAs III-V semiconductor layers with metalorganic vapor phase epitaxy (MOVPE)
  • beam projection lithography
  • wet and dry etching