x
Optical/IP Networks

Laser Blazers

1550 nanometers lies in the second low-loss window of optical fiber, which is mostly used for long-haul applications including DWDM. In DWDM, the price of the laser isn't a significant part of overall equipment costs, so the potentially low cost of a VCSEL doesn't have as much impact. To offer a competitive advantage, VCSELs will have to accommodate an additional requirement -- tunability (see Tune In!).

This means that 1550nm VCSEL makers have to achieve a double whammy. First they have to make a VCSEL that works, then they have to incorporate tunability. It's no wonder that 1550nm devices are still at a pretty early stage of development.

Only two companies look like they've got both sets of skills:

Bandwidth9 Inc.
  • Bandwidth9 announced its tunable VCSEL in September 2000 (see Bandwidth9 Claims Laser Breakthrough). The startup has ignored the problem of incompatible mirrors by simply growing GaAs on top of InP anyway. The result is a so-called "pseudomorphic" interface. The epitaxial layer sticks to the substrate "just like amorphous silicon sticks to glass in solar cells," according to Wupen Yuen, Bandwidth9's director of precision epitaxy.
  • Experts in the field question the reliability of the laser, saying that the dodgy interface might generate lots of defects in the structure that would kill light emission. Yuen counters by saying that though the structure does have defects, they don't matter because they don't occur in the active region.
  • Bandwidth9's laser is not particularly high power. The best reported to date is 0.45 milliamps.
  • Maximum reported lasing temperature is only 55 degrees C
  • Tuning is achieved with a micro-electro-mechanical system (MEMS) cantilever. The startup has experience in making these for short-wavelength VCSELs, and it should be relatively straightforward to translate this to a long-wavelength device.

    Coretek Inc.
  • Uses a separate optical pump to generate light. (Note, the device is not wafer bonded). The reason for optical pumping is it makes it possible to generate light from a larger volume of active material than is possible using electrical pumping. As a result, the VCSEL is high power -- at least 2 milliwatts.
  • Because there is more active material, the mirrors don't have to be as highly reflective. This eases the manufacturing requirements.
  • Fairly high pump powers (50 milliwatts at 980nm) are needed. The pump is expensive.
  • Tuning range of 50nm using MEMS
  • Nortel bought Coretek (see Nortel Gambles $1.43 Billion On Tunable Lasers).

    It's also worth mentioning that Novalux Inc. plans to make a tunable VCSEL. It says the device will be electrically tuned (not MEMS) but doesn't give any details. This device is probably still on the drawing board. Novalux says the development could take a couple more years (see Laser Startup Bags $109 Million).

    Although these are the only companies with concrete plans for tunable VCSELs, there is plenty of activity in universities and research institutes. Different methods for making 1550nm VCSELs are emerging, and commercial development is sure to follow. Some research groups worthy of note include:

    University of California, Santa Barbara
  • Larry Coldren's group is working a single-epitaxy approach for making long-wavelength VCSELs (see VCSEL Breakthrough at UCal). (Incidentally, Coldren is one of the founders of Agility Communications Inc., a tunable laser manufacturer).
  • John Bowers's group is using a wafer-bonding approach. However, unlike other wafer-bonded VCSELs, Bowers has managed to reduce the resistance of the bonded joint so that the laser can be electrically pumped. Adil Karim, who works in the group, says these devices have the best high temperature performance -- twice as much as the single-epitaxy device made in Coldren's group, he claims. Another innovation is to make arrays emitting at multiple wavelengths for use in DWDM systems. (Bowers is one of the founders of VCSEL manufacturer Alvesta Inc., which may provide a route to market for this technology.)

    Walter Schottky Institut, Technical University of Munich
  • Markus Amann's group has achieved the lowest threshold current and voltage for a 1550nm VCSEL to date. The VCSEL structure is made in a two-step epitaxial process. One mirror is InP-based, the other is made of low index dielectric. Maximum lasing temperature is around 50 degrees C.

    Previous Page
    8 of 8
  • Stockholm syndrome 12/4/2012 | 8:59:42 PM
    re: Laser Blazers Well, no mentioning of players like:
    - Altitun
    - NUFO
    I am not convinced that the picture is complete.
    Please fill me in!


    Peter Heywood 12/4/2012 | 8:59:40 PM
    re: Laser Blazers The article was an in-depth look at VCSELs - vertical cavity surface emitting lasers. Altitun (which was acquired by ADC) and NUFO don't make VCSELs.

    I did a more general survey of tunable lasers a while back. See:

    http://www.lightreading.com/do...

    Use our search engine on "tunable lasers", "altitun" or "NUFO" you'll find loads more info.

    HOME
    Sign In
    SEARCH
    CLOSE
    MORE
    CLOSE