The VCSEL Advantage

There are basically two types of lasers used in fiber optic transmission systems today:  edge-emitters and surface emitters.  The most prevalent high-speed edge-emitters are FP and DFB.  The beam emission for these devices is parallel to the substrate.  In the case of VCSELs, the light is emitted vertical to the substrate.

Short-wavelength VCSELs have been a part of the optical networking world since Honeywell introduced them in the early 1990s.  The devices were adopted quickly to replace unreliable and costly CD lasers in the datacom market.  Due to their inherent low cost, low power and small size, VCSELs became the light source of choice in enterprise networks.  They are also credited with enabling Gigabit transmission in that space and are currently being used in over 90 percent of Fibre Channel and Ethernet transceivers.   By taking what was learned at 850 nm and extending it to 1310 and to 1550-nm wavelengths, companies are now starting to show that they can drastically reduce the cost and size of transmitters.

Historically, longer wavelengths of 1300 to 1700 nm have been more difficult to produce in the VCSEL construction, because of the refractive index of InGaAsP.  This material is generally used for edge-emitters in these wavelengths and does not change very much with composition, which makes it difficult to produce components.  But in the early 2000s, development of different combinations of III-V elements led to long-wavelength VCSELs.  Several manufacturers such as Bandwidth9 with its tunable, 1550-nm VCSEL;  Cielo Communications, E2O and Picolight with their  1310-nm VCSELs had proven that lasers supporting wavelengths higher than 980 nm were possible to produce in volume. In fact, before JDSU acquired Picolight, it had several multi-Gigabit transceivers it produced with 1310-nm VCSELs. JDSU has since discontinued that line and, it seems, its support of 1310-nm VCSELs.

Key advantages of the VCSEL at production-level include the following:

• High Yields. VCSELs can be processed with as many as 20,000 individual lasers on a three-inch wafer.  Even if 20 percent of these are lost due to processing yields (a high number by VCSEL manufacturers standards), this is still a far higher yield than their edge-emitting cousins.
• Testing at the Wafer Level.  VCSELs can be tested before the wafer is diced.  Most edge-emitting lasers (FPs and DFBs) must be cleaved from the wafer and packaged before they can be tested, and are therefore tested individually, which increases processing costs and decreases yields significantly
• Easier Coupling and Packaging.  Another important advantage of the VCSEL structure is that its circular cross-section gives better control over beam size and divergence than for edge-emitters, allowing for much easier coupling of the fiber to the VCSEL output and easier alignment during packaging.

Table I shows a comparison of FP, DFB and VCSEL solutions, an availability status, and a list of some of the component manufacturers.

 

Table I:  Comparison of  VCSEL, FP and DFB Technologies
Attribute	VCSEL	FP	DFB
Cost	Low	High
Optical Output Power
Power Consumption
Size	Small (vertical construction)	Large (planar construction)
Mode Stability	Good	Fair
Testing	On chip	Packaged assembly
Manufacturing	Easy (20, 000 devices on 3 inch wafer)	Difficult
Packaging	Easy
Coupling to Fiber	Efficient	Inefficient
Modulation	Direct up to 12 Gbps	Direct only up to 2.5 Gbps then must be external
Drive circuitry	Simple	Complicated
Monolithic Integration	With receiver and electronic driver components	With other optical components
Suppliers	850 nm:  Agilent, Aerius Photonics, Applied Optoelectronics, Inc., Oclaro, Optowell, Emcore, EpiWorks, FCI/MergeOptics, Finisar, JDSU, Raycan, TE Connectivity, VI Systems 1310 nm:  Alight Technologies, Beam Express, JDSU, Raycan, Vertilas, VI Systems 1550 nm:  Raycan, Princeton Optronics, Vertilas	Agilent, Excelight, Finisar, JDSU, Modulight, Oclaro, OpNext	Agilent, Bookham,  JDSU, Excelight, Finisar, Fujitsu, Oclaro, OpNext

Long-wavelength VCSELs have started to emerge again mainly due to some new process technologies now being leveraged. Two companies stand out to me with their technology developments of long-wavelength VCSELs – Vertilas and VI Systems.