CML and EML see eye to eye

We believe that CML™ (Chirp Managed Lasers); can be a very complementary product (and in some cases even a substitute product) to EML (Electro-Absorption Modulated Lasers).

EMLs have been used as a primary transmitter type in many 10Gb/s module applications. Modules such as the current Finisar 40km/80km TDM and DWDM XFP today use EMLs.

CMLs have slightly different characteristics than EMLs. Specifically, CMLs have back-to-back eye diagrams with lower mask margin than EMLs. On the flip-side, CMLs offer higher output optical power, and much better performance after transmission through fiber.

However, the key point is that CMLs are fully interoperable with EMLs.

In fact, our Chief Scientist Daniel Mahgerefteh published a paper in Lightwave in November 2008 dedicated to this topic. The article, aptly named “CML and EML see eye to eye,” discusses how the two transmitter types are fully interoperable.

So, although CMLs may not win the beauty contest when looked at under a scope, CMLs will enable better performance over fiber. As Daniel so eloquently put it, “Using mask as a measure of performance is similar to trying to determine the speed of two race cars by comparing how clean their engines appear rather than by which of them crosses the finish line first.”

In addition, CML-based XFPs have very nice features not offered by EML:
• CMLs can work on 50GHz ITU grids (currently EMLs are only available on 100 GHz grid);
• CMLs offer 4x100GHz/8x50GHz narrow tunability;
• CMLs support L-band as well as C-band (EMLs only currently support C-band); and
• Inherent immunity to SBS (Stimulated Brillouin Scattering)

What are your thoughts on the differences between CMLs and EMLs? Feel free to let us know in the comments below.

Introduction to CML Technology

CML™ is a technology that I plan to discuss in many more postings.

CML stands for Chirp Managed Laser. CML technology was originally developed by a company called Azna which Finisar acquired in 2007. The original team remains largely intact and we are continuing to develop this technology in our Wilmington, MA facility.

One of the key advantages of CML technology is that it has extremely good dispersion properties over fiber. For example, Finisar has developed an XFP that is dispersion tolerant to over 200km of SMF at 10Gb/s. At OFC this year, we also published a post-deadline paper showing the results of some testing where we were able to make an SFP+ at 10Gb/s run without dispersion compensation over 360km of fiber. Of course, this 360km SFP+ is not a real product, only a lab experiment, but it puts into light some of the awesome capabilities of this technology.

We are developing several flavors of CML transmitters (including TOSA and butterfly packages) in our Wilmington, MA facility, and all of these will be manufactured in our Shanghai facility. Many of those will end-up in Finisar modules.

If you have any questions or thoughts about Finisar’s CML technology, feel free to comment here.