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High Speed VCSELs & Photodiodes

    Today’s datacom applications require sophisticated optical components. VCSELs provide high-speed data transmission at low power consumption. Due to their design, VCSELs have a circular beam profile with a low divergence angle, which ensures simple and efficient coupling to optical fibers. In addition, the vertical beam guidance allows the realization of emitter arrays on a monolithic chip.

    Philips Photonics offers a comprehensive portfolio of datacom VCSELs for optimal matching to all main applications, like Transceivers (TxRx), Active Optical Cables (AOC), and Embedded Optical Modules (EOM) within the standards of ,e.g., Infiniband, Ethernet, FibreChannel, and Thunderbolt. Depending on the requirements, the VCSELs may feature single-mode or multi-mode operation, optical output power up to 24 mW, or data rates up to 14 Gbps (25 Gbps in final development stage). Single channel devices are available as well as arrays with, e.g., 4 or 12 devices. Philips Photonics also supplies the corresponding photodiodes. Philips also works with customers on special solutions, like large scale 2D arrays or other wavelengths.

    Philips Photonics is committed to a high quality of the devices. All products are subject to extensive performance and reliability tests before shipment.

    Philips VCSEL Technology

    The structure and function of a Vertical Cavity Surface Emitting Laser (VCSEL) is illustrated by the figure on the right. It shows a combination of a Scanning Electron Microscope picture of the cross section of a single VCSEL, with a schematic diagram of the vertical structure.

    VCSELs are semiconductor laser-diodes, which emit light (1) perpendicular to the surface. The laser consists of active layers, with a thickness of a few nm. In these layers, the electrical carriers are converted into light. Above and below the active layer, multiple layers of alternating refractive index form resonant mirrors (4). The short laser cavity requires a high reflectivity of the mirrors in order to achieve sufficient gain. The doped semiconductor mirrors additionally provide electrical contacts (2) and (3) to the active layers. The size of the active area is defined by the width of an oxidized layer (6) near the active layer.

    The layer structure and the vertical light emission allow producing fully functional lasers in a single growth step. After this epitaxial growth process, standard semiconductor wafer processing steps define the emission area and provide electrical terminals to the individual laser-diodes.

    The vertical structure of the VCSEL allows building a large number of lasers next to each other to form 2D arrays. Depending on the application, the VCSELs in these arrays can be electrically connected either individually (by individual contacts for e.g. multi-channel datacom applications), or in parallel. In the parallel arrangement of a large number of lasers, the current is provided to the individual lasers by a thick conductor layer (5), to provide low resistance electrical terminals to the arrays.

    Philips VCSEL Array Technology
    for High Power Applications

    VCSEL arrays consist of thousands of micro lasers manufactured in a semiconductor process on GaAs wafers. Typical pitch between individual lasers is in the range of 40 μm. Wavelengths are between 800nm and 1100nm, most commonly used are 808nm, 850nm and 980nm, each with a line width of a few nm. Compared to infrared LEDs, VCSELs deliver a very narrow line width and an extreme forward emission characteristic.

    Chips are diced from the wafers and mounted onto carriers, thereby using assembly steps well known in the LED industry. For very high power applications, these carriers are mounted on water coolers to prevent excessive heating of the laser structure itself. Based on these building blocks, flexible and scalable solutions are possible, matching many different application requirements. VCSEL modules delivering infrared output powers of a few watts up to tens of kilowatts can be realized.

    The brightness of VCSELs ranges between traditional lasers and lamps or LEDs and the technology offers scalable system power. High power VCSEL systems are thereby an attractive solution in many application fields.

    Philips Laser Doppler Technology

    Philips Laser Doppler sensors employ laser Doppler self-mixing technology. Laser Doppler refers to the phenomenon that laser light, reflected on a moving target, contains information about the target’s velocity. Self-mixing refers to the fact that a single laser is used both for emitting and receiving the reflected light.

    Principle of Laser Doppler technology

    When the laser is aimed at a scattering object at a distance (see figure), a small portion of the scattered light reflects back into the cavity where it mixes with the strong laser field. When the movement of the object has a component along the direction of the laser beam, the phase of the reflected light continuously shifts with respect to the original laser light, resulting in a periodic variation of the feedback into the laser cavity at a frequency, equal to the Doppler frequency.

    The feedback from this moving object generates a changing interference signal inside the laser cavity with this Doppler frequency, and hence the laser output power is modulated with a frequency, from which the velocity of the scattering object can be derived, according to the equation above.

    Technology capabilities and application coverage

    Accurate speed measurement over 360km/h

    Nominal working distance from mm’s up to meters

    Working range from 30 to 60% of nominal distance

    Accuracy better than 0.01% (interferometric)

    Resolution from wavelengths to cm’s

    Works on virtually all surfaces

    Insensitive to environmental light

    Robust to smoke, mist or dust

    Philips Datacom Technology

    For highspeed data transfer, there is no way around optical datacom technologies. These allow data transmission with a bandwidth in the GHz regime over long distances, without cross-talk.

    For this purpose, electrical signals have to be translated into optical information - and back again. On the transmitter side, a driver processes the electrical signals and controls the electrical power going into the VCSEL, resulting in a fast modulation of the emitted light. In order to maintain clean signals, the electric characteristics of the VCESL, e.g. capacitance, resistance, ohmic contacts, have to be controlled precisely. Otherwise, signals are blurred, resulting in noisy data transmission. A common way to evaluate the quality of the data link is to record so-called eye diagrams. These are a superposition of randomly generated bit streams (signals), as monitored on an oscilloscope (see Fig 1). Eye opening and width provide information about any eventual presence of noise or jitter effects.

    The VCSEL light is then coupled into an optical fiber, usually collimated by a micro lens. The low divergence of the VCSEL, one of its key features, thereby facilitates the coupling considerably. A further feature of VCSELs, namely the possibility of fabricating 2D laser arrays on the chip, allows the efficient fabrication of multiple links with fiber bundles.

    On the receiver side, the incoming light is focused on photo diodes, generating an alternating photo current. A transimpedance amplifier then converts the current in a proportional voltage, thus the original electrical signal is reconstructed.