Optical Feedback in Laser Packages

John Spencer About The Author

Thu, Sep 01, 2011 @ 09:09 AM

Our customers specify Photodigm precision lasers for many reasons, including their narrow linewidth, tunability, spectroscopy-certified frequency, stability, and pulsing characteristics, among others.  To meet the requirements of our customers, Photodigm offers our lasers in several package types, including free space and fiber-coupled designs.  The optimal package depends on how it is used.

The butterfly package is favored for many applications, since the laser is sealed in a hermetic package aligned to polarization-maintaining (PM) single mode fiber.   An internal thermo-electric cooler maintains a constant operating temperature. The fiber can then be routed as needed to deliver the beam the product.  It is truly “plug and play.”  However, in certain cases the butterfly may not be the most suitable package. 

One of the differentiating features of the Photodigm DBR laser is its narrow linewidth.  For spectroscopic applications requiring narrow linewidth, feedback from anywhere along the fiber will be reflected right back into the laser, destabilizing it.  This well known effect is the subject of a Photodigm application note.  Even feedback as low as -55 dB from an FC/APC connector will set up external cavity modes and cause the laser to hop between these modes. These cavity modes may be separated by a few 10’s of MHz, and such instability can make the device unsuitable for applications requiring tuning over these frequency ranges. 

Butterfly packaged lasers used in telecom (near IR wavelengths between 1300 to 1550 nm) are packaged with an isolator inside the package. Located very close to the laser facet, the isolator completely eliminates feedback instabilities.  Isolators for the near IR are small and have low losses.  They fit very nicely into the butterfly package.  Low loss isolators for wavelengths in the 700 to 1100 nm range are large and bulky, and they cannot be mounted within the butterfly package. 

Photodigm recommends free space optical designs with or without external isolators for applications requiring the narrowest linewidths.  This includes resolving the hyperfine structure of alkali atoms. 

For other applications, including Raman spectroscopy and spectroscopy probing the natural absorption bands of alkali atoms, the butterfly package will be suitable, as will free-space designs.  Seed laser applications where the laser exhibits time dependent broadening are not affected by feedback, and the PM fiber butterfly provides the ideal package to connect to the fiber amplifier. 

A detailed discussion of feedback is available in a Photodigm application note, Optical Feedback in Photodigm’s Laser Diodes. 


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