Chirp-managed laser enables 10 Gbits/sec without DCF
By Matt Vincent
Available in a 13-pin/GPO butterfly package and an XFP pluggable transmission optical subassembly (TOSA) mini-package, Azna’s DM200 1550-nm chirp-managed directly modulated laser (CML) is designed for upgrading metro networks without using dispersion compensation. The device essentially pairs a DFB laser with the company’s optical spectrum reshaper (OSR), which filters FM to AM conversion with a high extinction ratio (ER) and low transient chirp.
“Chirp, essentially, is frequency which is changing in time. Until now, DFB performance has been masked by transient chirp,” explains Daniel Mahgerefteh, Azna’s chief technology officer at Azna (Wilmington, MA). “Transient chirp really distorts the signal after propagation-that’s why they’ve been limited to a few kilometers of transmission at 10G. What we’ve realized is that hidden in there, there’s also adiabatic chirp, which follows the optical power. Let’s say with a 1-0-1 bit sequence, the ones are going to have higher amplitude; they’ll be blue-shifted relative to the zeroes, which have lower amplitudes-they’ll be red-shifted. And the magnitude of that shift is called the ‘adiabatic chirp.’ The laser, at least at adiabatic chirp, kind of follows the bit sequence.”
According to the company, optical networks typically require some form of dispersion management for transport over more than very short distances; that frequently means dispersion compensation fiber (DCF). The company contends that in addition to being fixed in dispersion value, DCF is often bulky, has high loss, and is prone to cause nonlinearities at high optical powers. Moreover, when networks are reconfigured, DCF engenders a need to re-manage dispersion.
“Service providers are interested in upgrading their systems from 2.5G to 10G, but they don’t want high capital expenditure up front,” observes Mahgerefteh. “If they want to go to 10G, they’re going to have to put in DCF. The fiber’s lossy, so they’d have to add amplifiers to compensate for the loss. This would make sense if you’re putting in a system and populating it with 32 channels right up front; the cost would make sense, because compensating fiber compensates for all the channels at the same time.
“But there isn’t that much capacity upgrade at 10G. You’re going to have one channel, maybe two-you’re not going to have that many. So they’d much rather have a technology that says, okay, here’s a transmitter, you don’t have to put in DCF, just take out your 2.5G cards at the transmit-receive side, and you don’t have to take a truck roll. The upgrade cost now becomes the cost of changing your line cards.”
The DM200 is design to make such a scenario possible. The device supports extended long-reach (LR) and metro regional applications. According to the company, compared to lithium niobate duobinary products, its CML is more tolerant to low optical-signal-to-noise ratio and provides equipment vendors with footprint, power consumption, and cost savings advantages.
“I think one of the reasons the progress has been fast, and we’re getting deadlined in already, is that the technology lends itself to being relatively easy to manufacture. We don’t have fabrication, because we can use chips that are out there-the chip is a standard 10-Gbit DFB chip,” explains Mahgerefteh. “As far as performance, nobody else can beat this 200 km. There’s duobinary modulation that can also reach the same distance, but it uses lithium niobate and some pre-coating, and it’s complicated, and it can certainly never fit inside a TOSA. So as far as power consumption and size, our performance is a little bit better.”
At OFC/NFOEC in March, Azna demonstrated how its CML could generate continuous detectability of 10-Gbit/sec data transmission through 675 km of standard singlemode fiber, without relying on the use of DCF. “This was a collaboration with Lucent,” explains Mahgerefteh. “They were using our DM200. In addition to that, they were using electronic dispersion compensation. They also had their own tunable dispersion compensators at the receiver. With all of that together, they stretched [the transmission] to 675 km.”
The company’s other 1550-nm CML products include the DM80 for 10-Gbit/sec 100-km (1,600-psec/nm) LR, metro regional, and long-reach (LH) applications and the DM600 for 2.5-Gbit/sec 640-km metro regional and LH applications. The products are currently available for sampling.
Azna’s CML technology extends the reach of 10-Gbit/sec directly modulated lasers to beyond 100 km by managing the lasers’ various chirp properties.
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