Frequently Asked Questions

HIP is integration of multiple photonic components on a single chip for ruggedization, smaller packaged modules, and cost reduction. This enables a single photonic module to perform the same functions as multiple, discrete fiber components.

As interconnect speeds get faster (reaching the tens of GHz or Gbps range), electrons traveling in copper cables face increasing loss and consequently require additional amplification, which results in increased power consumption and heat generation. Optical communication over fiber is much more efficient and scalable to higher signal bandwidths with minimal transmission losses. Optical fiber cables also have zero susceptibility and emission of electromagnetic radiation.

APIC has designed extremely low noise lasers and high dynamic range photodetector optical receivers for use in demanding environments. These devices are fabricated to exacting standards to obtain the required performance.

Fiber optic photonic links are used by all the major telecommunications companies and an extensive supplier base has developed. Fiber links and networks are also being implemented on aerospace platforms to meet new and projected bandwidth requirements.

RFoF is the transmission of radio frequency signals (analog or digital) at through optical fiber. Optical transmitters modulate the coherent light from the laser with the RF signal which is then transmitted through fiber cables. At the receiving end a photodetector converts the light energy back into an electronic RF signal.

RFoF is used to consolidate and replace multiple electrical cables in systems with more compact fiber optic cables to take advantage of fiber’s higher bandwidth, EMI resistance, and ease of installation and maintenance. Application examples include:

• High speed signal transmission applications over 18 GHz and/or transmission lengths greater than 20 feet where copper cable losses are significant
• Passive optical networks (PON) such as for high bandwidth cable TV/Internet
• Distributed antenna systems (DAS), where high bandwidth fiber optics is used to preserve and re-use valuable RF spectrum

“High Fidelity” RF over fiber retains the highest quality signal transmission through fiber optics. It requires very low noise laser sources; a large dynamic range in the link; and highly linear response of the active components.

High fidelity links require optimization of a number of parameters in the link and link components:

• Eliminating/suppressing sources of noise, including optical and electrical elements
• Highly linear devices which do not distort the signal
• Operation of the link at high optical power which lowers noise floor and increases dynamic range to enable operation around the most linear point in the link response curve
• High responsivity detectors to improve link gain and dynamic range

The laser in the RFoF link requires the following:

• Low relative intensity noise (RIN); RIN < 160 dB/Hz^2/3
• High optical power; > 50 mW
• Single mode operation with good side-mode suppression ratio (SMSR > 35 dB) under operating conditions
• Low threshold current

The laser emits an RF modulated optical signal. A directly modulated transmitter varies the drive current of the laser according to the RF input signal, which causes the optical signal intensity to be modulated by the input signal (i.e. amplitude modulation).

The advantages of direct modulation are:

• Significantly lower cost
• Better dynamic range of the link
• Significantly smaller form factor; eliminates space needed for external modulator and modulator control electronics

• Capable of operating at high input power (at least 50 mW for 20 GHz, preferably higher)
• High maximum linear photocurrent (> 50 mA)
• High Responsivity > 0.8 for 20 GHz RF bandwidth