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Micron Optics has been helping customers use Fiber Fabry Perot filters since 1990.  In that time we have heard many common questions. Here are the top few:  

Q. What is the difference between your original FFP-TF tunable filter design and the newer FFP-TF2 design?

A. The main difference is that the PZT in the original design is oriented such that the actuator increases the cavity length with increasing voltage. In the newer FFP-TF2 design, the PZT is assembled in a fixture outside of the cavity in such a way that the actuator decreases the cavity length with increasing voltage, resulting in a more stable filter. For high speed tuning applications such as in a swept laser, the original FFP-TF design is still recommended.

Q. What is the difference between your FFP-I product and your FFP-TF product?

A. The FFP-I product is a fixed interferometer with a fixed cavity length. The FFP-TF and FFP-TF2 products have an air-gap inside the Fabry-Perot (FP) cavity to allow the cavity to be tuned through the use of a piezoelectric element (PZT). Changes in cavity length by modulation of PZT drive voltage yield corresponding changes to the FP resonance frequency.

Q. Is it possible to tune a FFP-I fixed interferometer?

A. Yes, by altering the temperature of the cavity. Through the use of an internal TEC package (option 080), FFP-I fixed interferometers can be slightly tuned (~0.8nm) if the free spectral range (FSR) of the filter is between 10 and 100 GHz. If the filter's FSR is outside the 10-100 GHz range, the user may opt to design an external heating and cooling system to tune the fixed filter.

Q. Can I get connectors installed on my filter?

A. Yes. Our standard connector offerings are FC/SPC (option 060), FC/APC (option 061), SC/SPC (option 062), and SC/APC (option 063). These connectors are actually connectorized pigtails which are fusion spliced onto the fibers exiting the filter. Another option (option 065) is to connectorized the fiber exiting the filter directly with FC/APC connectors. While this option is more expensive, it does avoid the fusion splice protection sleeve.

Q. What sort of voltage supply do I need to control the PZT in your tunable filters?

A. We recommend a 0-70 volt supply with the bias set around 35 volts. A FFP-TF filter should move across one free spectral range (FSR) for each 12 volts (max) applied. The FFP-TF2 should move at least one FSR with no more than 18 volts max. Operating the filter at higher bias voltages will cause the PZT's to be more efficient and therefore less voltage will be needed to move the filter one complete FSR. Micron Optics offers a filter controller (FFP-C) so that the first time user can quickly get up and running learning the behavioral characteristics of the tunable filter.

Q. Can I run the FFP-TF or FFP-TF2 tunable filter in open loop?

A. Because of the inherent drift properties of the PZT, Micron Optics recommends that the user will need to use the filters in a closed-loop manner. The closed-loop system design will most likely need to incorporate a known optical reference, and an optical switch to poll back and forth between the system signal and the reference signal in order to monitor and correct for any drift coming from the PZT.

Q. How much optical input power can your filters handle?

A. The amount of optical input power that our filters are capable of handling is inversely proportional to the finesse value of the filter. For instance, a filter with a finesse value of 200 will be capable of handling up to 100mW, while a filter with a finesse value of 1000 will be able to handle 30mW.

Q. Can I get filters from Micron Optics that operate in non-telecom regions of the spectrum?

A. Yes. Micron Optics has the ability to custom design and produce special mirrors that can operate anywhere from 400nm to 2000nm. A non-recurring engineering (NRE) fee is typically charged for these kinds of special designs.

Q. Why does the bandwidth change as I tune the filter?

A. The bandwidth variation is a function of the dielectric mirror layer structure. Alternating high and low index materials produces a reflector but only over a finite wavelength range. First generation filter designs allow operation centered at a specific wavelength (+/- 3% of center wavelength). Micron Optics has many newer and more complex designs that are able to accommodate much wider tuning ranges with minimal bandwidth variation. Contact Micron Optics if you have a specific bandwidth variation requirement.

Q. How fast can I operate the tunable filters?

A. For the FFP-TF2 design, Micron Optics guarantees that the filter can be scanned back and forth across 1 free spectral range at up to 800Hz. For the original FFP-TF design, Micron Optics guarantees that the filter can be scanned back and forth across 1 free spectral range up to 2500Hz. Many of our customers have achieved even higher scan rates, but Micron Optics does not guarantee error free operation of the filters at these higher rates.