Van den Hul The optocoupler MKII 4m

The optical link between source and converter remains an often overlooked weak point in an audio chain. Yet transmission quality directly affects the level of jitter received by the digital-to-analog converter. This is precisely the issue the Dutch company has tackled with the Optocoupler MKII, a Toslink cable whose design is based on an optical principle rarely exploited in this field: fiber ends cut into convex lenses.

An optical principle serving digital fidelity

In a conventional optical fiber, light bounces off the internal walls at different angles, thus traveling paths of varying lengths. This phenomenon, called modal dispersion, causes a temporal spreading of light pulses which results, once the signal is converted, in increased jitter.

The Optocoupler MKII circumvents this limitation thanks to its convex, spherical-lens-shaped ends. When light enters the fiber, it is focused rather than dispersed. Van den Hul claims a 50% reduction in internal reflections per unit length, which shortens the effective path of the light and decreases the time offset between the different rays. The D/A converter thus receives a cleaner signal, with fewer timing fluctuations.

This convex geometry also improves contact between the fiber and the optical components of the devices (transmitter and receiver). The surface of the lens better conforms to the interface, reducing insertion losses to a minimum.

Construction and handling

The Toslink connectors of the Optocoupler MKII feature fully metal bodies, ensuring a firm hold in the sockets and better durability than standard plastic plugs. The cable guarantees a transfer capacity of 250 Mbit/s, sufficient for all common digital audio streams.

The inherent optical isolation of the fiber naturally eliminates any electrical or magnetic interference: ground loops, mains noise, or surrounding radiation have no effect on the signal. Optical fibers also have the advantage of not degrading over time, unlike certain metal conductors that are sensitive to oxidation.

The trade-off of the lens-based design lies in its relative fragility. Van den Hul recommends handling the cable with care and never forcing when inserting the plugs. If mating proves a bit tight, a tiny amount of lubricant (without contact with the lens itself) can make the operation easier. For cleaning the ends, only a soft optical cloth is suitable.

Technical characteristics

Optical fiber technology

• Optical fiber with convex ends and spherical lenses
  • Reduction of internal reflections and modal dispersion
  • Minimized signal loss even when the cable is bent
  • Optimized optical insertion thanks to precise control of contact surfaces

Technical advantages

• Data transmission capacity: > 250 Megabits/second
• Optimized focusing of the light beam
  • The light beam entering at an angle is focused, reducing reflections against the fiber walls by 50%
  • Shorter light travel distance, reducing attenuation and propagation time between different light rays
• Reduced modal dispersion
  • Less jitter at the digital-to-analog converter
  • Optimized end contacts to minimize optical losses
• Complete protection against interference
  • Transmission completely immune to electrical or magnetic noise and interference
  • Stable optical fibers that remain free from deterioration over time

Technical specifications

• Cable type: digital optical interconnect
• Technology: optical fiber with convex ends
• Interference protection: complete, no electrical or magnetic noise