Jack --
Here's a good description of the RAMAN modules:
sdli.com
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SDL Leads the Way with Telco-Grade Raman Sources
SDL, the market leader in pump laser sources for Erbium Doped Fiber Amplifiers (EDFAs), is now supplying a new 1455 nm 1.5 W pump laser module specifically designed to address advanced optical systems that utilize distributed Raman amplification for dense wavelength division multiplexed (DWDM) systems.
Today's long-haul networks transmit data at rates up to 10 Gbits/sec using wavelength-division multiplexing technology. Overall system distances are typically thousands of kilometers. Traditionally, fiber-optic networks have used architectures that consist of terminal equipment at each end with a series of repeaters to regenerate signals. Repeaters are vital to a network's ability to transmit data over very long distances at low bit error rates. However, optical amplifiers are increasingly used in place of repeaters. The amplifiers allow for a simpler design since, unlike repeaters, they do not contain significant amounts of active high-speed electronics.
While EDFAs are used today to increase transmission distance, another novel approach is now available to increase the range of repeaterless technology. Distributed Raman amplification uses the transmission fiber to provide gain. The Raman shift of silica fiber is about 13 THz above the pump wavelength, which corresponds to approximately 60 to 100 nm, depending on the pump wavelength. One very important benefit of Raman amplification is that it can be used over a wide range of wavelengths, rather than within a fixed wavelength region as with erbium-doped fiber technology.
For transmission in the 1550-nm range, Raman amplification occurs when the transmission fiber is pumped with very high optical power at approximately 1455 nm. Therefore, by using a high-powered pump source at the receive terminal (in excess of 1 W), the transmission fiber itself will provide gain in the 1550-nm window. Additional gain of up to 15 dB, equivalent to approximately 100 km extra distance, can be realized. This technique is referred to as "distributed Raman amplification" due to the fact that unlike a conventional EDFA, where amplification occurs in a single discrete amplifier module, the gain is spread out, or distributed, over a significant fraction of the transmission wavelength.
One of the first practical applications of distributed Raman amplification in telecommunications networks is in repeaterless submarine systems. Future applications could include deployment in very high bit rate (40Gb/sec) DWDM systems. In addition, Raman amplification offers the potential of increased system data capacity by allowing amplification between 1300 and 1650 nm, thereby supporting a greater number of wavelength channels.
The critical technology for the distributed Raman amplification approach is the availability of a very high power pump laser source. Discrete pump laser modules today are only capable of supplying hundreds of milliwatts into fiber. Telecommunication-grade frequency multiplexed single-mode pumps, utilizing multiple pump modules, offer fiber coupled power up to 500 mW. However, this output is insufficient for the needs of longer-range repeaterless applications. For technical and economic feasibility, a source with output power greater than 1 W is required. Recently developed Raman fiber lasers, such as the SDL-RL30-1455, however, do deliver the required power at the necessary wavelengths.
The SDL-RL30-1455 Raman fiber laser contains several key elements to achieve the high output power required for these two applications (See schematic diagram below). Naturally, the multimode diode laser pump sources, which pump the fiber laser, are one of these key elements. Each diode operates at a wavelength of approximately 920 nm and has an output power on the order of 1 W. Use of multiple diode pumps is an important facet of the design, since this allows for redundancy to meet the possibility of "soft fail" (non-catastrophic failures) within the unit.
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Even though reliability requirements for terminal equipment are less severe than for submerged equipment, it is extremely important that the laser sources meet telecommunication reliability standards.
The laser source used in the SDL-RL30 is the SDLO-4000 series broad area pump laser. The diode used in the SDLO-4000 series multimode laser module has undergone extensive lifetesting. Estimated FIT rates for the multimode LD chip are 1500 FIT at Pout of 1.0 W/25øC and 2500 FIT at Pout of 1.2 W/25øC. Laser failures typically can be grouped into two broad categories: gradual degradation or wearout and sudden random catastrophic failure. For high-powered pump lasers in the 920 nm wavelength region, the sudden catastrophic failure mechanism is the dominant failure mode.
Packaging of the laser module must also meet telecommunications standards. The SDLO-4000 series module has a standard telecommunications-style 14-pin butterfly package. The packaged modules meets Bellcore standards for shock, vibration, temperature cycling, thermal shock, 5000 hours of accelerated aging and 2000 of high temperature storage as described in Bellcore TR-NWT-000468.
In addition to the laser modules, other elements play important roles in the performance of the pump laser source. For example, the control electronics provide the interface to the user's system and include laser diode drivers for pump sources. The individual outputs from the broad-area laser diode modules are fed into a multimode combiner, which merges the light into a single multimode fiber for pumping the fiber laser. The diode lasers pump a Yb-doped double-clad fiber laser that delivers high optical power into a single-mode fiber at approximately 1100 nm. The output of the fiber laser is fed into a cascaded Raman resonator cavity that employs a low-loss fiber Bragg grating. The resonator and gratings perform successive Raman shifts of the output wavelength of the fiber laser. Depending on the desired output wavelength, the Raman fiber laser can be designed to operate at 1455 nm for Raman applications.
Typically, the SDL-RL30 can be run at an output power of 1.5 W, but can be scaled to lower power to enable better overall system reliability. For example, when the unit is operated at 1 W output, the pumps are driven at lower power levels, which increases their lifetime significantly. The SDL-RL30 Raman laser source also provides excellent power conversion efficiencies, typically 45% from the 1110-nm to the 1455/1480-nm output bands and approximately 20% from the 920-nm output bands of the pump lasers. (Typical output characteristics of the RL30 are shown below.)
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SDL is now shipping initial production volumes of the SDL-RL30-1455. Donald R. Scifres, SDL, Inc.'s chairman and chief executive officer, has stated "This new product supports our efforts to target growing market opportunities and leverage our technology to broaden our product offerings for communications networks. In the development of the SDL-RL30-1455, we utilized existing SDL expertise to rapidly introduce what we believe is a reliable, state-of-the-art product in this emerging market. This introduction also continues our efforts to expand our product line to respond to the expected growing demand for telecommunications and Internet traffic bandwidth which distributed Raman amplification addresses."
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