1. What is the power limit in polished couplers?
We have input 3 watts of argon ion laser light to polished couplers. Customers report use of pulsed lasers at various powers and duty cycles that have not resulted in damage. It is assumed that powers approaching that of non-linear effects causing index changes could result in loss of guidance and potential damage due to heating.
2. What is between the polished coupler halves?
Nothing is between the fibers; they are in optical contact. The substrates are held in position by an epoxy.
3. How stable are polished couplers?
Very stable. Optical contact is dimensionally stable, i.e. there are no air gaps, and it is resistant to motion due to contact static friction. Coupling ratio is relatively constant with temperature as the coupling region is all glass and low expansion. Cross coupling due to stress induced birefringence can occur at very low temperatures with some fabrication techniques.
4. Can couplers be made in M by N arrays?
As coupler halves can be made on a continuous length of fiber any arrangement of pairs can be assembled from any number of strings of half couplers. A Mach Zehnder interferometer made from two 3 dB couplers is a simple example. 2 x 4 arrays using 3 couplers and 2 x 8 arrays using 7 couplers can also be made splice-free.
5. What is polarization isolation or cross-coupling?
This is the conversion of light from one axis of a polarization maintaining fiber into the orthogonal axis. It is caused by local stress induced birefringence that shifts the fast and slow axes or in the case of a coupler, a misalignment of the polarization axes in the coupling region.
Cascaded components with cross coupling used with long coherence length sources can display amplitude modulation unless they are thermally controlled.
6. What is the loss in polished couplers?
Loss is defined as "excess loss" the loss internal and specific to the component, and "insertion loss", the loss in the line due to the insertion of the component. i.e. when a 3dB (50/50) 2 x 2 coupler is inserted into a line 50% or 3dB is transmitted and 3dB is "lost" to the second port.
Insertion Loss = 10 Log Tr/Inp where Tr is the transmitted power and Inp is the input power.
Excess loss is the loss within the device itself. i.e. a 1% loss due to loss of guidance, scatter or absorption gives 10 Log .99 = 0.05 dB loss.
A perfect coupler with a 50/50 split ratio (3dB) has zero excess loss and 3dB insertion loss. If the coupler also had 0.5 dB excess loss the actual insertion loss would be 3.5dB.
7. Why is splice-free good?
The best splices have some loss and some cross coupling. Splice free certainly helps with critical applications.
8. What is over-coupling?
As two cores are brought closer or the interaction distance is increased.
The coupling increases to a maximum of 100%, and if the fibers are matched, reduces to zero and then begins to increase again. This cyclic coupling function shows a beating of the power from one fiber to the other, the number of beats being dependent on the positioning of the cores. A 3dB, 50/50 coupler is set to mid point of the first upbeat. Setting to the mid point of the next downbeat gives greater change with respect to wavelength.
9. What is the wavelength dependence?
A 50/50 or 3dB coupler has a change in ratio dependent on wavelength of 2.5% increase in coupling for 1% change in wavelength. This is typical for all fibers and wavelengths in the 0.98um to 1.55um range. (eg. 50/50 at 1.550 is 47.5/52.5 at 1.565).
Below 900nm the relationship is nominally 3% increase for 1% change in wavelength.
A high ratio coupler such as a WDM where the signal coupling is near 100% has a flatter response by a factor of 5 due to the overcoupling position.
Please feel free to contact us with any further questions.